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1.4896665.pdf | Effect of tunneling layers on the performances of floating-gate based organic thin-film
transistor nonvolatile memories
Wei Wang, Jinhua Han, Jun Ying, Lanyi Xiang, and Wenfa Xie
Citation: Applied Physics Letters 105, 123303 (2014); doi: 10.1063/1.4896665
View online: http://dx.doi.org/10.1063/1.4896665
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/12?ver=pdfcov
Published by the AIP Publishing
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128.143.1.222 On: Fri, 12 Dec 2014 14:07:45Effect of tunneling layers on the performances of floating-gate based
organic thin-film transistor nonvolatile memories
Wei Wang,a)Jinhua Han, Jun Ying, Lanyi Xiang, and Wenfa Xie
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin
University, 2699 Qianjin Street, Changchun 130012, China
(Received 31 July 2014; accepted 15 September 2014; published online 24 September 2014)
Two types of floating-gate based org anic thin-film transistor nonvolatile memories (FG-OTFT-NVMs)
were demonstrated, with poly(methyl methacryla te co glycidyl methacrylate) (P(MMA-GMA)) and
tetratetracontane (TTC) as the tunneling layer, respectively. Their device performances were measured
and compared. In the memory with a P(MMA-GMA) tunneling layer, typical unipolar hole transportwas obtained with a relatively small mobility of 0.16 cm
2/V s. The unidirectional shift of turn-on
voltage ( Von) due to only holes trapped/detrapped in/from the floating gate resulted in a small memory
window of 12.5 V at programming/erasing voltages ( VP/VE)o f6100 V and a nonzero reading voltage.
Benefited from the well-ordered molecule orien tation and the trap-free surface of TTC layer, a
considerably high hole mobility of 1.7 cm2/V s and a visible feature of electrons accumulated in
channel and trapped in floating-gate were achiev ed in the memory with a TTC tunneling layer. High
hole mobility resulted in a high on current and a large memory on/off ratio of 600 at the VP/VEof
6100 V. Both holes and electrons were injected into floating-gate and overwritten each other, which
resulted in a bidirectional Vonshift. As a result, an enlarged memory window of 28.6 V at the VP/VE
of6100 V and a zero reading voltage were achieved. Based on our results, a strategy is proposed
to optimize FG-OTFT-NVMs by choosing a right tunneling layer to improve the majority
carrier mobility and realize ambipolar carrie rs injecting and trapping in the floating-gate.
VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4896665 ]
Recently, floating-gate based organic thin-film transistor
nonvolatile memories (FG-OTFT-NVMs) have attractedconsiderable attention from both academia and industry for
many advantages over their inorganic counterparts such as
low cost, light weight, mechanical flexibility, and low-temperature processing. FG-OTFT-NVM is considered as a
promising candidate for the realization of the ultimate goal
of organic flash memory because of its nondestructiveread-out, complementary integrated circuit architectural
compatibility, and single transistor realization.
1,2
The basic mechanism is that charges trapped/detrapped
in/from the floating-gate by the programming/erasing (P/E)
operations are utilized to modulate the channel conductance
through a thin tunneling layer, defining a Boolean “1” or “0.”Up to now, various metal nanoparticles
1–7and polymer
electrets7–11have been used as floating-gate layers to realize
FG-OTFT-NVMs. As a structure component of a FG-OTFT-NVM, the tunneling layer is very important because that its
electrical properties and physical and chemical properties
determine the overall device performances. The tunnelinglayer should have a smooth surface morphology and matched
physical/chemical properties, which would benefit the growth
of organic semiconductor and the reduction of defects at thetunneling layer/active layer interface, that thereby favor
charge carriers to accumulate and transport in the channel
and favor the overall device performances to be improved.
Polyvinyl alcohol (PVA) is well known as a polymer
electret
12and has many advantages: processing with anon-harmful solvent (water), low-cost materials and process-
ing, compatibility with flexible substrates and good resist-ance to be damaged by the solvents involved in the lift-off
process.
13In our previous work, the hysteresis mechanism of
OTFTs based on PVA dielectric has been researched, whichwas attributed to charges trapping/detrapping in/from the
PVA dielectric, and the low-voltage operating, free-
hysteresis and high mobility OTFT was demonstrated bypreventing the charges injecting and trapping in the PVA
dielectric.
14In this letter, we fabricated and characterized
pentacene-based FG-OTFT-NVMs by controlling chargestrapping/detrapping in/from the PVA dielectric. Two types
of FG-OTFT-NVMs were obtained with poly(methyl meth-
acrylate co glycidyl methacrylate) (P(MMA-GMA)) and tet-ratetracontane (TTC) as the tunneling layer, respectively,
which were denoted as device A and device B, respectively,
in the following description. The effect of different tunnelinglayers on the device performances of FG-OTFT-NVMs was
researched. Benefited from the well-ordered molecule orien-
tation and the trap-free surface of TTC tunneling layer, ahigh hole mobility of 1.7 cm
2/V s and a visible feature of
electrons accumulated in channel and trapped in floating-
gate were obtained in FG-OTFT-NVM, which obviouslyimproved memory performances of device, including larger
memory window ( DV
on), higher memory on/off ratio, 0 V
reading voltage ( VR), and longer data retention time.
The schematic illustrations of both pentacene-based
FG-OTFT-NVM structures were shown in Figs. 1(a) and
1(b), respectively. The FG-OTFT-NVMs were fabricated on
patterned indium tin oxide (ITO) coated glass substrates
which acted as gate electrode. After the substrates werea)Author to whom correspondence should be addressed. Electronic mail:
wwei99@jlu.edu.cn
0003-6951/2014/105(12)/123303/5/$30.00 VC2014 AIP Publishing LLC 105, 123303-1APPLIED PHYSICS LETTERS 105, 123303 (2014)
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128.143.1.222 On: Fri, 12 Dec 2014 14:07:45routine cleaned, the block layer poly-(methylmethacrylate)
(PMMA) and the floating-gate layer PVA were spin coatedon the ITO gate electrode in sequence from the solution in
butyl acetate and deionized water, respectively. Each poly-
mer layer was baked at 120
/C14C for 1 h. For device A, a thin
P(MMA-GMA) tunneling layer was spin coated on the PVA
layer from the dilute solution in butyl acetate and cross-
linked at 120/C14C for 1 h. The resulted thicknesses of PMMA,
PVA, and P(MMA-GMA) films are about 600, 130, and
15 nm, respectively, measured by an ellipsometer. For device
B, a 30 nm thick TTC tunneling layer was deposited on thePVA layer by thermal vacuum evaporation at a rate of
0.4–0.7 A ˚/s, and then was annealed at 75
/C14C for 3 h in a con-
vention oven. The thickness of TTC can be precisely con-trolled by a quartz crystal oscillation monitor, and this 30 nm
thickness was an optimized result. For both devices, penta-
cene active layer (40 nm) and MoO
3(5 nm)/Cu (60 nm)
double-layer source-drain electrodes were deposited in
sequence by thermal vacuum evaporation, at the rate of
1A˚/s, 0.2 A ˚/s, and 2 A ˚/s, respectively, and patterned by the
corresponding shadow masks. The channel length ( L) and
width ( W) were defined as 140 and 2000 lm, respectively.
The electrical characteristics of both devices were measuredby using two Keithley 2400 source-measure units in the dark
at room temperature in ambient air.
The atomic force microscope (AFM) images of the
spinning-coated PMMA film, PVA film (on the surface of
PMMA layer), and P(MMA-GMA) film (on the surface of
PVA layer) were shown in Figs. 2(a)–2(c) , respectively. All
three films exhibited smooth surface morphologies, with the
root-mean-square (RMS) surface roughness of 0.397, 0.565,
and 0.315 nm for PMMA, PVA, and P(MMA-GMA), respec-tively, suggesting these films were continuous and free-
pinhole, which was important for getting a good memory
performance of device, especially for the P(MMA-GMA)tunneling layer, which can reduce the leakage of charges
stored in the floating-gate. The 40-nm-thick pentacene layer
deposited on the P(MMA-GMA) surface exhibited dendriticgrains, with an average grain size of about 1.5 lm/C20.6lm
and a RMS surface roughness of 4.40 nm, as shown in
Fig. 2(d). After annealing, the 30-nm-thick TTC tunneling
layer deposited on the PVA surface also exhibited a continu-
ous and free-pinhole morphology, with a RMS surface
roughness of 0.86 nm, demonstrated by AFM measurement,as shown in Fig. 2(e). It was flat for the most of area, and
there was a visible small low-lying area, on the surface of
TTC tunneling layer. The step height ( /C256.0 nm), along the
red solid line in the TTC AFM image, nearly correspondedto a molecular length of TTC,
15suggesting that the TTC
molecules had a well-ordered orientation and stood up with
full coverage of the PVA layer. Fig. 2(f)shows the measured
AFM image of a 40-nm-thick pentacene layer deposited on
the TTC surface. Both height profiles of pentacene grains
along the red solid lines in AFM images of Figs. 2(d) and
2(f) exhibited a clear terrace morphology, with each step-
height corresponding to monolayer of pentacene molecular.However, it exhibited distinct morphologies for pentacene
deposited on the TTC surface and the P(MMA-GMA) sur-
face, respectively, with a smaller size island-like grain and arougher surface (RMS of 5.49 nm) observed in the former.
The different morphologies of pentacene layers were attrib-
uted to the different surface properties of TTC and P(MMA-GMA), demonstrated by the measurement of water contact
angle, as shown in the insets of Figs. 2(c) and2(e). The
results indicated that the TTC film showed more hydropho-bic than the P(MMA-GMA) film. So, there was a different
interface quality between TTC/pentacene and P(MMA-
GMA)/pentacene, which had an important effect on thecharge carrier accumulation and transport in the channel.
The typical output characteristics of device A are shown
in the inset of Fig. 3(a). The drain-source current ( I
DS) was
measured while the drain-source voltage ( VDS) was varied at
different gate-source voltages ( VGS). In the low VDSregion, a
linear relationship between IDSandVDSwas observed, sug-
gesting efficient hole injection from MoO 3/Cu source elec-
trode into the pentacene. The IDSsaturated at high VDS
because conducting channel of pentacene was pinched-off.
The transfer characteristics ( IDS–VGS) of device A in the
saturation region are shown in Fig. 3(a), with VGSscanning
fromþ10 V to /C050 V and back to þ10 V. With the increase
of negative VGS, the IDSincreased due to more holes accumu-
lated in the channel. These results indicated that device A
exhibited standard p-channel field-effect operation. At the onstate, negligible hysteresis suggested that few charges were
trapped/detrapped in/from the PVA layer, because 15 nm-
thick P(MMA-GMA) layer prevented the injection/rejectionof charges in/from the PVA layer at the relative low V
GS
(</C050 V). The field-effect mobility ( l) of 0.16 cm2/Vs, the
on/off current ratio of larger than 104, the subthreshold slope
(S) of 4.8 V/decade, and the threshold voltage ( VT)o f
/C012.6 V were extracted from Fig. 3(a).
To verify the programmable and erasable property of de-
vice A, an original transfer curve of device A operated in the
linear region ( VDS¼/C05 V) was first recorded as the initial
state. Then, a series of large positive or negative VGSpulses
were supplied to gate electrode in sequent for 1 s while the
source and drain electrodes were shorted and grounded, and
the corresponding transfer curves in the linear region wererecorded, respectively. There was no visible change for the
transfer curves until the supplied positive pulses reached
100 V, as shown in Fig. 3(b). On the other hand, the transfer
curve had a visible negative shift, compared with the initial
state, after a negative V
GSpulse of /C070 V was supplied to
the gate electrode. With more negative VGSpulses supplied,
FIG. 1. The schematic illustrations of present pentacene-based FG-OTFT-
NVM structures with (a) P(MMA-GMA) and (b) TTC as the tunneling layer,
respectively.123303-2 Wang et al. Appl. Phys. Lett. 105, 123303 (2014)
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128.143.1.222 On: Fri, 12 Dec 2014 14:07:45the transfer curves were further shifted in negative direction.
After supplying a positive VGSpulse of 100 V again, the
transfer curve returned to the initial state, as shown in Fig.
3(c). The results demonstrated that device A had a good pro-
grammable and erasable property, with the supplied positive
VGSpulses defined as programming (P) operations and the
supplied negative VGSpulses defined as erasing (E) opera-
tions. We defined the turn-on voltage, Von, as the VGSat
which the IDSreaches 10 nA. The Vonwas estimated to
/C09.7 V at the initial state. Von/C0denoted the turn-on voltage
after a series of E pulses ( VE) were supplied, and was esti-
mated to be /C011.1,/C013.4,/C016.3, and /C020.5 V correspond-
ing to VEof/C070,/C080,/C090, and /C0100 V, respectively. Vonþ
denoted the turn-on voltage after a series of P pulses ( VP)
were supplied and were estimated to be /C08.7,/C08.0, and
/C08.4 V corresponding to VPof 90, 100 (before VE), and
100 V (after VE), respectively. All Von(Vonþ,Von/C0) were
plotted as function of VPorVEin the inset of Fig. 3(b).
Limited by the negative Vonand the unidirectional shift of
transfer curves, the VRhad to be defined as /C015 V rather
than 0 V. As a result, a memory window ( DVon, defined as
the difference between Von/C0andVonþ) of 12.5 V and a mem-
ory on/off ratio of about 25 (at VR¼VGS¼/C015 V) were
obtained at VE/VPof6100 V, respectively.
The output and transfer characteristics of device B are
shown in Fig. 4(a). The whole performance of device B had
a considerable improvement compared with that of device A,
including higher hole field-effect mobility of 1.7 cm2/V s,higher on/off current ratio of about 8.0 /C2104, smaller sub-
threshold slope (S) of 4.0 V/decade and smaller threshold
voltage ( VT)o f/C010.9 V. And, a slight electron transport fea-
ture was observed at positive VGS. The programmable and
erasable property of device B is shown in Fig. 4(b). The
transfer characteristic in the linear region was first recorded
as the initial state before any VP/VEwas supplied. Then the
corresponding transfer curves were recorded after a serial of
P/E operations. Compared with the initial state, the transfer
curves exhibited visible positive or negative shifts corre-sponding to supplied V
PorVE, respectively, which was dif-
ferent from that in device A. The extracted Von(orVonþor
Von/C0) was 11.7, /C00.9,/C04.9,/C011.7, and /C016.9 V, corre-
sponding to VP/VEof 100, 80, 0 (initial), /C080, and /C0100 V,
respectively. As a result, DVonof about 28.6 V was obtained
atVE/VPof6100 V, which was larger than 2 times of that in
device A. Benefited from large positive VonþatVP¼100 V,
VRcan be defined as VGS¼0 V, which is important for
reducing the power consumption of nonvolatile memory.Benefited from high on current due to high hole mobility,
high memory on/off ratio of about 600 (at V
R¼VGS¼0V )
was achieved in device B, which is much larger than that indevice A.
Compared with device A, higher hole mobility and better
memory performances were obtained in device B, whichshould be attributed to the effect of tunneling layer. Although
the grain size of pentacene deposited on the TTC surface
was smaller than that deposited on the P(MMA-GMA), the
FIG. 2. AFM images of (a) PMMA
film, (b) PVA film on the surface of
PMMA layer, and (c) P(MMA-GMA)
film on the surface of PVA layer,
respectively. AFM images and corre-sponding height profiles along the red
solid lines of (d) pentacene film on the
surface of P(MMA-GMA), (e) TTC
film on the surface of PVA layer and
(f) pentacene film on the surface of
TTC layer, respectively. The insets of
(c) and (e) show the images of thewater contact angles and their values
of P(MMA-GMA) and TTC,
respectively.123303-3 Wang et al. Appl. Phys. Lett. 105, 123303 (2014)
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128.143.1.222 On: Fri, 12 Dec 2014 14:07:45well-ordered molecule orientation of TTC layer resulted in a
trap-free surface and a better interface quality of semiconduc-
tor/dielectric to promote charge carriers transport, which notonly dominated a high hole mobility but also exhibited a
visible electron transport feature in device B. The similar
ambipolar carrier transport in pentacene-based OTFTs hasbeen shown in some reports with a TTC/pentacene
interface.
2,15,16
Combined with our previous work,14the memory mech-
anism in device A is attributed to that holes accumulated in
the channel were injected into the PVA layer by Fowler-
Nordheim (F-N) tunneling mechanism6at the VEsupplied
high enough. Holes, trapped in the PVA layer, induced a
built-in electric field ( Ei), which led to visible negative shifts
of transfer curves. By supplied enough high VP, holes, stored
in the PVA layer, were completely removed. As a result, the
transfer curve returned to the initial state. As for device B,
the negative shifts of transfer curves after supplied VEcan
also be explained by that holes accumulated in the channel
were injected and trapped in PVA layer. More holes were
trapped in the PVA layer at higher negative VGS, which
induced a larger Ei. This Eihad the same direction with the
electric field of supplied positive VGS. Both fields superim-
posed and induced more electrons transport in the channel,as shown in Fig. 4(b). While, the positive shifts of transfer
curves after supplied V
Pshould be attributed to that electrons
accumulated in the channel were injected and trapped inPVA layer. The injected electrons neutralized and overwrote
the trapped holes, and vice versa. That is to say, one type of
charge carrier, trapped, was overwritten by the other,injected, one. So, bidirectional shifts of transfer curves were
obtained in device B. Both high hole mobility and ambipolar
carriers injected and trapped in the PVA layer lead to promi-nently enhanced memory performances in device B, com-
pared with that in device A. Normally, holes are majority
carriers and electrons are minority carriers in pentacene-based transistor. Based on our results, a strategy is proposed
to optimize FG-OTFT-NVMs by improving the majority car-
rier mobility and realizing ambipolar carriers injecting andtrapping in the floating-gate, which is expected to be realized
by choosing a right tunneling layer.
The data retention capability is an important parameter
of the nonvolatile memory, which was measured at room
temperature in ambient air, with the corresponding I
DSat “1”
and “0” states (denoted as IDS,1andIDS,0, respectively) as a
function of time. Fig. 3(d) shows the data retention charac-
teristics of device A, at the reading state of VDS¼/C05 V and
VR¼VGS¼/C015 V, after supplied VP/VEof6100 V for 1 s,
respectively. In the measurement range, both IDS,1and the
IDS,0were slight decay with time. According to experiment
data, we proposed a numerical simulation to describethe retention property of “1” and “0” states, as shown in
following equation:
I
DS;T¼IDS;Initial/C2expðaTÞ; (1)
here, IDS,Tis the IDS,1orIDS,0at the time T; IDS, Initial is the
initial IDS,1orIDS,0with T ¼0 s; T is the retention time; and
theais the loss probability of the stored charges in an unit
time. The fitted result of a1¼/C06.0/C210/C05was in good
agreement with the experiment data at “1” state, as shown by
the blue line in Fig. 3(d). The simulation result indicates that
it is about 7.5 h for the IDS,1to decay to 50% of the initial
value. While, IDS,0 had a relatively quick decay, which
increased to 4.4 /C210/C09A at T of about 2600 s from the ini-
tial value of 2.2 /C210/C09A. The retention property of “0”
state was well fitted by a2¼2.0/C210/C04(red line). The
slightly faster increase of IDS,0 in the early stage can be
explained by that some shallow hole traps at the P(MMA-
GMA)/pentacene interface were filled at the VRof/C015 V.
The simulation result that a2was larger than a1indicated
that the data retention capability of device A was dominated
by the retention time of holes trapped in the PVA layer.
The data retention capability of device B is shown in
Fig. 4(c), at the reading state of VDS¼/C05 V and
VR¼VGS¼0 V, after supplied VP/VEof6100 V for 1 s,
respectively. In the measurement range, the IDS,0was always
maintained at about 3.0 /C210/C09A, suggesting a good reten-
tion capability for holes trapped in the PVA layer. After
FIG. 3. For device A: (a) Transfer
characteristic in the saturation region.
(b) Transfer characteristics in the lin-ear region recorded at the initial state
and after V
Pat 90 and 100 V, respec-
tively. (c) Transfer characteristics in
the linear region recorded at the initial
state, after VEat/C070,/C080,/C090, and
/C0100 V, after VPat 100 V, in
sequence. (d) The data retention char-acteristics. The inset of (a) presents
output characteristics. The inset of (b)
presents V
onþand V on/C0as a function
ofVPandVE, respectively.123303-4 Wang et al. Appl. Phys. Lett. 105, 123303 (2014)
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128.143.1.222 On: Fri, 12 Dec 2014 14:07:452200 s, the IDS,1reduced to 0.85 lA from 1.04 lAa t T¼0s .
The relatively quick decay of IDS,1than that of IDS,0indicated
that the data retention capability in device B was dominatedby the retention time of electrons trapped in the PVA layer,
which was different with device A due to the different tun-
neling layer. According to experiment data, the I
DS,1can be
simulated by following equation:
IDS;1/C0T¼IDS;1/C0Initial/C2expðb1TÞ/C25:4%þIDS;1/C0Initial
/C2expðb2TÞ/C294:64% ; (2)
here, IDS,1-T andIDS,1-Initial mean IDS,1at the time of T and
0 s, respectively, the b1andb2mean loss probability of the
electrons trapped in the PVA layer in an unit time. The fitted
result of b1¼/C02.5/C210/C03andb1¼/C06.5/C210/C05were in
good agreement with the experiment data, as shown by thered line in the inset of Fig. 4(c). Equation (2)included two
parts, which indicated that the thickness of tunneling layerhad a prominent effect on the data retention capability of
FG-OTFT-NVMs. There were two different thicknesses for
the whole TTC layer. In the thinner region (about 4 mono-
layers thickness) of TTC layer, the probability of electronstunneling was larger, corresponding to b
1. In the thicker
region (about 5 monolayers thickness) of TTC layer, the
probability of electrons tunneling was smaller, correspond-ing to b
2. Based on the simulation result, it was about 2.7 h
for the IDS,1to decay to 50% of the initial value and it was
about 15 h for the IDS,1to decay to 3.0 /C210/C08A, which was
one order of magnitude higher than IDS,0. High memory on/
off ratio is favorable to enhance the data retention capabilityof FG-OTFT-NVMs.
In summary, two types of FG-OTFT-NVMs were dem-
onstrated, with P(MMA-GMA) and TTC as the tunnelinglayer, respectively. Their device performances were meas-
ured and compared. Benefited from the good interface prop-
erty of TTC/pentacene, device B exhibited a considerablyhigh hole mobility and a feature of ambipolar carriers trap-
ping in the floating-gate, which resulted in remarkably
enhanced memory performances. Based on our results, astrategy can be proposed to optimize FG-OTFT-NVMs by
choosing a right tunneling layer to improve the majority
carrier mobility and realize ambipolar carriers injecting andtrapping in the floating-gate.
This work was supported by the National Natural
Science Foundation of China (Grant Nos. 61177028 and
60937001), the Natural Science Foundation of Jilin provincein China (Grant No. 201115028), the Independent Project of
State Key Laboratory on Integrated Optoelectronics (Grant
No. IOSKL2012ZZ16), and Scientific Frontier and CrossDisciplinary Innovation Project of Jilin University in China.
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16S. Ogawa, Y. Kimura, M. Niwano, and H. Ishii, Appl. Phys. Lett. 90,
033504 (2007).
FIG. 4. For device B: (a) Transfer characteristic in the saturation region. (b)Transfer characteristics in the linear region recorded at the initial state, after
V
P/VEat 80, /C080, 100, and /C0100 V, in sequence. (c) The data retention
characteristics. The inset of (a) presents output characteristics. The inset of
(d) presents the experimental data and the fitted data by Eq. (2)forIDS,1as a
function of time.123303-5 Wang et al. Appl. Phys. Lett. 105, 123303 (2014)
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1.4897309.pdf | Electro-hydrodynamic shooting phenomenon of liquid metal stream
Wen-Qiang Fang, Zhi-Zhu He, and Jing Liu
Citation: Applied Physics Letters 105, 134104 (2014); doi: 10.1063/1.4897309
View online: http://dx.doi.org/10.1063/1.4897309
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/13?ver=pdfcov
Published by the AIP Publishing
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141.212.109.170 On: Tue, 23 Dec 2014 15:58:28Electro-hydrodynamic shooting phenomenon of liquid metal stream
Wen-Qiang Fang,1Zhi-Zhu He,2,a)and Jing Liu2,3,a)
1Department of Engineering Mechanics, School of Aerospace Engineering, Tsinghua University,
Beijing 100084, China
2Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
3Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, China
(Received 9 September 2014; accepted 23 September 2014; published online 1 October 2014)
We reported an electro-hydrodynamic shooting phenomenon of liquid metal stream. A small
voltage direct current electric field would induce ejection of liquid metal inside capillary tube and
then shooting into sodium hydroxide solution to form discrete droplets. The shooting velocity haspositive relationship with the applied voltage, while the droplet size is dominated by the aperture
diameter of the capillary nozzle. Further, the motion of the liquid metal droplets can be flexibly
manipulated by the electrodes. This effect suggests an easy going way to generate metal droplets inlarge quantity, which is important from both fundamental and practical aspects.
VC2014
AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4897309 ]
In recent years, the room temperature liquid metal has
attracted much attention because of their versatile applicabil-
ity in energy management,1,2chip cooling,3and printed
electronics.4A lot of unique characters involved are thus
increasingly investigated.5–9Among the many issues ever
tackled, the production of liquid metal droplets or particleswith controlled size has been identified to be very useful in a
wide variety of important areas. Typical examples can be
found in MEMS,
10liquid marble preparation,11or microflui-
dic pump.12,13So far, several important approaches have
been developed to produce the liquid metal droplet in micro-
channel.14,15In those works, the droplets take shapes by flow
focusing, and the key factors for controlling the fabrication
include fluid velocity, viscosity, and surfactant properties.
Due to pre-requisite in the manufacture of the micro-fluidicchannels, such method is still somewhat expensive and
technically complex. For a smaller size, the liquid metal
microspheres can even be prepared to the nanoscale basedon ligand mediated self-assembly method.
16In a latest work,
a straight forward way was found for large-scale fabrication
of liquid metal micro-droplets and particles.17The mecha-
nism there lies in the Plateau–Rayleigh instability,18where a
liquid jet would break up into smaller packets because of the
high surface tension of the liquid metal inside the matchingsolution. As it is noted, the mechanical manipulation mecha-
nism is still not convenient enough for a continuous fabrica-
tion of the metal droplets.
Through continuous trials, we found in the present work
that the mechanical ejection can in fact be replaced by an
electro-hydrodynamic effect. It is based on this fundamentaldiscovery that we reported an alternative way of generating
liquid metal droplets through the electrically controlling
mechanism. The disclosed process and device are rather flex-ible and easy going. Given automatic control, this methodwould significantly improve the fabrication efficiency of the
liquid metal droplets.
To carry out the experiments, we have set up the test
platform as shown in Fig. 1(a) with working mechanisms
illustrated in Figs. 1(b)and1(c), respectively. Here, the cap-
illary tube serves as the channel connecting the liquid metaland the sodium hydroxide (NaOH) solution container, where
the cathode and anode are arranged as depicted in the figure.
It is well known that a conductive object with induced-charge in liquid phase would cause the formation of electri-
cal double layer on its surface, i.e., bipolarization of the
liquid metal. Therefore, an external non-uniform electricfield will break up the symmetrical surface tension of the liq-
uid metal. It is this effect that leads to the unconventional
ejection phenomena as will be disclosed later. For compara-tive purpose, two diameters of the capillary tube as 1 mm
and 0.7 mm were studied. The cathode and anode, made of
stainless steel are both linked with the direct-current (DC)voltage controller. Regarding the test liquid metal, it was
chosen as galinstan (made of 67%Ga, 20.5%In, and
12.5%Sn by volume), which has a broad temperature rangeof liquid phase with a melting point at 10.5
/C14C.5Such alloy
might have undercooling point to be around /C019/C14C under
certain measurement circumstances. Readers are referredelsewhere for more physical properties of the galinstan
material.
5,19
We first adjust the height of liquid metal level of the
container, so that the liquid metal can be infused into the
capillary tube, which however cannot flow out of the nozzle
due to its pretty large surface tension. The practical distanceof the two electrodes from the capillary nozzle to the anode
is about 82 mm because of the conductive characteristics of
the liquid metal. The voltage controller is turned on to applythe DC electric field on the electrolyte solution. Then, an
unconventional phenomenon was discovered that the liquid
metal would automatically eject from the capillary nozzle,which then shoots into the electrolyte solution, and forms
droplets until finally moves to the anode. The whole process
is recorded by a high speed camera (IDT, NR4.S3). The a)Authors to whom correspondence should be addressed. Electronic
addresses: zzhe@mail.ipc.ac.cn, Tel.: þ86-10-82543766, Fax: þ86-10-
82543767 and jliu@mail.ipc.ac.cn, Tel.: þ86-10-82543765, Fax: þ86-10-
82543767.
0003-6951/2014/105(13)/134104/4/$30.00 VC2014 AIP Publishing LLC 105, 134104-1APPLIED PHYSICS LETTERS 105, 134104 (2014)
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141.212.109.170 On: Tue, 23 Dec 2014 15:58:28velocity of droplets can thus be calculated from the videos
via image processing. Through altering the voltage, concen-
tration of solution, and aperture size of capillary tube, wecould systematically evaluate the effects of various typical
factors on the droplets generation behavior.
When electric field is applied to the electrolyte solution,
the force balance between pressure and surface tension on
the interface of liquid metal and NaOH solution at the capil-
lary nozzle is broken immediately. The traction forceinduced by the external electric field would then promote the
liquid metal to eject from the capillary nozzle and shoot into
the electrolyte solution. Due to large surface tension of theliquid metal, the stream then splits to form a large amount of
the droplets continuously. Fig. 2shows the snapshots of typi-
cal ejections in NaOH solution of 0.25 mol/l under voltagesfrom 2.5 V to 20 V. The intensity of the electric field can be
considered as linear dependence on the applied voltage. For
the too much low voltages (below 2.5 V), we did not observethe liquid metal droplet generation due to its high surface
tension. When raising the voltage strength, the injection
velocity of the liquid metal increases evidently. Overall, theejection direction of the liquid metal is along the central axis
of the nozzle for the voltages below 5 V (Figs. 2(a) and
2(b)). However, it is interesting to notice that such ejection
direction becomes unstable, which is affected by the high
voltage (Figs. 2(c)–2(e) ). Turbidity around the cathode was
seen when the voltage strength increases to about 5 V.According to our comparative experiments, these dark-grey
matters might be composed mainly of compounds containing
In and Sn ions due to the electrochemical reaction at theinterface between liquid alloy and NaOH solution. If only
using the liquid gallium to perform the same actuation
experiments, no such dark-grey matters were observed. Ittherefore can be inferred that the varied activity between the
GaInSn alloy and the gallium (Ga) may lead to the different
electrochemical reaction. A complete characterization on theproduct components is beyond the current work and needs
tremendous measurements in the future. It should be men-
tioned that, since the liquid metal is connected to the cath-ode, the reaction occurring here is a kind of electrochemistry
reduction one. Further, from Fig. 2, it is also observed that
the droplets are attracted to the anode under external electricfield. In a former research,
20it has been found that applying
the electricity on the liquid metal sphere immersed in NaOH
solution would induce its planar locomotion. The discreteliquid metal sphere as focused there could be used to clarify
the actuation mechanism. One particularly important
FIG. 1. (a) The schematic diagram of
the experimental setup. Both of the liq-
uid metal injection (b) and droplet
motion (c) are driven by electro-
hydrodynamic force.
FIG. 2. The snapshots of liquid metal shooting in NaOH solution of
0.25 mol/l under different voltages: (a) U ¼2.5 V; (b) U ¼5 V; (c) U ¼10 V;
(d) U ¼15 V; (e) U ¼20 V.134104-2 Fang, He, and Liu Appl. Phys. Lett. 105, 134104 (2014)
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141.212.109.170 On: Tue, 23 Dec 2014 15:58:28discovery and advancement as made by the current work lies
in that it disclosed that a large pool of liquid metal in the
container could be continuously ejected from the capillary
tube into the surrounding solution, which is quite useful forfuture large scale fabrication of metal particles with con-
trolled sizes.
The liquid metal droplets’ generation behavior is mainly
dominated by the size of the capillary nozzle, the voltage,
and the concentration of the NaOH solution. Fig. 3depicts
the relation between the applied voltage and liquid metal
droplet velocity for different aperture sizes of capillary noz-
zle and positions in 0.125 mol/l NaOH solution, where theposition is denoted by the distance from the capillary nozzle.
From the measurements, one can conclude that the ejection
speed of the droplets goes up rapidly with the increment ofthe voltage. The velocity of the droplet decreases due to
viscous resistance effect applied on it during traveling along
the solution. Fig. 3also indicates that the dependence of the
droplet velocity on the voltage is less affected by the aper-
ture size of the capillary nozzle considered here.
Fig. 4presents the relationship between velocity and
concentration of NaOH solution under applied voltage from
5 V to 20 V at the position (2 cm away from the capillary
nozzle). The concentration of NaOH solution has no signifi-cant effect on the velocity of the droplets. In fact, increasing
the concentration of NaOH would lead to the decrease of the
electric permittivity of the electrolyte solution,
21and weaken
the electro-hydrodynamic driving force. However, this effect
is not evident in the present experiments. Besides, the rela-
tionship between voltage and velocity does not generateprominent difference with different capillary nozzles.
According to the experiments, the size of the liquid metal
droplets is mainly determined by the capillary nozzle diame-ter. When using capillary of diameter 1 mm, the average size
of liquid metal droplet is about 2 mm. And, for the case of
diameter 0.7 mm, the average size of droplet is about1.6 mm. Given micro or even nano meter capillary tube,
much smaller droplets can still be obtained which will be
reported later. Further, we also observed that increasing thevoltages can slightly lead to smaller droplets. In addition, theinner surface roughness of the nozzle also affects the
droplet size.
Theoretically speaking, the present finding regarding the
metal droplet generation and manipulation can be attributedto the fundamental electro-hydrodynamic mechanism of the
interaction between liquid metal and electrolyte solution.
Overall, the whole process can be divided into three phases:liquid metal ejection induced by electric field, the liquid
metal stream breaking into droplets, and the droplets loco-
motion in the base solution.
The first phase provides the initial momentum of the
ejection in analogy to external mechanical force.
17Without
losing any generality, the Young-Laplace equation Dp¼
2c=Rcan be used to characterize the force balance on the
interface of liquid metal at initial stage, where Dpis the pres-
sure difference between liquid metal and base solution anddetermined by the both liquids level, cis the surface tension
of liquid metal (0.718 N/m for galinstan
4), and Rthe radius
of liquid metal sphere. For R¼1 mm, the liquid metal sphere
can sustain a large pressure difference 1436 Pa. When the
electric field is applied, an electrical double layer (EDL) is
formed at the interface of the liquid metal (Fig. 1(b)). The
induced electric force can be denoted by eE2, where eis the
electric permittivity of NaOH solution and E the electric
filed strength, acted on the liquid metal interface along itsnormal direction. As a result, the equilibrium of surface
tension and pressure is broken. Then, the interface deforms
and tends to move toward the side of base solution. Afterthis acceleration process, the liquid metal ejects out from the
nozzle. Obviously, increasing the applied voltage will result
in a larger electric force.
In the second phase, a liquid metal stream breaks into
droplets due to Plateau–Rayleigh instability. It should be
mentioned that we did not observe here the continuous thinstream travelling phenomenon as found in the mechanical
force controlled liquid metal injection.
17The reason lies in
that the electro-hydrodynamic force has much stronger effecton the flow instability, which thus enhances the liquid metal
droplet generation and leads to the disorder of the injection
direction.
FIG. 3. The relationship between applied voltage and liquid metal droplet
velocity for different aperture sizes of capillary nozzle (D) and positions (Ldenotes the distance from the capillary nozzle) in 0.125 mol/l NaOH
solution.
FIG. 4. The relationship between velocity and concentration of NaOH
solution under different applied voltages at the position 2 cm away from the
capillary nozzle.134104-3 Fang, He, and Liu Appl. Phys. Lett. 105, 134104 (2014)
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141.212.109.170 On: Tue, 23 Dec 2014 15:58:28For the third phase, the liquid metal droplet is driven by
electro-hydrodynamic force to move along the proposed
direction through the electrodes layout, which can be
adopted for precise manipulation of the droplets. The basicphenomenon can be understood from Fig. 1(c). Immediately
after external electric filed is applied, the current then drives
positive (Na
þ) and negative ions (OH/C0) to move towards the
corresponding side of the liquid metal droplet, which induces
an equal and opposite surface charge on the conducting sur-
face. The tangential electrical stress exerted within the elec-
tric double layer (shown in Fig. 1(c)) leads to the imposed
shear stress on the liquid metal surface, which can induce theflow inside droplet and drive its motion. Besides, droplets
carrying negative charges when ejecting out of the capillary
at cathode further contribute to the electric field force. Basedon the electro-hydrodynamic theory,
22the velocity of the liq-
uid metal droplet can be deduced as
U¼9kD
40 1þlL=lW ðÞeDE2
lW; (1)
where, k/C01is the Debye length (about 5.0 /C210/C09m),Dis the
diameter of the liquid metal (about 2 /C210/C03m),lLis the vis-
cosity of galinstan (2.4 /C210/C03Pa/C1sa t2 0/C14C5),lWfor NaOH
aqueous solution (about 1.0 /C210/C03Pa/C1sa t2 0/C14C), and eis
the electric permittivity of NaOH aqueous solution (about6.75/C210
/C010Fm/C01for 0.125 mol/L21).Edenotes the electric
field strength chosen as 61 V/m for voltage 5 V. Thus, the ve-
locity of the liquid metal droplet estimated from Eq. (1)is
about 13.3 cm/s, which is higher than the experimental
results about 3 cm/s. The reason for this deviation lies in that
the Eq. (1)is derived from the balance between viscous force
and electric field force in free space. However, the friction
from the current substrate impedes the droplet motion. In
addition, the electrochemical reaction on the liquid metaldroplet surface could induce the surrounding flow disorder,
and thus weaken its directional motion. According to Eq. (1),
the velocity of the droplet depends linearly on the electricpermittivity. For 1 mol/l NaOH solution, its electric permit-
tivity is:
215.70/C210/C010Fm/C01, which does not have too
much difference with that of concentration 0.125 mol/l.Thus, droplet velocity depends less on the concentration of
the NaOH solution as considered here.
The velocity of metal solid particle induced by the exter-
nal electric field in electrolyte solution is given by
U¼eDE
2=lW, and estimated as 5.0 mm/s according to the
above parameters, which is much smaller than that for liquidmetal droplet. The reason lies in that the tangential electric
field vanishes at free surface of liquid metal droplet. The vis-
cous stress associated with Debye-scale shear within theelectrolyte must be balanced by the electric stresses, which
leads to amplified velocity scaling about kDcompared with
the metal solid particle. It is noteworthy that the NaOH solu-tion plays a key role for droplet motion with high velocity.
The liquid metal surface tends to come into being Ga
2O3due
to electrochemical reaction under electric field, whichdecreases the surface tension and liquidity, and weakens the
electro-hydrodynamic effect. However, NaOH solution can
effectively deoxidize Ga 2O3. For NaCl solution, the velocity
of the liquid metal appears smaller than that for NaOHsolution.
In summary, we have discovered a fundamental electro-
hydrodynamic phenomenon that low magnitude electric fieldwould easily induce liquid metal ejection from a capillary
tube. The subsequent shooting of the metal stream into the
solution would generate a large amount of discrete droplets.
The carried out experiments disclosed the major factors to
dominate the events. Several important conclusions can bedrawn as follows. First, the ejection velocity of galinstan
droplets has positive correlation with the applied voltage.
Second, the concentration of NaOH solution has no signifi-cant effect on the ejection velocity. Third, the size of the
galinstan droplets depends mainly on the aperture diameter
of the capillary nozzle. The present finding opens an efficientstrategy to flexibly fabricate liquid metal droplets in large
amount and with controlled size via a rather rapid, easy, and
low cost way. It also raised important scientific issues worthof investigation in the coming time.
This work was partially supported by the Research
Funding of the Chinese Academy of Sciences (Grant No.
KGZD-EW-T04-4) and NSFC under Grant No. 81071225.
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1.4897491.pdf | Generation of circularly polarized radiation from a compact plasma-based extreme
ultraviolet light source for tabletop X-ray magnetic circular dichroism studies
Daniel Wilson, Denis Rudolf, Christian Weier, Roman Adam, Gerrit Winkler, Robert Frömter, Serhiy Danylyuk,
Klaus Bergmann, Detlev Grützmacher, Claus M. Schneider, and Larissa Juschkin
Citation: Review of Scientific Instruments 85, 103110 (2014); doi: 10.1063/1.4897491
View online: http://dx.doi.org/10.1063/1.4897491
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/10?ver=pdfcov
Published by the AIP Publishing
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138.87.160.11 On: Tue, 09 Dec 2014 16:51:05REVIEW OF SCIENTIFIC INSTRUMENTS 85, 103110 (2014)
Generation of circularly polarized radiation from a compact plasma-based
extreme ultraviolet light source for tabletop X-ray magnetic circulardichroism studies
Daniel Wilson,1,2,a)Denis Rudolf,1,2,a),b)Christian Weier,3Roman Adam,3Gerrit Winkler,4
Robert Frömter,4Serhiy Danylyuk,5Klaus Bergmann,6Detlev Grützmacher,2
Claus M. Schneider,3and Larissa Juschkin1,2
1RWTH Aachen University, Experimental Physics of EUV , Steinbachstraße 15, 52074 Aachen, Germany
2Forschungszentrum Jülich GmbH, Peter Grünberg Institut (PGI-9), JARA-FIT, 52425 Jülich, Germany
3Forschungszentrum Jülich GmbH, Peter Grünberg Institut (PGI-6), JARA-FIT, 52425 Jülich, Germany
4Institut für Angewandte Physik, Universität Hamburg, Jungiusstraße 11, 20355 Hamburg, Germany
5RWTH Aachen University, Chair for Technology of Optical Systems, JARA-FIT, Steinbachstraße 15, 52074
Aachen, Germany
6Fraunhofer Institute for Laser Technology, Steinbachstrasse 15, 52074 Aachen, Germany
(Received 1 September 2014; accepted 27 September 2014; published online 16 October 2014)
Generation of circularly polarized light in the extreme ultraviolet (EUV) spectral region (about
25 eV–250 eV) is highly desirable for applications in spectroscopy and microscopy but very chal-lenging to achieve in a small-scale laboratory. We present a compact apparatus for generation of
linearly and circularly polarized EUV radiation from a gas-discharge plasma light source between
50 eV and 70 eV photon energy. In this spectral range, the 3 pabsorption edges of Fe (54 eV), Co
(60 eV), and Ni (67 eV) offer a high magnetic contrast often employed for magneto-optical and elec-
tron spectroscopy as well as for magnetic imaging. We simulated and designed an instrument for gen-eration of linearly and circularly polarized EUV radiation and performed polarimetric measurements
of the degree of linear and circular polarization. Furthermore, we demonstrate first measurements
of the X-ray magnetic circular dichroism at the Co 3 pabsorption edge with a plasma-based EUV
light source. Our approach opens the door for laboratory-based, element-selective spectroscopy of
magnetic materials and spectro-microscopy of ferromagnetic domains. © 2014 AIP Publishing LLC .
[http://dx.doi.org/10.1063/1.4897491 ]
I. INTRODUCTION
Extreme ultraviolet (EUV) and soft X-ray spectral
region extends from about 25 eV to 12 000 eV and is domi-
nated by strong light-matter interaction.1The presence of ab-
sorption edges of every element allows strong elemental andchemical selectivity. In particular, linearly and circularly po-
larized EUV and soft X-ray radiation is highly desired for
applications, such as reflectometry, ellipsometry, lithography,magneto-optical spectroscopy, and photoemission studies. In
reflectometry, linearly polarized light is used for the charac-
terization of EUV optics, such as multilayer Bragg mirrors.
2
In interference lithography, the contrast between the high-
est and lowest intensity in the resist is considerably better
with linearly polarized light compared to unpolarized light.3
Another application of polarized EUV and soft X-ray radi-ation, magneto-optical polarization spectroscopy,
4–6provides
valuable information about magneto-optical constants and en-
ables studies of element- and layer-selective magnetization.
For magneto-optical spectroscopy, both linearly and circu-larly polarized light is required.
In particular, X-ray magnetic circular dichroism
(XMCD) is frequently used for magneto-optical and
a)D. Wilson and D. Rudolf contributed equally to this work.
b)Author to whom correspondence should be addressed. Electronic mail:
d.rudolf@fz-juelich.dephotoemission spectroscopy at the 2 p(700 eV–860 eV)
and 3 p(50 eV–70 eV) absorption edges of Fe, Co, and
Ni.4–8
Linearly and circularly polarized EUV and soft X-ray
radiation is routinely generated at large-scale facilities such
as electron storage rings and free-electron lasers – unique
sources of high energy photons in terms of intensity, pho-
ton energy range, spectral bandwidth, pulse duration, andpolarization. Therefore, a complete polarization analysis is
usually performed at synchrotrons with sophisticated polari-
metric and ellipsometric instruments.
9–11
In a small-scale laboratory, various EUV and soft X-ray
light sources are available, but only few of them are ap r i -
oripolarized. While linearly polarized EUV light is rou-
tinely generated by intense ultrashort laser pulses,12only few
attempts to either polarize EUV radiation circularly13,14or
directly generate circular EUV radiation from a femtosec-ond laser
15have been reported from laboratory-based exper-
iments. A straightforward concept for conversion of linear to
circular EUV polarization is to exploit the phase shift betweenthe s- and p-components of light upon reflection from a flat
surface. For that purpose, a phase shift of ±90
◦between the
s- and p-components and identical reflectivity for the s- and p-
components are required.13,14,16,17Laboratory-based instru-
ments for generation of circularly polarized EUV light em-ploy up to four mirrors. Due to the low overall reflectivity of
a few percent in the EUV spectral range a sufficiently intense
0034-6748/2014/85(10)/103110/9/$30.00 © 2014 AIP Publishing LLC 85, 103110-1
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EUV light source is required to obtain a reasonable photon
flux after the conversion.
For our studies, we employ an intense gas-discharge
plasma-based EUV light source.18–20The multiply ionized
atoms, in our case oxygen and nitrogen ions, emit narrow-
bandwidth spectral lines ( /Delta1E/E=10−3−10−5, where Eis
the photon energy) in the photon energy range between vac-
uum ultraviolet and soft X-rays. We optimized the EUV light
source for operation above 50 eV photon energy. To linearly
polarize the initially unpolarized EUV light and simultane-
ously select emission lines around the 3 pabsorption edge of
Co (60 eV), we designed a Bragg mirror linear polarizer op-
erating close to the Brewster angle. Behind the linear polar-
izer, we placed a broadband triple-reflection circular polar-izer, which covers the 3 pabsorption edges of Fe, Co, and Ni
between 50 eV and 70 eV .
To our knowledge, for the first time in a laboratory-
based experiment with a plasma-based EUV light source, we
demonstrate XMCD measurements on Co/Pt-multilayer films
at the Co 3 pabsorption edge.
Our paper is structured as follows. In Sec. II, we present
the simulation and design of the instrument for generation of
circularly polarized EUV radiation covering the 3 pabsorp-
tion edges of Fe, Co, and Ni. In Sec. III, we describe the
spectral reflectivity of the Bragg mirror linear polarizer de-
signed to reflect emission lines around the Co 3 pabsorption
edge. In Sec. IV, we analyze the performance of linear and
circular polarizer around 60.5 eV extracted from polarimetricmeasurements. Finally in Sec. V, we present X-ray magnetic
circular dichroism measurements on Co/Pt-multilayers at the
Co 3 pabsorption edge.
II. SIMULATION AND DESIGN OF THE INSTRUMENT
FOR GENERATION OF CIRCULARLY POLARIZED EUVRADIATION AT THE 3
pABSORPTION EDGES OF
IRON, COBALT, AND NICKEL
A. Simulation based on Stokes formalism
The concept of experimental apparatus to generate circu-
larly polarized light at the 3 pabsorption edges of Fe, Co and
Ni and to measure the degree of circular polarization is pre-
sented in Fig. 1. We refer to the angle notation of Fig. 1for
further discussion of the simulation and experimental results.
To simulate and design the instrument for generation of
circularly polarized light between 50 eV and 70 eV , we ap-plied the Stokes polarization formalism
13,14,21for our specific
case. The four Stokes parameters are
S0=E2p+E2s,
S1=E2p−E2s,
S2=2EpEscos(/Delta1),
S3=−2EpEssin(/Delta1),(1)
which include the amplitudes of s- and p-polarized light Es
andEpand the phase shift /Delta1between them describing the
complete polarization state of light.21To measure the degree
of linear ( pL) and circular ( pC) polarization, we define
pL=S1
S0(2)
FIG. 1. The EUV light at the 3 pabsorption edge of Co (60 eV) emitted by
the oxygen plasma first passes through an aperture and Al/Parylene N filter.
Then the light is linearly (s) polarized by a Bragg mirror placed at the Brew-ster angle θ
LP. The linearly polarized light is reflected by three mirrors whose
rotation angle ϕCPis adjusted to create circular polarization. The polarization
state of light is observed by the Bragg mirror analyzer together with a photo-diode, both rotatable around the beam axis (angle ϕ
A).
and
pC=S3
S0. (3)
Equation (2)assumes that the light is either completely s-
or p-polarized ( S2=0). Moreover, Eqs. (2)and(3)distinguish
between s ( pL=−1)- and p ( pL=+1)-polarization as well as
between positive ( pC=+1) and negative ( pC=−1) helicity
(right- and left-circularly polarized light), respectively.
Each optical element, represented by a 4 ×4 so-called
Müller matrix, has a different reflection or transmission for
the s- and p-component and, in addition to that, it causes
a phase shift between the two components. To characterizethe polarization state of light, in our case after reflection, the
two parameters ψ
r(rotation of the main ellipse axis) and /Delta1r
(phase shift between the s- and p-component) are fundamen-
tal quantities for ellipsometric studies.21For complex-valued
s- and p-reflectivities rsandrp, the ellipsometric relationship
reads
tan(ψr)×ei/Delta1r=rp
rs. (4)
The s- and p-reflectivities, readily calculated using Fres-
nel equations, depend on the refractive index (and thus the
photon energy) as well as the angle of incidence. For our sim-ulations, we extracted the complex-valued refractive indices
for the EUV spectral range from the database of the Center
of X-Ray Optics (CXRO).
22Since the emitted radiation of
our plasma discharge source is initially unpolarized, the light
has to be polarized linearly first and then polarized circularly.
To this end, we designed multilayer Bragg mirrors for Brew-ster angle operation and peak reflectivity at 53.9 eV (Fe 3 p),
60.5 eV (Co 3 p), and 67.0 eV (Ni 3 p). The requirements for
the linear polarizer are high reflectivity on one hand and high
degree of linear polarization on the other hand, both at the
3pabsorption edges of Fe, Co, and Ni. In addition to that, a
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FIG. 2. The inset shows a comparison of reflectivity for s- and p-polarized light between the Mo/Si (a) and B4C/Si (b) Bragg mirror linear polarizer at
60.5 eV photon energy (3 pabsorption edge of Co). The simulated reflectivity for s- and p-polarized light (black and red lines) is plotted for different angles /Theta1
with respect to normal incidence. The resulting degree of linear polarization pL(blue line) is higher for B4C/Si linear polarizer ( |pL|=1.00) compared to Mo/Si
linear polarizer ( |pL|=0.94).
spectrally sufficiently narrow reflectivity peak is required to
select single emission lines.
We simulated the Bragg mirror reflectivity based on the
iterative algorithm from Ref. 21. In our simulations, we com-
pared the s- and p-reflectivity of two material combinations,namely, Mo/Si and B
4C/Si at the 3 pabsorption edges of Fe,
Co, and Ni (Fig. 2for Co 3 p). For the B4C/Si Bragg mirror
linear polarizer, our simulation shows a higher ratio betweenthe s- and p-reflectivity in the vicinity of the Brewster angle
compared to the Mo/Si linear polarizer. Due to the high de-
gree of linear polarization p
Lclose to 1 and also due to a suf-
ficiently narrow bandwidth (1.36 nm for Co 3 p, see Table I),
we used the B4C/Si Bragg mirror linear polarizer in our ex-
periments. We summarize the most important parameters ofthe B
4C/Si Bragg mirrors in Table I.I nF i g . 2, we display the
simulation results for the 3 pabsorption edge of Co (60 eV).
For the 3 pabsorption edges of Fe and Ni, the simulation re-
sults are similar in terms of magnitude and shape of the pL-/Theta1
graph.
In the next step, we simulated the degree of circular
polarization due to the phase shift /Delta1between the s- and
TABLE I. Parameters of the B4C/Si multilayer mirror linear polarizers for
the 3 pabsorption edges of Fe (53.9 eV), Co (60.5 eV), and Ni (67.0 eV).
Edenotes the photon energy, θBthe Brewster angle (with respect to normal
incidence), RsandRpthe s- and p-reflectivities, and pLthe degree of linear
polarization after reflection.
[B4C (5.36 nm)/ [B4C (6.06 nm)/ [B4C (6.72 nm)/
Si (8.04 nm)]x50Si (9.09 nm)]x50Si (10.08 nm)]x50
E(eV) 67.0 60.5 53.9
θB(deg) 42.79 42.52 40.81
Rs0.41 0.42 0.39
Rp0.96×10−30.26×10−30.72×10−3
FWHM 1.05 1.36 1.71
ofRs(nm)
pL−0.999 −0.995 −0.996p-components of the electric field upon reflection. Reported
concepts of a circular polarizer between 50 eV and 70 eV
photon energy14,16are based on four non-rotatable mirrors to
create a phase shift /Delta1=±90◦.
To fulfill the requirements of high degree of circular po-
larization pC, high overall reflectivity of the circular polarizer
between 50 eV and 70 eV , and a simple rotation around the
beam axis without any beam movement, we used three insteadof four mirrors.
23
For 20 nm Mo on Si, we found the triple-reflection at
20◦–40◦–20◦grazing incidence to give the highest |pC|of
>0.99 at 60.5 eV . The total phase shift after three reflections
amounts to /Delta1CP=− 90.9◦. The degree of circular polariza-
tion pCdepends on the rotation angle ϕCP(see Fig. 1) and
amounts to pC=+ 1 (right circular) for ϕCP≈70◦(250◦)
and to pC=−1 (left circular) for ϕCP≈110◦(290◦)f o rt h e
3pabsorption edges of Fe, Co, and Ni (Fig. 3). Therefore,
the circular polarizer offers two main advantages. First, the
helicity of light is readily changed between left and right cir-
cular polarization. Second, only one circular polarizer cov-ers all 3 pabsorption edges of the 3 dferromagnets and there-
fore enables magneto-optical polarization spectroscopy and
microscopy of Fe, Co, and Ni taking advantage of the XMCD
effect. The overall reflectivity R
CPof the circular polarizer
at the 3 pabsorption edges of Fe, Co, and Ni starting from
fully s-polarized light is displayed in Fig. 3(b).F o r pC=1
(ϕCP=70◦), it amounts to about 1%. Our result is compa-
rable to the overall reflectivity obtained with the four mirrorconfiguration.
14,16
To measure the degree of linear and circular polarization,
we used a Bragg mirror analyzer and a photodiode, which werotated around the beam axis in the Rabinovitch polarimeter
configuration.
24The requirements for the Bragg mirror ana-
lyzer are the same as for the linear polarizer (high reflectiv-ity, high degree of linear polarization, narrow bandwidth) and
therefore, we used two identical Bragg mirrors for linear po-
larizer and analyzer.
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FIG. 3. (a) Simulated degree of circular polarization pCafter the circular polarizer as a function of rotation angle ϕCPfor three photon energies corresponding
to the 3 pabsorption edges of Fe, Co, and Ni. The circular polarizer was optimized for the Co 3 pabsorption edge, thus showing the highest degree of circular
polarization for ϕCP≈70◦, 250◦(pC=+1), and ϕCP≈110◦, 290◦(pC=−1). (b) Overall reflectivity RCPof the circular polarizer for pC=+1 around the 3 p
absorption edges of Fe, Co, and Ni with a strong magneto-optical signal6(grey boxes).
B. Design of the instrument for generation and
analysis of polarized EUV light and circular magnetic
dichroism measurements between 50 eV and 70 eVphoton energy
The entire optical setup is mounted on an optical bread-
board inside a vacuum chamber. For the Bragg mirror
linear polarizer, we used a commercial piezo-driven, vacuum-
compatible holder for 1 in. mirrors (Smaract STT-25). Duringthe alignment, the holder allows for tilting the mirror around
the vertical and horizontal axes by ±2.5
◦.
The subsequent optical element, the triple-reflection cir-
cular polarizer, required a specially designed holder for fixed
mounting of three mirrors (two mirrors 10 mm ×30 mm,
one mirror 10 mm ×23 mm) for 20◦–40◦–20◦grazing
incidence.23Thus, no further relative alignment of the mir-
rors is necessary. The holder consists of two parts se-cured together using screws (Fig. 4). Inside the cutouts of
the two parts (two cutouts in one part, one cutout in the
other part) the mirrors consisting of 20 nm Mo layer ther-mally evaporated on Si substrate are attached using glue.
The entire device is mounted on a piezo-driven, vacuum-
compatible rotational stage (Smaract SR-7012-S, minimumstep size of 0.2 ×10
−3◦) with positioning control. For po-
larization analysis using a Rabinovitch polarimeter,24we de-
signed a special Bragg mirror holder for 1 in. optics allowing42
◦incidence angle with respect to the normal (Fig. 4) cor-
responding to the Brewster angle at 60.5 eV (Co 3 pabsorp-
tion edge). Similar to the triple-reflection circular polarizer,
the holder is mounted on a piezo-driven, vacuum-compatible
rotational stage (Smaract SR-5714-S, minimum step size of0.16×10
−3◦) equipped with positioning sensors. We also
designed a holder for the AXUV 100G photodiode (10 mm
×10 mm active area) with mechanical support for the electri-
cal SMA connector. The photodiode holder is attached to the
analyzer holder and rotates with the analyzer around the beam
axis.
All optical elements were prealigned using visible light.
We performed the angular alignment of the Bragg mirrorlinear polarizer optimizing the reflectivity at 60.5 eV . For that
purpose, we rotated the vacuum chamber around the centralaxis in the surface plane of the linear polarizer simultaneously
recording EUV spectra. Once the intensity of emission lines
FIG. 4. (a) Holder for the triple reflection circular polarizer consisting of
two halves. All mirrors comprising a Mo (20 nm) layer on Si substrate are
glued inside the milled parts. The whole device is attached to a piezo-driven
rotational stage with positioning sensors. (b) Analyzer consisting of a Bragg
mirror holder for 42◦with respect to the normal incidence (Brewster angle
at 60.5 eV) and a photodiode holder. The whole device mounted on a piezo-
driven rotational stage with positioning control is rotated around the beam
axis (Rabinovitch polarimeter24).
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at 60.5 eV was highest, the alignment goal was reached. We
note that the linear polarizer accepts a wide angle range be-
tween /Theta1=35◦and/Theta1=50◦without significant loss of the
degree of linear polarization (Fig. 2(b), blue curve).
For X-ray magnetic circular dichroism measurements at
the Co 3 pabsorption edge, we removed the Bragg mirror an-
alyzer and placed a Co/Pt multilayer sample between the pole
shoes of a ferromagnetic yoke to magnetize it. We designed
the yoke with a bore hole of 2 mm diameter for transmission
measurements and magnetized the soft ferromagnetic yoke by
a coil wound around the yoke. When supplying 4 A current tothe coil, the magnetic field reaches a maximum of 320 mT
between the pole shoes wit ha2m mg a p .
For the Co/Pt multilayer sample (10 mm ×10 mm) with
aS i
3N4window (0.5 mm ×0.5 mm), we designed a separate
holder for transmission experiments. To align the Co/Pt mul-
tilayer sample between the pole shoes of the yoke, we placedthe sample on a two-dimensional piezo-driven linear stage for
vertical and horizontal movement.
III. CHARACTERIZATION OF THE PLASMA EMISSION
SPECTRUM BEHIND THE BRAGG MIRRORLINEAR POLARIZER
For our measurements, we employed a gas-discharge
plasma EUV light source developed at the Fraunhofer Insti-
tute for Laser Technology18–20and specifically designed for
water window operation (280 eV–530 eV). In our experi-ments, the source parameters were 3.5 kV discharge voltage,
2μF total capacity and 20 Hz pulse repetition rate. In order
to produce high intensity radiation between 50 eV and 70 eV
photon energy, we used EUV radiation emitted from highly
ionized nitrogen (N
3+,N4+) and oxygen atoms (O4+,O5+).
A typical spectrum measured by a grazing-incidence EUV
spectrometer with a blazed, spherical, gold-coated grating
(1200 lines/mm) and a back-illuminated CCD camera (AndoriKon-M) is presented in Fig. 5. The measured relative spectral
bandwidth of single emission lines /Delta1λ/λ≈10
−3is limited by
the spectral resolution of our spectrometer. We note that emis-sion lines of multiply ionized nitrogen and oxygen atoms are
well in the range of 3 pabsorption edges of Fe, Co, and Ni.
Furthermore, based on reported synchrotron measurements,the magneto-optical resonances are known to occur in the
vicinity of the absorption edges and to be spectrally broader
than the absorption edge itself, having a spectral width of feweV for the 3 pabsorption edges of Fe, Co, and Ni
5,6(grey
marked regions in Fig. 5extracted from Ref. 6). Therefore,
it is possible to use nitrogen and oxygen plasma radiation for
element-selective magneto-optical polarization spectroscopy
and microscopy at the 3 pabsorption edges of the 3 dferro-
magnets. In the case of Co, multiple emission lines of oxygen
plasma are within the 3 pmagneto-optical resonance used in
our magnetic circular dichroism measurements.
We selected oxygen and nitrogen emission lines by
a Bragg mirror linear polarizer with peak reflectivity at
60.5 eV (Fig. 5(b)). The pulse energy of the oxygen and nitro-
gen emission lines around 60.5 eV measured with a calibrated
EUV photodiode after the Bragg mirror is about 1.3 mJ/sr
(1.4×1014photons/sr) for the oxygen and 0.8 mJ/sr (0.8
×1014photons/sr) for the nitrogen plasma. To obtain the
highest possible magnetic contrast in our XMCD studies, we
used the oxygen instead of the nitrogen lines located veryclose to 60.5 eV (grey box in Fig. 5(b)). The intense oxygen
line at 64.3 eV only contributes to the overall intensity but not
to the magnetic signal.
IV. POLARIMETRIC MEASUREMENT OF THE DEGREE
OF LINEAR AND CIRCULAR POLARIZATION AT 60 eV
We measured the polarization properties of EUV light
around the 3 pabsorption edge of Co (60 eV) behind the lin-
ear and circular polarizer. Below we first describe the mea-surements of the degree of linear polarization and then that of
circular polarization.
Similar to Eq. (2)in Sec. II A, we define the degree of
linear polarization p
Lafter the linear polarizer as
pL=S1,L
S0,L, (5)
FIG. 5. (a) Spectra of highly ionized nitrogen (N3+,N4+) and oxygen (O4+,O5+) atoms between 53 eV and 70 eV as emitted from the gas-discharge plasma-
based EUV light source. The grey boxes indicate the spectral region of magneto-optical resonances around the 3 pabsorption edges of Fe, Co, and Ni.6The
dashed line corresponds to the reflectivity of the Bragg mirror linear polarizer for 60.5 eV at 42◦normal incidence (Brewster angle). (b) Spectra of highly ionized
nitrogen and oxygen atoms measured directly after the Bragg mirror linear polarizer for 60.5 eV comprising [Si(9.09 nm)/B4C(6.06 nm)]50x.
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where S0,LandS1,Ldenote the first two values of the Stokes
vector after the linear polarizer ( S2,L=0). For completely s-
polarized light, pL=− 1. The photodiode signal ILPcan be
readily derived using the Stokes formalism and it reads
ILP(ϕA)=ILP,0[1−pL×cos(2ψA)×cos(ϕA)], (6)
where ILP,0is a constant factor.
The influence of the analyzer is taken into account by
ψA=tan−1/parenleftbigg|rp,A|
|rs,A|/parenrightbigg
, (7)
with rp,Aandrs,Abeing the complex reflectivities for p- and
s-polarized light. For our analyzer placed at the Brewster
angle, we assume |rp,A|/|rs,A|≈0. Therefore, according to
Eq.(7),ψA≈0 and thus cos (2 ψA)=1. We define the ana-
lyzer rotation angle ϕA(Fig. 1) in such a way that for ϕA=0
the s-p-coordinate systems of the linear polarizer and analyzer
are identical and therefore ILP(ϕA=0) is at maximum.
To measure the degree of linear polarization pL
(Eq. (5)), we placed the Bragg mirror analyzer directly be-
hind the Bragg mirror linear polarizer and then rotated the an-alyzer clockwise with respect to the beam propagation direc-
tion around the beam axis by an angle ϕ
A(Fig. 6(a)). In order
to suppress photon energies below 30 eV being also reflected
by the Bragg mirror, we inserted an Al (100 nm)/Parylene N
(100 nm) spectral filter into the beam path. We detected thesignal by an EUV photodiode (AXUV 100G) and amplified
with a low-noise current amplifier (FEMTO DLPCA-200) by
a factor of 10
9V/A. Finally, we measured the time-integrated
voltage by a voltmeter (HP 3457A). For each angle position
ϕA, we took an average of four measurements. In addition,
we separately recorded the offset voltage for every angle po-sition in order to correct our data for rotational stage position-
dependent voltage variations. A representative measurement
is shown in Fig. 6(a). We fitted multiple data sets accord-
ing to Eq. (6)and obtained an average value of p
L=−(0.94
±0.04). Although the measured degree of linear polarizationis slightly lower than predicted by simulations, it is sufficient
for conversion of linearly to circularly polarized light.
To analyze the polarization properties of our triple-
reflection circular polarizer, we modeled the Müller matrix
of the circular polarizer as
MCP=⎛
⎜⎜⎜⎜⎝ab 00
ba 00
00 c×cos/parenleftbig
/Delta1
CP/parenrightbig
c×sin/parenleftbig
/Delta1CP/parenrightbig
00 −c×sin/parenleftbig
/Delta1CP/parenrightbig
c×cos/parenleftbig
/Delta1CP/parenrightbig⎞
⎟⎟⎟⎟⎠.(8)
The ansatz for the matrix M
CPstems from the multi-
plication of three standard Müller matrixes for all three Mo
mirrors.21Here, the parameters a,b, and cdepend on ellip-
sometric parameters ψof the three Mo mirrors, whereas the
parameter cdepends on aandb, and can be readily deter-
mined from these parameters (see the Appendix). The phase
shift/Delta1CPdenotes the total phase shift between the s- and p-
component after all three reflections. The circular polarizer
was designed for /Delta1CP=−90.9◦at 60.5 eV for efficient con-
version of linearly to circularly polarized light (Sec. II A).
The photodiode signal ICP(ϕA) behind the analyzer follows
the equation
ICP(ϕA)=ICP,0×[a−b×cos(2ϕCP)−b×cos(2ϕA+2ϕCP)
+0.5×a×cos(2ϕA+4ϕCP)+0.5×a×cos(2ϕA)].
(9)
Similar to Eq. (6),ICP,0is a constant factor, ϕAthe rota-
tion angle of the analyzer (clockwise with respect to the beam
direction), and ϕCPthe rotation angle of the circular polarizer
(counterclockwise with respect to the beam direction). Equa-
tion(9)assumes a completely s-polarized light behind the lin-
ear polarizer, i.e., S1,L=−S0,Land as before in Eq. (6),ψA
≈0. Furthermore, we fixed /Delta1CP=− 90.9◦and neglected
small terms proportional to c·cos (/Delta1CP)i nE q . (9).
FIG. 6. (a) Measurement of the degree of linear polarization with emission lines from oxygen (Fig. 5(b)) around 60.5 eV photon energy (blue dots) and fit
(black line) according to Eq. (6). The signal of the photodiode is plotted versus the analyzer rotation angle ϕA. The degree of linear polarization extracted
from the fit amounts to pL=−0.96. (b) Measurement of the degree of circular polarization with emission lines from oxygen around 60.5 eV behind the triple
reflection circular polarizer. The photodiode voltage is shown as a function of the analyzer rotating angle ϕA(dots) for different rotation angles ϕCPof the circular
polarizer. The lines are fits according to Eq. (9).F o rϕCP=70◦(red dots), the signal is independent of ϕAconfirming that the degree of circular polarization is
at maximum.
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Fitting the measured intensity ICP(ϕA)f r o mE q . (9)for
various fixed rotation angles ϕCP(Fig. 6(b)), we extracted the
parameters aandband then calculated the dependent param-
eterc. The details of our analysis are summarized in the Ap-
pendix. In the next step, we calculated the degree of circular
polarization according to
pC=c×sin(/Delta1CP)×sin(2ϕCP)
b×cos(2ϕCP)−a. (10)
To measure the Müller matrix parameters of our circular
polarizer, we placed it between the linear polarizer and an-alyzer and independently rotated both circular polarizer and
analyzer around the beam axis. We applied the same mea-
surement technique as for the linear polarizer, i.e., the pho-todiode signal was amplified with a low-noise current am-
plifier by a factor of 10
9V/A and the resulting voltage was
measured by a time-integrating voltmeter. We fixed the ro-tation angle ϕ
CP(counterclockwise with respect to the beam
direction) and scanned ϕA(clockwise with respect to the beam
direction) between 0◦and 360◦in 20◦steps. For accurate po-
larization analysis, we separately measured the voltage off-
set for each ϕAwith the EUV light source being off. A rep-
resentative result for ϕCP=30◦,7 0◦, and 330◦is displayed
in Fig. 6(b). The periodic voltage modulations present for
ϕCP=30◦andϕCP=330◦disappear when ϕCP=70◦, i.e.,
the photodiode signal does not depend on the analyzer angle
ϕA. For this rotation angle, we efficiently convert linearly to
circularly polarized light at 60.5 eV as expected from simula-tions (Sec. II A). From measurements at six different angles
ϕ
CP, we determined the parameters a,b, and cand derived
a maximum value of pC=0.81±0.15 for ϕCP=70◦and
/Delta1CP=−90.9◦.
V. XMCD MEASUREMENTS ON A Co/Pt-MULTILAYER
FILM AT THE COBALT 3 pABSORPTION EDGE (60 eV)
For the XMCD studies, we placed the coil and magnetic
yoke with a bore hole for transmission measurements directly
behind the Bragg mirror linear and circular polarizers and
mounted the sample on two piezo-driven linear stages for ver-
tical and horizontal movement in the center between the pole
shoes of the yoke. As a suitable test sample for our XMCDmeasurements we chose [Co (0.8 nm)/Pt (1.4 nm)]
16xlayers
grown on Si3N4(50 nm)/Pt(5 nm) by ion beam sputtering25,26
and capped with Pt (0.6 nm). A Co/Pt multilayer film exhibits
a large perpendicular uniaxial anisotropy25,26and therefore,
can be magnetized out-of-plane, which, for normal incidence
of light, ensures a strong XMCD signal at the Co 3 pabsorp-
tion edge. From the scientific perspective, a Co/Pt multilayer
film is a highly interesting ferromagnetic layer system for
several reasons. Most importantly, within a certain thicknessrange of Co and Pt layers, the magnetization oriented perpen-
dicular or even canted with respect to the film plane tends to
split into many alternatingly oriented ferromagnetic domainswith the average domain size of about 100 nm at zero exter-
nal magnetic field.
25,26The latter property renders Co/Pt mul-
tilayer films an ideal model system for studies of laser heat-
ing effects on the ferromagnetic domain structure employing
high harmonics27as well as for studies of femtosecond mag-netization dynamics of nanometer scale domains with a free-
electron laser.28,29
The magnetization curve of the Co/Pt multilayer sample
measured with polar magneto-optical Kerr effect (P-MOKE)
is shown in the inset of Fig. 7. In order to saturate the mag-
netization, we applied 320 mT magnetic field perpendicularto the sample surface. In our measurements, we first set the
circular polarizer to ϕ
CP=70◦(pC=+1), alternately applied
±320 mT magnetic field, and then recorded the transmitted
signal I±(±320 mT) on the CCD camera for both magnetic
fields. We note that the actual image on the CCD camera isthe beam profile after the Co/Pt multilayer sample including
all oxygen spectral lines reflected by the Bragg mirror lin-
ear polarizer (Fig. 5(b)). For further data analysis, the signal
was binned along one spatial direction in the region of inter-
est. The difference of the transmitted intensity averaged over
50 measurements for each magnetic field (10 s or 200 pulsesper measurement), is shown in Fig. 7. For the background-
corrected XMCD asymmetry A
XMCD calculated according to
equation
AXMCD =I+(+320 mT) −I−(−320 mT)
I+(+320 mT) +I−(−320 mT), (11)
we obtained AXMCD =+ (2.7±0.1)%.
After that, we changed the rotation angle of the circu-
lar polarizer to ϕCP=110◦(pC=− 1) and repeated the
above described measurement procedure. As expected forthe XMCD effect, the difference signal (Fig. 7) and thus
the asymmetry keeps the same magnitude but changes its
sign. We measured A
XMCD =−(2.8±0.1)%. To validate our
data, we calculated the expected XMCD asymmetry from
the magneto-optical absorption /Delta1β of the refractive index
FIG. 7. XMCD difference signal I+(+320 mT) −I−(−320 mT) as recorded
by the CCD camera for ϕCP=70◦(pC=+ 1) and ϕCP=110◦(pC=
−1). We note that the graph displays a beam profile including all oxy-
gen spectral lines reflected by the Bragg mirror linear polarizer (Fig. 5(b)).
For the background-corrected XMCD asymmetry, we obtained AXMCD
=+ (2.7±0.1) % and AXMCD=− (2.8±0.1) % for different helicities.
The inset shows the magnetization curve (Kerr rotation /Theta1Kerr)o fa[ C o( 0 . 8
nm)/ Pt (1.4 nm)]16xmultilayer measured by polar magneto-optical Kerr ef-
fect (P-MOKE) with visible light (350 nm wavelength). Using our magnetic
yoke with a maximum field of 320 mT the sample can be magnetized to sat-
uration.
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138.87.160.11 On: Tue, 09 Dec 2014 16:51:05103110-8 Wilson et al. Rev. Sci. Instrum. 85, 103110 (2014)
FIG. 8. XMCD spectrum for Co (total thickness d=12.8 nm) calculated by
Eq.(12) from experimentally determined imaginary part of magnetic refrac-
tive index6(black curve) and oxygen spectrum of the gas-discharge plasma-
based EUV light source behind the Bragg mirror linear polarizer (blue curve).
n=1−(δ+/Delta1δ)+i×(β+/Delta1β) and a total Co thickness
ofd=12.8 nm according to4
AXMCD =2E
¯cd×/Delta1β, (12)
where Edenotes the photon energy, ¯the reduced Planck con-
stant, and cthe speed of light. Contrary to other magneto-
optical effects like the MOKE or the Faraday effect, the
XMCD asymmetry depends on one single magneto-optical
parameter /Delta1β. To simulate AXMCD in the vicinity of the Co
3pabsorption edge, we inserted the photon energy-dependent
parameter /Delta1β recently measured by Valencia et al.6in
Eq.(12). The expected XMCD asymmetry is plotted together
with the oxygen spectrum behind the Bragg mirror linear po-
larizer in Fig. 8. The expected asymmetry of a few percent
agrees well with our data. We note that our XMCD signalrepresents an average over several spectral lines of the oxy-
gen plasma (Fig. 5(b)).
To further confirm our results, we studied the magnetic
field dependence of the intensity difference signal I+(μ0Href)
−I−(μ0H) defining a fixed reference magnetic field μ0Href
=+320 mT and six variable magnetic fields μ0H(−320 mT,
−200 mT, −100 mT, 0 mT, 100 mT, and 200 mT). The inten-
sity difference averaged over 10 measurements, each 10 s or
200 pulses, is displayed in Fig. 9.
As expected for μ0Href=+320 mT from the magnetiza-
tion curve (inset in Fig. 7), the difference signal is present for
μ0H<0 and disappears for μ0H>0.
In summary, we generated circularly polarized light with
both helicities from a laboratory-based plasma EUV lightsource and measured XMCD asymmetry values at the Co
3pabsorption edge comparable to the reported synchrotron
studies.
5,6The changing sign of the XMCD asymmetry upon
helicity reversal confirms the magnetic origin of the sig-
nal. Moreover, we changed the magnetic field and observed
a magnetic signal, which follows the magnetization curve.Our results, to our knowledge, are the first laboratory-based
XMCD measurements at the Co 3 pabsorption edge with a
plasma-based EUV light source.
VI. CONCLUSION AND OUTLOOK
We simulated, designed, and characterized an instrument
for generation of circularly polarized EUV light at the 3 pab-
sorption edges of Fe, Co, and Ni (50 eV–70 eV) employ-ing a compact gas-discharge plasma-based EUV light source.
For the first time in a laboratory-based experiment with a
plasma-based EUV light source, we successfully measuredthe XMCD effect at the Co 3 pabsorption edge (60.5 eV)
that previously was only possible at synchrotrons, at free-
electron lasers and with laser-generated high harmonics due
FIG. 9. Magnetic field dependence of the XMCD difference signal I+(μ0Href)−I−(μ0H) (blue: data points, red: smoothed curve) for μ0H=− 320 mT,
−200 mT, −100 mT, 0 mT, +100 mT, and +200 mT for a fixed reference magnetic field μ0Href=+320 mT. Here, we fixed ϕCPto 110◦. The XMCD difference
signal is only present for μ0H<0, i.e., for different signs of μ0Hrefandμ0Has expected from the magnetization curve (inset in Fig. 7). Forμ0H>0, i.e., for
the same sign of μ0Hrefandμ0H, no magnetic signal is expected according to the magnetization curve.
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138.87.160.11 On: Tue, 09 Dec 2014 16:51:05103110-9 Wilson et al. Rev. Sci. Instrum. 85, 103110 (2014)
to the lack of circularly polarized EUV radiation in the small-
scale laboratory. Our results open the perspective for transfer
of some synchrotron capabilities to the home laboratory us-ing full time available high power plasma-based light sources.
These light sources offer the advantages of photon energies in
the EUV and soft X-ray spectral range at high intensity withmulti-kHz repetition rate. The spectral position and intensity
of the emission lines can be tuned by gas species and elec-
trical discharge energy, which immediately allows elemental
and chemical contrast at elemental absorption edges. In ad-
dition, our work demonstrates that the initially unpolarizedlight is easily converted to linearly and circularly polarized
light required for magneto-optical polarization spectroscopy
and microscopy. In future studies, it is straightforward tocombine our polarization optics and a Fresnel zone plate
EUV microscope.
30In our concept, the polarization optics
will be placed between the collector and the ferromagneticsample, which domain structure will be imaged by the Fres-
nel zone plate. The proposed microscope will significantly
advance the current imaging techniques and allow element-selective imaging of ferromagnetic domains at the 3 pabsorp-
tion edges of Fe, Co, and Ni in the small-scale laboratory
environment.
ACKNOWLEDGMENTS
L.J. acknowledges financial support by the Helmholtz
Association for a Helmholtz Professorship as a part of the
Pact for Research and Innovation. D.W., D.R., R.A., and L.J.
also acknowledge financial support by JARA-FIT Seed Fundsthrough the Excellence Initiative. Moreover, we thank Stefan
Braun (Fraunhofer IWS Dresden) for Bragg mirror design and
fabrication as well as Konstantin Tsigutkin for careful proof-reading of the paper.
APPENDIX: CALCULATION OF pC
The parameters a and b from Eq. (8)depend on the ellip-
sometric quantities ψ(see Eq. (4)) of all three Mo mirrors of
the circular polarizer in the following way:
a=1+2×cos(2ψ40)×cos(2ψ20)+cos(2ψ20)2,
b=−2×cos(2ψ20)−cos(2ψ40)−cos(2ψ40)×cos(2ψ20)2,
(A1)
where ψ20andψ40denote the ellipsometric quantities for 20◦
and 40◦grazing incidence, respectively. We solved this sys-
tem of two nonlinear equations numerically for a fixed range
ofψ20andψ40and obtained ψ20andψ40from measured pa-
rameters aandb(Sec. IV). Then, we calculated the parameter
caccording to equation
c=sin(2ψ20)2×sin(2ψ40)( A 2 )
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138.87.160.11 On: Tue, 09 Dec 2014 16:51:05 |
1.4872813.pdf |
Structural Transformations in Reactively Sputtered
Alumina Films
P. Nayar * and A. Khanna
Department of Physics, Guru Nanak Dev University, A mritsar – 143005, India
*E-mail: priyankanayar_26@yahoo.co.in
Abstract. Thin films of amorphous alumina of thickness ~350 n m were prepared on silicon wafer by DC cathode
reactive sputtering. The effects of thermal anneali ng on the structural properties were investigated a t annealing
temperatures of 600 oC, 800 oC, 1100 oC and 1220 oC. X-ray diffraction showed that crystallization sta rts at 800 oC
and produces δ and θ alumina phases, the latter phase grows with heat t reatment and the film was predominantly
δ-phase with small amount of α-phase after annealing at 1220 oC. AFM studies found that the surface of thin films
smoothened upon crystallization.
Keywords: Alumina films; XRD; AFM
PACS : 61.05.cp, 68.37.Ps, 68.60.Dv
INTRODUCTION
During the last three decades significant efforts h ave been
made towards the development of thin film technique s for
growing crystalline alumina films due to its severa l useful
properties like high resistivity (10 13 -10 15 Ω-m), high
hardness (>22 GPa), high dielectric constant value of 9-11,
high thermal conductivity (40 W K -1 m-1), and excellent
chemical and thermal durability. Alumina films find
applications in microelectronic devices as dielectr ic layers,
refractory, anticorrosive and antireflective coatin gs [1],
wave-guide sensors [2] and as buffer layers [3]. Alumina
exists in several transient phases such as γ (cubic spinel), η
(cubic spinel), δ (tetragonal), θ (monoclinic) and κ-alumina
(orthorhombic) [4]. α-alumina, commonly known as
corundum has rhombohedral hcp crystal structure and is the
most desirable form of alumina [4].
Crystalline phases of alumina require high
substrate temperatures of 700 oC which are difficult to
maintain during deposition and also deteriorates ch amber
vacuum. Therefore, one method that has attracted at tention
for crystallization of amorphous films is thermal a nnealing
after the deposition of amorphous alumina [5]. The
microstructures which form in alumina films depend on the
deposition technique and on the growth conditions; the
crystalline phase formation is rarely achieved in a lumina
films prepared at substrate temperatures below ~ 30 0 oC.
For crystallization of the films, post deposition a nnealing and/or insitu substrate heating are usually required in the
transformation sequence (amorphous-Al 2O3 ⇒ γ- Al 2O3 ⇒
α− Al 2O3) in which the crystallization of γ- Al 2O3 is often
observed at 300-800 oC [6,7]; and above 1000 oC,
crystallization of α− Al 2O3 is achieved [8].
In the present study, we have studied the effects o f heat
treatment on the structural properties of amorphous alumina
coatings. Amorphous alumina films were prepared on
silicon substrates by DC cathode reactive sputterin g
technique and subjected to heat treatment in the
temperature range of 600 oC to 1220 oC in ambient air.
EXPERIMENTAL
Alumina films were deposited using reactive sputter ing of
aluminum target in Ar and O 2 atmosphere using AMAT
Endura metal sputtering system. Silicon wafers of 2 inch
diameter were used as substrates. The substrates we re
cleaned in acetone and ethanol and dried using nitr ogen gas
prior to deposition. Before deposition the depositi on
chamber was evacuated to a base pressure of 8 x 10 -8 Torr.
High purity Ar and O 2 oxygen were used as sputtering and
reactive gases with a total flow of 35 sccm. Using Ar:O 2
ratios of 30:70, films were deposited at a substrat e
temperature of 300 oC.
Solid State Physics
AIP Conf. Proc. 1591, 948-950 (2014); doi: 10.1063/1.4872813
© 2014 AIP Publishing LLC 978-0-7354-1225-5/$30.00
948
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202.177.173.189 On: Mon, 28 Apr 2014 06:06:31The process parameters used for deposition of alumi na
films are as below:
Target Voltage: 226 V
Target Current: 6.59 A
Power: 1.495 kW
One alumina film (Sample Code: RS1) deposited on Si
wafer at a substrate temperature of 300 oC was then
sequentially annealed at different temperatures of 600 oC,
800 oC, 1100 oC and 1220 oC. The sample was annealed at
these temperatures for 6 h each. XRD measurements w ere
performed on thin film samples before and after hea t
treatment on Bruker D8 Focus X-ray powder
diffractrometer in the grazing incidence geometry w ith Cu-
Kα radiation (λ =1.54056 Ǻ). Measurements were done by
keeping the incident angle fixed at 2 o and by scanning the
scintillation counter detector in the 2 θ range of 10-70 o.
AFM studies was done on one amorphous film (Sample
Code: RS1) and crystalline film (Sample Code: RS1-H -
1220) using Parks Intrument XE-70 AFM in the contac t
mode at a scan rate of 1 Hz.
RESULTS AND DISCUSSION
Figure 1 shows the evolution of XRD patterns of alumina
film on silicon substrate after heat treatment in t he
temperature range of 600-1200 oC. For the sample (RS1),
after annealing at 600 oC for 6h, film remained amorphous
and no crystalline peaks were detected in XRD scans . On
further annealing at 800 oC, crystalline peaks were observed
at 2 θ of 19.6 o, 31.8 o, 37.5 o, 46.0 o and 67.0 o. The peaks at
19.5 o, 31.7 o and 37.5 o could be the superposition of δ [ 9], γ
and θ phases [10] of alumina. It should be mentioned her e
that γ-alumina (cubic–spinel structure with lattice
parameter, a=0.79 nm) has two peaks of equal intens ity at
45.9 o and 67.1 o [PDF File # 10-0425], while the δ and θ− -
alumina have multiple peaks in the diffraction angl e ranges
of: 45 o to 47 o and 65 o to 67 o [11].
After annealing at 1100 oC, some other peaks were observed
at 39.4 o, 45.6o, 46.5 o, 66.7 o and 67.3 o and all these peaks
were attributed to δ-phase of alumina. The intensity of the
peaks at 19.5 o, 31.7 o and 37.5 o increased after annealing the
sample to 1100 oC. It can be seen from Fig. 1 that intensity
of all the peaks decreased after annealing the samp le to
1220 oC and one new peak was observed at 35.2 o that can be
attributed to crystalline α-phase of alumina.
FIGURE 1. GIXRD patterns of alumina films before and after
annealing.
Figure 2 and Figure 3 display the AFM images of rea ctively
sputtered amorphous (RS1) and crystalline sample (R S1-H-
1220) which were recorded on 0.5 x 0.5 µm2 area in contact
mode.
FIGURE 2. AFM image of amorphous alumina film on Si
wafer (Sample Code: RS1).
949
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202.177.173.189 On: Mon, 28 Apr 2014 06:06:31
FIGURE 3. AFM image of crystalline film heat treated
upto 1220 oC (Sample Code: RS1-H-1220).
The root mean square (rms) surface roughness of the
amorphous film was 14 nm which decreased to 7 nm af ter
heat treatment and crystallization.
CONCLUSIONS
Amorphous alumina films were prepared by DC cathode
reactive sputtering technique. The influence of ann ealing
temperature was studied. XRD measurements confirmed
that an annealing temperature of 800 oC was necessary for
the crystallization of amorphous films and only aft er
annealing the sample at high temperature of 1200 oC, a
small amount of α-alumina phase was detected. Therefore
it was concluded that alumina thin films are quite resistant
to transformation to the α-phase. This is contrary to the
properties of alumina nanoparticles which transform
completely to the α-phase at 1100 oC. Finally while
amorphous films are rough, crystalline samples smoo then
with heat treatment.
REFERENCES
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Minami and M. Tatsumisago, Thin Solid Films 516, 4526-4529
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950
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1.4896365.pdf | Nano-scale NiSi and n-type silicon based Schottky barrier diode as a near infra-red
detector for room temperature operation
S. Roy, K. Midya, S. P. Duttagupta, and D. Ramakrishnan
Citation: Journal of Applied Physics 116, 124507 (2014); doi: 10.1063/1.4896365
View online: http://dx.doi.org/10.1063/1.4896365
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/12?ver=pdfcov
Published by the AIP Publishing
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(Received 18 August 2014; accepted 12 September 2014; published online 24 September 2014)
The fabrication of nano-scale NiSi/n-Si Schottky barrier diode by rapid thermal annealing process is
reported. The characterization of the nano-scale NiSi film was performed using Micro-Raman
Spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The thickness of the film (27 nm) hasbeen measured by cross-sectional Secondary Electron Microscopy and XPS based depth profile
method. Current–voltage (I–V) characteristics show an excellent rectification ratio (I
ON/IOFF¼105)
at a bias voltage of 61 V. The diode ideality factor is 1.28. The barrier height was also determined
independently based on I–V (0.62 eV) and high frequency capacitance–voltage technique (0.76 eV),
and the correlation between them has explained. The diode photo-response was measured in the
range of 1.35–2.5 lm under different reverse bias conditions (0.0–1.0 V). The response is observed to
increase with increasing reverse bias. From the photo-responsivity study, the zero bias barrier height
was determined to be 0.54 eV. VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4896365 ]
I. INTRODUCTION
There have been a number of reports concerning the
design, fabrication, and test of Near Infra-Red (NIR) detec-
tors. The conventional photo-detector for 1.5 lm application
is based on In xGa1/C0xAs hetero structures on InP or GaAs
substrate.1–4The device fabrication is via Molecular Beam
Epitaxy (MBE) or Metal Organic Chemical VapourDeposition (MOCVD) process. With a few exceptions there
is, in general, a lattice mismatch problem involving thick,
multiple hetero-structure layers and the substrate which arerequired for efficient photo-response (8 A W
/C01at 1.5 lm).4
There exist specialized techniques such as buffered or lateral
growth for reducing lattice mismatch, however this result indecreased throughput and increased cost.
Bandhyopadhyay et al. have demonstrated NIR detector
based on photo responsive capacitance based on GaSb nano-wires.
5,6As a result of tunability of capacitance, a shift in
resonant peak frequency (in an LC circuit) is observed and
accordingly a change in the power delivered to the load. Thedetectivity is reported to be 3 /C210
7Jones. The process is
potentially low cost and the device characteristics are
observed to be reproducible and with a satisfactory shelf-life. However, this process is not silicon CMOS compatible.
Further, the device testing scheme requires an in-built, high
frequency, on-chip ac source (100 kHz and above) whichadds to system complexity and cost.
Liuet al. have reported InAs nano-structures based on a
cost-effective thermal CVD process. The nano-wires are sub-sequently suspended in anhydrous ethanol and transferred
onto a silicon (or silicon dioxide) substrate. The responsivity
was reported to be 4.4 /C210
3AW/C01at 532 nm (visible
region).7In contrast, Miao et al. have demonstrated InAsnano-wires grown by MBE process on GaAs substrate. The
maximum responsivity in this case was reported to be
5.3/C2103AW/C01in the visible region; however, photo-
response was observed up until 1470 nm.8
Although the devices discussed above are quite efficient;
however, the fabrication processes are mostly not CMOS com-patible and cost-effective. Nevertheless, in opto-electronic
devices silicon technology is c onsidered inappropriate due to
the indirect nature of the band gap. One way to resolve thisdrawback is to apply Silicide/Silicon Schottky Barrier Diodes
(SBDs) for infra-red detection. The primary advantages of
such diodes are a low (suitable for IR) and a tunable barrierheight (depends on silicide type) formation. Of the possible sil-
icide–silicon combinations, the PtSi/ p-Si SBDs are widely
used in the semiconductor industry. Due to the extensive appli-cation of PtSi SBDs in imaging technology, it has been widely
used in Focal Plane Array.
9The Schottky Barrier Height (SBH)
of PtSi/ p-Si has been reported in the range of 0.22–0.26 eV,10–12
which corresponds to a cutoff wavelength of 4.77–5.64 lm. For
lower cutoff wavelengths (8–10 lm)IrSi/ p-Si SBDs had been
proposed with a barrier height of 0.125–0.152 eV.10,13In con-
trast, for higher cutoff wavelengths ( /C243.7lm), Pd 2Si SBD with
SBH of /C240.33 eV has been used.14,15Hence, such diodes are
operable in the mid and far infrared regions.
This study aims at developing and optimizing SBDs for
detection of NIR. For this purpose, nano-scale nickel silicide
onn-Si diodes was fabricated. Previously, Zhu et al.16have
demonstrated the utility of NiSi 2/n-Si SBDs for NIR
(1.5lm) region with a photo-responsivity of /C242 mA/W. It
was observed that the barrier height of nickel silicide (NiSi)n-Si SBDs is /C240.66 eV;
17–20hence, the cut off wavelength is
/C241.87lm. Therefore, such diodes are suitable for optical
communication application ( k¼1.3–1.5 lm)21and also for
detection of hydrocarbon gases.22The Ni–Si phase diagram
predicts six stable inter-metallic compound (Ni 3Si, Ni 31Si12,a)Electronic mail: sdgupta@ee.iitb.ac.in
0021-8979/2014/116(12)/124507/6/$30.00 VC2014 AIP Publishing LLC 116, 124507-1JOURNAL OF APPLIED PHYSICS 116, 124507 (2014)
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137.44.187.65 On: Tue, 09 Dec 2014 13:01:10Ni2Si, Ni 3Si2, NiSi, and NiSi 2).23NiSi is considered to be
the most promising candidate for electronic devices since it
is stable with very low specific resistivity (of the order of7–10 lX-cm),
23which should result in high photo-
responsivity of nano-scale NiSi/ n-Si SBDs.
In this paper, we have investigated performance of NiSi
SBD. The diode was fabricated by deposition of Ni on Si fol-
lowed by Rapid Thermal Annealing (RTA). The device has
been characterized to investigate the optical response, and itis observed that the cutoff wavelength is around /C242.3lm.
Hence, such devices opened up the possibility in the field of
IR sensor in NIR region. The photo-responsivity of thedeveloped diode is observed to be better than the earlier
reported works.
16However, improvements presumably
results for the improvement of silicide-silicon interfaces.
II. EXPERIMENTAL DETAILS
The device has been fabricated using n-type Si (100) wafer
of resistivity 1–10 X-cm. First of all, Radio Corporation of
America cleaning was performe d to remove native oxide and or-
ganic contaminants from the surface of the wafer. A 100 nmSiO
2layer was grown by wet oxidation process for contact pad
deposition. Back side SiO 2of the wafer was etched by Buffered
Hydro Fluoric (BHF) acid after then nþregion was made by ion
implantation followed by 30 s RTA at 950/C14C. A 0.5 /C21m m2
window was constructed by optical lithography process, and
selective removal of SiO 2w a sd o n ef r o mt h es u r f a c eb yt h e
BHF. Pattering for top electrode was performed on the SiO 2
window for Ni deposition. After patterning, wafer was dipped
into BHF to remove native oxide formed during the process.Following the removal of native oxide, the wafer was immedi-
ately loaded in electron beam evaporator chamber for Ni deposi-
tion. Deposition was performed at a base vacuum of 5 /C210
/C06
mbars. A 10 nm Ni film was deposited on the patterned Si sub-
strate followed by lift-off. Subs equently, RTA was performed at
500/C14C for 60 s for silicide formation. The unreacted Ni was
removed by treating with an acid mixture (HNO 3:HCl¼1:5 for
60 s). Finally, Au was deposited for top contact (1 /C21m m2)
and Ti/Au was deposited for back ohmic contact.
The electrical characterization of diode was performed
using Keithley 4200 instrument. Optical response was meas-
ured using Keithley 2400 under illumination of a tungsten
lamp with a mono-chromator arrangement. Cross-sectional
Secondary Electron Microscopy (SEM) (Raith-150) techniquewas used to investigate the thickness of the silicide. X-ray
Photoelectron Spectroscopy (XPS) (PHI5000VersaProbe-II)
and Raman spectroscopic measurement (RAMNORHG-2S)were performed to get material signature. The area of top sili-
cide contact has been measured using microscope and was
found to be 8.4 /C210
/C04cm2. The schematic diagram of cross-
sectional view of the device is shown in Fig. 1.
III. RESULTS AND DISCUSSIONS
A. Materials characterizations
Raman spectroscopic analysis was performed to verify
the phase composition of the silicide film (Fig. 2) using
514.5 nm argon ion laser (10 mW power) source. The intensepeak observed at 522 cm/C01is attributed to silicon wafer.
This Si peak is significant for our study, which indicates that
all the compositional information of film has been gathered
till the substrate. Another set of four peaks (shown in theinset of Fig. 2) at 199, 217, 294, and 363 cm
/C01are attributed
to the NiSi phase.24,25The peak at 217, 294, and 363 cm/C01
are assigned to A gmode whereas 199 cm/C01assigned to the
B1gmode.26A slight sift ( /C241c m/C01) of peak compared to as
reported by the Karabko et al.26has been observed. A small
shoulder peak observed at 371 cm/C01is attributed to a forma-
tion of NiSi 2phase in the film.27
Peak corrections of XPS spectrum were performed by
carbon (C 1s) peak (at 284.5 eV) position. The spectrum ofthe film is shown in Fig. 3. The peak position at 853.9 eV and
871 eV of Ni2p
3/2and Ni2p 1/2(shown in the inset of Fig. 3(a),
respectively, corresponds to NiSi phase.23Along with that a
small overlapping peak of Ni2p 3/2position has been observed
at 854.6 eV which corresponds to NiSi 2phase. From the low
peak intensity at 854.6 eV, it is concluded that the fraction ofNiSi
2phase present in the film is less than NiSi phase. This
validates the observation of Raman analysis shown in Fig. 2.
The Si 2p spectrum is shown in Fig. 3(b). The peak position
found at 99.4 eV also attributes to NiSi phase. It is verified
from both XPS and Raman analysis as that NiSi phase has
been formed along with a small fraction of NiSi 2.
Cross-sectional SEM imaging was performed to investi-
gate the thickness of silicide film. The image is shown in
Fig. 4indicates that the NiSi film is uniform and the thick-
ness has been found to be 27 nm (shown in the inset of
Fig.4).
FIG. 1. Cross-sectional diagram of device.
FIG. 2. Raman analysis spectrum silicide film by 514.5 nm Ar ion laser
source.124507-2 Roy et al. J. Appl. Phys. 116, 124507 (2014)
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137.44.187.65 On: Tue, 09 Dec 2014 13:01:10The atomic concentration of Ni and Si in nickel-silicide
was calculated by peak intensities using the following
equation:
Cx¼Ix=SxðÞP
iIi=SiðÞ; (1)
where Cx,Ix, and Sxare the atomic concentration, peak inten-
sity, and sensitivity, respectively, of xth element. Thesensitivity value is determined by the instrument manufac-
turer (Ni 2p3/2: 4.04 and Si 2p: 0.339). Argon plasma etch-
ing (etch rate of 2.4 nm/min) was performed to investigatethe depth profile of film. The change in atomic fraction of Ni
and Si with the variation of nano-film thickness is shown in
Fig.5.
It is observed from Fig. 5that the ratio of Ni and Si is
constant for approximately 27 nm. Then, the atomic fraction
of Ni decreases to zero and Si fraction increases to 1. Thisindicates that NiSi phase formed and the composition is uni-
form till 27 nm. The variation of Ni and Si compositional ra-
tio with depth is shown in the inset of Fig. 5. This
observation correlates with the results obtained from SEM
image. Since the volume fraction of NiSi
2is much less in
comparison to NiSi phase, NiSi 2formation is considerable
insignificant.
B. Electrical characterization
1. I-V characterization
The current–voltage (I–V) characteristics of NiSi/n-Si
Schottky diode at different temperatures are shown in Fig.
6(a). The results indicate that the diode is Schottky in nature.
The rectification ratio ( Ion=Iof f) has been observed to be
/C24105at61 V (at room temperature). The forward bias I–V
relation of Schottky diode is expressed as28–30
I¼I0ðexpðeðV/C0IRSÞ=nkTÞ/C01Þ; (2)
where
I0¼A/C3AT2expð/I/C0V
B=kTÞ: (3)
I0is the reverse saturation current which has been calculated
by I–V plot by considering I /C1Rsvalue is very small (R s/C2450
Xfor our device).
The electrical parameter of Schottky diode was
extracted when V>3kT=e. ln(I) vs V plot is shown in the
inset of Fig. 6(b). The Richardson plot (ln(I 0/T2) vs 1000/T)
is shown in Fig. 6(b). Barrier height ( /I/C0V
B) has been
FIG. 3. (a) Ni 2p3/2 XPS spectrum for NiSi film. Inset shows Ni2p1/2 spec-
trum for NiSi film. (b) Si2p XPS spectrum of the film to investigate NISi
phase.
FIG. 4. Cross-sectional SEM image of NiSi/Si interface to investigate film
thickness as well as the interface of the metal semiconductor junction.
FIG. 5. Depth profile of NiSi film to investigate the atomic fraction of the
film with the variation of depth. Inset shows the Ni and Si compositional ra-
tio of the film with variation of depth.124507-3 Roy et al. J. Appl. Phys. 116, 124507 (2014)
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137.44.187.65 On: Tue, 09 Dec 2014 13:01:10calculated from the slope of Richardson plot and it is found
to be 0.62 eV. The barrier height is comparable to as reportedby Chang and Erskine.
18The ideality factor (n) has been cal-
culated at room temperature which is determined to be 1.28.
2. C–V characterization
Capacitance–voltage (C–V) measurement is another
well-established technique to calculate barrier height ( /C/C0V
B)
of the Schottky diode. The 1/C2vs V characteristic of NiSi/
n-Si Schottky diode in the reverse bias voltage (0 V–1 V) at afrequency of 1 MHz is shown in Fig. 7. The Schottky Mottmodel and abrupt junction approximation are implemented
to determine the carrier concentration ( N
d).Ndhas been cal-
culated by following equations:28,31
1
C¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2Vbi/C0V ðÞ
2Ndeess
; (4)
Nd¼2
ees1
d1=C2ðÞ =dV/C20/C21
: (5)
/C/C0V
Bhas been calculated by calculating the intercept ( Vbi)
of 1/C2(¼0) at voltage axis, and using the following
equation:28,29
/C/C0V
B¼VbiþVnþkT
e; (6)
Vn¼kT
elnNc
Nd/C18/C19
: (7)
The value of Ndhas been derived and it is found to be
5/C21015cm/C03. Accordingly, /C/C0V
Bvalue is found 0.76 eV.
For low doped ( /C241015cm/C03) substrate where tunnelling
current is not significant, the relation between the /C/C0V
Band
/I/C0V
Bhas been proposed by Broom et al.32The relation is
expressed as
/I/C0V
Bcal¼/C/C0V
BþVnn/C01ðÞ
n; (8)
where /I/C0V
Bcalis calculated value of zero bias barrier height
(/I/C0V
B) and it has been found to be 0.64 eV which closely
matches to /I/C0V
B(0.62 eV).
C. Optical measurement
The photo-responsivity (R) of NiSi/ n-Si SBD, with
wavelength ( k) under different reverse bias, is shown in Fig.
8(a). The value of R is found to be increasing with decrease
illumination wavelength. Similar characteristics are observed
for different bias conditions. It is observed from Fig. 8(b)
that the responsivity is promising (2.6 mA/W for zero biascondition at 1.5 lm). The photo-responsivity of the SBDs is
approximated by Fowler equation, expressed as
33,34
R¼C11/C0/opt
B
h/C23/C18/C192
; (9)
where C 1is the constant, /opt
Bis barrier height of SBD, and
h/C23is the energy of incident photon. The characteristic of
photo-responsivity at zero bias is shown in Fig. 8(b). Fowler
plot ( h/C23ffiffiffi
Rp
vsh/C23) was made for zero bias condition to calcu-
late the zero bias barrier height (inset of Fig. 8(b))./opt
Bwas
calculated at the intersection of extrapolation of h/C23ffiffiffi
Rp
to the
h/C23axis, and the value has been found to be 0.54 eV. The bar-
rier height value observed in this case is much less than thatderived by I–V and 1/C
2–V method. Such behaviour attrib-
uted to presence of acceptor like trap state at the interface.35
With the incidence of photons on the silicide, the valence
band electrons at interface region are excited and trapped by
acceptor like trap state. Hence, those trap states becomes
FIG. 6. (a) I-V characteristics of NiSi/n-Si SBD at different temperature. (b)
Richardson plot of NiSi/n-Si diode to find out barrier height. Inset shows the
ln(I) vs V to find out I 0value.
FIG. 7. 1/C2vs V plot of NiSi/n-Si Schottky diode measured at 1 MHz.124507-4 Roy et al. J. Appl. Phys. 116, 124507 (2014)
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137.44.187.65 On: Tue, 09 Dec 2014 13:01:10negatively charged. These negatively charge states contribute
to the Fermi energy band. In other word, the Fermi level shift-
ing towards the conduction band occurs, which effectively
reduces the band bending and hence, the barrier height reduc-tion of the SBD occurs. The estimated electrical parameters at
room temperature are listed in Table I.
The variation of photo-responsivity with reverse bias at
different irradiation wavelength is shown in Fig. 9.I ti s
observed that the photo-responsivity increases monotonically
with increase in reverse bias, and the diode response for 1.35
and 1.5 lm has been found to be similar.
The relation between photo cur rent (photo-responsivity) to
the bias voltage for Metal-Sem iconductor-Metal (MSM) diode
has been proposed by Nejad et al.
36which can be expressed as
R¼Roexp/C0B
V/C18/C19
; (10)
where Roand B are constants, which depends on the irradiat-
ing photon energy. The plot of ln(R) vs 1/V plot (shown in
inset of Fig. 9), the linearity of the plot indicates that thephoto-response with bias voltage of this device obey the rela-
tion expressed in Eq. (10).
IV. CONCLUSION
This study demonstrates fabrication of a nano-scale
NiSi/n-Si Schottky infrared detector SBD, fabricated by RTAprocess with top bottom contacts. The formation of NiSi
phase has been confirmed by Raman and depth sensitive XPS
technique. The silicide film thickness has been measured bySEM, which is found to be 27 nm and verified by XPS tech-
nique. The barrier height has been measured by I–V, C–V,
and optical process. The barrier height obtained from I–V isclosely matched with reported values, whereas that evaluated
from optical process differs. The variations of barrier height
have been explained by the presence of acceptor like inter-face trap states. Such trap states capture the photo–exited the
electrons form valence band which further contribute to the
Fermi energy level. Therefore, it eventually lowers the bandbending and reduces the barrier height. The device photo-
responsivity has been observed and found to be promising
comparable to the reported values. The responsivity wasmeasured at different reverse bias conditions and it has been
found that the response follows the relation as proposed by
the earlier works for MSM diode. The responsivity can beenhanced by improving the interface and creating an optical
cavity. Hence, it can be concluded that this diode has exten-
sive potential application in the field of gas detection by IRabsorption method and optical communication.
ACKNOWLEDGMENTS
We would like to express thanks to Mr. H. Singh Bana,
Department of Electrical Engineering, Indian Institute ofTechnology Bombay for his assistance in chemical process and
V. K. Bajpai, Department of Energy Science, Indian Institute
of Technology Bombay for cross-sectional SEM imaging.
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n /I/C0V
B(eV) /C/C0V
B(eV) /I/C0V
Bcal(eV) /opt
B(eV)
1.28 0.62 0.76 0.64 0.54
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FIG. 8. (a) Photo-response of NiSi/n-Si Schottky diode measured at differentreverse bias condition. (b) Photo-response of NiSi/n-Si Schottky diode
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bias barrier height.124507-5 Roy et al. J. Appl. Phys. 116, 124507 (2014)
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1.4896025.pdf | Dual mode acoustic wave sensor for precise pressure reading
Xiaojing Mu, Piotr Kropelnicki, Yong Wang, Andrew Benson Randles, Kevin Tshun Chuan Chai, Hong Cai, and
Yuan Dong Gu
Citation: Applied Physics Letters 105, 113507 (2014); doi: 10.1063/1.4896025
View online: http://dx.doi.org/10.1063/1.4896025
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/11?ver=pdfcov
Published by the AIP Publishing
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129.12.234.99 On: Mon, 15 Dec 2014 00:44:38Dual mode acoustic wave sensor for precise pressure reading
Xiaojing Mu,1,a)Piotr Kropelnicki,1Yong Wang,1,2Andrew Benson Randles,1
Kevin Tshun Chuan Chai,1Hong Cai,1and Yuan Dong Gu1
1Institute of Microelectronics, Agency for Science, Technology and Research (A*STAR), Singapore 117685
2The School of Electrical and Electronic Engineering, Nanyang Technology University, Singapore 639798
(Received 7 August 2014; accepted 3 September 2014; published online 17 September 2014)
In this letter, a Microelectromechanical system acoustic wave sensor, which has a dual mode
(lateral field exited Lamb wave mode and surface acoustic wave (SAW) mode) behavior, is
presented for precious pressure change read out. Comb-like interdigital structured electrodes on topof piezoelectric material aluminium nitride (AlN) are used to generate the wave modes. The sensor
membrane consists of single crystalline silicon formed by backside-etching of the bulk material of
a silicon on insulator wafer having variable device thickness layer (5 lm–50 lm). With this princi-
ple, a pressure sensor has been fabricated and mounted on a pressure test package with pressure
applied to the backside of the membrane within a range of 0 psi to 300 psi. The temperature coeffi-
cient of frequency was experimentally measured in the temperature range of /C050
/C14C to 300/C14C.
This idea demonstrates a piezoelectric based sensor having two modes SAW/Lamb wave for direct
physical parameter—pressure readout and temperature cancellation which can operate in harsh
environment such as oil and gas exploration, automobile and aeronautic applications using the dualmode behavior of the sensor and differential readout at the same time.
VC2014 AIP Publishing LLC .
[http://dx.doi.org/10.1063/1.4896025 ]
Pressure sensors can be found in several harsh environ-
ment application areas, like automotive, aeronautic, or oil-
drilling industry.1–10Different approaches have been used to
sense pressure at higher temperatures. One of these
approaches is represented by piezoresistive SiC pressure sen-
sors, which can be used to monitor the pressure of the inter-nal combustion engine with temperatures greater than
300
/C14C.11,12Unfortunately, the accuracy of the piezoresistive
sensor decreases when the temperature is higher than 100/C14C
due to its drop of resistivity.13High fabrication costs and up
to 300/C14C temperature required in harsh environment require-
ment, creates great demand for new sensor solutions withhigher reliability compared to the aforementioned ones.
