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1.1140879.pdf

Undulator engineering for synchrotron radiation applications J. M. Slater, S. C. Gottschalk, F. E. James, D. C. Quimby, K. E. Robinson, and A. S. Valla Citation: Review of Scientific Instruments 60, 1881 (1989); doi: 10.1063/1.1140879 View online: http://dx.doi.org/10.1063/1.1140879 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Progresses of synchrotron radiation applications at the NSRL Rev. Sci. Instrum. 66, 1836 (1995); 10.1063/1.1145798 Infrared synchrotron radiation instrumentation and applications Rev. Sci. Instrum. 63, 1535 (1992); 10.1063/1.1143014 Circularly polarized synchrotron radiation from the crossed undulator at BESSY Rev. Sci. Instrum. 63, 339 (1992); 10.1063/1.1142750 New undulator and conventional lines at the Wisconsin Synchrotron Radiation Center (invited) Rev. Sci. Instrum. 60, 1441 (1989); 10.1063/1.1140959 Perspectives on micropole undulators in synchrotron radiation technology Rev. Sci. Instrum. 60, 1796 (1989); 10.1063/1.1140907 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.120.242.61 On: Sat, 22 Nov 2014 07:35:19Undulator engineering for synchrotron radiation applications J. M. Slater, S. C. Gottschalk, F. E. James, D. C. Quimby, K. E Robinson, and A.S. Valla Spectra Technology, Inc .• 2755 Northup Way. Bellevue. Washington 98004-1495 (Presented on 29 August 1988) Six undulators have been designed and built by STI since 1980 for synchrotron and FEL applications. Several design concepts, producing successively higher fields, have been developed during this period. A wedged-pole hybrid design has been demonstrated to yield the highest field to date for a given gap-to-wavelength ratio. A simple method of reducing fieid errors has been demonstrated on the wedged-pole hybrid, and it may lead to significant cost reduction through relaxation of mechanical and magnet tolerances. INTRODUCTION Spectra Technology, Inc. (STI) has been actively involved with FEL technology since 1979, when a major U.S. pro gram series began in Seattle, Washington. These programs have been directed toward the development of efficient visi ble and IR PELs in a series of technology demonstration experiments. During the course of this work, extensive capa bility has been developed in FEL physics, systems engineer ing, undulators and optical cavities. There has been special emphasis on undulator (or wiggler) engineering. Six undu Iators have been delivered to various customers with one more currently in construction. These devices range from 50 em to 10 m in length, have periods from 2 to 8 em, and fields to 10 kG. They are used in both FEL and synchrotron emis sion applications. During the continual improvement of the undulator over these nine years, STI has concentrated on obtaining the highest possible magnetic field strength with simultaneous high field quality. This has lead from development of pure permanent magnet systems, \ to hybrids of permanent mag net with vanadium permendur poles,2 to a new wedged-pole hybrid.3 The wedged-pole hybrid produces the highest fields to date for a given gap-to-wavelength ratio. Both radiation resistant samarium cobalt and the new higher-strength neo dymium-iron-boron magnets have been used. The measurement capability necessary to certify undu lator coherence over the full device length has been devel oped. Coherence is a strict requirement for FELs and is de sirable for synchrotron emission when low-emittance beams are used, but it is not easily achieved due to material and mechanical imperfections. Typically, field adjustment after assembly is necessary to achieve full coherence, and an inex pensive, but accurate, tuning technique for adjusting the magnetic field to the ideal values under each pole has been developed. This article highlights the high field strength wedged pole design and a tuning method, called shim tuning, to sub stantially reduce field errors. I. HIGH FIELD STRENGTH WEDGED~POLE CONCEPT The rare-earth permanent magnet (REPM) hybrid un dulator (or wiggler) was originally proposed by Halbach4 as a means for achieving high-quality high-strength periodic magnetic fields. This concept is gaining widespread ac ceptance both as an insertion device for synchrotron radi ation generation and for use in free-electron lasers. The prin cipal advantages of the REPM-steel hybrid relative to the pure-REPM undulator include higher magnetic field strength at small gap-to-period ratio and higher field quality by making the field distribution less sensitive to magnet in homogeneities. The use of wedged poles has now been demonstrated as a means for increasing the field strength of the hybrid. The wedged-pole3 configuration can cause the magnet surface which faces the gap to be driven to the full magnet coercivity He' thus resulting in higher on-axis field strength. Pole satu ration is avoided by increasing the cross-sectional areas of the pole tip without sacrificing magnet volume. Thus, the design concept has the potential for both higher on-axis field strength and improved field uniformity by operating the poles farther from saturation. In addition, widening the pole tips reduces the harmonic content of the field distribution. It should be noted that wedged poles have previously been put to use,5 but the geometric configuration of the permanent magnets was not modified to exploit the advantages of the wedged-pole shape. The geometry of the wedged-pole concept and its field is compared with the more conventional pure-REPM and hy brid undu!ator concepts in Fig. 1. The pure-REPM undula tor, used as the reference, consists of an array of permanent magnet blocks, whereas the magnets are sandwiched between highly permeable steel poles of rectangular cross section in the conventional hybrid geometry. In the pure REPM device, the field distribution is determined by the strength and magnetic orientation of the magnet blocks. The wedged-pole concept shown is an improvement which is intended to alleviate some of the limitations that occur in the basic hybrid geometry. In the conventional hy brid, the on-axis field strength is maximized when the poles are considerably narrower than the magnets. This not only leads to considerable higher-order harmonic content in the field distribution, but also implies that the achievable field strength is limited by pole tip saturation. In Fig. 1, the pure-REPM reference system is assumed to have square blocks with unity fill factors. For both hy brids the average magnet operating point is taken to be ap proximately O.2B, (see Ref. 3 for additional detail). For each full gap (g) to wavelength ().) ratio, the relative advan- 1581 Rev. Sci. Instrum. 60 (7), July 1989 0034-6748/89/071 8tU -04$01.30 @ 1989 American Institute of Physics 1881 ." .-.. ,." ""·.·.".7.-•.•.• , ••.•.• :.~.:,:.:.~ •••• ' •• .:.:-:,:.;.:.;.: •.• ,";'.:.:.:.:,:.;: .•• ' •••.• ~.:;:.;.:-;.; •..••••• > ....... :.;.:.: •••••••••• ;.:.;.:.:.;.; ••••• ,..'.:.:.:.:.:.: •••••••• ~.~.:.;.:.:-:., •••••••• :.:.:.;.:.;-:.:, ••••• '.~.:.:.:.:.;.;.;0.', •.•.. ,.;0 ..• ;.: . .'..... ..;-; .... _._ ',' .,"'" •. , .....•.. "._ •...... ' .•....... _ ..... ; ..... -; .....•...• "._._ .• ! .•...•..... This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.120.242.61 On: Sat, 22 Nov 2014 07:35:19--- o ELECTROMAGNETIC {19BSt PAlADiN I Pure-REPM Conventional Hybrid Wedged-Pole Hybrid I) ___ L __ i ___ -" ___ L __ -'---.--' 0.2 0.4 0,6 9/AW tage of the hybrid and wedged hybrid is shown, The STI Nos. I and 2 undulators indicated are the pure-REPM ge ometry with the latter using oversize blocKs. THUNDER and NISUS are STI undulators with conventional and wedged-hybrid geometries, respectively. Also shown is an electromagnetic undulator of the Paladin experiment. 6 At a typicalg/ A ratio of 0.35, the conventional hybrid has a 28% advantage over the reference, and the wedged hybrid has a 45% advantage over the reference. The reason for the advantage of the wedged pole is shown in the calculated plots of Fig. 2, exploiting the quarter-period boundary conditions. The field in the con ventional hybrid is limited by pole tip saturation. This prob lem is reduced with the wide pole tip ofthe wedged geometry while the magnet thickness is increased at the opposite end. An additional benefit of the wider pole tip is a reduction of third harmonic content of the field. FIG. 2. Field plots show pole tip saturation is reduced with wedged pole. 1882 Rev. ScLlnstrum., Vol. 60, No.7, July 1989 EUI1ElwEl 8w8m8 FIG. L Comparison ofundulator geome tries and relative field strengths. The pure-REPM with square blocks and unit fill factor is taken as a reference for each full gap (g) to wavelength (It) ratio. II. SHIM TUNING FOR FIELD ERROR REDUCTION In practice, the undulator field quality is limited by the presence of several undesirable factors, most notably trajec~ tory (steering) errors, phase-shift errors and higher-order moment errors, such as improper quadruple moments or excessive sextupole. These imperfections are caused in part by inhomogeneities in the permanent magnets, imperfect poles and mechanical misplacements. The errors become more critical for longer systems, leading disproportionately higher costs. Discussions of the allowable magnetic field error toler ances can be found in Refs. 7 and 8. In those papers, dipole errors that lead to trajectory errors are considered; in Ref. 7, these errors are shown to have different tolerances depend ing on whether the undulator radiation is required to be co herent or incoherent. For the FEL application, coherence is required, whereas in synchrotron applications, the electron emittance in some cases precludes coherence, independent of the undulator errors. For the coherent case, there is the gen eral requirement that the wiggler errors be sufficiently small so that the phase space occupied by the electron beam is less than the phase space occupied by a diffraction-limited pho ton beam. Also in Ref. 7, it is shown that the dipole error tolerance, if expressed in terms of the error of the integrated dipole field, is dependent on the number of wiggler periods and in many cases independent of the photon wavelength and e-beam energy. These considerations are for dipole errors which lead to trajectory errors oflow spatial frequencies, that is, for orbit errors that occur over a substantial fraction of the wiggler length. A separate consideration is required for high spatial frequency errors. An example of such an error would be the errors remaining after the overall electron trajectory is cor rected at several points along the wiggler length. In the limit that the trajectory is corrected at very frequent intervals, every few periods, for example, these remaining errors are essentially phase errors (or time-of~flight errors for the elec tron) rather than trajectory errors. Separate ca1culations9 have shown the RMS errors at each pole as small as several tenths of a percent can be important even when the trajector ies are otherwise perfect. High Power beamlines 1882 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.120.242.61 On: Sat, 22 Nov 2014 07:35:19There is particular emphasis on reducing the errors of the wedged-pole hybrid wiggler, since it produces roughly at 15% higher field than the more conventional straight-pole hybrid and 40% higher than the samarium cobalt systems without poles. The higher-field strength is important since the FEL gain-extraction productlO scales as B2. Up to now, there has been no simple scheme for elimi nating, or tuning out, the hybrid's field errors, partly because individual errors are not easily traced to specific magnets or poles. If specific errors can be identified, then magnets and poles can be relocated compensating locations. Such musi cal-chair tuning schemes are poody suited to high precision assemblies with special fix turing for the large forces involved and are labor intensive. Methods are dearly needed for achieving substantially lower error levels than what has been demonstrated to date. The techniques used to get from the first lO-ttm devices to the long-undulator 0.5-and I-flm FELs (Ref. 2) are not suitable for further extrapolation. These methods consisted of ( 1) use of more stringent mechanical tolerances through precision grinding and thermal control, and (2) a narrowing of the acceptance criteria for the permanent magnets. Me chanical tolerances are already in the O.OOl-in. range for these large structures, and further magnet selection will be come prohibitively expensive due to decreased yield. What is needed is a simple, inexpensive method of tuning a wiggler to the desired fields after it has been assembled. A promising candidate for tuning is the newly demon strated field shimming technique. Thus far, it has been ap plied only on a short wiggler prototype to tune out dipole errors in the plane of the primary field, although with devel opment, any moment in either plane might be corrected. The basic concept is that thin iron shims are used to selectively shunt a small fraction of the field lines from regions where the field is higher than desired. Proper placement of the shims results in a unifonn field of slightly lower strength, about 1 %, than the average initial field. The geometry is shown in Fig. 3 using one wavelength of the wedged-pole configuration, although the concept has general applicability to aU wiggler types. For this geometry, the shims are placed in the shallow recess on the flat tips of the magnets, shunting field lines from one pole to another as indicated in the figure. Clearly the effect of shunting field lines between poles is to reduce the field on axis. Depending on the local field, the shi.ms vary in thickness from 0 to ap proximately 0.5 mm. With the large scalar potential differ ence between the poles, the shims are completdy saturated and the number of field Hnes shunted is determined simply by their thickness. The field signature from a single pair of shims (Le., top and bottom), away from the ends ofa iong FtG. 3. Shim placement in wedged-pole hybrid wiggler. The primary field component (hol low arrows) can be controlled with the shunted field (solid ar rows). 1883 Rev. SCi.lnstrum., Vol. 60, No.7, July 1989 300. · lPolQ fIo~a · t t " • 200. ~ " .,...+~ .., 0 100. ;;; !i ;Ji .300 -too. i -4.00 -2.00 .000 2.00 4.0n Z (em) FlG. 4. Shim signature for single pair of shims (as in Fig. 3) in a long wiggler assembly. wiggler, is shown in Fig. 4. The effect is confined largely between two poles, and it has been shown experimentally that this signature is approximately linear in the shim thick ness and additive with that of shims on neighboring poles. Given that the effect is predictable, one can clearly gen erate alogrithms that modify the field in some predeter mined way. Thus far, the shims have been used successfully to modify the RMS level of field errors in a short wedged pole undulator with a 3.9-cm period, 1.4-cm fun gap, and 5.6-kG on-axis peak field. That data is used here as an exam ple. It was desired to reduce the level of kick errors, defined as the error in half period field integrals under each pole, so that their RMS deviation could be reduced from the initial 1.3% to a much lower value. A computer alogrithm was devised to use the measured, uncorrected field and then identify the proper location and thickness of shims to counteract the measured errors. The shims are easily hand placed and self-attaching in the loca tions indicated in Fig. 3. Afier one shim set plus one iter ation, the result is shown in Fig. 5 for the central 18 poles of the 26-pole undulator. The initial kick errors are shown as the points connected by the dashed lines. The large sinusoi dal field and any offset has been taken out and only the resid ual errors are shown. The corrected field is shown by the solid line connected by the solid line. In this case, the kick error went from an initialleve1 of 1.3% to a value of 0.11 %. It is interesting to note, from the shim symmetry of Fig. 3, that there will be no net dipole movement created by the 200 RESIDUAL E~ROAS After SI'i!r.lmlng / " ,..../ I 200 / / ,\ A f \ I ' '.00 W.O fI. ! \ I / Inlilal Error .. t.3% RMS ShlIfifOOIJ Error,. O.IV. RMS t4.0 HAU' • P~J;IOO NUMS£R 18.0 FIG. 5. Measured comparison of field errors before and after shimmmg of IS·pole section ncar the center of a NISUS prototype module. Crosses are half period field integrals before shimming, solid line are after shimming. Solid and dashed lines are for visual reference only. High Power beamlines 1883 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.120.242.61 On: Sat, 22 Nov 2014 07:35:19CROSS SECTION shims, and as a consequence one wonders if the shims can be used to correct dipole errors. The answer is yes, and the explanation makes a constructive point concerning various length scales, or spatial frequencies, of errors and various error correction schemes. The shimming algorithm can be used to redistribute any arbitrary distribution of kick errors to a new distribution, In the simplest case, this may be move ment of a dipole error, associated with a single pole, to a location of a dipole correction coil, so that the correction and error then occur at the same location. In some systems, it is convenient for the correction coils to have large axial extent, and in that case the shims are used to redistribute the errors to a new distribution, which is simply a constant (position independent) error, This constant error is then removed with an externally applied field oflarge axial extent, Le., low spatial frequency. Thus, the shims are a means of converting the high spa tial frequency errors to lower spatial frequencies, where they can be dealt with by other methods, In the case of steering errors, the other method would be a long steering coil and, in the case of phase errors, the other method might be an ad justment of taper or gap in each module of the undulator; these modules being perhaps I-m long. With this in mind, we recall that for both curves of Fig. 5, the constant offset, i.e., the average dipole error, has been removed before the RMS errors were calculated. The shim algorithm adjusted whatever dipole errors are present to be a constant, axially uniform. This can easily be cancelled by constant bias field, and the RMS errors were calculated in this spirit. One demonstrated method of applying this bias field is the use of correction coils lying between the vacuum system and poles as shown in Fig. 6, To allow for e-beam diagnostics in the particular vacuum system considered, it is convenient that these steering coils be restricted to approxi mately 0.5 m in length and be repeated at meter intervals. 1884 Rev. SCi.lnstrum., Vol. 60, No.7, July 1989 FIG. 6. Vacuum system with imbedded steering correction. Under these circumstances, the shim algorithm would be adjusted to move all of the dipole errors to the locations under the correction wires, and leave no errors in the areas that have no correctors. The short undulator section for the measurements reported here was treated as ifit were entirely within a portion of constant correction field. III. SUMMARY The wedged-pole hybrid geometry has been demon strated to produce higher fields than the conventional straight-pole hybrid, having an advantage of approximately 15% at ag/ A of 0.35, as well as lower harmonic content. An inexpensive tuning method for the hybrid systems has also been demonstrated. 'J. M. Slater, J. Adamski, D. C. Quimby, T, L. Churchill. L. Y. Nelson, and R. E. Center, IEEEJ. Quantum Electron. QE-19, 374 (1983). 2K. E. Robinson, D, C. Quimby, J. M. Slater, T. L. Churchill, and A. Valla, ill Proceedings of the Eighth International Free Electron Laser Conference, Glasgow, UK, September 1986 [Nue!. lnstrum. Methods A 259, 62 (1987)]. 'D. C. Quimby and A, L. Pindroh, Rev. Sci. lnstrum. 58, 339 (1987), 4K, Halbach. J. Phys. (Paris) 44, Cl (1183). 5G. A. Kornyukkin. G. N. Kulipanov, V. N. Utvinenko, N. A. Mesentsev, A. N. Skrinsky, N. A, Vinokurov, and P. D, Voblyi, Nuc!. lnstrum. Meth ods A 237, 281 (1985). "G. A. Dies, in Proceedings of the Ninth International Free Electron Laser Conference, Williamsburg, VA, September 1987 (to be published). 7J. M. Slater, in Proceedings afthe 1987 IEEE Particle Accelerator Confer ence, Washington, D.C., March 1987 [IEEE Catalog No, 87CH2387-9, p. 479 (1987)]. "B. M. Kincaid. J. Opt. Soc, Am. B 2,1294 (l9SS). 9S. C. Gottschalk, Spectra Technology, Inc. and others (unpublished cal culations) . lOJ. M. Slater, AlP Conf. Proc. 130,505 (I985). High Power beamlines 1884 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.120.242.61 On: Sat, 22 Nov 2014 07:35:19

1.101524.pdf

Microstructure of epitaxial ErBa2Cu3O7−x thin films grown on MgO(100) substrates by rf magnetron sputtering J. Chang, M. Nakajima, K. Yamamoto, and A. Sayama Citation: Applied Physics Letters 54, 2349 (1989); doi: 10.1063/1.101524 View online: http://dx.doi.org/10.1063/1.101524 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/23?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Superconducting YBa2Cu3O7− x thin films on metallic substrates prepared by RF magnetron sputtering using BaTiO3 as a buffer layer AIP Conf. Proc. 251, 96 (1992); 10.1063/1.42061 The effects of secondary particle bombardment on ion beam sputtered thin films of Y1Ba2Cu3O x deposited on MgO (100) AIP Conf. Proc. 200, 102 (1990); 10.1063/1.39062 Microstructure of epitaxially oriented superconducting YBa2Cu3O7−x films grown on (100)MgO by metalorganic decomposition Appl. Phys. Lett. 55, 286 (1989); 10.1063/1.102406 Superlattice modulation and epitaxy of Tl2Ba2Ca2Cu3O1 0 thin films grown on MgO and SrTiO3 substrates Appl. Phys. Lett. 54, 1579 (1989); 10.1063/1.101387 Microstructures of YBa2Cu3O7−x superconducting thin films grown on a SrTiO3(100) substrate Appl. Phys. Lett. 52, 841 (1988); 10.1063/1.99302 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.193.242.161 On: Tue, 09 Dec 2014 21:02:42Microstructure of epitaxial ErBa2CUa07_X thin films grown on MgO (100) substrates by rf magnetron sputtering J. Chang, M. Nakajima, K. Yamamoto, and A. Sayama Yokohama R&D Laboratories, The Furukawa Electric Co., Ltd .. 2-4-3, Okano, Nishi-ku. Yokohama 220, Japan (Received 21 February 1989; accepted for publication 4 April 1989) The microstructural properties of superconducting ErBa2Cuj07 _ x films on single-crystal MgO substrates are studied by transmission electron microscopy. The as-grown films are single-crystal-like and are composed of subgrains of 0.1-0.2 pm in size. Due to annealing, the dislocations at the sub grain boundaries disappeared. The annealed films are epitaxial with either the a or the b axis of the ErRa2Cu,07 _ x unit ceil along < 100) directions of the MgO substrate. The stress caused by lattice mismatch is relaxed by the formation of misfit dislocations at the film/substrate interface. Various techniques have been applied to the preparation of epitaxial high-temperature superconducting oxide films. The best results for superconducting YRa2Cu307 x films have been obtained using single-crystal SrTi03 substrates due to the good lattice match between the substrate and film, H and consequently, most of the microstructure studies ofYBa1Cu307 x mms were made on SrTi03 substrates. S-7 However, despite the fact that MgO (100) substrates have a rather large misfit (-8 %) with RE Ba2Cul07 _, [rare earth metal (RE) 1 films, they are used because the films produced are suitable for real applications. 4 In this letter, we report the successful epitaxial growth of ErBa1Cu}07 _ x (hereafter referred to as ErBCO) films on MgO (100) sub strates by using rf magnetron sputtering. The annealed films have a zero resistivity temperature 1:. of 82 K and a critical current density Je > 105 A/cm] at 77 K. Furthermore, we studied the microstructure of these as-grown and annealed films by transmission electron microscopy. The as-grown films are single-crystal-like and are composed of subgrains of about 0.1-0.2 pm in size. After the 900 °C heat treatment, most of the dislocations at the subgrain boundaries disap peared. The films are epitaxially grown with either the a or b axis of the ErRCO cell parallel to the (100) of MgO sub strates, without an in-plane rotation ';,9 ofthe (001) planes of the ErRCO unit cell. The stress caused by lattice mismatch is relaxed by the formation of misfit dislocations at the film/ substrate interface. We have grown almost completely c-axis oriented ErRCO films on MgO (100) substrates by using rf magne tron sputtering, In order to reduce the res puttering effect, the sputtering was carried out under high pressure (Ar/02 = 111,80-100 mTorr). The substrate was heated to around 650 "C during the deposition and the deposition rate was about 2 nm/min. From x-ray 2e diffraction analysis, as grown films are oriented with the c axis perpendicular to the suhstrate surface. The c-axis lattice parameter was measured to be 11.75 A( ± 0.02 A). After annealing at 900°C for 2 h in oxygen flow, the c-axis lattice parameter of a 0.3-0.4 /-lm thick film became smaller and reached 11.68 A. By using a standard dc four-probe transport method, films with T: = 82 K and.le > 105 A/cm2 (with a best value 4X 105A/ em2) at 77 K were obtained. In order to further investigate the microstructure of these c-axis oriented films, transmission electron micro scopy (TEM) observations for both the as-grown films and the annealed films were carried out. Figure 1 shows the plan view image of an as-grown film. Figures 1 (a) and 1 ( c) arc the bright field image and the selected area diffraction pat tern from the area shown in Fig. 1 (b), respectively. In Fig. 1 (a) a granular structure is observed. However, the diffrac tion pattern shows dear diffraction spots, corresponding to (100) and (010) planes of the ErBCO unit cell. No ring patterns characteristic of polycrystalline films are observed. Furthermore, bright field/dark field TEM images showed that dislocations occurred at the boundaries between the granular structure of Fig. 1. However, twins did not occur in the as-grown film. Figure 2 shows the cross-sectional image of the as-grown film. Boundaries between c-axis oriented FIG, 1. TEM plan-view images of an as-grown film: (a) bright field image (RF.I): (b) B.EI with selector aperture: (c) ,elected area ditrraction pat tern from (b). 2349 Appl. Phys. Lett. 54 (23), 5 June 1989 0003-6951/89/232349-03$01.00 @ 1989 American Institute of Physics 2349 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.193.242.161 On: Tue, 09 Dec 2014 21:02:42FIG. 2. TEM cross-sectional image of an as-grown film. The inset shows the selected area diffraction pattern from an area which includes the MgO sub strate. domains can be observed. The diffraction pattern of the inset of Fig. 2 shows that neighboring grains are oriented parallel to the c axis. From these TEM observations, we can con clude that the as-grown films are single-crystal-like and are composed of small subgrains with an average size of 0.1--0.2 pm. The c-axis orientation spreads within a small deviation angle. The standard deviation angle of the c axis measured by the x-ray rocking curve is sharp and reaches a constant value of 0.45° (resolution angle ~0.3°) for films thicker than 50um. However, Tc of the as-grown films is below 77 K. In order to provide a proper amount of oxygen into the films, post-annealing was employed. Figure 3 shows the cross-sec tional image of the annealed film. Comparing Figs. 2 and 3, the boundaries of c-axis domains are seen to have disap peared due to annealing. The lattice fringes parallel to the interface correspond to the (002) planes ofthc ErRCO film. Furthermore, bright field/dark field TEM images showed that the twins of 20 to 30 nm in width occurred due to the tetragonal-orthorhombic phase transformation as the film was cooling down from 900 °C to room temperature. The inset of this figure shows the selected area diffraction pattern from the area which includes the MgO substrate; the inci- FIG. 3. TEM cross-sectional image of an annealed film. The inset shows the selected area diffraction pattern from an area which includes the MgO sub strate. 2350 Appl. Phys. Lett., Vol. 54, No. 23, 5 June 1989 FIG. 4. High-resolution image of the film/substrate interface. dent electron beam is parallel to (010) direction ofthe MgO substrate. The diffraction spots of Er BCO (100) can be ob served clearly. This result means that instead of an in-plane rotation of the (00l) planes of ErBCO to fit the lattice pa rameter ofMgO (100), the film grew epitaxially with either the a or b axis along the MgO < 100) direction. Figure 4 shows a TEM high-resolution image of the interface. It indi cates that the ErBCO film was epitaxially grown aligned with the MgO {IOO} lattice. Arrows point to the misfit dislo cations which occurred regularly along the interface. The stress caused by lattice mismatch has been relaxed, presum ably by the formation of these misfit dislocations. However, an amorphous layer was not formed at the film-substrate interface. Before annealing, we found that most of the sub strate surface was rather rough with hills and valleys of depth about 0,1 pm. However, apart from the roughn~ss at the interface with depth ~ 12 A as shown in Fig. 4, most of the interface of the annealed film became smooth. We believe that the formation of a smooth interface is attributed to a reaction that may have been taken place at the interface between the MgO substrate and ErBCO film during anneal ing. In summary, epitaxial ErBCO films have been success fully grown on MgO (100) substrates by rfmagnetron sput tering. As-grown films are single-crystal-like and are com posed of small subgrains. With proper annealing, films with 1~ = 82 K and Je > 105 A/cm2 at 77 K were obtained. These films were epitaxially grown with their a or b axis along the MgO < 100) direction. The stress caused by lattice mismatch is relaxed by the formation of misfit dislocations at the inter face. The authors wish to express great appreciation for fruit ful technical discussions and funding bestowed by a group of Japanese electric power companies-Tokyo Electric Power Co., Tohoku Electric Power Co., and Hokkaido Electric Power Co. Iy' Enomoto, T. Murakami, M. Suzuki, ane! K. Moriwaki, Jpn. 1. AppL Phys. 26, L1248 (1987). 2T, Tcrashima and Y. Bando, Appl. Phys. Lett. 53, 2232 (1988). .Ip. Chaudhari, R. H. Koch, R. B. Laibowitz, T. R. McGuirt:. and R. J. Gambino. Phys. Rev. Lett. 58. 2684 (19X7). Chang eta/. 2350 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.193.242.161 On: Tue, 09 Dec 2014 21:02:424J. K wo, M. Hong, D. J. Trevor, R. M. Fleming, A. E. White, R. C. Farrow, A. R. K01'tan, and K. T. Short, App!. Phys. Lett. 53, 26R3 (1988). 'C. II. Chen. H. S. Chen, and S. H. Liou, App!. I'hys. Lett. 53, 2339 ( 198R). "D. M. Hwang, 1.. Nazar, T. Vcnkatesan, and X. D. Wu, App!. Phys. Lett. 52, 1834 (1988). 2351 Appi. Phys. Lett., Vol. 54, No. 23, 5 June 1989 .•.••••••• X"; •••••••••••••••••• :.':':.:.:.~.:.:.:.:.:.:;;;-.: O;.:.:.:.: ••• ;.:.;';".O;-".· ••• ·.·;·;>.·.·.O;~.v.·.·.·.·.·.· ................................. ,. •. ;-0:.:.-;0:.:.:-;.;.;.; •••• ' ••••••••••••••••••••••••• ' ••••••• ' •.••••• -; •••••• 'lB. M. Clemens, C. W. Nieh, J. A. Kittl, W. L. Johnson, J. Y. Josefowicz, and A. T. Hunter, App!. Phys. Lett. 53, 187l (1988). "I. Bloch, M. Hciblum, and Y. Komem, Appl. Phys. Lett. 46,1092 (1985). "M. Eizenberg, D. A. Smith, M. Heiblum, and A. Segmuller, App!. Phys. Lett. 49, 422 (1986). Chang etal. 235i This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.193.242.161 On: Tue, 09 Dec 2014 21:02:42

1.343793.pdf

Rate equation analysis of microcavity lasers H. Yokoyama and S. D. Brorson Citation: J. Appl. Phys. 66, 4801 (1989); doi: 10.1063/1.343793 View online: http://dx.doi.org/10.1063/1.343793 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v66/i10 Published by the American Institute of Physics. Related Articles Emitter injection in terahertz quantum cascade lasers: Simulation of an open system Appl. Phys. Lett. 100, 102102 (2012) Random laser action in dielectric-metal-dielectric surface plasmon waveguides Appl. Phys. Lett. 95, 231114 (2009) Modal characteristics of terahertz surface-emitting distributed-feedback lasers with a second-order concentric- circular metal grating J. Appl. Phys. 106, 053103 (2009) Dynamic modeling of a midinfrared quantum cascade laser J. Appl. Phys. 105, 093116 (2009) Heisenberg algebra, umbral calculus and orthogonal polynomials J. Math. Phys. 49, 053509 (2008) Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors Downloaded 07 Oct 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsRate equation analysis of microcavity lasers H, Yokoyama8) and S. Do 8rorson Department 0/ Electrical Engineering and Computer Science and Research Laboratory afElectronics, Massachusetts Institute a/Technology, Cambridge, Massachusetts 02139 (Received 21 October 1988; accepted for publication 24 July 1989) We describe the light output properties of single mode lasers having cavity dimensions on the order of the emitted wavelength. A simple rate equation formula is derived for a four-level laser assuming enhanced spontaneous emission into the cavity. These rate equation analyses show that increasing the coupling of spontaneous emission into the cavity mode causes the lasing properties to become quite different from those of usual lasers having cavity dimensions much larger than a wavelength. We find that the lasing threshold disappears, the light emission efficiency increases, relaxation oscillations do not occur, and the dynamic response speed is improved. It is shown that the spontaneous emission rate alteration caused by the cavity plays an essentially important role for these characteristics. L INTRODUCTION The alteration of a material's spontaneous emission rate in a cavity 1,2 has recently attracted much attention as a fun damental means of studying the interaction of matters with vacuum field fluctuations. To date, many experiments have demonstrated such effects, using Rydberg atoms,3-9 a solid state laser material,1O organic dyes,11.12 and semiconduc tors. 13, 14 Altering the spontaneous emission, however, is also interesting from the device point of view. For example, Yab lonovitch has proposed the utilization ofinhibited spontane ous emission in semiconductor lasers for extremely low cur rent operation.15 On the other hand, Kobayashi et al. proposed the concept of thresholdless lasers with the full confinement of spontaneously emitted photons in closed mi~ ero-optical cavities (microcavities) .16 Although the concept of spontaneous emission rate alternation has not been taken into account in his idea, enhanced, rather than inhibited, spontaneous emission should occur in that situation. For recent surface emitting semiconductor lasers, very short cav ity structures have been fabricated. 17,18 Changes in sponta neous emission properties could play an important role in these devices. In this paper we describe an analysis for light output properties of microcavity lasers, based on rate equations which are simply derived taking into account the enhanced spontaneous emission caused by a microcavity. It is shown that, if the coupling ratio of spontaneous emission into the one cavity mode is sufficiently high, the laser oscillation characteristics are greatly changed, including the threshold behavior, the influence of nonradiative processes on input output conversion efficiency, and the dynamic modulation response. iI. EMISSION RATE ENHANCEMENT First, we discuss the enhancement of spontaneous and stimulated emission rate in a closed microcavity. Here, we assume that only one resonant cavity mode overlaps the gain bandwidth (free space transition width) ofthe laser medium 3) Presently on leave from Opto-Electronics Basic Research Laboratory, NEC Corporation, Miyukigaoka, Tsukuba 305, Japan. because of the very small cavity volume (of wavelength di mension) ; thi.s is the origin of the spontaneous emission rate alteration. Two cases of microcavity operation exist. In the first case, the gain bandwidth is much less than the cavity mode band width. According to Fermi's golden fule, the spontaneous emission rate Ac in a cavity is represented in this case by2 (1) with (2) (3) where A is the spontaneous emission rate in free space (here after, we use the word "free space" as the meaning of "with out cavity"), Pc (vo) [PI (Va) ] is the mode density for a final photon state in a cavity (in free space) at transition frequen cy Va. Qis the cavity quality factor, cis the velocity oflight, V is the mode volume (in this case, cavity volume), H is an interaction hamiltonian, Ii) is the initial state without pho tons, and I f) is the final state with one photon. In (1), 1/ is the enhancement of the spontaneous emission caused by the cavity, Although recent interest has been focused on sponta neous emission, (1) is also valid for stimulated emission. This becomes obvious with the quantization of electromag netic field. In this procedure, the overall photon emission rate Rc for an atom (or a molecule) in a cavity is expressed as (4) where s represents the number of photons in the cavity mode in the initial state. The second case of micro cavity operation occurs when the cavity resonance peak is sharper than the gain band width. This often occurs in atomic systems and causes the "golden rule" to break down. In this situation, coherent ef fects, such as Rabi oscillations,S or "one atom maser" oper ation6 occur. Expressions (1) and (4) may also be adapted to such broad transition linewidth systems as organic dyes, certain 4801 J. Appl. Phys. 66 (10), 15 November 1989 0021-8979/39/224601-05$02.40 @ 1989 American Institute of PhySiCS 4801 Downloaded 07 Oct 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionssolid-state laser materials, and semiconductors, as long as the cavity mode separation width is much broader than the transition linewidth in free space, and the cavity resonance width is broader than the inverse of the radiative lifetime. For example, in the case of a semiconductor material, based on the discussion in Ref. 19, the spontaneous emission rate enhancement ratio 11 can be expressed as 1/ = f" fc(v)dv 1100 ff(v)dv == l"'Pccv)P(v)dv /1'0'0 Pf(v)P(v)dv, (5) where fc (v) [rf (v)] is the spontaneous emission rate for emitting photons of energy hv with (without) a cavity, Pc (v) [PI (v) 1 is the mode density for photons, and P( v) represents the transition rate per mode. In (5), as a practical approximation, the free-space emission rate is given by PfCvo)P(vo)t:.P, where Vo is the photon frequency at the emission peak, and AP is the FWHM of P( l'). In a micro cavity, Peev) is more sharply peaked than P(v), and the spectrally integrated emission rate can be expressed as Pc (v~) )P( vb )¥c> where vb is the photon energy at a reso nancepeak, APe is the FWHM ofPe (v). However, it should be noted that when absorption loss is negligible Pc (v)!::.pc is nearly equal to PI (v)1.\1', where t:.v is the cavity mode sepa ration width. Thus, the ratio ?J is roughly expressed by (6) If v,; = vo, the ratio is ~ av/ AP. This shows that the spec trally integrated emission enhancement depends on the cav ity mode separation. Note that if PC vo)/ P( vb) > /::"v/ /::,.p (i,e., off resonance cavity), 1/ becomes less than 1; thus the spontaneous emission is inhibited instead of enhanced. The discussion based on (5) is also applicable to a homogeneous ly broadened two-level system, if the phase coherence time is much shorter than the population lifetime. Furthermore, if we assume that there are several cavity modes within bandwidth, it can be easily found by carrying out the integration of (5) that the spontaneous emission rate does not change. Therefore, from the mode density point of view, it is understood that we do not have to take into ac count the spontaneous emission rate alternation for a con ventionallargc-size (compared to the wavelength) cavity laser. However, even in that case, it can be seen that spectral ly partial emission enhancement occurs within each cavity mode resonance width, and emission inhibition takes place between cavity resonance peaks. Classically, the spontaneous emission rate alteration can be understood as caused by the change in radiation resis tance experienced by a classical dipole when inserted in a cavity. A complementary view is that it is the result of reso nant enhancement of the electromagnetic field by multiple reflections in the cavity, when the roundtrip time oflight is much shorter than the phase coherence time of a dipole. This effectively increases the coupling of the dipole to the field. Drexhage adopted this method to calculate the spontaneous emission rate modification of thin dye films. I I Furthermore, semiclassical laser equations may be able to describe the be havior of microcavity lasers if spontaneous emission pro- 4802 J, Appl. Phys., Vol. 66, No. 10, 15 November 1989 cesses are properly involved.20 However, as outlined by ex pressions (1)-(6), a description based on mode density alteration simplifies the discussion, and consistently treats both spontaneous and stimulated emission in laser rate equa tions, as long as we are not concerned with the laser's fre quency and phase. m. RATE EQUATIONS To use rate equations based on Fermi's golden rule, we must insure that an adiabatic approximation is valid. That is, no transient coherent effects occur. The phase coherence time of organic dyes and semiconductors are in the femto second range, while the inverse of the Rabi frequency in a cavity will be on the order of 1-10 ps for usual optical pump ing rates ( < 1 MW cm-2). Thus, such transient coherent phenomena as superradiance, optical nutation, etc., will not occur for these materials, and a simple rate equation ap proach is valid. To begin, we may study the rate equations of a single mode microcavity laser, which is completely enclosed by the reflector. For such a device, the spontaneous emission rate is given by (4). Assuming an ideal four-level laser material (the decay rates of the highest state to the upper laser state, and of the lower laser state to the lowest state are extremely fast), with no nonradiative processes and no inversion satu ration, the rate equations can be written as dn -=p~Ac(s+ On, dt ds ~ = Ac (s + l)n ~ ys, elt (7) (8) where n is the number of excited atoms (molecules) in the cavity of volume V, p represents the pumping rate, and y is the damping rate for photons from the passive cavity. The static solution of these equations is simple but noteworthy: s=p/yand n=yp/[Ac(p+r)]. We see that the light output increases linearly with increas ing pumping for all pumping rates. In other words, this de vice works as a "thresholdless laser." As we will show, this occurs because all photons are emitted into the one single cavity mode. Note that n does not proportionally increase with pumping increase, and this behavior is different from that of ordinary spontaneous emission, in which the excited state population n linearly increases with pumping increase. This thresholdless nature is not necessarily the same as the concept of one atom maser,5 in which at most only one atom exists in the cavity at a time and whose behavior is not simply described by an argument based on the golden rule. Although enhanced spontaneous emission CAe >A), is not the necessary condition for the lack of a threshold, the consequent increase in the spontaneous emission rate has some great advantages from the device point of view. For one thing, the response speed of the device to dynamic modula tion will be improved, as a result of the increased spontane ous emission rate. Furthermore, the influence of nonradia tive depopulation processes will be decreased since the spontaneous emission lifetime will be much shorter than the nonradiative lifetime. Another interesting feature of the H. Yokoyama and S. D. Brorson 4802 Downloaded 07 Oct 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsthresholdless laser is that relaxation oscillations will not oc-4.-----.-----.------.------:::. cur. This happens because there is no threshold, so the pumping energy is always immediately converted to laser output. Thus, there is no mechanism for storing energy in the laser medium, which is necessary for relaxation oscilla tions. This is confirmed by a standard small-signal analysis, which reveals that there is no resonance frequency for relax ation oscillations. So far, we have considered the case of a completely closed cavity resonator. Now we would like to generalize to the case of an open resonator. We assume there is still one cavity mode, but now other modes exist which correspond to photons leaving the open cavity. We assume that the sponta neous emission into the cavity mode is stilI enhanced, but the free-space modes have the free-space spontaneous emission rate. This corresponds to the case discussed in Ref. 8. We take the ratio of the solid angle subtended by the cavity mode to the free space modes to be po Thus, /3 is proportional to the inverse of the mode volume V; from another view point, it is the light-material interaction strength, If a concentric cav ity9 is assumed, the value of /3 simply corresponds to the solid angle which an atom sees the cavity mirrors at the cav ity center. Also taking into account nonradiative depopula- tion processes, the rate equations can be represented as dn -=p -(I-f3)An +/3A;O +s)n -rn, (9) dt ds - = tU ; (1 + s) n -ys. (10) dt Here, s is the number of photons coupled to the cavity mede, and r is the nonradiative depopulation rate. In this case, A ; represents the enhanced spontaneous emission rate for the cavity mode, and is related to the free space rate by A ; = FA, where the enhancement factor is F. In the limit /3 -> I, we have a closed cavity, and A ; reduces to Ac in Eq. (1).8 Note that in a broad bandwidth material, F depends on the cavity mode separation width as discussed in Sec. II. Thus, F de pends on the cavity size, as does/3. Therefore, to get an large /3A ~ value, the cavity should be quite small, and to avoid the photon lifetime (lIy) decrease, the reflectivity of cavity mirrors should be quite high. For an open microcavity with wavelength dimensions. although a plane mirror Fabry Perot configuration could provide a rather large value for f3 ( ~ 0.1), the achievement of a microscopic confocal or con centric Fabry-Perot configuration would improve the value of /3. Full confinement of spontaneous emission into the cav ity mode might be realized with microsphere or microcube cavity structures. IV. NUMERICAL RESULTS AND DISCUSSIONS We have carried out numerical analysis using (9) and (10). Steady-state solutions of (9) and (10), for an ideal four level laser (r = 0), are shown in Fig. 1. In the figure, to compare the output properties for cavities with different /3 (I.e., different mode volume), light (photon) output SOUl excited state population N, and pumping P are, respectively, normalized as 4803 J. AppL Physo, Vol. 66, No.1 0, 15 November 1989 3 0 4 z 2 z 0 !-(3-000001 <:I: 0,01 -l ::J 001 13 CL -~ 0 2 :3 4 PUMPING P FIG. L Light output SO,,! and population inversion Nvs pumping Pofmi cTOcavity four-lcvellaserso A co, 109 S -', F = 10, r = 10'2 s -', and r = o. r /3A; Sout = ---s = (JF5, A Y /3A' NT C =--11, Y and 1 (J A; /3F P=---p=-p. A Y r It is seen that as (J increases, the threshold disappearso In the mode point of view, for the (J.( 1 open cavity case, even though there is only one cavity mode, excited atoms are mostly coupled with free-space modes, and the cavity mode photons can only increase rapidly above "threshold" be cause of intensive stimulated emission. Thus, the phase tran sition (threshold) appears in the cavity mode output. (Note that in actual semiconductor laser devices, the spontaneous emission coupling ratio (J is 10-5_10-6 per cavity mode.) On the other hand, in the case of /3 = 1 (closed micro cavity), all the photons emitted couple into the single micro cavity resonance mode. Therefore, the emission process gradually changes from the spontaneous emission dominant one to the stimulated emission dominant one without a phase transition (threshold) 0 Although it may not be mean ingful to distinguish spontaneous emission and stimulated emission if there is no threshold, for convenience, we distin guish the emission rate proportional to s in the equation as stimulated emission. Therefore, for the pumping level shown in Fig. 1, the light emission process is dominated by the "en hanced spontaneous emission," because the unnormalized number of photon s = 0.4 at P = 4 is less than 1. The behav ior of the excited state population fl is also notable. For the case of an ordinary laser, with increasing pumping, n in creases until the lasing threshold level and then is clamped there. On the other hand, 11 of a thresholdless laser very slow ly increases with a pumping increase, and it reaches a con stant value at infinitely large pumping (the condition for s> 1 ) . As is shown in Fig .. 2, another noteworthy feature of a Ho Yokoyama and S. D. 8rorson 4803 Downloaded 07 Oct 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions4~----~--------'-------r-----~ ... " <> (f) :3 ~ 2 a... ~ ::> o o PUMPING P FIG. 2. Light output Sout vs pumping P of microcavity four-level lasers involving nonradiativc processes. A = 109 s-', F= 10, Y = 10" s-', and r = 1098-'. thresholdless laser is to maintain high output conversion ef ficiency, even if the nonradiative population lifetime is com parable or less than the free-space spontaneous emission life time. This occurs even while the lasing threshold of small /3 case markedly increases. This is easily understood, since in a thresholdless laser, the ratio of radiative depopulation rate to nonradiative one is greatly increased because of the en hanced spontaneous emission. It has also been found that for fixed p, increasing F also gradually removes the threshold. This is because of the sub stantial increase in the amount of spontaneous emission cou pled into the cavity mode. Concerning the dynamic properties of microcavity la sers, as discussed in Sec. III, higher frequency response is expected in thresholdless laser, because of the enhancement of spontaneous emission rates. Figure 3 shows a calculated result for microcavity four leve11asers with sinusoidal pump- 3r-----------------------------~ 2 ~ /3 " I (J) I-0 ;:) :3 a... I- ;:) 0 :2 ,ez 0.0001 o 4 TIME (ns) FIG. 3. Dynamic light output properties of microcavity fom-levellasers. A = 1095-', F= 10, r = 5 X 10" s-', f' = 0, Po O~ 2. The modulation fre quency of pumping is taken as! = 2 X 10'" s '. 4804 J. Appl. Pl1ys., Vol. 66, No. 10, 15 November 1989 ing modulation of P = Po( 1 -cos 211'ft). To clearly extract the effect of spontaneous emission coupling, the nonradia tive depopulation rate r is taken to be zero in this calcula tion. The enhancement factor is taken to be F = 50, since this value can be realized by a halfwavdength size cavity for semiconductors. In the small /3 case, a time delay for lasing and relaxation oscillations are observed. On the other hand, when f3 = 1, there i.s no relaxation oscillation (this is pre dicted from standard small-signal analysis), and the modu lation depth in steady state is much larger than that when/3 is small. It is noted here that the unnormalized average pho ton number in the cavity s = Soutl/3F is much larger for {3= 0.0001 case (s = 200) thanfor,8 = 1 case (s = 0.04). It is emphasized, therefore, that the response speed improve ment in the case of p = 1 is dominantly due to the decrease in spontaneous emission lifetime by a factor of F for the pumping level shown in Fig. 3. Although (9) and (10) are valid forfour-levella.';er sys tem, they are also approximately applicable to intrinsic semiconductors (with bimolecular radiative recombina tion). There, the spontaneous emission rate is represented by A = B r n (under the Boltzmann carrier distribution approxi mation), where Br is the bimolecular carrier recombination coefficient. When the calculation is performed using this expression for A, the features are qualitatively the same as for the case offour-levellasers. V. CONCLUSION In summary, rate equation analyses have been imple mented on static and dynamic output properties of micro cavity lasers, based on the concept of spontaneous emission rate enhancement in a cavity. Although our simple rate equation analyses can bring information only about output power, some attractive features of microcavity lasers have been predicted. Among these are the lack of threshold, the efficiency increase, and the high-speed response improve ment, when the coupling ratio of spontaneous emission into the cavity is sufficiently large. In these characteristics, the increase in spontaneous emission rate plays an essentially important role, Thus, it should be noted that the operational properties of single-mode microcavity laser are not correctly explained by simply counting the number of cavity modes, without taking into account the spontaneous emission rate alteration. Other aspects of microcavity lasers (oscillation frequency, linewidth, etc.) are also interesting subjects to study, but they should be discussed in the context of a semi dassical or a fully quantum mechanical analysis. ACKNOWLEDGMENTS The authors are grateful to Professor T. Kobayashi of Osaka University, Professor E. P. Ippen, Professor H. A. Haus, Professor D. Kleppner, and Dr. J. Wang of Massa chusetts Institute of Technology for their stimulating discus sions. The valuable comments of Dr. R. Lang, Y. Nambu, M. Suzuki, K. Nishi, T. Hiroshima, and T. Anan of NEC Corporation are also gratefully acknowledged. This work was supported in part at M.I.T. by the Joint Services Elec tronics Program Contract No. DAAL03-86-K-0002. H. Yokoyama and S. D. Brorson 4804 Downloaded 07 Oct 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions'E. M. Purcell, Phys. Rev. 69, 681 (1946). 2D. Kleppner, Phys. Rev. Lett. 47,233 (1981). 3A. G. Vaidyanathan, W. P. Spencer, and D. Klcppner, Phys. Rev. Lett. 47,1592 (1981). 4p. Goy, J. M. Raimond. M. Gross, and S. Haroche, Phys. Rev. Lett. 50, 1903 (1983). 'D. Meschede. H. Walther. and G. Muller, Phys. Rev. Lett. 54, 551 (1985). 6G. Rempe and H. Walther, Phys. Rev. Lett. 58, 353 (1987). 7W. Jhe, A. Anderson, E. A. Hinds, D. Meschede, L Moi, and S. Haroche, Phys. Rev. Lett. 58, 666 (1987). 'D. J. Heinzen, J. J. Childs, 1. E. Thomas, and M. S. Feld, Phys. Rev. Lett. 58,1320 (1987). 9D. J. Heinzen and M. S. Feld. Phys. Rev. Lett. 59, 2623 (1987). lOp. DeMartini, Phys. Lett. 115, 421 (1986). "K. H. Drexhage, in Progress in Optics, edited by E. Wolf (North Holland, Amsterdam, 1974), VoL XII, p. 165. '2F. DeMartini, G. Innocenti, G. R. Jacobovitz, and P. Mataloni, Phys. Rev. Lett. 59, 2995 (1987). "E. Yablonovitch, Phys. Rev. Lett. 51!. 2059 (I9!l7). I4H. Yokoyama, K. Nishi, T. Anan, and H. Yamada, Tech. Dig. of Topical 4805 J. Appl. Phys., Vol. 66, NO.10,15 November 1989 Meeting on Quantum Wells for Optics and Optoelectronics, Salt Lake City, March 1989, paper MD4 (unpublished). "E. Yablol1ovitch, T. J. Gmitter, and R. Bila!, Phys. Rev. Lett. 61, 2546 (1988). J('T. Kobayashi. T. Segawa, A Morimoto, and T. Sueta, Tech. Dig. of 43rd Fal! Meeting of Japanese Applied Physics Society, paper 29a-B-S, Sep tember 1982 (unpuhlished); T. Kobayashi, A Morimoto, and T. Sueta, Tech. Dig. of 46th Fall Meeting of Japanese Applied Physics Society, pa per 4a-N-l, October 1985 (unpublished) (both ill Japanese). 17J. L. Jewell, K. F. Huang, K. Tai, Y. H. Lee, R. Fischer, S. L. McCall, and A. Y. Cho, App!. Phys. Lett. 55, 424 (1989). "s. W. Corzine, R. S. Geels, R. H. Yan, J. W. Scott, L. A. Coldren, and P. L. Gourly, IEEE Photonics Tech. I,etL 1,52 (1989). '''n. c. Casey, Ir. and M. B. Panish, Heterostructure Lasers (Academic, New York, 1978), Chap. 3. 20 A semiclassical description of spontaneous emission in a cavity has been done in the following paper: J. J. Childs, D. J. Heinzen. J. T. Hutton, and M. S. Feld (unpublished). 21M. Sargent III, M. O. Scully, and W. E Lamb, Jr., Laser Physics (Ad dison-Wesley, Boston, MA, 1974), Chap. 8. H. Yokoyama and S. D. Brorson 4805 Downloaded 07 Oct 2012 to 152.3.102.242. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

1.343077.pdf

Experimental vortex transitional nondestructive readout Josephson memory cell Shuichi Tahara, Ichiro Ishida, Yumi Ajisawa, and Yoshifusa Wada Citation: Journal of Applied Physics 65, 851 (1989); doi: 10.1063/1.343077 View online: http://dx.doi.org/10.1063/1.343077 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in 3 ns single-shot read-out in a quantum dot-based memory structure Appl. Phys. Lett. 104, 053111 (2014); 10.1063/1.4864281 Experimental and theoretical response of distributed read-out imaging devices with imperfect charge confinement J. Appl. Phys. 107, 083917 (2010); 10.1063/1.3327412 Fundamental criteria for the design of highperformance Josephson nondestructive readout random access memory cells and experimental confirmation J. Appl. Phys. 50, 8143 (1979); 10.1063/1.325955 Meter ReadOut for Vibroscopes Rev. Sci. Instrum. 35, 232 (1964); 10.1063/1.1718787 Fast ReadOut Chronotron System Rev. Sci. Instrum. 28, 1010 (1957); 10.1063/1.1715790 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Tue, 25 Nov 2014 15:00:15Experimental vortex transitional nondestructive read .. out Josephson memory ceU Shuich! Tahara, ichiro Ishida, Yumi Ajisawa, and Yoshifusa Wada l.ficroelectronics Research Laboratories, NEC Corporation, 4-1-1, Miyazaki, Miyamae-ku, Kawasaki, Kallagawa 213, Japan (Received 7 April 1988; accepted for publication 20 September 1988) A proposal vortex transitional nondestructive read-out Josephson memory cell is successfully fabricated and tested. The memory cell consists of two superconducting loops in which a single flux quantum is stored and a two-junction interferometer gate as a sense gate. The memory cell employs vortex transitions in the superconducting loops for writing and reading data. The vortex transitional memory operation of the cell contributes to improving its sense discrimination and operating margin. The memory cell is activated by two control signals without timing control signals, Memory cell chips have been fabricated using a niobium planarization process. A ± 2i % address signal current margin and a ± 33% sense gate current margin have been obtah1cd experimentally. Successful memory operations of a cell driven by two-junction interferometer gates has been demonstrated. The single flux quantum operations of this memory cell makes it an attractive basic element for a high-speed cache memory. I. INTRODUCTION Josephson devices, with their high intrinsic switching speed and low-power dissipation, are promising circuit ele ments for future ultrahigh performance computer applica tions. In the Josephson computer, a high-speed cache mem ory is indispensable to complement the Josephson logic circuits which have picosecond-switching characteristics. In general, signal delay time through a memory array line is first-order proportional to the amount of stored flux quanta in the memory cell. I A single flux quantum memory cell, therefore, is an attractive basic element for a high-speed cache memory. Various kinds of single flux quantum mem ory cells have been proposed and examined experimental ly.2-4 A quantum loop cell proposed by Henkels et al.2•5 has been the object of particular study for a high-speed memory. This memory cell, however, has several problems. Interfer ometer gates with two control signals are used as write and sense gates in the memory cell. Since tolerances on all of a gate current and two control currents are equal for a maxi mum margin in these gates, it is difficult to approach a theo reticallimit of the operating margin. In the memory array, memory cell selection results from a coincidence of X and Y address signal currents. U nselected cells in the X and Y lines suffer from half-selected disturbance. Therefore, a large op erating tolerance in the memory cell is very important for increasing the discrimination between selected and unselect ed cells. In addition, a second problem for this cell is the fact that the necessity of timing sequence for address signals makes high-speed memory operations difficult. Supply of a timing signal requires a large timing margin, which prevents the circuits from reducing its cycle time. AdditionaHy, cell driving current levels are not equal in the memory cell, re quiring a different current level for signals such as address, data, and read/write conditions, This often reduces an inter connection margin between the cells and peripheral circuits. It is an important problem when memory circuits with large operating margins are constructed. In this paper, we discuss a single flux quantum memory cell, called a vortex transitional memory ceU.6 Its main fea tures are a large operating margin, a memory operation with no timing control signals and an almost equal current level for control signals. This memory cell is activated by X, Y address signals and a sense signal, and employs vortex transi tions in the superconducting loops for writing and reading data. The vortex transition in the superconducting loops coupled with the sense gate permits that operating margins for address signals are almost independent of a sense current margin. Therefore, the operating margin for the memory cell can be optimally designed to be its theoretical Hmit. High speed memory operations are possible because timing con trol signals are not necessary. The memory cell contributes to improve a margin for a total memory circuit since the applied control current levels are almost equal. The memory cell consists of two superconducting loops each of which stores a persistent circulating current corre sponding to a single flux quantum. The sense gate couples with one of the superconducting toops. The cell is fabricated by using Nb/ AIOJNb junctions and the niobium planari zation technique.7 The surface flatness for each layer results in high reliability. The basic circuit configuration is de scribed in Sec. II. The fabrication process and experimental results are presented in Sec. III. Conclusions are finally giv en in Sec. IV. II. CIRCUIT CONFIGURATIONS An equivalent circuit for the vortex transitional nondes tructive read-out (NDRO) Josephson memory ce!l is shown in Fig. 1. The cell consists of two superconducting loops (loop 1 and loop 2), each of which stores a single flux quan tum. The superconducting loop contains a Josephson june- 851 J. Appl. Phys. 65 (2), 15 January 1989 0021-8979/89/020851-06$02.40 @ 1 9S8 American Institute of Physics 851 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Tue, 25 Nov 2014 15:00:15ly Is IDC -----'l'""-4-...ro1fP---- I )( Mi ._~( '-' ""","'--.J-""""""""",,, LI JI RI FIG. 1. Equivalent circuit of a vortex transitional NDRD memory cell. L, = 5 pH, L2 = 4 pH, L, = 7 pH, L4 = 1 pH, I, = 0.2 rnA, 12 = 0.1 rnA, and I, ,= 14 = 0.1 rnA. (/,-1.,: critical currents of J,-J •. ) tion and inductance elements. Damping resistors R I and R2 are connected in parallel to junctions JI and J2, respectively, and provide suitable damping conditions.6 The sense gate is a two-junction interferometer gate, magnetically coupled with loop 2. Loop 1 stores information in the foml of a single flux quantum. The Josephsonjunction, J" included in loop 1 has a function in which a single flux quantum is caused to enter loop 1 when X and Y address signals are coincident. Reading of stored data can be accomplished by the loop 2 vortex transition, which depends on the stored data in loop 1, and the switching of the selected sense gate caused by the transition. Optimum design parameters are listed in Fig. 1. Here let us investigate the variation of the quantum phase differences B\ and 82 of the Josephson junctions J1 and J2 in the memory loops against the address signal currents Ix coupled with inductances L I and L2, and Iy injected into the loop 1 in Fig. 1. The flux quantization condition and the current continuation condition yield equations for I" [I and the quantum phase difference BI, 82 as (tPo/21T)( 8J + 2m1T -82 + 2n1T) = L2Iy + (Lj + L2) (lx -I, sin 8,) + L3I2 sin 82 , (1) (<1>01211')(82 -2mr) = L4ly -L4Ij sin 81 -(L3 + L4)I2 sin 82 , (2) where II and 12 are the Josephson critical currents of junc tion J1 and J2, respectively, <Po is the magnetic flux quantum, L1, L2• L3, and L4 are inductances in Fig. 1, and m and n are integers for the quantities of magnetic flux in loop 1 and loop 2, respectively. In these equations, we assume mutual induc tances MJ and M2 equal L I and L2, respectively. The stability condition produced by potential energy minimum is A JlIz cos ()\ cos 82 + A2I( cos Bj + A312 cos B2 + A4>0, (3) where Al = (L(L3 + LjL4 + L2L3 + L2L4 + L3L4)/L2L4, A2 = (<J>o/21T) (LI + L2 + L4)/(L2L4) , A3 = (tPo/21T)(L3 + L4)/(L2L4) , A4 = (<P0I21T)2/(LzL4) . From Egs. (1 )-( 3), we can obtain the threshold charaeter- 852 J. Appl. Phys .• Vol. 65, No.2. 15 January 1989 noise band -0.4 0.8 FIG. 2. Threshold curves of the memory loops on the (0,0), (1,0), and (0, 1) modes, along with hypothetical ± 10% variation of critical cur rents and inductance values. The dotted areas indicate the operating margin of Ix and I,. istics (Fig. 2). Figure 2 shows several parts of the threshold curves for the memory loops in the memory ceil. The hori zontal and vertical axes represent the address signal cur rents, Ix and Iy' respectively. The numbers in parentheses correspond to flux quanta in the memory loops. That is, (m,n) means m flux quanta in loop 1 and n flux quanta in loop 2. Point "0" is the memory operating origin and is defined by de powered current Ide' The (0,0) mode and (1,0) mode in the memory loops are respectively correspon dent to data" 1" and "0". Cell operations and the stability of the dynamics were established in Ref. 6. As shown in Fig. 2, the operating point moves from "0 "to "A" or"B" according to data on writing. On reading the stored information, the operating point moves from "0" to "e." The vortex state for the memory loop changes into the (0,1) mode only when the data" 1" is stored, and then the sense gate switches into a voltage state. After that, the vortex mode can return to the (0,0) mode at point "0" under the suitable damping conditions. In Fig. 2, the dotted areas indicate the operating regions for I~ and Iy for" 1", "0," writing and reading. The shaded areas illustrate thermal noise bands in a fashion similar to that described in Ref. 6. Minimum operating margins Ix = 0.16 rnA ± 14% and Iv = 0.18 rnA ± 14% are achieved, along with a hypo thetical ± 10% variation in the Josephson critical current and in the inductance value, while the optimally designed cells have address signal current margins Ix = 0.17 rnA ± 33% and Iv = 0.2 mA ± 33%. On reading the stored data, a vortex transition in the loop 2 causes the sense gate to switch. Figure 3(a) shows calculated characteristics in terms of the external current Ie ofloop 2 and the quantum phase difference 82 of junction J2• Figure 3(b) shows the threshold characteristics of the sense gate, along with hypothetical ± 10% variations in Joseph son critical current and inductance values, The shaded areas in Figs. 3(a) and 3(b) illustrate the thermal noise bands. In these calculations, we assumed a coupling factor between loop 2 and the sense gate is approximately 0.5. When the memory cell conserves data "1" ("0"), the operating point Tahara eta!. 852 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Tue, 25 Nov 2014 15:00:15Ie (mAl (a) I noise K / 0.5 band ,..-.. , L~ I "~- f: f..~l /1 ' H ll' J '--' 82 -411" Is (mAl (b) o FlG. 3. (a) Characteristics of the external current and the phase difference in loop 2. (b) Threshold curves of the sense gate. :±: 10% fabrication toler ances of circuit parameters were assumed. stays at "D" ("E") in Fig. 3 (a). The operating point moves from "D" to "H" through "G" with the supply of positive current Ix and negative current [y. On the other hand, when data "0" is conserved, the operating point moves from "E" to "F", and vortex transition does not occur. The operating point for the sense gate coupled with loop 2 moves to "M" or "N" depending upon the applied magnet ic field, as illustrated in Fig. 3 (b). The sense gate margin is determined by the characteristics of the sense gate and the input magnetic field at points "G" and "R" of Fig. 3(a). The designed sense gate has the product LI of ~o/6, where L is the total inductance value for the sense gate and I is the critical current for one junction (J3 = J4 in Fig. 1). A sense gate current margin has been designed Is = 0.12 rnA + 42% nominally, and Is = 0.13 rnA ± 13% assuming ± 10% parameter variations and thermal noise distur bance. Since the fiux mode in loop 2 changes only for read- ing, the sense gate current Is can be applied as a dock-pulse like gate current. As mentioned above, the operating margins for Ix and I is almost independent of the sense gate margin, because the sense gate detects the vortex transition in loop 2. There fore, the operating margins of ±: 33% for Ix and Iy are nom inally designed. And then the memory cell has capability for high-speed memory operation, since the cell is activated by the address signals and the sense signal without a timing sequence. Moreover, the designed memory cell contributes to improving the operating margin for a total memory cir cuit because the applied control current level for Ix and ly are designed to be nearly equal. 853 J. Appl. Phys., Vol. 65, No.2, 15 January 1989 TABLE 1. Layem for ,he vortex transitional memory cell. Layer Material GP Nb GIl Nb2O, GI2 Sial IL, 1\0 II SiO, RS Mo ILl Nb JJ Nb/AIO./Nb 1L3 Nb III. EXPERiMENT A. Fabrication Thickness (nm) Function 200 Ground plane 30 Ground insulation 300 Ground insuiation 200 Interconnection layer I 200 Interconnection insulation 1 70 Resistor layer 200 Interconnection layer 2 200/6/200 Junction trilayer 200 Interconnection layer 3 A test chip with four-level interconnections having less than about 50 nm planarity wa~ fabricated by a lift-off plan arization technique,7 using sputtered Nb films for all metalli zations except for Mo resIstors, and sputtered SiOz films for insulation layers. Table I describes the layers for the de signed cell circuits. A cross section of the cell is illustrated in Fig. 4. It is composed of a ground plane, three interconnec tion layers, Nbl AlO x Ir-..o Josephson tunnel junctions, resis tors, insulation between interconnections, and contact holes between interconnections. To heighten the reliability, the lift-off planarization technique was applied to each level in the cell structure because of its high layer-thickness control lability, low-temperature process ability, and pattern-size adaptability. The fundamentallift-offplanarization process flow con sisted of the foHowing five basic steps: (a) A sputtered lower film was patterned by a reactive ion etching technique using a CF4 gas plasma. (b) A second film was deposited over the entire surface, including resist masks. (c) The second film on the side waH of the etching mask is etched away selective ly with a slight wet etching. (d) The resist and the second film on the resist are removed with a solvent in an ultrasonic treatment. (e) An upper layer is sputtered over the planar ized surface. An of the interconnection layers and contact holes are planarized with the above technique. A microphotograph of the memory cel! is presented in Fig. 5. The cell size was 49X49 pm2, and the minimum line width, minimum layer to-layer registration, and minimum junction dimensions RESISTOR I~TERCCNNECT :ON :3) ''1"----~COUNTE.Ri ,,-JUNCTION fT?+-,¥v?jt>--"-..L...-'-1:;;..;r'--'''17~_ s _1~~~~~?N"ECTiON (2) ~~~~~~=r~~$Lij~~· CONTACT KOLE --INTERCONNECT:ON (\) H"';"444J.".-r~++r-~ CONTAC; HOLE '$"-GROUND-PLANE \¥,-rr-r>-rrrrrn-rrr.rrn.,.,-r,,,..,,rrr.r-rn77T,'777, Tl-.s--gUeST RATE FIG. 4. Vertical structure of the vortex transitional memory cell. Tahara et al. 853 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Tue, 25 Nov 2014 15:00:15FiG. 5. Microphotograph of the 2 X 2 bits vortex transitional memory edt were 1.5, 1.0, and 3.0 pm, respectively. In this designed cell, the coupling of two control lines to loop 1 consumes a large area. However, these two control lines can be changed into one control line by improving driver circuits because one of the two control lines is dc current line. Therefore, the mem ory cell has the ability to be designed with a smaller size. A Mo sheet resistance of 1.5 010 was achieved, almost the same as the designed value. The critical current of the 3,um Josephson junction was 0.08 mA, which was 20% smaller than the optimally designed value. B. Results and discussions Low-frequency measurements were carried out to evaluate the operation of the memory cello Figure 6 shows a properly executed quasistatic test pattern, including NDRO, for fuIl-and half-selected conditions. Current nota tions are the same as those in Fig. 1. Sense signal Vou, is the voltage across the sense gate. The sense gate current is ap plied on both writing and reading. The first half of the pat tern indicates the corresponding operation for writing and nondestructive reading of data "I," indicated by voltage v.,U( across the sense gate. The second half shows the same for data "0," indicated by zero voltage across the sense gate. Iy Is Vout U >( H H H H "0" H H H H WRRSRSRSRSRWRRSRSRSRSR -0 -0 -0 -0 FIG. 6. Quasistatic fUllction test patterns demonstrating successful NDRO memory operations. (W: write operation, R: read operation, and lIS: half .. selected disturhance;1,: 0.2 mA/div,I,,:0.2mA/div,I,:O.15 mA/div. ~'"': 4mV/div.) 854 J. Appl. Phys., Vol. 65, No.2, 15 January 1989 Iy(mA) "O·W(:!:24%) I){(mAl -0.3 i W ~p....=oc~~ (:!:33%) R(!21%) -0.5 FIG. 7. Measured operating region (shaded areas) for "0" writing, ''1'' writing, and reading. Circle points are presented the threshold curve oftlle vortex transitional memory cell. As shown in Fig. 6, the memory cell operates successfuHy even after encountering half-selected disturbance. In the memory cell, the necessary information such as address, data, and read/write conditions is transmitted only by I, and Iy-The sense current is simply applied as a clock-pulse like gate current. It improves the construction for the peri pheral circuits. The memory cell switching threshold. deduced from the function test, is plotted in Fig. 7. A 0,2 mA dc powered current was applied to the cell to set up an operating origin. The circles in Fig. 7 illustrate the threshold values for "1" writing, "0" writing, and reading. The operating regions, shaded in Fig. 7, show the address current margins Ix = 0.14 rnA ± 24%, 0.21 mA ± 33%, and 0.14 rnA ± 21 %, and 1v=0.15 mA ±24%, 0.17 rnA ±33%, and 0.18 rnA ± 21 %, corresponding to "0," "1" writing, and reading, respectively. The sense gate margin was measured Is = 0.14 rnA ± 33%. When the data was read, a single flux quantum entered loop 2 of the cell. This vortex transition was detected by the sense gate. Therefore, the sense gate margin is almost independent of the address currents I, and Iv' Each of the operating current margins is smalier than its designed value because the Josephson critical current and inductance values in fabricated chips were, respectively, 20% smaller and 10% larger than their designed values. The characteristics of the sense gate for loop 2 are ex perimentally measured to examine the vortex transitions of FrG. 8. Threshold characteristics oCthe sense gate measured on an isolated monitor gate. Vertical axes: the sense gate current (0.1 mA/div). Horizon tal axes: the external current (0.2 mA/div). Tahara et al. 854 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Tue, 25 Nov 2014 15:00:15FIG. 9. Estimated circuit parameter by measuring stray and mutual induc tallce value on isolated gates. loop 2. In order to measure these characteristics, a monitor gate consisting of the sense gate and loop 2 is fabricated. This gate has two gate lines: one is loop 2 external gate line and the other is the sense gate line. Figure 8 shows the characteristics in terms of the sense gate current I, and loop 2 external current Ie' The external gate current values at points "A '," "B '," "e '," and "D '" in this figure correspond to those at Points "G" "I" "J" a d "K'" F' 3 ( ) Th b , , ,n In ·lg. _ a. e a rupt changes in the sense gate current values at these points indi cate that the magnetic field entering the sense gate increases abruptly; that is to say, there is a vortex transition occurring in loop 2. From estimates of the characteristics of the moni tor gate, the circuit parameters were determined as fonows: L] = 7.5 pH, L4 = 1.5 pH, and 12 = 0<08 rnA. One of the reasons for the differences between these parameters and their designed values is the existence of stray inductance at contact holes, Josephsonjunctions, and so o~. Stray and mu tual inductance measurements at isolated monitor gates pro duced the circuit parameter estimates shown in Fig. 9< The calculated threshold characteristic curves of the memory loop for the experimental parameters are in good agreement with the experimental data, as may be seen in Fig. 10. Figure 11 illustrates the quasistatic results of the test for the memory operation with timing sequence" The patterns (a)-(d) in Fig. 11 show the writing and nondestructive reading operations for the four combinations of the se quences of setting and resetting for Ix and Iv' In pattern (a), for example, proper operations are demonstrated in the case Iy (rnA) 0.5 FIG< 10< Threshold curves (circle points) deduced from the quasistatic function tests. Solid lines show calculated threshold characteristic curves of the memory loops for the experimental parameters. 655 J. Appl. Phys .• VoL 65. No.2, 15 January 1989 Iv I, I, Vout (.) (b) "1" "cr W 11 R A R - -~- -, --- --- --- ~ -- -- ---- ----","",- ~----- {d) FIG. I!. Execution of quasistatic pulse test pattern demonstrating propel' operation with timing sequellce of a designed cell. (I,: 0.2 rnA/div, I,.: 0.4 mA/div, I,: 0.2 mA/div, V;",,: 4 m V /div.) . of setting Ix earlier than ly and resetting Ix earlier than Iy. The memory cell is successfully worked regardless of se quence for Ix and Iy" These results show its capability of reducing a cycle time of the memory operation. In the actual memory circuit, the address signal cur rents to the memory cell are applied from driver gates. In order to test the operation of the cell with driver gates, the test circuit illustrated in Fig. 12 was examined. The driver gate switching time is estimated to be approximately 20 ps from digital simulations. In this test, the dynamic stability of the cell is also measured. Test elements consisted of two in terferometer gates, the cell, and reset gates" In this circuit, the gate currents and the input current for the interferometer gates, the sense gate current and the reset gate currents were supplied from room-temperature pulse generators. The two interferometer gates drove the cell for X and Yaddress cur rents. The reset gates returned the applied currents from the cell to the interferometer gates" The quick pulses from the driver gates apply to the celL Figure 13 shows the results of a successful test of the test circuit, including nondestructive read-out operations and half-selected conditions. The mem ory ceH dynamicaHy operated propedy< IV. CONCLUSION A single flux quantum memory cell, caned a vortex tran sitional nondestructive read-out Josephson memory cell, was experimentally tested, using an isolated cell. The cell employs vortex transitions in the superconducting loops, for writing and reading information to improve operating toler ance. Test chips were fabricated using a lift-onplanarization techniqu.e with Nbl AIOxlNb junctions. Sputtered Nb, linx Memory Cell Ir Is 2 JJ Interferometer Vout FIG. 12. Test circuit diagram for the vortex transitiollal memory cell with driver gates. Tahara et al. 855 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Tue, 25 Nov 2014 15:00:15FIG. 13. Resu1tsofth~success ful measurements of the cdl with driver gates, including NDRO operation and half-se lected condition (W: write op eration, R: read operation, and HS: half-selected disturbance). Si02, and Mo films were used for interconnections, insula tions, and resistors, respectively. Successful quasistatic test patterns were obtained. Cell switching threshold character istics were deduced from a function test. There was good agreement with calculated threshold characteristics using experimental circuit parameters. The experimentally ob tained current margins were Ix = 0.14 rnA ± 21 %, ly = 0.16 mA ± 21%, and Is = 0.14 mA ± 33%, in spite of Josephson critical currents 20% smaner and indictance values 10% larger than their designed values. It was experi mentally shown that timing sequence is not necessary in the memory cell operation. The cell driven by interferometer 856 J. Appl. Phys., Vol. 65, No.2, 15 January 1989 gates was successfully operated, and the dynamic stability of the cell was evaluated also. ACKNOWLEDGMENTS The authors would like to thank H. Abe for his contin uous encouragement during this work, and J. S. Tsai, H. Tsuge, M. Hidaka, and S. Nagasawa for their helpful techni cal comments. The present research effort is part of the Na tional Research and Development Program on "Scientific Computing System," conducted under a program set by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry. 'w. H. Henkels, J. AppJ. Phys. 50, 8143 (1979). "W. H. Henkels and J. H. Greiner, IEEEJ. Solid-State Circuits SC-14, 794 (1979). 3K, Kojima, T. Noguchi. and K. Hamanaka, IEEE Electron Devices Lett EDL-4. 264 ( 1983). 4H. Bena, IEEE Trans. Magn. MAG·IS, 424 (1979). -'w. H. Henkels, L. M. Gappcrt, J. Kadlec, P. W. Epperlein, W. H. Chang, and H. Jaeckel, J. App!. Phys. 58, 2379 (1985). "5. Tahara and Y. Wada, lpn. J. App!. Phys. 26,1463 (1987). 71. Ishida, S. Tahara, Y. Ajisawa, and Y. Wada, Extended Abstract.s of the 19th Conferellce 011 Solid State Device alld Materials. Tokyo 1987 (The Japan Society of Applied Physics, Tokyo, 1987), p. 443. Tahara et al. 856 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Tue, 25 Nov 2014 15:00:15

1.1140379.pdf

Lowtemperature bolometer array M. Boninsegni, C. Boragno, P. Ottonello, and U. Valbusa Citation: Review of Scientific Instruments 60, 661 (1989); doi: 10.1063/1.1140379 View online: http://dx.doi.org/10.1063/1.1140379 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The lowtemperature energy calibration system for the CUORE bolometer array AIP Conf. Proc. 1185, 677 (2009); 10.1063/1.3292432 Rapid temperature variation of hopping conduction in GaAs lowtemperature bolometers Appl. Phys. Lett. 42, 685 (1983); 10.1063/1.94072 Low Temperature Silicon Thermometer and Bolometer Rev. Sci. Instrum. 41, 547 (1970); 10.1063/1.1684573 LowTemperature Lab Phys. Today 17, 70 (1964); 10.1063/1.3051382 LowTemperature Conference Phys. Today 9, 65 (1956); 10.1063/1.3059842 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 138.51.164.120 On: Thu, 27 Nov 2014 22:26:33Low .. temperature bolometer array M. Boninsegni,a) C. Boragno, P. Ottonello, and U. Valbusa Dipartimento di Fisica, Universita' di Genova, Via Dodecaneso 33. 16146 Genova. ltafy (Received 14 June 1988; accepted for publication 27 December 1988) The implementation of a 16-channel, low-temperature bolometer linear detector array is described. The detectors are silicon samples, whose surfaces are doped with phosphorus by the technique oHonic implantation. A single digital processor implemented on a common PC both provides the scanning of the array and performs synchronous signal detection from the different bolometers. The actual system has been tested with a broad infrared source, and some possible improvements are indicated. INTRODUCTION Thermal detectors are widely used in detecting infrared OR) radiation. In this class of sensors, the energy of the absorbed radiation raises the temperature of the detecting element and, as a result of it, changes the properties of the detector. Bolometers belong to this class of detectors. They are resistive elements fabricated with a material with a large temperature coefficient so that the absorbed radiation changes the value of its electrical resistance. In order to obtain the ultimate performance from this class of detectors, Low! was the first to develop a bolometer operating in liquid helium. Cryogenic bolometers are made of superconducting materials2 which have a large tempera ture coefficient. However, semiconducting bolometers are more widely used than the superconducting ones because they do not require critical temperature control. Recently, cryogenic bolometers have been used for detecting IR radi ation,2 molecular beams,3 ballistic phonons,4 and single par ticles5; they have been largely used in astrophysics, laser spectroscopy, surface science, atomic and molecular phys ics, and solid state physics. In all these applications, either the position of the detector is fixed with respect to the source or the detector itself is mechanically displaced through successive angular positions. Most current far-infrared and millimeter imaging systems, for instance, depend on a single detector with mechanically scanned optics, whereas in mo lecular-beam scattering experiments, the bolometer can ro tate around the target in order to record the angular distribu tion of the scattered molecules? For many applications, however, this approach is inadequate. The required integra tion time may be in some cases too long, the events can occur too quickly, or the construction is too complicated. There fore, the development of a bolometer array becomes particu larly important, for instance, in constructing ground-based, airborne, and balloon-borne telescopes6 for infrared astron omy or in the field of surface science for imaging the diffrac tion pattern of a molecular beam from a crystal surface as done in a similar way by a low-energy electron diffraction (LEED) screen. Multichannel bolometers 7,8 have been recently used in plasma diagnostic, In this case the bolometers work at room temperature and this simplifies the design of the array. In the present paper we describe a cryogenic array made of 16 phosphorus-implanted silicon bolometers driven by a microcomputer-controlled system which allows the collec tion of data. Section I describes the experimental setup with emphasis on the construction of the array (Sec. I A), on the calibration procedure (Sec. I B), and on the electronic sys tem controlling the imaging procedure (Sec. Ie). Section II reports the results. The array is used to detect the angular distribution of the radiant intensity of an IR light-emitting diode (LED) located in front of the array. A discussion on the performance of the array concludes the paper. I. EXPERIMENT A. Bolometer Each bolometer of the array is realized by ion implanta tion of phosphorus in a n-Si (100) wafer 300 pm thick and with a resistivity p = 103 n cm. The implant doses and ener gies are reported in Table L This procedure allows one to obtain a surface region uniformly doped for a depth of = 5600 ;"',9 as resulting from the Lindhard-Scharff-Schiott (LSS) method; this region has a net donor concentration of n = lAX 1018 cm-3, close to the critical value n" = 3.74X 1018 cm-3 for the metal-insu lator transition. lO Each bolometer of 4 X 2 X 0.3 mm 3, has been cut out from the wafer and provided with electrical contacts. The resulting detector is sketched in Fig. 1 (a). Two gold wires (150 f-lm in diameter) are soldered to the device by using the following procedure: two gold pads 500 A thick are first realized at both sides of the bolometer by thermal evaporation; the device is next maintained at a temperature of about 150"C and then, by flowing current through the wires, the temperature is locally raised up to the eutectic temperature of the Au-Si alloy (370 ·C) to produce the soldering. The procedure is carried out in inert and slightly reducent atmosphere (90% Nz + 10% Hz) in order to avoid formation of oxides at the Au-Si interface. The anal- TABLE L Implant doses and energies of phosphorus in the .'I-Si( 100) wafer." Ion energy (keV) 65 Doses (1013cm2) 0.53 lOS 0.83 160 1.26 265 1.99 370 3.32 'The silicon wafers, after the ion-implantation procedure, have been an nealed for l5 min in N 2 gas at 920 "C and immediately after, for 15 min, in O2 gas at 920 "Co 661 Rev. Sci. Instrum. 60 (4), April 1989 0034-6748/89/040661-05$01.30 @ 1989 American institute of Physics 661 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 138.51.164.120 On: Thu, 27 Nov 2014 22:26:33COPPEH DISK FIG. 1. (a) Schematic view ofa single bolometer. The sensitive area is I X2 mm2• (b) Schematic view of the bolometer array. The bolometers are locat ed close together in order to form a strip of 32 mm in length and ~ m~ m height. The G-IOeR substrate is 40X IOX2 mm3. The cop~er dls.k IS m good thermal contact with the bath. The electrical ~onnectlOns Wlt~ the external electronic system arc thermally connected with the copper disk. ysis of the interface, carried out with Auger spectroscopy, did not reveal oxides within the sensitivity of the method. After calibration (see next section), each bolometer is attached by General Electric 7031 (G E) varnish onto a glass-cloth/epoxy-Iaminate (G-lOCR) substrate 2 m~ thick to form a linear array of 16 bolometers; the substrate IS fixed with a silicon grease onto a copper disk in thermal contact with the liquid-helium bath. The complete arrange ment is shown in Fig. 1 (b). The gold wires on each bolometer are soldered by indi um alloy onto the copper pads evaporated on the substrate. These 32 pads are electrically connected to as many copper wires of 0.5 mm in diameter which link the array to the external electronic system. Care has been taken in reducing the input of heat through the copper wires by thermally con necting them to the copper disk. The array is inserted in a cryostat schematically shown in Fig. 2; the working temperature is fixed at 1.2 K by pump ing onto the liquid-helium bath. In front of the array, at a distance of5.5 cm, is located an infrared LED (Texas Instru ment, TIL 903) which is used to test the capability of the device in detecting the angular distribution of the emitted radiation. B. Calibration The single component of the array has been tested by measuring the R-T curve in the 4.2/1.2-K temperature range. For all bolometers we found that the a( T} coeffi- 662 Rev. SCi.lnstrum., Vol. 60, No.4, April 1989 LED light Li<\N" FIG. 2. Schematic view ofthe experimental setup. The bolometer array and the radiation source are maintained at a pressure of 10-5 mbar. The LED array distane is 5.S cm. The tube diameter is 16 mm. The LED is located along the tube axis. cient, defined as [ 1/ R (T ) ] [dR ( T ) I dT ], is the same with in 1 % and its value at T= 1.2 K is a = -2.9 K-1• After the realization of the array, we measured, for each bolo meter, the responsivity S, the response time T, and the noise equivalent power (NEP). The responsivity has been determined from the load curve, as suggested in Ref. 1. The measured values of S are reported in Table II. The current at the working point is fixed at 18 /lA. The responsivity can be also calculatedl by the follow ing equation: S = aiR I ( G -ai 2 R), (1 ) once the thermal conductance G between bolometer and thermostat has been evaluated. A simple model of the bolo meter has been made by assuming that the sensitive element can exchange heat with the copper pads across the gold wires and the copper disk of Fig. 1 (b) across the substrate (Gen eral Electric varnish, G-IOCR-silicon grease). To calculate G we took into account both these contributions, Gwires and G to the thermal conductance. G was calculated by substrate' . using the values of thermal conductivity and dimenslOns re- ported in Table III, considering that (a) Gwires and GS~bstrate are two conductance in "parallel," and (b) Gsubstrate IS the "series" of G2, G3, and G4• With these considerations and by using the values of Table III, we obtained for each bolometer a value of G = 3.1 X 10-5 W IK. This value is predicted on the base of the values of Table III and is only a first approximation of the real situation. Each bolometer, in fact, differs from the others for several reasons (length of the wires, goodness of the thermal contact, etc.), and therefore the values of Table III can vary up to 20% from one bolometer to the other. By using the values of G determined with this model, Eq. (1), and the values of resistance (see Table II), we ob tained the values ofresponsivity S * reported in Table II. The agreement between the experimental values S and the pre dicted ones S * confirms the goodness of the model. Figure 3 reports the response of a bolometer (No.8 of Table II) to an impinging modulated (20 Hz) square-wave Bolometer array 662 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 138.51.164.120 On: Thu, 27 Nov 2014 22:26:33TABLE H. Responsivity S and resistance R as measured from the load curve. a Bolometer 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 S (104V/W) 5.3 5.7 5.7 5.7 5.8 5.9 5.3 5.3 5.1 5.7 5.1 5.9 5.5 5.0 5.9 S.7 R (kH) 18 19 19 19 18 18 14 18 15 19 15 18 20 24 18 19 S* b (10' V /W) 6.7 7.5 7.5 7.5 6.7 6.7 4.1 6.7 4.6 7.5 4.6 6.7 8.5 14.9 6.7 7.5 "The working point is fixed at i= 18 11A. The temperature is 1.2 K. b S * is the responsivity calculated by using Eq. (l) and assuming G = 3.1 X 10-5 W /K. radiation. The response time r is 5 ms. According to Ref. 1, r is given by r= CI(G-ai2R), (2) where C is the thermal capacity of the bolometeL C can be calculated as This formula takes into account the contribution which arises from the thermal1inks of the sensor element. 13 Assum ing for the specific heat (at 1.2 K) ofthe materials constitu ent the different parts of the detector the following values: CSi = 4.5 X 10-7 11K g,14 CAu = 8.3 X 10-6 11K g,14 and CG _ IOCR = 2 X 10' 6 J/K g,15 C results equal to 3 X 10'8 JI K. By using Eq. (2) and the values of G and C previously calculated, one obtains for the bolometer No.8, r = 2 ms. which is in close agreement with the observed one. ' The NEP for all the bolometers is of "'" 10-12 W Imi. No care has been taken in minimizing it since it is lower than the noise input equivalent power of the electronic-acquisi tion system developed in the present work. c. Electronics When no radiation is impinging on the detector surface, a constant voltage Vo = Roi appears across the bolometer, where Ro is its resistance at the working temperature and i is the current supplied by a suitable constant-current gener ator. A change t:..R in the bolometer resistance occurs when ever radiation is absorbed, and the corresponding change in voltage, 11 V, is a measurement of the intensity of the imping ing radiation. The array is controlled by the electronics shown in Fig. 4, which allows the selection of each bolometer as well as low-noise amplification of the signal from the detectors. The TABLE III. Values of thermal conductivity gi' length Li' area A" and ther rna] conductance Gi of the materials forming the bolometer array." gi (W/cmK) L, (em) Ai (cml) G, (W/K) Gold + 0.07% Fe 5 X 10-3 b single wire GE 7031 varnish 5X 10" c G-lOCR 10-4 d Silicon grease 10-5 e "The temperature is 1.2 K. h From Ref. 11, extrapolated at 1.2 K. C From Ref. 11, extrapolated at 1.2 K, d From Ref. 12, extrapolated at 1.2 K. e From Ref. 11. 0.5 0.01 0.2 0.01 8XlO--1 4XIO I 8XlO'2 4XlO' 8Xl0 2 8X1O' 663 Rev. SCi.lnstrl.lm., Vol. 60, No.4, April 1989 IBM PC is equipped with an analog (12-bit resolution) and digital I/O interface board (LabMaster TM-30). The over all gain is distributed along the amplification chain in order not to saturate the analog-to-digital converter whose input range is switched from unipolar (0/5 V) to bipolar ( ± 2.5 V) passing from Il;) to ~ V measurement. Throughout a mea surement run, the chopper supplies an interrupt each time the incoming radiation is turned off. At these instants the constant current is switched from a detector to the next one along the array and a measurement is performed within a period of the modulation. Two successive steps can be out lined corresponding to the different halves of this period: 0) in absence of radiation, the dc component Vo is analog-to digital converted and stored in the main memory of the PC, and (ii) when the radiation is on, the voltage across the de tector, Il;) -fj. V, appears at one input of the instrumentation amplifier, while the previously stored Vo value is fed, via the DI A converter, to the other input. The large dc component being removed by subtraction, the small Ll V can be amplified with a gain factor G2 = 500 to a level suitable for AID con version. Also all voltage contributions from the on resistance of the different switches making up the analog multiplexers are strongly reduced. The subtracting procedure is, in fact, very effective because passing from step (i) to step (ii) noth ing in the system changes but impinging radiation. As a re sult of the two steps, a sample (i.e., a term in the sums yield ing the averaged values) of both Vo and 11 V is obtained for the selected bolometer. It is worth noting that the stored value of b.. Vis formed, as well as the Il;) value dl,ring the first step, by numerical FIG. 3. Picture of the oscilloscope output for a square-wave radiation irn pinging on bolometer No.8. Horizontal axis scale i, !O ms/cm; vertical axis scale is 50 m V / ern. Bolometer array 663 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 138.51.164.120 On: Thu, 27 Nov 2014 22:26:33r--------------, out 61 G = 10 I LAB MASTER I ~ FIG. 4. Block diagram of the control and data-acquisition electronics. Both tem perature and long-term drift ofthe current generator are actually limited to 5 ppml"C and 50 ppm/lOoo Hr, respectively, by us ing high-precision voltage sources and operational amplifiers, L ______________ J averaging over 32 readings performed within the corre sponding halves of the modulation period. This operation reduces the input noise level, although the gain in the signal to-noise ratio is lower than that possible with fully indepen dent events. A test carried out with the actual instrument when no radiation is impinging on the array gave a noise growth factor of about 1.8.jn, where n is the number of sam ples. A complete run consists of a preselected number N of measurement cycles whose length is equal to the number of detectors ( 16) times the period r of the modulated incoming radiation. All the bolometers are cyclically selected and the ratio LlV(j)IVo(j) is measured for each one (j= 1,2, ... ,16) during the measurement cycle. The intensity pattern is ob tained after having averaged over N cycles. The back~up procedure employed to remove the dc component assures the measurement of the comparatively large values Vo(j ) as well as the small Ll V(j ) without in troducing any time constant. In fact, the alternative simpler way, based on ac coupling between the detector and amplifi er chain, causes memory effects, preventing the fast scanning ofthe array. Also the 16 time sequences Va [j ; (16k + j) r] (j = 1,2, ... ,16; k = measurement cycle index = O,1,2, ... ,N -1) can be stored for a later check of the sta- bility of individual bolometers and of the system in its entire ty. Currently, no further exploiting ofthese large amount of data is foreseen. A few parameters (number N of cycles, option of storing the Vo time sequences, or their undersampled versions, etc.) can be passed to the machine-coded, interrupt-driven rou tine which, as already said, allows the synchronous scanning 664 Rev. Sci. (nstrum., Vol. 60, No.4, April 1989 of the array, the acquisition of data from the different chan nels, and the updating, cycle after cycle, of the different aver ages. II. RESULTS AND DISCUSSION Figure 5 reports the radiation-intensity pattern of the LED source as measured by the linear array. This pattern has been obtained accumulating 6400 samples (N = 200) per bolometer. The LED source is supplied with a square wave at a repetition rate of 10 Hz whose intensity level is set >f- (f) Z W f Z w > f <: .J w cr: ~ '. lli~ -ID D 10 DISPLACEMENT FROM OTPICAL AXIS (mm) FIG. 5. Angular distribution of the radiation emitted by the LED source as detected by the 16-bolometer array compared with the theoretical one (squares). Bolometer array 664 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 138.51.164.120 On: Thu, 27 Nov 2014 22:26:33in order to have a signal-to-noise ratio = 1 for the central bolometer. In the same figure we report a simulation of the pattern based on the LED characteristics and on the as sumption that the detected radiation pattern is formed by two contributions: the first stems directly from the LED source and the second from a single reflection on the wall of the tube. Because of the low resolution of the employed AID and D/ A converters, the quantization noise is the main limiting factor for the sensitivity of the apparatus. In fact, the rms equivalent noise voltage across the bolometer (referred to the system bandwidth of 200 Hz) due to the independent contributions ftom the AID and D/ A conversions is 11 D/A voltage output range _1 __ 1_=50 V. 212 ,-G /1 2~3 1 A first, considerable reduction (to = 3 fl V) is easily achieved by using a 16-bit analog-to-digital interface board. A further step towards lower noise level is possible because any quantization noise is generally considered to be white whenever the AID converter input (signal + analog noise) changes by at least a few quantization levels between sam plcs.16 That being the case in our operating conditions, the input equivalent noise voltage, i.e., the input 110ise power, can be reduced by lowering the system bandwidth, which can be obtained by simple averaging. 17 For instance, with reference to the above figure, a factor of 100 can be gained by accumulating 10 000 independent samples for each bolo meter (which requires a 25-min-long measuring run). 6S5 Rev. SCi.lnstrum., Vol. 60, No.4, April 1989 ACKNOWLEDGMENT We are grateful to Dr. F. Mod who realized the ion implants. a) Present address: Physics Dept., Florida State University, Tallahasse, FL. 'F. J. Low. J. Opt. Soc. Am. 51,1300 (1961). 'E. H. PUlley, in Optical and In/rared Detectors. edited by R. J. Keyes (Springer, Berlin, 1977), pp. 71-100. IG. Scoles, Ed., Atomic and Molecular Beam Methods (Oxford University, New York. 1988), Vol. 1. 4c. Boragno, U. Valbusa, and G. Pignatel, App!. Phys. Lett. 50, 583 (1987). 5S. M. Moseley, J. C. Mather, and D. Me Cammon, 1. App!. Phys. 56, 1257 (1984). "F. J. Low, T. Nishimura, A. W. Davidson, and M. Alwardi, inProcredillgs of Workshop all Ground-based Astronomical Observations with Illfrared Array Defectors, Hila, Hawaii, 1987. 7B. Joyc, P. Marmillod, and S. Nowak, Rev. Sci. Instrum. 57, 2449 (1986). gpo E. Young, D. P. Neikirk. P. P. Tong, D. B. Rutledge, and N. C. Luh mann, Jr., Rev. Sci. Instrum. 56, 81 (1985). "c. Boragno, U. Valbusa, G. Gallinaro, D. Bassi, S. Iannotta. and F. Mori, Cryogenics 24, 6R I (1984). "'T. F. Rosenbaum. R. F. Milligan. M. A. Paalanen, G. A. Thomas, R. N. Bhatt, and W. Lin, Phys. Rev. B 27, 7509 (1983). "G. E. Childs, L. J. Ericks, and R. L. Powell, Thermal cOllductiuityofsolids at room temperature and below, NBS Monograph No. 131, 1973. 12M. B. Kasen, G. R. MacDonald, D. H. Beekman. and R. E. Schramm, in Advances in Cryogenics Enginel'ring, edited by A. 1'. Clark and R. P. Reed (Plenum, New York. 1980), Vo!. 26, p. 235. I3G. Gallinaro, C. Salvo, and S. Terreni, Cryogenics 26, 9 (1986). I4JJandhook of Physics and Chemistry, 64th ed. (CRC, Boca Raton, 1983). 151'. Fabbricatore (pl'ivate communication). ;6B. Allen Montijo, Hewlett Packard J. 1988, 70 (June 1988). i7J. Max, it/ethodes et Techniques de Traitement du Signal et Applications (lUX Mesures Physiques (Masson, Paris, 1981). Bolometer array 665 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 138.51.164.120 On: Thu, 27 Nov 2014 22:26:33

1.1140027.pdf

Paralleled transconductance ultralownoise preamplifier Robert B. Hallgren Citation: Rev. Sci. Instrum. 59, 2070 (1988); doi: 10.1063/1.1140027 View online: http://dx.doi.org/10.1063/1.1140027 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v59/i9 Published by the AIP Publishing LLC. Additional information on Rev. Sci. Instrum. Journal Homepage: http://rsi.aip.org Journal Information: http://rsi.aip.org/about/about_the_journal Top downloads: http://rsi.aip.org/features/most_downloaded Information for Authors: http://rsi.aip.org/authors Downloaded 05 Oct 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissionsParalleled transconductance ultraiow .. noise preamplifier Robert B. Hallgren School o/Electrical Engineering, Cornell University, Ithaca, New York 14850 (Received 21 September 1987; accepted for publication 27 May 1988) A simple NJFET preamplifier was constructed from commercial parts using parallel input devices in a cascode configuration. The equivalent input noise resistance was 8.5 n (0.38 nV /~Hi) at 1 kHz, and 12 n (0.45 nV /,fHZ) at 100 Hz, measured at room temperature, independent of the source resistance. For SO-H sources, a gain of29 dB was achieved from 3 Hz to 13 MHz. The input noise equivalent resistance is verified by measuring the thermal noise oflow valued wire-wound resistors. Circuit utility is demonstrated by noise measurements performed on GaAs Ohmic contacts at room temperature, under various bias conditions. Design considerations for using parallel input devices, the bias criteria for them, and possible design extensions arc discussed. INTRODUCTION Much research has recently been devoted to the noise behav ior and noise mechanisms of various physical systems.] For accurate measurements, the noise of the system of interest should be greater than the residual noise of the amplifier used for the measurement. Whatever the source ofthe noise, it is limited by the thermal noise of the dc resistance present in the system. Flicker noise, or any nonthermal sources pres ent in the system, appear as excess noise above this thermal background. For detailed measurements of any nonthermal, or excess, noise in the system, it is necessary, first of all, to be able to measure the thermal noise present. In this way, the thermal component can be removed, leaving only the noise of interest. This measurement of the thermal noise is limited by the residual noise of the instrumentation used. In those systems, where the de resistance is quite low, the instrumentation must have an input noise resistance (Reg) which is corre spondingly lower. Such systems are gallium arsenide MESFETs used in microwave circuits, where the mean channel resistance is often less than 10 n. The excess noise in these devices is flicker noise, and the point at which the flicker component dominates the thermal noise ( 1/1 corner) is usually at a frequency above 1 MHz.2 To study accurately the excess noise in these systems, and the bias dependence of it, the midband noise of the amplifier must be less than the thermal noise of the channel, and the bandwidth must ex ceed the 1/1 corner. The preamplifier presented here ad dresses these needs by paralleling commercial JFET devices, in a cascode configuration, operating at room temperature. I. BACKGROUND The input noise of an amplifier is usually dominated by the device noise of the input stage. By careful selection of the devices, the bias points, and the temperature of operation, the input noise can be reduced. Silicon NJFETs are usually used as the input devices,3 where the noise power generated is inversely proportional to the transconductance of the FET, gm' as given by4 0) where a is a constant, gm = (2/1 Vp I) (lnss1ns ) 1/2, and Vp is the pinch-off voltage of the channel. Increasing gm reduces the noise, but necessitates in creasing lDss (the saturated drain current at zero gate bias). For maximum circuit gain, the FET must remain in satura tion during the entire voltage swing, and this poses a lower limit to the noise reduction possible. The minimum drain source voltage for saturation, VnsAT, and the drain current used, must generate less than the allowed power dissipation of the device, thus limiting the amount by which loss can be increased. The quantities loss, VOSAT' andgm are related to the gate dimensions. By making the PET physically larger, the noise can be reduced. This increase in the FET size is at the expense of a larger gate capacitance, limiting the useful frequency range of the device. What is needed is a way to reduce the noise of an FET, while remaining within the max imum power dissipation and allowing a sufficiently wide bandwidth. Motchenbacher and Fitchen have done this5 by paralleling separate devices at the input to an amplifier, thereby creating a much larger FET with a greatly increased power-handling capacity. This parallel connection has been used in FET amplifiers6 and op-amps circuits,? all having similar results, though for different applications. II. CIRCUIT THEORY A. Theory of operation Consider an FET as shown in Fig. 1 (a), biased at some current IDS' at a voltage greater than VosAr' The noise is dependent upongm' as given by (2), and for this single PET, the transconductance is gm, = (2/j v," I) (loss los ) 1/2 (A/V). (2) If N such identical devices were connected in parallel lFig. 1 (b)], biased to a total current of los, each individual FET would carry a current of Ins = los/No The transcon j ductance of an individual FET is thus (3) 2070 Rev. SCLlnstrum. 59 (9), September 1988 0034-6748/88/092070-05$01.30 @ 1988 American instItute of Physics 2070 Downloaded 05 Oct 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions(a) (b) ~~ IDS Making the transconductance of the parallel combina tion to be the sum of the individual contributions gives (A/V). (4) In this connection, each FET is biased well below 1 DSS' and contributes a transconductance that is less than the maximum available for that device. The sum of the separate contributions, however, is greater than the maximum trans conductance of the single FET considered above, using the same power dissipation in both cases. The ratio of the gain increase is N 1/2, so connecting more devices can give less noise. The input capacitance of the paranel combination in creases with N, making the largest number of devices that can be paralleled limited by the source resistance and the desired bandwidth. For wide-bandwidth circuits, a cascade connection is generally employed to reduce the effects of the increased feedback capacitance. The cascode FET then becomes the limiting device, as it is biased to the total drain current bias ing all of the input devices, and must itself remain in satura tion. As the number of paralleled FETs increases, the total bias current increases, so that the power dissipated by this cascode FET eventually exceeds the rated power dissipation. B. Circuit description The circuit tested is shown in Fig. 2, Six input FETs were used (Q l-Q6), with a single cascode FET (Q7) and a resistor load (RL ). The input stage (Q 1-Q7) is connected to a simple source follower (Q8) circuit, biased to give an ap proximate SOon. output impedance. The circuit operates ALL TRANSISTORS RRE 2N5434 as 790 UF I- 1329 UF -j 4.84K 10011 Rl-RS FIG. 2, Preamplilier schematic, 2071 Rev. Sci. Instrum., Vol. 59, No.9, September 1988 FIG, L (a) Single FETbiased ailps' (b) N paralleled FETs with total current of los, from a single 24-V supply obtained from commercial12-V wet cells. The input stage uses capacitive coupling from six paral lel tantalum capacitors to reduce the series resistance of the capacitors, and to allow adequate coupling to an 8-H source for frequencies below 10 Hz. Each input FET uses a separate resistor (RI-R6) for source degeneration, which allows for variations in the input device characteristics. These resistors use large bypass capacitors (CI-C6) for extending the low er-frequency limit. The bias point selected for the input devices was the largest current that the cascode FET could handle, while remaining at a drain-source voltage of 3 V, with a power dissipation less than one-half the maximum rating (to mini mize heating). This amounted to 10 rnA per input FET. At this drain current, the input FETs needed a gate-source vol tage of approximately -2 V, which was supplied by the appropriate source resistors, allowing the gate to be biased through a lOO-MH resistor to ground. The transconduc tance of the input transistors at this operating point was over 30 mS each, making the total close to 200 mS. C. Device description An FETs used in this circuit are commercial parts, ob tained as samples from Siliconix, and each device was tested on an HP 4145B semiconductor parameter analyzer. The 2N5434 are low ON resistance, N-channeI JFETs used com monly for switching applications, Other devices have been tested, though not in the current design; among these devices are 2N6550, U3 11, 2N4416, and 2SK1161. These transistors have all been reported as having low noise,8 but the commer cial availability and frequency response are unknown. The saturated drain current was over 100 rnA for all devices, giving a transconductance greater than 100 mS at a drain current of 100 rnA. Breakdown voltages are specified at 25 V and the maximum power dissipation is 300 m W ambient. Device geometry is quite large, to give the low channel resis tance for switching, and this results in a gate capacitance of around 50 pF per device. The saturation voltage is over 3 V at IDss' and the output conductance is rather large, even when operating in saturation. The 2N5434 devices are extremely rugged, due to a maximum forward gate current of 100 rnA, and at no time during construction or testing did a device faiL The reverse gate current was measured to be below 100 pA at room tem perature and 2-V reverse bias. Low-noise preamplifier 2071 Downloaded 05 Oct 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissionsIII. CIRCUIT PERFORMANCE A. Amplifier measurements The gain of the amplifier was tested by providing a -70-dBm input signal from an HP 3336C signal generator, and r-eading the output signal from an HP 3586C selective level meter. Additional amplification was provided by a low noise bipolar postamplifier, which had approximately 1.6 n V / JHi input noise and 33 dB of gain. The gain of the preamplifier was measured to be 29 dB from 3 Hz to over 10 MHz. The upper frequency limit of the preamplifier depends upon the source resistance and the total input capacitance. The 300-pF total input capacitance is high for an rf circuit, but using a 50-0 source, the circuit operated to 13 MHz, and only for a source resistance of over 300 n was the bandwidth reduced to a few MHz. The lower-frequency corner depends upon the input source impedance/coupling capacitor and the FET degeneration pole. The values used gave a cutoff of 3 Hz, which can be reduced by using larger bypass compo nents. B. Noise measurements To quantify the noise performance of the preamplifier, measurements were made of the thermal noise from 5%, wire-wound resistors at room temperature. The noise tests were made using an HP 3586C selective level meter, with the bipolar postamplifier providing additional gain. Multiple readings of the noise power at various frequencies were taken and the average computed. The power spectral density was calculated by dividing the average noise power by the band width of the filter, which was set to 20 Hz. The noise spec trum was referred to the input of the amplifier by subtracting the total gain, and the resultant value expr~ssed as a voltage spectrum in dBV (V2/Hz). The noise voltage spectra, as obtained from the resistors and from a shorted input to the preamplifier, are shown in Fig. 3. The noise from a lOon resistor is seen to be more than 3 dB greater than the shorted input noise, indicating that the amplifier equivalent input noise resistance is less than 10 ·n. N :r: "> co " <Il The noise measured from the various resistors can be -170.-------------------------------, ]30 OH~1 lSl OHM 56 OHM ~ -180 o > <Il " o Z 10 .__---------+ 8. 5 OHMS -190L-~~~L-~~~L-~~~L-~~~ ! 02 103 I (] 4 105 ICE Frequency (Hz) FIG. 3. Measured noise of wire-wound resistors. The shorted input noise floor of the preamplifier is shown along with the thermal noise expected from 8.5 n at room temperature. 2072 Rev. Sci. Instrum., Vol. 59, No.9, September 1988 normalized by subtracting the shorted input noise at each frequency of measurement. This normalized noise is plotted in Fig. 4. The right ordinate shows the expected value of the noise as calculated from the measured dc resistance of each sample resistor. The agreement is excellent in all cases, and only for resistances below 70 n can the instrumentation 1/1 noise be seen. Bandwidth reduction is evident only for the measurement of the 330-H resistor. C. Equivalent input noise resistance The input noise resistance can be defined as the resis tance value that contributes an amount of noise equal to the amplifier residual. This value is determined by using the data from the various resistor noise measurements. At each fre quency, the noise from a resistor is plotted versus the value of the resistance. These data are plotted in Fig. 5 in terms of log[e~(/)/e6(/) -1] vs logR, (5) where e~ ( I) is the noise of resistance R at frequency J, and e6 (f) is the shorted input noise at! The regression line from a least.squares fit for the data is plotted through the data points at two sample frequencies: 150 Hz and 400 kHz. The log R-axis intercepts give the val ue of the input noise resistance for these two frequencies. In a similar manner, ReG for each frequency can be found, and the values obtained are plotted versus frequency in Fig. 6, where the mean value is around 8.5 fl. This value is very close to the resistance equivalent of the shorted input noise, which shows that the preamplifier noise characteristics are not affected by the source resistance, as expected for FETs. This fact is also seen from the linearity of the noise plots yersus resistance value (Fig. 5). If the noise were dependent upon the source resistance, some nonlinearity would be seen, and the shorted input noise would be less than that generated by the equivalent noise resistance. This manner of testing for noise behavior is particularly useful, as it gives the noise equivalence very accurately in terms of a resistance for dif ferent frequencies. Included in the equivalent resistance is the noise due to any parasitics, such as the series resistance of the input coupling capacitance, or any frequency depen dence due to this capacitor. N :r: " > rn '0 " OJ '" +' o > .-o Z -1~0L-~~~~~~~~~~~~~ i02 11,3 104 10" 1116 Frequency (Hz) FIG. 4. Noise of resistors minus amplifier residual. The right ordinate labels the resistor values and the dashed lines indicate the expected thermall).oise of this resistance value at room temperature. Low-noise preamplifier 2072 Downloaded 05 Oct 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions2.5r---------------------------~ 7 2 * JS800flHZ 150HZ L1 • ~j -o ..J ?,5 LOG (R,nputl FIG. 5. Normalized resistor noise as a function of resistance value at two sample frequencies. The R-axis intercept gives the equivalent input noise resistance at the frequency plotted. IV. CIRCUIT UTILITY As an example of one possible application for this preamplifier, the noise from a series of Ohmic contacts was measured as a function of the voltage across them. The mea surements were made on a test pattern from a production gallium arsenide wafer. The pattern consisted of a series of minimum-sized Ohmic contacts with metal interconnects. The dc characteristics were measured on an HP 4145B semi conductor parameter analyzer. The current-voltage plot was linear, the intercept was through the origin, and the slope was calculated to be 12.5 n. The noise voltage from this sample was measured at bias voltages of 0, 10, and 50 m V across the sample. The results are plotted, in normalized form (with the amplifier residual subtracted), in Fig. 7. In dicated in the figure is the expected thermal noise floor from the 12.5-0, resistance at room temperature, and the agree ment is good. The low 1/ f noise corner of the preamplifier allows the excess noise of the contacts to be seen increasing as the bias voltage increases. At a bias of 10 mY, the 11/ comer of the amplifier is already below the 1// noise of the sample. V. CIRCUIT EXTENSIONS Extensions of the present design are possible, allowing some ability to tailor it to any specific application. The low \2 :1 E ..c 0 10 ij) 9 - u c 8 -(C +' (/) 7 (~ (j) b 0::: 5 102 103 \04 Frequency FIG. 6. Equivalent input noise resistance of the preamplifier circuit as a function of frequency as found from the intercept values. The mean value is seen to he around 8.5 n. 2073 Rev. Sci. Instrum., Vol. 59, No.9, September 1988 N I "> rn "1J --160 r--------------~ .;; -180 -o Z ~""""'~F .... -...... __ .._!-12.5 OHliS -I '30 ~J...-I..J...U.'_'_":_-J...-'-'-'-U-l.<L... ........ -J,.Wu.uL .......... --'-'..u.u.J ill 103 11')4 10" Frequency (Hz) F.IG. 7. No~se of Ohmic contact test pattern (minus instrument residual) at different bIas voltages. The sample measured 12.5-H dc resistance, and this value of thermal noise is indicated by a dashed line, value of the total input gate current to the preamplifier al lows direct coupling to the noise source, provided the sample can supply the one-half of a nanoampere required by the gates. If the bias voltage needed for the sample is small enough, direct coupling would be a distinct advantage. The 2 V dropped across the source resistors (RI-R7) biases the input PETs and allows for some deviation in the de input voltage from the sample, without appreciable gain devia tions in the preamplifier. Direct coupling places the low frequency limit at the FETs' source resistorlbypass pole fre quency, which is as low as the capacitor can be made large. An additional FET gain stage can be added directly to the follower output, and if the total gain were sufficient, no additional amplifiers would be needed. The present design is useful as a buffer between existing amplifiers and any smail signal, low-impedance noise source. The gain of almost 30 dB, and noise floor ofO.44nV I~Hz, anow postamplifiers to be used that have an input noise floor under 6 n V I[Hz, with no degradation in the noise performance, Commercial, low noise op-amps that typically have noise floors of around 4 n V I/Hz-would provide ample additional gain for lower frequency designs. Cooling the input devices would further reduce the noise,9 with some additional concern for high~frequency sta bility. The number of devices used at the input can be adjust ed to effect a compromise among the bandwidth, sample impedance, and the noise floor desired. To accommodate additional input FETs, the cascode FET could itself consist of two parallel devices, thereby doubling the power dissipa tion allowed. ACKNOWLEDGMENTS . . The author would like to express gratitude to and appre CIatIOn for the staff and all concerned at the Microwave Technology Center of Hewlett-Packard, and to Siliconix Inc. Special thanks are due to Nicole Bute! for patience and assistance in preparing much of this effort. 'See, for example, Proceedings of the International Conferences on Noise and Physical Systems (North-Holland, Amsterdam, 1983, 1984). low-noIse preamplifier 2073 Downloaded 05 Oct 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions2B. Hughes, N. G. Fernandez, and J. M. Gladstone, IEEE Trans. Electron Devices ED-34, 733 (1987). 3c. D. Motchenbacher and F. C. Fitchen, Low Noise Electronic Design (Wiley, New York, 1973), Chap. 6. 4A. Van der Ziel, Noise in Measurements (Wiley, New York, 1973). Sc. D. Motchenbacher and F. C. Fitchen, Low Noise Electronic Design 2074 Rev. Sci. Instrum., Vol. 59, No.9, September 1988 (Wiley, New York, 1973), Chap. 12. fiP. Bardoni and G. V. Pallotino, Rev. Sci. lnstrum. 48, 757 (1977). 7B. Sundvquist and G. Back.strom, Rev. Sci. lnstrum. 46, 928 (1975). 8D. Bloyet and r. Lapaisant, Rev. Sci. lustrum. 56,1763 (1985). 9S. Klein, W. Innes, and r. Price, Rev. Sci. lnstrum. 56,1941 (1985). Low-noise preamplifier 2074 Downloaded 05 Oct 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://rsi.aip.org/about/rights_and_permissions

1.342547.pdf

Electrical effects of atomic hydrogen incorporation in GaAsonSi J. M. Zavada, S. J. Pearton, R. G. Wilson, C. S. Wu, Michael Stavola, F. Ren, J. Lopata, W. C. DautremontSmith , and S. W. Novak Citation: Journal of Applied Physics 65, 347 (1989); doi: 10.1063/1.342547 View online: http://dx.doi.org/10.1063/1.342547 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Depth dependence of hydrogenation using electron cyclotron plasma in GaAs-on-Si solar cell structures J. Vac. Sci. Technol. A 17, 453 (1999); 10.1116/1.581605 New mechanism for Si incorporation in GaAsonSi heteroepitaxial layers grown by metalorganic chemical vapor deposition Appl. Phys. Lett. 55, 1674 (1989); 10.1063/1.102232 Heterointerface stability in GaAsonSi grown by metalorganic chemical vapor deposition Appl. Phys. Lett. 51, 682 (1987); 10.1063/1.98333 Formation of the interface between GaAs and Si: Implications for GaAsonSi heteroepitaxy Appl. Phys. Lett. 51, 523 (1987); 10.1063/1.98386 Activation characteristics and defect structure in Siimplanted GaAsonSi Appl. Phys. Lett. 50, 1161 (1987); 10.1063/1.97949 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.94.16.10 On: Sat, 20 Dec 2014 23:59:19Electrical effects of atomic hydrogen incorporation in GaAs",on .. Si Jo Mo Zavada u.s. Army European Research Office, London NWl 5TH, United Kingdom S. J. Pearton AT& T Bell Laboratories, Murray Hill, New Jersey 07974 R. G. Wilson Hughes Research Laboratories, Malibu, California 90265 C. S. WU,a) Michael Stavola, F. Ren, J. Lopata, and W. C. Dautremont-Smlth AT&T Bell Laboratories, Murray Hill, New Jersey 07974 S. W. Novak Charles Evans and Associates, Redwood City, California 94063 (Received 11 July 1988; accepted for publication 13 September 1988) ~ e have introduced atomic hydrogen by two methods into GaAs layers epitaxially grown on SI substrates, namely, by exposure to a hydrogen plasma or by proton implantation. In both cases, when proper account is taken of shallow dopant passivation or compensation effects, there is a significant improvement in the reverse breakdown voltage of simple TiPtAu Schottky diodes. Proton implantation into un doped (n = 3 X 1016 em -3) GaAs-on-Si leads to an increase in this breakdown voltage from 20 to 30 V, whereas plasma hydrogenation improves the value from 2.5 to 6.5 V in n-type (2 X 1017 cm-3) GaAs-on-Si. Annealing above 550'C removes the beneficial effects of the hydrogenation, coincident with extensive redistribution of the hydrogen. This leaves an annealing temperature window of about 50·C in the H-implanted material, in comparison to 150·C for the plasma-hydrogenated material. The hydrogen migrates out of the GaAs to both the surface and heterointerface, where it shows no further motion even at 700 ·C. Trapping in the GaAs close to the heterointerface is shown to occur at stacking fautts and microtwins, in addition to extended dislocati.ons. INTRODUCTION There has been an extensive effort in recent years to grow and characterize GaAs layers on Si substrates. 1-3 The reasons for this interest are wen documented, but briefly they relate to the advantages of replacing brittle, small-di ameter GaAs substrates with larger-diameter Si substrates of superior thermal and mechanical properties. At some point i.n the future, it may also be possible to combine the functions of GaAs-based photonic devices with those of very-large-scale integration (VLSI) Si electrical circuits, all on the same chip. At present this optoelectronic integration is hampered by the fact that aU GaAs layers grown on 8i substrates exhibit high densities of extended defects, in par ticular threading dislocations.4 These defects result from the 4% lattice constant difference between GaAs and 8i and appear in the initially coherently strained GaAs after a few hundred angstroms of growth. Regardless of the lattice mis match between the III-V layer (GaAs, GaP, InP) and the group-IV element substrate (Si or Ge), there appears to be an almost invariant defect density of 107_108 cm·2 at dis tances of -1 /Lm from the heterointerface.4 This may well be an interaction-distance argument in the sense that near the interface an initially high density of defects can tangle and terminate. This leads to a reduction in defect density with distance from the heterointerface until the remaining defects become far enough apart that their probability for interact ing with each other becomes small. This appears to occur at a a) Permanent address: Hughes Aircraft Co., Torrance, CA 90S09. distance of 1-10 p,m, corresponding to a defect density of 107_108 cm·--2• The performance of electrical devices fabricated on GaAs-on-Si is characterized by the presence of high reverse bias voltage leakage currents, whose origin is clearly related to the high defect density in the materiaLS The mechanism for production of these excess leakage currents is not, how ever, quite so clear. Intuitively, one might expect that recom bination at the extended defects would be a major contribu tor, although there is some evidence that defect-assisted tunneling may in fact be the dominant mechanism for the leakage current. (, The presence of the defects in the GaAs layer is perhaps even more deleterious to the performance of photonic devices, especially lasers. The defects tend to be mobile under minority-carrier injection and agglomerate in the active region of the laser, forming nonradiative areas. This obviously degrades the light output from the device and eventually leads to the termination of lasing action.7 It is clearly of interest to examine the effects of atomic hydrogen incorporation into this highly defected material system. Hydrogenation has previously been shown to passi vate or neutralize the electrical activity of a wide range of impurities and defects in semiconductors and might be ex pected to reduce the defect-related leakage currents in GaAs-on-Si diode structures. x We have previously reported this effect for the case in which the hydrogen was i.ntroduced by exposure of the GaAs-on-Si to a hydrogen plasma.9 In some respects a more controlled method of incorporating the hydrogen is by ion implantati.on, in which a known dose can be placed at known depths in the GaAs. The disadvan- 347 J. Appl. Phys. 65 (1), 1 January 1989 0021-8979/89/010347 -07$02.40 © 1988 American Institute of Physics 347 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.94.16.10 On: Sat, 20 Dec 2014 23:59:19tage of this technique, of course, is the introduction oflattice damage by the implanted ions, which requires annealing at elevated temperatures. The major question is whether this damage can be annealed out without removing the beneficial effects of the hydrogen. A related point of interest is the extent of any redistribution of the hydrogen during the post implant anneal. The GaAs grown on Si is highly strained and defective, and the motion of hydrogen within it might be expected to be somewhat different from conventional GaAs. In this paper we compare hydrogenation of GaAs-on-Si by both plasma exposure and ion implantation, compare the annealing required to restore the original conductivity, and examine the redistribution of the hydrogen during post-hy drogenation annealing. We observe a correlation of the amount of hydrogen incorporated during plasma exposures with the amount of initial disorder in the GaAs layer and note a somewhat surprising thermal stability of hydrogen located around the heterointerface region. We have predom inantly used leakage current-voltage (1-V) measurements as a qualitative indication of the concentration of electrically active defects. Deep-level transient spectroscopy is not sensi tive to the types of defect in GaAs-on-Si, and so this is the reason we use the somewhat indirect 1-V measurements. EXPERIMENTAL DETAilS The GaAs layers were deposited onto the Si substrates using a three-step technique which is basicaUy standard these days. We used 2-in.-diam, I-n em, p-type (B-doped) Si cut 4· off (l00) toward the [011], which was solvent cleaned and lightly etched before being loaded into a vertical geometry metalorganic chemical vapor deposition (MOCVD) reactor. 10 The Si substrates were then heated at 900 ·C for 10 min under AsH3 to thermally desorb native oxide from their surfaces. The substrate temperature was then lowered to 450°C to nucleate the growth ofGaAs, with deposition of ~ 100 A of material. Following this, the wafer temperature was raised to the growth temperature of -650 ·Cfor deposition of the GaAs at a rate of -4,um h.-I The final layer thicknesses varied frem 1.5 to 10 pm. Capaci tance-voltage (C~ V) profiling showed that all of the un doped GaAs layers were n type with net carrier densities in the range 1-3 X 1016 cm--3. Companion samples were exam ined by both plan-view and cross-sectional transmission electron microscopy (TEM). The defect structures and den sities observed in the material were similar to those reported previously by many authors,4 and discussed earlier in this paper. TABLE I. Types of GaAs-on-Si investigated. Structure No. GaAs layer sequence Doping We investigated hydrogenation in three basic layer structures summarized in Table 1. The first was simply to implant protons into the undoped GaAs. This was done both at high doses (1016 cm -2 at 100 keY), for the purpose of monitoring the redistribution of the implanted species upon annealing, and at low doses (5X 1013 cm-2 at 100 keY) to try to passivate the electrical activity of some of the defects in the material. The second type of structure consisted of a 0.15 ,um-thick n-type region (n~3 X 1017 cm-3) formed by im plantation of 29Si ions at a dose of5 X 1012 cm -2 (100 keY of energy), into the undoped GaAs. As we discussed in a pre vious paper," this simulates the depletion region of field-ef fect transistor structures, the most common electrical device used in GaAs technology. The implanted Si was activated by proximity rapid annealing at 900 ·C for 10 s. This annealing treatment reduced the microtwin and stacking fault density in the GaAs layer, but the threading dislocation density re mained essentially unchanged.!! These n-implanted GaAs structures on Si were hydrogenated by exposure to a 30-kHz, O.08-W cm--2 plasma contained within a parallel plate, ca pacitively coupled reactor. The samples were held at 250°C, and the exposure time varied from 0.5 to 3 h. After each plasma treatment, these samples were annealed at 400 ·C for 5 min in N2 to restore the electrical activity of the shallow donor impurities in the material. This anneal is necessary to ensure that we can make valid comparisons of hydrogenated GaAs-on-Si with unhydrogenated material of the same dop ing density. We have previously demonstrated that such an anneal is sufficient to remove shallow-donor passivation in GaAs grown on GaAs or Si. !2 The third type of structure consisted of -2.um ofSi-doped n+ (2X 1018 cm-3) GaAs grown on the Si, followed by 8 f.lm of undoped GaAs. Some samples were given an in situ anneal under AsH3 at 750·C for 10 min after deposition of the n -{-layer, in order to reduce the interfacial disorder. Companion samples were grown with a similar structure, but without the annealing step. The doping concentration in the undoped GaAs was similar in all samples (n-3X 1016 cm-3). The purpose of both types of samples was to examine the effect of the presence of varying degrees of lattice disorder on the total amount of hydrogen incorporated into the GaAs-on-Si. The electrical effects of hydrogen incorporation were examined by current-voltage (I-V) measurements in TiPtAu Schottky diode structures. The TiPtAu contacts were deposited by electron-beam evaporation through a shadow mask, to a total thickness of2S00 A. Ohmic contact was made by a low-temperature (_325°C) anoy of In on Hydrogenation method 1 2 3-,um undoped 0.15-,um n-type 2.85-,um undoped n~3x 1016 em-J n-3 X 1017 ern -3 n-3X1016cm 3 H+ implant; sx 10"_10IE em 2,IOOkeV H plasma 250 'c, 0.5-3 h 3 348 8-,um undoped 2-Pfi n+ GaAs J. Appl. Phys., Vol. 65, No. i, 1 January 1989 n-3XlO '6cm' n-2X 10'8 em-, H plasma 250 ·c, 0.5-3 h Zavada et at. 348 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.94.16.10 On: Sat, 20 Dec 2014 23:59:19lOp.m Gata-ON-Si AS -GROWN lOf'!'I'l GaAs -ON-Si IN -SITU ANNEALED '? 1020 D PLASMA O.5h, 250"C 106 ~ r-------~----, o PLASMA 0.5h, 250°C 106 FIG. 1. SIMS profiles of deuterium in GaAs-on-Si samples grown either with or without an in situ anneal and subsequently exposed to it D plasma for 0.5 h at 250°C, o 2 4 6 8 10 12 DEPTH (f\-m) o 2 4 6 8 10 12 14 DEPTH {fLml the front face of the samples. The atomic profiles of hydro gen or deuterium in the implanted or plasma-treated materi al were obtained using negative secondary ion mass spec trometry (SIMS) measurements with Cs-+ -ion bombardment in a Cameca IMS 3fsystem.13 The concentra tions obtained in this way were calibrated by comparison with implanted standards and the depth scales established by stylus measurements of the sputtered crater depths. The former are usually quoted to be accurate to within a factor of 2, while the latter are generally accepted to be accurate to ±7%. RESULTS AND DISCUSSION The amount of hydrogen or deuterium incorporated into semiconductors depends on a number off actors related to the density of sites to which it can bond. These sites in clude dopants, defective bonds, and regions of strain in the material associated with line and point defects and certain types of impurities.8 The high level of lattice disorder near the heterointerface of GaAs-ou-Si might be expected to at tract a significant density of hydrogen. To examine this we exposed the to-,um-thick GaAs layers on Si (layer structure 3 in Table 1) to a deuterium plasma for 0.5 h at 250"C. Figure 1 shows the SIMS profiles obtained from samples that either had or had not received the 750 "C, lO-min an neal. There are two components to the D profile in each sample. The ben-shaped distribution within the first 5,um is typical of that observed in plasma-exposed GaAs. It does not correspond to a classical error-function profile for unimped ed, one-species diffusion. Based on our current understand ing of the permeation of hydrogen into semiconductors, it is possible that the SIMS profile in this region represents deu terium present in at least two forms. The first is deuterium complexed with the shallow-donor impurities in the GaAs. These are present at a concentration of only _1016 cm-3, and therefore there must be at least one other form of deuter ium present at a concentration ofS X 1017 cm-3• This almost 349 J. Appl. Phys .• Vol. 65, No.1. 1 January 1989 certainly includes some form of dusters of deuterium, possi bly as simple as deuterium molecules, or may be larger asso ciates such as the extended platelets observed in proton-im planted GaAs which has been annealed above 200 QC. 14 The spike in the distributions near 3.5 pm corresponds to a growth-interruption step during the GaAs deposition and probably represents deuterium accumulation at interfacial defects or impurities. The second component in each D profile occurs at depths between 8 and 10 pm. In both samples this region is Si doped to a level higher than that of the overlying 8 lim of GaAs, and so one might expect more deuterium to accumu late there. However, there is clearly less deuterium between 8 and 10 pm in the sample that received an in situ anneal. This sample contained less disorder near the heterointerface than the un annealed sample, as measured by He-ion chan neling and cross-sectional TEM. There was a complete ab sence of stacking faults and microtwins in the in situ an nealed material, and the backscattering yield at a depth of 8 /-lm was 36%, compared with 49% in the unannealed GaAs on-Si. This is consistent with the previously observed char acteristics of in situ annealed material. ]5 The increased con centration of deuterium near the heterointerface in the latter sample therefore represents the combined influence of stack ing faults, microtwins, and other defects which can bond deuterium. Capacitance-voltage profiling of the GaAs-on-Si after hydrogenation showed reductions in the carner density in the first -1 lim from the surface in all samples. Typically, there was a reduction of approximately an order of magni tude within this region, corresponding to passivation of the shanow donors in the material. The initial doping levels were restored by annealing at 400 ·C for 5 min but even after this treatment we observed significant reductions in diode re verse leakage current in hydrogenated material. Figure 2 shows reverse J-V characteristics from the Si-implanted GaAs-on-Si material, hydrogenated for 3 h at 250 ·C, an nealed at 400"C to restore the shallow-donor doping, and Zavadaefal. 349 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.94.16.10 On: Sat, 20 Dec 2014 23:59:19'10-10 OL.L.L-~2--!3~-..J,4----'5~--:!6'---±-7---!S VR (VOLTS) FIG. 2. Reverse-bias 1-V characteristics from TiPtAu diodes fabricated on S1-implanted (n~ 3 X IO{7 em -3) GaAs-on-Si either untreated or plasma hydrogenated (3 h, 250 'C), followed by annealing at 400 'C for 5 min to restore the shallow-donor activtiy. then processed into diode structures. These diodes show breakdown voltages, defined as the reverse bias at which the leakage current is 1 rnA, of -6.5 V, compared with 2.5 V for unhydrogenated diodes. We emphasize that c-V measure ments showed that doping concentrations were identical in the two types of samples, with the only difference being that hydrogen is still presumably bound at defect sites in the plas ma-treated material, even after the anneal to restore the shal low doping level. Diodes formed in exactly the same fashion on homoepitaxial GaAs of the same doping density showed reverse breakdown voltages of ~ 8 V and displayed no im provement upon hydrogenation and annealing at 400 °e. We varied the plasma exposure conditions for the GaAs-on-Si over the temperature range 125-250°C, and from 30--180 min, but were unable to achieve diode breakdown voltages as high as in the homoepitaxial diodes. This could be due to several factors, including the possibility that some passivat ed defects were reactivated by the 400°C anneal to restore the shallow doping concentration, or that not all of the elec trically active defects were passivated by hydrogen. We have no way to distinguish these possibilities, although it is typical of many hydrogenation experiments to observe only partial passivation of defects or impurities. Passivation of the intrin sic defect levels in molecular-beam cpitaxially grown GaAs16 and of DX centers in AIGaAs, 17 all of which showed complete passivation to the deep level transient spectroscopy (DLTS) detection limit, are notable exceptions. It is worth mentioning at this point that not only was the reverse breakdown voltage in the GaAs-on-Si altered by hy drogen-plasma exposure, but the Schottky barrier height de termined from the J-V characteristics was also changed, as shown in Table II. In untreated samples the barrier height was measured to be 0.67 V, while after a 3 h, 250°C plasma exposure, followed by 400 °C annealing and deposition of the TiPtAu, the barrier height was reduced to 0.52 V.9 In sam ples hydrogenated by proton implantation, we observed im- 350 J. Appl. Phys., Vol. 65, No.1, 1 January 1989 TABLE II. Average ideality factors (n), barrier heights (,pe). and break down voltages (VB) in GaAs-on-Si diodes, obtained from 1-V measure- ments. Layer structure 2 Untreated n ifJB (eV) VB (V) 1.35 ± 0.08 0.67 ± 0.02 2.44 ± 0.07 Hydrogenated n ifJB (eV) VB (V) 1.32 ± 0.Q1 0.52 ± O.Ql 6.48 ± 0.27 Layer structure 3 Untreated n ,pB (eV) VB (V) 1.28 ± 0.06 0.71 ± 0.02 19.54 ± 0.58 Hydrogenated n o/B (eV) VB (V) 1.29 ± 0.04 o.n ± 0.Q3 30.30 ± 0.95 provements in reverse breakdown voltage, but no change in barrier height, as also shown in Table II. This is consistent with our previous assumption that the change in barrier height in plasma-treated material is due to removal of free As and its oxides from the surface as AsH) and water vapor.9,tH The improvement in reverse breakdown voltage was not stable for annealing above 550°C, decreasing to 3.5 V after a 600°C, 5-min treatment. Figure 3 shows the atomic profiles of deuterium in a plasma-treated sample after annealing at 400°C for 5 min. There was no motion of the deuterium up to 400 °C, at which temperature some redistribution is evi dent, with the onset of pileup at the heterointerface. After 600 ·C annealing there is diffusion of deuterium both toward the surface and to the heterointerface where there is a sub- 1020 4p.m GaAs -ON -Si lOS D PLASMA O.5h, 250°C ..., 5 MIN ANNEALS I E '" 1019 to5 m z I- 0 Z I-::l 0 <1 AS -TREATED ~ u 0::: I-104 Z Z Q lJ.! (,) . >-Z 0::: 0 400QC ! <1 u 1017 103 0 'f 1 :'--Ga-+ z :ii: 0 ::l '...., e e U a::: \I,...L! w , I' m w : \ I- ::l 10i6 .. 102 W 0 600·e 1015 101 0 2 4 6 8 DEPTH (ftm) FIG. 3. SIMS profiles of deuterium in undoped GaAs-on-Si treated in a plasma for O.S h at 250'C and subsequently annealed for 5 min at 4OO·C or 600 'C. Zavadaetal. 350 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.94.16.10 On: Sat, 20 Dec 2014 23:59:19II') \ e Q 1021 Wi 9 >- f- Cf) Z W o Z W (!) o 0:: o )0- J: 300"C DEPTH (,urn) FIG. 4. SIMS profiles of hydrogen in proton-implanted (10'6 em -2, 100- ke V) GaAs-on-Si as a function of post-implant annealing temperature (20- min anneals). stantial accumulation. This is the region of maximum disor der in the material and emphasizes once again that hydrogen and deuterium are attracted to any site of strain in semicon ductors. It is interesting that the hydrogen (or deuterium) must be in an atomic state, since molecules show no evidence of significant motion or trapping in any semiconductor.19 After trapping, however, the hydrogen is strongly bound and upon annealing shows no ability to passivate dopants. It is therefore in an apparently inactive state. The accumulation of hydrogen at the heterointerface upon annealing was even more evident in proton-implanted material (structure 1 in Table I). Figure 4 shows SIMS pro files of hydrogen in a sample implanted with IOO-keV H+ ions to a dose of 1 X 1011> cm-2, followed by annealing up to 700 ·C for 20-min periods. In this case there was little motion at 200 ·C, but some slight redistribution at 300 ·C, especially on the tail of the implanted profile. With increasing anneal ing temperature, hydrogen is lost to the surface, but there is a tremendous accumulation at the heterointerface. The sur prising result is that this accumulation is stable to 700·C annealing, and even in the original implanted region as hy drogen is lost by diffusion, the remaining hydrogen retains the profile shape of the implanted distribution. This indi cates that there is still some remnant damage in the GaAs even after 700°C annealing, and that the hydrogen is decor ating this damage. The enhanced accumulation near the he terointerface in the implanted material compared with the plasma-treated GaAs-on-Si may be slightly misleading, be cause it must be remembered that the implanted layer was only 2.5 f..tm thick and therefore had poorer crystalline quali ty than the 4-,um-thick sample that underwent plasma expo- 351 J. AppL Phys., Vol. 65, No. i, 1 January 1989 2,um GaAs-ON-GaAs .. Ht 5)\ 1013 cm-2 .. 3He+3xIO '3cm-2 .... I \ \ '" \ \ $ \ \\ ~ \ lOT \ 1O't \ : INITIAL SHEET RESISTANCE 1021 I I I ! I I o 100 200 300 400 500 600 700 800 ANNEALING TEMPERATURE (OC) FIG. 5. Sheet resistance of n-type (\0.7 em -3) GaAs layers implanted with multiple-energy (30-, 100-, and 20G-keV) H+ or 'He ~ at doses of 5 X 10'3 or 3 X JO l:l em -2, respectively, as a function of post-implant annealing tem perature. sure. The anneals in the former case were also for 5 min only, while in the latter case they were for 20 min, and deuterium was used in the plasma exposure compared with implanted hydrogen. Taking an these factors into account, there ap pears to be a roughly similar rate of accumulation at the interface for the two methods of hydrogen introduction. The obvious problem with the use of conventional high energy implantation as a technique for hydrogenating GaAs-on-Si is the introduction of the lattice damage so evi dent from the results in Fig. 4. The question is whether, for the dose levels that might actually be used in device struc tures, this damage can be annealed while still retaining the beneficial effects of hydrogen. The first thing to determine is the annealing temperature required to remove the hydrogen implant damage. Figure 5 shows the sheet resistance of 2- pm-thick n-type (_1017 cm-3) GaAs layers grown on semi-insulating GaAs substrates, after proton implants at multiple energies (30, 100, and 200 ke V) at a dose of 5 X 10 \3 em -2, and then annealed for 5 min at the indicated tempera tures. The evolution of the sheet resistance with annealing temperature can be explained by the introduction of dam age-related deep levels which trap electrons in the GaAs, increasing the resistivity of the material after implantation. However, the damage sites are close enough that electrons can hop from one to another, leading to a low-mobility con duction. The hopping conductivity is reduced as some ofthe damage is annealed out with increasing annealing tempera ture, leading to an increase in the resistivity. At some tem perature (around 300 ·C for proton implants) the deep-level density falls below that of the donor concentration and eIec- Zavada et at. 351 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.94.16.10 On: Sat, 20 Dec 2014 23:59:191018 H 5 x 1013 cm~2 100 keY ...... GaAs -ON-Si ---'--GaAS! Si 1017 ", , E t) <{ Z i 0 Z 1016 DEPTH (fLm) FIG. 6. Carrier profiles in GaAs-on-Si implanted with tOO-keY H+ ions at a dose of 5 X 10 13 em -2, and subsequently annealed for 5 min at either 400 aT 500 "C. trons are returned to the conduction band, lowering the re sistivity until eventually it reaches its unimplanted vaIue.20 This occurs at 500 ·C for this particular dose of protons into GaAs. It is worth noting that even the use oeRe +-ions, also shown in Fig. 5, shifts the annealing curve somewhat to higher temperatures, and therefore the use of another defect passivating species, such as Li, is probably precluded by the extra annealing required for heavier ions. As a further check that 500·C annealing restores the initial condition of the GaAs lattice for proton implants at doses around 5 X 1013 cm -2, we made electrochemical C-V measurements on implanted GaAs-on-Si samples after sev eral annealing treatments. Figure 6 shows the initial carrier profile and after a 5X lOLl cm-2, tOO-keY H+ implant. In the latter case the reduction in doping will be due predomi nantly to the damage introduced with possibly a small com ponent due to donor passivation by hydrogen. After anneal ing a 400 ·C this latter effect will be removed, as will some of the damage (refer to Fig. 5). Finally, we see that 500·C annealing restores the carrier density to its initial value. Based on this information, we can look for the beneficial effects of hydrogen incorporation by implanting protons into the GaAs-on-Si, annealing at 500 ·C to restore the initial carrier density, and comparing the 1-V characteristics of a diode structure with that of an unimplanted companion. The reverse-bias J-V data from TiPtAu diodes fabricated on un doped (n = 3X 1016cm-3) 2-3-.um~thickGaAslayersonSi are shown in Fig. 7. The untreated sample had a reverse breakdown voltage of ~ 19.5 V, whereas the hydrogenated diode shows a value of -30 V. An unimplanted sample that 352 J. Appl. Phys., Vol. 65, No.1, i January 1989 10-3 10-4 10-5 10~r '< It: 10-7 .... 10~8 10-9 10-10 10-11 0 ;0 TIPIAu A"2.08x10-:~cm-2 i5 20 25 30 35 VR (VOLTS) 40 FIG. 7. Reverse-biasl- Vcharacteristics from undoped (n-3 X 10'6 em-3) GaAs-on-Si samples processed into TiPtAu Schottky diodes. One of the samples was implanted with lOO-keY H+ ions (5 X 1013 cm-2) and an nealed at 500·C for 5 min prior to metallization. also underwent a 500 ·C anneal showed a similar breakdown voltage as the control diode (i.e., 19.5 V). Therefore, the proton implant appears to be effective in improving the di~ ode characteristics of GaAs-on~Si structures by passivating the electrical activity of some of the defects in the material. Once again, however, the breakdown voltage of even a hy~ drogenated diode was inferior to one fabricated on homoepi taxiaI GaAs of the same doping density. In the latter case we observed a diode breakdown voltage of -43 V. We note also that the thermal stability of improvement in performance of the implanted diodes was similar to that of plasma~exposed samples. The only difference between the two methods of hydrogen introduction was the fact that there was no lower ing of the Schottky barrier height for proton-implanted sam ples. CONCLUSIONS AND SUMMARY We have compared hydrogenation of GaAs-on-Si by two different methods: Hz-plasma exposure and proton im plantation. In both cases there is a significant improvement in reverse breakdown voltage of TiPtAu Schottky diodes, compared with unhydrogenated diodes. This improvement is presumably due to a reduction in the number of electrical ly active defects in the GaAs-on-Si upon hydrogenation. There is extensive redistribution of hydrogen to the heteroin terface at annealing temperatures above 500 °C, and the hy drogen appears to be in a strongly bonded form when it is in the interface region because of its subsequent thermal stabil ity. It could indeed be in several forms, such as bound to defects or dangling bonds, or in a clustered state. Since after high-temperature annealing there is no apparent dopant pas sivation during stimuli such as minority~carrier injection, the hydrogen is apparently in an inactive state. I t is worth emphasizing that the defect passivation in the Zavada at al. 352 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.94.16.10 On: Sat, 20 Dec 2014 23:59:19material is incomplete, a frequent feature of hydrogenation experiments. Therefore, the incorporation of hydrogen is not a panacea for the high defect density in GaAs-on-Si, but rather it indicates the important role these defects play in degrading the electrical quality of material. As is widely re cognized, the future utility of GaAs-on-Si depends on mak ing real progress in reducing the defect density from the cur rent value of ~ 108 to 104 cm--2 or less. ACKNOWLEDGMENTS The authors acknowledge the supply of some of the GaAs-on-Si material from So Mo Vernon and V. E. Haven (Spire Corporation), and the interest of A S. Jordan. The ion-channeling results were provided by K. T. Short (AT&T Bell Laboratories). 'R. M. F1etcher, D. K. Wagner, and J. M. Ballantyne, App!. Phys. Lett. 44, 967 (1984). 2R, J. Fischer, W. F. Kopp, J. S. Gedymin, and H. Morko,<, IEEE Trans. Electmn Devices ED-33, 1407 (1986). 3T. H. Wind hom, G. M. Metze, R-Y. Tsauf, and J. C. C. Fan, App!. Phys. Lett. 45,309 (1984). 353 J. Appl. Phys., VoL 65, No.1, 1 January 1989 'See, for example, papers in Proc. Mater. Res. Soc. Syrup. 91 (1987). 5N. Chand, F. Ren. S. J. Pearton, N. J. Shah, and A. Y. Cho, IEEE Trans. Electron. Device Lett. EDL .. S, 185 (1987). 6N. Chand, R. Fischer, A. M. Sergent, S. J. Pearton, D. V. Lang, and A. Y. Cho, App!. Phys. Lett. 51, 1013 (1987). 7J. P. Van del' Ziel, R. D. Dupuis, R. A. Logan, R. M. Mikulyak, C. J. Pinzone, and A. Savage, Appl. Phys. Lett. SO, 456 (1987). "S. J. Pearlon, J. W. Corbett, and T. S. Shi, App!. Phys. A 43,153 (1987). 95. J. Pearton, C. S. Wu, M. Stavola, F. Ren, J. Lopata, W. C. Dautremont .. Smith, S. M. Vernon, and V. E. Haven, App!. Phys. Lett. 51, 496 (1987). IllS. M. Vernon, V. E. Haven, S. P. Tobin, and R. G. Wolfson, 1. Cryst. Growth 77,530 (1986). lIS. M. Vernon, S. J. Pearton, J. M. Gibson, K. T. Short, and V. E. Haven, AppL Phys. Lett. 50, 1161 (1987). 12J. Chevallier, W. C. Dautremont-Smith, C. W. Tu, and S. J. Pearton, App!. Phys. Lett. 47,108 (1985). 13Charles Evans & Associates, Redwood City, CA. 14H. C. Synman and J. H. NecthHng, Radiat. Eff. 69, 199 (1983). "Y. Ito, Appl. Phys. Lett. 52, 1617 (1988). I·W. C. Dautreruont-Smith, J. C. Nabity, V. Swaminathan, M. Stavola, J. Chevallier, C. W. Tu, and S. J. Pearton, Appl. Phys. Lett 49, 1098 (1986). 17J. C. Nabity, M. Stavola, J. Lopata, W. C. Dautremont-Smith, C. W. Tu, and S. J. Pearton, Appl. Phys, Lett. 50, 921 (1987). '"F. Capasso and G. F. Williams, J. Electrochem. Soc. 124, 821 (1983). 19S. J, Pearton, J. W. C:orbett, and T. S. Shi, AppL Phys. A43, 153 (1987). 2OS. J. Peart on, J. M. Poate, F. Sette. J. M. Gibson, D. C. Jacobson, and J. S. Williams, Nucl. lnstrum. Methods B 10/20, 369 (1987). Zavada et al. 353 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 131.94.16.10 On: Sat, 20 Dec 2014 23:59:19

1.1140557.pdf

High performance xray area detector suitable for smallangle scattering, crystallographic, and kinetic studies J. Widom and H.P. Feng Citation: Review of Scientific Instruments 60, 3231 (1989); doi: 10.1063/1.1140557 View online: http://dx.doi.org/10.1063/1.1140557 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Real-time studies of gallium adsorption and desorption kinetics on sapphire (0001) by grazing incidence small- angle x-ray scattering and x-ray fluorescence J. Appl. Phys. 103, 103538 (2008); 10.1063/1.2936969 High pressure-jump apparatus for kinetic studies of protein folding reactions using the small-angle synchrotron x- ray scattering technique Rev. Sci. Instrum. 71, 3895 (2000); 10.1063/1.1290508 Performance of a highresolution, synchrotronbased, smallangle xray scattering instrument Rev. Sci. Instrum. 67, 3021 (1996); 10.1063/1.1147424 Evaluation of a linear photodiode array detector for synchrotron smallangle xray scattering measurements Rev. Sci. Instrum. 60, 3224 (1989); 10.1063/1.1140556 SmallAngle XRay Scattering J. Appl. Phys. 27, 620 (1956); 10.1063/1.1722443 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16High performance x .. ray area detector suitable for smaU .. angle scattering, crystallographic, and kinetic studies j. Widom Departments ajChemistry and Biochemistry, University oj Illinois at Urbana-Champaign, Urbana, Illinois 61801 H.-P. Fang Department 0/ Chemistry, University a/Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 31 March 1989; accepted for publication 9 June 1989) An x-ray area detector has been constructed that has the following capabilities: lower noise and/ or higher spatial resolution than previous opto-electronic designs; higher spatial resolution and no significant countrate limitations as compared to multi wire designs; and capability of acquiring millisecond- or microsecond-wide snapshots of a kinetically evolving x-ray pattern. An important feature of the present detector is that an key components are commercially produced; this detector can readily be duplicated in other laboratories. INTRODUCTION X-ray scattering studies oflarge, weakly diffracting systems require a detector that has good spatial resolution and very low noise levels. Moreover, whether samples are anisotropic or not, it is most efficient to collect a full two-dimensional diffraction pattern at once (with an "area detector"). rather than simply measuring the intensities at one point or along one line at a time. X-ray film is commonly used as an area detector for CuKa (8 keY) x rays, but it has several signifi cant limitations: it has a high background noise level (the "chemical fog" of development) which leads to a very poor detective quantum efficiency for weak signals; it must be developed and then digitized before the data may be ana lyzed quantitatively; and it has a very limited dynamic range, so that a typical pattern must be recorded on several films that are exposed for different times and then scaled together. These and other deficiencies of x-ray film have prompt ed the development of alternative technologies for x-ray area detection. One class of such devices are multi wire propor tional counters 1-4; two of these are commercially available. 1.2 These devices can have vanishingly low noise levels, but they are capable only of modest spatial resolution; strict count rate limitations generally prohibit their use in kinetic studies (see below). A second class of detectors use a phosphor to convert the x-ray pattern to a dim visible light pattern, and then use one of several technologies to acquire the visible image.5-12 One such detector is commercially available.5,11 This general approach seems quite promising; but, each of the devices described until recently (see below) suffered ei ther from relatively pocr noise levels or relatively poor spa tial resolution or both. Additionally, many of them required extensive and sophisticated custom electronics that could not readily be duplicated in another laboratory. A third class of detectors have recently been commercialized; these are called "imaging plates," 13 and are used in a manner analo gous to film. Incident x rays are absorbed in a "storage phos phor" which is subsequently read out by scanning with a focused laser. The noise, resolution, and dynamic range of imaging plates are all very good. They have only two modest disadvantages: because the plates must be read out in a sec ond instrument, they do not anow one to see the diffraction pattern in "real time, " which is often desirable; and, it would be difficult to use such a detector for rapid kinetic studies. We set out to develop an x-ray detector that would have noise levels, spatial resolution, and dynamic range similar to those of the imaging plate detectors, but that would also allow essentially real-time observation of the diffraction pat terns. Importantly, our detector would utilize commercial products for all key components, so that it could readily be duplicated by other laboratOlies. While our work was in progress, another detector was described which met similar design criteria. 10.12 Our detector is related to this and other opto-electronic detectors,5-12.14 but differs significantly in detail. Compared to this other recent design, we find that the present design yields competitive performance with regard to noise levels and spatial resolution, and that it provides a unique kinetic capability. I. SYSTEM OVERViEW A. Hardware A block diagram of the detector is illustrated in Fig. 1; key components are listed in Table I. The detector head con sists of two separately refrigerated modules, which are opti. cally coupled by a lens. The x-ray diffraction pattern passes through a layer of black paper and impinges on a phosphor which converts the x rays to visible tight. The phosphor is GdOzS (Tb) CP43) deposited at a density of 10 mg em -2 on a fiberoptic faceplate. \5.16 The faceplate is optically coupled, via direct mechanical contact, to the fiberoptic input win dow of a proximity focused microchanneI plate image inten sifier tube having a 40-mm-diam aperture. The intensifier tube operates at unity optical magnification and in the un saturated (linear gain) mode, at 15 ooaX luminescent gain. The phosphor and intensifier are mounted in a thermoelec- 3231 Rev. Sci. Instrum. 60 (10), October 1989 0034·6148/89/103231-08$01.;30 ® 1989 American Institute of Physics 3231 •••• , •••••••••••••••••••••••••••••••• n •••••••••••••••••••••••••• ' ••••••••••••••• ,. ~ ••••• ~ •• This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16PHOR MACRO LENS OPTIC FACEPLATE I WINDOW. SHUTTER IMITY- FOCUSED MICROCHANNEL I IMAGE INTENSIFIER I ceo COOLED HOUSINGS---l------. WINDOW ~----Ic---~ CCD READOUT ELECTRONICS ON I OFF SHUTTER CONTROL ON I OFF CAMERA CONTROLLER IMAGE PROCESSOR PARALLEL iNTERFACE HIGH VOLTAGE SUPPLIES L-. __________ . ______________________________ --l HOST COMPUTER INTENSIFIER PHOTOCATHODE FIG. 1. Block diagram of the x-ray detector. tricaUy cooled housing and cooled to -20 0c. The output of the image intensifier tube is imaged at 3: 1 demagnification by a macro camera lens, onto a charge coupled device ( CCD) image sensor that is thermoelectrically cooled to TABLE I. Key components of x-ray area detector. Component L Phosphor 2. Fiberoptic faceplate 3. Image intensifer 4. Refrigerated housing 5. Lens 6. CCD camera 7.CCD Vendor GTE/Sylvania Specification P43 type 1820; 6-,um grainsizc. Galileo Electro-Optics No. l324-0390; NA 1 Corp. fibers, 6 f.1m diam, withEMA. ITT Electro-optical Products Div. F4113 Proximity fo cused microchannel plate intensifier tube with fiberoptic input, S20ER photocathode, P20 output phosphoron HV-NESA fiberoptic; 40-mm aperture. Products for Research TE316, modified for F4113 tube. Alpa 50 mm! 11.8 Macro Switar Photometries Ltd. CH220 camera head, CE200 camera electronics unit with 14 bit 1m .. noise ADC, CC200 controller. Thompson CSF TH7882CDA; 576X384 pixels format; pixel size 23 /-lm X 23 !Lm. 3232 Rev. ScI. 'nstrum., Vol. 60, No. 10, October 1989 -53°C. The CCD has a format of576X 384 pixels, each 23 jIm square. The demagnified image of the intensifier tube aperture is centered on the rectangular CCD format, with the diameter of the demagnified image equal to the long di mension of the CCD, 576 pixels. The active area on the phos phor is thus a rectangle having dimensions of 40 mm X 26.7 mm. The CCD is read out by a 14 bit low noise analog to digital converter, at a gain of 1 analog to digital converter unit (ADU) per 31 photoelectrons. The CCD controller communicates with a host computer over a standard IEEE 488 parallel interface. The detector is gated on and otfunder computer control either by a shutter in front of the CCD, or by electronic switching of the intensifier tube photocathode voltage. B. Software noise rejection During preliminary evaluation ofthe detector, we found that numerous bright spots accumulated in the CCD images even when the detector was not exposed to x rays. These spots apparently occurred at random in space and in time, and were much more frequent when the intensifier tube pho tocathode was switched on (photocathode at -180V) than when the photocathode was switched off ( + 10 V) while leaving the intensifier tube otherwise activated (microchan nel plate and output phosphor high voltage supplies operat ing as IIsual). While the source ( s) of this noise is (are) not known, it is reasonable to suppose that the extra spots found when the photocathode is on may be due to cosmic rays and to radioactive decay near the phosphor; the spots found with the photocathode off may be due to ionization events in the very high voltage field between the microchannel plate and the (proximity focussed) output phosphor. Quantitative X-ray detector 3232 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16n short exposures i 576 ~--·---I l C1DD n I 2 n ~ For each pixel: Examine ei! n exposures; Delete anomalous intensity in pixel (i, j) of exposure K; Replace with average of remaining exposures for that same pixel. ! DOD D l FIG, 2. Noise-rejection algorithm t<'1r the detector. The figure illustrates n exposures of equal duration in which, for some par ticular pixel (shaded), exposure No.2 has an anomalously high intensity. The algo rithm detects the anomalous value and re places it by an average of the other mea surements for that pixel. Other corrections are applied subsequently, Sum noise-rejected exposures. Subtract background exposure; Correct for pixel-sensitivity voriations. output exposure measures of detector noise averaged over whole images (see below) showed that these bright spots dominated the noise, and that they significantly degraded the detector's perfor mance. We therefore developed an algorithm for detecting and removing these spots, and implemented this algorithm in a program that runs on the host computer. The algorithm is outlined in Fig. 2; it takes advantage of the random nature of the noise source (s). Instead of taking a single exposure of duration T, one takes a set of n shorter exposures each of duration t = Tin. The algorithm makes three passes over the n intensity measurements, for each CCD pixel in turn. In the first pass, a mean and standard deviation are computed using aU n measurements, In the second pass, individual exposures are identified in which a pixel has an intensity that differs from the mean for that pixel 3233 Rev. SCi.lnstrum., Vol. 60, No. 10, October HiSS by some small multiple a of the standard deviation for that pixel; the aberrant intensity is then simply deleted. In the third pass, the m snrviving measurements are summed to gether, scaled by thefactor (nlm), and placed in the appro priate location in the output image. The parameters a and n could presumably be optimized; we have arbitrarily chosen the values a = 1.0 and n = 8 for our preliminary studies, which leads to a typical value for m """ n -1. This algorithm was used in ali of the work described below, except where specifically noted. It provides a great improvement in the detector's signal to noise ratio. at a very low "cost" in lost information: if one measurement out of 8 is deleted fOf every single pixel, the cost (discounting the readout noise-see below) is simply that 8 units oftime have been spent to ac quire only 7 units of time's information. The algorithm has X-ray detector 3233 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16been implemented on two different host computers, It takes 30 min on an 8-MHz Dell 286 computer, and 4 min on a Hewlett Packard 16,7-MHz 68020 based workstation. II. PERFORMANCE EVALUATiON A, Sensitivity The sensitivity of the detector was evaluated by measur ing the response to a 55Fe (5.9-keV x rays) calibration source. The detector was found to respond linearly to the incident x-ray dose. Each incident x ray produced on aver age a signal of 4.0 ADU's, leading to saturation at 4096 de tected x rays per CeD pixel for a single exposure, Using the manufacturer's specified ADC sensitivity of3 Ie--per ADU, this means that each x-ray incident on the phosphor pro duced 124e'-in the ceD. The uniformity of the detector's response was evaluated with a uniform flood of 5sFe x rays. Pixel to pixel variation in sensitivity was observed; correcting for variation owing to counting statistics, the standard deviation in sensitivity was found to be 6.6%-a small value, easily corrected with a pixel sensitivity lookup table (see Fig. 2). B. Noise The detective quantum efficiency of a counting detector is limited in part by random noise introduced by the detector into the measured signal. We have used the methods devel" oped by Gruner and colleagues8•17 to quantitate this noise for the present detector system. Pairs of exposures of equal du ration (T) are taken in the absence of any incident x rays, for a series of values of T. For each pair, one image (exposure) is subtracted from the other, and the mean and variance are then measured over the entire difrerence image. If the detec tor introduces random noise, then the mean over all pixels in the difference images will be zero, independent of T, but the variance per pixel will increase linearly with T. We have carried out this analysis of the present detector system, with and without the noise rejection algorithm. In all cases, the mean intensity in the difference images did not differ significantly from zero. The measurements of variance are plotted in Figs. 3 (a) (no noise rejection) and 3 (b) (with noise rejection). The measured sensitivity and the known pixel size and optical demagnification are used to convert the measured variance (ADU pixel-1)2 into an equivalent vari ance expressed in (x rays mm -2f for x rays incident at the phosphor. In both graphs, the variance is seen to increase linearly with time, from an initial value (at T = 0) which is not zero, These observations are consistent with a time-inde pendent readout noise superimposed on other noise sources that accumulate over time. The noise rejection algorithm is extremely effective. As discussed above, for a very low cost in lost information, it reduces the rate of variance accumulation by a factor of -100. After noise rejection, the data for variance are fit by the following equations: (1; (ADU pixel-I)2 = 10.6 + 0.0621' (s), a} (x rays mm-2at phosphor) 2 = 139+0.81T (s), (1) where the subscript s in the variance cJl indicates that this is 3234 Rev. SCi.lnstrum., Vol. 60, No. 10, October 1989 U) :::", 2 I >< 10 r--'-'----'-------'_.-'-------'-'------I t b I 8 t f d i t r.::_-----o~'-'----11 : ( 0.-.--0.'- --0--- ~ f---E.1 .. - I x r o ~ ~~ -.'-~ ~-~ ~~~--~~~ __ ~ ~ _~_._J 100 20D JOO 4DO 500 time (sec) FIG. 3, Accumulation of variance in the detector output as a fUllction of exposure time, in the absence ofincidcnt x rays, Each point is obtained from the sum of eight difference images (see text), each of duration 118 of the indicated time. In (a) the 8 difference images are simply summed together; in (b) the images arc summed together after running the noise rejection algorithm. Note that the two ordinates differ in scale by a factor of 100. the variance introduced by the system. Compared to the re lated detector described by Gruner and colleagues,IO,]2 the present detector has a readout variance that is higher by a factor of 3.3, and a time-dependent variance that accumu lates more slowly by a factor of 4.9, when expressed as (x rays mm2 at phosphor).2 (See Table II.) TABLE II. Properties of x-ray area detector. Experimental values Conversion efficiency (CCD e -lx-ray photon): 124 (ADU/x-ray photon): 4,0 Pixel saturation level, single frame (x rays pixe\-I): 4096 multiple frames: unlimited RMS noise, (<T;) 112 (x rays mm-2 at phosphor): 1139 + 0,81 T (8)]'/2 Dynamic range for single pixel, single frame: 5032: 1 mUltiple frames: unlimited Spatial resolution (FWHM, microns at phosphor): 120 Temporal resolution (single frame, sec): 10-3 Active region of phosphor: 4OmmX26,7 mm X-ray detector 3234 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16The dynamic range of a detector is often defined as the maximum signal divided by the minimum rms noise, which in the present case is simply the apparent readout noise, u = [~ (T = 0)] 112; thus u = 3.26 ADU pixel -I, or 0.81 x ray at phosphor per pixel. Taking the value of 4096 x-ray pixel -1 at saturation gives a dynamic range for a single pixel of5032:1; this value is slowly reduced as the time required to saturate a pixel is increased. The dynamic range can be made arbitrarily large by summing multiple exposures in the host computer, or by integrating the signal over many pixels from a single frame. A useful measure of detector performance is the relative uncertainty in a measured signal, 14 p, given by (2) where So is the signal output from the detector and Uo is the rms noise in that signa!. Generally, one counts the incident x rays until p reaches the desired precision. Following Eiken berry, Gruner, and Lowrance,8 we assume that (To is given by the relation (3) where 07 is given by Eq. (1) and (if is the variance in the signal incident on the detector owing to Poisson counting statistics. For a mean incident signal Si' u7 = Si' For an ideal detector, 07 = 0 and So = Si; thus p = l/uu independent of the incident x-ray flux (i.e., of the time required to count Si) and independent of the area on the detector face over which Si has been integrated. For a nonideal detector, a nonzero time-dependent contribution to a; causes p to depend on the incident flux, and a nonzero time-independent contribution to ~ causes p to depend on the area of integration. Calculated values of p as a function of Si are plotted in Figs. 4(a} and 4(b) for the present detector, for two differ ent areas of integration and several different incident signal fluxes. For comparison, results are also plotted for an ideal detector and for x-ray film. Film has a large time-indepen dent contribution to a; ("fog") but essentially no time-de pendent contribution. For an incident flux of 100 x rays mm -2 S -1 or greater, and an integration area of 1 mm2, the present detector closely approximates the behavior of an ideal detector. For lower incident fluxes and larger integra tion areas, the present detector is worse than ideal, but still vastly better than film, and better than previously described optn-electronic detectors (see Sec. IIO. For example, for an incident fiux of 0.1 x ray mm 2 s -1 and an integration area of 100 mm2, the detector reaches 5-10% precision 20-30 fold faster than film. Another quantitative measure of detector performance that is useful for comparing different designs is the detective quantum efficiency (DQE, or D), 17 defined as '2 D=(So/u o) (4) (S;I O'i) 2 This property too will depend on the incident flux and on the area of integration, for nonzero time-dependent and time independent contributions to a:, respedively. Results for one area of integration and various fluxes are illustrated in 3235 Rev. Sci. Instrum., Vol. SO, No. 10, October 1989 jOO.----- __ ------------------~ a ~ 10 2...- Q... I I 102 103 104 Id' Si IOO~r-------------~----------~ 10 105 Si FIG. 4. Thc relativ<! uIIcertainty p is plotted as a function of the signal S" the number of photons incident on an integration area at the phosphor of (a) I mm/or (h) 100 mm'. (A) Ideal detector; (_) x-ray film. Open symbols are calculated for the present dete;:;tor: (\) flux incident on phosphor ,= 0.1 x rays mm 's '; (0) flux = 1 x ray mm- 2 s-'; (V) flux ",100 x-ray mm -'0 s -'. Data for film calculated as described in Ref. 8. 3235 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16iLl " Q FIG. 5. Detective quantum efficiency is plotted as a function of Sj' the num ber of photons incident on an integration area of I mm" at the phosphor. An ideal detector has DQE =, 1. Curve (c) illustrates the behavior of x-ray mm"; curves (a) and (b) illustrate the behavior of the present detector for incident x-ray fluxes of (a) 100 x rays mm-o s-I and (b) 1 x ray mm-2s-l. Fig. 5 as a function of St. An ideal detector has D = 1. Plain ly, film is a very poor detector except when large numbers of x rays are incident on an integration area. C. Resolution We anticipated that the detector's spatial resolution would be dominated by the CCO pixel size, 23 flm square, which maps onto the phosphor with 3:1 magnification. All other optical elements are expected to have a much higher spatial resolution. The phosphor consists of 5-6-.um grains in a ~ 14-,um-thick layer; the fiber optics use 6-flm fibers; the microchanne1 plate in the image intensifier has a 15-,um cen ter to center channel separation; and, the lens is designed specifically for macro imaging applications. One typically measures the resolution by quantifying the extent to which the image of a pinhole mask pressed against the detector face exceeds the known dimensions of the pinhole. We used a 50-ftm-diam pinhole in a 2-mm-thick platinum disk. Twelve images were obtained from various positions on the detector face. Each image appeared radially symmetric. Therefore, for each image, the centroid was lo cated and the data were radially averaged about the centroid. The twelve radial integrals were summed together, and de convoluted for the diameter of the pinhole to yield the detec tor point spread function. This is illustrated in Fig. 6 as a weighted least squares fit to a Gaussian curve.15 The fun 3236 Rev. Sci. Instrum., Vol. 60, No. 10, October 1989 1.0 \ 0.0 L-+~----f=~-----1 o 1 2 3 r (pixe:s) FIG. 6. Point spread function of the detector. The full width at half maxi mum is l. 7 pixels, corresponding to 120 pm at the phosphor. width at half-maximum is 1.7 pixels, or 120 pm at the phos phor. D. Kinetic capability We have not yet experimentally characterized the kinet ic capability of the present detector: however, manufac turer's specifications and previous work by others allows us to anticipate the detector's kinetic behavior. Note that it takes several seconds to read out each image, so the kinetic behavior we refer to is the ability to capture a single narrow window in time in a kinetically evolving x-ray pattern. The detector can be gated on/off electronically by switching the intensifier tube photocathode voltage between -180 and + 10 V. The manufacturer specifies a tube re sponse time of 5 ns or less. IS This property can be used as follows to capture an x-ray pattern during a narrow time window. The detector is initialized by switching the intensi fier photocathode off, and clearing the CCD of stored charge. The x-ray beam is then turned on, and at t = 0 an experiment is initiated, e.g., by stopped flow mixing. After waiting some desired time 7dclay' the intensifier photocath ode is switched on for a brief period 7' acquisition' and then switched off again. Only during the period 7 a<:quisition are elec trons from the intensifier photocathode able to excite the intensifier tube output phosphor, leading to a signal in the CCD. Therefore, even though the output phosphor (P20) has a slow decay time, the output image reflects only x rays that were detected during the period Tacqui,ition' convoluted by the decay characteristics of the "front end" phosphor used to convert the x rays into visible light. The Gd02S(Tb) phosphor used here is known to decay to 1 % in 1 ms when excited by 8-ke V x rays. 15 CsI has a decay time to 1 % of 1 flos, and has an intrinsic sensitivity for 8-keV x rays that is only 2-3X lower than that of Gd02S(Tb).15 We conclude that the present detector has a kinetic resolution of 1 msec and that this could easily be extended to 1 flS. Single x-ray patterns captured in this manner cannot be processed by the noise rejection algorithm; but this is unlike ly to pose a problem in practice, for several reasons. First, the X-ray detector 3236 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16frequency of bright spots is sufficiently low that images inte grated over several seconds or less will usually contain none of them. Second, if desired, one could carry out three or more identical experiments, and then apply the algorithm. Final ly, if an image is to be radially integrated, one could apply an equivalent algorithm to all of the pixels in each radial zone of that single image. III. DISCUSSION The present detector should be compared to the three classes of x-ray area detector that are currently in use: multi wire counters, other opto-electronic detectors, and imaging plates. The present detector has a better spatial reso lution than any of the previously described detectors, al though the imaging plateD and the opto-electronic detector of Templer et al. 10,12 are only slightly worse (150f-l and 160 fL FWHM, respectively, versus our measurement of 120 fL). The present detector's noise level is somewhat worse than that which obtains with multiwire counters, the imag ing plate, and with an apia-electronic detector that can be used in a photon counting mode. (, However, as shown in Fig. 4(a), even for countrates corresponding to extremely weak scattering (e.g., 1 x-ray mm-2 s-I incident on an integra tion area of 1 mm2) the present detector is only 2-3 fold worse than an ideal detector for counting to 5% uncertainty. It is also useful to compare the noise level of the present detector with the alternative recent detector of Templer and co-workers. [0,12 Using their amended values of noise ex pressed as e-per pixel,12 we caiculate the variance for that device expressed as (x rays mm -2 at phosphor) 2. We calcu late for their detector a readout variance of 41.6 (x rays mm -2)2 and a time dependent variance of4.0T(s) (x rays mm-Z)2. That detector therefore has a readout variance that is 3.3 X lower (better) than the present one's, but it has a time-dependent variance that accumulates more rapidly (worse) by a factor of 4.9. We calculate that, for an integra tion time of 30 s and an integration area of 1 mm2, the two detector's noise levels are identical. For longer integration times, the noise (variance) i.s dominated by the time-depen dent component, and the lower rate of variance accumula tion ofthe present detector could be an advantage. It is possi ble that the detector of Templer and co-workers could be improved with a noise rejection algorithm such as that de scribed here. Possible methods for improving the present detector are discussed below. Compared to other existing detectors, the detector de scribed here, and that of Templer and co-workers, 10.12 will be of particular use in ordinary scattering experiments when one requires both high spatial resolution and immediate dis play of the diffraction pattern. For example, with the present system it is practical to scan along partially oriented sam ples, looking at the diffraction pattern in real time, to find the best oriented region; many studies of fiber or liquid crystal line samples could benefit from this approach. The present detector has another capability which makes it unique among area detectors described to date: the ability to capture narrow windows in time in a kinetically 3231 Rev. Sci. Instrum., Vol. 60, No. 10, October 1989 evolving diffraction pattern. With the Gd02S(Tb) phos phor, the kinetic resolution is expected to be one millisecond or better; with CsI this is extended down to one microsec ond. The only existing detector which approaches this capa bility is a special multi wire area detector that has a deadtime of 470 us (Ref. 4); such a device would require 5-10 ms to count 104 x rays distributed over an entire diffraction pat tern. It takes our detector severa! seconds to read out a com plete image, so we cannot monitor kinetic processes continu ously in time. Nevertheless, by doing a series of experiments in which one starts some process, waits a variable delay time, and then takes a snapshot, one can obtain valuable informa tion about the kinetics. It has been pointed out that gateable detectors such as the present one can be synchronized with a pulsed electric field and used to record diffraction from elec tric field-oriented samples. [9 It is also useful to consider how the present detector might be improved. One possibility is to reduce the detector noise levels by operating either the CCO or the image inten sifier or both at a lower temperature. The results discussed above were obtained with the CCD operating at -53 "C and the image intensifier operating at -20 'C. We have also carried out experiments with the image intensifier operating at 5 °C (data not shown). We find that the rate of dark vari ance accumulation drops by a factor of -3.5 when the tem perature of the image intensifier is reduced from 5 to -20°c' Even with the intensifier at -20 °C, the overall system noise is dominated by the image intensifier; therefore it would not be of significant help to operate the CCO at lower temperatures. It is possible that greater ~ooling of the image intensifier would provide a further decrease in system noise. It would also be useful to increase the active area of the present detector, which currently is a rectangular window having dimensions 40 mm X 26.7 mm. This area is restricted by the 40 mm aperture of the image intensifier. ITT markets a similar image intensifier tube with a 75 mm diam. This could easily be used in place of our 4O-mm tube. However, if the 75-mm tube were used with the same 576 X 84 pixel CCD, one would have to optically demagnify the image in tensifier output by a factor of 5.6 instead of 3.0. This would result in some loss of sensitivit yl4; also, it would degrade the spatial resolution since each CCD pixel would then map to 130 fLm at the phosphor. Both of these problems can be over come by using a larger CCD such as the 2048 X 2048 pixel CCD, with 27-fLm pixel width, produced by Tektronix. Both the larger image intensifier and the larger CCO could easily be incorporated into the present detector; their only disad vantage is much higher cost. Finally, it is of interest to consider hybrid detectors which combine some of the technology used in the present detector together with previously used technology. One de vice which is particularly likely to be useful would have a phosphor and image intensifier tube combination such as ours, except with the image intensifier operating in the satu rated (photon counting) mode, and having a resistive car bon anode rather than an output phosphor. Commercial po sition-sensing electronics, similar to those used with some wire-based detectors,20 could then be used to locate, dis- X-ray detector 3237 .......•.•. ;:.;.; ......•••••••••• :.:<.;-;.; ................... -•.•••.••...•.. -.'0" ••••••••••••••• ; •••••• :.;.; •••••••••• '.:-:.;.:.;.;.; •••••• ~ ••••••• :.:.:.:-; •••••••••• .,-••••• :.:.:.:.; •••••••••••• -;;:; •• :.:.:-:.~ •••••••• :.~.;;:.:.:.;.: •••••• :.:.:.:.:.:.;.:.: •••••• ' ••••••••••••• _ •••• _ ••••••• This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16criminate and count each x ray. Such a device would lack the kinetic capability of ours; but it should otherwise combine performance equal to that of the imaging plate with realtime display of the diffraction pattern. ACKNOWLEDGMENTS We are grateful to U. W. Arndt and A. R. Faruqi for valuable discussions at an early stage of this work, and to J. Chappell and M. Brines for advice concerning phosphor de position. J.W. acknowledges research support from the NIH, the Searle Scholars Program of the Chicago Commu nity Trust, and from an NSF Presidential Young Investiga tor Award. IR. Hamlin, Methods in Enzymology 114, 416 (1985). 2R. M. Durbin, R. Burns, J. Moulai, P. Metcalf, D. FJ'eymann, M. Blum, J. E. Anderson, S. C. Harrison, and D. C. Wiley, Science 232,1127 (1986). 'A. R. Faruqi, NucL lnstrum. Methods A 273, 754 (1988). 4A. Gabriel, C. BouEn, and M. H. J. Koch, Nucl. lnstrum. Methods (to be published) . 'u. W. Arndt, Methods in Enzymology 114,472 (1985). 6K. Kalata, Methods in Enzymology 114, 472 (1985). 3238 Rev. ScI. Instrum., Vol. 60, No. 10, October 1989 7R. L Dalglish, V. J. James, and G. Tubbenhauer, Nue!. lnstrum. Methods 227,521 (1984). 8E. F. Eikenberry, S. M. Gruner, and J. L. Lowrance, IEEE Trans. Nue!. Sci. NS·33, 542 (1986). OM. G. Strauss,!. Naday, I. S. Sherman, M. R. Kraimer, and E. M. West brook, IEEE Trans. Nuc!. Sci. NS·34, 389 (1987). lOR. H. Templer, S. M. Gruner, and E. F. Eikenberry, Adv. Electron, Elec tron Phys. 74, 275 (1988). "u. W. Arndt, G. A. and In'l Veld, in Adv. Electron. Electron Phys. 74, 285 (1988). 12S. M. Gruner, in Proceedings of the Third International Conference on Synchrotron Radiation Instrumentation [Rev. Sci. Instrum, 60, 1545 (1989) ]. "Y. Ameniya, K. Wakabayashi, H. Tanaka, Y. Uena, and J. Miyahara, Science 237,164 (1987). 14H. W. Deckman and S. M. Gruner, Nucl. lnstrum. Methods A 246,527 ( 1986). ISU. W. Arndt, Nuel. lnstrum, Methods 201,13 (1982). "'J. H. Chappell and S. S. Murray, Nue!. lnstrum. Methods 221, 159 (1984). 17S. M. Gruner, J. R. Milch, and G. T. Reynolds, IEEE Trans. Nuc!. Sci. NS.25, 562 (1978). I"ITT Electro-optical Products Division, F4113 Technical Bulletin, 1986. ,oM. H. J. Koch, E. Dorrington, R. Kliiring, A. M. Michon, Z. Sayers, R. Marquet, and C. Roussier, Science 240,194 (1988). 20U. W. Arndt, J. App!. Cryst. 19, 145 (1986). X-ray detector 3238 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.49.59.195 On: Fri, 12 Dec 2014 22:02:16

1.576238.pdf

Particle bombardment effects on thinfilm deposition: A review D. M. Mattox Citation: Journal of Vacuum Science & Technology A 7, 1105 (1989); doi: 10.1116/1.576238 View online: http://dx.doi.org/10.1116/1.576238 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/7/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Effect of energetic bombardment on the magnetic coercivity of sputtered Pt/Co thinfilm multilayers Appl. Phys. Lett. 56, 2345 (1990); 10.1063/1.102912 Lowresistivity Cu thinfilm deposition by selfion bombardment Appl. Phys. Lett. 56, 198 (1990); 10.1063/1.103024 Composite thinfilm production by ion bombardment J. Vac. Sci. Technol. A 5, 1250 (1987); 10.1116/1.574783 Summary Abstract: Theory of thinfilm orientation by ion bombardment during deposition J. Vac. Sci. Technol. A 5, 1792 (1987); 10.1116/1.574498 Theory of thinfilm orientation by ion bombardment during deposition J. Appl. Phys. 60, 4160 (1986); 10.1063/1.337499 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:05Particle bombardment effects on thin .. film deposition: A review D.M. Mattox Surface and Interface Technology Division, Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received 25 July 1988; accepted 5 September 1988) In many atomistic film deposition processes, concurrent energetic particle bombardment (ions, atoms, molecules, atom citlsters) may occur inadvertently and uncontrollably or bombardment may be used to deliberately modify film propert~es. These energetic particles can arise from (i) the acceleration of charged particles, (ii) high-energy neutrals from reflection from bombarded surfaces, or (iii) charge exchange processes. Particle bombardment effects that can affect film formation and growth include (a) modifying the substrate surface (cleaning, defect formation), (b) momentum transfer processes in the surface region (sputtering, desorption, recoil implantation, defect formation), (c) addition of heat to the surface region, and (d) formation of secondary elelctrom; that can affect chemical reactions. These in turn affect film properties such as adhesion, resi<iual film stress, film morphology, density, grain size and orientation, surface coverage, pinhole d~~sity, and surface area. The understanding of these effects and how to use them advantageously is important to those utilizing processes where concurrent energetic particle bombardment is occurring or can be made to occur. I. INTRODUCTION , In the atomistic deposition of inorganic thin films in a vacu um or low-pressure environment films are formed by the controlled addition of condensable atoms (ada toms) to a surface (substrate). The source of the ada toms may be from (i) thermal vaporization, (ii) physical sputtering, (iii) from a gaseous species (reactive gas or chemical vapor precur sor), or (iv) from other vaporization sources such as vacu um or plasma arcs. Major processing variables that may be used to modify the various stages of atomistic film formation include (a) substrate temperature, (b) deposition rate, (c) angle ofinci dence of the depositing particles, and ( d) the use of energetic particle bombardment. Energetic particle bombardment may be used to modify the substrate surface or influence the nucleation and growth of the depositing film material. Gen erally these energetic particles are ions of either a gaseous or condensable species formed in a plasma, 1,2 but they may also be an energetic neutral species or atom clusters such as are used in ion duster beam (ICB) deposition? Argon g;tS is the most commonly used inert gas used for plasma formation since it is the least expensive inert gas. However, heavier gas species such as krypton and mercury vapor have a number of advantages such as better momentum transfer to he~vi.er tar get atoms and decreased gas incorporation in the deposited film. Ions of reactive gases may be used in reactive film depo sition processes, reactive etching, or surface modification processing. The properties of films formed by atomistic deposition processes are generally very process and process parameter dependent and in order to understand the role that process variables may play in the film properties we must consider the way that a film is formed. The stages of film formation are (i) surface preparation, (ii) condensation and nucleation of the adatoms, (iii) inter face formation, (iv) film growth and, in some cases, (v) postdeposition treatments. 1105 J. Vac. Sci. Techno!. A. 1 (3), May/Jun 1989 Surface preparation may be defined as the treatment of a surface in order to obtain satisfactory processing, stability, and functionality.4 Surface preparation may be in the form of (a) cleaning, (b) modification of surfa~ chemistry, (c) modification of the morphology or physical properties of the surface, (d) formation of nucleation sites, (e) addition of nucleating agents (sensitization), or (0 "activation" of the surface to make it more chemically reactive. Cleaning of the surface allows intimate contact between the surface and depositing adatoms of film material. Plasma species, and ions accelerated from the plasma, may be used to clean the surface by physical sputtering or by chemical reaction (0, Cl, F) to form a volatile species (reactive plas ma cleaning). 5 Plasmas and bombardment may also be used to texture surfaces6 and activate surfaces, particular polymer surfaces.7 Surface chemistry may be changed by bombard ment, e.g., bombardment of a carbide surface by hydrogen ions from a plasma has been shown to cause carbon depletion in the carbide surface to an appreciable depth.8 When adatoms impinge on a surface they may have a degree of mobility on the surface before they nucleate and condense.9•10 The nucleation density of adatoms on a sub strate surface (and mode of growth) determines the interfa cial contact area and the development of interfacial voids; generally a high nucleation density is desirable for good film adhesion. The nucleation density depends on the kinetic en ergy and surface mobility of the adatoms, chemical reaction and diffusion with the surface material, and the nucleation sites available. Plasmas and energetic particle bombardment may (i) increase the adatom surface mobility, (ii) promote chemical reaction and diffusion by heating, by introduction of surface defects, and by changing the surface chemistry, and (iii) may introduce nucleation sites by lattice defect for mation, adsorption of activated species, implantation of im pinging energetic species, generation of electric charge sites, and the recoil implantation of surface species. II Interface formation will begin during nucleation of the adatoms on the surface and may proceed throughout the 1105 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:051106 D. M. Mattox: Particle bombardment effects on thin-film deposition 1106 deposition and even during postdeposition processing, sub sequent processing, and in-service usage depending on con ditions. The interfacial types may be categorized as12 abrupt, mechanical, diffusion, compound, and "pseudodiffusion." The abrupt interface is formed when there is no diffusion and thus the interface is an abrupt transition from one material to another in the space ofa lattice parameter (Au on NaCl). In this case the gradient of materials properties is also large and the nucleation density will generally be low. Due to the lack of reaction and the method of nuclei growth interfacial voids may be formed at the abrupt interface. The mechanical inter face is an abrupt interface with mechanical interlocking. This type of interface may provide good adhesion if the sur face roughness is "filled in" and interfacial voids are avoid ed. The diffusion-type interface is formed when there is in terdiffusion of the film and substrate materials. A problem with this type ofinterface may be the development of voids in the interfacial ("interphase") material if the diffusion rates of the film and substrate materials are different (Kirkendall voids). In the compound interface diffusion is accompanied by reaction to form a compound material. The interphase material thus formed may be brittle, have Kirkendall voids, and develop microcracks due to the stresses developed in forming the compound material; all of which reduce the fracture strength of the interface region and hence lower the film adhesion.13 The pseudodiffusion type of interface may be formed under low-temperature deposition conditions, or where the materials are insoluble, by physically mixing the depositing materials during deposition or by implantation or recoil implantation of at011ls into the substrate surface. A major concern in the development of interfacial re gions is the properties of the interphase materials. If the ma terial has voids, microcracks, and is brittle, then it will have a low fracture toughness. This low fracture toughness materi al will degrade the adhesion. In many cases it is best to limit the extent offormation of the interphase material in order to obtain good adhesion. 13 Energetic particle bombardment processes affect the in terface formation by affecting the nucleation processes (cleaning, changes in surface chemistry, nucleation sites), by increasing the contact area, decreasing the interfacial voids, and by providing a high thermal input into the sur face. Bombardment can also promote the formation of the pseudo diffusion type of interface by implantation and recoil implantation. Film growth occurs by nucleation on a "like material" and the same considerations as for nucleation on a foreign surface apply. In addition, larger-scale effects must be con sidered. In particular, geometrical effects may lead to the development of a columnar growth morphology 14 that often leads to undesirable film properties such as microporosity, low film density, high chemical etch rates, contamination retention, and other such effects. This columnar morpholo gy can be dependent on the angle of incidence of the deposit ing flux of film material. 15.16 The addition of a gaseous envi ronment where there is adsorption of gaseous species 01} the surface and energetic particle bombardment can also change J. Vac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989 the growth morphology.17-21 Bombardment can also alter other film properties such as the residual growth stresses, film density, gas incorporation surface coverage, chemical reaction rates, etc., which will be discussed in greater detail later. For reactive film deposition processes two general cases exist. In the first case there is a condensable species and a gaseous reactive species (Ti + N). In the second case both species are condensable and reactive under the proper condi tions but may not react or only partially react under other conditions such as a low deposition temperature (Ti + C). In reactive film deposition processes, in the absence ofbom bardment effects, the rate and degree of reaction is depen dent on the chemical reactivity of the reactive species, the temperature, the extent of the reaction, and the availability of the reactive species to the depositing species which in tum is dependent on system geometry, process parameters, and relative surface areas. The presence of a plasma and concur rent energetic particle bombardment may enhance chemical reactions on the surface by providing activated and energetic reactive species (radicals, ions) and utilizing bombardment to enhance chemical reactions on the surface. 22-24 Energetic particles for bombarding surfaces and grow ing films may arise from 0) biasing (dc or rf) a substrate immersed in a plasma so that it is bombarded by particles from the plasma, (ii) extraction of ions from a confined plas ma and accelerating them to a high energy into a vacuum environment (ion beam),25 (iii) reflected high-energy neu trals which arise from ions bombarding a surface in a low pressure environment26,27 such that the reflected neutrals are not thermalized by collisions in the gas phase, (iv) accel eration of negative ions from a negatively biased sputtering target,28.29 or (v) special ion sources such as field emitters.3o Biasing of a substrate immersed in a "processing plasma" is probably the most common application of particle bombard ment processing but the same effects are to be found with any source of bombarding particles. Figure 1 shows some configurations whereby a surface may be bombarded from a plasma. Figure 2 shows some configurations which may be used to bombard surfaces using ion beams. In many cases the configurations for bombard ment of a surface are very similar to configurations for sput ter deposition (plasma or ion beam) where the substrate is now the sputtering target and there is another source of de positing material. In many instances the complex substrate configuration or substrate fixturing leads to nonuniform electric fields and nonuniform plasma densities over the sur face and hence nonuniform bombardment over the surface. This leads to film property variations over the surface. The ion plating process31-34 uses energetic particle bom bardment just prior to and during film deposition to modify surface and film properties such as adhesion, surface and bulk morphology, density, residual stress, crystallographic orientation, grain size, and chemical composition, etc. Fig ure 3 shows a simple ion plating configuration using a dc diode plasma and a thermal evaporation source. In some configurations the substrate is immersed in the plasma and the part is completely coated. Figure 4 shows one such con figuration called the sputter ion plating (SIP) system35 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:051107 D. M. Mattox: Particle bombardment effects on thin-film deposition 1107 DC/IIF \-r-SUBSTRATES PLASMA I~ !- TH£RAlAL SPUTTERING V APORIIA nON a DCi/IF 1- SUBSTRATE ~ _____ ----'GR,O ~ c ~ ~ CHEMICAL VAPOR PRECURSOR GAS DC RF 1- SUBSTRUE FI CrJ El DC. !IF I DCdlf t d I SUBSTRATE I B .~~~ SUBSTRATE G:) ~-j.-!lEAM V ... CUUM e FIG. 1. Some configuratiol1s for bombarding a surface from a plasma by using accelerated or reflected high-energy particles: (a) diode, (b) "downstream configuration" using a remote plasma source, (c) grid to allow bombardment of complex surfaces or insulators, (dl thermo electron sustained plasma with magnetic en hancement/confinement, (e) electron beam evaporation with a diiferentiaiiy pumped vacu um cham her, (f) utilizing reflected high-energy neutrals and sputtering, (g) magnetron sputter ing source, and (h) moving magnetron plasma to allow uniform bombardment of substrate surface. SPUTTERED ATOMS REFLECTED NEUTRALS 6 6 -PLASMA TARGET J I SUBSTRATES -OC'RF I PLASM"'-~- lot 5 S N SUBSTRATE 1·- f which uses a grounded sputtering cathode to provide the depositing material. The term ion plating may also be modi fied to indicate specific variations in environment, source of depositing material, or source of bombarding particles, namely, "sputter ion plating" which uses a sputtering target source, "chemical ion plating" which uses a chemical vapor precursor gas as a source of depositing material, "reactive ion plating" which uses a reactive gas plasma, or "vacuum ion plating" which uses a vacuum environment. Most re cently the term ion plating is being applied to processes where the substrate is in contact with a plasma and the term ion assisted deposition (lAD) or ion beam enhanced depo sition (IRED) is used where the substrate is bombarded by an ion beam in a vacuum environment during deposition.36 When energetic ions traverse environments where there is an appreciable density of gaseous species, charge exchange processes result in a spectrum of energetic neutrals as well as energetic ions.37,38 These energetic neutrals interact with surfaces in the same way as energetic ions but are not affect ed by electric or magnetic fields. Physical collisions will also thermalize the energetic particles in a gaseous environ ment,39.40 In some cases, ions of the film material (condensable or J. Vac. Sci. Techno!. A, Vol. 1, No.3, May/Jun 1989 h noncondensable) may be used to bombard the surfaces. These "film ions" do not represent the introduction of a "foreign" species into the film and thus have many attractive aspects compared to the use of inert gaseous ions for bom bardment. Ions of noncondensable, but reactive, film species (N,O) may be formed in a plasma by conventional tech niques. These species may then be used to bombard the growing film. High ion densities of condensable species can be expect ed in regions having a high density oflow-energy (HID eV) electrons41.47 and in vacuum arcs on solid cathodes4~--50 or above molten anodes.51•52 Many sources for the generation of high fluxes of condensable ions have been developed for the isotope separation programs.53-5 f> Film ions may also be formed by the fragmentation of chemical precursor species either in a plasma57•58 or in a plasma source chamber. 5'1.60 Ions of these condensable species can be used to bombard the substrate and depositing film. Plasma enhancement may also be used to locally in crease the plasma density. This plasma enhancement may be accomplished by using local rfflelds,61 thermo electron emit ting surfaces,62 hollow cathode electron emitters,63 deflec tion of secondary electrons, or localized higher gas pressure. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:051108 D. M. Mattox: Particle bombardment effects on thin-film deposition 1108 SUBSTRATE HOLMA SHUTT£II tON GUN C'. SPUTTERED REflECTED ~ ATOMS NEUTRALS ~~' (VAPORANT ....... a GAS kEf \ (--flEA" FIG. 2. Some configurations for bombarding a surface during deposition by using ion beam system(s): (a) single beam giving both sput tered particles and reflected neutrals, (b) single beam combined with an evaporation source, (c) dual beam given both ions and high-energy neu trals for bombardment, and (d) plasma chamber with extraction grid (s) and a gaseous chemical precursor species. b SUBSTIIATE CHEMICAL VAPOR PRECURSOR GAS c The plasma confinement and enhancement may also be in creased by the use of magnetic fields which cause the elec trons to spiral around the magnetic field lines thus increas ing their path length (magnetron configurations). Some of the most dense plasma sources have been developed for the magnetic fusion community.64 Many of these sources use rf c VARIABLE LEAK GAS INSULATOR MOVEABLESHUTTER~~~~;r==~~r===~r=~ EVAPORATOR FILAMENT CHAMBER HIGH CURRENT -__ -' FEEDTHROUGHS FILAMENT SUPPLY CURRENT MONITOR FIG. 3. An ion plating configuration using a de diode discharge and a sput tering vapor source at ground potential (SIP) (Ref. 35). J. Vac. ScI. Technol. A, Vol. 7, No.3, May/Jun 1989 d power input or thennoe1ectron emitting surfaces65 along with confining magnetic fields. PUMPING PORT GAS INLE;..:T __ --. +900V + 1000V BIAS/ION HT CLEANING I , -;<J GETTER CHAMBER FIG. 4. Sputter ion plating (SIP) system (Ref. 35). Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:051109 D. M. Mattox: Particle bombardment effects on thin-film deposition 1109 II. BOMBARDMENT EFFECTS ON SURFACES AND FILM GROWTH The physical effects of energetic particles on surfaces and depositing films bombardment is very dependent on the mass, flux, and energy of the bombarding particles. Also of importance is the incident flux of non energetic particles, i.e., depositing or absorbing species. In many cases these fluxes are not determined or controlled except by the deposition parameters. Figure 5 depicts the effects on the surface and the sub surface region by bombardment by energetic species. Sur face effects include (i) desorption of weakly bonded surface species, (ii) ejection of secondary electrons, (iii) reflection of the energetic species as high-energy neutrals, (iv) sputter ejection ("physical sputtering") of surface atoms by mo mentum transfer through "collision cascades," (v) sputter ing and redeposition of sputtered species by collisions in the gas phase, by ionization and acceleration back to the surface, and by "forward sputter deposition" due to the ejection an gle on a rough surface, (vi) enhanced surface mobilities of atoms on the surface, and (vii) enhanced chemical reaction of adsorbed species on the surface to produce condensed spe cies ("reactive deposition") or volatile speciesl7 ("reactive ion etching" (RIE) J. In the subsurface region: (i) the impinging particles may be physically implanted, Oi) the collision cascades cause displacement of lattice atoms and the creation of lat tice defects, (iii) surface species may be recoil implanted into the subsurface lattice, (iv) mobile species may be trapped at lattice defects, and (v) much ofthe particle kinet ic energy is converted into heat. Lattice channeling pro cesses can carry these effects deeply into the surface. The desorption of weakly bound surface species is im portant to plasma cleaning and may be used to reduce incor porated contaminants in deposited films.66•67 The desorption may also be useful in desorbing unreacted species in reactive ENERGETIC deposition processes giving rise to a more stoichiometric de posit. Secondary electrons are emitted from surfaces bom barded by energetic particles. These secondary electrons are accelerated away from the cathode and are necessary to sus tain the discharge in the de diode plasma configuration. These electrons may also play an important role in the chem ical process that occurs on the surface. When surfaces are SUbjected to bombardment by high energy ions a portion of the particles are reflected as high energy neutrals.26•27 If these high-energy particles are not thermalized by collisions in the gas phase, they bombard the growing surface of a depositing material giving film property modification.68•69 The physical sputtering of a surface may lead to a sur face texturing to give a roughened surface.6 Preferential crystallographic sputtering will result in some crystalline orientations being etched at a faster rate than are others (sputter etching). Preferential physical sputtering can cause changes in the chemical composition of aHoy and compound surfaces.70,71 If a reactive species is used for bombardment the surface may be etched (reactive ion etching, chemical sputtering) if the resulting chemical species is volatile or the surface may be converted to a compound if the chemical species is not volatile ("surface modification," e.g., plasma nitriding, plasma anodization). Concurrent energetic particle bom bardment enhances chemical reactions at the surface. The nature of this enhancement is poorly understood since heat ing, physical collisions, molecular fragmentation, formation of intermediate species, and the presence of energetic elec trons (secondary electrons) may each playa role. Energetic bombardment of surface can also introduce lattice defects into surfaces. In semiconductor surfaces these defects may act as electron traps when an interface is formed.72 In semiconductor device fabrication these types of PARTICLE REFLECTED SURFACE REGION NEAR SURFACE !lEGION o ENHANCED IONS/NEUTRALS SECONDARY E ..... ·NCED \ CHEMICAL 0 ELECTRONS "'.... REA"'TIONS _ SPUTTERED SURFACE MOBILITY" e ~ / / 0.0 ATOMS liONS) SPUTTERED-I ADSORBED" . REDEPOSITED . ~ SURFACE " AD ATOllS SPECIES '0 ~ ') A I:,} (BACKSCATTEREO) !U!COIL- V LATTICE IMPLANTED DEFECTS 'r-T~----~~--------~----'k-- _SURFACE DISPLACEMENT ~~ lh:.ALTEREOO @ I I t·· REGION t I ? TRAPPING IO~ IMPLANTED I i ~ COLLISION CASCADE CHANNEliNG FIG. 5. Schematic depiction of the energetic particle bombardment effects on surfaces and growing films. See the text for discussion. J. Vac. Sci. Technol. A, Vol. 7, No.3, May/Jun 1989 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:051110 D. M. Mattox: Particle bombardment effects on thin-film deposition 1110 defects must be avoided during surface preparation and film formation.73 The implantation of bombarding species into a surface increases the chemical potential between the surface region and the bulk thereby increasing the diffusion rate of mobile species (such as hydrogen) into the bulk of the material. III. BOMBARDMENT EFFECTS ON FILM PROPERTIES A. Film adhesion The adhesion of a deposited film to a surface depends on the deformation and fracture modes associated with the fail ure.12,13 Energetic particle bombardment prior to and dur ing the initial stages of film formation may enhance adhesion by (i) removing contaminant layers, (Ii) changing the sur face chemistry, (iii) generating a microscopically rough sur face, (iv) increasing the nucleation density by forming nu cleation sites (defects, implanted and recoil-implanted species), (v) increasing the surface mobility of adatoms, (vi) decreasing the formation of interfacial voids, and (vii) by introducing thermal energy directly into the surface re gion thereby promoting reaction and diffusion. Film adhe sion may be degraded by the diffusion and precipitation of gaseous species at the interface, The adhesion may also be degraded by the residual film stress due either to differences in the coefficient of thermal expansion of the film and sub strate material in high-temperature processing or the residu al film growth stresses developed in low-temperature pro cessing. B. Film morphology I density Physical sputtering and redeposition, increased nuclea tion density, and increased surface mobilities of adatoms on the surface under bombardment conditions may be impor tant in disrupting the columnar microstructures that devel op during low-temperature atomistic deposition pro cesses, \6.17 Figure 6 shows the fracture cross section and surface morphology of rf sputter deposited chromium films at zero bias and with a --500 V bias during deposition. Note that the bombardment completely disrupted the columnar microstructure and the surface morphology, The bias also improves the surface coverage and decreases the pinhole po rosity in a deposited film. This increased density is reflected in film properties such as better corrosion resistance, lower chemical etch rate, higher hardness, lowered electrical resis tivity of metal films, and increased index of refraction of optical coatings. However, it has been found that jfthe bom barding species is too energetic and the substrate tempera ture is low, high gas incorporation gives rise to voids.74 C. Residual film stress Invariably atomistically deposited films have a residual stress which may be tensile or compressive in nature and may approach the yield or fracture strength of the materials involved. The origin of these stresses is poorly understood although several phenomological models have been pro posed?' Generally, vacuum deposited films and sputter-de posited films prepared at high pressures ( > 5 f.1) have tensile stresses which may be anisotropic with off-normal angle of J. Vac. Sci. Techno!. A, Vol. 7, No, 3, May/Jun 1989 PIG, 6. Fracture cross section (bottom) and surface morphology (top) ofa thick rfsputterdeposited chromium deposit (Ref. 17). Ca) without bias (no bombardment) and (1:» with concurrent bombardment ( --500 V bias on the substrate). incidence depositions,76,77 In low-pressure sputter depo sition and ion plating, energetic particle bombardment may give rise to high compressive film stresses due to the recoil implantation of surface atoms.17,69,7K-Sj This effect is some times called "atomic peening." Studies of vacuum evaporat ed films with concurrent bombardment have shown that the conversion of tensile stress to compressive stress is very de- 150 ,---.---..---,---.--'-~-~--~ 8 100 \RF SPUTTER-DEPOSITED 7 50 \. CHROMIUM 6Z 'iii 0 .lit. I -50 II) 13 -100 ~ -150 (I) -200 01 I 500 ~5g /~4~ f// 3~ V 2~ ! I j: ~ -100 -200 -300 -400 -500 BIAS (volts DC) FIG, 7. Residual stress and gas content of a rf sputter deposited chromium deposit as a function of substrate bias during rfsputter deposition (Ref. 17), Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:051111 O. M. Mattox: Part/cle bombardment effects on thin-film deposition 1111 J sr z N Q II) S Z L!J " i-Z 3 ro 0 0 ... m -3 (J) I.IJ z a: -6 0 t- ii) (J) III :! ~9 w a: ...I !l.. u: :!i: -12 0 u 2 5 I i j 6 7 11 PRESSURE, microns !J = VERT. (II) 6= HORIZ, (J.) 1"· 100 50 1-50 -j -100 ~ -150 J -200 FIG. 8. Residual stress and stress anisotropy in molybdenum films deposited by post cathode magnetron sputtering as a function of sputter ing pressure (Ref. 69). The stress anisotropy is probably due to an anisotropy in the flux of high-energy neutrals formed by reflection from the post cathode. -15 ~ __ ~ __ ~ __ ~ __ ~ __ ~~I~!~~~ __ L-__ L-__ ~ __ ~I 0 o 0 0 ."oo~." 0 0 ~ II) ci ~ ~ ~ ~~~~~ ~ ~ ~ o 0 0 00000 0 0 ci MOLYBDENUM THICKNESS, microns pendent on the ratio of bombarding species to depositing species, 82,8~, The residual film stress anisotropy may be very sensitive to the sputtering target configuration and gas pres sure69 during sputter deposition. Film stress is typically measured by the deformation of a substrate.84 If the total film stress is sufficiently high, the film may fail by buckling from the surface (compressive stress) or microcracking and peeling from the surface (ten sile stresses).13 Where rather thick films of high modulus materials are involved, these stresses must be controlled or spontaneous failure (adhesion, cracking, blistering) will oc cur. Figure 7 shows the residual stress and gas content in sputter deposited chromium films as a function of substrate bias.17 Figure 8 shows the residual film stress and stress ani sotropy in post magnetron sputter deposited molybdenum films as a function of sputtering pressure, 69 The lattice strain associated with the residual film stress represents stored energy and this energy along with a high concentration oflattice defects may lead to 0) lowering of the recrystallization temperature in crystalline materials, (iO a lowered strain point in glassy materials, (iii) a high chemical etch rate, (iv) electromigration problems in metal lization, (v) room-temperature void growth in films, and other such mass transport effects. D. lattice defects Energetic particle bombardment of surfaces and grow ing films may lead to the creation of a high population of lattice defects (1-20 at. %). The concentration and trap ping energies of such defects have been studied by the trap ping and thermal desorption spectroscopy of mobile spe cies.85-87 The creation oflattice defects in the surface region may increase the adatoms nucleation density by forming nu cleation sites. In the extreme, the increased lattice defect density may lead to a decreased grain size and to the forma tion of an amorphous surface region. J. Vac. Sci. Technol. As Vol. 7, No.3, May/Jun 1989 E. Crystallographic orientation Under proper bombardment conditions the crystallo graphic orientation of the deposited material is developed such that the more dense crystallographic planes are parallel to the bombarding direction. 8~.89 This effect is attributed to the channeling of the bombarding species into the film thus decreasing the sputtering rate under this orientation. Under more energetic bombardment conditions, however, the crys tallographic orientation is disrupted due to the formation and consolidation of defects. F. Gas incorporation When a depositing film is bombarded during deposition by energetic gaseous particles the incorporated gas content is dependent on the particle energy, substrate temperature, film material, and bombarding species. Generally low atom ic mass bombarding particles may be more easily incorporat ed than are large mass particles. The gas incorporation gen erally increases with energy of the bombarding species up to the point where heating aids gas desorptionY Under some conditions very high concentrations of normally insoluble gas may be incorporated into the depositing film by concur rent bombardment during deposition. An example is the in corporation of 20 to 40 at. % hydrogen and helium in gold90•91 and the incorporation of krypton in amorphous metals films. '12 This incorporation is probably due, in part, to the high lattice defect concentration in the bombarded mate rial which traps mobile species. At very high gas contents the gas will precipitate into voids. Gas incorporation can be minimized by using low-energy bombarding species (i.e., < 100 eV for instance), an elevated substrate temperature during deposition (300-400 °C) , and/or using higher atom ic weight bombarding species (Kr, Xc, Hg). G. Surface coverage The macroscopic and microscopic surface coverage of a deposited film on a substrate surface may be improved by the Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:051112 D. M. Mattox: Particle bombardment effects on thin-film deposition 1112 use of concurrent bombardment during film deposition. The macroscopic ability to cover complex geometries depends mostly on scatteting of the depositing material in the gas phase. If gas scattering is extensive, then gas phase nuclea tion ("gas evaporation") will occur forming ultrafine parti cles93 and the resulting deposit will be poorly consolidated. If a plasma is present and the substrate is at a negative poten tial, the gas phase nucleated materials will become negative ly charged and be repelled from the substrate. In addition, bombardment will heat, densify, and consolidate the depos ited material into a high-quality film over the whole surface. On a more microscopic scale, the random deposition direc tion resulting from gas scattering and the sputtering and re deposition of the depositing film material will lead to better coverage on micron- and submicron-sized features. 74.94·-98 On the atomic scale the increased surface mobility, increased nucleation density, and erosion/redeposition will disrupt the porous columnar morphology. 16.17 In total the use of gas scattering, along with concurrent bombardment, increases the surface covering ability and decreases the microscopic porosity of the deposited film material as long as gas incor poration does not generate voids. Ii. Compound deposition In reactive deposition processes concurrent bombard ment enhances chemical reactions. This enhanced chemical reaction, along with the desorption of weakly bonded species and film densification, can produce films of compound ma terials that have better properties than those formed by just heating alone. I. Unique materials The plasma environment allows the deposition of amor phous inorganic films such as amorphous silicon.9'1 Plasma deposited silicon may be deposited in the amorphous form by the incorporation of hydrogen into the lattice from in complete decomposition of the precursor chemical vapor species SiH4• Amorphous carbon and boron may also be de posited from a plasma. 100 Diamond and diamondlike films can be deposited from a plasma using concurrent bombard ment during the deposition. 100.101 High-voltage pulsing of substrates immersed in plasmas is also being studied as a way to modify surf~ces by ion bom bardment. 102 This technique could be used as a means for modifying an the stages of film deposition by bombardment. IV. PROBLEM AREAS A major problem area in using energetic particle bom bardment to modify film properties is how to obtain a uni form and controlled bombardment over a surface. In the utilization of ion beam techniques this is usually done by rotation of the substrates and using multiple-beam sources. In plasma techniques nonuniformity can arise from a num ber of sources including (i) geometrical arrangement of power input electrodes and substrate fixturing, (ii) substrate geometry, (iii) the presence of surfaces that allow recombi nation and loss of species in the nearby plasma, and (iv) in J. Vac. Sci. Technol. A, Vol. 7, No.3, May/Jun 1989 the case of reactive deposition, reactive surfaces that deplete the supply of reactive gas at the growing film surface. As a general rule the best plasma system design is one that is geometrically symmetric. The SIP system shown in Fig. 3 is a good example of this approach. However, in many instances a symmetric geometry is difficult to attain. The use of magnetron configurations are one example. It is difficult to obtain a uniform magnetic field over a large or complex surface and small changes in magnetic parameters can give large changes in target erosion; in addition the use of a mag netic field to confine electrons and increase the local plasma density in one region leads to a decrease in plasma density in some other region. For complex surface geometries the electric field around points and corners on a substrate focus the bombardment giving high erosion rates. The low cross section of a thin region gives poor thermal conductance and results in local heating. Holes and reentrant features give low electric field gradients giving high plasma densities but low bombard ment. In these regions heating will be high and erosion will be low given poor cleaning and allowing reaction with conta mination. In some cases, high transparency grids at the substrate potential may be used to surround the substrate giving a more uniform bombardment over a complex surface. This is the basis of the equipment developed for the "ion vapor de position" (lVD) process 103 and in the "barrel-plating" ion plating configuration.104 A grid configuration may also be useful in coating dielectric materials where charge buildup may be a problem or in the coating of moving substrates where electrical contact may be a problem. When using plasmas and bombardment effects there are many processing variables that are unknown. Processing unknowns include (i) the portion of the measured substrate current that is due to secondary electron emission, (ii) the flux and energy spectrum of the bombarding ions and ener getic neutrals, and (iii) the flux and adsorption of neutral gaseous (reactive) species. Generally, no attempt is made to determine these process variables during the processing but rather they are controlled by controlling other processing variables such as (a) gas pressure, (b) gas composition, (c) gas flow rate (s), (d) substrate and system temperatures, (e) contaminants in the plasma. and (f) substrate power input per unit area (voltage and current). ACKNOWLEDGMENT This work was supported by the Department of Energy under Contract No. DE-AC04-76DP00789. 'Glow Discharge Processes, edited by B. Chapman (Wiley New York, (1980). 2J. A. Thornton, Thin Solid Films 107, 2 (1983). 3T. Takai, 1. Yamada, and A. Sasaki, J. Vae. Sci. Technol.12, 1128 (1975); see also I. Yamada, Thin Solid Films 80, 105 ( 1981), and references from proposal. 4D. M. Mattox, in Deposition Technologiesfor Films and Coatings, edited by R. F. Bunshah etal. (Noyes, City, 1982). Chap. 3. and also a chapter in the revised edition (in preparation). 'G. J. Kominiak and D. M. Mattox, Thin Solid Films 40,141 (1977). Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.238.33.43 On: Mon, 01 Sep 2014 02:49:051113 D. M. Mattox: Particle bombardment effects on thin-film deposition 1113 "Z. W. Kowalski, J. Maler. Sci. Lett. 6, 69 (1987). 71. A. Kelber, in Adhesion in Solids. MRS Symposium Proceedings, edited by D. M. Mattox, J. E. E. Baglin, R. E. Gottschall, and C. D. Batich, 1988, VoL 11<), p. 255. "D. J. Sharp and J. K. G. Panitz, Surf. Sd. 118, 429 (1982). 9Surface Mobilities on Solid Materials--Funclamental Concepts and Appli cations, edited by V. T. Binh. NATO ASI Series, Series B: Physics (Ple num, New York, 1983), Vol. 86. to Nucleation and Growth of Thin Films, edited by B. Lewis and J. C. Ander son (Academic, New York. 1979). I 'J. E. Greene, "Low energy ion/surface interactions during film growth from the vapor phase: Effects on nucleation and growth kinetics, defect structure and elemental incorporation probabilities," in NATO ASI Se ries, Proceedings of the NATO Advanced Studies Institute on P!asma Surface Interactions and Processing of Materials, Alicante, Spain, 1<)88 (to he published). 12D. M. Mattox, in Adhesion Jl,feasuremenf of Thin Films, Thick Films and Bulk Coatings, edited by K. L. MiHal, ASTM STP 640 (American So ciety for Testing and Materials, Philadelphia, (1978), p. 54. l.lD. M. Mattox and R. E. Cuthrell, "Residual stress, fracture and adhesion in sputter-deposited molybdenum films," in Ref. 7. 14B. A. Movchan and A. V, Demchishin, Fiz. Met. MctalJoved. 28, 653 (1969). ISJ. W. Patten, Thin Solid Films 63, 121 (1979). \61. A. Thornton, Thin Solid Films 40,335 (1977). !7R. D. Bland, G. J. Kominiak, and D. M. Mattox, J. Vac. Sci. TechnoL n, 671 (1974). "1. A. Thornton, Anrlu. Rev. Mater. Sci. 7, 239 (1977). i9J. A. Thornton, J. Vae. Sci. Techno!. A 4,3059 (1986). lOR. Meissier, A. P. Giri, and R A. Roy, J. Vac. Sci. Techno!. A 2,500 (1984). l'R. Meissier and J. E. Yehoda, J. AppL Phys. 58, 3739 (1<)85). nG. A. Lincoln, M. W. Geis, S. Pang, and N. Efremow, J. Vae. Sci. Tech nol. B 1, 1043 (1983). 23H. F. Winters, J. W, Coburn, and T. J, Chuang, J. Vac. Sci. Techno!. B 1, 469 (1983); also, 1. W. Coburn and H. F. Winters, Nue!. lnstrum. Meth ods Phys. Res. B 27. 243 (1987). 24J. M. E. Harper, J. J. Cuomo, and H. T. G. Henzel!, App!. Phys. Lett. 36, 456 (1980); also, AppJ. Phys. Lett. 37, 540 (1980). 7°H. R. Kaufman, J. Vac. Sci. Techno!. 15, 272 (1978). 20W. W. Y. Lee and D. 0])la5, J. App!. Phys. 46,1728 (1975). J7B. D. Hagstrum, in Inelastic Ion Surface Com~ions. edited by N. H. Talk, J. C. Tully, W. Heiland, and C. W. White (Academic, New York, 1977), pp.I-25. 2"J. J. Cuomo, R. J. Gambino, J. M. E. Harper, J. D. Kuptsis, and J. C. Webber, J. Vac. Sci. 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1.101217.pdf

Highfield perpendicular conduction in GaAs/AlAs superlattices A. Sibille, J. F. Palmier, C. Minot, and F. Mollot Citation: Applied Physics Letters 54, 165 (1989); doi: 10.1063/1.101217 View online: http://dx.doi.org/10.1063/1.101217 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highfield domain formation in GaAs/AlGaAs superlattices Appl. Phys. Lett. 66, 1120 (1995); 10.1063/1.113832 Highfield magnetooptical study of typeII GaAs/AlAs shortperiod superlattices Appl. Phys. Lett. 59, 96 (1991); 10.1063/1.105535 Electroluminescence and highfield domains in GaAs/AlGaAs superlattices Appl. Phys. Lett. 56, 1356 (1990); 10.1063/1.102513 Optical detection of highfield domains in GaAs/AlAs superlattices Appl. Phys. Lett. 54, 1757 (1989); 10.1063/1.101282 Electronic structure and transport properties of GaAsGaAlAs superlattices in high perpendicular electric fields J. Appl. Phys. 62, 558 (1987); 10.1063/1.339782 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Mon, 22 Dec 2014 20:33:41Highafield perpendicular conduction in GaAsl AlAs superlaUices A. Sibille, J. F. Palmier, and C. Minot Centre National d'Etudes des Tfdecommunications, 196 Avenue Henri Ravera, 92220 Bagneux, France F. Mollat Laboratoire de Microstructures et de lWicroidectronique, Centre National de fa Recherche Scientifique, 196 Avenue Henri Ravera, 92220 Bagneux, France (Received 13 Apri11988; accepted for publication 26 October 1988) Miniband conduction in undoped GaAs/ AlAs superlattices (SLs) has been investigated through current~voltage measurements on n+ -SL-n+ structures. From the comparison with simulations based on an effective medium approximation for the conduction through the superlattice, we directly obtain the field dependence of the electron velocity perpendicular to the layers. Our data show strong evidence of negative differential velocity in a 35.5/20 A (wellibarrier width) SL. Electronic transport perpendicular to the layers of sup perlattice (SL) structures has been the subject of several experimental and theoretical efforts in the past, partly moti vated by the hypothetical possibility of achieving high fre quency Bloch oscillations. I Although a few works show evi dence of miniband conduction,2,3 the only observed negative differential resistance effects in SLs are due to the formation of highly localized high-field domains,4-6 or to wen to well tunneling in multiquantum wells. 7 We show here that a uni form SL can electrically behave as a new tailorable bulk ma terial and, in particular, exhibit negative differential velocity (NDV) effects which are strongly dependent on the SL pa rameters. The samples studied in this work (Fig. 1) were grown by molecular beam epitaxy on n + -GaAs:Si substrates. The "active" superlattice section ("'" 1.3 pm thick, hereafter re ferred to as SL) was undoped and therefore lightly p-type because of residual acceptors (O.5-2x 1015 cm-3). The rest of the structure involved GaAs contact (2000 A) and buffer layers on one hand, and a Gao.> Alo.s window (3000 A) for optical time~of-ftight investigations8 on the other. Except for the SL, all the layers were heavily Si doped (= 1.5 X lOts atoms/cm3). The three heterojunctions were smoothed out by gradual composition layers. The latter (as well as the window) were conveniently obtained in the form of small period superlattices (3 monolayers of AlAs; 3 or m.ore layers ofGaAs). The above thicknesses and doping levels were checked by secondary-ion mass spectrometry (SIMS) analysis, using a CAMECA-IMS 3F and found to be very close to the ex pected values. Excellent simple x-ray diffraction spectra were also obtained, which yielded precise values of the per~ iods. From the latter and the knowledge of the number of periods, the SL thicknesses were thus very accurately deter mined. We also deduced the individual well and barrier widths from the average SL composition measured by dou ble diffraction. Two superlattice samples were grown for a fixed GaAs well width (13 mono layers, =36 A) and AlAs barrier widths of 3 and 7 monolayers. From now on they will be referred to as (13/3) SL and (13/7) SL. The SL periods and thicknesses were, respectively, 45.85 A and L334pm for the first and 55.50 A and 1.248 pm for the second. Transmis sion electron microscopy revealed the presence of a single slightly enlarged barrier in the (13/3) SL, presumably due to the accidental successive growth of two AlAs barriers. The consequences of this "defect" will be discussed later. In contrast, the (13/7) SL looked perfectly regular. A refer ence sample was also grown, in which the SL was replaced by a 3 pm un doped p-GaAs layer, the rest of the structure being unchanged. Sample processing involved recess of the window, mesa etching up to the n f buffer layer, and Au-Ge-Ni alloying at 450°C of ohmic top and back contacts. These operations were performed by standard photolithographic techniques, and yielded devices of variable area, in a proportion ranging from 1 to 3. Thus, the validity of all the measurements pre sented here could be ensured by checking the proportional ity of current to device area. Current~voltage (/-V) measurements at 300 K are shown on Fig, 1 for the three devices. They exhibit only little asymmetry. The rectifying behavior is inherent to the p-type character ofthe undoped layer which induces a built-in ener gy barrier for the electrons, which can be almost totally can celled by applying a bias.') As is evident on Fig. 1, we could obtain an excellent agreement between J-V data and numeri- E ..0::: :5 >I- Vi 15 10-2 Cl I Z UJ a:: 0:: => w 10-4 .. .. GaAs (referencel quasi-alloy Ga~.sAIO.5As In<) .. AuCieNi \ ~~/ CiaAs substrate (n<) ~ ... _.L~~Ge~i- _ .. ,_~_~~ o 1 2 3 4 VOLTAGE (V) FIG. I. 1-V data at 300 K for the three samples studied (crosses. do is. and circles), and the corresponding simulations (full curves). The data below 1. g V for GaAs and 0.3 V for ( 13/7) SL are not proportiol1al to device area and should not be compared to the calculated current. 165 Appl. Phys. Lett. 54 (2), 9 January 1989 0003-6951/89/020i 65-03$01.00 © 1989 American Institute of Physics 165 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Mon, 22 Dec 2014 20:33:41cal simulations. The latter rest on the self-consistent resolu tion of drift diffusion and Poisson equations, using a finite difference scheme and an iterative algorithm. Each layer, including the SL, is precisely defined by all the relevant pa rameters, i.e., thickness, effective densities of states, band gap, and electron affinity, velocity-field (V-F) relations for both carriers, and shallow impurity concentrations. In this approach the SL is thus regarded as an effective medium, Le., a new bulk material with tail arable parameters. The main underlying assumption is that of local equilibrium which neglects overshoot effects. In view of the length scales and mobilities involved, we consider this approximation as ade quate. A fuH description of these simulations win be present ed in another publication (see also Ref. 9). The only un known parameters of real importance are in fact the acceptor concentration NA and the V-F relation for electrons in the undopcd layer. Furthermore, NA and I-l influence computed /-V curves in a strongly different way, which means that (at least in principle) a unique set ofthese parameters can yield a satisfactory fit. In practice we estimate the uncertainty to 10% for NA and a factor 2 for p (since the low-voltage data is insensitive to the high-field part of the V-F curve, a linear relation V = pF was assumed in the simulations of Figs. 1 and 2). The situation is even better in the GaAs reference sam ple where only NA is unknown. The excellent agreement we could obtain over 6 decades of current for NA = 7.7 X 1014 cm 1 demonstrates the validity of our measurement tech nique (see also Fig. 2). One first interesting result of these experiments is the absolute value of the perpendicular mobility, namely =400 cm2/V s for (13/3) SL (NA = 1.6x 1015 em 3) and "",040 cm2/V s for (1317) SL (NA = l.4x 1015 cm-3). Here, the measurement procedure (low enough negative bias, current limited by injection) ensured that the presence of the above mentioned enlarged barrier (situated close to the anode) did not cast doubt on the determination of /1, in the ( 13/3) SL We have calculated the theoretical perpendicular mobil- 2000 ,.---,-,~, :;==':-:-:: rr _-----or---i N E , « >- ~ 1000 z w co 0- Z w 0:: 0:: ::l LJ r t:i1 o -' WJ > Z CJ a:: 0-w W w:j 0" v 2 ! -- B FIG. 2. High-field data (circles) for GaAs together with three simulations (fullline~) differing only by the V-F curves; the latter are shown in the inset. Only the true V-F relation with NDV yields a sublinear J-V, in agreement with the data. 166 Appl. Phys, Lett., Vol. 54, No.2, 9 January 1989 ity in two extreme cases. In one, pure miniband conduction occurs and the mobility is limited by polar-optic phonon scattering; we then obtain 2700 and 230 cmz /V s, respective ly, for the two samples. In the other, the electron wave func tion is assumed entirely localized in the well, and conduction occurs by phonon-assisted hopping from wen to welL lO We find in this case 0.25 and 0.04 cm2/V s. Since the experimen tal values are in between these two cases but closer to the first, we can conclude that the electron perpendicular trans port occurs mainly through miniband conduction, with, however, the prevalence of some additional scattering mech anisms. In particular, we can guess that the unavoidable in terface roughness will seriously contribute to reducing the mobility just as in parallel transport. I 1.12 When a large bias is applied on the present devices, the macroscopic space-charge barrier has been almost complete ly eliminated and a mixed regime of diffusion near the cath ode and drift near the anode occurs for the electrons. Since the injected electron concentration then exceeds N4, the cur rent is space-charge limited. Consequently, one expects a superlinear but not exponential dependence of Ion V ( V2_ like law). However, this is true only if the velocity-field ( V F) curve is monotonic, as we systematically found in all our simulations in which we imposed a simple saturation ( VI) at high fields without NDV. Such a V-F dependence is, however, in stark conflict with the experimental data for the GaAs reference sample (Fig. 2). On the other hand, if the true V-F curve for GaAs is taken (inset of Fig. 2), we obtain a qualitatively and quanti tatively excellent agreement. Particularly important is the negative curvature of the J-V, compared to that of a pure saturation law, which has indeed the same sign as experi mentally found (see also Fig. 4). We can therefore safely state that a sublinear 1-V in the present device structures gives strong evidence of NDV; in fact, such a behavior is a precursor to negative differential resistance which would oc cur in an isotype structure. These conclusions can addition ally be analytically proven from a careful analysis of trans port equations. 13 The data concerning the superlattice samples are shown in Figs. 3 and 4, with the corresponding simulations. For SL ( 13/3) a simple saturation law yields a fairly good fit to the data with Vi = 1.5 X 107 em s 1. It might be argued that the enlarged barrier present in this sample will influence the cur rent under large biasing. In the field range investigated in Fig. 3, however, no effects such as observed in Ref. 5 are found and there is no obvious ground to reject the effective medium description adopted here. On the other hand, SL (13/7) requires the incorporation of NDV, as is evident from the previous discussion and the negative curvature of the /-V. An excellent fit to the data is obtained with a phe nomenological V-F law of the form: V(F) = pF / (1 + F2/ F~), where f.l = 40 cm2 V--1 s -I is the low-field mobility and Fc = 16.5 k V cm -I is the critical field, corresponding to a peak velocity Vp = 3.3 X 105 em 5-1. The strong sensitivity of this fit to Fe is exemplified in Fig. 4, as well as the complete inadequacy of a simple saturation law. For completeness we want to point out that it cannot be at present conduded that NDV does not exist in SL ( 13/3). Sibille et al. 166 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Mon, 22 Dec 2014 20:33:41L..-....... __ ............. ""'----- L _____ ~ o 1 2 VOLTAGE (VI FIG. 3. High-field data (crosses) for sample SL (13/3) and the corn! sponding simulation using the V-F curve shown in the inset. The simulated band diagram under 3 V is also presented. Because oftne larger mobility compared to (1317), smaller voltages and, therefore, smaller electric fields have to be ap plied to avoid permanent damage. NDV is SL ( 13/7) may have different origins: (i) It can be a direct consequence of the negative effec tive mass for electrons heated by the field beyond the inflec tion point of the miniband dispersion relation. I With a co sine approximation of this relation and by imposing the true low-field mobility 40 cm2/V s, we calculate Fe = 128 kV I em (from the simple model of Ref. 1). This value ]s obvious ly much too large compared to experiment. On the other hand, more sophisticated Monte Carlo simulationsl4 yield markedly smaller critical fields, so that no definite answer can yet be given. (ii) It can occur because of r-x transfer offield-heated electrons. This effect is harder to estimate precisely; how ever, the low peak velocity is not necessarily in contradiction with this mechanism since the X band edge in AlAs is close to the r band edge of GaAs. 15 (iii) It can result from a Bloch to hopping transition due to electric field induced localization of the wave functions. In In this case Fe =IJ. (qd) -1 = 21 kVem·· 1, where t:. is the mini band width (12 meV from envelope function calcula tions), q the electron, and d the period. The measured criti cal field is quite close to this value, which renders this mech anism quite plausible. In conclusion, we have demonstrated the existence of NDV in a regular GaAsl AlAs superlattice, from J-V mea surements on nl -SL-n + structures under high bias, Since the critical field and peak velocity are SL parameters depen dent, they are tailorable. If, as suspected, this phenomenon results from electric field induced localization or is a nega tive mass effect, it may be intrinsically faster than phonon mediated electron transfer. In the frame of possible fast os cillator design, it is interesting that large voltage and large current density operation at room temperature and above are then possible. Note added in proof True negative differential resistance at 167 Appl. Phys. Lett., Vol. 54, No.2, 9 January i 989 "'i .:::: :5 :>0-f-200 \2100 ~ Cl I Z W '" 0:: => w 2 a:F,=10kVfcm i b:F,=16.5kV fern r with NOV cF,=20kVfcm ) j d:vl"3x105cm/s 1 I I wit~O\Jt ~OV i 4 5 6 VOL TAGE IV) 7 8 9 FIG. 4. High-Held data (dots) for sample SL (lJ/7) and several simula tions using the ,arne low-field rnohility p = 40 em'IV s; a. b, c: V-Frelation with NDV for various critical fieJds F, (sublinear 1-V); d: V-Frelation (in set) without NDV (superlinear J-V). An excellent agreement is obtained with NDV and F, ~ 16.5 kV lem. 300 K has recently been observed in several n-type (13/7) superlattices, from which a V-Frelation very close to that of Fig. 4 was directly deduced [A. Sibille, J. F. Palmier, F. MoUot, H. Wang, and J. C. Esnault (unpublished) ] . The authors wish to thank H. Le Person for fruitful dis cussions, F. Glas, M-C. Joncour, and Y. Gao for expert structural and chemical characterization of the samples, and J. C. Esnault and S. Vuye for device processing. 'L. Esaki and R. Tsu, IBM J. Res. Develop. 14,61 (1970). 'J. F. Palmier, C. Minot, J. L. Uevin, F. Alexandre, J. C. Harmand, J. Dangla, C. Dubon-Chevallief. and D. Allkri, App]. Phys. Leu. 49, ]260 (1986). 3D. Deveaml, J. Shah, T. C. Darnen, B. I,amber!, and A. Regreny, Phys. Rev< Lett. 58, 2582 (1987). 4L Esaki and L. L Chang, Phys. Rev. l.ett. 33, 495 (1974). 'R.. A. Davies, M. J. Kelly, and T. M. Kerr, Phys. Rev. Lett. 55, 1114 ( 1(85). "K. K. Chot, B. F. Levine, R. J. Ma.lik, J. Walker, and C. G. Bethea, Phys. Rev. B 35,4172 (1987). 'F. Capasso, K. Mohammed, and A. Y. Clio, IEEE J. Quantum Electron. 22,1853 (1986). "e. MinoL H. Le Person, F. Alexandre, and J. F. Palmier, App!. Phys. Lett. 51, ! 626 (1987). "J. F_ Palmier, H. Lc Person, e. Minot. and A. Sibille, J. Phys. (Paris) 48, Suppi., C5-443 (1987); J. F. Palmier,.T. Dangla, E. Caquot, and M. Cam pana, NASECODE IV Proceedings, edited by J. J. H. Miller (Hoole, Duh lin, 1(85). IOD. Calecki, 1. F. Palmier, and A. Chomctte, J. Phys. (Paris) en. 5017 (1984): see also 1. F. Palmier and A. Chomettc, ibid., 45,381 (1985). II A. Sibilic, J. F. Palmier, C. Minot, J. e. Harmaml, and e. Dubon-Cheval lier, Superlatt. Microstruct. 3,553 (1987). IOH. Sakaki, T. Noda, K< Hirakawa, M. Tanaka, and T. Matsusue, App!. Phys. Lett. 51, 1934 (I98i). "J. F. Palmier (lmpublished). 14M. Artaki and K. Hess, Superlatt. Microstruct. 1,489 (\985). "D. 1. Wolford. T. F. Kueeh. 1. A. Bradley. M. A. Gell, D. Ninno, and M. Jaros, J. Vac. Sci. Techno!. B 4,1043 (1986). IhR. Tsu and G. Diihkr, Ph)s. Rev. B 12, 680 (1975). Sibille et a/. 167 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Mon, 22 Dec 2014 20:33:41

1.342695.pdf

Molybdenum deposition from the decomposition of molybdenum hexacarbonyl C. C. Cho and S. L. Bernasek Citation: Journal of Applied Physics 65, 3035 (1989); doi: 10.1063/1.342695 View online: http://dx.doi.org/10.1063/1.342695 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Control of the outer diameter of thin carbon nanotubes synthesized by catalytic decomposition of hydrocarbons AIP Conf. Proc. 544, 242 (2000); 10.1063/1.1342509 Processing tungsten single crystal by chemical vapor deposition AIP Conf. Proc. 504, 1454 (2000); 10.1063/1.1290965 A design of a nanometer size metal particle generator: Thermal decomposition of metal carbonyls Rev. Sci. Instrum. 70, 4366 (1999); 10.1063/1.1150081 Electrical resistance of electron beam induced deposits from tungsten hexacarbonyl Appl. Phys. Lett. 62, 3043 (1993); 10.1063/1.109133 Lowtemperature chemical vapor deposition of tungsten from tungsten hexacarbonyl J. Vac. Sci. Technol. 20, 1336 (1982); 10.1116/1.571599 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44Molybdenum deposition from the decomposition of molybdenum hexacarbonyl c. C. Choal and S. L. Bernasek Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received 21 March 1988; accepted for publication 9 December 1988) Molybdenum metal deposition from the decomposition of Mo (CO) 6 adsorbed on Si ( 100), Mo. and Cu surfaces was studied by x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, thermal desorption spectroscopy, and low-energy electron diffraction. Pyrolytic, photolytic, and electron-induced MO(CO)6 decomposition were observed and indicated different dissociation mechanisms, Thermally decomposed MO(CO)6 was found to leave metallic Mo in the presence of C and O. Electron-induced decomposition resulted in the formation of molybdenum carbide on the surfaces. Ultraviolet (UV) irradiation of adsorbed Mo (CO) 6 induced new peaks in XPS and TDS spectra, suggesting the formation of an unsaturated molybdenum carbonyl adsorbate. MO(CO)6 was found to form a multHayer on these surfaces at low temperatures, and desorb with zero~order kinetics. Although both adsorbate desorption and decomposition took place when the samples were heated. desorption was the dominant reaction path, UV irradiation of gaseous and coadsorbed Mo (CO) 6 and O2 was also investigated. UV irradiation of the gas-phase mixture leads to Mo02 and Mo03 deposition; however, UV irradiation of coadsorbed Mo (CO) 6 and O2 resulted in unsaturated molybdenum carbonyl. The effects of annealing and Ar + bombardment on the Mo~deposited Si ( 100) surface are also reported. !. INTRODUCTION The adsorption and reactive properties of organometal lic compounds on surfaces have attracted increasing atten tion recently. Some studies have been motivated by the at tempt to further understand the chemical bonding between metallic atoms and the ligands of organometallic complexes, the interactions between adsorbate and substrate, and the transitions between the homogeneous chemistry of organo metallic dusters and the heterogeneous chemistry of transi tion-metal surfaces. 1-3 Other studies have been motivated by more practical requirements: metallization processes from organometallic compounds have exhibited strong potential for their application in various fields such as catalyst forma tion and semiconductor device fabrication. The understand ing and control of the metallization processes will be essen tial for the improvement of these productS.4,5 In the present paper, a study of molybdenum deposition from the decomposition of molybdenum hexacarbonyl on Si ( 100), polycrystalline Mo, and Cu surfaces is reported, Molybdenum is an interesting material for depositing on semiconductor surfaces because it has exhibited good poten tial as a new material for metallic gates and interconnects in microelectronics devices,6.7 PolycrystaUine Si and Al have been used as the gate and interconnect material for most integrated circuits. However, as the feature dimensions of devices continue to decrease, the resistance of polycrystal line Si starts to impede high-speed performance, and Al films deteriorate due to electromigration problems.8,9 The low re- a) Current address: Department ofChemis,ry, University of Toronto, Tor onto, Ontario M5S iAI, Canada. sistance, higher thermal stability, and good pattemability of molybdenum thus makes it an attractive substitute. The de position of molybdenum on Si also offers a route for the formation of molybdenum sUicide. which is another promis ing candidate of low resistivity and high thermal stability, and is useful in integrated circuits as a Schottky barrier. 10 In this work. different substrates were employed to investigate the effect of adsorbate-substrate interactions and the influ ence of the deposited layer upon further adsorption. Prior to recent studies of the interaction of transition metal carbonyls with surfaces,; 1-2l the decomposition of or~ ganometallic compounds in the gas phase,22-28 in liquid solu tion,29 or in low-temperature matrices, 3{~ 32 has been extensively investigated. Among these works, photon in duced metal carbonyl decomposition has been most widely studied. Metal carbonyls absorb radiation in the ultraviol.et (UV) range and exhibit high quantum yield for photodisso ciation reactions. In the gas phase, these carbonyls are be lieved to absorb one or more photons and to dissociate one CO ligand before the other CO ligands are eliminated se quentially. Various photofragments, such as neutral atoms M,22 excited atoms M", 2J ion fragments like M +, and M(CO),,+ _.~,24 have been observed. In CH4 or Ar matrices, Mo(CO)s, as well as two secondary photolysis products, MO(CO)4 and Mo(CO)" have been produced sequentially by UV irradiation ofMo(CO)6,:lo.31 Laser flash pyrolysis of refractory hexacarbonyls in perfluoromethylcyciohexane generated highly reactive coordinatively unsaturated penta carbonyls, which complex with CO, M(CO)6' or cyclohex ane, with large rate constants,29 Surface sensitive techniques such as Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and high-resolution electron energy loss spectrosco- 3035 J. AppL Phys. 65 (8), 15 April i 9813 0021-8979/89/083035-09$02.40 @ 1989 American Institute of PhYSics 3035 ' ••••• -.c ••••••••• '>..,. .. ~, ........ ~ ' ••••• "~"'.' •• '~"""". ...~ •••••• ,. ';,0,' ...................................... ~.'. , ..... _ ........... ....,..;-••••••••• rr r •••••• ~. '~ ••• +.", ••• -.-.................... ii ••••••••• '.~;:.;: •• , .............................. ..-...... ' •••••••••••••• <; •• • ••• ·.v.· •. o; .............. ; ••••• o,o ••••••••• <; ••••• ;.-;.o.-.. r •••••••••••••• v .... v ...... 0; •••• <;>; •••• -0;.< ••••••••••••••• <; •••• """'.~ •••• <;" ............... , ...... ~ [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44py (HREELS) have been applied to analyze some UV irra diated metal carbonyls adsorbed on surfaces, Mo (CO) 6 was found to adsorb as a multilayer on Si(100) and desorb at 210-230 K. KrF laser irradiation (248 nm) partially de composed the adsorbed MO(CO)6 and released gas phase CO in the process. Mo (CO) 5 was proposed as the product of the photodissociation reaction. 13 The splitting of the main c O stretching mode and the formation of a new C-O stretch ing mode in HREELS, after adsorbed Mo (CO) 6 on Si ( 111 ) was irradiated by a 257-nm laser light, 18,19 also suggested the existence of a surface stabilized photoreaction product. On the other hand, no partially decarbonylated iron carbonyl fragment was detected by infrared spectroscopy subsequent to exposure to UV photons.2' By comparing the results from laser irradiation at two different wavelengths, UY irradia tion was believed to decompose adsorbed MO(CO)6 by the same metal-ligand charge transfer dissociation mechanism as observed in the gas phase, 14 while visible light apparently induces thermal desorption ofMo(CO)6' UV laser induced photodeposition from refractory hex acarbonyls at room temperature has achieved high depo sition rate, good deposit resistivity, and conformal step cov erage.Il,l2,I5 However, C and 0 contamination was found in the deposited layers. The deposit layer produced by irradiat ing Mo (CO) 6 adsorbed on Si ( 100) and flashing the irradiat ed crystal also exhibited C and 0 contamination with a stoi chiometry of MoCOO.3 .13 Recently, photodeposited Fe from Fe(CO)s without C or 0 residue has been reportedY Studies of thermally decomposed metal carbonyls are relatively fewer. A laser pyrolysis study of metal carbonyl showed that, once the first metal-CO bond is broken, ther mal decomposition of Mo (CO) 6 proceeds to completion, re sulting in metal particulate products. The first bond scission is usually the rate determining step for most metal car bonyl28 photodissociations. In the present study, various methods such as thermal heating, UV irradiation, and electron bombardment have been used to decompose molybdenum hexacarbonyl. XPS was used as the major tool to identify the chemical states of the adsorbate and the reaction products. TOS, AES, and low-energy electron diffraction (LEEO) were also em ployed. Both a wide band UV lamp and a nitrogen laser (337 nrn) have been used as the light sources. In addition to the study of the decomposition reactions, the UV irradiation ef fect upon mixed gaseous and coadsorbed Mo (CO) 6 and O2 has been studied, The structure and composition of the de posited layers and the effects of thermal heating and Ar+ bombardment on these layers were also investigated. II. EXPERIMENT Since the detailed experimental setup has been reported elsewhere,33 only a bri.ef description will be given here. The experiments were performed in two ultrahigh vacuum chambers. One chamber was equipped with a Mg anode x~ ray source, a double-pass cylindrical mirror analyzer for x ray photoelectron spectroscopy (XPS) and AES, a quadru pole mass spectrometer, and a differentially pumped 0- 5-ke V ion sputtering gun. The second chamber was fitted 3036 J. Appl. Phys., Vol. 65, No.8, 15 April 1969 with four-grid LEED optics, a single-pass cylindrical ana lyzer for AES, a 0-3-keV ion sputtering gun, and a quadru pole mass spectrometer. Each chamber was connected with a reaction chamber, which could be isolated from the main chamber by a valve and permitted studies at low vacuum or at atmospheric pressure. In both systems, samples could be transferred between the reaction chamber and the main chamber under ultrahigh vacuum conditions, cooled to be low 110 K by liquid nitrogen, and heated to above 1270 K by either resistive heating or electron bombardment. Chromel alumel thermocouples were used to monitor the sample tem perature. Si( 100) samples were obtained from AT&T Engineer ing Research Center in Princeton. They were cleaned by rinsing with 1 % HF solution and pure methanol in air, fol lowed by Ar+-bombardment, and flashing to 1170 K in the UHY chamber. The samples were characterized by Auger spectra and the observation of a sharp (2 Xl) LEED pat tern. Polycrystalline Mo and eu foils were obtained from Alfa Products with purities of99.97% and 99.999%, respec tively. They were also prepared by standard bombardment and annealing processes and characterized by AES. Since the vapor pressure ofMo(CO)6 powder, obtained from Al drich, is high (-0.2 Torr) at room temperature, gaseous MO(CO)6 could be produced in a sman chamber containing solid MO(CO)6 after several vacuum pumping cycles for cleaning. Gaseous MO(CO)6 was then introduced into the main chamber or reaction chamber by way of a leak valve. A mercury lamp and a Molectron UV22 pulsed N laser were used as UV sources in the photon reaction experiments. The laser beam was focused slightly to the sample size ( ~ 1 cm2) with pulse duration of IOns, repetition rate 20 Hz, and average laser power 6 mJ/pulse. Sapphire windows were used in these experiments. III. RESULTS A.. Adsorption of Mo(CO)s Because the sticking coefficient of MO(CO)6 on solid surfaces at room temperature was found to be very smail, the studies of adsorbed Mo (CO) 6 were performed by exposing the samples at 110 K to gaseous molybdenum carbonyl. In the adsorption ofMo(CO)(', on Si( 100), the binding energies of the Mo(3d3!z), Mo(3d 5IZ)' Si(2p), COs), and O(1s) photoelectron peaks were monitored. These binding ener gies were calibrated using the CU(2P3/2) peak (939.4 eV) and the Cu L3 VV Auger peak (334.9 e V). The binding ener gies ofthese peaks were found to be 232.6, 229.4, 99.0, 291.7, and 534.3 eV, respectively. The peak intensity ratio for Mo, C, and 0 is 1.0:0.68:2.2. TDS experiments have shown that Mo(eO)" desorbs from Si(100) 13 and SiC 111) 18 surfaces with zero-order ki netics. Similar results were found upon the desorption of Mo (CO) 6 from the polycrystaHine Mo surface. The thermal desorption spectrum of Mo (CO) 6 from polycrystalline Mo is shown in Fig. 1. The asymmetric peak shape and the fact that higher desorption peak positions were observed with increased surface adsorbate coverage are consistent with zero-order desorption. Arrhenius analysis of the leading edge desorption yield showed that the activation energy for C, C. Cho and S. L. Bernasek 3036 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44..l <C ;z '" 00 '" E " <I> '" Mo(COls-Mo I ! ! ! ! ! 150 160 170 180 190 200 210 220 230 240 250 260 TEMPERATURE (K) FIG.!. TDS spectra ofMo(CO)6 from a polycrystalline Mo surface. (a) (e) areMo(CO)" exposures in Langmuir. The CO " mass signal wasmoni tared. desorption was 15.9 kcallmol. The peak area is linearly de pendent on the dosage, suggesting the formation of multi layers. The heating rate for these desorption spectra was 10 K/s. With this heating rate, only a trace amount of Mo was detected by AES or XPS on the surface after the desorption. However, when the samples were heated more slowly, signif icant Mo deposition was observed. Other chemical states during the heating process were also detected by XPS and win be discussed in the following section. Et Thel'mal~jnduced decomposition of Mo(CO)e After adsorption of Mo( CO) 6 at low temperature, the samples were heated at about 1 K/min while XPS spectra were taken, Figure 2 shows the XPS spectra of the Mo 3d region at various temperatures for Mo(CO)r, adsorbed on a polycrystaHine Mo surface. The spectrum of clean Mo foil is shown at the top for comparison. The positions of the Mo(3d 3!2) and (3ds!2) peaks for the clean Mo surface are 227.4 and 230.6 eV, respectively, Following MO(CO)6 ad sorption at 110 K, the Mo(3d) peaks appeared at 228.6 and 231.8 eV. These peak positions remain constant at low tem perature for multiple XPS scans indicating that the adsorbed layer is not affected by the x-ray radiation. The peak posi tions remain constant until the temperature is increased to about 190 K and shifted to 227.7 and 230.9 eV thereafter. The adsorption of Mo ( CO) 6 on polycrystalline Cn was also studied by the same procedure (Fig< 3) and exhibited results very similar to those for adsorption on Mo< The Mo( 3d) peaks had binding energies of 232.1 and 228.9 eV at 109 K which remained constant until around 190 K. At higher temperatures, the Mo peaks shift to 231.6 and 228.4 eV. The peaks shift back to slightly higher binding energies at yet higher temperatures (above 310 K). Without the interfer ence of the substrate Mo peaks as seen in Fig. 2, deposited Mo could be seen clearly from 194 to 450 K on the Cu sub strate. The spectra of the adsorbate at 110 K are due to physi cally adsorbed MO(CO)6< The adsorbate decomposes and 3037 J. Appl. Phys., Vol. 65, No.8, 15 April 1989 '" ... 'iii 1'-----c 1111-----S 236 234 232 230 228 226 224 Binding Energy (eV) FIG. 2. XPS spectra of the Mo( 3d) region for Mo( CO)6 adsorbed on poly crystalline Mo at low temperature and heated to variolls temperatures. forms metallic Mo in the presence of C and 0 when the substrate temperature is increased above 190 K. High-tem perature heating (above 310 K) oxidizes the deposited Mo and converts 0 and C into oxide and carbide, as evidenced by the peak shifting toward higher binding energy at this tem perature on the eu surface. On the Mo surface, the high- Mo(3c!) Binding Energy (eV) FIG. 3. XPS spectra of the MoOd) region for MoC COlo adsorbed Oil. po\y crystalline eu at low temperature and heated to various temperatures. c. C. Cho and S. L. Bernasek 3037 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44temperature peak shift due to the Mo oxidization process is obscured by the background peaks of the Mo substrate. However, by monitoring the C( Is) region, species corre sponding to these chemical states can be clearly observed (Fig. 4). The initial C peak at 291.5 eV is due to physisorbed MO(CO)6' The peak at 290.3 eV results from thermally de composed Mo(CO)6' When the sample is heated to 450 K, molybdenum carbide is formed as indicated by shifting the C( Is) peak to 287.8 eV. The XPS spectra of MO(CO)6 on Si(lOO) at various temperatures are shown in Fig. 5. Although the ratio be tween the peak areas of Mo, C, and 0 is similar to that ob served on Mo and Cu substrates, MO(CO)6 adsorption at 140 K exhibited a slightly higher Mo(3d) binding energy (232.6 and 229.4 eV) on Si(100) than the other two systems. The transition from molecular MO(CO)6 to Mo deposition on Si( 100), occurring at about 160 K, was approximately 25 K lower than on the two metal substrates. Beside the differ ences of peak positions and transition temperatures, the ad sorptionofMo(CO)6 on Si( 1(0) exhibited an unusual peak shifting from 149 K to 168 K. The Mo peaks shifted toward higher binding energy with diminishing peak amplitudes as the sample temperature is increased over this range. This shifting may be caused by a different decomposition mecha nism or by a different surface intermediate in this case. It is also possible that the peculiar peak shifting might be due to charging of the overlayer on the Si substrate. Although the constancy of the spectra wi.th varying x-ray beam fiux argues against charging for these low coverage layers, it has been observed that if the adsorbed Mo (CO) 6 layer is thicker than 50A, a charging effect forces all the peaks to shift toward higher binding energy_ 2 4~OK 26,K (SIr< 151K (6TK 145K all 29' 291 lIB; l!8'1' 1I./J 1!!J3 201 B!ndlng Energy CaV) FIG. 4. XPS spectra of the C(ls) region of Mo(CO)" adsorbed on poly crystalline Mo at low temperature and heated to various temperatures. 3038 J. Appl. Phys., Vol. 65, No.8, 15 April 1969 ::.. -'iii c: ~ ~ Mo(CO)a -Cu I 238 236 Binding Energy (eV) 226K 187 K 178 K l68K FIG. 5. XPS spectra of the Mo(3d) region for Mo( CO)" adsorbed on SiC 100) at low temperature and heated to various temperatures. c. Photon~induced decomposition of MO(CO}6 When the physisorbed Mo (CO) 6 layer on polycrystal line Mo and Cu surfaces was irradiated by UV light, a new species was observed. This new species, probably unsaturat ed molybdenum carbonyl, was produced on these surfaces only after UV irradiation. This species left significant metal lic Mo on the substrates after the irradiated systems were heated to room temperature. The chemical states of the physisorbed Mo (CO) 6' the photoproduct layer, and the deposited Mo layer were moni tored by XPS. These spectra are shown in Fig. 6. Curve (a) is physisorbed MO(CO)6 on polycrystalline Mo at 120 K. The spectra of the clean Mo substrate are shown in (e) for comparison. Spectrum (b) is the spectrum obtained after the adsorbed Mo(CO)6 is irradiated by a wide band mercury lamp. The UV irradiation broadened the Mo(3d) peaks of the physisorbed Mo (CO) 6 toward the lower binding energy side. This peak results from the overlap of the unreacted Mo ( CO) 6 adsorbate and a new species (designated the a adsorbate for convenience in the following discussion) pro duced after the UV irradiation. Spectrum (d) was taken after the physisorbed Mo (CO) (,IMo was heated to 450 K, with a heating rate of 10 Kls, and recooled to 120 K. Since the Me (CO) 6 adsorbate was desorbed by the heating pro cess, the peak positions in this spectrum are the same as those from the Me substrate. Since curve (d) showed that flashing the adsorbed MO(CO)6 did not induce any reaction except desorption from the surface, heating the irradiated Mo (CO) 6 should also remove the original Mo (CO) 6 adsor bate. Spectrum (c) is thus obtained by heating the a adsor bate. The peak positions of this spectrum suggest formation of oxidized molybdenum. The irradiation and heating of Mo (CO) (, adsorbed on C. C. CM and S. L. Bernasek 3038 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44MoreO). Mo lel cleM Mo ~ tel l,(a) heofedio4 en z 1&1 ~ z: 238 BINDING ENERGY (to< 1 FIG. 6. XPS spectra of the Mo(3d) region for Mo(CO)6 adsorbed on poly crystalline Mo at 120 K (a) after Mo exposed to 90-L Mo(CO),,; (b) sur face of Ca) irradiated by wide band UV lamp; (c) surface of (b) heated to 450 K a.nd recooIed; (d) surface of (a) heated to 450 K and recooled; (e) is from clean Mo. ell has also been studied (Fig. 7). Without the interference of the substrate Mo atoms, Fig. 7 (b) indicates that only trace amounts of Mo can be detected after adsorbed Mo(CO)6 is heated rapidly (10 K/s) to 450 K. Spectrum (c) results from a mixture of both reacted and unreacted MC(CO)6 adsorbate after UV irradiation. Spectrum (d) shows that after flashing the UV irradiated Mo (CO) 6 adsor bate, significant amounts ofMo were left on the surface. The peak positions in this spectrum suggest the formation of oxi dized Mo. Peak positions for C( Is) and O( Is) spectra of MO(CO)f on all three substrates (Si, Cu, Mo) for various experimental conditions are summarized in Table L Molyb denum carbide and oxide peak positions from previous stud ies are included for comparison. The nature of the a adsorbate is further illustrated by Mo(COle-Cu FlG. 7. XPS spectra ofthe Mo( 3d) region forMo(CO)6 adsorbed on poly crystalline Cu at 100 K (a) afterCu exposed to90-LMo(CO)h; (b) surface of (a) heated to 450 K and recooled; (c) surface of (a) irradiated by wide band UV lamp; (d) surface of (c) heated to 450 K. thermal desorption spectroscopy (TDS) results. As men tioned previously, MO(CO)6 desorbed with a sharp asym metric peak at about 170 K [Fig. 8 (a) ]. The peak shape and position are the same whether CO+, Mo-+-, or Mo(CO)3+ mass signals are monitored. However, after the adsorbed MO(CO)6 was irradiated by a Nzlaser (337 nm) (or with the mercury lamp), a second peak was observed at around 230 K when CO+ was monitored in thermal desorption. [Fig. 8(b)J. In contrast, this new second peak can not be detected by monitoring the Mo + signal [Fig. 8 (c) ], This new CO peak was examined further in the following way: The UV irradiated surface was heated to 250 K to desorb the new CO peak. This surface was then cooled to 130 K and redosed with Mo(CO)6 or CO, and the thermal desorption spectrum recorded. This spectrum did not show the new CO TABLE I. XPS peak positions for Mo(CO)" adsorbed on various substrates. T* is transition temperature discussed in text. Substrate Mo Cu Si(lOO) " Reference 41. b Reference 42. < 130K 291.5 291.3 291.7 T* 290.3 290.1 C( Is) >450K 287.8 288.2 3039 J. Appl. Phys., Vol. 65, No.8, i 5 April 1989 Peak positions in cV carbide 282,9" <l30K 534.3 534.4 534.3 T* 532.6 532.5 532.5 O(1s) >450K 531.5 531.4 531.6 C. C. Cho and $. L. Bernasek oxide 530.8h 3039 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44I- Z ::> (a l a:i a: <t .J <t Z :!) iii !b) (c) .00 150 Mo[CO)e-Mo Before UV{~31nml Irradialio!l CO, Mo,Mo{C013 Irradiation CO Afillf Irradiallan Me fOC 250 TEMPERATURE! K) FIG. 8. TDS spectraofMo(CO)6 on Mo (a) before irradiation by N2 iaser (337nm); (b) afterirradiationmonitoringCOf mass signal; (c) afterirra diation monitoring Mo + mass signal. peak (at 230 K), suggesting that this peak is not due to MO(CO)6 or CO adsorption on a UV deposited Mo layer. Since no Mo desorption was detected at the temperature of the new peak and it is still quite low (below the desorption temperature for CO on Mo), the decomposition ofunsatu rated molybdenum carbonyl with the Mo atom bonding to the substrate is likely to be the origin of the second TDS peak. When the unsaturated carbonyl is heated, the bonds between central Mo atom and CO ligands break and Mo is left on the substrate, with the CO desorbing directly. D. Photonainduced reaction of Mo(CO)e and O2 The effect of UV irradiation of gaseous and coadsorbed Mo(CO)6 and O2 mixtures has also been studied. After the cleaning and characterization process, polycrystalline Mo, Cu, and SiC 100) samples were transferred to the reaction chamber, dosed with Mo(CO)6 and O2 mixtures (typically 1:3 MO(CO)6:0Z' total pressure 1-5 Torr) with the surface held at room temperature or 120 K, and irradiated by a mer cury lamp. The samples were then transferred back to the main chamber and analyzed by XPS and AES. Molybdenum oxide was found to be deposited on these samples after UV irradiation in the gas phase of the MO(CO)6/02 mixture with the substrate at room tempera~ turc. Both Mo02 and MoO} are detected (see Fig. 9, lowest trace), with more MoO} present at higher 02:Mo(CO)6 ra tios. In contrast, when the substrate was held at 120 K and a Mo(CO)(J/O z coadsorbed layer was formed, molybdenum oxide was not observed to form on the surface following di~ 3040 J. Appl. Phys., Vol. 65, No.8, i 5 April 1989 MoO. 51! 100) MO!3d) )-i- fJ ~' It! i- !!: 920K ~4 2~ BINDING ENERGY (IV I FIG. 9. XPS spectra of the Mo(3d} region from deposited MoO, on Si ( 1(0)' after annealing to various temperatures. rect UV irradiation of the coadsorbed layer. The peaks in the Mo(3d) region were the same as the irradiated Mo(CO)6 adsorbate without 0 coadsorption, suggesting the formation of unsaturated molybdenum carbonyl. E. Electronuinduced Mo(CO)& decomposition The electron gun in the Auger spectrometer was used as an electron source to study the electron-induced decomposi tion ofMo(CO)6. Two types of electron-beam induced de composition experiments were carried out. The MO(CO)6 multilayer deposited an the substrate at 120 K was irradiat ed with the electron beam, and the room~temperature sub strate exposed to 10-7 Torr ofMo(CO)6 vapor was electron bombarded as well. It was found that electron beam induced metal deposition occurred from either gaseous or adsorbed MO(CO)6' and resulted in similar film compositions. With 3-keV incident electron collisions, the deposited layer showed a Mo, C, 0 ratio of 1: 1.2:0.4. The C composition is a factor of 2 higher than those layers deposited thermally or UV photolytically (1:0.6:0.4 and 1:0.7:0.5, respectively). By analyzing the C peak shape of the deposits, the Auger spec~ tra suggested that molybdenum carbide was formed during the electron bombardment deposition. Based on the sample current, electron spot size and the amount of Mo deposited on the surface, the decomposition cross section of the adsorbed Mo (CO) 6 is estimated to be about to-15 cm2 which is on the same order as the decompo sition cross section of gaseous Mo (CO) 0,24 The high decom position cross section indicates that AES is highly destruc tive of the low-temperature deposited MO(CO)6 overIayer. c. C. Cho and S. L. Bernasek 3040 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44a b Although extensive studies were not carried out, electron beam~induced changes in the UV or thermally deposited lay ers were not observed, in contrast to the effects described for the low~temperature-deposited MO(CO)6 overlayer. The nondestructive feature ofXPS thus offers a great advantage for the investigation of this system. F. Surface treatment after deposition LEED patterns were examined after MO(CO)6 was de posited on the Si( 100) surface at low temperature. No new ordered structure was observed. The original (2 Xl) LEED pattern gradually faded away with increasing Mo( CO) 6 ex posure. At low coverage, the LEED pattern could be recov ered by flashing the sample to about 1170 K . At high cover~ age, the intermixing of Mo and Si from heated samples permanently disordered the surface. In most cases, du.ring the annealing of UV~irradiated surfaces, which exhibited no LEED pattern, the original (2 Xl) pattern started to regrow with dim, broad LEED spots after the sample was heated to 1070 K and cooled for LEED observations. The LEBD spots became sharper after the crystal was heated to higher temperature. If the Si sam ple with UV-irradiated Me(CO)" overlayer was flashed to t 170 K for a short time, a (4 X 2) LEED pattern and a very clean Si Auger spectrum could then be seen after the sample was cooled. Since this LEBD pattern is believed to be asso~ dated with defects on the clean Si(100) surfaces,34 the LEED and AES results consistently suggested that Mo atoms removed C, 0, or even Si atoms as a scavenger during the desorption process. However, if the Sf surface was cov ered by multilayer Mo or if the sample was heated up slowly, Si atoms apparently segregate to the surface and become mixed with the Mo layer, which could then be detected by AES. In some cases, a rotated LEED pattern has been ob served after flashing the Si ( 100) surface covered by submon ~ olayer Mo to 1120 K. Figure lO(a) shows a normal (2 Xl) LEED pattern. Figure lO(b) is a rotated pattern, the rota tion angle being 36" ± 10 in this case. Rotation angles of 18° ± 1 cor 10° ± 10 have also been observed. LEBD patterns with different rotation angles have been observed in different regions of the same sample. A uger spectra showed that no detectable Mo was left on this surface in any region. Figure 10 (c) is a (4 X 2) pattern observed after the surface of (b) was further annealed to 1270 K. The rotation pattern is be lieved to be caused by the formation of microfacets. Facet formation from clean SiC 100) at temperatures ranging from 3041 J. Appl. Phys., Vol. 65, No.8, 15 April 1989 c FIG. 10. LEED patterns ofSi( 100) with 45-eV in cident elcctron energy. (a) Clean surface; (b) after flashing Mo deposited Si( 100) to about 1130 K; (e) after (b) was heated to 1270 K. 1375 to 1500 K has been observed by high-energy electron diffraction (RHEED),J5 LEED,3n and scanning tunneling microscopy (STM).34 Our results showed that the facet structure could appear at lower temperatures when Mo was deposited on the surface. This may be due to the removal of Si atoms with desorbing Mo, leaving the surface with high step density and thus offering an appropriate condition for facet formation. Wen-ordered Sic 100) could be recovered by annealing the surface at about 1270 K. The deposited molybdenum oxide film on SiC 100), re sulting from the UV irradiation of gaseous mixtures of Mo (CO) 6 and 02> was treated by thermal heating and Ar + bombardment. The XPS results are shown in Figs. 9 and 11. When the molybdenum oxide was bombarded by 3.G-keV Ar l-, it was reduced efficiently and formed metallic Mo atoms on the surface before they were gradually sputtered At Spytl~ll!g MOOl~Si{iOO) FlO. 11. XPS spectra of the Mo(3d) region after Mo( CO)6(g) and O2(.0) were (a) irradiated by a UV lamp; (b)-(d) after 2.0-keV Ar+ bombard ment ofsurface (a) for 6, 16, and 26 min, respectively. C. C. Cho and S. L. Bernasek 3041 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44away. During the annealing process, molybdenum oxide was reduced to metallic Mo after 1 170-K heating. The dimin ished peak area in Fig. 11 indicated that MoOx either de sorbed from the surface or dissolved in the substrate during the heating process, which is consistent with the LEED study discussed above. IV. DISCUSSION Generally speaking, Mo(CO)6 behaves quite similarly on different substrates, reflecting the nature of a weakly ad sorbed layer. Nonetheless, slightly different interactions be tween substrate and adsorbate can be inferred from the dif ferent peak positions and transition temperatures. The detection of thermally desorbed species by mass spectrosco py and the observation of thermally deposited Mo showed that both desorption and decomposition reactions occurred during the sample heating process. However, the desorption reaction overwhelmed the decomposition reaction in most cases. Since CO adsorbs dissociatively on Mo and associative lyon Cu at room temperature, the great similarity between the adsorptions ofMo (CO) 6 on these two substrates, indud ing peak positions, transition temperatures, and adsorbed species at various temperatures, suggests that the bond breaking between C and 0 of the CO ligand is not involved in this decomposition process. Rather, Mo-CO bond break ing appears to occur during the thermal dissociation reac tion, followed by immediate CO desorption. On the other hand, the C and 0 contamination found on the deposited layers in previous reports12." is likely to be formed by the dissociation of CO induced by the decarbony lated Mo atom on the surface. The fact that MO(CO)6 is highly reactive with O2 at room temperature under (IV irra diation indicates that the extremely high 0 fraction in some deposited films may be caused by the reaction of the irradiat ed Mo(CO)6 with background O2 gas in the reaction chamber. In the photoelectron spectra of the adsorbed Mo (CO) 6' shake-up peaks around the Mo(3d) and C( Is) regions have been observed. After UV irradiation, the shake-up peak at 237.2 eV shifted to 236.4 eV [Figs. 7(a) and 7 (c)]. The fact that the shake-up peak can still be seen after irradiation and that it exhibits a different peak position provides further evi dence that unsaturated molybdenum carbonyl is the photon induced product. The shake-up peaks in these metal car bonyl compounds are proposed to be from a metal-ligand electron transfer final state.2 A substituted Mo(CO).,L or unsaturated MO(CO)6 __ x thus still tend to exhibit this shake-up structure with slightly different energy levels. In contrast, the fact that our study of thermally decomposed MO(CO)6 did not show the shake-up peak (Figs. 2-5) indi cates the formation of a different product in this case, which appears to be Mo deposited with C and 0 atoms, as discussed previously. Using IR and Raman spectroscopy, UV photolysis of Mo (CO) 6 isolated in O2 doped Ar or CH4 matrices at 10K has been studied. 37 Mo (CO) 5 and CO were produced initial ly but further irradiation yielded MoOl and Mo03 as final products. Mo (0) 2 (CO) 4 with both 0 atoms from the same 3042 J. Appl. Phys., Vol. 65, No.8, 15 April 1989 O2 molecule was suggested to be the possible intermediate structure. These results indicated that Mo(CO)6 exhibited quite different reaction mechanisms under irradiation de pending on the environment of the complex molecule. In gas-phase photoreactions "naked" Mo atoms can be formed rapidly and react with adjacent O2 molecules because of the high mobility of the molecules. When MO(CO)6 was ad sorbed on the surface, photodissociated ligand(s) were re placed by surface atoms and thus lost the ability to react with coadsorbed O2 molecules. While in a low-temperature inert matrix, Mo (CO) 610st one CO ligand under UV irradiation, and was stablized by the surrounding inert atoms or mole cules until it was further dissociated by UV irradiation and then reacted with the O2 in the matrix. Electrochromic devices have been produced by various deposition techniques such as evaporation, sputtering, ano dization, spray pyrolysis, and colloidal oxide gel technology. Lately Mo03 has been deposited for electrochromic material by plasma deposition from a Mo ( CO) 6 and O2 mixture as a new deposition technique.38 The observation here that Mo03 can be formed efficiently by the UV photoreaction of Mo (CO) 6 and O2 mixtures offers another possibility for for mation of electrochromic films. The fact that electron induced metal deposition resulted in a higher C fraction than 0 has also been observed in the studies ofironlO•1I•3J and osmium15 carbonyls. It is interest ing to note that electron stimulated desorption (ESD) stud ies have shown a similar trend.19 The cross sections for the rupture of an internal bond in a weakly adsorbed molecule are usually found to be larger than the cross sections for the rupture of a metal---atom bond under the electron bombard ment ofESD experiments. For example, considering the ad sorption of CO molecules on a Ni surface, the C-O bond is easier to break than the C-Ni bond. Consequently, more C tends to be left on the surface after the electron bombard ment. These results indicate the possibility of a similar elec tron excitation mechanism in this case. The ion sputtering effect on molybdenum oxide deposit ed Mo metal has been studied previously with various ion voltage, current, and incident angle.40 Reduction of molyb denum oxide has been observed. However, the fact that in our study a reduced metallic Mo thin layer can be produced before the deposited MoOx is sputtered away might offer a process to create Mo metal from Mo03 insulator covered device structures. Yo CONCLUSIONS Molybdenum deposition from the decomposition of MO(CO)6 on SiC 100), polycrystalline Mo, and Cu has been studied by XPS, AES, TDS, and LEED. The adsorption, decomposition, and desorption of adsorbate Mo( CO) 6' the photoreactions of gaseous and adsorbed Mo (CO) fl'Oz mix tures, and the properties of the deposited layers were investi gated. The results are summarized as foHows: ( 1) Mo (CO) 6 adsorbs weakly on Si ( 100), polycrystal line Mo, and eu surfaces, and desorbs with zero-order kinet ics. For each substrate, the adsorbed layer exhibits similar thermal decomposition processes, indicating weak adsor bate-substrate interactions. Although both desorption and c. C. Cho and S. L. 8srnasek 3042 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44decomposition take place at low temperature when the sam ples are heated, the desorption process dominates the adsor bate decomposition reaction. Decomposition can be en hanced by slow heating of the substrate. (2) During the thermal decomposition process, the mo lecular MO(CO)6 adsorbate is first converted to metallic Mo in the presence of C and O. Higher temperature heating induces the formation of molybdenum carbide and oxide. (3) UV irradiation of the Mo (CO) 6 adsorbate produces unsaturated molybdenum carbonyl with the Mo atom at tached to the surfaces. The Iiberted CO is desorbed in a new TDS peak from this surface. Heating of the irradiated adsor bate dissociates the other CO ligands and leaves molyb denum carbide/oxide on the surfaces. ( 4) UV irradiation of a gaseous Mo (CO) 6/02 mixture leads to the deposition of Mo02 and Mo03; however, UV irradiation of a coadsorbed MO(CO)602 mixture results in unsaturated molybdenum carbonyl. This result indicates that UV irradiation of adsorbed Mo (CO) 6 does not result in the formation of reactive Mo atoms which are readily oxi dized, in contrast to what is observed in the UV irradiation of gas phase Mo (CO) 6' ( 5 ) Electron collisions with gaseous or adsorbed MO(CO)6 result in a high C composition deposit which ap pears to be molydenum carbide. (6) Mo deposition on SiC 100) obscures the LEED pat tern of the substrate. The LEED pattern can be recovered from a low coverage Mo/Si ( 100) surface by flashing the crystal to a high temperature, but thermal heating of a high coverage Mo/Si( 100) surface induces intermixing of Mo and Si and creates surface disorder. Rotated LEED patterns have been occasionally observed after the low coverage Mo/SiC 100) surface is fiashed. BothAr-~ bombardment and high-temperature heating reduce the molybdenum oxide film on SiC 1(0). Metallic Mo atoms are formed on the sur face by Ar + bombardment before they are gradually sput tered away. ACKNOWLEDGMENT This work has been supported by the Air Force Office of Scientific Research. 'E. Muetterties, Science 194,1150 (1976); 196, 839 (1977). E. L. Mutter ties, T. N. Rhodin, E. Band, C. F. Brucker, and W. R. Pretzer, Chem. Rev. 79,91 (\979). 2E. W. Plummer, W, R. Salaneck, and 1. S. Miller, Phys. Rev. B H!, 1673 ( 1978). 3M. R. Albert and J. T. Yates, Jr., TheSurfaceScientist's Guide to Organo metallic Chemistry (American Chemical Society, Washington, DC, 1987). 3043 J. Appl. Phys., Vol. 65, No.8, 15 April 1989 4See, for example, G. C. Bond, in Metal Support and Metal-Additive Effect in Catalysis, edited by B. Irnelik, C. Naccache, G. COllcturier, H. Praliaud, 1. Meriaudeau, P. Ga\lezot, G. A. Martin, and J. C. Vedrine (Elsevier, New York, 1982), p. l. 'See, for example, A. w. Johnson, D. J. Ehrlich, H. R. Schlossberg, Mater. Res. Soc. Pmc, 29, 1984. "~Po L Shah, IEEE Trans, Electron Devices ED-l6, 631 (1979). 7N. E. Miller and I. Beinglass, Solid State Techno!. 23, 78 (1980). 'w. C. Benzing, Electronics 116 (1982). 9A. J. Learn, J. Electrochcm Soc. U3, 894 (1976). lOS. I'. Murarka,J. Vac. Sci. Techno!. A 17, 775 (1980). "R. Solanski, P. K. Boyer, and G. J. Collins, AppL Phys. Lett. 41, 1048 (1982), :2R. So\anski, P. K. Boyer,.r. E. Mahan, and G. J. Collins, Appl. Phys. Lett. 38,572 (1981), I3J. R. Creightoll, J. App!. Phys. 59, 410 (1986). '4J, R. Creighton, J. Vac. Sci. Techno!' A 4,669 (1986). 150. K. FlYllll and J. I. Steinfeld, J. App!. Phys. 59, 3914 (1986). '''J, S. Foord and R. B. Jackman. Chern. Phys. Lett. 112, 190 (1984). 171. S. Foord and R. B. Jackman, Surf. Sci. 171,197 (1986). "c. E. Bartosch, N. S. Gluck, W. Ho, and Z. Ying, Phys, Rev. Lett. 57, 1425 (1986). I"N. S. Gluck, Z. Ying, C. E. Bartosh, and W. Ho, Phys. Rev. Lett. 86, 4957 (1987). l0c. C. Cho and S. L Bernasek, 1. Vae. Sci. Techno\. A 5, 1088 (1987). 2'J. R, Swanson, C. M, Friend, and Y. J. Chabal, J. Chem. Phy~. 87,5028 (1987). 21D. P. Gerrity, L. J. Rothberg, and V. Vaida, Chern. Phys. Lett. 74, 1 (1980), 23Z. Kamy, R. Naaman, and R. N. Zare, Chem. Phys. Lett. 59, 33 (1978). 24M. A. Duncan, T. G, Dietz, and R. E. Smalley, Chem. Phys. 44, 415 (1979), 25A. Iverson and B. R. Russel, Chern. Phys. Lett. 6, 307 (1970). 26G. J. Fisanick, A. Gedanken, N. Kuebler, and M. B. Robin, J. Chern. Phys, 75, 5215 (l98\). nT. M, Mayer, G. J. Fisanick, and T. S. Eichelberger IV,], AppJ, Phys. 53, 8462 (1982). 2sK. E. Lewis, D. M. Golden, and P. Smith, J. Am. Chem. Soc. 106, 3905 (1984). 29J. M. Kelly, C. Long. and R. Bonneau, J. Phys. Chem. 87, 3344 (J 983). "'M. A. Graham, M. Poliakolr, and J. J. Turner, J. Chern, Soc. A 18,2939 (l97I). JlR. N, l'erutz and J, J. Turner, J. Am. Chern. Soc. 97, 4791 (1975). 32}. J. Turner, J. K. Burnett, R. N. Perutz, and M. Poliakoff, Pure AppL Chem. 49,271 (1977). )lC. C. Cho, Ph.D. thesis, Princeton University, New Jersey, 1987. 34R. J. Hamers, R. M, Tromp, and J. E. Demuth, Phys. Rev. B 34, 5343 (1986). 35N. G. Norton, Vacuum 33, 62 I (1983). "'P. M. Gundry, R. Holton, and V. Leverett, Surf. Sci. 43, 647 (1974). 37J. A. Crays tOil, M. J. Almond, A. J. Down, M. Poliakoff, and J, ]. Turner, Inorg. Chern. 23, 3051 (1984). 3Xc. E. Tracy and D. l(, Benson, J, Vac. Sci. Technol. A 4,2377 (1986). 3"T, E. Madey and R. Stock bauer, in Methods a/Experimental Physics, edit ed by R, L Park and M. G. Lagally (Academic, New York, 1985), Vol. 22. 4!JG. A. Vervon, G. Stucky, and T. A. Carlson, Inorg. Chern. 15, 278 ( 1976). 41 L. R. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman, and C. Nordling, 1. Phys. Chem. Solids 30, 1835 (1969) (for C in Mo2C). 42T. A. Peterson, J. C. Carver, D. E. Leyden, and D. M, Hercules, J. Phys. Chern. 80, 1702 (1976) (for 0 in MoO,). C. C. Cho and S. L. Bernasek 3043 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Wed, 26 Nov 2014 03:03:44