A promising approach for high temperature operation
fell on quartz based resonators, which have been well knownas high pressure sensors in harsh environment for a long
time.
14,15A commonly used film bulk acoustic resonator
(FBAR) structure has top and bottom electrodes, which helpto generate a bulk acoustic wave (BAW) within the quartz
material. The pressure information is derived by its resonant
frequency read out, which is strongly dependent on stress andtemperature of the piezoelectric material. For this reason, a
temperature dependent and pressure independent reference
sensor is highly needed to calibrate out the temperatureeffect, which complicates the whole system. Thus three reso-
nators are indispensable for a whole system to extract temper-
ature and pressure separately at the same time.
In this letter, a piezoelectric material AlN based dual
mode acoustic wave sensor including surface acoustic wave
(SAW) and Lamb wave is developed. This sensor is capableof operating at large temperature ranges from /C050
/C14Ct o300/C14C and larger pressure ranges from 0 psi to 300 psi. The
temperature behaviors of the sensor among these dual modes
are almost the same, whereas the pressure sensitivity behav-iors are totally different.
With the assistance of a external digital circuit, the tem-
perature effects of the dual mode acoustic wave sensor arelikely cancelled out, which results in the sole physical pa-
rameter (pressure change) readout.
The fabrication process is Complementary Metal-Oxide-
Semiconductor (CMOS) compatible. 8 in. SOI (100) wafers
with device layer of 5 lm and 50 lm with buried oxide
(BOX) layer of 1 lm were employed. A 100 nm SiO
2layer
was first deposited on the SOI substrate by plasma enhanced
chemical vapor deposition (PECVD). After that, a 2 lmA l N
piezoelectric layer was deposited by physical vapor deposi-tion (PVD). Then, a 600 nm Al film was grown on AlN and
patterned by dry etch to form the Interdigitated Transducer
(IDT) structure. A 200 nm SiO
2was deposited by PECVD
and served as hardmask for IDT patterning. After front side
process, the silicon substrate layer was thinned down to
400lm by mechanical grinding. Next, a 2 lm SiO 2hardmask
layer was deposited on the backside of the wafer for release
process. Finally the silicon membrane structure was released
by deep reactive ion etching (DRIE). Finally, front side SiO 2
is removed by vapor hydrogen fluoride (HF) for contact open.
Figure 1shows the SEM of the IDT electrodes and the cross-
sectional structure of the acoustic wave pressure sensor.
Finite element method (FEM) simulations have been
carried out by using COMSOL to investigate the perform-
ance of the dual mode sensors. 2D simulation was performedusing periodic condition on left and right side of the device
in order to simulate an infinite, ideal resonator plate. Based
on the prior arts,
16,17the material properties that are used for
simulations are summarized in Table I.a)Author to whom correspondence should be addressed. Electronic
addresses: mux@ime.a-star.edu.sg and mxjacj@gmail.com
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129.12.234.99 On: Mon, 15 Dec 2014 00:44:38As observed from Table I, the elastic constants are
strongly dependent on temperature, which are almost homo-geneously, especially for the first order temperature coeffi-
cient. The elastic coefficients are inherently, in the same
time, strain/stress dependent.
Based on these assumptions, the resonance frequency of
both modes is dependent on the change of the elastic coeffi-
cients due to temperature and strain
f
s¼vph
k;vph¼ffiffiffiffi ffic/C3
pr
;c/C3¼fct P ;TðÞ ; (1)
where fsis the resonance frequency, vphis the phase velocity,
kis the wavelength, pis the density, and c*is the elastic
coefficient of AlN.
According to theory, with thinner membrane, the SAW
mode is moving into a S 0Lamb wave mode as can be seen in
Fig.2. This behavior is normally defined as phase velocity
dispersion. Meanwhile, a higher order mode Lamb wave also
presents in thick Si membrane devices with strong energy
behavior. This higher order Lamb wave exhibits a stablephase velocity throughout a large range of the h(Si)/k.
(Lamb wave high mode as shown in Fig. 2).
From the simulation predictions (as shown in Fig. 2),
the SAW mode can be excited at 480.87 MHz when the
membrane thickness is larger than one wavelength ( >10lm
in this case). Most energy of the SAW is concentrated in thedepth of one wavelength ( k). Simultaneously, a strong Lamb
wave high mode Lamb wave of 973.79 MHz is also observed
in this sensor. In this higher order mode, the silicon serves as
a transmitting medium, but the energy of the wave is obvi-ously decayed throughout the silicon thickness.
The dual mode pressure sensor with 50 lm thickness
silicon membrane is fabricated out for experimental testing.The acoustic wavelength of the device is designed to be
10lm, which corresponds to an IDT electrode finger width
of 2.5 lm. The length and the amount of the IDT electrodes
are 1280 lm and 128 pairs, respectively. To serve as pres-
sure sensor, a 1 mm diameter membrane is formed in thecenter to support the IDT structure. To determine the tem-
perature dependency of resonance frequency of the acoustic
wave pressure sensor, high temperature measurements in arange of /C050
/C14C to 300/C14C were carried out. By using a
Cascade PMV200 vacuum probe station and an Agilent
E5071B network analyzer, S-parameters were measured ata series of increasing temperatures. Short-Open-Load-
Through (SOLT) method was performed to calibrate the
measurand in network analyzer. The chuck of the probe sta-tion was heated up with 20 min dwell time before measure-
ment data were collected. In order to reduce the overall
measurement noise, an average factor of 10 was selectedduring the measurement.
Figure 3(a)indicates a second order relationship between
resonance frequency and temperature of the SAW modewithin a range of /C050
/C14C to 300/C14C. As it can be obtained
from this figure, the approximated first order and second
order temperature coefficient of frequency is extracted to beTCF¼/C021.14 ppm/
/C14C and TCF2 ¼/C023.53 ppb//C14C2respec-
tively, which shows comparable behavior with what have
been reported in previous literatures.17Experimentally, the
resonance frequency peak of SAW mode is found at
478 MHz, and this measurement data have a good agreement
with FEM simulations (480.87 MHz). At the same time, ahigher frequency peak of 988 MHz (Higher order Lamb wave
mode) (Fig. 3(b)) is also observed with a strong energy,
which shows similar temperature characteristics with theSAW mode due to likewise stiffness coefficients as described
before.
With respect to pressure coefficient of frequency (PCF)
characterization, pressure was applied on the backside of the
membrane in a range of 0 psi to 300 psi using pressurized sil-
icone oil (as shown in Fig. 4(c)). The devices were mounted
to an adapter with liquid epoxy and cured at 170
/C14C. The
adapter was then connected to a pressure controller by a pipe
to facilitate coupling the pressurized silicone oil flow to themembrane. Metal wires were bonded to the contact pads on
the MEMS device to measure pressure dependent resonance
frequency change by the network analyzer.
The relationship between resonance frequency and pres-
sure of the SAW and Lamb wave mode within the range of
0 psi to 300 psi are demonstrated in Figs. 4(a) and 4(b),
respectively. Obviously, a positive PCF of þ0.227 ppm/psi
is derived from Fig. 4(a) for SAW mode, while a negative
PCF of /C00.617 ppm/psi is obtained for Lamb wave mode.
This can be explained: like mentioned above, stress induced
frequency shift by external applied pressure on different
modes is dominated by different elastic constants.
FIG. 1. SEM of (a) top view and (b) the cross-sectional view of the fabri-
cated high temperature acoustic wave pressure sensor.113507-2 Mu et al. Appl. Phys. Lett. 105, 113507 (2014)
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129.12.234.99 On: Mon, 15 Dec 2014 00:44:38In the previous sections, temperature and pressure sen-
sitivity of our sensor were discussed. Due to the fact that
the temperature behavior is almost the same for the bothmodes while the pressure behavior differs, this behavior
leads to possible temperature compensation methods for
readout designs. Dual-mode MEMS resonator is driven byexternal connected oscillator circuits,
18the two resonant
frequencies ( fLamb andfSAW) of which are generated and fur-
ther quantized into digital signal through the digital coun-ters. Once resonant frequencies are read out, a ratio ncan
be calculated asTABLE I. The material used for simulation.
AlN Si SiO 2 Al
Elastic constants,
cij[GPa]c11 410.06 Young’s modulus,
E[GPa]170 70 70
c12 100.69
c13 83.82
c33 386.24
c44 100.58
c66 154.70
First order temperature coefficient
of elastic constants, Tcij[10/C06/K]Tc11 /C010.65 first order temperature coefficient of
Young’s modulus, TCE [10/C06/K]/C063 204 …
Tc12 /C011.67
Tc13 /C011.22
Tc33 /C011.13
Tc44 /C010.82
Tc66 /C010.80
Second order temperature coefficient
of elastic constants, T2cij[10/C09/K2]T2c11 /C020.61 second order temperature coefficient of
Young’s modulus, TCE2 [10/C09/K2]/C052 221 …
T2c12 /C019.51
T2c13 /C019.88
T2c33 /C020.03
T2c44 /C020.36
T2c66 /C020.39
Piezoelectric stress coefficients, eij[C/m] e15 /C00.48 … … …
e31 /C00.58 … … …
e33 1.55
Relative permittivity, eij e11 9 11.7 4.2 …
e33 11 11.7 4.2 …
Thermal expansion, aij[10/C06/K] a11 5.27 2.6 0.55 18
a33 4.15 2.6 0.55 18
Mass density, q[kg/m3] 3260 2329 2200 2700
FIG. 2. Simulation on the wave behavior for SAW mode and Lamb wave
mode.
FIG. 3. Measured temperature behavior for: (a) SAW mode; (b) Lamb wavemode.113507-3 Mu et al. Appl. Phys. Lett. 105, 113507 (2014)
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129.12.234.99 On: Mon, 15 Dec 2014 00:44:38n¼fLamb
fSAWwith n/C252:066661 ;
where fLamb and fSAWare the resonant frequencies of the
Lamb mode and SAW mode of the dual mode resonator at25
/C14C, respectively. Beat frequency is defines asDf¼fLamb/C0n/C2fSAW: (2)
This frequency can be obtained by feeding the two frequen-
cies into subtractor and multiplier circuits. The fSAWis multi-
plied by the frequency ratio nand then has a subtraction
calculation with fLamb. Figure 5depicts that beat frequency
Dfvaries separately versus temperature and pressure, in
range of /C050/C14C to 300/C14C and 0 psi to 300 psi, respectively.
As shown in Fig. 5,Dfis approximate constant within the
temperature range, implying that it is insensitive to tempera-ture; whereas, the ramp line corresponding to the pressure
(ranges from 0 psi to 300 psi) indicates a superior sensitivity.
Thus, a precise pressure reading is realized by this dualmode sensor-digital circuit system.
We present prototype of a MEMS dual mode resonator
for precise pressure monitoring. Comb-like interdigital electro-des on the top of piezoelectric material-Si stack membrane is
employed to generate waves. The waves generated in this sen-
sor mainly have two different modes (SAW and Lamb wave).The TCF of these two modes have been experimentally veri-
fied almost the same, while th e PCF of them differs. Benefits
from dual mode feature, the temperature induced frequencyshift is likely suppressed through the logical operation on two
readout frequencies by the integrated digital circuit. All in all,
more precise pressure readout is realized by utilizing such dual
mode resonator-oscillator-digital circuits system.
This work was supported by the Agency for Science,
Technology and Research (A*STAR) under Science andEngineering Research Council (SERC) Grant No. 1021650084.
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mode; (c) the setup of pressure measurement.
FIG. 5. Relative change of beat frequency for temperature and pressure.113507-4 Mu et al. Appl. Phys. Lett. 105, 113507 (2014)
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1.4895635.pdf | Cat-doping: Novel method for phosphorus and boron shallow doping in crystalline
silicon at 80°C
Hideki Matsumura, Taro Hayakawa, Tatsunori Ohta, Yuki Nakashima, Motoharu Miyamoto, Trinh Cham Thi,
Koichi Koyama, and Keisuke Ohdaira
Citation: Journal of Applied Physics 116, 114502 (2014); doi: 10.1063/1.4895635
View online: http://dx.doi.org/10.1063/1.4895635
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/11?ver=pdfcov
Published by the AIP Publishing
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58Cat-doping: Novel method for phosphorus and boron shallow doping
in crystalline silicon at 80/C14C
Hideki Matsumura, Taro Hayakawa, Tatsunori Ohta, Yuki Nakashima, Motoharu Miyamoto,
Trinh Cham Thi, Koichi Koyama, and Keisuke Ohdaira
Japan Advanced Institute of Science and Technology (JAIST), Asahidai, Nomi-shi,
Ishikawa-ken 923-1292, Japan
(Received 7 August 2014; accepted 2 September 2014; published online 16 September 2014)
Phosphorus (P) or boron (B) atoms can be doped at temperatures as low as 80 to 350/C14C, when
crystalline silicon (c-Si) is exposed only for a few minutes to species generated by catalytic
cracking reaction of phosphine (PH 3) or diborane (B 2H6) with heated tungsten (W) catalyzer. This
paper is to investigate systematically this novel doping method, “Cat-doping”, in detail. The electri-
cal properties of P or B doped layers are studied by the Van der Pauw method based on the Hall
effects measurement. The profiles of P or B atoms in c-Si are observed by secondary ion mass spec-trometry mainly from back side of samples to eliminate knock-on effects. It is confirmed that the
surface of p-type c-Si is converted to n-type by P Cat-doping at 80
/C14C, and similarly, that of n-type
c-Si is to p-type by B Cat-doping. The doping depth is as shallow as 5 nm or less and the electri-cally activated doping concentration is 10
18to 1019cm-3for both P and B doping. It is also found
that the surface potential of c-Si is controlled by the shallow Cat-doping and that the surface recom-
bination velocity of minority carriers in c-Si can be enormously lowered by this potential control.
VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4895635 ]
I. INTRODUCTION
Impurity doping to crystalline silicon (c-Si) at low tem-
peratures is required for fabrication of various devices such
as ultra-large scale integrated circuits (ULSI), thin film tran-
sistors for displays, and solar cells. Shallow doping of impur-ities is also required in fabricating ULSI and other devices,
apart from the control of surface potential of c-Si solar cells.
We have discovered that phosphorus (P) atoms can be
doped into c-Si at substrate temperatures lower than 350
/C14C
and that the surface of p-type c-Si is converted to n-type byP doping
1,2when c-Si surface is exposed to the ambient of
species generated by catalytic cracking reaction of phosphine
(PH 3) gas with heated tungsten (W) catalyzer. Then, we have
attempted to dope boron (B) atoms into c-Si similarly by
using diborane (B 2H6) instead of PH 3. However, the detailed
story about this novel low temperature doping method,named “Cat-doping”, has not been clearly mentioned. This
paper is to demonstrate systematically the results of investi-
gation on Cat-doping in detail.
After confirming that metal contamination originating
from heated W catalyzer is negligible, the electrical proper-
ties of P or B doped layers are studied by the Van der Pauwmethod based on the Hall effects measurement. The profiles
of P or B atoms in c-Si are observed by secondary ion mass
spectrometry (SIMS), mainly from back side of samples toeliminate influence of knock-on effects of probing ions. The
P profiles after doping through thin oxide layers on c-Si are
also observed to confirm the doping depth. It is found thatthe surface of p-type c-Si is converted to n-type by P Cat-
doping at 80
/C14C, and similarly, that of n-type c-Si is to p-type
by B Cat-doping. The doping depth is as shallow as 5 nm orless and the doped carrier concentration is 10
18to 1019cm/C03
for both P and B doping. In addition, as possible applicationof this novel technology, the control of c-Si surface potential
is attempted by the shallow Cat-doping, and it is found that
the surface recombination velocity of minority carriers in
c-Si is enormously lowered by this potential control.
II. FUNDAMENTALS FOR EXPERIMENTS
A. Apparatus and process parameters
The Cat-doping experiments were carried out by using
the conventional apparatus for catalytic chemical vapor dep-
osition (Cat-CVD), often called Hot-Wire CVD. A schematic
view of the typical Cat-doping apparatus is shown in Fig. 1.
A stainless steel chamber with a diameter of 30 cm and a
height of 30 cm was used as the apparatus and tungsten (W)
wires with a diameter of 0.5 mm and a length of about 2 mwere used as catalyzers. Experimental parameters of Cat-
doping are summarized in Table I. In the table, the tempera-
ture of catalyzer, the surface area of the catalyzing wire, thesubstrate temperature, the gas pressure during Cat-doping
process, the flow rate of gas X and the distance between the
catalyzer and the substrates are referred to as T
cat,Scat,Ts,
Pg, FR(X) and D cs, respectively. Both PH 3and B 2H6gases
FIG. 1. Schematic view of Cat-doping apparatus.
0021-8979/2014/116(11)/114502/10/$30.00 VC2014 AIP Publishing LLC 116, 114502-1JOURNAL OF APPLIED PHYSICS 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58were diluted to 2.25% by helium gas, however, here, the
flow rate of doping gas is expressed by net values.
For both Cat-CVD and Cat-doping, T catis one of the
most important parameters. However, contrary to CVD, to
suppress the surface etching due to hydrogen (H) atoms, pro-duced at catalyzer from H
2gas or hydrogenated doping gas
such as PH 3or B 2H6,Tcatfor Cat-doping is lowered from
about 1800/C14C of Cat-CVD to about 1300/C14C as mentioned
below. In addition, the surface contamination of impurities
originating from the catalyzer should be carefully avoided.
Thus, at first, the effect of surface etching on T catwas inves-
tigated, and then, the contamination from the catalyzer was
studied.
B. Surface roughness due to etching during process
Figure 2shows the surface roughness of c-Si as a func-
tion of T cat, after exposure to H atoms generated from H 2at
W catalyzer. P g, FR(H 2), and the process times were 1 Pa,
20 sccm and 60 s, respectively. The surface roughness was
measured on an atomic force microscope (AFM) of Digital
Instruments NanoScope, IIIa, and the roughness itself isexpressed by root mean square (RMS) of measured values.
The surface roughness is likely to increase as T
catincreases.However, it is only 0.2–0.3 nm for T catof 1300/C14C, although
the RMS of original c-Si surface is also around 0.2 nm. Fromthe figure, it is known that the surface roughness is negligible
up to T
catof 1300/C14C. Thus, in Cat-doping, T catis mainly
fixed at 1300/C14C for P Cat-doping and kept at lower than
1300/C14C for B Cat-doping, except for experiments of other
purposes.
C. Surface contamination during process
Next, the surface contamination was studied by the total
reflection X-ray fluorescence (TXRF), using Rigaku,TXRF3750S and the Rutherford back-scattering (RBS),
using an accelerator of Nisshin-Highvoltage, NISSHIN-
1700 H. Since direct observation of contaminants on surfaceof c-Si is not easy, the contamination inside a-Si and silicon
nitride (SiNx) films both deposited by Cat-CVD was
observed to know flux density of contaminants emitted fromheated catalyzers. Figure 3demonstrates the W concentra-
tion in deposited a-Si films as a function of T
cat. In the figure,
apart from our data taken by TXRF and RBS, SIMS datareported by other two groups
3,4are plotted together. In RBS,
2.0 MeV helium ions were used as incident probing ions.
The W concentration in a-Si is likely to increase as T cat
increases .In this case, the deposition rate (DR) of variousTABLE I. Parameters for Cat-doping of P and B atoms into c-Si.
P Cat-doping B Cat-doping
Temperature of catalyzer, T cat RT-1800/C14C RT-1800/C14C
mainly 1300/C14C mainly <1300/C14C
Surface area of catalyzer, S cat 31 cm231 cm2
Temperature of substrate, T s RT–350/C14C RT–350/C14C
Gas pressure during process, P g 0.5–3 Pa 0.5–3 Pa
Flow rate of PH 3, FR(PH 3)P H 3is diluted to 2.25% by helium 0–0.6 sccm —
Flow rate of B 2H6, FR(B 2H6) B2H 6is diluted to 2.25% by helium — 0–4 sccm
Flow rate of H 2, FR(H 2) Sometimes H 2is added to doping gas 0–20 sccm 0–20 sccm
Distance between catalyzer and substrate, D cs 12 cm 12 cm
Process time 0.5–240 min 0.5–240 min
FIG. 2. Surface RMS roughness of c-Si by H etching vs. T cat.
FIG. 3. W concentration in a-Si films vs. T cat.114502-2 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58a-Si films is 0.5–4.0 nm/s and T s¼150–400/C14C. The deposi-
tion conditions for each a-Si film are not same, however,
they are in a certain range, that is, P g, FR(SiH 4) and D cswere
1–5 Pa, 3.6 sccm, and about 1–5 cm, respectively.
Figure 4demonstrates the sheet density of all impurities
which can be detected by TXRF for SiNx films prepared atT
catof 1800/C14C by 40 runs of deposition with same deposi-
tion conditions.5In this case, S cat, FR(SiH 4), FR(ammonia,
NH 3), P g,T s, and D cswere 88 cm2, 15 sccm, 300 sccm,
10 Pa, 400/C14C and about 4 cm, respectively. DR was about
70 nm/min.
In the films, apart from W, impurities such as iron (Fe),
vanadium (V), nickel (Ni), manganese (Mn), zinc (Zn), chro-
mium (Cr), copper (Cu), and titanium (Ti) were detected.
Real origin of these contaminants is not clear, however, it isbelieved that the most of impurities come from W wires as
impurities contained in W. In the present experiments, high
purity W wires supplied by Allied Materials Corp. wereused. The total densities of various impurities are less than
10
11cm/C02for T cat¼1800/C14C. In TXRF, the measured depth
is believed to be about 10 nm from penetration depth ofX-ray. That is, the atomic density of impurities is roughly
estimated to be 10
17cm/C03at T catof 1800/C14C. The value is
also equivalent to that shown in Fig. 3for a-Si deposition.
Since DR of the film was 70 nm/min, the total number of
impurities in a newly grown layer per a minute was esti-
mated to be 7 /C21011cm/C02. This means that the flux density
of sum of all impurities emitted from the catalyzer is
7/C21011cm/C02min/C01for T catof 1800/C14C, D csof 4 cm, and
Scatof 88 cm2. The value of S catis larger and D csis shorter
than the present conditions for Cat-doping, and thus, the data
appear to make the contamination more serious than the
present Cat-doping.
From Fig. 3, it is found that the contamination is low-
ered by about 5 orders of magnitudes when T catis lowered
from 1800/C14C to 1300/C14C. That is, the flux density of contam-
inants is around 7 /C2106cm/C02min/C01at maximum. As men-
tioned later, it is found that the sheet carrier density of
doping impurities such as P and B atoms is on the order of10
12cm/C02for process time of 60 s. This means that the influ-
ence from contamination of any impurities is negligible for
Cat-doping when T catis kept at around 1300/C14C.D. Measurement of electrical properties for Cat-doped
samples
The electrical properties of Cat-doped c-Si were meas-
ured by the Van der Pauw method, based on the Hall effects
measurement. The size of rectangular samples was usually
10 mm /C210 mm, and four electrodes with a diameter of
1 mm were formed at the four corners of rectangular sam-
ples. In the Van der Pauw method, the size of electrodes is
required as small as possible. However, the present configu-ration appears enough to obtain the data within error of 10%,
according to the model measurements using a simple c-Si
sample. The measurement was carried out by using the Halleffects measuring system, Bio-Rad, HL5500PC, mainly at
room temperature, but, in some cases, to know the activation
energy, it was carried out in various temperatures from200 K to 310 K.
When the doping depth is shallow, the carrier concentra-
tion has a possibility to be influenced by surface defects ofc-Si samples. If surface potential of c-Si is forced to be bent
because of the surface defects, the measurements of doped
carrier density may be affected. Therefore, here, Cat-dopedc-Si samples were coated with an intrinsic (i-) amorphous-
silicon (a-Si) film, since the interface between a-Si and c-Si
is known as almost perfect
6and a-Si films are often used as
passivation films for c-Si surface.7In addition, electric con-
duction through a-Si is possible, although good conduction
will not be expected if insulating films such as silicon-dioxide (SiO
2) are used as passivation instead of a-Si.
Figure 5shows two types of electrodes used in the
experiments. One is n-type-a-Si/aluminum (Al) stacked elec-trodes A, Fig. 5(a), and the other only Al but on thin i-a-Si
layer, electrodes B, Fig. 5(b).
Figure 6shows the sheet carrier density and the carrier
mobility of P Cat-doped samples, as a function of the thick-
ness of coated a-Si films. The Cat-doped samples were pre-
pared with T
s¼150/C14C, P g¼1 Pa, FR(PH 3)¼0.43 sccm and
process time of 10 min. The sheet carrier density of the sam-
ple without a-Si coating film could be measured only by the
stacked electrodes A. The figure shows that coating by a-Simakes measured data stable at about 3–5 /C210
12cm/C02for
case of both electrodes A and B, although the value,
1010cm/C02, obtained without i-a-Si coating appears ambigu-
ous due to the effect of surface defects. The mobility of
doped layer is about 200 to 300 cm2/Vs, and the value
FIG. 4. Sheet concentration of contaminants in Cat-CVD SiNx films.
FIG. 5. Structure of electrodes for Van der Pauw measurements: (a) i-a-Si
(0-10 nm)/nþ-a-Si/metal and (b) i-a-Si(0-10 nm)/metal.114502-3 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58appears reasonable for P doped c-Si. That is, the influence of
i-a-Si itself does not affect measured data of sheet carrierdensity. From the figure, here, the electrical properties of
Cat-doped samples were evaluated after coating c-Si surface
with an about 10 nm-thick i-a-Si layer and evaporating Alwith a diameter of 1 mm on them.
E. Measurement of profiles by SIMS
As demonstrated below, Cat-doped P atoms distribute at
the depth of about several nm. For observation of such shal-low doped impurities, some methods of surface analysis such
as X-ray photo-emission spectroscopy (XPS) and Auger
electron spectrometry would be considered. However, asalso demonstrated below, the concentration of P atoms is
usually less than 1 atomic %, that is, less than detection limit
of ordinary designed measuring equipment. Thus, here, wechose SIMS to know profiles of doped atoms, although the
precise measurements of P profiles by SIMS are not so easy
when the doping depth is as shallow as several nm.
For instance, the measurement of P atoms with a mass
number of 31,
31P, is likely to suffer from the interference of
fragments with the same mass number, formed by the combi-nation of Si isotope of a mass number 30,
30Si, with H atom
of a mass number of 1,1H. Here, for distinguishing the mass
difference between31P and30Siþ1H, we used a high mass
resolution SIMS system with a magnetic mass-analyzer
using 5 kV primary ions of cesium (Cs), CAMECA, IMS-7F.
However, since the relatively high energy is used as primaryions to increase mass resolution, the depth resolution is
sacrificed for it. Thus, for the m easurements with high depth re-
solution, we used a different SIMS system, PHI, ADEPT 1010,using a quadrupole mass analyze r with probe ions of 1 keV.
Figure 7shows the P profiles observed by both IMS-7F
and ADEPT 1010, to know the difference of these two sys-tems. P Cat-doping was carried out with T
s¼80/C14C,
Pg¼1 Pa, FR(PH 3)¼0.43 sccm and process time of 5 min,and the Cat-doped c-Si was coated with a 60 nm-thick i-a-Si
prepared by Cat-CVD with T cat¼1750/C14C, T s¼90/C14C,
Pg¼0.5 Pa and FR(SiH 4)¼10 sccm. From the figure, it is
known that the depth profiles of P atoms measured by twosystems show the different penetration depth and that the
penetration depth by the high mass resolution SIMS looks
more than 2 times deeper than that by the high depth resolu-tion SIMS. Contrary to it, the peak density of P atoms at
interface between c-Si and coated a-Si for the high mass re-
solution system is about 1/10 smaller than that for the highdepth resolution system.
In addition, SIMS has another problem originated from
the knock-on effect by probing ions. Figure 8shows the
SIMS profiles of P atoms for the Cat-doped sample prepared
with T
s¼80/C14C and process time of 60 s. In the figure, two
profiles are shown. One is conventional and measured fromFIG. 6. Sheet carrier density and Hall mobility as a function of thickness of
inserted i-a-Si layers for P Cat-doping.
FIG. 7. Concentration of P atoms vs. depth, observed by two types of SIMS
systems for high depth resolution and high mass resolution.
FIG. 8. P Concentration vs. depth, observed by high depth resolution SIMS
from both front side and back side of samples.114502-4 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58the front side of sample. The other is measured from the
back side of sample by etching. It is clear that when the pro-
file is observed from the front side, it shows the exponentialdistribution, but when it is observed by the back side, the
measured profile looks different. It appears to follow com-
plementary error function ( erfc)o r Gauss distributions.
Thus, here, we discussed the profiles mainly by using the
SIMS data measured from the back side of samples, although
the data taken from the front side were often used to knowthe number of total doped atoms.
III. RESULTS OF CAT-DOPING
A. Electrical properties
At first, the conduction type of Cat-doped samples was
checked by the Hall effects measurement. The Hall voltage
of the samples were measured under the magnetic flux of
0.32 T, applied normally to the sample surface. Figure 9
demonstrates the Hall voltages as a function of applied cur-
rents for both P and B Cat-doped samples. P Cat-doping was
carried out to B-doped p-type c-Si with hole concentration of10
13to 1014cm/C03, and B Cat-doping was also to P-doped n-
type c-Si with electron concentration of 1013to 1014cm/C03.
In the figure, similar Hall voltage of an original p-type c-Siis demonstrated for comparison. T
sfor Cat-doping was
350/C14C in this case, however, the results for the sample of
Ts¼80/C14C were not so different from those shown here.
From the figure, it is confirmed that the conduction type of P
Cat-doped sample is converted to n-type from original
p-type, and also that n-type c-Si is converted to p-type by BCat-doping. This demonstrates that the conduction type can
be converted by Cat-doping for T
smuch lower than tempera-
tures for the conventional impurity doping by thermaldiffusion.
Figure 10shows the sheet carrier concentration and the
conduction types as a function of T
catfor P Cat-doping into
p-type c-Si mentioned above. In this case, T swas 80/C14C. Asshown in the figure, the sheet carrier density is likely to
increase as T catincreases. However, the conduction type is
kept p-type for T catlower than 800/C14C, and it is converted to
n-type when T catexceeds 1000/C14C. According to the recent
reports by Umemoto et al.,8PH3is decomposed to P and H
by cracking on W catalyzer heated over 1000/C14C, and the
amount of such cracked species increases exponentially as
Tcatincreases. This clearly demonstrates that the existence of
species generated by cracking of PH 3is essentially necessary
for this low temperature Cat-doping.
As explained below, P atoms distribute at the depth of
several nm in c-Si. If doping depth of P atoms is approxi-
mated to 5 nm, since the sheet carrier concentration is at theorder of 10
12cm/C02as shown in Fig. 10, the carrier concentra-
tion of doped P atoms at near to c-Si surface is estimated to
be 1018–1019cm/C03. The value appears enough to convert the
conduction type.
In case of B Cat-doping, the situation is a little bit com-
plicated. W surface is easily converted to W-boride duringprocess, and this appears to reduce the reproducibility of B
Cat-doping. In addition, B
2H6is easily thermally decom-
posed. For instance, when T sis 350/C14C, the surface of n-type
Si is converted to p-type by B doping even if the catalyzer is
not heated. B 2H6is thermally decomposed for T sover about
300/C14C and the simple thermal diffusion appears to occur for
B doping at T s¼350/C14C. However, when T sis 80/C14C, B dop-
ing cannot be detected when the catalyzer is not heated.
Figure 11demonstrates the sheet carrier density of B
Cat-doped samples as a function of T cat, taking T sas a pa-
rameter. B Cat-doping was carried out into n-type c-Si with
electron carrier concentration of 1013to 1014cm/C03as similar
as the case mentioned in Fig. 9. When T sis kept at 80/C14C, the
effect of B Cat-doping is clear, and the conduction type is
converted from original n-type to p-type as T catincreases at
over 500/C14C. T catrequired for the conversion of conduction
type appears different from that for P Cat-doping and much
lower than that. In addition, when T sis 350/C14C, the surface of
c-Si is already converted to p-type by the simple thermal dif-
fusion in addition to Cat-doping. For T catover 1000/C14C, the
FIG. 9. Hall voltages vs. applied currents for P Cat-doped, B Cat-doped and
original p-type c-Si samples.FIG. 10. Sheet carrier density of P Cat-doped c-Si as a function of T cat.114502-5 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58sheet carrier density for the samples at both T s¼80/C14C and
350/C14C is likely to decrease. This may be caused by forming
W-boride on W catalyzing wires and losing sufficient species
to control the carrier density of c-Si. The reproducibility of
B Cat-doping for T catover 1000/C14C appears quite low. The
sheet carrier density for T catover 1000/C14C is likely to fluctu-
ate depending on how long the catalyzer is used. For B
Cat-doping, T catlower than 1000/C14C appears better.
B. Profiles of Cat-doped atoms
In SIMS measurements, if a real P profile is so sharp,
approximately a delta function, the observed profile becomes
aGauss distribution ( Gaussion ) due to the depth resolution
of measuring system.
Figure 12demonstrates the plots of P profile which have
been already shown in Fig. 8as a profile observed from the
back side of a sample. When the surface of c-Si is exposed
to P-related species with constant concentration, doped Pprofile should be expressed by erfc as a solution of the con-
ventional diffusion equation. The profile in the figure appears
to follow erfc. However, the difference between erfc and
Gaussian is quite small, and the profile appears also to fol-
lowGaussian as shown in Fig. 12. This may mean that the P
Cat-doped depth is so shallow, several nm or less, and equiv-alent to the value of depth resolution.
Figure 13also demonstrates two P profiles for Cat-
doped samples with T
s¼80/C14C, P g¼1 Pa, process times of
1 min and 4 min. When the process times increases, the pro-
file appears to spread slightly. However, the difference of
two profiles is quite small and sometimes depends on the dif-ference of depth resolution for each measurement. Actually,
when the process times increases to 16 min, the profiles are
not so different from that of 4 min, although the expansion ofdistribution is expected depending on the root of times. The
time dependence of the profiles will be discussed later.
C. Activation energy of carrier density
From the above results, it is known that the doping depth
is limited at a region adjacent to the surface. In that case,
how P atoms are incorporated in c-Si structure? To know it,
next, we measured the temperature dependence of the sheetcarrier density of P Cat-doped sample prepared with
T
s¼80/C14C, P g¼1 Pa and process time of 60 s. Figure 14
shows the sheet carrier density of such a sample as a functionof reciprocal of measured temperatures for range from 200 K
to 310 K. Since the absolute values of sheet carrier density
are likely to fluctuate for sample to sample, it is expressed byarbitrary unit to avoid confusion. The activation energy for
this temperature range is believed to show the effect of dop-
ing impurity. The value is about 0.045 eV and appears nor-mal for the c-Si in which P atoms are substituted into Si-sites
and working as donors. That is, P atoms appear to work as
similar as those incorporated into c-Si by high temperaturethermal processes.FIG. 11. Sheet carrier density of B Cat-doped c-Si as a function of T cat.
FIG. 12. P concentration vs. depth by SIMS from the back side. Fitting
results using erfcandGaussian are also shown for comparison.
FIG. 13. P concentration vs. depth by SIMS from the back side for Cat-
doped samples with process times of 1 and 4 min.114502-6 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58D. P Cat-doping through thin oxide layer
Since P Cat-doping depth is so shallow and it does not
appear easy to know exact doping depth from SIMS data, we
attempted to dope P atoms through very thin silicon-oxide
(SiOx) layers formed on c-Si surface and to measure thesheet carrier density after removing SiOx layers of various
thicknesses. The SiOx layer is prepared by dipping c-Si in
boiled hydrogen peroxide (H
2O2) solution for several
minutes at about 90/C14C. Since the control of SiOx thickness
is not easy, we selected the c-Si samples with various thick
SiOx layers. The thickness of SiOx layers is measured on anellipsometer of Woolam, V-VASE. After P Cat-doping, the
SiOx layer is removed by 2% hydro-fluoric acid (HF) solu-
tion and after that the surface is immediately covered with a10 nm-thick Cat-CVD i-a-Si layer for Van der Pauw
measurements.
Figure 15demonstrates the sheet carrier density and the
conduction types as a function of the process times, after
p-type c-Si samples of the original hole concentration of
10
13to 1014cm/C03are Cat-doped by P atoms through SiOxlayers with thicknesses of 0 to 7 nm. In the figure, the thick-
ness of SiOx layers is taken as a parameter. P Cat-doping
was carried out with T s¼350/C14C and P g¼1 Pa. When c-Si is
not covered with the SiOx layer, after process time of 1 min,
the surface of p-type c-Si is converted to n-type and the sheet
carrier density becomes to about 2–3 /C21012cm/C02. However,
when c-Si is covered with 4 nm-thick SiOx, the surface of
p-type c-Si is not converted to n-type at the process time of
1 min but converted after the process times over 5 min.When the thickness of SiOx is 7 nm, no conversion is
observed any more. In addition, even after the conduction
type is converted, the sheet carrier density cannot reach thevalue of the sample without SiOx.
These results clearly demonstrate that Cat-doping phe-
nomena are not caused by simple adsorption of unknownspecies on c-Si surface, P atoms can diffuse through a thin
SiOx layer, and that P atoms reaching to c-Si are working as
donors. Although the penetration depth of P atoms inside c-Si is still not known from the present experiment, the results
shown in Figs. 12–14suggest that it is the depth where P
atoms are surrounded by many Si atoms in c-Si, but shal-lower than several nm. However, at the same time, it should
be also noted that the sheet carrier density is likely to satu-
rate after the process times over several min. If the phenom-ena are attributed to a simple diffusion process, the value
should increase monotonically as the process time increases.
This is discussed later.
IV. DEVICE APPLICATION OF CAT-DOPING
The Cat-doping is a newly developed technology, and
the study on the mechanism of low temperature impuritydoping is still under the way. However, the feasibility of de-
vice application is apparent. The shallow doping can be used
to control the surface potential of various semiconductordevices. For instance, the surface passivation for c-Si solar
cells can be improved by the electric field effects due to shal-
low Cat-doping.
The c-Si surface can be passivated with i-a-Si or SiNx
layer prepared on it. We have already reported the improve-
ment of passivation quality of Cat-CVD i-a-Si
9and
SiNx10,11films by introducing Cat-doping, prior to the depo-
sition of such passivation films. However, to demonstrate the
usefulness of Cat-doping and also to confirm P atom dopingat low temperatures, we summarize the reported results of
passivation by both i-a-Si and SiNx layers.
Figure 16demonstrates the carrier lifetimes of i-a-Si/c-Si
and SiNx/c-Si samples as a function of FR(PH
3) for Cat-
doping. The c-Si is n-type with electron density of 1013to
1014cm/C03. Cat-doping was carried out at T s¼150/C14C,
Pg¼1 Pa and process time of 60 s for i-a-Si passivation
samples9and at T s¼80/C14C, P g¼1 Pa and process time of 60 s
for SiNx passivation samples.10,11In Cat-doping for i-a-Si
passivation samples, H 2gas of FR(H 2)¼20 sccm was added
to PH 3. The i-a-Si films were deposited at T cat¼1700/C14C,
Pg¼0.64 Pa, FR(SiH 4)¼20 sccm, T s¼150/C14C and SiNx
films at T cat¼1800/C14C, P g¼10 Pa, FR(SiH 4)¼8 sccm,
FR(NH 3)¼150 sccm, T s¼100/C14C but after deposition theFIG. 14. Sheet carrier density (arbitrary unit) as a function of T catto derive
activation energy.
FIG. 15. Sheet carrier density of samples P-Cat-doped through 1.5 nm-thickSiOx, as a function of process times, taking thickness of SiOx as a
parameter.114502-7 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58SiNx passivation samples were annealed at 350/C14C for
30 min.
The carrier lifetime was measured by micro-wave
photo-conductivity decay ( l-PCD) method using Kobelco,
LTA-1510P. In the method, 10 GHz micro-wave is used to
detect the photo-induced carriers in c-Si, and a laser with a
wavelength of 904 nm is used to generate photo-carriers in c-Si. It is clear from the figure that the carrier lifetimes can be
easily improved by Cat-doping of P atoms prior to deposition
of i-a-Si or SiNx. Taking account of the thickness of c-Siwafers, about 280–290 lm, the maximum surface recombi-
nation velocity is evaluated under the assumption that all
carriers are not recombined in bulk at all but only at the sur-face. The values are about 3 cm/s for 100 nm-thick i-a-Si
passivation samples
9and 2 cm/s or less for 100 nm-thick
SiNx passivation samples.11
SiNx layers are widely used as anti-reflection coating
for c-Si solar cells. However, it is not so easy to obtain high
carrier lifetimes for the direct deposition of SiNx on c-Siwith the resistivity of several Xcm suitable to solar cells.
The maximum surface recombination velocity estimated to
be 2 cm/s or less is one of the best records for solar-cell-usa-ble c-Si with the resistivity of 1–5 Xcm for single SiNx
passivation.
The results demonstrate the positive effect of Cat-
doping. When P atoms with carrier concentration of
10
18–1019cm/C03are incorporated at near to c-Si surface of
original doping concentration of 1013–1014cm/C03, the band
near to c-Si surface is likely to bend down about 0.2 eV.
Holes are repulsed from c-Si surface by this band-bending,
and the surface recombination is suppressed. Cat-doping is anew useful tool for controlling surface potential of
semiconductors.
V. DISCUSSIONS
A. Features of Cat-doping
It is known from above experiments that (1) P and B
atoms are incorporated into c-Si at the temperatures as low
as 80/C14C, however, that 2) the doping depth is as shallow as4–5 nm or less. Since the doping depth is almost equivalent
to a scale of depth resolution in SIMS analysis, the exact
estimation of doping depth appear ambiguous. It is also
known that the extension of doping depth appears slow evenif the process time is prolonged although P and B atoms can
be incorporated in the times as short as 60 s.
Figure 17shows SIMS P profiles which were measured
from the front side of samples in this case. The samples are
the same ones whose carrier density is demonstrated in Fig.
15. The c-Si samples coated with 1.5 nm-thick SiOx were
used for measurements. Since the profiles were taken from
the front side, all profiles were expressed in exponential
shapes due to the knock-on effects as mentioned already.The SIMS profiles were taken in high mass resolution system
with the probe ions of 5 keV. In the figure, P Cat-doping was
carried out at T
s¼350/C14C, P g¼1 Pa for various process
times. Although the correct information on the shape of pro-
files can not be obtained due to the knock-on effects, the total
number of incorporated doping atoms can be evaluated bythe integral of profile along depth.
Figure 18shows the relationship between the total P
atoms evaluated by the integral of profiles in Fig. 17and theFIG. 16. Carrier life times for i-a-Si and SiNx coated c-Si samples which
are both P-Cat-doped prior to deposition of coating film, as a function of
FR(PH 3).
FIG. 17. P concentration vs. depth by SIMS from the front side of samples,after P Cat-doping through 1.5 nm-thick SiOx layer.
FIG. 18. Total sheet density of Cat-doped P atoms vs. process times.114502-8 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58process times. In the figure, results of some other experi-
ments are demonstrated together. The evaluated values from
SIMS profiles, taken by high depth resolution system withthe probe ions of 1 keV, are plotted with open circles or open
squares. The results by high mass resolution system with the
probe ions of 5 keV are also plotted with closed circles orclosed squares. The results for both T
s¼350/C14C and 80/C14C
are demonstrated. It is known that the total number of P
atoms observed in high depth resolution system is alwayslarger than those in high mass resolution system since the
effect of
30SiþH fragments is included in high depth resolu-
tion data. It is also known that total number of incorporatedP atoms is likely to saturate as the process time increases. If
the phenomena simply follow thermal diffusion, the number
of incorporated atoms should be proportional to the root ofprocess times. When the process time is shorter than 25 min,
such relation appears to hold. If we estimate the diffusion
constant of Cat-doping in such a short process time, it wouldbe by several to 10 orders of magnitudes larger than that of
the conventional thermal diffusion. However, even if the
process time is prolonged over 25 min, it does not increaseany more and the phenomena are not likely to follow simple
diffusion theory. We have to consider some new mechanisms
for understanding Cat-doping phenomena.
B. Activation ratio of incorporated atoms
From the results shown in Figs. 15and 18, and also,
from Figs. 10and18, the activation ratio of doped P atoms
can be estimated. Here, the activation ratio is defined as the
ratio of electrically activated impurities to the total numberof incorporated impurities. Since the sheet carrier density of
P Cat-doped c-Si without SiOx coating shown in Fig. 15and
the sheet density of incorporated P atoms shown in Fig. 18
are about 2–3 /C210
12cm/C02and 0.5–1 /C21014cm/C02, respec-
tively, the activation ratio is simply evaluated to be about
2–6% for Cat-doping at T s¼350/C14C. Similarly, for Cat-
doping at T s¼80/C14C, it is evaluated to be 7–10%. We have
also observed SIMS profiles and measured the sheet carrier
density for some other samples not shown here. The fluctua-tion appears quite large, the activation ratio distributes from
2% to 10% even for Cat-doping at the same T
s. Thus, at the
moment, we do not particularly conclude from the data forthe present range of temperatures that the activation ratio is
depending on T
s.
C. Mechanism of Cat-doping
As mentioned above, P and B atoms are incorporated
into c-Si at temperatures as low as 80/C14C and with process
times as short as 60 s. When the process time is shorter than
25 min, the incorporation of atoms appears to follow the sim-ple diffusion theory, but after that, it is likely to saturate.
This may suggest that there is an unknown special region at
near to c-Si surface. In the region, foreign atoms can be eas-ily incorporated until their concentration exceeds 10
20cm/C03,
judging from Fig. 18and assuming the doping depth of
5 nm. We have obtained no direct evidence of the existenceof such special region. Therefore, at the moment, the exact
mechanism of Cat-doping can not be clearly revealed.However, we have already discovered other phenomena sim-
ilar to the present Cat-doping.
We have reported on low temperature thermal oxidation
of c-S.
12,13When c-Si is exposed to species generated by cat-
alytic cracking reaction of H 2diluted oxygen (O 2) gas with
heated W catalyzer, the surface of c-Si can be oxidized andconverted to SiO
2even at the temperatures as low as 200/C14C.
The SiO 2appears to have sufficient electrical properties as a
gate insulator. At that time, we attempted to increase the oxi-dized thickness, however, the thickness of SiO
2appeared to
be limited at about 4 nm.13
On the other hand, we have also discovered that nitrogen
(N) atoms are sometimes incorporated into c-Si during deposi-
tion of SiNx using NH 3and SiH 4gases, and that such a N
incorporated layer forms a defect layer in c-Si to degrade passi-vation quality for SiNx/c-Si system.
14Observation by transmis-
sion electron microscope (TEM) demonstrates that the depth of
such defect layer is again at about a few nm to several nm.14
All these experiments including the present Cat-doping
demonstrates that foreign atoms can be incorporated into
c-Si at low temperatures when it is exposed to species gener-ated by catalytic cracking reactions with heated W catalyzer.
Although the mechanism is not clearly explained, it is clear
that the phenomena concerned with low-temperature dopingsurely exist.
D. Effect of hydrogen
Another thing we have to consider is the existence of H
atoms at the vicinity of incorporated P atoms. From allexperiments, incorporation of atoms into c-Si at low temper-
atures always requires the cracked species. In all experiments
concerned with P and B Cat-doping, low temperature oxida-tion and N incorporation, high density H atoms are also gen-
erated during the experimental process. Figure 19shows the
SIMS profiles of P atoms and H atoms for Cat-doping atT
s¼80/C14C, Fig. 19(a) , and T s¼350/C14C, Fig. 19(b) . c-Si sam-
ples were coated with i-a-Si layers. The profiles were taken
from the back side of the samples by high depth resolutionsystem with the probe ion energy of 1 keV. In this measure-
ment, since the isotope fragments of
30SiþH are included, P
profile itself is strongly affected by H profile.
However, the shape of P profiles is not always same to that
of H profiles, particularly in the region of coated a-Si layer.
This suggests that P profile itself is believable although the Pprofile suffers from the isotope fragments and, thus, the abso-
lute value of density is not correct. The figure demonstrates that
P atoms distribute at the same region where H atoms distribute.The H profiles are almost overlapped with P profiles. This sug-
gests that low temperature Cat-doping of P atoms might be
affected by the existence of H atoms.
According to our ab initio calculation for a model of
c-Si system consisting of 216 Si atoms and an additional sin-
gle P atom and a single H atom in them, the P atom has ener-getically stable 4 possible configurations corresponding to
4 sites in c-Si lattice when the H atom exists just adjacent to
the P atom. And such a P atom can hop to another site of0.1–0.2 nm far from the initial site with activation energy of
about 0.8 eV or less. When the P atoms move into c-Si by114502-9 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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131.230.73.202 On: Wed, 17 Dec 2014 23:45:58substitutional diffusion, the activation energy is about
3.0 eV. The ab initio calculation suggests that P atoms can
diffuse easily when H atoms exist in c-Si. This again sug-
gests that there may be a help of H atoms in low temperatureP diffusion or low temperature incorporation of foreign
atoms into c-Si. The result of ab initio calculation will be
reported in detail elsewhere.
15
The unknown region existing at near to c-Si surface,
speculated above, may be also concerned with incorporation
of H atoms. That is, there may be a special region where Hatoms can be easily incorporated and other foreign atoms
can be incorporated by following such H atoms. However, at
the moment, everything is only under speculation. Furtherefforts to reveal mechanism of Cat-doping are required,
although the phenomena are clearly revealed and the feasi-
bility of application are demonstrated in the present paper.
VI. CONCLUSIONS
As mentioned above, Cat-doping is studied in detail in
the present paper. There are some unknown matters includ-ing the mechanism. However, so far, the following conclu-
sions are obtained.
(1) When c-Si is exposed to species generated by the cata-
lytic cracking reaction of PH
3or B 2H6gas with heated
W catalyzer, P or B atoms are doped in c-Si at the tem-
peratures as low as 80/C14C. This novel doping method is
called “Cat-doping”.
(2) By Cat-doping of P atoms, p-type c-Si is converted to
n-type, and similarly by Cat-doping of B atoms, n-typec-Si is converted to p-type, even at the substrate tempera-
tures as low as 80
/C14C.
(3) Cat-doped layer is formed at the depth as shallow as
5 nm or less.
(4) By using Cat-doping technology, the surface potential of
c-Si can be easily controlled, and through this control,the surface recombination velocity of carriers in c-Si can
be enormously lowered for both i-a-Si and SiNx passiva-
tion on c-Si. Further device application is expected.ACKNOWLEDGMENTS
This work was supported by CREST Research Program
of Japan Science and Technology Agency of Government
(JST). The authors are grateful to advisory committeemembers of the CREST for their discussions and to students
of Japan Advanced Institute of Science and Technology
(JAIST) for their experimental supports. The authors are alsograteful to Mr. S. Osono and his co-workers at ULVAC
Corporation for providing TXRF data.
1T. Hayakawa, Y. Nakashima, M. Miyamoto, K. Koyama, K. Ohdaira, and
H. Matsumura, Jpn. Appl. Phys., Part 1 50, 121301 (2011).
2T. Hayakawa, Y. Nakashima, K. Koyama, K. Ohdaira, and H. Matsumura,
Jpn. J. Appl. Phys., Part 1 51, 061301 (2012).
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79, 2699 (1996).
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(1991).
5S. Osono, Y. Uchiyama, M. Kitazoe, K. Saitoh, M. Hayama, A. Masuda,A. Izumi, and H. Matsumura, Technical Digest of 2002 Fall Meeting of
Japan Society of Applied Physics (JSAP), Niigata, Japan, Sept., 2002,
(JSAP, 2002), 26a-C-5, p.732.
6K. Koyama, K. Ohdaira, and H. Matsumura, Appl. Phys. Lett. 97, 082108
(2010).
7G. Citarella, M. Grimm, S. Schmidbauer, K. H. Ahn, M. Erdmann, T.Shulze, M. Plettig, B. Gruber, J. Hausmann, R. Bohme, W. Stein, D.
Muller, M. Winkler, T. Zerres, E. Vetter, D. Batzner, B. Strhm, D.
Lachenal, G. Wahli, F. Wunsch, P. Papet, Y. Andrault, C. Guerin, A.
Buchel, and B. Rau, Proceedings of the 26th EU Photovoltaic Specialists
Conference, Hamburg, Germany, 2011 (IEEE, 2011), p. 865.
8H. Umemoto, Y. Nishihara, T. Ishikawa, and S. Yamamoto, Jpn. J. Appl.
Phys., Part 2 51, 086501 (2012).
9H. Matsumura, M. Miyamto, K. Koyama, and K. Ohdaira, Sol. Energy
Mater. Sol. Cells 95, 797 (2011).
10T. C. Thi, K. Koyama, K. Ohdaira, and H. Matsumura, Tech.
Dig. - Photovoltaic Sci. Eng. Conf. 2013 , 1-O-24.
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044510 (2014).
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State Lett. 2, 388 (1999).
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J. Vac. Sci. Technol. B 30, 031208 (2012).
15D. H. Chi, (private communication).FIG. 19. SIMS profiles of P and H
atoms, observed from the back
side, for P-Cat-doped samples at (a)
Ts¼80/C14C, and (b) 350/C14C.114502-10 Matsumura et al. J. Appl. Phys. 116, 114502 (2014)
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1.4896724.pdf | Effect of acoustic emission on the critical velocity for the transition to turbulent flow in He
II
I. A. Gritsenko and G. A. Sheshin
Citation: Low Temperature Physics 40, 802 (2014); doi: 10.1063/1.4896724
View online: http://dx.doi.org/10.1063/1.4896724
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On: Tue, 23 Dec 2014 19:40:08Effect of acoustic emission on the critical velocity for the transition to turbulent flow
in He II
I. A. Gritsenko and G. A. Sheshina)
B. I. Verkin Institute of Low-temperature Physics and Technology, National Academy of Sciences of Ukraine,
pr. Lenina 47, Kharkov 61103, Ukraine
(Submitted March 13, 2014; revised April 10, 2014)
Fiz. Nizk. Temp. 40, 1028–1034 (September 2014)
The conditions for the transition from laminar to turbulent flow in superfluid4He are investigated
experimentally, and the effect of acoustic emission with variable power on the critical velocity forthe transition is studied. The quartz tuning fork method is used at temperatures of 2–0.3 K. The
experiments are done over a wide range of pressures, from the saturated vapor pressure to 24.8 atm.
It is found that at high temperatures ( T>0.9 K) the critical velocity is determined by viscous
friction and at low temperatures ( T<0.5 K) by the effect of acoustic emission, which leads to a
significant increase in the critical velocity for the transition to the turbulent state. The critical
velocity depends on the power of the acoustic emission and the transition to the turbulent state ofthe superfluid is similar to that in ordinary liquids or gases. In the absence of any effects of acoustic
emission, the critical transition velocity is essentially independent of temperature and the driving
power is mainly determined by ballistic scattering of thermal excitations.
VC2014
AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4896724 ]
1. Introduction
In recent years the quartz tuning fork method has been
used extensively to study quantum turbulence and the prop-erties of superfluid liquids. This method is highly sensitive
to any dissipative processes owing to superfluid flows. It can
be used for research at ultralow temperatures over a widerange of frequencies from 6 to 250 kHz. Quartz tuning forks
are in active use for studying the kinetic properties of super-
fluids, viscous friction at T>0.7 K, ballistic scattering of
thermal excitations at T<0.5 K,
1the transition from laminar
to turbulent flow,2–4the absorption of3He on quantized vor-
tices,5and phase transitions.6It has also been found that a
quartz tuning fork in a superfluid can emit first sound waves7
and in3He-4He solutions, second sound waves.8
The conditions for excitation of acoustic waves by tun-
ing forks in He II have been studied.9–12The power of the
acoustic emission can be reduced by decreasing the size and
frequency of the tuning fork,9,12as well as by increasing the
speed of first sound, which can be controlled by varying the
pressure of a superfluid.10,11It was also discovered that
reducing the size of the cell that contains a tuning fork alsolowers the acoustic emission power.
10The influence of the
size of the cell on the acoustic emission becomes especially
important under acoustic resonance conditions in a cylindri-cal cell filled with He II.
11
Studies of the transition from laminar to turbulent flow,
as well as of the evolution of the turbulent flow, have beenconducted in parallel with research on various dissipative
processes in laminar flows of He II. The temperature depend-
ence of the critical transition velocity t
cwas measured,4a
transition regime with development of a turbulent flow was
found,13and hysteresis in the transition to quantum turbu-
lence in He II was found at T¼10 mK.14In addition, it was
found that if the main dissipative process in the laminar flow
regime was acoustic emission, then the transition to a turbu-
lent flow took place at higher flow velocities of the liquidand without the previously observed intermediate flow
regime.13,15This was usually explained by the fact that,
because of the high power expended in the acoustic emis-
sion, the onset of the transition to a turbulent flow was not
noticeable. And only when the dissipative processes in theturbulent flow become comparable to the dissipation of the
energy of the tuning fork owing to the acoustic emission is
the transition to a turbulent flow observed, while the meas-ured critical velocity of the transition is then significantly
higher.
12
The critical velocity tcof the transition to the turbulent
state has been studied under various conditions, but the
effect of acoustic emission on tcis still little studied. This
paper is a continuation of the earlier work. Here we studythe effect of acoustic emission on the velocity at which the
transition to turbulent flow takes place in He II.
2. Experimental technique
The behavior of eight quartz tuning forks immersed in
superfluid4He at different pressures was studied. The tuning
forks all had the same resonance frequency ( /C2432 kHz) but
had different geometrical dimensions (Table 1). They were
attached so that their axes coincided with or were perpendic-
ular to the axis of the cylindrical cavity.12
The experiments were done at temperatures of
0.3–2.1 K, which were reached using a solution refrigera-
tor.16The tuning forks were placed in a copper cell equipped
with a heat exchanger made of ultradispersed (700 A ˚) silver
powder. The cell was in thermal contact with the solution
chamber. The temperature of the cell was measured with aRuO
2resistance thermometer that was calibrated based on
the temperature dependence of the crystallization pressure of
3He and installed in the liquid being investigated. The tem-
perature of the solution chamber was kept constant using a
temperature stabilizer with feedback coupling to a resistance
1063-777X/2014/40(9)/5/$32.00 VC2014 AIP Publishing LLC 802LOW TEMPERATURE PHYSICS VOLUME 40, NUMBER 9 SEPTEMBER 2014
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On: Tue, 23 Dec 2014 19:40:08thermometer in the solution chamber. The temperature was
measured with an accuracy of 61m K .
A pressure was created in the cell with the aid of a
cooled volume with an adsorbent and measured with a ma-nometer kept at room temperature. The accuracy of the pres-
sure measurement at 24 atm was 60.05 atm, and the stability
of the pressure during the measurement process was lessthan the accuracy of the measurements.
Two types of experiment were done:
– at a constant temperature of 370 mK and different pres-
sures ranging from the saturated vapor pressure to 24.8
atm, and
– at various temperatures ranging from 0.3 to 2 K at the satu-
rated vapor pressure.
Since the acoustic emission power W
adepends strongly
on the sound speed c(Wa/c/C05(Refs. 9and10)) and the
sound speed, in turn, depends strongly on the pressure, the
experiments were done over a wide range of pressures,which made it possible to vary the acoustic emission power
over almost an order of magnitude.
The experiments were done in the following way. First
the cell was cooled to 0.5 K and the piezoelectric constants a
of all the tuning forks were measured in vacuum. Then
4He
was condensed at a rate such that the cell was not heatedabove 0.6 K. After the
4He was condensed, a pressure close
to the crystallization pressure was created in the cell which
was stabilized at 370 mK. Then a cycle of measurements atconstant pressure was carried out, after which the pressure in
the cell was reduced and the next cycle of measurement was
carried out at another pressure.
The quartz tuning fork method described in Refs. 10–13
was used to measure the amplitude-frequency characteristics
of the tuning forks at different driving voltages Uunder con-
stant thermodynamic conditions ( PandT). The amplitude of
the alternating current Iflowing through a tuning fork was
determined from the voltage drop on a standard 1 k Xresist-
ance using a 5208 two-phase lock-in analyzer.
11–13The ampli-
tude I0and frequency f0of the tuning fork resonances, as well
as the half width Dfof the resonance line, were determined
from the amplitude-frequency characteristics. The dependence
of the velocity t¼I=aat which the tuning fork tines oscil-
lated on the exciting force F¼1
2aUwas also determined.
3. Critical velocity for development of a turbulent flow. The
role of acoustic emission
Typical experimental data on the dependence of the
velocity of the oscillations of a tuning fork on the excitingforce at 370 mK are shown in Fig. 1. This figure shows that
for small F, there is a linear dependence F/C24t(the smooth
and dashed lines), which is typical of laminar flow in liquids.
With increasing Fthere is a deviation from this linear
dependence, and in one case (curve 1) a quadratic depend-
ence ( F/C24t2) shows up at once (dotted-dashed curves) and
in another (curve 2), an intermediate regime between linearand quadratic is observed. In both cases the curves have
characteristic deflections (indicated by arrows) correspond-
ing to the critical velocity t
cfor the transition from laminar
to turbulent flow.
It should be noted that the curves in Fig. 1correspond to
different values of the ratio R/k: for the black data points
R/k>1/4 and for the hollow points, R/k<1/4. For this rea-
son, the tuning forks used in the experiment are distinguished
arbitrarily. The physical reason is related to different mecha-nisms for dissipation in laminar flow with different R/k. For
R/k>1/4 the main dissipation mechanism can be acoustic
emission,
9–12while for R/k<1/4 thermal excitations of He II
undergo ballistic scattering on the oscillating tines of the tun-
ing forks. This mechanism is essentially independent of pres-
sure;10thus, in Fig. 1the data are the same for different
pressures. tðFÞhas been calculated using kinetic equations
for ballistic scattering of thermal excitations18and the results
of these calculations, shown as the solid curve in Fig. 1, are
in good agreement with experiment.TABLE 1. Major parameters of the tuning forks used in the experiments.
Tuning fork No. L(mm) H(mm) M(mm) D(mm) f0(Hz) Df0(Hz) R(cm) R/kSVP
K1 3.79 0.3 0.6 0.3 32708.35 0.035 1.1 1.48
K5 3.79 0.3 0.6 0.3 32709.95 0.08 0.13 0.18K8 3.79 0.3 0.6 0.3 32709.88 0.05 0.13 0.18K9 2.53 0.1 0.25 0.13 32708.25 0.032 0.07 0.095K19 3.41 0.33 0.38 0.2 32719.5 0.04 0.4 0.52K20 3.41 0.33 0.38 0.2 32704.65 0.1 0.4 0.52K21 3.41 0.33 0.38 0.2 32720.3 0.075 0.4 0.52K22 3.81 0.34 0.6 0.3 32711.7 0.044 0.4 0.52
FIG. 1. The oscillation velocity of the tuning fork tines as a function of
exciting force. Tuning fork K5 at He II pressures of 7.6 ( D) and 22.3 atm
(/H17034) and T¼370 mK. Tuning fork K21 at He II pressures of 8.35 ( /H17033) and
24.1 atm ( /H17009). The smooth curve is a calculation for ballistic scattering of
thermal excitations; the dashed curve is for laminar flow with acoustic
emission; the dot-dashed curve is an F/C24t2dependence corresponding to
turbulent flow.Low Temp. Phys. 40(9), September 2014 I. A. Gritsenko and G. A. Sheshin 803
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On: Tue, 23 Dec 2014 19:40:08It has been noted10that in laminar flows, the re-reflection
of acoustic waves has the greatest influence on the quantities
measured in the experiments near R/k¼1/2, when the first
sound wave is resonant in the cylindrical cell. For tuningforks K20-K22 this regime should be observed for P/C242 atm
in He II.
Figure 2shows the critical velocities t
cas functions of
pressure for both cases on the same scale. As noted above,
when R/k<1/4, tcis essentially independent of pressure
(Fig. 2(a)), while when R/k>1/4 a large spread in the data
is observed (Fig. 2(b)) owing to re-reflection of acoustic
waves from the cell walls. Here, as in Ref. 10, the spread in
the critical velocity is considerably greater, especially at lowpressures, because of the large value of kand the possibility
of attaining resonance conditions.
4. Dependence on excitation power and temperature
In order to trace the influence of acoustic emission on
the critical velocity for the transition from laminar to turbu-
lent flow, it is convenient to present the data in the form of a
plot of the critical velocity on the corresponding power driv-ing the oscillations of the tuning fork tines. Figure 3shows a
plot of this kind for tuning forks of different sizes and for
different constant He II pressures. The open plot pointscorrespond to tuning forks for which the main dissipation
mechanism in laminar flow is ballistic scattering of thermal
excitations on the vibrating tines of the tuning forks. Thesolid plot points indicate the critical velocities for tuning
forks for which acoustic emission dominates ( R/k>1/4).Figure 3shows that, depending on the driver power,
there are two mechanisms for excitation of a turbulent
flow. At higher emission powers W/C2110
/C08W, the data are
described by a single dependence of the form W/C24tc3that is
typical of turbulence in ordinary liquids or gases (see
Section 5). At lower powers W<10/C08W, a significant devi-
ation from this dependence is observed. Here tcis essentially
independent of Wc. As noted above, in this case a transition
regime was observed that is typical of the rapid rise in den-sity of quantized vortices.
13This kind of behavior has been
found for tuning forks of different sizes; that is, the major
factor is not the size of a tuning fork, but the absence ofacoustic emission.
In order to trace the influence of temperature on the
excitation of turbulent flow by acoustic emission, we havemeasured the temperature dependence of the critical velocity
for five of the tuning forks immersed in He II at the saturated
vapor pressure. Figure 4shows these data for R/k<1/4
(open plot points) and for R/k>1/4 (solid points). For
T>1.2 K, the experimental data essentially coincide in both
cases because of the dominant influence of viscous dissipa-tion. For T<1.2 K, when R/k>1/4 the main mechanism for
dissipation is acoustic emission and the critical transition
FIG. 2. The critical velocity for the transition from laminar to turbulent flow
as a function of pressure for T¼370 mK: (a) tuning fork K8 ( /H17034); (b) K21
(/H17004) and K22 ( /H17010).FIG. 3. The critical velocity for the transition to turbulent flow in He II as a
function of exciting signal. K5 ( /H11623); K9 (/H17006); K8 (/H17034); K1 (/H17004)[15]; K20 ( /H17039);
K21 (/H17009); K22 ( /H17010); K19 ( /H17033) for T¼370 mK and different pressures (see
Fig. 2); K8 (
); K19 (
); K20 (
); and K21 (
) for the saturated vapor
pressure and different temperatures (see Fig. 4).
FIG. 4. Temperature dependence of the critical velocity for the transition from
laminar to turbulent flow: K19 ð/H17004Þ;K 2 0ð/H17033Þ; K21ð/H17039Þ;K 8ð/H17005Þ;K 9ð/H17006Þ.804 Low Temp. Phys. 40(9), September 2014 I. A. Gritsenko and G. A. Sheshin
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On: Tue, 23 Dec 2014 19:40:08velocity is substantially higher than for R/k<1/4, where, as
before, the main mechanism for dissipation is viscous fric-
tion. For T<0.6 K, tcis essentially independent of tempera-
ture in both cases. But when R/k<1/4, the main mechanism
for dissipation of the kinetic energy of the vibrating tines of
the tuning forks changes from viscous friction to ballistic
scattering of thermal excitations.
For the following analysis, the temperature dependence
tc(T) was converted to W(tc), as shown in Fig. 3by the gray
plot points. This figure shows that these data also follow asingle dependence and coincide with the data obtained at
T¼370 mK. At high temperatures ( T>0.9 K) the experi-
mental data are also described by a W/C24t
c3dependence. For
T<0.9 K when R/k>1/4 (i.e., for the tuning forks with
strong acoustic emission), the experimental data remain close
to this dependence but when R/k<1/4,tcceases to depend
on the power, which also indicates that the mechanism for ex-
citation of turbulence changes in this temperature range.
The experimental data for T<0.9 K, when qnis low,
can be interpreted in terms of the classification of superfluid
turbulence given in Refs. 19–21. In this case, quasiclassical
turbulence is observed at high excitation powers (Fig. 3)
when the vortices are polarized and joined together into
bunches, while turbulence develops through reconnection of
vortices inside a bunch or the reconnection of bunches. At
powers <10/C08W, two mechanisms for the excitation of tur-
bulence are observed. Quasiclassical turbulence is evidentlyexcited when the acoustic emission dominates. When the
acoustic emission is negligible, we can speak of the onset of
a transition region toward quantum turbulence, and a turbu-lent clump forms as a result of quantized vortices that show
up on roughnesses and recombine among and with
themselves.
5. Drag coefficient
To compare the experimental data for the different flow
regimes with the behavior of ordinary liquids or gases, it is
convenient to express the observed dependences in terms of
the dimensionless drag coefficient Cd, which is defined as17
Cd¼2F
qt2S; (1)
where qis the He II density at the corresponding pressure
andSis the area of the head cross section of the tuning fork
tine.
The different flow regimes can be represented intuitively
in terms of drag. Since F/C24tin laminar flow, Cd/C241=t,
which corresponds to the experimental data for low veloc-ities tand is shown in Fig. 5. In turbulent flow, where
F/C24t
2,Cddoes not depend on t. In this case Cdis deter-
mined by the geometry of the object: for a flat, rectangularplate with its plane perpendicular to the flow, C
d/C252, for a
cylinder Cd/C251, and for a sphere Cd/C253.
Figure 5shows that when R/k>1/4 (solid plot points),
Cd/C250.5 over an extremely wide range of velocities. In the
opposite case of R/k<1/4,Cdhas a nonmontonic depend-
ence on twith a minimum at Cd/C281. The subsequent rise in
Cdwith increasing velocity coincides with the transition
regime and is apparently caused by an increasing density of
quantized vortices and, as a consequence, their mutualfriction. It has been shown4that quantized vortices in this
case can develop in the surface layer at roughnesses. Thedensity of quantized vortices continues to rise with increas-
ing velocity until it reaches a high enough value for devel-
oped turbulence. This regime corresponds to C
d/C250.3.
The curves in Fig. 5also illustrate the effect of acoustic
emission. With increasing power, the transition between
laminar and turbulent flows becomes smoother, which isqualitatively similar to the behavior of C
din ordinary liquids
or gases.
As in ordinary liquids and gases, the scaled dependence
oftconW(smooth curve in Fig. 3) agrees well with Eq. (1)
when Cd/C251, a condition which corresponds to the area for
the tuning forks ( S) and He II density ( q).
6. Conclusion
This series of experiments with tuning forks immersed
in superfluid4He has shown that there are two mechanisms
for the transition from laminar to turbulent flow which arerelated to acoustic emission from a tuning fork and viscous
dissipation at high temperatures and to ballistic scattering of
thermal excitations of He II at low temperatures. It has beenfound that when the effect of the acoustic emission domi-
nates, the drag coefficient C
d/C250.5 and the behavior of the
velocity of the oscillations of the tuning fork tines is close tothe case of a classical liquid. When ballistic scattering pre-
dominates, the critical velocity decreases significantly and
an intermediate mode appears between the laminar and tur-bulent states. The transition to the turbulent state depends on
the driving power: at low powers (below 10
/C08W) the two
mechanisms for the transition are distinctly separate. Whenacoustic emission predominates, quasiclassical turbulence
seems to occur in He II, but when the acoustic emission is
negligible, a transition to quantized turbulence sets in.
We thank E. Ya. Rudavskii, E. E. Nemchenko, and S. S.
Sokolov for discussing the data and for helpful advice. We
also thank L. Skrbek for providing the tuning forks used inthe experiments.
This work was supported in part by a Ukrainian-
Japanese grant (Project No. F52.2/005).FIG. 5. Drag coefficient as a function of He II flow velocity at T ¼370 mK:
tuning fork K5 for He II pressures of 7.6 ( D) and 22.3 atm ( /H17034), tuning fork
K21 for He II pressures of 8.35 ( /H17033) and 24.1 atm ( /H17009), and tuning fork
K21 for He II pressures of 8.35 ( /H17004) and 24.1 atm ( /H17039); the smooth curve is
Cd/C241/tc.Low Temp. Phys. 40(9), September 2014 I. A. Gritsenko and G. A. Sheshin 805
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On: Tue, 23 Dec 2014 19:40:08a)Email: sheshin@ilt.kharkov.ua
1M. Bla /C20zkov /C19a, M. /C20Clovec ˇko, E. Gazo, L. Skrbek, and P. Skyba, J. Low
Temp. Phys. 148, 305 (2007).
2R. Blaauwgeers, M. Bla /C20zkov /C19a, M. /C20Clovec ˇko, V. B. Eltsov, R. de Graaf,
J. Hosio, M. Krusius, D. Schmoranzer, W. Schoepe, L. Skrbek, P.
Skyba, R. E. Solntsev, and D. E. Zmeev, J. Low Temp. Phys. 146,5 3 7
(2007).
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025302 (2007).
4G. A. Sheshin, A. A. Zadorozhko, E. Ya. Rudavskii, V. K. Chagovets, L.Skrbek, and M. Bla /C20zkov /C19a, Fiz. Nizk. Temp. 34, 1111 (2008) [ Low Temp.
Phys. 34, 875 (2008)].
5V. Chagovets, I. Gritsenko, E. Rudavskii, G. Sheshin, and A. Zadorozhko,
J. Low Temp. Phys. 158, 450 (2010).
6M. Bla /C20zkov /C19a, D. Schmoranzer, and L. Skrbek, Fiz. Nizk. Temp. 34, 380
(2008) [ Low Temp. Phys. 34, 298 (2008)].
7D. O. Clubb, O. V. L. Buu, R. M. Bowley, R. Nyman, and J. R. Owers-
Bradley, J. Low Temp. Phys. 136, 1 (2004).
8A. Salmela, J. Tuoriniemi, E. Pentti, A. Sebedash, and J. Rysti, J. Phys.:
Conf. Ser. 150, 012040 (2009).
9D. Schmoranzer, M. La Mantia, I. Gritsenko, A. Zadorozhko, G.
Sheshin, M. Rotter, and L. Skrbek, J. Low Temp. Phys. 163, 317
(2011).10I. A. Gritsenko, A. A. Zadorozhko, and G. A. Sheshin, Fiz. Nizk. Temp.
38, 1395 (2012) [ Low Temp. Phys. 38, 1100 (2012)].
11I. Gritsenko, A. Zadorozhko, and G. Sheshin, J. Low Temp. Phys. 171,
194 (2013).
12I. Gritsenko, G. Sheshin, D. Schmoranzer, and L. Skrbek, Fiz. Nizk.Temp. 39, 1062 (2013) [Low Temp. Phys. 39, 823 (2013)].
13I. Gritsenko, A. Zadorozhko, V. Chagovets, and G. Sheshin, J. Phys.:
Conf. Ser. 400, 012068 (2012).
14D. I. Bradley, M. J. Fear, S. N. Fisher, A. M. Guenault, R. P. Haley, C. R.
Lawson, G. R. Pickett, R. Schanen, V. Tsepelin, and L. A. Wheatland,
J. Low Temp. Phys. 175, 379 (2014).
15I. Gritsenko and G. Sheshin, J. Low Temp. Phys. 175, 91 (2014)
16E. Ya. Rudavskii, V. K. Chagovets, and G. A. Sheshin, Fiz. Nizk. Temp.
15, 568 (1989) [Sov. J. Low Temp. Phys. 15, 320 (1989)].
17L. D. Landau and E. M. Lifshitz, Hydrodynamics (Nauka, Moscow, 1986).
18I. A. Gritsenko, A. A. Zadorozhko, A. S. Neoneta, V. K. Chagovets, and
G. A. Sheshin, Fiz. Nizk. Temp. 37, 695 (2011) [ Low Temp. Phys. 37,
551 (2011)].
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Translated by D. H. McNeill806 Low Temp. Phys. 40(9), September 2014 I. A. Gritsenko and G. A. Sheshin
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On: Tue, 23 Dec 2014 19:40:08 |
1.4891855.pdf | Linear temperature behavior of thermopower and strong electron-electron scattering in
thick F-doped SnO2 films
Wen-Jing Lang and Zhi-Qing Li
Citation: Applied Physics Letters 105, 042110 (2014); doi: 10.1063/1.4891855
View online: http://dx.doi.org/10.1063/1.4891855
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/4?ver=pdfcov
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128.252.67.66 On: Sun, 21 Dec 2014 03:27:12Linear temperature behavior of thermopower and strong electron-electron
scattering in thick F-doped SnO 2films
Wen-Jing Lang and Zhi-Qing Lia)
Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology,
Department of Physics, Tianjin University, Tianjin 300072, China
(Received 11 April 2014; accepted 20 July 2014; published online 30 July 2014)
Both the semi-classical and quantum transport properties of F-doped SnO 2thick films ( /C241lm)
were investigated experimentally. We found that the resistivity caused by the thermal phonons
obeys Bloch-Gr €uneisen law from /C2490 to 300 K, while only the diffusive thermopower, which
varies linearly with temperature from 300 down to 10 K, can be observed. The phonon-drag ther-mopower is completely suppressed due to the long electron-phonon relaxation time in the com-
pound. These observations, together with the fact that the carrier concentration has negligible
temperature dependence, indicate that the conduction electrons in F-doped SnO
2films possess free-
electron-like characteristics. At low temperatures, the electron-electron scattering dominates over
the electron-phonon scattering and governs the inelastic scattering process. The theoretical predica-
tions of scattering rates of large- and small-energy-transfer electron-electron scattering processes,which are negligibly weak in three-dimensional disordered conventional conductors, are quantita-
tively tested in this lower carrier concentration and free-electron-like highly degenerate semicon-
ductor.
VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4891855 ]
F-doped SnO 2(FTO) is one of the typical transparent
conducting oxides (TCOs). Remarkable progress has been
made in FTO film deposition techniques recent years.1–3
Currently, both the electrical conductivity and optical trans-
parency in visible frequencies of FTO film are comparable to
that of Sn-doped In 2O3(ITO) film.3,4Comparing with the
most widely used ITO film, FTO film has its own special
advantages, such as chemically stable in acidic and basic sol-
utions,5thermally stable in oxidizing environments at high
temperatures,6,7and inexpensive (do not include rare ele-
ments). Hence, FTO films are widely used in photoelectric
and electro-optic devices such as solar cells and flat paneldisplays.
8–11Although FTO film has been one of the major
commercial TCO products, the current understanding of the
origins for the combined properties of high electrical con-ductivity and high optical transparency of FTO film is
mainly based on ab initio energy bandstructure calculations
and optical properties measurements.
4,12–15Pure SnO 2is a
wide-gap semiconductor with direct bandgap /C243.6 eV and
possesses high transmittance in visible light range.12,13The
introduction of F in pure SnO 2shifts the Fermi level up into
the conduction band and enlarges the optical band gap
(known as Burstein-M €oss effect).13–15As a result, the con-
duction band of FTO is mainly composed of Sn 5 sstate and
thus FTO is a free-electron-like degenerate semiconductor or
alternatively a free-electron-like metal in energy bandstruc-
ture.15However, the free-electron-like feature of conduction
electrons in FTO has not been tested experimentally.
Moreover, the carrier concentrations in FTO films are often
/C241020cm–3,1,2which is /C242 to 3 orders of magnitude lower
than that in typical metals.16The low-carrier-concentration
metal characteristic of FTO may give us opportunities to testthe validity of some theoretical predications that is difficult
to be achieved in conventional metals. In this Letter, we
measured the temperature dependence of resistivity and ther-mopower from 300 K down to liquid helium temperatures,
and the results indicate that the transport processes of con-
duction electrons in FTO films can be approximately treatedusing free-electron-like model. Then, we show that thick
FTO film provides a valuable platform to test the three-
dimensional (3D) electron-electron ( e-e) scattering theory
due to its inherited weak electron-phonon ( e-ph) coupling
nature. It should be noted here that the predications of 3D e-
escattering theory have not been fully tested though the
theory has been proposed for about four decades.
17,18
FTO (SnF 0.06O1.94– d) films prepared by the chemical
vapor deposition method were provided by Zhuhai KaivoOptoelectronic Technology Corporation. Two series of films,
one is the as-deposited (donated as No. 1) and the other is
the film annealed in O
2at 300/C14C for 1 h (denoted as No. 2),
were measured. The thickness of the films ( /C241lm) was
determined by a surface profiler (Dektak, 6 M). (We inten-
tionally selected the /C241lm thick films to make sure they are
3D with respect to e-escattering and weak-localization
effect.) Crystal structures of the films were measured in a
powder x-ray diffractometer (D/max-2500, Rigaku) with CuK
aradiation. The results indicated that the films have tetrag-
onal rutile-type structure, which is the same as that of rutile
SnO 2(powder diffraction number: 46–1088), and no second-
ary phase was observed. The resistivity and magnetoresist-
ance (MR) were measured in a physical property
measurement system (PPMS-6000, Quantum Design) by astandard four-probe technique. During the MR measure-
ments, the applied field was perpendicular to the films. Hall
effect measurements were also performed in the PPMS withthe four-point method. The thermopower measurements
were carried out with the thermal transport option of the
a)Author to whom correspondence should be addressed. Electronic mail:
zhiqingli@tju.edu.cn
0003-6951/2014/105(4)/042110/5/$30.00 VC2014 AIP Publishing LLC 105, 042110-1APPLIED PHYSICS LETTERS 105, 042110 (2014)
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128.252.67.66 On: Sun, 21 Dec 2014 03:27:12PPMS by a four-probe leads configuration method, in which
two calibrated Cernox 1050 thermometers were used to mea-
sure the temperature of the hot and cold probes, respectively.
The pressure of the sample chamber was less than5/C210
/C04Torr during the measurements.
Figure 1shows the variation in the normalized resistiv-
ityqðTÞ=qð300 K Þwith temperature between 2 and 300 K
for the two FTO films. Upon increasing temperature from
2 K, the resistivities decrease initially, reach their minimum
atTmin(Tminis the temperature at which qreaches its mini-
mum value, and Tmin’50 and 90 K for films, Nos. 1 and
2), and then increase with further increasing temperature.The inset (a) of Fig. 1shows the variation of normalized
conductivity Dr=rð50 KÞ¼½rðTÞ/C0rð50 KÞ/C138=rð50 KÞas a
function of T
1=2f r o m2t o5 0 K .C l e a r l y , Drvaries linearly
with T1=2at this temperature regime. In a 3D disordered
metal, the electron-electron interaction (EEI) is strong and
leads to the T1=2correction to the conductivity.19,20The
characteristic length for both EEI and e-escattering effect
isLT¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
/C22hD=ðkBTÞp
,w h e r e Dis the electron diffusion con-
stant, /C22his the Planck constant divided by 2 p,a n d kBis the
Boltzmann constant. The thermal diffusion length of elec-
tron at 2 K is /C2531 nm in film No. 1 and /C2560 nm in film No.
2; both of them are much shorter than the thickness of thefilms. Hence, our FTO films are 3D with regard to EEI
effect, and the behavior of DrðTÞ/T
1=2at low temperature
regime is then attributed to EEI effect. The qðTÞdata at
high temperature regime are compared with Matthiessen’s
rule,16q¼q0þqðTÞ,w h e r e q0is the residual resistivity
andqðTÞis the resistivity caused by thermal phonons and is
expressed by the Bloch-Gr €uneisen (B-G) formula.21The
s o l i dc u r v e si nF i g . 1are the least-squares fits to the B-G
formula. Clearly, the experimental data are consistent withthe theoretical predications, indicating that FTO films pos-
sess typical metallic properties in electrical transport prop-
erties. The Debye temperatures h
Dobtained from the fitting
processes are 1096 and 1174 K for films, Nos. 1 and 2,respectively. According to the Hall effect measurement, the
main charge carriers in the FTO films are electrons. The
inset (b) of Fig. 1shows the temperature dependence of car-
rier concentration nH(we denote the carrier concentration
obtained through Hall effect measurement as nH)f r o m2t o
300 K. Clearly, the magnitudes of nHare almost invariable
with temperature over the wh ole measured temperature
range. For metals or degenerate semiconductors, activation
energy is not required to donate to the charge carriers.22
Hence, the result that nHis almost independent of Tfrom
liquid helium temperatures to 300 K confirms the metallic
transport nature of FTO films.
Figure 2displays the thermoelectric power S(thermo-
power or Seebeck coefficient) as a function of temperature
for the two FTO films from 10 to 300 K. Clearly, the thermo-powers are negative and vary linearly with temperature over
the whole measured temperature range. The negative thermo-
power means the main charge carrier is electron instead ofhole, which is identical to the result obtained from Hall effect
measurements. In a typical metal, the thermopower generally
contains contributions from two separate mechanisms: ther-mal diffusion of electron and phonon-drag.
23When a temper-
ature gradient is present in a sample, the electrons from the
hotter end will tend to diffuse towards the colder one, then athermoelectric potential difference between the hotter and
colder ends DVwill be generated. Then, the electric field in
the sample can be written as ~E¼SrT, where S/C25/C0DV=DT
is the thermopower. The phonon-drag thermopower origi-
nates from the e-ph interaction. When the temperature gradi-
ent is present, the phonon distribution will no longer be inthermodynamic equilibrium (there will be a heat flow carried
by phonons), and this asymmetry characteristic of the temper-
ature will influence the diffusion by the phonon-electron col-lisions. This is the phonon-drag effect. According to free-
electron model, the diffusive thermopower of pure metals at
low temperatures ðT/C28h
DÞis given by23
Sd¼/C0p2k2
BT
3jejEF; (1)
where eis the electron charge and EFis the Fermi energy.
Since the phonon-drag thermopower does not vary linearly
FIG. 1. Normalized resistivity as a function of temperature for our two FTO
films, Nos. 1 and 2. The symbols are the experimental data and the solid
curves are the least-squares fits to B-G formula. For clarity, the data for film
No. 1 have been shifted by þ0.03. Inset (a): variation of normalized conduc-
tivity ½rðTÞ/C0rð50 KÞ/C138=rð50 KÞas a function of T1=2, and inset (b): carrier
concentration obtained from Hall effect measurements vs temperature
between 2 and 300 K.FIG. 2. Thermopower Sas a function of temperature for the two FTO films.
The solid straight lines are least-squares fits to Eq. (1).042110-2 W.-J. Lang and Z.-Q. Li Appl. Phys. Lett. 105, 042110 (2014)
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128.252.67.66 On: Sun, 21 Dec 2014 03:27:12with temperature (see further remarks below) and the meas-
uring temperatures are far less than hD, we compare our
measured S(T) data with Eq. (1)and the least-squares-fitted
results are plotted as solid lines in Fig. 2. Using the fitted val-
ues of EF, we can obtain the carrier concentration nof the
samples through n¼ð2m/C3EFÞ3=2=ð3p2/C22h3Þ, an expression
also based on the free-electron model. Here, m* is the effec-
tive mass of the carrier and is taken as 0.3 me(meis the
free-electron mass) for FTO.24The values of nare listed in
Table I. The values of nare smaller than that obtained from
Hall effect measurements, nH. Specifically, nis about one
half of nHfor film No. 1 and three-fourths of nHfor film No.
2. In Sb doped SnO 2, considering that the conduction band is
not strictly parabolic and the effective mass increases
slightly with increasing occupation of the conduction band(carrier concentration), Egdell et al.
25,26found that the width
of the occupied part of the conduction band calculated by
using a modified free-electron model can be quantitativelycompared with that obtained from ultraviolet photoelectron
spectroscopy measurement.
27While assuming that the con-
duction band is strictly parabolic and m* is fixed, they
obtained a width that is much less than the experimental one.
For FTO, besides the Sn 5 sstates, both the Sn 5 pand F 2 p
states also have a little contribution to the conductionband.
14Hence, the energy-momentum dispersion relation in
the vicinity of the conduction band minimum is not strictly
parabolic either. The underestimate of nin FTO could partly
arise from neglecting the variation in m* with carrier concen-
tration. On the other hand, Eq. (1)and the relation between
EFand nare both derived from the standard free-electron
model, hence a slight deviation to the parabolic curve for the
conduction band itself could lead to a discrepancy between n
andnH.
At low temperatures, the phonon-drag thermopower Sg
can be approximately written as23
Sg’/C0CL
3njejsph
sphþsph–e; (2)
where CLis the heat capacity of the lattice per unit volume,
sph–eis the phonon-electron (ph- e) relaxation time, and sphis
the phonon relaxation time for all the other phonon scattering
processes. Assuming sph/C28sph–e,28one can obtain the equation
Sg’Sdsph=ð2se–phÞby using the energy-balance relation
sph–eCe¼se–phCL,29where se-phis the e-ph relaxation time and
Ceis the heat capacity of electron s per unit volume. Using the
relation sph¼3j=ð/C22v2
sCLÞ(where jis the thermal conductivity
of phonons and /C22vsis the mean velocity of sound) and the exper-
imental data of CL,30j,31and/C22vs(Ref. 32)f o rS n O 2,w eo b t a i n
the values of sphare 2.3 /C210/C011,2 . 2/C210/C012,1 . 5/C210/C012,a n d
9.8/C210/C013s at 10, 50, 100, and 200 K, respectively. Thetheoretical values of se-phfor film No. 1 (No. 2) are 1.0 /C210/C08
(3.7/C210/C08), 4/C210/C010(1.5/C210/C09), 1/C210/C010(3.7/C210/C010),
2.6/C210/C011s( 9 . 4 /C210/C011s), at the corresponding tempera-
tures, respectively. Here, we take se–ph/C25se–t;ph(se/C0t,phis the
relaxation time of electron-tran sverse phonon scattering) since
the electron scattering by tran sverse phonons dominates the e-
ph relaxation (see further remarks below). Thus, the contribu-tion of S
gto the total thermopower is no more than 2% of that
ofSdat the whole measured temperatures, and can be safely
ignored. We note in passing that in addition to what we have
found in the previous discus sion that FTO shows metallic
behaviors in electrical transport p roperties, the linear tempera-
ture behavior of S(T) and the comparability between nandnH
further indicate that the charge carriers in FTO possess the fea-
tures of free-electron-like Fermi gas.
Now we investigate the quantum transport properties of
the samples. We note that the drop of the resistivity from
300 K down to Tminis only /C245% (6.5% for film No. 1 and
4.0% for No. 2), which indicates the presence of a high level
of disorder in the films. This is confirmed by the slight incre-
ment of the resistivity below Tmin. The values of disorder pa-
rameter kF‘, deduced from free-electron-like model, are
/C256.6 and /C2524.1 for films, Nos. 1 and 2, respectively, where
kFis the Fermi wave number and ‘is the mean free path of
electrons. This indicates that the films fall into the weak-
localization region.29In dirty metals and alloys, a lot of
investigations have been carried out to detect the electronscattering processes and it has been established that the e-ph
scattering is the sole dominant inelastic dephasing process in
3D weakly disordered conductors.
29,33Recently, Zhang
et al .34found that the small-energy-transfer e-escattering
can govern the dephasing process in thick ITO films. Their
observation demonstrated the validity of the Schmid-Altshuler-Aronov theory of 3D small-energy-transfer e-e
scattering rate in disordered conductors.
17,18However, the
predication of the theory of 3D large-energy-transfer e-e
scattering rate has not been clearly observed and quantita-
tively tested up to now. According to Schmid,17the total e-e
scattering rate in 3D disordered conductors can be written as
1
see¼p
8kBTðÞ2
/C22hEFþffiffiffi
3p
2/C22hffiffiffiffiffiffiEFpkBT
kF‘/C18/C193=2
: (3)
The first term on the right hand side of Eq. (3)(denoted as
1=sL
ee) represents the contribution of large-energy-transfer e-
escattering process and would dominate at kBT>/C22h=se,
while the second term (denoted as 1 =sS
ee) stands for the con-
tribution of small-energy-transfer process and would domi-
nate at kBT</C22h=se, where seis the electron elastic mean free
time. Inspection Eq. (3)indicates that 1 =sS
eeis proportional
toðkF‘Þ/C03=2while 1 =sL
eeis independent of kF‘. We notice
that the kF‘values of the ITO films used in Ref. 34range
from 1.7 to 3.5, which are much less than that of the FTO
films. While the carrier concentrations nH(orEF) of the FTO
films are close to that of the ITO films. We expect the large-energy-transfer e-e scattering process, which was not
observed in ITO films, would dominate over the small-
energy-transfer one at higher temperatures in our FTO films.Then the theoretical predication of the total e-escattering
rate in Eq. (3)would be tested.TABLE I. Parameters for the two FTO films, Nos. 1 and 2. qis resistivity
andnis carrier concentration deduced from thermopower measurements. AS
ee
andAL
eeare defined in Eq. (4)andðAS
eeÞthandðAL
eeÞthare predicted by Eq. (3).
q(300 K) nAS
ee ðAS
eeÞthALee ðAL
eeÞth
Film (m Xcm) (1020/cm3)( K/C03=2s/C01)( K/C03=2s/C01)( K/C02s/C01)( K/C02s/C01)
1 1.07 0.95 1.38 /C21089.41/C21075.25/C21071.03/C2107
2 0.28 1.36 4.69 /C21071.32/C21076.16/C21079.77/C2106042110-3 W.-J. Lang and Z.-Q. Li Appl. Phys. Lett. 105, 042110 (2014)
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128.252.67.66 On: Sun, 21 Dec 2014 03:27:12To obtain the temperature dependence of electron
dephasing rate 1 =suof the FTO films, we measured the low
field MR at different temperatures from 2 to 35 K. The
dephasing rate 1 =suðTÞwas then extracted by least-square
fitting the MR data to 3D weak localization theory.17The
details of the fitting procedure have been discussed in the
previous study.34Figure 3shows the variation in 1 =suwith T
for the two FTO films, as indicated. We found that only the
second term in Eq. (3)cannot describe experimental 1 =suðTÞ
data, i.e., the small-energy-transfer e-escattering effect alone
cannot explain the electron dephasing process in the FTO
films. Here, we consider the contributions of both small- andlarge-energy-transfer e-escattering processes and compare
our measured 1 =s
udata with the following equation:
1
su¼1
s0
uþAS
eeT3=2þAL
eeT2; (4)
where the first, second, and third terms on the right hand side
stand for T-independent contribution, small-, and large-
energy transfer e-escattering rates, respectively. The solid
curves in Fig. 3are the least-squares fits to Eq. (4). Clearly,
the experimental dephasing rate can be well described by
Eq.(4). The fitted values of AS
eeandAL
ee, together with their
theoretical values ðAS
eeÞthandðAL
eeÞthdeduced from Eq. (3),
are listed in Table I. The experimental values of AS
eeðAL
eeÞare
within a factor of /C244(/C246) of the theoretical ones. This level
of agreement is acceptable. The value of AL
eeis about one
half of that of AS
eefor film No. 1 and the two values are very
close for film No. 2, which indicates the large-energy-trans-fere-escattering process has already played important role
at liquid helium temperatures in the FTO films.
Besides the large-energy-transfer e-escattering, the e-ph
scattering process also give a T
2temperature dependent contri-
bution to the electron dephasing rate. Theoretically, the elec-
tron scattering by transverse phonons dominates the e-ph
relaxation. In the quasi-ballistic limit ( qT‘>1, where qTis the
wavenumber of a thermal phonon), the relaxation rate is
expressed as35,361=se–t;ph¼3p2k2
BbtT2=½ðpFutÞðpFlÞ/C138,w h e r e
bt¼ð2EF=3Þ2NðEFÞ=ð2qmu2
tÞis the electron–transverse pho-
non coupling constant, pFis the Fermi momentum, utis the
transverse sound velocity, qmis the mass density, and N(EF)i sthe electronic density of states at the Fermi level. For FTO,
using ut/C253120 m/s,32one can readily obtain qT‘/C25
kBT‘=/C22hut/C250:15T and 0.53 T for films, Nos. 1 and 2 (the val-
ues of ‘are derived using free-electron-like model). Hence,
our films lie in the quasi-ballistic region above /C247K . T h e
electronic parameters can also be obtained using free-electron-
like model; we take qm/C256950 kg/m3,37the theoretical values
of 1=se–t;phare computed and approximately 9.7 /C2105T2s/C01
and 2.7 /C2105T2s/C01for films, Nos. 1 and 2, respectively.
Inspection of Table Iindicates the values of 1 =se–t;phare/C242
order of magnitudes less than the contribution of large-energy-
transfer e-escattering term. Hence, the contribution of e-ph
relaxation can be safely ignored in the FTO films. In fact, the
e-ph scattering rate 1 =se–t;phis proportional to the carrier con-
centration n,35,36while Eq. (3)predicts 1 =sS
ee/n/C04=3and
1=sL
ee/n/C02=3. The carrier concentrations in our FTO films
are/C242/C21020cm3,w h i c hi s /C242 to 3 orders of magnitude
lower than that in typical metals. Thus, the magnitudes of both1=s
S
eeand 1 =sL
eeare greatly enhanced over the magnitude of e-
ph scattering rate in the FTO films, which then give us the op-
portunity to fully demonstrate the validity of the theory of e-e
scattering rates in 3D disordered conductors.
In summary, both Boltzmann and quantum-interference
transport properties of thick FTO films were investigatedexperimentally in the present Letter. We found that the resis-
tivity q(T) can be well described by the Bloch-Gr €uneisen
law over a wide temperature range from 300 K down to T
min,
while the carrier concentrations are independent of tempera-
ture from 2 to 300 K. These results, together with linear tem-
perature dependence of thermopowers, demonstrate that theconduction electrons in the FTO films possess free-electron-
like characteristics. We also found that both the large- and
small-energy-transfer e-e scattering effect dominate the
dephasing process in the measured temperature range (2 to
35 K) in the FTO films. Both the linear temperature behavior
ofS(T) and strong e-escattering effect in FTO film are
related to the slow e-ph relaxation rate, or equivalently, low
carrier concentration characteristic of this highly degenerate
semiconductor.
The authors are grateful to Xin-Dian Liu and Pei-Jen
Lin for valuable discussions. This work was supported by
the NSF of China through Grant No. 11174216 and Research
Fund for the Doctoral Program of Higher Education throughGrant No. 20120032110065.
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128.252.67.66 On: Sun, 21 Dec 2014 03:27:12 |
1.4893540.pdf | Dynamic control of local field emission current from carbon nanowalls
Ying Wang, Yumeng Yang, and Yihong Wu
Citation: Journal of Vacuum Science & Technology B 32, 051803 (2014); doi: 10.1116/1.4893540
View online: http://dx.doi.org/10.1116/1.4893540
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/32/5?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41Dynamic control of local field emission current from carbon nanowalls
Ying Wang, Yumeng Y ang, and Yihong Wua)
Information Storage Materials Laboratory, Department of Electrical and Computer Engineering,
National University of Singapore, 4 Engineering Drive 3, Singapore 117583
(Received 29 April 2014; accepted 7 August 2014; published 21 August 2014)
The authors report on a systematic study of modulation of the field emission current from carbon
nanowalls using a sharp probe as the anode in an ultrahigh vacuum system. Modulation of the local
emission current was achieved by either varying the anode–cathode distance ( d) with the aid of an
AC magnetic field or superimposing a small AC bias on a DC bias during the field emission
measurement. Current modulation ratio of over two orders of magnitude was achieved with the
modulation becoming more efficient at a smaller d. The experimental results are discussed using
the Fowler–Nordheim theory in combination with a simple cantilever model to account for the
modulation effect. The experimental results demonstrated good static stability and dynamic
controllability of local field emission current from the carbon nanowalls. VC2014 American Vacuum
Society .[http://dx.doi.org/10.1116/1.4893540 ]
I. INTRODUCTION
Vertically aligned two-dimensional (2D) carbon with
self-supported network structures, such as carbon nanowalls
(CNWs) or carbon nanosheets (CNSs) have drawn much
attention as potential emitter materials for nanoscale fieldemission devices due to their large height-to-thickness ratio,
rigidity, and endurance.
1–3So far, various experimental
efforts have been made to improve the field emission charac-teristics (such as turn-on electric field and stability of emis-
sion current) of CNW/CNS; these include but are not limited
to (1) reducing the screening effects among adjacentCNW/CNS flakes through selective growth,
4–8(2) improving
the structure and morphology of CNW/CNS via fine tuning
of the synthesis conditions, such as the types of carbonfeedstock,
9gas flow ratio,10–13deposition temperature,14
substrate temperature,12and growth time,13(3) chemical
doping to reduce the turn-on field,14–17and (4) surface
treatment to improve the field emission characteristics of the
as-grown CNW/CNS, such as selective coating of a thin
layer of Mo 2C,18Au, Al, and Ti,19plasma surface modifica-
tion,20and thermal desorption of absorbed hydrocarbons.21
Most of the experimental results can be successfullyexplained by the Fowler–Nordheim (F–N) model,
22which
predicts a linear relation between emission current ( I) and
applied electric field ( E) in the F–N plot [i.e., ln( I/E2)v s
1/E], though slight modification is sometimes needed to bet-
ter account for the experimental observations. So far, very
low turn-on field (i.e., the macroscopic electric field for an
emission current density of 10 lA/cm2) in the range
/C240.23–6 V/ lm has been reported on large-area samples (typ-
ical sample area larger than 1 mm2) using a parallel plate
configuration.9,10,13,23–28A stable milliampere-level field
emission current for a duration of 1–200 h has been achieved
with both dand macroscopic applied electric field being kept
constant. These results demonstrate the great potential ofCNW/CNS as an efficient electron emitter for various appli-
cations. In addition to field electron emission sources,nanosized carbon emitters may also find applications in
nanoscale vacuum electronic devices. For the latter purpose,in addition to static stability, good controllability over the
emission current in a large dynamic range is also of crucial
importance, such as those demonstrated in the gated fieldemitter design.
27,29,30Considering the fact that practical
nanoscale vacuum electronic devices are to be based on
electron emission from nanosized emitters with the anode–cathode distance in the nanometer range, it is of great
importance to study both the static and dynamic emission
characteristics of CNW/CNS in an experimental configura-tion, which resembles the actual device design and at the
same time allows to perform the experiments in a controlla-
ble fashion. In this sense, the nanoprobe setup reported inour previous work is an ideal platform to carry out the
intended studies.
31,32
In the previous work, we have investigated systematically
the relationship between turn-on field and the anode–cathode
distance for localized field emission from CNW/CNS samples.
In this work, we study the dynamic properties of local fieldemission current from the CNW via three different
approaches. In the first approach (or approach I), we used an
in-situ AC magnetic field to periodically alter the distance
between the CNW cathode and a sharp magnetic anode (i.e., a
Ni probe) under a constant bias voltage. A schematic illustra-
tion of the experimental setup and energy diagram are shownin Figs. 1(a)and1(d), respectively. In the second approach (or
approach II), a small AC modulating bias is superimposed on
a DC bias to modulate the overall voltage bias across the ano-de–cathode gap [Figs. 1(b)and1(e)]. This provides a variable
macroscopic electric field between 0.3 and 3 kV/ lmi nt h e
direction along the emission gap. As a variation of approach I(hereafter referred to as approach III), the magnetic tip is
replaced by a nonmagnetic one, and instead, the CNW is in-
situcoated with a thin layer of Fe [Figs. 1(c)and1(f)]. As we
will discuss in Sec. III, in approaches I and III, current modu-
lation is mainly achieved through varying the anode–cathode
gap. As for approach II, although both bias voltage and anode-distance variations are expected to play a role, experimental
results suggest that distance variation induced effect is
a)Electronic mail: elewuyh@nus.edu.sg
051803-1 J. Vac. Sci. Technol. B 32(5), Sep/Oct 2014 2166-2746/2014/32(5)/051803/9/$30.00 VC2014 American Vacuum Society 051803-1
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41dominant. The first two approaches are effective in modulat-
ing the emission current by over two orders of magnitude and,
more importantly, the emission current is stable during theentire duration monitored.
II. EXPERIMENT
A. CNW and probe preparations
The CNWs were grown on Cu substrates using micro-
wave plasma-enhanced chemical vapor deposition. Thegases used were a mixture of CH
4and H 2with typical flow
rates of 40 and 10 sccm, respectively. Before the CH 4gas
was introduced to the quartz tube to commence the growthof nanowalls, the substrate was preheated to about
650–700
/C14C (limited by the microwave power) in hydrogen
plasma with a bias of 50 V for 10–15 min. The typicalgrowth time was 1–5 min. Details about the growth condi-
tions, morphology, and structural properties of the as-grown
CNWs can be found elsewhere.
1,24For local field emission
measurements, sharp W and Ni probes were used as the
anode. These probes were fabricated by electrochemical
etching of W and Ni wires in NaOH and KCl solution (2M),respectively, using the drop-off lamellae method.
33During
each round of etching process, two probes are formed above
and below the electrolyte lamellae, respectively. The lowerprobe with a larger taper length is always used for the pres-
ent work. The Ni or W probes were loaded into the vacuum
chamber immediately after the preparation to minimize airexposure. The CNW/Cu sample was fastened onto a sampleholder, which itself forms part of an in-situ electromagnet
that is described below.
B. Experimental setup for field emission
All the field emission measurements were performed in an
Omicron UHV nanoprobe system with a base pressure better
than 2.2 /C210/C010mbars at room temperature. The nanoprobe
system is equipped with four independently controllablenanoprobes and each probe module uses a piezoelectric iner-
tia drive to achieve step motion with nanometer precision.
Furthermore, the autoapproach capability of the probesensures safe and nondestructive approach of probe to the
sample surface. The whole measurement system is installed
on a vibration isolation table using air legs, which itself isplaced on the ground floor of a building to further minimize
external disturbances. All these are critical for achieving
precise control of din the field emission measurements. The
in-situ scanning electron microscope (SEM) allows for site
specific field emission measurements down to nanometer
scale. The sample stage is fitted with an electromagnet, whichis able to supply a vertical field up to 2000 Oe near the sam-
ple surface. The field can be controlled by an external bipolar
power supply (Keithley 6221). Figure 2shows a photo of the
sample stage and schematic of the probe and electromagnet
setup. All measurements were carried out in a
LABVIEW -based
program, which synchronises all source meters and allowsreal-time monitoring of the field emission current.
C. Calibration of probe step height
Prior to field emission measurements, calibration measure-
ments were performed to determine the step size of the probe
(i.e., anode) using gold pads of different heights (0.2–2 lm),
formed on a flat and heavily doped silicon substrate using
standard optical lithography (Fig. 3). The exact height (h) of
these patterned structures was measured by atomic forcemicroscope. The detailed calibration procedure is as follows.
First, a sharp probe was first approached (or lowered) to tun-
neling regime of the surface of a gold pad using the autoap-proach function of the nanoprobe controller (step 1 in Fig. 3).
After a high-resistance electrical contact was achieved, the
feedback loop of the controller was deactivated to allow man-ual control of the probe position. The probe was then lowered
further manually while the differential contact resistance is
closely monitored until an ohmic contact was formedbetween the probe and the gold pad. Second, the probe was
manually moved horizontally ( <2lm) to above the trench
between the gold pads, upon which the probe was loweredstep-by-step at a preset speed till a contact with similar differ-
ential resistance was achieved (step 2 in Fig. 3). The total
number of steps ( N) was recorded. The step size for down-
ward probe motion was then calculated as S
down¼h/N. The
above steps were repeated for many times ( >15) to obtain
the average downward step size ( hSdowni).
The lifting or upward step size ( Sup) was obtained by lift-
ing the probe step-by-step at a preset speed for a certain step
number ( Nup) and then bringing it back into contact with the
silicon substrate with a total number of steps ( Ndown); this
FIG. 1. (Color online) Schematic diagram of dynamic control of field emis-
sion current from (a) bare CNW with a Ni anode in an AC magnetic field,
(b) bare CNW with an AC electric field, and (c) Fe/CNW with a W anode in
an AC magnetic field. The corresponding energy diagrams for (a), (b), and
(c) are shown in (e), (f), and (g), respectively.051803-2 Wang, Yang, and Wu: Dynamic control of local field emission current from CNW 051803-2
J. Vac. Sci. Technol. B, Vol. 32, No. 5, Sep/Oct 2014
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41gave the step size for upward motion: Sup¼Ndown/C2hSdowni/
Nup. This process was then repeated for many times to obtain
the average lifting step size ( hSupi), which turned out to be
/C241.38 nm/step for the chosen speed.
D. Shape formation of anode probe
A ball-shaped probe (W or Ni) of desired size
(100 nm–2 lm) is subsequently prepared through a three-step
in-situ local electrical melting process by first applying a bias
of appropriate amplitude between a sharp probe apex and the
body of a relatively blunt W probe and then bringing them intocontact for a self-limited di scharge [steps 1 and 2 of Fig. 4(a)].
Upon formation of contact between the two probes, a closed
current loop is immediately estab lished and within a very short
interval, the local heat generated by electrical discharge melts
the apex of the anode probe into a sphere and automatically
opens the circuit [step 3 of Fig. 4(a)]. Figure 4(b)shows some
typical SEM images of probes as prepared (sub-100 nm in apex
size) and after (600–2200 nm) the local electrical melting pro-
cess. It should be emphasized that this process is necessary to
create a smooth anode surface without sharp protrusions, which
is in turn crucial in determining accurately the anode–cathodedistance and obtaining good reproducibility.
E. Procedures of performing local field emission
measurements
The blunt W probe was firmly pressed onto the CNW
sample to form a low resistance electrical contact with theCNW cathode, while the anode was carefully approached tot h et o pe d g eo fas i n g l eC N Wfl a k et h r o u g hm o n i t o r i n gt h e
differential contact resistance using a lock-in amplifier setup.
31
After the electrical contact between the CNW and anode was
achieved, the anode was then lifted by a certain Nupwith the
precalibrated hSupito serve as an anode for field emission
measurements at determined d¼Nup/C2hSupi. Despite the fact
that the edge of 2D carbon is not flat microscopically, emis-
sion of electrons occurs at 2D carbon sites that protrude along
the direction of the applied electric field. These sites areexpected to form contacts with the metallic anode first during
the distance determination process described above. Thus, dis
naturally the distance between the emission sites and the an-ode bottom surface. A typical SEM image taken during a field
emission measurement is shown in Fig. 4(c).
The measurements always began with ramping up the
bias voltage till an emission current setpoint (typically 1–10
nA) was reached. The bias voltage was then kept constant
for monitoring the emission current at a sampling rate of8.3 Hz. The static stability of emission current was moni-
tored in the first few hundred seconds. Upon reaching a sta-
ble emission, an external sinusoidal magnetic field H¼H
0
sint[for approach I, Fig. 1(a)] or a small superimposed sinu-
soidal voltage bias DV¼DV0sint[for approach II, Fig.
1(b)] was manually applied to examine the response of field
emission current to the AC magnetic or electric field.
Typical period of AC field is 15.3 s. The magnetization of
all Ni probes was saturated in a large magnetic field alongthe emission gap direction prior to measurements.