1.341583.pdf

Limit cycle oscillation in negative differential resistance devices E. S. Hellman, K. L. Lear, and J. S. Harris Jr. Citation: Journal of Applied Physics 64, 2798 (1988); doi: 10.1063/1.341583 View online: http://dx.doi.org/10.1063/1.341583 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Study on molecular devices with negative differential resistance Appl. Phys. Lett. 103, 033506 (2013); 10.1063/1.4813844 Graphene nanoribbon as a negative differential resistance device Appl. Phys. Lett. 94, 173110 (2009); 10.1063/1.3126451 Negative Coulomb damping, limit cycles, and self-oscillation of the vocal folds Am. J. Phys. 74, 386 (2006); 10.1119/1.2173272 Twomass model of the vocal folds: Negative differential resistance oscillation J. Acoust. Soc. Am. 83, 2453 (1988); 10.1121/1.396326 Interactions of limit cycle oscillators AIP Conf. Proc. 27, 187 (1976); 10.1063/1.30358 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.102.42.98 On: Mon, 24 Nov 2014 22:12:58limit cycle oscillation in negative differential resistance devices E. S. Hellman,a) K L. Lear, and J. S. Harris, Jr. Solid State Electronics Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California 94305 (Received 21 March 1988; accepted for publication 19 May 1988) We present experimental current-voltage curves for GaAsl AIGaAs resonant tunneling diodes which show complicated multiple step structures when biased into negative differential resistance. We show that these results can be explained as limit cycle oscillations in the nonlinear dynamical system consisting of a negative differential resistance device loaded with a resonant circuit. Two circuit models, a resistor-capacitor-inductor load, and the dispersionless transmission-line load, are discussed. The limit cycles in the second model exhibit a variety of behaviors characteristic of nonlinear systems, such as bifurcations, period doubling, and Devil's Staircases, resulting in good qualitative agreement with experiment. The observation of nonlinear conductance at frequen cies above 1 THz in GaAsI AIGaAs resonant tunneling di odes 1 has caused a resurgence ofinterest in high-frequency negative differential resistance (NOR) devices. Both funda mental research and practical applications of these devices depend on the stabilization of their conductance characteris tics when they are biased in the negative differential resis tance regime. This can be particularly difficult for devices which have high cutoff frequencies, because they must be properly loaded over such a large frequency range. In view of this difficulty, it is often convenient to measure current-vol tage characteristics of devices without proper loading. Un der these circumstances, spontaneous osciHations can arise when the device is biased to have a negative differential resis tance. The onset of oscillations often appears as discontin uity in the measured dc current-voltage characteristics, even if oscillating voltages do not appear at the voltmeter. Several recent studies have focused on fundamental aspects of the resonant tunneling process for diodes biased into NDR. For example, Goldman, Tsui, and Cunningham2 attributed step structure in the negative differential resis tance region of GaAsl AIGaAs resonant tunneling diodes to quantum-mechanical interactions between the central quan tum well of the device and the quantized states in the accu mulation layer at the cathode barrier interface. Berkowitz and Lux3 have proposed a similar effect. Goldman et af. claim that a lO-nF capacitor connected in parallel with the resonant tunneling diode is effective in suppressing oscilla tions in their devices, although this claim is controversial. 4 In this communication we will present current-voltage measurements of resonant tunneling diodes made in our lab oratory, some of which show remarkably complex step structure in the NDR region. This structure is entirely due to spontaneous oscillations in the devices. We win then discuss the behavior of two simple model circuits in which limit cy cle oscillations result in similar current steps. These models consider the coupling of a nonlinear negative differential re sistance element to a resonant circuit. Resonant tunneling diodes are fabricated in our labora tory using GaAsl AIGaAs epitaxial layers grown by molecu lar-beam epitaxy. dc current-voltage characteristics of both 0) Also at the Department of Applied Physics. wafer probed and bonded devices are measured using an Hewlett-Packard 4145 semiconductor parameter analyzer. Figure 1 shows two examples of the negative differential re sistance current-voltage characteristics for resonant tunnel ing diodes made in our laboratory. Note that both curves are characterized by multiple "stair" regions within which the conductance is less negative or even positiveo In particular, note the pair of substairs in the "legs" and "back" of the main stair in Fig. 1 (a). Observation of the diode current on an oscilloscope confirmed the existence of oscillations with frequencies in the 10-100-MHz range in the stair regions. Figure 2 shows oscilloscope traces with two different time scales taken from the device of Fig. 1 (b). Figure 2(a) shows a typical current oscillation waveform. The maximum and minimum current in the waveform are larger and smaller than the peak and vaHey current afthe device, while some of the local maxima and minima correspond closely to the peak and valley currents. This suggests that a voltage oscillation is occurring at the device. Figure 2 (b) shows a bUl'st of oscilla tions, a phenomenon which seems to occur in the substair regions. The oscillation frequency in this device did not change more than 5% over the range of stair biases. The simplest circuit in which a negative differential re sistance device might be expected to sustain oscillations would be the simple resistor-capacitor-inductor (RLC) load attached to a NDR element with current bias depicted in Fig. 3(a). For a NDR current-voltage characteristic given by i = iNOR (v), the differential equation governing the vol tage v across the NDR element is 25D (a, 300 350 voitage (mV) 400 200 200 (b) 250 300 voltage (mV) FIGo 1. Experimental current-voltage characteristics for two GaAsl AIGaAs resonant tunneling diodes. 2798 Jo AppJ. Phys, 54 (5), 1 September 1988 0021-8979/88/172798-03$02.40 @ 1988 American Institute of Physics 279B [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.102.42.98 On: Mon, 24 Nov 2014 22:12:58soc ~ 400 <:: 300 <» t: 200 :! 0 100 0 0 ~o 20 30 40 50 (a) time (ns) 500 ~ <:: 3ea ~ 200 :! <:.> 100 100 (b) time (ns) FIG. 2. Oscilloscope traces of the cllrrent for the device of Fig. l(b): (a) oscillations in diode biased at 260 m V; (b) oscillation burst in diode biased at 300 m V. The oscillations in the burst resemble the oscillations shown in (a) on a faster time scale. --rii = V + iNDR (v)R -IR + ro[Zif..DR (v) + ~), (1) where Z =,Jr Ie, .. = ..[Le', I is the bias current, and i' = dildv. This equation can display a variety of asymptotic behaviors depending on the values of the parameters 1, R, and Z for a given NDR characteristic. Dynamical systems described by this sort of equation are very well understood, as it can be considered to be a variant of the van der Pol equation.s Integration ofEq. (1) can be done to compute a current voltage characteristic for the RLC model. Figure 4 (a) shows the results of such a computation for models with R = 1, Z = 1.4,2.0, and 4.0, and a negative resistance device characteristic given by iNDR (/J) = 3v3 -6.Sv2 + 4v. The or dinate in Fig. 4 is the total applied bias current which flows both through the diode and the load resistor R, in parallel. The current in the stairs is given by the average current drawn by the NDR element as it undergoes limit cycle oscil lations. The widening and flattening of a single stair with increasing Z is clearly seen. The RLC model fails in several respects to explain the more complicated aspects of the oscil lations apparent in the data of Figs. 1 and 2. While the stairs are wen explained, the mUltiple stairs seen in the experimen- ! ~ Imas I 1~lbias Z H 1\ ~ 'In i ',1'1 ? c T i nd!V) R .1 (b) ~ FIG. 3. Schematics ofmooels for interaction of It nonlinear negative resis tance device with a resonant circuit. (a) RLC load model. (b) Transmis sion-line load model. 2799 J. Appl. Phys., Vol. 64, No.5, i September 1988 1.0r---------- 0.9 0.7 0.6 la) -~~\ U , .. " L~ z_, 2 "" ~.4 ~.2 <,.3 1.4 1.5 ~.6 tQtai tmts cummt 1.4 1.5 1.8 (b) total bi&s ciJfTenl FIG. 4. Calculated current bias characteristics for an idealized nonlinear negative resistance device interacting with resonant loads. The abscissa is the average cllrrent drawn by the negative resistance load, and the ordinate is the total bias current. The negative resistance device characteristic is giv. en by I(u) = 3v' --6.Sv' + 4v. (a) RLCioad, withR = LandZ = 1.4,2.0, and 4.0. (b) Dispersionless transmission-line load, with termination resis tance R = 1 and transmission-line impedances, Z = 1.33,2.5, and 6.0. The numbers adjacent to the curves indicate the limit cycle period. Curves for Z = 2.5 and 6.0 are shifted upward for claIity. tal data cannot be produced using the simple RLC model. Calculation of de current-voltage curves using this model is also computationally expensive because the differential equation must be integrated for each bias current until the system converges to an asymptotic behavior. A natural ex tension of the RLC load model to allow more complicated behavior would be the addition of one or more LC stages. The system with two LC stages is described by two coupled equations similar to Eq. (1), resulting in a four-dimensional phase space. The extraction of current-voltage curves from such a model would be computationally very expensive, par ticularly since no asymptotic behavior is insured.5 Ifwe instead consider the case ofa circuit with infinitely many identical LC stages (the ideal transmission line) the computational difficulties are circumvented. The model is depicted in Fig, 3 (b). It consists of an ideal transmission line terminated at one end by the nonlinear NOR device and at the other end by a resistor. A current bias is applied at the resistor. The impedance of the transmission line, Z, and the load resistance R are used as parameters for the model. Re flections from the unmatched resistor provide feedback to the negative differential resistance device, allowing it to sup port oscillations. The transmission lines are assumed to be dispersionless, which simplifies the calculation immensely. The calculation is reduced to a simple iteration of a function describing the amount of current which is reflected by the NOR load and terminati.on resistor in one round trip of the transmission line. This model has been implemented numerically. Typical average load current versus bias characteristics produced using the model are shown in Fig. 4(b) for transmission-line impedances, Z = 1.33, 2.5, and 6, and R = 1. The NOR characteristic is the same as that used in the computations using the RLC model. The conditions on Z and R required for oscillatory behavior are found to be identical to those found for the RLC model. For Z > !.R~:-, where r min is the minimum negative resistance of the NDR characteristic, os cillations (convergence to a cycle) are observed. for a range of bi.ases around the midpoint of the negative resistance re gion. As Z is increased, increasing the feedback, the limit Hellman, lear, and Harris, Jr. 2799 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.102.42.98 On: Mon, 24 Nov 2014 22:12:58cycle stair becomes more pronounced and in fact develops a positive slope, with discontinuities at either edge, For even larger Z, substairs and higher periodicity limit cycles appear. The character of the apparent discontinuities is found to de pend on whether or not the reflection matching condition has more than one solution. When the reflection matching condition is single valued, the average current in the NDR element changes continuously with bias. Transitions in the cyclic behavior occur by bifurcations of the recurring vol tages. Biases at which these occur are characterized by cusps in the load current characteristics. Period doubling (simul taneous bifurcation of all of the recurring solutions) is ob served for some parameter ranges. This period doubling be havior is universal in smooth mappings with extrema, in the limit of many period increases, and is important for transi tions to chaotic behavior, (, For larger Z, when the reflection matching condition has three solutions, the transitions in the cyclic behavior result in true discontinuities in the load cur rent characteristics. Examination of the discontinuities on a finer bias mesh typically reveals multiple discontinuous transitions to regions of higher periodicity. For example, the curve for Z = 2.5 in Fig, 4(b) shows regions of cycle 3, 5, 7, etc. between the main two-cycle stair and the one-cycle curve. In this way, the current bias curve reflects a Devil's Staircase in the limit cycle period. In fact, the "bistable," discontinuous mapping which appears in our model in this range has some similarities to maps of the circle known to generate Devil's Staircases.7 The tendency of negative differential resistance devices to oscillate is, of course, one reason for their usefulness; the fact that these oscillations show up in current-voltage char acteristics has been widely known, at least since the early work on tunnel diodes, g In fact, the behavior of tunnel diodes attached to transmission-line loads was reported by Nagumo and Shimura9 in 1961, Although the possibility of very high frequency oscillations in resonant tunneling diodes may complicate matters, the effect of oscillations in this case is also widely recognized.4,lo Aperiodic behavior and chaos in nonlinear dynamical systems involving coupled tunnel di odes have also been studied by several groups.! 1.12 Chaos in circuits with piecewise linear negative resistance devices ("Chua's circuits") has also been studied in detail. Both circuit models discussed here use a current source and load resistor to bias the NDR elements, In real current voltage measurements, voltage biases are typically used. This corresponds to the use of a very sman R with current bias in these models. Thus, it is very likely that the stray inductances and capacitances in the measuring circuit will result in enough feedback of the conect sign for oscillations (Z>~Rrmin)' Careful circuit design should be ahle to use the known stability requirements to suppress oscillations. 2800 J. Appt. Phys .. Vol. 64, No.5, 1 September 1986 The shape of the current bias characteristic predicted by the transmission-line model strongly resembles the experi mental curve in Fig. 1 (a), especially if the parameters Rand Z are optimized to fit the size and slope of the substair re gions. The experimentally observed oscillations in Fig. 2 are considerably more complicated than would be predicted by the simple models, The actual measurement circuit consists of inductance and capacitance of the probe tip, wiring induc tance, and coax running into an unmatched voltmeter, The transmission-line model should be thought of as an approxi mation of a complicated multidimensional dynamical sys tem which describes the actual circuit by a one-dimensional nonlinear map. The strong similarities between the current bias characteristics predicted by the RLC and transmission line models suggest that this approximation may be a good one. In this work, we have shown experimental measure ments of resonant tunneling diodes which show complex be havior in the negative differential resistance region. We point out that limit cycle oscillations can occur when nonlin ear negative differential resistance elements interact with resonant loads, and although the multiple steps in the experi mental data cannot be explained using an RLC load, they are reproduced in computationally efficient models which use an idea! transmission line a.') a resonant load. The authors would like to thank P. Hadley, S. Diamond, and G. Sollner for helpful comments on the manuscript. This work was supported by the Joint Services Electronics Program, Contract No. DAAG29-84-K0047. E. S. H. ac knowledges the support of an IBM Fellowship. IT. C. L. G. Soilner, W. D. Goodhue, P. E. Tannenwald, C. D. Parker, and D. D, Peck. App!. Phys. Lett. 43, 588 (1983). 'V. J. Goldman, D. C. Tsui, and J. E. Cunningham, Phys, Rev. Lett. 58, 1256 (1987). 'n. L. Berkowitz and R. A. Lux, J. Vac. Sci. Techno!. B 5, 967 (1987). ·T. c. L G. Sollner, Phys. Rev. Lett. 59, 1622 (1987). 'L. O. Chua, C. A. Desoer, and E. S. Kuh, Linear and Nonlinear Circuits (McGraw-Hili, New York, 1987), pp. 426-439. OM. j, Feigenbaum, Physica 7D, 16 (1983). 7B. B. Mandelbrot, The Fractal Geometry of Nature (Freeman, San Fran cisco, CA, 1982). "W. F. Chow, Principles of Tunnel Diode Circliits (Wiley, New York, 1964), pp. 151-182. 9J. Nagumo and M. Shimura, Proc. IRE 49,1281 (1961). lOT. 1. Shewchuk, J. M. Gering, P. C. Chapin, p, D. Coleman, W. Kopp, C. K. Peng, and H. Morko,!, AppL Phys. Lett. 47, 986 (1985). "M. t Rabinovich, SOy. Phys. Usp. 21, 443 (1978). 12J. P. Gollub, T. O. Bnmner, and B. G. Danly, Science 200, 48 (1978). uS. Wu, Proc. IEEE 75,1022 (1987). Hellman, Lear, and Harris, Jr. 2800 ........................................... :.:.:.:.:.:.:.:.:.: .... :.: ........... . [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.102.42.98 On: Mon, 24 Nov 2014 22:12:58

1.37620.pdf

Radiation generated by rotating electron beams Y. Y. Lau Citation: AIP Conference Proceedings 175, 210 (1988); doi: 10.1063/1.37620 View online: http://dx.doi.org/10.1063/1.37620 View Table of Contents: http://aip.scitation.org/toc/apc/175/1 Published by the American Institute of Physics210 RADIATION GENERATED BY ROTATING ELECTRON BEAMS Y. Y. Lau Naval Research Laboratory, Washington, DC 20375-5000 ABSTRACT It is shown that the degree of bunching in a rotating electron beam depends sensitively on the manner in which the equilibrium rotation is supported. Maximization or elimination of small signal growth can be achieved by adjusting the radial electric field and the axial magnetic field which are needed to support the equilibrium rotation. A simple dispersion relation is given for general combinations of electric and magnetic fields, and for arbitrary electron energy and beam current. The model encompasses a large class of radiation sources currently under active investigation. Some potential applications and proof-of-principle experiments are indicated. I. INTRODUCTION There has been sustained interest in the interaction between a rotating electron beam and its surrounding structure. This seemingly old subject still plays a major role in the recent developments in high power microwave electronics 1'2 and high current cyclic accelerators. 3'4 Depending on the device, the electron rotation is supported either by an axial magnetic field, or by a radial electric field, or by a combination of both. Even for the case of a thin beam, the crucial dependence of the beam dynamics on the equilibrium type was noted only in the last few years. Here, we summarize some of these recent findings. We shall show that, for a given geometry and a given kinetic energy of the beam, highest small signal growth is obtained if the rotation is supported by a radial electric field alone. We shall also give the condition under which the dynamical instabilities and the resistive wall instabilities are minimized. As we shall see, these are the basic properties of space charge waves on a rotating electron beam. © 1988 American Institute of Physics 211 In this paper, we adopt a highly simplified model to mimic a wide class of radiation sources (Fig. i). The analysis is self- contained. We state from the outset that we focus mainly on the longitudinal modes, i.e., on radiation generated by the bunching of the electrons along their rotational orbits. The transverse modes, which do not involve bunching but are also efficient modes of operation (especially when wall corrugations are introduced) will be addressed only briefly toward the end of this paper. 2. EOUILIBRIUM For simplicity consider a thin cylindrical layer of electrons which rotates concentrically about the z axis at velocity v = e O Vo(r) = e r ~o(r) between two coaxial metallic pipes. Whatever axial magnetic field B and radial electric field E which are O O required to support the equilibrium rotation, v must satisfy the O radial force balance: 2 V O e YO r-- = - m-- (Eo + VoBo)" (i) O Here e < 0 is the electron charge, m is the electron rest mass, O Yo = (I - v2/c2) -I/2 is the relativistic mass factor, and c is O the speed of light. It is important 5 to include the relativistic effects once the electron kinetic energy exceeds 5 keV. To label various types of equilibrium corresponding to different devices, we introduce a dimensionless quantity h, defined by -er E h - o 3 2" (2) moYoV O 2 is the ratio of the electric force to the Physically, yo h centripedal force in equilibrium. From Eqs. (1) and (2) we may characterize various electron devices according to the value of h [Fig. i]: 212 9 MAGNETRON --LIKE r O 1/7o 2 h I / , ORBITRON INVERTED-MAGNETRON --LIKE LARGE ORBIT GYROTRON, PENIOTRON, GYROMAGNETRON Fig. I. Correspondence between various electron devices and the values of h. A detailed comparative study of these devices was given in Ref. 6. 7-12 (i) h d 0 corresponds to the large orbit gyrotron, peniotron,13-17 _ 18-21,8 22-24,7,12 gyromagne[ron, Astron, etc., where the equilibrium rotation is supported by an axial magnetic field alone, and the only electric field is due to the beam's, own charge (E ° = 0). (ii) h = I/y~ corresponds to the orbitron model, 25-29'6 in which the rotation is supported solely by a radial electric field [B ° = 0]. O (iii) h > > I/y~ corresponds to an inverted magnetron, with the cathode at the outer conductor and the anode at the inner conductor. 30 The rotation is approximately given by the E x B drift (centrifugal force is small). (iv) h << - I/y~ corresponds to a conventional magnetron, with the cathode at the inner conductor and the anode at the outer conductor 31'32 Again, the rotation is approximately given by the E x B drift and the centrifugal force is small compared with either the electric or Lorentz force in equilibrium. (v) The planar limit is recovered formally as r ~ = (fixing Eo, Vo). That is, ]h[ ~ = corresponds to the planar limit. 213 The governed equation metallic 3. DISPERSION RELATIONSHIP self-excited modes in the system described in Sec. 2 are by a rather complicated second order ordinary differential subject to the appropriate boundary conditions at the walls. 26'33 For a thin beam, the growth rates have been obtained analytically to two orders in z/R, where ~ is the beam thickness and R is the mean radius of the beam. The dispersion relation takes into account of the effects of the DC self field of the beam, and has passed various tests, including comparisons with a direct numerical integration of the governing equation. 26 Given below is a heuristic derivation of just the leading term (in ~/R) of the dispersion relation, intending to illustrate the dominant physical processes and the salient features. A more detailed discussion of various issues is given in Ref. 6. As usual, we shall first calculate the density response of the beam to some imposed electric field. A dispersion relation is obtained when this electric field is required to be excited by the density perturbation. Ignoring axial motion and axial variation, one anticipates that the rotating thin beam interacts most strongly with the azimuthal component of the perturbed electric field (Ele) which the beam experiences. This interaction would be very strong if this field co-rotates with the beam. Conservation of energy gives e VoEle = d ~/dt (3) where e is the total energy (kinetic and potential) of a beam electron. Upon using the chain rule, we express de/dr = (de/dt)/(d e/d~) = (~/R)/(de/dc) (4) in terms of the linear azimuthal displacement ~ of an electron from its unperturbed position. Thus, (3) becomes, upon linearization, ~" = e R VoEle(dmo/d~) = e Ele/Mef f, (5) where the equilibrium value e = m is expressed as a function of the o particle energy. 34'35 In analogy with the force law "F = ma", we define in (5) an effective mass Mef f = (R Vod~o/de)-l. It is not difficult to show from (I) and (2) that, with 8 ° = Vo/C, 214 I; o + "eff= mo o which can either be positive or neEative [Fig. 2]. (6) I~O0" MASS ill I MASS I I 1 -1 _~o 2 o 2 -1 / Meff "toMo "yo 2 '- "NEGATIVE" MASS i 11.,,o 2 I I I i Fig. 2. The normalized effective mass (Meff/y ° mo) as a function of h. For a perturbation proportional to exp(i~t - i~e), d/dt stands for i(m - ~o ) where ~ is the azimuthal mode number. Associated with the azimuthal displacement ~ is a surface charge density perturbation al, given by % ~_~ -i~% e Ele (7) ~i = R ae - R (~ -~ ~o)2Heff ' in which a ° is the unperturbed surface charge density and EIe is evaluated at the beam radium r = R. In writing the last expression, we have used (5). Equation (7) expresses the surface charge density perturbation on the beam in response to some imposed azimuthal 215 electric field. Note that the various equilibrium types and the dynamics enter in Mef f (Fig. 2) and that the electromagnetic properties of the structure have thus far not entered into consideration. To complete the analysis, we need to determine what kind of perturbed electric field would be excited if there is a charge perturbation a I on the beam. Since Maxwell equations are linear the field Ele at the beam radius is proportional to al; the proportionality constant is ig: EIe = ig ~i' (8) where g is proportional to the "impedance" which depends only on e,~ and on the surrounding structure. 34'35'7'26 Inserting (8) into (7), we obtain the dispersion relationship for space charge waves on a rotating electron beam: :_ r 1. This is the dispersion relationship to the lowest order in ~IR, if we represent the surface density a ° = Po ~' Po being the volume density of the beam. 4. SMALL SIGNAL GROWTH OF SPACE CHARGE WAVES The dispersion relation (9) is valid for general container geometry and for arbitrary energy Vo and h, as long as the beam is sufficiently thin. 26 For convenience of discussion of the beam dynamics, we shall assume g to be real and positive, (except if otherwise stated). We may draw the following conclusions regarding the growth of the space charge waves on a rotating electron beam: (a) When h = 0, Eq. (9) indicates that enhanced bunching of the beam occurs as a result of the "negative mass" instability. This instability growth is a result of the relativistic effect 34. Physically, the electron rotation frequency, [e l Bo/Yomo, is a decreasing function of energy (yo). A test electron in front of some charge condensation on the beam is accelerated; its mass increases but its rotational frequency decreases. Thus, the test 216 particle in effect falls back to the condensation and the condensation grows. This "negative mass" instability [cf. Eqs. (5), (6)] has been known to place a limit on the beam current in cyclic accelerators. It is shown to be identical 10 to the cyclotron maser instability when this growing space charge wave is synchronized with _ 37,38 the waveguide structure. The radiation generated in gyrotrons is a product of this interaction. (b) The "negative mass" effect persists as long as h > - 8~/2. It is maximized with respect to h when h = i/~, as is readily demonstrated from Eq. (9). This case corresponds to the orbitron configuration [Figs. 1,2]. In other words, for a given rotational energy and a given geometry, the instability is most pronounced when the equilibrium rotation is supported by a radial electric field alone ( as in the orbitron model given in Refs. 25,26), regardless of the beam energy. From Fig.2, one sees that the orbitron growth is particularly pronounced at low energies (low go ). (c) We digress to remark that the negative mass effect disappears if [ cf. Eq.(9), Fig. 2 ] - ~2o/2. (iO) h < That is, negative mass instability may be stabilized by a negative radial DC electric field of a suitable magnitude. 26 In terms of an external potential V imposed between the inner conductor at r = a and the outer conductor at r = b, the stability condition reads levl > (m c2/2)e4y3~n(b/a). (ll) o oo Note that this stabilization mechanism is independent of the beam velocity spread, and is insensitive to the beam current or container geometry, or mode number. Although it is impractical to stabilize a high energy electron beam against the negative mass instability by this method due to the 7~ dependence in (ll), it becomes attractive, however, if this method is applied to cyclic acceleration of high energy ions (~ 500 MeV) of intermediate atomic mass (atomic number of order twenty). 217 (d) If the wall is lossy, g becomes complex and the resistive instabilities would result whether the effective mass is positive or negative. 39 However, even this resistive instability can be stabilized if h = - ~/2, as is evident in the dispersion U relation (9). Physically, when h = - ~/2, the effective mass of a rotating electron is infinite 40 [cf. Fig." 2]. The beam is very rigid azimuthally and is reluctant to transfer its rotational energy to the resistive wall, which is the physical mechanism for the excitation of the resistive instability. (e) The negative mass instability should disappear in the planar geometry limit. This intuition is also reflected in the dispersion relation (9). In the planar limit, h ~ ~ by (2) and the right hand member of (9) tends to zero. What remains is then the diocotron instability (i.e., Kelvin-Helmholtz instability) which arises from the velocity shear in the equilibrium E ° x B ° drift. This shear is due to the DC self electric field of the beam and its effect enters only 26 in the higher order term (in T/R) not displayed in the dispersion relation (9). Thus, the diocotron instability is the residual instability when the curvature effect is absent. DRIFT TUBE 1 E BEAM Fig. 3. Schematic drawing of an amplifier configuration to test the response of an electron beam to an external input signal. 218 All of above predictions regarding the dynamical dependence of beam bunching on the equilibrium type could be tested in a controlled experiment such as the one proposed in Fig. 3. The response of the electron beam may be monitored at the output cavity, after an external radio frequency (rf) signal is impressed upon the beam at the input cavity. The orbit of the beam is bent either by a magnetic field B ° or by an electrostatic field, or by both. The polarity and the magnitude of the externally imposed voltage (V) and the external magnetic field B ° may be adjusted to correspond to various values of h [Figs. 1,3]. Such an experiment may be carried out with an electron beam of energy < 10 KeV, B ° < 100G, IEol < 5 KeV/cm, and beam current < 0.1A. Note that under suitable conditions, the configuration in Fig. 3 serves as a power amplifier. 36 The input rf signal modulates the beam. The space charge wave grows as the beam propagates along the circular drift tube. Accompanying a propagating density perturbation is a strong rf current which excites the output cavity, where the amplified signal is extracted. A high power amplifier 41 experiment based on this configuration is currently being planned. 5. DISCUSSION In this section we shall address several issues of current interest. As we have seen above, the cyclotron maser derives its energy from the rotational motion of the electrons. A variation of this device, the cyclotron auto-resonance maser (CARM), utilizes both the rotational energy and the considerable axial kinetic energy of the spiralling electrons. 42-45 CARM is attractive in producing high frequency (~ lO0's GHz) radiation, using only a modest magnetic field and electron beams of several hundred KeV. A simple argument would show, however, that the successful operation of CARM would require a rather good quality electron beam, and, if an ordinary wavegulde circuit is used, the avoidance of the absolute instabilities near waveguide cut-off. 46 A CARM employing a quasi- optical configuration has been proposed 47 to alleviate these difficulties. 219 The strong negative mass behavior exhibited in the orbitron configuration implies that a beam may easily be bunched when its rotation is supported only by an outward radial electric field. In spite of its impressive small signal gain9 especially at low energies, the radiation generated in an orbitron derives mainly from the potential energy of the system. The reason follows. At non- relativistic energies, v ° = ~o r = constant [cf. Eq. (1)]. As the rotating electrons lose energy to the rf, they fall to a smaller radius, maintaining the same linear velocity v . Moreover, as the O electrons release their potential energy by falling to smaller radii, their angular frequency m ° increases, leading to a gradual detune from the mode of operation. Thus, unless some form o£ phase focusing is introduced, the operation efficiency of orbitron may be limited to only a few percent. The nonlinear theory 28 showed that this is indeed the case. Efficiency enhancement in the orbitron configuration remains to be demonstrated. So far, we have focused only on the radiation resulting from the bunching of a rotating electron beam along its orbit. It should be emphasized that a rotating beam may also yield its energy to radiation without a bunching process, especially when ridges or periodic structures are introduced on the walls of the waveguide circuits. 6'38'48 In that case, under suitable conditions, the rf field extracts the energy of thebeam by causing the beam electrons to migrate in the transverse direction, into a region of stronger fringing fields set up in the ridges. Magnetrons and peniotrons are prime examples where this transverse migration plays a dominant role in the energy conversion. 13-17 In the case of peniotron, there is a general consensus that the transverse mode operation may reach a very high theoretical efficiency. In some examples, efficiencies reaching a hundred per cent have been simulated. One should also remember, however, that the presence of waveguide ridges or periodic structure may also 18,19,21 encourage harmonic generation of the longitudinal modes. When there is a large number of periodic structures on the circumference, it might not be a simple matter to predict~ or even 220 to identify, the mode of operation, especially when there are axial variations (in the tuning magnetic field, for instance). The transverse modes might compete with the longitudinal modes. The former ones might be more efficient but the latter ones might have a lower starting current. Nevertheless, efficient operation using either class of modes would lead to a substantial reduction of the magnetic field requirement. ACKNOWLEDGMENT This work was supported by the Office of Naval Research. It was an outgrowth of my earlier collaboration with David Chernin (Ref. 26). 221 REFERENCES I. High Power Microwave Sources, Eds., V. L. Granatstein and I. Alexeff, Artech House, Inc., Norwood, MA (1987). 2. Special Issue on High Power Microwave Generation, IEEE Trans. Plasma Science, Vol. 16, April, 1988 (Guest Eds., S. H. Gold and J. H. Baird). 3. C. Kapetan~kos and P. 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Infrared MM Waves 3, 619 (1982). 19. W. Namkung, Phys. Fluids 27, 329 (1984); W. Namkung, J. Y. Choe, H. S. Uhm, V. Ayres, in Ref. 2, p. 149. 20. K. R. Chu and D. Dialetis, Int. J. Infrared MM Waves 5, 37 (1984). 21. W. W. Destler, R. Kulkani, C. D. Striffler and R. L. Weiler, J. Appl. Phys. 54, 4152 (1983); W. W. Destler et al., Appl. Phys. Lett. 38, 570 (1981). 222 22. R. J. Briggs and V. K. Neil, Plasma Phys. 2, 209 (1967); H. S. Grewal and J. A. Byers, Plasma Physics ii, 727 (1971). 23. Y. Y. Lau and R. J. Briggs, Phys. Fluids 14, 967 (1971). 24. P. Sprangle, J. Appl. Phys. 47, 2935 (1976). 25. I. Alexeff and F. Dyer, Phys. Rev. Lett. 43, 351 (1980); I. Alexeff, IEEE Trans. PS-12, 280 (1984), Phys. Fluids 28, 1990 (1985); Also, Chapter 8 of Ref. I. 26. Y. Y. Lau and D. Chernin, Phys. Rev. Lett. 52, 1425 (1984); D. Chernin and Y. Y. Lau, Phys. Fluids 27, 2319 (1984). 27. J. M. Burke, W. M. Manheimer and E. 0--tt, Phys. Rev. Lett. 56, 2625 (1986); Also, Chapter 7 of Ref. I. 28. A. K. Ganguly, H. P. Freund and S. Ahn, Phys. Rev. A36, 2199 (1987). 29. R. Stenzel, Phys. Rev. Lett. 60, 704 (1988); R. W. Schumacher and R. J. Harvey. Bull. Am. Phys. Soc. 29, 1179 (1984). 30. R. A. Close, A. Palevsky and G. Bekefi, J. Appl. Phys. 54, 4147 (1983). 31. G. Bekefi and T. J. Orzechowski, Phys. Rev. Lett. 37, 379 (1976); A. Palevsky and G. Bekefi, Phys. Fluids 22, 986 (1979). 32. J. Benford, in Ref. 2, Chapter 10. 33. R. C. Davidson and K. T. Tsang, Phys. Fluids 29, 3832 (1986). 34. C. E. Nielsen, A. M. Sessler and K. R. Symon, Proc. Intl. Conf. on High Energy Accelerators and Instrumentation (Geneva, Switzerland). Geneva: CERN, p. 239 (1959); A. A. Kolomenskii and A. N. Lebedev, ibid., p. 115. 35. V. K. Neil and W. Heckrotte, J. Appl. Phys. 36, 2761 (1965); R. W. Landau and V. K. Neil, Phys. Fluids 9, 2412 (1966). 36. Y. Y. Lau, Phys. Rev. Lett. 53, 395 (1984). 37. V. L. Granatstein, in Ref. I, Chapter 5. 38. J. M. Baird, in Ref. I, Chapter 4. 39. A. M. Sessler, Private communication (1983). 40. I. Alexeff, Private communication (1984). 41. J. Pasour, Private communication (1988). 42. M. I. Petelin, Radiophys. Quantum Electron. 17, 686 (1974); V. L. Bratman, N. S. Ginsburg, G. S. Nusinovich, M. I. Petelin and P. S. Strelkov, Intl. J. Electron. 51, 541 (1981); I. E. Botvinnik et al., Sov. Tech. Phys. Lett. 8, 596 (1982). 43. A. T. Lin, Intl. J. Electron. 57, 1097 (1984); K. R. Chu and A. T. Lin, in Ref. 2, p. 90. 44. B. G. Danly, K. D. Pendergast, R. J. Temkin and J. A. Davies, Proc. SPIE, vol. 873, p. 143 (1988); K. D. Pendergast, B. G. Danly, R. J. Temkin and J. S. Wurtele, in Ref. 2, p. 122. 45. A. W. Fliflet, Intl. J. Electron. 61, 1049 (1986). 46. Y. Y. Lau, K. R. Chu, L. R. Barnett and V. L. Granatstein, Intl. J. Infrared MM Waves 2, 373 (1981). 47. P. Sprangle, C. M. Tang and P. Sera£im, Appl. Phys. Lett. 49, 1154 (1986); Nucl. Inst. Methods A250, 361 (1986). 48. G. Dohler and W. Friz, Int. J. Electron. 55, 505 (1983). 223 DISCUSSION MARSHALL: You mentioned the low efficiency of the orbitron. What about its coherence? Also, do you understand theoretically the mechanism of how coherence is obtained, or is that obscure? LAU: Alexeff, kept telling me that he saw clean spectrum in his orbitron oscillators. Much work remained to be done in the theory of mode control, however. TRIVELPIECE: To some extent, orbltrons look llke an electrostatic mass analyzer, a 127-degree mass spectrometer. Has anybody tried to inject and use that effect for spatial focusing as well as bunching along the beam direction? REISER: During the electron ring work we did in the early '70s at Maryland it came upon me that an electric field produced by an inner conductor inside of the ring could stabilize the negative mass instability. I was trying to write something out, but I did not publish it, and so I was very pleased to hear that Y. Y. Lau is now developing a theory on this effect. The other day he told me about this 127 degree spectrometer, which matches the parameters of our electron-beam experiment in Maryland. It would be very nice first to grow the instability by choosing a good combination of electric and magnetic fields; and second to suppress it, so that in one case you get bunching and in another case you don't. It is a nice academic experiment. As a comment, I think the major problem is that for high power you need stronger focusing than the weak focusing. DAVIDSON: I would like to agree with your enthusiasm for the CARM as a microwave source concept. I think one point you made was very impor- tant. It is very sensitive to axial momentum spread. There are a number of experiments being planned around the country, and I think it is quite important that these experiments have very high beam quality. Otherwise, the experiments may fall even though it is still a good concept.

1.584514.pdf

Etching of GaAs for patterning by irradiation with an electron beam and Cl2 molecules K. Akita, M. Taneya, Y. Sugimoto, H. Hidaka, and Y. Katayama Citation: Journal of Vacuum Science & Technology B 7, 1471 (1989); doi: 10.1116/1.584514 View online: http://dx.doi.org/10.1116/1.584514 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/7/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Electron beamenhanced etching of InAs in Cl2 gas and novel in situ patterning of GaAs with an InAs mask layer Appl. Phys. Lett. 63, 1789 (1993); 10.1063/1.110663 Si and GaAs dry etching utilizing showered electronbeam assisted etching through Cl2 gas Appl. Phys. Lett. 59, 2284 (1991); 10.1063/1.106044 Novel in situ pattern etching of GaAs by electronbeamstimulated oxidation and subsequent Cl2 gas etching J. Appl. Phys. 69, 2725 (1991); 10.1063/1.348626 GaAs pattern etching with little damage by a combination of Ga+focusedionbeam irradiation and subsequent Cl2 gas etching J. Appl. Phys. 68, 6415 (1990); 10.1063/1.346862 Electron beam induced modification of GaAs surfaces for maskless thermal Cl2 etching J. Vac. Sci. Technol. B 8, 1830 (1990); 10.1116/1.585168 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 140.254.87.149 On: Thu, 18 Dec 2014 22:52:38Etching of GaAs for patterning by Irradiation with an electron beam and CI2 molecules K. Akita, M. Taneya, Y. Sugimoto, H. Hidaka, and Y. Katayama Optoelectronics Technology Research Laboratory, 5-5 Tohkodai, Trukuba 300-26, Japan (Received 30 May 1989; accepted 7 July 1989) Etching of GaAs for patterning by an electron beam (EB) and C]' molecules is described. When a GaAs substrate is exposed to a 10 ke V EB and C12 molecules, etching of GaAs is observed only in the ED-scanned area. Etch rates are obtained as a function of substrate temperature. Morphologies of the etched surface are rather smooth and the photoluminescence intensity indicates that this etching process introduces much less damage to the sample than some processes using ions. I. INTRODUCTION There has recently been great interest in the in situ wafer processing of III-V compound semiconductors in an ultra high vacuum (UHV) environment. Considering the lateral patterning process for in situ wafer processing, it is obvious that an ordinary organic resist film cannot be used because of its incompatibility with the subsequent crystal growth. Maskless etching using some energetic ions or electrons seems a hopeful candidate for such patterning processes. To date, some studies on the mask less etching for lateral pat terning have been reported. For instance, Ochiai et al. re ported the focused ion beam (FIB) -assisted Cl2 etching of GaAs/ AlGaAs using Ga+ ions.1 Also, Temkin et al. indi cated that FIB sputtering was effective for patterning an ul trathin InGaAs layer which would act as a mask for dry etching of the underlying InP layer.2 Although such FIB processes have the advantage of maskless patterning, they create a severe problem, that is, ion-induced subsurface damage of the processed sample.3 The ion-induced damage is extended rather deeply into the sample even when the ion energy is reduced to 100 eV.4 Accordingly, some patterning processes using electrons are the most likely alternative be cause they appear to introduce much less damage than ion beam processes.;) There have been reports about the electron beam (EB)-induced etching of Si using SF6 gas6 and XeF2 gas.7•X In IU-V compound semiconductors, we showed EB induced etching of GaAs using Cl2 gas for the first time.9 In this paper we describe some details of the characteris tics of GaAs etched by an EB and Cl2 gas. The etch rate is measured under various conditions of substrate tempera ture. Surface morphologies and photoluminescence (PL) spectra of the etched sample indicate that this EB-induced Cl2 etching is one of the desired damage-free etching pro cesses. II, EXPERIMENTAL Figure 1 shows the etching system utilized. The etching chamber was pumped with a sputter ion pump (SIP) and a turbomolecular pump (TMP), which kept the base pressure below 1 X 10-6 Pa. A load-lock chamber was installed for exchanging samples without exposing the etching chamber to the air. The EB column consisted of two chambers which were differentially pumped for the purpose of preventing the ZrO/W electron emitter from corrosion caused by the Cl, gas. The EE was accelerated to 10 keY and focused on th~ sample surface at a spot size of about 1 ,11m. The EB was raster scanned in areas such as 200 X 260 ,11m2 with a scan speed of 10 ms/line and 500 lines/frame. To introduce the Cl2 gas into the etching chamber, a thin stainless-steel noz zle, which was located 1 mm above the sample was used and directed toward the EB-scanning area. A sample stage with a heater was used to control sample temperatures in the range from room temperature to 300°C. The samples in this experiment were n-type GaAs sub strates with a (00 1) orientation and a carrier concentration of 4 X 1017 cm -3. The usual preparation of the sample for EB-induced Cl2 etching was as follows: First, the substrate was rinsed in an organic solvent and etched in a solution of H2S04:H202:H20 = 3:1:1 at 80°C for 60-90 s to remove the layer damaged by polishing. The substrate was then treated by trichloroethane, acetone, methanol, and de-ionized wa ter, and dried with N2 gas. This treatment is believed to form an adsorbate and/or a chemically reacted thin layer at the substrate surface. Samples prepared in this way were loaded through the load-lock chamber into th~ etching chamber, where EB-induced Cl2 etching was carried out. The average electron flux was varied from 1 X lOLl to 1 X 1015 elcc- EB COLUMN c:> SIP -C BUFFER -7 , CHAMBER "'-----7 TO LOAD LOCK CHAMBER TMP/S!P c:> TMP/SIP CI2 PRESSURE CONTROLLER j FIG. 1. Schematic illustration of the etching system. The EB column is differentially pumped by TMP and SIP to protect the ZrO/W elect roll emitter from corrosion. 1471 J. Vac. Sci. Technol. B 7 (6), Nov/Dec 1989 0734-211X/89!061471-04$01.00 Cc) 1989 American Vacuum Society 1471 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 140.254.87.149 On: Thu, 18 Dec 2014 22:52:381472 Akita et sl.: Etching of GaAs for patterning by irradiation trons/cm2 s by changing the beam current and the EB scanned area. Chlorine flux was varied from 2 X 1015 to 4 X 1017 molecules/cm2 s using a el2 pressure controller with a mechanical leak valve. Substrate temperature was varied between 25 and 150°C. Photoluminescence spectra of the etched area were ob tained to evaluate the etching-induced damage. In these measurements, the 514.5 nm line of an Ar+ ion laser whose intensity was 50 m W was focused onto the sample surface at a diameter of -200 ,urn. A Jovin-Yvon HR-320 monochro mator with a 600 groove/mm grating and a cooled S 1-type photomultiplier were used for the measurement. III. RESULTS AND DISCUSSION The photograph of a typical sample directly patterned by the EB-induced Cl2 etching is shown in Fig, 2, which con firms that only the area exposed to both EB and Clz mole cules was etched and resulted in pattern etching. The EB induced elz etching of GaAs is affected by substrate temperature, the Clz flux, and the EB flux as described be low. A. Etch rate dependence on substrate temperature The time dependence of etch depth measured by a Tencor Alpha Step is shown in Fig. 3 (a). The substrate temperature was varied between 50 and ! 50 "C. Cl2 flux and EB flux were fixed at 7XlOl6 molecules!cm2s and, ~2XlO14 elec trons/ cm" s (~3 nA for a scanning area of 120 X 90 I1m,2) respectively. Etch depth increases linearly with etching time, although there is a lag time before the etching begins when the substrate temperature is lower. This lag time reflects the existence of some adsorbate and/or a chemically reacted thin layer on the GaAs surface. The etch rate of GaAs increases as the substrate tempera ture increases. An Arrhenius plot of the etch rates is shown in Fig. :3 (b). The open circles indicate the data obtained in this study and the closed ones indicate the etch rate in a Cl2 i ETCH~D 300 ~m AREA FIG. 2. Microphotograph of the sample etched at 70·C for 60 min, which was taken using a differential interference microscope. J. Vac. Sci. Technol. S, Vol. 7, No.6, Nov/Dec 1989 (a) '2 E ..... E S W I-« Ct: ::z:: U I- W (b) 1.5 ,...-----------, o 102 10 .e12 • 7)(10" molecules/em2. sec .EB • _2)(1014 electrons/em!. sec 150 'C 100 'C o 10 20 30 40 50 60 TIME (min) TEMPERATURE ('C) 100 50 20 o EB induced CI, etchin9 • el. gas etching (ofler Furuhola et al) 10~~--~---------L------~ 2.5 3.0 3.5 1472 FIG. 3. (al Time dependencc of etch depth at the various substrate tempera tures. The CI2 flux .pC12 and the averaged EB flux .pEB were fixed at the values listed in the iigure. (bl Arrhcnius plot of the etching rate. (O) Re sults of this study; (.) etching rates reported in the Cl2 gas phase etching of a (001) GaAs by Furuhata et at. (see Ref. 10). gas phase etching of a GaAs epitaxial layer with a clean surface, which was reported by Furuhata et al. 10 These data indicate the same activation energy of about 7 kcal/mol. The coincidence of etch rate between this EB-assisted Clz etching and the Clz thermally activated etching suggests that the main mechanism of the EB-induced el2 etching is the same as that of the C12 thermally activated etching as de scribed in Sec. III B. An important difference of the result is that etch patterns can be formed on GaAs by this EB-in duced Cl2 etching. B. Lag time ofEB~induced CI2 etching and adsorbate As described in previous Sec. in A, there is a lag time before the etching begins. To make this lag time clear, an etching experiment was performed. In this experiment, the GaAs substrate was chemically etched and treated by trich loroethane, acetone, methanol, and de-ionized water, and Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 140.254.87.149 On: Thu, 18 Dec 2014 22:52:381413 Akita et sl.: Etching of GaAs for patterning by Irradiation E .a- Z i-n.. ~ Z U i- !.oJ 1.5 ,----------- ............ 1.0 0.5 o ~EB (eleclrons/cm2. sec) 2)(1014 3)(10'3 o 10 20 30 40 50 60 TIME (min) FIG. 4. Time dependence of etch depth for samples held in air for 60 min prior to loading into the vacuum chamber. Lag time becomes shorter as the EB flux increases. exposed to air for 60 min prior to loading into the vacuum chamber. The EB-induced Cl2 etching was carried out with a C12 flux of 3,6X 1017 molecules/cm2 s and a constant elec tron current of ~ 3.5 nA. Substrate temperature was fixed at 100 "C. The EB scanning area was 40 X 55, 90 X 120, or 200 X 260 p..m2, which corresponded to an EB flux of 1 X 1015, 2x 1014, and 4X 1013 electrons/cm2 s, respectively. The time dependence of the etch depth obtained in this experiment is shown in Fig. 4. The time at which the EB induced el2 etching begins depends on the EB-scanned area, i.e., the smaller the area is, the shorter the delay time be comes, while the etch rates are almost the same. In the smaller scanned area, the surface was irradiated by a dense EB flux. This dependence of lag time on EB flux can be understood by assuming the existence of an adsorbate and/or a chemically reacted thin layer on the surface, which is removed by the combined irradiation with an EB and C12 molecules. The lag time is thought to be the time required for the removal of this layer from the GaAs surface, which pre sumably depends on the EB flux. An adsorbate and/or a chemically reacted thin layer ap pears to be the origin of patterning in this EB-induced Cl2 etching. The reasons are that CO etching occurs only in the area irradiated by both the EB and C12 and (ii) etching char acteristics are similar to elz thermally activated etching as described in Sec. IlIA. In Auger electron spectroscopy mea surements, oxygen and carbon were detected on the sample surface before the EB induced C12 etching, suggesting that the adsorbate and/or the reacted layer likely consists of oxy gen and/or carbon-containing substances. Co Morphologies of the etched surface One of the important features ofEB-induced etching is the quality of the morphologies that can be obtained. Figure 5 shows microphotographs taken with a Nomarski micro scope of the samples etched at 100 ·C. In all the samples shown in this figure, the etch depth was constant at ~ 50 nm. When the el2 flux is small, the etched surface shows a slight ly rough morphology. However, the larger the e12 flux, the J. Vac. Sci. Techno!. EI, Vol. 7, No.6, Nov/Dec 1989 (bl tC!! .. 2 x,d' moleculesl em:!! • S T s '" 100 "C 1473 (c) flCI! .. 7 x,cf molecules/em:;!' S FIG. 5. Microphotographs of the sampl~s etched at a substrate temperature of 100 T. The etch depths of these samples arc about 50 nm. smoother the surface becomes. The morphology of the sam ple shown in Fig. S(c) (C12 flux = 7X 1016 molecules/ cm2 s) is excellent. Such good morphologies would be desir able for subsequent crystal growth. D. Photoluminescence measurements To evaluate the damage induced by the etching process, PL spectra were obtained at room temperature in the initial sample and in the sample etched by EB-induced Cl2 etching. The PL spectra of the near-band-edge emission were mea sured. These spectra were much the same at the peak wavc length, the fun width of half-maximum intensity, and the total PL intensity, which indicates that the BB-induced el2 etching introduces less damage in the sample. In the case of Ga + FIB-assisted elz etching, the PL inten sity of the etched region decreased. For example, when the energy ofGai FIB was 10 keY, the PL intensity decreased to l/30th-l/ 40th of that of the unetched region? This dif ference of PL intensity in samples processed by EB-induced elz etching to that in FIB-assisted el2 etching is thought to be due to the difference of momentum transfer to the lattice. From this point of view, EB-induced el2 etching seems promising as a damage-free etching technique. IV. SUMMARY Etching of GaAs for patterning by an EB and a Cll gas was described. When a GaAs substrate was exposed to a 10 ke V EB and Clz molecules, etching of GaAs is observed only in the EB-scanned area. The dependence of the etch rate on the sample temperature suggests that the main mechanism ofEB-induced elz etching is the same as that of Cl2 gas phase etching. Patterning in this method is considered to occur by the removal of an adsorbate and/or a chemically reacted thin layer by the combined EB and el2 irradiations. The merit ofEB-induced ell etching lies in the lower damage to the etched sample, which renders it more suitable for subse quent epitaxial growth. ACKNOWLEDGMENT The authors would like to thank Dr. 1. Hayashi for discus sions. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 140.254.87.149 On: Thu, 18 Dec 2014 22:52:381474 Akita et al.: Etching of GaAs for patterning by irradiation Iy. Ochiai, K. Garno, andS. Namba, J. Vac. Sci. TeclmoL B3, 657 (1985). 'H. Temkin, L. R. Harriott, and M. B. Panish, AppL Phys. Lett. 52, 1478 (1988). 'M. Taneya, Y. Sugimoto, and K. Akita, J. App!. Phys. 66,1375 (1989). 4H. Miyake, Y. Yuba, K. Gamo, S. Namba, R. Mimura, and R. Aihara, Jpn. J. App!. Phys. 27, 1.2037 (1988). 'R. R. Kunz and T. M. Mayer, J. Vae. Sci. Techno!. B 5, 427 (1987). 6D. J. Oostra, A. Haring, and A. E. DeVries, Nue!. Instrum. Methods B 16, Jo Vac. Sci. Techno!. S, Vol. 7, No.6, Nov/Dec 1939 364 (1986). 7J. W. Coburn and H. F. Winters, J. App\. Phys. 50, 3189 (1979). "S. Matsui and K. Mari, App!. Phys. Lett. 51,1498 (1987). 1474 oM. Tancya. Y. Sugimoto, H. Hidaka, and K. Akita, lpn. J. App!. Phys. 28, L429 (1989). ION. Furuhata, H. Miyamoto, A. Okamoto, and K. Ohata, J. App!. Phys. 65,168 (1989). Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 140.254.87.149 On: Thu, 18 Dec 2014 22:52:38

1.342603.pdf

The hole photoionization cross section of EL2 in GaAs1−x P x P. Silverberg, P. Omling, and L. Samuelson Citation: Journal of Applied Physics 65, 3721 (1989); doi: 10.1063/1.342603 View online: http://dx.doi.org/10.1063/1.342603 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Simple measurement of 300 K electron capture cross section for EL2 in GaAs J. Appl. Phys. 80, 3590 (1996); 10.1063/1.363233 Metastable transformation of EL2 in semiinsulating GaAs: The role of the actuator level and the photoionization of EL2 Appl. Phys. Lett. 68, 2959 (1996); 10.1063/1.116368 Minority carrier capture cross section of the EL2 defect in GaAs Appl. Phys. Lett. 61, 2452 (1992); 10.1063/1.108149 Hole photoionization cross sections of EL2 in GaAs Appl. Phys. Lett. 52, 1689 (1988); 10.1063/1.99020 Identification of EL2 in GaAs Appl. Phys. Lett. 47, 970 (1985); 10.1063/1.95947 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.180.142.23 On: Sat, 13 Dec 2014 09:18:16I A. Chiang, M. W. Gels, and L. Pfeiffer, Eds., Mater. Res. Soc. Symp. Free. 53 (1986). 'K. Furukawa, Ed., Silicon on Insulator, Its Technology and Applica tions(KTK, Tokyo, 1985). 3J. R. Davis, R. A. McMahon, and H. Ahmed, J. Phys. C 3,337 (1983). 4A. Ishitani, H. Kitajima, K. Tanno, and H. Tsuya, Microelectron. Eng. 4, 333 (1986). 'L. Jastrzebski. J. Cryst. Growth 70.253 (1984). 6L. Karapiperis, G. Garry, and D. Dieumegard, Mater. Res. Soc. Symp. Proc. (to be published). 71. R. Davis, R. A. McMahon, and H. Ahmed. J. Electrochem. Soc. 132. 1919 (1985). "D. A. Williams, R. A. McMahon, H. Ahmed, and W. M. Stobbs, Inst. Phys. Conf. Ser. 87, 415 (1987). 91. Knapp. 1. Appl. Phys. 58, 2584 (1985). The hole photolonization cross section of El2 in GaAs1_X P x P. Silverberg, P. Omling, and L. Samuelson Department of Solid State Physics, University of Lund, Box 118, S-221 00 Lund, Sweden (Received 31 October 1988; accepted for publication 3 January 1989) The hole photoionization cross section d:, of EL2 is determined at T = 80 K for different x in GaAs 1 _ x p x' From these data, the energy position ofthe EL2 level relative to the valence band is determined for different alloy compositions. The results are compared with the previously determined energy positions of the EL2 level relative to the conduction band and with the corresponding change in the direct band gap with aHoy composition. In the attempt to identify the notorious EL2 defect in GaAs, various techniques and methods have been used. I One such method is the investigation of the EL2 level as a function of aHoy composition. The information obtained from these measurements includes electronic localization, influence of band structure on the defect energy position, and, sometimes, identification of the defect by tracing it from one binary compound to the other where the defect might already have been identified. In the case of EL2, only a few attempts have been made to trace the energy level through an aUoy system.2-8 The most detailed investigations, so far, are in the GaAsl _ x p x aHoy system where the properties ofEL2 have been investi gated using different space-charge techniques.6•7 Using deep-level transient spectroscopy CDLTS) and photocapa chance measurements, the EL2 level (Ec -EEL2) was traced from GaAs (direct band gap) to the indirect band gap side (x>0.46). From an extrapolation of the deduced energy values for different values of x to x = 1.0 (GaP), the energy position for the corresponding EL2 candidate in GaP was suggested. Unfortunately, the spread in the data result ed in quite a wide "possible" energy range for the EL2 level in GaP, if at all present there. It would, therefore, be of great importance to reduce the spread in the data, making a more accurate prediction of the energy position in GaP possible. Recently, a method of measuring the hole photoioniza tion cross section 0-;; of GaAs:EL2 at low temperatures was presented. I;> This method is, however, also suitable for mea surements of the d:, cross sections of EL2 in alloy systems, such as the GaAsl _ x P x :EL2 system. If these data could be obtained, the complementary binding energy of the EL21ev el (i.e., relative to the valence band (EEL2 -E v) J for differ ent values of x could be deduced and the accuracy in the determined energy dependence of the EL2 level with alloy composition could be increased. The purpose of this communication is to report on ex perimental ~ data for different aHoy compositions, deduce the EL2 binding energy relative to the valence band as a function of aHoy composition x, and to compare these new data with those previously obtained from measurements on the a;: cross sections. The experimental photocapacitance measurements were performed using Schottky diodes fabricated on n-type GaAs1 _ -' P x' grown by metalorganic vapor phase epitaxy (MOVPE), with free-carrier concentrations in the range 5 X lOl5_2 X 1016 em -~3 and grown-in EL2 concentrations in the range 5X 1Ou-2x 1014 cm-3• A description of the ex perimental equipment and a review of the photocapacitance technique in general can be found in Refs. 6 and 10, respec tively. The measurements of the hole photoionization cross section (T~ were performed using the initial slope technique and the optical filling procedure presented in Ref. 9. The initial occupation of the EL2 levels (EL2 levels empty) was set by illumination with 1.38-eV photons. Since (7~ ~!7~, al most ail EL2 levels are empty, and the initial slope technique can be used directly at low temperatures. The experimental data from such a~ measurements at T = 80 K are presented in Fig. 1 for x = 0.0, 0,04, 0.08, 0.11, and 0.20. The EL2 origin of the signal was verified by the quenching properties. After iHumination with intense light centered around 1.1 e V, the signal was quenched. Also shown in Fig. 1 are data ob tained at T = 150 K for the electron photoionization cross section (T~ from Ref. 7. Since the energy position of the EL2 level in GaAs has previously been determined, and since, furthermore, the shape of the d:, spectrum does not change significantly with 3721 J. Appl. Phys. 65 (9),1 May 1989 0021-8979/89/093721-03$02.40 ® 1989 American institute of Physics 3721 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.180.142.23 On: Sat, 13 Dec 2014 09:18:1610-16 10-15 p~& -.. 'f )( ti:t*.., * '" 10-17 ~ ,;:< c" " ... ~ 10-16 E ~ ):( ~ ,a;sf!IP "if x ~ 111111'" '" ~ " II '" 'J ,," e.,8 *** c: 0 )( 0 &~ * 13 :'X f!:$~ ~ " .",. is >I< (I) 1 O-~8 ". · TOO 10-17 tJ) " " .,.0 to * c 0.04 (f) x .... 008 Ul , 1. : 0" .,:" 0 ... ".. * p" 020 0 (0 " o '" lef! axis () : *"'" 10-18 a 10-19 0.. '" 0 .. ~ * I {a.oo ~: t crO is 0.04 n ... 0.08 .. A '" 0.27 10.20 I '" ,Ight axis " , , . '10-19 0.7 0.9 1.1 1.3 Photon energy (eV) FIG. 1. Optical cross sections a:, (this work) and d,; (from Ref. 7) for EL2 in GaAs"I\. The d; data were obtained at 80 K, and the d,; data were obtained at 150 K. alloying, it is possible to determine the EL2 energy-level po sition for different values of x by measuring the shift of the measured d}, (x) spectra relative to the (J"~ (x = 0) spectrum. The shift with alloying is interpreted as the shift ofthe ener gy position orEL2 from the position in GaAs, and the energy position of EL2 relative to the valence-band edge, can thus be determined. In the same way, shifts in the d;, cross sec tions were interpreted as energy shifts of the EL2 level rela tive to the conduction-band edge. To a first approximation the energy position determined in this way is independent of the temperature used for measurements (i.e., the a~ cross sections can be obtained at a different temperature than the if,; data). The energy position of EL2, determined from the a~ measurements (this communication) and from (J";: measure ments (from Ref. 7), are plotted as a function of the aHoy composition in Fig. 2. The composition dependence of the energy difference between the r conduction band and the valence band is given by Eg = 1.508 + 1.366x + O.174x(x ~ 1) eV,l1 which for x lower than 0.46 corre- sponds to the band gap. The good agreement between the two measurements reduces the previously obtained uncer tainty in the slope of the ELl level with alloy composition. The slope is determined to be 6 meV /% (solid line in Fig. 2), a value which is considered accurate, at least on the direct band-gap side. I t should be noted that the spread in these optical data is much smaller than the spread in the data determined from thermal methods, even though the effect of alloy broadening was accounted for in the latter case. A complication with the thermal data is the capture processes. The thermal emission rate e;, is related to the capture cross section (J";, by the de tailed balance relationship 12 e;, = O':,utl1Nc X exp( -AGJkT), where Un, is the thermal velocity, Nc is the effective density of states, and AGn is the change in Gibb's free energy when an electron is emitted into the con- 3722 J. Appi. Phys., Vol. 65, No.9, 1 May 1989 > ~ -0 15 c: CIl .D <D <..:> c (!) <ii > 1.0 '0 0. .9 E ,g 0.5 >. e> <D c: UJ Valence Band 0.0 0.0 0.1 0.2 0.3 X in GaAs1•x Px FIG. 2. EL2 energy position plotted as a function of x in GaAs, -xP X' The circles (0) denote values obtained from ~ data and show the energy posi tion from the valence band. The crosses ( + ) denote data obtained from a;; data and show the energy position relative to the conduction band. The hand structure is plotted at 77 K. duction band from EL2. The thermal energy has not been compensated for changes in the capture cross section due to alloying. This might explain the divergence between the thermal and optical data especially on the indirect band-gap side (x> 0.46).7 In case an EL21evel does exist in GaP, the present data [with EEU-EV=O.78 eV for x=O and a(EEI.2 -Ev )/ax = 6 meV 1% PJ give the energy position of such a level at EEL2 -E v = 1.38 eV (or Ec -EEI.2 = 0.97 + 0.05 e V) for x = 1.0. Further work on the indirect band-gap side is, however, needed in order to see if this pre diction will be helpful when one tries to identify the EL2 counterpart in GaP. This work was supported by the Swedish Natural Science Research Council and the Swedish Board for Tech nical Development. We would also like to thank Khalil Ah mad for assistance with the photocapacitance measure ments. 's. Makram·Ebied.l'. I"anglade, and G. M. Martin, Semi-Insulating III-V Materials, edited by D. C. Look and J. S. Blakemore (Shiva, Nantwich, 1984), p. 184. 2A. Mircea, A. Mitonneau. 1. Hallais, and M. Jaros, Phys. Rev. B Hi, 3665 (1977). 'E. E. Wagner, D. A. Mars, and G. Holm, J. Appl.l'hys. 51, 5434 (1980). '1'. Matsumoto, P. K. Bhattacharya, and M. J. Ludowi~c, App!. Phys. Lett. 41, 1 (1982). 'P. K. Bhattacharya, J. W. Ku, S. J. T. Owen, G. H. Olsen, and S-H. Chiao. IEEE J. Quantum Electron QI<:-17, 150 (1981). "P. Om ling. L. Samuelson, and H. G. Grimmeiss, Phys. Rev. B 29, 4534 (1984). 7L. Samuelson and P. Dmling, Phys. Rev. B 34. 5603 (1986). Silverberg, Omling, and Samuelson 3722 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.180.142.23 On: Sat, 13 Dec 2014 09:18:16"A. B. Cherifa, R. Azoulay, A. Nouilhat, and G. Guillot, GaAsand Related Compo!lnd~, edited by A. Christou and H. S. Rupprecht, lust. l'hys. Conf. Ser. No. 90 (Institute of Physics, London, 1987), p. 235. 0p. Silverberg. P. Omiing, and L. Samuelson, AppL Phys. Lett. 52, 1689 (1988). lOH. G. Gl'immciss and C. Ovren, J. Phys. E 14, 1032 (1981). "D. E. Aspnes, Phys. Rev. B 14, 53]l (1976). 120. Engstrom alld A. Aim, Solid-State Electron. 21, 1571 (1978). Doping of InP and GalnAs with S during metalorganic vapor .. phase epitaxy R. A. Logan, T. Tanbun-Ek, and A. M. Sergent AT&T Bell Laboratori<?s, Murray Hill, New Jersey 07974 (Received 26 September 1988; accepted for publication 5 January 1989) The doping characteristics of S in the metalorganic vapor-phase epitaxial growth of InP and GalnAs are studied using three different but consistent methods of determining the doping level in the crystal, Hall effect, electrochemical C-V profiling and by C-Vbias over distances of ~O<5 p.m. It is shown that under the same growth conditions the doping level in InP is 3.5 times lal"ger than in GaInAs and increases lOO-fold when the growth temperature is decreased from 625 to 525 "c. S is shown to be a useful donor, essentially completely ionized at room temperature, easily incorporated at levels .s 1018 em--3 into metalorganic vapor-phase epitaxy growth at 625 ·C where crystal morphology is optimized and at high levels _1020 cm-3 to form low-impedance n-contact layers with growth at 525 "C. The donor sulfur may be conveniently introduced as a dopant in vapor-phase epitaxy since it is avaiJable as a stable mixture ofH2S in Hz, with Hz being the carrier gas generally used in metalorganic vapor-phase epitaxy (MOVPE). The molecule H2S readily dissociates at the growth temperature (500-650 ·C). S is a donor residing on the group-V subIat tice so that it competes with the group-V constituent in crys tal growth. The growth ofInP from phosphine (PHJ) and trimethylindium (TMln) is nearly independent of the PH3 flow rate (above a minimum level) but depends linearly on the TMIn flow rate. 1 The competition between the two vola tile constituents P and S for the group-V sub lattice sites in the growth process has not been studied in the GalnAsP MOVPE system. In addition, the utility of this growth pro cess requires control of the doping levels to provide low resistance contacts to high-current density devices such as lasers. This requirement often dictates the growth of an addi tional contacting layer where the dopant solubility is high and the contact resistance can be optimized. In GaAsl AIGaAs lasers, aGe-doped GaAs contacting layer is rou tinely used to provide a low-resistance contact to the AIGaAs cladding layer since the Ge ionization energy in creases rapidly with Al concentration, reducing the carrier concentration and hence the layer conductivity. 2 The objective of this study is therefore to characterize the dopant properties of S in MOVPE growth of lnP and InGaAs as a function of growth temperature and H2S level in the growth ambient. MOVPE was performed at atmospheric pressure on InP substrates oriented in a ( 100) direction. The constituents are AsH3, PH3, trimethylindium (TMln), and trimethylgal Hum (TMGa), with the latter two held at 30,0 and -15.0 ·C, respectively. The dopant source was 200 ppm H2S in H2• The H2 carrier gas at 5000 seem dominates the gas flow. lnP was grown with the addition of 20 seem PH3 and a H2 flow of 65 seem through the TMln source bubbler. The growth rates ranged from 10 A'/s at 625°C to 6 A/s at 525 "C. GaInAs, lattice matched to InP with f:.a/a-1O~-4, was grown at 625 ·C with additions to the carrier gas of 5.75 sccm AsH, and H2 flows of 5 and 65 seem through the TMGa and TMln sources, respecti.vely. The flow of 200 ppm HzS in H2 ranged from 0 to 20 secm and its variation caused a negligible effect on the concentra tion ofthe other constituents of the gas stream, especially the ratio of group-V to group-III elements, which could affect the impurity solubility. The doping level was determined by van der Pauw HaH effect measurements, by electrochemical C-V profiling, and by C-V profiling in lightly doped samples where the space charge could be swept appreciable distances (-0.5 ,urn). The net donor concentration in undoped layers was ~ 1015 cm---3• The carrier concentration as a function of dopant mole fraction in the H2 carrier gas stream is shown in Fig. 1 for growth of InP and GalnAs at 625°C. The reduced doping level of GalnAs compared to InP at a given level of HzS in the growth ambient is much larger than the difference in layer growth rates and is in marked contrast to the usual behavior of increased doping in the ternary cladding layer such as exhibiting by the acceptor Zn.3 The agreement between the Hall effect which measures carrier concentra tion and the capacitance measurements of net donor concen tration, IND -l'(~ jcm-3, implies that the dopant S has a shallow energy level which is essentially fully ionized at room temperature. This agreement in measurements also obviates the confirmation of these results by SIMS analysis. A p-n junction was constructed, as shown in the inset of 3723 J. Appl. Phys. 65 (9),1 May 1989 0021-8979/89/093723-03$02.40 @ i S8S American Institute of Physics 3723 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.180.142.23 On: Sat, 13 Dec 2014 09:18:16

1.2811122.pdf

New Books Citation: Physics Today 42, 8, 70 (1989); doi: 10.1063/1.2811122 View online: http://dx.doi.org/10.1063/1.2811122 View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/8 Published by the American Institute of PhysicsKruer's small book. The Physics of Laser Plasma Inter- actions grew out of lecture notes from a course Kruer taught in the applied science department of the University of California, Davis. As the foreword states, such books tend to have a rough, informal style, and this is true of Kruer's book. While Kruer states that the book does not assume an extensive knowledge of plasma phys- ics and that the treatment is based onsimple physical models, I suspect that a student coming to the subject cold would have trouble learning from this book. Many plasma theorists who are used to more formal, detailed treat- ments of plasma processes may find the style too rough for their tastes. However, Kruer is presenting a broad-brush picture of the physics of laser-plasma interactions; he pre- sents us with the forest and not detailed, leaf-by-leaf descriptions of 5.4 Gigabytes of Unattended Backup ////////////W/ttf/WIIIIIII HIGHEST CAPACITY HIGHEST DATA RATE HIGHEST INTEGRITY HIGHEST SEARCH SPEED GIGASTORE™, by Digi-Data, is a complete line of high capac- ity tape drives. Employing videocassette tape, GIGASTORE fea- tures three models offering capacities of 2.5, 3.7 or 5.4 Gigabytes of formatted data, on a single tape. Systems are available for DEC VAX™ and MicroVAX™, the IBM PC/XT/AT™ and compatibles as well as 386 machines. A Novell LAN compatible system is also available. And Digi-Data's product line includes 1600 and 6250 bpi 9-track tape drives and systems. Digi-Data is an organization with a 27 year history of manufac- turing quality tape drives. '•'GIGASTORE is a trademark of Digi-Dat a Corporation. VAX and MicroVAX are trademarks of Digital Equip- ment Corporation. PC/XT/ATare trademark s of IBM Corporation. DIGI-DATA CORPORATION 8580 Dorsey Run Road Jessup, MD 20794-9990 ^ (301)498-0200 FAX (301) 498-0771 ... First In Value In Europe contact: Digi-Dat a Ltd • Unit 4- Kings Grove* Maidenhead, Berkshire England SL6 4DP • Telephone No. 0628 29555/6 -Telex 847720 Circle number 40 on Reader Service Cardthe trees. His broad physical pictures are supported by a large amount of work on numerical models and by experimental results. Kruer dis- cusses these, but I suspect that the reader will not get a true feel for the imposing bulk of the supporting work. Some hint of its sheer magnitude comes through in the large number of cited references. If the book is used in a course, the instructor can undoubt- edly fill in details and answer ques- tions students have on the support- ing material. This is an excellent book, filled with insights into many of the complex phenomena involved in the interac- tions of intense electromagnetic waves with plasma. I recommend it to any student of the subject or, for that matter, to anyone interested in nonlinear processes in plasma. JOHN M. DAWSON University of California, Los Angeles NEW BOOKS Acoustics Acoustical Imaging, Vol. 16. L. W. Kessler, ed. Plenum, New York, 1988. 658 pp. $115.00 he ISBN 0-306-43011-8. Com- pilation Acoustical Measurements. Revised edi- tion. L. L. Beranek. AIP, New York, 1988. 841 pp. $30.00 he ISBN 0-88318-590-3. Monograph Engineering Noise Control: Theory and Practice. D. A. Bies, C. H. Hansen. Unwin Hyman, London, 1988. 414 pp. £40.00 he ISBN 0-04-620021-5; £17.95 pb ISBN 0-04-620022-3. Text Astrophysics The Atmosphere of the Sun. C. J. Dur- rant. Adam Hilger, Bristol, UK (AIP, New York), 1988. 168 pp. £23.50 ($64.00) Ac ISBN 0-85274-375-0. Monograph Classical Novae. eds. Wiley, New $163.00 he ISBN graph compilation Dark Matter. Rencontres de Moriond 23; M59. Proc. Mtg., Les Arcs, France, March 1988. J. Audouze, J. Tran Thanh Van, eds. Editions Frontieres, Gif-sur-Yvette, France, 1988. 498 pp. 410 FF ($63.00) Ac ISBN 2-86332-057-2 The Fundamentals of Stellar Astro- physics. G. W. Collins. Freeman, New York, 1989. 494 pp. $47.95 he ISBN 0- 7167-1993-2. Monograph Gravitational Lenses. Lecture Notes in Physics 330. Proc. Conf., Cambridge, Mass., June 1988. J. M. Moran, J-N- Hewitt, K. Y. Lo, eds. Springer-Verlag, New York, 1989. 238 pp. $31.40/icISBNO- 387-51061-3. Festschrift for Bernard BurkeM. F. Bode, A. Evans, York, 1989 . 341 pp. 0-471-92058-4. Mono- 70 PHYSICS TODAY AUGUST 1989Highlights in Gravitation and Cosmol- ogy. Proc. Conf., Goa, India, December 1987. B. R. Iyer, A. Kembhavi, J. V. Narli- kar, C. V. Vishveshwara, eds. Cambridge U. P., New York, 1988. 441 pp. $59.50 he ISBN 0-521-36125-7 Hot Spots in Extragalactic Radio Sources. Lecture Notes in Physics 327. Proc. Wksp., Tegernsee, FRG, February 1988. K. Meisenheimer, H.-J. Roser, eds. Springer-Verlag, New York, 1989. 301 pp. $37.10 he ISBN 0-387-50993-3 Knowledge-Based Systems in Astron- omy. Lecture Notes in Physics 329. A. Heck, F. Murtagh, eds. Springer-Verlag, New York, 1989. 280 pp. $31.40 he ISBN 0-387-51044-3 Large-Scale Motions in the Universe: A Vatican Study Week. Princeton Series in Physics. Proc. Mtg., Vatican City, Novem- ber 1987. V. C. Rubin, G. V. Coyne, eds. Princeton U. P., Princeton, N. J., 1988. 604 pp. $95.00 he ISBN 0-691-08524-2; $39.50 pb ISBN 0-691-08525-0 Nuclear Astrophysics. Research Re- ports in Physics. Proc. Sch., La Rabida, Spain, June 1988. M. Lozano, M. I. Gal- lardo, J. M. Arias, eds. Springer-Verlag, New York, 1989. 355 pp. $55.90 pb ISBN 0-387-50751-5 Particle Physics and Astrophysics: Current Viewpoints. Proc. Conf., Schladming, Austria, February 1988. H. Mitter, F. Widder, eds. Springer-Verlag, New York, 1989. 309 pp. $50.80 he ISBN 0-387-50699-3 Supernova 1987A, One Year Later; Re- sults and Perspectives in Particle Physics. Les Rencontres de Physique de la Vallee d'Aoste. Proc. Mtgs., La Thuile, Italy, February 1988. M. Greco, ed. Edi- tions Frontieres, Gif-sur-Yvette, France, 1988. 825 pp. 520 FF ($80.00) he ISBN 2- 86332-058-0 White Dwarfs. Lecture Notes in Physics 328. Proc . IAU Colloq. 114, Hanover, N. H., August 1988. G. Wegner, ed. Springer-Verlag , New York, 1989. 524 pp. $58.30 he ISBN 0-387-51031-1 Biophysics Electric Field Phenomena in Biologi- cal Systems. IOP Short Meetings 21. Proc. Mtg., London, March 1989. R. Paris, ed. IOP, Bristol, UK (AIP, New York), 1989. 87 pp. £17.50 ($32.00) pb ISBN 0- 85498-521-2 Exposure of the US Population from Diagnostic Medical Radiation. NCRP Report 100. Natl. Council on Radiation Protection and Measurements, Bethesda, Md. (20814), 1989. 103 pp. $14.00 pb ISBN 0-92600-1-0 Laser Picosecond Spectroscopy and Photochemistry of Biomolecules. Adam Hilger Series on Optics and Optoe- lectronics. V. S. Letokhov, ed. Adam Hilger, Bristol, UK (AIP, New York), 1987. 309 pp. £48.00 ($130.00) he ISBN 0-85274- 469-2. Monograph compilationMacro-Evolutionary Dynamics: Spe- cies, Niches and Adaptive Peaks. N. Eldredge. McGraw-Hill, New York, 1989. 226 pp. $28.95 he ISBN 0-07-019474-2; $14.95 pb ISBN 0-07-019476-9. Monograph Non-Ionising Radiation: Microwaves, Ultraviolet and Laser Radiation. Medi- cal Physics Handbooks 18. H. Moseley. Adam Hilger, Bristol, UK (AIP, New York), 1988. 293 pp. £37.50 ($102.00) he ISBN 0-85274-166-9 The Physics of Medical Imaging. Medi-cal Science Series. S. Webb, ed. Adam Hilger, Bristol, UK (AIP, New York), 1988. 633 pp. £65.00 ($176.00) he ISBN 0-85274- 361-0; £19.50 ($53.00) ISBN 0-85274-349-1. Compilation A Primer on Theory and Operation of Linear Accelerators in Radiation Ther- apy. C. J. Karzmark, R. J. Morton. Medi- cal Phys. Publ., Madison, Wis. (53706), 1989. 41pp. $12.00p6 ISBN 0-944838-07-3 Tissue Substitutes in Radiation Dosi- metry and Measurement. ICRUReport No Noise Is Good Noise The SR560 Low-Noise Preamplifier is the ideal voltage amplifier for the most demanding applications. With a low 2 nV/VHz of input noise, even the smallest signals won't get lost. Two adjustable signal filters, each configurable as high or low pass, attenuate unwanted interference. Internal batteries provide operation isolated from the AC line. And the best news of all, the SR560 is priced at only $1695, including remote interface. Whether you need lower noise, higher gain, or greater bandwidth, call Stanford Research Systems and take a closer look at the SR560.$1695 2 nV/VHz input noise 1 MHz bandwidth Gain variable to 50,000 AC or DC coupled True differential or single- ended input 2 configurable signal filters Selectable gain allocation 120 dB CMRR Line/Internal battery operation Remote interface SRS! Stanford Research Systems 1290 D Reamwood Avenue, Sunnyvale, CA 94089 TEL (408) 744-9040 FAX 4087449049 TLX 706891 SRS UD Circle number 41 on Reader Service Card PHYSICS TODAY AUGUST 1989 71A NEW DIMENSION IN GRAPHICS Plotworks revolutionizes its PLOT88 package to include three- dimensional contour maps at a fixed level. You can now project your two-dimensional contours in- to the third dimension for a unique visualization of your data. PLOT88 is a library of more than 50 subroutines to construct grids, contour maps, and mesh drawings that outputs to printers, plotters and displays. A device-indepen- dent graphics package, it includes PLOT, SYMBOL, AXIS and LINE— just to name a few.Now your mainframe graphics programs can run on your person- al computer at your convenience and at a fraction of the cost. Call (619) 457-5090 today for a free poster PLOTWORKS, Inc. Department P-13 16440 Eagles Crest Road Ramona, CA 92065 "Toolmakers for the Information Age" Circle number 42 on Reader Service Card ... with the Versatile ITC-4 Temperature Controller Explore Oxford flexibility with the ITC-4 cryogenic temperature controller ... with up to 3-sensor input and display, 3-term control, 256-step lineariza- tion and up to 18 preprogrammed and selectable sensor curves allowing the use of almost all commonly used temperature sensors. With thousands of cryogenic systems in operation- worldwide and worlds ahead- Oxford provides a most worthy companion for your quest . Write for details on "^ - ' • • • • the ITC-4. OXFORDOxford Instruments North America Inc. 3A Alfred Circle, Bedford, MA 01730, USA Tel: (617) 275-4350 Oxford Instruments Limited Osney Mead, Oxford OX2 ODX, EnglandA Member oi the Oxford Instruments Group pic Tel: (0865) 241456 Circle number 43 on Reader Service Card 72 PHYSICS TODAY AUGUST 198944. Intl. Comm. on Radiation Units and Measurements (7910 Woodmount), Bethes- da, Md., 1989. 189 pp. $22.00 pb ISBN 0- 913394-38-6 Fluids and Plasmas llth International Conference on Nu. merical Methods in Fluid Dynamics, Lecture Notes in Physics 323. Proc. Conf., Williamsburg, Va., June 1988. D. L Dwoyer, M. Y. Hussaini, R. G. Voigt, eds. Springer-Verlag, New York, 1989. 622 pp $65.00 he ISBN 0-387-51048-6 From Particles to Plasmas: Lectures Honoring Marshall N. Rosenbluth. Proc. Symps., Austin, Tex., February 1987- San Diego, Calif., April 1987. J. W. Van Dam, ed. Addison-Wesley, Redwood City Calif., 1989. 368 pp. $44.25 he ISBN 0-201- 15680-6. Festschrift Fusion Energy and Plasma Physics. Proc. Energy Independence Conf., Rio de Janeiro, August 1987. P. H. Sakanaka, ed. World Scientific, Singapore (Teaneck, N. J.), 1988. 892 pp. $96.00 he ISBN 9971-50-749-8 An Introduction to Alfven Waves. Adam Hilger Series on Plasma Physics. R. Cross. Adam Hilger, Bristol, UK (AIP, New York), 1988. 221 pp. £19.50 ($53.00) he ISBN 0-85274-245-2. Monograph Numerical Simulation and Optimal Control in Plasma Physics with Appli- cations to Tokamaks. Wiley/Gauthier- Villars Series in Modern Applied Math- ematics. J. Blum (translated from French by D. Chillingworth). Wiley, New York, 1989. 363 pp. $77.95 he ISBN 0-471- 92187-4. Monograph Physicochemical Hydrodynamics: In- terfacial Phenomena. NATOASISeries B: Physics 174. Proc. Inst., La Rabida, Spain, July 1986. M. G. Velarde, ed. Ple- num, New York, 1988. 1111 pp. $165.00 he ISBN 0-306-42905-5 Physicochemical Hydrodynamics: An Introduction. R. F. Probstein. Butter- worths, Boston, 1989. 353 pp. $65.00 he ISBN 0-409-90089-3. Monograph The Physics and Technology of Ion Sources. I. G. Brown, ed. Wiley, New York, 1989. 444 pp. $52.95 he ISBN 0-471- 85708-4. Compilation Plasma Diagnostics. Plasma-Materials Interactions. O. Auciello, D. L. Flamm, eds. Academic, San Diego, Calif., 1989. Vol. 1: Discharge Parameters and Chemistry. 456 pp. $89.50 he ISBN 0-12- 067635-4. Vol. 2: Surface Analysi s and Interactions. 337 pp. $79.50 he ISBN 0- 12-067636-2. Compilation Plasma Physics for Nuclear Fusion. Revised edition. K. Miyamoto (translated from Japanese). MIT P., Cambridge, Mass., 1989 [1987]. 618 pp. $27.50 pb ISBN 0-262-63117-2. Text Plasma Waves. D. G. Swanson. Aca- demic, San Diego, Calif., 1989. 422 pp. $39.95 he ISBN 0-12-678955-X. Text Reviews of Plasma Physics, Vol. M. B. B. Kadomstsev, ed. (translated fromDOOKb Russian by J. G. Adashko). Consultants Bureau (Plenum), New York, 1989. 252 pp. $85.00 he ISBN 0-306-11004-0. Com- pilation The Riemann Problem and Interaction of Waves in Gas Dynamics. Pitman Monographs and Surveys in Pure and Ap- plied Mathematics 41. Tung Chang, Ling Hsiao. Wiley, New York, 1989. 272 pp. $64.95 he ISBN 0-470-21014-1. Monograph Small Plasma Physics Experiments. Proc. Symp., Trieste, Italy, May 1987. S. Lee, P. H. Sakanaka, eds. World Scientif- ic, Singapore (Teaneck, N. J.), 1988 383 pp $55.00 he ISBN 9971-50-768-4 Theoretical Physics Applications of Self-Adjoint Exten- sions in Quantum Physics. Lecture Notes in Physics 324. Proc. Conf., Dubna, USSR, September 1987. P. Exner, P. Seba, eds. Springer-Verlag, New York, 1989. 273 pp. $31.40 he ISBN 0-387-50883-X Connections Among Particle Physics, Nuclear Physics, Statistical Physics and Condensed Matter. Proc. Sch., La Plata, Argentina, July 1987. J. J. Giam- biagi, G. G. Dussel, L. N. Epele, C. A. Gar- cia Canal, H. Wio, eds. World Scientific, Singapore (Teaneck, N. J.), 1988. 618 pp. $78.00 he ISBN 9971-50-405-7 Frontiers and Borderlines in Many- Particl e Physics. Enrico Fermi Interna- tional School of Physics 104. Proc. Sch., Varenna, Italy, July 1987. R. A. Broglia, J. R. Schrieffer, eds. North-Holland, New York, 1988. 460 pp. Dfl 325.00 ($171.00) he ISBN 0-444-87113-6 Gauge Fields: Classification and Equa- tions of Motion. M. Carmeli, K. Huleihil, E. Leibowitz. World Scientific, Singapore (Teaneck, N. J.), 1989. 136 pp. $24.00 Ac ISBN 9971-50-745-5. Monograph Schrodinger's Mechanics. World Scien- tific Lecture Notes in Physics 28. D. B. Cook. World Scientific, Singapore (Tea- neck, N. J.), 1988. 150 pp. $32.00 Ac ISBN 9971-50-760-9 Superstring s and Grand Unification. Proc. Sch., Puri, India, January 1988. T. Pradhan, ed. World Scientific, Singapore (Teaneck, N. J.j, 1988. 136 pp. $42.00 he ISBN 9971-50-527-4 Texts and Popularizations Advanced Calculus for Users. A. Rob- ert. North-Holland, New York, 1989. 364 pp. Dfl 115.00 ($60.50) he ISBN 0-444- 87324-4. Student text The Art and Science of Lecture Demon - stration. C. Taylor. Adam Hilger, Bris- tol, UK (AIP, New York), 1988. 181 pp. £7.50 ($21.00)pb ISBN 0-85274-323-8. Pop- ularization The Crumbs of Creation: Trace Ele- ments in History, Medicine, Industry, Crime and Folklore. J. Lenihan. Adam Hilger, Bristol, UK (AIP, New York), 1988.157 pp. £12.50 ($34.00) he ISBN 0-85274- 390-4. Popularization A Course in Mathematics for Students of Physics, Vol. 1. P. Bamberg, S. Stern- berg. Cambridge U. P., New York, 1988. 405 pp. $49.50 he ISBN 0-521-25017-X. Student text Data Analysis for Research Designs: Analysis of Variance and Multiple Regression/Correlation Approaches. G. Keppel, S. Zedeck. Freeman, New York, 1989. 594 pp. $42.95 he ISBN 0- 7167-1991-6. Student textThe Path of No Resistance: The Story of the Revolution in Superconductivi- ty. B. Schechter. Simon and Schuster, New York, 1989. 200 pp. $18.95 he ISBN 0-671-65785-2. Popularization Polynomials. Problem Books in Math- ematics. E. J. Barbeau. Springer-Verlag, New York, 1989. 441 pp. $59.00 he ISBN 0-387-96919-5. Student handbook Taking Risks: The Science of Uncer- tainty. P. Sprent. Pelican (Penguin Books), New York, 1988. 264 pp. $8.95 pb ISBN 0-14-022777-6. Popularization U Put the puzzle together with superconductive thin film systems from Balzers. For reproducible sandwich or alloy coatings of Y-Ba-Cu-0 on various substrates, Balzers ultra-high vacuum equipment gives you the control you need to achieve, and repeat, critical parameters for superconductive thin films. Several leading research labs use these Balzers systems to produce thin film superconductor coatings. Balzers systems reach UHV rapidly and allow precise regulation of E-beam evaporation at low-to- medium rates. For high-temperature coating materials (including UHV epitaxy for silicon), systems include a 900° C heater for substrate prepa- ration. The heater is suitable for operation with high partial pressure of oxygen. Choose from a broad range of sputtering or co-evaporation systems with process chamber diameters from 400-650mm (16-25 inches) to handle a variety of processes. Put a decade of thin film experience to work for you. Contact our New Hampshire headquarters or call your local Batzers representative for a free brochure or to arrange a presentation. f-L 9496Balieis Tel 1075) 44111 Tele* B89 788 bvo II Tdeia> (0751 444138 S.icj.imof.' P;irk H Hudson, NH030S1 Tet 16031 889 6888 Tele»294 041 Fax (603)889-8573 Circle number 44 on Reader Service Card PHY5IC5 TODAY AUGUST 1969 73

1.2810979.pdf

Laser Analytical Spectrochemistry and Laser Photoionization Spectroscopy and Photoacoustic and Thermal Wave Phenomena in Semiconductors V. S. Letokhov , Vladilen S. Letokhov , and A. Mandelis Richard Lee , Citation: Physics Today 42, 4, 66 (1989); doi: 10.1063/1.2810979 View online: http://dx.doi.org/10.1063/1.2810979 View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/4 Published by the American Institute of PhysicsQuick & Easy Superconductivity Measurements <MMMM» LR-400 Four Wire AC Resistance & Mutual Inductance Bridge Ideal for direct four wire con- tact resistance measurements with 1 micro-ohm resolution Ideal for non-contact trans- former method measurements where superconducting sam- ple is placed between primary & secondary coils and flux ex- clusion causes a change in mutual inductance Direct reading Low noise/low power Double phase detection Lock-in's built in LR-4PC accessory unit avail- able for complete IBM-PC computer interfacing Proven reliability & perfor- mance. In use world wide. LINEAR RESEARCH INC. 5231 Cushman Place, Suite 21 San Diego, CA 92110 U.S.A. Phone: 619-299-0719 Telex: 6503322534 MCI UW Circle number 32 on Reader Service Carding the generation of chemically reac- tive species. The predominance of neutral atoms and molecules, com- bined with the attendant myriad of excited states, makes low-tempera- ture plasmas very complicated. Un- derstanding their kinetics is essential to improving and extending their applications. The book begins with an introduc- tory chapter on basic plasma phenom- ena, including Debye shielding, equi- librium conditions and transport. The succeeding chapters go on to develop the theme of the book: the elementary collisional and radiative processes that exist in low-tempera- ture nonequilibrium plasmas, and the kinetics that result. The kinetics of the population of excited states, ioni- zation and recombination are consid- ered in detail. One interesting and very useful chapter is on radiative transport. Radiation trapping, which is often important for weakly ionized plasmas, is thoroughly discussed. The electron energy distribution func- tion is derive d with the relevant inelastic processes included. The fi- nal chapters include brief treatments of transient effects in nonequilibrium plasmas and of the kinetics of molecu- lar plasmas. The book gives a useful understand- ing of the fundamental processes that govern low-temperature nonequilibri- um plasmas. As such, it would be of value for developing a collisional- radiative model of a plasma or for quantitative spectroscopy. The lack of an index is unfortunate; however, the book contains useful appendices. A number of topics of practical importance are covered only cursorily or not at all. Topics such as sheath effects and plasma-material interac- tions, which are important in, for instance, reactive-io n etching, are clearly beyond the scope of this book. Also, very little attention is given to radiofrequenc y plasma excitation: No specific discharge configurations are discussed. The book would be a useful refer- ence text for a graduate course in the fundamentals of low-temperature plasmas; the rather abbreviated deri- vations would make it difficult to use the book as the primary text. The authors make extensive use of energy- space diffusion models to discuss the kinetics of excited states and ioniza- tion-recombination. In doing so they provide an excellent physical picture for the onset of the departure from equilibrium in terms of competition among processes. Although the ana- lytical techniques the authors discus s can be quite useful, they largely neglect the more powerful numericalmethods of analysis, which are now relatively easy to implement with the wide availability of computers. There is much current interest in gaining a better understanding, thor- ough diagnostics and modeling, of the plasmas used in various technological applications. Despit e some shortcom- ings, Kinetics of Nonequilibrium Low- Temperature Plasmas makes an im- portant contribution toward achiev- ing this objective. JOSEPH L. CECCHI Princeton University Loser Analytical Spectrochemistry Edited by V. S. Letokhov Adam Hilger, Bristol, UK (US dist. Taylor and Francis, New York), 1986 [1985]. 412pp. $109.00 he ISBN 0-85274-568-0 Laser Photoionization Spectroscopy Vladilen S. Letokhov Academic, San Diego, Calif., 1987. 353pp. $57.50 he ISBN 0-12-444320-6 Photoacoustic and Thermal Wave Phenomena in Semiconductor s Edited by A. Mandelis North-Holland, New York, 1987. 480pp. $75.00 he ISBN 0-444-01226-5 The Institute of Spectroscopy of the USSR Academy of Sciences, located in Troitsk, outside Moscow, was estab- lished in 1968, and since its inception researchers there have been engaged in the development of new techniques of laser spectroscopy. Important re- sults have been achieved in fields such as hole-burning spectroscopy, laser detection of single atom s and mole- cules, and laser cooling of atoms. They have had particularly impres- sive successes in ultrahigh-resolution and ultrasensitive spectroscopies. Based on their successes they have developed analytical methods that use photoacoustics, induced fluorescence and multiphoton resonance coupled to other techniques such as gas chroma- tography or mass spectrometry. Laser Analytical Spectrochemistry, edited by Vladilen S. Letokhov, direc- tor of the institute, contains eight separately authored chapters pre- senting tutorial reviews of several laser analytical techniques that were developed at the institute. The book 66 PHYSICS TODAY APRIL 1989A selection of North-Holland publications on STATISTICAL PHYSICS J.D. van der Waals: On the Continuity of the Gaseous and Liquid States edited with an introductory essay by J.S. Rowlinson 1988 xiii + 280 pages US $ 84.25 /Dfl. 160.00 The core of this book is a new edition of the English translation of the classic Leiden thesis of 1873 of J.D. van der Waals. A long introductory essay explains the historical context of the work and why it is still of great interest to physicists and chemists working in the fields of statistical mechanics, phase transitions and properties of liquids. The thesis is followed by a translation of Van der Waals' first great paper on the theory of liquid mixtures, which is also frequently quoted, but which has not hitherto been available. Simple Models of Equilibrium and Nonequilibrium Phenomena edited by J.L. Lebowitz 1987 xii + 272 pages US $92.00/Dfl. 175.00 This book consists of two articles of particular interest to researchers in the field of statistical mechanics. Its appeal is, however, not limited to this group. The first article is based on the premise that the best way to understand the qualitative properties that characterize many-body (i.e. macroscopic) systems is to study "a number of the more significant model systems which, at least in principle , are susceptible of complete analysis." The second article deals exclusively with nonequilibrium phenomena. It reviews the theory of fluctuations in open systems to which the authors have made important contributions. Like the first article it emphasizes simple but interesting model examples. The Wonderful World of Stocha sties A Tribute to Elliott W. Montroll edited by M.F. Shlesinger and G.H. Weiss 1985 xiv + 382 pages US $47.75 /Dfl. 140.00 Elliott W. Montroll had a profound influence on physics, beginning with his classical works on imperfect gases, the Ising model and the latticedynamics in the early 1940's. His innovative research continued over the next four decades with work ranging from the flow of electrons in amorphous semiconductors, to the flow of traffic on highways . This memorial volum e contains ten original contributions by noted scientists to statistical and mathematica l physics , a bibliography and review of Montroll's works, plus reprints of twelve of Montreal's classic papers. The Kind of Motion We Call Heat A History of the Kinetic Theory of Gases in the 19th Century by S.G. Brush Book 1: Physics and the Atomists 1976 (1st reprint 1986) xiv + 326 pages US $30.00/Dfl. 75.00 Book 2: Statistical Physics and the Irreversible Processes 1976 (1st reprint 1986) xiv + 494 pages US$34.00/Dfl. 85.00 Comprising two volumes this work provides a particularly comprehensive account of the development of kinetic theory and statistical mechanics up to the beginning of the 20th century. Book 2 is completed by an unusually comprehensive bibliography. Stochastic Processes in Physics and Chemistry by N.G. van Kampen 1981 (3rd reprint 1985) xiv + 420 pages US $ 30.00 / Dfl. 75.00 Although the number of articles on fluctuations and the stochastic method for describing them must run to thousands, the physicist or chemist who wants to become acquainted with the field cannot easily find a suitable introduction. This book is an attempt to fill this gap in the literature. Simulation of Liquids and Solids Molecular Dynamics and Monte Carlo Methods in Statistical Mechanics edited by G. Ciccotti, D. Frenkel and I.R. McDonald 1987 (paperback) xii + 481 pages US $ 32.50 / Dfl. 75.00 (also available as hardcover) Circle number 33 on Reader Service CardThis book is a collection of key reprints of papers on the computer simulation of statistical-mechanical systems, introduced and commented upon by the editors. Statistical Physics Invited papers from STATPHYS 16 edited by H.E. Stanley 1986 xvi + 432 pages US$15.00/Dfl. 50.00 The 52 papers in this volum e are based on the principal invited talks presented at STATPHYS-16. These papers form a concise but coherent summary of the "state of the art" of statistical physics in 1986. In Preparation: Hydrodynamics of Dispersed Media edited by AM. Cazabat, F. Carmona, E. Guyon and J.P. Hulin This book is part of the series Random Materials and Processes' series editors: E. Guyon and H.E. Stanley This book is based on the 4th EPS Liquid State Conference on the Hydrodynamics of Dispersed Media. It includes an extended general introduction presenting the various aspects of the hydrodynamics of disperse d media, followed by five main chapters from a microscopic to a macroscopic description: I. Wetting Phenomena and Interfacial Effects. II. Particle Dynamics in Dispersed Media. III. Statistical Descriptions of Multiple Scale Processes in Porous Media. IV. Macroscopic Description of Transport Processes in Dispersed Media. V. Experimental Approaches of Porous Media. For more information on the above-mentioned books, please contact: Eugene P.M. Wijnhoven, North-Holland Physics, P.O. Box 103, 1000 AC Amsterdam, The Netherlands. US $ prices are valid only In the USA and Canada In all other countries the Dutch Guilder (Dfl.) price Is definitive. 404/B/332 PHYSICS TODAY APRIL 1969 67NORLAND MULTICHANNEL ANALYZERS •••si amass BBBS0BQ0 0Q POWERFUL FULL FEATURED MCA'S ARE NORLAND'S FORTE MODEL 7800 NORLAND MCA'S are complete systems: from 4K to 32K data memory, built-in Amplifier (5700), linear or log displays, PHA, MCS, Data Processing, simultaneous I/O & data acquisition, and PEAK SEARCH (7800, with Gamma Library). In the classroom and the laboratory , NORLAND'S feature-packed MCA's offer what you need in multichannel analyzers. WE OFFER MORE FOR LESS Call Us! We Respond! NORLAND Norland Drive, Fort Atkinson, Wl 53538 Tel: US Toil-Free (800) 333-8456 In Wl: (414) 563-8456 Telex: 25-1776 FAX: (414) 563-9501 Circle number 34 on Reader Service Cardopens with Letokhov's introduction to the properties of lasers and to prob- lems of laser spectroscopy. The suc- ceeding chapters cover applications in analytical chemistry including atom- ic fluorescence, photoionization, in- frared absorption, photoacoustic and desorption spectroscopies . Although the book is not a step-by- step guide, the novice can read it easily. This book presents the materi- al with even more of a practical emphasis than does Nicolo Omenet- to's Analytical Laser Spectroscopy (Wiley, New York, 1979) or Edward H. Piepmeier's Analytical Applications of Lasers (Wiley, New York, 1986). Over the past decade laser photo- ionization spectroscopy—also known as resonantly enhanced multiphoton ionization—has become a powerful technique for the study of atomic and molecular structure as well as for the detection of atoms and molecules in various environments. Stirred by suc- cesses such as G. S. Hurst's experi- mental demonstration in 1977 of sin- gle-atom detection, this distinct sub- field of laser spectroscopy is being used by researchers in fields ranging from atomic and molecular physics to geochemistry. Letokhov, one of the early pioneers in the field, has written Laser Photoionization Spectroscopy, a monograph geared to this diverse audience. The book can be separated into three sections : fundamental physics of ionization, techniques for produc- ing free atoms and molecules, and applications. Letokhov presents first an introduction to resonant interac- tions between laser light and atoms or molecules . He follows this with de- scriptions of photoionization, field ionization and collisional ionization as well as an introduction to the comparative sensitivities of different excitation schemes: absorption, flu- orescence, resonant deflection and photoionization. Letokhov presents fairly complete descriptions of the current experimen- tal configurations in which photo- ionization studies are done. He in- forms the reader of the ultrasensitive and selective nature of photoioniza- tion measurements. He also presents some noteworthy results obtained at the Institute of Spectroscopy, such as the achievement of a sensitivity of a few parts in 1011 for the detection of Na in semiconductors. In addition Letokhov presents a comprehensive survey of applications. Laser Photoionization Spectroscopy is a valuable source book for anyone interested in resonance ionization spectroscopy. While books such as S. H. Lin, Y. Fujimura, H. J. Schlagand E. W. Neusser's Multiphoton Spectroscopy of Molecules (Academic, Orlando, Fla., 1984); Multiphoton Pro- cesses (Springer-Verlag, New York, 1984) edited by Peter Lambropoulos and S. J. Smith; S. L. Chin and Peter Lambropoulos's Multiphoton Ioniza- tion of Atoms (Academic, Orlando, Fla., 1984); and Joseph Berkowitz's Photoabsorption, Photoionization and Photoelectron Spectroscopy (Aca- demic, Orlando, Fla., 1979) cover var- ious aspects of the field, none intro- duces the field, provides the funda- mental physics needed to understand and appreciate the various processes, or presents as many of the key devel- opments, both experimental and theo- retical, as Letokhov's book does. Of- ten awkward English, coupled with some misprints, make reading Laser Photoionization Spectroscopy slow and difficult the first time through. Also, Letokhov seems to have over- looked such serious problems as how to vaporize (laser ablate) a sample while maintaining its chemical integ- rity. All in all, Letokhov has done an admirable job, producing a valuable source of information that is profuse- ly illustrated and includes a large number of references. Following the introduction of tech- niques by books such as Laser Optoa- coustic Spectroscopy by Letokhov and Vladimir P. Zharo v (Springer-Verlag, New York, 1986), Photoacoustics and Photoacoustic Spectroscopy by Allan Rosencwaig (Wiley, New York, 1980) and Optoacoustic Spectroscopy and Detection by Yoh-Han Pao (Academic, New York, 1977), interest in photo- acoustic techniques has spread over the past decade to fields ranging from gas chromatography to semiconduc- tor characterization. Photoacoustics provides a useful nondestructive tech- nique for measuring various proper- ties of materials. Photoacoustic and Thermal Wave Phenomena in Semiconductors, edited by Andreas Mandelis of the Univer- sity of Toronto, provides insight into the use of photoacoustics, specifically in applications to semiconductors. The book's five sections consist of reviews of established and emerging thermal-wave microscopies; treat- ments of imaging using thermal-wave techniques (with concentration on measurement of parameters), of novel photothermal-wave techniques, and of techniques for monitoring phenome- na at the electronic level; and reviews of progress in photothermal spectro- scopic techniques. Among the specific examples mentioned are Tsuguo Sawada's measurement of subsurface defects in GaAs 50 //m below the surface and Rosencwaig's identifica- 68 PHY5ICS TODAY APRIL 1989tion of differences in the depth of surface-state annealing in silicon wa- fers: Wafers with few defects showed the annealing effect 50-100 fj,m be- yond the irradiated region, whereas in wafers with a high level of structural damage (such as those that were heavily implanted) the annealing ef- fect was confined to the irradiated region. Each chapter contains many illus- trations and a bibliography . This book augments the texts already published and will find a place on many a pro- fessional's bookcase. I recommend it. RICHARD LEE Amperex Slatersville, Rhode Island Lasers, Spectroscopy and New Ideas: A Tribute to Arthur L Schawlow William M. Yen and Marc D. Levenson Springer-Verlag, New York, 1987. 337pp. $45.00 he ISBN 0-387-18296-9 This book allows the reader to enjoy, at least remotely, the experience of physics research with Art Schawlow. Its 19 short articles, whose authors all have been students of Art's at Stan- ford University over the past 25 years, cover the three primary areas to which Schawlow has richly contribut- ed—lasers, spectroscopy and "new ideas." Each article includes reminis- cences of Art's humor, his excellent physics intuition and, most important- ly, the immense joy and enthusiasm he brings to his research, teaching and lectures. It is interesting that many physics concepts can be more clearly grasped and understood with the in- formal writing styles used in this book. As one might expect, the style and scientific content of the brief articles in this volume vary widely. Some are detailed and will serve as excellent references and reviews. Examples are the three articles on solid-stat e spec- troscopy, by Roger M. Macfarlane ("Optical Spectral Linewidths in Sol- ids"), Satoru Sugano ("Spectroscopy of Solid-State Laser Materials") and George F. Imbusch and William M. Yen ("Ruby Solid-State Spectroscopy: Serendipitous Servant"). Sugano dis- cusses the early history of the laser. This is an ideal time to look back at the development of the laser, one of the major advances of this century, but I was disappointed that Schawlow's ear- ly laser research and that of his collaborators is not covered in this book. 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BOX 2565 • PRINCETON, NJ 08543-2565, USA • 609/452-2111 • TELEX: 843409 Circle number 35 on Reader Service Card"•'•» New Lower Price LEV881 fir VACUUM GAUGES: • Outstanding performance, long life, low maintenance costs • 10 ranges from 10"6 torr to atmosphere • Thermocouple, cold cathode, and diaphragm type gauges • Rugged, corrosion-resistant gauge tubes • Single or dual vacuum controllers • Vacuum recorders • Compact, stable, dependable; rapid response HASTINGS Request FREE CATALOG, #300. Teledyne Hastings-Raydist "W^TELEDYNE Hampton, VA 23661 U.S.A. HASTINGS-RAYDIST Telephone (804) 723-653 1 Circle number 36 on Reader Service Card PHYSICS TODAY APRIL 1989 69

1.342922.pdf

Diffusive processes in the crossfield flow of intense plasma beams B. Newberger and N. Rostoker Citation: Journal of Applied Physics 65, 1874 (1989); doi: 10.1063/1.342922 View online: http://dx.doi.org/10.1063/1.342922 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in CrossField Particle Diffusion in a Collisionless Plasma: A Nonresonant and a Resonant Mechanism AIP Conf. Proc. 703, 123 (2004); 10.1063/1.1718446 Relativistic theory of crossfield transport and diffusion in a plasma Phys. Plasmas 3, 804 (1996); 10.1063/1.871781 Nonlinear evolution of a strongly sheared crossfield plasma flow Phys. Fluids B 5, 3163 (1993); 10.1063/1.860653 Crossfield energy transport by plasma waves Phys. Fluids 19, 815 (1976); 10.1063/1.861547 CrossField Diffusion of Quiescent Potassium Magnetoplasmas Phys. Fluids 9, 2294 (1966); 10.1063/1.1761609 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Sat, 22 Nov 2014 12:45:42Diffusive processes in the cross .. field flow of intense plasma beams 8. Newberger and N. Rostokera) Institutefor Fusion Studies, The Uniuersity afTexas at Austin, Austin, Texas 78712 (Received 23 September 1988; accepted for publication 2 November 1988) We consider magnetic field diffusion in the presence of strongly magnetized electrons (wceTco> 1) as a mechanism for the rapid penetration observed in cross-field flows ofhigh-,B plasma beams. The diffusion has been investigated in several cases which are amenable to analytic solution. The flux penetration times are found to be insensitive to the particular configuration. Comparison with two experiments is made. Agreement within the limits of the experiments is found. Both require an anomalous collision rate which is consistent with observed fluctuations in one case but apparently not the other. I. INTRODUCTION Active injections of intense beams of neutral plasmas (sometimes alternatively called plasmoids or plasma jets) into the near earth space environment are of interest in the simulation of phenomena associated with naturally occur ring events. These events include auroras, magnetospheric substorms, and comets, and the phenomena include wave generation and emission, particle precipitations associated with these! and the interaction ofthe solar wind with come tary bodies.2 In order to interpret these active injection ex periments, the dynamics of the flow of the jet across the geomagnetic field must be understood. To this end, laborato ry experiments investigating the cross-field flow of plasma beams are also being conducted.3.4 Of particular interest, both in space and the laboratory, are jets of sufficient intensity that the ram kinetic energy, plf2 exceeds the magnetic pressure B 2/ 41T of the ambient field; this is called the high-,B regime. In this case, the convention al picture holds that the external field wiH be excluded from the interior of the jet and the jet propagates by plowing the ambient field aside. A significant finding in both the space based and laboratory experiments is an anomalously rapid penetration of the ambient field into the plasma beam. The penetration rates considerably exceed those based on a clas sical collision frequency. Some evidence exists for enhanced levels of turbulence in some of the active injection experi ments,S and mechanisms for an anomalous collision fre quency have been suggested." Recent laboratory experi ments4 also have observed field fluctuations although their interpretations are not yet complete. An anomalous resistiv ity could give an enhanced field diffusion. However, it has been noted by Rostoker and co-workers that even with an anomalous collision frequency v~ which could be expected based on observed fluctuations, the electrons arc magne tized, nc/v~ > 1. In this paper, we obtain estimates of the diffusion times to be expected, based on the anomalous resistivity in the presence of magnetized electrons. These estimates are ob tained from analytic solutions of the magnetic diffusion equation in several cases. In general, the electron magnetiza tion makes the diffusion equation nonlinear. We have ob tained an approximate solution in the slab limit. By compar- "' Permanent address: University of California, Irvine, CA. ing the diffusion times from this solution with those obtained by imposing linearity on the diffusion equation in both the slab and cylindrical limits, we can independently get some measure of influence of geometry and nonlinearity. It is found that both increase the diffusion time over the linear theory in the slab. The differences are measurable in princi ple but probably not within experimental uncertainties in practice, In considering some particular experimental situa tion, it is likely to be reasonably safe to use the simple linear slab estimates. The differences are expected to be compara ble to the factors of 1.5-2.5 found here. We have also evalu ated the absolute diffusion times for several cases of interest to space and laboratory experiments. The diffusion times are consistent with observations. We will now describe the mod el and present our solutions. II. DiFfUSION MODEL The transport in the beam is taken to be standard Bra ginskii,7 with collision times reduced by turbulence from the Coulomb value. The transport mode! neglects the effect of inertia and this imposes a constraint on the solution which, because we are not considering time-harmonic phenomena, takes the form ne l'D > 1, where ne is the electron gyrofre quency and 1'lJ is the diffusion time. Diffusion times based on Coulomb collisions alone are too fast to satisfy this condi tion, and the model breaks down. With an anomalous colH sion rate, the inequality can be satisfied. The jet is taken to be homogeneous so the V P term does not appear and thermo electric terms are neglected. In this case, the currents in the beam are related to the electric fields by J=(jlE~ +uHE'Xi1, where E '= E + 73 X B, fj = v/c'i is the jet velocity and taken to be along the z axis, (7i = (Jo/[l + (n,T~)2], the Pedersen conductivity,S the Hall conductivity, where We further define E l = E (\ + E/= and we see J!r = O. (1) (2) (3) (4) It is observed in the experiments that the motion of the 1874 J. AppL Phys. 65 (5). i March 1989 0021-8979/89/051874-06$02.40 @ i 989 American Institute of Physics 1874 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Sat, 22 Nov 2014 12:45:42beam particles are essentially force free in the beam frame. ___ A _ _ This implies E,\ = -[3 X B and therefore E, is parallel to the z axis. Ampere's law in cylindrical geometry gives aE,. 4rrJr + --= 0, at . aE!) 4rrJ(J + --= 0, at 4rrJz 1 a 1 oRr --=--(rB o) ----. c r or r ae (5) (6) (7) The z component of displacement current has been ne glected relative to the conduction current. This constrains the solution and must be checked a posteriori. In the rand e component equations, the terms in the spatial derivatives of B vanish identically by virtue of the symmetry. The current responsible for the diamagnetism is a Pedersen currentS driv en by the electric field induced by the penetrating flux. The self-consistency is obtained from Faraday's law which closes the system. After some straightforward algebra a pair of cou pled nonlinear diffusion equations for the r, f) components of R are obtained: where dimensionless variables have been introduced by the definitions p=rla, T=t IrD, b=BIB o, Tn =4rra2uolc2, K = 01eor; )", and neO = eRolme. Here Bo is the magnitude of the applies! magnetic field far from the beam where B = Bx; the initial condition is then b r = (1 -11 p2) cos e) . (10) be = -(l-1I/-J2)sin8IP> 1. (11) The boundary condition at P = 1 is found by matching to a vacuum solution for p > 1 which satisfies for all time b,. --cos e } _ . P~ 00. be --. -sm () (2) (13 ) In the slab limit, Eqs. (g) and (9) reduce to a single equation for bx• a ( 2 Jbx) Jb, -l+Kb --=--. ax ax ar . (14) subject to the boundary condition bx = 1 at x = ± ~ and the initial condition b x = 0, -i < x < ~. WewiU now consider the solution of the cylindrical sys tem in the linear approximation and the solution of the non linear slab model. m. LINEAR SOLUTION IN CYLINDRICAL GEOMETRY The solution of the problem in cylindrical geometry can be more readily obtained by introducing a scalar function X 1875 J. Appl. Phys., Vol. 65, No.5, 1 March 1989 through the definition 0=Z·V1·· (15) (x is essentiaHy the z component of the vector potential,) In terms of X, Eqs. (8) and (9) become of [1 a( Jv) 1 J2X]1 -. (l+KVrVx) --p~ +--1 1 Jp t L P ap Jp p 00- a ax --ap Br' 1 J { [! a (ax) 1 a 2r 1 } --(l + KVrVx) ---+-_/ J p ae p Jp Jp p pO 2 _~Jx pJ() ar (16) (17) These equations can each be integrated once. Because the integrated equations resulting from Eqs. (16) and (17) are the same and X must satisfy given boundary conditions, the arbitrary functions, otherwise resulting from the integra tion, vanish. Thus X satisfies the nonlinear diffusion equa tion (18) The boundary condition at p = 1 is obtained from a solution in the vacuum region of Laplace's equation, 'i72x = 0, which satisfies X...... -P sin e as p -" 00 • (19) (20) That is, our boundary condition is imposed by the essentially Cartesian nature of our magnetic field system. Hence, this problem is different from the standard mixed problem solved in the textbooks. Thus the solution is only obtained by solving simultaneously Eq. (18) for p < 1 and Eq. (19) for p> 1 and matching the solutions at p = 1. We do not know how to do this in general but will obtain the solution in the linear approximation where Eq. (18) is replaced by av A"? -f; = ;':v-X, K=l + K::::::K, since typically K~ 1. (21) In the outer region (p> 1), a solution of Laplace's equation satisfying the condition (20) is X = --p sin () + I a/ (r')p-I sin Ie, r' = K7. (22) I (The cos Ie terms turn out to be unnecessary.) In the interior (p < 1), the solution is found by taking the Laplace transform of Eq. (21). The solution is X(s,p,8) = I c/(s)sin 1011 cJsp) , (23) I w here II is the modified Bessel function of the first kind, and the transform variable is s. Taking the Laplace transform of Eq. (22) gives for p > 1 X(s,p,O) "-"" p sin e + )' aj(s)p- I sin lB. (24) s .., B. Newberger and N. Rostoker 1875 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Sat, 22 Nov 2014 12:45:42Now matching aX/ap, ai/ae from Egs. (23) and (24) at p = 1 gives I pairs of equations for the a{,c1 which are satis fied by al =Ci =0 for I> 1 and Cj(S) = -2/sjSlu(/s), (25) ptefs) -{sI; ({s)] al (s) = -....::..-....:-.-------.=:- ~12Io(fs) (26) where in al the prime denotes differentiation with respect to the argument of the Besse! function. These can be substituted back into their respective equa tions for X and the standard Bromwich inversions of the Laplace transforms done. The solutions are then, for p < 1, ( ~~~ ) e 0" J (a p) X = -p + 4L 2 I Un sin e, n aOnJ!a On (27) X sin 0, (29) where aOj is the jth zero of Jo and the Bessel function nota tion is standard. Forp> 1, . e-a5'/ sin e X= -psmO-4 2:-2---' n aOn P (30) 2 , e -' au"T cos () b, = cosO +42:-2---2-' n ao" p (31) . e --a5nr' sin () bfj = -smB +42:----. (32) a~n p2 We will use these to obtain numerical estimates of diffusion times fonowing our discussion of the nonlinear diffusion in the slab. IV. NONLINEAR DIFFUSION IN A SLAB In this case, the evolution of the field inside the beam is givenbyEq. (14): ~((1+Kb2)~)=!!!!..o (14) ax ax a7 Nonlinear diffusion equations of this kind are known to pro duce solutions with frontIike behavior. Because typical cases ofinterest have K). 1, we neglect the constant term relative to Kb Z in the diffusion coefficient. This is not quite correct just at the front but the effect is the elimination of a small foot (~lIiK) right at its leading edge. The finite thickness of the jet means self-similar solutions do not exist for our problem, However, until the fronts from the two edges meet, neither edge can know about the other and the problem is the same as the semi-infinite one. Thus we use the self-similar form of the solution b = [1 -x/8( 7') J 112, where 8C 7) will be deter mined by imposing an integral constraint on the solution; the method of moments. 9 The resulting constraint equation is an 1876 J. Appl. Phys., Vol. 65, No.5, 1 March 1989 ordinary differential equation in time for fj ( 1"). It will be found to be proportional to 71/2 which is consistent with self similarity. Putting r' = K7and integrating Eq. (14) inxover (G,li) we have a4> = b2~lb a7' ax 0' where <1>=: f bdx. (33) With b(x,r') = ba[1 -x/oCr') J 112, this becomes d82(r') 3b 6 ----;;;;;-= -2-. (34) The boundary condition at x = O=>bo = 1, at the solution, IS o(r') = ,fl-r\12 (35) and h(x,r') = (1 -/fT'X) 112, 0 <x < 1/2. (36) This solution breaks down when the fronts meet in the center of the slab at x = 1. This occurs at a time r; = i. After this time, the solution is no longer self-similar. We will again use the moment method to obtain an approximate solution in the case 7' > 7p' It is convenient in this case to shift the coordinate axis by half a unit and place the origin in the center of the slab. The solution is symmetric about the origin. It is also convenient to rescale so that the boundary conditions are again at x = ± 1. To return to the original system, x ...... 2x -1 in the solution. We also shift the time origin by r; and scale by a factor of 4, to have, for 7 = 47> 0, ~=~(b2~). dr ax ax (37) We look for a solution of the form b = [lioer) + PI (1')X2]a. (38) This is the simplest solution with the appropriate symmetry about x = 0, the required smoothness at x = 0, and which win let us impose the necessary physical constraints. One constraint is the moment integral. This will give a differen tial equation in time for either (30(1') or /3\ (7'). The other is determined from the boundary condition b(x = ± 1) = 1, which implies /3oFf) + /31 (1') = 1 for all 7';;:>0. (39) Furthermore, at r = 0, b(x = 0) = O. That is, just as the diffusion fronts meet, the field at the slab center is zero. This implies PI (0) = 1. The second constraint is a condition on 3b / ax at the slab edges at 7 = O. This is essentially a con straint on the "flux" (actually edge current density) which does not instantly change when the diffusion fronts meet. This will fix the value of a as we now show. From Eq. (38), ~! = 2a[3rx[{3oCr) + PI (1')x21a-I = -2ap\ (0) at T = 0, x = -1. (40) B. Newberger and N. Rostoker 1876 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Sat, 22 Nov 2014 12:45:42From the similarity solution, written in the present coordi- 1.0f'e::':::'-r---r---.....,r----..,r----., nate system, for -1 <;x<;O, b(x,r = 0) = ( -x) 1/2, (41) Therefore, -2aPl (0) = -1; :=}a = 1/4. (43) In fact, with a = ;\, the second derivatives at the edges also match. Thus we have b(x,r) = [1-f31Cr)(1-x2)]II4, The moment integral gives JL t bdz=b2~11 = {31(r) . ar Jo Jz 10 2 Now .c b dz = f [1 + /31 (r)(x2 -1)J !/4dx. (44) (45) (46) This does not have a closed-form expression. However, we know that for r> 0, PI < 1, and also dearly for XE (0,1 ), 1 -X2 < 1, so we will approximate the integrand by the first two significant terms in its Taylor expansion. The differen tial equation which results is _ 1., df31 _ i!.L!!.L _ [31 = O. 6 dr 10 dr 2 (47) This can be solved by quadratures and a transcendental equation for PI (r) results: PI exp [~(p't -1) ] = exp( -37). (48) To solve this, we again expand to 0([31)2 and find, on solv ing the resulting quadratic, This solution has the following appropriate limiting values: Forr= 0, .BI(O} = H -1 + (16)1/21 = 1, and as r-> 00 ,/31->0. The fun solution is then, in the original system of units with 0 < x < 1: ForO<T< lIOK, b(x,1") = P -OK1") 1/2X] 112, o<x <1 (50a) t,K<7, b(x,r) = {I -H -1 + (l + 15e--1ZK(T-1I6K»I12j (SOc) These solutions are sketched in Fig. 1 as a function of x for several values of 1"'. In the next section, we discuss nu merical estimates of diffusion times from the models consid ered. We will also compare these with a solution of the slab model in the linear limit. 10 ia77 J. Appl. Phys., Vol. 65, No.5, 1 March i989 0.8 0.6 b 0.4 0.2 r-O.25 FIG. 1. Normalized magnetic field as a function of position for several val ues of r'. The field is symmetric about x = 0.5. V. NUMERICAL RESULTS Here we will use our results of the previous sections to obtain some estimates of the penetration times of the field. First, we will make some relative estimates simply to have some measure of the effect of the different physics we have been considering. We will then use parameter values appro priate for the laboratory experiments of Ref. 4 and space experiment of Ref. 5 to obtain some absolute diffusion times. In the following experiments, the penetration of the field is determined by the decay of the diamagnetic signal and a concurrent rise of the polarization field as measured on fioat ing Langmuir probes.4 The experiments in space generally observe the field within the jet by means of satellite-borne instruments.l.; As a basis of comparison, both with experi ment and the different theoretical solutions, we adopt the central field 90% return time [the time at which b(x = D = 0.9) as a measure of the diffusion time. There is no particular justification for this. Given the experimental uncertainties (finite beam rise times, approximate geome tries, etc. ) and the relative insensitivity of the diffusion times to the central field fractional value if it is sufficiently near to one, this seems to be as reasonable measure as any. From Eq. (2) afRef. 10, for example, for the slab in the linear approxi mation and Eq. (49) in the nonlinear case, we find 1"~.90 = 0.26 (linear), 1"0.90 = 0.30 (nonlinear). The closeness of these values is striking considering the dif ference in the diffusion profiles at early times. [Compare Fig. 1 with Fig. 8 of Ref. 10, for example. This also shows that the evolution of the linear problem at late times and the nonlinear solution, Eq. (50), at late times have very similar profiles.] In the cylindrical case, Eqs. (28) and (29) give 1"0.90 = 0.48. The diffusion in this case is significantly slower, by about a factor of 2, than that in the linear case. (This is a conse quence of the planar geometry in the large p limit. If a dipo lar field were composed at the edge of a cylindrical plasma B. Newberger and N. Rostoker 1877 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Sat, 22 Nov 2014 12:45:42jet, the diffusion times would be much closer to the slab values.) In view of the slab results, it seems reasonable to expect that, even in the cylindrical case, the nonlinear diffu sion times will not differ substantially from the linear casco Numerical solutions of the nonlinear diffusion have proved to be frustrating and difficult in that parabolic solvers typi cally are based on an iterative application of elliptic integra tors. The frontlike solutions of the nonlinear diffusion are a problem for these methods. We now return to the dimensional form of the diffusion times by reintroducing the scaling 1" -> Kt /71) and evaluating these for the U C Irvine experiments,3,4 and the AMPTE arti ficial comet experiment. The diffusion model is not applica ble to the Porcupine experiments in that the Pedersen cur rent does not dominate the displacement current in that case, even with an anomalous collision frequency. In the experiments at the University of California- Irvine (U C-1) , a neu tralized ion beam of several hund red ke V cner gy and density n = 3 X 10 11 em -3 was injected across a mag netic field whose value was varied from several tens to sever al hundreds of gauss. The details of these experiments have been reported elsewhere. 4, 11--13 For the plasma parameters there, the classical collision frequency ~'ei::::: let /s and the displacement currents are not negligible compared to the conduction currents in the modeL If diffusive processes are to be responsible, an anomalous transport must be taking place and evidence of electrostatic turbulent fluctuations has been observed in the experiments.l 1,!3 The fluctuations are higher frequency than would be expected of the lower hy brid; they are more in the ion acoustic range. The role of these turbulent fluctuations has not yet been investigated in detail experimentally. However, turbulence ofthe ion acous tic type could be expected to produce an anomalous collision frequency v~ ~(JJpi which is, 14 for the parameters of the UC I experiments, somewhat larger than the anomalous coni sion frequency of Ref. 6. If we use this value of v~, with Bo = 200 G, a = 10 cm, we find r D = 1. 5 X 10 -7 s, K=24, to.90 =3 ns. This diffusion is very fast but is within the condition imposed by the neglect of the z component of the displacement cur rcnt. This result is qualitatively consistent with the expcri mental observation in which fast diffusion was seen relative to the 0.5 f-ls duration of the beam. We now consider the AMPTE artificial comet experi ment. 2 Here a rapid penetration of the interpianetary mag netic field was observed as well. If a diffusive process is to be responsible, some turbulent enhancement of electron-ion collisions is also required here, too. If we again look to an ion-acoustic enhanced resistivity, reasonable agreement with the experimental return time is obtained. The param eterslS are n c= 1.2X 104 em --3, Eo = 1.3 X 10 -3 G, a = 80 km, and a plasma composed of B u-t (A = 137). If we take 'j/~ -(i}pi' then 1878 K=3.5, to_90 =30 s, J. Appl. Phys., Vol. 65, No.5, 1 March 1989 compared to an experimental field return time of 17 s. The value of If only marginally satisfies our requirement K> 1. But, as the anomalous collision frequency is only an order of magnitude estimate in any case, the values derived there must be considered in the same way. Nevertheless, it would appear on the face of it that diffusive transport of the field would be a plausible mechanism for the observed rapid field penetration. Unfortunately the real puzzle (and problem) for the model lies in the observation of fluctuations in the ion-acoustic range with amplitUdes too small to be expected to lead to anomalous resistivity. This has led to the sugges tion that hydromagnetic mechanisms may be responsible. 15 However, it seems that these should be subject to inter change instabilities which have been found to be able to grow even in the case of unmagnetized ions. Indeed, an inter change instability of this general kind is now believed to be responsible for the fast field penetration in numerical simu lationsl6 done by our colleagues of high-;1 plasma beams crossing a magnetic field in vacuum. In these simulations, the plasma temperature was sufficiently high (ion masses were artificially low, a standard technique in simulations) that ion acoustic instability was not expected and no evi dence of it was found. VI. SUMMARY AND CONCLUSIONS In this work, we have considered the diffusive transport of magnetic fields in the limit in which the electrons are strongly magnetized and applied the results to the problem of field penetration into high-;1 plasma beams. Because the magnetization of electrons makes the diffusion coefficient nonlinear, the solution of the problem is nontrivial and nu merical methods can have difficulties even in simple geome try. 17 We have constructed analytic solutions under different approximations and have shown that while there are differ ences in the details of the solutions, the gross measures ob tained are quite similar. In particular, measures of the diffu sion times are the same to within any reasonably expected experimental determination and are about 50% of the values simple scaling arguments would give. We have applied these to two experimental cases of plasma beams in a transverse field. The diffusion times in the laboratory experiment qual itatively agrees with the observations which essentially es tablish an upper bound to the field penetration time. There is reasonably good quantitative agreement with the space ex periment (AMPTE) penetration time. Both require the ex istence of an anomalous collision frequency. Given the strong diamagnetic currents which must flow to shield the field, it is not unreasonable to expect such. This is consistent with the laboratory observations but not the experiment in space. The fluctuations in the laboratory experiments need to be investigated in more detail in order to come to quantita tive conclusions. These could be supported with some nu merical simulations as wen. The space experiment remains a puzzle. We anticipate that some simulations extending the work of Ref, 16 will provide some insight i.nto this problem. In general, the microscopic dynamics in the boundary layer between the plasma beam and field is likely to be where the action is, The physics of this region is complex and needs considerably more work Interestingly, this was an area of B. Newberger and N, Rostoker 1676 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Sat, 22 Nov 2014 12:45:42interest early in the magnetic confinement fusion program and is again becoming the focus of increased attention. ACKNOWLEDGMENTS We wish to thank Dr. Frank Wessel for many discus sions of his experimental results, One of us (R.N.) would like to thank Dr. W. R. Shanahan for several helpful discus sions and the results of his unpublished research. This work was supported by the U.S. Department of Energy Contract No. DE-FG05-80ET-53088, the Air Force Office of Scien tific Research, and NASA Grant No. NASW-846. 'G. Haerendel and R. Z. Sagdeev, Adv. Space Res. 1, 29 (1981). 2A. Valenzuela, G. HaerendeI, H. Poppl, F. Melzner, H. Neuss, E. Rieger, J. Stocker, O. Bauer, H. Hofner, and J. LQidi, Nature 320,700 (1986). 'F. J. Wessel, R. Hong, J. Song, A. Fisher, and N. Rostoker, Proc. Sl'IE- Int. Soc. Opt Eng. 828, paper 38 (1988). 4R. Hong, F. J. Wessel, J. Song, A. Fisher, and N. Rostoker, J. Appi. Phys. 64,73 (1988). 5B. Hausler, R. A. Treumann, O. H. Hauer, G. Haerendel, R. Bush, C. W. Carlson, B. Theile. M. C. Kelley, V. S. Dokukill, Yu. Ya. Ruzhill, J. Geophys. Res. 91, 287 (1986). 1879 J. Appl. Phys., Vol. 65, No.5, 1 March 1989 6E. V. Mishin, R. A. Treumann, and V. Ya, Kapitanov, J. Geophys. Res. 91, 10183 (1986). 7S. I. Braginskii, til RelJiews in Plasma Physics, edited by M. A. Leontovich (Consultants Hureall, New York, 198;), Vol. 1, Chap. 3, pp. 205--311. 8E, Rossi and S. Olbert, Introduction to the Physics a/Space (McGraw-Hili, New York, 1970), Chap. 13, p. 394. "W. F. Ames, Nonlinear Partial Differential Equations in Engineering (Academic, New York, 1965), pp. 249-256. IOH. S. Carslaw and J. C. Jaeger, Conduction 0/ Heat in Solids (Oxford, London, 1947}, Chap. 3, p. 83. If]. Song, F. J. Wessel, A. Fisher, and N. Rostoker. Conference Record, 1988 IEEE International Conference on Plasma Science, Seattle, WA, 1988, (IEEE, New York, 1988), p. 107. l2F. J. Wessel, R. Hong, 1. Song, A. Fisher, N. Rostoker, R. Li, and R. Y. Fan, Phys. Fluids 31,3778 (1988). uF. J. Wessel, A. Fisher, N. Rostoker, and J. Song. Proceedings of the 7th International Conference on High-Power Particle Beams, Karlsruhe, 1988 (to be published). 14A. A. Ga!eev and R. Z. Sagdeev, in Handbook of Plasma Physics-Basic Plasma Physics, edited by A. A. Galeev and R. N. Sudan (North-Holland, Amsterdam, 1984), Vol. 2, Chap. 6.1, pp. 271-303, "G. Haerendel, G. Paschmann. W. Baumjohann, and C. W. Carlson, Na ture (Paris) 320, no (1986). '''T. Tajima, J. Koga, and T. Fujinami, 68, 1400 (1987) Trans. Am. Geophys. Union; J. Koga, M. A. thesis, University of Texas, Austin, 1984. 11R, D. Richtmyer and K. W. Morton, Difference Methods/or Initial-Value Problems, 2nd ed. (!nterscience, New York, 1967), pp. 201-206. B. Newberger and N. Rostoker 1879 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.120.242.61 On: Sat, 22 Nov 2014 12:45:42

1.101280.pdf

Observation of apparent inelastic tunneling between Landau levels in superlattices T. K. Higman, M. E. Favaro, L. M. Miller, M. A. Emanuel, and J. J. Coleman Citation: Applied Physics Letters 54, 1751 (1989); doi: 10.1063/1.101280 View online: http://dx.doi.org/10.1063/1.101280 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Zener tunneling between Landau orbits in two-dimensional electron Corbino rings Appl. Phys. Lett. 100, 251602 (2012); 10.1063/1.4729590 Novel microwave resonance around integer Landau level fillings in unidirectional lateral superlattices AIP Conf. Proc. 1399, 619 (2011); 10.1063/1.3666530 Interference between wave modes may contribute to the apparent negative dispersion observed in cancellous bone J. Acoust. Soc. Am. 124, 1781 (2008); 10.1121/1.2953309 Formation of rotationinduced superlattices and their observation by tunneling spectroscopy Appl. Phys. Lett. 59, 570 (1991); 10.1063/1.105389 Radiative transitions of photoexcited electrons between Landau levels in nInSb Appl. Phys. Lett. 29, 169 (1976); 10.1063/1.89011 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 160.36.178.25 On: Sun, 21 Dec 2014 14:53:14Observation of apparent inelastic tunneling between Landau ~evels in superlattices T. K. Higman, M. E. Favaro, L. M. Miller, M. A. Emanuel, and J. J. Coleman Coordinated S'cience Laboratory and Compound Semiconductor Microelectronics Laboratory, University of Illinois at Urbana-Champaign, 1406 West Green Street, Urbana, Illinois 61801 (Received 14 December 1988; accepted for publication 13 February 1989) Evidence of the elastic and inelastic components of sequential resonant tunneling in AIAs/GaAs superlattices is presented. Magnetic field data (B paranel to current flow) show that as the energy spacing between the Landau levels in the quantum wells is changed, the corresponding density of states available for tunneling via inelastic scattering paths is changed and thus the magnetic field influences the inelastic portion of the current. Resonant tunneling through AIGaAs superlattices (SLs), observed as early as 1974,1 generally manifests itself as tunneling via a narrow El miniband with multiple nega tive differential resistance regions due to an expanding high field domain breaking the EI miniband coupling, with a re sulting transition to E, -E2 tunneling followed by a relaxa tion to l!.\ in the region of the high field domain. 1,2 In this work we show resonant tunneling results in a GaAsl AlAs superlattice, grown} by metalorganic chemical vapor depo sition (MOCVD), which demonstrates only one negative differential resistance region corresponding to the EI -E2 tunneling path being present over the entire superlattice. This is due to the combined effects of a high r -point conduc Hem-band offset in GaAsl AlAs (1.04 e V AlAs barriers) and the relatively wide (40 A) barriers incorporated in this structure. This results in an EI miniband with width4 I:.l.EI = 0.15 meV, which is narrow compared to the t::..EI 's of 5 and 0.4 me V of Refs. 1 and 2. This narrow miniband is essen tially equivalent to isolated EI states, with no observable E; to E] tunneling current. Hence resonant tunneling occurs only when the applied bias is such that EI levels are aligned with the next E2 level towards the cathode. Current versus magnetic field data (B parallel to current flow, with the re sulting Landau levels in the two-dimensional density of states in the GaAs quantum wells) show that as the magnet ic field and hence the energy spacing between Landau levels is changed the corresponding density of states available for tunneling via inelastic scattering paths involving both small energy acoustic phonons and larger energy optical phonons changes with corresponding structure in the I vs B trace. For this experiment the structure of Fig. l(a) was grown by metalorganic chemical vapor deposition. A 1 p-m GaAs:n + buffer layer was grown on a [100] GaAs:Si (n = 2x 1018 cm-3) substrate, followed by a 17 period 50 A/40 A GaAsl AlAs undoped superlattice (p < 1014 cm -3), 1300 A of nominally undoped GaAs (p < 1014 cm -:1), and a 0.5 pm Alo.2 Gao.8 As:nl cap layer. The layer thicknesses, determined from growth rate parameters obtained from bulk layers, are well controlled and routinely verified by transmission electron microscopy. Mesa diodes (150-pm diam dots) were fabricated by standard wet chemical proce dures and AuGel Agi Au top and substrate ohmic contacts were employed. A negative potential was applied to the top contact such that there was a two-dimensional accumulation layer of electrons at the interface of the superlattice and the undoped region [see Fig. 1 (b) ] . The experimental data were obtained with the device immersed in liquid helium in a su perconducting magnet and an data were taken with a Hewlet Packard 4145B semiconductor parameter analyzer. Several single quantum well tunneling experiments have shown that it is the r -point potential which governs the reso nance levels in AlAs/GaAsl AlAs quantum well resonant tunneling structures, with some speculation that the X point may playa role.4-6 The results shown here support the con tention of r -point profile confinement, as is illustrated in Fig. 1 (b), which corresponds to the electric field distribu tion at resonance (peak current) with V = 7.84 V in the J-V trace of Fig. 2. In this device, the zero bias mini band levels occur at E1 = 154 meV, width I:.l.EI = 0.15 meV and E2 = 573 meV, llE2 = 2.8 meV, with a center to center separa tion of 419 me V. Under bias, at resonance, the Stark shift to E I can be estimated by straightforward means due to the depth of the E\ subband and the resulting long lifetime of the state.7 Using the method of Ref. 7, the Stark shift to EI is 32 meV, resulting in El = 112 meV. A (a) (b) n+substrate 1.04 .v FIG. 1, (a) Conduction band edge diagram ofsllperlattice resonant tunnel ing diode at zero bias showillg the two minibands defined by the r-point profile. (b) Detail of (aJ under bias showing the accumulation layer at point A plus two quantum wells. Note thal the minibands arc 110 longer defined, 1751 Appl. Phys. Lett 54 (18), 1 May 19S9 0003-6951/89/181751-03$01.00 (c) 1989 American Institute of Physics 1751 ~ ••• --.-.-.-•••••••• -•• -•• ~ ••• " .... -.............. H ••.•. ~,T.~.-.;< •• ;O;>;<;--;.; •• ~ ••••• ';" ••• :-• ..-••• :.;.:.:.:.:.:~.:.-:o:.:.;:;~.:.:.~.:.:o:o:.~.:o: ••• -.-•.•.•..• -••. -. -- This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 160.36.178.25 On: Sun, 21 Dec 2014 14:53:141.0 0.8 <' ..IS 0.6 0.2 0.0 T=4.2k 6.5 7.5 Voltage (V) 8.5 PIG. 2. Current vs voltage trace of device pictured in Fig. I with no magnet ic field applied. The oscillations in the NDR region are an artifact of the digitizing measurement system. The true vaHey current is at 0.55 rnA. At resonance ( V = 7.84 V in the trace of Fig. 2) there is a 460 m V drop per superlattice period, which corresponds to a 460 meV separation between E, and E2• Since the Stark shifted E\ = 1 12 meV at resonance, this implies El = 572 meV, essentially equal to the zero bias miniband. Thus, this device shows almost no energy lowering to E2 due to Stark shift in this sample. Since E2 at resonance is strongly coupled to propagating states, the Stark shift cannot be calculated by the usual perturbation methods, and a good correlation to theory cannot be presented. It is also important to note that the sample to sample variation of total applied voltage at peak resonance is a few hundred m V, making the E2-EJ sepa ration somewhat imprecise. In estimating tunneling time out of the E2 level at resonance, the WKB approximation along with a classical attempt frequency yields tunneling times (out of E2 and into propagating statcs) on the order of20 ps, which is long compared to the various intra-and interband processes. In addition to the superlattice states, the electrons inci dent on the first barrier form a two-dimensional accumula tion layer of sheet carrier concentration ns, which forms two subbands, designated E ; and E ~, where the energies E; and E; are proportional to n;/3." At resonance, this sheet con centration is ns = 4.3 X 1012/cm2 in our sample. The tunnel ing rate for this large number of two-dimensional carriers is self-limited by the charge accumulation in the first well, and thus, it is the Ej -E2 process in the SL which limits the cur rent in the device. Under the influence of a magnetic field parallel to the current flow (perpendicular to the plane of the SL layers) the E II (energy due to propagating states parallel to the plane of the SL layers) is quantized into Landau levels such that the total energy of the electron is of the form ElI,m = En + (m + U2)hOJ c' n = 1,2; m = 0,1,2, ... , (1) where n is the subband, In is the Landau level, and ()Jc = en / In is the cyclotron resonance frequency. Since the effective mass in the We term refers to the k Ii' the parabolic effective mass from the bottom of the GaAs r conduction band (m" = 0.067 mo) can be used in the calculation of all of the Landau levels. In this case of We = 1.732 me V IT, and peak resonance, the (m + 1I2)1Uvc Landau levels also line up for each m (see Fig. 3). This means that for purely elastic tun- 1752 Appl. Phys. Lett., Vol. 54, No. 18, 1 May 1989 E2•2 E2,1 ":2,0 ~ E1,2 FIG. 3. Detail of Fig. 1 (a) showing the effects of a magnetic field applied perpendicular to the interfaces (parallel to the current) with the resulting Landau levels in the quantum wells. Also shown is one possible elastic and one possible inelastic tunneling path for 11 r electron. neling in the bulk of superlattice the tunneling would pro ceed from an arbitrary Landau level E l.m to E2,m of the next well. Using this simple model, a device biased at resonance undergoing purely elastic tunneling processes should show no magnetic field dependence for the tunneling current. If inelastic processes are involved, however, several interesting possibilities arise. As magnetic field is increased above a few TesIa, the energy separation between Landau levels (1.732 me V IT) begins to preclude acoustic phonon emission by the tunneling electrons, effectively limiting the tunneling to the elastic components and thereby reducing the total tunneling current as seen in the low magnetic field portion of Fig. 4. As can be seen in Fig. 4, eventually the current recovers and then increases slightly with two peaks in the data, one at 8 T and the other at 12 T, or, in terms of Landau level separa tion h())" = 13.9 and 20.8 meV, respectively. These peaks can be interpreted in the fonowing way: as the magnetic field, and therefore, the Landau level energy separation, is in creased, after the initial current reduction, the energy sepa ration between the levels becomes so large that a small in teger number of Landau levels can equal the energy of an optical phonon involved in an inelastic r -r scattering. The more complex intervalley scatterings would not be account ed for in this way due to the change to three dimensionality which the electrons undergo when scattering into theX or L valleys. The X and L valleys have lower conduction-band offsets than r and an associated lack of quantum confine ment at high electric field. In the situation of r -r inelastic tunneling involving emission of optical phonons (of energy Eph)' however, the density of states available to tunnel into would effectively be increased by including both elastic and inelastic paths. If one interprets the 8 T (hw, = 13.9 meV) Higman et at. 1752 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 160.36.178.25 On: Sun, 21 Dec 2014 14:53:140.955 « 0.945 g "E 0.935 ~ :l () 0.925 0.915 20 40 60 80 100 120 B(kG) FIG. 4. Current vs magnetic field (the magnetic fieid is parallel to current flow) for the resonant tunneling dcvice of Fig. 1 Cal at resonance, V C~ 7.84 V, 1'=4.2 K. peak as 3h(uc = Eph and the 12 T (h(j)c = 20.9 meV) as 2hwc = Epll' the resulting optical phonon would have an en~ ergy of Eph ~42 meV or wave number of 339 cm--l. This energy corresponds quite well to the theoretically predicted and experimentally observed "AlAs like" transverse optical (TO) phonon modes in GaAsl AlAs superlattices9~l2 (for a review of the general properties of phonons in superlattices see Ref. 12). The energies at these modes are predicted and observed to be smaller than the bulk AlAs values (ETo for bulk AlAs = 360 cm-1 or 44.6meV). In this typeoftunnel~ ing process, inelastic scattering paths such as E1,rn to E2,,,, _ 3 at 8 T or EJ,rn to E2,m 2 at 12 T would be allowed, thereby realizing a net increase in tunneling current. When the de vice is biased slightly below resonance (V = 7.70 V), the peaks in the current versus magnetic field are suppressed. It should be noted that the entire span of current observed in Fig. 4 is only about 5% of the total current, indicating that the total inelastic contribution is very small. This result is consistent with other experimental results 13 which show that the elastic portion of the tunneling current is dominant. This experiment can be contrasted with the results of Bockenhoff et al.14 in which the effects of a magnetic field on tunneling from an accumulation layer through a single barrier are studied. In this experiment with a very thin single tunneling barrier the change in the density of states and energy separa tion of the Landau levels in the accumulation layer causes a change in the potential f-l between the bottom of the lowest subband and the bulk Fermi level. This in turn causes a 1753 AppL Phys. Lett., Vol. 54, No. 18, 1 May 1989 change in the potential across the barrier and a correspond~ ing change in the device current. This results in an oscma~ tory behavior in the magnetoresistance with a periodicity corresponding to the Landau levels passing through the Fer mi level (analogous to Shubnikov--de Haas oscillations). With the comparatively wide barrier region of the device in this experiment this effect should be greatly diminished due to the small net change in electric field. In addition, the ex perimental current versus magnetic field trace of Fig. 4 does not show enough oscillations to correspond to this effect. We have shown that the dominant sequential resonant tunneling process in AIAs/GaAs superlattices is ela..~tic tun neling by r valley electrons in subbands defined by the large r -point conduction-band offset in AIAs/GaAs heterostruc~ tures. Magnetic field data have shown that the inelastic tun neling current component can be affected by quantizing and in effect limiting the number of allowed energy transitions of inelastic scattering paths involved in tunneling. The authors gratefully acknowledge K. Hess, J, M. Hig man, and J. P. Leburton for technical discussions. This work was supported by the Joint Services Electronics Program (NOOO14-84-C~0149) and the National Science Foundation (CDR 85-22666). 'L. Esaki and L. L. Chang, Phys. Rev. Lett. 33, 495 (1974). 2K. K. Choi, B. F. Levine, R. J. Malik, J. Walker, and e. G. Bethea, Phys. Rev. B35, 4172 (1987). 'L M. Miller and J. J Coleman, CRC Crit. Rev. Solid State Mater. Sci. 15, 1 (1988). 4T. K. Higman, M. E. Favaro, L. M. Miller, and J. J. Coleman, 15th Inter national Symposium on GOlAs and Related Compounds, September 11- 14, Atlanta, GA, Inst. Phys. Conf. Scries (in press). 'E. E. Mendez, W. I. Wang, E. Calleja, and C. E. T. Goncalves da Silva, AppL l'hys. Lett. 50,1263 (1987). 6A. R. Bonnefoi, R. T. Collins, T. e. McGill, R. D. Burnham, and F. A. Ponce, Appl. Phys. Lett. 46, 285 (1985). 7D. Ahn and S. L Chuang, Phys. Rev. B 34, 9034 (1986). "T. Alldo, A. B. Fowler, and F. Stem, Rev. Mod. Phys. 54, 437 (1982). "S.·K. Yip and Y.-C. Chang, Phys. Rev. B 30,7037 (1984). 10K. Kubota, M. Nakayama, H. Katoh, and N. Sallo, Solid State Commun. 49,157 (1984). lie. Colvard, T. A. Grant, M. V. Klein, R. Merlin, R. Fischer, H. Morko<;, and A. C. Gossard, Phys. Rev. B 31,2080 (1985). 12M. V. Klein, IEEE]. Quantum Electron. QE-22, 1760 (1986). IJE. E. Mendez, E. Calleja, and W. I. Wang, Appl. Phys. Lett. 53, 977 (1988). '4E. Biickenholf, K. v. Klitzing, and K. Ploog, Phys. Rev. B 38, 10120 (1988). Higmanetal. 1753 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 160.36.178.25 On: Sun, 21 Dec 2014 14:53:14

1.338782.pdf

Coherence in heavy fermion compounds: Effect of impurities M. Cyrot Citation: Journal of Applied Physics 61, 3391 (1987); doi: 10.1063/1.338782 View online: http://dx.doi.org/10.1063/1.338782 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/61/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of impurity on electron-phonon interaction in some alloyed heavy fermion (HF) systems AIP Conf. Proc. 1447, 825 (2012); 10.1063/1.4710257 Magnetism and superconductivity in heavyfermion compounds (abstract) J. Appl. Phys. 75, 6747 (1994); 10.1063/1.356871 Twoimpurity Kondo problem: Relevance to heavy fermions (invited) (abstract) J. Appl. Phys. 63, 3902 (1988); 10.1063/1.341161 Hall effect in heavy fermion compounds (abstract) J. Appl. Phys. 61, 4397 (1987); 10.1063/1.338436 Heavy fermions in Kondo lattice compounds (invited) J. Appl. Phys. 57, 3054 (1985); 10.1063/1.335212 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Sat, 29 Nov 2014 05:22:26Coherence in heavy fermion compounds: Effect of impurities M. Cyrot Laboratoire Louis Nee!, CN.R.S., 166X. 38042 Grenoble Cedex, France We present a model for heavy fermion compounds where the active atoms (U or Ce} create virtual bound states at the Fermi level. We describe the appearance of coherence between these virtual bound states at low temperatures by the formation of a band whose width is strongly reduced by correlations. We study the effect of normal impurities on the coherence and show that a small concentration can destroy it. This explains the different behaviors of the resistivity. The coupling ofthis heavy fermion band to the conduction band is responsible for both magnetic and superconducting properties. Impurities affect drastically the magnetic and superconducting states. This is a direct explanation of the welloknown experimental effects of normal impurities in these compounds, It is well known that single impurity models or, more correctly, models with active sites behaving incoherently are adequate to explain the high-temperature properties of heavy fermion compounds. 1.2 This type of model works even at rather low temperatures. At still lower temperatures, the rapid decrease of resistivity is interpreted as an effect due to coherence between active sites. This behavior has been exo plained by a Kondo lattice modeL The mathematical aspece is to transform to a renormalized hybridization model where the df hybridization ~atrix elements are all reduced by a common factor Vkf -+ Vkf = Vkf ( 1 -5), where 5 is the frac tional occupation of the magnetic configuration. Transport properties4 and coherence effect5 have been studied by intro ducing fluctuations in this df hybridization. However, some points remain unclear in this model. First the room-tem perature resistivity is very large, much larger than in many magnetic Kondo lattices, Second, the effect of impurities, which is very important in these compounds, is not well un derstood. Third, the interplay between magnetism and su perconductivity is not understood, as a phonon mechanism has to be invoked i.n that case. Recently we put forward a new approach where the high density of states is due to a virtual bound state band narrowed by correlation. These vir tual bound states explain the high resistivity at room tem perature and they behave coherently at low temperature in order to make a narrow band. This narrow band interacts with a conduction band by a Kondo-like interaction J s S. We show that at low temperature, this interaction was reo sponsible both for magnetism or singlet superconductivity, In this paper we describe the effect of normal impurities on the coherence between the virtual bound states. We show that impurities destroy the coherent scattering. Even at zero temperature, the lattice of active sites gives a contribution to the resistivity greater than the contribution of the impurities themselves. This explains the puzzling problem of a resistiv ity greater than the unitary limit. The mean free path of the electrons at the Fermi level is strongly reduced. As in the proposed model, magnetic and superconducting properties are strongly dependent on the mean free path; this is also an explanation of the drastic effect of a small amount of normal impurities on magnetism and superconductivity. We start from our model of active atoms (Ce or U) which are on a lattice whose spacing is N times that of the conduction electrons. This avoids direct overlap between d or f states of the active atoms and permits the hybridization parameter to hybridize not with a single k vector but with Nk vectors and thus to describe virtual bound states on each Ce or U atoms. This will describe the high-temperature region which is known to be wen explained by a collection ofimpur ities creating a virtual bound state. At low temperatures, since they are on a lattice, they behave coherently and form a band. In a first part we consider the residual resistivity at zero temperature. The key point is that a very small amount of impurities creates a large residual resistivity whose value is much larger than the maximum value that one can calculate in the unitary limit for these impurities. Our point will be that these impurities cannot create by themselves such a re sistivity but will make the lattice of active atoms contribute to the resistivity at zero temperature, These atoms will not only diffuse coherently but also incoherently and give an additional resistivity which will be important because the active atoms are strong scatterers since they create virtual bound state at the Fermi leveL In order to show that point, we consider the Green func tion of the conduction electrons G(k,k ') and expand it as a function of the hybridization parameter V. Let Gu(k) be the Green function of the conduction electrons before hybridi zation. We have G = Go + GoWGo + GoWGoWG o + '''' where W is an effective potential defined by V2 Wkk, = I /tk-.k')R;. iE-Ed For a perfect lattice, this series sums up to the usual result for the hybridized band. There is no umklapp term except near the Brillouin zone of the active atoms. If now we introduce a finite lifetime for the conduction electrons and write Go= l/[E-Ek +i(hlr)), this will give an uncertainty in the conversion of momentum as one can see using the following transfomlation: 3391 J. AppL Phys. 61 (8),15 Aprii 1987 0021-8979/87/083391 -02$02.40 @ 1987 American Institute of Physics 3391 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Sat, 29 Nov 2014 05:22:26where I = v F1"' is the mean free path of the conduction elec trons before hybridization. Now the sum over the active sites does not give a strict conservation of the momentum. The active sites can scatter in another part of the reduced Bril louin zone exactly as the umklapp processes of the phonon do. This new scattering is incoherent. Thus part of the scat tering of the active atoms contributes to the resistivity. This additional scattering, which in principle always exists in a hybridized system, is large only when the active atoms are strong scatterers and lead to a resonant scattering. Each ac tive atom behaves as a scatterer of order: (k;.d31IH V21E -Ed)' The additional resistivity is thus, if C is the concentration of active atoms: Ap~(k}d31l)CApA' where ApA is the resistivity created by 1 at. % of active atoms in the matrix. When the coefficient in front of CAp A is of order unity, it means that the lattice of active atoms be haves completely incoherently. The criterion for such a total incoherence is thus k~d311_1. This gives a rather small value for I except for d). k J.'-]. How ever, since Ap A can be rather large, Ce in CeAl:, will give a contribution of the order7 of7.5 flU cm %; this contribution is important. Now if we consider the behavior as a fUllction of tem perature, we introduce the coupling between the two bands that we derived in our model Hamiltonian: H= IJ;s;'S;, i Ifwe consider a nonmagnetic ground state, such a coupling is well known to give a ]'2 dependence for resistivity. The characteristic temperature Td, Le., R (T) -( T lTd) 2, being related to the width of the narrow bandwidth, is of order Tic in heavy fermion compounds. We ask the question at what temperature the lattice of active atoms can behave as a collection of impurities? An electron with a k vector must diffuse on the reciprocal lattice of the active atom. Let d = Na, the distance between active atoms; we must have 4nk 2dk( lid 3) ~ 1, where k~kp ~ (1la) and dk is the uncertainty on the k vector which is due to thermal diffusion dk=kT Ihvp. We will lose coherence between active atoms at temperature T cO with 1~=(~rhvF ~ -(~rTd' Thus the temperature at which coherence disappears is 1!5th-1I1Oth of the bandwidth which is often taken as the Kondo temperature. 3392 J. Appl. Phys., Vol. 61, No.8, 15 April 1987 For temperature of order (ald)3Td• coherence between active atoms begins to be destroyed. The resistivity is no longer determined by the relaxation rate of the coherent state, but by the relaxation rate of the conduction electron state scattered into the virtual bound states of the active atoms. The resistivity increases rapidly. We can have either a maximum or a slower increase depending on the possibility to the Kondo effect on one impurity to manifest. lfthe coher~ ence is very rapidly destroyed, for instance, helped by impur ities or defects in the materials,s the one impurity Kondo effect can manifest itself and we have a maximum of resistiv ity of order C t:J.PA' If not, no maximum can be observed. We now consider the effect of impurities on the magnet ic and superconducting ground state. In our model both are due to the RKK Y interaction, The effect of a mean free path I on this interaction is well known. Indeed, the coupling between nearest neighbors at distance d is reduced by J = Joe-dI1• Thus the value of TN is exponentially reduced by normal impurities. As we have shown tnat, due to the resonance at the Fermi level, a small concentration c of impurities reduces drastically the mean free, path we must have TN = T7ve --ac. A rapid decrease has been observed in U2Zn17•9 Superconductivity is also strongly reduced as the inter~ action responsible for it depends on the impurities contrary to the usual one. We must also obtain an exponential de crease of superconductivity with the concentration of nOf mal impurities. This is in contrast with the BCS case, where thermodynamics is not affected by them. This is also a much more severe reduction than in the anisotropic BCS case where normal impurities have an effect on To. Such a strong effect has been observed in UBeI3.!O We have benefitted from discussions with Professor P. Noziere and Professor T. M. Rice. 'Por an experimental review, see G. R. Stewart, Rev. Mod. Phys. 56, 755 (1984). LFor a theoretical review, see P. A. Lee, T. M. Rice, J. W. Serene, L. J. Sham, and J. W. Wilkins, Comm. Solid State Phys. 12, 99 (1986). 'c. Lacroix and M. Cyrot, Phys. Rev. B 20, 1969 (1979); T. M. Rice and K. Veda, Phys. Rev. Lett. 55, 995 (1985). 4M. Lavagna, C. Lacroix, and M. Cyrot, J. Appl. Phys. 53, 2055 (1982). 'C. Lacroix, J. Magn. Magn. Mater. 60,145 (1986). OM. Cyrot, Solid State Comrnun. (1986). 7K. Andres,J. E. Grabner, andH. R. Ott, Phys. Rev. Lett. 35,1779 (1975). SA. de Visser, J. C. P. Klaase, M. Van Sprang, J. J. M. Franse, A. Mer ovsky, and T. T. M. Palstra, J. Magn. Magn. Mater. 54-51, 375 (1986). 9J. O. Willis, Z. Fisk, G. R. Stewart, and H. R. Ott, J. Magn. Magn. Mater. 54-51,395 (1986). lOA. L. Giorgi, Z. Fisk, J. O. Willis, G. R. Stewart,and J. 1. Smith, Proceed ings of the i 7th international Conference on Low Temperature Physics (North-Holland, Amsterdam, 1984). p. 229. M. Cyrot 3392 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 69.166.47.134 On: Sat, 29 Nov 2014 05:22:26

1.344041.pdf

On the correlation between highorder bands and some photoluminescence lines in neutronirradiated FZ silicon Lei Zhong, Zhanguo Wang, Shouke Wan, and Lanying Lin Citation: Journal of Applied Physics 66, 3787 (1989); doi: 10.1063/1.344041 View online: http://dx.doi.org/10.1063/1.344041 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Origin of infrared bands in neutron-irradiated silicon J. Appl. Phys. 81, 1645 (1997); 10.1063/1.364020 Multivacancy clusters in neutronirradiated silicon J. Appl. Phys. 78, 6458 (1995); 10.1063/1.360530 ANNEALING OF LIFETIME DAMAGE IN NEUTRONIRRADIATED SILICON Appl. Phys. Lett. 16, 346 (1970); 10.1063/1.1653220 11.6 μ OxygenAssociated Absorption Band in NeutronIrradiated Silicon J. Appl. Phys. 40, 4679 (1969); 10.1063/1.1657265 Thermally Stimulated Conductivity of NeutronIrradiated Silicon J. Appl. Phys. 36, 2968 (1965); 10.1063/1.1714620 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 141.209.100.60 On: Sun, 21 Dec 2014 00:21:20On the correlation between hlgh~order bands and some photoluminescence lines in neutron .. irradiated FZ SmCon lei Zhong, Zhanguo Wang, Shouke Wan, and Lanying Lin Institute of Semiconductors, Chinese Academy of Sciences, Beijing, People's Republic of China (Received 20 March 1989; accepted for publication 5 July 1989) The defects in float-zone silicon irradiated by fast neutron with fluences up to 4.0 X 1018 n/ cmz, followed by various heat treatments, have been studied by low-temperature photoluminescence (PL) and infrared absorption measurement with emphasis upon the high order band (HOB) and its relationship with the commonly observed PL-lines such as II (1.018 eV) and 13( 1.039 eV). It has been shown that band 1124 em-I, unlike the other higher order bands, is considerably broader for the sample annealed at low temperature (for example, 385 ·C) with FWHM as large as 3 meV and is apparently narrowed as the anneal temperature was increased. We have obtained the 13 line and its phonon replicas in the near-infrared absorption measurement, further proving the transition involved in the 13 defect center to be electronic in nature. The combination of luminescence and absorption experiment results demonstrated that the HOB could be wel! developed after PL lines such as II and Ii disappeared completely, or vice versa. PL lines could be observed before the HOB emerged, therefore ruling out the possibility proposed by earlier authors that the HOB could be correlated with some PL lines. INTRODUCTION High-order bands (HOBs) are a series of26 bands in the wavelength region 6-15 pm which arise from defects intro duced by 40-50-MeV electron irradiation or reactor neutron bombardment rangi.ng in fluence from 5 X 1016 to 1019 11/ cm2 (E> 1 MeV) after annealing for about 15 min between about 350 and 600 0c.1-12 In order to observe the entire spec trum of bands, the sample must be cooled down to tempera tures less than 100 K, and the full energy spectrum of light is required to be incident on the sample. HOBs are indepen dent of initial oxygen content, but are strongly dependent upon the boron or phosphorus impurity concentration, since they are not observed in silicon which is chemically doped before irradiation to give a resistivity less than] n em (n type) or 0.1 n em (p type).9 The transitions involved in the HOBs have been proved to be electronic in nature mainly by the so-called dual-beam method, II All the bands could he characterized by applying uniaxial compressive stress with polarized light into eight types, each exhibiting a different response to stress and hence a different symmetry.~ The de fects giving rise to the bands were supposed to be clusters of vacancies andlor interstitials, formed only after the disap pearance of the defects like, for example, divacancies, inter stitials, vacancy-impurity pairs, etc. However, there is much left to do for its identification. <) On the other hand, radiation-induced defects in silicon have been studied using low-temperature photolumines cence (PL) as the probe since the mid-1960s. 13 A large body of line systems have been brought to light roughly spread over a spectral range from near-band gap to almost 2 pm. Sharp peaks have been investigated at 0.97 eV (G), 0.79 eV (C), 1.108 eV (J), 1.018 eV (11)' 1.080 eV ([2)' 1.039 eV (11), and 1.034 eV ([4)' which are the lines reported until now for the silicon subjected to ion-implantation, electron irradiation, or fast neutron bombardment, and then to an annealing at room temperature to about 500 "C. 11-27 Tkachev and Mudryi23 also reported four lines located at 1.0037,0.988,0.7667, and 0.761 eV for the samples irradiat ed by fast neutrons and annealed at about 400 "c. Although both the infrared absorption and photoluminescence study of neutron-irradiation-induced defects have been highly de veloped for about a quarter of this century, little correlation has been made between the two fields. Only recently, Vi dinski, SteckI, and Corelli 12 have selected of all the lines mentioned above, I]> 13, 1.0037,0.988,0.7667. and 0.7610 eV, as the candidates in an attempt to correlate the PL peaks with HOB state. They have abandoned the G-line because the incorporation of carbon in this center has directly been demonstrated in high-resolution PL reexamination ofthe G line by isotope effect of its vibrational sidebands, and anneal ing experiments have indicated that this defect center an neals completely in the range 250-300 "C. In this work, we are going experimentally to examine the hypothesis pro posed by Vidinski and co-workers. 12 EXPERIMENTAL METHOD The samples used here were float-zone silicon with resis tivity of 1000 n em (base boron and base phosphorus) irra diated with 4.0X lOIS n/cm2 fast neutrons. During the irra diation, the sample temperature was held at about 40°C. They were then isochronally annealed in a pure argon-ft.ow ing furnace at temperatures from 100 to 600°C in various steps for 45 min. Photoluminescence was excited with the 5145-A line of an argon-ion laser (Spectra Physics model 263, 40 mW) chopped at 273 Hz. Samples were immersed in liquid heli um. The luminescence was analyzed by a O.25-m grating monochromator (Jobin YVON H25), detected by a liquid nitrogen-cooled germanium detector (North Coast) and 3787 J. Appl. Phys. 66 (8), i 5 October 1989 0021-8979/891203787-05$02.40 (i',) 1989 American Institute of Physics 3787 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 141.209.100.60 On: Sun, 21 Dec 2014 00:21:20amplified in a conventional lock-in technique. The samples for infrared absorption measurement were cut to dimensions lOX20X2 mm3 and double-side polished. They were mounted with clips onto a copper block attached to a heli tran cold finger. An Air Product cryostat was used to attain sample temperature in the range 10 K to room temperature. While near-infrared absorption measurements were per formed on the Perkin Elmer NIR/UV Lambda 9, the medi um infrared absorption were measured on the Nicolet FTIR 170SX. RESULTS AND DISCUSSION In Fig. 1 are shown the photoluminescence spectra re corded at 4.2 K with a Ge detector of the fast-neutron-irra diated samples annealed at different temperature. Curve (a) is the spectrum for the sample as-irradiated without further heat treatment. The spectrum consists of the well-known G (0.970 e V) and II (1.018 e V) lines and their phonon replicas together with intrinsic luminescence. In this work, intrinsic luminescence lines and PL lines due to acceptor- or donor bound excitons and bound multiexciton complexes are not individually identified.2x After the sample was annealed at 280 "C, the I( line grew stronger and the 13 line appeared, as seen in Fig. 1 (b). While the I, line was enhanced with its transverse acoustic phonon replicas overlapping upon the I(line, the G-line was totany eliminated for the sample an· nealed at 300°C, as shown in Fig. 1 by curve (c). We could see from curve (d) that the I( line was completely sup pressed when the annealing temperature was increased to 385°C. Nothing but intrinsic luminescence could be ob· served when the annealing temperature was elevated above 470°C. The dependence of the intensity of the I( and 13 lines with annealing temperature is given in Fig. 2. There exists a reverse effect for the I( line between 240 and 280°C which was first reported by Kirkpatrick, Noonan, and Street· 11 :0 .. a ...J « z C> b (f) ...J a.. C d 132 1.28 L24 1.20 1.16 LI2 WAVELENGTH (OJ "') FIG. 1. PL spectra recorded at 4.2 K for samples Ca) as-irmdiated followed by heat treatment at (b) 280 'C, (e) 300 'C, and Cd) 385 'C, respectively. 3788 J. Appl. Phys., Vol. 66, No.8, i 5 October 1989 man.14 The mechanism for this reverse effect is still un known. The I, line decreased with temperature in the range from 280 to 385°C. We have measured the anneal deactiva tion energy of the 1\ line. The result is 1.34 eV, which could be compared with that of divacancy, 1.2-1.5 eV,2''J although it is currently considered that the II line arises from a five vacancy cluster, (5,20 The I, line decreased but more slowly in the temperature range 300--470°C. Lacking sufficient data, we cannot give the anneal deactivation energy for the I\ line. We see from Fig. 1, in contrast to the near-band-gap PL due to bound excitons at shallow donors or acceptors where coupling of momentum-conserving transverse acoustic (TA) phonons leads to sharp replicas of the principl.e non phonon (NP) line, that the spectra for the deep I, and 13 lines defects are of phonon density-of-state features indica tive of coupling of phonon from the whole phonon spectrum to the defects. The transverse acoustic phonon replicas II (TA) and Ii (2TA) resemble a Poisson distribution with a small Huang-Rhys factor indicating the weaker electron phonon coupling. In comparison of spectrum 13 with II' it can be clearly seen that Huang-Rhys factor is larger for the 13 than for the 1, line. It is the moderate Huang-Rhys factor together with the large stress parameters that led the authors of Ref. 27 to the suggestion that 13 (1.039 eV) center is va cancy cluster. We have measured the near-infrared absorp tion spectra about peak 13' The results are shown in Fig. 3. The phonon replicas 13 (TA) and 13 (2TA) appear on the higher-energy side ofthe zero-phonon line, whereas in emis sion they are on the low-energy side, which further supports the fact that the transition associated with the I, line is elec tronic in nature. Shown in Fig. 4 are spectra recorded at temperatures 10, 80, and 110 K, respectively. It is clearly TEMPERATURE (c) 2 500 400 300 200 100 10 , X X 101 II IIX Xx -X X ::0 ~ X II !III II! !III >-f-100 (f) z: II! X w .... Z --' 10-1 a.. X II 102 I 2.0 3.0 lOOOtT ( K-1, FIG. 2. Dependence of PL lines I, (X) and I, (III) IIpon the isochronal annealing temperature, recorded at 4.2 K. Lei etal. 3788 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 141.209.100.60 On: Sun, 21 Dec 2014 00:21:201.0 1.2 1.4 1.6 WAVELENGTH (PM) FIG. 3. AbsorptiOil spectra about the 1, line recorded at 10 K of NTD-Si samples isochronally annealed at (a) 385 'C, (b) 470 'C, and (c) 550 'C, respectively. Cd) is the spectrum of as-grown sample. demonstrated that the zero-phonon line position has been shifted to 1.036 eV when the sample temperature was in creased to 80 K, which could be caused by the temperature induced band-gap shrinkage. Here the absorption data are again in agreement with photoluminescence, which were re ported by Thewalt, Steiner, and Pankove21 for silicon im planted with In and/or Tl. 13 luminescence was almost quenched at about 110 K. '" ~ <Ii{ III II: 0 !II ID < I!:I w '" ::; c Ii! II: 0 Z Shown in Fig. 5 are the medium infrared spectra reeord- 1.10 1.15 WAVELENGTH a 1.20 (jJ III!) FIG. 4. Absorption spectra of samples neutron irradiated and then an nealed at 385 'C, recorded at (a) 10K, (b) 80K, and (c) 110 K, respective ly. 3789 ed at 10 K for as-grown sample and fast-neutron-irradiated samples annealed at temperatures 385, 470, and 550"c' Only observed in the as-grown sample between the wave number 700-1300 em -I is the peak 1136 cm-I, the wen known interstitial oxygen local vibration absorption. Be cause the oxygen content is very low (about 6X lOIS cm-3 estimated from the absorption coefficient measured at liq uid-nitrogen temperature), the other weaker oxygen vibra tion absorption peaks 3(),ll such as at 1134, 1132, 1130, 1129, 1128, and 1205 em -I have not been distinguished. The as irradiated sample for which the spectrum has not been given exhibited no new absorption peak in the wave-number re gion mentioned above. After being annealed at 385 DC for 45 min, HOB peaks 1124,776,741, and 709 em-1 began to step up. All the high-order bands, a series of 26 bands in total, have been observed apparently in the sample annealed at 550 DC, although the initially appearing bands 1124 and 741 cm -I had tended to decrease in comparison with spectrum for the sample treated at 470 ·C. lt is interesting to note that band 1124 cm-I is consider ably broader for the samples treated at low temperature. The FWHM for the 385 ·C sample was as large as 24 cm I as measured from curve (b) in Fig. 5, so that the preexisting 1136-cm -I peak and the emerging I! 0 I-em -I band appear just like two narrow shoulders on the both sides. However, there is no fine structure in the 1124-cm .. I band. A sample damaged by fast neutron and subsequently annealed at high er temperatures gave rise to narrower FWHM, as seen from UJ U z « III 0:: 0 f/) ro <{ Cl UJ ~ -' <{ ;!; oc 0 z 1205 WAVENUMBER 'CM~' FIG. 5. Infrared absorption spectra recorded at 10 K of (a) as-grown crys tal and NTD-Si samples isochronally annealed at (b) 385 'C, (c) 470 'C, alld (d) 550 'C, respectively. 3789 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 141.209.100.60 On: Sun, 21 Dec 2014 00:21:20the spectra for samples treated, respectively, at 470 and 550°C. It is well known that the sharpness of the absorption lines increases as the measurement temperature is lowered from 100 to 45 K,9 But to our knowledge, this is the first report about the dependence of FWHM of HOB upon the anneal temperature. Random lattice strains may be respon sible for this broadening. This kind of strain would he re leased as the annealing temperature was increased. But this explanation is not satisfying as it seems implausi.ble that ran dom lattice strains could give rise to FWHM as large as 3 meV without significantly changing the electronic proper ties of the center. Besides random strain, this broadening occurs perhaps even from the defects having the same core but different surroundings. It needs further work to eluci date the mechanism fo, the broadening. Since such broaden ing has never been observed for the other HOBs, for exam ple, the initially appearing bands 776, 741, and 709 cm -I of curve (b) in Fig. 5; the reported phenomenon has given band 1124 cm -1 a feature to distinguish from the others. Now we are going to examine the correlation between the HOB and the photoluminescence lines. It is currently considered that the defect giving rise to the HOB has three different charge states within the band gap of silicon. These charge states are T(E" + 0.17 eV), T*(E u + O.42eV), and T**. Here Trepresents the empty defect state, T* the inter mediate defect state after capture of one electron, and T** the defect state (energy location < 0.72 e V below conduc tion band) after capture of two electrons from which the optical transitions of the HOBs take place. Vidinski and co workers 12 have proposed that the state T could be the final state to which the electrons decay and emits PL of one of the four peaks II (1.018 eV), r" (1.039 eV), 1.0037 eV, and 0,988 eV, and that the T* may be the final state to which the electrons decay hence giving rise to PL lines 0,7667 and 0.7610 eV. Photoluminescence peaks 1,0037, 0.988, and 0.7610 e V have never been detected or reported in float-zone silicon, indicating that these radiative recombination centers must involve oxygen which may exist in large quantities in CZ-silicon crystals.2,\ Peak 0.767 e V, currently called as "P" line, has been proven to originate from radiative recombina tion of an exciton bound at an isoelectronic center which comprises thermal donors or oxygen donors.32-,\4 As it is generally accepted that oxygen does not participate in the defect center responsible for HOB,9 it seems impossible to correlate the HOBs and PL lines 1.0037,0,988,0.7610, and 0.767 eV. In fact, we have never observed these PL peaks in our float-zone samples used in this work. Now we are going to focus our attention on the PL lines II (1,018 eV) and 13 (1.039 e V). In comparison of Fig. 2 with Fig. 5, it can be dearly seen that I) has almost disappeared for the sample annealed at 385 "C, while HOB just emerged. So, we could rule out the possibility that II could be identified with HOB center. 13 seems to be the most promising candidate for the correlation with HOB state. 13 (1.039 eV) coexists with HOB in samples annealed at 385 or 470°C, as shown by comparing Fig. 2 or 3 with Fig. 5. But, unfortunately, the harmony was interrupted by the annealing at 550°C. Al though HOB have been observed apparently in the sample annealed at 550·C as shown in Fig, 5, the 13 (1.039 eV) line 3790 J. AppL Phys., Vol. 66, No.8, 15 October 19B9 disappeared as demonstrated by both PL and NIR absorp tion measurement. It should be pointed out that the NIR spectrum curve (c) in Fig. 3 and the MIR spectrum curve (d) in Fig. 5 were measured on the same sample. As it is unlikely that there are other processes such as Auger nonra diative recombination to come out suddenly and dominate the radiative recombination process when the sample an nealed at 550 DC, it is implausible to consider the state T (Ec + 0.17 eV) as the final state to which the electron de cays and emits PL [3' On the other hand, from curves (a) and (b) in Fig. 1, we could see that PL line II existed in as irradiated sample and I; could also be observed for the sam ple treated at 260°C. However, HOB could be detected only when the annealing temperature was above 300 °C. In short, the analysis of impurity (oxygen) dependence and anneal behavior lead us to object to the suggestion to correlate HOB and PL peaks II' /3' 1.0037,0.988,0.7667, and 0.7610 eV, although the relative energy position permits it. As It (l.018 eV) and 13 (1.039 eV) are generally consid ered to arise from five-vacancy or vacancy cluster, our find ings may be helpful to the identification of HOB. Our con clusion was also supported by the results of uniaxial stress on the II and 13 lines obtained by Minaev, Mudryi, and Tka chev20 and Ciechanowska, Gordon, and Lightowlers,27 re spectiVely. According to Corelli et al.'s work,8,9 the stress data of HOB can be fit in terms of treating the observed pattern as belonging to two defect symmetry groups, (1) tetragonal and (2) rhombic, However, the luminescence centers responsible for the II zero-phonon line have a tri gonal symmetry. 20 On the other hand, although the 13 center is a defect with tetragonal symmetry,27 band 1171 cm -I, the only one HOB having the Td symmetry,8.') did not emerge in sample annealed at 470°C as seen clearly in Fig. 5, when 13 has already disappeared, as shown in Fig. 2, 'M. E. Rolli and], C. Carelli, J. AppL Phys. 47, 37 (1976). 2R. C. Newman and D. H. J. Totterdell, J. Phys. C 8,3944 (1975). 'V. N. Mordkovich, S. P. Solovev, E. M. Temper, and V. A. Kharchenko, SOy. Phys. Semicond. 8, 666 (1974). ·Y. I'. Koval, V. N. Mordkovich, E. M. Temper, and V. A. Kharchenko, Sov. Phys. Semicond. 6, 1152 (1973). 'c. S. Chen, R. V. Lowell, and J. C. Corelli, Radiation Damage Defects in Semiconductors (Institute of Physics, London, 1973), p. 210 OM. T. Lappo and V. D. Tkachev, SOy. Phys. Semicond. 5, 1141 (1972). 7J. C. Corelli. R. C. Young, and C. S. Chen, IEEE Trans. Nne!. Sci. NS-17, 126 (1970). 8J. C. Carelli, D. Mills, R. Gotiver, D. Cuddeback, Y. Lee, and j, W. Cor bett, in Radiation Effects ill Semiconductors, 1976, edited by N. B. Urli and J. W. Corbett (Institute of .Physics, London, 1977), p. 215. oJ. C. Corelli and J. W. Corbett, in Neutran Transmutation Doped Silicon, edited by J. Guldberg (Plenum, New York, 1981), p. 35. 10K. Sahu Abha, T. R. Reddy, and A. V. R. Warrier, J. App!. Phys. 54, 706 (1983). 1'M. T. Mitchel!, J. C. Corelli, and J. W. Corbett, in Defects and Radiation Effects in Semiconductors, edited by J. H. Albany (Institute of Physics, Bristol, 1979), p. 317. I2W. J. Vidinski, A. J. Steckl, and J. C. Corelli, J. AppL Phys. 54, 4097 (1983). 13R. Sauer and J. Weber, Physica 1168, 195 (1983). Lei etal. 3790 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 141.209.100.60 On: Sun, 21 Dec 2014 00:21:20"c. G. Kirkpatrick, J. R. Noonan, and B. G. Streetman, Radia.t. Elf. 30, 97 (1976). 15c. G. Kirkpatrick, D. R. Myers, and R O. Streetman, Radial. Eir. 31, ! 75 (1977). 16c. E. Jones, E. S. Johnson, W. D. Compton, 1. R. Noonan, and B. G. StreetmanJ AppL Phys. 44, 5402 (1973). I?E. S. Johnson, W. D. Compton, J. R. Noonan, and B. G. Streetman, J. App!. Phys. 44, 5411 (1973). 181. R. Noollan, C. G. Kirkpatrick, and B. G. Streetman, RadiaL Elf. 21, 225 (1974). 19 A. V. Mudryi and Y. Khnevich, SOy. Phys. Semicond. 8, 875 (1975). 2°N. S. Mineav, A. V. Mudryi, and V. D. Thachev, Phys. Status Solidi B 108, K89 (1981). 21M. L W. Thewalt, T. Steiner, and J. I.Pankovc, J. Appl. l'hys. 57, 498 (1985). 220. F. Swenson, T. E. Luke, and R. L Hellghold, 1. App!. Phys. 54, 6329 (l9tB). 21V. D. Thachev and A. V. Mudryi, in Radiation Effects in Semiconductors, edited by N. B. Urli and J. W. Corbett Cillstitute of Physics, London, 3791 J. Appl. Phys., Vol. 66, No.8, 15 October 19139 1977), p. 231. 24M. S. Skolnick, A. G. Cullis, and H. C. Webber, J. Lumin. 24/25, 39 (1981). 2'K. P. O'Donnell, K. M. Lee, and G. D. Watkins, Physica 116B, 258 (1983). 2"J. Weber and M. Singh, App!. Phys. Lett. 49,1617 (1986). 27Z. Ciechanowska, D, Gordon, and E. C. Lightowlers, Solid State Com mun, 49, 427 (1984). 2"M, L. W. Thewalt, in Excitons edited by E. I. Rashba and M. R, Sturge (North-Holland, Amsterdam, 1982), Vol. 2, p. 393, 29L, S. Smimov, A Survey afSemiconductor Radiation Technology (MIR, Moscow, 1983), p. 23 . .loB. Pajot, J. Phys. Chern. Solids 28,73 (1967). 31R, C. Newman, Infrared Studies a/Cry-defect (Taylor and Francis, Lon- don, 1973), p. 91. 32N. Magnea, A. Lazrak, and J. t Pautrat, AppJ. Phys, Lett. 45, 60 (1984). "'J. Wagner, M. Domen, and R. Sauer, Phys. Rev. B 31,5561 (1985). 34J. Weber and R. Saller, Defects in Semiconductors 11, Vol. 14 of Materials Research Symposia (North-Holland, New Y ork,1983), p. 165. Lei etal. 3791 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 141.209.100.60 On: Sun, 21 Dec 2014 00:21:20

1.341728.pdf

Spacecharge behavior near implanted contacts on infrared detectors Nancy M. Haegel Citation: Journal of Applied Physics 64, 2153 (1988); doi: 10.1063/1.341728 View online: http://dx.doi.org/10.1063/1.341728 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Generalized space-charge limited current and virtual cathode behaviors in one-dimensional drift space Phys. Plasmas 20, 103122 (2013); 10.1063/1.4826590 Spacecharge effects in photovoltaic double barrier quantum well infrared detectors Appl. Phys. Lett. 63, 782 (1993); 10.1063/1.109906 Transient electric field and spacecharge behavior for unipolar ion conduction J. Appl. Phys. 45, 2432 (1974); 10.1063/1.1663610 Increasing SpaceCharge Waves J. Appl. Phys. 20, 1060 (1949); 10.1063/1.1698275 Spacecharge wave Phys. Today 2, 20 (1949); 10.1063/1.3066394 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.234.114.177 On: Tue, 25 Nov 2014 05:06:15Space .. charge behavior near implanted contacts on infrared detectors Nancy M. Haegei Department 0/ Materials Science and Engineering, University a/California. Los Angeles. Los Angeles, California 90024 (Received 12 October 1987; accepted for publication 26 April 1988) A phenomenological model for the steady-state distribution of electric field and potential near an implanted contact on a low-temperature infrared detector is presented. The model assumes a linear distribution of trapped space charge and calculates the resulting electric field gradient and potential barrier that are established. The change in these distributions for changes in incident photon flux, applied electric field, or compensating donor concentration is determined. An increase in photon flux under constant applied electric field results in no change in the space-charge distribution for low-field situations. A change in field for constant flux conditions, by contrast, results in a change in potential barrier width and height. These results indicate that transient phenomena should be strongly dependent on operating conditions and may explain why a wide range of apparently unrelated transient phenomena are observed in practice. INTRODUCTION Infrared detectors that are based on extrinsic semicon ductor materials and operate at low temperatures are used in both astronomical and spectroscopic investigations."] The understanding of transient effects as a result of abrupt changes in either incident photon flux or applied bias in these devices has traditionally been one of the most challenging and frustrating tasks in the field. A large number and wide variety of effects are observed, although rarely fully explored or documented. This is not surprising, when Olle considers that infrared (IR) detector operation encompasses a wide parameter space including photon flux, electric field or cur rent bias, temperature, materials parameters, and device configuration. Contact design and fabrication techniques have also been suspected as playing an important role in determining transient behavior. Anomalous behaviors that have been reported in IR de tectors include very slow transient response to changes in photon flux levels, l-3 complex instabilities at high applied electric fields,4.5 and an overshoot behavior of the current, which is commonly known as the "hook effect." Haegel and Haller3 have documented a transient response on the time scale of seconds in Ge:Be and Ge:Zn photoconductors. Be cause this response time is orders of magnitude longer than would be expected from transport behavior in the bulk, there have been attempts to explain such phenomena by consider ing the dynamics of trapped space charge near the injecting contact.6 This requires a good model for the contact and near-contact region. The modeling of IR-detector behavior must take into account several factors that differentiate the behavior of im planted contacts on high-resistivity semiconductors at low temperature from that of metal-semiconductor contacts or implanted contacts on low-resistivity semiconductors at room temperature. Infrared detectors, particularly those de signed for detection of far-IR wavelengths (20-200 pm), must be operated at low temperatures to reduce the noise associated with thermal generation of carriers and, in the case of extrinsic detectors, to assure that a large fraction of the dopant atoms are in the neutral, and therefore, optically active state. Applied electric fields must be kept low to pre vent impact ionization of dopant atoms, and the devices of ten operate under very low photon fluxes, resulting in low free-carrier concentrations and resistivities often in excess of 1010 n cm. This means that much of the modeling of room temperature devices is not applicable to IR detectors. Modeling of transient response offar-infrared detectors has proceeded through several stages. A simple model based on a space-charge neutral device and considering only bulk phenomena leads to the conclusion that the transient re sponse under low photon background should be determined by the free-carrier lifetime, which is usually a function ofthe compensating impurity concentration.2 Perturbations in this case for varying levels of photon excitation and various degrees of trapping states in the material have also been ex plored.7 At low temperatures, the dielectric relaxation time (the time required to neutralize trapped space charge and maintain charge neutrality in the bulk) can become longer than the free-carrier lifetime and can therefore playa role in transient behavior. The dielectric relaxation phenomena and model was identified by Williams in Ge:Hg detectors~-10 and is observed in cases where the electric field is high enough that the drift length of free carriers is comparable to the detector length. There is a growing consensus that transient behavior in photoconductors cannot be fully understood without an im proved model for the near-contact region, which is a region in which space-charge neutrality is not maintained. The space-charge distribution that is set up is due to the diffusion of holes or electrons from a heavily doped (usually implant ed) contact region. This space charge results in field gradi ents and a potential barrier to free-carrier flow, which is self consistent with the current passing through the device. A recent model that includes this space-charge region6 shows that slow transient response may be associated with space charge rearrangement near the contact. This model, how ever, also uses a simplified contact picture which eliminates consideration of the region in which diffusion current, nega tive field gradients, and the potential maximum of the energy 2153 J. Appl. Phys. 64 (4), 15 August 1988 0021-S979 fSS 1162153-07$02.40 © 1988 American Institute of Physics 2153 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.234.114.177 On: Tue, 25 Nov 2014 05:06:15barrier for free carriers exist. The purpose of this paper is to present an analytical model of the near-contact region that includes the complete contact barrier. The model allows for the calculation of the steady-state field and potential barrier distribution, assum ing a linear distribution with variable spatial extent for the space charge. This model can then be used to determine the change in near-contact field and potential for changes of photon flux or bias under conditions that simulate the actual use of these devices. The model contributes a phenomeno logical picture of the contact which allows for a clearer phys ical understanding of the important processes occurring in this region for low-temperature implanted contacts on oh mic devices. BACKGROUND A schematic diagram of a p-type photoconductor with implanted contacts is shown in Fig. 1. It is common to depo sit a metal layer on top of the implanted layer to facilitate the making of electrical contact to the rest of the circuit, al though this is not done in the case of arrays where planar detectors are illuminated from the top through the implant ed layer. The goal is to produce an ohmic contact, generally defined as a contact that does not itself affect device perfor mance and can supply whatever current is required with an associated voltage drop that is small with respect to the vol tage drop across the rest of the device. One way to achieve this is to form a heavily doped surface layer in the semicon ductor. This bends the bands (Fig. 2) so that carriers can tunnel through the metal-semiconductor barrier. As a re sult, the ability of the contact to supply current is indepen dent of the barrier height and independent of temperature. Most of the data on ohmic contacts have been obtained for room-temperature contact to Si and HI-V and II-VI de vices, where either a low barrier height or a tunneling con tact can lead to ohmic characteristics. For cooled IR detec tors, however, the available thermal energy is very small (0.36 meV at 4.2 K), so the contact must be a tunneling contact. Thermionic emission over a metal-semiconductor barrier could never supply sufficient current at such low temperatures and is not the type of process that will concern us here. Instead, the heavily doped layer allows carriers to --PHOTON FLUX METALLIZATION • .1 !lllllilllll~ T'50o-aoooA \ B implanted 0";0,, Ge:Ga I B Implanted ae:Ga 1.. iiiiiiliiiiiiii Tl000A i I\'IETALL!ZATiON FIG. l. Schematic diagram of a far-IR photoconductor fabricated from bulk p-type Ge, 2154 J. Appl. Phys., Vol. 64, No.4, 15 August 1988 \...------EC METAL iMPLANTED SULK REGiON MATERIAL FIG. 2. Effect of a heavily doped near-surface layer on band bending at a metal-semiconductor interface. move from the metal to the semiconductor by passing through rather than over the metal-semiconductor barrier. The impurity concentration in the implanted layer must be high enough to exceed the Mott transition, i.e., the Fermi level must lie within the valence (conduction) band at aU temperatures for an ohmic contact to a p-type (n-type) de vice. This will lead to an athermal tunneling process that produces a reservoir of free holes in the semiconductor, which can be replenished from the external circuit_ Photoconductors may be operated in either a constant current or constant-voltage mode. Although both modes of operation are feasible and, at least to first order, analogous, it is most common to operate the devices at a constant bias voltage and measure the change of current or the integrated current with time. The detector is operated at sufficiently low temperature that bulk resistivity is dominated in large part by the photon flux incident upon the device. Variations in the intensity of incident radiation result in changes of re sistivity, measured as a resulting change of either current or field. We will concern ourselves here with detectors operated under low background conditions, Le., conditions in which the incident photon flux is not sufficient to deplete any sig nificant number of the neutral dopant species. If one consid ers a typical doping concentration of 10 15 cm -3 in a Si or Ge extrinsic detector and an average free-carrier lifetime of 10-6-10-9 s, then afiux ratdn excess of 102°_1023 photons/ s would be required to ionize 10% of the dopants. At an arbitrarily chosen wavelength of 20 pm, this corresponds to a power of more than 1 W on the device. Astronomy-related applications and most all laboratory and detection opera tions fall wen below this limit. In the case of detectors for astronomy applications, it is more common to fall in the other limit of attempting to detect the smallest signals possi ble. A phenomenological model for the near-contact region of a metal-semiconductor contact was published in 1958 by Lampert and Rose, II and extensive modeling of both ther mionic and diffusion models has been done since that time. 12 Often attempts are made to apply this analysis to the tran sient behavior of implanted contacts at low temperatures as well. However, there are basic differences in the two cases. For the low-temperature implanted contact, one assumes that the tunneling contact from the metal to the semiconduc tor is adequate at an temperatures and provides no barrier, Nancy M. Haegsl 2154 ..................... ; ........ -.. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.234.114.177 On: Tue, 25 Nov 2014 05:06:15The space-charge region and the barrier that must be consid ered are between the p + -implanted layer and the non-degen erately doped bulk p-type material. This is in contrast to the metal-semiconductor case where one considers the bound ary between two different materials, and the effect of differ ences in work function and electron affinity playa role. The result is that, in the metal-semiconductor case, a depletion region of free carriers establishes itselfin the semiconductor. For the implanted contact, as we shall see, diffusion of free carriers from the implanted region leads to an enhancement of the majority-carrier type in the bulk material near the contact. This basic difference between the two cases can lead to very different contact properties. PHYSICS OF THE NEAR~CONT ACT REGION We will consider the physics of the near-contact region near the interface between the implanted layer and the bulk material. For a typical detector application (B implant to p type Ge) this process leads to the production of a O.l-pm region with a concentration of substitutional B of approxi mately 1019_1020 cm-3• Because this exceeds the Mott tran sition concentration for Ge, this region is metallic in nature and the Fermi level lies in the valence band. Consider now the effect of the holes that diffuse from the heavily doped region into the bulk region, which has very few holes at low temperature. They bring a positive charge into a previously neutral region, regardless of whether they remain as free carriers or are trapped by an ionized impurity. In this case, because of the low temperature, they will be trapped by the negatively charged acceptors and establish a positive space charge. This space charge causes an electric field to exist, which then produces a drift component for free holes back toward the heavily doped region, exactly counteracting the diffusion component at equilibrium and at zero current through the photoconductor. Figure 3 shows a schematic representation of the field and potential that are produced, assuming a linear distribu tion for the space charge. This simplifying assumption will be used throughout this paper. Exact calculation of the shape of the space-charge distribution requires the solution of the complete set of differential equations governing trans port, including the diffusion terms. This is a very difficult time-consuming calculation. Since the goal here is to deter mine the change in steady-state distributions for changing flux and field, rather than the exact shape of the distribu tions, this assumption seems acceptable, In addition, the modeling results of Westervelt and Teitsworth,6 which yielded the steady-state distributions by neglecting the diffu sion term in the current equation and then matching a diffu sion solution near the boundary, can be well approximated by a linear distribution. The diagrams in Fig. 3 show the transition from the implanted material to the bulk material. The space charge sets up a field gradient and a potential barrier. With the application of a bias field, one sees that the electric field distribution contains a negative and a positive region, and a point exists where E = 0 and purely diffusive current flow exists. The potential step becomes a potential maximum, which occurs at the point whereE = O. The goal of the mod- 2155 J. AppL Phys" Vol, 64, No.4, 15 August 1988 ---t----=:.~- " ----'-t-'~--- " (a) (b) FIG. 3. Space charge, electric field, potential, and band diagram as a func tion of distance ill the near-contact region (a) without applied external bias; (b) with applied external bias. eling is to calculate these distributions and determine the barrier height, the width of the space-charge region and the barrier, and finally the effect of changing photon flux or ap plied field. It should be noted that the model used by Wester velt and Teitsworth6 has neglected the negative-drift compo nent region and has considered the space-charge effects beginning only from the point where E = O. This may be justified, both physically and in terms of computational sim plification, but it leaves out a very important aspect of the near-contact transport, namely the point of purely diffusive current flow and the existence of a maximum in the potential distribution. It is this aspect of the contact behavior that will be clarified and included here. MODELING OF THE NEAR-CONTACT FIELD AND POTENTIAL As a prototype system, we will consider the ca..<;e of a p type semiconductor which contains an intentionally intro duced shallow level dopant (such as Ga in Ge or Si) and a much smaller concentration of residual donor impurities, N D' To calculate the distribution offield and potential in the near-contact region, we assume a linear distribution of space charge that has a value of N D at the point x = 0 and de creases linearly to 0 at some value Xmax' which is allowed to vary. Physically this means that the holes which diffuse from the implanted region will be trapped by the negatively charged ionized acceptors. These exist as a result of compen sation, the process in which the donors, the minority impuri ty species in this case, give up their electrons to the acceptors and are therefore funy ionized without contributing free electrons to the conduction band. The space charge exists because the holes neutralize the ionized acceptors, upsetting the space-charge balance between ionized acceptors and do- Nancy M. Haegel 2155 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.234.114.177 On: Tue, 25 Nov 2014 05:06:15nors. Because of the lqw temperature of device operation, we can assume that these holes are trapped and therefore repre sent a localized space charge. Making the assumption of the linear distribution, the space-charge density, often indicated as p(x), is given by p(x) =ND(1-x!xmax)' (1) where x is the distance from the implanted-region-bulk-re glon interface and Xmax is the point in the bulk where the space charge goes to zero (i.e., the end of the distribution of trapped charge). From the work of Westervelt and Teits worth6 one sees that this distance is on the order of 10-4_ 10-5 cm for typical operating parameters of Ge:Ga photo conductors. Using Poisson's equation, dE e -=-[p(x)], (2) dX €€o where e is the electron charge and €€o is the dielectric con stant, one can calculate the electric field E(x), E(x) = -e!uoND(x-x2!2xmax) +Emax + Ebias' (3) The quantity Emax + Ebias is the constant of integration; Emax is the field that results from diffusion at the implanted region-bulk-region interface, and Ebias is the externally ap plied field. This must meet two criteria: (1) E(O) = Emsx + Ebias and (2) E(xmax) = Ebias• This means that Emax can be evaluated and the final expression written as E(x) = --ND x---+ Ebias +----. e ( X2) eND (Xmax ) €€o 2xmax €Eo 2 (4) Integration of the field distribution leads to the expression for the potentia]; Vex) = _~ND(X2 _~) €Eo 2 6xmax [eND (Xmax)] + Ebias +----x + Vc' EEl) 2 (5) The boundary condition here is V(O) = 0, and the expres sion for V(x) can be written simply as [eND (Xmax)] + Ebia• + ---- X. EEo 2 (6) Of the three variables in the expression (ND, Ebias, and Xmax ), two, N D and Ebias, are determined by the detector material and the operating bias condition. Therefore, we must find a self-consistent way to determine Xmax' i.e., to determine the degree to which the space-charge region ex tends into the device. Additional constraints on the system can be obtained by considering the requirement that the current flow, under steady-state operation, must be constant throughout the en tire device. The current density consists of three components J(x,!) = e!1P(x,t)E(x,t) aE(x,t) D ap(x,t) + €E() at -p e -'-":'a-x'--'-- (7) 2155 J. Appl. Phys., Vol. 54, No.4, 15 August 1988 which simplifies in the steady state J(x) = e!1P(x)E(x,t) -Dpe a~~) (8) to include only the drift and diffusion terms. J(x,t) is the current density as a function of position and time, p is the hole mobility, and Dp is the diffusion constant for holes. At the point in the near-contact region where the elec tric field is equal to zero, the current must be a pure diffusion current. This means that a value for dp! dx can be calculated at this point. Also, an expression for x(E = 0) can be uniquely determined as a function of Xmax by solving Eq. (4) for the case of E = O. Finally, a general expression for the value of P. the free-hole concentration, can be written in terms of the optical generation rate y, recombination coeffi cient r, and the concentration of neutral (A) and ionized (A *) acceptors. These three equations are given below: dp = -J = -J at E = 0, (9) dx Dpe pkT x = Xmax -~ -2Ebi •• Et:r,xmax leND at E = 0, (10) p=y(A-A*)= yA (A>A"'). rA!Ie rA '" (11 ) Consider the physical meaning of Eq. (11). It simply states that the steady-state hole concentration is a balance between the optical generation process and the recombina tion into ionized acceptor states. We neglect any thermal generation contribution because of the low temperature. Also, the equation is simplified by the fact that concentra tion of ionized acceptors, A "', is usually less than 1 % of the neutral acceptor concentration A. In the bulk, p is a constant as a function of distance for a uniformly doped detector. In the near-contact region, however, p is strongly dependent on position because the concentration of ionized acceptors de creases as one approaches the contact, due to the effect of the trapped holes, which has already been discussed. The hole concentration then is a function ofxbecauseA '" is a function of x, i.e., p{x) = yA IrND (x!xmo.x) = yAxmaJrNDx. (12) Using this as an estimate for p(x), the slope function dp!dx can also be determined by simply taking the derivative dp yAxmax dx = -rNDx2' (13) Now the final approach can be outlined. We wish to solve for Xmax • Equation ( 13) gives an expression for dp/ dx as a function of x and Xmax' Since at the point E = 0 we also have an expression for Xmax as a function of x, these can be combined to give dp! dx, which is determined by the current, as a fuction of x. We solve for the appropriate x that satisfies both E = 0 and dp!dx = J !(;.tkT) and then solve for Xmax from the relationship between x and Xmax at E = O. This value of Xmax is then used to calculate the field and potential distributions of Eqs. (4) and (6). RESULTS A series offield and potential distributions as (i) a func tion of bias for a fixed detector doping and incident flux rate Nancy M. Hasgel 2156 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.234.114.177 On: Tue, 25 Nov 2014 05:06:15(accomplished in practice by an increase of the applied vol tage on the same detector) and (ii) a function of minority donor concentration for a fixed bias and incident fiux rate (accomplished in practice by considering different detector materials under the same operating conditions) are shown in Figs, 4 and 5, The fixed parameter values used are typical of Ge:Ga bulk detectors under a relatively low photon back ground and are summarized in Table I. The current (J), generation coefficient (r), electric field bias (E b ), and do nor concentration (Nn) are the variables of interest. The parameters selected, however, must satisfy the foHowing expression: (14) to assure self-consistent agreement for bulk behavior, The results show that an increase in bias across the de tector results in a change in the space-charge region and a displacement of the point of pure diffusion current (E = 0, -E ~ -Q ...I W u: 0 a: b W ...! W :; E -25 20 15 10 5 (a) 0 -5 0 2 3 4 5 3.-----------------------~ (b) 2 3 4 5 FIG, 4, (a) Electric field and (b) potential distribution as a function of applied electric field, Y= 5x 10-5 a-' and ND = LOX 10.2 cm-,3, 0 Bias 1: -LO V/ern, J=4,8XIO--1O A/em2,. Bias 2: -1.5 V/cm, J = 7,2X 10-,10 A/em2, III Bias 3: -2,0 V/em,J = 9,6X 10--.0 A/cm2, 0 Bias 4: -2,5V/cm,J= L2XlO-9 A/cm', 2157 J, Appl. Phys .• Vol. 64. No.4. 15 August 1988 16 13 -E (.) >; 10 -Q 7 ....I W ii: (.) 4 a: I-(a) (J W ...! W 2 4 6 8 10 DISTANCE (em x 10·4) 2.-----------------------~ -1+---~r_~_r--~,_~--r_~~ o 2 4 6 8 10 DISTANCE (em x 10.4) FIG. 5, (a) Electric field and (b) potential distribution as a function of compensating donor concentration, Eb = -2,0 V /crn and y = 5 X 10-5 S -t, DDonod: 1 X IOncm··3,] = 9.6X 10-' lO A/cm2• tDonor2:7X 10" em}, J = lAX 10-9 A/cm2, III Donor 3: 4X 10" cm-'. J = 2AX 10-9 A/cm', 0 Donor 4: I X 10" em-3• J = 9,6x 10-.9 A/cm2, v = V,nax) toward the implanted region, This means that the requirement for a larger diffusion current at this point is satisfied by moving into a region of higher hole concentra tion where the value of dp/dx is also higher. In a similar fashion, increasing donor concentration reduces the pene tration of the space-charge region by increasing the number of trapping centers for the positive space charge. The poten tial maximum moves toward the implanted region and be comes larger, reflecting the fact that more compensating do- TABLE 1. Modeling parameters. Temperature Acceptor concentration Dielectric constant Hole mobility Recombination coefficient T=3K A = 2x 10'" ern E = 16 ;. = 3x 10' cm1/V s f= 1 X 10-" cm"/s Nancy M, Haegel 2157 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.234.114.177 On: Tue, 25 Nov 2014 05:06:15nors results in a smaller current flow for a fixed bias and flux due to the shortened free-hole lifetime. If we consider now the case of increasing the incident photon flux on a detector at fixed bias, a different situation arises. Increasing the flux increases the hole concentration throughout the entire sample, assuming uniform illumina tion of the entire sample including the contact region. This results in an increase of both the diffusion and drift compo nents of the current. This is illustrated in Fig. 6, where the flux is increased by a factor of 10. One sees by considering Eq. (8) that the increased hole concentration leads to the increase of current and that, to first order, no change in the space charge or field distribution occurs. This is independent of the magnitude of the flux change, as long as the absolute photon flux is low enough to remain in the low background regime, i.e., it does not cause any significant ionization of the dopant levels. In considering this last case it is important to point out that the increase of free holes near the contact is a result of the increased lifetime in this region due to the decrease in the number of ionized acceptors which act as recombination centers. If the detector is operated at sufficiently low tem perature, then the excess holes which enter the bulk are trapped at the ionized shallow acceptor levels. In the absence of a photon flux, this prevents further diffusion and sets up the steady-state situation. Now, when photons are absorbed in the contact region as wen as in the bulk, the hole concen tration distribution reflects the distribution ofionized accep tors. Therefore. there is an increase in free-hole concentra tion that is proportional to the increase flux in the space-charge region as wen as in the bulk. This conclusion is no longer valid only at a point very dose to the implanted layer where the model breaks down because the hole concen tration cannot be infinite, but is in fact limited by the concen tration of the implanted region. Therefore the two curves must actually coincide in the limit of the contact. This occurs so very close to the interface however, that it does not affect results if they are calculated at equidistant points over the space-charge region. DISCUSSION The results presented give a self-consistent picture of the physical situation in the near-contact region and provide es timates of the width of the space-charge region and the field and potential distributions which result. In applying these results to the issue of transient behavior in far-IR detectors, one can see that the transient behavior due to a change in photon flux should be fundamentally different than to a change in electric field bias. These results are in agreement with constant-current modeling, which does show space charge rearrangement and resulting long transient behavior as a result of changing electric field due to changing incident flux. Standard operating procedure for detectors, however, is more often a constant electric field condition, in which case the model would predict that the same transient phe nomena due to near-contact space-charge rearrangement should not exist. These results may explain why the characterization of transient phenomena has been difficult. It is difficult in any 2156 J. Appl. Phys., Vol. 64, No.4, 15 August 1988 -"'l E U -Z 0 ~ a: I-:z w (,) z 0 0 W ....J 0 x: -E u -. :> '-" Q -1 W Ii: 0 iX I-(.) W ...I w >' E -103 (a) 102 101 10° 10-1 0 2 3 4 5 DISTANCE (em x 10.4) 25 15 (b) 5 -5 0 2 3 4 5 DISTANCE (em x 10-4) 3~-----------------------' (c) 3 4 5 DISTANCE (em x 10-4) FIG. 6. (a) Hole distribution (X 10-4) (b) electric field and (c) potentia! distribution as a function afflux rate. Eb = -1.0 V /crn and N D = 1 X 10'2 cm-3_XFlux 1: y=5XlO-s S·-I, J=4.8XlO- w A/cm2• 0 Flux 2: Y= 5x 10-4 s' " J = 4.8X 10--9 A/em2 • Nancy M. Haegel 2158 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.234.114.177 On: Tue, 25 Nov 2014 05:06:15case to prove that transient phenomena are due to space~ charge readjustment, and these results would indicate that the same detector operated at constant bias or at constant current may have different transient responses if space charge rearrangement was playing an evident role in the lat~ ter case. Experimentally it is observed that not aU photocon ductors display long transients on the time scale predicted by the space-charge rearrangement model, and this would be consistent with this result which says that, for an ideal con tact, no major space-charge rearrangement occurs under constant field bias. For constant~current bias, or, perhaps even more commonly, for constant~field bias with a poor contact (i.e., nonuniform, not sufficiently doped, not suffi ciently illuminated), contact-related transient phenomena may be observed. This would be consistent with the fact that many anamolous transient phenomena have disappeared as contact fabrication procedures have improved. CONCLUSION The electric field and potential distribution near an im planted contact to a low-temperature photoconductor have been calculated, using an analytical model which assumes a linear distribution of space charge. The results show that a change in space-charge distribution occurs when the applied field is changed. but that no space-charge adjustment occurs when the hole concentration is changed by increasing pho ton flux at a constant electric field bias. The potential goes through a maximum at the point where the electric field goes through zero (i.e., changes from a negative to a positive drift component for hole flow), but the height of this maximum reflects the balance between hoie concentration distribution and electric field and does not directly reflect the amount of 2159 J. Appl. Phys., Vol. 64, No.4, 15 August j 988 current flow. In this sense the model for the low-temperature implanted contact is different than the thermionic diffusion model for a metal~semiconductor contact because the poten~ tial barrier results from the diffusion process rather than from a material-dependent difference in work functions. The model calculates only the steady-state distributions. It shows. however, that the path from one state to another is different for the operating conditions of constant current or constant field, suggesting that transient phenomena in the two cases may not be fully analogous if and when space charge readjustment plays a role. ACKNOWLEDGMENTS The author gratefully acknowledges helpful discussions on this topic with E. E. Haller, R. M. Westervelt. and S. W. Teitsworth. This work was supported in part by the Califor nia Space Institute under Contract No. CS-64-87. Ip. L. Richards and L. T. Greenberg, in Infrared and Millimeter Waves, edited by K. J. Button (Academic, New York, 1982), Vol. 6, p. 150. 1p. R. Bratt, in Semiconductors and Semimetais, edited by R. K. Willard SOil and A. C. Beer (Academic, New York, 1977), Vol. 12, p. 54. 3N. M. Haegel and E. E. HaUer, Infrared Phys. 26,247 (1986). 4S. W. Teitsworth, R. M. Westervelt, and E. E. Haller, Phys. Rev. Lett. 51, 825 (1983). 5K. Aoki, K. Miyame, T. Kobayashi, and K. Yamamoto, Physica (Utrecht) 117&118, 570 (1983). 6R. M. Westervelt and S. W. Teitsworth, J. Appl. Phys. 51, 5451 (1985). 'See, for example, A. Rose, Concepts in Photoconductivity and Related Problems (Krieger, Melbourne, FL, 1978). ~R. L. Williams, 1. Appl. Phys. 40, 184 ( 1969). 9A. F. Milton, Appl. Phys. Lett. 16, 285 (1970). lOA. F. Milton and M. M. Blouke, Phys. Rev. B 3, 4312 (1971). "M. A. Lampert and A. Rose, Phys. Rev. 113,1236 (1959). 12C. R. Croweli and S. M. Sze, Solid State Electron. 9,1035 (1966). Nancy M. Haegel 2159 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.234.114.177 On: Tue, 25 Nov 2014 05:06:15

1.343240.pdf

Effect of thermal history on oxygen precipitates in Czochralski silicon annealed at 1050°C Chung Yuan Kung Citation: J. Appl. Phys. 65, 4654 (1989); doi: 10.1063/1.343240 View online: http://dx.doi.org/10.1063/1.343240 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v65/i12 Published by the American Institute of Physics. Related Articles Metastable ultrathin crystal in thermally grown SiO2 film on Si substrate AIP Advances 2, 042144 (2012) Domain growth kinetics in La0.89Sr0.11MnO3 single crystal studied by piezoresponse force microscopy J. Appl. Phys. 112, 052019 (2012) Understanding on the current-induced crystallization process and faster set write operation thereof in non-volatile phase change memory Appl. Phys. Lett. 100, 063508 (2012) Growth of single-crystalline cobalt silicide nanowires with excellent physical properties J. Appl. Phys. 110, 074302 (2011) Growth of thick heavily boron-doped diamond single crystals: Effect of microwave power density Appl. Phys. Lett. 97, 182101 (2010) Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsEffect of thermai history on oxygen precipitates in Czochralski silicon annealed at 1050 °C Chung Yuan Kung Electronics Research and Service Organization, Clw-Tung, Hsinchu, Taiwan, Republic oj China (Received 30 September 1987; accepted for publication 22 February 1989) The precipitation of oxygen in bulk Czochralski silicon wafers subjected to single-step isothermal, two-step (low-high), two-step (high-high), and three-step (high-law-high) annealing is studied by means of infrared spectroscopy and by preferential etching. Comparisons are made with transmission electron micrographs obtained on similar wafers. With the same amount of precipitated oxygen, the IR spectra near 1230 cm -1 and the precipitate morphology are different for samples that undergo different thermal cycles. For the single-step and three-step annealed samples, platelet precipitates are the dominant defect type and a peak is observed at 1230-cm-l• For the two-step annealed samples, the majority defects are polyhedral precipitates and stacking faults, no platelet precipitates are found and the 1230- cm -I peak is absent. It is believed that the stacking faults generated during two-step annealing have a strong effect in converting the oxygen precipitate morphology from platelets to polyhedra. A model is proposed to explain this phenomenon according to the coarsening concept. I. iNTRODUCTION The two primary, electrically neutral impurities found in Czochralski (CZ) silicon are oxygen and carbon. Their roles in oxygen precipitation and other oxygen-related de fects, such as thermal donors, have been intensively studied during the last decade. 1-5 Besides carbon and oxygen, micro defects (including point defects and their clusters) formed during crystal growth have also been suspected of playing an important role as nucleation centers for oxygen precipi tates.6--12 These grown-in microdefects are too sman in size to detect; even high-resolution electron microscopy cannot detect the existence of these microdefects, not to mention their chemical nature. Whether they are interstitial or va cancy types is still unknown. The question of micro defect formation is extremely complex, and one must consider the interactions between oxygen, carbon, silicon intcrstitials, and vacancies. Although there is little information as to the chemical nature of these grown-in defects, their character and their effect on oxygen precipitation have been tentative ly depicted through various experimental observa tions.6,8,12.13 HU,6 Schaake, Baber, and Pinizzotto,14 and Oehrelein, Lindstrom, and Corbett13 found that oxidation reduces oxygen precipitation in the silicon bulk compared to the same heat treatment in a nitrogen atmosphere. Hu6 be lieved that vacancy clusters were nucleation centers for oxy gen precipitation, and these types of nuclei were annihilated or shrunk by silicon interstitials generated during oxidation, so that the precipitation rates were reduced. Several experiments have shown that thermal history can affect the character of "grown-in defects" and therefore affect the oxygen precipitation behavior. A short, high-tem perature preanneal in nitrogen (1-2 h at 1000 °C or higher) was found to retard the precipitation rate during subsequent lower-temperature anneals. 11.15 This kind of retardation has been attributed to the breakup of vacancy dustersl5 or the dissolving of the silicon-interstitial-enriched swirl defects II that would otherwise act as nuclei for oxygen precipitation. In contrast to a high-temperature preanneal, a short, low-temperature (2-10 h at approximately 750°C) cycle can increase the density and size of nuclei and thereby acce lerate the oxygen precipitation rate.IO-12 It should be noted that the short preannealing cycle mentioned above does not measurably change the initial oxygen concentration in the matrix, but sometimes causes more than an order of magni tude variation in the precipitation rate. This clearly demon strated that thermal history has great influence on the oxy gen precipitation rate. Recently, high-resolution transmission electron microscopy (TEM) studies have shown that the morphology of the oxygen precipitates is quite dependent on the anneal temperaturel6 as well as ther mal history. 17.18 With the same amount of oxygen precipitat ed, the high-temperature isothermal anneal generated plate let precipitates while the low-high two-step anneal generated the polyhedral precipitates. It is also found that the appear ance of an IR spectra! peak around 1230 em -1 varies with thermal history. After 1050·C isothermal annealing, sam ples showed a clear peak around 1230 em -I . However, in samples with about the same amount of oxygen reduction, which were preannealed at 750·C for 8 h or longer, no 1230- cm -I band was detected following a subsequent 1050 °C an neal. 19 The samples that went through a short, wet oxidation before the 750 and 1050°C anneals, on the other hand, showed a very strong 1230-cm-1 peak. Although many studies of oxygen precipitation have been made, no satisfac tory explanation has yet been given regarding the mecha nisms that cause these differences in precipitate morphology and IR absorption at 1230 em .. I. In this research, the oxy gen precipitation behavior for both low carbon and carbon enriched silicon was studied. Extensive data from oxygen concentration variation and from IR spectral analysis for samples given four different thermal treatments are present ed. The effect of carbon atoms and the influence of a wet oxidation cycle on the oxygen precipitation behavior are dis- 4654 J. Appl. Phys. 65 (12), 15 June 1989 0021-8979/89/124654-12$02.40 @ 1969 American Institute of Physics 4654 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionscussed. An attempt is made to correlate the IR spectra with electron microscopic results. From the analysis, conclusions are drawn as to the nature of precipitation kinetics occurring during annealing at 1050 °C after different prior thermal treatments. I!. EXPERIMENT A boron-doped, 1-3-0 em, 75-mm-diam, I5-in.-long, < HI) silicon crystal was grown by the flat-shoulder CZ growth process, with counterclockwise seed rotation of 30 rpm and clockwise crucible rotation of 10 rpm. The total growth period from seeding to power-off was about 5 h. Wa fers were laser marked in numerical order to maintain wafer location identity. Wafers used i.n this experiment were taken from four different locations in the ingot, designated as sec tions A, B, C, and D. Grouping of the wafers allowed us to use those in each group with almost identical oxygen and carbon content as wen as thermal history. The oxygen and carbon concentrations, resistivity, and locations of wafers in the crystal (in terms of g, the melt fraction solidified) are shown in Table I for aU wafers used. To conduct the experi ments, nine or more nearly adjacent wafers from each sec tion were selected and each wafer was cleaved into 2 to 3 pieces. For this research, six similar pieces from each section of ingot were used. One piece from each wafer group was given an isothermal anneal in dry nitrogen at 1050"C. This anneal was stopped at 4, 8, 16, 24, and 40 h; after each inter val the oxygen and carbon concentrations were determined from Fourier transform infrared (FTIR) measurement us ing the calibration constants in ASTM F121-79 and, F123- 74, respectively. A second piece from each section was given a low-high two-step anneal cycle. This cycle consisted of a lOO-h anneal in dry nitrogen at 750"C foHowed by the iso thermal anneal described above. A third piece from each wafer was given a 165-min wet oxidation at 1000·C before passing through the low-high two-step anneal described above. A fourth piece from each wafer was given the 165-min wet oxidation at lO00"C before passing through the inter rupted 4O-h isothermal anneal in dry nitrogen at 1050 "C. For the 1000·C wet oxidations and 1050·C dry nitro gen anneals, temperatures were ramped up from 850 ·C with a heating rate of about 17 °C/min and ramped down to TABLE I. Position within ingot, initial oxygen, and carbon concentrations for wafers. 0, C , P g (ppma)" (ppma)' (ohmcm) A 33.2 ~O.2 2.1 -0.15 High-I! 31.5 -·0.5 1.9 ~(J.3 carbon C 30.0 1.0--1.5 1.7 -0.6 ingot D 30.2 1.6-2.2 1.6 ~O.9 Low-A 32.2 < (J.OS 2.1 -0.2 carbon D 30.0 ~O.15 1.7 -0.7 ingot a 1 ppma = 5X 1016/cm3. 4655 J. Appl. Phys., Vol. 65, No.12,15 June 1989 TABLE H. Thermal cycles. Isothermal anneal Two-step anneal (low-high) Three-step anneal Two-step anneal (high-high) Dissolution anneal Wet Oxidation 750 'C at 1000 'C annealing (min) (h) 100 165 100 165 100 &200 4. 8, 0, 4, 1050'C annealing 16, 24, 40 h 8, 16, 24, 40 h 0, 4, 8, 16, 24, 40 h 4, 8, 16, 24, 40 h lO S, 1 min, 5 min (}.5 h, 1 h, 2 h, 4 h 850 ·C with a cooling rate of about 12 ·CI min. The total ramping time is very short and does not have substantial effect on the precipitation nuclei. The thermal cycles used in this research are summarized in Table II. A fifth piece from each section was not heat treated; this piece was used as a reference for measurement of differential IR (DIR) absorp tion spectra of samples annealed for 16 and 40 h at 1050 "C. Following the heat treatments and IR measurements, the samples were cleaved and etched in Wright etchantl° for 1 min to reveal the defects. Selected samples were also studied by TEM; these results have been published elsewhere by Tsai, Carpenter, and Pengo [8 One group of wafers was also annealed at 750 fiC for times up to 200 h to allow the interstitial oxygen concentra tion to drop to close to the solid solubility at this tempera ture. These samples were then subsequently annealed at 1050 °C for short times; 10 s, 1 min, 5 min, etc., up to 4 h, to observe precipitate dissolution. Samples were pushed in and pulled out manually, with the total operation time less than 30 s. (Note: the data presented for the very short annealing times are not good for quantitative analysis, since the sam ples may not reach thermal equilibrium in such a short time.) The oxygen concentration and changes in the IR spectra were recorded after each intervaL A similar series of tests was also performed on a crystal with carbon concentration less than 0.2 ppma throughout the whole ingot for the purpose of comparison. The initial condition of the low carbon crystal is also shown in Table 1. III, eXPERIMENTAL RESULTS AND DISCUSSION The experimental results and discussion are presented in two parts. First, the effect of thermal history on the oxy gen precipitation rates is considered; and second, the effect of thermal history on the precipitate morphology and IR spectra around 1230 cm -1 is discussed. A. Effect of thermal history on oxygen precipitation rate 1. Wafer /ocationmdependent precipitation rate Fig. 1 shows the oxygen concentration (Oi) after each heat treatment step for samples from the different sections of the ingot. The left-hand side of Fig. 1 shows the variations of oxygen concentration after the different heat treatments pri or to the 1050·C annealing, and the right-hand side shows Chung Yuan Kung 4655 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions.--.. « :;:: <L CL z 0 f- '" '" F-;z W U ;z 0 u z I.!.i 'C) >-x c ~ « :;:: CL a.. z 0 f-« a:: I-z UJ u z 0 u z w (!) >-x a z o I-«: a:: fz: UJ u z: o u z w (!) >x o 4656 A -SEen ON 0 ISOTHERMAL ~~-::~:~:==~~=~~ -...... -----II TWO-STEP (LO-H! ) 30 (fll-H [ ) • TWO-STEP 0 THREE-STEP 20 f : ooooe 750°C 10 165 min 100 h [,Jet 02 Dry N2 OL-____ -L ______ ~ ____ ~ ______ _L ______ ~ ____ ~ ______ _L ______ ~ __ leI B -SECTION 30 ~,:-------&~~ :~~ ::-~ ~ ~~ " , " " " , " , 20 "-, " " " '!II------- 10 1 DOOoC 7~)OO C 165 min 100 h Wet 02 Dry N2 a Ib) C -SECTION o 0 8 16 24 32 ANNEALING TIME (H) AT 10500C o ISOTHERMAL • TWO-STEP (LO-HI) o TWO-STEP (HI-HI) III THREE-STEP 3 15 24 32 ANNEALING TIME (H) AT 10500C o I SOTf-lERMAL • TWO-STEP (LO-HI) o TWO-STEP (Hl-HI) 40 • 40 , ~\ • THREE-STEP 20 I -'--~ -----1\ - I lOOOoC 750°C 10 I--165 mi n 100 h Wet 02 Dry N2 OL-----~------~----~ ______ _L ______ ~ ____ ~~ ____ _L ______ ~ ___ a 8 16 24 32 40 Icl ANNEALING TIME (H) AT 10500e J. Appl. Phys., Vol. 65, No. 12, i 5 June 1989 FIG.!. Oxygen concentration after various anneals for (a) section A, (b) section B, (e) section C, and (d) section D. The left side of each figure shows the results prior to the 1050 'C anneal, the right side ofthe figure shows oxygen con centrations after various an nealing times at 1050 'C. Chung Yuan Kung 4656 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsz o I- 0« c:: ; z u..) u z o u z OJ '" >-X o D -SECTION 30 -::::: -::. -=-a..-::. -:.. -:. -::.. -.:.. -----=----::.- 20 1 0 '.... -'-" -........... ---- "-.... , t loaooe 165 min \~et 02 .... "- ''111-----, 750°C 100 h Dry N2 o ISOTHERMAL II o TWO-STEP (LO-Hl) TWO-STEP (HI-Ht) THREE-STEP FIG. l. (continlled). (dl ANNEALING TIME (H) AT I0500C the concentrations in the matrix versus different 1050 ·C an nealing time. Oxygen reduction ~Oi can be evaluated by subtracting the measured concentration after every step from the original concentration; this reduction represents the amount of interstitial oxygen lost as a function of time. The oxygen lost due to out-diffusion under the thermal cycle described in this research is negligible, therefore, the AOi measured roughly represents the amount of oxygen precipi tated. Figure 2 shows oxygen reduction for single-step and high-high two-step annealing. The precipitation rate for dif ferent sections of both high carbon and low carbon concen tration wafers can be easily compared in same figure. Table III shows that oxygen reductions after annealing at 750·C for 100 h with and without a prior wet oxidation at 1000 0c. With about the same initial impurity level (carbon and oxy gen), the A section wafers showed higher oxygen reduction rates than the B section wafers, which in tum had higher reduction rates than C and D section wafers for all four heat treatments schemes. (With the exception of two and three step cases, in which D section wafers showed a slightly high er precipitation than the C section wafers.) This slightly re versed trend may be attributed to the higher carbon concentration in D section wafers. In the early precipitation stage of both 1050 and 750°C anneals, the precipitation rates of the seed-end wafers were at least twice as fast as that of the tail-end wafers. See Fig. 2 for the 1050 °C and Table III for the 750°C anneaL It is noted that the faster precipitation rate may be due to the 10% higher oxygen concentration in the seed-end wa fers. However, according to the classical nucleation ap proach (theory) described by Inoue, Wada, and Osaka,21 assuming the oxygen concentration dominates the nuclea tion rate, this 10% difference in initial oxygen concentration should only generate about 20% difference in nuclei density and therefore cannot account for the doubled precipitation rate observed. It is therefore believed that seed-end wafers have much more grown-in nuclei to start with than the tail- 4657 J. Appl. Phys., Vol. 65, No. 12, 15 June 1989 end wafers. This is not surprising, since seed-end wafers have gone through a longer low-temperature cooling period than the tail-end wafers during the ingot pulling process. Thus more nuclei in the seed-end wafers can grow to reach critical size and survive at subsequent high-temperature heat treat ments. The fact that a higher density of swirl defects is ob served in seed-end wafers also seems to support this line of reasoningo (Such swirl defects have long been suspected22•Z3 to act as nucleation centers for oxygen precipitates.) ZEffectofheatueat;nentcycle Table III shows the impact of a short 1000 °C cycle on the subsequent 750 °CI IOO-h anneal. The short 1000 °c wet oxidation cycle does not measurably change the carbon and oxygen concentrations, but substantially retards the oxygen precipitation rate during the subsequent low-temperature 750 DC cycle. It is very likely that some grown-in defects that are smaner than the critical size at 1000 DC but larger than that at 750·C are dissolved during the WOO °c oxidationo In the absence of the 1000 DC oxidation, these defects would grow at 750°C. The 1000 ·C anneal also reduces the differences between seed-and tail-end wafers on the precipitation rate during the subsequent 750·C anneal (see Table III), indicating that in the seed-end wafers more grown-in nuclei are dissolved. The short oxidation may also dissolve the nucleation centers of the rodlike defects, or change the point defect concentration such that rodlike defects do not grow during the 750°C an neaL This would be responsible for part of the retardation, if a large portion of oxygen precipitates (crystalline coesite) grow on the rodlike defects as has been reported. 24 3. Role of carbon In oxygen precipitation The role of carbon in oxygen precipitation, both nuclea tion and growth, has been a highly interesting topic. Some groups have reported that carbon enhances oxygen prccipi. Chung Yuan Kung 4657 • -.-.-,",-. ,-.-••••• >.<;'~';O.'.v.-.;".'.' •• ~ •••••• ';'.~.';'.'.'~.,-"'.-.".~ ••••••••••••••••••••• ~;.o.v •• ~.~ ••• ;o •••••••• ;-.~ •••• ;<.-.; .................. <;> .......... v .......... _.~ ••••• '''-';< •••••••• _... • • • • • • • • •• • • .. • • _ Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions,--. <l: ::e:: CL CL z 0 r-u ::::l Q L1.! a: Z w C.9 >-x 0 (0) ,....., « ::0:: CL 0... Z 0 -r-l) ::::l Q LJ.J a: z w t!J >-x 0 (b) 30 ISOTHERMAL ANNEAL ..---------~ 20 HIGH CARBON 10 / A-SECTION 0 ".11/ B-SECTION <> C-SECTION A D-SECTION 0 0 8 16 24 32 40 ANNEAL TIME (H) AT lC500C HIGH lOW CARBON CARBON A-SECTION 0 • 30 B-SECTION <> C-SECTION A D-SECTION 0 II 20 10 TWO-STEP (HI-HI) 8 16 24 32 40 ANNEAL TIME (H) AT le50Ge LOW CARBON • FIG. 2. Oxygen reduction (L\.Oj) during 1050·C annealing for (al as-received wafers, and (b) samples after an initial lO00'C wet oxidation. tation,9,12,14,25,26 especially when the carbon concentration is very high. Other groups have asserted that the correlation between carbon concentration and oxygen precipitation is very weak, especially when the annealing temperature is above 1000 "c.ll Some27,28 observed a strong dependence of oxygen precipitation on carbon concentration for low oxy-gen content samples (Oi -25 ppma) but less or no depend ence for the medium oxygen samples (28 to 30 ppma). It is generally true that the presence of a third element would make the phase boundary stable no matter whether it is re jected or absorbed by the growing phase. This is the ternary effect.29 The carbon atoms are believed to act as a ternary 4658 J. Appl. Phys., Vol. 65, No. 12, 15 June 1989 Chung Yuan Kung 4658 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsTABLE HI. The change of oxygen and carbon concentrations after an anneal at 750 'C for lOOh. (II: 750 'C/l00 h; HI: 1000 'C/165 min + 750 'C/lOOh.) Initial concentration Section Thermal 0, C, cycle (ppma)" (ppma) II A HI 33.2 0.2 II 31.5 0.5 B III 30.9 0.8 high-carbon ingot II 30.0 0.9 C III 29.9 1.1 II 30.3 1.7 D III 30.1 1.6 low-carbon ingot II 32.1 <0.05 A III 32.5 <0.05 n 30.2 0.3 D III 30.4 0.2 a 1 ppma = 5x 10M/ern'. element that decreases the interfacial energy18 and, hence, stabilizes the nuclei and thus increases the precipitation rate. Besides, the supersaturated carbon atoms (or even the sili con interstitials) in solid solution increase the instability of the matrix and therefore facilitate the precipitation of the other supersaturated element (oxygen) from the matrix, For the carbon-rich wafers, especially those from the D sec tion, the carbon concentration ( ~ 2 ppma) is about 20 times higher than the solubility limit of carbon at 1050°C (Ref. 30); and the oxygen concentration, 30 ppma, is about 4 times higher than the solid solubility limit of oxygen at the same temperature.2() In such a highly supersaturated ternary sys tem, the impact of the ternary element, carbon, on the oxy gen precipitation should be very strong. However, compar~ ing the results between high-carbon and low-carbon ingots in this experiment, a carbon concentration of 2.2 ppma did not produce a significant effect in high~temperature heat treatments. It is possible that some other factors, for exam ple grown-in defects as described previously, may have a more pronounced effect, overshadowing whatever impact carbon has on the oxygen precipitation. The effect of carbon at low temperatures is not very dear as shown in Tab!e Ill, the wafers from the A section of high-carbon ingots show a slower oxygen reduction rate than that of the low-carbon ingot. However, a wafer from the D section shows an oppo site effect. Table III also shows the carbon concentrations after annealing at 750°C for 100 h. The carbon concentra tions dearly dropped to below the detection limit on the wafers as received (without preannealing at high tempera ture); however, the carbon reduction is not very pronounced when the wafers are preannealed at high temperature. 4659 J. Appl. Phys" Vol. 65. No. 12, 15 June 1985 .-.-.'.~.' •.. -.-.•.... ; •...........•.•.•.•.• ; •.•.•.•.•. ' .•........... ;.:·;o;·;·;·.·.·.·.·.·.·.·.·.·.·.v.~.'.~.:.".·.-.- .. -.•.....• ;...... . ..• ;-.y ... ' ....... " ....... :.;.; ..•....••.•...•.•.•.•• : .•. ~.:.:.:.:.:<.:.;" .•.•.•.• '.' .• > ~ •••• Final Concentration concentration change 0, C, AO, ACs (ppma) (ppma) (ppma) (ppma) 31.3 0.2 -1.9 -0 IS.2 <0.05 -16.3 -0.5 28.2 0.6 -2.1 ~O.2 22.5 0.1 -7.5 -0.8 28.5 1.0 -1.4 -0.1 20.9 0.2 -9.4 -1.5 26.5 l.5 -3.6 -0.1 25.8 <0.05 -6.3 -0 28.9 0.1 -3.6 +0.1 25.1 0.2 -5.1 -0.1 29.3 0.4 -1.1 +0.1 Some researchers"·12 believe that carbon atoms directly form the nucleation centers for oxygen precipitates, which results in the observation of carbon reduction during heat treatment. In this experiment, and in similar parallel experi ments, 1l.3! we have also shown that carbon atoms (in the matrix) can only be reduced by a low-temperature anneal of the wafers as received, but not on the wafers preannealed at high temperatures no matter how much oxygen is precipitat ed. This kind of carbon reduction behavior can be explained by a gettering model,31.32 in which the reduction of carbon is the consequence of defect (rodlike defect) formation but not a direct result of the formation of nucleation centers. The carbon reduction as well as a resistivity shift, can be used as an index for checking whether the rodlike defects grow or not if the proposed gettering mechanism is correct. B. Effect or thermal history on precipitate morphology and IF! spectra Before proceeding with the discussion, it should be not~ cd that the results obtaiI~ed on the microdefects features and DrR spectra at the 1230~cm-1 band are consistent for all the groups of wafers used in this research, [consistent results are also obtained on N-type (-6.5 n em), phosphors~doped, medium and high oxygen concentration (0; > 30 ppma) materials]. The following results are reported on a typical group of wafers from a high-carbon ingot. 1. Correlation between microdefects and etch pits Etching is the easiest and fastest way to reveal silicon defects and to provide data on defed density. On the other Chung Yuan Kung 4659 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions4660 (a) (b) 20pm I.---.-.t (a) fsothermal (c) Two-step (low-high) FIG. 3. SEM pictures for (a) single-step and (b) low-high two-step an nealed samples. Samples were etched in Wright etch for about 1 min. hand, TEM results provide the microstructure in detail, but only for limited volume. Etching results and TEM observa tions can complement each other to obtain a more complete picture of oxygen precipitation. As shown in theSEM picture in Fig. 3(a), three types of etch pits are observed on single-step annealed samples. The majority ofthem are dimplelike and triangular etch pits. The density of the linear etch pits is less than 5%. TEM observa tions on a similar sample showed that the majority defects are (100) platelets of amorphous 8i02 and precipitate-dislo cation complexes. J8 It is reasonable to assign the triangular pits to dislocations and therefore the dimplelike pits to the platelet precipitates. Similar results are observed in the three-step annealed samples. The high-high two-step an nealed samples showed the same precipitate features as the isothermal annealed samples, but the defect densities were lower. For the low~high two-step annealed samples, only a high density of linear etch pits is observed, as shown in the SEM picture in Fig. 3 (b). Since they are all on (111) planes and their size is quite uniform, these pits are associated with stacking faults. However, TEM results on this sample show that the predominant defects are high-density tiny polyhe dral particles with diameters of about 0.05 pm and a high density of stacking faults. It; There arises a question why etch pits other than those associated with stacking faults are not developed by the etching. Ponce, Yamashita, and Hahn, 17 using high-resolution TEM, also observed both stacking (b) Two-step (high-high) FIG. 4. An optical photomicrograph of a wafer cross section following (a) single-step isothermal anneal, (bl high-high two-step anneal, (el low-high two-step anneal, and ( d) three-step-annealed samples. Samples were cleaved and etched in Wright etch for about 1 min. :".". ..·.u .. · ..... ./'.· ...•.. " . . ." ....... SOpm (d) Three-step J. Appl. Phys., Vol. 65, No. 12, 15 June 1989 Chung Yuan Kung 4660 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsfaults and polyhedral precipitates in similarly two-step an· nealed samples. Moreover, they found these tiny polyhedral particles are essentially strain-free. It is very likely that tiny strain-free particles are not revealed by the preferential etch ing technique and therefore only the linear pits are observed. Figure 4 shows optical photomicrographs of etch fig ures associated with bulk defects generated during typical isothermal, (low-high) two-step, (high-high) t\VO-stcp, and three-step anneal cycles. Although the scanning electron mi crographs reveal a better view of detail, the optical picture provides a quantitative measure of the density of defects. These optical pictures in Fig. 4 show roughly the densities of defects which are larger than a certain size and reflect the oxygen precipitation rates, except in the two-step annealed cases, since the two-step 8.nnealed samples only reveal the density of stacking faults. As can be seen, the three-step an nealed sample shows a higher density than the single-step annealed samples, and in turn higher than the high~high two-step annealed samples, The etch pit density observa tions are consistent with the precipitation rates estimated from the oxygen reduction, 2< Correlation between IR spectra and microoefects The IR spectra near the 1230-cm --, band also varied when different thermal cycles were applied, This 1230-cm-l band indicates the precipitate morphology. Fig. 5 (a) shows the differential IR absorption spectra for three different samples in the 1000-1300 cm -1 range. All three curves are on the same scale. The DIR spectra are obtained using an as received wafer (w hieh only reveals the 1107 -em -1 peak in this range) as reference. The spectra obta.ined are the differ entiations before and after heat treatments. Therefore the amplitude of the l107-cm -I (vaHey) in Fig. 5(a) approxi mately represents the amount of interstitial oxygen precipi tated. As can be seen an three samples lost the same amount of interstitial oxygen to precipitation after three different thermal cycles. However, the appearance of the spectra near 1230 em-l is not the same, The single-step annealed samples showed a moderate, broad peak near 1230 em-I, and the three-step annealed samples showed a very strong peak near 1230 em -I, but the low-high, two-step annealed samples showed no peak in that range. The spectrum of the high-high two-step annealed samples is very similar to that of single-step annealed sam ples, but the amplitUde of 1230-cm -! peak is consistently loweL The relationship between the IR spectrum around 1230 em --l and oxygen precipitate morphology has been theoreti cally studied by HU.33 The 1230-cm -J band ofSi02 arising from its longitudinal optical phonon is normally infrared inactive, but can become infra-;ed active for particles in the platelet (disk) shape and with size smaller than 0.36 ,urn, Therefore, the appearance of the 1230-cm -J peak is depen~ dent on both the size and the shape of pa-;ticles in the silicon matrix but has nothing to do with the chemical bonding of the Si02• Based on Hu's theory, the observed variations on the amplitUde of the 1 230-cm -I band and the corresponding changes in the morphology of the precipitates in this re search can be consistently explained. 4661 J. Appl. Phys., Vol. 65, No. 12, 15 June 1989 '.-.-.-.~ ••••••••••.• ; •.•••••••• ""7. •• -.;." ••••• -••••• " ••••••• --.. '~ •.•••.•••• ~ •• ; •• • ••••• ·.v.·."'":".:.;.; •• -•••• .-•••• ;> ••• ' ••••• :.:.:.:.:.:.:.; •••••••• ' ••••••••• .-; •••• :.:.:;:.:.;.:.:.:o;.:.;.~ •...•.•.•.• ;".o; ••••••••• " •• 1480 '" o -' (b) 1400 THREE-STEP ISOTHERMAL TWO-STEP (LO-HI) 1200 1000 WAVE NUMBERS (CM-1) clOC 1200 1000 800 rIG. 5. (a) DFTIR absorption spectnull in 800--1400 em -I range for sam pIes after single-step isothermal anncal, low-high two-step anneal, and three-~tep anneal. All three curves refer to the same scale. {b) DFTIR ab sorption spectra t()[ three-step annealed sample showing a clear peak at JI25 cm--I. The appearance of a sharp 1230-cm -I peak in the three step annealed samples indicates the majority of defects in these samples are tiny platelet precipitates. It is noted that a I12S-em -I band in the three-step annealed samples is also quite clear, as shown in Fig. 5 (b), This is because some pla telets grow to become thicker. Such thick platelets resemble Chung Yuan Kung 4661 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsoblate-shaped particles and have strong absorption near 1125 cm-i as in Hu's calculation.33 In the single-step annealed samples, platelet particles are also observed, but, their density is lower in comparison with that of the three-step annealed wafers. Also, many of the precipitates are tangled with dislocations and are not in the shape of platelets. Therefore, the 1230-cm -1 peak is not as sharp as it is in the three-step annealed case, In the two step annealed samples, no platelet particles are observed and therefore no 1230-cm -i band occurs. Thus, correlation between IR spectra and observed precipitate morphology observed can be consistently explained by Hu's theory, and the IR spectra therefore provide a fast means for checking precipitate morphology. 3. Two different precipitation behavior patterns From the observations described above, one can deduce two different behavior patterns for oxygen precipitation at 1050 °e with different prior thermal cycles. (1) The single-step isothermal anneal, high-high two step anneal, and three-step anneal reveal a pattern in which platelet precipitates are the dominant type of defect and a clear 1230-cm -I peak is observed. Some stacking faults are observed, but their density is very low. No apparent reduc tion in carbon concentration can be detected during anneal ing. (2) The low-high two-step annealed samples reveal a pattern in which stacking faults and polyhedral particles are the dominant types of defects; no 1230-cm -1 peak is ob served. Carbon concentration always drops to the detection limit ( -0.05 ppma or below) after a lOO-h anneal at 750 ·C. The occurrence of these two different precipitation patterns is believed to be intimately related to the development of two different types of grown-in nuclei with different thermal his tories. Previously, Kung, Forbes, and Pengll and Kung~H have proposed a dual nucleation mechanism and an interac tion model to explain how the precipitation pattern convert ed from one type to the other. Two types of defects included in this model have been observed by high-resolution TEM after prolonged annealing at low temperatures, 650- 850 "C.24,34 They are platelet precipitates and rodlike defects with crystalline silica (oxygen precipitates) in them. High density dislocation dipoles or loops are also observed in this temperature range.24 When the annealing temperature is higher than 850"C , only platelet precipitates and disloca tion dipoles are observed, indicating that rodIike defects can not grow at high temperatures31•32 because the nucleation centers of the rodlike defects in the as-received wafers are smaller than the critical size. KungJ1,32 has pointed out that a short, high-temperature anneal would suppress the growth of rod like defects at a subsequent low temperature. This has been recently proven with high-resolution TEM observa tions.35 However, if the high temperature is applied after a prolonged low-temperature anneal, the defects observed are stacking faults and polyhedral particles. 17,18 Ponce, Yama shita, and Hahn 17 believed that small platelet precipitates seen after the low-temperature first stage evolved into poly hedra at the high-temperature second stage. This would ex plain the disappearance of the 1230-cm -I peak at the second 4662 J. Appl. Phys., Vol. 65, No. 12, 15 June 1989 stage. Further discussion will be presented in the dissolution test result, Comparing the result of the two-step and single-step an neals, it is reasonable to believe that stacking faults observed in the two-step annealed samples are introduced during low temperature annealing. Kung, Forbes, and Peng" have as serted that the high density of bulk stacking faults observed at the second stage evolved from silicon-interstitial-rich de fects (a-type defects), which can only grow at temperatures below a certain critical temperature Tc. These defects are now believed to be rodlike defects or stacking faults with oxygen precipitated in them. These defects can grow at low temperature to a larger size and then are able to survive at a higher temperature, but become stacking faults. This argu ment is supported by in situ TEM observations36 which show a conversion of rod-shaped defects (formed at 800 "C) into stacking faults during a subsequent higher-temperature an nealing ( 10 17°C), In the two-step annealing case, no plate let precipitates (dimplelike etch pits) were observed when the high-density stacking faults were present. Kung31,37 has further argued that the growth of rod like defects at low tem perature suppresses the growth of platelets, leading to pre cipitation retardation. Two problems remain: (1) Why are the morphology and types of precipitates influenced by the pre-heat-treat ment thermal cycles, but not by the 40-h anneal at 1050 °C in which most of the oxygen precipitation activity takes place? (2) Why does prolonged annealing at 750°C influence the two-step and the three-step annealed samples differently? To answer these two questions, let us consider the dissolution results and examine the approaches proposed previous ly.l],3l,37 IV. DISSOLUTION TEST RESULTS AND THE COARSENING MODEL Figure 6 shows interstitial oxygen concentration for two identical samples, annealed at 750°C for 100 and 200 h, re spectively, followed by annealing at 1050 °C for various per iods up to 4 h. Figure 7 shows differential IR spectra between samples annealed after 750 °CI200 h and after 750°C/200 h + 1050 °C 12 h. (The DIR spectra results are identical for the counterpart of the 750 °e /lOO-h cases,) Figure 8 shows the microdefect features (a) after 750 °C/200h and (b) after 750 °e/ 200 h + 1050 °C / 4 h. Figure 8 (c) is an enlarged picture of Fig. 8 (b) < The etch pits examined after 4 h at 1050 de are linear pits (stacking faults) with drastically re duced density as compared to the 750"C annealed samples. The results of the lOO-h case, in terms ofDIR spectra and the microdefect features, are identical to the 200-h case. For the 750 °C/lOO-h sample, the oxygen concentration dropped to 14 ppma which is still above the solid solubility limit at 1050 0c, For the 750 °C/200-h annealed sample, oxygen dropped to about 5.5 ppma, which is below the solid solubil ity at 1050°C, These two samples showed about the same density of etch pits, and both show a 1230-cm -1 band after the 750°C anneal. During the first 5 min of the 1050 °C an neal, both samples shew an increase in oxygen concentra tion, the 750 ·C/200-h sample increasing about 5 ppma. However, no noticeable change of the 1230-cm-1 peak is Chung Yuan Kung 4662 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions'< 30 :E 0.. 0.. z o ! <t et: r z w u z o u z UJ (,!J >x o 20 10 lmin O.SH o , AS RECEIVED 100 200 (H) lOs 5min ANNEALING TIME observed during this period, and no detectable change of the etch pit density was noticed. After a ~-h anneal at 1050 °C, the oxygen concentrations start to drop, and the 1230-cm-' peak gradually disappears, vanishing after 2 h at 1050 °C As described above, the precipitate density (in terms of etch pits) decreased during the ~-4-h anneal at 1050 "C, while in this same period the oxygen concentration also con stantly decreased (Fig. 6), indicating that more oxygen atoms precipitated. Ponce, and co-workers 17 using high-res olution TEM have also observed a drastic reduction in den sity from 5 X 1012 /crn3 at 750°C to lOl2/em" after a second stage anneal at 1175°C; while the oxygen concentration dropped from 26 ppma (after 7500e) to 12 ppm a (after 1175 ·C ). This observation indicated again that oxygen pre cipitation activity is very strong during the second stage, but the oxygen precipitate density is less 0 The results described W -.-l ~ 1230 0,:-1 (j) L~ o -..l 1400 1200 7500C/200h +1050oC/2h 1000 WAVE NUMBERS (0\-1) FIG. 7. DIRspectrum after 750 'C!200h and 750 'C/200h + 1050 'C/2h. 4663 J. Appl. Phys., Vol. 65, No. 12, 15 June 1989 2 3 4 (H) FIG. 6. Oxygen concentration after pro longed annealing at 750 'C and then short annealing at 1050 'c, showing that p1'ecip itatc dissolution has occurred. above indicate that a very strong coarsening phenomenon takes place in the two-step annealed samples. The coarsen ing3S is driven by the difference of interfacial chemical poten tial (energy) between two precipitates (of different size, shapes, or types). The nonequilibrium in microstructure would drive the coarsening, even if the concentration of so! ute has already reached equilibrium (solid solubility). Since the rodlike defects (with. an oxygen predpit<lte inside it) and (a) ZOpm (b) <.c) 4011m FIG. 8. Bulk defect features induced after (a) 750°C!200·11 anneal, (b) 750 'C/200 h + 1050 T/2 h, and (c) enlarged picture of (b). Samples were cleaved and etched in Wright etch for about 3 min. Chung Yuan Kung 4663 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsplatelet precipitates are extremely different in shape and chemical composition, they have a great difference in surface energy. The interaction (coarsening) between these two dif ferent types of defects should be very strong. The energeti cally unfavorable type may be suppressed or may be forced to change shape to gain stability. With the same volume, the polyhedral shape has a lower surface area than the platelet and is more stable than the platelet. Hi Thus the polyhedral precipitate becomes the final product after a second-stage anneal. From the dissolution test results and precipitation patterns described in the previous section, it is logical to believe that the growth of stacking faults during the second stage causes the platelets to convert into polyhedra. It is not clear how the interaction proceeds. The growth of stacki.ng faults may generate two effects on the nearby platelet precip itates. First, it assists strain to relaxation around the precipi tate and helps the emission of silicon atoms at the interface of precipitates and silicon matrix into the silicon lattice (in the matrix) as silicon interstitials. 39 Second, the crystalline silica in the rodIike defect may be transformed to low-energy poly morphs and commence the coarsening, forcing the nearby platelets to change shape. All the mechanisms described above may help the dissolution (or retrogression) of the cor ner section of a platelet where the surface/volume ratio is largest. The dissolved oxygen may be redeposited at the oth er facet of the platelet forming the energetically favored po lyhedron. The dissolved oxygen may also reprecipitate het erogeneously on the edge of stacking faults, as observed by Ponce, and co-workers. 17 In situ TEM studies are needed to verify the detailed evolution process. In the three-step an nealed samples, very few stacking faults are observed, even though the samples are annealed at the same low-high, two step anneal after the 165-min, 1000 °C cycle. This is because the first-step, high-temperature anneal suppresses the growth of rod like defects. 31,35 Hence the conversion of plate lets to polyhedra induced by rodlike defects observed in two step annealed samples, is not seen in the three-step-annealed samples. VoSUMMARY In this study, the effects of thermal history on oxygen precipitation behavior, both during crystal growth and sub sequent heat treatment cycles, are demonstrated. Seed-end wafers, which have gone through a longer low-temperature history, showed a much faster precipitation rate than the tail-end wafers. The two to three ppma higher carbon con centration in the tail-end wafers does not have any detectable impact on the oxygen precipitation rate nor on the oxygen morphology and IR spectrum near 1230 em--I. However, a short, 1000 °C preannealing cycle reduces the oxygen precip itation rate and changes the microdefect features. For isoth ermally annealed wafers, platelets and precipitates with dis location tangles are the major defects, and an IR peak at 1230 cm -1 is observed. The results of the high-high two-step anneal and three-step anneal are very similar to that of the single-step anneal. For the low-high two-step anneal, high density polyhedral particles were observed together with a high density of stacking faults. No 1230-cm -I peak is ob served. The 1230-cm-1 peak, generated by platelets at pro- 4664 J. AppL Phys., Vol. 65. No. 12, 15 June 1969 longed low-temperature annealing, fades away gradually during the second-stage high-temperature anneal, indicating that platelet precipitates are converted into polyhedra. A model is proposed to explain how platelets convert to poly hedra. However, the details of this model must await in situ TEMstudy. ACKNOWLEDGMENTS The author is grateful to Dr. Murray Bullis for his criti cal review and suggestions on this work. Also to Fairchild, Materials Division, for their support on the investigation of this study. Preliminary reports of the results of this work have been presented in the May, 1983 and May, 1984 Elec trochemical Society meetings. 'H. R. Huff, R. J. Kriegler, and Y. Takeishi, Eds., Semiconductor Silicon 1981 (The Electrochemical Society, Pennington, NJ, 1981). 2J. Narayan and T. Y. Tan, Eds., Defects in Semiconductors, Materials Re" search Society Symposia Proceedings (North-Holland, New York, 1981), Vol. 2. "s. Mahajan and J. W. Corbett, Eds., Dejects in Semiconductors II. Materials Research Society Symposia Proceedings (North-Holland, New York, 1983), Vo!' 14. 'W.M. Bullis and Le. Kimerling, Eds., Defects in Silicon (The Electro chemical Society, Pennington, NJ, 1983). 'W.M. BulliR and S.Broydo, Eds. (The Electrochemical Society, Penning ton, NJ, 1985). "S. M. Hu, App!. Phys. Lett. 36, 561 (1980). 7F_ Shimura, H. Tsuya, and T. Kawamura, App\. Phys. Lett. 37, 483 ( 1980). SA. J. R. de Kock and W. M. van de Wijgert, App!. Phys. Lett. 38, 888 (1981). oS. Kishino, Y. Matsushita, M. Kanamori, and T. lizuka, Jpn. J. App!. Phys. 21,1 (1982). ION. Inoue, J. Osaka, and K. Wada, J. Electrochem. Soc. 129, 2780 (1982). J 'e. Y. Kung, L. Forbes, and J. D. Peng, in Defects in Silicon edited by W. M. Bullis and L. C. Kimerling (The Electrochemical Society, Penning ton, NJ, 1983), p. 185. '2S. Kishino, M, Kanamori, N. Yoshihiro M. Tajima, and T. Iizuka, J. App\. Phys. SO, 8240 (1979). "G. S. Oehrlein, J. L. Lindstrom, and J. W. Corbett. App\. Phys. Lett. 40, 241 (1982). 14H. F. Schaake, S. C. Baber, and R. F. Pinizzotto, in Semiconductor Silicon 198], edited by H. R. Huff, R. J. Kriegler, and Y. Takeishi (The Electro chemical Society, Pennington. NJ, 1981), p.273. ISS. M. Hu, J. Appl. Phys. 52,3974 (1981) J6W. A. Tiller, S. Hahn, and F. A. Ponce, J. App!. Phys. 59, 3255 (1986). PF.A. Ponce, T. Yamashita. and S. Hahn, in De/ects if/Silicon, edited by W. M. Bullis and L. C. Kimerling (The Electrochemical Society, Penning" ton, NJ. 1983), p. 105. '"H. L. Tsai, R. W. Carpenter, and J. D. Peng, in Proceedings of the 42nd Annual Meeting of the Electron Microscopy Society 0/ America, edited by G,W. Bailey (San Francisco Press, San Francisco, 1984), p. 344. toe. Y. Kung, L Forbes, andJ. D. Peng, Mater. Res. Bull. 18, 1437 (1983). "'M. Wright-lenkins, J. Electrochem. Soc. 124,757 (1977). "N. Inoue, K. Wada, and J. Osaka, in Semiconductor Silicon 1981, edited by H. R. Huff", R. J. Kriegler, and Y. Takeishi (The Electrochemical So ciety, Pennington, NJ, 1981) p. 282. 22T. Y. Tan and U. Gosele, App!. Phys. A 37,1 (1985). 2'A. J. R. de Kock, in Aggregation Phenomena of Point De/ects in Silicon, edited by E. Stirtl and J. Goorissen (The Electrochemical Society, Pen nington, NJ, 1983), p. 58. 24A. Bourret, J. Thibault-Desseaux, and D. N. Seidman, J. App!. Phys. 55, 825 (1984). "F. Shimura, J. App!. Phys. 59. 3251 (1986). lOR. A. Craven, in Semiconductor Silicon 1981, edited by H. R. Huff, R. J. Kriegler, and Y. Takeishi (The Electrochemical Society, Pennington, NJ, 1981 J, p.254. Chung Yuan Kung 4664 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions27H. D. Chiou, Solid State TechnoL, March, 77 (1987). 1'R. F. Pinizzotto and S. Marks, in Defects in Semiconductors II. 111aterials Research Society Symposia Proceedings. edited by S. Mahajan and J. W. Corbett (North-Holland. New York, 1983), VoL 14, p. 147. ZOp. G. Shewmoll, Trans. Metall. Soc. AIME 233, 736 (1965). }() A. R. Bean and R. e. Newmann, J. Phys. Chem. Solids 32, 1211 (]971). "e. Y. Kung, in VLSI Science and Technology 1985, edited by W. M. Bullis lmd S. Broydo (Thc Electrochemical Society. Pennington, NJ, 1985), p. 446. 32c. Y. Kung, J. App!. Phys. 61, 2817 (1987). 33S. M. HII, J. App!. Phys. 51, 5945 (1980). 4665 J. Appl. Phys., Vol. 65. No. 12, i 5 June i 989 34K. Tempelhoif, F. Spiegelberg, R. Gleichmann, and D. Wruck, Phys. Sta tllS Solidi A 56, 213 (1979). 35W. Bergholz, J. L Hutchison, and G. R. Booker, in Semiconductor Silicon 1986, edited by H. R. Huff, T. Abe, and B. Kolbesen (The Electrochemi cal Society, Pennington, NJ, 1986), p. 874. 36W. K. Wu and J. Washburn, 1. Appl. Phys. 48, 3742 (1977). 37c. Y. Kung ill Extended Abstracts (The Electrochemical Society, Pen nington, NJ, 19~4). Vol. 84-1, p. 162. '"I. M. Lifshitz and V. V. Slyozov, J. Phys. Chern. Solids 19, 35 (1961). '"T. Y. Tan and C. Y. Kung, J. AppL Phys. 59, 917 (1986). Chung Yuan Kung 4665 Downloaded 17 Dec 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