After all measurements on bare CNW, the samples were
in-situ evaporated with a thin Fe layer of a nominal thickness
FIG. 2. (Color online) Photo of the sample stage and schematic of the probe and electromagnet setup used in this work.
FIG. 3. (Color online) Schematic diagrams showing the process of calibrating the downward step size of probe on patterned gold features.051803-3 Wang, Yang, and Wu: Dynamic control of local field emission current from CNW 051803-3
JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41of 5 and 26 nm for approach III. A large magnetic field along
the emission gap direction was first applied to saturate themagnetization of the Fe coating. The same field emission
measurements as in approach I (except for the Ni probe
replaced by a W probe) were then repeated on the Fe-coatedCNW samples [Fig. 1(c)].
III. RESULTS AND DISCUSSION
A. Stability of local field emission
The local field emission measurements were monitored
closely using the in-situ scanning electron microscope. No
visible shift of the relative positions of the studied CNWflake and probe was observed. The good stability has further
been confirmed by the current–time plot [Fig. 5(a)], in which
the emission current stays very stable under static conditionsthroughout the whole monitoring process except for theinitial training period of the first few tens of seconds. The
spike at 454–456 s is presumably caused by isolated externaldisturbance after which the emission current recovered to its
original value. The measurement was manually stopped after
/C2410 min, which is the typical duration used for one round of
modulation measurements. Nevertheless, this does not mean
that the emission current is only stable for this period of
time. It should be noted that Fig. 5(a)is obtained at a much
higher emission current (i.e., /C24150 nA) than the typical cur-
rent ( <10 nA) in the dynamic response study at zero modu-
lation field. The purpose is to show that the emission currentremains sufficiently stable even for a larger current than the
preset sourcemeter compliance (i.e., 100 nA) during the
current modulation investigation. For example, Fig. 5(b)
shows a small stable local field emission current over a time
span of over 40 min, obtained with the W probe shown in
the inset.
It should be emphasized that screening effect from neigh-
boring CNW flakes should be negligible since the spacing
between adjacent flakes is normally /C241lm, which is much
larger than the investigated range of distance (i.e.,
/C241n m<d<12.4 nm). If this was not the case, the measured
field emission characteristics should depend on the anodesize. To confirm this, we have performed field emission
measurements on the CNW sample with different anode
sizes ranging from 600 to 2200 nm. Considering that a d
larger than 150 nm requires a field emission ignition voltage
higher than the maximum output voltage of the source meter
(Keithley 2400), the emission current is predominantly com-ing from the CNW flake directly under the anode. As seen in
Fig. 5(c), the relationship between dand the electric field
required for an emission current of 1 nA obtained withdifferent anode sizes closely overlap with each other. The
increase of the required field with decreasing distance is due
to a smaller field enhancement factor (defined as the ratiobetween the actual local electric field at the emitter surface
and the macroscopic field) at a smaller d.
32These results
strongly show that the effect of neighboring flakes on bothfield distribution and emission current is negligible in our
experimental setup.
B. Dynamic control of local field emission current with
a Ni anode in an AC magnetic field
Figure 5(d) shows the typical response of the emission
current to 15 cycles of an external sinusoidal magnetic field
of amplitude H0¼80–158 Oe, obtained from the location
shown in Fig. 4(c). The measurement was performed at zero-
field distance d0¼11 nm (i.e., distance in zero magnetic
field), and the relation between emission current and electric
field at a fixed distance was found to be in good agreementwith the F–N model, in consistence with our previous
work.
32Figure 6(a) is a color contour plot of emission
current as a function of time where the color scale has beennormalized with respect to the zero-field current ( I
0, defined
as the emission current at t¼0 without magnetic field).
Superimposed with the color contour plot are the emissioncurrent in one cycle of a sinusoidal magnetic field of
FIG. 4. (Color online) (a) Three-step schematic of the local electrical melting
process. (b) Typical SEM images of probes as prepared and after the electri-
cal melting process. All scale bars are 1 lm. (c) SEM image for local field
emission measurements on CNW/Cu using a Ni probe as an anode atd¼11 nm. The lower inset is a close-up view of the as-grown CNW (scale
bar: 500 nm).051803-4 Wang, Yang, and Wu: Dynamic control of local field emission current from CNW 051803-4
J. Vac. Sci. Technol. B, Vol. 32, No. 5, Sep/Oct 2014
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41amplitude H0¼37 Oe (white solid curve) and 136 Oe (black
solid curve), respectively; the dotted line indicates the time
when the emission current returns to I0. When H0<50 Oe,
the time-dependence of emission current exhibits approxi-
mately a sinusoidal shape in the positive half-cycle but a
rather flattened shape in the negative half cycle. This is adirect consequence of the combined effects of the variation
ofdcaused by the magnetostatic interactions between the Ni
probe and the applied magnetic field, and the exponential d-
dependence of field emission current. In a weak applied
magnetic field, the magnetization of the Ni probe is oriented
along the probe axis direction due to strong shape anisot-ropy. Further, the magnetic flux in the probe is expected to
be concentrated to the probe apex since it is magnetostati-
cally unflavored for the magnetic flux to leak out from theside walls.
34When a weak magnetic field is applied, the Ni
probe is magnetostatically deflected downwards (upwards)
depending on the direction of applied field, as illustrated inthe upper (lower) inset of Fig. 6(b). In turn, the attracted
(repelled) state of the Ni probe reduces (increases) the ano-
de–cathode distance [Fig. 1(e)], resulting in a larger
(smaller) emission current than I
0. To further elaborate this
point, typical I–tcurves corresponding to H0¼12.5–74.5 Oe
were fitted using the F–N relation22with the local electric
field replaced by F¼bV=½d0ð1/C0a@Hz=@zÞ/C138
I¼Sab2
UV
d01/C0a@Hz
@z/C18/C192
643
752
exp /C0bU3=2
bd01/C0a@Hz
@z/C18/C19
V0
B@1
CA;
(1)where ais a constant in unit of nm /C1Oe/C01characterizing the
strength of the probe-field interaction, Sis the emission area
(in the order of hundreds of nm2), and a¼1.54/C210/C06A
V/C02eV,b¼6.83 eV/C03/2Vn m/C01,A¼5 eV, V ¼53.6 V, and
d0¼11 nm. bis the field enhancement factor, which also
depends on d.32However, the change of bis typically less
than 1.5% for all investigated H0and contributes insignifi-
cantly to the observed change in the emission current. Thus,
a constant bof 1.15 (calculated from the slope of the F–N
curve at d0¼11 nm) has been used to simplify the following
discussion. In Eq. (1),@Hz=@zis the field gradient near the
apex of the probe. In the specific magnet design used in thiswork, the gradient is approximately given by 5 /C210
/C07H(in
unit of Oe /C1nm/C01), where H (in unit of Oe) is the field
strength at the top surface of the central magnetic pole. Theexperimental data (symbols) are plotted together with the op-
timum fitting curves (blue solid curves) in Fig. 6(c).A l l
curves but the lowest one have been shifted vertically forclarity, and the figure beside each curve is the corresponding
H
0value in unit of Oe. It can be seen that the fitting results
are satisfactory for H 0/C2049.8 Oe. Furthermore, inset of Fig.
6(c) compares the extracted maximum deflections of the
probe [ Dd¼d0a(@Hz=@z)] at different H0(symbols) and the
simulation result (solid line) from a simple relation derivedfrom a cantilever model
35
Dd¼/C0 sinhL ðÞ3l0Msvcosh
3EmI/C3@Hz
@z; (2)
where l0is the permeability of free space, L¼8.5 mm is the
probe length, h¼45/C14is the angle between the probe axis and
the normal of the sample surface, I*¼1.92/C210/C04mm4is the
FIG. 5. (Color online) Typical field emission stability measurement with constant bias voltage at (a) large and (b) small emission current. The current co mpliant
was set to 400 nA. Inset of (b) shows the W probe used for the measurement. (c) Dependence of the electric field required for 1 nA emission current on ano-
de–cathode distance, obtained from CNW with probe of different sizes indicated in legend. (d) Typical response of the emission current to 15 cycles of AC
magnetic field of different amplitudes (H 0).051803-5 Wang, Yang, and Wu: Dynamic control of local field emission current from CNW 051803-5
JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41inertia, v/C253.27/C210/C02mm3is the volume estimated from the
probe shape and dimensions, Ms¼5.12/C2105A/m is the satu-
ration magnetization of fcc Ni, and E m¼2.07/C21011N/m2is
the modulus of elasticity for Ni. Good agreement is obtained
between experimental data and simulation results. It is worthmentioning that although the effect of magnetostriction cannot
be ruled out completely, it does not play a significant role in
modulating emission current in the present work mainly dueto two reasons: (1) the negative magnetostrictive strain of pol-
ycrystalline Ni (Ref. 36) would result in an increase of dand
in turn a smaller emission current in an applied magnetic field,in contradiction to Fig. 6(b),a n d( 2 )t h e I–H dependence
would be symmetric with respect to zero Hif magnetostriction
was the mechanism of the observed current modulation. Thelatter argument also excludes electron focusing as the domi-
nant mechanism.
ForH
0>62.1 Oe, a second peak in the emission current
is observed ( t¼/C2412 s) in the I–tcurves [Figs. 6(c) and
6(d)]. The origin of this second peak can be understood more
intuitively through the I–Hcurve shown in Fig. 6(e). When
the amplitude of the magnetic field is sufficiently large,
switching of the magnetization occurs in the Ni probe, lead-
ing to downward probe deflection at both positive and nega-tive half-cycles of the AC magnetic field [upper insets inFig.6(e)]. Interestingly, a few fine features in the I–Hcurves
are constantly observed due to the very sensitive exponentialdependence of the current on d. First, the steep increase in
emission current in the range from /C24/C070 to /C0100 Oe is cor-
responding to the reversal of the net magnetization in the Niprobe. One or a few small jumps before this magnetization
reversal are constantly observed at /C24/C060 Oe. This can be
seen more clearly in Fig. 6(f)where typical normalized I–H
curves corresponding to different H
0in the range from 87 to
148 Oe are shown. These jumps suggest that the magnetiza-
tion reversal of the Ni probe used in this work consists ofreversal of some small domains followed by a rapid reversal
of the magnetization of the entire probe. Second, the kink at
/C24/C090 Oe is believed to indicate the completion of the rever-
sal process of the net magnetization [Fig. 6(e)]. Further
increase of emission current ( H</C090 Oe) is presumably
caused by the increase of applied magnetic field gradient andby rotation of the net magnetization of the probe off the
probe axis toward the applied field direction. Lastly, the
emission current does not normally return to I
0when His
swept from H0to 0 Oe, but will recover to I0after a complete
cycle of magnetic field sweeping. This may be understood as
being caused by a certain degree of inelasticity of the probeunder a large magnetic field, though more in-depth analyses
FIG. 6. (Color online) (a) Response of field emission current to one cycle of sinusoidal magnetic field of different H0atd¼11 nm. Color scale is normalized
with respect to the emission current magnitude in zero magnetic field ( t¼0 s). Dotted lines indicate the time when the emission current returns to its zero- H-
field value. Superimposed with the color contour plot is the typical response of the emission current to a small (large) AC magnetic field in white (black ). (b)
and (e) Typical normalized I–Hcurves at small and large H0, respectively. Black arrows indicate the sweeping direction of the magnetic field. Insets illustrate
a simple cantilever model. (c) and (d) The response of emission current (symbols) to small and large AC magnetic fields in I–tplot, respectively. Solid curves
are the optimum fitting curves, and H0are indicated in unit of Oe beside the respective curves. Inset compares the maximum experimental probe deflection
(symbols) with simulation results (solid line) at different AC magnetic fields. (f) Kinks in the I–Hcurves constantly observed before reversal of net magnetiza-
tion of the Ni anode. H0is shown as figures beside the curves in unit of Oe.051803-6 Wang, Yang, and Wu: Dynamic control of local field emission current from CNW 051803-6
J. Vac. Sci. Technol. B, Vol. 32, No. 5, Sep/Oct 2014
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41are needed in order to reveal on the true behavior of this
nanoelectromechanical system. What is of importance hereis that these observations demonstrate strongly the excellent
stability of field emission current from CNW emitters.
Figure 6(d)shows the results of optimum fitting to the I–t
curves corresponding to larger H
0(/C2199.1 Oe) using Eq. (1).
The two peaks are fitted separately in similar procedure
described previously and with the same set of parametersexcept for a, which is weakly dependent on H
0and in the
range from 1.9 /C2104to 3.1 /C2104nm/C1Oe/C01. Comparisons
between the extracted Ddand the simulation result using Eq.
(2)show good agreements [inset of Fig. 6(d)]. In addition, it
is found that the probe deflection in the maximum investi-
gated magnetic field (148.1 Oe) is only /C242.5 nm. Even with
such a small deflection, a large current modulation ratio
(Imax/Imin) of over two orders of magnitude can be achieved
[inset of Fig. 7(a)]a td0¼11 nm.
C. Scalability of dynamic control of field emission
current
To further explore the scalability of dynamic control of
field emission current from CNW, similar field emissionmeasurements were performed at different dwith constant
H0¼40.46 Oe, which gives a probe deflection of /C240.7 nm.
The normalized I–trelation in three continuous cycles of si-
nusoidal magnetic field sweeping is shown in Fig. 7(b). The
superimposing lower and upper curves are I–tcurves at a dis-
tance of 1.38 and 11 nm, respectively. It can be seen that the
response of the emission current from 2D carbon to modula-
tion is well reproduced in all three cycles and shows a strongdependence on d. For the sake of clarity, Fig. 7(a)shows the
dependence of the I
max/Iminratios on different d(symbols)
and the averaged Imax/Iminratio is shown as the solid curve as
a visual guide. Clearly, the emission current modulation
becomes more efficient at a smaller d, suggesting that
dynamic control of local field emission current by varying d
is scalable in nanoscale field emission device applications.
D. Dynamic control of local field emission current with
a superimposing AC voltage bias
We next turn to modulating local field emission current
with an AC electric voltage of variable amplitude superim-
posed on a DC bias. The amplitude of the AC electric field(DE
0¼0.2–3.0 kV/ lm) is relatively small as compared to the
typical bias field (27.6 kV/ lm) at the investigated distance of
/C241.3 nm. A current modulation ratio of 1.3–123 has been
achieved [Fig. 8(a)]. To have a more in-depth understanding
of the modulation mechanism, Fig. 8(b)shows the typical ex-
perimental I–tcurves (symbols) corresponding to three differ-
entDE0¼0.5, 1.6, and 2.6 kV/ lm [dotted lines in Fig. 8(a)]
together with their optimum fitting curves (dotted curves)
using Eq. (1)with experimental parameters V¼38.1 V,
d0¼1.32 nm, H¼0 Oe, b¼0.2, and S¼/C24500 nm2. The
upper two curves have been vertically shifted for clarity. At
this point of discussion, it should be noted that bhas been
extracted from the slope of the F–N curve. The small value
ofbhas been discussed in detail in Ref. 32. Apparently, vari-
ation of bias voltage alone is unable to fully account for thelarge current modulation observed at such a small dof
1.32 nm, where effects arise from electrostatic interactions
can be significant under a large electric field. The most likelyexplanation for the enhanced current modulation is that dis
modulated too due to capacitive effects between the probe
and CNW [inset of Fig. 8(c)]. This argument is supported by
the significant improvement in the agreement between the ex-
perimental data and the fitting curves (solid curves in [Fig.
8(b)] with Eq. (1)with Hbeing replaced by the amplitude of
the AC electric field ( DE). To quantify the electrostatically
induced probe deflection and further examine the understand-
ing, the dependence of Dd
E(defined as d0aDE0)o nDE0is
shown in Fig. 8(c)(symbols). Assuming a simple capacitor-
and-cantilever model, the initial deflection of the probe under
a macroscopic electric field E0and zero AC electric field
(i.e.,DE¼0) can be estimated as
DdE0¼Ae0E2
0
6EmI/C3sinhL ðÞ3; (3)
where e0is the absolute permittivity and A¼3.48/C2105nm2
is the area of the bottom surface of the anode estimated from
FIG. 7. (Color online) (a) Dependence of current modulation ratio on dwith
H0¼40.46 Oe. Inset shows the current modulation ratio obtained in differ-
entH0atd¼11 nm. (b) Response of emission current to three continuous
cycles of AC magnetic field ( H0¼40.46 Oe) at different d. Color scale is
normalized with respect to the emission current magnitude at t¼0s .
Typical response of emission current to the magnetic field at a small (large)
dis shown as the superimposing lower (upper) curve.051803-7 Wang, Yang, and Wu: Dynamic control of local field emission current from CNW 051803-7
JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41the probe size. During the derivation, both the anode surface
and the CNW emitter have been assumed to be flat metallicsurfaces extending to infinity to simplify the discussion.
Taking Dd
E0as a reference deflection, the net deflection
caused by the superimposing AC electric field is given by
DdE¼Ae0E2
6EmI/C3sinhL ðÞ3/C0DdE0; (4)
where Eis the total macroscopic electric field. The simulated
dependence of DdonDE0is shown as the solid curve in
Fig.8(c). It can be seen that the simulation result agree withthe experimental data in the general trend for DE0¼0–2.3
kV/lm, though the experimental Ddincreases much faster
with the increase of DE0for larger superimposing AC elec-
tric field ( DE0>2.3 kV/ lm). The reason for the latter obser-
vation is still not clear yet. The difference betweenexperimental and simulated values is attributed to the large
morphological and electrical difference between CNW and a
flat metallic plate. Further systematic investigations arerequired in order to understand the true behavior of the elec-
tromechanical system involving the probe and the CNW in
this regime.
E. Dynamic control of local field emission current
from Fe/CNW with a W anode in an AC magnetic field
In view of potential difficulty with the use of magnetic
anode in certain applications, we have also investigated thepossibility of emission current tuning using a magnetic field
without resorting to a magnetic probe. To this end, the same
the field emission measurements with an AC magnetic fielddescribed Sec. III B were repeated on Fe-coated CNW at
d¼11 nm. A W anode was used instead of a Ni one so that
it does not respond to the variation of the applied magneticfield.
Although thin metal coating will inevitably change the
intrinsic field emission properties of CNW, it provides analternative route for emission current modulation which may
appear to be more attractive for certain applications. As
shown in Fig. 9(a), the emission current decreases with
increasing the magnetic field, indicating that dis increased
by the elastic deformation of the Fe-coated CNW. Based on
our previous work,
31,37it is understood that the coated Fe
has a large thickness near the edge. Therefore, when a mag-
netic field with gradient in the vertical direction is applied,
the CNW will be deformed due to attractive (repulsive)interactions between the Fe layer at the top edge and the
applied field. This will lead to a decrease (increase) of dand
hence an increase (decrease) in the emission current [Figs.1(c) and1(f)]. Current modulation ratio up to only 4.3 was
obtained for the maximum magnetic field amplitude investi-
gated (i.e., 591 Oe) for a 5 nm thick Fe layer and with a133 nm probe [Fig. 9(b)]. The relatively small modulation
ratio is presumably caused by both the rigidness of CNW
and wide spread of Fe on the CNW surface. This is furtherreflected in the fact that a thicker Fe layer (26 nm) reduces
the modulation ratio, which is more obvious when the meas-
urements were repeated with a larger probe (1.8 lm). This is
because in addition to generating a magnetostatic force
through interaction with the magnetic field, the Fe coating
also increases the rigidity of CNW. It is worth noting thatthis is just a proof-of-concept experiment; a much larger
modulation ratio is expected once the emitter structure is
optimized including the ferromagnetic coating layer.
IV. CONCLUSIONS
In summary, systematic experiments have been per-
formed to modulate the local field emission current fromCNW by varying the anode–cathode distance, and by
FIG. 8. (Color online) (a) Response of the field emission current to one cycle
of sinusoidal electric field of different magnitude ( DE0) superimposed on a
constant DC bias field at d¼/C241.3 nm. Color scale is normalized with
respect to the emission current magnitude at t¼0s . I–tcurves with three
typical DE0(indicated by dotted lines) are shown in (b). Solid (dotted) solid
curve is the fitting curve with (without) electrostatic interactions between
the anode and CNW taken into considerations. (c) Experimental (symbols)
and simulated (solid line) maximum electrostatically induced probe deflec-
tion at different DE0. Inset is a schematic of the capacitor-and-cantilever
model.051803-8 Wang, Yang, and Wu: Dynamic control of local field emission current from CNW 051803-8
J. Vac. Sci. Technol. B, Vol. 32, No. 5, Sep/Oct 2014
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.206.27.24 On: Thu, 02 Oct 2014 19:29:41varying the electric field in an UHV environment. Stable
field emission current was obtained and current modulationratio up to 105 and 123 has been achieved for the former and
latter case, respectively. The experimental results have been
explained by the F–N model in combination with a simplecantilever model to account for the change in either electric
field or anode–cathode distance. Our results have demon-
strated good stability of the local field emission current fromCNW during the emission current modulation process and
good scalability of current modulation at nanoscale, suggest-
ing that CNW is a reliable emitter material for nanoscalefield emission electronic devices. Although we have used a
probe as the anode in this work, in practical applications, the
CNW-probe configuration may also be replaced by a micro-electromechanical system involving 2D carbons.
ACKNOWLEDGMENTS
This work was supported by the National Research
Foundation of Singapore under Grant Nos. NRF-G-CRP2007-05 and R-143-000-360-281, and Agency for Science,
Technology and Research (A*STAR), Singapore, under
Grant No. R-398-000-020-305.
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FIG. 9. (Color online) (a) Response of the field emission current from Fe
(5 nm) coated CNW to one cycle of sinusoidal magnetic field of different H 0
atd¼11 nm. Color scale is normalized with respect to the emission current
magnitude at t¼0 s. Typical response of the emission current to a small
(large) AC magnetic field is shown as the superimposing dotted (solid)
curve. (b) Current modulation ratio obtained from CNW coated with two
different Fe layer thicknesses (5 and 26 nm) and using W probes of two dif-
ferent sizes (0.13 and 1.8 lm) as an anode.051803-9 Wang, Yang, and Wu: Dynamic control of local field emission current from CNW 051803-9
JVST B - Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena
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1.4898129.pdf | Grain size modification in the magnetocaloric and non-magnetocaloric transitions in
La0.5Ca0.5MnO3 probed by direct and indirect methods
M. Quintero, S. Passanante, I. Irurzun, D. Goijman, and G. Polla
Citation: Applied Physics Letters 105, 152411 (2014); doi: 10.1063/1.4898129
View online: http://dx.doi.org/10.1063/1.4898129
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134.117.10.200 On: Sat, 29 Nov 2014 23:09:12Grain size modification in the magnetocaloric and non-magnetocaloric
transitions in La 0.5Ca0.5MnO 3probed by direct and indirect methods
M. Quintero,1,2S. Passanante,1,3I. Irurzun,1,3D. Goijman,1and G. Polla1
1Departamento de F /C19ısica de la Materia Condensada, GIyA, GAIANN, Comisi /C19on Nacional de Energ /C19ıa At /C19omica,
Buenos Aires, Argentina
2Escuela de Ciencia y Tecnolog /C19ıa, Universidad Nacional de General San Martin, Buenos Aires, Argentina
3Departamento de F /C19ısica, Facultad de Ciencias exactas y naturales, Universidad de Buenos Aires,
Buenos Aires, Argentina
(Received 24 September 2014; accepted 1 October 2014; published online 17 October 2014)
The influence of grain size in the magnetic properties of phase separated manganites is an
important issue evidenced more than a decade ago. The formation of long range ordered phases is
suppressed as the grain size decreases giving place to a metastable state instead of the ground state.
In this work, we present a study of the magnetocaloric effect in the prototypical manganiteLa
0.5Ca0.5MnO 3as a function of the grain size. The differences obtained using direct and indirect
methods are discussed in the framework of domain walls in the ferromagnetic phase of the system.
VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4898129 ]
The discovery in 1997 of giant magnetocaloric effect
(MCE) near room temperature in Gd based compounds1trig-
gered a constant growth in the number of scientific publica-tions dedicated to the study of the mentioned effect. The
main motivation is the high cost of production of Gd which
difficult the production of magnetic refrigeration systems incommercial scale. A large number of compounds has been
proposed to replace Gd such as As based compounds,
2heus-
ler alloys,3and manganites.4
The MCE in solid materials is produced by the magnetic
entropy change induced when an external magnetic field is
applied. In standard ferromagnetic systems, an increase inthe magnetic field reduces the magnetic entropy and if the
field is applied adiabatically, the lattice thermal entropy
increases, giving rise in the sample temperature change.
In more complex systems, the above simplified scenario
may not be enough to describe the behavior of the entropy
change. A strong coupling between different degrees of free-dom (magnetic, electronic, etc.) is usually responsible for
such a mixed change of the state of the system by the appli-
cation of a magnetic field. Depending on the characteristicsof the different degrees of freedom the corresponding terms
in the first law of thermodynamic may increase the heat
change. But it can also be compensated, leading in a reduc-tion, the suppression, or even the inversion of the tempera-
ture change (the so called inverse magnetocaloric effect
(IMCE) (Ref. 5)). Because of this reason, a large number of
scientific works has been devoted to the understanding of the
MCE in cases beyond the standard ferromagnetic systems.
6
The most commonly used methods to study MCE can be
divided in two well distinguished groups, according to the
physical quantity that is measured to take account the effect.
The direct methods are those where the temperature
change or the heat exchanged with the environment is
directly measured. Once determined any of these magni-
tudes, the total entropy change can be estimated in non adia-batic conditions.
In the indirect methods, MCE is obtained through ther-
modynamic relations between the entropy and othermeasured magnitude such as magnetization or resistivity.
7
The most accepted way to obtain the entropy change is using
a Maxwell’s relation (MR)
@S
@H¼@M
@T:
Then, the entropy change can be estimated performing a nu-
merical integration of a set of magnetization loops at differ-
ent temperatures as
DST ;HðÞ ¼1
DTðH
0MT þDT;H0ðÞ /C0MT ;H0ðÞ ½/C138 dH0:
The main advantage of this approach is the use of a standard
experimental technique to reach the entropy values, instead
of a specific setup designed to measure the sample tempera-
ture change.8
During early years a lot of work was devoted to demo-
strate that, under certain circumstances, the results obtained
by MR were in good agree with those extracted from directmethods.
8But the use of the MR in cases where the system
is out of equilibrium can lead to an overestimation of
MCE.6,9In the last few years, due to the increase of the com-
plexity of the studied compounds, the validity of the MR
approach has been revised by a growing part of the scientific
community.6,10–12
The continuous search for materials with large MCE
stimulated further research in complex magnetic oxides,4,13
including mixed valence manganese based compounds, com-
monly named as manganites . One of the most interesting
properties of manganites is the spatial coexistence of regions
with different magnetic ordering, the so called phase separa-tion phenomena.
14In systems with phase separation (PS), it
is possible to tune the magnetic and structural properties by a
variety of parameters such as electric and magnetic field,strain, doping, confinement, and grain size.
15–17
In most of the cases of phase separation coexists an
insulating antiferromagnetic (AFM) charge ordered phase
0003-6951/2014/105(15)/152411/5/$30.00 VC2014 AIP Publishing LLC 105, 152411-1APPLIED PHYSICS LETTERS 105, 152411 (2014)
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134.117.10.200 On: Sat, 29 Nov 2014 23:09:12(CO) and a metallic ferromagnetic one (FM).18One of the
most studied systems with phase separation is La 0.5Ca0.5
MnO 3.19,20In this system, the coexistence between the dif-
ferent magnetic phases can be controlled by external stimuli(radiation, electric field) or by the modification of synthesis
parameters, particularly modifying the grains size (GS) in ce-
ramic samples.
17The increase in the GS favor the long range
ordering of the CO state over the FM. The low temperature
CO ground state of the system is strongly suppressed for
small GS, and when it is increased the system can reach the
CO state. The influence of GS in the MCE was recently stud-
ied in phase separated systems revealing a complex scenario,where the validity of the methods used to estimate the mag-
nitude of the effect must be carefully revised.
21,22
In this work, we present a study of MCE in the manganite
La0.5Ca0.5MnO 3which presents phase separation. The study
will be performed as a function of GS. We will compare the
results obtained from differential thermal analysis (DTA) andfrom indirect measurements with particular focus on the use
of the MR relation. The hysteresis of the magnetization loops
will be also analyzed and described in the framework of do-main walls displacement and related with the differences
observed between direct and indirect methods.
It has to be noted that the understanding of the phase
separation in the La
0.5Ca0.5MnO 3system escapes to the
aim of this work. We will assume the phase separated sce-
nario accepted and widely discussed in previous works,17,23
and we will not deal neither with the origin of the phase
separation nor with the possibilities of any alternative
description.
Polycrystalline samples of La 0.5Ca0.5MnO 3were synthe-
sized following a citrate/nitrate decomposition method using
99.9% purity reactants. To increase grain size, sub sequentialthermal treatments have been performed to the samples as is
decrypted in Levy et al.
16The grain size of the samples was
estimated from SEM microphotographs.
Magnetization measurements were made in a Quantum
Design VersaLab with the VSM and the heat capacity acces-
sories. For the DTA measurements, we used a home madesystem formed by two Cernox CX-1080-SD thermometers
(manufactured by Lake Shore Cryotronics) on a Teflon piece
to ensure thermal insulation between the sample and the ref-erence thermometers. The reference used was a piece of
alumina.
The whole system was mounted in a VersaLab’s trans-
port puck, allowing us to perform magnetization, Cp and
DTA measurements in the same range of magnetic field and
temperature.
As it was previously reported, the change of the GS of
the samples induces important changes in the magnetic
behavior. We can see those changes in Figure 1, where we
show magnetization measurements of the entire set of sam-
ples with an applied field of 1 T on cooling. The grain size of
the samples goes from 180 nm in sample A to 1300 nm insample E (see table in Figure 1for details).
All the samples present an FM ordering at around the
same temperature Tc ¼250 K but, while the sample with
smallest GS (A) remains FM in all the temperature range
below Tc, a clear FM to anti ferromagnetic transition is
observed in the rest of the samples at T ¼150 K.Measurements are performed with H ¼1, which is
enough to saturate the FM phase but not strong enough to
induce a ferromagnetic fraction enlargement.
24
Because of that the FM fraction at low temperature can
be estimated as the ratio between the magnetization at 50 K
of the sample and the same value on the sample A (fully
FM).
In the inset of Figure 1, we show the FM fraction at low
temperature as a function of the GS, being close to 20% in
the sample with the largest GS. This change in the magneticbehavior can be interpreted as an evidence of the frustration
of the CO state (associated with the AFM ordering) due to
small GS. The localization of the charges implies the pres-ence of a long range Jahn-Teller distortion that is suppressed
by the disruptive change in the lattice due to the grain bound-
ary.
17Similar behavior has been reported in other com-
pounds,23,25,26indicating that the GS is an extra ingredient to
take into account when the magnetic properties are studied.
To analyze how GS affects the MCE, we used two inde-
pendent methods to estimate the magnitude of DSandDT.
In the first method, we used isothermal magnetization
curves and the above mentioned Maxwell’s relation to obtainthe adiabatic entropy change due to the application of the
magnetic field. In Figure 2, we present the temperature de-
pendence of the entropy change for the different sampleswith an applied magnetic field of 3 T.
In all the samples, we observe a negative peak close to
Tc that can be associated to the paramagnetic (PM) to FMtransition. The maximum entropy change remains almost
constant at 2–3 J/kg-K for the entire series of samples.
An additional (positive) peak is observed at a lower tem-
perature, around 150 K. The maximum entropy change in
this peak increases as the grain size became larger.
According with the magnetization data, this peak can beassociated with the FM to CO transition.
The obtained value of DSfor the sample E (largest GS)
for H ¼3 T is 10 J/kg-K, similar to the obtained for pure Gd
around room temperature
27and in other half doped mangan-
ites such as Pr 0.5Sr0.5MnO 3(Ref. 28) and Nd 0.5Sr0.5MnO 3
(Ref. 29) measured using the same method.
FIG. 1. Magnetization as a function of temperature with an applied magnetic
field of 1 T for samples with different grain size. Inset: ferromagnetic frac-
tion at 50 K as a function of grain size. Table: grain size for each sample.152411-2 Quintero et al. Appl. Phys. Lett. 105, 152411 (2014)
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134.117.10.200 On: Sat, 29 Nov 2014 23:09:12According with the presented data, we can conclude that
the MCE has been enhanced increasing the grain size, since
an additional peak in the entropy change is observed and its
magnitude is controlled by the GS.
To complete the picture we performed differential ther-
mal analysis measurements, allowing us to determine the sam-
ple temperature change during the application of the magneticfield. In all the cases, the sample was zero field cooled to the
target temperature and then the field was applied with a con-
stant rate of 200 Oe/s and the heat exchanged with the envi-ronment has been taken into account.
30
In Figure 3, we present the adiabatic temperature change
(DTAD) extracted from DTA measurements for samples A, C,
and E. A positive peak can be observed around 225 K. This
is consistent with the expected behavior from the entropy
change associated with the PM to FM transition.Surprisingly, we do not observe any peak related with the
FM to CO transition. It has to be noted that according to the
entropy change values obtained from magnetization, theexpected temperature change should be three times larger
than the observed from the PM/FM transition.
Another important aspect to consider is the presence of
hysteresis in the magnetization as a function of magnetic
field curves. To examine these feature in depth, we calcu-
lated the magnetic work (W) defined as the area enclosedbetween the curves obtained increasing and decreasing the
magnetic field (between 0 and 3 T). In Figure 4, we present
W as a function of temperature for all the measured samples.
FIG. 2. Entropy change as a function of temperature for all the samples with
a magnetic field of 3 T. Inset: Intensity of both peaks in the entropy change
as a function of GS.
FIG. 3. Adiabatic temperature change ( DTAD) for samples A, C, and E as a
function of temperature when the magnetic filed is increased from 0 to 3 T.
The values of DTADwhere extracted from DTA measurements taking into
account the heat exchange between the sample and the sample holder.
FIG. 4. Magnetic work defined as the area enclosed by the increasing anddecreasing magnetic field curves as a function of temperature for differentsamples. In the insets we show magnetization loops at 250 K (black), 170 K
(red), and 60 K (blue).152411-3 Quintero et al. Appl. Phys. Lett. 105, 152411 (2014)
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134.117.10.200 On: Sat, 29 Nov 2014 23:09:12In all the samples, W is almost zero above the Curie
temperature, indicating the absence of hysteresis in the para-
magnetic phase. But when the FM phase is present we
observe a strong relation between the temperature depend-ence of W and grain size.
For smaller grain size (samples A, B, and C), the mag-
netic work presents an increase on cooling giving rise to aconstant value below 175 K. Samples D and E present a
maximum at 175 K, decreasing its value and keeping con-
stant below 75 K. It is interesting to note that in the tempera-
ture range in which W peak occurs coincides with the range
where the CO phase appears.
The temperature behavior of W can be explained consid-
ering the Jiles-Atherton model
31to describe the magnetiza-
tion curves. In this model, the hysteresis is produced byimpedances to domain wall motion caused by pinning sites
encountered by the domain walls as the move. Because of
that the system at a given field H cannot reach the globalminimum energy state, giving place to a hysteretic magnet-
ization loop.
The pinning sites could be grain boundary or any kind
of inhomogeneities within a grain, for example, tangles of
dislocation and precipitates or nonmagnetic inclusions.
The model consider that the domain walls are flexible so
that they not only can move but also can bend. When the do-
main walls bend while being held by a pinning site, it results
initially in a reversible change in the magnetization.
In our case, the formation of the CO phase increase the
amount of pinning centers in the sample, enhancing the re-
versible change in the magnetization.
On cooling, at 200 K, the CO phase start a nucleation
process, increasing the density of pinning sites in the mate-
rial which is reflected in the increase of W. Once the COphase is nucleated, the nuclei start to grow in size, decreas-
ing the amount of FM phase present. As a consequence, W is
reduced as the magnetic signal decreases.
The origin of the peak in the DSis the presence of a re-
versible component to the magnetization in this temperature
region.
The energy associated with the magnetization difference
is not exchanged with the environment because is used inter-
nally to bend the wall domain and recovered when the mag-netic field is turned off. Because of that we did not observe a
temperature change in the sample in this temperature region.
The reversible nature of the bending of domain walls makesthe entropy change calculated by Maxwell’s relation con-
vertible in magnetic work and not in heat exchanged with
the environment.
In summary, we presented a study about the influence of
grain size in the magnetic and magnetocaloric properties of
La
0.5Ca0.5MnO 3. The system is characterized by two well
distinguished magnetic transitions, a PM to FM one at 225 K
and a FM to a phase separated CO þFM at 150 K. The MCE
associated with the first transition do not present a significantdependence with grain size, and results extracted from mag-
netization measurements are in good agreement with those
obtained from DTA measurements.
The second transition, related with the formation of the
CO phase, presents a strong dependence with grain size. The
entropy change obtained from magnetization measurementsis not consistent with the temperature change extracted from
DTA measurements.
The hysteretic behavior in the magnetization loops
which was explained using a Jiles and Atherton model ofdomain walls in the FM phase. In this framework, the for-
mation of the CO phase modifies the density of pinning
sites increasing the hysteresis in the magnetization loops.
This additional pinning site increases the magnetic en-
tropy calculated by Maxwel l relation, but this entropy
cannot be used for applications since it is not converted in
heat and is related with the reversible bending of the do-
main walls.
It is just an example of how the inadequate use of the
Maxwell’s relation can l ead to a fake conclusion. Even
when the entropy change observed was larger than theobserved in Gd based compound it is not possible to use
this change in applications. The presence of hysteresis in
the magnetization vs magnetic field curves is indicative ofthe presence of an additional term in the first law of thermo-
dynamics that must be considered before any conclusion. In
the studied case, this feature was observed in the FM to PStransition; meanwhile, it is not present in the FM transition,
where the entropy change is converted in a temperature
change as expected.
This work has been done with the support of ANPCyT
PICT 1327/2008, Conicet PIP 00889, and UNSAM SJ10/13.
We are grateful with Leticia Granja and Roberto Zysler for
fruitful discussion and to Joaquin Sacanell for carefulreading of the manuscript. M.Q. is also member of CIC
CONICET.
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134.117.10.200 On: Sat, 29 Nov 2014 23:09:12 |
1.4896879.pdf | Nanosecond laser-induced phase transitions in pulsed laser
deposition-deposited GeTe films
Xinxing Sun,a)Erik Thelander, Pierre Lorenz, J €urgen W. Gerlach, Ulrich Decker,
and Bernd Rauschenbach
Leibniz Institute of Surface Modification, Permoserstr. 15, D-04318, Leipzig, Germany
(Received 15 July 2014; accepted 12 September 2014; published online 1 October 2014)
Phase transformations between amorphous and crystalline states induced by irradiation of pulsed
laser deposition grown GeTe thin films with nanosecond laser pulses at 248 nm and pulseduration of 20 ns are studied. Structural and optical properties of the Ge-Te phase-change films
were studied by X-ray diffraction and optical reflectivity measurements as a function of the number
of laser pulses between 0 and 30 pulses and of the laser fluence up to 195 mJ/cm
2. A reversible
phase transition by using pulse numbers /C215 at a fluence above the threshold fluence between 11
and 14 mJ/cm2for crystallization and single pulses at a fluence between 162 and 182 mJ/cm2for
amorphization could be proved. For laser fluences from 36 up to 130 mJ/cm2, a high optical
contrast of 14.7% between the amorphous and crystalline state is measured. A simple model is
used that allows the discussion on the distribution of temperature in dependency on the laser
fluence. VC2014 AIP Publishing LLC .[http://dx.doi.org/10.1063/1.4896879 ]
I. INTRODUCTION
Phase changes in chalcogenide-based alloys have been
widely studied in terms of the application in optical data
storage,1and the class of phase change materials is a promis-
ing candidate for further applications in non-volatile memo-ries.
2The basic principle of the optical memory storage
relies on the reversible transformation between the disor-
dered amorphous and ordered crystalline states.3For rewrit-
able optical storage, the recording (amorphization) process is
often achieved by an intense and short laser pulse which
induces rapid melting followed by quenching. In the erasing(crystallization) process, a moderately intense and long laser
pulse is used which heats the material above the crystalliza-
tion temperature for a sufficient amount of time. Utilizingthe significantly different optical reflectivity between the
amorphous and crystalline states, data bits can be read by
monitoring the local changes in the reflectivity of the mediawith a low power laser beam.
Among various phase-change materials, the Ge-Sb-Te
family (GST) is the most investigated one, both theoreticallyand experimentally in relation to their application in optical
phase-change data storage. Recently, the binary compound
GeTe has attracted great attention for such applications,since it offers a significant improvement of crystallization
speed, data retention at high temperature and an excellent
contrast in terms of electrical resistivity between the twostates, when compared to the more commonly studied
GST.
4–6In a previous work, it was demonstrated that phase
transitions of GST films could be induced using a singleultraviolet (UV) laser pulse of 20 ns pulse duration.
7
Compared to GST films, however, GeTe films exhibit rela-tively long incubation times and a delayed nucleation. Raouxet al. have reported that the shortest crystallization time ofmelt-quenched GeTe was 30 ns.
6In order to achieve the
crystallization of GeTe films with an ultrashort pulsed laser,
multiple pulses ( /C24500 pulses) are used.8On the other hand,
ultraviolet nanosecond pulses have been identified to besuperior in terms of minimizing feature sizes and surface
roughness of the films.
9
For the preparation of GeTe thin films, magnetron sput-
tering is typically employed.2,5,6As an alternative deposition
method, pulsed laser deposition (PLD) has been widely used
for the deposition of various complex oxides and chalcoge-nide films.
10,11Furthermore, PLD is capable of depositing
films with complex chemical composition and of stoichio-
metric transfer of the target material to the films. In this pa-per, PLD is employed as a deposition technique to prepare
GeTe thin films on Si substrates. A reversible phase transfor-
mation of GeTe films upon UV nanosecond pulsed laser irra-diation is demonstrated. The effect on optical and structural
properties of these GeTe films in the as-deposited state as
well as in the crystalline state are presented, which are ofcrucial importance for the performance of a phase-change
material.
II. EXPERIMENT
GeTe films were deposited on Si(100) substrates at
room temperature by pulsed laser ablation of a GeTe ceramic
target. A KrF excimer laser (LPXpro240) was used and oper-
ated at a wavelength of 248 nm, pulse duration of 20 ns, andrepetition rate of 10 Hz. The laser beam was focused on the
target at an incident angle of 60
/C14with respect to the target
normal to maintain a laser fluence of 1.5 J/cm2. The working
pressure was 5 /C210/C06Pa during deposition. The substrates
were ultrasonically cleaned by ethanol and deionized water
prior to deposition and were positioned parallel to the targetsurface at a target-substrate distance of 7.4 cm inside the vac-
uum chamber. Both target and substrates were rotated in
order to avoid deep damage of the target and to improve thea)Author to whom correspondence should be addressed. Electronic mail: xin-
xing.sun@iom-leipzig.de.
0021-8979/2014/116(13)/133501/5/$30.00 VC2014 AIP Publishing LLC 116, 133501-1JOURNAL OF APPLIED PHYSICS 116, 133501 (2014)
thickness homogeneity of the films, respectively. As a result,
as-deposited films with 60 nm thickness were obtained with
a chemical composition of GeTe 1.6as measured by energy
dispersive x-ray analysis.
Following the deposition, UV nanosecond pulsed laser
irradiation was performed on the GeTe films in order toinduce a phase change process. To study the switching
behavior of the films, the laser beam from the KrF excimer
laser was focused by a convex lens on the surface of theGeTe films using nanosecond laser pulses (20 ns) in order to
induce local phase changes. The spot size of the rectangular-
shaped laser beam was about 24 /C26m m
2with almost a top
hat intensity distribution and the laser fluence was varied
between 0 and 195 mJ/cm2by adjusting the laser pulse
energy from 0 to 300 mJ. The number of irradiating laserpulses was varied between 0 and 30 pulses. The topography
of the as-deposited and irradiated GeTe films was studied by
scanning electron microscopy (SEM). The optical reflectivityof the films was measured by a UV-Vis spectrophotometer
(Varian Cary 5000) in a wavelength range from 400 to
700 nm by use of an integrating sphere. Crystalline structureanalysis of the laser-irradiated films was performed by x-ray
diffraction (XRD) using Cu K aradiation ( k¼0.15418 nm)
in a parallel beam geometry using a 0.11
/C14parallel slit ana-
lyzer (Rigaku Ultima IV). In order to yield a high diffracted
intensity from the GeTe films despite the small film thick-
ness, grazing incidence diffraction (GID) at a fixed incidenceangle of 1
/C14with respect to the sample surface was employed.
III. RESULTS AND DISCUSSION
A. Crystallization process
The phase transitions in GeTe films are strongly deter-
mined by the temperature rise due to the laser irradiation.
The spatial distribution of the temperature T was calculatedby a finite element method
12as function of the pulse laser
fluence Uby solving the heat conductivity equation13,14
qGeTe;Si/C1cp;GeTe;Si/C1_Tt;~rsðÞ /C0r jGeTe;Si/C1rTt;~rsðÞ/C0/C1
¼Qt;~rsðÞ
¼Ut;~rsðÞ /C11/C0Ropt ðÞ
Dtp/C1a/C1exp/C0a/C1zðÞ
0 atSiat Ge Te8
<
:;
where Q is the laser beam induced heat source, qis the den-
sity ( q¼6.18 g /C1cm/C01), c pis the heat capacity at constant
pressure (here c p¼250 J/C1kg/C01/C1K/C01),jis the thermal conduc-
tivity ( j¼80 W/C1mK/C01), R optis the reflectivity (R opt¼74%),
ais the absorption coefficient ( a¼0.053 nm/C01) and r sis the
spatial coordinate (r s¼rs(x,y,z), z is the depth, x ¼distance
from the laser spot center).
Figure 1shows the temperature distribution in GeTe af-
ter 20 ns single pulse laser irradiation with a fluence of
14 mJ/cm2, which represents the threshold fluence for crys-
tallization of GeTe. It is obvious that the GeTe film is char-
acterized by a uniform temperature field up to a thickness of
about 60 nm (corresponds to the film thickness in this study).Consequently, it is assumed that also for higher laser fluen-
ces the complete films can be described by a definedtemperature. At these low laser fluences, the temperature
increases by about 30 K only. Considerably, higher tempera-ture rises can be expected for higher laser fluences.
Figure 2(a) shows the optical reflectivity of an as-
deposited GeTe film and of films after 20 ns laser pulse irra-diation with pulse numbers between 0 and 30 at a constant
fluence of 130 mJ/cm
2. The optical reflectivity of the as-
deposited film is in the range from 72 to 74% between 400and 700 nm. The reflectivity of the film is slightly increased
(3%) when the number of applied laser pulses increases to 3.
A more pronounced effect is discernable after irradiation
FIG. 1. Modeled 2D temperature distribution of a 60 nm thick GeTe film on
Si substrate after single laser pulse irradiation with a fluence of 14 mJ/cm2
for 20 ns. z is the depth, x is the distance from the laser spot center.
FIG. 2. (a) Optical reflectivity of as-deposited GeTe-film and films after20 ns laser pulse irradiation with a fluence of 130 mJ/cm
2as a function of
number of pulses. The inset shows the corresponding optical microscope
image of the irradiated area after 5 pulses. (b) Corresponding X-ray diffrac-
tion patterns of GeTe-films as function of the pulse numbers.133501-2 Sun et al. J. Appl. Phys. 116, 133501 (2014)with 5 pulses. For this film the reflectivity is in the range
from 85 to 88%, which is significantly higher than that of the
as-deposited film. Therefore, it can be concluded that at least5 laser pulses are needed to induce the crystallization pro-
cess. Upon further increase of the number of applied laser
pulses the reflectivity is further increasing, but the effect isnot so evident anymore. The corresponding results from
XRD measurements are shown in Fig. 2(b). The as-deposited
film and the film irradiated with 3 laser pulses only show aseveral degrees broad and weak peak centered around 28.3
/C14,
typical for the amorphous state. In contrast, when the film is
irradiated by /C215 pulses, clearly visible Bragg peaks located
at 26.4/C14, 30.2, and 43.7/C14appears, which correspond to the
(111), (200) and (220) lattice planes of cubic phase GeTe,
respectively. The XRD results are in good agreement withthe optical reflectivity measurements. Consequently, it can
be assumed that an increase in the number of laser pulses
continuously increases the crystalline fraction of the films. Itcan be accepted that the crystallization process after 20 ns
single pulse laser irradiation is incomplete,
15since the diffu-
sional processes involved in crystallization require sufficienttime above the threshold temperature typical of the GeTe
material. With that the superposition of pulses leads to an
increasing crystallization and to the increase of the opticalcontrast.
In order to extract the detailed effect of the laser fluence
on the phase transition, a series of fluence-dependent opticalreflectivity and XRD measurements of GeTe films was
investigated. A laser pulse number of 20 pulses was chosen
for all samples in order to obtain comparable data. The opti-cal reflectivity was evaluated at a wavelength of 650 nm, as
presented in Figure 3(a). With fluences /C2011 mJ/cm
2, only a
slight increase in reflectivity is observed. However, thereflectivity rises abruptly to 86% after laser irradiation with afluence of 14 mJ/cm
2. The reflectivity then increases only
slowly upon a gradual fluence increase from 14 and 130 mJ/
cm2, indicating a relatively large stability region of the high
optical contrast (up to 130 mJ/cm2). From these results, it
can be deduced that the threshold for crystallization of films
irradiated with 20 pulses lies between 11 and 14 mJ/cm2.
Above this fluence threshold, the series of laser pulses heats
the films above its crystallization temperature. The atoms in
the GeTe films thus become increasingly mobile and reachthe energetically favorable crystalline state, leading to a par-
tial crystallization of the film. With an increase of laser flu-
ence, the degree of crystallization increases, correspondingto the increase of film reflectivity. The highest resulting opti-
cal contrast between an as-deposited GeTe film and laser
irradiated GeTe film is calculated to be /C2414.7%, which is
much higher than the optical contrast value for laser induced
phase transition of GST.
7In the view of application, the
higher reflectivity contrast, the higher signal-to-noise ratioand the high absolute reflectivity of the films are independent
of the irradiation fluence in the range between 36 and 130
mJ/cm
2, which indicates a high stability and as a conse-
quence a long data retention. Also demonstrated in Figure
3(a), the reflectivity of the laser-irradiated films with flu-
ences /C21130 mJ/cm2drops to 82%. This is attributed to abla-
tion of the film surface as verified by the SEM image in the
inset of Figure 3(a). Holes in the film with sizes in the range
from 200 nm to 1 lm are clearly visible.
Figure 3(b) presents the evolution of XRD patterns of
films irradiated with an increasing laser fluence between 0 to
130 mJ/cm2after irradiation with 20 pulses and a pulse dura-
tion of 20 ns at a wavelength of 248 nm. The diffraction pat-
tern of the as-deposited film shows only a broad bump
located around 28/C14, which is characteristic of the amorphous
state, as shown before. No apparent differences in the
FIG. 3. (a) Optical reflectivity of
GeTe-films at the wavelength of650 nm after irradiation with 20 pulses
(pulse duration: 20 ns) as a function of
the laser fluence between 4 and 162
mJ/cm
2. The films are amorphous in
the as-deposited state. The inset shows
an SEM image of the film topography
after laser irradiation with 20 pulsesand a fluence of 162 mJ/cm
2. (b) XRD
patterns of GeTe-films after irradiation
with 20 pulses as function of the laser
fluence between 4 and 130 mJ/cm2.
The Miller indices correspond to the
rhombohedral crystal structure of
GeTe with the space group R3m(JCPDS no. 47-1079). (c) Details of
the XRD patterns around the 2 hposi-
tion of the rhombohedral (202) or/and
cubic (200) diffraction peaks, respec-
tively. (d) Corresponding interplanar
spacings d of the rhombohedral (202)
plane or/and cubic (200) plane as a
function of the laser fluence. The
dashed line is intended as a guide forthe eye, only.133501-3 Sun et al. J. Appl. Phys. 116, 133501 (2014)diffraction spectra can be seen until 11 mJ/cm2. It is also
visible that the crystalline diffraction peaks at 26.1/C14, 30.0/C14,
42.3/C14, 43.6/C14, which correspond to the (021), (202), (024) and
(220) lattice planes of the rhombohedral phase (the rhombo-
hedral structure can be interpreted as a slightly distorted
cubic phase), emerge in the GeTe films irradiated with alaser fluence >11 mJ/cm
2. This is in good agreement with
the results of the optical reflectivity measurements regarding
the threshold of the optical contrast. The intensity of peaks isslightly increased when continuously increasing the laser
irradiation fluence from 14 to 36 mJ/cm
2. Simultaneously,
the diffraction peak intensity of the (024) reflection graduallydisappears (as described in Figure 3(b)) because of the trans-
formation from the rhombohedral phase into the rocksalt
phase. Above a fluence of 36 mJ/cm
2only the latter cubic
phase is found. The measured fluence dependence correlates
with the assumption that a lower fluence is necessary for
construction of a low-symmetry rhombohedral unit cell thanfor a high-symmetry cubic unit cell, because there are large
atomic displacements required.
16Nevertheless, it is interest-
ing to point out that a peak shift to higher diffraction anglescan be observed for fluences between 14 and 36 mJ/cm
2.
Exemplary, the shift of the 2 hposition of the rhombohedral
(202) reflection at 30/C14toward cubic (200) at 30.2/C14is plotted
in Figures 3(c)and3(d). The corresponding interplanar spac-
ing d changes from about 0.297 nm to about 0.294 nm after
this fluence raise (Figure 3(d)). The peak shift to higher dif-
fraction angles or smaller interplanar spacings is the result of
the rhombodedral to cubic phase transformation induced by
fluences between 14 and 36 mJ/cm2. It is assumed that this
reduction of the interplanar spacing is probably due to the
release of film stress as a result of the transformation process
in the fluence range from 14 to 36 mJ/cm2.17With further
increase of the irradiating fluence from 36 to 130 mJ/cm2,
not surprisingly, the interplanar spacing slightly increases
caused by the higher thermal expansion, resulting in the peakshifting back to a relatively lower angle, as shown in Figs.
3(c)and3(d). To summarize, the crystallization phenomena
require several laser pulses instead of only one even at thehighest laser fluence used in this study. As the interval
between pulses, i.e., the delay time, is quite long (0.1 s) in
comparison to the cool-down time, a laser irradiationinduced temperature rise is not sufficient to explain the crys-
tallization behavior. Instead, the results indicate the presence
of incubation effects after each pulse, which accumulatewith the number of pulses. The physical nature of those
effects remains unclear without further local microstructure
investigations.
B. Amorphization process
The crystallized GeTe thin films were re-amorphized by
a single nanosecond laser pulse (20 ns) irradiation at the
wavelength of 248 nm and at different laser fluences up to195 mJ/cm
2. It was demonstrated that amorphization of a
crystalline film can be achieved with a single laser pulse irra-
diation with 8 ns pulses.8In Fig. 4(a), the reflectivity spectra
of single pulse irradiated GeTe films in the spectral region
from 400 up to 700 nm are shown. For comparison, thereflectivity of as-deposited GeTe films and of a GeTe film af-
ter furnace annealing at a temperature of 300/C14C for 20 min
is also shown. The reflectivity of this latter crystalline film
ranged between 90 and 93%. This figure also exhibits that no
obvious reflectivity change after irradiation by a single laserpulse at 98 mJ/cm
2can be observed. However, with an
increase of the laser fluence up to about 160 mJ/cm2, a con-
tinuous and gradual decrease in reflectivity is detected.Gawelda et al. have also observed such a decreasing trend of
the reflectivity of GeTe films after 800 nm single pulse irra-
diation for fluences >50 mJ/cm
2which corresponds to for-
mation of amorphous regions but also to an increased
ablation.8
With further increase of the fluence from 160 to 182 mJ/
cm2, a dramatic decrease in reflectivity can be discerned. It
seems that the atoms in the GeTe films are increasingly dis-
placed from their lattice sites above a laser fluence of 98 mJ/cm
2. The displaced atoms are quenched during the very short
laser pulse interaction with the film, which leads to a partial
amorphization of the film. With the increase of laser fluence,the degree of amorphization increases, corresponding to the
decrease of film reflectivity. However, when the fluence
increases to much higher values ( /C21195 mJ/cm
2), the reflec-
tivity of the laser-irradiated films shows only a slight further
decrease due to the increased fraction of amorphization and
FIG. 4. (a) Optical reflectivity of GeTe-films in the wavelength range
between 400 and 700 nm after irradiation with a single 20 ns laser pulse as
function of the laser fluence. For comparison, the optical reflectivity of an
as-deposited GeTe-film and a crystalline GeTe-film after thermal annealing
at 300/C14C for 20 min, both produced by PLD, are shown. The inset displays
an SEM image of the surface after single laser pulse irradiation with a flu-
ence of 195 mJ/cm2. (b) XRD patterns of nanosecond laser pulse irradiated
GeTe-films as a function of the laser pulse fluence.133501-4 Sun et al. J. Appl. Phys. 116, 133501 (2014)a significant ablation, as visible in the inserted SEM images
in Figure 4(a). In order to confirm the structural transition
from crystalline to amorphous state, those films with fluencesbetween 98 and 182 mJ/cm
2were studied by X-ray diffrac-
tion, as shown in Fig. 4(b). It can be observed again that with
an increase of the fluence from 98 to 182 mJ/cm2, the inten-
sity of diffraction peaks is gradually decreased. Thus, the
amorphous fraction in the crystalline films increases with
increase of the laser fluence. These results demonstrate thatamorphization of crystalline GeTe films can be achieved
with single UV nanosecond (20 ns) laser pulse irradiation of
sufficient fluence.
IV. CONCLUSION
This study provides a contribution to the elucidation of
the amorphous-to-crystalline a nd vice versa phase transi-
tion of GeTe 1.6films grown by pulsed laser deposition on
silicon substrates. The phase transformation processes are
g e n e r a t e db yi r r a d i a t i o nw i t h2 0 n sl a s e rp u l s e sa taw a v e -
length of 248 nm and are investigated in dependence on thenumber of pulses and the laser fluence. A reversible phase
transition is realized by using pulse numbers /C215 at fluences
above the threshold fluence between 11 and 14 mJ/cm
2for
crystallization and single pulses at a fluence between 162
and 182 mJ/cm2for amorphization. Moreover, a detailed
structural evolution of rhombohe dral-to-cubic phase transi-
tion is shown. The reflectivity contrast between the laser
induced and as-deposited films is studied and achieves a
maximum of 14.7% for laser fluences between 36 and 130mJ/cm
2.ACKNOWLEDGMENTS
This work has been supported by the Leipzig School of
Natural Sciences BuildMoNa (Grant No. GS 185/1).
1M. Wuttig and N. Yamada, Nat. Mater 6, 824 (2007).
2M. H. Lankhorst, B. W. Ketelaars, and R. A. Wolters, Nat. Mater 4, 347
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9J. Siegel, D. Puerto, J. Solis, F. J. Garc /C19ıa de Abajo, C. N. Afonso, M.
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of
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1.101065.pdf | Tunable twinguide laser: A novel laser diode with improved tuning performance
M.C. Amann, S. Illek, C. Schanen, and W. Thulke
Citation: Applied Physics Letters 54, 2532 (1989); doi: 10.1063/1.101065
View online: http://dx.doi.org/10.1063/1.101065
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/25?ver=pdfcov
Published by the AIP Publishing
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130.113.86.233 On: Mon, 22 Dec 2014 20:16:41Tunable twin~guide laser: A novel laser diode with improved tuning
performance
M.-C, Amann, S, Iliek, C. Schanen, and W, Thulke
Siemens AG, Research Laboratories, D-8000 1l1Tmchen 83, Federal Republic a/Germany
(Received 14 February 1989; accepted for publication 11 Apri11989)
A new wavelength tunable laser diode with a basically continuous tuning behavior is presented.
This essential progress is achieved by transversely tuning the effective index of a distributed
feedback laser using a twin waveguide. Due to the built-in synchronization of the Bragg
wavelength and the optical cavity length, the wavelength is controlled by only a single current.
The device technology and preliminary experimental results demonstrating the transverse
tuning mechanism are presented.
Tunable single-frequency laser diodes in the I,3-L55
pm wavelength region are key devices for future fiber optical
communication systems based on wavelength division mul
tiplexing or coherent optical techniques. For these applica
tions, several monolithic device structures have been devel
oped 1-6 enabling a continuous tuning up to 4.4 nm.4 Tunable
lasers are commonly made by longitudinaHy integrating an
amplifying section with a phase shifter and a Bragg reflec
tor. 3-5 A major disadvantage of these three-section devices is
the rather complicated mutual adjustment of the currents
flowing into the phase control and the Bragg reflector sec
tions. The nonlinear recombination processes together with
the inevitable fabrication tolerances require each laser to be
calibrated individually. 7 Moreover, degradation may cause a
deviation from this adjustment after some time of operation.
In practice, therefore, a built-in self-synchronization
between the phase condition and Bragg wavelength would
be highly desirable in order to reduce the expense for the
frequency control and the evaluation of the indivudual de
vice performance. It is hence the objective of this letter to
report on a novel tunable laser diode in which such a syn
chronization with the corresponding continuous tuning be
havior is realized. In addition, preliminary experimental re
sults are presented proving the efficiency of the new
approach.
The basic concept underlying the novel device is to tune
both the optical length of a distributed feedback (DFB) la
ser and the Bragg wavelength ofi.ts grating synchronously by
transversely varying the effective index of the laser cavity.
Since the Bragg wavelength and the optical length of the
resonator scale equally with the effective index, the longitu
dinal mode order remains unchanged while tuning the free
space wavelength, which yields a continuous tuning behav
ior. The InGaAsP/lnP twin guide (TG) as shown
schematically in Fig. 1 is a well-suited waveguide structure
for this purpose. For independent biasing, the active layer of
index II a and the tuning layer of (variable) index n tare
decoupled electrically by a thin n-InP layer of index no' p
type InP confinement layers and contacts on both sides of
the TG and an n-type contact connected to the central n-InP
layer complete the essential structure of the tunable twin
guide (TTG) laser, By i.ncorporating a (.-1. Ifour shifted)
Bragg grating, e.g" on top of the tuning layer, single longitu
dinal mode emission can be achieved. Laser operation and wavelength tuning are controlled by the currents II and It
flowing into the active and the tuning layer, respectively.
The intensity of the optical field S( x) in the TTG laser is also
shown in Fig. I to illustrate the strong optical coupling
between the active and the tuning layer.
A cross-sectional end view of a TTG laser structure
based upon the planar buried ridge structure (PBRS) laserS
is shown schematically in Fig. 2, As compared to Fig. 1 the
Bragg grating can also be located in an additional InGaAsP
layer at the bottom of the active layer. Using higher band
gap InGaAsP than in the tuning layer, the recombination
tosses at the grating interface can thereby be reduced by re
taining a high tuning efficiency, The p contact for the laser
current is on the bottom side of the p-InP substrate, whereas
both the p contact for the tuning current and the common n
contact are placed on top of the chip. For a low-resistive
ohmic contact, the n-type metallization is applied onto a
heavily n-doped en = 2 X 1019 em -3) InGaAs layer located
outside the PBRS region. The p-type tuning contact is made
low resistive and connected to the p-InP confinement layer
above the tuning layer by zinc diffusion. Excess leakage cur
rents across the forward-biased InP homo junction are sup-
FIG. L Fundametnal structure of a TTG laser ill InGaAsP IIllP material
system.
2532 Appi. Phys. Lett. 54 (25), i 9 June i 989 0003-6951/89/252532-02$01.00 (c) 1989 American Institute of Physics 2532
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130.113.86.233 On: Mon, 22 Dec 2014 20:16:41~ oxide
m metallization
FIG. 2. Schematic crass-secti()nal end view of a TTG laser in planar buried
ridge structure technology.
pressed by etching a 40-,um-wide mesa stripe. The longitudi
Ilal homogeneity of the TTG laser allows an arbitrary choice
of the cavity length by cleaving. This makes possible a flexi
ble control of essential laser parameters, particularly of the
cpticallinewidth.
In a first and nonoptimized experimental structure, the
thicknesses of the active layer (A~ = 1.52 jim), the tuning
layer (Ag = 1.3 pm), and the n-InP separation layer are
0.13,0.16, and 0.1 !tm, respectively, and the cavity length is
400 ,urn. The width of the buried ridge is about 3.5 jim. The
p-t -InGaAsP (Ag = 1.3 ,urn) and n +--InGaAs contact
stripes are 4 and 6,um wide, respectively. The fl-and p-InP
layers are doped with 1 X 10' 1\ em --3 and 5 X 10 l7 em --\ re
spectively. Since the main objective of the present investiga
tion is to get a first insight into the transverse tuning mecha
nism rather than to study the single-mode emission, we used
a simple Fabry-Perot laser made by two-stage liquid phase
epitaxy and omitted the Bragg grating and the quaternary
layer below the active region.
Figure 3 shows the transverse tuning characteristic of
this TTG laser diode, I.e., the wavelength shift
t:.A (It ) = A (It ) -A (1t = 0) which has been obtained by
investigating the wavelength of a specified longitudinal
mode at various tuning currents It. The optical output pow
er perfacet was about 2 m W (1/ = 60 mA). With increasing
I" an longitudinal modes shift to smaner wavelengths
(t:.A. < 0) indicating the electronic tuning. In close agree
ment with results on multi sectional tunable lasers,5,6 the
wavelength shift LVl is approximately proportional to the
square root of It. The efficiency of the tuning is at least com
parable to that of the usual multisection devices.5 By using a
tuning current as low as 30 mA (2.1 kAI cm2), a wavelength
shift of about 2 nm is achieved. Similar to other tunable laser
diodes/) the output power at constant II decreases with in
creasing It. This dependence is rather weak for I, < 30 rnA.
At I, = 30 mA the output power is reduced by about 50%
and for I, = 35 rnA the threshold current exceeds 60 rnA.
Hence in these nonoptimized TTG laser I, is limited to
about 30 rnA and the maximum tuning range is around 2
nm, corresponding to more than two longitudinal mode
spacings of the 400-,um-long cavity. However, model ca1cu
lations9 using experimental An versus current density char-
2533 App\. Phys. Lett, Vol. 54, No. 25, 19 June 1989 o
E -1 r::
..<
<l
-2 L:::;400~m
o 10 20 30
FIG. 3. Wavelength shift vs tuning current measured at a light output pow
er around 2 m W per facet. The schematic shape of the mode spectrum is
shown in the inset for two values of .he tuning current.
acteristics5 indicate that an optimized 2-pm-wide and 400-
,urn-long A = 1.55 !-lm TTG laser with a Ag = 1.38 /-tm
tuning layer may yield 8 nm continuous tuning range at
I, = 80 rnA (10 kA/cm2).
In conclusion, we have presented a novel tunable laser
diode, the tunable twin-quide (TTG) laser, which utilizes a
transverse tuning mechanism. This kind of tuning is realized
with a twin-waveguide structure and exhibits an inherent
continuous tuning behavior. Furthermore, the wavelength
control is facilitated since only a single current is required
for wavelength tuning. We have developed a fabrication
technology based on the planar buried ridge structure and
demonstrated the transverse tuning mechanism by using
Fabry-Perot laser diodes. Although being preliminary,
these first TTG lasers show a high tuning efficiency and a
maximum continuous tuning range of2 nm at a tuning cur
rent of 30 rnA. The optimization of the laser and the incorpo
ration of a DFE grating are expected to result in single-mode
devices with an estimated continuous tuning range up to
about 8 nm.
The authors gratefully acknowledge the helpful discus
sions with B. Stegm'liHer and J. Heinen and are also grateful
to L. lunker, G. Ehrlinger, and H. Lang for preparation and
evaluation of the lasers,
'N. K. Dutta, A. B. Piccirilli, T. Celia, and R. L. Brown, App!. Phys. Lett.
48, 150l (1986).
2L. D_ Westbrook. A. W. Nelson, P. J. Fiddymcnt, and J. R Collins. Elec
tron, Lett. ::m, 957 (1984).
"D. Lederc, J. Jacquet, D. Sigogne, C. Labourie, Y Louis, C. Artiguc, and
J. Benoit, Electron Lett. 25,45 (1989).
4S. Murata, T. Numai, S, Takano, t Mito, and K. Kobayashi, Digest 11th
IEEE Semiconductor Laser Conference. 3! August--l September, 1988,
Boston (IEBE/LEOS, Piscataway, NJ, 1988), p. 122.
'Yo Kotaki, M. Matsuda, H. ishikawa, and H. Imai, Electron. Lett. 24, 503
(1988).
6K. Kobayashi and 1. Mito, J. Lightwave TechnoL LT-6, 1623 (1988).
'p, I. Kuindersma, T. \I. Dongen, G. L A. v. d. Hofstad, W. Dijksterhuis,
and J, J. M_ Binsma, Proceedings of 14th European Conference on Optical
Communications (EeOC), 11-15 Sept., 1988, Brighton, UK (lEE, Lon
don, UK, 1988), p. 368.
"W. Thulke, A. Zach, alld H.-D. Wolf, Siemens Forsell. Entwic!dungsber.
17, I (1988).
"M_-C Amann (unpublished model calculations).
Amann eta/. 2533
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1.576931.pdf | The influence of ion bombardment on reactions between Ti and gaseous N2
R. A. Kant and B. D. Sartwell
Citation: Journal of Vacuum Science & Technology A 8, 861 (1990); doi: 10.1116/1.576931
View online: http://dx.doi.org/10.1116/1.576931
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 150.135.239.97 On: Fri, 19 Dec 2014 10:35:32The influence of ion bombardment on reactions between Ti and gaseous N2
R. A. Kant and B. D. Sartwell
Code 4675, Naval Research Laboratory, Washington, D. C. 20375-5000
(Received 17 February 1989; accepted 7 October 1989)
Simultaneous Auger electron spectroscopy and ion bombardment were used to study the
influence of ion bombardment on the amount of nitrogen accumulated on a Ti surface in a low
pressure nitrogen atmosphere. The nitrogen level was measured as a function of time, ion flux,
nitrogen partial pressure, and Ti temperature both directly before and during bombardment with
150 keY Ti+ ions. As a result of the ion bombardment, the steady-state nitrogen level increased
(and in some cases decreased) by an amount determined by the balance between the ion flux and
the flux of nitrogen gas. The experimental results were interpreted in terms of a multiple-step
process involving ion beam induced dissociative chemisorption of physisorbed molecular
nitrogen. The time dependence of the nitrogen concentration at the Ti surface was modeled with a
pair of differential equations that described the time rate of change of the physisorbed and
chemisorbed contributions to the surface nitrogen in terms of both ion beam induced and
thermally activated processes. These equations were solved numerically and the solutions agreed
with the experimental behavior. In addition, the results of this study were used to explain
observed variations in properties of thin film TiN grown by ion-assisted reactive deposition.
I. INTRODUCTION
Although ion bombarment during thin film deposition can
have a dramatic and often advantageous influence on film
properties, very little is known about the details of the phys
ical processes induced by the ion beam. For example, ion
bombardment can affect the kinetics of chemical reactions
between elements on a condensing surface and the constitu
ents of the surrounding atmosphere resulting in modifica
tions to the composition of compound films grown by reac
tive deposition techniques. While such phenomena have
been noted in the literature and have been the subject of some
speculation, there has been little direct and systematic ex
perimental investigation of such phenomena. Previously
Kant et al.I reported the qualitative features of ion beam
induced modification of the composition of TiN films depos
ited by ion beam assisted deposition. Subsequently, Baba2
studied nitrogenation of metals during Ar+ bombardment
and concluded that it occurred through a chemical reaction
and that the extent of the nitrogenation correlated with the
Gibbs free energy of nitride formation. The investigation re
ported here is designed to elucidate the mechanism and to
help provide a quantitative description of the influence of ion
bombardment on surface reactions that control the composi
tion and, hence, the properties of films grown by reactive ion
beam assisted deposition (IBAD). For these experiments, a
low partial-pressure of nitrogen (N) was introduced into a
vacuum system and the time dependence of the accumula
tion of N on a titanium (Ti) surface was measured during
ion bombardment as a function of ion flux, sample tempera
ture, and N pressure. The data was interpreted with the aid
of a numerical model and the resulting conclusions were
used as the basis for a new explanation of the relationship
between the TiN deposition parameters and the resultant
mechanical properties. II. EXPERIMENTAL METHOD
Figure 1 is a schematic of the ultra-high vacuum chamber
that was equipped with a cylindrical mirror analyzer
(CMA) with a coaxial electron gun for Auger electron spec
troscopy (AES). The chamber also contained a low-energy
ion gun (indicated by the letter B) that was oriented 62° out
of the plane of the figure, a precision leak valve for introduc
ing controlled amounts of gases, and a residual gas analyzer
for monitoring the partial pressures of the gaseous species
present in the chamber.
The chamber was connected to a 200 k V, medium-current
Varian/Extrion ion implanter through a differentially
pumped antechamber3 that contained a 3 mm diam beam
defining aperture and a rotating wire for measuring relative
beam current entering the analysis chamber. For these stud
ies isotopically pure 150 ke V Ti + ions were used, with cur
rent densities up to 20 flA/cm2• This Ti + beam was electro
statically rastered both horizontally and vertically to ensure
laterial uniformity of the average ion current density at the
sample for time intervals greater than I s.
Disks, 1 cm2, of high-purity, mechanically polished Ti
were mounted on a heating stage that was attached to a pre
cision XYZ manipulator. Heating was provided by electron
bombardment on the reverse side of the sample, with tem
perature measured using a chromel/alumel thermocouple
that was clamped to the front surface. Also attached to the
manipulator was a small Faraday cup (not shown in the
figure) with an entrance aperture plate that was positioned
in the same vertical plane as the surface of the Ti sample. The
size of the aperture was 1 mm2• This Faraday cup provided
for alignment of the electron and ion beams from the CMA,
low-energy ion gun, and ion implanter.
The base pressure in the target chamber was 1.3 X 10-7
Pa, which rose to LOX 10-6 Pa when the chamber was
861 J. Vac. Sci. Technol. A 8 (2), MarlApr 1990 0734-2101/90/020861-07$01.00 (e; 1990 American Vacuum Society 861
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opened to the implanter and the Ti + beam was incident on
the sample. For the experiments, N gas was intentionally
introduced into the chamber to produce pressures ranging
from 1.3 X 10-5 to 1.3 X 10-4 Pa.
The procedures for each experiment were as follows. The
Faraday cup was positioned to verify that the electron and
ion beams were coincident at the focal point of the CMA,
with the entrance aperture plate oriented such that it made
an angle of 50 ° with respect to the CMA axis and 40 ° with
the Ti + beam vector. The Ti + beam current density mea
sured with the Faraday cup was then correlated with the
current measured on the rotating wire. Next, the Ti sample
was placed in the analysis position at the same angle given
above, the temperature was set to a preselected value, and
the surface was sputter-cleaned using 2 keY Ar+ ions from
the low-energy ion gun. The N gas was introduced into the
chamber to the desired pressure with the time dependence of
the surface composition monitored with AES. Spectra were
acquired digitally on a personal computer every 25 s with the
C KLL (272 eV), N KLL (379 eV), Ti LMM (387 eV), Ti
LMV( 418 eV), and OKLL (503 eV) transitions being mea
sured. Once a steady-state composition had been achieved,
ion bombardment with 150 ke V Ti + was initiated, ensuring
that the current remained constant on the rotating wire, with
AES data continuing to be acquired until a steady-state sur
face composition was again achieved. Finally, the N gas flow
was stopped and the surface composition was monitored un
til all adsorbed species had been sputter removed by the Ti +
beam.
The AES data was analyzed using relative sensitivity fac
tors obtained by analyzing spectra from bulk samples of TiN
and Ti02. These factors were 0.45 for N KLL, 0.45 for Ti
LMV, and 0.32 for 0 KLL. Since it was not possible to re
solve the N KLL from the Ti LMM transitions, a deconvolu
tion technique was applied4 to determine the Ti and N con
centrations.
4" 10
VALVE
® --CMA
&1
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 Experiments were performed with ion current densities of
7 and 20 J1A/cm2, for N partial pressures of 1.3,2.6,4.0,6.6,
and 13.3 X 10-5 Pa and for substrate temperatures of250 °C,
350°C, and 450 °C.
III. EXPERIMENTAL RESULTS
The qualitative behavior ofthe N KLL AES intensity con
firmed the occurrence of ion beam induced effects on reac
tion kinetics, which had been inferred from previous studies
of TiN grown with concurrent ion bombardment. In the
present case, the net effect of the Ti + ion beam was to change
the steady-state N level at the Ti surface. This level could be
shifted up or down by adjusting the experimental conditions.
In addition to these net changes of the N levels, the transient
behavior of the N signal following the onset of bombardment
was also significant. Initially, the N level decreased under
bombardment but then it began to increase. The implica
tions of this increase are far reaching and are discussed at
length below. Carbon was not detected for any of the experi
ments and oxygen gettered from the ambient atmosphere
was quickly removed under ion bombardment.
A typical time dependence for both the Nand 0 concen
trations is shown in Fig. 2. The lines connecting data points
are included as guides to the eye only. This data is for a
sample that was at a temperature of250 °C and in a N partial
pressure of 1.3 X 10-5 Pa. The figure is divided into three
phases. During phase I, the ion beam was off and N 2 gas was
introduced into the chamber. N accumulated rapidly at first
and then more gradualy as a steady-state level was ap
proached. The 0 level behaved similarly and reached a rela
tively large value considering that the ratio of the partial
pressures of 0 to N was 3 X 10-3• Note that the 0 level
continued to increase after the N level was nearly saturated
and that this growth was not at the expense of the N level.
This indicated that the absorbed 0 was not simply replacing
RESIDUAL GAS
ANALYZER
FIG. I. Schematic diagram of experimen
tal apparatus. (A) sample heating
(1000 °C) and cooling ( -150°C) stage,
(B) sputter ion gun (5 kV) directed attar-
get 62° out-of-plane.
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the N. The behavior of the 0 may be explained by 0 entering
Ti to the subsurface.s-7 The 0 level was included here pri
marily to illustrate the dramatically different behavior ex
hibited by 0 and N during the ion bombardment phase of the
experiment. However, the cause of the 0 behavior is beyond
the scope of this paper and will not be discussed further.
During phase II, the sample was bombarded with 150 keY
Ti + ions and for the case shown here at a current density of7
f1A/cm2• Initially, both the N and the 0 levels decreased
rapidly. Although the 0 concentration continued to de
crease during bombardment, the N concentration stopped
decreasing and began to increase gradually. The final con
centration reached depended on the ion flux and N partial
pessure. For most of the cases examined, the steady-state N
level under ion bombardment exceeded the steady-state level
without bombardment. An exception was the case with both
the highest ion flux (20 f1AI cm2) and the lowest N pressure.
In this case, the steady-state N level under bombardment
was less than it was without ion irradiation. Once a steady
state condition had been reached, the third and final phase
was initiated by discontinuing the supply of N gas, which
resulted in the sputter removal of the N by the Ti + ions.
During these experiments, it was difficult to determine
exactly when the N signal was saturated and, thus, data ac
quisition may have been discontinued before a steady-state
condition was actually reached. Therefore, we decided not to
use the last value acquired as the steady-state level. Instead,
the steady-state levels were obtained numerically by fitting
the N concentration data as a function of time to a simple
exponential function of the form
(1)
where Y m is the N steady-state level, t is the time, to is the
time at the beginning of the phase, and tc is the characteristic
time constant for the process. Each phase of each run was
treated independently. This function was selected because it
6 0.20
i= () « c: u.
g
:;E o
~ 0.10 II
NITROGEN
TIME (seconds)
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 was expected to provide a reasonably good description of the
asymptotic approach to steady state exhibited by the N con
centration data. Each phase of each run was treated individ
ually and thus the behavior of the steady-state N levels was
determined for each combination of ion bombardment flux,
sample temperature, and N partial pressure.
The dependence of the steady-state level on experimental
conditions is shown in Figs. 3 and 4. Figure 3 is a plot of the
N steady-state levels as a function of sample temperature for
a Ti + current density of7 f1A/cm2 and for three values ofN
partial pressure. Figure 4 shows the dependence of steady
state level on the N pressure for a sample temperature of
350°C with no ion bombardment and with 7 and 20 f1AI cm2
Ti + ion fluxes. While the temperature dependence is weak,
there is a consistent trend toward a maximum accumulation
of N at temperatures between 250 and 450°C for all cases
examined. Moreover, this steady-state level increases with
increasing partial pressure of N. The net effect of the bom
bardment is most clearly evident from the behavior of the N
concentration as a function of ion flux at fixed N partial
pressure (Fig. 4). Here, the steady-state N concentration
obtained for no ion flux can be compared to that observed for
ion current densities of20 and 7 f1A/cm2• The N steady-state
levels obtained during ion bombardment at 7 f1A/cm2 were
substantially larger than observed for no ion bombardment
for all N pressures tested. At a higher current density (20
f1AI cm2), similar but smaller increases in steady-state levels
were oserved to result from Ti + bombardment for all pres
sures except the two lowest values. This behavior can be
understood in terms of a new kinetic model, which accounts
for the various particle fluxes at the Ti surface and includes
effects arising from ion bombardment.
IV. KINETICS MODELING
The nitrogen level during phase I (no ion bombardment)
can be modeled approximately by a single ordinary differen-
Ti+
500 III
FIG. 2. Surface concentrations of nitrogen
and oxygen on Ti as a function of time. N,
gas flow was on during phases I and II.
The ion beam was off during phase I and
was on during phases II and III. Sample
temperature was 250°C, current density
was 7 flA/cm', and nitrogen pressure was
1.3 X 10-5 Pa.
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 150.135.239.97 On: Fri, 19 Dec 2014 10:35:32864 R. A. Kant and B. D. Sartwell: Ion bombardment between Ti and gaseous N2 864
:§. 40 r
Z w
II:
8 II: I-Z
w
~ 20
II:
::;)
(J) NITROGEN
PRESSURE
("-'~i o?~~
3.9 +
1.3a /------
200 300 400
TEMPERATURE (0C)
FIG. 3. Temperature dependence of the steady-state nitrogen levels during
bombardment with 7 /lA/cm' of 150 keY Ti+ ions.
tial equation that describes the time rate of change of the N
in terms of the incident gas flux, the degree of surface cover
age, and thermally activated desorption. However, such a
simple model is totally inadequate to give even qualitative
agreement with the behavior exhibited during the bombard
ment phase of the experiments (phase II). Moreover, no
significant improvement in agreement with the experiment
is realized if the only ion beam induced effect included in the
model is sputtering. The problem is that such a simple model
fails to provide any mechanism that can account for ob
served increases during bombardment. There must be a
mechanism which overcomes the loss ofN due to sputtering
and which can eventually lead to a steady-state level that is
-~ 0
Cij -z 40
w
(!)
0 a: t: z
w
U < 20 u.. a:
::> en
1 3 larger than it was without bombardment. To explain the ob
served behavior, it was necessary to postulate that the sur
face N exists in two forms or states, that the two forms be
have differently under bombardment, and that ion
bombardment activates a conversion from one form to the
other.
The qualitative behavior of the N signal as a function of
time was described successfully with a mathematical model
consisting of a pair of coupled ordinary differential equa
tions [Eq.(2) and Eq.(3) 1 that determine the time rate of
change of the concentration per unit area of each of two, as of
yet, unspecified forms of N, which are identified here as n
and n'.
!!!!.... = J [1 -(n + n')INT] -(nFP INT -nFS)INT dt
-nK) exp( -Q/B) -nK2 exp( -Q2IB) , (2)
dn' = nFSINT + nK2 exp( -Q2IB) -n'FP'IN T dt
(3)
These equations take into account the quantities normally
expected to influence the composition: the flux of N gas im
pinging on the Ti surface, the fraction of the surface covered
with N, thermal desorption, and sputtering. In addition, a
term has been included to account for a newly proposed pro
cess which is an ion beam induced reaction that converts the
unprimed form, n, to the primed form, n'. In Eq. (2), the first
term describes the accumulation of n due to the gas flux, J,
and accounts for gas rejected because a site was already oc
cupied. NT is maximum number of available sites. The sec
ond and third terms account for sputtering and for an ion
induced conversion of n into n', respectively, where Fis the
ion flux, Pis the probability of sputtering, and S is the proba-
10 FIG. 4. Steady-state level of surface nitrogen
as a function of N, partial pressure for no
ion bombardment, and for low and high ion
fluxes.
N2 PARTIAL PRESSURE (10-5 Pa)
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990
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bility of the postulated ion induced reaction that converts n
to n'. The last two terms deal with the loss of n by thermally
activated desorption and by a thermally activated reaction
which converts n to n', respectively. QJ and Q2 are the activa
tion energies, KJ and K2 are the preexponential factors for
these thermally activated processes, and B is kT, i.e., the
product of the Boltzmann constant and the absolute tem
perature. Similarly, Eq. (3) describes the time rate of change
of the n' form of N. The first and second terms account for
the ion induced and the thermally activated reactions to
form n', respectively. The third term describes sputtering of
n' and the last term describes loss of n' due to thermal de
sorption. Here P' is the probability of sputtering n', and Q3
and K3 are the corresponding parameters for thermal de
sorption of n'.
Note that contrary to common practice, we have inten
tionally avoided casting our equations in terms of sputtering
yields and sticking coefficients since these quantities only
assume their tabulated values for a narrowly defined and
limited range of conditions. Strictly speaking, such coeffi
cients are expected to be complex functions of the N concen
tration we seek to calculate. Thus, use of these quantities
would have only added an unnecessary complication to the
model. Instead, we have cast our equations in terms of more
fundamental quantities, the probabilities for ion induced
processes, and in terms of the activation energies and pre
exponentials for thermally activated processes. The proba
bilities are proportional to the cross sections for the corre
sponding events. This formalism was selected because the
probabilities and activation energies were expected to be rea
sonably independent of the N level throughout the range of
conditions encountered here.
Solutions to Eqs. (1) and (2) were obtained numerically
using the experimental values for gas flux, J, and ion flux, F.
The values used to define the magnitudes of sputtering pro
babilities, Pand P', and the probability, S, of an ion induced
n to n' reaction were treated as the principle adjustable pa-
II
N2
20
# ... o 0
o o o
3
z w
<!l
0 a:
!::: z 10
200 400
TIME (s)
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 o
o rameters. The values used to describe the thermally activat
ed processes, desorption, and the thermally activated reac
tion to form n', were treated as adjustable parameters subject
to constraints outlined below.
A detailed knowledge of the nature of the two hypotheti
cal forms ofN is not necessary to investigate the qualitative
features of solutions to Eqs. (2) and (3). However, the fol
lowing assumptions were made concerning the two forms
and about the processes influencing their rates of accumula
tion. The unprimed form of N, n, is assumed to be derived
from the ambient atmosphere and is only weakly bound to
Ti. Thus, this form of N is expected to be easily removed by
thermal desorption or by sputter erosion. Moreover, the ac
tivation energy for the thermally induced reaction, which
converts it to the second form, is taken to be large compared
to kT. The second form of N is, by contrast, strongly bound
to the surface and, thus, is more strongly resistant both to
thermal desorption and to removal by sputtering. While
these properties were found to be necessary, solutions to the
model equations based upon these assumptions alone could
not be made to agree, even qualitatively, with the N signal
observed during the ion bombardment phase of the experi
ments. However, the required results could be obtained by
postulating that the ion beam induces transformations from
the loosely bound form, n, to the tightly bound form n'. With
this process incorporated into the model, the qualitative be
havior of the time dependence of the N level calculated by
the model agreed well with experimentally observed behav
ior.
An example of this agreement between qualitative fea
tures of the calculated and observed behaviors is shown in
Fig. 5. The solid line is the calculated behavior of n + n' and
the open circles are the same experimental data points plot
ted in Fig. 2. We have superimposed the theoretical and ex
perimental values on one plot to illustrate the qualitative
agreement only. One must be cautious in making a quantita
tive comparison because the two quantities plotted are, in
III
Ti+
600 FIG. 5. Results of model calculation (solid
curve) of total surface nitrogen plotted
against experimental data (open circles)
for the same run data, as shown in Fig. 2
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fact, slightly different. They differ because the AES analysis
is sensitive to N contained in the first three to four mono
layers, but the model deals with only surface N. However, a
direct comparison of the qualitative behavior of the two
quantities is justified if the amount of surface N scales with
the amount of N averaged over the first few monolayers.
With this assumption in mind, a comparison of the experi
mental and theoretical curves indicates that this model,
which uses two forms ofN, can correctly predict the rise in N
level that occurs following its initial drop at the onset of ion
bombardment (beginning of phase II).
The values of the parameters used in the calculation of the
solution for the case shown in Fig. 5 are listed in Table I.
Because the temperature was constant throughout this run,
it was possible to reduce the number of adjustable param
eters by replacing each of the exponentials, together with
their prefactors, by a single parameter. Then, aside from the
fluxes of N2 and ions, which assumed their experimentally
measured values, the parameters were determined by an iter
ative process in which the parameters were systematically
varied to achieve qualitative agreement between experimen
tal data and the model. Since the main intent of this phase of
the study was to establish that qualitative agreement with
the experimental data was possible with equations of the
form ofEqs. (2) and (3), the iteration process was terminat
ed when this point was reached. It is expected that an ex
haustive computer study could be used to refine the values
determined thus far, but such a study is beyond the scope of
this investigation. However, the values of P, S, and P' deter
mined by the fit were used to obtain estimates of the cross
sections c for the corresponding ion beam induced events.
For example, the cross section for sputtering of an n-type
atom is given by cp = PINT. Similarly, the cross sections for
sputtering of n' atoms, c;, and for ion induced conversion of
n to n', c" are PINT and S INT, respectively. These cross
sections values are also listed in Table I.
The good agreement found between the model and experi
ment lead to the following speculations about the detailed
nature of the two forms of N postulated above and of the
processes responsible for the observed behavior. The un
primed form of N is presumed to be molecular N. This is
known to have an exceptionally strong bond. x Thus, while
molecular N would be expected to accumulate rapidly on a
freshly cleaned Ti surface, initially it would be bound to the
surface by only the relatively weak van der Waals forces and
must dissociate before it can become tightly bound to Ti at
TABLE l. Parameters used to obtain the solution shown in Fig. 5.
J
Nr
F
P
S
K, exp( -Q/B)
K, exp( -Q,/B)
K, exp( -QJB)
P' 2XlO'4 em-'s-'
7 X 1015 em'
4.4X 1013 em~-' s-'
25 (el' =3.6XIO-15 em')
7 (c, = l.OX 10-15 em')
5 X 10 '
2x 10-5
2x 10 1
1.5 (cp =2.1XlO 16 em::)
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 the surface. However, at low temperatures the activation en
ergy for this dissociative chemisorption of N is large8 com
pared to kT. Therefore, in the absence of ion bombardment,
the surface N is expected to be primarily in the form of this
physisorbed molecular N.
However, under ion bombardment the energy needed to
drive the dissociative reaction could be supplied by an ion
beam and the amount ofN in the reacted form, n', would be
expected to increase. However, the ion beam also removes n
by sputtering. Thus, the steady-state surface compositions
depend on the balance between the incident gas and ion
fluxes which, in turn, determine the balance between the
competing ion beam induced effects, sputtering, and ion
induced conversion of n to n'.
V. DISCUSSION AND IMPLICATIONS
We have shown experimentally that ion bombardment of
a Ti surface exposed to N gas has a strong influence on the
surface composition and that successful modelling of the
time dependence of this composition can be achieved if it is
assumed that two forms ofN exist at the Ti surface. We have
speculated about the nature of the two forms, i.e., initially
the N is in the form of physisorbed molecular N and in its
final state, it is bound to Ti in the form of TiN. Moreover, our
interpretation of the experimental data suggests that ion
beam induced surface reactions are required to explain the
observed behavior. However, many of the details of the pro
cess have yet to be established. Examples of the remaining
questions include: Is the dissociation process a direct conse
quence of ballistic processes or is the molecule simply excit
ed to an elevated energy level from which it can more readily
react? What are the roles of ion beam mixing and other irra
diation induced and or chemically guided segregation ef
fects?
Although a full understanding of the details of the pro
cesses must await further study, the understanding already
gained from these experiments has served as the basis for a
proposed explanation for dramatic variations of the proper
ties of TiN coatings grown under different deposition condi
tions. TiN produced commercially by magnetron sputter
ing, for example, is a hard and brittle material, whereas, the
same compound grown in the laboratory by reactive ion
beam assisted deposition has been soft and ductile. J Our cur
rent results suggest that an explanation for the soft and duc
tile behavior may be that unreacted molecular N accumu
lates on, and becomes trapped within the film during
growth. Based upon the results presented above, such an
accumulation of physisorbed N would be expected unless the
ion flux is sufficiently large relative to the N gas flux to limit
such an accumulation. It is reasonable to expect that if most
of this N is not removed by ion bombardment (or by ther
mally activated processes at elevated temperatures), it may
be incorporated into the grain boundaries of a growing film.
This could lead to incomplete bonding across grain boundar
ies which, in turn, would decrease the yield strength and
account for the observed reduction in hardness. In addition,
such incomplete bonding at grain boundaries would be ex
pected to affect other properties as well. For example, poor
intergranular bonding could restrict electron conduction
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 150.135.239.97 On: Fri, 19 Dec 2014 10:35:32867 R. A. Kant and B. D. Sartwell: Ion bombardment between Ti and gaseous N2 867
between grains and lead to decreases in both electrical con
ductivity and optical reflectivity. It is interesting to note that
in the commercial magnetron sputtering process which pro
duces hard TiN, the N partial pressure is carefully con
trolled, ostensibly to limit poisoning of the Ti sputtering tar
get which, in turn, reduces the deposition rates. However, we
propose that this limitation on N pressure may also be re
sponsible for controlling the film properties as described
above.
The results presented here indicate that the presence of
large quantities ofunreacted N at a growth surface should be
expected for certain film preparation parameters. Further
more, we proposed that if this N were to be incorporated into
a growing film, it could significantly alter the properties of
that film. It was also suggested that because ion bombard
ment could be used to control both the amount and form of
the surface N, it should be possible to control or adjust film
properties by controling the balance between the ion and
atomic fluxes at the growth surface. To test these ideas, we
prepared a series of TiN samples with ion beam assisted de
position using a range of conditions which were expected to
vary the amount of molecular N at the growth surface and to
vary the rate of dissociative chemisorption ofN to form TiN.
The results of these experiments were consistent with the
view presented above and they are summarized below. The
details of this work are beyond the scope of this paper and are
presented elsewhere.9
Measurements of hardness, reflectivity, and electrical
conductivity were made for TiN grown using various rela
tive values of ion and atomic fluxes. The results indicated a
strong dependence of these properties on the relative N pres
sure during film growth. That is, as the ratio of ion flux to the
N gas flux was increased, the hardness, reflectivity, and elec
trical conductivity all increased. These results were com
pletely consistent with the views presented above.
VI. SUMMARY
In situ AES measurements of surface composition of Ti
exposed to a N atmosphere were made as a function of ion
bombardment flux, partial pressure of N, and Ti tempera-
J. Vac. Sci. Technol. A, Vol. 8, No.2, Marl Apr 1990 ture. We have shown that the steady-state N level could be
controlled (increased or decreased relative to the thermody
namic equilibrium level) by adjusting the balance between
the ion flux and the flux of N gas. Results of model calcula
tions showed that the observed behavior could be under
stood if it is assumed that N accumulates first in the form of
molecular N which is initially weakly bound to the surface
and that, under ion bombardment, this N is either removed
by sputtering or undergoes ion beam activated reaction to
form a stronger bond with Ti. N -Ti bonds formed as a result
of this ion beam activated dissociative chemisorption are
stronger and thus, this reacted form ofN is less susceptible to
being removed by sputtering or by thermal desorption. In
this way, ion bombardment can be used to control both the
nature and the amount ofN at a Ti surface. The understand
ing gained from these experiments was used as the basis for a
new explanation for variations of some of the physical prop
erties of TiN films made by reactive ion assisted deposition.
That is, the physical properties are thought to be controlled
by physisorbed molecular N which is incorporated within
such a TiN film during growth. Examples drawn from other
experimental studies were cited that support our view that
since the extent and nature of this absorbed gas can be con
trolled, the physical properties of TiN films can also be con
trolled by adjusting the relative magnitudes of the atomic
and ionic fluxes at the growth surface.
'R. A. Kant and B. D. Sartwell, Mater. Sci. Engin. 90, 357 (1987).
2y. Baba, T. A. Sasaki, and 1. Takano, 1. Vac. Sci. Technol. A 6, 2945
( 1988).
'D. A. Baldwin, B. D. Sartwell, and 1. L. Singer, Nucl. Instrum. Methods
Phys. Res. B 7/8, 49 (1985).
4D. A. Baldwin, B. D. Sartwell, and 1. L. Singer, Appl. Surf. Sci. 25, 364
( 1986).
5A. Olivia, R. Kelly, and G. Falcone, Nucl. Instrum. Methods B 19, 101
(1987).
"P. H. Dawson, Surf. Sci. 57, 229 (1976).
7M. 1. Pellin, C. E. Young, D. M. Gruen, Y. Aratono, and A. B. Dewald,
Surf. Sci. 151, 477 (1985).
KV. N. Kondratiev, Bond Dissociation Energies, Ionization Potentials and
Electron Affinities (Nauka Publishing House, Moscow, 1974).
OR. A. Kant, S. A. Dillich, B. D. Sartwell, and 1. S. Sprague, in Proceedings
of MRS Symposium A, Nov 28-Dec 2,1988, Boston, MA (unpublished).
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1.576939.pdf | Metal contacts on Hg1−x Cd x Te
W. E. Spicer
Citation: Journal of Vacuum Science & Technology A 8, 1174 (1990); doi: 10.1116/1.576939
View online: http://dx.doi.org/10.1116/1.576939
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Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.209.6.50 On: Thu, 18 Dec 2014 11:12:53Metal contacts on H91_xCdx Te
w. E. Spicer
Stanford Electronics Laboratories. Stanford University. Stanford. California 94305
(Received 4 October 1989; accepted 3 November 1989)
The available literature concerning metal contacts on Hg, _" Cd" Te is reviewed in order to obtain
a systematic overview. The theory and models available predict that, for metal in intimate contact
with Hg, _ xCdx Te, "ohmic" contact will be easily formed on n-type and rectifying on p-type
Hg1 _ xCdx Te. This is in agreement with experimental results. There is evidence that metal atoms
can move into the Hg, _ xCdx Te, doping it and thus changing the electrical properties of the
contact. Photoemission spectroscopy shows that Hg, _ x Cd x Te can be strongly disrupted by the
deposition of the metal and correlation is found between heats of formation and heats of solution
and the extent of this disruption. However, even for cases such as Ag where these bulk
thermodynamic parameters are small, evidence is found for disruption of the lattice and
movement of the metal into the Hg, _ x Cdx Te, producing doping. Formation of diffusion barriers
and cooling to low temperature ( lOOK) have been explored as ways to limit this disruption.
I. INTRODUCTION
Metal interfaces with Hg1 _ x Cdx Te are ofinterest due to the
necessity of making electrical contacts ("ohmic" contacts)
and rectifying contacts (Schottky barriers) on
Hg1 _ x Cdx Te. For most semiconductors of practical inter
est, such as Si or GaAs, one can easily find descriptions in the
published literature of how to form ohmic contacts or
Schottky barriers. However, this is not the case for
Hg1 _ xCdx Te. In fact, Hg, _ "Cdx Te is the only semicon
ductor on which an electronics business of over $1 billion per
year is based for which such knowledge is not easily avail
able. The bulk of published work concerning metals on
Hg1_xCd x Te does not concern the direct measurement of
the electrical characteristics (for instance, 1-V and C-V
measurements) of thick metal contacts but are photoemis
sion spectroscopy (PES) measurements of thin metal over
layers on Hg, _ xCd" Te. There is also available at least one
paper' which applies various theoretical work and/or mod
els to the electrical properties ofmetal!Hg, _ "Cdx Te inter
faces in order to draw conclusions concerning the electrical
behavior of the contacts. This paper suggested that it should
be easy to form ohmic contacts on n-type Hg, _ xCd" Te but
not on p-type Hg'_xCdx Te, and conversely easy to form
Schottky barriers on p-type but not on n-type Hg, _ x Cdx Te.
Subsequent to unpublished discussions it became clear that
this suggestion was in agreement with most of the practical
experience in industry. As one attempts to develop a more
sophisticated Hg, _ xCdx Te detector array technology, the
lack of knowledge and control of metal contacts may prove a
serious limitation. The purpose of this paper is to summarize
the available literature.
Because of the lack of published literature on the electrical
properties of metals on Hg, _ x Cdx Te, this paper will draw
strongly on the PES results and the results of theory and
modeling. The PES experimental results are also of value
because they allow one to examine the very complex chemis
try and/or intermixing which takes place between the metal
overlayer and the Hg, _ xCdx Te substrate. An important
consequence of this intermixing is the possible doping of the
Hg, _ xCdx Te by the metal atoms. We will first examine the use of theory and models to
predict the electrical characteristics of the metal contact
without considering effects due to metal doping. We will
then consider changes which might be produced by metal
doping.2 PES work will then be considered, with emphasis
on the following two aspects: (1) the position of the Fermi
level EI at the interface and the consequences for the electri
cal properties of the contact, and (2) disruption of the
Hg, _ "Cdx Te by the metal as well as overlayer-substrate
intermixing and/or possible indiffusion of the metal where it
could act as a dopant. Methods developed using PES and
other techniques to minimize intermixing between the metal
and Hg, _ x Cd, Te will be discussed.
II. ATTEMPTS TO PREDICT THE ELECTRICAL
PROPERTIES OF METALS ON Hg,_xCdx Te
Spicer, Friedman, and Carey' have used models and theo
ry to extrapolate from the established results of metals on
CdTe to those for the Hg, _ xCdx Te alloy of any composi
tion. Several different approaches were shown to give essen
tially the same final result, presented in Fig. 1. One approach
was to use the results from the "metal induced gap states"
(MIGS) model.3 The other approaches considered a defect
mechanism, using theoretical results from Kobayashi et al.4
and from Zunger5 consisting of calculating the defect energy
level as a function of alloy composition in order to extrapo
late from the position of EI at the metal interrace with CdTe
to the metal!Hg, _ x Cdx Te interface. As described by Spicer
et al., the electrical properties ofthe contact can be predicted
from the Fermi level position EI provided that the doping
profile in the Hg, _ x Cd, Te near-surrace region is known.
We will describe this briefly here for the case where the semi
conductor doping at the interface is sufficiently low that tun
neling through the barrier is not an important factor. If E(
lies above the conduction-band minimum (CBM), one will
have ohmic contacts on n-type Hg1 _ x Cd, Te and rectifying
contacts on p-type; conversely if EI lies below the valence
band minimum (VBM), one will have ohmic behavior on p-
1174 J. Vac. Sci. Technol. A 8 (2), Marl Apr 1990 0734-2101/90/021174-04$01.00 © 1990 American Vacuum SOCiety 1174
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type and rectifying on n-type. If Ef lies within the band gap,
one will have Schottky barriers for both n-and p-type doping
with the barrier height determined by the energy difference
between the VBM and Ef for p-type or by the difference
between the CBM and Ef for n-type. If tunneling becomes
important, rectifying contacts become increasingly "soft" or
ohmic in nature.
Figure 1 shows how both Ef and the CBM are predicted to
change from their position in CdTe as a function of alloy
composition. In terms of defects, Spicer et al. assumed that
the dominant defect was unchanged as one goes from CdTe
to Hgl_xCd x Te. Using the results of Kobayashi et al.,4
Spicer et al. I concluded that the dominant native defect was
a Te atom on a Cd site, i.e., a Teed antisite defect.
The key result from the extrapolation of Spicer et al. is
that, for less than about 40% Cd in Hgl_xCd x Te (x <0.4),
the Fermi level will lie above the CBM, i.e., the metal would
form an ohmic contact on n-type Hgi _ x Cdx Te and a recti
fying contact on p-type Hgl_xCd x Te. From unpUblished
discussions with many in the Hgi _ x Cdx Te industry, it ap
pears that this prediction is consistent with industrial experi
ence. For example, it appears easy to form ohmic contacts on
n-type but not p-type Hgl_xCd x Te. To the best of my
knowledge, all Hgi _ x Cdx Te systems in production only re
quire such ohmic contacts on n-type Hgi _ x Cdx Te. In con
trast, practical experience seems to indicate that it is difficult
to form ohmic contacts on p-type Hgi _ x Cdx Te.
Even though the work of Spicer et al. suggested that native
defects should be taken into account, it did not focus on the
possible doping of the Hgi _ xCdx Te by the metal overlayer.
Prior to the work of Spicer et al., Friedman et aU showed a
correlation between the movement of the Fermi level and the
dopant type of the metal deposited; i.e., if the metal atoms
doped the Hgl_xCd x Te n-type, the Fermi level moved
_1 >
~
>. e'
Q) 0.8 c:
UJ
0.4
0.3 +-x CdTe
1.0
FIG. 1. Lower limit of the Fermi level position relative to the VBM at the
interface, Eft as a function of alloy composition. Two models (MIGS and
defect) were used for extrapolation. Near x = 0.4 Eft moves into the con
duction band providing intrinsic ohmic contacts on n-type material. [From
Ref. 1).
J. Vac. Sci. Technol, A, VOl, 8, No.2, Mar/Apr 1990 1175
towards the conduction band, and, if they doped it p-type, Ef
moved toward the valence band. To translate these results to
electrical properties of metal contacts, metals which dope n
type would make the contact even "more ohmic" on n-type
Hgi _ x Cdx Te. Furthermore, metals which dope p-type
might reduce the barrier height or even produce ohmic con
tacts on p-type Hgi _ x Cdx Te. The fact that PES showed so
much disruption at the interface supported this concept.
This will be discussed in the next section.
To summarize, ohmic contacts were predicted on n-type
material and rectifying on p-type for Cd concentration
x < 0.4 if the interface was sufficiently perfect. However, if
the metal moved into and doped the Hgl_xCd x Te suffi
ciently p-type this could change the situation by moving the
Fermi level at the interface toward the VBM.
III. PHOTOEMISSION SPECTROSCOPY STUDIES
Two quantities can be obtained from PES measurements
which are of particular importance in understanding the be
havior of metal contacts on Hgi _ x Cdx Te. One involves the
interfacial chemistry, intermixing, interdiffusion, and relat
ed phenomena. The other is the position of the Fermi level
Ef at the interface. As mentioned above, Ef can be closely
related to the electrical properties of the metal contact. In
1983 Davis et al.6 reported the first study of a metal, AI, on
Hgi _ x Cdx Te. They found that deposition of approximately
a monolayer of Al resulted in the loss of over half of the Hg
from the first few layers of the Hgi _ x Cdx Te lattice. This
was followed by other work by the same group 7 on AI, In and
Au in which Fermi level movement as well as interfacial
chemistry and morphology was studied. The groups of Fran
ciosi8 at the Univ. of Minnesota (Cr) and Spicer9•10 at Stan
ford (Ag, AI, and Cu) also became involved. This work
clearly established that Hgl_xCd x Te was much more dis
rupted by the deposition of the metal than other semicon
ductors such as Si and GaAs. Another key point was that the
nature and extent of the disruption varied according to the
metal used. For example, Ag did not react strongly but
seemed to move deep into the Hgi _ x Cdx Te lattice over tens
to hundreds of A. In contrast, Al reacted very strongly with
Te, tearing up a few layers ofHgl _ xCdx Te but then appar
ently forming a diffusion barrier localizing the damage very
near the surface.
This pioneering work was followed by more comprehen
sive studies in which correlations were found between the
observed "chemical" reactions and the heats offormation of
metal-Te compounds and the heats of solution between the
metal and Cd and! or Hg.2 Clear evidence was also obtained
concerning the importance of kinetic phenomena such as
selective atom motion and diffusion barriers. II By 1986, a
large number of metals had been studied on Hgi _ x Cdx Te in
addition to those mentioned above, Pt,12 Pd,15 and Sm. II
Table I from the thesis of Friedman 13 shows the Fermi
level position Ef for the as-cleaved Hgi _ x Cdx Te surface
and after deposition of the metal. As can be seen from Table
I, AI, In, and Cr would be expected to provide strong ohmic
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T ABLE I. Cleaved surface Fermi level pinning El, and subsequent deposition-induced surface Fermi level motion b.Et.,. for metals on Hg, , Cd, Te. The
bulk doping properties of the metal in MCT are also shown.
Eg E -CBM" b.Ej., h Dopant j.'
Metal x (eV) (eV) (eV) type' Reference
Cu 0.25 0.22 0.0 ±O.I +0.1 ± 0.05 P 10
Ag 0.23 0.20 0.0 ±O.I + 0.05 ± 0.05 p 10.12
Au 0.28 0.26 0.15 ± 0.2 + 0.2 ±O.I p 6
AI 0.23 0.20 0.0 ±O.I -0.5 ±O.I n 10
0.28 0.26 0.15±0.2 -0.6 ± 0.2 7
In 0.28 0.26 0.15±0.2 -0.4 ± 0.2 n 7
Cr 0.22 0.18 0.3 ± 0.15 0.0 ± 0.05 d 8
Pt 0.30 0.29 -0.1 ±O.I 0.0 ± 0.05 d 12
Pd 0.39 0.41 -0.15 ± 0.1 0.0 ± 0.05 d IS
Ti 0.20 0.15 0.0
" Assuming surface and bulk bandgaps equal.
"Plus means EI, shifts down towards the VBM.
'From Ref. 21.
d Not tabulated.
strong ohmic contacts on n-Hg, xCdx Te and rectifying on
p-Hg, x Cd, Te. The other metals will form less ohmic con
tacts on n-type Hg, ,Cd, Te and less rectifying on p-type
Hg, ,Cd,Te.
Recently more attention has been paid to these interface
reactions 14 and their control. Carey et al. 15 explained the use
oflowering the substrate temperature to reduce reaction and
intermixing. Raisanen et al.'6 demonstrated the use of
strongly reactive metals to produce diffusion barriers to con
trol intermixing.
In 1988, Carey et al.,'7 deposited AI, Pd, and Ag on
Hg, ,Cd,Te held at low temperature (100 K). This work
is the best test of the predictions of Spicer et al. as to Fermi
level behavior (and thus the electrical properties of the metal
contact) since at low temperature the disruption of the
Hg, ,Cd, Te by the metal is minimized. The results of
Carey et al. are summarized in Fig. 2. Essentially they found
that at 100 K the Fermi level moved upon metal deposition
up to approximately 0.6 eV above the VBM independent of
the metal being deposited. This movement was completed
for relatively low metal coverages, about 0.2 of a monolayer.