1.342590.pdf

Shallow melting of thin heavily doped silicon layers by pulsed CO2 laser irradiation R. B. James and W. H. Christie Citation: Journal of Applied Physics 65, 3655 (1989); doi: 10.1063/1.342590 View online: http://dx.doi.org/10.1063/1.342590 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Free carrier absorption in heavily doped silicon layers Appl. Phys. Lett. 84, 2265 (2004); 10.1063/1.1690105 Optical studies during pulsed CO2 laser irradiation of ionimplanted silicon J. Appl. Phys. 57, 4727 (1985); 10.1063/1.335335 Pulsed laser melting of amorphous silicon layers Appl. Phys. Lett. 44, 35 (1984); 10.1063/1.94594 Stress relief in heavily doped silicon layers J. Appl. Phys. 54, 1033 (1983); 10.1063/1.332121 Resistivity reduction in heavily doped polycrystalline silicon using cwlaser and pulsedlaser annealing J. Appl. Phys. 52, 3625 (1981); 10.1063/1.329097 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.64.175.185 On: Wed, 03 Dec 2014 16:38:12Shallow melting of thin heavily doped silicon layers by pulsed CO2 laser irradiation R. 8. James Theoretical Diuision, Sandia National Laboratories, Livermore, California 94550 W. H. Christie Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 9 September 1988; accepted for publication 3 January 1989) We show that an extremely shallow ( S 800 A.) melt depth can be easily obtained by irradiating a thin (-200 A.) heavily doped silicon layer with a COzlaser pulse. Since the absorption of the CO2 laser pulse is dominated by free~carrier transitions, the beam heating occurs primarily in the thin degenerately doped film at the sample surface, and there is little energy deposited in the underlying lightly doped substrate. For CO2 pulse-energy densities exceeding a threshold value of about 5 J/cm2, surface melting occurs and the reflectivity ofthe incident laser pulse increases abruptly to about 90%. This large increase in the reflectivity acts like a switch to reflect almost aU of the energy in the remainder of the CO2 laser pulse, thereby greatly reducing the amount of energy available to drive the melt front to deeper depths in the material. This is in contrast to the energy deposition of a laser pulse that has a photon energy exceeding the band gap, in which case the penetration depth of the incident radiation is only weakly affected by the free-carrier density. Transmission electron microscopy shows no extended defects in the near-surface region after CO2 laser irradiation, and van def Pauw electrical measurements verify that 100% of the implanted arsenic dopant is electrically active. Calculated values for the melt depth versus incident pulse-energy density (EL) indicate that there exists a window where the maximum melt-front penetration increases slowly with increasing EL and has a value ofless than a few hundred angstroms. For smcon specimens having a thin degenerately doped film at the surface and a lightly doped substrate, the two primary reasons for using a CO2 laser pulse to achieve very shallow melt depths are (1) the pulse energy is deposited only in the thin surface layer and (2) the melting of this layer causes the reflectivity to jump abruptly to a value of almost unity. INTRODUCTION beam reproducibility and spadal inhomogeneities. Pulsed laser processing of ion-implanted silicon has been applied extensively to the fabrication of high-efficiency solar cells. I It has been demonstrated that pulsed laser an nealing is superior to thermal annealing for the removal of lattice damage caused by ion implantation, electrical activa tion of implanted dopants, and preservation of the minority carrier diffusion length in the base region of the solar cell 2 Most of the advantages oflaser annealing over conventional thermal processing result from the localization of thermal effects associated with the laser pulse and the increased con~ trol of several critical solar cell parameters (e.g., junction depth and free~carrier concentration). Several investigators have used a pulsed CO2 laser pulse to anneal ion-implantation damage in heavily doped silicon crystals (see, for example, Refs. 4 and 5). These studies COD firmed that absorption by free carriers can be used to melt degenerately doped samples, Furthermore, the measure~ ments showed that deep melt depths (;(; 1 pm) could be achieved by irradiating the heavily doped samples with CO2 laser pulses.4,5 Similar attempts to anneal large areas oflight ly doped silicon without substrate heating were found to be unsuccessful. 6, 7 There exist numerous reports on the use of pulsed lasers to melt ion-implanted silicon layers? Almost all of these in vestigations have been conducted with a laser that has a pho ton energy greater than the band gap, such as a ruby or ex cimer laser. Unfortunately, the energy deposited from a ruby or excimer laser always occurs in the near-surface region of the specimen, and one has little control over the penetration depth for a fixed photon energy, In order to melt extremely thin (S 800 A.) layers with a ruby or excimer laser, one has to precisely control the pulse~energy density at a value close to the melt threshold, which i.s generally difficult due to In this paper we show that a pulsed CO2 laser is also particularly suitable for forming very shallow ( S 800 A.) melt depths, which is a new application of C021aser process ing of silicon. These shallow melt depths are possible by con trolled heating of only a thin ( S 200 A) degenerately doped layer at the sample surface. The particular specimens used in our measurements were lightly doped silicon that had been implanted with low-energy (5-ke V) arsenic. The arsenic im plantation produced an extremely thin heavily doped film, which has a large absorption coefficient for CO2 laser radi ation. Comparable results are expected for other specimens which have a thin dopant film of sufficiently high free-car rier density on top of a lightly doped substrate. (Note that much different results for the melt depth versus incident 3655 J. Appl. Phys. 65 (9),1 May 1989 0021-8979/89 (093655-07$02.40 @ 1989 American Institute of Physics 3655 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.64.175.185 On: Wed, 03 Dec 2014 16:38:12pulse-energy density are predicted for samples which have a large absorption coefficient throughout the near-surface re gion. For example, there would be considerable difficulty in obtaining reproducible melt depths of less than 800 A using the ion-implanted specimens studied in Refs. 4 and 5.) For COzlaser radiation (A-lO ,urn), the absorption of the pulse energy in the shallow arsenic-implanted Si layer is dominated by free-electron transitions within the conduc tion band. Since the free-electron concentration in the thin surface film is several orders of magnitude greater than the underlying substrate, the energy ofthe laser pulse is prefer entially deposited in the thin film at the surface, If the dura tion of the pulse is short compared to the time required to conduct the heat out of the shallow surface layer, then the CO2 laser-induced heating occurs only in the thin arsenic implanted layer where the free-carrier density is large. The onset of surface melting causes the reflectivity of the sample to increase abruptly to about 90%, which is much larger than the reflectivity increase of a ruby or excimer laser pulse. This large increase ofthe reflectivity upon melting acts like a switch to reject most of the energy in the remainder of the CO2 laser pulse, thereby reducing the amount of energy available to further heat the surface and drive the melt front to considerably deeper depths in the materiaL Furthermore, for applications where one has both heavily doped and un doped areas in the near-surface region, one can spatially se lect the heavily doped regions for beam heating, without causing significant heating of the adjacent undoped materi al.s,s Thus, one can use a relatively large CO2 laser pulse to simultaneously process many smaller heavily doped regions on the same or on different silicon wafers. EXPERIMENT A gain-switched, transversely excited atmospheric (TEA) C02iaser was used to generate the pulses at a wave length of 10.6 pm. The laser was operated with low nitrogen content in the gas mix, so that the amplitude of the long tail on the pulses could be greatly suppressed. About 80% of the energy in each pulse was contained in the form of a nearly Gaussian peak of 60-ns duration (FWHM). The remaining 20% of the pulse energy was in a second pulse that was de layed by about 300 ns from the first pulse and had a duration of250 ns (FWHM). For the energy densities considered in this report, our theoretical results show that this second pulse makes a negligible contribution to the calculated melt depths and durations of surface melting. As a result, the pulse-energy densities quoted in this paper are for the energy in the 60-ns primary pulse only. The output pulse was diverged by a spherical convex mirror with a I-m radius of curvature. The diverging beam was later collimated by a spherical concave mirror with a 2- m radius of curvature. The collimated beam then impinged on a CO2 laser beam integrator which spatially homogenized the beam to within ± 10%, The dimensions of the laser pulses were 12 X 12 mm2 in the target plane of the integrator. The energy density at the sample surface was adjusted by using additional lenses to change the pulse size and linear 3656 J. Appl, Phys" Vol. 65, NO.9, 1 May i 989 attenuators to change the total energy in each pulse, A pho ton-drag detector and volume absorbing calorimeter ,,,ere used to measure the intensity and energy of the laser pulses, respectively. The samples used in the experiment were 340-,um-thick, boron-doped silicon (100) wafers which, prior to implanta tion, had an electrical resistivity of 2-3 n cm at room tem perature. This resistivity corresponds to a free-hole concen tration of approximately 6 X 1015 em -3 and hole mobility of about 350 cmz /V s. The samples were implanted on one side with 75 As·j ions at an energy of 5 ke V to a dose of 2 X 1015 cm -2, resulting in an arsenic profile that is peaked at about 70 A from the surface with a standard deviation of about 25 A. The samples were next thermally annealed at 873 K for 12 min to increase the fraction of electrically active arsenic and thereby increase the coupling of the CO2 laser radiation to the near-surface region. The concentration of arsenic near the peak of the implanted profile exceeds the solid solubility limit, <) resulting in the formation of arsenic-rich precipitates in part of the implanted layer. van der Pauw measurements on the thermally annealed samples showed a carrier density of6.2X 1014 cm 2, carrier mobility ono cm2/V s, and sheet resistivity of 335 010. The free-electron concentration in the first 200 A is greater than 1020 em 3, so that the absorption coefficient (a) of the CO2 laser radiation is large (~2 X 104 em -I) near the surface. 10,1 1 The lightly doped substrate is relatively transparent to lO.6-pm radiation and has a value ofless than 10 em -I at room temperature. Thus, the pulse-energy depo sition is primarily in the thin film at the surface. The samples were irradiated in air by CO2 laser pulses at different energy densities. van cler Pauw measurements were used to determine the changes in the carrier concentration, carrier mobility, and sheet resi.stivity. A Fourier transform infrared spectrometer was utilized to study the laser-induced modifications in the optical properties of the near-surface region. The microstructure of the near-surface region was investigated by cross-section transmission electron micros copy. Secondary ion mass spectrometry (SIMS) was uti lized to measure the redistribution of the implanted arsenic and to investigate the possibility of controlling the dopant profiles by varying the energy density of the laser pulses. EXPERIMENTAL RESULTS van der Pauw measurements were performed on the la ser-irradiated samples, and the results showed that for pulse energy densities (EL) greater than about 5 J/cm2, signifi cant changes in the electrical properties of the ion-implanted layer occurred (see Table O. For values of E[" between 5.0 and 7.5 J/cm2, there was an increase in the carrier concen tration (Ns) and a decrease in the sheet resistivity (p) with increasing E L' The increase in N, results from the partial melting of the arsenic-implanted layer and subsequent elec trical activation of the arsenic upon solidification of the mol ten layer. At pulse-energy densities greater than about 7,5 J/ cm2, the melt front penetrated to a depth exceeding the im plantation-damaged surface layer, and 100% activation of the implanted arsenic was observed. At these higher values R. 8. James and W, H. Christie 3656 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.64.175.185 On: Wed, 03 Dec 2014 16:38:12TABLE I. Elecerical properties of the arsenic-implanted silicon samples as a function of the incident pulse-energy density. Here, EL is the energy den sity, N, is the carrier concentration, p is the sheet resistivity, and fJ, is the carrier mobility. EE, N, P f.L Olcml) (1015 em .2) (fi/D) (cml;V s) 0.0 0.62 335 30 4.4 0.65 318 30 5.3 0.96 233 28 6.1 1.62 135 29 6,8 1,81 104 33 7.5 2.00 92 34 8.4 2.01 76 41 9.2 2.00 51 59 9,9 1.99 48 65 of EL, the implanted arsenic redistributes to deeper depths, and the electron mobility begins to increase monotonically with E L due to the reduced rate of carrier scattering by the ionized arsenic dopants. The CO2 laser-induced melting of the near-surface re gion also causes significant changes in the infrared optical properties of the silicon samples. A Fourier transform in frared spectrometer was used to obtain transmittance and reflectance spectra before and after laser irradiation. Figure 1 shows the total reflectance and transmittance spectra in the 400-2400 cm -I range for an unirradiated sample and a sample irradiated at EL = 8.1 J/cm2• The presence ofinter~ stitial oxygen in the unirradiated and laser-treated samples causes the infrared absorption at 1106 cm -1 as a result of the antisymmetric vibration of the Si20 complex. The narrow transmittance dip at about 610 cm -1 is due to phonon exci tations in the samples, although the local vibration mode of substitutional carbon (at 607 cm -1) may also playa role. The most significant changes in the optical properties in the WAVELENGTH (,.m) 6 B ~--El ~ 0.0 Jlcm2 -El ~ 8.1 J!cm2 40 i---:R:l..-. __ 25 4(J ;::: z 30 ~ a: w !!:o I- w '-' :z 20;::: t:: ::IE "" "" c a: 0- lD FIG, 1. Reflectance and transmittance spectra for an unirradiated sample and a sample after irradiation at EL = 8.1 J/cm2• 3657 J. Appl. Phys., Vol. 65, No, 9, 1 May 1969 FIG. 2. The top photograph shows a cross-section TEM micrograph of a sample after irradiation by a CO2 laser pulse having an energy density of 8.! Jlcm2• The bottom photograph shows a different sample that was heated in a furnace at 973 K for 10 min after being irradiated by a laser pulse at El. ,= 8.1 J/cm2• The location of the surface is shown by an arrow in each photo. 2400-400 cm-' I range are caused by the large increase ( > 200%) in the free-electron concentration within the ar senic-implanted layer (see Table 1). For example, after irra diation at E L = 8.1 J / cm2, the total reflectance was found to change from 45% to 38%, and the total transmittance changed from 22% to 14% for light having a wavelength of 1O.6,um. We llsed cross-section transmission electron micros~ copy (TEM) to investigate the presence of extended defects in the ion-implanted-damaged layer, Figure 2(a) is a micro graph of a specimen that has been irradiated by a pulse hav ing an energy density afS.! J/cm2• At this pulse-energy den- I I I 2 ARSENIC ATOMS IN SILICON AFTER CO2 LASER ANNEALING 1021 El iJlcm21 5 A-0,0 '7 @-7,6 ~ Ii! ~-9,1 <I) x-10,2 ::IE "" l-« U '" I.U '" '" 5 « 1019 0 2 4 10 12 DEPTH (10-2 I'm! FIG. 3. SIMS measurements of arsenic atoms as a fUllction of depth for samples irradiated at different energy densities, The four curves show the arsenic profiles for the following samples: A..; an unirradiated sample; 8; a sample irradiated atE[, = 7.6 J/cm1; III; a sample irradiated at EL = 9.1 J/ cmz; X; II sample irradiated at El. ,= 10,2 J/cmJ• R. B. James and W, H, Christie 3657 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.64.175.185 On: Wed, 03 Dec 2014 16:38:12sity, the surface layer contained no extended defects with a size larger than 20 A. which is the smallest size that can be clearly resolved in the micrograph. In addition, the van der Pauw measurements on the sample showed that all of the implanted arsenic was electrically active. The TEM micro graph, together with the electrical measurements, indicates that the entire implantation-damaged layer was melted by the laser pulse and that liquid~phase-epitaxial regrowth of the molten layer occurred. Another sample was also irradiated at 8.1 J/cm2 and then heated in a furnace to 973 K for 10 min to study the precipitation of the implanted arsenic. Figure 2(b) shows a cross-section TEM micrograph of the specimen after fur nace treatment. Arsenic-rich precipitates are observed to depths of about 250 A throughout the near-surface region. SIMS was used to measure the arsenic profiles before and after laser irradiation. The results are shown in Fig. 3 for several different energy densities. For EL less than 5 J/cm2, no redistribution of the implanted arsenic was observed. For higher pulse~energy densities, the arsenic was found to dif fuse to deeper depths due to the penetration of the melt front and subsequent liquid-phase diffusion in the near-surface re gion. For values of EL between 5.0 and 7.6 J/cm2, the maxi mum depth of arsenic diffusion is in the range of 500-800 A, indicating the existence of a window in the incident pulse energy density whereby one can easily obtain junction depths ofless than 1000 A. For EL > 8 J/cm2, the maximum depth of As diffusion begins to increase much more rapidly with increasing ELand reaches depths of wen over 1000 A for pulse-energy densities greater than 10 J/cm2• No surface segregation behavior was observed in any of the laser-irra diated arsenic-implanted specimens. THEORETICAL RESULTS We now present results of calculations using a finite dif ference (FD) method for solving nonlinear heat conduction equations. This FD method emphasizes the fundamental TABLE II. Input data for the melting model calculations. In the table N, is the concentration of ionized arsenic dopants, OJ is the angular frequency associated with the incident laser radiation, Pm is the magnetic permeability, c is the speed oflight, and Tis the lattice temperature. Here, Tis in units of Kelvin. Quantity Thicknesses of finite difference cells Density Thermal conductivity Solid Liquid Specific heat Solid Liquid Electrical conductivity ofliquid (u L ) Intrinsic free·carrier concentration Absorption (10.6I"m) Free-electron cross-section Free-hole cross section Extrinsic absorption coefficient at 293 K Absorption in liquid Reftectivity 00.6 ~m) Solid Liquid Latent heat of melting Melting temperature Substrate temperature Laser pulse First triangular pulse Second triangular pulse Total energy density 3658 J. AppL Phys., Vol. 65, No.9, 1 May 1989 Value used and comments 25 A for first cell; 50 A for remaining cells 2.33 gm/cm3 (-10% change on melting ignored) Temperature dependent; see Refs. 13 and 14 Temperature dependent; see Refs. 13 and 14 Temperature dependent; see Refs. 13 and 14 Temperature independent; see Ref. 13 Temperature dependent; value obtained using Wiedemann-Franz law with coefficient L equal to 2.612x 10-8 W H/K2; see Ref. 15 3.87X 10°(1)15 exp( -7020/1) (cm-3); see Refs. 6 and 7 See Refs. 10 and II 5X 10-17 (T/300K) + 3XlO-37 N/ 1.3 X lO~ 16 (T /300 K) + 3 X 10-37 NI See Fig. 4 0.45 1-[3.6X 1O-"/(ULA)'/2}; see Ref. 17 1799.1 J/gm 1683 K 293 K Two triangular pulses separated by 300 ns 60 ns (FWHM) with 80% of total energy 250 ns (FWHM) with 20% of total energy Varied R. B. James and W. H. Christie 3658 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.64.175.185 On: Wed, 03 Dec 2014 16:38:12role of enthalpy in a phase change process and uses tempera tures only to determine the heat fluxes in the sample. The sample is modeled as a slab extending in the positive x direc tion and composed of silicon layers with different free-car rier densities. The laser pulse is assumed to have a large radi us as compared to the penetration depth into the sample, so that melting occurs homogeneously i.n the y-z plane for a fixed pulse-energy density. While the original program, de scribed in more detail in Refs. 12 and 13, allows for most parameters to be dependent on the temperature, phase, and state of the material, it does not account fully for the depen dence of temperature and free-carrier concentration on the physical parameters relevant to this study. Therefore, the program was modified and the values we used for the various thermal and optical parameters are shown in Table n.6,7-17 A further discussion of the role of the temperature- and phase-dependent parameters is presented in Refs. 12-14. In order to accurately account for the free-carrier ab sorption, we must provide as input a depth profile of the free electrons resulting from the arsenic implantation and ther mal annealing. From the van der Pauw measurements, we found that approximately 31 % of the implanted As atoms were electrically active after the furnace annealing. Taking the solubility limit to be 3.6X 1020 cm-3 at a temperature of 873 K,9 we use the results of the SIMS measurements for the depth profile of total arsenic to obtain an approximate depth profile of the electrically active As dopants. These electrical ly active dopants donate free electrons to the conduction band, which can absorb the COzlaser radi.ation by intracon duction-band transitions. The electrons concentrated in the arsenic-implanted layer and the holes in the p-type substrate would like to diffuse to fill the crystal uniformly. As soon as a small charge transfer takes place, there is left behind an ex cess of positively charged arsenic atoms in the epiIayer and an excess of negatively charged acceptor atoms in the sub strate. This charge transfer by diffusion creates an electric field that inhibits further diffusion and attempts to maintain the separation of electrons in the heavily doped implanted layer and holes in the substrate. Because the built-in electric field only allows for a relatively small diffusion of the major ity carriers, we assume that the free-electron profile is given by the depth profile of electrically active arsenic. Using this approximation together with the measured values for the absorption cross sections, 10,11 the room-temperature absorp tion coefficient (ao) was calculated, and the results are shown by the solid curve in Fig. 4. At higher temperatures there is an increase in the intra conduction-band absorption cross section and an increase in the intrinsic, temperature-dependent free-carrier concentra tion. This causes the total absorption coefficient to increase with lattice temperature, where the relative amount of in crease at a particular depth depends on the extrinsic, free carrier density at that depth. Figure 4 shows the calculated absorption coefficient at a wavelength of 10.6 f.tm for lattice temperatures of750, 1000, and 1250 K. The rapid increase of ao with temperature at depths greater than about 600 A is primarily due to the thermal generation of electron-hole pairs, and the increase at depths less than about 500 A is primarily due to the increase in the intraconduction-band 3659 J. AppL Phys., Vol. 65, No.9, i May 1989 , E .!.! ABSORPTION COEFFICIENT VS DEPTH '\ ,~ II l~ Ii \ ~ II \\\ , \' \ i \~ I \~ I \" TEMPERATURE IKI -- 293 ---- 750 --~1000 ---1250 \~" \ ........... I \\ ------------ i,' \~ ,\ \\ \ \ \ '---------\ \ \ \ \ \ \ \ 102 o!J--.J----'--...J....~-l.B-~~-1J..2---'-----I16 DEPTH 1100 A ) FIG. 4. The solid curve shows the extrinsic absorption coefficient that was used as initial values (To = 293 K) in the melting model calculations. The other curves show the calculated absorption coefficient for lattice tempera tures of 750, 1000, and 1250 K. absorption cross section. Because of the temperature dependence of the absorp tion coefficient and the large increase in the surface reflectiv ity upon melting, the tpelt depths and the kinetics of melt front penetration and solidification depend on both the pulse-energy density and the shape of the incident CO2 laser pulse envelope. The calculated results presented in this pa per assume a triangular laser pulse, because this pulse enve lope closely resembles the pulses used in our measurements, In Fig. 5 the calculated results for the melt depth are shown as a function of the laser-energy density. The thresh old energy density ET for surface melting is computed to be 5.3 J/cm2, which is in good agreement with the melt-thresh old value of about 5 J/cm2 inferred from the van der Pauw electrical measurements (see Table I). Bel.ow the calculated threshold energy density, the enthalpy of the first finite dif ference cell (25 A thick) at the surface is too low for melting to occur, The calculated results show that for pulse-energy densi ties between 5.9 and 8.4 J/cm2, the maximum melt-front penetration (x m ) increases slowly with increasing E L' Upon melting of the first few finite difference cells at the surface, the surface reflectivity increases to about 90%, and almost all of the energy in the remainder of the laser pulse is reflect ed from the surface. For example, an effect of increasing El, from 6 to 8 1/ em 2 is to cause the onset of surface melting to decrease from 69 to 55 ns; however, once the hi.gh-reflectiv ity (molten) state of the surface occurs, there is little energy available in either pulse to drive the melt front to a much deeper depth in the material. For each pulse the rate of ener- R. B. James and W, H, Christie 3659 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.64.175.185 On: Wed, 03 Dec 2014 16:38:12MELT DEPTH VS PUlSEH,jERGY [)ENSITY A ~ 10,6 "m Tp~6ans 4 .< <:> ~ i= 3 ::b 0 ::. w ::;; 2 l 05-1 6 9 10 E[ IJ;cm2) FIG. 5. Calculated values for the maximum penetration of the melt front as a function of the incident pulse-energy density fo!' a 60-ns (FWHM) trian gular pulse, gy delivered during the high-reflectivity phase is comparable to the rate of energy removed from the molten cells by way of heat conduction, and the value of Xm remains almost con stant for a substantial portion of the laser pulse. During this portion of the pulse when Xm is varying slowly with time, the solidification velocity is small « 0.2 m/s) and much less than the calculated solidification velocities obtained for a ruby or excimer laser pulse. 14 The smaller solidification ve locity allows for more time for the arsenic to diffuse within the molten layer and may reduce the concentration of de fects observed after rapid liquid-melt quenching. 18 For pulse-energy densities exceeding 8.5 J/cm2, the on set of surface melting occurs at a time that significantly pre cedes the maximum intensity of the 60-ns (FWHM) trian gular pulse envelope. Although the high reflectivity of the molten surface rejects most of the pulse intensity, there is stilI enough energy absorbed in the first few FD cells to drive the maximum melt front (xm) to considerably deeper depths in the near-surface region, and x'" begins to increase much more rapidly with EL• The calculated threshold for surface melting of 5.3 JI cm2 is much larger than the melt threshold of 1.0 J/cm2 for a 70-ns (FWHM) triangular XeCl excimer laser pulse. IX Most ofthe difference in the melt thresholds results from the non-negligible temperature-dependent transmission of the CO2 laser pulses through the ion-implanted layeL In order for laser processing techniques to be commercially viable, it is important that the process be optimized for energy effi ciency. One way to significantly decrease the threshold for surface melting is to decrease the duration of the CO2 laser pulse, which can be easily achieved by using mode-locked laser pulses. 19,20 For the shorter CO2 laser pulses, the surface heating is much more rapid because there is less transport of heat out of the ion-implanted layer during the time the pulse 3660 J, Appl. Phys" Vol. 65, No.9, 1 May 1989 is incident on the sample. Since much less of the heat is con ducted into the underlying substrate ~uring the shorter pulses, the coupling of the laser energy is also more efficient due to the increased absorption coefficient of the CO2 laser radiation at the elevated lattice temperatures (see Fig. 4). Figure 6 shows the calculated results for the maximum melt depth (xm) as a function of the incident pulse-energy density for a I-ns (FWHM) triangular pulse. The melt threshold is calculated to be 0.62 J/cm2, which is more than eight times smaller than the threshold for a 60-ns pulse. Between about 0.74 and 0.90 J/cm2, the maximum melt front penetration is almost constant at a value of about 150 A. The window in the incident pulse-energy density for ob taining extremely shallow melt depths is narrower for the I ns pulse; however, the window is stm wide enough for spatial inhomogeneities of ± 15% without causing significant var iations in Xm over the laser-irradiated region. For E[. greater than 0.90 J/cm2, Xm begins to increase much more rapidly with E1, in a similar way as the results shown in Fig. 4 for a 60-ns pulse. The calculations also show that the velocity of solidification depends on the pulse duration and is generally much larger for the i-ns pulse than the 60-ns pulse. The larger regrowth velocities result primarily from the higher rate of pulse-energy deposition and larger temperature gra dients associated with the shorter pUlses. CONCLUSIONS We have shown that extremely shallow melt depths can be obtained by CO2 laser annealing anow-energy ( < 5-ke V) arsenic-implanted silicon layers. Similar results are expected for other silicon samples having a thin degenerately doped surface layer and an underlying lightly doped substrate. The primary advantages of using a COzlaser to achieve very shal low melt depths, as compared to a ruby or excimer laser, are 5 - MELT DEPTH VS PULSE,ENERGY DENSITY A ~ 10,6 "m T p -1 n8 4 1,3 FIG. 6, Calculated values for the maximum penetration ofthe melt front as a function of the pulse-energy density for a I-ns (FWHM) triangular pulse. R. B. James and W. H, Christie 3660 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.64.175.185 On: Wed, 03 Dec 2014 16:38:12that the pulse energy is deposited only in the thin heavily doped layer at the surface and the CO2 laser-induced melting of the surface layer causes the reflectivity to jump abruptly to a value of about 90%. The large and sudden increase in the reflectivity upon melting acts like a switch to reflect most of the energy in the remainder of the laser pulse and thereby greatly reduce the amount of pulse energy available for driv ing the melt front to deeper depths. For a 60-ns pulse and pulse-energy densities (EL) greater than about 7 J/cm2, we find that aU of the ion-implanted arsenic is electrically active and the near-surface region is free of any extended defects. The calculated melt depths versus incident pulse-energy density show that there exists a range in E L where the maxi mum melt-front penetration is less than a few hundred ang stroms and increases slowly with increasing Ev which is consistent with our measurements. ACKNOWLEDGMENTS We would like to thank R F. Wood, J. Narayan, G. A. Geist, D. H. Lowndes, P. H. Fleming, H. L. Burcham, Jr., D. C. Lind, J. R Adams, W. G. Wolfer, and M. 1. Baskes for many useful discussions. We would also like to acknowledge su.pport from the U. S. Department of Energy, Office ofBa sic Energy Sciences, Division of Materials Sciences. ISee,forexamplc, R. T. Young, G. A. van derLeeden, R. L. Sandstrom, R. F. Wood, and R. D. Westbrook, App!. Phys. Lett. 43, 666 (1983). 366i J. Appl. Phys., Vol. 65, No.9, i May 4989 2R. T. Young and R. F. Wood, Ann. Rev. Mater. Sci. 12, 323 (1982). 'See, for example, Pulsed Laser Processing a/Semiconductors, edited by R. F. Wood, C. W. White, and R. T. Young (Academic, New York, 1984), Vol. 23. JR. B. James,J. Narayan, W. H. Christie,R. E. Eby, O. W. Holland,lIndR. F. Wood, in Energy Beam-Solid Interactions and Transient Thermal Pro cessing, edited by D. K. Biegelsen, G. A. Rozgonyi, and C. V. Shank (Ma terials Research Society, Pittsburgh, FA, 1985), p. 413. 'R. B. James, J. Narayan, R. F. Wood, D. K. Ottesen, and K. F. Siegfriedt, J. App!. Phys. 57, 4727 (1985). OM. Blomberg, K. Naukkarinen, T. Tuomi, V. M. Airaksinen, M. Luoma jarvi, and E. Rauhala, J. Appl. Phys. 54, 2327 (1983). 7R. B. James, ill Pulsed La.~er Processing a/Semiconductors, edited by R. F. Wood, C. W. White, andR. T. Young (Academic, New York, 1984), VoL 23. pp. 555-625. "R. B. James, G. A. Geist, R. T. Young, W. H. Christie, and F. A. Greulich, J. Appt. Phys. 62, 298l (1987). "F. A. Trumoore, Bell Syst. Tech. J. 39, 205 (1960). lOW. G. Spitzer and H. Y. Fan, Phys. Rev. 106, 882 (1957). llW. G. Spitzer and H. Y. Fan, Phys. Rev. 108, 268 (1957). 12R. F. Wood and G. E. Giles, Phys. Rev.S 23, 2923 (1981). !JR. F. Wood and G. E. Geist, Phys. Rev. B 34, 2606 (1986). t4R. Po Wood, in Pulsed Laser Processing a/Semiconductors, edited by R. F. Wood, C. W. White, and R. T. Young (Academic, New York, 1984), Vol. 23, pp. 164-251. "C. Kittel, in Introduction to Solid State Physics, 4th ed. (Wiley, New York, 1971). pp. 263-264. 16J. D. Jackson, in Classical Electrodynamics, 2nd ed. (Wiley, New York, 1975), pp. 29(j....298. 171. M. Poate andJ. W. Mayer, in Laser Anl!ealillgo/Semicol!ductors (Aca demic, New York, 1982), pp. 47--48. 'S}. Narayan, in Defects in Semiconductors ll, edited by S. Mahajan and J. W. Corbett (Elsevier, New York, 1983), p. 491. 19 A. J. Alcock and A. C. Walker, Appl. Phys. Lett. 24, 306 (1974). 20e. R. Phipps, Jr. and S. J. Thomas, Opt. Lett. 1, 93 (1977). R. B. James and W. H. Christie 3661 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.64.175.185 On: Wed, 03 Dec 2014 16:38:12

1.2811535.pdf

New Products Citation: Physics Today 41, 8, 91 (1988); doi: 10.1063/1.2811535 View online: http://dx.doi.org/10.1063/1.2811535 View Table of Contents: http://physicstoday.scitation.org/toc/pto/41/8 Published by the American Institute of PhysicsNEW PRODUCTS The descriptions of fhe new products listed in this section ore based on information supplied to us by the manufacturers, and in some cases by independent sources. PHYSICS TODAY can assume no responsibility for their accuracy. To facilitate inquities about a parricular product, a Reader Service Card is attached inside the back cover of fhe magazine. Software System for Symbolic Computation Wolfram Research, a new computer software firm headed by physicist Stephe n Wolfram, has introduced a sophisticated softwar e system for symbolic and numerical calculations and graphical display. The software, called Mathematica, is the most ex- tensive of the so-called computer alge- bra systems (in the tradition of MAC- SYMA and Wolfram's own earlier sys- tem, SMP) to become available for microcomputers as well as for larger computers. It is available for the Apple Macintosh line, but because of the memory limitations of the MS- DOS operating system, Wolfram says the company currently has no plans to develop a front end for the IBM-PC or AT family of personal computers and their clones. (The system will be available for the IBM PS/2 computers once the operating system for the PS/ 2 is ready.) Mathematica can do numerical cal- culations to arbitrary levels of preci- sion. Given inputs of specified finite precision, it will deliver only as many significant figures as make sense. The system incorporates a complete «ParametncPlot3D r SpheriealPlot30[Abs[SphericalHarmonicY[3.1,theta,phil], {theta. 0, Pi, Pi/501, (phi, 0, 2 Pi, Pi/201, Boxed->Falae]set of standard mathematical func- tions, including the Bessel functions, spherical hamonics, elliptical func- tions and hypergeometric functions. It also incorporates efficient numeri- cal algorithms for matrix manipula- tion and numerial integration. Mathematica's most innovative ca- pabilities involve symbolic, as distin- guised from numerical, operations. All the standard operations of algebra and calculus are built in, including polynomical factorization and sym- bolic integration. Equations are solved in terms of analytic expres- sions rather than just numbers. Mathematica also offers impressive graphical capabilities. It can plot function s and data in two or three dimensions, in color. Given a symbol- ic description of an arbitrary geomet- rical object, the system produces a three-dimensional color picture. Two terse commands, for example, gener - ate a stellated icosahedron. The graphics output is in a resolution- independent format suitable for a wide range of graphics hardware. The architecture of the program allows it to be adapted for a variety of computers. The code for the basic computational routines is confined to the kernal of the program, which is designed to be independent of the machine on which it runs. To tailor the program to the input-output re- quirements of a specific computer, the manufacturer need only supply a "front end." We are told that the system is also designed for the straightforward addition of new com- putational modules written by other softwar e developers. The system is interactive, and it incorporates a high-leve l programming language. Mathematica has already been adopt- ed by a number of computer manufac- turers for personal and larger com- puters. In general, the manufac- turers will distribute the new software system to the user. An exception is the version of Mathema- tica written for the Apple Macintosh, which is being distributed directly byQuick & Easy Superconductivity Measurements LR-400 Four Wire AC Resistance & Mutual Inductance Bridge Ideal for direct four wire con- tact resistance measurements with 1 micro-ohm resolution Ideal for non-contact trans- former method measurements where superconducting sam- ple is placed between primary & secondary coils and flux ex- clusion causes a change in mutual inductance Direct reading Low noise/low power Double phase detection Lock-in's built in LR-4PC accessory unit avail- able for complete IBM-PC computer interfacing Proven reliability & perfor- mance. In use world wide. LINEAR RESEARCH INC. 5231 Cushman Place, Suite 21 San Diego, CA 92110 U.S.A. Phone: 619-299-0719 Telex: 6503322534 MCI UW Circle number 46 on Reader Service Card PHYSICS TODAY AUGUST 1988 91SCIENTIFIC GRAPHICS C02 SAT. VAPOR and SA1 N °~-^ 1 10 PRESSURE. aLm. LIQUID LINES 10 ' apheren 10 ' Linear-linear, log-linear, linear-log, and log-log graphs may de displayed with GRAPHER' Over 20,000 points may be displayed on one graph With GRAPHER' you may place axes and text anywhere Eacn graph component may De rotated to any angle and scaled to any size SURFERS quickly and easily creates contour maps Irom your irregularly spaced XYZ data You may specify axes with tic marks and labels posting, irregular contour intervals, and multiple shaped boundaries SURFER" has Ihe most impressive 3-D surfaces available Your 3-D surfaces will brilliantly visualize your data You may use your own XYZ data or enter an equation to generate a surface For the IBM PC & compatibles GRAPHER™ $199 SURFER~ $399 Demo/Tutorial Disks $10 FREE Brochure Give us a call for a free graphics brochure. 1-800-972-1021 (or 303-279 1021) GOLDEN SOFTWARE, INC. 807 14th St., Golden, CO 80401 *K~| Purchase orders are welcomeWolfram Research. The price is $495 for the original Macintosh and $795 for the Macintosh II. Eductional in- stitutions, we are told, will be granted substantial discounts. Addison-Wes- ley is publishing Wolfram's 768-page book Mathematica: A System for Do- ing Mathematics by Computer, a thor- ough introduction to the use of Math- ematica on a variety of computers. Wolfram Research, P. O. Box 6059, Champaign, Illinois 61821 Circle number 140 on Reader Service Card Automated Computer Clock Setting Service from NBS Computer users who need accurate time can now automatically set com- puter clocks with a new service of- fered by the National Bureau of Standards. Seismographic and astro- nomical data, for example, can be tagged by means of this service. NBS has initiated this Automated Computer Time Service (ACTS), which allows automated checking and setting of clocks through commercial telephone lines. The service provide s an accuracy of 1/10 to 1/1000 second, depending on the mode of operation. The new service will compensate for telephone-line delays, and it will pro- vide advance notice of daylight sav- ings time changes and leap seconds. The equipment needed to receiv e the signal is a 300- or 1200-baud modem and a computer (or simple processor chip). The service is now in a test phase, during which NBS is soliciting com- ments on format and operation. The telephone number for modem dial-up is 303-494-4774. There are three modes for checking or setting comput- er clocks: In the simplest mode, at 1200 baud, the user receive s a time code and on-time marker that has been advanced by a fixed period to account for modem and telephone- line delays. This should provid e 0.1- second accuracy. In a second 1200- baud mode, the user's modem will echo all characters back to NBS, allowing NBS to measure the delay and compensate for it more precisely. This mode should provid e an accuracy of better than 10 milliseconds. A third mode, at 300 baud, yields sub- stantially the same service as in mode 2, but with less modem asymmetry, so that accuracy is improve d to about a millisecond. More extensive documentation of the service is available on a 5-1/4 inch, 360-kilobyte DOS diskette for $35, prepaid, from the NBS Office ofStandard Reference Materials, B311 Chemistry Bldg., National Bureau of Standards, Gaithersburg, Maryland. 20899. Specify Automated Computer Time Service, RM 8101. Circle number 141 on Reader Service Cord Dual-Channel Joulemeter and Ratiometer Molectron has introduced its new Model JD2000, a microprocessor- based, dual-channel joulemeter and ratiometer. This laser instrument is designed for use with a variety of pyroelectric and silicon detector probes to cover a range from pico- joules to joules. The JD2000 can capture a single pulse or up to 40 pulses per second, displaying measurements in absolute volts, calibrated joules or average watts. Its large, backlit LCD display gives the value for channel A or B and the ratio A/B or B/A in linear or logarithmic units. The instrument can also average up to 100 pulses and display the standard deviation. One controls the operation of the JD2000 manually or through standard IEEE488 or RS232 interfaces. Molec- tron Detector, 1520B Dell Avenue, Campbell, California 95008 Circle number 142 on Reader Service Card Dual-Channel Digital Oscilloscope LeCroy's new model 9450 is a porta- ble, dual-channel 350-MHz digital os- cilloscope with high-speed analog-to- digital converters, wide bandwidth, long non-volatile acquisition memo- ries and a sophisticated trigger sys- tem. The 9450 offers the first com- mercial "flash" ADC. Each of the instrument's dual channels uses an independent ADC capable of digitiz- ing single-shot events at 400 mega- samples per second. This flash tech- nology yields a better signal-to-noise ratio, we are told, than do charge- coupled devices . Circle number 47 on Reader Service Card 92 PHYSICS TODAY AUGUST 1988NEW PRODUCTS The new oscilloscope has 50 K of memory per channel, and an addi- tiona l 200 K for storage. The acquisi- tion memory is also non-volatile, so that important waveform informa - tion can be stored indefinitely and used for future reference and analy- sis. Such long memories allow the capture of complex signals with im- proved timing resolution and more pre- and post-trigger information. Fine signal details can be examined with expansion functions. The Le- Croy 9450 lets one expand waveforms up to 1000 times horizontally and 10 times vertically. Long memories also provide higher sampling rates. Therefore the 9450 maintains a high single-shot bandwidth over a very wide range of time-base settings. Le- Croy, 700 Chestnut Ridge Road, Ches- nut Ridge, New York 10977. Circle number 143 on Reader Service Card Semiconductor Doping Profiles Solid State Measurements has intro- duced a new spreading-resistance sys- tem, the SSM 150, for performing detailed resistivity and carrier-con- centration depth profiles of existing or future silicon device structures. Applications of the SSM 150 include profiling ultrathin layers involvin g ion implants or diffusions. The spreading-resistance technique is also used in process development and con- trol for bipolar and CMOS devices, IMPATT structures, power devices and crystal growth studies. The SSM 150 is claimed to be "the most advanced system available for characterization for the electrical properties of semiconductor materials and processes." New hardware and software, we are told, make for high speed, flexibility and precision. Soft- ware written for the SSM 150 in the c language includes new Berkowitz- Lux correction algorithms to increase profiling accuracy. 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1.584174.pdf

Thermal stability of polyimidesiloxane (SIM2000) S. P. Sun, S. P. Murarka, and C. J. Lee Citation: Journal of Vacuum Science & Technology B 6, 1763 (1988); doi: 10.1116/1.584174 View online: http://dx.doi.org/10.1116/1.584174 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/6/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in The thermal stability and fragmentation of C60 molecule up to 2000 K on the milliseconds time scale J. Chem. Phys. 100, 8542 (1994); 10.1063/1.466755 XNR2000—A near term nuclear thermal rocket concept AIP Conf. Proc. 271, 1743 (1993); 10.1063/1.43035 Thermal stability of oxide films on Cd0.2 Hg0.8Te: A combined SIMS, AES, and XPS study J. Vac. Sci. Technol. A 1, 657 (1983); 10.1116/1.572203 Summary Abstract: Lithium compound identification in thermally activated batteries by ISS and SIMS J. Vac. Sci. Technol. 18, 750 (1981); 10.1116/1.570940 Thermal Diffusion and Convective Stability Phys. Fluids 15, 379 (1972); 10.1063/1.1693920 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 158.42.28.33 On: Mon, 22 Dec 2014 09:19:39Thermal stability of polyimidesiloxane (SIM-2000)S) s. P. Sun and S. P. Murarka Center for Integrated Electronics/Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180 C.J. Lee Occidental Chemical Corporation, Technology Center, Grand Island, New York 14072 (Received 27 May 1988; accepted 26 July 1988) Polyimides are finding increased use in integrated circuits as a dielectric and protective layer. Its low dielectric constant, ease of application, and ability to planarize the surfaces, permit their incorporation into very large scale integrated and ultra-large scale integrated circuit processing. However, there is no single polyimide available which possesses high-temperature stability at temperature > 300°C. A newer class of polymers called polyimidesiloxane (SIM-2000), resulting from the modification of polyimides by special equilibrated silicone blocks, has been found superior to commercial polyimides especially with respect to their high-temperature stability. In this paper, we present the results of our investigation of the high-temperature stability of a few polyimidesiloxane materials spun on various substrates including Si, Si02• and AI. I. INTRODUCTION The increases in the packing density and the resulting shrinkage of the device dimensions have required reduction in the interconnection resistance in the integrated circuits. Multilevel metallization schemes of a thin-film conductor and dielectrics (such as polyimide) can meet these needs to achieve high-density interconnections. 1,2 Polyimide is uniquely used as an interIevel dielectric because of its excel lent planarizing effect,3 its low thermal cures,4 and its good dielectric performance and ease of patterning vias.5 These properties have made it an attractive material in microelec tronics applications,6- 12 At present, only a limited number of polyimides are available for these applications. Momma et al.13 investigated modified polyimide-isoindaloquinazoline dione (PIQ-LlOO) because of its good planarization, and lower plasma and sputtering damage, Gildenblat et al. 14 sug gested silicone containing polyimide (SiPI) because of its good adhesion and resistance to water, and showed that it provided a higher dielectric strength. However, there is no single polyimide available which possesses aU the desired properties, particularly high-temperature resistance, A polyimide with thermal stability at 450 "C will be extremely useful in silicon integrated circuits. In this paper, a novel polyimidesiloxane (SIM-2000) used as spun-on dielectric has been found to provide good high-temperature stability. It is a block copolymer made from reaction products of a proprietary dianhydride with an organic diamine and an a, w-diamino siloxane. It is then fully imidized and is soluble in a polyether solvent (diglyme). Figure 1 shows the basic structure of this polyimidesiloxane. II. EXPERIMENTAL SIM films are deposited by spinning a solution of poly imidesiloxane onto different substrates. The films were then softbaked in air up to a temperature of 220·C for ! h to evaporate the solvent. Film thickness was measured by using Dektak profilometer. Film thicknesses up to 2 pm have been achieved by employing multiple spin coating and softbaking steps. Annealing (commonly called curing) was carried out in a cQnventional furnace as well as a rapid thermal anneal (RT A) unit over a range of temperatures in nitrogen am bient for short durations to investigate its thermal stability. A microbalance was used to monitor the progress of the film decomposition by measuring weight loss. Both thermogravi metric analysis (TG A) and R T A and fumance anneals were carried out during high-temperature cures. Infrared trans mission spectroscopy was then used to follow the changes in the functional chemical groups resulting due to high-tem perature treatments. III. RESULTS AND DISCUSSION SIM-2000 dissolved in diglyme (9% solids content) was spun on various substrate surfaces, namely, Si, Si02, AI, doped polysilicon, and previously prepared polyimidesilox ane. For as spun-on films very small «5%) variation in film thickness across the substrates was observed. As expect ed film thickness was a function of spin rate as shown in Fig. 2. For films softbaked at various baking temperatures, the film thickness stabilized at 180·C as shown in Fig. 3. The films heat-treated at 180 ·C are, henceforth, called softbaked films. Both spun-on and softbaked films adhered extremely wen with all types of substrates as determined by the qualita~ tive Scotch-tape-peel tests. To determine the high-temperature stability of the soft baked films, weight loss was measured as a function of time and/or temperature using a microbalance. Films annealed at 150 or 260·C for 24 h did not show any weight loss indicat ing the thermal stability up to a temperature of 260 ·C. Sub sequent to this observation, films were annealed at 450 ·C in a conventional furnace and weight change measured. Figure FIG. 1. General structure of the polyimidesHoxane. 1763 J. Vac. Sci, Technol. B 6 (6), Nov/Dec 1988 0734-211X/88/061763-0SS01.00 © 1988 American Vacuum Society 1763 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 158.42.28.33 On: Mon, 22 Dec 2014 09:19:391764 Sun, Murarka, and Lee: Thermal stability of polyimidesiloxane (SIM-2000) 1764 6000 ~------------------; o~ en en ~ 5000 -'" .~ .r: f- ~ 4000 a! x .2 '0; ~ 3000 'E >- "0 160°C CL 2000L--~--L--~-~-~--~50~0~O-~~6000 2000 3000 4000 Spin Rate (rpm) FIG. 2. Polyimidesiloxane thickness as a function of spin rates at various softbaked temperatures. 4 shows the weight loss as a function of time at 450·C in this furnace. For comparison the weight loss after a 60 s rapid thermal anneal at the same temperature is also shown. It is apparent that at 450°C there is practically an initial weight loss. After this initial loss, the film is stable and no further measurable loss with time is seen. The desired thermal treat ment to stabilize the film ofthis material will be a short time offurnace anneal or a short (1-2 min) RTA at 450 ·C. To compare these results obtained in the anneal-and weigh sequence with in situ weight change measurements, TGA of the SIM-2000 powder was carried out. Figure 5 shows the weight loss as a function of temperature. The re sults were obtained using dynamic TGA, with a heating rate of 10 'C/min. This figure clearly shows the stability of the material to about 300 'c in the nitrogen ambient supporting the early results of anneal for 24 h at 260°C. As the tempera ture increased above 300 ·C, the weight loss occurs first at a rapid rate, then at a slower rate, and finally again at a rapid rate which is followed by stabilizing at -600"C. The rela tively slower rate appears to prevail approximately between 400 and 480 ·C. The results of Fig. 5 indicate the possibility of more than one process that may be responsible for the weight loss in this material. Figure 6 shows the result ofthe isothermal TGA at 450 'c 5000 0$ 4800 (fj II) 4600 Q) c -" 4400 .2 .s:: f-4200 Q) C 4000 «I x 3800 E. 'iii 3600 Q) "0 'E 3400 >- "0 3200 0.. 3000120 140 160 180 200 220 240 Softbaking Temperature (O C) FIG. 3. Polyimidesiloxane thickness as a function of softbaked temperature at a fixed spin rate of 3000 rpm. Softbaking time: 30 min. J. Vac. Sci. Technol. B, Vol. 6, No.6, Nov/Dec 1988 100 Q) ~ 90 >< E. 80 'iii Q) 70 "0 'E 60 >-"0 500 RTA Unit Conventional Furnace ~ 4~ / /' ~O-~ :g 30 ..J 20 g 10 of. Annealing Time (min) FIG. 4. Percent weight loss of polyimidesiloxane as a function of annealing time at 450°C in nitrogen ambient for rapid thermal annealed (60 s) and furnace annealed films. after sample has attained 450°C. The initial loss occurred during heating to this temperature. Thus the weight loss of nearly 30% at a time very close to time = 0 in this plot corre sponds to the weight loss that is associated with a tempera ture rise from 300 to 450 °C in a time of 15 min. The rate of loss then decreases very rapidly and only a 38% weight loss is recorded after a 30 min anneal. TGA results seem to cor roborate the results in Fig. 4 obtained using a microbalance. They also indicate that the weight losses are similar in the softbaked films and the starting powder material. Rapid thermal anneal of the films deposited on silicon or oxidized silicon substrates was carried out at various tem peratures in the range of 350 to 550°C and weight loss was measured as a function of time. Figure 7 shows the results for films on silicon substrates. Similar results were obtained on SiO lSi substrates. At 350 ·C, a weight loss of nearly 5% is me:sured after 1 min of anneal and then the film stabilizes with no further loss. At 400, 430, and 450°C a gradual in crease in the weight loss up to 4, 4, and 2 min of anneal. The film is stable after nearly 22%, 33%, and 39% loss in the weight, respectively. At 500 and 550°C, once again, the film loses about 50% and 58% of weight, respectively, after first 1 min anneal and then is stabilized. The weight loss behavior observed during isothermal anneals (as shown in Fig. 7) is consistent with the TGA results of Fig. 5 where the weight loss occurred rapidly at temperatures between 300 and 380'C and above 480°C and rather sluggishly at tempera tures between 400 and 480°C. Figure 8 shows a plot of the film thickness as a function of the RT A temperature and for 1 min anneals. The ordinate is normalized with respect to the film thickness after softbake. The thickness, after 1 min R T A, decreases as the R T A tem perature is increased. The reduction in thickness is nearly 5%, 10%, 35%, 50%, and 65% at 350, 400, 450, 500, and 550 ·C, respectively. These numbers correspond very well with the % weight loss, after 1 min anneals, obtained from Fig. 7. This one to one correspondence in thickness reduc tion and weight loss would suggest that there was no change in the density of the films as a result of the thermal treat ment. More work is carried out at present time to understand this behavior. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 158.42.28.33 On: Mon, 22 Dec 2014 09:19:391765 Sun, Murarka, and Lee: Thermal stability of polyimldeslloxane (SIM-2000) 1765 100 90 80 --. ~ 0 70 .....; ~ 60 -.c 50 Ol Q.) ~ 40 30 20 10 100 200 300 400 Temperature (0 C) The IR spectra of the annealed polyimidesiloxane films and softbaked films are compared in Fig. 9. There are no major changes in the peak positions. However intensities of some peaks decreased after heat treatment. Table I summar izes the main observed changes. The changes occurred in the absorption peaks at (1) 1360, 1720, and 1775 em t, (2) 1310 em -I, (3) 725 cm-1, and (4) 800, 845, 1030, 1100, and 1260 cm-t which, respectively, could be associated with (1) the imide carbonyl group (C= 0). (2) methyl group ( -CH3) attached to the benzene ring, (3) methylene group -( CH2) n -between the siloxane segments and the imide. and (4) Si-CH3 or Si-O-CH, in the siloxane seg ments. The transmission infrared spectrum offuUy imidized 100 I- 90 t- 80 ;- -70 r:--~ Q ~ 60 - ~ -50 - .s::. OJ 'ID 40 r-S 30 ~ 20 - 10 I- 0 I L 1 ~ J I 0 ,0 20 30 40 50 60 Time (minutes) J. Vac. Sci. Technol. S, Vol. 6, No.6, Nov/Dec 1988 •• ".-,".".-.-.- •• ',' •••••••• ',. ••• ~ ••• < ••• :.:.:-:.:-; ••• ~ ••• ; •• ' •••••• ;> ••••••• :.:.:.;.:.:.:.:.;.;.;.~ •••••••• > •••••• ,:.'.'.' •• -.-.-••••••••• ,...... .-500 600 FlG. 5. Dynamic TGA of polyimidesi loxane powder in nitrogen ambient, heating rate is 10 "C/min. polyimidesiloxane, after soft bake ( 180 "C) and anneal (450°C) temperatures, indicated the intensity changes of absorption peaks for rigid imide segments. In addition. the change in the intensity levels of some flexible siloxane seg ments in cured polyimidesiIoxane measured was very large when compared with the levels of absorption measured for softbaked polyimidesiloxane. One can speculate that most of the initial weight loss at high temperatures occurs due to volatilization of the film and decomposition within the flexible siloxane segments which is connected to the rigid imide structure. Upon heat ing, the molecular motion of the silicones is relatively rapid and the side chains connected to it (i.e.,-CH3) become weak 1 70 60 90 FIG. 6. Isothermal TGA ofpolyimidesi loxanc powder at 450 OC (after the sam ple has attained this temperature) in ni trogen ambient. The initial loss occurs during the heating of the material from 300 to 450·C (at a 10 'C/min rate). Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 158.42.28.33 On: Mon, 22 Dec 2014 09:19:391766 Sun, Murarka, and lee: Thermal stability of polyimidesiloxane (SIM-2000) 1766 ~ 100r-------------------------------~ c: <a )( .Q 80 'iii ~ "U 'e 60 >.. (5 0.. '0 40 I/) I/) .3 20 ~ ~>-----o_____o 550·C •• -•• -•• -._ ..... .........,--- .... --_. 500· C ~-<>--¢---<)------¢----,---<> 450· C ~~_a---o---- ...... _-__ . 430· C .~-+----..... :----..... :---: ::::: "iF-O~~--~~~--~~--~~_~_L_~~ o 4 6 8 10 12 Annealing Time (min) FIG. 7. Percent weight loss of polyimidesiloxane/Si as a function of tem perature for rapid thermal anneals up to 10 min. in the early stage of the decomposition. The intensities of some imide segments decrease due to the breaking of C = 0 and CHr(benzene ring) bonds as observed in the IR spec tra. Further heat treatment would not result in more weight loss due to increasing rigidity of the molecular chain of the residual polyimidesiloxane. Therefore, most of the weight losses are speculated to be caused by thermal decomposition of the flexible siloxane segments. Identification of the species in the gas phase, at various temperatures, will help elucidate the mechanism more conclusively. Experiments, employing mass spectrometer and chemical analysis are in process to identify the gaseous species and elemental content, respec tively, of the softbaked and heat-treated material. IV. CONCLUSION In this investigation we have examined the stability of a polyimidesiloxane (SIM-2000) material at temperatures in the range 180-550·C, using both furnace and RTA meth ods. The stability was followed by measuring the changes in the thickness and weight of the softbaked film. TGA was performed on the parent material in powder form. The results indicate that the films and the powder are sta ble up to 300°C, and lose weight and thickness at higher m m ~ c: ..>< .2 1.0 ~ I- Q) c <a 0.6 )( .2 'V,j Q) "U 0.6 'e >.. (5 0.. "U 0.4 ~ .!::! m E 0.2 <; 350 450 550 Z Annealing Temperature (0 C) FIG. 8. Remaining polyimidesiloxane thickness after annealing at each tem perature for 1 min in RT A. J. Vac. Sci. Technol. B, Vol. 6, No.6, Nov/Dec 1988 c o ·00 .!!? E (/) c co f: Anneaied (SIM-2000) at 4500 C, 30 min, N2 Softbsked (SIM-2000; at 180°C, 30 min, air CM-1 FIG. 9. IR spectra of soft-baked and annealed films. temperatures. The loss in weight corresponds well with the thickness change at each temperature. There was an initial weight loss that occurred in .;;;4 min resulting in a film which did not lose any weight on continued annealing at the same temperature. The TGA results show three ranges: (a) a high weight loss rate region at 300-380 T, (b) a lower loss rate region between 400 and 480°C, and (c) another high-loss region between 480 and 600 °C. More experiments are in progress to understand this behavior. IR spectra of the soft baked and the 450 ·e annealed films clearly indicate changes in the intensities of several functional groups. At the present most of the weight loss is postulated to be associated with the volatilization of the film and the decomposition of the flexi ble siloxane segments. At 450°C the film loses about 35% of its thickness and then remains at the new thickness up to 30 min. This indi cates the 450 ·C-treated films have a potential for use as in terlevel dielectric film if other processing criteria are met. Weare in the process of acquiring the understanding and usefulness of this material. TABLE 1. Typical IR spectra ofSIM-2000. Characteristic Intensity Intensity infrared bands 180·C (softbaking) 450 'C (annealing) 1260 cm-I (Si-CH,) strong weak 1100 and 1030 em -I CR, (Si-d -) strong weak 845 and 800 em -I (Si-CR,) strong very weak 1775 em-I (carbonyl, C = 0) strong medium 1720 and 1360 cm .. , (carbonyl, C = 0) strong weak 72S cm-I -(CR2),,- strong medium 1310 cm I (CR3-O) strong medium Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 158.42.28.33 On: Mon, 22 Dec 2014 09:19:391767 Sun, Murarka. and lee: Thermal stability of polylmldesiioxane (SIM-2000) 1767 ACKNOWLEDGMENTS Authors SPS and SPM are thankful to Occidental Chemi cal Corp. for supporting this investigation and Dan Pulver for help in carrying out initial experiments. • ) SIM-2000 is a trademark name of Occidental Chemical Corporation. 'M. Teresawa, S. Minami, and J. Rubin, Int. J. Hybrid Microelectron. 6, 607 (l983). 2J. F. McDonald. A. J. Steckl, C. A. Neugebauer, R. O. Carlson, and A. S. Bergendahl, J. Vac. Sci. Techno!. A 4,3127 (1986). 'L. B. Rathman, J. Electrochem. Soc. 127,2216 (1980). 4A. M. Wilson, in Polyimides:Synthesis. Characterization, and Applica tions, edited by K. L. Mittal (Plenum, New York, 1984), Vol. 2, p. 715. J. Vae. ScI. Techno!. S, Vol. 6, No.6, Nov/Dec 1988 ••• " •• ".-.-••••••• >? ••••••••• ~ ••••••••• -.~.;.;.;-;.; ••• , •••••••••••••••••••• : ••••••• 50. Samuelson, Org. Coat. PJast. Prepr. 43 (2),446 (1982). 60. A. Brown, in IEEE Reliability Physics Symposium (IEEE, New York, 1981), p. 282. 7A. M. Wilson, Thin Solid Films 83,145 (1981). sp. W. Schuessler, Int. J. Hybrid Microelectron. 6 (l), 342 (1983). 9S. D. Senturia, R. A. Miller, D. D. Denton, F. W. Smith, and H. J. Neu haus, in Ref. 4, p. 107. "'D. R. Day, D. Ridley, and S. D. Senturia, in Ref. 4, p. 767 . "D. R. Sato, S. Harade, A. Saiki, T. Kimura, T. Okubo, and K. Mukai, IEEE Trans. Parts Hybrid Packag. 9 (3),176 (1973). I2A. M. Fraszer, in Polymer Reviews (Wiley, New York, 1968), Vol. 17. Chap. 7, p. 315. 13M. Yataka, K. Noriyuki, H. Mitsuru, N. Shunichi, and M. Naohiro, IEEE Trans. Electron Devices 34 (3), 621 (1987). 14G. C. Davis. B. A. Heath, and G. Oildenblat, in Ref. 4, p. 847. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 158.42.28.33 On: Mon, 22 Dec 2014 09:19:39

1.343709.pdf

Temperatures in the plume of a dc plasma torch D. A. Scott, P. Kovitya, and G. N. Haddad Citation: J. Appl. Phys. 66, 5232 (1989); doi: 10.1063/1.343709 View online: http://dx.doi.org/10.1063/1.343709 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v66/i11 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsTemperatures in the plume of a de plasma torch D. A. Scott, P. Kovitya, and G. N, Haddad Commonwealth Scientific and Industrial Research Organisation (CSIROJ Division of Applied Physics, P. O. Box 218, Lindfield, N.S. W. 2070, Australia (Received 22 March 1989; accepted for publication 26 July 1989) A magnetohydrodynamic model of a plasma torch that describes the complete torch system from the gas injection, through the arc region, and out into the plasma plume is presented. It is a two-dimensional model but includes a swirl component of the flow and the K-E model of turbulence and assumes local thermodynamic equilibrium. Temperatures i.n the plume of a plasma torch have been determined spectroscopically by measuring the emission from neutral argon. atoms, and these temperatures have been compared with the predictions of the model. I. INTRODUCTION The use of plasma torches as clean and efficient heat sources in industry has been common for many years, but many of the applications, such as materials processing, would benefit from a better understanding of the physical processes governing the interaction of the plasma and the injected feedstock. The temperature and velocity fields in the arc plume as it exits from the nozzle are of special interest as the interaction generally occurs within this region. Previous theoretical studies1-1 of plasma torches have described the exit region of the torch. Turbulent flow was included in these studies using theK-emodel,4 which is espe cially suitable for two-dimensional calculations. An obvious disadvantage in these approaches is the necessary specifica tion of the temperature and velocity profiles across the noz zle exit as boundary conditions. The results can be in fluenced greatly by the upstream boundary conditions. For example, McKelliget et al. l assumed flat temperature and velocity profiles at the nozzle exit, while Dilawari and Szeke ly2 assigned modified parabolic temperature and velocity profiles; their computed results showed significant differ ences in the decay of temperature as the distance from the nozzle exit is increased. Lee and Pfender~ studied the effect on the flow of two different temperature profiles at the noz zle exit. They concluded that while the profiles had the same mass-and energy-flow rates, the largest temperature differ ence in their region of calculation 'N1iS 3000 K and the largest velocity difference was 300 m s 1. These differences repre sent variations in the temperatures of about 30% and in the velocities of about 100%. As these models do not include the arc region, the tem~ peratures and velocities of the plasma in the plume region can only be predicted from gross inputs such as mass-and energy-flow rates, which themselves require knowledge of the voltage-current and voltage-flow characteristics. These problems can. be overcome ifthe whole torch, which includes the region behind the cathode, the arc region, and the plasma plume region, can be modeled as a single entity. The bound ary conditions at the nozzle exit then result naturally from conditions in the arc and pre-arc regions, and the gas flow at the nozzle exit is fully specified. Hence the exit assumptions can be dropped. In this paper we describe a model of the complete torch system from gas injection behind the cathode, through the anode nozzle, and out into the plume. It is a two-dimensional model but includes a swirl component of the flow and the K e model of turbulence and assumes local thermodynamic equilibrium. The equations of this model and the numerical techniques used to solve them are presented in Sec. n. Previous experimental temperature measurements in plasma torches have included the use of probes and optical techniques. 1.5.6 Calorimetric probes are limited to measuring temperatures less than 10 000 K and have poor spatial reso lution. Several different spectroscopic techniques may be employed in which the following are measured: absolute in tensities of atomic lines. relative intensities of atomic lines, Stark width and Doppler width of atomic lines, and, in cases where the plasma gas includes diatomic molecules, the rela tive intensities of rotational and vibrational lines of molecu lar spectra. The present experiments were designed to mea sure the excitation temperature in the plume of the plasma torch using spectroscopic techniques and are outlined in Sec. HI. In Sec. IV we compare the measured temperature fields with the predictions of the modeL II. THEORY In the present model of the plasma torch a magnetohy drodynamic approach is employed in which it is assumed that the collision times of the particles in the plasma are much smaller than the time constants of the flow field so that the particles can be considered as localized. Thus the Boltz mann distribution function can be replaced by the number density and the mean velocity, and the plasma behaves like a fluid, which can be described accurately by the equation of state and the con.servation equations. Axial symmetry is assumed, so cylindrical coordinates (r,e,z) are used and all variations in the e direction are ne glected. The plasma is assumed to be in local thermodynam ic equilibrium (L TE) and optically thin. The flow is as sumed to be subsonic, and density variations due to pressure changes caused by the flow are neglected. The ambient pres sure is taken to be 1 atm (10 1. 3 kPa) in all cases. The equa tions that result are similar to the equations used by Kovitya and Cram 7 for the two-dimensional model of the gas-tung sten welding arc. The K-E turbulence model of Launder and Spalding4 is used to estimate the effect of turbulence on ener gy and momentum transfer. Density fluctuations caused by 5232 J. AppL Phys. 66 (11), 1 December 1989 0021 -8979/89/235232-08$02.40 @ 1989 American Institute of Physics 5232 Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsturbulence are ignored. As the plasma and the ambient gas are both assumed to be argon, mixing effects that result from the difference in gas enthalpies, say, for an argon plasma jet operating into air, cannot arise, This is unlike the models of Dilawari and Szekdy2 or Lee and Pfender3 which assume the gas and plasma to be a binary mixture of "frozen" (i.e., nonreacting) components consisting of the plasma at a high er temperature and the gas at a lower temperature. Thus a single temperature (and pressure) defines the thermody rw.rnic state of the present torch system. The above assumptions are the standard assumptions for plasma flow models. The equations of the model are based on the equations given in Bird, Stewart, and Lightfoot8 for the cylindrical coordinate system and are as follows. Mass conservation: 1 d a --(rPVr) + -(pv?) = O. r dr c1z " . (1) Radial momentum conservation: ( aUr V~ aUr \ p u ---+u -;; \ r dr , Z az dP. 1 a ( aUr) = ---JzB o +--;-2r7J-a, r ar cf' a [(au z aUr)] v, +- 11 -+- -211-· az ar aZ ,2 (2) Azimuthal momentum conservation: -+-~ (17 avlI). dz az (3) Axial momentum conservation: ( aVz auz) v -v -= P r dr + Z az 1 a [ (av. aVr)] +-- rTf --+-. (4) r ar ar az Energy conservation: }; + j; U 5k (. aT . aT) =---+-Jr-+Jz-u 2e \ ar az + ~i!.. (rK aT) + ~ (I( aT) r ar ar (JZ az ap ap +Vr-+V,-, ar -aZ (5) Electric current continuity: ~~ l/m. a¢;) +~ (u a¢;) = 0, r a, (1r az dz (6) . a¢;. a¢ Jr=U-' Jz=U-. ar dz (7) In these equations the variables are pressure (P), radial ve locity (vr), azimuthal velocity (ve), axial velocity (vz)' plasma temperature (T), electric potential (¢), and radial 5233 J. Appi. Phys., Vol. 66, No. 11, 1 December 1989 and axial current density (j,. and}z). The plasma properties are density ( p), heat capacity (cp ), radiative loss ( U), elec trical conductivity (0'), and the azimuthal magnetic field (Be)' The viscosity (11) and thermal conductivity (K) in clude both laminar and turbulent components, rj=r!t+Yf" K=K,+K t• (8) The laminar components are derived from kinetic theory while the turbulent components are determined using the K E turbulence model. The other terms are the Boltzmann con stant (k) and the electron charge (e). The magnetic field is obtained using Maxwell's equation, 1 d , .D ) • -- ~rn(i =PoJz' , ar (9) where Po = 4rrX 10 7 H m . I is the permeability of free space. In the K-E turbulence model, which is incorporated in the present calculations, the eddy viscosity Tft and the eddy thermal conductivity Kt are obtained from (10) where K is the turbulence kinetic energy, e is the dissipation rate of the turbulence kinetic energy, Pr, is the turbulent Prandtl number, and C'I is one of the constants of the mod e1.4 The two turbulence variables K and E are obtained by solving ( dK. BK) d [( Yft) dK] P Ur-<-;-V z-=G-pE+- Yf,+- - \. dr Jz dz S K dz 1 d [ ( Yft \ /JK] +-- r 1l! +-)-r dr SK ctr (11 ) and ( rh; ae) p v,-+v z-dr az =CIG~-C2P'; +~i!..[r(711+~)ae] K K r ar SF 81' + ~[("tll + 17,) de] , Jz s. dz (12) where G is the product of the eddy viscosity and viscous dissipation terms. The constants in the turbulence model equations are given in Table I. These are the suggested values of Launder and Spalding4 for use in the case of flow in pipes. In the exit region, where the flow is freely expanding, Rodi9 suggested the following modifications to the param eters e'l and C2 that we have adopted: C'I = 0.09 -0.04J, C2 = 1.92 --0.0667/, (13) TABLE I. Table of turhulence parameters. C 1 C• 2 Pr, 1.44 1.92 1.0 1.3 0.09 0.9 Scott. Kovitya, and Haddad 5233 Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionswhere/is given by /= I~(a(vz)o _I a(Vz)O 1)1°.2 , 2f'>.u Jz aZ (14) where 8 is the diameter of the jet, !:s..u is the difference between the center-line velocity and the free-stream velocity, and (vz)o is the center-line velocity. The maximum Mach number for a 200-A arc with a 30-t' min -I gas flow is 0.35 inside the nozzle. For this case the assumption that pressure variations are small compared to the absolute pressure is justified. For higher flows the veloc ities remain subsonic, so that for the worst case in our calcu lations (400 A, 50 {'min -1) the Mach number is less than 0.8. The Reyno!ds number for a 200-A arc at a flow rate of30 11min -I is about 2000 in the current-carrying region and drops to 600 at the nozzle exit. The reason is that the viscos ity increases by an order of magnitUde as the plasma tem perature drops from 20000 to 12000 K, while the density only increases by a factor of 2. Gravity is not important, as the Froude number is about 3 X 106• All equations are then solved iteratively using the con trol-volume approach of PatankarlO with nonuniform grids to give Ur> VB' Vz' T, cj;, P, K, and E, This method has been widely used by McKelliget et al., I Dilawari and Szekely,2 and Lee and Pfender.3 The advantage of the Patankar meth od i~ that it is highly stable and converges relatively easily, but It does suffer from numerical diffusion and is thus less accurate than higher-order finite-difference methods. On the other hand, higher-order methods tend to have conver gence problems and use more computer time (see the review article of Patankarll). To generate the temperature profiles presented here a grid of 30 X 30 control volumes is used. The ratio of the largest grid to the smallest grid is about 15, al though the stretching ratio between adjacent grids is always less t~an 2. To test the accuracy the calculations were repeat ed wIth double the number of control volumes in both direc tions, that is, 60 X 60 control volumes. For temperatures greater than 100C{) K the differences amount to less than 4%. In the region where we are making a direct comparison of experiment and theory (that is, in the nlume where T> 6000 K) the differences are less than 10%. Where the temperature gradients are large, for example, near the wall of the nozzle and in the mantle of the plasma plume (at temperatures < 3000 K for z < 25 mm), the differences are up to 20%. However, because of the steep gradient of the t~rnperature contours, even this figure represents a spatial dIfference of the temperature contours of less than 0.2 mm. The plasma properties are obtained using the method of Gordon and McBridel2 to calculate the plasma density and heat capacity, the method of Devoto 13 to calculate the vis cosity and the thermal and electrical conductivities, and the me,thod of Cram 14 to calculate the radiation loss. The prop e.rttes are then calculated for each node by linear interpola tlOn from a table with temperatures spaced at 1000-K inter vals. Convergence is attained when Eqs. (1 )-( 6), (10), and ( 11) ,are satisfied to within 1 % of the largest contributing term in each of the equations for every control volume. Fur thermore, the integrated mass flow through the nozzle is checked to be the same (to four significant figures) at each 5234 J. Appl. Phys" Vol. 66, No. 11, 1 December 1989 axial position. The calculation for the 30 X 30 control vol umes takes 300 central processing unit (CPU) seconds on a CYBER 205 for 600 iterations. The plume region and the nozzle region are calculated concurrently. Figure 1 shows typical results of this calculation, illus trating the temperature contours [Fig. 1 (a) J and the mass flow contours [Fig. 1 (b) J predicted by the model with oper ating parameters of 600 A current at an argon flow rate of 30 t'min -I. (In this paper all volume-flow rates are in standard t' min -1.) A radial cross-sectional view of the torch is pre sented (the z axis is the axis of cylindrical symmetry) show ing the cathode (shaded) in the lower left corner with an included angle at the tip of 60·, and the hollow anode nozzle (also shaded) above and to the right of the cathode. The contour steps for the temperature are f'>.T = 2000 K with the outermost contour 1000 K and the innermost 23 000 K. The exit temperature of the gas on the axis is calculated to be approximately 15 000 K. The (integrated) mass-flow rate as a function of radius is defined as the rate at which mass (that is, argon) flows through a circle of radius r centered on the axis. The mass-flow rate is shown here as it illustrates the effects of entrainment outside the nozzle of the plasma torch. The contour steps for the mass-flow rate are f'>.m = 0.2 g s -! and the center-line mass-flow rate is zero (as r is zero). The turbulence model is applied from the entrance of the nozzle (where the arc root occurs) through to the region outside the nozzle in the plasma plume. The flow upstream of the nozzle entrance is assumed to be laminar as the gas has not been accelerated by arc heating. Outside the nozzle the ef fects of turbulence can be seen: the temperature profiles are flared and there is considerable entrainment of gases from th~ external atmosphere into which the plasma torch is oper atmg. 20 40 60 80 z (mm) 20 40 60 80 z (mm) FIG. I. (a) Temperature contours for a 6OO-A arc in argon at 30 t'min-I, The ,contour steps are AT = 2000 K, the outermost contour is T = 1000 K, the mnermost T = 23 000 K. (b) Integrated mass-flow contours for the same arc, with the contour step Am = 0.2 g 8-1, the center-line mass-flow rate IS () and the outermost contour is 2.8 g s -I. Scott, Kovitya, and Haddad 5234 Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsOne of the major problems in the modeling of the plas ma torch is the assumption of axial symmetry. It is known that the anode root moves in a circular path assisted by the swirling gas.S,IS If the assumption of axial symmetry is dropped, the problem becomes three-dimensional in nature and its solution is not feasible at present because of computer time considerations. However, a simplifying assumption can be made to keep the problem two-dimensional. The diffi culty that arises is that the electrical conductivity of argon falls rapidly below about 8000 K, and the gas is essentially nonconducting below about 5000 K. If the electrical conduc tivity as a function of temperature is calculated assuming L TE, then the resistance of the incoming gas between the anode and the cathode is very large. As a result ohmic heat ing of the gas occurs, the electrical conductivity rises quick ly, heating decreases, the conductivity falls quickly, and the solution becomes oscillatory. Because of the anode spot mo tion, the plasma and the gas flow near the anode become mixed and the average temperature is lower than the plasma temperature. However, the arc current is still carried through the anode spot and the effective electrical conduc tivity remaIns high. As an approximation to these complicat ed electrode effects we have assumed that for temperatures below 9000 K the electrical conductivity 17 (in units of S em I) varies according to a power law given by 17 = O.2eT12000 (15) and for temperatures above 9000 K the conductivity is equal to the calculated (LTE) values. While the arc voltage is par ticularly sensitive to the choice of the /7-T relation, experi ments show that the anode root attaches itselfto the point on the anode closest to the cathode, and therefore the choice of any 17-T relation is limited to those that permit the arc cur rent to flow between these two points. Moreover, if the total nylon Fl G. 2. Diagram of the plasma torch. 5235 J. Appl. Phys., Vol. 66, No. 11,1 December 1989 power input for the same plasma current remains the same for any changes in the (T-T function, then the temperature profile at the nozzle exit is insensitive to the changes. There fore this method represents an advance over previous meth ods because the plasma temperatures in the plume depend only on the total power input. These assumptions are sup ported by comparing the predicted and experimental volt age-current and voltage-gas-flow characteristics (see Sec. IV A) as these parameters are very sensitive to the near electrode approximations. m. EXPERIMENTAL ARRANGEMENT The plasma torch used in this series of experiments con sists of a water-cooled thoriated tungsten cathode and a wa ter-cooled copper anode nozzle of 6 mm internzl dizmeter. A scale diagram of the plasma torch appears in Fig. 2. The arc is vortex-stabilized by the injection of the plasma gas via a coaxial swirler mounted behind the cathode. Argon (99.999% pure) is used as the plasma gas, and the maximum power at which the torch may be run is 25 kW, with typical operating conditions being 600 A, 30 V at a gas flow of 30 {min -I. The experimental arrangement used to measure the temperatures in the plume of the plasma torch (shown in Fig. 3) is essentially that used by Haddad and Farmerl6 for temperature measurements in free-burning arcs. The plasma torch is mounted in an x-y-z computer-controlled table. The light from the plasma plume passes through an aperture and is imaged by a pair of lenses onto the entrance slit of a I-m monochromator. The effective spatial resolution of the opti cal system is 10'0 X 200 pm. The monochromator spectral resolution is set wide enough ( 1.2 nm) to pass the full width of the spectral line being measured, and the signal level from the photomultiplier is recorded by the computer as the x-y-z table is scanned in the horizontal direction (that is, the x direction across the diameter of the arc). At each point in the plasma the signal level is measured over a period of 0.4 s, so the emission intensity obtained in this experiment is the average value over a time that is long compared with the turbulence period, As the plasma plume is axially symmet ric, these integrated line-of-sight intensities can be converted into radial intensity distributions using the Abel inversion lens Iei'lS aperture plasma torch FIG. 3. Experimental arrangement used to measure excitation tempera tures in the plume of a plasma torch. Scott, Kovitya, and Haddad 5235 Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsprocess. Horizontal scans are taken at various vertical posi tions, thereby building up a map of the radial intensity pro me emitted by the torch. The intensity of a spectral line as a function of tempera ture can be written as neT) (-E\ SleeT) =K--exp --), u(T) kT (16) where K is a constant dependent on the atomic properties, u (T) is the internal partition function, n (T) is the atom number density, and Eis the energy ofthe upper level of the transition. For a high-temperature plasma in local thermo dynamic equilibrium, the intensity of a spectral line initially increases with temperature as the number density of atoms in the upper level of the transition increases, but then de creases as the number density is reduced due to both expan sion and ionization of the plasma. The temperature at which the emission passes through a maximum is called the "nor mal" temperature, For the 696.5-nm line of neutral argon the "normal" temperature at a pressure of 1 utm is 15 200 K. Although this temperature is not reached in the plume of the plasma torch used here, it is reached in the positive column of a free-burning argon arc, Therefore by substituting a free burning argon arc [in the form of a tungsten inert gas (TI G) welding torch] for the plasma torch, the optical system may be calibrated. This procedure allows radial temperature dis tributions to be derived from measured radial intensity dis tributions, The temperatures measured using this technique were checked by using a line ratio method. Emission intensities from several neutral argon lines were measured, and, assum ing a Boltzmann distribution of states, by taking the ratio of these intensities the temperatures may be derived. This method yielded temperatures that were in good agreement (within 5%) with those derived using the emission tech nique. However, in order to measure temperatures with enough dynamic range and with acceptable precision, the difference in the transition energy of the two lines used in the line ratio technique needs to be large. It is difficult to find two lines in the argon neutral spectrum that satisfy this crite rion and do not have overlapping lines or suffer from self absorption in the plasma. Ratios between argon neutral and ion lines could also be used but the ion line intensities in the plume of the plasma torch are very weak. For these reasons the emission intensity technique is used in preference to the Ene ratio technique. IV. RESUl1S A. VoltagEH:urrent and voltage-flow characteristics The two most readily controllable operating parameters ofthe plasma torch are the current and the gas flow. In order to check the near-electrode assumption of the model out lined above, the predicted voltage-current and voltage-flow characteristics were compared with the measured values. Figures 4(a) and 4(b) illustrate these comparisons, The model does not include the potential falls associated with the electrodes and so only the functional dependence of the two voltage characteristics should be compared, rather than the absolute values. It can be seen that the V-J curves are in fair 5236 J. Appl. Phys., Vol. 66, No. 11, 1 December 1989 40 30 ~ III ell ('II .... 20 '0 :0 10 () 0 la! 50 40 E 30 <II eo 0:1 "" "0 '" 20 10 0 0 (bl .... ---·---8-- __ & __ --0.---0 theory 100 200 300 400 500 600 current (A) expt ..... ~ ...$" .A'" .. ~ theory ..N~ ~ A' ~ ... _ ....... ~.A1' 10 20 30 40 50 flow II min-') 60 PI G. 4. (11) Voltage-current characteristics of the plasma torch operating in argon at 30 f min .-" showing experimental results (-) and model predic tions (---). (b) Voltage-flow characteristics of the plasma torch operating at a current of 400 A, experiment (-) and theory (---). agreement, both curves indicating that the voltage is largely independent of the current. The theory predicts a V-I char acteristic that has a slightly negative slope, whereas the ex perimental data indicate a slightly positive slope for most of the range of current. The sum of the anode and cathode fans appears to be about 12 V, The predicted V-flow characteris tics are in quite good agreement with the measured values, and again the difference between them indicates the elec trode faUs total around 12 V. As yet we have no dear expla nation of the minor discrepancies between theory and exper iment illustrated in Fig. 4, although the model calculations indicate that these characteristics are very sensitive to the changes in the boundary condition on the electrical conduc tivity, as discussed in Sec. II. However, in general, these re sults indicate that the simplifying assumptions incorporated Scott, Kovitya, and Haddad 5236 Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsinto the model to account for complicated electrode effects are adequate and that the other predictions ofthe model may be treated with reasonable confidence. B. Temperature profiles We have investigated the effects of the two major oper ating parameters, gas-flow rate and current, on the tempera ture profiles in the plume of the plasma torch. Figures 5 (a) and 5(b) show the experimental and theoretical tempera tures as functions of radius 1 mm from the nozzle exit of a 400-A arc. Data are shown for gas-flow rates of 20, 30, and 40 t'min-I, and it is clear that the temperature profiles are essentially independent of flow rate. Figures 5 (c) and 5 (d) show profiles 15 mm from the nozzle exit and indicate the agreement between experiment and theory is reasonable. The experimental results show that the temperatures near the axis are relatively unaffected by changes in the gas-flow rate and that the plasma appears to become marginally broader with increasing flow rate. The plasma current has a larger effect on the tempera ture profiles. Figures 6(a) and 6(b) show experimental and theoretical temperatures as functions of radius 1 mm from the nozzle exit for a plasma torch operating with a gas flow of 30 t'min- 1 and currents of 200, 400, and 600 A. In this case the temperatures near the axis are considerably higher for the higher currents, and further from the nozzle exit, 15 mm in Figs. 6(c) and 6(d), the plume becomes both longer and broader for the higher currents. Two effects are present here: an increase in the gas velocities (as the effective nozzle area is reduced by the larger arc) and an increase in the power delivered to the plasma, both caused by the increase in cur rent. Both of these will contribute to the lengthening of the plasma plume. However, if turbulence is not included in the model, no significant increase in the width of the plasma 14 lal z=1 mm '4 \"" 13 "-u,,1 13 --~ 12 .~ 12 -__ " !h .. ",¥ 1~ 11 10 10 ~ 9 9 ~ 10 13 S ~\ 10 ~ C 7 1 K' >< Go 2 3 aO 2 3 ~ '" 14 14 .... (0) a;uHi ml'ft (ell •• 15 m", 1:: il.I 13 up! 13 I~ .. ",y 12-12 12 ~ IE: ! 11 , 11 '~ , HI ',"':: HI 9 " "<::, 9 e " "':: a 7 '" "'" 7 50 2 3 5 0 2 :; radius (mm} FIG. 5. Radial temperature distributions for different gas-fiow rates at 400 A current: 20 imin-I (---),30 f'min-I (---), and 40 fmin-' (-). (a) Experiment and (b) theory for z = I mm; (c) experiment and (d) theory forz= 15 mm. 5237 J. Appl. Phys., Vol. 66, No. 11. 1 December 1989 14 13 12 11 10 9 13 7 a()!;--~--!2:---.L3 ~~ "'~(b) .,1 Mm ~ _" t~ .. "ry 12 '-, "- 11 ", "- H) \ \ II \ \ \ 13 '-\ \ . 7 \ \ \ radius (mml FIGo 6. Radial temperature distributions for different currents at a gas-flow rate of30 fmin I: 200 A (---), 400 A (---), and 600 A (-). (a) Experi ment and (b) theory for z= I mm; (e) experiment arId (d) theory for z=ISmm. plume is predicted as the cu.rrent is increased (only the length increases), and hence we infer that it is largely the increase in gas velocities and the consequent increase in the turbulence that cause the broadening of the plume. Again the model predictions are found to be in agreement with the trends demonstrated by the experimental results. Figure 7 shows a comparison of experimentally deter mined temperatures and temperatures predicted by the model outlined above. The torch parameters arc 400 A, 33.2 V, and 30 t'min --1 argon gas flow. There is reasonable agree ment between experiment and theory, especially in the hot ter regions of the torch; however, there are clear discrepan cies in the cooler regions. This divergence is better illustrated by the temperatures as a function of radius at different dis tances from the nozzle exit for fixed current and gas-flow --- Ellipt ------ theory ::: i 2 ! 1 '" 0 ::I 1 15 III 2 .. ::: 0 10 15 20 z (mm) FIG. 7. Experimental (-) am! theoretical (---) temperature contours for a plasma torch with parameters 400 A, 33.2 V at 30 (min-I. The plasma torch nozzle exit is at z = 0 mm and the nozzle radius is 3 mm. Scott, Kovitya, and Haddad 5237 Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions18 ~ 16 --- expt ------ theo~l' 0 14 0 0 ... >( 10 (I) a .. ::3 .... (; «II .. CD 4 Q, E CP 2 ... ------ CI 0 4 radius (mm) FIG. 8. Temperatures as a function ofmdius at different distances (z = 5, 10, and 20 mm) from the nozzle exit. Torch parameters are 400 A, 33.2 V, and 30 {min I rate (see Fig. 8). It can be seen that the difference between the model predictions and the experimental results becomes larger for temperatures below about 10000 K. These discrepancies may be explained via a number of qualifications that should be made with regard to these mea surements. First, no account is taken in the model for the variation in number density in plasma jets due to turbulence. The present spectroscopic measurements determine the "unweighted" time-averaged temperatures. If number-den sity fluctuations had been included in the model (by the inclusion of the density in the time-averaged integral), a "mass-weighted" average temperature would have been ob tained; this temperature can be measured by using an enthal py probe. Lee and Pfender3 have noted that there will always be a significant difference between time-averaged unweight ed temperatures when compared with the mass-weighted values due to the presence of turbulence. The mass-weighted temperatures will always be lower than the unweighted tem peratures, and this difference increases as the temperature decreases below 10 000 K. Second, previous work3,17 has shown that there are sig nificant differences between the temperature predicted by models depending on whether the atmosphere into which the argon plasma plume expands is argon or air. The diatom ic molecules of air dissociate at low temperatures, and as this process absorbs large amounts of energy the plume is cooled much more rapidly than when the surrounding gas is argon. The present experiments were performed in air, whereas the model results presented here assume a surrounding gas of argon. Instead of increasing the number of parameters and the complexity of the model to account for this difference, we believe the best way to proceed is to measure the tempera tures in an argon atmosphere, and we are planning such a series of experiments. The assumption that the plasma plume is optically thin would also be brought into question if an argon-air system was being modeled, but the experimen tal results of Farmer and Haddadl8 show that absorption is minimal in free-buring argon arcs. The modeling of radi ation absorption in the plasma mantle is very difficult, espe cially as optical paths have to be taken into account. 5238 J. Appl. Phys., Vol. 66, No. 11, 1 December 1989 Third, in free-burning argon al'es Farmer and Haddad 18 have shown that for temperatures above about 10 000 K lo cal thermodynamic equilibrium exists in the plasma and that the excitation temperature is equal to the atomic argon or heavy-body temperature, but at argon plasma temperatures below 10 000 K the plasma must be described by two tem peratures, an excitation temperature and an atomic tempera ture that is significantly lower than the excitation tempera ture. The present spectroscopic technique gives a measure of the excitation temperature and themodel calculates the tem perature of the ground-state argon atoms. Cram 14 suggested that departures from L TE in these types of arcs might arise because the argon atoms in the outer regions of the arc are exposed to intense resonance line radiation from the arc core. Radiative excitation of argon ground-state atoms to the upper level of the 696. 5-nm transition can result in very high spectroscopically derived temperatures for this level when compared with the LTE populations.l9 v. CONCLUSIONS In this paper we have outlined a magnetohydrodynamic model of a plasma torch and compared the calculations of this model with experimental measurements. The theoreti cal description outlined here represents the first report of a plasma torch modeled in its entirety from the gas injection behind the cathode, through the arc region in the nozzle of the torch, and out into the plume. It also includes the effects of swirl and turbulence. Torch parameters such as the volt age-current and voltage-flow characteristics have been com pared with the theoretical calculations, and this has high lighted the difficulties of modelin.g electrode phenomena. This model has also been used to predict the temperature fields in a plasma torch operating with argon, and the tem peratures in the plume of the plasma torch have been com pared with temperatures measured spectroscopically by monitoring the emission from neutral argon lines. These comparisons have highlighted the simplifying assumptions made in the model of the plasma torch and the consequent differences between predicted and measured temperatures. For plasma spraying and plasma processing applica tions, an understanding of the interaction of the plasma and the injected feedstock relies upon a knowledge ofthe thermal histories of the injected particles. Since these particles are heated almost exclusively by collisions with the hot argon atoms, it is clearly very important to measure the argon tem perature below 10 000 K, since the majority of the particle trajectories will be in these regions. Our future experiments are aimed at a more thorough understanding of temperatures in the argon plasma plume. The measurement of temperatures in the plume of a plasma torch operating into an argon atmosphere will allow the di rect comparison of the model calculations and empirical re sults. The contribution of turbulence to temperatures may be studied by varying the pressure of the surrounding atmo sphere. We also plan to measure devi.ations from L TE by measuring the temperature of the argon atoms using Ray leigh scattering techniques. Scott, Kovitya, and Haddad 5238 Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsACKNOWLEDGMENT The authors wish to thank K J. Powell for his technical assistance. IJ. McKelliget, 1. Szekely, M. VardelIe, and P. Fauchais, Plasma Chern. Plasma Process. 2, 317 (1982). "A. H. Dilawari and J. Szekely, Plasma Chern. Plasma Process. 7, 317 (1987). 3y. C. Lee and E. Pfender, Plasma Chem. Plasma Process. 7, I (1987). 4B. E. Launder lind D. B. Spalding. in Mathematical Models a/Turbulence (Academic, London, 1972), Chap. 4. sp. Fauchais, A. Vardelle, M. Vardeile, J. F. Coudert, and B. Pateyron, Pure App!. Chern. 57,1171 (1985). "e. Boffa, J. Heberlein, and E. Pfender, Wiirme- Stoffiibertrag. 4, 213 (1971). 5239 J. Appl. Phys., Vol. 66, No.1 i, 1 December 1989 'P. Kovitya and L. E. Cram, Welding J. 65, 34 (1986). 8R. B. Bird, W. E. Stewart, and E. N. Lightfoot, in Transport Phenomena (Wiley, New York, 1960). - oW. Rodi, in Turbulence Models and their Applications in Hydraulics (111- temational Association for Hydraulic Research, Delft, The Netherlands, 1980), pp. 27-29,44, and 45. lOS. V. Patankar, in Numerical Heat Transfer and Fluid Flow (Hemisphere, Washington, D.e., 1980). liS. V. Patankar, J. Heat Transfer 110,1037 (1988). 12S. Gordon and B. J. McBride, NASA Special Publication No. SP-273 (1971). 13R. S. Devoto, Phys. Fluids 10, 2105 (1967). 14L, E. Cram, J. Phys. D 18,401 (1985). "D. Apelian, M. Paliwal, R. W. Smith, and W. F. Schilling, Int. Met. Rev. 28,271 (1983). lOG. N. Haddad and A. J. D. Fanner, J. Phys. D 17,1189 (1984). 17M. Vardelle, A. Vardelle, P. Roumilhac, J. M. Leger, J. F. Coudert, and P. Fauchais, in Proceedings of the National Thermal Spray Conference, Cin cinnati, 1988 (to be published). I"A. J. D. Farmer and G. N. Haddad, J. Phys. D 21, 426 (1988). 19L. E. Cram, L, Poladian, and G. Roumeliotis, J. Phys. D 21, 418 (1988). Scott, Kovitya, and Haddad 5239 Downloaded 31 Aug 2013 to 128.103.149.52. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

1.341305.pdf

Properties of liquidphase epitaxy grown Pb1−x Sn x Te homostructure diode lasers with Gadoped cladding layer A. Shahar and A. Zussman Citation: Journal of Applied Physics 64, 4306 (1988); doi: 10.1063/1.341305 View online: http://dx.doi.org/10.1063/1.341305 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electronic properties in Gadoped CdTe layers grown by metalorganic vapor phase epitaxy J. Appl. Phys. 72, 3406 (1992); 10.1063/1.351412 Diffusion length and lifetime in highly Gadoped PbSnTe layers grown by liquidphase epitaxy J. Appl. Phys. 66, 2455 (1989); 10.1063/1.344256 Electrical and optical properties of Cu and Gadoped Hg1−x Cd x Te layers grown by liquidphase epitaxy J. Appl. Phys. 65, 672 (1989); 10.1063/1.343102 Long wavelength Pb1−x Sn x Te homostructure diode lasers having a galliumdoped cladding layer Appl. Phys. Lett. 42, 344 (1983); 10.1063/1.93927 Liquidphaseepitaxy homostructure Pb0.85Sn0.15Te diode laser with controlled carrier concentration Appl. Phys. Lett. 37, 7 (1980); 10.1063/1.91710 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55Properties of liquid-phase epitaxy grown Pb1_XSnX Te homostructure diode lasers with Ga .. doped cladding layer A. Shahar and A. Zussman Solid State Physics Department, Soreq Nuclear Research Centre, Yaune 70600 Israel (Received 13 April 1988; accepted for publication 19 July 1988) The properties of homostructure Ph) __ x Xu" Te diode lasers with tin compositions x = 0.126, 0.182, 0.210, and 0.238 fabricated from liquid-phase epitaxy grown p+ -p-n + layer structure with Ga-doped n-type cladding layer were investigated. Threshold current density (J th ) and quantum efficiency ('TJexl ) was measured as a function of temperature in the range lO.;;;T.;;; 140 K. Current versus voltage (1-V) and product derivative I dV /dI vs I characteristics were measured at T = 10 and 40 K. J th was evaluated from basic principles taking into account intrinsic radiative and nonradiative Auger recombination lifetime, where the last was calculated using both parabolic and nonparabolic energy band structures. Satisfactory agreement was obtained between the calculated and measured Jth for temperatures above 40 K, while at low temperatures the theory underestimates J th significantly. Using aJl electrical equivalent model of the diode laser and best fit procedure the 1-V and I d V / dI vs I characteristics were analyzed and the various current components and diode laser parameters were obtained. It was qualitatively shown that the discrepancy at low temperatures follows from the presence oflarge leakage and tunneling currents. The temperature dependence of the observed 'TJext' which exhibits a maximum between 40 and 50 K, was shown to be related to a filamentary lasing process. I. INTRODUCTION Homostructure diode lasers of lead salt compounds have been intensively used in various molecular spectrosco py related applications. I These lasers possess various attrac tive properties such as low threshold current density, high quantum efficiency and very slow degradation during ther mal cycles of cooling and heating.2•3 These properties stem from the crystallographic matching present in these devices. Another important advantage is their relatively simple pro duction process. Optical guiding is achieved in homostruc ture lasers by the free-carriers dispersion which is very strong at iong wavelengths:~·5 Carrier confinement is very efficient in lead salt lasers due to their low density of states which give rise to band filling eitects.2•6 An appropriate car rier concentration profile in homostructure diode lasers can be achieved by either a diffusion'·? or annealingS process or, in a more controlled manner, by epitaxial growth techniques such as molecular beam epitaxy (MBE) 9, I () and liquid-phase epitaxy (LPE).J·!1.12 In a previous letterl2 we reported preliminary results on the optical and electrical properties of p f--p-n + Pbl x Sn~ Te homostructure diode lasers, grown by LPE with gallium-doped n-type c1addng layer. The use ofGa as n type dopant in this device has a significant advantage on that of other n-type dopants of Pb] _ x SUx Te such as indium (used in similar devices) sincePbl. xSnx Te epilayers with a much higher electron concentration can be achieved, in par ticular in a material with higher tin concentration. This property enables the extension of the range of efficient oper ation of LPE grown homostructure Pbl _ x Snx Te lasers with Ga-doped cladding layer to composition x;;'0.24, which corresponds to wavelengths longer than 19 /-tID. Similar ho mostructure lasers in which indium, is used for this purpose are efficient only up to x < 0.15.3 In this work we present a detailed report of the experi mental results and of the corresponding theoretical analysis of the properties of Pbl_ x Snx Te homostructure diode la sers with the four compositions x = 0.126,0.182,0.210, and 0.238. In Sec. II, the theoretical background is outlined. It includes a description of the model used for the evaluation of the threshold current density. In the calculation both radia tive and nonradiative Auger recombination lifetimes are tak en into account. The Auger lifetime is calculated using the two different models ofEmtageU and Rosman and Katzir. 14 The experimental results and their numerical evaluation are presented and discussed in Sec. III. These include measure ments and calculation of threshold current versus tempera ture, measurements of J-Vand I d V / dI vs I characteristics at low temperatures and their best fit analysis according to the equivalent circuit model. Also dealt with in this section is the temperature dependence of the quantum efficiency. In the last section the results are summarized and the limitations of the model used are discussed. II. THEORETICAL BACKGROUND A. Threshold current The threshold current density is calculated assuming a complete carrier confinement and optical guiding. Since the diffusion length of the carriers in the active layer is assumed to be much larger than the layer thickness d, the threshold current density is given by'5 4306 J, Appl. Phys, 64 (9),1 November 1988 0021-8979/88/214306-12$02.40 @ 1988 American Institute of Physics 4306 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55J ttl = q onthd fr. (1) For a given layer thickness d, the threshold current is thus determined by the threshold density of injected carriers {mth and the lifetime r of the carriers in the active layer. The total minority-carrier lifetime is given by (2) where rr and rnr are the radiative and nonradiative lifetime, respectively. The radiative lifetime rr of the excess electrons was calculated using the relation between the spontaneous transition rate and the absorption (or gain) coefficient as derived by Lasher and Stem. 16 The absorption was calculat ed using the expression derived by Anderson 17 from the six band model ofPb1 _ x Snx Te. In the limiting condition of Eg > kT, which is valid in our case, the expression for the radiative lifetime reduces to 18 r, = 5.2XlO-9INEgI(7jp)' (3) where N is the refractive index, Eg is the energy gap and rtp = QplkT is the normalized quasi-Fermi level. I( Tip) is an integral given by l~' x!/2 dx I(rtp) = . o p + exp(x -llp )]exp(x) (4) Nonradiative lifetime in Pbj _ x Snx Te is dominated by band-to-band Auger recombination. Using a two band mod e{ parabolic dispersion relation and non-degenerate carrier concentration the foHowing expressions for the Auger re combination in Pbl _ x Sn~ Te (p-type material) was ob tained by Emtage13; lIrAlIg = Cp2, (5) where the majority-carrier concentrationp is the sum of the equilibrium density Po and the injected concentration 6,p. Cp is a constant given by Cp = 4.7X 1O-29(T lEg) 1/2E ~ sexp( -rEglkT), (6) where r = m 11m I -O. 1 is the anisotropic ratio between the transversal m, and longitudinal In, effective masses of Pb! _ xSnx Te. Recently, Rosman and Katzirl4 evaluated the Auger process in PbI __ x Snx Te using nonparabolic dis persion relation and the six band model. Their calculation is complicated and requires numerical integration. Under COIl ditions similar to those applied by Emtage, i.e., r-< 1, nonde generate carrier concentration and a temperature sufficient ly low so that kT -<rEg, they obtained explicit expressions for the lifetime which is larger by a factor of2,i2exp(-Egi kT) than that derived by Emtage. At T= 125 K and x = 0.220, for instance, this factor is about to. In the present work the calculation of the lifetime and of the corresponding threshold current density were carried out using both mod els, The threshold density of the injected carriers nIh is de termined by the requirement that at lasing threshold the gain equals the cavity losses 15: g(13n'h) = a (Dna. ) +L -lln[R(8n'h)- I] (7) where a and (L --1 )In(R-1) are the internal and mirror losses, respectively. The gain function depends on the inject ed carrier concentration which can be described in terms of the quasi-Fermi levels of the valence (Qp) and conduction 4307 J. Appl. Phys., Vol. 64, No.9, 1 November 1988 , •• ,.-•••• ..-.-•. "'? •• "" ••• -••• -., •• ! .•... '.~ ... -... -.......• , ............ -.-. .•. ~ •... '.> ••••••• ,_ •• -.' •••• -". _ ••. -,. •• .-••• ,. ',' ., •••••• -••••• ~ ••••• ~ ••••••• -.:O;':.: ••••• ; •••• ~.:.:.:.z·;· .•. ·.;.:.:·:·:·;·;-;·,·.·.·.·.-.·.·.-···-,··,····" (Qn ) bands. The calculated quasi-Fenni level Qi (i = n,p) versus carrier concentration for the relevant Pbl_ x Sn~ Te compounds at various temperatures is shown in Fig. 1. At high level ofinjection the terms in the right-hand side ofEq. (7) also depend on the injected carrier concentration. The origin of the losses is mainly due to free-carrier absorption (FCA) in both the active and guiding layers and is therefore given bylS 3 a = apeA = I rjaFCAi' i =-1 (8) where ri and aj are the corresponding confinement factors and FeA in the region i, respectively. The free-carrier ab sorption in Pb] _ x Snx Te is given byl7-19 (9) where n is the carrier concentration, N the refractive index, f-1 the mobility, and m* the effective mass. The reffectivity R = (N -1)2/(N + 1)2 depends on the level of injected carrier concentration through the dispersion relation4,5,19 N2=€=€oc +.6.€(A)- (Nq2/41TC2m€o)A2, (10) where £00 is the contribution from bound carriers at wave lengths well beyond the absorption edge and t:.£(A) that due to regions close to the absorption edge. Since both sides of Eq. (7) depend on the total carrier concentration the thresh old density ofinjected carriers 8n,h was calculated by solving numerically, using an iterative procedure, the equation gm [Qn (no + on),Qp (Po + 8P)'(;)m J 3 = 2: r j [Qp (Po + oP),{jjm ] ai (Po + 8p,no + {m,w", ) i~ 1 In this expression the energy wUrn is limited to the interval Eq < W", < Eq + Qp (p + op) and is close to the position of the gain maxima. The corresponding wavelength Am satis fies the Fabry-Perot condition. B. Electrical characteristics In the present work both 1-Vand I dV I dI vs I character istics were measured. In the product derivative method an ac current is applied and the modulated response of the diode is measured with a phase detector. Therefore, this technique is more sensitive than the simple 1-V method. 20-22 The product representation has the advantage that, in contrast to J-V re lationship, it delineates the saturation of the junction voltage at threshold. Furthermore, for the cases of a diode or a diode in series with a resistor, it yields a linear plot. The derivative technique is very useful in determining the distribution of the current among the various parallel paths of the laser device and its variolls current components. It is, therefore, very helpful in the study of the mechanism of the lasing pro cess. 1. Equivalent circuit mode! The determination of the electrical equivalent circuit model for homostructure mesa diode laser follows from a consi.deration of the observed electrical derivative results, the device structure and the assumed current transport A. Shahar and A. Zussman 4307 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:5540 30 ;; (al )i: 12.6 % '" 2C.! 10 Q. q 0 <: 0 -10 0; ". OJ -20 = E -30 of , -40 ';;; '" ::> -50 a 1016 1011 Corrier concentration, n,p! cm-3) 40 (bl X'IEl2 % 30-:> ... 20.§ 10 ~ 0 c 0 --10 .. > -20 :: ~ E E -30~ Q. , 0 -40 'g CO :> 0 -50 0 0 1016 1011' Corrier concenlrolion,n,p!cm-~) ~ E a. 0 c 0 .. ~ 0 ~ E <>. 0 c 0 0; ,. .!!! 0 ~----------------------------~ 40 !c) 1016 v:fl Carrier concentration, n,p I cm-~) (d) x: 23.S% 1016 1011 Carrier concentration, n,p (em-;,) 30> 2O.§ 10 ~ o & -10 .. > .. -20 ~ -30~ 50 40> OJ 30.§. 20 ~ 10 c a 0 .. ,.. .. -10 :g -ZO~ . -30 'g :> -40 0 -50 FIG. 1. Quasi-Fermi levels vscarrier concentration at various temperatures for Pbl _ "Snx Tecompounds. (a) x c= 0.126, (b) x ,= 0.182, (c) x = 0.210, Cd) x = 0.238. mechanisms in the diode. In Pb] __ , Snx Te diodes diffusion and tunneling are considered to be the dominant current mechanisms, In addition, leakage curent through parallel shunt path plays an important role and must be taken into account. The presence of this current is apparent at the re gions of small currents of the I dV IdI characteristics. Its magnitude depends on the surface treatment. This current path is linear and will therefore be represented in the equiva lent circuit by a resistor. The diffusion current, which is responsible for the lasing process, is given by the relation23 Id = [dO [exp(qV IkT) -lj. The diffusion saturation current is given by23 [dO = qnp (Lplrp )coth(d ILp)' (12) (13) where np' Lp' andTp are the minority-carrier concentration, 4308 J. Appl. Phys., VoL 64, No.9, i November 1988 diffusion length, and recombination lifetime, respectively, in the p-type active layer of thickness d. The saturation current can be calculated using known experimental values of carrier concentration24 and mobility25 of Pbl_ x Snx Te and calcu lated values of the radiative and Auger recombination life time. For temperatures below 40 K the lifetime in Pb] __ x Snx Te is dominated by the radiative mechanism. Since this mechanism is well understood reliable values of r can be obtained. Tunneling in Pbl _ x Snx Te diodes is a multistep recom bination tunneling process taking place via intermediate states (trapping levels) in the forbidden gap of the depletion region.26 The forward tunneling current is given by26 I, = Itoexp(f3V) = BN,exp[ -a(J 1/2( Vd -KV)], (l4) where A. Shahar and A. Zussman 4308 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55(15) B is a constant, N, the density of the trapping levels, Vel the built-in voltage of the homostructure diode, m* the effective mass of Pbl _ x Snx Te, E the dielectric constant of Pb] xSn~ Te, KV = N"/(N" + N,,) V the voltage drop on the p -Pbl x Sn~ Te active region side of the junction, and 13 is a constant. For highly doped n-type cladding layer N" >,pNa and thus K = 1. In this case 1/13 is equal to the number of tunneling steps. In homostructure diode Vd ~EJq, therefore, 1'0 =BN,exp[ -aeI/2Eg(x)] and {:J~aeI12 Thus, the saturation current 1'0 is expected to increase strongly with the decrease in the energy gap. The depen dence of B on material parameters is not clear since e is not known. The total current that flows through the diode is 1= ld + It + IR, where IR is the leakage current. In a broad area diode laser the lasing process can be inhomogeneous and take place via filaments having different threshold current density. An equivalent circuit describing a laser device with two filaments is shown in Fig. 2. Each fila ment is represented by paralIel diodes Dt and D d and a resis tance Rs representing tunneling, diffusion and leakage cur rents, respectively, and a zener diode Dz representing the voltage saturation of the corresponding filaments at thresh old. The two filaments are coupled by the cladding layer resistance R ~ and R c is the contact resistance. The equiva lent circuit can alternatively be described by the circuit shown in Fig. 3. The resistors in the two circuit are related by Rc =R;2/(2R:. +R1.), R\ =Ri.R;/(2R; +Ri.). (16) This representation is more suitable for numerical analysis. 2. Analysis of the !~ V and product characteristics There are three basic states of the equivalent circuit de pending on the excitation states of the two filaments: (1) both filaments are in a sublasing condition, (2) one of the filaments is above threshold, (3) both filaments are above threshold. The corresponding current versus voltage rela tionships is given in state 1 by ~~ l--_I'l""'L\e_---I R~---1 .--.-+-.. 'Ie '---'--t~~--.-_-.J __ ~~_D~2 J FIG. 2. Electric equivalent circuit representing a broad area diode laser with two filaments. Rc C~ contact resistance, R;_ ,-= lateral resl.stance of the dallding layer, R, = junction leakage resistance. D" and D, represent dif fusion and tunneling currents through the junction, respectively. Dc, and D,2 are zener diode representing diode !illturation voltage at lasing thresh old. 4309 J. Appl. Phys., Vol. 64, No.9, 1 November 1988 .. -......... ~ •••. -..•• ,. .... -•.....•.. " .-.-..........•• n~"' ••..•• _ . .-................ -;-." ......•.•.........•.•.•••...•. -........................... '7;,-••••• 7 •.•.•.•.•.••..• ;.< ••••• -.v ••• -;-•••••••• r II2 = Jdo(exp{(qlkT) [V -J(Rc + RIll) P -1) + Iwexp{f3 [V -J(Rc + RI/2)]} + [V -[eRe + RJ2) ]IRs' 07a) in state 2 by 1-(V --IRe --Vthl )IR1 = Ido(exp{(qlkT) [2V -J(2Rc + R1) -Vth] ]} -1) + I,oexp{f3 [2V -J(2Rc + R, ) -V.hl J}, (17b) where VI'" is the threshold voltage of the filament which lases first, and in state 3 by (17c) The product! dV IdI can be obtained directly by differ entiating the above current versus voltage relations. Alterna tively, since dV IdI represents the dynamic resistance of the whole circuit, it can be obtained by applying the rules of resistance addition to the dynamic resistance of its various circuit components. In particular, the dynamic resistance of the diffusion and tunneling diodes are given by r" = [(qlkT) (ld + Ielo)] -l~ [(qlkT)ld] -I and r, = [{:J(l, + 1,0) 1-1-({3I, )-1, respectively. This yields for state 1, --=1 /-+-I d -rIJI, IRe, I dV [( 1 . q . ) -1/2, 1 dl \Rs kT where, (18a) ld =Ido(exp{Cqlkn[V-I(R c +R,/2)]}-1), and I, = l,oexp{B [V -1(Re + Rjl2) p. For state 2, +2R\ , I }-I (lSb) where Rc Vc ,- v -.l PIG. 3. Alternative representation of the equivalent circuit shown in Fig. 2. A. Shahar and A. Zussman 4309 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55ld = l,iO(exp{(q/kT) [2V -1(2Rc + R1) -V.hl ]} -1), and I, = I;oexp{f3 [2V -J(2Rc + R,) -Vth1 p. For state 3, IdV =I(R.+!2). dI c 2 (lSc) C. Quantum efficiency The external quantum efficiency defined by the rela tion,15 q dpl 1]<,1 =--/ fUu dl hI", can be written in the form 1]ext = 'lJint7J)n(1/R)/[a + In( lIR)]. (19) The last factor in the right~hand side ofEq. (19) is the frac tion of the total radiation power produced in the laser cavity and emitted through the mirrors. 1](nt is the internal quan tum efficiency given by llint ='1,-1/(7,-1+ 7,;;:1), where 7 - 1 and 7 - 1 are the radiative and nonradiative transition r~tes. Abo~~ threshold the radiative recombination is domi nated by stimulated transitions, i.e., 7nT,tim ~r[l<' therefore 1]int -1. This relation is valid only in the case where the las ing process is homogeneous and takes place over the entire active layer width. In a filamentary lasing 71t"t = 1 for the lasing filament while 7J,nt < 1 in the nonlasing volume. 7Jc is the injection efficiency given by the ratio between the incre ment of the diffusion current, which is responsible to the lasing process, to the increment of the total current, i..e., 1Jc = !lId/(A1d + aI,), where Id and It are the diffusion and tunneling currents, respectively. The injection efficiency thus depends on the relative contribution of the current transport by diffusion and tunneling above I til' It also de pends on the homogeneity of the lasing process. Let us assume that the lasing takes place ever the entire active region. Above threshold the junction voltage is clamped at V ~ Egi q, and the diffusion current is given by ld ~ldOexp(Eg/kT). Although the diode voltage is pinned at Vg the diffusion current above Vg will continue to grow. This growth can be described by the increase in the satura tion current/dO -q!lflth CD /r) 1/2 due to the increase in the stimulated transition rate '1-1_ Tstim -I. On the other hand, it is not clear whether or not the tunneling current It = Ito exp(f3V) will saturate above threshold. This depends on the behavior of Ito above I th' According to the tunneling model discussed in Sec. II B, ItO is independent of the life time parameter and therefore the tunneling current is ex pected to saturate above Ith and thus, 1]c = 1. However, since the details of the tunneling mechanism are not known, this conclusion is not wen established. The situation is differ ent for filamentary lasing. In this case the external quantum efficiency is given by where the indices I and 2 correspond to the lasing and non- 4310 J. Appl. Phys., Vol. 64, No.9. i November 1988 lasing regions, respectively. This expression can be written in the form 1fI -IlII ?1.'p lIn (.l) faL + In (l..)] . (21) 'Iext -All + AI2 ·,mt! Ie R R Assuming saturation of the tunneling current in the lasing filament then, 71cl = 1 and the injection efficiency of the en tire laser device becomes !lIDl (22) Thus, in a filamentary lasing 1J, < 1 and depends on the amount of tunneling in the non-lasing regions. It is impor tant to note that filamentary lasing may foHow from rather small inhomogeneities in the active region and device inter faces. Since below threshold the current distribution over the entire laser area is almost uniform, the threshold current density defined as the ratio between the threshold current and the laser area is almost equal to that of the lasing fila ment. m. EXPERIMENTAL RESULTS AND DISCUSSION 140 Laser fabrication and characterization Homostructure Pb1_ x Sn~ Te lasers were prepared from Ll>E layer structures grown lattice matched to p -t - Pbl _ x Snx Te substrates. The substrates were 1 X I cm2 wa fers cut along the (100) plane from Pb] x Snx Te single crystals grown from the vapor phase.27 The wafers were pol ished mechano-chemicaHy, first in a 10% and then in a 2% Br2 solution in concentrated HBr. Just before the start of LPE growth the wafers were given a lO-s Norr etch. The layers were grown in a horizontal LPE system containing a graphite boat in a quartz tube flushed with ultrahigh-purity hydrogen and heated by a semitransparent gold coated fur nace. The LPE layers were grown under super-cooling of AT = 3 ·C and a cooling rate of2 ·C/min. The first (active) layer, 1.5-3 pm thick, was grown in the temperature range 500-480°C. The second, n+ -cladding layer, about 2 11m thick, was subsequenHy grown. The doping was obtained by adding Ga to the growth solution in an amount determined by the composition x and the required carrier concentra~ tion.12,28 The carrier concentration in the active undoped layer was assumed to follow the equilibrium phase dia~ gram.24 Stripe geometry mesa structure lasers, about 200 pm wide and 500 /Lm in length, were fabricated using standard techniques. The contacts to the device were made by electro plating In on the n + -cladding layer and Au/Ni-Cr/ln on the p+ substrate. The laser was cold bonded in a copper holder of the type used by Laser Analytic!> and was mounted in a closed cycle refrigerator. The threshold current was mea sured using current pulses about 2 ps wide applied at a rate of 102-103 Hz. The laser emitted power was collected using! /2 ZnSe optics and measured by means of HgCdTe photocon ducting detector having an active area of 1 mm2, a response time shorter than 1 ps and a cut-off wavelength of 20 /Lm. The spectral response of the detector and its absolute sensi tivity were calibrated using a blackbody source. The external quantum efficiency defined by Tlext = (q/"lku)(dP IdI) was obtained from the single mirror A. Shahar and A. Zussman 4310 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55104 (ol ~t- -~~---1 _---JR ~--~ ~ ,/'~ I '" I02.r ~ r X·12.6 % « . :E -:> ,Olf- 10° - 10' 0 20 40 60 80 140 T(K) ~ 2: ~IO ~- <t t "'-" 1 161~-L~ __ L--L __ ~J-~ __ L--L~ o 20 40 60 80 100 T (K) '" E <.) ..... :::'02 -:: -, IOI~ ~ 10J 0 20 40 20 40 60 80 T(K) 60 80 T (K) 100 100 120 FIG. 4. Pulsed threshold current density vs temperature for homostructure Ph, x 511, Te diodelasers. J M = measured, J E' J R = threshold current calculat ed using Auger lifetime according to Emtage" and Rosman and Katzir, '4 respectively. (a) x = 0.126, (h) x~, 0.182, (c) x"~ 0,210, (d) x =. 0,238. 4311 J. Appl. Phys., Vol. 64, No.9, 1 November 1988 A. Shahar and A. Zussman 4311 .............................. : .. ,.: .... ;.; ............ "...··· .. ···-·-··.··.· ... · .. ,:.·.·~-.·;:·-····.·.·.·.·s-,..·.·.· .. ;.".··.·.'.·.·;·.·.·.·.·.-.·-;;: .... -·.·.·s •. ";" .•. ·.·.v.-.-> ....• ;< ••••• 0;.,. .•... -.. . [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55power versus current (P-I) characteristics assuming equal power from the two laser facets. The measured power was corrected for the transmission losses of the optics. However, since the half power beam width of the emitted laser radi ation is somewhat larger than that collected by the f 12 op tics, the results obtained represent rather a low limit of the laser efficiency. The J-V characteristics were measured using a simple system consisting of a current source and resistor in series with the laser used for measuring the applied current flow. In this arrangement the voltage drop across the leads is eliminated but not the IRe potential drop on the contact resistance Rc. The first derivative dV Idl and the product I d V / dI were measured using an analog system similar to that described previously by Dixon20 and Barnes and Pao li.2i The method relies on ac modulated current superim posed on a slowly varying dc bias current applied to the diode laser. The amplitude of the ac current is kept small relative to the dc current. The modulated voltage which de velops across the laser device terminals is measured by means of a phase detector. The rms of the first harmonics of the laser response voltage is proportional to the first deriva tive dV /dI. The product I dV /dI is obtained by using an analog multiplier. The product I dV /dl and the dc current are supplied to the y and the x inputs, respectively, of an x-y recorder. The system response is calibrated against a known resistor. Similar arrangement is used to obtain the laser pow er derivative versus current, dP /dI vs I, characteristics. B. Threshold current density The threshold current density of the Pb! _ x Snx Te ho mostructure lasers with compositions x = 0.126, 0.182, 0.210, and 0.238 was measured as a function of temperature in the interval 10 < T < 140 K using current pulses. For each x several diodes fabricated from the same grown layer struc ture were tested and the best one was used to represent that x. The results are shown in Figs. 4(a)-4(d). Also shown in these figures is the threshold density calculated according to the procedure described in Sec. n A, whereJ E andJ R corre spond to Auger lifetime derived from the models of Em tagel> and Rosman and Katzir, 14 respectively. The results of Fig. 4 show that in general at low temperaturesJ th increases rather slowly with T. At temperatures above about 40 K the observed Jth increases almost exponentially with tempera ture and in a limited range of temperatures it can be de scribed by the relation J til = Jo exp ( T 1 To), where To is an empirical parameter. This temperature dependence is not due to a single mechanism but follows from a combination of several mechanisms. The results of the characteristic tem perature ~) obtained from the high-temperature exponential region of J th VS T are summarized in Table 1. The ~) mea sured in the present homostructure lasers with Ga-doped cladding layer are similar to those observed in single or dou ble heterostructure ternary PbSnTe-PbTeSe diode lasers6•29 and in quarternary DRS PbSnTe Se-PbTeSe lasers.30 How ever, PbSnTe homostructure diode laser grown by LPE with indium-doped cladding layer exhibited a lower characteris tic temperature of To = 12-13 K. 3 The relatively high To observed in this work demonstrates the advantage of Gal lium as a dopant for n-type cladding layer of this device. The 4312 J. Appl. Phys" Vol. 64, No.9, 1 November 1988 TABLE I. Values of T" for homostmcture Pb, _ x Su, Te diode lasers. x 0,126 0,182 0.210 0.238 22.0 17.2 16.3 15.5 lower ~) in the In-doped device may be a consequence of insufficient carrier confinement due to low potential barrier for the injected carrier and/or short nonradiative lifetime in the In-doped confinement iayer31 or in the active layer (due to diffusion of indium into this layer). The calculation of the threshold current relies on values of active layer thickness and carrier concentration deduced from the LPE growth conditions using equilibrium phase diagram.24 These values may be erroneous due to a diffusion during LPE growth of acceptors and donors from the p +-- substrate and n +--PbSnTe cladding layer, respectively, into the p-PbSnTe active layer. In the calculation of the Auger recombination lifetime a nondegenerate carrier concentra tion was assumed. A comparison between the observed and calculated J th vs Tshows that the agreement is satisfactory above ]';=40 K. For diode lasers with the compositions x = 0.126,0.182, and 0.210 a good agreement is obtained for Auger lifetime calcu lated according to the two band model of Emtage, [3 while for x = 0.238 a much better agreement is obtained when the Auger lifetime is calculated using the model of Rosman and Katzir 14 based on a non parabolic energy dispersion for Pb[ _ x Sn, Te. If the model of Emtage is more appropriate for PbSnTe, as implied by the results of the three lowest x values, then the discrepancy observed for x = 0.238 diode lasers may be related to a large overestimation of the carrier concentration in the active layer. Another explanation which may account for this effect is related to a process of diffusion of ions from the p -+ substrate into the p-active layer. Such a process may lead to a diffused (graded) p +--p junction and to a narrowing of the active tayer effective width, and therefore to a significant reduction in the calcula tion J tit. This effect is stronger for diode lasers with higher x due to the higher carrier concentration in the corresponding as grown bulk Pbl _ x Snx Te substrate. As can be seen from Fig. 4, the increase in the characteristic temperature To with the decrease in the tin fraction x fonows from the threshold current theory. The low-temperature discrepancy between the calculated and measured J th is attributed to current transport by tunneling and leakage which do not involve carrier injection and therefore do not contribute to the lasing process. These effects are discussed in the next section. C. Electrical derivative and current versus voltage characteristics The results of the electrical derivative measurements which demonstrate more clearly certain properties of the diode laser that are relevant to the mechanism determining the threshold current density are discussed first. Measured A. Shahar and A. Zussman 4312 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:550 (Q) 60 ;;:-102 E « E >, ... ...... 30 ..... I-< IO! 10° 0 40 SO 120 160 200 240 I(mA) 0 (b) I i02~ ) <t :> E E i-I ~I;j 101 .... R"'O.07'o' 40 80 I (rnA) FIG. 5. Electricall dV Idlvslandlvs V-IR,. characteristics of homos true ture Ph, . xSn, Tc diode la.~er with x = 0.238 at (a) T= 40 K, (b) T~~ 10 K, I d V I dI vs I characteristics: broken line, measured; unhroken line, cal culated; dash-dot line, calibration curvc measured with a resistance. and calculated I d V I dl vs I characteristics of a laser diode with a tin fraction x = 0.238 at T = 40 K are shown in Fig. 5. The calculations were performed according to the equiva~ lent circuit shown in Fig. 3. Also shown in Fig. 5, I vs V~lR plot of the same diode. The straight line is a calibration curve obtained from I dV /dI measurements on 0.33-0 resistance. The discontinuities of 1 dV I dI at I, = 145 mA and 12 = 230 mA are due to lasing onsets at these currents. In the current interval between I, and 12 the voltage is not completely satu rated. The behavior of the laser in this region can be attribut ed to either a transition from one longitudinal mode to an~ other one, or from a lasing in one filament to lasing in two filaments. In the first case the entire active region of the laser operates as a single filament. The increase in the current above II causes a change in the refractive index of the active region and therefore to a variation in the lasing wavelength so that the Fabry-Perot condition Nd = A.m is satisfied. The heating also gives rise to a shift in the energy gap and in the position of the energy Emax at which the gain attains its max imum. The variation in A. and Emax are not identical. There fore, in order to maintain the lasing conditions the gain and 4313 J. Appl. Phys., Vol. 64, No.9, 1 November 1988 ........... ..........•..••............•..•....•.................... ~ ... ~ .•..• " ....... .w ........... . 2 15 !lsI;; 0.5 ~I;; 160 175 190 205 220 235 [ (mA) FIG. 6. Electrical characteristics of Pb" Sn, Tc diode (x = 0.238) at T= 40 K in the current interval between thc two discontinuities I, and I,. (a) Analytical current derivative of the product I dV idl shown in Fig. 5. (b) Calculated current flow through the lasing filament above the first las ing point. (c) Current derivative of I, relative to the derivative at 1,= 145 rnA. (d) Current derivative of the emitted power versus current relative to its value at I,~ 145 mAo the height of the quasi-Ferm level should increase. This ef fect is associated with a super linear variation of the J dV /dI characteristics above II' The second discontinuity will occur when the wavelength of the next longitudinal mode coin cides with the position of the gain maximum. Experimental ly, however, the I dV /dI characteristics were found to be sublinear in the current interval II < I < 12, as shown in Fig. 6 where the derivative (d /dI) [/(dV /dJ) J, calculated nu~ merically from the product derivative, is plotted versus the applied current The second derivative decreases slowly with the increase in the current indicating that, in fact, there is no increase in the quasi-Fermi level for II < 1< 12, The observed behavior is, thus, in a better agreement with the model of a filamentary lasing process; a situation most likely in lasers with relatively wide active region (about 200 ,urn). The las ing which starts first in a part of the volume of the active region gives rise to the first discontinuity. Though the junc tion voltage of the lasing filament is saturated the voltage at the non-lasing filament can continue to grow since the two are separated by the lateral resistance (R ;, in Fig. 2) of the active layer. The voltage drop on the device is therefore not completely saturated and increases until the lasing onset of the second filament. A comparison between the I d V / dI vs I and log I vs ( V IRe) characteristics, also shown in this Fig. 5, demonstrates the advantage of the derivative method in revealing lasing phenomena. While in the derivative curves two discontinui ties are clearly evident this effect is obscured in the J-V char~ acteristics. It is evident from Fig. 5 that a good agreement can be obtained between the measured I dV IdI characteris tic and that calculated using the model represented by the equivalent circuit and best fit procedure. The electrical pa rameters of the diode thus obtained are summarized in Table U. dV /dJ is, by definition, the dynamic resistance of the diode and I dV /dI is therefore the voltage which develops across this resistance. This concept is very useful in the anal ysis of the product derivative characteristic. The product A Shahar and A. Zussman 4313 ..-.. -.. -."' .. -.-.~ ••.• =.-.. -.-.-.-.. -.-.-.-.. -..• -.-.-•.•... ~ •.• [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55TABLE n. Pho.762 Sn023• Te diode parameters obtained from best fit analy- sis of the equivalent circuit and the electrical derivative measurements. T fJ qlkT 1'0 1'/0 Rc RI (K) (V-I) (V-I) ({tAl (Al (n) (n) 41 120 280 5.0 2,8XlO-1O 0.24 0.08 10 120 1156 1.5 0.1 0.0025 derivative versus current plot can be divided into five dis tinct regions. (n 1<,10 rnA. The I dV Idl characteristic is linear, In this region the diodes representing current transport by tun neling and diffusion have a much larger resistance than the leakage shunt resistance. The total dynamic resistance of the device is equal to the contact resistance R : in series with half the shunt resistance R, of each diode, therefore, dV RI +Rs -=R +--=--~ dI c 2 (n) 10 rnA < 1< 30 rnA. In this region the voltage drop across the shunt resistance R, increases until the current flow first through the tunnel diode and at higher currents also through the diffusion diode becomes significant. The dynamic resistance of the device starts to decrease and there fore the product derivative reaches a maximum observed around 10 mA.l dV /dI decreases from this maximum to a local minimum around I = 30 rnA determined by the diodes and the passive resistive components of the circuit. Around this minimum the diffusion and tunneling currents are al most equal, ID = 7 rnA and It ;:::;;6 mAo Above this point the diffusion current becomes predominant since q/kT>/3. (III) 30 mA < 1< 145 rnA. The diode resistance contin ues to decrease with the increase in the current. When it becomes lower than the passive resistance of the device the I dV /dI vs I characteristic attains a linear form until the lasing onset of the first filament at 145 mAo At this point I d V / dl abruptly drops. (IV) 145 rnA < I < 230 rnA. The second filament is be low threshold and the dynamic resistance is given by (23) where rd (1) = dVd/dld is the dynamic resistance of the nonlasing diode. r d is already very small at the first lasing point [rd (145 mAl = 23 mOl and it continues to decrease at higher currents. Since r d <t"Rc the effect of the reduction in rd on the total dynamic resistance of the device is not appar ent in the product derivative characteristics. For the same reason the discontinuity at the second lasing point is much smaller than that of the first. These effects can be observed in the second derivative on the current can be obtained by ex panding dV IdIinto a series in the variable rdlRi which to second order yields IdV -I(R RJ)\ 1 1 dI - C +2 + 2A -4RtA2J' where A = qlkT. The second derivative is then given by 4314 J. Appl. Phys., Vol. 64, No.9, 1 November 1988 ~(IdV\ -R +~ 1 dI drJ -c 2 + 4RiA 212' showing a sublinear behavior, in agreement with the experi mental results of Fig. 6. Also shown in this figure are the calculated current increment I, = 1-Ithl flowing through the lasing filament, its derivative drdldI and the measured power derivative dPolJt / dl, where the last two are normal ized with respect to their value at the first lasing point 1\. The sublinear behavior of Ir (l) and the decrease of the deriva tives d/,ldJ and dPou\ldJ with the applied current I are caused by the Joule heating of the device. The coincidence of the relative derivatives shows that dPoutldl is proportional to dI,ldI. Since Pout = nexJr Vth, it follows that in the cur rent interval Ith! <I <llh2 the voltage across the diode is saturated at Vthl and 1Jextl is constant. (V) 230 mA<I. At 12 = 230 rnA a second discontin uity occurs due to the lasing onset of the second filament. Above I th2 the dynamic resistance of the device is given by dV =R +~ dI c 2 and the I dV IdI characteristic is linear in the current. The electrical characteristic of this diode measured at T = 10 K exhibits, as shown in Fig. 5 (b), only one discon tinuity (atI = 30 rnA). The device behaves homogeneously as a single filament. The calculation based on a diffusion current and the optical model yields a threshold current of 1 rnA at this temperature. Moreover, the diffusion forward current given by lei = IdoCexp(qV IkT) -1] with a calcu lated saturation current of IdO = 10-80 A is much smaller than the observed current in the entire voltage interval in cluding Vlh• This implies that at low temperatures diffusion is negligible and the current transport is dominated by tun neling and leakage currents. The diffusion current was there fore neglected in the best fit analysis the results of which are shown in Fig. 5 (b). The observed discontinuity in the prod uct characteristics at the lasing point is much larger at T = 10 K than at 40 K. This effect can be explained qualita tively as follows. The discontinuity of I dV / dI at threshold is equal to the inverse of the exponent of the dominant current component. At T = 10 K tunneling is dominant hence the jump is 8( 10 K) = 11/3 = 0.0083 V, whereas at T = 40 K diffusion is dominant and 8(40 K) = kT Iq = 0.0047 V. Thus, 8( 10 K) > 8( 40 K) as observed. If, however, diffusion current was dominant also at T = 10 K then the magnitUde of the discontinuity would be larger at T = 40 K than at 10 K. The value of R! = 0.08 n obtained at T = 40 K corre sponds to a lateral effective resistance R L = 0.16 0 of the cladding n-type layer. Using the layer parameters n = 1.1 X 1019 cm --3, p = 1.5 X lcr cm2 IV S,28 layer thick ness of 3 f.tm and active layer width of 300 f.tm, a value of 55 pm for the distance between the two filaments is obtained. 1. 1-V characteristics Measurement of the 1-V characteristics were carried out on diode lasers with an four compositions X. The electrical parameters were derived from these measurements assum ing lasing over the entire laser active area and using one branch of the equivalent circuit shown in Fig. 3. The J-V A. Shahar and A. Zussman 4314 ............................. ·····1 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55(a) 4: .s 101 ~-!/ lot~LI~ __ k--L_~ __ L-~I __ L-~I __ ~-L~~ a 20 40 60 80 100 120 140 V(mV) FIG. i. /-V characteristics of hmnostructure Pbl _, SUx Te diode lasers with four compositions. (a) T = 10 K, (b) T= 40K. Experimental: dotted circles: x -, 0.126; crosses: x~· 0.182; circles: x= 0.210; triangles: x'· 0.238. Calculated: nonsolid lines lying along the lines joining the ex perimental data. characteristics exhibit a relatively small dependence on tem perature and much stronger on the tin fraction x. Therefore, the 1-V curves were plotted with x rather than Tas a varying parameter. Measured and calculated J-V characteristics for the four compositions x are shown for T = 10 and 40 K in Figs. 7(a) and 7(b), respectively. The device parameters obtained from the best fit analysis are given in Table n. A typical voltage dependence of the various current compo nents obtained from the parameters of Table II is demon strated in Fig. 8 for a PbO.R74SUO.126 Te diode laser. From these results it follows that at low current the 1-V character istic is dominated by a current flow through the shunt leak age resistance R,. (R, is in fact, equal to the zero voltage resistance Ro of the diode). The shunt resistance is in general (excluding x = 0.182) larger for the diodes with the higher energy gap, At intermediate currents tunneling is predomi nant. The tunneling current is higher for diodes with lower energy gap. This follows from the exponential dependence of the tunneling saturation current on the built-in potential de scribed in Eq. (15). The current interval where tunneling is dominant is wider at 10 K than at 40 K since at higher tem peratures diffusion becomes significant. At higher currents the effect of contact resistance increases and it is completely dominant above threshold, It can be seen from Tables II and III that the exponent (J is independent of temperature. This property is a character istic of tunneling mechanism. f3 decreases slightly with the tin fraction x. According to the multistep tunneling model 4315 J. Appl. Phys., Vol. 64, No.9, 1 November 1988 102 r--------------, 40 80 120 V (mV) FIG. 8. Various current components vs app\ted voltage in a PbO"74SnOI2' Te diode laser. 1, 0' leakage current, I, = tunneling current, In = difIusi(ll1 current. f3 = a() liZ, where a -(m*E! Na ) 1/2 rEq. (16) J. Since the effective mass m* decreases with the increase in x and the acceptor concentration Na in the LPE grown p-type active layers increases with x (according to the phase diagram), 24 a is expected to decrease with the increase in x. In order to confirm with the observation that f3 increases with x, () should increase with x. That is, the number of tunneling steps R-1/ e decreases with the energy gap. This is a reason able conclusion since one expects the magnitude of single step to be independent of the energy gap. 2. Current components at threshold The threshold voltage and the various current compo nents at threshold were derived from the device parameters given in Tables n and III and the observed threshold current I tn (Fig. 4) using the relation lin = I D ( V th ) + II ( V th ) + 1, ( V th ). Since below threshold the voltage dependence of the various current component is known from the best fit of the electri cal model to the experimental 1-Vand 1 d V I d! vs I results, this equation can be solved at threshold to yield Vth as well as the various current components at Vth. At T = 10 K the diffusion current is negligible compared with the other cur rent channels, The current components at threshold derived from the! dV Idl andJ-V characteristics are given in Tables IV and V, respectively. Also shown in these tables are the observed threshold current! til and the threshold current I D 1 calculated from optical lasing theory. It is evident from these tables that at T = 40 K there is a good agreement between the diffusion current at threshold 1D(th) derived from the electrical measurements and the threshold current 1m cal culated from optical lasing theory. At this temperature the percentage of the diffusion current is between 55% to 70% of the total current, depending on x. The fraction of the leak age current increases with x from Is (th)llth = 0.03 for x = 0.126 to 0.14 for x = 0.238. At T = 10 K, where diffu sion is negligible, this ratio is larger and varies from I,(th) /l th = 0,16 for x = 0.126 to 0.95 for x = 0.238. A. Shahar and A. lussman 4315 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55TABLE IIt Parameters ofPb, ., Snx Tc diode lasers obtained from a best fit of the electrical model to the observed 1-V characteristics. T fJ q/kT 1'0 Ido Rc X (K) (V· ') (V·') (pA) CA) (n) 0.238 41 120 0.238 10 120 00210 42 112 0.210 10 112 0.182 43 108 0.182 10 !O8 0.126 43 WO 0.126 10 100 Do Quantum efficiency The power versus current characteristics P-I was mea sured at various temperatures. At low applied current the p I characteristics are linear. At currents where longitudinal mode transitions take place the power saturates or even drops. The external quantum efficiency 'TJexl derived from the P-I characteristics at a laser power of p = 15 fl W versus temperature for diode laser with tin fraction x = 0.126, 0.210, and 0.238 is shown in Fig. 9. An obvious feature of Fig. 9 is the occurence of a maximum for an three x values in the temperature interval 40 to 50 K. The maximum effi ciency is about 2.7% for x = 0.126 and 1.5% for x = 0.21 and 0.238. Quantum efficiency of the order of 1 % is com mon in heterostructure PbSnTe-PbTeSe or homo structure PbSnTe diode lasers in the long wavelength range.29,K In ex ceptional cases a quantum efficiency above 15% was ob served.3,32,33 The temperature dependence of the external quantum efficiency, in particular the presence of the maximum, can be explained by a model of filamentary lasing. The temperature dependence of 11e" is contained in all the four factors of Eq. (21). The factor In(R -l)/faL + In(R -l)] decreases monotonically with temperature due to the in crease in the free carrier absorption a with T. If one assumes that the tunneling current of the lasing filament (No. I) is saturated above threshold, j.e., 17inll = 1 and 'TJcl = 1, then Eq. (22) holds. Since according to Fig. 4 and the analysis of the electrical characteristics the fraction of the tunneling current reduces with the increase in temperature 11", given byEq. (22), wil1increasewith T 17cxt which is expressed as a product of two independent factors, the relative emitted TABLE IV. Current components at lhreshold of a l'b" 7(,2 SnO.23' diode laser obtained from electrical derivative and optical measurements, T lib ID, Eo I, I, (K) CA) (Al (A) (A) (Al 41 0.142 0.098 0.104 0.021 0.020 10 0.026 0.0015 O.OtH O.02l 4316 J. Appl. Phys., Vol. 64, No.9, 1 November 1986 280 1156 275 1156 270 1156 270 1156 3.0 2.8X 10-J<J 0.140 2.5 0.120 3.0 I.OxIO·" 0.165 1.0 0.165 1.6 l.Ox 10 11 0.095 1.2 0.060 0018 3.0X 10-15 0.155 0.17 0.168 power which decreases with T, and the device injection effi ciency which increases with T, must therefore attain a maxi mum at a certain temperature. This model accounts for the low value of "flex! , the presence of a maximum in the "f7ext vs T plot and is consistent with the electrical measurements. If one assumes, on the other hand, homogeneous lasing then 'Thnl = 1. The presence of a maximum in the 17ext versus T curve implies an increase in l1c with T. This is possible only if the tunneling current is not saturated above threshold, a re quirement which is not easy to justify. Moreover, the homo geneous model does not account for the observed low quan tum efficiency. IV. SUMMARY AND CONCLUSION LPE grown homostructure Pb) _ x Snx Te diode lasers with Ga-doped n + -cladding layer exhibit low threshold cur rent density and efficient power emission over a wide range of wavelengths extending beyond 19 pm. These devices are therefore useful for various spectroscopic applications in the long wavelength range, Threshold current density and elec trical 1-Vand I d V / dI vs I characteristics were measured as a function of temperature in homostructure Pbl _ x Snx Te diode lasers with four compositions in the range 0.13 <x < 0.24. The electrical measurements were found to be very useful in determining quantitatively the current components contributed by the various transport mecha nisms and the diode laser passive parameters. The results of the best fit analysis based on electrical equivalent circuit TABLE V. Current components at threshold of Ph, _" Sn, Te diode lasers obtained from 1-V characterislics and optical measurements. T l'h IDI In 1, I, X (K) (A) (A) (A) (A) (A) 0.238 41 0,142 0.098 0.101 0.021 00020 0.238 10 0.026 0.0015 0.008 O.Q18 0.210 42 0.139 0.072 0,080 0.048 0.011 O.2lO 10 0.DI9 0.0009 0.01l 0.008 00182 43 0.119 0.075 0.064 0.051 0.(J04 0.182 10 0.031 0.0003 0.025 0.005 0,126 43 0.098 0.078 0.070 O.Q2S 0.003 0.126 10 0.Q28 0.0003 0.024 0.004 A. Shahar and A. Zussman 43i6 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55\ 2 + 20 40 60 80 100 ,20 140 T(K) FIG. 9. Temperature dependence of the external quantum eflkic.ncy ofho mostructure Phi .,Sn, Te diode lasers measl.lr~d at a power P" 15 p.W. Crosses: x = 0.126; triangles: x = 0.210; circles: x = 0.238. model of the diode laser clearly show that at low tempera tures Jth is dominated by tunneling current. At lower cur rents leakage through a shunt resistance path is very signifi cant. At high temperatures diffusion becomes the dominant cnrrent mechanism. At temperatures above 40 K good agreement was obtained between the observed J th VS T of the three lowest x lasers and that evaluated from basic theory of lasing threshold, assuming current transport by diffusion and using Auger recombination lifetime derived from the models suggested by Emtage.13 The magnitude of the ob served external quantum efficiency and its temperature de pendence, in particular the existence of a maxima, was shown to be consistent with a filamentary lasing. In the calculation of J til a uniform carrier concentration profile in the active and passive regions of the laser device was assumed, neglecting possible inter diffusion. This model can be considered only as an approximation since in LPE growth of Pbl x Snx Te at temperatures around 500°C a significant diffusion of acceptors from the p + substrate and donors (Ga) from the n+ -cladding layer into the active re- 4317 J. Appl. Pl1ys., Vol. 64, No.9, 1 November 1988 .... -.-.-.•.............. ~ ... -.• "." --'".-.-.-.-.. -. , .. ", ..•..... --. ....... -.-..... -, .........•.•.•.•.•.............. gion is expected giving rise to a nonuniform graded active region. In a more realistic and accurate model these effects must be taken into account. 'H. Prier, App!. Phys. 20,189 (1979). 2R, G. Grisar, W. J. Riedel, and H. M.Prier, IEEE J. Quantum Electron. QE-17, 586 (1981). ]Mo Oron and A. Zussman, App!. Phys. Lett. 37, 7 (1980)0 4M. Oron, A. Zussman, and A. Katzir, Infrared Phvs. 22, 317 (1982). 'Y. Shani, R. Rosman, and A. Katzir, IEEEJ. Quan:tum Electron. QE-20, 1100 (1984). 6A. Zussman, D. Eger, M. Oron, S. Szapiro, A. Shachlla, and B. HailS, IEEE Proc. 129, 203 (1982). 7W. Lo, App\. Phys. Lett. 28,154 (1976). "W. Lo and D. E. Swets, Apr\. Phys. Lett. 33, 938 (1978). 9J. N. Walpole, S. H. Groves, and T. C. Harman, IEEE Trans. Electron Devices ED-24, 1214 (1977). IOD. L. Partin and W. Lo, J. AppJ. Phys. 52,1579 (1981). "J. N. Walpole, S. H. Groves, and T. C. Harman, Solid State Research Report, Lincolll Lab. MIT (1977:4) p. 3. 12 A. Zussmall, Z. Feit, D. Eger, and A. Shahar, App!. Phys. Lett. 42, 344 (1983). 13p. R. Emtage, J. Apr!. Phys. 47,2565 (1976). 14K Rosman and A. Katzir, IEEE J. Quantum Electron. QE-18, 814 ( 1982). ISH. C. Casey and M. B. Panish, Heterostructure Lasers (Academic, New York, 1978). JbG. Lasher and F. Stern, Fhys. Rev. 133, 553 (1964). 17W. W. Anderson, IEEE J. Quantum Electron QE.13, 532 (1977). '"T. S. Moss, G. J. Burrell, and Bo Ellis, Semiconductor Opto-Electronics (Butterworths, London, 1(73). 19G. Diorme and J. C. Wolley, Phys. Rev. B 6,3898 (1972). 20R. W. Dixon, Be1l Sys. Tech. J. 55, 973 (1976). 21p. AoBarnes and To L. Paoli, IEEE J. Qualltum Electron. QE-12, 633 (1976). 22p. D. Wright, W. B. Joyce, and D. C. Craft, J. App\. Phys. 53, 1364 ( 1(82). 23S. M. Sze, Physics a/Semiconductors (Wiley and Sons, New Yark, 1981). 24T. C. Harman, J. NOllmetals 1,183 (1973). 25A. Zemel, N. Tamari, D. Eger, and O. Yaniv, J. App!. Phys. 50, 5549 (1979). lbA. Zemel and D. Eger, Solid State Electron. 23, 1123 (1980). 27N. Tamari and H. Shtrikman, J.Cryst. Growth 47,230 (1979). 28Z.Feit, Ph.D. thesis (Hebrew University of Jerusalem, 1984) (unpub lished). 29D. Kasemset and C. G. Fonstad, International Electron Devices meeting (IEDM), 130 (1979). JOy. Horikoshi, M. Kawashima, and H. Saito, Jpn. J. App!. Phys. 21, 77 (1982) . 31 A. Shahar and A. Zussman, J. App!. Phys. 54, 2477 (1983). 32J. N. Walpole, A. R. Calawa, R. E. Ralston, alld T. C. Harman, J. Appl.Phys. 44, 2909 (1973). "K. Shinohara, Y. Nishijima, and H. Fukuda. Proc. SPIE, 4311, 21 (1983). A Shahar and A. Zussman 4317 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.24.51.181 On: Mon, 01 Dec 2014 00:03:55

1.1684418.pdf

An Electron Beam Method for Measuring High Sheet Resistances of Thin Films A. N. Chester and B. B. Kosicki Citation: Review of Scientific Instruments 41, 1817 (1970); doi: 10.1063/1.1684418 View online: http://dx.doi.org/10.1063/1.1684418 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/41/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Measurements of the sheet resistance and conductivity of thin epitaxial graphene and SiC films Appl. Phys. Lett. 96, 082101 (2010); 10.1063/1.3327334 Resistance and sheet resistance measurements using electron beam induced current Appl. Phys. Lett. 89, 241919 (2006); 10.1063/1.2405886 Three-beam interference method for measuring very thin films Appl. Phys. Lett. 83, 260 (2003); 10.1063/1.1591234 Measurement of the sheet resistance of resistive films on thin substrates from 120 to 175 GHz using dielectric waveguides J. Appl. Phys. 91, 2547 (2002); 10.1063/1.1430534 A Method for Measuring High Resistance Rev. Sci. Instrum. 7, 44 (1936); 10.1063/1.1752027 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:40THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 41. NUMBER 12 DECEMBER 1970 An Electron Beam Method for Measuring High Sheet Resistances of Thin Films A. N. CHESTER* AND B. B. KOSICKI Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey 07974 (Received 27 May 1970; and in final form, 23 July 1970) Measurement of sheet resistances of high resistance (up to 10'612/0) thin films using conventional methods is both difficult and subject to error. We have developed a method for making sheet resistance measurements in which a low velocity electron beam is used to inject charge into the film. The injected charge drifts under the influence of its self-induced field to metal electrodes evaporated on the film, where it is collected and measured externally as a time dependent current. We give examples of thin GaAs films for which the measured current can be fit with a current predicted by a simple model over many orders of magnitude by adjusting only one parameter, the sheet resistance of the film. The good agreement between experimental and theoretical currents provides reliable values for the sheet resistance. This allows an estimation of a lower limit on the carrier density native to our GaAs films. INtRODUCTION IT is well known that the leakage resistance of the sur- face of insulating materials can be strongly affected by surface contaminants, such as adsorbed water. In order to minimize the possibility of such effects, it may be desirable to make sheet resistance measurements of high resistance thin films under vacuum conditions. Standard two-or four-probe resistance measurements on high resistance (lOlL 1016 Q) samples are difficult in a vacuum, since the leakage resistance of the vacuum elec trical feed through must be made much larger than the sample resistance. There is also the possibility that poor contacts, space charge accumulation, or other contact effects could cause error in the measurement of sheet resistance. We have developed a low velocity electron beam method which surmounts some of these difficulties. A low velocity electron beam for measurement of elec tron transport in a direction normal to the insulating film plane has been recently used by several workers.I-3 It has been recognized that use of an electron beam in place of a top contact in the usual MIM or MIS structure offered freedom from the effects of pinhole shorts and poor injecting contacts.I However, the actual voltage profile in the insulating thin film under study cannot be measured directly, and indirect determinations of this quantity re quire questionable2 simplifying assumptions1.3 about the secondary emission coefficient of the thin film.2 In this paper we introduce a technique for measurements of lateral sheet resistance of high resistance thin films using low energy electron beam injection. This technique is similar in concept to previous measurements of sheet resistance of thin films deposited on Si diode array vidicon targets.4 We employ a ground plane closely spaced ('" 1 !J.) to the thin film under measurement, in order that voltages on the surface of the film in this lateral transport measure ment be well defined in contrast wIth the longitudinal transport measurements referred to above,l-a As a conse quence, if care is taken during the measurement, the experimental conditions can be made to approximate 1817 closely a simple and easily soluble problem. The agreement between theory and experiment gives confidence that the quantity measured is in fact physically meaningful. Finally, it should be ~entioned that by limiting the energy of the incident electron to ;S 5 eV, the chance of electron bombardment induced conductivity is kept small; that is, the thermalized transport properties of the thin film are measured. We will discuss the specific assumptions used in de veloping the model. Experimental application is then made in the measurement of GaAs polycrystalline thin films whose sheet resistance varies from 1011 to more than 1016 Q/O. Finally, a variation of this technique is described in which the electron beam is eliminated and only an external voltage supply is used to charge the film under measurement, although for very high sheet resistance films charging the sample in this way is not as convenient as using an electron beam. 1. DESCRIPTION OF METHOD AND SAMPLE GEOMETRY As stated above, it is desirable to perform high sheet resistance measurements in vacuo in order to minimize extraneous effects due to adsorbed species. Furthermore, a nearby ground plane is necessary so that the potential in the film depends only on the local charge density in the resistive film under examination. It is then possible to consider making sheet resistance measurements with no external connections to the sample; for example, charge could be deposited non uniformly, by an electron beam, and the spreading of charge due to differences in surface po tential could be monitored by later electron scans. How ever, we have elected to add a collector electrode to act as a sink for the deposited charge. The more complex sample geometry is offset by greater convenience and more cross checks in the consistency of the measurement, and, in fact, as discussed later, it is possible to eliminate the electron beam and perform a one contact resistance measurement in This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:401818 A. N. CHESTER AND B. B. KOSICKI Si/Si02 SUBSTRATE RESISTIVE LAYER EVAPORATED METAL CONTACT TO Si ~ METAL /ELECTRODES FIG. 1. Typical sample geometry for resistive film measurement (not drawn to scale). which contact space charge effects should be negligible for at least one polarity of the measurement. The sample geometry is shown in Fig. 1. The substrates are n-type low resistivity silicon wafers, which are ther mally oxidized to a thickness of about 7000 A. 5 This thickness provides sufficient charge storage in a convenient area ('" 1 cm2) and at low enough voltages (5 V) so that discharge currents can be easily measured. A resistive film is deposited on this substrate, and the gold collector elec trode is evaporated in two steps through appropriate evaporation masks. The exposed area of the resistive film is uniformly charged by an electron beam to a potential close to the cathode potential. The beam is then turned off and a current flow is monitored as a function of time in an ammeter attached between the collector electrode and the 5i ground plane as the deposited charge drifts from the film onto the collector electrodes, which are held at the silicon ground plane potential. The linear geometry of many parallel stripes was chosen for ease of fabrication (a spacing of about 0.5 mm between electrodes will be shown to be convenient for sheet resistances greater than 1011 0/0), as well as ease of analysis. II. ANALYSIS OF THE MEASUREMENT TECHNIQUE The problem that must be analyzed is as follows: Long narrow stripes of resistive film of width s and thickness d (d«s) lie on oxidized silicon, having capacitance Ca per unit area to the silicon (see Fig. 1). The stripe width s is much greater than the oxide thickness, and there are N stripes, each of length L»s. The silicon substrate will be taken as the zero of potential. Both edges of each film stripe are attached to a metal contact stripe; these contacts. are all connected to the substrate through a current measuring device of resistance RL• We shall generally be interested in the case in which RL is sufficiently small that the edges of the film stripes are maintained at essentially zero potential. At t=O the film is at uniform negative potential V and the metal contacts are at ground poten tial; it is required to find the current flowing to the sub strate as a function of time from then on. We assume, of course, that charge transport in the 5i02 film will be completely negligible. In the absence of any injected charge, let the charge density per square centimeter due to mobile electrons and holes in the film be q. and qh with effective mobilities of JJ.. andJJ.h. Let the injected electronic charge density be qi, with effective mobilitY)J.i. The current flow is in the x direction, which is perpendicular to the long dimension of the film stripe, and in the plane of the film. The current in amperes per unit length of film is . age aqh aqi J=[JJ..q.+JJ.hqh+JJ.iqiJE-D.--Dh--D j-, (1) ax ax ax where q. and gh may now depend on position, and the D's are the diffusion coefficients. Here we consider fJ.., fJ.i, D., Di, q., qi<O, andfJ.h, Dh, qh>O. In several cases we may simplify this expression and treat only unipolar conduction: (1) If the majority carriers in the film are electrons, we may neglect the term JJ.hqh. The total mobile charge is then (q.+qJ=q. We assume that the injected electrons will occupy states with the same effective mobility fJ.'=fJ.. and diffusion constant D'=D. as the mobile electrons already present. In the absence of any injected charge, the native electrons in the film are distributed homogeneously over the area of the film, with charge density q.=qo. (2) If, on the other hand, the majority carriers in the film are holes, we neglect the term JJ..q •. We may assume that the injected electrons will rapidly combine with these holes, as long as there are sufficient holes available (we restrict I qi I < I qh I here in order to make the problem tractable). We then have a total mobile charge density q=qh+qi, all with mobility l =JJ.h and diffusion coefficient D' =Dh• Again in the absence of injected charge the native carriers have a uniform charge density qh=qO' In either case we may write j=fJ.'qE- (D'aqjax) (2) and the equation of motion for the mobile charge may be written aq(x,t)jat= -aj(x,t)jax. (3) We therefore have a simple unipolar form [Eqs. (2) and (3)J which describes the charge motion, regardless of This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:40RESISTANCE OF FILMS 1819 whether the film is p type or n type, and regardless of whether the conductivity behavior is Ohmic (j ex: E) or not. Because of the thinness of the oxide layer compared with the stripe width, it may be shown that the film po tential can be determined in the capacitance approxima tion; that is, as a function only of the local excess (un compensated) charge density (q-qo), and unaffected by the distribution of charges elsewhere, V (x,t) = [q(x,t) -qoJ/C a• (4) The transverse drift field is therefore E(x,t)= -aV(x,t)jax, (5) which is assumed to be homogeneous throughout the thickness of the film. Thus, using Eqs. (2)-(5), and the fact that aqo/ax=o, aq(x,t) = _~[(P.'q _D,)aq]. at ax Ca ax (6) In order that the problem be soluble we require the silicon ground plane to be of sufficiently high conductivity so that the capacitance Ca is independent of the surface potential, and therefore independent of x and t. In general, the quantities /J.' and D' may depend upon the local electric field, since they may be controlled by field dependent trapping processes. Equation (6) allows for this possibility; however, at sufficiently long times that the electric field does not exceed 104 V / cm such effects are probably not important. 6 Equation (6) is not only nonlinear in q but also involves effective mobilities and charge densities that are not, in general, known for high resistance thin films. It is therefore desirable to obtain an initial linear solution depending upon as few unknown parameters as possible. The excess (uncompensated) charge density, which we now denote by q' (x,t), q' (x,t) =q(x,t) -qo, (7) gives rise to the net transverse electric field as expressed by Eqs. (4) and (5). In order to further simplify Eq. (6), we consider the condition I q' (x,t) 1« I qo I, (8) that is, the injected charge density is small compared to the native charge density in the film. As long as the material under study is not intrinsic, this assumption can always be satisfied in principle by injection of only an infinitesimal amount of charge. Moreover, we will discuss later a possible model for conduction in semiconductor thin films which suggests that this condition should easily be satisfied for semiconductor films even with moderate injected electron densities. The size of the diffusion term may be estimated with the help of the Einstein relation D'=/J.'kT/e. (9) Since q' /Ca is initially several volts, but even then is much less than qo/Ca according to Eq. (8), D'«/J.'q/C a and diffusion may be neglected. In addition to Eq. (8), we now consider times suffi ciently long and therefore E sufficiently small so that J.t' does not depend upon E. Then Eq. (6) becomes (neglecting the diffusion term) (10) where the second form arises by defining an effective sheet resistance RO = (qOp.')-I. (11) In this approximation the current flowing into the film electrode, which is also the current measured, per unit length of film boundary, is i = -[qoJ.t' EJ.,=8/2 = + (ROCa)-I[ aq' (x,t)/ ax ],"-8/2, (12) where x=o in the center of a film stripe of width s. Thus the boundary conditions applicable to Eq. (10) are q' (x,O) = VC a, (13) q'(s/2,t)/C a= (RdRoCa)[aq'(x,t)/aXJ"=8/2, (14) [aq'(x,t)/axJ,,=o=O, (15) where V is the initial film potential. In the special case that the external resistance, RL~O, which is obtained in practice, the solution is found to be 00 (-1)n4 (2n+ 1)1I'X q'(x,t)=VC a L ---cos--- n=O (2n+l)1I' s Xexp[ -(2n+ 1)2t/fJ (16) with an effective time constant of (17) Equations (12) and (16) lead to a total measured current of. 00 I=2NL·i=I o L exp[ -(2n+1)2t/rJ, (18) n=() where In this equation, Qtotal=CaNLsV is the total charge density deposited on the film, in coulombs. For t> l' one may take I ~Io exp( -t/f). (20) The current of Eq. (18) is indefinitely large at 1=0; this is due to the small amount of charge near the metallic This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:40lR20 A. N. CHESTER AND B. B. KOSICKI 104r-~--r-~~r-----------------------' '" t02 ~ Q ,.: ~ 10' a: a: G TOTAL INITIAL CHARGE/IO'sec TIME, SECONDS FIG. 2. Current I predicted by Eq. (18), as a function of time t, for various values of time constant 'T. Experimental conditions are: V= -5 V, C,,=4XIQ-9 F fern!, area=NLs=O.73 em!, Qtotal= -1.45 X lO-s C. contact, which flows into the contact quickly as soon as the potential of that contact is changed. At short times, the current may be approximated by an integral, 10 f exp[ -(2n+l)21/f]::doj'" exp[ -(2n+l)2l/fJdn n~ & , = (10/4) (n·f/t)!, t«f. (21) The specific predictions of this Ohmic solution will now be shown for a typical set of geometrical and physical quantities used in this work, which are as follows: V = -S V (initial potential applied); /,=0.877 cm (length of resistive film stripes); N= 15 (number of resistive film stripes); s=0.555 mm (spacing between metallic contacts, that is, equal to the width of the resistive film stripes) ; Ca=4X10-9 F /cm2 (silicon dioxide layer 7000 A thick). UJ II: g tot5 '" a: UJ a. If) IOt4 ::. :t:: o 102 101 10° 101 10' 10° TIME CONSTANT T, SECONDS UJ <!) '" >-..J 0 > t:: z :0 0 Z .. .. UJ a: .. N § -a: 0 "- vi UJ a: UJ a. ::!; .. 0 u 5: S FIG. 3. Predicted sheet resistance RO vs observed time constant 'i for various electrode spacings (left scale); current 10 for 1 em! area and unit voltage, as a function of observed time constant T (right scale). Capacitance assumed: C.=4Xlo-a F/cm2• For these values, Qtotal = -1.45 X 10-s C, (22) and 1o(nA)=5.9/f (sec). (23) The current 1 as a function of time predicted by Eq. (18) for a typical case is shown in Fig. 2 for various values of time constant f. Note the regions of validity for the short and long time functional forms [Eqs. (21) and (20)J given above. The considerations going into choosing the spacings (an experimental geometry) are displayed in Fig. 3. The time constant f given by Eq. (17) is shown as a function of sheet resistance Ro for various electrode spacings s. From this plot and Eq. (23) it may be seen that the experimental geometry referred to in Fig. 2, when used to measure sheet resistances between 1011 and 1016 n/o, gives measured values of 10 ranging from 10 nA to 1 pA and characteristic time constants ranging from 0.1 to 104 sec. This makes the currents acceptable within the sensitivity and response times of available meters having appropriate input impedances. III. EXPERIMENTAL It is clear that a demountable electron gun is highly desirable for performing electron beam injection experi ments. We have used a commercial vidicon gun,1 sealed into a flange with low vapor pressure epoxy cement,S as shown in Fig. 4. During sample changing, the oxide coated cathode is protected from atmospheric moisture by flowing dry N 2 through the stem. Elevated filament voltage was required during operation (8-9 V), but the tube could be repeatedly opened and closed without serious deterioration of the cathode. Al though O-ring seals were used, with continuous pumping by a titanium SUblimation-sputter ion pump combination, the vacuum was sufficiently good that ion damage to the cathode was not a problem. SAMPLE AND MOUNTING RING (Si CONTACT -TO . SECOND VACUUM FEED THROUGHl REMOVABLE FRONT FLANGE + TYPE BNC VACUUM ELECTRICAL FEED THROUGH EPOXY SEAL VACUUM PORT FIG. 4. Schematic diagram of demountable electron gun. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:40RESISTANCE OF FILMS 1821 Since the currents measured were often less than 10-12 A (see Fig. 2), careful shielding and high leakage resistance were required. We found glass sealed, Teflon insulated type BNC connectors satisfactory as electrical vacuum feed throughs.9 Although two electrical feedthroughs were used, one for the collector electrode and the second for the Si ground plane, only the first of these is shown in Fig. 4. During an initial charging period, the target was elec trically disconnected from the sensitive current meter in order to prevent serious overload to the meter, and to allow the entire sample surface to become fully charged. By using a relay instead of a manual switch the timing can be made precise enough to observe the current pulse as close as 10 msec after the start of current flow, thus allowing measure ment of films with time constants as short as 0.1 sec. Furthermore, the switch can be repetitively opened and closed rapidly enough for convenient observation of rapidly changing current pulses on an oscilloscope. During the charging cycle, the following sequence of events occurs: (1) The metal collector electrode is disconnected from the meter input and connected to a voltage source (a voltage pulser with an adjustable pulse length); the pulse height is set equal to the applied cathode voltage (this prevents beam pulling to the electrode during the charging cycle); at about the same time the beam is turned on for many frames (0.5 sec total). (2) After 0.5 sec, the entire face of the sample is ap proximately at cathode potential, and the beam is turned off. No charge flows onto the collector electrodes, since these electrodes are still maintained at approximately the same potential as the resistive film by the voltage source. (3) The voltage on the metal collector is now set tQ ground potential. This defines the start of the current flow [i.e., the boundary conditions given by Eqs. (11)-(13) at t=O]. (4) Very shortly thereafter (,....,10 msec), the relay con nects the meter again and the measurement of current decay begins. Because of the small currents to be measured good shielding and a sensitive, stable current meter are required. The electrometer which we have used10 measures current as a voltage drop across a grid resistor, which is adjustable from 10 to 1011 12. As mentioned earlier, this grid resistor RL should be large enough to sense the voltage drop, yet small compared to the effective total source resistance (RT) of the sample measured. In practice, it was found satisfactory to choose RL so that the voltage drop across the meter input was always limited to 1 m V or less. In order that sheet resistance will be the only unknown parameter in our measurement, it is necessary to measure the total charge Qtotal deposited on the resistive film. ELECTRON BEAM SCANNED AREA ~~~STlVE:'-~~~~S~IO~2!2m,zM~ Si (b) COLLECTOR ELECTRODE FIG. 5. (a) Arrangement for measurement of deposited charge. (b) Approximate equivalent circuit for (a). Qtotal can be accurately measured by the experimental arrangement of Fig. Sea), whose approximate electrical equivalent is shown in Fig. S(b). CE and CR represent the total capacitance of electrode and resistive film, respec tively; RT is an effective film resistance; Q E and Q R are the charges deposited on electrode and film when the electron beam is turned on; and C is a large external series capaci tor. The voltage V M across C measures the total charge deposited by the electron beam. If both the film and the contact are uncharged before the beam is turned on, then QR=Qtotal and the measurement gives Qtotal+QE. If CE is then discharged (by shorting the electrode to the silicon substrate for a time short compared with RrCR), a second electron beam charging will deposit only charge QE. This allows a determination of Qtotal without accurate knowledge of C E, C R, or RT• In general the voltage to which the resistive film is charged, V = Qtotal/C R, is not as large as the cathode voltage Ve. This comes about because of a voltage drop in the cathode and the contact potential difference between the resistive film and the oxide coated cathode. Since C R can be estimated from a knowledge of the Si02 thickness, the measurement of Qtotal allows the potential drop ~= Vo -V to be determined, as will be discussed in the following section. If the gold electrode is connected to the Si substrate after charging, and the current decay curve monitored for some time, it is obvious that the charge QR required to recharge the film at the end of this time will be equal at the integral of the current decay curve up to that time; this has been verified experimentally. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:401822 A. N. CHESTER AND B. B. KOSICKI SA~~LE IS~CI Roln/o) QRICOULI ... -GoAs-83 -.4-3.5 X 10" '1.4 X 10'8 0-Go As -34 14 1.1 X 10'3 -.65XIO,8 • -GoAs -67 110 aox 10'3 -,76XIO,8 0-Go As -38 630 4.7X 1014 -c79XIO'8 e-GoAs-37 _10' -7XIO'6 (c8 X 10-8) X -BARE Si02 Co 4000 pflcm2 S= .555mm X X X ,62 ':;-L....U.J..1J..1I<~..J....UwuJ.--'--U.I.WJ..I..-'--L.J.J..LwL.,,w...u..u.w.....-L..J...u..uuI ~ ~ 10 ~ ~ ~ TIME. SEC. FIG. 6. Measured current as a function of time for five GaAs films (points) compared with calculated current (solid curves). IV. RESULTS We repeat here that the objective of this work is to characterize lateral sheet resistance in high resistance thin films. If the current flow is Ohmic like (not an unreasonable assumption for the small electric fields which prevail after the initial bit of charge adjacent to the electrodes has decayed) and if the density of injected carriers is much less than the density of native carriers in the film, then there should exist a current decay curve, calculated for some sheet resistance, to which the measured current decay curve corresponds reasonably well over a large range in current and time. All physical parameters other than the resistance (i.e., electrode spacing and film area, capacitance per unit area of the substrate, and initial charge deposited) can be determined independently of the current measure ment. In this section we will show that the single parameter characterization is valid for carefully prepared GaAs thin film samples, and we will point out some of the practical aspects of making such measurements. Most of our sheet resistance measurements have been taken on very thin GaAs films (S~400 A, measured by optical and mechanical techniques), We will use the results of five of these samples, collected in Fig. 6, to illustrate the capability and limitations of the technique discussed above. Each experimental curve (points) can be fitted well with a theoretical curve (see Fig. 2) by varying only one parameter, the time constant f. The agreement between theory and experiment generally extends over many orders in current and time . Samples 83 and 37 were chosen since they represent approximately the limiting times over which current can be measured. As explained above, the shortest time is limited by the time accuracy with which the electromechanical relay can be made to close. By careful adjustments of the timing sequence discussed above, it is possible to make measurements down to less than 10 msec (not shown in Fig. 6). On the other hand, it is obvious that measurement times greater than 104 sec (",,3 h) are inconvenient. This last reason, together with the fact that the measured currents are becoming extremely small, is the reason that no knee appears in the current decay curve for sample 37. In order to estimate the time constant of this sample, current was allowed to decay overnight, but was not mea sured during this time. At the end of this time, the charge needed to restore the sample was measured as described above. The time constant f could then be estimated by integrating Eq. (18). The value of QR=-0.8X10- sC obtained from the current decay curve together with the overnight recharging measurement is consistent with values of Q R measured for the other samples. Note in Fig. 6 that the sample with the shortest time constant (No. 83) has a total charge QR = Qtotal close to that expected if the resistive layer were charged to -5 V, the cathode potential [measured value of QR, 1.4X1Q-s C, expected value from Eq. (22), 1.4SX1Q-s CJ. This comes about because this sample has such a short time constant (0.4 sec) that it can be almost fully charged by the fixed voltage applied to the contacts for 0.5 sec; thus its charge reflects this voltage rather than the potential produced by the electron beam.ll For the other samples, the time constant T greatly ex ceeds 0.5 sec, so that their total charge reflects only the influence of the electron beam charging. As previously discussed, the charge deposited on these samples permits a calculation of the potential drop.:l= Vc-V due to cathode potential drop and contact potential. Using the estimated resistive film capacitance of 2900 pF (4000 pF /cm2 and film area NLs=0.73 cm2) , a charge QR of 0.8X1Q-s C implies an actual initial potential of the film of 2.8 V. Thus the potential drop .:l is about 2.2 V. This calculation can be independently confirmed by varying the cathode voltage. Since the capacitance of the sample is approximately independent of applied voltage, an increase in the cathode voltage from 5 to 10 V should (if .:l is constant) cause the initial potential of the resistive film to rise by 5 V, to about 7.8 V. The observed decay current from the sample using 10 V on the cathode should thus be QR(lOV)/QR(S V)=2.8 times the current measured with 5 V applied to the cathode, but with the same functional dependence on time. This measurement was performed with sample No. 37. The current ratio was found to be This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:40RESISTANCE OF FILMS 1823 approximately constant as a function of time, ranging from 2.6 to 2.8. This is considered satisfactory agreement with the predicted value of 2.8. As indicated in the introduction, it is very difficult to carry out conventional, two-or four-probe resistance measurements under vacuum conditions for such high resistance samples being considered here. Such two-probe resistance measurements, carried out in a dry atmosphere, were made on GaAs films similar to the ones described here, with largely unsatisfactory results.12 Furthermore, direct resistance comparisons were made on a series of TaHfN samples, each of which was measured both by our technique and by a conventional two-probe technique, again carried out in a dry atmosphere.13 The conventional technique always gave lower sheet resist ances, which differed from the sheet resistances given by our technique by widely varying amounts. This may be due to the effects of adsorbed species in the two-probe method, which may be acting to short out the high film resistance, and points up possible difficulties with the conventional method, on samples not contained in a vacuum. There are several small but persistent discrepancies be tween the model predicted curren ts and the measured ones, which occur for many samples. These are (1) a larger than expected current for t«f (see Fig. 6, samples 67, 38, and especially 37), and (2) a larger than expected current for t> f (see Fig. 6, samples 34 and especially 83). The larger than expected current observed at short times is thought to arise from nonideal contact geometry, caused by gold creeping under the shadow evaporation mask, therefore causing nonsharp electrode edges, or even a lower effective sheet resistance near the contact. In cases where severe masking problems have occurred, evidence for an invisible conducting region extending an appreciable dis tance from the electrodes has been obtained from video pictures of the sample produced by scanning. the electron beam as in a conventional vidicon tube. It is impossible to obtain a good fit with the current decay curves of these samples, which show very large excess currents for t;:; f. The excess current in Fig. 6 for sample 37 near 1 sec is not severe, when it is realized that this occurs at a time about five orders of magnitude below the characteristic time constant. A tail similar to that in Fig. 6 for sample 83 occurs for the current decay curves of many samples and is thought to be due to either thermal effects or adsorbed species on the surface, since generally this tail diminishes in time, after the sample has been mounted and is under vacuum. In fact, the data fit to the predicted curve for some samples has been seen to improve more than an order of magnitude simply by allowing the sample to remain under high vacuum overnight. Adsorbed species could possibly modu late the current as suggested here by adding deep traps which change the effective mobility of the film. These considerations again underline the appropriateness of performing high sheet resistance measurements under high vacuum conditions. Charging by the contacts alone was mentioned earlier as a way to perform resistance measurements on samples of our geometry without the use of an electron beam. The contact charging method is less convenient and flexible than using electron beam charging [for example, the sample must charge through the electrodes for many time constants, a time interval which is initially unknown, whereas, the charging time required with the electron beam (much less than 0.5 sec) is a function only of the beam current, capacitance per unit area of the substrate Ca, and cathode voltage Vc]. On the other hand, the contact charging method has the advantage that the sample can be easily charged either negatively or positively with respect to the ground plane, and can be charged to a high voltage (we have charged some samples to 100 V, a limit deter mined only by the breakdown field of the Si02 layer) without exposing the sample to high energy electrons. The capability of charging to either polarity is quite useful as a test for space charge accumulation effects at the contact, since this effect can only limit the current in at most one polarity.I.6 We have used the contact charging method in conjunction with electron beam charging and have verified that no observable contact effects occur in our GaAs thin film samples (the same current decay curve is obtained with either polarity) and that contact injected carriers are equivalent to beam injected electrons in our film (the same current decay curve results from beam charging and contact charging). If, varying only f, a good fit cannot be obtained between experimental data on a given film and the theoretical curves, then either the assumptions of the theory are violated for this film, or nonideal experimental conditions, such as those outlined above, are hiding the true curve shape. The two main assumptions of the model are (1) that the mobility f.I is field independent, which is probably true for the small field conditions « 104 V jcm) under which all data of Fig. 6 were taken, and (2) that the number density of injected electrons is much less than the number density of native charge carriers, which may be violated for very high resistivity, nearly insulating films. It is possible to discriminate between what we call nonideal experimental conditions and real breakdown of the model assumptions, which can give information about the physical conduction processes occurring in the film. For example, the excess current at t < f observed in Fig. 6 for the high resistance sample 37 might be thought to be due to either nonconstant mobility (too great an initial field) or too great an initial number of injected charges, either of which may relax to values satisfying the model at later times. A test of this hypothesis is given by simply taking a second current curve at a different, say, larger initial voltage (and, of This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:401824 A. N. CHESTER AND B. B. KOSICKI course, charge density). If the model assumptions have been violated, the new current curve will not be simply displaced upward in current, since the time at which model assumptions are met will also be displaced to a later time. The results of this experiment on sample 37 showed, how ever, a simple displacement in current, proportional to the increased amount of charge deposited on the sample. Collecting together the information given above, we can estimate, under the conditions of our experiments, the carrier concentrations in our GaAs films. We have injected up to 1012 electrons/cm 2 (corresponding to about 1018 electrons/cm3 for 100 A thick films), and as shown above we have observed no strong departures from the predicted curves. The arguments above imply therefore that these are lower limits on the native carrier concentrations in our films. This relatively large concentration, and the at tendant low mobility (lO-L 10-7 cm2/V· sec) necessary to account for the high sheet resistances observed (lOlL 10140/0) may be explained by a model in which all carriers reside in deep traps either on the surface o~ in the bulk, and conduction takes place by hopping or tunneling between neighboring trap centers.6 The fields are generally too small « 104 V /cm) to observe any field dependent mobility behavior.6 Surface trap densities of greater than 1012/cm2 may be expected for GaAs and other semicon ductor surfaces,9 and volume trap densities greater than 1018/cm3 are not unreasonable for such polycrystalline films; therefore, either type of trap could reasonably account for the apparent agreement with the initial as sumptions of the theory. These techniques can obviously be extended to give some information about samples in which there is observed a current increase which is nonlinear in deposited charge. To discriminate between field dependent mobility or in jected charge effects, a second sample would be necessary, either with different substrate capacity (the surface voltage being held constant) or with different electrode spacing (the deposited charge density being held constant in this case). Finally, it is obvious that the tail observed at long times (as, for example, in sample 83) cannot be caused by breakdown of the model assumptions, since the first part of the current decay, in which the charges and fields are the highest, apparently satisfies the assumptions quite well. ACKNOWLEDGMENTS For helpful discussions we are grateful to D. Kahng, F. J. Morris, M. Kuhn, Y. S. Chen, and M. H. Crowell. We thank D. A. Brooks and A. G. Timko for capable assistance in preparation and measurement. We also ap preciate mechanical design work by R. P. Hynes and resistive film deposition by F. J. Morris, E: W. Chase, R. A_ Furnanage, and L. D. Locker. .. Present address: Hughes Research Laboratory, Malibu, Calif. 90265. I W. Tantraporn, J. Appl. Phys. 39, 2012 (1968). 1 K. A. Pickar, Solid State Electron. 13,303 (1970). 3 J. Lampert, J. Vac. Sci. Technol. 6, 753 (1969). 4 M. H. Crowell and E. F. Labuda, Bell System Tech. J. 48, 1481 (1969). • A. S. Grove, B. E. Deal, E. H. Snow, and C. T. Sah, Solid State Electron. 8, 145 (1965). 6 P. M. Hill, Thin Solid Films 1, 39 (1967); A. K. Jonscher, ibid., p. 213; A. K. Jonscher, Thin Film Dielectrics, edited by F. Vratny (Electrochemical Society, New York, 1969), p. 3. 7 Griffiths Electronics, Inc., Linden, N. J. 8 "Torr-Seal," Varian Associates, Palo Alto, Calif. 9 Gremar model 912AjV bulkhead receptacle, Gremar Mfg. Co .• Inc., Woburn, Mass. 10 Keithley model 602, Keithley Instruments, Inc., Cleveland, Ohio 44139. 11 This effect could have been avoided by adjusting the fixed voltage to the value V c-il. 12 F. J. Morris (private communication). 13 L. D. Locker (unpublished data). 14 D. R. Frankl in Semiconductors, edited by H. K. Henisch (Pergamon, New York, 1967), Vol. 7, p. 269. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.42.202.150 On: Sat, 22 Nov 2014 13:10:40

1.98375.pdf

Implantation damage and the anomalous transient diffusion of ionimplanted boron A. E. Michel, W. Rausch, and P. A. Ronsheim Citation: Applied Physics Letters 51, 487 (1987); doi: 10.1063/1.98375 View online: http://dx.doi.org/10.1063/1.98375 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/51/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Suppression of anomalous diffusion of ionimplanted boron in silicon by laser processing J. Appl. Phys. 71, 3628 (1992); 10.1063/1.350923 Influence of implant condition on the transientenhanced diffusion of ionimplanted boron in silicon J. Appl. Phys. 71, 2611 (1992); 10.1063/1.351082 Implantation damage and anomalous diffusion of implanted boron in silicon Appl. Phys. Lett. 54, 1433 (1989); 10.1063/1.101353 Transient boron diffusion in ionimplanted crystalline and amorphous silicon J. Appl. Phys. 63, 1452 (1988); 10.1063/1.339926 The diffusion of ion−implanted boron in silicon dioxide AIP Conf. Proc. 122, 20 (1984); 10.1063/1.34817 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Sat, 29 Nov 2014 04:51:15~mp~antation damage and the anomaloBJs transient diffusion of ion-imp~anted boron A. E. Michel IBM Thomas J. Watson Research Center. Yorktown Heights. New York 10598 W. Rausch and P. A. Ronsheim IBM East Fishkill Facility. Hopewell Junction. New York 12533 (Received 13 April 1987; accepted for publication 17 June 1987) The effect of the implantation of silicon ions on the anomalous transient diffusion of ion implanted boron is investigated. It is found that silicon ion fluences well below that necessary to amorphize the lattice substantially reduce the anomalous transient diffusion of subsequently implanted boron. The sheet resistance, however, is increased by the additional silicon implant. The implantation of silicon ions into activated boron layers causes additional anomalous diffusion at substantial distances beyond the range of the silicon ions. The anomalous motion is also reduced in regions where the damage is greater. The effects can be explained in terms of the generation of point defect clusters which dissolve during anneal and the sinking of point defects in the regions of high damage by the formation of interstitial type extended defects. The anomalous diffusion of ion-implanted boron during the thermal anneal and activation process is a phenomenon of considerable importance in the fabrication of shallow p-n junctions in silicon. A definitive explanation of the phenom enon is still being debated. The proposed mechanisms fall into two categories: one which involves a fast diffusing inter stitial component of the boron I· 3 and the other an enhanced point defect concentration related to the implantation dam age.4-6 In this letter we present data that demonstrate two dis tinct effects of ion-implantation damage. The first is that lattice damage generated by the implantation of silicon ions prior to the boron implant and in the region of high boron concentration retards the anomalous diffusion in the tail re gion of the boron distribution. This is believed to be caused by an increase in the density of extended defects which act as a sink for the point defects.4 The other effect is that the lattice damage resulting from implanting silicon ions into a stabi lized, i.e., substitutional, boron distribution causes anoma lous diffusion of the boron at considerable distances from the implanted silicon distribution. Both of these effects strongly support a model in which the implant damage is the source of point defects which are trapped in small clusters7 after implantation and during subsequent thermal treatment, de pending on the density, either grow and generate extended dislocations or dissolve and rapidly distribute through the lattice enhancing the boron diffusion. It is further speculated that the point defects involved are interstitials, since the ex tended defects are of the interstitial type and it is known that boron diffusion is substantially enhanced by a supersatura tion of silicon interstitials.8 Several fluences of 130 keY silicon ions were implanted at a 7° offset into (100), 10 n cm, n-type, silicon wafers. The wafers were subsequently implanted with a fluence of 2 X 1014/cm2, 60 keY boron ions. The energies were selected so that the ranges of the boron and silicon ions are about equal. The secondary ion mass spectrometry (SIMS) pro files of the boron after a 35-min furnace anneal at 800 DC are shown in Fig. 1. The solid curve is the as-implanted profile for the case with no silicon implantation. The lowest silicon ion dose of 2 X 1014!cm2 is wen below that necessary to amorphize the silicon lattice. The intermediate dose of 5 X 1014/cm2 is at the amorphization threshold at the con centration peak. The highest dose of 2 X 1015/cm2 is suffi cient to amorphize the lattice approximately 100 nm on ei ther side of the peak. It is apparent that, in a11 cases where the silicon is initially damaged, a significant reduction in the tail diffusion is observed. At 800 °C there is little motion in the region of the boron peak, with or without the silicon implan tation. It might be argued that some of the reduction in the tail diffusion is caused by a decrease in the boron channeling tail. However, in practice the boron channeling tail is not substantially reduced unless the crystal is amorphized to a sufficient depth to contain the majority of the boron profile.9 In the highest dose case the displacement is less in the con centration range of 10 17_101 scm 3, i.e., in the upper part of the taiL This may be due to a difference in the channeling tail produced by the rather thick amorphous region. The dis placement in the :lower concentration part of the tail is some what greater than for the samples with the lower silicon ~'8s,~ N'I~ on1 • NONE 0-2Xl014/C1"!""2, UC .ell 0-5>::10'''/crrl, 1JC -ell t,,-2xl0'5/cm2. 13G ~,ev zooo 4C:;0 60C~ DEPH-(AlJGS TRC \AS) FIG. 1. Reduction by silicon ion implantation ofthe anomalous boron dif fusion during an 800 ·C furnace anneal. 487 Appl. Phys. Lett. 51 (7). 17 August 1987 0003-6951/87/330487-03$01,00 @ 1987 American Institute of Physics 487 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Sat, 29 Nov 2014 04:51:15""1 1 2 u 1 [119 ~ Z 10'B o f-« 0:: f- Z lj 10" Z o U 2eSi+ mplant .,. NONE 0-2X10'4/cm2, 130 keY 0-5xl014/cm2, 130 keY ~-2xlo'5jC'T12• 130 keY 2000 4000 6000 DEPTH (ANGSTRO~S) FIG. 2. Reduction by silicon ion implantation of the anomalous boron dif fusion during a 1000 'C rapid anneal. doses; however, it is still considerably less than the sample with no silicon implant. The transient diffusion is not elimin ated by the additional damage but is reduced by an amount comparable to that achieved by a short high-temperature rapid thermal anneal (R T A). 10 Shown in Fig. 2 are the SIMS profiles for wafers pre pared with the same implant conditions as the sample for Fig. 1, but given a rapid thermal anneal at 1000 DC for 10 s instead of the 35-min 800 °c furnace anneal. At a tempera ture of 1000 DC the entire boron distribution is involved in the anomalous diffusion 10 and, as in the case of the low temperature furnace anneal, a retardation of the displace ment is observed for the samples with implanted silicon ions. The curve with the highest silicon dose has nearly the same anomalous displacement in the very low concentration re gion as that for the sample with no silicon implant. The sheet resistance measurements for these samples are shown in Table I. Note that while the anomalous diffu sion is diminished by the silicon damage, the sheet resistance is increased in proportion to the dose of silicon ions. Also in all cases the sheet resistance values for the furnace anneal are greater than those for the corresponding sample with the RTA. This presumably is related to the improvement of the defect removal at the higher temperature. 11 The retardation of the anomalous diffusion is difficuh to reconcile with the mechanism involving a fast diffusing in terstitial boron component, since it is unlikely that the sili con implantation would reduce the amount of boron intersti tials. It may be understood, however, by the effect of the implantation damage on the point defect concentration. For TABLE I. Sheet resistance values for 35-min 800 'C furnace anneal and IO-s lOOO'C RTA for various silicon implant fluences. 488 Silicon dose (fern') none 2X 1014 5X 1014 2X 10" Sheet resistance (n) Furnace anneal Rapid anneal 481 555 569 703 414 436 466 496 Appl. Phys. Lett .. Vol. 51 , No.7, 17 August 1987 the two cases where the silicon implant dose is insufficient to amorphize the lattice, the increase in lattice disorder genera ted by the silicon implant may enhance the formation of extended defects in the initial part of the anneal. The addi tional extended defects absorb the silicon interstitials4 which are emitted as the small point defect clusters dissolve during the anneal cycle.7 This mechanism is also present in the case without the silicon implant since there are extended defects formed in the region of the boron peak. The sheet resistance data are also consistent with an increase of damage which may act as trapping centers for boron atoms thus reducing the electrical activity. For the highest dose silicon implant another mechanism must be considered. Here the lattice is amorphized and re grows by solid phase epitaxy in a fraction of a second at the beginning of the anneal. 12 Since the majority of the implant damage is concentrated in this region, the density of defect clusters may be substantially reduced by the epitaxial re growth process, causing a reduction in the source of the dif fusion enhancing silicon interstitials. In the case where the boron implant is entirely within the amorphous region there is no substantial anomalous diffusion. 13.14 Since the boron concentration is well below the solubility limit, one might expect the boron to be all electrically active in the region of the peak and hence have a very low sheet resistance. The measured sheet resistance values in Table I show the oppo site trend. It is thus likely that the recrystaUized region is heavily defected. The effect of silicon implantation on the motion of stabi lized boron distributions is shown in Fig. 3. In this experi ment the boron implant was annealed for 30 s at 950 DC dur ing which the boron is activated, i.e., moved into substitutional sites and the anomalous diffusion effect is completed. to For this condition the boron diffuses only by the normal mechanisms during a subsequent heat treatment. Since the boron concentration only slightly exceeds the in trinsic level, the concentration enhanced diffusion is mini mal and the diffusion length for a subsequent anneal for 30 min at 800 DC is only 3 nm. In fact no significant displace ment is observed during such a heat treatment on a stabilized - G 2000 4000 6D)0 JEPlr1 (ANGSiROMS; FIG. 3. Generation of anomalous diffusion of a stabilized boron distribu tion by a 13O-keY silicon ion implantation. Michel. Rausch, and Ronsheim 488 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Sat, 29 Nov 2014 04:51:15Z ,('8 o \ I \ 16Si"' ImpiO"'lt .~ NONE , ,)_. 1 )(1014/Cm2, 50 loreV - - - o 1CI'6 '--___ -.l. __ ~_-'-____ _L_ _ _'____J o 2000 4000 DEP 1 H (ANGSTROI;1S FIG, 4, Generation of anomalous diffusion of a stabilized boron distribu tion by a 50-keV silicon ion implantation, boron distribution. One wafer was subjected to a 2x 1014/ cm2 silicon implant prior to the anneal. The solid line is a Gaussian representation of the silicon ion distribution using the range and standard deviation tables of Gibbons et al.ls The sample that received the silicon implant, however, shows a displacement of the boron in the tail region of over 100 nm, similar to the anomalous transient displacement of a boron implant for such a heat treatment. 10 A similar effect on a stabilized boron distribution was reported by Cho et al.6 during RTA after a boron 10 implantation. Figure 4 demonstrates that an anomalous displacement of the same magnitude is obtained with a much lower energy silicon implant. The solid curve is again the Gaussian repre sentation of the implanted silicon ion distribution, The dis placed boron atoms in the tail of the boron distribution are about 500 nm deeper than the average penetration of the primary silicon ions or the secondary displaced lattice atoms. This phenomenon clearly requires a species that has an extremely high diffusion coefficient and the only possible candidates are point defects. It is thus consistent with the picture that the anomalous boron diffusion is a result of the 489 AppL Phys, Lett.. Vol. 51. No, 7. 17 August 1987 breakup during anneal of interstitial clusters which rapidly diffuse and enhance the boron diffusion. A comparison of the displacement in the surface region for the cases with the different energy silicon implant (the curves with open circles in Figs. 3 and 4) shows that there is much less boron motion for the low-energy case. This is again consistent with the concept that in the region of high damage the motion is retarded by the increased formation of extended defects, 'w, Hofker, H. Werner, D, Osthoek, and H. deGrefte, AppL Phys. 2, 265 ( 1973), 'R. T Hodgson, V, Deline, S, M. Mader, F. E Morehead, andJ, C Gelpey. Mater. Res, Soc. Symp, Proc, 23, 253 (1984); Appl. Phys, Lett 44. 589 ( 1984), 'L. C Hopkins, T, E. Seidel, J, S, Williams, and J, C Bean. Electrochem, Soc. 32, 2035 (1985), 4R. B. Fair, J J, Wortman, and J, Liu, J, Electrochem. Soc, 131, 2387 (1984). 'G, S, Oehrlein. R, Ghez. J. D. Fehribach. E, F, Gorey. T, O. Sedgwick. S. A Cohen, and V. R, Deline. Proceedings of the 13th International Confer ence on Defects in Semiconductors. edited by L. C. Kimerling and J. M, Parsey. Jr. (Metallurgical Society of AIME, Warrendale. PA, 1984), P 539. "K. Cho. M. Numan, T. Finstad. W, Chu, J. Lui, and J, Wortman, AppL Phys, Lett. 47.1321 (1985). 'F Cembali. M. Servidori, E. Landi, and S, Solmi. Phys, Status Solidi A 94. 315 (1986), "S. M, Hu, J, App!. Phys, 45,1567 (1974), "c. I. Drowlwy, J. Adkission. D, Peters, and S, Chiang, Mater, Res, Soc. Symp, Proc, 35, 375 (1985), lOA, E. Michel. W. Rausch, p, A Ronsheim. and R. H. Kastl, Appl. Phys, Lett SO, 416 (1987), liT. 0, Sedgwick. presented at Symposium on Reduced Temperature Pro cessing for VLSI, Fall Meeting ofthe Electrochemical Society, Las Vegas. NV, Oct \3-18, 1985, "G. L. Olson, in Energy Beam -Solid Interactions and Transient Thermal Processing (1984 MRS Symp, Proc.), edited by D. K. Biegelsen, G, A. Rozgonyi, and C. V. Shank (Elsevier. North-HolIand, 1985), "T. 0, Sedgwick. Mater. Res, Soc, Proc. 71. 403 (1986), 14T. E. Seidel. IEEE Electron Device Lett. EDL-4. 353 (1983). "J, F. Gibbons. W, S, Johnson, and S. W, Mylroie, in Projected Range Sta tistics (Halstead, Straudsburg, PA. 1975). Michel. Rausch. and Ronsheim 489 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Sat, 29 Nov 2014 04:51:15

1.1140328.pdf

Determining the integrated cavity emissivity of blackbody furnaces Quansheng Wu, Yinghang Chen, Zaixiang Chu, and Bijuan Li Citation: Review of Scientific Instruments 60, 1140 (1989); doi: 10.1063/1.1140328 View online: http://dx.doi.org/10.1063/1.1140328 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A new compact fixed-point blackbody furnace AIP Conf. Proc. 1552, 300 (2013); 10.1063/1.4821382 Comparison of the emissivity uniformity of several blackbody cavities AIP Conf. Proc. 1552, 757 (2013); 10.1063/1.4819637 Design of an isothermal cavity with nonuniform local instrinsic emissivities to give true blackbody radiant characteristics Rev. Sci. Instrum. 63, 3213 (1992); 10.1063/1.1142581 Comment on Blackbody Radiation in Small Cavities Am. J. Phys. 42, 505 (1974); 10.1119/1.1987762 Blackbody Radiation in Small Cavities Am. J. Phys. 40, 1337 (1972); 10.1119/1.1986827 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.216.129.208 On: Mon, 01 Dec 2014 00:37:56Determining the integrated cavity emissivity of blackbody furnaces Quansheng Wu and Yinghang Chen Institute No. 303, Ministry of Aerospace Industry, Beijing, China Zaixiang Chu and Bijuan U Harbin Institute of Technology, Harbin, China (Received 6 June 1988; accepted for publication 20 November 1988) An apparatus has been constructed for determining the integrated cavity emissivity of blackbody furnace simulators near 500 K and for calibrating infrared detectors. A difference-ratio method is described for measuring the radiant emission characteristics of the simulator by using a high quality heat-pipe blackbody as a standard radiant source operated at 500 K and a water-immersed cavity as a reference source operated at a lower temperature. It is verified that the total uncertainties of the method are less than 0.6% for determining the integrated cavity emissivity and 0.5% for determining irradiance from the 500-K standard blackbody on the detectors. INTRODUCTION Radiant sources at temperature near 500 K are increasingly used for calibrating industrial blackbody furnaces (hereafter referred to as simulators), infrared detectors, infrared sys tems and instruments, infrared radiation thermometers, and so on. It is usual to calculate the radiant emission character istics of a cavity, that is, the distribution of effective emissi vity along the inner wall of the cavity, by theoretical meth ods.1-3 The values of effective emissivity within the cavity depend upon the geometrical configuration, the temperature distribution along the inner wan of the cavity, and the intrin sic emissivity of the material of the cavity wall. It is very difficult to measure the emissivity of the cavity wall and the temperature distribution. Further, the temperature distribu tion within the cavity and the material emissivity may change with time of use. Consequently, it is valuable to set up an apparatus and employ a method for determining and monitoring many blackbody simulators with a total uncer tainty less than ± 0.5%--0.6%. A system that includes a heat pipe with a 500-K stan dard blackbody whose emissivity is readily calculated, a wa ter-immersed reference blackbody with stable and uniform temperature distribution, a pyroelectric detector integrated with a high-quality amplifier or a thermoelectric pyrometer, a mechanical reset to assure correct positioning of the stan dard and calibrated blackbodies, a temperature controller, and a microcomputer for controlling operation of the system has been set up in our laboratory. The schematic diagram of the system is shown in Fig. 1. i. DIFFERENCE-RATIO METHOD FOR DETERMINING BLACKBODY FURNACES A difference-ratio method is proposed to calibrate simu lators in order to eliminate the effect of surrounding radi ation and assure an accuracy within ± 0.6%. It can be seen from Fig. 2 that the net flux received by the detector from the standard blackbody is (1) FlG. 1. Schematic diagram of 500-K standard blackbody and radiation comparison system. 1,12: Leveling instruments; 2: cross mark plate; 3: de tector; 4: precise stop; 5: flowing water switch; 6: water-cooled shield; 7: chopper; 8: standard blackbody; 9: reference blackbody; 10: blackbody fur· nace; 11: He-Ne laser; 13: microcomputer; 14: digital voltmeter; 15: radi ation shield; 16: radiation comparison operating board. where t'; is the integrated cavity emissivity of the standard blackbody. Ed and ad are the emissivity and absorptivity of the detector, respectively. Tj and TO! are the temperatures of the standard blackbody and the detector, respectively. F;)l and Fd" are the geometry factors of the detector to the stan dard blackbody and to the surroundings, respectively. WI,s is the net radiant contribution from the surrounding to the detector and ()' is the Stefan-Boltzmann constant. If a pyro electric detector is used and a chopper is located between blackbody and stop, it is reasonable to suppose WI•s to be zero. Similarly, we can write FIG. 2. Geometry and temperatures of the blackbodies, simulator, and de tector. 1140 Rell. Sci. Instrum. 60 (6), June 1989 0034-6748/69/06 i 140-03$01.30 @ 1989 American institute of Physics 1140 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.216.129.208 On: Mon, 01 Dec 2014 00:37:56W2 = ~adoTiFo2 -Cd aT 62 Fd,s + W2•s, (2) W3 = E~adO'T1Fo3 -£dO'T63Fd.s + W3•s• (3) where W2 and W3 are the net fluxes received by the detector from the simulator and the reference blackbody, respective ly. £~ and £~ are the integrated cavity emissivities of the simulator and the reference blackbody. T2 and T3 are the temperatures of the blackbody furnace and the reference blackbody, respectively. F02 and Fo} are the geometry fac tors of the detector to the blackbody furnace and the refer ence blackbody, respectively, Wz,s and W3., are the radiant contributions from the surrounding to the detector corre sponding to the cases in which the detector views the simula tor and the reference blackbody, respectively. Tm and T03 are the temperatures of the detector when viewing the black body furnace and the reference blackbody, respectively. In order to eliminate the radiant effect from the sur rounding to the detector, we utilize a parameter which is the ratio of the radiant flux differences as (4) The temperature of the detector can be controlled to be con stant in the calibration procedure and the temperatures of the detecting surface in three cases are nearly the same, hence TOl :::':: I;J2 :::':: T03, ad =€d' (5) (6) Because the geometrical configuration of the standard blackbody and the reference blackbody are identical and the two cavities are both isothermal, it follows that €'; = £L FOI = F02 = Fm, WI,s = W2•s = W3,s' (7) (8) (9) The integrated cavity emissivity can be calculated according to the method described in Ref. 4. Combining Eqs. ( 1 )-(9), we can obtain finally the basic equation for calibrating the industrial blackbody functions, (10) We limit the area of effective radiant surface on which the temperature distribution is sufficiently uniform, so T2 is the temperature of the radiant surface of the simulator viewed by the detector. It APPARATUS DESCRIPTION A photograph ofthe experimental apparatus is shown in Fig. 3 and the major components are described in the follow ing section. A. Standard blackbody A heat pipe, cylindro-double cone cavity with effective emissivity greater than 0.9995 is used for the standard black body and reference blackbody in order to assure demands for high effective emissivity and uniform temperature distribu tion. The specifications for the geometry of the cavity are: 1141 Rev. Sci. Instrum., Vol. 60, No.6, June 1989 FIG. 3. Experimental apparatus. the half-angle of the cone is 36°; the frustum is part of a cone having half-angle of 30°; the radius and the length of the cylinder are 15 and 150 mm, respectively; and the radius of the aperture is 10 mm. The zonal approximate method pro posed by Bedford and Ma3 has been developed and used here to calculate the effective emissivity distribution of the cavity. The effective emissivities of the portion viewed by the detec tor are 0.9995 ± 0.0003. When the cavity is used in the infrared region, the specu lar reftection component within the cavity must be consid ered. In this condition, the directional effective emissivity can be defined from the work of On05; the half-angle of the cone is taken as 36° in order to maintain higher effective emissivity in both the visible and infrared regions, The direc tional effective emissivity of the cavity has been calculated using the Monte Carlo method. The working fluid selected for the heat pipe is deter mined by the desired operating temperature. The design principle is that the pressure of saturated vapor be less than 2 X 106 Pa (20 atm) at 500 K, so octane is used. The material of the tube and wick is stainless steel which is compatible with octane. Three tubes with length 195 mm and inside diameter 6 mm for measuring the temperature are arranged in the vapor space. They accommodate a platinum resistance thermometer and copper versus Constantan thermocouple for temperature measurement and control. The measured results show that the temperature uniformity and the tem perature stability of the heat pipe cavity are better than 0.1 K and 0.1 K/4 h, respectively, B. Reference blackbody In order to eliminate the effect of surrounding radiation and improve accuracy of calibration, an isothermal refer ence blackbody at 323 K is provided. The cavity is immersed in flowing water whose temperature is well controlled by the microcomputer. To simplify the calibration equation, the reference blackbody is designed with the same geometry and thus has the same values of effective emissivities as the stan dard blackbody. It can be seen from the measured data that the temperature stability of the reference blackbody is better than 0.1 K12 h. Blackbody furnace simulator 1141 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.216.129.208 On: Mon, 01 Dec 2014 00:37:56c. Mechanical reset system This system is designed to measure the radiant fluxes emitted from the standard blackbody, the reference black body, and the simulators. The microcomputer collects and processes the detected signals and completes the determina tion of the integrated cavity emissivity of the blackbody fur naces. The radiation comparison system is composed of a later al comparison guide, a vertical measurement guide, a data acquisition system, a controlling unit, and a beam alignment system as shown in Fig. 1. The standard blackbody, the reference blackbody, and the simulator are set on an operating board which can be moved by a stepping motor. The resct accuracy of the oper ating board is within 0.05 mm. Another operating board for locating the detector is as sembled on the vertical guide on which a water-cooled radi ation shield is also set to prevent the influence of stray radi ation from the surroundings. The beam alignment system uses a helium-neon laser and a leveling instrument to adjust I Substituting the experimental data listed in Table I into Eq. ( ! 1), we find that the total uncertainty of the integrated cavity emissivity determination is less than 0.6%. The method proposed in our previous article'~ can be used to calculate the irradiance from the standard blackbody onto the detector. An approximate expression for estimating this irradiance is (12) where (yis the Stefan-Boltzmann constant, E~ is the integrat ed cavity emissivity of the standard blackbody, 1'1 is the tem perature ofthe standard blackbody, Td is the temperature of TABLE l. Values of the rc1evanl errors of the system. Error sources M~,M; Ll r,tan ~T> !:J.D b.r Diffraction loss Absolute values of error O.OCll 0,3 K 0.4 K O.5mm D,003 mm 0.1% 1142 Rev. Sci. Instrum., Vol. 60, No.6, June 1989 the axes of the three blackbodies parallel with one another and vertical to the receiving plane of the detector. The entire system is automatically controlled by the computer. iii 0 UNCERTAINTY ANALYSIS The main errors for determining the integrated cavity emissivity of the blackbody furnaces are due to uncertainties associated with: (1) the isothermal calculation of the effec tive emissivity of the standard blackbody and the reference blackbody, (2) temperature measurement of the standard and reference blackbodies, (3) temperature control of the reference blackbody, (4) position reset of the operating board, (5) instability of the radiation detecting system, and (6) the distance measurement between the stop and the de tector. When the apparatus is used for absolute calibration, the error due to the stop radius measurement, the tempera ture increase of the stop, and the loss of diffraction must be considered. We can derive the following expression for the total un certainty of the system from Eq. (10): (11 ) the detector, r is the radius of the source stop, and D is the distance between the stop and the detector. The total'uncer tainty can be expressed as (13) Substituting the measured data into Eq. (13), we can calcu late that the total uncertainty in the irradiance is less than 0.5%. I R. E. Bedford and C. K. Ma, J. Opt Soc. Am. 64, 339 ( 1974). 'R. E, Redford and C. K. Ma, J. Opt. Soc. Am. 65, 565 (1975). 'R, E Bedford and C. K. Ma, J. Opt. Soc. Am, 66, 724 (1976). ·S. Chen, Z. Chu, and H. Chen, Mctrologia 16, 69 (1980). 'A. Ono, J. Opt. Soc. Am. 70,547 (1980), Blackbody furnace simulator 1142 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.216.129.208 On: Mon, 01 Dec 2014 00:37:56

1.2810992.pdf

Mylar Giri Moses Chan and Milton Cole Attilio Stella Citation: Physics Today 42, 4, 94 (1989); doi: 10.1063/1.2810992 View online: http://dx.doi.org/10.1063/1.2810992 View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/4 Published by the American Institute of PhysicsBoth men earned PhDs from Har- vard: Ewen in 1951 and Purcell in 1938. Purcell shared the 1952 Nobel Prize with Felix Bloch for the discov- ery of nuclear magnetic resonance. The Tinsley Prize was awarded to Ewen and Purcell at the Boston AAS meeting in January. —PHA PHYSICISTS ELECTED FOREIGN MEMBERS OF SOVIET ACADEMY The Soviet Academy of Sciences elect- ed 16 US scholars as foreign members last December. The scholars consti- tute the largest group of foreign members ever electe d at one time. Eight of the 16 new members work in physics or a closely related field: Roald Hoffman, professor of chemis- try at Cornell University; Peter David Lax, professor of mathematics at the Courant Institute of Mathematical Sciences at New York University; Edward N. Lorenz, professor at the Center for Meteorology and Physical Oceanography of MIT; Wolfgang Pan- ofsky, director emeritus of SLAC; David Pines, professor of physics at the University of Illinois at Urbana- Champaign; Frank Press, president of the National Academy of Sciences; J. Robert Schrieffer, director of the In- stitute for Theoretical Physics at the University of California, Santa Bar- bara; Samuel Ting, professor of phys- ics at MIT; and Peter Wyllie, head of the department of geological and pa- leontological sciences at Caltech. IN BRIEF Steven Kivelson and Sudip Chak- ravarty, former assistant professors at the State University of New York, Stony Brook, have been named profes- sors of physics at UCLA. Alan Lightman, formerly a staff member at the Harvard-Smithsonian Center for Astrophysics, has been appointed professor of science and writing at MIT, teaching in the de- partments of physics and humanities. Kevin D. Pang, a physicist at Cal- tech's Jet Propulsion Laboratory, has been awarded Dudley Observatory's Herbert C. Pollack Award for re- search in the history of astronomy and astrophysics. Pang received the $10 000 prize for his studies of an- cient Chinese eclipse records aimed at determining the past rotation rate of the earth.Leon J. Radziemski, former head of the physics department at New Mexi- co State University, has been appoint- ed associate dean and director of the research center in the university's college of arts and sciences. Ethan T. Vishniac and Donald E. Winget have been promoted from assistant professors to associate professors with tenure in the astron- omy department at the University of Texas at Austin. OBITUARIES William Savage William R. Savage, a professor in the department of physics and astronomy at the University of Iowa, died on 28 May 1988 following a brief illness. Born in Cedar Rapids, Iowa, on 12 September 1926, Savage received his BS in 1951 and his PhD in 1956, both in physics, from Iowa State Universi- ty. His thesis, done under Donald Hudson and Frank H. Spedding, was a study of the heat of sublimation of rare-earth metals by an extension of the Knudsen and Taylor-Langmuir methods. From 1956 until 1958 he worked as a research physicist at the Honeywell Research Center in Hop- kins, Minnesota. In 1958 he moved to the Central Research Laboratories of Texas Instruments at Dallas, where he contributed to investigations of surfac e properties of semiconductors using the field emission microscope. Savage joined the University of Iowa as an associate professor of physics in 1963. He established a solid-state physics laboratory there and supervised many MS and PhD students on research projects involv- ing measurements of the specific heats, resistivities and magnetic sus- William R. Savageceptibilities of dilute magnetic alloys and intermediate-valence compounds. During the year before his death he turned his attention to the prepara- tion and study of materials related to the high-rc superconductors. After volunteering several years ago to teach a course in acoustics for music and speech pathology students, Savage became seriously interested in musical acoustics. He collaborated on studies of harpsichords and other instruments. From 1976 until 1979 he was chairman of the technical com- mittee on musical acoustics of the Acoustical Society of America. Be- tween 1974 and 1987 he organized six conferences on acoustics and the phys- ics of sound and music that were held at the University of Iowa. William Savage was an enthusiastic teacher and researcher who will be fondly remembered by his students, colleagues and friends. JOHN W. SCHWEITZER University of Iowa Iowa City, Iowa Mylar Giri Mylar Giri died suddenly on 1 July 1988, at the age of 37. This brought to a premature end a promising career as a physicist and educator. He was an associate professor of physics at the Hazleton campus of The Pennsyl- vania State University, where he had taught for eight years. Giri was educated at Bangalore University, the Indian Institute of Technology, New Delhi, and Rutgers University. He had been a research scientist with E. I. du Pont de Ne- mours & Company Inc and a visiting professor at the University of Padua. His research interests and publica- tions were wide ranging, and included percolation, phase transitions, poly- mers, films and fractals. He brought to these problems great intensity, curiosity and enthusiasm. He pub- lished only the most important contri- butions (to his collaborators' occa- sional dismay). Giri was appreciated enormously also for his dedication and skill in teaching both researchers and un- dergraduates. His generosity with time to his students was legendary. Those who knew him will miss his wit, his warmth, his openness and his intelligence. MOSES CHAN MILTON COLE Pennsylvania State University University Park, Pennsylvania ATTILIO STELLA University of Padua Italy' 94 PHYSICS TODAY APRIL 1989

1.2810989.pdf

Physicists Elected Foreign Members of Soviet Academy Citation: Physics Today 42, 4, 94 (1989); doi: 10.1063/1.2810989 View online: http://dx.doi.org/10.1063/1.2810989 View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/4 Published by the American Institute of PhysicsBoth men earned PhDs from Har- vard: Ewen in 1951 and Purcell in 1938. Purcell shared the 1952 Nobel Prize with Felix Bloch for the discov- ery of nuclear magnetic resonance. The Tinsley Prize was awarded to Ewen and Purcell at the Boston AAS meeting in January. —PHA PHYSICISTS ELECTED FOREIGN MEMBERS OF SOVIET ACADEMY The Soviet Academy of Sciences elect- ed 16 US scholars as foreign members last December. The scholars consti- tute the largest group of foreign members ever electe d at one time. Eight of the 16 new members work in physics or a closely related field: Roald Hoffman, professor of chemis- try at Cornell University; Peter David Lax, professor of mathematics at the Courant Institute of Mathematical Sciences at New York University; Edward N. Lorenz, professor at the Center for Meteorology and Physical Oceanography of MIT; Wolfgang Pan- ofsky, director emeritus of SLAC; David Pines, professor of physics at the University of Illinois at Urbana- Champaign; Frank Press, president of the National Academy of Sciences; J. Robert Schrieffer, director of the In- stitute for Theoretical Physics at the University of California, Santa Bar- bara; Samuel Ting, professor of phys- ics at MIT; and Peter Wyllie, head of the department of geological and pa- leontological sciences at Caltech. IN BRIEF Steven Kivelson and Sudip Chak- ravarty, former assistant professors at the State University of New York, Stony Brook, have been named profes- sors of physics at UCLA. Alan Lightman, formerly a staff member at the Harvard-Smithsonian Center for Astrophysics, has been appointed professor of science and writing at MIT, teaching in the de- partments of physics and humanities. Kevin D. Pang, a physicist at Cal- tech's Jet Propulsion Laboratory, has been awarded Dudley Observatory's Herbert C. Pollack Award for re- search in the history of astronomy and astrophysics. Pang received the $10 000 prize for his studies of an- cient Chinese eclipse records aimed at determining the past rotation rate of the earth.Leon J. Radziemski, former head of the physics department at New Mexi- co State University, has been appoint- ed associate dean and director of the research center in the university's college of arts and sciences. Ethan T. Vishniac and Donald E. Winget have been promoted from assistant professors to associate professors with tenure in the astron- omy department at the University of Texas at Austin. OBITUARIES William Savage William R. Savage, a professor in the department of physics and astronomy at the University of Iowa, died on 28 May 1988 following a brief illness. Born in Cedar Rapids, Iowa, on 12 September 1926, Savage received his BS in 1951 and his PhD in 1956, both in physics, from Iowa State Universi- ty. His thesis, done under Donald Hudson and Frank H. Spedding, was a study of the heat of sublimation of rare-earth metals by an extension of the Knudsen and Taylor-Langmuir methods. From 1956 until 1958 he worked as a research physicist at the Honeywell Research Center in Hop- kins, Minnesota. In 1958 he moved to the Central Research Laboratories of Texas Instruments at Dallas, where he contributed to investigations of surfac e properties of semiconductors using the field emission microscope. Savage joined the University of Iowa as an associate professor of physics in 1963. He established a solid-state physics laboratory there and supervised many MS and PhD students on research projects involv- ing measurements of the specific heats, resistivities and magnetic sus- William R. Savageceptibilities of dilute magnetic alloys and intermediate-valence compounds. During the year before his death he turned his attention to the prepara- tion and study of materials related to the high-rc superconductors. After volunteering several years ago to teach a course in acoustics for music and speech pathology students, Savage became seriously interested in musical acoustics. He collaborated on studies of harpsichords and other instruments. From 1976 until 1979 he was chairman of the technical com- mittee on musical acoustics of the Acoustical Society of America. Be- tween 1974 and 1987 he organized six conferences on acoustics and the phys- ics of sound and music that were held at the University of Iowa. William Savage was an enthusiastic teacher and researcher who will be fondly remembered by his students, colleagues and friends. JOHN W. SCHWEITZER University of Iowa Iowa City, Iowa Mylar Giri Mylar Giri died suddenly on 1 July 1988, at the age of 37. This brought to a premature end a promising career as a physicist and educator. He was an associate professor of physics at the Hazleton campus of The Pennsyl- vania State University, where he had taught for eight years. Giri was educated at Bangalore University, the Indian Institute of Technology, New Delhi, and Rutgers University. He had been a research scientist with E. I. du Pont de Ne- mours & Company Inc and a visiting professor at the University of Padua. His research interests and publica- tions were wide ranging, and included percolation, phase transitions, poly- mers, films and fractals. He brought to these problems great intensity, curiosity and enthusiasm. He pub- lished only the most important contri- butions (to his collaborators' occa- sional dismay). Giri was appreciated enormously also for his dedication and skill in teaching both researchers and un- dergraduates. His generosity with time to his students was legendary. Those who knew him will miss his wit, his warmth, his openness and his intelligence. MOSES CHAN MILTON COLE Pennsylvania State University University Park, Pennsylvania ATTILIO STELLA University of Padua Italy' 94 PHYSICS TODAY APRIL 1989

1.2810991.pdf

William Savage John W. Schweitzer Citation: Physics Today 42, 4, 94 (1989); doi: 10.1063/1.2810991 View online: http://dx.doi.org/10.1063/1.2810991 View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/4 Published by the American Institute of PhysicsBoth men earned PhDs from Har- vard: Ewen in 1951 and Purcell in 1938. Purcell shared the 1952 Nobel Prize with Felix Bloch for the discov- ery of nuclear magnetic resonance. The Tinsley Prize was awarded to Ewen and Purcell at the Boston AAS meeting in January. —PHA PHYSICISTS ELECTED FOREIGN MEMBERS OF SOVIET ACADEMY The Soviet Academy of Sciences elect- ed 16 US scholars as foreign members last December. The scholars consti- tute the largest group of foreign members ever electe d at one time. Eight of the 16 new members work in physics or a closely related field: Roald Hoffman, professor of chemis- try at Cornell University; Peter David Lax, professor of mathematics at the Courant Institute of Mathematical Sciences at New York University; Edward N. Lorenz, professor at the Center for Meteorology and Physical Oceanography of MIT; Wolfgang Pan- ofsky, director emeritus of SLAC; David Pines, professor of physics at the University of Illinois at Urbana- Champaign; Frank Press, president of the National Academy of Sciences; J. Robert Schrieffer, director of the In- stitute for Theoretical Physics at the University of California, Santa Bar- bara; Samuel Ting, professor of phys- ics at MIT; and Peter Wyllie, head of the department of geological and pa- leontological sciences at Caltech. IN BRIEF Steven Kivelson and Sudip Chak- ravarty, former assistant professors at the State University of New York, Stony Brook, have been named profes- sors of physics at UCLA. Alan Lightman, formerly a staff member at the Harvard-Smithsonian Center for Astrophysics, has been appointed professor of science and writing at MIT, teaching in the de- partments of physics and humanities. Kevin D. Pang, a physicist at Cal- tech's Jet Propulsion Laboratory, has been awarded Dudley Observatory's Herbert C. Pollack Award for re- search in the history of astronomy and astrophysics. Pang received the $10 000 prize for his studies of an- cient Chinese eclipse records aimed at determining the past rotation rate of the earth.Leon J. Radziemski, former head of the physics department at New Mexi- co State University, has been appoint- ed associate dean and director of the research center in the university's college of arts and sciences. Ethan T. Vishniac and Donald E. Winget have been promoted from assistant professors to associate professors with tenure in the astron- omy department at the University of Texas at Austin. OBITUARIES William Savage William R. Savage, a professor in the department of physics and astronomy at the University of Iowa, died on 28 May 1988 following a brief illness. Born in Cedar Rapids, Iowa, on 12 September 1926, Savage received his BS in 1951 and his PhD in 1956, both in physics, from Iowa State Universi- ty. His thesis, done under Donald Hudson and Frank H. Spedding, was a study of the heat of sublimation of rare-earth metals by an extension of the Knudsen and Taylor-Langmuir methods. From 1956 until 1958 he worked as a research physicist at the Honeywell Research Center in Hop- kins, Minnesota. In 1958 he moved to the Central Research Laboratories of Texas Instruments at Dallas, where he contributed to investigations of surfac e properties of semiconductors using the field emission microscope. Savage joined the University of Iowa as an associate professor of physics in 1963. He established a solid-state physics laboratory there and supervised many MS and PhD students on research projects involv- ing measurements of the specific heats, resistivities and magnetic sus- William R. Savageceptibilities of dilute magnetic alloys and intermediate-valence compounds. During the year before his death he turned his attention to the prepara- tion and study of materials related to the high-rc superconductors. After volunteering several years ago to teach a course in acoustics for music and speech pathology students, Savage became seriously interested in musical acoustics. He collaborated on studies of harpsichords and other instruments. From 1976 until 1979 he was chairman of the technical com- mittee on musical acoustics of the Acoustical Society of America. Be- tween 1974 and 1987 he organized six conferences on acoustics and the phys- ics of sound and music that were held at the University of Iowa. William Savage was an enthusiastic teacher and researcher who will be fondly remembered by his students, colleagues and friends. JOHN W. SCHWEITZER University of Iowa Iowa City, Iowa Mylar Giri Mylar Giri died suddenly on 1 July 1988, at the age of 37. This brought to a premature end a promising career as a physicist and educator. He was an associate professor of physics at the Hazleton campus of The Pennsyl- vania State University, where he had taught for eight years. Giri was educated at Bangalore University, the Indian Institute of Technology, New Delhi, and Rutgers University. He had been a research scientist with E. I. du Pont de Ne- mours & Company Inc and a visiting professor at the University of Padua. His research interests and publica- tions were wide ranging, and included percolation, phase transitions, poly- mers, films and fractals. He brought to these problems great intensity, curiosity and enthusiasm. He pub- lished only the most important contri- butions (to his collaborators' occa- sional dismay). Giri was appreciated enormously also for his dedication and skill in teaching both researchers and un- dergraduates. His generosity with time to his students was legendary. Those who knew him will miss his wit, his warmth, his openness and his intelligence. MOSES CHAN MILTON COLE Pennsylvania State University University Park, Pennsylvania ATTILIO STELLA University of Padua Italy' 94 PHYSICS TODAY APRIL 1989

1.2810990.pdf

In Brief Citation: Physics Today 42, 4, 94 (1989); doi: 10.1063/1.2810990 View online: http://dx.doi.org/10.1063/1.2810990 View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/4 Published by the American Institute of PhysicsBoth men earned PhDs from Har- vard: Ewen in 1951 and Purcell in 1938. Purcell shared the 1952 Nobel Prize with Felix Bloch for the discov- ery of nuclear magnetic resonance. The Tinsley Prize was awarded to Ewen and Purcell at the Boston AAS meeting in January. —PHA PHYSICISTS ELECTED FOREIGN MEMBERS OF SOVIET ACADEMY The Soviet Academy of Sciences elect- ed 16 US scholars as foreign members last December. The scholars consti- tute the largest group of foreign members ever electe d at one time. Eight of the 16 new members work in physics or a closely related field: Roald Hoffman, professor of chemis- try at Cornell University; Peter David Lax, professor of mathematics at the Courant Institute of Mathematical Sciences at New York University; Edward N. Lorenz, professor at the Center for Meteorology and Physical Oceanography of MIT; Wolfgang Pan- ofsky, director emeritus of SLAC; David Pines, professor of physics at the University of Illinois at Urbana- Champaign; Frank Press, president of the National Academy of Sciences; J. Robert Schrieffer, director of the In- stitute for Theoretical Physics at the University of California, Santa Bar- bara; Samuel Ting, professor of phys- ics at MIT; and Peter Wyllie, head of the department of geological and pa- leontological sciences at Caltech. IN BRIEF Steven Kivelson and Sudip Chak- ravarty, former assistant professors at the State University of New York, Stony Brook, have been named profes- sors of physics at UCLA. Alan Lightman, formerly a staff member at the Harvard-Smithsonian Center for Astrophysics, has been appointed professor of science and writing at MIT, teaching in the de- partments of physics and humanities. Kevin D. Pang, a physicist at Cal- tech's Jet Propulsion Laboratory, has been awarded Dudley Observatory's Herbert C. Pollack Award for re- search in the history of astronomy and astrophysics. Pang received the $10 000 prize for his studies of an- cient Chinese eclipse records aimed at determining the past rotation rate of the earth.Leon J. Radziemski, former head of the physics department at New Mexi- co State University, has been appoint- ed associate dean and director of the research center in the university's college of arts and sciences. Ethan T. Vishniac and Donald E. Winget have been promoted from assistant professors to associate professors with tenure in the astron- omy department at the University of Texas at Austin. OBITUARIES William Savage William R. Savage, a professor in the department of physics and astronomy at the University of Iowa, died on 28 May 1988 following a brief illness. Born in Cedar Rapids, Iowa, on 12 September 1926, Savage received his BS in 1951 and his PhD in 1956, both in physics, from Iowa State Universi- ty. His thesis, done under Donald Hudson and Frank H. Spedding, was a study of the heat of sublimation of rare-earth metals by an extension of the Knudsen and Taylor-Langmuir methods. From 1956 until 1958 he worked as a research physicist at the Honeywell Research Center in Hop- kins, Minnesota. In 1958 he moved to the Central Research Laboratories of Texas Instruments at Dallas, where he contributed to investigations of surfac e properties of semiconductors using the field emission microscope. Savage joined the University of Iowa as an associate professor of physics in 1963. He established a solid-state physics laboratory there and supervised many MS and PhD students on research projects involv- ing measurements of the specific heats, resistivities and magnetic sus- William R. Savageceptibilities of dilute magnetic alloys and intermediate-valence compounds. During the year before his death he turned his attention to the prepara- tion and study of materials related to the high-rc superconductors. After volunteering several years ago to teach a course in acoustics for music and speech pathology students, Savage became seriously interested in musical acoustics. He collaborated on studies of harpsichords and other instruments. From 1976 until 1979 he was chairman of the technical com- mittee on musical acoustics of the Acoustical Society of America. Be- tween 1974 and 1987 he organized six conferences on acoustics and the phys- ics of sound and music that were held at the University of Iowa. William Savage was an enthusiastic teacher and researcher who will be fondly remembered by his students, colleagues and friends. JOHN W. SCHWEITZER University of Iowa Iowa City, Iowa Mylar Giri Mylar Giri died suddenly on 1 July 1988, at the age of 37. This brought to a premature end a promising career as a physicist and educator. He was an associate professor of physics at the Hazleton campus of The Pennsyl- vania State University, where he had taught for eight years. Giri was educated at Bangalore University, the Indian Institute of Technology, New Delhi, and Rutgers University. He had been a research scientist with E. I. du Pont de Ne- mours & Company Inc and a visiting professor at the University of Padua. His research interests and publica- tions were wide ranging, and included percolation, phase transitions, poly- mers, films and fractals. He brought to these problems great intensity, curiosity and enthusiasm. He pub- lished only the most important contri- butions (to his collaborators' occa- sional dismay). Giri was appreciated enormously also for his dedication and skill in teaching both researchers and un- dergraduates. His generosity with time to his students was legendary. Those who knew him will miss his wit, his warmth, his openness and his intelligence. MOSES CHAN MILTON COLE Pennsylvania State University University Park, Pennsylvania ATTILIO STELLA University of Padua Italy' 94 PHYSICS TODAY APRIL 1989

1.857545.pdf

A periodic grain consolidation model of porous media R. E. Larson and J. J. L. Higdon Citation: Physics of Fluids A: Fluid Dynamics (1989-1993) 1, 38 (1989); doi: 10.1063/1.857545 View online: http://dx.doi.org/10.1063/1.857545 View Table of Contents: http://scitation.aip.org/content/aip/journal/pofa/1/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Heat Transfer Characteristics in Consolidated Porous Media AIP Conf. Proc. 1254, 27 (2010); 10.1063/1.3453825 Acoustics in granular porous media and the consolidation continuum. J. Acoust. Soc. Am. 125, 2639 (2009); 10.1121/1.4784094 Theory of compressional and transverse wave propagation in consolidated porous media J. Acoust. Soc. Am. 106, 575 (1999); 10.1121/1.427026 A new approach to seismic wave propagation in unconsolidated and consolidated porous media J. Acoust. Soc. Am. 104, 1787 (1998); 10.1121/1.423507 Thermal Conductivity of Porous Media. II. Consolidated Rocks J. Appl. Phys. 32, 1699 (1961); 10.1063/1.1728420 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:38A periodic grain consolidation model of porous media R. E. Larson and J. J. L. Higdon Department o/Chemical Engineering, University 0/ Illinois, Urbana, Illinois 61801 (Received 1 August 1988; accepted 22 September 1988) Calculations are presented for a periodic grain consolidation model of porous media. The model is an extension of previous work on lattices of spheres, in which the radius of the spheres is allowed to increase past the point of close touching to form a consolidated medium. A collocation method is used for the solution of Stokes How in terms of Lamb's general solution in spherical coordinates. Excellent accuracy is achieved with only moderate computational effort. At low concentrations up to the close touching limit excellent agreement is found with the earlier calculations of Zick and Homsy [J. Fluid Mech. 115, 13 (1982)]. For high concentrations above the close touching limit, an asymptotic theory is presented that agrees within a few percent with the numerical computations over the entire consolidated range. I. INTRODUCTION Fluid How through a porous medium plays an impor tant role in many engineering systems such as packed beds of particles, fibrous materials used in filtration, and naturally occurring materials such as permeable rock in petroleum reservoirs. Theoretical attempts to model such materials generally fall into two classes. The first class employs net works of capillaries and pores of various sizes with simple rules for How resistance based on the size of the capillaries. The simplified mechanics allows consideration of a large dis tribution of capillary sizes and network topologies yielding information on the statistical relationship between network structure and macroscopic How properties. Such informa tion may be combined with empirical data to achieve good descriptions of real porous materials. In the second class of model, detailed How fields are calculated for idealized media typically involving lattices of simple particle shapes such as spheres or cylinders. These models may provide highly accu rate results, but their usefulness depends on the extent to which they are a faithful representation of the actual materi al of interest. As an example, a lattice of close touching spheres might be an excellent model of a packed bed of smooth particles, but perform poorly as a model of fibrous materials or permeable rock. In the present effort, we pre sent a new model of porous media, which falls in this second class, but which captures certain features of real media not previously represented in lattice-type models. The simplest type of lattice model one might consider consists of two-dimensional arrays of circular cylinders. Such models were widely studied during the 1950s and 19608 using ad hoc "cell models" with artificial boundary condi tions. Results from this approach are not rigorous and need not be discussed further. The first rigorous results for the permeability, or alternatively the friction coefficient, of a regular lattice of circular cylinders were presented by San gani and Acrivos. I These authors gave results for both square and hexagonal arrays and discussed their results in the context of heat transfer in porous media. A further study of two-dimensional lattices of cylinders was conducted by Larson and Higdon,2.3 who considered How in both the axial and transverse directions. A variety of different lattice geom-etries and inclusion shapes were analyzed to study their ef fect on the concentration dependence and anisotropy of the permeability. The most significant contribution of this work was its use of these lattice models to investigate the How near the boundary of a porous medium. Arrays of two-dimensional inclusions present a simple model for analysis and may prove useful in modeling fibrous materials, but they fall short when three-dimensional media, such as packed beds, are to be modeled. Hasimot04 gave the first results for three-dimensional lattices of spheres, though his results were limited to small concentrations. Snyder and Steware and Sorensen and Stewart6 calculated the perme ability for a few close touching lattices of spheres represent ing the high concentration limit. The first results for a full range of concentrations for spheres in regular arrays were given by Zick and Homsy7 and by Sangani and Acrivos.8 These results for the friction coefficient (or permeability) were obtained by independent research groups using differ ent methods and may be considered as the definitive results for this classic model. With reliable results for lattices of spheres, one may ask whether further studies based on idealized periodic lattices are worthwhile. It is certainly feasible to calculate the per meability for other particle inclusion shapes, perhaps to in vestigate anisotropy, but there is a limit beyond which further permutations lose interest. We believe that there are certain extensions to existing models that do add significant new physical insight, even within the context of isotropic media. Flow through lattices of spheres involves the concept of How around objects rather than How through narrow con strictions found in many porous media. Only in the close touching limit does the lattice begin to mimic this type of How. Owing to the kinematic constraints of the lattice, ar rays of spheres are limited to relatively modest maximum concentrations (0.5236, 0.6802, and 0.7405 for the classic geometries), which fall significantly below the solid volume fraction of many real porous materials, especially in geologi cal applications. One final limitation of sphere models is that they represent unconsolidated media, that is, the solid inclu sions are separated from each other by the Huid. In a consoli dated medium, including essentially all real media, the two phases are both interconnected with neither phase isolated 38 Phys. Fluids A 1 (1), January 1989 0899-8213/89/010038-09$01.90 © 1988 American Institute of Physics 38 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:38GRAIN CONSOLIDATION MODEL UNIT CELL 1m W FIG. I. Schematic view of three-dimensional consolidated medium based on overlapping spheres on a simple cubic lattice. Inset shows unit cell used for computations. into distinct inclusions in a space filling medium. While not essential for accurate estimates of permeability, the solid connectedness is important for various other transport prop erties. These include the effect of heat conduction through the solid matrix and the deformation of the solid in an elastic medium. The propagation of acoustic waves or vibrations may be markedly different through a consolidated model of a porous medium than through a lattice of isolated spheres. To address these issues, we present calculations for flow through a model of a consolidated porous medium. The simplest way to achieve a consolidated medium is through a trivial extension of work on classic sphere lattices. We retain the same lattice geometries and spherical inclu sions, but allow the sphere radii to increase beyond the point of touching (see Fig. I). The overlapping spheres then form a consolidated medium whose volume fraction may increase up to one, completely filling the space. All channels for fluid flow are closed off at concentrations just a few percent below unity. At volume fractions just below these critical concen trations, the fluid spaces consist of very narrow constrictions connecting somewhat larger pores. The shape and sizes of the pores and constrictions vary according to the lattice ge ometry and solid concentration. While this overlapping sphere model is chosen primarily for its mathematical sim plicity, it is also appealing because it mimics certain features of real porous media in which grains of solid are forced to gether under high pressure and temperature, flattening their contact surfaces. To solve for the velocity field and permeability of the consolidated grain lattices, we require a technique for the solution of the Stokes equations in periodic domains. There 39 Phys. Fluids A. Vol. 1. No.1. January 1989 are a number of possible choices dictated by the representa tion of the velocity field and the manner in which periodicity is enforced. If one employs the periodic form of the funda mental solution as given by Hasimoto,4 then this condition is satisfied implicitly and the only remaining boundary condi tion is the no-slip condition on the sphere under the flow produced by a specified mean pressure gradient. This was the approach chosen by both Zick and Homsy and by San gani and Acrivos. The former used an integral equation based on the fundamental solution, while the latter group employed a harmonic expansion based on the fundamental solution and its derivatives. Owing to the complexity of the periodic fundamental solution, we have chosen to work with the free space form of the fundamental solution, sometimes called the Stokeslet. Under such conditions, periodicity must be enforced expli citly on the boundaries of the unit cell. In our initial attempts at this problem, considerable effort was devoted to the solu tion in the form of an integral equation using refinements of the method first proposed by Youngren and Acrivos.9 De spite the success of this approach in the work of Zick and Homsy and in our own two-dimensional calculations, we were unable to achieve satisfactory results using the integral method. The primary difficulties were the extremely long computational times required for accurate integral evalua tions over the complicated body surfaces and the construc tion of an accurate interpolant for the surface force over the intersecting spherical surfaces. Zick and Homsy were able to avoid these difficulties by performing analytical integrations using a global expansion based on spherical harmonics. These simplifications did not apply for the truncated spheri cal surfaces in our domain. After abandoning the integral equation approach, we formulated the problem as a collocation method based on the harmonic expansion of the velocity field using Lamb's general solution in spherical coordinates (see Happel and BrennerlO). Such collocation methods have been used wide ly to solve Stokes flow problems in a variety of geome tries.II-13 One difficulty that is often encountered is the ex treme sensitivity to the placement ofthe collocation points. This problem was avoided by using an excess number of collocation points and finding a least squares solution of the resulting overconstrained linear system. The details of the method are described in Secs. II and III. We note that excel lent accuracy was achieved with modest computation times. Typical runs on a single processor of the Cray XMP-48 ranged from a few seconds for nonoverlapping spheres to several seconds for the overlapping consolidated media. An asymptotic approximation for high concentration is de scribed in Sec. IV. Detailed results of the numerical compu tations are given in Sec. V along with comparisons with the results of Zick and Homsy at low concentrations and the asymptotic theory at high concentration. II. MATHEMATICAL FORMULATION Under the assumption of vanishingly small Reynolds number, the velocity field is described by the solution of the Stokes equations representing the momentum balance and the continuity equation representing the mass balance, R. E. Larson and J. J. L. Higdon 39 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:38-Vp + ftV2U = 0, VoU = o. (1) (2) In spherical coordinates, Lamb's general solution to these equations is written (Happel and Brenner, 10 p. 62) Negative orders are identical with n replaced by -(n + I). In these expressions, p, X, and ~ are the coefficients multi plying each harmonic. They will, of course, take on different values for each value of m, n. The factor (1/a)" has been pulled out of each constant for convenience; a will be defined later as the radius of a sphere. The argument of the associat ed Legendre function P '; is of course cos 0 in all cases. In the tP dependence, we have included only the term sin mtP or cos mtP, which is used in satisfying the boundary conditions in the present problem. In all cases, the general solution in cludes both terms. 00 p= L PIt' (3) n = -00 00 u= L [VX(rx,,) +V<I>" +aJ2Vp" +p"rp,,], II = -00 (4) where Pn' X" and <l>n are solid spherical harmonics. The numerical coefficients an and PIt are defined for conven ience. Their values are given by The above expressions for the velocity are valid at all values of r, however, they are not the most convenient forms for satisfying the no-slip condition on a sphere of radius a. In this case, we follow Happel and Brenner and define the ex pressions a" = (n + 3)/2(n + 1)(2n + 3)ft, p,,= -nl(n+I)(2n+3)ft· (5) Each of the harmonics p", X,,, and <1>" is written in terms of associated Legendre functions in the form 00 (na n) L PIt +-<1>" , ,,= _ 00 2ft (2n + 3) a r = a rnp,;(cos O)eim¢. (6) With these expressions for the harmonics, the three components of velocity associated with each harmonic may be written explicitly as f (_n.....;.(_n_+_I_)a_ p" + n(n -I) <I>/l) , (8) 40 ur = (ria)" cos mtP [~P';(nlr) +pP';(na" +p,,)r], Uo = -cosmtP Xp';-.-+<I>-P';-,,= _ 00 2ft (2n + 3) a r = a 00 [ro(Vxu)],=a = L [n(n + l)X,,]r=a· n = -00 ( r)" (-m -d I a sm 0 dO r -d pIt ) +p- ma"r , dO ( r)n (-d -m (7) For velocity equal to zero on a sphere r = a, each of these expressions must be zero. This result is used to advan tage in Sec. III. u'" = -sin mtP -X -r;: -<l>P'; --.-a dO r sm 0 -pm rm) -p "a" -.-. sm 0 It is convenient to write the stress tensor as the sum of a pressure contribution and a viscous contribution such that O"i) = -p8i) + 'Ti). The explicit form of the viscous stress T in spherical coordinates becomes 'Trr = (ria)" cos mtP P';{~[2n(n -l)/r] + p(2)(n + I )(na" + P It}' 'T rO = ( : r cos mtP P ~n (X m; :i; ol) ) + ( : r cos mtP :0 P'; (~ (2n;; 2) + p( 2na" + p" ) ) , (r)". d m(-(1-n») (r)n. A.pm( A;.2(n-l)m _(2na/l+p,,)m) 'Tr¢ = -sm mtP -P n X + -sm m'l' n -'V _2· 0 -P . 0 ' a dO r a r sm sm 'TOO = ( : r cos mtP [X C :i: 0) (:0 P'; -cot OP '; )] + ( : r cos mtP [ ~ (~ ) (:02 2 P'; + nP:7) ] + (: r cos mtP [P(2) (a" :022 p~n + (nan + PIt )P,;)] , 'To", = (: r sin mtP[x (+ ) (cot 0 :0 P'; -Si:220 P'; -:022 p,;)] + (: r sin mtP[~ + p(anr)] (r-s~mo) (:0 P'; -cot OP';) , 'T "'''' = ( : r cos mtP [X C~i!~) (:0 P ~n -cot 0 P ~n ) ] + ( : )" cos mtP [ ~ (~ ) ( nP ~n + cot 0 :0 P '; -Si:220 P '; ) ] + (:)" cos mtP [P(2) (nan + PIt )P'; -a" Si:220 P'; + an cot 0 :0 p,;)]. Phys. Fluids A, Vol. 1, No.1, January 1989 R. E. Larson and J. J. L. Higdon (9) 40 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:38As in the case of the velocity field, the negative orders are found by replacing n with -(n + 1). This completes the specification of the velocity, pressure, and stress fields. III. GEOMETRY SPECIFICATION AND BOUNDARY CONDITIONS A three-dimensional periodic lattice is characterized by a set of three independent base vectors Si' In standard nota tion, we specify the base vectors, the center-to-center dis tance d, and the volume of the unit cell Vo for the three classic lattices: (a) simple cubic (SC), s, = d( 1,0,0), S2 = d(O,I,O), S3 = d(O,O,1), Vo = d3 ; (b) body centered cubic (BCC), SI = 3-1/2d(1,I, -1), S2 = 3-1/2d( -1,1,1), S3 = 3-1/2d(1, -1,1), Vo = p-1/2d3; (c) face centered cubic (FCC), SI = 2-1/2d(1,1,0), S2 = 2-1I2d(0,1,1), S3 = 2-1/2d(1,0,1), Vo = 2-1/2d3• (10) (11) (12) For spheres of radius a centered on the lattice points, the volume fraction of solids is given by c = ~1Ta3 / Vo. For over lapping spheres, the volume fraction is given by a slightly more complicated expression that accounts for the volume of the spherical caps in the overlapping region. The optimal choice for the origin of the coordinate sys tem and the boundaries of the unit cell requires some careful consideration. The primary concern is to minimize the num ber of surfaces on which the boundary conditions must be enforced. With the origin at the center of a spherical inclu sion, the no-slip boundary condition on that sphere may be satisfied implicitly as shown below. The obvious choice for cell boundaries is the parallelepiped corresponding to a unit lattice cell centered on the origin. This is indeed the best choice for a SC lattice and is illustrated in Fig. 1. Unfortu nately, for BCC and FCC lattices, this choice leads to addi tional solid surfaces intruding into the domain. This occurs because the FCC spheres have 12 nearest neighbors-six along lattice vectors and six additional neighbors at the same distance corresponding to combinations oflattice vectors. A sphere in the BCC lattice has eight nearest neighbors. The additional solid surfaces that intrude in each case would re quire additional collocation points to enforce the no-slip condition, and hence would increase the computational ef fort required. To avoid this problem, we consider vectors drawn to the 12 nearest neighbors in the FCC case and to the eight nearest and six next nearest neighbors in the BCC case. Planes perpendicular to these vectors at the midpoint between sphere centers form the periodic boundaries of the unit cell. The unit cells thus defined are polyhedrons with 12 faces for the FCC lattice and 14 faces for the BCC lattice. While this choice of geometry may appear obtuse, it proves the most efficient from a computational point of view. Having specified the geometry of oui' system, we tum to the question of boundary conditions. The no-slip condition on the sphere r = a requires the three velocity components 41 Phys. Fluids A, Vol. 1, No.1, January 1989 (7) to be zero. As noted earlier, this condition is equivalent to the requirement that the three expressions (8) equal zero. Comparing spherical harmonics of positive and negative or der, it is easy to show that this condition is satisfied identical ly if the negative orders are chosen as (13) A comment is in order. In exterior flow problems, such as flow around a single particle, only harmonics of negative order may be employed, or else the velocity will become un bounded at infinity. Conversely, for interior flows, only posi tive orders may be employed if the origin lies in the fluid domain, or else the velocity will be unbounded at r = O. In the present case, neither r = 0 nor r = 00 in the fluid do main, and there is a degree of redundancy. This redundancy is exploited as shown above to satisfy the boundary condi tion on the sphere r = a implicitly. With the no-slip condition on the sphere satisfied, we require only that the velocity and stress satisfy the periodic ity conditions at the cell boundaries. Specifically, assuming a mean pressure gradient Vp through the medium, the velocity and stress at points A and B on cell boundaries must satisfy and (14) fA + fB = Vp·(XAOA + XBOB), where points A and B are related by a linear, integral combi nation of lattice vectors. Here 0A and 0B are unit normal vectors pointing out of the unit cell. For certain faces perpendicular to a coordinate axis, these conditions may be further simplified owing to symme try. Without loss of generality, assume that the mean pres sure gradient lies along the z axis. On a surface normal to the z axis, the simplified boundary conditions are u -(u·o)o = 0 and (15) foo = -Vp·x = const. This applies to the top and bottom surfaces in the SC and BCC lattices. On a surface perpendicular to the x or y axis, the bound ary conditions become uoo = 0 and (16) f -(foo)o = O. This applies to all four side surfaces in the SC lattice and the four vertical side faces in the BCC lattice. Finally, note that all lattices possess 16-fold symmetry with all quantities being either symmetric or antisymmetric about the planes z = 0, x = 0, y = 0, and x = y. This greatly reduces the number and area of the surfaces that must be covered by collocation points. There is a proportionate de- R. E. Larson and J. J. L. Higdon 41 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:38TOP VIEW SC FRONT VIEW (a) ::.~::'::.: .. TOP VIEW . . '" .' ... . . ..... : .. · '" : :::' . :-: : .'. . ......... . · .... . .. . : .. : .. <.~::': .' : : . ..... .... . " ." ., .' .' . · . '.' ' .. BCC FRONT VIEW (b) crease in the number of basis functions in each harmonic expansion. Careful consideration of these symmetries leads to the result that each of the harmonic functions contain only the following terms: Xm,n: r;: sin me, n = 4,6,8,10, ... , m = 4,8,12, ... , n>m; (17) ct> m.n: P';: cos mt/J, n = 1,3,5,7, ... , m = 0,4,8,12, ... , n>m; (18) Pm.n: P';:cosmt/J, n = 1,3,5,7, ... , m = 0,4,8,12, ... , n>m. (19) With these specifications, the solution for the velocity field reduces to the problem of solving a linear system of equations for the unknown coefficients of the basis func tions. The equations consist of the boundary conditions ( 14 ) or ( 15) and (16) applied at discrete collocation points. As noted earlier, the solution of such a system is often sensitive to the layout of the collocation points. To avoid this prob lem, we overspecify the problem by including an excess num ber of collocation points and seek a least squares solution. A few examples of the distribution of collocation points are shown in Fig. 2. SC BCC FCC FIG. 3. Layout of collocation points for numerical computations: (a) SC, (b) BCC, and (c) FCC. 42 Phys. Fluids A, Vol. 1, No.1, January 1989 '. TOP VIEW FCC FRONT VIEW (e) FIG. 2. Channel cross sections for use in asymptotic theory; length scale I as defined in Eq. (22). The least squares solution of a system oflinear equations Ax = b consists of a solution vector x*, which minimizes the sum of the residuals r = b -Ax*. For an overconstrained system, A is an M X N matrix with M > N. The solution of the least squares problem is simply the solution of the modified linear system A TAx = A Tb. The direct solution of this sys tem is sensitive to roundoff error owing to the poor condition of the matrix AT A. To avoid such problems, we follow the standard procedure of finding the QR decomposition of A and solving the right triangular system Rx = QTb. The QR decomposition was computed via a modified Gram Schmidt routine run on a SUN 3/160 workstation or using the UNPACK routine LLSIA on a Cray XMP-48. This latter routine effects the QR decomposition by Householder trans formations. IV. ASYMPTOTIC THEORY As the concentration of solids increases toward the maximum concentration for which the fluid may flow, the resistance to flow is dominated by the resistance in the small constrictions where the fluid cross section reaches its mini mum. Near the maximum concentration, these constrictions are of simple shape whose size changes slowly on a length scale based on a characteristic diameter. Thus it is possible to predict the resistance in these channels by a lubrication-type theory based on the resistance of a straight channel with the same cross section. In the lubrication limit, the cross section of the SC con strictions is simply a square, while the BCC limit is an isos celes triangle with height v2 times its base, and the FCC limit is an equilateral triangle. These cross-sectional shapes are illustrated in Fig. 3. For flow in a straight channel of fixed cross section the pressure drop may be related to the volume rate Q by an expression of the form R. E. Larson and J. J. l. Higdon 42 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:38(20) where /3 is a dimensionless numerical constant and I is a characteristic length scale for the cross section. Values for the three channel shapes in the lattices are given below. For a channel of slowly varying cross section with I(s), sbeing the distance measured along the channel, this equation may be used to find an integral for the pressure drop in the form t:.. -fLQ + fL, ds P -/3 - -L, [I(S)]4 . (21) The limits of the integral L.,L2 are determined by the lattice geometry and are representative of the length of the constriction. With this integral expression for the pressure drop through a constriction, we may estimate the pressure drop associated with a given volume flow rate and calculate the friction coefficient or permeability for the lattice. For the lattice and channel geometries required here, we have the following geometric expressions: SC: /3 = 0.5623 = (111/2) d -~, L. = -d /2, L2 = d /2; BCC: /3=0.85651 = (V6/4)d-~, L. = -(1I2V6)d, L2 = (1I2V6)d; FCC: /3 = 0.77942 = (lIv3) d -~, L. = ( -lIv6)d, L2 = (1I2V6)d. (22) The definition of the length scale 1 in each case is indicat ed on Fig. 3. The values of /3 for the square (SC) and for the equilateral triangle (FCC) may be found in Happel and BrennerlO (see pp. 38 and 39). The value for the BCC case was found by solving the channel flow problem using a sim ple collocation method. V.RESULTS In this section, we present the numerical results for the calculation offriction coefficients and permeabilities for the SC, BCC, and FCC lattices. Following previous authors and consistent with Zick and Homsy,7 we define the friction co efficient K as K = F /61TfLaU, (23) where Fis the force on a single inclusion, a is the radius of the sphere, U is the superficial velocity through the lattice, and fL is the fluid viscosity. This definition is quite useful for spheres because it has the value 1.0 in the limit as concentra tion approaches zero. It is somewhat artificial for overlap ping spheres, but we retain it for convenience. To demonstrate the convergence of the numerical meth od, Table I gives the values of the friction factor for two concentrations for each of the three lattices. The higher con centration corresponds to the close touching limit for spheres, while the lower concentration corresponds to the value used by Zick and Homsy in their convergence tests. Excellent accuracy is achieved in all cases; the results agree with those of Zick and Homsy to within the stated accuracy 43 Phys. Fluids A. Vol. 1, No.1. January 1989 TABLE I. Friction coefficient K as a function of concentration for different lattices. Order indicates highest-order harmonic used in expansion. Higher concentration for each lattice represents close touching limit. Lattice se Bee FCC se Bee FCC c 0.027 0.125 0.216 0.5236 0.6802 0.7405 Order 14 2.0044 4.442 7.766 38.825 124.9 127.7 20 2.0074 4.446 7.758 40.369 144.0 315.0 28 2.0077 41.357 158.4 385.8 38 2.0077 42.388 42 41.951 162.3 428.7 50 41.995 162.6 430.7 of those earlier results. The order number in the first column refers to the highest -order harmonic appearing in the expan sion. As an example, a computation up to 28th order would have 49 basis functions for each variable with a total of 3 ( 49) = 147 total unknowns; 100 collocation points were used for a total of 300 equations. Typical Cray XMP-48 computation times would be under 1 sec for the SC lattice, about 12 sec for the BCC, and 5 sec for the FCC lattice. For an order 50 computation, there would be 468 total un knowns with approximately 1000 equations. Cray CPU times were of order 11 sec, 42 sec, and 22 sec for the SC, BeC, and FCC lattices, respectively. Table II presents similar convergence tests for consoli dated media with overlapping spheres. As before, excellent accuracy was achieved for all lattices. The higher concentra tion in each case was the maximum for which three or four figure accuracy was achieved. Slightly higher concentra tions could be run at somewhat higher computational ex pense. Such calculations were unnecessary owing to the ex cellent performance of the asymptotic model developed in Sec. IV. This behavior is described below. Having demonstrated the reliability of the numerical method for a few selected concentrations, we present in Ta ble III, a comprehensive listing of the friction coefficients for the entire range of concentrations covering both sphere lat tices and consolidated media. At the highest concentrations, TABLE II. Friction coefficient K as a function of concentration of various lattices of consolidated media of overlapping spheres. Order indicates high- est-order harmonic used in expansion. Highest concentration for each lat- tice type is the highest concentration for which three or four significant figure accuracy was confirmed. Lattice se Bee FCC SC BCC FCC c 0.70 0.85 0.85 0.90 0.94 0.92 Order 20 1178 8648 28 1395 1076 1192 3186 9404 2267 42 1398 1074 2170 4096 11400 17640 50 1398 1073 2209 4291 11450 24180 58 2210 24670 64 2210 24810 R. E. Larson and J. J. L. Higdon 43 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:38TABLE III. Friction coefficient as a function of concentration for lattices of spheres and overlapping spheres. All numbers are accurate to within ± I in the last place, except where the last digit is underlined in which case the uncertainty is ± 2 in that place. Values indicated by * are those predicted by the asymptotic theory in Sec. IV. Concentrations for which the resistance becomes infinite are SC: C = 0.965 069, BCC: c = 0.994 500, FCC: c = 0.964 103. Concentration SC BCC FCC 0.000 125 1.096 1.098 1.098 0.001 1.212 1.217 1.217 0.008 1.5247 1.539 1.539 0.027 2.0077 2.044 2.044 0.064 2.8102 2.889 2.889 0.125 4.292 4.446 4.446 0.216 7.4423 7.739 7.758 0.343 15.402 16.34 16.64 0.450 28.09 31.7 33.39 0.5236 41.99 51.7 57.38 0.53 43.6 0.55 48.8 0.60 66.10 88.87 108.0 0.65 93.36 0.68018 162.6 229.5 0.70 139.8 191.0 0.72 346.0 0.74048 431.0 0.75 228.3 299.2 480.0 0.80 426.9 520.5 913.8 0.85 1.020 X 10' 1.0i3x IO' 2.210X IO' 0.90 4.29X 10' 3.05X 10' 9.343 X 10' 4.22X 10'* 0.92 1.20 X 10" 5.47 X 104 2.48 X 104 1.114 X 10"* 2.523 X 10"* 0.94 5.726X 104* 1.14 X 104 1.5 X 10-' 1.23 X 104* 1.307 X 105* 0.95 2.583X 105* 6.189X 105* 0.96 1.447 X 10"· 3.2X 10' 3.06x 107* 3.425 X 10'* 0.98 2.6X 105 2.667 X 105. 0.99 5.944X 10"· predictions based on the asymptotic theory are indicated by *. Examining the results for c = 0.92 for the SC and FCC lattices and 0.94 for the BCC lattice, we see that the comput ed results mesh smoothly with the asymptotics at these high concentrations. In fact, the asymptotic results provide a quite adequate model over the entire range of concentrations for the consolidated media. This is seen clearly in Figs. 4 (a)- 4( c), where the friction coefficients are plotted as a function of concentration. The dashed lines show the results of the asymptotic theory; the agreement is good even down to the close touching limit. Figure 5 shows the friction factors for all three lattices on a single graph. We note the qualitative similarity among all three curves. The fact that the simple cubic ° crosses over the body centered!:::" at high concentra tion is due to the fact that it will reach its maximum concen tration for flow at a smaller value than the other lattice. Finally, we note the smooth behavior of the curves in cross ing the limit of close touching spheres, marked by the sym bols O,!:::", and 0 for SC, BCC, and FCC, respectively. There is no qualitative change in the slope or curvature at this point. In fact, the location of the crossover point would be 44 Phys. Fluids A, Vol. 1, No.1, January 1989 12.0 (a) SC g 10.0 3 I-8.0 z w U u: u.. 6.0 w 8 z Q 4.0 b a: u.. 2.0 0.2 0.4 0.6 0.8 1.0 CONCENTRATION c 12.0 (b) BCC 10.0 g 3 8.0 I-z W /. U , /. u:: 6.0 , /. u.. , , w 0 () Z 4.0 0 i= () a: 2.0 u.. 0.0 0.0 0.2 0.4 0.6 0.8 1.0 CONCENTRATION c 12.0 (e) FCC g 10.0 z ...J !Z 8.0 w U u:: 6.0 u.. w 0 () z 4.0 Q Q a: u.. 2.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 CONCENTRATION c FIG. 4. Dimensionless friction coefficient K as a function of concentration c. Vertical lines mark the close touching limit for spheres and maximum concentration. (a) SC, (b) BCC, and (c) FCC. R. E. Larson and J. J. L. Higdon 44 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:383 ffi s;2 u. u. w 8 ~ 5 if u. 0.2 0.4 0.6 0.8 1.0 CONCENTRATION c FIG. 5. Combined friction coefficient figures for the three lattices. Symbols identify the individual curves and mark the concentration at which the spheres reach the close touching limit. SC: 0, BCC: £:,., and FCC: D. quite indistinguishable on the curves if it w~re not explicitly marked. Although the data presented on friction coefficients completely specify the medium's resistance to flow, it is sometimes more convenient to examine the permeability k, defined in Darcy's law, flU = -kVp. (24) The permeability thus defined has units of length squared. For a given superficial velocity U, we need to know the mean pressure gradient Vp. This is simply given by the force on an inclusion divided by the volume of the unit cell containing the inclusion, F / Vo. With the force defined in terms of the friction coefficient K, this leads to k= Vo/61TaK. (25) With this relationship, the nondimensional permeabil ity (61Tak / Vo) is simply the reciprocal of the friction coeffi cient. This definition is not the best choice because the factor a/ Vo changes as a function of concentration. A more natural scaling is to nondimensionalize k with respect to d 2, where d is the interparticle distance in the lattice. The nondimen sional permeability may then be written (26) The permeability thus ca1culated is presented in Figs. 6(a)-6(c) for each of the three lattices. The curves for all three lattices are plotted in Fig. 7 for comparison. The ap pearance of these curves is exactly as one would infer from the previously shown results for friction coefficients. 45 Phys. Fluids A, Vol. 1. No.1. January 1989 2.0 (al SC 0.0 1 -2.0 z -4.0 -I 5 -6.0 iii < -8.0 w ::i a: w ~ -10.0 -12.0 \ \ -14.0 0.0 0.2 0.4 0.6 0.8 1.0 CONCENTRATION c 2.0 (b) 0.0 BCC ,.. -2.0 I 3 -4.0 5 -6.0 iii ~ -8.0 :::E , , a: , , w -10.0 ~ -12.0 -14.0 0.0 0.2 0.4 0.6 0.8 1.0 CONCENTRATION c 2.0 (el FCC 0.0 1 -2.0 3 -4.0 5 -6.0 ~ w -8.0 ::i a: w ~ -10.0 -12.0 -14.0 0.0 0.2 0.4 O.S 0.8 1.0 CONCENTRATION c FI G. 6. Dimensionless permeability kid' as a function of concentration c. Vertical lines mark the close touching limit for spheres and maximum con centration. (al SC, (b) BCC, and (c) FCC. R. E. Larson and J. J. L. Higdon 45 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:382.0 r------------------, 0.0 ,.. I 3 -4.0 ~ ::::i -6.0 iii ~ ::t -8.0 a: w !l.. -10.0 -12.0 -14.0'--__ .L-. __ -'-__ -'-__ --.l._---'J...LJ 0.0 0.2 0.4 0.6 0.8 1.0 CONCENTRATION c FIG. 7. Combined permeability figures for the three lattices. Symbols iden tify the individual curves and mark the concentration at which the spheres reach the close touching limit. SC: 0, BCC: 6, and FCC: D. ACKNOWLEDGMENTS The authors would like to thank L. Schwartzi 4, 15 and D. Johnson ofSchlumberger-Doll for suggesting this problem 46 Phys. Fluids A, Vol. 1, No.1, January 1989 in connection with acoustic propagation in porous media. This problem has been studied by P. Shen of Exxon Corpora tion with a collocation method based on a different set of expansion functions. Computations were performed on the Cray XMP-48 at the National Center for Supercomputing Applications at the Unversity of Illinois. This work was supported by the National Science Foun dation. 'A. S. Sangani and A. Acrivos, Int. J. Multiphase Flow 8,193 (1982). 2R. E. Larson and J. J. L. Higdon, J. Fluid Mech. 166,449 (1986). 'R. E. Larson and J. 1. L. Higdon, 1. Fluid Mech. 178, 119 (1987). 4H. Hasimoto, J. Fluid Mech. 5, 317 (1959). 'L. J. Snyder and W. E. Stewart, AIChEJ.12, 167 (1966). oJ. P. Sorensen and W. E. Stewart, Chern. Eng. Sci. 29, 819 (1974). 7A. A. Zick and G. M. Homsy, J. Fluid Mech. 115,13 (1982). KA. S. Sangani and A. Acrivos, Int. J. Multiphase Flow 8,343 (1982). "G. K. Youngren and A. Acrivos, J. Fluid Mech. 69, 377 (1975). IOJ. Happel and H. Brenner, Low Reynolds Number Hydrodynamics (Pren- tice-Hall, Englewood Cliffs, NJ, 1975). "P. Ganatos, R. Pfeffer, and S. Weinbaum, J. Fluid Mech. 84, 79 (1978). 12p. Ganatos, R. Pfeffer, and S. Weinbaum, 99, 755 (1980). IJZ._Y. Yan, S. Weinbaum, P. Ganatos, and R. Pfeffer, 1. Fluid Mech. 174, 39 (1987). 14J. N. Roberts and L. M. Schwartz, Phys. Rev. B 31, 5990 (1985). "L. W. Schwartz and S. Kimminau, Geophys. 52,1402 (1987). R. E. Larson and J. J. L. Higdon 46 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.100.58.76 On: Fri, 06 Mar 2015 21:59:38

1.342804.pdf

Continuous Al5 Nb3Ge superconducting tapes via the amorphous state Tariq Manzur and Mireille Treuil Clapp Citation: Journal of Applied Physics 65, 2384 (1989); doi: 10.1063/1.342804 View online: http://dx.doi.org/10.1063/1.342804 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Strain effect in the critical current densities of superconducting Nb3Ge tapes J. Appl. Phys. 75, 2131 (1994); 10.1063/1.356275 Fabrication of Nb3Al and Nb3(Al,Ge) superconducting composite tapes by electron beam irradiation Appl. Phys. Lett. 49, 46 (1986); 10.1063/1.97079 Flexible A15 superconducting tape via the amorphous state J. Appl. Phys. 57, 4672 (1985); 10.1063/1.335326 Hightemperature superconducting tape via the amorphous state Appl. Phys. Lett. 33, 262 (1978); 10.1063/1.90326 Chemical vapor deposition of Nb3Ge on continuous stainlesssteel tapes Appl. Phys. Lett. 33, 105 (1978); 10.1063/1.90171 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 138.251.14.35 On: Mon, 22 Dec 2014 13:34:48Continuous AI5 Nb3Ge superconducting tapes via the amorphous state Tariq Manzur Metallurgy Depanment, University of Connecticut, Storrs, Connecticut 06268 Mireiile Treuii Clapp Department of Mechanical Engineering, Universiiy oJIVlassachusetts, Amherst, Massachuseits 01003 (Received 6 October 1988; accepted for publication 22 November 1988) Alloys of Nb75Ge24.5 Bo.s were rapidly solidified into amorphous ribbons using the melt spinning technique. After annealing continuous A15 tapes were obtained with an average grain size of 30 nm. It was not possible to form a continuous tape by melt spinning directly into the Al5 structure since only brittle fragments were obtained. The amorphous annealed A15 phase was close to stoichiometry with a lattice parameter of 0.514 nm and a superconducting transition temperature of 18 K. The critical current densities were 5 X lO\() and 8 X 10M A/m2 at magnetic fields of a and 15 T, respectively. This processing technique could be the initial step in the fabrication of multifilamentary Nb3Ge superconducting composites. I. INTRODUCTION Large-scale applications of superconductivity require the use of multifilamentary composites consisting of thin superconducting filaments embedded in a Cu matrix. A15's have the highest critical current densities at high magnetic fields, but since they are extremely brittle, unusual tech niques have had to be devised for their fabrication, such as the bronze route for Nb3Sn, and in situ and powder metal lurgy. Stoichiometric Nb3Ge is one of the AlS's with the highest superconducting critical parameters, but it is diffi cult to synthesize. Sputtering1,2 and chemical vapor depo sition} have been successful in forming thin films. If this material is to be used for large-scale applications, a method must be found for forming continuous filaments of A15 Nb3Ge near stoichiometry. A novel processing technique has been discovered recently4-6 that has been successful in forming thin Al5 tapes with improved superconducting and mechanical properties, The basic approach was to melt spin the desired Al5 compound into an amorphous ribbon and then to anneal to form an ultrafine-grained A15 tape. This was very successful in the processing of Nb3(AlSiB) alloys, In this study the same processing technique has been applied to Nb3Ge, It EXPERIMENT Samples of Nb3Ge1 _ x Ex were prepared from high-pu rity elemental powders: Nb 99.8, Ge 99.99, and B 99.9 wt. % metallic purity. The powders were weighed out stoichiome trically on a microbalance; the total weights of the samples were between 1.5 and 2 g. The samples were compacted un der high pressure (100 MPa) with a hydraulic press to form pellets. They were then arc melted for 10 s in a water-cooled copper hearth with a titanium getter under an argon atmo sphere ( -17 kPa) using a nonconsumable tungsten elec trode. To ensure homogeneity, the samples were turned over and remelted for the same amount of time. A Nb wire 1.0 mm diam was attached to one end of the pellets during melt ing, for reasons to be described shortly. Different composi. tions of Nb and Ge were chosen ranging from Nb73Ge27 to Nb79Ge21• Boron (2-0.5 at. %) was added to replace Ge and promote glass formation. A. Melt spinning The melt-spinning machine consisted of six essential components (Fig, 1): (1) a vacuum chamber with a gas inlet allowing the quenching to take place in a controlled atmo sphere; (2) a three-way solenoid valve; (3) an induction coil for melting the sample; (4) a sample holder to suspend the sample inside the quartz crucible and to adjust the position of the crucible inside the chamber; (5) an infrared detector to measure the relative temperature of the sample and to activate the solenoid valve for ejection of the melt through the orifice ofthe crucible; and (6) a solid 17 -cm -diam copper disk capable of rotating at over 10 000 rpm. A ferrofiuidic coupling was used for sealing between the wheel shaft and the chamber, and for minimizing vibration at high wheel velocities. The chamber argon pressure was adjusted and kept between 34 and 68 kPa. The ejection pressure was set between 7 to 35 kPa. After melting, a stream of molten alloy was ejected from the orifice of the crucible onto the surface of the wheel rotating with a tangential velocity up to 75 m/s. The velocity of the ejecting melt depended on the difference between ejection and chamber pressure, An individual regu lator valve was set and monitored in both the pressure and 2 2 0) heat up 2 -Ci-. _-1._ .. b) eject;on FIG. 1. Melt-spinning apparatus consisting of (1) vacuum chamber, (2) solenoid valve, (3) induction heating coil, (4) sample and crucible holder, (5) temperature sensor, and (6) ell wheel. 2384 J. Appl. Phys, 65 (6), 15 March 1989 0021-8979/89/062354-05$02.40 © 1989 American Institute of Physics 2364 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 138.251.14.35 On: Mon, 22 Dec 2014 13:34:48vacuum gauges with an accuracy of 3 kPa. During heat-up the sample out-gassed and a pressure built up in the crucible. Since there was no outlet for this entrapped gas, premature ejection of the melt could occur. To prevent this unregulated ejection of the melt and also to control the exact melt jet velocity a three-way solenoid valve was used (Fig. 1). The valve kept the pressure in the crucible and the chamber equal during heat-up (a); this prevented pressure buildup in the crucible and premature ejection of the melt. At ejection (b), the three-way valve was activated, isolating the vacuum chamber from the crucible and pressurizing the crucible for ejecting the melt. This solenoid valve was activated by a tem perature control detector. A LepeI 25-kW rf generator was used to induction heat the Nb-based alloys. The A-gauge o.d. copper tubing was used to make the induction coil which had five turns and a 22 mm o.d. When the sample approached its melting temperature (~2000 CC), radiation heating of the quartz crucible began to occur. This resulted in two side effects, one bad and one good. If the sample was too close to the crucible side wall it could melt the crucible. On the other hand, the tip of the crucible was heated, which made it easier for the molten alloy to flow through the orifice without so lidifying. It was found that a 16-mm-i.d. crucible worked best to avoid the side-wall melting and to provide optimum heating of the tip. To control the temperature of the crucible tip, the position of the sample inside the quartz crucible and the induction heating coil and also the time of each heating step was accurately recorded. The sample was suspended vertically by means of the attached Nb wire and was con tained within a quartz crucible with an orifice 0.8-0.4 mm in diameter. The sample was suspended towards the top of the induction coils, thus ensuring that the wire did not melt be fore the sample. Since there was no contact between the sam ple and the crucible, the sample remained free of contamina ti.on from the crucible. The sample suspension device was capable of moving horizontally and vertically, and had a rotation in the y-z plane to adjust the melt jet inclination angle and the distance from the crucible tip to the wheel surface. Temperature measurement for purposes of monitor ing and control was done by an infrared detector, which measured the relative temperature of the melt. The output voltage of the detector corresponded to the relative melting temperature. When a preset voltage was reached, the detec tor activated the three-way solenoid valve and the molten sample dropped due to the applied ejection pressure. By in troducing this system it was possible to keep the melt super heat under control and the reproducibility rate of good-qual ity ribbons was increased from 20% to 80%. B.Annealing The samples of melt-spun ribbons were cleaned in ace tone, wrapped in Nb foil, and placed inside a vacuum-sealed (1 flPa) quartz tubeo These were then annealed, at tempera tures between 650 and 730 ·C, in a temperature-controlled furnace for 12 to 24 h. C. Mechanical test The flexibility of the sample was determined by a simple bend test. The annealed ribbons were attached between the 2385 J. API'\. Phys., Vol. 65, No.6, 15 March 1989 two jaws of a micrometer with a very small amount of epoxy so that the sample remained fixed. The micrometer was slowly closed and when the sample fractured the distance between the two jaws was measured. It was assumed that the ribbon bent in a semicircle with a diameter equal to the dis tance between the two jaws. The bend strain to fracture was calculated from the equation t I(d -t}, where t is the sam ple thickness. D. Crystal structure An x-ray diffractometer and a Gandolfi camera with CuKa radiation were used to determine the phases present before and after annealing. The lattice parameter ao of the Al5 Nb3Ge phase was calculated by plotting Qo values for the individual diffraction planes as a function of cos2 e and ex trapolating to cos2 9 = O. Grain size was estimated from broadening of the x-ray peaks and the Scherrer formula. E. Superconducting properties A four-point probe resistive technique was used to mea sure the superconducting transition temperature. The tran sition was characterized by recording the temperatures at which the transition started (Tc onset) and at which it was half completed (Tc midpoint). Sample preparation for the critical current density mea surement by transport current is very critical. Values of Jc are limited by the heating problem due to contact resistance Rc between the sample and the current leads. To achieve a better contact between the sample and the current leads, samples -2 em long were gold or copper electroplated, with platings -20 flrn thick. The sample was then coated with a thin layer of indium, and placed in the sample holder on a thin copper strip which acted as a shunt and supported the sample mechanicaHy. The Cu strip was then soldered to the two thick current leads. Two thin Au voltage leads were soldered to the sample. The critical currents were measured at 4.2 K as a function of applied transverse magnetic field up to 15 T. The critical current was defined as the current at which a voltage of 2 fJ'v appeared across 1 cm of the super conductor at a specific value of a magnetic field. When the samples were plated prior to annealing, Rc was very low and was estimated from the slope of the V-I curves to be less than 10-8 n. For calculating current density, the cross-sectional areas of the samples were measured by means of an optical microscope with an accuracy of 1 f.lm. l!IoRESULTS The initial meltospinning tests on Nb75 ± x Gezs ± x al loys, where x = 0-3, resulted primarily in brittle fragments< However, for the Nb7SGe25 aHoy, some very ductile shiny pieces were formed. Although the quantity of ductile frag ments was small, it was a clear indication of the possibility of forming amorphous ribbons. To make a continuous ribbon boron had to be added to enhance the glass forming tenden cy; 0.5 at. % B was found to be optimum. A simple test of amorphousness was to see if the ribbons survived the 180· bend test without breaking. Optimization of the melt-spino ning parameters was essential, and to increase the reproduc- T. Manzur and M. T. Clapp 2385 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 138.251.14.35 On: Mon, 22 Dec 2014 13:34:48p. I .,. FIG. 2. Critical melt-spinning parameters: (a) position of the induction coil; (b) position of the sample; (c) distance between the tip of the crucible and the wheel surface; (P,) chamber pressure; (Pe) ejection pressure; and (a) ejection angle. ducibiHty rate it was necessary to accurately control the melt super heat temperature, the crucible shape and orifice diam eter, the position of the induction coil (a) and of the sample inside the coil (b), the distance between the tip of the cruci ble and the wheel surface (c), the chamber Pc and ejection pressure P,", and the ejection angle (a) (Fig. 2), Increased cooling rates were possible by reducing the orifice diameter and the ejection pressure, i.e., by controlling the melt flow through the orifice. Increasing the orifice diameter from 0.36 to 0.52 mm with an ejection pressure of 14 kPa in creased the thickness from 10 to 20 {lm. Thickness is inverse ly proportional to cooling rate. The thinner ribbon was com pletely amorphous whereas the thicker one was a combination of amorphous plus crystalline. Quartz was used as the crucible material and different nozzle geometries were tried. For longer nose crucibles, there were problems of solidification of the molten alloy inside the crucible. For fiat crucibles, the melt jet was not stable. To have continuous amorphous ribbons, short-nose crucibles gave the best re sults with orifice diameters of 0.36-0,52 mm. Ribbon thick ness varied inversely with substrate speed. As the wheel speed increased from 4000 to 7000 rpm (37 to 65 m/s) the thickness of the ribbons decreased from 50 to 10 pm and the cooling rates increased from ~ 2 X 104 to 106 "Cis. 7 The ver tical position of the sample within the crucible had to be controlled to ± 0.1 cm for good reproducibility. As the an gle of ejection of the melt (a) increased, the length of the melt puddle increased due to the larger tangential momen tum, and thus thicker ribbons were formed. At O· the ejected melt did not have enough surface area to spread on to, and it solidified in the gap. For a more than 10°, instability of the melt puddle occurred. In the NbGe alloys the optimum a was found to be g •. To obtain good-quality smooth-edged ribbons, the chamber pressure was optimized at -50 kPa. Before annealing, all the melt-spun ribbons that were suspected of being amorphous were x-ray analyzed by using 2386 J. Appl. Phys., Vol. 65, No.6, 15 March 1989 44 42 40 38 36 34 DIFFRACTION ANGLE ( 2 e } FIG. 3. Portions of x-ray diffraction patterns of (a) amorphous Nb75GC24.5 EO<5' (b) annealed at 700 OC for 24 h, (c) annealed at 730 'C for 24h. the x-ray diffractometer or the Gandolfi camera, and no evi dence of crystallinity was found. The ribbons were then an nealed. The x-ray diffraction patterns of amorphous Nb3Ge annealed at different temperatures and times are shown in Fig. 3. The intensity is plotted as a function of 2e, the dif fracted angle. Figure 3(a) shows the melt-spun completely amorphous structure. After annealing at 675·C for 12 h there was no crystallization. Figure 3 (b) shows that after annealing at 700 °C for 24 h, partial crystallization occurred. The x-ray peaks were broad and low, and the samples were amorphous + A15. Figure 3(c) shows that after annealing at 730·C for 24 h the A15 peaks intensified. From the Gan dolfi camera films, it appeared that small amounts of second phases began to precipitate out at this temperature, The sec ond phases were identified as tetragonal and hexagonal NbsGe3. After annealing at 730°C the lattice parameter of the A15 was 0.514 nm and the average grain size was -30 nm. The amorphous ribbons could be bent 180· without breaking. After annealing at 730 ·C the average bend strain to fracture was -1 %, Material that was melt spun directly into the A15 structure was extremely brittle, very fragment ed, and had a bend strain to fracture ofless than 0.1 % No superconducting transition above 4.2 K was ob served for the samples annealed at 675 ·C for 12 h. As an nealing temperatures and times increased from 700 to 730 ·C and 12 to 24 h, Tc onset increased from 9 to 18 K and To midpoint from 10 to 16 K. The transiti.on increased with annealing temperature and became sharper, no doubt due to further crystallization of the A15 phase. It should be noted that not all samples annealed at 700°C showed transitions, indicating that there was a fair variety in crystallization be havior from sample to sample. A plot of critical current density as a function of applied T. Manzur and M. T. Clapp 2386 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 138.251.14.35 On: Mon, 22 Dec 2014 13:34:48;p- f- U5 Z W Cl I-~ ZN wE 0::, 11::« a~ ..J « 2 f iE u APPliED MAGNETIC FIELD (Teslo) FIG. 4, Critical current density as a function of applied magnetic field for amorphous Nb75Ge245 B05 annealed at 730 'C for 24 h. transverse magnetic field can be seen in Fig. 4 for a sample eu plated before annealing at 730 ·e, 24 h. At 0 T, Je was 5 X 1010 A/m2 and at 15.2 T, Jc was 8 X lOR A/m2• Although theJc 's of most samples were consistently high at 0 T, Jc 's at 15 T varied by an order ofmagnit1.lde. This is further indica tion that the crystallization and microstructure varied from sample to sample for a given annealing condition. IV. DISCUSSION Amorphous ribbons of Nb7sGe24.5 Bo.s were made by optimizing and accurately controlling the melt-spinning pa rameters. According to the NbGe phase diagram, the eutec tic trough occurs near the stoichiometric composition Nb3Ge and eutectics promote the glass forming tendency because the melting temperature is lower. However, it was found that the addition of 0.5 at. % B was necessary to form an amorphous ribbon. This is consistent with the empirical rule for glass forming tendencies in ternary alloys based on the relative size of the atomic constituents8; the rule (where ra, rh, and Yc are the radii ofr-.;'b, Ge, and B, Cb is the concentration of Ge, and C ~ill is the minimum concentra tion of B required to fonn a glass) predicts C~in to be 0.5 at. % B. Annealing produced continuous tapes of A15 Nb3Ge. According to the NbGe phase diagram at 25 at. % Ge, the stable phases are Al5 Nbs1Ge19 and tetragonal or hexagonal NbsGe3' Stoichiometric Nb3Ge is highly metastable. The lattice parameter of the AI5 decreases from 0.520 to 0.513 nm as the Ge content increases from 19 to 25 at. %.9 In this study the lattice parameter was 0.514 nrn corresponding to 24 at. % Ge. Hence the initial A15 that crystallized out of the amorphous matrix was the highly metastable "stoichiome tric" phase. This polymorpholls transformation required no long-range diffusion. As the annealing times and tempera tures increased a sman amount of the NhsGe) phase formed. It would be very interesting and informative to do a detailed microstructural analysis of the crystallization process dur ing the entire transformation. It was not possible to form a continuous tape by melt 2387 J. Appl. Phys., Vol. 65, No, 6, 15 March 1989 spinning directly into the Al.5 structure, since only brittle fragments were obtained, The bend strain to fracture of the A 15 annealed from the amorphous state was a factor of 10 higher. eu plating of the ductile amorphous ribbons prior to annealing not only decreased the contact resistance in Jc measurements but further enhanced the flexibility of the Al5 tape. This processing technique could therefore be the initial step in the fabrication. of multi filamentary ~b3Ge supercon ducting composites. The highest Tc measured was 18 K T" of A15 Nb3Ge increases rapidly as the Ge content approaches stoichiome trylO and is strongly correlated to lattice parameter, increas ing from 8 to 23 K as ao decreases from 0.518 to 0.513 nm. For our samples ao was 0.514, which corresponds to a Tc of 19 K. This is consistent with our observed value and is further proof that our samples were close to stoichiometry. The B which is a small interstitial atom no doubt had a role in expanding the lattice and decreasing Tc. The addition of 0.5 at. % B has been shown to decrease Tc to 19 K.lI The values of critical current densities as a function of magnetic field were consistent with those of other research ers,3,12-1n who have observed that flux pinning in Nb3Ge alloys is due to grain boundaries and second-phase precipi tates. It has been proposed that to have the highest Jc 's an optimum amount of the NbsGe3 0' phase is required. For example, Thompson et ai, 14 reported Je values at 18 T and found that as the amount of Nb5Ge3 phase in.creased the critical current density initially increased, went through a peak, and then decreased. Braginski and co-workers 12 re ported that for single-phase Al5 the critical current density was very low and that the optimum concentration of NbsGe3 waS -5 vol % for a Je of ~ 1010 A/m2 at 6 T. Jc is also a function of precipitate configuration and should increase as the dispersion becomes finer. It should be possible with our processing technique to accurately control the amount and distribution of the a phase. Furthermore, our samples exhib ited a very high Je at 0 T followed by a sharp drop at very low magnetic fields which is thought to be due to incomplete crystallization of the amorphous phase. For these reasons a detailed microstructural study of the crystallization process is required, after which we believe it may well be possible to further increase J" of our samples. ACKNOWLEDGMENTS The authors gratefully acknowledge the Francis Bitter National Magnet Laboratory for providing the facilities for measuring the critical current densities and magnet fields. They are very grateful to L. Rubin for his advice and help throughout. This work was sponsored by the National Science Foundation under Grant No. MSM-8610814. 'I. R. Gavaler, M. A. Jancko, A. I. Braginski, and O. W. Roland, IEEE Trans. Magn. MAG·ll, 192 (1977). 2J. R. Gavaler, M. A. Janocko, and Co K. Jones, J. App!. Phys. 45, 3009 (1974). 'A. I. Braginski, I. R, Gavaler, G. W. Roland, M. R. Daniel. M. A. Jan ocko, and A. T. Sanathanam, IEEE Trans. Magn. MAG·13, 300 (1977). 4M. T. Clapp and D. Shi, J. App!. Phys. 57,4672 (1985). 'M. T. Clapp and D. Shi, Adv. Cryog. Eng. Mater. 32,1067 (1986), OM. T. Clapp and D. Shi, App!. Phys. Lett. 49, 1305 (1986). T. Manzur and M. T. Clapp 2387 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 138.251.14.35 On: Mon, 22 Dec 2014 13:34:487H. Hillmann and H. R. Hi\zinger, in Rapidly Quenched Metals Ill, Third International Conference, edited by B. Cantor (University of Sussex, Brighton, 1978), Vol. 1. "E. J. Kabel, Jr., in Metal Progress [Research Institute of Mineral Dressing and Metallurgy (SENKEN) Sendai, Japan], p. 61, May 1986. 9B, Letellier and J. C. Renard, IEEE Trans. Magn. MAG.IS, 498 (1979). lOR. F!ukiger, in Superconductor MaTerials Science, edited by S. Foner and B. Schwartz (Plenum, New York, 1(81), p. 576. "J. D. Thompson, M. P. Maley, L. R. Newkirk, F. A. Valencia, and K. C. Kim, Physica l07B, 267 (1981). 2388 J. Appl. Phys., Vol. 65, No.6, 15 March 1989 [2 A.!, Braginski, G. W. Roland, and A. T. Santhanam, IEEE Trans. Magn. MAG-iS, 505 (1979). DR. T. Kamwirth, IEEE Trans. Magn. MAG-IS, 502 (1979). '4J. D. Thompson, M. P. Maley, L. R. Newkirk, and R. V. Carlson, IEEE Trans. Magn. MAG· is, 516 (1979). ISS. A. Alterovitz, J. A. Woollam, J. J. Engelhardt, and G. W. Webb, IEEE Trans. Magn. MAG.IS, 512 (1979). "J. D. Thompson, M. P. Maley, L. R. Newkirk, F. A. Valencia, R. J. Bart· lett, and R. V. Carlson, Solid State Commun. 28, 729 (1978). T. Manzur and M. T. Clapp 2386 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 138.251.14.35 On: Mon, 22 Dec 2014 13:34:48

1.343970.pdf

Absorption peaks at 2663 and 2692 cm− 1 observed in neutrontransmutationdoped silicon Lei Zhong, Zhanguo Wang, Shouke Wan, and Lanying Lin Citation: Journal of Applied Physics 66, 4275 (1989); doi: 10.1063/1.343970 View online: http://dx.doi.org/10.1063/1.343970 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Athermal annealing of neutron-transmutation-doped silicon AIP Conf. Proc. 429, 981 (1998); 10.1063/1.55511 Infrared absorption study of neutrontransmutationdoped germanium J. Appl. Phys. 64, 6775 (1988); 10.1063/1.342011 The nature of a 2050–2150cm− 1 infrared band in neutrontransmutationdoped silicon grown by the floatingzone method in a hydrogen atmosphere J. Appl. Phys. 63, 5606 (1988); 10.1063/1.340342 Spatially resolved lifetime measurements in neutrontransmutationdoped polycrystalline silicon J. Appl. Phys. 60, 1681 (1986); 10.1063/1.337258 Annealing characteristics of neutrontransmutationdoped germanium J. Appl. Phys. 55, 1437 (1984); 10.1063/1.333397 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.174.21.5 On: Thu, 18 Dec 2014 08:24:49Absorption peaks at 2663 and 2692 em -1 observed in neutron .. transmutation ... doped silicon Lei Zhong, Zhanguo Wang. Shouke Wan, and Lanying Lin Institute a/Semiconductors, Chinese Academy a/Sciences, Beijing, People's Republic a/China (Received 13 March 1989; accepted for publication 26 July 1989) Two absorption peaks at 2663 and 2692 cm-I are reported which were observed by Fourier transform infrared at a temperature below 77 K in aU fast-neutron-irradiated samples investigated. These peaks are very weak and obscured by the nearby divacancy 3.61-,um band in most cases. However, they are obviously enhanced by the presence of impurity hydrogen. They anneal out at about 200 'C. It is proposed that a single defect center, whi.ch may be the di-interstitial, gives rise to the two peaks. iNTRODUCTION Many radiation-induced infrared absorption bands have been observed in silicon for the past three decadeso 1-9 In Ref. 8, as many as 114 bands known to exist in silicon irra diated by neutrons, hydrogen ions, and electrons of energy > 2 MeV were listed. These bands could be produced in one of two ways: First, the pairing of an impurity atom with vacancies and/or interstitials or a high degree of lattice dis order; in these cases one observes predominantly the vibra tional modes ofthe defect in the infrared wavelength region. Second, defects that give rise to transitions which are purely electronic in nature and whose energies are in the infrared region of the spectrum. Of these bands, the well-known 3.3- /-tm band has been unambiguously identified with the diva caney. 1-7 The 3.3-,um band in fact consists of a broadband with two distinguishable sharp subbands at 3.45 and 3061 ,um.2-7 However the 3.45-and 3.61-pm sharp bands were observed only at low temperature ( < 150 K) and only if the Fermi level position is above Ee -0.21 eV.1-7 In addition, while the knowledge of electronic levels and atomic configu rations for the divacancy is quite complete, the mechanism of divacancy formation is still under discussion.9 On the other hand, recent demonstrations of the ability of atomic hydrogen to bond to and passivate electrically ac tive deep-level centers such as transition-metal impurities, lattice defects, and even group-III acceptors in silicon have created considerable interest. 10-13 Thanks to the pioneering work of Stein, 14 a large body of absorption bands associated with hydrogen in silicon has been brought to light roughly spread over the spectral range from 1600 to 2500 em -1. These absorption bands arise from the local-mode vibration of the hydrogen-silicon bond. 14.15 A number of calculations have been devoted to the determination of the position of hydrogen atoms in the silicon crystal. 16-18 All calculations show that hydrogen atom does interact and form a complex with the vacancy, although there are still arguments about its exact position in the crystal. In this work, for the first time two new absorption peaks are reported, which were seen in aU our silicon samples irradiated with fast neutrons. In most cases, the two new peaks are very weak when compared with the nearby divacancy 3.61-llm band and are obscured by this bando However, we found that the new peaks are obviously enhanced by the presence of hydrogen impurity. EXPERIMENTS AND RESULTS Crystals used in this work are described in Table I. The concentration of oxygen and carbon were estimated from the absorption bands at 9 and 16 pm measured at liquid-nitro gen temperature, No crystals used here had received heat treatment before irradiation. The crystals were irradiated at a temperature of about 40°C with a total fast-neutron flux of 4.0X 1018 n/cm2• After irradiation, an extensive isochronal annealing study was done in the range from room tempera ture to 400 "C. Samples were optically polished on both sides with thickness 2.00 ± 0.01 mm. The infrared absorption of the samples was studied with a Nicolet 170SX Fourier trans form spectrometero Detailed measurements of temperature dependence of the absorption bands were performed from 10 K to room temperature with an Air Products heIitran dewar. We display part of the IR spectra in Fig. 1 for sample B irradiated with fast neutrons which were recorded at 10 K and liquid-nitrogen temperature. A broadband at 3022 em-1 (33 pm) together with two sharp bands at 2891 em-1 (3.45 pm) and 2766 em-I (3.61 ,um) was observed at both temperatures. The earlier reported 3.9-,um bandl.? was never observed in this work. It is noticeable that two new absorp tion peaks located at 2692 cm-l (3.715 !-lm) and 2663 cm-I (3.755 pm) have been discovered at 10 K, although the in tensity of both new peaks is much weaker than that of the nearby divacancy 3.61-,um band. The half-width of peak 2663 em-I (FWHM = 6 cm-I) and 2692 cm-~I (FWHM = 4 cm -1) are much narrower than that of the divacancy 3.61-,um band (18 cm-~l) or 3.45-,u.m band (54 cm -1 ). The two new absorption peaks completely disappear at liquid-nitrogen temperature. These two new peaks were present in all types of samples measured in this work which are listed in Table I. Their respective IR spectra are shown in Fig. 2 on an expanded scak Comparison of the spectrum of control sample A with that of sample D and E leads us to suggest that impurity oxygen or the presence of the A center (oxygen vacancy, with its absorption peak located at 829 cm ~-l) tends to anni hilate the defect center, giving rise to the peaks at 2663 and 2692 cm-I. The dependence upon the impurity oxygen of the new peaks is totally different from that of the 3.61-!-lm divacancy band. We have also observed in our work that the presence ofinterstitiaI oxygen and the A center enhances the 4275 J. Appl. Phys. 66 (9). 1 November 1989 0021-8979/89/214275-04$02.40 @ 1989 American Institute of Physics 4275 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.174.21.5 On: Thu, 18 Dec 2014 08:24:49TABLE I. Parameters of samples used. [0] Resistivity Sample Growth Ambient (1017 cm-3) [C] Type (Hem) A FZ Ar I" I P >1000 B FZ H G.lb I P 400 C FZ H 0.2 I N 250 D CZ Ar 16 I P 100 E CZ Ar II I P 12 a Below the detection limit. bEstimated with ASTM Fl2!-79, VoL 43. formation of the divacancy, which is consistent with the findings of Oehrlein et al. 9 The possible mechanism for the enhancement in the production ofthe divacancy by intersti tial oxygen suggested by Oehrlein et al.9 is that interstitial oxygen prevents interstitial-vacancy recombination by the capture of silicon self-interstitials and therefore increases the steady-state vacancy concentration for the formation of the divacancy via an agglomeration of two single vacancies. The presence of hydrogen in samples Band C was sug gested by the fact that both crystals had been grown in am bient hydrogen and was further proved by our IR data between 1600 and 2500 em -1, which revealed hydrogen vi brational peaks at 1838 em -I, 1986 cm -I, etc. Therefore, comparing the spectrum data of samples Band C with that of control sample A, as seen from Fig. 2, we arrive at the conclusion that the two new absorption peaks are obviously enhanced by the presence of impurity hydrogen. The absorp tion intensity of the crystal grown in ambient hydrogen is -" .. .. w U Z <:( a:l Il: )111 0 (/) II! <:( x2 3300 3100 . 2900 2700 WAVENUMBER (e",-l) FIG. L Infrared spectra of sample B after neutron bombardment with a dose of 4.0X 10'8 em--2, measured at 10 K (above) and liquid-nitrogen temperature (below). 4276 J. AppL Phys., Vol. 66, No.9, 1 November 1989 2710 2690 2670 2650 2630 FIG. 2. Infrared spectra at 10 K from samples A, B, C, D, and E after neutron bombardment with a dose of 4.0X lOtS cm·2• about eight times larger than that ofthe control sample. This result was surprising as it is generally considered that hydro gen should passivate lattice defects. We conclude, however, that impurity hydrogen does not participate directly in the defect center responsible for the new peaks at 2663 and 2692 em -1 since the two new peaks do appear in samples contain ing no hydrogen such as control sample A and samples D andE. One-hour isochronal-annealing experiments have been run on the new 2663-and 2692-cm _0 I peaks together with the 3.3-,um band. The results are shown in Fig. 3. The an- 100 X !IIi .. X Z 80 0 f- 0.. ex: 60 0 X (f) II! .. « IJJ 40 > I-« ...J ~ 20 X .. 0 " » 0 100 200 300 400 TEMPERATURE ("C) FIG. 30 The plot for sample B of relative absorption measured at 10 K vs isochronally annealing temperature. (III) peak 2663 cm-t and (X) diva caney 3. 61-ttm band. The annealing time interval is j h. Zhongetal. 4276 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.174.21.5 On: Thu, 18 Dec 2014 08:24:49nealing behavior of the two new peaks is about the same. Their recovery is similar to but faster than that of the 3.61- {-tm divacancy band. The amplitude of each ofthe new peaks starts to decrease sharply when the annealing temperature is about 150·e and completely anneals out at about 200 ·C, whereas the amplitude of the 3.61-,um divacancy band de creases more slowly and disappears at a temperature of about 300 "e, which is in agreement with the results reported in Ref. 3. Detailed measurements of the temperature dependence of the intensity have been made for the two new peaks in the region from 10 K to room temperature. A line-shape change or frequency shift of the peaks has not been observed. Such changes frequently occur in the vibrational modes of a defect in the infrared wavelength region due to the thermal occupa tion of closely spaced levels that comprise the vibrational ground state. The transition involved in the two new peaks are therefore temporarily supposed to be electronic in nature just like most irradiation-induced IR absorption bands and secondary bands (such as the wen-known high-order bands).8 The variation of absorption with measurement temperature is plotted i.n Fig. 4 for the peaks at 2663 and 2692 cm-i• The two new peaks could be observed only if the temperature was less than 70 K. In addition, the absorption of the two new peaks exhibited an exponential rather than a linear temperature dependence. The thermal deactivation energy was estimated to be about 34 me V for the 2663-cm-1 peak and 20 me V for 2692-cm -I peak. From their annealing TEMPERATURE (K l 10-1 7050 30 10 , II III I!II iii II III III If II I!II • • • • II • II Z • 0 III fi: 102 II • IX: 0 (j) II m ..:( • III • • 10-3 0.2 0.4 0.6 008 1.0 1.2 1/KT PYlllv-1, FIG, 4. The plot for sample B of absorption vs measurement temperature. (III) 2663-cm- I peak and (+) 2692-cm -1 peak. 4277 J. App!. Phys" Vol. 66, Noo 9, 1 November 19S9 behavior and impurity dependence discussed above, the two new peaks may be associated with same defect center. But the temperature dependence of the ratio ofthe relative inten sity of the 2663-cm- I peak to 2692-cm --1 peak rules out the possibility of a single ground-state to various excited-state transitions. The investigation of the nature of the transitions giving rise to these peaks is still in progress. DISCUSSIONS Now, we could give a possible explanation for the reason why the two absorption peaks have not been found by earlier workers. First, although the two new peaks do occur in all NTD-silicon samples, they are very weak in most cases and obscured by the nearby strong divacancy 3.61-,um band. In fact, it was in the samples containing hydrogen that we found for the first time the existence of the peaks at 2663 and 2692 cm -'. And second, most measurements were done by the earlier authors at temperatures above 70 K. Finally, we would like to discuss the possible identifica tion of the defect center responsible for the two new peakso The observation of the two peaks in widely different sample types argues that no impurity is involved in the defect center responsible for these absorptions. We also conclude that the new peaks at 2692 and 2663 cm--I cannot be associated with a divacancy, as evidenced by the difference in the annealing behavior and the different dependence upon the impurities hydrogen and oxygen for the 3.61-pm di.vacancy band and the two new peaks. The impurity dependence of the two peaks suggests that they may be attributed to a self-intersti Hal-related defect. If that is so, the enhancement of the new peaks by hydrogen may result from the capture of vacancies by hydrogen,'6-18 which prevents the recombination of in terstitials and vacancies, and therefore increases the concen tration of interstitials. On the contrary, interstitial oxygen captures interstitial atoms9 and decreases the intensity ofthe two peaks. It is generaUy believed that interstitial silicon atoms are unstable and migrate until trapped at impurities even at tem~ perature as low as 4.2 K.19 Theoretical calculations have shown that the lowest energy configuration of an interstitia! is not found with an atom in the normal tetrahedral intersti tial site but rather in bonded interstitial configurations. 19 Lee, Gerasimenko, and Corbett20 and Brower2! have studied an EPR spectrum (Si-P 6) which has been suggested to in volve di-i~terstitia!s with < 100) dumbbell configurations . The P 6 center together with the four-vacancy P 3 center and the divacancy center dominate what can be seen immediate ly after irradiation.22 P 6 anneals at 170°C, 20 with a similar annealing behavior to that of our new absorption peaks, as can be seen from Fig. 3 . SUMMARY New absorption peaks located at 2663 and 2692 em-o ' are reported for silicon after fast-neutron irradiation. It is demonstrated that the presence of hydrogen obviously en hances the new absorption peaks. The annealing behavior and temperature dependence of the peaks have been studied. We tentatively propose that a single defect center, which may be the di-interstitial, gives rise to the two peaks. Zhong etal. 4277 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.174.21.5 On: Thu, 18 Dec 2014 08:24:49'H. Y. Fan and A. K. Ramdas, J. App!. Phys. 30,1127 (l959), 2L. J. Cheng, J. C. Corelli, J. W. Corbett, and G. D. Watkins, Phys. Rev. 152,761 (1966). 3C. S. Chen and J. C. Corelli, Phys. Rev. B 5, 1505 (1972). ·E. N. Lotkova, SOY. Phys. Solid State 6,1500 (1964). 5J. C. Corelli, G. Oehler, J. Becker, and J. Eisentraut, J. Apr!. Phys. 36, 787 (1965). 6c. S. Chen. J. C. Corelli, and G. D. Watkins, Bull. Am. Phys. Soc. 14, 395 (1969). 7L. J. Cheng and P. Vajda, Phys. Rev. 186, 816 (1969). "J. C. Corelli and J. W. Corbett, in Neutron Transmutation Doped Silicon, edited by J. Guldberg (Plenum, New York, 198i), p. 35. 9G. S. Oehrlein, J. L. Lindstrom, 1. Krafesik, A. E. Jaworowski, and J. W. Corbett, Physica 116B, 230 (983). 10M. Stavola, S. J. Pearton, J. Lopata, and W. C. Dautrement-Smith, Apr!. Phys. Lett. 50,1086 (1987). l1c. T. Sah, I. Y.-c. Sun, andJ. J.-T. hou, Appi. Phys.Lett.43, 204 (1983). !ZG. G. Deleo and W. B. Fowler, Phys. Rev. Lett. 56,402 (1986). 4278 J. App!. Phys., Vol. 66, No.9, 1 November 1989 13M. L. W. Thewalt, E. C. Lightowlers, and J. 1. Pankove, Appl. Phys. Lett. 46,689 (1985). !4H. J. Stein, J. Electron. Mater. 4,155 (1975). ISS. T. Picraux, F. L. Vook, and H. J. Stein, lust. Phys. Conf. Sec. 46, 31 ( 1979). 16J. M. Baranowski and J. Tatarkiwiez, Phys. Rev. B 35,7450 (1987). (7W. E. Pickett, Phys. Rev. B 23, 6603 (1981). ISG. G. Deleo, W. B. Fowler, and G. D. Watkins, Phys. Rev. B 29, 1819 (1984). '"G. D. Watkins, J. R. Troxell, and A. P. Chattetjee, Inst. Phys. Conf. Ser. 46, 16 (1979). 20y. H. Lee, N. N. Gerasimenko, and J. W. Corbett, Phys. Rev. B 14, 4506 (1976). 21K. L. Brower, Phys. Rev. B 14, 872 (1976). 22J. M. Meese, M. Chandrasekher, D. L. Cowen, S. L. Chang, H. Yousif, H. R. Chandrasekhar, and P. McGrail, in Neutron Transmutation Doped Si· licon, edited by J. Guldberg (Plenum, New York, 1981), p. 101. Zhong eta/. 4278 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.174.21.5 On: Thu, 18 Dec 2014 08:24:49

1.344004.pdf

ntype (Pb)Te doping of GaAs and Al x Ga1−x Sb grown by molecularbeam epitaxy S. M. Newstead, T. M. Kerr, and C. E. C. Wood Citation: Journal of Applied Physics 66, 4184 (1989); doi: 10.1063/1.344004 View online: http://dx.doi.org/10.1063/1.344004 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Defects in GaAs grown by molecular-beam epitaxy at low temperatures: stoichiometry, doping, and deactivation of n-type conductivity J. Appl. Phys. 86, 1888 (1999); 10.1063/1.370984 Sidoped and undoped Ga1−x In x Sb grown by molecularbeam epitaxy on GaAs substrates J. Appl. Phys. 80, 6556 (1996); 10.1063/1.363678 Photoluminescence study of Al doping in GaAs grown by molecularbeam epitaxy J. Appl. Phys. 80, 5932 (1996); 10.1063/1.363588 Optical characterization of Sidoped InAs1−x Sb x grown on GaAs and GaAscoated Si by molecularbeam epitaxy J. Appl. Phys. 69, 2536 (1991); 10.1063/1.348694 Trap suppression by isoelectronic In or Sb doping in Sidoped nGaAs grown by molecularbeam epitaxy J. Appl. Phys. 64, 3497 (1988); 10.1063/1.341486 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.189.170.231 On: Fri, 19 Dec 2014 10:42:34n .. type (Pb)Te doping of GaAs and AlxGa1_XSb grown by molecular .. beam epitaxy s. M. N ewstead ,a) T. M. Kerr, and C. E. C. Wood b) GEC Hirst Research Center, East Lane, Wembley, Middlesex HA9 7PP, United Kingdom (Received 29 June 1988; accepted for publication 10 July 1989) A PbTe flux has been used for n-type (Te) doping of GaAs, GaSb, and AIGaSb. The effects of surface accumulation and Te desorption were noticeable in secondary-ion mass spectroscopy profiles of GaAs layers grown at temperatures in excess of 54O·C. Te accumulation was not apparent in GaSb layers grown at temperatures up to 630 ·C, but Te desorption occurred from GaSb at temperatures above 540"C. The donor ionization energy of Te in At Gal _ x Sb is 44 meV for 0.4 <X < 0.5, i.e., significantly lower than the ionization energies of S or Se in similar material. iNTRODUCTION The alloys of GaSh with AI, Ga, and As offer consider able potential for use in long-wavelength optoelectronic de vices. 1,2 Unfortunately, the conventional GaAs molecuiar~ beam epitaxy (MEE) donors Si and Sn are essentially amphoteric in GaSb and AIGaSb,3,4 and so cannot be used to produce p-n junctions or to dope laser cladding layers. The group-VI chalcogens S, Se, and Te are reasonably effective as MBE donors in GaSb,4-7 with carrier concentrations of up to 5X 1017 cm--3 (Se doping)6 and 2.5X 1018 cm-3 (Te dop ing) 7 having been reported. Producing n-type AIGaSb by MBE is more difficult, with heavy S doping yielding highly compensated n~ of p-type materiaL 8 However, it has been reported that Te is a viable MBE donor for AISb (Ref, 7) and AIGaSb, \,2,7 although only limited information is avail able on the incorporation behavior of Te or the electrical properties of the doped layers. This paper reports preliminary results on the use of Te as a donor in GaSb and AlGaSb. Te-doped GaAs was also grown. Dopant volatility problems were avoided by the use of a PbTe "captive doping source.,,9 The effect of growth temperature on the incorporation of Te from PbTe was in vestigated by using secondary ion mass spectroscopy (SIMS) to profile abrupt changes in the doping level in GaAs and GaSh. The Hall properties of several GaSh and AIGaSb epilayers are also presented. EXPERIMENT GaSh and AIGaSb epilayers were deposited on semi insulating undoped GaAs substrates (for Hall measure~ ments) or n+ Te-doped GaSh substrates [for capacitance~ voltage (CV) analysis] in a VG V80H MBE system. The use ofindium bonding allowed simultaneous deposition on both types of substrate when required, The GaAs substrates were precleaned in 7H2S04:1HzOz:IH20, and the GaSb sub strates in a 1 % solution ofbromille in methanol. On heating a) Present address: Department of Physics, University of Warwick, Coven try CV 4 7 AL, United Kingdom. b) Present address: North East Semiconductors, Inc" 134 Lexington Drive, Itacha, New York 14850, to 580·C under an Sb flux of 1 X 1015 to 1 X 1016 atoms cm -2 S -\ a diffuse (2 X 4) reconstruction was ob tained from the GaAs substrates and bulk streaks or, less commonly, a (3 Xl) reconstruction from the GaSb sub~ strates. Growth was initiated under Sb4:Ga flux ratios of between 3:1 and 5:1, these being close to minimum permit ting stoichiometric growth at the temperatures of interest. The growth rates used centered on 1 jimlh. A clear (3 Xl) reconstruction was usually obtained within the first 300 A of GaSh overgrowth on both GaAs and GaSb substrates, PbTe FLUX CALIBRATION The interpretation of Hall data for n~type GaSb and GaAISb is complicated by the effects of two valley conduc tion,7,1O.1l Therefore, the PbTe cell was calibrated by C-V profiling a doping staircase grown in a GaAs epilayer depos ited at 54O·C (negligible Te desorption occurs at this tem perature). In addition Hall measurements were made on uniformly doped GaAs epilayers grown under similar condi tions. The results of these calibrations are presented in Fig. 1. EFFECTS OF Ts ON Te INCORPORATION IN GaAs Te incorporation in GaAs was investigated by growing two O.5~,um~thick doping spikes (corresponding to doping r. cell temperature I·C) PIG, I. PbTe doping cell calibration in GaAs: (0): SIMS secondary-ion yields from GaAs epilayers grown at T, < 570 ·C; (e): c-V derived carrier concentrations; ( + ): SIMS secondary-ion yields in GaSh. 4184 J, Appl. Phys, 66 (9),1 November 1989 0021 -8979/89/214164-04$02.40 © 1989 American Institute of PhySics 4184 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.189.170.231 On: Fri, 19 Dec 2014 10:42:34levels of 1 X 1016 cm-·3 and 5 X 1017 em ·-3 at 540 'C) into an epilayer at each of three growth temperatures (570,600, and 630 ·C). These spikes were separated by O.25~llm-thick nominally undoped layers. The Te doped layers were co doped with Si to facilitate identification when profiling. Im mediately after growth the epilayer was cleaved into two pieces. One piece was reloaded into the MBE system and annealed at 680 ·C for 30 min to investigate the eirects of Te diffusion at elevated processing temperatures. SIMS profil ing failed to reveal any differences betwen the annealed and nonannealed pieces, indicating that the phenomena de scribed below may be attributed to incorporation effects. Figure 2 is a SIMS profile through the (nonannealed) GaAs epilayer. Note that identical Te secondary-ion yields were obtained in layers 5 and 13, indicating that any pertur bation of the data by SIMS depth effects was negligible to a depth of about 3.5 f-lm. However, a slight loss in depth reso lution may have occurred in the deeper layers, as indicated by the apparent diminution of the Si doping spikes in layers 3 and 1, and the inexact coincidence of the 8i and Te dOpng spikes at the interface between layers 4 and 5. Some asymmetrical smearing is apparent in the layers grown at Ts = 570°C (layers5-7,12,13). However, the effect of severe surface accumulation only become noticeable in the profiles of the layers grown at 6OO·C and above. Signifi- T Sub @C 570 600 510 i 630 T Te ·C ~IO 3131~ 0 0 0 0 6 F 369 F 313 F 369 F 313 F 369 9:F F F F F r Lay~r 13112 11 po 9 8 7 6 5 4 3 2 1 As 5i n r1 i ! , j Te o 50 100 150 200 250 Depth (Arb. uniul FIG. 2, SIMS profile through a 5-,um-thick GaAs epllayer doped with Te and Si steps. 4185 J. AppL Phys., Vol. 66, No.9, 1 November 1989 cant Te desorption is also apparent at 600 and 630°C. In creased Te desorption is probably responsible for the de crease in doping level on closing the PbTe shutter being more rapid at 630 than 6OO·C (Fig. 1), whereas incorporation! segregation modelsl2 predict the opposite behavior. The steady-state secondary-ion yields obtained (with extrapolation) from layers 1,5,9, and 13 are plotted in Fig. 3 as a function of reciprocal substrate temperature (curve d). Representative data from other studies into the desorption ofS (curve C),B Se (curve a),14 and Te (curve b)'5 from GaAs are included for comparison. Within experimental er ror, an activation energy of 70 kcallmol (3.0 e V) character izes the desorption of each of the three chalcogens. This be havior has been attributed 14 to the formation and immediate sublimation of Ga2S, Ga2Se, and Gaz Te at high growth tem peratures (the availability offree gallium, controlled by the sublimation of arsenic, being the dopant-indepenent rate~ limiting step). EFFECTS OF T" ON Te INCORPORATION IN GaSb Figure 4 is a SIMS profile through a GaSb epilayer doped from a constant PbTe flux at a series of substrate tem peratures in the range 540-627 ·C. O.25-Jlm-thick undoped layers were grown between the doping spikes. Identical steady-state secondary-ion counts were obtained from the doping spikes grown at Ts = 540 ·C at the beginning and end of the epilayer, indicating that SIMS depth effects did not significantly convolute the data. It is apparent that little if any accumulation of Te occurred on the GaSb surface at 540 < T, < 627°C. Loss of stoichiometry above 627°C made estimation of the activation energy of desorption difficult to quantify. However, Te loss became significant at 570·C and above. The effects of surface accumulation and Te desorp~ Hon were not noticeable in any GaSb epilayers grown at tem peratures below 550 "C. The increase in the Te secondary-ion yield in the Alo.ls GaO.85 Sb layer in Fig. 4 is possibly a SIMS artifact due to matrix effects. It is also possible that a higher incorpora tion efficiency may be achieved on AIGaSb than GaSb due to CQ:qft<eS c(!(tt!1nldl! 10'932_~_~ _W-3!Q ____ 6_~-,Z5 .6~O. -E1 !", .. , ,. ,. -//~ 'l:::, 1""1 g:~-. j"'~ l ""r /;/" 1'",j 10150";;--- ~_.--' ___ l-___ -----L....-. ____ J101 , to U 1.2 U 1.4 Re-ciprccal substrate hm;>eratur€ x ~O:s. FIG. 3. Arrhenius plot of the rate ofloss of (a) Se (Ref. 14), (h) Te (Ref. 15), (e) S (Ref. 13), and (d) Te (this work). (el shows the loss ofTefrom GaSb measured in this study. Newstead, Kerr, and Wood 4185 .•.• -•.••• -.-. -•.• ,., ••••••• s.~-.; •••••• ·.·.·.·.·.--.·;-.·.·.·.·.·.-.·.-. -•.• :.:.:.: •.••• ;, ••••••••••••••••• <;<; •••••••••••••• ~ ........ , ••••••• o:.:.~ •.• ;.;.;.; •.•.•.•••.••.•.•••••••••• ; •••• :.:.~.:.~.:.:.:.:.:.:.:.:.:.: ... "... •...• . ... ,., .•. , ••• "" T •• _T • -~ ',", .-.".-.-.-.-.- ••••••• ' ••••••••••••••••• : ••• :.~.:.:.:.:.:.:.:.:--:.:.;.;.; ••• ;.: ••••••••••••• ~.:.:.;.:.:.:.;.-='~.:.:;;:.:.:.-;O:.;.;.:-; •.• ; •.••••••••••••• ; •••••• :.:.:;:-••• ;.:.;.;-:-.;.:.;.;.;.; •. ' .• ; •.•.•...•••.• ;.... . .•....••••.•.• ;.-.;.:.;.;.; •.•.•.•.•.•.•.• ;0 ••••••••••• v;".T ••••••• _ .... ;-••••••• "._. > .-. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.189.170.231 On: Fri, 19 Dec 2014 10:42:34!05r---------------------------~I~Th~OO-Pe~d~G~Q7Snb substrate 54\ 627101101590 101510 1 01540 10FFtTe F F F F F F F f 102~--~---L----~--~--~----~--~--~ o 10 20 30 40 50 60 70 80 Depth (Arb. units! FIG, 4. SIMS profile through a 5-ftm-thick GaSb epilayer doped with Te steps. a decrease in the influence of the Gaz Te desorption mecha nism previously described,14 Pb was sought in the GaSb and GaAs epilayers using SIMS, but was not unambiguously detected due to matrix interferences at mle = 204, 206, 207, and 208. ELECTRICAL PROPERTIES OF Te DOPED AIGaSb EPILAYERS Several uniformly doped GaSb and AlxGal _xSb epi layers were grown for electrical assessment (Table I, Fig, 5). All were deposited onto 300-A.-thick undoped GaSh buffer layers (unintentionally doped GaSb and AIGaSb grown in parallel experiments were p type with NA -N D 5 X 1016 cm-J), The Hall samples were contacted by alloying in Sn beads in a reducing atmosphere, as indium contacts became highly resistive at temperatures below 200 K. The layers included in Table I were all doped to nominal 2X 1018 cm-3, i.e., this would be the free-electron concen tration obtained in GaAs grown at 540°C under the same doping fiux and at the same growth rate. However, Hall carrier concentrations of2 X 10 17 cm .-:> or less were obtained from the AIGaSb epilayers, This can largely be attributed to the effects of two valley conduction resulting from the small energy separation of the r -L minima in the AI, Gal _ x Sb alloy systems.7,lO,ll Indeed, Poole has shown that a single valley Hall analysis can underestimate the true free carrier TABLE I. 300-K Hall properties of MBE-grown Alx Gal _ xSb. Growth Thickness Material No, X (/Lm) GaSb MB1076 0.00 3.0 AlxGa, xSb MBI078 0.42 2.4 AlxGa, xSb MBlO80 0.42 3.7 AlxGa, xSb MB1082 0.48 5.0 AtGa'_x Sb MB1090 0.70 g,O 4186 J. Appl. Phys., Vol. 66, NO.9, i November 1989 ! ! ! It! I l ; 2_0 4.0 6.0 B.O 10.0 12.0 14!l 16.0 18.0 20.0 22.0 24.0 26.0 Reciprocal temperctllr~ )( 103 FIG. 5. Hall-derived electron concentrations of the AIGaSb epilayers de scribed in Table I as a function of reciprocal temperature. concentration by up to an order of magnitude in GaSb, in agreement with the results presented here. The exceptionally low carrier concentration obtained in the high Al content layer MB 1090 is consistent with close compensation or deep-level formation. Table J shows the Han mobilities of the epilayers to fall with an increasing Al content. In part, this behavior refiects the influence of increased aHoy scattering, higher effective masses, and different r-L valley distributions. Nonetheless, these mobilities are much lower than calculated lattice mo bilities,? indicating that optimization of the MBE growth conditions is required. The donor ionization energies derived from the near room-temperature freeze-out data for layers MB1078, MSW80, and MB1082 (Fig. 5) range from 35 to 53 meV. These ionization energies are lower than those of S in AIGaSb.8 The apparent increase in the free-carrier concen tration at temperatures below 150 K in MB 1078 is not incon sistent with the onset of hopping conduction which, al though usually observed at lower temperatures, is to be expected in a heavily compensated semiconductor having relatively deep impurities. Growth temperature N,300K 300-K mobility Symbol in (Ge) (ern -3X 1016) (cm2;V-1 S-') Fig.S 580 10 1950 610 11 193 X 560 15 212 + 560 20 130 • 560 0.3 59 D. Newstead, Kerr, and Wood 4186 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.189.170.231 On: Fri, 19 Dec 2014 10:42:34SUMMARY AND DISCUSSION The incorporation ofTe from PhTe in GaAs is severely affected by both surface accumulation and desorption at growth temperatures in excess of 570 ·C. Conversely, little if any surface accumulation occurs on GaSh up to a tempera ture of at least 627 ·C, although appreciable Te loss occurs from this compound at temperatures in excess of 540 ·Co The differing accumulation behavior ofTe on GaAs and GaSh is most probably related to atomic size effects. 16 The neutral atomic radiusl7 of Te (1.37 A.) is larger than those of Ga (1.25 .A) and As (1,21 A.), and strain effects will provide a driving force for Te segregation on GaAs. However, the neu tral atomic radii ofTe and Sb (1.41 A) are closely similar so that it is reasonable to assume that the Te atom can occupy a group V lattice site on GaSb without causing excessive strain, In practical terms, PbTe derived Te has been shown to be an effective MBE donor in GaSb and, even though the 77- K activation is limited, the shallowest donor thus far identi fied in Alx Gal .. "Sb, ACKNOWLEDGMENT We would like to thank R. Nichols for performing the Han measurements, 4187 J, AppL Phys,. Vol. 66. No.9, 1 November 1989 'w. T. Tsang and N, A. Olsson, App!. Phys, Lett. 43, 8 (1983), 2T. H. Chui, W. T, Tsang. J. A. Ditzenberger, and J. van dey Zeil, Appl. Phys. Lett 49,1051 (1986), 3C. A. Chang, R. Ludeke, L. L. Chang, and L, Esaki, AppL Phys. Lett. 31, 759 (1977). 4H. Gotoh, K. Sasamoto, S. Kuroda, T. Yamamoto, K. Tamamura, M. Fukushima, and M. Kimita, Jpn. J, Appl. Phys. 20, L893 (1981). 5M. Yano, y, Suzuki, T. Ishii, y, Matsushima, and M, Kimita, Jpn. J. App!. Phys. 17,2091 (1978). "T. D. McClean, T. M. Kerr. D. I. Westwood, C. E. Co Wood, and D. E J. Howell, J. Vae. Sci, Techno!. B 4.601 (1986). 7S. Subbanna. G. Tutie, and H. Kraemer, Presented at the 1987 Electronic Materials Conference, University of California Santa Barbara, June 24- 26. 81. Poole, M. Lee, K. Singer, J. Frost, T. M. Kerr, C. E. C. Wood, D. An drews, W. J. M. Rothwell,andG.J. Davies,I. AppLPhys.63, 396 (1988). 9C. E. C. Wood, Appl. Phys. Lett. 33, 770 (1978). BOA. Sagar, Phys. Rev. 117,93 (1960). 1II. B. Poole, Ph.D. thesis, University of Manchester. 1989. I2A. Rockett, T. J. Drummond, J. E. Greene, and II, MorkOf, J. Appl. Phys, 53, 7085 (1982), nO. A. Andrews, R. Heckingbottom, and G. J. Davies, J. Appl. Phys. 54, 4421 (1983). "G. J. Davies, D. A. Andrews, and R. Heckingbottom, J. Appl. Phys. 52, 724 (1981). lSD, M. Collins, Appl. Phys. Lett, 35, 67 09i9). 16G. Patel, R. A. A. Kubiak. S. M. Newstead, and P. Woodruff (unpub lished). 17The atomic radii are quoted from J. A. Dean, Ed., Lange's Handbook of Physical Chemistry, 13th ed. (McGraw-Hili, New York, 1985). Newstead, Kerr. and Wood 4187 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.189.170.231 On: Fri, 19 Dec 2014 10:42:34

1.576136.pdf

Adsorption of CO, O2, and H2O on GaAs(100): Photoreflectance studies E. G. Seebauer Citation: Journal of Vacuum Science & Technology A 7, 3279 (1989); doi: 10.1116/1.576136 View online: http://dx.doi.org/10.1116/1.576136 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/7/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Theoretical Study of the Nitrogen Adsorption on Si and GaAs (100) Surfaces AIP Conf. Proc. 893, 321 (2007); 10.1063/1.2729897 Adsorption/desorption kinetics of H2O on GaAs(100) measured by photoreflectance J. Chem. Phys. 99, 7190 (1993); 10.1063/1.465435 Photoreflectance study of surface photovoltage effects at (100)GaAs surfaces/interfaces Appl. Phys. Lett. 58, 260 (1991); 10.1063/1.104682 Oxidation and annealing of GaAs (100) studied by photoreflectance J. Appl. Phys. 66, 4963 (1989); 10.1063/1.343769 Adsorption of O2 and CO on cleaved GaAs(110) at low temperatures J. Vac. Sci. Technol. A 1, 679 (1983); 10.1116/1.571977 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:01Adsorption of CO, O2, and H20 on GaAs{1 00): Photoreflectance studies E. G. Seebauef"l> Laser and Atomic Physics Division. Sandia National Laboratories. Albuquerque. New Mexico 87185 (Received 11 May 1989; accepted 13June 1989) Adsorption of CO, O2, and H20 on semi-insulating GaAs( 1(0) has been examined with photoreflectance (PR). This work represents the first use of PR for quantitative adsorption measurements on semiconductors. In PR, the laser-induced change in surface reflectance is monitored as a function of wavelength. The resulting spectra are sensitive to changes in both the surface potential and the nature of surface states. Results at the EI (3.0 eV) and Eo (1.4 eV) transitions are complementary to each other. Sticking coefficients S were obtained from EI data for these gases, and S at low coverage was found to increase in the order CO.( O2 < H20. S decreases by at least four orders of magnitude for all the gases as saturation is approached. The results suggest that oxygen has two binding states that fill sequentially. Gas adsorption generally improves the communication between isolated surface states and the bulk. I. INTRODUCTION Photoreflectance (PR) is one of a class of modulation spec troscopies in which a semiconductor sample is periodically perturbed, and the resulting change in dielectric constant is detected by reflectance. 1.2 PR accomplishes the modulation with a chopped laser beam having hv>Eg, where Eg is the fundamental band gap energy.3 Photogenerated minority carriers migrate through the space-charge region (SCR) and recombine with charge in surface states. The resulting change in the built-in surface field changes the reflectance. To date, PR has been used primarily as a probe of bulk band structure, with emphasis most recently on the characteriza tion of superlattices. Scant attention has been paid to the fact that since PR is a con tactless technique, the interaction of a free semiconductor surface with gases (and with transparent liquids) may be investigated. That PR is sensitive to adsorption seems to have been re cognized only in early work in the late 1960s by Wang et 01.2, who briefly examined the effects of air, wet O2, and dry O2 on the PR spectra of CdS. Major changes in line shape were observed by changing the ambient gas, but little explanation was attempted, and the changes were not further quantified. However, semiconductor growth and processing by direct gas-surface reaction has become widespread in recent years. The associated surface chemistry is understood quite poorly, particularly for compound semiconductors such as GaAs. The presence in the substrate of two elements (Ga and As) having different chemical reactivities complicates matters considerably. It has become clear that the interactions between semicon ductor surfaces and many gases are often quite weak or in volve activated adsorption, so that relatively high ambient pressures (10-3 to 103 Torr) are sometimes needed to pro duce significant surface coverages. At these pressures, tradi tional electron or ion-based spectroscopies such as low-ener gy electron diffraction and Auger spectroscopy, which have been quite useful in studies of surface chemistry on metals, cannot be applied. Transfer of the adsorbate-covered surface from high pressure to ultrahigh vacuum for analysis risks desorption of the weakly bound species characteristic of gas-semiconductor systems. A few optical methods such as Ra man spectroscopy, infrared spectroscopy, and ellipsometry may be used successfully, but they focus only on a limited set of surface characteristics. More techniques are needed to provide a complete and coherent picture. In this paper, PR of CO, O2, and H20 adsorption on the Ga-rich (4 X 6) GaAs ( 1(0) surface is described. Results for exposure sequences at 200 K are presented for both the E I (3.0 eV) and Eo (1.4 eV) transitions. Sticking coefficient data are extracted from the EI data for all three adsorbates. II. EXPERIMENTAL The apparatus was similar to that used for clean surface studies on GaAs( 1(0)4 and is shown in Fig. 1. The main components were a small ultrahigh vacuum chamber in which the samples were mounted, and the various optics and detection electronics used for PR. The optics were arranged to direct tunable monochromatic light and a laser beam of uniform intensity onto the same spot on the sample surface. Ar+ loser beam I '" , , -[Chopper t , , : I Monochromator ~Lamp Monochromator I I r--- I I L _________ ...J FIG. 1. Schematic diagram of the apparatus for photorefiectance. 3279 J. Vac. Sci. Technol. A 7 (6), NovlDec 1989 0734-2101/89/063279-08$01.00 © 1989 American Vacuum Society 3279 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:013280 E. G. Seebauer: Adsorption of CO, O2, and H20 on GaAs(100) In particular, white light from a quartz-halogen lamp (for Eo spectra near 1.4 eV) or a xenon arc lamp (for EI spectra near 3.0 eV) passed through a ~-m monochrometer and a lens system onto the sample at a 45° angle of incidence. This monochromatic light was reflected from the sample at 90° to the incident beam through another lens system into an iden tical monochromator that scanned coincidently with the first. For Eo spectra, color glass filters were placed in the incident and reflected beams to remove second-order violet light that the source monochromator transmitted and to block stray laser radiation which sometimes interfered with long-wavelength measurements. The spot size on the sample was -0.16 X 8 mm for the Eo transition and 0.32 X 8 mm for EI• The signal was detected with a Si photodiode, amplified, and analyzed with a lock-in amplifier at the chopping fre quency of the laser (326 Hz). The output of the lock-in was divided by the amplified input signal using an analog voltage divider, so that the quantity t::.R / R was given directly; varia tions in source intensity and absolute reflectance canceled out. The spectra of t::.R / R vs A. were processed and stored by a computer. Typical resolutions in these experiments were 1.40 meV for Eo spectra and 1.64 meV for EI spectra. The modulating laser light was obtained from a chopped and expanded argon ion laser beam operating at 5145 A. Upon reaching the sample, the laser beam had an essentially uniform intensity of 25 mW /cm2 over the entire reflection spot. The laser was polarized parallel to [110]; the probe beam was unpolarized. The sample was suspended in a small turbomolecularly pumped ultrahigh vacuum chamber. The system consisted primarily of an optical cube with Pyrex windows and was equipped with an ionization gauge and with two capacitance manometers for pressure readings between 1 X 10-4 and 1000 Torr. Both the ionization gauge and the pump could be valved off from the main chamber, which had a total volume of -1.5 1. The sample was tightly mounted by clips onto a piece of tantalum or stainless-steel foil (0.051 mm), which was in turn spot welded to an electrical feed through and liquid-nitrogen cryostat. Heating was accomplished by pass ing electrical current through the mounting foil, while tem peratures as low as 90 K could be attained by using liquid nitrogen in the cryostat. Temperatures were monitored with a chromel,...alumel thermocouple spot welded to the mount ing foil. In the constant-temperature experiments described here, the sample temperature could be determined independently through analysis of the Eo PR spectra as discussed in Sec. III below. Precise band-gap energies Eg could be obtained this way. The temperature dependence of Eg for GaAs is well known.5 With Eg measurable to within ± 1.5 meV in these experiments, the temperature could be calculated to within ± 3 K. The temperatures obtained from the thermocouple and from the PR spectra agreed to within this precision at temperatures between 90 and -500 K. Between 500 and 600 K the PR spectra gave readings 5 to 7 K higher than the thermocouple. Above 600 K the PR spectra became unmea surably small, so that no calibration was possible. Since the temperatures derived from PR were considered most reli- J. Vac. Sci. Technol. A, Vol. 7, No.6, Nov/Dec 1989 3280 able, the difference between the two methods was extrapOlat ed to correct the thermocouple readings above 600 K. Experiments were performed on several GaAs ( 1(0) sam ples cut from undoped, (LEC) -grown wafers obtained from Cominco Electronic Materials. The wafers were n type with a resistivity of 8.4 X 107 n cm and room-temperatutre mobil ity of 6100 cm2 IV s. The calculated carrier density was 1.2x 107 carriers/cm3• The samples were rectangles -12 mm long X 5 mm wide by 0.5 mm thick, and were oriented 2° toward [110] . Before being mounted in the vacuum chamber, the samples were degreased with hot trichloroeth ylene and then rinsed with acetone and methanol. Subse quently, an acid etch was performed for 30 s in a 10:1:1 mixture ofH2S04:H202:H20 at -40 °c, followed by rinsing with running deionized water and drying with a stream of dry nitrogen. This procedure appears to minimize the forma tion of excessively thick oxide layers.6 No HCI etch was em ployed because Massies et al. have shown 7-9 that the above procedure leaves essentially no oxide on the surface; oxida tion takes place during subsequent handling in air. The sam ples were mounted and the chamber pumped down within 5 min after drying. After system bakeout, the samples were annealed in vacu um for 10 min at 845 K to remove any residual oxide. This treatment has been shown to reliably yield a Ga-rich surface with a (4 X 6) electron diffraction pattern,10-12 with a Ga coverage (J Ga of -O. 7 monolayers (ML). It should be noted that Auger spectroscopy and electron diffraction were not available in the present experiments, so that the nature of the GaAs surfaces could not be verified directly. The gases were research grade (02: 99.9995%, CO: 99.99%) and were further purified by passing them through a trap filled with zeolite at 100-120 K. The H20 used for adsorption experiments was deionized and then subjected to between five and eight freeze-pump-thaw cycles to remove dissolved gases. Exposure sequences were performed at 200 K by admit ting a fixed gas pressure into the system for specified periods of time. Illumination of the surface with the chopped laser beam during dosing had no detectable effect on subsequent PR spectra. During exposure, pressures were monitored with the capacitance manometers; the ion gauge was turned off. For the highest exposures used, the dosing pressure ap proached 300 Torr. PR spectra were taken after the residual gas had been pumped away. Successive spectra taken after gas shutoff and spectra taken during adsorption both showed that desorption subsequent to system pumpdown was negligible on a time scale of2-5 min for all adsorbates at the doses used in these experiments. However, for H20 above 3 X 106 L (1 L = 10-6 Torr s), tbe dosing pressure and the delay time between pumpdown and recording of spectra did influence the results. Isothermal desorption was indeed important in these cases, so that the data presented here are confined to exposures < 3 X 106 L. It must be emphasized that the system pumpdown was employed only to shorten the time required for the overall experiment, not to eliminate spurious effects associated with PR itself. Since -2 min were required to take a PR spec trum, excessively high pressure would cause the total dose to Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:013281 E. G. Seebauer: Adsorption of CO, O2, and H20 on GaAs(100) change substantially while a spectrum was being taken. Lower pressure would solve the problem but also require long waiting times for a given dose. At high exposures (P:::::300 Torr) where the coverage changed only slightly with increasing exposure, experiments performed with sys tem pumpdown and with the gas left running gave identical results for all adsorbates (except for H20 as noted above). It was found that adsorption as manifested in the PR spec tra was drastically altered when the ion gauge was on, even when the gauge was throttled and highly baffled from the main chamber. The hot filament appeared to speed adsorp tion, consistent with previous studies of oxygen on GaAs( 110).13 Preliminary studies of the equilibrium of H20 with GaAs ( 1(0) were also performed. PR spectra were taken in the presence of between 10-6 and 1 Torr of H20. The results showed that turning on the filament of the ion gauge in creased the equilibrium surface coverage 0 at a fixed pres sure and surface temperature. However, the effects were re versible; shutting the filament off would cause 0 to revert (after -10 min) to its unperturbed value. In all adsorption studies, the spectrum of the clean surface could always be recovered by flashing the sample to -840 K in vacuum for a few seconds. III. RESULTS All spectra for thejth transition were analyzed in terms of the third derivative functional form (TDFF) given by Aspnes' application of Franz-Keldysh theory to modulation spectral4: Il.R (E) = Re{c.i8j[E _ E. + IT.] -n} R J J J (1) where Cj is an amplitude factor, OJ is a phase factor, rj is a phenomenological broadening parameter, and Ej is the ener gy ofthejth transition. Cj is proportional to Vs for PR spec tra,4,14 although Cj also depends on other physical quantities (such as momentum matrix elements) that are not known precisely. Hence, only relative values of Vs may be extracted from PR spectra. The parameter n depends on the local elec tronic band structure and, following conventional theory, 14 was set equal to 3 for the MI transition at E1• The Eo transi tion at the fundamental band edge is known to be Mo, corre sponding to n = 5/2. However, it was found in these experi ments that setting n = 3 produced narrower structures that generally gave more satisfactory fits to the data. Aspnes and Rowe have developed a curve-fitting schemel5 to obtain Cj, OJ, rj, and Ej from experimental spectra. The scheme is essentially a three-point fit to the absolute maximum and minimum in a spectrum, together with the baseline at Il.R / R = O. This procedure was em ployed to obtain many of the parameters described in the next section. Typical spectra at 200 K are shown with theoretical fits for a clean GaAs( 100) surface in Figs. 2 and 3, respectively. J. Vac. Sci. Technol. A, Vol. 7, No.6, Nov/Dec 1989 -4 IxlO t.R If 0.5 -0.5 GaAs (100) EI T =200K --Experimental + Res. ---Theoretical Franz-Keldysh 3281 ----- FIG. 2. TypicalEt PR spectrum of clean GaAs( 1(0) at 200 K with theoreti cal line shape according to Eq. (1). For EI spectra, the three-point method yielded simulations that were notably asymmetric because of the shallowness of the negative lobes. Much better fits could be obtained by setting 01 ~ -90· and then using Eq. (1) to obtain the re maining parameters. This could be done easily because the energy of the transition EI depends on the position of the peak center, Cion the peak amplitude, and r 1 on the full width at half-maximum. Near 01 = -90·, the other param eters are largely independent of 01 so that small asymmetries in the line shape are of little consequence. The Eo line shape is strongly modified by an interference phenomenon between reflections from the surface, where ex citons are quenched by the electric field, and the interface between SCR and the bulk, where excitons appear. Often it is still adequate to model the experimental spectra phenom enologically with a single line shape given by Eq. (1). How ever, Co is then no longer easily related to Vs' EI spectra do not show interference because of a lack of substantial exci tonic effects. See Ref. 4 for a more detailed discussion of these phenomena. Another noteworthy effect was observed in these mea surements that involved the phase difference between the t.R R 2 GaAs(IOO) Eo T = 200K -Experimental - -Theoretical Franz-Keldysh Or-~----~------ -I --II--Res. -2~--~~----~~--~~--~~----~ 1.40 1.42 1.44 1.46 1.48 1.50 E(eV) FIG. 3. Typical Eo PR spectrum of clean GaAs( 1(0) at 200 K with theoreti cal line shape according to Eq. (I). Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:013282 E. G. Seebauer: Adsorption of CO, O2, and H20 on GaAs(100) reflectance signal I:!.R / R and the reference signal from the laser chopper. One would normally expect these signals to be perfectly in phase. However, adjustment of the phase control on the lock-in amplifier to maximize the output showed that I:!.R /R at both E, and Eo generally lagged the reference sig nal. At E, the delay was roughly 10°, but for Eo it was as much as 40°. Previous work in this laboratory has shown that the phase lag depends on temperature as well.4 The exact phase delay 0 was determined by centering the wavelength on the spectral maximum (always the positive lobe), adjust ing the phase control to null out the signal, and then shifting by exactly 90°. The phase lag arises from poor communication between surface states and the bulk.4 Thus, Vs and the electric field ~ do not attain their equilibrium values instantaneously after the laser is turned on or off. The time variation in ~ gives rise to a concurrent variation in I:!.R. Demodulation with a sinewave reference in the lock-in amplifier then yields non zero values for O. At Eo, 0 is magnified by the interference effect described above. A.CO As might be expected, the interaction of CO with GaAs(lOO) was rather weak, and exposures> lOS L were required to see substantial effects. Above lOS L, however, the spectra began to narrow gradually, as indicated by the de crease in r, for the E, transition in Fig. 4. By 1010 L, r, decreased from a clean surface value of 60 to 54 meV. The amplitude C1 also decreased by -15% in this range, as shown in Fig. 5. The phase factor 01 varied slightly from -93 to -87° with increasing exposure, and the delay 01 decreased by 1°_2°. E, remained constant. Sticking coefficients S as a function of the relative change in C1 are shown in Fig. 6. The quantity C * is defined here as the ratio of the actual change in C1 to the maximum change C1,max; C1,max for CO was approached at 1011 L. Equivalent ly, C * represents the fractional change in Vs relative to satu ration. How C * varies with absolute surface coverage 0 is unknown without independent calibration, since adsorp tion-induced changes in the density of surface states may be important. However, in this work C1 was assumed to be lin- 65 60 CO > ] 55 ~ O2 50 GoAs (100) EI T = 200K 45 101 103 105 107 109 lO" Exposure (L) FIG. ~'. Variation of the broadening parameter r, with exposure for the E, transitIOn. J. Vac. Sci. Technol. A, Vol. 7, No.6, Nov/Dec 1989 3282 2.0 1.8 • CO • . . 1.6 <.> ~" .... =' )( 1.4 "Q 1.2 H20 • GoAs(IOO) 1.0 EI T=200K 0.8 1 10 10 10 lO" Exposure( L) ~IG. 5. VariationoftheamplitudefactorC, with exposure for the E, transi tIon. ear in O. The sticking coefficients were then obtained by dif ferentiating the data in Fig. 5 with an assumed saturation coverage equal to the surface atom density of the unrecon structed substrate (6.26X 1014 atom/cm2). These satura tion coverages have evidently never been measured on GaAs(lOO) , although data for 0217,18 and H20'9 on GaAs( 110) indicate that one adsorbate molecule per sur face atom is a good approximation on that surface. For CO, it appears that no such data exist, so that this assumption is made primarily for convenience. For CO, S decreases mono tonically from a low-coverage value of 2 X 10-8 to near 3 X 10-12 at saturation. It should be noted that although Eo spectra are larger and have better signal-to-noise ratios than EI spectra, the extrac tion of even relative values of Vs from Eo spectra is not straightforward. Hence, Eo spectra are presented and inter preted here in a qualitative fashion only. Changes in Eo spec tra were rather slight. Eo and 00 were unaffected, while the spectra narrowed from 5.6 to 4.6 meV as shown in Fig. 7. Figure 8 shows how the amplitUde Co first increased slightly and then decreased. The phase delay 00 decreased from 43 to 38°, as shown in Fig. 9. s GoAs(IOO) T = 200K 1012'------'-_--'--_.L........l._--1 o 0.2 0.4 0.6 0.8 1.0 C· FIG. 6. Sticking coefficient vs fractional change in ampli tude C· = C,/C ',max at the E, transition. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:013283 E. G. Seebauer: Adsorption of CO, O2, and H20 on GaAs(100) 6 5 4 > ., 3 E ~ GaAs(iOO) 2 Eo T = 200K Exposure (L) FIG. 7. Variation of the broadening parameter ro with exposure for the Eo transition. 8.02 E) spectra displayed a stronger dependence on O2 adsorp tion, with r) beginning to decline at exposures of only 103 L as shown in Fig. 4. The amplitude C) decreased rapidly at first, but then much more slowly at large exposures until _107 L, where it decreased more quickly again. The exact exposure where this dropoff took place varied somewhat from sample to sample, but it was generally within a factor of 2 of 107 L. Typical results are given in Fig. 5. The phase factor varied slowly from -93° at low doses to -86° at the highest doses, while D) decreased by 3°-4°. E) was not signifi cantly affected by adsorption. Sis shown in Fig. 6. Again, the saturation value for C) was taken to be near 1011 L. At low coverage, S was roughly constant at 4 X 10-4, which is several orders of magnitude higher than for CO. However, S decreased precipitously near C * = 0.5 until reaching 3 X 10-9 at C * = 0.6. From this point, S decreased much more slowly until near C * = 0.9, where it began to decline more sharply again to near 10-)2. Eo spectra were drastically affected by oxygen adsorption, with measurable effects beginning near 103 L. Experimental spectra are shown in Fig. 11, while numerical parameters are exhibited in Figs. 7-10. For exposures < 107 L, the spectra 6 5 4 u )( 3 =0 2 GaAs(lOO) Eo T = 200K 0 10' 103 Exposure (L) ~IG. 8. Variation of the amplitude factor Co with exposure for the Eo transi tIon. J. Vac. Sci. Technol. A, Vol. 7, No.6, Nov/Dec 1989 -20" -40· -60· B -80· -100· -120· -140" 101 GoAs (i00) Eo T = 200K 10 Exposure(L) 3283 10 lOll FIG. ~ .. Variation ofthe line shape phase factor eo with exposure for the Eo transitIOn. narrowed from 6 to 4 meV, and eo decreased (in absolute value) from -137° to -100°. Co decreased by a factor of5 in this range, while Do decreased from 43° to 35°. Experimen tal spectra for this exposure range are shown in the top three drawings in Fig. 11. At exposures of 2-4 X 107 L, the (TDFF) ceased to be a good representation of the experimental spectra, which are shown as the next two drawings in Fig. 11. The linewidth varied unpredictably and sometimes irreproducibly in this regime so that few data for r 0 are shown in Fig. 7. Likewise, few reliable eo data could be obtained. However, Do and Co (using interpolated values of ro) were reproducible, and both quantities went through minima in this regime. At exposures above 108 L, the bottom two spectra in Fig. 11 show how the spectra inverted nearly completely from their original shape, and grew to be large. The inversion is shown quantitatively in Fig. 9 for eo; eo reached -10° from a starting point of -138° fOf the clean surface. Figure 8 shows how Co became large again, while Fig. 7 shows a COf responding increase in r o' The phase delay Do also increased (Fig. 10) but did not reach the clean surface value of 43°. For both E) and Eo spectra, annealing the oxygen-covered surface for up to 60 s did not affect the 200 K PR spectra unless the annealing temperature exceeded 420 K. Above this temperature, the spectra began to revert to that of the clean surface. However, evidence of oxygen adsorption re mained until the surface was heated to at least 620 K. 500 400 CO 300 O2 8 20° GaAs(lOO) Eo 10° T = 200K 101 lO" Exposure (L) FIG. 10. Variation of the phase delay tio with exposure for the Eo transition. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:013284 E. G. Seebauer: Adsorption of CO, O2, and H20 on GaAs(100) 1lli R 1.5 x 107 L (x 2.5) lAO 1.44 E (eV) C.H20 1.48 FIG. 11. Line shape variation of Eo spectra after various exposures of 02' H20 spectra followed the same pattern at lower exposures. Photorefiectance spectra at both EI and Eo indicated that H20 interacted most strongly with GaAs( 1(0). EI results for r 1 and C1 in Figs. 4 and 5 show substantial decreases in both parameters even at 100 L. Lower exposures could not be obtained reliably with the capacitance manometers used in this experiment, and as mentioned above, the ion gauge modified adsorption substantially. Thus, 100 L was the low est exposure achievable. The rate of adsorption leveled off near 3 X 106 L (five or ders of magnitude lower than for O2 and CO). However, the saturation value of C1 depended not just on the exposure (product of pressure and time), but on the pressure as well. That is, even at 200 K the adsorption was measurably rever sible at P> 10-3 Torr. Experiments showed that _10-1 Torr was required for saturation, and the value of C1 (1.18 X 10-8) at this pressure was used as the saturation value. Both (JI and 81 decreased slightly over the coverage range much as they did for O2, while EI was constant. Figure 6 shows that S for H20 at the lowest measured C * (-0.45) was near 1 X 1O-2-quitehigh. S decreased to -5 X 10-7 as saturation was approached, but remained several orders of magnitude higher than for O2 or CO. Eo spectra for H20 evolved through a sequence of shapes and amplitudes similar to that of O2, although much lower exposures of H20 were required to achieve a given line shape. Likewise, the various parameters in Eq. (1) mirrored J. Vac. Sci. Technol. A, Vol. 7, No.6, Nov/Dec 1989 3284 the behavior of O2, as shown in Figs. 7-10. However, for H20 the "critical" dose where Co, r 0' (Jo, and 80 underwent strong variations was at about 5 X 105 L instead of 2 X 107 L as for O2, IV. DISCUSSION A.CO CO adsorption on GaAs( 1(0) apparently has been exam ined only by Dvoryankin et ai.,2o who employed LEED, AES, and soft x-ray spectroscopy. The CO was admitted into the vacuum system though a "heated" leak valve and was probably thermally activated. Although the interaction seemed to be weak, several ordered and partly ordered struc tures were observed between 300 and 600 K. Pretzer and Hagstrom21 determined only that the initial sticking coeffi cient was very small on (111), (III), and (110) surfaces. Frankel et ai. 22 have physisorbed CO on GaAs ( 110) at 50 K with up to 2 L exposures, but found that desorption was complete by 100 K. The present experiments appear to confirm the general notion that the interaction of GaAs with CO is weak in the absence of activation. However, once the CO adsorbs, it in teracts strongly enough to remain on the surface for at least several minutes at 200 K. The narrowing of both EI and Eo spectra upon adsorption is noteworthy. The CO evidently removes surface states that are responsible for rapid trapping and therefore decreased carrier lifetime. The Eo spectra were not very sensitive to CO adsorption, and Co actually increased instead of decreasing as for the other adsorbates. This increase does not necessarily imply that CO behaves in a way fundamentally different from H20 and O2, however. The results for H20 and O2 suggest that for these adsorbates, PR does not distinguish between species; the effects of adsorption on Vs and surface states were simi lar. In both cases, as (J increased from zero, Co increased. If the effects of CO on the surface resemble those of H20 and O2, then the PR spectra for CO should follow a similar pat tern with increasing exposure. The weakness of the CO-sur face interaction might make these spectra mirror over the entire exposure range the behavior of H20 and O2 below 100 L. Such exposures were below the possible range of the pres ent experiments, so direct verification of this behavior was impossible. 8.02 The sticking coefficient data presented in Fig. 6 strongly suggest the presence of two binding states for oxygen. The first saturates near C * = 0.6 while the second state fills se quentially at higher coverages. UPS evidence for two binding states of oxygen on GaAs( 1(0) has been presented by Ranke and Jacobi.23 O2 was postulated to adsorb molecularly on a Ga-deficiency site. Oxygen in this state could dissociate (under unknown conditions), thus freeing the deficiency site for more molec ular adsorption. The net result would be a Ga-depleted sur face covered with both molecular and atomic oxygen. These workers found that O2 adsorbed more readily on Ga-rich Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:013285 E. G. Seebauer: Adsorption of CO, O2, and H20 on GaAs(100) (100) surfaces than on As-rich, with So-1.5 X 10-5. Lu deke and Koma observed the same trend,24 although they placed So at -1.5 X 10 -4. Furthermore, they considered the surface to be saturated at () = 0.5, corresponding to an expo sure of -106 L. This finding does not disagree with the pres ent work because S was quite low at 106 L. At least 1011 L were required to reach saturation, consistent with more re cent work on GaAs( 110).25 Moreover, ()sat for O2 on GaAs( 110) seems to be > 0.5.17.18 The results shown here cannot unambiguously distinguish molecular and atomic oxygen. However, the fact that annealing to 420 K is re quired to change the PR spectra suggests that the species responsible is adsorbed fairly strongly and is therefore prob ably atomic. The large line shape variations for Eo spectra constitute the spectral "rotation" referred to in Sec. III. This rotation has been observed by PR for GaAs as the temperature in creases,4 although in that case C1 in El spectra and therefore V. appeared constant over much of the temperature range. Rotation was attributed to the small (as low as 4.5%) change in Vs required to induce full spectral rotation, and to the additional effect of the smearing out of the excitons in the bulk by thermal quenching. In the present constant-temperature experiments, rota tion akin to that in Ref. 4 was observed, but only in the presence of a fairly large (15%-20%) change in CI and hence Vs. Thus, the importance of thermal quenching of excitons in the experiments of Ref. 4 is confirmed. C.H20 The results presented here show that H20 interacts far more strongly with GaAs( 100) than CO or O2. Substantial effects were observed even for 100 L exposures, and satura tion effects became apparent at -3 X 106 L. Both r I and r 0 decreased upon adsorption, following the behavior of the other two gases. The behavior of the Eo spectra for H20 follows that of O2 fairly closely, indicating that the primary cause for rotation is the reduction in Vs. Small deviations may be observed, however. For example, ~o decreases more for H20 than for O2, and for H20 the "critical" exposure where the Eo spectra undergo rapid changes corresponds to a different CI than for O2. These effects probably reflect differences in the way the two adsorbates modify the surface states, but additional techniques such as surface photovoltage or photoemission will be required for a more definitive interpretation. H20 adsorption on GaAs( 100) does not appear to have been studied previously. However, Buchel and Luth have exam ined H20 adsorption on cleaved GaAs( 110) by photoemis sion.26 At 180 K they detected a single physisorbed molecu lar phase. Effects were seen for doses as small as 10-2 L. At 300 K additional features corresponding to chemisorption through the oxygen lone pair were observed. At high cover age, the room-temperature spectrum showed evidence of the physisorbed H20 seen at 180 K. Both phases reduced the band bending from its clean surface value, although the ex act results depended on whether Vs was obtained from the widths of the spectra or the position of the valence band J. Vac. Sci. Technol. A, Vol. 7, No.6, Nov/Dec 1989 3285 onset. This reduced band bending corresponds to the de crease in CI observed in the present experiments. Evidence for two adsorption phases of H20 on GaAs( 110) also comes from the photovoltage measure ments of Liehr and Luth.27 These workers suggested that H20 induces creation of extrinsic surface states 0.18 e V be low the conduction band edge. Webb and Lichtensteiger28 have observed the two states and in addition have seen by XPS and UPS the formation of Ga-OH bonds from H20 dissociation above 109 L. The dissociation was further con firmed by SIMS. Mokwa et al.19 have examined H20 on GaAs(1lO) by thermal desorption and LEED, and determined a low-cover age sticking coefficient of 1.3 X 10-3. S decreased to 7 X 10-5 near 0.1 ML, but increased sharply by at least two orders of magnitude near () = 0.4 before returning to the 10-5 range at high coverage. The singularity at () = 0.4 was attributed to substrate reconstruction to the unrelaxed con figuration. No such singularities in S are evident in the present data, although the GaAs( 1(0) surface may be expected to behave in a manner entirely different from GaAs (110). Multiple binding states are not discernible either, although low cover ages could not be investigated here. V. CONCLUSION The results presented here show that PR is quite sensitive to adsorption, even when the interaction is rather weak as it is for CO on GaAs. Because PR is an optical technique, adsorption measurements could be made even in the pres ence of several hundred Torr of ambient gas without diffi culty. Such measurements at high pressure were critical for obtaining equilibrium results and may be crucial to the study of adsorption/desorption/reaction of gases on semiconduc tors in general, since such interactions are usually weak. PR showed little species specificity for the adsorbates studied, however. The data obtained at the El (3.0 eV) and Eo (1.4 eV) transitions are complementary. Spectra at both transitions respond to changes in the surface potential Vs; however, the nature of this response is quite different. Adsorption serves to change the amplitude of El spectra in direct proportion to Vs. Relative (not absolute) spectral amplitudes can be ade quately described with standard Franz-Keldysh theory. For Eo spectra the line shape changes as well because of varia tions in the width of the SCR that cause optical interference effects between the surface and excitons at the edge of the SCR. Eo spectra are larger than E 1 spectra and therefore have better signal-to-noise ratios, but the line shape changes make unambiguous interpretation more difficult because of the complexity of the interference effects. PR also responds to adsorption-induced changes in the nature of surface states. The widths of both E I and Eo spectra decrease upon adsorption, indicating increased carrier life time and therefore a reduction in recombination rate at the surface. Moreover, adsorption changes the phase lag between AR / R and the reference signal from the laser chop per. This phase lag results from slow equilibration of carriers Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:013286 E. G. Saebauer: Adsorption of CO, O2, and H20 on GaAs(100) in the space-charge region with the surface states. Gas ad sorption generally improves this communication. Low-coverage sticking coefficients on GaAs( 1(0) in crease in the order CO<02 < H20. The data show evidence for two binding states of O2 on this surface. In all cases, S decreases by at least four orders of magnitude as saturation is approached. ACKNOWLEDGMENT This work was supported in part by the U.S. Department of Energy under Contract No. DE-AC04-76DP00789. a) Present address: University of Illinois, Department of Chemical Engi- neering, Urbana, IL 61801. 'M. Cardona, K. L. Shaklee, and F. H. POllak, Phys. Rev. 154,696 (1967). 2E. Y. Wang, W. A. Albers, Jr., and C. E. Bleil, in II-VI Semiconducting Compounds, edited by D. G. Thomas (Benjamin, New York, 1967), p. 136. 30. J. Glembocki, B. V. Shanabrook, N. Bottka, W. T. Beard, and J. Co mas, Proc. SPIE 524,86 (1985). ·E. G. Seebauer, J. Appl. Phys. (in press). 'C. D. Thurmond, J. Electrochem. Soc. 122,1133 (1975). "G. Laurence, F. Simondet, and P. Saget, Appl. Phys. 19, 63 (1979). J. Vac. ScI. Technol. A, Vol. 7, No.6, Nov/Dec 1989 3286 7A. Saletes, J. Massies, and J. P. Contour, Jpn. J. Appl. Phys. 25, U8 (1986). 8J. Massies and J. P. Contour, J. Appl. Phys. 58, 806 (1985). 9J. P. Contour, J. Massies, and A. Saletes, Appl. Phys. A 38, 45 (1985). lOR. Z. Bachrach, R. S. Bauer, P. Chiaradia, and G. V. Hansson, J. Vac. Sci. Techol. 18,797 (1981). "R. Z. Bachrach, R. S. Bauer, P. Chiaradia, and G. V. Hansson, J. Vac. Sci. Technol. 19, 335 (1981). '2p. Drathen, W. Ranke, and K. Jacobi, Surf. Sci. 77, 1162 (1978). 13p. Pianetta, I. Lindau, C. M. Gamer, and W. E. Spicer, Phys. Rev. B 18, 2792 (1978). '40. E. Aspnes, Surf. Sci. 37, 418 (1973). "D. E. Aspnes and J. E. Rowe, Phys. Rev. Lett. 27, 188 (1971). 16V. A. Kiselev, Phys. Status. Solidi B 111, 461 (1982). 17G. Landgren, R. Ludeke, Y. Jugnet, J. F. Morar, and F. J. Hirnpsel, J. Vac. Sci. Technol. B2, 351 (1984). 18F. Bartels and W. Monch, Surf. Sci. 143, 315 (1984). I~. Mokwa, D. Kohl, and G. Heiland, Surf. Sci. B 9,98 (1984). 2"V. F. Dvoryankin, A. Yu. Mitayagin, and T. O. Uustare, Sov. Phys. Solid State 22, 1000 (1980). 210. D. Pretzer and H. D. Hagstrom, Surf. Sci. 4, 265 (1966). 220. J. Frankel, Y. Yukin, R. Avci, and G. J. Lapeyre, J. Vac. Sci. Technol. A I, 679 (1983). 23W. Ranke and K. Jacobi, Progr. Surf. Sci. 10, 1 (1981). 24R. Ludeke and A. Korna, Phys. Rev. Lett. 34, 817 (1973). 2'W. E. Spicer, P. W. Chye, C. M. Gamer, I. Lindau, and P. Pianetta, Surf. Sci. 86, 763 (1979). 26M. Biichel and H. Liith, Surf. Sci. 87, 285 (1979). 27M. Liehr and H. Liith, J. Vac. Sci. Technol. 16, 1200 (1979). 28C. Webb and M. Lichtensteiger, J. Vac. Sci. Technol.21, 659 (1982). Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 155.33.120.209 On: Sat, 22 Nov 2014 07:32:01

1.1139685.pdf

Simple balance technique for determining the transition temperature of high T c superconducting powders Takeshi Takamori and Derek B. Dove Citation: Review of Scientific Instruments 59, 1430 (1988); doi: 10.1063/1.1139685 View online: http://dx.doi.org/10.1063/1.1139685 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/59/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Quasiequilibrium determination of highT c superconductor transition temperatures Am. J. Phys. 58, 642 (1990); 10.1119/1.16424 Oxidized treatment of high Tc superconducting thin films by plasmaion doping technique AIP Conf. Proc. 200, 122 (1990); 10.1063/1.39053 Photoacoustic detection of the superconducting transition in high T c superconductors J. Appl. Phys. 65, 2568 (1989); 10.1063/1.342785 Thinfilm high T c superconductors prepared by a simple flash evaporation technique Appl. Phys. Lett. 53, 1663 (1988); 10.1063/1.100468 Superconducting oxide films with high transition temperature prepared from metal trifluoroacetate precursors Appl. Phys. Lett. 52, 2077 (1988); 10.1063/1.99752 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.18.123.11 On: Thu, 18 Dec 2014 23:02:39mating surfaces of the metals were sanded with 2000 Japa nese rate emery paper which corresponded to a roughness of 10 p. Maximum torques which did not break threads were applied to make each joint. When a 5~mm-diam stainless steel socket head cap screw was used for fastening two pieces with a torque of 12 N m, the best result of 0.005 lIn was obtained for the contact. A joint with a 5~mm-diam hexag onal brass bolt had a resistance of 0.011 /-in. When a pair of smaller screws of 3 mm in diameter were used for the same contact area with smaller torques, the electrical resistance increased to 0.26 pO for stainless-steel bolts and 0.087 pn for brass bolts, respectively. The electrical resistance, the size of screws, and the applied torques concerning the con tact between the different kinds of materials are summarized in Table I for convenience, together with typical values for the contact between the same kinds of materials. Only the electrical resistances at 4.2 K are shown for other workers. The electrical resistance is in rough inverse proportion to the applied torque. One can easily apply larger torques to larger bolts. From these measurements it is concluded that fasten ing two pieces with bolts as large as 5 mm in diameter is very effective and satisfactory for good thermal contact, which is comparable to contact between the same materials. Incidentally an EB (electron beam) welded joint ofsil ver rods having a diameter of 10 mm and welding cross sec tion of 0.5 X 31 mm showed a resistance of 0.018 pD.. This value is of the same order of magnitude as copper-to-copper TIO welding and the best values of threaded joints. We thank the Technical Division of the Institute of PI as rna Physics, Nagoya University for EB welding. 'M. C. Veura, Ph. D. thesis, Helsinki University of Techuology, Finland, 1978. 2K. M. Lau and W. Zimmermann, Jr .• Rev. Sci. lustrum. 50, 254 (1979). 'K. Muething, G. G. Ihas, and J. Landau, Rev. Sci. lostrum. 48, 906 (1977). 4R. 1. Boughton, N. R. Brubaker, and R. J. Sarwinski, Rev. Sci. lustrum. 38, li77 (1967). 'M. Suomi. A. C. Anderson, and B. Holmstrom, Physica 38,67 (1968). OM. Manninen and W. Zimmermann, Jr., Rev. Sci. lnstrum. 48, 1710 (1977). 7D.I. Bradley, A. M. Guenault, V. Keith, C. J. Kennedy, I. E. Miller, S. G. Mussett, G. R. Pickett, and W. P. Pratt, Jr., J. Low Temp. Phys. 57, 359 (1984). Simple balance technique for determining the transition temperature of high Tc superconducting powders Takeshi Takamori and Derek B. Dove IBM Research Divisiofl, T. J. Watson Research Center, Yorktown Heights, New York 10598 (Received 2S January 1987; accepted for pUblication 29 April 1988) A simple technique is described that permits the measurement of the transition temperature of high Tc superconductors in powder form. The method employs a modification of a single-pan balance and has sensitivity for measurements on powder amounts as low as a few mg. High Tc superconductors have been the subject of intensive study, foHowing the report by Bednarz and Muller on the La-Ba-Cu oxide system. I A wide range of structural and compositional modification has been explored through the past year in a search for superconductors with even higher Tc.2-4 Because these polycrystalline superconductors are prepared through typical ceramic powder processing tech niques, characterization of the as-prepared powder is as im portant as that of sintered or pressed material. 5 For example, by determining the transition temperature of the powder, it is possible to follow development of the superconducting property from the starting powder mixture through the fir ing process and other processing steps. Since it is exceedingly difficult to make electrical conductivity measurements on typical powder samples, we have set up a simple balance utilizing the Meissner effect in order to determine the transi tion temperature. This method is sensitive, nondestructive, and readily permits measurements on powder amounts as low as a few mg. The arrangement, shown in Fig. 1, consists of a single hanging-pan balance,6 so-called student balance, to which is attached a small permanent magnet in place of the pan. The powder sample to be measured is placed in a copper cup arrangement that allows easy cooling and heating between room temperature and liquid-nitrogen temperature, and the copper cup is placed in dose proximity beneath the magnet. As the sample is cooled through the superconducting transi tion, a repulsive force is exerted on the magnet due to the Meissner exclusion of magnetic nux from the sample. It has been found convenient to measure the small displacement from the equilibrium position of the magnet by means of a linear variable differential transformer (L VDT) .7 As shown in Fig. 1, the core of a differential transformer is attached to the suspension carrying the magnet and the windings were securely located relative to the balance. Any small motion of the magnet displaces the core resulting in a signal from the differential transformer. An electrical signal obtained by this means is proportional to the displacement of the magnet and, hence, to the force exerted on the magnet as the sample becomes superconducting. Details of the sample holder are shown schematically in Fig. 2. The sample holder consists of three layers of copper cups with good thermal contact to each other. A typical size of the outer cup is 25 mm deep and 25 mm in diameter with a wall thickness of 1.6 mm. The size 1430 Rev. Sci. Instrum. 59 (3), August 1988 0034·6748/88/081430-03$0 1.30 @ 1938 American institute of Physics 1430 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.18.123.11 On: Thu, 18 Dec 2014 23:02:39Balance -1----- LVDT -+-+-+--- COfe \ \:::::==~g To Recorder --t----- Compensating Weight / Magnet To Recorder Sam pie Powder -+--++- Sample Holder ~~~~~~~, +-----Liquid Nitrogen ~============~====~ Container FIG. 1. Schematic representation of the present technique. and number of cups used should be adjusted depending on the measurement condition desired. For example, if a heat ing or cooling rate slower than actually measured is desired, an additional outer cylinder with a good thermal contact is used. The outer cup also prevents the sample and the magnet from wetting by the liquid nitrogen in the cooling bath, so that the balance is not disturbed by the buoyancy. The mid dle cup holds the sample container with a thermocouple hole at the bottom, and a slot on the outside wall to guide the thermocouple. The sample container can be made of any nonmagnetic materials, such as the alumina containers sup- Somple Powder Thermoeouple FIG. 2. Details of the sample holder. ++t<~<'+--- Inner Cup t+f?~?!--- Middle Cup \W<\l--- Outer Cup =EfI#~(IN--- No..,mo9~etic Thermal Insulation Container 1431 Rev. Sci. instrumo, Vel. 59, No.8, August 1988 plied with commercial DT A instruments. The inner cup is a cover for the sample to ensure thermal uniformity across the sample. Its bottom should be thin enough to retain the de sired sensitivity, however. The thickness ofthe bottom of the inner cup we use is 0.6 mm. When the magnet is close to the bottom of the inner cup and the sample container is filled with the powder, the magnetic field exerted on the top of the sample is found to be 2500 G. At this maximum field of the present arrangement, flux may penetrate the sample if the applied field is greater than Hcl. 8 Even at this field, however, we did not find any difficulty in determining the transition temperature. Since the present arrangement is designed pri marily to provide a convenient method for measuring the superconducting transition temperature of powder samples, the magnetic field applied to the sample is kept relatively low, typically 1000 G, to avoid appreciable influence of the field upon the transition temperature.9 The measurement can be performed in room atmo sphere without difficulty. To minimize frosting, however, it would be preferable to use a dry box. A typical operation is as follows. First, the sample powder is loaded as shown in Fig. 2, and the sample holder sitting in the cooling bath is lifted up toward the magnet, until the magnet is slightly above the bottom of the inner cup, and is hanging freely so that the balance is free from frictional disturbance. Then liquid nitro gen is carefully poured into the cooling bath until the surface of the liquid reaches a level just short ofthe top of the sample holder. After the temperature of the sample reaches close to that of the liquid nitrogen, the liquid is drained off or evapo rated. If the sample powder is cooled through a supercon dueting transition, the magnet is pushed upwards and the displacement is recorded via the L VDT signal. It is possible to determine the Tc of the sample during cooling, but with the present arrangement it is preferable to measure it during heating since control of the heating rate is usually easier to achieve than control of the cooling rate. After the liquid nitrogen is drained off, the sample wanns up by exposure to the room atmosphere. Because of the large heat capacity of the sample holder, the heating occurs gradually enough to determine the Tc with a reasonable accuracy< As the super conducting state disappears, the magnet comes down to its FIG. 30 Typical displacement-temperature curves for two samples. Notes 1431 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.18.123.11 On: Thu, 18 Dec 2014 23:02:39biJ r.----,-----.-----' --,-~~-.-, ~ .. ~-, S ::6 iJ:I ... C) , "0 ~ o (,_, P-. " :> • ..0 C) ~ i~li ;;... ... // .. /. '00 /' -;: ~/ ~ l ./y ~ ~.~I ____ ~ __ ~ ____ -L ____ L-__ ~ ____ ~ __ --u 4U 120 lUI I'owd(~r B in Mixture' (mg) FIG. 4. Repulsive force vs weight of powder B dispersed in alumina. original position. This motion of the magnet is recorded to gether with the sample temperature. The present arrange ment has been found to provide a simple alternative to the use of more elaborate cryostat systems. Figure 3 shows results obtained by the present tech nique on superconducting powders ofYBa2Cu30x prepared by two preparation procedures. Subtle differences in shape of the curves at the onset of T,. were quite reproducible, indicating that this technique may also be useful for moni toring such difference in transition in sample-to-sample comparison. Even though the strength of the magnetic field exerted upon the sample depends upon the relative distance between magnet and sample, little difference was seen in the transition curves obtained with the magnet placed at heights ranging from 1 to 3 mm above the sample. As shown in Fig. 3, about 1.2-1.3-mm displacement of the magnet by the Meissner effect (corresponding to 60-65 mg of the force) was recorded for sample powders of 120-160 mg in weight. One of the advantages of the present technique over the direct conductivity measurement is that the material to be measured does not have to be in a form that ensures electrical continuity. Powder particles can also be measured when dis persed in an inert matrix without contacting each other. This is demonstrated in Fig. 4. The powder B in Fig. 3 was diluted by fine alumina powder (0.5 ?lm) over a wide range of weight ratios, and subjected to the same measurement. Be cause of the density difference between alumina and YBa2Cu30x, the weight of the mixture filling the sample container varied from 44 mg for the 9.9 weight % mixture to 160 mg for the powder B only. This resulted in the recorded displacement varying by more than an order of magnitude. It was found that the observed variation of the magnitude of 1432 Rev. ScLlnstrum., Vol. 59, No.8, August 1988 displacement was proportional to the weight of supercon ducting powder in each mixture measured. In Fig. 4, the repulsive force due to powder B is plotted versus weight of powder B in the mixtures. The least amount of powder B in Fig. 4, 4.4 mg in the 9.9 weight % mixture, is less than one half the nominal sensitivity limit (10 mg) of the balance used. In conclusion, a simple arrangement has been described that provides a convenient method for determining the tran sition temperature of high Tc superconductors that are in powder form. It has been found that the method permits measurements on a few mg of powder and for a given powder it is at least semiquantitative in estimating the superconduct ing fraction. We may note, however, that the interaction between magnetic field and sample is such as to make truly quantitative measurements very difficult, since the effective field acting on the sample is due both to the applied field and to the field exclusion due to the sample itself. For measure ments on powders, however, semiquantitative comparative measurements can be made as shown in the present work. Magnetic field penetration depth, particle size effects, and interaction with the applied field need to be considered, how ever, if more quantitative estimates of super conducting vol ume fraction are to be obtained. We thank M. W. Shafer and J. J. Cuomo for supplying the high Tc superconducting materials used in developing the present technique. 'I. G. Bednorz and K. A. Muller, Z. Phys. B 64,189 (1986). 2M. K. Wu, J. R. Ashburn, C. 1. Torng, P. H. Hor, R. L Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 58, 908 (1987); P. H. Hor, L. Gao, R. L. Mcng, Z. J. Huang, Y. Q. Wang, K. Forster, J. Vassi lious, C. W. Chu, M. K. Wu, J. R. Ashburn, and C. T. Torng, ibid. 911. 'X.-D. Chen, S. Y. Lee, J. P. Golben, S.-I. Lee, R. D. McMichael, Y. Song, T. W. Noh, and J. R. Gaines, Rev. Sci. Instrum. 58, 1565 (1987). 4Several proceedings are available, see for example, Current Research on Ceramic Superconductors (Am. Ceram. Soc., 1987); Chemistry of High Temperature Superconductors, Am. Chern. Soc. Symposium Series 351, 85 (1987). SR. A. Hein, Phys. Rev. B 33,7539 (1986). "DIAL-O-GRAM, OHAUS, Florham Park, NJ 07932. 7Type 500HR, Schaevitz Eng., Pennsauken, NJ 081l0. BF. Hellman, E. M. Gyorgy, D. W. Johnson, Jr., H. M. O'Bryan, and R. C. Sherwood, J. App\. Phys. 63, 447 (1988). 9S. S. P. Parkin, V. Y. Lee, and E. M. Engler, Chemtronics 2, 105 (1987). Notes 1432 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.18.123.11 On: Thu, 18 Dec 2014 23:02:39

1.344302.pdf

Trapdominated breakdown processes in an insulator bridged vacuum gap R. G. Bommakanti and T. S. Sudarshan Citation: J. Appl. Phys. 66, 2091 (1989); doi: 10.1063/1.344302 View online: http://dx.doi.org/10.1063/1.344302 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v66/i5 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsTrap .. dominated breakdown processes in an insulator bridged vacuum gap R. G, Bommakanti and T. S. Sudarshan Department of Electrical and Computer Engineering. University of South Carolina, Columbia, South Carolina 29208 (Received 13 February 1989; accepted for publication 16 May 1989) Measurements of coordinated, time-resolved breakdown current and luminosity phenomena associated with a pulsed surface flashover event in an insulator bridged vacuum gap are presented. It was observed that the luminosity and current waveforms differ vastly in their temporal character. The luminosity profile has a sharp pulse with no counterpart in the current waveform. A significant afterglow activity is also observed after the cessation of the breakdown current. Further, the profiles of the luminosity and current waveforms changed with successive breakdowns, The rise times and decay times of the luminosity waveform, the time delay between the onset of the luminosity and current waveforms, and the rise time of the current waveform changed with successive breakdowns showing regular trends. The above modifications in the temporal profiles of luminosity with successive breakdowns are analyzed on the basis of carrier trapping and recombination processes within the localized levels of the forbidden gap associated with the insulator-vacuum interface. The experimental results reported here qualitatively support the surface flashover model based on carrier trapping for low mobility, large band-gap insulators. The analysis ofthe results is suggestive of a breakdown model in which hot-electron generation culminates in impact ionization-induced breakdown in the subsurface layers of the insulator. I. INTRODIJCTION The degradation of the excellent voltage hoidoffproper ties of a plain vacuum gap due to the insertion of an insulator has been researched extensively for over three decades. Var ious models based on prebreakdown and breakdown investi gations have been proposed to explain the physics of pulsed surface flashover phenomena. 1 The principal elements of the models proposed earlier incorporate electron emission from the cathode triple junction, cascade multiplication on the insulator surface, and ionization of gas desorbed from the insulator surface, Recent models, however, suggest that the surface flashover phenomena can be explained using an en ergy-band perspective. Insulator surface charge generation and neutralization has been experimentally demonstrated to be due to recombination processes in the localized levels in the forbidden gap of the insulator.2 An electronic cascade within the surface layers of an insulator where the defects are concentrated has been proposed as the cause of surface flashover in bridged vacuum gaps.3 Based on detailed pre breakdown and breakdown measurements on a wide range of ceramic and polymer materials with varying surface and bulk properties, a model invoking the energy-band structure at the insulator-vacuum interface has been proposed for de surface flashover. 4 The model proposes that the breakdown occurs in a thin surface sublayer within the insulator by a process of collision-ionization of trap emptied charge carri ers from defect sites within the forbidden band. The validity of these new models for pulsed surface flashover is being examined in our present investigations. The focus of this paper is the description and interpreta tion of time coordinated breakdown luminosity and current measurements of the pulsed surface flashover event in a bridged vacuum gap. The observation of predischarge and discharge luminosity associated with the surface flashover event using high~speed camera techniques has been the sub ject of past investigations.5-7 However, these techniques do not provide quantitative information pertaining to the spa tially integrated, temporal evolution of the luminosity ava lanche, essential for understanding the electron and photon generation processes and their consequent roles in the sur face flashover process, The luminosity diagnostics employed in our studies offer quantitative data with nanosecond tem~ poral resolution and an extended spectral response not pre viously reported. The application of time coordinated stud ies of breakdown luminosity and current with these enhanced diagnostic capabilities, to an insulator which has not been prestressed, has led to severa] novel experimental observations. It was observed, for instance, that not only do the current and luminosity profiles differ vastly from each other, but that both waveforms undergo significant modifi cations with successive breakdown. Also, while the luminos ity avalanche precedes the breakdown current avalanche, the time delay between the two is not constant, contrary to earlier reports.5 The time delay is actually found to decrease steadily within the first few breakdowns. These and other results described later are suggestive of changes in the details of the flashover process with successive breakdowns. As a coronary, the study of preconditioned insulators, assuming the breakdown event to be amenable to statistical analysis, seems questionable. Additionally, the emphasis in the earlier studiesS-s was to comprehend the evolution of the breakdown phase from prebreakdown events such as gas desorption and secondary electron emission. An analysis of our data does not, how~ ever, require any assumptions regarding gas desorption or secondary emission prebreakdown processes. In the present report, the observed changes in the temporal profiles of lu minosity with repeated voltage breakdowns are examined by taking into consideration carrier recombination and trap- 2091 J. Appl. Phys. 66 (5). 1 September 1989 0021-6979/89/172091-09$02.40 @ 1989 American Institute of PhYSics 2091 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsping processes within the forbidden gap associated with the dielectric-vacuum interface. The breakdown event is per ceived as a perturbation of the thermal equilibrium distribu tion of the electrons and holes in the localized levels of the forbidden gap. The resultant nonequilibrium situation and the final relaxation process towards the thermal equilibrium are believed to be characteristic of the insulator-vacuum in terface. The observed modifications in the luminosity pro files with successive breakdowns are found to be adequately described as manifestations of the relaxation and associated trap dominated processes. Specifically, the dominant role of the trapped carriers in influencing the luminosity rise times, decay times, and afterglow behavior is emphasized. Experi mental confirmation of physical processes influencing the prebreakdown phase are beyond the scope of the currently reported results. However, no unduly restrictive assump tions regarding the prebreakdown processes culminating in the flashover event are required excepting experimentally well-established charge injection and transport phenomena in low mobility, large band-gap insulators such as alumina. A. Experimental setup Figure 1 shows the overall schematic of the experimen tal test arrangement. The voltage source for the experiments is a six-stage Marx impulse generator with a nominal output of 600 kY, 15 kJ. The output is a double exponential voltage waveform. For the investigations reported here, a O.5/15-,us wave was chosen. The Marx generator was enclosed in a Faraday cage (using mesh screen) to ensure that the noisy breakdown of the spark gaps do not interfere with the mea sured signals at the test chamber. A high-voltage cable (rat ed at 3OO-kV dc) with a surge impedance of 67 n connects the output of the generator to the test chamber. The cable sheath is grounded at the Faraday cage and at the test chamber to retain the transmission line characteristics of the Capaciliv6_ divider ~ Photo!ube Mechanical and diffusion pump system ~~~=~ Bottom electrode manipulator FIGo L Schematic of experimental setupo 2092 Jo AppL Physq Vol. 66, Noo 5, i September 1989 cable. The cable terminates at the vacuum chamber in a high-voltage Delrin ™ bushing. The stainless-steel chamber itself is 46 em tall and 31 em in diameter and is evacuated by an oil diffusion pump backed by a mechanical pump. Apart from the port connected to the diffusion pump, the chamber is provided with several ports to enable optical diagnostics and to facilitate the changing of samples. AU the ports are provided with quartz windows, the useful optical frequency response for which is 170 nm to 2.2 flm. The test samples are held in a butt-contact between plane parallel stainless steel electrodes with rounded edges providing a quasiuniform field. The top electrode is mounted at the high-voltage feed through end of the vacuum chamber while the bottom elec trode is mounted on a linear motion feedthrough with a dial gauge arrangement to measure the gap spacing to an accura cy of 0.01 mm. t. Electrical and optical diagnostics The breakdown voltage across the test gap, the break down current through the gap, and the luminosity of the breakdown arc were aU recorded using a dual channel digi tizing oscilliscope (2S0-MHz single-shot capability, and a usable rise time of 1.5 ns). The breakdown voltage was mea siued using a self-integrating coaxial capacitive divider in Hne with the test gap. The linear sensitivity of this divider was measured to be 3.87 mV/kV as calibrated against the Tektronix P6015 high-voltage probe. The breakdown cur rent was measured using a commercially available current viewing resistor (T&M Research Products, model SSDN- 015,15 mn and a usable response time of I ns) in series with the test gap on the ground side. The luminosity signal from the breakdown arc was measured using a vacuum phototube (Hamamatsu model R 1193U -02, useful spectral response of 185-650 nm with a peak at 340 nm, a usable rise time 270 ps and a fall time lOOps). This signal was terminated with 50 a at both ends while the voltage and current signals were matched with 50 n at the oscilloscope alone. Low noise RG 223/U, 50-0 coaxial cables were used for aU signal measure ments. Additionally, the cables were doubly shielded using copper braid. All the cables were matched equal in length to within 3 in. using a time domain refiectometer to ensure that the temporal phase relationships between the measured sig nals are always preserved. 2. Specimen and electrode preparation The insulators studied were 99.9% pure polycrystaUine alumina specimens procured from Frenchtown Alumina (FA 7258-Bl) in the form of right circular cylinders, lOmm thick, 25.4 mm diam. The samples were rinsed with analyti cal grade methanol and placed in the test gap. The stainless steel (No. 304) electrodes were polished after each test by 9. 5-pm alumina abrasive followed by 0.05-,um alumina abra sive. The electrodes were subsequently ultrasonically cleaned in Buehler Ultramet cleaning solution followed by cleaning in hot distilled water for! h. The electrodes were assemble.d in the test chamber after drying and rinsing with methanol. All sample and electrode handling was done using surgical gloves. R Go Bommakanti and To So Sudarshan 2092 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions4 .;::' 3 (f) 0 2 .- E :J -.I --I o 2 3 4 ~) 6 7 !3 CJ 10 TirrJe FIG. 2. Typical osciHograms of breakdown current and luminosity. Ap plied voltage = 35 kV, time = 500 ns/div. Sensitivity: current at 267 A/ div., luminosity at 45.3 lm/div. 3. Experimental procedure The test chamber was pumped down for about 10 h to a level of 5 X 10-6 Torr. The voltage was then applied begin ning at 5 kV. The time interval between successive voltage applications was about 15 min. Three voltage applications were made at each voltage level and the voltage increased by 5 k V if none of the applications resulted in a breakdown. The current-luminosity measurements reported here were all ob tained at a 35-k V peak at which the sample flashed over first and repeatedly thereafter on each successive voltage applica tion at the same voltage. Data reported here pertain to ten such voltage applications. Typical breakdown current and luminosity data are shown in Fig. 2. The Marx generator waveform is a 0.5/15 fl,s, provided the sample does not breakdown on voltage application. In case of the occurrence of a breakdown, the Marx generator is essentially a capacitor bank discharging across a shorted gap. The voltage across the bank collapses completely and in case the load matches the source, only one pulse can be seen. However, since the gap impedance during the flashover event is not matched to the generator impedance, the breakdown current consists of a train of reflections. The time for the current to decay to zero would depend on the extent of the impedance mis match. In the ensuing discussion, only the pulse correspond ing to the first two-way transit time of the high-voltage (hv) cable (-50 ns) is considered. AU the signal measurements were made using external triggering of the oscilloscope with the test gap voltage divider output as the trigger. Pretrigger information of 100 ns was acquired in order to ensure that the breakdown precursors were not missed. II,RESULTS Figures 3(a) and 3(b) show oscHIographs of the time resolved data of breakdown current and the associated lumi nosity for the second and tenth breakdowns, respectively. The summarized results for a total of ten breakdowns are presented in Fig. 4. The fonowing observations could be made from Fig. 3 and 4. ( 1 ) The temporal structure ofthe luminosity waveform 2093 J. Appl. Phys., Vol. 66, No.5, 1 September 1989 (bl Time FIG. 3. (-a) Luminosity and breakdown current profiles for the second breakdown. Applied voltage: 35 kV, time = 10 IIs/div. Sensitivity: current at 105 A/div., luminosity at 3.8 Im/div. (b) Luminosity and breakdown current profiles for the tenth breakdown. Applied voltage: 35 kV, time = 10 ns/div. Sensitivity: current at 103 A/div., luminosity at 11.81m/dlv. 20 15 c~ 10 (f) C <V E 5 i-- 0 No. of breakdowns FIG. 4. Summary of observed modifications ill the luminosity and current profiles with successive breakdowns. a, Rise time ofIuminosity pulse, b, rise time of current pulse, and c, time delay between the onset of photonic and electronic avalanches. R. G. Bommakanti and T_ S. Sudarshsn 2093 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions(al Time " I 7 6 6 ~ 5 ~ 4 c 4 .;'>;> Q) j " \.. :J U 2 if) 3 c c 2 E :~ _J 0 0 (hi lime FIG. 5. (a) Oscillograms showing details of the slowly varying and expo nential components of the luminosity profile for the second breakdown. Ap plied voltage: 35 kV, time = 5 ns/div. Sensitivity: current at lOS A/div., luminosity at 3.8Im/div. (b) OscilIograms showing details of the slowly varying and exponential components of the luminosity profile for the tenth breakdown. Applied voltage: 35 kV, time = 5 us/div. Sensitivity: current at 103 A/div., luminosity at 11.8 Im/div. is very different from that of the current waveform. It is noted that while the current waveform matches the output of a SPICE circuit simulation program for a short-circuited test gap, the luminosity rise and faU times (10%-90%) do not have any counterpart in the current waveform. The !uminos ity rise time and the FWHM is short compared to the current waveform. (2) With successive breakdowns, the structure of the luminosity is altered in terms of rise and fall times. The rise and decay times of the luminosity decrease with successive breakdowns (Fig. 4). The luminosity rise and decay profile on the first few breakdowns can be divided into two por tions-a slowly varying portion reaching a plateau and a steeply lising portion. In Fig. 5(a), the slowly varying por tion can be seen up to 8 ns for the rising profile and from 26 ns onwards for the decay profile. The plateau region extends from 8 to 14 ns. The steeply varying portion can be seen in Fig. 5(a) from 14 to 21 ns on the rising profile and from 22.5 to 26 ns on the decay profile. Wi.th successive breakdowns, the slow portion shortens in time and almost disappears both on the rising and falling edges of the luminosity pulse [Fig. 5(b)]. However, the current rise times increase with succes sive breakdowns as can be seen from Fig. 4. 2094 J. Appl. Phys., Vol. 66, No.5, 1 September 1989 (3) In the first breakdown event, the luminosity ava lanche (photon avalanche) precedes the electronic ava lanche by a few nanoseconds [Fig. 3(a)]. A quantitative measurement of the time delay between the two signals is nontrivial since both the signals have vastly differing fre quency contents. Specifical1y, the luminosity signal consist ed of two components, which varied significantly with successive breakdowns, as described above. In view of this, a choice was made to measure the time delay between the 0% point on the luminosity rise and the 0% point on the current rise. As can be seen from Fig. 4, this time delay decreases from the second to the tenth breakdown. Other choices such as measuring the time delay between the 10% point (of the peak value) on the luminosity waveform and the 10% point on the current waveform were explored. For instance, it was found that the absolute value of the delay for the second breakdown was 8 ns using the 10% measurement instead of 6 ns with the 0% measurement. However, it was found that the delay decreases significantly after ten breakdowns, simi lar to that of the 0% measurement. A similar trend was con firmed using values corresponding to 20% of the peak value. It was thus confirmed that the trend of decreasing time delay with successive breakdowns is not a measurement artifact. Also, it may be added that the true delay after ten break downs may have been limited by the oscilloscope resolution (1.5 ns). It was also noticed that subsequent to the significant decrease of the time delay between the luminosity and cur rent avalanche inception, any breakdown would lead to sim ilar luminosity waveforms in terms of rise times, fall times, and delays (again within the resolution of the oscilloscope). (4) A pronounced second peak, lower in magnitude (about a third of the initial peak) and with a decay many orders longer than the initial luminosity (several microse conds) is observed (Fig. 2). This activity is similar to an "afterglow" since it occurs after the cessation of the break down current pulse train. It was observed that the duration of afterglow decreased significantly with successive break downs. It is shown in the following that these experimental re sults can be qualitatively accounted for, by elucidating the role of the trapped charges within the discrete levels of the forbidden gap. As a consequence, an explanation based on the energy band-gap model is proposed, to account for the observed trap-dominated breakdown processes. IU, DISCUSSION Figure 6 shows a conceptual energy-band diagram of a wide band-gap insulator between two electrodesY The var ious localized levels shown in the forbidden gap are due to both the bulk and surface defects in the atomic structure of alumina. The nature and distribution of these levels in the bulk have been investigated for Czochralski-grown sap phire. 10 The surface of alumina is an abrupt discontinuity of the periodicity of the crystal structure and abounds in var ious proportions of point, line, and 3D defects.11 In view of this, the vacuum-dielectric interface is the largest solid-state defect structure within the bridged vacuum gap and it is logical that flashover occurs at the interface rather than in R. G. Bommakanti and T. S. Sudarshan 2094 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionso METAL eledrode INSULATOR !C diffusion trapping rafter elaclronic injection /, r belore elaclronic injection EFI ! FIG. 6. Energy-band diagram showing the localized levels, trapping, and associated space-charge effects (see Ref. 9). the bulk, at a field which is a fraction of the bulk breakdown strength of alumina. The distributed levels for holes and electrons have been identified as trapping or recombination states. 12 The occupancy of the recombination states is dictat ed by the kinetics of recombination and that of the trapping states by charge injection and trapping and detrapping mechanisms, for insulators with long dielectric relaxation times. The nature of the charge injection and transport pro~ cesses in insulators has been studied extensively and is briefly reviewed in the fonowing to emphasize the validity of the energy~band model and the consequent dominance of the trap-dictated processes operative during the surface flashover process. On application of an electric field, the con duction mechanism in alumina has been identified to be pri marily electronic since the transport properties, as indicated by the mobility lifetime product of the electrons, is many times that of the holes. 10 Electrons are injected into the insu lator conduction band from the cathode by Schottky barrier reduction at moderately low fields and by Fowler-Nord heim tunneling at high fields. 13 Nonmetallic injection asso ciated with prebreakdown activity has also been reported. 14 Irrespective of the mode of injection, the injected electrons are quickly trapped after a few scatterings because of the short mean free path and large concentration of localized states. The average distance It for electron trapping by emp ty traps, of capture cross section ap and density Nt is given by [l/(Nta,)]. In insulators I, is sman (-0.5-2 nm) and smaller than Ii' the mean distance for impact ionization. The magnitude of the trapped charge is a complex function of field, temperature, trap density, insulator thickness, elec trode material, and duration of charge injection.15 As a re sult of the electron trapping, a macroscopic negative space charge is generated near the cathode which tends to reduce further electron injection from the cathode. Low-density re gions are then formed by mechanisms which are still under investigation.9•16 This permits a large electron mean free 2095 J. Appl. Phys., Vol. 66, No.5, 1 September 1989 path (5-10 nm) for subsequently injected electrons to gain energy from the field and cause impact ionization leading to a local avalanche. Bulk breakdown in alumina has been proposed to be due to electron trapping succeeded by impact ionization-induced current runaway.15 Spectroscopic studies of prebreakdown luminescence for Lucalox alumina bridged vacuum gaps have identified the emission as a result of transitions from P+ defect centers. 17 Based on these observations, it has been proposed for de stresses that the electrons in the subsurface layer produced by the field emptying of traps cause positive charge generation by internal ionization processes leading to breakdown.4 It can thus be concluded that experimental evi dence exists for analyzing the surface flashover as an elec tronic cascade involving localized levels in the forbidden gap. More significantly, a11 the models mentioned above re quire the trapping of electrons as a precursor, to produce a space charge, for further impact ionization processes. The method of determination of trap levels and densities are well developed for insulators in photoconductor applications us~ iog the energy-band model. IB,19 The interpretation and anal ysis ofthese concepts for determining occupational statistics for complex polycrystalline insulators has been treated elo quently, thereby justifying the use of the energy-band model and the associated nonequilibrium Fermi levels even for dif ferent classes of traps and trap densities.20 Consistent with the above observations, the modifications in the temporal profiles of breakdown luminosity, observed in our study, are examined with emphasis on the electron trapping and de trapping processes in the localized states within the forbid den gap. A. On the observed luminosity rise times, decay times, afterglow, and the time deiay The observed luminosity profile (Fig. 2) has three dis tinct portions-a rising edge, a falling edge, and a relatively longer afterglow. Physically, the rising edge corresponds to the time during which there is a net increase in the radiative recombination. This can be brought about in this short time scale by an increase in the population densities in the higher energy levels, due to the excitation processes. The decaying edge corresponds to the regime in which there is a net de crease in the radiative recombination. This regime also cor responds to the system relaxation towards its equilibrium position. Both these regimes occur in time scales on the order of a few nanoseconds in contrast to the ensuing afterglow which lasts for a few microseconds. As will be shown later, the afterglow is a phenomenon dominated by the shallow trapped carriers. 1. Rise times While the current waveform profile is dictated primarily by the discharge arc parameters, the luminosity signal is an indicator of radiative recombination processes. It is known that the chief effect of the shallow trapping states is to cause the rise time afthe observed signal, in response to an applied excitation, to exceed the free lifetime of the carriers. 18 Dur ing recombination, the excited electrons are captured by the R. G. Bommakanti and T. S. Sudarshan 2095 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionstrap states thus clogging recombination processes. It has been shown that'S (1) where robs is the observed luminosity rise time, 7 is the life time in the absence of the traps, flte is the number density of trapped carriers, and nf is the number density of free carri ers. The observed rise time is therefore lengthened by the ratio of the trapped to free carriers and is a multiple of 7. It is emphasized that ntr and nf correspond to the carrier statis tics in the particular breakdown under consideration as the trapping and detrapping processes are dynamic and compete with each other. Also, it is to be expected that in insulators with long relaxation times, the trapping times are signifi cantly longer than the detrapping times. Hence, ntr and nf represent a snapshot of tile carrier statistics for each break down event. The theoretical determination of the free lifetime 7 is intractable in this situation as the trap state densities and occupation densities are not available for commercial poly crystalline alumina. A decrease in the observed rise time in the data reported here, however, indicates a decrease of the ratio of trapped to free carriers with successive breakdowns (Fig. 4). This implies either a decreasing density of trapped carriers or an increasing density offrec carriers with succes sive breakdowns. As discussed later, it is more likely that the number of trapped carriers decrease, as the available traps get filled, with successive breakdowns. The slowly rising portion in Fig. 5(a) could be entirely due to the trapping of electrons in the trap states at low free-carrier concentrations. As the number offree carriers increases (due to the decrease in the number of carriers trapped as compared to the pre vious breakdown), the exponential rise dominates with successive breakdowns. This is indicated by the observation that from the fifth application, the luminosity profile does not have the slowly varying portion at all on the rising edge. Also, consistent with the reduction in the ratio of trapped to free carriers with successive breakdown, the rise time on the seventh voltage application is 1.4 ns. It may be noted that subsequent decreases in the rise times may not have been perceivable as the measurement would be limited by the us able rise time of the oscilloscope. Radiation-induced conductivity experiments (RIC) have identified trap dominated conduction mechanisms in sapphire. 10 An electron lifetime of about 2 ns was observed subsequent to irradiation with x rays in experiments to deter mine electron transport properties. An increase in the ob served lifetime was found with increased electron trapping. The observed electron mobility was also found to be trap dominated. The induced conductivity, which is dominated by the electron mobility-lifetime product had two por tions-a prompt portion and a slow portion. It was addition ally found that the slow portion was strongly temperature dependent, disappearing at a low temperature of 110 K and below, clearly indicating the role of the shallow trapping states. The prompt portion, on the other hand, dependend on temperature only weakly and was not trap dominated. It is believed that the influence of shallow trapping states re ported in this paper and in the above RIC experiments is 2096 J. Appl. Phys., Vol. 66, No.5, 1 September i 989 Ul ~, o ; 345 6 7 8 Time (ns) FIG. 7. Modifications in the exponential component of the luminosity de cay profile with successive breakdowns. Slopes on the second, fourth, and seventh breakdowns are --0.09, -·0.17, and -0.29, respectively. similar, for example, to that of modulating the mobility of the electrons by the presence of the traps. 2.Decay times The effect of the shallow trap states on the observed decay time is phenomenologically similar to that of the rise time. The decay portion of the luminosity profile com mences on a net decrease in the rate of recombination. The intensity of emission is proportional to the density N of excit ed electrons available for recombination, 21 dN L=-= -aN. d! (2) Therefore, L = K exp( -at), where a is the rate con~ stant and K the constant deciding the initial condition. The decay of luminosity is therefore exponential. As can be seen in Fig. 7, the observed decay profile is exponential for the first few nanoseconds. It is also to be noted that the exponent a varies from 0.09 in the second voltage application to 0.29 in the seventh voltage application. This implies that the ob served decay time of the carriers (lie of the peak value) has changed from 11.11 to 3.45 ns between the second and se venth breakdown. The observed decay times are consider ably longer than the free-electron lifetime, and are again lengthened by the ratio of trapped to free carriers. A ratio of 106 has been reported previously for Vidicon-type television pickup tubes.18 With successive breakdowns, there is a de crease in the decay times. The decrease is again proportional to the ratio of trapped to free carriers similar to the case of rise times. It is interesting to note that the slow portion on both the rise and decay profiles observed on the second vol tage apPlication decreased significantly by the tenth voltage application. Hence, with repeated breakdowns, it can be conduded that there is a decreasing ratio of trapped to free carriers consistent with the rise-time data. R. G, Bommakanti and T. S. Sudarshan 2096 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions3. Afterglow The afterglow phenomenon is essentially due to ther mal-trap-rei.ease processes from the shallow traps resulting in a long tail oflow amplitude. 18 At room temperature, these shallow trapped carriers are released relatively slowly lead ing to an afterglow. An afterglow lasting up to 6 ps after the cessation of the current signal has been observed here with a pronounced peak as shown in the Fig. 2. This afterglow gen erally decreases in duration with successive breakdowns. As seen from Fig. 6, the trapped carriers induce a space charge near the cathode, modifying the field distribution. It can be noticed that after electron injection, the trap states are re moved farther away from the bottom of the conduction band. Stated differently, the electron Fermi level moves towards the bottom of the conduction band with successive breakdowns, thus relegating the previous shallow traps to recombination centers. It can thus be concluded that, with successive breakdowns, there is a decrease in the density of carriers available for slow thermal release at room tempera ture. The modification of the trap states is believed to be the primary cause for the reduction in trapped carriers with successive breakdowns as observed from 7 nbs for both the rise and decay profiles as the rise and decay times are affect ed only by shallow traps and not by deeper traps. In other words, in the absence of the shallow traps within the forbid den gap, no changes would have been observed from one breakdown to another, as the shallow trapping effects, Le., rise and decay time lengthening, would not vary with succes sive breakdowns. 4. TIme delay The time delay between the inception of photonic and electronic avalanches cannot be explained comprehensively without invoking the specifics of carrier and photon genera tion, which are, as already noted, beyond the scope of the present investigations. An Auger-type process, which gener ates hot electrons, has been proposed to be likely in wide band-gap insulators2 as explained in the foHowing. Electrons with energy greater than 3Eg (Eg is the band gap) partici pate in a transition from a ground state in the valence band to an ionized state in the conduction band leading to carrier recombination (band to band), radiative or nonradiative. Trap to band recombination could occur similarly. Either type of recombination (trap to band or band to band), would lead to emission of photons andlor electrons through an Au ger-type process. Radiative transitions are relatively more efficient compared to nonradiative transitions in crystalline insulators.8,21 Figure 8 shows the SEM of the alumina sam ples studied here, from which the polycrystalline structure is evident. Hence the emitted photons could cause hot-electron generation by an Auger-type energy transfer.2 The hot-elec tron generation has been proposed to result in the creation of low-density regions favoring impact ionization leading to an electronic avalanche.9 Temporally, this would imply that the recombination radiation precedes hot-electron genera tion, which is the precursor for the impact ionization-in duced breakdown. Stated differently, the photonic ava lanche inception precedes the electronic avalanche inception in time, in this model of hot-electron generation. 2097 J. AppJ. Phys., Vol. 66, No.5, 1 September 1989 FIG. B. Scanning electron micrograph of FA 7258-JB1 alumina sample. Figure 3(11.) which shows a delay of 6 ns in the second voltage application between the photonic and electronic ava lanche inceptions is supportive of this model for surface flashover. With successive breakdowns, however, this delay decreases from 6 to about 1 ns. Figure 9 shows the curves of time-integrated luminosity for both the second and the tenth breakdowns. Since, L = (dN Idt), the integrated luminosity should correspond to N, the number of excited species in volved in the radiative transitions. This should again be pro portional to the number of photons generated. It can be seen from Fig. 9 that in the second breakdown, there is a very slow rise in the i.ntegrated Hght during the first few nanose conds compared to the tenth breakdown, where there is a sharp rise in the integrated light during the first few nanose conds. Based on this, it appears as though there is a critical t: '---"" ~. if) () c ~~ ~, "_"J bOU :l[lO 400 Joe 700 1 DO () f l3-BfHHl 1 " h~·r·l8kd(·Jwr"', breakdmvn Time (ns) FIG. 9, Time-integrated luminosity for the second and tenth breakdown. R. G, Bommakanti and T. S. Sudarshan 2097 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsvalue of the integral required for the generation of hot elec trons leading to electron avalanche processes. In the tenth breakdown, this number was reached earlier whereas the number was reached slowly in the second breakdown, thus resulting in a significantly large time delay between the in ception of photonic and electronic avalanches. A possible explanation for the increasing luminosity with successive breakdowns as seen in Fig. 9 is proposed here. The hot-electron generation is dependent on the total energy imparted to the cold electron,15 which is a strong function of the photon energies and the number of photons generated. It is possible that the transitions in the earlier breakdowns being trap to band, require a greater number of photons (since they are oflower energy) to reach the critical value of the integrated luminosity. Since the traps initially empty would fill up after several breakdowns, band to band transitions are more probable in the later breakdowns. These band-to-band transitions require a lesser number of photons to reach the same critical value (since they are of higher energy). It has been noted that the carrier lifetimes decrease with increasing energy of transition. 2! As observed from the rise time and decay time data, there is a definite decrease in the lifetime of the carriers with successive breakdowns as pointed out earlier. Thus an increase in the energy of trans i tions or an increase in the number of such transitions with successive breakdowns would result in an increased wave length-integrated light activity with repeated breakdowns. This would be consonant with the trends shown in Fig. 9. This also lends additional support to the model ofhot-elec tron generation as explained earlier. While spectroscopic in vestigations would confirm this quantitatively, the prelimi nary evidence presented here qualitatively supports the Auger-type transfer of energy causative of hot-electron gen eration. The above discussion addresses the observed changes in the luminosity profiles and the time delays between the pho tonic and electronic avalanches. It has already been noted that the discharge current rise time increases with successive breakdowns (Fig. 4). The current rise time is determined by the (L / R) ofthe breakdown are, where L is the inductance of the arc and R is the resistance of the arc. An increase of the rise time of the arc with successive breakdowns is thus indi cative of an increasing L and/or a decreasing R. It was ob served that flashover would occur at a voltage as low as 15 k V after ten breakdowns for the samples tested here. This apparent "deconditioning' of the insulator is consistent with the deterioration of the resistance offered by the insulator surface with successive breakdowns. The trends observed in Fig. 4 in the luminosity and cur rent profiles and the luminosity-current delays with succes sive breakdowns seem to suggest that the observed phenom ena are independent of the precise location of the breakdown around the sample periphery. It is not possible to ascertain in this experiment whether this is indeed the case, since the observed luminosity is spatially integrated and is detected whenever the breakdown occurs within the solid angle of the phototube. Since the observed trends in Figs. 4 and 9 reveal a regular pattern with successive breakdowns, it is speculated that the observations are not influenced by the spatialloca- 2098 J. Appl. Phys., Vol. 66, No.5, 1 September 1989 tion of the breakdown arc. An explanation is offered here, if it is indeed true that the observed trends are independent of the arc location. It is possible that the observed luminosity before the current inception is a spatially uniform glow in the subsurface layers of the insulator leading to a local break down at the surface at a statistically random point. This would be coherent with the results reported for Lucalox alu mina in which a uniform glow in the sample (visible to the naked eye) would precede a local breakdown.4 This would also explain the fact that the phototube detected luminosity trends irrespective of the specific arc location. It may be noted that such a possibility is supportive ofthe trap-domi nated surface flashover model proposed earlier.2,3,4 In summary, it may be pointed out that the analysis offered for the observed modifications associated with the luminosity profiles was based on a band-gap model assuming that the localized levels within the forbidden gap do not change with repetitive breakdowns due to the x-ray, uv, and electron bombardment of the sample surface. It is entirely possible that new defects are, indeed, created with each breakdown event and that the created defects play an impor tant role in the carrier generation and multiplication pro cesses. It is worthwhile pointing out that both cases (preex isting or created defect structure) would provide the same trends observed here and hence cannot be separated out in our experiment. 22 It is known that an Auger-type process is inherently inefficient compared to the competing mechanism of impact ionization for producing an electronic cascade. Also, for in sulators with several localized levels in the forbidden gap, electron-phonon interactions cannot be ignored. Dielectric polarization, due to the application of electric stress, leads to electrostriction and changes in electron-phonon interac tions. In imperfect insulators, this could alter the trapping! release equilibrium at the defect sites. 3 The observed changes in the luminosity profile reported here could also be inter preted as a manifestation of the above phenomena, since the resultant trapped charge varies with successive voltage ap plications. Nevertheless, the results presented here highlight the significance of the carrier trapping mechanisms in the surface flashover process and of conducting experiments to identify the trapping/ detrapping mechanisms in order to aid the formulation of a comprehensive surface flashover model. IV. CONCLUSIONS The temporally resolved breakdown luminosity and current measurements offer new insights into the physics of the pulsed surface flashover process. It is observed that not only are the temporal profiles ofluminosity and current vast ly different, but that both change with successive break downs. The luminosity profile has a sharp pulse which has no counterpart in the current waveform. The rise times and decay times of the pulse change dynamically in the first few breakdowns. A significant afterglow activity is observed after the cessation of the breakdown current, which de creases in duration and magnitude with successive break downs. Also, the luminosity pulse precedes the current pulse by a few nanoseconds in the first few breakdowns, the delay R. G. Bommakanti and T. S. Sudarshan 2098 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsdecreasing with successive breakdowns. All the above obser vations have been explained satisfactorily by invoking the energy-band model at the dielectric vacuum interface, with out making any assumptions about prebreakdown processes regarding gas desorption or secondary electron emission mechani.sms. The analysis of the experimental results pre sented here emphasizes the dominant role of one carrier trapping processes in the surface flashover of a vacuum gap bridged by wide band-gap, low mobility insulators. The rise times, decay times, and the delay between the onset of the luminosity and the discharge currents are explained satisfac torily by invoking the modifications in the ratio of the trapped to free carriers, which is modified during successive breakdowns. Since most practical insulators are polycrystalline or amorphous in nature, their performance in vacuum-based applications would be strongly dependent on the band struc ture as dictated by the defects at the dielectric vacuum inter face. The findings of this work suggest that additional ex periments are required to characterize the dominant parameters in the different but interdependent processes of carrier generation, trapping, multiplication, and annihila tion, in order to formulate a quantitative model for the pulsed surface flashover process i.n terms of the energy-band model. ACKNOWLEDGMENTS This work was sponsored by SDIO/IST and managed by ONR. The authors wish to thank Dr. C. Le Gressus, Commissariat a l' Energie Atomique, France, and Dr. H. C. 2099 J. AppL Phys., Vol. 66, No.5, 1 September 1989 Miller, Principal Physicist, GE Neutron Devices Dept., Lar go, Florida for their careful scrutiny of the results and for the suggestions offered. 1 H. C. Miller, in XIII International Symposium all Discharges alld Electri cailnsulation in Vacuum, edited by J. M. Buzzi and A. Septier (Les Edi tions de Physique, Paris, France, 1988), p, 27. 2J. P. Vigoroux, O. Lee-Deacon, C. 1£ Gressus, IEEE Trans. Electr, Insul. EI-lS,287 (1983). 3e. Le Gressus, in XIII International Symposium on Discharges and Elec ericallnsulation in Vacuum, edited by J. M. Buzzi and A. Septier (Les Editions de Physique, Paris, France, 1988), p. 57. 'N. C. Jaitly and T. S. Sudarshan, J. App!. Phys. 64, 3411 (1988). 5S, P. Bugaev, A. M. Isko!'dskii, and G. A. Mesyats, Sov. Phys.-Tech. Phys., 12, 1358 (1968). 6J. D. Cross &\'ld K. D. Srivastava, Appl. Phys. Lett. 21, 549 (1972). 7A. A. Avidenko and M. D. Malev. SOy. Phys.-Tech. Phys. 24, 581 (1979). ·P. H, G1eichauf. J. Appl. Phys 22,535 (1951). oK. C. Kao, J. Appl. Phys. 55, 752 (1984). lOR. C. Hughes, Phy. Rev. B 19,5318 (1979). i IW. Hayes and A. M. Stoneham, Defects alld Defect Processes in Non-me tallic Solids (Wiley-Interscience, New York, 1985). i2A. Rose, Pmc. IRE 43, 1850 (1955). i3N. C. Jaitly and T. S. Sudarsnan, IEEE Trans. Electr. IlIsu\. EI-23, 231 (1988). 14R. V. Latham, Vacuum 32, 137 (1982). 15N, Klein and M. Albert, J. Appl. Phys. 53, 5840 (1982). 16J. J. O'Dwyer, in Conference on Electrical Insulation and Dielectric Phe- nomena, Amherst, MA, 1982, IEEE Cat. No. 82CH1773-1. I7N. C. Jaitly and T. S. SlIdarshan, J. Appl. Phys. 60, 3711 (l986). !gA. Rose, Phys. Rev. 91,322 (1955). !9R. C. Bube, Photoconductivity of Solids (Wiley, New York, 1960). 2QJ. G. Simmons and G. W. Taylor, Phys. Rev. B 4,502 (1971). llH. W. Leverenz, An Introductioll to LuminescellceofSolids (Wiley, New York, 1950). nC. Le Gressus (private communication). Fl. G. Bommakanti and T. S. Sudarshan 2099 Downloaded 07 Sep 2013 to 131.211.208.19. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

1.1139774.pdf

Null input current SQUID magnetometer for the measurement of the transition temperature of highT c superconducting samples S. Barbanera, M. G. Castellano, and V. Foglietti Citation: Review of Scientific Instruments 59, 1031 (1988); doi: 10.1063/1.1139774 View online: http://dx.doi.org/10.1063/1.1139774 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/59/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Disposable sample holder for high temperature measurements in MPMS superconducting quantum interference device magnetometers Rev. Sci. Instrum. 78, 046101 (2007); 10.1063/1.2720722 A sample holder design for high temperature measurements in superconducting quantum interference device magnetometers Rev. Sci. Instrum. 76, 083910 (2005); 10.1063/1.2006267 Monolithic low-transition-temperature superconducting magnetometers for high resolution imaging magnetic fields of room temperature samples Appl. Phys. Lett. 82, 3487 (2003); 10.1063/1.1572968 Integrated hightransition temperature magnetometer with only two superconducting layers Appl. Phys. Lett. 63, 559 (1993); 10.1063/1.110781 A low field SQUID magnetometer system for magnetic characterization of highT c superconducting samples Rev. Sci. Instrum. 62, 2271 (1991); 10.1063/1.1142348 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 11:12:14Null input current SQUID magnetometer for the measurement of the transition temperature of high~ Tc superconducting samples s. 8arbanera Istituta di Elettronica della Stato Solido del C.N.R., Via Cineto Romano 42,00156 Rama, Italy M. G. Casteliano Istituto di Fisica della Spazia I nterplanetario del C. N. R., P. O. Box 2 7, 00044 Frascati, Italy V. FoglieUi Istitutodi Elettronica della Stato Solido del CN.R., Via Cineto Romano 42, 00156Roma, Italy (Received 1 February 1988; accepted for publication 28 March 1988) In this article we describe a SQUID-based system used to measure the superconducting transition temperature in the range 4.2-300 K. The apparatus has been tested using a high-critical temperature single crystal ofYBa2 CU3 07 _ x (= 10 -I mm3 volume). The system is based on mutual inductance variations measurements, performed in a low-frequency ac magnetic field of 2 X 10-5 T. A feedback loop is closed on the input circuit in order to null out the current flowing in the pickup coil. This is achieved using an electronic scheme which does not involve any modification of the commercial SQUID electronics. Our experiment is performed using a copper wire pickup coil, but the scheme can also be used with a superconducting input circuit, thus allowing measurements in a dc regime. The obtained sensitivity is 5 X 10-5lvHz. This figure can be further improved by optimizing the circuit parameters. INTRODUCTION In order to investigate the magnetic properties of the new, high-criticaI-temperature superconducting materials, it is necessary to have an apparatus both sensitive and working at about 100 K. For the first requirement, the use of supercon ducting quantum interference devices (SQUIDs) is almost mandatory, Using rfSQUIDs with a superconducting trans former as the input circuit, the most sensitive commercial susceptometers can measure the magnetic susceptibility of tiny samples even at zero frequency. On the other hand, the classical mutual inductance method, though simpler from the experimental viewpoint, requires the use of alternating magnetic fields, thus ruling out the possibility of performing de measurements. Building in the laboratory a dc-SQUID based superconducting susceptometer can pose some con structive problems because of the requirement of varying the sample temperature up to 100 K while keeping the SQUID and the input superconducting transformer at liquid-helium temperature and maintaining a high coupling coefficient be tween the sample and the primary of the transformer. To test the YBa2 CU3 07 _ x monocrystal samples grown at the lsH tuto di Elettronica della Stato Solidol (CNR, Rome), we developed an apparatus for measuring mutual inductance changes by means of a SQUID and a nonsuperconducting input circuit, using a feedback system to null the input cur rent. The importance of this latest point has been recently brought into evidence,2 especially when dealing with the large susceptibility changes at the superconducting transi tion point. Our apparatus has the further advantage that the current nulling is obtained with a feedback system which does not require any modification of the SQUID electronics, which was a drawback of the system described in Ref. 2. In our scheme, only the SQUID is held at liquid-helium tem-perature, while both the sample and the external coils of the input circuit follow the temperature variations. The nonsu perconducting input circuit imposes the limitation of using only alternating magnetic fields; nevertheless the feedback scheme can be applied to a superconducting input circuit, which responds also to static flux variations, allowing true Meissner effect measurements. I. EXPERIMENTAL SYSTEM In our experimental configuration we use a commercial SHE rfSQUID to measure the changes in the mutual induc tance M rn between the excitation and the pickup coil of the input circuit, due to the superconducting transition of the sample inserted in the pickup coil. The experimental setup is shown in Fig. 1. The bottom end of the insert consists of a vacuum chamber which can be easily dipped into a 2-in. neck storage liquid-helium Dewar. The top end of the insert con tains a vacuum gauge and a valve used to anow for exchange gas into the experimental chamber as well as to pump it out. Temperature is varied using a heater to warm up the sample held in vacuum and exchange gas to cool it down. The heat er, the silicon diode thermometer, and the circuitry are glued onto a piece of "coil foil,,,3 using thermal GE 7031 varnish, in order to provide thermal contact and stability. The piece of "coil foil" is thermally detached from the liquid-helium bath and its use ensures minimal temperature gradients over the sample holder. The circuit is made of various coils. The excitation coil (E) consists of 40 turns of copper wire, 24 mm inner diameter, The pickup coil (P), located concentri cally with E is made of 50 turns of copper wire wound on a 5- mm-diam form. The sample is put inside the coil P, in corre spondence with the middle section and held in place by 1031 Rev. Sci.lnstrum. 59 (7), July 1988 0034-6148/83/071031-04$Oi .30 @ 1988 American Institute of Physics 1031 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 11:12:141 em ] < vacuurn SL: ~~~ I r flange I vacuum _J feed through -\wt=;eater s -p "coil foil" -----sample FIG. 1. Bottom end of the experimental insert. P is the pickup coil, E is the excitation coil, F is the feedback coil, and S is the coil connected to the slave generator (see text); R is the Evanohm wire resistance and TR is the match ing transformer. T is the silicon diode thermometer. The sample is not drawn to scale. means ofCryocon, which also provides thermal contact. The feedback coil (F) is made of a few turns of copper wire cou pled to a larger coil (S) fed by an external generator. Coil F can allocate a reference sample of known superconducting transition temperature. The circuit terminals exit the vacu um chamber via a feedthrough connector, reaching the SQUID input by means of a superconducting matching transformer T, with nominal transforming ratio 10: 1 and the primary consisting of ISO turns Nb wire wound on a 6-mm diam form. For practical purposes we introduced a resis tance (R) of 1.5 0., made of Evanohm wire, serially connect ed to the primary of the transformer and located inside the liquid-helium bath. Ii. CIRCUIT OPERATION To operate the system as a null detector for the total magnetic flux in the input circuit, a flux controlled by a feed back loop is supplied to F. In this way the apparatus becomes sensitive only to unbalances of the two fluxes due to mutual inductance changes. In particular, this configuration is to tany insensitive to resistance variations of the input circuit due to temperature changes, and avoids eddy currents and the resulting image fields. Figure 2 shows the schematic of the circuit. An ac current im is supplied to coil E through a 1- kn resistance using a Hewlett-Packard model 3325A syn thesizer, producing a typical magnetic field of :::::2 X 10-5 T. This results in a magnetic flux coupled into the SQUID and proportional to the mutual inductance M m between E and P. Similarly, a magnetic flux at the same frequency as the exci tation fiux is coupled to the circuit by means of the mutual inductance Msl between F and S, fed by an external gener ator through resistance RF• The amplitude ofthis generator 1032 Rev. Sci.lnstrum., Vol. 59, No.7, July 1988 is controlled by the feedback loop to ensure the nulling of the two fluxes. The two generators are phase locked to each oth er; for simplicity we will call "master" the excitation gener ator, "slave" the other. In the following, we will also call ¢Jm the magnetic flux in the SQUID due to the excitation gener ator, and ifJsl that due to the slave generator. Initially, one works with the master alone and sets its amplitude to a con venient value. Then, with the master turned off, one sets the slave amplitude in such a way as to achieve roughly the same SQUID output as in the previous step. With the two genera tors turned on at the same time, the relative phase is carefully adjusted in order to null out the two resulting fluxes ¢Jm and ¢JsJ in the SQUID. The feedback loop can now be closed: the SQUID output is passed through a lock-in amplifier refer enced at the working frequency (70-180 Hz in our case) and the lock-in output is connected to the amplitude-modulation input of the slave generator. As far as ¢Jm and ¢lsi are bal anced, the lock-in output and then the feedback signal are zero. Temperature is varied. At the point of the supercon ducting transition, due to flux expulsion, there is a change in 1'4 m 0 The resulting unbalance between the fluxes in the input circuit causes a signal to be detected by the lock in, and this is used to adjust the slave amplitude to a new equilibrium val ue. The output signal of the lock-in amplifier is also the out put of the whole feedback system. The transfer function of the system in closed-loop configuration is determined only by the reverse transfer function due to the high value of the open-loop gain Vout/l1Mm = RFimlaM s1' where Vout is the lock-in amplifier output; 11M m is the mutu al inductance variation; a is the amplitude-modulation coef ficient of the slave generator. The value of 1j,ls1 can be deter mined, for example, at open loop, turning off the master generator, measuring the amplitude of the slave generator and the SQUID output; the same holds for Mm. m. SENSITIVITY The device sensitivity can be evaluated by recalling that the system is sensitive to changes of the magnetic flux in the pickup coil and that the minimum detectable variation is master ref slave osc. osc. :j ! ! RF ~ : I FIG. 2. Schematic of the circuit. High-Tc superconductor from temperature controtier amp]. mod, in -, I I U X -Y r recorder I ~ ou t ,..------, --'--Ioc k--in ref amplifier 1032 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 11:12:14related to the total flux noise at the SQUID input t:Ptot. This is the result oftwo contributions, namely the SQUID intrinsic noiset:Pn "",2 X 1O-4¢o/",Hz, t:Po being the flux quantum, and the Johnson current noise of the input circuit, which can be kept reasonably low by increasing either the resistance R, or the superconducting transformer primary inductance. The most convenient choice must be done on the basis of signal to-noise ratio optimization. In our case taking out the Evan ohm resistance in the input circuit would result in SQUID operating performance degradation. Even if we did not make special efforts to optimize the matching conditions, we were able to achieve a value for t:Ptot ::::::;4 X 1O-4¢o/,{Hz. The minimum detectable change in mutual inductance is With the experimental value of t:Pm ::::::: 10 t:Po, in a I-Hz band width we have a sensitivity of = 4 X 10-5. The sensitivity of the system can be increased by reducing the bandwidth, but a reasonable tradeoff must be achieved between system perfor mances and the time required for the measurement. IV. EXPERIMENTAL RESULTS A measurement was carried on without any sample and the output of the system in the closed-loop configuration recorded as a function of temperature. In this case we did not take care of varying the temperature slowly. As a result the curve was fiat from 300 down to 4,2 K, thus proving that temperature dependencies of l'ifm and Msi' if any, do not affect the system response. Furthermore, we checked the appropriate response of the system by inserting a tiny piece of lead (similarly shaped to the YBa2 Cu] 07 _ x sample) in the pickup coil and ob serving a sharp transition at 7.7 ± 0.5 K. Figure 3 shows the system output as a function of tem perature obtained for a YBa2 CU3 07 __ x single-crystal sam ple, sized 1 mm2 area and 0.1 mm thickness. We take as the reference level the high-temperature portion of the curve, where the system has been initially balanced. Any deviation from this level corresponds to mutual inductance variations and, therefore, to susceptibility changes. The sensitivity for these measurements was 5 X 10 -5 in a bandwidth of about 1 Hz, in agreement with the predicted value mentioned above. This means that we do not have any interference from the ambient magnetic noise, thanks also to a metglass shield around the Dewar. The temperature was varied cooling the sample by means of a small quantity of exchange gas, starting from room temperature and attaining 4.2 K in about 1 h. The temperature was monitored using a Lake Shore Cryotronics mode191C controller, with a O.l-K resolution. The superconducting transition of the YBa2 CU3 07_ x single crystal shown in Fig. 3 has an onset temperature of =92 K. The transition width is very large (ranging from =92 down to "",20K) as is not unusual in such compounds: in fact this parameter as well as the transition temperature is 1033 Rev. Sci.lnstrum., Vol. 59, No.7, July 1988 80, -40 -80 L--r---'I--r-~--~~, --.---r 20 40 60 80 T (K) FIG. 3. System output vs temperature for a YB~ eu, 07. x single crystal. The onset of the superconducting transition for this slllnple is = 92 K. The sharp step at 7.7 K is due to the superconducting transition of a test lead sample, located inside the feedback coil. The signals for Pb and YBaz Cu, 07 x are in opposite directions due to the circuit configuration (see text for details). related critically to the oxygen content in the crystal struc ture, which can be quite different depending on the anneal ing treatment the samples have undergone." Figure 3 also shows a sharp step at 7.7 ± 0.5 K. This was due to the transition of the test lead sample that we inserted in the feedback coil in order to make a rough comparison between the two superconducting transitions. On the other hand, making coils P and E exactly equal to F and Sand comparing the amplitUde of the two steps at the transition temperatures would provide a quantitative measurement of the YBa2 CU3 07 __ x susceptibility variation with respect to that oflead, which is well known. The fact that the two steps are in opposite direction is a direct consequence of the sys tem configuration. The agreement between the measured value of the transition temperature oflead and that reported in the literature for pure lead (7.23 K.) (Ref. 5), is within the experimental error and indicates that temperature gradients over the sample holder are not important. It is also possible to calculate directly the susceptibility variations of the sample from measurements of mutual in ductance changes. However, the proportionality constant between these two quantities depends critically on the de magnetizing factor, which, due to the geometry of our YBa2 Cu) 07 _ x monocrystaI (an irregular, thin platelet in our case), can be evaluated only with heavy approximations. Vo DISCUSSION The SQUID-based system described can be used to mea sure the superconducting transition temperature of very sman samples ( ::::: 10 -2 mm 3). The apparatus can operate in the temperature range 4.2-300 K. This range of operation is particularly useful for the new high-critical-temperature su per conducting materials. Furthermore, the obtained sensi tivity of 5 X 10 -51 [Hi can be further improved by optimiz ing the circuit parameters. The same feedback scheme can also be applied to superconducting input circuits, allowing de susceptibility measurements. Hlgh-To superconductor 1033 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 11:12:14ACKNOWLEDGMENTS The authors wish to thank G. Balestrino, P. Carelli, P. Paroli, and G. Paterno for helpful discussions. They are also grateful to S. D'Angelo for his technical assistance and guid ance in making the experimental insert. 1034 Rev. Sci. Instrum., Vol. 59, No.7, July 1988 'G. Balestrino, S. Barbanera, and P. Paroli, J. Cryst. Growth 85, 585 ( 1987). 2D. Dummer and W. Weyhmann, Rev. Sci. lnstrum. 58,1933 (1987). 3A. C. Anderson, G. L. Salinger, and J. C. Wheatley, Rev. Sci. lnstrum. 32, 1110 (1961). 4A. Junod, A. Bezinge, T. Graf, J. L. Jorda, J. Muner, L. Antognazza, D. Cattani, J. Cars, M. Decroux, 0. Fischer, M. Banovski, P. Genoud, L. Hoffmann, A. A. Manuel, M. Peter, E. Walker, M. Frangois, and K. Yvon, Europhys. Lett. 4, 247 (1987). 'G. Gladstone, M. A. Jensen, and J. R. Schrietfer, in Superconductivity, edited by R. D. Parks (Marcel Dekker, New York, 1969). High-Tc superconductor 1034 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 11:12:14

1.1137330.pdf

Probe for studying wall charges in electrodeless discharges at 60 Hz RuiLin Ma and F. L. Curzon Citation: Review of Scientific Instruments 54, 1767 (1983); doi: 10.1063/1.1137330 View online: http://dx.doi.org/10.1063/1.1137330 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/54/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Charging and discharging of graphene in ambient conditions studied with scanning probe microscopy Appl. Phys. Lett. 94, 233105 (2009); 10.1063/1.3149770 Hydrogenated amorphous silicon films by 60 Hz glowdischarge deposition J. Appl. Phys. 74, 668 (1993); 10.1063/1.355228 Electromagnetic Field in Electrodeless Discharge J. Appl. Phys. 42, 5460 (1971); 10.1063/1.1659964 Numerical Study of the Inductive Electrodeless Discharge J. Appl. Phys. 41, 3621 (1970); 10.1063/1.1659481 Noise Picked Up by External Probes in Electrodeless Gas Discharges J. Appl. Phys. 34, 2613 (1963); 10.1063/1.1729779 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.83.63.20 On: Thu, 27 Nov 2014 11:58:11Probe for studying wall charges in electrodeless discharges at 60 Hz Rui-Lin Maa) and F. L. Curzon Physics Department, University of British Columbia, Vancouver, British Columbia, Canada V6T 1 W5 (Received 4 April 1983; accepted for publication 1 August 1983) A probe is described, which together with a high-resistance b~ffer and ?ifferential amplifier, has been used to study wall charge effects in a 60-Hz electrodeless dIscharge In neon at a pressure of ~ 0 Torr. The device makes use of the external fields produced by the wall charges and shows that, In typical conditions, the wall charges take several milliseconds to come to an equilibrium after a breakdown has occurred. A major feature of the device is the use ofthe small external electrodes which localize the region where the wall charges tend to accumulate. PACS numbers: 52.80.Dy; 51.50. + v INTRODUCTION The study of electrodeless breakdown of gas is of interest because of its possible application in measuring environmen tal electric field. 1,2 As is well known, wall charges tend to create fields which oppose the applied field inside the bulb and playa very important role in the electrodeless discharge. The most popular method used to study the wall charges is by integrating the current pulses which flow in the outside circuit at the instant of breakdown.3,4 This method is only sensitive near the breakdown interval when the field inside the bulb and the wall charges change rapidly and hence, the external current is large enough for measurement. It cannot respond to the process of deionization, Since the deioniza tion process lasts the order of a millisecond, the current in duced in the external circuit is too small to measure and it is obscured by the noise. Recent studies of wall charge effects, relevant to plasma display panels, have been reported,5.6 Previous analyses of the electrodeless discharges tend to neglect the variation of wall charges during the process of deionization. However, to study the phenomena further in low-frequency electrodeless discharges it would be desirable to have an instrument which can indicate directly the time dependence of wall charges and the field inside the bulb, The above purpose has been achieved by means of a voltage mea suring probe outside the bulb together with a high-input re sistance buffer and a differential amplifier. The principle of this method and the practical apparatus are reported in Secs. I and II. Some new phenomena were found and a prelimi nary discussion appears in Sec, III. I. THE PRINCIPLE OF THE PROBE The principle of the probe is shown in Fig, 1. The elec trodeless discharge is produced in neon contained in a 40- mm-diam Pyrex glass bulb by applying a 60-Hz alternating field to two external spherical electrodes (6.4-mm diam). The voltages applied to the electrodes are equal and opposite, so that the equatorial plane of the bulb is a ground plane. Charges produced by a breakdown in the gas will tend to migrate towards the electrodes and are therefore deposited in well defined locations. As shown in Fig. 1 a probe P and a ground plate are placed outside the bulb. P is very small and is close to the equatorial plane of the discharge gap, so that the influences of the probe and ground plate on the discharge is small and can be neglected. We study at first the case when there are no charges on the inner surface of the glass, The charges on the upper exter nal electrode produce an electric field which causes a poten tial difference between the probe and the ground plate. The influences from the lower electrode are screened by the ground plate and can, therefore, be ignored. The potential of the probe up is, therefore, given by the following expressions: Up = US C1/2(CI + C2), (1) = AQ 12Co, (2) A = CI/(CI + Cz), (3) where Us is the electrode voltage, CI is the coupling capaci tance as shown in Fig. 1, C2 is the total capacitance between the probe and the ground, including the lead capacitance and the input capacitance of the buffer, Co is the capacitance between two external electrodes, Q is the amount of charge on the external electrode. In our experiment CI is very small (less than 0.01 pF). It is always true that Co> > CI, so that the feedback from the probe to charges on the electrode can be neglected, If Co, CI, and C2 remain constant the signal picked by the probe is only determined by Q. When there are charges ( -q) on the upper inner sur face of the bulb (Fig. 1) and they are very close to the eIec- FIG. I. Diagram illustrating the principle of the probe. u, = applied vol tage; P = probe; Q = charges on the external electrode; q = charges on the inner surface of glass. 1767 Rev. Scl.lnstrum. 54 (12), December 1983 0034-6748/83/121767-04$01.30 1767 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.83.63.20 On: Thu, 27 Nov 2014 11:58:11Cg +Q r-Q CG +(Q-q) r-(Q-q) CG +(Q-q) I-(Q-q) Cg +Q -Q FIG. 2. Equivalent circuit of the discharge gap. Cg = capacitance between the electrode and the charged inner surface; CG = capacitance between the charged inner surface and the middle plane of the bulb. trode, Up can be expressed approximately as up = A (Q -q)l2Co. (4) Since glass is a very good insulator, the leakage of -q in about a 20-ms period of the applied field can be entirely ne glected. This implies that once the charges have been placed on the glass surface they stay there until they are neutralized by charges of opposite sign coming from the discharge space. If the wall charges -q are distributed in an equipotential plane through which the most electric flux from the external electrode passes, then Fig. 2 can be used as an equivalent circuit for Fig. 1. It is a reasonable approximation in our experiment. In Fig. 2, Cg is the capacitance between the electrode and the charged inner surface of the bulb, CG is the capaci tance between charged inner surface and the middle plane of the discharge gap, Cg > > CG• It follows from the equivalent circuit (Fig. 2) that, Us = 2(ug + uG) = Usm sin OJt, (5) uG = (Q -q)lCG, (6) ug = QICg, (7) Co = 1/2[(l/C g) + (1/CG )], (8) Q = Co [us + (2qICG )], (9) Q -q = Co[us -(2qICg)], (10) FIG. 3. Sketch of experimental apparatus. P = probe; G = ground plate; E E' = external electrodes; B = bulb; F = optical fiber; d = distance of the probe from the bulb. 1768 Rev. Scl.lnstrum., Vol. 54, No. 12, December 1983 4 3 2f- Usm (400)2 Vl I 1.0 1.5 FIG. 4. Plot of up vs u,. up = output of the buffer (signal picked by the probe); u, = applied voltage. where ug and UG are potential differences across the capaci tances Cg and CG. From Eqs. (4), (6), and (10) and using Co;::::; CG 12, we get, Up =A(CGI2Co)uG,;::::;Au G, (Au,l2) -Up = AqlC g• (11 ) ( 12) Two useful results can be derived from Eqs. (11) and (12). (a) The signal up picked up by the probe directly indi cates the potential difference and, hence, the electric field inside the bulb. (b) If up and Aus 12 are fed into a differential amplifier, then the output Ud directly indicates the amount of charges placed on the upper inner surface of the bulb Ud = BAqICg, (13) where B is the gain of the differential amplifier. I AO/A L5i- 1.0- I // / / /' / / / / o Lo~~ -~ ------- .l ____ ~_ -20 -10 / / / o FIG. 5. Plot of A,IA vs Cr. Cr = capacitance connected additionally across the input ofthe buffer; A = gain of the probe defined in Eq.(3);A" = value of A when C, = O. Electrodeless discharges 1768 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.83.63.20 On: Thu, 27 Nov 2014 11:58:11~NWM r~ I I 1 1 1 I a b c d FIG. 6. Scope traces of Up (t ) and ud (t) when U = 1130 V. Up = output of the buff:'; ud = output of differen tial amplifier; time base = 10 ms/ diy. a) up(t). 50 mV /diy. absence of breakdown; b) Ud (t). 0.5 V /diy. ab senceofbreakdown; c) up(t). 50 mV / diy. two breakdowns per cycle; d) Ud (t). 0.5 V /diy. two breakdowns per cycle. II. DESCRIPTION OF THE EXPERIMENTAL APPARATUS Figure 3 is a sketch of the experimental apparatus. The details of the gas-filled bulb and external electrodes have been given above. The electrodes are connected across the secondary of a transformer which has a balanced output vol tage. The primary is fed from an autotransformer which per mits the voltage across the electrodes to be varied from 0 to 4 kV. A copper ground plate is placed on the equatorial plane of the bulb. The probe is a copper wire with a diameter of 1.5 mm and a length of 20 mm. The distance between the probe and the ground plate is 4.5 mm. The tip of the probe is nor mally 7 mm from the bulb surface. The buffer is an integrat ed circuit chip (LHOO33G) which has a high input resistance (exceeding 1010 [}) and a voltage gain near one. The differen tial amplifier is a 3626 integrated circuit with a voltage gain of B = 9.6. A variable resistance is used to adjust the input of the differential amplifier to ensure zero output when there is no charge on the inner surface of glass. All leads are screened in order to eliminate spurious signals. The noise picked by the probe is less than 3 m V which is less than 4% of the typical value of up' In order to obtain more information we also observe the flashes of light emitted from the gas when breakdowns oc cur. The light pulses are conveyed to a photomultiplier by a glass fiber bundle. More details of the system appear in Ref. 2. III. EXPERIMENTAL RESULTS Two experiments were performed to verify that the probe operated as expected. In the first the probe, signal up was measured as a function of Us when the breakdown is f= f\MNVV I 1 II I I 1 I I I I a b c d FIG. 7. Scope traces ofup(t land Ud (t) when there are 4 and 6 breakdowns in a cycle. Up = output of the buffer; Ud = output of the differential am plifier; time base = 10 ms/diy. a) up(t). 100mV /diy.4breakdownsper cycle; b) ud(t). I V /diy. 4 break downs per cycle; c) up(t). 100 mV/ diy. 6 breakdowns per cycle; d) ud(t). I V /diy. 6 breakdowns per cycle. 1769 Rev. Scl.lnstrum., Vol. 54, No. 12, December 1983 I I 1lJI-J'-.J 11 n 11'\ 1\11 WV a b c d FIG. 8. Scope traces of Ud (t) simulta neously with light pulses emitted by the gas. Time base = 5 ms/diY. a) light pulses. 2 breakdowns per cycle; b) U d (t). 2 breakdowns per cycle; c) light pulses. 4 breakdowns per cycle; d) ud(t). 4 breakdowns per cycle. absent. As Fig. 4 shows, the plot of Up vs Us is a very good straight line with a slope of A = 1.54 X 10-4• In the second experiment a variable capacitor C T was ~onnected acro~s the input of the buffer, and A -1 was determmed as a functlOn. of CT' The straight line plot (Fig. 5) is in good agreement wIth Eq. (1) with C 1 = 4.2x 10-3 pF and C2 = 27 pF. Figures 6(a) and 6(b) show the waveform of Up and u~ (the output of differential amplifier) when the breakdown IS absent. Up is a sine wave while Ud is a horizontal line (noise<4%). The typical wave form of Up and Ud when breakdowns occur are shown in Figs. 6(c) and 6(d), where Usm = 1130 V (peak value) and there are two breakdowns per cycle of the applied voltage. Since Up represents the electric field inside the, bulb when breakdown occurs, up suddenly goes to zero then tends to follow the Us waveform again. The waveform for Ud is roughly rectangular in shape and can be divided into three parts. First, a jump occurs in the moment of breakdown. This results from space charges which rapidly migrate to the inner surface of the glass. These charges build up an opposite field and stop the discharge. When breakdown occurs with up > 0, the jump is caused by the accumulation of electrons near the upper electrode 0.8 Ji(,\0~ 6q/6ql . --0 ____ o X'A ~x~ I x~x 0.6 0.4 0.2 d (mm) 10 20 30 FIG. 9. Plot of .Jq/.Jq, and A/A, as the function of d. d = distance of the probe from the bulb (see FIG. 3);.Jq = charges deliyered by one breakdown; .Jq, = yalueof.Jq whendis 7 mm;A = gain factor of the probe; A, = yalue of A when d is 7 mm. Electrodeless discharges 1769 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.83.63.20 On: Thu, 27 Nov 2014 11:58:11(q < 0). When Up < 0 ions accumulate near this electrode (q > 0). The initial jump in Ud is larger if q < 0 than if q > O. In the second part of the waveform, Ud changes continuously but with a slower rate. The duration of this stage is about 2-3 ms. The slower rate of change of Ud means that wall charges are still being altered for a rather long time after the break down. The amplitude of this variation is about 20% ~ 30% when q < 0 and 50% or more when q > O. The third portion of the Ud waveform is flat and indicates that the wall charges are constant and that the field inside the bulb follows the applied field completely. The total amount ofthe wall charges delivered by every breakdown (Llq) can be determined by the difference between two successive flat regions (Llud ).Llq = Llud Cg/(AB ).IfCg is estimated as 1.6 pF the typical value of Llq is 1.1 X 10-9 C and it is independent of the sign of the wall charge. Figure 7 shows the waveforms of Up and Ud when there are four and six breakdowns in a cycle. In Fig. 8 the light pulses are shown simultaneously with Ud. When the probe is at a distance (d, see Fig. 3) from the bulb the corresponding values of A and Llq can be written as A (d ) and..:::1q(d ). With this notation, one would expectLlq(d ) to be constant irrespective of distance d. As d is increased from 7 to 30 mm, A (d) decreases by a factor of three, however, Llq(d) decreases by 20%. If it is noted that spurious signals greatly affect the accuracy of measurement in the case of small values of A, then this result shows that the probe does indeed give a good measure of Llq, the charges deposited by 1770 Rev. Scl.lnstrum., Vol. 54, No. 12, December 1983 successive breakdowns. Figure 9 shows a plot of Llq(d) and A(d). The above results indicate that the voltage probe and differential detection system provides a convenient method of studying the properties of wall charges generated in elec trodeless discharges. The method is particularly beneficial if the wall charges are localized by means of small external electrodes. Finally, since the probe is remote from the charged areas, it has very little influence on the properties of the quantities being measured. ACKNOWLEDGMENTS This work was financed by a grant from the National Science and Engineering Research Council of Canada. The authors are indebted to A. Cheuck, R. Keeler, and R. Mor gan for their assistance in devising some of the apparatus. "iCultural exchange visitor from South China Institute of Technology. Guangzhou. China. IF. L. Curzon. D. E. Friedmann. and M. Feeley. J. App\. Phys. 54. 86 (1983). 2D. E. Friedmann. F. L. Curzon. M. Feeley. J. F. Young. and G. Auchin leck. Rev. Sci. Instrum. 53.1273 (1982). 'w. L. Harries and A. von Engel. Proc. R. Soc. London. Ser. B 64.951 (1951); and A 222. 491 (1954). 4J. M. El-Bakkal and L. B. Loeb. J. App\. Phys. 33,1567 (1962). 'E. Kindel and R. Arndt. Beitr. Plasmaphys. 21. 411 (1981). 6E. Kindel and R. Arndt, 13th International Conference on Phenomena in Ionized Gases. Part I (Physics Society German Democratic RepUblic. Leipzig. Berlin. 1977). p. 219. Electrodeless discharges 1770 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 128.83.63.20 On: Thu, 27 Nov 2014 11:58:11

1.94224.pdf

Propagation loss of the acoustic pseudosurface wave on (ZXt)45° GaAs M. R. Melloch and R. S. Wagers Citation: Applied Physics Letters 43, 1008 (1983); doi: 10.1063/1.94224 View online: http://dx.doi.org/10.1063/1.94224 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/43/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in X-ray diffraction study of surface acoustic waves and pseudo-surface acoustic waves propagation in La3Ga5.5Ta0.5O14 crystal J. Appl. Phys. 113, 144909 (2013); 10.1063/1.4801527 Surface acoustic wave properties of ZnO films on {001}cut 110propagating GaAs substrates J. Appl. Phys. 75, 7299 (1994); 10.1063/1.356639 Trap emission rates in GaAs in the presence of surface acoustic waves J. Appl. Phys. 67, 6315 (1990); 10.1063/1.345150 Low reflectivity surface acoustic wave transducers on GaAs Appl. Phys. Lett. 43, 915 (1983); 10.1063/1.94178 Charge transport by surface acoustic waves in GaAs Appl. Phys. Lett. 41, 332 (1982); 10.1063/1.93526 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Sun, 30 Nov 2014 03:07:32Propagation loss of the acoustic pseudosurface wave on (ZXt)4S0 GaAs M. R. Melloch and R. S. Wagers Central Research Laboratories, Texas Instruments, Incorporated, Dallas, Texas 75265 (Received 22 August 1983; accepted for publication 12 September 1983) Measurements of propagation loss for the leaky surface acoustic wave on (l(JO)-cut GaAs with < 110) propagation direction are reported. The measurements were made in the frequency range 200-900 MHz. The propagation loss was determined with a novel technique using a delay line with four interdigital transducers. The effect of a hydrogen implant of dose 1014 cm -2 and energy 120 keY (technique for producing high resistivity isolation regions in GaAs) on propagation loss and macroscopic piezoelecticity are reported. PACS numbers: 43.35.Cg, 61.70.Tm, 61.80.Jh, 77.60. + v In efforts to develop monolithic surface acoustic wave (SA W) devices which incorporate semiconductors in the de vice structure sputtered ZnO on an oxidized silicon wafer has attracted much attention. 1-4 With a semiconductor in corporated in the device structure, interactions of the SAW with the semiconductor and fabrication of other electronic components on the same chip with the SAW device are now possible. In recent years SAW structures employing GaAs with ZnO overlays5-7 and without ZnOX-IO have been inves tigated as candidates for monolithic SAW devices. The structures using the piezoelectric properties of the GaAs have an added advantage over the ZnO/GaAs structures in that the propagation loss is much lower when the ZnO is absent from the propagation path; thus, higher frequencies and bandwidths are possible. The most promising crystal orientation on GaAs is that of the leaky surface acoustic wave (LSA Wi, the (100) cut with < 110) propagation direction. The electromechanical coupling coefficient for this LSA W mode is considerably higher than for the normal SAW modes on GaAs. Fortu nately the LSA W has zero leaky mode attenuation for the exact (100) cut with propagation in the exact (110) direc tion.11 In this letter we report propagation loss measurements for the LSA W. The measurements were made in the frequen cy range 200-900 MHz. The effect of a hydrogen implant (common technique for producing high resistivity isolation regions in GaAs 12) on propagation loss and macroscopic pie zoelectricity are reported. The device structure utilized for propagation loss mea surements is depicted in Fig. 1. The aluminum interdigital transducers consisted of 40 wavelengths of double electrodes which produced a beam of width 100 wavelengths. The cen ter-to-center spacing for adjacent transducers was L = 0.889 cm. Devices were fabricated for operation at 200, 300, 600, and 900 MHz. Using the transducer labeling illustrated in Fig. 1 the ABC D FIG. 1. Propagation loss measurement device configuration. delay line insertion loss between various pairs of transducers can be written as ILAB = TLA + TLB + a(L ), ILAC = TLA + TLC + a(2L ), ILAD = TLA + TLD + a(3L ). ILBC = TLB + TLC + a(L ), ILCD = TLC + TLD + a(L ), (1) where TLn is the transduction loss of the nth transducer, ILmn is the insertion loss between transducers m and n, and a is the propagation loss in dB/em. The set of Eq. (1) is a consistent set of five equations in five unknowns which can be solved for a, a = (1/2L )(!LAD -ILCD + !LBC -ILAB ). (2) Hence by measuring the insertion loss between various pairs of transducers one can determine the propagation loss. Chrome-doped ( ~ I X 1017 cm -3) horizontal Bridgman grown substrates were used for all experiments. The LSA W propagation was in the < 110) direction; the crystal was cut 2° from a (100) cut with the 2° tilt toward the nearest < 110) equivalent direction. Hence the LSA W propagation loss would have both leaky mode and viscous damping attenu ation components. Alternative devices on each wafer were implanted with hydrogen (energy = 120 keY, dose = 1014 cm-2). This is a common technique for producing high resistivity isolation regions in GaAs and hence it is important to determine the effect of the implant on viscous damping and macroscopic piezoelectricity. Typical results for propagation loss are tab ulated in Table I. The symbol I next to the device frequencies TABLE I. Rayleigh wave attenuation on 45' (ZXt) GaAs. The symbol I designates hydrogen implanted devices. a a a Frequency (dB/cm) (dB/flS) 200 1.13 0.323 2001 1.60 0.460 300 1.98 0.567 300 1 3.14 0.90 600 5.37 1.54 6001 6.35 1.82 900 11.5 3.29 9001 15.3 4.40 al Hydrogen implant: energy = 120 keY; dose = \014 cm 2 1008 Appl. Phys. Lett. 43 (11),1 December 1983 0003-6951/83/231008-02$01.00 © 1983 American Institute of Physics 1008 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Sun, 30 Nov 2014 03:07:32.1 U / ill "-iii ~ . (J) / (J) 9 1.0- -z 0 ~ t!) . ~ / 0 II: 0-• 1 0.1 I 100 500 1000 2000 FREQUENCY (MHz) FIG. 2. Propagation loss vs frequency. in Table I indicates devices which were hydrogen implanted as described above. As can be seen in Table I, the above described hydrogen implant had a noticeable effect on the viscous damping at tenuation. However, measurements of transducer radiation conductance and susceptance showed no appreciable differ ence for transducers fabricated on hydrogen implanted and nonimplanted regions. Hence no noticeable affect on the macroscopic piezoelectric activity was observed due to the hydrogen implant. The measured propagation loss versus frequency data are plotted in Fig. 2. As mentioned above there are two com ponents to the measured propagation loss. There is a leaky mode attenuation component which is proportional to fre quency II and a viscous damping component which is pro portional to the frequency squared. 13 The results shown in Fig. 2 of the logarithm of propagation loss versus the loga rithm offrequency have a slope of -1.5. Measurements of nonleaky mode SAW viscous damp ing attenuation have been made for other orientations of 1009 Appl. Phys. Lett., Vol. 43, No. 11, 1 December 1983 GaAs 14 using a laser probe technique.15 Those measure ments were made in the vicinity of 1 GHz:3.62 dB/Jls was obtained for the (211) cut with < 111) direction SAW propa gation and 4.22 dB/ JlS for the (110) cut with < 1(0) direction SAW propagation. Extrapolating our results for (100) cut GaAs with < 110) direction LSA W propagation loss to 1 GHz, we obtained -3.6 dB/Jls for the propagation loss which includes both leaky mode and viscous damping com ponents. In summary, the propagation loss for the LSA W on (1 OO)-cut GaAs with < 110) propagation direction from 200 to 900 MHz has been measured and found to be nearly the same as for non leaky SAW modes on other cuts of GaAs. The effects of a typical hydrogen implant (used for high resis tivity isolation in GaAs) on viscous damping and macro scopic piezoelectricity have also been determined. This work was sponsored by the Defense Advanced Re search Projects Agency and monitored by the Office of Na val Research. 'B. T. Khuri-Yakub and G. S. Kino, App!. Phys. Lett. 25, 188(1974). 'H. C. Tuan and G. S. Kino, App!. Phys. Lett. 31, 641 (1977). 'J. K. Elliott, R. L. Gunshor, R. F. Pierret, and A. R. Day, App!. Phys. Lett. 32, SIS (1978). 4F. C. Lo, R. L. Gunshor, and R. F. Pierret, App!. Phys. Lett. 34, 725 (1979). 'T. W. Grudkowski, G. K. Montress, M. Gilden, and J. F. Black, 1980 Ultrasonics Symposium Proceedings, IEEE Cat. No. 8OCH1602-2 (IEEE, New York, 1980), p. 88. OK. W. Loh, D. K. Schroder, and R. C. Clark, 1080 Ultrasonics Symposium Proceedings, IEEE Cat. No. 80CH 1602-2 (IEEE, New York, 1980), p. 98. 7G. R. Adams, J. D. Jackson, and J. S. Heeks, 1980 Ultrasonics Symposium Proceedings, IEEE Cat. No. 80CHI602-2 (IEEE, New York, 1980), p. 109. HM. R. Melloch, R. S. Wagers, and R. E. Williams, App\. Phys. Lett. 42, 228 (1983). OM. R. Melloch, R. S. Wagers, and R. E. Williams, 1982 Ultrasonics Sym posium Proceedings, IEEE Cat. No. 82CH1823-4 (IEEE, New York, 1982), p. 442. "'M. R. Melloch and R. S. Wagers, App\. Phys. Lett. 43, 48 (1983). II D. Pen un uri and K. M. Lakin, 1975 Ultrasonics Symposium Proceedings, IEEE Cat. No. 75CH995-4SU (IEEE, New York, 1975), p. 478. 12K. Wohlleben and W. Beck, Z. Naturforsh A 21, 1057 (1966). "B. A. Auld, Acoustic Fields and Waves in Solids (Wiley, New York, 1973). "A. J. Siobodnik, Electron. Lett. 8, 307(1972). I5A. J. Siobodnik, P. H. Carr, and A. J. Budreau, J. App\. Phys. 41, 4380 (1970). M. R. Melloch and R. S. Wagers 1009 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Sun, 30 Nov 2014 03:07:32

1.1140693.pdf

Boxcar photography G. J. Greene, G. Cutsogeorge, and M. Ono Citation: Review of Scientific Instruments 60, 2690 (1989); doi: 10.1063/1.1140693 View online: http://dx.doi.org/10.1063/1.1140693 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Boxcar Integrator Attachment for Oscilloscopes Rev. Sci. Instrum. 41, 545 (1970); 10.1063/1.1684572 ``Boxcar'' Integrator with Long Holding Times Rev. Sci. Instrum. 32, 1016 (1961); 10.1063/1.1717602 Photography Phys. Today 8, 32 (1955); 10.1063/1.3062033 Streak Photography J. Appl. Phys. 21, 445 (1950); 10.1063/1.1699682 The Teaching of Photography Am. J. Phys. 7, 394 (1939); 10.1119/1.1991491 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sat, 22 Nov 2014 03:15:38Boxcar photography G. J. Greene, G. Cutsogeorge. and M. Ono Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543 (Received 15 February 1988; accepted for publication 18 April 1989) A si:nple, inexpensive diagnostic has been developed for time-resolved imaging of repetitive self lummescent phenomena. An electro-optic birefringent ceramic shutter is employed to perform photographic sampling and has demonstrated time resolution of 60 f1s. Photographic film was used for imag~ detection in ~his system, and methods of image enhancement in the presence of background hght from. a fi?lte-contrast shutter are discussed. The system has been applied to the study of plasma evolutlOn 10 the CDX plasma confinement device. INTRODUCTION In the investigation of transient plasma phenomena, visual imaging is often a useful supplement to other macroscopic diagnostics. In hot plasmas, such observation gives informa tion primarily about the plasma edge, while in cooler plas mas light may be emitted from the entire plasma volume. Information can be obtained from the spectrum of the emit ted light, from its time history, and from its spatiallocaliza tion. In particular, photographic imaging has long been uti lized as primary diagnostic of magnetically confined plasma boundaries. I When the time evolution of the plasma is of interest, a number of imaging techniques are available. If the relevant time scales are on the order of seconds, as is often the case with present-day tokamaks, a standard video or motion pic ture camera can provide significant information.2 Many events, however, occur on faster time scales. For investiga t~on o~ submillisecond plasma phenomena, high-speed mo tion picture cameras and image-intensified framing cameras have been used. Both techniques are quite expensive, and the latter, while capable of extremely fast time resolution, can not directly provide a color image. This article describes a very simple and inexpensive diagnostic that has been recently devised to obtain time-re solved color or black -and-white images of a plasma with par ticular application to the Current Drive Experiment (COX). The goal of the COX is to investigate novel means of current drive via various forms ofhelicity injection. Initial experiments have studied formation of a toroidal plasma dis charge by a circulating electron beam, injected from a heated cathode, and subsequent radial current penetration and evo lution of a tokamaklike field topology. 3 This approach offers the eventual possibility of a steady-state tokamak device. The COX discharge is presently pulsed with a repetition rate of up to lO Hz, and current penetration and evolution ofthe plasma shape occur with time scales on the order of millisec onds or less. Important information can be obtained by com 'paring the experimentally observed images with the plasma evolution predicted by a numerical magnetohydrodynamic (MHO) model. I. DESIGN CONSIDERATIONS The light available from a self-luminescent object (such as the COX plasma) is insufficient, in many cases where a short exposure is desired, to adequately expose the particular image detector employed (typically photographic film or a solid-state array). An image intensifier can be used for light amplification at the expense of a loss of spectral information and a rather high cost. Since the COX discharge can be re producibly pulsed (the time evolution of the total plasma current, for instance, varies by only a few percent from one shot to the next), our approach is simply to expose photo graphic film, on successive shots, to light from a selected time interval during the discharge until the film is sufficient ly illuminated. The film can then be changed, a different time interval during the discharge selected, and the procedure repeated to obtain images of the plasma showing its evolu tion as a function of time. Since light from the same portion of many successive discharges is used to form one "hoto graphic image, the result represents an optical ave;age of these shots and the approach is analogous to the electronic technique of boxcar integration4; hence, we have termed this technique "boxcar photography." This approach, of course, is subject to the fundamental limitations of any sampling system. Features associated with fluctuation phenomena oc curri?g on time scales shorter than twice the sampling peri od wIll tend to be averaged out unless phase coherence is maintained from shot to shot, in which case aliasing may occur. This procedure requires a shutter which can be electron ically triggered and which has adequately short opening and closing times (submillisecond resolution was desired for the CDX application). Although electromechanical shutters have been recently produced with effective exposure times as short as 0.25 ms (for example, in certain 35-mm single-lens reflex cameras), substantial timing jitter associated with the solenoid-triggered mechanisms prevents their use here. In addition, these fast shutters cannot be repeatedly fired at rates of more than a few hertz, and a lO-Hz repetition rate was required for the CDX diagnostic. Liquid crystal shutters are rapidly evolving,5 but currently available products are to~ slow for ~he application considered here. Shutters using flUld suspenslOns of metallic dipoles aligned by electric fields have been developed by Marks,6 but are also currently too slow. Shutters can be constructed using an electrically con trollable, optically birefringent material placed between crossed polarizers. Implementations such as the Kerr cell or Pockel ceU7 can be switched in time scales of nanoseconds. Their primary drawbacks are a small useful angular aper- 2690 Rev. Sci. (nstrum. 60 (8), August 1989 0034-6748/89/082690-07$01.30 ® 1989 American Institute of Physics 2690 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sat, 22 Nov 2014 03:15:38ture, relatively high cost, and the complexity of the drivers needed to provide pulses of multikilovolt amplitude. Prox imity-focused diode image intensifier tubes are also relative ly costly but can provide large useful apertures, fast shutter ing (nanosecond time scale), and moderate luminous gains (30-100). They require switching large voltages (6-7 kV), however, and can provide only a monochrome image. The switching voltage can be reduced to 150 V using a micro channel plate image intensifier, which also increases the lu minous gain to _104 and may double the cost of the device compared to a diode intensifier tube. The electro-optic shutter used for the CDX diagnostic was originally developed for flash-blindness protection gog gles for pilots and is now commercially available.8 The de vice uses a transparent ceramic material (lanthanum-modi fied lead zirconate titanate, or PLZT) which exhibits an electric-fieId-dependent optical birefringence.9 The material is manufactured in large wafers ( 1-and 2~in. diameter) with interdigital electrode arrays deposited on both surfaces in order to generate a uniform electric field in the ceramic. The wafer is sandwiched, as with a Kerr cell, between two linear polarizers whose axes are perpendicular, and the optic axis of the PLZT is oriented at 45° with respect to the polarizer axes. These devices can be switched on and off with an ap~ plied voltage of less than 700 V (a range easily accessible to transistor switches), provide submillisecond response, and are relatively inexpensive. The model used in this work costs only 4% of the price of a proximity-focused diode intensifier tube (without a microchannel plate) of comparable size. For the particular composition of PLZT used in this shutter, the effective birefringence (L~n) is a quadratic func tion of the applied electric field, and there is zero effective birefringence when the applied field (E) vanishes: an = K/iE 2, where K is the Kerr constant and..t is the wave length of incident light. Therefore, with no applied voltage the shutter is essentially closed and residual transmission depends primarily on the polarizer quality. If the polarizers are ideal (providing perfect linear polarization for all wave lengths), then the ratio of the light intensity transmitted through the modulator assembly to the incident intensity is proportional tosin2( 17KTE2), where T is theeffectivethick~ ness of the birefringent layer and K is, in general, a function of ..t. 10 For a fixed incident wavelength, the transmission in creases from zero with no applied field to a first maximum at an applied field of magnitude Em = (2KT)~-lf2. Over the range of visible wavelengths, Em varies by some 30% for the shutter used here. Typically, Em is chosen to maximize white-light transmission and this choice results in somewhat reduced transmission at the blue end of the spectrum. The field Em is reached in these devices for ap plied voltages on the order of 400-700 V. The contrast ratio of the shutter (defined as the ratio between transmission of white light with applied field Em and transmission with zero applied field) is approximately 2000, and absolute transmis sion at E = Em is approximately 15%. II. EXPOSURE AND CONTRAST CONSIDERATIONS The use of photographic film as an image detector re- 2691 Rev. ScI. Instrum., Vol. 60, No.8, August 1989 quires that care be taken in the interpretation of the results. The eye is approximately a logarithmic detector, and film attempts to mimic this response. The reflected density of a developed print (the log of the ratio of intensity reflected from an unexposed portion to intensity reflected from an exposed portion) has a total range of only -1.5 for most papers, and the range over which the density is a linear func tion . of the log of the exposure is considerably smaller (-0.5). Thus the dynamic range over which a print can be used as an absolute measure of incident flux is at most -3D, and then only if the characteristic curve of the film is accu rately known. If quantitative measurements are needed, much more sensitivity can be obtained with the use of nega~ dve film and measurements of transmission (using a densi tometer) rather than reflection (a dynamic range of -1000 is then feasible). In the experiments described here, quanti tative analysis of the images was not required, and Polaroid type 57 print film was typically used for convenience. This black and white film has a relatively high speed (ASA 3000), a spectral range of 350-650 nm, a total reflected den sity range of 1.54, and a linear density range of approximate lyO.7.1l Photographic film does not act as an ideal integrator of incident fiux. As the intensity ofincident light decreases, the effectiveness of forming a photographic latent image (in terms of photons absorbed per density increment on the de veloped film) also eventually decreases, i.e., the exposure (incident intensity X time) required to produce a constant density in the developed film is not independent of intensity. This is a statement of reciprocity failure which is related to the fact that formation of a latent image requires more than one photon to be absorbed by a particular grain in the film within a certain period. 12 In addition, a series of N exposures ofintensity 10 and duration to, separated by time tw' do not, in general, produce the same photographic effect as a single exposure of intensity 10 and duration Nto. This effect, known as the intermittency effect, is of importance in photographic sampling systems. It was found by Webbl3 that the photo graphic effect of an intermittent exposure is identical (in cluding reciprocity failure) to the effect of an exposure of duration N(to + tw) and constant intensity Ie = Iotol (to + tw), provided the frequency of the intermittent expo sure [J; = 1/ (to + tw )] exceeds a critical frequency of fu sion,.fc (typical1y./c -0.1-1 HZI4). A significant loss of sen sitivity of the photographic system can therefore occur due to reciprocity failure even if the intensity during each indi vidual exposure is high. For a typical CDX experiment, to = 0.5 ms, t", = 200 ms, and the average intensity 10 is suf ficient that a single exposure of duration ts = 50 ms during the plasma shot is adequate to expose the film. Assuming for simplicity that the intensity does not change greatly during the shot, the above relations would predict that a sampled exposure of N:::::: tsl to = 100 shots would be required to pro duce an adequate image. However, the duration of the equiv alent exposure ( -20 s) is of sufficient length that reciproc ity failure is important, II and in fact more than twice the predicted number of shots are required to produce a suitable image. Note that if the shot repetition rate (.t;)is lowered below the frequency of fusion, a further loss of sensitivity Boxcar photography 2691 .•• . •................. -..•.....• -•.... -•...•.• ~ •.•.•.•.•. <.:.:.:.:.;.;.;.:-;.:-; ..• :.:0;' •••••••••• ; •••••••••••••••••••••••••• >.<;.;<;<;".-.;..;> ••• ' ••••••• : ••••••• :.:.:.:.:-:.:.:.~.:.:.:.:.:.~.:.:.:;:.:.:.:.:.:.:.:.:-:.:<.; •. • •••••• -. •••••••••••••••••••••••••••••••••••••••••••• ' ••••• ~'.T ••••••• ' ••• '.' •.••• :.:.:<.:.~.:.:.:.:.;.:.:.:.:.;.;.:-:.:.-. ,:,-,:,:,:,:,;,z,:,;,;,x,;,z,;,;",-.,:",,-, .-•••••••••••• , ..... ,. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sat, 22 Nov 2014 03:15:38will occur, requiring correspondingly more shots for an ade quate exposure. The contrast ratio of the shutter together with the film characteristics determine the ability to discern features in the final photographic image that arise from events occur ring in the interval during which the shutter is open. The effect of the finite contrast of the PLZT shutter is that very low-level light from the nonsampled part of the discharge is incident on the film. If the leakage is of sufficient intensity, an image can be formed in regions of shadow on the primary image. Also, a sublatent image due to the leakage can, at the particular locations where it is produced, enhance the sensi tivity of the film to formation of the primary image (the effect oflatensification12) and thus, upon development, dis tort that image. These concerns become more serious when the ratio of the discharge duration to the exposure period becomes large or when imaging a portion of the discharge cycle that is much less intense than the remainder of the cycle. It is advantageous to arrange the intensities oflight in the system so that light leakage through the closed shutter is reduced in effectiveness for image formation through reci procity failure to a greater extent than is the primary image. Experimental checks of image contamination due to leakage through the shutter are discussed in the following section. The contrast ratio of2000 specified for the shutter used here assumes that light is normally incident on the polar izers. The efficiency of polarization, and hence the contrast ratio, decreases as the angle of incidence (OJ) increases from zero (normal incidence). For (Ji = 15°, the contrast ratio is reduced to a value of approximately 600. Hence limiting the numerical aperture of the optical system can be an important consideration. In the CDX diagnostic, the numerical aper ture was limited to a value of 0.045, which ensured that all rays passed through the PLZT shutter at angles OJ of less than 2.6° and avoided any observable degradation of the con trast ratio. Another method employed to limit the effects of a finite contrast shutter is to back the device with an electromechan ical shutter that has effectively infinite contrast. Such electri cally triggered shutters, designed for laboratory use, are available with clear aperture diameters of 2.5 cm and mini mum exposure periods of 6 ms. 15 In operation, the timing of the slow mechanical shutter is arranged so that its open peri od brackets the open period of the fast PLZT shutter. If the average intensity of the discharge does not vary greatly during the time that the backing shutter is open (t B), the ratio of the exposure due to the sampled period to that from the nonsampled period is eto/ (t B -to), where C is the contrast ratio (and the different reciprocity effects on the sampled and nonsampled images have not been taken into account). The minimum sampled exposure time that can then be used without danger ofimage contamination can be /C lO(lJ .--D·) h roughly estimated as tmin = (t B) max moo, were Dmax and Dillin are the maximum and minimum values of reflected density (taken from the characteristic curve of the film). For typical CDX experiments, evaluation of the above yields tmill ::::::: 100 j..ls. Exposures of this duration or shorter should be accompanied by careful experimental verification of the lack of image contamination. 2692 Rev. Sci. Instrum., Vol. 60, No. 8, August 1989 III. IMPLEMENTATION AND OPERATION A diagram of the diagnostic built for the CDX experi ment is shown in Fig. 1. A standard 4 X 5 view camera 16 is used to record the image as it allows convenient interchange of sheet and Polaroid film backs. The PLZT shutter is mounted in a light-tight phenolic housing with an adapter that screws into the camera lens. An electromechanical shut ter is, in turn, mounted to the phenolic housing. The camera views the plasma cross section tangentially through two mir rors placed in a vacuum chamber port, and, due to the long optical path length, a 300-mm focal length lens is used to provide adequate image size on the film. The signal to initi ate the boxcar photography cycle (from a manual push but ton) enables a shot counter gate which allows the CDX dis charge trigger pulses to pass to the subsequent electronics for a selected number of sequential shots. The PLZT shutter appears electrically as a capacitance of approximately 0.02 J.1F in series with an equivalent elec trode resistance of ~ 25 n. The shutter driver must be able to provide relatively fast rise and fall times into this largely capacitive load. A schematic diagram of a high voltage pulse amplifier designed for this application is shown in Fig. 2. The input stage utilizes a VMOS FET inverter (Q 1) and accepts TTL-level pulses. The output stage consists of two Darling ton-connected high voltage transistor pairs. The pair direct ly driven by the FET (Q2, Q3) is an inverter and the other pair (Q4, Q5) acts as a fonower. On the positive-going edge of the input pulse, the shutter capacitance is charged by the follower stage. When the input signal returns to zero, the Shottky barrier diode (D 1 ) conducts and the shutter capaci tance is discharged through the inverter portion of the out put stage. The circuit is designed to operate at a low duty cycle. The l-kfl resistor in series with the high-voltage input limits the maximum current drawn, and the 2-j..lF capacitor provides the peak currents needed for fast rise and fall times. The input to the high voltage amplifier is a pulse of adjusta ble width that is synchronized and delayed with respect to the CDX discharge trigger. The time delay and the pulse width are adjustable in lO-llS increments using inexpensive, commercially available modules. 17 An example of the switching speed of the shutter and high voltage amplifier is shown in Fig. 3. Here a white light source (a dc-powered tungsten lamp) illuminated the shut ter through a collimator, and a photomultiplier tube (rise time < 1 j..ls) was used as a detector. The output from the detector and the voltage across the shutter are shown for three exposures of different widths. The rise time of the vol tage signal is approximately 50 j..ls and the fall time, 20 J.1s. The shutter optical closing time is also approximately 20 j..ls, but the opening time is significantly longer. The opening characteristic appears to be a two-stage process, an effect which has been previously reported18 and which appears to be related to the establishment of birefringence in the ceram ic. The initial rate of opening is rather fast (the shutter reaches 20% of its ultimate transmission within -40 j..ls) but subsequently the rate decreases considerably. The shut ter attains some 66% of its ultimate transmission in about Boxcar photography 2692 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sat, 22 Nov 2014 03:15:38Polaroid Film Back 4 x 5 View Camera COX Vacuum Vessel (Top View) High Voltage Swi tch Solenoid Driver Exp08ur~ Initiah Trigger Time Delay &. Gate Modules Repetitin Discharge Trigger FiG. 1. Diagram of the boxcar photography diagnostic. 1 k!l 1W 400 -700 V 2111 + Lens PLZT Shutter Electromechanical Shutter Window 0.01 I! f 1 kV T T1kV 100 kO 2W Gats Input (TIL) 01 10kO iOO kO 01:VNS 10KM Q2-5: MJ8503 D1: 1/2 MBR1535 5.1 kO 02 1 kO FIG. 2. Schematic diagram of the high-voltage PLZT shutter driver. 2693 Rev. ScLlnstrum., Vol. 60, No. e, August 1989 Dl lOOn lW Boxcar photography PlZT Shutter Assembly 2693 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sat, 22 Nov 2014 03:15:38SHUTTER 1000l; VOLTAGE 500 """' ............................... """"'l (Volts) 0 E ...... _I ......... \:.-... 2 ............ t: ... _3 ... SHUTTER 100lk TRANSMISS!ON _ ...... __ -, (% of Fully 50 r 2 \-3 Open Value) -I l \ o --~------~-- L! ~ __ L-_~~~! "~ __ L-~~~~ o 500 1000 TiME <P.Sl)C) FIG. 3. Example of the switching speed of the high-voltage amplifier and the corresponding spatially averaged optical transmission through the ~hutter (collimated white light illuminated the shutter aperture). Signals from three exposures of different widths are superimposed. The traces show full width, half-maximum exposures of approximately 50 f.ls (trace I), 300 its (trace 2), and 600 ItS (trace 3). 600 f..ts and requires several hundred ms to reach the final steady-state, fully open level. The optical switching speed tests described above uti lized a light source that illuminated most of the shutter aper ture. The transmission curves therefore represent the spa tially-averaged response of the PLZT device. Since it is conceivable that different areas of the shutter might open asynchronously, particularly for very short exposures, a di rect test of the effective shutter speed that demonstrates the imaging qualities of the shutter was performed. A test pat tern, consisting of black squares, radial lines, and the letters "CDX" drawn on a white background, was affixed to a met al disk, 12.7 cm in diameter, and was rotated at high speed using an electric motor. A small hole near the edge of the disk allowed a synchronization pulse to be generated using a slotted optical switch. A photograph was taken of the rotat ing disk using a signal derived from the synchronization pulse to trigger the PLZT shutter at a rate of 3 Hz. The disk rotated at 51 revolutions per second, corresponding to an edge speed of20.3 m/s. The electromechanical backing shut ter was employed, and 2000 exposures were required to pro duce the photograph shown in Fig. 4. The black rectangle at the bottom of the disk is the slotted optical switch (its blurred outline arises from the shadows it casts on the disk). The synchronization hole is visible as a black dot on the right-hand side of the photograph. The lines of the test pat tern are clearly defined in the center of the disk, but blurring of the radial lines is observed toward the edge of the disk. This blurring is due to the motion of the disk while the PLZT shutter is open, and its magnitude gives a measure of the effective shutter speed. The blurring of the radial lines at the edge of the disk corresponds to an angular rotation of about 1.10, from which an effective shutter speed of 60 f.1S is calcu lated. No other distortion of the test pattern is visible, and the image density is relatively uniform across the face of the disk. This test demonstrates that the PLZT shutter opens uniformly, even at high speed. In the application discussed, a perfectly square shutter characteristic was not a requirement, and effective exposure times as short as 40 f.1s were feasible at reduced contrast. For even shorter exposures, several techniques are possible. La- 2694 Rev. Sci.lnstrum., Vol. 60, No.8, August 1989 FIG. 4. Photograph ofa 12.7-cm-diam test pattern, rotating at 51 revolu tions per second, taken with the boxcar system. The shutter was synchro nized to the rotation of the disk, and 2000 exposures were used to produce this image. The extent of the blurring of the radial lines at the edge of the disk indicates an effective shutter speed of 60 its. guna19 has demonstrated a method of charging a PLZT shutter with a constant current (rather than R-C exponen tial charging) which yielded a significant improvement, and Wolfram20 has discussed the approach of overdriving the ceramIC. Usbg the techniques discussed earlier, the maximum ratio of the discharge length to the shutter open period for which contrast degradation is not significant can be in creased. Experimental checks of image contamination due to finite shutter contrast should be performed, however, and the characteristics of the particular film in use must be con sidered. Since a plot ofthc relative log exposure versus opti cal density produced in a film is not linear but rather sigmoi dal, 12 simply repeating a series of exposures with the shutter closed may not provide an accurate indication of contamina tion of the desired image by light from the rest of the dis charge. For a more realistic test, the film should be exposed to a uniform source of light for a period sufficient to yield an optical density comparable to that in the experimental im age. This will ensure that the sensitivity of the film to the low light levels penetrating the closed shutter is comparable to its value in the actual experiment. A subsequent series of expo sures to the discharge made with the shutter closed should then correctly indicate if image contamination occurs. The system described above has recently been used to provide the first indication of the formation of a tokamaklike plasma by electron beam injection in the CDX device.3 For Boxcar photography 2694 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sat, 22 Nov 2014 03:15:38#87XOO86 ~- J E "... e .3 u '-' v , )- )-J ~_~_-L -5 0 5 ~5 0 5 -5 0 5 X (em) X (em) X (em) FIG. 5. (a)-(c) Results ofa numerical model showing the path ofthe electron beam in CDX (single cross hatch shaded regions), projected onto a poloidal cross section of the toroidal device, and areas of closed magnetic field lines containing the main CDX plasma (double cross hatch shaded regions), (d)-(f) Photographs taken of the CDX discharge, with a tangential view, using the boxcar system. Plasma currents common to model and experiment were 10 A [parts (a) and (d)], 50A [parts (b) and (e)j,and 330A [parts (el and m], this experiment, type 57 Polaroid print film was used, the shutter exposure period was 500 p.s, and 200 successive dis charges were used to produce the images shown in Fig. 5. Since the plasma duration for each shot was only ~ 20 ms and the luminosity did not vary greatly during the shot, the contrast was adequate without need for an auxiliary shutter; no image contamination was discemable using the test de scribed above. The graphs displayed in Figs. 5(a)-5(c) show the re sults of a 2-D current transport simulation for the CDX plas ma at increasing electron beam currents. In each case, the corresponding image of the plasma (viewed tangentially, as indicated in Fig. 1) is shown in the photograph beneath the figure. The lightly shaded regions (single cross hatch) in Figs. 5(a)-5(c) show the path of the electron beam used to create the plasma. The beam eminates from a heated cathode (the large rectangle at the bottom of the figure). The more heavily shaded areas (double cross hatch) in Figs. 5(b) and 5 (c) indicate regions of closed magnetic field lines contain ing the main CDX plasma. The photographs show qualita tively some of the general features of the simulation result, especially the change in shape from a diffuse, vertical object 2695 Rev. Sci. Instrum., Vol. 60, No.8, August 1989 at low beam current to a wen-delineated, brighter, rounded plasma object at high beam current. Magnetic probe mea surements have confirmed that the plasma, in the high cur rent case, has developed a tokamaklike closed magnetic field structure. The probe measurements generally require a sig nificant amount oftime to complete and analyze. The boxcar photography system allows rapid observation of the plasma shape; the immediate feedback is routinely used to adjust the CDX device for optimum performance. ACKNOWLEDGMENT This work was supported by U.S. DOE contract DE AC02-76-CHO-3073. IR. H, Huddlestone and S. L Leonard, Plasma Diagnostic Techniques (Academic, New York, 1965), pp. 27-520 2S, S. Medley, D, L Dimock, S. Hayes, D. Long, J. L. Lowrance, V. Mas trocola, G. Renda, M. Ulrickson, and K, M. Young, Rev, Sci. Instrum. 56, 1873 (1985). 3M. Ono, G, J. Greene, D. Darrow, C. Forest, H. Park, and T, H. Stix, Physo Rev, Lett, 59,2165 (1987), 4D. W. Swain, Rev. Sci. Instl'um, 41, 545 (1970). 5J, J. McCormick, Electron. Des. 33, 1i7 (1985). BOl(car photography 2695 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sat, 22 Nov 2014 03:15:386 A. M. Marks, App!. Opt. 8, 1397 (1969). 7E. A. Enemark and A. Gallagher, Rev. Sci. lnstrum. 40, 40 ( 1969). 8The shutter used in these experiments was the model A20CE40BA PLZT Modulator, manufactured by the Motorola Communications Systems Di- vision, Ceramic Products Group (Albuquerque, NM). 9G. H. Haertling and C. E. Land, J. Am. Ceram. Soc. 54, II (1971). '"A. Yariv, Introduction to Optical Electronics (Holt-Reinhart, New York, 1971),pp.230-236. I I Polaroid Corporation, Polaroid Black and White Land Hlms (Focal, Bos ton, MA, 1983), passim. "T. H. James and C. A. E. Mees, Eds., The 1heory of the Photographic Process (Macmillan, New York, 1966), passim. 2696 Rev. Sci. Instrum., Vol. 60, No.8, August 1989 IJJ. H. Webb, J. Opt. Soc. Am. 23, 157 (1933). 14R. E. Maerker, J. Opt. Soc. Am. 44,625 (1954). 15For example, the model 225L shutter, manufactured by Vincent Asso ciates, Inc. (Rochester, NY). '''The Cambo model SCX view camera was used (manufactured by Cambo Fotografische Industrie. Kampen, Holland). 17Modc14145 programmable time delay and gate modules were used (man ufactured by Evan~ Electronics, Berkeley, CA) . ISJ. T, Cutchen, J. O. Harris, Jr., and G. R. Laguna, App!. Opt. 14, 1866 (1975). 19G. R. Laguna, Ferroelectrics 50,73 (1983). It'G. Wolfram, Ferroelectrics 10, 39 (1976). Boxcar photography 2696 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 155.247.166.234 On: Sat, 22 Nov 2014 03:15:38

1.100192.pdf

Unified planar process for fabricating heterojunction bipolar transistors and buried heterostructure lasers utilizing impurityinduced disordering R. L. Thornton, W. J. Mosby, and H. F. Chung Citation: Applied Physics Letters 53, 2669 (1988); doi: 10.1063/1.100192 View online: http://dx.doi.org/10.1063/1.100192 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/53/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Laterally injected lowthreshold lasers by impurityinduced disordering Appl. Phys. Lett. 59, 3375 (1991); 10.1063/1.105679 Buriedheterostructure lasers fabricated by in situ processing techniques Appl. Phys. Lett. 57, 1864 (1990); 10.1063/1.104042 Properties of closely spaced independently addressable lasers fabricated by impurityinduced disordering Appl. Phys. Lett. 56, 1623 (1990); 10.1063/1.103145 Buried heterostructure lasers by silicon implanted, impurity induced disordering Appl. Phys. Lett. 51, 1401 (1987); 10.1063/1.98689 Low threshold planar buried heterostructure lasers fabricated by impurityinduced disordering Appl. Phys. Lett. 47, 1239 (1985); 10.1063/1.96290 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 142.157.129.16 On: Wed, 10 Dec 2014 16:11:43Unified p~anar process for fabricating heterojunction bipolar transistors and tUJrieobheterostructure iasers utilizing impuritYftinauced disordering R. L. Thornton, W. J. Mosby, and H. F. Chung Xr?rox Palo Alto Research Center, 3333 Coyote Hill Ruad, Palo Alto, California 94304 (Received 9 August 1988; accepted for pUblication 18 October 1988) We describe results 011 a novel geometry of heterojunction bipolar transistor that has been realized by impurity-induced disordering. This structure is fabricated by a method that is compatible with techniques for the fabrication of low threshold current buried-heterostructure lasers. We have demonstrated this compatibility by fabricating a hybrid laser/transistor structure that operates as a iaser with a threshold current of 6 rnA at room temperature, and as a transistor with a current gain of 5. There is currently great interest in device design issues for the realization of integrated optoelectronics. One desir able goal of integration is to realize a low-threshold laser structure and a transistor structure on the same substrate. The ability to integrate these two components would be greatly enhanced if techniques arc developed for fabricating both structures within the same set of epitaxial layers. There have been initial demonstrations of this concept 1.2; however, simultaneous high performance of hoth the laser and the transistor devices has not yet been reported. In addition, for high-density integration applications, it is highly desirable that the fabrication process for the devices be planar, and therefore that there be no etching and regrowth steps in volved. In this work we introduce a novel transistor structure, which we call a lateral hcterojunction bipolar transistor (L HBT). fmpurity-induced disordering (UD) 1,4 via silicon diffusion is used to selectively convert a buried p-type GaAs layer into n-type AIGaAs regions that serve as the emitter and collector of the heterojunction bipolar transistor. The resulting transistor structure is completely planar, and cur rent now from emitter through base to collector occurs in the plane of the base layer as opposed to perpendicular to the plane of the base layer, as is the case in conventional hetero junction bipolar transistors. Due to the no effect acting on the base layer to form the wide band-gap emitter and collec tor regions, a buried heterostructure is formed that is similar to structures previously used to make high performance lID buried-heterostructure (BH) lasers.5,1> We have in fact ob served room-temperature laser operation from our transis tor structure with threshold currents as low as I) rnA. The basic structure for the transistor is shown schemati cally in Fig. I. The multilayer heterostructure is grown 011 a p-type GaAs substrate, and the layers grown subsequently are (l) a GaAs buffer layer, (2) an AI(12 Ga()~ As buffer layer, (3) an AIoA Gao"As burying layer, (4) a GaAs active base layer, (5) an AloA Gaol; As base burying layer, and (6) 11 GaAs capping layer. All layers except the active GaAs base layer arc intentionally doped p type to a concentration of 1 X 101~. Outdiffusion ofMg during the subsequent process ing steps is relied upon to dope the active base layer to an estimated concentration on X 1017• After growth of the epi taxiallayers, a chemical vapor deposited (CVD) Si1N4 film is deposited and patterned to serve as a mask for the diffusion ofSi. A bilayer film ofSi capped by Si3N4 is then CVD depos ited to serve as a source for Si diffusion. As a result of the Si,N4 capping layer, the diffusion can be performed in a hy drogen ambient without the need to provide an overpressure nf As to the surface to avoid As outdiffusion. The diffusion is performed at 850°C for 8 h to reach a junction depth of 1.4 f~m. During the process of the Si diffusion, the buried p-type GaAs layer is dispersed into the adjacent 40% aluminum layers by impurity-induced disordering. As a result, the lat eral npn structure that is formed by the Si diffusions has higher alloy compositions in the emitter :lnd collector re gions than in the buried GaAs stripe that forms the active portion of the base. Figure 2 shows a scanning electron mi croscope cross section of the transistor structure after the Si diffusion step, in which the emitter and collector diffusion profiles can be clearly seen as well as the lID action on the base layer to make a wide-gap emitter and collector. Proton bombardment is used to isolate the individual devices on the chip, followed by metallization of the 4-flm-wide emitter and collcctor contacts. All of the metallizations on this structure are Cr-Au. The high 5i doping level in the Si-diffused regions is relied upon to make ohmic contact to the emitter and col lector. The base contact is made to the p-type substrate. A shallow ion implantation is performed aftcr the metalliza tion in order to prevent the GaAs cap layer at the surface from functioning as a parasitic base channel which would introduce substantial surface recombination. The devices arc cleaved into 250-,um-long strips in the direction perpen dicular to the stripe emitter and collector contacts. GaA.Ooli'm Alo.4GaO.6As O.9p.m: GaAs Ool/Lm I Alc.4GaQ.eAs 1.4j.Lml p.GaA==~:stra!" : : ;»;;7"/ C:~Proton !mpiant [(ijSi D;ffU~ FIG 0 I. Schematic diagmm of the lateral heterojunction bipolar transistor structure (L-HBTl fabricated by impurity-induced disordering. 2669 Appl. Phys. Lett. 53 (26), 26 December 1988 0003-6951/88/522669-03$01000 @ 1988 American Institute of Physics 2669 ............ ' •••••••••••••••• ;.;.;.;.:.;':.: •••••• ,. ••••••••• -;0;>; ••••••••• -.-.-....... -.-••••• ";".~;-••••••• ~->;~ •• ; •••• , .-.-.-.-.' " •••• "0',",-. r •••• " •• This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 142.157.129.16 On: Wed, 10 Dec 2014 16:11:43Unstained 4 microns Stained 6 microns FIG. 2. SEM cross section of tilt' transistor structure after performing of disordering silicon diffusions. We have fabricated this transistor structure and ob served transistor action with significant gain. Figure 3 shows a typical transistor characteristic for one of these devices, indicating a current gain of approximately 5. This device has a base length of 1.4 pm. It can be seen also that the turn-on voltage for the transistor is on the order of 2.5 V. This is a result of the high contact resistance associated with the emit ter and collector contacts. The Si concentration in the Si diffused region results in a net n-type doping of 7 X 10 III n type. This level is not sufficient to yield a very low value of contact resistance. The use of a more standard alloyed Au Ge contact would result in a substantial improvement in the quality of the ohmic contact, at the expense of increased device fabrication complexity. The collector base break down voltage of 8 V is also evident in this figure. The heavily doped p-n junctions resulting from the Si diffusion result in this relatively low reverse breakdown. As has becn previously mentioned, the active base layer <C 500 ~ 400 E ~ 300 ... ::l 0 .... .2 200 (.) .!!! "5 100 0 0 0 1.6 3.2 4.8 6.4 8.0 Collector Emitter Voltage (volts) fIG. 3. Transistor characteristics of the lateral helcrojunctlOIl bipolar tran sistor with a base width of 1.4 fLm. Base current is incremented in 10 11A steps. 2670 Appl. Phys. Lett., Vol. 53, No. 26,26 December 1988 Lr=l: 8932 11936 8940 8944 I Wsvelength (Angstroms) I '---t---; --f-· --r--1 o 5 10 15 20 25 Current (Milliamperes) FIG. 4. Properties of the laser emission of the transistor structure when operated in the saturation mode. of this transistor can also function as a BH laser under ap propriate conditions of device bias. Specifically, when the transistor is driven into saturation by forward biasing both the emitter-base and the collector-base p-n junctions, the carrier density in the base region can build to sufficiently high levels to exhibit stimulated emission gain and lasing action. We refer to this laser structure as a heterotransverse junction (HTJ) laser, due to the fact that, similar to the normal transverse junction stripe laser,7 current is injected along the plane of the epitaxial layers. The important differ ence is that the disordered heterojunctions provide strong optical waveguiding and carrier confinement in this device. Single transverse heterostructure lasers using Zn disorder ing have been previously reported,X We have operated this transistor in the saturation mode and observed lasing action with threshold currents as low as 6 rnA pulsed, and 10 mA cw, both at room temperature. This threshold current value compares favorably to those rou tinely obtained in our more conventional geometry oflascrs. In Fig. 4 we show the light versus current characteristics for a typical laser/transistor structure. As shown by the optical emission spectrum in the inset, the device operates predomi nantly in a single longitudinal mode. The transverse injection geometry employed in this de vice can result in substantial reduction in the parasitic ca pacitance of the EH laser, as well as a potential path for contacting both sides of the lasing p-n junction from the top surface. For these initial device demonstrations we have sim plified the processing by contacting the base from the p-type substrate; however, it is relatively straightforward to modify the structure to allow for the laser structure to be grown on a semi-insulating substrate with a buried p-type layer to be used for accomplishing the base contact. The reduced ca pacitance in this geometry oflaser may be preferred for ap plications involving high-speed modulation and/or integra tion with other optoelectronic components. In conclusion, we have successfully fabricated a novel Thornton, Mosby, and Chung 2670 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 142.157.129.16 On: Wed, 10 Dec 2014 16:11:43transistor structure, which we refer to as a lateral hetero junction bipolar transistor. The lateral heterojunctions are formed by impurity-induced disordering of the buried GaAs base layer. These transistor structures exhibit current gains of 5 for base widths of 1.4 ,urn. The transistor device also functions as a buried-heterostructure laser, with a threshold current as low as 6 rnA for a 1.4 fl1TI stripe. The ability to fabricate both high performance lasers and transistors in the same set of epitaxial layers, via a technology that is com pletely planar and diffusion based, has the potential for a great impact on the future ability to realize complex opto electronics on a common substrate. 2671 Appl. Phys. Lett., Vol. 53, No. 26, 26 December 1988 'Y. Hasumi, A. Kazen, J. Temmyo, and H. Asahi, iEEE Electron Device Lett. EDL·8, 10 (1987). 1J. Shibata, Y. Mori, Y. Sasai, N. Hase, H. Scrizawa, and T. Kajiwara, Eke tnm. I"ett. 21, 99 (1985). 'W. D. Laidig, N. Ho]onyak, Jr. M. D. Camras, K. Hess, J. J. Coleman, P. D. Dapkus, and J. Bardeen, AppJ. Phys. Lett. 38, 776 (1981). 4K. Meehan, J. M. Brown, M. D. Camras, N. Ho]orlyak, Jr., R. D. Burn ham, T. L. Paoli, an W. Streifer, App!. Phys. Lett. 44, 7()O (1984). 'R. L. Thornton, R. D. Burnham, T. L. Paoli, N. Holonyak, Jr., and D. G. Deppe, App!. Phys. Lett. 41, 1239 (1985). "R. L. Thornton, R. D. Burnham, T. L. Paoli. N. Holonyak, Jr., and D. G. Deppe, App!. Phys. Lett. 48, 7 (1986). ·IH. Namizaki, IEEE J. Quantum Electron. QE-ll, 427 (1975). xY. J. Yang, Y. C. Ln, G. S. Lee, K. Y. Hsieh, and R. M. Kolbas, Appl. Phys. Lett. 49,835 (1986). Thornton, Mosby, and Chung 2671 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 142.157.129.16 On: Wed, 10 Dec 2014 16:11:43

1.100873.pdf

Surface passivation of GaAs H. H. Lee, R. J. Racicot, and S. H. Lee Citation: Applied Physics Letters 54, 724 (1989); doi: 10.1063/1.100873 View online: http://dx.doi.org/10.1063/1.100873 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of passivation on different GaAs surfaces Appl. Phys. Lett. 103, 173902 (2013); 10.1063/1.4826480 Passivation of GaAs(111)A surface by Cl termination Appl. Phys. Lett. 67, 670 (1995); 10.1063/1.115198 Sulfur passivation of GaAs surfaces AIP Conf. Proc. 227, 118 (1991); 10.1063/1.40657 The chemistry of sulfur passivation of GaAs surfaces J. Vac. Sci. Technol. A 8, 1894 (1990); 10.1116/1.576822 Photoelectrochemical passivation of GaAs surfaces J. Vac. Sci. Technol. B 1, 795 (1983); 10.1116/1.582680 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.210.2.78 On: Wed, 26 Nov 2014 05:00:25Surface passivation of GaAs H. H. Lee,a) R J. Racicot, and S. H. Lee Department of Chemical Engineering, University of Horida, Gainesville, Florida 32611 (Received 29 July 1988; accepted for puhlication 15 December 1988) We have successfully passivated the surface of n-type (100) GaAs on the basis of P2SS1 NH40H treatment of the surface. A fivefold increase in the photoluminescence (PL) intensity results at room temperature when the surface is passivated and the PL intensity remains the same even after ten days' exposure to room air. Current-voltage characteristic also corroborates the PL measurements and shows that the GaAs surface retains its integrity when passivated with P 2SS and its electronic characteristic remains invariant with time even after exposure to air for one month. The results are indications of the robust stability of the passivated GaAs surface. Recently, there has been a renewed interest in improv ing the poor electronic quality of GaAs surface 1-4 which is caused by the high density of surface states on GaAs formed by segregated arsenic atoms5 via oxidation reactions. fi These arsenic atoms are the main eause for the deep level traps which pin the Fermi level and increase the nonradiative rc combination.7-'i This inherent problem has limited the per formance of existing GaAs-based electronic and opto electronic devices and 1ms stilI prevented successful develop ment of GaAs-bascd metal-insuIator-semiconductor tech nology. The root cause of arsenic segregation lies in the presence of oxygen that is always present in a minute amount. The presence of oxygen causes oxidation of GaAs to arsenic ox ide and gallium oxide and then the gallium atoms in the vicinity of the arsenic oxide gradually extract oxygen from arsenic: oxide to form gallium oxide, leading to the segrega tion of arsenic atoms.10,ll This is due to the fact that the thermodynamic equilibrium composition prohibitively fa vors gaHium oxide over arsenic oxide because of the higher heat of formation of gallium oxide than that of arsenic oxide. Therefore, the success of any passivating technique hinges on choosing a species that makes the surface repel approach ing oxygen and at the same time has a higher heat of oxide formation than that of gallium oxide. The first requirement can be satisfied by choosing a chemical species that adsorbs strongly on the GaAs surface as an impenetrable passivating barrier. Thus, a species that chemisorbs strongly has to be chosen. as is well known in catalytic reactions (e.g., Ref. 12) where a chemisorbing species such as phosphorus com pounds occupies the active surface sites, thereby preventing adsorption of other species. Excellent candidates that meet the two requirements are phcsphorus compounds since the heat offormation of phosphorus oxide is higher than that of Ga203• The samples used in the experiments are silicon-doped, liquid-encapsulated Czochralski grown GaAs with a (100) orientation. The doping level is 1,5-2.3 X 101'/cm3 with a resistivity of 0,0015-0,0021 n cm and a mobility of 1475- 1834 cm:? IV s. The sample wafers were first cleaned, with a subsequent rinsing in trichloroethane, acetone, methanol, and de-ionized (Dl) water. Various etching solutions were "' To whom correspondem:(' should be addressed. i.nvestigated based on sulfuric acid or ammonium hydroxide mixtures. Best results were obtained from a mixture of NH.!OH;H202:H20 in a reaction rate limited composition. This is believed to be due to the GaZ01 being more readily soluble in alkaline-based etchants.13 Wet etching usually leaves behind a layer of a mixture of lower oxides and other compounds. To remove the interfacial oxides of gallium and arsenic, the samples were immersed in concentrated NH40H/H20 (1:2) solution, followed by a brief dip of the sample in a diluted HN03/H20 ( 1: 19) solution for removal of arsenic. For further removal of arsenic, the samples were subjected to photochemical activation (254 nm) of the sur face while squirting with de-ionized water. [.14 The samples thus pn~pared were then dipped directly into a passivating ~,olution and blow-dried with nitrogen after removal from the solution. The solutions used were PCl] and PzSs dis solved in NH40H. The best result was obtained at a P 2S5 concentration of 0.1 g/ml. It is noted that P 2SS may hydro lyze in NH40H at high concentrations or upon heating. Photoluminescence (PL) intensity measurements and cur rent-voltage (f-V) profiles were obtained for botb the passi vated <!lid unpassivated (but cleaned) samples. The room temperature PL measurements 1 were made with an argon laser at a wavelength of 488 nm and an output power of 350 m W. The J-V profiles were obtained with an electrochemical semiconductor profiler system (Polaron 4200 from Bio Rad). The system uses an electrochemical cell containing an electrolyte. It employs an electrochemical Schottky contact, which is equivalent to the conventional metal Schottky con tact, on the polished side of GaAs wafer while ohmic con tacts were made by pressure on the other side. The results of the PL measurements before and after various treatments are summarized in Table I. The PL inten sity from unpassivated (but cleaned) samples is the refer ence for the normalization. As shown in the table, the PL intensity for the P 2Ss/NH40H-treated sample increases by a factor of 5 as a result of the passivation. Further, the PL intensity does not decrease even after exposure to room air for ten days. The sample was left as originally mounted in air and the PL intensity was periodically measured from the same spot. The result for pel} shows that the PL intensity increases by a factor of 3.5 bilt then gradually decreases to 2.5 after exposure to room air for tell days, indicating some segregation of arsenic atoms. Because of recent passivation 724 AppL Phys. Lett. 54 (8). 20 February 1989 0003-6951/139/080724-03$01.00 @ 1989 American Institute of PhysiCS 724 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.210.2.78 On: Wed, 26 Nov 2014 05:00:25TABLE t Increase (lfthe relative room-temperature PL intensity after var ious chemical and photochemical treatments. Sample Un passivated c Unpassivated C, e LJnpassivated c, e, NH40H Unpassivated c, e, HNO, Ullpassivated c, e, Dr Passivated, 1\S, Passivated, PC!, Passivated, Na2S'9H20 c-c1eaning. e--ctching. DI-activated D1 washing. Normalized PL intensity after treatment 1 I 1 1.4 1.4 5 3.5 5 Normalized PL inkllsity after ten days in room air 5 2.5 3.5 success reported in the literature based on sodium sul fides, [-3 the samples treated with Na2S'9H20 were also test ed. The result in the table shows that the PL intensity in creases by a factor of 5 but then decreases to 3.5 after ten days. More details on the treatments based on PCl3 and Na2S'9H20 can be found in Ref. 15. The fact that the PL intensity remains invariant with time for P2Ss-treated GaAs is evidence that the surface remains the same with time. The purpose of any passivation is to retain the integrity of the surface under the passivating layer. Therefore, passi vation is successful ifthe electronic state of the surface being passivated is invariant with time. An example of unsuccess ful passivation is shown in Figs. 1 and 2. The /-V profile for a sample treated with PCl3 that was obtained right after passi vation is shown in Fig. 1. It is seen that the usual 1-V profile for a Schottky junction results. When the same sample was exposed to air for seven days, the J-V profile changed to that shown in Fig. 2. It is apparent that there are two modes of conduction mechanism and that the electronic state of the surface changed. It is noted in this regard that for n-type samples, the negative region of the proflle corresponds to the forward-bias region. It> It is also noted that the 1-V profile of VOLTAGE FIG. 1. J-V profiles of pel,-passivated GaAs right after passivation. 725 AppL Phys. Lett., Vol. 54, No.8, 20 February 19S9 FIG. 2. 1-V profile ofPClrpassivated GaAs after exposure to room air for seven days for the same sample as in Fig. 1. the unpassivated sample, when exposed to air after cleaning, shows the same dip (negative resistance) as in Fig. 2 around the same forward bias (--0.8 V). The 1-Vprofiles obtained for the P2Ss-treated sample right after the passivation and after exposure to air for one month are almost identical with that in Fig. 3, which is the profile after exposure to air for seven days, and thus are not shown here. It can be conduded that the passivation is successful in that the 1-V profile is invariant with time for the P2Ss-treated samples. The dip in the I-V profile for unpassivated sample and that in Fig. 2 may be explained in terms of tunneling effect dominating at low voltage level due to the surface states created by segregated arsenic atoms and oxides, and then the usual thermionic emission dominating at high voltage level. The point, however, is that regardless of the exact nature of the J-V characteristics, the electronic state of the P 2Ss-treat ed samples is invariant with time. It can be concluded from the PL and J-V measurements that the GaAs surface can be successfully passivated with p 2SS' Further, the passivated surface is very robust in the o~.') GaAs 0..1. 0.3 0.2 0.1 c-/ 0.0 E u ! -0. [ -0.2 -0.3 -o,t, -0.5 -2.0 (PASS) }",)S t -l.~ -1.0 -O.S G.U l.0 1.5 VOl.TAGE (\I) I I;; : 2.0 FIG. ~1. J-V profile of l'oS,-passiva,ed GaAs after exposure to air for seven days. Lee, Racicot, and Lee 725 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.210.2.78 On: Wed, 26 Nov 2014 05:00:25sense that the increase in the PL intensity remains the same and the J-V characteristic shows no sign of change with time, even after more than seven days' exposure to room air. Work is progressing on the nature of the passivation. We are grateful to Arnold Howard for the J-V measure ments, to our Microfabritech and DARPA programs for partial support of the work, and to Paul Honoway for his interest in the work. Our special thanks are to Louis Figue roa, who as an electrical engineering faculty then and now a researcher at Boeing has given constant encouragement and advice in difficult times. IS. D. Oftsey, I. M. Woodall, A. C. Warren, P. D. Kirchner, T. I. Chappel, and G. D. Pettit, App!. Phys. Lett. 48, 475 (1986). 'c. J. SandroJf, R. N. Nottenburg, J. C. Bischoft', and R. Bhat, Appl. Phys. Lett. 51, 33 (1987). 726 Appi. Phys. lett., Vol. 54, No.8, 20 February 1989 3D. J. Skromme, C. J. Sandrofl', E. Yablonovitch, and T. Gwitter, App!. Phys. Lett. 51, 2022 (1987). 4E. Yablonovitch, C. 1. Sandrof!', R. Bhat, and T. Gwitter, App!. Phys. Lett. 51, 439 (1987). 'w. E. Spicer, P. W. Chye, P. R. Skeath, C. Y. Su, and l. Lindau, J. Vac. Sci. Techno!. 16, 1422 (1979). "H. H. Lee and L. Figueroa, J. Electrochcm. Soc. 135, 496 ( 1988). 7D. E. Aspnes, Surf. Sci. 132,406 (191\3). Xc. H. Henry, R. A. Logan, and F. R. Merritt, J. App!. Phys. 49, 3530 (1978). oR. P. H. Chang, T. T. Sheng, C. C. Chang, and J. J. Coleman, App!. Phys. Lett. 33, 341 (1978). HIe. O. Thermond, G. P. Schw:l.rtz, G. W. Kammlot, and B. Schwartz, J. Electrochem. Soc. 127, 1366 (1980). "E. Capasso and G. F. WilIiams,J. Electrochem. Soc. 121), 921 (1982). I.'H. H. Lee, Heterogeneous Reactor Design (Butterworth, Stoneham, MA, 1985), Chap. 5. uS. D. Mukherjee and D. W. Woodard, Gallium Arsenide (Wiley, New York, 1985), Chap. 4. l4N. A. rYeS, G. W. Stupin, and M. S. Leung, App!. Phys. Lett. 50, 256 ( 1(87). /SR. J. Racicot, M.S. thesis, University of Florida, Gainesville, FL, 1987. "'Bio-Rad Company (persona! communication). Lee, Racicot, and Lee 726 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.210.2.78 On: Wed, 26 Nov 2014 05:00:25

1.2810980.pdf

Lasers, Spectroscopy and New Ideas: A Tribute to Arthur L. Schawlow William M. Yen and Marc D. Levenson R. E. Slusher , Citation: Physics Today 42, 4, 69 (1989); doi: 10.1063/1.2810980 View online: http://dx.doi.org/10.1063/1.2810980 View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/4 Published by the American Institute of Physicstion of differences in the depth of surface-state annealing in silicon wa- fers: Wafers with few defects showed the annealing effect 50-100 fj,m be- yond the irradiated region, whereas in wafers with a high level of structural damage (such as those that were heavily implanted) the annealing ef- fect was confined to the irradiated region. Each chapter contains many illus- trations and a bibliography . This book augments the texts already published and will find a place on many a pro- fessional's bookcase. I recommend it. RICHARD LEE Amperex Slatersville, Rhode Island Lasers, Spectroscopy and New Ideas: A Tribute to Arthur L Schawlow William M. Yen and Marc D. Levenson Springer-Verlag, New York, 1987. 337pp. $45.00 he ISBN 0-387-18296-9 This book allows the reader to enjoy, at least remotely, the experience of physics research with Art Schawlow. Its 19 short articles, whose authors all have been students of Art's at Stan- ford University over the past 25 years, cover the three primary areas to which Schawlow has richly contribut- ed—lasers, spectroscopy and "new ideas." Each article includes reminis- cences of Art's humor, his excellent physics intuition and, most important- ly, the immense joy and enthusiasm he brings to his research, teaching and lectures. It is interesting that many physics concepts can be more clearly grasped and understood with the in- formal writing styles used in this book. As one might expect, the style and scientific content of the brief articles in this volume vary widely. Some are detailed and will serve as excellent references and reviews. Examples are the three articles on solid-stat e spec- troscopy, by Roger M. Macfarlane ("Optical Spectral Linewidths in Sol- ids"), Satoru Sugano ("Spectroscopy of Solid-State Laser Materials") and George F. Imbusch and William M. Yen ("Ruby Solid-State Spectroscopy: Serendipitous Servant"). Sugano dis- cusses the early history of the laser. This is an ideal time to look back at the development of the laser, one of the major advances of this century, but I was disappointed that Schawlow's ear- ly laser research and that of his collaborators is not covered in this book. There is an interesting anecdote from the first session at a conferenceImprove your Low Level Light Measurements .. .Two Ways! MODEL 197 LIGHT CHOPPER • Single/Dual Frequency operation • Simple Frequency Control using Pushbuttons or External Oscillator • 15Hz-3KHz Operation for your Detector MODEL 113 LOW NOISE PREAMP • Battery Operation Eliminates Line Interference I Single Ended/Differential Input Allows Uncompromised Detector Matching • AC or DC Coupling provides Optimized Low Frequency Operation To improve your low level light measurements, consider our high perform- ance components. For free literature, call or write today. PRINCETON APPLIED RESEARCH P.O. BOX 2565 • PRINCETON, NJ 08543-2565, USA • 609/452-2111 • TELEX: 843409 Circle number 35 on Reader Service Card"•'•» New Lower Price LEV881 fir VACUUM GAUGES: • Outstanding performance, long life, low maintenance costs • 10 ranges from 10"6 torr to atmosphere • Thermocouple, cold cathode, and diaphragm type gauges • Rugged, corrosion-resistant gauge tubes • Single or dual vacuum controllers • Vacuum recorders • Compact, stable, dependable; rapid response HASTINGS Request FREE CATALOG, #300. Teledyne Hastings-Raydist "W^TELEDYNE Hampton, VA 23661 U.S.A. HASTINGS-RAYDIST Telephone (804) 723-653 1 Circle number 36 on Reader Service Card PHYSICS TODAY APRIL 1989 69SPEAKEASY SPEAKSHI LANGUAGE For physicists who want to use computational tools put together for physicists by physicists. For researchers who want to con- centrate on the mathematics of a problem rather than learning to program. For fast accurate results. Speakeasy meets your needs! Speakeasy is a high level problem solving tool that has been used for many years by physicists in laboratories and universities throughout the world. It is a well tested system that enables you to bring together the tools you need to carry out exploratory calcula- tions and rapidly analyze data. Matrix algebra, interactive graphics, and the many special functions used by physicists combine with a natural language to provide a truly interactive environment in which you can concentrate on your calculations rather than the mechanics of running a computer. The same Speakeasy is avail- able for IBM computers with TSO and CMS, for DEC VAX's under VMS and Micro VMS, and for Micro computers using MS-DOS and PC-DOS. You can even transfer results from one type of computer to another to exploit specific machine capabilities. Want to learn more about the package that is already satisfying many of your colleagues? • Speakeasy *« Computing Corporation 222 west Adams street Chicago, IL 60606 312 346 2745 UK Numerical Algorithms Group Ltd. Maylield House 256 Banbury Road OxlordOX2 7DE Uniled Kingdom 0865 511245 Circle number 37 on Reader Service Card 70 PHYSICS TODAY APRIL 1989on lasers in June 1959: Schawlow suggested that the laser would most likely be used as an oscillator and so should be named "Light Oscillation by Stimulated Emission of Radiation"— or LOSER. I would love to have heard more about this early period from the laser pioneers. Several histories of this period will soon be available [see the article by Joan Bromberg in PHYSICS TODAY, October 1988, page 26], and some important figures, such as Charles Townes in his remarks in his 1986 Beckman lecture at the Universi- ty of Illinois, have already begun to recount their versions. The first section of this book on lasers and laser spectroscopic tech- niques includes a contribution from Theodor Hansch that describes the beautiful work on Doppler-free preci- sion spectroscopy and the original ideas on laser cooling. Today cooling of atoms well below 10 fiK has been attained, a breakthrough that can be directly traced to the Stanford group. Of course, it was the brilliant research in spectroscopy for which Schawlow received the Nobel Prize in 1981. The book includes a song written for Art's 65th birthday and a picture of the Stanford group taken the day the Nobel Prize was announced. Again , the joy of physics research is the book's primary focus. But it also manages to convey Schawlow's deep humanity, scientific achievements and broad influences on physics through his graduate students in a thoroughly enjoyable, though ran- dom, manner. R. E. SLUSHER AT&T Bell Labs Murray Hill, New Jersey Exercises in Astronomy Edited by Josip Kleczek Reidel (Kluwer), Boston, 1987. 339 pp. $64.00 he ISBN 90-277-2409-1; $19.50pb ISBN90-277-2423-7 In all fields legends abound, and a laboratory book by Marcel G. J. Minnaert has been an astronomical legend. Since its publication in 1969, following 25 years of development at Utrecht University in the Nether- lands, Minnaert's Practical Work in Elementary Astronomy has been wide- ly respected. Just as Minnaert's Na- ture of Light and Color in the Open Air (Dover, New York, [1948]) led many people to a new appreciation of the sky, Minnaert's laboratory book provided a wide range of interesting experiments to astronomy students on all levels . Josip Kleczek, a Czech astronomerwho is coordinator for the Interna- tional Astronomical Union of the International Schools for Young As- tronomers, has edite d a new edition of Minnaert's laboratory book. He has retained Minnaert's organization, with its split between "The Planetary System" and "The Stars," as well as 20 of Minnaert's 34 planetary exer- cises and 33 of 40 stellar exercises. To these, he has added 2 planetary and 19 stellar exercises, credited to astron- omers from countries around the world, including the US, Belgium, Australia, the UK, Brazil, France and Czechoslovakia. As in Minnaert's original, the exercises are labeled "S" for observations carried out on the real sky and "L" for laboratory exer- cises carried out indoors. The ideal is to start work on a given night out- doors, and then continue indoors, though weather will rarely permit this ideal to be met. Astronomical observing often is dis- parate from the astrophysics taught in class, because it is easier to observe the locations of objects in the sky than to analyze their radiation. Many of the exercises in this book aim at combining theory and observation. In the first two of six exercises con- tributed by David A. Allen (Anglo- Australian Observatory), for exam- ple, one learns about blackbody radi- ation—as theoretical a concept as there is. The first is a straightfor- ward plotting exercise leading to Wien's displacement law. The second concerns infrared photometry—Al- len's specialty. The third exercise is about interstellar reddening. Allen's three other exercises consider studies of the atomic spectral transitions known as forbidden lines and show how to calculate abundances. Be- cause such calculations make use of recombination coefficients, statistical weights and collision strengths, some sophistication is called for on the student's part. Surely the lengthy exercise on magnetohydrodynamics entitled "Practice with MHD," by Donat G. Wentzel (University of Maryland), is accessible only to stu- dents with advanced physics experi- ence. For such students, however, it is an unparalled opportunity to learn about an important field. Some of the exercises are quite up to date. Jean Surdej (University of Liege, Belgium) even provides one about gravitational lenses. But the bulk of the exercises are unaltered since the first edition, so the refer- ences have dated. Too, the telescope described in the opening pages of the technical notes is not of a type likely to be in current use. Nor are comput- ers or even calculators put to explicit

1.575054.pdf

Surface temperature determination in surface analytic systems by infrared optical pyrometry Donald R. Wheeler, William R. Jones Jr., and Stephen V. Pepper Citation: Journal of Vacuum Science & Technology A 6, 3166 (1988); doi: 10.1116/1.575054 View online: http://dx.doi.org/10.1116/1.575054 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/6/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Hugoniot temperatures and melting of tantalum under shock compression determined by optical pyrometry J. Appl. Phys. 106, 043519 (2009); 10.1063/1.3204941 In situ determination of the surface roughness of diamond films using optical pyrometry Appl. Phys. Lett. 72, 903 (1998); 10.1063/1.120931 Neutral beam interlock system on TFTR using infrared pyrometry Rev. Sci. Instrum. 57, 2063 (1986); 10.1063/1.1138739 Effect of scattered light on temperature measurement by optical pyrometry Rev. Sci. Instrum. 47, 1547 (1976); 10.1063/1.1134577 Optical Pyrometry J. Appl. Phys. 11, 408 (1940); 10.1063/1.1712789 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 69.166.47.134 On: Mon, 08 Dec 2014 00:15:55D. S. Blair and G. L. Fowler: Thermocouple mounting on semiconductor samples The method of mounting a thermocouple to a semicon ductor described above has clear advantages over previously applied techniques. These advantages include (i) proven re liability and reproducibility over a broad temperature range, (ii) applicability to a variety of sample sizes and configura tions, and (iii) ease of implementation. Acknowledgment: This work was supported by the United States Department of Energy under Contract No. DE AC04-76DP00789. 'M. J. Bozack, L. Muehlhotf, J. N. Russell, Jr., W. J. Choyke, and J. T. Yates, Jr., J. Vac. Sci. Techno\. A 5, I (1987). Surface temperature determination in surface analytic systems by infrared optical pyrometry Donald R. Wheeler, William R. Jones, Jr., and Stephen V. Pepper National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio 44135 (Received 13 May 1988; accepted 30 July 1988) I. INTRODUCTION The measurement of specimen surface temperature is impor tant for many experiments in surface science. Since modern ultrahigh vacuum surface analytic systems with load locks have the capability to change samples rapidly for routine analysis, the technique of temperature measurement should not compromise that capability. However, a thermocouple spot welded to or near the specimen surface and hard wired to an electrical feed through requires opening and rebaking the analytic chamber to change specimens, effectively re moving the system from routine analytic service. Therefore, other methods must be developed if the system is to be capa ble of both surface science experiments as a function oftem perature and routine analytical service. Arrangements that continue to use a thermocouple but employ sliding electrical contacts on the sample mounts have been recently described.I•2 However, these arrange ments require custom-made sample mounts and internal modification of the analytical chamber. Optical pyrometry is a technique that offers an attractive alternative, because it requires no mechanical or electrical contacts to the speci men. In the temperature range above 700 DC, either disap pearing filament or two-color optical pyrometers are com monly used. Optical pyrometry, between room temperature and 700 DC, requires infrared (lR) pyrometry, however, and is seldom used. The major barriers to its use are the usually unknown and possibly variable emissivity of the specimen surface, the poor IR transmission of the vacuum chamber viewport and the large viewing area required by most in frared pyrometers. In this note we describe our method of using infrared optical pyrometry to measure specimen tem peratures in the range 70-500 DC in which these difficulties are largely overcome. 3166 J. Vac. Sci. Technol. A 6 (6), Nov/Dec 1988 II. APPARATUS AND MATERIALS Our method is based on a commercially available infrared microscope3 with a right-angle, long working distance objec tive that has a focal spot diameter of 1 mm at a focal distance of 53 cm. The instrument has a liquid-nitrogen-cooled, indi um-antimonide detector with a 1.8-5.5 pm bandwidth and can detect temperatures close to room temperature. The in strument was operated as a radiometer rather than in its calibrated direct temperature measurement mode. Since the signal from the pyrometer varied by several orders of magni tude over the temperature range used here, it was processed by a logarithmic amplifier before reading. Transmission of the IR radiation was improved by replacing the metal coated glass vacuum chamber viewport with a quartz viewport cov ered with a tantalum mesh. The molybdenum specimen stubs for our system (VG ES CALab, Mk II.) have an internal resistive heating element. Our specimens were disks 6.35 mm in diameter and 6.35 mm high with a 3-mm-wide flat ground on the side facing the viewport. The problem of unknown specimen emissivity was dealt with by focusing the microscope on a film of graphite on this flat. The graphite film was formed by painting an alcohol dispersion of colloidal graphite (DAG) onto the flat. The DAG has a high emissivity that is independent of the surface on which it is painted, is insensitive to the com mon specimen treatment of ion bombardment and un changed by exposure to most gases. The particular specimen used for the initial calibration is depicted in Fig. 1. For initial calibration, the specimen had a type-K thermocouple spot welded to its side, diametrically opposite the DAG coated flat, and 0.5-mm-diam beads of indium and tin soldered to its top surface. The In and Sn melting points were used as fixed temperature points, as described below, and these materials 3166 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 69.166.47.134 On: Mon, 08 Dec 2014 00:15:553167 Wheeler, Jones, Jr., and Pepper: Surface temperature determination in surface analytic systems 3167 VIEWPORT- FIG. I. Specimen for calibration: (a) heatable molybdenum stub, (b) tanta lum disk, (c) flat side ofTa disk with DAG, (d) indium and tin beads, and (e) thermocouple spot welded to side ofTa disk, removed after initial cali bration. were chosen for their low vapor pressures in the temperature range used here. For clarity, the method of clamping the Ta disk to the heated specimen stub is not shown in Fig. 1. How ever, it could be easily replaced, and after initial calibration, the top surface of other such disks (without the In and Sn beads or thermocouple, but with the DAG coated flat) were available for surface analysis. III. CALIBRATION A. Initial calibration Our approach requires initial calibration of the pyrometer response against known specimen temperatures, because the DAG emissivity and viewport transmission are unknown. Before calibration, the initial calibration specimen was cy cled between room temperature and 500 ·C several times to equilibrate both the DAG and the metal beads. Then the calibration curve of pyrometer response versus thermocou ple reading shown by the circles in Fig. 2 was obtained. Our thermocouple readout gave temperature readings to the nearest degree, and it is evident from the curve that the preci sion of the calibration is at least that good. The accuracy of the thermocouple readings was assessed by determining the melting points of the In and Sn beads. The measured melting points were 154 and 227 ·C for In and Sn, respectively. The corresponding handbook values are 157 and 232 ·C. The discrepancy is probably due to heat conduction in the thermocouple leads and appears to be pro portional to the difference between the specimen tempera ture and room temperature. Accordingly, a linear correction of the thermocouple readings was applied. The corrected calibration curve is given by the crosses in Fig. 2. From 50 ·C to the melting point ofSn, the calibration is probably correct to ± 1 ·C. The extrapolation of the thermocouple correction to 500·C introduces more uncertainty at higher tempera tures, but we feel that the calibration is always within 5 ·C of the true temperature. B.Spotchecks Visual observation of the melting ofIn and Sn can be used to check the temperature of other specimen configurations J. Vac. Sci. Technol. A, Vol. 6, No.6, Nov/Dec 1988 .70 .60 c:ffJ m ([]<iIP .50 ~ > ()l] ,.; ()l] :::l .40 <III "-em .... :::l 0 <III t :30 <III , em is ...I G .20 # 0 UNCORRECTED T .C. READING 0 CORRECTED T. C. READ I NG .10 a a 0 100 FIG. 2. Pyrometer calibration curves: 0 uncorrected thermocouple readings on abscissa and D thermocouple readings linearly transformed for correct room-temperature and indium and tin melting points. and to check the stability of the initial temperature calibra tion. Specimens of the form shown in Fig. 1 allow the use of poly-or single-crystalline materials commonly available in the form of bars or rods. Although thin films to be studied can be deposited on these disks, it is often more convenient to deposit films directly on the surface of the heatable molyb denum stub. Therefore, we have fabricated such a stub with a DAG coated Ta foil tab spot welded to the edge of the top surface and protruding 2 mm below the edge. An In bead on the top surface of this stub melted at a temperature of 154 ·C as measured by the pyrometer. Thus, the Ta tab was 3 ·C cooler than the top surface at 157·C. Assuming that the error is proportional to the increase in temperature above room temperature, we expect the error to be < 10·C at 450·C. This crude calibration is sufficient for our purposes but could be improved by the use of both In and Sn beads or by mounting a thermocouple on the stub and repeating the initial calibration. In any case, the precision of the readings is still better than 1 .C. After the initial calibration, the thermocouple was re moved from the calibration specimen of Fig. 1. We have found it reassuring, however, to keep the specimen, with its In and Sn beads, mounted on a heatable stub and easily avail able. By verifying that the melting points measured with the pyrometer agree with the handbook values, the pyrometer calibration can be verified whenever desired. During six weeks of intermittent use, there was no change in the calibra tion. We have removed and repainted the DAG on the cali bration specimen and used the melting points of the metal beads to assure that the calibration did not change. The cali bration also did not depend critically on the angle between the pyrometer axis and the DAG coated flat on the specimen or on the angle between the pyrometer axis and the quartz window. Both angles could be within ± 1 SO of perpendicular Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 69.166.47.134 On: Mon, 08 Dec 2014 00:15:55Wheeler, Jones, Jr., and Pepper: Surface temperature determination in surface analytic systems and were easily set, by eye. In general, this method of tem perature measurement has been notably stable and repro ducible. ACKNOWLEDGMENTS The authors wish to thank Ralph D. Thomas for soldering the In and Sn beads and Dennis P. Townsend for assistance in the initial stage of this work. IJ. M. Lindquist and J. C. Hemminger, J. Vac. Sci. Techno!. A 5, 118 (1987). 2G. S. Chottiner, W. D. Jennings, and K.1. Pandya, J. Vac. Sci. Techno!. A 5,2970 (1987). 3Barnes Engineering Company, Stamford, CT 06904, model RM-2A Mi croscope with model no. RM-164 right-angle, long working distance ob jective. An easily constructed, inexpensive cold stage for use in ultrahigh vacuum G. Nelson, T. Ohlhausen, E. Hardegree, and P. Schulze Departments of Physics and Chemistry, Abilene Christian University, Abilene, Texas 79699 (Received 28 April 1988; accepted 30 July 1988) A liquid-nitrogen-cooled sample holder for use in ultrahigh vacuum systems is described. The holder is well suited for either resistive or electron-beam heating, and requires less coolant than some other designs. The holder can be constructed from commercially available parts for about $350, using commonly available tools. Many experiments which are performed in ultrahigh vacu um require cooling of a sample to liquid-nitrogen tempera tures. Some traditional designs for liquid-nitrogen-cooled sample holders utilize a hollowed-out block of material, usually oxygen-free high-conductivity (OFHC) copper, to which the sample is attached.I-5 The liquid nitrogen is fed through the block by stainless-steel capillaries which pass through a vacuum flange. Typically, the block must be elec trically isolated from the sample, but must still make good thermal contact. This is often accomplished by placing a thin sapphire wafer between parts which are to be electrically but not thermally separated. The overall design usually requires a number of custom-made pieces and may be expensive and time intensive to construct. We have designed and tested a new cold stage which is easier to construct and is somewhat less expensive. In addi tion, it uses far less liquid nitrogen and does not clutter the region of space around the sample. The sample holder allows cooling the sample to -80 K, and is well suited for resistive or electron beam heating to above 2300 K. The holder is mechanically sound and is easily adjusted for different sys tem geometries. The sample holder is shown in Fig. 1 and the parts used in the construction are listed in Table I. The total cost of the parts, including the stainless-steel liquid-nitrogen lead-in capillaries and the tantalum rods and wires for mounting the sample, was about $350. The major items used in the design consisted of commercially available A-in. stainless-steel tub ing, elbows, crosses, and tees, and three Ceramaseal electri cal insulators. As shown in Fig. 1, the capillaries (a) are attached to the V-shaped sample holder (g) which serves as a flow-through liquid-nitrogen reservoir. The three insula tors (d) provide electrical isolation for resistive heating or biasing of the sample. The holder is mounted to the center shaft of a manipulator by set screws threaded into a stainless- a b c d e f 9 FIG. I. Liquid-nitrogen-cooled sample holder consisting of (a) feed-in cap illaries, (b) adjustable center support mounting cross, (c) adjustable cold stage support mounting tees, (d) Ceramaseal electrical isolators, (e) adjus table or fixed sample mounting elbows, (f) tantalum sample mounting rods, and (g) stainless-steel reservoir tubes. For scale purposes, diameter of the tubes is ! in. 3168 J. Vac. Sci. Techno!. A 6 (6), Nov/Dec 1988 0734-2101/88/063168-02$01.00 © 1988 American Vacuum Society 3168 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 69.166.47.134 On: Mon, 08 Dec 2014 00:15:55

1.98249.pdf

Highly stable indium alloyed TbFe amorphous films for magnetooptic memory Tetsuo Iijima Citation: Applied Physics Letters 50, 1835 (1987); doi: 10.1063/1.98249 View online: http://dx.doi.org/10.1063/1.98249 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/50/25?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structure analysis of TbFe amorphous films using extended xray absorption fine structure J. Appl. Phys. 68, 4035 (1990); 10.1063/1.346240 Magnetic and magnetooptic properties of Inalloyed TbFe amorphous films Appl. Phys. Lett. 54, 2376 (1989); 10.1063/1.101086 Domain size measurements on GdTbFebased thinfilm structures for magnetooptical recording J. Appl. Phys. 63, 2141 (1988); 10.1063/1.341070 High performance magnetooptical TbFeCo disks prepared by coevaporation Appl. Phys. Lett. 51, 288 (1987); 10.1063/1.98475 Magnetic and magnetooptical properties of TbFeCo amorphous films J. Appl. Phys. 61, 2610 (1987); 10.1063/1.337889 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.180.142.23 On: Sat, 13 Dec 2014 10:17:48Highly stable indium alloyed Tbfe amorphous fUms for magneto-optic memory Tetsuo lijima NTT Electrical Communications Laboratorie.l; Tokai, lbaraki 319-11. Japan (Received 9 February 1987; accepted for publication 27 April 1987) Indium ulioyed TbFe amorphous films for use as a magneto-optic memory are proposed and studied. These TbFeIn films show strong resistance to corrosion and oxidation. Indium is effective in suppressing oxygen diffusion into the films. An oxygen diffusion coefficient of 5 X 10-25 mlls is calculated for ThFeln films incubated at room temperature. Activation energy is 1.3 eV. This value is over 1.5 times larger than that of TbFe films, where the value is obtained with ellipsometry measurements by R. Allen and G. A. N. Conn eli [J. Appl. Phys. 53, 2353 (1982)J. Amorphous rare-earth transition meta! films have been regarded as very promising materials for high-density mag neto-optic memories.I•2 However, these materials exhibit some intrinsic problems in resisting corrosion and oxidation. Many studies have been done to improve corrosion resis tance through anoying.3 This letter shows that TbFe amor phous films aHoyed with indium (In) have an extremely high resistance to corrosion and oxidation. The TbFeln and TbFe films used were prepared by rf sputtering from composite targets at 10--25 urn/min onto a glass (Corning No. 0211) substrate under 3--4 X 10--2 Torr Ar pressure. A protective layer was not deposited. Average film compositions were determined by both x-ray fluores cence analysis and inductively coupled plasma emission spectrometry, while composition depth profiles were mea sured by Auger electron spectroscopy. Reflectivities at 830 nrn, Kerr hysteresis ioops, and saturation magnetizations M, at 15 kOe field were also measured. Finally, accelerated aging tests were carried out in air at temperatures from 40 to 80 "C under constant 85% relative humidity (% R.H.). The reflectivities of (Tbo.2gFeo.7i )OQ5IuOOS and Tbo24 Feo.76 film surfaces are shown in Fig. 1 versus aging time in air at 70°C and 85% R. H. The TbFeln film's reflec tivity showed only a 10% decrease from its initial value of 63% even after 4000 h of aging. Few corrosion sites were observed on the film's surface, as Fig. 2(a) shows. On the contrary, the ThFe film without In showed an abrupt de crease in reflectivity within a few hours. Many corrosion sites occurred as shown in Fig. 2 (b). Composition depth profiles of TbFeIn and TbFe films are shown in Figs. 3 and 4. The TbFeln film profiles are shown before and after 150 and 2000 h of aging at 70 'C and 85% R. H. in Fig. 3. The TbFe film profile is shown after 20 h of aging under the same conditions in Fig. 4. The abscissa indicates film thickness normalized according to etching time until the etching reached the substrates of both films. The oxide layer etching rate was assumed to be half that of the inside film layer from measurements on the homogen eously oxidized film. Three characteristics were evident as can be seen in Fig. 3. First, Tb, Fe, and In concentrations were uniform inside the film. Second, oxygen diffused from the surface into the inside of the film as aging time increased. The surface oxide layer increase suggests a co-diffusion reaction between oxy gen in the air and the constituents of the ThFeln film. Third, increases in Tb and oxygen in the surface oxide layer were caused by more intense preferential oxidation ofTb than the other atoms. As a result, the Fe concentration in the surface layer was smaner than that inside the film. On the contrary, the TbFe film profile in Fig. 4 shows that preferential Tb oxidation depth increased about 35 nm in thickness after only 20 h at 70 "C, 85% R. H. It is also evident that oxygen completely diffused into the TbFe film where Tb and Fe concentrations were uniform. These results are qui.te different from those of the TbFeln films. As stated above. in TbFeln !Hrns, oxygen remains near the surface and homogeneous oxygen diffusion does not oc cur in the same way as in TbFe films. Both results show that low rate preferential Tb oxidation is dominant in TbFeln films and that oxygen diffusion into the films is rather slow. The differences in both the velocity of preferential Tb oxida tion and oxygen diffusion into the films between the TbFeln and TbFe films are caused by the presence ofIn atoms. These effects of In differ from those of AI, which does not suppress homogeneous oxidation from the film surface,> The TbFeln film Kerr hysteresis loops aged at 70 "C, 85% R. H. were measured. Neither changes in Kerr rota tion, Ok' nor in its polarity were found on the bottom surface side. On the contrary, with increased aging time, Ok de- 100 ~ 80 >-... .; '£ w ;;: OJ 0:: 0 ! 1 10 70°C, 85% R,H. ;1..=830 11m (Tbo,29 FeO.l1 }IUlS1nO.05 Aging time (hoursl FIG. I. Aging time dependences on reflectivity for (Tbo.,', Feo." )0>5 11lo-05 and Tbo.I.Fe,,76 films. 1835 Appl. Phys. Lett. 50 (25). 22 June 1987 0003-6951/87/251835-03$01.00 @ 1987 American Institute of Physics 1835 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.180.142.23 On: Sat, 13 Dec 2014 10:17:48(a) (b) 100.urn FIG. 2. Comparison of material degradation between ThFeIn and TbFe films: (a) ThFeIn film surface after 4000 h of aging at 70 'C, 85% R. II.; (b) TbFe film surface after 2 h of aging at 70 'C, 85% R. H. creased and its polarity changed in Tb-rich compositions somewhere from 80 to 280 h on the film side. When the film surface was etched to 50 nm, both Ok and the Kerr hysteresis loop polarity on the surface recovered to their initial values. These results indicate that oxygen diffused halfway through the film. These phenomena correspond to the results of the composition depth profiles described above. Aging time dependences of Ms were examined for (Tbo.30Feo.7o )097 Ina.o3 films 100 urn thick in the same man- "#. 60 200 (a) 20 In o--o-~.....,.o--o-o..o.""'" -tk.o--0-..0.-0-"'" ..a. o 50 100 150 200 (hI . C !n '!"~..o-o--O" ...o,...-O--«--o-"O--o---O.o-O.'1:r-CI--<f.-G. O~~~'---~r---~----~~~ o 50 100 150 200 (c) Film thickness (nm) FIG. 3. TbFeln film composition depth profiles before (aJ and after 150 h (b) and 2000 h (c) of aging time at 70 'C, 85% R. H. 1836 Appl. Phys. Lett.. Vol. 50, No. 25. 22 June 1987 ~ 60 -c:: 0 /c Fa .~ Cl! / 0 ... ... 40 r.: w (J r:.: 0 g (J 20 'E () ... 4: (I 0 50 100 Film tilicknes;s (nm) FIG. 4. ThPe film composition depth protile after 2(J h of aging at 70 'C, 85% R. H. ncr as in Fig. 1. The aging test was carried out with tempera ture as a parameter. The 11.( for TbFeIn films showed a gra dual increase for each aging temperature, as a result of the preferential Tb oxidation from the film surface. When the medium life is defined as the time for Ms to become 1.2 times larger than its initial value, the Arrhenius plot shown in Fig. 5 is obtained. These medium life values correspond to the times when the oxide layers reached a certain thickness at each temperature. The result shows that the oxidation process is activated at 1.3 eV. Oxygen diffusion coefficients Dv were determined from oxygen diffusion depth ( = 2,/i5.,1) at time interval t, which was obtained from the depth profiles mentioned previously. These values were calculated to be 10-22 m2/s at 70 OC, 85% R. H. and 5 X 10-25 m2/s at room temperature. Activation energy, as determined by these diffusion coefficients, was calculated as 1.1 e V. This value is almost the same as that calculated from the 111, changes mentioned above. Using data from Allen's and Connell's study,4 which used ellipsometry measure ments, medium lifetimes in which the TbFe film oxide layers increased up to certain thicknesses at several temperatures were calculated. A O.84-eV activation energy for TbFe films 105 100 8070605040 25 -104 '" ~ :':I 0 ~ w 103 ~ E ,;lEa:;;"; ',3eV ~ i ::iE 102 10 I I 2.6 2.8 3.0 3.2 3.4 FIG. 5. Medium life Arrhenius plot vs liT for (Tbo.3o Fea,o )097 Irt003 films. Tetsuo !ijima 1836 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.180.142.23 On: Sat, 13 Dec 2014 10:17:48was obtained by these calculations, The TbFeh film activa tion energy of 1.3 eV was over 1.5 times larger than that of TbFe films, and the medium life of TbFeIn films was esti mated to be 10 years longer at room temperature. The authors would like to thank Dr. Akihiko Yamaji and Dr. Iwao Hatakeyama for their valuable discussions. 1837 Appl. Phys, Lett, Vol. 50, No, 25, 22 June 1987 'p, Chaudhari, ], J. Cuomo, and R, J. Gambino, AppJ. Phys, Lett, 22, 337 (1973), 2N. Imamura, S. Tanaka, F, Tanaka, and Y. Nagao, IEEE Trans. Magn. MAG-21. 1607 (1985). 3K. Aratani, T, Kobayashi, S. Tsunashima, and S, Uchiyama, J. App!. Phys, 51, 3903 (1985), "R. Allen and G. A, N. Connell, J, App!. Phys, 53, 2353 (1982), Tetsuo lijima 1837 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.180.142.23 On: Sat, 13 Dec 2014 10:17:48