However, for Ag and Pd, the Fermi level was found to move
back towards the VBM when interface "disruptions" took
place. For Ag, this occurred most clearly when the sample
was warmed up to room temperature and allowed to stay at
that temperature for an hour. As Fig. 2, shows, this resulted
in a movement of E1 back toward the VBM. In fact the final
position of E1 was similar to that obtained for room tempera
ture deposition of Ag. The core level spectra of Carey et al.'7
showed clearly that Hg was depleted from the surface even
during the low temperature deposition; however, after the
room temperature "anneal," it was found that Hg had clear
ly moved back into the Hg, . x Cd, Te near-interface region.
Other work 14 suggested that the Ag moved into the
Hg, x Cd, Te doping it at the same time that the Hg moved
into the near-interface region. Since Ag dopes Hg, _ x Cdx Te
p-type, the movement of the E1 toward the VBM was consis
tent with doping of the lattice.
In contrast to the results for Ag and Pd, reducing the
J. Vac. Sci. Technol. A, Vol. 8, No.2, Marl Apr 1990 ±O.I 0.0 ±O.I d 7
temperature had little effect for AI. It was found that AI
reacted strongly at 100 K as well as at room temperature.
Even at 100 K, the PES spectra showed clear evidence of Hg
loss for coverages as low as 0.3 or 0.7 monolayers of AI. The
band bending, i.e., E1 movement, was completed at even
lower coverage, 0.1 monolayer at both temperatures. It ap
pears that Al dopes the Hg, ,Cd, Te n type.
For Pd, the band bending was maximized at a coverage of
I
0.6 f-
:2 0.5 I-
m >
w-0.41-
0.3f-
I
cleave I
I
little
disruption I
I AI (x ~ 0.30)
n-type -
Pd (x ~ 0.39L
unknown
-
Ag (x ~ 0.23)
p-type _
chemistry,
intermixing
FIG. 2. Systematics of Fermi level E, movement for the Hg, ,Cd, Te crys
tals (x value indicated on each curve) with deposition of AI. Pd. and Ag
(from Ref. 17). The ordinate gives the position of E, above the VBM. The
E( positions labeled "cleave" on the left are the values of E, after the crystal
was cleaved in silu to form a clean surface and before any metal was deposil
ed. The position labeled "little disruption" agrees with the extrapolated
prediction of Fig. I, i.e., Ef lies above the CBM. The set of points to the right
indicate the effects of strong disruption. The metals were deposited with the
crystals held at 100 K to minimize disruption. The middle points labeled
"little disruption" were taken after enough metal was deposited to ohtain
the maximum band bending (movement of Ef) but before disruption of the
Hg, ,Cd, Te became apparent. This movement was completed by a cover
age of a third of a monolayer of the metal. The right hand set of points
indicates the Ef position after disruption. interdiffusion (for Ag) and inter
facial chemical reactions (for Al and Pd). has taken place. For I'd this
disruption is achieved by continued deposition of the metal at 100 K. and for
Ag by raising the Hg, ,Cd, Te to room temperature after deposition at low
temperature; for AI disruption occurs due to reaction at low temperature.
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.209.6.50 On: Thu, 18 Dec 2014 11:12:531177 W. E. Spicer: Metal contacts on Hg,_xCdx Te
0.3 monolayer. With further metal deposition, EJ moved
back toward the valence band maximum, where clear evi
dence of a HgI _ x Cdx Te-Pd reaction was seen. It is not
known whether Pd dopes HgI _ ,Cdx Te n-or p-type.
IV. DISCUSSION AND OVERVIEW
The work of Carey et al. 17 gave data which was consistent
with the predictions of Spicer et aI., I that ohmic contacts on
n-type and rectifying contacts on p-type would occur if the
disruption of the HgI __ xCdx Te by the metal was kept to a
minimum. By cooling the substrate and using submonolayer
metal coverages the disruption and doping by the metal were
minimized, and the three metals AI, Ag, and Pd all gave
Fermi levels at the interface well above the CBM. In con
trast, at room temperature and/or at higher metal coverages
where disruption occurred more diversity was found. For
metals which do not dope HgI _ xCdx Te n-type, the Fermi
level moved toward (but did not reach) the VBM. Also, for
Au, Ag, and Cu which are known to be p-type dopants, Ef
usually lies in the upper half of the band gap. This behavior
agrees with the suggestion of Freeman et al.2 relating Ej
movement to doping by the metal insofar as the dopant type
of the metal is known.
It is encouraging that such a correlation was found
between the simple ideas reviewed here and the PES results
and, as far as one can ascertain, with practical experience in
making metal contacts to HgI ,Cd, Te. However, one
should also recognize that the HgI ,Cd, Te/metal contact
problem is very complex due to the fragility of
HgI _ xCd, Te. The situation is not helped by the fact that
there are still gaps in our fundamental understanding of met
al contacts on semiconductors. We have mentioned here de
fects, the metal used, doping, and the MIGS theory for per
fect interfaces; however, we have not discussed the effect of
metal electronegativity or the original Schottky model for
formation of rectifying contacts.I,IH.19 In considering de
fects, we have not addressed key questions such as whether
there are several different native defects of importance each
of which might be dominant under various conditions. Rath
er, we have only examined the situation where a single native
defect is important and examined how its energy varies with
composition. I We have also not considered here tunneling
contacts in which nominally rectifying contacts become oh
mic due to high majority doping of the semiconductor adja
cent to the contact. To summarize, the models for contacts
outlined here correlate well with the available experimental
data. However, much more work is needed before compre
hensive understanding and control of metal contacts on
HgI ,Cdx Te can be achieved.
As outlined in this article, the most demanding contact
problem is that of providing an ohmic contact on p-type
HgI x Cd, Te. Krishnamurthy, Simons, and Helms20 have
very recently reported success in forming such contacts with
a structure in which a thin oxide or chloride (i.e., nonmetal
lic) structure was superimposed between the metal (Au)
and HgI .. x Cdx Te. There the results were explained in terms
of a reduction of interface state density. Within the context
of this paper, it is clear the MIGS could be eliminated by the
insulating layer, and that the same layer could reduce defect
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 1177
production by metal deposition. One might then think of the
Schottky type mechanism 18 applied, with the large electron
egativity of the Au moving the EJ in the metal near or below
the VBM; however, a tunneling mechanism cannot be ruled
out at this time. The work of Krishnamurthy et al. illustrates
the importance of novel approaches to development of con
tacts on HgI _ x Cdx Te. However, ultimate control of the
electric properties of metal contacts on HgI __ x Cd, Te will
only be made possible by sufficient fundamental understand
ing. Without this insulating layer, rectifying contacts were
found; with it, satisfactorily low resistance ohmic contacts
were formed.
ACKNOWLEDGMENTS
Fruitful discussion with G. P. Carey, D. J. Friedman, and
A. K. Wahi are gratefully acknowledged. This work was
funded by DARPA under Contract No. N00014-86-K-
0854.
'w. E. Spicer. D. J. Friedman. and G. P. Carey, J. Vac. Sci. Technol. A 6.
2746 (1988).
'D. J. Friedman, G. P. Carey. I. Lindau, and W. E. Spicer. J. Vac. Sci.
Techno!. A 5.3190 (1987).
IF. FloresandC. Tejidor,J. Phys. C20.145 (1987);J. Tersoff. Phys. Rev.
Lett. 56, 2755 ( 1986).
"A. Kobayashi. D. F. Shankey. and J. Dow. Phys. Rev. B 25, 6367 (1982)
'A. Zunger, "Electronic Structure of 3d Transition Atomic Impurities." in
Solid State Physics. edited by D. Turnbull and H. Ehrenreich (Academic.
New York, 1987), Vo!' 39; M. J. Caldas. A. Fazzio. and A. Zungcr. App!.
Phys. Lett. 45, 671 (1984); A. Zunger, Phys. Rev. Lett. 54. 849 (19R5)
"G. D. Davis, N. E. Byer, R. R. Daniels, and G. Margaritondo. J. Vac. Sci.
Techno!. A 1, 1726 (1983); 2, 546 (1984).
'G. D. Davis. N. E. Byer, R. A. Reidel. and G. Margaritondo. J. App!.
Phys. 57,1915 (1985); G. D. Davis, W. A. Beck, D. W. Nib, E. Calavita.
and G. Margaritondo. ibid. 60, 3150 (1986).
"D. J. Peterman and A. Franciosi, App!. Phys. Lett. 45, 1305 (1984)' P
Philip, A. Franciosi, and D. J. Peterman. J. Vac. Sci. Techno!. A 3. 1007
(1985); Phys. Rev. B 32.8100 (1985).
"D. J. Friedman, G. P. Carey, C. K. Shih, I. Lindau. W. E. Spicer, and J. A.
Wilson, App!. Phys. Lett. 48. 44 (1986); J. Vac. Sci. Techno!. A 4.1977
(1986).
'liD. J. Friedman, G. P. Carey, C. K. Shih, I. Lindau, W. E. Spicer, and J. A.
Wilson, J. Vac. Sci. Techno!. A 41977 (1986); D. J. Friedman, G. P
Carey, I. Lindau, and W. E. Spicer, Phys. Rev. B 34, 5329 (1986).
"A. Wall, A. Raisanen, S. Chang, P. Philip, N. Troullier, A. Franciosi. and
D. J. Peterman, J. Vac. Techno!. A 5, 3193 (1987).
'"D. J. Friedman, G. P. Carey, I. Lindau, and W. E. Spicer. Phys. Rev. B 35.
1188 (1987).
"D. J. Friedman. Ph.D. dissertation. Stanford University. 1987 (unpuh
lished).
'"G. D. Davis, W. A. Beck, M. K. Kelly, D. G. Kilday, Y. W. Mo. and G.
Margaritondo, J. Vac. Sci. Techno!. A 6. 2732 (1987).
"G. P. Carey, D. J. Friedman, A. K. Wahi. C. K. Shih, and W. E. Spicer. J.
Vac. Sci. Techno!. A 6, 2736 (1988); G. P. Carey. Ph.D. dissertation.
Stanford University, 1988 (unpublished).
If'A. Raisanen, A. Wall, S. Chang, P. Philip, N. Trouillier. and A. Francimi.
J. Vac. Sci. Techno!. A 6, 2741 (1988).
"G. P. Carey, A. K. Wahi, D. J. Friedman. C. E. McCants, and W F.
Spicer, J. Vac. Sci. and Techno!. A 7. 483 (1987).
"E. H. Rhoderich and R. H. Williams, J\1etal-Semiconductor ConWels
(Clarendon, Oxford. 1988).
''!w. E. Spicer, R. Cao. K. Miyano, C. McCants. T. T. Chiang. C. J. Spindt.
N, Newman, T. Kende1ewicz. I. Lindau, E. Weber. and Z. Li1ental-We
ber, in Metallization and Metal-Semiconductor Interfaces. edited hy I. P.
Batra, NATO ASI Series. B 195 (Plenum. New York, 19R8).
'''V. Krishnamurthy, A. Simons. and C. R. Helms. J. Vae. Sci. Techno!. A 8.
1147 (1990).
"E. S. Johnson and J. L. Schmit. 1. Electron. Mater. 6. 25 (1977).
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.209.6.50 On: Thu, 18 Dec 2014 11:12:53 |
1.38545.pdf | AIP Conference Proceedings 190, 106 (1989); https://doi.org/10.1063/1.38545 190, 106
© 1989 American Institute of Physics.RF current drive and heating in
JT-60
Cite as: AIP Conference Proceedings 190, 106 (1989); https://
doi.org/10.1063/1.38545
Published Online: 16 June 2008
K. Uehara , H. Kimura , and JT-60 Team
106
RF CURRENT DRIVE AND HEATING IN JT-60
K. Uehara, H. Kimura and JT-60 Team
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute,
801-1, Naka-machi, Naka-gun, Ibaraki-ken, 311-01 Japan
ABSTRACT
RF current drive and heating experiment with Lower Hybrid Range of Frequencies
(LHRF) and Ion Cyclotron Range of Frequencies (ICRF) in JT-60 are presented. In
LIq_RF, high efficient current drive and profile control with various N/./are demonstrated
by the multi-junction launcher in successful. In ICRF, optimizatmn of the second
harmonic heating with various methods, and beam acceleration and heating by third
harmonics are presented.
I. LHRF RESULTS
1. INTRODOUCTION
JT-60 is the only machine among the four large tokamaks (TFTR, JET, T-15 and
JT-60) 1 that the Lower Hybrid Wave stressed to the first priority for the rf heating. In
JT-60, 24 MW LHRF at 2 GHz band are installed,in which we have already performed
2 MA for 2.5 see steady current drive and high efficient current drive of 1.7-2.8 x 1019
AW-lm -2 with the conventional 4 x 8 phased array waveguides launcher. 2 We have
improved some controlling system and one of the 4 x 8 waveguide launchers is
changed into 4 x 24 multi-junction type launcher. For the sake of above
improvements, we can get relatively quick performance of launcher aging. We can
further enlarge the driving efficiency of LHCD up to 3.4 x 1019 A W -1 m "2 by using
the multi-junction launcher and can get successful profile control with various Nil. In
section 2 and 3, system description and LHRF heating results are given,respectively.
Discussion and conclusion are presented in section 4.
2. SYSTEM DESCRIPTION
In JT-60,a new divertor coil was installed to produce a lower X point configuration
in early 1988. 3 Experiments described in this part were performed in the hydrogen
plasma in the range of the average plasma density ~e= (0.8-3.0) x 1019 m "3, plasma
current Ip = 1 -1.5 MA, toroidal magnetic field Bt =4 -4.5 T and the effective q value
qeff = 3 -4, respectively.
RF heating system 4 has performed some improvements of the control system in
order to obtain efficient launcher conditioning and good operational maintenance.
Especially,the notching circuit of the LHRF reflection power are equipped. When the
reflection power to the klystron is higher than the setting value,then rf power is cut off
and we cannot retry until the shot is over, however, the operational efficiency becomes
worse when we perform the high power experiments. So,we improve the system so as
to retry again in a sequence. By these improvement of the control system we can
contribute to shorten the conditioning time and the efficient operation can be realized.
© 1989 American Institute of Physics
107
The multi-junction launcher with the sharp wave spectrum and good directivity to
improve the current drive efficiency is newly installed in JT-60. Previously we have
two launchers of wide width waveguide for the plasma heating and one launcher of
narrow waveguide width for the current drive. We have changed one of the heating
launchers into the current drive launcher. The new current drive launcher is multi-
junction type 5 with 4 x 24 phased array consisted by 8 modules. Each waveguide of
the conventional launcher is divided into three sections at the top of the launcher to
form one module. The geometrical phase shifters with taper type are equipped at the
middle position of the launcher. The top of the waveguide piece is carbon coated to get
a low secondary emission and the copper is plating inside the waveguide. The most
difficult points during manufacturing the launcher are reduction of distortion due to the
welding and the difficulty of copper plating inside waveguides. 6 The phase difference
between adjacent waveguide is set 70 degree. Directivity is improve by 50 %
compared with the conventional one.
Improvement of the control system and new launcher lead to quicken the aging time
,that is, we can get about 2 MW injection by performing I0 hours in vacuum
conditioning,16 hours in TDC conditioning and 30 shots plasma injection,whereas we
need about 100 shots vacuum and plasma injection in the former operation. The
reflection is reduced by 20 - 25 % compared with the conventional one. 6
3. CURRENT DRIVE EXPERIMENT
3.1 Current drive efficiency
Figure 1 shows the obtained driving current of ~eRIRF vs the LH power, where ne is
the line average density,R is the major radius and IRF is the driving current. We can see
that JT-60 LHCD results exceeds more than eight times than other tokamaks and that
the continuous progress of the currnt drive is obtained with increase of the rf power
and ten times progress has been performed since the initiation of LHCD experiments in
the world. Figure 2 shows the tic D vs We ,where the experimental current drive
efficiency is defined as
~eIRFR (1)
qCD= PLH
We can see the increasing ofl]c D with Hc up to 2.7 x l019 m -B for plasma current Ip =
1 MA and the higher plasma current tends to have higher efficiency for the same he.
The hil~hest efficiency of 3.4 x 1019AW - lm-2 is obtained at Hc = 1.5 x 1019 m -3 with
N//P eaK: !.3 and <Te> = 2 keV,where N/p eak means the peak value of rf spectrum in
the new multi-junction launcher. The efficiency is compared with various N// by
varying the phase difference between adjacent modules of the multi-junction launcher.
The efficiency clearly increases than the conventional one and changes with N/bin
which we can see that 1]CD increases with decrease of N#P eak and decreases beyond
N//P eak = 1.3. The optimum rlCD is obtained at N#P eak = 1.3 and the value of ~CD
shows a similar behaviour to the Fisch prediction qualitatively. 7 The hard X ray
emission (E> 200 keV) normalized by the LH power shows the same behaviour as that
of ~]CD with N//peak , that is,the signal from higher energy electrons behaves in
corresponding to the variation of TIED. The temperature dependence of qCD iS shown
in Fig.3,in which we can see that the higher temperature may lead the higher efficient
current drive.
It should be noted that the conventional theoretical prediction may give the
maximum value,because the theoretical efficiency is usually defined as
108
gJj 27~rdr
TICD = (2)
IPd 2rtrdr
where IPd 2~rdr is the absorbed power by plasmas to hold the current 7 which differs to
Prf in eq.(1) and is rather smaller than this depending on the extent of power
absorption. 8 Strictly speaking,we cannot observe Pd by the experiment. Experimental
results in Figs 2 and 3 suggest that the extent of power absorption may be a function of
<Te> and We. The effect of Zeffmay be also considered ,since Zeff becomes small with
increase of the density.
3.2 Volt-See saving experiment
Volt-see saving by the LHCD was performed during the plasma current ramping up
from 0.7 MA to 1.5 MA with Bt = 4.5 T and We =1 x 1019 m -3 by varying the ramp-up
speed of Ip. The loop voltage of 1.9 V without LHCD is reduced to 0.9 V during 2
sec of LHCD for Ip= 0.4 MA/s discharge. The saved voh-sec is 0.9 V x 2 sec =1.8
V-sec and the saving volt-sec is proportional to the injected LH energy, PLHAt, and is
higher for larger current ramping up rate.
3.3 Profile control with N//
Using the advantage of sharp N//and high directivity for wide N// range,we
demonstrate the profile control experiments with various N//in successful. Time
derivative of internal inductance li against N//shows the higher decreasing rate for the
larger N//,which indicates that the higher N//wave may flatten the current profile more.
Correspondingly,the spatial distribution of the hard X ray signal vs N//shows the same
behaviour. We equipped four channel X ray diagnostics in the radial direction as
shown in Fig.4 (a). The Abel transformed radial profile using emission signals
measured at r/a=0.23(chl),r/a=0.57(ch.2) and 0.86(ch.3) are shown in Fig.4 (b) and
the dependence on N//P eak is shown in Fig.4 (c),in which we can see that the relatively
larger number of higher energy electrons are localized at the center with small N//and
small number of the tails are in the outer with large N//.
We also observe the increase of the coherent m=2 and m=3 oscillation with large
N//which is accompanying the decrease of li and the suppression of the sawtooth
oscillation of NB heated plasma with small N//of 1.3,which is characterized by the
delay of the starting time of the sawtooth oscillation. The suppression period xst of
sawtooth oscillation increases with PLH and Xst = 1.8 sec is obtained for PLH = 2MW.
The decay time of the stored energy after the LHCD cut off shows the twice as much
that of NB or LH alone.
4. DISCUSSION AND CONCLUSIONS
Dependence of the electron temperature and the density must be further refined to fit
the theoretical understanding of LHCD. The effects which are not consider in the
quasi-linear theory may be included such as the multi-path, the density fluctuation, the
forbbiden condition of mode conversion,the non-linear effect and so on. 9 Many
experimental results obtained in JT-60 can be expected to refine the theory of LHCD.
The success of the profile control by the LHCD can open new frontier in tokamaks
for the various possibility such as the controlling the plasma disruption,the
improvement of plasma confinement and so on. It is stressed that the higher energy
109
electron tail caused in LHCD is independent on the bulk plasma and it is confirmed that
the LHCD can really affect the plasma profile by varying N//,whereas it may be very
difficult to vary the current profile by affecting bulk electrons as is shown by the
scheme of profile consistency.10
In conclusions,multi-junction launcher successfully brought the further
imp.rovement of the driving efficiency and the profile control is demonstrated with
varmus N//. Many experimental results on the LHCD in JT-60 can give the
informations on the further verification on the LHCD including the quasi-linear theory.
II. ICRF RESULTS
1. INTRODUCTION
An experimental study of the second harmonic and even much higher (up to 4th)
harmonic ICRF heating on a large tokamak is being carried out in JT-60. Up to now, we
have investigated most intensively the second harmonic heating with ohmic and NBI-
heated plasmas. Phase control in the toroidal direction has been found to play an important
role in optimizing the second harmonic heating I 1. Significant enhancement of the plasma
stored energy associated with strong beam acceleration has been observed in combination
with high power NBI heatingl2. Combination with pellet fuelling has also been
examined 13. In Section 2, system description is presented. In Section 3, optimization of
the second harmonic heating is discussed from the point of view of phase control, species
effects and dependence on plasma current. Most recently, we have observed significant
beam acceleration and effective heating via third harmonic resonance in combination with
NBI. These results are described in Section 4. Conclusions are given in Section 5.
2. SYSTEM DESCRIPTION 4
The total generator output is 6MW in the frequency range of 108-131MHz, which is
delivered by eight lines of amplifier chains. The frequency is set at 131MHz for the
present experiment. The phased 2x2 loop antenna array is used. The maximum injected
power so far is 3MW. The corresponding power density at the antenna is 1.6kW/cm 2.
New functions, reflection power limiter and frequency feed-back control, have been
introduced. The former is useful to continue power injection without cut-off even in the
Case of bad matching due to rapid change of the antenna loading. The latter is effective to
maintain good matching against practical change of the antenna loading, although long line
effect is not applied.
3. OPTIMIZATION OF SECOND HARMONIC HEATING
3.1 Phase Control
Up to now, two phasing modes, (0,0) mode and (~,0) mode, have been mainly
investigated. The former in the parenthesis is the toroidal phase difference and the latter is
the poloidal one, respectively. (0,0) mode is characterized by a large coupling resistance
but moderate heating efficiency. (n,0) mode has a smaller coupling resistance but excellent
heating efficiency. Figure 5 shows incremental energy confinement time ,~(--AW/AP,
AW is incremental stored energy due to additional power AP) as a function of the line
averaged electron density ~c for various heating conditions. Circles and squares denote
(0,0) mode and (,0) mode, respectively. Apparently, "c~ nc of (zt,0) mode is much larger
than that of (0,0) mode. "~ of (0,0) mode tends to decrease with increasing electron
110
density. Significant scattering of'c~ of (0,0) mode is not only due to random error but also
due to appearance of two distinct modes 14. "t~ ¢ of (0,0) mode is kept at -50ms even in the
high density regime with pellet fuelling as shown in Fig.5.
3.2 H Minority Second Harmonic Heating in He Discharge
Heating efficiency of the second harmonic heating has been further improved when
(re,0) mode was applied to the hydrogen minority second harmonic regime in the helium
discharge• In this experimental run, mixture gas of 90% He and 10% H was used. The
operational range for ICRF experiment has been extended significantly with helium
discharge, i.e., the highest ~c and lp and the lowest qeff achieved so far are 8.3x1019
m "3, 2.8MA and 2.2, respectively. Part of the data corresponding to this scheme are also
indicated in Fig.5. "r~of 100-120ms was obtained in the wide range of the electron
density. Giant sawteeth were observed during the heating even in the high density regime
(~e-7x1019 m -3) . Period of the giant sawteeth seems to be independent of~e. Typical
waveforms of the hydrogen minority second harmonic heating is shown in Fig.6.
3.3 Dependence on Plasma Current
We have observed that "~ of (0,0) mode increased with Ip unlike the NBI heating in
im JT-60. xE reached about 100 ms at Ip=2MA, whereas typical value ofx~ at Ip=I.5MA is
50ms. Therefore, x~: of lOOms obtained at Ip=2MA means considerable good
confinement. However, the good confinement shots showed some strange behaviours in
their time evolutions. Typical example is shown in Fig.7. A minor disruption (M.I.D.)
took place twice in the course of the ICRF pulse. The plasma stored energy increased
dramatically just after the second M.I.D. Both electron and ion temperatures increased in
the plasma core, but ~e at r= 0.5a did not change appreciably after the second M.I.D. It
seems that M.I.D. produces some favourable conditions for the heating of (0,0) mode.
4. THIRD AND FOURTH HARMONIC BEAM ACCELERATION
We have examined whether third and fourth harmonic beam accelerations occur in the
central region, varying the toroidal magnetic field, B T. The beam acceleration is measured
in the incremental tail ion temperature, AT] ail 15, which is the difference of the slope of the
.... tail 1on energy spectra above the m.lect~on energy between NBI only and NBI+ICRF. AT i.
was measured by a charge exchange neutral analyzer, whose line of sight intersected with
specific beam lines of NBI in the plasma core, so that we could obtain the ion energy
spectra in the plasma core 16. From the data of AT~ air, we have confirmed that the third and
fourth harmonic beam accelerations actually occur in the plasma core. De~ee of the beam
acceleration becomes weak with increasing order of harmonics.
Heating effects on the bulk plasma by the third harmonic beam acceleration are found
to be as strong as that of the second harmonics. Figure 8 illustrates time evolutions of the
plasma stored energy, the central electron temperature, the charge exchange neutral flux at
92 keV and ~e at r= 0.5a in the case of the third harmonic beam acceleration• Enhancement
of the central electron temperature and sawteeth period was seen with increasing population
of the energetic ions. x~ of ICRF of this shot is 80ms, which is comparable to the one of
the second harmonic heating. Heating effects by the fourth harmonics is not so strong up
to now,
111
5. CONCLUSIONS
Recipes for improving the second harmonic heating has been elucidated. Phase
control, pellet injection, helium discharge, higher plasma current as well as beam
. ine acceleration tmprove x E of the second harmonic heating. Beam acceleration with third and
fourth harmonics has been observed for the first time. Heating effects by the third
harmonics are as strong as those by the second harmonics.
III. RF PLAN FOR JT-60 UPGRADE
We are planning the up-grade programme for JT-60 (named JT-60U). 17 RF plan for
JT-60U is in the following. In LHRF, two units of RF lines are jointed to form one rf
injection with horizontal direction and one unit keeps with oblique injection. The
horizontal launcher consists of 4 x 4 module multi-junction with four 18 waveguide
columns at the center and eight 12 waveguides columns at the top, bottom and side. 18 We
also expect further power up of klystrons with some improvements. In ICRF, present
antenna will be replaced by two new antennae, which are also 2 x 2 loop array and have
larger width ( - 90 cm) to ensure large coupling for (~,0) mode and H-mode. The
generator output will be increased up to 10 MW by replacing the present tetrode 8973 with
X-2242.
ACKOWLEDGEMENTS
The continuing support of Drs. M. Yoshikawa and M. Tanaka is greatly appreciated.
REFERENCES
1. A.H. Spano (compiler), Nucl. Fusion 15 909 (1975)
2. T. Imai et al., Nucl.Fusion 28 1341 (1988)
3. JT-60 Team presented by H. Kishimoto et al., Plasma Phys. and Contr. Fusion A-
I-4
4. T. Nagashima and K. Uehara et al., Fusion Eng.& Design 5 101 (1987)
5. T.K. Nguyen et al., Fusion Tech. 2, 1381 (1882) and G. Gormezano et al.,
Nucl.Fusion 25 419 (1985)
6. Y. Ikeda et al, "First operation of multi-junction launcher on JT-60" this conference
7. N. Fisch, Phys. Rev. Letters 41,873 (1978)
8. G. Tonon, Plasma Phys.Contr.Fusion 26 45 (1984)
9. K Uehara, M. Nemoto et al., Nucl. Fusion 29 May 1989 (in press)
10 F. Wagner., et al., Phys. Rev. Lett. 56 2187 (1986)
11. H. Kimura et al., in Contr. Fusion and Plasma Phys. (Proc. 14th Europ. Conf.
Madrid, 1987) EPS, vol.11D, Pt.3 p.857
12. T. Fujii et al., in Plasma Phys. and Contr. Nucl. Fusion Research 1988 (Proc.
12th Int. Conf. Nice, 1988) IAEA, Paper IAEA-CN-50/E-2-4
13. JT-60 Team, Japan Atomic Energy Research Institute Report JAERI-M 89-033
(1989) p.185
14. ibid., p.181
15. M. Yamagiwa et al., Plasma Phys. Controlled Fusion 30 943 (1988)
16. H. Kimura et al., Japan Atomic Energy Research Institute Report, JAERI-M 88-
123 (1988)
17. M. Kikuchi et al., 15th SOFT,Utrecht
18. M. Seki et al., "Design of new launcher on JT-60 Up-grade" this conference
112
98 LT4~A
,
=., _ .,111
i~" ,., #%~Y,~_F~ /W o_','~/ I /
i ,-j ,ICO ~'~ i ]
Current drive efficiency vs average density Fig.2
00 I 2 3 4 .... i .... I ' ' ' ' i . . , , i , , • , i
Pt. (MW } ;.6~M~,
h, t~AII Otv [ t l~ I~LIOWI~ )~ULT I - ,~UNCT ~ON _"""~'-~
o.~ - I ,, I -- "1 4,.~ e°o// Rg.l Driving current is demonstrated vs PLH ~ L, * | • I + | /. O ~/
Z' ~ ,.~ i,t-I~.e__j /.t.
(~)
~2 /, 0, , 4 ., CONVENTIONAL
• o %,
"~ / o 8 o~"
~'-_,_ _ ,t- pL !
or~ PEIULX 150[I ,~ .;_~_~
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|
1.343944.pdf | Relaxation phenomena of image sensors made from aSi:H
M. Hoheisel, N. Brutscher, and H. Wieczorek
Citation: Journal of Applied Physics 66, 4466 (1989); doi: 10.1063/1.343944
View online: http://dx.doi.org/10.1063/1.343944
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/9?ver=pdfcov
Published by the AIP Publishing
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128.248.155.225 On: Sat, 22 Nov 2014 19:28:09Relaxation phenomena of image sensors made from a-Si:H
M. Hoheisel, N. Brutscher, and H. Wieczoreka)
Siemens AG, Corporate Research and Development, Otto-Hahn-Ring 6, D-8000 Mfmchen 83,
Federal Republic of Germany
(Received 17 February 1989; accepted for pUblication 6 July 1989)
Image sensors made from amorphous silicon ( a-Si:H ) are under development. Their elements
consist of back-to-back Schottky diodes. For practical operation, long-term stability is of great
importance. We investigated dark conductivity and photoconductivity, capacitance-voltage
characteristics, and response behavior after switching off illumination. Even after light soaking
for many hours, no change in photocurrent occurred, whereas dark current, capacitance, and
response time increased. These changes are metastable and can be reversed by annealing above
200 DC. Contrary to the Staebler-Wronski effect, [App1. Phys. Lett. 31, 292 (1977) J, the dark
current increase disappears at room temperature after several hours. We investigated the time
dependence of this relaxation and calculated the energetic depth of the states involved. The
contact between a-Si:H and indium-tin-oxide is described as a Schottky-Bardeen-metal
insulator-semiconductor junction. Its properties are strongly dependent on interface states, in
particular on the position of the neutrality energy of the interface states with respect to the
Fermi energy. We show that besides the well-known Staebler-Wronski effect, a new
degradation process is observed. We suggest a model where holes are trapped in interface states
about 1.0-1.4 eV above the valence band. Their thermal emission governs the relaxation
behavior of the dark current.
t INTRODUCTION
Easier reading of documents for communication and of
fice automation calls for large-area scanners that can read
A4-size documents without optical reduction. Therefore,
large-area thin-film photoconductors are required that can
be fabricated at least 21 cm wide at low cost. The most prom
ising way to implement such a device is an arrangement of
amorphous silicon (a-Si:H)sandwiched between two elec
trodes forming Schottky-type contacts. A review of such im
age sensors has been given by Kempter. 1
OUf sensors are built in the sequence eel a-Si:H/ITO.
Their elements meet the most important requirements for
image sensors: high photocurrent, low dark current, fast re
sponse behavior, and long-term stability.2 In this paper we
present an investigation of the stability of the sensor based on
the physics of the junction involved.
As the elements are reverse biased ( ITO negative) dur
ing operation, the properties of the a-Si:H/ITO junction are
crucial for the performance of the sensing element. To obtain
a low dark current and an enhanced chemical stability, an
intermediate oxide layer is introduced between a-Si:H and
indium-tin oxide (ITO). Thus, strictly speaking, the contact
is a metal-insulator-semiconductor (MIS) junction, On the
one hand, this junction can be described by the Schottky
theory, but on the other hand, interface states play an impor
tant role in the performance of the junction; so the Bardeen
theory should also be applied. We will therefore call it a
Schottky-Bardeen- MIS junction (SEMIS junction).
a) Permanent address: Philips GmbH. Research Laboratories, Weisshaus
strasse, D-5100 Aachell, FRG. II. THEORY OF THE SCHOTTKY-BAROEEN-MIS
JUNCTION
To explain a SBMIS junction, we start from an ideal
metal-semiconductor contact. The Schottky theoryJ-5 pre
dicts the barrier height 4> B of the Schottky contact from the
work function of the metal ~ M and the electron affinity of
the semiconductor X:
etl»B=e<l»M-ex· 0)
The difference between the barrier height and the acti
vation energy Ee -Ep in the bulk of the semiconductor
leads to a charge transfer from the semiconductor to the
metal, resulting in a positive space charge and a band bend
ing with a diffusion voltage VD:
eVD = ~B -(Ee -EF)· (2)
ITO shows an almost metal-type conductivity in excess of
104 (0 cm)-l. Although it is a highly doped semiconduc
tor, it can be treated as a metal for our investigations.
It is well known that interface states play an important
role in the vicinity ofthe metal-semiconductor contact. This
is explained by the Bardeen theory." It leads to an expression
for the barrier height that depends on the energy gap Eo of
the semiconductor, not on the work function of the metal
used:
e$B = EG -e<l>o· (3)
e<Po denotes the energy difference between the neutrality lev
el of the interface states and the Fermi level. Equation (3)
can be understood as follows: The interface states are filled
up to the Fermi level Ep. Suppose the neutrality level En of
these states is Ev and thus lower than EF by an energy e<I>o.
The interface states are then negatively charged. An equal
amount of charge of the opposite sign forms the space-
4466 J. Appl. Phys. 66 (9),1 November 1989 0021-8979/89/214466-08$02.40 ® 1989 American Institute of PhysiCS 4466
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128.248.155.225 On: Sat, 22 Nov 2014 19:28:09charge region inside the semiconductor. With a high inter
face-state density and a low density of states inside the semi
conductor, this leads to a pinning of the Fermi energy at the
interface.
The theories discussed above outline the two limiting
cases. The barrier height of a real diode must be described by
a combination of both the Schottky and the Bardeen bar
riers.
Introduction of an intermediate oxide layer leads to an
MIS junction. Without a space charge inside the insulator
and without interface states, the electric field will be con
stant throughout the insulator and at the interface. This
leads to a voltage drop ~U across the insulator which de
creases the barrier height:
e<PB = e<l>M -e.¥ -e~U. (4)
In an SBMIS, the interface states result in a partial pinning
of the Fermi energy at the interface, reducing the influence
of the metal's work function. The electric field at the inter~
face has a kink due to the interface charge. A change of the
charge dQ per unit area A causes an equivalent modification
ofthe electric field dU /dj where Ei is the dielectric constant
of the insulator and di its thickness:
d. dU= +-'-dQ.
E€iA (5)
The plus sign applies to the voltage drop as depicted in Fig.
1 (a); Le., a negative charge at the interface leads to a lower
voltage drop ~ U and thus to a higher barrier.
We can now discuss the different changes that can occur
at an SBMIS junction during degradation. Two different
mechanisms will be considered. Either new states are created
by degradation or existing states become negatively or posi
tively charged.
Let us discuss the first case. New states can be created
either in the bulk of the semiconductor or at the interface.
What will happen to the barrier? When the density of states
in the bulk is increased in the energy interval between E F and
e<P B [Le., the shaded area marked as positive space charge in
Fig. 1 (a)], it is obvious that these states have to be re
charged positively and the band-bending profile will change
accordingly. Hence the barrier will become narrower. If in
terface states are increased, their influence upon the barrier
depends on the position of their neutrality energy. For sim
plicity we assume that the newly created states are of the
same type as the existing ones. Suppose their neutrality level
En is lower than the Fermi level EF [Fig. 1(a)J, then the
newly created states have to be fined with electrons, thus
increasing the negative interface charge. This diminishes the
voltage drop au along the insulator according to Eq. (5).
As Eq. (4) shows, the barrier e<l> B will increase. The oppo
site will happen when states with En> EF are created (Fig.
1 (b) 1. These states have to be fined with holes, thus increas
ing the positive interface charge. Hence ~ U will increase and
<I> lJ will decrease.
We will now discuss the case of recharging existing
states. When electrons are trapped in bulk states, they com
pensate the positive space charge. This leads to a lower band
bending and thus to a diminished barrier height. Corre
spondingly, the capture of holes leads to a stronger band
4467 J, AppL Phys" Vol. 66, No.9, 1 November i 989 1J
Metal
I
LL-.L-.
Metal Oxide
Oxide (a)
Semiconductor
(b)
_ . ...E
Semiconductor
FIG. I. Band diagram of a Schottky-Bardeen-MIS junction. The barrier
height amounts to ~8 = e4>M -ex -etJ.U, (a) Interface states with a
neutrality energy below EF are negatively charged; (b) those with a neutra
lity energy above EF are positiveJy charged.
bending and to an increased barrier height. If trapped elec
trons are localized in interface states, this negative interface
charge win increase the barrier [Eqs. (4) and (5) ], whereas
hole capture at the interface decreases it. A summary of all
cases is given in Table I.
m. EXPERIMENT
The samples were produced on glass substrates, as can
be seen from Fig. 2 (a). The bottom electrode consisting of
Hoheisel, Brutscher, and Wieczorek 4467
••• n' ••••••••••••••••• -•••• '. ,".0;.-."," •••••• n .......................... -••••• : ••••••••••••••••• <;.";.' ••••••••••••••••••••••••••••••••••••• .-•••••••• " •• " ••••• _ •••••••••• T .............................................................. ' •••••• ' •••••• '. •••••••••••••••• •••••• .' '~ ••••• " •••••••••••••••• •••••• "'''' ..................... '.~'~'" .n~ ••••••••
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128.248.155.225 On: Sat, 22 Nov 2014 19:28:09T ABLE I. Influences upon the barrier height.
States are Located The barrier becomes
created in the bulk narrower
created at the interface (En <E,,) higher
created ai the interface (En> EF) lower
negatively charged in the bulk lower
positively charged in the bulk higher
negatively charged at the interface higher
positively charged at the interface lower
sputtered chromium was foHowed by an undoped a-Si:H lay
er, which was deposited by plasma-enhanced CVD at 220°C
and with about I-pm thickness. Its surface was treated in an
oxygen plasma to develop an oxide layer. The thickness of
the oxide can be determined from the cross-sectional TEM
image [Fig. 2(b)] to be about 3 nm. Then a lOO-nm-thick
ITO film was evaporated by means of an electron gun as a
transparent upper electrode. Additional gold strip lines were
used to form a low-resistive interconnection between the
sensing elements and the readout circuit. Details have been
described elsewhere. 7
The dark conductivity of the a-Si:H material was mea
sured in a gap-cell geometry in order to determine the posi
tion of the Fermi energy. The samples were n-type, and the
Fermi level was typically about 0.8 eV below Ec. The barrier
height of the SBMIS junctions was determined from tem
perature-dependent J-V characteristics. It amounted to 0.86
eV.8 The Schottky barrier at the bottom of the samples
between chromium and a-Si:H is rather low and has little
{al
Gold 250nm ITO 10()nm
1111
~llliSiOX -30m ~ .~SOH '000._
0.5 mm
FIG. 2. (aJ Schematic cross section through a sample. The deposition se
quence of a typical element is glass!chromium/ a-Si:H/oxide/ITO. An ad
ditional gold layer is supplied for low-resistive wiring. (b) Shows a TEM
image of the oxide layer between a-Si:H and ITO (glue is necessary for pre
paring the TEM cross section).
4468 J. Appl. Phys., Vol. 66, No.9, 1 November 1989 influence on the performance of the device. Therefore, an n+
layer is not necessary.
From every sample we measured current-voltage char
acteristics in the dark and under illumination. The latter was
usually 1014 photons/cm2 s of green light (550 nm). Photo
current transients were measured by switching off steady
state illumination by a Bragg cell. The current was fed
through a fast current-voltage converter into a waveform
recorder. The decay ofthe current could thus be monitored
from 1 Jls to 1 s after switching off the lightY To determine
the junction capacitance, a quasistatic capacitance-voltage
meter (Keithley model 595) was used. It superimposes a
voltage step on a fixed bias and integrates the charge during a
delay time td ranging from 70 ms to 200 s. An internal leak
age-current correction takes care of a constant-current con
tribution to the integrated charge and substracts it automati
cally. 10
The capacitance-delay time measurement is equivalent
to a conventional capacitance-frequency measurement. The
total amount of charge that is thermally emitted from occu
pied states is integrated during the delay time. Hence the
contribution of all states closer to Ec than a certain demarca
tion energy Ed is included. In capacitance-frequency experi
ments, the same states above Ed can be charged and re
charged, and thus foHow the applied alternating voltage.
Likewise, the density of states N(E) can be computed from
the CUd) data. To obtain the N(E) curves shown below
(Fig. 9), we used the computer program developed by Glade
et al,u·'2
Most of our investigations were performed under vacu
um of about ! mPa. The influence of different ambients on
the degradation behavior was shown by experiments in N2,
O2, or air under atmospheric pressure or in saturated water
vapor.
IV. RESULTS
Typical current-voltage characteristics of our SBMIS
junctions are shown in Fig. 3. The annealed sample ( curve
A ) exhibits a dark-current density at reverse bias of only
5 X 10-10 A/cm2• The photocurrent is independent of ap
plied voltage in the negative-bias regime. This clearly shows
its primary nature. Its absolute magnitude corresponds to a
quantum efficiency of unity which is diminished only by re
flection losses at the ITO top electrode. The oxide layer at
the interface is thin enough to have no influence on the pho
tocurrent.
Subsequently, the degradation behavior of the devices
was studied. The diodes were light soaked for 12 h with 100
m W Icm2 white light. Under open-circuit or short-circuit
conditions or with a + 5-V bias applied to the samples dur
ing iHumination, only slight changes were observed. Light
soaking of the diodes under negative bias ( -5 V) causes the
dark current at -5 V to increase strongly (curve B). The
open-circuit voltage under illumination, recognizable by the
sharp dip in the logarithmic plot, decreases, as does the
short-circuit current.
Resting the sample for several days (curve R3 = 3 days,
curve R30 = 30 days) at room temperature leads to a recov
ery of the dark-current and photocurrent characteristics.
Hoheisel, Brutscher, and Wieczorek 4468
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128.248.155.225 On: Sat, 22 Nov 2014 19:28:09i Current density
lO-l =r---------,.-------------,
A Annealed
B Afte~ light soaking
R3 Rested (3 days)
R30 Rested (30 days)
-5 [V}
Applied bias --
FIG. 3. Current-voltage characteristics in the dark (lower curves) and un
derillumination ( uppercufves). A = annealed state, B = after light soak
ing (12 h, 100 mW /cm2), R3 = rested 3 days at room tempemture, and
R30 = rested 30 days at room temperature.
The dark current approaches its low value in the annealed
state (A). The open-circuit voltage and the short-circuit
current also recover. When the sample is annealed again,
state A is reestablished. The whole degradation cycle can be
executed repeatedly.
The dark current under forward bias does not follow
this trend. It decreases slightly after illumination, but does
not increase during resting. This can be easily understood, as
the forward current is limited by the bulk resistance of the a
Si:R It is subject to the normal Staebler-Wronski degrada
tion, which remains stable at room temperature. As the Fer
mi energy in the bulk material already lies near midgap, light
soaking causes only a weak shift, and therefore the dark cur
rent decreases just slightly.
We studied the degradation behavior of the dark current
at -5 V bias in detail. Figure 4(a) shows its increase with
illumination time in a double-logarithmic plot. The light was
interrupted from time to time and the dark current recorded.
After a very steep rise during the first 1000 s (not shown),
the current increases with the square root of time up to 10 h.
In comparison, the spin density Ns rises proportionally to
t1/3•13 However, the reverse current depends only indirectly
on the spin density and is therefore not expected to exhibit
the same relationship ( _ t1/3).
Then, starting from the annealed state, we degraded the
sample several times with different photon fluxes F for 45
min at each intensity [Fig. 4(b)]. This led to a very strong
4469 J. Appl. Phys., Vol. 66, No.9, 1 November 1989 i Dar: cu~rer.: denSi!,!
1O-0--r------------------~
Dark current aller light soaking
[sJ 104
(a) Light soaking time ----+
r Dark current d:~ _________ . ________ ___,
Dark current rise after light soak;ng !or 45 m;n
10-1
10--8
[photonsicm2sl 1015
(b! Photon flux -
FIG. 4. Degradation behavior of the dark current Id at-· :5 V bias. (a)
Shows the increase of Id with time during light soaking. (b) Shows fa after
light soaking with different photon fluxes for 45 min in each case.
dependence of the dark current on F with an exponent of
1.25. For comparison, the intensity dependence of the rise of
Ns has an exponent of 2/3. 13,
The relaxation of the dark current under reverse bias
Hoheisel, Brutscher, and Wieczorek 4469
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128.248.155.225 On: Sat, 22 Nov 2014 19:28:09( - 5 V) was investigated at different temperatures. The
samples were light soaked as described above at room tem
perature. After switching off" the illumination, they were
heated quickly to the desired temperature, and the dark cur
rent was monitored over times up to 270 h. Figure 5 shows
the decreasing current which reveals the features depicted in
Fig. 3. The decay ranges from 1 min up to 270 h. If we as
sume that the basic process is a thermal emission of trapped
carriers from localized states, we can convert the time scale
to an energy scale by the relation
E = kTln(vot). (6)
For the attempt-t~-escape frequency vo, we take a value of
1014 S-·I. Hence the energies span a range of about 0.93-1.4
eV. In Fig. 6 the transients of Fig. 5 are replotted with the
curves shifted relative to each other to yield a continuous
trend. In the case of a diffusion-limited saturation current
density js the barrier height can be written as
$B = -kTln(~) ,
e eNcpF (7)
To calculate t1> B the effective density of states in the conduc
tion band Nc' the carrier mobility p, and the strength of the
electric field F at the interface are required. As we know
neither the actual values of Nc' f..l, and F nor their depend
ence on temperature, degradation, or relaxation, Eq. (7)
only allows a general trend to be estimated. Nevertheless, the
change in current indicates an increasing barrier height.
From the energy interval in which the current decreases we
conclude that carriers are emitted from states between 1.02-
1,4 eV deep.
The photocurrent during illumination at reverse bias
( -5 V ) was monitored for several hours. It showed no
significant degradation and remained constant within
± 0.5% due to experimental scatter.
On the other hand, short-circuit photocurrent decay
transients show significant differences between the annealed
and light-soaked states. The curves recorded after switching
off steady-state illumination are shown in Fig. 7(a). Elec
trons are trapped in localized states in the a-Si:H during
illumination. Subsequently, these electrons are thermally
emitted and extracted by the built-in field determining the
i Dark current denSity
iO-5,,-------------------,
Relaxation al various temperatures
10-"
297 K
10-1
[s] 10·
Relaxation time -
FlGo 50 Relaxational behavior of the dark current ld at -5 V bias at differ
ent temperatures. Id decreases up to two orders of magnitude within 270 h.
4470 J. Appl. Phys., Vol. 66, No.9, 1 November 1989 i Dark current
10° ,,----""""--------------,
[arbitrary units]
10-1
10-1
10-5
0.9 1.0 1,1 1.2 i.3 leV] 104
Energy
FIG. 6. Logarithm of the dark current in arbitrary units as in Fig. 5, plotted
vs emission energy. The curves were shifted relative to each other, yielding
the general shape of the emission process.
current decay. 14 The emission time can be converted into an
energy by means of Eq. (6). The energy spectrum of the
trapped charge n (E) can then be calculated from the current
J(t) by
icurrent (a)
10-5.~--~---------------_,
[A]
10-8 ,
10-7 10-2 Isl 10-1
Time-
i Charge density (b)
1017 ::r-----------------------,
Charge emitted alier illumination
1014 -t-----.----.-----r-----.----,.-----'
OJ 0.4 0.5 0.6
FlG. 7. (a) Photoeurrent transient after switching off steady-state illumi
nation (sample area 0.2 em2). (b) As the transient current is due to thermal
emission of trapped electrons, the energy-resolved charge density neE) can
be evaluated. Starting from the annealed sample (state A), neE) rises
strongly after light soaking (state B).
Hoheisel, Brutscher, and Wieczorek 4470
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128.248.155.225 On: Sat, 22 Nov 2014 19:28:09nCE) = I(t)t lekTV, ( 8)
where V is the sample volume [Fig. 7 (b) ] .
It should be pointed out that the measured charge den
sity n(E) is the product N(E)f(E), i.e., bulk density of
states N(E) and occupancy of these statesf(E) , re<;pective
ly. nCE) shows an increase in the energy range from 0.45 eV
down to about 0.7 eV below Ee. The trend is mainly due to
fCE) which is dominated by a Boltzmann distribution. The
shape of N(E) itself cannot be resolved by this method in
detail, but it reveals no distinct structure. The measured
charge density rises strongly after 64 h illumination with 100
mW/cm2 white light (state B). Asf(E) is expected to be the
same in the light-soaked state, N(E) must increase by about
one order of magnitude. This can be easily understood by
assuming additional dangling bonds created by the Staebler
Wronski effect.
Figure 8 shows the capacitance of the space-charge re
gion of our SBMIS junction measured by the quasistatic
method plotted versus delay time. At short times, the capaci
tance is equal to the geometrical capacitance of the diode.
Within a certain delay time td, only states that are closer to
Ee than the corresponding demarcation energy Ed are ther
many emi.tted and can thus contribute to the integrated
charge. In analogy to Eq. (6), Ed is given by
(9)
For td = 70 ms, the shortest time interval used, and Vo
= 1014 S I (Ref 12), we obtain Ed = 0.76 eV. As in our
samples, the position of the Fermi level, Ee -EE' >0.76 eV,
and the states above the demarcation energy Ed are empty
and thus do not appear in the capacitance. Toward longer tel
we observe the contribution of deeper states, and the capaci
tance therefore increases. From this curve, the density of
states N(E) can be calculated. 12
Figure 9 shows a fit to N(E) around 0.8 eV. As our
samples are undoped, we are restricted to a narrow energy
interval. The density of states near midgap amounts to about
2X 1016 and 3X 1017 em -3eV 1 in the annealed and light
soaked samples, respectively. These results are comparable
with those obtained by Glade, Reichler, and MeWS on un-
t Capacitance
. 150~------------
100
50
Delaytlme---
FIG. 8. Junction capacitance measured by the quasistatic method at zero
bias. Delay times range from 70 ms to 200 s. Starting from the annealed
sample (state A), the capacitance rises strongly after light soaking (state
B).
4471 J. Appl. Phys., Vol. 66, No.9, 1 November 1989 i N{E)
1018'::r-~ ________________ ---,
1015 +.-r-r-r-,-..,-r-r-;-.,..-..,.--,,-,-,-..,.--,,-,-,-,--i
0.95 0.90 0.85 0.80 [eVj 0.75
Ec-E
FIG. 9. Density of state.~ N(E) calculated from the junction capacitance
(Fig. 8). N(E) rises strongly from state A to B according to the normal
Staebler-Wronski effect.
doped a-Si:H/Pt Schottky barrier diodes. The change of the
capacitance spectrum after illumination, i.e., the rise from
state A to state B, reflects the creation of states due to the
Staebler-Wronski effect.
V. DISCUSSION
In our measurements we can see two different processes
taking place: We observe the well-known Staebler-Wronski
effect (SWE), which is metastable at room temperature. It
can be annealed as usual above 200 ·C. Additionally, we find
a new phenomenon, a marked increase in the dark current of
the SBMIS junctions after intense illumination, which is not
stable at room temperature and disappears after several
hours. Earlier investigations by Jousse et al.16 did not con
centrate on reverse-biased diodes. They found the normal
SWE in the forward characteristics, but could not explain
the features or the reverse current.
Let us discuss the Staebler-Wronski effect first. Illumi
nating an a-Si:H sample for a long time with intense white
light leads to the creation of dangling bonds near midgap.
This can be understood by the breaking of weak Si-Si bonds
and a rearrangement of the hydrogen associated with these
bonds. l:l As a consequence, the Fermi energy may shift, de
pending on its position in the annealed state. As in our un
doped samples, E F is located near midgap, and there will be
no significant shift of E p. So changes in the properties of our
junctions have to be interpreted in terms of a risen density of
states in the vicinity of midgap.
Capacitance measurements offer relatively easy access
to the N(E) dependence. The results show an increase of
more than one order of magnitude at 0.85 eVbelow Ec. This
corresponds to the neutral dangling bond state in agreement
with the interpretations of Kocka, Vanecek, and Schauer. 17
Measurements of the photocurrent decay reflect an in
crease in charge density nCE) after illumination. As noted
above, neE) depends on the occupancy f(E) of the states
involved.f(E) for its part depends on illumination and tem
perature. Thus n (E) gives only a lower limit for the density
of states N(E). As the neutral dangling bonds do not emit
Hoheisel, Brutscher, and Wieczorek 4471
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128.248.155.225 On: Sat, 22 Nov 2014 19:28:09electrons into the conduction band within the measuring
time ( < 1 s) at room temperature, we cannot observe their
contribution. That is why we attribute the rise in neE) to an
increase in negatively charged dangling bonds. It is likely
that at least part of these states are occupied under illumina
tion and that they will take part in the emission processes
during the transient.
The Staebler-Wronski effect reduces the photoconduc
tivity18 since the additionally created dangling bonds act as
recombination centers. Nevertheless, we see no effect on the
photocurrent of our samples. At a bias voltage of -5 V, the
electric field across thea-Si:H layer amounts to about 5 X 104
V Icm. This leads to a Schubweg of the order of 100 pm.
Compared to the thickness of the sample of 1 pm, the proba~
bility for recombination is negligible. Even a pronounced
diminution of the Schubweg by the SWE has no influence on
the photocurrent if it is greater than the a-Si:H thickness.
Only in extreme cases of low fields that are found in the
interior of the sample at zero bias is there recombination in
our SBMIS diodes. This can be seen in the photocurrent
voltage characteristics (Fig. 3) where the photocurrent at
zero bias reflects the influence of recombination on the
short-circuit current by the Staebler-Wronski effect.
The behavior ofSBMIS devices with respect to junction
capacitance, steady state, and transient photocurrent can be
understood on the basis of the norma] Staebler-Wronski ef
fect. But this effect does not predict the considerable rise in
dark current observed. A possible explanation could be a
reduction in the width of the barrier due to an increased
density of states. The tunneling current through a-Si:H bar
riers is limited by thermionic field emission, as pointed out
by Jackson et ai. 19 Therefore, additional states should lead to
a decrease of the effective barrier height and thus to an in
crease of Jd• But since the observed rise is not stable and
disappears at room temperature with time constants in the
order of hours, the explanation on the basis of the Staebler
Wronski effect can be mled out.
Looking at Table I, severa! other possibilities can ex
plain the barrier lowering observed. In our opinion, it is un
likely that new states are created at the interface by light
soaking. Interface states are caused by the polarizability of
the chemical bonds between semiconductor and metal or
insulator, respectively. Although great efforts have been un
dertaken to reduce the interface-state density, a certain num
ber of such states cannot be avoided. Therefore, these inter
face states are not expected to disappear quickly at room
temperature.
Negatively charged bulk states could arise from trap
ping of photogenerated electrons in the space-charge region.
However, they are inconsistent with the situation in our
SBMIS junctions. From the relaxational behavior (Fig. 5)
we conclude that the energetic depth of the states involved is
between 1.0 and L4 eV. With reference to the barrier height
of 0.86 eV,8 these charged states would be below the Fenni
energy. Therefore, an emission of electrons into the conduc
tion band cannot take place.
Finally, we propose that interface states become posi
tively charged, resulting in a barrier lowering. These states
are located at the interface between the semiconductor and
4472 J. Appl. Phys., Vol. 66, No.9, 1 November 1989 the insulator or inside the insulator, respectively. Their ener
getic position is 1.0-1.4 e V above the valence-band edge.
Holes generated by prolonged illumination are trapped in
these states. This leads to a barrier lowering as described
above (see Table I).
Thermal excitation of trapped holes into the valence
band can explain the barrier relaxation described above.
However, we have to assume that holes can leave the inter
face states only thermally. The tunneling transitions from
the hole traps to the metal or to the a-Si:H as wen as recombi
nation with electrons should be negligible.
A striking feature of the dark current increase is its de
pendence on the applied bias during light soaking. The deg
radation effect is strongly enhanced by a negative voltage at
the ITO electrode. This reverse bias draws the photogenerat
ed holes towards the a-Si:H/ITO interface and intensifies
the trapping process. The Staebler-Wronski effect, on the
contrary, is suppressed by a negative bias which prevents
recombination within the a-Si:H layer.
One further observation points to the fact that interface
states are responsible for the barrier degradation and relaxa
tion: Samples measured under vacuum show a smaller effect
than those investigated in air. Experiments with different
ambients show a weak degradation and relaxation in dry
ambients (N 2' (2) and a strong effect in moist ambients
(air, water vapor). So it is obvious that water is the most
likely cause of the interface states mentioned above. We sug
gest that additional interface states are created by the pres
ence of polar molecules, i.e., water. They change their charge
state by trapping holes during prolonged illumination. This
leads to the barrier lowering observed. From the slow relaxa~
tion process described above, we estimate their energetic po
sition to be 1.0-104 eV above Ev'
The upper electrode of our diodes made from ITO has a
porous structure.20 Molecules can therefore diffuse to the
interface and induce the formation of interface states. Once
formed, these states remain even under vacuum. They can be
annealed away at an elevated temperature. These ambient
induced defect states are still not fully understood and will be
investigated more closely in the near future.
Vt CONCLUSIONS
In this paper, the influence of the creation and recharg
ing of defect states in SBMIS diodes on the barrier height is
investigated. An overview is given in Table II. The creation
of bulk states (dangling bonds) by light soaking leads to
TABLE II. Effects of light-soaking and their origin.
Staebler-Wronski effect
Light soaking causes rise
of bulk density of states
Forward current decreases
Transient photocurrent increases
Junction capacitance increases
Staebler-Wronski effect
anneals above 200 'C Interface-state elreet
Light soaking causes barrier
lowering by hole trapping
Reverse current increases
Open~circuit voltage decreases
Short-circuit current decreases
Barrier lowering recovers
at room temperature
Hoheisel, Brutscher, and Wieczorek 4472
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128.248.155.225 On: Sat, 22 Nov 2014 19:28:09changes in transient photocurrents as well as in space-charge
capacitance. The results are consistent with the normal
Staebler-Wronski effect.
Water vapor diffusing through the ITO electrode
towards the a-Si:H gives rise to interface states. They be
come positively charged after light soaking, which lowers
the SEMIS barrier considerably. After resting the samples in
the dark from several minutes up to a few days at room
temperature, the barrier recovers. Hence we conclude that
the interface states are about 1.0-1.4 eV above the valence
band edge.
ACKNOWLEDGMENTS
The authors would like to thank R. Primig, H. Doneyer,
W. Mtiller, and E. Scheuermeyer for preparing the samples,
and A. Kiendl for carrying out the CAD. The cross-sectional
TEM pictures taken by S. Schild are gratefully acknowl
edged. We are indebted to A. Glade for stimulating discus
sions about the capacitance measurements and their inter
pretation, and to W. Fuhs for his helpful comments.
4473 J. Appl. Phys., Vol. 66, No.9, 1 November 1989 'K. Kempter, Proc. spm 617, 120 (1986).
2M. Hoheisel, G. Brunst, and H. Wieczorek, J. Non-Cryst. Solids 90, 243
(1987).
'w. Schottky, Z. Phys. 113, 367 (1939).
"s. M. Sze, Physics a/Semiconductor Devices (Wiley, New York, 1969), p.
363
5K J. Ncrnanich. in Semiconductors and Semimetals, edited by J. I. Pall
kove (Academic, Orlando 1984), Vol. 21, p. 375.
6J. Bardeen, Phys. Rev. 71,717 (1947).
'K, Rosall and G. Brullst, MRS Symp. Proc. 70, 683 (1986).
8M. Hoheisel, N. Brutscher, H. Oppoizer, and S. Schild, J. NOIl-CrysL Sol
ids 97&98,959 (1987).
9H. Wieczorek, thesis, Marburg, 1987.
"'T. J. Mego, Rev. Sci. lnstrum. 57, 2798 (1986).
II A. Glade, thesis. Marburg, 1987.
12A. Giade, W. Fuhs, and H. Mell, J. NOll-Cryst. Solids 59&60, 269 (1983).
13M. Stutzmann, W. B. Jackson, and C. C. Tsai, Phys. Rev. B32, 23 (1985).
I4H. Wieczorek and W. Fuhs, Phys. Status Solidi A 109,245 (1988).
"A. Glade, J. Beichler, and H. Mel!, J. Non-Cryst Solids 77&78, 397
(1985) .
16D. Jousse, R. Basset, S. Delionibus, and B. Bourdon, Appl.l'hys. Lett. 37,
208 (1980).
'7J. Kocka. M. Vanecek, and F. Schauer, J. Non-Cryst. Solids 97&98,715
(1987).
"D. L. Staebler and C. R. Wronski. App!. Phys. Lett. 31, 292 (1977).
lOW. B. Jackson, R. J. Ncmanieh, M. J. Thompson, and B. Wacker, Phys.
Rev. B 33,6936 (1986).
2U A. Mitwalsky, M. Hoheisel. W. Miiller, and C. Mrotzek, Inst. Phys. Conf.
Ser. 93,107 (1988). .
Hoheisel, Brutscher, and Wieczorek 4473
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1.342749.pdf | Sodiumfluoride discharge for fast Zpinch experiments
B. L. Welch, F. C. Young, R. J. Commisso, D. D. Hinshelwood, D. Mosher, and B. V. Weber
Citation: Journal of Applied Physics 65, 2664 (1989); doi: 10.1063/1.342749
View online: http://dx.doi.org/10.1063/1.342749
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/7?ver=pdfcov
Published by the AIP Publishing
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38Sodium .. fluoride discharge for fast Z ... pinch experiments
B. L. Welch,a) F. C. Young, R. J. Commisso, D. O. Hinshelwood,b) D. Mosher,
and 8. V. Weberb)
Naval Research Laboratory, Washington, DC 20375-5000
(Received 24 June 1988; accepted for publication 18 November 1988)
A capillary-discharge plasma source has been developed to produce a sodium-bearing plasma
for fast Z-pinch implosion experiments. Peak currents of 40-50 kA from a O.5-kJ capacitor
bank were driven through a O.S-mm-diam, few cm long capillary drilled in packed sodium
fluoride powder to form the source. A nozzle was used to collimate plasma ejected from one
end oftne capillary to produce a 1-2-cm-diam, several em long cylindrical plasma. Ions with
velocities of 2.2-3.4 cm/f1s and densities of up to 5 X 1015 cm-3 were measured with biased
charge collectors located at least 5 cm from the nozzle. Measurements of visible light from
neutrals near the nozzle exit gave velocities of 1. 5-1. 7 cm/ f.ls. Indications of axial and radial
nonuniformities of the plasma were observed in framing photographs of visible-light emission
and in spatially resolved spectral measurements. Neutral-sodium and neutral-fluorine lines
were identified in the spectral range from 2300 to 6700 A. Also, impurity lines of carbon,
copper, and hydrogen were identified and used to characterize the plasma. Stark broadening of
the Balmer alpha line of hydrogen was used to deduce a peak electron density of 8 X 1016 cm -3
at the exit of a 2-cm-diam nozzle. Electron temperatures of 1.4-1.6 e V at the nozzle exit were
inferred from relative intensities of the C I and C II lines. At this density and temperature,
Saha-equilibrium-model calculations indicate that the plasma consists primarily of singly
ionized sodium and neutral fluorine. A total mass per unit length (sodium and fluorine) of at
least 15 pg/cm is deduced from this analysis of the plasma constituents. This capillary
discharge has been used to produce 50-100 GW of sodium K-shell x rays in fast Z-pinch
experiments.
I. INTRODUCTION
The possibility of creating population inversion by
matched line phoropumping has been suggested I and inves
tigatedV by a number of authors. An attractive scheme4
employs the Na x Is2 ISo-ls2p IP1 line at 11.0027 A to pump
the Ne IX ls2 !So-ls4p IPI line at 11.0003 A. This scheme is
attractive because the line coincidence is excellent (2 parts in
104), but it requires an intense source of ll-A Na X pump
radiation. soft x-ray source10 or for thermonuclear fusion II has been
examined. In this report, the development of a capillary dis
charge to produce a plasma appropriate for high-power Z
pinch implosions is described.
Intense x-ray sources are produced by Z-pinch implo
sions driven by fast (<; 100 ns), high-current Dd MA)
pulsed power generators. For example, neon gas-puffimplo
sions driven by the Gamble II generator at the Naval Re
search Laboratory have produced up to 4 kJ of neon K-shell
radiation with as much as 70% of the energy in the Lyman
alpha (Ly-a) and heliumlike resonance (He-a) lines.s A
similar approach is being taken to produce heliumlike sodi
um in a Z-pinch implosion. This paper reports the develop
ment of a sodium-fluoride (NaF) capillary discharge to pro
vide a sodium-bearing plasma for such implosion
experiments. A peak power of 25 GW with a total radiated
energy of 600 J has been measured in the He-a line for impio~
sians of this NaF plasma driven by a peak current of 1.2
MA.6 Recently, this plasma source has been used in sodium
pump/neon-lasant photopumping experiments.7
Discharges through dielectric capillaries that vaporize
the wall are well known. They have been developed as stan
dard light sourccs8 and have been studied as plasma
sources.9 More recently, the use of capillary discharges as a
a) Also at the University of Maryland, College Park, MD 20742.
b) Also at Jaycor, Vienna, VA 22180·2270. II. DESCRIPTION OF THE CAPILLARY SOURCE
The NaF plasma was produced by discharging a capaci
tor through a capillary which was drilled in packed NaF
powder. The geometry of the capillary source is shown in
Fig. 1. The NaF powder was supported in a Teflon (CF2)
dielectric separating the center electrode and the outer con
ductor. The powder was packed to a density of approximate
ly 1 g/cm-' in a 5-mm-diam hole in the dielectric, either 1.25
or 2.S cm long. A O.S-mm-diam capillary was drilled
through the packed NaF powder. Previous work12 with cap
illaries ranging in diameter from 3 to 0.3 mm indicated that
more energy was coupled to the plasma when smaller diame
ter capillaries were used. Current was driven through the
capillary from the negative high-voltage center conductor to
the grounded outer conductor. Powder from the capillary
TRIGGERED
SWITCH DIELECTRIC NOZZL:c
\
NoF CAPILLARY (I 250m xOo5mmi
FIG.!. Geometry of the sodium fluoride capillary-discharge source.
2664 J. Appl. Phys. 65 (7), 1 April 19S9 0021-8979/89/072664-09$02.40 @ 1989 American Institute of Physics 2664
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38TRIGGERED
SWITCH r: : L
'WIMI\I
R " ~~ Le
'I
/: Re
FIG. 2. Electrical circuit used to power the capillary. CAPILLARY
walls was heated to the plasma state by the electrical dis
charge and subsequently ejected from the capiIIary by
overpressure of the heated NaF. Measurements were also
made with a CF2 capillary which was formed by drilling a
O.'-mm-diam hole in the CF2 dielectric.
A nozzle was required to restrict radial expansion of the
plasma and to collimate the plasma into a cylindrical col
umn. Three different anodized aluminum nozzles were used.
Two nozzles had a 1.2-em exit diameter and either a 2 or 5
cm length, and the other nozzle had a 2-cm exit diameter and
a 5 cm length.
m. ELECTRiCAL MEASUREMENTS
The current waveform of the capacitor discharge was
measured to determine the electrical characteristics of the
discharge. A schematic diagram of the discharge circuit is
given in Fig. 2. The 1.8-IlF capacitor was charged to 25 kV
providing a 560-J energy store. The current waveform was
modeled by an RLC circuit, and the resistance, inductance,
and amplitude of the current were determined by fitting the
calculated waveform to the measured current. The capillary
was initially replaced by a short circuit to determine the re
sistance (R) and inductance (L) of the driving circuit. The
short circuit was made by replacing the NaF with a 3.2-mm
diam brass rod. Once values of Rand L were known, the
waveform of the current through the NaF capillary gave an
indication of the total resistance (R + Rc ) and inductance
(L + Lc) of the circuit with a capillary load. Figure 3 pre
sents waveforms for (i) the capillary replaced by a short
circuit, (ii) a US-em-long NaF capillary, and (iii) a 2.5-
em-long NaF capillary. For the short-circuit load, the cur
rent oscillates in an underdamped fashion and the waveform
is in good agreement with the RLC circuit-model current.
With a NaF capillary load, the current is further damped
due to the additionai resistance of the capillary. The agree
ment with the RLC model current is not as good with a
capillary load because the resistance and inductance of the
capillary are time dependent. Even so, the RLC model cur
rent agrees with the measured waveform over three-fourths
of the first period of the current.
The circuit characteristics obtained from the RLC mod
el with and without NaF loads are given in Table 1. The 1.25-
cm capillary adds 33 mn and 25 nH to the circuit, while the
2.5-em capillary adds 77 mn and 54 nH. The uncertainties in
these resistance and inductance determinations of ± 20 mn.
2665 J. App!. Phys., Vol. 65, No.7, 1 April 1989 FIG. 3. Current traces for different capillary conditions.
and ± 20 nR, respectively, are based on estimating the
goodness of the fits to the measured current waveforms. The
scaling of the resistance with the length of the capillary is
consistent with interpreting this resistance as the capillary
plasma resistance. The scaling of the inductance with the
length of the capillary suggests that this inductance is asso
ciated with the current path in the capillary plasma. How
ever, if the 25-nH increase in inductance observed for the
1.2'-cm capillary arises from a linear current path in the
capillary, the current must be confined to an unrealistically
small diameter (0.2 f.1m). The large increases in inductance
observed with the capillary loads are not understood. Possi
ble explanations are that the current path in the capillary is
much longer than the length of the capillary, for example,
either a spiral path within the capillary, or current distribu
tion in plasma within the exit nozzle.
The energy ddivered to the capillary was determined by
using the measured current and resistance to calculate the
ohmic power dissipated in the capillary. Integrating this
power in time gave the energy coupled to the capillary. This
energy is 180 ± 50 J for the 2.S-cm capillary and 50-200 J
for the 1.25-cm capillary. The uncertainties in these values
are due to the uncertainties in the resistance determinations.
More than 90% of this energy is delivered to the capiIlary
during the first period of the current pulse.
IV. FARADAY CUP MEASUREMENTS
Faraday cups were used to determine the net ion-cur
rent density ofthe plasma from the capillary as a function of
time. Negatively biased (50-V) cups with O.25-mm-diam en-
TABLE I. RLC circliit characteristics for various capillary configurations.
Peak First
R f-R, L + Lc current period
Configuration (mf!) (uH) (kA) (IlS)
Short-circliit capUlary
(Rc=Lc=O) 75 135 77 3.1
1.25-cm NaF capillary 108 160 53 3.5
2.S-cm NaF capillary 152 189 41 3.9
Welch eta!. 2665
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38,
LE~S.,,-~
CAPILLARV
W:iH
NOZZLE , ,
'<"
PHOTQOIOOES \
F"ARADAY
CUPS FIG. 4. Experimental ar
rangement of diagnostics on
the capillary source.
trance apertures were placed in the vacuum chamber oppo
site the capillary source, as shown in Fig. 4. The measured
signals gave an indication of the local positive-ion-current
density as a function of time. By using two Faraday cups at
different distances, an ion drift velocity was determined
from the time difference between the signals at the two dif
ferent locations on the same shot. The velocity can also be
determined by varying the distance between the Faraday
cups and the capillary on multiple shots. Faraday cup signals
(see Fig. 5) were obtained for a 2.5-cm-Iong NaF capillary
with a 2-cm exit diameter nozzle. The times of arrival of the
three peaks of the Faraday cup signals are plotted versus
distance for these two shots along with two other shots in
Fig. 6. Measurements were taken on the same shot at 10 and
15.6 em, at 15 and 20.6 em, and at 20 and 25.6 em to mini
mize variations due to lack of reproducibility. The slopes of
these plots indicate velocities of 3.4, 2..7, and 2.2 em/,us for
the three peaks with standard deviations of ± 0.2 em/,us.
Once the velocity is known, the measured Faraday cup
signal, V, can be used to estimate the ion density according to
FIG. 5. Signals from a Faraday cup located 15 em from the nozzle on one
shot and from Faraday cup~ located 20 and 25.6 em from the nozzle 011
another shot.
2666 J. Appt. Phys., Vol. 65, No.7, 1 April 1989 20~----------------------------------~
!I+
~IJ
\.L o
w :a:
f= 5
5 PEAK #3
\
I
PEAK #1
10 15 20 25
DISTANCE FROM NOZZLE (em) 30
FIG. 6. Plot uftlle time of arrival for the peaks of the Faraday cup signals in
Fig. 5 vs the distance from the nozzle. The lines are least-square fits to the
data.
11, = V /(evAR), (1)
where v is the ion velocity, A is the area of the Faraday cup
aperture, e is the electronic charge, and R (50 n) is the
termination resistance of the Faraday cup signal. It is as
sumed that the ions are singly charged, that electron emis
sion from ion impact in the cup is negligible, and that the
Faraday cup does not perturb the plasma flow. The Faraday
cup trace in Fig. 7 was measured with a detector located 5 em
in front of the nozzle. The first three peaks in this trace corre
spond to those in Figs. 5 and 6. The third and largest peak is
associated with the 2.2-cmllls velocity and represents a den
sity of about 5 X 1015 cm-3. Smaller densities were deter
mined for the first two peaks. If the Faraday cup was located
closer to the capillary, this signal was saturated. Increasing
the detector bias to prevent saturation lead to electrical
breakdown when plasma impinged on the detector. An Of
der-of-magnitude estimate of the total number of charged
particles arriving at 5 em was made by integrating this signal
in time and using the velocity associated with the largest
peak. For this estimate, the area of the ion beam was as-
FIG. 7. Faraday cup signal measured at 5 em from a 2-cm-diam nozzle for a
2.5-cm-long NaF capillary.
Weich etai. 2666
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38sumed to be given by the diameter ofthc plasma at the Fara
day cup as indicated by framing-camera pictures (see Sec.
V). If all these ions are assumed to be singly ionized sodium,
the estimated total mass of sodium is 23 f-lg. For comparison,
the total mass ofNaF powder from the capillary, determined
by weighing the capillary before and after the discharge, was
approximately 100 mg. Clearly, only a small fraction of the
totai N aF is ionized sodium. The total number of singly ion
ized sodium ions corresponds to a total ionization energy of
0.5 J and a total kinetic energy of 5.6 J. These estimates
indicate that only a small fraction of the energy delivered to
the capillary (6 out of 180 J) is carried by sodium ions in the
plasma, and it is primarily kinetic rather than ionization en
ergy.
v. VISIBLE~LIGHT MEASUREMENTS
The light emitted from the plasma was recorded in a
number of ways. Open-shutter photographs and framing
camera pictures were used to give an indication of the spatial
distribution of light-emitting plasma. Figure 8(a) is an
open-shutter photograph of the light from the discharge of a
1.25-cm-long NaF capillary without a nozzle. For this mea
surement, a 50-A bandpass filter centered at 5890 A was
used to transmit the sodium "doublet" lines at 5890 and
5896 A. The elliptical image indicates that plasma is expand
ing from the capillary into less than 2rr Sf.
A framing camera was used to record time-resolved im
ages of the visible light as shown in Fig. 8 (b). Images were
recorded for 0.2 f1s at I-fis intervals during the discharge.
The frames are numbered sequentially in time, beginning in
the lower left-hand corner and ending in the upper right. The
first frame begins 0.5 .us after the discharge is initiated, un
less otherwise specified. In all cases the capillary source is on
the right. The framing-camera pictures show well-defined
luminosity fronts that progress from frame to frame. The
discharge in Fig. 8(b) is for a O.5-mm-diam capillary in a
CF~ dielectric without a NaF fill and without a nozzle. Also,
the ~odium "doublet" filter was not used. The ejected plasma
(oi OPEN SHUTTER
I--5cm--t--7.5cm-.1
SCREENS _J _______ _ I -~
CAPILLARY
(tll FRAMING
2 6 8
5 7
FIG. 8. (a) Time-integrated photograph of sodium "doublet" light for a
1.25-cm-long NaP capillary, and (b) framing-camera pictures with no Na
line filter for a L25-cm-Iong CF, capillary.
2667 J. Appl. Phys_, Vol. 65, No.7. 1 April 1989 (a) OPEN SHUTTER
SCREEN---<"
(bl FRAMING
2 \<>--------- 7.5 em --------1
6
5 CAPILLARY
WITH NOZZLE
8
7
FIG. 9. (al Time-integrated photograph and (h) framing-camera pictllres
for a 1.25-cm-long NaP capillary with a 1.2-cm-diam, 2-cm·long nozzle.
impinges onto two screens located 7.5 and 12.5 cm, respec
tively, from the capillary. Frame 1 shows plasma just begin
ning to exit the capillary. In later frames, two successive
luminosity fronts are observed to expand from the capillary
and interact with the screens. The luminosity at the exit of
the capillary is peaking in frames 3 and 5. In frames 4 and 7,
the luminosity peaks behind the first screen as plasma passes
through this screen. The divergence of the ejected plasma
into nearly 217 Sf is evident in these frames.
The effect of 1.2-and 2-cm-diam nozzles on redirecting
the plasma can be seen in the photographs in Figs. 9 and 10,
respectively. For the open-shutter photographs in Figs. 9 (a)
and W(a), the sodium "doublet" mter was not used because
spectral measurements indicated that the total visible-light
emission was predominantly from sodium. The light-emit
ting plasma is limited to a diameter comparable to the nozzle
diameter. The emission from this plasma is axially and radi
ally nonuniform. Near the exit of the nozzle, the plasma is
confined to the nozzle diameter, Several centimeters from
the nozzle, the plasma appears to expand radially in a cone
originating near the nozzle exit.
(0) OPEN SHUTTER
'". .
$t .' .. :;?[
.... .... ... , \ --~-
(bl FRAMING --------I
:2 4 6
3 5 (CAPILLARY
WITH NOZZLE
FIG. 10. (a) Time-integrated photograph and (b) framing-camera pictures
for a 1.25-cm-long NaF capillary with a 2-cm-diam, 5-cm-long nozzle.
Welch etal. 2667
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38Framing-camera photographs for discharges with these
same diameter nozzles are shown in Figs. 9(b) and lOCb).
Light is recorded in the first frame in Fig. 9 (b) because the
frame times have been delayed by 2 j1s for this measurement
Radial confinement of the plasma by the nozzle can once
again be seen, but now with time resolution. The velocity of
the luminosity front was estimated by measuring the dis
tance the front travels between frames. For frames 3-5 in
Fig. 9 (b), the velocity of the luminosity front is about L 7
cm/j1s. In Fig. 10, a screen is located only 5 cm away from
the nozzle. The frames in Fig. 10 (b) show intense light com
ing from the outer edge of the nozzle suggesting that plasma
is redirected toward the axis by the walls of the nozzle. Also
shown is a cone of intense light originating on-axis at the exit
of the nozzle and extending toward the screen. Both of these
features are also evident in the open-shutter photograph in
Fig. lO(a). These photographs indicate that the capillary
discharge can be used to produce a column of sodium-bear
ing plasma about 2 cm in diameter and at least 4 cm long, as
required for Z-pinch implosion experiments.
Photodiodes (EG&G Model FND-IOO) were used io
record time histories of the light intensity in the geometry
shown in Fig. 4. Flexible black tubing was used to restrict the
detector field of view to a 1.8 em length of plasma, and a filter
was used to limit the detector signal to only sodium "doub
let" emission. By using two photodiodes to record the plas
ma light at two different distances from the nozzle exit, a
velocity was determined. Figure 11 shows two photodiode
traces corresponding to a 4-cm separation of the diodes. Ve-
10cities determined from the two peaks are 1.7 and 1.2 cm/
f-ls. These velocities are up to a factor of2 smaller than veloc
ities deduced from the Faraday cup measurements, but are
in agreement with the velocity determined from framing
photography. The 5-1O-ps duration of the sodium-light
emission combined with the measured velocities indicates
that sodium from the capillary should form a column longer
than the 4 em required for implosion experiments on the
Gamble n generator. The successive peaks observed in the
ion-and visible-light emissions may be associated with the
periodicity of the current driving the capillary.
Oo:-~-'---':-------L--L--1--L---'-~
4 6 8 10 12 14 16 18
TIME 'fLO)
FIG. 11. Photodiode signals for a 1.25-cm-Iollg NaP capil1ary measured at
the exit of a 2-cm-diam nozzle (solid line) and at 4 cm from the nozzle
(dashed line).
2668 J. Appl. Phys., Vol. 65, No.7, i April 19S9 illl l---5 em--\-- 5 cm--t
FARADAY __ oO<:,.::::SJ ~
CUPS (;// L~::,u,,,
( ( WITH NOZZLE
2 4. 6 8 3. 5 7.
!o)
Sr--r~'-~~-r--?-~--~--r-~
O~LL~~-u~-L~~~ __ ~~L-~ o 23456789
riME (/&1)
FIG. 12. (a) Framing camera pictures for a L25-cm-Iong CF2 capillary
with a L2-cm-diam, 2-cm-Iong nozzle. Faraday cups were located 5 and 10
em from the nozzle. (b) Faraday cup signal (solid line) for the detector in
(a) located 5 em from the nozzle. The dashed curve shows the frame times
corresponding to the bottom TOW of framing pictures in (a).
dated with the periodicity of the current driving the capil
lary.
A comparison of visible-light measurements, i.e., fram
ing-camera pjctures, and a Faraday cup signal is made in
Fig. 12. The framing duration and the timing interval are as
in Figs. 8-10. Frame times corresponding to the bottom row
of framing pictures in Fig. 12 (a) are indicated by the dashed
traces in Fig. 12(b). The arrival of ions at the Faraday cup
occurs before the luminous front reaches the same location.
These observations indicate that the larger velocities deter
mined from Faraday cups (which measure ions) are not as
sociated with the smaller velocities obtained from photo
diodes (wYich measure light mainly from neutrals). Light is
observed, t the locations of the Faraday cups in Fig. I2(a)
after the luminous front exits the nozzle. This light may be
due to either reflection oflight from the front of the Faraday
cup or to emission from plasma which stagnates on the Fara
day cup. No light emission was observed to be associated
with the first peak in the Faraday cup signal in either the
framing images or the photodiode traces. These data suggest
that the discharge produces a fast ion component followed
by a slower, luminous, neutral component.
Welch eta!. 2668
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38VI. VISIBLE AND NEAR UV LIGHT SPECTROSCOPY
A O.S-m spectrometer with a 1200-line/mm grating was
used to study light emitted from the capillary-discharge
plasma both photographically and with time resolution. The
spectrometer viewed the plasma perpendicular to its exit
from the nozzle, and 11 lens was used to image the source onto
the entrance slit of the spectrometer, as shown in Fig. 4. For
the photographic work, the spectrum was spatially resolved
in the radial direction at the exit of the nozzle. In spectral
scans from 2300 to 6700 A, lines of neutral sodium and neu
tral fluorine were identified, as well as impurity lines of car
bon, copper, zinc, aluminum, and hydrogen. Spectral lines
from Na I for two different discharges, recorded with a 100-
fim entrance slit, are compared in Fig. 13. In each case, the
intense and broadened portion of the lines corresponds to the
nozzle diameter. Measurements with various neutral density
filters and entrance slit widths indicated that this line broad
ening is not due to overexposure of the film. The line broad
ening is indicative of a higher electron density on-axis than
elsewhere in the plasma, The Na I emission is radially more
uniform for the 1.2-cm-diam nozzle in Fig, 13(b) than for
the 2-cm-diam nozzle in Fig. 13 (a), This suggests that radial
variations of the electron density are less severe for the
smaller diameter nozzle,
The sodium "doublet" emission, labeled 3s-3p in Fig.
13, appears to be reabsorbed at line center, which indicates
that the plasma is opaque at these wavelengths. This absorp
tion is attributed to neutral sodium in the ground state
between the light-emitting sodium and the spectrometer.
This reabsorption can be used to estimate a minimum sodi
um ground-state density in the plasma, Measurements were
"0 Q,
V rt')
I !
Q, <II
f') i<"I
(a) ~ +
(b)
i 0/1
i.O
I
n
r0
+
t .i
nozzle
diameter
T
nOllie
diameter
T
FIG, 13. Spectrallines from Na I for a L25-cm-Iong NaFcapillary (a) with
a 2-cm-diam nozzle and (b) with a 1.2-cm-diarn, 5-cm-long nozzle. The
short lines along the bottom of the photographs are reference spectra.
2669 J. Appl. Phys., Vol. 65, No.7. 1 April 1989 :>
i-
::'l a..
~2 o
cr.::
ILl
-l a..
5
::'l
~ o
b J: a.. AT NOZZLE EXIT
/"" 5cm FROM
I "\ /NOZZLE EXIT , ,
i ... _.., t - .... ,
; '~,
""'" "'" -~
°o~~~~~~--~--~--~--~--~--~
FIG. 14. Photomultiplier signals of the Na I 3s-3p line (5890 A) at the
nozzle exit (solid line) and 5 cmfrom the nozzle (dashed line) for a 2.S-cm
long NaP capillary and 2-cm-diam nozzlc.
made of higher members of the Na I resonance series because
the smaner osciliator strengths of these lines, compared to
the 3s-3p line, lead to much larger minimum densities. A
photomultiplier was coupled to the exit slit of the spectrom
eter to record the time histories of weak lines. The Na I line at
2852 A (3s-5p) was scanned in wavelength and found to be
reabsorbing at line center. Reabsorption occurred for a dura
tion of more than 5 !1s beginning about 6,as after the start of
the capinary current. To account for this absorption, an opti
cal depth of at least unity is required, which corresponds to a
minimum sodium ground-state density of about 3 X 1OJ6
cm-3.
Velocities of sodium atoms were determined by measur
ing the Na I line emission at two different distances from the
nozzle. Time histories of the Na I line at 5890 A, recorded at
the nozzle and 5 em from the nozzle, are shown in Fig. 14,
Velocities corresponding to the two peaks in these signals are
1.7 and 1,5 cm/j.ts, respectively. These values agree with the
velocities from the visible-light photodiode measurements.
Time histories of the eu I line at 5218 A are presented in
Fig. 15, The solid trace gives an indication of the impurity
:;; 1.6
I-
::>
Q.
I-
::> o 1.2
0<
!:':!
...1 a.
S
1"1
"OT
FIG. 15, Photomultiplier signals ofthe 5218-A eu I line for a 2.S-em-Iong
NaF capillary and 2-cm-diam uonle with bra:;.,> electrodes (solid line), with
an aluminum outer electrode (short dashed line), and with an aluminum
ollter electrode and tantalum inncr electrode (long dashed line).
Welch eta/. 2669
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38emission in a typical discharge. The dotted trace indicates
the relative copper-impurity conteni when the hole in the
brass outer electrode was covered with an aluminum insert.
The dash-dot trace indicates the relative copper-impurity
content when the aluminum insert was used and the brass
center electrode was covered with tantalum foiL These re
sults demonstrate that the copper impurity is from the brass
electrodes and can be changed by modifying the electrodes.
The electron density, no in the plasma wa" determined
by measuring the Stark broadening ofthe Balmer alpha line
(6563 A) from the hydrogen impurity. Measurements of
this line were carried out at the exit of the nozzle and were
restricted in the direction perpendicular to the line-of-sight
to within 2 mm of the axis in order to minimize uncertainties
due to nonuniformities in ne' Time histories of the intensity
of this line were measured from 9 A below line center to 7 A
above line center. Histories at seven different wavelengths at
and above line center are given in Fig. 16. From these mea
surements, Hne profiles were extracted at various times dur
ing the discharge. The profiles were fit to Lorentzian func
tions, and the half-width at half-maximum was determined
for each profile. An example of one of the profiles is shown in
Fig. 17. There is no indication of self-absorption in this line
shape. Stark broadening is the major contributor to the 2.2-
A half-width of this line. For the various times of these mea
surements, the half-widths ranged from 1.0 to 3.5 A. An
instrumental width of only 0.3 A for the 20-,um entrance slit
was determined by scanning the 5461-A line of a mercury
vapor lamp. The thermal Doppler width is only 0.4 A for a
temperature of2 eV. An upper limit on the Doppler width
due to macroscopic motion was estimated to be 0.3 A based
on a velocity of 1.5 cm//1s. Therefore, the instrumental and
Doppler contributions to the linewidths are small and were
neglected in determining nc. The measured half-widths were
compared with tabulated Stark widths 13 to determine the
negative-charge density. This density is attributed to elec
trons, not negative ions, because the negative-ion density is
negligible at the measured electron density and temperature,
FIG. 16. Photomultiplier signals of the Balmer alpha line of hydrogen at
line center (6563.4.) and at six longer wavelengths lor a 1.25-cm-Iong NaF
capillary with a 2-cm-diam nozzle.
2670 J. Appl. Phys., Vol. 65, No.7, I April 1989 > -0.4 >-;-
<n z w
~ 03
..J
'" 0:: fu :t 0.2
Ul
w >
~ ;;j 0.1
IX "
oL-L-J-~-L~~ __ L-~~~-L~~ __ L-L-J-~ -8 -6 -4 -2 0 2 4 6
RELATIVE WAVELENGTH (A) 8
F1IG. 17. Hydrogen Balmer alpha line profile at 2.75 liS near the peak of the
signals in Fig. 16. Zero on the horizontal axis corresponds to line center.
The curve is the fit of a Lorentzian function with a linear background.
as will be shown. The Stark widths were evaluated for a tem
perature of 2 eV, based on measurements to be described.
The Stark width at this temperature and density is not
strongly dependent on temperature. A temperature of 1 eV
would only cause a 5%-15% decrease in this density, Values
of ne, determined from the measured linewidths, 14 are given
in Fig. 18 along with the time history of the line at line center.
These are radially averaged electron densities at the nozzle
exit. Recent calculations of Stark broadened profiles of the
Balmer alpha line in this temperature and density range
would imply slightly higher electron densities. J5 The uncer
tainty in each measurement arises from uncertainty in the
linewidth due to shot-te-shot variations in the signal and to
small signal-to-noise ratios at late time. The peak value of 11.
is 8 X 1016 em -3. This peak is associated with the second
peak in the Faraday cup signal shown in Fig. 7, It was not
possible to extend this measurement to a later time corre
sponding to the maximum ion density recorded by the Fara-
r5
~04
~ ;
~O.3*
j: ~
5
~:~I
00~~~·-L---3LI --J~---~5---L~~
TIME (p.')
FIG. 18. Electron densities determined from the measured linewidths (data
points), and the photomultiplier signal of the Balmer alpha line of hydrogen
at line center (curve).
Welch eta!. 2670
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38day cup because the hydrogen line emission is too weak.
However, the Faraday cup measurement suggests that ne
may be a factor of2larger about 3 tJ'i" later (see Fig. 7).
The electron temperature, T",' was inferred from the rel
ative intensities of the C I Hne at 2478 A and the C II lines at
2509-2512 A. The relative populations of these ionization
states of carbon can be calculated with the Saha equation for
a plasma in local thermodynamic equilibrium (LTE) if both
ne and Te are known. 16 Then, the relative population of the
upper states of these two transitions is determined using the
Boltzmann relationship for level populations. Oscillator
strengthsl7 of 0.094 for the 2p2_2p3s transition of C I and
0.14, 0.16, and 0.016 for the 2s2p2_2p3 transitions of C II
(J -J' = 3/2-5/2, 112-3/2, 3/2-3/2) were used to evaluate
the relative intensity of these lines as a function of ne and Te'
Because this line ratio depends on both Te and n" Te was
determined only over the time interval for which ne was also
determined, as described in the previous paragraph. For the
time interval from 2.25 to 5.25 f.Js, Te = 1.4-1.6 eV. This
measurement represents an upper limit on Te because the
weak C II line intensity may be overestimated due to other
contributions to this signal.
The plasma conditions required for L TE ofthe C I and C
II ionization states and for equilibrium between these ioniza
tion states were examined. For a particular ionization state
in a homogeneous static plasma, complete L TE is estab
lished if the collisional excitation rate is a factor of 10 larger
than the radiative decay rate of the resonance transition. IS
Applying this criterion to a 1.5-eV carbon plasma leads to
lowerlimits on ne of5 X 1016cm-3 forC I and 4x 1017 em -3
for C u. Thus, complete LTE is valid at the time of measured
peak fie (8 X 1016 cm--') for C I but not for C II. Partial LTE
with respect to transitions from higher principal quantum
number does not require as large a density as complete L TE.
If an effective principal quantum number of 3 is used for the
C II transition, Ii< partial LTE is valid because the minimum
density is only 1 X WIt> cm-3• The time required for equili-
80
~
§ 70
~
:5 60
a.. o a. w 50
~
~ 40['
~ 30 t:! ,
~ 2+
It
o '---_l ... _~_~t;;;;;;:~~:::!::::::::!::=1
0.25 0_50 0_75 1.00 1.25 1.50 1.75 2.00
TEMPERATURE leV)
FIG. 19. Populations of the ionization states of sodium and fluorine for a
NaF plasma with a total ion density of2X 1017 ern -3.
2671 J. Appl. Phys., Vol. 65, No.7. 1 April 1989 bration between these ionization stages of carbon was esti
mated from inverse reaction rates. 19 An ionization rate coef
ficient of 8 X 10-12 cm3/s, based on a l.S-eV C I plasma,
indicates an equilibrium time of 1.5 f-ls for an electron den
sity of 8 X 1016 cm -3. For a plasma that goes through a se
quence of near L TE states, the time required for relaxation
of transient effects can be estimated from the inverse colli
sional excitation rate of the ground state multiplied by the
fraction of atoms to be excited. This time is < 11 ns for C I.
These comparisons suggest that the L TE analysis is margin
ally appropriate for the C ! and C n states used for the tem
perature estimates.
A model20 based on the Saha equation, charge neutra
lity, and conservation of particles was used to estimate the
population of various ionization stages of sodium and fiu
orine in the plasma assuming a total density for each species
of 1017 em -3, This density was selected because it leads to an
electron density at 1.5 eV that is comparable with the mea
sured fl,. The populations of each ionization state relative to
the particular species, Na I, Na n, F I, and F n, are given in
Fig. 19 as a function of plasma temperature. At the mea
sured temperature of < 1.5 eV, the sodium is almost entirely
ionized and the fluorine is 86% neutral. At 1.5 eV, popula
tions of the discharge constituents are approximately 1 % N a
I, 49% Na II, 43% F I, and 7% F n. The population of F
ions is negligible, and becomes significant only for tempera
tures less than 0.6 eV. The population of Na In is negligible
over the temperature range in Fig, 19 due to the large ioniza
tion potential (47 V) of neonlike sodium.
The total particle density in the plasma was determined
by combining the electron density with the population frac
tions deduced from the Saha model. This procedure is appro
priate because the population fractions are only weakly de
pendent on the electron density. With only single-stage
ionization, the value ofne (8 X 1016 cm-3) determined from
the hydrogen linewidth is the total positive-ion density at the
exit ofthe nozzle. This result is more than an order of magni
tude larger than the density determined from the Faraday
cup located 5 em in front of the nozzle (see Sec. IV). This
difference may result from expansion and/or recombination
as the plasma moves away from the nozzle. For a total posi
tive-ion density of 49% Na nand 7% F n, the total density
(sodium and fluorine) is approximately 1.4 X 1017 em -3.
This result is relatively insensitive to the temperature in the
range from 0.7 to 1.6 eV because the sodium is more than
90% singly ionized for temperatures greater than 0.7 eV,
and the F II contribution is less than 12% for temperatures
below 1.6 eV. For a 2-cm-diam nozzle, this total density
corresponds to a mass per unit length of 15 J1.-g/cm at 4 f-ls
after the start of the current (peak electron density in Fig.
18). This mass may be a factor of 2 larger 3 f.ls later in the
current pulse.
The Na I density of l.4X 1015 cm-3, based on impurity
line emission and the Saha-equation analysis, is not inconsis
tent with the minimum ground-state density of 3 X 1016
em -3 estimated from absorption at line center of the 2852-A
line of Na I. These densities are obtained at different times
and possibly at different radial sites within the plasma at the
exit of the nozzle. The smaller density is recorded at 4 J-ls
Welch eta/. 2671
.... _._._.-. ••• '~.'.'.v.~.'._._._"._._., ................ ~._ ............ ~ .•.•. ";";' •••.. _ .••.••••••. ~ .• , ... ~ •• ~ .......... , .•.•. ~ .•.•.•. "' •• "'_ ....... _,_._ •.• ;"; •.•.• ,
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130.70.241.163 On: Tue, 23 Dec 2014 01:48:38(peak of ne in Fig. 18), while the larger density is not ob
served until at least 6 JIS after the discharge is initiated. Be
cause both measurements are averaged along a radial line-of
sight through the plasma, the emission may occur at a
different location in the plasma than the absorption occurs
due to radial non uniformities, as observed in Fig. 10.
VII. CONCLUSIONS
A sodium fluoride plasma for Z-pinch implosions has
been produced from a capillary-discharge source. The capil
lary is driven with peak cun'ents of 40-50 kA for a few mi
croseconds to produce the plasma. Approximately 180 J of
energy is delivered to the capillary to produce a plasma of
sodium and fluorine which is ejected from the capillary.
Nozzles are necessary to confine the plasma to a cylindrical
geometry, either 1.2 or 2 em in diameter. Ions are ejected at
velocities of 2.2-3.4 em/ fis and neutrals at velocities of 1.5-
1.7 em/JIs. Plasma is emitted for several microseconds so
that a 4-cm length may be fined with plasma as required for
Z-pinch implosion experiments.6
The plasma has non uniformities in both the radial and
axial directions. Measurements suggest that the plasma is
emitted in successive fronts which may be associated with
the periodic nature of the driving current. It may be possible
to smooth these non uniformities by using a slower period
current. Visible-light framing photographs suggest that the
radial nonuniformities are due to reflection of plasma from
the walls of the nozzle and from radial expansion of the plas
ma as it propagates away from the nozzle. Observed broad
ening of spatially resolved spectral lines is consistent with a
higher electron density on-axis, particularly for the 2-cm
diam nozzle. Reducing the nozzle diameter seems to smooth
the radial variations by producing a more uniform electron
density throughout the plasma. Copper impurities in the
plasma originate from both electrodes of the capillary, and
the impurities can be altered by changing the composition of
the metal electrodes.
Time-resolved spectral intensity measurements from
carbon and hydrogen impurities were made to determine the
electron density and temperature. An electron density of
about 8 X 1016 cm-3 at the exit of the nozzle was determined
from Stark broadening of optically thin hydrogen line emis
sion. Temperatures of 1.4-1.6 eV were inferred from the rel
ative intensity ofC I and C II line emissions. Interpretation of
these results with a Saha model indicates that the plasma
consists mainly ofNa II and F I at the exit ofthe nozzle, and a
total (sodium and fluorine) density of about 1.4 X 1017 cm-'
is deduced from the electron density. Faraday cup measure
ments 5 em in front of the nozzle indicate that this density
may be a factor of 2 larger a few microseconds later in time.
This density represents a mass loading of 15 Jig/cm which
compares favorably with values of 14--33 p,g/cm that have
been inferred from observed implosion times6 and analyses
of spectroscopic measurements2! in experiments with this
source on the Gamble II pulsed power generator.
2672 J. Appl. Phys., Vol. 65, No.7, 1 April 1989 ACKNOWLEDGMENTS
We are grateful to R. Boller and G. Cooperstein of the
Naval Research Laboratory (NRL) for their suggestions
and to A. T. Robinson and G. Langley for technical assis
tance. We are grateful to M. J. Herbst for making available
the photodiodes used in these measurements. H. R. Griem,
J. S. Wang, J. Moreno, E. Iglesias, S. Daniels, S. Goldsmith,
and R Grober of the University of Maryland assisted in the
interpretation and presentation of the spectral measure
ments. This work was supported in part by the Innovative
Science and Technology Office of the Strategic Defense Ini
tiative Organization and directed by the Naval Research
Laboratory.
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Welch eta/. 2672
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1.101497.pdf | Generation of thick Ba2YCu3O7 films by aerosol deposition
T. T. Kodas, E. M. Engler, and V. Y. Lee
Citation: Applied Physics Letters 54, 1923 (1989); doi: 10.1063/1.101497
View online: http://dx.doi.org/10.1063/1.101497
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Published by the AIP Publishing
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Correlation of structural and superconducting properties of Ba2YCu3O7−δ thin films
AIP Conf. Proc. 251, 44 (1992); 10.1063/1.42100
Superconducting properties of Ba2YCu3O7−x thin films prepared by chemical vapor deposition on SrTiO3 and a
metal substrate
Appl. Phys. Lett. 55, 1581 (1989); 10.1063/1.102311
Oxygen diffusion into oxygendeficient Ba2YCu3O7−x films during plasma oxidation
Appl. Phys. Lett. 53, 811 (1988); 10.1063/1.100152
Plasma oxidation of Ba2YCu3O7 − y thin films
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Appl. Phys. Lett. 52, 2183 (1988); 10.1063/1.99763
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141.218.1.105 On: Mon, 22 Dec 2014 14:05:23Generation of thick Sa:;!! YCUa07 fUms by aerosol deposition
T. T. Kodas,a) E. M. Engler, and V. Y. lee
IBM Research Division. Almaden Research Center, 650 llarrp Road. San Jose. California 95120-6099
(Received 21 December 1988; accepted for pUblication 17 March 1989)
Thick superconducting films were fabricated by producing high-purity Ba2 YCu30, particles by
aerosol decomposition in a gaseous flow system, depositing the particles directly from the gas
phase onto surfaces by thermophoresis, and then sintering and annealing the deposited
particulate films in an oxygen flow. Particulate films with thicknesses of 1 mm were deposited
on the inside surfaces of copper tubes and sintered to provide uniform adherent coatings with
sharp superconducting transitions above 91 K. High-purity powders based on the Bi-Sr-Ca
Cu-O and TI-Ca-Ba-Cu-O systems were also produced and sintered to form bulk ceramics with
transitions at 80 and 110 K, respectively, suggesting that the process is general and can be used
for a variety of materials. Advantages of the process include the ease of obtaining the correct
oxygen content and the ability to fabricate thick films of fine grained material while
minimizing exposure to carbon and other contaminantso
A number of methods have been developed for the fabri
cation of ceramic superconductor wires and tapes. These
methods include packing superconducting powder into met
al tubes, 1 solid-state diffusion/reaction processes,2 extrusion
of a powder/organic mixture,3 application of a slurry onto a
surface,4 molten oxide drawing,5 sol-gel6 and other methods.
Although higher critical current densities have been ob
tained in materials composed of elongated single crystals,7
the highest critical current densities that have been reprodu
cibly demonstrated for polycrystaIline material are on the
order of 10 000 A/cm2 with most techniques providing
much lower values and less than optimum 1~ values. For the
case of metal-coated wires, the low critical current densities
and transition temperatures may be due in part to the diffi
culty in obtaining the correct oxygen content In addition,
the low critical current densities and 1:. values may be
caused by impuritiesB and secondary phases9 at the grain
boundaries, grain orientation effects,1O and by microcrack
ing. Thus, critical current densities and Tc values may be
improved by achieving the correct oxygen content, minimiz
ing exposure to contaminants, using chemically homoge
neous particles, and minimizing microcracking by control
ling the size ofthe grains.
In an effort to overcome the impurity, chemical homo
geneity, oxygen content, and microcracking problems, we
have developed a new method for generation of thick super
conducting ceramic films. In this method, which is illustrat
ed schematically in Fig. 1, superconducting particles are
formed in a gaseous fiow system and then deposited directly
from the gas phase onto a surface. Wires can be formed by
depositing particles on the inside of tubes. The deposition
process can be controlled to provide a uniform deposit along
the inside of the tube. Similarly, unifoml coatings can be
formed on fiat surfaces. The desired oxygen content can be
obtained because oxygen can be flowed over the fiat sub
strate or through the tube (which is not entirely filled) dur-
,,) Present address: Center For Micro-Engineered Ceramics and Chemical
and Nuclear Engineering Department, University of New Mexico. Albu
querque, NM 87131 ing sintering and annealing. Chemically homogeneous parti
cles with a submicron number average diameter can be
applied to surfaces of tubes and tapes while minimizing ex
posure to carbon-containing specieso
In this work, the Ba2 YCu3 07 system was used to dem
onstrate that thick superconducting films can be formed on
the inside of copper tubes to produce wires with high transi
tion temperatures and reasonable critical current densities.
Results are also presented for the TI-Ca-Ba-Cu-O, Bi-Ca-Sr
Cu-O, and La-Sr-Cu-O systems which demonstrate that
high-purity powders can be produced and then fabricated
into superconducting ceramics, a necessary requirement for
the use of these chemical systems for film and wire fabrica
tion using aerosol deposition.
Figure 2 shows the experimental apparatus. The aerosol
generation apparatus has been described previously, 1I so
only a brief description will be given hereo A solution of the
powder precursors is passed through an aerosol generator to
form fine droplets of the solution with an average diameter in
the micron range. The use of aqueous solutions of the nitrate
salts ofY, Ba, Cu, Ca, Sr, Hi, TI, and La minimizes carbon
contamination from either the solvent or precursors. The
particles are then carried through a furnace at temperatures
of900-1100 °C with reactor residence times of 1-25 s, where
the precursor compounds react with the oxygen carrier gas
to form superconducting powder. Since the particles come
into contact only with water and the materials composing
the aerosol generator, contamination problems can be mini
mizedo
Some of the properties of the Ba2 Y Cu} 07 aerosol pow-
)) (!Q,.r!ll}138:ll )) ~
>e- -I>()~ a -~
,,,,,I CIi,micoi Q () ~ 2~ [v-oporation Reaction """\ Ir3lll_«_
Particl. )) Particle ;)
F' orrr:o{!on Deposi'ion
FIGo i 0 Schematic of overall wire fabrication process, Sintering
or.d
Annea;ing
1923 AppL Physo Lett 54 (19), 8 May i 989 0003-6951/89/191923-03$01000 @ 1989 American Institute of Physics 1923
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141.218.1.105 On: Mon, 22 Dec 2014 14:05:23Carrier
Ga. ,-:---,-, PlZ77hZ???????????21
FIG. 2. Experimental system. Q'l/VV7Z22ZV22Z2ZA
Depositicln Zone With
Heated Reactor Tube Controlled Temperature
Gradier:~
der have been examined in earlier work. 11 This along with
additional work has shown that powders with the following
characteristics can be formed: completely reacted [thermo
gravimetric analysis (TGA) L single phase [x-ray diffrac
tion (XRD)], superconducting with Tc > 90 K (magnetic
susceptibility), single-crystal particles (electron diffrac
tion), uniform composition from particle to particle [energy
dispersive spectroscopy (EDS)], solid particles l transmis
sion electron microscopy (TEM) and BET surface area
measurements 1, funy oxygenated (iodometric titration),
1:2:3 ratio of Y:Ba:Cu (wet chemical analysis), and
equiaxed and sphedcal shapes.
Powders based on the La1.8SSrO.15Cu04' Bi-Sr-Ca-Cu-
0, and TI-Ca-Ba-Cu-O systems were also produced. The
La 1.85 SrO.15 Cu04 system gave single-phase material while
multiphase material was obtained for the Tl and Hi systemso
Powders collected on glass fiber and silver membrane filters
were sintered and annealed to produce material with super
conducting transitions at 35, 110, and 80 K, respectively.
These results demonstrate the feasibility of using this aerosol
deposition method for the production of tapes and wires
based on chemical systems other than Ba2 YCU~07 since the
collected powders can be converted into bulk superconduc
tors.
Thick superconducting films were formed on the inside
of a copper tube by sending the aerosol particles in the oxy
gen carrier gas exiting the reactor into a copper tube with a
controlled wall temperature profile where deposition took
place by thermophoresis to coat the inside surface of the
tube. Both straight and coiled tubes were coated. In addi
tion, thick films were formed on flat substrates to demon
strate the feasibility of producing tapes by this process. The
temperature of the copper tube walls and flat substrates was
100-200 ·C, far below the temperature required for reason
able sintering rates.
Deposition of the micron-sized particles in the system
can, in general, take place by gravitational settling, inertial
impaction, Brownian diffusion, and thermophoresis, the rel
ative contributions of these mechanisms being dictated by
the operating conditions. Calculations based on the average
particle diameter, density of the particles, and geometry of
the system indicated that gravitational settling and impac
tion were not important. Estimated deposition efficiencies
for thermophoresis (which depend on the temperatures in
the heated zone and downstream from the heated zone, and
the product PrK where Pr is the Prandtl number and K is the
thermophoretic coefficient12) were much higher than those
for diffusional deposition. This conclusion was supported by
the observation that particle deposition was not visible in
sections of the system where temperature gradients were not
encountered. An advantage of exploiting thermophoretic
1924 Appl. Phys. Lett., Vol. 54, No. 19.8 May 1989 o 50 '00 150 200
Temperature (K)
FIG. 3. Resistivity as a function of temperature for thick film of
Haz Yeu, 07 on inner surface of copper tube.
deposition as opposed to other deposition mechanisms is
that the location of the deposition zone along a tube or flat
surface can be controlled by varying the temperature profile
in the gas phase, thereby allowing deposition of uniform
coatings as in optical fiber production. [2 Limitations on de
position efficiencies and rates are currently being investigat
ed.