1.101219.pdf

Arsenicdoped CdTe epilayers grown by photoassisted molecular beam epitaxy R. L. Harper Jr., S. Hwang, N. C. Giles, J. F. Schetzina, D. L. Dreifus, and T. H. Myers Citation: Applied Physics Letters 54, 170 (1989); doi: 10.1063/1.101219 View online: http://dx.doi.org/10.1063/1.101219 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in ptype arsenic doping of CdTe and HgTe/CdTe superlattices grown by photoassisted and conventional molecular beam epitaxy J. Vac. Sci. Technol. A 8, 1025 (1990); 10.1116/1.577000 Properties of doped CdTe films grown by photoassisted molecularbeam epitaxy J. Vac. Sci. Technol. A 6, 2821 (1988); 10.1116/1.575608 Growth and properties of doped CdTe films grown by photoassisted molecularbeam epitaxy J. Vac. Sci. Technol. B 6, 777 (1988); 10.1116/1.584373 Controlled substitutional doping of CdTe thin films grown by photoassisted molecularbeam epitaxy J. Vac. Sci. Technol. A 5, 3059 (1987); 10.1116/1.574216 ptype CdTe epilayers grown by photoassisted molecular beam epitaxy Appl. Phys. Lett. 49, 1735 (1986); 10.1063/1.97231 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Fri, 19 Dec 2014 08:38:19Arsenic~doped CdTe epnayers grown by photoassisted molecular beam epitaxy R. L Harper, Jr., S. Hwang, N. C. Giles, and J. F. Schetzina Department of Physics. North Carolina State University, Raleigh, North Carolina 27695-8202 D. L. Dreifus Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695-7911 T. H. Myers Electronic Laboratory, General Electric Company, ,Syracuse, New York 13221 (Received 1 August 1988; accepted for publication 26 October 1988) We report the successful p-type doping of CdTe films with arsenic using the photoassisted molecular beam epitaxy growth technique. These doped epilayers were grown at substrate temperatures as low as 180 0c. The room-temperature hole concentrations in the CdTe:As layers ranged from 7X 1015 to 6.2X lOIS cm-1 as determined by van der Pauw-HaH effect measurements. We propose a doping mechanism responsible for the highp-type doping levels observed in the films. The arsenic acceptor ionization energy was found to be ~58-60 meV using low-temperature photoluminescence measurements. Cadmium telluride (CdTe) has many potential applica tions in the fabrication of electronic and optoelectronic de vices. 1 Interest in this wide-gap H-VI material also stems from its chemical compatibility and close lattice match to the important infrared detector material HgCdTe. However, the use of CdTe in device applications has been limited due to the tendency of this materia! to self-compensate. As a consequence, the introduction of dopants in CdTe, using conventional bulk or thin-film growth techniques, usually leads to low activation of the impurity and results in highly resistive as-grown materials, In order to realize device appli cations similiar to those developed in III-V materials, a thin film growth technique with demonstrated control over hoth n-type and p-type conductivities must be utilized. We are studying the enhancement of substitutional dop ing in CdTe thin films using a nonequilibrium technique, photoassisted molecular beam epitaxy (MBE)o By illumi nating the substrate during film growth, energies beyond those accessible by thermal means can be transferred from the impinging photons to the atoms and molecules present on the growth surface. This doping technique has already been successfully employed to activate two different types of impurities in CdTe: indium for n-type conduction (n.;;6X 1017 cm-3),2 and antimony for p-type conduction (p.;;2X lOI~ cm-3).1.4 Using the photoassisted MBE tech nique, the first growth of an all-thin-film p-n junction of CdTc was realized using In and Sb as the dopant materials.5 In addition, n-type CdTe:ln epitaxial layers prepared by photoassisted MBE have been used to fabricate CdTe metal semiconductor field-effect transistors.6 In this letter we report the successful preparation by photoassisted MBE of highly conducting p-type CdTe epi layers in which arsenic is used as the p-type dopant. We present the doping mechanism which is responsible for the high p-type doping levels we observe. We have also deter mined the ionization energy associated with the substitu tional arsenic acceptor in CdTe. In this work, an argon ion laser with broadband yellow-green optics (488,0-528.7 um) was used as an illumination source. The laser power density at the substrate during the film growth was approximately 90mW/cm2• The MBE system in which the CdTe:As epilayers were grown has been described in previous publications.7•8 In the present work, three MBE ovens were employed. Two of the ovens contained high-purity CdTe, while the third oven con tained As. The CdTe:As epiIayers were grown on semi-insu lating chemimechanically polished (100) CdTe substrates. Prior to insertion into the photoassisted MBE system, the substrates were degreased using electroni.c grade standard solvents. Next the substrates were etched in a weak (1 % ) solution of bromine in methanol and rinsed in methanol. And finally, they were dipped briefly in a 1:1 solution of hydrochloric acid and water to remove the native oxide, and rinsed in de-ionized water. Immediately prior to film growth, the substrates were preheated briefly in the MBE system to 300 °C. The CdTe:As films were grown using sub strate temperatures ranging from 180 to 230 0c, The arsenic even temperature was varied over the range 160-200 "c. Pri or to the growth of the As-doped layer, a l!Lm semi-insulat ing CdTe buffer layer was first deposited by conventional MBE (no substrate illumination). The CdTe:As layers grown under illumination were approximately 1.5,um thick. Electrical characterization of the cpilayers was carried out by means of van der Pauw-HaH effect measurements. Ohmic contacts were made to the CdTe:As epilayers by evaporating Ni/Cu/ Au (100 A/600 A./2000 A) over a pho toresist mask. The excess metals were removed by using a standard lift-ofl:' process. After deposition of the metal con tacts, the CdTe:As epilayers were annealed at 200°C for up to 30 min in a nitrogen atmosphere. Contacts prepared in this manner allowed Hall measurements to be performed over the temperature range 230--300 K for heavily doped epilayers. However, for lightly doped samples only room temperature measurements could be made, due to their higher resistance. Weare investigating alternate means to make electrical contact to p-type CdTe, and hope to report more extensive temperature measurements in the future. The photoluminescence (PL) characterization was per- 170 AppL Phys. Lett 54 (2), 9 January 1989 0003-6951/89/020170-03$01.00 © 1989 American Institute of Physics 170 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Fri, 19 Dec 2014 08:38:19formed using a He-Ne laser (6328 A) as the excitation source with the samples cooled to liquid-helium tempera tures in a Janis SuperVaritemp optical cryostat. The laser output was mechanically chopped and focused onto the sam ple surface giving a power density of ~ 2 W Icm2• A SPEX double grating monochromator with a GaAs photomulti plier tube, along with a lock-in amplifier, was used to mea sure the PL signal. The room-temperature electrical properties of a series of CdTe:As epilayers are summarized in Table 1. The initial CdTe:As films were prepared using a substrate growth tem perature of T, = 230 °C, since this temperature produced highly conducting films when Sb was used as ap-type dopant in the photoassisted MBE system. 3.4 CdTe:As epilayers Nos. 15-19, grown using 1'., = 230°C, exhibited hole concentra tions ranging from 7X 1015 to 1.9x 1016 em 3. Increasing the arsenic oven temperature from 160 to 180°C for films Nos, 20 and 21 did not result in a significant increase in the hole concentration of these films. However, the hole mobil ity did decrease, as a result of nonoptimum growth param eters. Instead of raising the arsenic oven temperature, in or der to increase the hole concentration in the epilayers, T, for subsequent CdTe:As films was decreased. This lowering of substrate temperature led to a drastic enhancement of do pant activation in the layers. The highest hole concentration obtained from this study was 6.2 X 1018 em -3 for an epiluyer (No. 30) grown with T, = 180 cC and an arsenic oven tem perature of 200"C. As a result of the improvement of the electrical properties of the CdTe:As films, the hole mobilities for layers grown at 180°C are comparable to the highest hole mobilities observed in bulk p-type CdTe at 300 KY A plot of mobility and carrier concentration versus tem perature for the heaviest doped CdTe:As epilayer (No. 30) is shown in Fig. 1. The mobility ofthis sample at 290 K is 74 cm2 IV s and increases with decreasing temperature to a val ue of 157 cm2/V s at 230 K. The hole concentration of this film remains above 2 X 101 g em 'even at 230 K, and, in fact, is close to the degenerate doping level for CdTe. In contrast, p-type doping levels in bulk CdTe rarely exceed 5 X 1016 em 3 at 300 K.1O It is a significant accomplishment that p type films grown by photoassisted MBE have hole concen trations ~ 100 times larger than hole concentrations in TABLE 1. Growth parameters and room-tcmperature electrical properties of CdTe:As epilayers grown by plJOtoassisted MBE. Hole Substrate As oven concentration Sample temp. temp. (xtOI" Hole mobility numbcr ee) ee) cm-') (crn1/V s) 15 230 160 0.70 69.1 16 230 160 1.1 66.2 18 230 160 1.4 6().4 19 230 160 1.9 34,5 20 230 180 1.7 34.6 21 230 180 2.3 27.2 24 200 180 1.8 49.0 33 200 200 29.0 65.0 29 180 200 70.0 7G.O 30 180 200 620.0 74.0 171 Appl. Phys. Lett., Vol. 54, No.2, 9 January 1969 FIG.!. Mobility and carrier concentration for ap-type CdTe:As film grown by pholoassisted MBE. CdTe samples grown by conventional thermal-equilibrium bulk methods. Here we propose a mechanism which is responsible for the high hole concentrations that have been achieved in both CdTe:Sb4 and CdTe:As films grown by photoassisted MBE. It has recently been reported, on the basis of reflection high energy electron diffraction studies, that the introduction of light from a He-Ne laser increases the rate ofTe desorption from CdTe surfaces.ll The existence of these vacant Te sites thus favors the incorporation of substitutional acceptors such as SbTc and AsTe. This, coupled with increased atomic surface mobility brought about by the impinging photon beam, is what we believe gives rise to the highly doped p-type layers that have been produced by photoassisted MBE. In the absence of a p-type dopant, and at high laser illu mination intensities, one might thus expect n-type layers to be produced, since the film growth surface would be rich in Cd, thus favoring the incorporation of interstitial Cd and/or Te vacancies, either of which act as an n-type dopant. We have recently observed this type of behavior in CdTe films grown under high laser illumination (greater than 100 mW Icm2) with no dopant present. These n-type films had carrier concentrations of over 1017 em -3 at 300 K. From low-temperature PL measurements we can rule out uninten tional donor impurities such as Cl,12 as the source of the n type conductivity. The mechanism described above is also consistent with growth of highly activated n-type CdTe:In by photoassisted MBE. In this case, we attribute the high degree of dopant activation (100% in some films) to suppression of the sec ond-phase defect structure 1n2 Tc3, which we believe to be the p-type compensating defect in CdTe:ln samples grown by conventional means. In the case of CdTe:ln films grown by photoassisted MBE, we suggest that formation ofIn2 Te3 is suppressed or absent because the number density of Te atoms available at the growth surface has been reduced by the incident light. As a consequence, tetrahedral bonding of Incd occurs. The low-temperature PL measurements from the CdTe:As films are used to determine optical quality and ac ceptor ionization energy. In general, the PL signal from the CdTe:As films is brighter than that previously observed from CdTe:Sb films. Figure 2 shows a low-temperature PL Harper, Jr. et at. 171 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Fri, 19 Dec 2014 08:38:19o 1.585 1.590 1.595 1.600 ENERGY (eV) FIG. 2. Low-temperature \5 K) PL spectrum for a heavily doped CdTe:As film grown by photoassisted MBE. spectrum for the CdTe:As epilayer (No. 30) whose electri cal properties are shown in Fig. 1. The PL spectrum of this heavily doped CdTe:As film is dominated by a sharp (full width at ha!fmaximum = 0.5 meV) bright (Ao,X) peak at 1.5907 eV. We associate this feature with an exciton bound to a neutral As acceptor. The free-exciton (X) transition is observed at 1.5967 eV in Fig. 2. NormaHy, X recombination is seen at 1.5964 eV in undoped CdTe films. The slight decrease in free-exciton binding energy is believed to be due to the increase in dielec tric constant E which follows from an increase in free-carrier concentration. The X binding energy is proportional to 1/ i', where E is 10.6 for high quality CdTe. (The increase in E leading to the change seen in Fig. 2 is only-O. I 5, less than 2 % change.) PL emission at energies below and above the free exciton transition energy occurs at energies associated with the n = 2 and the n = 3 excited states of the (Au,X) transition. The peak at 1.5937 eV occurs in the range where donor transitions are normally observed in CdTe. We identi fy the donor as residual el introduced during the substrate preparation. The (Ao,X) peak was seen to occur from 1.5901 to 1.5907 eY in the CdTe:As films studied here. In general, the recombination energy increased with increasing hole con centration. This is believed to occur due to a decrease in acceptor impurity level ionization energy. This exciton ener gy range is slightly higher than (A!>,X) recombination nor mally observed for acceptors in CdTe.13 However, acceptor bound exciton recombination has been reported at 1.5901 ey14 about a shallow complex acceptor. Because both the (Au,X) and X recombinations can be clearly seen in Fig. 2, one can approximate the acceptor ionization energy using Haynes' rule's as applied to acceptors in CdTe:16 Ell lEA -0.1, where E B is the binding energy of the free-exciton to the acceptor impurity level. Using this formalism, one ob tains EA (arsenic) = 60 meV, which is in close agreement with the ionization energy associated with an effective-mass acceptor in CdTe (56.8 meV).13 Also, our value for £4 is in agreement with the 62 ± 4 meV As-acceptor ionization en ergy obtained using resistivity measurements on CdTe:As films grown by organometallic vapor phase epitaxy (OMVPE).17I n that report, an (Ao,X) PL line at 1.591 eV was also observed. However, our value for the binding ener- 172 Appl. Phys. Lett" Vol. 54, No.2, 9 January 1989 gy of a hole to the AsT< level differs from an earlier reported value of 92.0 meV,13 obtained from studies of As + + ion implanted bulk CdTe samples. To elimi.nate uncertainty in the arsenic ionization energy, we have recently completed temperature-dependent PL studies on a series of CdTe:As films. These studies have produced spectra in which the elec tron-tn-neutral acceptor (e,Ao) peak is present, correspond ing to an acceptor ionization energy of 58 me Y for low dop ing concentration. The results of those detailed studies will be published elsewhere. In summary, we report the growth of p-type CdTe:As films using the photoassisted MBE technique. The CdTe:As epilayers exhibit hole concentrations as high as 6.2X 1018 em 3 and mobilities as high as 74cm2jV s at room tempera ture. The epilayers were grown at substrate temperatures as low as 180 "Co We propose a dopi.ng mechanism responsible for the extremely high p-type doping concentrations ob served in layers grown by photoassisted MBE. The As ac ceptor ionization energy as detennined by low-temperature PL measurements was found to be -58-60 meV, in close agreement with the effective-mass acceptor ionization ener gy. This work was supported by the Defense Advanced Re search Projects Agency j Army Research Office contract DAAL03-86-K-0146. The authors wish to acknowledge the assistance of J. Matthews with substrate preparation, J, Tas sitino with deposition of electrical contacts on p-type layers, and K. A. Bowers for performing some oftne photolumines cence measurements. 'K. Zanio, in Semiconductors and Semimetals, edited by R. K. Willardson and A. C. Beer (Academic, New York, 1978), Vol. 13. JR. N. Bicknell, N. C. Giles, and J. F. Schetzina, App!. Phys. Lett. 49, 1095 (1986). 'R. N. Bicknell, N. C. Giles, and J. F. Schctzina, Appl. Phys. Lett. 49,1735 ( 1986). is. Hwang, R. L. Harper, K. A. Harris, N. C. Giles, R. N. Bicknell, J. F. Schctlina, D. L. Dreifus, R. M. Kolbas, and M. Chu, J. Vae. Sci. Tec!mol. B 6,777 (1988); or N. C. Giles, R. N. Bicknell. R. L Harper, S. Hwang, K, A. Harris and I. F. Schetzina, J. Cryst. Growth 86.,348 (1988). 'R. N. Bicknell, N. C. Giles, J. F. Schetzina, and C. Hitzmal1, J. Vac. Sci. Techno]. A 5,3059 (! 987). "D. 1,. Dreifu;;, R. M. Kolbas, K. A. Harris, R. N. Bicknell, R. L. Harper, alld J. F. Schetzina, Apr!. Phys. Lett. 51, 931 (1987). 'T. H. Myers, Yawchcng Lo, R. N. Bicknell, and J. F. Schdzina. App!. Phys. Lett. 42, 247 (1983). "T. H. Myers, J. F. Schctzina. T. J. Magee, and R. D. Ormond, J. Vae. Sci. Techno!. A 1, j 598 (1983). 9S. Yamada, J. Phys. Soc. lpn. 15, 1940 (1960). ,oJ. Gu. T. Kitahara, K. Kawakami, and T. Sakaguchi, 1. AppJ. Phys. 46. 1184 (1975). "I. D. Benson and C. 1. Summers, J. Cryst. Growth 86,354 (1988). I2n-type conductivity (n-W" em· ') in CdTefilms grown under low laser illumination was attributed to shallow residual donor impurities; S. Hwang, R. L. Harper, K. A. Harris, N. C. Giles, R. N. Bicknell, J. W. Cook, Jr., J. F. Schetzina, and M. Chu, J. Vac. Sci. Techno!. A 6,2821 ( 1988). "E. Molva, J. L. Pautrat, K. Saminadayar, G. Milchberg, and N. Magnea, I'hys. Rev. B 30,3344 (1984). '4B. Monemar and E. Molva, Phys. Rev, B 31, 6554 (J985}. "J. R. Haynes, Phys. Rev. Lett. 4, 361 (1960). "'R. E. Halsted and M. Aven, Phys. Rev. Lett. 14,64 (1965). !"IS. K. Ghandhi, N. R. Taskar, and 1. B. Bhat, AppL Phys. Lett. 50, 900 (1987). Harper, Jr. et al. 172 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Fri, 19 Dec 2014 08:38:19

1.338347.pdf

Superconducting AgMo6S8 thin films prepared by reactive sputtering G. B. Hertel, T. P. Orlando, and J. M. Tarascon Citation: Journal of Applied Physics 61, 4829 (1987); doi: 10.1063/1.338347 View online: http://dx.doi.org/10.1063/1.338347 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/61/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dependence of critical current density on microstructure in the SnMo6S8 Chevrel phase superconductor J. Appl. Phys. 77, 6377 (1995); 10.1063/1.359110 Specific heat measurements of pressureinduced reentrant superconductivity in Eu0.9Ho0.1Mo6S8 J. Appl. Phys. 73, 1886 (1993); 10.1063/1.353176 High critical current densities reproducibly observed for hotisostaticpressed PbMo6S8 wires with Mo barriers J. Appl. Phys. 72, 1180 (1992); 10.1063/1.351799 Poor intergrain connectivity of PbMo6S8 in sintered Mosheathed wires and the beneficial effect of hotisostatic pressing treatments on the transport critical current density J. Appl. Phys. 70, 1606 (1991); 10.1063/1.349525 Reactive sputtering of superconducting thin films AIP Conf. Proc. 182, 2 (1989); 10.1063/1.37970 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 11:06:19Superconducting AgNhlsSa thin fUms prepared by reactive sputtering G. 8. Hertela) and T. P. Orlandob) Massachusetts Institute a/Technology, Cambridge, jilfassachusetts 02139 J. Mo Tarascon Bell Communications Research, Red Band, New Jersey 07701 (Received 26 August 1986; accepted for publication 14 January 1987) Preferentially oriented thin films of the Chevrel-phase superconductor AgMo6Sg were prepared by reactive sputtering. Ag and Mo were simultaneously sputtered from separate guns onto sapphire substrates held at about 850°C with H2S gas injected near the substrate. The films have superconducting critical temperatures up to 9.2 K and narrow-phase transitions. The reactive sputtering process chosen for the preparation of our films makes it possible to change the superconducting properties and the microstructure of the samples in a systematic way by changing individual preparation parameters and to study which of the preparation conditions are the most crucial for the formation of the Chevrel phase. We find that the superconducting transition temperature of the Chevrel phase is very sensitive to both substrate temperature and to the flow of H2S but insensitive to the background pressure in the chamber before deposition. The microstructure can be changed by controlling the H]S pressure. X-ray measurements show that the films are preferentially oriented with the rhombohedral 001 planes parallel to the surface of the substrate. I. INTRODUCTION Ternary molybdenum sulfides with the stoichiometry Mx Mo6Sg, where M is a metal, have attracted considerable interest since their discovery in 1972. I Many of these com pounds, also caned Chevrel phases, are superconductors with interesting physical properties. They have, together with the superconducting rhodium borides, allowed the study of interactions between magnetism and superconduc tivity, and more recently, were found to exhibit field-in duced superconductivity.2 Their very high upper critical fields make them excellent candidates to be used for the next generation of high-field magnets.3-5 The main reason pre venting the development of Chevrel phases for commercial use is the fact that they are extremely difficult to prepare. Although a few single crystals have been grown,6 most of the work has been done on sintered samples, which have some cbvious disadvantages. In this paper we describe the prep aration of AgMo6Ss films by a reactive sputtering process, In Sec. II, we first review previous work on sputtering Chevrel-phase materials and then describe the reactive sput tering technique which we used. Section III describes how the physical properties of thin films are correlated to the deposition conditions, and Sec. IV briefly discusses the crys tal structure of the films. It PREPARATION CONDiTIONS The preparation of Chevrel-phase thin films is fairly dif ficult. The relative concentration of the three constituents has to be controlled with a very high degree of accuracy because of the narrow-phase range of most Chevrel phases. Straightforward thermal evaporation or dc sputtering from elemental sources is difficult because the possible target ma terials have a melting point that requires electron-beam a) Department of I'hysics. Present address: Pacific Telesis International, Piscataway, NJ 08854. b) Department of Electrical Engineering and Computer Science. evaporation (Mo), can only be evaporated with special tech niques (S), are not sufficiently conducting so that rf sputter ing has to be used (MoS "' PbS), or cannot be sputtered at all (S). In this section we first review previous multistep meth ods of fabricating thin films of ChevreI-phase materials in order to provide the rationale for the single-step reactive sputtering technique which we have developed. One way to prepare Chevrel-phase thin films is by sput tering from a composite target that contains all elements in the desired composition. Good thin films with critical tem peratures comparable to bulk values have been prepared this way.7 The targets were pressed tablets consisting of a mix ture ofMoSz powder, and powders of the ternary metal or its sulfides. Because the target was net conducting, rf sputtering was used. Whereas CUx M06S8 could be directly formed by sputtering onto substrates held at 850 ·C, all other com pounds had to be deposited onto room-temperature sub strates. The Chevrel phase was then obtained by subsequent annealing in sealed quartz tubes under argon atmosphere. Very similar is another method in which a reacted Chev rel phase is used as a sputtering target. In this case,dc sput tering can be used and the problems associated with rf sput tering can be avoided. Good films of PbM06Sg,8-1l AgMo6Sg, 12 and BaM06SS13 have been obtained. Again, the Chevrel phase was formed by annealing the films in quartz tubes after the deposition process, because the material sput tered onto room-temperature substrates was amorphous and not superconducting. Both techniques described above require the prepara tion of a new target every time a change in stoichiometry is desired. This problem can be avoided if a more modular tar get is used. In the first effort to prepare Chevrel-phase thin films, a MoS2 target overlayed with wedges of Mo sheet and sheets of the ternary metal was used. 14 In this case the com position can be varied much easier between runs by simply changing the dimensions of the metal sheets rather than manufacturing a whole new target. But because parts of the 4829 J, Appl. Phys. 61 (10), 15 May 1987 0021 -8979/87/1 04829-06$02.40 (e) 1987 American Institute of Physics 4829 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 11:06:19target are not conducting, rfsputtering has to be employed, and the homogeneity of the sputtered films may also become a problem because the target consists ohones with different compositions and very different sputtering rates. Again, the Chevrel phase was formed by annealing the films in quartz tubes after the deposition process. Another technique to obtain thin films is to react a sput tered or evaporated molybdenum film or a molybdenum tape at high temperature in a sealed quartz tube with sulfur and lead or tin. Layers ofPbMo6Sg with good superconduct ing properties have been grown with this method.IS-17 Al though very homogeneous films can be grown and the stoi chiometry can be fairly easily controlled through the vapor pressures, very little control of the microstructure is possi ble. Because the molybdenum film has to significantly ex pand its volume during the reaction in order to accommo date the sulfur and the ternary metal, large amounts of material have to be displaced and the films are rough on a micron scale. It would be very useful to control the microstructure in such a way that smooth surfaces (for example, for tunneling experiments) can be obtained. It is possible to control the grain size up to a certain degree through the annealing tem perature. Sufficiently smooth films to prepare tunnel junc tions were obtained by annealing films sputtered from a composite target at fairly low temperatures1H or by anneal ing at the usual temperature, but reducing the substrate tem perature during the sputtering process. 19,20 Unfortunately, the lower temperatures did indeed lead to a smoother sur face, but also to a reduced critical temperature and resis tance ratio. So far, neither vacuum tunneling experiments on single crystals21-23 nor thin-film tunneling experiments have been of good enough quality to extract the phonon spectrum a2 F( OJ) of any Chevrel phase. The multistep processes described above have many dis advantages: First, because of the many steps involved, the turnaround time is long. Every time a change in stoichiome try is desired, a new composite target has to be fabricated. Even if the same target is used, it is only good for a few runs because it changes its surface appearence and its composi tion as a function of time, thus causing uncontrollable changes in sputtering rates and stoichiometry. In addition, MoS2 and Chevrel-phase targets are difficult to use because of their brittleness, porosity, and poor thermal conductivity. The best chance of controlling the microstructure in such a way that either smooth surfaces for tunneling experi ments or sman grain sizes for high critical current densities can be obtained is provided by a single-step process that eliminates both the preparation of a composite target and the subsequent annealing step. This can be realized by reactive evaporation or reactive sputtering. These single-step pro cesses also provide opportunities to control the stoichiome try and microstructure of Chevrel phases in a reliable and . reproducible way, and to study correlations between prep aration conditions and sample properties. Good CUx Mo6Sg films have been made by reactive evap oration using an electron-beam heated source for the molyb denum and a resistively heated source for the copper.24•25 The sulfur was introduced as HzS gas or hot sulfur vapor. 4830 J. Appl. Phys., Vol. 61, No. 10, 15 May i 987 The films were directly grown on heated substrates. In a similar experiment, both CUx Mo6SS and HoMo6Sg films were prepared using electron-beam heated sources for both metals and a Knudsen cell for the sulfur.26•27 CuxMo6Sg could be directly prepared on a hot substrate, whereas Ho Mo6Sg films required additional annealing. We prepared AgM06Sg films by sputtering silver and molybdenum onto sapphire substrates held at 850"C. The sulfur was introduced by directing H2S gas through a copper tube directly onto the heated substrate. The sputtering guns and deposition system are described in detail elsewhere.28 Briefly, the system has an I8-in. UHV cryopumped chamber. Three sputtering guns are mounted at an angle in the bottom of the chamber so that each gun focuses onto the substrate area. This tilted arrangement allows the substrate area to be coated relatively homogeneously. Ali the guns are magnetically enhanced triode guns which allow separate control over the sputtering voltage and the target current, Each gun is independently operated with its own gas supply, and its own plasma and target power supplies. Ag and Mo were sputtered at the same time but from separate guns; this allowed for individually controlling their deposition rates. We were able to obtain single-phase AgM06Sg films with critical temperatures up to 9.2 K and resistive transitions only 0.2 K wide. The resistance ratios were found to vary between 1.2 and 20, depending on the microstructure [the resistance ratio is R (300 K) I R (15 K), where R (T) is the resistance at temperature T]. The next section demonstrates that the single-step reactive sputtering process described above does indeed allow accurate and reproducible control of the sample properties by changing the preparation condi tions. III. CORRELATIONS BETWEEN PREPARATION CONDITIONS AND SAMPLE PROPERTIES Over two hundred sputtering runs were made to test for correlations between preparation conditions and sample properties because of the many sputtering parameters that had to be varied. However, each ofthe following figures con tains a particular set of samples for which all preparation conditions are identical except for the one parameter being varied. Molybdenum has a much lower sputtering efficiency than silver, and the molybdenum sputtering rate, therefore, turned out to be the factor limiting the thickness of the sput tered films. Therefore, the molybdenum gun was operated at the highest convenient power of 500 V and 1.5 A, corre sponding to the sputtering voltage and the target current, respectively. These parameters were kept constant through out all depositions, and the silver sputtering rate, the H2S flow rate, and the substrate temperature were varied in order to obtain optimal superconducting properties. Figure 1 shows the influence of the substrate tempera ture during deposition on the critical temperature T,. of the films, if all other preparation conditions are kept constant. Because the thermocouple did not reliably perform in the H2S atmosphere, the current through the graphite tape used to heat the substrate holder was used as a measure for the substrate temperature. Figure 1 shows that to consistently Hertel. Orlando. and Tarascon 4830 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 11:06:19FIG. 1. Critical temperatures of several reactively sputtered AgMo6S, films as a function of the substrate heater current. obtain films with high critical temperatures, the heater cur rent had to be around 73 A. If the heater current was raised to 76 or 77 A, the critical temperature decreased by several degrees. Although the exact temperature during each run is not known, it can be estimated from tests of the thermocou ple before it was exposed to H2S. The optimum temperature is about 850 ·C, and a change in the heater current of I A corresponds to a temperature change of 10 ·C. This mea~s that an increase in the substrate temperature by 30 or 40 C above the optimum temperature reduces the critical tem perature by several degrees. A similar but less dra~tic reduc tion in Tc is observed if the substrate temperature IS lowered below the optimum value. . Figure 2 shows that Tc is also sensitive to changes in the H2S flow rate. If the H2S flow rate is reduced from 17 to 16 secm (standard cubic centimeters per minute), a change of about 5%, Tc again drops from 9 to around 5 K. Although T. very quickly decreases once the H2S flow drops below a critical value, fairly good samples can be obtained if the H2S flow is increased up to twice the optimum value, Although for very high flow rates Tc is only slightly depressed, the increased HzS pressure during deposition has a drastic effect on the resistance ratio and the microstructure, 10 9 8 "52 7 ;2 6 5 4 15 16 17 18 19 20 21 HYDROGEN SULFIDE FLOW (SeeM) FIG. 2. Critical temperature of several sputtered films as a function of the hydrogen sulfide flow rate. 4831 J, Appl. Phys., Vol. 61, No, 10, 15 May 1987 15 20 25 30 35 HYDROGEN SULFIDE FLOW (SeeM) FIG. 3. Resistance ratios ofseveral AgMo6S. films as a function ofthe H2S flow rate during deposition. Figure 3 shows that very high resistance ratios can be obtained with the optimal H2S flow. The resistance ratio drops very quickly to values between 1 and 2 if the flow rate deviates from the ideal value in either direction. Electron micrographs were made of four samples pre pared under identical conditions but with different H2S flow rates.28 A film made with the ideal flow rate (which has a resistance ratio of about 20) shows no grain structure on a scale of 1 pm. As the hydrogen sulfide flow is increased, the resistance ratio drops and the films develop a grainy surface, down to a grain size of about 0.1 {-lm for the film with the highest HzS flow rate. This correlation between ~he micro structure and the resistance ratio suggests that m the film with the highest resistance ratio the measured resistivity is more representative of the intrinsic resistivity of the Chevrel phase which is strongly temperature dependent. In.t~e ~l~s with the loweroresistance ratios, the measured resIstIVIty IS mostly due to grain boundaries, and, therefore, does not vary much with temperature. The least critical of the sputtering conditions is the silver sputtering rate. Figure 4 shows that there is a maximum in the critical temperature if the target current, which at con- 10 9 I 8 g 7 1-0 6 5 4' 0.9 1.0 Lf 1.2 1,3 1.4 1.5 1.6 TARGET CURRENT (Al FIG. 4. Critical temperatures of the sputtered films as a fnnction of the target current for the silver gun. The target voltage was 350 V for aU samples in this graph. Hertel, Orlando, and Tarascon 4831 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 11:06:198 7 0 f= 6 <1: a:: w 5 u Z 4 « f- (f) [ji 3 w 0::: .. A 2 .. .. O~ __ L-__ ~ __ ~ __ ~ __ ~~~~~ 0.9 1.0 1.1 1.2 1.3 14 1.5 1.6 TARGET CURRENT (A) FIG. 5. Resistance ratio for the same films as in Fig. 5 as a function of the target current for the silver gun. stant voltage is proportional to the sputtering rate, is varied. But even a change in the sputtering rate of 60% or more still allows the preparation of samples with critical temperatures above 8 K. It is interesting to note that the sputtering param eters used for the silver and molybdenum guns, 350 V -1.35 A and 500 V-1.5 A, respectively, are of the same order, al though silver has a five times higher sputtering yield than molybdenum, and only one sixth as much silver as molyb denum is needed to obtain the correct stoichiometry for AgM06Sg. This means that it is necessary to sputter approxi mately 30 times more silver than one would expect from a simple comparison of the sputtering rates, The reason for this is probably that most of the silver does not stick to the substrate because the substrate temperature during deposi tion is too close to the melting point of silver. The sticking coefficients of silver, as wen as sulfur, seem to depend very strongly on the preparation conditions. Sputtering condi tions that result in films with good superconducting proper ties usually lead to a film thickness about 5000 to 10 000 A after a deposition time of 15 min. If any of the sputtering parameters significantly varies from its op~imal value, the resulting film thickness is only about 2000 A after the same 10 8- S ::.:: f-'" 4 2 - 0 0 .. "I .. 123 RESISTANCE RATIO .. 4 FIG. 6. Critical temperatures and resistance ratios of most of the supercon ducting AgMo6SS films prepared for this paper. 4832 J. Appl. Phys., Vol. 61, No.1 0,15 May 1987 deposition time. This thickness roughly corresponds to the thickness one would expect, if only the molybdenum sticks to the substrate. Silver and sulfur are apparently only incor porated into the films at the right stoichiometry if the prep aration conditions are exactly right for the formation of the Chevrel phase. Although the superconducting properties of the sputtered AgM06SS films did not strongly depend on the silver sputtering rate, it was not possible to obtain supercon ducting films if the sputtering voltage for the Ag target was lower than about 300 V, indicati.ng that there is a threshold voltage and, therefore, a threshold energy for the sputtered silver particles in forming the Chevrel phase. Figure 5 shows that there is a maximum in the resistance ratio occurring at the same silver sputtering rate that led to the highest critical temperature. This indicates that the high resistance ratios in the reactively sputtered films are indeed intrinsic to the Chevrel phase, and are not caused by sHver precipitates in the samples, Silver precipitates could be very clean because they are deposited so dose to the melting point of silver, and could therefore show a large resistance ratio, But in that case, the resistance ratio would be likely to in crease for higher silver sputtering rates. It should also be pointed out that some of the prepara tion conditions can be played off against each other to some degree. A higher substrate temperature, for example, will shift the optimum HzS flow rate to higher values, and vice versa. In order to find the optimal preparation conditions it was therefore necessary to first find the optimal substrate temperature for a given H2S flow rate, then vary the flow rate and find its optimum value, and if it is different from the original value, vary the temperature again. The background pressure in the vacuum system before deposition does not seem to have any influence on the super conducting transition temperature of the films, although the Chevrel phases are known to be very sensitive to impurities, especially oxygen.29,30 This may be because the system is continuously pumped with a dynamic gas flow during sput tering. There is a loose correlation between the critical tem peratures and resistance ratios of the sputtered films, Figure 6 shows that samples with high critical temperatures gener any also have high-resistance ratios. Similar correlations have been observed in other superconducting systems.31-34 IV, CRYSTAL STRUCTURE Because the Chevrel phases have a rhombohedral crys tal structure which possesses only a very low symmetry, the diffraction pattern is very complicated and one would expect to see about 35 lines below e = 30° in an x-ray experiment using CuKa radiation. A diffractometer scan for a typical reactively sputtered film is shown in Fig. 7, The Bragg peak at 29 = 40° is generated by the sapphire substrate, and the only Chevrel phase Bragg peak ever observed in any of the films is the one at 20 = 13.6°, which is due to the 001 plane in the rhombohedral notation or the 101 plane in the hexagonal notation. The fact that only one Bragg peak associated with the Chevrel phase is observed in any of the samples suggests that the films might be preferentially oriented. (We note that thin films with randomly oriented grains that we have pre- Hertel, Orlando, and Tarascon 4832 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 11:06:1925.0 22.5 20.0 : 7.5 ,. , 5.0 u '" "' 12 5 (J) D-10.0 u -'.5 5.0 2.5 FIG. 7. Diffractometer scan using CuKa for a typical reactively sputtered AgMo6Sg film. pared by a multistep reaction process show all the Chevrel phase lines.) The preferential orientation is indicated by measuring a pole figure as shown in Fig. 8. The pole figure was taken using both reflection and transmission geometries. A pole figure can be visualized as a contour map that plots the height of a particular diffraction peak (in this case the one at 28 = 13.6°) as the sample is tilted in various directions. The shaded region A in the center of Fig. 8 is caused by a cluster ing of diffraction peaks at tilt angles around 4°. This indicates that a significant number of grains have 001 planes nearly parallel to the surface of the thin film. Such oriented grains should also give rise to regions on the pole figure at tilt angles at 88° and 92° from region A. However, these additional re gions are not evident, probably because of the low signal-to noise ratio inherent to the diffuse diffraction pattern of Chevrel phases and the small amount of material in our thin films. Note that the shaded region B at tilt angles near 27° is independent of region A and indicates that not all grains have the 001 orientation. Although we were able to determine the crystal struc ture of the thin films, we were not able to determine the 6°f 30r of- -30 - -60 FIG. 8.001 pole figure for a reactively sputtered AgMo6S, filmo The vertical scale is the tilt a..'1g1e. The azimuthal angle is marked on the circumference of the pole figure. 4833 J. Appl. Phys., Vol. 61, No.1 0, i 5 May 1987 chemical composition of the sputtered AgMo6Sg films. At tempts to measure the composition with an electron micro probe failed because the primary x-ray lines of sulfur and molybdenum are so dose together that they partially over lap. Therefore, it was necessary to use secondary lines which, especially in the case of sulfur, have very low intensity. In addition, the thickness of our films was only 1 pm or less, so that the intensities were very low. The measured sulfur to molybdenum ratio, therefore, has a large experimental error and did not provide any useful information. X-ray fluores cence studies also provided no compositional information, again because the sulfur and molybdenum x-ray lines were too close to be resolved. It was also attempted to determine the chemical composition of the films by Rutherford back scattering. In this case the peaks generated by molybdenum and silver overlap if the films are thicker than 50 A, and no superconducting films thinner than 50 A could be prepared. 'V,SUMMARY AgMo/iSg films with good superconducting properties were prepared in a one-step reactive sputtering process that does not require the preparation of a composite target or additional annealing. Although the superconducting prop erties of AgM06SS are very sensitive to changes in the prep aration conditions, especially the substrate temperature and the H2S flow, the reactive sputtering process can be con trolled accurately enough to reproducibly influence the su perconducting properties and the microstructure of the AgMo6SS films. X-ray measurements show that the films are preferentially oriented with their rhombohedral 001 planes parallel to the substrate. ACKNOWLEDGMENTS We would like to thank L. H. Greene for doing Ruther ford backscattering and D_ Bucholz for the pole figures and electron microprobe_ We also acknowledge useful discussion with J. Remcika. This work was supported by the National Science Foundation under Contract No. DMR-8403493. 'B. T. Matthias, M. :Marezio, I~. Corenzwit. A. S. COOPCl-, and H. E. Barz, Science 175, 1465 (1972). 'H. W. Meul, C. Rosse!, M. Decroux, (j). Fischer, G. Remenyi. and A. Briggs, Phys. Rev. Lett. 53. 497 (1984). ,.(/). Fischer, R. Odermatt, G. fiongi, H. Jones, R. ChcvJ'd. and M. Sergent, Phvs. Lett. 45A, 87 ( 1973). 4S. Poner, E. J. McNiff, Jr., and E. J. Alcx~lnder, Phys. Lett. 49A, 269 (l974). sK. Okuda. :\1. Kitagawa, T. Sakakihara, and M. Date, J. Phys. Soc. Jpn. 4!l, 2157 (1980). "R. Fhikiger, K. Baillif, and E. Walker, Mater. Res. Bull. 13, 743 (1975). 7p. Przyslupski, R. Horyn, 1. Szyrnaszek, and B. Gren. Solid State Com mUll. 28, 869 (1975). 'K. Ddesc1efs o l'h.D. thesis (University of Geneva. 1976). 'G. Hertel. H. Adl'ian, J. Biegel', C. Niilscher, and G. Saemann-Ischenko, Phys. Rev. B 27, 212 (]983). 10K. Hamasaki, T. Yamashita, T. Komata, K. Nota. and K. Watanabe, Adv. Cryog. Eng. 30. 715 (19R4). "Y. Quere, A. Perrin, Ro Horyn, and M. Sergent, Mater. Lett. 3, 340 (1985). ,on. Adrian, F. Ptirscho R. Behrle, and G. Saemmm-lsehenko. Supercon ductivity in d-and j-Band AfetaL~, edited by W. Buckel and W. Weber (Kernfarschungszcntrurn, Karlsmhe, 1982), p. 193. uH. Adrian, F. Pfirsch, and R. Behrle, Phys. Ref. B 29.1447 (1984). ,-Ie. K. Banks, L. Kamrnerdincr, and H. L. Lua, J. Solid Stat~ Chern. 15, 271 (1975). Hertel, Oriando, and Tarascon 4833 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 11:06:19I5N. E. Alekseevskii, M. Glinski, N. M. Dobrovolski, and V. 1. Tsebio, JETP Lett. 23, 413 (1976). 16M. Decroux, (J), Fischer, and R Chevrel, Cryogenics 17, 291 (1977). 17K. Hamasaki, T. Inoue, T. Yamashita, T. Komata, and T. Sasaki, AppL Phys. Lett. 41, 667 (1982). "P. Przyslupski and U, Poppe, Solid State Comrnun, 53, 703 (1985). '9R. Ohtaki, B. R. Zhao, and H. L. Luo, Mater. Res. Bull. 17,575 (1982). 2°R. Ohtaki, B. R. Zhao, and H. L. Lua, J. Low Temp. Phys. 54, 119 (1954 ). 2'U. Poppe and H. Wiihl, J. Phys. (Paris) 39, C6-361 (1978). 22U. Poppe and H. Wtihl, J. Low Temp. Phys. 43, 371 (1981). 2'U. Poppe and H. Schroder, in 17th International Conference on Low Tem perature Phy.~ics, edited by U. Eckern, A. Schmid, W. Weber, and H. Wiihl (North Holland, Amsterdam, 1984), p. 835. 24K. C. Chi, R. O. Dillon, R. F. Bunshah, S. A. Alterovitz, D. C. Martin, and J. A. Woollam, Thin Solid Films 47, L9 ( ! 977). 15K. C. Chi, R. O. DiIlon, R. F. Bunshah, S. A. Alterovitz, and J. A. Wool lam, Thin Solid Films 54, 259 (1978). 4834 J. Appl. Phys., Vol. 61, No.1 0,15 May i 987 2°R. J. Webb, A. M, Goldman, J. H. Kang, J. Maps, and M. F. Schmidt, IEEE Trans. Magn. MAG-21, 835 (1985). 21J. Maps, J. H. Kang, and A. M. Goldman, l'hysica 13SB, 336 (1985). 2"G. B. Hertel, T. P. Orlando, and J. M. Tarascon, Physica 135B, 168 (1985). 29D. G. Hinks, J. D. Jorgensen, and H. C. U, Phys. Rev. Lett. 51, 1911 (1983). JOD. G. Hinks, J. D. J0rgensen, and H. C. Li, Solid State Commun. 49, 51 ( 1984). "R. C. Dynes, J. M. Po ate, L. R. Testardi, A. K. Stann, and R. H. Ham mond, IEEE Trans. Magn. MAG-13, 640 (1971). :12L. T. Testardi, R. L. Meek, J. M. Poate, W. A. Royer, A. R. Storm, and J. H. Wemick, Phys. Rev. B 11, 4304 (1975). "J. M. Rowell, R. C. Dynes, and P. II. Schmidt, Solid State Commun. 30, 191 (1979) . . HJ. M. Rowell, R. C. Dynes, and P. H. Schmidt, in Superconductivity in d andf-Band Metals, edited by H. Suhl and M. B. Maple (Academic, New York, 19t1O), p. 409. Hertel, Orlando, and Tarascon 4834 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 11:06:19

1.37600.pdf

Turbulent magnetohydrodynamic density fluctuations David Montgomery Citation: 174, 60 (1988); doi: 10.1063/1.37600 View online: http://dx.doi.org/10.1063/1.37600 View Table of Contents: http://aip.scitation.org/toc/apc/174/1 Published by the American Institute of Physics 60 TURB~MAGNETOHYDRODYNAM~C DENSITY FLUCTUATIONS David Montgomery, Dartmouth College MHD turbulence theory has developed mostly be generalizing Navier- Stokes results, almost always incompressible ones. It has recently been possible to develop I a slightly-incompressible theory of MIID density fluctuations by what is essentially a generalization of Lighthill's method. An approximately incompressible MHD turbulence field drives a parasitic density field, the fluctuation spectrum of which can be expressed in terms of kinetic and magnetic spectra. If the incompressible MHD spectrum is Kolmogoroff-like, the inertial- range results can be summarized by saying that the Fourier- transformed density fluctuation, 6pk, is proportional to --(B2)k, the k TM Fourier component of the square of the variable magnetic field. 2 Making the quasi-normal approximation on expectations of products of four v and B field Fourier coefficients, a k -5/3 omni-directional (k -11/3 modal) density fluctuation spectrum. Even without enough spatial scale separation for the Kolmogoroff assumptions to apply, it is still possible to demonstrate by numerical solution 2 of the MHD equations that the connection between 6pk and (B2)k is valid for low enough Mach number and high enough beta (ratio of mechanical to magnetic pressure). The most difficult assumption to justify, for the magnetohydro- dynamics of the interstellar medium, is the use of an equation of state, Pmechanlcal p(p), which uniquely connects the density and mechanical pressure. Even assuming the existence of an equation of state, it will in general have to be of the form p=p(p,s), where s is the specific entropy. The assumption is thus basically one of isentropic (or isothermal) MHD flow. At high Reynolds numbers, the entropy is produced mainly in' the dissipation range, due to the action of thermal conductivity and resistivity. If the inertial- range hydrodynamic time scales are faster than the times necessary for the entropy to travel back up to the inertial length scales, the approximation would apparently be justified. It would also be justified (via an isothermal equation of state) if the inequality were sharply reversed; but not unless one of the two inequalities were satisfied would any equation of state be plausible. Until we know more about the thermodynamic parameters of the interstellar medium, this will remain an open question. I D. Montgomery, M.R. Brown, and W.H. Matthaeus, J. Geophys. Res. 92, 282 (1987). J.V. Shebalin and D. Montgomery, "Turbulent Magnetohydrodynamic Density Fluctuations", to be published in J. Plasma Phys., 1988 (in press). © 1988 American Institute of Physics

1.1139611.pdf

New device for measuring postarc currents in circuit breakers V. Vokurka, U. Ackermann, and E. Schade Citation: Review of Scientific Instruments 58, 1087 (1987); doi: 10.1063/1.1139611 View online: http://dx.doi.org/10.1063/1.1139611 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/58/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The influence of electrode erosion on the air arc in a low-voltage circuit breaker J. Appl. Phys. 106, 023308 (2009); 10.1063/1.3176983 Mechanical arcless dc circuit breaker by current zero operation Rev. Sci. Instrum. 63, 3993 (1992); 10.1063/1.1143252 Simulation of highvoltage breakdown in the postarc column J. Appl. Phys. 53, 4695 (1982); 10.1063/1.331297 New magnetic command system for a 200kA circuit breaker with 10μs current transfer time Rev. Sci. Instrum. 50, 464 (1979); 10.1063/1.1135852 New Device for Current Measurement in Exploding Wire Circuits Rev. Sci. Instrum. 39, 90 (1968); 10.1063/1.1683118 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:42New device for measuring post .. arc currents in circuit breakers v. Vokurka, U. Ackermann, and E. Schade Brown Boueri Research Center, CIl-5400 Baden, Switzerland (Received 23 December 1986; accepted for pUblication 25 February 1987) This paper describes a new technique for measuring post-arc currents in circuit breakers. The measurements are performed with a specially designed current monitor. In contrast to commonly used current monitors saturation of the core of the transformer at high pulses of current is prevented by shunting the turns of the transformer by antiparallel fast-recovery diodes. This new device offers several advantages in comparison to other known techniques for post-arc measurements: there is no galvanic coupling to the network, the system is easy to handle, and the costs of the components are low. The two post-are-current monitors (PACM) described have a sensitivity of 0.1 V / A and 10 m V / A, and are linear within 1 % in the range of ± 4 and ± 100 A, respectively, The first PACM has an upper cutoff frequency of 18 MHz, the second one 3.8 MHz and are designed for sinusoidal fault currents of 6.3 kA and 56 kA, respectively, at a frequency of 50 Hz, Examples of applications to axially blown arcs in SF 6 and to vacuum circuit breakers are presented. INTRODUCTION In a circuit breaker an arc of plasma is initiated by separation of the current-carrying contacts, The plasma forms a con ducting bridge between the parting contacts until the separa tion is sufficient to extinguish the arc and to insulate the disconnected parts of the network. At higher network vol tages, current interruption is only possible when the current passes through zero which occurs periodically in ac net works. 1 Current interruption takes place when the residual charge carriers are sufficiently reduced after current zero. Up to this moment a so-called post-arc current is driven between the disconnected network parts under the influence of the recovering network voltage. In high-pressure arcs collisions between the charge carriers are very frequent and deionization occurs by volume recombination as the arc cools down. Post-arc currents up to some tens of amperes might flow immediately after current interruption for about 1 f-ls up to several microseconds depending on the experi mental conditions and the arc-quenching medium. In low-pressure arcs such as vacuum arcs the behavior of the post-arc current is determined by separation and col lection of residua! charges. Immediately after current inter ruption and application of voltage a positive space-charge sheath and associated electrical fields are built up and spread out from the cathode to the anode. The propagation of the sheath is reflected by the shape of the post-arc current. Cur rents of up to some tens of amperes occur, the magnitude depending on the rate of decay of power current and the rate of rise of recovery voltage. The measurement of post-arc currents of only a few am peres is very difficult. The detection system must withstand short-circuit currents of several kiloamperes for 5--10 ms during arcing. Under these circumstances most standard measuring techniques cannot be applied. An accurate measurement of post-arc current requires the following: (a) high sensitivity at low current (less than 1 A), (b) highresolutionoftime (better than I,us), (c) capa bility to withstand fault currents of several tens of kA's for several cycles, (d) high safety standards to protect personnel and delicate instruments, and (e) no interference between network and measurement. Because of these considerable difficulties, few measure ments afpost-arc current have been made (e.g., Refs. 2-14) although such measurements are particularly important in improving the understanding of the physical phenomena in volved in current interruption. The old techniques are reviewed in Sec. I A. The new device is presented in Sec. I B. The application to circuit breakers is demonstrated by the two examples of Secs. II A and II B. I. MEASURING TECHNIQUES FOR POSTsARC CURRENTS A. Previously used principles of measurement Hitherto three different principles for measuring post arc currents have been described in the literature which more or less fulfill the requirements mentioned above: ohmic shunts, electron beam tubes, and ordinary current trans formers. In the first case, an ohmic resistance i.s placed in series with the circuit breaker and the potential drop due to the current is monitored, A disadvantage common to all measurements with such shunts is galvanic coupling to the network which can lead to risks to personnel and equipment which can only be avoided by optical links. Additionally EMC problems due to transient groundrise may occur. Such an ohmic shunt must have a high tolerance to thermal over load because of the high arcing current preceding post-arc current. Consequently a small resistance must be chosen, leading to poor sensitivity. Only in networks with low peak current can satisfactory sensitivity be achieved in this way? However, shunts may be used successfully when the fault currents are simulated with synthetic networks. These net works consist of two circuits: one generating the main cur rent and the other injecting a current with lower amplitUde and short duration at a predefined time before the main cur- 1087 Rev. Sci. Instrum. 58 (6), June 1987 0034-6148/87/061087-09$01.30 © 1961 American Institute of Physics 1087 ····-·;·~·.·.·.·.·.·.·,.·····:·:·z·.·.·;·.'."'·.:.··:·:-;·;.; •.••.••• -•.•.• :.;.;.;.; •.•. ' .•. ' •.•.• : ••• :.:-; •.•.••••.••• :.:.:;:.:.:.: •.• :.; •••••••••• :.~.:.:.;.:.:.: ............ ;.:.:.;.:.; •.•.•.•• ' ............. , ••• -...•.•.•. ".".". This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:42rent vanishes and thus simulating the rate of decrease of a high sinusoidal fault current. Recently St. Jean et al. 3 placed a shunt in the low curren t part of sueh a network thus obviat ing the need for the shunts to have high tolerance to over load. The disadvantage of this arrangement is that not only the current flowing through the breaker is measured such that the post-arc current has to be evaluated. To overcome overload problems the shunt may be short-circuited by some switch or electronic components while high current flows. Thus a higher resistance may be used resulting in better sensitivity. Since the shunt is de signed for lower power it may have smaller dimensions lead ing to better frequency response. However, if the test breaker fails, additional half-cycles of the fault current can flow whieh may damage the resistor. The main problem of this technique is the realization of such a bypass system. Murano et a/" 4 employ two vacuum switches in parallel to their high resistance shunt. While high current flows one of the switches is closed and the other is open. After peak current the first switch is opened and an arc is initiated between the two contacts of the vacuum circuit breaker. Most of the current flows through this bypass until commu tation to the shunt occurs shortly before current zero, when the potential drop across the vacuum arc falls below a limit ing value of about 20 V at which the vacuum arc can exist no longer. The value of the current commutating to the shunt can be controlled by the potential drop in the bypass which is controlled, for example, by the number of vacuum switches connected in series. To protect the shunt from damage if the test breaker fails the second vacuum switch is closed within a defined time after current zero" Because this arrangement is independent of the peak power, current allows the use of this technique for post-arc-current measurements in high-power test circuits. A new bypassing technique has been described recently by Mahdavi et al. 6 In this device the shunt is inserted into the network only for a time of approximately 600 fts. The central part of the device consists of a moving conducting rod par tially coated with an insulating layer. Current flows over the rod and stationary tulip contacts. These short-circuit the shunt unless the insulating layer of the rod forces the current to flow through the shunt. The rod is appropriately acceler ated by a magnetic coil, a spring provides the necessary re turn force. The main difficulty with this device is mechanical wear which eventually reduces reproducibility. Another bypassing method, is described by Stokes" 7 He uses an electrical discharge near the Paschen minimum which ignites at a potential drop of about 200 V and extin guishes at low voltages. This discharge needs no synchroni zation but the handling of high fault currents is not triviaL Schade and Molls use diodes in parallel to the shunt. During the high-current period, the current flows through the diodes" As soon as the current becomes so small that the potential drop across the shunt is smaller than the drop across the semiconductor barrier the current begins to com mutate onto the shunL After current zero the diode blocks and the post-arc current flows entirely through the resistor. The coaxially arranged diodes have to satisfy the following requirements; high peak forward current, short recovery 1088 Rev. Sci" Instrum., Vol. 58, No.6, June 1987 time, low reverse currents, and low capacitance in the junc tion" The system does not need synchronization but its fre quency response is limited by the capacitance of the diodes. In addition to the described methods there are many other ways to insert temporarily a high-resistance shunt into the network, but the problems arising are similar to those described above. A completely different technique for measuring post arc currents was developed by Roth et al. 10 The electromag netic field of a current-carrying conductor is used. An elec tron-beam tube is placed near the conductor. The electromagnetic field perpendicular to the electron beam causes a deflection of the beam and, therefore, a change of the current detected at ihe anode of the tube. This results in a voltage variation across a resistor in the anode circuit. In order to cancel geometric distortions two electron-beam tubes are used. An advantage of this device is decoupling from network and independence from high-power currents. A current transformer has been used first by Spruthl2 for measuring post-arc currents. During the high-current period the core of the transformer which consists of high permeability material is saturated" As the current goes to zero the energy of the magnetic field is absorbed in a choke which is connected to an auxiliary winding of the current transformer. When the post-arc current starts to flow the unsaturated part of the hysteresis curve is reached and the integral of the magnetic flux is measured. Because of the physical properties of cores this technique allows resolution of time in the order of only 10-100 J.ls depending on the peak fault current. B. A novel post~arc·current monitor Figure 1 shows the simplified circuit diagram of the nov el post-are-current monitor (PACM) to be described here. The down transformation of the current to be measured to lower values makes it easier to fulfill the requirements for measurement of post-arc current. While the heavy current flows through the primary circuit, the secondary of the transformer is shunted by the antiparallel configuration of fast-recovery diodes. Within the linear range of measure ment, these diodes turn off and the transformed post-arc current produces a proportional voltage drop across the shunt which is proportional to current. Compared to a clipping of overvoltage with diodes placed directly in parallel with a shunt in the primary circuit, FIG. L Principal circuit diagram of the new post-are-current monitor (PACM)" Post-arc currents 1088 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:42the new PACM offers a number of advantages for the user, as well as for the designer. The insertion impedance of the primary circuit can be kept very low, thus imposing practically no additionallimi tations on the voltage/current rating of the circuit contain ing the circuit breaker. The output circuit of the PACM is isolated from the circuit carrying heavy current. Thus EMC problems due to the transient ground rise are minimized. With proper screen ing and a balanced arrangement of differential output, the disturbing effects of the differential-mode coupling through the various stray capacitance in the assembly can be sup pressed to an acceptable level without special precautions or excessive EMC engineering during the setting up of the ex periment. In the design, the down transformation of the current allows for an optimized trade-off between the maximum surge current and the reverse recovery time of available sem iconductor diodes. The shunt resistor Rs across the secondary winding of the transformer has a transient power dissipation in the or der of 1 W, allowing easy broadband design as wen as selec tion of the full-scale range ofthe post-arc current by chang ing or switching the value of Rs. If the impedance of the secondary circuit of the trans former is well matched to the impedance of the two-wire screened rf cable, any length of cable necessary in practical experiments may be used without reflections or limitations of the bandwidth. 1. Theory of operation Figure 2 shows the equivalent circuit diagram of the P ACM with all parameters transformed in thc usual way from the primary to the secondary side. The primary circuit is assumed to consist of one single conductor. For analysis, two basic operational modes have to be distinguished: a. High-current operation: The high current, ranging from several to several tens of kA, flows through the circuit breaker and the primary circuit of the PACM. The second ary winding of the transformer is shunted by the diodes, the output voltage is given by the equivalent forward turn-on voltage UF of the diode and the potential drop on its incre mental series resistance rF• b. Post-arc operation: Close to the moment of current zero of the high primary current, the voltage on the second ary drops below the diode voltage U F and the diodes turn off. The PACM returns to the linear range and the secondary current flows now through the shunt Rs and the zero-bias capacitance CD of the diodes in paralleL FIG. 2. Equivalent circuit of P ACM. 1089 Rev. Sci.lnstrum., Vol. 58, No.6, June 1987 In the high-current mode of operation, the maximum permissible primary current is determined by the saturation conditions of the toroidal transformer and thus by its rat ings. Referring to Fig. 2, the saturation voltage is, for a sine wave, given by (1) wheref denotes the frequency, A Fe the cross section of the magnetic transformer, Bmax the flux-density at saturation, and II the number of secondary turns. The maximum permis sible primary current is then ltmax = n(Uls --uF)/Z2, (2) where U F is the forward voltage across the diode at the maxi mum current, Ilmax and Z 2 is the total impedance of the secondary circuit Z2 = jU1Ls2 + r2 + rF. By substitution into (2), the maximum primary current can be expressed as 2rrfAFeBmal<.n -UF llmax = . n. jOJLS2 + r2 + rr (2a) Clearly thc linear current range is lUll = n(uFi IR,), (3) where Up] denotes the forward voltage across the diode, at which the current, IF, through the diode is considered negli gible in comparison to I lin' i.e" IF < 0,005 I Lin' In the low-frequency range the P ACM behaves as a first-order high-pass filter with a cutoff frequency ( -3 dB) of (4) where R I is the resistance of the primary circuit and R ; =n2R!. The pulse response of such a high-pass filter always ex hibits a droop of the top of the pulse, followed by a corre sponding undershoot of the output voltage at the end of the input pulse. To minimize this undershoot the cutofffrequen cyofthePACMmustbesufficientlylow (fgL <50Hz) if the post-arc current, following a high-current half-wave at 50 Hz, is to be measured with high resolution and accuracy. Within the linear range, the upper cutoff frequency of the PACM will be given mainly by the secondary leakage inductance LS2' the resistance r2 of the secondary of the transformer, and the resistance R ll' which represents all losses in the magnetic core. This simplification is permissi ble, since for correct design the conditions '2<RH; l/(,;C[)~Rs; R; <WL'SI; always hold. The upper cutoff frequency is then hu = CR, + Ru)/2-rrLs2' (5) In many experiments the half-wave high-current pulse is generated synthetically by discharging capacitors through shaping networks. In such experiments high current of mostly one polarity flows through the P ACM. Especially in cases where the amplitUde of current comes dose to the max imum rating of the PACM the core of the transformer has to be demagnetized after each high-current pulse, which is clear from consideration of the hysteresis curve. Starting in the fully demagnetized state the flux density will increase Post-arc currents 1089 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:42during the first half-wave ofthe high current up to the satu rated fiux Bmax, returning then to the remanent fiux density Brem after the circuit is broken. For the next experiment the significantly reduced swing of the fiux density between Brem and Bmax would result in a much lower equivalent induc tance and thus transient distortion of the output voltage. A complete demagnetization of the core or premagnetization into the opposite remanence -Bren> avoids these problems. 2. Design of the PACM The design considerations for a high limit to maximum primary current are contradictory to the demand for a maxi mum upper cutoff frequency (or fast response) since higher current limits require a larger core, which necessarily has a larger leakage inductance. For this reason, two different PACM's have been devel oped, each optimized for specific requirements. For both PACMs, toroidal cores were chosen, made of the Ni-Fe aHoy PERMAX M (54%-68% NO from the German Vacuumschmelze Co. The high-saturation flux density of Bmax = 1.5 T together with a high permeability (/14 = 40 000; Pmax = 125000) allow a design with high current-carrying capacity and sufficiently low lower cutoff frequency of the PACM's. A fast PACM for measurements on SF 6 -insulated cir cuit breakers allows maximum currents of 6.3-kA peak, of fering a resolution of about 0.1 A and a rise time of 20 ns within the linear range. Another PACM, designed for studies on vacuum circuit breakers, has a full-scale range of 56-kA peak with 90-ns rise time in the linear range. Figure 3 shows a complete circuit diagram of this PACM, including the device for demagnetization of the core. By remote control a power relay switches the second ary winding of the transformer from the diode/shunt config uration to the output of a demagnetizing generator, which produces a sinewave voltage with exponentially decaying amplitude. Alternatively, a dc premagnetization into the op posite remanence can be done. More detailed data of both PACM's are summarized in Table I. 3. Performance tests At first the current-carrying capacity of the PACM's was tested under well-defined and monitored conditions. A FIG. 3. Cllrrent monitor (2) with demagnetization circuit; linear range 50 A (100 A). 1090 Rev. Sci.lnstrum., Vol. 58, No.6, June 1987 TABLE I. Specifications of the post-are-current monitors. l'ACM (1) PACM (2) Dimensions of toroidal cores (mm) 120/80X20 180/120X30 Number of secondary turns 50 100 Resistance of secondary winding, r2 14 40 (mO) Low-frequency inductance of 0.15 1.1 secondary, Ln (H) Leakage inductance, LS2 (pH) 13 70 Secondary saturation voltage U1S (V) 8.5 39.6 Saturation primary current, J I max (kA) 6.3 56 (1/2 sine wave, 10 ms) Peak surge current of diodes, IFSH (A) 140 600 (112 sine wave, ]0 illS) Reverse recovery time of diodes, 35 100 t" (ns) Slope resistance of diodes, rf (mn) 15 7 Resistance ofslmnt, Rs eu) 10 2 Primary insertion impedance (mn) 4 0.2 Linear primary current range, fUN (Al 4.2 60 Sensitivity into 95 n, S (mV/A) 100 10 Lower cutoff frequency, f (Hz) 7 0.3 Upper cutoff frequency,f (MHz) 18 3.8 Rise time, t (ns) 20 90 FIG. 4. Current pulse half sine wave 20-kA peak; upper trace: PACM (2). (a) 100 A/div, (b) 2 A/div; lower trace: Transfoshunt LEM 5 kA/div. Post-arc currents 1090 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:42FIG. 5. Circuit diagram for testing transient response of P ACM. Trig IN fast-recovery thyristor controlled by a logic circuit, chops a single half-wave of 50-Hz current from a power-line powered transformer and feeds up to 1.25-kA peak into the PACM's primary winding of 20 tUnIS, thus simulating an equivalent current surge of 25-kA peak. The primary cur rent of the PACM is monitored by a dc-coupled current monitor, a "Transfoshunt" from LEM Co. The results obtained with the 56-kA PACM are shown in Figs. 4(a) and 4(b), where the upper trace always gives the response of the PACM, the lower trace the output of the current monitor. The undershoot of the PACM's response, caused by the high-pass behavior (lower cutoff frequency) of the P ACM, is 1 A. To test the high-frequency behavior of the PACMs an arrangement according to Fig. 5 was used. Only a single conductor formed the primary of the PACM in order to simulate the real experimental conditions to the best possible extent. An HP 214A pulse generator was used as a source, delivering a current of about I-A peak through 50 n into the primary circuit. This current was monitored by the de to 50- MHz Tektronix current probe P 6302. In Figs. 6(a) and 6(b) the input current with a rise time of -15 ns as monitored by the Tektronix current probe is shown on the upper trace. The lower trace is the output of the 6.3-kA peak PACM. In Fig. 6(c) the bandwidth of the scope was limited to 5 MHz. The difference in time between both traces is caused by the difference in the delay times of the devices. The experimental results are in very good agreement with the theoretical predictions. II. APPLICATION TO CIRCUIT BREAKERS A. Highapressure arcs Separation of current-carrying contacts leads to an elec tric arc between them which results in a highly conductive plasma between the contacts of the breaker. For current in terruption it is necessary to reduce the electrical conductiv ity of the plasma, which is equivalent to a reduction in the number of charge carriers. In collision-dominated plasmas deionization predominantly occurs by volume recombina tion. For this to occur the plasma has to be strongly cooled to reduce its temperature quickly. In one kind of circuit breaker this is done by imposing a flow of gas generated by a pressure gradient across a nozzle. When the current goes to 1091 Rev. ScI. Instrum., Vol. 58, No.6, June 1987 FIG. 6. Dynamic ac performance ofPACM (1) vertical 0.2 A/div; Ca) and (b) transient response: lower trace P ACM, upper trace Tektronix P 6302; (e) bandwidth of the scope limited to 5 MHz. zero the arc plasma is cooled down, especially in the region of the nozzle where a strong, turbulent heat exchange oc curs. The influence of cooling on the conductivity of the plasma can be seen in Fig. 7, where the calculated conductiv ity16 for an arc in SF6 at a pressure of 4X 105 Pa is plotted as a function of the temperature of the plasma. During the high-curreni period the temperature of the arc is 20000 K and the conductivity is in the order of 104 S/m while it is one Post-arc currents 1091 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:424000 10000 2.0000 TEMPERATURE ( K ) FIG. 7. Conductivity of SF" at 4 X 105 Pa as afunction ofthe temperature of the plasma. order of magnitude lower at 8000 K, which is a typical tem perature of the region where interruption occurs at current zero. However, immediately after current zero the residual arc plasma cannot decay freely because a post-arc current flows under the influence of the recovering voltage of the network The resultant input of electrical energy counteracts the energy loss caused by cooling. Depending on which way the balance tips either reignition or interruption of the power current occurs. The time during which this energy balance is reached is called the thermal regime. Afterwards, the input of electrical energy can be ignored because the post-arc cur rent drops to very low values (micro-and miIli-amperes). However, there still exist regions of high temperature and consequently high electrical conductance. In these cases the post-arc current mainly consists of a current component which is proportional to the derivative of the voltage, Depending on the arc-quenching medium the ability of thermal recovery may be quite different. Modern high-vol tage circuit breakers use SF6 which is an eminently suitable quenching and insulating medium because of its high dielec tric strength, great thermal stability, and chemical inertness. In comparison to air-blown arcs recovery is much faster. Consequently, the post-arc period is different. While air blown arcs have post-arc currents in the order of some tens of amperes with durations of several tens of microseconds, SF 6 breakers usually have post-arc currents of a few amperes or less which flow for about 1 its. Therefore, measurements of post-arc currents for blown arcs in SF6 are much more difficult and have been rarely performed.2-4,8,9 For investigations of post-arc current we use a model breaker blown with SF 6 by a dual-nozzle configuration. The pressure value upstream is 3.4 X 105 Fa and 1.2 X 105 Pa downstream at the moment of interruption. The nozzle di ameter is 18 mm to prevent clogging ofthe nozzles due to the arc. By means of the explosion of a wire between two fixed contacts an arc is initiated and a nearly constant current of about 2.5 kA flows for about 10 ms. The pulse of current is generated by the discharge of capacitors. To simulate high 1092 Rev. Sci. Instrum., Vol. 58, No.6, June 1987 -1.1=0 ~i=O u=O 1=0 FIG. 8. Examples of measured post-arc currents from a blown arc in SF6 stressed with a rate of change of current of 14 AI fis in the power circuit; upper traces: recovery voltage--400 V Idiv, lower traces: post-arc cur rents-l A/div. fault currents a synthetic network similar to that of Frind et aJ.2 was used to produce the corresponding current decay rates. The equivalence between direct and synthetic inter ruption test has been shown, e.g., by St-Jean and Fu Wang. 15 In the thermal regime the highest stress for circuit breakers is the short-line fault. At this fault condition recov ery voltage rises linearly for several microseconds beginning immediately after current zero. The rate of change of current decreases to zero and the rate of rise of transient recovery voltage (TR V) can be freely chosen for the range of param eters in which we are interested. The exact geometry of the model breaker will be published in another paper. The first measurements of post-arc currents, monitored with one of the current transformers developed, are shown in Figs. 8(a) and 8 (b). A fast resistive capacitive voltage divider mea sures the voltage across the arc. The rate of change of current at interruption of 14 All1s corresponds to a 50-Hz sinusoidal current of 31.5 kAeif• Figure 8( a) shows a successful inter ruption where a post-arc current with a peak amplitude of 1 A flows for about 1.5,1s, measured with PACM (1). During this time the TR V is fairly linear. When the TR V is increased interruption fails as shown in Fig. 8(b). The second oscillo graph is an example for a process of interruption at the boundary between successful interruption and failure of the breaker. The balance between cooling of the arc and reheat ing due to the post-arc current is impressively evident. About 1 flS after current zero the post-arc current has an amplitude of 1.3 A and is nearly constant for the next 2 f-ls. If Post-arc currents 1092 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:42~ W U z <:: I- (f) (J'j W et: ...J « I- 0 I-104 103 -2 -1 0 1 2 3 4 TIME (IJ,S) FIG. 'l. Total resistance between the breaker contacts as a function of time, for the two examples given in Fig. 8, counted from current zero (0). the input of energy to the arc is too high heating exceeds cooling and current increases within a microsecond by a fac tor of 3. From the measurement of post-arc current and vol tage across the contacts of the breaker the total resistance of the arc close to current zero can be determined. This is shown in Fig. 9 for the two examples given in Fig. 8. When current is out of the range of the P ACM values measured by an additional current transformer are used. Before current zero the resistances of the arcs for the two cases differ be cause of statistical variations in the arc-quenching process by less than 15%. At current zero the resistance is about 120 n. In the case of successful current interruption the resis tance of the arc increases within 1.5 f-ls by two orders of magnitude. If interruption fails the resistance first increases similarly, but then more slowly up to 2.81ls after current zero where a maximum value of about 1.7 kn is reached. Thereafter, resistance decreases rapidly leading to the onset of heavy current flow. Obviously analysis of post-arc currents gives important information about the arc-quenching process. Standard tests on circuit breakers detect whether interruption fails or not, but by measuring post-arc currents during tests it can be estimated how close an interruption is to failure. This allows the predictions oflimiting conditions for interruption in cir cuit breakers and spares time and costs required for their development. Another motive for making post-arc current measure ments is the possibility to assess modelsl6,21 of arcing which describe interruption. These models are based on the com bined solution of the three equations for the equilibria of energy, momentum, and mass. However, the different ap proximations and assumptions made in the models lead to different results, the calculated post-arc currents being very sensitive to the assumptions made. A comparison between the measured post-arc currents and the calculated ones will be published later. 10S3 Rev. Sci. Instrum., Vol. 58, No.6, June 1987 .••• -•••••••• ',';o ••••••• -.~.; •••••• ' ••• -.'.-.~.: ••••• , •••••••••• :.;.:.: •••••••••••••••• ;.:.:.; ••••• , •••• ' •• ;:-:.:.: •• >; •••• '.' ••• r ••••• ;.; ••••••••• ,o;"; •••• -,-.·. '-'-'.' B. Vacuum arc Different physical processes determine the behavior of the post-arc current and its significance for the process of interruption in a vacuum arc which consists of an arc in metal vapor surrounded by vacuum. For high-pressure dis charges the value of the post-arc current reflects the conduc tivity of the decaying arc and is proportional to voltage. In vacuum arcs the post-arc current results from the separation and collection of the residual charge carriers. The energy balance between heating and cooling of the plasma is no longer of importance . In the case of vacuum arcs the charge carriers in the plasma have to be conti.nually renewed during arcing be cause of the steady loss due to outflow and condensation at the surrounding walls. The material ionized to form the plas ma originates from the metal electrodes. It is vaporized by the footpoints of the arc. When the power current ap proaches current zero the rate of ionization simultaneously drops to zero, the evaporation stops, and the decay of the plasma commences. At the prevailing low densities colli sions between electrons and ions are very rare and conse quently the process of volume recombination of charge car riers is of no importance for the decay of the plasma after a vacuum arc. Instead, neutralization of charge carriers main ly occurs when they come into contact with the surrounding walk Thus the velocities of the ions determine the rate of decay of plasma. While charge carriers remain, a so-caBed post-arc cur rent is driven by the voltage increasing across the electrodes. The residual charge carriers are separated and collected at the electrodes. Due to formation of a positive space-charge sheath in front of the cathode a strong increase of the electric field strength at the cathode results. The space-charge sheath spreads out to the anode and finally reaches the anode when the charge carriers are collected. All these processes are reflected by the temporal behavi.or of the post-arc cur rent. An example is given in Fig. 11, which demonstrates the measurement of post-arc current performed on a vacuum model breaker with the new current monitor. The corre sponding experimental arrangement, outlined in Fig. 10, is t<o \>0 \=::: I . orc c+l~+ e HV .~ ~ FIG. 10. Experimental setup. Post-arc currents i~"HkA ~OA -_. -----__ t I 40ms I 5,5ms 1 1093 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:42r-~.----r---'r-~~==~120 U 6.0 kV i A 4.0 2.0 100 " i = 4kA 80 60 ... -j shield 40 0'-----'---'----'-----'----'-----'0 o 10 20 t/J-LS FIG. 11. Examples for two post-arc currents in a vacuum circuit breaker, carrying 4 kA. The current~ to the anode and the shield were measured separately. described in detail elsewhere.22 The interruption unit con sists of two flat OFHC Cu contacts of 25 mm diameter mounted in a spherical UHV enclosure and separated to 7.5 mm. The vacuum enclosure was connected to earth by a diode so that it assumed floating potential during the arcing, but the potential of the anode during the application of the HV pulse. A rectangular pulse of current of 5.5-ms duration and peak values up to 11 kA was used. The high-voltage pulses of exponential shape applied to simulate the increas ing network voltage are switched on at different delay times after current interruption. In the case of Fig. 11 this is after about 20 Jis. The new current measuring device [PACM (2)] monitored the post-arc current (mode to the lower elec trode, the HV anode, while ishield' the current to the metal enclosure or shield, was measured with an ordinary current transformer. The post-arc current to the anode ianooe lasts for about 3.5 Jis, that is until all charge carriers are collected which were contained in the space between the contacts be fore the HV pulse was applied. In order to obtain the number of ions, the post-arc cur rent has to be integrated over time assuming that the loss of plasma during the collection can be neglected. This is the case if the applied HV pulse quickly reaches high enough values as in the present experiment. However, the post-arc current signal does not only result from the collection of residual plasma. It includes a considerable flux of secondary electrons emitted when the residual charges butt the elec trodes. This additional current was evaluated and subtracted from the original signal. When the number of ions is known, the density can be estimated. For the measurement of the decay of the residual plasma the moment of triggering of the HV pulse was varied. An important conclusion from these measurements could be drawn. The amount of slow ions with velocities correspond ing to temperatures of only some thousand degrees assuming a Maxwellian velocity distribution was much higher than previously expected. Correspondingly the dielectric recov ery of the present experiment was mainly determined by the influence of residual plasma. That no ionization occurred 1094 Rev. Sci. Instrum., Vol. 58, No.6, June 1987 during the collection of charges was a prerequisite for the measurement of the decay of residual plasma. However, the technique described can also be used to study the onset of amplification of current in case of a dielectric breakdown. III. DISCUSSION A new device for measurement of post-arc current has been presented which has special importance for the under standing of current interruption in circuit breakers. The de vice consists of a current transformer with short-circuited secondary turns while the heavy current flows through the primary circuit. This is done by antiparallelly arranged di odes. Within the range of parameters for which the post-arc current monitor is designed core saturation is prevented and the post-arc current provides a linear potential drop across the resistor. The device offers several advantages: no galvan ic coupling to the power circuit and consequently minimized EMC problems, low-insertion impedance of primary cur rent, 110 synchronization, and high safety standards. Two post-are-current monitors, designed for different applica tions, are presented. The first one is applied to a blown arc in SF 6' This P ACM is designed for one half sine wave (10 ms) with a maximum peak current of 6.3 kA and has an upper cutoff frequency of 18 MHz. The sensi ti vi ty of detection of post-arc current is 100 m V I A. The device has a primary insertion impedance of only 4 mD.. The second PACM was designed for vacuum circuit breakers. The saturation current of pri mary circuit is calculated to 56 kA (one half sine wave 10 rns). With an upper cutoff frequency of 3.8 MHz post-arc currents can be detected with a sensitivity of 10m V I A. All requirements for an accurate measurement of post-arc cur rent are fulfilled. To achieve optimum performance the PACM must be designed for specific applications. Unfortunately the upper cutoff frequency decreases if the PACM is designed for high er saturation currents. For this reason the applicability of this technique of measurement is restricted. Applied to circuit breakers, both PACM's showed ex cellent performance. lK Ragaller, A. Pless!, W. Hermann, and W. Egli, International Confer ence on Large High Voltage Electric Systems, Paris, CIGRE paper 13-03, 1984. 2G. Frind, L. E. Prescott, and J. H. van Noy, IEEE Trans. Power Appar. Syst. PAS-99, 268 (1980). "G. St-Jean, M. Landry, M. Leclerc, and A. Chenier (to be published). 4M. Murano, H. Nishikawa, A. Kobayashi, T. Okazaki, and S. Yamashita, IEEE Trans. Power Appar. Syst. P AS-94, 1890 (1975). 5G. Frilld, L. E. Prescott, and J. H. van Noy, J. Phys. D 12, 133 (1979). 6J. Mahdavi, A. Schaffer, C. Velo, L. Hompa, and I. Gatcllet, lEE l'roc. 132,285 (1985). 7 A. D, Stokes, lEE 4th International Conference on Gas Discharges, Swansea (The Whitefriars Press Ltd, London, 1976), p. 75. SR. Moll and E-Schade, J. Phys. (Paris) Colloq. C7 SuppL No.7, C7, 309 (1979). 9A. Kobayashi, S. Yanabu, S. Yamashita, S. TomimuTO, and E. Hagino mori, IEEE Trans. Power Appar. Syst. PAS·97, 1304 (1978). lOH. Roth, F. Brischnik, and H. Notz, BBC Mitteilungen 49, 119 (1962). lIN, AI. Husayni and G. Voisin, Rev. Gen. Electr. (France) 86, 833 (1977). 12W. Spruth, thesis, RWTH Aachen, 1956. Post-arc currents 1094 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:4213S. Yanabu, Y. Satoh, M. Homma, T. Tamagawa, and E. Kaneko, IEEE Power Society, Winter Meeting, 86 \'>'M 139-0, 1986. 145. E. Childs, A. N. Greenwood, and J. S. Sullivan, IEEE Trans. Plasma Sci. PS·ll, 18! (1983). 150. St-Jean and R. Fu Wang, IEEE Trans. Power Appar. Syst. PAS·IIll, 2216 (1983). 16e. B. Ruchti, Proceedings of the 5th International Symposium on HSwitching ARC Phenomena" Lodz (Polen) (1985), p. 39. I7W. Hermann and K. Ragaller, IEEE Trans. Power Appar. SyM. PAS·96, 1546 (1977). l"W. Hermann, U. Kogelschatz, L. Niemeyer, K. Ragaller, and E. Schade, 1095 Rev, Scl.lnstrum., Vol. 58, No.6, June 1987 IEEE Trans. Power Appar. Syst. PAS·95, 1165 (1976). 19B. W. Swanson, R. M. Roidt, and T. E. Browne, Jr., Elektrotech.-Z .. Ar chiv 93,375 (1972). 2nB W. Swanson, R. M. Roidt, and T. E. Browne, Jr., IEEE Trans. Power Appar. Syst. PAS-90, 1094 ( 1971). OJF. EI-Akkari and D. T. Tuma, IEEE Trans. Power AppaL Syst. PAS-96, 1784 (1977). 22E. Dullni, E. Schade, and B. Gellert, IEEE Proceedings, XfIth Interna tional Symposium on Discharges and Electrical Insulation in Vacuum, Is rael (Tel Aviv University, Tel Aviv, 1986), p. 214. Post-arc currents 1095 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.248.9.8 On: Mon, 22 Dec 2014 06:13:42

1.102144.pdf

Timeresolved measurement of tunneling and energy relaxation of hot electrons in GaAs/AlGaAs double quantum well structures N. Sawaki, R. A. Höpfel, E. Gornik, and H. Kano Citation: Appl. Phys. Lett. 55, 1996 (1989); doi: 10.1063/1.102144 View online: http://dx.doi.org/10.1063/1.102144 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v55/i19 Published by the AIP Publishing LLC. Additional information on Appl. Phys. Lett. Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 28 Aug 2013 to 137.99.26.43. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsTimearesolved measurement of tunnenng and energy relaxation of hot electrons in GaAsl A~GaAs double quantum weH structures N. Sawaki,a) R. A. Hopfel, and E. Gornik Institutfur Experimentalphysik. Universitat Innshruck, Technikerstrasse 25, A-6020 Innsbruck. Austria H. Kano Toyota Central Research and Development Laboratories, Inc., Nagakute-cho, Aichi 480-11, Japan (Received 25 May 1989; accepted for publication 1 September 1989) The tunneling and cooling times of photoexcited hot electrons in AlGaAs/GaAs double (one narrow and the other wide) quantum well structures have been measured using photoluminescence excitation correlation spectroscopy. The tunneling time was of the order of 20C ps for a 60 A barrier. The tunneling is the indirect process assisted by the emission of optical phonons. The relaxation time of electrons as a function of the kinetic energy shows a threshold for cooling via the emissicn of optical pnonons. The tunneling structure with coupled quantum wens via a thin potential barrier is interesting for the fundamental studies of the low-dimensional system as well as for the ap plication to electron and electro-optical devices. I~ In the application to high-speed devices, the dynamics of carriers such as tunneling through the potential barrier and cooling of hot electrons is the main scope of the studies. Several au thors have shown that negative differential resistance (NDR) devices can be obtained by utilizing the double quantum wen (DQW) structure with two quantum wells of different width, where the tunneling transfer afhat electrons from a wide wen into a narrow well plays an essential role.4,5 As has been discussed previously,"; the tunneling transfer in DQW systems could be of the same mechanism as of the intersubband transitions in a quantum well. If the two quan tized levels are not at the same energy (not resonant), the tunneling transfer must be assisted by impurity or phonon scattering. Thus, the interest is on the mechanism and the speed of the tunneling transfer. In this letter, we have investigated the carrier dynamics of photoexcited hot electrons by picosecond luminescence spectroscopy. The samples were made by molecular beam epitaxy on semi-insulating (OOn GaAs. Between 500-700 A thick nondoped Ala.3 Gao. 7 As layers, ten sequences of DQW were embedded. The width of the wide and narrow wells is 140 and 60 A, respectively. The potential barrier is due to 60 A Alo.3 G~1.7 As. (See the inset in Fig. 1.) The DQW region was doped uniformly with Be. The hole density isp = 2X lOll cm-2. Time-resolved photoluminescence spectroscopy was performed by using the two-beam correlation method.6-H For the excitation we used a colliding pulse mode-locked (CPM) dye laser at 620 nrn (pulse width 150 fs, repetition rate 100 MHz). The laser beam (pulse train) split into two beams, one of which gave a certain delay T. The two beams were focused on the sample, which was immersed in liquid nitrogen. Two electron and hole populations n 1 (t), PI (t), and n2 (t + r), P2 (t T r) are generated. The laser power at the sample surface was < 2 m Wand the total excited carrier ") Permanent address: Department of Eicl'tronics, Nagoya University, Chi kusa-ku, Nagoya 464, Japan. density was estimated to be < 5 X 1010 cm-2• The two pulse trains were chopped at 250 and 225 Hz, respectively, and the cross-correlation signal of the photoluminescence intensity was measured at the difference frequency of25 Hz by a lock in amplifier. The signal intensity is proportional toB: Correlation signal o::f[fl1U)P2U+r) +PI(t)n 2U+r)]dt. (1) Obviously, by measuring the correlation signal at various wavelengths as a function of the delay time r, we can deter mine the decay time constant of the photoexcited hot carri ers at various kinetic energies. R Figure 1 shows the typical photoluminescence spectra at 77 K. The inset shows the schematic structure of the DQW. The main peak at A I = 810 nm is by the electron-hole A g ... e..,. B >-l- ll) 10-1 Z W r- Z W U 10"2 Z w U lfl W Z ~ 10-3 ::::> ...J 0 I- 0 :r: a.. "Aex= 620 nm 1()"4 750 770 790 810 WAVELENGTH (nm) FIG. l. Time-integrated photoluminescence spectrum of sample No. 378. The inset shows schematically the DQW structure and the processes con cerned. 1996 Appl. Phys. Lett. 55 (19), 6 November 1989 0003-6951/89/451996-03$01.00 @ 1989 American Institute of Physics 1996 Downloaded 28 Aug 2013 to 137.99.26.43. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsrecombination in the wide well, and the small peak at /1.2 = i76 nm is that in the narrow well. The processes in volved are also shown by the inset. Since the excitation energy (620 am, 2.0 e V) is larger than the energy gap of the AIGaAs barrier layer, the carriers are excited in the whole region. After the capture into the QW's (A =? B), the hot electrons in the QW relax in each QW by emitting phonons (B=}C). Those electrons captured in the narrow QW escape into the wide QW by phonon-assisted tunneling ( C =? D =? E). Because of the energy difference Ilt 12 between the two lowest subbands, direct tunneling is forbid den due to momentum conservation. The same applies for the holes but the time constants are different from those for the electrons. Figure 2 shows the correlation signal as a function of the delay time at several wavelengths. The curves are described by an exponential decay exp( -Tire)' from which the de cay time constant'e is determined. 8 The results are shown in Fig. 3. At the peak wavelength A I' the time constant was too long to be measured (longer than 1 fiS). As the wavelength decreases, or as the kinetic energy of electrons andlor holes increases, the time constant decreases rapidly and reaches a value of ~ 80 ps with a maximum at A ~ Az. At the shortest wavelength measured, the correlation signal becomes very weak and we have two time constants. The shorter one is as short as 15 ps. The capture time of electrons and holes is < 1 pS,9 Be cause ofthe larger effective mass, the relaxation of hot holes :;; d ....l \ <! Z ~ (,!) 99.5 750nm If) z Q I-12 ps <! ....l X 5 / w a:: a::: 0 !\ 124ps u DELAY TIME Cps) FIG. 2, Correlation signal ofphoto!uminescence intensity at vairous wave lengths, The intensity is not to scale. At 1"'~ 0, e.g., the intensity for A = 800 nrn is 590 times stronger than that for A = 750 nrn. At A = 750 nm, the decay curve includes two components. The resolution of the measuring sys tem is estimated to be < 0.3 ps. 1997 Appl. Phys. Lett., Vol. 55, No. 19,6 November, 9S9 1000 #378 T= 77 K Ae,('620 nm ... ~ 0. W 0 ~ 100 0 I- >- <i t i u w Cl i1wLO "2 nwLO "1 I" O>j ~ .. ! 10 750 770 790 810 VvAVELENGT H (nm) FIG. 3. Correlation time or the decay time of photo excited hot electrons in sample No. 378. to a thermally steady state takes place very quickly « 1 ps), l{) and the tunneling time of holes is expected to be much longer than that of electrons. II Therefore, the time constant obtained above is mainly determined by the electron process. The time constant longer than 1 ns at /1. I should be the life~ time of electrons at the bottom of the wide weli. Similarly, the time constant at Jl2 should be the lifetime of electrons at the bottom of the narrow well. We cannot explain the tre~ mendous difference in the magnitUde of the lifetime by its quantum well width dependence. i2 An additional relaxation process for the electrons in the narrow well is the tunneling escape into the wide weIL From the experimental data the tunneling time in this sample is of the order of 200 ps. This tunneling time is nearly equal to the tunneling es~ cape time obtained for a double-barrier (DB) structure. Us~ ing the results by Tsuchiya and co-workers13 and assuming Wentzel-Kramers-Brillouin (WKB) approximation, we estimated the tunneling escape time in a DB structure having 60 A AIIJ.3 Gao.? As barriers. For the calculation we assumed the potential barrier height V = 0.23 eV and the effective mass m*lmo = 0.07. We obtained a value -100 ps. The time constant obtained experimentally is slightly longer than the estimated one. This i.s probably due to the fact that the tunneling escape in DQW is to a quantized level in the wide wen, while in DB structures there is a three-dimensional continuum outside the barriers. Recently Jackson and co workersll studied the tunneling escape time in DB struc tures. Their results were four times longer than those by Matsusue and co-workers. The precise discussion on this point, therefore, needs further experimentation including the exact determination of the width and height of the poten tial barrier. As pointed out earlier, the tunneling transition in DQW can be considered as a kind of intersubband transition. For the phonon-assisted case, the transition probability is pro portional to4,14: Sawaki et al. 1997 Downloaded 28 Aug 2013 to 137.99.26.43. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsPi; a:: if r/>'l (Z)CP,K (z)Cq (z,z')eiqZZ'dZ'dzl" x 8 (k ~ --k 1 + q 1 ), (2) where ¢a, (z) is the electron wave function along the z axis, the subscript 1 stands for the two-dimensional vectors in the x-y plane parallel to the heterointerface, and Cq is the cou pling constant In DQWs, tPiK' (z) has its maximum in the wide well, while ¢,K (z) is maximum in the narrow well, Thus the overlap is weak Seilmeier and co-workersl5 measured the decay time constant of electrons in the upper level in MQWs, They ob tained a value of the order oflO ps for LlEl2 = 150 meV. Ifwe consider the overlap integral between the two associated subband states, the longer time constant of the order of 200 ps for sample No, 378 is attributed to the reduction of the overlap integral in the DQW structure. The precise estima tion including phonon localization will be the subject of fu ture work. By analyzing the temperature dependence of the photo luminescence spectra, the energy separation of the two quan tized levels, LlEl2, can be determined experimentaBy.4 For sample No. 378 we got LlE12 = 38 meV. Since this is larger than the LO phonon energy of GaAs (36 meV), the tunnel ing can be assisted by the emission ofLO phonons. For com parison, we performed similar measurements about a sample having a 35 A A10 2(,Gao 74As barrier (sample No. 21 I). In this sample we also got a similar wavelength dependence of the decay time constant. By decreasing the potential barrier thickness from 60 to 35 A, simple WKB calculation gives the tunneling time of the order of 3 ps. But the tunneling time was as long as 300 ps. For this sample the energy separation Ll.€!2 was 24 meV,4 so the tunneling was assisted by acoustic phonons. Oberli and co-workers [6 measured the intersub band relaxation time 712 in a quantum well and obtained 570 ps for the acoustic phonon process. If this is the case, the acoustic phonon process is 60 times slower than the LO phonon process. Thus, the possibility of the reduction of the tunneling time in No, 211 is canceled out by the reduction of the phonon emission rate. Finally let us focus our attention on the relaxation phe nomena in the narrow quantum welL As was shown in Fig. 3, in the wavelength range 755-770 nm the time constant (~100 ps) is rather insensitive to the wavelength, At shorter wavelengths, we observe two time constants, The shorter one is as short as 15 ps, At this wavelength, the kinet ic energy of electrons in the narrow well is larger than the LO phonon energy ofGaAs. So the appearance of the short time constant is attributed to the onset of the energy relaxation via the interaction with LO phonons in the narrow welL The slow component of the order of lOOps might be due to the combination of the energy relaxation time via the acoustic phonon process and the tunneling escape time. The observa tion of the LO phonon threshold has been scarce in the study 1998 AppL Phys, Lett" VoL 55, No, 19,6 November 19S9 of energy relaxation in QWs. Since sample No. 378 is p-type doped, we have negligible density of equilibrium electrons in the DQW. By the ratio of the photoluminescence intensity at the two maxima (AI and ..1.2), the electron density in the narrow well is estimated to be as low as 5 X lOR em -,2. There fore, the hot phonon effect in the narrow well is weak and we might expect to see the intrinsic character of the electron phonon interactions. 17 In order to confinn the interpretation given above, we need further experiments on the excitation intensity dependence of the time constant, which is the sub ject of future work. In conclusion, we have measured the tunneling time constant and the cooling time constant in AIGaAs/GaAs DQW structures. The tunneling time constant is of the same order as the escape time constant in a double-barrier tunnel ing diode structure. It has been shown that the tunneling is assisted by the emission of optical phonons and/or the acoustic phonons. The relaxation time of electrons as a func tion of the kinetic energy shows a threshold of the cooling via the emission of optical phonon:'>. This work is partly supported by the Fonds zur Forde rung del' Wissenschaftlichen Forschung, Austria, under project No, P 6184. Partial support for the stay of N.S. at Universitat Innsbruck by the Murata Science Foundation, Japan, is acknowledged. lN, Yokoyama, K. Imamura, H, Olmishi, T, Mod, S, Muto, and A, Shiha tomi, Solid-State Electron, 31, 577 (1988), 2M, N, Islam, R. L Hillman, D, A, B. Miller, 0, S, Chernia, A. C. Gossard, and J, C. English, AppL Pbys, Lett, 50, 1098 (1987), 'K K Choi, B. F. Levine, C. G. Bethea, J, Walker, and R. J, Malik, AppL Phys, Lett 50, 1i!14 (1987), 'N, Sawaki, M, Suzuki, E. Okuno, II. Goto, I. Akasaki, H. Kano, Y. Tan aka, and M, Hashimoto, Solid-State Electron, 31,351 (1988), and refer ences therein, 5J, M. Pond, S, W, Kirchoefer, and E J, Cukauskas, App!. Phys, Lett. 47, 1175 (1985), "0, von der Linde, J, Kuhl, and E. Roscngart, J, Lurnin, 24/25, 675 (1981) , 'D, Rosen. A, G, Doukas, y, Budansky, A. Katz, and R. K Alfano, App!. Phys, Lett, 39. 935 (1981), 'R. Christanell and R. Harfel, SupcrlatL Microstruct. 5, 193 (19B9), "B, Deveaud, J, Shah, T, C. Damen, and W, T, Tsang, App\. Phys, l,ett. 52, 1886 (1987), [()j, Shah, B. Deveaud, T. C. Damen, W, T, Tsang, A, C. Gossard, and p, Lugli, Phys, Rev, l,ett. 59, 2222 (1987), "M, K, Jackson, M, n. Johnson, D, H, Chow, T, C. McGill, and C. '\N, Nieh, AppL l'hys, Lett. 54, 552 (1989), 111j, Cebulla, G. Bacher, G, Mayer, A, FOfChcl, W, T, Tsallg, and M, Ra zeghi, Supleratt, Mierostruct. 5,227 (1989). 1'M, TS\lchiya, T, Matsusue, and H. Sakaki, Phys, Rev, Lett. 59. 2356 (l9Xn 14N, Sawaki and L Akasaki, Physica B 134, 494 (1985), ISA, Seilmeier, H. J, Hubner, G, Abstreitcr, G, Weimann, and W, Schlup]), Phys, Rev, Let(, 59, 1345 (1987), "'D, y, Oberli, D, R. Wake. M, V, Klein,./, Klcm, T. Henderson, and H, Morkoc, Phys, Rev, Lett. 59, 696 (1987), lIK. Leo. \'ii, W, Riihle, and K, Ploog, Phys, Rev, B 38. 1947 (]988), Sawaki eta/. 1998 Downloaded 28 Aug 2013 to 137.99.26.43. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

1.458431.pdf

Collisioninduced angular momentum reorientation and rotational energy transfer in CaF(A 2Π1 / 2)–Ar thermal collisions Jeffrey B. Norman and Robert W. Field Citation: The Journal of Chemical Physics 92, 76 (1990); doi: 10.1063/1.458431 View online: http://dx.doi.org/10.1063/1.458431 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/92/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Depolarization of rotational angular momentum in CN(A2Π, v = 4) + Ar collisions J. Chem. Phys. 136, 164306 (2012); 10.1063/1.4705118 Collision-induced Rotational Energy Transfer in an Atom-Diatom System Chin. J. Chem. Phys. 21, 457 (2008); 10.1088/1674-0068/21/05/457-462 The role of angular momentum in collision-induced vibration–rotation relaxation in polyatomics J. Chem. Phys. 121, 169 (2004); 10.1063/1.1758696 Angular momentum reorientation in CO(A 1Π)–He rotational energy transfer studied by optical–optical double resonance multiphoton ionization spectroscopy J. Chem. Phys. 98, 9487 (1993); 10.1063/1.464380 Monte Carlo trajectory calculations of the energy of activation for collisioninduced dissociation of H2 by Ar as a function of rotational energy J. Chem. Phys. 74, 6709 (1981); 10.1063/1.441126 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40Collision-induced angular momentum reorientation and rotational energy transfer in CaF(A 2ll1/2)-Ar thermal collisions Jeffrey B. Norman Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Physics and Astronomy, Vassar College, Poughkeepsie, New York 12601 Robert W. Field Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 5 September 1989; accepted 25 September 1989) We have carried out an experimental study of collision-induced rotational angular momentum laboratory frame reorientation and energy transfer in CaF(A 2II1/2, v = 0) in thermal collisions with ground state Ar atoms. An optical-optical double resonance (OODR) technique has been used in which the J = 1/2, M = + 1/2,j-symmetry level of the CaF A 2II1/2 state is initially populated. from the ground X 2!, + electronic state, using circularly polarized cw dye laser radiation at 606 nm. Collision-induced population of nearby magnetic sublevels of the A state, belonging to both e and/symmetry components of J' = 1/2 and 3/2, is probed with a second circularly polarized cw dye laser via the E 2!, + +-A 2II I 12 transition at 560 nm while monitoring subsequent E 2!, + -X 2!, + ultraviolet fluorescence at 290 nm. This experiment has yielded M-dependent thermal rate constants and velocity-averaged cross sections, ratios of which are in partial agreement with those predicted by Alexander and Davis [M. H. Alexander and S. L. Davis, J. Chem. Phys. 79, 227 (1983)] in an infinite-order sudden, irreducible tensor treatment of the collision dynamics of an open-shell diatomic molecule and a structureless collision partner. I. INTRODUCTION This paper describes an experimental study of angular momentum reorientation and energy transfer in CaF (A 2II1/2, J = 1/2-J' = 1/2 and 3/2) in thermal collisions with Ar( IS) atoms. The experiment uses an optical-optical double resonance technique. A specific electronic, vibration al, rotational. parity, magnetic sublevel of CaF (A 2II1/2, v = 0, J = 1/2,J, M = + 1/2) is populated by a single mode (spectral bandwidth -2 MHz) cw dye laser via the A 2II1/2_X2!,+ transition. A second cw dye laser probes neighboring rotational and magnetic sublevels, through the E2!,+ -A 2III/2 transition, for collision induced popula tion. A pressure-dependent study of the resulting E 2!, + -X 2!, + fluorescence yields rate constants and veloc ity averaged cross sections for rotational energy transfer and reorientation. This type of OODR technique was first used for the study of atom-diatomic molecule collisions by Silvers etaU in their work on BaO(A I!,+ )-Ar and BaO(A I!,+)_ CO2 collisions in which M-state selectivity was achieved. The present experiment is modeled after that work. The key difference here is the presence of nonzero spin and electronic orbital angular momentum. Similar methods have recently been used in rotational and vibrational energy transfer stud ies of N2, Nt, and CN2 without M selectivity. Reference 3 describes a related form of collision-induced double reso nance using cw and pulsed infrared lasers and transient ab sorption detection. Other double resonance experiments in the infrared, microwave, and visible regions have yielded information on M-changing collisions in polyatomic mole cules.4 Dufour et al. 5 have measured orientation-averaged cross sections (no M selectivity) for rotational energy transfer within theA 2[11/2 and A 2II3/2 states ofCaF in thermal colli sions with Ar and He. Their initial (J) and collisionally pop ulated final (J') rotational levels were J = 1/2, 3/2 and J' = 1/2,3/2,5/2,7/2, and 9/2. The CaF molecules were excited into the A 2II1/2 state by a cw dye laser and the flu orescence back into the ground state was analyzed with a high-resolution spectrometer. Their results confirmed the propensity for e//symmetry conserving collisions predicted earlier by Alexander.6 They also showed that their experi mental CaF-Ar cross sections obeyed the sudden limit scal ing relation of Ref. 6. Alexander and Davis 7 have derived explicit integral cross section expressions for rotational energy transfer and laboratory-frame M-changing (reorientation) collisions in an infinite-order-sudden, irreducible tensor treatment of the collision dynamics of an open-shell diatomic molecule (one with a nonzero internal electronic angular momentum) and an inert collision partner, in a thermal cell environment. The theory of Alexander and Davis 7 was chosen for comparison with the present experimental data for two reasons: (1) the dynamical approximations which were made, in particular the sudden approximation (in which it is assumed that the inverse of the collision duration is much larger than any in ternal frequency spacing between collisionally coupled lev els of the diatomic molecule) were satisfied in this experi ment, and (2) detailed predictions were available which could be readily related to measured quantities. The deriva tion in Ref. 7 was an extension of the results for closed-shell molecules which were developed earlier, also by Alexander and Davis.8 The open-shell results retain the features of the closed-shell case while adding to them the consequences of the coupling of diatomic molecule internal electronic angu lar momentum to the relative orbital motion of the collision. 76 J. Chern. Phys. 92 (1).1 January 1990 0021-9606/90/010076-14$03.00 @ 1990 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40J. B. Norman and R. W. Field: Angular momentum reorientation 77 For example, when the total internal angular momentum of the molecule has a nonzero projection along the internuclear axis (0.=10), resulting in two near degenerate levels (A doublets) of opposite parity, interference effects occur in the squared atom-molecule interaction potential matrix ele ment expression which lead to propensities for and against collision induced transitions among these levels. These inter ference effects are due to the fact that each ofthe A-doublet wave functions is a linear combination of + 0. and -0. basis states. The relevant results of the Alexander-Davis theory will be outlined in Secs. IV and V. A few words are needed concerning the notation which has been adopted in this paper. J is used here to denote the diatomic molecule internal angular momentum and L is used for the relative collisional orbital angular momentum of the collision partners. Unprimed quantum numbers denote the initial state of the collision, that is, before the collision takes place. Primed quantum numbers will be used to denote thefinal states, that is, after the collision. Mis used here for the space-jixed projection of J. In the Appendix, where the formalism is extended to include diatomic molecule hyper fine structure, MF will be used for the space-fixed projection of F, the total angular momentum, including nuclear spin. E = + I and E = -I are used to denote e and/symmetry levels, respectively, of the molecule. Details of the experiment are presented in Secs. II and III. Theoretical results of Alexander and Davis,? along with relevant predictions, are outlined in Secs. IV and V. An ex planation of the method of data analysis is given in Sec. VI and the experimental results appear in Sec. VII. Section VIII presents a summary and discussion. The Appendix is a dis cussion of the hyperfine structure of CaF and its influence on the present experiment. II. EXPERIMENTAL METHOD This experiment is based on an optical-optical double resonance (OODR) scheme which is pictured schematically in Fig. 1. In this figure only t::..J = ° collisional processes are shown, for clarity of explanation. The case of t::..J = + 1 is discussed separately below. Referring to Fig. I, we see that only the J = 1/2, M = + 1/2, / symmetry sublevel of the A 211,/2(v = 0) state is populated from the ground state using a+ light when the pump laser is tuned to the A-X Q, (1/2) (0,0) transition. This population is then transferred by thermal collisions with Ar atoms into other J', M', el/sublevels. The collision ally transferred population is probed by a second laser which selects specific J " M ' , elf sublevels by appropriate choices of frequency and polarization. For example, as shown in Fig. I, exciting the E2};+_A 211,/2 Q,(12)(O,O) transition with a+ light probes the A 211, /2 J' = 1/2, M' = -1/2, f state (this population resulting from purely elastic reorienta tion). The E2 };+_X2};+(0,0) ultraviolet fluorescence which results is then detected (unpolarized detection) and, as shown below, its intensity is related to the relative popula tion in the collisionally populated level. The capability of uniquely pumping and probing individual M sublevels, the UV fluorescence, which provides an unambiguous indica tion of collision induced population, and the sub-Doppler M -1/2 +1/2 -1/2 +1/2 E2I+ '4(1/2) 2 A lTl/2 '4(1/2) o· 606 nm X2I+ --e FIG. 1. OODR level diagram for laser pumping and probing of magnetic sublevels of the CaF A 201/2, J = 1/2 states. All levels shown correspond to u = 0, J = 1/2. The curved, dashed arrows denote collisional transfer of population. For clarity, only t::..J = 0 collisional processes are shown here. nature of the signal comprise the chief advantages of this experimental technique. We have benefitted from the extensive previous spectro scopic studies of the CaF A 211_X 2}; + and E 2}; + -A 211 transitions.9•10 Nakagawa et al.'o and Bernath and Field9 performed rotational analyses of the A 211_X 2}; + transition while Bernath et al.9 did optical-optical double resonance spectroscopy (E 2}; + -A 2113/2-X 2}; + ) and rotationally analyzed the E 2}; + state. Bernath et al. did not analyze E-A transitions which originate in the A 211 state 0. = 1/2 spin orbit component. This spectroscopy had to be performed here before the collisional studies could be started. Table I displays a line list of the relevant E 2}; + -A 211, /2 transitions used in this experiment as well as a number of other low-J E A lines which were observed. Unfortunately, theA 211'/2-X2};+ (O,O)Q, (1/2) pump transition turned out to be blended with unrelated and con siderably more intense higher-J lines from the (1,1) band. This was exacerbated by the fact that the J = 1/2 level pos sesses relatively little thermal population compared to the rotational levels from which the (1,1) band lines originate (Jz20.5). It was not possible, therefore, to tune the pump laser directly to the peak of the Q, (/2) line in a Doppler limited spectrum. It was necessary first to resolve the Q, ( 1/2) line using sub-Doppler intermodulated fluores cence spectroscopy I , and to measure its line center relative to lines in the 12 Fourier transform spectrum,I2 which was used for absolute frequency calibration in the experiment. Subsequently, the pump laser frequency could be placed at the center of the Q I ( 1/2) line using the 12 spectrum along with relative frequency calibration fringes from the output of a 300 MHz free spectral range Fabry-Perot etalon, through which a small percentage of the power of the pump and probe laser beams was directed. The Doppler-limited and intermodulated fluorescence spectra of the Q, ( 1/2) line are shown in Fig. 2. In addition to the four J' = 1/2 sublevels, the eight J I = 3/2 magnetic sublevels of the A 211 '/2 state have been J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:4078 J. B. Norman and R. W. Field: Angular momentum reorientation TABLE I. Line list ofCaF E2 l:+ -A 2 n'/2 (0,0) low-J transitions." ell ell (lower E-A v (lower E-A v J" level) transition (±0.01 cm-') J" level) transition ( ± 0.01 cm-') 1/2 I Q,(O.5) 17678.109 7/2 I Q,(3.5) 17677.436 R2, (0.5) 680.195 R2, (3.5) 683.710 e Q,.(O.5) 678.740 P2,(3.5) 677.230 R,(0.5) 678.825 e Q2,(3.5) 679.938 3/2 I Q.(1.5) 677.850 R,(3.5) 680.179 R2, (l.5) 681.345 P,(3.5) 675.036 P2,(1.5) 677.764 9/2 I Q,(4.5) 677.251 e Q,.(1.5) 679.105 R2,(4.5) 684.937 R,(1.5) 679.246 P2,(4.5) 676.999 P,(1.5) 677.010 e Q2,(4.5) 680.384 5/2 I Q,(2.5) 677.623 R,(4.5) 680.692 R2,(2.5) 682.515 P,(4.5) 674.102 P2, (2.5) 677.481 e Q2' (2.5) 679.505 R,(2.5) 679.700 P,(2.5) 676.016 "CoIlisional cross sections were determined in this experiment using the transitions shown in boldface. probed in this experiment, though not uniquely. Figure 3 shows the J' = 3/2, M' sublevels which are probed upon E ..... A excitation via the QI (3/2) and Q21 (3/2) transitions using u+ and u-light. In principle, it would be possible to Doppler Broodened Ftuorescence Intermodulated Fluorescence ( I,ll (1,1) Q(2 (20.5) PI (21.5) crossover H 300MHz I (0,0) Q( (0.5) FIG. 2. Doppler-limited and sub-Doppler fluorescence spectra in the region ofthe OODR A 2n'I2-X2l:+ (0,0), Q,(l/2) transition. Because of the se vere blending of the Q, (1/2) line with unrelated (1,1) lines, the frequency ofthe pump laser was tuned to the Q, (1/2) line using reference 12 spectra (Ref. 12) and fringes from a 300 MHz Fabry-Perot etalon. The intermodu lated fluorescence spectra allowed precise measurement of the frequency of the Q,(l/2) transition. extract collisional rate constants into individual J' = 3/2, M' sublevels by probing via PI (3/2) and P21 (3/2) transi tions as well and with both helicities of the probe laser circu lation polarization. The four relative intensities obtained in this manner would yield the relative populations in the four J' = 3/2, M' sublevels through a simple solution off our si multaneous linear equations. This approach was unsuccess ful owing to inadequate signal to noise ratio for the P branch excitations (an alternative approach is outlined in Sec. VIII). Therefore, only the E ..... A Q branches were used to probe these levels. III. EXPERIMENTAL DETAILS A diagram of the experimental setup is shown in Fig. 4. Two cw dye lasers with spectral bandwidths of -2 MHz were used. The pump laser wavelength is 606 nm (dye: Rho damine 6G) and the probe wavelength is 560 nm (dye: Rho damine 560). The beams are initially linearly polarized in the vertical direction. Both beams are passed through calcite polarizers in order to better specify their polarizations. The pump beam is amplitude modulated at -1 kHz by a chopper to allow for phase sensitive detection. It is focused by lens L 1 if = 150 cm. ) and reflected at the edge of mirror M 1. It then propagates through the Fresnel rhomb (labeled Il. /4), whose optical axis is at 45° to the laser polarization direction, there- -1/2 +1/2 +3/2 J = 3/2 f or e J = 3/2 e or f FIG. 3. Magnetic sublevels in the A 2n'/2 state sampled by the 0'+ or 0'--. polarized probe laser tuned to the E2l:+ -A 2n'/2' Q,(3/2) or Q2,(3/2) transitions. J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40J. B. Norman and R. W. Field: Angular momentum reorientation 79 M2 '/2 POL2 lasers M1 spectrometer 606nm (A-X) i"'~"--'-"'-"--'--'-"'-'-:~"~-" _~~-:: __ J.. J laser power no.rmalization FIG. 4. Diagram of the CaF OODR experiment. The experimental setup is described in detail in the text. by producing circularly polarized light. The beam enters the cell through antireflection coated, quartz windows mounted nearly perpendicular to the laser propagation direction. Typical laser intensities were l(pump) ;::;1(probe) ;::;0.1 W / cm2• After passing through the calcite polarizer POL2, the probe laser goes through a half-wave plate (A. /2) which ei ther rotates the laser polarization by 90° or leaves it un changed, depending on the experiment. It is focused by lens L 2 (j = 150 cm.) and reflected by mirror M 2. The probe beam then bypasses mirror M 1, barely missing its edge. The pump and probe beams are then nearly copropagating. The probe beam also goes through the Fresnel rhomb (A. /4) and emerges circularly polarized. The relative directions of cir cular polarization (same or opposite) of the pump and probe beams is determined by the half-wave plate (A. /2) orienta tion. The two beams are overlapped at a small crossing angle (-4 mrad) within the cell. It was determined that the cell window did not measurably depolarize the beams. Although they are not shown in Fig. 4, a number of -1 mm apertures were placed in the path of the beams in order to ensure the preservation of optical alignment as the half-wave plate was rotated. To test the purity ofthe circular polarization of the two laser beams after emerging from the Fresnel rhomb, a rotat able calcite polarizer and a power meter were placed in the beam path. As the polarizer was rotated, the minimum and maximum powers of the transmitted beams were noted. The relationship between these two measurements and the purity of circular polarization can be specified as follows. The laser power is proportional to the square of the electric field am plitude. Let E ~in and E ~ax be the minimum and maximum squared electric field amplitUdes at the detector as the polar izer is rotated, and let a = E ~in / E ~ax' which is the ratio of minimum to maximum measured laser powers. The two cal cite polarizer orientation directions, corresponding to Emin and Emax, are perpendicular to each other regardless of the polarization state of the beam. We can use these directions, described by the unit vectors Emin and Emax, to express the electric field vectors in terms of left and right circularly po larized basis vectors, (1..]2) (f" min ± if" max) : Solving for ELand E R , we get E=I+.Ja E L 2 max and The fraction of laser power in the left circular polarization state is then given by Ei +E~ a = 0 corresponds to pure linear polarization and a = 1 cor responds to pure circular polarization. In this experiment, a was found to vary approximately between 0.40 and 0.70, corresponding to 95%-99% pure left circular polarization. The amplitude modulated CaF A 2nI/2->x2~+ laser induced fluorescence at -606 nm excited by the fixed fre quency pump laser is focused (using suitable detection op tics) into a one meter monochromator (SPEX Model 1802, 1200 lines/mm, first order) equipped with a Peltier-cooled photomultiplier tube (RCA C31034). This signal is detect ed at the 1 kHz chopper frequency by lock-in No. 1. The fluorescence is detected in first order with a 500p slit width, which corresponds to a spectral bandwidth of -10 cm -I. This bandwidth assured that all A 2n-x 2~ + (0,0) fluores cence was detected, making the signal insensitive to rota tionallinestrength effects. The OODR signals are produced by fixing the pump laser frequency on the center of the A-X QI (112)(0,0) line and scanning the probe laser frequency. When the probe frequency coincides with an E 2~ + -A 2n 1 /2 transition for which lower state population exists, due either to direct exci tation from the ground state by the pump laser or to colli sional transfer, UV E2~+ ->X2~+ fluorescence at -290 nm is detected by a solar blind photomultiplier tube (Hama matsu R166UH side-on, operated at -600VDC; spectral response: A. = 160-320 nm) through a Corion UG-ll inter ference filter. This signal is sent to lock-in No.2, whose refer ence frequency is also the pump laser amplitude modulation frequency. These two fluorescence signals, along with a vol tage proportional to the probe laser power, are sent to the computer. The OODR (E-X) signal is divided (point by point) by the A-X fluorescence signal and by the probe laser power signal. This procedure normalizes the data to the pump and probe laser powers and to the total CaF popula tion encountered by the beams, which changes slowly due to varying oven conditions. This procedure is successful only if the probe laser does not saturate the E-A transition. Relative frequency calibration of the spectra is done by sending a small portion of the probe beam through a semi confocal Fabry-Perot etalon with a free spectral range of 300 MHz. Simultaneously, 12 spectra are recorded for abso lute frequency calibration (± 0.01 cm -I). The Fourier J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:4080 J. B. Norman and R. W. Field: Angular momentum reorientation transform spectra of Gerstenkorn and Luc IZ are used for this purpose. The CaF molecules are produced in a resistively heated oven by sublimation and dissociation ofCaFz crystals (melt ing point: 1360 'Cl3), just as was done by Dufour et al.5 Optical grade CaFz (Morton Thiokol, Alfa Products No. 30109, > 97% purity, -7g) and Ca metal (Baker, purified, 99% purity, -0.5g) are placed in a graphite crucible (Ultra Carbon Corp., UPG grade graphite) and resistively heated by a tungsten coil (R. D. Mathis Co., tungsten filament No. B1O-3X.04OW, modified), surrounding the crucible (l = 40-70 A, V = 3-6 VAC) to 1200-1400 'C, as mea sured by an optical pyrometer focused onto the CaFz crys tals. At these temperatures, the CaFz and hence the CaF pressure is less than 10-4 Torr.l3 The laser beams are direct ed to cross immediately above the crucible in order to maxi mize the observed CaF density. The total cell pressure is measured using a capacitance manometer (MKS Instru ments, Inc., Baratron, model No. 220BA-0001OB2A) to a precision of 5J.l and ranges between 0.5 and 1.5 Torr. The oven is a modified Broida-type oven. 14 The changes are made to accommodate the geometry and relatively high temperatures of this experiment. Three modifications are made to the basic Broida oven design: (1) the stainless steel cone, which is normally used to direct the argon flow in order to form a compact flame in the traditional Broida-type oven, was removed (no flame is evident in the present oven arrangement), (2) the water-cooled copper electrodes are extended in length in order to bring the top of the crucible close to the laser beams, and (3) it is found necessary to purge the windows, through which the fluorescence signals are observed, with argon, to prevent Ca from condensing on them and thereby degrading the signal. The argon purge serves also as the source of argon collision partners (99.996% purity). IV. THEORY OUTLINE Presented in this section is a brief summary of the rel evant theoretical results of Alexander and Davis.7 The formulation of Alexander and Davis 7 derives its predictive power from two sources: (1) a rotationally invar iant multipolar expansion of the cross sections in terms of irreducible tensorial components, IS each rank of which cor responds to a definite amount of angular momentum trans ferred in the collision, and (2) the application of dynamical approximations (infinite-order-sudden and first Born) which permit the separation (decoupling) of translational and internal diatomic molecule degrees of freedom. In this section and the next we state the essential results of the Alex ander-Davis analysis which allow a direct comparison of the theory with the experimental data. Results are presented for the case where diatomic molecule hyperfine structure is ab sent. As shown in the Appendix, this is nearly correct for 40CaI9F(A zIIl/z), in spite of the 19F nuclear spin of 1/2. Modifications of these results to include hyperfine structure are also presented in the Appendix. Alexander and Davis have adapted an irreducible tensor formalism originally formulated by Grawertl6 and well known in the theory of atomic collisions. In this formula-tion, the ,£o»ision frame scattering amplitude fJMnE.J'M'n'E' (R,R lab) for transitions between molecular states with well-defined quantum numbers J, M, n and elf symmetry is expressed in terms ~f .£roducts of irreducible tensorial componen~,J~gE.J'n'E' (R,Rlab) and M-dependent 3j coefficients. Here R denotes scattering angles in the colli sion frame with the z axis defined to lie along the initial relative velocity vector and R lab describes the orientation of the collision frame relative velocity vector with respect to the laboratory Z axis. To obtain the corresponding M-depen dent integral cross sections appropriate to thermal cell ex periments, this formalism is combined with the standard Arthurs and Dalgarnol7 expression for the collision frame scattering amplitude. The resulting scattering amplitude is squared, integrated over all scattering angles, and averaged over all possible orientations of the collision frame with re spect to an external laboratory frame Z axis to which the M quantum numbers refer (corresponding, in the present case, to the laser propagation direction). The result1 for the laboratory-frame M-resolved inte gral cross section appropriate to thermal cell type experi ments is, in a case (a) basis, where J' K )2 M' -Q (1) (2) k ~nE = 2J.lEJfllfzz is the wave number of the initial state, where EJflE is the translational energy. K and Q are the con tributing tensor order and component, respectively, of the scattering amplitude, and (JLnEII TK IIJ'L 'n'E') is the re duced matrix element of the T matrix, 18 which contains all aspects of the dependence of the collision upon the interac tion potential, reduced mass, and collision energy of the sys tem. The sum in the definition of P~nE.J'n'E' is over initial and final relative collisional orbital angular momenta. These P~nE.J'n'E' quantities are similar to Grawert coefficients, which have been defined in the theory of atomic collisions. 16 The most important feaure of Eq. (1) is the separation of orientational and dynamical effects, as expressed in the 3j symbol and tensor opacities, respectively. This permits the derivation of propensity rules for collisional energy transfer and reorientation which are independent of the specific in teraction potential and depend only on angular momentum coupling factors, as shown below. The non vanishing ofthe 3j coefficient in Eq. (1) requires that J, J', and K satisfy an angular momentum triangle relation, IJ -J' I <,K <,J + J'. The allowed values of K correspond therefore to the amount of angular momentum transferred during the collision. This means that [2 min (J,J') + 1] tensor orders, at most, will contribute to the sum in Eq. (1). This result applies only to laboratory frame quantization.7 Alexander and Davis7 have shown that the number of non vanishing tensor orders is further reduced if either the infinite-order-sudden (lOS) or first-order Born approximation is applicable. J. Chem. Phys., Vol. 92. No.1. 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40J. B. Norman and R. W. Field: Angular momentum reorientation 81 The sudden approximation may be shown to be valid in the present CaF-Ar collision system based on simple classi cal arguments, especially at the relatively low CaF rotational angular momentum and in the range of temperatures ofthis experiment. Its validity has been experimentally confirmed by Dufour et al.,5 through a successful fit of their CaF-Ar collision data to a sudden scaling relation. Taking the typical collision duration to be 1"c ;:::,2~ohTrv;:::,5 X 10-13 s, where (T = 5 A 2 is a typical cross section and v is the mean thermal relative speed, and taking the rotational period of the mole- cule to be 1"rot = 21TI/ [h ~J(J + 1)] = 5 X 10-11 s, where I is the moment of inertia of CaF and J = 112, we have 1" c /1" rot ;:::, 10 -2• This demonstrates the validity of the sudden approximation in the present experiment. The remaining condition required for the validity of the lOS approximation is that the translational energy be large compared to the energy separations between collisionally coupled states of the molecule. 19 The average translational energy is on the order of 600 cm -1 and the rotational and A doublet energy gaps in this experiment are on the order of 1 cm -I, so that this condition is clearly satisfied. Since this experiment monitors only collisional pro cesses fot which 0. = 0.', the following discussion will be restricted to this case. Application of the lOS approximation leads to the fol lowing expression for the tensor opacities in Eq. (2), for collisions which are elastic in 0. (0. = 0.'): X 1 " IR K.IOS 12 (3) (2K + 1) ft, JW •. J'L'n." where Ffn •. J'n .. =Hl +EE'( _1)J+J'+K+2n] (4) and all the dependence on the interaction potential, reduced mass, and collision energy is now contained in the RK fac tors. Important propensity rules for K are found in the r factor of Eq. (4) and will be discussed in the next section. These arise from the interference effect mentioned in Sec. I, which occurs in the squared matrix element of the interac tion potential. A similar term occurs in inelastic cross sec tion expressions for all states possessing A doubling (Le., non-~ states). The propensity rules implied in Eq. (4) and the restric tions on K and aM dictated by the 3j symbols ofEq. (1) are the key results. These are explored in Sec. V. Here we state Alexander's and Davis' final expression for the laboratory frame M-state-resolved lOS integral cross sections, within a case (a) 211 state, for collisions in a thermal cell environment and for the case of 0. = 0.': =_1T I(J k]n. K.Q -M M' K )2 _ Q Ffn • .J'n .. J' x(2J+ 1)(2J'+ 1) " IRK•IOS 12 (5) 2K + 1 Ct, JLn •. J'L'n.'· v. PREDICTED RESULTS This section contains a description of some observable consequences of Eq. (5) and how they relate to the present work. These features are discussed in greater detail in refer ence 7. A significant simplification of the problem results when one transforms from the collision frame treatment of the scattering to a laboratory frame treatment, independent of any particular dynamical approximations. In this transfor mation the directions of the initial relative velocity vectors of the collision pair are averaged with respect to the laboratory fixed quantization axis, assuming an isotropic distribution. In the collision frame, interference occurs between the var ious possible contributing tensor orders K which appear in the orientation-dependent integral cross sections.20 In the transformation to the laboratory frame these effects are washed out, i.e., the tensor orders are decoupled from each other and the expressions are simplified. The essential differ ence between the orientation-dependent cross sections in the collision and laboratory frames is that in the collision frame the cross sections consist of the square of a sum of K-depen dent T-matrix elements (thereby producing interference) whereas the laboratory frame cross sections consist of a sum of squares of the same. Naturally, the degeneracy-averaged cross sections are identical in both frames. The important result of this decoupling of tensor orders in the laboratory frame is a profound reduction in the number of independent measurements which are required to determine the entire matrix of JM --J' M' cross sections. When combined with the triangle relation between J, J', and K, the decoupling of the tensor orders implies that all JM--J'M' integral cross sections (for fixed J and J'), of which there are (2J + 1 )(2J' + 1), are determined by a much smaller set of2 min(J,J') + 1 parameters, the tensor opacities of Eq. (2). For example, the 12 J = 112, M --J' = 5/2, M' cross sections are reduced to 2 tensor opac ities, with K = 2 and 3. Similar reductions in the number of parameters required to model inelastic processes have been shown to apply in atomic collisions.16 It is important to re member that this reduction of independent parameters ap plies only to laboratory frame quantization. An important additional propensity rule, which con trols the tensor orders which contribute to the cross sections, is implicit in the r factor of Eq. (4). The condition for the nonvanishing of the r factors, and therefore for the non vanishing of the tensor opacities and cross sections in CaF (A 2111/2) is, from Eq. (4), (6) This implies that for each set of JME --J' M' E' cross sections, for fixed J,E, and J' ,E', only even or only odd tensor orders will contribute. Table II summarizes this propensity rule. There are two remaining features of Eq. (5) which de serve to be highlighted. First, the 3j symbol implies that a strict aM = 0 collision selection rule will never be valid in a thermal cell environment.7 It would require that the 3j sym bol vanish whenever Q is not equal to zero, which does not happen. The second feature involves the RK factor in the sum of Eq. (5). As already mentioned, this factor contains J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:4082 J. B. Norman and R. W. Field: Angular momentum reorientation TABLE II. Nonvanishing tensor orders K contributing to cross sections. J+J' even J+J' odd E= E' odd K even K even K odd K all interaction potential, reduced mass, and collision energy information about the system. Alexander and Davis7 have shown that in the lOS approximation, for any state in which O;fA (such as 2nl/2), 1 KIOS 12 (J' K J)2 R JLnE;J'L'nE' a: _ 0 0 0 (7) The full expression for the RK factor can be found in Ref. 7. In the high-J limit, the 3j symbol in Eq. (7) will be non van ishing only if K -J' + J is even. This, combined with the condition for the nonvanishing ofthe pc factor [Eq. (6)], implies that, in the high J limit, there is a propensity toward the conservation of the € index, i.e., collisional transfer between A-doublet levels of different elfsymmetry is unlike ly. This rule holds for both M-dependent and degeneracy averaged integral cross sections. It can show up clearly even in the lowest rotational levels of a 2n 1/2 state, as demonstrat ed in the present experiment (see Fig. 5), and has been veri fied in a number of other experiments.5,9,21 The relevant aspects of the Alexander and Davis analy sis which have been discussed in this section are summarized in Table III below. The table shows each feature of Eq. (5) and its source. The successful application of these results to the CaF A 2n state relies on this state being well described by case (a) coupling. 22 That this is the case can be readily seen by noting that the ratio of the diagonal spin-orbit and rotational con stant in the v = 0 level is much greater than unity.22 For CaF A 2n we have AoJBo:::::215. Since J, J', and K must obey a triangle relation, in the case of J = 1/2 there are only two possible values of K for each J = 1/2 ..... J' collision induced transition, K = J' ± 1/ 2. Table II can then be used to eliminate one of these, either the odd or the even one, for collisions into a specific elf symmetry level. In other words, for collisions out of J = 1/2, regardless of the final rotational quantum number (at least up to the validity limit of the lOS and first Born approxima tions) only one parameter, the only surviving tensor opacity pK is needed to determine all 2 ( 2J' + 1) cross sections, for a givenJ',€'. TabelIVsummarizestheseresultsuptoJ' = 9/2. Extension to higher J' is trivial. An interesting feature which is displayed in Table IV, in addition to the extreme reduction in the number of tensor orders, is that for a given J = 1/2 ..... J' process elf symmetry conserving and changing collisions are governed by different values of K, i.e., these two dynamical processes are complete ly independent of each other, as initially recognized by Alex ander and Davis.7 The consequence of there being only one non vanishing tensor order for J = 1/2 ..... J' ,€' collisions is that the pK fac- Co F Collision Induced OODR: E2 ~+:A2 nl/2-x2~+ PAr = 1.5Torr E2!+-A2n 01 9/2 7/2 5/2 3/2 1/2 1/2 I I I I I G> U c: G> U VI G> ... o ::J lJ... > :J "0 G> N o E ... o Z * 2 A nl/2 f P21 * 7/2 512 3/2 I I * * 17678 porent 112 3/2 5/2 ~+2 I I I ~I !-~ nll2 2 A nl12e RI 1/2 I * * * 17679 3/2 I pump: 0"+ probe: 0" pump: 0"+ probe: 0" + Probe Loser Frequency (cm-I ) J. Chem. Phys., Vol. 92. No.1. 1 January 1990 FIG. 5. Collision-induced OODR fluorescence spectra in CaF. The pump laser is u+ circularly polar ized and its frequency is fixed on the A'n'/,-X'1:+ (0,0), Q, (112) transition, while the probe laser fre quency is scanned in the region of the E '1: + -A 'n 1/2 transition. E 21: + - X '1: + ultraviolet fluorescence is de tected. The lines marked by asterisks were used to obtain the reported cross sections. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40J. B. Norman and R. W. Field: Angular momentum reorientation 83 TABLE III. Features of theoretical laboratory frame cross sections [see Eq. (5)]. Feature Only odd or only even K's contribute to a given cross section Il.M = 0 not strictly valid Source (/-M ~, ~Q) e-e and/-/at high I. (I' K I) -!l 0 !l and pK Only 2min (/,J') + I tensor orders required to determine all 1M -I'M' cross sections. collision -laboratory frame averaging tors will cancel in ratios of cross sections into (1) different M' sublevels of the same J' ,E', or (2) adjacent J' levels of opposite elfsymmetry but equal K values, as in the case of J' = 112 elf changing and J' = 3/2 elf conserving pro cesses, for both of which K = 1. As an example of case (1) above, the ratio of cross sections for collisionally induced transitions from J = 112, M = + 112,/ into the J' = 112, M' = ± 1/2, e levels is predicted to be UJ = 112.M= + 112j-J' = 112.M' = -112.e UJ = 112,M= + 112j-J' = 112,M' = + 112,e which can be seen from Eq. (1). (8) Table V summarizes the predicted proportionalities among the M-dependent cross sections which are derived fromEq. (l),forJ= 1I2,M= + 1I2,f-+J' = 112 and 3/2 processes. In using this table, it must be remembered that only those cross sections which involve equal K values can be related to each other in the way described above, since only then will the tensor opacities cancel. Table V contains all of the predictions which are experimentally tested here. TABLE IV. 1= 1/2-contributing tensor orders. I' 1/2 3/2 5/2 7/2 9/2 # ofo's to be determined 4 8 12 16 20 K, ell conserving 0 2 3 4 K, ell 2 3 4 5 changing VI. METHOD OF DATA ANAI,.YSIS This experiment has yielded thermally averaged rate constants, from which velocity-averaged cross sections were obtained by dividing the former by the average relative ther mal speed of the sample. The steady state rate equation for a collisionally populated (daughter) level is nd ( -) p CaF _ 0 nArCTU nCaF - 2 -, 'TR (A 111/2) (9) where n Ar is the Ar density, u is the cross section of interest, n~aF and n&F are the steady state densities of the parent and daughter levels, and 'TR (A 2111/2) is the radiative lifetime of the A state. That is, at steady state the rate of collision-in duced population transfer from the parent to the daughter level is equal to the daughter (A -+ X) radiative decay rate (in terms of molecules Is). A more useful form ofEq. (9) is d [( _ ) ] nCaF v 2 -p-= - 'TR (A 111/2) UPAr nCaF kT [( 8 )112 2 ] = 1Tp,kT 'TR(A 111/2) UPAr> (10) where PAr is the Ar pressure and the ideal gas law has been used. It remains to relate this population ratio to the ratio of intensities of laser induced fluorescence lines corresponding to p~obe transitions out of the daughter and parent levels (ld and Ip). TABLE V. Predicted cross section proportionalities for I = 1/2, M= + 1/2./ M' I' -3/2 -1/2 1/2 3/2 K 1/2 I-I 0 0 1/2 I-e 2 3/2 I-I 0 2 3 3/2 I-e 4 3 2 2 J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:4084 J. B. Norman and R. W. Field: Angular momentum reorientation There are two main assumptions inherent in Eqs. (9) and ( 10) above: ( 1 ) on the average, at most one collision per radiative lifetime of the parent A 2n 1/2 state occurs, and (2) the rate of probe laser depopulation of the A state sublevels is small compared to the A-state radiative decay rate. In other words, both the collisional transfer and probe laser pumping rates (per molecule) must be small compared to the A -state radiative decay rate. The A-state radiative decay rate is -4.6X 107 S.-I 23 Assuming an upper limit of U= 10 A2, the maximum collisional transfer rate per molecule at PAr = 1 Torr is approximately nAroV = 5 X 105 s-1, which validates the first assumption. The second assumption can be shown to be true by a simple comparison of the Rabi frequency of the E-A excitation at the laser intensities of this experiment to the known A-state radiative decay rate.24 The expression for the fluorescence intensity depends upon the transition moments of the excitation and fluores cence transitions, the laser and fluorescence polarizations, and the detection geometry. The general theoretical treat ment of resonance fluorescence can be found in Refs. 25, 26, and 27. For the present case of circularly polarized incident radiation and unpolarized detection, it can be shown that the fluorescence intensity for the A -+ E -+ X process has the fol lowing form, in terms of the 2K multi pole moments of the scattered light: [a:n(JA)S(JA,JE)S(JX,JE) L (_I)JrJx(2K+l) K=O.2 (1 1 K\2 {11K} {11K} X 0 0 0) JE JE Jx JE JE JA XPK (cos (J), (11) where n (JA ) is the population in the initial A state level, P K is a Legendre polynomial, (J being the angle of detection mea sured relative to the incident laser propagation direction (90· in this case), J A ,J E' and J x are the angular momentum quantum numbers of the A (initial), E (intermediate), and X (final) states, and S(JA,JE) and S(JX,JE) are the line strength factors for the probe excitation and fluorescence transitions, respectively. The sum in Eq. (11) contains all of the angular and polarization dependence of the fluorescence intensity. Since all of the probe transitions in this experiment are between the same electronic and and vibrational states and lie close in frequency, no Franck-Condon, electronic linestrength, or frequency corrections to the intensity ratios are needed. Integrated rather than peak intensities were measured in order to account for some velocity equilibration (inhomogeneous broadening) which accompanies the ener gy transfer events. These intensities were also normalized to the probe laser power and to the A -+ X fluorescence signal, as discussed in Sec. III. The latter provided corrections for var iations both in pump laser power and CaF density. Combining Eq. (10) and (11), we obtain ~; = [Sd(J~'J~) ~ Sd(J~,J~)¢d(J~,J~,J~) 1 SP(J~,J~) L SP(J"x,J~)¢JP(J~,J~,J"x) J~ X [(1T'J.l8kTY1\-R (A 2n1/2) ]uPAT' (12) where the ¢ factors represent the sum in Eq. (11). The ex plicit sums in Eq. (12) are over all E-X fluorescence chan nels and are necessary since the UV fluorescence is not dis persed. According to Eq. (12), a plot of integrated and normalized daughter to parent intensity ratios as a function of Ar pressure will yield straight lines, the slopes of which determine the thermal rate constants and the velocity-aver aged cross sections, if the temperature of the sample and the radiative lifetime of the collisional daughter state are known. VII. EXPERIMENTAL RESULTS Figure 5 displays representative data obtained at fixed Ar pressure (PAr = 1.5 Torr) for both relative helicities of the circularly polarized pump and probe lasers. E 2I, + -+X 2I, + ultraviolet fluorescence intensity, normal ized to the A 2nI/2-+X2I,+ fluorescence signal and to the probe laser power is plotted against probe laser frequency. The OODR fluorescence excitation spectra have sub Doppler linewidths (-200 MHz FWHM for the parent line, > 300 MHz for the collisional lines), since the active molecules are velocity selected by the pump laser. The par ent line is labeled in the upper trace and corresponds to prob ing the population in the originally pumped A 2n 1/2' J = 1/2, M = + 1/2,/ symmetry sublevel (see Fig. 1). All E --A QI transitions probe/symmetry levels (corresponding to e//symmetry conserving collisions), whereas E--A Q21 transitions probe e symmetry levels (corresponding to e/ / symmetry changing collisions). The transitions used in this experiment are marked in Fig. 5 with asterisks. Precise wave numbers of all transitions shown in Fig. 5 are listed in Table I. The cross sections result experimentally from a pressure dependent study of relative daughter and parent line intensi ties (integrated and normalized). This analysis is presented below. The spectra of Fig. 5, obtained at afixed argon pres sure, provide a qualitative picture of the relative amounts of population transferred in these collision processes. Each daughter level studied receives less than 6% of the partially relaxed steady-state parent population in the range of pres sures ofthis experiment (0.5-+ 1.5 Torr). The most important feature of Fig. 5 is the relative in tensities of a single line in the upper and lower scans, corre sponding to u+ and u-polarizations of the probe laser. For example, a dramatic effect is seen in the E-A QI ( 1/2) transi tion recorded with u+ probe light (lower trace), whereby the popUlation of the J' = 1/2, M' = -1/2, / symmetry sublevel is probed. Collisions which populate this level are rotationally elastic and correspond to a pure reorientation of J without any change of magnitude. It can be seen from Table V and the discussion in Sec. V that this cross section is predicted to vanish. The nonzero intensity ofthis line and its pressure dependent behavior can be qualitatively accounted for by two experimental defects: ( 1 ) the imperfect u+ polar ization of the pump laser, and (2) partial saturation of the A --X transition. These defects, along with a simple model designed to explain the data, are discussed below after the data are presented. Another large reorientation effect is re vealed in the P21 (3/2) transition, which is labeled in Fig. 5. Since the lower state of this transition (J = 3/2) has four J. Chem. Phys., Vol. 92. No.1, 1 Jjanuary 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40J. B. Norman and R. W. Field: Angular momentum reorientation TABLE VI. Pressure-dependent ratios of daughter to parent line intensities8•b (integrated and normalized, as described in the text). Initial CaF level: A 2n'/2' J = 1/2, M = + 1/2,/ I:J.J 0 +1 +1 E+-A Line Q,(0.5) Q2,(0.5) Q,(\.5) Q2' (\.5) Daughter symmetry f e f e Probe laser polarization q+ q+ q q+ q q+ q - Pressure 0.50 Torr 0.1l97 0.0204 0.0108 0.0072 0.0144 0.0021 0.0018 0.62 0.1l84 0.0216 0.0122 0.0082 0.0150 0.0027 0.0024 0.75 0.1202 0.0219 0.0129 0.0090 0.0213 0.0027 0.0027 0.87 0.1l36 0.0268 0.0150 0.0098 0.0038 0.0026 1.00 0.1093 0.0274 0.0167 0.0110 0.0246 0.0035 0.0029 1.25 O.llll 0.0343 0.0195 0.0132 0.0324 0.0045 0.0036 1.50 0.0314 8 The estimated experimental uncertainty in these ratios is 5%. b Each data point corresponds to the ratio of the intensity of the stated E-A line to the parent line, the Q, (0.5) line with q-probe laser polarization. (a) 0.14 0.12 ~ -I: 0.10 .. .. ., CI. -0.08 -.. .. 0.06 -..c .. = ., 0.04 "d ;:- 0.02 0.00 0.4 0.6 (see text) " \ " 0.8 1.0 1.2 magnetic sublevels (M = -3/2 -+ + 3/2) and the upper state (J = 1/2) has two sublevels (M = -1/2, + 1/2), 0'+ light probes only the J' = 3/2, M' = -3/2 and -1/2 sublevels and 0'-light probes only the J' = 3/2, M' = + 1/2 and + 3/2 sublevels. According to Table V, the ratio of cross sections and therefore of line intensities for exciting this transition with 0'-vs 0'+ light is 5 (i.e., [3 + 2] / [0 + 1]). It is seen in Fig. 5 that this is qualitative ly in agreement with the data (the P21 ( 1.5) line is barely discernible above the noise level in the lower trace) . Another propensity rule from the Alexander-Davis theory7 which is readily apparent in Fig. 5 is that e//symmetry conserving collisions are more probable than e/ / symmetry changing ones. This is seen by noting that the intensities of Ql lines (with/symmetry lower levels) are always larger than those of Q21 lines (with e symmetry lower levels) at the same J (the rotational linestrength factors for these two branches are nearly identical at J = 1/2 and 3/2). Finally, it is worth noting that, according to Table V and the data of Fig. 5, larger reorientations seem to correlate with changes in elf symmetry. Argon Pressure (Torr) (b) =: c .. .. .. CI. -.. .. .c .. :I co ... -0.05 0.04 0.03 0.02 0.01 1.4 85 1.6 02 t(O.5) (IV) QI(1.5) «n <b1(1.5) (a+) The pressure-dependent daughter/parent line intensity ratios are displayed in Table VI and plotted in Fig. 6. Figure 6(b) shows only the inelastic data, which are labeled and shown on an expanded vertical scale relative to Fig. 6(a). The ratios of collisional daughter to parent line intensities (integrated and normalized) are plotted against argon pres sure. Eqmiti(;m (12) predicts the form of this dependence. The solid lines are the linear least squares fits to the data. The nonzero intercepts ofthe linear fits to the inelastic data are due to collisions of CaF with residual gas (other than Ar) in the cell. The ambient cell pressure (before Ar was added) was typically 20 mTorr. The thermal rate constants and ve locity-averaged cross sections, which are derived from the slopes of these lines, are displayed in Table VII, along with o.oof~;;~:::!=:.:::::=:::::::: <b1(1.5) «n 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Argon Pressure (Torr) FIG. 6. Pressure-dependent daughter/parent line intensity ratios from Ta ble VI. The solid lines are linear least squares fits to the data. The nonzero intercepts of the linear fits are due to collisions of CaF with residual gas in the cell. (a) complete data set, (b) inelastic data only. J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:4086 J. B. Norman and R. W. Field: Angular momentum reorientation TABLE VII. Measured thermal rate constants (k) and velocity-averaged cross sections (u).· Initial CaF level: J = 1/2, M = + 1/2,/ M' probe sublevels transition Ie" l' t:J probed symmetry (E2I,+ -A 20,/2) ( X to-II cm3/s) U(A.2)b 1/2 0 -1/2 e Q2,(0.5)u+ 8.1(1.8) 9.1 (2.0) + 1/2 e Q2,(0.5)u- 5.1(.98) 5.7(1.1) 3/2 +1 -3/2, -1/2, t Q,{ l.5)u+ 3.4(.71 ) 3.8{0.8 ) + 1/2 -1/2, + 1/2, t Q,(l.5)u- 8.2(2.0) 9.2(2.2) + 3/2 -3/2, -1/2, e Q2,(l·5)u+ 1.3(.36) 1.5(0.4 ) + 1/2 -1/2, + 1/2, e Q2'( l.5)u- 0.89(.18) 1.0(0.2) + 3/2 ·Constants used: J.l=23.877 amu=3.994xto-23 grams. 'TR(A20) = 21.9(4.0) X to-9 s (Ref. 23). kT( T= 900 K) = 1.249 X to-13 ergs. b 1 u uncertainties in parentheses. These arise primarily from the uncertainty in the experimental value of'T R (A 20). the constants used to arrive at these results. The linear rela tionship predicted in Eq. (12) is well satisfied by the data. The assumption of single collision conditions made in Sec. VI is experimentally validated. The experimental uncertain ties in the intensity ratios of Table VI are estimated to be 5%, based on the reproducibility of the data over a number of runs. The elastic data, the uppermost data set in Fig. 6 (a), are anomalous. They yield an unexpectedly large intercept and an unphysical negative cross section. This behavior can be qualitatively explained by taking into account the two pre viously mentioned experimental defects: (I) the a+ polar ized pump laser beam had approximately 5% of the wrong (a-) circular polarization state, so that unwanted AM = -1 transitions occurred in the preparation step (see Fig. I), and (2) the pump laser partially saturates the A ..... X transition, so that the ratio of the prepared population in the A state J = 1/2, M = + 1/2 sublevelto that in the J = 1/2, M = -1/2 sublevel is not linearly proportional to the a+ 1 a-polarization ratio of the pump beam. Under conditions of saturation of the A-X transition, the M-sublevel popula tions tend towards equalization. With increasing pressure, as the mean relaxation time of the A state decreases and hence the degree of saturation decreases, the M = + 1/21 M = -1/2 population ratio asymptotically approaches the a+ la-intensity ratio, giving the observed negative slope of Fig. 6 (a). This simple model neglects collisional coupling between these two sublevels and between those of the ground state, treating the a+ and a-transitions as independent of each other. It also neglects loss of population due to relaxa tion to other levels. We have made an attempt to model the data using this picture and have obtained qualitative agree ment with the observation. Naturally, the nature of these data prevented us from extracting quantitative information concerning the probability of elastic reorientation. In Fig. 7 the experimental cross section ratios are com pared with those predicted in Table V. These three predicted inelastic cross section ratios shown lie within the experimen-tal range of uncertainty. The experimental uncertainties are primarily due to two sources: (I) the large experimental uncertainty in 7'R (A 2n1/2) (_18%23), and (2) the propa gation of errors due to the compound nature of the cross section ratios (they are ratios of line intensity ratios). In contrast to this favorable agreement, the predicted propor tionalities between theJ = 1/2,J-J' = 3/2,/andJ = 1/2, /-J' = 1/2,e cross sections, as derived from Table V, are in very poor agreement with the experimental values. These proportionalities should obey an energy sudden scaling rela tion for 2n states similar to that derived by Alexander [Ref. 28, Eq. (60)] and observed by Dufour et a/.s for the CaF (A 2n 1/2) orientation-averaged cross sections. This predicts the J = 1/2, /-J' = 3/2,Jorientation-averaged cross sec tion to be twice that ofJ = 1/2,/ -J' = 112, e, which agrees with Table V if one sums over M'. It can be seen from Table 0(0+) 0(0 -) 3 2 o 2.0 If-7fllf~el 1.6(5) • predicted ~ experiment 1.5 1.5(5) FIG. 7. Experimental and theoretical cross section ratios. The initial state is A 20'/2 V = 0, J = 1/2,/ u+ and u-refer to probe laser circular polariza tion states. J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40J. B. Norman and R. W. Field: Angular momentum reorientation 87 VII that the measured cross sections into the J' = 3/2,fsub levels are comparable to those into J' = 112, e, in disagree ment with the predictions. Nothing aside from a systematic error in the present measurements or analysis seems capable of accounting for this discrepancy. VIII. SUMMARY AND DISCUSSION We have measured thermal rate constants and velocity averaged cross sections for collision-induced angular mo mentum reorientation and rotational energy transfer within the A 201/2 state of CaF, in collisions with Ar atoms. An optical-optical double resonance (OODR) configuration was used with two circularly polarized cw dye lasers. A spe cific A 201/2 magnetic sublevel, J = 112, M = + 112, was chosen as the initial state in these collisions. We have compared our results to the theoretical predic tions of Alexander and Davis,7 who have used the infinite order-sudden approximation along with spherical tensor techniques to describe collisions of open-shell molecules with inert partners. Satisfactory agreement of the experi mental results with the Alexander-Davis theory has been found when comparing ratios of cross sections correspond ing to collision-induced transitions into different magnetic sublevels ofa given final rotational level, as shown in Fig. 7. Experimentally obtained ratios of cross sections correspond ing to collision-induced transitions into different final rota tionallevels agree poorly with the predicted values. An at tempt to measure the cross section for elastic depolarization in the A 20 1/2' J = 112 level failed due to imperfect circular polarization and the partially saturating intensity of the pump laser. As mentioned in Sec. I, Dufour et al.5 have measured orientation-averaged (and velocity-averaged) cross sections within CaF(A 20) under nearly identical sample condi tions, through analysis of dispersed fluorescence. Among their measured cross sections were those corresponding to the same initial and final rotational levels as in our experi ment. These orientation-averaged cross sections, OjE_J'E" are related to the present M-dependent ones, ° JME_J' M'E' , by 1 °JE_J'E' = -2J 1 L °JME_J'M'E' , + MM' (13) which represents a sum over M' and an average over M. The only orientation-averaged cross section which is possible to predict from our data is 0J= 112J-J' = 112,e' since this experi ment lacked complete M-state specificity in the J = 112, j-J' = 3/2 cross sections, making it impossible to perform the sum in Eq. (13). Our prediction of 0J= 1/2J-J' = II2,e is obtained by summing the measured cross sections of Table VII corresponding to transitions into the J' = 112, M' = + 112 and -112 sublevels. The result is °2 14.8(3.1)A, compared to the result of Dufour et al., 5.2(0.5)A2.5 This disagreement is disturbing, but we have not found any errors in either experiment which might ex plain it. A possible method of achieving complete M-state speci ficity for J' > 112 levels is to apply a small magnetic field. Since the CaF A 201/2 state is close to the case (a) limit, it is not magnetically sensitive. Its magnetic sublevel degeneracy would therefore not be split in the field. The X 2 1: + state magnetic sublevels, on the other hand, will tune in the mag netic field. This would allow individual magnetic sublevels of theA 20112 state to be selectively populated by a circularly polarized laser through appropriate choices oflaser frequen cy. M-specific cross sections could be determined through analysis of the dispersed A-X fluorescence, as in the work of Dufour et al. A preliminary calculation29 has shown that the required magnetic fields are quite modest ( < 200 Gauss). Since the A 201/2 state is not affected by the magnetic field, the collision dynamics within this state would be unchanged from the zero field case. ACKNOWLEDGMENTS This work was supported by a grant from the National Science Foundation(CHE86-I4437). We would like to ac knowledge M. Alexander for encouraging this work. Assis tance from D. Baldwin and S. Cameron is gratefully ac knowledged. APPENDIX: THE EFFECT OF CaF HYPERFINE STRUCTURE It was stated in Sec. I, and the subsequent analysis was based on the assumption, that the hyperfine structure in the CaF A 201/2 state, due to the 19F nuclear spin of I = 112, could be neglected, as far as the collisional propensity rules of Sec. V are concerned. The justification for this will be presented in this section. The condition for neglecting the hyperfine structure is that the A 20112 state be "well described" by the quantum numbers J and M and not by F and M F (F = J + I). The important parameter for determining whether CaF is de scribed by J or by F in the A 20112 state is the ratio of the characteristic coupling time between I and J (given by the inverse of the hyperfine splitting) to the lifetime (deter mined by the inverse of the homogeneous linewidth of the A X transitions) . If this ratio is large, then I has very little effect on the angular momentum coupling and J is the correct quantum number for describing the state. It will be argued below that the hyperfine splitting in the CaF A 201/2, and 3/2 levels is less than about 6 MHz, giving a ratio of I,J coupling time to A-state relaxation time of -200 MHz/6 MHz> 30. This implies that J is the important quantum number in the A state and that the nuclear spin has a negligi ble effect on the collision dynamics. Detailed and accurate molecular beam, laser-rf double resonance experiments have been performed by Childs et al.30 in which the hyperfine structure of the X 2 1: + state of CaF was determined. Even though they used the A-X transi tion in die laser excitation step, their double resonance tech nique was totally insensitive to the hyperfine structure of the A 20 state. Bernath et al. 31 determined an upper limit on the hyperfine splitting of the A 203/2 state using sub-Doppler intermodulated fluorescence spectroscopy. They found a + (b + c) < 10 MHz, 2 (AI) where a, b, andc are the nuclear spin-electron orbital, iso tropic nuclear spin-electron spin, and anisotropic nuclear J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:4088 J. B. Norman and R. W. Field: Angular momentum reorientation spin--electron spin hyperfine parameters, respectively. It is well known32 that this compound parameter determines the hyperfine splitting in a case (ap) 2113/2 state. In a 211 1/2 state the hyperfine splitting is described by the same three con stants plus an additional constant, called d, which arises from the part of the nuclear spin--electron spin Hamiltonian which obeys the selection rules Il.A = ± I, ± 2. It connects the + nand -n basis functions of 2111/2, but does not connect those of 2113/2, Up to first order in J (neglecting Il.J =FO matrix elements), the expression for the hyperfine energies in a case (ap) 211 1/2 state are32 Ehf(elj) = [a-(b;C) ±d(J++)] F(F + I) -J(J + 1) -1(1 + I) X 4J(J + 1) , (A2) where the plus sign refers to the e component and the minus sign to the/component ofthe A doublet. Treatment only to first order is certainly justified, since the hyperfine splitting is much smaller than the rotational spacing. CaF is known to be well described by an ionic molecule picture.33,34 There is one unpaired electron which is well.lo calized on a Ca2+ center and which has 4P1T and 3d1T orbItal character35 in theA 211 state. The implications of this picture for the hyperfine structure of CaF are straightforward. Since the only unpaired electron in CaF is located on the opposite nuclear center from that of the nuclear spin, a very weak hyperfine interaction is expected. Both the nuclear spin electron spin and nuclear spin--electron orbit angular mo mentum interactions are expected to be small based on these considerations. This is consistent with Eq. (AI). There are two well known relations among the hyper fine parameters of a diatomic molecule, which apply under certain conditions that are well satisfied here.32 The first is b = -c/3. c is a spin-spin parameter and b includes spin spin and Fermi contact interactions. If the Fermi contact term vanishes, (as it does for a p1T or d1T orbital) the above expression results. The other relation is c = 3 (a -d), which applies when both the spin and orbital angular momenta are carried by only one electron. This is clearly the case for CaF(A 211). If these expressions are used in Eq. (A2) (with 1= 1/2), the result is E (el'j) = d [1 + (J + 1/2)] hf 4J(J + I) X [F(F + I) -J(J + I) -!] . (A3) Table VIII displays the hyperfine splittings in the A 2111/2 state for J = 1/2 and 3/2, which are derived from Eq. (A3). The most striking feature 0/ Table VIII is that the hyperfine splitting in the J = 112, / symmetry level 0/ the CaF A 2[[//2 state is predicted to vanish. This is the parent level in this experiment, populated by the pump laser. An upper l!~it for d can be estimated from the results of Bernath et al. They have shown that the hyperfine splittings in the highly ionic calcium monohalides increase in going from CaF to Cal and have measured an upper limit of Id I < 6 MHz for both CaBr and Cal. 34 This should be compared to the homogeneous broadening in this experiment, which is -200 MHz. TABLE VIII. Hyperfine splittings in CaF (A 20,/2)' d = 6p(#1 (sin2 8 I rl)ca< (4p7T) ,a J ell F Ehf 1/2 e 0 -d I (1/3)d I 0 0 I 0 3/2 e 0 ( -9/1O)d I (-1/2)d I 0 (3/1O)d I (l/6)d a Po = Bohr magneton, PI = '9F nuclear magnetic moment, (J = angle be tween molecular axis and radius r from nucleus to electron. See Ref. 32. Alexander and Dagdigian have extended the spherical tensor calculation of cross sections in thermal cell environ ments to include molecular hyperfine structure.36 They ar rived at an expression which is analogous to Eq. (I): = ~(2F+ 1)(2F' + 1) k2 {J J' K}2(F Xh F' F I -MF K )2 -Q XPJflE,rfl'E' , (A4) where PJflE,J'fl'E' was defined in Eq. (3). Since the same J dependent tensor opacity appears here as in the case of zero hyperfine structure, the same propensity rules on K will ap ply, as given in Tables II and IV. An additional propensity rule can be obtained from Eq. (A4), however. That is, the largest cross sections will correspond to transitions for which Il.J = Il.F, and this propensity will become increasing ly strong as J and J' increase.36 Details can be found in Ref. 36. This propensity is independent of dynamical limit and of the atom-molecule interaction potential. It has been the purpose of this section to demonstrate the inappropriateness of applying hyperfine collisional pro pensity rules to the CaF A 2111/2 state. Equation (A4) pro vides a quantitative test of this assertion. For example, con siderEq. (A4) as applied totheJ = 1/2,F,MF,j .... J' = 1/2, F' ,M F', e collisional transitions, that is, to the Il.J = 0, el/ symmetry changing transitions. If hyperfine propensity rules are relevant, the predicted ratio of cross sections corre sponding to probing the J' = 1/2, F', M F' , e levels with a'+ and (7'-probe laser polarization is 0.75. If hyperfine struc ture is neglected this ratio becomes 1.5. The measured value is 1.5 ± 0.5. As Fig. 7 shows, the experimental results are consistent with the zero hyperfine case. 's. J. Silvers, R. A. Gottscho, and R. W. Field, J. Chern. Phys. 74, 6000 (1981 ). 2D. H. Katayama, J. Chern. Phys. 84,1477 (1986); 81, 3495 (1984); Phys. Rev. Lett. 54 657 (1985); D. H. Katayama and A. V. Dentamaro, J. Chern. Phys. 85, 2595 (1986); G. Jihua, A. Ali, and P. J. Dagdigian, ibid. 85,7098 (1986); N. Furio, A. Ali, and P. J. Dagdigian, ibid. 85, 3860 (1986); A. Ali, G. Jihua, and P. J. Dagdigian, ibid. 87, 2045 (1987); A. Ali and P. J. Dagdigian, ibid. 87, 6915 (1987). J. Chern. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40J. B. Norman and R. W. Field: Angular momentum reorientation 89 3Ph. Brechignac, A. Picard-Bersellini, and R. Charneau, Chern. Phys. 53, 165 (1980). 4S. Halle, S. Coy, and R. W. Field (unpublished results); F. Matsushima, T. Shimizu, and Y. Honguh, J. Chern. Phys. 87, 6995 (1987); R. L. Shoe maker, S. Stenholm, and R. G. Brewer, Phys. Rev. A 10, 2037 (1974); J. W. C. Johns, A. R. W. McKellar, T. Oka, and M. Riimheld, J. Chern. Phys.62, 1488 (1975); W. K. BischelandC. K. Rhodes, Phys. Rev. A 14, 176 (1976). 5C. Dufour, B. Pinchemel, M. Douay, J. Schamps, and M. H. Alexander, Chern. Phys. 98, 315 (1985). oM. H. Alexander, J. Chern. Phys. 76, 3637, 5974 (1982). 7M. H. Alexander and S. L. Davis, J. Chern. Phys. 79, 227 (1983). 8M. H. Alexander and S. L. Davis, J. Chern. Phys. 78, 6754 (1983). '1'. F. Bernath and R. W. Field, J. Mol. Spectrosc. 82,339 (1980). 10J. Nakagawa, P. J. Domaille, T. C. Steimle, and D. O. Harris, J. Mol. Spectrosc. 70, 374 (1978); R. W. Field, D. O. Harris, and T. Tanaka, ibid. 57,107 (1975). "M. S. Sorem and A. L. Schawlow, Optics Comm. 5, 148 (1972); M. S. Sorem, Ph.D. thesis, Stanford Univ., 1972. 12S. Gerstenkorn and P. Luc, Atlasdu Spectred'Absorption de la Molecule d'/ode (CNRS, Paris, 1978); Rev. Phys. Appl. 14, 791 (1979). "G. D. Blue, J. W. Green, R. G. Bautista, and J. L. Margrave, J. Phys. Chern. 67, 877 (1963). 14J. B. West, R. S. Bradford, Jr., J. D. Eversole, and C. R. Jones, Rev. Sci. Inst. 46,164 (1975). 15D. M. Brink and G. R. Satchler, Angular Momentum, 2nd ed. (Oxford University, Oxford, 1975). lOG. Grawert, Z. Phys. 225, 283 (1969); R. H. G. Ried, J. Phys. B 6,2018 (1973); F. H. Mies, Phys. Rev. A 7, 942 (1973). 17 A. M. Arthurs and A. Dalgarno, Proc. R. Soc. London, Ser. A 256, 540 (1960). 18A. S. Davydov, Quantum Mechanics, Volume 1, Chap. XI (Pergamon, London, 1965). 19G. A. Parker and R. T Pack, J. Chern. Phys. 68,1585 (1978); R. T Pack ibid. 60, 633 (1974); G. A. Parker and R. T. Pack, ibid. 66, 2850 (1977). 2"T. Orlikowski, Mol. Phys. 59, 1215 (1986). 210. Nedelec and J. Dufayard, Chern. Phys. 71, 279 (1982); C. Linton, J. Mol. Spectrosc. 69, 351 (1978); R. Copeland and D. R. Crosley, J. Chern. Phys. 81, 6400 (1984). 22G. Herzberg, Molecular Spectra and Molecular Structure. /. Spectra of Diatomic Molecules (Van Nostrand-Reinhold, Princeton, New Jersey, 1950). 23p. J. Dagdigian, H. W. Cruse, and R. N. Zare, J. Chern. Phys. 60, 2330 (1974). 24J. B. Norman, Ph.D. thesis, Massachusetts Institute of Technology, 1988. 25R. J. Ballagh and J. Cooper, Astrophys. J. 213, 479 (1977). 26A. Omont, E. W. Smith, and J. Cooper, Astrophys. J. 175, 185 (1972). 27C. H. Green, and R. N. Zare, J. Chern. Phys. 78, 6741 (1983). 28M. H. Alexander, J. Chern. Phys. 76, 5974 (1982). 29David P. Baldwin (private communication). 3OW. J. Childs, G. L. Goodman, and L. S. Goodman, J. Mol. Spectrosc. 86, 365 (1981); W. J. Childs and L. S. Goodman, Phys. Rev. A 21, 1216 (1980). 31p. F. Bernath, P. G. Cummins, and R. W. Field, Chern. Phys. Lett. 70, 618 ( 1980). 32R. A. Frosch and H. M. Foley, Phys. Rev. 88,1337 (1952); C. H. Townes and A. L. Schawlow, Microwave Spectroscopy, Chap. 8 (Dover, New York, 1975). 33S. F. Rice, H. Martin, and R. W. Field, J. Chern. Phys. 82, 5023 (1985). 34p. F. Bernath, B. Pinchemel, and R. W. Field, J. Chern. Phys. 74, 5508 (198\). 35p. F. Bernath, Ph.D. thesis, Massachusetts Institute of Technology, 1981. 36M. H. Alexander and P. J. Dagdigian, J. Chern. Phys. 83, 2191 (1985). J. Chem. Phys., Vol. 92, No.1, 1 January 1990 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.212.109.170 On: Tue, 16 Dec 2014 15:21:40

1.100649.pdf

Catastrophic loss of superconductivity in ionirradiated films of YBa2Cu3O7−δ D. B. Chrisey, G. P. Summers, W. G. Maisch, E. A. Burke, W. T. Elam, H. Herman, J. P. Kirkland, and R. A. Neiser Citation: Applied Physics Letters 53, 1001 (1988); doi: 10.1063/1.100649 View online: http://dx.doi.org/10.1063/1.100649 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/53/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Terahertz radiation from superconducting YBa2Cu3O7−δ thin films excited by femtosecond optical pulses Appl. Phys. Lett. 69, 2122 (1996); 10.1063/1.117175 Secondary ion mass spectroscopy study of Au trapping and migration in the Auirradiated YBa2Cu3O7−δ film Appl. Phys. Lett. 68, 2738 (1996); 10.1063/1.115582 Microstructure of superconducting YBa2Cu3O7−δ thin films on Si and alumina substrates with buffer layers J. Appl. Phys. 66, 4886 (1989); 10.1063/1.343807 Single target sputtering of superconducting YBa2Cu3O7−δ thin films on Si(100) Appl. Phys. Lett. 54, 859 (1989); 10.1063/1.101557 Formation of YBa2Cu3O7 superconducting films by ion implantation Appl. Phys. Lett. 52, 1729 (1988); 10.1063/1.99714 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 136.165.238.131 On: Fri, 19 Dec 2014 01:01:09Catastrophic loss of superconductivity in lon-irradiated fUms of YBa2CU301_ 8 D. B. Chrisey,a) G. P. Summers, and W, G, Maisch Naval Research Laboratory, Washington, DC 20375-5000 E A, Burke Mission Research Corporation, San Diego, California 92123 W, To Eiam Naval Research Laboratory, Washington, DC 20375-5000 H Herman ,YUNY Stony Brook, Stony Brook, New York 11794-2275 J, p, Kirkland and R A Neiser Sachs/Freeman Associates Inc" Landover. Afaryland 20785 (Received 9 May 1988; accepted for publication 11 July 1988) We have investigated the effects oflaw fluence ( < 1014 em -2) 63 MeV He and 65 MeV He2 t irradiation of prototype thin films of YEa2 CU.l 07_ 15 produced by a plasma-arc spray technique, The observed change in the resistance versus temperature behavior is much more dramatic than that observed for films produced by other techniques and resembles qualitatively a bond percolation threshold. The radiation sensitivity ofthcse plasma-arc spray films is conduded to be due to poor intergrarmlar characteristics. This information is being used to modify the processing steps to improve the properties of films produced by, this technique. The discovery of high Tc supercondl.lcting materials l.2 has prompted much research activity worldwide. Since suc cessful applications in harmful radiation environments win require that the superconducting properties be maintained during irradiation, understanding the radiation sensitivity of these materials is an important consideration, This is espe cially the case since preliminary radiation damage measure ments of e-beam (electron-beam) deposited,-5 and laser evaporated6 thin films (~·ll-lm), and sintered peUets7 of YBa2 Cu} 07 (j indicate a 1-2 order of magnitUde increase in sensitivity as compared to the old high Tc compounds, i.e" theA-15 compounds, This increased sensitivity has been as cribed to the granular nature ofthe films and the presence of insulating behavior in other phases, and not to the intrinsic sensitivity of the bulk material:l,7 More recent measure ments by our group on thick films (-200 pm) of YBa2 Cu] 07 _ b produced by a plasma-arc spray technique7,g indicate a sensitivity much greater than that observed for the thinner e-beam and laser-evaporated films. Although the plasma-arc spray technique was in the eady stages of devel opment as applied to superconductors, tests of radiation sen sitivity were performed to determine directions for improve ments, In this letter we present new data showing this increased radiation damage sensitivity and account for it in terms of recent ideas about radiation-enhanced decoupUng of the granules and a qualitative application of bond percola tion theory, The preparation of thin films of YBa2 eu, 0, 00'" by the plasma-arc spray technique is described in detail in Ref. 9. A large piece of plasma-arc spray film was cut on a low-speed diamond saw into seven samples of approximately the same ,,) Office of Naval Technology Postdoctoral Fellow. size and geometry (2.3 mm X 7.4 mm), Three of these sam ples were irradiated with 63 MeV H ' ions to fiuences of 7.5 X 1012, 2,06 X 1013, and 7.5 X lOllcm -2, respectively, and another three were irradiated with 65 MeV He2 + ions to fiuences of 3,76XlOlI, l.03XW12, and 3075XlOl2 cm,2, respectively, Irradiations were performed at room tempera ture and in air at the UC-Davis cyclotron. After irradiation, silver paint electrical contacts were made and the resistance temperature behavior was measured for each of t.he films. The data were then compiled to produce a single plot show ing the progression of radiation damage for each type of irra diation, i.e., minor differences in resistance measurement ge ometry were ignored, The resistance measurement was made between room temperature and liquid-helium tem perature using a four-point ac technique with a measure ment current of 10 pAc The results for the H + -and He2t--irradiated samples are shown in Figs, 1 and 2, respectively. Similar conclusions can be drawn from both sets of results, These are that ( 1) the Tc onset does not change with increasing particle iluence, (2) at low ftuences '(. completion does not change signifi cantly with increasing particle fluence with the exception of the most heavily irradiated samples, (3) the transition from the nonnal to the superconducting state occurs in one step, Le" there is no sign of a second transition, (4) the room temperature CRT) resistivity is very sensitive to radiation damage and increases linearly with increasing particle fiuence, and (5) at approximately the same value of nonion izing energy deposition between the second and third irra diations, for both H' and He2l , the zero-resistance state of the film as measured was lost catastrophically. All of the changes produced by the irradiations were due to displace ment damage effects characteristic of ion bombardment be cause irradiation of a piece of the same materia! to a dose of 1001 Appl. Phys. Lett. 53 (11),12 September 1988 0003-6951/88/371001-03$01.00 (i;) 1988 American Institute of Physics 1001 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 136.165.238.131 On: Fri, 19 Dec 2014 01:01:09; I \ 63 MeV H + -YBa2 CU3 0)( 20 30 50 100 150 200 250 300 TEMPERATURE (K) FlG. L Resistance vs temrcratnre for YBa,Cu,O, pl:lsma-arc-sprayed tllms (x;::;7) irradiated with 63 MeV H' ions, Note that the temperature axis is actually the temperatur~-seflsing diode voltage which becomes ex tremely nonlinear helow 40 K, 200 Mrad (Si) with 53 MeV e caused no appreciable change in either the RT resistivity or Tc r the largest 63 Me V H + and 65 MeV Hel ~ irradiations produced a dose of 14 and 8 Mrad (Si), respectively 1. Furthermore, this increased radiation damage sensitivity is not seen for the laser-evapo rated samples fOf similar fiuences,6 The major differences in the radiation hehavior between plasma-<!rc spray films and e-beam-deposited and laser evaporated films are observations (3) and (4) above. The R T resistance increases faster than would be expected based OIl results on thinner films, and yet no second transition gradualty lowering 1:. completion with increasing fiuence is seen. This means that an unperturbed superconducting path ~I ~ r-!D, ~I w' ~I ~~ LU, re 10 I I! I I 65 MeV He2 + -YBa2 CU3 Ox UNIARAD!ATED ()( 10) I I I 20 30 50 100 150 200 250 300 TEMPERATURE (K) FIG. 2. Resistance vs temperature for YBa, eu, 0., plasma-arc sprayed films (x::::; 7) irradiated with 65 M{'V He' I ions. Na!e that the temperature axis is actually the temperature sensing diode voltage which becomes ex tremely nonlinear below 40 K. 1002 AppL Phys. L.ett., Vol, 53, No. 11, 12 September 1988 continues to exist through the sample even after the first two irradiations. This path shorts out the rest of the sample be low Tc as the RT resistivity is rapidly increasing with fiuence above Te• We attribute the difference in the radiation behavior between the plasma-arc spray films and the e-beam-deposit ed and laser-evaporated films to the different morphology of the films. The plasma-arc spray films studied were relatively open, cracked, and irregular structures less than 80% dense and consisting of randomly oriented granules approximately 10,um across, e-beam-deposited and laser-evaporated films consisl of mere uniform and closer packed granules about 1 ,u.m in diameter and have the c axis preferentially oriented perpendicular to the substrate and thus to the sample resis tance measurement geometry. The critical current Jc, a property which is strongly dependent on the intergranular characteristics of the films, is evidence supporting the micro scopic differences in fUm morphology. e-beam-deposited and laser-evaporated thin films have values of J, on the Of der of !Of> A/cm2, whereas these plasma-arc spray fiims have values of Jc less than 102 A/cm2. The existence of an unperturbed, continuous supercon ducting path through the sample, which is present until it is destroyed by a small incremental particle ftuence (or energy deposition), suggests that it would be instructive to view the damage results in the light of bond percolation theory. In particular, the observed catastrophic loss of superconductiv ity is strongly reminiscent of a percolation threshold. Treat ing the superconducting path between the voltage electrodes as forming an infinite cluster, a bond percolation system can be defined. Broken bonds between the clusters are formed by nonsuperconducting junctions between individual grains due to an insulating amorphous layer, structural defects, lowered oxygen content at the grain boundaries, or the pres ence of a different phase. Particle irradiation increases the number of nonsnperconducting or broken bonds until the superconducting path is finally destroyed by a small incre mental fiuence. At this point the zero resistance of the sam ple WOUld be lost catastrophically, as is observed. However, a drop in resistance with decreasing temperature would still occur at 1:, onset because most of the final unperturbed su perconducting path would stili be intact. Indeed the magni tude of the resistance drop might be expected to be compara ble to that observed just prior to the incremental fluence that finally breaks the superconducting path, This effect can be seen in Fig. 2 and to a lesser extent in Fig. 1. Data suggesting that the volume fraction of superconducting material re mains unchanged, even though radiation destroys a zero resistance state, have also been seen elsewhere.4•b,7 The similarity between the sudden loss of superconduc tivity observed in these films ~md other percolation phenom ena is very compelling, \(}. 13 What is particularly interesting is that the percolation threshold appears to be reached by the deposition of a certain nonionizing energy, independent of the incident particle type. Assuming there is a percolation threshold, calculated values of the energy loss can be used to narrow the range of critical ftuences. The calculated nonion izing energy loss for 63 MeV H "-is 3.40 keV cm2/g (see Ref. 14) . Current estimates of the ratio of nonionizing energy loss Chrisey et al. 1002 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 136.165.238.131 On: Fri, 19 Dec 2014 01:01:09by 65 MeV He2 t iOilS relative to 63 MeV H C ions is 5.4-5.7, and from Figs, 1 and 2 the measured minimum critical fiuence ratio was 2.06 X 1013/3.75 X 1012 = 5.5. Using 5.5 from above as a minimum and 5.7 from the energy loss as a maximum, the range of critical fiuences required to destroy superconductivity for incident 63 MeV H + and 65 MeV He! + can be narrowed to 2.06-2.14 X 1013 em 2 and 3.61- 3.75>< 1012 em ~2, respectively. This means that the thresh old was reached just after the second H f-irradiation, where as it was reached just before the end of the third He2 + irra diatiem. The degradation of the resistive drop of the largest t1uence H + irradiation at 7:. onset with respect to that for the He2 + irradiation is evidence supporting this idea, In heavier ion irradiation of YBa2 CUI 07 _ b thin films Clark et al.3 demonstrated a higher sensitivity based on the mean amount of nonionizing energy deposited per atom when the zero-resistance state was lost. Clark et aI.'s value of approximately 1 e V I atom has since been obtained eIsew here with a wide range of ions and energies.4-6 On the other hand, the amount of energy deposited for the largest fiuence on the plasma-arc spray films is on the order of 10-5 eV/atom! We have chosen to apply the bond percolation theory to explain our results because of its success in describing trans port phenomena in a wide range of situations such as com posite media, iO amorphous solids, II and granular supercon ductors.12 Furthermore, the percolation-like threshold in the effect of radiation on the transport properties of the plas ma-arc spray films reemphasizes the importance of obtain ing good electrical conductivity in the intergranular regions. With this in mind, the plasma-arc spray technique is being 1003 AppL Phys, Lett.. Vol. 53, No. 11, 12 September 1988 improved to produce denser and mechanically stronger films with full oxygenation at the grain boundaries. 'J. G. Bednorz a.nd K. A. Miiller, Z. Phys. B 64, 1S9 (1986). 2M. K. Wu. 1. R. Asburn, C, T. Tomg, P. H, Hm, R. L. Meng, L Gao, Z. J. Huang, Y. Q. Wang. and C. W. Chu, Phys. Rt:v. Lett, !m, 90S (1987). 3G. J. Ciark, A. D. Mafwkk, R. H. Koch, and R. B. Laibowitz, Apr!. Phys. Lett. 51, 139 (1987). 4A. E. White, K. T. Short, D. C. Jacobsen, J. M. Poat<:, R. C. Dynes, P. M. Maukiewich.. W. J. Skocpol, R. E. Howard, M. Anzlowar, K. W. Baldwin, A. F. Levi, J. R. Kwo, T. Hsieh, and H. Honig, Phys. Rev. B 37. 3755 (l98g). 'G. J. Clark, Po K. LeGnues, A. D. Marwick, R. B. Laibowitz, and R. Koch. AprL Phys, Lett. 51,1462 (987). nD. B. Chrisey, W. C. IVhisch, G. P. Summers, A. R. Knudson, J. C. Ritter, E. A. Burke, M. L. Mandich, A. M. DeSantolo, M. F. JarroJd, J, E. Bower, and S. Sunshine (unpublished). 'IJ. R. Cost, J. O. Willis, J. D. Thompson, and D. E. Peterson, Phys. Rev, B 37,1563 (1988). "W. G. Maisch, G. P. Summers, A. B. Campbell, C. J. Dale, J, C. Ritter, A. R. Knudson, W. T. EIam, H. Hermall, J. P. Kirkland, R. A Neiser, and M. S. Osofsky, IEEE Trans. Nuc!. Sci. NS-34. 1782 (1987). 9R. A. Nciser. J. P. Kirkland, H. Herman, W. T. Elam, and E. F. Skelton, Mater. Sci. Eng. 91, U3 (1987). ,oR. B. Laibowitz, E, t. Alessandrilli, and G. Deutscher, Phys. Rev. H 25, 2965 (19x2). ' t 'R. ZalIen, The Physics a/Amorphous c'l'olids (Wiley, New York, 1983). 12G. Deutscher, O. Elltin-Wnhlman, S. Fishman, and Y. Shapira. Phys, Rev, B. 2t, 5041 (1980). "D. U, Gubser, T, L Francavma, S. A. Woli: and J. R. Laibowitz, cds., Inhomogeneous Superconductors-J979 (Al1letican Institute of Physics, New York, 1980). 14Th.: calculated value for the nonionizing energy loss includes both elastic ,md inelastic effects as well as the Lindhard partition. Details of the calcu lation can be found elsewhere. E. A. Uurke. D. B. Chriscy. G. P. Summers, and W, G. Maisch (unpublished). Chrissy at al. 1003 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 136.165.238.131 On: Fri, 19 Dec 2014 01:01:09

1.584561.pdf

The electrical characteristics of metal/SiO2/InSb capacitor fabricated by photoenhanced chemical vapor deposition TaiPing Sun, SiChen Lee, and ShengJenn Yang Citation: Journal of Vacuum Science & Technology B 7, 1115 (1989); doi: 10.1116/1.584561 View online: http://dx.doi.org/10.1116/1.584561 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/7/5?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Comparison of electrical and electro-optical characteristics of light-emitting capacitors based on silicon-rich Si- oxide fabricated by plasma-enhanced chemical vapor deposition and ion implantation J. Appl. Phys. 111, 053109 (2012); 10.1063/1.3692082 Compositional and electrical properties of Si metal–oxide–semiconductor structure prepared by direct photoenhanced chemical vapor deposition using a deuterium lamp J. Vac. Sci. Technol. A 13, 237 (1995); 10.1116/1.579404 InSb pchannel metaloxidesemiconductor fieldeffect transistor prepared by photoenhanced chemical vapor deposition Appl. Phys. Lett. 63, 3622 (1993); 10.1063/1.110068 Studies of InSb metaloxidesemiconductor structure fabricated by photochemical vapor deposition using Si2H6 and N2O J. Appl. Phys. 69, 2335 (1991); 10.1063/1.348967 Highperformance metal/SiO2/InSb capacitor fabricated by photoenhanced chemical vapor deposition J. Appl. Phys. 68, 3701 (1990); 10.1063/1.346334 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.193.164.203 On: Mon, 22 Dec 2014 19:57:02The electrical characteristics of metai/Si0 2/1nSb capacitor fabricated by photoenhanced chemical vapor deposition Tai-Ping Sun and Si-Chen Lee DepartmentojElectrical Engineering, National Taiwan University. Taipei, Taiwan, Republico/China Sheng-Jenn Yang Chung Shan InstituteojScienceand Technology, Lung-Tan, Taiwan, Republica/China (Received 9 February 1989; accepted 26 Apri11989) The AuCr/Si02/lnSb metal-oxide semiconductor capacitor was fabricated using photo enhanced chemical vapor deposition. The Si021ayer with a thickness of 1000 A was deposited on InSb substrate at 200 °Co The electrical and structural properties were analyzed by capacitance voltage and Auger electron spectroscopy, respectively. The high-frequency (I-MHz) capacitance-voltage measurements were usually performed after positive or negative bias temperature stressing. Both the flatband voltage shift and the change of hysteresis of capacitance voltage curve indicate the existence of enormous negative mobile charges in the bulk Si02• These negative charges can move in Si02 freely even in the room temperature. Auger depth profile reveals that these negative mobile charges are metallic indium and antimony ions, t INTRODUCTION Among many compound semiconductor materials, InSb has the important characteristics of narrow band gap and high electron mobility.l This material can absorb infrared radi ation in the 3-5 J-lm region that passes through the at mosphere easily.1 Recently, metal-oxide semiconductor (MOS) deviees formed on InSb are finding wide application in the fabrication of advanced discrete and integrated de vices for infrared detector.3,4 The most critical step in fabri cating this type of MOS devices is the deposition of the thin oxide film on the InSb. It is important to look for a low temperature method, since the melting point of compound InSb is low, and the material may dissociate at elevated tem peratures.s Several low-temperature methods have been ap plied to deposite oxide on InSb, such as direct anodic oxida tion on surface,6-13 low-temperature chemical vapor deposition (LTCVD),14-18 plasma enhanced vapor depo sition (PECVD), I CJ and photo-enhanced chemical vapor de position (photo-CVD). 20,21 Among these the oxide made by L TCVD has been analyzed by high-frequency (I-MHz) ca pacitance-voltage (C-V) measurement. 14-16 It is found that after the samples were prepared, most ofthem exhibited hys teresis phenomena arid a shift of the flat band voltage as compared to the ideal case in the C-V curve.8.14.18 There have been a few discussions on detail mechanismss,22.23.24 which cause this kind of C-V curve. In this paper, we present our detailed studies on the elec trical properties of the AuCr/SiOl/lnSb MOS capacitor. The lOOO-A-thick Si02 layer is deposited on an-type InSb substrate by photo-CVD. Evidence is provided to show that the C-V characteristics of the MOS capacitor are affected most severly by the negative mobile charges in Si02 layer which are identified to be indium and antimony atoms using Auger depth profiling technique. The results are important for the interpretation of electrical characteristics of this MOS capacitor. II. EXPERIMENTS The substrates used for fabricating capacitors are (111) oriented, n-type InSb with a carrier concentration of 1.S X 1014 em 3. In this experiment, the Si02 films were pre pared by mercury-sensitized photo-CVD. This photo-CVD system PCVD 1000 is made by Tylan. Prior to the deposition of Si02, the growth chamber was pumped down to a few mTorr and purged with nitrogen before introducing the reactant gases. When the desired work temperature 200°C was reached, the SiH4 + N20 gases mixed with mercury va por were introduced into the reaction chamber. The flow rates of SiH4 and N20 were 2 and S5 standard cml/min (seem), respectively. The operation pressure is 1 Torr. When the gas flow was stabilized, the ultraviolet (UV) lamp was turned on and the Si02 film started to grow. After depo siting -1000 A, the UV lamp was turned off. This complet ed the entire growth processes. The oxide thickness was mea sured by ellipsometry. A 200 A of Cr and 3000 A of Au were evaporated on the oxide layer to form the metal gate. The area of the gate is 2X 10-3 cm2• Finally, each MOS device was scribed into an individual piece and bonded to a To-5 header. Indium was used as the alloy contact at the back side of InSb substrate. The device is cold shielded from back ground radiation in order to prevent photogeneration of electron-hole pairs in the InSb, thus one may assume that the device is in thermal equilibrium. The device structure is shown in Fig. 1. After the bonding process, the sample is ready for measurements. First, the metal and InSb substrate of the MOS capacitor were shorted at room temperature to do the discharge test. Second, the high-frequency (l-MHz) capacitance are mea sured at 77 K with an automatic capacitance meter (HP 4280). Third, the leaky current through the oxide was mea sured at 77 K to characterize the oxide quality. Sometimes, before the C-V measurement, the bias temperature stressing (BTS) i.e., the device temperature was raised to 50 °C for a 1115 J. Vac. Sci. Technol. B 7 (5), Sep/Oct 1989 0734-211X/89/051115-07$01.00 @ 1989 American Vacuum SoCiety 1115 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.193.164.203 On: Mon, 22 Dec 2014 19:57:021116 Sun, Lee, and Yang: Electrical characteristics of metai/Si0211nSb 1116 Au ~Cr InSb SUb. / FIG. 1. Schematic structure of a InSb MOS capacitor. period of time with a bias applied on, was performed to study the mobile charge effect in the oxide. The block diagram of the measuring system is shown in Fig. 2. The automatic ca pacitance meier is controlled directly by a HP computer 9836. The depth profile of the MOS device was analyzed by Auger electron spectroscopy (AES). The AES measure ment were performed using a Perkin-Elmer electron spectrometer with a 3-KeV, O.5-flA electron beam. III. RESULTS A. Electrical measurements 1. Current transient characteristics Figure 3 shows the current transient characteristics of the discharge test for the sample No. 1027. Apparently after the metal and semiconductor contacts of the MOS capacitor are short circuited at room temperature a current of ~ 30 nA is flowing out of the metal contact for ~ 100 s, then it gradually dies down. This phenomenon strongly suggests that the MOS capacitor is under nonequilibrium for a long period of time after short circuit. The leak current through the oxide measured at 77 K is 5.1 pA (at 2.5 V), which corresponds to an oxide resistivity ofO.98X 1014 n cm. 2. C-V characteristics All the C-V measurements were done at 77 K. The C-V scan was selected from -20 to + 20 V and then back to -20 V with a sweep rate of 100 mV Is. First, the Auer/ Si02/InSb capactor is biased to strong inverstion region ( -20 V) for a hold time of 5 min. Since the hold time has some effect on the occupation of slow states near Si02/lnSb 6205 ua\ DC power supply FIG. 2. Block diagram showing the bias temperature stressing test system for MOS capacitor. J. Vac. Sci. Technol. S, Vol. 7, No.5, Sep/Oct 1989 « 10' c - I ~ I- Z w 100~ 0:: InSb '" 0:: ::J t u I 100 10' 102 103 T 1 ME, t (sec) FIG. 3. The short current as a function of time for MOS capacitor No. 1027 at room temperature. interface and thus the C-V curve, it must be selected proper ly. For measurement with gate voltage VG being scanned from negative, 5 min is good enough to stabilized the rising part of the C-V curve. Figure 4 shows the typical measured and ideal characteristics for a MOS capacitor No. 1105. The ideal one is calculated by assuming zero interface state and mobile charges. The major characteristics of these C-V curves are described as below: (i) Since all C-V curves exhibit a large hysteresis, they appear to have two flatband voltages, i.e., VFB(i) and VFB(r) corresponding to the left forward and right returning C-V curves. It is clear that both ftatband voltages are more nega tive than the ideal one, and the semiconductor surface must be accumulated with electrons at zero applied voltage. This indicated that there were large numbers of positive fixed charges trapped in the oxide probably near the Si02/InSb interface. H,IS.19,24 (ii) The magnitude of the return voltage affects the return C-V curve significantly. Figure 5 shows the return voltage dependence ofthe C-V curve of a MOS capacitor No. 1128. It is first biased at -15 V for ~ 5 min, then the voltage is LL a. 80 u w" 60 u z « I-40 u (f <l: u EXP ------ I DEAL B I AS, V (vo It) FIG. 4. The ideal and experimental C-V curve ofa MOS capacitor No. 1105 fabricated on n-type InSh with 1000 A SiO, layer as the insulator. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.193.164.203 On: Mon, 22 Dec 2014 19:57:021117 Sun, Lee, and Yang: Electrical ch.,racterlstlcs of metallSI0 2/1nSb 1117 lJ.. 80 0. U,60 w U Z ;:: 40 u « ~ 20 u RETURN VOLTAGE (V) --10 ----20 ~-------.-. _._.-30 OL--~2-4~---1~2~--~O--~~12~~~2~4~ BIAS. V (volt) I'IG. 5. The C-V characteristics of an InSb MOS capacitor No. 1128 under different return voltages. swept from 15 to 10 V, and then immediately back to -15 V without a hold time. Before the second sweep the capacitor is biased at --15 V for a hold time of 5 min to restore the capacitor to the same initial state as that of the first trace. Then the return voltage is increased to 20 V for the next C-V sweep. As can be seen from this figure, the forward C-V curves for both traces are similar, but the re turning C-V curve shifts toward more positive voltage for the second trace. When the return voltage is increased to 30 V. the same trend is also observed. This is consistent with the theoryX that the density of interface slow states in the oxide is so high that when the gate voltage is made more positive (in the accumulation regioIl), more electrons, are trapped by the slow states which causes the right Hatband voltage to shift more positively. On the other hand, most of the elec trons trapped at positive voltage can be removed by applying a large negative voltage at the gate for a hold time of 5 min, thus resulting in a hysteresis in the C-V curve. (iii) The C-V curve shown in Fig. 4 does not have a sharp transistion between the accumulation region and the deple ticm region as that of the ideal one indicating a large density of fast surface states at the oxide semiconductor interface. B. Bias"temperature stressing (BTS) test To study the mobile charge effect, the MOS capacitors were subjected to BTS test before C-V measurement. During the test the caDacitor was treated at 50°C with positive and negative bias ;emperature stressing each for 5 min and the cu;rent-time characteristics of the MOS capacitor are re corded. Then aU the C-V surves were measured at 1 MHz at 77 K. The return voltage during the C-V sweep is kept at + 20 V. Hysteresis voltage Ll V1'B was defined to be the dif ference between the flatband voltages VFIl( I) and VFB( r) • From it, the equivalent slow state density EoxLlVFB Nss = qd can be calculated, where d and fox are the thickness and dielectric constant of the oxide, q is the electron charge. J. Vac. Sci. Techno!. 13, Vol. 7, No.5, Sep/Oct 1989 ~ ~ 0 16 ~ III w.. 8 >. w (9 « ~ 0 0 > 0 -B z « en I-« -16 ...J ll... -30 -24 -18 -12 -6 o BIAS,V (volt) FIG. 6. The fiat band voltage shift ofa MOS capacitor No. 1166 after higher und higher negative IlTS tests. 1. Negative bias~temperature stressing test Figures 6 and 7 show the f1athand voltages and equivalent slow state density N ss ofMOS device No. 1166 after various negative bias temperature stressing. It is found that when the device is successively bias-temperature stressed from -0.5 to --30 Veach for 5 min, the flatband voltage shifts first positively and then moves back to more negative voltage and the equivalent slow state density increases monotonically, i.e., from 6.5X 1011 to 2.5x 1012 cm-2• Figure 8 shows the measured current during negative BTS at --6 V as a func tion of time for a typical device No. 1179. Similar to the case shown in Fig. 3, the current is large (4.5 llA) initially and then rapidly decreases to 1 nA after 300 s. This indicates that the MOS device is under extreme non equilibrium after short circuit. Therefore, the low-temperature (77 K) C-V mea surement is done only after the short circuit current during BTS test reduces to below 1 nA, after the negative bias tem perature stressing for 5 min. If the G"-V measurement is done earlier, i.e., short circuit current during BTS test is still at the uA level, the C-V curve tends to display large distortion, as 'shown in Fig, 9 for a MOS capacitor No. 1195, indicating a large nonuniformity being created inside the Si02 layer. ~ 103 -'-,--- ~---~--, -~---r ----'-~-l z ~ O~----~8 ______ ~ ____ ~ j -6 o BIAS ,v (volt) Fit •. 7. The equivalent slow slate density as a fUllction of higher and higher negative 31'S ("SIs. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.193.164.203 On: Mon, 22 Dec 2014 19:57:021118 Sun, Lee, and Yang: Electrical characteristics of metai/Si0211nSb 1118 ~1 101~0~O----~~10~'~--~--'~OA2----~~103 TiME. t (sec) PIG. 8. The discharge current of a MOS capacitor No. 1179 as a function of time at 50 °C during negative BTS at - 6 V. 100 U-80 0. U w 60 u z « I-40 u rt « 20 u 0 -16 -8 0 8 16 B 1 AS. V (vo It) FlG. 9. The distorted C-V curve of an luSh MOS capacitor No. 1195 after negative BTS of -6 V at 50 'C for 3 min. ~ --16 "" VFB(r) 0 Z-0 VF8(J} J 8 w ~ ~ 0 0 > 0 z -8 « ell I- :3 -16 I.L 0 2 3 4 BIAS.V(volt) FIG. 10. The flathand voltage shift of aMOS capaeitor No. 1223 after higher and higher positive BTS tests. Jo Vac. Sci. Techno!. e, Vol. 7, Noo 5, Sep/Oct 1989 ~ 103r-~---.---r--~--r-~---r--~ I./l Z W o v v v 1 2 3 4 B I AS, V (vo I t) FiG. 11. The equivalent slow state density as a function higher and higher positive BTS tests. 2. Positive bias-temperature stressing test Figures 10 and 11 show the fiatband voltages and equiva lent slow state density N ss of a capacitor No. 1223 after various positive bias-temperature stressing. It is clear that the fiatband voltage shifts to more negative voltage after suc cessively higher and higher positive bias-temperature stress ing. The equivalent slow state density, however, increases only slightly, i.e., from 6.5 X 1011 to 7.8 X 1011 cm-2• Figure 12 shows the short circuit current during the positive bias temperature test ( + 3 V) as a function of time for aMOS capacitor No. 1255. The current is 5 pA initially and lasts for 80 s, then something drastical happens. The current first reduces to 1 f.lA then rapidly rises to 3 rnA in 10 s. If the 77 K C-V curve is measured at this stage, it exhibits a large distor tion as shown in Fig. 13. The oxide capacitance at strong accumulation region apparently decreases indicating that a parallel conducting channel has been formed. C. AES measurement The AES was used to study the chemical composition of the oxide film including the mobile charges. The measured 104 ,..... 103 « .3 ~. 102 w a:: 0:: ::J 101 U 10° 10° 101 102 103 TlME,t (sec) FIG. 12. The discharge current of a MOS capacitor No. 1255 as a function of time at 50"C during positive llTS at + 3 V. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.193.164.203 On: Mon, 22 Dec 2014 19:57:021119 Sun, Lee, and Yang: Electrical characteristics of metai/Si02/1nSb 1119 u:-80 Cl u w 60 u Z <t: I-40 u :. <t: 20 u 0 -16 ORIGINAL DEGRADED BY STS -8 0 8 B I AS, V (vo It) 16 t"lG. 13. The original C-V curve of all luSb MOS capacitor No. 1288 and that after positive BTS of + 3 V at 50 "(' for 5 min. intensities of various elements were corrected to get their atomic percentage. The error was within 10%. Three sam ples were used for this purpose; sample No. 1301 without BTS test (Fig. 14), sample No. 1302 measured immediately after the negative BTS at -6 V (Fig. 15), sample No. 1303 measured immediately after the positive BTS at 3 V (Fig 16). As shown in Fig. 14, significiant concentration of both In and Sb are detected at the surface and in the bulk of the Si02 film indicating that In and Sb atoms outdiffuse from the InSb substrate during the Sial deposition. Hg signal was not detected so the deposition films were free of Hg contamina tion ( < 1.0 at. % ). After negative bias-temperature stress ing, as shown in Fig. 15, the In and Sb atoms were driven back to the Si02/InSb interface such that the interior ofSiO} is free ofln and Sb. After positive bias temperature stressing, however, as shown in Fig. 16, the In and Sb atoms spread to entire Si02 layer and even to the AuCr metal electrode and thus form a conducting channel in the oxide layer. This indi cates that the negative mobile charges are both In and Sb atoms, they seem to move in pairs. .-.. ~100 .. _ .. _ .. --....., Au '. Z \ 0 -80 ~ 0:: \ t-60 z UJ u z 40 0 u u 20 ~ 0 t-0 <t: 0 20 40 60 80 SPUTTER TIME (min) FIG. 14. Depth profile of a lOoo-A-thick oxide on n-type InSb without BTS tests. J. \lac. Sci. Techno!. B, Vo!. 7, No.5, Sep/Oct 1989 ~ ~100 z Q 80 I-00_"-Au .\ 0:{ 0:: I-60 z w u z 40 0 u u 20 ~ 0 I- °0 <t: 20 SPUTTER TIME (min) fiG. 15. Depth profile of a 1000-A.-thick oxide 011 IHype InSh after negative BTS of 6 V at 50 T for 5 min. IV. DISCUSSION Based on the results of the AES analysis and the C-V characteristics of the MOS capacitor, we propose the follow ing model to describe the oxide and interface properties of a photo-Si02 film on InSb. The initial charge distributions in photo-CVD MOS capacitors can be described by the sche matic diagram shown in Fig. 17. QF are the positive fixed charges at the Si02/lnSb interface, QAt are the negative mo~ bile charges (In and Sb atoms) in silicon oxide. Referring to the Fig. 17, the left flat band voltage VI/BU) can be written as25 _ .l. _ XOQF _ X1Q~f ~,_ dQss VFB(I) -<i'M~ , . , ,. ofox €ox cux where Xo and Xl are the distance between metal and weight center of the fixed charges and mobile charges, respectively, d is the oxide thickness, Qss the net interface slow state charges, ¢MS the work function difference between metal AuCr and InSb. The work function difference ¢MS is -001 V for the MOS capacitor with a AuCr gate.8020 The fixed charge QF is positive, situated near the Si02/InSb interface, so a large Xo contributes a large negative value to flatband voltage. The mobile charges QM is negative, so a large Xl contributes a large positive shift to the flatband voltage. It is ~100 -.!' ~ z 80 0 ~ 0:: _ .. _"'" Au \ \ I-60 z w u z 40 0 u u 20 L 0 l-<t: °0 20 40 60 80 SPUTTER TIME (min) FIG. 16. Depth profile of a 10oo-A-thick oxide on n-type InSb after positive HTS of + 3 V at 50 T for 5 min. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.193.164.203 On: Mon, 22 Dec 2014 19:57:021120 Sun, Lee, and Yang: Electrical characteristics of metai/Si02/1nSb 1120 Xl ~--il\ ~ d j ~ ~ QM Au Cr Si02 InSb FlU. 17. Schematic diagram of the initial charge distribution in aMOS structure, where Q, is the fixed charges, QM is mobile charge, and Qss the net interface slow state charges. clear from Fig. 4 that all four terms sum up to give negative contributions to the fiatband voltage of the MOS capacitor before the BTS test indicating the initially dominant role of positive fixed charge. During the negative BTS aging, the mobile charges are gradually driven to the SiOz/InSb interface, by applying higher and higher negative biases, this results in an increas ing Xl and thus an increase of the flatband voltage (Fig. 6) since QM is negative. However, when the mobile charges are driven so close to the interface that it can communicate elec trons with the semiconductor, part ofthe mobile charges will convert to slow interface states. This decreases the mobile charge QM and causes the left flatband voltage to shift back toward more negative voltage. This is consistent with the observation that the equivalent interface slow state increases monotonically (Fig. 7). When the negative voltage is applied to this MOS capaci tor during BTS test, the electric field inside the Si02 layer is altered, the negative mobile charges will respond to this change and move, which induces a large current flow in the outside circuit. So an initial current of approximate 5 J1A is flowing for -20 s, as shown in Fig. 8. When the mobile charge gradually approaches a new steady-state distribu tion, the current will decrease to very small value. The same thing happens, when the InSb and metal electrode were shorted, at room temperature (Fig. 3) indicating the free movement of the negative mobile charge in Si02 layer even in the room temperature. During the positive bias stressing the negative mobile charges QM are attracted to the metal side, so XI decreases and the flatband voltage shifts to more negative voltage (Fig. 10). It is also found that the MOS capacitor breaks down easily (Fig. 13) at very low positive voltage (3 V). This is because the negative mobile charges (In and Sb) have been attracted to the metal electrode, and probably form a fila mentary conducting channel inside the Si02 which degrades the electrical properties of the MOS device. This is consis tent with the observation in Fig. 10 that during the positive BTS test, the current suddenly increases to abnormally high value of 3 rnA after applying 3 V bias for 80 s. If the C-V characteristics of the MOS capacitor is measured before see- J. Vac. Sci. Techno!. 8, Vol. 7, No.5, Sep/Oct 1989 ing this abnormally high current, it looks normal, otherwise, it is seriously degraded as shown in Fig. 13. The InSb surface seems to be dissociated by the applying positive bias, so huge number of negative In and Sb pairs are attracted to the metal gate. During the process, only a few convert to interface slow states. (Fig. 11). This indicates most of them are stilI in direct contact with the substrate when they move toward the metal contact, so they don't convert to a slow state. This is unlike what happens during negative BTS test, when the small number of residual In and Sb pairs after photo-CVD depostion (they don't form conducting channel) are driven back to the Si02/lnSb interface. They first convert to slow state, i.e., communicate electrons with substrate, then to fast states as they gradually approach the InSb surface. How ever, the interface slow state traps slightly increase rather than decrease as the negative mobile charges left the inter face (Fig. 11). It suggests that there are certain number of intrinsic interface slow states formed during the photo-CVD process which won't move under any bias condition. The slight increase of slow state density is due to the conversion of small amount ofIn and Sb pairs when they are dissociated from the substrate. Notice, however, most of the In and Sb pairs form filamentary conducting channels which are in direct contact with substrate. v. CONCLUSIONS We have studied the chemical and' electrical properties of photo-Si02 oxide on InSb by means of AES analysis and MOS device characterization. The results indicate that the C-V characteristics of the photo Si02/lnSb MOS capacitor exhibits flatband voltage shift and hysteresis behavior due to positive fixed charges, negative mobile charges, and enor mous intrinsic interface slow states. Using the positive and negative BTS technique in conjunction with AES analysis, we clearly demonstrated that the negative mobile charges are In and Sb atoms in pairs, which can be driven to move easily by electric field in the oxide even at room temperature. Therefore, when we short circuit or apply a bias to the MOS device at room temperature a large current will flow for a period of time reflecting the fact that mobile charges are redistributed to reach a new steady state. When those mobile charges are driven back to the SiOzilnSb interface by nega tive bias, they convert to slow interface states. Those charac teristics suggest that the photo-oxide grown on InSb is not ideal. Therefore, proper modifications in the oxide growth technique may be required to improve the oxide and inter face property. JR. K. Willardoll and A, C. Beer, In/rared Detectors (Academic, New York, 1970), pp.15-17. ~R. D. Hudson, Jr., In/rard System Engineering (Wiley, New York, 1968), p.265. 3R. J, Stirn and Y. C. M. Yeh, IEEE Trans. Electron Devices 24, 476 (1977). 4M. Gibbons and S. Wang, Proc. SPIE 443, 151 (1984). 'C. Y. Wei, K. 1,. Wang, E. A. Taft, J. M. Swab, M. Gibbons, W. Davern, and D. M. Brown, IEEE Trans. Electron Devices 27, 170 ( 1980). "Y. Shapira, J. Bregman, and Z. Calahorra, App!. Phys. Lett. 46, 48 (1985). Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.193.164.203 On: Mon, 22 Dec 2014 19:57:021121 Sun, Lee, and Yang: E!ectrlcal characteristics of metai/Si02/1nSb 1121 'R. Y. Hung and E. T. Yon, J. Appl. Phys. 41, 2185 (1970). sA. Etchells and C. W. Fisher, J. Appt. Phys. 47, 4605 (1976). 9R. Fujisada and T. Sasase, Jpn. J. App!. Phys. 23, L46 (1984). lOR. Hujisada, T. Kakagawa, and T. Sasase, Jpn. J. App!. Phys. 22, L525 (1983). "J. Bregman and Y. Shapira, J. Vae. Sci. Techno!. B 3,959 (1985). I2Z. Calahorra, J. Bregman, and Y. Shapira, J. Vac. Sci. Techno!. B 4, 1195 (1986). I3J. Bregman, Y. Shapira, and Z. Calahorra, J. Vac. Sci. Techno!. A 5,1432 (1987). 14G. W. Anderson. W. A. Schmidt, and}. Comas, J. Electrochem. Soc. 125, 424 (1978). 150. N. Pocock, Ikasai, D. E. Nutall, C. It Chen, and R. N. Ting, SPrE. J. 227,129 (1980). 16J. D. Langan and C. R. ViSWlinathan, J. Vac. Sci. Techno!. 16, 1474 (1979). 17M. Okamura and M. Minakata, J. App!. Phys. 57, 2060 (1985). J. Vac. Sci. Techno!. S, Vol. 7, No.5, Sep/Oct 1989 I"y. Avigal, J. Bregman, and Y. Shapira, J. App!. Phys. 63, 430 (1988). 19U. Kachens and Umerkt, Thin Solid Film. 97,53 (1982). 2°K. F. Huang, J. S. Shie, J. J. Luo, and J. S. Chen, Proc. SPIE. 558, 11 (1985). 21 A. Bahraman, Extended Abstract of the 18th International Conference on Solid State Devices and Materials (Japan Society of Applied Physics, Tokyo, 1986), p.189. 22p. P. Heiman and G. Warfield, IEEE Trans. Electron Device 12. 167 (1965). 21J. Buxo, D. Esteve, J. Farre, G. Sarrabayrouse, and J. Simonne, Appl. Phys. Lett. 33, 969 (1978). 24D. N. Pocock, C. H. Chen, J. B. Underwook, E. 1. Dillard, SPIE. J. 267, 31 (1981 ). 25E. H. Nicollian and J. R. Brews, MOS Physics and Technology (Wiley, New York, 1982). 265. M. Sze, Physics of Semiconductor Devices, 2nd. (Wiley, New York, 1981), Chap. 7. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.193.164.203 On: Mon, 22 Dec 2014 19:57:02

1.575857.pdf

The decomposition of [Mn(CO)5]2(μSiH2) G. T. Stauf, P. A. Dowben, K. Emrich, W. Hirschwald, and N. M. Boag Citation: Journal of Vacuum Science & Technology A 7, 634 (1989); doi: 10.1116/1.575857 View online: http://dx.doi.org/10.1116/1.575857 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/7/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Effect of substitution of In for Co on magnetostructural coupling and magnetocaloric effect in MnCo1-xInxGe compounds J. Appl. Phys. 115, 17A911 (2014); 10.1063/1.4863255 Large magnetocaloric effects over a wide temperature range in MnCo1−xZnxGe J. Appl. Phys. 113, 17A922 (2013); 10.1063/1.4798339 Magnetization and structure of MnNi and MnCo layered magnetic thin films J. Appl. Phys. 53, 2436 (1982); 10.1063/1.330969 Sealed RoomTemperature CO–CO2 Laser Operating at 5 or 10 μ Appl. Phys. Lett. 20, 436 (1972); 10.1063/1.1654006 The Infrared Emission Spectra of OH, CO, and CO2 from 3 μ to 5.5 μ J. Chem. Phys. 20, 1178 (1952); 10.1063/1.1700698 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.69.47 On: Sun, 23 Nov 2014 00:33:57The decomposition of [Mn(CO)sh(J.I,-SiH 2) G. T. Stauf and P. A. Dowben Laboratory for Solid State Science and Technology. Syracuse University, Syracuse. New York 13244-1130 K. Emrich and W. Hirschwald Institutfiir Physikalische Chemie, Freie Universitiit Berlin. Takustrasse 3, 1000 Berlin 33, Federal Republic of Germany N.M. Boag Department of Chemistry and Applied Chemistry. Salford University. Salford. England, M5-4 WT (Received 16 August 1988; accepted 29 August 1988) The prospect of using chemical vapor deposition to deposit mixed metals and silicides from a single source compound is attractive but largely uninvestigated. Studies of decomposition energies of such compounds are nearly nonexistent. One such compound which has successfully been used to make a silicide coating is [Mn(CO)512 (,u-SiH2). We have used electron impact mass spectroscopy, photoionization mass spectroscopy, and photoabsorption to determine bond energies within this compound. The combination of methods allows a high degree of confidence in the resultant ionization and fragment appearance potentials. Some possible mechanisms of decomposition are discussed. A complete ionic decomposition thermodynamic cycle has been generated, and the results are used to illuminate the coating processes previously observed. I. INTRODUCTION Chemical vapor deposition (CVD) from organometallic compounds is a common way of creating metal and metal silicide coatings. I Deposition may be induced by pyrolysis, plasma processes, or by photolysis. Silicides, metal/silicon compounds in various phases and combinations, have found favor of late in the semiconductor industry when low-resis tance interconnects capable of withstanding high tempera tures are needed. While some commonly used organometal lic source compounds such as alkyis, chlorides, and hydrides have been heavily investigated, little research has been done on more unusual sources. At the moment, when a multicom ponent coating is desired, a two-or three-gas flow-metering system is used. If a molecule such as [Mn (CO) 5 ] 2 (II-SiR2 ) which contains both the Mn and Si in a 2: 1 stoichiometric ratio could be induced to decompose, however, wasteful ex cesses which are now used to compensate for differing reac tion rates would be avoided. Such reactions may not be as simple as they would seem on the surface, however. For example, recent work on the one-photon photolysis of gaseous Mn2 (CO) 10 has shown that there are two different decomposition pathways: sepa ration ofa CO or the cleavage of the Mn metal-metal bond.2 Another complication is that decomposition may be a mul tistep process. This sequential removal of ligands3 has been observed to be the case with the pyrolysis of Ga(CR3)3 ( Ref. 4) and In ( CH 3 ) 3 •5 These elimination reactions can be a considerable problem, for example, with the release of si lane from metal silyl carbonyls.6 2[Mn(CO)s (SiH3)] -SiH4 + [Mn(CO)s b (SiR2) • Alternatively, tl1e pyrolysis process can result in more com plex elimination reactions,7 such as in the case CO(CO)4 (SiR3) .... OeSiR3)2 +? , which leads to disiIoxanes when R = H,8 methyl,9 or ethyl. 10 Coatings formed from this reaction have a cobalt-to-silicon ratio of 5:3, while the pyrolysis of Fe(CO)4 (SiR3)2 which follows a similar mechanism resulted in films with an iron to-silicon ratio of 1:0.9.7 In order to control the decomposition of such a molecule, an understanding of the energetics of decomposition and bond breaking is certainly necessary. With few excep tionsl1.12 these are poorly understood. With the aim of understanding decomposition energetics we have therefore undertaken an electron impact mass spectroscopy, pho toionization mass spectroscopy, and gas-phase photoab sorption investigation of r Mn (CO) 5 ] 2 (II-SiR2 ). We have also demonstrated the actual feasibility of silicide coating formation with this compound via pyrolysis. The use of x-ray electron spectroscopy (XES), Auger electron spec troscopy (AES), and Rutherford back scattering spectros copy (RBS) confirmed that the material formed was Mn2 Si. Such a metal silicide thin film is difficult to form without the use of metal organic CVD (MOCVD). The microstructures of the films were also studied via x-ray diffraction (XRD) and scanning electron microscopy (SEM). II. EXPERIMENTAL The [Mn (CO) 5 J 2 (,u-SiR2 ) complex was prepared as de scribed previously13.14 and purified by crystallization fol lowed by low-pressure sublimation. The electron impact mass spectroscopy experiments were undertaken using a molecular beam of sample vapor genera ted in an alumina Knudsen cell. This beam was directed into the electron impact ion source of a Varian MAT single-sec tor magnetic field mass spectrometer, as described previous ly.ls.lt> Calibration, data reduction, evaluation procedure, and analysis of the fine structure of the ionization efficiency curves (lEC's) were undertaken using procedures outlined elsewhere. 17.18 634 J. Vac. Sci. Technol. A 7 (~), May/Jun 1989 0734-2101/89/030634-06$01.00 Cc) 1989 American Vacuum Society 634 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.69.47 On: Sun, 23 Nov 2014 00:33:57635 Stauf et at.: Decomposition of [Mn(CO)5],(p-SiH2) The fragmentation of [Mn(CO)s12 (,a-8iR2) was also studied inside a high-temperature photo ionization system using synchrotron radiation in the photon energy region 6- 24 eV.15 The synchrotron radiation source was the electron storage ring BESSY (Berliner Elektronenspeicherring Ge sellschaft fuer Synchrotronstrahlung mbH) in Berlin, Fed eral Republic of Germany. The ions were detected using a Balzers QMG 511 quadrupole mass spectrometer. Photoabsorption data on the gaseous species was collected at the Tantalus Storage Ring at the University of Wisconsin Synchrotron Radiation Facility, as has been discussed else where.ls Pyrolysis of gaseous [Mn(CO), L (,a-SiHz) was under taken in a glass vacuum system pumped by a three-stage oil diffusion pump, capable of reaching a base pressure of 1 X 10 -5 Torr. Solid crystals were allowed to sublimate at room temperature on one side of the deposition chamber, while on the other side the line to the vacuum pump carried away excess reactants and products. The substrates were pure nickel foils (Driver Harris Company) resistively heat ed with an alternating current. The temperature of the foil was monitored via a Chromel-Alumel thermocouple spot welded to the back of the foil. Coatings were examined while still on their substrates by SEM, XES, AES, and RBS. The instruments and procedures used have all been previously described. 19 X-ray diffraction studies were made in transmission mode. The coating was removed from the substrate and placed on cellophane tape, then exposed for up to 15 h to ensure that any crystal structure would be revealed. The x ray source was Cu Ka radiation (1 ,542-A wavelength). ill. RESULTS The mass spectroscopy revealed the fragmentation behav ior to be expected from this molecule. Basically, it lost successive carbonyls until it was down to a Mn2 Si core. The >'- ~ -e o ::>:1 -I 'Vi C III C ..... c o H 400 250 200 m/e rat io, amu J. Vac. Sci. Techno\' A, Vol. 7, No.3, May/Jun 1989 -,".-,".-.-.-.- •••••••••• n> ".' •• Y •••••••••• , .-...... ~ •• -•• , ••••••••••••••••••••••••••••• ~ •• ,.-.-... -.: ••• -.-••••••••••••••••••••• v .•...•.•..• ~.· .•• ·.·.·..-.-.-.·.·.-.- ..••• _rH •• _.~~ ••• , • 150 635 complete electron impact mass spectra at 25 e V can be seen in Fig. 1. It can be seen in this figure that the ion signals for (Mn2 SiR2 ) (CO) 7+ and (Mn2 8iR2 ) (CO) 6-1 were too small to obtain accurate appearance potentials CAP's), so AP energies for these fragments were derived from the pho toionization experiments. We did not find fragments differ ing only by one or two hydrogens to have very different ap pearance potentials. Even at the highest energy setting available on the mass spectrometer (70 e V) no additional fragments were seen, although more hydrogen loss was ob served. Total Mn2 Si fragment abundance at this energy. with or without H2 attached, was 27%. The ionization po tentials (IP's) and appearance potentials we found are sum marized in Table I. In most cases electron impact ionization and photoionization results were averaged to find the AP's listed in the table. Good agreement between the two tech niques was found, with the largest difference of 0.4 eV pres cnt only in the case of (Mn2 SiHl )(CO) 3-+ • It may be noticed that slope changes are mentioned in the caption of Table I. These come from the IEC of the parent ion. The IEC is a plot ofthe molecular or fragment ion inten sity versus the electron (or photon) impact energy employed to ionize the gaseous species, It rises linearly until impact energy becomes high enough to access a new molecular orbi tal, which causes an abrupt slope increase as a new ionization pathway is reached. Of course, higher energy may also cause bond breakage and a downward slope change. These slope changes, or "breaks," also occur in fragment IEC's, but are more difficult to interpret and so have been left off Table 1. More detail can he found in Ref. 15, It should be noted that while photon impact data provide very accurate IP's of the parent and first AP's of the frag ment ions due to its high degree of monochromacity, it is not as useful for obtaining higher appearance potentials (slope changes) from lEC plots due to poorer signal-to-noise ra tios. Fortunately, a combination of the two ionization tech- !OO 50 F1G, 1. Mass spectra from electron im pact ionization 25-eV electron energy, for lMn(CO), 12 (25-eV electron ener gy) (p-SiH2)· Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.69.47 On: Sun, 23 Nov 2014 00:33:57636 Stauf et sl.: Decomposition of [Mn(C0lsMwSiH,) TABLE I. The ionization and appearance potentials based on the ionization efficiency curves of [Mn(CO),12 (/l.-SiH2). Arrows designate relative in creases (I) and decreases ( ,) of slope in the parent lEC's. Initial AP's are given both for electron and photoionization data, which are then averaged below, while higher AP's are from electron impact data only. Values are given in eV and rounded to tenths, as estimated error is ± 0.1 eV. Species [Mn(CO), h(SiH2)+ (parent ion) Mn2SiH2 (CO) ': MnzSiH2(CO)/ Mn2SiH2(CO)/ Mn2SiH2 (CO) 0" Mn2SiH2 (CO) ,~ Mn2SiH2(CO)/ Mn2SiH2(CO)t Mn2SiH2 (CO) t Mn2SiHz (CO) t Mn2SiH.+ AP (eV) 7.9 (electron) 7.9 (photo) 7.9 (avg.) 8.5 8.7 9.0 10.4 12.7 13.8 14.8 15.9 17.4 8.4 (avg.) 9.4 (avg.) 10.7 (photo) 11.0 (photo) 12.6 (avg.) 13.6 (avg.) 14.7 (avg.) 16.4 (electron) 17.1 (avg.) 18.9 (avg.) Relative increase ( !) or decrease ( I) of ml e slope in parent IEC ratio 420 392 364 336 308 280 252 224 196 168 140 niques acts to eliminate the uncertainties associated with ei ther one alone. The relative intensities of the parent and fragment ions are seen plotted in Fig. 2. They are based on electron impact data, because the magnetic sector mass spectrometer used 636 for the electron impact studies was capable of better mass resolution than the quadrupole mass spectrometer used for photoionization, and was able to easily distinguish a 2-amu separation. The gas-phase photoabsorption curve for this compound exhibited several peaks or absorption bands. The energies at which these were observed served to confirm the AP values in Table 1.15 Having collected thermodynamic information from var ious sources, we made coatings by pyrolysis of (tt-SiH2) [Mn (CO) 512 as described above. Our results have been reported in the literature. 19 This particular organome tallic compound has not been previously studied for pur poses of coating production. Pyrolysis of our complex was found to start at -225 ·C. Coatings were black to the eye as in the case of pure metal coatings we have previously made by pyrolysis. 16 Scanning electron microscopy showed the coatings to be smoothly granular and slightly cracked. We believe the cracks to be the result of different thermal expansion coeffi cients between the manganese-silicide coating and the Ni substrate, as the coatings were rather thick (over 2ttm). Despite this, the coatings showed fair adhesion. X-ray electron spectroscopy showed by attenuation of the Ni substrate signal that the manganese silicide coatings were well over 1 pm thick after a deposition period of 1 h. This means the deposition rate was at least 170 A/min. XES also indicated a ratio of Mn:Si of approximately 2:1, based on relative peak heights. 19 A coating formed at high temperatures (;:::; 550°C) was examined by transmission x-ray diffraction, as described in the experiment section. Even with long exposure only a cou ple offaint, broad, evenly spaced rings became visible on the photographic film, indicating that the coating had either an amorphous microstructure or very fine microcrystallites « 10 A).20 100 X.-----------------------Io~------------------------------~6 9 Relative 8 _ --'-.-..... li· /x Abundance 7 ~. ~ 6 X-x ~ _-0----0 O (%) 5 x--.. 0--/' --0, / 0<:' /0 ..... 0..... 4 /x ~,,-6:':'-_0--0--0 '0 3 X ....... ~q)-:::-7 I , 2 /' ___ ,.~ .... N Q) 0 " I ;X' 1) .... ---g/ .. 6····· g 60 I 0, ~16~~lg7~1~8~~19~~20~~2~1~2~2~2~3~~24~~2~5~26· o 2! 50 '0 Electron Impact Energy (eV) E I "0 « 40~ I ..... ,0...... .~ I x '0... § 30 I \ "0_ --0_-,", &!. Iv--. -0--0 20~ I x, /6'"'--A--:-:~:-::-:'"'~-~·..:.l!~.:·:.:.g:.;.:·:..:.Z:.::..:.:.X:.:.:.:.:.X x............. CJ.--·-O·-·-:;~:'_._ ... 'i/.... I I Or x-:;;-< "," 0"""'-0. _._ r 0 x_ .. , .. 1il 0--·-0-'-0 . . / ... L>-X~v __ v..x -'-0'-'-0'-'-0-'-0 I ." , ." ~ --x--x--x~_x I OL6I O-·CJ! I 4'" I'i/ ... r··'i/I 1 I 1 1 I I 1 1 -I-X-I-X 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 ~ 70 90 80 Electron Impact Energy (eV) J. Vac. Sci. Techno/. A, Vol. 7, No.3, MaylJun 1989 .... '1 .. FIG. 2. The breakdown diagrams for [Mn(CO)s], (WSiH,) derived from elec tron impact lEC's. Relative intensities are plotted as a function of electron impact ener gy, with an intensity of 100% implying that this is the only observed fragment. Main plot: X-parent (Mn2SiH2)(CO),b ion, 0- (Mn2SiH2)(CO).;', D-'-(Mn2SiH2)(CO),;', /':,.---(Mn2SiH2) (CO),+, and \7'.' (Mn,SiH1)(CO)4~' Corner inset plot: x (Mn2SiH2) (CO)t, 0--(Mn2SiH2) (COlt, D---(MnzSiHz)(CO), ,/':,.---(Mn2SiHz) '. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.69.47 On: Sun, 23 Nov 2014 00:33:57637 Stauf et al.: Decomposition of [Mn(CO)5MwSiH2) Auger electron spectroscopy was the primary method used for compositional analysis of the thin films. AES spec tra were collected for coatings made at a variety of different substrate temperatures. Argon ion sputtering was used in each case to remove the top 200-300 A of the surface in order that the underlying material composition could be assessed without surface contamination, providing of course that ion induced mixing and preferential sputtering are negligible. 21 Auger analysis was performed at intervals during the sput tering process, thus producing a depth profile of elements. With the aid of these spectra, a plot of average bulk composi tion of the films versus substrate temperature at formation was created. This plot can be seen in Fig. 3. Rutherford backscattering spectroscopy also provided very useful information on coating structure and composi tion. When He f incidence angles 0[7° and 45° were used, the resulting spectra were virtually identical, indicating homo geneity throughout the bulk of the coating. Computer mod eling of the spectra suggests that the thin films ~eposited at 5OO"C are composed of a surface region, 775 A thick, for which the composition (at. %) is 38% Mn and 19% Si, the rest being oxygen. Below this surface layer, the bulk of the sample is estimated from modeling to be 44% Mn, 22 % Si, and 33% 0 for the films deposited at 400, 450, and 500°C, respectively. The Mn, Si coating was found by RBS to be 3.4 {lm thick, following ~ I-h exposure of the nickel foil at 500 0c. This corresponds to a deposition rate of 570 A/s. The films made at lower temperatures (though still above 250°C) were thicker than this. Carbon concentration was found to be 5% or less for films deposited at 400, 450, and 500 ec, while oxygen was < 33%. The manganese edge was found to become less abrupt and cornerlike at higher film deposition temperatures. This is explained by an increasing amount of oxygen ~ with increas ing temperature from 400-500 DC) in a 2000-A selvedge re gion near the surface. 100 x __ x ______ i~/~ x __ x_l!. .i?j 60 Co ~:~ x\ :-D~a<-D ~ 20 0 y ----6 &->~~~ ~ I 0 +"r;:/o~+ "'--+ o o -L...-L.. f I I ! J ' ! I 1.0 200 300 400 500 Foil Deposition Temperature (degrees C) FIG. 3. The composition (AES) of films formed via the thermal decomposi tion of (WSiH2) [Mn(CO), 12 as a function of the foil temperature. Sput· tering as mentioned previously was performed to remove surface contami.n ation, but no correction has been made for ion mixing or preferential sputtering. The attenuation of the XES nickel signal has also been plotted ( >< ) to provide an indication of the coating thickness. )< -inverse Ni seen (right axis), 0-Mn signal seen, 0-Si signal, 6-carbon signal, and + oxygen signal. J. Vac. Sci. Technol. A. Vol. 7, No.3, May/Jun 1989 637 IV. DISCUSSION Table I shows both the higher appearance potentials of the parent IEC and the AP's of the fragment ions. An increase in the slope ofthe parent ion IEC is seen at 8.5 eV, close to the 8.4 e V at which the (Mnl SiHz ) (CO) 9+ ion appears. This indicates that the fragmentation mechanism involves a pre dissociation excitation, i.e., dissociation associated with an electronic excitation to an antibonding orbital. This partial filling of an antibonding orbital would weaken the carbonyl metal bond leading to creation of the (Mn2SiH2) (CO)9+ ion. A downward slope change would indicate a direct bond breakage between the parent and the associated carbonyl fragments, since it decreases the amount of parent ion pres ent while increasing the amount of the fragment ion seen. These conclusions have been presented in more detail pre viously,15 and have been supported by our photoabsorption experiments. From the IP and AP information in Table I, we were able to construct an ionic thermodynamic cycle for this com pound. 15 This thermodynamic cycle is shown in Fig. 4. We can then conclude that the total energy to go from the parent carbonyl to the Mnl SiH, core is D [(Mn2SiH2)(CO) l~ -IO(CO)] = AP [MN2SiHt ] -IPl (Mn2SiH2)(CO) HI] = 18.9 -7.9 = 11.0 eV . From the relative abundances of the parent ion and differ ent fragment ions (shown in Fig. 2) the (Mn2 SiH2 ) (CO) lb Mn2SiHZ(COllo-....:.7:.,::.9-_. Mn2SiH2(CO);o ----, I ~O.5 MnzSiH2(CO); 18.9 ~ 1.0 Mn2SiH2(COl: ~IA Mn2SiHz (COl; !0.2 Mn2SiH2(COl: ~ 1.6 Mn2SiH2(COl; ~ 1.0 MnzSiH2(COl: ! 1.1 MIl2SiH2(COl; I .1.7 MnzSiH2(COl; !O.7 Mn2SiHz (cot ~ 1.8 L-_____ .-Mn 2SiH 2+ 11.0 FIG. 4. The decomposition thennodynamic cycle for ionic fragmentation of [Mn(CO), L (Il·SiR2), constructed from the AP/IP information in Table I. The average AP and IP values listed (average between electron and pho toionization data) were used. All numbers are in units of e V. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.69.47 On: Sun, 23 Nov 2014 00:33:57638 Stauf et at.: Decomposition of [Mn(CO)5],(WSiH2) parent ion is seen to readily dissociate at energies greater than the appearance potential at-8 eV. The first (and at low energies certainly the major) fragment formed is (Mnz SiR2 ) (CO) 9+ • Other fragments put in appearances at higher energies, and for the most part remain relatively con stant in percentage composition. An exception is the Mnz SiR2t fragment, which increases in percentage abundance with increasing electron impact energy range near 26 eV. This indicates that even in the high electron impact energy environment of a plasma, the core manganese-silicide clus ter would survive the stripping away of carbonyl ligands, offering hope of deposition as a thin film with a wen-defined metal-to-silicon ratio of 2: 1. The bonds between the two Mn's and the Si are obviously stronger than the bonds be tween each Mn and its CO ligands. The activation energy for CO cleavage from r Mn (CO) 5 J 2 (fL-SiH2 ) is similar to that observed for other metal carbonyl species,22-z6 indicating that this molecule is fairly representative of the metal car bonyls despite having a three-atom metal center (p-SiMn2 ). The application of thermodynamics to thin-film deposition is obvious in this instance; with the right deposition param eters, the CO ligands will be removed while leaving the prop er stoichiometric Mn:Si ratio. Results of pyrolytic coating experiments support these statements. The primary means used to analyze composition and contaminants in these thin films were AES and RBS. Table II provides a summary of these results for coatings made at various temperatures. Discussion of the apparently rather high contamination levels follows. From AES, the onset of thermal decomposition of (,u-SiR2) Mn (CO) 5 ] 2 on the surface of a nickel foil is at ~225 0c. At this temperature it seems that [Mn(CO)s]2 (SiRz ) is undergoing disproportionation with loss ofSi, pos sibly in the form of silane, as discussed previously. This is leading to films with considerable carbon incorporation, and a manganese-to-silicon ratio of ;::::4:1. Raising the tempera ture of pyrolysis eliminates this effect, however, producing the ratio 2: 1 which is the same as in the source molecule. Pyrolysis of a similar compound, Mn (CO) 5 (SiR3 ), at 773 K in a flow system, has been seen to result in a mixture of the MnSi-Mns Si3 phases.7 The presence of carbon and oxygen in the MIlz Si coatings at higher deposition temperatures could be the result of sev eral different processes. One is incorporation of methane and T AR!E n. Summary of Rutherford hack scattering and Auger electron spec troscopy results for contamination and Mn:Si ratios. Mn-to-Si ratio Contamination Temperature -"---~"----"-~-.----- (,Cl AES RBS AES RBS 200 1.94 13% O. 37% C 225 4.33 9% O. 76% C 250 3.30 17% O. 27% C 300 2.09 13% 0,18% C 400 2.16 2.0 15% O. 37% C <33%O,<5%C 450 2.67 2.0 24% 0, 20% C <33%O,<5%C 500 2.10 2.0 22% 0.18% C <33%0.<5%C J. Vac. Sci. Technol. A, Vol. 7, No.3, May/Jun 1989 638 carbon monoxide from background gases in the chamber, quite possible at a vacuum of only 10 5 Torr. The absence of an XRD pattern suggests that the thin film is amorphous, so impurity incorporation could occur readily without disturb ing the silicide lattice structure. Diffusion is another potential source of contamination. At high temperatures, the diffusion of carbon and oxygen from the thin film-nickel substrate interface will occur.27 Given the cracked and granular nature of the thin film as seen in SEM photographs, we would expect a large number of inter faces akin to grain boundaries within the coating. These in terfaces could provide excellent diffusion pathways for car bon and oxygen for the films prepared at 400-500 °e. The high metal content of the thin films results in a very reactive surface, particularly with the metal manganese which oxidizes readily. The cracks and grain boundaries of the film provide additional surface area. Exposure to air, following preparation of the sample, is probably thus the origin of the heavily oxidized surface layer observed in RBS. The change in the manganese edge shape, mentioned pre viously, indicates that there is a gradual change in the oxy gen concentration from the surface to the bulk in a selvedge region 2000 A thick. This monotonically changing oxygen concentration is representative of oxygen diffusion from the surface after the film is deposited. If contamination were taking place during coating deposition, a uniform distribu tion would be expected. If it were a result of direct CO incor poration, the RBS spectra would be expected to show com parable amounts of carbon. The diffusion of oxygen into the thin film does depend on the deposition temperature at which the thin film is made. The oxygen diffusion layer was seen to have a higher concentration of oxygen near the sur face with films deposited at 500°C than at 400 °C. As previously mentioned, Table II presents our contamin ation analysis. While the Auger spectroscopy did detect oxy gen and carbon impurities, RBS indicated a much lower lev el « 5%) of ear bon than did AES (lowest number 18%). The system in which the AES was carried out was pumped with an oil diffusion pump, and it is possible that some of the carbon detected was introduced by ion mixing of surface impurities during the sputtering process. Since RBS is a less surface sensitive process, we believe it to be more representa tive of the true bulk composition of this coating. V. CONCLUSION It seems then that pyrolysis of (/l-SiR2) [Mn(CO)s]2 does create the desired Mn2 Si compound, if the right range of substrate temperatures is maintained. The thin films that result are isotropic and uniform in content, apart from the oxygen impurities diflusing into the films from the surface. The high manganese content renders them very susceptible to oxidation upon exposure to air. If deposition and analysis were performed in URV in situ, contamination levels might prove to be less than the 5% carbon and 18% oxygen indi cated by our RBS and AES analyses. The consistent 2: 1 ratio of manganese to silicon from both AES and RBS at reaction temperatures over 300 °C is also very encouraging, as is the stability of the metal three-atom Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.69.47 On: Sun, 23 Nov 2014 00:33:57639 Stauf et al.: Decomposition of [Mn(COlsMwSiH2) center. It indicates that pyrolysis of this complex results in loss of CO, with no sHyl or silane by-products created. ACKNOWLEDGMENTS This work was funded by the U.S. DOE through Grant No. DE-FG-02-87-ER-45319, Bundesministeriun filr Fors chung und Technology (BMFT) Contract No. 05313FXB3TPl, the Freie Universitat Berlin, and the Syra cuse University Senate. We would also like to thank the staff of the Synchrotron Radiation Center (SRC) at Stoughton, WI and BESSY (Berlin, FRG). The SRC is supported by the National Science Foundation through Grant No. DMR- 86-01349. IPA. Dowben, J.T. Spencer and G.T. Stanf, Mater. Sci. Eng. B (in press). 2D. A. Prinslow and V. Vaida, J. Am. Chern. Soc. 109, 5097 (1987). 'J. Haigh, J. Vac. Sci. Technol. B 3, 1456 (1985). 4M. G. Jacko and S. J. W. Price, Can. J. Chern. 41,1560 (1963). 5M. G. Iackoand S. J. W. Price, Can. I. Chern. 42,1198 (1964). 6S. J. Aylett and J. M. Campbell, J. Chern. Soc. A 1969,1916. 7.13. J. Aylett and H. M. Colquhoun, 1. Chern. Soc. Dalton Trans. 1977, 2058. sB. Aylett and J. M. Campbell, J. Chern. Soc. A 1969, 1910. "Y. L. Baay and A. G. MacDiarmid, Inorg. Nue!. Chern. Lett. 3, 159 (1967). J. Vac. Sci. Technol. A, Vol. 7, No.3, May/Jun 1989 639 lOA. J. Chalk and J. F. Harrod, J. Am. Chern. Soc. 89,1640 (1967). lly. J. Kime, D. C. Driscoll, and P. A. Dowben, J. Chern. Soc. Faraday Trans. 283,403 (1987). l2G. T. Stauf, D. C. Driscoll, P. A. Dowben, S. Barfuss, lmd M. Grade, Thin Solid Films 153, 421 (1987). l3B. J. Aylett and H. M. Colquhoun, I. Chern. Res. Synopses 6, 148 (1977). 14K. M. Abraham and G. Urry, Inorg. Chern. 12(2),2850 (1973). ISG. T. Stauf, D. A. LaGraife, P. A. Dowben, K. Emrich, S. Barfuss, W. Hirschwald, and N. M. Boag, Z. Naturforsch. Tei! A 43,758 (1988). !6G. T. Stauf, P. A. Dowben, K. Emrich, S. Barfuss, W. Hirschwald, a.l1d N. M. Boag, J. Phys. Chern. (in press). 17M. Grade, J. Wienecke, W. Rosinger, and W. Hirschwald, Ber. Bun senges. Phys. Chern. 87, 355 (1983). 18W. Rosinger, M. Grade, and W. Hirschwald, Ber. Bunsenges. Phys. Chern. 87,536 (1983). 19G. T. Stauf, P. A. Dowben, No M. Boag, L. Morales de la Garza, and S. L. Dowben, Thin Solid Films 156, 327 ( 1988). 2°D. B. Cullity, Elements a/X-Ray Diffraction, 2nd ed. (Addison-Wesley, Reading, MA, 1978). 2'A. W. Czanderna, "Methods and Phenomena: Their Application in Science and Technology," in MethodsofSuiface Analysis, edited by A. W. Czanderna (Elsevier, Amsterdam, 1975), Vol. 1. 22C. M. Melliar Smith, A. C. Adams, R. H. Kaiser, and R. A. Kushner, J. Electrochem. Soc. Solid State Sci. Technol. 121, 298 (1974). 2.lH. E. Carlton and J. G. Oxley, AIChEJ. 13, 86 (l967). 24H. E. Carlton and J. G. Oxley, AIChE J. 11,79 (1965). "R. K. Chan and R. McIntosh, Can. J. Chern. 40, 845 (1962). 2"J. S. Foord and R. B. Jackman, Chern. Phys. Lett. 112,190 (1984). 27p. A. Dowben and M. Grunze, J. Electron Spectrosc. Relat. Phenorn. 28, 249 (1983). Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.69.47 On: Sun, 23 Nov 2014 00:33:57

1.100232.pdf

Sensitive, highly linear lithium niobate interferometers for electromagnetic field sensing C. H. Bulmer Citation: Appl. Phys. Lett. 53, 2368 (1988); doi: 10.1063/1.100232 View online: http://dx.doi.org/10.1063/1.100232 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v53/i24 Published by the AIP Publishing LLC. Additional information on Appl. Phys. Lett. Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 29 Jun 2013 to 130.102.42.98. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsSensitive, highly linear lithium niobate interferometers for electroma.gnetic field sensing c. H. Bulmer Na/lal Research Laboratory, Optical Sciences DilJisiofl, Washington, DC 20375-5000 (Received 29 July 1988; accepted for publication 3 October 1988) An asymmetric Mach-Zehnder interferometer in Ti:LiNbOJ has been demonstrated to have a 84 dB linear dynamic range and L 1 flY sensitivity, for a 3 kHz detection bandwidth and a 50 n resistance, at the 1.3;tm wavelength. This device is useful for electric and magnetic field sensing. Optimum linearity is achieved with a 90° intrinsic phase bias. The dependence of dynamic range and sensitivity on optical power, phase bias, and modulation voltage is reported. The reasons for, and magnitudes of, deviations from optimum linear behavior are described for many fabricated interferometers. An integrated-optical modulator is of use for electro magnetic field sensing as the active device can be smail, mini mally perturbing, and highly sensitive. A linear modulator is desired, which is passively biased for optimum performance. The behavior of such a Ti-indiffused LiNb03 modulatorl sensor has previously been reported. 1-3 In this letter we re port linear dynamic range and sensitivity measurements at the 1.3/im wavelength. We also describe the dependence of dynamic range and sensitivity on modulator parameters and optical power. The effect and causes of fabrication errors which produce deviation from perfectly linear behavior are discussed. The device is a channel waveguide Mach-Zehnder in terferometer, as in Fig. 1, where a difference in interferome ter arm lengths, 6.L, gives rise to an intrinsic phase bias 4;0 given by (1) where nell' is the mode effective index which is the same in each arm. The two arms differ in length but are otherwise nominally identical; the branches are symmetric with a 0.60 half-angle. For optimum linearity ¢() = 90", which corre sponds to 6.L = 152 nm at the operating wavelength.,{ = 1.3 11m. The total arm length is approximately 20 mm. The de vice is formed in X-cut, Y-propagating LiNb03 in order to both avoid pyroelectric effects causing temperature instabil ity4 and also to obtain good voltage sensitivity. Electrodes are placed symmetrically to either side of the waveguide arms and a dipole antenna mighi be connected to the elec trodes in order to modulate the optical output as a function of electric field. Here we describe operation for a modulating voltage V directly input to the electrodes. For unity input power the output power Po is defined by PI' = (/2) [1 + cos(tpo + 1TVIVr,»), (2) with the modulation voltage V1T given by V". = .,{gl(2on;~r33L), (3) where 15 is an electrode efficiency facior, depending on the overlap of the mode optical field with the applied electric field, ffe is the extraordinary refractive index, r~B is the ap propriate electro-optic coefficient, and Land g are the elec trode length and gap width, respectively. For sensing applications the input voltage Vis desired to produce linear modulation of the optical output fundamen- 2368 AppL Phys. Lett 53 (24), 12 December 1968 tal frequency component. The lower end of the linear dy namic range is defined as the point where V = VL produces a fundamental signal equal to the noise due to the unmodulat ed power level and the upper end as the point where V = Vu produces any higher harmonic equal to the noise. The dy namic range (DR) is then 20 log 10 ( VjVL). This is the spurious-free dynamic range.s If rPo = 90° exactly, only odd harmonics are produced by the input modulating voltage; otherwise, even harmonics are also present in the output. For tPo#90c the upper limit is generally set by the second harmonic (V"2); the third harmonic magnitude (Vu3) is dominant only for an error in tPo of S; OS. A given detector is either thermal or shot noise limited, depending on the opti cal power Ieve1.2 Figure 2 shows theoretical plots of DR as a function of the optical power at the detector, for varying error in the desired value of rPo = 900 and for bandwidth B = 3 kHz, detector load resistance R = 50 n, and detector responsivity r= 0.6 A/W (as for a typical GalnAs pin di ode). Values are determined by expanding Eq. (2) as a har monic series using V = Vc sin illt. For optical powers of < 1.7 m W, the detector is thermal noise limited. Powers of -1 mW at the detector are typically achievable with presently available laser diodes. For thermal noise limited detection and an error in ¢o such that linearity is second harmonic limited, DR is directly proportional to the optical power at the detector. The dynamic range decreases rapidly with in creasing errror from ¢;o = 900, especially for small errors. For instance, for 1 mW power, DR is 86.3 dB for zero error, SINGLEMODE CHANNEL WAVEGUIDE FIG. 1. Asymmetric interferometer with horizontal fidd electrode struc ture. 2368 Downloaded 29 Jun 2013 to 130.102.42.98. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions~ 10 ----------------r-----r---------r-----r-----'-"--r ---~l ' fl THERMAL NOISE lIMITED--J ----~ SHOT NOISE lIMITW I I 0' 1 lOOt- I __ -----j I _--- O.~ I -___ 3'd KARMONIC LIMITED ---- ~~10 --2nd HARMONIC LIMITED _--:--_---- BOr /~------==--====-~ :.: Bof /~~----===--~ I ~------~~~ I F-~-/---...__--- -----I ~ 70~ ~ ~--- I I ~~: I 6C~- ------>-----'------'-- ----__ . .L ____ .1 _________ __ L ____ J C.l 1.0 10 100 OPTICAL POWER AT lJETEC';'OR ImW) FIG. 2. Linear dynamic range as a function of optical power at the detector and varying error [rom ,po = 90', for B == 3 kHz, R .= 50 n, and r =,0.6 A/W. 81.4 dB for 1° error, 78.4 dB for 2° error, and 74.4 dB foy 5° error. The modulator sensitivity is VL, which is the lowest in put voltage which can be detected. The lower VI., the greater the sensitivity. The sensitivity degrades only very slightly with error in ¢o and depends mainly on the optical power level at the detector and on the "electro-optic efficiency" of the modulator, as determined by the modulation voltage V iT' (In contrast the dynamic range is independent of V". as both VL and V" vary by proportionate amounts for changes in V".) The sensitivity VL is plotted in Fig. 3 as a function of optical power for.po = 90e and V7T = 2 V (typical ViT for our sensors; corresponding to L = 14 mm, g ~ 8 pm, and 8-0.6). For 1 mW power, the voltage sensitivity is -1 tlV; for $0 = 900, VL = 1.06 /-l V and for $0 = 900 ± 100, VL ~ L07 f1. V. The negligible change in VL for a 10° error indicates how tolerant device sensitivity is to deviations from .po = 90°. Interferometers were fabricated from Ti strips, ~ 6.5 f1.ID wide and -46nm thick, diffused at 1000 °C for 10 h in a wet oxygen atmosphere. Al electrodes, 0.5 fim thick and 14 mm long, were defined to either side of the channel wave guides with a gap of ~ 8 pm. The TE (extraordinary) mode was launched in the LiNbO, using a high-power laser diode. The modulation voltage V7T was measured as 2.0 V. The an gle .po was determined as 89S by measuring the voltages 5~ I 21 1.0~ I THER'IIIAL NOISE LIMITED 0.1 . I SHOT NOISE L1MIYED :::[1'_ o.a~.l ----~--,:0 -J--~-1O_-------'-Ma OPTICAL POWER AT DETECTOR (mW) FIG. 3. Sensitivity as a function of optical power at the detector for </>" = 90', Vr, =~ 2 V, B = 3 kHz, R 0= 50 n, and r = 0.6 A/W. 2369 App1. Phys. Lett., Vol. 53, No. 24, 12 December 1988 ..... -.: ......•.•. -;-: ... ;;.: .... '.~.'~.'7 .-.';.;.;O;'."' •••• ~.: •••••• ·.v.·.·.·.;·; •.•.••••••••. ;-•.• -:0 .... '~.'. Vmax and V;,,;n required to drive the output to a maximum and minimum, respectively; then V7T = i v'nax I + I Vrnin I and .po = rrVmax/VTT• The total loss from input to detector was 9 dB and 0.85 m W light was incident on the detector. The loss includes 3 dB loss intrinsic to the linear interferometer com pared to an interferometer with symmetric arms or.po = 0°. The dynamic range was determined by applying a sinusoidal voltage at 8 kHz to the electrodes and measuring the output harmonic magnitudes using a spectrum analyzer and lock-in amplifier. (The modulator 3 dB bandwidth was separately measured, using the swept frequency technique,6 as 570 MHz). The detector (0.65 A/W responsivity) was thermal noise limited at Vth = -146 dB V for the terminating resis tance R = 50 n and bandwidth B = 3 kHz. ( Vo. = f4k~-tB.if, where T = 300 K and kb = 1.38 X 10-23 J/ K, was both calculated and checked experimentally). The corresponding DR was 84 dB, limited by the second har monic, and the sensitivity was 1.15 Ii V, as shown in Fig. 4. These values are in excellent agreement with theory. Scaling them for 1 m W incident light gives DR = 85 dB and a sensi tivity ofO. 98!-t V. Scaling to any power or any detector can be achieved by noting that VL a: Pth I P, VU2 a: (PthIP)1/2, v,t3 0:. (PthIP)l!3,wherePth is the optical power at the detector corresponding to thermal noise (PtJ; = Vth / Rr) and P is the incident optical power. The high dynamic range ( > 80 dB) and fi V sensitivity were achieved by maximizing the output power and by maxi mizing the modulator efficiency (8 = 0.65) or minimizing V Also an interferometer with only a OS error from the 90° phase bias was used; this smail error was critical for high DR although not for good sensitivity. We aim to achieve good linearity passively withollt, for instance, using a dc bias or any subsequent fabrication tuning.7-9 Perfectly linear oper ation requires a small but precise degree of asymmetry in the interferometer geometry and errors in photolithographic processing or mask making produce errors in ¢o . Using a mask set with 25 interferometers spaced at 0.5 or 0.75 mm intervals across the LiNbO J substrate, we have measured and compared f/Jo errors for given interferometers on different substrates. For a given device .po can be mea sured repeatedly to ± 0.40 accuracy. In our case random fabrication processing errors seem to predominate rather than mask errors. Many experimental variables can result in :rf~5~·~··cr~·~;/~l ,of 1 ~ ~ -i ~ -90 L o -110 I -13G 1- -:501 ------.-----84d8---------., IR~50D g~o3kH'1 Ii 1.15,N 18.2 mV i ' ~ ___ L __ ....! ___ .l. ___ L ___ L __ -L ___ L ___ J __ L -' ___ ...L ___ L ___ L-...-: -'20 -'00 -80 -60 -40 -20 G INPUT IdB\I) FIG. 4. Experimental interferometer response; ¢e'= 89S; 0.85 mW at de tector. C. H. Bulmer 2369 Downloaded 29 Jun 2013 to 130.102.42.98. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsnon uniformity between the two interferometer arms which are spaced by 80 11m. For the 20 mm arm length, a mode effective index difference between the arms of i:>.nctr = 1.8 X 10--6 produces a 10° change in ¢o' Such a slnaH dndf might be caused by variations in channel width, Ti thickness, electrode dimensions and alignment, LiNb03 dif fusion parameters, or LiNb03 material composition. We achieve a-10% fabrication yield for devices with 88°~<p()<92°, corresponding to linear dynamic ranges of > 78 dB, for 1 m W power and the detection parameters pre viously stated. On several substrates, after initial ¢() measurements, the electrodes were etched away, new electrodes were defined using the same mask, and the devices were remeasure(t This procedure was repeated up to four times and changes in ¢;o from one electrode set to another were noted. The average magnitude of 6.60 was 2.6". Although this variation is small it can significantly affect dynamic range. Any source of stress can cause small variations in ¢;o and should be avoided for optimum dynamic range. The addition of an insulating layer over the electrodes results in small changes in ¢;o' A O.5-,lm thick photoresist overlayer caused ¢(J of 50 interferometers to change by an average ofl.O°. The addition ofLiNb03 edge blocks used in end polishing has sometimes caused ¢() of some interferometers to change by up to 6". Dicing a 25-mm wide LiNbO-, substrate with many interferometers into 2370 AppL Phys. Lett., Vol. 53, No. 24, 12 December 1988 pieces approximately 3 mm wide caused no change in <Po of half the devices. Overall the average change in <Po was 2.80 and the maximum change was g". In conclusion, the theoretical dependence of inter fer om eter linear dynamic range and sensitivity on optical power, intrinsic phase bias <Po, and modulation voltage has been described. An experimental device with 84 dB dynamic range and 1.1 J.i V sensitivity has been reported. These char acteristics make the interferometer useful as an electric field sensor. Factors causing <Po to vary from the ideal asymmetric 90" value were described. They should be avoided, if possible, for optimum linear performance of interferometric sensors. Grateful acknowledgment is made to S. C. Hiser for technical assistance, W. K. Burns for useful discussions, and M. L Rebbert for photolithographic work. 'e. H. Bulmer, W. K. Burns, and R. P. Moeller, Opt. Lett. 5, 176 ( 1980). 2e. H. Bulmer and W. K. Hums, 1. Lightwave Techno!. I,T-2, 512 (1984). 'e. H. Bulmer and S. C. Hiser, SPIE 517, 177 (1984). -'c. H. Bulmer. W. K. Burns, and S. C. Hiser. AppL Phys. Lett. 48, 1036 ( 1986). 'B. H. Kolner and D. W. Dolfi, Appl. Opt. 26, 3676 (1987). (,S. Uehara, App!. Opt. 17. 68 (1978). 70. Mikami, J. Noda, S. Zembutsu, and S. Fukunishi, App!. Phys. Lett. 31, 376 (lCJ77). '0. Mikami and S. Zt'mbutsll, Appl. Phys. Lett. 35, 38 (1979). "M, J. Ahmed and L. Young, App!. Opt. 22, 40~2 (1983). C. H. Bulmer 2370 Downloaded 29 Jun 2013 to 130.102.42.98. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

1.97966.pdf

Abnormalglowdischarge deposition of tungsten K. E. Greenberg Citation: Applied Physics Letters 50, 1050 (1987); doi: 10.1063/1.97966 View online: http://dx.doi.org/10.1063/1.97966 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/50/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optogalvanic effect and measurement of gas temperature in an abnormal glow discharge Appl. Phys. Lett. 89, 131502 (2006); 10.1063/1.2352793 Deposition of trimethylsilane in glow discharges J. Vac. Sci. Technol. A 19, 773 (2001); 10.1116/1.1365137 A model of dc glow discharges with abnormal cathode fall J. Appl. Phys. 67, 154 (1990); 10.1063/1.345294 A CathodeAnode Configuration for an Abnormal Glow Discharge with Long Term Stability Rev. Sci. Instrum. 43, 703 (1972); 10.1063/1.1685737 The Deposition of Atoms Sputtered in an Abnormal Glow Discharge J. Appl. Phys. 38, 2960 (1967); 10.1063/1.1710032 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 138.251.14.35 On: Mon, 22 Dec 2014 20:16:28Abnormai~glow .. discharge deposition of tungsten K. E. Greenberg Sandia National Laboratories, Laser & Atomic Physics Division, P. O. Box 5800, Albuquerque, New Mexico 87185 (Received 3 December 1986; accepted for publication 20 February 1987) a-tungsten films that adhere well to silicon dioxide were deposited using a dc abnormal-glow discharge through WF6, H2, and Ar. Film resistivities on the order of 30 I-lfi cm and deposition rates as high as 200 A/min were obtained without heating the substrate externaHy. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and transmission electron microscopy measurements indicate that electron scattering at the grain boundaries has limited the conductivity ofthc plasma-deposited films. Tungsten films having resistivities within a factor of two times that of bulk tungsten were produced with a two-step process utilizing plasma and conventional chemical vapor deposition. Tungsten is one of the refractory metals being consid ered as a repiacement for aluminum in integrated circuits. It has a higher melting point than aluminum and is more resis tant to electromigration, hillock formation, and spiking. While it has been possible to selectively grow tungsten on silicon by ehemicalvapor deposition, 1-3 it has been difficult to obtain high quality tungsten films that adhere well to sili con dioxide, a necessity for broad-area metallization. Here we describe the plasma deposition of tungsten films that ad here wen to silicon dioxide, aluminum, silicon, and galIium arsenide. The films were deposited at temperatures below 70°C (purely a plasma deposition process) using a de abnor mal-glow discharge4 through a mixture ofWFIi, H2, and Ar, Although the 2s-depositcd film resistivities typically have been greater than 30 pH em (the deposition process has yet to be optimized), we have found that low resistivity materia] ( < 10 pO. em) that adheres well to silicon dioxide can be produced by plasma depositing 30-100 A of tungsten and growing a subsequent layer by conventional chemical vapor deposition (CVD). Figure 1 is a schematic diagram of the experimental ap paratus. The cathode, a stainless-steel plate, was separated from a stainless-steel cross (1.375 in. Ld.) by an insulating ring. The cathode was driven negative while the remainder of the system was maintained at ground potential, A turbo molecular pump evacuated the cell to a base pressure of 5X 107 Torr before WF" (Spectra Gas 99.88% purity), H2, and argon were introduced into the system through mass flow controllers. The total gas pressure was typically 100- 300 mTorr with the individual flow rates being less than or equal to 10 seem. While tungsten was deposited on all of the materials listed above, the remainder of this paper will per tain to oxidized silicon substrates (8000 <h.. of thermal oxide on top of silicon) unless noted otherwise, A number of workers have reported that the adhesion of thin metal films can be greatly improved by high-energy eiectron, photon, or ion hombardment.'-7 Similarly, when the substrates were placed directly on the cathode of our abnormal-glow discharge in order to receive ion bombard ment during film growth, it was possi.ble to obtain tungsten films that adhered weH to the underlying materiaL The ener gi.es of the ions impinging on the substrates in our experi ments were much lower than those used previousIy7 since the maximum energy an ion eouid attain in our configura tion was 2000 eV, corresponding to the 2 kV discharge vol tage. In addition, the majority of the ions reaching the cath ode probably had energies less than 1 keY due to charge exchange collisions in the cathode fall region.8 Although the energies were much lower than those in the previous studies, the ion bombardment significantly increased the adhesion of the deposited films. Furthermore, the substrates had to be placed on the cathode for the films to adhere. The cathode was not heated externally and, for all ofthe discharge conditions reported here, the highest temperature measured for the cathode was 70 DC, immediately after a 30- min deposition, Since thermal tungsten CVD does not occur at 70 ·C, aU of the deposition could be attributed to plasma processes. This was also confirmed by the fact that the depo sition rate was independent of the deposition time (see be low). Any thermal contribution to the deposition process would have been reflected by a nonlinear deposition rate with respect to time due to the high thermal capacity and long thermal equilibration time constant of the cathode. (It could be possible, of course, that the substrate surface was at a higher temperature than the cathode. However, simple cal culations based on thermal conductivity indicate that the temperature of the substrate surface should not have been significantly elevated such that thermal CVD would occur.) The adhesion of the deposited films was determined us ing the adhesive tape test. As an added test, a section of the FIG 0 I. Schematic diagram of the experimental apparatus, 1050 App!. Physo Lett 50 (; 6), 20 April 1987 0003-695 i /87/161050-03$01000 @ 1987 American Institute of Physics 1050 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 138.251.14.35 On: Mon, 22 Dec 2014 20:16:28film was ablated using a Kr F laser (248 urn) and the remain der was tested again with tape. For films that adhered well, low KrF laser ftuences on the order of 300 mJ/cm2 would cleanly ablate the tungsten from the oxide without damaging the underlying layer. In aU cases, the part of the deposited film which was not irradiated by the laser still adhered well. In addition, post heating at 500°C in vacuum did not signifi cantly alter the adhesion. Figure 2 shows the thickness and resistivity of the de posi.ted film as a function of deposition time for a discharge voltage of2000 V, a current of5.0 rnA, and WF6, H2, and Ar partial pressures of 10, 150, and 100 mTorr, respectively. The resistivity was determined using a four-point probe and the film thickness was obtained by ablating part of the depos ited film with the KrF laser and measuring the resultant step height with a stylus profiIometer. Chemical etching of the films confirmed that the laser cleanly removed the tungsten without affecting the underlying oxide. As shown, deposi tion began immediately (zero induction time) at a rate on the order of 150 A/min, Unlike the resistivity of cOllvention al CYD tungsten, which may decrease with increasing film thickness due to an increase of the grain size, ') the resistivity of the plasma-deposited fUm was independent of the film thickness. In addition, for these discharge conditions, the resistivity was on the order of70 pU em which is significant ly greater than the bulk resistivity of tungsten, approximate ly S.5pn em, The properties of these films were investigated using a number of surface analysis techniques in order to explain the high. film resistivities. X-ray diffraction patterns were ob tained for a number of 70 ,til em films with thicknesses between 700 and 1300 A. In all cases, the diffraction peaks corresponded to a-tungsten and no trace of the h.igher resis tivity, metastable p phase oftungsten was detected, This is in sharp contrast to the results obtained by Tang and Hess for the plasma-enhanced CVD of tungsten. 10 In the plasma· en hanced CVD studies, an rf discharge was used and sapphire substrates were placed on a temperature-controlled elec trode. For electrode temperatures less than 250°C, Tang and Hess detected only p-tungsten and their films had resistivi ties about 120 pO em. Since the phase of the deposited film can be greatly influenced by ion bombardment, we believe 30CO G3 :<500 III III 2000 W :Ii: :.; l) :i: 11;00 !-V~2()(laV.1 ·S.O rnA ./' I CJ __ Ij 0 _~-'['-~ _~ """'":r-........ _. 175! o 10 .:;. >- -5<) ~ 1= III 00 p(WF5) ~ iC mT~" 25 ~ l '" p{Ha} =~50 mTorr peAx) ~ 1'00 m70rr I '0 15 20 25 T!ME (millutee) FIG. 2. Film thickness and resistivity as a function of deposition time. 1051 Appl. Phys. Lett., Vol. 50, No, 16,20 April 1987 that a-tungsten films were obtained wi.th the abnormal-glow discharge due to the fluxes and energies of the ions imping ing on the substrates. The cleanliness of our vacuum system may have also contributed to the production of a-tungsten as the presence of oxygen or other impurities enhances the for mation of the beta phase, II Auger analyses of the plasma-deposited films show the material to be relatively free from impurities. Films deposit ed on SiOz and GaAs substrates were analyzed and, in both cases, the Auger spectra indicated that less than 1 at. % of carbon was incorporated in the tungsten. In addition, no oxygen or fluorine was detected. A comparison was made between plasma-deposited tungsten and conventional CVD tungsten using scanning and transmission electron microscopies. The conventional CVD film was produced by depositing 100 A of tungsten using the plasma and then doing a conventional CVD growth (no plasma) at 400 "c. The resistivity of the 4S00-A.. thick conventional CYD film was about 14 pH em while that of the 3000-A-thick plasma-deposited film was 70 pn em. Figure 3 shows a scanning electron micrograph of the con ventional CVD film and Fig. 4 shows a transmission electron micrograph and diffraction pattern for the plasma-deposited film. The grain size of the conventional film was approxi mately 3000 A, more than an order of magnitude larger than 2000 A grain size of the plasma-deposited film, Consequent ly, it appears that electron scattering at the grain boundaries has limited the conductivity of the plasma-deposited films. This inference is consistent with the fact that the resistivities of the plasma-deposited. films were independent of the mm thickness since the grain size did not seem to increase signifi cantly with thickness. It was possible to obtain different resistivity films by altering the various discharge parameters (partial gas pres- FIG. 3. Scanning electron micrograph of conventional CVD tungsten grown on top of plasma-deposited tungsten. K. E. Greenberg 1051 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 138.251.14.35 On: Mon, 22 Dec 2014 20:16:28FIG. 4. Transmission electron micrograph and diffraction pattern for a plasma-deposited film. sures, discharge current, etc.). Deposition rates as high as 200 A/min were achieved and films with resistivities as low as 30 fl!l em were obtained. It is believed that by optimizing the discharge deposition process it will be possible to pro duce films with resistivities much closer to that of bulk tung sten. Even if it is not possible to obtain device quality materi al purely by plasma processing, this could stiH be a viable technique for broad-area metallization since we were able to produce a lOflD em tungsten film that adhered to the under lying oxide by plasma depositing 100 A of tungsten and then growing an additional 5000 A by conventional CVD. A typi- 1052 Appl. Phys. Lett., Vol. 50, No. 16,20 April: 987 cal metallization scheme might include a conventional sili con selective tungsten deposition in the contact windows fol lowed by the plasma deposition of a thin layer of tungsten and the conventional CVD growth of an additional layer. This scheme would produce low resistivity tungsten that ad heres well to the oxide, ensure low contact resistance, and cause minimal damage due to ion bombardment. In summary, tungsten films that adhere well to silicon dioxide were deposited using an abnormal-glow discharge. The metal was deposited at low temperatures and prelimi nary studies indicate that electron scattering at the grain boundaries, rather than impurity incorporation or film mor phology, was responsible for the higher than bulk resistivi ties of the plasma-deposited films. Tungsten films having resistivities within a factor of two times that of bulk tungsten and that adhere well to silicon dioxide were produced by the plasma deposition of a thin layer of tungsten foHowed by a conventional CVD growth, The author wishes to thank L. Maestas and M. Eatough for the x-ray diffraction work, W. Wallace for the Auger studies, R. Lujan for the scanning electron micrograph, and C. Hills for the TEM studies. This work was performed at Sandia National Laboratories and supported by the U.S. De partment of Energy under contract number DE-AC04- 76DPOO789 for the Office of Basic Energy Sciences. .- ly' Pauleau and Ph. Lami, J. Electrochcrn. Soc. 132, 2780 (1985). lEo K. Broadbent and W. T. Stacy, Solid State Techno!. 28, 51 (1985). 'E. K. Broadbent and C. L. Ramiller, J. Electrochem. Soc. 131, 1427 (1984 ). 4A. von Engle, Ionized Gases, 2nd ed. (Oxford University, New York, 1965), Chap. 8. 5H. Dallaporta and A. Cros, App!. Phys. Lett. 48,1357 (1986). "J. V. Mitchell, G. Nyberg, and R. G. E!Iiman, App!. Phys. Lett. 45, 137 (1984 ). 'J. E. Griffith, Y. Qui, and T. A. Trombello, Nuc!. Instrum. Methods 198, 607 (1982). "W. D. Davis and T. A. Vanderslice, Phys. Rev. 131,219 (1963). Y A. J. Learn and D. W. Foster, I. App!. l'hys. 58, 2001 (1985). ICc. C. Tang and D. W. Hess, App!. Phys. Lett. 45, 633 (1984). "S. Basavaiah and S. R. Pollack, J. AppL Phys. 39,5548 (1968). K. E. Greenberg 1052 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 138.251.14.35 On: Mon, 22 Dec 2014 20:16:28

1.37211.pdf

AIP Conference Proceedings 170, 498 (1988); https://doi.org/10.1063/1.37211 170, 498 © 1988 American Institute of Physics.Bolometers as high-resolution gamma spectrometers Cite as: AIP Conference Proceedings 170, 498 (1988); https:// doi.org/10.1063/1.37211 Published Online: 29 May 2008 George Simpson 498 BOLOMETERS AS HIGH-RESOLUTION GAMMA SPECTROMETERS George Simpson Space Science Center, University of New Hampshire, Durham, NH 03824 ABSTRACT A significant advance in nuclear gamma-ray spectroscopy could be made if detectors capable of measuring Doppler shifts at MeV energies were available. With this goal in mind, we have investigated the prospects for constructing gamma-ray bolometers. We discuss the advantages and disadvantages of this approach, drawing on recent progress in their application to X-ray astronomy and neutrino detection. INTRODUCTION Bolometers, which measure radiant energy by converting photons into heat, are interesting to study because they raise the possibility of achieving very high resolution. This has been demonstrated at X-ray energies by the GSFC group who, using a silicon boule at 304 mK, have achieved a resolution of 17 eV at 6 keVl,2, 4. This is more than a factor of 10 improvement over the best resolution previously achieved with solid state detectors. Their detector is unsuitable for gamma-ray work, having an active volume of only 2.5 x 10 -6 cm 3. Bias Cabrera 3 also took up the bolometer concept, proposing that a neutrino detector be built using many 1 kg blocks of silicon at 1 inK. He suggested that the required thermometry be performed using either superconducting thin films or SQUIDs. This paper discusses the potential of bolometers as spectrometers for gamma- ray astronomy. The scope of the discussion is restricted to defining one feasible design for a single bolometer unit, which corresponds to the individual detectors of conventional gamma-ray spectrometers (eg: NaI or Ge crystals). A brief discussion of the background conditions which will be encotmtered in orbit, and some possible background suppression strategies are given. We are trying to answer the question: "Is it possible, in principle, to use a bolometer to perform high-resolution astronomical spectroscopy at gamma-ray energies?" SCIENTIFIC PROMISE Recent solid-state detector gamma-ray results have shown that the widths of the galactic 5115,6,7 and 18098 keV lines are less than the resolution of these devices (N1 keV). It is reason,'tble therefore to expect that narrow widths will be a characteristic of many other astrophysical lines, its yet undiscovered. Some of the questions which could be addressed if a spectrometer with sufficient sensitivity and resoh, tion to measure Doppler shifts at gamma-ray energies became available include the following: SOLAR: Sites of nuclear activity on the stm: If the line-of-sight velocities of the excited nuclei emitting gamma-rays could be determined, then by correlating the velocities with those fotmd at other wavelengths, the interaction regions could be uniquely identified. Different lines, such as those front positron annihihttion and neutron capture, may well be produced at different places in the solar atmosphere. Electron temperature at the annihilation site: If we could measure the width of the /e) 1988 American Institute of Physics 499 positron annihilation line, we could determine the electron temperature, and thus open the door to gamma-ray and radio correlation studies BURSTS: Neutron star mass: It would be possible to make precise measurements of gravitational red-shifts in gamma-ray burst spectra, if we had high enough resolution. These data would pin down the ratio of object mass to emission radius. Assuming that the emission comes from the surface, and assuming that we know the density from the equation of state, we could then determine the radius of the star. PULSARS: With a very high resolution spectrometer, we might find Doppler shifts in pulsar emission which are a function of the phase. This would give a very strong clue as to the emission geometry. PHYSICAL PRINCIPLES In this section, we discuss the physical principles which govern bolometry at MeV energies. One of the key property of bolometers which makes them attractive for gamma-ray astronomy is the fact that they use the bulk properties of matter, rather than surface effects. This is important because the photon cannot be confined to a small volume, A second property is the very low specific heat which some substances show near OK. 1. Specific Heats: The Debye-Somerfeld Equation shows that the specific heat of the lattice approaches zero as the third power of the temperature, while that due to electrons is linear in the temperature, For very pure Germanium and Silicon, the electronic specific heat is negligible. At .0001K (0.1 mK), the specific heats of these elements actually fall as low as 1 electron volt per degree kelvin per gram. The figure below shows the thermal capacity as a function of temperature in this range, for the Germanium detector described below. electron- volts/mK 10000 Thermal Capacity of Germanium Gamma-Ray Bolometer Detecting Element 1000 100 10 Germanium, 55 gms I I 2.41 cm height x 2.32 cm diam => 1 radiation length x 20-1 MeV electron ranges 2 4 6 mK 2. Gamma-RCw Absorption: We require that gamma-rays of the energies of interest have a probabi-lity (>0.5) of being totally absorbed in the detector. To achieve this, we 500 need a thickness of at least one interaction length, and we also need the linear dimensions to be large compared to the range of recoil electrons. A cylindrical element of Germanium, 2.3 cm in diameter and 2.4 cm in height, while only one interaction length long, meets these requirements. ,~, Recovery: Because the thermal resistance from an object to a Helium bath is a very strong function of surface quality, one usually provides thermal links between the element and the cooling surface. This strategy not only promotes a uniform response over the volume of the bolometer element, but also allows the recovery rate to be controlled. The parameters of the thermal link must be chosen to provide heat flow adequate for the bolometer element to recover quickly. They should have the highest possible thermal conductivity, combined with the lowest possible specific heat. The next figure shows a configuration which could be used in a gamma-ray telescope. Evacuated Refrigeration Cell, also acts as Faraday cage and magnetic shield coolant flows over surface Bolometer Element: 3ermanium Therr uor resistance Link thermometry) 4. Thermometry: The thermometry requirements for this instrument pose a challenge. We must measure the total heat flow of a pulse which lasts only microseconds. We will assume, for the present discussion, that equipment is available which can measure the temperature of the bolometer element with an accuracy of lmK across the range from 1 mK to 1K, within the instrument response time. One strategy to achieve this requirement is to use noise thermometry to establish the absolute temperature baseline, and resistance thermometry to follow the thermal pulse. Since it is common to use doped Germanium as the resistor .element, the possibility exists to combine the resistor with the bolometer itself. This would resolve the often troublesome issue of thermal contact between the sample and the thermometer. 501 A PRELIMINARY DESIGN The preliminary design given in this section shows that reasonable parameters can be found allowing such a device to function as a gamma-ray spectrometer. The next step, a detailed optimization making use of engineering data for existing devices, will be published in a follow-on paper. The parameters of this design are as follows: 1. Material: Germanium is chosen, because of its low specific heat in the millikelvin regime, and its high gamma-ray absorption coefficient. Silicon, for example, requires more than 5 times as much mass as Germanium, to achieve the same stopping power for gamma-rays. 2. Mass: 55 gms provides one radiation length. 3. Shape: A right cylinder, of diameter approximately equal to the height, is chosen for this design exercise. 4. Operating Temperature: 1-5 mK 5. Link Properties: Pure copper links are chosen to give the highest possible thermal conductivity combined with the lowest thermal capacity. These quantities are a function of temperature, so the net instrument response will be non-linear. DESIGN MODELS 1. Response model: The response model describes what happens to the detector when it is hit by a gamma-ray or other particle. It models the transfer of energy from the infalling photon into heat, and it predicts the response of the instrument as a function of time after the pulse. We have used STELLA, an interactive graphical modelling tool, for this purpose. The STELLA model of our system is shown below. Each of the graphical elements contains a relation specifying its response to its inputs. The boxes integrate the flows into and out of them, which are shown as outlined arrows with valves. The circles contain relations governing the local properties, and the arrows show the functional dependencies. For example, heat enters the bolometer element (U_bolo_element) from gamma-ray energy deposits (power_input), and leaves it due to cooling via the thermal links (cooling_power). The specific heats and conductivities of the links and the bolometer elements are represented graphically. The most important of the equations are shown below the figure. We used this simple model to predict the response of our system to gamma-rays of given energies, adjusting parameters such as link area and bolometer mass until we achieved acceptable perfomaance. 2. Gamma-Ray transport model: We have used a gamma-ray transport program to determine the photofraction and distribution of energy deposits in our bolometer element. 3. Res~l~l~i~n and Backgro~md Models: For purposes of the present discussion, we assume that the achievable resolution limit is given by the value already achieved with X-ray detectors. An investigation of the sources of the various background contributions, and what can be done to suppress each of them, will be completed in the near future. 502 Log Cp_bolo T bolo dT bolo Thermal_Capac ~._ Net_Cooling_Energy Net_bolo power_t " ~ U_bolo_element '~," " Power_Input Cooling_ Link_Tem Power Gamma ~ Energy Gamma_ Gamma Pulser Interval Link Conductance Key Model Equations Net Cooling_Energy -- NcLCooling_Energy + dt * ( Cooling_Power ) INIT(Net_Cooling_Energy) = 0 T_bolo = T_bolo + dt * ( dT_bolo ) INIT(T_bolo) = 25 { inK} U_bolo_elcment = U_bolo element + dt * ( -Cooling_Power + Power_Input ) INIT(U_bolo_element) = 0 {eV: arbitrary reference level} Cooling_Power = IF(Link_Temp_Diff>0) THEN Link_Temp_Diff*Link_Conductance ELSE 0 {eV per microsecond} dT_bolo -- Net bolo power flow,q'hcrmal_Capac {mK} Gamma_Pulser = PULSE(1,3,Gamma_Intcrval) Link_Temp_Di ff = T_bolo-Tsink {inK } Nct_bolo_power_flow = Powcr_Input-Cooling_Powcr (ev/microsecond} MODEL PREDICTIONS T_bolo Link Area Log_T_bolo Length 1. Response model: The themaal pulse time constants, as a function of time, were optimized by adjusting the properties of the themlal link in the model. Pulse profiles such as that seen below were typical. The important point is that reasonably fast pulses can be achieved, with the conductivities and specific heats which exist at these temperatures. 503 mK 1000, ,0eL ,0I Bolometer Response to 511 keV Energy Deposit 0 1 2 3 4 5 6 microseconds 2. Gamma-Ray Transport Model: The figure below shows the efficiencies for the various interaction types, as a function of photon energy, for a single bolometer element. It is clear from the figure that we would like to have a thicker detector, or a material with a higher photopeak efficiency. efficiency 60% 50% \ 40%, 30% < 20% 10%• 0%13 200 PROCESS EFFICIENCIES for GAMMA-RAY BOLOMETER DETECTING ELEMENT. (Preliminary Design) •-- , [] , , [] j ~rh 400 600 800 1000 1200 1400 1600 1800 2000 Photon Energy, keV O. photoelectric O. single Compton • . multiple Compton []. pair production 3. Background: The small bolometer element size defends it against excessive dead- time due to charged particle events. Our detector has a geometrical factor of ~82 cm -2 sr. The expected rate of charged particle events through the element is therefore -10/second. Recovery times faster than 1 millisecond are required, to avoid having gamma-ray events ride on the tail of charged particle events. Activation background is the most serious problem for this system; our models do not yet address this issue. 504 4. Resolution Model: The resolution model is not yet complete. But considering that with higher temperatures, McCammon et al. achieved 17 eV at 6 keV, it should be possible to achieve similar values in the gamma ray regime. SUMMARY The advantages of the bolometer for gamma-ray spectroscopy are its very high ultimate resolution and its simplicity of concept. Some disadvantages are the relatively modest sensitivity which each individual element may have (too avoid pileup due to charged particle events), the necessity of cooling to millikelvin temperatures, and the need for extremely good electrical, acoustical, and magnetic isolation, to achieve the ultimate resolution. REFERENCES 1. S.H. Moseley, J.C. McCammon, and D. McCammon, J.Appl.Phys. 56(5),1257 (1984) 2. D. McCammon, S.H. Moseley, J.C. Mather, and R.F. Mushotzky, J.Appl.Phys. 56(5), 1263 (1984) 3. B. Cabrera, L.M. Krauss, and F. Wilczek, Phys. Rev. Letters, 55 (1), 25 (1985) 4. S.H. Moseley, R.L. Kelley, R.J. Schoelkopf, A.E. Szymkowiak, D. McCammon, and J. Zhang (in press) 5. M. Leventhal, C.J. MacCallum, and P.D. Stang, Ap.J. 225, L11 (1978) 6. M. Leventhal, C.J. MacCallum, A.F. Huters, and P.D. Stang, Ap.J. 240, 338 (1980) 7. G.R. Riegler, J.C. Ling, W.A. Mahoney, W.A. Wheaton, J.B. Willett, A.S. Jacobson, and T.A. Prince, Ap.J. 248, L13 (1981) 8. W.A. Mahoney, J.C. Ling, W.A. Wheaton, and A.S. Jacobson, Ap.J. 286, 578 (1984)