Deposition of the powders on the inside of the copper
tube to form a particulate film was followed by sintering and
annealing to produce an adherent superconducting layer on
the inside ofthe copper tube. Sintering was achieved by heat
ing the deposited particulate films at 880°C for 1-2 h in the
presence of oxygeno Resistive transition temperatures were
measured by four-probe electrical measurements. Critical
current densities were measured by sawing out a 1.5 X 005
mm piece of the tube, applying silver paste contacts, and
using a pulsed current technique. Coatings up to 1 mm in
thickness were deposited on the inside of 4.8-mm-i.d., 6.4-
mm-o.d. copper tubing, 15 cm longo The coatings were thick
enough to minimize problems associated with the reaction
between the copper tube and the superconducting powder.
The fabrication of thicker coatings and the use ofbuff'er lay
ers of other materials is currently being investigated. Since
the tubes were not totally filled in, sintering and subsequent
annealing of the superconductor could be carried out by
passing heated oxygen through the tubes. This overcomes
the need for oxygen diffusion through the copper tube walls
in order to achieve the correct oxygen content in the super
conductor. Also, since the material can be easily heated in
the presence of oxygen once deposited, the particles that are
used to form the deposit do not have to be superconducting.
This allows the formation of superconducting films using TI
Ca-Sa-Cu-O and Bi-Ca-Sr-Cu-O, in which the aerosol parti
cles themselves are not necessarily superconducting but can
be converted into superconducting material by diffusion and
reaction in the deposited film.
Figure 3 shows that a sharp superconducting transition
was observed at 92 K for the sintered and annealed
Ha2 YCu) 07 on the inside of the Cu tube. Critical current
densities were on the order of those measured for sintered
and annealed pellets of the same aerosol powder, roughly
10-100 A/cm2• However, the processing conditions have
not yet been optimized with respect to the critical current
densities and improvements are expected as the influence of
Kodas, Engler, and Lee 1924
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141.218.1.105 On: Mon, 22 Dec 2014 14:05:23the powder production, deposition, and sintering/annealing
conditions on the superconductor properties are investigat
ed further.
In summary, an aerosol deposition method has been de
veloped for the fabrication of thick superconducting ceramic
films. The method relies on a novel combination of pro
cesses: formation of high-purity particles in an oxygen flow
system, the controlled deposition of the particles from the
gas stream onto surfaces, and sintering and annealing of the
deposited particulate fiims in an oxygen flow. Ba2 yeu3 07
coatings were deposited on the inside surface of copper tubes
to give wires with a sharp superconducting transition at 92
K. Weare currently examining limitations on the deposition
rate, working to increase the critical current densities, exam
ining the formation of thick films based on the TI-Ca-Ba-Cu
o and Bi-Sr-Ca-Cu-O systems and are investigating the role
of the aerosol dynamics in the particle formation process.
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kagawa,Jpn.J. App\. Phys. 26, Ll653 (1987); M.Okada, H. Okayama, T.
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Kodas, Engler, and Lee 1925
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141.218.1.105 On: Mon, 22 Dec 2014 14:05:23 |
1.584373.pdf | Growth and properties of doped CdTe films grown by photoassisted molecularbeam
epitaxy
S. Hwang, R. L. Harper, K. A. Harris, N. C. Giles, R. N. Bicknell, J. F. Schetzina, D. L. Dreifus, R. M. Kolbas, and
M. Chu
Citation: Journal of Vacuum Science & Technology B 6, 777 (1988); doi: 10.1116/1.584373
View online: http://dx.doi.org/10.1116/1.584373
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/6/2?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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
Arsenicdoped CdTe epilayers grown by photoassisted molecular beam epitaxy
Appl. Phys. Lett. 54, 170 (1989); 10.1063/1.101219
Properties of doped CdTe films grown by photoassisted molecularbeam epitaxy
J. Vac. Sci. Technol. A 6, 2821 (1988); 10.1116/1.575608
Lowtemperature photoluminescence study of doped CdTe films grown by photoassisted molecularbeam epitaxy
J. Vac. Sci. Technol. A 5, 3064 (1987); 10.1116/1.574217
Controlled substitutional doping of CdTe thin films grown by photoassisted molecularbeam epitaxy
J. Vac. Sci. Technol. A 5, 3059 (1987); 10.1116/1.574216
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.59.226.54 On: Wed, 10 Dec 2014 13:31:22Growth and properties of doped CdTe films grown by photoassisted
molecular-beam epitaxy
S. Hwang, R. L. Harper, K. A. Harris, N. C. Giles, R. N. Bicknell, and J. F. Schetzina
Department of Physics, North Carolina State University. Raleigh. North Carolina 27695
D. L. Dreifus and R. M. Kolbas
Department of Electrical and Computer Engineering, North Carolina State Univel:sity. Raleigh. North
Carolina 27695
M. Chu
F'ermionics Corporation, Chatsworth. California 91311
(Received 9 September 1987; accepted 17 November 1987)
Photoassisted molecular-beam epitaxy (PAMBE), in which the substrate is illuminated during
film growth has been successfully employed to prepare n-type CdTe:ln andp-type CdTe:Sb films.
The n-type layers exhibit large electron mobilities at low temperatures. Field effect transistors
have been fabricated from selected CdTe:ln layers grown by PAMBE which show good device
characteristics. The p-type CdTe:Sb films exhibit bright photoluminescence of excitonic origin at
low temperatures. At 300 K, hole mobilities of81 cm2 IV s and carrier concentrations in excess of
1018 cm -.l have been achieved by the P AMBE technique.
At North Carolina State University we have developed a
new technique for controlled substitutional doping of com
pound semiconductor films, photoassisted molecular-beam
epitaxy (PAMBE), in which the substrate is illuminated
during the deposition process.I-3 In the present work, an
argon ion laser operating with yellow-green optics (488.0-
528.7 nm) was used as an illumination source for the growth
of doped CdTe films. In and Sb were used as n-type and p
type dopants, respectively. Semiconductor field effect tran
sistors were successfully fabricated from CdTe:ln layers,
Unlike undoped CdTe films grown by conventional MBE,
which usually exhibit high resistivity (p> 105 H cm) due to
compensation effects, CdTe films grown by P AMBE are 11-
type with low carrier concentrations and high mobilities.
These desirable characteristics are illustrated in Fig. 1 which
shows Hall effect data for an unintentionally doped CdTe
film (M291) grown by P AMBE. It is seen that film 291
exhibits a room temperature mobility of -800 cm2 IV s. The
mobility increases with decreasing temperature and reaches
a maximum value of 6600 cm2 IV s at about 40 K. At 297 K,
7000 Wi6
CdTe Epiiayci'
~ 6000 (M291) til ... , ;; 5000 S ---~ ....
S 4000 c
~ Ts = 230 0(: 1015 .:§
:E' 3000 Ol
t" 5.05 1m. ... c :.c 2000 Q,j
~ 0 I:
~ 0
1000 U
0 :W14
II 50 100 150 200 250 300
Temperature (K)
FIG.!' Hall effect data for an unintentionally doped CdTe film (M291)
grown by F AMBE. the electron concentration is 3 X 1015 em 3 and decreases to
about 2 X 1014 em -3 at 30 K.
Films grown by the P AMBE technique show a high de
gree of structural perfection, as manifested by sharp double
crystal x-ray diffraction rocking curves. This is illustrated by
the rocking curve shown in Fig. 2 for film M287
( 1 mm X 1 mm Cu K" beam at sample surface; detector full
open) for which the FWHM( 400) peak is 32 arc sec, Rock
ing curves as sharp as FWHM ( 400) = 18 arc sec have been
obtained for thin CdTe layers (t~0.4--0.6 ,urn) grown by
PAMBE.
n-type CdTe:ln films grown by PAMBE show essentially
100% activation of the dopant, as determined by Hall effect
and secondary ion mass spectroscopy (SIMS) measure
ments. Carrier concentrations ranging from 8 X 1015 to
8 X 1017 cm-3 can be reproducibly obtained using different
In oven temperatures during film growth. Hall data for a
CdTe:ln film (BCTCT12) are shown in Fig. 3. At 77 K, the
carrier concentration is ~ 1016 cm ···3 and the electron mobil
ity is > 2000 cm2 IV s. This particular film exhibited an x-ray
diffraction rocking curve FWHM ( 400) = 48 arc sec.
T X-RAY DIFI,'RACTION
'B 12
'2 HI fI = 1 CoIT. "LM ,.Q (M287) .. Il ~WHM(400) ..:! = 32 arc sec ..s 6 Ts=2~O°C
'" t = 5.3 Itm c:: 4" Q,j
C .... 2
0 -400 ·200 0 200 400
Angle (arc sec)
FIG. 2. Double crystal x-ray rocking curve for a 5.3-ftm-thick CdTe film
(M287) grown by PAMBE.
777 J. Vac. Sci. Techno!. B 6 (2), Mar/Apr 1988 0734-211X/88/020777-02$01.00 @ 1988 American Vacuum Society 777
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 128.59.226.54 On: Wed, 10 Dec 2014 13:31:22778 Hwang et al.: Growth and properties of doped CdTe films
3000 WI7
CdTe:ln Epilayer
(BCTCT12) ~ ~ 2500 ":, <Il ;;; e --2000 ~
N ::: e i 0
~ 1500 ~ Hll6 ::
& 1 ~ 1000 :0 I ~ 0
~ t " 0.60 ~n
" 10150
50 100 150 200 250 300
Temperature (K)
FIG. 3. Hall effect data for a O.6-flm-thick CdTe:ln film (BCTCTl2) grown
byPAMBE.
A number of CdTe:In films grown by PAMBE, including
BCTCT12, have been used to fabricate metal-semiconduc
tor field effect transistors (MESFET's). This achievement
has important technological significance since, with the ad
vent of transistor structures that are lattice matched with the
HgCdTe materials system, a monolithic technology involv
ing the integration of infrared focal plane detectors with on
board signal processing electronics may be possible. The epi
taxial growth for the MESFET layers proceeded as follows.
First, a I-,um-thick insulating CdTe buffer layer was deposit
ed onto a (100) CdTe substrate by conventional MBE fol
lowed by a O.4-0.8-,um-thick n-type CdTe:In layer depositcd
by P AMBE. Transistors were fabricated photolithographi
cally using a three-level masking sequence for device isola
tion, Ohmic contacts (indium), and Schottky gates (gold),
respectively. Transistor action was observed for all six of the
CdTe:In samples processed to date. The best results were
achieved for a sample having a channel doping density of
6 X 1016 em -3. Typical forward turn-on for the gate-source
Schottky barriers is ~O.8 eV, with reverse bias breakdown
occurring at ~ 8 V. Some diodes exhibited reverse break
down at voltages as large as 14 V. Depletion mode transistor
action was observed for most of the devices tested. The best
MESFET's (5,um gate length, 50,um gate width) exhibited
transconductances of -10 mS/mm and pinch-off voltages
of ~4 V.
Antimony has been successfully employed to grow p-type
CdTe films using the PAMBE technique. At 300 K, hole
J. Vac. Sci. Technol. B, Vol. 6, No.2, Mar/Apr 1988 778
CdTe:Sb EPILAYER
>-I-
UJ T=1.6 K p-type
HeNe 5 W/cm2 t= 1.9 p'm
Z
W l-
~
w
U z
w
U
UJ
W
Z -
~
;:)
~
1.53
FIG. 4. Low-temperature photoluminescence spectrum for a CdTe:Sb film
grown by PAMBE. The sharp line at 1.5894 eV is an acceptor-bound exci
tOil. Peaks at higher energies are excited states of the bound exciton; while
the peak at 1.5683 is a phonon replica. The feature at 1.5414 is thought to be
associated with the Sb acceptor level.
concentrations of up to 2 X 1018 em3 with accompanying
hole mobilities of 81 cm2/V s have been achieved. Electrical
measurements at low temperatures have not been successful
ly completed to date, because of problems encountered in
obtaining Ohmic contacts to the p-type CdTe:Sb films. Low
temperature (1.6-4.2 K) photoluminescence from the
CdTe:Sb films is very bright and consists principally of a
single sharp line at the acceptor-bound exciton energy
(1.5894 eV) for CdTe, as shown in Fig. 4.
In summary, controlled substitutional doping of CdTe
has been achieved using PAMBE. n-type CdTe layers exhib
it high structural perfection and large electron mobilities at
low temperatures. MESFET's have been successfully fabri
cated from CdTe:In films grown by PAM BE. Antimony has
been used to prepare p-type CdTe films which exhibit bright
photoluminescence at low temperatures and excellent elec
trical properties at room temperature.
'R. N. Bicknell, N. C. Giles, and J. F. Schctzina, App!. Phys. Lett. 49, 1095
(1986).
2R. N. Bicknell, N. C. Giles, and J. F. Schetzina, App!. Phys. Lett, 49, 1735
(1986).
3R. N. Bicknell, N, C. Giles, and J. F. Schetzlna, AppL Phys. Lett. 50, 691
(l987),
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1.99349.pdf | Highpower gainguided coupledstripe quantum well laser array by hydrogenation
G. S. Jackson, D. C. Hall, L. J. Guido, W. E. Plano, N. Pan, N. Holonyak Jr., and G. E. Stillman
Citation: Applied Physics Letters 52, 691 (1988); doi: 10.1063/1.99349
View online: http://dx.doi.org/10.1063/1.99349
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Published by the AIP Publishing
Articles you may be interested in
Twodimensional array of highpower strained quantum well lasers with λ=0.95 μm
Appl. Phys. Lett. 54, 2637 (1989); 10.1063/1.101020
Anomalous dependence of threshold current on stripe width in gainguided strainedlayer InGaAs/GaAs quantum
well lasers
Appl. Phys. Lett. 54, 2521 (1989); 10.1063/1.101081
Highpower nonplanar quantum well heterostructure periodic laser arrays
Appl. Phys. Lett. 53, 1159 (1988); 10.1063/1.100044
Lowthreshold gainguided coupledstripe quantum well diode lasers by laserassisted processing
Appl. Phys. Lett. 51, 558 (1987); 10.1063/1.98346
Broadband operation of coupledstripe multiple quantum well AlGaAs laser diodes
Appl. Phys. Lett. 47, 779 (1985); 10.1063/1.96035
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128.118.88.48 On: Tue, 04 Nov 2014 19:17:39Highapower gainaguided coupled~stripe quantum wen laser array
by hydrogenation
G. S. Jackson, D. C. Hail, L. J. Guido, W. E. Plano, N. Pan, N. Holonyak, Jr., and
G. E. Stillman
Electrical Engineering Research Laboratory, Center for Compound Semiconductor Microelectronics, and
Alaterials Research Laboratory~ University of lllinois at Urhana-Champaign, Urbana, Illinois 61801
(Received 14 October 1987; accepted for publication 21 December 1987)
High-power coupled-stripe (ten-stripe) Alx Gal _ x As-GaAs quantum wen lasers that are
fabricated by hydrogenation are described. Continuous (cw) room-temperature thresholds as
low as l'h = 90 rnA and internal quantum efficiency as high as 85% are demonstrated.
Continuous 300 K laser operation generating 2X 375 mW (0.75 W) at 910 rnA (WIth) or
57% efficiency is described (8-,um-wide stripes on 12pm centers). Minimal heating effects are
observed up to the point of catastrophic failure.
The semiconductor laser has become an important and
convenient source of high optical power. To overcome the
problems of high-power emission (i.e., catastrophic facet
damage and heating), large p-n junction areas are required
and, of course, a uniform distribution of the injection cur
rent. This can be accomplished with an array of closely
spaced active stripes. Optical coupling between very closely
spaced laser stripes creates a narrowing of the far-field (FF)
emission pattern and a corresponding increase in optical
power density in the output beam. 1~·1 Both gain-guided and
index-guided laser arrays can be fabricated. Index-guided
laser arrays are usually produced either by etching and some
type of crystal regrowth,4 or by layer disordering with an
impurity (c.g., Zn or Si) in the case of an
Al.,Ga; _ ,As-GaAs quantum weB heterostructure
(QWH).H Gain-guided laser arrays usually are fabricated
by some form of current segregation at the contact layer.
Shallow proton implants create highly resistive regions that
channel current into the conducting stripes. H Insulators on
the surface with stripe openings2 and mesa stripes with
Schottky-barrier contacts between them achieve similar re
sults.9 All of these schemes fer gain-guided arrays allow sig
nificant current spreading at the stripe active regions, which
is a limitation making gain-guided arrays vulnerable to gain
profile changes as operating conditions change. In fact, the
current spreading is so large that usual gain-guided lasers
can appear almost like broad-area devices.Q,lO A different
form of gain-guided coupled-stripe laser array is described in
this letter, a coupled-stripe array fabricated by hydrogen
compensation of the dopants, i.e., hydrogenation. The hy
drogenation process is effective in eliminating current
spreading at the active region and allows broad area metalli
zation over the entire p side, thus providing excellent heat
sinking for high-power operation.
The coupled-stripe laser arrays described here are fabri
cated on a QWH crystal grown by metalorganic chemical
vapor deposition (MOCVD) in an EMCORE GS 3000 reac
tor.1! The separate confinement heterostructure (SCH)
consists of a single 140-A GaAs QW centered in an
AI, Gal xAs waveguide layer (x-O.25, O.18pm). The en
tire undoped active region is sandwiched between two
Alx·Gal x,As (x' -0.75, 1 {tm) confining layers, the bot-tom one doped n type with Se (nSe ~2x 1018 em 3) and the
top doped p type with carbon (C) (nc-9X 1017 em 3).
The use of C as a p-type dopant has been described else
where.12 The fabrication of these laser diodes is similar to the
process used previously for single stripe lasers. l:' Prior to the
hydrogenation step a shallow Zn diffusion step (550°C, 15
min), in a stripe array pattern, is carried out on the top-side
GaAs contact layer to improve the p-side contact. The Zn
diffused regions are then masked with -1000 A ofSiOz, and
the wafer is placed in a hydrogen plasma (750 Torr, 0.4
W /cm2) at 250·C for 8 min. Hydrogenation of the C in the
nonmasked top regions creates highly resistive stripes in the
p-type AIo.7s GaO.25 As confining layers.12 After hydrogena
tion the oxide mask is removed, the wafer is thinned to -100
pm thickness, and contacts CGe-Au for n type, Cr-Au for p
type) are evaporated onto the wafer. For cw operation the
devices are mounted p side down on Cu heat sinks with In.
The laser array consists of ten 8-,um-wide p-type con
ducting stripes on 121lm center-io-center spacing. A scan
ning electron micrograph of the ten-stripe wafer is shown in
Fig. 1. Conventional A-B etch is used to stain the cleaved
facet and enhance the contrast between the conducting and
the resistive (4-.um-wide hydrogenated) p-type regions. In
Fig. 1 (a) no metallization is present, and the conducting
stripes (811m wide) are completely etched down to the QW
active region. This allows easy identification of the hydroge
nated areas and the ten active stripes on 12 pm centers. The
two outside stripes of the array are marked with vertical
arrows 1 and 10 to denote the extent of the array. There
appears to be little or no "undercutting" ofthe oxide mask in
this device, which is in contrast to earlier results on single
stripe lasers. 13 This difference may result from the use ofC as
the p-type dopant, as wen as from confining the Zn diffusion
to a stripe pattern. A metallized cleaved section is shown in
Fig. 1 (b) that also is stained with the A-B etch. The 8-l1m
wide conducting stripes appear as dark regions separated by
lighter (4 pm) hydrogenated areas. Again, the two vertical
arrows in Fig. 1 (b) point to the two laser stripes 1 and ! 0 at
the edges of the array.
The results of pulsed operation of these ten-stripe lasers
are summarized in Fig, 2. Excitation is by 5 f.1.s pulses at a 10-
kHz repetition rate. Diodes with different lengths are tested
691 Appl. Phys. Lett. 52 (9), 29 February 1988 0003-6951/88/090691-03$01.00 (c) 1988 American Institute of PhysiCS 691
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128.118.88.48 On: Tue, 04 Nov 2014 19:17:39FIG. I. Scanning electron micrographs of (a) an unmetaIlized and (b 1 me
tallized ten-stripe laser array. In (a) the 8.urn Ilnhydrogenated conducting
p-type stripes are etched away but not the 4-.um-wide high-resistivity hydro
genated coupling stripes. In (b) a metallized cleaved (and etched) section
of the ten-stripe wafer of Ca) is shown with the as-grownp-type stripes ap
pearing as dark areas under the Au contact.
to anow determination of the internal differential quantum
effidency 17t and the internal absorption a. In Fig. 2(a) the
pulsed threshold current density J'h is displayed versus in
verse length, 1/ L, Diodes that are 250 lIm long exhibit an
average threshold of J'h = 400 A/crn2. The longest diodes
tested have an average threshold of Jth = 237 A/cm2• The
values of Jth for all the diodes fall along a line
J'h =p-!(a +L-'ln[l/R]), (1)
where {3 is a constant depending on the diode and R is the
reflectivity of one facet. In Eg. (1) a linear dependence of
gain on current density is assumed. 14 This agrees qualitative
ly with other experimental results on single stripe single QW
lasers.15 In Fig_ 2(b) the external differential quantum effi
ciency 17eXI is displayed versus the diode length. The best
value measured is 17"xt = 79% for a 400-,um diode operated
just above fth. The linear fit [Fig. 2(b)] is based on the
formula
N 600 Ten-Stripe (H) p-n AlxGa l·xAs-GaAs QWH
E
~ 400
(a)
00 10 20 30 40 50
FIG. 2. Performance data (pulsed excitation) on various length ten-stripe
hydrogenated laser arrays. The average threshold current density vs inverse
length is plotted in (a). In (b) a plot of 1/17"t (external differential effi
ciency) -l vs length reveals an internal quantum efficiency 71, ~, 85%.
692 Appl. Phys. Lett., Vol. 52, No.9, 29 February i 988 from which T/i can be obtained.14 In this case 7Ji = 0.85.
These high efficiencies result from the effective carrier col
lection of the QW and the strong overlap of the gain and the
optical mode in a SCH laser. Also the lack of absorption in
the tails of the transverse mode profile due to the carrier
confinement by hydrogenation may improve the efficiency
of these lasers. Another parameter that can be obtained from
Fig. (2b) is a. For these lasers a = 8.5 em -1, which can be
attributed predominantly to free-carrier absorption.
Near field (NF) patterns for one of the laser arrays un
der cw operation are presented in Fig. 3. Operation at Fig.
3(a) 100 rnA, just above Ith, and Fig. 3(b) 220 rnA are
shown, with a trace of intensity versus position along the
Fabry-Perot facet shown just below each NF image. The 12
,um periodicity of the ten-emitter array is easily resolved in
Fig. 3(a) and can, in fact, be seen below flit < This pattern of
ten emitters is stable with increasing current as seen in Fig.
3(b), and even up to 500 rnA cw excitation, indicating a
stable gain profile. Also the NF image shows there is signifi
cant optical overlap in the hydrogenated regions between the
emitters. This is confirmed by observations of the FF pat
terns (not shown), which exhibit the common two-lobe pat
tern observed for many coupled-stripe laser arrays. 1,6.7.9
Of special importance, the hydrogenated laser arrays
are capable of high-power emission. The inset of Fig. 4
shows, for a 250-,um-Iong hydrogenated ten-stripe diode un
der cw excitation at room temperature, recombination radi
ation spectra at threshold (lth = 90 rnA), and just above
(f = 100 mA). A relatively broad gain spectrum with a nar
row lasing region at the lowest confined-particle transition
of the QW, A ~ 8590 A, is observed. The optical power L
versus current f (L-I) characteristic of Fig. 4 agrees with the
spectral behavior and indicates a threshold I'll = 90 rnA for
stimulated emission. The small kinks present in the L-I char
acteristic above Ilh are caused by the onset of various trans
verse modes of the stripe array. These can be seen in both the
NF and FF patterns as the current is changed (not shown).
Even with these kinks, theL-l characteristic is predominant
ly straight up to 375 rnW single-facet output (I = 910 mA),
at which point catastrophic failure occurs. If the uncoated
facets are assumed to emit equally, a total efficiency of 57%
FI G. 3. Near field (NF) images of the laser array of Fig. 1 under continuous
room-temperature operation at (a) 100 rnA and (b) 220 rnA. All ten emit
ters are easily resolved under both conditions.
Jackson et a/. 692
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128.118.88.48 On: Tue, 04 Nov 2014 19:17:39400
~300
E-
8200 -
~
(1) s o
t:L 100 Ten-Stripe (H) C-Doped p-n AlxGa1-xAs-GaAs QWH
L-I
300K, cw
00 0.4 0.8
Current, I (Ai
FIG. 4. Power vs current (L-l) and spectral data for continuous room
temperature laser operation of the ten-stripe array of Fig. I. The nearly
linear L-J reveals only slight heating up t(l 910 rnA and 2 X 375 mW (0.75
W) where catastrophic failure occurs. The spectra show lasing on the first
confined-particle transition (rUt) = 1.443 eV).
is found for operation at 910 rnA. Other diodes under pulsed
operation have emitted 1.24 W peak power at 1.3 A for an
efficiency of 66%. These results illustrate the excellent heat
sinking of these diodes. An estimate of the thermal imped
ance of these lasers is made by comparing the shift in wave
length between pulsed and cw operation at 500 rnA, yielding
a value of 5.6 ·C/W. The ability to remove heat from a large
area on the p side results in the low thermal impedance. No
statistically meaningful li.fetime data exist on these diodes;
however, they have been operated cw abovel'h for over 12h.
In conclusion, a high-power coupled-stripe laser array
of ten gain-guided emitters fabricated by hydrogenation is
693 Appl. Phys. Lett., Vol. 52, Nc. 9, 29 February 1988 demonstrated. The good current confinement results in sta
ble operation and high quantum efficiency. A!so the hydro
genation process does not cause crystal damage, thus leading
to potentially better heat dissipation.
The authors are grateful to R. T. Gladin and B. L. Payne
for technical assistance. This work has been supported by the
Army Research Office contract DAAG-29-85-K-0133, and
National Science Foundation grants CDR 85-22666 and
DMR 86-12860.
'D. R. Scifres, R. D. Burnham, and W. Streifer, Apr!. Phys. Lett. 33,1015
( 1978).
2D. R. Scifres, W. Streifer, and R. D. Burnham, IEEE J. Quantum Elec
tron. QE·!5, 917 (1979).
'1. K Buller, D. E. Ackley, and D. Botez, Appl. Phys. Lett. 44, 293 (1984).
4D. F. Welch, W. Streifer, P. S. Cross, and D. R. Scifres, IEEEJ. Quantum
Electron. QE-23, 752 (1987).
'P. Gavriiovic, K. Meehan, J. E. Epler, N. Holonyak, Jr.. R. D. Burnham,
R. L. Thomton, and W. Slreifef, App!. Phys. Lett. 46,857 (1985).
"D. G. Deppe, G. S. Iackson, N. Holonyak. Jr., R. D. Burnham. and R. L.
Thofr.ton, App!. Phys. Lett. 50, 632 (1987).
7L. J. Guido, W. E. Plano, G. S. 1ackson, N. Holonyak, Jr., R. D. Burn
ham, and J. E. EpJer, Apr!. Phys. Lett. 50, 757 (1987).
"D. R. Scifres, C. Lindstrom, R. D. Burnham, W. Streifcr, and T. L. Paoli,
Electroll. Lett. 19, 169 (l983).
9J. P. vall der Ziel, R. M. Mikulyak, H. Temkin, R. A. Logan, alld R. D.
Dupuis, IEEE J. Quantum Electron. QE-20, 1259 (1984).
tOJ. E. Epler, N. Holollyak, Jr ,R. D. Burnham, T. L. Paoli, R. L. Thornton,
and M. M. Blouke, Appl. Phys. Lett. 47, 7 (1985).
1'K D. Dapuis, L. A. Moudy, and P. D. Dapkus, in Proceedings a/the 7th
lntern:ltionai Symposium on GaAs and Related Compounds, edited by C.
M. Wolfe (Illstitute ofPhy.k" London, 1979), pp. 1-9.
12L. J. Guido, G. S. Jackson, D. C. Hall, W. E. Plano, and N. Ho!onyak, Jr .•
Appl. Phys. Lett. 52, 522 (1988).
"G. S. Jackson, N. Pan, G. E. Stillman, N. Holonyak, Jr., and R. D. num
ham, Apr!. Phys. Lett. 51, 1629 (1987).
14H. Kresscl and J. K. Butler, Semiconductor Lasers and Heterojullction
Ll:;J)s (Academic, Orlando, FL, 1977), p. 270.
"N. K. Dutta, R. L. Hartman, and W. T. Tsang, IEEE J. Quantum Elec
tron. QE-i9, 1243 (1983).
Jackson et al. 693
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128.118.88.48 On: Tue, 04 Nov 2014 19:17:39 |
1.102161.pdf | Homoepitaxial films grown on Si(100) at 150°C by remote plasmaenhanced chemical
vapor deposition
L. Breaux, B. Anthony, T. Hsu, S. Banerjee, and A. Tasch
Citation: Applied Physics Letters 55, 1885 (1989); doi: 10.1063/1.102161
View online: http://dx.doi.org/10.1063/1.102161
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/55/18?ver=pdfcov
Published by the AIP Publishing
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137.149.200.5 On: Sun, 23 Nov 2014 20:29:08Homoepitaxial films grown on Si(100) at 150°C by remote plasma .. enhanced
chemical vapor deposition
L. Breaux, B, Anthony, T, Hsu, S, Banerjee, and A. Tasch
Electrical and Computer Engineering, Unillersity a/Texas, Austin, Texas 78712
(Received 12 June 1989; accepted for publication 21 August 1989)
Low-temperature silicon epitaxy is critical for future generation ultralarge scale integrated
circuits and silicon-based heterostructures. Remote plasma-enhanced chemical vapor
deposition has been applied to achieve silicon homo epitaxy at temperatures as low as 150 ·C,
which is believed to be the lowest temperature reported to date, Critical to the process are an in
situ remote plasma hydrogen cleaning of the substrate surface in an ultrahigh vacuum growth
chamber prior to epitaxy, and substitution of thermal energy by remote plasma excitation via
argon metastables and energetic electrons to dissociate silane and increase adatom mobility on
the surface of the silicon substrate. Excellent crystallinity with very few defects such as
dislocations and stacking faults is observed.
There has been a sustained effort in recent years to lower
the temperature at which silicon homoepitaxy and hetero
epitaxy can be achieved for future generation ultralarge scale
integration and novel Si-based heterostructure devices. In
order to remove contaminants such as oxygen and carbon
from the Si surface prior to epitaxy and to dissociate the
reactant species, and also to provide adequate ada tom mobil
ity at low temperatures, it is necessary to employ ultrahigh
vacuum (URV) systemsl--3 and nonthermal energy such as
plasma excitation.4 We present results of very low tempera
ture (150°C) Si homoepitaxy using a novel technique, re
mote plasma-enhanced chemical vapor deposition
(RPCVD).5
The details of the RPCVD process and system have
been described before.6 Briefly, the deposition is carried out
in an UHV chamber (partial pressures of 10-9 Torr for wa
ter vapor and 5 X 1O-1l Torr for oxygen) that is equipped
with a variety of analytical equipment such as reflection
high-energy electron diffraction (RHEED) and residual gas
analysis. Ultrahigh purity gases are used (99.9999% for hy
drogen, and comparable values for the other process gases).
Furthermore, the gases flow through Nanochem gas purifi
ers which reduce oxygen and H20 levels to the parts per
billion level.7 Unlike in a conventional plasma CVD
chamber, a noble gas (argon) rfplasma is generated remote
ly from the wafer so that the plasma-induced damage is mini
mized or avoided. The plasma-generated excited species,
such as long-lived noble gas metastables and energetic elec
trons, are transported to the sample where they interact with
and selectively excite the reactant gas (2% silane in He)
which is introduced through a gas ring placed between the
sample and the plasma column. Therefore, in this technique
the downstream plasma excitation rather than elevated tem
perature is used to provide the energy needed for the depo
sition reaction and to increase adatom mobility on the sur
face of the substrate. This allows greatly improved control
over the reaction pathways and therefore, the crystal mor
phology, layer thicknesses, and sharpness of interfaces and
doping transitions. The RPCVD system is also equipped
with a load-lock chamber for sample introduction and a sur
face analysis chamber containing Auger electron spectros
copy (AES).
p-type (100) Si substrates with resistivities of 10-15 n cm were used in these experiments. Ex situ wet chemical
cleans to remove contaminants from the wafer surface were
performed. The best results were obtained using ultrasonic
degreasing [trichloroethane, acetone, methanol, and de-ion
ized (D1) water] foHewed by a modified RCA elean.R There
is a final dilute 1:40 HF:H20 dip fonowed by a 30 s DI water
rinse and N2 dry prior to wafer loading into the vacuum
chamber in order to remove as much of the native oxide as
possible.
An in situ remote plasma-excitL'<l hydrogen clean in the
URV growth chamber was employed prior to epitaxy to re
mov,e any residual oxygen and carbon. Hydrogen, intro
duced through the plasma column, is rf excited such that the
plasma glow does not engulf the wafer.9 Hydrogen plasma
cleans have been attempted over a range of hydrogen flow
rates (5 and 200 sccm), hydrogen partial pressures (3-200
mTon), rfpowers (10-70 W), and substrate temperatures
during clean ( 150-325 ·C) for durations of 5-60 min. Typi
cal AES results for a 45 min hydrogen plasma clean at 45
mTon, with 10 W plasma power, and with the substrate at
300°C are shown in Fig. 1. The as-loaded sample shows
strong Si LMM and KLL peaks and small C and 0 KLL
peaks [Fig. 1 (a) J. After hydrogen plasma cleaning, the Au
ger spectrum [Fig. 1 (b) ] shows a reduction of the C and the
o peaks and an increase of the Si peaks, which is evidence of
the effectiveness of the in situ clean in terms of reduction of
both carbon and oxygen. We believe that the hydrogen plas
ma produces atomic hydrogen which, in turn, produces a
reducing environment and has an etching effect on Si and
Si02 by converting them to volatile byproducts.9•10
Corroborating evidence of achieving an atomically
clean, smooth Si surface by plasma dean has been obtained
from in situ RHEED analysis. 6 A typical RHEED pattern of
the as-loaded (l 00) Si surface from the [011] direction
shows integral order streaks but no half-order lines, indicat
ing a smooth but unreconstructed (1 Xl) Si surface, pre
sumably due to surface contamination. After in situ hydro
gen cleaning at low pressures for 60 min at a substrate
temperature of 31 0 ·C, we observe both stronger integral or
der streaks compared to the as-loaded sample and the ap
pearance of faint half-order lines indicative of a (2 X 1) re
construction pattern (Fig. 2). If the sample is briefly baked
for 5 min at 400 "C, strong integral and half-order streaks are
1885 App!. Phys_ Lett. 55 (18), 30 October i 989 0003-6951/89/441885-03$01_00 @ 1989 American Institute of Physics 1685
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137.149.200.5 On: Sun, 23 Nov 2014 20:29:08Auger Analysis of ill Situ Clean
"
:c 10 AFTER CLEMI
~ ____ ).. ___ ~ _. _J ____ l-<~l ~ ,_
300 400 500 600 15sa 165()
!ELECTRON !::NERGV (eV) FIG. 1. Auger spectrum of
as-loaded Si surface and after
in situ plasma clean.
visible due to Si H bond breaking and hydrogen desorp
tion, leading to the ccnversion of a silicon dihydride surface
to a monohydride termination. 11 This is significant because
adsorbed hydrogen impedes the motion of silicon-bearing
silylene (SiH2) on the silicon surface, thereby hampering
low-temperature epitaxy. 2
Silicon epitaxial films were deposited by flowing 250
sccm of Ar through the plasma column and exciting it with
10--15 W ofrfplasma power. A silane mixture (2% SiH4 in
He) was introduced through the gas ring and excited by Ar
metastables and energetic electrons from a remote plasma in
order to dissociate the silane nonthermally into precursors
responsible for 5i epitaxy. The substrate was heated from the
back during deposition by tungsten-halogen lamps, and sub
strate temperatures were monitored by a calibrated thermo
couple in contact with the back of the wafer. Although the
deposition depends on a variety of process parameters (Ar
and silane flow rates, chamber pressure, rf plasma power,
and substrate temperature), this study was focused on the
dependence of epitaxial quality on substrate temperature.
The Ar flow rate and rf power were kept fixed. Some varia
tions were made in the silane flow rates (5-38 sccm) and
deposition pressures (450 and 200 mTorr) in order to obtain
insight about the substrate temperature dependence rather
than to achieve an optimized deposition. The deposition
temperatures were systematically lowered from 305 to
110 °C in separate experiments involving different wafers in
order to determine the lowest temperature at which single
crystal growth could be achieved.
The film crystallinity and morphology were assessed by
in situ RHEED analysis, transmission electron microscopy
(TEM). and electron difrraction.12 The defect microstruc
ture of the layers in terms of stacking faults and voids was
evaluated by preferential etching in a dilute Schimmel etch n
followed by Nomarski microscopy. The etch rates were ap
proximately 70 A/s. Since the grown film thicknesses were
between 170 and 350 A, etch times of2-3 s were used so that
approximately half of the film thickness would be etched.
TEM analysis was used to complement the defect etching
analysis a.nd was used to detect dislocations.
Silicon epitaxy was attempted at 305 "C and 450 mTorr
chamber pressure, where the silane flow was varied between
1886 Appl. Phys. Lett.. Vol. 55, No. 18, 30 October i 989 FIG. 2. RHEED pattern of (100) Si surface along [011] direction after
hydrogen plasma clean.
5 and 38 sccm. High crystalline quality films were obtained
at around 15 scem, as indicated by integral order streaks and
Kikuchi lines in the RHEED pattern. A short bake at 400 °c
results in half-order lines appearing in the RHEED pattern
which provides evidence of a smooth, single-crystal film. At
higher flow rates (38 sccm), a polycrystalline RHEED pat
tern is observed, while at lower flow rates (5 secm) a slight
degradation of the single-crystal RHEED pattern is ob
served. TEM analysis and Nomarski microscopy of defect
etched samples confirm that the defect density increases for
low (5 scem) and high (38 sccm) flow rates compared to the
15 sccm case. Even for the film grown at IS seem, TEM
micrographs reveal a few very small "pinhole" defects which
may be due to voids or dislocation loops in the film.
As mentioned earlier, one problem with 5i epitaxy at
low substrate temperatures is insufficient adatom (SiH2)
mobility on the Si surface as well as the difficulty with hydro
gen desorption, which has been identified as the rate-limiting
step in low-temperature epitaxy.2 It is possible that at 38
scem, the growth rate is too high to anow the SiH2 species to
migrate to appropriate sites and hydrogen to desorb, leading
to polycrystalline growth. Lower silane flow rates are likely
to be more favorable for epitaxy at lower substrate tempera
tures. However, if the silane flow is too low (5 seem), residu
al oxygen and H20 contamination in the deposition chamber
may compete with the Si deposition process, leading to high
defect density.
To test this hypothesis, Si films were grown at the same
temperature (305 °C) and silane flow rate (15 secm) as be
fore, but at a lower pressure (200 mTorr) so as to reduce the
oxygen and H20 partial pressures relative to the silane par
tial pressure. Excellent crystallinity is achieved, as seen from
RHEED patterns. In situ Auger analysis ofthe surface of the
deposited film reveals lower levels of carbon Hnd oxygen
than on the surface of the starting material. A TEM micro
graph (not shown) of the film is featureless and electron
diffraction shows a single-crystal pattern [Fig. 3 (a) ]. De
fect etching and Nomarski microscopy also show a smooth,
defect -free film [Fig. 3 (a) J. In contrast, films grown at 450
mTorr, although single crystal, reveal a few pinhole-type
defects under TEM and some surface texture under No
marski microscopy. Thus, it appears tha.t lower deposition
pressures Clfe preferable for low-temperature epitaxy.
Epitaxial films were grown at 200 mTorr and 15 scem
Breaux eta/. 1886
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137.149.200.5 On: Sun, 23 Nov 2014 20:29:08{hI
Ie)
FIG. 3. Nomarski picture of epitaxial film after dilute Schimmel etching
and selected area electron dlti'radion pattern of film grown at (a) 3()S "C,
(b) 150 "C, and (e) 125 "c. The magnification of the Nomarski views ;s
400x.
silane with ~ 10 W plasma power at various temperatures
down to 110 0c. Excellent single-crystal RHEED patterns
were achieved at 220, 180, and 150 cC, while lower tempera
tures ( < 125°C) resulted in all amorphous pattern. The re
sults of defect etching and electron diffraction for the films
grown at 200 mTorr at 150 and 125 DC are shown in Figs.
3(b) and 3(c), respectively. The diffraction pattern for the
film grown at 150°C is single crystal as for the one grown at
305°C [Hgs. 3(a) and 3(b)]. A TEM micrograph ofthc
film grown at 150 DC appears defect -free, indicating that if
1887 Appl. Phys. Lett., Vol. 55, No. 18,30 October 1989
.".~.-.-,-.-.' ••• '."~'~";:'"'"!':'"'!'.'7'?' ••• '.'.'.'.'.'.'.'.'.'~ ........... . defects such as voids or small dislocation loops exist, their
density is extremely low. The absence of stacking faults in
the film should be noted. It should also be pointed out that
TEM and Nomarski analysis of substrates, which had un
dergone the in situ hydrogen plasma clean only, do not show
any defects such as stacking faults, dislocations, or hydrogen
bubbles. 14 Therefore it appears that any defects in the epitax
ial films do not propagate from the substrate but are nuclea
ted during growth, The TEM micrograph of the sample
grown at 125°C is featureless because it is amorphous, as
shown by the faiDt, diffuse ring electron diffraction pattern
from the film (superposed on the single-crystal pattern from
the substrate) [Fig. 3(c) 1. Nomarski microscopy reveals
some surface texture, but no macroscopic defect features
l Fig. 3 (e) ] . One factor that should aid single-crystal growth
at lower temperatures is the fact that the heating of the depo
sition chamber walls during deposition is reduced, leading to
less outgassing and lower oxygen and H20 partial pressures.
In conclusion, single-crystal Si growth with low defect
density has been demonstrated by RPCVD at temperatures
as low as 150 cc. To the best of our knowledge, this is signifi
cantly lower than that reported by any other technique. 3 The
growth rates observed in our study range from -1 to 12 A/
min. Single crystallinity has been retained even after 4 h of
sustained deposition resulting in 100 nm films. These low
growth rates in RPCVD are ideally suited for Si-based heter
ostructures because of the control provided over layer thick
nesses.
This work was supported through Office of Naval Re
search/Strategic Defense Initiative Organization contract
N00014-87-K-0323.
'v. 01a, Thin Solid Films 106,1 (1983).
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ota, Extended Abstracts afthe 19th Conference Oil Solid State Devices and
llilaterials, Aug. 25-27, 1987, Tokyo. Japan (Business Center for Aca
demic Societies, Tokyo, Japan), Pl'. 239-242.
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3519 (1986).
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of the Industry-Uniuersity Aduanced Materials Conference 1989, March 6·-
9, 1989, Denver, CO, edited by Fred W. Smith (Advanced Materials Insti
tute, in press).
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purifiers afe manufactured under license by Semi-Gas Systems. Inc., San
Jose. CA.
'w. Kern. Semiconductor Intcnmtional April, 94 ( ] 984).
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Cryst. Growth 95. 91 (1989).
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Technol. B 2, 359 (1984).
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the 1989 Electronic Materials Conf., June 21 23, Boston, MA. to be pub
lished in the J. Electron. Mater. See abo abstract No. UlO in Technical
Program of 19S'J Electronic Materials Conf. in J. Electron. Maler. 18 July
(1989).
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;,'S. J. Jeng, G. S. Oehdein, and G. J. Scilla, App!. Phys. Lett. 53, 1735
( 1988).
Breaux et ai. 1887
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1.100563.pdf | Superconducting coatings in the system BiCaSrCuO prepared by plasma spraying
A. Asthana, P. D. Han, L. M. Falter, D. A. Payne, G. C. Hilton, and D. J. Van Harlingen
Citation: Applied Physics Letters 53, 799 (1988); doi: 10.1063/1.100563
View online: http://dx.doi.org/10.1063/1.100563
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155.33.16.124 On: Sun, 30 Nov 2014 14:12:21Superconducting coatings in the system Bi",Ca",Sr~Cu .. O prepared
by p~asma spraying
A. Asthana, P. D. Han, L. M. Falter, and D. A. Payne
Department ofJI.fateria!s Science and Engineering and Materials Research Laboratory, University of
Illinois at Urbana-Champaign, Illinois 61801
G. C. Hilton and D. J. Van Harlingen
Departmellt of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign,
Illinois 61801
(Received 3 June 1988; accepted for publication 28 June 1988)
Superconducting coatings in the system Bi-Ca-Sr-Cu-O were deposited on alumina substrates
by plasma spray methods. The coatings were superconducting in the "as-sprayed" condition
and improved with heat treatment. The best results were for coatings with resistivity values
near 10 mn cm at room temperature and zero resistance at 96 K. The coatings had a magnetic
transition near 80 K, with a weak diamagnetic signal up to 112 K. Superconductivity in the
coatings was associated with two distinct phases, one of which was not identified. Scanning
electron microscopy, x-ray diffraction, electrical resistivity, and magnetic measurements were
used to characterize the coatings.
The discovery of high-temperature superconductivity in
the Y-Ba-Cu-O system, 1.2 and more recently in the Bi-Oi-Sr
Cu-O system,3.4 has stimulated an intense effort to produce
high Tc thinS-7 and thick films.!! Our results demonstrate
that plasma spraying is a simple and effective method for
producing high quality superconducting coatings from
powders. The high plasma temperatures obtained give a
method of "quenching-in" novel phases. In addition, high
deposition rates, unique thermal history (rapid quench),
controllable crystallinity, and low preferential evaporation
of high vapor pressure constituents make this method tech
nologically attractive.
The feedstock powder for plasma spraying was prepared
by mixing Si20" CaC03, SrCO}, and CuO in the cation
ratios of2:1:2:2. So as to minimize excessive Bi loss, a two
stage calcination process was adopted. The powder was pre
calcined at 760°C in oxygen for 6 h and then reground to a
fine powder. The final calcination was carried out in oxygen
at 820°C for 12 h before quenching in air to room tempera
ture. The product was then ground into powder suitable for
plasma spraying. Polycrystalline alumina substrates were
used. Plasma spraying was carried out in air using a METeO
type 7 MB system with a G-type nozzle and external powder
injection. Plasma spray para8eters are listed in Table L The
coatings were then heat treated in oxygen under various con
ditions and quenched in air from the annealing temperature.
S08e of the salient features of the coatings are presented in
Table II.
A dual stage ISI-130 scanning electron microscope
(SEM) was used for examination of the coatings. X-ray
powder diffraction was carried out in a Philips APD 3520
diffractometer using eu radiation. Magnetization measure
ments were obtained from a superconducting quantum il1-
terference device at 1 Hz. Resistivity measurements were
made using a standard four-point probe method of 1000 Hz
and an excitation current ofO,l /.lA.
The deposited coatings were observed to be black and
typically 100 /.lID in thickness. SEM examination of the coat-iugs revealed a spherulitic morphology, typical of plasma
sprayed material. Flat, plate-like crystallites were observed
within the spherulites. A needle-like phase was also ob
served.
Figure 1 illustrates the phase development in the coat
ings for different heat treatment conditions, as determined
by x~ray diffraction. The feedstock powder was determined
to be multiphase and contained the orthorhombic 2122
phase, reported by Hazen et aC The "as-sprayed" coatings,
and all coatings annealed at 780°C, contained an unidenti
fied. phase (1) characterized by a strong reflection near
d = 3 A (29.8°). Hcat treatment at 860"C resulted in recrys
tallization of the 2122 phase, marked by "0" in Fig. 1. Heat
ing at 900°C resulted in partial melting of the material, and
conversion of the 2122 phase to another unidentified phase
(2) .
Figure 2 illustrates the temperature dependence of resis
tivity for coatings, heat treated under various conditions. So
as to allow for ease in comparison between various samples,
resistivity values were normalized by their room-tempera
ture values. Annealing for various times at 780 °C resulted in
progressively higher transition temperatures [Fig. 2 (a) ] .
The best results in this case were for coatings heat treated at
780°C for 40 h (1~ -60 K). Figure 2 (b) illustrates the sub-
TABLE L Process parameters for plasma spraying,
Are voltage
Arc current
Primary gas
Secondary gas
Primary gas flow rate
Secondary gas flow rate
Powder feed rate
Carrier gas
Spray distance 60V
400 A
AI" at 0,5 MPa
He at 0.5 MPa
800 eels
120 eels
8-10 g/min
Ar
10-12 ern
799 App\. Phys, Lett. 53 (9), 29 August 1988 0003-6951/88/350799-03$01,00 @ 1988 American Institute of Physics 799
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155.33.16.124 On: Sun, 30 Nov 2014 14:12:21TABLE H. Important features of plasma-sprayed coatings.
Sample Anneal condition Superconductivity Major phase
Feedstock powder Yes 2122
2 As-sprayed Yes Unidcntified (!)
3 4 h!780"C Yes Unidentified (1)
4 8 h!780°C Yes Unidentified (!)
5 16 h!780 'C Yes Unidentified ( I)
6 40 h/780'C Yes Unidentified ( I )
7 16 h/860'C Yes 2122
8 16 h/900'C No Unidentified (2)
stantial improvement in Tc for samples heat treated for 16 h
at 780 and 860°C. The material heat treated at 860 "C had
zero resistance at 96 K, with a room-temperature resistivity
of approximately 10 run em, whereas materia! heat treated
at 900°C was semiconducting (inset).
Magnetization versus temperature data are illustrated
in Fig. 3, All measurements were made at a field of 20 Oe.
The sharpness of the transition increased with annealing
time for samples heat treated at 780°C [Fig. 3 (a)1. Heat
treating at 860°C for the same time gave a sharper transition
[Fig. 3 (b) ]. A small diamagnetic signal was detected up to
28 (deg)
FIG. I. X-ray spectra for plasma-sprayed coatings, after various heat treat
ments. An unidentified superconducting phase (I) was present for all coat
ings heat treated at 780 'c. Heat treatment above 860 'c resulted in the
formation of thc 2122 phase (marked by 0). A still higher annealing tem
perature resulted in the formation of all unidentified semiconducting phase
(2).
800 Appl. Phys. Lett" Vol. 53. No.9, 29 August 1988 112 K (inset), but the major magnetic transition occurred in
the vicinity of 80 K.
The above data suggest that at least two different phases
were responsible for superconductivity in the Bi-Ca-Sr-Cu
o system, The lower Tc phase present in the as-sprayed coat
ings remained stable at 780°C for all annealing times. The
improvement in resistive and magnetic transitions with an
nealing time at 780 cC was attributed to an increased phase
development of an unidentified lower T, phase (1). The
higher Tc material, present in samples heat treated at 860°C,
was identified as the 2122 phase. An interesting feature of
these coatings was the unusually high zero resistance tem
perature of96 K, compared with temperatures reported pre
viously in the 60---90 K range.3,9,!O One possible explanation
is that the 80 and the 112 K phases were similar in structure,
with minor modi.fications (perhaps, only in atomic order),
as suggested by Hazen et al.3 Another possibility is that the
intergrowth characteristics of the various superconducting
phases in the Bi-Ca-Sr-Cu-O system preclude "ideal" phase
formation. Hence, the individual phases grow as syntactic
intergrowths, as suggested by Morgan et al. 11 In either case,
,-..., 2.0
(fl +-
C
:J 1.5 0: as SRr!lyed
b: 16h/780 C
c: 4011/780 C
(l: •
...0 !~ '~". .: .............. .
L b \ .... "... '.
~ 0:':0: r'; r?:::~:::::' .. ""-···1
-jj! ,,! , I
,......"
(fl +-
C
:J
...0
L « "-'
Q 1.5
1.0
0.5 o
(a)
J
0
(b) 100 200 300
T(K)
~'~I 2:.:~" ...... . a 100 200
~~ ... ,.
" ....... ;;; 16h/. 780 C
: b: 16h/S60 C
: c: lSh! 90~.~.
.,/ /'
'" a· b!
.d --i,
50 100 150 200
T(K)
FIG. 2. Resistivity vs temperature characteristics of pi as rna-sprayed 2122
coatings 011 alumina. Sllperconducting properties of the coatings improved
with longer anlleaiing time (a). The best heat treatment condition was de
lermined to he 16 h at 860'C (b). Heat treatment at 900'C resulted in
semiconducting behavior (inset).
Asthana et al. 800
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155.33.16.124 On: Sun, 30 Nov 2014 14:12:210.5
(10-3)
0.0 , ~ nnn***~$flr!" .,c a .. :;-<! " .. E -0.5 ,," 2- " .. " " c:: [] ..
0 -1.0 c " .~ I>
.~ CC " a; " " c -1.5 ..
OJ .. " '" As sprayed ro " 4 hi 780 C ~ II! ..
-2.0 .. + 8 hi 78C C
c 16 h1780 C .. 40 hi 780 C
-2.5 L.---<--<--L-J:::::r::::;::::r=:::=.J
o 20 40 60 80 1 00
(a) Temperature (K)
0.0 .-------.iII1l'\!!ra,......-..--,
3 ...J'
(10 ) .1 ",,' 16h1780C I ",' I 16 hi 880 C " 01
-0.5 ",'"
~ '" E " ..
" .. 2-.. " ..
"""~(~;~)"[".":J' .. .. -3 .
-6
80 90 100 110 120
-2. 0 L.....--'-___ -'-~-'---........,;'--------'--'--'
o
(b) 20 40 60 80 100 120
Temperature (K)
FIG. 3. Magnetization vs temperature data for plasma-sprayed 2122 coat
ings on alumina. Annealing (a) forlonger times, and (b) at higher tempera
tures gave progressively sharper magnetic transitions. A weak diamagnetic
signal, with an onset lIear 112 K, was also detected (inset).
80\ Appl. Phys. Lett., Vol. 53, No.9, 29 August 1968 the unique features of the plasma spray method allow nu
merous opportunities in selectively quenching and recrystal
lizing high Tc phase by suitable heat treatment methods.
Work in progress is directed at understanding the phase de
velopment and maximizing the high Tc phase by careful con
trol of composition and processing conditions.
The authors gratefully acknowledge NSF-DMR~12g60
(AA, GCR, DJV) and U.S. DOE DMR DE-AC02-
76EROl198 (PDH, LMF, DAP) for support of this re
search. We also wish to express our thanks to J. C. Grindley
and P. T. McGuire for their invaluable assistance with plas
ma spray equipment.
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9M. A. Subramaniam, C. C. Torardi. J. C. Calabrese, J. Gopalakrislman,
K. J. Morrissey, T. R. Askew, R. B. Flippen, U. Chowdhry, and A. W.
Sleight, Science 239. 10 l5 (1988).
oJ. H. Kang, R. T. Kampwirth, K. E. Gray, S, Marsh, and E. A. Huff,
Phys. Lett. A 211, 102 ( 1988).
"P. E. D. Morgan, J. J. Ratto, R. M. Housley. and J. R. Porter, presented at
the MRS Spring Meeting, Reno, NV, April 1988.
Asthana et al 801
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155.33.16.124 On: Sun, 30 Nov 2014 14:12:21 |
1.38832.pdf | AIP Conference Proceedings 194, 123 (1989); https://doi.org/10.1063/1.38832 194, 123
© 1989 American Institute of Physics.Specific heat of liquid 3He
bubbles in solid matrices
Cite as: AIP Conference Proceedings 194, 123 (1989); https://
doi.org/10.1063/1.38832
Published Online: 16 June 2008
E. Syskakis , Y. Fujii , M. Gebhardt , and F. Pobell
123
SPECIFIC HEAT OF LIQUID 3He BUBBLES IN SOLID MATRICES
t
E. Syskakis, Y. Fujii , M. Gebhardt, and F. Pobell
Phys. Inst., Universit~t Bayreuth, D-8580 Bayreuth, FRG
ABSTRACT
We report on specific heat measurements of liquid 3He bubbles in Ag
foils as well as in a matrix of solid 4He at millikelvin
temperatures. - For the first part we have implanted about o.I % 3He
in Ag foils. The foils have been annealed between 900 K and 1112 K.
This results in high-pressure 3He gas bubbles of diameters between
about 40 A and Ii0 A; the 3He liquifies when the metal foils are
cooled to low temperatures. After each annealing step we have
measured the thermal relaxation time of the foils at 13 mK ~ T ~ 1.2
K, and have determined in this way the specific heat of the 3He
bubbles; it differs from the specific heat of bulk liquid 3He. - In
the second set of experiments we have measured the specific heat of
a o.75 ~ 3He-4He mixture at 25.3 bar (27.1 bar) and 20 mK g T~ 300
mK; simultaneously we have measured the pressure of this mixture.
Our data indicate that at the lower (higher) pressure there are
bubbles of a liquid mixture (of liquid 3He) in a solid 4He matrix.
Permanent address: Okayama Univ. of Science, Okayama, Japan
@ 1989 American Institute of Physics
124
INTRODUCTION
The influence of dimensionality and of finite size on magnetism,
superconductivity, superfluidity, or other ordered states is of
fundamental importance in physics. In the studies of size effects,
helium has played an outstanding role because of its homogeneity and
purity, and because of the advanced state of low temperature
thermometry. One can study phase transitions of liquid and solid
helium with a temperature stability, resolution and homogeneity much
better than for any other material. In addition, samples of helium
can be produced in almost any size, and their quantum behavior adds
additional interest. Therefore, the most detailed studies of the
influence of finite size and of dimensionality on phase transitions
1 have been done using in particular liquid helium. The experiments
have been performed on thin helium films or on helium confined to
the pores of compressed powders or porous glasses. The measured
properties have been mostly the specific heat, the superfluid
density, or the onset to superflow.
Even though these studies have profoundly influenced, for example,
our understanding of the superfluid state as well as the influence
of a restricted geometry on phase transitions in general, there are
severe shortcomings of many of them. The reason being that the
confining geometry of porous systems usually is a tangled,
interconnected, often insufficiently known structure with a
distribution of sizes (Fig. la). These not well-defined geometries
can influence the experimental results and sometimes have made their
interpretation difficult. In this paper we discuss investigations of
size effects on liquid and solid helium at low temperatures in
alternative, well defined confining geometries: microscopic,
isolated, helium filled bubbles in metal foils or in a matrix of hcp
4He2,3
125
b.
Fig. I. a) Typical structure of a porous glass, one of the favorite
matrices for studies on liquid helium in finite size
geometries
b) TEM picture of underfocused helium bubbles in Cu. The
bubbles appear as light areas and are clearly faceted
(from Ref. 2,3).
MICROSCOPIC HELIUM BUBBLES IN METALS
The samples for these studies are produced by shooting 4He or 3He
ions with a cyclotron into metal foils where the atoms come to
2 rest. We wobble the ion beam and degrade its energy periodically
from 0 to E (some MeV) to get a homogeneous distribution of about max
o.I at % He in typically o.I mm thick metal foils of io mm diameter.
-3 The solubility of He in metals is extremely low, typically Io at
ppm. By annealing the He doped metal, the He precipitates in gas
bubbles whose pressure and size can be adjusted by the annealing
temperature. The resulting sample can be examined by transmission
electron microscopy; we find isolated He bubbles which are not
interconnected (see Fig. ib).
126
The He pressure P in a spherical bubble of radius r at thermal
equilibrium is given by P = 2 • ~metal/r (~metal = surface tension
of the metal, 1.17 J/m 2 for Ag). This results in a helium density of
order o. 1 g/cm 3 for a typical bubble radius of 5o A (or about lo 4
atoms). Such a density corresponds to the low temperature density of
solid or liquid helium, which means that we obtain bubbles filled
with solid or liquid helium when we cool the sample to low
temperatures. The advantages of this geometry are that the
microscopic systems are isolated from each other and not
interconnected, it allows a simple variation of size, and we can
study it visually by electron microscopy. Unfortunately, we still
have a size distribution (see Fig. 2} and, as a new disadvantage,
the pressure or density in the bubbles change when we change their
size. - In Fig. 2 we show the mean bubble radius r of 4He in Cu and
of 3He in Ag, respectively, as a function of annealing temperature
T . For the latter combination r seems to be roughly 15 % smaller at a
the same T . a
12
10
(/)
< 8 p,,.
I.¢J
-J
CO
z <c
MJ s- I, i ! I
~0 1000 1100 1200
ANNEALING TEMPERATURE (K)
Fig. 2. Average radius of 4He - @ (3He - m)bubbles in Cu (Ag) for
isochronal annealing experiments. The samples were annealed to
progressively higher temperatures Ta (ta = 2 h). The bars show
typical half-widths of the corresponding size distributions.
127
In our former studies 2 we had measured the heat capacity of about 1
Hg of 4He confined to bubbles in Cu foils (7 mm diam., o.I mm
thickness) in a sensitive microcalorimeter at 1.5 K E T E 7 K after
annealing the samples at temperatures between 930 K and 1220 K,
resulting in 4He bubbles of 25 A to llO A diameter. By these
experiments we could investigate the influence of this new confining
geometry on the superfluid transition of 4He. In this paper we
present our new results on the heat capacity of 3He filled bubbles
in Ag foils at 13 mK _c T _c 1.2 K.
SPECIFIC HEAT OF MICROSCOPIC LIQUID 3He BUBBLES IN SILVER
The measurements were performed in a relaxation microcalorimeter
consisting of a o.5o g Ag sample holder with three different carbon
layers (o.8 mg) as thermometers, and about 6 mg further addenda(see
Fig. 3). Support of the calorimeter is by four o.I mm nylon threads
and the electrical leads are I0 Nm NbTi wires. The thermal link to
the dilution refrigerator are two Ag wires (50 Mm diam., 12 cm
length) with a thermal resistance R = 5.2 • lo5/T (K/W). Silver with
its nuclear spin I = I/2 was used for the calorimeter and for the
samples to avoid possible nuclear quadrupole contributions to the
specific heat. The heat capacity C of the samples were obtained by
observing the exponential decay of their temperature after applying
a heat pulse and by calculating C = T/R from the measured thermal
time constant T.
Our first samples were Ag foils (9 mm diam., o.25 mm thick) into
which about o.2 Z 3He were shooted with an energy up to E = 36 max
MeV. When we tried to measure their heat capacity, it turned out
that the samples showed a "large" heat leak of about 5 nW resulting
from radioactivity induced by the 3He implantation. We could not
cool the samples below T = 6o mK. We then produced a second set of
samples by implanting 3He ions with an energy up to only E = 12 max
MeV, which substantially reduced the heat leak. These Ag foils had a
9.5 mm diameter and were 58 Hm thick with an implanted depth of 42
128
~m. The II foils used for calorimetry had a Ag mass of mAg = 0.47 g
or 4.4 mmole. The implanted volume (3.2 mmoles Ag) contained about
o.I % 3He, giving about 3 pmoles or about 9 ~g of 3He.
Fig. 3 shows the measured thermal relaxation times T of our
calorimeter, of the Ag foils without 3He, of the Ag foils implanted
with 3He, of these foils after annealing them for 2 h at the
indicated temperatures, as well as calculated relaxation times.
The data show that we can measure the tiny heat capacity of our
calorimeter and foils, and that the data agree reasonably well with
the specific heat of Ag at o.I K ~ T ~ 1 K. We do not have an
obvious explanation for the increase of T~ C/T of our calorimeter
and bare foils at T z o.i K, because there is no nuclear quadrupole
interaction in Ag; possibly it results from the addenda or from
impurities in the Ag. The data for the foils plus 3He annealed at T a
g 9oo K show an extra contribution from the 3He below o.I K, which
is independent at T . But after annealing the foils at T ~ 940 K we a a
see a contribution from the 3He at all T which increases with T .
-i a Because R ~ T we should have T = constant if C ~ T; this is
clearly not the case for our 3He data.
In Fig. 4 we have plotted the heat capacity C of the 3He bubbles
(calculated from the data in Fig. 3) and compared them to the
specific heat of 3 ~moles of bulk 3He at SVP. 4 For T = ii12 K, for a
example, the data at o.I g T g 1K coincide with the specific heat
of bulk liquid 3He if we shift the Fermi temperature of the liquid
in the bubbles to T F = 1.2 K (whereas TFbUlk'SVP = 1.8 K) and
reduce the amount of 3He from 3 ~moles to about half this value.
Already in our former measurements on 4He in Cu, we had found that
the sample showed only about half the heat capacity of the
corresponding amount of bulk helium after annealing at Ii00 K. But
there is clearly an extra contribution at T g o.I K for the data at
T = 1112 K as well as for the data at the other T . a a
129
-\
30
25
o
v
20
.n .$.l
c
0
x m15
o
m £
ilo
I- i I I T T i I I i ~ , , ~ 11[ I
, -- catorimeter
. -- catorlmeter • Ag foit
• -- Tq = 750K
...... 900 K
........ 9kO K
..... 960 K
...... 976 K
'~ ..... 1003 K
...... 1033 K
...... 1069 K
...... 1112 K
4 ...........................
' ' ' '''"' 260 ....... ' 20 50 100 500 1000
T (inK)
Fig. 3. Thermal relaxation time T = R s" C as a function of
temperature with R = 5.2 Io /T (K/W), the thermal
resistance to our calorimeter, and C
a) heat capacity of the calorimeter
b) heat caRacity of the calorimeter plus Ii Ag foils
(without 3He)
c) specific heat of the calorimeter plus Ii Ag foils
implanted with 3He, after annealing the Ag foils for 2 h
at the indicated temperatures.
The dashed lines are calculated values for the
calorimeter and calorimeter plus Ag foils,
respectively.
130
1(: t I ~ t t t I t I t ( i I t t I ( I
GreywaLt, 3 x 10 -6 mole..."
P= 0 bor --,. ""
o....----*
• " ""~" ,./-"'"'"'"'" ..... ~o.~%%~~/
_ ,,'""'~ /
;/
0.1 I I I ' I , Ill / , I J I , ,ll
10 20 50 100 200 500 1000
T (mK)
Fig. 4. Heat capacity of 3He bubbles in Ag calculated from the data
of Fig. 3 for the given annealing temperatures. The data are
compared to the specific heat of 3 8moles bulk 3He at SVP
(from Ref. 4).
For T z o.I K we discuss the data in terms of the "layer model"
usually applied to analyze data on helium in restricted geometries,
1 particularly helium films. The first layer of helium near a metal
wall is a two-dimensional solid at P ~ 400 bar, I'5 giving negligible
lattice and magnetic contributions to the total specific heat of the
helium bubbles in the investigated T-range. 5'6 In Ref. 6 it was
shown that the second layer of 3He on a Ag substrate shows a
constant specific heat C 2 = 0.2 N2k B at o.3 mK g T g 7 mK. Higher
layers behave like bulk liquid 3He. For our analysis we make the
crude approximation that the first layer of d I ~ 3.0 A does not
contribute to our data, the second layer of d 2 ~ 3.5 A gives the
131
result of Ref. 6, C 2 = 0.2 R, also at temperatures of 13 mK g T g
loo mK(!), and that the remaining liquid in the center of the
bubbles have the specific heat of bulk liquid 3He at SVP. 4 With
these crude assumptions we have calculated the specific heat for 2.2
pmoles 3He in bubbles of 5o A radius (corresponding to T ~ 11oo K) a
and plotted the result as C/T together with our measured data in
Fig. 5; the agreement seems to be remarkably good for this crude
model. - The dip near T = 0.2 K in the data at T = 94o K may result a
from melting of solid 3He in small bubbles still present at this low
annealing temperature. - More data, in particular from our planned
magnetic measurements are necessary for a more detailed discussion.
4O
v
I-- 60 ..... I I I I I ! I I a=1112K",
940 K
_
010 I 1 I I I I I 50 100
T (mK)
Fig. 5. Heat capacity C divided by temperature T of 3He bubbles in
Ag calculated from the data of Fig. 3 for the given
annealing temperatures. The broken line are the values for
2.2 pmoles 3He in r = 50 A bubbles as calculated with the
model described in the text.
132
SPECIFIC HEAT OF LIQUID HELIUM BUBBLES IN SOLID HCP 4He
Liquid as well as solid 3He-4He mixtures phase-separate when cooled
to millikelvin temperatures. 3He and 4He have different melting
pressures. Therefore there exists a pressure range between about 25
bar and about 28 bar where hcp 4He coexist with liquid 3He or liquid
3He-4He mixtures depending on pressure. Many studies of the
7 complicated phase diagram at these pressures have been published.
In Refs. 8 and 9 it was shown that with appropriate experimental
conditions, one may succeed in creating liquid helium bubbles in hcp
4He. This was particularly obvious from the specific heat
measurement at Grenoble which showed Fermi liquid behavior C = ~ T
below phase separation, but with a coefficient ~ larger than the
bulk 3He value. Unfortunately, the pressure was only approximately
known for this measurement. We have performed a first set of
measurements of the specific heat of a o.75 % mixture at 20 mK z T z
3oo mK, and at ig.o bar, Z5.3 bar, and 27.1 bar, respectively. The
3 calorimeter contained a liquid volume of 1.9 cm , was equipped with
carbon thermometers, and linked to a dilution refrigerator by a
superconducting heat switch. The measurements are at constant volume
because a plug was formed in the fill capillary during cooldown
under pressure.
At 18. o bar (not shown) we see a Fermi liquid specific heat linear
in T for T z 60 mK and temperature independent at 6o mKx T z 180 mK
indicating that our whole mixture is in the liquid state. For P =
27.1 bar we find a linear behavior, C = 0.03 T (H/K), for T z 80 mK
and a strong signature of phase separation at 8o mK z T z 220 mK
(see Fig. 6a). This result is expected if the mixture separates
almost completely into liquid 3He and hcp solid 4He. If the liquid
phase would remain pure 3He after we reduce the pressure from 27.1
bar to 25.3 bar, we should observe a decrease of the specific heat
by a few percent. Instead we see a dramatic increase to C = o. o7 T
(J/K) for T z 6o mK, then a slight flattening off, and eventually an
onset to phase separation at T ~ 2oo mK (see Fig. 6a). This large
linear specific heat indicates that the liquid phase at 25.3 bar is
a mixture containing probably about 8 % 3He.
133
The drastic change of the behavior of the liquid phase in an hcp 4He
matrix by changing the pressure by less than two bar is even more
obvlous from the "excess" pressures at phase separation (see Fig.
6b). Actually, the behavior at the pressure of 25.3 bar may lndlcate
that the sample was in a three-phase region consisting of hcp 4He, a
liquid mlxture phase, and possibly some pure liquid 3He. - From the
behavlor of our samples - in particular the fast response to
[J .01
• 001
20 a. ~m
50 100 200
T [rrt<]
27.2
c,.
27.0
359 Im m
bo
' ' ' ' ' ' ' I
50 i00
T [~] W i
m ~7
25.6
%. ~ 25.5
25.4
25.3 2OO 2O e
e
o
e ~ 2
e
e
e ¢
e • ~
o
5O I O0 2O0
T [mK] I
J
Fig. 6. a) Heat capacity at constant volume of 1.9 cm 3 of a 0.75 %
3He-4He mixture at about 25.3 bar (8) and at about 27.1
bar (~), respectively
b) Pressure in the closed sample cell during the specific
heat measurements of which the data are shown in the
upper part of the figure (for details see text).
134
temperature changes - we conclude that liquid bubbles are
distributed in the hcp 4He matrix and that the two phases are not
totally separated in space. Of course, much more studies are
necessary to understand the behavior of this interesting system.
CONCLUDING REMARKS
The discussed data demonstrate the remaining problems but also the
possibilities of the two investigated confining geometries to
understand the influence of size effects on the Fermi properties of
liquid 3He.
There is the unsolved question of the extra contribution to the
specific heat of microscopic liquid 3He bubbles in Ag which may
result from the second layer, and which we hope to understand better
when the results from our planned magnetic measurements on this
system are available. But the size distribution of bubbles, the
dependence of the helium pressure on the radius of the bubbles, and
different contributions from different 3He layers close to the
substrate may make quantitative interpretation of experimental
results difficult. - There is also the question of the possible
existence of superfluidity of 3He in this restricted geometry,
possibly of another symmetry than for the bulk superfluid states. We
have to remember that the typical 3He bubble radii in a metal matrix
are smaller than the coherence length of the bulk superfluid state
of 3He.
The measurements on liquid helium bubbles in hcp 4He will first be
extended to more pressures to understand the phase diagram and
eventually we are interested in the bubble size and bubble
formation, and the dynamics of phase separation. The advantage of
this system is the fact that the surface tension of hcp 4He is much
smaller than the surface tension of metals. Hence in principle one
has access to substantially smaller liquid bubbles. Unfortunately,
very little is known about the dynamics, size, and isotopic
concentration of the bubbles in this system.
135
ACKNOWLEDGEMENT
We gratefully acknowledge performance of the 3He implantation as
well as the TEM measurements by Dr. P. Jung, Dr. H. SchrSder and
Prof. H. Ullmair (KFA JOlich). - This work was partly supported by
the Deutsche Forschungsgemeinschaft.
REFERENCES
I.
2.
3.
4.
5.
6.
7.
8.
9. D.F. Brewer,J. Low Temp. Physics 3, 2o5 (1970); and in "The
Physics of Liquid and Solid Helium", Part II, p. 573, ed. K.H.
Bennemann and J.B. Ketterson; J. Wiley and Sons, New York
(1978).
E.G. Syskakis, F. Pobell, and H. Ullmaier, Phys. Rev. Lett.
55, 2964 (1985); E.G. Syskakis, Ph.D. Thesis, KFA J~lich,
Report JOL - 2012 (1985).
A preliminary report of this work is given in E. Syskakis, M.
Gebhard, and F. Pobell, Proc. Int. Conf. on Polarized Quantum
systems, Torino, June 1988.
D.S. Greywall, Phys. Rev. B 27, 2747 (1983).
D.F. Brewer, A. Evenson, and A.L. Thomson, J. of Low Temp.
Phys. 3, 603 (1970).
D.S. Greywall and P.A. Busch, Phys. Rev. Lett. 60, 1860
(1988).
P.M. Tedrow and D.M. Lee, Phys. Rev. 181, 399 (1969); V.L.
Vvedenskii, JETP Lett. 24, 132 (1976); B. v.d. Brandt, W.
Griffioen, G. Frossati, H.V. Beelen and R. de Bruyn Ouboter,
Physica II4B, 295 (1982); V.N. Lopatnik, Sov. Phys. JETP 59,
284 (1984); D.O. Edwards and S. Balibar,Phys.Rev.B39,4083(1989)
A.S. Greenberg, W.C. Thomlinson, and R.C. Richardson, J. Low
Temp. Phys 8, 3 (1972).
B. Hebral, A.S. Greenberg, M.T. Beal-Monod, M. Papoular, G.
Frossati, H. Godfrin, and D. Thoulouze, Phys. Rev. Lett. 46,
42 (1981).
|
1.345721.pdf | Measurement of ion energy distributions at the powered rf electrode in a variable
magnetic field
A. D. Kuypers and H. J. Hopman
Citation: Journal of Applied Physics 67, 1229 (1990); doi: 10.1063/1.345721
View online: http://dx.doi.org/10.1063/1.345721
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/67/3?ver=pdfcov
Published by the AIP Publishing
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33Measurement of ion energy distributions at the powered rf electrode
in a variable magnetic field
A. D. Kuypers and H. J. Hopman
FOM Institute/or Atomic and Molecular Physics, Kruislaan 407, NL-1098 SJ Amsterdam,
The Netherlands
(Received 10 August 1989; accepted for publication 23 October 1989)
High-resolution energy distributions of ions, accelerated by the sheath at the powered electrode
of a low-pressure 13.56-MHz gas discharge, have been measured. The observed spectra are
compared to existing models. Excellent agreement between measured and calculated spectra is
obtained. Detailed information on rf sheath behavior is derived from the observed energy
profiles and from the measured total ion current densities towards the electrode surface.
Analogous to the case of dc discharges, a decrease of sheath thickness is observed when a
homogeneous variable magnetic field (O<B<315 G) is applied. However, the product of
magnetic-field strength B and sheath thickness d is found to be independent of sheath voltage.
This leads to the conclusion that in rf discharges, sheath contraction under influence of a
magnetic field proceeds by a different mechanism than in de discharges. It is suggested that the
value ofthe product Ed is determined by the (virtually constant) temperature of the plasma
electrons, rather than by the energy of secondary electrons that have been liberated from the
electrode surface by ion bombardment. The decrease of sheath thickness d with magnetic-field
strength B leads to a changing capacitive-voltage division of the applied generator voltage over
the discharge. When the magnetic-field strength is sufficiently high, this may result in a sign
reversal of the electrode self-bias voltage.
I. INTRODUCTION
A. Motivation of this study
The use of high-frequency discharges for surface modifi
cation of semiconductor materials is still a relatively new
field. The development of micron-and submicron-scale elec
tronic circuits, where the demand for improved pattern de
finition implied the need for highly anisotropic etching pro
cesses, has led to a rapid development of this technique. 1.2
Reactive-ion etching combines the selectivity of chemi
cal processes with the anisotropy of ion and electron bom
bardment of the surface. From beam experiments it is known
that the energetic particles can influence gas-surface reac
tions in several ways. }"ossibilities are, for example, the cre
ation of active surface sites by sputtering, the supply of a
threshold energy for the chemical reaction, or the removal of
reaction products from the surface.' The plasma etching
process is generally a complicated (and for most cases unre
solved) combination of such mechanisms. However, it is
clear that energetic ion bombardment plays an important
role in the etch behavior.
The energy of the ions is largely determined by the de
voltage difference between the plasma and the substrate. In
the case of capacitiveiy coupled rf discharges, negative sub
strate potentials of several hundreds of volts arc typical. 2
These high values give rise to substantial radiation damage
in the substrate surface, deteriorating electrical properties of
underlying layers and contact surfaces.4-6 Therefore it is im
portant to have better control of the ion energies and to be
able to measure them, in order to study how ion energy is
related to etch rate, substrate damage, anisotropy, and selec
tivity.
This article describes the use of a dedicated energy ana-lyzer at the powered rf electrode to analyze the energy distri
butions of ions, which have been accelerated by the sheath
potential. In addition, the sheath potential can be varied by
application of a homogeneous magnetic field. After an intro
duction, where the experimental apparatus is presented,
some necessary theoretical background is given before the
experimental results are shown. The theory consists of two
parts. In the first part, a model is presented to calculate the
energy distributions of ions after their acceleration by an rf
modulated sheath potential. In the second part, it is shown
how the sheath potentials are related to the electrode vol
tages. Then the model is tested, and a comparison between
measured and calculated spectra is made. Having estab
lished the validity of the model describing the energy spec
tra, this model will be used to extract detailed information
about sheath potentials and ion flux from the measured data.
Experimental observations of sheath behavior, both with
and without application of a variable, homogeneous magnet
ic field, are reported and discussed.
B. rf sheath generation
When the rf power supply of a discharge is coupled to
the electrodes in series with a capacitor, a large dc electrode
voltage develops in addition to the applied generator voltage.
This effect is referred to as self-biasing. Self-bias is caused by
the difference in mobility between electrons and Ions. Ions
are too heavy to respond to an electric field that is oscillating
at rf frequencies, while the electrons are able to follow the
field fluctuations and thereby oscillate in energy. In the case
considered here (generator frequency (j) = 2trX 13.56X 106
1(1), the plasma is operated in a regime where (t);<{J) <We
(with (I.), and w" representing the ion and electron plasma
1229 J. Appl. Phys. 67 (3),1 February 1990 0021-8979/90/031229-12$03.00 (~) 1990 American Institute of Physics 1229
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
131.170.6.51 On: Sun, 17 Aug 2014 23:15:33frequency, respectively). Because of their higher velocity,
the electrons will tend to leave the discharge much faster
than the ions. This causes an excess of negative charge on the
electrode surfaces, giving rise to a negative dc offset voltage.
In the case of capacitively coupled discharges no net direct
current can flow through the circuit, so the total electron
and ion currents toward the electrode must cancel. There
fore, an equilibrium will be reached, where positive ions are
almost continuously being accelerated towards the powered
electrode by the negative self-bias potential. On the other
hand, electrons are repelled by this potential. Only during a
short fraction of an rf period will the sum of de and rf poten
tial be close to zero, such that electrons can reach the elec
trode.
When a magnetic field of a few hundred gauss is applied,
parallel to the electrode surface, the mobility of the electrons
in radial direction will be decreased by their Larmor preces~
sion, while the ions are virtually unaffected by the magnetic
field. This means that, due to the magnetic field, current
equilibrium will take place at smaller self~bias voltages.7•8
Therefore, variation of the strength of such a magnetic
field gives control over the potential difference between plas~
ma and electrode, which in turn determines the energy of the
ions hitting the surface.
II. EXPERIMENT
A. The cylindrical magnetron reactor
The plasma chamber used in this experiment is shown
schematically in Fig. 1. It consists of two coaxial aluminum
cylinders, 30 em long and of 10-and 20-cm radius, respec~
tively. Two opposing sides of the inner cylinder are flattened
so that on each surface a 3-in. wafer can be mounted vertical
ly. The outer cylinder is grounded, the inner is capacitive1y
coupled by a matching network to a 5-kW rf source of 13.56
MHz. Gas discharge takes place between the two cylinders.
The plasma chamber is pumped to an operating pressure of
Coils
Wafer
Powered electrode
FrG. 1. Schematic cross section of cylindrical discharge geometry with
magnetic Ilelds (cylinder axes are horizontal in the figure) .
1230 J. Appl. Phys., Vol. 67, No.3, 1 February 1990 typically several mTorr, while the volume inside the inner
electrode is differentially pumped down to 10-6 Torr.
A variable magnetic field is generated along the cylin
drical axis by two sets of coils in a Helmholtz configuration.
Field strength can be varied from 0 to 315 G.
The combination of the radial electric field Err with an
axial magnetic field B causes a Larmor precession of the
charged particles in the discharge. This prevents the elec
trons from moving directly to the electrodes, as they would
when only the rf field were present. Thus, the lifetime of
these electrons is enhanced considerably, along with their
ability to ionize the etch gas. This effect of the magnetic field
on plasma density and etch rates has been described in a
previous articleY
In addition to this homogeneous variable magnetic
field, a multi pole field along the surface of the grounded
electrode is generated by permanent magnets. Its construc
tion and consequences have been described previously.9 The
point of relevance to the work discussed here is primarily
that it results in a higher plasma density. In addition, it may
modify the sheath properties at the grounded electrode.
However, under most conditions the sheath potentials there
will be low compared to those at the powered electrode.
B. The parallel~plate energy analyzer
Through a hole in the substrate surface, that is, at the
powered rf electrode, incident ions are collected for direct
energy analysis by an analyzer that has been mounted inside
the inner cylinder. 10 Although additional information on ion
mass is desirable, an electrostatic parallel-plate analyzer was
chosen because of the complicating axial magnetic field. The
analyzer could not be screened against this field, because the
use of mu metal or compensating B fields would directly
influence the orientation of the field lines in the plasma itself.
This would cause unacceptable nonuniformities both in the
discharge and at the substrate surface. The consequences of
the magnetic field for the interpretation of the measured data
are discussed below. Screening against disturbing electric
fields is provided by the construction of the electrode itself:
the closed inner cylinder acts as a Faraday cage, and the
analyzer is at the same potential as the electrode.
In order to control the voltage applied to the plates and
to measure the ion current, a connection from the powered
electrode to ground had to be provided. For this purpose
optical fiber coupling was chosen because it made electrical
filtering against the rf and de electrode voltages unnecessary.
A more detailed description of the analyzer setup has been
given elsewhere. 10
The applied axial magnetic field strength is varied from
o to 315 G. Combined with the fact that the voltage differ
ence Ve between plasma and wall under normal conditions is
limited to about 500 V, Larmor radii rL of typically a meter
or less are obtained in the case of singly charged Ar. There~
fore, the influence of the applied magnetic field on the ion
trajectories through the analyzer has to be taken into ac
count. (The path length of the ion trajectory through the
analyzer is in the order of 10 cm.) To be able to relate the
field strength between the analyzer plates to the actual kinet
ic energy ofthe ions being transmitted, an analytical expres-
A. D. Kuypers and H. J. Hopman 1230
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33sion has been derived for the ion trajectory through the ana
lyzerl! as a function of ion mass Iv.!, magnetic field B, and
electric field strength E between the analyzer plates. Using
this expression, the measured values of E are converted to
absolute ion energies by a personal computer.
m. THEORY
A. Model for the ion energy distribution
Most of the work that has been done on the analysis of
ions reaching the electrode surfaces of an rf discharge has
been performed at the grounded electrode. The first attempt
to model the ion acceleration in an rf sheath was made in
order to explain the unexpected behavior of ions extracted
from a Thoneman rfion source. 12 The energy spread of the
ion beams was one or two orders higher than thermal, and
mean ion energies several hundreds of volts higher than the
extraction voltage were detected, 13 The higher mean energy
was attributed to the large de sheath potentials developed in
rf discharges. Theoretical work by several authors 14-16
showed that in addition to the dc accelerating term, the ions
are also sensitive to the rf modulation of the sheath potential.
The time it takes an ion to cross the sheath is of the same
order as an oscillation of the rf field. Therefore, the final
energy of the ion will be determined by the phase of the field
at the moment that it entered the sheath. This causes a
broadening of the ion energy distribution. Assuming a sinu~
::;oidal time dependence of the sheath potential V, the total
width !1E of the energy profiles was shown 14 to be given by
!1E= (8eAVe/3wd)(2eV,jM)!l2. Here Vc' d, M, and (u
represent time-averaged sheath voltage, sheath thickness,
ion mass, and the angular frequency of the rf field, respec
tively. (The parameter J., describing the relative magnitude
of the rf and dc components of the electric field, will be dis~
cussed below.) This theoretical result has been confirmed
experimentally in rf glow discharges at the grounded elec
trode by other authors. 17-21 However, the assumptions un
derlying these models were not self-consistent,22 Recently,
the model has been improved by Vallinga and Meijer.22•23 It
will be used here to interpret the measurements.
The following assumptions are made in this model:
( 1) The ion acceleration is predominantly determined
by the time-averaged sheath potential, and the rf contribu
tion can be considered as a perturbation.
(2) The ion sheath thickness is constant in time.
(3) Free falI of ions through the sheath, Le., the ion
mean free path I> d.
( 4-) Contribution of electrons to the total space charge
in the sheath can be neglected.
(5) The number of ions entering the sheath is constant
in time.
( 6) The initial velocity of ions entering the sheath can be
neglected.
(7) The ion transit time r across the sheath is approxi
mately constant, Le., independent ofthe phase of the electric
field upon entering the sheath.
( 8) The sheath potential can be approximated by
vex,t) = Ve [1 + A sin ((ut) H (x/d)n -1], (l)
1231 J. App!. Phys., Vol, 67, No.3, 1 February 1990 where x denotes the distance perpendicular to the electrode
surface, and J. and n represent parameters that will be dis
cussed below. Under these assumptions, the equation of ion
motion was solved analytically, and the following relation
for the ion energy at the electrode was obtained:
(2)
where to and t[ are the moments of entering the sheath and
reaching the electrode surface, respectively. Thus the theo~
retical broadening of the energy distribution is given by
!1E=Emax -Emin =4a[An(eV,,)3/2/ wdy2M], (3)
where a = maxlsinmt, -sin lVtol. Under assumption 7, giv
en above, the ion transit time T = t I -to is constant. Then,
a<2lsin (wr/2) I. When it takes an ion several rffield oscil
lations to cross the sheath (1""> 21T/W), it will be assumed 23
that, on the average, a = 1.
As a final result, the ion energy distribution is given byl4
(4)
for (eV. -tl.E/2)<E«eV,. +flE/2)andF:CE) = o else
where. Here No represents the number of ions entering the
sheath per unit time. The profiles described by Eq. (4) are
symmetric around the mean energy value E = eVe. An ex
ample of a profile as described by Eq. (4) is given in Fig. 2 for
a typical choice of parameters. The applicability of the as
sumptions 1 to 8, given above, to the discharge under consi~
deration will be discussed below. However, further assump~
dons have to be made about the constants A and n in Eqs.
(1)-(3).
B. Model for parameter A
The constantA determines the relative magnitude of the
dc and the rf component of the sheath voltage. The sheath
potential Ve can be measured directly, but A has to be esti-
Ion energy reV]
FIG. 2. Energy profile, calculated from Eq. (6) for a typical choice of pa~
rameters (compare with measured spectrum in Fig. 5).
A. D. Kuypers and H. J. Hopman 1231
••••••••••• ".-.-••• -••• ;.:.;.;.; ••• ;.; ••••••••••••••••••••••••••••••••••• " •••••••••••••• -;. ••••••••••••••••••••••••• , •• ,.,. .................. '.'.' ••••• :.~.;.:.;.:-;.:-: ••••••••••••••••••• <; •••• ~ ••• :.-••• ---. ••• -.-.-••• , •• , ••• ~ ••••••••••••• _.'O;'._" ••••• ; ... " .. ;:.~.:.:.:-; • .-.~.: •••••••• ~ •• ; .......................... '..-o:o:.;.:.:-;.;.:.-l': •••••••••••••• ~ ••••••• O;O;O;'7.-.;" ••••••• -.. •••• -.-••• :.-.·.·.·.·.·.·.·.·.·.·.·.v.v.·.".·."'-.-.·.-.- ••• --.;-.-;o, •• " ••••••• .-.>;>;~ •• ? •••••• "' •• = ........ -... -.~ .. ;:.;.:.,o;.-.••• , •. .-..;.-.< ••••••• "' •••••••• --; ••••.•.•. ..-.-•.• -
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33mated. To do this, a model will be used here, as presented by
Keller and Pennebaker.24 Assume that the sheath potential
is given by V = Ve + A Ve sin (wt) [Eq. (1) J. Then, if the
electrons have a Maxwell-Boltzmann energy distribution of
temperature Te, the time-averaged electron current density
(Ie) t through the sheath is given by25
(JJ, =J,?fJo(eAVjkTe), (5)
where J<f,) is the current density which would be drawn if
A=O,
(6)
and 10 is the zeroth-order modified Bessel function of the
first kind. In the steady-state situation, the time-averaged
ion and electron currents across the sheath must cancel.
Thus, the average electron current density must be equal to
the ion current density Ji• Together with Eqs. (5) and (6),
this gives
exp (-eVjkT e) = JesaJo(eJ.VjkTe)IJj' (7)
When A = 0, this equation reduces to the equation for the
floating potential Vf' as will be discussed below. Defining
(8)
the change in dc potential due to the presence of the rf vol
tage, .1 Ve, is given by24
(9)
which for eA VelkTe > 1 reduces t024
.1 Vjlc v" = -1 + (kT,J2e.tlVe)ln(21TeAVelkTe).
( 10)
For a typical electron temperature Tc:::::3 eV and ion tem
perature Ti::::: 0.04 e V (room temperature), the floating po
tentia126 V'r is in the order of 10 to 20 V. On the other hand,
the observed sheath potentials Ve at the powered electrode
are typically in the order of a few hundred volts. Thus,
Vf"~ Ve, and it follows from Eq. (8) that Ve::::: Ii Ve' Physical
ly speaking, it means that the dc component of the sheath
potential is determined primarily by the rf-induced term.
Further, for this kind of large value for Vo it follows from
Eq. (10) that Ii Ve::::: -AVe' It is concluded that, as long as
v" > v,., A::::: -1. However, in the case of small sheath vol
tages, combination of Eqs. (8) and (10) shows that A is
given by
(11)
To interpret the measurements, this model has to be
extended with a relation between the sheath potential Vand
the applied generator voltage Va at the powered electrode.
For this purpose, a sheath model developed by Kohler27 will
be used here. Assume that the electron current in the sheath
can be divided into a dc and an rf term:
JeU) = (Ie), +Jd(wt) =Jj +Jd sinew:). (12)
(As above, the fact that the average electron current density
(Je ) , is equal to the ion current density Jj has been used
here). Jd stands for the displacement current density, asso
ciated with the oscillatory electron movement in the rffield.
When Ji <t.Jd, the sheath essentially acts as a capacitor. In
1232 J. Appl. Phys., Vol. 67, No.3, 1 February 1990 Capacitive model
Cp Cg
rj_ _~r
FlG. 3. Equivalent electrical circuit for the discharge, where the sheaths are
assumed to be purely capacitive, and the plasma bulk is assumed to be per
fectly conducting. As a result, the potential of the plasma is equal to the
potential drop across Cg,
that case, the discharge can be modeled by an equivalent
electrical circuit, where the electrode sheaths are represent
ed by capacitors (Fig. 3).
C. Capacitive sheath approximation
Let Cp and Cg be the capacitances of the sheaths at the
powered and the grounded electrode, respectively (Fig. 3).
The plasma bulk is considered to be a perfect conductor with
zero resistance. The potential difference between the
powered and the grounded electrode is capacitively divided
over both sheaths. Consequently, when the applied gener
ator voltage is given by
Va et) = Vdc + Vrr sin(wt), (13)
the plasma potential Vp (t) will also show a purely sinusoidal
behavior:
Vp (I) = V pelc + Vprf sin (M). (14)
(See Fig. 4.) The sheath potential at the grounded electrode
is then given by the voltage drop over Cg, which is just Vp (t).
One implication of Eq. (11) is that the plasma always
exceeds the electrode potentials by at least an amount Vf.
i +--o T
Time (arb. units)
FIG. 4. Applied electrode potential V" (t) and plasma potential Vp (t) rela
tive to ground, according to the capacitive sheath model.
Va·= Vdc + V,r sin({ut), and Vp(t) = V""c + Vpri' sinC(u!). The potential
drop across the sheath is just Vp (0 -V" (t) = Vet) = Ve + A. Ve sin(wt).
A. D. Kuypers and H. J. Hopman 1232
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33(Because Vf is defined as being positive, it means that the
plasma is always more positive than any surface in contact
with it. This is a direct consequence of the fact that the drift
velocity of electrons is much higher than that of ions.) Using
this implication, it follows from Eqs. (13) and (14) that27
Vp (1) = Vr + -!( Vdc + Vrf)[ 1 + sin (wt)]. (15)
(See also Fig. 4.) The sheath potential at the powered elec
trode is equal to the potential drop over Cp (Figs. 3 and 4):
Vet) = Ve[l +A sinC(,;t)] = Vp(t) -Va(t).
This leads to the following results:
Ve =!( Vrf -Vdc) + VI
and
A= F Vrf -Vee) -v( (16)
(17)
Note that, generally, V,lc is negative. Now that both sheath
potentials V(t) and Vp (t) can be directly related to the gen
erator voltage Vrr and the electrode offset Vdc' it is useful to
have a relation connecting the two last mentioned. It follows
directly from the capacitive voltage division of the applied-rf
amplitude Vrf that the rf component of the plasma potential
is given by27 Vprf = Vrr[ Cpl (Cp + Cg) ]. Combining this
with Eq. (15) then gives
Vdc = Vrr[(Cp -Cg)/(Cp + Cg)]. (18)
A larger electrode area results in a higher electrode sheath
capacitance.28 Thus, in the reactor considered here, the
sheath at the grounded electrode has the largest capacitance:
Cg >-Cp' It follows from Eq. (18) that Vdc;::;:;; -Vrf• Neglect
ing Vf in cases that Vrf;::;:;; I Vdc I >-1 VII, it then follows from
Eq. (17) that again ),;::;:;; -1, as was obtained earlier above.
D. Value of parameter n
The constant n [in Eqs. (3 )-( 5)] determines the de
pendence of the sheath potential on the distance x to the
electrode surface. When the gas pressure is so low that the
ion mean free path is larger than the sheath thickness (I> d,
assumption 4), a free-fall model can be used. In this case,26
n = 1-When in addition A = -1 anda = 1, Eq. (5) reduces
to
(19)
In the following, this formula will be used as a first attempt
to interpret the measured energy profiles, and all other re
gimes and choices of constants will be considered as devia
tions from this ideal case. In this regime (n = j) the Lang
muir-Child space-charge-Iimited current equation for the
total ion flux towards the electrode is also valid26:
J _ 4£0 2e ; /jVV2 ;-9 Md 2' (20)
When both E( = eVe) and J; are measured, thi.s can be used
to check the validity of Eq. (19) because both equations
have to be consistent: the value of the time-averaged sheath
thickness d, which can be obtained from Eq. (16), has to
give the right value for Jj when inserted into Eg. (20).
1233 .J. AppL Phys., Vol. 67, No.3, 1 February 1990 IV. MEASUREMENT OF ENERGY DISTRIBUTIONS FOR
8=0
Ao Shape of the ion energy distribution
In the following experiments, a O.S-mm-thick alumi
num dummy wafer with a 200 pm hole with knife edges was
used to extract the substrate bombarding ions for energy
analysis. Instead of operating as an energy analyzer, the
complete analyzer can also be used as one big Faraday cup.
The total ion current collected by the diaphragm can then be
measured. From this, the ion flux Jj towards the surface of
the powered electrode can be derived.
Parallel to these measurements, the dc offset of the
powered electrode has been recorded using an oscilloscope.
A probe with an attenuation factor of 1000 was connected to
the electrode, and the signal was measured relative to ground
potential. From the dc shift of the sinusoidal oscilloscope
trace the value of I!;k was obtained.
A typical result of an energy spectrum, obtained from a
discharge at 2.4-mTorr argon gas pressure, is shown in Fig.
S. The shape ofthe measured profile resembles the calculat
ed distribution given in Fig. 2. However, two differences be
tween both figures are obvious. First, the measured profile is
slightly asymmetric, and second, the slope of the edges is not
infinite. These observations can be accounted for by the lim
ited energy resolution of the analyzer. The measured profile
is a convolution of the calculated profile and the response
characteristics of the analyzer. When the analyzer is scanned
to measure the ion distribution, not only ions with energy E
will be collected, but also particles of slightly different ener
gy. This explains the finite slope at the edges of the profile. In
addition, it is assumed that the energy analyzer has a Gaus
sian energy window of full-width-half-maximum t.. w. It is
known29 that t.. WI E is constant for a given analyzer geome
try. This means that the sensitivity of the apparatus increases
with E, because ions from a larger energy window are col-
Ion energy [e Vj
FIG. 5. Measured ion energy distribution in 2.4-mToIT argon (B ~~ 0, rf
power 1 kW). Mean ion energy E= 203eV. energy width IlE = 54eV. The
smooth line has been obtained by convoluting the calculated profile in Fig. 2
with a Gaussian energy window of FWHM Il W(E), to account for the ener
gy dependence of the analyzer detection efficiency. Plotted line corresponds
to best fit, obtained for AW(E) = O.016E.
A. D. Kuypers and H. J. Hopman 1233
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:333 I l
~t 02
_~_L
.~ 2-
i·1 '" i
" !\ ~
is
t::
oj ,~ .Q
,0
.... -----~ 0 _. ,..-
0 200 400
Ion energy reV]
FIG. 6. Measured iOIl energy distribution ill 3-mTorr oxygen (lJ = 0, rf
power 1 kW). (AE,/6.E,)2;::;2.
lected. Therefore, a symmetric ion energy distribution will
result in a measured spectrum with higher intensity at the
high-energy side. This explains the observed asymmetry.
(When the curve, given in Fig. 2, is convoluted by a Gaus
sian energy window with a fitted FWHM of
tl. W( E) = 0.016E, the calculated line in Fig. 8 is obtained. )
Residual differences may be attributed to small deviations
from sinusoidal time dependence of the sheath potential.
B. Mass effect
From Eq. (19) it is expected that the width tl.E of the
energy profile scales with M 1/2. To check this, profiles have
been measured in different molecular gases. A typical result
obtained in an oxygen discharge is shown in Fig. 6. Two
profiles of widths /lEI and t:..E2 are superimposed. Their rel
ative magnitude is given by (IlE2/ IJ.E, )2;::::2, soMI ;::::2M2• It
is conduded that the inner peak represents 02i molecules,
4
3
2
F 70
OIl
<" 60 'E
~
<U
~ 50
<=-40 E £
13 30
> 'E 20 '0
" E 10 " S
0
0 10 20 30 40 so 60 70
A~signed ion mass [amu J
FIG. 8. 1011 mass as obtained from the observed energy spJittings ill Fig. 7,
plotted as a function of assigned ion mass.
and the outer peak 0+ atoms, produced by dissociation in
the discharge. This leads to the remarkable result that, al
though the analyzer only measures energy, also mass selec
tion is obtained, because of the different response to the rf
component of the sheath potential with ion mass.
A similar effect is observed in a CF4 discharge. Figure 7
shows a spectrum measured at a relatively high rf power of 3
kW, Because the value of din Eq. (19) is not known, abso
lute values for the different ion masses can only be obtained
by tentatively attributing one peak to a certain mass, and
then verifying whether the other peaks correspond to masses
that are to be expected from a CF4 discharge. The assign
ment of the peaks in Fig. 7 was obtained by attributing the
largest splitting to C f ions. (The second C+ peak is missing
in the observed spectrum in Fig. 7, because of the limited
scan range. The distance of the first C+ peak to the middle of
the profile was used to find b.Ec' .) The mass thus calculated
from the measured splittings has been plotted as a function
of assigned ion mass in Fig. 8. It is concluded that the corre-
F FIG. 7. Measured iOIl ellergy distribu
tion ill 3-mTorr CF4 (lJ = 0, rfpower
3kW).
o 4--------.--------,--------.--------,--------~------~
300 500 700 900
Ion energy [eVJ
1234 J, Appl. Phys., Vol. 67, No.3, 1 February 1990 A. D. Kuypers and H, J. Hopman 1234
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33lation is very good, in contrast to observations by other au
thors.18•30
The spectra in Figs. 5, 6, and 7 are all symmetric around
a mean energy value E. This confirms the assumption that
acceleration by the rf component of the sheath potential can
be treated as a second-order effect, in addition to the accel
eration by the dc potential. However, ions collected from a
discharge in a mixture of argon and hydrogen show an ener
gy distribution as given by Fig. 9. Peak assignment was per
formed as above. The middle of the Ar profile is taken as the
mean energy E. Then for aU three hydrogen profiles, it is
observed that the high-energy peaks are located further
away from E than the corresponding low-energy peaks. This
is explained by the low mass of the H atoms and molecules.
From the Ar profile, a sheath thickness d = 3.9 mm is ob
tained, using Eq. ( 19). When the traversion time r for an Ar
atom of energy E( = 334 eV) is calculated by a computer
trajectory calculation, it appears that it takes more than four
rf oscillations to cross the sheath. The same calculation for
an H atom gives values from 0.3 up to about 0.1 oscillations.
This means that the Ar atom predominantly experiences a
time-averaged sheath potential, while the H atom responds
to an almost instantaneous potentiaL Clearly, assumptions 1
and 7 in the analytical treatment above break down for the
case of hydrogen. When the H atom enters the sheath at a
moment that the sheath potential becomes high, it crosses
the sheath very fast. In principle, it can be accelerated to an
energy E<,2E. Note that in Fig. 9 the H+ energy distribution
extends almost exactly to this ultimate value. However,
when the H atom enters the sheath when the potential be
comes low, it will take a considerable part of an rf oscillation
to cross the sheath. Therefore, its final energy wiII be closer
to the time-averaged value E.
V.ION ENERGY MEASUREMENTS AT CONSTANT
POWER FOR B> 0
At a constant absorbed power of 500 W, the influence of
the variable axial magnetic field on electrical discharge char-
Ion energy [e V J
FIG. 9. Measured ion energy distribution in a 3-mTorr mixt\lre of argon
and hydrogen (5 seem H2 + 2 seem Ar, B = 0, rfpower 2 kW).
1235 J. Appl. Phys., Vol. 67, No.3, 1 February 1990 ::f l<
"
300
<5 r:c
2: III"
] 200 III"
" ~ " " " " [! )( " r&: [!
100 " '" " " " C I:l " 0 M iii III III III
-100
0 100 200 300
Axial magnetic field l Gauss I
FIG. 10. de (black sq\lares) and rf (crosses) components of applied elec
trode voltage, together with time-averaged sheath voltage Ve (open
squares), measured in 2A-mTorr argon (constant rf power 500 W).
acteristics has been studied. Field strength was varied from 0
to 315 G. Measured values of VdC' Vrr, and Vee =E/e)
have been plotted as a function of B in Fig. 10.
The physical meaning of Fig. 10 is clear: the electron
diffusion in the direction of the electric field decreases with
increasing magnetic-field strength. Therefore, equilibrium
between the time-averaged ion and dectron currents
towards the electrode will be reached at a lower sheath po
tential. This will be discussed in more detail below, together
with the measurements performed at constant amplitUde
Vrf·
The total ion current density, both as measured and as
calculated from the ion-energy profiles [using Eqs. (19) and
(20)], is given in Fig. n. It should be noted that the mea
sured current densities at magnetic-field strengths above 100
G, are higher than reported in a previous publication.3l In
the case of a 200-pm diaphragm in front of the energy ana
lyzer it was observed that, under certain conditions, not only
ions, but also electrons, were able to enter the detection vol
ume. Therefore, the measurements of ion current density
10
.. .. ..
N 8 ..
..§
::s ..
c· 6
':,n III
'" "' .. "0 0 ;: III
""' 4 III " c t: tl " " u 0 0 D <: .. 0 ,s 2 !l!e
0
0 100 200 300
Axia[ magnetic field [Gauss 1
FIG. 11. Filled squares: measured ion flux to the powered electrode surface.
Open squares: nux as calculated from observed energy splittings, assuming
A = I (discharge conditions as in Fig. 10).
A. D. Kuypers and H. J. Hopman 1235
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33have been repeated with a smaner (501tm-diam) dia
phragm. After this modification, the measured ion current
densities for B < 100 G were unaltered. However, for B > 100
G, now a continued increase ofJ is observed with B (instead
of the saturation mistakenly reported earlier). 31
This observed increase of J with magnetic field strength
is obviously related to a similar increase of the ion density in
the plasma bulk, which has been measured with a Langmuir
probe.
Although the calculated current density gives good
agreement with experimental values at low-magnetic-field
strengths, above 100 G the calculated values are clearly
much too low. This apparent discrepancy can be explained
as follows. First, it should be noted that there is no reason to
assume that the equation for space-charge-limited current
[Eq. (20) j does not hold anymore. At higher B values, ion
movement is still collision less and also, the Lorentz force
acting on the ions is much smaller than the Coulomb force
due to the sheath electric field. Therefore, Eq. (20) remains
valid. To calculate Ji from Eg. (20), values for V" and dare
substituted. The sheath potential Ve is weB known, because
it is directly obtained from the energy profiles. Therefore,
the observed discrepancy must be due to an error in the de
termination of the sheath thickness d. So far, an values for d
have been derived from the observed energy splitlings tl.E,
using Eq. (19). However, in this equation it is assumed that
A = I, and this is only justified when V" > V: lEq. (8)].
Apparently, this condition is violated as Ve decreases with B
(Fig. 10).
For O<B < 50 G, good quantitative agreement is ob
tained between the measured ion fiux and the fluxes which
are calculated with Eqs. (19) and (20) lsee Figs. 11 and
15 (a), and also a previous publication 10 which deals exclu
sively with the case B = O}. Thus, it has been established
that the parameter d is indeed the same in both equations.
Therefore, it is now allowed to reverse the procedure: In
stead of calculating d from Eg. ( 19) in order to obtain Ji, the
measured values of Ji and Vc can be used to find d [Eq.
(20) ]. To calculate A from d, Eq. (3) should be used instead
of Eq. (19), The parameters n and a, appearing in Eq. (3),
are 110t influenced by the magnetic field, because the frce-fall
approximation remains valid (n = j) and it takes an argon
iOI1 several rf oscillations to cross the sheath (a = I). Fol
lowing this procedure, substitution of the data presented in
Figs. 10 and 11 leads to the conclusion that A is somewhat
smaller than 1, which implies that the rf component AVe of
the sheath potential is smaller than the dc component V".
This was predicted by Eq. (8), where it has been shown that
the difference between the dc and the rf component is ap
proximately equal to the floating potential VI'
The values of VI' thus calculated from the experimental
data, are plotted in Fig. 12. The substantial scattering in
these data is due to the fact that they are obtained by sub
tracting two relatively large numbers. However, fitting a
straight line to these data points, it is conduded that the
floating potential has a value somewhere between 13 and 17
V. These are realistic values for a discharge with an electron
temperature of a few eV (see Sec. HI B), and thus support
the statement that A can be obtained as given above.
1236 J. Appl. Phys., Vol. 67, No.3, 1 February 1990 30
'=' "0 !Ii
2: 20 4-< ..
!Ii
:>
~ 5 r--__ ------- .. --....,:r-___ - .. " c 0-
M 10 " .~ .. 0 !Ii fl:
0
0 100 200 300
Axial magnetic field [Gauss]
FIG. 12. Floating potential V;, as calculated from measured ion flux and ion
energy distributions (discharge conditions as in Fig. 10).
VI. MEASUREMENTS AT CONSTANT GENERATOR
VOLTAGE
In the experiments presented in Sec. V, the rf power
absorbed by the discharge was kept constant in order to
study the response of the system to the applied axial magnet
ic field. However, when a detailed study is made of the effect
of this magnetic field on the potentials and offset voltages, it
is more convenient to keep the applied generator voltage
constant. The following results have aU been obtained by
keeping the rf amplitUde Vrf (as observed on the oscilloscope
connected to the powered electrode) at a constant value of
232 V. This, of course, has the consequence that now the
total input power is varying with B.
For a discharge in 2.4-mTorr argon, the mean energy of
ions arriving at the surface of the powered electrode is given
in Fig. 13. Instead of the monotonic decrease in energy, ob
served in the case of constant power, an initial increase as a
function of magnetic-field strength is seen here. Above 100
G the ion energy becomes lower.
Also the measured de offset voltage on the powered elec
trode shows a different dependence on magnetic field now
~~ ~
5 B
200 5B ~
~ B
D • DO Ii G II ;-, m c .. ", c b Ii B t;:j " " " .s 10(1 f- a c
~ c c
~ c c
"
0 c
()
0 100 200 3D!)
Axial rnap!letic field (Gall~sJ
FI G. 13. Measured mean ion energy E (fiiled squares) and energy spliUings
t:.E (open squares) in 2.4-mTorr argon, at a constant electrode voltage
Vcr = 232 V. (rfpowerbetwecn 180 and 3100 W).
AD. Kuypers and H. J. Hopman 1236
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33that the generator voltage is kept constant. From Fig. 14 it is
seen that up to 100 G, the dc offset remains remarkably con
stant. Only at higher field strengths again a monotonic de
crease is observed, down to 0 V, and even changing sign.
As shown above, the time-averaged plasma potential
Vp = V" + Vdc' The result is also plotted in Fig. 14. It ap
pears that above 100 G, Vctc and v., = E Ie (Fig. 13) de
crease at about the same rate, resulting in an almost constant
plasma potential.
In addition, the total ion flux has been measured under
the same conditions [Fig. lS(a)]. From the measured ener
gy distributions, the splittings t:.E have been determined
(Fig. 13). Substituting these values into Eq. (19) it is found
that the sheath thickness shows a monotonic decrease from 5
to 1 mm. Substituting d Eg. (20) again, the ion flux can be
calculated. The values thus obtained are also plotted in Fig.
lS(a) for comparison. Just as in the measurements at con
stant power (Fig. 11) a large difference between measured
and calculated flux is observed for higher magnetic-field
strengths. Note that here the deviation starts at a higher
magnetic-field strength (;:::; 100 G) than in the constant
power case ( ;:::; 40 G). This supports the conclusion that the
deviation is due to a breakdown of the condition Ve ~ V;,
because in the former situation the sheath potentials only
start decreasing near 100 G, whereas in the latter V" de
creases monotonically for B> O. Following the same proce
dure as in Sec. V, A and VJ can be derived from the measure
ments in Figs. 13 and 14. Values for Vr, scattering around 20
V ( ± 15 V), are obtained.
Langmuir-probe measurements have been performed
under the same discharge conditions. II The ion density as a
function of magnetic-field strength is given in Fig. 15 (b). It
is observed that both the ion density ni and the ion current
density Ii show qualitatively the same behavior. At zero
magnetic-field strength, an rfamplitude of232 V is obtained
at 180 W input power. Going from 0 to 130 G, the absorbed
rf power has to be increased up to 3.1 kW to maintain the
same rf voltage on the powered electrode. In this domain,
both the ion current density towards the electrode surface
and the ion density in the bulk of the plasma show a linear
150 t
m .... S l1lil1li l1li
B
I:! "0
'''1-.. o 0 D o D 0 0
" c
C 0 • t::, CO ..
" ;:
CC lJ .. c
"" B
"
-50 L-_~ __ --'-__ ~ __ '---_~ __ -'-_---'
o wo 2()O .Ion
Axial mJgnetic field [Ci-auss;
FIG. 14. Measured dc offset of the powered electrode (filled squares), to
gether with de component of the pj,tsma potential (open square,). as ob
tained from measured Velo and E by v,,,,, .~ E + V"e' Discharge conditions
as in Fig. 13.
1237 J. Appl. Phys., Vol. 67, No.3, 1 February 1990 30 -I ('-;-'>
1': 20 ~" " " ~ 5
:s. " 5 C
~.
0 C " ~
" 0 " .g .. " " E 0
~ 10 2 " " " ~ " ..
"" " 0
0 J ~
e ~~
0
() 100 20u 300
Cal Axial magnetic field (Gaussj
'"
'" ..
" " "
" ..
"
DL---~-----L----~----~--~----~--~
o 100 200 :JOO
(bl Axial m:ignetk fiek! ~(jallssl
FIG. IS. (a.) Measured ion flux to the powered electrode surface (filled
squares), and flux as calculated from observed energy spiittings, assuming
A ,.= 1 (open squares). Discharge conditions as in Fig. 13. (b) Ion density at
a distance of 4. 5 em from the surface of the powered electrode, measured by
Langmuir probe. Discharge conditions as in Fig. 13.
increase with power. A further increase of the magnetic-field
strength allows the supply of power to be lowered again,
down to a value of 1.9 kW at 315 Go However, ion current
and ion density show a further increase with B, despite the
lower power levels. Thus, the knee in Figs. 15 (a) and 15 (b)
can be explained: Below 100 G there is a combined increase
of power input and electron confinement. Above this value,
the effect of a further improvement of confinement is almost
compensated for by a decreasing power input.
VII. DISCUSSION
A. Validity of the assumption of a capacitive sheath
First, it will be shown here that the capacitive sheath
model is indeed valid under the current experimental condi
tions. To prove this, it has to be shown that the displacement
current la = Ap dQ Idt is larger than the condition current
Ii'
The capacitance C of two surfaces of equal area A at a
mutual distance d is given by
C=Eo(Ald). (21)
For the sheath at the powered electrode, A = Ap' while dis
given by space-charge-limited current equation (20). Sub
stitution into Eq. (21) gives
(22)
A. D. Kuypers and H. J. Hopman 1237
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33This relation is also given by Keller and Pennebaker. 24 Be
cause d was shown to be independent of power (B = 0 and
constant pressure), it foHows from Eq. (21) that Cp is also
power independent. Substitution of the values measured at
P = 500 W (Ve = 280 V, Ji = 1.6 A/m2, Ap = 0.163 m2)
gives Cp = 0.3 nF. Assuming Cp to be time independent, Jd
can now be estimated:
dV J" = Cp-= Cp{;) V" cos(cot). (23) dt
Taking the time average, it foHows that
dQ/dt = 2CpwVJ(rrA). For P= 500 W, this gives a dis
placement current of88 A/m2• Thus it has been shown that
indeed Ji ~Jd' and the capacitive approximation is justified.
B. Influence of magnetic field on sheath thickness
For the nonmagnetized case, it has been shown that the
sheath thickness d, derived from the observed ion energy
splittings, is the same as the value determining the space
charge-limited current. In addition it has been shown that
for B > 0, the values of d, calculated from the measured ion
current densities Ji, lead to consistent results if deviations of
A from unity are taken into account. It is concluded that at
the low pressures considered here, the value of d thus ob
tained gives a reliable absolute measure of the sheath thick
ness. This will be used in the following to study rf sheath
behavior in a magnetic field.
For both experiments in argon, at constant power (500
W) and constant generator voltage (Vrf = 232 V), the reci
procal value of d (obtained from the measured Ji ) is plotted
as a function of B in Fig. 16. A striking coincidence of the
results of the two experiments is observed. This is remark
able, because the sheath potentials and ion fluxes involved
are entirely different for the two cases. For B> 30 G, a con
stant value of the product Bd ( = 0.134 G m) is obtained.
A similar decrease of sheath thickness has been ob
served in magnetized dc discharges. At gas pressures be
tween 0.3 and 20 Torr, the cathode dark-space thickness was
measured visually in different gases by Giintherschulze>2.
He observed a gradual decrease of sheath thickness as a func-
o
300
Axial magnetic field [GaussJ
FIG. 16. Reciprocal sheath thickness, as calculated from measured ion flux,
for the cases of constant power (500 W, filled squares) and constant rf vol
tage (232 V. open squares), respectively.
1238 J. Appl. Phys .• Vol. 67, No.3, 1 Feoruary 1990 tion of E, from 1-2 cm down to a minimum value d = 0.7
mm. In addition, the current density appeared to be given by
Jj = const/d 2. No change in sheath potential was observed
here. However, earlier work by WillOWS33 at lower gas pres
sures had shown a decreasing cathode faU potential, indicat
ing a pressure dependence. A theoretical explanation for the
shrinking of the dark space was given by Thomson and
Thomson?4 It is based on the fact that the dc discharge is
sustained by secondary electrons, liberated from the cathode
surface by charged-particle bombardment. This theory has
been adopted by many workers in the field of dc magne
trons.35-37 Here Thomsons' results are given as reproduced
by Francis.38 The equations of motion for an electron in
crossed electric and magnetic fields are given by
d2x dy m--=eE-eB-dt2 dt (24)
and
(25)
A linear decreasing electric field is assumed across the
sheath (case n = 2 above), giving
E= (2Ve/d)[1- (x/d)]. (26)
With the boundary condition that for x = 0, dx/dt = dy/
dt = 0, the solution of Eqs. (25) and (26) is given by
2VJd x = [1 -cos(yt)] (27)
2Vc/d2 + eB2/m
with
r = 2eVe/d2 + e2B2/m .
m (28)
Under the assumption of collisionless movement, the maxi
mum distance Xmax the electron can reach, relative to the
electrode surface, is then given by Eg. (27) with
cos (yt) = O. As long as xmax > d, the path length the elec
tron travels through the sheath will only be slightly affected
by B. However, whenxmax < d, the electron will be bent back
towards the surface, and the path length increases drastical
ly. This causes an increasing excitation and ionization in the
sheath region, reducing the observed sheath thickness. Thus
a critical value Be can be defined, above which the magnetic
field will modify the sheath. It is given by the condition
Xmax = d. From Eqs. (27) and (28) it then follows that
(29)
Recent calculations by Maniv39 lead to a comparable result.
Physically, Eq. (29) means that the Larmor radius of the
electron, corrected for the accelerating field E(X), is equal to
d.
It is tempting to apply the same reasoning to the rf case.
Substitution of relevant numbers shows that the observed rf
discharge behavior might also be due to secondary elec
trons.31 However, there is growing evidence that secondary
electrons only play an additional role in sustaining 13.56-
MHz plasmas.11•24,40 The experimental evidence for fast
electrons, originating from the sheath regions, can also be
A. D. Kuypers and H. J. Hopman 1238
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33accounted for by acceleration at the oscillating plasma
sheath boundaryY-43
From Fig. 16 a value Bd = 0.134 G m is obtained. Sub
stituting in Eq. (26), this gives a potential V = 8 V. It should
be noted that a constant value is found, although the sheath
potential Ve depends on B. This suggests that d is not deter
mined by the ability of secondary electrons to reach the plas
ma, but by the condition that electrons from the plasma must
be able to reach the electrode, in order to satisfy the condi
tion Ji + (Je) t = O. As stated above, Te is a few eV, and will
be fairly constant under the given conditions. Most of the
electron current towards the electron surface flows when
V(x,t) is closest to zero. Then, electrons from the tail of the
Maxwell-Boltzmann distribution have to cross the sheath
against the remaining negative sheath potential (which is in
the order of Vr). Only when d scales with B as given in Eq.
(29), will it be possible to maintain zero net current.
The ion current density Ji is determined by the ion drift
towards the plasma-sheath boundary, as argued above.
From Figs. 15(a) and 15(b), it follows that Ji a: nj• There
fore, J~ depends on B via the growing plasma density, which
in turn is a consequence of improved charged-particle con
finement. Then the space-charge-limited current Eq. (20)
explains the behavior of Ve, and thus E, with B.
Co Capacitive~voltage division and sign reversal of de
offset on powered electrode
The development of the dc component of the plasma
potential, V pck' is determined by the sheath behavior at the
grounded electrode. In order to say something quantitative
about this, two assumptions have to be made. First, it is
assumed that the ion drift towards both electrodes is equal.
This assumption is supported by the knowledge that Ji is not
depending on the sheath potential. Second, it is assumed that
there is no dc potential drop across the plasma volume. Then
the plasma potential V pelc' measured at the powered elec
trode, is equal to the dc sheath potential at the grounded
electrode. Thus, knowing both Ji and V pelc' the sheath thick
ness dg can be calculated [Eq. (20)]. Substituting the ex
perimental data obtained at constant power (500 W), values
for dg in the order of 1.5 mm are found. Only small devia
tions ( ± 0.15 mm) from this val ue are observed as a func
tion of B. It follows that at the grounded electrode the sheath
thickness is not related to the axial magnetic field by a rela
tion Bd = constant, as was observed at the powered elec
trode. However, this may be ascribed to the influence of the
cusp field. It generates a magnetic-field strength at the sur
face of the grounded electrode, which is much larger than
the axial field. Therefore, variation of the axial field B may
have little influence on the sheath thickness.
As reported above, at the powered electrode a value
dp = 5 mm was observed when B = 0, Thus dg <dp-Be
cause in addition A g > A p' it follows from Eq. (19) that at
B = 0, Cg is considerably larger than Cpo According to Eq.
(HI) [which states that Vdc = Vrr(Cp -Cg)/(Cp + Cg)],
this in good qualitative agreement with the observation of
large negative dc offset voltages VdC' which are of the same
order of magnitude as the applied generator voltage Vrf (see
Fig. 17, and also Fig. 14). Going towards larger Bvalues, Cg
1239 J. Appt. Phys., Vol. 67, No.3, 1 February i 990 SUO
400 ~
x
300 s
~ c~
~ 200 c
~ l: " x " ;,; " " d: 100 If
~ ~ IL 0 ~ c C lJ .. []
-lO(!
0 100 200 :JOO
Axial nlagnetic field IGauss~
FIG. 17, Measured electrode voltage Vcr (crosses) and electrode-offset V.k
(open squares), together with calculated values of Vd,. (filled squares). Cal
culated data are based on estimated sheath capacitances at the grounded
electrode. Measured data taken from Fig. 10.
remains virtually unchanged, whereas the decrease of dp
with B(Bd = c) results in increasing values for Cpo In Fig.
17, the 500-W data for Vdc and Vcf (from Fig. 10) are replot
ted, together with the values for Vdc which are calculated by
substituting (.~ and Cg into Eq. (18). Although Vrr increas
ing above 100 G, the calculated values for Vdc show a mono
tonic decrease with B, finally resulting in sign reversal. Thus
it is shown that the observed monotonic decrease of Vdc, and
also its reversing sign, are a direct consequence of the fact
that the relative magnitude of both sheath capacitances is
reversed by the magnitude field.
VIII. CONCLUSIONS
Ion energy distributions have been measured with an
electrostatic energy analyzer at the powered electrode of a
13.56-MHz discharge. Plasma confinement by magnetic
cusp fields permits low-pressure discharge operation, result
ing in collisionless acceleration of ions in the sheath. Thus,
high-resolution energy spectra are obtained. This has been
used for an experimental verification of the theory for ion
acceleration in rf sheaths, as developed by Vallinga and
Meijer. For this purpose, experiments have been performed
in absence of an axial magnetic field. Excellent agreement
between measured spectra and calculated energy distribu
tions is obtained. From the ion energy distributions, sheath
thickness and ion flux towards the wall can be derived. The
ion flux, thus calculated, corresponds very well with mea
sured values. In case of high-rf voltages, ion mass resolution
is also obtained, resulting from the rf modulation of the
sheath potential.
Having established the validity of the model, it has been
used to interpret experimental results as a function of axial
magnetic-field strength. Application of a variable axial mag
netic field results in lower mean ion energies and higher cur
rent densities. Both are related to the sheath thickness by the
space-charge-limited current equation. The reciprocal
sheath thickness behaves linearly with magnetic field, with a
slope that is independent of sheath potential and discharge
power. This is explained by assuming that the sheath thick
ness is determined by the Larmor radius of plasma electrons,
A. D. Kuypers and H. J. Hopman 1239
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131.170.6.51 On: Sun, 17 Aug 2014 23:15:33and thus only by the electron temperature in the plasma
bulk. It implies that rf sheath behavior is principally differ
ent from the corresponding dc behavior, which is governed
by secondary electrons.
Estimated thickness of the sheath at the grounded elec
trode lead to the conclusion that the observed sign reversal of
the dc offset voltage at the powered electrode is directly re
lated to the relative magnitude of both sheath capacitances.
At B = 0, the largest sheath capacitance is found at the
grounded electrode (which has the largest area). This ca
pacitance changes little upon variation of the axial magnetic
field. At sufficiently high magnetic-field strengths (between
200 and 300 G) the capacitance at the powered electrode has
increased so much that it exceeds this constant value, result
ing in the observed reversal of the offset voltage.
ACKNOWLEDGMENTS
The authors wish to thank P. M.Meijer and W. J. Goed
heer (FOM-Institute Rijnhuizen, Nieuwegein) and A.
Manenschijn (Delft University of Technology) for stimulat
ing discussions. The work described here was performed as
part of the research program of the Stichting voor Funda
menteel Onderzoek der Materie (POM), with financial sup
port from the Nederlandse Organisatie voor Wetenschap
pelijk Onderzoek (NWO) and the Dutch Ministry of
Economic Affairs within the framework of the lOP-Ie pro
gram.
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A. D. Kuypers and H. J. Hopman 1240
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1.343999.pdf | Investigation of the probabilistic behavior of laserinduced breakdown in pure water and
in aqueous solutions of different concentrations
H. SchmidtKloiber, G. Paltauf, and E. Reichel
Citation: Journal of Applied Physics 66, 4149 (1989); doi: 10.1063/1.343999
View online: http://dx.doi.org/10.1063/1.343999
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/9?ver=pdfcov
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131.193.242.21 On: Thu, 27 Nov 2014 22:18:51Investigation of the probabilistic behavior of laser .. induced breakdown
in pure water and in aqueous solutions of different concentrations
H. Schmidt-Kloiber, G. Paltauf, and E. Reichel
Abteilung Biophysik, Institutjur Experimentalphysik. Karl-Franzens-Universitat Graz. Universitc41splatz 5.
A-80l0 Graz, Austria
(Received 27 March 1989; accepted for publication 20 June 1989)
In this paper we report on experiments to inve.~tigate the laser-induced breakdown properties
of saline solutions of different concentrations and of highly deionized water, using a Q
switched Nd:Y AG laser. The observation of the dependence of the breakdown probability on
the pulse energy gives informati.on about the influence of the ion concentration on the
breakdown occurrence. It has turned out that the generation of initial electrons for the
avalanche by the ions determines the breakdown threshold in saline solutions. In extremely
pure water, with no ions as electron donors, the first free electrons have to be produced by
multiphoton ionization of the water molecules, which leads to a very sharp threshold. The
region of pulse energies, where breakdown occurs only with a certain probability, has its
minimum width in pure water, shows its maximum extension in low concentrated solutions
and is again getting narrower with increasing concentrations.
I. INTRODUCTION
Pulsed, high-power laser sources are commonly used to
produce mechanical effects in various medical applications.
The conversion of light into mechanical energy is achieved
by the effect of laser-induced breakdown. The optical field
initiates the buildup of an electron avalanche, foHowed by
the rapid expansion of the resulting plasma due to absorp-
tion of the incident light by inverse bremsstrahlung.1,2 In a
later phase of the breakdown a shock wave is emitted from
the plasma region. The peak pressure of this shock wave is
sufficiently high to cause different kinds of destructive ef
fects such as urinary stone fragmentation. 3 The most impor
tant feature of this treatment is the ability to deliver the laser
radiation dose to the stone through an optical fiber. For a
safe and efficient application care must be taken that the
breakdown is released in the irrigation liquid near the sur
face of the stone and not in the optical components of the
delivery system. It is therefore of great importance to choose
a liquid that exhibits a low breakdown threshold. To under
stand the mechanisms that lead to a lowering of the thresh
old, we examined the breakdown properties of aqueous solu
tions paying our special attention to the effect of varying the
concentration.
The generation of an electron avalanche by an optical
field is governed by statistical processes. The consequence of
this statistical nature is the formation of a more or less wide
region of incident laser pulse energies, where breakdown oc
curs only with a certain probability. This region separates
the energy values where breakdown is always produced from
those where it never occurs.
Bass and Barrett have shown in their "lucky electron"
model that the reason for the probabilistic nature of optical
breakdown lies in the first stage of the electron avalanche.4
From that model they derived a formula describing the de
pendence of the breakdown probability on the optical elec
tric field strength.
In this paper we show how this formula can be used to
calculate the probability that a single laser pulse with a eer-tain energy win cause breakdown. The parameters that are
obtained by fitting the function to the experimental data are
suitable to give an explanation for the influence of the solute
content in the liquid on the breakdown probability. An im
portant observation in our experiments was that extremely
pure deionized water behaves in a significantly different way
than saline solutions. This effect seems to be un explicable in
terms of the model and may be due to unfavorable starting
conditions for the avalanche in the absence of ions as a
source of initial electrons. It will be tried to show how the
breakdown probability formula has to be modified consider
ing the generation of starting electrons by multiphoton ioni
zation as the effect that determines the breakdown probabili
ty.
II. EXPERIMENT
The experimental setup for the breakdown probability
measurements is shown in Fig. 1. The laser used for our
experiments is an actively Q-switched Nd:YAG laser
(JK HY750) operating at the fundamental wavelength of
1064 nm. It emits pulses with a duration of 8 ns (FWHM), a
beam diameter of9 mm in multimode operation, and a pulse
to-pulse energy reproducibility within I %. The adjustment
of the energy was obtained by means of neutral glass filters
FlLTERS,
ATTENUATOR CELL
LASER t: It -:-:.:-~--l-l~-- r' -~ L_~ ~ -~:::--::-< '--____ .....1_ L _0 ________ -'
CALORIMETER \
81 -01 l_ ~
COUNTER
_ _ _ PHOTODIODE
FIG. 1. Experimental setup for the measurement of the laser-induced
breakdown probability in aqueous solutions, using a Q-switched Nd:Y AG
laser at 1064 nm. The calorimeter is inl>erted into the beam tc calibrate the
attenuator and the filters.
4149 J. Appl. Phys. 66 (9).1 November 1989 0021-8979/89/214149-05$02.40 @ 1989 American Institute of Physics 4149
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131.193.242.21 On: Thu, 27 Nov 2014 22:18:51and of a polarizing attenuatar. A plano convex lens with a
focal length of 50 mm in air was used to focus the beam
through a quartz window into a stainless-steel cell contain
ing the liquid.
To detect the occurrence of a breakdown, a portion of
the laser radiation scattered by the plasma was collected
onto a photodiode that was located at an angle of 90· to the
beam axis. In order to detect the breakdown rate, the electri
cal signal from the diode was then guided to a counter. The
probability was calculated by dividing the number of break
downs that had occurred by the number of the incident laser
pulses. The latter was 600 in our experiments and was ob
tained by running the laser at a pulse repetition rate of 10
pulses per second for 1 min.
Pulse energy measurements were made using a laser cal
orimeter (Scientech 380101). The energy at the focal plane
was corrected for reflection and absorption losses. The tem
poral shape of the laser pulses was measured by means of a
combination of a fast photodiode (RCA 971 E) and an oscil
loscope (Iwatsu TS-8123, Tektronics 7834).
Temporal fluctuations in the laser pulse not resolved by
the oscilloscope were measured with a boxcar averager
(EG&G) with a resolution of 400 ps. Histograms taken at
single instants of the pulse showed a standard deviation of
7%.
Earlier experiments have shown that the presence of
NaCl reduces the breakdown threshold of pure water.' For a
further examination of this effect we used the following li
quids for our experiments:
Highly deionized water (Nanopure quality) with a spe
cific resistance of 18 MD cm and aU organic contaminations
removed and saline solutions with concentrations of 0.01,
0.1, and 1 mol! twith the same deionized water as solvent.
III. RESULTS
Figure 2 shows a plot of the breakdown probability ver
sus the energy of the incident laser pulses at the location of
breakdown for the four liquids listed above. The energy lev
els required to cause breakdown increase with decreasing ion
concentration. The pure water shows almost thresholdlike
behavior, meaning that the breakdown probabilities lie in a
very narrow energy region. The 0.01 moll t'solution is char-
D.S
2"" 0.6
£
0
D
2 0.4
Il.
0.7 1 / // j ;-,1 IX ! / / x . / / / ;' l x p'-1re water
I r , O.O~ rnol/l
/' / . 0.1 '1'101/1
!J () 1 mol/I
,/.{ ;'
/' J/x [)
() 2 4 6 ~ 10
Loser pulse energy (me)
FIG. 2. Breakdown probability vs pulse energy in highly deionized water
and in saline solutions with concentrations of 1, 0.1, and 0.01 mol/I:
4150 J. Appl. Phys., Vol. 66, No.9, 1 November 1989 acterized by a small slope at the onset of the curve and by an
approach to the H20 values at higher energies.
IV, DISCUSSION
First the derivation of the breakdown curve form from a
theoretical model will be shown. Bass and Barrett intro
duced a model to explain qualitatively the electrical field
dependence of the damage probability in transparent solids.4
This model is based on the assumption that the probability of
breakdown occurrence is governed by the first stage of the
electron avalanche, where some starting electrons are accel
erated by the optical field to produce ionization. To undergo
an ionizing collision, a starting electron has to gain an excess
energy over the ionization energy of the surrounding medi
um. The remaining energy is divided between the two sec
ondary carriers, which therefore have much better starting
conditions to be accelerated than the first electrons. So it
remains to consider the probability for the first ionizing
event, involving electrons that start from rest. One difficulty
for this calculation arises from the fact that the electrical
field oscillates at optical frequencies. For an effective accel
eration the electrons have to keep in phase with the field,
undergoing elastic collisions at each time the field reverses.
If there are N free starting electrons present in the focal vol
ume prior to the onset of the avalanche, the probability per
unit time u (1) that the avalanche is released at the instant t
during the laser pulse is given by
u (t) = A exp [ -K IE (t) ],
(1)
where.,. c-;;lIl is the collision frequency of the electrons, f the
fraction of the favorable collisions that reverse the momen
tum of the electron, M the approximate number of half-cy
cles of the field required for the electron to reach the ioniza
tion energy Wi' q and I the electron charge and mean free
path, and E( t) the rms electric field strength at the time t
To get the exact expression for the probability that a
single laser pulse will produce breakdown, we must take into
account the temporal behavior of the irradiation. The proba
bility per unit time s(t) that breakdown starts at a time t
during the laser pulse is given by6
s(t) = u(t)v(t), (2)
where v(t) is the probability that breakdown has not started
until t and u(t) is given by Eq. (l). s(t) is also the time
derivative Ofp(t),6 which is the probability that breakdown
has already started until t.
pet) = 1 -v(t),
s(t) = dp(t) = _ du(t) = u(t)v(t).
dt dt
Solving the equation yields
v(t) = exp ( -f U(t')dt'). (3)
(4)
(5)
If the integral in Eq. (5) is expanded over the whole
pulse duration 1~ we get p( n, the probability that the laser
pulse win cause breakdown.
Schmidt-Kloiber, Paltauf, and Reichel 4150
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131.193.242.21 On: Thu, 27 Nov 2014 22:18:51p( T) = 1 -exp ( -iT uU)dt ) . (6)
Figure 3 shows again the breakdown curves of the 1
moll t and the 0.1 moll.f saline solutions. The dots repre
sent the measured data; the solid curves are least-squares fits
off unction (6) to the experimentai values. Since the geomet
rical focusing conditions in our experiments were held con
stant, we used the relation
E(t)2_p(t), (7)
where P(t) is the power to substitute E(t) by P(t) 1/2 in Eq,
0). Tocomputep(n (without A), u(t) is integrated nu
merically over a measured pulse shape, using an estimated
value for K. Following that, Eq. (6) is calculated for the
experimental values ofp( n to get a mean value for A .. This
procedure is repeated while altering the value of K untIl the
best fit is achieved.
The values of A and K for the solutions under study and
for the deionized water are listed in Table 1. It can be seen
from the data that K nearly remains constant for the 1 mol! /
and the 0.1 moll t solutions, while A increases approximate
ly proportional to the concentration, This behavior can be
explained in the foHowing way.
Since the factor A is proportional to the starting eiectron
density, its increase with the concentration indicates that the
ions in the solution act as electron donors. On the other
hand, K, which depends on the ionization energy and the
mean free path of the electrons in the liquid, is not influenced
by a variation of the ion density. Since the main component
oftne two solutions is water, the conditions under which one
of the starting electrons is accelerated by the optical field and
the amount of energy it must gain to produce ionization,
both described by K, are mainly those of water and therefore
must remain constant as long as the ion concentrations are
not too high. So the analysis of the curve parameters suggests
that the mechanism that causes a decrease of the breakdown
threshold in saline solutions is the delivery of starting elec
trons for the avalanche by the ions. This is in agreement with
the interpretation of breakdown data by other authors, who
explain the role of impurities in a similar way. 7,8
If we now look at the parameters of the breakdown
curve in extremely pure water, we find that they are as large
y---
0,8 ( //
>. ;;:: 0.6 I :D
0 n j 0 0.4
(l- I 1 mol/I
0 0,1 mol/I
0,2
0
0 2 3 4
Loser pulse energy (mJ)
FIG. 3. Breakdown probability vs puise energy in saline solutions. The solid
curves are least-squares fits of function (6) to the experimental data (dots).
4151 J. Appl. Phys., VoL 66. No.9, 1 November 1989 TABLE r. Parameters obtained by fitting Ell.. (6) to the experimental data.
Concentration A K
(molll) (ns -I) (WIlt)
481 2350
0.1 57 2410
0.Q1 4,8 2800
(first 3 points)
0 3><107 13 900
(pure water)
as K = 13 900 and A = 3 X 107• The increase afK is in con
tradiction to the explanation we found for the concentration
dependence of the breakdown probability in the saline solu
tions. If the K value of aU the samples is characteristic of the
water, it should not change in such a drastic way in the ab
sence of the ions. This discrepancy can also be seen in the
shape of the curves. Figure 3 shows that the slopes of the
breakdown curves are getting smaller with decreasing ion
content. Therefore, one would expect a very flat slope for the
H20 curve. The steep slope that is actually seen corresponds
to the high K and A values shown above.
To explain this contradictory behavior we have to re
consider one of the basic assumptions of the theory that was
used to derive u (t) in Eq. (l). This assumption was that
there are enough free starting electrons present in the focal
volume before the breakdown starts to initiate the ava
lanche. This may hold if these electrons are delivered by the
ions with low ionization energy in the saline solutions. In this
case we may assume that the electrons are delivered right at
the beginning ofthe laser pulse, thereby providing the neces
sary starting conditions for the avalanche throughout the
pulse, In other words, the threshold for the generati.on of
initial electrons lies below the threshold for the formatIOn of
the avalanche in the case of the more concentrated saline
solutions. (The breakdown curve of the 0.01 mol!.f solution
will be discussed later.)
In the absence ofious the first free electrons have to be
delivered by the water molecules by an intrinsic effect. We
suggest that a multiphoton ionization effect is responsible
for the generation of initial electrons in this case as it has
910 l'h .. been proposed for gases and solids,' Mu tIP oton lomza-
tion requires high levels of irradiation and shows a strong
field dependence of the ionization rate proportional to
P(tV, where z is the number of photons required to ionize a
water molecule.'! If the avalanche cannot start unless a suffi
cient amount of initial electrons has been generated by multi
photon ionization, then the breakdown probability must be
governed by the field dependence of this effect. In terms of
our previous argumentation this can be interpreted by a
higher threshold for the formation of starting electrons than
for the avalanche.
To take into account this fact, we substituted the con
stantA in Eq, (1) by a factor BP(W. UsingK = 2400 (asin
the saline solutions) and foHowing the same procedure as
described above to get B, we found that z = 9 yielded the best
Schmidt-Kloiber, Paltauf. and Reichel 4151
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131.193.242.21 On: Thu, 27 Nov 2014 22:18:51fit to the experimental data. This is very close to the theoreti
cal value z = 11 for gaseous H20 and a laser wavelength of
1064 nrn. The resulting curve form is shown in Fig. 4. It can
be seen that this modified formula fits quite well to the ex
perimental data. This confirms that the sharp threshold that
is observed in the breakdown curve of water might be due to
the supposed field dependence of the starting electron den
sity.
Next we want to discuss the breakdown curve of the
saline solution with the least concentration of 0.01 moll i'.
Obviously this curve represents a superposition of the two
effects described above. There we stated that a solution with
a small concentration and consequently a small starting elec
tron density should be characterized by a flat slope and a
wide statistical region. This relation seems to be realized by
the first three data points of the curve. Figure 4 shows that
the beginning of the 0.01 moll c curve coincides with a cal
culated curve with A = 4.8 and K = 2800 (dotted curve).
As expected, A is about one order of magnitude smaner than
in the 0.1 mol! t solution. It can further be seen that the
experimental values begin to depart from this curve right at
the onset ofthe H20 curve, indicating the enhanced electron
supply by multiphoton ionization.
The question whether fluctuations in the laser output
could give rise to the statistical nature of breakdown has
already been discussed in Ref. 6. There the authors empha
sized that the probabilistic behavior does not disappear even
with very smooth laser pulses. The fluctuations of the laser
used in our experiments were so small that they could only
contribute to a vanishing part of the observed statistical dis
tributions. This is evident since the quantity p( T) is the over
all probability that an electron anywhere in the focal vc1ume
causes the start of the avalanche at any instant during the
laser pulse. Therefore,p( 1') describes an effect that is mainly
dependent on the temporal and spatial integral over the in
tensity, given by the total energy, which was stable within
1%.
Finally the dependence of the relative width of the sta
tistical region on the concentration will be discussed. The
relative width will be defined as [W(O.95) -W(O.OS)]I
0.8
£-0.6
J:'i c
..Q
0 0.4 ,.
0...
0.2
0
0 2 4 x pure W(1t~r
o 0.01 me,/I
8
Loser puise energy (mJ) 10
FIG. 4. Breakdown probability vs pulse energy in 0.0! mol/l saline solu
tion and in highly deionized water. Dots: experimental; solid curve: calcu
lated using z = 9 and K = 2400; dotted curve: calculated using A = 4.8 and
K=2800.
4152 J. Appl. Phys., Vol. 66, No.9, 1 November 1989 W(O.95), where W(O.95) and W(O.05) are the pulse ener
gies at probability values of 0.95 and 0.05, respectively. A
semischematic plot of the situation in the investigated li
quids is shown in Fig. 5. The measured relative width in pure
water that exhibits the most thresholdlike behavior is indi
cated by the horizontal dotted line. At the same time, this
value gives a limit, to which the values of saline solutions
with decreasing concentration should asymptotically ap
proach (dashed line). The experimental values connected
with a solid line show that there exists a maximum width
near a concentration of 0.1 mol! t: The extrapolated point at
0.01 mol/I' is obtained from the dotted curve in Fig. 4 and
suggests that without the influence of multiphoton ioniza
tion no maximum would be formed, but rather a steady in
crease of the width should be expected with decreasing con
centration (dot-dash line) .
A similar connection between the number of starting
electrons, the breakdown probability, and the statistical
width has previously been suggested for the focal volume
dependence of the breakdown probability in solidso and
proved in the case of liquids. l2 In principle, it makes no dif
ference whether the number of starting electrons is changed
by varying the focal volume at a given electron density or by
changing the concentration of electron donors at constant
focusing conditions. Nevertheless, the second method offers
a possibility to influence the breakdown probability in liq
uids in a reproducible way, without changing the beam ge
ometry.
v. CONCLUSION
The observation of the statistical behavior of the laser
induced breakdown can lead to an understanding of the
physical effects associated with this phenomenon. We have
shown that the functional dependence of the breakdown
probability in aqueous solutions on the pulse energy can be
accurately described according to the lucky-electron model
of Bass and Barrett, who assumed the initiation of the elec
tron avalanche as the effect that is responsible for the statisti
cal nature of breakdown. An analysis of the parameters that
O.S
pure waier
Hi' 10.-2 10
Concentration (mol/I)
FIG. 5. Semi schematic plot oftbe relative width ohlle statistical region vs
the NaCI concentration. Dotted line: pure water, determining the lower
limit of the relative width; dashed line: estimated asymptotic trend with
decreasing concentration; points, connt:eted with solid curve: experimental;
single point and dot-dash line: shows the trend without the influence ofmul
tiphoton ionization.
SChmidt-Kloiber, Paitauf, and Reichel 4152
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131.193.242.21 On: Thu, 27 Nov 2014 22:18:51are obtained by fitting the theoretical function to the experi
mental data, has suggested that the influence of the ion con
centration on the breakdown threshold can be explained in
terms of a supply of starting electrons for the avalanche.
Another proof for this assumption is the significant change
of the statistical behavior in deionized water that we have
tried to explain with the necessity of an intrinsic effect, the
multiphoton ionization, to generate the initial electrons.
The difference to solids consists in the fact that in solu
tions the density of electron donors can easily be changed by
varying the concentration, what may be important for prac
tical application, where a reproducible adjustment of the
breakdown threshold is often required.
We have shown that at high concentrations as well as at
very low concentrations the relative width of the statistical
region has the tendency to decrease. This means that in these
solutions and especially in pure water the breakdown occur
rence may rather be characterized by a sharp threshold than
at the intermediate concentrations.
4153 J. Appl. Phys., Vol. 66, No.9, 1 November i 989 ACKNOWLEDGMENT
This work was supported by the FWF Austria, project P
6127.
'Yu. P. Raizer, SOy. Phys. JETP 21,1009 (1965).
2c. E. Bell and J. A. Landt, App!. Phys. Lett. to, 46 (1967).
'H. Schmidt-Kloiber, E. Reichel, and H. Schiiffmann, Biomed. Tcchnik
30, 173 (1985).
.IM. Bass and H. H. Barrett, IEEE J. Quantum Electron. QE-8, 338
(1972).
'u. Schmidt-Kloiber and E. Reichel, Acustica 54, 284 (1984).
10M. Bass and H. H. Barrett, A ppl. Opt. 12, 690 (1973).
7F. Docchio, P. Regondi. M. R. C. Capon. and J. Mellerio, ApI. Opt. 27,
3661 (1988).
"S. Hunklinger and P. Leiderer, Z. Naturforsch. 26a. 587 (1971).
9C. De Micheiis, IEEE J. Quantum Electron. QE-5, 188 (1969).
"'N. Bloembergen. IEEE J. Quantum Electron. QE-IO, 375 (1974).
ilL. V. Keldysh, SOY. Phys. JETP 20,1307 (1965).
12F. Docchio and C. A. Sacchi, Lasers Surg. Med. 6, 520 (1987).
Schmidt-Kloiber, Paltauf, and Reichel 4153
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131.193.242.21 On: Thu, 27 Nov 2014 22:18:51 |
1.576203.pdf | Effects of anodic fluorooxide on the thermal stability of Hg1−x Cd x Te
photoconductive arrays
Nili Mainzer, Eliezer Weiss, Daniel Laser, and Michael Shaanan
Citation: Journal of Vacuum Science & Technology A 7, 460 (1989); doi: 10.1116/1.576203
View online: http://dx.doi.org/10.1116/1.576203
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/7/2?ver=pdfcov
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fluoro-oxides does not depend much on the bath composition, as long as it is a solution of
hydroxyl and fluoride ions. Both secondary ion mass spectroscopy and low-energy proton
induced nuclear reaction, which is very sensitive to fluorine atoms, were used as depth profile
probes. It was found that the fluorine concentrated at the anodic film-semiconductor interface ~s
well as on the film surface. A mechanism by which fluorine is deposited in such a manner IS
advanced.
I. INTRODUCTION
The narrow-gap semiconductor Hg1 _ x Cdx Te has become
the most widely used infrared detector material today. 1,2
Hg1 _ x Cdx Te photoconductive detectors have become an
accepted standard in the 8-12 J-lm spectral range.3 It has
been pointed out, however, that the volatility and rapid dif
fusion of Hg may present some difficulties in processing and
degrade device performance.4
An anodic oxide film on Hg1 _ x Cdx Te is considered to be
an effective means for passivating the surface of photocon
ductive5 and metal-insulator semiconductor 1 (MIS) de
vices. As an anodic growth, the anodic oxide formation
causes only a small perturbation of the crystal lattice at the
interface. It also protects the semiconductor during the de
position of the insulating and antireflecting ZnS coating.
Furthermore, the large, fixed positive charge contained in
the anodic oxide causes a strong accumulation on the surface
of n-type Hg1 _ x Cdx Te photoconductors, which signifi
cantly reduces the surface recombination velocity.6 The ma
jor drawback of this technology is the poor thermal stability
of the oxide, which is a practical requirement for vacuum
packaging. 7
In a previous paper8 we reported on the development of a
novel process for the anodic growth of native insulating films
on Hg1 _ x Cdx Te containing both oxides and fluorides. A
careful selection of the hydroxyl-to-fluoride ion ratio in the
anodization bath enables the tuning of the band bending at
the semiconductor-dielectric interface. Furthermore, the
use of this anodic fluoro-oxidation yields interfaces with low
surface state densities which are stable up to -105 0c.
We report here on the use of anodic fluoro-oxidation in
the fabrication of Hg1 _ x Cdx Te photoconductor arrays.
These arrays show an improved thermal stability as com
pared to arrays made with unmodified anodic oxide.
II. EXPERIMENTAL
Anodic films were grown on a wafer of Hg1 _ x Cdx Te
(x-0.2) using either a carbon or a gold counterelectrode and constant current density. The anodization was accom
plished in an aqueous solution of either KP + KOH (fluoro
oxidation) orKOH (oxidation) in ethylene glycol. Theano
dization process was characterized by measuring voltage
time (V-f) characteristics.
Secondary ion mass spectroscopy (SIMS) profiles were
obtained on a PHI SIMS-I attached to a PHI 590A system
with 1.5-keV Ar+ primary ion beam. The beam was rastered
over a square region 3 mm on a side, with negative ion analy
sis restricted to a central region of -100 J-lm on a side. A
low-energy proton induced nuclear reaction having a high
sensitivity to fluorine atoms was also used as a depth profile
probe. The fluorine content in the film was measured by
counting the y rays (using a BGO detector) produced by the
resonant reaction9 19P(p,ay) 160 at increasing proton ener
gies. The measurements were carried out using the 5-MeV
Van de Graaffnuclear analysis facility of the Ecole Normale
Superieure. The beam currents were of the order of 60 nA,
on a I-mm2 spot. The energy spread of the incoming protons
was negligible so that the natural width of the resonance was
not broadened. The target was placed perpendicular to the
proton beam and the y rays were measured along the ion
beam. Under these conditions, the depth resolution was
-100 A.
The photoconductive devices were fabricated on slices of
n-type Hgl_xCdx Te with an x value of 0.213 (77 K cutoff
wavelength of 12.1 J-lm) purchased from Cominco, Ltd.
Neighboring wafers from the same ingot were used. The ar
rays were fabricated using either anodic flu oro-oxidation or
anodic oxidation. The same anodic film was grown on both
sides of the photo conductors. The detectors were heat treat
ed under high vacuum (10-7 Torr) at various temperatures.
Each annealing cycle lasted 17 or 48 h. The same array was
annealed at several elevated temperatures, yielding an accu
mulated annealing effect.
Arrays passivated by either the anodic fluoro-oxide or the
anodic oxide were characterized in wafer form by measuring
their resistance and photoconductivity at 77 K under the
460 J. Vac. Sci. Technol. A 7 (2), MarlApr 1989 0734·2101/89/020460-04$01.00 © 1989 American Vacuum SOCiety 460
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photon flux of a 300 K background (180' field of view). The
photoconductivity was measured as the relative change in
device resistance, resulting from the reduction of the photon
flux by ~30%.3
Photoconductive arrays passivated by anodic fluoro-oxi
dation using various solutions were packaged with a 60' field
of view and operated at liquid-nitrogen temperature. They
were evaluated by measuring their responsivity at peak
wavelength and 1000 K detectivity at 10 kHz.
III. RESULTS AND DISCUSSION
Arrays fabricated using either anodic fluoro-oxidation or
anodic oxidation were compared in wafer form after various
heat treatments. Figure 1 shows the differences between the
performance of the devices fabricated using the two types of
anodization. The photoconductors passivated with anodic
oxide show a monotonic increase of their resistance [Fig.
1 (a) ]. The resistance of the fluoro-oxidized photoconduc
tors, on the other hand, remains relatively constant up to
~ 105 'C. A similar trend is seen in the photoconductivity
[Fig. 1 (b) ]; there is a monotonic decrease of the photocon
ductivity of the devices passivated with the anodic oxide
while in those passivated with the anodic fluoro-oxide, the
photoconductivity remains relatively constant on annealing
at temperatures of < 100 'c.
Figure 2 shows the effect of annealing temperature on the
responsivity at peak wavelength and the 1000 K blackbody
detectivity at 10 kHz of photoconductive arrays passivated
with fluoro-oxides grown from various solutions. The data
shown are averages of the results of all 100 elements in each
array operated at liquid-nitrogen temperature. There is little
or no change in both the responsivity and the detectivity up
70 80 90 100 110 120 130
ANNEALING TEMPERATURE ('C)
FIG. I. Effect of annealing temperature on the average normalized resis
tance Ca) and relative photoconductivity (b) of 50 element photoconduc
tive arrays, passivated with either an anodic oxide CO) or an anodic fluoro
oxide (e). Measurements were carried out on the arrays still in wafer form
at liquid-nitrogen temperature under the photon flux of a 300 K back
ground (180' field of view). The photoconductivity was measured as the
relative change in the device resistance caused by reducing the photon flux
by -30%. Each annealing cycle lasted 17 h. Each array was annealed in
each experiment at all temperatures.
J. Vac. Sci. Technol. A, Vol. 7, No.2, Mar/Apr 1989 500'r-----------------------------~(a~)--
f= 10kHz.
•
80 90 100 110 120 130
ANNEALING TEMPERATURE (oC)
FIG. 2. Effect of annealing temperature on the responsivity at peak wave
length (a) and the 1000 K blackbody detectivity at 10 kHz (b) for photo
conductors passivated using various fluoro-oxides. Shown are average re
sults of all 100 elements in each array packaged with 60' field of view and
operated at liquid-nitrogen temperature. Each array was heated in all 48-h
long cycles. KF concentration in the anodization bath: 1M. KOH concen
tration: • 0.05M; • 0.075M, and e 0.15M.
to 105 'c. The thermal stability of the arrays does not de
pend much on the bath composition as long as it is a solution
of both hydroxyl and fluoride ions. However, performing the
anodization in the fluoridic bath with the highest hydroxide
concentration yields an array in which the degradation
caused by heating is greater than in the other arrays.
The characterization of MIS devices8 has shown that the
density of the fast surface state at the fluoro-oxide
Hg1 _ x Cdx Te interface is very low. Furthermore, this negli
gible density is stable up to 105 'c. This is in accordance with
the improved thermal stability of the photoconductors re
ported here.
It was suggested in Ref. 8 that the electrical behavior of
devices passivated with fluoro-oxides may be due to fluoride
ions dispersed in them, and mainly near their interface with
the semiconductor. However, Auger electron spectroscopy
(AES) measurements have shown that anodic layers grown
from such baths are composed of anodic oxides having only
traces of fluorine ( < 0.1 at. %).!O
To monitor their fluorine content, the layers were ana
lyzed using both SIMS and the 19F(p,ay) 160 nuclear reac
tion which are both very sensitive to fluorine atoms. Figures
3 and 4 show the depth profiles obtained for films grown
from various solutions. In the SIMS profile (Fig. 3), the
accumulation of fluorine at the oxide-semiconductor inter
face is clearly observed. As regard to Fig. 4, although there is
considerable scatter of the nuclear reaction data, it is clearly
seen that the anodic films contain fluorine. Furthermore, it
is concentrated at the anodic film-semiconductor interface
as well as at the surface of the film. The ratio of fluoride to
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~ 6
Z
:::>
~ g: 4
iii a::
S
« il! 2 «
~
UJ o
o
F
~ °O~~~--~8--~--~1~6---L--~2~4~~--~32~--L-~40
SPUTTER TIME (MIN)
FIG. 3. Typical SIMS depth profiles (1.5-keV Ar+ primary ions) of a
~ 500-A fluoro-oxide film showing the higher concentration of fluorine at
the anodic film-Hgi _ x Cdx Te interface.
hydroxyl ions in the anodization bath does not affect the
amount of fluorine at the interface, whereas the higher the
hydroxyl ion concentration, the lower the fluorine concen
tration at the surface. Figure 4 shows that using the films
depicted in the insets, the calculated profiles fit the measured
data. We assume the anodic fluoride [Fig. 4(a) 1 composi
tion to be as suggested in Ref. 8. The composition proposed
100
100
100 (a)
1M KF
(b)
(e) ~...J Ilh (~2CdO.8TeHCdF2)Y -i-MCT
:X'0.2
~ .01 I
"'.001 c:=J o 05 1.0 DEPTH (arb)
I b (HIl06 Cdo.4Te )~Fy -:-MCT
W I I X-0.2
~ 01 --_....Ir:=]
'" .0010 10
W :3 .I
;; 01
1M KF+0.15M KOH
1.0
350 360 370 380
PROTON ENERGY (keV)
FIG. 4. The yield of r rays counts from the 19F(p,ar) 160 resonant nuclear
reaction for films grown from various baths: (a) 1M KF in nonaqueous
ethylene glycol, (b) 1M KF + 0.05M KOH in 90% ethylene glycol and
10% water, and (c) 1M KF + 0.15M KOH in 90% ethylene glycol and
10% water. Solid lines are fits to the measured profiles calculated using the
films depicted in the insets. Film thickness: ~ 500 A.
J. Vac. Sci. Techno!. A, Vol. 7, No.2, Mar/Apr 1989 by Stahle et al. II and confirmed by us 12 was used for the
fluoro-oxide. It can be seen that fluorine atoms exist
throughout the layers. However, at the interface the concen
tration is higher.
This distribution of fluorine within the oxide implies for
an initial interaction between the F-in solution and the bare
Hgi _ x Cdx Te according to
(1)
with M probably CdS or Hg. MF2 stands for monolayers of
fluoride being absorbed and incorporated at the
Hgi _ x Cdx Te surface, while the following oxide growth and
thickening proceeds mainly independent of the presence of
F-.
The fact that this small amount of F-is found at the
semiconductor-oxide interface after the growth of the oxide
is completed may suggest a growth mechanism of the oxide
in which cations migrate out toward the electrolyte and the
main oxide being built on top of the original fluorinated in
terface. However, according to Strong, I3 the anodic oxide
growth on Hgi _ x Cdx Te consumes the surface instead of
being deposited on top of the outer surface. Thus, one would
expect to find fluorine at the oxide-electrolyte interface, un
less it is swept toward the substrate by oxygen vacancies.
Their movement toward the solution was shown to be the
primary growth mechanism of the oxide. 13
The film grown from the F-bath containing O.15M KOH
grew differently from those grown from either fluoride baths
which contain smaller amounts of OH-ions, or from
fluoride-free KOH solutions. This can be seen in Fig. 5 in
which the V-t curve shown for this film [Fig. 5 (b)] is non
linear. This indicates partial dissolution of the film during its
growth. Besides, this film contains smaller amounts of
fluorine in comparison with the other films studied (Fig. 4).
Also, the photoconductors fabricated using this film exhibit
a greater degree of degradation when heated (Fig. 2).
The reason for the smaller concentration of fluorine at the
oxide-semiconductor interface of this film is not exactly
known, but may be due to the large concentration of hy
droxyl ions which compete with the reaction in Eq. (1) for
an initial formation of an oxide.
As for the thickening of this film, we propose a process in
which the oxide growth is accompanied by its enhanced dis
solution due to the combined action of hydroxyl and fluoride
ions which are both present in large concentrations. This is
because neither the presence ofKOH nor that ofKF without
KOH yields the V-t curves shown in Fig. 5 (b).
According to this mechanism, a soluble hydroxy-complex
of at least one of the Hgi _ x Cdx Te constituents serves as a
necessary precursor for the interaction with the fluoride ions
in solution to form irreversibly a soluble fluoro-complex.
The partial dissolution which accompanies the film growth
is summarized as follows:
K
MOn + OH-~ HMO;+ I (sol) ,
HMO;+ 1+ mF-+ nH20
--.MF;;;(m-2n)(sol) + (2n + l)OH
(n = 1 for Cd and Hg, and 2 for Te.) (2)
(3)
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7 (0)
6
5
4
3
2
0
7
6
>5
~4 « I-cP
>
2 (b) _2
O.O~",A· c"'
1M KF+ 0.05M KOH
O.OS
~~--==---------
o.o~",A-c",-2
1M KF +0.15M KOH
O~~~~~~~~~~~~
7
6
5
3
2
0.15M KOH
012
TIME (MIN.)
FIG. 5. Voltage vs time characteristics at constant current for anodizations
carried out in various baths. Solvent composition: 90% ethylene glycol and
10% water.
J. Vac. Sci. Technol. A, Vol. 7, No.2, Mar/Apr 1989 Two of the Hg1_xCd x Te constituents (Cd and Te) are
amphoteric14 and thus dissolve to some extent in alkaline
solution. When the dissolved species is transformed (irre
versibly) into its more stable fluoride (e.g., TeF~ -) or oxy
fluoride, reaction (2) will be shifted to the right and film
dissolution will proceed.
IV. CONCLUSIONS
The use of fluoro-oxidation in the processing of
Hg1_xCd x Te (x~O.2) photoconductor arrays has been
demonstrated. This technology has the advantage of yield
ing devices which have an improved thermal stability rela
tive to arrays passivated by anodic oxidation.
ACKNOWLEDGMENTS
The authors are grateful to L. Carmiel, G. Moses, and M.
Saar for technical assistance, and to Dr. R. Brener for carry
ing out the SIMS analysis. Special thanks are due to Profes
sor G. Amsel for permission to use the nuclear analysis fa
cility of the Ecole Normale Superieure, Paris.
'Semiconductors and Semimetals, edited by R. K. Willardson and A. C.
Beer (Academic, New York, 1981), Vo!' 18.
2R. Oornhaus and O. Nimtz, in Narrow Gap Semiconductors, edited by C.
Hoehler (Springer, Berlin, 1985), p. 119.
3R. M. Broudy and V. J. Mazurczyk, in Ref. I, p. 157.
4K. Takita, T. Ipposhi, K. Murakami, K. Masuda, H. Kudo, and S. Seki,
App!. Phys. Lett. 48,852 (1986).
sp. C. Catangus and C. T. Baker, U.S. Patent No.3 997 018 (24 August
1976).
6y' Nemirovsky and I. Kidron, Solid State Electron. 22, 831 (1979).
7c. M. Stahle, C. R. Helms, and A. Simmons, J. Vac. Sci. Techno!. B 5,
1092 (1987).
"E. Weiss and N. Mainzer, J. Vac. Sci. Techno!. A 6,2765 (1988).
90. Oieumegard, B. Maurel, and O. Amsel, Nuc!. Instrum. Methods 168,
93 (1980).
ION. Mainzer and E. Weiss (unpublished results).
"C. M. Stahle, O. J. Thomson, C. R. Helms, C. H. Becker, and A. Sim-
mons, App!. Phys. Lett. 47, 521 (1985).
12M. Shaanan and O. Laser (unpublished results).
I3R. L. Strong, J. Vac. Sci. Techno!. AS, 2003 (1987).
'4Inorganic and Theoretical Chemistry, edited by J. W. Mellors (Longmans,
London, 1960), Vols. IV and XI.
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1.584108.pdf | Characteristics of optical components for soft xray microscopy and xray holography
using an undulator radiation optical system
M. Kakuchi, H. Yoshihara, T. Tamamura, H. Maezawa, Y. Kagoshima, and M. Ando
Citation: Journal of Vacuum Science & Technology B 6, 2167 (1988); doi: 10.1116/1.584108
View online: http://dx.doi.org/10.1116/1.584108
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
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.21.35.191 On: Mon, 22 Dec 2014 17:07:43Characteristics of optical components for soft x-ray microscopy
and x~ray holography using an undulator radiation optical system
M. Kakuchi, H. Yoshihara, andT. Tamamura
NIT Laboratories, Atsugi-shi, Kanagawa, 243-01, Japan
H. Maezawa. Y. Kagoshima, and M. Ando
Photon Factory, National Laboratory for High Energy Physics, Tsukuba-shi, Ibaraki, 305, Japan
(Received 1 June 1988; accepted 18 August 1988)
X-ray optical components, including Fresnei zone plates and transmission gratings, have been
fabricated using finely focused electron beam lithography and Ta-on-SiN x-ray mask fabrication
technology. The optical components for x-ray microscopy and x-ray holography were evaluated
in an undulator radiation optical system.
!.INTRODUCTION
Nanometer lithographY is now regarded as an important
technology to create new devices on very small structures
such as quantum wires or boxes 1 and new scientific probe
elements such as x-ray optical components. Optical compo
nents such as Fresnel zone plates have been investigated as
focusing and imaging elements in soft x-ray microscopy2.3
and astronomical spectroscopy. Transmission gratings have
been used in spectrophotometry.
Optical components have been obtained by using an x-ray
mask fabrication technology. In our study, we have found
that Ta-on-SiN x-ray mask fabrication technology is useful
for x-ray lithography.4 Finely focused electron beam lithog
raphy has enabled us to obtain Ta-on-SiN x-ray masks with
patterns under a quarter micron in size.
An undulator source which generates highly brilliant and
highly coherent soft x rays, is available at the Photon Fac
tory in Tsukuba.5 It is important to fully and best utilize
those capabilities. Therefore investigation of an undulator
radiation optical system and its application to soft x-ray mi
croscopy6 and holography has begun.
This paper describes new fabrication technology of Fres
nel zone plates and transmission gratings, and discusses
those optical characteristics for soft x-ray microscopy and
holography using an undulator radiation source.
II. FABRICATION OF XmRAY OPTICAL COMPONENTS
Reflective mirrors, transmission gratings, and Fresnel
zone plates have been investigated for x-ray optical compo
nent. Fresnel zone plates and transmission gratings have
been obtained with an x-ray mask fabrication process using a
gold absorber. In this study, we have established a new fabri
cation process for x-ray optical components using a Ta-on
SiN x-ray mask fabrication process. Tantalum is used for the
x-ray absorber, because it has the same absorptivity as gold.
A thin SiN film, which has a high transparence for soft x
rays, acts as a membrane supporting the Ta absorbing pat
terns.
The main steps of the fabrication process for Ta-on-SiN
structure optical components are shown in Fig. 1. SiN, Ta,
and Si02 were deposited by plasma chemical vapor depo
sition (CVD) or by a sputtering process. Electron beam (e
beam) lithography was carried out using phenylmethacry-late-methacryIic acid copolymer (tP-MAC) e-beam resist,
using a focused e-beam exposure machine. ¢-MAC has a 95
f-L C/cm2 sensitivity to a 30-keV e-beam on a Si02/Ta/SiNI
Si laminated substrate. Both Si02 and tantalum are etched
by a dry etching process. Tantalum in particular can be so
easily patterned by reactive ion etching (RIE), that we are
able to produce high aspect ratio Ta--Ta structures. Further
more, tantalum has an excellent physical property, of a me
chanical strength four times greater than that of gold. Ac
cordingly, the Ta-on-SiN fabrication process enables us to
obtain probe elements, which have a fine structure and a
large area useful for shorter soft x rays.
The fabricated outermost zone width is 0.25 p.m with up
to 2-mm diameter. The patterned pitch width of the trans
mission grating is O.4?lm with up to I.O-mm-square area. A
scanning electron microscope (SEM) picture of the Ta zone
plate with 1-mm diameter with O.25-f-Lm outer zone width is
shown in Fig. 2. The roundness of each zone is over 99%.
The inner five zones are apodized to decrease the undiffract
ed x rays passing through near the center.
m. UNDULATOR RADIATION
It is wen known that undulator radiation has such excel
lent properties of (i) high brilliance, (ii) quasimonochroma
ticity, (iii) wavelength tunability, (iv) high coherency, and
-----_E-beom resist !40MACI -----_Si02 -----....__Ta -----_SiN
-51 substrale -----------5iN
1 Fabrication process
~I E -Beom lithography
5102 RiE
To RIE ® SI bock etching
D D Ta
SiN .J-"--, ...... C,...-SI
FIG, I. Fabrication process of Ta-on-SiN x-ray optical components.
2167 J. Vac. Sci. Techno!. B 6 (6), Nov/Dec 1988 0734-211X/88/062167-03$01.00 @ 1988 American Vacuum Society 2167
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la)
(b)
FIG. 2. SEM pictures of Fresnel zone plate: (a) apodized zone plate with a
diameter of 1 mm and (b) outer zone with 0.25-p;m width.
(v) symmetrical radiation distribution with respect to the
radiation propagation axis. The first harmonic of the Photon
Factory (PF) undulator radiation can be tuned over the
wavelength range from 1.3 to 3.0 nm by varying the gap
width of the 60-period parallel permanent magnets.5 The
associated brilliance is -1014 photons/s mm2 mrad2 0.1 %
b.w .. Its spatial intensity has a Gaussian distribution, and
has -1 X2 mm area truncated by lie of the peak intensity.
The x-ray spectrum is measured using a reflective grating
mirror. The fractional bandwidth (the wavelength disper
sion degree) of the quasimonochromatic first harmonic was
observed to be zb--i\. 7 For x-ray microscopy and x-ray holog
raphy, quasimonochromatic x rays with a first harmonic
wavelength of2.7 nm were used.
IV. IMAGE CHARACTERISTICS WITH X-RAY ZONE
PLATES
The beamline for the soft x-ray undulator has two
branches: a straight branch and a deflection branch. For
evaluating soft x-ray optical elements or studying the soft x-
SR
UR Mirror
t/) imm
pinhole Film or
chonnel plate
PIRhOle:-j with object
~~
\ Micro \
Condenser zone plate
zo~------"
Zone plate
X-ray Microscope
FIG. 3. Undulator optical system for soft x-ray microscopy.
J. Vac. Sci. Technol_ a, Vol. 6, No.6, Nov/Dec 1988 2168
TABLE L Numerical parameters of zone plates.
Condenser zone plate Object zone plate
Radius of innermost zone
Number of zones
Diameter
Width of outermost zone 15.8 f-lm
1000
1.0 mm
0.25 flIT! 5.0 lim
100
0.1 mm
O.251lm
ray microscope and soft x-ray holography, an optical bench
with a high-precision linear translator is installed in the de
flection branch 25 m distant from the center of the undula
tor. As shown schematically in Fig. 3, a plane deflection
mirror is inserted upstream of the beamline to cut off unde
sired harmonics of the undulator radiation. A water-cooled
copper diaphragm of I-mm diameter is also inserted up
stream of the mirror for rejecting any components which
diverge too far from the axis.
The optical system of the zone plate soft x-ray microscope
is shown schematically in Fig. 3. The optical system is com
posed of a condensor zone plate, pinhole, objective zone
plate, and screen. In determining numerical parameters of
the optical components, it is considered that the source point
is supposed to be not the pinhole inserted in the beam line,
but the undulator itself. The numerical parameters of the
condenser and the objective zone plates were determined, as
shown in Table I. The diameter is 1 mm for the condenser
zone plate, and 0.1 mm for the objective zone plate. The
width of the outermost zone is 0.25 f1m for both the zone
plates. The focal1engths are 111 mm for the condenser zone
plate, and 11 mm for the object zone plate.
The microscope system consists of four parts, i.e., a con
denser chamber, object chamber, photodiode chamber, and
photographic chamber. Each of the chambers, except the
photographic one, has a laterally adjustable stage. The con
denser chamber can be scanned along the beam axis with a
high-precision linear translator constructed on the optical
bench. The object chamber has one manipulator on each
side; one is for the object zone plate and the other, for sam
ples. The optical system can be aligned by these manipula
tors in the object chamber.
la) (bl
FIG. 4. Image in soft x-ray microscopy: (a) object (SEM picture) and (b)
Ta grating image with x-ray microscope.
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Undulator radiation distance m., pd 12 A )
----i_-~.c.::::::--+I d ~'-'-'':;-';''
--_!oM!i!~- -I
I
a pair of gratings
(pitch: p ) Inferference fringes
( pictch : p 12 )
FIG. 5. Schematic draw of divided wavefront interferometer.
An microchannel plate (MCP) is mounted in the photo
graphic chamber so that the magnified image can be moni
tored through a view port at the end of the chamber. A cam
era is mounted on a movable stage in the same chamber next
to the MCP. After completing the alignment of the optical
system, the camera can be inserted on the beam axis in the
place of the Mep and the magnified images can be captured
on the film.
The resolving power of our x-ray microscope was mea
sured to be 0.3 11m with a magnification of 150, which is very
close to Rayleigh's limit of zone plates for an outermost zone
width of 0.25 11m. A Ta-imaged pattern with I.O-Ilm grating
period width is shown in Fig. 4. The bright space near the
center of the image field is the area illuminated through the
pinhole. The dark lines show the tantalum grating.
v. INTERFEROMETER USING XsRA Y
TRANSMISSION GRATINGS
By using a transmission grating as an x-ray beam splitter,
several kinds of interferometers could be constructed. As
schematically shown in Fig. 5, a divided wavefront interfer
ometer is constructed with a pair of gratings. Each grating
has O.4-flm grating pitch and lOO-l1m-square area. Each +
and -first-order diffracted x ray from a grating is superim
posed on an x-ray resist several em distant from the grating
plane. The mechanical stages of gratings and resist sample
holder were installed in a holography chamber. The third
harmonics (0.9 nm) of the undulator radiation through alu
minum foil with 4-,um thickness was used for illumination.
The spatial distribution of diffracted x rays of third har
monic was observed in a poly-hexafluoromethacrylate
(FIlM) x-ray resist. Only an undulator radiation dosage
changed interference colors in FBM films on substrates. The
J. Vac. Sci. Techno!. S, Vol. 6, No.6, Nov/Dec 1988 2169
exposure time for color changes with diffracted x rays is
several seconds. The exposed area is larger than the grating
area because of diffraction of quasi monochromatic (low dis
persion) x rays. The amount of spread perpendicular to grat
ings in excess of the initial grating aperture size is almost the
same as the numerical spread by quasimollochromatic x rays
with a 2b wavelength dispersion.
The superimposed exposure of + and -first-order dif
fracted x rays is also examined; however, interference fringes
have not been observed, perhaps due to the use oflow-disper
sion x rays and insufficient vibration prevention.
VI. CONCLUSiON
In conclusion, soft x-ray Fresnel zone plates and soft x-ray
transmission gratings have been successfully fabricated us
ing finely focused e-beam lithography and Ta-ou-SiN x-ray
mask fabrication technology. The optical components were
evaluated in x-ray microscopy and x-ray holography which
employed a brilliant and highly coherent undulator radi
ation source.
The resolving power of the zone plate x-ray microscope
was estimated to be 0.3 pm, which is very close to the Ray
leigh limit of 0.25 ,urn for the outermost zone width.
Finally, we constructed a divided wavefront interferome
ter with x-ray transmission gratings, and estimated the dis
persion of undulator radiation to be i~'
ACKNOWLEDGMENTS
The authors thank Professor S. Aoki of Tsukuba Univer
sity for his useful discussion on x-ray optics, The authors are
grateful for the help of x-ray components fabrication from
A. Ozawa and T. Ohkubo, NTT, and the help of undulator
examinations from Y. Toyoshima of the Photon Factory.
I Microcircuit Engin~eril1g 86, edited by H. W. Lehmann and Ch. B1eiker
(North-Holland, Amsterdam, (986).
'G. Schmahl, D. Rudolph, and B. Niemann Ann. N.V. Acad. Sci. 342, 368
( (980),
3J, Kirz and H. Rarback Rev. Sci. lnstrum. 56, 1 (1985).
'M. Sekimoto, A. Ozawa, T. Ohkllbo, and I-L Yoshihara, ill Extended Ab
stracts of 16th International Conference on Solid State Devices and Mate
rials, Kobe, Japan, 19R4, p. 23.
'II. Maezawa, Y. Suzuki, H. Kitamura, and T. Sasaki, Appl. Opt. 25, 3260
( 1986).
'Y. Kagoshima et al. in Proceedings of the International Symposium on X
ray Microscopy (Brookhaven National Laboratory. NY, 1987).
7H. MaezawR, A. MikllUi, M. Ando, and T. Sasaki, Jpn. I, App\. Phys. 26,
U(1987).
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1.576122.pdf | Highporosity coated getter
E. Giorgi, B. Ferrario, and C. Boffito
Citation: Journal of Vacuum Science & Technology A 7, 218 (1989); doi: 10.1116/1.576122
View online: http://dx.doi.org/10.1116/1.576122
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/7/2?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Tue, 23 Dec 2014 20:46:22High-porosity coated gettera)
E. Giorgi, B. Ferrario, and C. Boffito
SAES Getters S.p.A., Via Gallarate, 215/217, 20151 Milano, Italy
(Received 23 May 1988; accepted 5 November 1988)
The importance of nonevaporable getters is recognized by the increasing number of various
applications in vacuum technology. To meet some specific requirements of certain applications
(e.g., special vacuum tubes) a nonevaporable getter layer (from a few tens to 100,um or more)
having high porosity (50%-60%) and very good gas sorption and mechanical characteristics has
been developed. The special method used for the preparation of these getters produces gettering
surfaces on conductive substrates of many different preformed shapes (very thin strips, cylinders,
wires, etc. ). This allows accomodation of the getters in special or restricted spaces. These results
are reached by depositing on the substrates a mixture of powdered Ti (or Zr) and a nonevaporable
getter alloy of the types: Zr-Fe-V or Zr-Al. The getter material is then partially sintered to obtain
a good mechanical stability, keeping a relatively high porosity. The present paper describes the
characteristics of the recently developed high-porosity coated getters in terms of their physical
features (porosity, mechanical behavior in different working conditions, such as high H2 gas
load, etc.) and sorption performances for some main gases under different activation conditions
(from 300 to ~ 750°C), also making some comparisons with an evaporated Ti layer.
I. INTRODUCTION
The importance of nonevaporable getters is recognized by
the increasing number of various applications in vacuum
technology. For these reasons the Zr-AII-5 and Zr-V-Fe2-6
alloys have been extensively studied in their basic character
istics. The former can be considered a "wide spectrum" get
ter material, used in most applications where activation at
high temperature (above 700°C) is possible. The latter has
been found to have a satisfactory degree of activation even
after heating at moderate temperatures ( < 500 °C). These
gettering materials are available in different forms such as
pills, coated strips, etc., which are suitable for most applica
tions. However, in some cases, it is necessary to further im
prove the sorption characteristics of the getter (e.g., pump
ing speed at room temperature). High porosity in
nonevaporable getters provides a high adsorption pumping
speed even at room temperature. 7.8
Severe limitations for the use of nonevaporable getters in
many different applications have been caused by the difficul
ties in finding suitable accomodation for these getters in spe
cial or restricted spaces. A further problem in sophisticated
applications are loose particles released from the getter ma
terial, so that good mechanical stability became an impor
tant characteristic.
The new kind of getter described here combines high po
rosity, good mechanical characteristics and high flexibility,
an optimum practical thickness of ~ 70 ,urn, and a variety of
the shapes and the metallic bases usable, with very few limi
tations.
II. GETTER MATERIALS AND STRUCTURE
The new getter material is based on a powder mixture ofTi
and Zr 84--AI 16 or Zr70-V24.6-Fe5.4 alloys in the weight
ratio of ~ 7:3. The mixture is deposited onto a metallic sub
strate from a water suspension by means of a special process
which allows a very precise control of the thickness of the
getter coating (typically ranging from 50 to 150,um). After drying, the getter structure is sintered in high-vacuum sys
tems (in the temperature range 800-900 °C). The presence
of the Zr alloys acts as a getter and, at the same time, suitably
controls the sintering rate of the powder. This results in a
getter coating with a very porous structure and large surface
area, combined with high mechanical stability. The total po
rosity, measured by means of a mercury porosimeter, is
around 50%, with an average pore diameter of ~ 5 ,urn as
shown in Fig. 1. The histogram shows the pore radius distri
bution in percent (right-hand ordinate), whereas the curve
shows the pore volume occupied by pores having a radius
greater than that read on the abscissa. The pore structure
characteristics of the getter are shown by the scanning elec
tron microscope (SEM) photomicrographs in Fig. 2.
The getter coating exhibits a strong adhesion to the sub
strate, due to the formation of a diffusion layer at the inter
face during sintering at high temperature. With nickel and
nickel chrome substrates the anchoring of the getter material
Ol -... C'lE150----- ____ _
E
...J
~100
o
~5 Q.
~ ()
() o 10 100
90
80
70
60
50
40
30
20
10
106 " 0
:Xl m
:Xl »
0
c
VI
0
VI -t
:Xl
OJ c -t
0 z
~ -
FIG. 1. Porosity measurements ofSt 121. Average pore radius 3 Jim, B.E.T.
surface area 0.25 m2jg.
218 J. Vac. Sci. Technol. A 7 (2), MarlApr 1989 0734-2101/89/020218-05$01.00 © 1989 American Vacuum Society 218
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50 jJm
A
8
A
MAPPED AREA Ti MAP Zr MAP
FIG. 2. 8EM photomicrograph of cross section of8t 121 coated strip showing component distribution. A: getter (Zr-Al + Ti) and B: support (Nichrome).
is even stronger due to the formation of a low-temperature
melting eutectic with titanium. Evidence of this phenome
non is given by the SEM micrographs in Fig. 3 showing a
cross section of a getter sample and the x-ray maps of the
components in the same area. III. SORPTION CHARACTERISTICS
The room-temperature sorption characteristics of the new
porous getters have been investigated for H2 and CO, mea
suring the pumping speed as a function of the sorbed quanti
ty by means of the dynamic method.9 The getter activation
20 jJm
A
8
C
MAPPED AREA Ti MAP Ni-Cr MAP
FIG. 3. 8EM photomicrograph of cross section of8t 121 coated strip after sintering showing diffusion layer. A: getter (Zr-Al + Ti), B: diffusion layer, and C:
support (Nichrome).
J. Vac. Sci. Techno!. A, Vol. 7, No.2, Mar/Apr 1989
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H2
--I-a
-
r.::::::::: ~ ---~ r-.... O-Cl
........... ---~
10-5
10-6 10-5 10-4 10-3 10-2
SORBED QUANTITY (Pa m3/ cm2)
FIG. 4. Sorption tests of H2 on St 121 (Ti/Zr-Al) and St 122 (Ti/Zr-V
Fe) getters at room temperature and a pressure of 4 X 10 -4 Pa. a: St 121 act:
750°CX 10 min; a, :St 122 act: 750°CX IOmin;b: St 121 act: 350°CX3 h;
b,: St 122 act: 350 °CX 3 h; C: St 121 act: 500 °CX 10 min; andc,: St 122 act:
500 °c X 10 min.
was carried out at three temperatures: 350, 500, and 750°C
for 10 min referring to different possible conditions ofpracti
cal use. The sorption tests were performed at room tempera
ture at a constant pressure of 4 X 10 -4 Pa. The results ob
tained are shown in Figs. 4 and 5, respectively, for the
Ti/Zr-AI (St 121) and for the Ti/Zr-V-Fe (St 122) getter
compositions. The former exhibits poor sorption character
istics after activation at the lowest temperature while it
shows excellent performances at high activation tempera
tures. The behavior of the latter seems to be less dependent on
the activation temperature. In comparison with the Ti-Zr-
(fj
c w
~ 10-3
CI)
C!I
Z
~ 10-4
;:)
Q.
10-5 CO
~ ~ ~a i'-" '\~ \ c~\
~ 1\
\ \ \
~ ~\
10-6 10-5 10-4
SORBED QUANTITY 10-3 10-2
(Pa m3/cm2)
FIG. 5. Sorption tests of CO on St 121 (Ti/Zr-Al) and St 122 (Ti/Zr-V
Fe) getters at room temperature and a pressure of 4 X 104 Pa. a: St 121 act:
750 °CX 10 min; a,: St 122 act: 750 °CX 10 min; b: St 121 act: 350 °CX 3 h;
b,: St 122 act: 350 °CX 3 h; c: St 121 act: 500 °CX 10 min; andc,: St 122 act:
500 °c X 10 min.
J. Vac. Sci. Technol. A, Vol. 7, No.2, Mar/Apr 1989 220
Al material it is found to be more efficient after activation at
350°C, but less active at 700 °e. At 500 °e both materials
practically achieve the same degree of activation. These re
sults are in agreement with the well-known possibility of
being able to activate the Zr-V-Fe alloy at a lower tempera
ture with respect to the Zr-AI alloy.3 In Fig. 6 the pumping
characteristics of 70 wt. % Ti/30 wt. % Zr-AI (St 121) are
shown in comparison with a Ti film. The Ti film was ob
tained by evaporation from a Ti/Ta alloy filament in the
glass bulb used as a test chamber for the sorption tests. The
film so obtained was of 4 to 5 J.1g/cm2• The same tests have
been performed with thicker films of titanium but only negli
gible differences have been found with respect to the results
reported in Fig. 6. These data are in good agreement with the
results reported by Gupta et al.1O The nonevaporable getter
described here shows better room-temperature sorption per
formances per cm2 than the Ti film. The particularly high
sorption performance for H2 depends on the larger getter
material quantity per cm2 available in the nonevaporable
getter. The CO sorption differences, in fact, reflect the sur
face area differences (since no diffusion takes place at room
temperature for this gas). The similar trend of the CO sorp
tion on both getter materials indicate the relatively high gas
accessibility of the structure of the Ti/Zr-AI getter (due to
its high porosity).
IV. H2 EQUILIBRIUM PRESSURE
The H2 equilibrium pressures of both materials have been
investigated over the temperature range 100-800 °e and for
hydrogen concentrations from 102 to 2.103 Pa m3/kg, by
means of a typical static sorption apparatus. The results ob
tained in the present work are shown as the solid points in
Fig. 7 for the Ti/Zr-AI composition and in Fig. 8 for the
Ti/Zr-V-Fe composition. The lines are the calculated equi
librium isotherms using the weight ratio of the composition
components and the previously published data for titan
ium, II Zr-AV and Zr-V-Fe.5 The effect produced by mix-
::' 10-1
E
U
(fj
10.5 do '" .~
\ l~
" a
'\. A-
\
\
~ ~~
\ V W V1
10-6 10-5 10.4 -I-
10-3 10.2
SORBED QUANTITY (Pa m3/cm2)
FIG. 6. Sorption tests ofH2 and CO; comparison between St 121 getters and
a Ti film. a: St 121 act: 750 °CX 10 min, b: Ti film (4--5 ,ug/cm2).
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Tue, 23 Dec 2014 20:46:22221 Giorgi, Ferrario, and Boffito: High-porosity coated getter
FIG. 7. St 121 (Ti/Zr-Al) H2 equilibrium pressure vs quantity sorbed: solid
points = present work and lines = calculated from published data.
FIG. 8. St 122 (Ti/Zr-V-Fe) H, equilibrium pressure vs quantity sorbed:
solid points = present work, and lines = calculated from published data.
J. Vac. Sci. Technol. A, Vol. 7, No.2, Mar/Apr 1989 221
ing the two components is to produce an equilibrium pres
sure curve which, at any given temperature, lies between
those of the individual components. It is seen that there is
good agreement between the experimental results and the
calculated values. At H2 concentrations lower than 103 Pa
m3/kg for the Ti/Zr-V-Fe composition and 5.102 Pa m3/kg
for the Ti-Zr-AI composition the getter materials form H2
solid solutions which obey Sieverts' law in the following
forms:
Ti/Zr-V-Fe: log P = 1.98 + log q2 -5540/T,
Ti/Zr-AI: log P = 2.75 + log q2 -671O/T,
where P is the H2 equilibrium pressure in Pa, q is the H2
concentration in Pa m3/kg, and Tis the temperature in Kel
vin.
As it is seen the H2 equilibrium pressure for St 122 is
somewhat higher than that for St 121 (as expected from the
characteristics of the alloys present in the two cases).
V. MECHANICAL CHARACTERISTICS
Embrittlement phenomena could occur on getter coatings
especially due to the sorption of high loads of H2 and to
thermal fatigue. The resistance to high loads of H2, for
Ti/Zr-AI, has been tested in a glass system similar to that for
sorption tests. The getter at room temperature, after the acti
vation process, has been gradually doped with known
amounts of H2 until the embrittlement point was reached
(detachment of plates was observed). This point has been
found to be ~ 1.2 X 104 Pa II? kg -1 which is very high if
compared with usual nonevaporable getter coatings.12 Re
sistance to thermal fatigue has been evaluated submitting
some Nichrome (80/20) strips coated with Ti/Zr-AI getter
material to thermal cycles in a specially designed appara
tus. 13 The cycles were as follows: from 25 to 700°C in 5 min,
40 min at 700 DC, cooldown in 15 min to 25°C, and the cycle
is started again. The test was stopped after 400 cycles with
out observing any peel-off phenomena.
VI. CONCLUSIONS
In summary the main characteristics of this new type of
getter are high porosity and specific surface area that allow a
good sorption performance even at room temperature. The
chemical-physical characteristics of substrates and coatings
involved and the sintering process performed on them allows
very good mechanical stability of the getter materials even
under severe working conditions (thermal cycles or high H2
loads). Moreover the gettering surfaces can be well con
trolled as far as thickness is concerned and be obtained in a
variety of shapes with different substrate materials.
a) This paper was presented at the 34th National Symposium of the A VS,
Anaheim, CA, 1987.
IA. Barosi, in Proceedings o/the 4th International Symposium on Residual
Gases in Electron Tubes, Florence, 1971 (Academic, London, 1972), pp
221-235.
2K. Ichimura, M. Matsuyama, and K. Watanabe, J. Vac. Sci. Techno!' A 5,
220 (1987).
'C. Boffito, B. Ferrario, P. della Porta, and L. Rosai, J. Vac. Sci. Techno!.
18, 1117 (1981).
4C. Boffito, B. Ferrario, and D. Martelli, J. Vac. Sci. Techno!. A 1 1279
(1983).
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.174.255.116 On: Tue, 23 Dec 2014 20:46:22222 Giorgi, Ferrario, and Bottito: High-porosity coated getter
'c. Boffito, F. Doni, and L. Rosai, J. Less Common Metals 104, 149
(1984).
6K. Ichimura, K. Ashida, and K. Watanabe, J. Vac. Sci. Techno!. A 3,362
(1985).
7N. Hansen, Supp!. Nuovo Cimento 1, 627 C 1963).
HB. Ferrario, A. Figini, and M. Borghi, Vacuum Vo!. 35(1),13 (1984).
9 ASTM Standard F 798-82.
J. Vac. Sci. Technol. A, Vol. 7, No.2, Mar/Apr 1989 222
lOA. K. Gupta and J. H. Leck, Vacuum 25,8 (1975).
"T.A. Giorgi and F. Ricca, Supp!. Nuovo Cimento 2, 472 (1967).
12B. Ferrario and L. Rosai, in 7th International Vacuum Congress, Wien,
1977 CR. Dobrozemsky, Vienna, Austria, 1977), Vo!. I, pp. 359-362.
13B. Ferrario, M. Borghi, J. L. Cecchi, and J. J. Sredniawski, in Proceedings
of 11th SOFT, Oxford, 1980 (Pergamon, New York, 1981), pp. 375-383.
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1.575763.pdf | The deposition rate and properties of the deposit in plasma enhanced chemical vapor
deposition of TiN
Dong Hoon Jang, John S. Chun, and Jae Gon Kim
Citation: Journal of Vacuum Science & Technology A 7, 31 (1989); doi: 10.1116/1.575763
View online: http://dx.doi.org/10.1116/1.575763
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/7/1?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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J. Vac. Sci. Technol. A 4, 2726 (1986); 10.1116/1.573714
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.216.129.208 On: Tue, 25 Nov 2014 01:11:49The deposition rate and properties of the deposit in plasma enhanced
chemical vapor deposition of TiN
Dong Hoon Jang and John S. Chun
Department 0/ Materials Science and Engineering. Korea Advanced Institute o/Science and Technology.
p. O. Box 131, Chongryang. Seoul 131-00. Korea
JaeGon Kim
Technical Center. Daewoo Heavy Industry Company Ltd .• Mansukdong6. Donggu. Incheon.134. Korea
(Received 7 December 1987; accepted 19 September 1988)
Titanium nitride (TiN) films were deposited onto tool steels and cemented carbide cutting tools
by plasma enhanced chemical vapor deposition (PECVD) using a gaseous mixture of TiC14 , N 2 ,
H2, and Ar in order to find out the effects of the deposition temperature and rf power density on
the deposition rate and properties of deposited TiN. The deposition rate and crystallinity of the
deposited TiN was affected by the deposition temperature as well as the plasma power density.
The deposition rate was decreased with an increase in deposition temperature between 270 and
430°C. The crystallinity of deposited TiN was improved by an increase in deposition temperature
as well as rf power density. Crystalline TiN was obtained above 300 °C and showed a strong
crystallographic preferred orientation of (200). TiN layers deposited by PECVD using TiC14 as a
reactant contained chlorine, the content of which was increased with a decrease in deposition
temperature. Oxygen at the interface between the TiN deposited layer and the substrate excluded
nitrogen and chlorine. The surface morphology of the deposited TiN is a dome-shaped cluster
composed of many fine grains.
I. INTRODUCTION
TiN overlay coatings find numerous applications on steel
tools where they improve the mechanical properties of the
surface, increase hardness and wear resistance, lower fric
tional forces, and improve corrosion resistance. I Because of
its attractive golden yellow color, another application is as a
decorative coating. In recent years, a rapid development has
been achieved in the field of wear resistant coatings for steel
tools. Ever since the invention of the high-temperature
chemical vapor deposition (CVD) process, consistent ef
forts have been made to lower the deposition temperature.
Over the past few years three relatively low-temperature
physical vapor deposition (PVD) processes (namely, ion
plating,2 cathode arc plasma deposition,3 and reactive sput
tering4) have become commercially available for applying
TiN thin films on the surface of tools. However, there are
some restrictions to the practical application ofPVD process
for wear and corrosion resistant coatings due to nonuniform
ity and poor step coverage found with complex-shaped ob
jects. Recently, there have been some investigations of plas
maenhancedchemical vapordeposition5-7 (PECVD) in the
hope of producing more uniform deposits on substrates with
complicated shapes than are obtained from the line-of-sight
PVD processes. 8
Although Archer9 confirmed that TiN can be obtained by
PECVD at a low deposition temperature and Hilton8 inves
tigated the properties of TiN deposited by PECVD using a
TiC14 and NH3 gas mixture, detailed analytical experiments
have not been performed on the effect of process variables. In
the present investigation, TiN has been synthesized by a
PECVD process using a TiC14, N2, H2, and Ar gas mixture.
The effect of deposition temperature, rf power, and system
pressure on the deposition rate and the properties of the de
posited TiN were investigated. II. EXPERIMENTAL DETAILS
Titanium nitride was deposited onto tool steels and ce
mented carbide (WC-6Co) by means of a PECVD tech
nique using a gaseous mixture ofTiC14, N2, H2, and Ar. The
deposition took place between parallel horizontal plates in a
vacuum chamber. A glow discharge was ignited by rf excita
tion between the two circular electrodes of 150-mm diame
ter. A rf generator operating at 13.56 MHz provided power
to the upper electrode, while the lower electrode and reactor
wall maintained ground potential. The automatic matching
network had a 1T configuration with no blocking capacitor.
Specimens were held on the lower electrode which could be
heated to 450°C by a resistance heater adjacent to its under
side. A schematic diagram of the experimental apparatus is
shown in Fig.1.
Plain carbon steel (AISI WI), high-alloy tool steel (AISI
D2), high-speed steel (AISI M2), and cemented carbide
(WC-6Co) were used as substrates (dimensions of
12X18x3 mm). The chemical compositions of the steels
are listed in Table I. The specimens were cleaned in an Ar
and H2 gas mixture plasma for 20 min before the deposition
process. During the heating of the substrate, bubbled TiC14
in Ar carrier gas bypassed the reaction chamber and other
reactant gases were fed into reaction chamber. When the
substrate had reached the deposition temperature and the rf
power stabilized, TiC14 and other reactant gases were simul
taneously fed into the reaction chamber through a gas mix
ing box. After the deposition reaction had been terminated,
the substrates were cooled in vacuum.
The thickness of the deposited layer was calculated from
the weight gain of the specimen. A calibration was made by
measuring the cross section of the deposited layer by scan
ning electron microscopy (SEM). Since there was no dis-
31 J. Vac. Sci. Technol. A 7 (1), JanlFeb 1989 0734-2101/89/010031-05$01.00 © 1989 American Vacuum Society 31
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8
,--___ 12
~+=::::;::::>J.C::: 11
FIG. I. Schematic diagram of the experimental apparatus for PECVD of
TiN: I TiCI4 bubbler, 2 ice and water, 3 air operated bellows valve, 4 fine
metering valve,S gas mixing box, 6 heater, 7 specimen, 8 automatic match
ing network, 9 rf generator, 10 throttle valve, II vacuum system, and 12 oil
diffusion pump.
crepancy in deposition rate for the three kinds of steel sub
strates, the rate on steel was taken as the average value.
However, the deposition rate of TiN on WC-6Co alloy is
more rapid than that on steels.
X-ray diffraction and electron diffraction analysis were
carried out to identify the structure, and to evaluate the de
gree of crystallinity and preferred orientation of the deposit
ed TiN. X-ray diffraction analysis was performed using a Cu
Ka radiation with a graphite (0001) crystal monochro
meter. The chemical composition of the deposited layer was
analyzed by Auger electron spectroscopy (AES), which was
performed by a scanning Auger microprobe (Perkin-Elmer,
PHI 610) with a base pressure of 1 X 10-10 Torr. The single
phase cylindrical mirror analyzer was used with a primary
electron beam current of 0.15 f-1.A at 3 keY. The sputter etch
ing was performed with argon at a pressure of 5 X 10-8 Torr
at an accelerating potential of 3.5 kV and an ion current
density of 50 f-1.A/cm2•
The surface morphology and fracture surface of the de
posited layer were observed by scanning electron micros
copy. The grain size of the deposited TiN was investigated
using a transmission electron microscope (TEM) operated
at 200 kV. The specimens for TEM were mechanically
thinned to obtain wafers of ~ 30 f-1.m thick. Electron trans-
TABLE I. Designation and chemical composition of the substrate steels used
in this study.
Designation Chemical composition (wt. %)
AISI lIS C Cr Mo V W Fe
WI SK3 1.0 Balance
D2 SKDII 1.50 11.50 0.60 0.50 Balance
M2 SKH51 0.90 4.10 5.00 1.85 6.35 Balance
J. Vac. Sci. Technol. A, Vol. 7, No.1, Jan/Feb 1989 parent specimens could be obtained by ion beam milling. Ion
beam milling was carried out on the polished wafer surface
opposite to that covered by the TiN coating using argon ions
at a voltage of 5 kV.
III. RESULTS AND DISCUSSION
A. Effect of the deposition temperature on the
PECVDofTiN
The dependence of the TiN deposition rate on the depo
sition temperature is shown in Fig. 2. Above 270 T, the de
position rate decreases as the deposition temperature in
creases. The TiN deposited at 400 °C is a golden yellow
color; it tends toward a black color with a decrease in depo
sition temperature. Although TiN can be deposited below
270 °C, the coatings readily flaked off the substrate. The x
ray diffraction pattern of TiN deposited at 400 °C is shown in
Fig. 3. A distinct x-ray peak of the (200) crystallographic
plane corresponds to the lowest energy plane of the TiN
crystal lattice, which is a NaCl-type face-centered-cubic
(fcc) structure. x Improvement in crystallinity of the depos
ited TiN is observed with increasing deposition temperature.
This is confirmed by an increase in the peak height and a
decrease in the half-width of the (200) x-ray diffraction
peak. The x-ray diffraction patterns also show that the TiN
deposited by PECVD has a strong preferred orientation of
(200). The AES spectra of TiN deposited by PECVD at 270
and 400 °C are shown in Figs. 4 and 5, respectively. From the
AES spectra, it is seen that TiN deposited at 270 °C retains
more chlorine atoms than TiN deposited at 400 °C. The in
corporated chlorine atoms may contribute to the increase of
deposition rate as decreasing deposition temperature be
cause the chlorine is difficult to desorb at the lower tempera-
.r:::
"-1.5
5.1.0
w
~ a::
z o
f=
Ui
~ w 0.5 o
o 200 300 • Steel
owe-Co
400
DEPOSITION TEMPERATURE (t)
FIG. 2. Dependence of deposition rate on deposition temperature (system
pressure 3 Torr, total flow rate 200 sccm, TiCl4 inlet fraction 0.01, N2 inlet
fraction 0.25, rfpower 25 W, and d 5 em)
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.216.129.208 On: Tue, 25 Nov 2014 01:11:4933 Jang, Chun, and Kim: Deposition rate and properties of the deposit 33
>t::
(f) z w
f-
Z
(a' 30
>f--
iii z w
f-
~
z ;:: 40 .~ A
50
29 (degree) o
'" '"
~ .A
60 70
~
z ;::
70
FIG. 3. A portion of the x-ray diffraction pattern of TiN films deposited by
PECVD at a temperature of 400 'C: (a) rfpower 25 Wand (b) rfpower 50
W.
ture. Kikuchi et al. 10 investigated the effects of chlorine con
tent on the wear resistance of TiN deposited by PECVD.
They proposed that the chlorine atoms induce some lattice
defects in TiN and thus its crystallinity partially deterio
rates. Therefore, an increase in chlorine content may induce
more strains in the deposited layer. The induced strain may
be the cause of flaking of the TiN deposited at low tempera
tures.
Figure 6 shows Auger depth profile spectra of TiN depos
ited at 400 0c. A thin oxide layer is present at the interface.
Helmersson et al. 11 investigated the adhesion of reactive dc
magnetron-sputtered TiN on high-speed steels. They report
ed that the thin oxide layer at the interface has an important
effect on the adhesion of the deposited layer. Sputter etching
of the substrate prior to deposition improves the film adhe
sion even though a complete removal of the oxide layer is not
achieved. The AES spectra of the interface is shown in Fig. 7.
7
6
5 w '0
"--4 w C g3 z o
'0
2
418
0 CI 383
160 240 320 400 480 560
ELECTRON ENERGY, eV
FIG. 4. AES spectrum of TiN deposited at 270 'c.
J. Vac. Sci. Technol. A, Vol. 7, No.1, Jan/Feb 1989 7
6
5
w
~4 ~
f-
f-
f-
o
30 c
CI
230 ~ / -\-1
0
-
418
383
430 630 830 1030
ELECTRON ENERGY, eV
FIG. 5. AES spectrum of TiN deposited at 400 'C.
100
80
~60
<.5
« 40
20
o N1
Tl2 --.l!L
~ 01
FE3 II... CI1
o 8 r
-
FE3
~
01 )\
16 24 32 40 48 56 64 72 80
SPUTTER TIME (MIN.)
FIG. 6. In-depth distribution of composition of TiN layer deposited at
400'C.
7
6
5
w "04
:::::
w 0 Fe
~3
Z
"02
418
ELECTRON ENERGY, eV
FIG. 7. AES spectrum of the interface after sputter etching the specimens of
Fig. 6 for 68 min.
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The ratio of the peak height of 383 to 418 e V at the interface
(Fig. 7) is smaller than that of the deposited layer (Fig. 5).
According to Dawson and Tzatzov, 12,13 the decrease in this
ratio means that the atomic ratio of nitrogen to titanium is
decreased.12,13 The CI 181-eV peak height at the interface
(Fig. 7) is also less than that in the deposited layer (Fig. 5).
In addition, such results can be seen in the Auger depth
profile (Fig. 6). Therefore, it is suggested that the oxygen
atoms in the interface replace both nitrogen and chlorine
atoms in the deposited layer.
B. Effect of the rf power on the PECVD of TiN
The dependence of TiN deposition rate on the rfpower is
shown in Fig. 8. The deposition rate is affected by rf power
and electrode spacing. The deposition rate reaches a maxi
mum value and then decreases with an increase in rf power.
At low rfpower, the deposited TiN has a golden yellow color
but tends toward a brown color at power levels greater than
that which produces the maximum deposition rate. At low rf
power, an increase in power improves the growth kinetics by
generation of more reactive radicals; however, the depo
sition processes may be inhibited by ion bombardment or
plasma-etching processes as the power is further increased.
In addition the depletion of reactant gases by the plasma
between cathode and reactor wall is also regarded as a factor
for the low deposition rate observed at high rf power. From
Fig. 8, it is seen that the rf power needed to achieve a maxi
mum deposition rate increases with increasing electrode
spacing. As a result, the deposition rate may depend upon
the plasma-power density, which is defined by dividing the
applied power by the plasma volume. The crystallinity of
deposited TiN improves with an increasing rf power. The
.c
"-1.5
E
2-1.0
W
~ a:::
z o
i=
Vi
&::0.5
W o
o • d-3cm o d z3cm
• d -5cm a d -5cm
• d-7cm
tJ. d -7cm
RF POWER (W) Steel
WC-Co
Steel
WC-CO
Steel
WC-CO
60
FIG. 8. Dependence of deposition rate on the rf power (temperature 400 "C,
system pressure 3 Torr, total flow rate 200 sccm, TiCI. inlet fraction 0.01,
and N2 inlet fraction 0.25).
J. Vac. Sci. Technol. A, Vol. 7, No.1, Jan/Feb 1989 half-width of the (200) x-ray diffraction peak becomes nar
rower, as shown in Fig. 3. The crystallization of deposited
TiN must be enhanced by ion bombardment. Generally, the
ion bombardment energy increases witIr"an increase in rf
power because the sheath electric field increases as the
square root of the rfpower density. 14,15
C. Effect of the system pressure on the PECVD of TiN
Figure 9 shows the deposition rate as a function of the
system pressure with a total flow rate of 120 sccm. The depo
sition rate is slightly affected by the system pressure with a
dependence similar to that of the rf power. Generally, for
chemical vapor deposition the deposition rate increases with
an increase in system pressure. However, in the case of
PECVD, as the system pressure increases at constant power
density the collision rate will also increase, but the average
electron energy will eventually decrease simply because the
existing field will have less time to accelerate an electron
between collisions. Since the generation rate for the active
species depends on the electron energy distribution, this will
determine the number of electrons in an appropriate energy
range for generating a particular active species or its precur
sor. Therefore, a pressure increase must have a similar effect
on the plasma characteristic as does a decrease in rf power. 16
Therefore at a constant rfpower density, the TiN deposition
rate decreases at high system pressures.
D. Electron micrographs of TiN deposited by PECVD
The microstructures and a selected area electron diffrac
tion pattern of TiN deposited on D2 steel by PECVD at
430°C are shown in Fig. 10. The diffraction pattern
[Fig.lO(a)] shows that the deposits have a fcc polycrystal
line structure. A typical bright-field transmission electron
micrograph as well as (200) and (220) dark-field transmis
sion electron micrographs are shown in Figs. 11 (b), II (c),
and 11 (d), respectively. The transmission electron micro-
1.0
z 0.5 o t:
(f)
~
W o
o •
2 • Steel
OWC-Co
3
SYSTEM PRESSURE (torr) 4
FIG. 9. Dependence of deposition rate on the system pressure (temperature
400 "C, total flow rate 120 sccm, TiCl4 inlet fraction 0.01, N2 inlet fraction
0.33, rfpower 25 W, and d 5 em).
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la) Ib)
Ic) Id)
FIG. 10. Transmission electron micrographs and the diffraction pattern of
TiN films prepared by PECVD at 430 'C: (a) ring pattern of de posited TiN,
(b) a typical bright field micrograph, (c) (200) dark-field micrograph, and
(d) (220) dark-field micrograph.
graphs show that TiN deposited by PECVD consists of ex
tremely fine grains.
Scanning electron micrographs of the surface morphology
and fracture surface of TiN deposited onto D2 and WC-6Co
alloy are shown in Fig. 11. The deposited TiN has a smooth
dome-shaped surface with a dense structure regardless of rf
power density and substrate. The dome-shaped cluster is not
a single grain but is composed of many grains of extremely
fine size.
IV. CONCLUSIONS
The deposition rate of TiN deposited by PECVD is affect
ed by the deposition temperature, rf power, and electrode
spacing. TiN deposited at temperatures > 300°C has a fcc
polycrystalline structure with a strong preferred orientation
of < 200 > and contains chlorine. In the range 270--430 °C, the
deposition rate decreases with an increase in temperature.
The deposited layer holds more chlorine atoms at the lower
deposition temperatures. This incorporated chlorine in
duces strains in the deposited layer, which results in flaking
at temperatures < 270°C. Oxygen atoms at the interface ex
clude nitrogen and chlorine atoms. The crystallinity of the
deposited TiN improves with increasing temperature as well
as rf power. The deposition rate reaches a maximum value
and then decreases with increasing rf power. The rf power
needed to achieve a maximum deposition rate increases with
an increase in the electrode spacing because the plasma char
acteristics are controlled by the plasma-power density. The
dependence of the deposition rate on the system pressure is
like that on the rf power because an increase in the system
pressure induces a similar effect on the plasma characteris-
J. Vac. Sci. Technol. A, Vol. 7, No.1, Jan/Feb 1989 lal b)
Icl Id)
FIG. II. Scanning electron micrographs of surface morphology and fracture
surface of TiN films deposited onto D2 steel and WC-6Coalloy: (a) and (c)
on D2 steel, (b) and (d) on WC-6Co alloy.
tics as a decrease in rf power. The surface morphology of
TiN deposited by PECVD is a smooth dome shape, regard
less of deposition parameters and substrate, but the dome
shaped cluster is composed of many fine grains.
ACKNOWLEDGMENTS
This work was supported by Ministry of Science and
Technology, Korea. The authors thank Professor K.-T. Rie
for his valuable discussions and J. D. Park for the Auger
measurements.
'K. K. Yee, Int. Metal!. Rev. 1,19 (1987).
2R. Buhl, H. K. Pulker, and E. Moll, Thin Solid Films 80, 265 (1981).
3H. Brandolf, P. Flood, and P. Walsh, Cutting Tool Eng. 34, 4 (1982).
4W. D. Muenz, D. Hofmann, and K. Hartig, Thin Solid Films 96, 79
(1982).
'J. L. Hollahan and R. S. Rosier, in Thin Film Processes, edited by J. L.
Vossen and W. Kern (Academic, New York, 1978), p. 335.
oS. M. Ojha, in Physics o/Thin Films, edited by G. Hass (Academic, New
York, 1982), Vo!. 12, p. 237.
7T. D. Bonifield, in Deposition Technologies/or Films and Coatings, edited
by R. F. Bunshah (Noyes, New Jersey, 1982), p. 365.
HM. R. Hilton et al., Thin Solid Films 139, 247 (1986).
9N. J. Archer, Thin Solid Films 80,221 (1981).
ION. Kikuchi and Y. Oosawa, in Proceedings o/the 9th International Con
/erence on Chemical Vapour Deposition (Electrochemical Society, New
Jersey, 1984), p. 728.
"u. Helmersson, B. O. Johnansson, J.-E. Sundgren, H. T. G. Hentzell, and
P. Billgren, J. Vac. Sci. Techno!. A 3, 308 (1985).
'2p. T. Dawson and K. K. Tzatzov, Surf. Sci. 149, 105 (1985).
13B. J. Burrow, A. E. Morgan, and R. C. Ellwanger, J. Vac. Sci. Techno!. A
4,2463 (1986).
14c. B. Zarowin, J. Electrochem. Soc. 130, 1144 ( 1983).
"C. B. Zarowin, J. Vac. Sci. Techno!. A 2,1537 (1984).
"'C. M. Melliar-Smith and C. 1. Mogab, in Ref. 5, p. 531.
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.216.129.208 On: Tue, 25 Nov 2014 01:11:49 |
1.38028.pdf | AIP Conference Proceedings 184, 1830 (1989); https://doi.org/10.1063/1.38028 184, 1830
© 1989 American Institute of Physics.Introduction
Cite as: AIP Conference Proceedings 184, 1830 (1989); https://
doi.org/10.1063/1.38028
Published Online: 16 June 2008
D. A. Edwards
1830
INTRODUCTION
D. A. Edwards
Fermi National Accelerator Laboratory
P. 0. Box 500, Batavia, IL 60510
TABLE OF CONTENTS
1 Evolution of the Tevatron Design ............................ 1831
I.i Context and Initial Decisions .......................... 1831
1.2 Development of the Fixed-Target Design ................. 1832
1.3 Transformation to a Collider ........................... 1833
2 Components of the Tevatron .................................. 1834
2.1 Superconducting Magnets ................................ 1835
2.2 Cryogenics ............................................ 1837
2.3 Power System and Quench Protection ..................... 1838
2.4 Vacuum System .......................................... 1839
2.5 Conventional Systems ................................... 1840
3 Performance and 0utlook ..................................... 1841
3.1 The Fixed-Target Program ............................... 1841
3.2 Colliding Beams ........................................ 1842
3.3 0utlook ................................................ 1843
1831
INTRODUCTION
D. A. Edwards
Fermi National Accelerator Laboratory
P. 0. Box 500, Batavia, IL 60510
1 EVOLUTION OF THE TEVATRON DESIGN
1.1 Context and Initial Decisions
When the Fermi Natlonal Accelerator Laboratory and its
accelerators were designed in the late 1960's, it was too early to
give serious consideration to the use of superconducting magnets in
the main proton synchrotron, for the necessary technology was in its
infancy. However, space was reserved in the Main Ring enclosure and
in the service buildings for the eventual addition of a
superconducting synchrotron. Because the "supermagnets" might be
expected to reach twice the field of their room-temperature
counterparts, the superconducting ring was called the Energy Doubler
at that time.
After high energy physics experiments were underway in 1972, it
became possible to devote some attention to the Doubler idea. A
major factor in the design context was already settled: the
superconducting synchrotron would occupy the same tunnel as the Lain
Ring. A number of other design principles were established at the
very outset, a few of which deserve mention here. The decision to
use a cold beam tube was controversial at the time; today it is the
natural choice. A warm iron magnet design was chosen, but the
debate over the relative virtues of cold or warm iron still
continues and successful magnets have been constructed with both
approaches. The decision to use NbTi was a recognition of the state
of materials technology.
The superconducting magnet designs of that time were quite
unsuited to the needs of a large synchrotron. Nor was there an
established production base for filamentary NbTi in the volume or
quallty required. A substantial research and development program
was necessary, focused almost exclusively on the magnets and their
cryogenic system. The evolution and verification of a successful
magnet design for a mass production environment led to the
constuction of over 200 full scale prototypes.
In 1978, after six years of magnet development, it was possible
to expand the Doubler effort to include the design o~ the entire
accelerator-collider synchrotron. As a result of this research and
development program, a design report was published in 1979 that
served as the basis for the superconducting accelerator construction
project. A parallel effort on an antlproton source followed close
behind, with the intent that colliding- beam physics commence soon
after the beginning of fixed- target physics in the new energy
region.
© 1989 American Institute of Physics
1832
The "Tevatron" projects included
synchrotrons, but common usage has
superconducting ring itself, and we
here. the construction of three new
attached that name to the
will follow that convention
1.2 Development of the Fixed-Target Design
Among questions of overall accelerator design, one concern
dominated all others. Could beam extraction be performed so
efficiently that beam particles striking the magnets would not cause
a superconducting-to-normal transition?
For use in high energy physics experiments, the beam is removed
from the synchrotron by a process called resonant extraction. After
the beam of over 1013 protons has been accelerated, the particles
are gradually spilled from the ring at a rate of about 10 7 per turn.
The delicate control needed to coax such a small fraction of the
protons out on each orbit is provided by nonlinear magnetic fields
that excite a resonance in their motion.
Of course, other operational situations involve beam loss, and
so the possibility of quenches. But in most cases, the risks
attendant on beam loss can be eliminated or reduced by component or
systems design. With resonant extraction the situation is
different. In any version of the process devised to date, the
particles leaving the circulating beam encounter the entrance to an
extraction channel, and the particles that will depart are separated
from those that will remain within the ring by a material boundary.
Inevitably, some particles strike this boundary, or "septum."
Protons will scatter or interact with the nuclei in the septum, and
the secondary particles from these processes can deposit their
energy in superconducting magnets downstream. Experimental studies
confirmed the anticipated sensitivity of accelerator-style
superconducting magnets to beam loss.
The fact that the Tevatron would occupy the same enclosure as
the gain Ring implied that the magnet disposition of the Tevatron
would be a near-replica of the resident synchrotron. An exact
reproduction of the ~ain Ring optics would lead to energy deposition
exceeding allowances by a factor of fifty to a hundred.
Fortunately, it was not necessary that the superconducting ring
duplicate its predecessor exactly bend-for-bend and lens-for-lens.
By introducing conventional bending magnets in the neighborhood of
the extraction septa, local orbit modifications were made that
reduced losses from inelastic interactions by an order of magnitude.
Similarly, a modification in the focusing order of the quadrupole
lenses spread the beam out as it approached the extraction septa and
so improved the extraction efficiency. Collimation was introduced
within the arcs to intercept particles that had undergone elastic
scattering in the primary septum.
Finally, the lattice was modified to permit the introduction of
an efficient beam abort system. If beam losses in the magnets
during extraction should exceed an operationally determined level,
the remainder of the beam would be kicked out of the ring into an
external beam dump. Again, use was made of conventional magnets to
1833
the degree possible in the critical region near the beam exit
channel. A longitudinal gap was left in the beam so that no
particles would be deflected during the abort kicker risetime.
Thus, the ring as constructed was a hybrid: mainly composed of
superconducting magnets, but with a vital admixture of traditional
steel and copper hardware.
1.3 Transformation to a Collider
In the mid-lgVO's, the growing appreciation of the potential of
hadron colliders added an additional function to the requirements
for the superconducting ring. It should perform as a storage ring
as well. Though there was a period during which proton-proton
collisions involving the ~ain Ring and the Tevatron was
contemplated, the decision finally fell on the side of proton-
antiproton collisions in the Tevatron.
Interestingly enough, the potentially vexing question of magnet
aperture had been settled by the slow extraction requirement. The
7.5-cm inner diameter of the coil was fixed in 1975 following an
analysis of the aperture and field quality needed for resonant
extraction. It was judged that magnets of the resulting design
would be adequate for single-particle stability during beam storage.
Rather, the main issue raised by the additional application was
a good deal less subtle; namely, where yet another set of functions
was to be put within the existing tunnel. A major colliding-beam
experiment needs space surrounding the beam tube for a detector some
20 meters in length, and on either side there must be strong
quadrupole lenses to reduce the beam area at the interaction point.
In short, at least one of the six long straight sections permitted
by the tunnel geometry would have to be reserved exclusively for
colliding-beam physics. A plan was devised which compressed the
accelerator and fixed-target physics systems into the other five
regions in both the Main Ring and the Tevatron.
Use of a second straight section for collisions requires the
removal of some function used in the fixed-target physics program.
Fortunately, this can be done relatively easily, for the primary
extraction septum and the nearby conventional magnets that protect
the superconducting magnets from beam loss occupy an entire straight
section. Thus, that straight section can be cleared by moving a
limited number of components. To be sure, the installation of the
focusing optics and the detector is a task of somewhat greater
magnitude, but still of reasonable scale.
Production of the principal missing ingredient - the p's - was
the mission of the antiproton source. The design called for
targetting of 120- GeV protons from the Kain Ring, followed by
capture of the resulting p's at 8.9 GeV/c in a debuncher
synchrotron. Radiofrequency system gymnastics in the Main Ring
deliver short duration but broad momentum-spread bunches to the
target. Since the momentum distribution of the ~'s is inherently
wide, this procedure provides the optimum use of the longitudinal
acceptance of the system. After momentum-spread reduction by
debunching, the p's transfer to an accumulator synchrotron, where
stochastic cooling and stacking take place along the lines of the
approach pioneered by CERN.
1834
The layout of the Tevatron facility is shown schematically in
Fig. 1, and a short s-mm~ry of the principle design parameters is
given in Table I. The ratio between peak beam energy and injection
energy may look surprisingly low, but this is simply a consequence
of the Tevatron sharing the tunnel with the Main Ring and so having
a high energy injector available.