1.343156.pdf

Photoluminescence of CuAl x Ga1−x Se2 crystals grown by chemical vapor transport Koichi Sugiyama, Satoshi Iwasaki, Tamio Endo, and Hideto Miyake Citation: Journal of Applied Physics 65, 5212 (1989); doi: 10.1063/1.343156 View online: http://dx.doi.org/10.1063/1.343156 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photoluminescence studies in CuAlSe2 epilayers grown by lowpressure metalorganic chemicalvapor deposition J. Appl. Phys. 77, 1225 (1995); 10.1063/1.358990 Excitonic photoluminescence in a CuAlSe2 chalcopyrite semiconductor grown by lowpressure metalorganic chemicalvapor deposition J. Appl. Phys. 74, 6446 (1993); 10.1063/1.355129 2.51 eV photoluminescence from Zndoped CuAlSe2 epilayers grown by lowpressure metalorganic chemical vapor deposition Appl. Phys. Lett. 62, 3306 (1993); 10.1063/1.109054 Photoluminescence characteristics of CuAl x In1−x Se2 solid solutions grown by iodine transport technique J. Appl. Phys. 72, 3697 (1992); 10.1063/1.352314 Electrical and optical properties of CuAlSe2 grown by iodine chemical vapor transport J. Appl. Phys. 70, 1648 (1991); 10.1063/1.349531 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 142.157.129.8 On: Sat, 13 Dec 2014 22:20:311ITO 10-11 '--_.L..-_..L..-_-'-_-'-_~ o ~ ~ ~ ~ ~ APPLIED VOLTAGE[V) FIG. 6. J-V characteristics of Mo!Zr02(30.5 nrn)! Si02!p-Si( 100) diodes for various oxidation times. where E is the electric field in the Si02 films. A Fowler Nordheim tunneling theory leads to the equation Ee = 4(2m*) 1/2tp3/Z13fxq, (2) where m* is the effective mass of holes at the Si surface, <I> is the barrier height, and q is the electric charge. From the observed value of Ee (1.18 X 108 V Icrn) and the average ef fective mass m* = O.28m (when m is the free-electron mass), the value ofct> is estimated to be 3.44 eV. The photoe mission measurement for Si into Si028 shows that the barrier height for holes is about 3.6 eV which is nearly equal to the above-mentioned value of <P. This shows that the current transport mechanism through the Mo/ZrOzlSi02/Si diode for a 600-min oxidation is predominantly detennined by the tunneling current through the Si02 film at higher electric fields (IV I> 8.5 V). The Frenkel-Poole plot of the I-V char acteristics at higher electric fields gives an abnormal dielec tric constant of 19.2 Eo for Si02 films, which invalidates the Frenkel-Poole emission process.9 In conclusion, thin zrOz films vacuum-deposited on Si( 100) were oxidized in dry oxygen at 800"C. This results in the growth of a thin SiOz layer at the Zr02/Si interface. From the C-V characteristics of Mo/Zr02(30.S nm)/Si02Ip-Si( 1(0) diodes, it was found that the static di electric constant of the zr02 film decreases from 21.2 eo to 15.5 Eo due to the crystallization of the amorphous ZrOz film during the oxidation. The leakage current of this diode was lowered by the formation of the Si02 layer at the Zr02/Si interface. 'M. Koyanagi, Y. Sasaki, M. Ishihara, M. Tazunoki, and N. Hashimoto, IEEE Trans. Electron Devices ED-27, 1596 (1980). 2K. Ohta, K. Yamada, K. Shimizu, and Y. Tarui, IEEE Trans. Electron Devices ED·29, 368 (1982). 3M. Morita, H. Fukumoto, T. {mura, Y. Osaka, and M. Ichihara, 1. Appl. Phys. 58, 2407 (1985). 4y' Osaka, Y. Nishibayashi, and T. Jmura, J. App!. Phys. 63, 581 (1988). ~Y. Nishibayashi, T. Imura, Y. Osaka, and F. Nishiyama, Proceedings of the 12th International Symposium at llosei University, edited by T. Sebe, and Y. Yamamoto (Hosei University Press, Japan, 1988), p. 493. oR. E. Pawel, J. Elcctrochem. Soc. 126, 1111 (1979). 7B. E. Deal and A. S. Grove, J. Appl. Phys. 36, 3770 (1965). "R. Williams, Phys. Rev. A 59,140 (1965). "J. Frenkel, Phys. Rev. 54, 647 (l938). Photoluminescence of CuAlx Ga1_XSea crystals grown by chemical vapor transport Koichi Sugiyama, Satoshi Iwasaki, Tamio Endo, and Hideto Miyake Department 0/ Electrical Engineering, Mie University, Kamihama·cho, Tsu-shi, Mie 514, Japan (Received 14 December 1988; accepted for publication 10 February 1989) The photoluminescence (PL) characteristics have been studied for the CuAlx Gal _ x Se2 chalcopyrite quaternary crystals grown by an iodine transport technique. The PL measurements at 17 K reveal the existence of three types of emission bands, which are attributed to transitions involving localized states: shallow acceptors, deep donors, and deep acceptors, respectively. The origin of the shallow acceptors is tentatively assigned to Se interstitials and that of the deep donors to iodine impurities. All the measured crystals are p type and the hole mobility at room temperature decreases with x ( S 0.3). Recently there has been considerable interest in I-III VI2 chalcopyrite semiconductor alloys for potential techno logical applications. CuAlxGa1_ xScz quaternary alloys have band gaps of 1.7-2.7 eV and are fairly well lattice matched to ZnSe, and hence high-quality heterostructures are expected to be fabricated from these materials for opto electronic devices. The preparation of the alloys by an iodine chemical vapor transport technique and their optical ab sorption characteristics have been reported by Bodnar and co-workers, 1.2 but more detailed information concerning the 5212 J. Appl. Phys. 65 (12}, 15 June 1989 0021-8979/89/125212-04$02.40 © i 989 American Institute of Physics 5212 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 142.157.129.8 On: Sat, 13 Dec 2014 22:20:31optical and electrical properties of the alloys is required for the applications. In this communication we describe the lu minescent and electrical properties of the CuAlx Gal _ xSe2 alloys with various compositions x. The CuAlxGa1_xSe2 quaternary crystals were grown by the chemical vapor transport method using iodine as a transport agent. As source materials for the growth, we used CuAlSez and CuGaSez ternary compounds, which were pre pared by direct chemical reaction of stoichiometric mixtures of the constituent elements. The purities of Cu, AI, and Ga were 6N grade and those of Se and I were 5N grade. For preparation of the quaternary crystals, a mixture of pre scribed amounts of powdered CuAISe2 and CuGaSe2 was charged with iodine (lOmg/cm3) in a partially carbon-coat ed quartz ampoule 10 mm Ld. and 120 mm long. The am poulewas sealed after evacuation to a pressure of about 10--6 Torr, and then inserted in a two-zone furnace. The crystal growth was carried out for 10 days by keeping the source and growth portions of the ampoule at 830 and 700 ·C, respec tively. The source material was placed in the carbon-coated portion of the ampOUle. The grown crystals were platelets 5 X 5 mm2 wide and 1 mm thick or needles 5 mm long and 1-2 mm wide for x -0, but the needlelike crystals were dominant for x> 0.6. The crystals were ascertained to have a chalcopyrite structure by x-ray ditrraction method. The alloy composition x was mea sured with energy-dispersive x-ray microanalysis (EDX) and the conventional atomic number, absorption, and flu orescence (ZAF) corrections were performed for the data. The photoluminescence (PL) measurements of the grown crystals were performed at 77 K using a setup com- Photon energy (eV) 2,0 1.5 1.2 600 17K x=0.53 )(=0.43.-/ ~ x=U33./ ~ :A;=O.2.1~ " )(=0.11 ~.:.:-l(=-=-O _--_ sao Wavelength (nm) 1000 FIG, 1. PL spectra at 77 K for CuAI,Ga, _ xSe2 crystals. 5213 J. Appl. Phys., Vol. 65, No. 12, 15 June 1989 c (} '0 2.5 f.. 1.5 1. T o 17K / ,/ ,,-/' ..-;" " " , ,. " " ,. / /' " " ", ", ", /' FIG. 2. Peak energies of ernission bands in the PL at 77 K vs composition x. The band-gap energies obtained from absorption oflight for E Ii C polariza tion by Bodnar and co-workers (sec Ref. 2) are plotted as a dashed line. Filled circles represent the band-gap energies for CuGaSe2 and CuA!Se1, estimated from the exciton absorption and emission data in Refs. 3-6, posed ofaXe lamp, combinations of appropriate optical filters, a grating monochromator, and a photomultiplier. The PL spectra to be presented were not corrected for the wavelength-dependent response of the measuring system. Typical examples ofthe PL spectra are shown in Fig. 1. The spectra generally consist of one or two emission bands. The peak energies of the emission bands observed in this experi ment are plotted as functions of the composition x in Fig. 2. Bodnar, Gil, and Lukomskii have derived the band-gap en ergies of the aHoys at 77 K from measurements of the funda mental absorption edge for E lie (the optic axis) polariza tion, corresponding to upper-valence-band to conduct ion-band transition.2 These data are exhibited as a dashed curve in the figure. The band-gap energies of CuGaSe2 and CuAISez at 77 K have been estimated to be 1.73 and 2.73 eV, respectively, from free-exciton absorption and emission data,3-6 and are also plotted for comparison. The values are 50-110 me V larger than those of Bodnar and co-workers because the absorption edges obtained by the latter research ers are presumably shifted to lower energies due to the impu rity-to-band transition effect. The PL emission bands in CuAlx Gal _ x Sez alloys can be classified into three types, and are labeled PI' P2. and P3• Band PI with the highest peak energy was observed for x < 0.6, and the peak energy for CuGaSez is about 1.68 eVo The band is considered to be the same as those reported in Refs. 3 and 4, and is attributable to the transition between conduction band to a shallow acceptor, whose origin may be Sugiyama et al. 5213 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 142.157.129.8 On: Sat, 13 Dec 2014 22:20:31300 K 120 0 15 1/1 :> -N E u 10 10~ ....... ;;>. M ~ I 'E :a u ~ co 0 1? ::i ~ 5~ ;;>. ~ 15 .... III ~ C. (\,I "0 J QJ 10 -0 J: 0 0.1 0.2 0.3 )( in CuAlx Ga.I-xSez FIG. 3. Hole concentration and mobility at room temperature in the as- grown CuAlxGa, .xSe2 crystals vs composition x (:S0.3). assigned to a Se interstitia1.4 The ionization energy of the acceptor is about 50 meV for CuGaSe" and those for CuAix Gal .. x Se2 alloys can be estimated fr~m the difference between band gap and Pj emission-peak energies, when the curve of the band gap used by Bodnar and co-workers is shifted upwards such that the value for CuGaSe" becomes 1.73 eV. The ionization energy increases with comPosition x and becomes 190 meV for x = 0.5. The increase is consid ered to be mainly due to the change in hole effective mass with x. Band Pz was observed for the samples with x> 0.6 and was dominant for x > 0.8. The band with a peak at L 76 e V observed in CuAlSe2 seems to have the same origin as that for the emission at 1.8 eV reported by Yamamoto.? Susaki et al. observed a band peak at 1.33 eV for CuGaSe2, and from the result of annealing experiments they concluded that the band is associated with an impurity incorporated on a Se site; thus they suggested that the 1.8-eV emission in CuAISe2 is also attributable to the same mechanism. H They considered that the iodine becomes a deep donor and that the emission band of 1.33 eV in CuGaSe2 is due to the transition between the donor and the valence band. In order to identify the origin of band P2' we performed annealing experiments in sealed quartz ampoules for crystals with x = 0.63 in vacuum and then in an iodine atmosphere. After annealing in vacu um at 600°C for iO h, band Pz disappeared but the spectrum shape for band P3 was nearly unaltered. Band P2 reappeared after the subsequent annealing in the iodine atmosphere at 600°C for 7 h using 5 mg/cm3 iodine in the ampoule, The results indicate that band P2 is associated with iodine impur ities and has the same nature as the band at 1.33 eV in 5214 J. Appl. Phys., Vol. 65, No. i2. 15 June 1989 CuGaSe2' If the emission mechanism is due to the donor-io valence-band transition, the donor ionization energy in creases from 0.38 to 0.97 eV for x = 0 to 1. The experimental result that bands PI and P2 are dominant for x<O.6 and x> 0.6, respectively, will be explained in terms of the foHow ing model. As-grown CuAl" Ga1_ x Se2 crystals with small x contain excess Se atoms entering into interstitial sites. They form acceptors and contribute to PI emission. The concen tration of the excess Se decreases with increasing Al compo sition x, and Se sites of the chalcopyrite lattice will have a tendency to become vacant and to be occupied by iodine atoms for larger x, which results in the formation of the deep donors involved in the emission of band Pl' Band P3 was observed for crystals with x < O. 8; the peak energy is 1.23 eV for CuGaSe2 and gradually increases with composition x. For CuGaSe2 single crystals, an acceptor lev el with 0.55 eV has been obtained from the temperature de pendence of the electrical properties,9.10 and for flash-evapo rated CuGaSe2 films a PL band peaking at L 16 eV has been observed at 80 K by Schumann et at. 1l and has been inter preted as the emission associated with the deep acceptor lev el (at 0.55 eV). Band p} observed in our study might be attributable to the same mechanism, and might be related to the deep acceptor level with an ionization energy of 0.5 eV in CuGaSe~. The electrical properties of the as-grown crystals were examined by using the van der Pauw method at room tem perature after formation of ohmic contacts by evaporation of Au. The Hall measurements could not be achieved for sam ples with x > 0.3 because of the very high contact resistance. AU the measured samples are p type and the hole concentra tions are of the order of wig cm-3 and decrease markedly with x as shown in Fig. 3. Since the p-type conduction has been related to the shallow acceptors, which are responsible for P1 emission band,4 the change in hole concentration with x can be explained as in the case of bands Pj and P2 on the basis of the change in concentration of the excess Se. The hole mobility decreases from 18 to 6 cm2jV s with increasing x from 0 to 0.3. This remarkable decrease in hole mobility with x might be due to either increase in hole effective mass or increase in the concentration of scattering centers. The centers might be related to Si impurities introduced from the CuAlSe2 source material, which was contaminated with Si as a result of the reaction of Al with quartz ampoules during i.ts preparation. In conclusion, the CuAt Gal . x Sez alloys grown by the iodine transport technique are shown to have three emission bands in the PL spectra. The three bands are considered to be related with shallow acceptors, deep donors, and deep accep tors, respectively. The as-grown alloy crystals have hole con centrations of the order of 1018 cm--3 for xSO.3, and the mobility decreases markedly with x. The authors wish to thank S. Ogawa for technical assis tance in the EDX work. II. V. Bodnar, A. A. Vaipoiin, and L. S. Unyarkha, bv. Akad. Nauk SSSR, Neorgan. Mater. 21, 1656 ( 1985). 2r. V. Bodnar, N. L. Gil, and A. 1. Lukornskii, SOy, Phys. Semicond. 17, 333 (1983). Sugiyama et at. 5214 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 142.157.129.8 On: Sat, 13 Dec 2014 22:20:313M. P. Vecchi, J. Ramos, and W. Giriat, Solid-State Electron. 21, 1609 (1978). • J. Stankiewicz, W. Giriat, J. Ramos, and M. P. Vecchi, Sol. Energy Mater. 1,369 (1979). '8. Tell and P. M. Bridenbaugh, Phys. Rev. B 12, 3330 ( 1975). "M. Bettini, Solid State Commun. 13, 599 (1973). 7N. Yamamoto, Jpn. J. Appl. Phys. 15, 1909 (1974). KM. Susaki, T. Miyauchi, H. Horinaka, and N. Yamamoto, Jpn. J. Appl. Phys. 17, 1555 (1978). 9L. S. Lerner, J. Phys. Chern. Solids 27, I (1966) . IOL. Mandel, R. D. Tomlinson, M. J. Hampshire, and H. Neumann, Solid State Commun. 32, 201 (1979). "B. Schumann, A. Tempel, G. Kiihn, H. Neumann, N. V. Nam. and T. Hansel, Kristall. Tech. 13, 1285 (1978). Creation of TICaBaCuO fine features by a wet process I. Shih and C. X. Qiu Electrical Engineering Department, McGill University, 3480 University St.. Montreal. P. Q. H3A 2A 7. Canada (Received 15 November 1988; accepted for publication 14 February 1989) During the study of the formation of TlBaCaCuO from rf-sputtered Ba-Ca-Cu-O films (Ba:Ca-Cu = 2:2:3 in target) it was found that the rf-sputtered materials were relatively unstable in water, in photoresist developer, and even in room atmosphere. To overcome the severe undercutting effect during the chemical etching process resulting from the instability, a Cu protective layer with a thickness of 100-150 A. was adopted for the fine-line patterning. Using a positive photoresist technique, 3-,um lines have been successfully produced. Lines with a width of 150 pm also have been produced on Zr02 substrates and superconductivity was observed after a heat treatment in an environment containing O2 and Tl gas. After the discovery of the new high Tc superconductor system TlBaCaCuO, I many experiments have been reported on the preparation of thin films of this materiaI2-s. Recently, it has been shown that high Tc films of Tl-Ba-Ca-Cu-O can be formed by diffusing Tl into rf-sputtered Ba-Ca-Cu-O films.6 Although the formation of the pure high Tc phase is complex, further development to process the sputtered films has been made in our laboratory. For electronic applica tions, it is required first to produce fine features of the high Tc films. In our previous experiments using a wet chemical method, we have successfully patterned YBaCuO thin films into fine lines.7 In the present work, experiments have been made to establish a procedure for the patterning of rf-sput tered Ba-Ca-Cu-O films. After the patterning, the lines were treated in an environment containing both Oz and TI. The treated lines were tested for superconductivity and the re sults are reported in this paper. Films of Ba-Ca-Cu-O with a thickness from 1 to 5 ,um were deposited from a single presintered Ba-Ca-Cu-O target (Ba:Ca:Cu = 2:2:3) using an rfmagnetron method.6 Glass slides, zrOz, and Si were used as substrates which were mounted on a water-cooled Al substrate holder. The compo sition of the as-sputtered films was determined by electron probe microanalyzer and the results showed that these films are Ca deficient (Ca/Ba = 0.76). Results of etching of the rf-sputtered Ba-Ca-Cu-O films at 25°C are shown in Table I. Here, we can see that the rate is relatively high for all acids with A/Hz 0 = 1/40 (A = HN03, HCI, or H3 P04). The rate decreases as the ratio is decreased and values of about 1 ,um/min is achieved for a ratio of 1/160. The solution with the above ratio will be used in fine-line etching experiments to be described below. By treating the films at a temperature above 750°C in O2 for a few minutes, the etching rates were also found to reduce. The variation of morphology of the as-deposited Ba-Ca Cu-O films was also examined and it was found that these were relatively unstable even under normal laboratory envi ronment as com pared to the YBaCuO material. Figure 1 (a) shows the top view of a film taken about 1 week after the deposition. Roughly circular areas (red color seen by eyes) are visible due possibly to the interaction with moisture or due to the slow crystallization process or both. In addition to the slow morphology variation in the air, it was found that the as-deposited films are very sensitive to water and the photoresist developer. "Etch pits" with irregular shape were found in Ba-Ca-Cu-O films after being immersed in deion ized H20 at 25°C for 1 min [see Fig. 1 (b)]. More severe T ABLE I. Etch rate (in jim/min) of BaCaCuO films in HCl, HNO,. and H,PO. solutions at 25 'C HCl HNO, H,PO. Ca:Ba:Cu 2:3:4 2:2:3 2:3:4 2:3:3 2:2:3 1/40 24 15 14 12 4.4 1/60 14 7.3 8.2 6.9 2.2 1/80 10 3.1 5.6 2.3 1.7 1/100 5.6 2.4 4.2 1.3 1.3 1/160 2.7 \.3 2.5 1.0 0.9 5215 J. Appl. Phys. 65 (12),15 June 1989 0021-8979/89/125215-03$02.40 © 1989 American Institute of Physics 5215 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 142.157.129.8 On: Sat, 13 Dec 2014 22:20:31

1.98280.pdf

Effects of passivating ionic films on the photoluminescence properties of GaAs B. J. Skromme, C. J. Sandroff, E. Yablonovitch, and T. Gmitter Citation: Applied Physics Letters 51, 2022 (1987); doi: 10.1063/1.98280 View online: http://dx.doi.org/10.1063/1.98280 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/51/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of passivation on different GaAs surfaces Appl. Phys. Lett. 103, 173902 (2013); 10.1063/1.4826480 Solvent effect on the properties of sulfur passivated GaAs J. Vac. Sci. Technol. B 14, 2761 (1996); 10.1116/1.588827 Excitation power dependence of photoluminescence enhancement from passivated GaAs Appl. Phys. Lett. 66, 3504 (1995); 10.1063/1.113778 Microcavity effects in the photoluminescence of GaAs microcrystals Appl. Phys. Lett. 62, 1958 (1993); 10.1063/1.109503 Effects of polyimide passivation on the photoluminescence of highpurity epitaxial GaAs J. Appl. Phys. 63, 962 (1988); 10.1063/1.340041 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.24.51.181 On: Sun, 30 Nov 2014 01:56:05Effects of passivating ionic films on the photoluminescence properties of GaAs B. J. Skromme, C. J. Sandroff, E. Yablonovitch, and T. Gmitter Bell Communications Research, 331 Newman Springs Road, Red lJank, New Jersey 07701 (Received 27 August 1987; accepted for publication 15 October 1987) The passivating effects of spin-coated films of Na2S' 9H20 on GaAs surfaces have been studied using room-temperature photoluminescence (PL) and low-temperature PL spectroscopy. After passivation, the 300 K PL efficiency is increased on both n-and p-type material; improvements of up to 2800 X are observed. The surface field and surface recombination related notch features in the free and bound exciton emission spectra at low temperature are eliminated, implying that the residual band bending under illumination is less than 0, 15 V. Oxygen-exposed GaAs surfaces possess a large density of extrinsic states near the middle of the forbidden energy gap, which effectively pin the surface Fermi level at that positioIl,l,2 These pinning states have hindered the develop ment of a successful GaAs metal-insulator-semiconductor technology.3 Moreover, the midgap pinning of the Fermi level greatly increases the nonradiative recombination rate at surface recombination centers.4 The latter effect is re sponsible for the "2kT" currentS which limits the perfor mance of minority-carrier devices such as light-emitting di odes, lasers, solar cells, and bipolar transistors. Chemica!O or photochemicaf treatments to passivate GaAs surfaces are consequently of great interest. Recently, a novel technique involving spin-coated films of wide band gap inorganic sulfides was reported. 8,'1 These mms produced 60-fold improvements in the gain of heterojunction bipolar transistors at low current levels,8 and reduced surface re combination velocities to values as low as 500 em/s.9 In this letter, we use photoluminescence (PL), one of the most sen sitive and widely employed methods of characterizing non radiative surface recombination, 5-7,10 to study these sur faces.l! The samples consist of various (100) oriented struc tures (described below in detail) grown by molecular beam epitaxy (MBE), organometallic chemical vapor deposition (OMCVD), and vapor phase epitaxy (VPE). Details of the passivation procedure are described elsewhere.8 Briefly, the sample is first lightly etched in a (1:8:500) solution of (H2S04:H202:HzO) to obtain a hydrophilic surface, then rinsed and spun dry. A few drops of -1.0 M aqueous solu tion of Na2S . 9H20 are applied to the surface and the sample is spun in air at about 5000 rpm until dry. Room-tempera ture PL measurements are performed with the sample in a sealed, He-fined chamber. Low-temperature measurements are performed with the sample freely suspended in a super fluid He bath at 1.8 K. The excitation is provided by an Ar+ ion laser operating at 5145 A. A fixed excitation level of ~ 10 W / cmz is employed at 300 K, while the 1.8 K spectra shown here were recorded at ~700 mW/cm2• A l.O-m double spectrometer, a GaAs photomultiplier tube, and a photon counting detection system are used. The data have been cor rected when necessary for the spectral response of the sys tem. Intensity measurements are generally reproducible to within ± 30% or better. The strength of the room-temperature PL signal is shown in the upper two curves in Fig. 1 for two n-type GaAs samples as a function of surface treatment. As the spectral shape of the luminescence is typical of band-to-band transi tions and does not change, we indicate only relative intensi ties. Application of Na2S'9H20 yields improvements of up to 2800 X in the PL intensity with respect to that of a freshly etched surface for the low-doped sample, restoring to within 15% the intensity obtained with the original AIGaAs clad- IOU ---------------------l SURFACE TREATMENT I I I I I FIG. 1. Relative PL intensities at 300 K for four GaAs samples as a function of surface treatment. (Open circles) OMCVD GaAs layer, 7.0 jim thick, f! = oX 10" cm-3, sandwiched between two O.l-p.m-thick A1osGIlosAs layers; the top AIGaAs layer is removed in the first etching step. (Open triangles) OMCVD GaAs layer, 0.31 p.m thick, n O~ 9X 10'6 cm-3, grown on a O.55-p.m-thick A1o,GIlo7As buffer layer. (Closed circles) a 9.1-p.m thick !ayer,p = 4X 1015 em""', grown Oil a semi-insulating (SI) GaAs sub strate. Initial "etched" state corresponds to Na2S rinsed off (no chemical etch). (Closed triangles) a 3.2-,um-thick layer, p = 1 X 10'" em -:" grown ou a S1 substrate. Initial "etched" state actually air exposed. Intensities are normalized to the etched condition for each sample; lower two curves are displaced down one decade for clarity. 2022 Appl. Phys. Lett. 51 (24), i 4 December 1987 0003-6951/87/502022-03$01.00 © 1987 American Jnstitute of Physics 2022 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.24.51.181 On: Sun, 30 Nov 2014 01:56:05ding layer. Improvements for the more heavily doped sam ple are smaller, primarily because this layer is thinner, which results in larger residual surface recombination after treat ment (The quantum efficiencies of the two samples are about the same before treatment.) Variations between successive NazS'9H20 applications are believed to result from changes in the wetting properties of the surface and resultant film quality. Residual improvements in PL efficiency are observed even after thoroughly rinsing off the bulk of the film in de ionized water, presumably due to the presence of a passivat ing sub monolayer surface phase.8,9 The data indicate a deg radation in the properties of the hygroscopic Nu2S'9HzO films when exposed ovemight to humid air; under drier con ditions we found essential!y no change on this time scale. Similar, though less pronounced improvements are ob served on p-type samples of low and high doping levels, as shown in the lower two curves in Fig.!. The maximum im provement obtainable in these samples is smaner, due in large part to the absence of underlying AIGaAs buffer layers and the consequent diffusion of carriers into the highly non radiative substrates. The improvements we observed after N a2S' 9H20 passi vation in the PL efficiency of both heavily doped p-and n type material in the low injection regime are consistent with a reduction in surface recombination center density and/or unpinning of the surface Fermi level by the treatment How ever, these observations alone do not rule out the alternative explanation that we have merely repinned the Fermi level closer to one of the band edges. Assuming low level injection and quasi-equilibrium between the surface and the electri cally neutral bulk, the surface recombination velocity S re sulting from recombination through a given trap is given by4 Here, S[IJ and SnlJ are proportional to the concentration and capture cross sections of the trap, no and Po are the equilibri um bulk electron and hole densities, lls and p, are the elec tron and hole densities at the surface under illumination, and n* and p* are the densities which would be present if the Fenni level were coincident with the trap level. Bare GaAs surfaces exhibit large values of S primarily because the Fer mi level is pinned near midgap, so that fls + p, is minimized (note that n,p, is constant).4 If a given surface treatment pins the Fermi level near one ofthe band edges, then either ns or Ps will be large, regardless of the bulk doping, and Swill be reduced on both n-and p-type material. An additional mea surement, such as the capacitance-voltage data presented by Off.sey et al., 7 is necessary to eliminate this possibility. In the following, we demonstrate that the low-temperature PL spectra provide the needed evidence. A low-temperature excitonic luminescence spectrum from a VPE GaAs sample is shown in Fig. 2 beth before and after passivation by Na2S·9H20. The spectrum of the un treated sample is typical of high-purity GaAs.12 In particu lar, notch or dip features are observed at the locations ex pected for both ground-state neutral donor-bound exciton CD o,X')and longitudinal free-exciton (FE) luminescence. The notch in the (D o,X) peak is usually observed in n-type 2023 Appl. Phys. Lett., Vol. 51, No. 24. 14 December 1987 8170 ENEIlGY (e\l) 1.5'i50 ~ .5125 1.5100 8180 {DO, Xl 'I (0'. Xl! (!P. h) I 8190 VPE GaAs T =1.8K PL ". 700 mW/cm2 Aexc=5145A 8200 8210 8220 INAVELENGTH (A) FIG. 2. Normalized eltcitonic luminescence spectra (intensity in arbitrary units) of !In n-type VPE GaAs layer (n = 4>< 10'4 em 3, 77 K mobil ity = 92 000 em2/V s) before and after NazS'9H20 surface passivation. The position of the ground state (D oX) peak is indicated by the tic mark at 81 8i A. Two excited states are also marked at shorter wavelengths. Incident laser intensity (Pl. ) and wavelength CAe,c) are indicated. material at a sufficiently high excitation intensity, and is as cribed to self-absorption in a region near the surface where the density of the (D 0 X) complexes is reduced by surface recombination. 12,13 The notch i.n the free exciton/polariton peak has been ascribed both to enhanced trapping of polaritons near the surfacel4-16 (i.e., nonradiative surface recombination) and to self-absorption in a portion of the space-charge layer at the surface where the polaritonic resonance is modified by the electric field.17 Irrespective of the chosen model, the presence of surface charge and/or surface recombination centers is known to be essential in producing the notch, since surfaces cleaved in vacuum do not display the notch until they are exposed to oxygen 18 and samples clad with an AIGaAs cap layer do not display the notch until the cap layer is removed. 17 After the NazS'9HzO coating is applied, both netches are eliminated. A smaH amount of inhomogeneous strain, resulting from. differential thermal contraction of the GaAs and the Na2S film, is believed to be responsible for the slight broadening of the (A 0,x) doublet and the (D o,X) excited states; the latter peaks are no longer resolved. The absolute PL intensity is reduced by a factor of about 3.5, which we tentatively attribute to scattering by the polycrystalline sur face film. The absence of improvements in the PL efficiency suggests that rec.ombination associated with the bare surface does not limit the lifetime at low temperature as it does at 300K. Similar data are presented in Fig. 3 for a p-type MBE sample. The spectrum of the sample before passivation is dominated by (A 0,x) and "defect" acceptor-bound exciton (d,x) features; a strong notch is present in the center of the Skromme et al. 2023 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.24.51.181 On: Sun, 30 Nov 2014 01:56:051 5150 MBE GaAs T ~ 1.81< ENERGY (eV) 1.5125 PL RI 700 mWlcm2 >-"xc = 5145;' 8170 No,S APPLIED I 8180 8190 CIA', X) ,.,.... 8200 WAVELENGTH (A) 1.5100 (d. X) 8210 8220 FIG. 3. As Fig. 2, but for ap-typeMBE GaAs sample (p = 4x 1015 cm-3), before and after Na2S'9HzO surface passivation. FE peak. After application of aN a2S' 9HzO film, the (A () ,x) and (d,x) peaks are broadened by the strain effect men tioned above. More important, the FE line shape no longer displays any notch. The presence of the FE notch in the PL spectrum of this untreated p-type sample has some interesting implications for the mechanism by which such notches are produced. In particular, Lee et al. and Steiner et ai. have argued that neu tral donor concentration is an important factor in producing the notch.15,16 The proposed mechanism involves elastic scattering of the polariton modes in the vicinity of the longi tudinal exciton energy by neutral donors, which in conjunc tion with the small group velocity of these modes slows the diffusive transport ofpolaritons from the bulk to the surface and thereby enhances the effects of inelastic scattering and surface trapping. In the p-type sample of Fig. 3, the neutral donor concen tration is essentially negligible, since the (D o,X) peak is vir tually absent. Therefore, the notch in the FE peak in this sample cannot be attributed to the effects of polariton scat tering by neutral donors. Elastic scattering of polaritons by neutral acceptors might be considered in this case, but the calculations of Lee et 01. 15 conclude that acceptor scattering is much less effective than donor scattering, and will produce only an asymmetric single peak rather than a notched "dou ble" peak. The presence of notches in our moderately doped n-and p-type material and their absence in some high-resistivity p type samples studied by Lee et al.15 is consistent with the 2024 Appl. Phys. Lett., Vol. 51, No, 24, 14 December 1987 role of residual surface fields on the spectral transmissivity ofthe surface for polaritons, as emphasized in the theory of Schultheis and Tu. 17 Surface fields are expected for moder ate illumination if the equilibrium bulk Fermi level is close to either band edge (since the surface Fermi level is pinned midgapl ), but not in high-resistivity samples where the bulk Fermi level is near midgap. Direct measurements of the sur face field under illumination would be highly desirable to test this model. The elimination of the surface-related notches in the spectra of both n-and p-type material rules out the possibil ity that the surface treatment has repinned the Fermi level near one of the band edges. In that case, a large electric field would exist at the surface under moderate illumination on either the n-or the p-type material, which is known to pro duce a notch in the FE peak. 17 An upper limit on the residual band bending in the illuminated n-type sample at low tem perature is estimated to be aboutO.15 V, based on a compari son with the data of Ref. 17. We therefore conclude that the Na2S' 9H20 treatment substantially reduces the density of pinning and/or recombination centers on oxygen-exposed GaAs surfaces. We would like to thank R. Bhat, H. M. Cox, J. Harbi son, and M. C. Tamargo for supplying the samples used in this study. We would also like to acknowledge helpful dis cussions with R. N. Nottenburg and a critical reading of the manuscript by D. E. Aspnes. 'w. E. Spicer, P. W. Chye, P. R Skeath, C. Y. Su, and I. Lindau, J. Vac. Sci. Techno!. 16,1422 (1979). 2T. E. Kazior, J. Lagowski, and H. C. Gatos, J. App\. Phys. 54, 2533 (1983), 3L. G. Meiners. 1. Vac. Sci. Techno!. 15, 1402 (1978). 4D. E. Aspnes, Surf. Sci. 132, 406 (1983). 'C. H. Henry, R. A. Logan, and F. R. Merritt, J. Appl. Phys. 49, 3530 (1978). 6R. J. Nelson, J. S. Williams, H. J. Leamy, B. Miller, H. C. Casey, Jr., B. A. Parkinson, and A. Heller, App!. Phys. Lett. 36, 76 (1980). 7S. D. Offsey, J. M. Woodall, A. C. Warren, P. D. Kirchner, T. I. Chappell, and G. D. Pettit, Appl. Phys. Lett. 48, 475 (1986). "c. J. Sandrofl', R. N. Nottenburg, J. C. Bischoff, and R Bhat, AppL Phys. Lett. 51, 33 (1987). 9E. Yablonovitch, C. J. Sandrafl', R. Rhat, and T. Gmitter, App!. Phys. Lett. 51, 439 (1987). IOH. C. Casey, Jr. and E. Buehler, App!. Phys. Lett. 30,247 (1977). liB, J. Skromme, C. J. Sandroif, E. Yablonovitch, T. Gmitter, L. A. Far row, and R. N. N ottenburg, presented at the Electronic Materials Confer ence, Santa Barbara, June 24-26, 1987. 12H. Venghaus, J. Lumin. 16, 331 (1978). "D. C. Reynolds, D. W. Langer, C. W. Litton, G. L. McCoy, and K. K. Bajaj, Solid State Commun. 46, 473 (\983). 14C. Weisbuch and R. G. Ulbrich, 1. Lurnin. 18/19, 27 (1979). 15Johnson Lee, Emil S. Koteles, M. 0. Vassel[, and I. P. Salerno, J, Lumin. 34, 63 (1985), and references therein. [('T. Steiner, M. L. W. Thewalt, E. S. Koteles, and J. P. Salerno, Phys. Rev. B 34, 1006 (1986). 17L. Schultheis and C. W. Tu, Phys. Rev. B 32, 6978 (1985). IRB. Fischer and H. J. Stolz, Appl. Phys. Lett. 40, 56 (1982). Skromme et al. 2024 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.24.51.181 On: Sun, 30 Nov 2014 01:56:05

1.2811402.pdf

Walter H. Brattain John Bardeen Citation: 41, (1988); doi: 10.1063/1.2811402 View online: http://dx.doi.org/10.1063/1.2811402 View Table of Contents: http://physicstoday.scitation.org/toc/pto/41/4 Published by the American Institute of Physics SCIENTIFIC GRAPHICS C02 SAT. 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I -800-972-1 021 (or 303-279-1021) GOLDEN SOFTWARE, INC. 807 14th St., Golden, CO 80401 CSHIPurchase orders are welcomeOBITUARIES Walter H. Brattain Walter H. Brattain, co-inventor of the transistor and a Nobel laureate, died 13 October 1987 at the age of 85 after a long illness. He spent the bulk of his career at Bell Telephone Labora- tories, returning to teach at his alma mater, Whitman College, in his retire- ment years. A descendent of a pioneer Western family, Brattain spent his childhood on a homestead cattle ranch in Tonas- ket, Washington, and was always proud of his Western heritage. He graduated from nearby Whitman Col- lege in 1924, one of a famous class of four students of Benjam in H. Brown, an exceptional teacher of physics. The other three also went on to distinguished careers: Walker Bleak- ney at Princeton, Vladimir Rojanski at Union College and at Harvey Mudd College, and E. John Workman as president of the New Mexico Institute of Mining and Technology. Brattain's parents had also attended Whitman and taken courses under Brown. After graduating in 1926 with an MS from the University of Oregon , Brattain took a sheep train east to attend the University of Minnesota. He was a research student of John T. Tate, doing his PhD thesis on "Effi- ciency of Excitation by Electron Im-pact and Anomalous Scattering in Mercury Vapor." While at Minneso- ta, he took one of the first courses in quantum theory given in the United States, under John H. Van Vleck, and he never lost interest in the subject. Brattain received his PhD in 1929. Before joining Bell Laboratories that year, he spent eight months with the radio divisio n of the National Bureau of Standards. Prior to World War II, Brattain worked at Bell Labs with Joseph A. Becker on thermionic emission and on semiconductor rectifiers. After the war he was selected to be an initial member of the newly formed solid-state division, founded to exploit the understanding of solids at the microscopic level made possible by quantum mechanics. His main inter- ests both before and after the war were on problems of surface physics. I first met Brattain in the early 1930s, when I was a graduate student at Princeton and he was working at Bell's West Street laboratories. When I joined Bell Labs in the fall of 1945, because of wartime crowding, I shared an office with him and with Gerald L. Pearson (who survivied Brattain by less than two weeks). Through them I became interested in semiconductors, and I worked closely with both of them during my six years at Bell. Brattain's first work with Becker Circle number 62 on Reader Service Card 116 PHYSICS TODAY APRIL 1988William Shockley, Walter H. Drottain and John Dardeen in 1948, shortly after the invention of the transistor. 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Telephone (415) 962-9620 Telex 184817 Fax (415) 962-9647 England AC Eleciro-Optics, Tarporley lapan: ABE Trading Company, Ltd., Osaka India: VS. Scientific Instrument Agencies, New Delhi Ask about our SEM Cold Stage System, Low Temperature Micro Probe for DLTS and X-Ray Diffractometer. Circle number 63 on Reader Service Card THE PS 120 C THE FIRST TRULY INTERACTIVE SUPERCONDUCTING MAGNET CONTROLLER Much more than just a power supply: • The PS120C provides remote interactive system control for all aspects of magnet operation. • All functions including ramp rate, set current, persistent mode switch operation, voltage limit, etc., are accessible via the RS232 or IEEE interface or by manual front panel control. • The PS120C is our standard 120 amp, 5 volt unit. Higher currents to 450 amps and voltages to 15 volts are also available. For further information on our microprocesso r controlled PS120C, contact: CRYOGENIC CONSULTANTS LIMITED Metrostore Building, 231 The Vale. London W3 70S. 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Box 5120 Stn"P' Circle number 64 on Reader Service CardLIMITED Telex: 053-4591 Fax: (613) 226-2802 Germany: MICROSCAN GmbH Hamburg 040/63 20 03-0 UK: LYONS INSTRUMENTS Hoddeson (0992) 467161 France: EQUIPMENTS SCIENTIFIQUES Sa Garches 741 90 90 Japan: MEISHO ELECTRONICS Tokyo (03) 816 6581 2 Circle number 65 on Reader Service Card Buyers Guid ... A revised and expanded di- rectory of equipment for physics research, offering easy access to over 1,500 "physics" products, many of which have not ap- peared elsewhere. Designed and edited for physi- cists, and counseled by a select Advisory Committee , the Guide will draw from the many thou- sands of items available from North American and European sources. The 1988 Guide will be published as a separate issue, not as a part of August PHYSICS TODAY. For information on listings (prod - uct and company), write to Ms. Elaine Cacciarelli, % American Institute of Physics. For adver- tising info, write or call the AIP Advertising Division. AMERI OF PHYSICS 335 East 45th Street New York, NY 10017 (212)661-9404was on effects of adsorbed layers on emission from tungsten cathodes. Later he did experiments to try to understand the physics of copper- oxide rectifiers. One of his last pro- jects before the war was a study of the oxidation of copper using a radioac- tive copper tracer. As the war was coming on, interest shifted to silicon cat's whisker detectors for radar. He was involved with some of the early work; in particular he noted a large photoeffect on the contact potential of a silicon surface. During the war Brattain worke d on the design of airborne magnetometers for subma- rine detection under the National Defense Research Committee at Co- lumbia University. The Bell solid-state division was formed in late 1945 as staff members returned from various wartime activi- ties. The semiconductor group, of which Brattain and Pearson were members, was one of several in a broad program of solid-state research. William Shockley was cohead of the division and head of the group. Other members in the initial semiconductor group were Robert B. Gibney, a phys- ical chemist, and Hilbert R. Moore, an electrical engineer. In the summer of 1945 Shockle y had suggested making a solid-state amplifier by exploiting the field-effect principle—namely by altering the conductance of a thin semiconducting film via application of a transverse field. In a simple form, the film is one plate of a parallel-plate condenser. Shockley's calculation indicated that if the induced charge in the film came from mobile carriers (conduction elec- trons or holes) the effect should be large enough to give amplification. When attempts to observe the effect failed, I suggested that the reason might be that the induced charge was in the form of electrons in states at the surface that shielded the interior of the film from the transverse field. This hypothesis of surface states led to several predictions that could be tested experimentally. In accordance with his background, Brattain decid- ed to concentrate on surface prob- lems, while Pearson studied bulk phenomena. Brattain's experiments helped to verify the existence of the surface states. In experiments done with Gibney, Brattain found that one could bypass the surface states if one applied the field through an electrolyte adjacent to the surface. He and I showed that the current to a cat's whisker point contact on a silicon surface biased in the reverse (high resistance) direction could be controlled by a voltage ap- plied through an electrolyte insulatedfrom, but surrounding, the cat's whisker. Later experiments showed even larger effects when silicon was replaced with germanium. To avoid the slow response time of the electrolyte, we tried to apply the field across a thin oxide layer on germanium. We found that the oxide, if present, was not insulating, but that there was a small effect on the reverse current in a direction opposite to what one expected from the field effect. We had discovered a new way to control the current flowing across a rectifying contact: the bipolar princi- ple, which involves flow of both types of carrier—conduction electrons and holes. It did not take long to create an amplifier that used the new principle. The point-contact transistor was dem- onstrated on 23 December 1947. A month later Shockley conceived of the superior junction transistor geome- try, in which all of the action takes place with the bulk of a semiconduc- tor rather than at metal-semiconduc- tor contacts. For these discoveries Brattain shared the 1956 Nobel Prize in Physics with Shockle y and me. Unfortunately Brattain did not live to see the 40th anniversary of his inven- tion. Through his remaining years at Bell, Brattain continued to work on surface problems. He devised meth- ods for measuring the energy distribu- tions of surface states and the cross sections for trapping of electrons and holes. His work was on "real" sur- faces—ones on which there is the usual thin oxide layer—rather than the "clean" surfaces that are of cur- rent interest. I collaborated with him for some time after I went to Illinois in 1951 , and he later worked with Charles G. B. Garrett and with Phillip J. Boddy. In the early 1960s, Brattain re- turned to Whitman College on a part- time basis, and he joined the faculty there after his retirement from Bell in 1967. One of his favorite courses to teach was one taught earlier by Ben- jamin Brown: "Understanding Science for Non-Science Majors." At Whitman, Brattain became interest- ed in problems of biophysics. He collaborated with scientists at Bat- telle Pacific Northwest Laboratories in Richland, Washington, on studies of ion flow through lipid bilayers. Very receptive to new ideas, Brat- tain was always ready to cooperate on suggestions for experiments on pro- posed new devices even when he had reservations about the outcome. He was one of the first experimenters with a good understanding of the Mott-Schottky theories of contact 118 PHYSICS TODAY APRIL 1988 See what you couldn't see before Olympus Remote Observation Systems Olympus Remote Observation systems let you see and analyze processes and equipment internals that are other- wise difficult—even impossible—to see. Watch from a re- mote location a magnified image of an evolving process . Inspect without disassembly inside vacuum chambers, particle beam lines and cavities, cryogeni c systems , magnet assemblies, and a host of other equipment. Put borescope probes through opening s as small as 0.9mm. Snake fiberscopes and image carriers around ob- stacles with no interference with procedure to remote obser- vation areas. Magnify high resolution images up to 200X. 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The Answer: Toll Free 1-(800) RECORDER'VBJ JIT ifff m i LINSEIS East: LINSEIS INC., P.O.Box 666, Princeton-Jet., NJ. 08550 Phone:(609)799-6282 Telex: 843 360 West: = LINSEIS CORP., P.O.Box 10991, — Marina Del Rey.CA 90295 THE RECORDER COMPANY Phone-. (213) 306-3112 Circle number 67 on Reader Service CardRAY TRACING on your PC, XT, AT, PS/2 or compatible! BEAM TWO for coaxial systems: • lenses, mirrors, irises • flats, spheres, conic sections • exact 3-D monochromatic trace • convenient built-in table editor • 2-D system and ray layouts • diagnostic plots • Monte-Carlo spot diagrams • least-squares optimizer • CGA, EGA, VGA, & Hercules $89 postpaid if prepaid, quantity 1-9 CAadd 7% sales tax BEAM THREE for advanced work: • all the above functions • 3-D optics placement • tilts and decenters • polynomials & torics • diffractio n gratings • refractive index tables • CGA, EGA, VGA, & Hercules $289 postpaid if prepaid, quantity 1-9 CAadd 7% sales tax STELLAR SOFTWARE P.O. BOX 10183 BERKELEY, CA 94709 Circle number 68 on Reader Service Card PHYSICS TODAY APRIL 1988119 Not by Design The Origin of the Universe Victor J. Stenger A strong, confident case supporting the hypothesis that the universe originated because of a series of spontaneous, random events, devoid of plan or design. Using the laws of physics, Stenger shows that order can and does happen every day—by chance. Victor J. Stenger is professor of physics at the University of Hawaii. He has been a visiting professor at Oxford University, Heidelberg University, and the National Insitute for Nuclear Physics in Italy. 200 pages (Illustrated) ISBN 0-87975-451-6 Cloth $22.95 A Physicist's Guide to Skepticism Milton A. Rothman The laws of physics provide clear-cut principles defining what is possible—and not possible—in the physical world. Rothman examines many widely held pseudoscientifi c beliefs in light of these laws. Milton A. Rothman is a former professor of physics at Trenton State College and a former research physicist at the Franklin Researc h Center in Philadelphia. 250 pages ISBN 0-87975-440-0 Cloth $19.95 Prometheus Books 700 East Amherst St., Buffalo, N.Y. 14215 Call toll free (800) 421-0351 In N.Y. State, call (716) 837-2475 Add $2.00 for postage and handling; N.V. Start rc.idt.-nt* add sak-s tax. • Circle number 69 on Reader Service Card 120 PHYSICS TODAY APRIL 1988rectification developed just prior to World War II. One has the feeling that if he had not engaged in the particular series of experiments that led to the transistor, he would have been involved in another series that would have been successful not much later. Brattain often expressed the view that the transistor radio might help bring the peoples of the world closer together: "All people can listen to what they wish independent of what dictatorial leaders might want them to hear and I feel that this will eventually benefit society." Brattain was a member of the Commission on Semiconductors of the International Union of Pure and Ap- plied Physics, and he served as its chairman in 1966. He was also a member of the Defense Science Board and of various advisory committees. While history will remember Wal- ter Brattain for his achievements, I will remember him as a close personal friend, golf and bridge partner, and colleague. JOHN BARDEEN University of Illinois at Urbana-Champaign Urbana, Illinois Arthur H. Cooke Arthur Hafford Cooke, born 13 De- cember 1912, died in Oxford, England, on 30 July 1987. He had recently retired as warden of New College, Oxford, a position he had held since 1976. Prior to this, he had been associated with the Clarendon Labo- ratory, Oxford, for more than 40 years as an undergraduate, graduate stu- dent, university demonstrator and lecturer, and finally as a reader in physics. His early work before World War II, under Frederick A. Lindemann (later Lord Cherwell ) and Francis Simon , was concerned with the production of low temperatures, and he helped to establish Oxford as one of the early centers of cryogenic research. Most of his later work was also devoted to low- temperature physics and, in particu- lar, to the magnetic and thermal properties of rare earth and transi- tion metal salts. His work on para- magnetic relaxation and hyperfine effects led in 1953 to the discovery of cerium magnesium nitrate, which soon became the accepted standard for the production and measurement of temperatures in the millikelvin range. His insight also led to the discovery of the first Ising-like mate- rial, cerium ethyl sulfate, in 1951 , and in 1959 to the recognition of the firstdipolar ferromagnet, dysprosium ethyl sulfate. In 1970, Cooke and his associates found another prototypical material, dysprosium vanadate—the first example of a crystal with a magnetically controllable Jahn-Tell- er distortion. All of these studies stemmed from the same common thread: a detailed understanding of the macroscopic properties in terms of microscopic interactions as revealed by micro - wave paramagnetic experiments then being developed in the Clarendon Laboratory. Cooke's contact with microwaves started during the Sec- ond World War, when he worked on radar for the Admiralty team at Oxford. He designed the "transmit- receive" cell, which played an impor- tant part in the battle against U- boats. For his wartime services, he received a royal award: He was made a Member of the Order of the British Empire. Cooke was an inspiring teacher. His enthusiasm for physics was infec- tious and he was unusually effective in persuading the student to think for himself, constantly checking the rea- sonableness of each idea. Cooke had a clear and intuitive feel for physics that never led him astray. Where others became bogged down in formal- ism he proceeded by common sense. He used the backs of many envelopes to explain observed effects. This same common sense also made him an outstanding administrator. He served from 1969 to 1983 as a member of the Hebdomadal Council of Oxford University and for ten years on the General Board of Faculties, including a period as de facto chair- man. His thoughtful and fair ap- proach to all matters and his tactful and witty manner earned the respect of all who knew him. Cooke was by nature a shy man, but he had many friends. Everyone around him appreciated his engaging sense of humor and his even tempera- ment. There must be some with whom he battled, but they would be hard to find. He cared for people and they cared for him. His terminal illness was diagnosed three months before the end, and during this time a constant stream of friends and col- leagues came to see him, some travel- ing from far away. His unfailing courtesy and his personal interest in each visitor continued up to the end, and his ex-students, now well on in their own professional lives, found they could still learn from this witty, wise and gentle man. WERNER P. WOLF Yale University New Haven, Connecticut U

1.341693.pdf

Some properties of bulk YBaCuO compounds containing SiO2 C. X. Qiu and I. Shih Citation: Journal of Applied Physics 64, 2234 (1988); doi: 10.1063/1.341693 View online: http://dx.doi.org/10.1063/1.341693 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Properties of laser ablated superconducting films of YBaCuO AIP Conf. Proc. 219, 569 (1991); 10.1063/1.40210 Characterization of YBaCuO superconducting thin films prepared by coevaporation of Y, Cu, and BaF2 Appl. Phys. Lett. 54, 1365 (1989); 10.1063/1.101402 Spectroscopic and ion probe measurements of KrF laser ablated YBaCuO bulk samples Appl. Phys. Lett. 53, 534 (1988); 10.1063/1.100628 Role of added fluorine to enhance the electromagnetic properties of superconducting YBaCuO compounds Appl. Phys. Lett. 52, 1528 (1988); 10.1063/1.99697 Magnetic properties of YBaCuO superconductors J. Appl. Phys. 63, 4167 (1988); 10.1063/1.340530 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 136.165.238.131 On: Wed, 24 Dec 2014 19:26:49Some properties of bulk y .. Sa .. Cu-O compounds containing 8102 C. X. Qiu and I. Shih Electrical Engineering Department, McGill University. 3480 University Street, Montreal, Quebec H3A 2A 7, Canada (Received 4 January 1988; accepted for publication 10 May 1988) Bulk Y-Ba-Cu-O samples containing Si with nominal compositions ofY:Ba:Cu:Si = 1:2:3:x (x = 0.2, 0.4, and 0.6) have been prepared by adding SiOz to the source materials. After a relatively short heat treatment, the resistance transition characteristics of the samples were found to improve as the SiOz content was increased. Results from ac susceptibility measurements also showed improved values as the Si02 content was increased. However, as the solid-state reaction was carried out at a lower temperature but for a longer time, the sample resistance increased and the superconductivity was found to degrade. Several recent attempts have been made to deposit thin films of the high Tc superconducting compound YBa2 Cu, 07 on Si or Si02/Si substrates. 1,2 These attempts were made in order to develop this material for future high speed electronic device and circuit application. In such film deposition experiments, a heat treatment at an elevated tem perature in oxygen was generally required. The heat treat ment, whieh was required to form the superconducting films, resulted in diffusion of Y, Ba, andlor Cu into Si or Si02 .2 For this structure, it was also a general belief that, in addition to the in-diffusion mentioned above which de stroyed the stoichiometry, the out-diffusion ofSi atoms from the substrates into the films also will result in nonsupercon ducting materials. Although the actual mechanisms were not known, all of the reported attempts on film deposition involving Si or SiOz were not successful. Recently, Y-Ba Cu-O films with a thickness of about 30 p,m have been pre pared in our laboratory using a paint-on method.3 A resis tance transition onset temperature of 103 K has been observed on a treated sample deposited on a Si substrate. However, the zero-resistance state was not reached even at 77 K. In order to develop a procedure for the thin-film depo sition on these substrates, it is necessary to know the effect of Si atoms on the superconducting properties of the Y -Ea-Cu° compound. In the present work, we have prepared Y -Ba Cu-O bulk samples containing SiOz and the results obtained are reported in this communication. Samples used in the study were prepared by three differ ent procedures. Procedure (A): Weighted amounts of Y203, BaCO}, and CuO (nominal purity 99.9%) were thoroughly mixed and then pressed to form disks with a di ameter of 2.1 cm and a thickness of about 0.4 cm. The disks were sintered at 900 °C in air in a horizontal furnace for a period of 5 h. Following this, the sintered materials were powdered again. Powder ofSi02 (99.9% purity) was then added to the sintered Y -Ba-Cu-O materials to a composition ofY:Ba:Cu:Si = 1:2:3:x (x = 0.2,0.4, and 0.6). Disks were prepared and heated at 1000 °C in air for a period of 8 h. After this heating, oxygen was allowed to flow through the furnace tube and the temperature was decreased to 600 °C (maximum cooling rate about 10°C/min) and maintained at this value for a period of 12 h. The temperature was finally reduced to room value over a period of 3 h. Procedure (B): Weighted amounts of Y20" BaCO, , CuO, and Si02 (x = 0.2,0.4, and 0.6) were mixed and pressed into disks. The disks were first treated in air at 600°C for about 1 h. After this short treatment, the furnace temperature was in creased to 1000 "C and maintained at this level for a period of 5 h. The temperature was then reduced to 600 °C and the low-temperature treatment was allowed for a period of 12 h. The treatment using the procedure (B) was carried out in air. Procedure (C): Weighted amounts of Y203, BaC03, CuO, and Si02 (x = 0, 0.2, 0.4, and 0.6) were mixed, pressed and heated at 900 °C for 2 h in air. The materials were reground and pressed again to form disks. The disks were then treated at 950°C for 12 h in air. After this, oxygen was introduced and the samples were treated for a period on hat 700°C. After the complete treatment using procedure CA) de scribed above, samples with x = 0 and 0.6 were found to be similar to conventional sintered Y-Ba-Cu-O samples with some voids, especially in the region near the bottom surface. However, the samples with x = 0.2 and 0.4 adhered tightly to the alumina plate with a deformed globular appearance, a sign ofme1ting during the sintering at l000"C. For the sam ples prepared using procedures (B) and (C), no special ex ternal features were observed. However, for x = 0.4 and 0.6 dark gray samples were resulted. X-ray diffraction was made on samples prepared using procedure (B) and the results obtained showed characteris tic peaks which were consistent with those obtained for the orthorhombic Y-Ba-Cu-O compounds. Several weak peaks for other phases were also observed, suggesting the possible formation of these phases or an incomplete reaction of the source materials. For the samples prepared using procedure (C), x-ray results were also obtained and are shown in Fig. 1. All of the major peaks for these samples can be identified to belong to the orthorhombic YBa2 Cu, 07 _ d' For the sam ple without Si02, there are several weak peaks resulting from Y203, Y2CU20S' or other unidentified phases.4 The Y 203 peak decreases as the Sial content increases. The magnititude of the two peaks due to Y 2 Cu} Os and the un identified peaks increase as the Si02 content is increased. Furthermore, a weak Si02 peak appears with the addition of Si02 and the magnitude increases as the Si02 content in creases. Samples with a typical thickness of 0.2 em and a length of about 2.0 cm were cut for temperature-dependent electri- 2234 J. Appl. Phys. 64 (4). 15 August 1988 0021-8979/88/162234-03$02.40 @ 1988 American Institute of Physics 2234 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 136.165.238.131 On: Wed, 24 Dec 2014 19:26:49(Q20.006) (005) (103.110) -(003) x::O 50 40 30 20 28 (d egrees) FIG. 1. X-ray diffraction results for the YBa2Cu,07 with different Si02 contents. Peaks marked with # are for the Y2CU20, phase and those marked with * are due to unidentified phases. cal measurements. Results obtained for four samples pre pared using procedures (A) and (B) were compared and it was found that the room-temperature resistivity decreased as the Si content was increased. Furthermore, the transition temperature also increased with the Si content. This Si-con tent effect on bulk samples was not expected from the pre viously reported thin-film results on Si substrates. H In or der to confirm this effect, ac susceptibility measurements were carried out at 77 K. The measurements were made at 1 and 100 kHz using an HP model 4274A multifrequency LCR meter. During the measurements, the sample (typical weight 0.6 g) selected was located inside an induction coil with a room-temperature low-frequency inductance of 220 T ABLE I. ac susceptibility results of the YBa2 Cll, Si, 0, samples prepared using YEa2 Cu, 07 _ y/SiO, sources. /j,Ln/L,u" b Sample" Si02 at I kHz No. content, x (%) 35-1 0 --1.64 35-2 0 -1.92 32-1 0.2 -0.92 32-2 0.2 ·--0.62 33-1 0.4 -7.00 33-2 0.4 -7.11 34-1 0.6 -9.62 34-2 0.6 -9.07 "All samples prepared in the same experimental run. b L300 is the inductance of the empty coil at 300 K. 2235 J. Appl. Phys., Vol. 64, No.4, 15 August 1988 /j,L77/L;()o at 100 kHz (%) -1.89 --1.90 -0.94 ----0.48 -6.95 -··6.97 -9.60 -9.07 ,uH (inner diameter about 0.5 cm and the effective length about 2.0 cm). The sample temperature was then reduced by immersing it slowly in liquid nitrogen. The ac susceptibility was obtained by taking -!1Ln 1 L300• Here ilL77 is the dif ference of inductance values taken at 77 K with and without the Y-Ba-Cu-Si-O sample. Several samples were also checked using an independent HP model 4192A LF Imped ance Analyzer, yielding results within 2% of those obtained with the LCR meter. The results obtained for eight samples prepared using procedure (A) are shown in Table 1. It was found that the I1L77 1 L300 first decreased and then increased as the Si02 content x was increased to 0.6. For the samples prepared using procedure (B) described above, the Si02 content effect was clearly observed. The room-temperature resistance was found to decrease and the onset temperature increased from 81 to 93 K with the addition ofSi02. ac sus ceptibility measurements were also carried out and the re sults are given in Table n. In Table II, it is seen that the inductance difference increases as the x value is increased. The values for the samples with x = 0.6 are about 10 times of that for the two samples without Si02• The susceptibility results are thus qualitatively consistent with the onset tem peratures. The low onset temperature of about 80 K for the sample without SiOz was due to an incomplete reaction of the materials [by procedure (B) ]. In order to obtain further information, electrical measurements were carried out for the samples prepared using procedure (C). The results have shown onset temperature values from 93 to 95 K for all of the samples, which seem not to be affected by the presence of Si02• However, the room-temperature resistivity was found to increase as the SiOz content was increased. In Fig. 2, the resistivity of two samples (x = 0, and 0.6) are plotted versus temperature. It can be seen that the onset temperature of the two samples was about 94 K. For the sample without Si02, zero resistivity occurs at about 86 K. A more sharp transi tion was found for the sample with x = 0.6. However, a very small resistivity remained even at 78 K. This residual resis tivity was not found for samples with x = 0.2 and 0.40 ac susceptibility measurements were also carried out for sam ples prepared by procedure (C). Results showed an average -ilL77 1L300 value of about 6.13% at 1 kHz and about 6.38% at 100 kHz for all samples. TABLE II. ac susceptibility results of the YBa2 Cu, Six Oy samples prepared using Y203/BaCO,/CuO/SiO o sources. !J.Ln/L"X) !J.L77/L"Hl Sample" Sial at I kHz at 100 kHz No. content, x (%) (%) 36-2 0 --0.47 -0.42 36-3 0 -0.45 -0.52 37-2 0.2 -1.16 -1.16 37-3 0.2 .--1.S1 -1.49 38-2 0.4 -4.36 -4.30 38-3 0.4 --4.37 -4.30 39-2 0.6 -4060 ---4.30 39-3 0.6 0004.76 -4.46 a All samples prepared in one experimental run. "Measured during the heating cycle. C. X. Qiu and I. Shih Tn onset" (K) 81 85 87 93 2235 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 136.165.238.131 On: Wed, 24 Dec 2014 19:26:49e u E .J:::: '" .!! ,... .... ...... ". .... .... IJ") .,-, '" w a: 30 20 10 0 50 iOO 150 Temperature (K) 200 250 FIG. 2. Temperature-dependent resistivity characteristics for a sample without SiO, and a sample with Si02 (x = 0.6) prepared using proce dure (C). The present results on bulk Y-Ba-Cu-O samples have demonstrated that the presence oflarge quantities ofSi02 in this compound material did not result in apparent detrimen tal effects on the superconducting properties when the sam ples were treated for a relatively short period of time [proce dures (A) and (B)]. In fact, the resistance transition and the ac susceptibility measurements at 77 K both showed an improvement in the material quality. The critical current density of the samples was also found to increase by about 3- 4 orders of magnitude as the Si content increased from 0 to 0.6. However, under near optimum treatment conditions with a long period of time, quality of the samples did not show any further improvement with the addition of Si02 , even though the resistance transition temperatures were similar. After prolonged heat treatment, the sample with high Si02 content (x = 0.6) did not reach zero resistance state even at 77 K. For the samples with Si02• preliminary experiments have been made by immersing them in boiling deionized water for 10 min. The results showed that these samples were more resistant to water at the elevated tem perature than the samples without Si02• Further experi ments are being made and the results will be reported. 'T. Aida, T. Fukazawa, K. Takagi. and K. Miyauchi. Jpn. J. App!. Phys. 26, Ll489 (1987). 2M. Gurvitch and A. T. Fiory, AppL Phys. Lett. 51, 1027 (1987). 'r. Shih and C. X. Qiu, App!. Phys. Lett. 52, 748 (1988). 'J. M. Tarascon, L. H. Greene, W. R. McKinnon, and G. Hull, Phys. Rev. B 35,7175 (1987). Entropy optimization in quantitative texture analysis H. Schaeben Department afGeology, University of Bonn, Nussallee 8, 5300 Bonn 1. West Germany (Received 5 February 1988; accepted for publication 12 April 1988) The mathematical model of entropy optimization is introduced into texture goniometry to provide a solution of the problem of quantitative texture analysis, i.e., of reproducing an orientation distribution function from its corresponding experimental pole distribution function data. Let (Zp)p= 1 •...• P be a partlt10n of the unit sphere S3 = {rER3111rll = 1}, i.e., S3 = UZp, Zp nZq = if; if p=j:.q, and set for each rES 3 A P Pit (r) = 2:>p][z/r)/s(Zp), p= 1 (l) with Yp = { P" (r)ds(r»O, p = 1, ... ,P, Jzp where s(Zp) denotes the two-dimensional area of Zp, and ds( r) denotes an infinitesimal areal element of S 3 containing r, and with {I ifrEZp, Iz (r) = .p 0 otherwise. Then Ph (r) is a Zp patchwise constant approximate of the mathematical pole distribution function (pdf) Ph (r) with respect to the reflection of the crystal form {h}, where {Ii} denotes the set of symmetrically equivalent crystal direc tions of direction 11., liES 3. Analogously, let (Gn) n = t •...• N be a partition of the three-dimensional space G of orientations g, i.e., G = UG n' Gn nGm = if; if n=j:.m, and set for each gEG, A N f(g) = L x"lIGn (g)/v(G n), n~l with XII = r f(g)dv(g) >0, n = Ip .. N, Jan (2) where v ( G n) denotes the three-dimensional volume of G n , and dv (g) denotes an infinitesimal volume element of G con- 2236 J. App!. Phys. 64 (4). 15 August 1988 0021-8979/88/162236-02$02.40 @ 1988 American Institute of Physics 2236 ............................................. -..... : ..... : ...•.....•.........•...•... ( •.............. ····································'·l·'·'····· [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 136.165.238.131 On: Wed, 24 Dec 2014 19:26:49

1.4873751.pdf

Determination of adhesion between thermoplastic and liquid silicone rubbers in hard- soft-combinations via mechanical peeling test C. Kühr , A. Spörrer , and V. Altstädt Citation: AIP Conference Proceedings 1593 , 142 (2014); View online: https://doi.org/10.1063/1.4873751 View Table of Contents: http://aip.scitation.org/toc/apc/1593/1 Published by the American Institute of Physics Articles you may be interested in Injection molding of high precision optics for LED applications made of liquid silicone rubber AIP Conference Proceedings 1713 , 040003 (2016); 10.1063/1.4942268 Determination of Adhesion between Thermoplastic and Liquid Silicone Rubbers in Hard-Soft-Combinations via Mechanical Peeling Test C.Kühr1*, A.Spörrer1 and V.Altstädt2 1Neue Materialien Bayr euth GmbH, Germany – Christin.Kuehr@nmbgmbh.de; Andreas.Spoerrer@nmbgmbh.de 2 Polymer Engineering, Unive rsität Bayreuth, Germany – Volker.Altstaedt@uni-bayreuth.de Abstract The production of hard -soft- combinations via multi injection molding gained more and more importance in the last years. This is attributed to different factors. One principle reason is that the use of two -component injection molding technique has many advantages such as cancelling subsequent and complex steps and shortening the process chain. Furthermore this technique allows the combination of the properties of the single components like the high stiffness of the hard component and the elastic properties of the soft component. Because of the incompatibility of some polymers the adhesion on th e interface has to be determined. Thereby adhesion is not only influenced by the applied polymers, but also by the injection molding parameters and the characteristics of the mold. Besides already known combinati ons of thermoplastics with thermoplastic elastomers (TPE), there consists the possibility to apply liquid silicone rubber (LSR) as soft component. A thermoplastic/LSR combination gains in importance due to the specific advantages of LSR to TPE. The faintly adhesion between LSR and thermoplastics is currently one of the key challenges when dealing with those combinations. So it is coercively necessary to improve adhesion between the two components by adding an adhesion promoter. To determine the promoters influence, it is necessary to develop a suitable testing method to investigate e.g. the peel resistance. The curre nt German standard “VDI Richtlinie 2019”, which is actually only employed for thermoplastic/TPE combinations, can serve as a model to determine the adhesion of thermoplastic/LSR combinations. Keywords : Liquid Silicone Rubber (LSR), hard-soft-combination, VDI Richtline 2019, peel test INTRODUCTION Since the development of liquid silicone rubber (LSR) in the 1970 [1], this material showed an extreme rapid increase. In 2010 LSR received 11 % in the world market of silicone elastomers [2]. This trend is remarkable due to the distinguished thermic and mechanical properties, like fast curing, excellent temperature stability or good resilience. Therefore liquid silicone rubber can be used in many applications such as soothers, baking molds or seals in automotive area. Liquid silicone rubber belongs to the high temperature curing silicones and consists of a component A, which contains a platinum catalyst, and a component B with a cross-linking agent. After mixing the components in a 1:1 ratio, the composition can be cured at high temperatures and in a few minutes via hydrosilylation. The main trial for LSR processing is the liquid injection molding (LIM). In this connection the LSR components A and B are mixed via a static mixer at 20 °C and then the mixture is injected in a hot mold (140 – 200 °C). Besides the manufacturing of LSR products via one- component injection molding, the combination between thermoplastics and LSR gains more and more important, e.g. rain sensors in automobile industry. The interconnection of thermoplastic and LSR is assembled by two-component injection molding. However for the production of this combination it is coercively necessary to consider the faintly adhesion between LSR and thermoplastic. To improve the adhesion between LSR and thermoplast ic it is possible to add an adhesion promoter. In order to determine the adhesion quality and of course the influence of the adhesion promoter, it is required to use a suitable testing method. But at the moment no standardized technique exists, so the achieved results can’t be compared. The current German standard “VDI Richtlinie 2019”, which is actually only employed for thermoplastic/TPE combinations, can serve as a model to determine the adhesion of thermoplastic/LSR compounds. This test method offers the potential to determine the adhesion quality and the fracture surface, but also a calculation of the peel resistance. The aim of the presented paper is the evaluation of the mechanical bonding strength of selected thermoplastic/LSR combinations (e.g. PBT or PA) characterized on an adapted peel test device, which is abutted to the “VDI Richtlinie 2019”. EXPERIMENTAL Material The cross-linked LSRs (in the following labeled as LSR 1 and LSR 2) used in this study are two- component commercial grade rubbers produced by Wacker-Chemie GmbH in Germany. The two components A and B were mixed in a 1:1 ratio. As carrier material a polybutylene terephthalate (PBT) by Ticona was disposed. Proceedings of PPS-29 AIP Conf. Proc. 1593, 142-145 (2014); doi: 10.1063/1.4873751 2014 AIP Publishing LLC 978-0-7354-1227-9/$30.00 142 Two-component injection molding The test specimen in Fig. 1 was produced via two- component injection molding at a Krauss Maffei Multinject CXV 65-180/55. The screw of the vertical injection unit (thermoplastic) had a diameter of 20 mm and the horizontal unit (LSR) 25 mm. The thickness of the thermoplastic was 2 mm and the LSR had a thickness of 2 mm, too. Figure 1 – Test specimen produced via two-component injection molding. The yellow plate depicts the hard thermoplastic carrier material and the white dog bone represents the soft LSR. In Tab. 1 and Tab. 2 the recommended (material sheets from material supplier) and the adjusted parameter values are shown. Table 1 – Recommended parameter values from the material sheets of the material suppliers. PBT LSR 1 LSR 2 Melting [°C] 230 – 250 - - Mold [°C] 65 – 93 165 (in a press) 165 (in a press) Heating time [s] - 300 (in a press) 300 (in a press) Table 2 – Adjusted parameter values, which were used at the test specimen production. PBT LSR 1 LSR 2 Melting [°C] 230 – 250 - - Mold [°C] 80 200 160 Heating time [s] - 140 170 The production of a thermoplastic/LSR-combination is more difficult than the multi-injection molding of a thermoplastic/TPE-combination, because the hot thermoplastic melt is injected in a cold mold (60 – 80 °C) and in contrast to this, the cold liquid silicone rubber in a hot mold, as already mentioned. The process started with filling the first mold cavity vertical with hot thermoplastic. After a short cooling of the first carrier plate, the mold is opened and rotated by 180°. Then the mold is closed again, the liquid silicone rubber is injected onto the carrier plate. After a sufficient cross-linking time, the test specimen is demolded. Peel-Test The peel-test was performed on an universal testing machine Zwick Z2.5 to determine the peel resistance, the adhesion quality and the fracture patter. For that purpose the test specimen was clamped in a test slide, which was fixed via a tension rod. Then the soft component (LSR) was pulled off at a 90° angle (Fig. 2). The haul-off speed was 100 mm/min [3]. Figure 2 – Experimental setup of the peeling test. Fig. 3 shows a typical slope of a peel-test. The outcome of the peel-test is the averaged force F. Once the average force F has been determined, the peel resistance Ws can be obtained [2], ܹ௦ൌி (Eq. 1) where b is the width of the soft component (LSR). 0 20 40 60 80 100 1200810121416force [N] traverse path [mm]F Figure 3 – Typical curve of a peel-test. 143 RESULTS AND DISCUSSION Peel resistance of PBT with LSR 1 After the injection molding of the test specimen the interface of PBT/LSR 1 was analyzed by SEM. As shown in Fig. 4 the contact surface was firmly bonded between the two materials. No entrapped air or defects were visible in the interface. So it was possible to determine the produced test specimen by the peeling test according to “VDI Richtlinie 2019”. Figure 4 – SEM image of interface PBT with LSR 1. Subsequent to the peeling tests the fracture surface was examined by SEM, too. As seen in Fig. 5 and 6 some residues of LSR 1 remained on the carrier material. Thereby it was obvious in Fig. 6a to attribute the LSR residues to the injection point of LSR onto the thermoplastic. The silicone was radially spread in fluid line of the liquid silicone rubber. Like in Fig. 5 depicted, the silicone residues appeared not only at the injection point of the LSR, but also on some other positions of the thermoplastic. So it could be assumed that there existed a good adhesion at selected points of the carrier material. Figure 5 – SEM pictures of LSR 1 residues on PBT after a peeling test. Figure 6 – SEM pictures of LSR 1 residues at the LSR injection point of LSR. Besides the estimation of the fracture surface it was also possible to calculate the peeling resistance of the produced combination. The peel resistance was 1.32 ± 0.06 N/mm. This value indicated that an adhesion between the thermoplastic as carrier material and the LSR 1 as soft component already exists. However it is still nece ssary to elevate the peel resistance of this material combination. Comparison of the peel resistance of PBT with LSR 1 and LSR 2 As shown in Tab. 3 the peel resistance for the two liquid silicone rubber types was diverse. The PBT/LSR1 combination achieved a higher peel resistance than the test specimen of PBT/LSR 2. This result was attributed to different additives in the LSR types. Table 3 – Peel resistance of the thermoplastic/LSR- combinations. peel resistance [N/mm] PBT/LSR 1 1.32 ± 0.06 PBT/LSR 2 1.05 ± 0.05 But, as already mentioned above, it is required to examine the adhesion between the material combinations even more closely to increase the adhesion forces. One possible approach is the addition of additives, which are able to raise the peel resistance. a b PBT LSR 1 144Quality assessment of the test method At last the used test method was assessed. Therefore this mechanical examination method had to achieve different points of criticism: 1.Is it possible to peel the soft component? 2.Is it feasible to examine the fracture surface, the adhesion quality and to calculate the peel resistance? 3.Does the test setup show some influence onto the peeling? 4.Is the used geometry of the test specimen practical? As the represented results indicated the soft component could be peeled off without any complications. Thus the peel force could be observed via the used test method. Above the averaged peel force it was possible to calculate the peel resistance and evaluate at the same time the adhesion quality. In addition the fracture surface was simultaneously assessed. In the conducted tests no visible influence of the experimental set-up was evident. Furthermore the selected geometry of the test specimen seemed useful. CONCLUSION In this study two hard/soft-combinations were determined by a mechanical test method. A polybutylene terephthalate was used as hard component and two liquid silicone rubbers were applied as soft component. Besides the examination of the adhesion quality and fracture surface, the peeling resistance was calculated. Thereby it was noticed that the two different liquid silicone rubbers showed an adhesion on the carrier material (PBT). Furthermore LSR 1 showed a higher peeling resistance than LSR 2 and so this LSR type had a better adhesion to the hard component. Moreover some silicone residues, which were examined via SEM, remained on the carrier material. At last the used testing method was assessed. The mechanical test technique was abutted to the German standard “VDI Richtlinie 2019”, which is currently applied for thermoplastic/TPE-combinations. It could be shown that this method is absolutely suitable to determine the adhesion of a thermoplastic/LSR- combination. ACKNOWLEDGEMENTS We gratefully acknowledge STMWIVT (Programm Neue Werkstoffe in Bayern, NW-1204-0003 Cluster Neue Werkstoffe) and the support of our project partner for providing the material. REFERENCES 1. J. LeFan; M. Eng Saint-Gobain Performance Plastics 2011, 1. 2. U. Wachtler in Fachtagung Silikonelastomere, Würzburg, 2013, CD. 3. Verein Deutscher Ingenieure, VDI Richtlinie 2019,2011. 4. M. Bräuer; B. Hupfer; J. Nagel; U. Reuter Kautsch. Gummi Kunstst. 2006, ??, 115. 5. E. Delebcq; F. Ganachaud Appl. Mater. and Interfaces 2012, 4, 3340. 6. E. Haberstroh; C. Lettowsky J. of Polym. Eng. 2004, 24, 203. 7. E. Haberstroh; C. Ronnewinkel J. of Polym. Eng. 2001, 21, 303. 145

1.4876767.pdf

Deformation Sensor Based on Polymer-Supported Discontinuous Graphene Multi-Layer Coatings G. Carotenuto, L. Schiavo, V. Romeo, L. Nicolais Institute for Composite and Biomedical Materials. National Research Council. Piazzale E. Fermi, 1, 800 55 Portici (NA), Italy. Abstract. Graphene can be conveniently used in the modification of polymer surfaces. Graphene macromolecules are perfectly transparent to the visible light and elec trically conductive, consequently these tw o properties can be si multaneously provided to polymeric substrates by surface coating with thin graphene layers. In addition, such coating process provides the substrates of : water- repellence, higher surface hardness, low-friction, self-lubricatio n, gas-barrier properties, and many other functionalities. Po lyolefins have a non-polar nature and therefore graphe ne strongly sticks on their surface. Nano-crystalline graphite can be used as graph ene precursor in some chemical processes (e.g., graphite oxide sy nthesis by the Hummer method), in addition it can be directly appl ied to the surface of a polyolefin substrate (e.g., polyethylene) to cove r it by a thin graphene multilayer. In particular, the nano-c rystalline graphite perfectly exfoliate under the applic ation of a combination of shear and frict ion forces and the produced graphene sing le- layers perfectly spread and adhere on the polyethylene substrate surface. Such polymeric materials can be used as ITO (indium-t in oxide) substitute and in the fabrication of different electronic de vices. Here the fabrication of transparent resistive deforma tion sensors based on low-density polyethylene film s coated by graphene multila yers is described. Such de vices are very sensible and show a high reversible and reproducible behavior. Keywords : graphene, sonsors, optically tr ansparent, electr ical conduction. PACS: 72.80.Vp 07.07.Df 78.66.Qn 85.40.Hp. 1. INTRODUCTION The use of graphene-on-polymer to fabricate transparent strain sensors and other stretchable resistive sensors has been described in the literature [1-4]. The surface of non-polar subs trates can be modified by coating with graphene macromolecules. Polymeric films like polye thylene are very adequate non-polar su bstrates for such graphene coating. Depending on the amount of deposited graphene, both few-layers thick coatings and several-layers thick coatings can be achieved. When the surface of the polymeric substrate has been uniformly coated by graphene, continuity is established in the coating layer since electrons can move through the co ntacting graphene sheets. In particular, electron transport between the contacting graphene sheets in the interconnect graphene network takes place because adjacent graphene - orbitals overlap, extending the molecular wavefunctions to the full graphene coating layer [1]. For such a reason, in the case of an iso-oriented multi-layer graphene coating, electr on transport is not possible between adjacent layers but only inside the same layer. Consequently, a multi-layer graphene coating can be considered as a sort of multi-channel electrical conductor, and it is equivalent to a combination of resistors in a parallel connection. The equivalent resistance of equal parallel resistors is given by R=r/N where r is the layer resistance and N the total number of layers present in the multilayer. Since the deformation of a graphene layer caus es an increase of the distance between each graphene unit, thus interrupting some percolative paths, an increase of the multilayer resistance is observed for all types of film deformation. In particular, the bending of the LDPE substrate causes a similar deformation of the discontinuous graphene layers coating the substrate surface with a consequent reduction of the electrically conductive paths present in each planar layer of this coating. Actu ally the graphene layers placed above th e median plane get far away each other and the graphene layers located below this plane get closer. However, electrical conduction re quires an alignment of the planes which is obstructed in both cases. Such property can be used to fabricate deformation sensors based on polymer supported graphene multi-layers. The sensibility of such conduc tive sensors is related to the number of graphene layers contained in the coating ( R/R 01/N) and it decreases with increasing of the multi-layer thickness. Promptness of the Times of Polymers (TOP) and Composites 2014 AIP Conf. Proc. 1599, 18-21 (2014); doi: 10.1063/1.4876767 © 2014 AIP Publishing LLC 978-0-7354-1233-0/$30.00 18 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:37:25 graphene-based sensor is quite high since a variation in th e percolative path number takes place as it deviates from the planar configuration. In particular, because of the low intens ity of the Van der Waals forces involved in the interaction between adjacent graphene layers, this movement of the graphene sheets induced by the substrate deformation is completely reversible and a recovery of the electrical properties can be achieved. Owing to the scarce mechanical properties of the LDPE subs trate, the reversibility of the sensor must be ensured by an external elastic polymeric packaging which has also the important function to prevent the graphene coating from contamination that may falsify the deformation measurement. Because the electrical conductiv ity of the graphene layer can be influenced by the absorption of non-polar organic molecule s (e.g., hydrocarbon compou nds) on its surface, the graphene-LDPE system requires to be protected by encapsulation in an elastic polymeric layer. Such encapsulation can be easily done by sealing the deforma tion sensor between two PET films (plastification pouches). Because PET is a quite elastic polymeric material, such encapsulation has also the effect to provide the device of adequate flexural elasticity which is strictly required to achieve a convenient reversibility in the sensor answer. A thermally sealed PET packaging may represent a convenient choice for the LDPE-graphene device. The sensor measurements should be quite reproducible and free from hysteresis problems for little deform ation of the device and the thermal sealing also helps to increase reproducibility of the sensor behavior. Little deforma tions should not change irreversibly the distribution of graphene sheets present on the LDPE surface, consequently this type of device does not show hysteresis phenomena at least under moderate deformations. 2. EXPERIMENTAL LDPE films were uniformly coated by a thin layer of graphene simply by gently rubbing an alcoholic suspension of nano-crystalline graphite (few-layer graphene, FLG) on its surface by using a piece of LDPE. The FLG alcoholic suspension was produced according to a literature process [5]. The resulting graphene-based coating layer had a very uniform thickness that could be varied from a few layers to several by changing the conc entration of the alcoholic suspension. Because the graphene unities are optically tr ansparent (the opacity of a singl e graphene layer is 2.3% [6]) the coating layer resulted transparent, and transparency was depending on the number of graphene coating layers. In particular, an evaluation of the film thickness was possible based on the opacity value of the single graphene sheet (2.3%). The full layer opacity is given by 0.023N where N is the layer number, however this value can be measured (it corresponds to 1-T, where T is the film transmittance) and therefore N=(1-T)/0. 023. In order to prevent contamination and obtain an elastic system, the obtained graphene/LDPE films were sealed between two PET films (plastification pouches) together with the device electrodes (two aluminum strips) (see Figure 1). FIGURE 1. Deformation sensor based on graphene-coa ted LDPE films encapsulated into PET. 3. RESULTS AND DISCUSSION The special surface morphology which resulted when the surface of a LDPE film has been rubbed out with a FLG suspension in order to generate a uniform graphene coating layer has been accurately imaged by Scanning 19 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:37:25 Electron Microscopy “SEM-FEG Jeol JS M-7001F”. As shown in Figure 1, th e substrate surface results completely covered by the graphene layers which form planar percolative paths. The FLG nanocrystals are not completely exfoliated and some crystals adhering to the LDPE substrate are still present. 15 20 25 30 35 40 45 50 55 60 6502000040000Intensity 2 (deg) gct = 16.7 nm gct = 14,5 nm gct = 16,0 nm Pure substrate (LDPE) (002)(004)B FIGURE 2. SEM micrograph of the graphene-based layer coating the LDPE surface (A) and XRD with indication of graphene coating thickness (gct) measured by the Scherrer equation (B). 20 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:37:25 The structure of the very thin gr aphene-based layer coating the surface of the LDPE substrate has been investigated by X-ray powder diffraction (XRD). Diffuse halo and peaks at 21.4°, 23.69°, and 36.16° belong to the LDPE substrate. The graphene coating layer has a crystallin e nature and its diffraction pattern results locate at a 2 value lightly different from that of the FLG precursor. In fact , the (002) peaks is located in the diffractogram at 26.45°, while the starting FLG sample has this signal at 26.51° (the same signal is located at 26.6° for hexagonal graphite). Therefore, the interspacing in the graphene coating is higher, probably because of a higher concentration of defects. The layer thickness has been evaluated by applying the Scherrer formula to the (002) peak and it resulted of 16.7 1nm (corresponding to 50 5 crystalline planes). The mechanisms involved in the electrical conduction of graphene-based coatings are depending on the applied voltage. At low voltages (e.g., 10V) only an ohmic conductivity is involved. In this case the film resistivity is not depending on the voltage, and only the planar percolative paths contained in the multilayer coating contribute to electron transport. Electrons move th rough the graphene-to-graphene contact s present in each layer because of a wavefunction extension trough the contacting graphene unities. However, at higher electrical potential values (e.g., 100V) also hopping and tunneling conduction mechanisms can be involved in the electrical conductivity of the modified surface. In this case, the electrons move from neig hbor planes in the graphene-based coating layer. Thus, the resistivity value results strictly depending on the applied voltage. However, because electronic devices are usually working at low voltages, only the ohmic regime is important for a deformation sensor from a practical point of view. The electrical conductivity of the coating layer was measured by a resis tivity meter (Monroe Electronics, Model 272, configured for measurements of surf ace resistivity) and it was strictly depending on the coating thickness and significantly decreased with thickness decreasing to a few nanometers. An exponential behavior described the dependence of the surface resistance on th ickness for these systems (the best da ta fitting was achieved by the following equation: R=R 0·exp(-N/a)+b; where R 0=(6.1±1.7)·109, a=1.12±0.02, and b=115±20). 4. CONCLUSION Optically transparent resistive deformation sensors can be based on graphene-coated LDPE films conveniently protected by encapsulation in PET. Owing to the percolative structure of the graphene coating layer, deformation may cause a variation in the conductive path number, and conseq uently in the surface coating resistance. The discontinuous nature of the coating layer is essential for the working of such a kind of device. The electrically conductive and optically transparent graphene/polyethylene films were si mply fabricated by rubbing alcoholic suspensions of nanocrystalline graphite on the LDPE surface. 5. ACKNOWLEDGMENTS Authors knowledge Nicola Bazzanella, labo ratory technician at Dept. of Phys ics, University of Trento for the microscopical analysis of the samples. We are gratef ul to the Research Project “ENAM - PHYSICAL-CHEMICAL- BIOTECHNOLOGY FOR ENERGY AND ENVIRONMENT” for financial supporting of this work. REFERENCES 1. Z. Jing, Z. Guang-Yu, S. Dong-Xia, Chin. Phys. B 22, 057701 (2013). 2. J. Wang, Y. Geng, Q. Zheng, J.-K. Kim, Carbon 48 1815-1823 (2010). 3.X. Li, R. Zhang, W. Yu, K. Wang, J. Wei, D. Wu, A. Cao, Z. Li, Y. Cheng, R.S. Ruoff, H. Zhu, Sci. Rep. 2, 870 (2012). 4. S.-H. Bae, Y. Lee, B.K. Sharma, H.-J. Lee, J.-H. Kim, J.-H. Ahn, Carbon 51 236-242 (2013). 5. G.Carotenuto, V.Romeo, S.DeNicola, L.Nicolais, Nano Res Lett 8 (94), 1-6 (2013). 6. R.R. Nair, P. Blake, A. N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, Science 320, 1308 (2008). 21 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:37:25AIP Conference Proceedings is copyrighted by AIP Publishing LLC (AIP). Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. For more information, see http://publishing.aip.org/authors/rights- and- permissions.

1.4876781.pdf

Structural FEM analysis of the strut-to-fuselage joint of a two-seat composite aircraft Erik Vargas-Rojas*, Diego Camarena-A rellano, Hilario Hernández-Moreno IPN, ESIME Ticomán; Av. Ticomán 600; Col. San José Ticomán; México, 07340 *Contact author: erikvargasrojas@hotmail.com; T. (+33)787085133 Key Words : aircraft structure analysis, composite materials, FEM, geometrical modeling Abstract. An analysis of a strut-to-fuselage joint is realized in order to evaluate the zones with a high probability of failure by means of a safety factor. The whole section is analyzed using the Fini te Element Method (FEM) so as to estimate static resistance beh avior, therefore it is necessary a numerical mock-up of the section, the mechanical properties of the Carbon-Epoxy (C-Ep) material, an d to evaluate the applied loads. Results of the analysis show that the zones with higher probability of failure are found around the wing strut and the fuselage joint, with a safety factor lower than ex pected in comparison with the average safety factor used on air crafts built mostly with metals. INTRODUCTION The approach for the development of aircraft structures usi ng composite materials is similar to a testing pyramid, i.e, mechanical properties of constituent materials require an extensive tests campaign, conforming the base of the pyramid; later on, a less broad series of mechanical tests of standa rdized coupons that consider the constituent materials mechanical properties and the stacking sequence follow. Next , simple structural elements require even fewer but more specialized experimentation, so do complete structures [1]. However, the evaluation of a structure following this approach becomes a complex and expensive task [2,3], spec ially for small aircrafts manufacturers for which several full scale test may be cost prohibited when taking into ac count a low unit production rate. Additionally, the complete structural integrity analysis should consider other tests su ch as static resistance, environmental effects, fatigue and damage tolerance [1]. For this study, the static resistance analysis requires to measure the material mechanical properties of the C-Ep composite material, and to compare them against the stresses produced by flight loads. Other approaches exist for the analysis of aerospace stru ctures, such as the development of technological evaluators proposed by Grunevald and Collombet [4], who consider the tes ting of representative specimens as an alternative, or a complementary method to the testing pyramid, so costs can be reduced by considering focused analysis of critical zones of a structure, thus developing representative specimens an d extensively using numerical simulations with FEM software. In previous works, structural critical zone s have been already identified for the Stela-M1 aircraft, as part of a joint collaborative effort between Mexican aircraft manufacturer, Aeromarmi; and Instituto Politécnico Nacional of Mexico (IPN) [5-7]. This research has for objective to present the ev aluation of the static strength of the joint of the fuselage with the main landing gear and with the supporting strut of the wing , as depicted in Fig 1. In or der to achieve this goal, the physical and mechanical characterization process of the comp osite material are presented, as well as the digitalization process of the geometry under study, so a Product Life Manage ment (PLM) and a FEM software can be used robustly in order to identify the most stressed zones under the actual st ress state [8] produced by the most critical flight load case according to normalized design criteria for this kind of aircraft [9]. FIGURE 1 . (a) Strut-to-fuselage, and landing-gear-to-fusela ge joint (circled). (b) Single representative part. Times of Polymers (TOP) and Composites 2014 AIP Conf. Proc. 1599, 74-77 (2014); doi: 10.1063/1.4876781 © 2014 AIP Publishing LLC 978-0-7354-1233-0/$30.00 74 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:42:23 MATERIAL CHARACTERIZATION Physical characterization deals with the estimation of density measurements; matrix, reinforcement, and void volume fractions; stacking sequence (number and fiber orientation) and thickness. Volume fractions are obtained with the matrix acid digestion method, following the procedure A as presented in ASTM D 3171 [10]. Void content V p is obtained using the test method ASTM D 2734 [11]. Density measurement is achi eved by using the hydrostatic principle according to the test method described in ASTM D 792 [12]. According to [ 13], results of the physical char acterization considering 5 test coupons cut with the water jet process show that the average fiber volume fraction V f reaches 43.4%, matrix volume fraction V m a slightly higher value of 47.4%, and a porosity volume fraction V p equal to 9.2%. Respective standard deviation s is 6.2, 6.3, and 0.7%; so coefficient of variat ion CV is 14.22, 13.26, and 7.08%, respectively. An analysis using a metallurgical microscope allows to meas ure the thickness and to visualize the stacking sequence of the composite material which consists of nine bidirection al layers, each one with two main directions: 0° and 90°. Outermost layers 1 and 9 are made with glass fiber and epoxy resin and serve as protection and as a sacrifice material if sanding is needed to remove paint and for a smoother finish , whereas layers 2 through 8 are fabricated with thicker carbon fiber fabrics (0.578 mm per layer). Test methods for tensile and shear properties using tension specimen are described by ASTM standards D 3039 [14], and D 3518 [15]. Values obtained are intended to be used in FEM modeling as input data aiming robustness in results. Specimen dimensions for tensile and shear test fulfill minimu ms specified in standards because it was not possible to obtain the preferred dimensions (25 u 250 mm) due to limited material availab ility. Both kinds of coupons were instrumented with strain gages oriented in the transverse dire ction with respect to the load axis; axial strain measurement was accomplished with a standard extensometer. Stress-strain pl ots obtained with tensile tests allow to determine a chord elastic modulus, as well as the axial tensile strength; the axial Poisson ratio is obtained from the transverse-axial strain plot. With respect to shear properties, the coupons are oriented during cutting proc ess so as to have a ±45° fiber pattern with respect to the axial axis of the spec imen; in this case, a stress-strain tran sformation is needed so the shear elastic modulus and shear maximum stress can be calculated with a sh ear stress-angular strain. Acco rding to [8], results show for tensile test averages values of 308.3 MPa for tensile strength, 29880.52 MPa for elastic modulus, and 0.06 for Poisson ratio; respective s is 74.10 MPa, 6500 MPa, and 0.05; and CV is 25.96%, 19.96%, and 83.06%, respectively. With respect to shear tests using tensile test coupon, the averag e shear strength is 59.52 MPa, and the shear modulus has a value of 6592.07 MPa; respective s is 0.7, MPa and 463 MPa; and CV is 1.18%, and 7.02, respectively. NUMERICAL MODELING Loads applied onto the structure are du e to aerodynamical forces, inertial forc es, and reactions during landing and taxing. For this study, the most critical condition on flight mane uvering is used for the FEM an alysis; this extreme case is identified using the so called maneuver diagram , which is the plot of the load factor as a function of speed [16,17], obtained from a previous resear ch [11] based on the official calculus memento of the Stela-M1 [18], and on the requirements specified in FAR Part 23 [9]. It is worth to remark that for this airplane the gust diagram is comprised within the maneuver diagram [16]. The most critical cases are present at load factors of +4.4 and 2.2, so for the analysis it is chosen the case of the +4.4 load factor (at maximum dive speed). Additionally, it is pertinent to emphasize the extreme rare condition that this value could be reached along the lifetime of the airplane. The lift distribution over the midspan of the wing is calcu lated by Morales in [19]. Once the total half-wing lift of 2002.46 N is obtained, the load on the wing strut is calculated reaching a numerical value of 4879.15 N. Considering that the biggest in-flight load factor is 4.4, the calcu lated wing strut load reaches a value of 21469.87 N. The strut-to-fuselage joint is a geometrically complex part; so in order to digitize the actual geometry, two methods are used: in the first one standard metrol ogy instruments are used an d several geometrical simpli fications are considered, but results are not sufficiently accurate for the FEM anal ysis, so it was decided to employ a geometrical image correlation technique by photogrametry using the 3D Rhinoceros software. According to the software documents [20], the maximal deviation of this method is about 0.02 mm for a 10.2 Megapixel camera, as used for this study. To obtain the spatial location of the surface points, a cali bration procedure is needed to register the specific targets, which are then pasted on the surface of the part as it is shown on Fig. 2a; then, several pictures of the part are taken from different angles. After processing, the digitized surface, presented on Figure 2b, is ready for FEM analysis. The digitized geometry 75 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:42:23 was exported to CATIA and then again to ANSYS so it could be meshed as presented on Fig. 2c. A shell element with 8 nodes, able to simulate layered compos ites was chosen (SHELL 99), [21]. FIGURE 2 . Digitizing technique. (a) Geometry with targets. (b) Digitized part on CAE software. (c) Digitized part on FEM software. FEM method for structural analysis requires several material properties. In this case, in-plane strength in tension corresponds to mean experimental values; the tensile strength considered along the stacking direction is the value solely of the resin, as obtained from literature for an epoxy resin [22]. In a similar way, the in-plane shear strength is the mean experimental value. The compressive strength values are es timated considering the same tensile/compressive strength ratio reported in literature [23] for a car bon reinforced composite, i.e., if the experimental tensile strength is 308 MPa, the estimated compressive value lays around 246 MPa (1.25 ratio). Boundary conditions are estab lished trying to simulate as best possible the real conditions of the real structure, thus all degrees of freedom (DOF) are restrained along the borders, excepting the corner radius which is free of constraints beca use it is part of the door peri meter. The load imposed by the strut, as previously calculated, is distributed uniformly over the surface of the bo lted metallic plate attached to the fuselage. The rotational DOF of this surface were constraine d so as to simulate the presence of the metallic fitting between the strut and the fuselage. RESULTS AND DISCUSSION According to FEM results, normal stress and shear stress fields , it can be corroborated that the most stressed zones are located in the lower border where the metallic fitting is in contact with the composite shell. On future studies these zones must be subjected to careful examinations because they are the most likely to present damages, if ever appear. In Table 1, the maximum and minimum values of the stress are presented. TABLE 1 . Maximum and minimum stress values on nodes. Stress Maximum (MPa) Minimum (MPa) Stress Maximum (MPa) Minimum (MPa) Normal in x (Vl) 375.88 – 269.67 Shear in xy 027.16 – 031.93 Normal in y 082.73 – 075.62 Shear in yz 112.73 – 145.53 Normal in z (Vl) 355.81 – 267.65 Shear in xz (Wlt) 044.04 – 034.05 Once the stress fields are obtained, the Tsai-Wu strength cr iterion is used according to the equation 2, in order to calculate the strength factor R, which is analogue to the static safety factor for metals. In equation 2 the coefficients Fij are the elements of the Tsai-Wu tensor which can be calculated using the expressions for a, b, and c, that depend on the material strength properties. For the set of equations, V stand for normal stresses, W for shear stresses, l for longitudinal (along the fiber direction), t for transverse, T for tension, and C for compression. The minimum strength factor calculated has a value of 1.343, which means that the analyzed structural component may not present structural failure even for the worst flight scenario according to the maneuver diagram. a ac b b R 2/42r (1) tl lt t l F F F Fa VVWVV122 662 222 11 2 , t lF Fb VV2 1 , 1 c ( 2 ) C lT l Fmax max 1 /1 /1 VV , C tT t Fmax max 2 /1 /1 VV , C lT l Fmax max 11 /1VV , C tT t Fmax max 22 /1VV , 2 max 66 /1lt FW (3) 76 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:42:23 CONCLUSIONS x The geometrical modeling of a complex geometrical shape, constituted by the strut-to-fuselage joint is achieved by photogrametry using the 3D Rhinoceros software, and properly treated so as to use it for FEM with ANSYS. x Physical and mechanical characterization is necessary so the material properties allow to work with a FEM model as accurate and robust as pos sible. However, certain values are used as published in respective literature. x After exploiting the FEM model, the most stressed areas are identified and the respective numerical values allow to establish an approximate strength factor for the worst in-f light load case. For such a case, this value which means that the analyzed structural component may not present structural failure even for the worst flight scenario according to the maneuver diagram, which is an extrem e improbable case not expected during normal operation. x This study is the first step of a process which aims to design a representative specimen that permits the detailed analysis of the evolution of the stress and material conditions during the lifespan of the structure, known as health monitoring, which can be impact the manufacture proc ess, inspection procedures, and maintenance practices. ACKNOWLEDGEMENTS Authors gratefully acknowledge the logistical support of Architect Nestor Romero-P., Head Director of Romfer Industries; of Engs. Gerardo Cortés-M. and Carlos Martínez-G. from Aeromarmi; of Dr. Jorge-Luis González-V., Head Director of GAID Research Group for his scientific advice; as well as the former directives of ESIME Ticomán, in particular to Director Eng. Miguel Álvarez-M., and Academical Sub-Director Eng. Porfirio Sarmiento-M. REFERENCES [1.] US Department of Defense. Handbook Polymer Matrix Composites Vol. 1, Guide lines for characterization of structural materials . MIL-HDBK-17-1E. DOD. 1997. p. 6 –1 through 6 –11. [2.] Karen E. et al. A history of full-scale aircraft and rotorcraft crash testing and simulation at NASA Langely Research Center , 15th-18th November 2004; 4th International Aircraft and Cabin Safety Research Conference. Lisbon, Portugal. [3.] Tomblin, J., Seneviratne, W., F AA Research on large-scale test substantiation, damage tolerance and maintenance workshop . Rosemont, Chicago, IL, July 19th-21st, 2006. [4.] M. Mulle, F. Collombet, B. Trarieux, J.-N. Périé and Y.-H. Grunevald. Démonstrateur technologique multi-instrumenté (réseaux de Bragg et mesure de champs). Revue des Composites et Matériaux Avancés. 15(1)33-51. 2005 [5.] Vargas-R., E. Project Aeromarmi –ESIME Ticomán: development of an utility aircraft fabricated with composite materials . In Spanish. Internal document. 2009. p6. [6.] Hernández, H. Proposal for the study of the structural integrity of the Stela-M1 aircraft . In Spanish. 2009. p10. [7.] Bello-Olvera, O. E. Conceptual design approach of a six-seat aircraft using composite materials . BSc thesis, ESIME-Ticomán, IPN. In Spanish. México. 2010. [8.] Camarena-Arellano D., FEM analysis of the strut-to-fuselage, and landing-gear-to fuselage joint of a composite, low-weight, two-seat, and single-engine aircraft . BSc thesis, ESIME-Ticomán, IPN. In Spanish. México. 2009. [9.] FAA. FAR Part 23. Airworthiness standards: Normal, utility, acrobatic, and commuter category airplanes . [10.] ASTM Standard D 3171-76. Method for fiber content of resin-matrix composites by matrix digestion . [11.] ASTM Standard D 2734 – 91; Method for void content of reinforced plastics . [12.] ASTM Standard D 792 – 91; Method for density and specific gravity (relative density) of plastics by displacement . [13.] Arellano, D., Vargas, E., Hernández, H. Medición de fracciones volumétricas en materiales compuestos C-Ep y G-Ep por digestión y calcinación de resina . 5º CIIES. México. 10-14 Noviembre 2008. [14.] ASTM D 3039M – 00. Method for Tensile properties of polymer matrix composite materials . [15.] ASTM D 3518M – 94. Method for in-plane shear response of polymer matrix composite materials by tensile test. [16.] Niu, M. Airframe structural design . Hong Kong, Conm ilit Press LTD. 1999. p.612. [17.] Bruhn, E. Analysis and design of aircraft structures . Cincinnati: Tri-State Offset Co., 1958. [18.] Stela-M1, Calculus Memento . In Spanish. Aeromarmi SA de CV. España, 2005. [19.] Morales-Hernández, A. Analysis of stresses in order to design an inspection access panel under the wing of the Stela-M1 aircraft . BSc thesis, ESIME-Ticomán, IPN. In Spanish. México. 2008. [20.] Rhinoceros 4.0 User’s guide . [21.] Release ANSYS 10.0 Documentation. [22.] Miravete A., Larrodé E., Castejón L., Materiales Compuestos Tomo I. Editorial Reverté, España, 2000. [23.] Gay, D., Matériaux composites . Édition Hermes – Lavoiser, France 2005. 77 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:42:23AIP Conference Proceedings is copyrighted by AIP Publishing LLC (AIP). Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. For more information, see http://publishing.aip.org/authors/rights- and- permissions.

1.4876779.pdf

Thermoplastic matrix composites for aeronautical applications – The amorphous/semi-crystalline blends option Michele Iannone, Floriana Esposito, and Aniello Cammarano Citation: AIP Conference Proceedings 1599, 66 (2014); doi: 10.1063/1.4876779 View online: http://dx.doi.org/10.1063/1.4876779 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1599?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Simulation of shrinkage and warpage of semi-crystalline thermoplastics AIP Conf. Proc. 1664, 050009 (2015); 10.1063/1.4918413 Time Issues in Semi‐Crystalline Thermoplastics Processing AIP Conf. Proc. 1255, 19 (2010); 10.1063/1.3455578 Application of Image And X‐Ray Microtomography Technique To Quantify Filler Distribution In Thermoplastic‐ Natural Rubber Blend Composites AIP Conf. Proc. 1202, 139 (2010); 10.1063/1.3295585 THERMOPLASTIC COMPOSITE MATERIALS FOR AEROSPACE APPLICATIONS AIP Conf. Proc. 1042, 276 (2008); 10.1063/1.2989032 REVERSE AGING OF COMPOSITE MATERIALS FOR AERONAUTICAL APPLICATIONS AIP Conf. Proc. 1042, 163 (2008); 10.1063/1.2988987 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.247.112.3 On: Thu, 13 Aug 2015 18:35:43Thermoplastic Matrix Composites for Aeronautical Applications – The Amorphous/Semi-Crystalline Blends Option Michele Iannonea , Floriana Espositoa and Aniello Cammaranob aAlenia Aermacchi, Viale dell’Aeronautica snc, 80038 Pomigliano D’Arco (Naples) Italy bIMAST SCaRL, Piazza Bovio 22, 80133 Napl es, Italy Abstract. Blends obtained by mixing high temperature applications thermoplastics have been investigated. Namely the blends considered in this work are made by semi -crystalline thermoplastics PEEK with amorphous PEI. The final goal is to analyse the mechanic al, chemical -physical and environmental resistance characteristics of these blends to evaluate their suitability as matrices of carbon reinforced composites for aeronautical structural applications . The first collected results are very promising. Keywords: Composites, Aeronautical Structures, Thermoplastic, PEEK, PEI, Blends. PACS: 81 INTRODUCTION Composites for structural aeronautical applications are generally based on carbon fiber reinforcement and Polymeric Thermo setting Matrix, mainly epoxy. Several research and development activity has been performed to evaluate the possibility to utilize thermoplastic based matrices. In fact thermoplastics appear very promising, due the high toughness, the potential re -processability, weldability and the easier recycling. Furthermore thermoplastics don’t require refrigerated storage and transport, which is required for the thermosetting which have also a limited storage and handling time. One of the reasons for limited thermoplastic applications for aeronautics are the severe requirements in terms of maximu m service temperature and environmental resistance. For the structures generally a Tg wet not lower than 110 °C is required; that means that some thermoplastic resin with satisfactory structural properties (e.g. Polycarbonate) cannot be considered due to relaxation phenomena [1 -4] correlated to a Tg too much closer to the maximum wet working temperature . As a consequence, the use of thermoplastics for aeronautical structural applications has been considered only when very high Tg amorphous thermoplastics (PEI, PES) and high Tf semi -crystalline thermoplastics (PPS, PEEK, PEKK) have been available. The evaluation of such materials allowed to realize that PEEK and PEKK are always suitable for structural applications, PPS is preferred for interior applications, PEI and PES give some concerns about resistance to environmental humidity and solvents. The usage of PEEK and PEKK based composites, which show very good mechanical properties, is limited by cost considerations. In fact the raw material cost of the resin is high. Also the prepreg fabrication process is expensive, due to the high melting temperature (above 350 °C) and the high viscosity of the melt PEEK, making very difficult the fiber impregnation. Part fabrication is also difficult, and the advantages of working with a material already polymerized (the long curing cycles needed for epoxy are not required for thermoplastics) are balanced by the disadvantages due by the need to work at high temperature and to control the cooling rate to obtain the correct crystallinity level in the performed items. TABLE 1. Some Thermoplastic Resins suitable for aeronautical stru c tural applications . Resin Molecular Structure Tg Tf PEI Amorphous 200 -- PEEK Semi -Crystalline 140-145 334-343 PPS Semi -Crystalline 85-90 275-290 Times of Polymers (TOP) and Composites 2014 AIP Conf. Proc. 1599, 66-69 (2014); doi: 10.1063/1.4876779 © 2014 AIP Publishing LLC 978-0-7354-1233-0/$30.00 66 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.247.112.3 On: Thu, 13 Aug 2015 18:35:43 Cost reduction can be pursued through the utilization of fabrication techniques peculiar of ther moplastics, like thermoforming. This technique is suitable for parts not too large, and cannot be used for large structures (e -g- wings and fuselages); for these latter automated lay -up and automated fiber placement, requiring an appropriate heating facility to melt the material, could allow an interesting cost reduction, because don’t require cu ring in autoclave, but they still need some improvements. An idea that could help, mainly the assembly process, is the amorphous bonding. It is based on the addi tion of a layer of amorphous material as external ply during the fabrication of a semi -crystalline matrix composite part. An example is the fabrication of parts made of PEEK -carbon composite with the addition of external PEI layers. Two parts made in this way can be joined at a temperature above the PEI Tg and below the PEEK Tf. If needed a PEI film can be put between the two parts during joining. The same concept can be used to simplify other process techniques. This approach, that in the following w ill be called “ amorphous bonding”, was already mechan ically tested with good results [5 ] , but needs some additional checks to be utilized in a reliable way. In fact, it must be demonstrated that blending doesn’t decrease the properties of the blend comp onents. The physical and mechanical properties of the blends must be evaluated, including also a check of the after blending properties of the amorphous component. In fact the structural suitability of the amorphous bonding requires that both materials are melt together; melting of the only amorphous gives a very poor bonding with the semi - crystalline part in the “solid” state. A point to be verified is the soundness of the amorphous resin molecular structure when processed at a temperature much above Tg. Also the crystallization behavior of the semi -crystalline resin in the blends must be compared with the one of the resin alone. BLEND CHARACTERIZATION PEEK -PEI blends have been performed through a high temperature mixing process utilizing an industr ial extruder. Tests have been performed on neat PEEK and PEI and on the following PEEK -PEI blends (percent by weight): 90-10; 80- 20; 70- 30; 50- 50; 30- 70; 20- 80; 10- 90. DSC tests performed on blends with different compositions show that the transiti on temperatures of the single components aren’t modified in a relevant way in the blend. The PEEK crystallization is affected by the cooling rate [6] but a constant level of crystallinity (about 30%) is obtained when cooling rate ranges between 10 and 30 0°C/min. For cooling faster than 300 °C/min th e crystals (generally aggregated in a spherulitic geometry) cannot form adequately, and for very high cooling rate an amorphous material is obtained. This is an unstable status, and when the quenched material is heated above Tg a crystallization occurs, producing crystals not arrange d in a spherulitic geometry (cold crystallization). For very slow cooling (e.g. 1°C/min) crystallinity higher than 30% is obtained. In fig. 1 the DSC scanning (repeated for two s amples) is shown for a quenched PEEK. In a first run a crystallization above Tg is observed; the formed crystals melt above Tf. After cooling at a speed in the window from 10°C/min , a correct crystallization occurs. In the following scanning no cold crysta llization is observed, and Tg and the melting heath of the crystalline phase can be measured. It can be also observed that the melting curve observed in the first scan, related to the melting of the cold crystallization phase , shows a shallow which gives a shape different from the one observed in the second curve (spherulites melting). Is also interesting to note that for all the PEEK/PEI blend s only a single Tg is observed with a value intermediate between the PEEK and the PEI Tg , and the Tg of the sin gle components aren’t observed, indicating a good level of blending . 67 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.247.112.3 On: Thu, 13 Aug 2015 18:35:43 Temperature, °C FIGURE 1. DSC scanning of a 50/50 PEEK/PEI blends . TABLE 2. DSC experimental results of PEEK/PEI blends with different composi tions . PEEK/PEI blend composition Tg PEI,°C Tg PEEK, °C Tf PEEK, °C Fusion Heat !H, J/g 0/100, first run 214.94 0/100, second run 100/0, first run 100/0, second run 214.87 146.53 151.83 343.58 342.08 44.99 49.19 PEEK/PEI blend composition Tg blend, °C Tf blend, °C Fusion Heat ! H, J/g 10/90, first run 10/90, second run 207.50 206.88 334.83 334.84 0.35 0.14 20/80, first run 198.81 334.80 6.63 20/80, second run 30/70, first run 30/70, second run 199.47 192.83 199.77 334.31 332.10 331.61 5.44 13.87 14.03 50/50, first run 177.03 339.21 24.04 50/50, second run 70/30, first run 70/30, second run 80/20, first run 80/20, second run 207.15 161.29 167.79 156.45 / 335.54 341.28 338.75 341.06 340.04 24.18 33.12 32.40 38.29 35.62 90/10, first run 90/10, second run 148.17 157.72 342.26 340.71 43.64 44.82 68 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.247.112.3 On: Thu, 13 Aug 2015 18:35:43The values indicate that no specific negative effect is induced on thermal properties by blending, including the high temperature dwell (10 minutes at 380°C) Further investigation is in progress on mechanical, D ynamical -mechanical and environmental properties. ACKNOWLEDGMENTS The reported activity has been performed in the research project TECOP, supported by MIUR (Italian Ministery of Research and University), leaded by IMAST (District for Polimeric and Composite Materials and Structures). of which Alenia Aermacchi is Member. REFERENCES [1] L. Grassia, A. D’Amore, Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 74, art. no. 021504 (2006) [2] L. Grassia, S. L. Simon, Polymer 53, 3 613-3620 (2012) [3] L. Grassia, M. G. Pastore Carbone , A. D’Amore, Journal of Applied Polymer Science 122, 3752- 3757 (2011) [4] J. Guo, L. Grassia, S. L. Simon, Journal of Polymer Science Part B: Polymer Physics 50, 1233 -1244 (2012) [5] C.Voto and M.Iannone , “ Environmental Resistance of Amorphous Bonded Thermoplastic Joints ” AGARD Report 785, 1991 [6] J. Kenny, A. D’Amore, L. Nicolais, M. Iannone and B. Scatteia, “Processing of Amorphous PEEK and Amorphous PEEK Based Composites” SAMPE Journal Vol.25, N°4, July/August 1989 . 69 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.247.112.3 On: Thu, 13 Aug 2015 18:35:43

1.4876788.pdf

Immobilization of natural anti-oxidants on carbon nanotubes and aging behavior of ultra-high molecular weight polyethylene-based nanocomposites Nadka Tzankova Dintchevaa, Rossella Arrigoa, Cristian Gambarottib, Monica Guenzib, Sabrina Carroccioc, Francesca Cicognad, Giovanni Filipponee aDipartimento di Ingegneria Civile, Ambi entale, Aerospaziale, dei Materiali, Università di Palermo, 90128 Palermo, IT bDipartimento di Chimica, Materiali ed Ingegneria Chimica "G. Natta", Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano, IT cConsiglio Nazionale delle Ricerche - ICTP UOS Catania, Via P. Gaifami 18, 95126 Catania, IT dConsiglio Nazionale delle Ricerche CNR - ICCOM UOS Pisa, Area della Ricerca, Via G. Moruzzi 1, 56124 Pisa, IT eDipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università di Napoli Federico II, Piazzale V. Tecchio 80, 80125 Napoli, IT Abstract. The use of natural antioxidants is an attractive way to formulate nanocomposites with extended durability and with potential applications in bio-medical field. In this work, Vitamin E (VE) in the form of α-tocopherol and Quercetin (Q) are physically immobilized on the outer surface of mu lti-walled carbon nanotubes (CNT s). Afterward, the CNTs-VE and CNTs-Q are used to formulate thermally stable ultra high molecular weight polyethy lene based nanocomposites. The obtained results in the study of the thermo-oxidation behavior suggest a beneficial effect of the natural anti-oxidant carbon nanotubes systems. The unexpected excellent thermo -resistance of the nanocomposite s seems to be due to a synergistic effect of the natural anti-oxidant and carbon na notubes, i.e. strong interaction between CNT surface and anti- oxidant molecules. Particularly, these interactions cause the formation of structural defects onto outer CNT surfaces, which, in turn, increase the CNT radical scavenging activity. Keywords: Natural antioxidants, Carbon nanotubes, α-Tocopherol, Quercetin, UHMWPE, Stabilization. PACS: 82.35.Lr; 61.82.Pv; 87.85.Jf INTRODUCTION Thermal- and photo-oxidative degradation of polymers and polymer-based nanocomposites is a key issue for these class of materials. By adding suitable stabilizing systems, such as anti-oxidants, light stabilizers, thermal stabilizers, and fire retardants, the prot ection against thermal- and photo-oxidativ e degradation is usually realized [1- 3]. However, the use of low molecular weight stabilizing sy stems is restricted because of their physical loss, i.e. volatilization, migration, and water extraction. One of the possible approaches in order to solve these matters is promoting an increase of the molecular weight of the anti- oxidant systems, for example through the introduction of long alkyl chains, which is the common i ndustrial practice. On the other hand, if the molecular weight is too high, a poor molecular distribution in the polymer matrix can occur. The appropriate alkyl chain length is strictly dependent on the kind and intrinsic molecular weight of polymeri c matrix and so the formulation of universal stabilizing systems is not easy. Another possibility is grafting of the stabilizing molecules onto the polymeric macromolecules [4]. Such an approach is a multi-st ep chemical modification way, hard to control. An innovative method to immobilize low molecular weight chemicals is their physi cal entrapment. This approach allows to preserve the active functionalities in the structure of the molecules, th at could be damaged through covalent linkage. Moreover, in recent years, several studies are fo cused on the use of stabilizing systems coming from natural sources because of Times of Polymers (TOP) and Composites 2014 AIP Conf. Proc. 1599, 102-105 (2014); doi: 10.1063/1.4876788 © 2014 AIP Publishing LLC 978-0-7354-1233-0/$30.00 102 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:44:42their presumed safety and bio-compatibility. In particular, natural vitamins and flavonoids, easy available in nature, are widely studied and considered for seve ral environmental applications. α-Tocopherol molecule (a com ponent of vitamin E) is a hindered phenol an ti-oxidant, able to react with the free radicals in the cell membranes and to protect polyunsaturated fatty acids against oxidative degradation. This natural molecule has been used as antioxidant and stabilizer fo r polymeric matrices such as polyolefins [5] and bio- polyesters [6]. In recent years, the α-tocopherol was successfully used as anti- oxidant in ultra high molecular weight polyethylene (UHMWPE) for the formulation of orthopedic components [7]. Quercetin is a flavonoid with numerous biological activities and used as a potent antioxidant, able to stabilize polyolefins, such as polypropylene, both against thermo-oxidation and action of UV radiation [8]. The quercetin anti-oxidant activity is related to its ability to scavenge free radicals and to reduce free radical formation. Therefore, the im mobilization of natural anti-oxidants, such as vitamin E and quercetin, can be considered as a promising way to overcome drawbacks arising from their easy volatilization at high temperatures during melt proce ssing of polymer matrices, as wellas migration issues during the manufacture life-time. In pa rticular, the physical imm obilization onto the outer surface of nanoparticles, such as nanosilica, carbon nanotubes, etc. allows to formulate multi-functional nanoparticles having in-build reinforcement and stabilizing actions. In this study the attention is focused on the formul ation and use of carbon nanotubes based multi-functional nanoparticles because of CNTs versatility and unique properties. An innovativ e approach in the immobilization of vitamin E and quercetin onto CNT surface has been performed. Long alkyl chains have been covalently linked to outer CNT surface; in a seco nd distinct step, the anti-oxidant molecu les have been entra pped between the long chains, aiming at preserving the integrity of active functionality of the stabilizing molecules. The thermo-oxidation behavior of the complex nanocomposite systems has been accurately investigated an d compared to the neat UHMWPE matrix. EXPERIMENTAL PART The materials used in this work were: - Ultra high molecular weight polyethylene (UHMWPE) supplied by Sigma-Aldrich, having average molecular weight 3÷6 MDa, softening point T=136°C (Vicat, ASTM D 1525B), melting point Tm=138°C (determinate by DSC) and density 0.94 g/mL at 25°C; - Multi-walled CNTs, bearing covalently linked -COOH groups, supplied Cheap Tubes, U.S.A. The main properties are: outer diameter OD=150÷200 nm, inner diameter ID=10÷20 nm, length L=10÷20 μm, purity >95 wt.%, ash <1.5 wt.%, specific surface area SSA>60 m2/g an d electrical conductivity EC>10-2 S/cm. - (±)-α-Tocopherol, a natural anti-oxidant vitamin E (VE) molecules coming from vegetable oil, supplied by Sigma- Aldrich. It has molecular weight 430.71 g/mol. - Quercetin hydrate, a natural flavonoid compound (Q), supplied by Sigma-Aldrich srl. Molecular weight: 302.24 g/mol; Formula: C 15H10O7 xH 2O; Purity: >= 95 %. The CNTs-COOH have been subjected to chemical modifi cation to obtain CNTs functionalized with long chain alkyl ester groups (alkyl- f-CNTs). In a second separate step, the VE or Q molecules have been immobilized/adsorbed on the outer surface of the alkyl- f-CNTs. The UHMWPE powder and 1 wt.% of CNTs were manually mixed at room temperature until a homogeneous black powder was obtained. The blends were then hot compacted at 210°C for 5 min under a pressure of 1500 psi to get thin films (thickness less than 100 μm) for the subsequent analyses. UHMWPE wa s subjected to the same procedure. The investigations have been performed by: - Spectroscopical characterization, pe rformed using a Spectrum One spectrometer by Perkin-Elmer. FT-IR spectra were obtained through 16 scans with a 4 cm-1 resolution. The carbonyl index (C I) was calculated as the ratio between the carbonyl absorption area (1850-1600 cm-1) and the area of a reference peak at about 1370 cm-1. The hydroxyl index (HI) was calculated as the ratio between the hydroxyl absorption area (3570-3150 cm-1) and the area of the same reference peak; - Thermo-Gravimetrical analysis (TGA), carried out using an Exstar TG/DTA Seiko 7200 instrument with a heating rate of 10°C/min from 30 to 750°C under nitrogen flow. - Rheological tests, performed using a stress-controlled rheometer SR5 by Rheometrics Scientific in parallel plate geometry. The complex viscosity ( η*) was measured performing frequency sweep tests at T=210°C from 10 -1 to 102 rad/s considering a maximum strain of 2.0%. The thermo-oxidation of the nanocomposites was performed at 120°C in air oven. The samples were then subjected to FTIR analyses at different annealing times. 103 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:44:42RESULTS AND DISCUSSION The presence of the immobilized VE and Q molecules onto outer CNT surface has been assessed by infrared spectroscopy and thermo-gravimetric analysis, although, obtai ned results suggest that, in this case, both techniques are qualitative but not quantitative. The rheology (see Figure 1) and morphology analysis (not reported for sake of conciseness) indicate a beneficial effect of the presence of the natural anti-oxidant molecules. As known, the rheology analysis is particularly sensitive to molecular architecture and it can be considered a valid technique to assess the occurrence of degradation phenomena. The complex viscosity curves ( η*) for neat UHMWPE and UHMWPE/alkyl-f-CNT without and with VE and Q molecules as a function of the frequency are shown in Figure 1. The viscosity values of the three alkyl-f- CNTs filled nanocomposites are higher than the unfilled matrix. In particular, the VE- and Q-additivated samples show even higher viscosity, suggesting a beneficial effect of both anti-oxidants against the thermo-oxidation phenomena which may occur during processing. FIGURE 1. Complex viscosity curves ( η*) for neat UHMWPE and all investigated complex nanocomposites systems as a function of the frequency. The thermo-oxidation behavior of the complex UHMWPE/alkyl-f-CNT/VE and UHMWPE/alkyl-f-CNT/Q systems has been studied and compared to that for the neat matrix and UHMWPE/alkyl-f-CNT one. In Figure 2, the trends of carbonyl and hydroxyl indices for all the investigated systems are reported as a function of the thermo-oxidation times. (a) (b) FIGURE 2. Carbonyl (a) and Hydroxyl (b) Indices for neat UH MWPE and complex nanocomposite s systems as a function of the thermo-oxidation times. The increases of the carbonyl and hydr oxyl species as a function of the aging times is related to the undergone thermo-oxidation. It is evident that the degradation process is slower for the nanocomposites containing VE and Q 104 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:44:42molecules. VE and Q molecules are both known to be effec tive in the protection of the polymeric matrices against thermo-oxidation, but the excellent stabilizing actions attained in our samples are definitely much higher than what commonly reported in literature [5, 8]. Such an unexpected finding could be explained considering some specific interactions between the used natural anti-oxidants and the carbon atoms of the outer surface of the CNTs. Such interactions could give rise to the formation, upon ther mal treatment, of several intermediate anti-oxidant radicals, which induce the formation of CNT surface defects. In this way, some carbon at oms change their hybridization from sp 2 to sp3, achieving radical scavenging properties. It is important to highlight that such kinds of interaction between the radicals and carbon atoms ar e favored if the radicals are close to the CNT surface, which satisfy the assumptions of the ab initio theoretical calculations based on the density functional theory [9]. Such a condition could be fulfilled thanks to the long alkyl chains covalently linked to the CNTs, which eventually promote the trapping of the anti- oxidant molecules in the close proximity of the CNT surface. ACKNOWLEDGMENTS This work was financially supported by the Ministry of University and Research in Italy, FIRB2010 Futuro in Ricerca (project title ‘‘GREENER—Tow ard Multifunctional, Efficient, Safe and Stable ‘Green’ Bio-Plastics Based Nanocomposites of Technological Interest via the Immobilization of Functionalized Nanoparticles and Stabilizing Molecules;’’ cod: RBFR10DCS7). REFERENCES 1. N.Tz. Dintcheva, F.P. La Mantia, Polym. Degrad. Stab. 92, 630-634 (2007). 2. N.Tz. Dintcheva, F.P. La Mantia, V. Malatesta, Polym. Degrad. Stab. 94, 162-170 (2009). 3. N.Tz. Dintcheva, E. Morici , R. Arrigo, F.P. La Mantia, V. Malatesta, J.J. Schwab, Polym. Degrad. Stab. 97, 2313-2322 (2012). 4. S. Al-Malaika (Ed.), “Reactive Modifiers for Polymers”, Bl ackie, Academic and Professiona l, an imprint of Chapman and Hall, London, ISBN 0-7514 0265 6 (1997). 5. Al-Malaika S, Goodwin C, Issenhuth S, Burdick D. The antioxidant role of α-tocopherol in polymers II. Melt stabilising effect in polypropylene. Polym. Degrad. Stab., 64(1) , 145-146 (1999). 6. Gonçalves CMB, Tomé LC, Coutinho JAP, Marrucho IM. Addition of α-tocopherol on poly(lactic acid): Thermal, mechanical, and sorption properties. J. Appl. Po lym. Sci., 119(4) , 2468–275 (2011). 7. Oral E, Muratoglu OK. Vitamin E diffuse d, highly crosslinked UHMWPE: a review. Int. Orthop., 35(2) , 215-223 (2011). 8. M. D. Samper, E. Fages, O. Fenollar, T. Boronat and R. Balart, J. Appl. Polym. Sci. 129, 1707–1716 (2013). 9. De Menezes VM, Fagan SB, Za nella I, Mota R. Carbon nanotubes interacting with vitamins: First principles calculations. Microelectr. J., 40, 877-879 (2009). 105 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:44:42AIP Conference Proceedings is copyrighted by AIP Publishing LLC (AIP). Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. For more information, see http://publishing.aip.org/authors/rights- and- permissions.

1.4876770.pdf

Shear Creep Compliance of Polyoxymethylene Copolymers with Different Molecular Weights Joamin Gonzalez-Gutierreza, Zerihun Mellese Megena, Bernd Steffen von Bernstorffb and Igor Emria aCenter for Experimental Mechanics, Faculty of Mechanical Engineering, University of Ljubljana, Pot za Brdom 104, Ljubljana, Slovenia, SI-1125 bBASF AG, G-CA/MT - J513, Ludwigshafen, Germany, D-67056 Abstract. Polyoxymethylene copolymer (POM) is considered a high performance engineering polymer with many applications due to its good ch emical resistance and very good mechanical properties. It is k nown that mechanical properties of polymers are greatly influenced by their average molecular weight ( Mw). This paper presents the shear creep compliance of new POM copolymers with a broad range of average molecular weights (10240 to 204400 g/mol). Master curves of creep compliance were constructed using the time-te mperature superposition principle. It was observed that at short time ( t = 0.25 s), creep compliance is independent of Mw. As the time increases ( t = 3.16x108 s ~ 10 years) shear compliance decreases as a power function of Mw, but only up to a critical Mw of a pproximately 92300 g/mol. After this critical Mw creep compliance becomes again independent of Mw. These results in combination with finite element analysis could be used for selecting a specific Mw according to suit the require ments of certain application. Keywords: polyoxymethylene, shear creep compliance, time-tem perature superposition, model, molecular weight PACS: 83.60.Bc, 83.80.Ab, 83.85.Ns, 83.85.Tz INTRODUCTION Polyoxymethylene (POM) is an engineering polymer of formaldehyde with hydroxyl ends stabilized by esterification or etherification. It is sometimes also referred as polyacetal or less commonly as aldehyde resins [1]. POM is distinguished from other engin eer polymers by its crystallinity level th at can be between 60 to 90% [2, 3]; such high crystallinity induces very go od mechanical properties su ch as high modulus, stiffness, fatigue and creep resistance [3, 4]. Other desirable technological properties of POM include dimensional stability, corrosion resistance, superior tribological properties and capability of operating at temperatures in excess of 90 °C [5, 7]. All of these properties combined with good moldability allow using POM as a structural material in many different applications. In many occasions, POM is used as a substitute for metals or ny lons [7, 8]. In fact POM can compete with nylons, which can show some serious deficiencies in dimensional stability in humid environments [9]. It is well known that many properties of polymeric systems in general are greatly influenced by their molecular weight [10-12]. Of particular interest is shear creep comp liance, which may help in the selection of an appropriate material for applications that require mechanical stability when constantly loaded. Thus, the aim of this paper is to investigate how creep compliance is affected by changi ng the average molecular we ight of POM copolymers. MATERIALS AND METHODS For this investigation eight POM copolymers with different average molecular weights ( Mw) were synthesized by BASF (Ludwigshafen, Germany) following a r ecently patented methodology [13] and traditional copolymerization methods for polyoxymethylene. Molecular weights were measured by the supplier using gel permeation chromatography and they were 10240, 26600, 52750, 81100, 92360, 109000, 129300 and 204400 g/mol. In order to perform creep compliance measurements on the selected POM copolymers, cylindrical specimens with diameter D = 5.8 ± 0.1 mm and length l = 29.0 ± 2.0 mm were prepared by gravimetrical casting as described in Times of Polymers (TOP) and Composites 2014 AIP Conf. Proc. 1599, 30-33 (2014); doi: 10.1063/1.4876770 © 2014 AIP Publishing LLC 978-0-7354-1233-0/$30.00 30 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:39:04[14]. The casting temperature was set to 200 °C and the heater travelling speed was 1 mm/min. After casting, specimens were cut to the required length and glued with acrylate-based glue (F524 black and activator B, Kemis plus d.o.o, Slovenia) to custom made metal holders for gripping the cylindrical specimen to the measuring device. Shear creep compliance measurements were performed in a HAAKE MARS II controlled stress rheometer fitted with solid clamps. Initial part of the creep measur ing procedure started with an annealing phase at high temperature (120 °C for 2 h) to erase mechanical stress–strain history of th e material. Annealing was followed by slow cooling to the first measuring temperature, -20 °C at a rate of 0.028 °C/min to minimize the effects of physical aging. After cooling down, shear creep measurements were performed in segmental form at eight different temperatures: -20, 0, 20, 40, 60, 80, 100, and 120 °C. Each specimen was loaded in shear with a constant shear stress ( ) of 30000 Pa for 1000 s, once the desired temperature had stabilized for approximately 15 min. The level of applied stress was previously determined to be within the linear viscoelastic region. The useful segment length was set from 1 to 1000 s. Three repetitions were performed on different samples for each molecular weight under consideration and their results at a given temperatur e were averaged. Finally, following the time-temperature superposition principle, averaged segments were shifted alon g the time-scale in relation to the segments measured at two nominal reference temperatures, Tref = 20 and 60 °C. Shifting was executed by using the closed-form shifting procedure [15]. RESULTS AND DISCUSSION As previously described in the research methodology, creep measurements were performed at eight different temperatures: -20, 0, 20, 40, 80, 100 and 120 °C. After the segments were collected they were shifted to two different temperatures 20 and 60 °C to construct master cu rves. The master curves for all the POM copolymers at the reference temperature of 20 °C can be se en in Fig. 1. Please notice that both ax es in Fig. 1 are in logarithmic scale. By looking at Fig. 1, one can see that during the first four decades the change in creep compliance is negligible. Moreover, after a period of ten decades the increase in compliance is only two d ecades. Therefore, Fig. 1 demonstrates that even POM copolymers with low av erage molecular weight have good creep resistance. FIGURE 1. Creep compliance master curves for POM copolymers with different Mw at Tref = 20 °C In order to compare the effect of molecular weight on the creep comp liance of POM copolymers, isochronal creep compliance curves at two reference times are shown in Fig. 2. Please notice that both axes in Fig. 2 are in logarithmic scale. At shorter times ( t = 0.25 s) the creep compliance is independent of the molecular weight at both selected reference temperatures. The hori zontal lines that run through the data points in Fig. 2 represent the average value of the measured creep compliances at the selected time, such line fits very well the experimental data. 6.E-106.E-096.E-08 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07 1.E+09 1.E+11Shear creep compliance [Pa-1] Time [s]Mw = 10240 g/mol Mw = 26600 g/mol Mw = 43090 g/mol Mw = 81100 g/mol Mw = 92360 g/mol Mw = 109000 g/mol Mw = 129300 g/mol Mw = 204400 g/molTref= 20 °C = 30000 Pa 31 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:39:04However at longer times ( t = 3.16x108 s ~ 10 years), it was observed that the creep compliance decreases until a molecular weight around 92300 g/mol, followed by a plateau until the maximum Mw investigated (Fig. 2) at both selected reference temperatures. The experimental data has been fitted using a combination of power and linear functions (i.e. constants) as shown in Table (1). FIGURE 2. Change in creep compliance ( J) of POM copolymers with different aver age molecular weight at two reference temperatures, Tref =20 °C and 60 °C and two reference times, t = 0.25 s and 3.16x108 s (10 years). Experimental data is fitted according to equations shown in Table (1) TABLE (1). Equations for shear creep compliance ( J) dependence on molecular weight ( Mw) Time [s] Equation Tref [°C] Values R2 0.25 J = Constant when 10240 g/mol Mw 204400 g/mol 20 J = 9.47 x10-10 Pa-1 1 60 J = 1.42 x10-9 Pa-1 1 3.16x108 (10 years) J = bMwc when 10240 g/mol Mw 92360 g/mol J = Constant when 92360 g/mol < Mw 204400 g/mol 20 b = 2.90 x10-7 c = -0.408 0.932 J = 2.60 x10-9 Pa-1 1 60 b = 9.55 x10-6 c = -0.643 0.981 J = 6.33 x10-9 Pa-1 1 The decrease in creep compliance w ith increasing average molecular we ight has been reported for other polymers such as polypropylene [16, 17], polyimides [ 18], poly(1,3-trimethylene car bonate) [19], fiber glass reinforced polyester [20] and even for polyoxymethylene homo-, co- and terpolymers [21]. However, current results show that after a critical average molecular weight ( Mw = 92360 g/mol) creep comp liance at longer times ( t ~10 years) becomes independent of Mw, at least up to Mw = 204400 g/mol. Also at very short times ( t = 0.25 s), shear creep compliance is independent of the average molecular weight (Fig. 2). Decrease in creep compliance could be attributed to an increase in crystallinity [22] and increase of entanglements [23] as the molecular weight increases. 6E-106E-096E-08 10000 100000Shear creep compliance [Pa-1] Average molecular weight, Mw [g/mol] J(10years,60°C) J(10years,20°C) J(0.25s,60°C) J(0.25s,20°C) FitJ(10years,60°C) FitJ(0.25s,60°C) FitJ(10years,20°C) FitJ(0.25s,20°C) 32 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:39:04CONCLUSIONS POM is an important engineering poly mer with a great variety of applica tions. In this study the shear creep compliance of POM copolymers with di fferent Mw was measured. It was fou nd that the shear creep compliance at short times appears to be independent of Mw. At longer times, the shear creep compliance decreases with Mw following a power law relationship with exponent values between -0.408 and -0.643, depending on the reference temperature. There seems to be a critical Mw after which creep compliance b ecomes once again independent of Mw, in this study it appears to be around 92300 g/mol. These resu lts in combination with finite element analysis could be used for selecting a specific Mw according to suit the requirements of particular applications. ACKNOWLEDGMENTS We will like to acknowledge the financial support of the Ad Futura Fund of the Republic of Slovenia ( Javni sklad Republike Slovenije za razvoj kadrov in štipendije ), the Slovenian Research Agency (ARRS) and the Erasmus Mundus Program; as well as, the support of the staff at BASF Ludwigshafen, Germany for the synthesis and molecular weight characterization of POM copolymers. REFERENCES 1. R. Zhao, International Nonwovens Journal 14, 20-24 (2005). 2. J. Masamoto, Prog. Polym. Sci. 18, 1-84 (1993). 3. M. Hasegawa, K. Yamamoto, T. Shiwaku and T. Hashimoto, Macromolecules 23, 2629-2636 (1990). 4. A. A. Edidin and S. M. Kurtz, J. Arthroplasty 15, 321-331 (2000). 5. D. Jauffres, O. Lame, G. Virg ier, F. Dore and C. Chervin, J. Appl. Polym. Sci. 106, 488-497 (2007). 6. K. Al Jebawi, B. Sixou, R. Seguela and G. Vigier J. Appl. Polym. Sci. 106, 757-764 (2007). 7. C. Pistor and K. Friedrich, J. Appl. Polym. Sci. 66, 1985-1996 (1998). 8. H. Benabdallah, Wear 254, 1238-1246 (2003). 9. T. Kongkhlang, K. Tashiro, M. Kotaki and S. Chirachanchai, J. Am. Chem. Soc. 130, 15460-15466 (2008). 10. X. Zhao, L. Ye and L. Hu, Polym. Adv. Technol. 19, 399-408 (2008). 11. S. Srivastava, S. Srivastava, S. Srivastava, S. J. La'Verne, I. Ali Khan, P. Ali and V. D. Gupta, J. Appl. Polym. Sci. 122, 1376- 1381 (2011). 12. W. Dziadur, Mater. Charact. 46, 131-135 (2001). 13. L. Pottie, and B. S. von Bernstorff. U.S Patent No. 2013/0203958 A1 (8 August 2013). 14. G. B. Stringari, B. Zupan čič, G. Kubyshkina, B. S. von Bernstorff and I. Emri, Powder Technol. 208, 590-595 (2011). 15. M. Gergesova, B. Zupan čič, I. Saprunov and I. Emri, J. Rheol. 55, 1-16 (2011). 16. L. Xialolin, H. Yajiang and D. Cong, Polym. Eng. Sci. 49, 1376-1384 (2009). 17. D. Drosdov and J. D. Christiansen, J. Appl. Polym. Sci. 88, 1438-1450 (2003). 18. L. M. Nicholson, K. S. Whitley and T. S. Gates, Int. J. Fatigue 24,185-195, 2002 19. A. P. Pego, D. W. Grijpma and J. Feijen, Polymer 44, 6495-6504 (2003). 20. A. Kouadri-Boudjelthia, A. Imad , A. Bouabdallah and M. Elmeguenni, Mater. Des. 30, 1569-1574 (2009). 21. Y. Tajima and T. Itoh, J. Appl. Polym. Sci. 116, 3242-3248, 2010 22. H. Jin, J. Gonzalez-Guti errez, P. Oblak, B. Zupan čič, I. Emri. Polym. Degrad. Stabil . 97, 2262-2272 (2012). 23. C.A. Tweedie and K.J. Van Vliet. J. Mater. Res. 21, 1576-1589 (2006). 33 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 220.225.230.107 On: Fri, 16 May 2014 06:39:04AIP Conference Proceedings is copyrighted by AIP Publishing LLC (AIP). Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. For more information, see http://publishing.aip.org/authors/rights- and- permissions.

1.4898148.pdf

Determination of constitutive parameters of homogeneous metamaterial slabs by a novel calibration-independent method U. C. Hasar, G. Buldu, M. Bute, J. J. Barroso, T. Karacali, and M. Ertugrul Citation: AIP Advances 4, 107116 (2014); doi: 10.1063/1.4898148 View online: http://dx.doi.org/10.1063/1.4898148 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Band diagrams of layered plasmonic metamaterials J. Appl. Phys. 116, 173101 (2014); 10.1063/1.4900532 Bandwidth enhancement in disordered metamaterial absorbers Appl. Phys. Lett. 105, 081102 (2014); 10.1063/1.4894181 Coherence of magnetic resonators in a metamaterial AIP Advances 3, 122119 (2013); 10.1063/1.4861028 A calibration-independent method for accurate complex permittivity determination of liquid materials Rev. Sci. Instrum. 79, 086114 (2008); 10.1063/1.2976037 Polarizabilities and effective parameters for collections of spherical nanoparticles formed by pairs of concentric double-negative, single-negative, and∕or double-positive metamaterial layers J. Appl. Phys. 97, 094310 (2005); 10.1063/1.1884757 All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54AIP ADV ANCES 4, 107116 (2014) Determination of constitutive parameters of homogeneous metamaterial slabs by a novel calibration-independent method U. C. Hasar,1,2,aG. Buldu,1M. Bute,1J. J. Barroso,3T. Karacali,2,4 and M. Ertugrul2,4 1Department of Electrical and Electronics Engineering, University of Gaziantep, Gaziantep 27310, Turkey 2Center for Research and Application of Nanoscience and Nanoengineering, Ataturk University, Erzurum 25240, Turkey 3Associated Plasma Lab., National Inst. for Space Research 12227-010 São José dos Campos, SP , Brazil 4Department of Electrical and Electronics Engineering, Ataturk University, 25240, Erzurum, Turkey (Received 2 July 2014; accepted 28 September 2014; published online 10 October 2014) A calibration-independent line-line method for broadband and simultaneous consti- tutive parameters determination of homogeneous metamaterial (MM) slabs is pro- posed. It is shown that the su fficient condition for parameters retrieval by the proposed method is to measure uncalibrated (raw) complex scattering parameters of measurement cells (di fferent air regions in free-space) which are completely and partially loaded by the two identical metamaterial slabs. The stability of derived equations for di fferent measurement uncertainty cases is analyzed. We have validated the proposed method by using simulated scattering parameters of a MM slab with split-ring-resonators and then by comparing the extracted electromagnetic param- eters with those of a general method used in the literature in the cases with and without a small o ffset in reference-plane positions (as well as other measurement errors). From this comparison, we note that while the general method does not eliminate those errors, the proposed method not only does not introduce the non- physical anti-resonance behavior but also removes the measurement errors arising from di fferent mechanisms such as inaccurate reference-plane positions and mis- matched connections. C2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http: //dx.doi.org /10.1063 /1.4898148] I. INTRODUCTION Materials characterization is an important research field involving an analysis of electromag- netic responses of various materials through the relative complex permittivity ( εr), the relative complex permeability ( µr), and other related parameters (e.g., magneto-electric coupling and chiral parameters). With the advent of artificial resonant-type structures (coined as metamaterials–MMs), it becomes possible to devise an engineered material with some exotic electromagnetic properties including negative refractive index ( n) so that otherwise unattainable applications such as perfect lenses1and invisibility cloaks2come into real. The first fabricated MM was composed of periodic arrangement of a metallic-dielectric cell with split-ring-resonators (SRRs) and a thin metal wires on opposite faces of a dielectric substrate.3After this MM structure, various MM slabs have been proposed for di fferent purposes, some of which are given.4–7 aElectronic mail: uchasar@gantep.edu.tr 2158-3226/2014/4(10)/107116/10 4, 107116-1 ©Author(s) 2014 All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-2 Hasar et al. AIP Advances 4, 107116 (2014) In artificially constructed MM structures, both the size of the composite particles and the period of lattices are generally made larger than the wavelength at the particle resonance, and so a homogenized description3applies thus allowing the description of the properties of MMs in terms of bulk or macroscopic material parameters ( εrandµr). In the literature, very interesting tech- niques have been proposed for retrieval of electrical properties of homogeneous MMs.3,8–20Many of these methods require some sort of calibration (named as calibration-dependent techniques) before accurate measurements are carried out.3,8–17However, calibration kits used for calibration of measurements may produce inappropriate material characterization due to inaccurate scattering (S-) parameter measurements arising from imperfect calibration kits. In recent studies, we have applied calibration-independent techniques18–20for accurate electromagnetic characterization of MMs. Nonetheless, these techniques are restricted to the measurement of either εrorµr. It was shown that the complex propagation constant ( γ) (as well as all the properties of an unknown transmission line and those of the junctions to the measurement lines) of a non-reflecting line can be determined from uncalibrated (raw) S-parameter measurements by a line-line (LL) method.21–24In this research paper, we propose a new calibration-independent method based on the LL technique for constitutive parameter ( εrandµr) measurements of isotropic MM slabs. II. THE METHOD A. Background Fig. 1 presents the measurement configurations of our method for retrieval of constitutive para- meters of a homogeneous3MM slab with di fferent lengths, L1andL2. Here, the length of a MM slab refers to the length of the substrate in the propagation direction (in our analysis, it is the x direction as shown in Fig. 2(a)). More information about definition of the length of MM slabs can be found in Section IV. S1andS2denote the two-port networks for the slab region with di fferent lengths (shorter and longer ones) to be determined; and XandYdesignate two-port (error) networks corresponding to transitions used for linking S1andS2to a vector network analyzer (VNA) or a calibrated reference plane. In Fig. 1(a), the shorter slab with length L1is placed between two horn antennas, while the longer slab with length L2is positioned between the same antennas in Fig. 1(b). The final configuration, Fig. 1(c), illustrates the case when the shorter slab at arbitrary distances L01andL02replaces the longer one. In each measurement configuration, it is assumed that the regions where the slabs are positioned are at far field (zone) of the antennas (plane wave assumption) and that XandYare asymmetrically spaced ( D1,D2) from each measurement cell. The networks XandYinclude source and load match errors, errors due to mismatched connections, transitions between connecting cables and horn antennas, contact structures, embedded devices, etc. The methodology in calibration-independent techniques is to characterize S1and/orS2without resorting to any knowledge of error networks XandY; that is, the e ffect of these networks has to be removed from the whole system. B. Propagation constant determination For the analysis of cascaded networks, either of the two transfer matrix forms, namely, ABCD25–27or wave cascading matrix (WCM)18–20,28–30can be employed. Because the WCM form is more useful for treating two-port calibration problems than the ABCD form,28in this paper, we will use the WCM form. The basis for propagation constant determination in the LL technique (discussed in Section I) relies upon using two identical unknown MM slabs with di fferent lengths Luwhere u=1 or 2 asymmetrically ( D1,D2) spaced from the antennas as schematized in Fig. 1(a) and Fig. 1(b).21–24 We use the matrices TX,TY,TL1andTL2for modeling, respectively, the transitions X,Y, and the measurement cells including slabs with di fferent lengths. The WCM presentation of the whole sys- tem in Figs. 1(a) and (b) for two measurement cells loaded with slabs can be expressed as18–20,28–30 Mu=TXTLuTY, (1) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-3 Hasar et al. AIP Advances 4, 107116 (2014) FIG. 1. Typical two-port measurement problem: unknown two-port network ( S1andS2) and two-port error networks, X andY. where Mu=1 S(u) 21S(u) 12S(u) 21−S(u) 11S(u) 22S(u) 11 −S(u) 221, (2) andS(u) klparameters ( k,l=1,2) are the uncalibrated (raw) measured complex S-parameters. Using Eq. (2), the theoretical WCM form for TLuin Eq. (1) can be expressed as18–20,29 TLu=1 (1−Γ2)TsuT2 su−Γ2Γ1−T2 su
−Γ1−T2 su
1−Γ2T2 su, (3) where Γ=z−1 z+1,Tsu=e−γLu,z=z′+iz′′= µr/εr,n=n′+in′′=√εrµr, (4) γ=γ0n, γ 0=−i2π λ0, ε r=ε′ r+iε′′ r, µ r=µ′ r+iµ′′ r. (5) Here,γ,z, and nare, respectively, the propagation constant, normalized wave impedance, and refractive index of the slab-filled cell, and γ0,µ0, and λ0correspond to the propagation constant, All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-4 Hasar et al. AIP Advances 4, 107116 (2014) FIG. 2. (a) Configuration of the unit cell for the analyzed SRR MM slab (for better visualization, the substrate thickness and the dimensions of the metallic inclusions are not drawn to the scale of the cubic cell) and (b) magnitudes of simulated S-parameters of two MM slabs with L1=4.0 mm (periodic arrangement of unit cell in yandzdirections with periods ay=4.0 mm and az=4.0 mm) and L2=8.0 mm (cascade connection in the propagation ( x) direction of two identical MM slabs with length L1), corresponding to the configurations in Fig. 1(a) and 1(b), respectively. permeability, and wavelength of free-space. In our mathematical analysis, exp (−iωt)time depen- dence has been assumed in phasor (complex) domain. The theoretical WCM form for TLugiven in Eq. (3) can be written in a di fferent form23as TLu=TΓT0uT−1 Γ, (6) where∗−1denotes the inverse of the matrix ‘ ∗’ and TΓ=1Γ Γ1,T0u=Tsu 0 0T−1 su. (7) Using Eqs. (1), (6), and (7), we can write M1M−1 2=(TXTΓ)T01T−1 02
(TXTΓ)−1. (8) It is obvious from Eq. (8) that M1M−1 2andT01T−1 02are similar matrices.18–20,29Using the fact that similar matrices have the same trace (denoted by Trin the remainder of the manuscript), which is the sum of diagonal elements in a square matrix, and hence both matrices have the same eigen- values, from Eq. (8) we obtain22,23 λ(1,2)=TrM1M−1 2
∓ TrM1M−1 2
2−4 2, (9) where λ(1,2)=exp(∓γ∆L),∆L=L2−L1, and λrepresents either forward or backward traveling waves inside the slab. Finally, the propagation constant will be γ(1,2)=∓lnλ(1,2)
∆L. (10) C. Constitutive parameters determination The technique for determination of εrandµrof isotropic homogenized MM slabs by the pro- posed method is based upon using raw S-parameters measured on a given cell completely filled with a longer slab and then partially filled with a shorter slab as shown in Fig. 1(b) and Fig. 1(c). The WCM form of the whole system for a measurement cell loaded by the shorter slab (Fig. 1(c)) can be written M21=TXT1TL1T2TY, (11) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-5 Hasar et al. AIP Advances 4, 107116 (2014) where Tu=αu 0 0 1/αu, α u=e−γ0L0u, (12) andL01andL02are arbitrary lengths of air-filled sections inside the measurement cell in Fig. 1(c). Then, using Eqs. (1) and (11), we obtain M21M−1 2=TXT1TL1T2T−1 L2T−1 X. (13) It is clear from Eq. (13) that Tr(M21M−1 2)=Tr(T1TL1T2T−1 L2).18–20,29Next, using Eqs. (3), (11), and (12), we find an objective function Fobj(Γ)=Λ1Γ4+Λ2Γ2+Λ3=0, (14) where Λ1=T2 s1 α1α2+α1α2T2 s2−Λ0,Λ0=TrM21M−1 2
Ts1Ts2, (15a) Λ2=(1+T2 s1T2 s2)α2 1+α2 2−α2 1α2 2−1 α1α2 −(T2 s1+T2 s2)α2 1+α2 2 α1α2 +2Λ0, (15b) Λ3=T2 s2 α1α2+α1α2T2 s1−Λ0. (15c) It is obvious from Eq. (14) and (15) that Fobjis a function of only Γsince using Eqs. (9) and (4), Ts1 andTs2can readily be expressed as Ts1=λ(1,2)
L1/∆L,Ts2=λ(1,2)
L2/∆L. (16) As a result, using Tr(M1M−1 2)andTr(M21M−1 2)we can determine Γfrom Eq. (14). Because there are two roots for Γin Eq. (14), the correct root can be chosen by imposing the constraint |Γ|≤1 (orℜe{Zs}≥0) for passive samples and continuity of Γover frequency. Finally, the εrand µrcan analytically be determined by using Eqs. (4), (5), (9), and (14) as µr=γ γ01+Γ 1−Γ , ε r=1 µrγ γ02 . (17) III. ON ACCURACY AND STABILITY OF THE PROPOSED METHOD In the derivation of the objective function in Eq. (14) it is tacitly assumed that α1,1 and α2,1. It is instructive then to evaluate what happens when a) α1=α2=1 and b)α1=α2=α0 whereα0,1. We will also demonstrate how the correct root for λ(1,2)andγ(1,2)from Eqs. (9) and (10) can be selected. Finally, we will investigate the stability of the derivations for γ(1,2)in Eq. (10) andΓin Eq. (14) for di fferent values of S-parameters. A. Analysis of special cases for α1andα2 We first consider the case when α1=α2=1, corresponding to the case in which the shorter slab with length L1fills completely the measurement cell in Fig. 1(c). Although this case is already taken into consideration in the determination of γin Subsection II B, it is important for two reasons: 1) it verifies the derivation of the objective function in Eq. (14), and 2) it underlines the basic idea behind the constitutive parameters ( εrandµrsimultaneously) determination. Substituting α1=α2=1 into Eqs. (14) and (15), we obtain with the assumption of Γ,∓1 Λ(1) 1=T2 s1+T2 s2−Λ0,Λ(1) 2=−2Λ(1) 1,Λ(1) 3=Λ(1) 1, (18a) Tr(TL1T−1 L2)=Ts1 Ts2+Ts2 Ts1 , (18b) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-6 Hasar et al. AIP Advances 4, 107116 (2014) where Λ(1) 1,Λ(1) 2, and Λ(1) 3, respectively, correspond to the Λ1,Λ2, and Λ3values when α1=α2=1. Here, it is clear that Eq. (18b) will reduce to Eq. (9), which accordingly verifies the derivation of Fobj(Γ), and that this case does not give any information about the value of a physical Γ(|Γ|≤1). We next consider the case when α1=α2=α0whereα0,1, corresponding to the case of the shorter slab symmetrically positioned into the measurement cell (the same air regions between the end surfaces of the shorter slab and the terminals of the cell) in Fig. 1(c). Reflecting the condition α1=α2=α0whereα0,1 into Eqs. (15a)-(15c) we determine Λ(2) 1=T2 s1 α2 0+α2 0T2 s2−Λ0, (19a) Λ(2) 2=−1 α2 01−α2 0
2+T2 s1T2 s21+α4 0
+2Λ0, (19b) Λ(2) 3=T2 s2 α2 0+α2 0T2 s1−Λ0. (19c) where Λ(2) 1,Λ(2) 2, and Λ(2) 3, respectively, correspond to the Λ1,Λ2, and Λ3values when α1=α2=α0 whereα0,1. Because using Eqs. (10) or (18b) we can determine at most either εrorµrvia the complex pro- pagation constant γ, we also need to measure Γto extractεrandµrsimultaneously. It is apparent from Eqs. (19a)-(19c) that Λ(2) 1,Λ(2) 2, and Λ(2) 3are all linearly independent from one another so that their substitution into Eq. (14) will not allow the factoring out of the Γterm. Therefore, this case gives some information about Γ. As a result, considering these two special cases, we conclude that the su fficient condition for simultaneous εrandµrdetermination in LL methods is to measure raw S-parameters of two measurement cells which are completely and partially (symmetrically or asymmetrically) loaded by the two identical unknown slabs with di fferent lengths ( L1,L2). B. Solving the ambiguity in the selection of the correct propagation constant It is obvious from Eq. (9) that λ(1,2)represents both forward and backward traveling waves inside the MM slab. In this circumstance, it is di fficult to discern which eigenvalue, λ1orλ2(in Green’s function terminology), corresponds to forward or backward traveling waves at a given frequency. To resolve this problem, a simple technique using the comparison of λ(1,2)at different frequencies was introduced.23In applying this technique, for the first two lowest frequencies in the band, the positive solution of Eq. (9) is arbitrarily retained and plotted in the complex plane. For each of next frequencies, the two possible solutions in Eq. (9) are tested and only the one which ensures a monotonic variation ofλ(1,2)in the complex plane is taken as the correct solution. This technique resembles to similar ones in the literature3,8,31–34and is based on the principle of causality of physical (non-anticipative) systems in which the system response is directly dependent upon past and present values of the input. C. Stability analysis for the complex permittivity and permeability determination Because the derivations for εrandµrdepend on M1M−1 2andM21M−1 2, we have to monitor the effects of measurement uncertainties on εrandµrdetermination. Using Eq. (2), we can express Tr(M1M−1 2)as Tr(M1M−1 2)=S(1) 12S(1) 21+S(2) 12S(2) 21−S(1) 11 S(1) 22+S(2) 11 −S(2) 22 S(2) 11+S(1) 22 S(1) 21S(2) 12. (20) When both S(u) 11andS(u) 22approach zero in Eq. (20), the value of Tr(M1M−1 2)will not go to zero or infinity as in the case of broadband constitutive parameters determination by the methods3,8,10 (i.e., inadequacy of retrieval procedures35,36), and thus will be stable. In addition, it is well-known that the phase uncertainty of measured reflection S-parameters ( S(u) 11andS(u) 22) increases significantly All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-7 Hasar et al. AIP Advances 4, 107116 (2014) when S(u) 11andS(u) 22simultaneously or separately approach zero. This situation results in a large ripple in the extracted εrandµr36(or anti-resonant behavior35) by most S-parameter techniques in the literature since the phase uncertainty in S(u) 11and/orS(u) 22is multiplied by 1 /|S(u) 11|and/or 1/|S(u) 22|in the process of extracting εrandµr.3,8,10Here, the vertical bars on each side of a quantity denote its magnitude. On the other hand, it is expected that the proposed method will extract more smootherεrandµrover a broad band since the e ffect of phase uncertainty is already mitigated by the amplitudes of reflection S-parameters in Eq. (20). Because the WCM and ABCD matrix representations preclude both S(u) 21andS(u) 12from approaching zero, the derivations of εrandµrwill be stable by the proposed method. However, when the amplitudes of S(u) 21and/orS(u) 12become less than approximately -30 dB (depending on the accuracy of measuring instrument), the uncertainties in amplitudes and phases of S(u) 21andS(u) 12will greatly increase, and this circumstance may adversely affectεrandµrextraction. As a result, the derivations for εrandµrfrom Eqs. (10) and (14) are generally e ffective and suitable for high-to-low-loss (but not very high-loss) MM slabs. IV. RESULTS For validation of the proposed method, in this section we consider a MM slab with SRRs in rectangular or circular form. This MM slab structure is one of the mostly studied MM slabs in the literature.3,4,8–10,17–20The cell of the analyzed slab, as shown in Fig. 2(a) – slightly di ffering from that in,3,4is assumed to be in cubical form ( ay=az=L=4 mm) and has the following features. The substrate onto which the metal strips of SRRs are placed has a complex permittivity ofεs=4.4−i0.15, a thickness of t=0.25 mm, and an area of 4 ×4 mm2. Strips are assumed to be copper [5 .8×107(S/m)] with a thickness of 17 µm. The outer ring length is l=2.2 mm and both rings have a linewidth of w=0.2 mm. While the slit gap is g=0.22 mm for both rings, the separation distance between inner and outer rings is s=0.15 mm. While the MM slab with length L1=4.0 mm is obtained by periodic arrangement of the cell in Fig. 2(a) in the yandzdirections with periods ay=4.0 mm and az=4.0 mm, the MM slab with length L2=8.0 mm is obtained by the cascaded connection of two identical MM slabs with lengths L1=4.0 mm in the xdirection (wave propagation direction). S-parameters simulations are performed by a commercial 3-D full electromagnetic simulation software (CST Microwave Studio C37). In the simulations, electric (magnetic) boundary conditions for which tangential components of electric (magnetic) fields are zero are applied at xy(xz) planes. To ensure periodicity of the unit cell (Fig. 2(a)) in the yzplane as well as a wave propagating in the xdirection, waveguide ports are positioned over yzplanes. Fig. 2(b) demonstrates magnitudes of the simulated S-parameters of the investigated SRR MM slabs with L1=4.0 mm and L2=8.0 mm (simulated S-parameters for the configurations in Fig. 1(a) and 1(b), respectively). For simplicity, phases of the these simulated S-parameters as well as S-parameters of the configuration in Fig. 1(c) are not shown. It is seen from Fig. 2(b) that both of the investigated SRR MM slabs with lengths L1 andL2resonate around 11.0 GHz, at which fast variation of S-parameters is notable. It is a well-known fact that for the analyzed wave incidence (wave propagation in xdirection and electric field in zdirection), the cell in Fig. 2(b) behaves as an isotropic MM slab (no coupling between electric and magnetic fields4,10), characterized by both εrandµror both zandn[Eqs. (4) and (5)]. For validation of our method, we first substitute the simulated S-parameters, corresponding to the configurations in Fig. 1(a)-1(c), into Eq. (2). Then, we calculate the matrix multiplications M21M−1 2andM1M−1 2. Next, we find γfrom Eq. (10) and Γfrom Eq. (14). Using Eqs. (17) and (4), we finally extract n,z,εr, andµrvalues, as shown in Figs. 3 and 4, of the analyzed MM slab by applying the proposed method (PM). The same figures also illustrate the extracted n,z,εr, andµrparameters of the MM slab with length L1=4.0 mm by the general method (GM) in.3,10,17 We have the following two important points drawn from the dependencies in Figs. 3 and 4. First, each extracted electromagnetic parameter by the proposed method is in good harmony with that by the general method. Second, while extracted ε′′ rby the general method is less than zero over approximately 10.3-11.0 GHz (as seen from the inset in Fig. 4(a)), that by the proposed method is greater than zero over the whole analyzed frequency band (5.0-15.0 GHz). It is a well-established All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-8 Hasar et al. AIP Advances 4, 107116 (2014) FIG. 3. Real and imaginary parts of the retrieved (a) refractive index ( n) and (b) the normalized wave impedance ( z) by the general method (GM) in Refs. 3, 10, and 17 and the proposed method (PM). fact that for a passive medium such as that we consider in our paper in Fig. 2(a), ε′′ r(andµ′′ r) must all be positive in the whole frequency range.3,8,10,16,17,35,38In a recent study,35it has also been shown that non-physical anti-resonant behavior [a decay in the electromagnetic property with increasing frequency (e.g., a negative slope of ε′ rbetween 10 .3-11.0 GHz in Fig. 4(a)) and an increase in power gain around the resonance band (e.g., ε′′ r<0 between 10 .3-11.0 GHz in Fig. 4(a))] of ex- tracted electromagnetic properties comes from not periodicity of MM slabs but from in-complete adequacy of retrieval procedures. In addition, in another study38it has also been demonstrated that this non-physical behavior of electromagnetic properties of MM slabs can be remedied by fitting the simulated S-parameters to those obtained from the Lorentz /Drude e ffective medium models. As a result, for the tested MM slab in Fig. 2(a) we note that our proposed method not only extracts physically acceptable electromagnetic properties but also eliminates the need for the fitting process for accurate electromagnetic properties. In the calculation of the frequency-dependent parameters in Figs. 3 and 4 we assumed that measurement error sources (incorrect reference-planes, source and load match errors, errors due to mismatched connections, transitions between connecting cables and horn antennas, contact struc- tures, embedded devices) have no e ffect on electromagnetic properties extracted by both methods. Now, we consider the e ffect of a shift in positions of reference planes [planes or surfaces to which the setup is assumed to be calibrated; e.g., to the left and right terminals of S1in Fig. 1(a)] on n,z, εr, andµrparameters. For simplicity, it is assumed in this analysis that while there is no shift in the reference-plane on the right surface of the MM slab, there is a −1 mm shift in the reference-plane on the left surface of the MM slab. In the extraction of electromagnetic properties by the GM method, the e ffect of a shift in the left reference-plane is reflected into the analysis by multiplying the reflection and transmission S-parameters for the configuration in Fig. 1(a) with exp (+2γ0)and exp(+γ0), respectively (please refer to Eqs. (11) and (12) for more details). Figs. 5 and 6 illustrate the retrieved electromagnetic parameters of the analyzed SRR MM slab using the GM method (from S-parameters of only the MM slab with length L1=4.0 mm) and the PM method. FIG. 4. Real and imaginary parts of the retrieved (a) relative complex permittivity ( εr) and (b) relative complex permeability (µr) by the general method (GM) in Refs. 3, 10, and 17 and the proposed method (PM). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-9 Hasar et al. AIP Advances 4, 107116 (2014) FIG. 5. Real and imaginary parts of the retrieved (a) refractive index ( n) and (b) the normalized wave impedance ( z) by the general method (GM) in Refs. 3, 10, and 17 and the proposed method (PM) when there is an o ffset (-1 mm) in the reference-plane position on left side of the MM slab. By comparison with the dependencies in Figs. 3 and 4, it seen from the dependencies in Figs. 5 and 6 that an absolute shift of 1 mm (a generally acceptable level of measurement error at micro- wave frequencies) drastically alters actual dependencies of electromagnetic parameters retrieved by the general method.3,10,17This is because MM structures are resonating structures whose response sharply changes around resonance frequency. The same amount of shift, nonetheless, does not a ffect the same frequency-dependent parameters extracted by the proposed method. Furthermore, we note that the o ffset in the left reference-plane position results in a non-physical ε′′ rextracted by the GM method over a broader band (10.3-15.0 GHz) than that (10.3-11.0 GHz) when there is no o ffset. As a result, the dependencies in Figs. 5 and 6 clearly indicate the importance of a retrieval method resistant to undesired measurement errors arising from a shift in reference-plane positions. To test the proposed method for other systematic measurement errors such as source and load match errors, errors due to mismatched connections, and imperfection of the used calibration kit, we have considered various TXandTYsquare matrices in Eqs. (1) and (11). We note from this analysis that our proposed method is resistant and immune to those errors and retrieves accurate constitutive parameters of the test MM slab because our proposed method eliminates the e ffect of unknown error matrices TXandTYin Eqs. (11) and (13) from determination of electromagnetic parameters,25,28,29 as discussed in Subsection II C. V. CONCLUSIONS We have proposed a calibration-independent method for determination of complex permittivity and complex permeability of isotropic homogeneous MM slabs. We think that such a method, capable of extracting the constitutive parameters, has been proposed for the first time in the liter- ature. Its advantage is that it removes or eliminates measurement errors of various origins arising FIG. 6. Real and imaginary parts of the retrieved (a) relative complex permittivity ( εr) and (b) relative complex permeability (µr) by the general method (GM) in Refs. 3, 10, and 17 and the proposed method (PM) when there is an o ffset (-1 mm) in the reference-plane position on left side of the MM slab. All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54107116-10 Hasar et al. AIP Advances 4, 107116 (2014) from incorrect reference-planes, source and load match errors, errors due to mismatched connec- tions, transitions between connecting cables and horn antennas, contact structures, embedded de- vices. The idea behind how the constitutive parameters are retrieved is through some illustrative cases. In addition, the accuracy of the proposed method is analyzed by letting the reflection S-parameters approach zero. We have tested the proposed method by using simulated S-parameters of a MM slab with SRRs and noted for the tested slab that while our proposed method removes the so-called antiresonance problem as well as the problem of measurement errors, the method widespreadly used in the literature has both of these problems in di fferent levels. ACKNOWLEDGMENTS Authors, U. C. Hasar, G. Buldu, M. Bute, T. Karacali, and M. Ertugrul, would like to express their thanks to the Scientific and Technological Research Council of Turkey (TUBITAK) under the project Grant Number 112R032 for supporting this study. U. C. Hasar also sends special thanks to the Science Academy of Turkey (the Young Scientists Award in 2014) for supporting his studies. 1J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000). 2J. B. Pendry, D. Schuring, and D. R. Smith, Science 312, 1780 (2006). 3D. R. Smith, S. Schultz, P. 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Bute, and M. Ertugrul, J. Electromagnetic Waves Appl. 28, 903 (2014). 37CST Microwave Studio. Version 2014. Darmstadt (Germany): CST GmbH. 38G. Lubkowski, R. Schuhmann, and T. Weiland, Microw. Opt. Technol. Lett. 49, 285 (2007). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license. See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 155.247.166.234 On: Sun, 23 Nov 2014 23:01:54

1.4820457.pdf

Low-temperature magnetic characterization of optimum and etch-damaged in-plane magnetic tunnel junctions Jimmy J. Kan, Kangho Lee, Matthias Gottwald, Seung H. Kang, and Eric E. Fullerton Citation: J. Appl. Phys. 114, 114506 (2013); doi: 10.1063/1.4820457 View online: http://dx.doi.org/10.1063/1.4820457 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v114/i11 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors Downloaded 23 Sep 2013 to 128.143.23.241. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsLow-temperature magnetic characterization of optimum and etch-damaged in-plane magnetic tunnel junctions Jimmy J. Kan,1Kangho Lee,2Matthias Gottwald,1Seung H. Kang,2and Eric E. Fullerton1 1Center for Magnetic Recording Research, University of California, San Diego, La Jolla, California 92093, USA 2Advanced Technology, Qualcomm, Inc., San Diego, California 92121, USA (Received 5 June 2013; accepted 20 August 2013; published online 19 September 2013) We describe low-temperature characterization of magnetic tunnel junctions (MTJs) patterned by reactive ion etching for spin-transfer-torque magnetic random access memory. Magnetotransport measurements of typical MTJs show increasing tunneling magnetoresistance (TMR) and larger coercive fields as temperature is decreased down to 10 K. However, MTJs selected from the high-resistance population of an MTJ array exhibit stable intermediate magnetic states when measured at low temperature and show TMR roll-off below 100 K. These non-ideal low-temperature behaviors arise from edge damage during the etch process and can have negative impacts onthermal stability of the MTJs. VC2013 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4820457 ] I. INTRODUCTION Magnetic tunnel junctions (MTJs) comprised of CoFeB/ MgO/CoFeB trilayers are the core components in today’s Spin-Transfer Torque Magnetic Random Access Memory (STT-MRAM) cells. For memory applications, many electri-cal and magnetic characteristics need to be optimized simultaneously. For instance, establishing a high tunneling magnetoresistance (TMR) ratio is important for securinglarge read speed/voltage margins while large magnetic energy barrier (EB) is essential to guarantee data retention and low bit error rates. 1All of these parameters are highly sensitive and can be easily degraded during device etching steps. In this paper, we describe the magnetotransport behav- ior of STT-MRAM MTJs and report the emergence of inter-mediate states and TMR roll-off at low temperatures in etching damaged MTJs and link these characteristics to degraded device performance and thermal stability at roomtemperature. II. EXPERIMENTAL DETAILS The in-plane MTJs investigated in this study are ellipti- cal 40 nm /C2110 nm devices based on CoFeB free and refer- ence layers and have been fully integrated into a 45-nm CMOS logic platform.2(a),2(b)Transport properties of the MTJs are measured by conventional two-probe techniques.Pulsed measurements of room temperature transport and probabilistic switching were performed using a National Instruments PXIe Pin Parametric Measurement Unit. Wehave studied an array of devices and characterized the distri- butions of resistance, coercive field, and magnetoresistance. The average room temperature, zero-field TMR, andresistance-area (RA) product measured from more than 30 MTJs are 132% and 10 Xlm 2, respectively. The TMR values described in this paper are given by the parallel resist-ance (R p) and antiparallel resistance (R ap) measured at zero field (TMR ¼[Rap0-Rp0]/Rp0) instead of at full saturation because the zero-field TMR value is more relevant for STT-MRAM devices. We further characterized selected devicesby variable-temperature magnetotransport measurements performed in a helium cryostat (Quantum Design Physical Property Measurement System with a temperature range of 2–400 K) using a custom designed insert probe to suppresselectro-static discharge events. To study the room-temperature switching characteristics and thermal stability we used time-dependent STT measure-ments. In the thermally activated spin-transfer torque switch- ing regime, which is for an applied voltage V /C28V c0(Vc0is the intrinsic switching voltage), the probability of switchingbetween the R pand R apstates can be modeled as Psw¼1/C0exp/C0tp s/C18/C19 ;s¼s0expD1/C0V Vc0/C18/C19/C20/C21 ;(1) where tpis the duration and amplitude of a voltage pulse, s0 is the thermal attempt time (approximately 10/C09s), and Dis the energy barrier for reversal of the free layer.3Assuming V/C28Vc0, this thermal activation model describes the small voltage probability of switching, often referred to as theread-disturbance rate (RDR) lnðP swÞ¼lntp s0/C18/C19 /C0D1/C0V Vc0/C18/C19 : (2) The EB values ( D¼KuV/k BT) are estimated by fitting the slope of the logarithmic portion of RDR at 20-ns pulse widths as shown in Fig. 1.4,5This method of extracting EB has been shown to provide more accurate estimates of thestatic EB than the widely used method which expresses the switching voltage (at P sw¼1-e/C01) as a function of pulse duration Vc¼Vc01/C01 Dlntp s0/C18/C19 /C20/C21 : (3) The application of Eq. (3)generally underestimates the static EB because V cis often beyond the assumed V /C28Vc0limit for the thermal activation model. In addition the high biases used can cause current induced heating and field-like torque 0021-8979/2013/114(11)/114506/4/$30.00 VC2013 AIP Publishing LLC 114, 114506-1JOURNAL OF APPLIED PHYSICS 114, 114506 (2013) Downloaded 23 Sep 2013 to 128.143.23.241. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionscontributions that reduce the measured EB.6In Fig. 1,w e have fitted the RDR probability for a MTJ that represents theaverage electrical and magnetic characteristics of the pri- mary population. For this cell with close to ideal electrical characteristics, we extracted D AP-P¼43.3 and DP-AP¼46.4 for antiparallel-to-parallel (AP-P) and parallel-to-antiparallel (P-AP) switching, respectively. III. RESULTS AND DISCUSSION The quasi-static magnetic properties of MTJs are char- acterized by measuring the transport behavior of the device during magnetic field sweeps. Temperature-dependentresults from the MTJ in the primary resistance population are shown in Fig. 2. This MTJ has an average D¼44.8. The magnetic field is swept at a rate of 10 Oe/s with a constant5-lA read current. R-H loops in Fig. 2(a) show decreasing H Cwith increasing temperature as thermal activation reduces the coercive field. Fig. 2(b) shows that H cAP-P changes over temperature much faster than H cP-AP . Combined with the observed asymmetry in EB values, this suggests that two different reversal mechanisms are responsible for AP-P andP-AP switching. If the P-AP reversal is through coherent rotation while nucleation and propagation is responsible forAP-P switching, the slope of H cAP-P vs. temperature is larger due to the significance of thermal activation in the domain wall depinning processes, and the EB is smaller as a result ofthe reduced effective volume. 7The origin of this asymmetri- cal reversal behavior is not fully understood but is possibly attributed to the stabilizing effect that the dipolar field emit-ted from the reference layer has on the AP state configura- tion. This explanation is consistent with the observation that the R-H loop offset field grows in the AP direction as tem-perature is decreased. This magnetostatic interaction may be stabilizing the spins in the free layer during an attempted AP-P switch, but conversely aiding in the P-AP directionreversal. The introduction of thermally induced magnetic disorder is responsible for the large drop in AP state resistance andzero-field TMR versus temperature as shown in Fig. 2(d). Upon increasing the temperature from 10 K to 300 K, the TMR decreases from 200% to 145%. This results inTMR(10 K)/TMR(RT) ¼1.4, comparable to ratios obtained in other reports on CoFeB/MgO/CoFeB junctions. 8This change in TMR is dominated by a steady increase in R apas temperature is decreased (R apchanges 27% from 300 K to 10 K while R pchanges only by 3%). A compounding of two effects is responsible for these changes. Due to asymmetry inwave functions for different spins across a single crystal MgO barrier, the P state conductance is dominated by tun- neling of majority spins while the AP state conductance isprimarily due to minority spin tunneling through interfacial states, which is more active at higher thermal energies. 9 However, higher thermal energies will induce magnetic dis- order that degrades the spin-polarization of the electrodes, resulting in a decrease in R apand an increase in R p. For these MTJs, a simple model combining elastic and inelastictunneling terms developed by Shang et al. can explain this temperature dependence. The conductance in the P and AP states can be expressed as 10 GPðTÞ¼GTð1þP1P2ÞþST4=3; GAPðTÞ¼GTð1/C0P1P2ÞþST4=3; TMRðTÞ¼DGðTÞ=GAPðTÞ;(4) where G Tis a prefactor for direct elastic tunneling dependent on the MgO barrier thickness and height, P 1and P 2are the effective spin polarizations of the tunneling electrodes (in FIG. 1. A fit of the logarithmic portion of the RDR measurement to a ther- mal activation model gives the energy barrier of this MTJ to be 43.3 (AP-P switching) and 46.4 (P-AP switching). FIG. 2. (a) Resistance vs. field loops measured at various temperatures. The magnetic field is swept at a rate of 10 Oe/s with a constant 5- lA read current. (b) Coercive fields for AP-P and P-AP switching vs. tempe rature. As temperature is decreased, H cAP-P increases faster than H cP-AP. (c) Anti-parallel and para llel resistance vs. tem- perature curves are obtained by extracting R apand R pat H¼0 Oe. (d) As temperature is decreased, zero-field TMR increases monotonically in ideal MTJ cells.114506-2 Kan et al. J. Appl. Phys. 114, 114506 (2013) Downloaded 23 Sep 2013 to 128.143.23.241. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsthese MTJs, the electrodes are symmetrical so P 1¼P2¼P), and S is a prefactor dependent on the density of localized states in the barrier and reflects the inelastic tunneling conductance. The temperature dependence of the TMR, shown in Fig. 2(d), is affected by spin-wave excitations of the magnetiza- tion and can be expressed by Bloch’s law as polarizationscales with magnetic moment PðTÞ¼P 0ð1/C0aT3 2Þ; (5) where ais a material dependent spin-wave parameter and P 0 is the full spin polarization. Fitting the experimental data using Eqs. (4)and(5)gives G T¼3.86/C210/C04(Xlm2)/C01, S¼6.8/C210/C012(Xlm2)/C01K/C04/3, the full spin polarization P0¼0.71, and a¼1.7/C210/C05K/C03/2, which is on the same order of fits previously reported for CoFeB electrodes.11The very small value of S compared to G Tis a good indicator that the transport mechanism is dominated by spin selective tunneling in these ideal MTJs. Switching state diagrams (SSD) give a quantitative picture of the effects of applied magnetic field and spin- polarized current on the MTJ magnetization as shown inFig. 3. For a given field, current with a pulse period of 500ls was swept to generate these maps in DR(H,I) space. By subtracting the up and down branches of the currentsweeps, we highlight the bi-stable region and the STT switching boundaries. These switching boundaries provide insight into parameters that affect the MTJ critical switchingcurrent density that can be estimated from a stability analysis of the Landau-Lifshitz-Gilbert (LLG) equation J c0¼2e /C22ha gMstH extþHkþHd 2/C18/C19 ; (6) where ais the damping parameter, gis the spin transfer tor- que efficiency parameter, t is the free layer thickness, and H k is the uniaxial anisotropy field. In the macrospin regime, the LLG equation relates the height of this region to H Kand the slopes of the boundaries to g/(aMst).12 The spin transfer torque efficiency term depends on the angle between the relative magnetization and polarization as g¼(p/2)/(1þp2cosh).13Because of the angular dependence ofg, the slopes for AP-P and P-AP switching boundariesshould not be the same. SSD measurements shown in Fig. 3 are made between 400 K and 10 K on MTJs representative of the normal population. The measured SSD at 400 K inFig.3(a)shows reduced coercive fields and low slope, asym- metric boundaries. Low slopes at high temperatures are attributed to poor charge to spin conversion due to thermallyinduced magnetic disorder. Boundary slopes for both switch- ing directions become more symmetric as temperature is decreased to 10 K, indicating that AP-P/P-AP spin torqueefficiencies are becoming balanced. This leads to a decrease in the measured I Casymmetry (I cP-AP /IcAP-P ) shown in Fig.3(b). Throughout the entire temperature range, the AP-P slope is steeper, qualitatively indicating a higher switching efficiency in the AP-P direction as expected. The fabrication process flow for MTJs typically involves multiple processing steps that must be optimized in order to minimize issues such as electrical shorts from redeposition, MgO lattice damage, and magnetic materialdegradation. Etching damage to the sidewalls can reduce the electrical size of MTJ cells, the MgO barrier, and intro- duce edge roughness. These etch damaged MTJs are identi-fied in our study by examining the H Cand R Pof an entire population of more than 30 MTJs and selecting cells with high R pand low Hc more than 2 standard deviations from the mean. When all other parameters are assumed to be equal, R pis inversely proportional to the cross sectional area of the MTJ,and H Cis proportional to the anisotropy. By focusing on the cells with high R Pand low H C, we are effectively selecting the low EB tail bits of the population. Fig. 4shows an example of an MTJ from this population. This MTJ has average room- temperature D¼30. Room temperature R-H loops of these cells show low to average H C, but stable intermediate magnet- ization states begin to appear at temperatures lower than 100 K. At this temperature, marked loss of squareness occurs, and TMR becomes reduced as the fully remnant AP and Pstates cannot be reached. This loss of squareness suggests that both AP-P and P-AP switching in these cells are accomplished incoherently by domain nucleation and propagation. A fitof the linear portion using Eqs. (4)and (5)gives G T ¼3.86/C210/C04(Xlm2)/C01,S¼1.2/C210/C09(Xlm2)/C01K/C04/3, the full spin polarization P 0¼0.66, and a¼1.7/C210/C05K/C03/2. The S parameter in this fit is markedly higher than the S of the FIG. 3. (a) SSD measurements of a primary population MTJ at various temperatures. The slopes of the tilted switching boundaries are reflective of the sp in transfer torque efficiency. Dashed lines are guides for the eye. (b) Ic asymmetry vs. temperature measured by extracting I cAP-P and I cP-AP from a series of SSD measurements at H ¼0 Oe.114506-3 Kan et al. J. Appl. Phys. 114, 114506 (2013) Downloaded 23 Sep 2013 to 128.143.23.241. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissionsprimary population MTJ and indicates a larger inelastic, spin- independent tunneling component. This is a general trend forthis population of damaged devices and R-H loops of these tail bits often show unexpected features such as stable intermediate states characterized by an intermediate resistance and poorsquareness. A similar loss of squareness in MTJ devices at low temperature has been associated with magnetic pinning sites brought on by the formation of a spin-glass-like phase that pre-vents full alignment of magnetic spins below a spin-freeze temperature. 14This results in an inhomogeneous distribution of the spins in the free layer and reduces spin polarization andTMR as shown in Fig. 4(c). Domain nucleation and propagation type switching is prevalent in these MTJs because sidewall etching damageaids nucleation of domains and provides pinning sites for domain wall motion. 15These states are unstable and cannot be observed at higher temperatures because thermal activa-tion can provide the additional energy necessary for depin- ning processes. However, it has been shown that initial magnetization conditions and specific switching modes havean influence on the observed energy barrier at room tempera- ture. 16With these metastable domain states present within the MTJ, a macrospin like picture cannot be used to com-pletely describe the MTJ. Sun et al. have shown that increas- ing the nominal size of the MTJ will induce a transition from macrospin to sub-volume activation type behavior. 17This transition comes with degradation in the spin-torque switch- ing efficiency as well as EB. IV. CONCLUSION In conclusion, we have shown the magnetotransport behavior of both normal and failure bit MTJs. MTJ failurebits damaged by the RIE process can exhibit a number of defects that cannot be observed in room temperature mag- netic characterization such as metastable intermediate statesand loss of spin-polarization. Low temperature measure- ments suggest that domain states in failure bits are stabilizedby edge pinning sites and have a negative impact on overall data retention. ACKNOWLEDGMENTS Research at UCSD was supported by NSF Award No. DMR-1008654 and by UC Discovery Grant No. 21294. 1A. Driskill-Smith, S. Watts, V. Nikitin, D. Apalkov, D. Druist, R. Kawakami, X. Tang, X. Luo, A. Ong, and E. Chen, Dig. Tech. Pap. - Symp. VLSI Technol. 2010 , 51–52. 2(a) K. Lee, W. C. Chen, X. Zhu, X. Li, and S. H. Kang, J. Appl. Phys. 106, 024513 (2009); (b) Specific details of the stack structure are not disclosed as it is considered proprietary information. However, the content of the pa- per is not specific to the materials system. 3I. N. Krivorotov, N. C. Emley, A. G. F. Garcia, J. C. Sankey, S. I. Kiselev, D. C. Ralph, and R. A. Buhrman, Phys. Rev. Lett. 93, 166603 (2004). 4R. Heindl, W. H. Rippard, S. E. Russek, M. R. Pufall, and A. B. Kos, J. Appl. Phys. 109, 073910 (2011). 5K. Lee, J. J. Kan, E. E. Fullerton, and S. H. Kang, IEEE Magn. Lett. 3, 3000604 (2012). 6Z. Li, S. Zhang, Z. Diao, Y. Ding, X. Tang, D. M. Apalkov, Z. Yang, K. Kawabata, and Y. Huai, Phys. Rev. Lett. 100, 246602 (2008). 7J. Cucchiara, Y. Henry, D. Ravelosona, D. Lacour, E. E. Fullerton, J. A. Katine, and S. Mangin, Appl. Phys. Lett. 94, 102503 (2009). 8X. Kou, J. Schmalhorst, A. Thomas, and G. Reiss, Appl. Phys. Lett. 88, 212115 (2006). 9A. A. Khan, J. Schmalhorst, G. Reiss, G. Eilers, M. Munzenberg, H.Schuhmann, and M. Seibt, Phys. Rev. B 82, 064416 (2010). 10C. H. Shang, J. Nowak, R. Jansen, and J. S. Moodera, Phys. Rev. B 58, R2917–R2920 (1998). 11H. D. Gan, H. Sato, M. Yamanouchi, S. Ikeda, K. Miura, R. Koizumi, F. Matsukura, and H. Ohno, Appl. Phys. Lett. 99, 252507 (2011). 12M. D. Stiles and J. Miltat, “Spin dynamics in confined magnetic structures III,” Top. Appl. Phys. 101, 225–308 (2006). 13Z. Diao, D. Apalkov, M. Pakala, Y. Ding, A. Panchula, and Y. Huai, Appl. Phys. Lett. 87, 232502 (2005). 14L. Yuan and S. H. Liou, Phys. Rev. B 73, 134403 (2006). 15O. Ozatay, N. C. Emley, P. M. Braganca, A. G. F. Garcia, G. D. Fuchs, I. N. Krivorotov, R. A. Buhrman, and D. C. Ralph, Appl. Phys. Lett. 88, 202502 (2006). 16J. Z. Sun, P. L. Trouilloud, M. J. Gajek, J. Nowak, R. P. Robertazzi, G.Hu, D. W. Abraham, M. C. Gaidis, S. L. Brown, E. J. O’Sullivan, W. J. Gallagher, and D. C. Worledge, J. Appl. Phys. 111, 07C711 (2012). 17J. Z. Sun, R. P. Robertazzi, J. Nowak, P. L. Trouilloud, G. Hu, D. W. Abraham, M. C. Gaidis, S. L. Brown, E. J. O’Sullivan, W. J. Gallagher, and D. C. Worledge, Phys. Rev. B 84, 064413 (2011). FIG. 4. (a) Resistance vs. field loops measured at various temperatures for an etch-damaged MTJ. The magnetic field is swept at a rate of 10 Oe/s with a con- stant 5- lA read current. Intermediate states are present below 100 K. (b) Anti-parallel and parallel resistance vs. temperature curves are obtained by extrac ting Rapand R pat H¼0 Oe. (c) Zero-field TMR vs temperature in etch-damaged MTJs show a characteristic roll off at around 100 K corresponding to the onset of intermediate states.114506-4 Kan et al. J. Appl. Phys. 114, 114506 (2013) Downloaded 23 Sep 2013 to 128.143.23.241. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

1.4895073.pdf

Proximity effect between a topological insulator and a magnetic insulator with large perpendicular anisotropy Wenmin Yang, Shuo Yang, Qinghua Zhang, Yang Xu, Shipeng Shen, Jian Liao, Jing Teng, Cewen Nan, Lin Gu, Young Sun, Kehui Wu, and Yongqing Li Citation: Applied Physics Letters 105, 092411 (2014); doi: 10.1063/1.4895073 View online: http://dx.doi.org/10.1063/1.4895073 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructures Appl. Phys. Lett. 105, 053512 (2014); 10.1063/1.4892353 Topological insulator Bi2Te3 films synthesized by metal organic chemical vapor deposition Appl. Phys. Lett. 101, 162104 (2012); 10.1063/1.4760226 Magnetically doped semiconducting topological insulators J. Appl. Phys. 112, 063912 (2012); 10.1063/1.4754452 Ferromagnetism in thin-film Cr-doped topological insulator Bi2Se3 Appl. Phys. Lett. 100, 082404 (2012); 10.1063/1.3688043 Epitaxial growth of Bi2Se3 topological insulator thin films on Si (111) J. Appl. Phys. 109, 103702 (2011); 10.1063/1.3585673 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.59.226.54 On: Wed, 10 Dec 2014 08:34:50Proximity effect between a topological insulator and a magnetic insulator with large perpendicular anisotropy Wenmin Y ang,1Shuo Yang,1Qinghua Zhang,2Yang Xu,1Shipeng Shen,1Jian Liao,1 Jing Teng,1Cewen Nan,2Lin Gu,1Young Sun,1,a)Kehui Wu,1,b)and Yongqing Li1,c) 1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China (Received 9 July 2014; accepted 27 August 2014; published online 5 September 2014) We report that thin films of a prototype topological insulator, Bi 2Se3, can be epitaxially grown onto the (0001) surface of BaFe 12O19(BaM), a magnetic insulator with high Curie temperature and large perpendicular anisotropy. In the Bi 2Se3thin films grown on non-magnetic substrates, classic weak antilocalization (WAL) is manifested as cusp-shaped positive magnetoresistance (MR) in perpendicular magnetic fields and parabola-shaped positive MR in parallel fields, whereas in Bi2Se3/BaM heterostructures the low field MR is parabola-shaped, which is positive in perpendicu- lar fields and negative in parallel fields. The magnetic field and temperature dependence of the MR is explained as a consequence of the suppression of WAL due to strong magnetic interactions at the Bi2Se3/BaM interface. VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4895073 ] The surface of a three-dimensional topological insulator (TI) hosts a fascinating Dirac electron system with momen- tum locked to real electron spins,1,2in contrast to the valley- related pseudospins in graphene.3The helical spin structure has been exploited theoretically as the basis for realizing top- ological magnetoelectric effects and spintronic applica- tions.4–12In many proposals, a key ingredient is to open an energy gap near the Dirac point via the proximity effect between a TI and a magnetic insulator (MI). In case of mag- netization of the MI parallel to the interface, obtaining a siz-able gap would require significant Fermi surface warping. 13,14In contrast, an MI with out-of-plane magnetic order can break time reversal symmetry, thereby opening alarge energy gap on any TI surface as long as the interfacial exchange interaction is sufficiently strong. Unfortunately, the easy magnetization axis in most known MIs, such as fer-romagnets EuO, 15EuS,16,17EuSe,18GdN,19and ferrimagnet yttrium iron garnet (YIG),20lies inside the thin film/plate plane. Magnetic insulators with perpendicular magnetocrys-talline anisotropy are very scarce. 21Thus far, strong proxim- ity effect between a TI and an MI with perpendicular anisotropy has not yet been reported, even though stronginterface interaction has been realized recently in a TI/mag- netically doped TI heterostructure. 22 Here, we demonstrate that Bi 2Se3thin films can be epi- taxially grown onto BaFe 12O19, a room temperature mag- netic insulator with large perpendicular anisotropy. When a magnetic field is applied perpendicular to the Bi 2Se3/ BaFe 12O19heterostructure, positive magnetoresistance (MR) is observed. It has quadratic field dependence in weak mag- netic fields and crosses over to logarithmic dependence instronger fields. Applying parallel magnetic field leads to neg- ative MR. The magnetotransport data suggest strongsuppression of weak antilocalization (WAL) due to the mag- netic proximity effect at the Bi 2Se3/BaFe 12O19interface. M-type Barium hexaferrites (BaFe 12O19, BaM), is an important magnetic material that has been studied for deca-des due to applications in magnetic recording and micro- wave devices. 23–27It is highly insulating and has a Curie temperature of 723 K.27In this work, the flat (0001) surfa- ces of nearly hexagon-shaped single crystalline thin plates (Fig. 1(a)) were used as the substrates for epitaxial growth of Bi 2Se3thin films. Fig. 1(b) shows magnetization curves FIG. 1. (a) Schematic diagram of a Bi 2Se3/BaM Hall bar device. The upper- left inset shows a micrograph of a 200 lm wide Hall bar device, and the upper-right inset is an optical image of a hexagon-shaped BaM single crys- talline plate with a size of /C246/C24/C21m m3. (b) Magnetization curves meas- ured at T¼2 K with an external magnetic field Happlied parallel (open circles) or perpendicular (solid squares) to the (0001) plane of BaM. (c)Cross-section TEM image of the interface region of a Bi 2Se3/BaM hetero- structure. (d) X-ray diffraction pattern of a Bi 2Se3/BaM heterostructure. Diffraction peaks can be indexed either (0,0,0,2n) for BaM or (0006) for Bi2Se3.a)Electronic mail: youngsun@iphy.ac.cn b)Electronic mail: khwu@iphy.ac.cn c)Electronic mail: yqli@iphy.ac.cn 0003-6951/2014/105(9)/092411/4/$30.00 VC2014 AIP Publishing LLC 105, 092411-1APPLIED PHYSICS LETTERS 105, 092411 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.59.226.54 On: Wed, 10 Dec 2014 08:34:50of a typical BaM sample with magnetic field Happlied per- pendicular and parallel to the (0001) plane at T¼2K . T h e magnetization Mreaches saturation at l0H¼0.5 T and 1.75 T for perpendicular and parallel field orientations,respectively. For both orientations, Mhas a nearly linear dependence on Hbelow the saturation. These features are in agreement with those previously reported for high qual-ity single crystals. 23,25The large perpendicular anisotropy, the simple M-H relationship, and the high Curie tempera- ture make BaM a valuable platform for investigation of the interfacial interactions between TIs and magnetic materials. Furthermore, the large remnant magnetization in some spe-cially engineered BaM thin films 28could be very useful for pursuing topological magnetoelectric effects without exter- nal magnetic fields. Fig.1(c)is a high resolution cross-section transmission electron microscopy (TEM) image of a Bi 2Se3/BaM hetero- structure. It shows that the 1 nm thick Se-Bi-Se-Bi-Se quin-tuple layers are parallel to the (0001) surface of BaM despite some minor ripples. The interface between BaM and Bi 2Se3 is quite sharp, even though the first 1/2 quintuple layer is imaged less clearly than the other layers. The crystalline structure of the Bi 2Se3/BaM heterojunction is further con- firmed with x-ray diffraction, as shown in Fig. 1(d). Low temperature electron transport measurements were used as a probe for interfacial magnetic interactions. A thick- ness of 10 nm was chosen for the Bi 2Se3thin films grown on the BaM substrates. Such a thickness is well above the 5 nm threshold, below which the wavefunctions of the top and bot- tom surfaces overlap substantially, resulting in a hybridiza-tion gap near the Dirac point. 29This would modify the Berry phase of the surface states, and produce transport characteris- tics similar to those brought by strong magnetic interac-tions. 30–32We also carried out transport measurements of the Bi2Se3thin films grown on SrTiO 3(STO) substrates in order to provide a reference system with non-magnetic substrates.Hall resistance R xyhas a nearly linear dependence on the magnetic field.33The extracted electron densities are in the range of 2–3 /C21013cm/C02, consistent with previous transport and photoemission studies.34–41Such high electron densities indicate that the Fermi level is located above the conduction band minimum. Both the bulk and the surface electrons areexpected to participate in the transport. In Fig. 2, we plot the main results of the electron trans- port measurements with magnetic field applied perpendicularto the Bi 2Se3thin films. As shown in Fig. 2(a), the magneto- resistance, defined as MR¼½qxxðHÞ/C0qxxð0Þ/C138=qxxð0Þ,i s positive for the Bi 2Se3thin film grown on BaM. The sign of the MR is same as that of the Bi 2Se3thin film on STO (Fig. 2(b)). However, the shape of the MR in the Bi 2Se3/BaM het- erostructure is drastically different from its STO counterpartat low fields. The latter is characterized by the cusp-shaped MR due to the WAL effect. 35The quantum correction to the conductivity of the Bi 2Se3thin films on non-magnetic sub- strates can be described by the Hikami-Larkin-Nagaoka (HLN) equation43 DrHðÞffi/C0ae2 2p2/C22hw1 2þHu H/C18/C19 /C0lnHu H/C18/C19"# : (1)Here, the magnetoconductivity is defined as DrðHÞ ¼rxxðHÞ/C0rxxð0Þ,wðxÞis the digamma function, Hu¼Bu l0 ¼1 l0/C22h 4el2uis the dephasing field, and luis the dephasing length. In single channel systems, the prefactor ais equal to 1/2 for WAL. As illustrated in Fig. 2(d), theDrðHÞdata of the Bi2Se3/STO sample can be fitted fairly well to the HLN equation. The obtained avalues are close to 1/2, which can be attributed to the strong scatterings between the surfaceand the bulk electrons. 37,39,42The magnetoconductivity of the Bi 2Se3/BaM heterostructure, however, cannot be reason- ably fitted to the HLN equation. As shown in Fig. 2(c),i t rather exhibits a quadratic dependence on magnetic field up to at least l0H¼0:3 T. At fields above the magnetization saturation of BaM (i.e., l0H>l0Hs¼0:5 T), however, the MR of the Bi 2Se3/BaM heterostructure crosses over to the HLN-like (or logarithmic) magnetic field dependence. This implies that the phase coherent transport may still be rele-vant in the magnetic heterostructure. In order to gain further insight into underlying physics in the Bi 2Se3/BaM heterostructure, we performed transport measurements in tilted magnetic fields at 1.7 K. Fig. 3(a) shows that, as the magnetic field tilts toward the thin films plane, the sign of the MR is reversed for h<9/C14, at least at H below the magnetization saturation. Here, his the tilting angle relative to the parallel field orientation. In contrast, the MR of the Bi 2Se3/STO remains positive for any field orientation, as shown in Fig. 3(b). In the STO case, the positive MR origi- nates from the WAL-related phase coherent transport if the magnetic field is not too strong. Our previous work44showed that both parallel and perpendicular components of the mag- netic fields Hcan cause destruction of the WAL, and hence positive MR. Therefore, the negative MR observed here oughtto originate from the influence of the magnetic substrate.FIG. 2. Transport properties of the Bi 2Se3thin films grown on the BaM ((a) and (c)) and STO ((b) and (d)) substrates in perpendicular magnetic fields. (a) MR of a 10 nm thick Bi 2Se3film on BaM at T¼1.7–35 K. (b) MR of a Bi2Se3thin film on STO with comparable longitudinal resistivity to that of the Bi 2Se3/BaM heterostructure. Shown in panels (c) and (d) are the corre- sponding magnetoconductivity data. The symbols represent experimentalvalues. The lines in (c) and (d) are the best fits to a quadratic function and the HLN equation, respectively.092411-2 Yang et al. Appl. Phys. Lett. 105, 092411 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.59.226.54 On: Wed, 10 Dec 2014 08:34:50Fig. 4(a) depicts the parallel field MR of the Bi 2Se3/ BaM sample at temperatures up to 70 K. The low field MR is negative, and has a parabolic shape. The MR reaches a mini- mum value at l0H’1:55 T, which is close to the in-plane saturation field ( l0HA¼1:75 T). At H>HA, the magnetiza- tion of the BaM is aligned parallel to the interface, and one would anticipate much weaker magnetic proximity effect onthe electron transport. This is evidenced by the resemblance of the parallel field MR of the Bi 2Se3/BaM heterostructure to the STO counterpart at H>HA(Fig. 4(b)). This further sup- ports that the negative MR observed at lower fields is related to the magnetism in the BaM substrate. The magnetotransport data presented above can be sum- marized in the following two key aspects. One is the parabola-shaped MR existing in a rather broad range of mag- netic fields for both parallel and perpendicular field orienta-tions. Such a quadratic field dependence has never been observed in either perpendicular or parallel fields in previous studies of TI/MI heterostructures such as Bi 2Se3/EuS, Bi 2Se3/ GdN, and Bi 2Se3/YIG.16,17,19,45The other is the strong T-de- pendence of both types of parabolic MR. This is further illus- trated in Fig. 5. For the perpendicular field orientation, the T-dependence of the MR is characterized by K?vs.Tshown in Fig. 5(a), where the coefficient K?is extracted from fitting the data in Fig. 2(c)toDrðBÞ¼/C0 K?B2with B¼l0Hup to 0.3 T. Correspondingly, K==is obtained by fitting the MR data to a similar parabolic function (Fig. 5(b)). Both K?andK== drop more than three times as Tincreases from 1.7 to 30 K, whereas the longitudinal resistivity qxxvaries only about 10% in the same temperature range. As mentioned above, the MR of the Bi 2Se3thin films on non-magnetic substrates can be viewed as a consequence of time reversal symmetry breaking by the externalperpendicular magnetic field, which introduces different Aharonov-Bohm phases to the time-reversed pairs of pathsalong any of the closed loops. 46Such symmetry breaking suppresses WAL, leading to the positive, cusp-shaped MR described by the HLN equation. The parabolic MR observedin the magnetic heterostructure therefore suggests the exis- tence of an extra source for the suppression of WAL. In literature, random magnetic impurities 43,46and mag- netic exchange interaction30are known to be able to break time reversal symmetry, and suppress the phase coherent effect. When the strength of magnetic scatterings is weak,the magnetoconductivity can also be described by the HLN equation, except that the extra dephasing due to the random magnetic scatterings needs to be taken into account. 43,46The low field MR would maintain the cusp-like shape. Such behavior has been observed in GdN/Bi 2Se3heterostructures as well as conventional metal films (e.g., Au thin films) withmagnetic adatoms. 19,46In case of very strong magnetic scat- terings, there is a crossover from the symplectic limit ( a¼1/2) to the unitary limit ( a¼0).43Transport close to the latter limit30was observed previously in Bi 2Te3thin films capped with 1 ML Fe, in which the strong magnetic scatterings from Fe nanoclusters are believed to be responsible for the para-bolic MR. 47In this classical diffusive regime, one would expect weak T-dependence of the MR at low temperatures. This is contradictory to the strong T-dependence of the MR in the Bi 2Se3/BaM heterostructure (Fig. 5(a)). Moreover, it is also unclear how the magnetic impurity scattering model can account for the negative MR in parallel fields. For the MR in perpendicular magnetic fields, the most pronounced deviation from the WAL behavior takes place in low magnetic fields. The magnetization of the BaM substrateis featured by micron-sized maze-like domains. 23Even though the global magnetization is small, the local magnet- ization has a large perpendicular component inside each do-main because of the large magnetocrystalline anisotropy. 23 The magnetic exchange interaction as well as the local strayfield at the interface breaks the time-reversal symmetry inthe Bi 2Se3layer, leading to the suppression of WAL. This interface proximity effect is much larger than the conductiv- ity corrections due to the external field. This can qualita-tively explain the much weaker field dependence of the MR in the Bi 2Se3/BaM heterostructure than that of Bi 2Se3/STO. On a quantitative level, Lu et al. calculated the quantum cor- rections to the conductivity of TI under the influence of per- pendicular magnetization. They found that in case of strong exchange interaction and weak magnetic impurityFIG. 4. The MR data taken in parallel magnetic fields. (a) MR of the Bi 2Se3/ BaM heterostructure at T¼1.7–70 K. (b) MR of the 7 nm thick Bi 2Se3film on STO measured at Tup to 20 K.FIG. 5. Temperature dependence of the magnitude of the quadratic magne- toconductivity (or MR) in the Bi 2Se3/BaM heterostructure in perpendicular (a) and parallel (b) magnetic fields. FIG. 3. MR data recorded in tilted magnetic fields at T¼1.7 K for the Bi2Se3/BaM heterostructure (a) and a 7 nm thick Bi 2Se3thin film grown on STO (b). Data from 10 nm thick Bi 2Se3samples on STO are similar, except with smaller dephasing fields.44092411-3 Yang et al. Appl. Phys. Lett. 105, 092411 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.59.226.54 On: Wed, 10 Dec 2014 08:34:50scatterings, the modified Berry phase in the surface states could result in the positive, parabolic MR.30 The negative MR observed in parallel fields can also be qualitatively explained within the picture of broken time re-versal symmetry in the phase coherent transport. Since His applied along the hard axis of BaM, it rotates the magnetiza- tion out of the perpendicular direction, and hence reducesthe (local) perpendicular magnetization approximately in the form of (1 /C0H 2=H2 A) when H<HA. This decreases the mag- netic proximity effect, resulting in the negative MR. Nevertheless, it should be noted that the negative parabolic MR has also been observed in magnetic multilayers andmagnetic granular systems, in which the MR is attributed to spin dependent scatterings. 48–50In these systems, however, the MR is negative for both parallel and perpendicular fieldorientations. Moreover, the resistance change due to spin de- pendent scatterings usually has weak T-dependence below 30 K. This is also inconsistent with the strong T-dependence of the MR of the Bi 2Se3/BaM heterostructure (Fig. 5). Therefore, we conclude that the MR observed in this work can be mainly attributed to the interplay between the mag-netic interactions at the interface and the phase coherent transport. Nevertheless, further work is needed in order to determine whether these properties mainly originate fromthe interface exchange interactions or from the local stray field induced effects on the quantum diffusive transport. In summary, we have demonstrated the strong magnetic proximity effect in the Bi 2Se3/BaM heterostructure. It is manifested as the parabola-shaped positive MR in perpendic- ular fields and negative MR in parallel fields. Such a uniquetype of MR has not been observed previously in any low dimensional magnetic system, including ferromagnetic thin films, magnetic multilayer structures, magnetic granular sys-tems, and TI/MI heterostructures. The strong proximity effect achieved in this work with the magnetic insulator that has a large perpendicular anisotropy and the Curie tempera-ture higher than room temperature may pave a way to realiz- ing many topological spintronic effects with potential for practical applications. We are grateful for stimulating discussions with P. Xiong and S. von Moln /C19ar. This work was supported by the National Basic Research Program of China (Grant Nos. 2012CB921703, 2013CB921702, and 2014CB921002), the National ScienceFoundation of China (Grant Nos. 91121003, 11374337, and 51332001), and the Chinese Academy of Sciences. 1M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010). 2X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011). 3A. K. Geim and K. S. Novoselov, Nat. Mater. 6, 183 (2007). 4X.-L. Qi, T. L. Hughes, and S.-C. Zhang, Phys. Rev. B 78, 195424 (2008). 5A. M. Essin, J. E. Moore, and D. Vanderbilt, Phys. Rev. Lett. 102, 146805 (2009). 6X.-L. Qi, R. Li, J. Zang, and S.-C. Zhang, Science 323, 1184 (2009). 7L. Fu and C. L. Kane, Phys. Rev. Lett. 102, 216403 (2009). 8A. R. Akhmerov, J. Nilsson, and C. W. J. Beenakker, Phys. Rev. Lett. 102, 216404 (2009). 9I. Garate and M. Franz, Phys. Rev. Lett. 104, 146802 (2010). 10W.-K. Tse and A. H. MacDonald, Phys. Rev. Lett. 105, 057401 (2010). 11T. Yokoyama, Y. Tanaka, and N. 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1.4817302.pdf

Nb2O5 nanofiber memristor A. M. Grishin, A. A. Velichko, and A. Jalalian Citation: Appl. Phys. Lett. 103, 053111 (2013); doi: 10.1063/1.4817302 View online: http://dx.doi.org/10.1063/1.4817302 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i5 Published by the AIP Publishing LLC. Additional information on Appl. Phys. Lett. Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 03 Aug 2013 to 129.93.16.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsNb2O5nanofiber memristor A. M. Grishin,1,2,a)A. A. Velichko,3and A. Jalalian1 1Department of Condensed Matter Physics, KTH Royal Institute of Technology, SE-164 40 Stockholm-Kista, Sweden 2INMATECH Intelligent Materials Technology, SE-127 45 Sk €arholmen, Sweden 3Department of Physical Engineering, Petrozavodsk State University, 185910 Petrozavodsk, Russia (Received 21 April 2013; accepted 17 July 2013; published online 31 July 2013) Non-woven bead-free 100 lm long and 80–200 nm in diameter highly crystalline orthorhombic T-Nb2O5nanofibers were sintered by sol-gel assisted electrospinning technique. Electrical and dielectric spectroscopy tests of individual fibers clamped onto Pt coated Si substrate were performed using a spreading resistance mode of atomic force microscope. Reproducible resistiveswitching with ON-OFF resistance ratio as high as 2 /C210 4has a bipolar character, starts with a threshold voltage of 0.8–1.7 V, and follows by continuous growth of conductivity. Resistive memory effect is associated with a voltage-driven accumulation/depletion of oxygen vacancies atNb 2O5/Pt cathode interface. Poole-Frenkel emission from the electronic states trapped at reduced NbO xcomplexes determines a shape of Nb 2O5/Pt diode I-Vcharacteristics. Simple thermodynamic model explains a threshold character of switching, relates experimentally observed characteristicsin low and high resistive states, and gives a reasonable estimate of the concentration of oxygen vacancies. VC2013 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4817302 ] During the last two decades, significant progress made in synthesis of nanowires and nanofibers raised an intensive quest for their applications as chemical and bio-sensors,nanophotonic, nanoelectronic, and energy storage devices, field emission and electrochromic displays, actuators, and neural interfaces. 1Simple and effective electrospinning tech- nique was patented in 1934 (Ref. 2), and since mid-1990s it attracts continuously growing interest for fabricating ultra- thin ceramic threads.3There were several reports on electro- spun nanofibers: magnetic,4luminescent,5multiferroic,6as well as various ferrites.7Despite a reach portfolio of avail- able materials, yet the characterization of fibers functionalproperties is severely limited to phase content, morphology by electron microscopy, field, and temperature dependencies of the magnetization in the case of ferromagnetic fibers.Among the most recent results, we observed broad band res- onant microwave absorption in ferrite Y 3Fe5O12(Ref. 8) and ferroelectricity in biocompatible (Na,K)NbO 3nanofibers.9 Herein, we report reproducible bipolar resistive switch- ing in continuous highly crystalline electrospun Nb 2O5nano- fibers. Fibers withstand without breakdown electric field ashigh as 1 MV/cm and demonstrate ON-OFF resistance switching ratio of 2 /C210 4. Opposite to nonvolatile unipolar resistive switching in amorphous anodic NbO x/Nb thin film cells (observed in 1965 (Ref. 10) and explored recently in Ref. 11), Nb 2O5nanofiber/Pt cathode memristor does not require electroforming process preceding to write-in andread-out operation but has a bipolar character while transi- tions from high to low resistive state occur as a continuous growth of conductivity when a bias exceeds a threshold volt-age. Following the classification of resistive storage mecha- nisms, 12all the observed features relate a switchable contact resistance in Nb 2O5fiber/Pt junction to the redox-based volt- age-driven oxygen migration memory.Nb2O5nanofibers were fabricated by ethoxide-rout sol-gel assisted electrospinning technique. To prepare 5 ml of precur- sor solution with 0.2M concentr ation, 0.251 ml of niobium(V) ethoxide Nb(OCH 2CH 3)5was mixed at room temperature with 2 ml of acetylacetone CH 3COCH 2COCH 3and magnetically stirred in a closed cap glass bottle for 15 min. Then, followingin succession, 1 ml of 2-methoxyethanol CH 3OCH 2CH 2OH was added to this solution and stirred for 10 min and then mixed with 2 ml of ethanol C 2H5OH and kept stirred for 10 min. Finally, 250 mg of polyvinylpyrrolidone (PVP) was added, and mixture was stirred 30 min until a viscous solution becomes homogeneous and highly transparent. Electrospinning stock solution was ejected from the sy- ringe pump that feeds PVP/Nb 2O5solution at a constant rate of 0.5 ml/h. 11 kV voltage was applied between the stainlesssteel needle and grounded conducting collector placed at 8 cm below the needle tip. Bead-free nanofibers’ mat on the collector was dried at 100 /C14C in nitrogen atmosphere for 12 h, then collected from the surface of the collector and annealed at 700/C14C for 1 h in air. Fibers morphology was observed with a field emission scanning electron microscope (FE-SEM, JEOL JSM-7500 FA). After drying at 100/C14C, as-spun mat consists of binder- contained jelly-like threads 350 nm in diameter (not shown).They have a smooth surface and amorphous structure. During annealing at 700 /C14C in air, threads experience strong shrinkage and transform to the crystallized non-woven dense100lm long and 80–200 nm in diameter fibers with a rough crystalline faceted surface (see Fig. 1). Phase content and crystalline structure of calcined Nb 2O5fibers were examined by x-ray diffraction (XRD) using a Siemens D-5000 powder diffractometer. All the lines inH-2Hscan in Fig. 2were indexed on the basis of an orthorhombic unit cell.13Diffraction angles are in the best correspondence with all the main and superstructure Bragg peak positions observed by Waring for Nb 2O5powdera)Electronic mail: grishin@kth.se 0003-6951/2013/103(5)/053111/5/$30.00 VC2013 AIP Publishing LLC 103, 053111-1APPLIED PHYSICS LETTERS 103, 053111 (2013) Downloaded 03 Aug 2013 to 129.93.16.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsspecimen heated to 700/C14C.14As for relative intensities, a no- ticeable enhancement of (00 l) Bragg reflections indicates Nb2O5orthorhombic cells that have dimensions a¼6.199 A ˚, b¼29.124 A ˚, and c¼3.938 A ˚are packed inside fibers pref- erentially along the b-axis direction. To assign sintered Nb 2O5nanofibers to one of eight known different polymorphs, we recorded their unpolarized backscattered Raman spectrum at room temperature using a confocal Jobin Yvon LabRam HR800 microscope with a CCD detector and 632.8 nm light pumping from a He-Ne laser. Nanofiber spectrum in inset to Fig. 2resembles the Raman spectrum of bulk niobium oxide calcined at 800/C14C.15 According to Tamura,16this orthorhombic T-Nb 2O5phase crystallizes at 700-800/C14C and forms distorted octahedral and decahedral Nb-ion sites with 6 and 7 oxygen neighbors, cor-respondingly. The strongest Raman band at 686 cm /C01is commonly assigned to the symmetric stretching mode of the niobia octa- and decahedrons. Weaker bands in the 200-400 cm /C01region, Jehng and Wachs17ascribed to the bending modes of the Nb-O-Nb linkages. Electrical tests of Nb 2O5fibers were carried out using a spreading resistance mode as an extension of the static force mode in atomic force microscope (AFM). To obtain reliable electric characteristics, fibers should be tightly clamped andhave a good electrical contact with a substrate used as a bot- tom electrode. For this purpose, samples for electrical tests were spun and then annealed directly on Pt-coated Si wafer. Very long immobilized fibers are easily located with an opti-cal microscope. Then, AFM probe scans the surface in a con- tact mode to produce a topography image of an individual fiber (not shown). Next, the cantilever tip is positioned ontothe top of the fiber, and a tip current is recorded whilst sweeping the voltage applied to the tip. We used wear-resistant conductive DCP20 AFM probes. Nitrogen-doped diamond-like carbon (DLC) 100 nm thick film is coated on the tipside of the cantilever. Tip’s curvature radiusis about 100 nm. I-Vcharacteristics were recorded in a constant voltage mode using Keithley 2400 SourceMeter with a serially connected ballast resistor. Keithley 6485 Picoammeter was used to increase accuracy in a low voltage-small cu rrent range. Prior to tests, measurement circ uit was calibrated using a kit of reference 1 k X–22 MXresistances. I-Vcharacteristic of the direct contact between the A FM DLC-probe and Pt layer on silicon wafer (not shown) is a non linear, symmetrical irrespec- tive of the voltage polarity and fitted to the power lawI½Amp/C138¼4:5/C210 /C05/C2½1þðV=0:205Þ1:2/C138/C2V½Volt/C138.F o r currents below 10 lAt h e DCP20 DLC-probe can be consid- ered as an ohmic contact with the resistance of 22 k X.U s i n g DLC film resistivity qtip¼0.5-1Xcm, asserted by a vendor, we estimated tip’s contact area to be (2–4) /C210/C010cm2. Current-voltage characteristics of immobilized fibers to a significant extent are reproducible and delineate similar features. Semilog I-Vplots of DLC/Nb 2O5fiber/Pt diode are collected in Figs. 3and4. A sequence of several resistive switching obtained after the reversal of bias voltage is shown FIG. 1. Scanning electron micrographs of Nb 2O5nanofibers calcined at 700/C14C for 1 h in air. FIG. 2. XRD Cu Karadiation pattern of Nb 2O5nanofibers annealed at 700/C14C in air. Nb 2O5Bragg reflections are notified by Miller indices for orthorhombic unit cell (Refs. 13and14). Inset shows the Raman spectrum of Nb 2O5fibers. FIG. 3. I-Vcharacteristics of DLC/Nb 2O5nanofiber/Pt junction recorded in aconstant voltage mode with a 100 k Xballast resistor. The voltage drop across the tip and the ballast resistor were subtracted, so hereinafter RandV stand for a fiber resistance and a voltage applied directly to the fiber, respec- tively. “Forward” and “Reverse” directions correspond, respectively, to a positive and a negative voltage applied to the DLC tip. Ascending branch of the curve 1 shown with triangular symbols Dwas recorded for a fresh “virgin” contact between AFM probe and the fiber. Continuous sequence of I-Vloops is denoted by Nos. 1-1 -2-2-3-4-5-6-6 -7. Underlined numerics mark loops traced in the reverse direction. Bias voltage was swept at firstwith the rate of 0.25 V/s within 61.5 V, then 0.5 V/s and 1 V/s in the forward and reverse directions, respectively.053111-2 Grishin, Velichko, and Jalalian Appl. Phys. Lett. 103, 053111 (2013) Downloaded 03 Aug 2013 to 129.93.16.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsin Fig. 3. The curve No. 1 with triangular symbols Dwas recorded for a fresh “virgin” contact between the probe and the fiber. Fiber appeared to be in the high resistive state(HRS) with the resistance R HRS¼40 GX. At threshold volt- ageVth¼þ0.8 V current starts to increase, grows 45 times at V¼þ1.5 V, and then keeps almost constant though voltage decreases down to þ0.8 V along the descending branch. This is a low resistive state (LRS). ON-OFF (LRS-to-HRS) switching occurred when voltage was reversed to /C01.5 V (curve No. 1 ). At positive voltage, HRS was restored and gradually switched to LRS along the ascending I-Vbranch (curve No. 2). After reversing the voltage to /C06 V (curve No. 2 ) HRS of a full value (curve No. 3) was achieved. When we cycled the voltage keeping the same positive polarity, the sequential descending branches of I-Vcurves (Nos. 4, 5, and 6) repeat each other whereas their ascending branch currents gradually decrease with a number of cycles. It demonstrates the degradation of LRS with a positive volt-age cycling. Finally, HRS (curve No. 7) was restored again after voltage reversal along the curve No. 6 . To summarize, at low voltages multiple change of polar- ity shows approximately the same resistance as high as RHRS¼30–50 G X. Using above calculated tip’s contact area of (2–4) /C210/C010cm2, we conclude in HRS the fiber has a re- sistivity as high as qHRS¼(1–3)/C2106Xcm. Then, in the forward direction when a positive bias on DLC probe goes beyond a threshold voltage Vth, varying between þ0.8 and þ1.7 V, current gradually increases and grows exponentially along the ascending branch fitted to the expression I¼ðV=RHRSÞexpðþ2a1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V/C0Vthp Þ: (1) When voltage decreases, a hyste resis occurs indicating a LRS. Experimental data fairly fo llow descending branch of I-Vcurve I¼ðV=RLRSÞexpðþ2a2ffiffiffiffi Vp Þ: (2) As seen in Fig. 4, ON-OFF current ratio reaches the value as high as 2 /C2104. LRS with a high current flow is maintained as long as a bias voltage is applied.Reverse current always remains several orders of magni- tude smaller than a forward one. It slightly increases at vol- tages beyond /C01 V, and there was no breakdown detected at voltages as high as /C06 V. Hysteresis is much smaller than in the forward direction, if any. Voltage reversal repeatedly returns DLC/Nb 2O5fiber/Pt junction back to the HRS. Contrary to abrupt switching in amorphous anodic Nb2O5(90 nm)/Nb film cells,11resistive switching in Nb 2O5 fibers does not require electroforming process, has a bipolarcharacter, and goes on continuously. Switchable contact re- sistance in DLC/Nb 2O5fiber/Pt junction we attribute to the NbO xredox process induced by electromigration of oxygen vacancies. Polarity of the resistive switching in our I-Vchar- acteristics coincides with those that Sawa observed in n-type oxide/high work function cathode junctions.18In our case, Pt template layer works as an active electrode while oxygen vacancies form donor-type reduced NbO xcomplexes thus render n-type conductivity in Nb 2O5matrix.19 At negative voltage applied to DLC tip, oxygen vacancies leave the fiber/Pt interface and disperse into Nb 2O5matrix. Depletion layer in Nb 2O5widens, contact resistance increases, and memory cell is reset to a HRS. Positive voltage at DLC brings vacancies back to Pt electrode, reduces resistance, and sets a LRS in the junction. Enriched concentration of oxygenvacancies near the Pt cathode remains stable as long as bias voltage is applied, and, being metastable at zero bias, it decays with a read-out (positive voltage) cycling. Since a LRS is associated with donors caught at Pt cath- ode, the descending branch of hysteretic I-Vcharacteristics in the forward direction Eq. (2)can be explained with a Poole-Frenkel emission from the localized electronic states j/Eexp/C0qð/B/C0ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qE=p/C15dp Þ kT/C20/C21 : (3) This standard expression relates current density jand electric field strength Ethrough the depth of the trap potential /Band dynamic dielectric permittivity /C15d.20qis elementary charge, k is the Boltzmann constant, and Tis the temperature. We fitted one of the experimental hysteresis loop (No. 7 in Fig. 4)t o Eqs. (1)and(2)with the solid lines and obtained the following parameters: RHRS¼40 GX,Vth¼1.56 V, a1¼5.8 V/C01/2;a n d RLRS¼490 MX,a2¼2.2 V/C01/2. Comparing Eqs. (3)and(2) with experimentally achieved constant 2 a2¼ðq=kTÞffiffiffiffiffiffiffiffiffiffiffiffiffiffi q=pd/C15dp ¼4:4V/C01=2and fiber thickness d¼80 nm, we can calculate /C15d:Obtained value /C15d¼5:54/C2/C15o(/C15ois the electric permittivity of free space) we consider as a reasonable esti-mate of dynamic dielectric permittivity of Nb 2O5fiber. To produce HRS-to-LRS switching, electric field Eapplied to the fiber should collect uniformly distributed oxygen vacan-cies back to Pt cathode producing a work against the entropy of mixing. Let a mole fraction g(z) to define the spatial distribu- tion of oxygen vacancies inside the DLC/Nb 2O5fiber/Pt capaci- tor. Then, Gibbs free energy Fextended over the capacitor thickness 0 <z<dcontains two competing terms F½J=mole/C138¼ðd 0dz d2qE /C15dzgþkTglng e/C18/C19 : (4) FIG. 4. Forward part of I-Vcharacteristics Nos. 1, 2, 3, and 7 from Fig. 3. Solid lines show fitting of experimental data No. 7 to Eqs. (1)and(2). Inset shows schematics of AFM spreading resistance test.053111-3 Grishin, Velichko, and Jalalian Appl. Phys. Lett. 103, 053111 (2013) Downloaded 03 Aug 2013 to 129.93.16.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsThe first is the electrostatic energy of O2þvacancies carrying an electric charge of þ2qwhile the second term is the en- tropy of an ensemble of vacancies considered as a non- equilibrium ideal gas.21To determine g(z) we should mini- mizeFfrom Eq. (4)with respect to g(z):dF=dgðzÞ¼0 and then normalize g(z) to the total amount of oxygen vacancies d/C2goinduced in Nb 2O5matrix during the growth process:Ðd 0dzgðzÞ¼d/C2go. Resultant distribution of O2þvacancies is governed by the Boltzmann distribution function gðzÞ¼go2qV /C15dkTð1/C0e/C02qV=/C15dkTÞ/C01e/C0ð2qV=/C15dkTÞðz=dÞ:(5) At zero bias V¼0, vacancies fill Nb 2O5fiber uniformly g(z)¼go. When voltage goes beyond þ100 mV, their con- centration in proximity of DLC anode drops exponentially as go2qV /C15dkTe/C0ð2qV=/C15dkTÞðz=dÞ¼13:94Volt/C01/C2goVe/C00:17Volt/C01nm/C01/C2Vz. Here we substituted fiber’s thickness d¼80 nm and dynamic permittivity /C15d¼5:54/C2/C15oobtained previously from Eqs. (3) and(2).A tV¼0.1, 0.5, 1, and 1.7 V, relative O2þvacancies’ concentration at the anode g(d)/gobecomes as small as 0.35, 6.6 /C210/C03, 1.2/C210/C05, and 1.2 /C210/C09. Meanwhile, at Pt cathode concentration of vacancies grows linearly asgð0Þ¼g o2qV=/C15dkT¼13:94Volt/C01/C2goV. Switching from a HRS to a LRS has a threshold charac- ter. It occurs when the NbO xcomplexes collected at the cath- odez¼0 completely replace all the undistorted Nb 2O5unit cells: gð0Þ¼1. This condition relates the threshold voltage Vthrequired to set a LRS to the parameter goas follows: Vth¼/C15dkT q1 2goifgo/C281; /C15dkT qð1/C0goÞifgo!1:8 >>< >>:(6) Lowering the temperature diminishes the role of the entropy thus linearly reduces a threshold voltage. At room tempera- ture, using experimental values Vth¼0.8–1.7 V, we found the mole fraction of O2þvacancies goranged from 8 :9 /C210/C02to 4:2/C210/C02. As an independent examination of a LRS, we employed impedance spectroscopy using IET/QuadTech 7600 Plus Precision LCR Meter operating in the range of 10 Hz–2 MHz. Alternating voltage Vac¼500 mV was superimposed on a bias voltage Vdc¼þ2V . F i g . 5presents -Im Z–Re ZCole-Cole diagram22for the DLC/Nb 2O5fiber/Pt cell in the LRS. The equivalent circuit comprises those connected serially: inductor L¼5 mH and parallel-connected resistor Rac¼680 kXwith capacitor C¼1.7 pF. The apparent resistance Racas low as 680 kXmatches well the quasistatic resistance in the LRS which can be obtained from Figs. 3and4atVdc¼þ2V . It is worth comparing our results with a charge carrier transport in commercial amorphous anodic NbO thin film capacitors.23They comprise three layers structure: semicon- ducting MnO 2, amorphous anodic Nb 2O5, and metallic NbO. Their I-Vcharacteristics also show threshold voltages de- pendent on barrier heights at MnO 2/Nb 2O5interface in one polarity and at Nb 2O5/NbO in opposite polarity. Within a rated voltage range, Poole-Frenkel is a dominant conduction mechanism and the best capacitor technologies, which giverelatively small or no change of leakage current after ageing, have the Rin Eq. (2)above 100 M X. In conclusion, highly crystalline orthorhombic T-Nb2O5 nanofibers were sintered by sol-gel assisted electrospinning technique. Reproducible bipolar resistive switching with ON-OFF resistance ratio as high as 2 /C2104was observed in individual fibers put in intimate contact with Pt coated Si substrate. Switching from a high to a low resistive state mimics some features of a 2ndorder field-induced insulator- to-metal phase transition: starts with a threshold voltage and continues gradually with a growth of conductivity. In electric field oxygen vacancies migrate to Pt cathode and form therereduced NbO xcomplexes. Hysteretic current-voltage charac- teristic of Nb 2O5/Pt diode is determined by Poole-Frenkel emission from NbO xcomplexes trapped at the interface between Nb 2O5fiber and Pt cathode. This work was partially supp orted by the Vetenskapsra ˚det (Swedish Research Council) through the Advanced Optics and Photonics (ADOPT) Linn /C19ec e n t e rg r a n t . 1H. Yan, H. S. Choe, S. W. Nam, Y. Hu, S. Das, J. F. Klemic, J. C. Ellenbogen, and C. M. Lieber, Nature (London) 470, 240 (2011); R. Yan, D. Gargas, and P. Yang, Nat. Photonics 3, 569 (2009); T. Palacios, Nature (London) 481, 152 (2012). 2A. Formhals, U.S. patent 1,975,504 (2 October 1934). 3Yu. Dzenis, Science 304, 1917 (2004); N. Tucker, J. J. Stanger, M. P. Staiger, H. Razzaq, and K. Hofman, “The history of the science and tech-nology of electrospinning from 1600 to 1995,” J. Eng. Fibers Fabrics Special Issue - Fibers 7, 63–73 (2012). 4H. Wu, R. Zhang, X. Liu, D. Lin, and W. Pan, Chem. Mater. 19, 3506 (2007). 5H. Wang, Y. Li, L. Sun, Y. Li, W. Wang, S. Wang, S. Xu, and Q. Yang,J. Colloid Interface Sci. 350, 396 (2010). 6S. H. Xie, J. Y. Li, Y. Qiao, Y. Y. Liu, L. N. Lan, Y. C. Zhou, and S. T. Tan, Appl. Phys. Lett. 92, 062901 (2008). 7Y.-W. Ju, J.-H. Park, H.-R. Jung, S.-J. Cho, and W.-J. Lee, Compos. Sci. Technol. 68, 1704 (2008); W. Ponhan and S. Maensiri, Solid State Sci. 11, 479 (2009); D. Li, T. Herricks, and Y. Xia, Appl. Phys. Lett. 83, 4586 (2003); X.-W. Zhang, J. Cryst. Growth 310, 3235 (2008). 8A. Jalalian, M. S. Kavrik, S. I. Khartsev, and A. M. Grishin, Appl. Phys. Lett. 99, 102501 (2011). 9A. Jalalian and A. M. Grishin, Appl. Phys. Lett. 100, 012904 (2012). 10W. R. Hiatt and T. W. Hickmott, Appl. Phys. Lett. 6, 106 (1965). FIG. 5. Symbols: experimental -Im Z/C0ReZplot for the low resistance state of DLC/Nb 2O5fiber/Pt memory cell. Solid line: Cole-Cole fit with serially connected inductor L¼5 mH and parallel connected resistor Rac¼680 kX with a capacitor C¼1.7 pF.053111-4 Grishin, Velichko, and Jalalian Appl. Phys. Lett. 103, 053111 (2013) Downloaded 03 Aug 2013 to 129.93.16.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions11T. V. Kundozerova, A. M. Grishin, G. B. Stefanovich, and A. A. Velichko, IEEE Trans. Electron Devices 59, 1144 (2012). 12R. Waser, R. Dittmann, G. Staikov, and K. Szot, Adv. Mater. 21, 2632 (2009). 13JCPDS-International Center for Diffraction Data, Card No. 30–0873. 14J. L. Waring, R. S. Roth, and H. S. Parker, J. Res. Natl. Bur. Stand. 77A(6), 705 (1973). 15S. Xie, E. Iglesia, and A. T. Bell, J. Phys. Chem. B 105, 5144 (2001). 16S. Tamura, K. Kato, and N. Goto, Z. Anorg. Allg. Chem. 410, 313 (1974); K. Kato and S. Tamura, Acta Crystallogr. B 31, 673 (1975). 17J.-M. Jehng and I. E. Wachs, Chem. Mater. 3, 100 (1991).18A. Sawa, Mater. Today 11(6), 28 (2008). 19NbO possesses metallic conductivity whereas NbO 2is a narrow gap semiconductor. 20J. Frenkel, Phys. Rev. 54, 647 (1938); S. M. Sze, Physics of Semiconductor Devices (John Wiley & Sons, 1981). 21L. D. Landau and E. M. Lifshits, Statistical Physics (Pergamon Press, Oxford, 1980). 22K. S. Cole and R. H. Cole, J. Chem. Phys. 9, 341 (1941). 23J. Sikula, V. Sedlakova, J. Hlavka, and Z. Sita, in CARTS EU (2006), pp. 189–196; J. Sikula, V. Sedlakova, H. Navarova, J. Hlavka, M. Tacano, and Z. Sita, in CARTS USA (2007), pp. 337–345.053111-5 Grishin, Velichko, and Jalalian Appl. Phys. Lett. 103, 053111 (2013) Downloaded 03 Aug 2013 to 129.93.16.3. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

1.4742150.pdf

ZnO-based graphite-insulator-semiconductor diode for transferable and low thermal resistance high-power devices ZhiKun Zhang, Jiming Bian, Jingchang Sun, Zhenhe Ju, Yuxin Wang, Fuwen Qin, Dong Zhang, Yingmin Luo, and Hongzhu Liu Citation: Applied Physics Letters 101, 052108 (2012); doi: 10.1063/1.4742150 View online: http://dx.doi.org/10.1063/1.4742150 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electrical properties of BaTiO3 based – MFIS heterostructure: Role of semiconductor channel carrier concentration AIP Advances 4, 057131 (2014); 10.1063/1.4880496 Electrically pumped random lasing in ZnO-based metal-insulator-semiconductor structured devices: Effect of ZnO film thickness J. Appl. Phys. 113, 213103 (2013); 10.1063/1.4808445 ZnO-based light-emitting metal-insulator-semiconductor diodes Appl. Phys. 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Downloaded to IP: 128.143.1.222 On: Thu, 11 Dec 2014 22:23:36ZnO-based graphite-insulator-semiconductor diode for transferable and low thermal resistance high-power devices ZhiKun Zhang,1Jiming Bian,1,a)Jingchang Sun,2Zhenhe Ju,3Yuxin Wang,1,2Fuwen Qin,1 Dong Zhang,1Yingmin Luo,1and Hongzhu Liu4 1School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China 2School of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, China 3New Energy Source Research Center of Shenyang Institute of Engineering, Shenyang 110136, China 4Dalian Zebon Fluorocarbon Paint Stock Co., LTD, Dalian 116036, China (Received 3 July 2012; accepted 18 July 2012; published online 31 July 2012) ZnO-based graphite-insulator-semiconductor (GIS) diode was fabricated on the high thermal and electrical conductive graphite substrate, with a SiO 2thin layer employed as the insulator layer. The current-voltage characteristics exhibit an excellent rectifying diode-like behavior with an obviousturn on voltage of 2.0 V and rather low leakage current of /C2410 /C04A. An interesting negative capacitance phenomenon was also observed from the GIS diode. The excellent heat dissipation performance of the GIS diode compared with conventional sapphire based devices wasexperimentally demonstrated, which was of special interest for the development of high-power semiconductor devices with sufficient power durability. VC2012 American Institute of Physics . [http://dx.doi.org/10.1063/1.4742150 ] ZnO has recently become one of the most attractive materials for a wide range of solid-state optoelectronic and electronic applications because of its distinctive optical andelectrical properties. It has been regarded as one of the most promising candidates for the next generation of short- wavelength light emitting diode and lasing devices. 1,2Recent improvements in the control of background conductivity of ZnO and demonstrations of p-type doping have intensified interest in this material for applications in optoelectronicfield. 3,4Although great progress has been made in related area, some difficulties remain challenging and unresolved. For example, one particular issue for application as high-power devices is the severe heat dissipation problem, which might significantly affect the power persistence of a high- power device based on ZnO structure. 5Hence, an important and key issue with the optoelectronics application of ZnO material is the selection of substrates, since the properties of ZnO based film and the subsequent device process are highlydependent on the employed substrates. So far, a variety of methods have been employed to fabricate high-quality ZnO films on various single-crystal substrates, such as GaAs, sap-phire, ZnO, ScMgAlO 4, and Si.6–10Nevertheless, the heat dissipation performance would not be satisfied due to the rel- atively high thermal resistance of these substrates. In addi-tion, for some special applications such as large area foldable and high-power devices, it is necessary to transfer crystalline ZnO films onto foreign substrates, such as flexibleplastic or metal substrates. 11,12However, it is difficult to sep- arate the ZnO film from the above mentioned single-crystal substrate because of strong bonding between them, thispresents one of the major limits for such applications. Our previous study demonstrates the feasibility of growing ZnO on graphite substrate. 13The advantage of graphite lies in its excellent mechanical and chemical stability, especially theelectrical and thermal conductivity even higher than cop- per,14as well as the potential advantage for transferable optoelectronics devices since it consists of multi-layer sys-tem with nearly decoupled 2D graphene planes. Therefore, direct growth of ZnO based semiconductor devices onto graphite substrates would be a good solution for transferableand high-power devices with sufficient power durability. Nevertheless, so far there has been little research on ZnO-based devices grown on graphite substrate and its heatdissipation performance compared with those devices on conventional substrates. Due to the well established difficulties with device qual- ity reliable and reproducible p-type ZnO, the fabrication of ZnO-based homojunction devices remains challenging and problematic. 15,16Fortunately, many of the advantages of ZnO can also be realized by the fabrication of ZnO-based graphite-insulator-semiconductor (GIS) devices. In this let- ter, ZnO-based GIS diode was fabricated on the high thermaland electrical conductive graphite substrate, with a SiO 2thin layer employed as the insulator layer. The carrier transport properties of the GIS device were analyzed based on current-voltage (I-V) and capacitance-voltage (C-V) characteristics. The excellent heat dissipation performance of the GIS diode compared with conventional sapphire based devices wasexperimentally demonstrated, which was crucial for the de- velopment of high-power semiconductor devices with suffi- cient power durability. A schematic diagram of the ZnO-based GIS diode struc- ture is shown in the left top inset of Fig. 3. The fabrication procedures for this GIS structure were as follows: (1) Priorto the deposition, the graphite substrate with the size of 10 /C210 mm 2was washed with acetone, ethanol, and deionized water for 3 min, respectively, then blown dry with nitrogen.(2) A SiO 2thin layer (150 nm) was deposited on the graphite substrate as the insulator layer by conventional electron beam (E-beam) evaporation technique with quartz as thesource. (3) ZnO film (50 nm) was deposited on top of thea)Author to whom correspondence should be addressed. Electronic mail: jmbian@dlut.edu.cn. 0003-6951/2012/101(5)/052108/4/$30.00 VC2012 American Institute of Physics 101, 052108-1APPLIED PHYSICS LETTERS 101, 052108 (2012) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.143.1.222 On: Thu, 11 Dec 2014 22:23:36SiO 2layer by radio frequency (rf) magnetron sputtering with a ZnO ceramic target and high purity argon (99.999 %) as the working gas, the working pressure and sputtering power were kept at 3.0 Pa and 120 W, respectively. Prior to deposi-tion, the target was pre-sputtered for 10 min in order to remove any contamination. (4) For electrode contact, a thin indium tin oxide (ITO, 50 nm) layer was deposited onto theZnO layer. (5) Then the device was annealed in nitrogen atmosphere at 400 /C14C for 3 min to reduce contact resistance as well as increase adhesive force. The crystalline quality and orientation of the as-deposited ZnO films on SiO 2/graphite were determined by x-ray diffrac- tion (XRD) using a D/Max-2400 (CuK a1:k¼0.154056 nm). The device temperature was measured by the infrared radia- tion thermometer (Fluke 561). The characteristics of current-voltage (I-V) and capacitance-voltage (C-V) of the ZnO-based GIS diodes were measured by KETHLEY semiconductor characterization system (4200-SCS). To designate the polarityof bias on the devices, the forward/reverse bias refers to the fact that graphite is connected to positive/negative voltage. One of the major advantage of graphite substrate is its excellent thermal conductivity K of /C242/C210 3W/mK at room temperature, which is better than copper ( /C24400 W/mK).17 To investigate the heat dissipation performance of the GIS diode, the device temperature under a certain working cur- rent was measured as a function of duration time and the results were shown in Fig. 1. For comparison, the conven- tional ZnO/P-GaN heterojunction LED grown on sapphire substrate under the same work conditions was also measured as reference. As can be seen in Fig. 1, the temperature of the conventional sapphire based device raised quickly beyond 35/C14C within a short time of 100 s as operated at 40 mA (Fig. 1(b)), it increased quickly over 40/C14C within 100 s as operated at 100 mA (Fig. 1(a)). After the rapid upstroke region, the ascendant trend became slow and then the tem- perature increased gradually. In contrast, for our ZnO basedGIS diodes, nearly undetectable temperature increase was observed under the same operating conditions (Figs. 1(c)and 1(d)). Therefore, the superb thermal conduction property of GIS structure was clearly demonstrated, which might beespecially beneficial for high power electronic applications where severe heat dissipation problem generally present. Although the electroluminescence (EL) under forward bias injection current (graphite positive) from the ZnO-based GISdiode was clearly observed by naked eyes in a dark ambient, it was not intense enough to be measured by the spectrome- ter. No emission was observed under revise bias conditionsand in device without a SiO 2insulting layer. Similar EL per- formance has been reported previously.18The mechanism of forward-bias EL in ZnO and other wide-band gap semicon- ductor (ZnSe,GaN) based metal-insulator-semiconductor (MIS) devices is still not well understood.19Here, the weak- ness of light emission under electrical injection was sup- posed to be attributed to the relatively poor crystalline quality of the as-grown ZnO-based GIS junction, i.e., a largeamount of defects were unavoidable present in the GIS junc- tion which may act as non-radiative recombination centers. This was in agreement with the following XRD analysis.Therefore, further studies on improving material quality as well as the optimization of the ZnO-based GIS junction pro- cess are required to achieve desired performance. Fig. 2shows the representative XRD patterns of the ZnO based GIS structure after rapid thermal annealing treat- ments. The dominant diffraction peak at /C2426.4 correspond- ing to the graphite substrate (002), the peaks at /C2442.4, 44.5, 54.6, 77.5 can be indexed to SiO 2(200), (008), (202), (220), respectively, while only one diffraction peak correspondingto ZnO (002) was observed at /C2434.4. No peak from other compounds is detected beside those of ZnO, SiO 2, and graphite. The results indict that wurtzite polycrystalline ZnOfilms with c-axis preferred-orientation have been grown on SiO 2/graphite. It should be noted that it is nearly impossible to grow epitaxial and single crystal ZnO based GIS structuredirectly on amorphous graphite substrate due to the extremely large lattice mismatch between ZnO, SiO 2, and graphite substrate, as well as the relatively low growth tem-perature of sputtering. It was well accepted that when the films were deposited at lower temperature, the reactive species on the substrate surface have lower energy and poormobility which trend to deteriorate the crystalline quality of as-grown ZnO films. 20Therefore, there are much room for FIG. 1. The variation of devices temperature under different operating cur- rent as a function of duration time. (a) P-GaN/ZnO LED at 100 mA; (b)P-GaN/ZnO LED at 40 mA; (c) GIS diode at 100 mA; and (d) GIS diode at 40 mA. FIG. 2. The representative XRD patterns of the ZnO based GIS structure af-ter rapid thermal annealing treatments.052108-2 Zhang et al. Appl. Phys. Lett. 101, 052108 (2012) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.143.1.222 On: Thu, 11 Dec 2014 22:23:36the improvement of crystalline quality of ZnO based GIS structure by optimizing the growth process and annealing treatments. The I-V characteristic of the ZnO-based GIS diode was shown in Fig. 3. According to the I-V curve in Fig. 3, the ZnO-based GIS diode shows an excellent rectifying diode- like behavior with a turn on voltage near 2.0 V and reversebreakdown voltage higher than 9 V. The good ohmic contact behavior between ITO electrode and the ZnO layer can be demonstrated by the perfect I-V linear dependence as shown in the right bottom inset of Fig. 3, which confirms that the rectification behavior arises from the ZnO-based GIS dioderather than the ZnO/ITO contacts. In addition, the current rectification ratio reached /C241100 at the bias voltages of 68.0 V with a rather low leakage current of /C2410 /C04Aw a s observed under a reverse bias, which may result from defects produced at the interface between SiO 2insulating layer and ZnO film due to a large difference in lattice constant andstructure. 21,22It should be noted that the deviation from that of the ideal MIS junction suggests that there might be several current transportation mechanisms in the ZnO-based GISjunction. Moreover, the I-V characteristic of the ZnO-based GIS junction was highly dependent on the thickness of SiO 2 insulating layer with the optimized thickness of 150 nm.23 To further investigate the carrier transport properties of the fabricated ZnO-based GIS junction. The C-V characteris- tics were measured in a wide range of frequency at roomtemperature, and the typical C-V characteristics of the ZnO- based GIS diodes were shown in Fig. 4. As shown in Figs. 4(a)–4(d), the present ZnO-based GIS diodes exhibit a roughly symmetric C-V characteristics under positive and negative biases in the measured frequency range, i.e., a re- markable rise of the value of capacitance with the increasingvoltage under both positive and negative biases within a low voltage range and then stabilized at a certain capacitance. Moreover, the values of the stabilized capacitance show anobvious increase with the measured frequency from 1 KHz to 10 MHz. This behavior was typical for MIS structures and the mechanism has been well elucidated. 24It should be notedhere that an interesting negative capacitance (NC) phenom- enon (i.e., negative values of the capacitance versus fre-quency) was observed from our ZnO-based GIS diode structure. Similar NC phenomena have been previously reported on a variety of devices based on crystalline or amor-phous inorganic semiconductors. 25,26Though numerous explanations for NC have been presented that involved mi- nority carrier flow, interface states, slow transient time ofinjected carriers, charge trapping, or space charge effect, the exact mechanism still remains an open issue. 27,28The charge and discharge process was even more complex for the GISdiode discussed here due to the multiple inter-grain scatter- ing by the metal islands embedded in the SiO 2layer.29Thus, it presently remains a challenge for us. The NC is tentativelyattributed to the nonradiative recombination of injected car- riers into the trap levels, or to the capture-emission of injected carriers between multilevels. We can write the fre-quency xdependent capacitance according to the following formula: 24 CðxÞ¼C0þ1 xdVð1 0/C0ddjðtÞ dt/C20/C21 sinxtdt : (1) Here, C 0is the geometric capacitance, and dj(t) is the relaxa- tion component, which results from the electron transport, trapping, impact ionization, and other physical processes, versus an applied small voltage variation dV(t). C( x) can be negative when the function /C0ddj(t)/dt is negative and monot- onically increasing to zero, i.e., NC effect can be obtained when the time-derivative of the inertial current is positive-valued or nonmonotonic with time. For high frequency elec- tronic and optoelectronic applications involving the GIS diode, the NC effect would be especially attractive due to itsdecreasing response time. 24 In conclusion, a high-quality ZnO-based GIS diode was fabricated. The excellent heat dissipation performance of theGIS diode compared with conventional sapphire based devi- ces was experimentally demonstrated, indicating the GIS structure reported here would be of special interest for thedevelopment of high-power semiconductor devices with suf- ficient power durability. This GIS structure exhibits an excel- lent rectifying diode-like behavior with rather low leakage FIG. 4. The typical C-V characteristics of the ZnO-based GIS diodes meas- ured under various frequency: (a)1 KHz; (b)100 KHZ; (c)1 MHZ; and (d) 10 MHz. FIG. 3. I-V characteristic of the ZnO-based GIS diode illustrating an excel-lent rectifying diode-like behavior. The left top inset shows the schematicdiagram of the ZnO-based GIS diode. The right bottom inset shows the I-V characteristics of ITO contact to ZnO film.052108-3 Zhang et al. Appl. Phys. Lett. 101, 052108 (2012) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.143.1.222 On: Thu, 11 Dec 2014 22:23:36current of /C2410/C04A. An interesting NC phenomenon was also observed from the GIS diode. In addition, the successful fab- rication of ZnO-based GIS diode on graphite substrate offers the significant opportunity to be readily transferred onto anyrigid or flexible foreign substrates, since the graphite sub- strates consist of weakly bonded layer structure. This work was supported by SRF for ROCS, SEM; the Fundamental Research Funds for the Central Univer- sities (DUT12ZD(G)01); Natural Science Foundation ofChina (11004092); and Science Found of Dalian (No.2011J21DW013). 1H. K. Liang, S. F. Yu, and H. Y. Yang, Appl. Phys. Lett. 97, 241107 (2010). 2J. M. Bian, W. F. Liu, H. W. Liang, L. Z. Hu, J. C. Sun, Y. M. Luo, and G. T. Du, Chem. Phys. Lett. 430, 183 (2006). 3D. P. Norton, Y. W. Heo, M. P. Ivill, K. Ip, S. J. Pearton, M. F. 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Choi, and S.-J. Park, Appl. Phys. Lett. 91, 121113 (2007). 22P. L. Chen, X. Y. Ma, and D. R. Yang, Appl. Phys. Lett. 89, 111112 (2006). 23Y.-P. Lin and J.-G. Hwu, J. Electrochem. Soc. 151, G853 (2004). 24M. Ershov, H. C. Liu, L. Li, M. Buchanan, Z. R. Wasilewski, and A. K. Jonscher, IEEE. Trans. Electron Devices 45, 2196 (1998). 25J. Werner, A. F. J. Levi, R. T. Tung, M. Anzlowar, and M. Pinto, Phys. Rev. Lett. 60, 53 (1987). 26V. Kytin, Th. Dittrich, F. Koch, and E. Lebedev, Appl. Phys. Lett. 79, 108 (2001). 27H. H. P. Gommans, M. Kemerink, and R. A. J. Janssen, Phys. Rev. B 72, 235204 (2005). 28I. Mora-Ser /C19o, J. Bisquert, F. Fabregat-Santiago, G. Garcia-Belmonte, G. Zoppi, K. Durose, Y. Proskuryakov, I. Oja, A. Belaidi, T. Dittrich, R. Tena-Zaera, A. Katty, C. L /C19evy-Cl /C19ement, V. Barrioz, and S. J. C. Irvine, Nano Lett. 6, 640, (2006). 29P. L. Chen, X. Y. Ma, D. S. Li, Y. Y. Zhang, and D. R. Yang, Opt. Express 17, 4712 (2009).052108-4 Zhang et al. Appl. Phys. Lett. 101, 052108 (2012) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.143.1.222 On: Thu, 11 Dec 2014 22:23:36

1.4899059.pdf

Rapid microfluidic solid-phase extraction system for hyper-methylated DNA enrichment and epigenetic analysis Arpita De, Wouter Sparreboom, Albert van den Berg, and Edwin T. Carlen Citation: Biomicrofluidics 8, 054119 (2014); doi: 10.1063/1.4899059 View online: http://dx.doi.org/10.1063/1.4899059 View Table of Contents: http://scitation.aip.org/content/aip/journal/bmf/8/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Modeling and validation of autoinducer-mediated bacterial gene expression in microfluidic environments Biomicrofluidics 8, 034116 (2014); 10.1063/1.4884519 An equipment-free polydimethylsiloxane microfluidic spotter for fabrication of microarrays Biomicrofluidics 8, 026501 (2014); 10.1063/1.4871935 A robotics platform for automated batch fabrication of high density, microfluidics-based DNA microarrays, with applications to single cell, multiplex assays of secreted proteins Rev. Sci. Instrum. 82, 094301 (2011); 10.1063/1.3636077 Dynamic in situ chromosome immobilisation and DNA extraction using localized poly(N-isopropylacrylamide) phase transition Biomicrofluidics 5, 031101 (2011); 10.1063/1.3637631 Thiolene-based microfluidic flow cells for surface plasmon resonance imaging Biomicrofluidics 5, 026501 (2011); 10.1063/1.3596395 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29Rapid microfluidic solid-phase extraction system for hyper-methylated DNA enrichment and epigenetic analysis Arpita De,1,a),b)Wouter Sparreboom,1Albert van den Berg,1 and Edwin T. Carlen1,2,a) 1BIOS Lab on a Chip Group, MESA þInstitute for Nanotechnology, University of Twente, Enschede 7522NH, The Netherlands 2Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan (Received 28 July 2014; accepted 10 October 2014; published online 21 October 2014) Genetic sequence and hyper-methylation profile information from the promoter regions of tumor suppressor genes are important for cancer disease investigation. Since hyper-methylated DNA (hm-DNA) is typically present in ultra-low concen-trations in biological samples, such as stool, urine, and saliva, sample enrichment and amplification is typically required before detection. We present a rapid micro- fluidic solid phase extraction ( lSPE) system for the capture and elution of low con- centrations of hm-DNA ( /C201n gm l /C01), based on a protein-DNA capture surface, into small volumes using a passive microfluidic lab-on-a-chip platform. All assay steps have been qualitatively characterized using a real-time surface plasmon reso-nance (SPR) biosensor, and quantitatively characterized using fluorescence spec- troscopy. The hm-DNA capture/elution process requires less than 5 min with an efficiency of 71% using a 25 ll elution volume and 92% efficiency using a 100 ll elution volume. VC2014 AIP Publishing LLC . [http://dx.doi.org/10.1063/1.4899059 ] I. INTRODUCTION The emerging field of epigenetics is primarily concerned with DNA modifications, such as DNA methylation, post-translational modifications of histone proteins, and chromatin remodel-ing, amongst others, without an actual change in the DNA sequence. 1Epigenetic assays are becoming increasingly important due to their potential for the early diagnosis of cancer, and new analysis tools have already increased the number of candidate oncogenes by the specific recognition of hyper-methylation patterns in the promoter regions of tumor suppressor genes.2 In particular high-throughput, small volume sample processing and analysis system will be im- portant, which are ideally realized in microfluidic lab-on-a-chip (LOC) platforms. In particular, hyper-methylated DNA (hm-DNA) is characterized as the abnormal methylation of cystosine residues in CpG dinucleotides of normally non-methylated CpG islands in promotor sequences,and is associated with the transcriptional inactivation of tumor suppressor genes, thus it is crit- ically important for epigenentic based assays. 3However, the amount of hm-DNA in typical samples is very small, e.g., 160 pg hm-DNA was recovered from human-genomic DNA usingmethyl-binding domain (MBD) protein enrichment and numerous PCR cycles, 4,5and hm-DNA has been reported to be present in concentrations less than 6 pM in 1 ml of homogenized stool.6 Therefore, hm-DNA enrichment is a necessary sample processing step for the analysis of bio- logical samples such as stool, urine, saliva.7–9Small volume LOC systems are well suited for processing small sample volumes, which is based on sample manipulation in microfluidic a)Authors to whom correspondence should be addressed. Electronic addresses: arpita.de@wsi.tum.de and ecarlen@ims.tsukuba.ac.jp b)Present address: Department of Molecular Electronics and Walter Schottky Institute, Technical University Munich,Germany. 1932-1058/2014/8(5)/054119/11/$30.00 VC2014 AIP Publishing LLC 8, 054119-1BIOMICROFLUIDICS 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29channels. LOC systems have been previously applied to clinical and molecular biology assays, where assay steps can be combined into a single miniaturized analytical system.10Although microfluidic LOC systems have been applied extensively to DNA extraction, to the best of our knowledge they have not been previously reported for hm-DNA extraction and enrichment. Conventional DNA extraction from silica-based resins have been reported to have extrac- tion efficiencies around 70%–80%,11and microfluidic solid phase extraction ( lSPE) systems have been reported to have extraction efficiencies around 40%–50% from genomic sam- ples.12–15There have been many reports of lSPE DNA extraction.12–27Previous reports describe DNA lSPE systems with integrated polymerase chain reaction21,22and electrophoretic separation.23–25Furthermore, DNA lSPE has been applied to forensics, where samples are typi- cally very dilute.26,27Recently, a hm-DNA analysis platform based on a temperature gradient microfluidic bisulfite assay was reported.28MBD protein-based hm-DNA extraction is particu- larly promising as it facilitates both enrichment and purification of hm-DNA.6,28–30 In this article, we present a passive hm-DNA lSPE system, where the captured hm-DNA is eluted into a reduced sample volume, thus resulting in enrichment and purification. ThelSPE system is comprised of a high surface area microfluidic chip that is functionalized with MBD proteins that serve as the capture agents. The capture and elution of hm-DNA from the MBD surfaces is based on modifying the electrostatic effects of binding through the variationof the ionic strength of the supporting buffer solution. The lSPE system is intended to process samples with low hm-DNA concentrations, in contrast to conventional hm-DNA enrichment systems that work best with more concentrated samples. The lSPE system is demonstrated with small hm-DNA concentrations ( <1n gm l /C01), which results in an enrichment factor of 28 /C2 using a small 25 ll elution volume. The main advantages of the lSPE system compared to the conventional hm-DNA enrichment methods include: a small volume sample enrichment step,simple operation with a reduced number processing steps, and potential for high throughput sample analysis. The details of the microfluidic chip fabrication, surface functionalization schemes, and assay capture and elution characterization protocols are presented. II. EXPERIMENTAL A. Materials and chemicals Amine functional monolayers are covalently attached to silicon dioxide (SiO 2) surfaces using 3-amino propyl trimethoxy alkyl silane (APTES, Sigma Aldrich). Biotinylated surfaces are formed using a 10 mM biotin N-hydroxysuccinimide ester (NHS) (Thermo Scientific) in aphosphate buffered saline (PBS) solution. The MBD-Biotin protein (MBD2b protein conjugated to biotin, MethylMiner TMKit, Life Technologies) is supplied in 0.5 mg ml/C01concentration and is diluted to 35 lgm l/C01for further use. Streptavidin (SA) (Life Technologies) is used for attachment to the biotinylated surfaces. The hm-DNA sample used for capture is 80 bp ds-DNA with eight symmetric 5-methyl-CpG islands, with sequence (methylated CpG dinucleotides are in bold): 50-GCTATACAG GG MGTGTTAA MGATATAA MGTTTTGGCT MGACCAGTGAC MGGACTCT MGTTCCTACCAG MGCAAMGCCCCC-30and 30-CGATATGTCCC GMACAA TTGMTATATT GMAAAACCGA GMTGGTCACTG GMCTGAGA GMAAGGA TGGTGGTC GMGTTGMGGGGG-50(Eurogentec).31The non-hm DNA used as control is identical in sequence to the hm-DNA without the methylated cytosines. The non-hm-DNA sequence is as follows (non-methylated CpG dinucleotides are in bold): 50-GGCC CGGCGGTCGCCACACCA ATTCGTTACTCAGGGA CGTTACCA CGGCTACTAT CGTCGCAATTCAGTCAGGGATCT CG–30and 30-CCGG GCCGCCAGCGGTGTGGTT AA GCAATGAGTCCCT GCAATGGT GC CGATGATA GCAGCGTTAAGTCAGTCCCTAGA GC–50(Eurogentec).31The 160 mM NaCl incubation and wash buffers, MBD-Biotin proteins, and elution buffer (2 M NaCl) were useddirectly from the Methyl Miner Kit. 31Fluorescence imaging was done using two different fluo- rescent dyes: Alexa Flour 488 (AF488) conjugated directly to SA (SA-AF488) (Streptavidin, Alexa Fluor 488 conjugate, Life Technologies), and the PicoGreen intercalating dye (Quant-iTTM Technology, Life Technologies). The DNA and MBD-Biotin concentrations were measured prior to each experiment using a spectrophotometer (Nanodrop 2000c, Thermo Scientific). The054119-2 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29surface plasmon resonance (SPR) imaging measurements were done using thiolated reagents conjugated to gold-coated sensor disks (SPRchipTM, GWC Technologies). 11-mercapto-undecyl- amine (MUAM, Dojindo Molecular Technologies, Inc.) was used to form amine functionalmonolayers on the SPR sensor disk surfaces. B. Surface functionalization The MBD-biotin protein conjugation to SiO 2surfaces is shown in Scheme 1. Following immersion in a 2% APTES solution, the glass-silicon chip was washed with ethanol and heated at 120/C14C for 15 min prior to further processing steps. The biotinylated surface was subsequently exposed to a 1 lM SA solution. The MBD-biotin protein is then attached to the available SA sites on the surface. All surface functionalization steps were performed for 30 min by first flowing the sample in the microchannel and then stopping the flow and allowing incubation for 30 min. Fluorescenceimaging with the AF488 dye is used to assess the surface coverage of the MBD-biotin function- alization in the microchannels, as shown in Figure 1, which demonstrates the attachment of the MBD protein to the biotinylated surface. SCHEME 1. Surface functionalization of MBD-biotin proteins to the SiO 2surface of the microchannel. FIG. 1. Fluorescence image (AF488) of MBD-biotin attachment to the SiO 2surface of the microfluidic channel. Inset shows a magnified view of the microfluidic channel.054119-3 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29C. Capture assay protocol and quantification Following the preparation of the surface with MBD-biotin capture proteins, the microfluidic chip was used to extract hm-DNA from the sample solutions. An 80 bp non-hm-DNA, ds- DNA, is used as a control. Briefly, the hm-DNA in incubation/wash buffer is loaded into themicrofluidic chip by flowing a 1 ml load volume through the microfluidic chip for 1 hr with a flow rate of Q¼18llm i n /C01(applied pressure: DP¼10 kPa). The microfluidic channels are then immediately washed with a 100 ll wash buffer, followed by a wash step with a 1 ml elu- tion buffer with a flow rate of Q¼200llm i n/C01(DP¼100 kPa). For the small enrichment vol- ume of 25 ll, the capture and elution flow rates are Q¼18llm i n/C01. The capture and elution experiments were done using different DNA concentrations. The eluted DNA was de-saltedwith ethanol precipitation and re-suspended in 1 ml wash buffer and the concentration was quantified with a fluorescent assay (PicoGreen intercalating dye) and spectrometer (Perkin Elmer). A calibration curve was created and used for quantification of the capture/elution assay(Fig. S3, supplementary material). 35Using the calibration curve, the output hm-DNA concentra- tion from the MBD-chip was quantified by dilution into a 2 ml volume. D. Real-time assay monitoring SPR imaging (SPRimagerVRII, GWC Technologies) is used to qualitatively assess to validate the overall capture and elution assay protocol, as well as to assess the specificity of the MBD protein for hm-DNA compared to non-hm-DNA. Prior to any surface functionalization, the SPRchips were cleaned in a fresh 3:7 piranha solution (H 2SO4:H2O2) for 3 min, and subsequently incubated in a 5 mM ethanolic MUAM solution overnight (12 h) to form the MUAM monolayer on the gold surface. The MUAM treated SPR sensor disks were then treated with NHS for30 min in 1 /C2PBS buffer pH 7.4, followed by incubation with 1 lM SA, which is used to con- jugate the MBD-biotin proteins to the SPR sensor disk surface. Scheme 2shows the complete surface functionalization to the SPR sensor disk surface. The formation of the MUAM-biotin-SA-biotin-MBD capture surface is done using a con- ventional procedure. Since the MBD-biotin protein availability is limited, the surface prepara- tion and assay steps were performed in a microchannel to reduce reagent consumption. E. Microfluidic chip design The capture of target hm-DNA from solution requires transport from the flow stream to the MDB protein capture moieties tethered to the surfaces of the microfluidic channels. Pressuredriven fluid transport in microfluidic channels is laminar with a parabolic velocity profile across the channel cross-section and transport by diffusion is the dominant mechanism at the micro- channel surfaces. 32Due to the parabolic velocity flow profile, the majority of the hm-DNA in the sample flow in the microchannel is not utilized because the interaction occurs only at the channel surfaces, where the flow velocity is small, and therefore, a balance of the diffusion-con- vection-reaction regimes is required. There are different ways to break the boundary condition SCHEME 2. Surface functionalization of MBD-biotin proteins on Au SPR sensor disks.054119-4 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29of static flow on the microchannel walls, such as microparticles,19,20sol-gels,11,19and pillar structures.12,13In this article, we use vertical pillar structures integrated into parallel microchan- nels to achieve the large capture surface area, as shown in Figure 2. The microfluidic lSPE sys- tem is implemented with 12 parallel microchannels, each with length Lc¼8.8 mm, width Wc¼0.25 mm, and height hc¼0.05 mm; each microchannel is packed with a two-dimensional array of vertical pillars. Square vertical pillars, 10 lm on a side, and spaced 10 lm apart, are integrated into the microfluidic channel. The pillar design is based on the need for high pillar density, for large surface area, while ensuring that the hydrodynamic resistance to fluid flow is not too large, which would require large applied pressures to achieve the desired flow rates forthe assay. A three-dimensional finite element model was constructed and used to simulate (Multiphysics, Comsol, Inc.) the flow velocity in microchannels with different pillar orienta- tions. A rotated pillar design was determined to facilitate a more efficient capture and elutionprocess (Fig. S1, supplementary material). 35 F. Microfluidic chip fabrication The microfluidic chip is comprised of a bonded glass-silicon structure that is implemented using conventional micromachining and anodic bonding microfabrication methods. The silicon layer is first processed where a 50 lm deep channel was first etched in a silicon substrate using deep reactive ion etching (AMS100-SE ICP, Adixen) to form the high aspect ratio pillar struc-tures using a patterned photoresist layer (OIR 907-17, Arch Chemicals, Inc.). Following the cleaning step, a 300 nm thick SiO 2layer was reactively grown (1000/C14C) on the silicon sub- strate. The thickness of the SiO 2layer is sufficient to avoid quenching of the fluorescent dyes. The inlet and outlet holes in the glass substrate are next microfabricated. A 1.1 mm thick glass wafer (Borofloat 33, Schott) is covered with a polymer foil (BF410, Ordyl) and subsequently exposed to UV light (EVG-620, EV Group) through a lithography mask to form the maskinglayer for the inlet and outlet holes. The exposed foil is then developed in a bicarbonate solution to form the inlet and outlet hole opening in the polymer foil. The exposed glass in the patterned mask is then removed using powder-blasting (29 lm diameter alumina particles), thus forming the through-wafer inlet and outlet ports. The remaining polymer foil is afterwards removed by sonication in deionized water. The silicon and glass wafers are aligned and anodically bonded (EV-501, EV Group). The bonded glass-silicon wafer is finally diced into 10 /C220 mm 2pieces (DAD-321, Wafer dicing saw, Disco Hi-Tec). The entire fabrication process for the glass- silicon MBD capture chip is shown in Figure S2 (supplementary material).35Figure 3shows images of the microfabricated chips. Figure 3(a) shows a scanning electron micrograph (SEM) image of the microchannel chip design etched in the silicon substrate. The inset in Figure 3(a) shows a single pillar with the scalloped pillar sidewall due to etch and passivation cycles of the Bosch etching process. Figure 3(b) shows an optical image of representative microfabricated glass-silicon chip with inlet and outlet holes aligned to the microchannels, which is used for sample inlet and waste outlet, respectively. Although we have used a silicon-glass microchip FIG. 2. Schematic diagram of the microchannels of the lSPE LOC system.054119-5 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29structure, which can be conveniently implemented in our research group, the entire microfluidic chip can be manufactured in low cost plastic, such as cyclic olefin copolymer (COC) or cyclic olefin polymer (COP), using injection molding since all feature sizes are 10 lm and greater. G. Assay test system The diced chips are clamped into a custom-made chip holder. Poly-ether-ketone tubing (PEEK tubing, Upchurch Scientific) with 150 lm inside diameter is connected to the microflui- dic chip with standard fittings (Nanoport, Upchurch Scientific). The sample fluids were trans- ported through the PEEK tubing and microfluidic chip assembly using hydrostatic pressurefrom a regulated pressure source and controller (MFCS-8C, Fluigent). The capture of hm-DNA by the MBD proteins is performed by flowing 1 ml of hm-DNA (concentration <1n gm l /C01) through the microchannel for 1 h (applied pressure drop: DP¼10 kPa), and followed by wash- ing and elution. The elution step is performed at a higher flow rate Q¼200ll min/C01 (DP¼100 kPa) using a 1 ml elution volume. This was used for concentrations of DNA higher than 1 ng ml/C01. For smaller enrichment volumes of 25 ll, both capture and elution flow rate is Q¼18llm i n/C01(DP¼10 kPa). A small enrichment volume was tested for concentrations of hm-DNA less than 1 ng ml/C01. Figure 4shows the MBD microchip assay test system. FIG. 4. Experimental setup for the MBD capture/elution microfluidic chip testing. (a) Complete test system. (b) Microfluidic chip mounted in chip holder. FIG. 3. Microfabricated LOC system. (a) SEM image of pillar array etched in the silicon substrate. (b) Optical image of glass-silicon bonded chips with inlet and outlet holes.054119-6 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29III. RESULTS AND DISCUSSION A. Real-time MBD assay monitoring Each step of the MUAM-biotin-SA-biotin-MBD protein capture layer formation was moni- tored using real-time SPR measurements, and the major steps are depicted in Figure 5. The MBD-biotin protein treated SPR sensor disks are exposed to the hm-DNA and non-hm-DNA samples in the wash buffer and then subsequently treated with the elution protocol with the elu-tion buffer (2 M NaCl). All sample handling, hm-DNA incubation, and washing steps are done in 1/C2wash buffer (160 mM NaCl) from the MethylMiner TMKit, Life Technologies. The real-time capture and elution sequences are demonstrated using the SPR experiments, shown in Figures 6and7. The real-time sensorgram trace is first recorded and demonstrates the conjugation of each component of the MUAM-biotin-SA-biotin-MBD complex. In Figure 6(a), the MUAM treated gold SPR sensor disks are biotinylated with Biotin- NHS. Starting with a biotinylated gold surface, SA is introduced to the surface at t¼180 s, which saturates in about 150 s. The MBD protein is injected at t¼500 s and washed with buffer att¼625 s. The baseline does not completely return to the starting level, as there is some bind- ing of the MBD-biotin complex on the SA layer. The MBD protein is diluted from the com- mercial stock, and is not prepared by reconstituting dry protein with the same buffer used for SA and hm-DNA, which results in a bulk refractive index shift. The hm-DNA injection to theMBD protein capture surface starts at t¼800 s, thus giving rise to the association binding curve, and subsequently exhibits no dissociation when washed with buffer at t¼900 s. The top left inset in Figure 6(a) shows the response of a SPR sensor surface not treated with MBD pro- teins, and thus shows no binding response to the hm-DNA sample injection. Figure 6(b) shows the response of the capture surface to the injection of non-hm-DNA samples (solid black trace) and to bare gold surfaces (solid red trace). Biotin-SA binding is very strong and saturates thesurface, as observed during the washing step at t¼350 s (solid black trace). The non-hm-DNA is injected on a MBD surface at t¼1150 s and most of the non-hm-DNA is removed with the FIG. 5. hm-DNA capture and elution protocol using a MBD capture surface. FIG. 6. Real-time SPR sensorgrams. (a) Monitoring the biotin-SA/MBD-biotin complex conjugation and subsequent bind- ing of hm-DNA. (b) Selectivity of non-hm-DNA that is minimally hybridized with the MBD moiety. The measured signals are reported in the instrument RU.054119-7 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29wash buffer injection at t¼1300 s, however, the response indicates a larger than expected sig- nal following the wash step and requires further investigation to determine if this is a function of the MBD protein or the gold surface. A bare gold surface, which is not biotinylated, does not bind any target, as shown in the red trace. Figure 7shows the capture and elution process of hm-DNA by the MUAM-biotin-SA- MBD surface during the assay protocol. Figure 7(a) shows the sensor response during the prep- aration of the gold surface with the MBD-biotin protein. In Figure 7(b), the elution process is demonstrated with a surface prepared with the MBD protein. The hm-DNA sample prepared in 1/C2wash buffer is injected at t¼200 s, followed by a wash step at t¼380 s. It is important to note that the starting response in Fig. 7(b) is lower than that shown in Fig. 6(a), which is due to baseline shift of the SPR instrument between different experiments. Despite this difference in baseline signal, the hm-DNA binding response in Fig. 7(b) at t¼200 s is of similar magni- tude in response units (RUs) to the binding response of hm-DNA in Fig. 6(a) at t¼800 s. There is a small amount of dissociation in the wash buffer injected at t¼380 s, followed by the elution buffer injection at t ¼580 s, where the baseline returns to the pre-injection response level after t ¼700 s, thus confirming that captured hm-DNA has been removed. The elution pro- cess produces a large bulk refractive index change response due to the difference in the compo- sition of the elution buffer and wash buffer. B. MBD microfluidic chip assay Table Ilists the capture/elution results quantified using the complete assay protocol and flu- orescence spectroscopy. The sample flow rate during the capture process is 18 ll min/C01. A cali- bration plot of the fluorescence intensity as a function of hm-DNA concentrations was meas- ured and used for all quantification experiments (Fig S3, supplementary material).35For input concentrations less than 1 ng ml/C01, the capture/elution assay performs with an efficiency of 89% (Table S1, supplementary material).35The surface appears to saturate with concentrations above 100 ng ml/C01. Considering that the capture surface area is approximately 1.7 cm2, the protein attachment density is approximately 3 /C21010MBD-biotin molecules per cm2(assuming 100% capture TABLE I. Measured average input and output capture/elution concentrations from the hm-DNA capture/elution assay with 1 ml elution volume ( n¼3). Input concentration (ng ml/C01) Output concentration (ng ml/C01) 1 0.9 60.01 123 4.0 60.14 200 3.8 60.14 FIG. 7. Real-time SPR sensorgrams. (a) MBD protein attachment step. (b) hm-DNA capture and elution steps. The meas- ured signals are recorded in the instrument RU.054119-8 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29efficiency), which is reasonable considering the mass of the MBD protein.33The assay results show a linear capture/elution relationship for concentrations less than 1 ng ml/C01(Figure 8). Table IIlists a summary of the small volume hm-DNA elution results using the same assay with 1 ng ml/C01hm-DNA concentration. We used three different elution volumes, 25 ll, 50 ll, and 100 ll, followed by ethanol precipitation and quantification with the fluorescence assay. For elution volumes larger than 100 ll, the elution efficiency is improved, at the expense of a reduced enrichment factor. The elution efficiency is 71% using an elution volume of 25 ll. A5 0 ll elution volume resulted in an increased efficiency of 90%, and increased only margin- ally to 92% using a 100 ll elution volume. A 28 /C2enrichment of input hm-DNA with 1 ng ml/C01 sample concentration was obtained for the lowest elution volume of 25 ll. The elution effi- ciency for the 100 ll elution volume is 92%, which is higher than the large volume 1 ml elution experiments (Table I). The large elution volume of 1 ml during post processing with ethanol precipitation could be the reason for loss of sample. MBD proteins can discriminate hm-DNA from non-hm-DNA, however, there is a larger than expected amount of non-specific attachmentof the non-hm-DNA to the MBD modified surface, as previously confirmed qualitatively with SPR measurements (Figure 6). A control experiment with 1 ng ml /C01non-hm-DNA, in a 200 ll elution volume, resulted in a 32% capture efficiency, which is similar to the non-specific attach-ment shown in the SPR results (Figure 6(b)). We are not certain if this higher than expected level of non-specific attachment of non-hm-DNA to the MDB surface is due to the MBD pro- tein or the surface as no blocking layers were used in either experiment, and more investigationis required to determine the cause of the non-specific adsorption, which will improve the per- formance of the assay. The efficiency was calculated as a ratio of the output concentration in the small volume eluent to the input concentration. Since the capture and elution of hm-DNAon MBD surfaces is based on modifying electrostatic binding effects by varying the ionic strength of the supporting buffer solution, the use of poly-MBD proteins may decrease the non- specific binding of non-hm-DNA. 34The hm-DNA lSPE system can be further miniaturized and optimized for integration with down-stream amplification or a detection chamber. The smallest volume used for elution, i.e., 25 ll, is limited by the ability to effectively reconstitute the hm- DNA into the elution buffer and can be further optimized by improving the target dissociationfrom the capture surface. We estimate the reproducibility of the protocol to be about 80%, which can be further improved with optimization to each of the protocol steps. Regardless, this FIG. 8. hm-DNA capture and elution profile from MBD chip. TABLE II. Summary of measurements from capture-elution MBD-chip-hm-DNA assay ( n¼3). hm-DNA (ng ml/C01) Elution vol. ( ll) Eluted hm-DNA (ng ml/C01) Efficiency % 12 5 2 8 627 1 15 0 1 8 659 0 1 100 9 629 2054119-9 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29first report of a hm-DNA lSPE system is promising for application in an analytical platform that combines an integrated system for hm-DNA purification, enrichment and subsequent detection IV. CONCLUSIONS A small volume lSPE system has been presented that is designed for capturing low hm- DNA concentrations ( <1n gm l/C01). The elution into a small buffer volume provides sample enrichment and purification. All assay steps have been qualitatively characterized using a real- time surface plasmon resonance imaging biosensor, and quantitatively characterized using cali-brated fluorescence spectroscopy. The lSPE system performs the capture-elution process with an efficiency greater than 90%, which is comparable to the efficiency of conventional large vol- ume extraction kits, such as Epimark (New England Biolabs), Methyl Collector Ultra (ActiveMotif), and Methyl Miner (Life Technologies). The commercially available hm-DNA enrich- ment kits use MBD-protein based capture of hm-DNA, where the MBD protein is functional- ized onto paramagnetic beads. Target hm-DNA capture and elution efficiencies are higher than95% through repeated wash-elution cycles in the commercial kits. The lSPE system has been characterized with a 80-bp ds-hm-DNA and a 28 /C2enrichment factor was obtained using a 25ll elution volume. In principle, we can reach an enrichment factor near 100 /C2with a 5 ll elu- tion volume, which requires further optimization of the elution protocol. The advantages of the lSPE system compared to the conventional hm-DNA enrichment methods include (1) small volume sample enrichment step, (2) simple operation and a reduction of sample handling steps,which facilitates automation, and (3) a clear path to high throughput sample analysis. ACKNOWLEDGMENTS This work was supported by a private cancer research foundation in the Netherlands, and NanoNextNL, a nanotechnology consortium with 130 partners that is funded by the Government ofthe Netherlands. The authors thank Lennert de Vreede, Johan Bomer, Jan van Nieuwkasteele, and the MESA þNanolab staff for helpful comments with device processing, and Mark Smithers for assistance with SEM imaging. 1A. P. Wolffe and M. A. Matzke, Science 286, 481 (1999). 2S. Mulero-Navarro and M. Esteller, Crit. Rev. Oncol. Hem. 68, 1 (2008). 3J. G. Herman, J. R. Graff, S. My €oh€anen, B. D. Nelkin, and S. B. Baylin, Proc. Natl. Acad. Sci. U.S.A. 93, 9821 (1996). 4L. Gebhard, L. Schwarzfischer, T. H. Pham, R. 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Norris, and J. P. Landers, Electrophoresis 23, 727 (2002). 20C. Breadmore, K. A. Wolfe, I. G. Arcibal, W. K. Leung, D. Dickson, B. C. Giordano, M. E. Power, J. P. Ferrance, S. H. Feldman, P. M. Norris, and J. P. Landers, Anal. Chem. 75, 1880 (2003).054119-10 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:2921J. Min, J. H. Kim, Y. Lee, K. Namkoong, H. C. Im, H. N. Kim, H. Y. Kim, N. Huh, and Y. R. Kim, Lab Chip 11, 259 (2011). 22C. Cady, S. Stelick, M. V. Kunnavakkam, and C. A. Batt, Sens. Actuators B-Chem. 107, 332 (2005). 23C. J. Easley, J. M. Karlinsey, J. M. Bienvenue, L. A. Legendre, M. G. Roper, S. H. Feldman, M. A. Hughes, E. L. Hewlett, T. J. Merkel, J. P. Ferrance, and J. P. Landers, Proc. Natl. Acad. Sci. U.S.A. 103, 19272 (2006). 24J. M. Bienvenue, L. A. Legendre, J. P. Ferrance, and J. P. Landers, Forensic Sci. Int-Gen. 4, 178 (2010). 25L. A. Legendre, J. M. Bienvenue, M. G. Roper, J. P. Ferrance, and J. P. Landers, Anal. Chem. 78, 1444 (2006). 26C. R. Reedy, J. M. Bienvenue, L. Coletta, B. C. Strachan, N. Bhatri, S. Greenspoon, and J. P. Landers, Forensic Sci. Int.- Gen. 4, 206 (2010). 27C. R. Reedy, K. A. Hagan, B. C. Strachan, J. Higginson, J. M. Bienvenue, S. A. Greenspoon, J. P. Ferrance, and J. P. Landers, Anal. Chem. 82, 5669 (2010). 28H. Zhang, L. Shan, X. Wang, Q. Ma, and J. Fang, Biosens. Bioelectron. 42, 503 (2013). 29Y. Zhao, S. Guo, J. Sun, Z. Huang, T. Zhu, H. Zhang, J. Gu, Y. He, W. Wang, K. Ma, J. Wang, and J. Yu, PLoS One 7, e35175 (2012). 30L. G. Acevedo, A. Sanz, and M. A. Jelinek, Epigenomics 3, 93 (2011). 31MethylMinerTMMethylated DNA Enrichment Kit Users Manual (Invitrogen, A11129, 09 September 2009). 32M. Zimmermann, E. Delamarche, M. Wolf, and P. Hunziker, Biomed. Microdevices 7, 99 (2005). 33W. Shu, E. D. Laue, and A. A. Seshia, Biosens. Bioelectron. 22, 2003 (2007). 34H. F. Jorgensen, K. Adie, and A. P. Bird, Nucleic Acids Res. 34, e96 (2006). 35See supplementary material at http://dx.doi.org/10.1063/1.4899059 for the three dimensional flow simulation results, the glass-silicon microchip fabrication process, the fluorescence assay calibration curve, and hm-DNA capture elution data.054119-11 De et al. Biomicrofluidics 8, 054119 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.69 On: Sun, 07 Dec 2014 19:38:29

1.4898095.pdf

Terahertz magnetic modulator based on magnetically clustered nanoparticles Mostafa Shalaby, Marco Peccianti, Yavuz Ozturk, Ibraheem Al-Naib, Christoph P. Hauri, and Roberto Morandotti Citation: Applied Physics Letters 105, 151108 (2014); doi: 10.1063/1.4898095 View online: http://dx.doi.org/10.1063/1.4898095 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temperature dependent dissipation in magnetic nanoparticles J. Appl. Phys. 115, 17B301 (2014); 10.1063/1.4853155 Spherical magnetic nanoparticles fabricated by laser target evaporation AIP Advances 3, 052135 (2013); 10.1063/1.4808368 Relaxation of biofunctionalized magnetic nanoparticles in ultra-low magnetic fields J. Appl. Phys. 113, 043911 (2013); 10.1063/1.4789009 Iron oxide nanoparticles fabricated by electric explosion of wire: focus on magnetic nanofluids AIP Advances 2, 022154 (2012); 10.1063/1.4730405 Detection of magnetic nanoparticle fusion by magnetic measurements J. Appl. Phys. 104, 074319 (2008); 10.1063/1.2996083 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.89.98.137 On: Sat, 29 Nov 2014 17:30:58Terahertz magnetic modulator based on magnetically clustered nanoparticles Mostafa Shalaby,1,2,a)Marco Peccianti,3Y avuz Ozturk,1Ibraheem Al-Naib,1,b) Christoph P . Hauri,2,4and Roberto Morandotti1,a) 1INRS-EMT, Varennes, Quebec J3X 1S2, Canada 2SwissFEL, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland 3Department of Physics and Astronomy, University of Sussex, Pevensey Building II, 3A8, Falmer, Brighton BN1 9QH, United Kingdom 4Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland (Received 17 September 2014; accepted 2 October 2014; published online 15 October 2014) Random orientation of liquid-suspended magnetic nanoparticles (Ferrofluids) gives rise to a zero net magnetic orientation. An external magnetic field tends to align these nanoparticles into clusters, leading to a strong linear dichroism on a propagating wave. Using 10 nm-sized Fe 3O4, we experi- mentally realize a polarization-sensitive magnetic modulator operating at terahertz wavelengths.We reached a modulation depth of 66% using a field as low as 35 mT. The proposed concept offers a solution towards fundamental terahertz magnetic modulators. VC2014 AIP Publishing LLC . [http://dx.doi.org/10.1063/1.4898095 ] Terahertz (THz) signal processing recently rose to promi- nence with numerous potential applications. On the one side,the increasing demand for high bandwidth and data rate in sub-THz wireless communication systems keeps pushing up the frequency limit, reaching the edge of the THz band. 1On the other hand, great efforts are dedicated to extend the well- established infrared materials and spectroscopic techniques to the THz regime. Terahertz is capable of superior matter com-position discrimination due to the inherently compound- dependent fingerprints exhibited in this bandwidth. 2–8 Although sources9–13and detectors14have evolved rap- idly over the past years, THz radiation is still difficult to manipulate mainly because of the lack of both suitable mate- rials and efficient modulation (control) techniques. Terahertzmodulation has been demonstrated by optical, 15–20elec- tronic,21–23and thermal24,25means. Those techniques differ in bandwidth, complexity, flexibility, and modulation depth,hence preferences are usually dictated by application con- straints. For instance, optical beams can dramatically change the electric current density in a semiconductor and modulatea propagating THz pulse in fractions of a picosecond, but this technique depends on the availability of intense ultra- short femtosecond laser sources. On the other hand, THzmodulation through a VO 2film can be triggered just heating up the sample with a small electric current flowing in a con- ducting wire. However, the material response is limited tothe scale of tens of a millisecond. Magnetic fields are an important tool to change the ma- terial response against a propagating electromagnetic wave,but their effect is generally weak. 26Efficient modulation, thus, requires a significant propagation length, which is pre- vented by the associated losses (generally increasing withfrequency). Hence, a practical THz magnetic modulator has not been realized so far.In this paper, we use liquid-suspended magnetic nano- particles (i.e., a Ferrofluid 27) to achieve an efficient modula- tion of short THz pulses using very low magnetic fields. A modulation depth as high as 66% is shown using a magnetic field as low as 35 mT. The concept proposed here may openinteresting alternatives and perhaps a paradigm for future THz modulation devices and systems. Our sample consists of a Ferrofluid-filled 10 mm-long cuvette. Such Ferrofluid is commercially available and con- sists of 10 nm-sized Fe 3O4particles suspended in a carrier liquid. The nanoparticles are coated with a stabilizing surfac-tant providing electrostatic resistance against agglomeration, in turn preserving their free movement as non-interacting particles. This property makes them sensitive to small (milli-Tesla level) magnetic fields. A non-uniform (spatially vary- ing) magnetic field can impose a strong force on those nano- particles that not only rotate them but can also sweep themalong the field gradient. On the contrary, uniform fields tend to simply align the nanoparticles along the field direction. Typically, two kinds of contributions to the magneticmoment reorientation can be recognized here: Brown and Neel type contributions. While the former tends to physically rotate the particles towards the field direction, the latter justrotates the magnetic moments without any physical rota- tion. 28The otherwise randomly oriented (Fig. 1(a)) particles appear to be organized in the form of clusters in the directionof the field lines (Figs. 1(b)and1(c)). Cluster formation is a basic mechanism responsible for many of the unique properties Ferrofluids exhibit. 29For example, if the particles get aligned along the direction of the wave propagation, this builds up net magnetization (M) in the same direction, which, in turn, leads to a difference inthe propagation velocity of the wave circular eigenmodes and thus in the rotation of the plane of polarization (Faraday rotation). 30,31An in-plane magnetic field induces a direc- tional absorption (linear dichroism) associated to a propagat- ing electromagnetic wave. This interesting phenomenon has exciting consequences. For example, following oura)most.shalaby@gmail.com and morandotti@emt.inrs.ca b)Present address: Department of Physics, Engineering Physics and Astronomy, Queen’s University, Kingston, Canada. 0003-6951/2014/105(15)/151108/4/$30.00 VC2014 AIP Publishing LLC 105, 151108-1APPLIED PHYSICS LETTERS 105, 151108 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.89.98.137 On: Sat, 29 Nov 2014 17:30:58measurements of the tunable in-plane magnetic properties of Ferrofluids at THz frequencies,32Chen et al. , measured the H-induced tunability of the in-plane real refractive index.33 Here, we show that it is possible to magnetically control the THz absorption to modulate broadband THz pulses. Magnetic particle alignment and cluster formation induce a variation in the absorption coefficient Da¼a(0) –a(H).a(H) and a(0) are the absorption coefficients in the presence and absence of an external magnetic field H, respectively. Dais strongly dependent on the angle between the cluster axis (external magnetic field direction) and the THz electric polarization. Two main absorption mechanisms can be responsible for the attenuation:34(a) Absorption by the propagating field induced imaginary magnetic polariza- tion (Eddy currents losses), which is ignored here because of the low macroscopic conductivity among magnetic nanopar-ticles, (b) Absorption by the field induced imaginary electric polarization. This latter component represents the current generated within the colloidal nanoparticles. Even in thepresence of weak magnetic fields, this component can lead to significant attenuation and is, thus, the main mechanism re- sponsible for the absorption of light considered here. A prop-agating wave with the electric field polarized parallel to the cluster orientation— the extraordinary wave , undergoes absorption (Fig. 1(c)), as opposite to the non-clustered(randomly oriented particles) case (Fig. 1(a)). At the same time, a wave polarized orthogonal to the cluster direction (Fig. 1(b))—the ordinary wave —undergoes a reduced attenuation and shows an increase in transmission relative tothe reference (isotropic) case (Fig. 1(a)). We performed our experimental measurements using a time domain terahertz spectroscopy setup. The laser pulses(energy ¼/C242 mJ, duration ¼130 fs, repetition rate ¼1 kHz, center wavelength ¼800 nm) were split between the tera- hertz generation—through optical rectification—and detec- tion—via electro-optical sampling—in two different ZnTe crystals. The sample is placed in the x-y plane and z is takento be the direction of propagation. We used the EFH ferro- fluid series (EFH1 and EFH3 with particles concentrations of 7.8% and 12.4%, respectively) because of their organic sol-vent that exhibits significantly lower absorption in the THz band when compared to water-based Ferrofluids. Unless oth- erwise stated, EFH1 was used. To demonstrate the magnetic field-induced dichroism, we consistently probed the transmission of the ordinary and extra- ordinary wave components. We placed the sample in anx-aligned (planar) magnetic field generated using an electro- magnet (GMW-3470). To measure the transmitted extraordi- nary wave, the THz was (horizontally) x-polarized. Two wiregrid polarizers (with wires aligned along y) were placed before and after the sample to ensure the THz horizontal polarization. In the case of an ordinary wave measurement, THz generation,detection, and wire grid polarizers were rotated by 90 /C14. Figure 2shows the transmitted pulses and the corresponding spectra of the extraordinary and ordinary waves for severalmagnetic field levels, specifically 0, 8, 17, 35, and 106 mT. With the increase in the magnetic field, the transmitted extraordinary/ordinary polarization decreases/increases con-firming the dichroism over the broad THz spectrum. This is accompanied by a magnetic field-induced birefringence. The magnetic field-induced absorption coefficients of the extraor-dinary ( k) and ordinary ( ?) polarizations are related by Da k¼/C02Da?: (1) This relation was verified using near infrared probing.29 However, it is purely related to the average domain reorienta- tion and does not depend on the frequency as long as the wave-length is greater than the nmscale of the particle chain. The measured change in transmissi on can originate from both the FIG. 1. Nanoparticles alignment with the external static magnetic field (H) and its effect on THz propagation. (a) In the absence of an external field, the nanoparticles are randomly oriented giving rise to a zero magnetic state andthe THz experiences isotropic absorption. (b) and (c) An external magnetic field tends to align the particles along its direction inducing THz linear dichroism. If the particles orientation is orthogonal/parallel to the THz elec- tric field (b)/(c), a lower/higher absorption is expected. FIG. 2. Transmitted THz waves underthe application of different external magnetic fields. When the THz field is polarized parallel to the applied field (a) and (c), a strong attenuation is observed. A THz polarized orthogonal to the external magnetic field shows anincrease in transmission (b) and (d) in comparison with the zero-field ran- domly oriented case. The rate of the induced attenuation decreases with the increase in the applied magnetic field.151108-2 Shalaby et al. Appl. Phys. Lett. 105, 151108 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.89.98.137 On: Sat, 29 Nov 2014 17:30:58change in the Fresnel reflection l osses at the interfaces and the bulk attenuation. The perturbation in the complex refractiveindex of the sample was found to be less than 3% for the levels of the magnetic fields used in our experiment. We, therefore, assume no change in the Fresnel losses and the bulk losses aresolely responsible for the change in the transmission. To extract the magnetic field-induced absorption ( Da kandDa?) from the experimental measurement, we first write the spectralcomponents of the THz field as E tðxÞ¼E0ðxÞeDad,w h e r e d¼10 mm is the sample thickness. EtandE0are the modu- lated and unmodulated fields, respectively. From this, Dacan be readily extracted using the logarithmic transmission t1¼ lnEtxðÞi E0xðÞ¼Daid;i2k ;?ðÞ . Figure 3(a) shows the extracted extraordinary- induced absorption for two levels of magnetiza-tion (17 mT and 35 mT). A good agreement over the wide THz bandwidth with the prediction obtained from Eq. (1)is also shown. To evaluate the efficiency of the modulation process, we calculate the energy spectral density modulation depth I mxðÞ¼jE0xðÞj2/C0jEtxðÞj2 jE0xðÞj2: (2) The frequency-resolved modulation intensity is presented in Fig.3(b) for the extraordinary wave at the two levels of the magnetic field (17 mT and 35 mT) shown in Fig. 3(a), where up to 66% modulation is found for a field of 35 mT. We stress here that in a perspective modulation device, therequired (very) low magnetic field can be locally obtained by a moderate current flowing in a wire. The modulation increases with both frequency and applied magnetic field. In principle, the induced magnetiza-tion and, thus, modulation should continue to increase until saturation ( /C241 T for EFH1). However, at higher magnetic field levels, the magnetization build up has a nonlinear trend(saturates). This behavior is described by the Langevin rela- tion M ¼cothðKHÞ/C01=KH, where K is a temperature- dependent parameter. 30AsDakis proportional to M, the induced absorption is expected to have a similar Langevian dependence. This is experimentally demonstrated in Fig. 4(a), where both M and the extraordinary Dakare shown for fields up to 600 mT. We would like to emphasize here that EFH1 requires 1 T to reach the magnetization saturation ðMs¼40 mT Þ. Yet, due to the nonlinear behavior, only 30 mT is required to reach M s=2 (with a magnetization approximately linear with the applied field up to this level). The attenuation process is mediated by an increase in the electrical conductivity proportional to the number of par- ticles aligned with the magnetic field. The modulation pro- cess is thus expected to be independent of the THz polarity.This is confirmed in Fig. 4(b), where the THz pulses are measured under two equal but oppositely polarized magnetic fields. Finally, the effect of the nanoparticles concentration FIG. 3. The induced absorption of the extraordinary (E) and ordinary (O) waves for applied fields of 17 mT (top) and 35 mT (bottom), respectively.The experimental measurements are shown in blue and red solid lines. The asterisks underline the E-wave measurement after applying Eq. (1)to calcu- late the attenuation in the O-wave. (b) Modulation depth of the E-wave cal- culated using Eq. (2)for the two magnetic field levels. FIG. 4. Langevian behavior of both the magnetization and the induced THz absorption of the E-wave. (b) Waveforms of the THz E-wave in the absence of an external field and in the case of two equal but oppositely polarizedfields. (c) The induced absorption of the E-wave in EFH3 (solid lines) and EFH1 after scaling by 1.5 (asterisks) to account for the difference in concen- tration. The agreement between the plot pairs demonstrates the scalability of the induced absorption with the concentration.151108-3 Shalaby et al. Appl. Phys. Lett. 105, 151108 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.89.98.137 On: Sat, 29 Nov 2014 17:30:58is briefly considered here by comparing two Ferrofluids from the same series, EFH3 and EFH1. The particle concentration of the former is 1.5 times higher than that of the latter. The induced absorption is directly proportional to the particle concentration29and so is DaEFH3¼1:5/C2DaEFH1: This last relation is experimentally verified and shown in Fig. 4(c), which demonstrates an excellent agreement with the theoretical predictions. This implies that a higher absorp- tion modulation can be obtained by increasing the concretion of the sample. However, this comes at the expense of higher absorption. The advantage of using higher concentration liquids over longer sample lengths can be seen if this liquidis coupled with other structures, where the thickness cannot be arbitrarily varied (like metamaterials) or where a higher thickness induces more losses associated with the structureitself (like waveguides). In conclusion, we demonstrated terahertz magnetic modu- lation using magnetic field-induced clustering of nanoparticlesin Ferrofluids. The demonstrated technique combines a high modulation depth and low magnetic field requirements, while preserving the flexibility given by the possibility of usingliquids. We believe that our results will pave the way to a class of THz modulators that can be integrated in other magnetic/ nonmagnetic systems such as metamaterials and waveguides. 1T. Kleine-Ostmann and T. Nagatsuma, J. Infrared Millimeter Terahertz Waves 32, 143 (2011). 2W. L. Chan, J. Deibel, and D. M. Mittleman, Rep. Prog. Phys. 70, 1325 (2007). 3B. B. Hu and M. C. Nuss, Opt. 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1.4894801.pdf

Isentropic wave propagation in a viscous fluid with uniform flow confined by a lined pipeline Y ong Chen, Yiyong Huang, Xiaoqian Chen,a)and Yuzhu Bai College of Aerospace Science and Engineering, National University of Defence Technology, 410073, Changsha, People’s Republic of China (Received 11 February 2014; revised 17 June 2014; accepted 19 August 2014) The axisymmetric wave propagation in a viscous fluid with the presence of a uniform flow confined by a circular pipeline is investigated. Particular considerations are imposed on the features of theacoustic wave propagating in the liquid where the thermal conduction is neglected. The boundary constraints at the wall are reasonably discussed for both lined-walled and rigid-walled pipelines. Numerical comparisons of the phase velocity and wave attenuation among three different boundaryconfigurations (rigid wall, steel-composed wall, and aluminum-composed wall) are presented. Meanwhile, the effects of the fluid viscosity and acoustic impedance are coherently analyzed. In the end, parametric analysis of the influence of the acoustic impedance is given in the case of a steel-composed pipeline. VC2014 Acoustical Society of America .[http://dx.doi.org/10.1121/1.4894801 ] PACS number(s): 43.55.Rg, 43.35.Bf, 43.20.Mv, 43.20.Hq [JDM] Pages: 1692–1701 I. INTRODUCTION Wave propagation in a moving fluid confined by a circu- lar pipeline is a common configuration existed in many industrial applications such as ultrasonic flow measure- ment,1,2noise attenuation,3,4and so forth. Present paper takes into consideration the effects of the fluid viscosity3,5–8 and acoustic impedance3,4,9–11of the wall as the two mecha- nisms bring about energy dissipation. Accounting the effects of the fluid viscosity and thermal conductivity in a stationary gas, Kirchhoff12first proposed a complex transcendental acoustic equation in the case of alossless rigid wall. Tijdeman 5gave a numerical solution to the Kirchhoff formulation and summarized consecutive work. In the case of a uniform pipeline flow, Dokumaci3,6 investigated the fundamental acoustic mode based on the Zwikker and Kosten approximation. Numerical study showed that the assumption of a uniform flow could closelypredict the features of an acoustic wave propagating in the shear flow. In the case of a stationary liquid, Elvira-Segura 13 assumed the acoustic wave to be isentropic, neglecting the process of thermal conduction. Chen et al.7expanded the problem in the case of a uniform flow profile. If the wall is not rigid, its influence may alter the propaga- tion speed and attenuation as well. Roughly speaking, two dif- ferent methodologies exist in the literature to analyze the influence of the wall on wave propagation. By expressing thedisplacement and stress of the wall and describing the bound- ary condition at the fluid-wall interface, the features of wave propagation in the stationary fluid can be numerically ana-lyzed. Such a conception was adopted by Grosso, 14Greenspon and Singer,15Lafleur and Shields,16Elvira-Segura,13Sinha et al. ,17Plona et al. ,18and Leighton’s group,10,11to name a few.On the other hand, many researchers prefer to establish the boundary condition at the fluid-wall interface through the acoustic impedance of the wall. In the framework of an inviscid fluid, the theory of the Ingard–Myers boundary con-dition 9,19–21was widely used in the literature. However, such a method neglects the energy loss due to the fluid vis- cosity.22By analyzing the features of wave propagation in the viscous boundary layer, different types of modified boundary condition were proposed by Brambley et al. ,19 Rienstra and Darau,9Auregan and co-workers,22–24and so on. Although the viscous dissipation was taken into consid- eration at the viscous boundary layer, the governing equation in the fluid was yet based on the inviscid assumption. The present paper coherently analyzes the effects of the fluid viscosity and acoustic impedance on the acoustic wave propagating in the uniform pipeline flow. Although the uni-form flow is controversial in reality, such an approximation can give a reasonable prediction of wave propagation in the shear flow as revealed by Dokumaci. 3,6As the present paper pays particular attention to the acoustic wave in the liquid flow, an isentropic acoustic assumption with the influence of thermal conduction omitted7,13is reasonable. II. MATHEMATICAL FORMULATION In this section, the comprehensive mathematical formula- tion of the isentropic wave propagation is deduced from the conservation of mass and momentum. A viscous fluid isassumed to move uniformly along a circular pipeline while the FIG. 1. Geometric configuration of the problem in the circular cylindrical coordinate system. r,h, and zdenote the radial, circumferential, and axial directions, respectively. yis the vertical axis.a)Author to whom correspondence should be addressed. Electronic mail: chenxiaoqian@nudt.edu.cn 1692 J. Acoust. Soc. Am. 136(4), October 2014 0001-4966/2014/136(4)/1692/10/$30.00 VC2014 Acoustical Society of America Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42thermal conduction is neglect ed. The isentropic acoustic wave is considered to be linear a nd axisymmetric. Figure 1presents the configuration of the problem in the cylindrical coordinate system. Specifically, RandZdenote the inner radius of the pipeline and the acoustic impeda nce of the wall, respectively. q,p,a n d vrepresent the fluid density, pressure, and velocity. The uniform flow profile is expressed by U0¼const. A. Governing equation The basic equations of the problem are the conservation of mass and momentum, expressed by @q @tþr/C1 qvðÞ¼0; (1) @v @tþv/C1rðÞ v¼/C0rp qþg qr2vþ1 qfþg 3/C18/C19 rr /C1 vðÞ ; (2) where gandfare the coefficients of the shear and bulk vis- cosity which are assumed to be constant.3,10,13When the viscous fluid experiences a small-amplitude acoustic disturb- ance ( q0,p0,a n d v0), its ambient physical variables change to q¼q0þq0;p¼p0þp0, and v¼v0þv0, where the varia- bles with the subscript 0 denote the steady mean flow. As the steady density and velocity satisfy the conditions of q0¼const and v0¼½0;0;U0/C138(expressed in the cylindrical coordinate system as shown in Fig. 1), one obtains v0/C1rðÞ v0¼/C0rp0 q0þg q0r2v0 þ1 q0fþg 3/C18/C19 rr /C1 v0 ðÞ ¼0: (3) If the acoustic wave is considered to be linear, Taylor expan- sion of Eqs. (1)and(2)can be simplified to @q0 @tþv0/C1rðÞ q0þq0r/C1v0¼0; (4) q0@v0 @tþv0/C1rðÞ v0þv0/C1rðÞ v0/C20/C21 ¼/C0 r p0þgr2v0þfþg 3/C18/C19 rr /C1 v0 ðÞ : (5)Due to the assumption of the isentropic acoustic wave, the acoustic pressure can be expressed by p0¼c2 0q0; (6) where c0represents the adiabatic sound speed which is assumed to be constant in this paper. Then Eq. (4)can be simplified to @p0 @tþv0/C1rðÞ p0þq0c2 0r/C1v0¼0: (7) If a harmonic axisymmetric wave is presumed, the acoustic variables can be expressed as exp ½iðxt/C0k0KzÞ/C138 with xð¼2pfÞ,K, and k0¼x=c0being the angular fre- quency, the dimensionless axial wavenumber, and the invis- cid wavenumber, respectively. Then Eqs. (7)and(5)can be reduced to7 ix1/C0KMðÞ p0þq0c2 0r/C1v0¼0)p0 ¼/C0q0c2 0 ix1/C0KMðÞr/C1v0; (8) ixq01/C0KMðÞ v0¼/C0 r p0þgr2v0 þfþg 3/C18/C19 rr /C1 v0 ðÞ ; (9) where M¼U0=c0represents the flow Mach number. Insertion of Eq. (8)into Eq. (9)yields ixq01/C0KMðÞ v0¼gr2v0þq0c2 0 ix1/C0KMðÞþfþg 3/C18/C19"# /C2r r/C1 v0 ðÞ : (10) By expanding Eq. (10) in the cylindrical coordinate system and non-dimensionalizing the radial coordinate by x¼r=Rwith x2½0;1/C138, the governing function of the acous- tic velocity v0¼½v0r;v0z/C138(due to the axisymmetric acoustic assumption, the circumferential component of the acoustic velocity is omitted) can be deduced into ixR21/C0KMðÞ v0r¼g q0@ x@xx@v0r @x/C18/C19 /C01 x2v0 r/C0k2 0R2K2v0 r/C20/C21 þc2 0 ix1/C0KMðÞþ1 q0fþg 3/C18/C19"# @ @x1 x@ @xxv0 rðÞ /C0ik0RKv0 z/C20/C21 ; (11) ixR21/C0KMðÞ v0z¼g q0@ x@xx@v0z @x/C18/C19 /C0k2 0R2K2v0 z/C20/C21 /C0ik0RKc2 0 ix1/C0KMðÞþ1 q0fþg 3/C18/C19"# 1 x@ @xxv0 rðÞ /C0ik0RKv0 z/C20/C21 : (12) Using the separation-of-variables principle, the components of the acoustic velocity can be expressed by J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline 1693 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42v0 r¼urðxÞexp½iðxt/C0k0KzÞ/C138;v0 z¼uzðxÞexp½iðxt/C0k0KzÞ/C138; (13) where the functions urðxÞanduzðxÞare stepwise and regular in the interval x2½0;1/C138. Insertion of Eq. (13)into Eqs. (11) and (12), respectively, gives ixR21/C0KMðÞ ur¼g q0d xdxxdur dx/C18/C19 /C0ur x2/C0R2k2 0K2ur/C20/C21 þc2 0 ix1/C0KMðÞþ1 q0fþg 3/C18/C19"# d dxd xdxxurðÞ /C0iRk0Kuz/C20/C21 ; (14) ixR21/C0KMðÞ uz¼g q0d xdxxduz dx/C18/C19 /C0R2k2 0K2uz/C20/C21 /C0ik0Kc2 0 ix1/C0KMðÞþ1 q0fþg 3/C18/C19"# Rd xdxxurðÞ /C0iR2k0Kuz/C20/C21 : (15) Obviously, the axisymmetric acoustic wave propagating in the uniform flow can be governed by a set of two second- order differential equations with the unknown functions urðxÞanduzðxÞplus the dimensionless axial wavenumber K. B. Boundary conditions In the rigid-walled pipeline, the non-invasive condition at the wall leads to the vanishment of the radial acoustic velocity3,7 v0 rðxÞ¼0)urðxÞ¼0a tx¼1: (16) Furthermore, the fluid viscosity promises the non-slip condi- tion3,6,13with v0 zðxÞ¼0)uzðxÞ¼0a tx¼1: (17) As a result, Eqs. (16)and(17)constitute the boundary condi- tion in the case of a rigid-walled pipeline. It should be noticed that the non-slip constraint on the steady flow is relaxed, which prevails in the literature.3,6 If the wall’s effect is taken into consideration, the non- invasive condition collapses but the non-slip condition [Eq. (17)] holds. According to the work of Auregan et al. ,22–24 the acoustic pressure and radial velocity in the viscous fluid satisfy @ @tþ1/C0b/C23 ðÞ c0M@ @z/C20/C21 p0 Z¼@v0r @t; (18) where b/C23represents the transfer of momentum into the lined wall induced by the fluid viscosity and Zis the acoustic im- pedance of the wall. If the acoustic frequency is largeenough, b /C23vanishes22–24and the Ingard–Myers boundary condition9,19recovers @v0r @t¼@ @tþc0M@ @z/C18/C19 p0 Z: (19) Under the assumption that the acoustic impedance is inde- pendent of the axial coordinate, one obtainsv0 r¼1/C0KMðÞp0 Z: (20) Substituting Eq. (8)into this equation results in v0 rþq0c2 0 ixZr/C1v0¼0)urxðÞ þq0c2 0 ixZd xdxxurðÞ /C0iRk0Kuz/C20/C21 ¼0: (21) Using the non-slip condition [Eq. (17)] yields q0c2 0 ixRZdurxðÞ dxþ1þq0c2 0 ixRZ/C18/C19 urxðÞ¼0;atx¼1:(22) As a result, Eqs. (17) and (22) constitute the boundary condition of wave propagation in the lined-walledpipeline. Furthermore, the axisymmetric wave promises the van- ishment of the radial acoustic velocity at the pipeline centerwith u rðxÞ¼0;atx¼0. Meanwhile, the axial acoustic ve- locity remains finite. III. SOLUTION BASED ON FOURIER–BESSEL THEORY According to the Fourier–Bessel theory,25the bounded functions urðxÞanduzðxÞmay be expressed by urðxÞ¼X1 n¼1Cr nJ1ðkr nxÞ; (23) uzðxÞ¼X1 n¼1Cz nJ0ðkz nxÞ: (24) The functions J0ðkz nxÞand J1ðkr nxÞare Bessel functions of the zeroth and first orders, respectively. In the rigid- walled pipeline, kr nandkz nare determined by Eqs. (16) and(17), 1694 J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42J1ðkr nÞ¼J0ðkz nÞ¼0: (25) In the lined-walled pipeline [Eq. (22)], the constraint equations are changed to q0c2 0 i2xZRJ0kr n/C0/C1/C0J2kr n/C0/C1/C0/C1kr nþ1þq0c2 0 ixZR/C18/C19 J1kr n/C0/C1¼0; (26) J0ðkz nÞ¼0: (27) According to the orthogonal property of Bessel function, the rigid-walled configuration leads to ð1 0J1kr nx/C0/C1J1kr mx/C0/C1xdx¼J2 2kr m/C0/C1 2dmn; (28) ð1 0J0kz nx/C0/C1J0kz mx/C0/C1xdx¼J2 1kz m/C0/C1 2dmn; (29) where the symbol dmndenotes the Kronecker delta function. Given specific acoustic velocity components [ urðxÞanduzðxÞ], the corresponding coefficients in Eq. (23)can be calculated by Cr n¼2 J2 2kr n/C0/C1ð1 0urxðÞJ1kr n/C0/C1xdx; (30) Cz n¼2 J2 1kz n/C0/C1ð1 0uzxðÞJ0kr n/C0/C1xdx; (31) which shows that these coefficients are independent of the radial coordinate. In the lined-walled pipeline, the orthogonal property of Bessel function can be expressed by ð1 0J1kr nx/C0/C1J1kr mx/C0/C1xdx¼1 8J0kr m/C0/C1/C0J2kr m/C0/C1/C0/C12þ1 21/C01 kr m/C0/C12 ! J2 1kr m/C0/C1"# dmn; (32) ð1 0J0kz nx/C0/C1J0kz mx/C0/C1xdx¼J2 1kz m/C0/C1 2dmn: (33) As in the case of the rigid-walled pipeline, the coefficients ( Cr nandCz n) are independent of the radial coordinate. If the Fourier–Bessel sequences [Eqs. (23)and(24)] are substituted into Eqs. (14)and(15), respectively, one obtains X1 n¼1ixR21/C0KMðÞ Cr nJ1kr nx/C0/C1¼X1 n¼1( /C0g q0kr n/C0/C12þR2k2 0K2/C16/C17 Cr nJ1kr nx/C0/C1 þc2 0 ix1/C0KMðÞþ1 q0fþg 3/C18/C19"# ikz nRk0KCz nJ1kz nx/C0/C1/C0kr n/C0/C12Cr nJ1kr nx/C0/C1/C16/C17) ; (34) X1 n¼1ixR21/C0KMðÞ Cz nJ0kz nx/C0/C1¼X1 n¼1( /C0g q0kz n/C0/C12þR2k2 0K2/C16/C17 Cz nJ0kz nx/C0/C1 /C0ik0Kc2 0 ix1/C0KMðÞþ1 q0fþg 3/C18/C19"# Rkr nCr nJ0kr nx/C0/C1/C0iR2k0KCz nJ0kz nx/C0/C1/C16/C17) : (35) Some rearrangements yield X1 n¼1ixR21/C0KMðÞ þg q0R2k2 0K2þc2 0kr n/C0/C12 ix1/C0KMðÞþkr n/C0/C12 q0fþ4g 3/C18/C19"# Cr nJ1kr nx/C0/C1 ¼X1 n¼1c0kz nRK 1/C0KMðÞþikz nRk0K q0fþg 3/C18/C19"# Cz nJ1kz nx/C0/C1; (36) J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline 1695 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42X1 n¼1ixR21/C0KMðÞ /C0ixR2K2 1/C0KMðÞþgkz n/C0/C12 q0þ1 q0fþ4g 3/C18/C19 R2k2 0K2"# Cz nJ0kz nx/C0/C1 ¼X1 n¼1/C0c0kr nRK 1/C0KMðÞþikr nk0RK q0fþg 3/C18/C19"# Cr nJ0kr nx/C0/C1: (37) Multiplying Eq. (36) byJ1ðkr mxÞxand Eq. (37) byJ0ðkz mxÞx, respectively, and then integrating over the interval x2½0;1/C138 lead to RRðÞmCr mþX1 n¼1RZðÞn m /C2c0kz nRK 1/C0KMðÞþikz nRk0K q0fþg 3/C18/C19"# Cz n¼0; (38) ZZðÞmCz mþX1 n¼1ZRðÞn m /C2c0kr nRK 1/C0KMðÞþikr nRk0K q0fþg 3/C18/C19"# Cr n¼0; (39) where RRðÞm¼ixR21/C0KMðÞ þg q0R2k2 0K2 þc2 0 ix1/C0KMðÞkr m/C0/C12þ1 q0fþ4g 3/C18/C19 kr m/C0/C12; (40) ZZðÞm¼ixR21/C0KMðÞ /C0ixR2K2 1/C0KMðÞþg q0kz m/C0/C12 þ1 q0fþ4g 3/C18/C19 k2 0R2K2; (41) ðRZÞn m¼ðHRÞmð1 0J1ðkz nxÞJ1ðkr mxÞxdx; (42) ZRðÞn m¼2 J2 1kz m/C0/C1ð1 0J0kr nx/C0/C1J0kz mx/C0/C1xdx; (43) withðHRÞm¼/C08ðkr mÞ2=½ðJ0ðkr mÞ/C0J2ðkr mÞÞ2ðkr mÞ2þ4ððkr mÞ2 /C01ÞJ2 1ðkr mÞ/C138in the lined-walled pipeline and ðHRÞm ¼/C02=J2 2ðkr mÞin the rigid-walled pipeline. If the number of the Bessel functions in Eqs. (23) and (24)isN, Eqs. (38)and(39)can be expressed by GðKÞX¼0; (44) where X¼½Cr 1;Cr 2;…;Cr N;Cz 1;Cz 2;…;Cz N/C138Tis the coefficient-composed vector. GðKÞis a matrix of 2 N/C22N whose element is a function of the dimensionless axial wave-number KifM,R, and xð¼2pfÞare specified. According to Eqs. (23) and(24), it can be learned that the coefficients of the Fourier–Bessel series do not vanish simultaneously dueto the non-zeros of the acoustic velocity, thus one mayobtain the constraint of X6¼0. Physically speaking, the con- dition of X¼0reveals that the acoustic velocity disappears. Consequently, the corresponding determinant of Eq. (44) vanishes, detðGðKÞÞ ¼ 0: (45) As a result, the dimensionless axial wavenumber Kcan be numerically solved. 7,8,26 IV. SPECIAL CASE: INVISCID FLUID In the framework of an inviscid fluid, the convected wave equation can be represented as a function of the acous-tic pressure p 0¼upðxÞexp½iðxt/C0k0KzÞ/C138.1,19If the mean flow is uniform, an analytical solution exists with upðxÞ¼J0ðk0Rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1/C0KMÞ2/C0K2q xÞ: (46) On the neglect of the fluid viscosity, substituting Eq. (46) into Eq. (9)gives the expression of the radial acoustic velocity, urxðÞ¼/C01 ixq0R1/C0KMðÞdupxðÞ dx: (47) In the lined-walled pipeline, insertion of Eqs. (46) and (47)into Eq. (19)yields the constraint function of the acous- tic pressure, dup dxþiq0xR1/C0KMðÞ2 Zup¼0a tx¼1: (48) In the rigid-walled pipeline with Z¼1 , Eq. (48) can be simplified to dupxðÞ dx¼0a tx¼1: (49) V. NUMERICAL STUDY In what follows, wave propagation in water is consid- ered. The constant parameters27areq0¼1000 kg =m3, c¼1500 m =s,g¼1/C210/C03kg=ðsmÞ,f¼2:4g,R¼4m m , andf¼1 MHz. If the lined wall is composed of Helmholtz resonators,9the acoustic impedance can be expressed by ZxðÞ¼Z0þix~m/C0iq0c0cotxD c0/C18/C19 ; (50) where Z0is the specific acoustic resistance of the wall, ~mð¼ 0:02q0þð1=3Þq0DÞis the damping inertance, and Dis the 1696 J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42liner depth. Equation (23) reveals that the number Nof the Bessel functions should be large enough to make sure that the numerical calculation of the axial wavenumber is con- verged. According to the previous research,7the selection of N¼50 can give an acceptable numerical result. In the nu- merical calculation, particular considerations are placed on the phase velocity ( cp¼c0=KR;) and attenuation coefficient (A¼j8:686k0KIj:dB=m), where the subscripts “R” and “I” denote the real and imaginary components, respectively. To get a normalized expression of the phase velocity, the rela-tive phase velocity is defined by c p=c0¼1=KR. A. Rigid wall and lined wall In this subsection, comparisons of the relative phase ve- locity and attenuation coefficient among three differentconfigurations (rigid wall, steel-composed wall, and aluminum-composed wall11) are given. As an example, the liner depth is assumed to be D¼2 mm. From Eq. (50), the acoustic impedance can be calculated as shown in Table I. Special concentrations are given to the features of the firsttwo modes while the discussions of other modes are omitted. Comprehensive analysis of higher order modes propagating in the uniform flow confined by the rigid wall can be foundin Chen et al. 7 1. Phase velocity Figure 2demonstrates the relative phase velocity of the first mode as a function of the Mach number propagating in the downstream (a) and upstream (b) directions. Meanwhile, Fig.3illustrates the corresponding relative phase velocity of the second mode. Obviously, the relative phase velocity of each mode increases along with the Mach number in the downstream propagation but decreases against the Machnumber in the upstream propagation. Physically speaking, as the downstream propagation is along the flow direction, the effect of the steady flow accel-erates the propagation speed. On the other hand, as the FIG. 3. The relative phase velocity of the second mode confined by the three different walls in the downstream (a) and upstream (b) propagation.TABLE I. The acoustic impedance of the two configurations. Material Z0:P as =m Absolute value of Z:P as =m Phase of Z: deg. Steel 4 :48/C21071:38/C210871:07 Aluminum 1 :73/C21071:32/C210882:47 FIG. 2. The relative phase velocity of the first mode confined by the three different walls in the downstream (a) and upstream (b) propagation. J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline 1697 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42upstream propagation is against the flow direction, the flow convection decelerates the propagation speed. If the flowMach number is higher, the effect of flow convection on the propagation velocity becomes more obvious. A careful comparison of the relative phase velocity among the rigid, steel-composed and aluminum-composed walls shows that the elastic vibration of the wall speeds up the propagation velocity. With the increase of the specificacoustic resistance ( Z 0), the relative phase velocity of each mode slows down in the downstream and upstream direc- tions. However, the difference of the relative phase velocityis minor between the steel-composed and aluminum- composed walls as the absolute values of acoustic imped- ance are nearly the same. Physically speaking, if the specificacoustic resistance ( Z 0) is higher, the absolute value of the corresponding acoustic impedance ( Z) becomes larger (see Table I). Then the rigid property of the wall shows more obvious (the absolute value of the acoustic impedance of the rigid wall can be assumed infinite). From Fig. 2, it can be learned that the impact of the three different walls on the relative phase velocity of the firstmode is very small, which applies to the second mode as shown in Fig. 3. Furthermore, comparison between Figs. 2 and3shows that the relative phase velocity of the second mode is larger than that of the first mode in the downstream and upstream propagation. Numerical comparisons of 1 =KR among different modes can be found in Chen et al.7 2. Wave attenuation While Fig. 4displays the attenuation coefficient of the first mode in the downstream (a) and upstream (b) propaga- tion, Fig. 5illustrates the scenarios of the second mode. With the increase of the Mach number, the attenuation coef- ficient of each mode decreases in the downstream propaga- tion but increases in the upstream propagation depending onthe configuration of the wall. Especially, the energy dissipa- tion due to the fluid viscosity and acoustic impedance becomes slight in the downstream propagation. Physicallyspeaking, the effect of steady flow accelerates the acoustic propagation, the processes of viscous dissipation in the fluid and wave absorption at the wall become less obvious FIG. 5. Attenuation coefficient of the second mode confined by the three dif- ferent walls in the downstream (a) and upstream (b) propagation. FIG. 4. Attenuation coefficient of the first mode confined by the three differ-ent walls in the downstream (a) and upstream (b) propagation. 1698 J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42compared with the case of the stationary fluid. With the increase of the uniform flow profile, the attenuation coeffi-cient caused by the two mechanisms becomes smaller and smaller. On the other hand, the energy dissipation in the upstream propagation is strengthened as the processes of theviscous dissipation and wave absorption are reinforced by the decelerated propagation speed. If acoustic impedance is considered, careful investiga- tion reveals that as the flow Mach number goes up, the incre- ment ratio of the attenuation coefficient in the upstream propagation is more rapid than the decrement ratio in thedownstream propagation. Such a phenomenon indicates that the influences of the flow convection on the acoustic wave between the downstream and upstream propagation areasymmetric with respect to the case of the stationary fluid. Among the three configurations, the attenuation coeffi- cient in the steel-composed wall is the largest while theattenuation coefficient in the rigid wall is the smallest. In the rigid-walled pipeline, the source of wave attenuation is only from the viscous loss. In the lined-walled pipeline, theenergy dissipation from the wall impedance is added, which leads to a greater attenuation coefficient. An interestingphenomenon is that the attenuation coefficient in the steel-composed wall is bigger than that in the aluminum- composed wall, even though the absolute value of the acous-tic impedance in the steel-composed wall is nearly identical to that in the aluminum-composed wall (see Table I). It can be seen from Table Ithat the phase of the acoustic imped- ance in the steel-composed wall is 71 :07 owhile the phase in the aluminum-composed wall is 82 :47o. This may be a possi- ble interpretation of the distinct difference between the steel-composed and aluminum-composed walls. It has been demonstrated that the effects of the fluid vis- cosity and wall impedance lead to the energy dissipation inwave propagation. An attractive question may be that whether the two mechanisms of energy dissipation take effect independently. Figure 6gives a numerical analysis in the case of the steel-composed wall. Specifically, Fig. 6dis- plays the attenuation coefficient of the first mode in the downstream [Fig. 6(a)] and upstream [Fig. 6(b)] propagation. Clearly, the attenuation coefficient in the presence of the fluid viscosity and acoustic impedance (“vis þimpedance”) FIG. 7. The absolute values of the amplitude (a) and phase (b) of the acous- tic impedance as functions of the liner depth. FIG. 6. Attenuation coefficient of the first mode due to the effects of the vis- cosity and acoustic impedance in the downstream (a) and upstream (b) propagation. J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline 1699 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42is not the sum of the coefficients contributed independently by the fluid viscosity (“vis þrigid”) and the acoustic imped- ance (“inv þimpedance”). The inequality is more obvious in the upstream propagation when a larger Mach number is present. These phenomena illustrate that one should considerboth the effects of fluid viscosity and wall impedance to get a precise prediction of wave attenuation in the lined-walled pipeline. B. The effect of the liner depth In this subsection, the effect of liner depth ( D) on wave propagation is discussed. Figure 7exhibits the amplitude [Fig. 7(a)] and phase [Fig. 7(b)] of the acoustic impedance [Eq. (50)] as functions of the liner depth. Numerical calcula- tion is proceeded for the first acoustic mode with M¼0:1 confined by the steel-composed wall. 1. Phase velocity Figure 8presents the effect of liner depth on the relative phase velocity in the downstream [Fig. 8(a)] and upstream[Fig. 8(b)] propagation. At the same time, the difference between the inviscid (“inv”) and viscous (“vis”) assumptionsis clarified. Specifically, the relative phase velocity in the inviscid fluid is larger than that in the viscous fluid in the downstream and upstream propagation. It then can berevealed that the existence of fluid viscosity decelerates the propagation speed. Although the variation trend of the relative phase velocity with respect to the liner depth is complex, the relationship between the relative phase velocity and the amplitude of the acoustic impedance [Fig. 7(a)] may be simple. Generally speaking, a larger amplitude value of the acoustic impedance corresponds to a smaller propagation velocity in the viscous and inviscid assumptions. Although the amplitude range ofthe corresponding acoustic impedance is large, the change interval of the relative phase velocity remains short. Such a phenomenon can also be found in Figs. 2and3.F u r t h e r m o r e , the influence of acoustic impedance on the relative phase ve- locity is more obvious in the viscous fluid than that in the inviscid fluid. FIG. 9. The effect of liner depth on the attenuation coefficient in the down- stream (a) and upstream (b) propagation. FIG. 8. The effect of liner depth on the relative phase velocity in the down-stream (a) and upstream (b) propagation. 1700 J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:422. Wave attenuation Figure 9illustrates the effect of liner depth on the attenuation coefficient in the downstream [Fig. 9(a)] and upstream [Fig. 9(b)] propagation. Comparison between the viscous and inviscid assumptions is simultaneously shown. The variation of attenuation coefficient as a function of the liner depth is sharper compared with the case of the relativephase velocity. If the amplitude of the acoustic impedance is high [Fig. 7(a)], the absolute value of the corresponding phase may reach the maximum point of 90 o[Fig. 7(b)]. The attenuation coefficient then goes down to the case of the rigid wall as shown in Fig. 9. Comparison between Figs. 7and9reveals that the tend- ency of the attenuation coefficient with respect to the abso- lute value of the phase [Fig. 7(b)] may be simple while the relationship between the attenuation coefficient and linerdepth is complicated. As the absolute value of the phase increases and finally goes to the maximum point of 90 o, the attenuation coefficient decreases and eventually simplifies tothe case of the rigid-walled configuration. Similar results can be found in Figs. 4and5. It should be noted that the differ- ence between the viscous and inviscid assumptions is moreapparent under the condition of a smaller phase. As a result, to get a comprehensive description of wave propagation with high quality, the effects of fluid viscosity and wall imped-ance should be taken into consideration synchronously. VI. CONCLUSIONS Present paper investigates the axisymmetric wave prop- agation in the viscous fluid with uniform flow confined by a circular pipeline. As particular considerations are given to the phase velocity and wave attenuation in the liquid, theeffect of thermal conduction can be neglected. The effects of acoustic impedance at the wall and fluid viscosity on phase velocity and attenuation are analyzed synchronously.Numerical calculations reveal the following results. (1) The phase velocity of each mode seems dominantly determined by the amplitude of the acoustic impedance.As the amplitude of the acoustic impedance goes up, the phase velocity decreases and finally goes down to the rigid-walled configuration (see Figs. 2and3, and 8). Furthermore, the phase plays a more important role on the wave attenuation of each mode compared with the amplitude. As the absolute value of the phase goes up to90 o, the attenuation coefficient goes down to the case of the rigid wall (see Figs. 4and5, and 9). (2) The energy dissipation due to the fluid viscosity and acoustic impedance should be considered synchronously to get a comprehensive description of wave propagation. The two processes coherently impose influences on thephase velocity and wave attenuation. With the increase of the acoustic impedance of the wall, its effect becomes small and the wall finally behaves rigid. ACKNOWLEDGMENTS The work described in this paper is funded by the National Natural Science Foundation of China (Grants Nos.11404405, 91216201, 51205403, and 11302253). The authors gratefully acknowledge the funding. 1Y. Chen, Y. Huang, and X. 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Chen, “Isentropic acoustic propagation in a viscous fluid with uniform circular pipeline flow,” J. Acoust. Soc. Am. 134, 2619–2622 (2013). 8Y. Chen, Y. Huang, and X. Chen, “Ultrasonic wave propagation in ther- moviscous moving fluid confined by heating pipeline and flow measure- ment performance,” J. Acoust. Soc. Am. 134, 1863–1874 (2013). 9S. W. Rienstra and M. Darau, “Boundary-layer thickness effects of the hydrodynamic instability along an impedance wall,” J. Fluid Mech. 671, 559–573 (2011). 10K. Baik, J. Jiang, and T. G. Leighton, “Acoustic attenuation, phase and group velocities in liquid-filled pipes: Theory, experiment, and examples of water and mercury,” J. Acoust. Soc. Am. 128, 2610–2624 (2010). 11J. Jiang, K. Baik, and T. G. Leighton, “Acoustic attenuation, phase and group velocities in liquid-filled pipes II: Simulation for spallation neutron sources and planetary exploration,” J. Acoust. Soc. Am. 130, 695–706 (2011). 12L. Rayleigh, Theory of Sound (Macmillan, London, 1896), Vol. 2, pp. 319–326. 13L. Elvira-Segura, “Acoustic wave dispersion in a cylindrical elastic tube filled with a viscous liquid,” Ultrasonics 37, 537–547 (2000). 14D. Grosso, “Analysis of multimode acoustic propagation in liquid cylin- ders with realistic boundary conditions. Application to sound speed and absorption measurements,” Acta Acust. Acust. 24, 299–311 (1971). 15J. E. Greenspon and E. G. Singer, “Propagation in fluids inside thick visco- elastic cylinders,” J. Acoust. Soc. Am. 97, 3502–3509 (1995). 16L. D. Lafleur and F. D. Shields, “Lo w-frequency propagation modes in a liquid-filled elastic tube waveguide,” J. Acoust. Soc. Am. 97, 1435–1445 (1995). 17B. K. Sinha, T. J. Piona, S. Kostek, and S.-K. Chang, “Axisymmetric wave propagation in fluid-loaded cylindrical shells. I: Theory,” J. Acoust. Soc. Am. 92, 1132–1143 (1992). 18T. J. Plona, B. K. Sinha, S. Kostek, and S.-K. Chang, “Axisymmetric wave propagation in fluid-loaded cylindrical shells. II. Theory versus experiment,” J. Acoust. Soc. Am. 92, 1144–1155 (1992). 19E. J. Brambley, A. M. J. Davis, and N. Peake, “Eigenmodes of lined flow ducts with rigid splices,” J. Fluid Mech. 690, 399–425 (2012). 20U. Ingard, “Influence of fluid motion past a plane boundary on sound reflection, absorption, and transmission,” J. Acoust. Soc. Am. 31, 1035–1036 (1959). 21M. K. Myers, “On the acoustic boundary condition in the presence of flow,” J. Sound Vib. 71, 429–434 (1980). 22Y. Renou and Y. Auregan, “Failure of the Ingard-Myers boundary condi- tion for a lined duct: An experimental investigation,” J. Acoust. Soc. Am.130, 52–60 (2011). 23Y. Auregan, R. Starobinski, and V. Pagneux, “Influence of grazing flow and dissipation effects on the acoustic boundary conditions at a lined wall,” J. Acoust. Soc. Am. 109, 59–64 (2001). 24Y. Renou and Y. Auregan, “On a modified Myers boundary condition to match lined wall impedance deduced from sever al experimental methods in presence of a grazing flow,” in 16th AIAA/CEAS Aeroacoustics Conference (2010). 25G. N. Watson, A Treatise on the Theory of Bessel Functions , 2nd ed. (Cambridge University Press, London, 1966), pp. 1–804. 26Y. Chen, Y. Huang, and X. Chen, “Fourier-Bessel theory on flow acousticsin inviscid shear pipeline fluid flow,” Commun. Nonlinear Sci. Numer. Simulat. 18, 3023–3035 (2013). 27S. M. Karim and L. Rosenhead, “The second coefficient of viscosity of liq- uid and gases,” Rev. Mod. Phys. 24, 108–116 (1952). J. Acoust. Soc. Am., Vol. 136, No. 4, October 2014 Chen et al. : Wave propagation in lined pipeline 1701 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 130.113.111.210 On: Wed, 24 Dec 2014 06:15:42

1.4895579.pdf

Copper oxide assisted cysteine hierarchical structures for immunosensor application Chandra Mouli Pandey, Gajjala Sumana, and Ida Tiwari Citation: Applied Physics Letters 105, 103706 (2014); doi: 10.1063/1.4895579 View online: http://dx.doi.org/10.1063/1.4895579 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Nanostructured magnesium oxide biosensing platform for cholera detection Appl. Phys. Lett. 102, 144106 (2013); 10.1063/1.4800933 The synthesis of CuO nanoleaves, structural characterization, and their glucose sensing application Appl. Phys. Lett. 102, 103701 (2013); 10.1063/1.4795135 Nanocrystalline Ni-Al ferrites for high frequency applications AIP Conf. Proc. 1512, 408 (2013); 10.1063/1.4791084 Synthesis of reduced graphene oxide and its electrochemical sensing of 4-nitrophenol AIP Conf. Proc. 1512, 400 (2013); 10.1063/1.4791080 Zirconia based nucleic acid sensor for Mycobacterium tuberculosis detection Appl. Phys. Lett. 96, 133703 (2010); 10.1063/1.3293447 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 120.117.138.77 On: Tue, 16 Dec 2014 02:27:31Copper oxide assisted cysteine hierarchical structures for immunosensor application Chandra Mouli Pandey,1,2Gajjala Sumana,1,a)and Ida Tiwari2 1Biomedical Instrumentation Section, CSIR-National Physical Laboratory, New Delhi 110012, India 2Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India (Received 13 July 2014; accepted 29 August 2014; published online 11 September 2014) The present work describes the promising electrochemical immunosensing strategy based on copper (II) assisted hierarchical cysteine structures (CuCys) varying from star to flower like morphology. The CuCys having average size of 10 lm have been synthesised using L-Cysteine as initial precursor in presence of copper oxide under environmentally friendly conditions in aqueous medium. To delin-eate the synthesis mechanism, detailed structural investigations have been carried out using character- ization techniques such as X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy. The electrochemical behaviour of self-assembled CuCys on gold electrodeshows surface controlled electrode reaction with an apparent electron transfer rate constant of 3.38/C210 /C04cm s/C01. This innovative platform has been utilized to fabricate an immunosensor by co- valently immobilizing monoclonal antibodies specific for Escherichia coli O157:H7 ( E. coli ). Under the optimal conditions, the fabricated immunosensor is found to be sensitive and specific for the detection of E. coli with a detection limit of 10 cfu/ml. VC2014 AIP Publishing LLC . [http://dx.doi.org/10.1063/1.4895579 ] The fabrication of hierarchically structured materials with desired properties and their controlled assembly is a key step for nanofabrication techniques and the realization of advanced nanodevices for biomedical applications.1–3In particular, implementing ordered nanostructures of biocom- patible organic-inorganic hybrid nanomaterials using transi- tion metal oxides for drug delivery systems, clinicaldiagnostics, and biosensing has lots of futuristic scope. 4–6To optimize the preparation conditions and processing of these materials tailored to specific requirements, a better under-standing of the reaction mechanism, facile and biocompati- ble synthesis conditions are very essential. 7,8 The discovery and development of biomaterials which can undergo self-assembly into well-ordered structures have shown burgeoning interest in nanotechnology.9,10The strat- egies for the refinement of nanomaterials also offer potentialbenefits in biological research and recently the study of mem- brane proteins and their applications in device fabrication is especially challenging. 11,12These membrane proteins may further organize in a 3D membrane lattice, thus opening ave- nues to their biochemical study, crystallization, and integra- tion into nanodevices.9,13Therefore, efforts should be focussed on the discovery, selection, and development of bio- materials for use in nanofabrication.14,15Research is under progress to control and organize the nanocrystals of the nano-fibres for their potential applications in biomolecular synthe- sis. 16,17Among the numerous biomolecules, L-cysteine (Cys) is of particular interest in biomedical applications, especiallyfor biosensing due to its favourable multifunctional groups (–SH, –NH 2,a n d– C O O/C0) which can be used for the conjuga- tion of metallic ions and bioanalytes.18–20 Previously, we have synthesized different cystine hier- archical structures at controlled pH and concentration, wherethe formation of microstructures took almost 8 h.21,22In the present work, the results relating to systematic one pot syn- thesis methodology for producing highly symmetric ordered structures of copper (II) assisted hierarchical 3D cysteinestructures (CuCys) using easy, reproducible, and rapid syn- thetic technique have been elucidated. In addition, we dem- onstrated that the CuCys based platform could serve as apromising probe material for fabrication of highly sensitive, specific, and stable immunosensor. The protocol relating to the synthesis of CuCys and the immunosensor fabrication has been shown in Figure S1. 36 The structural analysis of the prepared Cys, copper oxide(CuO), and CuCys were investigated by X-ray diffraction(XRD), and the results are shown in Figure 1(a). The diffrac- tion pattern shows that the CuCys exhibited clearly distinct diffraction pattern in comparison to that of pristine CuO andCys. The major peaks (2 /H9052¼24.57) for cysteine (curve (i)) and CuO (2 /H9052¼26.48, curve (ii)) diminished in CuCys (curve (iii)), which suggests the formation of new phaseprobably due to the complexation. To estimate the amount of Cu(II) entrapped within the CuCys, thermogravimetric analysis (TGA) was carried out inthe range 30 /C14C–800/C14C in inert nitrogen atmosphere at the heating rate of 10/C14C/min (Fig. 1(b)).20The TGA measure- ments of CuO show a mass loss of 10.35 wt. % (256.19/C14C), which indicates that the total mass of nanoparticles are com- posed of 89.65 wt. % copper oxide (curve (i)). The TGA curves of CuCys show that there was a distinct weight loss at237 /C14C inferring the melting of the cysteine. Further, there was a gradual and significant weight loss as the temperature was increased (237 to 450/C14C) above the melting point. The change in wt. % (48.03%) may be due to the expulsion of molecules of SO 2,N H 3, and CO 2from the cysteine chain (curve (ii)). Further, the TGA of Cys shows a completedecomposition temperature of carbon at about 680 /C14C and af- ter that no significant mass loss was detected (curve (iii)).18a)Author to whom correspondence should be addressed. Electronic mail: sumanagajjala@gmail.com, Tel: 91-11-42342439. 0003-6951/2014/105(10)/103706/5/$30.00 VC2014 AIP Publishing LLC 105, 103706-1APPLIED PHYSICS LETTERS 105, 103706 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 120.117.138.77 On: Tue, 16 Dec 2014 02:27:31The formed CuCys composite was analyzed using Fourier transform infrared spectroscopy (FT-IR) and com-pared to the spectrum of Cys (Table ST1). 36The FTIR spectra of the CuCys exhibit several significant spectral dif- ferences than the spectrum of the cysteine without copperoxide. The disappearance of the thiol (SH) stretch at 2552 cm /C01in the CuCys complexes indicates deprotonation of the thiol group and subsequent binding of Cu(II) to cyste-ine via the sulphur atom (Fig. 1(c)). The appearance of strong IR bands for the NH 2stretch at 3179 cm/C01and the disappearance of the bands in the CuCys signify their depro-tonation. The symmetric stretch, in plane bending, and out of plane bending frequencies of the carboxylate functionality remained unchanged in the CuCys complexes (curve (i))compared to the pristine cysteine (curve (ii)), indicating that the carboxylate functionality played little or no role in bind- ing Cu(II) to the cysteine. 23Further, evidence of this can be observed as the CO stretch and in plane bending of the OH associated with carboxylate functionality in the 1408 cm/C01 and 1338 cm/C01region were similar in both free ligand and the complex.24Moreover, the strong band of –CN at 1196 cm/C01present in the L-cysteine and the CuCys indicates that—CN did not take part in bonding. The prepared CuCyshave been further characterized using NMR spectroscopy and compared with the spectra of pristine L- cysteine (Fig.S2). 36It was observed that the addition of Cu(II) results in broadening of the a-CH resonance which get shifted towards a higher field. There was also broadening of the b-CH 2reso- nance of the Cys, which shifted towards a lower field (curve i) in comparison to the spectra of L-cysteine (curve ii). Moreover, the b0-CH 2andb00-CH 2resonances were also not equivalent, which suggest that the thiol group of Cys partici- pates in the binding to Cu(II).25 Transmission electron microscopy (TEM) investigations indicate the uniform granular morphology of copper oxide nanoparticles, with average diameter of /C248 nm (Fig. 1(d) (i)). The Cys hierarchical structures were grown in presenceof Cu(II) and it was observed that the symmetric structures were formed rapidly and the crystallization growth process was completed within 1 h. When the Cys concentration was100 mM, the structures were well oriented in uniform flower like structures having a petal diameter of 100 nm, where the size of each flower was found to be 10 lM (Fig. 1(d)(ii) and (iii)). However, on reducing the concentration of Cys (10 mM), star shaped structures having six arms of equal length ( /C241.5lM) were formed, which clearly indicate the FIG. 1. (a) X-ray diffraction pattern of (i) Cys, (ii) CuO nanoparticle, and (iii) CuCys. (b) TGA of (i) CuO, (ii) CuCys, and (iii) Cys. (c) FTIR spectra o f (i) CuCys and (ii) Cys, and (d) Transmission electron micrograph of (i) CuO nanoparticles, (ii) CuCys at lower magnification, (iii) single CuCys at higher magnifi- cation, and (iv) star like structure of CuCys at 10 mM concentration.103706-2 Pandey, Sumana, and Tiwari Appl. Phys. Lett. 105, 103706 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 120.117.138.77 On: Tue, 16 Dec 2014 02:27:31most symmetrical structures were grown in presence of Cu(II) (Fig. 1(d)(iv)). From the above observations, it may be incurred that cop- per (II) plays a major role in the oxidation of the thiol (RS)residues of the L-cysteine resulting in the formation of inter- molecular CuCys complex. 26,27The formation of a CuCys complex not only depend on the hard–soft acid–base classifi-cation of the ligand but also to the structure that plays an im- portant role in the complex stability. 28According to the structure of the peptides, inter-molecular copper complexes are easiest to form because they have higher enthalpy.29,30 Thus, Cu2þions enhance the inter-molecular reactions coming from an inner sphere electron transfer.31Further, when gold nanoparticles (AuNPs) were introduced into the Cys solution, no such intermolecular oxidation was observed (Fig. S3).36 These AuNPs have the potential to electro-statically interact with the hydrophilic zwitter-ionic layer, resulting in the higher density of polar residues on the hexagonal faces than on therectangular ones. 32This anisotropic decoration on the hexago- nal faces arise due to the attachment of conducive functional groups (–COOH, –NH 2) and lead to the attachment of AuNPs by other intermolecular interactions.32This assembly of AuNPs on the surface inhibits the interaction of other Cys flakes due to which there is no formation of star (Figs. S3(a)and S3(b)) and flower like (Figs. S3(c) and S3(d)) Cys struc- tures. 36On the basis of above observation, the plausible mech- anism for the interaction of Cu(II) with Cys may be givenas 33,34 H2O!H/C1þ/C1OH ; HO/C1þ/C1OH!H2O2; RSH! H2O2RS-SR :(1) CuðIIÞþRSH! H2ORS-Cu-SR ; R¼HOOCCH ðNH 2ÞCH2:(2) Scanning electron microscopy (SEM) was used to study the surface morphology of CuCys/Au electrode before and after antibody immobilization. Figure 2(a)shows the self as- sembly of CuCys on the Au surface. It can be seen that each flower comprises of several irregular flakes arranged in orna- mental manner (Fig. 2(b)). Contradictory to what obtained in the absence of Cu(II).22Further, change in morphology was observed after immobilization of antibody onto CuCys/Au electrode, which may be due to well-oriented functionalgroups present in the CuCys that help in covalent binding of the antibody (Fig. 2(c)). When the concentration was low- ered to 10 mM, there was formation of several stars likestructures, well separated from each other (Fig. 2(d)). The interfacial properties of the electrode after each modification step were investigated using electrochemicalimpedance spectroscopy (EIS). The impedance spectra fol- low the theoretical shapes and include a semicircle portion, observed at higher frequencies, which corresponds to theelectron transfer limited process, followed by a linear part characteristic of the lower frequency attributed to a diffusion limited electron transfer. 22Figure 3(a)shows the impedance plots for the bare Au electrode, CuCys/Au electrode, anti- bodies (Ab) immobilized on CuCys/Au electrode (Ab/CuCys/Au), and after incubation of Ab/CuCys/Au electrode with target E. coli cells. After modification of the Au surface with CuCys, the interfacial electron-transfer resistance (R ct) corresponding to the respective semicircle diameter increases from 30 X(curve iv) to 214.4 X(curve iii). This increase in R ctvalue is due to the presence of negative charges from –COO/C0groups of CuCys that perhaps perturb the interfacial electron-transfer rate between the electrodeand the electrolyte solution. 21Interestingly, in comparison to Cys/Au electrode (Fig. S4, curve i), there was a decrease in the R ctvalue, owing to the electron facilitation of the Cu(II) present in the hierarchical structure (Fig. S4, curve ii).36 After the immobilization of antibodies on CuCys/Au elec-trode, the increase in diameters of the semicircle wasobserved (Fig. 3(a); curve ii), which results in the generation of kinetics barrier for [Fe(CN) 6]3/C0/4/C0redox probe, leading to the increase in the corresponding R ct. Finally, on incubation ofE. coli O157:H7 cells on Ab/CuCys/Au electrode, further increase in the Rct was obseved (Fig. 3(a); curve i). When theE. coli cells are attached to the electrode surface, there is formation of antibody–bacteria complexes that could create a barrier for the electrochemical process, thereby hindering the access of the redox probe to the electrode surface, result-ing in increase in the R ctvalue.35To elucidate the interfacial electrochemical changes after each fabrication steps, the measured impedance spectra were analyzed using Randlesequivalent circuit fitting method (inset Fig. 3(a)), in which a solution resistance (R s), a constant phase element (CPE) instead of capacitance in parallel with an electron transferresistance, (R ct) and a Warburg resistance (W) are included. The fitting values of the equi valent circuit elements are shown in Table ST2.36Within the time domain, where a ki- netic semicircle is observed in the complex impedance plot, the electrode reaction is totally controlled by the electron- transfer kinetics.21Thus, the exchange current per unit FIG. 2. Scanning electron micrograph of (a) self assembled CuCys micro- structure on Au electrode, (b) single CuCys microstructure, (c) antibody im- mobilized on CuCys/Au electrode, and (d) star like structure of CuCys at 10 mM concentration.103706-3 Pandey, Sumana, and Tiwari Appl. Phys. Lett. 105, 103706 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 120.117.138.77 On: Tue, 16 Dec 2014 02:27:31geometric area may be given as: R ct¼1R T / i oF; and the apparent electron transfer rate constant (k app)o fe a c ho ft h e electrodes was obtained using equation: k app¼RT/ n2F2AR ctC; where n is the number of electrons transferred, F is the Faraday constant, R is gas constant, T is Kelvin temperature, A is experimentally determined area of the electrode, the R ctvalue is obtained from the fitted Nyquist plots, and C is concentration of the [Fe(CN) 6]3/C0/4/C0(in mol cm/C03). Table ST2 shows the various value obtained for the different modified electrodes, indicating that the k appvalues increase after each modification.36Similar results were also obtained using cyclic voltammetry (CV) as shown in Fig. S5.36 The immobilization of optimum concentration of anti- body on CuCys/Au electrode was determined to be 80 lg/ml (Fig. S6(a)) and the optimized incubation time between E. colicells and the antibodies was 25 min (Fig. S6(b)).36The quantitative assessment of the detection limit of the immuno- sensor was recorded using Nyquist plots for differentconcentrations of E. coli target cells (Fig. 3(b); 1/C210–1/C2109cfu/ml). It was observed that the semicircle of impedance spectra increases with increase in the E. coli cells. The linear increase in impedance indicates the genera-tion of a resistant and capacitive double layer between the surface and electrolyte. As the concentration of E. coli anti- body increases, more cells are captured on CuCys modifiedelectrode, thus generating a higher blocking effect. Moreover, the R ctvalues of these impedance signals dis- played a good linearity with the concentration of target bac-teria in the range from 10–10 9cfu/ml (Fig. 3(c)). The linear equation was calculated as R ct(X)¼17.56–22.67 (log cfu/ml ofE. coli cells) having a correlation coefficient of 0.996. The detection limit of the developed impedimetric immunosensor is estimated to be 10 cfu/ml, where R ctof antibodies- immobilized on CuCys/Au electrode, in PBS solution wastaken as the control background. The high sensitivity in this system was achieved by increasing the immobilization effi- ciency of antibodies and reducing the large open area of the FIG. 3. (a) Nyquist diagram (Z imversus Z re) for the Faradic impedance measured for (i) E. coli cells/Ab/CuCys/Au electrode, (ii) Ab/CuCys/Au electrode, and (iii) CuCys/Au electrode (iv) Au electrode. (b) EIS plot showing the immuno-sensing response of the Ab/CuCys/Au electrode. (c) EIS linearity plot sh owing the immuno-sensing response of the CuCys immunosensor with variation in the concentration of E. coli cells (10 cfu/ml to 1 /C2109cfu/ml) in PBS solution (pH 7.4) containing 5 mM [Fe(CN) 6]3/C0/4/C0, in the frequency range from 105to 0.1 Hz.103706-4 Pandey, Sumana, and Tiwari Appl. Phys. Lett. 105, 103706 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 120.117.138.77 On: Tue, 16 Dec 2014 02:27:31electrode, thereby effectively impeding ion transfer through system.5 The selectivity of the immunosensors against three other bacteria ( Salmonella typhi ,Shigella dysenteriae , and Vibrio cholera ) was evaluated by measuring the impedimetric responses at the same concentration level that of E. coli (1/C2103, 1.0/C2106, and 1 /C2109cfu/ml) (Fig. S7).36When the bioelectrode was incubated with these bacterial cells, there was negligible change in R ct, indicating no significant cross-reaction/interference. These results demonstrated that the electron-transfer resistance as recorded reflected the interaction between the antibody and the target E. coli cells, therefore showing the specificity of the immunosensor for E. coli. The comparison of the biosensing parameters such as linear range and detection limit of the present work with therecent reports are shown in Table ST4. 36 Real sample analyses have been carried out by inocula- tion of the cultured E. coli O157:H7 cells into water by com- paring the quantitative analysis of the water sample with the plate count method. A total of five measurements were made for each E. coli sample, and it was found that the average values were approximate to the standard results obtained from the plate count method, and the relative error maximum was less than 5.0% (Table ST3).36Further, EIS results dem- onstrated that the proposed immunosensor is stable for at least 30 days (Fig. S8(a)) and could be regenerated and used for at least 6 times (RSD 8.79%) (Fig. S8(b)).36 In conclusion, we reported the aqueous phase synthesis of ordered hierarchical microstructures of CuCys. The addi- tion of Cu (II) in L-cysteine enhances the kinetics of reac-tion, leading to the formation of stable and highly ordered flower like structure in a short duration of time. As a proof of concept, these formed CuCys were self assembled onto Auelectrode for the fabrication of highly sensitive and stable immunosensor for E. coli detection. The detailed investiga- tions on the growth of these hierarchical structures in pres-ence of other metal oxides are in progress, which will help in understanding the physical insights and structure property relations of these biomaterials and their applications in clini-cal diagnostics. We thank Professor R. C. Budhani, Director, CSIR- NPL, New Delhi, India for his interest and support in this work. C.M.P. is thankful to CSIR, India, for the award ofSenior Research Fellow. We thank Professor B. D. Malhotra (Delhi Technological University, Delhi) Dr. A. M. Biradar (NPL, New Delhi), Mr. Ashawani Singh, and Mr. PavneshMani (Delhi University, New Delhi, India) for interesting discussions. We thank Department of Science and Technology and CSIR for facilities.1P.-X. Gao, P. Shimpi, H. Gao, C. Liu, Y. Guo, W. Cai, K.-T. Liao, G. Wrobel, Z. Zhang, Z. Ren, and H.-J. Lin, Int. J. Mol. Sci. 13, 7393–7423 (2012). 2J.-S. Hu, L.-S. Zhong, W.-G. Song, and L.-J. Wan, Adv. Mater. 20, 2977–2982 (2008). 3P. R. Solanki, A. Kaushik, P. M. Chavhan, S. N. Maheshwari, and B. D. Malhotra, Electrochem. Commun. 11, 2272–2277 (2009). 4J. Yuan and A. H. E. M €uller, Polymer. 51, 4015–4036 (2010). 5P. R. Solanki, A. Kaushik, V. V. Agrawal, and B. D. Malhotra, NPG Asia Mater. 3, 17–24 (2011). 6C. Sanchez, G. J. D. A. A. Soler-Illia, F. Ribot, T. Lalot, C. R. Mayer, and V. Cabuil, Chem. Mater. 13, 3061–3083 (2001). 7P. Xu, X. Han, B. Zhang, Y. Du, and H.-L. Wang, Chem. Soc. Rev. 43, 1349–1360 (2014). 8R. K. Joshi and J. J. Schneider, Chem. Soc. 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1.4896692.pdf

Strain-assisted current-induced magnetization reversal in magnetic tunnel junctions: A micromagnetic study with phase-field microelasticity H. B. Huang, J. M. Hu, T. N. Yang, X. Q. Ma, and L. Q. Chen Citation: Applied Physics Letters 105, 122407 (2014); doi: 10.1063/1.4896692 View online: http://dx.doi.org/10.1063/1.4896692 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic tunnel junctions for magnetic field sensor by using CoFeB sensing layer capped with MgO film J. Appl. Phys. 115, 17E524 (2014); 10.1063/1.4868181 Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction Appl. Phys. Lett. 101, 122403 (2012); 10.1063/1.4753816 Reduction of switching current density in perpendicular magnetic tunnel junctions by tuning the anisotropy of the CoFeB free layer J. Appl. Phys. 111, 07C907 (2012); 10.1063/1.3673834 Spin transfer switching in Tb Co Fe ∕ Co Fe B ∕ Mg O ∕ Co Fe B ∕ Tb Co Fe magnetic tunnel junctions with perpendicular magnetic anisotropy J. Appl. Phys. 103, 07A710 (2008); 10.1063/1.2838335 Estimation of spin transfer torque effect and thermal activation effect on magnetization reversal in Co Fe B ∕ Mg O ∕ Co Fe B magnetoresistive tunneling junctions J. Appl. Phys. 101, 09A511 (2007); 10.1063/1.2713695 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.7.27.132 On: Fri, 05 Dec 2014 18:45:58Strain-assisted current-induced magnetization reversal in magnetic tunnel junctions: A micromagnetic study with phase-field microelasticity H. B. Huang,1,2,a),b)J. M. Hu,1,b)T. N. Y ang,1X. Q. Ma,2and L. Q. Chen1 1Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA 2Department of Physics, University of Science and Technology Beijing, Beijing 100083, China (Received 23 July 2014; accepted 16 September 2014; published online 25 September 2014) Effect of substrate misfit strain on current-induced in-plane magnetization reversal in CoFeB-MgO based magnetic tunnel junctions is investigated by combining micromagnetic simulations withphase-field microelasticity theory. It is found that the critical current density for in-plane magnet- ization reversal decreases dramatically with an increasing substrate strain, since the effective elastic field can drag the magnetization to one of the four in-plane diagonal directions. A potential strain-assisted multilevel bit spin transfer magnetization switching device using substrate misfit strain is also proposed. VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4896692 ] Spin transfer torque (STT) effect1,2arises from the trans- fer of angular momentums from the electrons of the spin- polarized current to the local ferromagnet when a current goesthrough a spin-valve nanopillar. One of the most attractive applications is high density magnetic random access memory (MRAM), 3,4which has the advantage of large storage density, high addressing speed, low energy consumption, and avoid- ance of cross writing. A memory cell of MRAM has two ferro- magnetic layers separated by a non magnetic conductive spacer or thin insulating interlayer. One of two layers has a fixed mag- netization along a predetermin ed direction, while the other magnetization of free layer could be reoriented by externalmagnetic field. Based on the STT effect, the magnetization reorientation can be induced by in jecting a spin-polarized cur- rent into the free magnetic layer. 5–7The current-induced switching eliminates crosstalk between neighboring cells dur- ing writing in using the external magnetic field.8Furthermore, STT-MRAM has practically un limited endurance and requires less energy, and faster than conv entional magnetic field control MRAM. However, the high critical switching current J cof STT-MRAM has to be reduced for achieving the compatibilitywith the metal-oxide-semiconductor technology. Many attempts have been made to reduce J c. For exam- ple, using CoFeB as the free layer to reduce M S;9using a dou- ble spin-filter structure,10an antiferromagnetic pinning structure,11or inserting a Ru spin scattering layer to increase spin scattering;12or using a composite free layer consisting of two ferromagnetic layers with various coupling types;13–16or using Heusler-based spin valve nanopillar.17–23Another possi- ble approach to increasing the storage density is to store multi-ple bits per cell. 24–26The combination of small critical current and multiple bits per cell in one device is the most desired path towards high density STT-MRAM. In magnetic thinfilms or islands, strain can be effectively utilized to tune the magnetic domain structures. 27–31For example, the magnetiza- tion can be switched between an in-plane and out-of-plane ori-entation under isotropic biaxial in-plane strains, 32,33or rotatewithin the film plane under anisotropic biaxial in-plane strains.34Recently, Pertsev and Kohlstedt35theoretically dem- onstrated that the critical current density needed for 180/C14mag- netization switching in a free magnetic layer of spin valve can be reduced drastically by the assistance of substrate misfit strain. The conventional micromagnetic simulations do nottake account of such effect of elastic energy and thus cannot be employed to investigate the assistance of substrate misfit strain in spin transfer magnetization switching. In this work, we propose to combine the phase-field microelasticity theory with micromagnetic simulations to understand the effect of substrate misfit strain in spin transferswitching. In particular, we investigate strain-assisted spin transfer switching in CoFeB-based magnetic tunnel junc- tions. First, we show the strain distribution to illustrate themechanism of strain-induced magnetization reorientation. Then, we discuss the effect of substrate strain assistance in spin transfer switching by showing magnetization trajecto-ries and magnetic domain evolutions. At the end, we present a potential strain-assistance multilevel bit spin transfer mag- netization switching by using substrate misfit strain. We investigated spin-valve nanoislands with the structure of CoFeB (40 nm)/MgO (2 nm)/CoFeB (20 nm) of square cross section area as shown in Figure 1(a).W ee m p l o y e da Cartesian coordinate system where the current is along the z axis in Figure 1(b). The two CoFeB layers are separated by a thin MgO layer, and the bottom CoFeB layer is the free layerwhose magnetization dynamics is triggered by a spin- polarized current. The top CoFeB layer is the pinned layer with its magnetization vector Pfixed in the direction along the positive x axis. The initial magnetization vector Mof the layer is along the negative or positive x axis. The free layer lateral length of spin valve magnetic island is fully constrained by astiff substrate. We generally define the substrate strain repre- sented by e ii(i¼1 and 2), and the positive current as electrons flowing from the free layer to the pinned layer. In this paper,the positive current will lead to the antiparallel structure (AP, “1”) between the free layer and the pinned layer while the negative current will lead to the parallel structure (P, “0”)according to the STT theory. To illustrate the mechanism of strain-assisted magnetization switching, Figure 1(c)shows the a)Author to whom correspondence should be addressed. Electronic mail: houbinghuang@gmail.com. b)H. B. Huang and J. M. Hu contributed equally to this work. 0003-6951/2014/105(12)/122407/5/$30.00 VC2014 AIP Publishing LLC 105, 122407-1APPLIED PHYSICS LETTERS 105, 122407 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.7.27.132 On: Fri, 05 Dec 2014 18:45:58in-plane strain distributions e11and e22in the y-z and x-z planes under the condition of isotropic in-plane substrate strain e11¼e22¼/C00.9%. We observe that the largest strain is located in the interface of substrate, and the strain in the mid-dle of nanoisland is larger than the strains in the corners. We use Figure 1(d)to illustrate the mechanical effect of substrate strain on the x-y plane. The strain will drag the magnetizationfrom its initial direction (axial) to the four corner directions. Therefore, we can obtain 45 /C14and 135/C14magnetization switch- ing. Due to four possible diagonal directions, we can obtainfour possible magnetization distributions which could be used in the multi-bit spin transfer magnetization switching. The magnetization dynamics i s described by using a gener- alized Landau-Lifshitz-Gilbert- Slonczewski (LLGS) equation 1,2 dM dt¼/C0 c0M/C2Hef f/C0ac0 MsM/C2M/C2Hef f ðÞ /C02lBJ 1þa2 ðÞ edM3 sgM;PðÞ M/C2M/C2P ðÞ þ2lBaJ 1þa2 ðÞ edM2 sgM;PðÞ M/C2P ðÞ ; (1) where Heffis the effective field, c0¼c/(1þa2),cis the elec- tron gyromagnetic ratio, and ais the dimensionless damping parameter. The effective field includes the anisotropy field, the demagnetization field, the external field, the elastic field,and the exchange field, namely, H eff¼HkþHdþHext þHelasþHex, given as Hef f¼/C01 l0dE dM; (2) where E is the total energy, expressed by E ¼EkþEdþEext þEelasþEex, where E k,Ed,Eext,Eelas, and E exare anisotropy energy, demagnetization energy, Zeeman energy, elastic energy, and exchange energy, respectively. The details forobtaining E k,Ed,Eext, and E excan be found in our previous papers.36–38Note that a finite size magnet magnetostatic boundary condition39is applied to calculate the demagnet- ization energy E d, to consider the influence of geometric size on the magnetic domain structures of such three-dimensional nanomagnets. In particular, the elastic energy E elasis calculated based on a previously developed phase-field model40for a three- phase system that is comprised of an isolated magnetic nano- island (the free layer herein), a stiff substrate, and the air. Inthis case, the stress-free boundary condition at the top and lateral surfaces of the magnetic nanoisland can be automati- cally incorporated by setting the elastic constants of the airphase as zero. Overall, the integration of such phase-field model with micromagnetic simulations allows us to study the effect of the spatially variant strains [that are obtained af-ter the mechanical relaxation of the substrate strain e ii, also see Fig. 1(d)] on the magnetic domain structure and magnet- ization dynamics. Mathematical expression and the detailednumerical solution of E elascan be found in Ref. 40. The cor- responding effective elastic field H elascan be expressed as Hx elas¼2B2 2mxm2z c44/C02B1e11mx; Hy elas¼2B2 2mym2z c44/C02B1e22my; Hz elas¼2B2 2mz1/C0m2 z/C0/C1 c44þ2B1 /C2c12e11þe22 ðÞ mzþ2B1mzm2 z/C01 3/C18/C19 /C20/C21 c11;(3) where B1¼/C01.5k100(c11–c12)a n d B2¼/C03k111c44,w i t h k100 andk111representing the magnetostrictive coefficients. From Eq.(3), it can be seen that the in-plane effective elastic field is FIG. 1. Schematics of (a) the high den- sity patterned bit array (b) the building block of CoFeB/MgO/CoFeB nanois- land spin valve in Cartesian coordi- nates. Spin-polarized current is applied perpendicularly to the island plane. (c)The strain distributions at different planes from phase-field simulations. (d) Illustration of strain distributions in 3D coordinate.122407-2 Huang et al. Appl. Phys. Lett. 105, 122407 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.7.27.132 On: Fri, 05 Dec 2014 18:45:58biaxial isotropic ( Hx elas¼Hy elas) under substrate-induced iso- tropic in-plane strains (i.e., e11¼e22). In this case, the magnet- ization vector is likely to align along one of the four in-plane diagonal axes if not switching out of the film plane.40 The last two terms on the right side of Eq. (1)describe STT that tends to drag the magnetization away from its ini- tial state to its final state. The scalar function is given by1,2 gðM;PÞ¼½ /C0 4þð1þgÞ3ð3þM/C1P=M2 sÞ=4g3=2/C138/C01;(4) where gis the spin polarization constant, MandPare the magnetizations of free and fixed layers in Figure 1(b), the angle between MandPish.M/C1P/Ms2¼cosh.HSTTis the corresponding effective field given by HSTT¼2lBJgðM;PÞM/C2P=ðcedM3 sÞ; (5) where lB, J, d, e, and M s, are the Bohr magneton, current density, thickness of the free layer, electron charge, and satu- ration magnetization, respectively.The magnetic parameters employed in the simulations are as follows: saturation magnetization M s¼9.549 /C2105A/m,41 Gilbert damping parameter a¼0.00439,42spin polarization factor g¼0.5,43magnetocrystalline anisotropy constants K1¼1.2/C2104J/m3and K 2¼0,35elastic constants c 11¼2.57 /C21011Nm/C02,c 12¼1.62 /C21011Nm/C02, and c 44¼1.05 /C21011Nm/C02,35magnetostrictive constants k100¼139ppm andk111¼22ppm.41We investigate the influence of normal substrate strain e11and e22on the magnetization state by assuming a zero shear strain. The dynamics of magnetization was investigated by numerically solving the time-dependentLLGS equation using the Gauss-Seidel projection method and the semi-implicit Fourier spectral method. 44,45The samples were discretized in computational cells of 2 /C22/C22n m3,a n d the total size is 80 /C280/C220 nm3.46 Figure 2(a)shows the temporal evolutions of magnetiza- tion components at the current density of 5.0 /C2106A/cm2. Three lines represent magnetization component hmxievolu- tions with different substrate biaxial strains ( e11¼e22¼0, FIG. 2. (a) Temporal evolutions of the average normalized magnetization components hmxiwith different sub- strate strains at the current density of 5.0/C2106A/cm2. (b) Magnetization tra- jectories at different strains. (c) Snapshots of magnetic domains evolu-tion with different substrate strain at J¼5.0/C210 6A/cm2. FIG. 3. (a) and (c) Temporal evolu- tions of effective fields along x axis at the substrate strains of /C00.5% and /C00.9%. (b) and (d) The magnetization trajectories projection on x-y plane at the strains of /C00.5% and /C00.9%.122407-3 Huang et al. Appl. Phys. Lett. 105, 122407 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.7.27.132 On: Fri, 05 Dec 2014 18:45:58/C00.5%, /C00.9%). At the low current density of 5.0 /C2106A/cm2, the magnetization cannot be switched by the spin transfer torque without substrate misfit strain. However, the magnet- ization switching (180/C14switching) can be achieved with the assistance of substrate misfit strain /C00.5%. In addition, the magnetization component hmxiwill switch from /C01.0 to 0.702 at the substrate biaxial strain /C00.9%, which we call 135/C14switching. The substrate misfit strain reduces effec- tively the critical current and the magnetization switching time. We show the magnetization precession trajectorieswith zero, /C00.5% and /C00.9% strains at the current density of 5.0/C210 6A/cm2. It is observed that three types of trajectories in Figure 2(b) show no switching, 180/C14switching, and 135/C14 switching at zero, /C00.5%, and /C00.9%, respectively. The evolution of magnetic microstructure is illustrated in Figure 2(c)with the numbers corresponding to those in Figure 2(a). The 180/C14magnetization switching can be accomplished under the substrate strain of /C00.5% ( e11,e22), while the 135/C14 magnetization switching is obtained at a higher substrate compressive strain of /C00.9%. As shown in Figure 3,w e show the evolutions of effective fields along x axis and the projection of magnetization trajectories on x-y plane at thebiaxial substrate strains of /C00.5% and /C00.9%. The elastic effective field plays a significant role during the magnetiza- tion switching from AP to P. With the assistance of elasticeffective field, the magnetization switching is easily accom- plished by a small current input since the elastic effective field will drag the magnetization to the diagonal directions.However, the large elastic effective field will impede the 180 /C14magnetization switching. A small current input cannot overcome the barrier of the elastic effective field, therefore,the 135 /C14magnetization switching is obtained at the large biaxial substrate strain /C00.9%. In the following, we focus on the 135/C14magnetization switching to achieve the strain- assisted four-state magnetization switching. We use Figure 4(a) to illustrate the process of strain- assisted spin transfer switching. Three-step magnetizationswitching has four resistance states that are useful in design- ing multi-bit MRAM. Figure 4(b)shows the hysteresis loops at different substrate strains ( e 11¼e22¼0,/C00.5%, /C00.9%). At the biaxial strain /C00.5%, we observe the decrease of criti- cal current for magnetization switching (blue hysteresis loop). For the strain /C00.9%, we observe two intermediate states (45/C14and 135/C14) at a low current density. If we continue to increase the current density, AP and P structures can be obtained at larger positive and negative current densities,respectively. Compared with previous multilevel bit spin transfer switching,16,28our results have several advantages. First, it may reduce the cost of magnetic devices because only one free layer is required during the design of multile-vel bit spin transfer switching magnetic devices, while two soft layers (one is hard layer and the other is soft layer) are needed in previous multilevel bit spin transfer switchingdevices. Second, certain transitions are prohibited in the pre- vious structures since the hard soft layer requires a large cur- rent to switch and the soft layer can be switched by a smallcurrent. For example, “11,” “10,” “01,” and “00” in Ref. 16 are four resistance states, where the first digit refers to the hard soft layer. Level 00 cannot be switched into 10 state byusing a single current. Only reversible transitions between 11 and 10, 01, and 00 can be achieved. However, all transi- tions among 0 /C14,4 5/C14, 135/C14, and 180/C14states can be obtained by adjusting the substrate strain. Third, the substrate strain can be produced by a piezoelectric substrate, and hence one can use voltage or electrical field to control the magnitude ofstrain through the converse piezoelectric effect. Despite these promising impacts, there are issues remain to be solved before the practical applications of such strain-assistedmulti-bit MRAM. For example, as the memory states along the diagonal axes are essentially stabilized by biaxial strains, the possible strain relaxation may somewhat affect the longtimescale device operation. In conclusion, we investigated strain-assisted spin transfer switching in CoFeB-based magnetic tunnel junctions by com-bining phase field simulations with micromagnetic simula- tions. An effective method of strain-assisted spin transfer magnetization switching is proposed to reorient the magnet-ization instead of using an external magnetic field. The critical current of spin transfer switching is shown to decrease with substrate biaxial strain. A potential strain-assistance multilevelbit spin transfer magnetization switching was proposed. This work was sponsored by the US National Science Foundation under the Grant No. DMR-1410714, and by the National Science Foundation of China (No. 11174030). Thecomputer simulations were carried out on the LION and Cyberstar clusters at the Pennsylvania State University. 1L. Berger, Phys. Rev. B 54(13), 9353 (1996). 2J. C. Slonczewski, J. Magn. Magn. Mater. 159(1–2), L1 (1996). 3J. A. Katine, F. J. Albert, R. A. Buhrman, E. B. Myers, and D. C. 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Lett. 105, 122407 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 152.7.27.132 On: Fri, 05 Dec 2014 18:45:58

1.4897247.pdf

Grazing incidence angle based sensing approach integrated with fiber-optic Fourier transform infrared (FO-FTIR) spectroscopy for remote and label-free detection of medical device contaminations Moinuddin Hassan and Ilko Ilev Citation: Review of Scientific Instruments 85, 103108 (2014); doi: 10.1063/1.4897247 View online: http://dx.doi.org/10.1063/1.4897247 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Design and characterization of a novel multimodal fiber-optic probe and spectroscopy system for skin cancer applications Rev. Sci. Instrum. 85, 083101 (2014); 10.1063/1.4890199 Fiber-optic Fourier transform infrared spectroscopy for remote label-free sensing of medical device surface contamination Rev. Sci. Instrum. 84, 053101 (2013); 10.1063/1.4803182 Chunk-shaped ZnO nanoparticles for ethanol sensing AIP Conf. 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Downloaded to IP: 137.99.31.134 On: Thu, 21 May 2015 15:04:45REVIEW OF SCIENTIFIC INSTRUMENTS 85, 103108 (2014) Grazing incidence angle based sensing approach integrated with fiber-optic Fourier transform infrared (FO-FTIR) spectroscopy for remote and label-freedetection of medical device contaminations Moinuddin Hassana)and Ilko Ilev Optical Therapeutics and Medical Nanophotonics Laboratory, Division of Biomedical Physics, Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, U.S. Food and DrugAdministration, Silver Spring, Maryland 20993, USA (Received 11 July 2014; accepted 23 September 2014; published online 9 October 2014) Contamination of medical devices has become a critical and prevalent public health safety concern since medical devices are being increasingly used in clinical practices for diagnostics, therapeu- tics and medical implants. The development of effective sensing methods for real-time detectionof pathogenic contamination is needed to prevent and reduce the spread of infections to patients and the healthcare community. In this study, a hollow-core fiber-optic Fourier transform infrared spec- troscopy methodology employing a grazing incidence angle based sensing approach (FO-FTIR-GIA)was developed for detection of various biochemical contaminants on medical device surfaces. We demonstrated the sensitivity of FO-FTIR-GIA sensing approach for non-contact and label-free detec- tion of contaminants such as lipopolysaccharide from various surface materials relevant to medicaldevice. The proposed sensing system can detect at a minimum loading concentration of approxi- mately 0.7 μg/cm 2. The FO-FTIR-GIA has the potential for the detection of unwanted pathogen in real time. [ http://dx.doi.org/10.1063/1.4897247 ] I. INTRODUCTION Healthcare associated infections (HAIs) in clinics and hospitals are a major concern for public safety and impose significant medical, social, and economic consequences. Ap- proximately, 1 in every 20 inpatients has an infection asso-ciated with hospital care. 1In 2002, 1.7 ×106HAI occurred in U.S. hospitals and approximately 99 000 deaths were asso- ciated with it.2Recently published CDC report showed that 2×106people in the United States become infected and at least 23 000 people die due to antibiotic resistant bacteria.3 Department of Health and Human service (DHHS) and asso- ciated organization including the U.S. Food and Drug Admin- istration (FDA) have already setup their action plan to identifythe reduction of HAI caused by any infectious agent, includ- ing bacteria, fungi, viruses, etc. 1,4 Although there are many factors related to HAI, medi- cal devices in clinical setting are one of the major risk factor as the devices are being extensively used in clinical practices for diagnostics, therapeutics, and indwelling devices such asmedical implants. There are several techniques available in healthcare facilities to validate cleaning process for prevent- ing the spread of infection to patients and healthcare com-munity caused by medical device contamination. These tech- niques are based on ex situ approaches such as swap/wipe sampling, which are complex, time consuming, and not ad- equate to monitor and detect pathogen contamination in real time. In order to reduce HAI for protecting public health, al- ternative methods for quantitative, accurate, easy-to-use and a)Author to whom correspondence should be addressed. E-mail: moinuddin.hassan@fda.hhs.gov. Tel.: +1 301-796-3089.real-time detection, and identification of microorganism con- taminations on medical devices surface in clinical settingare needed. We have recently presented a novel proof-of- concept platform for label-free, remote, and rapid detection of medical device surface contamination employing a fiber-optic Fourier Transform Infrared (FO-FTIR) spectroscopy methodology. 5FTIR has a potential for providing qualitative and quantitative spectral signature information about the tar-geted samples. 6–9Furthermore, the developed reflection based FO-FTIR method ensures some unique benefits such as intrin- sic biochemical specificity, non-destructive, non-contact, andsensitive contamination detection with potential for minia- turization for in situ on site applications. We demonstrated the feasibility and sensitivity of the FO-FTIR technology fordetecting and analyzing some reference low-concentration protein (such as ≤0.0025% or ≤4×10 11molecules of BSA) and bacterial endotoxins (such as 0.5% or 0.5 EU/ml endotoxin).5However, since the FO-FTIR design uses a re- flection sensor mode with a relatively small angle of incidenceof about 20 ◦, it is more effective for testing samples with highly reflected surfaces. In practice, medical device surfaces are made of different types of materials from metals to dielec-tric (such as vinyl, glass, etc.) with various surface finish qual- ity from smooth to rough, which provides lower surface re- flection modes. Therefore, to enhance the FO-FTIR sensitiv-ity for measurement of thin layer of samples on non-reflective or semi-reflective surface, a significantly increased sensor path-length through the tested sample is required, which canbe achieved using a grazing incidence angle (GIA) sensing approach integrated to the FO-FTIR methodology. Reflection spectroscopic measurement at GIA is a broadly employed sensing method for various applications. 10–14Currently, some commercially available 0034-6748/2014/85(10)/103108/5/$30.00 85, 103108-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 137.99.31.134 On: Thu, 21 May 2015 15:04:45103108-2 M. Hassan and I. Ilev Rev. Sci. Instrum. 85, 103108 (2014) GIA spectroscopic probes have been used in food and drug manufacturing plants for cleaning validation.11–15However, a major challenge of this technique is its applicability to anytarget area on medical device surfaces due to large and flat designs employed. As a continuation of the study, we have developed a novel simple sensing approach (FO-FTIR-GIA)using a flexible IR hollow-core fiber probe operating in a GIA mode that is integrated in a FO-FTIR spectroscopy platform. FO-FTIR-GIA sensing method can provide a non-contact, label-free tool for detection and identification of contami- nants on various remote target areas including tough-to-reachareas of different medical device surfaces such as metal and dielectric materials. In this study, we investigated the sensitivity of the proposed FO-FTIR-GIA sensing methodutilizing different types of target surfaces (substrates) relevant to medical device surfaces with biological contaminant such as lipopolysaccharide as an example. The results suggest thatthe FO-FTIR-GIA method provides reasonable sensitivity for in situ identification of pathogenic contaminant on medical device surface. II. MATERIAL AND METHODS A. Reagents and chemicals Lipopolysaccharide (LPS) from Pseudomonas aerug- inosa was commercially supplied by Sigma-Aldrich (St. Louis, MO) in powder form and endotoxin-free water was purchased from Fisher Scientific (Fair Lawn, NJ). B. Sample preparation All chemicals were used without further purification. A thin plastic film (0.1 mm thick) was used as a homogeneoussample to validate the sensitivity of the experimental setup with different types of sample substrates. LPS was used as a contaminant sample relevant to microorganism comminationon medical device surface. 100% stock solution of LPS was prepared by adding 1mg of LPS to 1 ml of endotoxin free wa- ter at room temperature. The solution was stirred for 10 min or until the LPS dissolved completely. Using the stock solution, different concentration samples (such as 50%, 25%, 10%, 5%,etc.) were prepared and stored at 4 ◦C until the measurements were completed, for a maximum of 1 day. C. Substrate Different types of substrates were selected relevant to medical device surfaces including metals (rough and smoothstainless steel, aluminum, etc.) and dielectric (vinyl and glass). The highly reflected surface (99.9%) of a 25 mm di- ameter gold mirror (ThorLabs Inc., Newton, NJ) was used asa standard sample substrate. D. Fiber-optic sensor system A schematic diagram of the measurement setup is shown in Fig. 1. We have designed and developed a prototype of GIA sensing probe that is integrated with the fiber-optic FTIR FIG. 1. (a) Schematic diagram of the measurement system and (b) grazing incidence angle. spectroscopy platform for remote and in situ detection of mi- croorganism. As shown in Fig. 2, the GIA probe includes two flexible hollow-core fiber arms to set incidence and detec- tion angles, respectively. The incidence angle can vary from 70◦to 85◦as compared to the normal incidence angle (0◦), which allows the sensitivity of IR reflectance measurements to be maximized for thin layers of biochemical contaminantson any types of metallic (such as steel, aluminum, etc.) and dielectric (such as glass, polymer, etc.) materials surfaces. In this study, we used a fixed incidence angle of 85 ◦to the surface for all measurements of contaminations on different types of reflecting surfaces. The FO-FTIR-GIA sensor head is connected to the external ports of the FTIR spectrometer (Ver-tex 70, Bruker Optiks, Ettlingen, Germany) by two mid-IR hollow-core optical fibers (Hollow Waveguide with Acrylate Buffer, HWEA7501200, Polymicro Technologies, Phoenix,AZ) with diameters of 750 μm and a numerical aperture of 0.05. One of the sensor fibers is employed for light delivery from a light source (Halogen) to the sample, and the otherfiber for transmitting the signal light to the detector (liquid Nitrogen cooled MCT) after absorption by the sample. By using an adjustable stage, the sensor head was placed abovethe sample at a distance where the signal intensity is maxi- mal. Each spectrum was averaged over 256 scans in the range of 850–5000 cm −1a ta4c m−1resolution. Preceding sample measurement, the background signal was collected from the corresponding surface (substrate) without the sample. Prior to FTIR experiments, the substrate of different types were cleaned with 70% isopropyl alcohol wipes and dried with scientific grade wipes. Surface finish of differ-ent types was characterized using a digital microscope (VH- Z500, Keyence Corp., Itasca, MA) with 2000 ×magnifica- tion. As representative model of a homogenous sample, aplastic thin film is placed on different types of substrates and measured. LPS solution of different concentrations were placed on each plate of different material composition in 2 μl drops of equal size ( ∼4 mm diameter) and allowed to dry for ∼30 min under a covered area to decrease the dust landing on FIG. 2. Proposed grazing incidence angle (GIA) sensing head. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 137.99.31.134 On: Thu, 21 May 2015 15:04:45103108-3 M. Hassan and I. Ilev Rev. Sci. Instrum. 85, 103108 (2014) the plates. Before each measurement, background spectrum of the clean surface was collected prior to sample deposition. Three trials were recorded and averaged at different positionson the sample. III. RESULTS AND DISCUSSION In our previous study,5we demonstrated the feasibility to use a fiber-optic FTIR (FO-FTIR) reflection sensing head for real time detection and analysis of surface contaminations.The method has a potential for detecting surface contamina- tion in order to control transmission of HAI in health care fa- cility. However, the sensor head was limited to use for highlyreflected surface with an incidence angle of 22 ◦. In this study, FO-FTIR-GIA sensing head using a flexi- ble IR hollow-core fiber probe operating in a GIA mode withan incidence angle from 70 ◦to 85◦was developed for FTIR spectroscopic measurement to improve the detection sensitiv- ity of various types of sample material surfaces (high-, semi- or non-reflective) relevant to medical device surface contam- ination detection. The angle of incidence of the FO-FTIR-GIA sensing head was optimized from any kind of distortion of the spectrum by comparing the spectrum of a plastic film (0.1 mm homogeneous thickness) on a mirror surface tothe spectra obtained from reflection sensing head. The spec- trum distortion factors could include peak shift, band shape changes, band splitting, etc., caused by various refractive in-dex of surface materials, wavelength, incidence angle, etc. 16 After fixing the incidence angle of FO-FTIR-GIA sensinghead at 85 ◦, the comparative absorption spectra of the ref- erence plastic film on a mirror surface are obtained using the FO-FTIR-GIA and FO-FTIR reflection sensing head. Figure 3 illustrates typical absorption spectra which are identical andreproducible. Moreover, we did not observe any incidence angle dependent distortion in these spectra. The effectiveness and sensitivity of the FO-FTIR-GIA sensing head at the 85 ◦incidence angle was further investi- gated by measuring the absorption spectra of the plastic filmon different types of metallic (steel and aluminum) and di- electric (clear vinyl and glass) surfaces as well as a gold mir- ror surface. As shown in Fig. 4, the measured spectra of the plastic film on different types of substrates are intense and FIG. 3. Comparative absorbance spectra of plastic film (0.1 mm thick) on mirror surface obtained by GIA sensing head at incidence angle 85◦and re- flection sensing head at incidence angle 22◦to the surface normal. FIG. 4. Absorbance spectra of plastic film on different types of metallic anddielectric surfaces. the peaks are readily identifiable of the material. The absorp- tion peaks from plastic film on metal surfaces were found to be highly reproducible as compare to mirror surfaces. In addi-tion, we did not observe any significant changes due to surface roughness for stainless steel substrates. Furthermore, in case of dielectric material surfaces, al- though these surfaces are generally not reflective enough to allow beam to successfully reflect off the surface, the pro- posed novel FO-FTIR-GIA sensing approach provides ade-quate sensitivity which enable the IR light to pass through the contaminant on a non-reflective surface and to be detected. In Fig. 4, the absorbance spectra of plastic film on vinyl surface is lower compared to the metallic surface, but the quality of spectra is good enough for the identification of peaks. How- ever, in the spectral range lower than 1500 cm −1wavenumber, the spectrum of plastic film on vinyl surface is distorted and high-intensity fluctuation effects are observed in comparison with metallic or mirror surfaces. Similar but more intense ef- fects are also observed for clear glass substrate in the same region as shown in Fig. 5. These effects on glass surfaces have been observed and reported in the literature.11However, in the region above 1500 cm−1in the spectra for the plastic film on glass substrate are identical to the metallic surface as shownin Fig. 5(inset). At wavenumber greater than 1500 cm −1, glass or vinyl materials are transparent to mid-IR radiation, but there is a strong absorption band at longer wavelengths FIG. 5. Absorbance spectra of plastic film on glass surface. (Inset) Enlarge- ment of the region containing the peak used for comparison. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 137.99.31.134 On: Thu, 21 May 2015 15:04:45103108-4 M. Hassan and I. Ilev Rev. Sci. Instrum. 85, 103108 (2014) FIG. 6. Absorbance spectra obtained from lipopolysaccharide (LPS) on alu- minum surface using incidence angle 22◦(reflection sensing head) and 85◦ (GIA sensing head). that is associated with refractive index of the materials of the surface. Medical device contaminations in clinical environment are mostly pathogen growing on the surface or harmful residue left on the device surface. As a representative sam- ple relevant to medical device microorganism contaminations,standard lipopolysaccharide (LPS) was used in this study. 17,18 LPS is the major component of the outer membrane of Gram- negative bacteria and consists of lipid and polysaccharide. LPS acts as endotoxins and an excessive amount of LPS (1 μg/kg) in blood may induce shock in human.19We tested the feasibility and sensitivity of the proposed FO-FTIR-GIA sensing head as compared to the reflection head (an inci- dent angle of 22◦) for detecting LPS on different type of surfaces. The comparative spectra of LPS on aluminum sur- face are shown in Fig. 6. As compared to the proposed FO- FTIR-GIA sensing head, the reflection sensing head is notsensitive enough to record spectra of LPS from aluminum surface (semi-reflective), but there is no significant differ- ence between the spectra of LPS for a higher reflecting sur-face such as smooth stainless steel or mirror surface. Typical GIA absorption spectra of LPS in dry condition are shown in Fig. 7. The LPS spectra can be sub-divided in accordance FIG. 8. Absorbance spectrum of lipopolysaccharide (LPS) on various types of surface. to the constituents of biological cells, for example, fatty acid region or lipid (3000–2800 cm−1), amide region (1800– 1500 cm−1), polysaccharide region (1200–900 cm−1), etc.8 Each region of the recorded spectra was found to be intense and identifiable thus enabling identification of the LPS. TheLPS spectral signatures observed in this study are similar to those published earlier. 20Repeated measurements of LPS on smooth stainless steel demonstrate a high reproducibil- ity as shown in Fig. 7(a)., whereas in the case of aluminum surface excellent reproducibility in the LPS spectra (posi-tion and peaks) is accompanied slight variations in the ab- sorbance magnitudes (Fig. 7(b)). This may be attributed to the higher degree of LPS homogeneity over the smooth stain-less steel surface relative to rough aluminum surface. Simi- lar characteristics in the reproducibility were also observed from other surface types such as rough stainless steel, vinyl,glass, etc. The acquisitions of LPS spectra from dielectric surfaces (vinyl, glass, etc.) are limited to observations above 1500 cm −1wavenumber due to high absorption of mid-IR light as mentioned in previous paragraph. The LPS signature spectra obtained from various metals and mirror surfaces are shown in Fig. 8. The spectra of LPS from various substrates FIG. 7. Typical absorbance spectrum of lipopolysaccrade (LPS) using three consecutive trials (a) on smooth stainless steel surface and (b) on alumin um surface. The region can be defined according to the components of the cell: lipid (3000–2800 cm−1), amide region (1800–1500 cm−1), and polysaccharide region (1200–900 cm−1). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 137.99.31.134 On: Thu, 21 May 2015 15:04:45103108-5 M. Hassan and I. Ilev Rev. Sci. Instrum. 85, 103108 (2014) FIG. 9. Typical absorbance spectra of lipopolysaccharide (LPS) of different concentrations on aluminum with surface loading 15 μg/cm2–0.7μg/cm2. are identical to each other. We also investigated the sensi- tivity of the FO-FTIR-GIA sensing head for LPS of differ-ent concentrations (from 1 μg/ml to 0.025 μg/ml) after de- positing on each plate in 2 μl drops, which provide average surface concentrations of LPS ranged from 0.7 μgc m −2to 15μgc m−2. Measured absorption spectra of LPS at differ- ent loading concentrations on aluminum surface are shown in Fig. 9. In this case, we did not observe a linear dependence in the absorbance intensity due to the inhomogeneous LPS dis- tributions after depositing on the surface in the dry condition. We identified a minimum detection limit of ∼0.7μgc m2us- ing the GIA sensing probe for detecting LPS surface residues on aluminum when the specific spectral signals are above the system noise level. As compared to other commercially avail-able devices, the proposed FO-FTIR-GIA sensing probe pro- vides a detecting sample area much smaller due to the sin- gle detecting hollow-core fiber design with a numerical aper- ture of 0.05 used for the system (spot size approximately 0.5 mm). The minimal detection threshold is also in agreementwith other surface materials such as metals and dielectric ma- terials used in this study. In addition, depending on the spe- cific quantitative applications or area of interest, the minimumthreshold level could be improved by adjusting the spot size of fiber-optic sensor system. IV. CONCLUSIONS Employing the mid-infrared FTIR sensing methodology, we have developed FO-FTIR-GIA sensing approach for non- contact, label-free identification of pathogen from various types of surface materials relevant to medical device surfaces in real time. Due to the fiber-optic advanced features, theFO-FTIR-GIA sensing head is flexible to fit at any target area on medical device surface. However, further work is required to determine the limits for in situ detection and identifica- tion of pathogen contamination from medical device in clin- ical environment. The proposed sensing system has the po- tential for real time identification of pathogen in conjunctionwith mathematical algorithm and possible to control transmis- sion of infection in healthcare industry as well as to address infectious disease threats for the nation determined by pub- lic health needs and Emergency Medical Countermeasures Enterprise. 21In addition, the proposed technique could be useful for regulatory agencies such as U.S. FDA as an alter- native test method to implement regulatory guidelines. ACKNOWLEDGMENTS This study is supported by the intramural research pro- gram of Medical Counter Measure initiative (MCMi) of Cen- ter for Devices and Radiological Health (CDRH), U.S. Food and Drug Administration (FDA). We like to thank Dr. DarrellTata for his useful discussions. The authors have no conflicts of interest or financial ties to disclose. The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either an actual or implied endorsement of such products by the U.S. Food and Drug Administration (FDA), Department of Health and Human Services. 1U.S. Department of Health and Human Services (DHHS), Health Care- Associated Infections (HAI), 2014, see www.hhs.gov/ash/initiatives/hai/ . 2R. M. Klevens, J. R. Edwards, C. L. Richards, T. C. Horan, R. P. Gaynes, D. A. Pollock, and D. M. Cardo, Public Health Rep. 122, 160–166 (2007). 3CDC, Threat Report 2013, 2013, see http://www.cdc.gov/drugresistance/ threat-report-2013 . 4Department of Health and Human Services, “Action plan to prevent healthcare-associated infections,” 2009, pp. 1–116, see http://www.hhs.gov /ash/initiatives/hai/actionplan/hhs_hai_action_plan_final_06222009.pdf . 5M. Hassan, T. Xin, E. Welle, and I. Ilev, Rev. Sci. Instrum. 84, 053101 (2013). 6K. K. Chittur, Biomaterials 19, 357–369 (1998). 7P. I. Haris and D. Chapman, TIBS 17, 328–333 (1992). 8D. Naumann, D. Helm, and H. Labischinski, Nature (London) 351, 81–82 (1991). 9N. A. Ngo-Thi, C. Kirschner, and D. Naumann, J. Mol. Struct. 661–662 , 371–380 (2003). 10O. M. Primera-Pedrozo, Y . M. Soto-Feliciano, L. C. Pacheco-Londono, and S. P. Herna´ndez-Rivera, Sensing Imaging 10, 1–13 (2009). 11B. B. Perston, M. L. Hamilton, B. E. Williamson, P. W. Harland, M. A. Thomson, and P. J. Melling, Anal. Chem. 279, 1231–1236 (2008). 12M. L. Hamilton, B. B. Perston, P. W. Harland, B. E. Williamson, M. A. Thomson, and P. J. Melling, Appl. Spectrosc. 60, 516–520 (2006). 13M. S. Robinson, G. Mallick, J. L. Spillman, P. A. Carreon, and S. Shalloo, Appl. Opt. 38, 91–95 (1999). 14B. B. Perston, M. L. Hamilton, P. W. Harland, M. A. Thomson, P. J. Melling, and B. E. Williamson, Appl. Spectrosc. 62, 312–318 (2008). 15P. J. Melling and P. H. Shelly, U.S. patent 6,310,348 (2001). 16R. G. Greenler, R. R. Rahn, and J. P. Chwartz, J. Catal. 23, 42–48 (1971). 17A. K. Rathinam and K. A. Fitzgerald, Nature (London) 501, 173–175 (2013). 18C. Raetz and C. Whitfield, Annu. Rev. Biochem. 71, 635–700 (2002). 19H. S. Warren, C. Fitting, M. Adib-Conquy, X. Liang, and C. Valentine, J. Infect. Dis. 201, 223–232 (2010). 20S. Kim, B. L. Reuhs, and L. J. Mauer, J. Appl. Microbiol. 99, 411–417 (2005). 21Public Health Emergency Medical Countermeasures, 2014, see http://www. phe.gov/Preparedness/mcm/enterprisereview/Pages/default.aspx . This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 137.99.31.134 On: Thu, 21 May 2015 15:04:45

1.4894105.pdf

Peeling-off of the external kink modes at tokamak plasma edge L. J. Zheng and M. Furukawa Citation: Physics of Plasmas (1994-present) 21, 082515 (2014); doi: 10.1063/1.4894105 View online: http://dx.doi.org/10.1063/1.4894105 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/21/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Generation of non-axisymmetric scrape-off layer perturbations for controlling tokamak edge plasma profiles and stabilitya) Phys. Plasmas 19, 056124 (2012); 10.1063/1.3702048 Edge plasma boundary layer generated by kink modes in tokamaks Phys. Plasmas 18, 062503 (2011); 10.1063/1.3596536 Interplay between ballooning and peeling modes in simulations of the time evolution of edge localized modes Phys. Plasmas 12, 012506 (2005); 10.1063/1.1832600 Stability analysis of H-mode pedestal and edge localized modes in a Joint European Torus power scan Phys. Plasmas 11, 1469 (2004); 10.1063/1.1668646 Models for the pedestal temperature at the edge of H-mode tokamak plasmas Phys. Plasmas 9, 5018 (2002); 10.1063/1.1518474 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 13:34:10Peeling-off of the external kink modes at tokamak plasma edge L. J. Zheng1and M. Furukawa2 1Institute for Fusion Studies, University of Texas at Austin, Austin, Texas 78712, USA 2Graduate School of Engineering, Tottori University, Tottori 680-8552, Japan (Received 15 April 2014; accepted 5 August 2014; published online 28 August 2014) It is pointed out that there is a current jump between the edge plasma inside the last closed flux surface and the scrape-off layer and that the current jump can lead the external kink modes to con- vert to the tearing modes, due to the current interchange effects [L. J. Zheng and M. Furukawa, Phys. Plasmas 17, 052508 (2010)]. The magnetic reconnection in the presence of tearing modes subsequently causes the tokamak edge plasma to be peeled off to link to the divertors. In particular, the peeling or peeling-ballooning modes can become the “peeling-off” modes in this sense. This phenomenon indicates that the tokamak edge confinement can be worse than the expectation basedon the conventional kink mode picture. VC2014 AIP Publishing LLC . [http://dx.doi.org/10.1063/1.4894105 ] I. INTRODUCTION The H-mode confinement—an operating mode with high energy confinement1—has today been adopted as a ref- erence for next generation tokamaks, especially for ITER. However, the H-mode confinement is often tied to the dam- aging edge localized modes (ELMs).1There is a concern that ELMs can discharge particles and heat into the scrape-off layer and subsequently to the divertors. The divertor plates can be potentially damaged by such a discharge. This is par-ticularly a concern for big devices like ITER. This concern has stimulated active researches in this field for clarifying the tokamak plasma edge instabilities, inorder to understand the ELMs. The most well-known theo- ries are the peeling and peeling-ballooning modes. 2,3 However, the peeling or peeling ballooning modes are of kink type. Without field line reconnection, the plasmas inside the last closed flux surface actually are not peeled off. The necessity to consider the tearing mode excitation and the coupling of the scrape-off-layer current were first pointed out in Ref. 4. Apparently, to understand the ELMs one needs to take into consideration the subtle feature oftokamak plasma edge, where the plasma on one side is con- fined by the closed flux surfaces and, on the other side, the plasma is linked to the divertors due to the open-field-linefeature in the scrape-off layer. Otherwise, one cannot explain why there is not any ELM-type of bursting at the internal transport barrier. The development of tearing modes caneffectively connect the pedestal plasma to the scrape-off layer. Taking into account this edge feature, Ref. 4proposed a current-driving-mode theory for ELMs. The magnetohy-drodynamic (MHD) modes at plasma edge can be amplified due to the nonlinear coupling with scrape-off-layer current. This coupling can be a positive feedback process and lead tothe ELM bursting. The theory explains many characteristic features of ELMs as observed in tokamak experiments, such as a sharp onset and initial fast growth of magnetic perturba-tions even when the underlying equilibrium is only margin- ally unstable for a MHD mode and also a quick quenching after the bursting peak. This work also points to the currentdriven modes—tearing type—as the ELM bursting explana- tion, although the kink type of modes, such as the peeling ballooning modes, can be a trigger. In this paper, we further explain how the external kink modes in tokamaks, such as the peeling ballooning modes, can become a trigger to the excitation of tearing modes. Wepoint out that there is a current jump between the plasmas inside the last closed flux surface and in the scrape-off layer. When there is a plasma perturbation at the edge, the currentson each side of the jump are carried over alternatively in the opposite direction to form a perturbed current sheet (see Fig.1). This current sheet can lead to the excitation of tear- ing modes. This mechanism reflects the extreme case of the current interchange tearing modes as pointed out in Ref. 6, with the tokamak edge and scrape-off layer specialties beingtaken into consideration. Note that the drive to the current interchange tearing modes, as pointed out in Ref. 6,i sp r o - portional to the current gradient. The current jump betweenthe plasma edge and the scrape-off layer makes the drive at the edge to be dramatically enhanced. As shown in the analy- sis later in this paper, the conversion of external kink modesto tearing modes at tokamak edge can therefore happen read- ily and cause the edge plasma to be peeled off. Note that this FIG. 1. The coordinate system for analyzing the current interchange effects. The axis zpoints out of the paper. The perturbed current directions are indi- cated. The edge plasma locates in the x<0 region, while the scrape-off layer in the x>0 region. The plasma displacement nis plotted by the dashed curve with n0¼0 assumed. 1070-664X/2014/21(8)/082515/6/$30.00 VC2014 AIP Publishing LLC 21, 082515-1PHYSICS OF PLASMAS 21, 082515 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 13:34:10process may be positively fed back, as pointed out in Ref. 4. This phenomenon indicates that the tokamak edge confine-ment can be worse than the expectation based on the conven- tional kink mode picture. We prove this peeling-off phenomenon by re-deriving the nonlinear tearing mode equation, which was originally developed by Rutherford. 7Note that Ref. 7intended to con- sider the resistivity/current gradient effects. However, it onlytook into consideration the thermal conductivity effects related to the current gradient, without including the current convective effect as pointed out in Ref. 6. The current con- vective effect at the plasma edge can be very significant due to the jump between the plasma edge and the scrape-off layer. This motivates us to examine this issue. This paper is arranged as follows: Following to Sec. I,i n Sec. II, the Rutherford’s equation will be rederived with the current jump between the plasma edge and scrape-off layerbeing taken into account. The results will be summarized and discussed in Sec. III. II. REDERIVATION OF RUTHERFORD’S EQUATION AT THE PLASMA EDGE In this section, we will rederive the Rutherford’s equa- tion in Ref. 7to include the effects of the current jump between the plasma edge and scrape-off layer. We first describe the Ohm’s laws for the edge plasma and the scrape-off layer. For the edge plasma inside the last closed flux sur- face, Ohm’s law is j k¼rEk; (1) where jis the current density, Erepresents the electric field, ris the conductivity, with resistivity being g¼1/r, and sub- script kdenotes the parallel direction. In the scrape-off layer, the field lines are connected to divertors at the both ends, indicated by Aand B. The generalized Ohm’s law in the scrape-off layer was derived in Ref. 8 jk¼rvEk/C0c0:85/C0a ðÞ jSATTB/C0TA TA; (2) where jk¼jSAT^jk; ^jk¼/C0c( e/0 TAþjþ0:85/C0a ðÞTB TA/C01/C18/C19 þln1þ^jk ð1/C0TB=TA ðÞ1=2^jkÞTB=TA2 43 5) ; jSAT¼1 23=2enC s; rv¼e2k11Lk meðB Adlk nesei"#/C01 ; c¼^rTA eLkJSAT; j¼1 2ln2mi pme/C18/C19 ¼3:89:Here, eis the elementary charge, mis the mass, nis the den- sity, Tdenotes the temperature, /is the electric potential, /0¼/B/C0/A,Csis the sound speed, a¼k12/k11,k11andk12 are the Spitzer-Harm coefficients,9seiis the electron-ion col- lisional time, Lkdenotes the connection length between both ends AandB,lkis the arc length along magnetic field line, subscripts eand irepresent, respectively, the electron and ion quantities, and subscripts AandBdenote quantities at the ends Aand B, respectively. Note here that the Ohm’s laws in Eqs. (1)and(2)are given the moving frame. In the laboratory frame, the electric field Eneeds to be replaced by Eþv/C2B. Here, we use the bold face to denote vectors, B denotes the magnetic field, and vis the fluid velocity. As Ref. 7, we use the slab model in the ( x,y,z) space, with x¼0 specifying the rational surface and zrepresenting the longitudinal direction. The coordinate system is shown in Fig. 1. The flux function wand the stream function uare introduced to represent the magnetic field Bx¼–@w/@y, By¼@w/@x,and the velocity vx¼/C0@u=@y;vy¼@u=@x. Here, the subscripts ( x,y,z) are introduced to denote the cor- responding projections. We also introduce the displacement n, which is related to the velocity by @n=@t¼v. We consider the equilibrium with magnetic shear, in which the poloidal magnetic field is represented by By¼B0 yx. Here, prime is used to denote the derivative with respect to x. The total magnetic flux can be written as7 wðx;y;tÞ¼w0ðxÞþdwðy;tÞ; (3) where w0ðxÞ¼B0 yx2=2 is the equilibrium value, dwðy;tÞ ¼dw1ðtÞcoskyis the perturbed value, and kis the poloidal wave number. We use subscript 0 to denote the unper- turbed quantities and “ d” to tag the perturbed quantities. Nevertheless, the subscript 0 is dropped as soon as there is no ambiguity with the total qua ntities. The purpose of this work is to prove that, if there is a free-boundary kinkmode, it can be converted to the tearing modes due to the current jump from the plasma region inside the last closed flux surface to the scrape-off layer. Therefore, we assumethat there is a kink perturbation at the plasma edge as follows: n¼n 0þn1cosky: (4) Here, n0is used to specify the distance between the last closed flux surface and the rational surface. Note that at the plasma edge, the magnetic shear is very large, the distance between the last closed flux surface and the rational surfacec a nb ev e r ys m a l l ,s ot h a to n em a ya s s u m e n 0!0. We also note that the kink modes have different parities from that of tearing modes. Although there is finite displacement n/C0n0 at the rational surface, the direct effect of ( n/C0n0)o ndwis negligible, since dw/C24x(n/C0n0). The effects of the dis- placement ( n/C0n0) to be considered in this work are the for- mation of current sheet due to the convective carrying-over of equilibrium current. In difference from Ref. 7,i nw h i c h then/C0n0turbulence effects on the tearing modes through the thermal conduction are considered; in this work, we consider the convective effect on the formation of current sheet.082515-2 L. J. Zheng and M. Furukawa Phys. Plasmas 21, 082515 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 13:34:10As usual, we use the Ampere’s law and the field diffu- sion equation to construct the basic set of equations. TheAmpere’s law gives d2dw dx2¼l0djz; (5) where l0is the magnetic constant. As for the field diffusion equation, we have to consider separately the edge plasma region ( x/C200) inside the last closed flux surface and the scrape-off layer ( x>0). We first consider the edge plasma region ( x/C200). The derivation of the field diffusion equation is similar to that in Ref. 7. Using Faraday’s law, one obtains dEz¼@dw/@t. Using this expres- sion and the velocity representation with du, the curl opera- tion of Ohm’s law in Eq. (1)yields @dw @t/C0@du @yB0 yx¼dgjzðÞ: (6) Here, as discussed previously, the v/C2Beffect has been added in the Ohm’s law Eq. (1). The perturbed quantity d(gjz) in Eq. (6)contains both the local inductive ( @/@t) and convective ( v/C1r) contributions due to the presence of the displacement nin Eq. (4)(see Fig. 1). We exclude the inho- mogeneity effects of the plasma resistivity both in the edge plasma region ( g) and in the scrape-off layer ( gv) from our consideration, since they are smaller than the effects from the current jump between the edge plasma and the scrape-off layer. In consistence with this, we also ignore the inhomoge-neity effects of other thermal quantities, such as nandT.I n the region, where the edge plasma is not taken over by the scrape-off-layer plasma, we then have dðgj zÞ¼gdjz: (7) Instead, in the region, where the edge plasma is replaced by the scrape-off-layer plasma, one has to include the convec-tive effects due to the displacement n. This yields dgj zðÞ¼gvjzþc0:85/C0a ðÞ gvjSATTB/C0TA TA/C0gjz0; ¼gvdjz/C0D^E; (8) where the electric field jump reads D^E/C17gjzp0/C0gvjzv0þc0:85/C0a ðÞ gvjSATTB/C0TA TA/C20/C21 :Here, jzp0andjzv0denote the equilibrium current densities, respectively, in the plasma edge and the scrape-off layer.Using Eqs. (7)and(8), the diffusion equation in the edge plasma region ( x<0), Eq. (6), can be expressed as @dw @t/C0@u @yB0 yx¼Hn/C0xðÞ gdjzþHx/C0nðÞ gvdjz/C0D^E/C0/C1 ;(9) where H(x) is the Heaviside step function. Similarly, the dif- fusion equation in the scrape-off layer ( x>0) can be obtained as @dw @t/C0@u @yB0 yx¼Hx/C0nðÞ gvdjzþHn/C0xðÞ gdjzþD^E/C0/C1 :(10) The current jump between the edge plasma and scrape-off layer and the inclusion of the convective effects make the diffusionequations (9) and (10) become different from that in Ref. 7. To proceed to derive the tearing mode equation, we still need to consider separately the edge plasma region ( x/C200) and the scrape-off layer ( x>0). We first treat the edge plasma region ( x/C200). Dividing by xand averaging over yat a constant wto eliminate the second term on the left, Eq. (9)becomes 1 l0@2dw @x2¼@dw=@t w/C0dwðÞ1=2*+ þHx/C0nðÞ D^E w/C0dwðÞ1=2*+ Hn/C0xðÞ gþHx/C0nðÞ gv w/C0dwðÞ1=2*+ ; (11) where h/C1 /C1 /C1i ¼ ð k=2pÞÐ2p=k 0f/C1 /C1 /C1g dy. Here, we have used Eq.(5)to express djzon the left hand side and noted that djz(w) is a function of wonly as required by the reduced vor- ticity equation B/C1rdjz¼0, proved in Ref. 7. Further integra- tion over xfrom/C01 ! 0 of Eq. (11)yields @dw @x/C12/C12/C12/C120 /C01¼/C0l0ffiffiffiffiffiffiffi ffi2B0 ypð0 /C01dw w/C0dwðÞ1=2 /C2@dw=@t w/C0dwðÞ1=2*+ þHx/C0nðÞ D^E w/C0dwðÞ1=2*+ Hn/C0xðÞ gþHx/C0nðÞ gv w/C0dwðÞ1=2*+ : Multiplying cos kyand averaging over y,this equation is reduced to @dw1 @x/C12/C12/C12/C120 /C01 dw1dw1¼/C02l0ffiffiffiffiffiffiffi ffi2B0 yp@dw1 @tð0 /C01dwcosky w/C0dwðÞ1=2*+2 Hn/C0xðÞ gþHx/C0nðÞ gv w/C0dwðÞ1=2*+ /C02l0ffiffiffiffiffiffiffi ffi2B0 ypð0 /C01dwcosky w/C0dwðÞ1=2*+ Hx/C0nðÞ D^E w/C0dwðÞ1=2*+ Hn/C0xðÞ gþHx/C0nðÞ gv w/C0dwðÞ1=2*+ :(12) Introducing the dimensionless quantities w¼w=dw1;DE¼l0D^E=ðgB0 yÞ;D0 /C0¼@dw1 @xj0 /C01=dw1, and the island width xT¼2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dw1=B0 yq , one obtains from Eq. (12)082515-3 L. J. Zheng and M. Furukawa Phys. Plasmas 21, 082515 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 13:34:10D0 /C0¼l0ffiffiffi 2p g@xT @tðþ1 /C01dwcosky w/C0cosky ðÞ1=2*+2 Hn/C0xðÞ þHx/C0nðÞ gv=gðÞ w/C0cosky ðÞ1=2*+ þ2ffiffiffi 2p xTðþ1 /C01dwcosky w/C0cosky ðÞ1=2*+ Hx/C0nðÞ DE w/C0cosky ðÞ1=2*+ Hn/C0xðÞ þHx/C0nðÞ gv=gðÞ w/C0cosky ðÞ1=2*+ : (13) Similarly, in the scrape-off layer ( x>0), one has D0 þ¼l0ffiffiffi 2p g@xT @tðþ1 /C01dwcosky w/C0cosky ðÞ1=2*+2 Hn/C0xðÞ þHx/C0nðÞ gv=gðÞ w/C0cosky ðÞ1=2*+ /C02ffiffiffi 2p xTðþ1 /C01dwcosky w/C0cosky ðÞ1=2*+ Hn/C0xðÞ DE w/C0cosky ðÞ1=2*+ Hn/C0xðÞ þHx/C0nðÞ gv=gðÞ w/C0cosky ðÞ1=2*+ ; (14) where D0 þ¼@dw1 @xjþ1 0=dw1. Combining Eqs. (13)and(14), one finally obtains the tearing mode equation D0¼2ffiffiffi 2p l0 g@xT @tA0/C02ffiffiffi 2p xTAc; (15) where D0¼D0 /C0þD0 þand A0¼0:5ðþ1 /C01dwcosky w/C0cosky ðÞ1=2*+2 Hn/C0xðÞ þHx/C0nðÞ gv=gðÞ w/C0cosky ðÞ1=2*+/C12/C12/C12/C12/C12 x<0þðþ1 /C01dwcosky w/C0cosky ðÞ1=2*+2 Hn/C0xðÞ þHx/C0nðÞ gv=gðÞ w/C0cosky ðÞ1=2*+/C12/C12/C12/C12/C12 x>02 6666643 777775; A c¼/C0ðþ1 /C01dwcosky w/C0cosky ðÞ1=2*+ Hx/C0nðÞ DE w/C0cosky ðÞ1=2*+ Hn/C0xðÞ þHx/C0nðÞ gv=gðÞ w/C0cosky ðÞ1=2*+/C12/C12/C12/C12/C12 x<0þðþ1 /C01dwcosky w/C0cosky ðÞ1=2*+ Hn/C0xðÞ DE w/C0cosky ðÞ1=2*+ Hn/C0xðÞ þHx/C0nðÞ gv=gðÞ w/C0cosky ðÞ1=2*+/C12/C12/C12/C12/C12 x>0: Equation (15) is the modified Rutherford equation with the current convective effects being taken into account at the plasma edge, where there is a current jump. Note that in Eq.(15),D0can be obtained from the outer solution, A0 specifies the inductive contribution, and Acis the convective contribution. Letting Ac¼0 (i.e., DE¼0) and g¼gv, Eq.(15) reduces to the usual Rutherford equation given in Ref. 7. From Fig. 1, one can see that in the region for H(x/C0n)¼1 and x<0, one usually has cos ky<0; and in the region, for H(n–1 )¼1 and x>0, one usually has cosky>0. Therefore, one usually has Ac>0. This shows that the convective contribution from the current jump is generally a driving term for tearing modes. Using the Ampere’s law, one can get the ordering esti- mate: DE/C24O ð 1Þ. Noting that the second term on the right hand side of Eq. (15) is inversely proportional to the island width xT, the convective driving contribution can be very large. In the case with the current varying smoothly without a steep jump, the convective driving term is proportional tothe displacement n 1as shown in Ref. 6. In the current case,the current jump significantly enlarges the convective driv- ing effects in Eq. (15). Note that the kink mode has a differ- ent parity from that of the tearing mode. However, the inclusion of the current convective effects causes the twotypes of modes to become coupled. This makes the kink mode to be prone to convert to the current interchange tear- ing modes at the plasma edge. To show the magnitudes and parameter dependences, we numerically compute the two parameters A 0andAc.I n the calculations, we make the transformation t¼cosky,s o that the integrations over kyare reduced, for example, as follows: ð dkycoskyffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiw/C0coskyp ¼ð dttffiffiffiffiffiffiffiffiffiffiffiw/C0tp ffiffiffiffiffiffiffiffiffiffi 1/C0tp ffiffiffiffiffiffiffiffiffiffi1þtp : The integrations of this type fit exactly the existing mathe- matical library and we then compute them using the NAG (Numerical Algorithms Group) library: D01APF. We con-sider the case with n 0!0. Figure 2shows the dependence082515-4 L. J. Zheng and M. Furukawa Phys. Plasmas 21, 082515 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 13:34:10ofA0on the resistivity ratio gv/g. The displacement n1is used as a parameter in this figure, which is normalized by xT. Figure 3shows the dependence of Acon the electric field jump DE, with the resistivity ratio gv/gas a parameter. Figure 4shows the dependence of Acon the normalized dis- placement n1with the resistivity ratio gv/gand the electric field jump DEas parameters. The calculations show that the dominant contributions come from the current inside the magnetic island. From Fig. 3, one can see that the bigger the current jump across the last closed flux surface, the stronger the island drive ( Ac). We can also see from Fig. 4thatAc!0, asn1!0. This agrees with the asymptotic behavior of the parameter Ac. We find that, when n1<1,Acmonotonically decreases as n1decreases. This is reasonable, since the cur- rent induced by the kink perturbation is a monotonic functionofn 1in this case. Note that, when n1grows from being less than unity to slightly greater than unity, the kink-mode- excited current involves from being solo inside the magneticisland to both inside and outside the island. Numerical results show that the currents excited outside and inside the magnetic island have the opposite effects on island drive.This leads to an A chump at the vicinity of n1/C241 in Fig. 4.Figure 4also shows that, when n1becomes even larger than unity, i.e., the peak kink perturbation becomes considerablylarger than the island width, the parameter A cbecomes insen- sitive with the magnitude of n1. This is because the island size is mainly determined by the current induced inside theisland and slightly outside the island. From the parameter scans in Fig. 4, one can see that A cis of order unity as n1/H114071, which is of the same order as A0.N o t e thatn1is normalized by the island width. The condition n1/H114071 tends to be satisfied for the small island width case, as the kink modes develop. Equation (15) therefore indicates that the current-convective contributio n (the second term on the right) can be very big as compared to D0, as the kink mode ( n1)g r o w s . To be specific, one can estimate the tearing mode growth rate c from the second term on the right hand side of Eq. (15) cxT/C24g l0Ac A01 xT: Note that Acis of order unity just as A0, as soon as n1/H114071( s e e Fig.4). This indicates that the tearing mode growth rate is very large, especially for a small island width and with the kink mode having developed ( n1/H114071). As an estimate, we use the electron Larmor radius as the estimate for xT. Assuming that at the edge g/C2410/C07Ohm –m;Te¼1k e v ;B¼1T , a n d n1/H114071, the electron Larmor radius is then of order 10/C04m and, there- fore, one has that c/C24104kHz. The large tearing mode growth rate indicates that the perturbations of kink type at the plasma edge tend to convert to the tearing modes readily, due to thecurrent jump between the edge plasma and the scrape-off layer. Nevertheless, we should point out that the current results are based on the resistive MHD theory, just as the Rutherford’s theory. 7Other small island effects, such as the finite Larmor radius effect10and the effects induced by the transport current,11are anticipated to be significant, espe- cially for the small xTcases. Also, the coupling of the neo- classical tearing modes12,13has not been taken into account. These need to be examined in the future. III. CONCLUSIONS AND DISCUSSION The release of thermal energy by the kink modes in tokamaks has been widely studied in this field. In this paper,FIG. 3. The parameter Acversus the electric field jump DE, with the resistiv- ity ratio gv/gas a parameter. The normalized displacement n1¼1i s assumed.FIG. 4. The parameter Acversus the normalized displacement n1, with the resistivity ratio gv/gand the electric field jump DEas parameters.FIG. 2. The parameter A0versus the resistivity ratio gv/gwith the displace- ment n1as a parameter.082515-5 L. J. Zheng and M. Furukawa Phys. Plasmas 21, 082515 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 13:34:10we show that the kink modes can carry over the equilibrium current and leads to the formation of current sheet at the sin-gular layer. Due to the vast difference between the equilib- rium currents in the edge plasma and the scrape-off layer, the current sheet can induce the tearing modes. This is anextreme case of the so-called current interchange tearing modes at the plasma edge as pointed out in Ref. 6, with the tokamak edge and scrape-off layer specialties being takeninto consideration. Due to the current jump between the edge plasma and the scrape-off layer, the driving effects for cur- rent interchange tearing modes at the plasma edge can bevery big. Practically, any kink perturbations on the plasma edge tend to induce the tearing modes. The direct conse- quence of the excitation of current interchange tearing modeat the plasma edge is that the confined plasma inside the closed flux surfaces can be peeled off to the scrape-off layer and then to the divertors. As an example, the peeling orpeeling-ballooning modes can become the “peeling-off” modes in this sense. What is more, Ref. 5points out that the pumping out of the confined plasma in the closed flux surfaces to the scrape- off layer can enhance the scrape-off-layer current, especially because the plasma edge usually carries the negativecharges, while the divertor sheets are excessive in the posi- tive charges. The scrape-off-layer current can further drive the tearing modes and cause the positive feedback process.Therefore, the current work can help to explain further the edge localized modes in the H-mode confinement. Note that there is a similarity between the edge localized modes and the tokamak major disruptions. In the edge local- ized mode case, the scrape-off layer current is excited; while in the disruption case, the halo current is induced. Both areexplosive nonlinear processes and involve plasma and wall interaction. One is in a small scale and the other is in a large scale. The peeling-off of the plasma confined in the closedflux surfaces to the scrape-off layer or wall due to the current interchange tearing modes at the plasma edge may also help to explain the disruption, especially the generation of thehalo current and its feedback. In passing, we note that the current work has not included the neoclassical tearing mode effects, 12,13although, in principle, the current interchange can include theinterchange of the bootstrap current. We also point out that the current work is based on the resistive MHD theory andother small island effects, such as the finite Larmor radius effect 10and the effects induced by the transport current,11 have not been included. These will be investigated in the future. In conclusion, the possible excitation of current inter- change tearing modes at the plasma edge due to the currentjump indicates that the tokamak edge confinement can be worse than the expectation based on the pressure driven (or kink) instabilities alone. ACKNOWLEDGMENTS This research was supported by U. S. Department of Energy, Office of Fusion Energy Science: Grant No. DE-FG02-04ER-54742 and by JSPS KAKENHI Grant No.23760805. 1F. Wagner, G. Becker, K. Behringer, D. Campbell, A. Eberhagen, W. Engelhardt, G. Fussmann, O. Gehre, J. Gernhardt, G. V. Gierke, G. Haas, M. Huang, F. Karger, M. Keilhacker, O. Kl €uber, M. Kornherr, K. Lackner, G. Lisitano, G. G. Lister, H. M. Mayer, D. Meisel, E. R. M €uller, H. Murmann, H. Niedermeyer, W. Poschenrieder, H. Rapp, H. R €ohr, F. Schneider, G. Siller, E. Speth, A. St €abler, K. H. Steuer, G. Venus, O. Vollmer, and Z. Y €u,Phys. Rev. Lett. 49, 1408 (1982). 2H. R. Wilson, P. B. Snyder, G. T. A. Huysmans, and R. L. Miller, Phys. Plasmas 9, 1277 (2002). 3P. B. Snyder, H. R. Wilson, J. R. Ferron, L. L. Lao, A. W. Leonard, T. H. Osborne, A. D. Turnbull, D. Mossessian, M. Murakami, and X. Q. Xu, Phys. Plasmas 9, 2037 (2002). 4L. J. Zheng, H. Takahashi, and E. D. Fredrickson, Phys. Rev. Lett. 100, 115001 (2008). 5H. Takahashi, E. D. Fredrickson, M. J. Schaffer, M. E. Austin, T. E.Evans, L. L. Lao, and J. G. Watkins, Nucl. Fusion 44, 1075 (2004). 6L. J. Zheng and M. Furukawa, Phys. Plasmas 17, 052508 (2010). 7P. H. Rutherford, Phys. Fluids 16, 1903 (1973). 8G. M. Staebler and F. L. Hinton, Nucl. Fusion 29, 1820 (1989). 9F. L. Hinton, in Handbook of Plasma Physics , edited by M. N. Rosenbluth and R. Z. Sagdeev (North-Holland, Amsterdam, 1983), Vol. 1, pp. 147. 10F. L. Waelbroeck, J. W. Connor, and H. R. Wilson, Phys. Rev. Lett. 87, 215003 (2001). 11R. Fitzpatrick, Phys. Plasmas 2, 825 (1995). 12R. Carrera, R. D. Hazeltine, and M. Koschenreuther, Phys. Fluids 29, 899 (1986). 13J. D. Callen, W. X. Qu, K. D. Siebert, B. A. Carreras, K. C. Shang, and D. A. Spong, Plasma Physics and Controlled Nuclear Fusion Research (International Atomic Energy Agency, Vienna, 1987), Vol. 2, p. 157.082515-6 L. J. Zheng and M. Furukawa Phys. Plasmas 21, 082515 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.113.111.210 On: Fri, 19 Dec 2014 13:34:10

1.4894003.pdf

In situ transmission electron microscopy of individual carbon nanotetrahedron/nanoribbon structures in Joule heating Yusuke Masuda, Hideto Yoshida, Seiji Takeda, and Hideo Kohno Citation: Applied Physics Letters 105, 083107 (2014); doi: 10.1063/1.4894003 View online: http://dx.doi.org/10.1063/1.4894003 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in In-situ high resolution transmission electron microscopy observation of silicon nanocrystal nucleation in a SiO2 bilayered matrix Appl. Phys. Lett. 105, 053116 (2014); 10.1063/1.4892658 Thermal conductivity measurement of individual Bi2Se3 nano-ribbon by self-heating three-ω method Appl. Phys. Lett. 102, 043104 (2013); 10.1063/1.4789530 Local structure of titania decorated double-walled carbon nanotube characterized by scanning transmission X-ray microscopy J. Chem. Phys. 136, 174701 (2012); 10.1063/1.4706515 Graphitization of amorphous carbon on a multiwall carbon nanotube surface by catalyst-free heating Appl. Phys. Lett. 99, 091907 (2011); 10.1063/1.3630132 In situ observations of carbon nanotube formation using environmental transmission electron microscopy Appl. Phys. Lett. 84, 990 (2004); 10.1063/1.1646465 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 69.166.47.144 On: Tue, 25 Nov 2014 08:31:59In situ transmission electron microscopy of individual carbon nanotetrahedron/nanoribbon structures in Joule heating Yusuke Masuda,1Hideto Y oshida,2Seiji Takeda,2and Hideo Kohno3,a) 1Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan 2The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan 3School of Environmental Science and Engineering, Kochi University of Technology, Kami, Kochi 782-8502, Japan (Received 12 June 2014; accepted 12 August 2014; published online 25 August 2014) Collapse of a carbon nanotube results in the formation of a nanoribbon, and a switching of the collapse direction yields a nanotetrahedron in the middle of a nanoribbon. Here, we report in-situ transmission electron microscopy observations of the behavior of carbon nanotetrahedron/nanoribbonstructures during Joule heating to reveal their thermal stability. In addition, we propose that the observed process is related to the formation process of the structure. VC2014 AIP Publishing LLC . [http://dx.doi.org/10.1063/1.4894003 ] Stability of nanomaterials such as nanotubes and nano- wires under Joule heating is crucial when they are utilized for electronic devices and wiring; therefore, the behavior of nanomaterials under Joule heating has been investigated bymeans of transmission electron microscopy (TEM) by many research groups. 1–4For example, we reported in-situ TEM observations of Joule heating of nanowires such as Sinanochains 5,6and SiC nanowires.7,8These studies show that both Si nanochains and SiC nanowires are converted into carbon-nanotubes by Joule heating. In the conversion of Sinanochains to carbon nanotubes, the carbon source is the sur- face carbon contamination, and the empty core of the nano- tube is formed by vaporization of the Si oxide of the chains.In the conversion of SiC nanowires to carbon nanotubes, the graphitization of SiC nanowires is induced by Si vaporiza- tion. One of the important points of the transformation byJoule heating lies in the possibility to convert a highly resis- tive nanostructure (Si nanochain) to an excellent conductor (carbon nanotube). The relative ease of Joule heating—asimple application of high current by microprobes—makes the nanostructures transformations a very important candi- date for nanowiring applications. It is therefore clear thatstructural changes of nanomaterials by Joule heating are an important topic with yet undiscovered possibilities. In the final analysis, both the good durability and the structuralchange can be utilized if the behavior is understood well. We previously reported the formation of carbon nano- ribbons by flattening of carbon nanotubes, and the formationof nanotetrahedra by switching of the flattening direction (see Fig. 1). 9The structure consisting of nanotetrahedra inside a nanoribbon host is interesting since it may modulatethe charge transport properties and could be useful for nano- devices. In addition, a junction of a nanotetrahedron and a nanoribbon could be utilized to change the direction of nano-wiring. All these possible applications require knowledge of the durability of the nanostructures against Joule heating. In this study, we investigate the structural changes and durabil-ity of the nanotetrahedron/nanoribbon structure by means ofin-situ TEM observation. We show that carbon nanotetrahe- dra have an excellent thermal durability and do not change their shape up to the temperature at which carbon nanorib- bons are broken off near the electrode. In addition, weobserved a process in which a carbon nanotetrahedron was absorbed in the tip of a W probe keeping its shape of tetrahe- dron. We propose that this could be the reverse process of itsformation, or provides a clue to the understanding of the for- mation mechanism of the carbon nanotetrahedra. We fabricated the carbon nanotetrahedron/ribbon struc- tures by the chemical vapor deposition (CVD) method. A Si (100) substrate was roughened with SiC powder, then a 20 nm thick film of iron was deposited on the substrate at apressure of 1.0 /C210 /C03Pa. The sample was sealed in an evac- uated silica tube (inner diameter 6 mm, length about 20 cm) with 0.8 mg of hexadecanoic acid [C 15H31C(¼O)OH] as the carbon source. The tube was heated to 1000/C14C for 30 min, followed by cooling down to room temperature. Grown FIG. 1. TEM image of a carbon nanotetrahedron formed in the middle of a flattened multiwalled carbon nanotube.a)kohno.hideo@kochi-tech.ac.jp. URL: http://www.scsci.kochi-tech.ac.jp/kohno/ . 0003-6951/2014/105(8)/083107/5/$30.00 VC2014 AIP Publishing LLC 105, 083107-1APPLIED PHYSICS LETTERS 105, 083107 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 69.166.47.144 On: Tue, 25 Nov 2014 08:31:59nanotetrahedron/ribbon structures were mounted on a Au wire. We used a commercial piezo-driven micromanipulator system, Nanofactory TEM-STM holder, to apply voltage and measure electric current, and the Au wire was set in theholder. The tip of a mobile W electrical probe was located near a nanotetrahedron structure using the micromanipulator, so the nanotetrahedron structure was situated between the tipof the W probe and the Au wire. Then a voltage was applied between the W probe and the Au wire, which increased as a linear function of time. Individual nanotetrahedron/ribbon structures were observed during Joule heating on a TEM. The CCD camera images were recorded at a rate of 2.6frames per second with a resolution of 512 /C2512 pixels. Figs. 2and 3show an in-situ TEM observation of a carbon nanotetrahedron/nanoribbon structure during Jouleheating. The nanoribbon was about 50 nm in width and a nanotetrahedron was located about 200 nm apart from the W probe. From Figs. 2(a) to2(c) as the applied voltage was increased, we did not observe any marked change in the structure of the nanotetrahedron/nanoribbon and the W probe. At the moment of Fig. 2(d), the tip of the W probe changes its shape presumably due to partial melting; how- ever, the nanotetrahedron/nanoribbon structure remained intact. The nanotetrahedron/nanoribbon just moved slightlytoward the W probe, possibly owing to enhanced contact with the molten tip of the W probe. Finally, as a result of Joule heating, a part of nanoribbon was broken near thecontact to the W probe Fig. 2(e). Nevertheless, the nanotetra- hedron did not change its shape. Just before the moment when the nanoribbon structure broke off, the tip of the W probe melted and noticeably changed its shape. Therefore, the temperature at which the nanoribbon structure was broken off was estimated to bearound the melting temperature of tungsten, which is 3695 K for bulk crystal. However the curvature radius of the tip of the W probe is of the order of 10 /C08m; therefore, we have to take account of the size effect which lowers the melting tem- perature below that of bulk W crystal of 3695 K. The follow- ing formula10can be used to estimate of the melting temperature of a nanoparticle: T¼T01/C04 qsLdrs/C0rlqs ql/C18/C192=3 ! ! ; in which Tis the melting point of a nanoparticle, T0is the melting point of the bulk, Lis the latent heat, dis the diame- ter of a nanoparticle, qsis the density of solid phase of a nanoparticle, qlis the density of liquid phase of a nanopar- ticle, rsis the surface tension of solid phase, and rlis the surface tension of liquid phase. The estimated melting tem-perature for a nanoparticle that has the same radius as that of the tip of the W probe was 1676 K, using the flowing values for the parameters: T 0¼3695 K, L¼35 kJ/mol, d¼10 nm, qs¼19 g/cm3,ql¼18 g/cm3,rs¼3.5 N/m, and rl¼2.5 N/m. This temperature is the lowest estimation for the tip of the W probe, since the tip of the W probe is not an isolated nano-particle. Therefore, the actual melting point of the tip of the W probe is considered to be between 1676 K and 3695 K. The local temperature of the tip of the W probe when thebreakdown occurred is considered to be higher than this melting point because the process was very fast and might not be in equilibrium with other part of the W probe. Thebreakdown of the nanoribbon at the contact suggests that the Joule heat was produced mainly at the contact due to the contact resistance, and the temperature of this part of the FIG. 2. A series of TEM images of a nanotetrahedron/ribbon structure dur- ing the first Joule heating. The position of the nanotetrahedron is indicated by the star. The tip of the W probe was attached to the right of the nanorib-bon. The nanoribbon was broken off at the moment between (d) and (e) at the position indicated by the arrow. FIG. 3. Movie of Fig. 2(Multimedia view). [URL: http://dx.doi.org/ 10.1063/1.4894003.1 ]083107-2 Masuda et al. Appl. Phys. Lett. 105, 083107 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 69.166.47.144 On: Tue, 25 Nov 2014 08:31:59nanoribbon is higher than at least the melting point of carbon, 3773 K. The nanotetrahedron was apart from the contact by about 200 nm; therefore, the temperature aroundthe nanotetrahedron should have been slightly lower than this temperature. The nanoribbon was about 800 nm in length between the two electrodes. Given that the contact resistanceof the left contact was very low and the Joule heating was negligible at the left contact, the simple linear temperature distribution gives the estimation of the temperature at thenanotetrahedron to be approximately 2900 K, which is the lowest estimation. After the first Joule heating, the tip of the W probe was moved to make a contact to the nanoribbon again near the nanotetrahedron structure for the second Joule heating as shown in Fig. 4(a). The second in-situ TEM observation revealed that the nanotetrahedron structure was absorbed with keeping its shape to the W probe during Joule heating as shown in Figs. 4(b)–4(d) . Then, a part of nanoribbon wasalso absorbed in the W probe [Figs. 4(d)–4(f) ]. Finally, the nanoribbon structure was broken off again [Fig. 4(h)], where the bias voltage was about 3.2 V and the current was about210lA [Fig. 4(i)]. The movie of the in-situ observation is in Fig.5. We speculate that the phenomenon in which the nanote- trahedron was absorbed in the probe tip might give a clue to the understanding of the formation process of nanotetrahe- dron/nanoribbon structures. In our previous paper, 9we pro- posed a formation mechanism of our nanoribbons and nanotetrahedra, which we call the origami mechanism; when a carbon nanotube is expelled from a Fe catalyst nanopar-ticle, its geometry forces the nanotube’s wall to converge, resulting in the immediate flattening in a superior direction, FIG. 4. A series of TEM images of a nanotetrahedron/ribbon structure duringthe second Joule heating. (b)–(d) The nanotetrahedron (indicated by the arrows) was absorbed to the W probe, then the nanoribbon was broken off between (g) and (h). (i) Current plotted as a function of time. The values of applied voltage and measured current:(a) [2.54 V, 108 lA], (b) [2.70 V, 151lA], (c) [2.92 V, 180 lA], (d) [2.97 V, 185 lA], (e) [3.03 V, 193 lA], (f) [3.08 V, 199 lA], (g) [3.14 V, 207lA], and (h) [3.19 V, /C240lA]. FIG. 5. Movie of Fig. 4(Multimedia view). [URL: http://dx.doi.org/ 10.1063/1.4894003.2 ] FIG. 6. (a) Before Joule heating of a nanoribbon/nanotube structure and (b) after Joule heating. The part of ribbon (upper part) in (a) changed to the tubular form in (b). The values of applied voltage and measured current: (a)[8.97 V, 248.5 nA] and (b) [9.61 V, /C240lA (due to the breakdown of the contact)].083107-3 Masuda et al. Appl. Phys. Lett. 105, 083107 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 69.166.47.144 On: Tue, 25 Nov 2014 08:31:59and a nanotetrahedron is formed if the flattening direction changes during the growth. It is also possible that the whole part of a nanotube is formed first, then it flattens. We think it is possible to build a hypothesis that the process shown inFigs. 4and5is approximately the reverse process of the for- mation process, in which the tip of the W probe worked as a catalyst. If a nanotetrahedron/nanoribbon structure can beabsorbed in a metal catalyst keeping its form, it would also be possible that it is expelled from a metal catalyst forming its shape immediately. Accordingly, the TEM observation in Figs. 4and5supports our origami mechanism for the forma- tion of our nanoribbons and nanotetrahedra. We also examined a nanoribbon/nanotube structure as shown in Figs. 6and7. Its wall number was estimated to be around 27 using its wall thickness. Before the Joule heating,the upper part of the structure was flattened, while the lower part had a tubular form: the inner wall was visible in the lower part showing that it was a tube (Fig. 6(a)). During the Joule heating, the flattened part expanded to take a tubular form then the whole part in the TEM view became a nano- tube Fig. 6(b). The change was so fast and within the frame rate that its details of the transition could not be observed. The W probe located at the upper part became molten by the Joule heating; therefore, the temperature must have been ashigh as the melting point of the tip of the W probe. The local melting point of the W tip shown in Fig. 6is approximately estimated to be 3500 K using d¼100 nm. The experimental fact that the nanoribbon/nanotube did not break suggests that the temperature of the nanoribbon/nanotube was below the melting point of carbon, 3773 K during the structural change.This result suggests that a nanotetrahedron/nanoribbon structure would be thermally more stable than a nanotube/ nanoribbon structure. We speculate that structural defectssuch as five-membered, seven-membered, and eight- membered rings are necessary to form a nanotetrahedron and they are produced simultaneously with its growth,while such structural defects are not necessary to form a flattened nanotube. Some defects would be generated at a nanotube/nanoribbon junction; however, it requires lessdensity of defects than a nanotetrahedron. Therefore, a nanotetrahedron is very stable once it is formed owing to the structural defects, while only the adhesion of the inmostwall to itself by van der Waals force needs to be broken to make a flattened nanotube take a tubular form, making a simple flattened nanotube not as stable as a nanotetrahe-dron. We note that the current measured in the experiment of Fig. 6was much lower than that of Fig. 4. We speculate that this was due to poor contact between the nanoribbonand the W tip in Fig. 6. Senga et al. 11also reported in-situ TEM observations of Joule heating of simple fla ttened multi-walled carbon nanotubes, not nanotetrahedra. When their flattened MWCNT was Joule-heated, a part of the ribbon expanded and took a tubular form. Furthermore, the interface of thetubular and the flattened parts moved in accordance with the intensity of the electric current, namely temperature. The transition between the tubular and the flattened stateswas reversible and as slow as it could be recorded using a CCD camera equipped with their TEM. This slow and re- versible transition between the two states indicates that theenergy barrier between the two states was relatively low and its height was not sensitive to the transitional structuresince the wall number of their MWCNT was only several layers, and also that the difference in energy of the tubular and the flattened states was small with a lower energy forthe flattened state. In contrast, the very fast structural change observed in the nanotube/nanoribbon structure in Figs. 6and7suggests that the energy barrier between the tubular state and the flattened state was relatively high and the height had strong dependence on its transitional struc- ture, and also that the tubular state had a much lower energythan the flattened state since our structure had a thicker wall. It is very likely that once a weak pinning at the nano- tube/nanoribbon interface is broken, it lowers the energybarrier and the structure falls down immediately to the deep ground state, namely the tubular form. It is also considered that the energy barrier was so high for our nanoribbon/nanotetrahedron structures owing to the dense structural defects that the Joule heating could not make them jump over the barrier. In summary, we investigated the behavior of carbon nanotetrahedron/nanoribbon structures during Joule heating by in-situ TEM. Our nanotetrahedron/nanoribbon structureswere thermally stable and did not transform into a tubular form up to a temperature at which they were broken off. This excellent durability implies a certain mechanism of stabiliza-tion of the structure, and promising for application in nanode- vices and nanowiring. We also proposed a hypothesis that the process in which a nanotetrahedron/nanoribbon was absorbedin the W probe was the reverse process of its formation. This work was supported in part by Adaptable and Seamless Technology Transfer Program through Target-driven R&D, Japan Science and Technology Agency and JSPS KAKENHI Grant Number 25246003. H.K. is grateful to Y. Ohno and I. Yonenaga for the support by the inter-university cooperative research program of the Institute for Materials Research, Tohoku University, and to D. J. Arenas for the critical reading of the manuscript. FIG. 7. Movie of Fig. 6(Multimedia view). [URL: http://dx.doi.org/ 10.1063/1.4894003.3 ]083107-4 Masuda et al. Appl. Phys. Lett. 105, 083107 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 69.166.47.144 On: Tue, 25 Nov 2014 08:31:591C. Jin, K. Suenaga, and S. Iijima, Nat. Nanotechnol. 3, 17 (2008). 2I. Park, Z. Li, A. P. Pisano, and R. S. Williams, Nano Lett. 7, 3106 (2007). 3J. Y. Huang, S. Chen, Z. F. Ren, G. Chen, and M. S. Dresselhaus, Nano Lett. 6, 1699 (2006). 4T. Westover, R. Jones, J. Y. Huang, G. Wang, E. Lai, and A. A. Talin, Nano Lett. 9, 257 (2009). 5T. Nogami, Y. Ohno, S. Ichikawa, and H. Kohno, Nanotechnology 20, 335602 (2009). 6H. Kohno, T. Nogami, S. Takeda, Y. Ohno, I. Yonenaga, and S. Ichikawa,J. Nanosci. Nanotechnol. 10, 6655 (2010).7H. Kohno, Y. Mori, S. Ichikawa, Y. Ohno, I. Yonenaga, and S. Takeda, Nanoscale 1, 344 (2009). 8H. Kohno, Y. Mori, S. Ichikawa, Y. Ohno, I. Yonenaga, and S. Takeda, Appl. Phys. Express 3, 055001 (2010). 9H. Kohno, T. Komine, T. Hasegawa, H. Niioka, and S. Ichikawa, Nanoscale 5, 570 (2013). 10J. Wang, H. L. Duan, Z. P. Huang, and B. L. Karihaloo, Proc. R. Soc. A 462, 1355 (2006). 11R. Senga, K. Hirahara, and Y. Nakayama, Appl. Phys. Lett. 100, 083110 (2012).083107-5 Masuda et al. Appl. Phys. Lett. 105, 083107 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 69.166.47.144 On: Tue, 25 Nov 2014 08:31:59

1.4894858.pdf

Atom-probe tomographic study of interfaces of Cu2ZnSnS4 photovoltaic cells S. Tajima, R. Asahi, D. Isheim, D. N. Seidman, T. Itoh, M. Hasegawa, and K. Ohishi Citation: Applied Physics Letters 105, 093901 (2014); doi: 10.1063/1.4894858 View online: http://dx.doi.org/10.1063/1.4894858 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optical and electrical properties study of sol-gel derived Cu2ZnSnS4 thin films for solar cells AIP Advances 4, 097115 (2014); 10.1063/1.4895520 Intermixing at the absorber-buffer layer interface in thin-film solar cells: The electronic effects of point defects in Cu(In,Ga)(Se,S)2 and Cu2ZnSn(Se,S)4 devices J. Appl. Phys. 116, 063505 (2014); 10.1063/1.4892407 Bandgap engineering of Cu2CdxZn1−xSnS4 alloy for photovoltaic applications: A complementary experimental and first-principles study J. Appl. Phys. 114, 183506 (2013); 10.1063/1.4829457 Impact of KCN etching on the chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers Appl. Phys. Lett. 99, 152111 (2011); 10.1063/1.3650717 Loss mechanisms in hydrazine-processed Cu 2 ZnSn ( Se , S ) 4 solar cells Appl. Phys. Lett. 97, 233506 (2010); 10.1063/1.3522884 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 75.102.255.44 On: Fri, 21 Nov 2014 19:15:40Atom-probe tomographic study of interfaces of Cu 2ZnSnS 4 photovoltaic cells S. Tajima,1,a)R. Asahi,1D. Isheim,2D. N. Seidman,2T. Itoh,1M. Hasegawa,1and K. Ohishi1 1Toyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan 2Northwestern University, Evanston, Illinois 60208-3108, USA (Received 16 June 2014; accepted 26 August 2014; published online 5 September 2014) The heterophase interfaces between the CdS buffer layer and the Cu 2ZnSnS 4(CZTS) absorption layers are one of the main factors affecting photovoltaic performance of CZTS cells. We have studied the compositional distributions at heterophase interfaces in CZTS cells using three-dimensional atom-probe tomography. The results demonstrate: (a) diffusion of Cd into theCZTS layer; (b) segregation of Zn at the CdS/CZTS interface; and (c) a change of oxygen and hydrogen concentrations in the CdS layer depending on the heat treatment. Annealing at 573 K after deposition of CdS improves the photovoltaic properties of CZTS cells probably becauseof the formation of a heterophase epitaxial junction at the CdS/CZTS interface. Conversely, segregation of Zn at the CdS/CZTS interface after annealing at a higher temperature deteriorates the photovoltaic properties. VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4894858 ] Recently, Cu 2ZnSnS 4(CZTS), a chalcopyrite system, is one of the candidate materials for new photovoltaic cells.1–5 This is because: (1) it has a high absorption coefficient; (2) its constituents are nontoxic and abundant in the earth’s crust; and (3) it has a band-gap energy ( Eg) of 1.4 eV, which is an ideal value for photovoltaic applications. The reported efficiency of CZTS cells is, however, below 10%,2–5 demanding further improvements. In the case of Cu(In,Ga)Se 2(CIGS), for which a thin- film based solar cell with a high efficiency has been demon- strated already, an absorber layer (CIGS) is usually coveredby a buffer layer (CdS is generally utilized) forming a p-n junction at the interface between these layers. Because the formation of the buffer layer affects greatly the conversionefficiency, 6it is critical to understand and control the hetero- phase interface between the buffer layer and the absorption layers as a key route for improving photovoltaic perform-ance. Recently, the interface between the buffer layer and the CIGS layers has been studied in detail. 7,8For example, it was suggested that a p-n homojunction was formed as aresult of diffusion of Cd into CIGS, leading to a reduction of the recombination at the interface. 6,7 In the case of CZTS cells, the CdS buffer layer and the CdS/CZTS interface are the important factors for improving the photovoltaic properties of CIGS cells. In contrast to the CIGS cells, however, the details of the CdS/CZTS interfacehave not been studied. Although CZTS and CIGS are at first sight similar, the cations are different, namely, Zn and Sn versus In and Ga, and the anions, S versus Se, which mayresult in significantly different interfaces with the CdS buffer layer. It is therefore absolutely imperative to study crystallin- ity, interdiffusion, and segregation at the CdS/CZTS inter-face to achieve a high conversion efficiency in CZTS cells. While conventional surface analysis tools, Auger elec- tron spectroscopy (AES) and secondary ionization massspectroscopy (SIMS) with sputte r etching, are often utilizedto study heterophase interfaces, these analyses cannot resolve the complicated atomic -scale distributions of atoms in the three-dimensional nm-scale interfacial region. Scanning transmission electron microscopy and energy dispersive spectroscopy (STEM-EDS) have nm-scale andeven atomic scale resolution, but they only provide two- dimensional information in a gi ven cross-section, and they do not determine directly elemen tary atomic distributions. In particular, Cu and Zn or Cd and Sn are generally indis- tinguishable from one another even using STEM high-angle annular dark-field imaging (STEM-HAADF). These limita-tions of conventional analyses have left the detailed CdS/ CZTS interface inaccessible. Herein, we present a study of the compositional distribu- tions at heterophase interfaces in CZTS cells using three- dimensional atom-probe tomography (3-D APT). 9,10The 3-D APT analyses, thanks to its unique capability to combineatomic scale resolution in three dimensions with quantitative chemical analyses, are the most suitable tool for this purpose. We report on direct observations of diffusion of Cd and Znat CdS/CZTS interfaces in CZTS cells, and correlations between the interfacial structure and composition, and photo- voltaic efficiency. We prepared CZTS cells as described earlier. 5Mo elec- trode layers were deposited on alkali glass substrates utiliz- ing sputtering. The CZTS absorber layers were formedby sulfurizing the CZTS precursor utilizing H 2S gas. The overall composition of the CZTS layer was measured by induction coupled plasma (ICP) spectroscopy. CdS bufferlayers were deposited onto the CZTS layers by a chemical bath deposition (CBD) method, and then the specimens were annealed at 473, 573, or 673 K in an N 2atmosphere. Then, ZnO:Ga (GZO) window layers and Al surface electrodes were deposited. The microstructure of the CZTS cells was observed by TEM. The photovoltaic properties weremeasured under a simulated AM 1.5 global spectrum and 1000 W/m 2illumination. The photovoltaic properties [con- version efficiency ( g), short-circuit current density ( JSC),a)e0954@mosk.tytlabs.co.jp 0003-6951/2014/105(9)/093901/4/$30.00 VC2014 AIP Publishing LLC 105, 093901-1APPLIED PHYSICS LETTERS 105, 093901 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 75.102.255.44 On: Fri, 21 Nov 2014 19:15:40open-circuit voltage ( VOC), and fill factor ( FF)] of the CZTS cells are given in Table I. The specimen microtips, which include regions of inter- est, were lifted out of the bulk specimens and sharpened to a radius of curvature of less than 50 nm, using a dual-beam focused ion-beam (FIB) microscope. APT analyses wereperformed using a local-electrode atom-probe tomograph LEAP 4000XSi (Cameca, Madison, WI). 9Laser-assisted evaporation of the surface atoms from a microtip wasachieved using an applied voltage of /C242 kV dc and picosec- ond ultraviolet laser pulses with a wavelength of 355 nm. During APT analyses, the samples were maintained at 30 K.The base pressure of the ultrahigh-vacuum chamber was approximately 10 –13Pa during the analyses, and a laser pulse energy of 25 pJ was employed at a pulse repetition rate of250 kHz to achieve a target evaporation rate of 0.002–0.01 atom pulse /C01. Three-dimensional reconstructions and data evaluation were performed using Cameca’s IVAS3.6.6 code.Proximity histogram concentration profile 9,10analyses of the CdS/CZTS interface are a superior approach for determining three-dimensional atomic distributions from a non-flat inter-face and it is analytically evaluated from the reconstructed data set of atomic positions normal to the defined 3-D iso- concentration surface of a given element. We note one difficulty for 3-D APT to distinguish among ions having the same mass-to-charge ratios ( m/n). 9In the case of CZTS cells, the following ions with the same m/nvalues may be detected: ( 32S2þand64Znþ), (32S34Sþand 66Znþ), (116Snþand116Cdþ), and (16O2þand32Sþ). In our study, the relative contributions to m/n¼64 and 66 from S2þand Znþ, respectively, were determined from the natural isotopic abundances. Alternatively, m/n¼116 was omitted from the analyses because of difficulty of distinguishingbetween 116Snþand116Cdþ, and the respective elemental compositions were corrected based on the natural isotopic abundances. For m/n ¼32, we assigned it to be Sþ. Additional overlaps between complex molecular ions con- taining32So r34S and64Zn or66Zn lead to underestimating or overestimating the Zn and S concentrations. The oxygenconcentration is based on the16O peak and could be underes- timated due to the16O2þand32Sþoverlap mentioned above. We investigated the relationship between photovoltaic properties and annealing temperatures after CBD of the CdSbuffer layer 11and the results are summarized in Table I. The photovoltaic efficiency of the CZTS cells increased with increasing annealing temperature and had a maximum valueat 573 K and then decreased at 673 K. A similar behavior was also reported for the CIGS system. 12To understand the reason for this behavior, we first investigated the microstruc- ture of the CdS/CZTS interface by TEM. Interestingly, an epitaxial junction was observed at the CdS/CZTS interfaceafter annealing at 573 K, while it was not observed after annealing at 473 K, Fig. 1. The crystallographic orientation relationship between CZTS and CdS was CZTS [100] // CdS[100] or CZTS {010} // CdS {010}. The epitaxial junction at the CdS/CZTS interface would reduce recombination at this interface and concomitantly improve the photovoltaic prop-erties. It is thus one of the reasons that annealing at 573 K improves the photovoltaic properties more than annealing 473 K. Alternatively, the photovoltaic properties of CZTScells decreased rapidly for annealing at 673 K. This decrease in the photovoltaic properties could be caused by a solid- state reaction between CdS and CZTS. To clarify this point,we investigated the 3-D atomic scale elemental distributions at the interface by 3-D APT. First, we evaluated the CZTS layer by 3-D APT. Figure 2displays 3-D elemental distribution of the intra-granular CZTS grain having a volume size of about 50 /C2300 nm 3. The elemental distribution was uniform over the CZTS grainand no secondary phases were detected, and hence the CZTS layers were homogeneous. Table IIgives the average compo- sition of the CZTS grain. The composition is approximatelyin agreement with the ICP results. The concentration of Na, which was assumed to diffuse from the glass substrate, was less than 40 atomic ppm. Recently, 3-D elemental distribu-tions of a Cu 2ZnSnSe 4(CZTSe) grain by 3-D APT were reported.13In contrast to CZTS, small variations in the Sn and Se concentrations in the CZTSe grain were observed,and this variation could have been caused by the formation of secondary phases. Next, specimen microtips were targeted near the CdS/CZTS interface and analyzed by APT. Figure 3displays proximity histogram concentration profiles of Zn and Cd with respect to distance from a 10 at. % Cu isoconcentrationsurface, representing the CdS/CZTS interface. The data of Cu and Sn were omitted because a diffusion of Cu and Sn into a CZTS layer was not observed. Table IIIindicates the relationships between the annealing temperature andTABLE I. Photovoltaic properties of CZTS cells after annealing at different temperatures following CBD-CdS deposition. ( g: conversion efficiency, JSC: short-circuit current density, VOC: open-circuit voltage, and FF: fill factor.) Annealing temperature (K) g(%) JSC(mA/cm2)Voc(V) FF(%) 473 6.6 17.5 0.62 61 573 7.0 19.7 0.61 58673 3.1 7.1 0.37 50 FIG. 1. Cross-sectional TEM micro- graphs and electron diffraction analysis of the interface between CdS andCZTS for a CZTS cell annealed at 573 K after deposition of CdS.093901-2 Tajima et al. Appl. Phys. Lett. 105, 093901 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 75.102.255.44 On: Fri, 21 Nov 2014 19:15:40elemental compositions near the CdS/CZTS interface, where the elemental compositions are evaluated by considering all possible atomic and molecular ions in the 3-D APT massspectra and also subtracting the background white noise from the mass spectra. The small concentration of Cd in the CZTS layer detected for the 473 K annealing sample wasconsidered to be the background count of mass spectrum, which was confirmed by cross-check measurements using AES, SIMS, STEM-EDS, and X-ray photoelectron spectros-copy (XPS). Alternatively, the apparent Zn concentration of about 8 at. % in CdS layer, Fig. 3, was concluded to reflect overlaps between complex molecular ions containing 32So r 34S and64Zn or66Zn, which could not be corrected for based on isotopic abundances, and does not mean the presence of significant amounts of Zn in the CdS layer. The Cd concen-tration, Fig. 3, was less than 50 at. % due to the elimination of the 116Cdþions whose m/n value overlaps with116Snþ, see above. For annealing at 473 K, the elemental distribution reveals a chemically sharp CdS/CZTS interface. Interdiffusion, such as diffusion of Cd into a CZTS layer, therefore did not occurat this temperature. Conversely, at 573 K and at 673 K, segre- gation of Zn occurred at the CdS/CZTS interface. Interfacial segregation of Zn could lead to the formation of impurityphases, such as ZnS and a solid solution of (Cd,Zn)S at the CdS/CZTS interface. 14This is because CdS and ZnS can form a continuous series of solid-solutions, while stable ter-nary compound phases involving Cu-Zn-S or Sn-Zn-S do not exist. The segregation of Zn increases with increasingannealing temperature. There is an optimum Zn concentration at the CdS/CZTS interface, as realized by annealing at 573 K,which yields a high photovoltaic efficiency. There is another intriguing situation at the interface. Table IIIalso indicates that there is a small concentration of Cd in the CZTS layer diffusing from the CdS/CZTS interface when the specimen was annealed at 573 and 673 K. This Cd may substitute for Zn in CZTS and thus considered to be aconcomitant diffusion with the segregation of Zn occurring at the CdS/CZTS interface. This is understood by the so-called kick-out mechanism. 15Maeda et al. reported, based on first-principles calculations, that coexistence of Cd at a Cu site and a Cu vacancy can be easily formed in the CZTS and CdS systems.16This result is inconsistent with our observations. Alternatively, Nagoya et al . showed that the dominant acceptor in p-type CZTS is Cu at a Zn site.17 The substitution of Cd at a Zn site in CZTS, if it occurs preferentially, could cause a decrease in the concentration of Cu at a Zn site and result in a decrease of the carrier concentration. Figure 4displays proximity histogram concentration profiles of oxygen for an isoconcentration surface of 10 at. % Cu at the CdS/CZTS interface as measured by 3-D APT.When annealing at 473 K, oxygen and hydroxide exist in the CdS layer as residual substances of CBD and segregate at the interface, forming Cd(S,O,OH). Therefore, the formationof Cd(S,O,OH) is also a reason why annealing at 473 K did not yield a sufficient photovoltaic efficiency. A trace amount of Cd(S,O,OH) and the resultant interfacial defects couldcontribute to recombination. 18After the 573 K annealing step, the concentration of oxygen decreased and Cd(S,O,OH) transformed to CdS. Consequently, the crystallinity of theCdS layers was also improved with the annealing as shown in Fig. 1. In case of 673 K, segregation of oxygen at the inter- face was observed, which was caused by the diffusion of aminute amount of residual oxygen from inner CZTS and CdS layers to the interface. We attribute this improvement of the photovoltaic prop- erties by annealing at 573 K mainly to the following effect: impurity phases such as Cd(S,O,OH) and oxides at the CdS layers and the interface of CdS/CZTS are eliminated. As a FIG. 2. Concentration profile of Cu, Zn, Sn, and S atoms along a cylinder in an APT reconstruction of a CZTS grain. Inset shows 3-D distribution of the atoms and the analysis cylinder. TABLE II. Composition of intragranular CZTS grain via ICP and 3-D APT measurements. Atomic % 3-D APT 3-D APTaICPa Cu 24.1 51.2 46.5 Zn 13.9 29.5 29.1Sn 9.1 19.3 24.4S 51.5 … …Na <0.004 … … O <0.004 … … aNormalized using Cu þZnþSn¼100%. FIG. 3. APT proximity histogram concentration profiles of Zn and Cd across a CdS/CZTS interface, with respect to a 10 at. % Cu isoconcentration surface (distance: 0 nm), after annealing at 473, 573, and 673 K, respectively. Insetdisplays a 3-D distribution and the 10 at. % Cu isoconcentration surface, representative of the CdS/CZTS interface.093901-3 Tajima et al. Appl. Phys. Lett. 105, 093901 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 75.102.255.44 On: Fri, 21 Nov 2014 19:15:40result, an epitaxial junction at the interface of CdS/CZTS was developed and the JSCincreased by 10% compared to the value at 473 K because of reduction in carrier recombina- tion at the interface of the CdS/CZTS. By contrast, the VOC stayed constant because the compositional modification at the CdS/CZTS interface was not significant. On the other hand, in case of 673 K, an excess Zn concentration at the interface may result in the precipitation of ZnS, which causesa high series resistance in CZTS cells due to the high electric resistivity of ZnS and a high conduction band offset (CBO) at the ZnS/CZTS interface. 19Consequently, the photovoltaic properties of the CZTS cells declined significantly leading to an optimum annealing temperature, at 573 K in our experiments. In summary, 3-D APT measurements have yielded a detailed picture of the CdS/CZTS interface in a photovoltaic cell. The results demonstrate diffusion of Cd into the CZTSlayer, segregation of Zn at the CdS/CZTS interface, and a change of oxygen and hydrogen concentrations in the CdS layer depending on the annealing temperature. Annealing at573 K, after deposition of CdS, improved the photovoltaic properties of CZTS cells because of the formation of a heter- ophase epitaxial junction between a solid-solution of(Cd,Zn)S and CZTS and elimination of Cd(S,O,OH). Alternatively, by annealing at 673 K, an excess segregation of Zn leads to the formation of ZnS at the CdS/CZTS inter-face and reduces the photovoltaic properties. Therefore, it is important to optimize the annealing temperature to improve the photovoltaic properties of CZTS cells. 1K. Ito and T. Nakazawa, Jpn. J. Appl. Phys., Part 1 27, 2094 (1988). 2H .K a t a g i r i ,K .J i m b o ,S .Y a m a d a ,T .K a m i m u r a ,W .S .M a w , T .F u k a n o ,T .I t o ,a n dT .M o t o h i r o , Appl. Phys. Express 1, 041201 (2008). 3B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey, and S. Guha,Prog. Photovoltaics 21, 72–76 (2013). 4H. Hiroi, N. Sakai, and H. Sugimoto, in 26th European Photovoltaic Solar Energy Conference and Exhibition (2011), p. 2448. 5T. Fukano, S. Tajima, and T. Ito, Appl. Phys. Express 6, 062301 (2013). 6P. Reinhard, A. Chirila, P. Bloesch, F. Pianezzi, S. Nishiwaki, S. Buecheler, and A. N. Tiwari, IEEE J. Photovoltaics 3, 572–580 (2013). 7T. Nakada and A. Kunioka, Appl. Phys. Lett. 74, 2444 (1999). 8K. Hiepko, J. Bastek, R. Schlesiger, G. Schmitz, R. Wuerz, and N. A. Stolwijk, Appl. Phys. Lett. 99, 234101 (2011). 9B. Gault, M. P. Moody, J. M. Cairney, and S. P. Ringer, Atom Probe Microscopy , Springer Series in Materials Science Vol. 160 (Springer, New York, Heidelberg, Dordrecht, London, 2012). 10O. C. Hellman, J. A. Vandenbroucke, J. Rusing, D. Isheim, and D. N. Seidman, Microsc. Microanal. 6, 437 (2000). 11M. Hasegawa, S. Tajima, T. Ito, and T. Fukano, in PVSEC-22 Technical Digest (2012), presentation number: 3-O-23. 12S. Kijima and T. Nakada, Appl. Phys. Express 1, 075002 (2008). 13T. Scwarz, O. Cojocaru-Miredin, P. Choi, M. Mousel, A. Redinger, S. Siebentritt, and D. Raabe, Appl. Phys. Lett. 102, 042101 (2013). 14N. Korozlu, K. Colakoglu, and E. Deligoz, Phys. Status Solidi B 247, 1214 (2010). 15R. W. Balluffi, S. M. Allen, and W. C. Carter, Kinetics of Materials (John Wiley & Sons, Inc., Hoboken, New Jersey, 2005), p. 168. 16T. Maeda, S. Nakamura, and T. Wada, Jpn. J. Appl. Phys., Part 1 51, 10NC11 (2012). 17A. Nagoya, R. Asahi, R. Wahl, and G. Kresse, Phys. Rev. B 81, 113202 (2010). 18S. Tajima, H. Katagiri, K. Jimbo, N. Sugimoto, and T. Fukano, Appl. Phys. Express 5, 082302 (2012). 19A. Nagoya, R. Asahi, and G. Kresse, J. Phys.: Condens. Matter 23, 404203 (2011).TABLE III. Composition of the position from CdS/CZTS interface with length of 30 nm at various annealing temperatures after CdS deposition. CdS/CZTS interfaceaCZTS grainb Annealing temperature (K) Znc(at. %) O (at. %) Cdc(at. %) O (at. %) Na (at. %) 473 10 0.2 0 <0.004 <0.004 573 16 0.1 0.7 <0.004 <0.004 673 22 0.2 1.5 <0.004 <0.004 aCu 10 at. % isosurface was identified as CdS/CZTS interface. bLocation of 30 nm from CdS/CZTS interface. cThe value was corrected for the influence of mass spectrum background. FIG. 4. APT proximity histogram concentration profiles of oxygen across the CdS/CZTS interface, here represented by a 10 at. % Cu isoconcentra- tion surface (distance: 0 nm), after annealing at 473, 573, and 673 K, respectively.093901-4 Tajima et al. Appl. Phys. Lett. 105, 093901 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 75.102.255.44 On: Fri, 21 Nov 2014 19:15:40

1.4896476.pdf

Equilibrium reconstruction based on core magnetic measurement and its applications on equilibrium transition in Joint-TEXT tokamak J. Chen, G. Zhuang, X. Jian, Q. Li, Y. Liu, L. Gao, and Z. J. Wang Citation: Review of Scientific Instruments 85, 103501 (2014); doi: 10.1063/1.4896476 View online: http://dx.doi.org/10.1063/1.4896476 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High resolution polarimeter-interferometer system for fast equilibrium dynamics and MHD instability studies on Joint-TEXT tokamak (invited)a) Rev. Sci. Instrum. 85, 11D303 (2014); 10.1063/1.4891603 Influence of plasma diagnostics and constraints on the quality of equilibrium reconstructions on Joint European Torus Rev. Sci. Instrum. 84, 103508 (2013); 10.1063/1.4824200 Measurement of the edge plasma rotation on J-TEXT tokamak Rev. Sci. Instrum. 84, 073508 (2013); 10.1063/1.4815824 Development of KSTAR ECE imaging system for measurement of temperature fluctuations and edge density fluctuationsa) Rev. Sci. Instrum. 81, 10D930 (2010); 10.1063/1.3483209 Observation of instabilities during density limit experiments in the Hefei Tokamak-7 Phys. Plasmas 14, 062504 (2007); 10.1063/1.2745305 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15REVIEW OF SCIENTIFIC INSTRUMENTS 85, 103501 (2014) Equilibrium reconstruction based on core magnetic measurement and its applications on equilibrium transition in Joint-TEXT tokamak J. Chen, G. Zhuang,a)X. Jian, Q. Li, Y . Liu, L. Gao, and Z. J. Wang State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan 430074, China (Received 24 July 2014; accepted 13 September 2014; published online 1 October 2014) Evaluation and reconstruction of plasma equilibrium, especially to resolve the safety factor profile, is imperative for advanced tokamak operation and physics study. Based on core magnetic measurement by the high resolution laser polarimeter-interferometer system (POLARIS), the equilibrium of Joint- TEXT (J-TEXT) plasma is reconstructed and profiles of safety factor, current density, and electron density are, therefore, obtained with high accuracy and temporal resolution. The equilibrium recon- struction procedure determines the equilibrium flux surfaces essentially from the data of POLARIS.Refraction of laser probe beam, a major error source of the reconstruction, has been considered and corrected, which leads to improvement of accuracy more than 10%. The error of reconstruction has been systematically assessed with consideration of realistic diagnostic performance and scrape-offlayer region of plasma, and its accuracy has been verified. Fast equilibrium transitions both within a single sawtooth cycle and during the penetration of resonant magnetic perturbation have been inves- tigated. © 2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4896476 ] I. INTRODUCTION In tokamak experiments, information of plasma equilib- rium, especially on safety factor profile, is extremely im- portant for physics study, as well as for plasma control and tokamak operation.1It is well known that the fine determi- nation of plasma equilibrium mainly depends on the detailed knowledge of core magnetic field. Laser polarimeter based on Faraday-Rotation measurement is considered as one ofthe most reliable diagnostics which is capable to offer the required information. 2Over a few decades in the past, re- searches on plasma reconstruction using polarimetric datahave been carried out on various machines under different conditions. 3–7 For tokamaks with non-circular cross-section, the recon- struction is usually approached directly from Grad-Shafranov equation, and polarimetric data are served as internal con-straint to yield a best fit of plasma equilibrium. 8,9For toka- maks with circular cross-section, the equilibrium flux surfaces of plasma can be well described by circles with shifted cen-ters, in which the trace of probe beam for polarimetric mea- surement can be geometrically divided into sections and de- termined, and the local information of plasma can be obtaineddirectly from line-integrated data by inversion. Of course, other information, for example, pressure profile and periph- eral magnetic field, is also demanded as the input for theplasma equilibrium reconstruction. Nevertheless, the possibil- ity to determine equilibrium flux surfaces by mainly using the line integrated data based on an appropriate modeling is alsoexamined, 10which is significant concerning on the next gen- eration of tokamak-type reactor that the available information may be very limited. Along with the line-integrated data from the laser polarimeter, a numerical algorithm based on a prede- a)Email: ge-zhuang@hust.edu.cnfined modeling should be developed to retrieve local informa-tion for reconstructing the plasma equilibrium. In general, the predefined magnetic flux surfaces modeling in plasma equi-librium state will put restriction on performance of an equi- librium reconstruction procedure. Consequently, the accuracy of the reconstruction in the process is of prior consideration. Besides correct modeling of plasma, a reliable reconstruction procedure should also take all possible effects into account,e.g., refraction of probe beam trace and plasma behavior in Scrape-Off Layer (SOL) region, which could introduce large error in some cases. Motivated to study of plasma equilibrium and Magneto- Hydro-Dynamics (MHD) instability, a high resolution laser polarimeter-interferometer system (POLARIS) has been es-tablished on J-TEXT tokamak recently. 11–15An Equilibrium Reconstruction Procedure (ERP) has been developed to de- termine the plasma equilibrium and offers profiles of safetyfactor, current density, and electron density. The procedure determines equilibrium flux surfaces for the J-TEXT plasma by only using data from POLARIS and peripheral magneticfield measurement. Refraction effect, which is considered as a major error source, is directly included and corrected in the model of probe beam trace. The reliability of ERP for physics study has been systematically examined, and the procedure has been successfully applied for several studies of fast equi-librium transition. The rest of the paper is organized as follows: in Sec. II a brief of J-TEXT POLARIS is given. The principles ofERP and the computing results are presented in Sec. III, along with consideration and examination of reconstruction error under various conditions. In Sec. IV, two equilibrium reconstruction examples, associated with the sawtooth cy- cle and resonance magnetic perturbation penetration, by im- plementing the ERP are reported. The paper is ended by asummary. 0034-6748/2014/85(10)/103501/9/$30.00 © 2014 AIP Publishing LLC 85, 103501-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15103501-2 Chen et al. Rev. Sci. Instrum. 85, 103501 (2014) II. DIAGNOSTIC SETUP J-TEXT POLARIS is based on three-wave technique,16 in which three laser beams in 432 μm with slight frequency shifts ( ∼1 MHz) are used for detection. Two of the laser beams propagating through plasma as probe beams arecounter circular-polarized and collinear, offering information of Faraday angle, while the third beam is served as local os- cillator to yield phase of line-integrated density. J-TEXT POLARIS is featured by its high resolution. In the system the probe beam is expanded using parabolic mir- rors to cover the whole poloidal cross section of plasma (a=25.5 cm). The expanded probe beams propagate through the plasma vertically and are received by a multi-chord mixer array (currently 17 chords with 3 cm chord spacing), offer-ing simultaneous measurements of Faraday angle ( α), and line-integrated density ( ϕ) profiles along the major radius of machine. This feature makes it feasible to determine equilib- rium flux surfaces reliably only with POLARIS data, as will be shown. Furthermore, the temporal resolution of the sys-tem, determined by the frequency shifts among the three laser beams, could reach 1–10 μs typically while the phase reso- lution of the system is ∼1m r a d( ∼0.05 ◦). The fast time re- sponse and high phase resolution enable J-TEXT POLARIS to catch detailed changes of plasma equilibrium. III. EQUILIBRIUM RECONSTRUCTION A. Principles The modeling of equilibrium flux surfaces is of impor- tance for a plasma equilibrium reconstruction procedure. In principle, the equilibrium of plasma in tokamak is governedby Grad-Shafranov equation. By assuming a J-TEXT plasma having large aspect ratio, low beta and circular cross section, the equilibrium flux surfaces can be geometrically consid-ered as circles with shifted centers, as describing by following equations in (R, Z) coordinates: 4 [R−R0−/Delta1(r)]2+Z2=r2, (1) where r is the radius of the flux surfaces, R0is the center of last closed flux surface (LCFS), /Delta1(r) is the displacement func- tion of the centers of flux surfaces. In this way, the geometryof equilibrium flux surfaces is fully determined by /Delta1(r), R 0, and a radius of LCFS. The latter two terms can be obtained from peripheral magnetic field measurement, and determina-tion of /Delta1(r) is therefore the key issue in the reconstruction. It has been shown that a polynomial function of r is a good approximation to /Delta1(r), 3so in ERP /Delta1(r) is depicted by /Delta1(r)=/summationdisplay i=k i=1ci(1−(r/a)2i),i=1...k, (2) where ciare coefficients to be determined. Normally, k =2o r 3 is accurate enough for J-TEXT case. With this model, the equilibrium reconstruction is equivalent to search the best fitof/Delta1(r). On the other hand, refractive effect of the probe beam propagating through a non-uniform plasma would also have a strong impact on the line-integrated measurement since the beam trace would be bent. Thus, the realistic trace of probe FIG. 1. Model of equilibrium flux surfaces and probe beam trace for J-TEXT. beam for J-TEXT POLARIS has to be taken into account. With knowledge of initial condition of beam propagation,traces of probe beam can be fully determined based on the principle of geometrical ray traces, 17 d dl(N(R,Z)/arrowrighttophalfl)=∇ N(R,Z), (3) where l and/arrowrighttophalfl are the path length and unit vector of probe beams, N (R, Z) is the refractive index, which can be con- sidered as a scalar here and determined by electron density profile. Based on Eq. (3), refraction of probe beam trace can be characterized once electron density profile is given. A scheme of the modeling of equilibrium flux surfaces and probe beam trace is depicted in Figure 1. As being shown in Figure 1, the equilibrium flux sur- faces can be divided into n circles with radius ri,i=1 ... n. When n is large enough, poloidal magnetic flux ψand electron density necan be approximated as constants in the region between neighbor circles. The expanded probe beamof POLARIS can be considered as a series of m parallel probe beams with impact parameters R j,j=1 ... m , p a s s - ing through the poloidal plasma cross-section. The intersec-tion between one probe beam and the poloidal circles divides the passing path of the beam into a sum of line segments with different lengths. Defining l ijas the segment length of the ith beam within the region between (j −1)th and jth circles (lij =0i ft h e ith beam does not pass through the jth circle), Fara- day angle αand phase of line-integrated density ϕcan be ex- pressed as follows: αi=cpol./integraldisplay Bl(R,Z)ne(R,Z)dl ≈cpol./summationdisplay j=n j=1ne(rj)ψ/prime(rj)·gij, (4) where gij=lij Rij(∂r ∂R/arrowrighttophalfeZ−∂r ∂Z/arrowrighttophalfeR)|r=rj·/arrowrighttophalflij, ϕi=cint./integraldisplay ne(R,Z)dl≈cint./summationdisplay j=n j=1ne(rj)lij, (5) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15103501-3 Chen et al. Rev. Sci. Instrum. 85, 103501 (2014) FIG. 2. Flow diagram of the equilibrium reconstruction procedure. in which cpol.and cint.are constants, Blis magnetic field along beam direction, gijis defined as geometric factor,/arrowrighttophalfeZand/arrowrighttophalfeR are the unit vectors of (R, Z) coordinates. Above equations can be further written in matrix way: gm×n(nen×1·ψ/prime n×1)=αm×1, (6) lm×nnen×1=ϕm×1. (7) When m >n, Eqs. (6)and(7)present an over-determined problem. For a given equilibrium flux surfaces and probebeam trace, g m×nand lm×ncan be calculated, then the least square solutions of Eqs. (6)and(7)can be resolved to give a b e s tfi to fne(r) and ψ/prime(r), while their residues are defined as δα=/bardblαm×1−gm×n(nen×1·ψ/prime n×1)/bardbl2 (8) δϕ=/bardblϕm×1−lm×nnen×1/bardbl2 describing the fidelity of given equilibrium flux surfaces and probe beam trace to the line-integrated data. The residues should be as small as possible, ideally zero, which implythat equilibrium flux surfaces and probe beam traces perfectly match the line-integrated data. Based on above-mentioned principles, the diagram of the flow of ERP can be drawn out, as illustrated in Figure 2. First, geometry of equilibrium flux surfaces is initialized by assigned values of c i. Then probe beam trace is obtained by an iteration loop: (a) initializes a straight beam trace with- out consideration of refraction; (b) calculates lm×nand solves Eq.(7)to obtain the electron density profile under tempo- rary conditions; (c) utilizes the electron density profile and computes Eq. (3)to obtain a new probe beam trace; and (d) updates the initial beam trace by the new values and iteratessteps (a)–(c) until the beam trace is converged. (In ERP, the iteration is stopped when difference between the new beam trace to the initial one is less than 0.1 mm.) Once the probe beam trace is determined, residues δ αandδϕcan be resolved. The total deviation of current equilibrium flux surfaces isdefined as χ2=ωαδα εα+ωϕδϕ εϕ, (9) where εis the error of measurement, ωis weight factor and its subscripts αandϕdenote the Faraday angle and phase of line-integrated density, respectively. (The weight factors are determined according to the importance of the residues and according measurement error. Normally, ωαis larger than ωϕ because profile of Faraday angle presents more information of core plasma, especially position of magnetic axis.) By op- timizing ciand repeating above steps to achieve a minimum value of χ2, equilibrium flux surfaces that match the exper- imental data best can be obtained. Consequently, best fits ofn e(r) and ψ/prime(r) are resolved from Eqs. (6)and(7), from which safety factor and toroidal current density can be calculated.3 Ideally, the accuracy of ERP is only determined by mea- surement error, as long as the modeling is accurate enough and the computation error is negligible. However, there are still some other items would affect the accuracy of ERP. First,to express the line-integral data into a matrix form, n should be large enough, normally n =30 in ERP. Therefore, m is larger than the actual chord number of POLARIS. Before thereconstruction, a fitting process is required to offer α m×1and ϕm×1used in Eqs. (6)and(7)based on the raw experimental data, respectively. Although the spatial coverage and resolu-tion of POLARIS guarantees the fitting process can be car- ried out properly, this is still a potential error source in the reconstruction. The second item is the evaluation of bound- ary conditions precisely. Practically, the electron density and magnetic field in SOL region is not zero, which will intro-duce additional error on phase of line-integrated density ϕand Faraday angle α. It is very difficult to estimate the contribu- tion related to the SOL region because plasma behavior in thisregion is usually highly asymmetric and the measurement of this region is very limited. As a result, additional phase con- tribution from SOL is not subtracted in the ERP. This is sup-ported by the fact that the additional phase on αis normally below or close to the measurement error and could be ignored. For this reason, the error coming from finite beam size, whichis smaller than the phase contribution from SOL, is also ig- nored. Third, Eq. (6)is mathematically unstable for the core plasma, so that ψ /prime(r) is forced to smooth across the region of magnetic axis to ensure it matches the condition ψ/prime(0)=0. Impact of these effects will be considered and examined in Sec. III B . B. Assessment of errors Three validation tests are done to assess the errors in ERP. The first and second tests mainly examine the accuracy of themodel for equilibrium flux surfaces and probe beam trace sep- arately, while the third test assesses total error of ERP with consideration of actual POLARIS performance and plasmaqualities in SOL region. To perform these tests properly, an equilibrium simu- lation code directly based on Grad-Shafranov equation is used to generate the target plasma. The typical parameters of J-TEXT plasma are used for the simulation (if not specifically This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15103501-4 Chen et al. Rev. Sci. Instrum. 85, 103501 (2014) FIG. 3. Test of the model of equilibrium flux surfaces. Simulated profiles (solid line) and reconstructed profiles (circles) are plotted in Figures 3(a)–3(d), while their relative errors are given in Figures 3(e)–3(h). mentioned): plasma current Ip=180 kA, toroidal field B0 =2 T, center safety factor q0=0.85, edge safety factor qa =3.6 and ne(r)=(ne0−nea)(1−(r/a)2)+nea, where the center electron density ne0=6×1019m−3and boundary electron density nea=3×1018m−3or nea=0, depending on the test purpose. The simulated plasma is used as inputs ofJ-TEXT POLARIS synthesizer to generate Faraday angle data αand phase of line-integrated density data ϕ. Depending on the purpose of test, beam refraction, real diagnostic perfor-mance and plasma properties in SOL region can be added into the synthesizer. Profiles of relative reconstructed errors, ε /Delta1,εq,εj, andεne, are used to characterize the performance of reconstruction, defined as the relative difference of the re- constructed profiles to the simulated profiles if without spec- ification. Under some circumstances the maximum and meanvalues of these error profiles are used to characterize the max- imum and mean errors for simplicity. Since the errors due to the relatively small absolute amplitudes of profiles in the plasma periphery would be incredible large, the maximum and mean relative error are calculated within the center regionof plasma: −0.6<r/a<0.6. In the first case, the Faraday angle and phase of line inte- grated density are synthesized without including real diagnos-tic performance and plasma in SOL region, while refraction of the beam trace is also not considered in both synthesizer and ERP. The test results are given in Figure 3. The simulated pro- files of /Delta1(r), safety factor, current density, and electron den- sity are plotted in Figures 3(a)–3(d) by red solid lines while the reconstructed results are shown in the corresponded fig-ures by blue circles. The relative errors of them are shown in Figures 3(e)–3(h). The results of reconstruction agree with simulated profiles very well, in which the maximum errorsfor all profiles are less than 2% while mean errors are less than 1%. In general these errors could be considered as in- trinsic numerical error of ERP, which is mainly introduced bythe modeling of plasma equilibrium. The test results convince that model of equilibrium flux surfaces used in ERP is reliable and accurate enough for application on J-TEXT. In the second case, the Faraday angle and phase of line- integrated density are synthesized with consideration of re- fraction to test the probe beam trace model, in which the realdiagnostic performance and plasma characteristics in SOL re- gion are not included. With the synthesized data, plasma equi- librium can be reconstructed, as shown by the green circle linein Figures 4(a)–4(d), and the relative errors for each inversed result are also plotted in Figures 4(e)–4(h) using green cir- cle line. It is shown that the errors are much closed to the results of first case. For comparison, reconstruction without considering refraction is performed by disabling the iterationof beam trace in ERP, and corresponding reconstruction re- sults and relative errors are shown in the same figures using blue dashed lines. As shown by the results, when refraction isnot considered, all reconstructed profiles are affected and rel- ative errors are increased to more than 20%. To systematically assessing the model, ERP runs under different density levelswith and without refraction effect, and the results are given in Figure 5. It is clear that without consideration of refraction, the errors of all reconstructed profiles are largely affected and This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15103501-5 Chen et al. Rev. Sci. Instrum. 85, 103501 (2014) FIG. 4. Test of the model of probe beam trace. Simulated profiles (solid line), reconstructed profiles with (solid line with circles), and without consi deration of refraction (dashed line) are plotted in Figures 4(a)–4(d), while corresponding relative errors of reconstruction with (solid line with circles) and without consideration of refraction (dashed line) are given in Figures 4(e)–4(h). increased approximately proportional to the level of electron density. For most cases the maximum relative errors can reach>10%. When refraction is considered, the errors are dramati- cally decreased to ∼3% or lower for all cases. Tests with dif- ferent shapes of density profile show similar results. The rela-tive errors are normally larger than 10% without consideringrefraction, and reduced to ∼3% when refraction is corrected. These results confirm that the model of realistic probe beamtrace is of importance to improve the accuracy of the ERP. The last case concerns on the assessment of reconstruc- tion errors with all effects included. Here, the plasma withinthe SOL region is modeled with uniform electron density FIG. 5. Test of the model of probe beam trace under different levels of electron density. The averaged errors of /Delta1(r) (stars), safety factor (circles), current density (squares), and electron density (diamonds) with (dashed line), and without considering refraction (solid line) are plotted in Figure 5(a), while the maximum errors of that are plotted in Figure 5(b). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15103501-6 Chen et al. Rev. Sci. Instrum. 85, 103501 (2014) FIG. 6. Assessment of error of reconstruction. Simulated profiles (solid line) and errors (shadow) are plotted in Figures 6(a)–6(d), while the relative errors are given in Figures 6(e)–6(h). 3×1018m−3with∼3 cm thick according to Langmuir probe measurements, and with poloidal magnetic field consistent with distribution of current density. Faraday angle and phase of line-integrated density, with respect to the realistic spa- tial coverage and resolution of diagnostic, are generated. The measurement errors, referring to the actual diagnostic perfor-mances, are taken as ε α=0.05◦,εϕ=1◦,εIp=5k A , εBt =0.01 T, εx=5 mm, and εa=5 mm, corresponded to er- rors of Faraday angle, phase of line-integrated density, totalplasma current, toroidal field, position of LCFS, and plasma minor radius, respectively, and are taken into account by adding equivalent random noises to the generated data. TheERP runs 50 times and thus the errors can be evaluated statis- tically. Hereafter, the mean value ε mean and stand deviation δε of the errors in the 50 times can be calculated and total error of reconstruction is assumed as εmean+2δε, i.e., sum of mean value and the 95% confidence region of the errors. The test results are shown in Figure 6. The simulated profiles of /Delta1(r), safety factor, current density, and electron density are plotted in Figures 6(a)–6(d) by red solid lines while the errors of re- construction are shown by the green shadow in Figures 6(e)– 6(h). It is seen that for all profiles the errors of reconstruc- tion are small comparing to the true values; for all profiles the maximum relative errors are around 10% while mean relative errors are around 5% except the boundary region, which is acceptable for most of physics studies. As a summary, results of the validation tests are listed in Table I, along with the conditions of tests. These tests con-firm the feasibility and accuracy of ERP as a tool for experi- mental research of plasma equilibrium, and offer a reference of reconstruction error under current J-TEXT conditions. It is helpful to discuss the impact of reduced chord number to the accuracy of reconstruction, which is probably the case for future reactor. It is no doubt that reduction of chord numberwill lead to larger error when reproduces the whole αandϕ profiles, but it is relievable by carefully choosing chord posi- tions to measure the characteristic spatial points, such as zero-crossing point and turning points of αprofile, to maximize available information. For J-TEXT, it is shown that cases with chord number down to 10 can provide similar performance ascase no. 3 shown in Table I, by optimizing chord positions. Similar result has also been reported elsewhere. 7 TABLE I. Summary of maximum and mean errors of reconstruction under given conditions. Test case no. 1 2 3 Cond. Refraction \√√ Interpolation \\√ SOL effect \\√ Instrum. error \\√ |ε/Delta1(r)|max,|ε/Delta1(r)|mean<2%,<1% 2%, 1% 5%, 3% |εq(r)|max,|εq(r)|mean<2%,<1% 2%, 1% 11%, 4% |εj(r)|max,|εj(r)|mean<1%,<1% 2%, 1% 11%, 6% |εne(r)|max,|εne(r)|mean<1%,<1% 1%, 1% 3%, 1% This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15103501-7 Chen et al. Rev. Sci. Instrum. 85, 103501 (2014) IV. STUDIES OF PLASMA EQUILIBRIUM TRANSITION A. Sawtooth Sawtooth is a Magneto-Hydro-Dynamic instability in core plasma, which could be usually observed by soft-X ray emission18and electron cyclotron emission.19The existence of sawtooth instability is considered associated with the value of center safety factor q0; it is widely agreed sawtooth appears when q0<1 so that this fact could be used as a judgment of the accuracy of inversion method.3–5 Figure 7shows a typical result of J-TEXT sawtooth dis- charge. Total plasma current, line-integrated density, and soft- X ray signal are given in Figures 7(a)–7(c), respectively. Evo- lution of q0,j0, and ne0reconstructed from ERP are shown in Figures 7(d)–7(f), while the evolution of profiles of safety factor, current density, and electron density are shown in Fig- ures 7(g)–7(i). The total plasma current reached the plateau at ∼0.06 s, and the sawtooth activity gradually appeared around ∼0.08 s, as shown on soft-X emission. It is evidenced that the safety factor profile became hollowed in this process, whilethe plasma current profile and electron density profile peaked up. The value of center safety factor q 0decreased to less than 1a f t e r ∼0.076 s, agreed with the soft-X emission results, and finally stabilized at the value of ∼0.85. The value of center safety factor remained below 1 during all the sawtooth cy- cles and did not follow the prediction by the full connection model.20It is worthy to mention that the J-TEXT POLARIS is ca- pable to observe sawtooth instability on both Faraday angle and phase of line-integrated density. To investigate equilib-rium changes during a single sawtooth cycle by ERP, 30 peri- ods of sawtooth under similar plasma conditions are extracted and averaged to smooth out noises of measurement and per-turbation of m =1/n=1 mode, yielding accurate perturba- tion of Faraday angle and phase of line-integrated density dur- ing one sawtooth period. Using the averaged sawtooth data, evolution of safety factor profile, current density profile, and electron density profile can be reconstructed with lower er-rors. For clarity, instead of plotting the equilibrium profiles, the relative changes of profiles of safety factor, current den- sity, and electron density are presented in Figures 8(a)–8(c) individually. τ stis the normalized sawtooth period, and crash of sawtooth happened at τst=0. It is shown that the electron density profile was hollowed at the crash moment, which indi-cated a fast loss of confinement in the core region of plasma. In addition, there was also a quick negative change on cur- rent density profile right at the crash, accompanied with acorresponded increase of safety factor profile. The decrease of center current density was ∼0.1 MA/m 2, about 4% to the equilibrium current density, and correspondingly center safetyfactor increased for ∼0.07, which was still below 1. During the ramp-up phase, the current density and electron density profiles peaked up, while safety factor profile also changed consistently. FIG. 7. Results of typical J-TEXT sawtooth discharge. Total plasma current, line-integrated density, and soft-X ray signal are given in Figures 7(a)–7(c), respectively. The reconstructed q0,j0,a n dne0are shown in Figures 7(d)–7(f), while the evolution of profiles of safety factor, current density, and electron density are shown in Figures 7(g)–7(i). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15103501-8 Chen et al. Rev. Sci. Instrum. 85, 103501 (2014) FIG. 8. Equilibrium reconstruction of a single sawtooth period. Temporal evolutions of safety factor, current density, and electron density profiles areshown in Figures 8(a)–8(c) in sequence.B. Resonant magnetic perturbation penetration External applied resonant magnetic perturbation (RMP) is now considered as a useful tool to control varied instabili- ties in tokamak. However, the nature behind the influence of RMP on plasma would be complicated, and still needed moreinvestigations, for example, the understanding of RMP pen- etration. It is found the RMP could excite tearing instability in tearing-free plasma when its amplitude is strong enough. 21 For a static RMP, the excited mode is also stationary in lab-oratory frame. This process always accompany with largedegrade of confinement and may lead to disruption. Since the phase stays stationary, routine diagnostics, especially the boundary magnetic measurement, for the tearing mode detec-tion is ineffective; it is normally characterized by the increas- ing radial magnetic field measured by saddle coils outside the vacuum vessel. With the capability to determine the plasmaequilibrium and J-TEXT DRMP coils, 12,22study of the im- pact of RMP penetration on plasma equilibrium in details be- comes possible. A typical discharge of RMP penetration is presented in Figure 9. Total plasma current, line-integrated density, soft- X ray signal, Br(n=1) signal, and Mirnov signal are given in Figures 9(a)–9(e), respectively. The reconstructed q0,j0, and ne0obtained from ERP are shown in Figures 9(f)–9(h), along with evolution of the profiles of safety factor, current FIG. 9. Equilibrium reconstruction of penetration of RMP. Total plasma current, line-integrated density, soft-X ray signal, Br(n=1) signal, and Mirnov signal are given in Figures 9(a)–9(e), respectively. Reconstructed q0,j0,a n dne0are shown in Figures 9(f)–9(h), along with the evolution of safety factor, current density, and electron density profiles in Figures 9(i)–9(k). The RMP current is shown by red dotted line in Figure 9(a). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15103501-9 Chen et al. Rev. Sci. Instrum. 85, 103501 (2014) density, and electron density in Figures 9(i)–9(k).T h eR M P was applied at 0.3 s and reached its plateau at ∼0.35 s, shown by red dotted line in Figure 9(a). The penetration began at ∼0.35 s, as seen from the Brsignal. At the meantime the sawtooth activity disappeared and confinement was degraded quickly as seen by soft-X emission. Since the mode waslocked, no apparent magnetic perturbation could be observed on Mirnov signal. As shown by the reconstructed results, from 0.3 s to 0.35 s the center safety factor was less than 1, agreed with the sawtooth on soft-X emission. When penetration started,strong perturbation could be seen on both safety factor and current density profile; however, unlike the drop on soft-X emission or fast increase on B r(n=1) started at 0.35 s, the significant changes on the center safety factor and cen- ter current density occurred at ∼0.36 s or later. At 0.40 s the Br(n=1) reached a constant level, ∼20 ms later the center safety factor and center current density reached another equi- librium state. This time scale is close to the resistive time. On the other hand, in the viewing of current density and elec-tron density profiles, the electron density profile began de- graded gradually, consistent with soft-X emission and B r, and later on became much flatten; but the current density pro-file tended to peak up when the penetration occurred. Such peaking-up profile lasted for about 20 ms and then dropped. From these results it is clear that the growth of island directly degraded the confinement by enhanced particle transport and modified the current redistribution. It took almost 60 ms (from0.36 s to 0.42 s) for the current density (and safety factor) and electron density profiles to reach a new equilibrium state, the whole transition involves particle transport and currentdissipation. V. SUMMARY Based on core magnetic measurement of high resolution POLARIS, plasma equilibrium on J-TEXT tokamak, includ- ing profiles of safety factor, current density, and electron den-sity has been reconstructed with high accuracy and temporal resolution. The equilibrium flux surfaces of J-TEXT plasma are modeled and accurately determined by the ERP. Refrac-tion of probe beam trace is modeled and corrected based on principle of geometrical ray tracing, which improves the ac- curacy of reconstruction for more than 10%. The error of re-construction has been systematically assessed with consider- ation of diagnostics performances and SOL region of plasma, and it is shown that the maximum error of reconstruction inthe center region for profiles of safety factor, current den- sity, and electron density is around 10% while the mean er- ror is around 5%. Using ERP, evolutions of plasma equilib-rium during sawtooth discharge and penetration of RMP have been studied. The center safety factor q 0varies from above 1 to below 1 when sawtooth appears and keeps below 1 dur-ing all the sawtooth cycles, consistent with former observa- tion on other tokamaks. Additionally, the changes of profiles of safety factor, current density, and electron density duringone single sawtooth period are also resolved. For the case of RMP penetration, when penetration begins, electron density profile decreases and flattens simultaneously, while profiles of current density and safety factor become flat with a delay approximately equal to the resistive time, accompanied withtransition of q 0from below 1 to above 1. ACKNOWLEDGMENTS The authors would like to thank Dr. D. Brower, Dr. W. X. Ding, and Dr. K. Gentle for their helpful discussions. This work is supported by the ITER Project Funds of Peo-ple’s Republic of China: Contract No. 2009GB107003, and partly supported by the National Natural Science Foundation of China (Grant No. 11105056) and the JSPS-NRF-NSFC A3Foresight Program in the field of Plasma Physics (NSFC No. 11261140328). 1A. J. H. Donné et al. ,Nucl. Fusion 47, S337–S384 (2007). 2A. J. H. Donné, Plasma Phys. Controlled Fusion 4, B137–B158 (2002). 3H. Soltwisch, Rev. Sci. Instrum. 57(8), 1939 (1986). 4B. W. Rice and E. B. Hooper, Nucl. Fusion 34, 1 (1994). 5L. Zeng, D. L. Brower, and Y . Jiang, Plasma Phys. Controlled Fusion 39, 591–608 (1997) 6F. P. Orsitto, A. Boboc, P. Gaudio, M. Gelfusa, E. Giovannozzi, C. Maz-zotta, A. Murari, and JET EFDA Contributors, Plasma Phys. Controlled Fusion 53, 035001 (2011). 7R. Imazawa, Y . Kawano, and Y . Kusama, Nucl. Fusion 51, 113022 (2011). 8L. L. Lao, J. R. Ferron, R. J. Groebner, W. Howl, H. St. John, E. J. Strait, and T. S. Taylor, Nucl. Fusion 30(6), 1035 (1990). 9J. Blum, E. Lazzaro, J. O’Rourke, B. Keegan, and Y . Stephan, Nucl. Fusion 30(8), 1475 (1990). 10H. K. Park, Plasma Phys. Controlled Fusion 31, 2035–2046 (1989). 11G. Zhuang et al. ,Nucl. Fusion 51, 094020 (2011). 12G. Zhuang et al. ,Nucl. Fusion 53, 104014 (2013). 13J. Chen, L. Gao, G. Zhuang, Z. J. Wang, and K. W. Gentle, Rev. Sci. In- strum. 81, 10D502 (2010). 14J. Chen, G. Zhuang, Z. J. Wang, L. Gao, Q. Li, W. Chen, D. L. Brower, and W. X. Ding, J. Instrum. 7, C01064 (2012). 15J. Chen et al. ,Rev. Sci. Instrum. 85, 10D303 (2014). 16G. Dodel and W. Kunz, Infrared Phys. 18, 773 (1978). 17M. Born and E. Wolf, Principles of Optics , 7th ed. (Cambridge University Press, 1999). 18S. V on Goeler, W. Stodiek, and N. Sauthoff, Phys. Rev. Lett. 33, 1201 (1974). 19Y . Nagayama, G. Taylor, M. Yamada, E. D. Fredrickson, A. C. Janos, K. M. Mcguire, Nucl. Fusion 36, 521 (1996). 20B .B .K a d o m t s e v ,S o v .J .P l a s m aP h y s . 1, 389 (1975). 21N. C. Wang et al. ,Nucl. Fusion 54, 064014 (2014). 22B. Rao et al. ,Fusion Eng. Des. 89, 378–384 (2014). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 132.174.254.155 On: Tue, 23 Dec 2014 12:36:15

1.4897710.pdf

The first principle study of Ni 2 ScGa and Ni 2 TiGa Mustafa Özduran, Kemal Turgut, Nihat Arikan, Ahmet İyigör, and Abdullah Candan Citation: AIP Conference Proceedings 1618, 178 (2014); doi: 10.1063/1.4897710 View online: http://dx.doi.org/10.1063/1.4897710 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1618?ver=pdfcov Published by the AIP Publishing Articles you may be interested in First principles study of hydroxyapatite surface J. Chem. Phys. 139, 044714 (2013); 10.1063/1.4813828 First Principle Study of Sodium Nanoclusters AIP Conf. Proc. 1393, 101 (2011); 10.1063/1.3653629 A first-principles study on the magnetocaloric compound Mn Fe P 2 ∕ 3 Si 1 ∕ 3 J. Appl. Phys. 105, 07A902 (2009); 10.1063/1.3056408 First principles study of magnetism in nanographenes J. Chem. Phys. 127, 124703 (2007); 10.1063/1.2770722 A First Principles Study of Alumina Nanoparticles AIP Conf. Proc. 712, 1583 (2004); 10.1063/1.1766755 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 14 Aug 2015 12:43:11The First Principle Study of Ni 2ScGa and Ni 2TiGa Mustafa Özdurana, Kemal Turgutb, Nihat Arikanc, Ahmet øyigörd, Abdullah Candand a Ahi Evran Üniversitesi Fen Edebiyat Fakültesi Fizik Bölümü, K Õrúehir-TÜRK øYE b Yüksek Lisans Ö ÷rencisi, K Õrúehir-TÜRK øYE cAhi Evran Üniversitesi E ÷itim Fakültesi ølkö÷retim Bölümü, K Õrúehir-TÜRK øYE dAhi Evran Üniversitesi Merkezi Ara útÕrma Laboratuvar Õ, KÕrúehir-TÜRK øYE Abstract. We computed the electronic structure, elastic moduli, vibrational properties, and Ni 2TiGa and Ni 2ScGa alloys in the cubic L2 1 structure. The obtained equilibrium lattice constants of these alloys are in good agreement with available data. In cubic systems, there are three independent elastic constants, namely C 11, C 12 and C 44. We calculated elastic constants in L2 1 structure for Ni 2TiGa and Ni 2ScGa using the energy-strain method. The electronic band structure, total and partial density of states for these alloys were investigated within density functional theory using the plane-wave pseudopotential method implemented in Quantum-Espresso program package. From band structure, total and projected density of states, we observed metallic characters of these compounds. The electronic calculation indicate that the predominant contributions of the density of states at Fermi level come from the Ni 3d states and Sc 3d states for Ni 2TiGa, Ni 3d states and Sc 3d states for Ni 2ScGa. The computed density of states at Fermi energy are 2.22 states/eV Cell for Ni2TiGa, 0.76 states/eV Cell for Ni 2ScGa. The vibrational properties were obtained using a linear response in the framework at the density functional perturbation theory. For the alloys, the results show that the L2 1 phase is unstable since the phonon calculations have imagine modes. Keywords: Band structure, Elastic moduli, DFT, Ductility PACS: 71.20.-b, 62.20.de, 71.15.Mb, 62.20.fk INTRODUCTION The Heusler compounds have attracted scientific and technological interest in the spintronics areas [1]. A large number of the Heusler alloys of X 2YZ stoichiometric composition, where X and Y are generally transition elements and Z is a main group metal, are known to exhibit half-metallic ferromagnetic behavior. These materials are good candidates for devices based on spin injection such as the huge tunnel magnetoresistance (TMR) and giant magnetoresistance (GMR) in magnetoelectronic devices. They can also be used as perfect spin filters and spin- injection devices as an alternative to ferromagnetic 3d metals. Their L2 1 cubic structure belonging to space group 225: Fm-3m consists of four interpenetrating face-centered-cubic (fcc) lattices, in which X atoms occupy the A (0,0,0) and C (1/2,1/2,1/2) sites, Y atoms the B (1/4,1/4,1/4) site and Z occupies the D (3/4,3/4,3/4) site in Wyckoff coordinates. The lattice constants of Ni 2TiGa and Ni 2ScGa compounds were calculated using LMTO-ASA code [2]. Zayak et al. [3] calculated structural, electronic and dynamical by using ab initio calculation of Ni 2TiGa. Their results show phonon anomalies for L2 1 phase Ni 2TiGa. The results of an investigation of the paramagnetic Ni 2TiGa are reported using stoichiometric annealed and quenched from 1000 0C by Kreissl et al. [4]. Magnetic measurements of this compound were made using a SQUID magnetometer in controlled field up to 5.5 T and temperature between 2 and 350 0K. The present article aims at investigating the ground-state properties such as lattice constants, bulk modulus, elastic constants, and band structure properties of both materials by density-functional theory (DFT). COMPUTATIONAL DETAILS The calculations were carried out using a plane wave pseudopotential scheme within density functional theory (DFT) as implemented in the Quantum-ESPRESSO package [5]. The electronic exchange-correlation potential was calculated by the generalized gradient approximation (GGA) using the scheme of Perdew–Burke–Ernzerhof (PBE) [6]. The wave functions were expanded in a plane-wave basis set with a kinetic energy cut-off of 40 Ry. Brillouin- zone integrations were performed using a 10x10x10 k-points mesh. Integration up to the Fermi surface was performed using the smearing technique [7] with smearing parameter r = 0.02 Ry. Elastic constants were obtained International Conference of Computational Methods in Sciences and Engineering 2014 (ICCMSE 2014) AIP Conf. Proc. 1618, 178-181 (2014); doi: 10.1063/1.4897710 © 2014 AIP Publishing LLC 978-0-7354-1255-2/$30.00 178 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 14 Aug 2015 12:43:11by calculating the total energy as a function of volume-conserving strains that break the cubic symmetry. Bulk modulus B, C44, and shear modulus ܥᇱ= (C11-C12)/2 were calculated from hydrostatic pressure e = (į, į, į, 0, 0, 0), tri-axial shear strain e = (0, 0, 0, į, į, į) and volume-conserving orthorhombic strain e = (į, į, (1+į)-2-1, 0, 0, 0), respectively [8]. Hence, B was obtained from 'ா ൌଽ ଶܤGଶ (1) where V is the volume of unstrained lattice cell, and ǻE is the energy variation as a result of an applied strain with vector e = (e1, e2, e3, e4, e5, e6). ܥᇱ was found from 'ா ൌ ܥᇱGଶ (2) The two expressions above yield C11= (3B + 4ܥᇱ)/3, C12= (3B - 2 ܥᇱ)/3, and C44 is given by 'ா ൌଷଶܥସସGଶ (3) We calculated 21 sets of 'ா by varying į from -0.02 to 0.02 in steps of 0.002. Then, we fitted these data to a parabola, and the elastic constants were obtained from quadratic coefficients. In general, hardness is known to be a material parameter that indicates resistance to elastic or plastic deformation, this parameter is the bulk modulus B or the shear modulus G. The shear modulus G of a cubic structure is given by: ܩൌభభିభమାଷరర ହ (4) The anisotropy factor A is given by: ܣൌଶరర ሺభభିభమሻ (5) As A approaches unity the crystal becomes isotropic. We also list some auxiliary quantities which are often quoted in the literature. The young’s modulus for an isotropic solid is related to B and G by the formula: ܧൌଽீ ଷାீ (6) Poisson’s ratio is also of interest ߪൌଵ ଷቀͳെா ଷቁ (7) RESULTS The energy values obtained were fitted to Murnaghan equation of state [9] in order to determine the lattice constant a, the bulk modulus B at zero pressure. The computed structural parameters of Ni 2TiGa and Ni 2ScGa compounds are listed in Table 1, compared with the available results [2-4]. The present computed lattice constants for Ni 2TiGa and Ni 2ScGa are in good agreement with the early calculations. The elastic properties of a cubic single- crystal are completely defined by three independents elastic constants, namely C11, C12 and C44. The obtained values of C11, C12, C44 and C’ for Ni 2TiGa and Ni 2ScGa compounds are presented in Table 1. For properties of ductility and brittleness for Ni 2TiGa and Ni 2ScGa compounds, the ratio of bulk modulus to shear modulus, B/G, was calculated. This ratio can be considered empirical criterion of the extent of the fracture range in the materials [10]. If the ratio of B/G is higher 1.75, then the material behaves in a ductile manner. If it is less than 1.75, then the material demonstrates brittleness. The B/G values are 2.515 and 2.128 for Ni 2TiGa and Ni 2ScGa compounds, respectively. Hence, these materials indicate a ductile nature. The electronic band structures of Ni 2TiGa and Ni 2ScGa compounds obtained using the plane wave pseudopotential method along the higher symmetry directions are shown in Figure 1. The band profiles of Ni 2TiGa and Ni 2ScGa compounds are quite similar to each other. There is no band gap at the Fermi level for both compounds, thus, Ni 2TiGa and Ni 2ScGa compounds exhibit a metallic behavior. The main contribution of Fermi level of Ni2TiGa and Ni 2ScGa compounds come from Ni 3d and Ti 3d states for Ni 2TiGa, and Ni 3d and Sc 3d states for Ni 2ScGa, respectively. From the computed total and partial DOS, it can be seen that there is one sharp peak above the Fermi level of both materials. These peaks are almost centered 1.78 eV for Ni 2TiGa and 2.73 eV for Ni 2ScGa, which are mainly dominated by the Ti 3d states for Ni 2TiGa and Sc 3d states for Ni 2ScGa. Under the Fermi level there are dispersive bands for two materials. These bands of two materials are mainly the contribution of Ni 3d states. 179 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 14 Aug 2015 12:43:11TABLE 1. Calculated lattice constants (in Å), bulk modulus, pressure derivative of the bulk modulus and second order elastic constants (all in GPa) for Ni 2TiGa and Ni 2ScGa in the L2 1 structure. a(A) B c' C 11 C12 C44 G B/G E V Ni2TiGa This work 5,897 163,1 24,4995 197,594 148,595 92,920 65,552 2,515 173,650 0,324 VASP [3] 5.8895 LMTO-ASA [2] 5.889 Ni2ScGa This work 6,050 132 35,139 182,074 111,795 82,441 63,520 2,128 164,762 0,296 LMTO-ASA [2] 6.041 FIGURE 1. The electronic band structure for Ni 2TiGa and Ni 2ScGa compounds along several lines of high symmetry in the Brillouin zone. FIGURE 2. Calculated partial and total DOS for Ni 2TiGa and Ni 2ScGa in the L2 1 phase. CONCLUSION In this article, structural, electronic and elastic properties of Ni 2TiGa and Ni 2ScGa compounds were investigated using the ab initio pseudopotential method within GGA of DFT. The calculated equilibrium lattice constants of Ni2TiGa and Ni 2ScGa compounds are in fairly good agreement with previous results. The electronic calculations predict that Ni 2TiGa and Ni 2ScGa compounds have metallic character for the L1 2 structure. The elastic constants of Ni2TiGa and Ni 2ScGa compounds were calculated. Ni 2TiGa and Ni 2ScGa compounds indicate the ductile manner. Γ KX Γ LX W L−12−8−404Energy (eV)Ni2TiGa (L2 )1 EF Γ KX Γ L X WL−10−505Enerji (eV)Ni2ScGa (L2 1) EF −12 −8 −4 0 4Energy (eV)−9−6−30369DOS (States/eV CELL)Total Ni 4s Ni 3d Ti 4s Ti 3d Ga 4s Ga 3d Ga 4p −10 −5 0 5 Energy (eV)−9−6−30369DOS (States/eV CELL)Total Ni 4s Ni 3d Sc 4s Sc 3d Ga 4s Ga 3d Ga 4p 180 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 14 Aug 2015 12:43:11ACKNOWLEDGMENTS This work was supported by the Ahi Evran University Research Project Unit under Project No. PYO- FEN.4010.14.001. REFERENCES 1. A. Fert, Reviews of Modern Physics 80, 1517-1530 (2008). 2. M. Gilleßen, "Maßgeschneidertes und Analytik-Ersatz: über die quantenchemischen Untersuchungen einiger ternärer intermetallischer Verbindungen", Ph.D. Thesis, Aachen University, 2009. 3. A.T. Zayak, P. Entel, M.K. Rabe, W.A. Adeagbo and M. Acet, Phys. Rev. B 72, 054113, 1-9 (2005). 4. M. Kreissl, K-U Neumann, T. stephens and K.R.A Ziebeck, J. Phys.: Condens. Matter 15, 3831-3839 (2003). 5. Giannozzi P, Baroni S, Bonini N, Calandra M, Car R, Cavazzoni C, et al. J Phys Condens Matter 21, 395502-? (2009). 6. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77, 3865-3868 (1996). 7. M. Methfessel, A.T. Paxton, Phys. Rev. B 40, 3616-3621 (1989). 8. S.Q. Wang, H.Q. Ye, Phys. Stat. Sol. b 240, 45-54 (2003). 9. F.D. Murnaghan, Proc. Natl. Acad. Sci. USA 30, 244-247 (1944). 10. S.F. Pugh, Philos. Mag. 45, 823-843 (1954). 181 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. 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1.4899207.pdf

The impact of disorder on charge transport in three dimensional quantum dot resonant tunneling structures B. Puthen-Veettil, R. Patterson, D. König, G. Conibeer, and M. A. Green Citation: Journal of Applied Physics 116, 163707 (2014); doi: 10.1063/1.4899207 View online: http://dx.doi.org/10.1063/1.4899207 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electromagnetically induced transparency of charge pumping in a triple-quantum-dots with Λ -type level structure Appl. Phys. Lett. 102, 163116 (2013); 10.1063/1.4803072 Reduction in dark current using resonant tunneling barriers in quantum dots-in-a-well long wavelength infrared photodetector Appl. Phys. Lett. 93, 131115 (2008); 10.1063/1.2996410 Finite-frequency current (shot) noise in coherent resonant tunneling through a coupled-quantum-dot interferometer J. Appl. Phys. 104, 033532 (2008); 10.1063/1.2967710 Spectral function and responsivity of resonant tunneling and superlattice quantum dot infrared photodetectors using Green’s function J. Appl. Phys. 102, 083108 (2007); 10.1063/1.2799075 Tunneling resonances and Andreev reflection through an interaction quantum dot coupled with two half metals and a superconductor J. Appl. Phys. 99, 08F713 (2006); 10.1063/1.2173625 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.30.242.61 On: Wed, 10 Dec 2014 00:19:27The impact of disorder on charge transport in three dimensional quantum dot resonant tunneling structures B. Puthen-Veettil,a)R. Patterson, D. K €onig, G. Conibeer, and M. A. Green Australian Centre for Advanced Photovoltaics, UNSW, Sydney 2052, Australia (Received 15 January 2014; accepted 12 October 2014; published online 30 October 2014) Efficient iso-entropic energy filtering of electronic waves can be realized through nanostructures with three dimensional confinement, such as quantum dot resonant tunneling structures. Large-areadeployment of such structures is useful for energy selective contacts but such configuration is sus- ceptible to structural disorders. In this work, the transport properties of quantum-dot-based wide- area resonant tunneling structures, subject to realistic disorder mechanisms, are studied. Positionalvariations of the quantum dots are shown to reduce the resonant transmission peaks while size var- iations in the device are shown to reduce as well as broaden the peaks. Increased quantum dot size distribution also results in a peak shift to lower energy which is attributed to large dots dominatingtransmission. A decrease in barrier thickness reduces the relative peak height while the overall transmission increases dramatically due to lower “series resistance.” While any shift away from ideality can be intuitively expected to reduce the resonance peak, quantification allows betterunderstanding of the tolerances required for fabricating structures based on resonant tunneling phe- nomena. VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4899207 ] I. INTRODUCTION Resonant tunneling (RT) structures—and double barrier structures (DBS) in particular1—have attracted significant attention for their use in quantum cascade lasers,2,3single electron transistors,4and next-generation photovoltaics.5–7 This is perhaps largely due to the ability of techniques such as molecular beam epitaxy (MBE) to fabricate layers in a va-riety of materials with precisely controllable thicknesses on the order of a monolayer. Quantum well (QW) RT structures allow electron confinement and tunneling transport in thegrowth direction, while the allowed energy and momenta of electronic states are continuous in the lateral directions. Electrons and/or holes at the resonant energy that are nor-mally incident on the plane of the contact will be transmitted effectively. Obliquely incident electrons and holes may be transmitted even if their total energy falls outside the desiredenergy range, though with a decreased probability. 8This limits the energy and momentum filtering offered by QW structures. By contrast, in quantum dot (QD) DBSs, effectivefiltering occurs irrespective of the direction of the electron/ hole momentum, making it a superior energy filter to a QW DBS. The increased confinement energy of QDs over QWsalleviates some fabrication constraints for the same level of confinement as compared to QWs. For a large area application, such as contacts for an advanced photovoltaic cell, RT structures involving QDs can be made by the solid phase crystallization of sub-oxides. 9 Ideally, this fabrication technique produces spherical quan- tum dots of uniform size situated in the center of the sub- oxide layer. Our prior modeling work10has focused on such ideal structures. However, non-idealities in dot shape, sizeuniformity, dot position and effective barrier thickness areinherent in solid phase crystallization techniques. 9,11–13 These aspects cannot be ignored in simulations if a realistic picture of device performance is to be obtained. A stabilizedmethod is needed for modeling complex three-dimensional (3D) nanostructures. Some of the popular methods based on non-equilibrium Green’s function, 14–16density matrix,17and Wigner distribu- tion function18can efficiently describe transport through sin- gle QDs or perfectly periodic QDs but becomescomputationally prohibitively in modelling large area struc- tures with disorders. Effective formalisms in the framework of Finite Difference method 19and Kronig-Penny like meth- ods20,21on envelop functions have been reported for a per- fectly periodic QDs in orthorhombic geometry. Generalised 3D scatter matrix method is an efficient tool for analysingspherical QDs that are not perfectly periodic. In this work, the impact of disorder in position, size and barrier thickness in spherical QD RT structures are investigated numerically usinga stabilized multi-mode scattering matrix technique. Initial implementations of similar methods used previously by Gomez et al. 22and B €oer23function adequately in quantum well structures that can be modeled in one dimension but tend to become unstable in higher dimensions. While any shift away from ideality can be intuitively expected to reduceresonances, quantifying this is essential for better understand- ing of the tolerances required to fabricate working devices. II. MODEL AND METHOD In this section, a three-dimensional model for analyzing a QD DBS is described. The scattering matrix modeldescribed in Ref. 24is extended to three dimensions with position-dependent effective mass. The instabilities inherent to the conventional scattering matrix (CSM) method inhigher dimensions are overcome by using a stabilized multi- mode scattering matrix method (SMSM method). a)Author to whom correspondence should be addressed. Electronic mail: b.puthen-veettil@unsw.edu.au 0021-8979/2014/116(16)/163707/7/$30.00 VC2014 AIP Publishing LLC 116, 163707-1JOURNAL OF APPLIED PHYSICS 116, 163707 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.30.242.61 On: Wed, 10 Dec 2014 00:19:27A. Scattering matrix formation Assuming a parabolic band near the [100] valley, the three-dimensional Schr €odinger equation may be discretized in space into P,M,andNpoints in x,y,andz(growth) direc- tions, respectively. Discretization and scatter matrix forma-tion are explained in detail in the Appendix. The DBS under consideration has scatter-free contact regions on the left and right and a nanostructure region inthe middle, where most of the electronic scattering occurs. The contact materials are assumed to be either degenerately doped semiconductors or metallic contacts. Electrons areassumed to be incident from the left contact. A fraction of t h ee l e c t r o n i cw a v ei sr e fl e c t e da n dt h er e m a i n i n gf r a c t i o n is transmitted in the elastic RT process. The free electronwave traveling from left to right at the left contact is con- s i d e r e dt oh a v e UandVmodes in xandydirections, respec- tively. A sample size of area 256 nm 2that contains at most 25 QDs was considered for sim ulation. Periodic boundary conditions (PBCs) in xandydirections and open boundary conditions (OBCs) in the zdirection were assumed for this sample. Transfer matrices can be formulated on the left and right contact region of the structure as U1 U2/C20/C21 ¼X211X212 10/C20/C21 U2 U3/C20/C21 ; (1) UN/C02 UN/C01/C20/C21 ¼XN/C0111XN/C0112 10/C20/C21 UN/C01 UN/C20/C21 ; (2) where Uiis the multimode wavefunction of the ith slab in thex-yplane and Xis the Hamiltonian. B. Multi-mode wavefunction method The wavefunction at each slab has U(V) modes in the x(y) direction. The wavefunction at point ( p,m,n) in mode (u,v) can be written as shown below. Wp;m;nu;v¼1ffiffiffiffiffiffiffi UVp ei2pup Pei2pvm Meikzu;vDzn; (3) where kzu;vis the wave vector of the mode ( u,v) in the z direction. For the left region where the material structure is assumed to be uniform, the effective mass does not changewith position (see Eqs. (A1)–(A3)). Thus, an expression for the wave vector can be obtained as k u;v¼1 Dzcos/C01/C261 2Gz/C20 /C0Yxyz/C0Gx2 /C2cos2pu P/C18/C19 /C0Gy2 cos2pv M/C18/C19/C21/C27 : (4) The wavefunction in mode ( u,v) on the left contact region consists of the incident wave in mode ( u,v) and the reflected wave in mode ( u,v) from the scattering region. The reflected wave consists of the components of scattered waves from all modes to mode ( u,v), as shown in Figure 1. The total wavefunction on the left of the scattering region isUzlef t¼XU;V u¼1;v¼1ðh^Wu;vðku;v zÞj^Iu;vþh^Wu;vðku;v /C0zÞjjru;viÞ;(5) where ^Iu;vis a column matrix of ones with dimension UV/C21, ku;vzrepresents the wave vector of the wave traveling in the þzdirection in mode ( u,v)a n d ku;v/C0zrepresents the wave vector of the wave traveling in the – zdirection. jru;viis a ma- trix whose elements consist of the sum of probabilities of each mode to be reflected back to mode ( u,v). All unphysical solu- tions (those corresponding to an imaginary ku;v) are neglected. Similarly, the wavefunction in mode ( u,v) on the right contact region consists of the transmitted wave in mode(u,v) from the scattering region. This consists of scattered waves from all modes into mode ( u,v), as shown in Figure 2. The total wavefunction to the right of the scattering region is U zright ¼XU;V u¼1;v¼1hWu;vðku;v zÞjjtu;vi; (6) where jtu;viis a matrix whose elements consist of the sum of probabilities of each mode to be transmitted to mode ( u,v). ku;vzrepresents the wave vector of the wave traveling in the z direction in mode ( u,v) towards the right contact. Using (1),(2),(5), and (6), a system of equations can be written for all the x-yslabs together in the structure as ½T/C138/C2jUi¼jKi; (7) where the matrices are as given below: FIG. 1. The incident and reflected waves in mode ( u,v) on the left contact region of the DBS. The reflected wave in mode ( u,v) consists of back- scattered waves from all modes to mode ( u,v).ru;v!u0;v0stands for the proba- bility of the wave traveling to the right in mode ( u,v) to be scattered back in mode ( u0,v0). FIG. 2. The transmitted waves in mode ( u,v) on the right contact region of the DBS. The transmitted wave in mode ( u,v) consists of the unscattered wave in mode ( u,v) and scattered waves from all modes to mode ( u,v)i n the same direction of propagation. tu;v!u0;v0stands for the probability of the wave traveling to the right in mode ( u,v) to be scattered to mode ( u0,v0)i n the same direction.163707-2 Puthen-Veettil et al. J. Appl. Phys. 116, 163707 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.30.242.61 On: Wed, 10 Dec 2014 00:19:27T½/C138¼1/C0ðhW/C01jhW/C02j/C01Þ 00 0 ::: 00 /C01 X211X21200 0 0 0 /C01 X311X3120 :: 0 ::: :: :: ::: ::: :: ::: 00 /C01XN/C0111XN/C0112 00 0 0 ::: 01 /C0hWN/C01jhWNj/C012 66666666666643 7777777777775: jUi¼U 1 U2 U3 ::: ::: UN/C01 UN2 6666666666643 777777777775 jKi¼hW 1jj^Ii/C0hW/C01jhW/C02j/C01hW2jj^Ii 0 00 ::: ::: 02 66666666666643 7777777777775 These matrices can be efficiently stored in sparse format using standard library routines. The memory requirement of this method is comparable to the conventional transfer matrix meth- ods. The matrix ½T/C138observed for a sample structure of dimen- sions P¼50,M¼50 and N¼50 has a sparseness of 99.92%. Solving the system of equations (7)is straightforward using standard library routines. Once jUiis found, the trans- mission coefficient and conduction G(Ref. 25) are calcu- lated as jti¼hW Nj/C01UN; (8) G¼2e2 2p/C22hTracejtiTCjti/C16/C17 ; (9) where jtiTCis the transpose conjugate of jti. The SMSM method is as efficient as the CSM method since the total number of multiplication steps needed to find the matrix jtiis the same. Most importantly, the instability in- herent to the CSM at higher dimensions were overcome using the SMSM method. A comparison is made between error from the CSM method26and the SMSM method for a two- dimensional (2D) problem in Figure 3. The step size in the z direction, az, is fixed at 0.2 nm. Once the transmission coeffi- cient ( T) and reflection coefficient ( R) are calculated, the error is calculated as j1/C0T/C0Rj. The conventional method works well in one-dimensional (1D) models27as well as 2D models with low yresolution (large step size in the ydirection).However, as the step size in the ydirection ( ay) becomes com- parable to that in the zdirection, the error in finding the solu- tion using the conventional model dramatically increases and the model itself becomes unstable. In contrast, the SMSM method is stable for all resolutions. The maximum error valuein the new method is less than 1 /C210 /C013in all cases. C. Models of disorder The model derived above can be used to model the disor- ders of QDs in a DBS. QDs grown by sputter-anneal methods9 are unlikely to have a narrow position and size distribution.1 This is due to the inherent randomness associated with thenucleation process by which these QDs are formed during the annealing step. These disorders also will depend on the stoi- chiometry of the sub-oxide as well as the effectiveness of thedielectric as a diffusion barrier. The deviations from periodic- ity can be detrimental to the conductivity and selectivity of the RT structures. These deviations may arise in the periodic-ity of the QDs in the structure (position anomaly), the size of the QDs from the expected diameter (size anomaly) or the thickness variation of the dielectric layer. Disorder is modeled by perturbing parameters such as QD position and size away from their mean values pseudo- FIG. 3. A comparison between the CSM method and the SMSM method developed in this work. azis taken to be 0.2 nm. Three different ay step sizes were considered: 10 az,2az, and az. As the resolution in the ydirection approaches that in the zdirection, the conventional method becomes unsta- ble. The error is calculated as j1/C0T/C0Rjwhere Tis the transmission coeffi- cient and Ris the reflection coefficient.163707-3 Puthen-Veettil et al. J. Appl. Phys. 116, 163707 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.30.242.61 On: Wed, 10 Dec 2014 00:19:27randomly with weighting factors given by a wrapped normal distribution. A periodically repeating array of randomly- distributed QDs was considered for the disorder simulations. Care was taken to eliminate situations where the QD transla-tion or size deviation resulted in the merging of two or more QDs together or with the contacts. Simulation was run S number of times and tunneling coefficients corresponding toeach random normal distribution event were calculated. The smoothed average of the S tunneling coefficients was taken. A high value of S is preferred to represent a realistic disor-der. In this work, a smoothed average of S ¼1000 simulation runs was taken for which a relative error of 0.72% was esti- mated for a maximum deviation ¼1 nm. As the maximum deviation increases, the relative error of computation for the same S value will also increase. However, considerable devi- ation in the transmission coefficients could be seen even forsmall deviations. For this work, the maximum deviation was limited to 1 nm in order to save on computational resources. A schematic illustration of the position and size disorder isdepicted in Figure 4. III. RESULTS AND DISCUSSION The resonant tunneling features of various QD-based SiO 2/Si/SiO 2DBS were analyzed. The thickness of SiO 2barriers in all cases was 1 nm, and spherical Si QDs of diame-ters 3, 2.4, and 2 nm were considered. The barrier thickness and the mean diameters of the QDs were chosen arbitrarily. In the first simulation, the impact of lateral size variation ofperiodically-arranged QDs in the DBS on resonant peak ener- gies was analyzed. Simulation runs were performed for three different QD diameters in the ydirection—3, 2.4 and 2 nm— keeping the diameter in the xandzdirection constant at 2 nm, resulting in oblong QDs. As shown in Figure 5, it was observed that as the QDs were elongated in the ydirection, the resonance was red-shifted due to the lowering of confinement energy. For this reason, a peak occurred at 1.5 eV for the QD with 3 nm lat- eral diameter, showing that the (3, 2, 1) wavefunction mode isof lower energy than the (2, 2, 2) mode. These well-understood quantum mechanical properties are d e s c r i b e di nd e t a i li nt h el i t - erature. 27,28The lateral effects are also seen from Figure 6, where the probability densities of electronic waves through a DBS made of QWs and QDs are compared. For QWs, no lat- eral scattering is apparent. In the case of QD DBS, waves arescattered laterally as well, cr eating waves with 3D modulation. Position disorder was evaluated on a DBS with 3-nm- diameter Si QDs in a 5-nm-thick SiO 2matrix. The QDs were centered at the midpoint between the two barriers. FIG. 4. An illustration of size and position disorder in a single layer Si/SiO 2 QD resonant tunneling structure. FIG. 5. RT features in Si QDs in a double barrier structure for varying lat- eral dimension. Diameter in xandzdirections was fixed at 2 nm. Three dif- ferent diameters in the ydirections—2, 2.4, and 3 nm—were considered. As the QDs were elongated in the y direction, the resonant energies decreased. FIG. 6. Probability density of electronwaves through a double barrier struc- ture made of (a) a 4 nm QW (b) 4 nm diameter QDs for an electron energy that is 10 meV above the barrier height.The white circles indicate the positions of the QDs.163707-4 Puthen-Veettil et al. J. Appl. Phys. 116, 163707 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.30.242.61 On: Wed, 10 Dec 2014 00:19:27The third resonant peak was analyzed by a smoothed aver- age of 1000 simulation runs with different maximum deviations from the mean position values of a normally-distributed position parameter. Figure 7(a) shows the con- ductance in the units of 2 e 2/hfor different values of maxi- mum deviation. For zero deviation—a perfectly orderedarray of QDs—the peak energy is found to be 1.072 eV. As the maximum deviation was increased from 0 nm to 0.5 nm, the peak position of the average response droppedby 27.5%; with 1-nm maximum deviation, it dropped by another 27%. However, in all three maximum deviation values, the mean position of the peak remained the sameat 1.072 eV. The fixed size of the QDs in all cases yields the same confined energy levels with no red-shifting. Since the deviation in position did not induce a breakdown ofeither tunneling barrier, the energy selectivity of the DBS decreases only as a result of disorder in position. The same structural model discussed above was used for simulating the size disorders. The center points of all QDs were kept exactly in the middle of the dielectric ma- trix. Assuming a wrapped norma l distribution for size devi- ations, the average distribution was found from 1000 simulation runs. The maximum deviation in size was assumed to be 0, 0.5, and 1 nm. Zero deviation representedan orderly array of QDs; the corresponding peak energy was 1.072 eV as in the previous case. Figure 7(b) shows the conductance in units of 2 e 2/has the smoothed average of the outcome of 1000 simulation runs with the different deviation values. As shown in Figure 7(b), the maximum deviation increased from 0 nm to 0.5 nm and the peak position of the average conduction red-shifted by 50.2%. From 0.5 nm to 1 nm, it red-shifted another 16.6%. In addition to the drop inconduction, as the extent of the disorder increased, the width of the resonant peak also increased. Since QDs with different sizes have different resonant energies, the average value ofthe response increased the width of the resonant peak, reduc- ing the energy selectivity of the structure. The peaks showed a shift towards lower energy with increasing size deviations.This red shift is attributed to larger QDs dominating the transmission through the structure as a result of the smaller barrier thicknesses. Compared to disorder in position, sizedisorder has a greater negative impact on the conductance and the selectivity of DBSs. The precise size control of the QDs deserved prime importance for achieving a DBS withgood selectivity in energy. This is challenging experimen- tally and has been a subject of substantial effort. 11 To evaluate the impact of the thickness of the barriers on the tunneling coefficient, the thickness of the SiO 2bar- riers was varied between 0.4 nm and 1.6 nm. The second FIG. 7. (a) Reductions in transmission for QD DBSs with position disorder are shown. Maximum deviations of the QDs from the midpoint of the DBS were 0 nm, 0.5 nm, and 1 nm. The QD positions were varied pseudo-randomly, with deviations weighted by a normal distribution. (b) Clear peak broadening and a slight red shift of the transmission are observed for the QD- DBS subjected to disorder in QD size. The size parameter was varied pseudo- randomly within maximum deviation limits of 0 nm, 0.5 nm, and 1 nm, weighted by a normal distribution. The mean QD diameter is 3 nm. All ener- gies are calculated with respect to the conduction band edge of silicon. FIG. 8. Orders-of-magnitude reductions in the transmission coefficient areobserved for a 2-nm diameter Si QD/SiO 2DBS with barrier thickness chang- ing from 1.6 nm to 0.4 nm. Substantial broadening of the peak and selectivity loss occurs for barrier thicknesses below 1 nm. A small red shift is again apparent for small barrier thicknesses. Energies are scaled relative to the conduction band edge of silicon.163707-5 Puthen-Veettil et al. J. Appl. Phys. 116, 163707 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.30.242.61 On: Wed, 10 Dec 2014 00:19:27resonant peak of periodically-arranged spherical Si QDs of 2 nm diameter was analyzed. The simulation results are shown in Figure 8, demonstrating an improvement of four orders of magnitude when the barrier thicknesschanged from 1.6 nm to 1 nm. It again rose by four orders of magnitude when the thickness was reduced to 0.4 nm, with a slight red-shift in the position of the peak.This is due to the increased leakage through the thin bar- riers which brings down the confined energy. While very thin barriers increase the t unneling current, they also decrease the energy selectivity of a RT structure based on a QD array. IV. CONCLUSIONS In this work, a numerically stable multi-mode three- dimensional scattering mat rix method was employed to analyze several types of dis order in QD-based resonant tunneling structures. As the maximum deviationincreased from 0 nm to 1 nm, the transmission peak was shown to drop by 55% for position disorder and 70% in the case of size disorder. It was found that the size disor-der impacts the performanc e of these structures more than other types of disorder, leading to broadened tunnel- ing peaks and a decrease in the peak conductance value.A suitable fabrication met hod for RT structures must show better control of nanostructure dimensions and positions than that which is presently demonstrated bysolid-phase crystallization, as deviations in the order of a few nanometers can have a subs tantial impact on the per- formance of these structures. ACKNOWLEDGMENTS This Program has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). The Australian Government, throughARENA, is supporting Australian research and development in solar photovoltaic and solar thermal technologies to help solar power become cost competitive with other energysources. The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for anyinformation or advice contained herein. APPENDIX: DEVIATIONS 1. Discretization of the Schr €odinger equation Assuming a parabolic band near the [100] valley, the one-electron Schr €odinger equation in a three-dimensional Cartesian coordinate system takes the form as shown in (A1) below. /C0/C22h2 2@ @x1 m/C3xðÞ@WxðÞ @x/C18/C19 þ@ @y1 m/C3yðÞ@WyðÞ @y ! þ@ @z1 m/C3zðÞ@WzðÞ @z/C18/C198 >>>>< >>>>:9 >>>>= >>>>; þUx ;y;z ðÞ Wx;y;z ðÞ ¼EWx;y;z ðÞ : (A1) Here, m /C3ðrÞis the position-dependent effective mass. WðrÞis the wavefunction at r,Uis the potential energy of the band above the conduction band, and Eis the total energy of the electron. Tunneling effective masses were used for electrons in all materials.23 The structure (defined by Uðx;y;zÞand m/C3ðx;y;zÞ)i s discretized in real space as shown in Fig. 9. Discretizing (A1) with P,MandNpoints in the x,yand zdirections respectively, WpWpþ1;m;nþGpWp/C01;m;nþWmWp;mþ1;nþGmWp;m/C01;n þWnWp;m;nþ1þGnWp;m;n/C01þYp;m;nðEÞWp;m;n¼0; (A2) where Wp¼Fp m/C3pþ1;m;n/C0Fp m/C3p;m;nþGp/C18/C19 ;Wm¼Fm m/C3p;mþ1;n/C0Fm m/C3p;m;nþGm/C18/C19 ;Wn¼Fn m/C3p;m;nþ1/C0Fn m/C3p;m;nþGn/C18/C19 ; Yp;m;nEðÞ¼Fp m/C3p;m;n/C0Fp m/C3pþ1;m;nþFm m/C3p;m;n /C0Fm m/C3p;mþ1;nþFn m/C3p;m;n/C0Fn m/C3p;m;nþ1 /C02Gp/C02Gm/C02GnþUp;m;n/C0E0 BBBBB@1 CCCCCA8 >>>>>>>>>>< >>>>>>>>>>: FIG. 9. A schematic representation of the discretization of the structure in to P, M and N points in the x, y and z directions, respectively. The inset shows a schematic representation of the discretized wavefunction in real space.163707-6 Puthen-Veettil et al. J. Appl. Phys. 116, 163707 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.30.242.61 On: Wed, 10 Dec 2014 00:19:27Fp¼/C0/C22h2 2Dx2;Fm¼/C0/C22h2 2Dy2; Fn¼/C0/C22h2 2Dz2;Gp¼/C0/C22h2 2m/C3p;m;nDx2; Gm¼/C0/C22h2 2m/C3p;m;nDy2;Gn¼/C0/C22h2 2m/C3p;m;nDz2: The wavefunction at the zpoint ( n/C01) can be written as a function of the wavefunction at nand that at ( nþ1) as shown below Wp;m;n/C01¼/C01 GnYp;m;nEðÞWp;m;nþWmWp;mþ1;n þGmWp;m/C01;nþWpWpþ1;m;n þGpWp/C01;m;n0 B@1 CA þ/C0Wn GnWp;m;nþ1 ðÞ : (A3) 2. Scattering matrix formation For any slab in the x-yplane, the wavefunction is repre- sented as Uz; it consists of modes as shown in the equation below: Uz¼Xu¼U;v¼V u¼1;v¼1h^Wzu;vj; (A4) where UandVare the mode numbers in the xandydirec- tions and ^Wzu;vis the wave function in mode ( u,v) in the zth x-yslab. The maximum number of modes in the xand y directions are PandM, respectively. For all z, each element of^Wzu;vconsists of Pnumber of yfingers, as shown in the following equation. Wzu;v¼W11uv W12uv ::: W1Muv2 666643 77775y finger :1 W21uv W22uv ::: W2Muv2 666643 77775y finger :2 ::: :::::: :::2 666643 77775y fingers WP1uv WP2uv ::: WPMuv2 666643 77775y finger :P2 66666666666666666666666666666666666643 7777777777777777777777777777777777775 z:: (A5) Using (A3) and (A4), the matrices Xz11andXz12were formed, so that the equation below holds true for any z. Uz/C01¼Xz11UzþXz12Uzþ1: (A6)Xz11andXz12are 7 diagonal matrices of size PM/C2PM. (A6) can be written in a transfer matrix form as Uz/C01 Uz/C20/C21 ¼Xz11Xz12 10/C20/C21 Uz Uzþ1/C20/C21 : (A7) 1I. Kamiya et al. , “Resonant tunneling through a single self-assembled InAs quantum dot in a micro-RTD structure,” Physica E 13(2–4), 131–133 (2002). 2C. Sirtori et al. , “Resonant tunneling in quantum cascade lasers,” IEEE J. Quantum Electron. 34(9), 1722–1729 (1998). 3J. Faist et al., “Quantum cascade laser,” Science 264(5158), 553–556 (1994). 4K. Likharev, “Single-electron transistors: Electrostatic analogs of the DC SQUIDS,” IEEE Trans. Magn. 23(2), 1142–1145 (1987). 5M. A. Green, “Potential for low dimensional structures in photovoltaics,” Materi. Sci. Eng. B 74(1–3), 118–124 (2000). 6M. Green et al. , “Hot carrier solar cells: Challenges and recent progress,” in35th IEEE Photovoltaic Specialists Conference (PVSC) (2010). 7Y. Feng et al. , “Non-ideal energy selective contacts and their effect on the performance of a hot carrier solar cell with an indium nitride absorber,” Appl. Phys. Lett. 100(5), 053502 (2012). 8H. C. Liu and G. C. Aers, “Resonant tunneling through one-, two-, and three-dimensionally confined quantum wells,” J. Appl. Phys. 65(12), 4908–4914 (1989). 9M. Zacharias et al. , “Size-controlled highly luminescent silicon nanocrys- tals: A SiO/SiO[sub 2] superlattice approach,” Appl. Phys. Lett. 80(4), 661–663 (2002). 10B. Puthen Veettil, D. K €onig, G. Conibeer, and M. A. Green, “2-Dimensional model of the resonant tunnelling through an ideal double barrier structure,” in 3rd International Solar Energy Society Conference, Asia Pacific Region, 46th ANZSES Annual Conference, Sydney (2008). 11G. Conibeer et al. , “Silicon quantum dot nanostructures for tandem photo- voltaic cells,” Thin Solid Films. 516(20), 6748–6756 (2008). 12A. Yurtsever, M. Weyland, and D. A. Muller, “Three-dimensional imaging of nonspherical silicon nanoparticles embedded in silicon oxide by plas- mon tomography,” Appl. Phys. Lett. 89(15), 151920–151920-3 (2006). 13O. I. Micic et al. , “Synthesis and characterization of InP quantum dots,” J. Phys. Chem. 98(19), 4966–4969 (1994). 14M. J. McLennan, Y. Lee, and S. Datta, “Voltage drop in mesoscopic sys- tems: A numerical study using a quantum kinetic equation,” Phys. Rev. B 43(17), 13846 (1991). 15S. Datta, “Nanoscale device modeling: the Green’s function method,” Superlatt. Microstruct. 28(4), 253–278 (2000). 16R. Lake and S. Datta, “Nonequilibrium Green’s-function method applied to double-barrier resonant-tunneling diodes,” P h y s .R e v .B 45(12), 6670 (1992). 17H. Mizuta and C. J. Goodings, “Transient quantum transport simulation based on the statistical density matrix,” J. Phys.: Condensed Matter 3, 3739 (1991). 18F. A. Buot and K. L. Jensen, “Lattice Weyl-Wigner formulation of exactmany-body quantum-transport theory and applications to novel solid-state quantum-based devices,” Phys. Rev. B 42(15), 9429 (1990). 19O. L. Lazarenkova and A. A. Balandin, “Electron and phonon energy spec- tra in a three-dimensional regimented quantum dot superlattice,” Phys. Rev. B 66(24), 245319 (2002). 20D. L. Nika et al. , “Charge-carrier states and light absorption in ordered quantum dot superlattices,” Phys. Rev. B 76(12), 125417 (2007). 21O. L. Lazarenkova and A. A. Balandin, “Miniband formation in a quantum dot crystal,” J. Appl. Phys. 89(10), 5509–5515 (2001). 22I. G/C19omez et al. , “Electron scattering on disordered double-barrier GaAs- AlxGa1-xAs heterostructures,” Physica E 18(4), 372–382 (2003). 23K. W. Boer, Survey of Semiconductor Physics- Electrons and Other Particles in Bulk Semiconductor (Van nostrand Reinhold, NY, 1990), Vol. 1. 24B. P. Veettil et al. , “Impact of disorder in double barrier QD structures on energy selectivity investigated by two dimensional effective mass approx- imation,” Energy Procedia 2(1), 213–219 (2010). 25D. S. Fisher and P. A. Lee, “Relation between conductivity and transmis- sion matrix,” Phys. Rev. B 23(12), 6851 (1981). 26I. Gomez et al. , “Transport in random quantum dot superlattices,” J. Appl. Phys. 92(8), 4486–4489 (2002). 27S. Datta, Quantum Phenomena , Modular Series on Solid State Devices Vol. 8 (Addison-Wesley, 1989). 28J. Singh, Quantum Mechanics: Fundamentals and Applications to Technology (John Wiley and Sons, inc., 1996).163707-7 Puthen-Veettil et al. J. Appl. Phys. 116, 163707 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 137.30.242.61 On: Wed, 10 Dec 2014 00:19:27