UNAC
SOURCE ~OOSTER
sw, .Y, R0
p EXTRACT j ., v
B0 DETECTOR "~ & LOW BETA \
ENERGY DOUBLER
( TEVATRON ) MR
~ ABORT
0 ~ CO
15/p TRANSFER ~'~ SAVER EXT:ACTION OR /
~ p ABORT
00 OVERPASS
Figure 1 Fermilab accelerator complex including the Antiproton
Source rings and the Tevatron. The straight sections are labeled
AO through FO. AO and DO are needed for extraction, with DO as a
colliding beam area as well. BO is the other dedicated colliding
beam area. Injection occurs at gO; rf acceleration at FO. Beam
abort is at CO.
2 COMPONENTS OF THE TEYATRON
The main special design features of a superconducting
accelerator are, of course, the magnets themselves and the cryogenic
system to cool them. But other systems are required to assume new
roles in the superconducting ring. For example, the magnet power
1835
Table I Tevatron Design Parameters
General
Acce-c-~ator radius
Peak beam energy
Injection energy
Bend magnetic field at I000 GeV
Beam emittance ~N (gt~)
Fixed target
Intensity
Acceleration rate
Cycle time
Slow spill duration
Fast spill
Collider
Intensity per bunch
Number of bunches
Luminosity
Storage time between fills
Amplitude function p* llm
800-1000 GeV
150 GeV
4.4 Tesla at 4400 Amperes
247 mm mrad
~2x1013 protons/cycle
50 GeV sec-"
50s
20 s
5 pulses at 2XlO 12 protons
6x1010 expected
3p. 3p
1030 cm-2 sec-1
"4 hr
1 meter (x,y)
system must take on the primary burden of protecting the magnets if
they quench. The vacuum system must provide thermal insulation for
the cryostats in addition to establishing the empty space in which
the beam circulates. Finally, a higher standard of reliability is
demanded of the conventional accelerator systems, in order to
compensate for the £nevitable maintenance demands of the components
at the frontier of technology.
2.1 Superconducting Magnets
The potential benefits of superconductivity for synchrotron
magnets are high field in a compact package with low power
consumption. As the Tevatron design effort began, high field meant
4.4 T, a figure twice that of the Main Ring and one that should be
achievable with the materials of the day.
The superconducting magnet complement of the Tevatron includes
772 bending magnets (dipoles), 224 quadrupoles, and 720 small
correction and adjustment elementsl The main bending magnets occupy
75~ of the perimeter of the accelerator - they are the dominant
magnetic element, and the discussion here will be limited to them.
The standard dipole is 8.4 m long and 38 ca by 25 ca in cross
section. The only exceptions are two half-length dipoles installed
in the vicinity of the beam abort. The evacuated beam pipe runs the
length of the magnet through its center, and the vertically oriented
magnetic field deflects the protons and antlprotons by an angle of
8.1 arad.
A transverse section of the magnet is shown in Fig. 2. Just
outside of the square beam tube, there is space for the liquid
helium which cools the inner edge of the coil. Each of the many
small rectangles in the coll represents the cross section of the
1836
cable. The coil is clamped by stainless steel collars in a highly
reproducible, accurate configuration that does not distort durinE
magnet excitation. There are additional spaces for liquid helium
flow between the outer surfaces of the collared coil assembly and
the enclosing tube. The next annular region contains two-phase
(liquid and gas) helium flowing in the direction opposite to that of
the single-phase fluid. The two-phase helium is at the lower
temperature and so extracts heat from the llquld, which in turn
extracts heat from the coil. Outside of the helium container is an
insulating vacuum space and then two concentric pipes. The narrow
space between these pipes contains liquid nitrogen, which intercepts
heat flow inward from room temperature. The insulating vacuum
region between the nitrogen shield and the room temperature outer
cryostat tube contains superinsulatlon (aluminized Mylar) as an
additional radiation shield. The whole magnet-cryostat assembly is
vacuum tight. It is held in a laminated iron yoke that contributes
some 18~ to the total magnetic field. The cryostat is precisely
adjusted relative to center with suspension blocks of epoxy-
fiberglass laminate and preloaded suspension cartidges that allow
for contraction and expansion during the thermal cycle.
Figure 2 Cross section of the Tevatron dipole magnet showing the
collared coil assembly, the cryostat, and the warm iron yoke.
1837
Measurements on superconducting cable for the Tevatron magnets
gave an average critical current density in the NbTi of 1800 A/mm 2
at 5 T and 4.2°K. Taking into account the magnet geometry, field,
and operating temperature of 4.6°K, one finds that this critical
current density should permit the magnets to achieve 4.6 T. All
magnets prior to their installation in the ring were measured under
two different excitation conditions. In the first test, magnets
were ramped at 200 A/s until a quench occurred. In the second test,
repetitive ramps approximating the accelerator cycle were used and
the peak current was gradually increased until the quench limit was
found. The results indicate that excitation of the ring in the
900-950 GeV range should be possible with only a few magnet
replacements, whereas to reach 1TeV will probably require either
very slow ramps or cryogenic modifications.
2.2 Cryogenics
The helium refrigeration system is the world's largest. It
consists of a large helium liquefier, a nitrogen reliquefier, a
distribution system for the cryogens around the ring, and 24
satellite refrigerators spaced around the ring. These components
can provide a total of 24 kW of cooling at 4.7°E for the magnets as
well as liquid helium for power lead cooling and liquid nitrogen for
the cryostat heat shields.
The satellite refrigerators are located directly above the
tunnel and feed helium and nitrogen directly into the magnet string
below. The flow is split and goes upstream and downstream (with
respect to the proton beam) through typically 16 dipoles and
associated components in each direction. Throughout this outward
flow, the helium is in a single subcooled liquid phase; it is this
helium that is in direct contact with the magnet coil. At the end
of the string, the helium passes through an expansion valve which
lowers its temperature and pressure. These new conditions are
adjusted so that the helium is a boiling liquid at a temperature of
about 4.5°K, one or two tenths of a degree lower than the
temperature of the single- phase fluid. The two-phase fluid is
directed back through the string of magnets and absorbs heat from
the outgoing stream. Thus each magnet is a counterflow heat
exchanger. The ratio of gas to liquid in the two-phase path
increases as the distance to the refrigerator feed point decreases,
but the temperature remains nearly constant. Energy generated in
the coil is thereby removed efficiently by the single-phase liquid
and absorbed as heat of vaporization in the two-phase region.
Nitrogen for the magnet shields makes a single pass to the ends of
each string, where it is discharged into a nitrogen header as 92°K
gas.
The central plant plus satellites arrangement offers a wide
variety of operating conditions, and provides the redundancy
necessary to the continuous operation of the synchrotron. In
'satellite mode,' the central plant supplies large amounts of cold
helium to the magnet strings and thus to the return side of the
satellite heat exchangers. The excess flow in the satellite heat
1838
exchangers results in 1 kW of refrigeration from the satellites
without use of their gas expansion engines. At the other end of the
spectrum is the "stand-alone mode." Here, without the availability
of helium from the central plant, each satellite is able to deliver
450-500 W of refrigeration plus 25 liters per hour of liquid helium.
This capability is adequate to compensate the static heat load of
the magnets. A variety of intermediate cases are possible depending
on the availability of helium from the central plant.
2.3 Power System and Quench Protection
The magnet power system plays the dual roles of powering the
main bend and quadrupole magnet string and protecting these same
magnets from the stored energy in the magnetic field should any
fraction of the superconductor in the whole ring become normal
(resistive) for any reason. Because this system requires quick
detection of quenches and consequent action of electrical components
in order to save the magnets from self-destruction, it is called
nactive" as opposed to one that might require little or no external
action, that is, a "passive" system.
The main bend and quadrupole magnets form a single series
circuit, in which 12 power supplies are uniformly distributed. Each
power supply is capable of ramping to 4500 A at 1 k¥. Since the
resistance of the circuit is small (but not zero, for there are
conventional magnets in the circuit), a single well-regulated supply
is able to supply during particle injection, flattop, or storage
conditions.
Our main interest here lies in the quench protection aspect of
the system. Consider what happens in the cable of a magnet coil
when a "normal zone" appears and current transfers from the NbTI to
the copper in which the superconductor filaments are embedded. The
copper, which now conducts most of the current, has too high a
resistivity to prevent further heating, and the cable will melt
unless some means is found to remove the current expeditlously. The
rate at which the cable temperature rises is difficult to calculate
because of the nonlinear behavior of the parameters (specific heat,
resistivity, thermal conductivity, etc) that describe the cable
consituents at low temperature. The system parameters were set as a
result of measurements made on the rate of temperature rise in
quenched cable. At the maximum operating current, there is less
than one-half second available for removing the magnet current to
prevent permanent damage.
The magnets and their interconnections are continuously
monitored for a resistive voltage component. Once the onset of a
quench is detected, the power supplies are turned off, and the
current is shunted through dump resistors at the supply locations.
The resulting exponential current decay has a 12- second time
constant that is too slow to protect the normal zone in the magnet
that has quenched, so locally the current must be reduced much more
quickly. To do this, the circuit is divided into 24 quench
protection units. A "safety lead" connects the superconducting bus
to a room temperature bypass circuit at the ends of each unit. This
1839
lead cannot carry steady state operating currents. If it were
designed to do so, the refrigeration load presented by the leads
would be unreasonably high. But the leads can convey the decaying
magnet current around the quench protection unit that contains the
quenching magnet. Current is switched into the safety leads by the
closing of thyrister switches.
The fate of the cable depends on the outcome of a race between
the cable temperature and the decay of the current in the quench
protection unit; the latter depends on the total resistance of the
normal zone. To insure that the race ends in favor of the cable,
heaters are energized in the dipoles of the protection unit to
quench a large quantity of superconductor. The resulting rapid
resistance growth drives the current down with a time constant
appropriate to an in-bounds temperature rise.
2.4 ¥acuumSystem
The vacuum system consists of three separate subsystems with
different characteristics and requirements. The cryostat insulating
vacuum system is the most complex and is completely isolated from
the high-vacuum cold beam tube system inside the magnets. The
straight sections and other noncryogenic regions have warm beam
tube, bakeable, conventional vacuum systems.
All in all there are about 1300 cryogenic interfaces between
magnets or between magnets and other components. A magnet-to-magnet
interface includes a beam tube seal, two liquid helium connections,
one liquid nitrogen connection, and a large external room
temperature insulating vacuum seal. Each of the cryogenic seals
must be able to be verified at room temperature with sufficient
sensitivity to assure that it will not leak liquid helium. By far
the most time-consuming aspect of installation is the interface
connection and leak checking. During initial installation each
interface took on the average one man-week; subsequent work has been
done in about half this time.
The static heat leak of the cryostat-magnet system is due to
thermal radiation and heat conduction through magnet supports and
other structural elements as well as through residual gas in the
insulating space of the cryostat. For the Tevatron geometry, the
static heat load doubles at a pressure of 2XlO -5 torr (He). An
upper limit of 10 -5 torr is set for operation, which corresponds to
a reading of 3×10 -8 on nltrogen-calibrated cold cathode gauges. In
operation, readings are typically at the 10 -7 level. The insulating
vacuum is pumped with turbo molecular pumps.
The pressure in the cold beam tube is very low if helium leaks
are absent. Pressures of 5XlO -11 torr cold (5×10 -10 torr as
measured warm) are normal. The cold beam tube provides an
economical way of obtaining the high vacuum required for beam
storage over the major fraction of the rink circumference. The main
concern in adopting the cold bore approach was the potential of
helium leakage to the beam tube; such leaks are extremely difficult
to detect or pump without warming the system up. Because of this
worry, the cryostat was designed so that the beam tube seam weld is
the only weld between the helium spaces and the bore tube vacuum.
1840
The vacuum systems, with their many flanges, seals, pumps,
valves, and gauges, have been remarkably trouble free and reliable.
This must, to a large extent, be due to the cryo-pumping ability of
the refrigerated surfaces. The fact that most of the circumference
of the ring (93~) is cold means that the pressure in the warm
regions need not be particularly low. With 5x10 -11 torr where cold
and 10 -8 in the warm regions, the reduction in luminosity during
storage due to interactions in the residual gas is expected to be
23% after 20 hours.
2.5 Conventional Systems
Under this heading, we comment briefly on other systems of the
Tevatron, which, though impacted by the superconducting design, are
basically required in any accelerator. Correction magnets, beam
diagnostics, and the radiofrequency acceleration system are
examples.
Correction magnets are used to compensate for field
imperfections or alignment errors of the main magnets, and to tune
the optics of the ring to desired operating conditions. With a few
exceptions, these are superconducting magnets configured in circuits
suitable for specific functions. For instance, there are two
circuits each containing 90 trim quadrupoles that are used to make
fine adjustments in the overall focusing characteristics of the
synchrotron. The correction magnet power supplies can be programmed
to produce virtually any waveform throughout the accelerator cycle.
The strengths of the superconducting elements are sufficient for use
at full excitation, a design feature that has proved particularly
valuable for steering corrections.
Under beam diagnostics, we will mention only the position and
loss monitoring systems; they are essential to the operation of the
synchrotron. The fact that recovery from a quench can take an hour
or more makes it imperative that the reasons for erratic beam
behavior be sorted out with as few beam pulses as possible. The
position monitoring system can measure the deviation of the beam in-
or-out or up-or-down from center at 200 locations around the ring.
The system has a wide dynamic range to permit start-up of operations
with a beam so low in intensity that a quench is unlikely. A large
amount of information is stored in the system memory, so that an
operator can ask for recall of position at every location of the
injected beam, position at specific locations for 1000 turns, orbit
deviation from center at arbitrary times throughout the accelerator
cycle, and detailed position profiles prior to an abort. A similar
number of loss monitors are distributed around the ring. These are
radiation detectors placed outside of but close to the magnets. The
electronics for each detector is designed so that the output signal
is related to the probability of quenching the magnets. Outputs of
the loss monitors are continuously checked and used to abort the
beam automatically (within 200 microseconds) if the signal is larger
than tolerances derived from experience.
The radiofrequency accelerating system consists o~ eight 53-MHz
resonant cavities, each of which can produce i/3 MV. The frequency
1841
must be changed by only 2 kHz from 150 to I000 to compensate for the
change in the protons' speed, since the proton is already moving at
99.998~ of the speed of light at injection. Because the required
modulation is small, the frequency program is a completely dead-
reckoned digitally generated function derived from the main magnet
excitation program. All eight cavities are used for acceleration of
protons during fixed-target operation. The cavities have been
positioned relative to one another so that, by appropriate phasing
of their radiofrequency excitation, they will ~unction as one set of
four for acceleration of protons and the second set of four for
independent acceleration of antiprotons. The colliding point of the
two beams can be moved circumferentially around the accelerator and
frozen at a particular point by frequency sad phase adjustment of
the two sets.
3 PERFORMANCE AND OUTLOOK
3.1 The Fixed-Target Program
The Tevatron was commissioned in the Summer of 1983. On
July 3, protons were accelerated to 512 GeV and the Tevatron became
the highest energy accelerator in the world. Operation for fixed-
target physics began in October of that year at an energy of
400 GeY; the following February the energy was raised to 800 GeY and
the goal of doubling the energy of the Main Ring had been reached.
To date, there have been four fixed- target runs. Generally
speaking, the beam dynamics behavior of the Tevatron has been
excellent. Of course, a small beam size at injection and an
extensive correction magnet system are a big help. But, more to the
point, some of the "ghosts" that had been attributed to the new
magnet technology failed to materialize. For instance, there had
been concern that the coils of magnets would gradually move as many
ramps accumulated or suddenly move as a result of quenches. Such
has not been the case; operating conditions are more stable and
reproducible from day to day than the older synchrotrons in the
accelerator chain.
In the fixed-target mode, the peak energy has been limited to
800 GeV, primarily due to energy deposition in the superconducting
magnets during the resonant extraction process. Typically, about
l.SXlO 13 protons are accelerated in each one-minute cycle, and
extracted throughout a twenty- second interval at peak energy.
Intensities as high as 1.8xlO 13 have been reached. Though the
intensity that can be achieved varies ~rom day to day, it usually
represents a balance between total protons accelerated versus a
tolerable level of beam aborts or quenches. During a week, 1000
aborts sad 20 quenches would not be unusual figures.
Cyclic operation provides a severe test of the new
superconducting magnets. In the 20 months of fixed-target physics,
the ring has been put through about three-quarters of a million
magnetic cycles, most of them to 800 GeV. During the first run, a
production error was uncovered that required repair of half of the
magnets in the S,,mmer of 1984. The coil leads at one end of these
1842
magnets had not been tied to prevent motion due to their mutual
magnetic repulsion; as a result, strands of the superconducting
cable began to break. Now, three years later, the present run has
been plagued by a succession of magnet failures that are likely due
in part to more subtle motions of the magnet leads. Another round
of repairs is in the offing.
Reliability is the big issue in the accelerator facility.
There are now four accelerators in the chain for flxed-target
physics ( for collidlng-beam physics, there are six). Actual uptime
for high energy physics is about 70~ of scheduled time, as opposed
to 80~ for gain Ring operation prior to the Tevatron era. It is
obvious that the long-term success of the Tevatroa rests on the
attainment of high operational reliability.
3.2 Colliding Beams
At the end of the fixed- target run in 1985, four weeks of
intensive effort concluded with the observation of a dozen pp events
in the detector facility, marking the beginning of colliding-beam
physics at Fermilab. A long shutdown for a variety of construction
projects followed, after which a four-month "engineering" run for
collider operation took place in early 1987. During this latter
period, a peak luminosity of 1029 cm -2 sec -1 and an integrated
luminosity of 70 nb -1 were achieved.
The collider mode is much more congenial to the superconducting
ring. The number of particles in the accelerator is almost two
orders of magnitude less, there is no slow extraction, and the
burden of frequent ramping is absent. The reduced beam loss in the
magnets permitted an increase in the energy to 900 Ge¥, only about
30 Ge¥ lower than the quench limit without beam.
The last major dynamical 'ghost" associated with
superconducting magnets vanished with the observation of single-beam
lifetime in excess of 100 hours. Thus far, the luminosity lifetime
is in the 5 to 10 hour range, and is determined by emittance growth
of unknown origin. A major goal of present collider studies is the
identification of the noise mechanism that produces the emittance
growth of both particle species.
The peak luminosity in the 1987 run was 10~ of the design
value. Roughly speaking, beam loss in the various transfer and
acceleration stages, and emittance dilution can be blamed in equal
proportion. Approximately a factor of two dilution in emittance
takes place in the transfer from the Hain Ring to the Tevatron as a
result of the vertical dispersion mismatch between the two rings and
the relatively large momentum spread of the proton or antiproton
bunch. That there is a vertical dispersion is a consequence of the
undulations added to the gain Ring to move the two accelerators
apart at the collislon points. This source of dilutlon will be
removed by a further modification to the gain Ring in early 1988.
During acceleration and turn-on of the collision optics in the
Tevatron, the p's suffer a further factor of two increase in
emittance. The p bunches are about an order of magnitude less
intense than the proton bunches, so the beam-beam tune spread
1843
differs for the two species. It is likely that inadequate tune
control throughout the many steps between the injection optics and
the low-beta optics is the source of much of this dilution; if so,
improvement will come with further study.
The non-cycllc character of colllder operation transformed a
hitherto innocuous characteristic of the superconducting magnets
into a major irritant. The magnets exhibit a broad spectrum of eddy
current time constants, from the second to many hour time scale. In
fixed target operation, multipole moments arising from these
currents could be lumped together with the persistent current
multipoles. But during the lengthy setup for a transfer in collider
mode, the variation in, for example, the chromaticity must be
compensated. An adequate model of the magnet does not yet exist to
account for these effects. In retrospect, it is again fortunate
that it was posslble to inject at relatively high energy; the
advantage of high injection energy for persistent currents was
recognized, but the additional advantage for eddy current phenomena
was not.
3.3 Outlook
Over four years after its co~issloning, the Tevatron is still
very much a prototype accelerator, as much a research instrument in
its own right as it is a high energy physics tool. The
superconducting magnets are not as robust as the better examples of
their conventional counterparts, and it is entirely possible that
extensive repairs and improvements will be necessary in the near
future to achieve adequate reliabillty of operation in the fixed-
target mode.
For the collider run scheduled to begin in April of 1988, the
goal is to achieve a peak lumlnosity of 3XlO 29 cm -2 sec -1. Steps
have been taken to reduce the beam loss at some of the points in the
transfer and acceleration process, and as noted earlier, a major
source of emittance dilution will be corrected. In reaching the
peak luminosity in the 1987 run, p* at the interaction point was
reduced to 0.7 z in contrast to the design value of 1 m listed in
Table I. For 1988, p* will be further reduced to 0.5 m. Operation
will be upgraded to 6 bunches of both particle species.
It is likely that the performance levels of Table I can be
reached with time. But to surpass those levels to a significant
degree will probably require some modification of the accelerator
facility. Both the fixed-target and collider programs would benefit
from a reduction in the proton beam emittance. The emittance growth
in the first few milliseconds of the Booster synchrotron cycle is
attributed to space charge; raising the energy of the Linac injector
would ameliorate that situation. The gain Ring lifetime at
injection is much less than that which would be predicted from gas
scattering; simulations suggest that there is a dynamic aperture
limitation. At 20 CeV however, beam lifetime is consistent with gas
scattering, and a Post-Booster synchrotron to inject at this level
would also eliminate the necessity to cross transition in the gain
Ring.
1844
An order of magnitude increase in peak luminosity beyond the
present design may be possible. It implies multi-bunch operation
with separated beams in the Tevatron. Many bunches are needed in
order to achieve the luminosity, and separation is necessary except
at the collision points in order to limit the beam-beam tune spread.
The total number of p's required is almost an order of magnitude
larger than the number that can be collected and stored in the
present Accumulator, so another ring, that might be called the
Depository, is implied. The Depository would accept p's from the
Accumulator, or receive and recool diluted p's from the Tevatron.
Hany variations of the foregoing scenario are obviously
possible. But the main point is that some major modification of the
existing Fermilab facility will be needed to keep the high energy
physics program abreast of demands until the SSC becomes available
in the middle of the next decade.
|
1.339501.pdf | Effects on InP surface trap states of i n s i t u etching and phosphorusnitride
deposition
YoonHa Jeong, Shinichi Takagi, Fusako Arai, and Takuo Sugano
Citation: Journal of Applied Physics 62, 2370 (1987); doi: 10.1063/1.339501
View online: http://dx.doi.org/10.1063/1.339501
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/62/6?ver=pdfcov
Published by the AIP Publishing
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129.105.215.146 On: Sat, 20 Dec 2014 19:54:41Effects on InP surface trap states of in situ etching and phosphorus-nitride
deposition
Yoon-Ha Jeong, Shinichi Takagi, Fusako Arai, and Takuo Sugano
Department 0/ Electronic Engineering. University o/Tokyo. 3-1. Bongo 7 chome. Bunkyo-ku.
Tokyo 113. Japan
(Received 15 January 1987; accepted for publication 12 March 1987)
Effects of in situ etching of InP surfaces with PCl3 followed by low-temperature in situ
chemical vapor deposition of phosphorus-nitride in a phosphorus-rich ambiance using NH31
PCl3/H2 on trap states of the interfaces were studied. The breakdown field of the phosphorus
nitride films was as high as 1 X 107 V cm - 1 and the films showed trap-assisted conduction in
high electric field with resistivity higher than 1 X 1014 n cm near the electric field of 1 X 107
V cm -I. Interface properties were found to be critically dependent upon PCl3 molar fraction,
both the etching and deposition time, and the etching and deposition temperature. The
frequency dispersion of capacitance-voltage characteristics in accumulation was about 3.3%
for the frequency range from 10 kHz to 1 MHz. The hysteresis was as low as 0.17 V for the
field electrode voltage swept between - 6 and + 6 V. The density of interface trap states, Nss'
was 2x 1011 cm-2 eV-1 at about 0.3 eV below the conduction-band edge ofInP and was
8X 1011 cm-2 eV-1 near the bulk Fermi level.
I. INTRODUCTION
Recently, thermal and photochemical vapor deposition
(CVD) techniques for the formation of phosphorus-nitride
(PN) films have been reported by other workers to improve
the electrical properties ofthe interface of the indium-phos
phide metal insulator (InP MIS) structures. 1-3 Phosphorus
nitride films are considered suitable for binding InP MIS
structures because they have a common constituent with
InP, and phosphorus (P)-rich ambiance for deposition also
provides an efficient protection of the InP surface during
good insulator deposition.4-8 The thermal CVD film shows a
large resistivity and a high breakdown voltage, but thermal
degradation of the InP substrate remains a problem in this
. film fabrication technology because of its high deposition
temperature of about 600 ·C.
Evidence for degraded electrical properties of InP MIS
structures is found in hysteresis in capacitance-voltage
curves, in hysteresis in low-frequency current-voltage char
acteristics of metal insulator semiconductor field-effect tran
sistors (MISFETs), and drift of the MISFETs drain cur
rent. The presence of the electrical interface instabilities is a
major problem impeding the development of reliable high
speed InP MIS integrated circuits.
Okamura and Kobayashi9 and FritzschelO suggested
that this problem may be associated with the unintentional
formation of a natural oxide between the deposited insulator
and the InP substrate surface. Wilmsen et al.l1-13 reported
that electrical interface instabilities were associated with
tunneling of electrons into the natural oxide.
We intentionally used in situ etching of InP substrates
by PC13/H2 for removal of natural oxides immediately prior
to in situ chemical vapor deposition of the phosphorus-ni
tride film on the InP surface at a low temperature of 450 ·C.
The chemical vapor deposition was carried out in a phos
phorus-rich ambiance using NH3/PCl3/H2 for suppression
of phosphorus evaporation. We determined in situ etching and deposition conditions as functions of in situ etching
time, PCl3 molar fraction in etching, and both in situ etching
and deposition temperature.
II. EXPERIMENTAL DETAILS
Our experimental strategy of in situ etching followed by
in situ deposition procedures can be described as follows:
(i) Phosphorus-rich ambiance for group V defects; The
hydrogen reduction of PCl3 as described in Eq. (1) is ther
modynamically favorable above about 200 ·C, although un
der certain conditions, unreacted PCl3 has been observed in
hydrogen at 650 ·C.14.IS
T>200·C
4PC13+6H2 P4+ 12HCI . (1)
(ii) Removal of natural oxides; From Eq. (1), in situ
etching process has been conducted by HCI and PCl3 in
phosphorus (P)-rich ambiance.
(iii) Group V-rich dielectrics; Phosphorus-nitride film
formation in P-rich ambiance can be obtained at low tem
peratures by the following procedure. From Eq. (1),
4 PCl3 + 12 H2 ~P4 + 12 HCI (2)
P 4 + 4x NH3 ~ 4 PNx + 6x H2 . (3)
Here, x is the atomic ratio of nitrogen to phosphorus in the
phosphorus-nitride film. From Eqs. (2) and (3),
2PCI3+2xNH3~2PNx +6HCI+3(x-I)H 2.
(4)
When we assume x = j and that the PN film is composited
with P3Ns,
T>2S0·C
3PC13+5NH 3 P3Ns+9HCI+3H 2. (5)
Here, the temperature of T> 250·C was determined from
our experimental results.
The strategy referred to above led to us to carry out in
2370 J. Appl. Phys. 62 (6). 15 September 1987 0021-8979/87/182370-06$02.40 © 1987 American Institute of Physics 2370
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
129.105.215.146 On: Sat, 20 Dec 2014 19:54:41NH, --r-ll-=-=---t.\
H.
o
BAFFLE
(a) -=---~H.
SUBSTRATE
e GOO,-----------,
w cr
~ 500 « cr
~ 400
~
W I-
~O eteh & deposit temp'
( substtate '\
position
20 40 60 80 100 120
DISTANCE (em)
( b)
FIG. 1. Schematic diagram of the apparatus (a) and the temperature profile
on distance from the left end of a quartz tube (b) for in situ etching and
deposition of phosphorus-nitride films on the .InP surface. The apparatus
allows quick conversion from the vapor etching mode to the chemical vapor
deposition mode of operation. FM: flow meter; R. P.: rotary pump; MFC:
mass flow meter.
situ etching and deposition of phosphorus-nitride films on
the InP substrate surface under P-rich ambiance.
Undoped InP wafers with electron concentration of
8.1-8.5 X 10'5 cm -3 for interface characterization, and
(100) oriented Sn-doped InP wafers with electron concen
tration of 1.6-1.7 X 10'8 cm-3 for phosphorus-nitride film
characterization were used as substrates, respectively. The
substrates were first cleaned with trichloroethylene, ace
tone, and ethanol and rinsed in deionized water prior to
chemical etching with H2S04:H202:H20 = 4:1:1 (in vol
ume) solution for 1 min followed by chemical etching in
0.5% bromine in methanol solution for 2 min. Finally, to
reduce the natural oxide to a minimum, the wafers were
dipped in HCI:H20 = 1:5 (in volume) solution for 10 s im
mediately prior to being loaded into the in situ process sys
tem.
The in situ process system was built to allow quick con
version from the in situ etching mode to the in situ deposition
mode of operation. The details of the reaction chamber and
the temperature profile are shown in Fig. 1. The purpose of
the bume is to prevent both back streaming of vapors from
deposits in the exhaust area during the in situ process, and air
contamination during dismantling for exchanges of InP sub
strates in nitrogen gas streaming. The magnetic sample lifter
is used to load the InP substrate prior to the in situ process at
the in situ process temperature to prevent thermal damage to
the substrate. In a typical vapor etching, a mixture of PCl3 in
H2 is passed over the InP substrate for 1 min at 450 ·C and
then phosphorus-nitride films were deposited in situ on the
InP substrate surface using the PCI3INH3/H2 at a total flow
rate of about 155 cm3/min. The flow rate of PCI3, whose
partial pressure was 16 Torr, in diluted H2 and NH3 as 3-6
and 30 cm3/min, respectively. These reactants were fed into
the reactor from two by-pass lines with 2.2-mm Ld. The re
sistance-heated furnace was constructed with the open tube
2371 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 ...... ...
~600
0<
~500 0::
ai400 -a
0::300
c:
.~ 200 -.~ 100
a. / ,.-<:
, ,
A.. .. I /, ....... ,
.--~-...... ~ \
/ " ... , ./
., 2.1 c
2.0 .
)(
CIJ
1.9 -g
1.8
CIJ
1.7 .~ ... u
1.6 E
Q;
~ O~~--~~--~~--~~--~--~~ 0::
100 200 300 400 500 600
Substrate Tempera ture. To (OC)
FIG. 2. Phosphorus-nitride deposition rate RD and refractive index n as a
function of the substrate temperature T D'
type. The reaction chamber was a 28-mm-i.d., 130-cm-long
quartz tube.
When the substrate temperature was raised to 450·C
with the flow rate of PCl3 for etching from 5-15 cm3/min,
the surface of the etched InP good mirrorlike smoothness.
The etching rate of PCl3 vapor with the flow rate of 5 cm3/
min on (100) InP substrates is in the range of about 200-300
A/min at 450 ·C etching temperature.
III. EXPERIMENTAL RESULTS AND DISCUSSION
A. Phosphorus-nitride film characterization
The dependence of the deposition rate R D and refractive
index n on the substrate temperature TD are shown in Fig. 2.
The film thickness and refractive index were determined
from ellipsometric measurement. The deposition rateRD in
creased almost linearly with the substrate temperature TD•
The refractive index n also increased with deposition tem
perature near 500 ·C. Near TD = 450 ·C, RD and n are stable
and reproducible with values of 450 AIh and 1.95, respec
tively. Therefore, the in situ etch/deposition CVD phos
phorus-nitride films were formed mainly at 450 ·C.
Figure 3 shows the results of conductivity defined by J /
E versus the square root of electric field E for various TD
n"=1.6-1. 7 xld' em'
CVO PN-lnP
300 K
.... To=300'C
........ (950 A)
....... -.... 4oo°C .,../' .'.~. (750l)
... .flo ............ ......... .... r
> -14 :;: 10 .............. .Aso'c
• ",.......... ... ..... ~ (1000 A)
..,Y"""-u
:J
"0 c: o u ........ ~-.
-16 10~~ __ ~~ __ ~~ __ ~
0.5 1.5 2.5 3.5
FIG. 3. Electrical conduction in phosphorus-nitride films at room tempera
ture. The parameter is deposition temperature.
Jeongetal. 2371
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129.105.215.146 On: Sat, 20 Dec 2014 19:54:41'iii ..-'§ 1.0
..ci ....
III ---PN InP ---'"" >--'iii
~ ...... ..- P .s • . .
0.5 • N .... • • QI • :i • • •• • • " ......... In · ...... • . . .
: P .. . .
" . . . .... ,. .. • QI · . • . ~ . In • ) N
0.0
a 50 100 150 200
Sputtering Time (min)
FIG. 4. In-depth profiling of in situ etched and in situ chemical vapor depos
ited phosphorus-nitride on an InP substrate by Auger electron spectrosco
py. The total thickness was about 1000 A. and the deposition temperature
was 450·C.
samples at room temperature, where J is the current density.
The measurements were performed for PN films on an Sn
doped n+ -lnP substrate whose electron concentration was
1.6-1.7 X 1018 cm-3 with aluminum (Al) field electrode
dots of about 10-3 cm2 in area. It is shown in Fig. 3 that the
resistivity defined by u-I is more than 1014 n cm at the elec
tric field of 107 V cm -I for the TD = 450·C sample at room
temperature. Electrical transport properties of the PN films
show the Poole-Frenkel type conduction in a high electric
field. At low field, current-voltage characteristics are seen as
ohmic conduction in this figure. The insulator resistivity in
creases as the deposition temperature increases, which is
mainly caused by decreasing the density of traps due to ap
proaching a more stoichiometric PN film.
Furthermore, the dielectric strength is found to be more
than 1 X 107 V cm -I at 300 K, which is at least one order of
magnitude higher than those of the conventional oxide gate
CVD films, although the PN film deposition temperature,
450 ·C, is lower than 600 ·C reported by Hirota and Kobaya
shi, who deposited film above 600 ·C.
B. In-depth proflilng of compositions by Auger electron
spectroscopy
The in situ etched and deposited PN film on an InP
substrate at 450 ·C was sputter etched by using Ar+ with a
30-mA emission current at 3-keV ionic energy, and the com
position was in-depth profiled by Auger electron spectrosco
py (AES) as shown in Fig. 4, in which the AES intensity is
normalized by a sensitivity factor for the P( 118 eV), the
In( 404 eV), and the N(382 eV) lines.
As shown in Fig. 4, P, N, and In have relatively uniform
distributions in this PN film. Large amounts of indium (In)
are incorporated into the PN film. The AES spectra of the
PN-InP structures contain three major In lines at 397, 400,
and 405 eV, phosphorus (P) double lines at 114 and 118 eV,
and a nitride (N) line at 382 eV.
The normalized AES intensity for the P(118 eV), the
In( 404 eV), and N( 382 eV) lines were plotted'! As shown in
2372 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 60
CVO PN-InP
2 sweep
IL. a. 0.1 Vlsec
1 MHz
QI 40
u
c:
.E 'u
0 a. 20 0 u To 450'C j .......-
300 K
o
-6.0 -4.0 -2.0 0.0 2.0 4.0 6.0
Bias Voltage (Volts)
FIG. 5. High-frequency C-VG plot for Al-PN-InP MIS structure measured
at room temperature.
Fig. 4, P, N, and In have relatively uniform distributions in
this insulator.
A very small amount of oxygen was detected for the first
few seconds only and a trace of carbon contamination was
found on the front surface only.
C. Capacitance-voltage characteristics
Typical capacitance C versus voltage V G characteristics
are shown for an AI-PN-InP MIS diode in Fig. 5, where the
PN film was deposited at 450 ·C, the thickness was about
1000 A, and the gate electrode area was 1 X 10-3 cm2• The
high-frequency (1 MHz) C-VG curve shows a clockwise
hysteresis loop for a field sweep rate of 0.1 V s -I at room
temperature.
As can be seen in the figure, small hysteresis appears
only in part of the depletion region and the weak inversion
region. The maximum hysteresis width, which is defined as
the hysteresis width at the half point of the C-V G curve, is as
low as 0.17 V in this region at room temperature in a light
tight enclosure. The maximum hysteresis width varied with
2.8
2.4
;2.0
~ 1.6 -" ~ 1.2
I/)
'ID 0.8 ....
QI
~0.4 :z: evo PN-lnP
VG: -6V-+6V
1 MHz
0.1 Visec
300 K
•
: I . :
• -j
I • , . :
O~~~~~~~~~
100 200 300 400 500 600
Etching and DepOSition Temperature("C)
FIG. 6. Hysteresis widths as a function of etching and deposition tempera
ture. The maximum hysteresis width was calculated from I MHz C-VG
characteristics ofPN-InP MIS diodes.
Jeongetal. 2372
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129.105.215.146 On: Sat, 20 Dec 2014 19:54:4160 ..... 300 K IL. a. sweep rale 0.1 V/sec
C1I u 40 10 kHz C 100 kHz 0 -1 M-tz
U 0 a. 20 0 u
0 ..... -____ ....... ______ .....
-6.0 -4.0 -2.0 0.0 2.0 4.0 6.0
Bias Voltage (Volts)
FIG. 7. Frequency-dispersion characteristics ofa PN-InP MIS diode (run b
from Table I).
the in situ etching and in situ deposition temperature, as
shown in Fig. 6. Typically, it is shown that the maximum
hysteresis width is very small, 0.17 V, near T D = 450 ·C. The
range of deposition temperature for the CVD phosphorus
nitride films used in this work are indicated by arrows in
Fig. 6.
Figure 7 shows the frequency dispersion of the in situ
etch CVD PN -InP MIS diode in the frequency range from
10kHz to 1 MHz. The frequency dispersion of the C-V hys
teresis curve has been greatly improved in comparison with
those obtained by other thermal or photochemical vapor de
position techniques. 1-3
Figure 8 shows the effect of etching conditions on the
density distributions of interface trap states obtained from
the 1 MHz C-VG characteristics for various samples. For the
field electrode voltage V G swept between - 6 and + 6 V,
the density of interface trap states Nss was about 8 X 1011
cm-2 eV-1 near the bulk Fermi level and the minimumNss
value was found to be about 2X 1011 cm-2 eV-1•
The detailed results are summarized in Table I for var
ious etching conditions. There is only a small amount of
frequency of dispersion of accumulation capacitance and in
version capacitance from 10 kHz to 1 MHz for runs a and b.
Furthermore, the maximum hysteresis widths for these runs
are as low as O.17--D.18 V. As previously mentioned, inter
face properties of the indium-phosphide surface prepared by
in situ etching with PCl3 and subsequent in situ deposition of
phosphorus-nitride are critically dependent upon the in situ 'I >
C1I 13 10
~E
u 12 10
1/1
1/1
Z
11
10 c
d in-situ etched & CVD PNl1nP
To: 450 'C
a 1 MHz
Yo:!6 V
0.1 Vlsec
300 K
M. F. _4 Etc:htnir\l
a :4.9xl0 =1
b:3.7xl0 5
c:4.9xl0 10
d: unetch
Ec 0.2 0.4 0.6 0.8 1.0 1.2 Ev
Energy (eV)
FIG. 8. Effects of etching on density distribution of interface trap states for
various in situ etched CYD PN-InP MIS diodes.
processes. It is observed that the in situ PCl3 etch treatment
of 10 min of etching time to run c introduces surface traps
resulting in a large Nss.2.16
In Fig. 9, these results are compared with the InP MIS
diodes fabricated with other various oxides at the same labo
ratory.17-21 The AI203-lnP MIS structure is fabricated so
that the aluminum film is deposited on the InP substrate and
then anodically oxidized in oxygen plasma. The fabrication
condition of InP MIS diodes with native oxide film inter
layed between plasma anodic Al203 film and the InP sub
strate has been done previously, except that the anodization
was done before the deposition of aluminum film as well.
The Si02 (a-Si)-lnP MIS diode is prepared by anodizing
amorphous silicon film which was deposited by plasma-en
hanced decomposition of SiH4 diluted in Ar, and the SiO
InP MIS device is fabricated by vacuum deposition of evapo
rated SiO. All other results show larger values for Nss than
these results. Although not shown here, in situ etched CVD
PN-InP MIS structures also show smaller frequency disper
sion characteristics than those of oxide structures.
Figure 10 shows the effect of deposition temperature on
the density ofinterface trap states. It is shown that the in situ
etching and in situ deposition processes with TD near 450·C
gives lower density of interface trap states near the conduc-
TABLE I. Measured electrical properties of CVD PN-InP MIS structures for various in situ etch and deposition conditions. The field electrode voltage was
swept between - 6 and + 6 Y.
Etch and Etch
deposit time M.F.· N", Nos
temp. PCI) (min) (near Ec) Hysteresis
Run ('C) (min) (X 10-4) (X 10" cm-2 ey-I) (X 1012 cm-2 ey-I) width ~Cmin (%)b ~Cm .. (%)C
a 450 1 4.9 2 0.8 0.17 2.8 3.3
b 450 5 3.7 6 2 0.18 3.1 3.7
c 450 10 4.9 2 6 0.41 25.0 3.9
d 450 unetched 2 4 0.29 14.2 4.2
• M.F.: Molar fraction.
b ~Cmin (%): Dispersion of inversion capacitance (ClOkHJCI MHz -1).
c ~Cmin (%):Dispersion of accumulation capacitance (ClOkHz/CI MHz -1).
2373 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 Jeong etal. 2373
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129.105.215.146 On: Sat, 20 Dec 2014 19:54:41'> QI ':'e 12
~ 10 Si02(a-Si)/lnP'" r rAl~3nnp(··")
'·'1 • ..,i' 11\
11\
Z ~ '. . I "')
'. '. ~ ." A'2O:Ilnatlve oxidellnP V ___ '1-a-o.o..~ SiO/lnF'>l
~ --r-in-situ etch PN/lnP
\ .
~ \: 1U~~--~-- ______ ~ __
Ec 0.2 0.4 0.6 0.8 1.0 1.2 Ev
Energy (eV)
FIG. 9. Comparison of density distribution of interface trap states between
these results and various gate oxide·lnP MIS structures from our previous
data.
tion-band edge and a lower minimum near 0.3 eV. The den
sity of interface trap states is seen to increase above 500 ·C,
probably due to the thermal degradation of the InP surface.
The distributution of interface states as plotted in Figs.
11(a) and 11(b) for various bias voltages VG from the 1
MHz C-V G characteristics. Similar results of increasing of
the peak of Nss distributions near the midgap were also re
ported by previous workers,22-24 who also included group V
rich materials.
Several models for the insulator-semiconductor inter
face trap states have been proposed, such as the dangling
bond model for Si-Si02 interface by Sakurai and Sugano,25
the unified defect model by Spicer et al.,26 and the surface
disorder model by Hasegawa et al.27•28 The energy levels of
the defects and their identity have been characterized by
Spicer et al. and by subsequent experimental and theoretical
work. 5.26.29-32
11\
11\
Z 13
10
11 in-situ etched & cve PN/lnP
etch time = 1 min
M.F. 4.9 x 10'
10~~~~ __ ~ __ ~~ __ ~~
Ec 0.2 0.4 0.6 0.7 1.0 1.2 Ev
Energy (eV)
FIG. 10. Deposition temperature TD dependence of the density distribution
of interface trap states.
2374 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 ~ ,
:; 13
10
""e 12
~10
11\
11\
Z cve PN-lnP
TD'450'C
Ro: 420 A/hr.
M.F.: 3.7.10
1 MHz
0.1 Vlsec
300 K
a.-4!f.~:!..l
b .-6~\I;".6
c .-"()~", •• 10
1~~~~ __ ~~~~~~ __ ~~
Ec 0.2 0.4 0.6 0.8 1.0 1.2 Ev
~
~
""e 13 10
v 12
~10
11\
11\
Z
11 Energy (eV)
(a)
cve PN-lnP
TD 450'C
RD 500 A/hr
M.F. 4.9xl(f'
IMHz
0.1 V/sec
300 K
e ..
d. -6.V.~6
e .. -J}sVGs+S
t • -lSoV ••• 5
10L-~ ___ ~~ ___ ~~ ___ ~~
Ec 0.2 0.4 0.6 0.8 1.0 1.2 Ev
Energy (eV)
( b)
FIG. 11. Density (N,,) distributions of interface trap states as a function of
the bias (Va): (a) PCl3 molar fraction (MF) = 3.7XIO-4; (b) PCl3
MF = 4.9X 10-4• .
Notice that the peak of Nss distributions near the mid
gap sharply increases and then decreases in Figs. 11 (a) and
11 (b), probably due to an antisite defect PIn according to the
unified defect model, although the energy levels in our ex
perimental results were varied with bias voltage VG' This
new experimental phenomena, which the peak of Nss distri
butions near the midgap sharply increases and decreases, is
first reported about InP MIS structures.
IV. CONCLUSION
We have demonstrated the improvement of interface
properties of InP MIS structures by in situ etching of InP
substrates with PCl3 and subsequent low-temperature in situ
CVD of phosphorus-nitride in P-rich ambiance using NH3/
PC13/H2' The phosphorus-nitride films and the in situ CVD
PN-InP interface characteristics are summarized as follows:
Jeong eta!. 2374
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129.105.215.146 On: Sat, 20 Dec 2014 19:54:41( 1) The resistivity of the PN films was in excess of
1 X 1014 n cm near the electric field of 1 X 107 V cm -I. The
dielectric strength of the PN film was as high as 1 X 107
V cm -I at room temperature. The films showed the Poole
Frenkel-type conduction in high field.
(2) The deposition rate ofPN film was about 450 A!h
and the dielectric constant of the film was estimated to be 5.9
at 1 MHz.
(3) The frequency dispersion of C-V G characteristics in
accumulation was about 3.3% for the frequency range from
10 kHz-l MHz. Hysteresis was as low as 0.17 V for the field
electrode voltage swept between - 6 and + 6 V.
( 4) The density of interface trap states Nss was 2 X 1011
cm -2 e V-I at about 0.3 e V below the conduction-band edge
of InP, and was 8X 1011 cm-2 eV-1 near the bulk Fermi
level. The peak of Nss distributions near the midgap sharply
increases and then decreases.
ACKNOWLEDGMENTS
The authors are grateful to Y. Takahashi for many fruit
ful discussions and T. Takahashi for assistance in film char
acterization, and also to Dr. B. B. Triplett for proofreading
the English manuscript. We would also like to thank Sumi
tomo Electric Industry Ltd. and Nippon Mining Company
for supplying InP wafers.
Iy. Hirota and T. Kobayashi, J. App!. Phys. 53, 5037 (1982).
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Spring Meeting of Japan Society of Applied Physics, Tokyo, April, 1983,
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Communication Engineers of Japan (May, 1987), Trans. Pt. C (to be
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Jeongetal. 2375
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1.339472.pdf | Selfconsistent analysis of resonant tunneling in a twobarrier–onewell
microstructure
K. F. Brennan
Citation: J. Appl. Phys. 62, 2392 (1987); doi: 10.1063/1.339472
View online: http://dx.doi.org/10.1063/1.339472
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Published by the American Institute of Physics.
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Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsSelf-consistent analysis of resonant tunneling in a two-barrier-one-well
microstructure
K. F. Brennan
School 0/ Electrical E,ngineering and Microelectronics Research Center, Georgia Institute o/Technology,
Atlanta, Georgia 30332
(Received 1 April 1987; accepted for publication 28 May 1987)
A self-consistent solution to the resonant tunneling problem is presented based on the
simultaneous solution of the time-independent Schrodinger equation with the Poisson
equation. The solution is obtained from a piecewise linear matching of Airy functions. The
model is used to explqre the effects of the self-consistent electron charge on the transmissivity
and current-voltage characteristics of a double-barrier single-well GaAs-AIGaAs device. It is
found that the self-consistent potential always acts to shift the negative differential resistance
onset voltage to large positive values. The self-consistent field effectively acts to screen the
positive applied voltage. Therefore, the effects of the self-consistent field can essentially be
modeled by a smaller applied positive bias. It is further found that the effects of the self
consistent field are most prevalent at high temperatures, -300 K, and at high dopings,
> l.Ox 1018. It is necessary to include the self-consistent effects then when designing resonant
tunneling structures within these constraints.
I. INTRODUCTION
Recent refinement of exacting crystal growth technolo
gies, particularly molecular-beam epitaxy, metalorganic
chemical vapor deposition, and chemical beam epitaxy, have
fomented the rapid development of microstructure devices
incorPorating multiple quantum wells and superlattices. Su
perlattices I can be loosely defined as multiquantum-well sys
tems in which each well is coupled to its nearest neighbor
such that any single electron has a nonzero probability den
sity within two or more wells. Alternatively, in multiquan
tum-well devices, the electronic wave function does not
overlap between adjacent wells. Localization of carriers can
occur. Therefore, the transport properties are very different
within the two systems.
Multiquantum-well systems, such as those used in new
avalanche photodiode structures2--6 and lasers,7 operate in
the semiclassical regime in that the transport is dominated
by drift and diffusion effects. The multiquantum-well geom
etry is exploited in avalanche photodiodes (APDs) to en
hance the electron ionization rate from that achievable in the
corresponciing bulk material. In these devices, carrier con
finement is detrimental to device performance. Therefore,
multiquantum-well APDs are designed such that trapping
of carriers within the wells is avoided. 8 Quantum-well lasers,
on the other hanci, take full advantage of the carrier confine
inent properties of narrow quantum-well systems in order to
reduce the lasing threshold current density. Carrier confine
ment is achieved in muitiquantum-welliasers by decoupling
adjacent wells through increasing the separating barrier
widths and heights. In addition, if the carrier temperature is
kept low, thermionic emission over the barriers can be effec
tively eliminated.
As the barrier widths and heights decrease, the elec
tronic states in adjacent quantum wells interact, leading to
the formation of a quasicontinuum of states or a miniband. 1,9 The electronic states become Bloch-like and cannot be con
sidered localized within anyone particular well. The system
undergoes a transition from a series of uncoupled, noninter
acting quantum wells to that of a superlattice. This transi
tion is physically identical to that which occurs in the forma
tion of a solid. As in a solid, the transport then proceeds via
mini band conduction provided that the carrier mean free
path appreciably exceeds the superlattice period. 10 Locali
zation,though, can occur under two different conditions:
broadening of the carrier states due to collisional and disor
der effects to greater than the minibandwidth, or localiza
tion due to the voltage drop in each cell exceeding the mini
band width.II,12 In either case, the conduction changes from
miniband transport to "hopping" conduction wherein the
carriers hop from one localized state to another via phonon
emission.
Transport within a supedattice or across a double-bar
rier single-well structure can be described using resonant
tunneling. Recently, much work, both theoretical13-17 and
experimental, 18-25 has been done on resonant tunneling in
double-barrier single-well microstructures. Two possible
mechanisms have been suggested for the underlying physics
of the tunneling process,9.17 a Fabry-Perot mechanism or
sequential tunneling. The two mechanisms can be differenti
ated by the ratio of the intrinsic resonance width to the total
scattering width.9 If collisions occur within the structure,
phase coherence cannot be established, thereby eliminating
Fabry-Perot-type tunneling. However, sequential tunnel
ing, that in which phase coherence is not preserved, may still
occur. Sequential tunneling, as opposed to Fabry-Perot tun
neling, does not require the resonant buildup of the electron
probability density within the well. Therefore, it is insensi
tive to the symmetry of the transmission coefficients of the
two barriers. 16.17 It is conceivable that the different mecha
nisms can be identified by exploiting the symmetry depen-
2392 J. Appl. Phys. 62 (6), 15 September 1987 0021-8979/87/182392-09$02.40 © 1987 American Institute of Physics 2392
Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsdence ofthe Fabry-Perot-type resonance. It is important to
identify which mechanism is present in the device since the
maximum frequency response is different in each case.
Fabry-Perot-type tunneling is of greatest significance at
low temperatures in single-well structures. Nevertheless, it is
important to consider the general physics of coherent reso
nant tunneling in order to identify its presence in various.
situations, i.e., multiple well structures. The transient re
sponse of coherent tunneling,26.27 as well as the effects of
multiple barriers,28-30 has been treated theoretically. None
of these treatments has included the effects of the self-consis
tent field. Recently, Ohnishi et aPI have compared experi
mental measurements25 to a self-consistent coherent reso
nant tunneling model. However, they did not compare their
model to an Airy function model without the self-consistent
potential. Therefore, it is not clear from their work what the
extent of the self-consistent potential has on the resonant
tunneling current.
Cahay et al.32 have also recently explored the effects of
the self-consistent potential. They have determined that the
self-consistent potential has two important effects; the peak
to valley ratio in the current density versus voltage curve is
reduced and the negative differential resistance "knee" is
shifted to higher applied voltages. We confirm these effects
below and offer an explanation as to their cause. In addition,
the model we present converges much faster than that re
ported by Cahay et al.32 As explained below, the faster con
vergence of our model is due to the selection of Airy solu
tions to begin with.
In this paper, we isolate the effects of the self-consistent
potential on coherent resonant tunneling. The details of the
model are outlined first in Sec. II. In Sec. III, calculations of
the transmissivity as a function of the incident carrier energy
and current-voltage characteristics are presented with and
without the effects of the self-consistent field as a function of
the temperature and doping of the device. The self-consis
tent calculation is compared to that corresponding to a re
duced positive bias voltage. It is found that the resonance
peak shifts in the presence of the self-consistent potential in a
similar way to that corresponding to a lower positive applied
bias. Finally, conclusions are presented in Sec. IV.
II. DESCRIPTION OF THE MODEL
The model is diagrammatically presented in Fig. 1. The
goal is to solve the Schrodinger equation simultaneously
with the Poisson equation. The first step in the calculation is
to solve the Schrodinger equation in the presence of a uni
form applied bias field neglecting the self-consistent contri
bution from the carrier electric charge. The solution of the
Schrodinger equation within each region under the applica
tion of a constant applied electric field is given by a linear
combination of Airy and complementary Airy functions. 28
The wave function and its first derivative are then matched
at each interface throughout the structure. The imposition of
these boundary conditions leads to a product of matrices
coupling the incident to the outgoing wave function from the
multilayer stack.28 The transmission coefficient can then be
found from the incident and transmitted wave vectors, k and
k', and transfer matrix elements as,28
2393 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 SELF-CONSISTENT SOLunON OF THE
SCHROEDINGER EQUA nON
!Input Device Structure I
i
Solve the Schroedinger Equation without the Self-Consistent
FIeld. Obtain the transmission coefficient as a
function of Incident energy. T (E)
COiculate n(Z) using
n(Z) =1: \I/I(Z, EI)\2.!!llllog(1 + e-(EI-£~,-1<T)
EI lTo!I2
Solve the one-dimenslonoi Polss~" equation.
-l} (E(Z)!) = qn(Z)
Obtain V(Z) and F(Z) everywhere
Use a piecewise linear approximation (5 A unit cells)
and solve the Schroedinger equation in each region.
Match the boundary conditions and recalculate
T(E)
No
No
FIG. 1. Diagrammatic flowchart detailing the computational method.
T = k!k'l/Mil'
where the matrix Mis, 28
1 (ik
M = 2ik ik (1)
-i~,)' (2)
S (O,L) is the resultant of the product of the transfer matrices
coupling the first barrier to the end of the superlattice struc
ture.
Once the overall transmission coefficient of the device
has been found, the steady-state wave functions can be deter
mined at each incident energy over the full range of interest.
The electronic charge density can then be determined using
p(z) = q L 1¢,(Ei,zWf[ E,Ej(zo)] , (3)
E,
whereJ1E,Ef(zo)] is the Fermi distribution. Note that the
K. F_ Brennan 2393
Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsdistribution function is taken as in equilibrium, despite the
fact that a current flows through the structure. The distribu
tion function,
I(E) = 1/(1 +exp{[E; + (fi1k2/2m*) -EF]lkT}j, (4)
is reasonably accurate since the x-y system is essentially de
coupled from the z direction and can then be considered in a
quasiequilibrium state. The evaluation of Eq. (3) with Eq.
( 4) is standard and yields
p(z) = q L Irp(z,E;) 12 m~!T 10g(1 + e -(E,-Ejl/kT) • (5)
E, 1T1T
Equation (5) accounts only for left-to-right incident
carriers. We have also included the charge density due to
carriers impinging from the right-hand contact similar to
Cahay et al.32 The total charge density is found as the sum of
the two streams. We have found that the right-to-Ieft inci
dent stream of electrons has a negligible effect on the current
density for the structures and applied voltages considered
herein. This can be easily understood as follows. The wave
functions of the right-to-Ieft carriers are evanescent at ener
gies less than or equal to the bias voltage. This is clear since
the band bending of the structure is greater than the incident
carrier energy at some place to the left of the contact until the
incident carriers have energies greater than the bias voltage.
Hence, the total transmissivity of the structure for these car
riers is zero. Due to the exponential decay of the probability
densities of these modes, their contribution to p (z) is small.
. At higher incident energies, the modes are no longer evanes
cent. Therefore, the probability densities are much greater.
However, the distribution function, Eq. (4), decays rapidly
since the energy must now be related to the right-hand con
tact. We find that the right-to-Ieft contribution to p(z) has a
less than 1 % effect. Therefore, the charge density is essen
tially that given by Eq. (5). The charge density is then sub
stituted into the one-dimensional Poisson equation,
!{.(E(Z) d(J) = p(z),
dz dz (6)
and both the potential (J(z) and the field F(z) are obtained.
The structure is partitioned into small, -5 A, cells in which
the potential is assumed constant. The Schrodinger equation
is then solved in each cell obtaining, once again, Airy func
tion solutions, but now of different argument than before.
From matching the boundary conditions, the transmission
coefficient as a function of incident carrier energy is once
again calculated. The program loops back to calculate the
wave functions, n(z), (J(z), andF(z) until excellent conver
gence is obtained.
The convergence is determined from calculating the
transmission versus incident energy as a function of the
number of iterations. Figure 2 presents a sequence of calcu
lations out to the fourth iteration using the Poisson solver.
Notice that the solution is fairly well converged after only
the second iteration with the Poisson equation. The rapid
convergence is due to the selection of Airy functions as the
zeroth order solution. In most instances, as we will discuss
below, the self-consistent field solution departs only weakly
from the uniform field approximation. Therefore, the uni-
2394 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 >f-4.00
0.00
~ -4.00
(!)
51
(!)
Z -8.00 «
0:: f-
-12.00 Zeroth Iterotion
0.20 volts bios
Temp = 300 K
Well width = 50 A
Borrler widths = 50 A
Borrier heights = 0.347 eV
Resonance:
E, = 0.009 eV
log T, = -3.505
-16.oo.j..--~-~-~-~--~-~-~-~
0.00 0.04 O.OB 0.12 0.16 0.20 0.24 0.28 0 . .32
>f-•. 00
0.00
'> Vi -4.00
(!)
51
(!)
Z -8.00 «
0:: f-
-12.00 First Iteration
Resonance: ENERGY (eV)
E, "0.017 eV
log T, = -5.3899
-16.00+---~-__'_-~-~--~-~-~-~
0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32
>f-4.00
0.00
~ -4.00
(!)
51
(!)
Z -8.00 «
~
-12.00 Second Iteration
Resonance:
E, = 0.017 eV
log T, = -4.1312
-16.00 +---~---.--~-~--~-~-~-~
0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0 . .32
>f-4.00
0.00
~ -4.00
(!)
51
(!)
Z -8.00 «
go
-12.00 Third Iteration
Resonance:
E, = 0.017 .V
log T, = -3.8851
-16.00.j..--~-~-~-~--~-~-~-~
4.00
0.00
~ -4.00
(!)
51
(!)
Z -8.00
<{
a:: .....
-12.00 0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.37
Fourth Iteration
Resonance:
E, = 0.017 eV
log T, = -3.7604
-16.00 '----~-__._-~-~--~-~-~-~
0.00 O.a.. 0.08 0.12 0.16 0.20 0.24 0.28 0.32
ENERGY (eV)
FIG. 2. Series of plots of the logarithm of the transmissivity vs incident
electron energy as a function of the number of iterations of the calculation.
The zeroth iteration does not include the self-consistent potential.
K. F. Brennan 2394
Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsform field Airy function solution is already close to the final
result.
Plots of the wave function probability amplitude with
and without the self-consistent field corresponding to the
calculations in Fig. 2 are presented in Fig. 3. The wave func
tion is plotted at an incident carrier energy of 0.009 eV corre
sponding to the resonant energy peak for the zeroth iteration
in Fig. 2. Notice that in the presence of the self-consistent
field, Figs. 3 (b) and 3 (c), that the wave function is moved
off resonance. The wave function is no longer symmetric
about the center of the structure, as in Fig. 3(a). As dis
cussed below, the self-consistent potential acts to screen the
positive applied voltage, thereby altering the symmetry of
the structure driving the wave function off resonance.
Finally, the current density is calculated at each applied
bias using13
0.20,,....-------,
~
I o
Zeroth Iteration
)( 0.16 E .. = 0.009 eV
Resonance- w o
::> I-
::J 0.12
11.
::;: «
~ 0.011
::J
iIi «
~ 0.0'
a:
11.
60.00 60.00 100.00 140.00
';' 0.25"...------.., POSITION (A)
o
)(
w o
::l I-0.20
~ 0.15
::;: «
~ 0.10
::J
iIi «
~ 0.05
a:
11. First Iteration
E, = 0.009 eV
Off Resonance
o.oo-l-- ....... ==-,.-.L..-r----r---+--.---...,..-I
I o 0.2~·,,00--2-0.-oo--4O.:.....oo:.:.....,
)( Second Iteration
w 0.20 E, = 0.009 eV
o Off Resonance
::> I-
~ 0.15
::;: «
~ 0.10
::J
iIi «
~ 0.05
a:
11. 60.00 100.00 120.00 140.00
POSITION (A)
FIG. 3. Series of plots of the wave function probability amplitude vs the
device geometry as a function of the number of iterations of the calculation.
The wave functions correspond to an incident energy of 0.007 eV. Notice
that the wave function becomes "detuned" in the presence ofthe self-consis
tent field.
2395 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 J =em*kT
2-rrff
xi'" 1'ln( l+exp[(E F-Ej)/kT] )dE,(7)
o l+exp[(EF-Ej-eVA)/kT] .
where Va is the applied voltage across the entire device. The
program steps through the full applied voltage range and the
current density as a function of applied voltage is output.
The current-voltage curve calculated from the above
does not necessarily match the experimental data precisely
since the effect of the equilibrium alignment of the Fermi
levels in the device is not a priori accounted for. In equilibri
um, the Fermi levels throughout the device are, of course,
aligned. However, for this to happen in a roughly intrinsic
double-barrier device, there must exist a zero field band
bending. Therefore, it is necessary to apply additional vol
tage from that calculated above in order to first obtain a
ftatband condition and subsequently observe the negative
differential resistance "knee." In order to determine what
additional voltage is needed, it is necessary to know the posi
tion of the Fermi levels in the well and barrier layers. This, in
tum, depends upon the degree of "intrinsicness" of the mate
rial. We have found that our calculations match the experi
mental data25 reasonably well if there is a significant offset
voltage arising from the Fermi levels. A more thorough in
vestigation of the effect of the Fermi level will be presented in
a future work where a systematic comparison to experimen
tal data will be performed.
III. RESULTS OF THE CALCULATIONS
We have analyzed the effects of temperature and doping
variation on both the transmissivity versus incident carrier
energy and current-voltage characteristics for a single-well
double-barrier, GaAs-Alo. 45 Gao. 55 As structure using the
above model. The barrier heights are chosen to be 0.347 eV
in magnitUde corresponding to the 60/40 rule conduction
to valence-band-edge discontinuity. 33.34 The effective
masses of the electrons in each region, as well as the nonpara
bolicity factors, are taken from the work of Adachi. 35 Ener
gy-dependent effective masses are used in the calculations .
which accounts for the nonparabolicities of the conduction
bands in both GaAs and AIGaAs.
We first analyze the effect of the self-consistent potential
on the transmissivity at various incident electron energies at
both 0.10 and 0.15 V applied bias. From Figs. 4 and 5, it is
clear that at higher temperatures the effect of the self-consis
tent potential on the location and magnitude of the reso
nance is more detrimental. At low applied bias voltage, 0.10
V, as the self-consistent field increases, arising from in
creased doping or temperature, the transmissivity of the
structure decreases abruptly. The self-consistent field acts to
screen the bias field. Therefore,' as the self-consistent field
becomes a greater fraction of the bias field, the incident car
riers see a much lower effective positive voltage and the
transmissivity decreases strongly.
Figure 5 clearly illustrates how the resonance peak
shifts in energy as a function of the self-consistent field at
various temperatures. It is important to notice that the reso
nance peak shifts monotonically in energy from 0.037 to
K. F. Brennan 2395
Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions0.00
-4.00
~
~ -8.00
U1
U1
~
~ -12.00
go
-16.00
-20.00
0.00
0.00
-8.00
>-...
5 -16.00
iii
U1
:iii
U1
~ -24.00
go
-J2.oo
-40.00
0.00
0.00
-4.00
~
~ -8.00'
~
~
~ -12.00
g:
-16.00 0.04-
0.04 Without Self-Consistency
0.10 vol ts bios
Well width = 50 A
Barrier widths = 50 .1.
Barriers heights = 0.347 eV
Resonance:
E, 0.055 eV
\ log T, = -8.0945
0.08 0.12 0.16 0.20
ENERGY (EV) 0.24- 0.28
With Self-Consistency
0.10 volts bios
Temp = 300 K
Resonance:
E, = 0.056 eV
log T, = -16.17
0.08 0.12 0.16 0.20 0.24 0.28 r
\·,~~".t~"
0.10 volts bios
Temp = 25K
Resonance:
E, =0.055 eV
log T, = -5.8865
-20.00 -I::---::-o::-:------=-~--:-r::--~_::_-~-~:---~-
0.00 0.04 0.08 0.12 0.18 0.20 0.24 0.28
ENERGY (EV)
FIG. 4. Series of plots of the logarithm of the transmissivity vs incident
electron energy at fixed bias, 0.10 V, and doping, 1 X 1O'81/cm3, but at dif
ferent temperatures, 300 and 25 K. The transmissivity decreases in ampli
tude strongly at 300 K in the presence of the self-consistent field.
0.041 eV in going from no self-consistent effects to increas
ing self-consistent potential, 25-300 K. This is easily ex
plained from Eq. (5), from which it is readily observed that
the charge density increases dramatically with increasing
temperature owing to both the exponential term in the loga
rithm and the linear prefactor term. The self-consistent po
tential increases with the charge density, thus at higher tem
peratures and doping concentrations, its effects are more
important. The self-consistent potential acts to screen the
positive applied voltage such that a smaller band bending
occurs throughout the device. Physically, this is equivalent
in part to applying a smaller overall positive bias. It is well
known that as the bias voltage increases, leading to larger
band bending, the confined quantum-well state energy ap
proaches the conduction-band minimum energy at the first
contact, 17,28,29 and the resonance peak shifts to lower energy.
If sufficient bias is applied, the resonance peak can disappear
altogether.29 Alternatively, as the bias voltage decreases, less
2396 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 >... 0.00
-4.00
? -8.00
U1
U1
~
~'-'2.00 g:
-16.00 Without Self-Consistency
0.15 volts bios
Well width = 50 A
Barrier widths = 50 "
Barrier heights = 0.347 eV
Resononce:
E, 0.037 eV
log T, = -3.0824 '--..... '
FIG. 5. Series of plots of the logarithm of the transmissivity vs incident
electron energy at fixed bias, 0.15V and 1 X 10'8 Vcm3 doping, but at differ
ent temperatures, 300 and 25 K. Notice that the decrease in the transmissi
vity is less at 300 K under 0.15 V bias than at 0.10 V bias.
band bending occurs and the resonance shifts to higher inci
dent carrier energies. From Fig. 5 it is apparent that the self
consistent potential, by screening the applied bias voltage,
shifts the resonance energy to higher values. Interestingly,
the transmissivity decreases in magnitude as the screening
potential increases. Figure 4 clearly shows that as the self
consistent potential becomes comparable in magnitude to
the applied bias, the transmissivity of the structure greatly
decreases. As discussed below, this is apparently due to the
fact that the self-consistent potential destroys the symmetry
of the structure and, hence, drives the wave function off reso
nance. The wave function no longer builds up from succes
sive reflections and transmissions. Therefore, the Fabry
Perot resonance buildup is avoided, leading to a much re
duced transmissivity.
The current density versus applied voltage at 25-300 K,
both with and without the effects of the self-consistent po
tential, are presented in Figs. 6 and 7. Notice that the nega
tive differential resistance "knee" is shifted, in both cases, to
K. F. Brennan 2396
Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions8.00
E
~ 4.00
"' a.
E o '-"" 0.00
>-f-
Vi
Q-4.00
o
f-
Z W-B.OO a:: a::
:::J
U Temp -25 K
Doping"" 1 ){ 10'81/cm3 --= Without Self-Consistency _._-= With Self-Consistency
-12.oo-\---.---...---r--"---~--~-""-----.
0.04 O.OB 0.12 0.16 0.20 0.24 0.2B 0.32 0.36
VOLTAGE (V)
FIG. 6. Logarithm of the current density vs applied bias voltage at 25 K and
I X 10'Bl/cm3 doping, both with and without the self-consistency. Notice
that the negative differential resistance "knee" is shifted to larger applied
voltage in the presence of the self-consistent field.
higher applied voltages when the effects of the self-consistent
potential are included. The shift in the "knee" is much
greater at 300 K than at 25 K, owing to the larger charge
density and, hence, self-consistent potential present in the
device.
Tht: unnormalized wave function probability amplitude
for left incidence carriers at an energy of 0.055 eV, corre
sponding to the resonance state in the absence of the self
consistent potential, is plotted versus the device geometry at
25 and 300 K in Fig. 8. The wave function is greatly distorted
from the resonance shape (symmetric form) at 300 K. The
wave function at 25 K is less disturbed, which gives a much
higher transmissivity than at 300 K. In fact, the potential is
such that the resonance at 25 K is stronger than the case
where the self-consistent field is absent for the particular
energy chosen. This is not truly surprising since we have not
finely tuned the resonance to begin with. Ifwe choose a finer
energy range, we can find the location of the resonance more
precisely. Therefore, the apparent increase in the transmissi
vity at 25 K is not a physical phenomenon, but is due to our
not having tuned the resonance in the absence of the self
consistent field more precisely in energy.
12.00
E u "0.00
"' a.
E o
~ 8.00
>-f-
Vi Q 6.00
o
f-
Z W 4.00 a:: a::
:::J
U Temp = 300 K
Doping -1 x 1018 1/ern3
- -With""t self_conSistenlcy ~ -- = With Self-Consistency
//
i
2.00 +---r--"---~--~-""-----.---r----.
0.04 O.OB 0.12 0.16 0.20 0.24 0.2B 0.32 0.36
VOLTAGE (V)
FIG. 7. Logarithm of the current density vs applied bias voltage at 300 K
and f X (018 Vern] doping, both with and without the self-consistent field.
Notice the dramatic shift in the negative differential resistance "knee" in the
presence of the self-consistent field to higher applied voltages.
2397 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 .. 0.2~ ,
o
W o
::J f-0.20
~ 0.15
:::<
<!
'C 0.10
..J
iIi
<!
gs O.O~
a:: n. --NO Self-Consistency
0.10 volts bias
Barrier height = 0.347 eV
El = 0.055eV
I I
0.00 -I===--..,--~-L-'T"""----'---+-_'T"""_---'--.l
0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00
POSITION (A)
~ 0.2~ ''''''=t-=n'S''''eiT-Conslstency
o 0.10 volts bill
0.20
w
0
:::J f-
::::i n. 0.1~
::E
<{
~ 0.10
::::i
iIi
<{
III 0.05 o
Q::
n. Temp = 300K
oping = 1 x 1018/cm3
El· 0.055 eV
0.00 +-----.--.....,....--"'--'T"""----.---+---...------.~-'
0.00 20.00 40.00 80.00 110.00 100.00 120.00 140.00
.. 0.2~ ,r:=-;::-::-:c-:-:::-7':1
0, With Self-Consistency
w o
::J f-0.20
i? 0.1~
::E <{
~ 0.10
::::i
iIi <{
gs 0.05
a:: n. 0.10 volts bias
Temp =25K
Doping = 1 )( 1018/cm3
El = 0.055 eV
0.00 ~=::;:::=----._L-'--_-r-_---'_-'::;:"'~--.-..J
0.00 20.00 40.00 80.00 110.00 100.00 140.00
POSITION (A)
FIG. 8. Series of plots of the wave function probability amplitude vs the
device geometry at fixed doping, but variable temperature. At 300 K with
the self-consistency present, the wave function is very far off resonance at
0.055 eV, while in the absence of the self-consistent field the wave function is
close to resonance.
The effect of the contact doping concentrations <;m the
transmissivity at both 0..10 and 0.20 V bias at 300 K is pre
sented in Figs.. 9 and 10. As expected, as the doping in
creases, the self-consistent potential increases owing to the
increase in the Fermi level. From Eq. (5), the charge density
increases. The self-consistent potential subsequently iIi
creases, leading to an increased screening of the applied posi
tive bias. The self-consistent field is again seen to have two
important effects: the resonance energy is shifted upwards
and the amplitude of the transmissivity decreases with in
creasing self-consistent field. Notice, in both cases, the trans
missivity amplitude decreases as the doping increases.
The logarithm of the current density versus the applied
K. F. Brennan 2397
Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions0.00
-4.00
~
5 -1.00
ill
~
~ -'2.00
~
-'8.00
-20.00 0.00
8.00
0.00
>-I-
5 -8.00 Vi
U1
~
U1
Z « -'8.00
~
-24.00
-J2.00
0.00
0.00
-8.00
~
5 -18.00
ill
~
~ -24.00
I"
-32.00 0.04
0.04 Without Self-Consistency
0.10 \/OIts bios
Well width -50 l
Barrfer widths = sol
Barrfer height. -0.347 eV
Resonance:
E, 0.055 eV
log T, = -8.0945
0.08 0.12 0.18 0.20 0.2' 0.28
ENERGY (EV)
With Self-Consistency
0.10 volts bios
Temp = 300 K
Doping = 5 x 10171/cnn3
Resonance:
E, = 0.055 eV
log Tt = -9.396
0.08 0.12 0.'8 0.20 0.24 0.28
Doping=1X10'·
With Self-Consistency
0.10 volts bios
Temp = 300 K
Resonance:
E, 0.056 eV
log Ti = -16.17 0.J2
0.32
-4Q.00 +---~--.,--~---.--~--~--~-~
0.00 0.04 0.08 0.,2 0.'8 0.20 0.24 0.28 0.32
0.00
-8.00
~
5 -'8.00 ill .
~
~ -24.00
I"
-32.00 With Self-Consistency
0.10 voIt8 blall
Temp = 300 K
Doping = 5 x 10181/cm3
-~.OO+--~--~-~r--~--r--~--"---' 0.00 0.04 0.08 0.'2 0.'8 0.20 0.24 0.28 0.32
ENERGY eV
FIG. 9. Series of plots of the logarithm of the transmissivity vs incident
carrier energy under fixed bias 0.10 V and variable doping concentrations,
5X 1017,1 X 1018, and 5X 1018 Vcm3 at 300 K. As the doping concentration
increases, the self-consistent field breaks the symmetry of the structure,
leading to ~ much lower transmissivity.
bias voltage at various doping concentrations and tempera
tures is presented in Figs. 11-13. The negative differential
resistance "knee" shifts again to higher voltages in each case
owing to the screening effect. At 300 K and 5 X 1017 Vcm3
doping, Fig. 11, the "knee" shifts slightly less than the case at
1 X 1018 Vcm3 doping and 300 K, Fig. 4. This is as expected
2398 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 >-t-
:>
Vi
U1 :;
U1
Z «
0:: r
>-f-
:>
Vi
Vl
~
U1 z «
0:: f-
>f-4.00
0.00
-4.00
-8.00
-12.00
~·16.00
0.00
4.00
0.00
-4.00
-8.00
-12.00
-16.00
0.00
4.00
0.00
~ -4.00
Vl
~ Vl
Z -8.00 «
~
-12.00
-16.00
0.00
0.00
-4.00
>-...
:> -8.00 en
VI
~
VI
~ -12.00
eo
-16.00
-20.00 0.00 Without Self-Conaistency
0.20 vol ta bios
Weli width = 50 A
Barrier widths = 50 l
Borriera heights = 0.347 eV
Resonance:
E, = 0.009 eV
log T, = -3.505
0.04 0.08 0.12 0.16 0.20
ENERGY (eV)
With Sel f-Consisten cy
0.20 volts bios
Temp = 300 K
10171/cm3 Doping = 5 x
Resonance:
E, 0.015 eV
log Tt = -.1475
0.04 0.08 0.12 0.16
With Self-Consistency
0.20 volts bios
Temp = 300 K
Doping = 1 x 10181/cm3
Resonance:
E, 0.017 eV
log T, = -4.1312
0.04 0.08 0.12 0.16 0.20
0.20 0.24
0.24
0.24
With Self-Consistency
0.20 volts bios 0.28
0.28
0.28
Temp = JOOK
10181/cm3 Doping = 5 x
Resonance:
E, 0.023 eV
log T, = -7.685
0.04 0.08 0.12 0.16 0.20 0.24 0.28
ENERGY (EV)
FIG. 10. Series of plots of the logarithm of the transmissivity vs incident
carrier energy under a 0.20 fixed bias and variable doping concentration
5 X 1017, 1 X 1018, and 5 X 1018 Vcm3 at 300 K. The transmissivity peak
shifts upwards in voltage monotonically as the doping increases. Notice that
the relative shift is greater at 0.20 V bias than at 0.10 V bias.
since less charge is present and the screening is less in the
lower doped device. Figure 13 presents an interesting case.
The negative differential resistance peaks decrease dramati
cally when the self-consistent field is added. Again, the peaks
K. F. Brennan 2398
Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions12.00
E
<,0.00
rn c.
E o
---B.OO
>-r--
Vi ti 6.00
o
r--z
W 4.00
0::
0::
::>
U Temp g 300 K
Doping = 5 x 10" 1/cmJ --= Without Self-Consistency --= With Self-Consistency ~
L
2.00+--__r--~--~-_,_--.,__-__r--~-__,
0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36
VOLTAGE (V) .
FIG. 11. Logarithm of the current density vs applied voltage at 300 K and
5 X 1017 Vcm3 doping, both with and without the self-consistent field. The
voltage shift of the negative differential resistance "knee" is slightly less
than at 1 X 1018 Vcm3 (Fig. 4).
8.00
N
E
< 4.00
III
C.
E o '-" 0.00
>-r--
Vi
Q-4.00
o
r--z W-8.00
0:: 0::
::>
U Temp = 25 K
Doping = 5 x 1018 l/crrr"l --= Without Self-Conslsteney --= With Self-Consistency
-12.00+--__r--~-_~-_,_--.,__-~--~-_
0.08 0.16 0.24 0.32 0.40 0.48 0.56 0.64 0.72
VOLTAGE (V)
FIG. 12. Logarithm of the current density vs applied bias voltage at 25 K
and 5 X 1018 Vcm3 doping in the presence and absence of the self-consistent
field. The voltage shift of the "knee" is similar to that at 1 X 1018 Vern3 (Fig.
6).
13.00
E ()
'-...'2.00
III c.
E o '-"11.00
>-r--
Vi
~ 10.00
o
W 9.00
0::
0::
::>
U
0.10 0.20 0.30 0.40 Temp"" 300 K
Doping _ 5 )(10'8 'fem3
•••. -... "" Wlthout Self-Consistency --= With Self-Consistency
0.50 0.60 0.70 0.80
VOLT A GE (\/) 0.90
FIG. 13. Logarithm of the current density vs applied bias voltage at 300 K
and 5X 1018 Vcm3 doping, both with and without the self-consistent field.
Interestingly, the negative differential resistance peak decreases dramati
cally owing to the much lower transmissivity of the structure when the self
consistent field is included. One must look carefully at curve 2 to see the
shifted features present in curve 1. Two very small peaks corresponding to
the "knees" in curve 1 appear near 0.20 V bias in curve 2.
2399 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 >r--4.00
0.00
~ -4.00
til
~ til
Z -8.00 « g:
-12.00 Without Self-Consistency
0.20 volts bios
Well width = 50 A
Barrier widths = 50 '"
Barriers heights = 0.347 eV
Resonance:
E, 0.009 eV
log T, = -3.505
-16.oo+---~-~--~--"__-_--_'_--_r_-
0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28
4.00
0.00
>-.-> -4.00 Vi
til
~
til
Z -8.00 «
0:: .-
-12.00 ENERGY (eV)
Without Self-Consistency
0.1775 volts bios
Resonance:
E, 0.023 eV
log T, = -1.8825
-'6.OO+-----~--~-_--_,_--_r_-~~-
>f-0.00
0.00
-4.00
~ -8.00
(f)
(f)
~
(f)
~ -12.00
f';
-16.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28
With Self-Consistency
0.20 volts bios
Temp = 300 K 18 :I
Doping = 5 x 10 ·1/cm
Resonance:
E, 0.023 eV
log T, = -7.685
-20.00 o;l;.0;;:0--:::0.~04:---:0:':.0:::-8-~0.C::'2:---:0~.':-6 --0~.2--:0--0.~24--0"".2-8-
ENERGY (EV)
FIG. 14. Sequence of plots of the logarithm of the transmissivity vs the
incident carrier energy illustrating the similarity between the self-consistent
field and a smaIler applied bias potential. Notice that a smaller bias pro
duces a resonance at the same energy as in a structure with a self-consistent
potential, but at a higher bias.
are shifted to higher voltages but the peak to valley ratios are
drastically reduced. The decrease is due to the much reduced
transmissivity of the structure owing to symmetry breaking
of the self-consistent field.
IV. CONCLUSIONS
From the preceding discussion, we have determined
that the self-consistent field acts to screen the applied posi
tive bias voltage, leading to an increase in the observed ener
gy of the resonance peak and a lowering of the transmission
amplitUde. Physically, the self-consistent field must be
roughly comparable to applying a smaller bias voltage. Fig
ure 14 compares the transmissivities as a function of incident
electron energy in three cases: no self-consistent field at 0.20
K. F. Brennan 2399
Downloaded 22 Mar 2013 to 142.51.1.212. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissionsV bias, with self-consistent field at 0.20 V bias, and rio self
consistent field at 0.1775 V bias. From the figure, we see that
applying a smaller positive bias results in positioning the
resonance peak at the same energy as in the self-consistent
case. The amplitude of the transmission resonance is much
larger when a uniform field is used (0.1775 V bias) as op
posed to the self-consistent case. It is not surprising that the
transmission amplitude is smaller in the self-consistent case
than in the Uniform field case since the symmetry of the
structure is more greatly disturbed by the self-consistent po
tential than the uniform potential. As we discussed in the
Introduction, coherent resonant tunneling arises from the
resonant buildup of the wave function within the well due to
successive reflections and transmissions from the barriers as
iIi a Fabry-Perot cavity oscillator. This type of tunneling is
extremely sensitive to the symmetry of the structure. 16 As
the potential becomes more distorted, the wave functions are
driven well off resonance leading to a decay in the transmis
sion resonance. Ricco and Azbel16 have shown that the ap
plication of a uniform applied field breaks the symmetry of
the structure, leading to a decrease in the coherent resonant
transmissivity. The transmissivity can be increased back to
unity by altering the geometry of the structure, i.e., changing
the widths of the barrier layers such that the transmissivities
of each barrier becomes equal at the desired bias voltage. In
the presence of the self-consistent potential, the distribution
of the charge and subsequent asymmetric potential profile
leads to· a gross breaking of the symmetry of the structure
and a subsequent "detuning" of the steady-state wave func
tions.
In summary, we have demonstrated, using a piecewise
linear solution of the coupled Schrodinger and Poisson equa
tions, the effects of the self-consistent potential on both the
transmissivity and current-voltage characteristics of a dou
ble-barrier structure. The self-consistent potential detunes
the coherent resonance by altering the symmetry of the po
tential, leading to Ii reduced transmissivity. In addition, the
self-consistent potential shifts the resonance to higher ener
gy due to the screening of the applied positive bias. It is found
that, as expected from the equilibrium distribution function,
that the self-consistent field is most important at high lattice
temperatures, -300 K, and high doping concentrations.
ACKNOWLEDGMENTS
The author would like to thank Dr. C. J. Summers and
Dr. W. R. Frensley for many helpful technical discussions
on this work. The author is also indebted to Professor K.
Hess and T. K. Gaylord for their technical suggestions and
comments. The assistance of Peggy Knight and Diana Fouts
at the Georgia Institute of Technology in preparing this
2400 J. Appl. Phys., Vol. 62, No.6, 15 September 1987 manuscript is gratefully acknowledged. This work was sup
ported in part by the Eastman Kodak Company under
Grant No. R6074-0AO and by the E. I. DuPont Young Fa
culty Investigators Program.
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[Sov. Phys.·Semicond. 6, 120 (1972)].
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D. D. Peck, Appl. Phys. Lett. 43,588 (1983).
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(1983 ).
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K. F. Brennan 2400
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1.101284.pdf | Electrooptic voltage profiling of modulationdoped GaAs/AlGaAs heterostructures
P. Hendriks, F. J. M. Schnitzeler, J. E. M. Haverkort, J. H. Wolter, Kees de Kort, and G. Weimann
Citation: Applied Physics Letters 54, 1763 (1989); doi: 10.1063/1.101284
View online: http://dx.doi.org/10.1063/1.101284
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/18?ver=pdfcov
Published by the AIP Publishing
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160.36.178.25 On: Sat, 20 Dec 2014 23:09:49Electro .. optic voltage profiling of moduialion .. doped GaAsl AIGaAs
heterostructures
p, Hendriks, F. J. M. Schnitzeler, J. E. M. Haverkort, and J. H. Woiter
Eindhoven University of Technology, Faculty of Physics, NL-5600 ME Eindhoven, The Netherlands
Kees de Kort
Philips Research Laboratories, NL-5600 JA Eindhoven, The Netherlands
G. Weimann
Forschungsinstitut der Deutschen Bundespost beim FTZ, D-6J ()() Darmstadt, Federal Republic of Germany
(Received 21 December 1988; accepted for publication 15 February 1989)
The electro-optic effect of GaAs is applied to profile the voltage distribution of the two
dimensional electron gas (2DEG) in a GaAsl AIGaAs heterostructure. In our setup we
reached a voltage sensitivity of2 mY. We used this technique to characterize the local
resistivity of the 2DEG. The results are consistent with those obtained from scanning electron
microscopy voltage contrast measurements,
Modulation-doped GaAsl AIGaAs heterostructures
are widely used to study two-dimensional transport phe
nomena. For the correct interpretation of experimental data
of electrical transport measurements knowledge of the local
potential of the two-dimensional electron gas (2DEG) at
the GaAsl AIGaAs interface is of great importance.
The linear electro-optic effect or Pockels effect is ex
tremely useful to measure voltages. 1-6 The measuring tech
nique is based on the fact that the birefringency of the elec
tro-optic crystal changes with the applied electric field. In
the right experimental geometry this effect leads to a change
of the polarization of light. This change of polarization can
be measured with great accuracy.
In this letter we describe how the electro-optic effect of
the semi-insulating (SO GaAs substrate can be used to pro
file voltages in the 2DEG of a modulation-doped GaAsl
AlGaAs heterostructure. We use this technique to deter
mine the homogeneity of the conductivity of the 2DEG,
which can show both abrupt and more gradual changes.7-9
The samples used in this study are selectively doped he
terostructures grown by molecular beam epitaxy (MBE) on
a SI GaAs substrate. The structures consist of a 5 pm GaAs
buffer layer, a 36 nm undoped Alo.38 Ga().62 As spacer, a 31
nm Si-doped Alo.3sGao.6zAs layer, and a 24 nm GaAs cap
layer. Themobilityofthe2DEGisO,82 (36) m2/V sand the
electron concentrations 2.5 X 1015 (1.9 X 1015) m -·2 at 300 K
(4.2K),
A Hall bar configuration was photolithographically de
fined and mesa etched [see Fig. 1 (a) ]. The ohmic contacts
were formed by alloying small In spheres into the surface
[ the black circles in Fig. 1 (a) ], We polished the rear of the
sample and subsequently evaporated a 100 A layer of Au on
it to maintain an equipotential plane as a reference for the
potential of the 2DEG. For a current flowing through the
2DEG two electric field components are present: one paral
lel to the 2DEG and one between the 2DEG and the Au
layer.
The experimental setup is depicted in Fig, 1 (b), As a
light source we use an InGaAsP diode laser with a wave
length). of 1.3 f.lm and a power of 1 m W. The light is polar
ized by a Glann-Taylor polarizer and is subsequently fo-cused on the sample to a spot of 40 f.lm. This spot can be
moved across the sample by displacing the total optical setup
with an xy stage.
The light polarized along the (100) axis is passed
through the GaAs heterostructure along the (00 1) axis in
the same direction as the electric field between the 2DEG
and the Au layer. Only this perpendicular electric field gives
a noticeable phase difference. In the described geometry the
phase difference t.r between the slow and fast axis is given
by 10
(1)
where no and Y4l are the refractive index and the component
of the electro-optic tensor of the GaAs, d is the thickness of
the substrate, Ez (x,y,z) is the electric field along the z direc
tion, and V( x,y) is the potential difference between the refer
ence electrode and the 2DEG at position (x,y). From Eq.
( 1 ) it follows that the phase difference and the potential are
directly proportional to each other and the potential of the
2DEG is measured. If a quarter-wave plate is used the inten
sity of the transmitted light depends linearly on the phase
difference. 10 To detect small variations in intensity we mod
ulated the potential V(x,y). This results in a modulated in
tensity of the transmitted light. This intensity variation is
2
9 8
(a) (b)
FIG. 1. Cal Geometry of the samples used in this study. The black circles
are the indium ohmic contacts. The dashed lines indicate the four voltage
line scans [Figs. 2(a) and 2(b) J. The box displays the area of the sample
which has been studied by SEM voltage contrast (Fig. 3). (b) The experi
mental setup for the electro-optic voltage probe experiment.
1763 Appl. Phys. Lett. 54 (18), 1 May 1969 0003-6951/89/181763-03$01.00 (0) 1989 American Institute of Physics 1763
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
160.36.178.25 On: Sat, 20 Dec 2014 23:09:49. • 0$
Q.20 .... II l jUl ". f ~lkH . E '.: t~3s:
L~LLLL~J~' '='~~~l · 1 2 3 40.S 1.0152.0 2.5
(a) y!mm) (b) .[mmJ
FIG. 2. (a) Potential of the 2DEG with respect to the Au layer at the rear,
for contact 2 at 0.7 V and contact 9 connected to the Au layer, scanned along
the dashed line! of Fig. I (a). Fo]]owingthelinesII, III, and IV of Fig. I (a},
we obtain voltage profiles as indicated in (b). I,incs II and III are almost
fiat, but line IV shows a step, which we associate with an interruption ofthe
2DEG.
detected with a photodetector and lock-in amplifier. r41 was
measured by applying a known voltage between the 2DEG
and the Au layer, without a current flowing through the
2DEG, and was about 1.48 pm/V. This result was also used
to calibrate the experimental setup.
With the described experimental setup we obtained sen
sitivities of2 mY, when the laser spot was kept at one posi
tion. When scanning over the sample some additional uncer
tainties of about the same order of magnitude in the
measured potential are introduced due to variations in the
transmission.
During the electro-optic experiments we kept contact 1
at 0.7 V and contact 9 at 0 V. When we scanned along line I of
Fig. I (a) we obtained the voltage profile given in Fig. 2 (a) .
One observes that the slope of the curve in the lower part of
the curve (0 < y < 2.4 mm) is steeper than in the upper part,
implying a change in the resistance of about a factor of 2. In
Fig. 2 (b) the voltage profiles of lines II, III, and IV are
depicted. A quite extraordinary step is found in line IV,
where a drop of almost 0.25 V is present.
Intuitively we associated this step with an interruption
of the 2DEG between contacts 8 and 9. This also then would
explain the two different slopes of curve I. The current then
would flow in the upper part through a 2-mm-wide region,
while in the lower part the current flows through an approxi
mately I-mm-wide regi.on with a higher resistance.
To check this interpretation we performed a scanning
electron microscopy (SEM) voltage contrast measure
ment,7.11 which in a different way also measures the electri
cal potential of the surface. The primary electron beam of the
scanning electron microscope generates secondary elec
trons. The number of secondary electrons detected strongly
depends on the surface potential. An area with a positive
potential appears dark on the monitor while a negative area
appears bright. In Fig. 3 a SEM voltage contrast image of the
part of the sample enclosed by the box [Fig. 1 (a)] is shown.
We held contact 2 at 0 V and contact 9 at 2 V. One immedi-
1764 Appl. Phys. Lett., Vol. 54, No. 18, 1 May 1989 FIG. 3. SEM voltage contrast image of the part ofthe sample within the box
which is indicated in Fig. I (a). Contact 2 is at zero voltage while contact 9
is kept at 2 V. The dark regions are associated with a large positive value
while the lighter regions have smaller values. Clearly visible is the step in
potential between contacts 8 and 9.
ately observes the sharp contrast between contacts 8 and 9
indicating a large potential difference. This contrast exactly
coincides with the potential drop we found with the electro
optic experiments. It is also clear that this interruption stops
almost 2 mm above the lower contacts, confirming that the
resistance of the lower part of the sample is larger than in the
upper part. It is also interesting to note that the interruption
of the 2DEG is exactly parallel to the (110) crystal axis. We
have already reported on this feature earlier.7 Since these
interruptions are both present in MBE and metalorganic
chemical vapor deposition material, they probably arise
from an imperfection in the substrate.
The main advantage of the electro-optic voltage profil
ing above SEM voltage contrast is that there is almost no
influence of the measuring system on the device. Further
more, the electro-optic measuring technique is extremely
well suited to be used at low temperatures and in high mag
netic fields. This makes it possible to tackle, for example, the
fundamental prol?lem of current and potential distribution
under quantum Hall conditions. 12.13
In conclusion, we used the electro-optic effect of the
GaAs substrate to profile the potential of the 2DEG of a
GaAsl AlGaAs heterostructure. Furthermore, we showed
how this technique is a powerful tool to characterize GaAsl
AIGaAs heterostructures.
The authors are grateful to J oris V rehen for performing
the SEM voltage contrast experiments and to Peter
Nouwens for the preparation of the samples. Part of this
work was supported by the Stichting voor Fundamenteel
Onderzoek der Materie.
'J. A. Valdmanis, G. A. Mourou, andC. W. Gabel, IEEE!. Quantum Elec
tron. 19,644 (1983).
'1. A. Valdmanis, G. A. Momou, and C. W. Gabel, App!. Phys. Lett. 41,
211 (1983).
Hendriks et al. 1764
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160.36.178.25 On: Sat, 20 Dec 2014 23:09:493J. A. Vaidmanis and G. A. Mourou, IEEE J. Quantum Electron. 22, 69
(1986).
4S. H. Kolner and D. M. Bloom, IEEE J. Quantum Electron. 22, 79
(1986).
'Y. H. Lo, Z. H. Zhu, C. L. Pan, S. Y. Wang, and S, y, Wang. Appl. Phys.
Lett. 50, 1125 (1987).
oz. H. Zhu, C. L. Pan, Y. H. Lo, M, C. Wu, S. Wang, B. H. Koiner, and S,
Y. Wang, Appl. Phys. Lett. 50,1228 (1987).
7P. Hendriks, K. de Kart, R. E. Horstman, J. P. Andre, C. T. Faxon, and J.
Wolter, Semicond. Sci. Techno!. 3,521 (1988).
1765 Appl. Phys. Lett., Vol. 54, No.1 S. i May i 989 'Po F. Fontein, P. Hendriks, J. Wolter, R. Peat, and D. E. Williams, J. App!.
Phys. 64, 3085 (1988).
"P. F. Fontein, P. Hendriks, J. Wolter, A. Kllcernak, R. Peat, and D. E.
Williams, SPIE Free. 1028, 197 (19R9).
IDA. Yariv. Quantum Electronics (Wiley, New York, 1967).
"H. P. Feuerbaum, Scanning 5, 14 (1983).
"G. Ebert, K. von Klitzing, and G. Weimann, J. Phys. CHI, L257 (1985).
liE. K. Sichel, M. L. Knowles, alld H. H. Sample. 1. Phys, C 19. 5695
( 1986).
Hendriks et al. 1765
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1.339812.pdf | Electrical properties of ion beam recrystallized and laser beam annealed arsenic
implanted silicon on sapphire
G. Alestig, G. Holmén, and J. Linnros
Citation: Journal of Applied Physics 62, 409 (1987); doi: 10.1063/1.339812
View online: http://dx.doi.org/10.1063/1.339812
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/62/2?ver=pdfcov
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132.206.7.165 On: Tue, 09 Dec 2014 21:32:15Electrical properties of ion beam recrystallized and laser beam anneaied
arsenic~implanted sUicon on sapphire
G. Alestig, G. Holmen, and J. Linnros
Department of Physics, Chalmers University afTechnology, S-41296 Goteborg, Sweden
(Received 18 November 1986; accepted for publication 27 February 1987)
A 300-ke V neon ion beam has been used to epitaxially regrow an amorphous surface layer in
silicon on sapphire at three different target temperatures, 350, 400, and 450"c' The layer was
produced by implantation of 40 keY, 1015 arsenic ions/cm2• After the ion beam induced
recrystallization, only a few percent of the dopants were electrically active. However, the
electrical activity increased to 70%-80% by a subsequent cw laser anneaL Channeling
measurements showed that the crystal quality of these samples was better than that for samples
subjected only to laser annealing. Measurements of the angular dependence of the
backscattering yield showed that, for the ion beam recrystallized samples, the arsenic atoms
were displaced from substitutional positions.
I. INTRODUCTION
In recent years, a number of investigations have been
devoted to ion beam induced epitaxial regrowth of amor
phous layers in semiconductors. The phenomenon has been
studied with respect to mass and energy of the annealing ion,
target temperature, crystal orientation, and for a channeled
or nonchanneled annealing beam, 1-14
Most experiments have been made on silicon, and the
amorphous layer has usually been created by a silicon ion
implantation. However, in some cases a doping element,
such as phosphorus, 15 arsenic, II or antimony,H-1O has been
used. For arsenic and antimony, the substitutional fraction
as determined by channeling measurements is reported to be
about 90% following ion beam induced recrystallization. An
interesting question is then, what are the electrical properties
of the materials, particularly since thermal or laser anneal
ing of them makes a high percentage of the dopants electri
canyactive.
In the work presented, electrical measurements on ion
beam recrystallized silicon on sapphire (SOS) have been
performed. The effect of a subsequent cw laser annealing of
the ion beam recrystallized material has also been investigat
ed. Channeling measurements were used to determine the
structural properties of the regrown layers.
II. EXPERIMENT
The material used in the experiments was intrinsic, 0.6-
11m (100) silicon on sapphire. A 3-in. wafer was implanted
with 40 keY, 1015 arsenic ions/cm2 at room temperature,
thereby creating an amorphous surface layer approximately
550 A thick. After the implantation, the wafer was cut into
samples measuring 5 X 7 mm2 for channeling analysis and
sheet resistivity measurements, and 9 X 14 mm2 for Hall ef
fect measurements.
The ion beam induced recrystallization was performed
with 300-keV ZONe I ions at three different target tempera
tures, 350, 400, and 450°C. A low dose rate, 3.9X 1012 ions/
cm2 S, was used to minimize the effect of beam heating. The
target was mounted with the surface normal 7° off the beam
direction. The neon dose required to regrow the amorphous layer was found to be 1.5, 1.0, and 0.5 X 1016 ions/cm2 for the
target temperatures 350, 400, and 450°C, respectively.
During the ion beam regrowth stage, a number of sam
ples were mounted in the same holder but masked from the
ion beam. In this way, we also obtained samples thermally
treated for 2 h 30 min, 2 h, and 1 h at 350, 400, and 450°C,
respectively. To enable comparison with conventional fur~
mace annealing, one SOS sample and one bulk silicon sample
were annealed at 850°C for 30 min.
Laser annealing was performed on the recrystallized
samples and also on the samples that had only been thermal
ly treated. The laser system consisted of a cw argon-ion laser
operated at 5145 A, a lSO-mm focusing lens, and a scanning
system. The focused beam was scanned over the samples at 1
cm/s and with 9 p.m between each line. The beam diameter
was approximately 40 j.tm at lie intensity. Laser powers
between 1.4 and 2.0 W were used in the investigation. Melt
ing of the silicon surface occurred at about 2.1 W.
The ion beam recrystallized and the laser annealed sam
ples were analyzed by the channeling technique using a beam
of 230-keV protons entering in the (100) direction, The
back scattered particles were detected at a scattering angle of
135° with a surface-barrier detector cooled to -50°C, A
description of the equipment and the procedure for the mea
surements is given in Ref. 7.
To investigate lattice location of the arsenic atoms, we
used the method of measuring the angular dependence of the
yield of back scattered particles.]1> Using this method, a com
parison was made between the angular dependence of the
yield for protons backscattered by the arsenic atoms and for
protons backscattered by the silicon atoms. If all arsenic
atoms are located on substitutional lattice sites, the two sig
nals will coincide except for scale, while other locations will
give a distorted arsenic channeling dip. A beam of 230-keV
protons was used and the target was tilted to a maximum
angle of 2.4° between the < 100) direction and the proton
beam.
Sheet resistivity measurements were made using a four
point probe. i7 On some samples, the effective mobility and
the effective sheet carrier concentration were determined by
Hall-effect measurements. IS For the Hall-effect measure-
409 J. Appl. Phys. 62 (2). 15 July 1987 0021-8979/87/140409-05$02.40 @ i 987 American Institute of Physics 409
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132.206.7.165 On: Tue, 09 Dec 2014 21:32:15ments, a laser power of 2.1 W had to be used to obtain sheet
resistivities previously observed at 2.0 W. The discrepancy
was due to a slightly increased spot size of the laser beam.
!!I. RESULTS
A. Channeling measurements
Figure 1 shows channeling spectra for the ion beam re
crystallized samples. It can be seen that the amorphous layer
has been completely regrown by the ion beam. However, the
backscattering yield has not resumed the virgin level, indi
cating that some damage remains in the material. Much of
this damage is probably caused by the neon implantation,
which produces defects, particularly at depths close to the
projected range.7 This agrees with the fact that the samples
annealed at 350°C, which received the highest neon dose,
have the highest damage level.
Figure 1 also shows a spectrum for a sample only sub
jected to a thermal treatment at 450 "C. The amorphous lay
er, initially 550 A thick, has regrown about 80 A. For the
samples treated at the lower temper:atures, 350 and 400°C,
this value was smaller. Thus, the thermally induced re
growth was small compared to the ion beam induced, Pre
vious investigations have shown a high initial thermal
growth rate due to regrowth of the partially damaged region
beyond the amorphous/crystalline interface. When this re
gion was regrown, a much lower rate was observed,7
Spectra for the samples which were both ion beam re
crystallized and laser beam annealed are shown in Fig. 2.
The laser annealing reduces the back scattering yield to a low
DEPTH (.4)
6000 4000 2000 c
18 ~
16 !
~ :14
z
~ 8 12
o g 10
"'""'
8
4 ' i
i
2 ~ ION BEAM ONLY
AS IMPLANTED1
Ijtr} THERMAL 450 ·c 11
: ~,~
,: \~" ~ II ....... \~., .~ !' ...... '.:\ ~ I
...... '\\, ~ I
...... ~, .. ~. I
..... ~ ~.'
········ ... ~V350 DC
400 oC/··.~'~
4500C~~
VIRGIN/··..::'
'"
50 100 150 200
CHANNEL NUMBER
FIG. 1. Backscattering spectra for SOS implanted with 4O-keV lOIS
As ions/em2 and ion beam recrystatlized with 300-keV Ne ions at different
target temperatures. Spectra for an as-implanted sample and for a sample
only subjected to a thermal treatment at 450 'C are also shown.
410 J. Appl. Phys., Vol. 62, No.2, 15 July 1987 U1 t-z
::J
0 u
0
0
0
"""
0
...J w
H
>-DEPTH CA)
6000 4000 2000 o
18
16
14
12 !-
I I ,
10
8
6
4
2
o ION BEAM t 2.0 W LASER
AS IMPl..ANTED~
/f"\ I
\\ I
' r'
¥-'~ \ """:\'\"
'~\
'~.
'~. 450·C
",.~
?:)(VIRGIN
350 ·C -'" :)
400 ·C/'·"-
50 100 i50 200
CHANNEL NUMBER
FIG. 2. Backscattering spectra for SOS implanted with 40-keV 1015 As
ions/crn2 and annealed, first with a 300-keV Ne beam at three different
target temperatur~s and then with a 2.0-W laser beam. As-implanted and
virgin spectra are also shown.
level, which shows that any remaining damage from the ar
senic implantation and the damage caused by the neon im
plantation has been removed. The lowest damage level is
obtained for samples ion beam recrystallized at 400 ·C, while
for 350 ·C the level is slightly higher. For both temperatures
the backscattering yield is actually below the yield for virgin
SOS, but since the virgin spectrum was taken on a sample
from a different wafer, the significance of this is perhaps not
clear. The spectrum for 450 DC is a little above the spectra for
the two other temperatures, probably due to the creation of
more stable defects during implantation at this temperature.
Figure 3 shows spectra for the samples treated at 400 dc.
Here it can be seen that the sample that had only a thermal
treatment before the laser annealing has a higher damage
level than if it also had been ion beam recrystallized. The
same was observed for the temperatures 350 and 450 "C.
The angular dependence of the yield for protons back
scattered in the depth interval 0-800 A is shown in Fig. 4. It
should be noted that due to planar channeling, unity normal
ized yield represents a lower yield than for a random direc
tion. It is clearly seen in Fig. 4 that for the sample which had
only been recrystallized by the ion beam, a large fraction of
the arsenic is not substitutional since the dip in the arsenic
signal is both shallower and narrower than the dip in the
silicon signaL
It is also seen in Fig. 4 that after laser annealing the
substitutional arsenic fraction increases. Samples which
were not regrown by the ion beam but were subjected to the
same thermal treatments showed a still higher fraction upon
laser annealing, and have an arsenic signal that almost coin-
Alestig. Holmen, and Linnros 410
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132.206.7.165 On: Tue, 09 Dec 2014 21:32:15DEPTH (A)
6000 4000 2000 a
i8 DIFFERENT TREATMENTS
AT 400 Cc
16
(,,') 14 AS IMPLANTED!\ I-z
:::::J
0 12 u ! .~.
0 / \
0 10 (('2\ \ I 0 I .,...,
<"~~':"" \
8 ',j,,\; \ \ \" ''\, 0 '\~, " ..-l
lJ.J 6 .. :'. ''\,
H "'~'" , ............... ION BEAM >-! ';~:'~"',:XVIRGIN 4 ~
iTHERMAL + LASER~'~ "-.,
/<~ 2 t ION BEAM + LASER '. '~
0
50 100 150 200
CHANNEL NUMBER
FIG. 3, Backscattering spectra for 50S implanted with 40-keV 1015 As
ions/cm2 and subjected to different treatments at 400 'CO As-implanted and
virgin spectra are also shown.
• ION BEAM
• ION BEAM ... LASER
A THERMAL + LASER
0 •
..-l 1.0
IJ...I
H
>-
0
lJ.J
N
H
....J
4.:
::E II a z 0,5
0.0
-3 -2 -1 0 i 2 3
ANGLE (DEG)
FIG. 4. Normalized backscattering yield as a function of angle between the
proton beam and the target. The angle is relative to the < ! 00) direction and
the SOS target was implanted with 4O-keV 1015 As ions/cm2, The yield for
protons backscattered by the arsenic atoms is shown for three different
cases: (a) ion beam recrystallized at 450 'C, (b) ion beam recrystallized at
450·C and laser annealed, and (c) thermally treated at 450 ·C and laser
annealed, The yield for protons backscattered by the silicon atoms is also
shown.
411 J. Appl. Phys., Vol. 62, No.2, 15 July 1987 cides with the silicon signal. This indicates that a substantial
part of the dopants are on substitutional sites and should be
electrically active.
B. Electrical measurements
The results of the sheet resistivity measurements are
shown in Fig. 5. Before laser annealing, all samples had high
sheet resistivities. The ion beam recrystallized samples had
sheet resistivities ranging from 3 kH/D for 450°C to 8 kH/D
for 350°C. The thermal reference samples had sheet resisti
vities between 3 kn/D for 450 ·C and 40 kfl/D for 350 "C.
After laser annealing the sheet resistivity decreases, but
in quite a different way for the ion beam regrown samples
compared with the thermal references. The latter reach a
low sheet resistivity, about 1400/0, at 1.7 Wand the sheet
resistivity is then approximately constant up to the highest
laser power used. This behavior agrees with earlier laser an
nealing studies.19 The ion beam recrystallized samples re
quire a much higher laser power to get low sheet resistivities.
Not until 2.0 W do the sheet resistivities decrease to about
160 H/D, which is still a bit higher than for the thermal
references.
Table I shows the results of the Hall-effect measure
ments. Ion beam recrystallized samples had effective mobili
ties between 31 and 46 cm2/V s, the highest value for the
highest temperature, 450"C. After laser annealing the mo
bilities increased slightly for the lowest temperatures and an
ANNEAL PRIOR TO LASER:
105
~ + LASER ONLY • THERMAL 350 DC
0 ION BEAM 350 ·C
• A THERMAL 400 ·C
0' ~ b-ION BEAM 400 ·C
....... • THERMAL 450 DC
(f) ION BEAM 450·C ;;;;;: A~ <)
::c 9.
o~~\ 10'
>-I-A~\ H >-
,.~~\\ H I-
(f)
t H
(f)
·\'o~ I.JJ
0:
I-j 03
I.JJ § ~ ~,o\ I.JJ ::c
(f)
\ ~\
\:~ \~
'6 ...... _i
:102 l ,
1
o 1.4 1.8 1.8 2.0
LASER POWER (W)
FI G. 5. Sheet resistivity before laser annealing (0 W) and after annealing at
different laser powers, The samples were implanted with 10" As ions/cm2
at 40 keY, Open symbols refer to ion beam recrystallized samples and filled
symbols to thermal reference samples.
Alestig, Holmen, and Linnros 411
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132.206.7.165 On: Tue, 09 Dec 2014 21:32:15TABLE I. Effective mobility, Peff' and effective sheet carrier concentration,
(N, jeff' determined by Hall-effect measurements for 50S implanted with
1015 As ions!cm2 at 40 keV. Results are given for samples ion beam recrys
tallized at different temperatures and for samples which have been subse
quently laser annealed. Values for SOS and bulk silicun furnace annealed at
850 'c are also shown.
f-ieff (Ns Jeff
Annealing (cm2/V s) (1015 em-2)
Ion beam 350'C 31 0.025
400'C 40 0.034
450'C 46 0.036
Ion beam 350'C 48 0.68
+ Laser 400'C 47 0.75
450'C 47 0.79
Furnace, SOS 8500C 57 0.68
Furnace, bulk Si 8500C 84 0.67
samples obtained mobilities of about 47 cm2 IV s, i.e., no ma
jor change was observed. The effective sheet carrier concen
tration, on the other hand, shows a drastic increase after
laser annealing. The number of electrically active arsenic
atoms rises from about 3% to values between 68% and 79%.