1.4898648.pdf

Hierarchical cobalt-formate framework series with (41263)(4966) n (n = 1–3) topologies exhibiting slow dielectric relaxation and weak ferromagnetism Ran Shang, Sa Chen, Ke-Li Hu, Ze-Chun Jiang, Bing-Wu Wang, Mohamedally Kurmoo, Zhe-Ming Wang, and Song Gao Citation: APL Materials 2, 124104 (2014); doi: 10.1063/1.4898648 View online: http://dx.doi.org/10.1063/1.4898648 View Table of Contents: http://scitation.aip.org/content/aip/journal/aplmater/2/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic, dielectric, and magneto-dielectric properties of rare-earth-substituted Aurivillius phase Bi6Fe1.4Co0.6Ti3O18 J. Appl. Phys. 116, 154102 (2014); 10.1063/1.4898318 Observation of a new cryogenic temperature dielectric relaxation in multiferroic Bi7Fe3Ti3O21 Appl. Phys. Lett. 103, 122901 (2013); 10.1063/1.4821435 Muon Spin Relaxation Study of an Impurity Doped Antiferromagnetic Triangular Lattice with a 1D Ferromagnetic Chain: Ca3(Co1−xZnx)2O6 AIP Conf. 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Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissions Downloaded to IP: 141.212.109.170 On: Fri, 21 Nov 2014 18:17:53APL MATERIALS 2, 124104 (2014) Hierarchical cobalt-formate framework series with (412·63)(49·66)n(n=1–3) topologies exhibiting slow dielectric relaxation and weak ferromagnetism Ran Shang,1Sa Chen,1Ke-Li Hu,1Ze-Chun Jiang,1Bing-Wu Wang,1 Mohamedally Kurmoo,2Zhe-Ming Wang,1,aand Song Gao1,a 1Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China 2Institut de Chimie de Strasbourg, CNRS-UMR 7177, Université de Strasbourg, 4 rue Blaise Pascal, 67000 Strasbourg Cedex, France (Received 16 September 2014; accepted 7 October 2014; published online 24 October 2014) The employment of linear di-, tri-, and tetra-ammoniums has generated a hierarchy in the binodal (412·63)(49·66)ntopologies with n=1, 2, and 3, respectively, for the cobalt formate frameworks with increasing length of the cavities to match the ammoniums. This indicates the length-directing e ffect of the polyammoniums. The dynamic movements of polyammoniums between favored sites or orientations within the cavities lead to slow dielectric relaxations. All materials are spin-canted antiferro- magnets in low temperatures and show reduced spontaneous magnetizations from di- and tri-, to tetra-ammoniums, because of the increased number of unique Co ions or the antiferromagnetically coupled sublattices. C2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http: //dx.doi.org /10.1063 /1.4898648] The last two decades have witnessed major developments in metal-organic frameworks (MOFs) through very active and intense studies.1–7Such materials, classified as the “middle” in Cheetham and Rao quote “There is plenty of room in the middle,”2have showed a very wide spectra of struc- tures, properties, functionalities, and possible applications. Despite the continued great interest in their chemical aspects,1–3MOFs have been exploited for the abundance in their physical properties and critical phenomena or phase transitions.4Magnetism has been a long and extensive research subject for MOFs,5but dielectric (DE) and ferro- /antiferro-electricities (FE /AFE) of MOFs have attracted even greater attention recently.6MOFs showing synergy through the coexistence of mag- netic and electric orderings have emerged as a field of MOF-multiferroics.7However, the examples are still few. The recent research on ammonium metal formate frameworks (AMFFs, mainly for 3 d metals or Mg, TM) has revealed not only the diversity in framework structures but more impor- tantly, promising magnetic and /or electric properties, phase transitions, and others.8The framework structures could be easily controlled, or templated, by the shape, size, and charge of the ammo- niums. For mono-ammoniums (AH+), the small ones9–14(e.g., NH 4+) led to the chiral frameworks of[AH ][TM (HCOO )3]with (49·66) topology. The larger sized ones15–18(e.g., (CH 3)2NH 2+) re- sulted in many metal-formate perovskites, with (412·63) topology. AMFF analogous to the niccolite (NiAs) could be obtained by using di-ammoniums, as [dmenH 2][TM (HCOO )3]2series19(dmenTM , dmenH 22+=CH 3NH 2(CH 2)2NH 2CH 3) and [bnH 2][Mg (HCOO )3]2(bnMg , bnH 22+=H3N(CH 2)4- NH 3),10or mono-ammonium in [(CH 3)2NH 2][FeIIIMII(HCOO )6](dmaFeM ),20and the framework topology is binodal (412·63)(49·66). Lanthanide21,22and uranyl23AMFFs have also been reported to show more complicated structures and framework topologies. The physics of AMFFs are found abun- dant, thanks to the combination of ammonium, metal ion, and the formate bridge, which provide the aElectronic addresses: zmw@pku.edu.cn and gaosong@pku.edu.cn 2166-532X/2014/2(12)/124104/8 2, 124104-1 ©Author(s) 2014 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissions Downloaded to IP: 141.212.109.170 On: Fri, 21 Nov 2014 18:17:53124104-2 Shang et al. APL Mater. 2, 124104 (2014) magnetic coupling, the H-bonding (HB) systems, and the order-disorder alteration of the ammonium components for creating various properties.8For example, coexistence or synergy of magnetic and electric orderings, large dielectric anomalies and relaxor behaviors, negative thermal expansion, and so on, have all been documented in several [AH ][TM (HCOO )3]series,9–18especially in the perov- skites. Some of them even have high phase transition temperatures comparable to the ferroelectric oxides.10,11,14Magnetic /dielectric relaxation and temperature /pressure-induced structural transitions have been observed in several lanthanide AMFFs.21,22The niccolite dmaFeFe displayed para- to antiferro-electric transition of unusual structural alternations and Néel N-Type ferrimagnetism.20It is noted that the order-disorder alternations of ammoniums and the triggered phase transitions are closely relevant to these properties, and could occur in many AMFFs, with di fferent patterns of the dynamics, such as vibration, flipping, or rotational motion of ammoniums, depending on the symme- try requirements and weak interactions between the ammoniums and the host frameworks, and the subtle balance of energies within the detailed structures.8 The development has been expanded to polyammoniums,8,24and we reported here three compo- unds, [bnH 2][Co(HCOO )3]2(bnCo ),[dptaH 3][Co(HCOO )3]3(dptaCo ), and [tptaH 4][Co(HCOO )3]4 (tptaCo ) (dptaH 33+=H3N(CH 2)3NH 2(CH 2)3NH 3, and tptaH 44+=H3N(CH 2)3NH 2(CH 2)3NH 2(CH 2)3- NH 3)having increasing length of the ammonium but retaining the width and most importantly, the flexibility. They form a family of hierarchical frameworks possessing the binodal (412·63)(49·66)n topologies of order n=1, 2, and 3. The length of the polyammonium defines the order of the topology and thus the cavities in which they are located. The loose fitting of the flexible polyammonium within the cavity space provides dynamical motion between di fferent sites or orientations, thus, results in slow dielectric relaxations. They are also spin-canted antiferromagnets (AF) or weak ferromagnets (WF), with the Néel temperatures ( TN’s) around 10 K, and the reduced spontaneous magnetizations from bnCo anddptaCo totptaCo . The crystals of the three compounds were prepared by the convenient solution methods and using commercial chemicals, as described before for other AMFFs,8in satisfactory yields. Anal., bnCo , calcd for C 10H20N2O12Co2: C, 25.12; H, 4.22; N, 5.86%; found: C, 25.04; H, 4.23; N, 5.75%; dptaCo , anal. calcd for C 15H29N3O18Co3: C, 25.16; H, 4.08; N, 5.87%; found: C, 25.42; H, 4.24; N, 5.78%; tptaCo , anal. calcd for C 21H42N4O24Co4: C, 26.05; H, 4.26; N, 5.79%; found: C, 26.16; H, 4.13; N, 5.80%. The single crystal X-ray di ffraction data for bnCo ,dptaCo , and tptaCo at room temperature were collected on a Nonius KappaCCD di ffractometer using graphite monochromated Mo K αradi- ation (λ=0.71073 Å). The structures were solved by direct method and refined by full-matrix least-squares on F2using program.25Crystallographic data are briefly listed in Table I, the full details and the selected molecular geometries are in Tables S1 and S2 of the supplementary material.26 The temperature-dependent alternative current (ac) dielectric permittivity measurements were performed against the capacitors prepared from powdered samples10,14on a TH2828 Precision TABLE I. The brief crystallographic data for bnCo ,dptaCo , and tptaCo at room temperature. Compound (CCDC number) bnCo (1024916) dptaCo (1024917) tptaCo (1024918) Formula C 10H20Co2N2O12 C15H29Co3N3O18 C21H40Co4N4O24 Fw 478.14 716.20 968.29 Crystal system Trigonal Trigonal Trigonal Space group P31c R 3c P 31c a=b(Å) 8.5322(2) 8.4069(2) 8.3617(1) c(Å) 13.3228(3) 61.921(3) 28.3983(5) α=β,γ(deg) 90, 120 90, 120 90, 120 V(Å3) 839.94(3) 3790.0(2) 1719.52(4) Z,DC(g cm−3) 2, 1.891 6, 1.883 2, 1.870 Total, uniq. and obs.[I ≥2σ(I)] refls. 15334, 649, 554 13053, 969, 518 25884, 1329, 873 R1, wR2[I≥2σ(I)] 0.0238, 0.0668 0.0300, 0.0710 0.0260, 0.0705 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissions Downloaded to IP: 141.212.109.170 On: Fri, 21 Nov 2014 18:17:53124104-3 Shang et al. APL Mater. 2, 124104 (2014) inductance-capacitance-resistance meter under dried N 2flow. Magnetic measurements were per- formed on a Quantum Design MPMS XL5 SQUID system using polycrystalline samples tightly packed. Diamagnetic corrections were estimated using Pascal constants27(−197×10−6,−309 ×10−6, and−401×10−6cm3mol−1forbnCo ,dptaCo , and tptaCo , respectively) and background correction for sample holders. The experimental details of element analyses, powder X-ray di ffrac- tion (PXRD), FTIR spectra, UV-Vis reflectance spectra, and thermal analyses are given in the supplementary material.26 The experimental PXRD patterns for the bulk samples and the pressed pellets of the three com- pounds match well the simulated ones based on the crystal structures (Fig. S1, see supplementary material26), confirming the phase purity and no pressure-induced structural phase transitions.21The three IR spectra (Fig. S2(a) and Table S3, see supplementary material26) are quite similar, with char- acteristic bands for polyammonium and HCOO−groups, indicating the similarity of the structures with similar components.28Three bands, 15 600 cm−1(sh), 19 300 cm−1(s), and 20 900 cm−1(sh) in the UV-Vis spectra (Fig. S2(b), see supplementary material26), correspond to the three transitions of 4T1g(F)→4A2g(F),4T1g(F)→4T2g(F), and4T1g(C)→4T1g(V), respectively, typical for the octahedral CoO 6moiety,29and similar to other reported Co-AMFFs.9,18The three materials were thermally stable up to ca. 200◦C, then the departure of polyammonium formates occurred and was closely followed by the subsequent pyrolysis (Fig. S3(a), see supplementary material26). The di fferential scanning calorimetry (DSC) trace of bnCo revealed a reversible phase transition around −30◦C, but fordptaCo andtptaCo , no anomalies were observed (Fig. S3(b), see supplementary material26). The three structures are closely related to one another. They are all trigonal, space group P31cforbnCo andtptaCo ,R3cfordptaCo with similar a/bdimensions but di fferent caxes (Table I; Table S1, see supplementary material26). They all possess binodal 3D metal-formate frameworks containing two kinds of Co nodes, octahedral (412·63) and trigonal prismatic (49·66), connected by anti-anti formate ligands (Fig. 1). The (412·63) node has appeared in perovskites of [AH ][M(HCOO )3]for larger AH =NH 2NH+ 3, CH 3NH+ 3, (CH 3)2NH+ 2, and so on,15–18and the (49·66) one in the chiral phases of [AH ][M(HCOO )3]for small AH =NH+ 4, HONH+ 3, and NH 2NH+ 3.9–14 InbnCo ,dptaCo , and tptaCo , the ratios of the two nodes, (412·63) to (49·66), are 1:1, 1:2, and 1:3, respectively, or the three metal-formate frameworks have topologies of (412·63)(49·66)n with n=1, 2, and 3 (Fig. 1, top). Such topologies for MOFs are still very rare and the hierarchy is unique. In fact, the topology of bnCo forn=1,(412·63)(49·66), was observed in dmenTM series,19the first MOF analogous to the mineral niccolite, then followed in dmaFeM20andbnMg .10 We are unaware of any MOF with topologies of (412·63)(49·66)nforn=2 and 3. These frame- works can also be considered as (4, 4) waved sheets linked along the normal direction, and in the sheet the same kind of nodes occupied the diagonal positions of the square grids to form arrays of (49·66) or (412·63) nodes. For bnCo ,dptaCo , and tptaCo , there are one, two, and three arrays of (49·66) nodes between two arrays of (412·63) nodes, respectively, within the sheet, and the (412·63) node links only (49·66) nodes. The octahedral CoO 6moieties in the three structures have Co–O distances: 2.086(2)–2.108(2) Å, cis- O–Co–O angles 85.46(5)◦–94.54(5)◦, and trans - ones 174.26(5)◦–180◦, and the Co···Co distances via the formato bridge are 5.927–6.015 Å (Table S2, see supplementary material26). The frameworks possess longer and longer shaped cavities for accommodating longer and longer polyammoniums (Fig. 1, middle and bottom; Fig. S4, see supplementary material26). For bnCo , the cavity is formed by two one-corner-missing cubanes twinned together by sharing the three opening corners. In dptaCo , the two one-corner-missing cubanes are connected via their six opening corners. Finally, in tptaCo , three additional (49·66) nodes link the openings of the two one-corner-missing cubanes. Therefore, from bnCo totptaCo , they show that the longer the ammoniums, the longer the cavity directed, and the accompanied addition of (49·66) nodes into the framework. The cations, [bnH2+ 2],[dptaH3+ 3], and [tptaH4+ 4]in the cavities are all trigonally disordered at room temperature. Most of the CH 2groups neighboring NH 2or NH 3groups locate on 3-fold axes, and other CH 2and ammonium groups are in three symmetry-related positions, except that the middle NH 2of[dptaH3+ 3] is still on the 3-fold axis, with a disk-like thermal ellipsoid (Fig. S4, see supplementary material26). The framework cavity of dptaCo looks staggered for the two half parts on both sides of the verti- cally central plane, but those in bnCo andtptaCo are symmetric. Consequently, the middle NH 2of This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissions Downloaded to IP: 141.212.109.170 On: Fri, 21 Nov 2014 18:17:53124104-4 Shang et al. APL Mater. 2, 124104 (2014) FIG. 1. The structures of bnCo (a),dptaCo (b), and tptaCo (c). For each column, the top is the topological view of the Co-formate framework, with spheres being metal atoms and bonds the anti-anti HCOO bridges, and one cavity highlighted in red; the middle and the bottom are the side and top views of the cavity with the disordered polyammmonium in space-filling model. Color scheme: green, (412·63)nodes; violet blue, (49·66)nodes; red, O; dark gray /white, C; cyan, N; white, H. [dptaH3+ 3]has di fferent dynamics from the terminal NH 3, and possesses smaller motion amplitude and looser binding. However, for [bnH2+ 2]and [tptaH4+ 4]cations, all ammoniums have same or similar dynamics and motion amplitudes. These are relevant to the dielectric properties. The NH 2and NH 3 groups of the cations form HBs to the oxygen atoms of anionic frameworks (N ···O contacts =2.88– 3.25 Å, but 3.42 Å for the middle NH 2of[dptaH3+ 3]indptaCo , Table S2, see supplementary mate- rial26) similar to that in the [(CH 3)2NH 2][TM (HCOO )3]anddmenTM series.15–19 The Co AMFF series with di fferent ammonium components now has more than 10 members. [NH 4][Co(HCOO )3],[HONH 3][Co(HCOO )3], and [NH 2NH 3][Co(HCOO )3]possessing the chiral (49·66) topology;9–14[CH 3NH 3][Co(HCOO )3], [(CH 3)2NH 2][Co(HCOO )3], [CH 3CH 2NH 3] [Co(HCOO )3],[C(NH 2)3][Co(HCOO )3], and [(CH 2)3NH 2][Co(HCOO )3]belong to the perovskite of (412·63) topology;15–18dmenCo19andbnCo have niccolite topology of (412·63)(49·66), and dp- taCo andtptaCo showing novel topologies of (412·63)(49·66)nforn=2 and 3. Running through this series, it is very clear that the structural evolution of AMFFs depends on the ammoniums, and the present three compounds clearly demonstrate the length-directing e ffect of the polyammoniums. This series, showing (412·63)m(49·66)n(m=0, 1; n=0, 1, 2, and 3) topologies, is one of the rare occasions that a 3D perovskite-related network can accommodate a progression in cation lengths by progressive change of framework structure. The temperature-dependence of the complex electric permittivity ( ε′andε′′) for the three mate- rials is shown in Figs. 2(a)–2(c) ( bnCo ,dptaCo , and tptaCo , respectively) and the characteristic data in Table S4 of supplementary material.26They all feature strong dielectric dispersion. At a repre- sentative frequency ( f) of 50 kHz, the ε′values were 32.2, 17.6, and 27.1 for bnCo ,dptaCo , and tptaCo , respectively. On cooling, ε′ofbnCo decreased continuously, first slowly to 280 K and then quickly, with 270 K as the fastest descending point ( Tm). Below 250 K, the decrease became slow again until a constant ε′value below 200 K. For lower /higher f’s, the traces shift to lower /higher temperatures but retain the same features, and the Tm’s ranged from 220 K to 315 K for 500 Hz to 1 MHz. The ε′traces of dptaCo andtptaCo show similar behaviors, and the descending are slower or flatter. The Tmranges, from 500 Hz to 1 MHz, are 180–240 K ( dptaCo ) and 180–250 K ( tptaCo ). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissions Downloaded to IP: 141.212.109.170 On: Fri, 21 Nov 2014 18:17:53124104-5 Shang et al. APL Mater. 2, 124104 (2014) FIG. 2. Temperature-dependent traces of the dielectric permittivities for bnCo (a),dptaCo (b), and tptaCo (c), and the Arrhenius plots for the dielectric relaxations (d). Below 150 K, the ε′values under all f’s seemingly converged to 5.0, 6.0, and 6.5 for bnCo ,dptaCo , andtptaCo , respectively. The ε′′traces clearly display strong f-dependence. For bnCo andtptaCo , the plots show single peaks corresponding to the fall in the ε′traces, and the temperatures of the peak positions ( TP) are close to the Tm’s of theε′traces, due to the Kramers-Krönig relations.30The fvs TPdata could be fitted by the Arrhenius law of τ=τ0×exp (Ea/kBT)(τ=(2πf)−1), resulting in the pre-exponential factor τ0=3.0×10−16s and the activation energy Ea/kB=6.3×103K∼0.54 eV for bnCo , andτ0=1.7×10−16s and Ea/kB=5.1×103K∼0.44 eV for tptaCo , respectively (Fig. 2(d)). FordptaCo , the broad peaks in the ε′′traces are composed of two peaks merged together, one in lower temperature (LT) and one in higher temperature (HT), or there are two dielectrics relaxa- tions. By fitting the peak regions using a double-peak model, the individual TPdata could be derived. Then the two sets of fvsTPdata could be fitted by the Arrhenius law, leading to the parameters τ0=1.6×10−14s and Ea/kB=3.7×103K∼0.32 eV for the LT relaxation, and τ0=1.4×10−15s andEa/kB=4.9×103K∼0.43 eV for HT one. These parameters of the dielectric relaxations are rational for dielectrics30and comparable to other AMFFs.10,14 At room temperature, the flexible polyammoniums are all trigonally disordered in the framework cavities. As observed in several reported AMFFs, such as dmenTM ,19bnMg ,10andtmenEr ,21these disorders are related to the motion of the polyammoniums, i.e., the rotating, twisting, or flipping of the constituent parts between several preferred sites or orientations. Such motions induce the dipoles or polarizations and their fluctuations within the lattices, thus, contribute the dielectric responses, high ε′but lowε′′in HT region.30It is expected that on cooling, the contraction of the frameworks and the increased HB interactions will slow or damp the movements and finally freeze them.9,10,12,14,21,22 The damped movements resulted in the decrease /increase inε′/ε′′and the strong dielectric disper- sion. The activation energies are 0.32–0.54 eV , or 31–52 kJ mol−1, seemingly rational for the alterna- tion of several N–H ···O HBs and C–H···O interactions required for the movements.10,14However, the dielectric data and behavior of bnCo are quite di fferent from those of bnMg10though they are isostructural at HT, indicating the di fferent characters in lattice dynamics, disorder-order transition pattern, and phase transition. The two dielectric relaxations of dptaCo corroborate with the two different dynamics of the middle NH 2and the terminal NH 3, as revealed by structural analysis. The former contributes the LT relaxation with the smaller Ea/kB=0.32 eV , and the latter corresponds to This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissions Downloaded to IP: 141.212.109.170 On: Fri, 21 Nov 2014 18:17:53124104-6 Shang et al. APL Mater. 2, 124104 (2014) the HT relaxation, with the slightly higher Ea/kB=0.43 eV , as those of bnCo andtptaCo . On further cooling, the final freezing of the motions of the polyammoniums led to the low dielectric responses in LT region. Such relaxation mechanism as observed here adds to the multitudes and complexities provided by the family of AMFFs which requires further specialized studies to reveal the true state of arts in these novel materials. The phase transition of bnCo has been confirmed by DSC anomalies, but whether the phase transitions occurred for dptaCo andtptaCo need further investigation. The materials will be of interest for MOF-multiferroics7because they have shown magnetic orderings, as below. The three compounds are all 3D spin-canted AFs showing WF in LT region but with interesting differences (Table S4, see supplementary material26). The plots of the temperature-dependent static susceptibilities of the three materials, measured under a field of 100 Oe, are shown in Fig. 3(a). Above 15 K, the three χTvsTplots are nearly overlapped. The χTvalues per mole Co are 3.21 (bnCo ), 3.16 ( dptaCo ), and 3.21 ( tptaCo ) cm3K mol−1at 300 K, typical for the Co2+ions.31,32 Upon cooling, the χTvalues decreased gradually. The HT susceptibilities obey the Curie-Weiss law (Fig. S5(a), see supplementary material26) with Curie constants ( C) and Weiss temperatures ( Θ) in cm3K mol−1/K: 3.85 /−60.5, 3.75 /−54.4, and 3.90 /−63.9 for bnCo ,dptaCo , and tptaCo , respec- tively. Assuming S=3/2, for Co2+, these Cconstants led to the Landé g-factors of 2.83–2.88, and the large negative Θvalues indicate AF exchange within the materials, though the values include the effect of spin-orbit coupling of octahedral Co2+ion, showing an e ffective S=1/2 at LT from a S=3/2 at HT due to the depopulation of the higher energy Kramers doublets ( ±3/2 and±5/2), be- ing equivalent to a Θof ca.−20 K.32When further cooled, the decreased χTvalues reach at minima around 10 K, then rise to maxima and after that they go down to 2 K. The minima are similar for the FIG. 3. Magnetism of the three compounds: (a) plots of χTvsTunder 100 Oe field, and inset, the ZFC /FC plots under 10 Oe field; (b) isothermal magnetization plots at 2 K, and inset, the zoomed part of the hysteresis loops in low fields. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://aplmaterials.aip.org/about/rights_and_permissions Downloaded to IP: 141.212.109.170 On: Fri, 21 Nov 2014 18:17:53124104-7 Shang et al. APL Mater. 2, 124104 (2014) three compounds but the maxima χTvalues are quite di fferent, and for tptaCo , it is small. Under 10 kOe field, the maxima were all suppressed (Fig. S5(b), see supplementary material26). All these observations indicate the occurrence of 3D long-range ordering (LRO) of spin-canted AF within the materials in LT. The materials were further characterized in LT region by measurements of zero-field-cooled (ZFC) and field-cooled (FC) magnetizations under 10 Oe field (Fig. 3(a), inset), isothermal magne- tizations at 2 K (Fig. 3(b)), and ac susceptibilities at 10, 100, and 1000 Hz (Figs. S5(c) and S5(d), see supplementary material,26ac data at 10 Hz only). The small spontaneous magnetizations and irreversibility observed in ZFC /FC plots clearly indicated the 3D LRO of spin-canted AF, and the TN’s were 9.9 K ( bnCo ), 12.5 K ( dptaCo ), and 10.9 K ( tptaCo ), by the negative peak positions in the dFC/dT (Fig. S5(b), inset, see supplementary material26). These are typical for Co-AMFFs.8,13,18,19 The second peak at 11.0 K in the dFC/dT plot of dptaCo is probably due to a spin-reorientation.8,19 The FC magnetizations below TN(Table S4, see supplementary material26) show bnCo>dptaCo ≫tptaCo . This should be due to the increased number of unique Co ions or the AF-coupled sub- lattices from bnCo totptaCo , resulting in the occurrence of hidden spin-canting.33The isothermal magnetizations at 2.0 K (Fig. 3(b)) all display hysteresis, with the coercive fields ( HC’s), being 0.97, 1.5, and 0.39 kOe for bnCo ,dptaCo , and tptaCo , respectively, and small remnant magnetizations (RM’s) of 0.033 ( bnCo ), 0.017 ( dptaCo ), and 0.0035 ( tptaCo ) Nβ. The magnetizations around 0.5 Nβat the highest applied field of 50 kOe, are significantly lower than the expected 2.2 N βassuming S=1/2 andg=4.3.32The spin-flop transition (AF-SP) occurred above ca .20 kOe. In the ac suscep- tibilities at 10 Hz, bnCo shows peaks at 9.9 K in both χ′andχ′′components, and the responses are strong. dptaCo exhibits double peaks (12.5 K and 11.3 K), but the responses are significantly weak. FortptaCo , there is only a broad cusp around 12 K in the very weak χ′response, and χ′′component noisy. The peak positions and the strengths of the χ′andχ′′are in agreement with the ZFC /FC data. Nof-dependences were observed. These results confirm the spin canting AF LRO in the three mate- rials whose structures, with the non-centrosymmetric bridges of anti-anti HCOO linking anisotropic Co2+ions, satisfy the requirement for the antisymmetric interaction.34Finally, the couplings ( J/kB) between Co2+ions via the anti-anti formato bridge, estimated from J/kB=3Θ/[2zS (S+1)],33are −4.0 (bnCo ),−3.6 (dptaCo ), and−4.3 K ( tptaCo ), similar to those of Co-AMFFs with anti-anti HCOO linkages reported before.8,11,13,14,18,19 In conclusion, the results of varying the length of linear polyammonium cations demonstrate the progressive structure-directing e ffect in the formation of binodal (412·63)(49·66)n(n=1, 2, and 3) topologies in Co AMFFs. This progressive development is a rare observation in the field of transition-metal perovskites chemistry. Due to the misfit of the polyammoniums in the spaces available that allow for their distortions and motions between crystallographically and energetically degenerate locations, a series of dielectric anomalies are observed as a function of temperature. These vary with the number of degrees of freedom in the motion of the polyammoniums. However, they all freeze at low temperature for the weak ferromagnetic ordering to set in at ca. 10 K. Thus, possible structural order-disorder is observed at high temperature while at low temperature 3D magnetic order is present. The present results add to the range of other properties already shown for AMFFs, which have proved very beneficial in the development of multifunctional MOF mate- rials. Further studies of these materials will certainly enhance our academic understanding of the multitude of properties as well as the subtle synergy of the coexisting properties. 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1.4876222.pdf

Magnetic transitions and electrical transport in Bi-doped lanthanum strontium manganites A. M. Ahmed, H. F. Mohamed, and Martin Šoka Citation: Low Temperature Physics 40, 418 (2014); doi: 10.1063/1.4876222 View online: http://dx.doi.org/10.1063/1.4876222 View Table of Contents: http://scitation.aip.org/content/aip/journal/ltp/40/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electrical properties of strontium doped yttrium manganite oxide AIP Conf. Proc. 1512, 948 (2013); 10.1063/1.4791354 Magnetoelectric behavior of sodium doped lanthanum manganites J. Appl. Phys. 106, 023707 (2009); 10.1063/1.3173285 Magnon drag effect as the dominant contribution to the thermopower in Bi 0.5 − x La x Sr 0.5 MnO 3 ( 0.1 ≤ x ≤ 0.4 ) J. Appl. Phys. 103, 113717 (2008); 10.1063/1.2938033 Thermopower and thermal conductivity of the electron-doped manganite La 0.9 Te 0.1 Mn O 3 J. Appl. Phys. 100, 123701 (2006); 10.1063/1.2402030 Transport mechanism and magnetothermoelectric power of electron-doped manganites La 0.85 Te 0.15 Mn 1 − x Cu x O 3 ( 0 x 0.20 ) J. Appl. Phys. 100, 073706 (2006); 10.1063/1.2356106 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 28 Nov 2014 11:26:31LOW-TEMPERATURE MAGNETISM Magnetic transitions and electrical transport in Bi-doped lanthanum strontium manganites A. M. Ahmed and H. F . Mohameda) Physics Department, Faculty of Science, Sohag University, Sohag 82524, Egypt Martin /C20Soka Slovak University of Technology, Faculty of Electrical Engineering and Information Technology Ilkovic ˆova 33, Bratislava 81219, Slovak Republic (Submitted September 2, 2013) Fiz. Nizk. Temp. 40, 539–544 (May 2014) The temperature dependence of the electrical resistivity q, thermoelectric power Sand the magnetic susceptibility vof La 0.7–xBixSr0.3MnO 3(x¼0.05, 0.10, and 0.15 at. %) manganites were investigated. La 0.7–xBixSr0.3MnO 3crystallizes in a single phase rhombohedral structure with para- sitic phase inclusions. With increasing Bi concentration, a systematic decrease in the ferromagnetic transition temperature ( Tc), the metal-semiconducting transition temperature ( Tms1) and also the values of activation energies EqandESfrom q(T) and S(T) were observed. On the other hand, in the high-temperature ( T>Tms) paramagnetic semiconductor regime, the adiabatic small polaron hopping model fit well, thereby indicating that polaron hopping might be responsible for the con- duction mechanism. In addition, the thermoelectric power data at low temperatures were analyzedby considering both the magnon and the phonon drag concept, while the high-temperature data were confirmed a small polaron hopping conduction mechanism. VC2014 AIP Publishing LLC . [http://dx.doi.org/10.1063/1.4876222 ] 1. Introduction Perovskite-type oxide LAMO (LaAMnO 3, where A is a divalent alkaline earth metal ion such as Sr2þor Ca2þ) exhibits colossal magnetoresistance (CMR) with a magnetic resistance ratio of more than 100%.1–4In particular, CMR appears near the point of transition from the antiferromag-netic insulator phase to the ferromagnetic metallic phase, and it is closely associated with Mn in the LAMO specimen having a large spin polarization based on strong Hund’s rulecoupling. 5In this case, the electrical conduction characteris- tics depend on whether a conduction electron enters an elec- tron orbit in terms of the Jahn /C0Teller (JT) strain which accompanies the symmetry of the crystal structure.6It is known that by doping the bismuth to the system LAMO, the electrical resistivity and magneto-optical effect change,7but the details are not clear. In order to reveal the mechanism of magneto-transport, Righi et al.8have investigated the Bi-doping effects on the structural, transport and magneticproperties of La 0.7–xBixSr0.3MnO 3, and have found that the dopant Bi cause structure change and decreases the Tms. However, interpretation of the temperature dependence ofthe thermoelectric power (TEP) S(T) for transition metal ox- ide is rarely reported 9–11due to the complexity of elucidating theS(T) apart from the diffusion TEP or temperature- independent TEP. As we know, there are many different properties in La1–xCaxMnO 3and La 1–xSrxMnO 3, such as metal-insulator transition temperature at optimal doping and the critical dop- ing concentration for the presence of ferromagnetism.12,13 So, we have investigated systematically the Bi-doping effect on the magnetic and electrical properties in La 0.7–xBixSr0.3MnO 3with the expectation that it will provide new insight and interesting physics. 2. Experimental All samples reported in the present study were synthesized by a standard solid-state reaction procedure. Stoichiometric compositions of La 0.7–xBixSr0.3MnO 3(x¼0.05, 0.10, and 0.15 at. %) were prepared by mixing equimolar amounts of La 2O3, Bi2O3,S r O ,a n dM n C O 3, respectively (all having greater than 99.99% purity). The powders of these oxides and the carbonate were mixed and were finely ground in an electric grinder for 30min. After grinding, the powders were pressed into pellets with a pressure of 2 ton/cm 2and calcined at 1173 K for 8 h followed by cooling to room temperature, they were reground and again pressed into pellets with a pressure of 7 ton/cm2and subse- quently calcined at 1373 K for 6 h.14Samples were checked by x-ray powder diffraction analysis indicating the presence of a unique phase with perovskite-type structure. Resistivity meas- urements were performed in a commercial variable tempera-ture liquid nitrogen cryostat. The resistivity was measured as a function of temperature using the standard four-probe method and air-drying conducting silver paste as in previousworks. 14,15The thermoelectric power measurements were car- ried using the sample two-heater method with copper electro- des see Refs. 15–17. The magnetic susceptibility measurement was performed, from room temperature to 700 K, using the Kappa Bridge KLY-2 with operating frequency 920 Hz. 3. Results and discussion The x-ray diffraction patterns of the La 0.7–xBixSr0.3 MnO 3(x¼0.05, 0.10, and 0.15 at. %) show that the 1063-777X/2014/40(5)/5/$32.00 VC2014 AIP Publishing LLC 418LOW TEMPERATURE PHYSICS VOLUME 40, NUMBER 5 MAY 2014 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 28 Nov 2014 11:26:31systematic substitution of La by Bi does not produce relevant effect on them. In general, all the peaks for samples satisfy theLa–Sr–Mn–O phase. In addition, some weak impurity peaks from SrMnO, BiSrMnO, and Bi 2O3phases were found.14The crystal structure for the compositions La 0.7–xBixSr0.3MnO y was found to be rhombohedral structure.18–20Lattice parame- ters and cell volume were calculated and tabulated in Table 1. As seen in Table 1the lattice parameter aand unit-cell volume slight increase with increasing Bi concentration, while parameter cslight decreases with x. This almost per- fect match can be explained considering the similar dimen-sion of the two cations La 3þ(ionic radius r¼1.032 A ˚) and Bi3þ(ionic radius r¼1.030 A ˚).21 Figure 1shows the variation of resistivity with tempera- ture for La 0.7–xBixSr0.3MnO 3. Obviously, the resistivity increases with Bi doping. We expect that when the Bi con- tent increases not only the La-content decreases but also thecharge carrier density 22which leads to a reduction of the double exchange which is proportional to bandwidth. Therefore, the La/Bi configuration plays a prominent role incontrolling the resistivity. Consistently, the figure shows that the transition temperature ( T ms1) for La 0.7–xBixSr0.3MnO 3 decreases with increasing Bi content. These compounds have a distinct metallic phase below the transition tempera- ture ( Tms1) and above this temperature they become semi- conducting (S). In addition both the change in carrier concentration and Tms/Tcwith Bi content can be interpreted as arising from the rather covalent character of the Bi–Obonds (which are shorter than the La–O due to the covalent character of the former). That, in turn, contributes to the localization of the oxygen electrons coupling the Mn 3þ/4þ ions, and could explain the increase of the antiferromagnetic interactions and the decrease of the metallic character for theBi-substituted compounds, ending up with the totally AF and insulating Bi–Sr–MnO 3. The resistivities data above Tms1(PM–S region) are ana- lyzed in view of small polaron hopping (SPH) are generallyused where the transition temperatures of our composites are high temperature. The data are fitted well with the SPH model of Mott 23viz., q=T¼qaexp Eq=kBT/C0/C1; (1) where Eqis the energy equal to WD/2þWH; for T>Tms1 (where WHis the polaron hopping energy and WDis the dis- order energy). Eqandqacalculated and tabulated in Table 2 Indeed, as a result of the fit, the adiabatic SPH model is used in the present investigation. As in Table 2both Eqandqa decrease with increasing Bi content. This behavior is explained by considering that increasing xcauses charge delocalization (due to decrease of small polaron coupling con- stant or el–ph interaction constant) in the system and therebythe energy required to liberate a free carrier is reduced. To discuss the nature of the conduction mechanism below T ms1(FM–M region), the resistivity data are fitted with three empirical equations derived by different previous work:24–26 q¼q0þq2T2; (2) q¼q0þq2:5T2:5; (3) q¼q0þq2T2þq4:5T4:5; (4) where q0represents the resistivity due to grain boundary effects. q2T2in term in Eqs. (2)and(4)indicates the resistiv- ity due to electron–electron scattering process and is gener- ally dominant up to 100 K. On the other hand, the term q2.5T2.5represents the resistivity due to electron–magnon scattering process in ferromagnetic phase. Finally, the term q4.5T4.5indicates the resistivity due to electron–magnon scat- tering process in ferromagnetic region, which may be likelyto arise due to spin-wave scattering process. Our data of the metallic (ferromagnetic) part of the temperature-dependent resistivity ( q) curve (below T ms) fits well with Eq. (4)(R2>99.9%). Indicating the importance of grain/domain boundary effects and electron–magnon scatter- ing processes in the conduction of our composites. As inTable 3the values of q 0>q2>q4.5, this means that both grain boundaries and electron–electron scattering process play a role besides an electron–magnon scattering process inconduction mechanism. The last term q 4.5is also found toTABLE 1. The lattice parameters and cell volume (V)with concentration of the La 0.7–xBixSr0.3MnO 3. Bi content, at. % Parameter 0.05 0.10 0.15 a,A˚ 6.043 6.050 6.052 c,A˚ 7.760 7.760 7.748 V,A˚3245.41 245.98 245.76 FIG. 1. ln qversus temperature for La 0.7–xBixSr0.3MnO 3.TABLE 2. The variation of Eq(meV), qa(X/C1cm), ES(meV), Band WH (meV) with concentration for T>Tms. Bi content, at. % Parameter 0.05 0.10 0.15 qa 6.35 6.33 6.31 Eq 77.50 76.60 76.24 ES 11.90 9.40 7.50 B 0.035 0.022 0.022 WH 65.40 67.50 68.74Low Temp. Phys. 40(5), May 2014 Ahmed, Mohamed, and /C20Soka 419 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 28 Nov 2014 11:26:31decrease with increasing Bi content; the observed behavior may be due to partial alignment of the spins which results inlowering their fluctuations. 27 The temperature dependence of magnetic susceptibility (v) were measured with a magnetic field of 300 A/m. Figure 2shows the v–T curves for La 0.7–xBixSr0.3MnO 3samples ( x ¼0.05, 0.10, and 0.15 at. %), demonstrating the presence of clear FM transitions, while the value of magnetic susceptibil-ity decreases with increasing the doping of Bi for these sam- ples. This phenomenon can be interpreted as the increased bending of the Mn–O–Mn bond with decreasing averageA-site ionic radius hr Aidue to the partial substitution of smaller Bi3þions for a bit larger La3þions. This substitution causes the narrowing of the bandwidth and the decreasing ofthe mobility of e gelectron resulting in the weakness of DE interaction magnetism28(this confirm the q(T) data). Based on these results, ferro- to paramagnetic transition tempera-tures ( T C) were determined from the inflection point of dv/dT. It is clear from the values of Tvalues are also follow- ing the same trend as those of Tms. Figure 3shows the dependence of Seebeck coefficient (S) on the temperature. The TEP of these samples, depicted in Fig. 3, is positive at low temperatures, suggesting hole conduction, but becomes negative at high temperatures ( T> 300 K). The transition from metallic to semiconducting behavior ( Tms2) is clearly seen in the figure. Below the Tms2, the value of Sincreases with increasing Bi doping, above the transition this is also true except for x¼0.15 at. %. In addi- tion, above the transition Sdecreases rapidly. When the re- sistivity is thermally activated, the thermopower may also be expected to show semiconducting-like behavior. The signchange in Sat high temperatures confirms that the coexis- tence of two types of carriers. The negative Sat high temper- ature is attributed to the electrons which are excited from thevalence band (VB) into the conduction band (CB). Because of the higher mobility of electrons within the CB, Sis nega- tive. At low temperatures, the electrons in the VB band are excited into the impurity band which generates hole-like car- riers, which is responsible for a positive S. 29The magnitude ofSincreases with increasing Bi-doping except in the case ofx¼0.15 above Tms2, and the observed behavior due to the fact that for every ion of Bi doping, double the hole centers,which are localized and causes narrowing of e gband, this have been confirmed by v(T) and q(T) measurements. As in many previous work30,31that phonon drag ( Sg) and magnon drag ( Sm) contributions to the diffusion ( Sd)i n the low-temperature region. In the low-temperature FM–M region, a magnon drag effect is produced due to the presenceof electron–magnon scattering, while the phonon drag is due to electron–phonon scattering. In general, we can analyzed S–Trelation as (note that n ph/C24T3,nmag/C24T3/2), S¼S0þS3=2T3=2þS4T4; (5) where S0is a constant and accounts the low-temperature var- iation of thermo-power. The second term S3/2T3/2is attrib- uted to the magnon scattering process, while the origin of the last term S4T4is related to the spin-wave fluctuations in the FM–M region.30We fitted our data using Eq. (5)and we found that it fit well only for a short range of low tempera- ture. Therefore, we refit our data using the modified Eq. (6), which modified by adding two more terms, phonon drag and diffusion drag and the resulting equation is given by31 S¼S0þS1TþS3=2T3=2þS3T3þS4T4; (6) where the term S1TandS3T3represent to the diffusion and the phonon drag contribution to the TEP, respectively. The lines in Fig. 4, indicate that Eq. (6)is in good agreement with the experimental results of magnon contribution fromTABLE 3. The resistivity data, fitted with empirical Eqs. (2)–(4), due to different scattering process. Sample code q¼¼ q0þq2T2q¼q0þq2.5T2.5q¼q0þq2T2þq4.5T4.5q0,X/C1cm q2,1 0/C05X/C1cm/K2q4.5,1 0/C011X/C1cm/K4.5 0.05 0.9801 0.9602 0.9977 5.8214 6.6775 1.3144 0.10 0.9921 0.9876 0.9921 7.2756 5.7660 0.05250.15 0.9690 0.9466 0.9950 9.4549 7.9799 2.1155 FIG. 2. Temperature dependences of susceptibility for samples La 0.7– x BixSr 0.3MnO 3.FIG. 3. Temperature dependences of thermoelectric power for La 0.7–x BixSr0.3MnO 3.420 Low Temp. Phys. 40(5), May 2014 Ahmed, Mohamed, and /C20Soka This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 28 Nov 2014 11:26:3183 up to 313, 273, and 263 K high of samples with Bi content x¼0.05, 0.10, and 0.15 at. %, respectively. But for the con- tribution of phonon is in good agreement with experimental results from 163 to 273, 103 to 253, and 113 to 263 K ofsamples with x¼0.05, 0.10, and 0.15 at. %, respectively (Fig. 4(b)). It follows the linear dependence of T 3through a broad temperature regime and becomes zero at T¼0K . This behavior reflects that the phonon drag effect disappears because the lattice is frozen at T¼0 K. Here, it deviates from the T3-dependence below 100 K. The magnon drag component shows T3/2-behavior in several regimes, espe- cially below 173 K. This indicates that the dominant contri- bution of TEP in low temperature due to magnon drageffect. The charge carriers in the semiconductor region are not it inerrant and the transport properties are governed by ther-mally activated carriers because the effect of JT distortions in manganites results in strong electron–phonon coupling and hence the formation of polarons. Therefore, the thermo-electric power data of the present samples in semiconductor regime are fitted to Mott’s polaron hopping equation, S¼6k B=eDES=kBTþB ðÞ ; (7) where kBis the Boltzmann constant, eis the electronic charge, ESis the activation energy obtained from thermo- electric power data, and Bis a constant. In Eq. (7),B<1 implies the applicability of small polaron hopping model, whereas B<2 indicates the large polaron hopping. From theslope and the intercept of Sversus 1 /Tcurves (Fig. 5), we obtain the values of activation energy ESand the constant B (Table 2). The estimated values of Bindicated B<1 for three samples. Therefore, the small polaron hopping conduc-tion mechanism is also strongly supported by the high tem- perature ( T>T ms) TEP data. From conductivity data also we have approved of the possibility of the formation of smallpolaron hopping conduction mechanism. Using the activa- tion energy values from q(T) plots E qand those from S(T) plots ES, the polaron hopping energy values of all the sam- ples have been calculated using the relation, WH¼Eq-ES, and are given in Table 2. The Eqvalues are found to be higher than those of ES. Such a large difference in the activa- tion energy is confirm also the applicability of the SPH model in the semiconducting region.30 FIG. 4. Variations of phonon drag component with T3(a) and magnon drag component with T3/2(b). The red lines represent the deviation of linear fit to experimental curve. FIG. 5. Variations of Svs 1/Tfor samples La 0.7–xBixSr0.3MnO 3. The red lines represent the best fit to SPH model.Low Temp. Phys. 40(5), May 2014 Ahmed, Mohamed, and /C20Soka 421 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 28 Nov 2014 11:26:31The Curie temperature TCand the metal-semiconducting transition temperatures Tms1andTms2were deduced from the derivatives of the magnetic susceptibility curves q(T) and S(T), respectively. The evolution of Tms1ofq(T) and Tms2of S(T) and TCare shown in Fig. 6. This figure shows the phase diagram of rhombohedral structure La 0.7–xBixSr0.3MnO 3 (x¼0.05, 0.10, 0.15 at. %) system, where the FM /C0M phase underlie Tms1(red line), the FM /C0S phase lie between Tms1 (red line) and TC(black line), finally, PM–S phase lie above TC(black line). One interesting feature concerns the value of both Tms1 (q(T)) and Tms2(S(T)) which is often smaller than the corre- sponding value of TC(about 40 K). These transition tempera- tures decrease as xincreases, as expected. Therefore, we can predict the composition which should lead to the maximum magneto-resistance at the room temperature32which is more suitable for applications. 4. Conclusion In conclusion, temperature-dependent (360–80 K) elec- trical conductivity and thermopower measurements of the Bi-doped La 0.7–xBixSr0.3MnO 3(x¼0.05, 0.10, 0.15 at. %) system have revealed metal-semiconducting transitions. In other side, the magnetic properties have showed FM–PM transition between 310 and 334 K. The high-temperatureconductivity data can be successfully fitted with the small polaron-hopping conduction theory like that of usual oxide semiconductors. The data of Seebeck coefficient supports the small- polaron hopping transport mechanism. Also, the large differ- ence between E qandESprovides evidence of small polaron transport mechanism in the high-temperature PM region. The metallic state below Tmshas been considered in terms of the electron–magnon or electron–phonon scatteringprocess depending on the ambient temperature. From the high difference between the values of T msandTC, we predict the maximum magnetoresistance is at room temperature. The authors would like to thank Professor Dr. Marcel Miglierini and Dr. Marius Pavlovic for they help insusceptibility measurements in Slovak University of Technology, Faculty of Electrical Engineering and Information Technology. a)Email: fikry_99@yahoo.com 1Y. Tokura, A. Urushibara, Y. Moritomi, T. Arima, A. Asamitsu, G. Kido, and N. Furukawa, J. Phys. Soc. Jpn. 63, 3931 (1994). 2T. Ogawa, A. Sandhu, M. Chiba, H. Takeuchi, and Y. Koizumi, J. Magn. Magn. Mater. 290, 933 (2005). 3A. M. Ahmed, A. Kattwinkel, K. B €arner, C. P. Yang, J. R. Sun, G. H. Rap, H. Schicketanz, P. Terieff, and I. V. Medvedeva, J. Magn. Magn. Mater. 242, 719 (2002). 4E. Rezlescu, C. Doroftei, P. D. Popa, and N. Rezlescu, J. Magn. Magn. Mater. 320, 796 (2008). 5S. Takemoto, R. Takumi, H. Takeuchi, and Y. Koizumi, J. Soc. Powder Metall. 48, 1107 (2001). 6L. E. Gontchar, A. E. Nikiforov, and S. E. Popov, J. Magn. Magn. Mater. 22, 175 (2001). 7T. J. A. Popma and M. G. J. 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Uhlenbruck, B. B €uchner, R. Gross, A. Freimuth, A. Maria de Leon Guevara, and A. Rvecolevschi, Phys. Rev. B 57, R5571 (1998). 20A. Pimenov, M. Biberacher, D. Ivannikov, A. Loidl, V. Yu. Ivanov, A. A. Mukhin, and A. M. Balbashov, Phys. Rev. B 62, 5685 (2000). 21G. Srinivasan, R. M. Savage, V. Suresh Babu, and M. S. Seehra, J. Magn. Magn. Mater. 168, 1 (1997). 22G. H. Rao, J. R. Sun, A. Kattwinkel, L. Haupt, K. B €arner, E. Schmitt, and E. Gmelinn, Physica B 269, 379 (1999). 23N. F. Mott and E. A. Davis, Electronics Process in Non-Crystalline Materials (Clarendon, Oxford, 1971). 24A. Banerjee, S. Pal, and B. K. Chaudhuri, J. Chem. Phys. 115, 1550 (2001). 25L. Pi, L. Zhang, and Y. Zhang, Phys. Rev. B 61, 8917 (2000). 26G. Jeffrey Snyder, R. Hiskes, S. Dicarolis, M. R. Beasley, and T. H. Geballe, Phys. Rev. B 53, 14434 (1996). 27V. Ravindranath, M. S. Ramachandra Rao, G. Rangarajan, Y. Lu, J. Klein, R. Klingeler, S. Uhlenbruck, B. Buchner, and R. Gross, Phys. Rev. B 63, 184434 (2001). 28J. B. Torrance, P. Lacorre, and A. I. Nazzal, Phys. Rev. B 45, 8209 (1992). 29J. Yang, Y. P. Sun, W. H. Song, and Y. P. Lee, J. Appl. Phys. 100, 123701 (2006). 30S. Battacharya, S. Pal, A. Banerjee, H. D. Yang, and B. K. Chaudhuri,J. Chem. Phys. 119, 3972 (2003). 31B. H. Kim, J. S. Kim, T. H. Park, D. S. Le, and Y. W. Park, J. Appl. Phys. 103, 113717 (2008). 32G. Srinivasan and D. Hanna, Appl. Phys. Lett. 79, 641 (2001). This article was published in English in the original Russian journal. Reproduced here with stylistic changes by AIP Publishing.FIG. 6. The phase diagram of rhombohedral structure La 0.7–xBixSr0.3MnO 3 (x¼0.05, 0.10, 0.15 at. %) system.422 Low Temp. Phys. 40(5), May 2014 Ahmed, Mohamed, and /C20Soka This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.174.255.116 On: Fri, 28 Nov 2014 11:26:31

1.4874848.pdf

A sessile drop setup for the time-resolved synchrotron study of solid-liquid interactions: Application to intermetallic formation in 55%Al-Zn alloys N. Bernier, G. B. M. Vaughan, D. De Bruyn, H. Vitoux, M. De Craene, H. Gleyzolle, B. Gorges, J. Scheers, and S. Claessens Citation: Applied Physics Letters 104, 171608 (2014); doi: 10.1063/1.4874848 View online: http://dx.doi.org/10.1063/1.4874848 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect and kinetic mechanism of ultrasonic vibration on solidification of 7050 aluminum alloy AIP Advances 4, 077125 (2014); 10.1063/1.4891035 X-ray nano-diffraction study of Sr intermetallic phase during solidification of Al-Si hypoeutectic alloy Appl. Phys. Lett. 104, 073102 (2014); 10.1063/1.4865496 Multiscale modeling of the influence of Fe content in a Al–Si–Cu alloy on the size distribution of intermetallic phases and micropores J. Appl. Phys. 107, 061804 (2010); 10.1063/1.3340520 Publisher's Note: In situ photoelectron emission microscopy of a thermally induced martensitic transformation in a CuZnAl shape memory alloy [Appl. Phys. Lett. 88, 091910 (2006)] Appl. Phys. Lett. 88, 179902 (2006); 10.1063/1.2202239 In situ photoelectron emission microscopy of a thermally induced martensitic transformation in a CuZnAl shape memory alloy Appl. Phys. Lett. 88, 091910 (2006); 10.1063/1.2177450 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.79 On: Sun, 07 Dec 2014 01:33:16A sessile drop setup for the time-resolved synchrotron study of solid-liquid interactions: Application to intermetallic formation in 55%Al-Zn alloys N. Bernier,1,a)G. B. M. Vaughan,2D. De Bruyn,1H. Vitoux,2M. De Craene,1H. Gleyzolle,2 B. Gorges,2J. Scheers,1and S. Claessens1 1OCAS N.V., ArcelorMittal Global R&D Gent, Pres. J.F. Kennedylaan 3, 9060 Zelzate, Belgium 2European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France (Received 23 March 2014; accepted 23 April 2014; published online 1 May 2014) We introduce a dedicated setup for measuring by synchrotron diffraction in-situ crystallographic and chemical information at the solid–liquid interface. This setup mostly consists of a double- heating furnace composed of a resistive heating for the solid surface and an inductive heating toproduce a liquid droplet. The available high energy and high flux beams allow the rapid reaction kinetics to be investigated with very good time resolution down to 1 ms. An application of this setup is illustrated for the growth mechanisms of intermetallic phases during the hot-dipping ofsteel in a 55%Al-Zn bath. Results show that the three g-Al 5Fe2,h-Al 13Fe4, and a-Al 8Fe2Si phases grow at different times and rates during the dipping process, whereas the face-centered cubic AlFe 3 phase is not formed. VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4874848 ] The understanding of reactions that take place in the interfacial region between a solid substrate and a liquidphase is critical for many applications such as hot-dip coat- ing, welding, intermetallic matrix composites, or liquid metal corrosion. In particular, the growth kinetics and mech-anisms of intermetallic phases are of particular interest due to their significant effect on mechanical, adhesive, or corro- sion properties of various materials such as hot-dip coatedsteels or solder joints. 1–3However, in the case of the Fe-Al system, for instance, the growth kinetics and mechanisms of the intermetallic phases, as well as the solidification phe-nomena are still a controversial topic, 4–6mainly due to a lack of in-situ investigations for solid/liquid interactions. The technical challenges in joining metallic coated steelswith Al sheets, 7especially due to the formation of brittle intermetallic phases, clearly shows the need for a better understanding of the growth mechanisms of the intermetallicphases. The reported studies of the interfacial region between a steel substrate and a solidified Al-based metal are mostlybased on post-mortem characterizations of intermetallics for samples subjected to immersion tests and in some cases to diffusion treatments. 8Therefore, much effort has been devoted toward in-depth characterization of the interfacial region morphology, such as, e.g., the interface profile between steel and the first intermetallic phase, or the thick-ness of the different interface layers as a function of reaction time. However, these analyses have not led to a unified theory regarding the intermetallic formation, since diffusionlaws alone cannot account for all the experimental results. In-situ X-ray investigations provide valuable information, but such studies have been restricted to the formation ofintermetallic phases during coated steel heating 9or to the imaging of solidification processes.10Therefore, the in-situ investigation of crystalline products between solid steel andmolten metal during growth and solidification processes becomes an inevitable step toward a greater understanding ofthese phenomena. In the present work, a sessile drop experi- mental setup is used for the in-situ identification using syn- chrotron X-ray diffraction of the intermetallics formedbetween solid steel and molten 55%Al-Zn metal during growth processes. This composition corresponds to the so- called Galvalume 11coating deposited by hot-dipping typi- cally used to improve the corrosion resistance at high tem- peratures, the abrasion resistance, and the thermal and light reflectivity of steels. Fig. 1shows the experimental setup used at the ID11 beamline at the European Synchrotron Radiation Facility (ESRF). The experiment is designed to reach the followingobjectives: (i) to follow the evolution of diffraction patterns at the interface between the drop and the steel, (ii) to ensure the complete absence of oxidized compounds at the surfaceof the drop and substrate, (iii) to control the temperature of the system. For that purpose, this setup, which was devel- oped by the sample environment group on the ESRF ID 11beamline, consists of a combined resistance and induction furnace (Fig. 1(a)) that allows the Galvalume drop and the steel substrate to be independently heated under controlledatmosphere. This furnace is equipped with induction heating coils surrounding the tip of a stainless tube containing at the tip a 3 mm long solidified Galvalume rod. This rod has beenextracted from a bath whose chemical composition is approximately 55% Al, 43.1% Zn, 1.5% Si, and 0.4% Fe (wt. %). Si is added to prevent the very strong exothermicreaction between the Al-Zn bath and the sheet steel. A pol- ished high-purity electrolytic iron plate placed on a resistive heating assembly is mounted under this tube. The selectiveoxidation of the substrate is controlled through the use of the electrolytic steel which prevents to a large extent the buildup of, e.g., silicon oxide. The temperature of both heating ele-ments is controlled by two K-type thermocouples. Thin Kapton windows are fixed in the furnace walls to allow inci- dent and scattered X-rays to pass through the furnace with a)Author to whom correspondence should be addressed. Electronic mail: n.bernier@yahoo.fr 0003-6951/2014/104(17)/171608/5/$30.00 VC2014 AIP Publishing LLC 104, 171608-1APPLIED PHYSICS LETTERS 104, 171608 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.79 On: Sun, 07 Dec 2014 01:33:16negligible absorption or contribution to the background. The other elements of the setup can be visualized in Fig. 1(b), including the beam tube and the 2D fast readout low noise (FReLoN) detector. The primary optics, a double bent Laue- Laue monochromator, and a compound refractive lens trans-focator are used to produce an approximately 65.31 keV 5/C250lm 2monochromatic beam. A 10% H 2/C090% Ar atmosphere was used in the furnace to prevent the formation of an oxide on the steel surface. Although H 2might have some effect on the interface reac- tions, this atmosphere is also fairly representative of most ofthe hot-dipping industrial lines, in which the annealing pro- cess is usually done in a protective atmosphere of nitrogen and hydrogen. Prior to each experiment, the furnace chamberwas flushed with both the 10% H 2/C090% Ar atmosphere and a turbomolecular pump three times for 5 min each. The steel substrate was then subjected to the following annealing cycleto simulate the hot-dipping process: heating up to 800 /C14Ca t 8/C14C/min, then soaking for 5 min and finally cooling down to 600/C14Ca t8/C14C/min. The tip of the stainless tube was then heated by the inductive coils up to the melting point of Galvalume ( /C24600/C14C). Fig. 2shows a series of photos of the inside of the fur- nace during the annealing of the tube. Interestingly, Ebrill et al.12demonstrated that an oxidation of the steel substrate necessarily leads to a non-wetting Galvalume droplet. Indeed,the latter author showed that contact angles of h/C2420 /C14were measured on clean substrates and increased to h>90/C14for oxidized substrates. Therefore, the equilibrium contact angleof/C2430 /C14shown in Fig. 2(c) proves that there is no or negligi- ble oxidation of the substrate in the present experiments. In addition, measurements are performed using grazing inci-dence so as to only detect the intermetallic layer. The use of a high flux, high energy micro focused beam, and a fast readout detector allows diffraction patterns to be recorded every10–20 ms in order to follow the high reaction kinetics involved in the intermetallic phase formation. 13The diffrac- tion patterns were calibrated using a CeO 2powder. Fig. 3shows a series of diffraction patterns acquired over time, with the time “t” referring to the last acquisition before the droplet contacts the steel substrate. At this time, aweak Fe signal is detected because the beam is positioned slightly above the steel surface to be more sensitive to theintermetallic layer formation. Moreover, the typical large grain size of the annealed electrolytic steel leads to very few diffraction spots. As seen from Fig. 3(b), the Fe diffraction peaks are almost completely suppressed after 35 ms, whereas a few rings can already been observed in the diffraction pat- tern. These results illustrate the high kinetics of the nuclea-tion and growth phenomena of intermetallic phases. For a clear identification of individual phases, the two-dimensional diffraction patterns are integrated along the azimuthal angleinto intensity profiles as a function of the Bragg angle, as shown in Fig. 4. All the diffraction peaks can be indexed using the three following intermetallic phases: the base- FIG. 1. Photo of (a) the inside of the combined resistance and induction furnace, (b) the set-up for in-situ observation. (1) Tube containing 55%Al-Zn at the tip, (2) induction coil, (3) steel substrate, (4) resistance heating, (5) K-type thermocouples, (6) Kapton window, (7) beam tube, (8) furnace gas inl et, (9) vacuum valve, (10) induction power supply, and (11) 2D camera. FIG. 2. Photos of the inside of the furnace (a) before, (b) during, (c) afterheating the 55%Al-Zn tip in the tube.171608-2 Bernier et al. Appl. Phys. Lett. 104, 171608 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.79 On: Sun, 07 Dec 2014 01:33:16centered orthorhombic Al 5Fe2, the base-centered monoclinic Al13Fe4, and the hexagonal Al 8Fe2Si phases, also known as theg,h, and aphases, respectively. The JCPDS file numbers are given in the legend of Fig. 4. Note that the face-centered cubic AlFe 3phase does not appear in the diffraction spectra. Interestingly, the intensities of diffraction peaks taken overtime are significantly different for each intermetallic phase: (i) as for the hphase, the diffracted intensity is very intense as soon as the droplet contacts the steel, but this intensitydecreases after around 700 ms (see, e.g., ( /C0402) and (221)peaks), (ii) as for the gphase, the diffraction peaks appear soon after those from the hphase, and they do not vary much over time (see, e.g., (002) and (331) peaks), (iii) as for the a phase, the diffraction peaks start appearing after approxi- mately 700 ms, except for the (330) peak, and show the high- est intensity after 50 s. After performing the in-situ synchrotron experiments, a transmission electron microscope (TEM) analysis of the post-mortem sample has been carried out in order to bothconfirm and localize the presence of the above mentioned intermetallic phases. Fig. 5(a) shows a TEM image of the cross-section of the interface layer prepared by focused ionbeam (FIB) milling. The TEM and FIB instruments used in the present work are a JEOL JEM 2200FS-CS and a JEOL- SEIKO SMI 3050 Triple Beam, respectively. Electron dif-fraction (ED) patterns have been acquired along the thick- ness of the transition layer and subsequently indexed using theg,h, and aphases. Again, the face-centered cubic AlFe 3 is not observed in the post-mortem sample. The indexation of ED patterns, given in Figs. 5(b)–5(d), shows that the g andaphases are, respectively, composed of an elongated and prismatic coarse-grain structure; they are, respectively, located on top of the steel surface and below the Al-Zn coat- ing. In contrast, the hphase exhibits an elongated fine-grain structure which extends over several microns from the gtoa phases. The synchrotron and TEM results show that the first phase to be formed is the h-Al 13Fe4as it shows the highest intensity after 35 ms. The gphase most likely starts growing as soon as this hphase is formed. This confirms the high FIG. 3. Measured 2D diffraction patterns acquired at different times. The time “t” refers to the last diffraction pattern acquisition before the drop touches the steel surface. FIG. 4. Integrated diffraction patterns from the 2D camera shown in Fig. 3, together with the main diffraction peaks of the g(Al5Fe2),h(Al 13Fe4), and a (Al8Fe2Si) phases; the JCPDS files are 29-0043, 29-0042, and 41-0894, respectively.171608-3 Bernier et al. Appl. Phys. Lett. 104, 171608 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.79 On: Sun, 07 Dec 2014 01:33:16kinetics involved in the growth of the intermetallic phases.13–15In addition, these results are in agreement with Durandet et al.15who reported that the first intermetallic phase to form is the hphase while the growth of subsequent intermetallics, such as the gphase, is based on the diffusion and build-up of Fe atoms at the substrate- hphase interface. As seen from Fig. 4, the intensities of the diffraction peaks corresponding to the gphase show a maximum for a dipping time of /C24700 ms. Therefore, the local equilibrium at the substrate–intermetallic layer interface is most likely reached after 700 ms, and no subsequent intermetallic phase at thesteel interface is formed. The present study also confirms that the local equilibrium between the hphase and the coat- ing generates the growth of the aphase, as shown in Fig. 5. Fig. 4shows that this phase becomes clearly visible in the diffraction patterns after 700 ms, although the strong (330) peak can be observed as soon as the droplet contacts the sub-strate, meaning that a small amount is formed immediately. This suggests that the nucleation of the prismatic agrains may be favored along preferential crystallographic direc-tions. The reason for this direction-dependent growth is unknown. Finally, note that the diffracted intensities of the a phase significantly increase from 700 ms to 50 s. However,this continuous agrowth may result from the low cooling rate of the droplet with respect to the industrial production, providing in turn a constant source of Al by thermaldiffusion. In summary, a setup has been developed for the in-situ synchrotron analysis of solid-liquid interactions. The ID 11beamline at the ESRF was fully appropriate for such experi- ments, since it meets the main technical requirements, such as an X-ray microbeam, to be precisely focused on the solid-liquid interface, high-energy x-rays to pass through the liquid drop, high flux beams with very good time resolutions down to 1 ms to investigate the high reaction kinetics, and thepossibility to work with different types of sample environ- ment. This setup has been used to understand the growthmechanisms of intermetallic phases during the hot-dipping of steel in a Galvalume bath. The results show that the inter- metallic phases start growing within 35 ms time, first withthe development of the hphase. The growth of the gphase is then controlled by the diffusion and build-up of Fe atoms at the substrate- hphase interface up to a dipping time of /C24700 ms for which the local equilibrium is reached. In addi- tion, the preferential growth of agrains on top of the hphase along a particular direction is also illustrated. Future experi-ments should be conducted both to investigate the latter result and to improve the current setup by, e.g., dividing the experiment into two parts after the drop touches the sub-strate: (i) drop and substrate held at high temperature to sim- ulate the reaction time and (ii) a high cooling rate system to simulate the solidification process. These experiments willprovide more insight into the solidification mechanisms of the overlay coating, which may explain in the case of the Galvalume coating the occurrence of different spanglesizes 16as a function of the process parameters. 1X. Wu, D. Weng, S. Zhao, and W. Chen, Surf. Coat. Technol. 190, 434 (2005). 2H. Springer, A. Kostka, J. F. dos Santos, and D. Raabe, Mater. Sci. Eng. A 528, 4630 (2011). 3B. Szczucka-Lasota, B. Formanek, and A. Hernas, J. Mater. Process Technol. 164, 930 (2005). 4R. W. Richards, R. D. Jones, P. D. Clements, and H. Clarke, Int. Mater. Rev. 39, 191 (1994). 5N. X. Zhang, J. Wosik, W. Fragner, R. Sonnleitner, and G. E. Lautner, Intermetallics 18, 221 (2010). 6K. Kee-Hyun, K. Van-Daele, G. Van-Tendeloo, and Y. Jong-Kyu, Mater. Sci. Forum 519, 1871 (2006). 7G. Sierra, P. Peyre, F. Deschaux Baume, D. Stuart, and G. Fras, Mater. Charact. 59, 1705 (2008). 8P. Dillmann, B. Regad, and G. Moulin, J. Mater. Sci. Lett. 19, 907 (2000). FIG. 5. (a) TEM image of the interface layer; (b) and (c, d) electron diffraction patterns acquired in areas marked in (a).171608-4 Bernier et al. Appl. Phys. Lett. 104, 171608 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.79 On: Sun, 07 Dec 2014 01:33:169F. Rizzo, S. Doyle, and T. Wroblewski, Nucl. Instrum. Meth. B 97, 479 (1995). 10R. H. Mathiesen, L. Arnberg, K. Ramsoskar, T. Weitkamp, C. Rau, and A.Snigirev, Metall. Mater. Trans. B 33, 613 (2002). 11J. H. Selverian, M. R. Notis, and A. R. Marder, J. Mater. Eng. 9, 133 (1987). 12N. Ebrill, Y. Durandet, and L. Strezov, Metall. Mater. Trans. B 31, 1069 (2000). 13M. L. Giorgi, J. B. Guillot, and R. Nicolle, J. Mater. Sci. 40, 2263 (2005).14M. Hamankiewicz, J. Krol, M. Talach-Dumanska, and J. Dutkiewicz, Arch. Metall. Mater. 51, 503 (2006), http://www.imim.pl/archives/vol- ume-51-issue-3-2006 . 15Y. Durandet, L. Strezov, and N. Ebrill, in 4th International Conference on Zinc and Zinc Alloy Coated Steel Sheet (1998), p. 147. 16J. F. Willem, H. Cornil, S. Claessens, C. Xhoffer, M. Fiorucci, and A. Hennion, in 5th International Conference on Zinc and Zinc Alloy Coated Steel Sheet (2001), p. 401.171608-5 Bernier et al. Appl. Phys. Lett. 104, 171608 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 216.165.95.79 On: Sun, 07 Dec 2014 01:33:16

1.4893606.pdf

An efficient light trapping scheme based on textured conductive photonic crystal back reflector for performance improvement of amorphous silicon solar cells Peizhuan Chen, Guofu Hou, QiHua Fan, Qian Huang, Jing Zhao, Jianjun Zhang, Jian Ni, Xiaodan Zhang, and Ying Zhao Citation: Applied Physics Letters 105, 073506 (2014); doi: 10.1063/1.4893606 View online: http://dx.doi.org/10.1063/1.4893606 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optimal design of one-dimensional photonic crystal back reflectors for thin-film silicon solar cells J. Appl. Phys. 116, 064508 (2014); 10.1063/1.4893180 Solar light trapping in slanted conical-pore photonic crystals: Beyond statistical ray trapping J. Appl. Phys. 113, 154315 (2013); 10.1063/1.4802442 Diffraction and absorption enhancement from textured back reflectors of thin film solar cells J. Appl. Phys. 112, 024516 (2012); 10.1063/1.4737606 Photonic crystal based back reflectors for light management and enhanced absorption in amorphous silicon solar cells Appl. Phys. Lett. 95, 231102 (2009); 10.1063/1.3269593 Effect of self-orderly textured back reflectors on light trapping in thin-film microcrystalline silicon solar cells J. Appl. Phys. 105, 094511 (2009); 10.1063/1.3108689 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.64.11.153 On: Mon, 29 Sep 2014 11:50:05An efficient light trapping scheme based on textured conductive photonic crystal back reflector for performance improvement of amorphous siliconsolar cells Peizhuan Chen,1Guofu Hou,1,a)QiHua Fan,2Qian Huang,1Jing Zhao,1Jianjun Zhang,1,b) Jian Ni,1Xiaodan Zhang,1and Ying Zhao1 1Tianjin Key Laboratory of Photoelectronic Thin-Film Devices and Technique, Institute of Photoelectronics, Nankai University, Tianjin 300071, People’s Republic of China 2Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, South Dakota 57007, USA (Received 24 June 2014; accepted 9 August 2014; published online 20 August 2014) An efficient light trapping scheme named as textured conductive photonic crystal (TCPC) has been proposed and then applied as a back-reflector (BR) in n-i-p hydrogenated amorphous silicon (a-Si:H) solar cell. This TCPC BR combined a flat one-dimensional photonic crystal and a ran- domly textured surface of chemically etched ZnO:Al. Total efficiency enhancement was obtainedthanks to the sufficient conductivity, high reflectivity and strong light scattering of the TCPC BR. Unwanted intrinsic losses of surface plasmon modes are avoided. An initial efficiency of 9.66% for a-Si:H solar cell was obtained with short-circuit current density of 14.74 mA/cm 2, fill factor of 70.3%, and open-circuit voltage of 0.932 V. VC2014 AIP Publishing LLC . [http://dx.doi.org/10.1063/1.4893606 ] One of the foremost challenges in designing thin film sil- icon solar cells (TFSC) is devising an efficient light-trappingscheme due to the short optical path length imposed by the thin absorber thickness. 1,2Forn-i-p solar cells, the strategy relies on the deposition of a thin ITO front contact which actsas an antireflection layer, and a randomly textured Aluminum-doped Zinc Oxide (AZO)/Ag back-reflector (BR) which is commonly used to reflect and scatter light within theabsorption layer. 3,4It is well known that a larger texture pro- vides superior light trapping.5However, there is a trade off between the suitable texture for the light scattering and theloss in the AZO/Ag BR, which originates from plasmon absorption on the rough surface of metallic layer. 6,7In addi- tion, cost reduction achieved by efficiency enhancement forusing AZO/Ag BR is counterweighed mostly for the expen- sive raw material silver. An alternative, highly promising approach is to use a dielectric one-dimensional (1D) photonic crystal (PC) to enhance total internal reflection at the back surface, which is a multilayer structure in which two different films with highrefractive index contrast are periodically stacked. 8,9A combi- nation of transparent conductive oxide (TCO) layer and 1D- PC can simultaneously serve as the back electrical contactand the BR in a TFSC, while avoiding unwanted intrinsic losses from surface plasmon modes. Besides high reflectivity and conductivity, another necessary issue for highly efficientBR is strong scattering of the incoming light back into the absorption layer. In order to introduce scattering into 1D-PC with TCO, some groups have adopted two-dimensional gra-tings on the TCO layer 10or even on the 1D-PC.11Although simulation results show a significant improvement of opticalabsorption with respect to the flat one, so far no experimental results have been reported. Actually, the steep valleys of thegratings would inevitably induce defects within the active layer and deteriorate the device performance. 12Some other groups suggest depositing 1D-PC on textured substrate (ran-dom 13or periodic grating14). However, it is uneasy to copy the waviness of the substrate texture profile from one period to another period due to the non-conformal growth during thedeposition process. 4,15Thus, the periodic symmetry of 1D- PC would be destroyed, leading to a reduction of reflectivity and an offset of reflection region. Here we developed a BR that can improve the perform- ance of the n-i-p TFSC via a Texture Conductive Photonic Crystal (TCPC). 1D-PC was first deposited on a flat glass,followed by a thick AZO film (800 nm) with a lower sheet re- sistance ( R sq)o f5X/sq. We called this structure Conductive Photonic Crystal (CPC). If the CPC was chemically etched in0.5% HCl acid to introduce a crater-like textured surface, then it can be called TCPC with a slight increase of R sqto 10X/sq. The 1D-PC structure is alternatively stacked with 155 nm SiO x(nffi1.5 at k¼650 nm) on top and 25 nm a-Si:H (nffi4a tk¼650 nm) at the bottom in five periods. Figure 1 shows the schematic diagram of the TCPC-based n-i-p a-Si:H TFSC. Note that the 1D-PC structure is dielectric, and the back electric current transports laterally in the AZO layer until to be collected at the electrode. Focus was put on n-i-p a-Si:H TFSC with structure of BR/n-a-Si:H (15 nm)/i-a-Si:H (300 nm)/p-nc-Si:H (15 nm)/ ITO (70 nm) as a convenient prototype. All of the p,i, and n layers as well as 1D-PC were deposited in a multi-chamber RF-PECVD system. The AZO film was deposited using radio frequency magnetron sputtering system with a sintered ce-ramic ZnO target with 2 wt. % Al 2O3. For comparison study, the sputtered AZO (100 nm)/textured Ag BR deposited on the stainless steel by the same sputtering system were fabricated,a)Author to whom correspondence should be addressed. Electronic mail: gfhou@nankai.edu.cn. Tel.: þ86-022-23508663. b)Electronic mail: jjzhang@nankai.edu.cn 0003-6951/2014/105(7)/073506/5/$30.00 VC2014 AIP Publishing LLC 105, 073506-1APPLIED PHYSICS LETTERS 105, 073506 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.64.11.153 On: Mon, 29 Sep 2014 11:50:05where the texture was controlled by changing the substrate temperature and film thickness. Surface morphology of BRs was characterized using scanning electron microscopy (SEM) and atomic force mi-croscopy (AFM). A UV-vis-near-infrared (NIR) spectropho- tometer (Carry 5000) was used to measure the total and direct reflection spectra in the range of 300–1000 nm for op-tical analysis. Current-density versus voltage ( J-V) character- istics and spectral response were measured with a Wacom solar simulator (WXS-156 S-L2, AM1.5GMM) and a quan-tum efficiency ( QE) system (QEX10, PV Measurement), respectively. In the a-Si:H TFSC with an intrinsic layer thickness of 300 nm, the BR will not start to work until the wavelength of the incoming light is above 550 nm, since light with k<550 nm can be efficiently absorbed by the intrinsic layer even during one single optical pass. 1It is well known that the upper absorption wavelength limit for a-Si:H locates around 750–800 nm due to its wide bandgap of over 1.6 eV.1So the light trapping spectral range needed for a-Si:H TFSC or highly reflective range needed for 1D-PC should at least cover the wavelength range of 550–800 nm. We have identi-fied before that light will couple into the top three periods of 1D-PC within the photonic bandgap (PBG). Little change can be observed for the reflection in the PBG area if the periodnumber higher than five. 16An average reflectivity of 98% in the above light trapping range can be achieved for our 1D-PC with only five periods, which is superior to Ag film of 93.4%.The reason we adopted SiO xas the top layer and a-Si:H as the bottom layer was that the total internal reflection will be easier to take place at the AZO/SiO x(refractive index of nAZO>nSiOx) interface than the AZO/a-Si:H ( nAZO<na-Si:H) interface, especially when light is obliquely incident. In order to reduce parasitic absorption, a thin TCO film inserting between 1D-PC and active layers was always adopted as back electric contact in previous simulation work.10But care should be taken, a high conductivity TCO layer is specially needed for the TCPC or CPC-based cell in order not to affect the cell fill factor ( FF), since the electric current transports laterally in the TCO layer as shown inFig.1. Thereby a trade off might arise since the Rsqof AZO decreases with the increase of layer thickness.1To quantify the influence of the AZO thickness on the short-circuit cur- rent density ( Jsc), we used finite difference time domain algorithm to calculate the quantum efficiency QE(k) of the solar cell at first. The Jscvalue can be obtained by integrating theQEcurves with AM1.5 solar spectrum (taken from ASTM G173–0317). Here we studied the CPC based a-Si:H TFSC instead, as it is uneasy to identify the thickness of a rough surface AZO layer in a TCPC. We assumed that all electron-hole pairs contribute to the photocurrent. Figure 2 illustrates the Jscvalues as a function of AZO thickness for the five periods of CPC based a-Si:H TFSC. The fluctuant morphology of the curve is mainly caused by the interference effect of the AZO layer. The Jscvalue for a-Si:H TFSC with a flat AZO (100 nm)/ flat Ag (100 nm) BR was also plotted as a reference. It is clear to see that the CPC-based solar cell offers a higher Jscthan the AZO/Ag-based of 13.01 mA/cm2. And no significant deterioration of Jsccan be observed by increasing the AZO thickness to 1 lm, which is thick enough to form the textured surface by post chemical etching. Forp-i-n TFSC, typical R sqvalues of /C2410X/sq (relative thick- ness of 400 nm in our lab) prove the high quality of AZO film used as front electric contact1(electric current transports laterally too). This value is more sufficient for CPC and TCPC-based cells due to the higher mobility of electron than hole. In a word, a relative thicker AZO film would be benefi-cial for CPC-based solar cells from both electrical and opti- cal (could be etched to provide scattering) point of view. The thickness of AZO film adopted in our CPC is 800 nm with R sqof 5X/sq. TCPC was formed after 40 s post etching in 0.5% HCl acid with a slight increase of Rsqto 10X/sq due to the decrease of film thickness. Figure 3shows the cross-sectional SEM images of CPC and TCPC BRs, respectively. It is clear to see that five periods of flat 1D-PC was deposited and the surface morphology was changedfrom flat to texture by chemical etching of CPC. Note that it is difficult to distinguish a-Si:H and SiO xin the SEM images due to the close nature of these two materials. It has beenshown that light trapping can be enhanced by using a tex- tured surface with relatively large features that produce a FIG. 1. Schematic diagram of n-i-p a-Si:H TFSC based on TCPC BR. FIG. 2. Short-circuit current density ( Jsc) values as a function of AZO thick- ness for five periods of CPC based a-Si:H TFSC. The value for the cell onflat AZO (100 nm)/Ag (100 nm) BR was plotted also for reference (dash line).073506-2 Chen et al. Appl. Phys. Lett. 105, 073506 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.64.11.153 On: Mon, 29 Sep 2014 11:50:05high haze factor.5A RMS value of 126 nm was obtained for our TCPC with a crater-like surface, as can be seen from the AFM images of Fig. 4(a). In traditional n-i-p TFSC, a BR with textured Ag and thin AZO layer (100–150 nm) has beenfound to give the best cell performance by United Solar before. 3,18It is noteworthy that a highly textured Ag inter- face could cause a significant plasmonic loss and some otherparasitic loss in narrow angle valleys, thus to counteract the gain from enhanced scattering. Yan et al. achieved a J scover 30 mA/cm2in hydrogenated nanocrystalline silicon solar cells based on an optimized textured AZO/Ag BR with a RMS–40 nm and lateral feature size /C24500 nm.18The RMS value (39 nm) and lateral feature size of the reference tex-tured AZO/Ag BR used in this letter is similar to the one in above literature, as illustrated in Fig. 4(b). With respect to AZO/Ag, a more moderate surface morphology can beobserved for TCPC, which might offer a favorable physical property for the growth of TFSC. 19 Figure 5shows the optical performance of TCPC, flat 1D-PC, CPC, and textured AZO/Ag, where (a) is the total re- flectance (TR) and transmittance (TT), and (b) is the haze fac- tor. Several interference fringes can be observed byincorporating a thick AZO film into 1D-PC (CPC), resulting in a reduction of average TR from 98% to 92.4% in the wave- length range of 550–800 nm. However, the interferencefringes disappear after chemical etching due to the diffuse reflection caused by the rough surface. An average TR of 90.4% can be obtained for TCPC, which is superior to AZO/Ag of 85.8%. Note that nearly no transmission can be observed in the wavelength range of 550–800 nm whether a chemical etching process is applied or not. As mentionedbefore that light will couple into the top three periods of 1D- PC. 16So the reduced TR from CPC to TCPC is mainly afforded by the increase free carrier absorption of AZO1and enhanced bulk absorption of 1D-PC, since the rough interface would lead to angled incidence which elongate the optical path length in the AZO layer and 1D-PC. Actually, the TRwould even be reinforced at the device level, because it will be easier to form total internal reflection at the reduced index matching of the rough a-Si:H/AZO interface. An averagehaze factor of 80.6% in the light trapping range (550–800 nm) can be obtained for TCPC, corresponding to a relative enhancement of 69% with respect to the AZO/Ag (47.8%). To further study the influence of electrical and optical properties of BRs on solar cell performance, the above- mentioned CPC, TCPC, and AZO/Ag BRs were used assubstrates into n-i-p a-Si:H solar cells. Figure 6shows J-V characteristics and external quantum efficiency (EQE) curves of a-Si:H solar cells. The J-Vcharacteristic parameters of these solar cells are inserted in Fig. 6(a). The J scvalues were calculated by integrating the measured EQE curves with the AM1.5 solar spectrum. Both the TCPC-based and FIG. 3. Cross-sectional SEM images of (a) CPC and (b) TCPC. FIG. 4. AFM images of (a) TCPC and (b) textured AZO/Ag.073506-3 Chen et al. Appl. Phys. Lett. 105, 073506 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.64.11.153 On: Mon, 29 Sep 2014 11:50:05AZO/Ag-based cells suffer a slight decrease of open-circuit voltage ( Voc), since the introduced textured interface tend to decrease the shunt resistance.20Compared to the highly con- ductive AZO/Ag-based and the flat CPC-based cells, no sig-nificant deterioration of FFcan be observed for the TCPC- based cell. This indicates that the TCPC BR not only can serve as a sufficient back electric contact but also can providea moderate textured surface, which is beneficial to reduce the cell defect densities. It is clear that the EQE values can be significantly improved in the wavelength range of550–800 nm by introducing textured morphology. Significant interference fringes are observed in the flat CPC-based cell in above wavelength range, signaling substantial directlyreflected light. With the increase of RMS value to 39 nm (AZO/Ag), the interference fringes are reduced, indicating a reduction of direct reflection. Further increase the RMS valueto 126 nm (TCPC) results in a smooth and an enhanced EQE curve, pointing to strong scattering of the incoming light and less BR parasitic absorption. The calculated J scincreases from 12.13 mA/cm2for the CPC-based cell to 14.74 mA/cm2 for the one with TCPC-based, with a relative increase of21.5%. The initial efficiency ( Eff.) increases from 8.24% to 9.66%, corresponding to a relative enhancement of 17.2%. Also, with respect to AZO/Ag-based cell ( J sc: 14.02 mA/cm,2 Eff.: 9.2%), an enhancement of 5% for both Jscand efficiency can be obtained for the TCPC-based cell. In summary, we have experimentally demonstrated that TCPC BR, combining flat 1D-PC and conductive randomtextured AZO, can provide significant light scattering while maintaining high reflectivity in the light trapping range from 550 nm to 800 nm. Compared with the a-Si:H solar cells on CPC BR and traditional AZO/Ag BR, the cell with TCPC BR showed an improved Jscwithout deterioration of VocandFF, resulting in a total efficiency enhancement of 17.2% and 5%,respectively. The TCPC BR overcame the issues of high electri- cal resistance and low scattering in conventional photonic crystal reflectors, which limited their appl ications in thin film solar cells. The work was supported by the National Natural Science Foundation of China (Nos. 61176060 and 61377031), The key Project of Natural Science Foundation of Tianjin (No. 12JCZDJC28300), The National High-TechR&D Program of China (No. 2011AA050503), The National Basic Research Program of China (No. 2011CBA00705, 2011CBA00706, and 2011CBA00707), and Major Scienceand Technology Support Project of Tianjin (No. 11TXSYGX22100). Also acknowledged are the funding supports from South Dakota BOR PIF Grant, NationalScience Foundation (Grants Nos. 1248454, 1248970, and 0903804) and the State of South Dakota. 1J. Muller, B. Rech, J. Springer, and M. Vanecek, Sol. Energy 77, 917 (2004). 2Z. F. Yu, A. Raman, and S. H. Fan, Opt. Express 18, A366 (2010). 3G. Yue, L. Sivec, J. M. Owens, B. Yan, J. Yang, and S. Guha, Appl. Phys. Lett. 95, 263501 (2009). 4T. Soderstrom, F. J. Haug, X. Niquille, and C. Ballif, Prog. Photovoltaics 17, 165 (2009). 5Y. Zhao, S. Miyajima, Y. Ide, A. Yamada, and M. Konagai, Jpn. J. Appl. Phys. Part 1 41, 6417 (2002).FIG. 5. Optical performances of TCPC, 1D-PC, CPC and textured AZO/Ag. (a) Total reflectance (TR) and transmittance (TT) curves and (b) haze factor.FIG. 6. (a) J-Vcurves and (b) EQE curves of a-Si:H TFSC with differing BRs (CPC, TCPC, and textured AZO/Ag).073506-4 Chen et al. Appl. Phys. Lett. 105, 073506 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.64.11.153 On: Mon, 29 Sep 2014 11:50:056F. J. Haug, T. Soederstroem, O. Cubero, V. Terrazzoni-Daudrix, and C. Ballif, J. Appl .Phys. 104, 064509 (2008). 7B. Curtin, R. Biswas, and V. Dalal, Appl. Phys. Lett. 95, 231102 (2009). 8L. Zeng, P. Bermel, Y. Yi, B. A. Alamariu, K. A. Broderick, J. Liu, C. Hong, X. Duan, J. Joannopoulos, and L. C. Kimerling, Appl. Phys. Lett. 93, 221105 (2008). 9J. Springer, A. Poruba, L. Mullerova, M. Vanecek, O. Kluth, and B. Rech, J. Appl. Phys. 95, 1427 (2004). 10D. Y. Zhou and R. Biswas, J. Appl. Phys. 103, 093102 (2008). 11J. G. Mutitu, S. Y. Shi, C. H. Chen, T. Creazzo, A. Barnett, C. Honsberg, and D. W. Prather, Opt. Express 16, 15238 (2008). 12H. Sai, Y. Kanamori, and M. Kondo, Appl. Phys. Lett. 98, 113502 (2011). 13O. Isabella, S. Dobrovolskiy, G. Kroon, and M. Zeman, J. Non-Cryst. Solids 358, 2295 (2012).14L. Zeng, Y. Yi, C. Hong, J. Liu, N. Feng, X. Duan, L. C. Kimerling, and B. A. Alamariu, Appl. Phys. Lett. 89, 111111 (2006). 15M. Sever, B. Lipovsek, J. Krc, A. Campa, G. Sanchez Plaza, F.-J. Haug, M. Duchamp, W. Soppe, and M. Topic, Sol. Energy Mater. Sol. Cells 119, 59 (2013). 16P. Z. Chen, G. F. Hou, J. J. Zhang, X. D. Zhang, and Y. Zhao, J. Appl. Phys. 116, 064508 (2014). 17P. Bermel, C. Luo, L. Zeng, L. C. Kimerling, and J. D. Joannopoulos, Opt. Express 15, 16986 (2007). 18B. Yan, G. Yue, L. Sivec, J. Owens-Mawson, J. Yang, and S. Guha, Sol. Energy Mater. Sol. Cells 104, 13 (2012). 19H. Sai, K. Saito, N. Hozuki, and M. Kondo, Appl. Phys. Lett. 102, 053509 (2013). 20J. Yin, H. Zhu, Y. Wang, Z. Wang, J. Gao, Y. Mai, Y. Ma, M. Wan, and Y. Huang, Appl. Surf. Sci. 259, 758 (2012).073506-5 Chen et al. Appl. Phys. Lett. 105, 073506 (2014) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. 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1.4896375.pdf

Influence of high magnetic field on the luminescence of Eu3+-doped glass ceramics Wei Jiang, Junpei Zhang, Weibo Chen, Ping Chen, Junbo Han, Beibei Xu, Shuhong Zheng, Qiangbing Guo, Xiaofeng Liu, and Jianrong Qiu Citation: Journal of Applied Physics 116, 123103 (2014); doi: 10.1063/1.4896375 View online: http://dx.doi.org/10.1063/1.4896375 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in New oxyfluoride glass with high fluorine content and laser patterning of nonlinear optical BaAlBO3F2 single crystal line J. Appl. Phys. 112, 093506 (2012); 10.1063/1.4764326 Optical properties of Dy3+ doped bismuth zinc borate glass and glass ceramics AIP Conf. Proc. 1447, 579 (2012); 10.1063/1.4710136 Ultraviolet and white photon avalanche upconversion in Ho 3 + -doped nanophase glass ceramics Appl. Phys. Lett. 86, 051106 (2005); 10.1063/1.1861975 Photostimulated luminescence in Eu-doped fluorochlorozirconate glass ceramics Appl. Phys. Lett. 83, 449 (2003); 10.1063/1.1593228 Role of the Eu 3+ ions in the formation of transparent oxyfluoride glass ceramics J. Appl. Phys. 89, 5307 (2001); 10.1063/1.1366658 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.230.243.252 On: Thu, 27 Nov 2014 01:03:27Influence of high magnetic field on the luminescence of Eu31-doped glass ceramics Wei Jiang,1Junpei Zhang,2Weibo Chen,1Ping Chen,1Junbo Han,2Beibei Xu,1 Shuhong Zheng,1Qiangbing Guo,1Xiaofeng Liu,1,a)and Jianrong Qiu1,3,a) 1State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China 2Wuhan National High Magnetic field Center, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China 3State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, China (Received 24 June 2014; accepted 11 August 2014; published online 24 September 2014) Rare earth (RE) doped materials have been widely exploited as the intriguing electronic configuration of RE ions offers diverse functionalities from optics to magnetism. However, thecoupling of magnetism with photoluminescence (PL) in such materials has been rarely reported in spite of its fundamental significance. In the present paper, the effect of high pulsed magnetic field on the photoluminescence intensity of Eu 3þ-doped nano-glass-ceramics has been investigated. In our experiment, Eu-doped oxyfluoride glass and glass ceramic were prepared by the conventional melt-quenching process and controlled heat treatment. The results demonstrate that the integrated PL intensity of Eu3þdecreases with the enhancement of magnetic field, which can be interpreted in terms of cooperation effect of Zeeman splitting and magnetic field induced change in site symme- try. Furthermore, as a result of Zeeman splitting, both blue and red shift in the emission peaks of Eu3þcan be observed, and this effect becomes more prominent with the increase of magnetic field. Possible mechanisms associated with the observed magneto-optical behaviors are suggested. The results of the present paper may open a new gate for modulation of luminescence by magnetic field and remote optical detection of magnetic field. VC2014 AIP Publishing LLC . [http://dx.doi.org/10.1063/1.4896375 ] I. INTRODUCTION Functional materials with both magnetic and optical properties have sparked considerable interest due to theirpotential applications in magnetic field detection, high accu- racy communications, and high magnetic field calibration. 1–3 Materials doped with lanthanide ions are ideal candidates to realize these functions because of their intriguing magnetic properties, rich 4f energy levels in optical frequency, and excellent photostability.4,5According to the Judd-Ofelt theory,6,7the effect of applied magnetic field on the photolu- minescence (PL) intensities of rare-earth (RE) ions is extremely weak because of the even parity of magnetic field.Some recent work, however, revealed that the external applied magnetic field strongly affected the PL intensities of RE doped materials. 1–3,8–10For example, Tikhomirov et al. reported that the intensity of the4S3/2!4I15/2emission of Er3þin nano-glass-ceramics decreased by two orders of magnitude in the presence of magnetic field up to 50 T.1 Similarly, Liu and his co-workers found that the PL intensity of NaGdF 4:Nd3þ,Yb3þ,Er3þnanocrystals could be effi- ciently tuned under external magnetic field by varying theconcentration of Nd 3þdoping.2 To make use of the magneto-optical effect in bulk mate- rials, transparent glass ceramics might be a better alternativefor optical application as large single crystals are not easily accessible. Among different types of glass, RE doped oxy- fluoride glass ceramics have been widely studied as they pos- sess not only relatively high chemical and mechanical stability but also relatively low phonon energy.11,12Such systems have in fact been employed for the studying of optical-magnetic effect,1but the poisonous nature of its main components PbF 2and CdF 2hinders its extensive application. Alkaline-earth fluorides are therefore better matrix due to their stability and high solubility for RE ions.13Concerning the type of RE ions, europium (Eu) ion has been used exten-sively as luminescence activator for its strong characteristic emission. Furthermore, Eu 3þion can serve as hypersensitive probe as its red emission is highly sensitive to the structureand site symmetry. This high symmetry sensitivity as well as the relatively high luminescent efficiency and narrow emis- sion bandwidth suggests that Eu 3þions might also facilitate the optical detection of magnetic field.14,15 In this work, the effect of strong pulsed magnetic field on the PL properties of Eu-doped glass ceramic containing SrF 2 nanocrystals was studied. The in tegrated PL intensity is reduced with the increment of magnetic field, possibly due to the change of symmetry of ligand environment and absorption triggered bymagnetic field. Both blue shift and red shift of peaks were found in the 5D0!7F4emission band, and the possible mecha- nisms are discussed. These remarkable changes indicate thatthe studied material can serve as a good optical-magnetic dual- functional material for various potential applications.a)Authors to whom correspondence should be addressed. Electronic addresses: xfliu@zju.edu.cn and qjr@zju.edu.cn. 0021-8979/2014/116(12)/123103/5/$30.00 VC2014 AIP Publishing LLC 116, 123103-1JOURNAL OF APPLIED PHYSICS 116, 123103 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.230.243.252 On: Thu, 27 Nov 2014 01:03:27II. EXPERIMENTAL PROCEDURE Oxyfluoride glass with composition of 50SiO 2–22Al 2O3–20SrF 2–6NaF–2EuF 3(in mol %) was pre- pared by the conventional melt-quenching process, usingSiO 2,A l 2O3, NaF, SrF 2, and EuF 3as raw materials. In a typi- cal process, the homogenized powder mixtures were melted in a covered corundum crucible at 1450/C14C for 45 min in air atmosphere, and then the melt was poured onto a cold brass plate and pressed by another plate to form the glass chips. Followed by a heat treatment at 590/C14C for 1 h in air, the transparent glass ceramics containing SrF 2nanocrystals were obtained. X-ray diffraction (XRD) measurements for the samples were performed with a D/MAX-2550 pc diffractometer with Cu K aas the radiation source. Transmission electron micros- copy (TEM) was carried out using a FEG-TEM (Tecnai G2 F30 S-Twin, Philips-FEI, The Netherlands). A FLS920 fluo- rescence spectrophotometer (Edinburgh Instrument Ltd., UK)was employed for the measurement of emission and excitation spectra without magnetic field. The PL spectra under pulsed magnetic field were measured using a similar fiber-optical sys-tem reported previously. 9,10A pulsed magnetic field generated by a liquid nitrogen-cooled resistive coil with a pulsed dura- tion of 270 ms was applied to the sample, which was locatedin the center of the magnet. The PL spectra were collected with a fiber optical probe under the excitation by a Ti:sapphire laser beam, which was focused onto the sample using a fiberoptical system. The PL signal was analysed by an EM-CCD (Andor DU970P) and a monochromator (Andor SR500). The absorption spectrum for the excitation light (392 nm) underdifferent magnetic field strengths was determined using the same optical setup. All the measurements were carried out at room temperature, except that the PL spectra recorded in mag-netic field were taken for samples cooled to 77 K. III. RESULTS AND DISCUSSION The as-made glass is completely amorphous as no dif- fraction peaks can be observed from the XRD pattern givenin Fig. 1(a). After crystallization, the XRD pattern shows prominent diffraction peaks of cubic SrF 2. By using the Scherrer formula,11the calculated size of SrF 2nanocrystals is about 6 nm. Figs. 1(b)and1(c)give the TEM and HRTEM (high resolution transmission electron microscope) images of the glass ceramics. The nanocrystals were homogeneouslydistributed inside the glass ceramic. From the TEM images, the size of SrF 2nanocrystals in the glass ceramic is about 5 nm, in accordance with the size calculated above.Moreover, HRTEM image reveals well-defined crystalline lattice and a interplanar spacing of 0.329 nm, corresponding to the (111) lattice plane of the SrF 2crystals. Fig.2(a)shows the excitation and emission spectra of the Eu-doped glass ceramics. The excitation spectrum (monitored at 615 nm) shows a highest peak centered at 392 nm, matchingthe Eu 3þ:7F0!5L6transition. Under 392 nm excitation, the emission spectra of both samples consist of many peaks (Fig. 2(b)), corresponding to transitions from the5D0to the7Fj (j¼0–4) levels of the Eu3þions. After partial crystallization, the reduction of the emission intensity is ascribed to theconversion of Eu3þto Eu2þ. In addition, the presence of better resolved Stark levels of each emission bands implies that Eu3þions enter into more ordered sites in the glass ceramics. On the other hand, it is well known that the5D0!7F2transi- tion is electric dipole transition which is highly sensitive to the surrounding ligand environment, while the5D0!7F1 FIG. 1. (a) XRD patterns of Eu-doped glass (red dashed line) and glass ceramics (black line). The standard pattern for SrF 2is shown as a reference. (b) TEM image of nano-glass-ceramics. (c) HRTEM of the glass ceramics containing SrF 2nanocrystals. The white circles highlight the area where SrF 2nanocrystals are precipitated inside the vitreous matrix. The inset is an enlarged image of a typical SrF 2nanocrystal. FIG. 2. (a) PLE (monitored at 615 nm) and PL (excited at 392 nm) spectra of Eu-doped glass ceramics. (b) The5D0!7Fj(j¼0–4) emission bands of both glass and glass ceramics at room temperature (excited at 392 nm). (c) Emission spectra (excited at 392 nm) of the Eu3þ-doped glass ceramics under various magnetic fields at 77 K. The peaks can be ascribed to the transitions from5D0to the7Fj(j¼0–4).123103-2 Jiang et al. J. Appl. Phys. 116, 123103 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.230.243.252 On: Thu, 27 Nov 2014 01:03:27transition is magnetic dipole transition which is not influenced by the ligand environment.14,16The ratio of the integrated intensities of the5D0!7F1to the5D0!7F2transitions is therefore associated with the crystal site symmetry around the doped Eu3þions.17We therefore define R ¼I(5D0!7F2)/ I(5D0!7F1), which can be used for the analysis of site sym- metry. The smaller the R value is, the higher the symmetry is.16According to the emission spectra, the calculated index R value of glass is 3.135, as compared to 1.086 for the glass ce-ramic. This change of R value demonstrates that the site sym- metry of the doped Eu 3þions in glass ceramic was much higher than that in glass, indicating that Eu3þions in the glass move into the cubic SrF 2nanocrystals after partial crystalliza- tion of the as-melt glass. In the presence of magnetic field, the same set of transi- tions is observed in Fig. 2(c). Interestingly, from the spectra we can clearly see that the intensities of the peaks decrease with the increase of the magnetic field. Fig. 3(a) shows the dependence of the total integrated PL intensities of the Eu3þ-doped glass ceramic on the mag- netic field at 77 K. It is quite obvious that the PL integratedintensities are reduced as magnetic field intensify. When the magnetic field is smaller than 14 T, the emission intensity drops slowly with the increase of magnetic field; in the rangeof 14 T–42 T, the PL intensity sharply decreases with the magnetic field. Totally, the PL integrated intensity decreased by about 26.1% at 42 T as compared to that of the originalvalue recorded at 0 T. For a detailed analysis, we found that the integrated PL intensities of transitions from 5D0!7F1, 5D0!7F2, and5D0!7F4are all reduced by 30.5%, 32.4%, and 31.9%, respectively, in magnetic field of 42 T, as shown in Figs. 3(b)–3(d) . In addition, similar field dependences of emission intensity are present for all the above transitions. The suppression of PL of RE ions by magnetic field has been reported in a recent work by Tikhomirov’s group.1In their system, the emission from Er3þ:4S3/2!4I15/2in the glass ceramic was suppressed by magnetic field, which wasattributed to the Zeeman splitting of4S3/2into four levels, in which the lowest level exhibited negligible transition proba- bility. In the present case, however, the excited5D0level is non-degenerate (J ¼0); therefore, it does not show Zeeman splitting. Du et al. has ascribed the reduction of PL in a Eu- doped YVO 4single crystal to the change in the site symmetry around Eu3þ,9which has been known to affect its PL. Under strong magnetic field, slight structural distortion around the paramagnetic Eu3þion can be possible, resulting in the change in site symmetry. This is because in the presence of magnetic field, magnetization of the material that involves the magnetic ordering of paramagnetic centers is not avoid-able. This magnetic effect can sometimes lead to a structural phase transition. For instance, the Gd 5(Si1.8Ge2.2) alloy showed a reversible field-induced first-order structural transi-tion from a P112 1/a monoclinic (paramagnetic) to a Pnma orthorhombic (ferromagnetic), resulting in the strong magne- toelastic effects due to the large difference in the cell con-stants of the two phases. 18In addition, it is widely known that magnet field can induce a shape change for a particular type of magnetic alloys. For instance, the NiCoMnIn alloy exhibitsa 3% deformation and almost full recovery of the original shape in magnetic field, due to the reversible martensitic phase transition transformation from the antiferromagnetic(or paramagnetic) to the ferromagnetic parent phase at 298 K in this system. 19Based on these previous observations, we believe that probably a similar structural influence can beinduced to the studied systems in the presence of strong mag- netic field up to 40 T, leading to a small disturbance of the local structure around Eu 3þions. This effect may associate with the strong magnetic interaction between the 4f electrons of Eu3þions doped in the glass ceramic sample, while the detailed mechanisms need to be elucidated further. As discussed previously, the change in site symmetry can be correlated with the spectra intensity ratio R, defined as the intensities ratio of the5D0!7F2to the5D0!7F1 transitions. The index R values at various magnetic fields were shown in Fig. 4(a). It is obvious that the index R value decreases with the enhancement of applied magnetic field, FIG. 3. (a) The dependence of the total integrated PL (excited at 392 nm) intensities of Eu3þ-doped glass ceramic on the magnetic field at 77 K; (b) the dependence of the integrated PL (excited at 392 nm) intensities of 5D0!7F1emission band, (c)5D0!7F2emission band, and (d)5D0!7F4 emission band of Eu3þon the magnetic field at 77 K. FIG. 4. (a) The index R values at various magnetic fields and (b) the depend- ence of /C0log (I/I 0) on magnetic field at 77 K. The blue curves are provided as guidance for eye.123103-3 Jiang et al. J. Appl. Phys. 116, 123103 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.230.243.252 On: Thu, 27 Nov 2014 01:03:27which indicates that the site symmetry of Eu3þions becomes higher. With the increase in the strength of magnetic field, an initial slow decrease ( <14 T) of R value is followed by a rapid drop in PL intensity from 14 T to around 35 T. Finally, a saturation effect seems to take for field higher than 35 T, where again a slow change of PL is found. Apparently, thistrend is consistent with the dependence of PL intensity on the magnetic field. Although the variation of the index R value from 1.086 to 1.040 is not large, the change of crystalsite symmetry of Eu 3þions is apparently related to the reduction of PL intensities. The reduction of PL intensity in magnetic field may also relate to the change in absorption in magnetic field. We examined dependence of absorption of the excitation light on magnetic field by recording the intensities of transmittedlight at different magnetic fields. The absorbance can be then calculated according to the Beer-Lambert law. 20As shown in Fig. 4(b), the absorption of the 392 nm laser decreased with the enhancement of magnetic field, which is in agree- ment with the remarkable reduction in emission intensity of both electric dipole and magnetic dipole transitions (Fig. 3), implying that the reduction of absorption is also responsible for the decrease of PL integrated intensities. Due to possible shift in the positions of the ground and the excited statesinduced by Zeeman effect, the reduced absorption could arise from the energy mismatch of the excitation light in higher magnetic field. In other words, the reduction inabsorption and suppression of emission can be of the same magnetic origin. Furthermore, Zeeman splitting results in the broadening of each 4f levels (J 6¼0) as shown below. This could be another reason for the suppression of PL because it may lead to enhanced cross-relaxation rate among nearby Eu 3þions as the energy gaps between lower and higher 4f levels are reduced. As discussed above, magnetic field splits 4f levels of RE ions due to Zeeman effect and this effect becomes prominentat higher magnetic field. We indeed observed shift of peak positions and it is most notable for the 5D0!7F4transition, as shown in Figs. 5(a)and5(b). The peaks of the5D0!7F4 transition of Eu3þions show clear blue shift at higher energy side and red shift at lower energy side. Fig. 5(d) shows the energy level diagram of Eu3þions. The excited state5D0is non degenerate (J ¼0); therefore, it does not show Zeeman or Stark splitting. The lower ground state7F4splits into several Zeeman levels under external magnetic field. Since the gapbetween the Zeeman levels becomes larger with the increase of magnetic field, the upper subbands shift to higher position in the energy level diagram while the lower subbands moveto lower position, leading to blue shift and red shift of peaks. The dependence of energy shift of these two peaks on the magnetic field was shown in Fig. 5(c). It is parabolic when the applied magnetic field is less than about 14 T. This dependence becomes linear when the magnetic field exceeds 14 T, which is in agreement with the theory of Zeeman effect.In a PbF 2-based glass ceramic, a similar phenomenon was observed in the4I13/2!4I15/2emission band of Er3þions by Saurel et al.21who ascribed the parabolic dependence on magnetic field to the quantum confinement as the optical gap of PbF 2is smaller than that of the glass network. Incomparison, it is the magnetic confinement that makes the de- pendence linear at stronger magnetic fields. However, in the present case, the optical band gap of the precipitated SrF 2is 6.9 eV,22larger than that of the surrounding glass matrix. Despite a similar field dependence is observed, their explana- tion based on quantum confinement may be not valid here. This parabolic field dependence may relate with the changeof effective g-factor under high magnetic field, which was predicted theoretically in a previous report. 23The detailed mechanism remains to be revealed in further studies. IV. CONCLUSION In conclusion, the influence of external magnetic field on the PL of Eu3þ-doped nano-glass-ceramics has been stud- ied. The PL integrated intensity of Eu3þdecreased with the enhancement of magnetic field, which is explained by thecooperation effect of the Zeeman splitting, the change in site symmetry of Eu 3þions, and the cross-relaxation effect between adjacent Eu3þions. Furthermore, Zeeman splitting also results in both blue shift and red shift of peaks with the strengthening of magnetic field. The detailed reason for the observed magneto-optical behaviors deserves furtherinvestigation. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51132004and 51102209), and the National Basic Research Program of China (2011CB808100). The authors thank the Pulsed High Magnetic Field Facilities at the Wuhan National HighMagnetic Field Center. 1V. K. Tikhomirov, L. F. Chibotaru, D. Saurel, P. Gredin, M. Mortier, and V. V. Moshchalkov, Nano Lett. 9, 721 (2009). 2Y. Liu, D. Wang, J. Shi, Q. Peng, and Y. Li, Angew. Chem., Int. Ed. 52, 4366 (2013). FIG. 5. (a) Emission from the5D0!7F4transition of Eu3þions; (b) the enlarged part of the rectangular region (dashed line) in Fig. 5(a); (c) depend- ence of energy shift for the two peaks (shown in Fig. 5(a)) on the magnetic field; and (d) energy level diagram of Eu3þions, the gray arrows stand for the non-irradiative transition processes.123103-4 Jiang et al. J. Appl. Phys. 116, 123103 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.230.243.252 On: Thu, 27 Nov 2014 01:03:273Z.-W. Ma, J.-P. Zhang, X. Wang, Y. Yu, J.-B. Han, G.-H. Du, and L. Li, Opt. Lett. 38, 3754 (2013). 4J. Zhou, N. Shirahata, H.-T. Sun, B. Ghosh, M. Ogawara, Y. Teng, S. Zhou, R. G. Sa. Chu, M. Fujii, and J. Qiu, J. Phys. Chem. Lett. 4, 402 (2013). 5S. Gai, C. Li, P. Yang, and J. Lin, Chem. Rev. 114, 2343 (2014). 6G. S. Ofelt, J. Chem. Phys. 37, 511 (1962). 7B. R. Judd, Phys. 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Kitakami, K. Oikawa, A. Fujita, T. Kanomata, and K. Ishida, Nature 439, 957 (2006). 20J. W. Suk, A. Kitt, C. W. Magnuson, Y. Hao, S. Ahmed, J. An, A. K.Swan, B. B. Goldberg, and R. S. Ruoff, ACS Nano 5, 6916 (2011). 21D. Saurel, V. Tikhomirov, V. Moshchalkov, C. G €orller-Walrand, and K. Driesen, Appl. Phys. Lett. 92, 171101 (2008). 22H. Ai-Min, Y. Xiao-Cui, L. Jie, X. Wei, Z. Su-Hong, Z. Xin-Yu, and L. Ri-Ping, Chin. Phys. Lett. 26, 077103 (2009). 23T. Ando and Y. Uemura, J. Phys. Soc. Jpn. 37, 1044 (1974).123103-5 Jiang et al. J. Appl. Phys. 116, 123103 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 169.230.243.252 On: Thu, 27 Nov 2014 01:03:27

1.4887796.pdf

Carrier transport properties of nanocrystalline Er3N@C80 Yong Sun, Yuki Maeda, Hiroki Sezaimaru, Masamichi Sakaino, and Kenta Kirimoto Citation: Journal of Applied Physics 116, 034301 (2014); doi: 10.1063/1.4887796 View online: http://dx.doi.org/10.1063/1.4887796 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characterization of metal contacts for two-dimensional MoS2 nanoflakes Appl. Phys. Lett. 103, 232105 (2013); 10.1063/1.4840317 Nanocrystal-based Ohmic contacts on n and p-type germanium Appl. Phys. Lett. 100, 142107 (2012); 10.1063/1.3700965 Electrical and photoelectrical characterization of undoped and S-doped nanocrystalline diamond films J. Appl. Phys. 103, 084905 (2008); 10.1063/1.2908884 Thermoelectric properties of n -type nanocrystalline bismuth-telluride-based thin films deposited by flash evaporation J. Appl. 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Downloaded to ] IP: 129.12.235.98 On: Mon, 01 Dec 2014 18:15:33Carrier transport properties of nanocrystalline Er 3N@C 80 Y ong Sun,1,a)Yuki Maeda,1Hiroki Sezaimaru,1Masamichi Sakaino,1,b)and Kenta Kirimoto2 1Department of Applied Science for Integrated System Engineering, Kyushu Institute of Technology, Senshuimachi, Tobata, Kitakyushu, Fukuoka 804-8550, Japan 2Department of Electrical and Electronic Engineering, Kitakyushu National College of Technology, 5-20-1 shii, Kokuraminami, Kitakyushu, Fukuoka 802-0985, Japan (Received 14 May 2014; accepted 26 June 2014; published online 15 July 2014) Electrical transport properties of the nanocrystalline Er 3N@C 80with fcc crystal structure were characterized by measuring both temperature-dependent d.c. conductance and a.c. impedance. Theresults showed that the Er 3N@C 80sample has characteristics of n-type semiconductor and an elec- tron affinity larger than work function of gold metal. The Er 3N@C 80/Au interface has an ohmic contact behavior and the contact resistance was very small as compared with bulk resistance of theEr 3N@C 80sample. The charge carriers in the sample were thermally excited from various trapped levels and both acoustic phonon and ionic scatterings become a dominant process in different tem- perature regions, respectively. At temperatures below 250 K, the activation energy of the trappedcarrier was estimated to be 35.5 meV, and the ionic scattering was a dominant mechanism. On the other hand, at temperatures above 350 K, the activation energy was reduced to 15.9 meV, and the acoustic phonon scattering was a dominant mechanism. In addition, a polarization effect from thecharge carrier was observed at low frequencies below 2.0 MHz, and the relative intrinsic permittiv- ity of the Er 3N@C 80nanocrystalline lattice was estimated to be 4.6 at frequency of 5.0 MHz. VC2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License .[http://dx.doi.org/10.1063/1.4887796 ] I. INTRODUCTION The endohedral fullerenes have attracted attention for their applications in optics,1bio-medicine,2electronics,3 magnetics,4and quantum information processing.5–8Such applications would require the fabrications of their crystal- line structures and metal electrode on the materials. One of their interest properties is a charge transfer from the endohe-dral atoms to the fullerene cage. The charge transfer has been widely investigated, 9–13as these materials are expected to display remarkable electronic and structural propertiesassociated with this charge transfer. Among these endohedral fullerenes, trimetallic nitride endohedral fullerenes (TNEFs), such as Sc 3N@C 80and Er 3N@C 80, can be obtained in large yield and evaporated onto heated substrates14because of their thermal stabilities.15,16After the extensive studies in theoretical calculations and experimental analysis for iso-lated molecule of the materials, few fundamental investiga- tions are now carried out on electrical properties of the endohedral fullerenes in condensation states, recently. The self-assembled island formations of Sc 3N@C 80and Er3N@C 80molecules on Au(111) and Ag/Si(111) surfaces have been investigated.17Charge transport properties of the Sc 3N@C 80film prepared by drop-casting its CS 2solution on the quartz substrate, such as carrier mobility and energy band structure, have also been studied.9The Sc3N@C 80thin film exhibits a low electron mobility of 5:7/C210/C03cm2V/C01s/C01under normal temperature andatmospheric pressure. However, it is not easy so far as to obtain enough amounts of the endohedral fullerenes to mea- sure physical and electrical characters. Therefore, the diffi-culties in fabricating crystals and actual devices still remain, and a discussion of the carrier transport properties through the TNEFs/metal contact was not carried out in detail. In this study, we prepared a nanocrystalline Er 3N@C 80 solid sample by pressing powder material to a pellet withtwo gold electrodes. The temperature-dependent conducti-vity of the Er 3N@C 80sample was measured in the condition of various applied electric fields. In addition, the resistance and capacitance of the Au/Er 3N@C 80/Au structure were obtained at various d.c. bias and a.c. voltages. The results obtained in this study indicate that the charge transfer leads to a high conductivity of the nanocrystalline Er 3N@C 80solid as well as a low contact resistance with gold electrodes. The energy levels at the Er 3N@C 80/Au interface and the trans- port properties of the charge carriers passing through thesample will be discussed. II. EXPERIMENTAL Er3N@C 80powder with purity >95 wt. % was pur- chased from LUNA Innovations to make a sample specimen for measurement.18,19The Er 3N@C 80powder was pressed into a pellet at room temperature at 1.25 GPa for 50 min. Theso formed pellet was 5.0 mm in diameter and 0.55 mm in thickness. Two gold electrodes on the surfaces of the sample were prepared using an Au nano-particle paste (NAU-K05B,Daiken), and the sample was annealed at temperature of 500 K in vacuum for 30 min. Prior to electrical measure- ments the powder and pellet samples were characterized bya)E-mail address: sun@ele.kyutech.ac.jp b)Present address: Department of Vehicle Production Engineering, NISSAN MOTOR CO. LTD., 560-2, Okatsukoku, Atsugi-city, Kanagawa-pref. 243- 0192, Japan. 0021-8979/2014/116(3)/034301/00 VCAuthor(s) 2014 116, 034301-1JOURNAL OF APPLIED PHYSICS 116, 034301 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.12.235.98 On: Mon, 01 Dec 2014 18:15:33an x-ray photoemission spectroscopy (XPS; AXIS-NOVA, SHIMATSU/KRATOS) and x-ray diffraction (XRD; JEOL JDX-3500 K). In the XPS analysis, the beam diameter ofAl Kaline was 55 lm, and the binding energy resolution was 0.15 eV. In the electrical measurements, the current passing through the sample was measured using a digital electrome- ter (ADVANTEST R8252) with a current resolution of 1.0 fA at various d.c. bias voltages from 0.001 to 3.0 V. Thepellet sample was set in a vacuum chamber of a cryostat dur- ing the electrical measurements. The base pressure of the vacuum chamber was less than 10 /C05Pa. The current meas- urements were carried out in the course of heating up or cooling down process between the temperatures from 100 K to 500 K. The rate of heating or cooling was 0.14 K min/C01 with a stepwise increment of 1.0 K. The impedance of the sample was measured at room temperature in atmosphere to separate the bulk and interfaceresistances in the sample by using a Cole-Cole plot method. The impedance Z ¼Z 0þjZ00was used to characterize both resistance and capacitance by plotting the imaginary part/C0Z 00¼/C0Im½Z/C138versus the real part Z0¼Re½Z/C138of the imped- ance. The important information pertinent to the Er 3N@C 80/ Au structure can be obtained. III. RESULTS Three x-ray photoemission spectra of the pellet sample at room temperature were show in Fig. 1. They were obtained from the surface of the Er 3N@C 80sample before and after Arþion sputtering for 10 and 30 s, respectively. Eight peaks at binding energies of 9, 56, 98, 167, 242, 285,531, and 999 eV were observed in the spectra. The 9 eV peak is attributed to a photoemission from 4 felectrons of Er atoms. The double peaks at 56 and 98 eV are the photoemis-sions from Er MVV, and the 167 eV peak is from Er 4 d. The peak around 285 and 531 eV comes from C 1 sand O 1 score level, respectively. The peaks around 999 eV correspond toO KLL Auger emission. Also, the peaks around 240 eV observed after the Ar þion sputtering are from Ar 2 p1/2and 2p3/2core levers. In the XPS spectra, the Arþion sputtering causes both the decrease in the O-related peaks and the increases in Er and C-related peaks. Namely, the oxygen atoms adsorb only on the surface of the pellet sample. Fromthe spectrum after the 30 s Ar þion sputtering, atomic ratio of Er/C is evaluated to be 3.64 at. %, close to the stoichio- metric ratio of 3.61 at. % for Er 3N@C 80. Also, the photoem- ission from the N atoms cannot be detected due to its smaller relative sensitivity factor (RSF, 0.505) and concentration as well as encapsulation in the C 80cage. Although the RSF of C1sis also small, 0.318, its XPS intensity is somewhat strong because of the abundant concentration of C atoms in the Er 3N@C 80molecules. The enlarged photoemission spectra from O 1 score level were shown in Fig. 2for various Arþion sputtering times. The Arþion sputtering results in the decrease of the peak intensity and the shift of the peak toward the low energy side. The results indicate that the oxygen atoms adsorbed only on the surface of the pellet sample as well asthere is an electronic interaction between the adsorbed oxy- gen atoms. The photoemission spectra from the Er 4 dcore level were enlarged in the energy scaling and they were plotted in Fig. 3. The peak at binding energy of 169.5 eV does not change with increasing sputtering time. This result suggestsa weak electronic interaction between the Er atoms with adsorption oxygen atoms on the surface of the C 80cage. On the other hand, the peak intensity increases after the Arþion sputtering due to desorption of the adsorbed oxygen atoms. Figure 4shows the photoemission spectra from the C 1 score level in the enlarged binding energy scale. The intensity ofthe C 1 speak increased after the Ar þion sputtering but no significant peak shift was observed. This may be related to the conjugation effect of pelectrons on the surface of the C80cage. XRD patterns of the as-received Er 3N@C 80powder sample were shown in Fig. 5. Several diffraction peaks can FIG. 1. X-ray photoemission spectra of the nanocrystalline Er 3N@C 80sam- ple prepared at a pressure of 1.25 GPa. The spectra are detected on the sur- face of the sample before and after Arþion sputtering for 10 and 30 s.FIG. 2. Enlarged x-ray photoemission spectra from O 1 score level before and after Arþion sputtering for 10 and 30 s.034301-2 Sun et al. J. Appl. Phys. 116, 034301 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.12.235.98 On: Mon, 01 Dec 2014 18:15:33be recognized for the pattern, a strong peak at 2 h¼9:30 deg and four broad peaks centered at 2 h¼18:00;25:70;32:95; and 50 :80 deg. The enlarged XRD pattern of the 2 h¼9:30 deg peaks was shown in the inset to Fig. 5. As seen in the inset figure, no significant asymmetry is observed for this diffraction peak. The 2 h¼9:30 deg peak was ascribed to the diffraction from (111) planes of a face-centered cubic(fcc) crystal structure with a lattice constant of 1.65 nm. The grain size of the as-received powder sample was estimated to be 4 nm from the full width at half-maximum (FWHM) ofthe (111) peaks. Cole/C0Cole plots of the a.c. impedance of the Au/ Er 3N@C 80/Au structure at room temperature at the peak voltage of 1.0 V at the d.c. bias voltage of 0.0 V was shown in Fig. 6. The Cole /C0Cole plot exhibits a semicircle, indicat- ing that the impedance is reflected only by both resistanceand capacitance of the bulk Er 3N@C 80sample and its inter- facial component can be ignored. The bulk resistance andcapacitance are defined from the real and image parts of the impedance, their values are 7 :28/C2105Xand 1 :08/C210/C012F at the frequency of 300 KHz. We must also point out that the bulk resistance of the sample in atmosphere increases due to the adsorption of gas molecules, which results in the local-ization of the charge carrier. The current-voltage ( I-V) characteristics of the Au/ Er 3N@C 80/Au sample at temperatures of 300 and 500 K were shown in Fig. 7. The currents passing through the sam- ple at 300 and 500 K can be fitted as a quadratic function of the d.c. bias voltage in the range of 0.001–3.0 V. The quad-ratic I-Vcharacteristic is related to a hopping conductance of the charge carrier in molecular materials 20and is distinctly different to an exponential I-Vcharacteristic of the Schottky barrier. The results in Figs. 6and7indicate that the contact between the nanocrystalline Er 3N@C 80sample and the Au electrode is ohmic and the electron affinity of the Er 3N@C 80 sample is larger than the work function of gold metal. Therefore, we can characterize directly the carrier transport properties of the sample by measuring its field andtemperature-dependent I-Vcharacteristics. In general, when FIG. 4. Enlarged x-ray photoemission spectra from C 1 score level before and after Arþion sputtering for 30 s.FIG. 5. X-ray diffraction patterns of the as-received Er 3N@C 80powder. The inset shows the enlarged patterns of the (111) diffraction peaks. FIG. 6. Cole-Cole plot of the impedance of the Au/Er 3N@C 80/Au structure at room temperature at a.c. voltage of 1.0 V at d.c. bias voltage of 0.0 V.FIG. 3. Enlarged x-ray photoemission spectra from Er 4 dcore level before and after Arþion sputtering for 30 s.034301-3 Sun et al. J. Appl. Phys. 116, 034301 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.12.235.98 On: Mon, 01 Dec 2014 18:15:33the electrical transport is governed by space charge limited conduction (SCLC) mechanism,21,22the current Iis repre- sented by IE;TðÞ ¼9Sere0lE;TðÞ E2 8L; (1) where Eis the strength of the applied electric field, Tis the absolute temperature, Sis the area of the electrode, Lis the thickness of the sample, ere0is the permittivity, and lðE;TÞ is the mobility of the charge carrier in the sample. Namely,the current Iis a quadratic function of the electric field E¼V=L. Here, the mobility lðE;TÞis field and temperature dependent and is described as follows: 23 lE;TðÞ ¼qR2/C23 kT/C20/C21 exp/C0/C15a/C0D/C15a kT/C26/C27 ; (2) where Ris the mean free pass of the charge carrier, /C23is the thermal vibration frequency of the host molecule, qis the unit of electronic charge, /C15ais the activation energy of the trapped charge carrier, and D/C15a¼ðE=4pere0qÞ1=2the change of/C15aafter the electric field Eis applied. Here, ere0¼e1e0is the permittivity at high frequency. One can notice from Eq. (2)that the /C23is dependent of temperature. Therefore, Eq. (2) can be written as follows: lE;TðÞ ¼Taexp/C0/C15a/C0D/C15a kT/C26/C27 ; (3) where ais a constant depending on scattering mechanism of the charge carrier during the electrical transport process. The current Iat various d.c. bias voltages were measured as a function of temperature during heating up and cooling down processes. Arrhenius plots of I/C241=kTat the d.c. bias voltage of 1.0 V were plotted in Fig. 8. The current I increases with temperature in the range of 100–500 K and cannot been fitted using single exponential function. The result indicates that there is different aand/C15aat high and low temperature sides. We have conformed from the Arrhenius plots of I/C241=kTthat the current I can be fitted by usinga¼/C01:5 for high temperature side and a¼1:5 for low temperature side, respectively. Arrhenius plots of the I/C2T1:5/C241=kTfor high temper- ature side and I/C2T/C01:5/C241=kTfor low temperature side at the d.c. bias voltage of 1.0 V during heating up and cooling down processes were shown in Figs. 9(a) and9(b). The good linear relationships in the Arrhenius plots indicate that the electrical transport properties of the nanocrystalline Er3N@C 80sample can be explained using Poole-Frenkel model.23The a¼/C01:5 at high temperature side and a¼ 1:5 at low temperature side suggest various scattering mech- anisms of the charge carrier in the sample. On the basis ofthe Arrhenius plots at various d.c. voltages, we obtained the activation energies of the trapped charge carrier to be /C15 a¼15:9 meV for high temperature side and /C15a¼35:5 meV for low temperature side. The D/C15ais in the range of 1 :6 /C210/C02/C248:8/C210/C01meV and can be ignored as compared with /C15a. The dielectric properties of the nanocrystalline Er3N@C 80sample were characterized by measuring its im- pedance spectra. In general, an equivalent electric circuit ofa metal/semiconductor/metal system can be represented by a parallel combination of the interfacial resistance (R i) and ca- pacitance (C i) in series with a parallel arrangement of the bulk resistance (R B) and capacitance (C B).24–26In this study, both R iand C iare small enough and can be ignored. The bulk resistances at frequencies of 6.25 KHz and 5.0 MHzwere plotted in Fig. 10(a) as a function of the d.c. bias volt- age. R Bis constant at frequency of 5.0 MHz but it decreases with increasing d.c. bias voltage at frequency of 6.25 KHz.On the other hand, the bulk capacitances at frequencies of 6.25 KHz and 5.0 MHz were plotted in Fig. 10(b) as a func- tion of the d.c. bias voltage. C Bis also constant at 5.0 MHz but it decreases with increasing d.c. bias voltage at frequency of 6.25 KHz. The dielectric properties as shown in Fig. 10 indicate that there are two kinds of polarization mechanismsin the nanocrystalline Er 3N@C 80sample. One is related to the conducting charge carriers, which contribute to theFIG. 7. Current-voltage characteristics of the Au/Er 3N@C 80/Au structure at 300 and 500 K. FIG. 8. The current passing through the Au/Er 3N@C 80/Au structure as a function of temperature during heating up and cooling down process.034301-4 Sun et al. J. Appl. Phys. 116, 034301 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.12.235.98 On: Mon, 01 Dec 2014 18:15:33sample polarization at lower frequencies only because of a low mobility of the carrier in the sample. Other one is related to the dielectric properties of the Er 3N@C 80crystal lattice, which contributes to the sample polarization in the higherfrequencies. The bulk resistance R Band capacitance C Bwere defined as these of the resistance and capacitance at maximum of theCole-Cole curve. The time of the charge carrier passing through the sample, the resonance time s, can be obtained from a relationship of xs¼1, where x¼2pfand s¼R BCB, and fthe frequency of the carrier passing through the sample. The relative permittivities of the nanocrystalline Er3N@C 80sample at various a.c. voltages at the d.c. bias voltage of 0.0 V were plotted in Fig. 11as a function of the a.c. frequency. The permittivity decreases rapidly withincreasing a.c. frequency from 8.5 at 6.25 KHz to 4.6 at 5.0 MHz. It becomes constant at higher frequencies. No sig- nificant difference due to the a.c. bias voltage is observed.The larger permittivities at low frequency side are related to the polarization from the charge carrier. On the other hand, FIG. 10. (a) Bulk resistances of the nanocrystalline Er 3N@C 80sample at 6.25 kHz and 5 MHz as a function of d.c. bias voltage. (b) Bulk capacitancesof the nanocrystalline Er 3N@C 80sample at 6.25 kHz and 5 MHz as a func- tion of d.c. bias voltage. FIG. 11. Relative permittivities of the nanocrystalline Er 3N@C 80sample at various d.c. bias voltages as a function of frequency. FIG. 9. (a) Arrhenius plots of I /C2T1:5/C241=kTat high temperature side during heating up and cooling down process. (b) Arrhenius plots of I /C2T/C01:5/C241=kT at low temperature side during heating up and cooling down process.034301-5 Sun et al. J. Appl. Phys. 116, 034301 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.12.235.98 On: Mon, 01 Dec 2014 18:15:33the smaller permittivities at high frequencies are due to the polarization of the Er 3N@C 80crystal lattice only. IV. DISCUSSION A. Energy band structure of the Er 3N@C 80/Au interface From the results in Figs. 6and7, we can conclude that the Er 3N@C 80/Au interface corresponds to an Ohmic con- tact, namely, there is not the Schottky barrier for the carriertransport passing through the interface. The Er 3N@C 80sam- ple is n-type semiconductor with the electron affinity larger than the work function of gold metal, 5.1 eV.27Tang et al. have calculated the energy levels of the C 80and Er 3N@C 80 molecules with Ihsymmetry by using density function theory (DFT).28The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap and the LUMO level are 0.05 eV and /C03:80 eV for C 80and 0.13 eV and/C05:40 eV for Er 3N@C 80molecule, respectively. In their theoretical results, the electron affinity of the Er 3N@C 80 molecule is larger than the work function of gold metal. Thisis consistent with our experimental results in this studybecause no Schottky barrier was observed at the Er 3N@C 80/ Au interface. Namely, the ohmic contact at the Er 3N@C 80/ Au interface indicates a large electron affinity of the nano-crystalline Er 3N@C 80solid. As far as we know, there are still no experimental results on the energy structure of the Er 3N@C 80crystal. At present, the surface potential analysis is an effective method to inves- tigate the electronic structures of fullerene-related materi- als.29–31Several experimental results indicated that the shift of the surface potential, the difference between the work function of metal and the electron affinity of fullerene- related materials, depends on film thickness of the materi-als. 29,32–34Therefore, the energy band structure of the Er3N@C 80material may depend on its crystallographic and interfacial properties. B. Dielectric properties As shown in Fig. 11, the relative permittivity of the nano- crystalline Er 3N@C 80sample decreases from 8.5 at 6.25 KHz to 4.6 at 5.0 MHz. At low frequencies, the resistance and ca-pacitance of the Er 3N@C 80sample decrease with increasing d.c. bias voltage as shown in Fig. 10. The results indicate that polarization properties of the nanocrystalline Er 3N@C 80sam- ple at low frequencies are related to its electrical properties such as the mobility and concentration of the charge carrier. For example, the time sis 1:44/C210/C06s at 6.25 KHz and 1:94/C210/C07s at 5.0 MHz, respectively. The period of the a.c. voltage, t,i s1 :6/C210/C04s for 6.25 kHz and 2 :0/C210/C07sf o r 5.0 MHz, respectively. It is clear that sð1:44/C210/C06sÞ /C28tð1:6/C210/C04sÞat 6.25 KHz and sð1:94/C210/C07sÞ ffitð2:0/C210/C07sÞat 5.0 MHz. This fact indicates that the polarization of the charge carrier affects the dielectric proper-ties of the sample at lower frequencies only. At present, the permittivity of the Er 3N@C 80solid has not been reported as far as we know. It is well known thatthe crystal C 60lattice has an intrinsic permittivity of 4.4.35,36 The dipole dynamics in the endohedral metallofullereneLa@C 82have been studied theoretically and experimen- tally.37,38In the solid state, pure La@C 82has a fcc structure at room temperatures. The C 82cage with C2vsymmetry is highly disordered in high-symmetry lattice. In the La@C 82 molecule three electrons transferred to the C 82cage from the endohedral La atom. Electrostatic interactions result in theendohedral La 3þion being located close to the cage edge and an important consequence of such an arrangement is a molecular electric dipole. At room temperature, the relativepermittivity of the La@C 82molecular solid is 40 at 100 Hz and 25 at 1.0 MHz. The large permittivity is due to a dynamic response of the ½La/C1383þ½C82/C1383/C0dipole in the La@C 80 molecule. In this study, the intrinsic permittivity of the Er3N@C 80sample, 4.6, is larger than that of C 60crystal, 4.4. This may be related to the electron transfer from Er 3N cluster to C 80cage because of the formation of three dipoles, ½ErN/C1383þ½C80/C1383/C0, between the cluster and the C 80cage. On the other hand, the permittivity of the Er 3N@C 80is smaller than that of La@C 80because of a high asymmetry of ½Er/C138þ½N/C138/C0 and½ErN/C1383þ½C80/C1383/C0as compared with ½La/C1383þ½C82/C1383/C0. In addition, the dielectric properties of the fullerene- related materials are strongly affected by the adsorptions of O and N atoms.39,40The fact that both C 60and oxygen mole- cules are non-polar, together with the evidence of reversibleoxygen diffusion into the C 60solid, strongly suggest that these dipoles arise from charge transfer between oxygen molecules and C 60cages. The amount of this charge transfer is bound to be very small, reflecting the fact that the electron affinities of both C 60and molecular oxygen are relatively high. Due to the large size of the C 60molecules, this small charge transfer creates large dipole moments. Since the elec- tron affinity of the C 60molecule, 2.65 eV,41is considerably higher than that of molecular oxygen, 0.45 eV,42one might expect oxygen to be the donor and C 60the acceptor of electrons. C. Electrical transport properties Based on the measurement results of temperature- dependent current as shown in Figs. 7–9, we can include that the conductivity of the Er 3N@C 80sample is governed by both mobility and concentration of the charge carrier. Thereare different temperature dependences on the mobility and concentration of the carrier at high and low temperature sides. At high temperature side, the activation energy ofthe trapped carrier is 15.9 meV as well as the temperature dependence of the mobility is l/T /C01:5. This temperature dependence suggests an acoustic phonon scattering mecha-nism 43during the carrier transport. On the other hand, at low temperature side, the activation energy is 35.5 meV as well as the temperature dependence of the mobility is l/T1:5. The activation energy of the trapped carrier becomes large and there is a dominant ionic scattering process44at low tem- perature side. It is well known that a phase transition between single cubic (sc) and fcc phases in the C 60crystal occurs when tem- perature varies passing through 260 K.45,46This transition is described to be due to a free rotation of C 60molecules on its crystal lattice. Because of the same molecular symmetry, Ih,034301-6 Sun et al. J. Appl. Phys. 116, 034301 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 129.12.235.98 On: Mon, 01 Dec 2014 18:15:33between the C 60and C 80cages, a similar phase transition may occur in the Er 3N@C 80crystal phase. This transition temperature may be above 350 K due to a large mass and di-ameter of the Er 3N@C 80molecule. It has also been reported that the energy band structure of the C 60crystal changes when the sc-fcc phase transition occurs.47Similar changes on the energy band structure may occur in the Er 3N@C 80crystal phase. This change results in the decrease of the activation energy of the trapped carrier inEr 3N@C 80solid at sufficiently high temperatures. In order to clarify the relationship between the energy band structure and the activation energy of the trapped carrier, furtherexperiments such as far infrared (FIR) absorption measure- ment on the Er 3N@C 80material are needed. V. CONCLUSION We have studied the carrier transport properties of the nanocrystalline Er 3N@C 80sample by measuring temperature- dependent conductivity and curre nt-voltage characteristics. The electrical transport in the nanocrystalline Er 3N@C 80sam- ple was governed by space charge limited conduction mecha- nism which is explained usin g Poole-Frenkel model. At temperatures above 350 K, the charge carriers during the trans-port were scatted mainly by acoustic phonon scattering pro- cess. On the other hand, ionic scattering was a dominant process in the charge carrier transport at temperatures below250 K. There were different activation energies of the trapped charge carrier in high and low temperature regions, 16 meV for temperatures above 350 K and 35.5 meV for temperaturesbelow 250 K. The differences on the scattering mechanism and the activation energy of the charge carrier can be explained on the basis of molecular crystal structure and van der Waalsinteraction between the Er 3N@C 80molecules. ACKNOWLEDGMENTS This work was partially supported by Project No. 15 /C0 B01, Program of Research for the Promotion ofTechnological Seeds, Japan Science and Technology Agency (JST). The work was also partially supported by Grant-in-Aid for Exploratory Research No. 23651115, JapanSociety for the Promotion of Science (JSPS). 1E. Xenogiannopoulou, S. Couris, E. Koudoumas, N. Tagmatarchis, T. Inoue, and H. Shinohara, Chem. Phys. Lett. 394, 14 (2004). 2D. W. Cagle, T. P. Thrash, M. Alford, L. P. F. Chibante, G. J. Ehrhardt, and L. J. Wilson, J. Am. Chem. Soc. 118, 8043 (1996). 3J. Park, A. N. Pasupathy, J. I. Goldsmith, C. Chang, Y. Yaish, J. R. Petta, M. Rinkoski, J. P. Sethna, H. D. Abruna, P. L. McEuen, and D. C. Ralph, Nature 417, 722 (2002). 4T. I. Smirnova, A. I. 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Gu, T. Tang, C. Hu, and D. Feng, Phys. Rev. B 58, 659 (1998). 41M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund, Science of Fullerenes and Carbon Nanotubes (Academic, New York, 1996). 42R. C. Weast, CRC Handbook of Chemistry and Physics (CRC Press, West Palm Beach, FL, 1992). 43J. Bardeen and W. Shockley, Phys. Rev. 80, 72 (1950). 44H. Ehrenreich, Phys. Rev. 120, 1951 (1960). 45K. Prassides, H. W. Kroto, R. Taylor, D. R. M. Walton, W. I. F. David, J. Tomkinson, R. C. Haddon, M. J. Rosseinsky, and D. W. Murphy, Carbon 30, 1277 (1992). 46T .M a t s u o ,H .S u g a ,W .I .F .D a v i d ,R .M .I b b e r s o n ,P .B e r n i e r ,A .Z a h a b ,C . Fabre, A. Rassat, and A. Dworkin, Solid State Commun. 83, 711 (1992). 47L. Pintschovius, B. Renker, F. Gompf, R. Heid, S. L. Chaplot, M. Haluska, and H. Kuzmany, Phys. Rev. Lett. 69, 2662 (1992).034301-7 Sun et al. J. Appl. Phys. 116, 034301 (2014) [This article is copyrighted as indicated in the article. 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1.4897452.pdf

First-principles study of structural, electronic, vibrational, dielectric and elastic properties of tetragonal Ba2YTaO6 C. Ganeshraj and P. N. Santhosh Citation: Journal of Applied Physics 116, 144104 (2014); doi: 10.1063/1.4897452 View online: http://dx.doi.org/10.1063/1.4897452 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in First-principles calculations of the electronic, vibrational, and elastic properties of the magnetic laminate Mn2GaC J. Appl. Phys. 116, 103511 (2014); 10.1063/1.4894411 First-principle calculation and assignment for vibrational spectra of Ba(Mg1/3Nb2/3)O3 microwave dielectric ceramic J. Appl. Phys. 115, 114103 (2014); 10.1063/1.4868226 Elastic properties of tetragonal BiFeO3 from first-principles calculations Appl. Phys. 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Santhosha) Low Temperature Physics Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India (Received 15 August 2014; accepted 27 September 2014; published online 13 October 2014) We report first-principles study of structural, electronic, vibrational, dielectric, and elastic properties of Ba 2YTaO 6, a pinning material in high temperature superconductors (HTS), by using density functional theory. By using different exchange-correlation potentials, the accuracy of thecalculated lattice constants of Ba 2YTaO 6has been achieved with GGA-RPBE, since many important physical quantities crucially depend on change in volume. We have calculated the electronic band structure dispersion, total and partial density of states to study the band gap originand found that Ba 2YTaO 6is an insulator with a direct band gap of 3.50 eV. From Mulliken population and charge density studies, we conclude that Ba 2YTaO 6have a mixed ionic-covalent character. Moreover, the vibrational properties, born effective charges, and the dielectric permittivitytensor have been calculated using linear response method. Vibrational spectrum determined through our calculations agrees well with the observed Raman spectrum, and allows assignment of symmetry labels to modes. We perform a detailed analysis of the contribution of the various infrared-activemodes to the static dielectric constant to explain its anisotropy, while electronic dielectric tensor of Ba 2YTaO 6is nearly isotropic, and found that static dielectric constant is in good agreement with experimental value. The six independent elastic constants were calculated and found that tetragonalBa 2YTaO 6is mechanically stable. Other elastic properties, including bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and elastic anisotropy ratios are also investigated and found that Poisson’s ratio and Young’s modulus of Ba 2YTaO 6are similar to that of other pinning materials in HTS.VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4897452 ] I. INTRODUCTION For many decades, transition metal oxides have received much attention owing to their diverse properties like colossaldielectric constant, 1magnetoresistance,2high temperature superconductors (HTS),3magnetocaloric effect,4and magne- toelectric effect.5Perovskite and its structurally related oxides have been widely studied due to their interesting physical properties and extensive structural diversity. Even though many compounds adopt the ideal cubic perovskitearistotype, most perovskites undergo a distortion away from high-symmetry cubic structure. 6Perovskite oxide has the general stoichiometry ABO 3and is composed of corner- sharing BO 6octahedra with A-site cation occupying void created by the three dimensional octahedral network. Thus, the B cations are at the centers of the octahedra, while the Acations occupy 12-fold- coordinated sites. This ideal struc- ture, which is usually described by lattice parameter a p, dis- plays a wide variety of structural instabilities in the variousmaterials. The low symmetry structure can be described as a distorted structure with respect to ideal cubic structure. These distortions often influence the physical properties andremarkable structural chemistry has been observed in these materials. These distortions may involve rotations and distor- tions of the BO 6octahedra as well as displacements of thecations from their ideal sites. The most common distortion consists of rigid unit modes (RUM), where BO 6octahedra remain almost rigid and the rotation of octahedra generates antiferrodistortive distortion. The second type of distortion is ferroelectric, where the A or B cations are displaced againstrigid BO 6octahedra. The third type of distortion involves de- formation of BO 6octahedra. A detailed study on the octahe- dral tilting in cation ordered double perovskite using numberof technique, including group theoretical analysis, 7has been examined by various groups. The family of double perov- skite Ba(B01/2B001/2)O3have been investigated in last two decades for their microwave and infrared dielectric proper- ties. Among double perovskites, Ba 2YTaO 6(BYTO) under- goes an equitranslational improper ferroelastic second orderphase transition from cubic ( Fm-3 m ) to tetragonal ( I4/m) structure without any unit cell multiplication, characterized by the tilting of the oxygen octahedra (Glazer’s notation:a 0a0c/C0) along c-axis, around 253 K.8A pressure induced phase transition from cubic to tetragonal structure is also observed between 4.3 and 5.6 GPa with a onset of an octahe-dral tilting distortion about c-axis. 9Recently, BYTO has been used in a form of nanocomposites with YBa 2Cu3O7,a s an artificial pinning centres immobilizing quantized vorticesin HTS, to enhance the ability of HTS to carry electrical cur- rent with zero resistance at high temperature and magnetic fields. 10Also, BYTO is a useful microwave dielectric resonator material with e/C2432 near room temperature with very low dielectric loss.8a)Email: santhosh@physics.iitm.ac.in. Tel.: þ91 44 2257 4882. Fax: þ91 44 2257 4852. 0021-8979/2014/116(14)/144104/10/$30.00 VC2014 AIP Publishing LLC 116, 144104-1JOURNAL OF APPLIED PHYSICS 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29In this paper, we present ab initio study of the structural, electronic, vibrational, dielectric, and elastic properties of tetragonal BYTO. We have calculated structural parameters, electronic band structure and electronic density of states(DOS), and nature of bonding of BYTO. Lattice dynamical properties, including zone-centre vibrations, Born effective charge, and electronic and static dielectric permittivitytensors, are calculated by first principles linear response method. A detailed analysis of the contribution of the various infrared-active modes to the static dielectric constant hasbeen done to explain its anisotropy. In addition to that, we have also calculated the elastic constants, bulk modulus of BYTO using first-principles method. II. COMPUTATIONAL DETAILS The calculations were performed based on density func- tional theory (DFT) using CAmbridge Serial Total Energy Package code (CASTEP) with local-density approximation(LDA) and generalized gradient approximation (GGA). 11 The interaction of electrons with ion cores was representedby norm conserving pseudopotential for Ba (5s 25p66s2), Y (4s24p64d15s2), Ta (5d36s2), and O (2s22p4). A plane wave basis set with cut off energy 690 eV was used to expand the valance electronic wave functions. Theexchange-correlation potential used in the calculations is LDA, GGA, GGA-PBE, GGA-PW91, and GGA-RPBE. The sampling of Brillouin zone was carefully tested and based onthese convergence tests, a k-point grid of 4 /C24/C23 was used. A1 2 /C212/C29 k-point grid was used for calculating vibra- tional properties. The Broyden-Fletcher-Goldfarb-Shanno(BFGS) minimization scheme was used in geometry optimi- zation. 12The geometry optimization is performed with the experimentally observed cell parameters and internal coordi-nates of ions, 13,14until the maximum energy, force, stress, and displacement on the system converge to the tolerance values of 10/C05eV, 0.03 eV/A ˚, 0.05 GPa, and 0.001 A ˚, respec- tively. The lattice dynamics of BYTO is studied by density functional perturbation theory (DFPT) within the theory of linear reponse.15This method includes calculations of charge response to the lattice distortions for the specified vectors in the first Brillouin zone. The frequencies and displacementpatterns of the phonon modes were calculated using the dynamical matrix method. III. RESULTS AND DISCUSSION A. Crystal and electronic structure BYTO possesses a tetragonal structure ( I4/m), with c-axis (tetragonal and optical axis) as a high symmetry axis, corresponding to an out of phase tilting of the octahedraaround four-fold c-axis (Glazer’s notation: a 0a0c/C0(Ref. 16)). I4/m has the structural equivalence between a-axis and b-axis as well as structural difference between a-axis/b-axisand c-axis. The optimized crystal structure along ac and ab plane is illustrated in Figure 1. Table Ishows the results of optimized lattice parameters obtained using the GGA-RPBEmethod along with LDA, GGA, GGA-PBE, GGA-PW91, and available experimental results. These results are consist- ent with the fact that LDA often underestimates lattice con-stant, while GGA reduces this error considerably. 18The optimized internal co-ordinates of BYTO are given in Table IIalong with available experimental values. Figure 2 shows the calculated band structure along the high symmetry directions and the observed bandgap is /C243.50 eV between the valance band maximum and the conduction band mini-mum along C-point. Unfortunately, we have been unable to find the experimental values of the energy gap in the litera- ture. But, it is known that the GGA calculation underesti-mates the band gap of semiconductors and insulators. 19The lowest valence bands occur between /C011 and /C010 eV and are essentially dominated by Ba 5p states with small pres-ence of O 2s and 2p states and non-negligible presence from O 3d to Y/Ta 3p states. The valence band lies between /C24/C04.5 and 0 eV (E F), which is derived mainly from O 2p states but also, there is a quite strong hybridization between FIG. 1. Crystal structure of tetragonal (I4/m) unit cell of BYTO projected along ac and ab planes. Yellow andRed balls are represents Ba and O ions. The violet and green octahedra repre- sent TaO 6and YO 6. This figure was drawn using VESTA.17 TABLE I. Calculated (LDA, GGA-RPBE, GGA-PBE, and GGA-PW91) and experimental lattice constant and volume of Ba 2YTaO 6. LDA GGA-RPBE GGA-PBE GGA-PW91 Exp. a, b (A ˚) 5.8374 5.9552 5.9301 5.9205 5.9551 c( A˚) 8.3452 8.4419 8.4155 8.4023 8.4482 V( A˚3) 284.36 299.38 295.94 294.52 299.60144104-2 C. Ganeshraj and P . N. Santhosh J. Appl. Phys. 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29O 2p states with d states of Ta and Y, due to the presence of Ta and Y atoms within the Ta(Y)O 6octahedra and to lesser extent with Ba d states. The nature of chemical bonding canbe elucidated from the total to partial density of states (DOS). By comparing the total DOS (Figure 2(b)) with angu- lar momentum projected DOS (Figures 3(a)–3(d) ), one can show that some electrons from p states of O, d states of Ta, and Y are transferred into valence bands (VBs) and contrib- ute to covalent interactions between Ta(Y)-O bonds and alsoTa-O bond has high covalency than Y-O bond. The conduc- tion band is situated above E Fat around /C243.4 eV to 6.6 eV. The conduction band around 5 eV is mainly contributed fromd states of Y, Ta, and Ba as well as from O 2p states. B. Mulliken band population In Sec. III A , it has been shown that there exists a signif- icant hybridization of Ta/Y 3d with O 2p states in BYTO indicating the bonding in this system cannot be purely ionicbut must exhibit a covalent part. In order to have a clear pic- ture about the nature of chemical bonding between constitu- ent of BYTO, we have calculated Mulliken band populationand Born effective charges. The Mulliken band population is essential for evaluating the bonding character in a material. A high value of bondpopulation indicates a covalent bond, and a low value indi- cates an ionic nature. Positive and negative values indicate bonding and antibonding states, respectively. The Mullikenpopulation reported in Table IIIshows that the Ba-O bond exhibits almost ionic nature with slight covalency, whereas Ta/Y-O bond has mixed covalent and ionic characteristics.Since Ta-O bond has higher Mulliken population than that ofY-O bond, it is slightly more covalent than Y-O bond. Since the overlap of t 2g(d0configuration) of Ta (Figure 3(b))i s higher than that of Y (Figure 3(c)) with ligand orbitals, Ta-O bonding energy is higher than that of Y-O bonds and this lead a higher covalent character Ta-O bond than that of Y-O bond. The two different wyckoff positions of O atoms lead asmall change in bond length between O 1and O 2with Ba, Ta, and Y ions. Further, the interatomic distance (Table III) between Ta (Y) and O is /C241.9522 A ˚(/C242.2689 A ˚) and that between Ba and O is /C242.9818 A ˚and 2.8870 A ˚(for O 1and O2) indicating the covalent bonding between Ta(Y) and O ions and ionic bonding between Ba and O ions. It is to bementioned that if we consider the self-consistently calculated valence charges (Table IV), the chemical formula for the system may roughly be written as Ba 21.22Y1.08Ta1.51O6/C00.84. Thus, we find a significant deviation from the charge distri- bution of charge balanced Ba 22Y3Ta5O6/C02. The strong hybridization between O 2p states and the Ta(Y) d statesreveals that the static Ta (Y) and O charges are significantly less than þ5(þ3) and /C02. According to the ionicity scale, 20 the population ionicity can be calculated as Pi¼1/C0exp½/C0jPc/C0Pj=P/C138; (1) where P is the overlap population of the bond, P cis the bond population, for a purely covalent bond (P c¼0.75). P iis equal to zero for purely covalent bond and to unity for purely ionic bond. The Ta- O bond exhibits high covalency than Y-Obond, whereas Ba-O bond exhibits high ionicity. C. Born effective charges Born effective charges (Z*) play a fundamental role in the dynamics of the crystal lattices. They govern the ampli-tude of the long-range coulomb interactions between nuclei and the splitting between longitudinal optic (LO) and trans- verse optic (TO) phonon modes. For an insulator, Z *is a measure of change in electronic polarization due to ionic dis- placements. The form of effective tensor for the constituents is determined by the site symmetry of ions. Z*is defined as the proportionality coefficient relating, at linear order, the polarization per unit cell created along the bdirection, to the displacement along the direction aof the atoms belonging to the sublattice j, under the condition of zero electric field ( e)TABLE II. Calculated (experimental) structural parameters of BYTO using GGA-RPBE. Wyckoff sites x y z Ba 4d ( /C04) 0 0.5 0.25 Y 2a (4/m) 0 0 0 Ta 2b (4/m) 0 0 0.5 O1 4e (4) 0 0 0.2685 (0.2687) O2 8h (m) 0.2520 0.2857 0.0 (0.2175) (0.2578) FIG. 2. (a) Electronic band structure along high symmetry directions and (b) DOS of BYTO.144104-3 C. Ganeshraj and P . N. Santhosh J. Appl. Phys. 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29Z/C3 abjðÞ¼X0@Pa @sbjðÞ/C12/C12/C12/C12 e¼0; (2) where X0is the unit cell volume. The components of these tensors reflect the effects of covalency or ionicity with respect to some reference ionic value. Z*related to the atoms constituting BYTO are reported in Table IV. In this case, the cartesian x, y, and z are aligned along the crystalline [100], [010], and [001] directions. Intetragonal symmetry, the Born effective charge tensor is diagonal and reduces to values Z * xx¼Z* yy¼Z* ?and Z* zz¼Z* ?, except Z*of O 2. The off-diagonal elements (Z* yx¼Z* yx) arise from the tetragonal distortion due to lower site symmetry of O.21Since O 2along [100] and [010] direction ((m) site symmetry, (x, y, 0)) and O 1along [001]- direction (4 site symmetry, (0, 0, z)) is bonded with Ta (Y) atoms, a large anomalous contributions to Z*, on Ta/Y and O2ions in the ab-plane and on Ta/Y and O 1ions in the apical direction (along [001] direction). This anomalouscontribution clearly indicates that a strong dynamic charge transfer takes place along the Ta/Y-O bond. When O atom isdisplaced closer to Ta/Y atom, the change in bond hybridiza- tion causes transfer of electrons from O to Ta (Y). This trans- fer of electrons results in an increase in Ta (Y) bondcovalency. The deviation in large anomalous contributions of Born effective tensors, along and perpendicular to [001] direction, reflects the sensitivity to the atomic displacementof the partially covalent character of Ta/Y-O bond, owing to the point site symmetry of the ions. The charge observed on Ba indicates the ionic covalent character of bonding along 3crystallographic directions. In the ab-plane, the value of Z * on O 1and along apical direction on O 2is close to nominal value ( /C02) indicating the ionic character of bonding. The fi- nite value of Z* yx¼Z* yxoccurs due to site symmetry (m) of O2ions. The anisotropic diagonal elements of Z*of O 2and finite off-diagonal element of O 2clearly reflect the presence of covalent bonding of interactions.22 In order to gain further insight into the nature of the bonding accurately, we calculate the electron density distri-bution for BYTO. The charge density in our calculation is derived from a reliable converged wave function and hence it can be used to study the bonding nature of solid. The totalcharge density plots of BYTO along [100] direction show the deviation of the spherically distributed charge density around O ions, as shown in Figure 4. Moreover, it also shows that the deviation of the spherically distributed charge den- sity is more towards Ta atoms than Y atom, indicating the slightly high covalent nature of Ta–O bond than Y-O bond.The spherical distribution of charges around Ba ions indi- cates a dominated ionic nature of interaction. FIG. 3. Partial DOS of (a) Ba, (b) Ta, (c) Y, and (d) of BTYO. The Fermienergy is set to zero. TABLE III. The calculated (experimental) bond population, population ion- icity, and bond length of BYTO using GGA-RPBE. Bond Bond population, P Population ionicity, P iBond length (A ˚) Ba - O 1 0.06 0.999 2.9818 (2.979) Ba - O 2 0.06 0.999 2.8870 (2.867) Y-O 1 0.42 0.564 2.2687 (2.210) Y-O 2 0.40 0.524 2.2689 (2.216) Ta - O 1 0.57 0.271 1.9522 (2.015) Ta - O 2 0.57 0.254 1.9516 (2.000)144104-4 C. Ganeshraj and P . N. Santhosh J. Appl. Phys. 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29D. Vibrational properties The linear response method was used to calculate vibra- tional properties of BYTO. The linear response method pro-vides an analytical way of calculating the second derivative of the total energy with respect to perturbation. A perturba- tion in ionic positions yields the dynamical matrix and pho-nons. The lattice vibration mode with q /C250 plays a dominant role in Raman scattering and infrared absorption. Therefore, the vibration frequency at the C(q¼0) is called as normal vibration mode. Using method of factor group analysis, we determine the distribution of the zone-centre vibrational modes in terms of the representation of C 4hpoint group CðI4=mÞ¼6Tð2AuþBgþEgþ2EuÞ þ2LðAgþEgÞþt1ðAgÞþ2t2ðAgþBgÞ þ2t3ðAuþEuÞþ2t4ðAuþEuÞ þ2t5ðEgþBgÞþt6ðEuÞ; where t1,t2, and t3are related to Y(Ta)-O stretching mode, andt4,t5, and t6are O- Y(Ta)-O bending modes. T and L are translational and librational lattice modes, respectively.The symbols A and B represent nondegenerate and E double degenerate vibrational modes; symmetric and antisymmetric modes with respect to a centre of inversion are denoted bysubscripts g and u, respectively. There are nine Raman- active and infrared (IR)-active modes that exist. In the Raman-active modes, Y and Ta atoms are at rest. Table V compares calculated frequencies at the Brillouin zone center with available experimental and theoretical results. Theagreement with experimental values is typically within 5%, which is good for a first principles calculation. 23Figure 5 shows the displacement patterns of allowed Raman modes of vibration of BYTO. Among Raman modes, three modes (95 cm/C01, 398 cm/C01, and 597 cm/C01) are presumably arises due to the descent to lower symmetry (cubic-tetragonal sym- metry).26The highest Raman A g(847 cm/C01) mode corre- sponds to the displacement of oxygen atom along Y-O-Taaxis, while all the cations are at rest. This frequency is mainly determined by the Ta-O and Y-O distances and bond- ing forces, and thus by the chemical nature of the Ta and Yoctahedral cations. In this mode, all the O atoms are moving outward/inward, known as breathing mode. 28In B g (602 cm/C01) mode, only in-plane O 2atoms are involved, with one diagonally opposite pair displaced outwards (inwards) and other pair displaced inwards (outwards), known as out- of-phase stretching.29Here, the Ba atoms are also displaced in opposite direction along c-axis. The A g(597 cm/C01) mode is an octahedral stretching mode (only oxygen atoms are vibrating), where the four in-plane O 2atoms are displaced FIG. 4. Charge density distribution of BYTO along [100] direction.TABLE V. Calculated and experimental frequencies (cm/C01), assignment of the Raman and IR modes of BYTO using GGA-RPBE and oscillator strength (Debye2A˚/C02(amu)/C01) of IR mode. The phonon frequencies are presented in increasing order. Symmetry Cal. Exp.a,bModeOscillator strength, Aab(m) Cal.cCal.d Eg168 R 76.4 Eg295 (105) 104 R 109.8 Bg197 R 109.8 Ag1114 (Soft mode) R Eg3398 (388) 384 R 390 403.7 Bg2403 R 403.9 Ag2597 (795e) 573 R 562 583 Bg3602 R 567 583.1 Ag3847 (838) 836 R 841 823 Au193 110 IR 24.58 125.3 Eu199 IR 17.11 124.7 Au2204 IR 112.54 210.4 Eu2211 IR 88.37 211 Eu3228 218 IR 26 225.3 Au3258 264 IR 0.20 270.9 Eu4260 IR 5.17 271.0 Au4546 540 IR 61.41 548.5 Eu5552 IR 59.53 548.3 aReference 24. bReference 25. cReference 26. dReference 27. eRaman mode due to local defects.TABLE IV. Components of the calculated Born effective charge tensors at the Ba, Ta, Y, and O sites of BYTO using GGA-RPBE and Mulliken charges of Ba, Ta, Y, and O. Z* xx Z* xy Z* xz Z* yx Z* yy Z* yz Z* zx Z* zy Z* zz Mulliken charges jej Ba 2.72 0.01 0.0 /C00.01 2.72 0.0 0.0 0.0 2.80 1.22 Ta 6.99 0.03 0.0 /C00.03 6.99 0.0 0.0 0.0 7.00 1.51 Y 4.81 /C00.12 0.0 0.12 4.81 0.0 0.0 0.0 4.84 1.08 O1 /C02.05 0.01 0.0 /C00.01 /C02.05 0.0 0.0 0.0 /C04.55 /C00.84 O2 /C03.42 /C01.22 0.0 /C01.25 /C03.15 0.0 0.0 0.0 /C02.08 /C00.84144104-5 C. Ganeshraj and P . N. Santhosh J. Appl. Phys. 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29inwards and the two apical O atoms are displaced outwards and vice versa, known as out-of-phase stretching.29The B g (403 cm/C01) is Raman active scissor mode, where O 2atoms are displaced in ab-plane and Ba atoms are displaced in op-posite direction along c-axis. The E g(398 cm/C01) mode is an octahedral tilt mode ([110] rotation), with Ba atoms are dis- placed in ab-plane.30The A g(114 cm/C01) mode is anti-phase rotations of the octahedra about [001] and O 1atoms are dis- placed opposite direction along c-axis. The B g(97 cm/C01) mode is a scissoring mode with Ba atoms are displaced op-posite direction along c-axis. Here, O 1atoms are not dis- placed.31In Eg (95 cm/C01) mode, diagonally opposite pair of O2atoms is displaced opposite direction along c-axis with O1and Ba atoms along c-axis are displaced in ab plane in opposite direction. The vibration of E g(68 cm/C01) mode issimilar to that of last mode (95 cm/C01). All E gmodes are dou- bly degenerate modes that involve the motion in the ab plane of the tetragonal BYTO have the same frequency and charac- ter for the displacement along either a or b axis. Figure 6shows the displacement patterns of allowed IR modes of vibration of BYTO. The E umodes are an inplane mode; all atoms are displaced only in ab-plane, while in A u modes, the atoms are displaced along c-axis. The IR modes 552 cm/C01, 546 cm/C01, 211 cm/C01, and 204 cm/C01(2E uand 2A u) are octahedral stretching modes.32The 260 cm/C01, 258 cm/C01, and 228 cm/C01modes (2E uand A u) are corresponding to octa- hedral bending and stretching modes. The lowest frequency modes 99 cm/C01and 93 cm/C01(Euand A u) consist of bending vibration of octahedra along with polar motion of Ba atoms against all atoms, known as last mode.33–35The 204 cm/C01 FIG. 5. Illustrations of the displacement patterns of the Raman modes of BYTO. Each displacement vector is obtained from the eigen vector by dividing e ach component by the square root of the corresponding atomic mass. This figure was drawn using VESTA.17144104-6 C. Ganeshraj and P . N. Santhosh J. Appl. Phys. 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29and 211 cm/C01modes are known as Slater mode33with a con- tribution from Ba displacements. E. Dielectric properties The electronic dielectric permittivity tensor, e1,i s related to second derivative of the electronic energy withrespect to electric field and has been computed using linear response method. 36For our calculations, no scissor correc- tion has been used. Because of the symmetry properties oftetragonal crystal structure of BYTO, this tensor is diagonal with two independent components, parallel ( e zz1¼ejj1) and perpendicular ( exx1¼eyy1¼e?1) to the tetragonal axis. The calculated values of these two independent components aree?1¼4.57 and ejj1¼4.61, indicate that BYTO is apositive uniaxial ( ezz1/exx1>1) tetragonal crystal with a quite isotropic electric response to a homogeneous field. Unfortunately, there are no available experimental or theo- retical data reported in the literature for electronic dielectricpermittivity tensor of BYTO for comparison purpose. Nevertheless, the magnitudes of the e 1components are simi- lar to those observed in BaTiO 3(e1¼5.60),37ZrO 2 (e1¼4.805), A-La 2O3(e1¼4.924),38KNbO 3(e1¼4.69), NaNbO 3(e1¼4.96), and ZrSiO 4(e1¼4.14).39 It is well known that DFT usually overestimates the absolute values of e1with respect to the experimental ones. This problem has been related to the lack of polarization de- pendence of the quasi-local (GGA) and local (LDA)exchange-correlation functionals. In spite of this error in the absolute value, the evolutions of the optical dielectric FIG. 6. Illustrations of the displacement patterns of the IR modes of BYTO. Each displacement vector is obtained from the eigen vector by dividing each com- ponent by the square root of the corresponding atomic mass. This figure was drawn using VESTA.17144104-7 C. Ganeshraj and P . N. Santhosh J. Appl. Phys. 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29permittivity tensor are, in general, qualitatively well described by GGA and LDA calculations. Since electronic dielectric tensor describes the response of the electron gas to a homogeneous electric field if the ions are taken as fixed at their equilibrium positions, one need to use a model, which assimilates the solid to a system ofundamped harmonic oscillator in order to include the response of the crystal lattice to the electric field. The static dielectric tensor, e 0, can be therefore decomposed into an electronic and an ionic part such as e0 ab¼e1 abþ4p X0X mAabmðÞ x2 m: (3) The infrared oscillator strengths, A, are a second–order ten- sor given by AabmðÞ¼X c;jZ/C3 acjðÞffiffiffiffiffiffi ffiMjp ecj;mðÞ"#/C3X c;jZ/C3 bcjðÞffiffiffiffiffiffi ffiMjp ecj;mðÞ"# ;(4) where the sums run over all atoms jand space directions !, Mkis the mass of the jth atom, and e !(j,m) and xmare, respectively, !jcomponent of the eigen vector and the fre- quency of the mth mode obtain ed from the diagonalization of the analytical part of the dynam ical matrix. The relevant com- ponents of the oscillator strengt h tensor (parallel-parallel com- ponent for A 2umodes and the perpendicular-perpendicular component for E umodes) are given in Table V. According to the symmetry of BYTO structure, e0is a diagonal tensor with two different components e0 ?¼36.58 and e0 jj¼45.51. From Table V, it is found that the IR modes A u1(93 cm/C01)a n dA u2 (204 cm/C01) have small (large) oscillator strength (frequency) and they almost equally contribute to e0 jj. In addition to that, Eu1(99 cm/C01)a n dE u2(211 cm/C01) have also nearly equally contribute to e0 ?,s m a l l e rt h a nt h a to fA u1and A u2and explains why e0 jjis slightly higher than e0 ?.B yc o n t r a s tt o e1,t h e inclusion of ionic contribution results in an anisotropic e0with e0 jjslightly higher than e0 ?. The dielectric response of BYTO is therefore mainly ionic with slightly higher value of e0along optical axis (c-axis) than in orthogonal plane (ab plane). Inorder to compare our results with experimental data, we aver- age the values of e 0parallel and perpendicular to c-axis. The average value of e0obtained by taking one third of the trace of respective dielectric tensors ([2 e0 jjþe0 ?]/3) is 39.55, close to experimentally observed value 32.5 around T c/C24253 K.8The calculated dielectric tensor component values are expected tobe overestimated due to underestimation of band gaps in DFT calculation and it is attributed to the lack of polarization dependence in the exchang e-correlation functional. F. Elastic properties Elastic constants characterize the ability of a material to deform under small stresses. The elastic tensor c ijisdetermined by performing six finite distortions of the lattice and deriving the elastic constants from strain-stress relation- ship.38Because of the symmetry of the I4/m structure of BYTO, these tensors have only 6 independent elements to be determined. The mechanical stability in a tetragonal crystal is determined by Born’s mechanical stability condition, therequirement that the crystal be stable against any homoge- nous elastic deformation, as follows: ðC 11/C0C12Þ>0;ðC11þC33/C02C13Þ>0 C11>0;C33>0;C44>0;C66>0; ð2C11þC33þ2C12þ4C13Þ>09 >>= >>;: (5) The elastic constants of our calculation in Table VIsatisfy all the stability conditions. In particular, C12is smaller than C11, and C 13is smaller than the average of C 11and C 33. The other elastic properties (bulk modulus (B) and shear modulus (G)) can be calculated based on the calculated elastic con- stants c ij. In the calculation of elastic moduli, there are two different theories, Reuss theory and Voigt theory.41The bulk BR(BV) and shear G R(GV) modulus using Reuss theory (Voigt theory) are given as follows: BV¼1 9c11þc22þc33 ðÞ þ2c12þc23þc31 ðÞ ðÞ ;(6) GV¼1 15c11þc22þc33 ðÞ /C0c12þc23þc31 ðÞ ð þ3c44þc55þc66 ðÞ ; (7) 1 BR¼s11þs22þs33 ðÞ þ2s12þs23þs31 ðÞ ; (8) 15 GR¼4s11þs22þs33 ðÞ /C04s12þs23þs31 ðÞ þ3s44þs55þs66 ðÞ ; (9) where c ijand s ijare the elastic stiffness coefficients and the elastic compliance coefficients, respectively. For tetragonal system, c 22equals c 11,c23equals c 13, and c 55equals c 44.sijis the inverse matrix of c ijand vice versa. It is known that the Voigt bound is obtained by the average polycrystalline mod- uli based on the assumption of uniform strain throughout a polycrystal and is the upper limit of the actual effective mod-uli, 42while the Reuss bound is obtained by assuming a uni- form stress and is the lower limit of the actual effective moduli.43The arithmetic average of Voigt and Reuss bounds is termed as the Voigt-Reuss-Hill approximations.41Bulk B H and shear G Hmoduli using Hill theory are given as follows: BH¼ðBVþBRÞ=2; (10) GH¼ðGVþGRÞ=2: (11) TABLE VI. Calculated properties of BYTO using GGA-RPBE including elastic constants c ij’s (GPa), bulk modulus B (GPa), and shear modulus G (GPa) in Voigt-Reuss-Hill approaches, B/G ration, Young’s modulus E (GPa), and Poisson ratio tin Hill approach. C11 C12 C13 C33 C44 C66 GV GR GH BV BR BH BH/GH E t 315.25 185.21 54.57 333 64.2 129.72 96.23 82.40 89.32 172.47 169.48 170.97 1.91 221.86 0.26144104-8 C. Ganeshraj and P . N. Santhosh J. Appl. Phys. 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29If B H/GH(Pugh criterion) is lesser (bigger) than 1.75, the material is considered brittle (ductile).44By combining me- chanical stability restrictions with Eq. (8), one can easily obtain1 3C12þ2C13 ðÞ <BV<1 3C11þ2C33 ðÞ . It implies that bulk modulus B V(172.47 GPa) must be larger than the weighted average of C 12and C 13(119.9 GPa), and smaller than the weighted average of C 11and C 33(302.03) and also the condition1 3C12þ2C13 ðÞ <BH<1 3C11þ2C33 ðÞ is sat- isfied. Since BYTO possesses tetragonal symmetry with c-axis as a high symmetry axis, the elastic stiffness constants C11,C22,C33can be directly related to the crystallographic a, b, and c-axis, and C 44,C55, and C 66indicates the shear elasticity applied to the two-dimensional rectangular lattice in the (100), (010), and (001) planes. The weakest elastic stiffness constant C 11and C 22than C 33represents weakness of lattice interaction along crystallographic a and b-axis. Moreover, the C 44and C 55found to have weakest shear stiff- ness constant, indicating the soft shearing transformationalong (100) and (010), respectively. The observed weakest stiffness constants and shear stiffness constant are mainly due to the out of phase Ta(Y)O 6octahedra tilting about c-axis. The analysis of elastic anisotropy is of great significance for understanding the mechanical properties of crystal.Young’s modulus and Poisson ratio are important parameters for selecting materials in engineering design. Young’s modu- lus E and Poisson’s ratio tare obtained by the following formulas: 40 E¼9BG=ð3BþGÞ; (12) t¼ð3B/C02GÞ=ð2ð3BþGÞÞ: (13) From the predicted value B Hand G Hin Table VI, the Young’s modulus and poisson’s ratio of BYTO are 221.86 GPa and 0.26, respectively. The calculated poisson ratio (0.26) is similar to that of other pinning materials BaZrO 3 (0.237),45Y2O3(0.257),46and BaCeO 3(0.294).47Also, the calculated Young’s modulus (221.86 GPa) is similar to that of BaZrO 3(22964 GPa)48and Y 2O3(198.562 GPa),46but higher than that of BaCeO 3(109.6 GPa).47 In the present work, we use universal elastic anisotropy index AU(AU¼5GV/GRþBV/BR/C06) for crystal with any symmetry to estimate the anisotropic characteristic of tetrag- onal BYTO. For isotropic materials, AU¼0. If the value of AUdeviates from zero, the material has larger anisotropy. For tetragonal BYTO, the predicted value is 0.856 slightly deviates from 0, indicating the small elastic anisotropy char- acteristics of BYTO. In addition, the anisotropy indexes ofbulk and shear moduli (A Band A G) proposed by Chung and Buessen49are used to estimate the anisotropic characteristics of the system are given as AB¼ðBV/C0BRÞ=ðBVþBRÞ; (14) AG¼ðGV/C0GRÞ=ðGVþGRÞ; (15) where A B¼AG¼0 represents elastic isotropic and AB¼AG¼1 represents the maximum anisotropy. For tetrag- onal BYTO, A Band A Gare 0.0087 and 0.0774, which are far away from 1, suggesting again existence of a smallcompression and shear anisotropy. This is the first prediction of elastic properties of tetragonal BYTO and yet to be veri- fied experimentally. Further, we also estimated the Debye temperature HD, which is an important fundamental quantity that related to many physical properties such as specific heat.50At low tem- peratures, vibrational excitations arise from acoustic vibra- tions and HDfrom elastic constants is the same as that from specific heat measurements. Debye temperature can be esti-mated from the average sound velocity t m:51 HD¼h k3n 4pNAq M/C18/C19 1 3 tm; (16) where h is plank constant, k is Boltzmann’s constant, N Ais Avogadro’s number, qis the density, M is the molecular weight, and n is the number of atoms in the unit cell. The av- erage sound velocity tmis approximately calculated from tm¼1 32 v3 lþ1 v3 t/C18/C19/C20/C21/C01 3 ; (17) where tlandttare longitudinal and transverse sound veloc- ities, respectively, can be obtained from Navier’s equation50 as follows: tl¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð3Bþ4GÞ=3qp ; (18) tt¼ffiffiffiffiffiffiffiffiffi G=qp : (19) The calculated values of tl,tt, and tmare 5.64 km/s, 3.54 km/s, and 4.46 km/s, which yield a HDof 537.7 K, close to those obtained for Ba 2MgWO 6(570 K),52SrRuO 3 (525.5 K),53BaSnO 3(522 K),54and other pinning material in HTS BaZrO3 (544 K)55and Y 2O3(533.42 K).48 IV. CONCLUSIONS In summary, the band structure, bonding analysis, vibra- tional, dielectric, and elastic properties of Ba 2YTaO 6were investigated using first principles calculations are based on density functional theory. The calculated electronic band structure results indicated that the compound is an insulatorwith a direct band gap of 3.50 eV. An inspection of Mulliken population, born effective charges, and distribution of charge density shows that this material has mixed ionic–covalentcharacter. The Raman and infrared active phonon modes are calculated and properly assigned. The calculated and meas- ured phonon energies are in good agreement. The electronicdielectric tensor of Ba 2TaYO 6is nearly isotropic, and the magnitude of its components is similar to those reported in ferroelectric materials. The static dielectric permittivity con-stants have been computed and found that the average static dielectric constant is in good agreement with experiment. A detailed analysis of the contribution of the different vibra-tional modes to the static dielectric constant has been per- formed, including the computation of oscillator strength. By contrast to electronic dielectric tensor, its static dielectrictensor is anisotropic in plane orthogonal to the optical axis of this material. The calculation of elastic constants has been144104-9 C. Ganeshraj and P . N. Santhosh J. Appl. Phys. 116, 144104 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.59.222.12 On: Sat, 29 Nov 2014 15:53:29performed and satisfies the Born mechanical stability criteria for tetragonal materials, indicates the fact that the Ba 2YTaO 6 is mechanically stable. Other elastic properties, including bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and elastic anisotropy ratios are also investigated. The calculated Poisson’s ratio is similar to that of other pinningmaterials. ACKNOWLEDGMENTS We gratefully acknowledge to Department of Biotechnology, India for the financial support to procureCASTEP. We also thank to Dr. A. Gopalakrishna, Department of Biotechnology- IIT Madras for his help to procure and use CASTEP. The authors thankfullyacknowledge the computer resources, technical expertise, and assistance provided by the High Performance Computing Environment (HPCE), IIT Madras. The crystal structure andvibrational modes are drawn using VESTA software. 1P. Lunkenheimer, S. Krohns, S. Riegg, S. G. Ebbinghaus, A. Reller, and A. Loidl, Eur. Phys. J. Spec. Top. 180, 61 (2009). 2B. Raveau, A. Maignan, C. Martin, and M. Hervieu, Chem. Mater. 10, 2641 (1998). 3C. N. R. Rao, Ferroelectrics 102, 297 (1990). 4M.-H. Phan and S.-C. Yu, J. Magn. Magn. Mater. 308, 325 (2007). 5M. Fiebig, J. Phys. D: Appl. Phys. 38, R123 (2005). 6M. W. Lufaso and P. M. Woodward, Acta Crystallogr., Sect. 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1.4894819.pdf

New Products Andreas Mandelis Citation: Review of Scientific Instruments 85, 099501 (2014); doi: 10.1063/1.4894819 View online: http://dx.doi.org/10.1063/1.4894819 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in New Products Rev. Sci. Instrum. 85, 129501 (2014); 10.1063/1.4903909 New Products Rev. Sci. Instrum. 85, 059501 (2014); 10.1063/1.4875057 New Products Rev. Sci. Instrum. 85, 019501 (2014); 10.1063/1.4862460 New Products Rev. Sci. Instrum. 83, 069501 (2012); 10.1063/1.4727880 CrossFertilization between Spallation Neutron Source and Third Generation Synchrotron Radiation Detectors AIP Conf. Proc. 705, 1013 (2004); 10.1063/1.1757969 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Thu, 18 Dec 2014 05:16:38REVIEW OF SCIENTIFIC INSTRUMENTS 85, 099501 (2014) New Products Andreas Mandelis Department of Mechanical and Industrial Engineering, University of Toronto,5 King’s College Road, Toronto, Ontario M5S 3G8, Canada (Received 25 August 2014; accepted 25 August 2014; published online 12 September 2014) In order to supplement manufacturers’ information, this Department will welcome the submission by our readers of brief communications reporting measurements on the physical properties ofmaterials which supersede earlier data or suggest new research applications.[http://dx.doi.org/10.1063/1.4894819 ] NEW INSTRUMENTS AND COMPONENTS X-ray detector for diffraction studies Rigaku designed its HyPix-3000 two-dimensional semiconductor detector to meet the needs of thehome laboratory diffractionist. The hybrid pixelarray detector features an active area of approx- imately 3000 mm 2, a pixel size of 100 μm2,a count rate of >106 counts per second/pixel, a fast readout speed, and essentially no noise. Withseamless switching from 2D-TDI (time delay andintegration) mode to 2D snapshot mode to 1D-TDI mode to 0D mode with a single detector, theHyPix-3000 is a versatile diffraction detector thatsaves time. Each pixel on the HyPix-3000 detec-tor has dual energy discriminators, which makesit possible to adjust the energy window width bysetting the energy threshold to “high” or “low.”The low-energy discriminator can eliminate elec-trical noise and reduce fluorescence background,and the high-energy discriminator can eliminatecosmic rays and white radiation, so data can bemeasured with an optimized signal-to-noise ratio.The HyPix-3000 detector is available as an optionto augment Rigaku’s SmartLab diffractometer. Itscapabilities enhance the flexibility of the diffrac-tometer, which can perform powder, thin film, re-flectivity, and small angle x-ray and in-plane scat-tering experiments. The HyPix-3000 was designedfor flexibility and minimal maintenance. For exam-ple, the compact angular enclosure allows for ex-cellent high angle accessibility. Unlike other types of detectors, it requires neither an external cooling device as needed on charge-coupled device detec-tors, or gas exchange and anode wire washing as onmulti-wire detectors.— Rigaku Corporation, 9009 New Trails Drive, The Woodlands, Texas 77381-5209. (281-362-2300) http://www.rigaku.com Nanoscale chemical mapping system Bruker has released Inspire, an integrated scan- ning probe microscopy infrared (IR) system for10-nm spatial resolution in chemical and materi-als property mapping. The novel system incorpo-rates Bruker’s proprietary PeakForce IR mode toenable nanoscale IR reflection and absorption map-ping for a wide range of applications, includingthe characterization of microphases and their in-terfaces in polymer blends, plasmons in the 2Delectron gas of graphene, and chemical heterogene-ity in complex materials and thin films. The In-spire features sensitivity down to molecular mono-layers, even on samples not amenable to standardatomic force microscopy (AFM) techniques. It isa scanning-probe-based nanoscale characterizationsystem that extends AFM into the chemical regimeby providing IR reflection and absorption imag-ing down to a spatial resolution of 10 nm usingscattering scanning near-field optical microscopy.All optics, detectors, and configurable sources andall AFM hardware and software needed for atomicresolution imaging are included in a compact, ro- bust, integrated package. Bruker’s PeakForce IR mode builds on the company’s PeakForce tap-ping direct force control technology. According toBruker, PeakForce IR overcomes the limitationsof contact and of TappingMode, and thus of tra-ditional near-field optical and photothermal ap-proaches to nanoscale IR imaging. It also avoidssample damage from lateral forces, retaining high-est resolution on soft polymers, and enables high-resolution imaging of polymer brushes and evenpowders. PeakForce IR includes ScanAsyst self-optimization and PeakForce quantitative nanome-chanical property mapping nanomechanics for in-stantly correlated nanomechanical data. The com-prehensive set of optional modes includes Peak-Force tunneling AFM (TUNA), which enables *Not to be confused with the famous sandwich. McAllister Technical Services Manufacturers of surface analytical instruments and devices Ph. + 208-772-9527 800-445-3688 www.mcallister.com Bellows–Sealed Linear Translator (BLT *) Model BLT86-4 Save with an online subscription! This international journal cuts across all fields of study to bring original research totheoretical and mathematical physicists. Providing a unique link among specialists, the Journal of Mathematical Physics applies mathematical solutions to problemsin physics, and develops mathematical methods suitable for the formulation of physical theories. Research is presented in context, often addressing a general physical or mathematicalproblem before proceeding to a specific approach. Keep current with the latest developments in this exciting field...Subscribe today! For rates and ordering information call toll-free: 1-800-344-6902 or 516-576-2270. or visit: www.aip.org 0034-6748/2014/85(9)/099501/4/$30.00 © 2014 AIP P ublishing LLC 85, 099501-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Thu, 18 Dec 2014 05:16:38099501-2 Andreas Mandelis Rev.S c i .I n s t r um.85, 099501 (2014) conductivity mapping on samples not amenable to contact mode AFM, and PeakForce Kelvin probeforce microscopy (KPFM), which employs FM de-tection for high spatial resolution work functionmapping while avoiding the mechanical cross-talkaffecting single-pass FM-KPFM.— Bruker Corpo- ration, 40 Manning Road, Billerica, MA 01821.(978-663-3660) http://www.bruker.com DC hypoid right-angle gearmotor Bison Gear and Engineering has launched a perma- nent magnet DC brushed motor version of its Pow-erSTAR hypoid right-angle product line. The newgearmotor offers the gearing advantages of the ACmotor PowerSTAR series in addition to easy vari-able speed control and high start-up torque. TheDC motor PowerSTAR series features permanentmagnet, totally enclosed, non-ventilated DC mo-tors with dynamically balanced armatures, tang-type diamond turned commentators, high-grade ce-ramic magnets, and oversized replaceable brushes.The motors are built to Class F insulation stan-dards. Each motor series will feature different volt-age options to align with specific applications:720 and 725 series—12, 24, 90, 130, and 180 Vand 730 series—24, 90, 130, and 180 V . Likethe AC motor version, the DC PowerSTAR comesequipped with advanced hypoid gearing technol-ogy that, according to the company, provides upto 4×the efficiency of a typical worm gearmo- tor and mounting configurations for both impe-rial and metric equipment. The DC motor versionis suitable for remote or battery powered applica-tions and equipment requiring high start-up torqueor variable speed control, such as gate operators, pumping stations, and food service conveyors.— Bison Gear and Engineering, 3850 Ohio Avenue,St. Charles, IL 60174. (800-282-4766 or 630-377-4327) http://www.bisongear.com Rotational rheometer ATS RheoSystems, a division of the Cannon In- strument Company, has introduced the Black Pearlcontrolled shear rate rotational rheometer. It isrugged, capable of both steady shear and yieldstress testing, and has a compact footprint. De-signed for performing routine quality control testsand complex research and development evalua-tions, the Black Pearl rheometer is suitable for in-vestigating the mixing, stirring, and process flowcharacteristics of fluid systems. It comes stan-dard with built-in Peltier temperature control forall measuring systems. Cone and plate, parallelplate, and concentric cylinder measuring systemsare included. The measuring systems employ novel“quick capture” mounting technology; the gap- ping mechanism is accurate and user adjustable.The Black Pearl is interfaced and controlled us-ing Windows-based software. Featuring real-timedisplay of test values, automated program con-trol, and “test definition” windows to rapidly cre-ate multiple step experiments, it provides instan-taneous viscosity flow curves. Interactive graph-ing allows full control over displayed, printed,and plotted variables. User-enterable data sets anddata comparison are available with simple dataexportation through Windows Clipboard. The de-vice is CE certified, denoting that it meets therequirements of the applicable European Uniondirectives.— ATS RheoSystems, 231 Crosswicks Road, Bordentown, NJ 08505. (609-298-2522)http://www.atsrheosystems.com Helium-3 cryostat The ultra-low temperature (ULT) group of JanisResearch has developed an enhanced top-loadinghelium-3 cryostat with the sample in a UHV en-vironment. The model HE-3-TLSUHV-STM ULTsystem is optimized for atomic resolution scan-ning tunneling microscope (STM) measurementsand can be integrated with a commercially sup-plied or user-built STM. A central UHV tube (32mm diameter or larger) and gate valve providetop-loading access to the low-temperature, high-magnetic-field region for sample and tip exchange.Vertical, horizontal, and 2D/3D vector field super-conducting magnets are available and fully inte-grated. ULT STM systems are frequently used toinvestigate the electronic and magnetic propertiesof materials. Applications include quantum com-puting and information, electronics, and nanoscale manufacturing. A ULT STM system can be used to observe or manipulate the topography and elec-tronic structures of atoms, islands, molecules, andsurfaces. Techniques such as magnetic force mi-croscopy and spin-polarized STM can be used toinvestigate a variety of materials in conjunctionwith variable temperature and magnetic field con-ditions. Typical materials studied include metals,semiconductors, thin films, and carbons such asgraphene and carbon nanotubes. The Janis HE-3-TLSUHV-STM provides an operating time of>80 h at a base temperature of <300 mK; longer operating times are also possible with the use ofadditional He-3 gas. Quiet operation is assuredthrough proprietary acoustic noise reduction fea-tures inside the 1 K pot and the vibration-reducingmechanical support structure on the He-3 pot. For further vibration reduction, it is possible to oper-ate the system without pumping on the 1 K pot.The system includes a mechanical heat switch forrapid precooling and cooled 4 K and 1.5 K radia-tion shutters for maximum hold time and minimumbase temperature. Other available features includea sliding seal, low- and high-frequency wiring, andpre-installed optical fibers.— Janis Research Com- pany, LLC, 225 Wildwood Avenue, Woburn, MA01801. (781-491-0888) http://www.janis.com NEW DETECTORS, MEASUREMENTS, ANDMATERIALS Two-component epoxy Master Bond EP112LS is a two-part epoxy suit- able for impregnation, potting, encapsulation, seal-ing, and coating applications, particularly in theaerospace and optoelectronics industries. Opticallyclear, EP112LS features reliable non-yellowingproperties and has a refractive index of 1.55. Theelectrically insulative system is resistant to chemi-cals including water, oils, fuels, acids, and bases.It is serviceable over the temperature range of −60 ◦Ft o+450◦F, has a working life exceed- ing two to three days at room temperature, andrequires oven curing. Post curing will enhance itsproperties. With a mixed viscosity of 50–200 cps,EP112LS bonds well to various substrates, in-cluding metals, composites, glass, ceramics, andmany rubbers and plastics. Bonds feature a tensilestrength, compressive strength, and tensile mod-ulus of 11 000, 20 000, and 400 000 psi at room This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Thu, 18 Dec 2014 05:16:38099501-3 Andreas Mandelis Rev.S c i .I n s t r um.85, 099501 (2014) temperature, respectively. EP112LS has high di- mensional stability and a shelf life of one yearin original, unopened containers. It is availablein half-pint, pint, quart, gallon, and five-galloncontainer kits.— Master Bond, Inc., 154 Hobart Street, Hackensack, NJ 07601. (201-343-8983)http://www.masterbond.com High-energy x-ray and neutron CMOS detector The latest addition to Andor’s scientific com- plementary metal-oxide-semiconductor (sCMOS)Zyla family, the Zyla 5.5 HF fiber-optic coupledsCMOS, is the company’s fastest and highest-resolution platform for high-energy x-ray and neu- tron “indirect” detection. It offers a 5.5-megapixel array with a 6.5- μm pixel, 100 frames/s sus- tained acquisition rate, and ultra-low 1.2 e −read- out noise. The fiber optic design maximizes pho-ton throughput and ensures high image fidelityrelay from the scintillator. A modular architec-ture allows a wide range of scintillators, fiber-optic tapers, and beryllium filters to be coupled to-gether. Therefore the Zyla HF configuration canbe adjusted to match the requirements of appli-cations such as x-ray materials imaging, noninva-sive x-ray bio- and medical imaging, or neutrontomography.— Andor Technology USA, 425 Sulli- van Avenue, Suite 3, South Windsor, CT 06074.(860-290-9211) http://www.andor.com Power supplies for precision measurement According to Keithley Instruments, its new series 2280S programmable DC power supplies are, un-like conventional power supplies, also sensitivemeasurement instruments. They have the speedand dynamic range needed to measure the standby current loads and load current pulses produced by battery-powered wireless, medical, and in-dustrial devices. Applications include characteriz-ing wireless sensors, radio-frequency identificationtags, intrinsically safe devices, low-power semi-conductor devices, and consumer electronics. Se-ries 2280S supplies can output up to 192 W oflow-noise, linear-regulated DC power. The model2280S-32-6 can output up to 32 V at up to 6 A,and the model 2280S-60-3 can output up to 60 Vat up to 3.2 A. For users who want to utilize thesame model of instrument for research, design, andproduction test, series 2280S provides a balanceof sourcing and measurement capabilities at eco-nomical cost. The power supplies can make volt-age and current readback measurements with up to6 1 2digits of resolution for maximum precision or 31 2digits for greater speed. V oltage output mea- surements can be resolved down to 100 μVa n d load currents from 100 nA to 6A can be accuratelymonitored. Four load current measurement ranges(10 A, 1 A, 100 mA, and 10 mA) support measur-ing full load currents, standby mode currents, andsmall sleep mode changes precisely. For monitor-ing fast-changing and pulse-like load currents, se-ries 2280S supplies can capture dynamic load cur-rents as short as 140 μs to observe load currents in all operating modes to determine the device’s to-tal power consumption. Each state of a power-upload sequence and a power-down sequence can bemeasured. Measurements of 2500 readings/s makeit possible to characterize and test the current drawat each of the start-up states. Testers of devices orsystems with high in-rush currents can program thevoltage output’s rise time to slow the voltage rampand avoid voltage overshoot, which could poten-tially damage the device under test. V oltage falltime is also programmable to prevent a fast rampdown of the output voltage. A bright, 4.3 in. thinfilm transistor screen displays a large amount ofinformation. To reduce the chance of test errors,source settings and other data appear next to themeasurement readings. Soft-key buttons and a nav- igation wheel provide an intuitive user interface with shallow, easy-to-navigate menus. The icon-based main menu simplifies configuring tests. Thepower supplies’ graphing function makes it easyto monitor the stability of the load current, cap-ture and display a dynamic load current, or viewa start-up or turn-off load current. The power sup-plies take measurements quickly, store up to 2500measurement points, and compute statistics on thestored data. Statistical calculation options includeaverage, maximum, minimum, peak-to-peak, andstandard deviation. The built-in “list mode” func-tion simplifies testing a design over its operatingvoltage range automatically or studying how thedesign responds to DC output changes. Up to ninelists of sequenced voltage levels can be created andsaved with up to 99 distinct voltages in each list. Asingle trigger automatically executes the list onceor multiple times. To minimize test times in au-tomated systems, an external trigger input allowsfor hardware synchronization and control by othersystem instruments. Series 2280S power suppliesfeature KickStart instrument start-up software thatpermits automated acquisition of large amountsof data. General purpose interface bus, universal serial bus (USB), and LXI LAN interfaces offer additional options for programming and control- ling series 2280S supplies. The LXI core compli-ant LAN interface and built-in web page supportremote control and monitoring, so users can al-ways access the power supply and view measure-ments. A choice of front or rear panel terminals en-hances connection flexibility. For maximum volt-age accuracy, rear panel four-wire remote sensingensures that the output voltage programmed is thelevel actually applied to the load. Series 2280S sup-plies are suitable for both benchtop and automatedtesting.— Keithley Instruments, Inc., 28775 Aurora Road, Cleveland, OH 44139-1891. (888-534-8453or 440-248-0400) http://www.keithley.com NEW FACILITIES AND HARDWARE Processor for DAQ systems A processor board for ADwin-Pro-II data acqui- sition and control systems recently introduced byCAS DataLoggers and Jager has a 64-bit floating-point unit (FPU) for math co-processing. The Pro-CPU-T12 processor module offers real-time com-puting and is largely software-compatible to pre-vious versions. The Pro-II system provides com-plex applications with a high data rate while the fast processor allows an intelligent pre-selection of relevant data, mathematical functions, or digitalchannel filtering. According to the company, it cansatisfy demanding applications including physicsexperiments, vibration monitoring, failure analy-sis, and high-speed data acquisition. With Ether-net support allowing for high-speed data transfer,ADwin’s Pro-CPU-T12 module enables stan-dalone data recording in applications where thereis no personal computer (PC) or connection toone. The processor module – the ADwin CPU –is the center of the ADwin-Pro II system. It exe-cutes the ADbasic programming instructions andaccesses the inputs, outputs, and interfaces of theother modules. The Pro-CPU-T12 processor of-fers a 1 GHz clock, 1 GB main memory for stor-ing code and data, and a 1 Gb Ethernet inter-face for communication to the PC. These featuresmake the 64-bit double-precision FPU 5 ×faster than its predecessor module, the CPU-T11. Thenew module has a trigger input and two digital in-put/outputs (I/Os). The board can run C code em-bedded into ADbasic code. The module enablesstandalone operation without a PC using the Boot-loader. Users can choose either USB or a serial ad- vanced technology attachment (SATA) storage de- vice on the module: the SATA device can either beintegrated into the module or removable. Accessto the memory is provided via processor moduleor via network file system, server message block,or file transfer protocol from the Ethernet network.ADwin data acquisition systems feature tightly-coupled analog and digital inputs along with coun-ters to provide users’ applications with very low-latency operation. The ADwin-Pro II series uses amodular form factor with plug-in modules to al-low up to 480 analog or digital inputs in a singlechassis, a 300-MHz digital signal processor, andan Ethernet communications interface. Other I/Ooptions include CANbus, SSI, Profibus/Fieldbus,RS-232/485, and signal conditioner modules.— This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Thu, 18 Dec 2014 05:16:38099501-4 Andreas Mandelis Rev.S c i .I n s t r um.85, 099501 (2014) CAS DataLoggers, Inc., 12628 Chillicothe Road, Chesterland, OH 44026. (800-956-4437 or 440-729-2570) http://www.dataloggerinc.com HD/SD USB audio/video encoder Sensoray has added the model 2263S USB au- dio/video encoder to its line of broadcast grade,low-latency, real-time video solutions. Support-ing multiple analog and digital input formats, themodel 2263S captures high definition (HD) orstandard-definition (SD) video and simultaneouslysends a compressed and an uncompressed (pre-view) stream to the host. Supported video inputsinclude digital visual interface (DVI), component(with a component to DVI-I adapter, not included),and composite. Audio is optionally captured from analog line input, compressed and multiplexed into transport stream. The device is suitable for the cap-ture of multiple video sources, as for example invideo pipeline inspection, radar and sonar process-ing, remote video surveillance, and traffic moni-toring. The model 2263S is designed as a USBvideo class device, which means it does not re-quire a device-specific driver. It is controlled usinga video application program interface (DirectShowor Video4Linux). Sensoray provides software de-velopment kits that speed up application develop-ment for several operating systems. A demo ap-plication illustrates the encoder’s capabilities andserves as a starting point for custom development.The device implements efficient H.264 video com-pression. The resulting data is output as a mov-ing picture experts group transport stream (MPEG-TS) or in MP4 or audio video interleave (A VI) fileformats. Audio compression is performed usinglow-complexity advanced audio coding. Hardwaretimestamps used for multiplexing help keep audioand video data in sync. Motion joint photographicexperts group (MJPEG) compression is supportedfor snapshots and A VI streams.— Sensoray, 7313 Southwest Tech Center Drive, Tigard, OR 97223.(503-684-8005) http://www.sensoray.com NEW LITERATURE AND SOFTWARE Supercontinuum laser software PicoQuant has released a new version of Easy- Tau software for its FluoTime 300, an automatedfluorescence lifetime spectrometer with a steady-state option. The EasyTau software now supportsmeasurements with an integrating sphere and thecompany’s Solea supercontinuum laser. The in-tegrating sphere can reproduce published litera-ture data of selected quantum yield standards suchas Rhodamin 6G, Coumarin 153, and Ru(bpy)3.A dedicated measurement wizard in EasyTau al-lows even untrained users to make precise quan- tum yield measurements in a few minutes. The Solea laser can now also be directly controlledfrom the software. The laser’s wavelength tun-ability, variable repetition rates, and short pulsewidths allow excitation scans and fluorescence life-time measurements to be performed over a broadspectral and temporal range using only a sin-gle laser.— PicoQuant GmbH, Rudower Chaussee 29, 12489 Berlin, Germany. (49-(0)30-6392-6942)http://www.picoquant.com Scientific graphics software According to Golden Software, its Grapher 11 isan accurate, efficient 2D and 3D graphing programthat meets graphing needs from simple to complex.Designed primarily for scientists, engineers, and business professionals, and featuring an interfaceclaimed to be easy to use, Grapher converts datainto more than 60 fully customizable graph types.It allows users to create publication-quality graphsquickly and easily. Among the new features in thisrelease of Grapher are the three new graph types:polar vector plots, ternary class scatter plots, and2D and 3D doughnut plots. 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Classes cannow be based on text or numbers, allowing eas-ier class scatter plots to be created from any data.In addition, the number of classes has increased to300 for better representation of all data. Grapher 11operates in a Microsoft Windows environment withWindows XP, Vista, 7, or 8.— Golden Software, Inc., 809 14th Street, Golden, CO 80401-1866.(303-279-1021) http://www.GoldenSoftware.com This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 129.174.21.5 On: Thu, 18 Dec 2014 05:16:38

1.4897241.pdf

High-pressure structural and elastic properties of Tl2O3 O. Gomis, D. Santamaría-Pérez, J. Ruiz-Fuertes, J. A. Sans, R. Vilaplana, H. M. Ortiz, B. García-Domene, F. J. Manjón, D. Errandonea, P. Rodríguez-Hernández, A. Muñoz, and M. Mollar Citation: Journal of Applied Physics 116, 133521 (2014); doi: 10.1063/1.4897241 View online: http://dx.doi.org/10.1063/1.4897241 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mechanical behaviors and phase transition of Ho2O3 nanocrystals under high pressure J. Appl. Phys. 116, 033507 (2014); 10.1063/1.4890341 Compression of scheelite-type SrMoO4 under quasi-hydrostatic conditions: Redefining the high-pressure structural sequence J. Appl. Phys. 113, 123510 (2013); 10.1063/1.4798374 High-pressure study of the structural and elastic properties of defect-chalcopyrite HgGa2Se4 J. Appl. Phys. 113, 073510 (2013); 10.1063/1.4792495 High pressure transport, structural, and first principles investigations on the fluorite structured intermetallic, PtAl2 J. Appl. Phys. 111, 013507 (2012); 10.1063/1.3673522 Pressure-induced amorphization in mayenite (12CaO·7Al2O3) J. Chem. Phys. 135, 094506 (2011); 10.1063/1.3631560 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 202.28.191.34 On: Fri, 19 Dec 2014 18:37:55High-pressure structural and elastic properties of Tl 2O3 O. Gomis,1,a)D. Santamar /C19ıa-P/C19erez,2,3J. Ruiz-Fuertes,2,4J. A. Sans,5R. Vilaplana,1 H. M. Ortiz,5,6,b)B. Garc /C19ıa-Domene,2F . J. Manj /C19on,5D. Errandonea,2 P . Rodr /C19ıguez-Hern /C19andez,7A. Mu ~noz,7and M. Mollar5 1Centro de Tecnolog /C19ıas F /C19ısicas, MALTA Consolider Team, Universitat Polite `cnica de Vale `ncia, 46022 Vale `ncia, Spain 2Departamento de F /C19ısica Aplicada-ICMUV, MALTA Consolider Team, Universidad de Valencia, Edificio de Investigaci /C19on, C/Dr. Moliner 50, 46100 Burjassot, Spain 3Earth Sciences Department, University College London, Gower Street, WC1E 6BT London, United kingdom 4Geowissenschaften, Goethe-Universit €at, Altenh €oferallee 1, 60438 Frankfurt am Main, Germany 5Instituto de Dise ~no para la Fabricaci /C19on y Producci /C19on Automatizada, MALTA Consolider Team, Universitat Polite `cnica de Vale `ncia, 46022 Vale `ncia, Spain 6CINVESTAV-Departamento de Nanociencia y Nanotecnolog /C19ıa, Unidad Quer /C19etaro, 76230 Quer /C19etaro, Mexico 7Departamento de F /C19ısica, Instituto de Materiales y Nanotecnolog /C19ıa, MALTA Consolider Team, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain (Received 23 July 2014; accepted 24 September 2014; published online 7 October 2014) The structural properties of Thallium (III) oxide (Tl 2O3) have been studied both experimentally and theoretically under compression at room temperature. X-ray powder diffraction measurements up to 37.7 GPa have been complemented with ab initio total-energy calculations. The equation of state of Tl 2O3has been determined and compared to related compounds. It has been found experi- mentally that Tl 2O3remains in its initial cubic bixbyite-type structure up to 22.0 GPa. At this pres- sure, the onset of amorphization is observed, being the sample fully amorphous at 25.2 GPa. The sample retains the amorphous state after pressure release. To understand the pressure-inducedamorphization process, we have studied theoretically the possible high-pressure phases of Tl 2O3. Although a phase transition is theoretically predicted at 5.8 GPa to the orthorhombic Rh 2O3-II-type structure and at 24.2 GPa to the orthorhombic a-Gd 2S3-type structure, neither of these phases were observed experimentally, probably due to the hindrance of the pressure-driven phase transitions at room temperature. The theoretical study of the elastic behavior of the cubic bixbyite-type structure at high-pressure shows that amorphization above 22 GPa at room temperature might be caused bythe mechanical instability of the cubic bixbyite-type structure which is theoretically predicted above 23.5 GPa. VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4897241 ] I. INTRODUCTION Thallium (III) oxide (Tl 2O3) is a sesquioxide which occurs naturally as a rare mineral named avicennite.1Tl2O3 crystallizes at ambient conditions in the body-centered cubicbixbyite-type structure with space group (S.G.) Ia-3, No.206, Z ¼16. 2–4Bixbyite-type Tl 2O3is isomorphic to the cubic structure of In 2O3and several rare-earth sesquioxides. Apart from the bixbyite structure, the corundum-type struc-ture has been reported to be synthesized at high pressures and high temperatures. 5 Tl2O3can be applied in many technological areas.6In particular, it has been used as an electrode in high-efficiency solar cells due to its very low resistivity.7,8It has also been studied for optical communication applications becauseof its strong reflectance in the near infrared region (1300–1500 nm); 9however, its most promising application is in thallium oxide-based high-temperature superconductors.10 Despite its interesting technological applications, Tl 2O3 is one of the less studied sesquioxides probably because ofthe poisonous nature of thallium. In particular, contact with moisture and acids may form poisonous soluble thalliumcompounds, like thallium acetate, whose contact with skin should be avoided. 11Consequently, many properties of Tl 2O3 are unknown. In particular, it was long thought that this com- pound behaves as a metallic conductor;12–14however, it has been recently shown that it is a degenerate n-type semicon- ductor.15This result is in good agreement with transport measurements which suggest that n-type conductivity comes from oxygen deficiency in the crystalline lattice.13,16–18It is also in good agreement with optical measurements providing a band gap between 1.40 and 2.75 eV.9,12,16 Very little is known about the structural and mechanical properties of Tl 2O3. The bulk moduli of both bixbyite-type and corundum-type structures are unknown. In this context, studies of Tl 2O3under compression could help in under- standing its physical properties. In this work, we report an experimental and theoretical study of bixbyite-type Tl 2O3at room temperature and high-pressure (HP) by means of angledispersive X-ray diffraction (ADXRD) measurements and ab initio calculations. Technical aspects of the experiments and calculations are described in Secs. IIandIII, respectively. Results are presented and discussed in Sec. IVand conclu- sions summarized in Sec. V.a)Author to whom correspondence should be addressed. Electronic mail: osgohi@fis.upv.es b)On leave from Departamento de F /C19ısica, Universidad Distrital “Francisco Jos/C19e de Caldas,” 110311 Bogot /C19a, Colombia. 0021-8979/2014/116(13)/133521/9/$30.00 VC2014 AIP Publishing LLC 116, 133521-1JOURNAL OF APPLIED PHYSICS 116, 133521 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 202.28.191.34 On: Fri, 19 Dec 2014 18:37:55II. EXPERIMENTAL DETAILS Commercial Tl 2O3powder with 99.99% purity (Sigma- Aldrich) was crushed in a mortar with a pestle to obtain a micron-sized powder. XRD measurements performed at 1atm and room temperature with a Rigaku Ultima IV diffrac- tometer (Cu K aradiation) confirmed the bixbyite-type struc- ture of Tl 2O3. HP-ADXRD experiments at room temperature up to 37.7 GPa were carried out at beamline I15 of the Diamond Light Source using a monochromatic X-ray beam(k¼0.4246 A ˚) and a membrane-type diamond-anvil cell (DAC). Tl 2O3powder was loaded in a 150– lm diameter hole of an inconel gasket in a DAC with diamond-culet sizes of 350 lm. A 16:3:1 methanol-ethanol-water mixture was used as pressure-transmitting medium. A strip of gold wasplaced inside the gasket and used as the pressure sensor. Pressure was determined using the gold equation of state (EOS): B 0¼167.5 GPa, and B0’¼5.79, whose parameters are obtained with a third-order Birch-Murnaghan equation.19 The X-ray beam was focused down to 30 /C230lm2using Kickpatrick-Baez mirrors. A pinhole placed before the sam-ple position was used as a clean-up aperture for filtering out the tail of the X-ray beam. The images were collected using a MAR345 image plate located at 350 mm from the sample.The diffraction patterns were integrated as a function of 2 h using FIT2D in order to give conventional, one-dimensional diffraction profiles. 20The indexing and refinement of the powder diffraction patterns were performed using the Unitcell,21POWDERCELL,22and GSAS23,24program packages. III. THEORETICAL CALCULATIONS We have performed ab initio total-energy calculations within the density functional theory (DFT)25using the plane- wave method and the pseudopotential theory with the Vienna ab initio simulation package (VASP).26We have used the projector-augmented wave scheme (PAW)27imple- mented in this package to take into account the full nodal character of the all-electron charge density in the core region. Basis set, including plane waves up to an energy cut-off of 520 eV were used in order to achieve highly converged results and accurate description of the electronic properties. The exchange-correlation energy was described with thegeneralized gradient approximation (GGA) with the PBEsol prescription. 28A dense special k-points sampling for the Brillouin zone (BZ) integration was performed in order toobtain very well-converged energies and forces. At each selected volume, the structures were fully relaxed to their equilibrium configuration through the calculation of theforces on atoms and the stress tensor. This allows obtaining the relaxed structures at the theoretical pressures defined by the calculated stress. In the relaxed equilibrium configura-tions, the forces on the atoms are less than 0.006 eV/A ˚, and deviations of the stress tensor from a diagonal hydrostatic form are less than 1 kbar (0.1 GPa). The application of DFT-based total-energy calculations to the study of semiconductor properties under HP has been reviewed in Ref. 29, showingthat the phase stability, electronic, and dynamical properties of compounds under pressure are well described by DFT. Ab initio calculations allow the study of the mechanical properties of materials. The elastic constants describe the mechanical properties of a material in the region of small deformations, where the stress-strain relations are still linear.The elastic constants can be obtained by computing the mac- roscopic stress for a small strain with the use of the stress theorem. 30Alternatively, the macroscopic stress can be also calculated using density functional perturbation theory (DFPT).31In the present work, we perform the evaluation of the elastic constants of Tl 2O3with the use of the DFT as implemented in the VASP package.32The ground state and fully relaxed structures were strained in different directions according to their symmetry.32The total-energy variations were evaluated according to a Taylor expansion for the total energy with respect to the applied strain.33Due to this fact, it is important to check that the strain used in the calculationsguarantees the harmonic behavior. This procedure allows us to obtain the C ijelastic constants in the Voigt notation. The number of independent elastic constants is reduced by crys-talline symmetry. 34 IV. RESULTS AND DISCUSSION A. X-ray diffraction and structural properties The crystalline structure of cubic bixbyite-type Tl 2O3 (see Fig. 1) has two different types of six-fold-coordinated thallium atoms. Thallium located at the 8 bWyckoff site has slightly distorted octahedral coordination whilst thallium located at 24 dWyckoff site has distorted trigonal prismatic coordination. Finally, oxygen atoms occupy 48 eWyckoff sites. From XRD measurements carried out at 1 atm and room temperature outside the DAC, we have made aRietveld refinement of the lattice parameter and relative FIG. 1. Schematic representation of the crystalline structure of cubic bixbyite-type Tl 2O3. The unit-cell and atomic bonds are shown. Oxygen cor- responds to small (red) atoms while Tl(1) located at 8 band Tl(2) located at 24dcorrespond to light blue and dark blue atoms, respectively.133521-2 Gomis et al. J. Appl. Phys. 116, 133521 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 202.28.191.34 On: Fri, 19 Dec 2014 18:37:55atomic positions of the bixbyite-type structure. The refine- ment R-values are Rp¼7.7% and Rwp¼10.2%. These results (summarized in Table I) are in quite a good agreement with those of Refs. 2–4, and with our calculations, all of them included in Table Ifor comparison. Figure 2shows ADXRD patterns of Tl 2O3up to 37.7 GPa. Diffractograms up to 22 GPa can be indexed with the cubic bixbyite-type Tl 2O3structure. The main difference between diffraction patterns up to 22 GPa is the shift ofBragg peaks to higher angles with pressure as the result of a unit-cell volume decrease. A typical peak broadening of XRD peaks 35is detectable above 11 GPa. In this respect, before continuing the discussion of the results, we would like to comment on possible non-hydrostatic effects in our experiments. We have checked that non-hydrostatic condi-tions above 11 GPa do not induce a tetragonal or rhombohe- dral distortion of the cubic structure. As an example, the Rietveld refinement of the powder XRD pattern measured at18.2 GPa is included in Fig. 2. The refined parameters were: the scale factor, phase fractions, lattice parameters, profile coefficients, xfractional atomic coordinate of the Tl(2) atom, the overall displacement factor, and the background. The high quality of the Rietveld refinement shows that Tl 2O3 remains in the cubic phase at 18.2 GPa; i.e., just before the onset of the amorphization process, as will be explained below. We note that the broadening of XRD peaks above 11 GPa could be due to the loss of quasi-hydrostatic condi-tions of the pressure-transmitting medium 36–38or to local distortions caused by the appearance of defects which could be precursors of the pressure-induced amorphization (PIA)that will be commented later on. From the refinement of the diffraction patterns up to 22 GPa, we obtained the pressure dependence of the Tl 2O3 lattice parameter. In the Rietveld refinement, the oxygen atomic coordinates were supposed not to vary with pressure due to its small X-ray scattering cross section in comparisonto that of thallium atom. Rietveld refinements carried out on HP-ADXRD data for Tl 2O3show that the xfractional atomic coordinate of the Tl(2) atom up to 22 GPa was similar to thatat 1 atm within experimental uncertainty. This result agreeswith the weak pressure dependence of this atomic parameter obtained from our theoretical calculations (according to sim- ulations, the xfractional atomic coordinate of Tl(2) in Tl 2O3 TABLE I. Structural parameters of bixbyite-type Tl 2O3at 1 atm. X-ray diffractionaAb initio PBEsolbNeutron diffractioncX-ray diffractiondNeutron diffractione a(A˚) 10.5390(4) 10.6074 10.543 10.5344(3) 10.5363 Tl(1) site: 8 bx ¼0.25 x¼0.25 x¼0.25 x¼0.25 x¼0.25 y¼0.25 y¼0.25 y¼0.25 y¼0.25 y¼0.25 z¼0.25 z¼0.25 z¼0.25 z¼0.25 z¼0.25 Tl(2) site: 24 dx ¼0.969(1) x¼0.9667 x¼0.971(4) x¼0.96815(22) x¼0.9657(8) y¼0 y¼0 y¼0 y¼0 y¼0 z¼0.25 z¼0.25 z¼0.25 z¼0.25 z¼0.25 O site: 48 ex ¼0.388(5) x¼0.3829 x¼0.397(5) x¼0.3824(17) x¼0.3897(10) y¼0.394(3) y¼0.3885 y¼0.377(6) y¼0.3905(15) y¼0.3982(11) z¼0.148(3) z¼0.1540 z¼0.157(5) z¼0.1542(18) z¼0.1431(12) aOur XRD measurements. bOur calculations. cReference 2. dReference 3. eReferernce 4.FIG. 2. Room temperature XRD patterns of Tl 2O3at selected pressures. The background has not been subtracted. The diffractogram measured at 18.2 GPa is shown as empty circles. The calculated and difference XRD pat- terns at 18.2 GPa obtained from a Rietveld refinement are plotted with solid lines. The residuals at 18.2 GPa are Rp¼2.3% and Rwp¼3.0%. Bragg reflections from Tl 2O3and gold are indicated with vertical ticks at 2.1 and 18.2 GPa. Gold reflections are marked with plus ( þ) symbols. The XRD pattern at 1 atm after releasing pressure is shown at the top.133521-3 Gomis et al. J. Appl. Phys. 116, 133521 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 202.28.191.34 On: Fri, 19 Dec 2014 18:37:55varies from 0.9667 at 0 GPa to 0.9664 at 22 GPa). The pres- sure evolution of the unit-cell volume of Tl 2O3is plotted in Fig. 3. We have fitted these data with second-order Birch- Murnaghan (BM2) and third-order Birch-Murnaghan (BM3) EOSs.39Weights derived from the experimental uncertain- ties in both pressure and volume were assigned to each datapoint in both fits. The fits were carried out with the EoSFit software (v.5.2). 40All the experimental and theoretical val- ues at zero pressure for the volume, V0, bulk modulus, B0, and its first-pressure derivative, B0’, are summarized in Table II. Our experimental values are in relatively good agreement with our calculated values. For the case of our ex-perimental data, the obtained value for the weighted chi- squared, v 2 w, in the BM2 and BM3 EOS fits is 6.2 and 6.7; respectively. We note that, the refinement of the B0’ parame- ter in the BM3 EOS fit does not improve the fit of the data because the v2 wincreases to a value of 6.7 and the standard deviation of B0increases with respect to that obtained with the BM2 EOS, thus indicating that an expansion of the EOS to third order is not required to fit the data. These results show that the second-order equation of state is an adequaterepresentation of the volume-pressure data of Tl 2O3. For comparison purposes, the EOS parameters for isostructural In2O3are also included in Table II.41–43It can be highlightedthat the experimental value for B0in Tl 2O3(B0¼156(3) GPa) is approximately 15% smaller than that obtained for In2O3(B0¼184(10) GPa).43In this comparison, we consid- ered the EOS parameters obtained with a BM2 EOS with B0’ fixed to 4 because the B0andB0’ parameters are strongly correlated.44The lower value of B0for cubic Tl 2O3when compared to that of In 2O3is consistent with the decrease of the bulk modulus of bixbyite-type sesquioxides when the ionic radius of the Acation increases in the series A¼In, Tl. We note that bixbyite-type sesquioxides like In 2O3and Tl2O3are much less compressible than sesquioxides of late group-15 elements in the Periodic Table like cubic a-Sb 2O3 (S.G.: Fd–3m, No. 227, Z ¼16)45and monoclinic a-Bi2O3 (S.G.: P21/c, No. 14, Z ¼4).46 Figure 2shows a drastic decrease of the intensity of the Bragg reflections of cubic Tl 2O3between 18.2 and 22 GPa. In addition, at 25.2 GPa, all the sharp crystalline peaks of cubic Tl 2O3disappear and two broad peaks appear at 8.75/C14 and 11.53/C14(noted with asterisk marks). These two peaks remain up to 37.7 GPa, the maximum pressure achieved in our experiment and exhibit a small shift to higher anglesbetween 25.2 and 37.7 GPa. These results can be interpreted as an amorphization of Tl 2O3above 22.0 GPa which is already completed at 25.2 GPa and will be discussed inSec. IV B . B. Amorphization It is commonly accepted that PIA in crystalline solids may occur if the crystalline structure becomes mechanical ordynamically unstable at a certain pressure; i.e., if mechanical stability criteria are violated or if the phonon dispersion curves contain imaginary frequencies for phonon modes at agiven pressure. 47PIA due to these instabilities usually occurs when the crystalline solid cannot undergo a phase transition to a HP crystalline phase at a smaller pressure than that ofamorphization. The hindrance of the pressure-driven phase transition between two crystalline phases is usually due to the presence of kinetic barriers between the low- and high-pressure structures. This barrier cannot be overcome if the temperature is not high enough and consequently the transi- tion is frustrated at low temperatures. Therefore, it is worthto investigate which could be the frustrated HP phase of Tl 2O3and at which pressure the phase transition is predicted to occur. In order to look for candidates of HP phases of Tl 2O3, we have performed total-energy calculations for Tl 2O3with the structures observed experimentally in In 2O3at different pressures and temperatures.48–52They include bixbyite-type (Ia–3), corundum-type (S.G.: R–3c, No. 167, Z ¼6), ortho- rhombic Rh 2O3-II-type (S.G.: Pbcn , No. 60, Z ¼4), and orthorhombic a-Gd 2S3-type (S.G.: Pnma , No. 62, Z ¼4) structures. We have also considered in our calculations the orthorhombic Rh 2O3-III-type structure (S.G. Pbca , No. 61, Z¼8), which is a high-temperature and low-pressure form of Rh 2O3,53and two of the structures commonly found in rare-earth sesquioxides (RES) under different pressure andtemperature conditions, 54–58like the monoclinic B-RES (S.G.: C2/m, No. 12, Z ¼6) and trigonal A-RES (S.G.FIG. 3. Evolution of the unit-cell volume with pressure. Symbols refer to ex- perimental data. Error bars are smaller than symbol size. Red dashed line and blue dotted line represent the fit of experimental data with a BM2 andBM3 EOS; respectively. Theoretical results are plotted with solid line. TABLE II. Experimental (Exp.) and theoretical (Th.) EOS parameters for cubic bixbyite-type Tl 2O3at zero pressure. Last column indicates the EOS type used (BM2 ¼Birch-Murnaghan of 2ndorder, BM3 ¼Birch-Murnaghan of 3rdorder). Results for isostructural In 2O3are included for comparison. Compound V0(A˚3) B0(GPa) B0’ Reference EOS type Tl2O3(Exp.) 1170.6(1) 147(13) 5(2) This work BM3 Tl2O3(Exp.) 1170.6(1) 156(3) 4 (fixed) This work BM2 Tl2O3(Th.) 1193.2(1) 125.0(4) 4.97(4) This work BM3 Tl2O3(Th.) 1191.5(5) 134.2(7) 4 (fixed) This work BM2 In2O3(Exp.) 1038(2) 194(3) 4.75 (fixed) 41 BM3 In2O3(Exp.) 1035.4(2) 178.9(9) 5.15 42 BM3 In2O3(Exp.) 1028(2) 184(10) 4 (fixed) 43 BM2133521-4 Gomis et al. J. Appl. Phys. 116, 133521 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 202.28.191.34 On: Fri, 19 Dec 2014 18:37:55P–3m1, No. 164, Z ¼1) structures. Finally, to complete the study, we have also considered as candidates for HP phases of Tl 2O3structures observed in transition-metal sesquioxides at different pressures and temperatures. These structures are the Sb 2S3-type (S.G.: Pnma , No. 62, Z ¼4) found in Ti2O3,59,60the distorted orthorhombic perovskite or GdFeO 3-type (S.G.: Pnma , No. 62, Z ¼4) found in Fe 2O3 (hematite)61and the orthorhombic post-perovskite or CaIrO 3-type (S.G.: Cmcm , No. 63, Z ¼4) found in Mn 2O3.62 The enthalpy difference vs. pressure diagram for the dif- ferent Tl 2O3polymorphs, taking as reference the enthalpy of the bixbyite-type phase, is plotted in Fig. 4. Our calculations predict a phase transition from the bixbyite-type phase (Ia–3) to the Rh 2O3-II-type phase ( Pbcn ) at 5.8 GPa, and from the Rh 2O3-II-type phase to the a-Gd 2S3-type phase (Pnma ) at 24.2 GPa. This sequence of pressure-induced phase transitions for Tl 2O3is the same as for In 2O3.48,49The main difference is the phase transition pressures predictedtheoretically in both compounds: 5.8 GPa (7–11 GPa) and 24.2 GPa (36–40 GPa) for Tl 2O3(In2O3).48,49The fact that our HP-ADXRD measurements in Tl 2O3up to 37.7 GPa do not show evidence of the phase transitions predicted at 5.8 GPa and 24.2 GPa, but an amorphization whose onset is around 22 GPa, suggests that kinetic barriers might be pres-ent in the phase transition to the HP phases at room tempera- ture in Tl 2O3. Note that the phase transition from the bixbyite-type to the Rh 2O3-II-type structure was observed in In2O3at room temperature above 30 GPa (Ref. 43) and the transition to the a–Gd 2S3-type structure in In 2O3was not observed at room temperature between 1 atm and 51 GPa.49 In fact, an amorphous halo was observed in In 2O3at 51 GPa and room temperature; thus suggesting a PIA in In 2O3at room temperature above this pressure. On the other hand, thephase transition in In 2O3from the Rh 2O3-II-type structure to thea–Gd 2S3-type structure was observed at HP and high temperature.49Furthermore, the phase transition pressures in In2O3were observed experimentally close to those theoreti- cally predicted only in HP and high temperatureexperiments.48,49Those works already showed that large ki- netic barriers are present in In 2O3at room temperature between the bixbyite-type, Rh 2O3-II-type and a–Gd 2S3-type structures; therefore, similar barriers are expected to occur for the same structures in Tl 2O3. In particular, the hypothesis of the kinetic frustration of the pressure-induced phase tran-sition from the bixbyite-type to the Rh 2O3-II-type structure in Tl 2O3will be explored in detail in future simultaneous HP and high temperature experiments on Tl 2O3, as it was al- ready done for In 2O3.48–52We note that our ab initio calcula- tions do not include kinetic energy barriers and therefore the theoretically predicted HP phases for bixbyite-type Tl 2O3 could be found experimentally in future HP and high temper- ature experiments where the kinetic energy barriers can be overcome. Finally, we want to stress that Tl 2O3remains amorphous after decompression from 37.7 GPa to 1 atm, i.e. PIA in Tl 2O3at room temperature is irreversible. C. Elastic properties In order to further understand the amorphization process in Tl 2O3, we have studied the mechanical stability of the cubic bixbyite-type (Ia-3) structure of Tl 2O3at HP. This structure belongs to the cubic Laue group CII with pointgroup m-3 which has three independent second order elastic constants: C 11,C12, and C44. Table IIIsummarizes the values of the three Cijin Tl 2O3at zero pressure as obtained from ourab initio calculations. The calculated elastic constants of bixbyite-type In 2O3taken from Ref. 63are also included in Table IIIfor comparison. The values of the three elastic con- stants of Tl 2O3are smaller than those of In 2O3. This result supports the smaller zero pressure bulk modulus of Tl 2O3 when compared to that of In 2O3as previously commented. A lattice is mechanically stable at zero pressure only if the Born stability criteria are fulfilled.64In the case of cubic systems, these criteria are C11þ2C12>0;C11–C12>0;C44>0: (1) FIG. 4. Theoretical calculation of enthalpy difference vs. pressure for Tl 2O3 polymorphs. Enthalpy of bixbyite-type phase is taken as the reference. Enthalpy is written per two formula units for all structures for the sake of comparison.TABLE III. Calculated Cijelastic constants and elastic moduli B, G, E (in GPa) and the Poisson’s ratio, /C23, for Tl 2O3at zero pressure. Elastic moduli and Possion’s ratio are given in the Voigt, Reuss and Hill approximations, labeled respectively with subscripts V, R, and H. The B/G ratio and the Zener anisotropy factor, A, are also given. Calculated data at zero pressure taken from Ref. 63for In 2O3are also added for comparison. Tl2O3aIn2O3b C11 177.0 234.3 C12 99.2 107.2 C44 32.8 62.7 BV¼BR¼BH 125.1 149.6 GV,GR,GH 35.3, 35.0, 35.1 63.0c EV,ER,EH 96.7, 96.1, 96.4 165.8c /C23V,/C23R,/C23H 0.37, 0.37, 0.37 0.32c BV/GV,BR/GR,BH/GH 3.55, 3.57, 3.56 2.37c A 0.84 0.99 aOur calculations with GGA-PBEsol prescription. bCalculated with the GGA approximation. cResults calculated in the Hill approximation from reported elastic constants.133521-5 Gomis et al. J. Appl. Phys. 116, 133521 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 202.28.191.34 On: Fri, 19 Dec 2014 18:37:55In our particular case, all the above criteria are satisfied for bixbyite-type Tl 2O3at zero pressure; therefore, cubic Tl2O3is mechanically stable at 1 atm (10/C04GPa), as it was expected. When a non-zero uniform stress is applied to the crystal, the above criteria to describe the stability limits of the crystal at finite strain are not adequate and the Born sta-bility criteria must be modified. In this case, the elastic stiff- ness (or stress-strain) coefficients are defined as B ijkl¼Cijklþ1=2½dikrjlþdjkrilþdilrjkþdjlrik–2dklrij/C138; (2) where the Cijklare the elastic constants evaluated at the cur- rent stressed state, rijcorrespond to the external stresses, and dklis the Kronecker delta.65–67In the special case of hydrostatic pressure applied to a cubic crystal, r11¼r22¼r33¼/C0P, and the elastic stiffness coefficients are:B11¼C11–P,B12¼C12þP, and B44¼C44–P, where Pis the hydrostatic pressure. Note that the BijandCijcoeffi- cients are equal at 0 GPa. When the Bijelastic stiffness coef- ficients are used, all the relations of the theory of elasticity can be applied including Born’s stability conditions which are identical in both loaded and unloaded states.66–69 The bulk ( B) and shear ( G) moduli of cubic Tl 2O3can be obtained in the Voigt,70Reuss,71and Hill72approxima- tions, labeled with subscripts V,R, and H, respectively, using the formulae73 BV¼BR¼B11þ2B12 3; (3) BH¼BVþBR 2; (4) GV¼B11/C0B12þ3B44 5; (5) GR¼5B11/C0B12 ðÞ B44 4B44þ3B11/C0B12 ðÞ; (6)GH¼GVþGR 2: (7) In the Voigt (Reuss) approximation, uniform strain (stress) is assumed throughout the polycrystal.70,71On the other hand, Hill has shown that the Voigt and Reuss averages are limits and suggested that the actual effective BandG elastic moduli can be approximated by the arithmetic mean of the two bounds.72The Young ( E) modulus and the Poisson’s ratio ( /C23) are given by74,75 EX¼9BXGX GXþ3BX; (8) /C23X¼1 23BX/C02GX 3BXþGX/C18/C19 ; (9) where the subscript Xrefers to the symbols V,R, and H.We summarize in Table IIIall the values obtained for B, G, E , and /C23in bixbyite-type Tl 2O3at zero pressure in the Voigt, Reuss, and Hill approximations. Note that our calculatedvalue for the bulk modulus in the Hill approximation (B H¼125.1 GPa) is in very good agreement with the value ofB0¼125.0(4) GPa obtained from our PBEsol structural calculations via a BM3 EOS fit. This result gives us confi- dence about the correctness of our elastic constants calculations. Table IIIalso includes the values of the ratio between the bulk and shear modulus, B/G, and the Zener anisotropy factor, A. The B/G ratio has been proposed by Pugh to pre- dict brittle or ductile behavior of materials.76According to the Pugh criterion, a B/G value above 1.75 indicates a tend- ency for ductility; otherwise, the material behaves in a brittlemanner. In our particular case, we found a value of B/G¼3.56 in the Hill approximation indicating that the ma- terial should be ductile at 1 atm. The Zener anisotropy factorAfor our cubic cell is defined as A¼2B 44/(B11/C0B12). IfAis equal to one, no anisotropy exists. On the other hand, the FIG. 5. Pressure dependence of the theoretical (a) Cijelastic constants and (b)Bijelastic stiffness coefficients of bixbyite-type Tl 2O3. Solid lines con- necting the calculated data points are shown as a guide to the eyes.133521-6 Gomis et al. J. Appl. Phys. 116, 133521 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 202.28.191.34 On: Fri, 19 Dec 2014 18:37:55more this parameter differs from one, the more elastically anisotropic is the crystalline structure. In cubic Tl 2O3,theA value (0.84) is slightly different from 1 and evidence a smallelastic anisotropy of our cubic cell at 1 atm. Figures 5(a) and5(b) show the pressure dependence of the three calculated C ijelastic constants and the three Bij elastic stiffness coefficients of bixbyite-type Tl 2O3, respec- tively. It can be seen that B11andB12increase monotonically as pressure increases, while B44decreases monotonically as pressure increases and at 23.5 GPa crosses the 0 GPahorizontal line. This fact is related with the mechanical insta- bility of bixbyite-type Tl 2O3and will be discussed in the next paragraphs. The knowledge of the behavior of the three elastic stiff- ness coefficients with pressure allows us to study the me- chanical stability of bixbyite-type Tl 2O3as pressure increases. The new conditions for elastic stability at a given pressure P, known as the generalized stability criteria, are obtained by replacing in Eq. (1)theCijelastic constants by theBijelastic stiffness coefficients, and are given by77 M1¼B11þ2B12>0; (10) M2¼B11/C0B12>0; (11) M3¼B44>0; (12) where B11,B12, and B14are the elastic stiffness coefficients at the considered pressure. These generalized stability crite- ria are plotted in Fig. 6. It is found that Eq. (12), related to a pure shear instability, is violated at 23.5 GPa while Eq. (11), called the Born instability,77is violated at 26.0 GPa. Therefore, our theoretical study of the mechanical stability of Tl 2O3at HP suggests that the bixbyite-type phase becomes mechanically unstable beyond 23.5 GPa. This pres- sure is slightly above but very close to the pressure at which the onset of PIA takes place experimentally. Consequently,this result suggests that shear instability could be involved in the PIA process of Tl 2O3at room temperature. We want to stress that our calculations are performed for a perfectFIG. 6. M10¼M1/10,M2, and M3stability criteria for bixbyite-type Tl 2O3as a function of pressure. The pressure for the onset of the amorphization pro- cess, Pam, in our experiments is indicated. FIG. 7. Pressure dependence of (a) BH, (b) GH, (c) EH, (d) /C23H, (e) BH/GH, and (f) A. Solid lines connecting the calculated data points are shown as a guide to the eyes. Results are shown in the Hill approximation.133521-7 Gomis et al. J. Appl. Phys. 116, 133521 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 202.28.191.34 On: Fri, 19 Dec 2014 18:37:55material, whereas our powder samples are very defective and contain a high concentration of O vacancies that make Tl 2O3 a degenerate n-type semiconductor. Therefore, we expect that PIA in our sample takes place at a lower pressure than that theoretically predicted since defects are known to induce amorphization and decrease the pressure at which PIAbegins in a number of materials. 78,79 We have also performed the study of the dynamical sta- bility in Tl 2O3in order to complement the study of the me- chanical stability of Tl 2O3and verify that PIA in Tl 2O3is caused by the mechanical instability of the cubic phase. To check the dynamical stability of the cubic phase, we havecarried out ab initio calculations of the phonon dispersion relations in bixbyite-type Tl 2O3. We have found that the cubic phase is dynamically stable up to 32 GPa and that pho-nons with imaginary frequencies appear above this pressure. This result thus indicates that bixbyite-type Tl 2O3becomes dynamically unstable above 32 GPa.80Since this pressure is higher than the pressure at which bixbyite-type Tl 2O3 becomes mechanically unstable, we conclude that PIA ofTl 2O3observed at room temperature at 22 GPa might be caused by the mechanical instability of the cubic lattice at pressures above 22 GPa. Finally, for completeness we have plotted the pressure dependence of the elastic moduli ( BH,GHand EH),/C23H Poisson’s ratio, BH/GHratio, and AZener anisotropy factor in Fig. 7. It is found that BHincreases with pressure and reaches the value of 216.0 GPa at 23 GPa. On the other hand, GHandEHdecrease with pressure approaching a value of 0 GPa near 23.5 GPa, pressure at which the mechanical insta-bility is predicted to occur. We note that the fact that the shear modulus decreases with pressure is compatible with the fact that the equation that first is violated (Eq. (12)) is the one related with the pure shear instability because of the decreasing of B 44with pressure. The Poisson’s ratio, /C23H, increases with pressure and reaches a value of 0.49 at23 GPa. The B H/GHratio increases with pressure, grows exponentially above 19 GPa, and reaches a value of 94.4 at 23 GPa. The increase of the BH/GHratio with pressure indi- cates that the ductility of Tl 2O3is enhanced under compres- sion. In the case of the Zener anisotropy factor, A, it is found that it increases with pressure reaching a maximum value ofA¼0.96 at about 11 GPa and afterward decreases quickly above 20 GPa indicating a strong increase of the elastic ani- sotropy above that pressure. V. CONCLUDING REMARKS We have studied both experimentally and theoretically the structural properties of Tl 2O3under compression at room temperature. The equation of state of Tl 2O3has been deter- mined and its bulk modulus has been found to be smaller than that of isostructural In 2O3.T l 2O3starts to amorphize above 22 GPa and retains the amorphous structure at 1 atmwhen decreasing pressure from 37.7 GPa. The theoretically predicted transitions to the Rh 2O3-II-type structure, near 6 GPa, and to the a-Gd 2S3-type structure, near 24 GPa, are not observed experimentally, probably, due to the kinetic hindrance of the phase transitions at room temperature.To understand the pressure-induced amorphization pro- cess of Tl 2O3, we have studied theoretically both the me- chanical and dynamical stability of the cubic phase at highpressures. In this respect, the mechanical properties of bixbyite-type Tl 2O3at high pressures have been commented. Our calculations show that amorphization might be causedby the mechanical instability of the bixbyite-type structure predicted above 23.5 GPa since this phase is dynamically sta- ble up to 32 GPa. ACKNOWLEDGMENTS This study was supported by the Spanish government MEC under Grant Nos. MAT2010-21270-C04-01/03/04, MAT2013-46649-C4-1/2/3-P, and CTQ2009-14596-C02-01,by the Comunidad de Madrid and European Social Fund (S2009/PPQ-1551 4161893), by MALTA Consolider Ingenio 2010 project (CSD2007-00045), and by GeneralitatValenciana (GVA-ACOMP-2013-1012 and GVA-ACOMP- 2014-243). We acknowledge Diamond Light Source for time on beamline I15 under proposal EE6517 and I15 beamlinescientist for technical support. A.M. and P.R.-H. acknowledge computing time provided by Red Espa ~nola de Supercomputaci /C19on (RES) and MALTA-Cluster. B.G.-D. and J.A.S. acknowledge financial support through the FPI program and Juan de la Cierva fellowship. 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Milling TimeandTemperature Dependence onFe 2TiO5 NanoparticlesSynthesizedby Mechanical Alloying Method R.Fajarin*,H.Purwaningsih,Widyastuti,D.Susanti,andR.Kurnia Helmy Material and Metallurgical Engineering, Institute of Technology Sepuluh Nopember Surabaya 60111, Indonesia * Email: fajar@mat-eng.its.ac.id Abstract. Fe2TiO5is one type of titanate oxides which has M xTiyOzcrystal structure. It has various kinds of applications due to its electric and magnetic properties such as spintron ics, electromagnetic devices, and gas sensor. In this study, Fe 2TiO5nanoparticles were synthesized bysimple mechanical alloying using plan etary ball milling machine with various milling times and sintering temperatures. TiO 2and Fe2O3powders obtained from coprecipitation process were used as starting materials. The resulted Fe 2TiO5powders were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy(SEM), and Vibration Sample Magnetometer ( VSM) in order to observe crystal quality, particles morphology, and magnetic propertiesrespectively. As the mill ingtime increases and the sinteringtemperature decreases,thecrystalsizeofFe 2TiO5phasedecreases.ThesmallestcrystalsizeofthesynthesizedFe 2TiO5nanoparticles was51 nmobtained bythe millingtime of 25 hours and sintering at1100oC.Thedistribution oftheresultedFe 2TiO5 nanoparticleswasnotsohomogeneousduetotheappearanceofsmallamount impurities.TheVSMmeasurementsshow thataparamagneticpropertywas observed whichshouldbeanalyzed morede tailsonthelowexternalmagneticfields. Keywords: Fe2TiO5, Nanomaterials,MechanicalAlloying,MillingTime,Sintering. PACS:81.07.-b;81.40.-z;81.20.Fw;81.20.Wk;81.20.Ev INTRODUCTION Most of electronic devices in the last decade are based on semiconductors. Among various oxidesemiconductors,then- type semiconductor α-Fe 2O3has many applications and also has been studied becauseof its electric and magnetic properties. The otheradvantages ofthis oxide are lowcost production, highcorrosion resistance, non-toxic and environmentally-friendly oxides [1]. Pure phase of α-Fe 2O3has very low conductivity. However, this electric property of α- Fe2O3can be increased by incorporation addition or doping. Improving electric properties means that it willincreasetheusabilityasagassensingmaterial[2]. Addition of TiO 2in α-Fe2O3and vice versa has attracted many researchers to investigate theirproperties, especiallyin optical properties [3]. Fe 2TiO5 is one ofthe pseudobrookite solid solution Fe yTi3-yO5 which has optical, electric and magnetic properties.This titanate oxide has many applications such as Li-ion battery and gas sensor [4, 5]. Researches onphotoelectrochemical, spin glass properties, and gassensitivity of both combination TiO 2-Fe2O3and Fe2TiO5hadbeen done[6,7,8]. Several synthesis methods have been applied in order to obtain pure Fe 2TiO5in polycrystals (bulk forms),nanoparticles,andthinfilms.Fe 2TiO5thinfilm on silica glass with crystal size 40 nm could beobtained from sol-gel method and Fe(NO 3)3.9H2Oa nd Ti(OC3H7)4as the raw materials [9]. Fe 2TiO5 nanoparticles could also be synthesized from TiCl 3, Fe(NO 3)3.9H2O, dan NH 2CONH 2as the raw materials with hydrothermal route. The particle size rangeobtained fromthismethod was50– 200nm[4]. In the present paper, the nanoparticles of Fe 2TiO5 are obtained by simple mechanical mixing technique as a synthesize method. The crystal structure,microstructure observation and room temperaturemagneticpropertiesareanalyzed. METHODOLOGY Iron (II) Chloride Tetrahydride, FeCl 2.4H2O, and Titanium Oxide, TiO 2(Rutile Phase), all purchased with analytical grade product, were used as startingmaterials. Pure α-Fe 2O3powders were synthesized from sintering process of Fe 3O4powders obtained by precipitation method of FeCl 2.4H2O. The solid solution with weight ratio of α-Fe2O3:TiO2= 4.7:5.3 was prepared to synthesize Fe 2TiO5nanoparticles using Planetary ball mill Fritsch Pulverisette P-5 as mechanical alloying method with 300 rpm of millingspeed, BRP 6:1, and various milling times for 15, 20,and 25 hours in air atmosphere. The obtained milling powders were compacted by pressing machine and then sintered with various temperatures at 1100, 1200, 3rd International Conference on Theoretical and Applied Physics 2013 (ICTAP 2013) AIP Conf. Proc. 1617, 63-66 (2014); doi: 10.1063/1.4897105 © 2014 AIP Publishing LLC 978-0-7354-1254-5/$30.00 631300°C for 1 hour in air condition of Carbolite furnace. The powder X-Ray Diffraction (XRD) patterns were recorded using PAN Analytical Diffractometer with wavelength 1.54056 Å of CuK αradiation, 10 – 90o2θrange, and scan rate 0.02o. Phase identification, lattice parametersandcrystal size were analyzed usingSearch-Match software and Peak-Profile analysis. TheScanningElectronMicroscope(SEM) FEI Inspect S50 was conducted to the surface of obtained powdersamples to investigate particle morphology. VibrationSample Magnetometer (VSM) was used to measurethemagnetizationofthesamplesinroomtemperature. RESULTS AND DISCUSSIONS Qualitative analysis based on XRD patterns of the obtained powders just after milling process withvarious milling times are observed four identifiedphases, which are Fe 2O3(hematite),T i O2(anatase), Fe3O4(magnetite)a n dT iO 2(rutile) with corresponded JCPDF Number of 79-0007 at 2θ33.19°; 21-1272 at 2θ25.28°; 89-0950 at 2θ30.10°; and 77-0441 at 2θ 27.38°; respectively as can be seen in Figure 1. Theformation of Fe 3O4phase after milling process is due to reversible reaction of α-Fe2O3to Fe3O4during millingprocess. ThisisrelatedtothebreakingpartsofFe-O bonding in α-Fe 2O3and reacts with oxygen inside the vial [10]. It is suggested that the millingprocess have enough energy needed to initialize thetransformation from α-Fe 2O3toFe3O4phase. FIGURE 1. XRD patterns for powders obtained after millingprocessfor15,20,and25hoursofmillingtime. Based on the peak profile analysis, the peak of anatasephaseat 2θ25.21° decreasesandit isfollowed bypeak broadeningwith increasing FullWidth at HalfMaximum(FWHM) valueindicatingtransformation tomore amorphous phase. The disappearance of anatase peaks is realized at 37.81° and 48.06° for the millingtime more than 15 hours. The decreasing anatase phase is followed by the increasing rutilephase forsample with 20 hours of milling time. Transformation ofanatasetorutilephase during mechanical alloying process is occurred below its transformationtemperature. This is due to the thermal energy ofcollision during milling process and the crystal defects, such as vacancy and crystal distortion that may increase the free energy of anatasephase, thus decreasethephasetransformationtemperature[11]. The diffraction peaks of Fe 2O3and TiO 2(anatase) phases increased in their intensities and broadened intheir peak widths with increasing milling time. Thismay occur because of the smaller particle size andaccumulationofmicrostrainsaftermechanicalalloyingprocess [12]. There isno solid solution detected in thesamples after milling process, even after 25 hoursmilling, indicating the milling energyis not enough toreactallelementseach othertoform other compounds. XRD patterns of samples after sintering process with various temperatures at 1100, 1200, and 1300°Cfor 1 hour show that there are new diffraction peaksincrease which corresponds to the pseudobrookite (Fe 2TiO5) phase as can be seen in Figure 2. The formation of this new phase indicates partial reactionbetween iron oxides and titanium oxides. Thediffraction peaks of Fe 2O3and TiO 2(anatase) decrease, suggesting the intermetallic diffusion takeplace and the solid solution reaction is occurred toformFe 2TiO5phase. This phase transformation was begun at the temperature higher than 900oC. The diffraction peaks ofFe2TiO5phaseincreasedaswellastheincreasingof sintering temperature. At high temperature, Fe 3O4 phase is oxidized toFe 2O3phase andreacts with TiO 2 to form Fe 2TiO5phase. As increasing thermal energy, diffusion of Ti atoms increases and fills vacancy sitesof Fe 2O3structure, thus, it increases the diffraction peaks of Fe 2TiO5phase as well as the crystal size. Sinteringprocess as aheat treatment affects the crystalgrowth of Fe 2TiO5phase as shown in Figure 3. The remaining anatasephase will transform to rutilephase at high sintering temperature. The peaks of TiO 2 (rutile) phase increased with increasing sintering temperature showing more crystalline phase wasformedinthesamples. (a) 64FIGURE 2. XRD patterns for obtained powders after sintering process with various sintering temperatures at1100,1200,1300 oCfor 1hour forsamples withmillingtime for:a)15hours,b)20hours,andc)25hours. 1100 1200 1300405060708090100CrystalSize(nm) SinteringTemperature(oC)Sinteringfor15hours Sinteringfor20hours Sinteringfor25hours FIGURE 3. Sintering temperature dependence of Fe 2TiO5 crystalsizeswithvariousmillingtimes. 15 20 25405060708090100Sinteringat1100oC Sinteringat1200oC Sinteringat1300oCCrystalSize(nm) MillingTime(hours) FIGURE 4. Milling time dependence of Fe 2TiO5crystal sizeswithvarioussinteringtemperatures.XRD analysis showed that Fe 2TiO5phase had not formed yet until the samples were sintered. For thesintered samples, as the milling time increased, thecrystal size of Fe 2TiO5phase decreased. Figure 4 showstheeffectofmillingtimewithcrystalsizeinthe sinteredsamples. The particle morphology of sintered samples was observed by SEM images. Figure 5 represents SEMimages of the obtained particles for sample with themilling time for 25 hours and various sinteringtemperatures for 1 hour. As increasing sinteringtemperature from 1100 – 1300 oC, the particle size increased. This condition was shown bydisappearanceof small particles and followed by increasing size ofthelarger particles. Based on theseimages,the particlesize distribution seemed to have no homogeneity inshape. However, the trend of particle size was consistentwithXRDdataanalysis. FIGURE 5. Particlesmorphologyofthesinteredsamplesat (a) 1100, (b) 1200, and (c) 1300oC with milling time for 25 hours. FIGURE 6. Particlesmorphologyofthesinteredsamplesat 1300oCwithmillingtime for(a)15hoursand(b) 20hours. 10m 10m(a) (b) 10m 10m 10m(a) (b) (c)(b) (c) 65The differences of particles morphology and their distributions can be seen in Figure 6 for sinteredsamples at 1300 oC with milling for 15 and 20 hours. Comparing these two images shows that increasingmilling time will make particle size reduction which means that the particle size will be smaller. This also agreedwiththeXRDdataanalysis. Magnetic properties of the obtained sintered Fe 2TiO5samples were measured by VSM at room temperature. Figure 7 shows hysteresis curves of 25hours milled sample with various sinteringtemperatures at 1100, 1200, and 1300 oC for 1 hour. It can be seen that powders before sinteringprocess havelargemagnetization because of the existence of Fe 2O3 phaseinthesamples.Fe 2O3phase,even inTiO 2phase, has ferromagneticbehavior, thusit will showmagnetichysteresiscurve. The values of magnetic properties according to their magnetization curves were summarized in Table1. After sintering process, it was realized that therewas magnetic properties changing. The magneticparameter values become smaller. This is related toformation of Fe 2TiO5phase and decreasing Fe 2O3 phase in the samples. TiO 2phases also have influence in the magnetic properties. The obtained samplesbehave more paramagnetic as increasing sinteringtemperature. -1 0 1-0.40.00.4 (d)(c)(b)M(emu/g) H(T)(a) FIGURE 7. Magnetic hysteresis curve of the sintered samplesfor 25hourswithvarioussinteringtemperatures:(a)before sintering;(b)1100 oC;(c)1200oC;and(d)1300oC. TABLE 1. Magneticparameters ofthesinteredsamples for 25hourswithvarioussinteringtemperatures. SamplesMagneticProperties Hc (T)Ms (emu/g)Mr (emu/g) Millingfor25hours0.069 0.385 0.149Beforesintering Millingfor25hours0.058 0.228 0.0084at1100oC Millingfor25hours0.043 0.176 0.0082at1200oC Millingfor25hours0031 0.129 0.015at1300oCCONCLUSION Nanoparticles of Fe 2TiO5have been successfully synthesized by simple mechanical alloying methodusingballmillingtechnique.TheformationofFe 2TiO5 nanoparticles were followed by TiO 2as secondary phase. The obtained Fe 2TiO5nanoparticles have crystal size 51 nm measured by XRD peak profile analysis and confirmed by SEM images observation. The magnetic properties of the resulted samples havesmall magnetization and more paramagnetic behaviorrealized as increasing sintering temperature due todecreasingFe 2O3phaseinthesamples. ACKNOWLEDGMENTS Ourteamresearchwouldliketosaythankyouvery much to all collaborators who help the experimentaland analysis works. 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