Furnace annealing at 850°C results in a mobility of 57 cm2 I
V s for SOS and 84 cm2 IV s for bulk silicon. The number of
electrically active arsenic atoms is about the same as for the
material subjected to the combined ion beam and laser treat
ment.
IV. DISCUSSION
The most striking result of the present investigation is
that even if an ion beam can induce a complete recrystalliza
tion of an arsenic implanted silicon layer, the resistivity of
the material remains high. The Hall-effect measurements
show that the high resistivity is not due to a low mobility, but
is explained by a low electrically active fraction of the do
pants. In Table I it can be seen that the mobility of furnace
annealed material is higher, particularly for bulk silicon,
That the mobility is lower in 50S than in bulk silicon is well
known and is explained by a higher defect density in the SOS
material. The lower mobility in the ion beam recrystallized
and laser beam annealed SOS compared with the furnace
annealed SOS may be due to an incomplete annealing by the
laser treatment of the defect complexes created during the
neon ion bombardment.6,7
All mobility values presented here are rather low, but
this is due to the high doping level of the samples. The mobil
ity in low doped or intrinsic SOS is, of course, much higher,
where values in the range 200-600 cm2 IV s can be ob
tained,2o,21
The conclusion about the low electrically active fraction
of the dopants is supported by the angular dependence of the
backscattering yield (Fig, 4), where the arsenic signal has a
shallower and narrower dip than the silicon signal. The re
duced depth of the dip indicates that a substantial part of the
arsenic is randomly distributed in the silicon lattice, i.e., not
on substitutional sites. That the dip for arsenic is narrower
than for silicon can be interpreted as some arsenic atoms
sitting close to silicon rows, but displaced from substitution
al positions.
412 J. Appl. Phys., Vol. 62, No.2, 15 July 1987 It should be pointed out that only one arsenic dose
(l015/cm2) and one mass and energy of the annealing ion
has been used in our investigation, Different beam param
eters might lead to an increased number of electrically active
dopants.22
The properties of the ion beam recrystallized silicon
somewhat resemble the properties of silicon implanted at a
high temperature, where the material has relatively little
damage but usually very few electrically active dopants.
Mayer et al.23 have, for instance, implanted 1015 arsenic
ions/cm2 in silicon at 500 cC and obtained only 1.5% of the
arsenic electrically active. It is also interesting to compare
with results from channeling measurements on silicon im
planted with arsenic at 450 °C.24•25 Here, an attenuation of
about 50% was seen in the yield for channeled helium ions
backscattered by the arsenic atoms. This result is similar to
what is seen in Fig. 4 for a sample ion beam recrystallized at
450 cC.
The arsenic in the ion beam recrystallized silicon can be
made el.ectrically active by a laser annealing treatment. It is
interesting to note that at the lowest laser power used, 1.4 W,
all ion beam recrystallized samples reach the same sheet re
sistivity. Obviously, laser annealing at this power produces
about the same number of electrically active arsenic atoms,
irrespective of the temperature used during ion beam recrys
tallization. However, to obtain low sheet resistivities, a high
laser power is required so that the crystal temperature is
raised to just below the melting point. By decreasing the scan
velocity and thereby increasing the time at high tempera
ture, it should be possible to use a lower temperature during
the laser anneaL This has been confirmed by some prelimi
nary measurements. The laser annealing of ion beam recrys
tallized material again resembles the situation for silicon im
planted at a high temperature, where a high annealing
temperature is needed to activate the dopants. 26
To further investigate the reason forthe high resistivity
of the arsenic implanted layers after the ion beam recrystalli
zation, the following experiment was performed. Some ofthe
laser annealed samples were again exposed to a neon beam.
The target temperature was 400°C and the ion dose was the
same as that used for the ion beam recrystallization. The
resistivity of these samples increased and resumed values
about the same as they had prior to the laser annealing. This
shows that the neon ion bombardment and the associated
generation of point defects have a tendency to move the ar
senic out of substitutional sites. Thus, if some arsenic atoms
are incorporated in the lattice during the ion beam recrystal
lization, there is a high probability that they will be moved
out of lattice sites by the continued ion bombardment. The
mechanism of such a displacement could be the formation of
arsenic defect complexes.
v. CONCLUSIONS
A 300-keV neon ion beam can induce regrowth of an
amorphous arsenic implanted silicon layer, but after the ion
beam induced recrystallization only a few percent of the do
pants are electrically active. This has been shown to be due to
a displacement of arsenic atoms from lattice sites. By a cw
laser annealing, the residual damage can be removed and the
Aiestig, Holmen, and Linnros 412
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132.206.7.165 On: Tue, 09 Dec 2014 21:32:15dopants activated. The required laser power is higher than
for samples not exposed to the neon beam, but the crystal
quality is better for samples that were regrown by the ion
beam before the laser annealing. No major difference in the
electrical properties was observed between samples which
were ion beam recrystallized at different temperatures.
ACKNOWLEDGMENTS
The authors would like to thank J. J acobsson for techni
cal assistance. Financial support was received from the Swe
dish Board for Technical Development and from the Swe
dish Natural Science Research Council.
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Channeling (Academic, New York, 1982), p. 76.
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land, NY, 1983), Vol. 13, p. 517.
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Alestig, Holmen, and Lir.nros 413
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1.100109.pdf | Extremely low resistance nonalloyed ohmic contacts on GaAs using
InAs/InGaAs and InAs/GaAs strainedlayer superlattices
C. K. Peng, G. Ji, N. S. Kumar, and H. Morkoç
Citation: Appl. Phys. Lett. 53, 900 (1988); doi: 10.1063/1.100109
View online: http://dx.doi.org/10.1063/1.100109
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v53/i10
Published by the American Institute of Physics.
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Downloaded 12 Sep 2012 to 128.148.252.35. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsExtremely low resistance nonaUoyed ohmic contacts on GaAs
USing InAsllnGaAs and inAs/GaAs strained .. layer superlaUices
c. K. Peng. G. Ji, N. S. Kumar, and H. Morkot;:
University a/Illinois, Coordinated Science Laboratory, } 101 West Springfield Avenue, Urbana,
Illinois 61801
(Received 22 April 1988; accepted for publication 29 June 1(88)
Employing a structure consisting of n+ -lnAs/lnGaAs and InAs/GaAs strained-layer
superlattices (SLS's) grown by molecular beam epitaxy on GaAs films, non alloyed contact
resistances less than 8.5 X 10-8 n cm2 have been obtained, Self-consistent simulations show
that these extremely sman nonalloyed contact resistances are due to the suppression of the
depletion depth in the GaAs channel and tunneling through the SI~S layer. Similar structures
on InGaAs channels have led to nonaHoyed specific contact resistances of about 1. 5 X 10 -8
n cm2, These results represent the smallest figures reported for these important material
systemso
It is wen known that parasitic resistances deteriorate the
overall performance of electronic and optical devices, with
near intrinsic device characteristics achievable only through
the minimization of parasitic resistanceso One important
source of parasitics is the specific contact resistance between
the metal contact and semiconductor, particularly for sub
micron devices. Since device dimensions arc continuously
being reduced for improved performance and higher density
in integrated circuits, contact resistances have taken a very
important role.
As devices are scaled down, the vertical and lateral dif
fusion of contact metal during high-temperature processing
becomes more criticaL These problems limit the utility of
low-resistance metal contacts obtained through convention
al thermal annealing. To alleviate the obstacles associated
with diffusion, while achieving low contact resistances, we
have demonstrated non alloyed ohmic contacts on InGaAs,
lattice matched to InP, with specific resistances as small as
2.7 X 10 -H n cm2, I In this letter, we describe a short-period
supcrlatticc structure which leads to extremely small nonal
loyed ohmic contact resistances on GaAs.
Samples investigated were grown on (100) GaAs(un
doped) substrates by molecular beam epitaxyo Details ofthe
sample preparation and growth procedure have been report
ed clsewhere.2 The structure consists of a 0.3 flm undoped
GaAs buffer, a 0.1 jJ,m Si-dopcd GaAs channel, five periods
of 10 Alto A Si-doped GaAs/InAs strained-layer superlat
tice (SLS), five periods of 20 A/20 A Si-doped InGaAsl
InAs SLS, and finally a 50 A Si-doped InAs cap layer. The 8i
doping concentration in doped films was about 4X lO'H
em -3 0 Transmission line geometry was defined using stan
dard photolithography and chemical etching in a Br-based
solution.
Contact resistances were measured using the well-estab
lished transmission line model (TLM).3 The test pattern
consists of rectangular pads, 100 pill long and 250 pm wide,
separated by a gap varying from 1 to 20 11m. Precise calibra
tion ofthe contact spacing was performed by optical micros
cOPYo A four-point probe arrangement was used to eliminate
any possible error introduced by the probe contact resis
tanceo Measurements were then performed with each datum
point being the average resistance measured from two near
by devices with identical gap spacing in each seL Thc contact and sheet resistances were derived from a
plot of the measured resistance versus gap spacing as shown
in Figo 1. The method of least squares was used to get a
straight line fit to experimental data. From the y intercept
and the slope, a contact resistance of 00 16 n and a sheet
resistance of 190 U/D were measured, respectively. These
values translate to a specific contact resistance of 8.5 X 10· . H
n cm2 with a correlation coefficient ofO,999, which is indi
cative of weB calibrated contact spacings and good sample
uniformity. To ascertain that lateral conduction is not domi
nated by the thin SLS layers, a similar sample without the
GaAs channel was grown for which a contact resistance of
0033 n and a sheet resistance of 2.75 X !O" n/o were ob
tained. This sheet resistance is 145 times greater than that
with the O. ! flm doped GaAs channel, indicating a generally
insignificant current flow in the thin, highly strained SLS.
It should be pointed out that the assumptions made in
the TLM method should be treated with caution.4 The basic
assumption that the contact metal be an equipotential plane
at the actual contact area is adequately satisfied for the pres
ent nonalloyed contactso Meanwhile, errors can be intro
duced from the structure of the sample and the fabrication
processo 5 These include the difference between the specific
contact resistance and the characteristic impedance, the lat
eral crowding resistance due to the difference in metal width
and mesa width, and the voltage drop of the crowding cur-
3484--1 InAs/GaAs nanalloyed
'''0-------'v. f-
ro t E 1000 ' .s::
~
OJ () c
1'l
.~ 5.0
tV
0::
500 10.0 1500
Contact spacing (microns)
FIG.!. Transmission line method data of the sample investigated, 20.0
900 Appl. Phys, Lett 53 (10),5 September 1988 0003-6951/88/360900-02$01.00 @ 1988 American Institute of Physics 900
Downloaded 12 Sep 2012 to 128.148.252.35. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsMetal/lnAs/SLS(tl a")/GaAs("b")
;-=ch,
~""'"-=-_£ g,--_. __ .
: t ------_,----------
'\.., Ee
Distance
FIG. 2. Schematic representation of the conduction-band profile. The low
ering of the bulk barrier height (<Ph) due to the strained-layer supcrlattice
(SLS) is clearly shown.
rent due to the finite vertical thickness of the conduction
channel (r;). A detailed description concerning all these
factors has been given in Ref. 1. In the present structure, no
significant effects from the aforementioned factors were ob
served except the crowding resistance r;.
The crowding resistance caused by the finite vertical
thickness (h) of the conduction channel has been derived, in
Refs. 3-5, to be
r;' ;::;O.2rJl. (1)
It should be noted that an effective bulk channel resistivity
has to be used for rb, which depends not only on the channel
doping concentration but also on band structure< The pre
factor 0.2 needs to be modified as wen since it was derived
under the assumption of a uniformly doped channel layer.
As discussed in Ref. 1, the contribution from r; on Yc was
found to be insignificant in the InAs/InGaAs heterojunc
tion. In the InAs/GaAs heterojunction, however, the deple
tion in the GaAs channel layer is important, leading to a
significant contribution of r; on "c < The calculated r; is
around the low to mid 10-H n cm2
<
The factors behind the small specific contact resistance
can be further revealed by looking into the detailed band
structure and the current conduction mechanism. In form
ing contacts between metal and semiconductors (in particu
lar, most of the III-V's), the Fermi level of the metal is typi
cally pinned in the energy gap. The resultant Schottky
contacts normally have barrier heights varying from approx
imately 0.5 to 1.0 eV leading to large depletion depths. Un
less samples are heavily doped and contact metal alloyed at
high temperatures, low-resistance contacts are usually diffi
cult to obtain. Although the ohmic behavior of InAs was
reported earlier by Mead and Spitzer,6 only recently has the
advantages of an InAs contact layer been explored with re
ports of extremely sman contact resistances in a metal!
InAs/lnGaAs structure. 1
With a band-gap difference between InAs and GaAs of
1.06 eV, and assuming that 70% of the difference occurs at
the conduction band, a conduction-band discontinuity of
0.74 eV is obtained. This large discontinuity at the InAs/
GaAs junction gives rise to a large depletion region which,
90~ Appl. Phys. Lett., Vol. 53, No.1 0,5 September 1988 unless remedied significantly, leads to large resistances.
When incorporating the short-pedad SLS, as shown in Fig.
2, a much smaner depletion region and effective barrier
height ~ b can be obtained. The advantages of the strained
layer supedattice are twofold: (1) At the heterojunction,
keeping in mind that the Fermi level in InAs is pinned in the
conduction band, the effective barrier height is determined
by the charge transfer from the large band-gap material
(GaAs) to the lower band-gap one (InAs). A great majority
of the electrons are provided by the GaAs layers of the short
period SLS, leading to the lowering of the effective bulk bar
rier height <l>b (marked "b" region). The larger the conduc
tion-band discontinuity (and therefore the barrier height)
between the contact layer and the channel layer, the larger
the number of extra carriers contributed by the SLS, and the
larger the degree to which barrier lowering can be achieved.
(2) Although electron transfer leaves the SLS region
(marked "a") partially depleted of carriers, the current con
duction is provided mostly by quantum mechanical tunnel~
iug, and to some extent conventional conduction. Solving
the Schrodinger's and Poisson's equations self-consistently,
the overall decrease in the specific contact resistance was
calculated to be one to two orders of magnitude in the InAs/
GaAs heterojunction when the short SLS is used. Further
more, the use of short-period superlattice may alter the na
ture of dislocations formed in a favorable manner for current
conduction< All these factors, we believe, collectively lead to
the achievement of these extraordinarily low nonalloyed oh
mic contact resistances.
To illustrate the extent of perpendicular current con
duction through the SLS, a structure similar to that reported
in Ref. 1 was grown, but with a five-period 15 AilS A InAs/
InGaAs superiattice, Si doped to 2X 101~ cm-3• Due to the
aforementioned arguments having to do with the unique ad
vantages of short-period SLS's, nonalloyed specific resis
tances afabout 1.5 X 10-8 n cm2 have been obtained, which
is almost one-half of that reported in Ref. L
In summary, strained-layer superlattices (SLS's) used
as part of the contact layer have led to extremely small non
alloyed contact resistances, less than 8.5 X 1O-~ n cmz on
n+ -GaAs and 1,5 X 108 n cm2 on InGaAs. The critical
role of crowding resistance r:< was investigated. Calculations
show that lowering of bulk barrier hieght by this SLS struc
ture is responsible for the small contact resistances obtained.
Since the doping concentrations used are standard and the
contact layers are thin, application of the present SLS struc
ture on real devices is easy and straightforward.
The authors would like to express their appreciation to
Sandie Norwood for preparing the manuscript. Helpful dis
cussions with J. Chen and D. Mui are also acknowledged.
This work is supported by the Air Force Office of Scientific
Research and NASA Lewis Research Center,
'e. K. Peng,J. Chen,J. Chyi,andH. Morko~J. App!. Phys. 64, 429 (1988).
"T. J< Drummond, H. Morko~, and A. Y. Cho, J. Cryst. Growth 55, 449
(1982).
3H. H. Berger, Technical Digest oflEEE, International Solid-State Confer
ence, Philadelphia, I' A, 19-21 FebrU!lfY 1969, p. 162.
1H. H. Berger, Solid-State Electron, 15,145 (1972).
'e. Y. Ting and e. Y. Chen, Solid-State Electron. 14,433 (1971)<
"e. A. Mead and W. G. Spitzer, Phys. Rev. A 134, 173 (1964).
Pang eta/. 901
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1.341393.pdf | Doping effects of 3D metal on singlephase YBa2Cu3O7−δ
Z. H. He, Z. Y. Chen, J. S. Xia, G. Q. Pan, Y. T. Qian, and Q. R. Zhang
Citation: Journal of Applied Physics 64, 3589 (1988); doi: 10.1063/1.341393
View online: http://dx.doi.org/10.1063/1.341393
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/7?ver=pdfcov
Published by the AIP Publishing
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128.189.205.30 On: Wed, 10 Dec 2014 20:35:59Doping effects of 3D metal on single-phase YBa2Cu307_S
Z.H. He
Department 0/ Physics, University a/Science and Technology a/China, Hefei, AnhUl; People's Repuhlic of
China
Z. Y. Chen
Department of Applied Chemistry, University of Science and Technology of China, He/ei, Anhui, People's
Republic o/China
J. S. Xia
Department 0/ Physics, University a/Science and Technology a/ China, He/et, Anhui, People's Republic 0/
China
G, Q, Pan and Y. T. Qian
Department 0/ Applied Chemistry, University of Science and Technology of China, He/ei. Anhui. People's
Republic 0/ China
O. R. Zhang
Center a/Condensed Matter and Radiation Physics CCAST (World Laboratory) Beijing, People's Republic
a/China and Department 0/ Physics, University 0/ Science and Technology a/China, Hefei, Anhui, People's
Republic 0/ China
(Received 22 January 1988; accepted for pUblication 23 May 1988)
The measurements of x-ray diffraction, the temperature dependence of the de resistance and
the ac susceptibility have been performed for the single-phase 3D-metal doping systems YBa1
CU3 xM" Oy (M = Fe, Co, and Ni; x = 0,025, 0.05, 0.075, 0.10, 0.25, and 0.50 for Ni and Co
and 0.05, 0.075, 0.10, 0.15, and 0.20 for Fe). With an increase of impurity content, two
structural transitions were observed for the Co and Fe dopants but only one for the Ni dopant.
The resistivity in the normal state changes from metaHic to semiconductinglike behavior and
the depression of Tc is linear with the impurity concentration (x) when x < 0.10. A weak
Curie-Weiss type paramagnetism, which is enhanced with impurity content, exists in the
samples studied. Incorporating other work on oxygen defects, we suggest that a change of
oxygen content induced by doping was the dominant effect on superconductivity in these
samples.
I. fNTRODUCTION
Soon after high Tv superconductivity (above 90 K) was
observed in single-phase YBa2Cuj07 _ I), the crystalline
structure was determined by both x-ray diffraction and neu
tron diffraction experiments. The band structure calculation
based on this structurel and the anisotropic character of sin
gle-crystal YBa2Cu307 __ Ii (Ref. 2) indicates that, in this dis
torted layer oxide, the Cu-O layer and/or Cu-O chain give
the dominant contribution to superconductivity. Further
more, each element in YBa2Cu307 -0.5 has been substituted
respectively to investigate which site of the elements is the
most important to the high Te. A large number of experi
ments have revealed that high Tc superconductivity would
not exist w:ithout the Cu-O layer or Cu-O chain. This conclu
sion can also be applied to the superconducting oxides (Tc
== 40 K) with K2 NiF 4 structure. Therefore, studies of the
effect of element substitution on the superconducting struc
ture will be very helpful in understanding the superconduct~
ing mechanism. In order to retain the original single phase,
doping is a feasible way for partial substitution. Some groups
have reported their initial work on 3D-metal doping, which
includes a discussion of the oxidation state,3 disorder in
duced by doping and two possible substitution sites for Cu
(Ref. 4) and magnetic depairing.5•6 Tarascon et aU pro
posed four interrelated factors-structural disorder, oxygen vacancy, different oxidation state in the copper introduced
by dopants. and magnetic pair breaking-to interpret the
influence of doping on superconductivity. To date, a clear
explanation has not resulted. We suggest that systematic ex
periments should be carried out before one can describe how
various factors affect the superconductivity. In this paper,
we present the effects on the superconducting transition
temperature, the reSIstivity, the susceptibility, and the lattice
parameters produced by doping single-phase YBa2Cu307 -I)
with the Fe, Co, and Ni. Incorporating other work on oxy
gen deficiency, we suggest that the change of oxygen content
induced by doping has the main effect on superconductivity
in the samples studied.
3589 J_ Appl. Phys. 64 (7}, 1 October 1988 II. EXPERIMENT
The samples were prepared by the solid-state reaction
technique. Mixtures of stoichiometric proportions of high
purity BaC03, Y 203' and CuO and Fe2 03, CO2 03, and Niz
(OH)2C03' for Fe, Co, and Ni doping, respectively, were
ground and heated at 930 "C in air for 24 h. They were then
reground and pressed into pellets, and sintered at 930°C in
flowing oxygen for 24 h. The samples were slowly cooled
down to 400 °C and maintained at this temperature within
furnace. During the cooling process, the samples were kept
in the flowing oxygen.
0021·8979/68 I i 93589-04$02.40 @ 1988 A.merican Institute of Physics 3589
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128.189.205.30 On: Wed, 10 Dec 2014 20:35:59The standard de four-probe technique was employed to
measure resistances. Voltages were read from a 181 N ANO
VOLTMETER made by Keithley Instruments Inc, The
temperature of the samples was detected with a calibrated
Pt-resistance thermometer, The ac susceptibilities were
measured with a mutual inductance bridge with a sensitivity
better than 10--1 f.lH. A Cu-constantan thermocouple was
used to determine the sample temperature.
iii. RESULTS
The powder x-ray diffraction patterns of the
YBa2CuJ_xCOXOy series (x = 0.025, 0,05, 0,075, 0.10,
0.25, and 0.50), shown in Fig. I, indicate that the samples are
single phase 1-2-3 compounds, As can be seen from the pat
terns, with the increase of Co content, the 1-2-3 phase under
goes two structural transitions: first, atx::::;0.05, from ortho
rhombic phase I (3a '" c) to orthorhombic phase II (3a = c)
indicated by the overlap of the (013) and (110) peaks, and
second, from orthorhombic phase to tetragonal phase ac
companied by the overlap of the (123) and (213) peaks. The
peak intensity of the (123) becomes weak and that of the
(213) becomes strong, giving a inversion of peak intensity
between these two peaks. The x-ray diffraction patterns for
YBa2Cu3 __ xFexOy (x = 0.05,0.075,0.10,0.15, and 0.20)
are quite similar to those for YBa2 CU3 _" Cox Oy. But from
the diffraction patterns for YBa2 Cu3_ xNixOy (x = 0.025,
0.05,0.075,0.10,0.25, and 0.50), the transition from ortho-
(010) (013) (110) x~O.025
(OO5) (020)
15 25 35 45 5~ 65
2Q
FIG.!. The powder x-ray diffraction patterns ofYBa,Cu, _xCOxOy series
(x = 0.025, 0.05, 0.Q75, 0.10, 0.25, and 0.50).
3590 J. AppL Phys., VoL 64, No.7, i October 1968 TABLE 1. The lattice parameters and superconducting transition tempera
ture.
M x a b c (A.) b/a b -a(J, .. ) V(A3) l~{K)
Co 0.00 3.813 3.882 11.656 L018 0.069 172.51 91.9
O.oz5 3.824 3.889 11.678 l.017 0.065 173.67 90.3
0.05 3.833 3.872 11.651 1.010 0.039 172.92 84.0
0.075 3.853 3.882 11.667 1.008 0.029 174.51 77.0
0.10 3.850 3.870 11.665 1.005 0.020 174.70 70.0
0,25 3.853 3.867 1l.619 1.004 0.014 173.12 39.9
0.50 3.870 3.870 11.624 1.000 0.000 174.10
Fe 0.05 3.822 3.886 11.650 L017 0.064 173.03 84.8
0.075 3.841 3.887 11.651 1.012 0.046 173.95 78.2
0.10 3.850 3.867 11.639 1.004 0.017 173.28 71.5
O.IS 3.861 3.880 11.643 1.005 0.019 174.42 56.0
0.20 3.859 3.874 11.636 1.004 0.Ql5 173.96 -50.0
Nt 0.Q25 3.815 3.882 11.654 l.018 0.067 172.59 90.8
0.05 3.804 3.871 11.629 1.018 0.067 171.24 88.9
0.Q75 3.807 3.878 11.641 1.019 0.071 171.86 87.0
0.10 3.813 3.879 lL646 1.017 0.066 172.25 85,5
0.25 3.808 3.888 11.648 1.021 0.080 172.45 69.0
0.50 3.818 3.880 11.633 1.016 0.062 172.33 60.5
rhombic to tetragonal phase is not found; only the transition
from orthorhombic I to orthorhombic II phase is observed at
x:::;0.50. The analysis of structure modification caused by
doping will be given in detail elsewhere. The lattice param
eters as a function of impurity compositions are shown in
Table 1. For the samples contained Co or Fe, a increases and
c decreases as x increases, while b remains almost constant.
That is, as the impurity content increases, the structure
tends to become tetragonal one. For the samples contained
Ni, the lattice parameters change little.
The temperature dependence of the resistance (R) for
YBa2 CU3 __ x Cox Oy is shown in Fig. 2. For the samples with
x < 0.10, the resistance in the normal state exhibits metallic
behavior. For the sample with x = 0.25, in which transition
from orthorhombic to tetragonal phase occurs, a metallic to
semiconductorlike transition appears as the temperature de
creases. The rapid drop of R in the superconducting transi
tion region means that the samples are single phase. For the
sample in the tetragonai structure, the behavior of R ( T)
~ ..
"' ~
'" ...
~ .. ;: ...
" .! ..
0
0 100 ISO
T(I!:) 200 250 300
FI G. 2. The temperature dependence ofresistance for YBa2 CU3 x Cox 0 y'
.: 0,025; 0: 0.05; A: 0.075; .: 0.10; V: 0.25; +: 0.50.
He etal. 3590
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128.189.205.30 On: Wed, 10 Dec 2014 20:35:59'" .:;
:S ...
'" ~
'" .~
'" " ~
"
0
() 50 100 150 200 250 300
T(K)
FIG. 3. The temperature dependence of resistance for YEa, Cu) _ x NixOy.
e: 0.025; 0: 0.05; A: 0.075; II: 0.10; "':0.25; +: 0.50.
exhibits a typical semiconductorlike character. The behav
ior of the resistance for YBa2 CU3 _ xFex Oy (x = 0.05,0.075,
and 0.10) is very similar to that for YBazCu3 _",CoxOy'
On the whole, the R(T) curves for YB~CU3_xNixOy
are similar to those for YBazCu3_XCOXOy, and are in ac
cord with the results reported by Adrian and Nielsens How
ever, the effect on the midpoint temperature of supercon
ducting transition (Tc) of Ni doping is weaker than that of
Co doping, even for x = 0.50, the sample doped with Ni is
still superconducting up to 60 K. Compared with those for
the Co and Fe dopants, the R (n curves for YBa2 Cu) _. x Nix
Oy have smaller superconducting transition widths and are
similar to that of single-phase YBaZCu307 _ 05 (see Fig. 3). It
seems easier to substitute eu by Ni than by Co or Fe. The
effect on Tc with each doping element is shown in Fig. 4. The
linear dependence of Tc on x, in the region of x < 0.10, can be
easily seen. But the straight lines for Co and Fe dopants,
shown by the broken line, do not pass the point which is
corresponding to the undoped sample. The values of Tc are
also given in Table L
0.0 0.05 0.10 0.15 0.20 0.250.50
FIG, 4. The variation of T, with each doping element, A, III, and. repre
sents Fe, Co, and Ni doping, respectively, The "error bars" show the super
oonducting transition regions from the onset temperature to the zero resis
tance temperature,
3591 J. Appl. Phys., Vol. 64, No.7, i October 1968 o
-2
A"
• • ;--4 f.
5- • •
ill • .. "I i. • • ~
-6
60eo , • • -3 $I; ~ a.1t /t"
50 100 200
TUO
FIG. 5. The ac susceptibilitie§ for YBa,Cu3_.xCoxOy' X: 0.025; e: 0.05;
A: 0.015; 0: 0.10; v: 0.25.
The ac susceptibilities for YBaz eU3 _ x Co", Oy are given
in Fig. 5. A weak Curie-Weiss-type paramagnetism, which
increases with Co content, can easily be seen.
IV. DISCUSSION
In general, a small amount of magnetic impurity, such
as a 3D metal, in a superconducting metal or aHoy matrix
will depress superconductivity strongly, due to the pair
breaking effect of the sod exchange interaction. 3 The stronger
the magnetic moment of the impurity is, the greater the pair
breaking effect will be. The band structure calculations for
YBa2 CU3 0, (Ref. 1) show that the metallic bands are asso
ciated with the Cu-O layers and Cu-O chains, but the Y
layers are insulating. As a result, the substitution of Cu by a
3D metal would be expected to result in magnetic depairing,
just as in conventional superconductors. Transmission elec
tron microscope (TEM) (Ref A ) and Mossbauer9 experi
ments have shown that Ni and Fe atoms enter the 1-2-3
phase, respectively. (Which sites the Ni or Fe atoms have
occupied are not given in those papers.) The x-ray diffrac
tion and neutron diffraction have also revealed that Cu
atoms can be substituted by Co atoms. 10 Incorporating our
diffraction results on the modification of lattice parameters
with impurity content, we conclude that the Cu sites have
been occupied by Fe, Co, and Ni atoms, respectively.
The linear relation between Tc and x in the low impurity
concentration region seems to be in qualitative agreement
with Abrikosov-Gorkov's theory (see Fig. 4), and the influ
ence of Fe or Co doping is more significant than that of Ni
doping. However, the character of both R (1) and Tc (x)
suggest that the effects of Co or Fe doping on superconduc~
tivity and electronic transport in the normal state are quite
similar. Thus, it is difficult to explain this phenomenon
based only on the magnetic pair breaking effect since the
moment of Fe is larger than that of Co. 6 For the same reason,
the phenomenon is also hard to explain, based on the rigid
band model involving a shifted Fermi level due to doping.
Another aspect of this problem is which eu site the im
purity would replace, since two possible Cu site are avail
able, i.e., that on Cu-O layer (Cu2+ A.) and that on Cu-O
He etal. 3591
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
128.189.205.30 On: Wed, 10 Dec 2014 20:35:59chain (Cu2 + ), In solid-state oxides, the stable valence ofNi
is usually 2 +, while that of Fe is 3 +; Co is generally 3 +
although Co 2 + is occasionally found. So, Ni probably re
places Cu on the Cu-O chains, while Fe or Co replaces Cu in
the Cu-O layers, Presuming that the Cu-O layer is more im
portant to high Tc superconductivity than the Cu-O chain,
suggests an answer to the questions of why the effect of Fe or
Co doping is stronger than that of Ni doping and why the
similarity of the effect of Fe and Co doping appears. Unfor
tunately, the experimental results do not seem to support
this point of view, since, at least two Fe configurations are
seen in M6ssbauer spectra, which can be assigned to the sub
stitution on the two Cu sites,9 and the Cu sites on Cu-O
chains can be occupied by Co as indicated by neutron dif
fraction. 10 We cannot exclude the possibility that Fe or Co
would partially occupy the Y sites, reducing the effective
impurity concentration on the eu sites which are sensitive to
superconductivity, [Note thatthe straight lines of Tc (x) for
the Co and Fe dopants in Fig. 4 are above the point of Tc (0),
as has been mentioned in Sec. III]. To clarify the problems
associated with preferential substitution, further experi
ments that allow a direct observation are necessary,
An explanation combining the valence state of the im
purity and the oxygen content seems to be more suitable. As
the stable valence state of Fe or Co is higher than that of Cu,
when Cu is substituted by Co or Fe (even in part), an equi
librium must be established by charge compensation. This
can be realized in two ways: first, lower the valence state of
the other elements; second, increase the content of oxygen,
The observed modification of the lattice parameters with x
favors the latter explanation. We have succeeded in prepar
ing single-phase YBa2 CU3 Oy with excess oxygen
(7 < x < 8). With the increase of oxygen content in these
samples, the transition in structure from orthorhombic I to
orthorhombic II phase is found and the lattice parameter c
becomes shorter while (J and b change little. The resistivity
measurement shows that Tc reduces as y increases. Details of
this work will be reported in Ref. 11, Apparently, the effect
of doping is in consistent with that of excess oxygen content.
Note there is a similarity between the variations in X ( T),
R(T), and Tc with oxygen deficiency parameter {j in the
formula YBa2Cu307 _ /j and those with impurity concentra
tion x in the formula YBa2 CU3 _ xMxOy. Besides, they all
undergo the structural transition from orthorhombic to te
tragonal phase. The difference is that c tends to increase with
increasing 8, but to decrease with increasing x.
With regard to the oxygen deficiency, the decrease of Tc
and the transition from orthorhombic to tetragonal phase in
YBaz CU3 Oy (y < 6.9) are closely related to the disorder in
troduced by oxygen vacancies.12 Incorporating the effect in
the case y> 7, we conclude that, the change of the lattice
3592 J. Appl. Phys., Vol. 64, No.7, 1 October 1988 parameter c is related to the oxygen vacancies between the
Ba-O layers. The bonding of oxygen makes the interaction
between the two Ba-O layers increase and, thus, c shorter,
The more oxygen in this site which is in the layer containing
Cu-O chain, the stronger the interaction and the shorter the
c. It is not surprising that superconductivity is found to be
hardly affected by the length of c. But the relative change
between a and b affects Tc strongly, it directly reflects the
ordering in the "conductive tunnel."
Because doping itself can be regarded as an introduction
of disorder and because the oxygen content changes with
doping simultaneously, the experimental phenomena may
be attributed to the effect of disorder, The inversion of peak
intensity of (123) and (213), as mentioned in Sec. III, im
plies the rearrangement of atoms and the introduction of
disordeL Further experiments are being carried out.
If this explanation is confirmed, a question remains as to
why the effect of a magnetic impurity is so weak in these high
Tc superconducting compound matrices including
Laz __ xSrxCu04, even though the impurities have occupied
sites in the "conductive tunnel." The clarification of this
problem would be helpful to understand the superconduct·
iug mechanism in these compounds.
ACKNOWLEDGMENT
This work was supported by the Natural Science Foun
dation of China.
'So Massidda, Jaejun Yu. A. J. Freeman, and D. D. Koelling, Phys. Lett.
A122, 198 (1987).
2S. W. Tozer, A, W. Kleinsasser, T. Penney, D. Kaiser, and F. Holtzberg,
Phys. Rev. LeU. 59,1768 (1987).
3J. Thiel, S, N. Song, J. B. Ketterson, and K. R. Peoppelmeier, in ACS
Symposium Series No. 35], Chemi~try of High-Temperature Superconduc
tors, edited by D. L. Nelson, M. S. Whittingham, and T. F. George (1987),
Chap. 17, p, 173.
·Y .Maeno, J. Nojima, Y. Aoki, M. Kato, K HashinG, A. Minami, and T.
Fujita, Jpn. J. App), Phys. 26, PPL774 (1987).
-'H. Adrian and S, Nielsen (private communication).
6Gang Xiao, F. H. Streitz, A. Gavrin, Y. W. Du, and C. L. Chien, Phys.
Rev. B 35.8782 (1987).
7J. M. Tarascon, L. H. Greene, P. Barboux, W. R. McKinnon, G. W. Hull,
T, P. Orlando, K. A. Delin, S. Foner, and E. J. McNiff, Jr., Phys. Rev, B
36,8393 (1987).
"G. Boato, G. Gal!inaro, and C. Rizzoto, Phys. Rev. 148, 353 (1966).
"c. W. Kimball, 1. L. Matykiewicz, J. Giapintzakis, A. E. Dwight, M, B.
Brodsky, M. Slaski, B. D. Dunlap. and F. Y. Fradin, Physica 148B, 309
(1987).
lOT, Kajitani, K. Kusaba, Y. Masana, and M. Hirabayashi (in Japanese)
(unpublished) .
"D. Yu, R Zhang, and S. Liu (private communication).
12E. Takayama-Muromachi, Y. Uchida, M. Ishii, T. Tanaka, and K. Kato,
Jpn. J. Appl. Phys. 26, PPLll56 (1987).
He etal. 3592
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128.189.205.30 On: Wed, 10 Dec 2014 20:35:59 |
1.576298.pdf | I n s i t u deposition monitoring for solar film production by roll coating
Stephen F. Meyer
Citation: Journal of Vacuum Science & Technology A 7, 1432 (1989); doi: 10.1116/1.576298
View online: http://dx.doi.org/10.1116/1.576298
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
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.193.242.165 On: Mon, 01 Dec 2014 19:55:45In situ deposition monitoring for sol~r film production by roll coating
Stephen F. Meyer
Southwall Technologies. Palo Alto, California 94303
(Received 12 December 1988; accepted 16January 1989)
Real-time measurement of critical coating properties is a necessity for corpmercial success in
large-scale roll coating. Both sheet resistance and optical measurements are used in modem
sputter roll coating plants. This presentation will introduce sputter roll coating and discuss in situ
film monitoring in that context. Some of the difficulties in interpreting the measurements and in
relating them to product specifications will be addressed.
I. INTRODUCTION
Process conditions in a vacuum deposition system are noto
riously time dependent. The background gases continue to
be reduced by pumping, source geometries evolve, and inter
nal structures may change temperature without air to con
duct heat. Many such changes have one or more correspond
ing process variables which can compensate for the changes;
e.g., source power or deposition time can compensate for
rate changes, reactive gas flows can offset background water
in a reactive process, and hardware can be heated or cooled.
In short, apparently different process conditions or recipes
can produce equivalent coatings. This paper reports on the
method used at South wall Technologies for ensuring consis
tent coating quality in a large commercial sputter roll coater
by using in situ optical monitoring of the coated product.
The examples will be from the manufacture of Heat Mir
ror™, a three-layer dielectric-silver-dielectric optical fil
ter, used for making efficient insulating windows. Heat Mir
ror provides a transparent window which reflects the IR to
retard heat transport across the window. It may be tailored
for both high solar gain or solar shading. The IR reflectivity
is provided by the silver layer, hence silver quality is a crucial
concern in manufacturing. The visible transmission through
the silver is substantially enhanced by the antireflecting ef
fect of the two dielectric layers. The optical thickness of the
dielectrics control the color of the transmission (and reflec
tion).
II. COATING AND MONITORING EQUIPMENT
A schematic of the essential internal parts of a sputter roll
coater is shown in Fig. 1. A supply roll of plastic film (called
a web, usually polyester) is loaded onto the payout roll,
threaded around a chilled drum past the sources, through
the monitors, and onto the take-up roll. ConceptuaUy this is
much like a large, wide tape recorder. From three to six or
more sources are typically arrayed around the drum, which
is cooled to prevent overheating the web. Speeds range from
less than one to many tens of m/min, with runs lasting for
many hours. Background gas pressure is constantly decreas
ing over this time as the supply roll is used up .
. Southwal1 has designed and built an optical monitor
(OM) specifically adapted for the manufacture oflarge rolls
of Heat Mirror on SO-,um-thick polyester. The monitor em
ploys a chopped quartz halogen light source, fiber optics to
carry the light into the vacuum system and back to the ana-lyzer. The web optics use lenses to spread the beam out to
l.S-cm diameter to avoid local effects. The analyzer section
consists of a wheel with a maximum of 32 filters rotating at 1
Hz. Filters were chosen because high wavelength resolution
is not needed for Heat Mirror, and because they eliminate
substr~te related interference fringes in the near IR. A two
color detecter (Si/PbS) feeds its output through a synchro
nous demodulator to an analog/digital (A/D) converter
and a computer. The current version of the monitor reported
here supports three channels across the web in transmission
only, and has only 18 filters installed. The monitor under
current development will also support three more channels
for reflectivity at the same points across the web.
The computer software converts the scan data in real time
into quantities directly comparable to product specifica
tions. The visible spectrum is converted to tristimulus coeffi
cients X, Y, and Z by the 1931 eIE ~ethod of weighted
ordinates at lQ-nm intervals using illuminant C. The tristi
mulus coefficients are then transformed into visible trans
mission (Tvis) and dominant wavelength (DmWl). The
dominant wavelength is used as a measure of color. Solar
CHILLED DRUM
FIG. I. Schematic diagram of the major internal components of a sputter
roll coater showing the payout roll, take·up roll, process monitor, and up to
seven sputter sources around the cold process drum.
1432 J. Vac. Sci. Technol. A 7 (3), May/Jun 1989 0734-2101/89/031432-04$01.00 @ 1989 American Vacuum Society 14
.... ·······················r
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.193.242.165 On: Mon, 01 Dec 2014 19:55:451433 Stephen F. Meyer: In situ deposition monitoring for solar film 1433
transmission eTsol) is calculated via the ASTM E42b meth
od of weighted ordinates at 50-nm intervals. These quanti
ties are measured on the finished product in quality control
(QC) with a computerized Perkin-Elmer Lambda 9 spec
trophotometer. The processed data are saved at intervals to a
disk file for later engineering review.
The computer can display data to the operator in several
modes. The first mode is a graph of spectral data for all
channels. The second is a table of the current processed data
(Tvis, Tsol, DmWv) compared to the product specifica
tions. The third is a graph of downweb processed data. Final
ly, a full optical model can determine stack optical param
eters by a least-squares fit to the spectral data. The model is
included to analyze discrepancies between desired and actu
al data.
The optical analysis model is based on the characteristic
matrix methodology of MacLeod. I The optical properties of
the materials are provided by a Drude-Lorentz dielectric
function2.:1 with adjustable parameters for each material (sil
ver, dielectric, substrate). The optical stack is represented by
semi-infinite, homogeneous layers with' discrete thicknesses.
The difference between the actual spectra and the spectra
calculated from trial layer thicknesses and material param
eters is minimized by a multidimensional gradient search
routine. The application of this model to spectrophotometer
data has been reported elsewhere.4 The application of this
model in situ can be a powerful diagnostic tool for machine
malfunctions.
The material properties are described by a complex, fre
quency-dependent dielectric function E( w). For simple ma
terials transparent in the visible and near infrared, the dielec
tric function contains wavelength-dependent terms
representing effects from above or below the region of inter
est, and a constant term to account for all other effects. This
is equivalent to saying that the solid contains only two or
three populations of electrons. Each wavelength-dependent
term in t:( w) is a Lorentzian function that describes a popu
lation of electrons interacting strongly with light near a reso
nant frequency outside the calculation region. Frequencies
in the UV are usually associated with a band gap, while those
in the IR are rotational or vibrational resonances:
t:( w) = "a (near UV electrons), (1 )
wi-oi -iyw
where w is the angular frequency of the light. WI is the reso
nant frequency or band-gap energy, a is proportional to the
population size, i is the square root of -1, and y is the relaxa
tion rate or linewidth.
Conduction electrons can be represented by the Drude
approximation which is the special case of Eq. (1) with a
resonant frequency of zero. The population amplitude is
usually expressed as a plasma frequency OJ; = ne2/m:
E((v) = -ne2
/m (conduction electrons), (2)
(JJ2 + irw
where n is the conduction-electron volume density.
A metal will also have UV interband transitions and a
constant term leading to the full expression for the dielectric
function of a metal:
J. Vac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989 a E(W) = Eo + -2--ry--.
(iJ I --ur -'rlu ne2/m
w2 + iW/7 ' (3)
where the relaxation rate of the population has been convert
ed to a reciprocal scattering time 1/7 (scattering frequency).
The traditional expression for the dc conductivity is recov
ered from the dielectric function in the limit ofzero frequen
cy:
(4)
The scattering time is the mean interval between collisions
of conduction electrons with crystallographic defects. This
time is inversely proportional to the density of defects in the
metal, including interstitials, vacancies, impurities, defects,
grain boundaries, etc. These defect sites can also create elec
tron traps which may reduce the electron density. The two
effects can be separated from the optical data, but not from
conductivity measurements. Disorder also provides addi
tional final states available for electronic transitions, thus
increasing the absorption of UV and blue photons. The fun
damental problem of producing good optical quality silver is
controlling these effects.
m. OPTICAL MONITOR RESULTS
Figure 2 is a graphics display from the center channel of
an entire roll of Heat Mirror 88. The three horizontal graphs
show Tvis, Tsol, and dominant wavelength versus downweb
meters. The finely dotted line on each trace is the average for
the entire screen. The heavily dotted line is the nominal QC
specification for the product. Clearly this is a very smooth
run. However, the operator was aiming for values other than
the nominal QC specification. In fact the yield from the roll
was 95%, and in the middle of the QC specifications. The
discrepancy between the OM data shown in Fig. 2 and the
QC data measured on samples taken following completion of
the run indicates that the product has changed. The change
is from room-temperature annealing of the silver film which
increases the conduction-electron density and decreases the
scattering time,
Figure 3 displays data from a roll of Heat Mirror 44 show
ing the same quantities (Tvis, Tsol, and dominant wave-
.89
. 86 -•• --. ---••• ---•• '-" •• --••• --.----•••• -••• --.-------.---•• ---•••• -•• -.-••••••••• -
--
.83
.7 .0'.=:." = ..... """"""""'=,.,... .... _:':: __ ::::_.""._-:::_.=. __ .r.'._:::_.:=:_.'='!._""._.!;:'._"" .. ~ •. :::: __ = .. ~---~-.~.-~-.... ==t
.57
565.
525. -.. ~-.. ---.---... -------.. --..... -.-.--.-----.. ---.. --.--...... --.. _--_ .. _--
500. Tvis
Tsol
DmWv
o. 200. 400 . GOO . 1100 . tooo. 1200.
0"""""0 METEAS
FIG. 2. Graphics display from the center OM channel of a run of HM-88.
The three Ilraphs are visible transmission, solar transmission, and dominant
wavelength shown vs downweb meters.
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.193.242.165 On: Mon, 01 Dec 2014 19:55:451434 Stephen F. Meyer: In situ deposition monitoring for solar film 1434
.49
Tv1 •
. 46~~~~~~~~~~~~~~~~~~~~
.43
550.
505.
O. 200. 400. 600. 800. 1000. 1200. 1400.
Downweb I4ETERS
FIG. 3. Graphics display of a run of HM-44. The increased "noise" region
between 685 and 800 m is a drum oscillation.
length) on the center channel. Two differences are immedi
atelyapparent: (i) the operator is having considerably more
difficulty maintaining a uniform run and (ii) something
happened at 685 m to increase the noise level for 100 m. The
abrupt change at 380 m in all three graphs is an operator line
speed change (which changes all layer thicknesses). The
abrupt change in Tvis and Tsol at 640 m with no color
change is a silver power change. The region from 660 to 720
m is expanded in Fig. 4 to show that the "noise" is in fact a
web drive oscillation with a period of exactly one revolution
of the chilled drum. The drum is slowing down, momentar
ily producing a locally thicker coating from all cathodes.
Then, the drive system overcomes the sticking and momen
tarily speeds up, thinning out the coating. As indicated ear
lier, the transmission is controlled by the silver thickness
which to first order causes no color change. Hence the trans
mission peaks and valleys correspond to thinner and thicker
silver only. The color changes due to changing dielectric
thicknesses are out of phase with the transmission because
the dielectric sources are rotated around the drum from the
silver. This form of graphical data display is a powerful hard
ware diagnostic tool.
A collection of five different samples of Heat Mirror were
analyzed in QC and then measured in the OM for calibra
tion. These data points are shown as squares in Fig. 5 which
Tv15
.43
Teol
.22
DmWv
505.
660. 670. 680. 690. 700. no. 720.
Ooomweb I4ETERS
FIG. 4. Expansion ofthe oscillation region of the HM-44 run shown in Fig.
3.
J. Vac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989 .9
.8
.7
~ .6
" !:! .5 .,
~
.4
.3
.2 .2 .3 .04 .5 .6 .7 .8 .9
Qt l4ea .. ur .... nt
FIG. 5. In situ OM vs QC measurements of visible and solar transmissions
for HM-88, HM·66, and HM-44 (diamond points). The square points are
calibration points from aged samples. Equal measurements will be on the
diagonal line.
displays both Tvis and Tsol. The diagonal line corresponds
to perfect agreement between the QC and OM measure
ments. The calibration points are all on or above the line
indicating that the OM systematically reads higher than QC.
This bias arises from the sparse nature of the data from 18
filters: it is impossible to cover the wavelength ranges at the
required intervals for the true algorithms. The data are inter
polated to generate the approximate algorithms.
The remaining clustered diamond-shaped points in Fig. 5
are comparisons between QC data and the ill situ OM data
for Heat Mirror 44,66, and 88. The scatter in the compari
son points is due to uncertainty in locating the exact areas
measured by QC within the run time data files and to vari
ation in process conditions which affect the annealing behav
ior of the silver film. The systematic difference between in
situ OM data and calibration OM data aged material is most
apparent for the high transmission end of the range, HM-88.
Figure 6 shows the HM-88 visible transmissiqn broken out
separately to exaggerate the effect. Notice that all of the in
situ HM-88 Tvis data is below the diagonal, whereas the
calibration square point is well above the diagonal.
Figure 7 shows the in situ dominant wavelength discrep
ancy for HM-88. Four calibration square points are shown,
all of which are on or above the diagonal line. All but one of
.89
.88
.87
z .86 0
" !:! .85 '"
~ :. '. .
.84
.83
.82 .82 .83 .84 .85 .81i .87 .88 .89
OC MfI •• ureMf't
FIG. 6. In situ OM vs QC measurements of visible transmission for HM-88
shown as diamond points. The square point is the calibration.
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.193.242.165 On: Mon, 01 Dec 2014 19:55:451435 Stephen F. Meyer: In situ deposition monitoring for solar film 1435
511t).
570.
560.
5150.
~ a"o.
" 530. /,// ...
en 520.
::. . " 510. . .
500. . .
450. . .
480.~--------------------------------------J ·4BO. 490. 000. 5iO. 520. 530. 540. !i50. !ISO. 570. 5S0.
GC Ml98i1Urtilfient
FIG. 7. In situ OM vs QC measurements of dominant wavelength for HM-
88 shown as diamond points. The four square points arc calibration.
the data points are on or below the diagonal. in a similar
manner to the visible transmission data. Figures 6 and 7 are
the most dramatic evidence for the time-dependent optical
properties of the silver layer in Heat Mirror. (No similar
changes are seen in the optical properties of the dielectrics
without the silver layer.) Diagnosis of the actual cause of the
change required application of the optical model to both in
situ OM and QC data.
As previously indicated, disorder in the silver layer can
have several effects on the optics: increased blue absorption
from interband transitions, increased free-carrier absorption
throughout the spectrum (117), and lower reflectivity from
a reduced free-carrier density (w!). Figure 8 shows a com
parison of the transmission spectra of a typical sample of
HM-88 in situ versus after room-temperature annealing as it
appears to the OM. The optical and electronic properties
inferred from the optical spectra are summarized in Table I.
It is an artifact of the solar spectrum that the solar trans mis-
i.
.9
. S .... .-.. ~.~D~ ...... ~ ..............
,7
c: .6
~ .. .5 !l
" .. .4 c: ..
L .3 ...
. 2
,\
o.~--~-- __ ------------------------------~ 350. !ISO. 750. 950. 1150. 1350. 1550. 1750. 1950,2100.
lIa.elength in N.no ... t ......
FIG. 8. The transmission spectra ofHM-88 shown in situ (dotted) and after
room-temperature annealing (solid). The difference is due to 30% fewer
conduction electrons from trapping and triple the electron scattering fre
quency from defects.
J. Vac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989 TABLE 1. Comparison ofHM-88 properties inferred from in situ (OM) data
and from aged samples (QC),
Quantity In situ Aged Units
Visible transmission 83 87 %
Visible absorption 11 7 %
Solar transmission 70 70 %
Solar absorption 16 9 %
Dominant wavelength 510 550 nrn
Sheet resistance 48 12 U/sq
Far IR reflectivity 65 88 %
w; = ne21m I.4X 10'2 1.8 X 1032 S-2
U= nf?rlm 2.5 X 10" 9.6X 10' W-cm) I
Scattering Frequency 1/ r 5X 1016 1.7xlO!6 S-l
Electron Density 4.4 X 1022 5,8x 1022 crn-]
sion does not change with annealing even though the solar
absorption changes by 7%. The substantial changes in visi
ble transmission and in dominant wavelength are in good
agreement with Figs. 6 and 7. In essence, the annealing pro
duces a 30% increase in free-carrier density and a factor of 3
decrease in scattering.
IV. SUMMARY
The application of an optical monitor to commercial sput
ter roll coating has been presented. The emphasis in the de
velopment of SouthwaU's optical monitor has been on the
advanced analysis of optical data as well as the traditional
"hold the process constant" and data logging applications.
An example of diagnosing a mechanical web drive malfunc
tion from the optics was presented. An analysis of the discre
pancies between in situ measurements and post deposition
measurements shows that instrument calibration is not the
issue. The optical properties of silver evolve rapidly as the
quenched-in disorder relaxes, Substantial changes in con
duction-electron density and scattering frequency directly
result from this relaxation. The conclusion from these obser
vations is that in situ measurements may change with rea
sonable predictability in the final product.
ACKNOWLEDGMENTS
The author wishes to thank Curt Peterson and Gale Allen
for the electrical engineering and Steve Pace for the optical
engineering of the optical monitor described here .
IH. A. Macleod, Thin-Film Optical Filters (Elsevier, New York, 1969).
2C. Kittel, Introduction to Solid State Physics (Wiley, New York, 1968).
3M, V. Klein, Optics (Wiley, New York, 1970).
's. F. Meyer, in Proceedings of the 31s1 Annual Technical Conference, So
ciety of Vacuum Coaters, 1988, pp. 113-132.
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1.100635.pdf | Annealing studies of YBa2Cu3O7−x thin films
S. I. Shah
Citation: Applied Physics Letters 53, 612 (1988); doi: 10.1063/1.100635
View online: http://dx.doi.org/10.1063/1.100635
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130.113.86.233 On: Mon, 22 Dec 2014 17:15:02AnneaUng studies of YBa2Cu307 -x thin fUms
s. i. Shah
Central Research & Development Department, Experimental Station, E. L du Pont de Nemours &
Company, Wilmington, Delaware 19898
(Received 12 April 1988; accepted for publication 20 June 1988)
In situ electrical resistance measurements, differential thermal analysis, and x-ray diffraction
studies were carried out between room temperature and 950°C on as-grown amorphous
insulating YBa2Cu307_ x thin films. Results for the phase transformation reaction path are
reported in order to optimize the post-deposition annealing process. Amorphous-to-crystalline
transformations were observed at 550°C along with a reversible orthorhombic-tetragonal
transition near 670 "c. Eutectic melting above 850°C was also noted, which restricts the
maximum annealing temperature to around 850 0c.
Thin superconducting YBa2CuJ07 ... x films have been
grown by various techniques including ion beam sputtering, I
rf diode sputtering,2 magnetron sputtering/ electron beam
evaporation,4 molecular beam epitaxy,S metalorganic depo
sition/' and laser evaporation.7 All these techniques require
either a post-deposition annealing of the films or growth at
elevated substrate temperatureH•9 to obtain superconducting
films. No standard post-deposition annealing procedure has
so far been developed despite the fact that the annealing con
ditions are crucial in obtaining optimum superconducting
properties.
In order to optimize post-deposition annealing condi
tions, it is necessary to understand the thermodynamics of
the structural and chemical changes that occur during an
nealing. These processes have been studied in great detail in
bulk YBa2Cu,07 _ x' There is an adequate understanding of
the effect of oxygen concentration on the superconducting
properties ofYBa2Cu307 _ x' 10.11 and structural transitions,
i.e., from tetragonal to orthorhombic and vice versa, have
also been studied extensively. 12.13 However, very little work
has been done on the annealing studies of thin films. David
son et al.14 have measured the resistance of thin-film samples
with different heating rates and oxygen partial pressures in
order to control the annealing process. They concluded that
a quick heating is preferable. In this letter we present differ
ential thermal analysis (DT A) along with high-temperature
in situ resistance measurements and x-ray diffraction
(XRD) analysis of thin YBa2Cu307 _ x films in order to
understand the thermodynamics of the annealing process
and to establish an optimum annealing sequence.
All the films were grown by magnetron sputtering from
a single stoichiometric YBa2Cu307 _ x target. Target fabri
cation has been described elsewhere.3 Typically, films were
grown at room temperature in a reactive sputtering atmo
sphere of Ar + 10% O2 with a target substrate separation of
4 cm. Power density on the target was kept low, 5-10 W /
cml, in order to increase the gas density at the surface of the
target. IS This helps decrease the negative ion resputtering of
the growing films. In our experiments, with a proper combi
nation of gas pressure, power density, and target-substrate
separation, we have almost completely eliminated the ener
getic particle bombardment effect3 and were able to consis
tently reproduce films of homogeneous composition and
uniform thickness. As-grown films had a room-temperature resistance of 20 MO and were amorphous, as confirmed by
XRD and transmission electron microscope analyses. For
DT A analysis, 3-5 J.lm films were grown on MgO substrates
and scraped off to obtain 20-30 mg samples. A Du Pont 1900
DT A cell was used with A1203 as reference sample. Thermo
grams were taken during both heating and cooling, and a
heating rate of 20 ·C/min was used. Cooling was uncon
trolled, causing peak shifts due to undercooling; thus only
heating thermograms are reported here. All the analyses
were done in air. Resistance measurements were carried out
on films grown on ( 100) MgO using four Pt probes. Contacts
were made with silver paste. This configuration withstood
repeated temperature cycles with maximum temperature of
more than 1000 ·C. XRD analysis was carried out in a Ri
gaku diffractometer modified for automated high-tempera
ture in situ diffraction analysis in flowing 02. A Cu tube
operated at 40 kV and 20 rnA was used as the x-ray source.
Figure 1 is a plot of resistance versus temperature for a
I-pm-thick as-grown YBa2Cu307 -x film on (lOO) MgO.
Both heating and cooling were carried out in flowing oxy
gen. Upon warming the resistance dropped continuously. A
precipitous drop at 200 °C was foHowed by a small change in
the slope at 500 °C and finally another big change near
700 "C. The resistance dropped from 900 to 90 n between
700 and 850°C. Some samples which were left at 850 °C
showed a slow increase in resistance, and all the samples that
were heated beyond 900 °C exhibited very poor post-anneal
ing superconducting properties. A short-time anneal at
YBo2 CU3 °Hi
on (iOO)MgO
FIG. L Film resistallce as a function of measurement temperature for a
YBa2Cu307_ A film 011 (lOO)MgO substrate.
612 Appi. Phys. Lett. 53 (7), 15 August 1988 0003-6951/88/330612-03$01.00 @ 1988 American Institute of Physics 612
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130.113.86.233 On: Mon, 22 Dec 2014 17:15:02850·C gave the best Tc. As the sample was cooled from
850 ·C, the resistance continuously decreased to a room
temperature resistance af20 n. Upon reheating the sample,
the change in resistance simply followed the cooling path
and never showed any of the features observed during the
first heating cycle. The initial drop in the resistance is due to
the oxidation during which the oxygen concentration COI1-
tinuously increases from a value of 6.3 at room temperature,
as measured by Rutherford backscattering (RBS).16 As the
sample is heated above 200 ·C, oxygen outdiffusion of the
sample occurs, causing a decrease in the rate of resistance
drop. Amorphous-to-crystaHine transition occurs at 500 ·C,
but this is also the temperature at which most of the oxygen
is lost from the film. As a result, the resistance drop due to
the amorphous-crystalline transition is not very sharp.
Figure 2 shows several XRD patterns at different an
nealing temperatures. The first crystalline peaks start to
show around 500 ce. Although, as a result ofthermal asym~
metry and peak shifts, it is difficult to detemline the struc~
ture of the phase which crystallizes first above 500 °C, films
annealed at 550°C and quenched showed semiconducting
behavior down to liquid He temperatures, indicating that the
majority of the crystallized phase was tetragonal along with
some orthorhombic phase. The coexistence of orthorhombic
and tetragonal phases was also observed in differential ther
mal analysis, which is discussed later. The semiconducting
tetragonal phase forms at this low temperature as a result of
low oxygen concentration. Equilibrium phase diagrams cal
culated by Wille et al. 17 and Khachaturyan et al. 18 show the
tetragonal phase to be stable above 230 "C when oxygen con
centration is between 6.2 and 6.4, which is what we expect
the oxygen concentration to be in our sample around 550°C,
Upon heating above 550 °C, grain growth continued up to
850 ce, indicated by a continuous increase in the peak inten
sities. Above 850 °e non-YBaZCu307 x peaks start to ap
pear, suggesting precipitation of other phases. Films an~
nealed at 850°C and slowly cooled in oxygen show a metallic
behavior going through a superconducting transition at 92
K with a complete resistance loss at 87 K.
2D.OO 24.40 28.80 3~.20 31.50 42.00
Flu. 2. X-ray diffraction pattern ofa YBa2Cu,07_., film on (lOO)MgO
substrate at (a) room temperature, (b) 500 "C, (c) 550"C, (d) 750 'C, (e)
850 'C. and (f) 9OO"c'
613 Appl, Phys. Lett., Vol. 53, No.7. 15 August 1988 The phase transformation reaction path during the an
nealing of the amorphous films was also studied through
differential thermal analysis (DTA). Figure 3 shows ther
mograms obtained during the heating of an as-grown amor
phous film and the reheating of the same film. Two exother~
mic and one endothermic peaks were observed in the first
heating cycle. The first exotherm was at 500 °e, correspond
ing to the temperature of annealing at which crystallization
was observed in the XRD patterns and the temperature at
which a change in I1R / tJ. T was also noted. We can, there
fore, conclude that this is the amorphous-to-crystalline
transformation temperature. The enthalpy of transforma
tion calculated from the integrated area under the peak, and
using Zn melting peak as a standard, was 48 kJ/g mol. This
was an irreversible transformation, as no peak was observed
at this temperature during cooling or reheating of the sam
ple. The second exotherm was at 672 "C. This is the tempera
ture for the orthorhombic~to-tetragonal transformation as
reported by several other authors. 12.13 Although this transi~
don was not seen during the cooling cycle, the second heat
thermogram does show the reappearance of this peak. The
structural transition was, therefore, reversible, but because
of the small enthalpy of transition, a high cooling rate, and a
small sample size, it was not observed during cooling, At
temperatures above 900 'C, a big endothermic peak was ob~
served, The shape of the peak, especially during cooling and
second heating, is very typical of a eutectic melting, A eutec
tic isotherm in the BaO + CuO and YBa2Cu307 _ x pseudo
binary system has been suggested by Keefer et al. ! 9 at 890 °e,
which agrees wen with our results. The occurrence of these
eutectic peaks suggests that the film composition might be
slightly off stoichiometry. Visual observation of the DTA
sample also showed partial melting of the sample. Upon
heating the sample through several heating cycles and to
higher temperatures, several endothermic peaks appeared,
signaling the decomposition of the sample.
In summary, the superconducting properties of
YBa2Ct1307 x are strongly dependent on the annealing
method. Initially, amorphous and insulating, as-grown films
go through two structural transitions upon heating, first an
amorphous-to-crystalline transition at around 500°C, fol-
E ·c
::l
i!' I
~ ~.£ E :e i~
::!. i~ <II ., it <::
~ U ~ -c ! .~
~
f~ .:::
!'! :.g
OJ
r~ c.
E
'" 10-2nd He~!
300 4no 500 son 700 SOO 900 1000
Temperature (C)
FIG. 3. DTA therrnograms of YBalCu.\07 x film obtained during two
successive neatifl.gs of as-grown YBaOCu,07 _ , film.
S.1. Shah 613
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130.113.86.233 On: Mon, 22 Dec 2014 17:15:02lowed by a tetragonal-to-orthorhombic transition at 670°C
The resistance of the sample drops abruptly as the sample is
warmed. This is due to the increase in the oxygen content at
low temperatures, but at temperatures above 300 DC, the rate
of resistance drop decreases as O2 is evolved instead of being
absorbed by the film. Further decrease in the resistance is
due to the crystallization of the sample at about 500 T and
grain growth beyond this temperature. Above 850°C the
sample resistance slowly increases as partial eutectic melting
is observed, indicated by eutectic peaks in the thermogram
and apearance of non-YBu2CU307_ x peaks in the XRD pat
tern. Cooling in oxygen does not change the resistance of the
crystallized film except for a small overall decrease in the
resistance due to the metallic nature of the sample. It is,
therefore, conduded that a post-deposition annealing
should be done above the orthorhombic-tetragonal phase
transformation temperature but below the eutectic iso
therm. Best results were obtained when films were annealed
in flowing O2 at 850°C followed by cooling in O2 to room
temperature.
The author wishes to acknowledge the technical assis
tance of Brian D. Jones, Allan D. Meinhaldt, and Glover A.
Jones. The author also wishes to acknowledge M. Subra
manian for the preparation of the target.
IS. L Shah and P. F. Carcia, MateI'. Lett. 6, 49 (1987).
1S. H. Liou. M. Hong, B. A. Davidson, R. C. Farrow. J. Kwo, T. C. Hsieh,
R, M. Fleming, H. S. Chen, L. C. Feldman, A. R. Kortan, and R. J. Felder,
614 Appl. Phys. Lett., Vol. 53, No.7, 15 August 11388 Am. Inst. Phys. Proc. 165.79 (1987).
'So I. Shah and P. F. Carcia, Appl. Phys. Lett. 51,2146 (1987).
4R. H. Hammond, M. Naito, B. Oh, M. Hahn, P. Rosenthal, A. Marshall,
N. Missert. M. R. Beasley, A. Kapitulnik, and T. H Geballe, in Extended
Abstracts for MRS Symposium on High Temperature Superconductors,
Anaheim, CA, 23-24 April 1987 (unpublished).
'J. Kwo, T. C. Hsieh, M. Hong, R. M. Fleming, S. H. Liou, B. A. Davidson,
and L. C. Feldman, MRS Symp. Proe. 99, 339 (1987).
"A. H. Hamdi, 1. V. Mantese, A. L. Micheli, R. C. O. Laugal, D. F. Dun
gan, Z. H. Zhang, and K. R. Padmanabhan, Appl. Phys. Lett. 51, 2152
( 1987).
7D. Dijkkamp, T. Vekatesan, X. D. Wu, S. A. Shaheen, N. Jisrawi, Y. H.
Min-Lee, w. L. McLean, and M. Croft, Appl. Phys. Lett. 51, 619 (1987).
RH. Adachi, K. Hirochi, K. Setsune, M. Kitabatake, and K. Wasa, Appl.
Phys. Lett. 51, 2263 (1987).
9D. K. Lathrop, S. E. Russek, and R. A. Buhrman, App!. Phys. Lett. 51,
1554 (1987).
"'I. M. Trascon, W. R. McKinnon, L. H. Greene, G. W. Hull, and E. M.
Vogel, Phys. Rev. B 36, 226 (1987).
"R, Kanno, Y. Takeda, M. Hasegawa, O. Yamamoto, M. Takano, Y.
Ikeda, and Y. Bando, Mater. Res. Bull. 22, 1525 (1987).
12M. O. Eatough, D. S. Ginley, B. Morosin, and E. L Venturni, Appl. Phys.
Lett. 51, 367 (1987).
Ill. K. Schuller, D. G. Hinks, M. A. Beno, D. W. Capone, L. Soderholm, J.
P. Locquct, Y. Bruynseraede, C. U. Segre, and K. Zhang, Solid State
Commun. 63, 385 (1987).
I4A. Davidson, A. Palevski, M. J. Brady, R. B. Laibowitz, M. Scheuer
mann, and C. C. Chi, Appl. Phys. Lett. 52,157 (1988).
15S. M. Rosnagel, J. Vae. Sci. TechnoL A 6,19 (1988).
16S. I. Shah, C. R. Fincher, M. W. Duch, D. A. Beames, K. M. Unruh, and
C. P. Swan, in Proceedings of the 15th International Conference on Me
tallurgical Coatings, San Diego, CA, 1988 (unpublished).
17L. T. Wille, A. Herera, and D. de Fontaine, Phys Rev. Lett. 60, 1065
( 1988).
ISA. G. Khachaturyan, S. V. Semenovskaya, and J. W. Morris, Jr., Phys.
Rev. B 37, 2243 (198B).
19K. Keefer (private communication).
S.1. Shah 614
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130.113.86.233 On: Mon, 22 Dec 2014 17:15:02 |
1.101467.pdf | Increased T c of bismuth strontium calcium copper oxide superconductor by
praseodymium substitution
S. Geller and K.Y. Wu
Citation: Applied Physics Letters 54, 669 (1989); doi: 10.1063/1.101467
View online: http://dx.doi.org/10.1063/1.101467
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/7?ver=pdfcov
Published by the AIP Publishing
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Fluorineimplanted bismuth oxide superconductors
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Amorphoustocrystalline transformations in bismuthoxidebased high T c superconductors
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Preparation of superconducting thin films of bismuth strontium calcium copper oxides by reactive sputtering
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130.216.129.208 On: Fri, 12 Dec 2014 04:03:01Increased Tc of bismuth strontium calcium copper oXide superconductor by
praseodymium substitution
s. Geller and K.-Y. Wu
Department afElectrical and Computer Engineering. University a/Colorado, Boulder,
Colorado 80309-0425
(Received 7 November 1988; accepted for publication 15 December 1988)
It is shown that a smaIl substitution of praseodymium for bismuth as in
Pro<os BiL,)5 Sr2CaCu2.2 08<2 -1 (j produces a significant increase, 13 K, in the onset temperature, 7
K in the temperature at which the whole specimen is superconducting, and 10 K in the
transition-midpoint temperature relative to B12Sr 2CaCu2 208<2 -t !j' Unfortunately the transition
is broadened, 20 K versus 14 K. Doubling the amount of praseodymium substitution causes
the occurrence of extraneous phase ( s), the effect of which is seen only in the resistance versus
temperature data, and does not indicate any additional significant increases in the
aforementioned temperatures.
The main purpose of this letter is to report an increase in
the transition temperature of the bismuth-strontium-cal
dum-capper-oxide (BSCCO) superconductor resulting
from a small substitution of praseodymium for bismuth,
Ever since the first report] of the BSCCO superconductors,
we have thought that the bismuth should be contributing
carriers, a result of previous work on other systems.2 The
transition temperature of the YBa2Cu307 b system does ap
pear to be a function of the CuH -ion content and the carrier
concentration from the copper must, it seems, be much low
er in the Bi compound than in YBa2Cu30,,<9' The only possi
ble source of additional carriers are the bismuth ions. How
ever, the crystal-structure analyses reported thus fBr'A do
not appear to permit the accurate determination of the Bi-O
distances, from which one might speculate on the valence of
the bismuth. It seemed appropriate to substitute some pra
seodymium for bismuth, because Pr has two stable valence
states, 3 + and 4-+ . If the Pr went into the compound in
both valence states, it might increase the transition tempera
ture; if it chose to go in as Pr3 -+, it would tend to reduce Tc.
From the behavior ofBfl+ in the garnets,S its size is between
that ofNdH and Prl+.
We made several specimens in which 0.1 Bi
was replaced by praseodymium; our starting oxide
was Pr"OII' Specimens made were in the system
Pro<] BiL9Sr2CaCu2 ,-"Os H j_,)' with several different val
ues of x. We noticed that the firing temperatures could be
raised substantially over those that we were constrained to
use for preparing the materials not containing Pr. The prep
aration technique was similar to that described elsewhere, I>
The first two 2 h firings were done at 840 and 880°C, respec
tively, and the 50 h firings were done at 900°C. In some
cases, the specimens were given an additional 50 h firing at
900"C. We found that although powder photographs of
some of the specimens indicated that the specimens were
single phase the relative resistance CRt) versus temperature
measurements did not, For example (Fig" I), the data for
the x = 0.2 specimen show four straight-line segments.
However, they do show a significant increase in the onset
temperature; it could be as high as 98 K. Although the sam
ple became completely superconducting at 75 K, potentially
it could be (Fig. 1) 83 K. There is no question that the smaH amount of praseodymium substitution had a large effect on
the transition.
Because the R, vs T data indicated that the material was
not single phase (even though this was not discernible in the
powder photograph), we reduced the Pr substitution to 0.05.
In a sample of total starting weight of 1.0493 g, the required
amount of dry Pr60] i was 8.5 mg. The firing temperatures
were somewhat lower for these specimens than for those
containing 0.1 Pro The three 50 h firings were done at 860,
860, and 880°C, respectively. The R, vs T data for
Pro 05 BiL95 Sr2CaCU220g2 f-ij are shown in Fig. 2. (The 2.2
eu per formula unit is a result of the work on the Pr-free
materials described in another paper.l» There is still a short
intermediate straight-line segment which indicates that this
material is still not completely single phase, although again
not discernible in the x-ray photographs. However, the data
are much improved over those of the 0.1 Pr specimen. In this
case, the onset temperature, taken as the intersection of the
two outer straight-line segments, is 99 K and the tempera
ture at which the specimen is completely superconducting is
79 K. This is an improvement on the specimen with 0,1 Pr,
but unfortunately, the transition width is increased relative
to the specimen not containing any IlL Nevertheless, the
:: ~ Pr 0.1 Bij <SSr2CaCuZ.20S.2 +8 ,
(I) j u co 0.8 i-t) ...-
Ul
'iii
<ll 0,6 a::
Q)
1 > I :;:;
0 0.4 OJ a:: t
0.2
0.0 0 100 200 300
Temperature (K)
FIG< I < Relative resistance vs temperature,
669 Appl. Phys, Lett 54 (7). i 3 February 1989 0003-6951/89/070669-02$01 <00 @ 1989 American Institute of Physics 669
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130.216.129.208 On: Fri, 12 Dec 2014 04:03:01:: ~
0.8 ~
0.6 t'
OA
0,2 ,
;00 200 300
Temperature (K)
FIG. 2. Relative resistance vs temperature.
very small substitution of Pr for Bi has unequivocally in
creased both temperatures, especially that of onset, signifi
cantly. The transition midpoints are 79 K for the 2.2 eu
specimen6 not containing Pr and 89 K for the analogous
specimen containing 0.05 Pr.
The results of the Pr substitution are difficult to under-
670 Appl. Phys. Lett., Vol. 54, No.7, 13 February 1989 stand, even based on a simple model of potential supercon
ducting carrier concentration. It is not easy to argue that
somehow the small substitution of Pr altered significantly
the concentration of Cu3+ ions. On the other hand, the con
clusion that the result implies that the bismuth ions are also
essential (not only because they give the right structure) to
the supefconducting behavior of these compounds may be
plausible.
"Quench-enhancement" of the transition temperature
of a bismuth compound has been reported by Ishida et a/.7
We wish to thank the Graduate School of the University
of Colorado for support of this research.
'ft Maeda, Y. Tanaka, M. Fukutomi, and T . .'\5ano, lpn. J. App!. Phys. 27,
209 (1988).
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Science 239, 1015 (1988).
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"S. Geller and K.-Y. Wu (unpublished).
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s. Geller and K. -Yo Wu 670
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1.341375.pdf | Doping dependence of the specific contact resistance of NiSi2 on (100) nSi
E. Sasse and U. König
Citation: J. Appl. Phys. 64, 3748 (1988); doi: 10.1063/1.341375
View online: http://dx.doi.org/10.1063/1.341375
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v64/i7
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Downloaded 24 Jul 2013 to 128.171.57.189. 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_permissionstical waveguide, and an uneven fiber space array have been
developed. They have been assembled into one body to form
a three channel demultiplexer. The flexible replica grating
was used to realize a concave grating, and it performed well
as a demultiplexer. The uneven fiber space array enabled the
input light to be demultiplexed into the three specific wave
length channels.
The authors are thankful to Dr. T. Shimomura, S. Hashizume, and Y. Shimada for their useful discussions and
technical advice.
JR. Kobrinski, R. M. Bulley, M. S. Goodman, M. P. Vecchi, and C. A.
B!"ackett, Electron. Lett. 23, 824 (1987).
2B. D. Metcalf and J. F. Providakes, Appl. Opt. 21, 794 (1982).
3y' Fujii and J. Minowa, Appl. Opt. 22, 974 (1983).
4E. G. Loewen, M. Neviere, and D. Mayster, App!. Opt. 16,2711 (1977).
5T. [zawa and H. Nakagome. App!. Phys. Lett. 11, 584 (1972).
Doping dependence of the specific contact resistance of NiSi2 on (100) n .. Si
E. Sasse and U. Konig
AEG Research Center Uim, Sedanstrasse 10, D-7900 Ulm, Federal Republic o/Germany
(Received 15 February 1988; accepted for publication 17 June 1988)
Nickel disilicide (NiSi2) was formed on (100) oriented n~type Si~molecular beam epitaxial
layers (Si-MBE) of various doping levels between 2 X 1016 and 13 X lOt8 cm -3 and on
substrates of 2 X 10!9 em -3. Very low contact resistances were found and a low Schottky
barrier of ¢an = 0.49 V was derived. A comparison with other commonly used contact
materials shows NiSi2 to be highly favorable in this doping range.
Silicides have been intensively investigated for their po~
tential as interconnect materials for very large scale integra
tion structures owing to their relatively low resistivity com
pared to the commonly used poly-Si. Another region of
application, which has hardly been explored, is the fabrica
tion of ohmic contacts to semiconductors. Usually the con
tact material is of secondary importance as long as it is che
mically stable at elevated temperatures, because good
contacts can always be obtained ifthe silicon surface is high
ly enough doped (> 1020 cm-:l) by implantation or other
wise.! If for certain reasons (e.g., an undesirably high
amount of defects at high doping concentrations, which are
detrimental for Impatt diodes2) such excessive doping con
centrations are undesirable, a slight leverage can be obtained
by using a contact metal having a very low Schottky barrier
tPBn' In the case ofImpatt diodes the Ti:Au couple is an often
employed contact since the Schottky barrier ofTi lies at O.S
V. Unfortunately the Ti:Au couple is not very stable to tem
peratures above 300·C owing to interdiffusion and subse
quent reaction.3-7
Tung and Gibson8 established for NiSi2 on (100) n-Si a
Schottky barrier height of 0.48 V making it highly probable
that specific contact resistances resulting therefrom would
be lower than for Ti. Furthermore NiSi2 just as CoS12 has
the potential of growing epitaxially on silicon. In this work
we have prepared NiSi2 contacts to (100) n-Si of various
doping levels and have for the first time determined the dop
ing dependent specific contact resistances.
The specific contact resistances have been evaluated by
two methods, For n-doped epitaxial layers the transmission
line model (TLM)9 was employed, whereas for substrate
material the method of Terry and Wilson in the adaptation by KuphallO was used. The samples used for the first method
were prepared by depositing Si-layers by molecular beam
epitaxy (MBE)!i doped with antimony on (100) p-Si of
10000 em. The MBE layers had thicknesses varying from
0.6 to 0.8 j.lm, whereas we had five different doping levels
varying between 2X 1O!0 and 1.3X 1018 em --3. For studying
a higher doped level a n~type Si substrate of 2 X }OI9 em -3
was used.
No (cm-3!
• 1020 10111 1011 2)(1016
l'O'r I
I Rcmcm2)
10-2 j I
I I I 10-41
J/ I
I , , , , ,
10 20 30 4-0 50 60 70
ViNe 110-10 cm3/2) -
FIG. 1. Specific contact resistance of NiSi, on (100) n-Si in dependence of
the doping concentration ND (top abscissa) or 1I,JN;; (bottom abscissa).
3748 J. Appl. Phys. 64 (7). j October i988 0021-8979/88/193748-02$02.40 © 1988 American Institute of Physics 3748
Downloaded 24 Jul 2013 to 128.171.57.189. 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_permissionsTABLE l. Comparison of the specific contact resistances in n cm2 of PtSi,
Ai, Ti, and NiSi2 to n-Si of varying concentration.
Contact material
Nl) cm -3 Type~
2Xl010
2X 10"
2X lOIS
2X 1019
"Derived from Ref. 16.
bDerived from Ref. 17.
cDerived from Ref. 18. PtSi-Si AI-Si
••• Ii< •• ,b
(111) (111)
1.5 X 10'
7X 10'
L6X10' 1.4XlO-1
4x 10-3 7x 10-5
dDerived from the present work. Ti-Si NiSiz-Si
, .. c •.• d
(-t-) ( 1(0)<
3XlO-1
l.5Xl0-1
7X 10-J lXlO-3
1.5X to-5 4XIO--6
eChanging from (111) to (lOO)-Si reduces the specific contact resistance by
a factor of2 (Ref. 19).
After depositing 100 nm of pyrox, windows for the long
TLM mesas were opened which were prepared by anisotrop
icaHy etching the epilayers with a mixture of KOH/pro
panol-2 at 80°C, After a second pyrex deposition contact
windows 100 J.lm wide and having spacings of 5, 15, 30, and
60 p..m were opened onto the TLM mesa and 40 nm of Ni
were deposited bye-beam evaporation (base pressure
5 X 10-7 mbar). The Ni silicide was prepared by rapid ther
mal annealing using an AET heating system. The annealing
temperature was slightly above 800 "C and the annealing du
ration 15 s in accordance with the work of ChevaHier and
Nylandsted Larsen12 and Nylandsted Larsen, ChevaUier,
and Sorensen.13 The ambient was an oil-pump vacuum
(;:::1O-3mbar).
We found a certain amount of epitaxially grown NiSi1
on (100) Si indicated by the X min value of about 0.5 as ob
tained by RES channeling measurements. This correlates
with the findings of Chiu et Ol.14 and FoIl et al.15 that the
disilicide is highly (111) faceted and that it contains disor
dered domains. The specific contact resistances are depicted
in Fig. 1 dependent of the doping concentration (top) as wen
as of 1I.,JN;; (bottom). When comparing these values with
those for PtSi-Si and the commonly used pure metals AI-Si
and Ti-Si in Table I we find a marked improvement, which is
consistent with the lower Schottky barrier height of the
NiSi2 -Si contact.
Using the consideration that at lower doping levels the
specific resistance can be expressed byl
Rc = k IqA *Texp(q¢Bn1kT),
A1\< =2.1 A,
3749 J. App\. Phys., Vol. 64, No.7, 1 October 1988
~.'.' .• '" '~".'.--;o.'.'.'.-.'.-., ••.• .' .•.•. ' .•. ' •••.• -.:.~.~ •. , .• -., ..................... _, and
A = 120 A cm-2 K-2,
we obtain ¢JBn ;:::;:0.49 V when using Rc = 0.2 n cm2• This
result is in good agreement with the Schottky barrier height
obtained for smooth and completely epitaxial NiSi2 by Tung
and Gibson.s The reason this result is obtained, although the
NiSi2 layer consists only partially of epitaxially grown lay
ers, is to be sought in the understanding that beside the epi
taxial (100) regions there are regions of (111) facets20
which because of their higher Schottky barriers (q;Bn: 0.65-
0.79 V) 8 are only responsi.ble for a minor part of the current
transport across the interface.2J A part of the interface cer
tainly consists of polycrystaIline material which also has a
high Schottky barrier. AU in all the main region through
which the current can flow is the (100) region, but with a
lower effective contact surface.
The authors would like to thank H. Kibbel, G. Kohn for
technical assistance and Dr. E. Kasper for stimulating dis
cussions. The financial support provided by the Federal
Ministry of Technology, FRO under the grant NT 2731 Dis
gratefully acknowledged.
's. M. Sze, Physics of Semiconductor Devices, 2nd ed. (Wiley, New York,
1981), p. 304.
21._P. Luy, private communication.
3J. Hersener, E. Sasse, and A. Wilhelm, unpublished.
4A. Hiraki, Jpn. J. AppI. Phys. 22, 549 (1983).
SR. Goronkiu, Solid-State Electron. 18, 891 (1975).
oJ. M. Poate, P. A. Turner, W.l. DeBronte,andl. Yahalom,J. Appl. Phys.
46,4275 (1975).
7p. Staecker, International Electron Devices Meeting, Washington, DC,
Dec. 1973 (The Institute of Electrical and Electronics Engineers, New
York, 1973), p. 493.
"R. T. Tung and J. M. Gibson. J. Vac. Sci. TechnoL A 3,987 (l985).
'G. K. Reeves and H. B. Harrison, IEEE Electron Device Lett. EDL-3, 111
( 1982).
WK. Kuphal, Solid-State Electron. 24, 69 (1981).
"H. Jorke, H.·]. Herzog, and H. Kibbe!, AppL Phys. Lett. 47, 511 (1985).
!2J. Chevallier and A. Nylandsted Larsen, Appl. Phys. A 39,141 (1986).
13 A. Nylandsted Larsen, J. Chevallier, and G. Sorensen, Mater. Res. Soc.
Symp. Proc. 23, 727 (1984).
14K. C. R. Chill, J. M. Poate, J. E. Rowe, T. T. Sheng, and A. O. Cullis,
App!. Phys. Lett. 38, 988 (1981).
!SF. Fol!, P. S. Ho, and K. N. Tu, J. App!. Phys. 52, 250 (1981).
l0e. Y. Chang and S. M. Sze, Solid-State Electron. 13,727 (I970).
17A. Y. e. Yu, Solid-State Electron. 13,239 (1970).
18L, E. Terry and R. W. Wilson, Proc. IEEE 57, 1580 (1969).
19C. Y. Chang, Y. K. Fang, and S. M. Sze, Solid-State Electron. 14, 541
(1971).
20J. M. Gibson, R. T. Tung, and J. M. Poate, Mater. Res. Soc. Symp. Pmc.
14,395 (1983).
211. Ohdomari and K. N. Tu, J. App!. Phys. 51, 3735 (1980).
E. Sasse and U. KOnig 3749
Downloaded 24 Jul 2013 to 128.171.57.189. 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 |
1.2811178.pdf | White House Global Climate Plan Calls for Research by 7 Agencies
Irwin Goodwin
Citation: Physics Today 42, 10, 52 (1989); doi: 10.1063/1.2811178
View online: http://dx.doi.org/10.1063/1.2811178
View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/10
Published by the American Institute of Physicsic fusion, which the House had
marked up earlier for $280 million—a
savage cut from the Bush Administra-
tion's request of $349 million. In the
amended bill, the Princeton lab,
which had suffered a $40 million
reduction in the earlier version,
would get back half of its loss. (In the
Senate bill, which passed in late July,
the mark for magnetic fusion was
$330.4 million and in the joint House-
Senate version the program wound up
with somewhat more, $330.8 mil-
lion—though the exact amount for
the Princeton Lab is not yet known).
The "you support mine and I'll sup-
port yours" trade-off gained Roe's
support for the SSC. Accordin g to
several lawmakers, Roe was able to
swing many undecided members on
the science committee and the public
works committee, which he once led.
What's more, they add, Roe's impas-
sione d speech at the climax of the SSC
debate contributed to the large vote
for the project. When Massachusetts
Republican Mario Conte, his voice
rising and his arms flailing, resorted
to some vintage Texas-bashing about
greedy state legislators demanding
money for the SSC and other projects,
Roe became indignant. "Do not pit
one section of the country against the
other," he told his colleagues. "If I
could have this [SSC] built in New
Jersey ... I would be fighting as hard
as the people in Texas are."
It wasn't only Texas politicians bat-
tling for the SSC. The day before the
House vote, Obey got a phone callfrom a physicist at the University of
Wisconsin urging him not to submit
his amendment to strike construction
funds. "The SSC is seen as so much
pork that will be divvied up every-
where," said Obey. "So many peopl e
think they have a piece of the action."
Spreading the money around
Indeed, about three-fourths of the
roughly $205 million appropriated for
SSC R&D in the past five years went
to three national laboratories—Law-
rence Berkeley, Brookhaven and Fer-
milab. The labs, in turn, have spread
the money to university researchers
and commercial contractors around
the country. DOE, for its part, has
awarded direct grants in 18 states.
Some lawmakers thought they
would be free of all outside pressures
once the site around Waxahachie,
Texas, was chosen for the machine. In
the past year, for instance, Don Rit-
ter, a Pennsylvania Republican who
had been an outspoken opponent of
the SSC for years, has lowered his
voice. That's not surprising, consider-
ing that Westinghouse Electric and
Air Products & Chemicals, both head-
quartered in his state, are competing
for contracts to build the $6 billion
machine. On the Senate side, J.
Bennett Johnston, chairman of the
energy and natural resources commit-
tee and powerful on budget and ap-
propriations committees, had been
cool to building the SSC in an era of
large deficits. But he became one of
its ardent proponents when GeneralDynamics let him know it would build
a plant in Hammond, Louisiana, if it
was selected to manufacture the su-
perconducting magnets that will hold
the beams in their oval course. Bab-
cock & Wilcox, another company that
wants to build SSC magnets and other
components, also is located in John-
ston's state. Johnston, in fact, effec-
tively led the Senate campaign for the
SSC. He met little resistance because
Texas's own Lloyd Bentsen and Phil
Gramm had already signed up more
than 60 senators.
In conference, Johnston, working
with Republican senators Pete Do-
menici of New Mexico, Mark Hatfield
of Oregon and Thad Cochran of Mis-
sissippi, got House members to agree
to add two key points to the SSC
section of the Energ y and Water
Development Appropriations Act:
One calls for $25 million in construc-
tion funds "to be available only to
initiate the first tunnel sector con-
tract and for no other purpose." The
other argues that while foreign parti-
cipation in the project could signifi-
cantly reduce its cost to the US, it is
likely that such contributions would
require sharing in its technological
development. Congress wants DOE to
report on the advantages and disad-
vantages of foreign partnerships be-
fore any agreement is made. The
agreement says, "Using this report,
Congress can then make a decision on
how much and what type of foreign
participation is appropriate."
—IEWIN GOODWIN
WHITE HOUSE GLOBAL CLIMATE PLAN
CALLS FOR RESEARCH DY 7 AGENCIES
With the abundance of scientific re-
ports that humans are altering the
basic chemistry of the Earth's atmo-
sphere, leaders of the most industrial-
ized nations are latching on to a hot
topic. Britain's Margaret Thatcher
and the Soviet Union's Mikhail Gor-
bachev speak forcefully on environ-
mental issues, though cynics argue
that their eloquence is shaped by
public opinions and actual events like
Chernobyl and summer droughts, not
by personal principles. In his cam-
paign for the US presidency last year,
George Bush promised to clean up
America and become the "environ-
mental President."
In his budget manifesto, "Building
a Better America," he declared he is
"committed to developing a better
understanding of the processes that
influence global climate." As he saw
it, "present understanding of complexEarth system processes is rudimen-
tary and substantial research will be
necessary before we can begin to
make reliable predictions of global
climate change." Considering the un-
certainty, the President is loath to
promise to limit or lower the levels of
atmospheric gases—notably, CO2,
S3O, CH4, N2O and chlorofluorocar -
bons such as CFC13 and CF2C12—that
seem to trap some of the sun's radi-
ation like the glass in a greenhouse.
'White House effect'
"The problem ... is international in
scope," Bush is quoted in "Building a
Better America" as saying. "Unila-
teral action by the US alone will not
solve it. In fact, some say the problem
is just too big to be solved.... I say
they are wrong. Those who think
we're powerless to do anything about
the greenhouse effect are forgettingabout the 'White House effect.' As
President, I intend to do something
about it." In fact, Bush's budget,
submitted last February along with
"Building a Better America," includ-
ed $191.5 million for a US Global
Change Research Program—a 43%
increase over fiscal 1989 research
activities, which amounted to $133.9
million spread through seven agen-
cies—amon g them, the National
Science Foundation, Environmental
Protection Agency, the Energy and
Agriculture departments and NASA.
Little more was heard about Bush's
global climate change program until
D. Allan Bromley was asked about it
in July by Senator Albert Gore Jr, the
Tennessee Democrat. At the Senate
science subcommittee's hearing on
Bromley's confirmation as the new
director of the White House Office of
Science and Technology Policy, Gore
52 PHYSICS TODAY OCTOBER 1989WASHINGTON REPORTS
said the time has come to find out just
what is the White House effect. Not
only did Gore want to see the report
by the Committee on Earth Sciences
of the interagency Federal Coordinat-
ing Council on Science, Engineering
and Technology, but he wanted to
know what Bromley would do to
reduce the threat of global warming.
Bromley's answer was ambiguous.
"There's every reason not to wait," he
said, to pursue certain actions, such
as preserving tropical rain forests,
practicing energy conservation and
planting more trees. But, he added ,
he had yet to see compelling scientific
evidence arguing for reducing or
eliminating most of the gases that
have been identified so far as culprits
in climate change.
The committee report from
FCCSET (ironically pronounced "fix-
it") came out on 31 August at a news
conference called by OSTP. It con-
tains a coordinated plan for a broad
government program that will in-
volve geophysicists, Earth scientists,
biologists, atmospheric modelers and
other specialists. One purpose of this
multidisciplinary study is to examine
the likelihood of global warming and
ozone depletion so that others may be
able to better determine their impli -
cations for public policies.
The report also carries some state-
ments guaranteed to puzzle and pro-
voke environmentalists. "Many glo-
bal changes can have tremendous
impacts on the welfare of humans," it
states at the start. "These events
may stem from natural processes that
began millions of years ago or from
human influence. Responding to
these changes without a strong scien-
tific basis could be futile and very
costly."
Priorities of study
Accordingly, says the report, research
needs to be done on the "interactive
physical, geological, chemical, biologi-
cal and social processes that regulate
the total Earth system." To help
achieve this end, the report sets
priorities for seven broad categories of
research to monitor, understand and
ultimately predict global climate.
Studies will be done, for instance, on
ocean circulation and on cloud cover.
At the top of the research list is
clouds, a major source of uncertainty
in models of the greenhouse effect.
Clouds act as both a blanket to trap
heat near the Earth and as a reflector
of the sun's rays to cool the planet.
Understanding the balance between
these actions is critical in predicting
climate change. (See article by Ra-
manathan and others in PHYSICS TO-
DAY, May, page 22). At the bottom ofthe list is the heading "solar in-
fluences," which includes studies of
ultraviolet light and solar radiation.
At the press conference announcing
the program, Robert W. Corell, NSF's
assistant director of geological
science, characterized its purpose as
answering questions about "how this
magnificen t planet works, how it
ticks." Corell, a member of the inter-
agency panel, explained that the re-
search categories had been ranked
according to questions that commit-
tee members believed needed answers
to help resolv e scientific uncertain-
ties. "This is the 1989 edition of the
key scientific questions," he said. He
then observed that the questions were
apt to change as scientists learned
Dallas Peck: 'Embarrassment of riches'
more about the interactions of hu-
mans, the planet and its atmosphere.
Once the causes and consequences are
understood and evaluated, the gov-
ernment's plan of actions presumably
will follow.
The picture of Earth's history that
scientists have assembled so far, the
report states, shows dramatic changes
in global conditions, such as warm
and cool epochs, continental shifts,
rising and falling sea levels , and
movements of deserts, marshes and
mountains. While humans have con-
tributed to environmental changes
for centuries, it is only since the
Industrial Revolution that conditions
have been seriously altered. With
increased burning of coal and oil to
run power plants and transport vehi-
cles, concentrations of CO2 in the
atmosphere has risen by something
like 30% in the past 100 years.
Still, evidence for global warming
remains largely conjectural, the re-
port observes. True, Earth scientists
seem to agree that the world's tem-perature has risen by about 0.5° C
since the beginning of the century
and that the six warmest years on
record came in the 1980s. But most
meteorologists are not sure about the
warming to come. Even if the world
stopped producing all greenhouse gas-
es today, some warming would still
occur. This is because of the Earth's
thermal inertia, due largely to the
way oceans and vegetation hold heat,
and because of the decades it would
take greenhouse gases to disperse.
The conventional belief is that the
global mean temperature may rise by
between 1° C and 2° C by 2030 and an
additional 0.5° C by mid-century. A 2°
rise is hardly a modest change, how-
ever, considering that the mean tem-
perature was lower by only 5° C dur-
ing the ice age, some 18 000 years ago.
The FCCSET committee's report
argues that it is essential to under-
stand natural systems so that unde-
sirable climate changes can be avoid-
ed before large parts of the US turn
into tropics or deserts and before the
ocean currents and sea levels make
some coastal regions unlivable and
some agricultural lands unproduc-
tive. In a statement accompanying
the report, Bromley observes that
global change "may well represent
the most significant societal, environ-
mental and economic challenges fac-
ing the US and the world."
Congress is likely to back the re-
search program, says Dallas L. Peck,
director of the US Geological Survey
and chairman of the FCCSET commit-
tee. Peck has been with USGS since
1951, when he received his BS degree
from Caltech, and served as a geolo-
gist with the agency while he earned
his MS from Caltech and his PhD
from Harvard. Over the years, he has
followed Congress's concern with the
subject. Peck is so sure the funds will
be appropriated for the research that
some agency programs may experi-
ence "an embarrassment of riches."
Dills before Congress
Indeed, several bills have been intro-
duced in Congress to deal with it—
notably, the World Environmental
Policy Act (S. 201), introduced by
Senator Gore, which calls for regulat-
ing and eventually phasing out anth-
ropogenic gases, improving fuel effi-
ciencies for vehicles, preserving the
world's biodiversity and developing
international controls on greenhouse
gases; and the Global Warming Pre-
vention Act (H. R. 1078), by Represen-
tative Claudine Schneider, a Rhode
Island Republican, which covers the
same matters. Schneider's bill, now
cosponsored by 150 House members,
would give highest priority, in her
PHYSICS TODAY OCTOBER, 198953words, "to reinvigorating the nation's
energy efficiency and renewable ener-
gy R&D programs," which suffered
severe budget cuts of 50% and 75%,
respectively, in the past decade.
During the Reagan years the White
House displayed little or no commit-
ment to such policies. Under Brom-
ley, OSTP is in charge of coordinating
the global climate research program,
but even sources in the Administra-
tion admit they are not convinced
that the President or any one agency
has the interest and influence to
bring it off. "It's not clear how the
Administration is going to exercise
authority over highly independent,competitive agencies that are all com-
peting for increasingly limited re-
sources," says a DOE official.
Despite this pessimism, Bush has
made some headway. He has taken a
stand that the US will not destroy any
more wetlands, the habitats of large
varieties of wildlife. The White
House Domestic Council recently
urged Bush to campaign for planting
10 billion trees in the US over the
next decade. He is virtually certain
to call on government agencies, corpo-
rate interests and individuals to do
that as an easy preventive action
against global climate change.
—IRWIN GOODWIN
NSAC BACKS BROOKHAVEN'S RHIC
AND SUGGESTS CLOSINGS TO COME
For the Nuclear Science Advisory
Committee, the conclusions reached
at Boulder, Colorado, last August
evoked a bittersweet taste. Commit-
tee members were absolutely over-
joyed to recommend as NSAC's top
priority that the Department of Ener-
gy should build the long-sough t Rela-
tivistic Heavy Ion Collider, but they
were saddened at what the project
would mean for other elements of
nuclear physics. They know, for in-
stance, that by starting RHIC the
department will need to make hard
choices about closing older facilities.
This disagreeable prospect was
raised in a letter to NSAC on 11 July
from Robert O. Hunter Jr, director of
DOE's Office of Energy Research, who
asked the committee to evaluate the
proposed facilities in the field.
Hunter's letter pointed out that cur-
rent "budget balancing activities,
competition with other highly regard-
ed scientific projects and other press-
ing and high-priority concerns within
DOE all indicate that even maintain-
ing ongoing levels of expenditures
will require substantial justification."
Accordingly, the letter went on, while
RHIC is a one of the "forefront
opportunities" in nuclear physics,
NSAC needs to consider whether it
should be built in light of its implica-
tions for nuclear physics and scientif-
ic manpower. Specifically, Hunter
wanted to know if the community
advocates going ahead with RHIC,
knowing that the budget for the field
would be virtually constant and that
the operation of some existing facili-
ties would be curtailed.
The response to Hunter's letter
came on 18 August from NSAC Chair-
man Peter Paul of the State Universi-
ty of New York at Stony Brook. Paul
made it clear that NSAC understoodHunter's message: that when it comes
to the economics of science there is no
free lunch. Paul explained that a
Long Range Working Group that met
for a week before NSAC's discussion
had rated "swift construction" of
RHIC second only to "timely comple-
tion" of the Continuous Electron
Beam Accelerator Facility at New-
port News, Virginia.
Engine of scientific change
CEBAF and RHIC symbolize the prog-
ress in nuclear physics as well as some
of the problems that afflict the field.
The new instruments are patently
engines of change. They drive new
physics. This makes experiments on
older facilities less interesting.
NSAC's endorsement of RHIC was
unambiguous. "After the long delay
which this project has already experi-
enced," Paul wrote in his letter, "we
urge swift start of construction, even
under an approximately constant
budget for nuclear science." Still,
with a DOE budget of around $300
million for all nuclear physics in 1990
and a likelihood that it will not go
beyond $315 million in fiscal 1991 ,
RHIC might seem a high price to pay.
The machine—a 2.5-mile ring in
which two beams of heavy ions will
collide with a center-of-mass energy
of 200 GeV per nucleon—and its
detectors and equipment are now
estimated to cost $328 million on
completion. According to knowledge-
able sources in the Bush Administra-
tion, DOE plans to include the first
year of RHIC's construction in its
1991 budget request, to be given to
Congress in January. The cost of
building RHIC would be spread over
five years—from 1991 through 1995.
The committee agonized over how
to build RHIC in a period whennuclear physics is unlikely to get
much more funding, Paul recalls.
Down deep, NSAC knew that however
distasteful the idea of closing facilities
was, it had to come to grips with the
budget dilemma. Assuming a con-
stant budget in nuclear physics,
Paul's letter says, RHIC could still be
built—but not without sacrifice: Its
construction would need to be
stretched out from five to six years;
Lawrence Berkeley's aging Bevelacl
which is capable of accelerating to 1
GeV per nucleon all ion species up to
uranium, would be phased out in the
mid-1990s; reductions would be made
in some programs no longer consid-
ered of great value; and Brookhaven's
combination tandem Van de Graaff
and Alternating Gradient Synchro-
tron might be curtailed "at an appro-
priate time." Of these, only the tan-
dem-AGS gives pause. When it was
linked with the tandem Van de Graaff
at Brookhaven a few years ago, the
AGS was transformed from a purely
proton synchrotron to a hybrid that
also accelerates heavy ions. The ma-
chine is now undergoing another
transformation that will enable the
AGS to accelerate all the heavy ions
the tandem can produce, including
those as heavy as gold, up to 15 GeV
per nucleon.
As for other machines of high
priority, NSAC informed Hunter it
favored US participation in the build-
ing of KAON, a high-intensit y 30-GeV
K-meson factory that the Canadian
government would like to add to the
TRIUMF cyclotron operating near Van-
couver, British Columbia (PHYSICS TO-
DAY, June, page 44). The committee
finds KAON "a very cost-effective and
timely opportunity" to investigate
important questions in physics. Still,
the committee admits that complet-
ing CEBAF and starting RHIC will
cause budget pressures requiring
DOE to look beyond a constant fund-
ing level for money to join up with
Canada. Canada has sought a total of
$75 million from the US to build
KAON over a five-year period and
possibly another $30 million for detec-
tors and other equipment. The total
cost of the project is estimate d at $450
million (in US dollars).
Even though DOE adds a 30%
contingency to the cost of building
projects, it remains uncertain if exist-
ing operations at laboratories and
universities can be fully funded in an
era of severe budgetary constraints.
CEBAF was originally figured to cost
$236 million. But then the start of
construction was delayed and the job
was stretched by a year. The machine
is now calculated to be completed for
$265 million. —IRWIN GOODWIN!
54 PHYSICS TODAY OCTOBER 1989 |
1.344172.pdf | Model of plasma immersion ion implantation
M. A. Lieberman
Citation: J. Appl. Phys. 66, 2926 (1989); doi: 10.1063/1.344172
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Downloaded 17 Jul 2013 to 131.170.6.51. 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_permissionsModel of plasma. immersion ion implantation
M. A. Lieberman
Department 0/ Electrical Engineering and Computer Sciences and the Electronics Research Laboratory,
University a/California, Berkeley, California 94720
(Received 3 April 1989; accepted for publication 15 June 1989)
In plasma immersion ion implantation, a target is immersed in a plasma and a series of
negative high-voltage pulses are applied to implant plasma ions into the target. We develop an
approximate analytical model to determine the time-varying implantation current, the total
dose, and the energy distribution of the implanted ions.
I. INTRODUCTION
In ion implantation, energetic ions are injected into the
surface of a solid material with the result that the atomic
composition and structure of the near-surface region is
changed. The process is routine in semiconductor device fab
rication. Metallurgical implantation is an emerging technol
ogy; in this application, new surface alloys are created with
improved resistance to wear, corrosion, and fatigue.
Conventional implantation is carried out in a high vacu
um environment, in which a thin beam of ions is extracted
from a plasma ion source, focused and accelerated through a
potential of tens to hundreds of kilovolts, and delivered to
the target material. Then the beam and target are manipulat
ed to expose the target surface to the beam until the desired
dose is accumulated. Some drawbacks of conventional im
plantation are ion source and beam scanning complexity and
maintenance, low beam current, nonuniform implantation
profile, and low-energy efficiency per implanted ion.
In plasma immersion ion implantation (PHI), the inter
mediate stages of ion source, beam extraction, focusing, and
scanning are omitted. The target is immersed in a plasma
environment, and ions are extracted directly from the plas
ma and accelerated into the target by means of a series of
negative, high-voltage pulses applied to the target. Both me
tallurgicall-5 and semiconductor6 implantation processes
have been demonstrated using PIlL
When a sudden negative voltage is applied to the target,
then, initially, in the time scale of the inverse electron plasma
frequency (J);~ 1, electrons near the surface are driven away,
leaving behind a uniform density ion "matrix" sheath. Sub
sequently, on the time scale of the inverse ion plasma fre
quency, ions within the sheath are accelerated into the tar
get. This, in turn, drives the sheath-plasma edge further
away, exposing new ions that are extracted. On a longer time
scale, the system evolves toward a steady-state Child law7•8
sheath. Generally, this is of no interest in PIlI, because the
sheath thickness exceeds the plasma size; hence the voltage is
returned to zero before the steady-state sheath forms.
The matrix sheath and its time evolution determine the
current J(t) and the energy distribution dN / dW of implant
ed ions. The structure of the initial matrix sheath in one
dimensional planar, cylindrical, and spherical targets9 and
two-dimensional wedge-shaped targetslO has been deter
mined. In addition, the self-consistent equations have been
solved numerically to find the time evolution of the matrix
sheath in planar geometry .11-15. However, it is desirable to
have an analytical estimate of J and dN /dW In this study, we develop an approximate analytical model for an applied
rectangular voltage pulse in one-dimensional planar geome
try and compare the results with the numerical solutions.
The model yields quantities, such as the peak implantation
current and time, and their scalings with system parameters,
that are useful in describing the PIU process.
II. BASIC MODEL
Figure 1 (a) shows the initial PHI geometry. The planar
target is immersed in a uniform plasma of density no. At time
t = 0, a voltage pulse of amplitude -Vo and time width tp is
applied to the target, and the plasma electrons are driven
away to form the matrix sheath, with the sheath edge at
x = so< As time evolves [Fig. 1 (b) J, ions are implanted, the
sheath edge recedes, and a nonuniform, time-varying sheath
forms near the target. The model assumptions are as foHows:
( 1) The ion flow is collisionless. This is valid for suffi
ciently low gas pressures.
(2) The electron motion is inertialess. This follows be
cause the characteristic implantation time scale much ex
ceeds (JJ p-; !.
(3) The applied voltage V;) is much greater than the
electron temperature T.; hence the Debye length AD -( So,
and the sheath edge at s is abrupt.
(4) During and after matrix sheath implantation, a qua
sistatic Child law sheath forms. The current demanded by
this sheath is supplied by the uncovering of ions at the mov-
TARGET
V o-_,,_J-<;II MATRIX r--"" (dS) L£- SHEATH \ cit 0
-vof-J ~ 0 So PLASMA
no -----,------r--------
J -v 0----5; I
I
I
I
CHILD LAW SHEATH ds dt (a)
t=o'
(I:»
FIG. 1. Planar PIlI geometry (a) just after formation of the matrix sheath
and (b) after evolution of the quasistatic Child law sheath.
2926 J. Appl. Phys. 66 (7),1 October i989 0021-8979/89/192926-04$02.40 @ 1989 American Institute of Physics 2926
Downloaded 17 Jul 2013 to 131.170.6.51. 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_permissionsing sheath edge and by the drift of ions toward the target at
the Bohm (ion sound) speed UB = (eTe/M) 1/2 (Te is in
units of volts).
(5) During the motion of an ion across the sheath, the
electric field is frozen at its initial value, independent of time,
except for the change in field due to the velocity ofthe mov
ing sheath.
Assumptions (4) and (5) are approximations that per
mit an analytical solution to the sheath motion. These as
sumptions are justified post hoc by comparison with numeri
cal results.
III. SHEATH MOTION
The Child law current density jc for a voltage Va across a
sheath of thickness s is 7,8
. _ ~E (2e)112 vii12
lc - 9 0 M S2' (1)
where Eo is the free-space permittivity and e and It1 are the
ion charge and mass. Equatingjc to the charge per unit time
crossing the sheath boundary,
(ds ). eno dt+UB =Jc'
we find the sheath velocity
ds 2 s5Uo -=----u B' dt 9 .12
where
So = (2EOVoleno) 1/2
is the matrix sheath thickness and
Uo = (2eVo/i\!) 1/2 (2)
(3)
(4)
(5)
is the characteristic ion velocity. Integrating (3), we obtain
tanh-1(s/s,,) -s/s" = uBt /s" + tanh -1 (so/sc ) -so/s",
(6)
where
sc=so[~(U()IUB)]1/2 (7)
is the steady-state Child law sheath thickness. Since Sc >so
and assuming Sc >s, we obtain from (6) that
(8)
where wpi = /.Lolso is the ion plasma frequency in the matrix
sheath. Substituting (7) into (8), we note that the time scale
t" for establishing the steady-state Child law sheath is
tc;::::; (~2/9) Wpl 1 (2 VolT. )3/4, and we assume that the pulse
width tp <te in the development that follows,
IV. MATRIX SHEATH IMPLANTATION
Because the initial charge density in the matrix sheath is
uniform, the initial electric field varies linearly with
x: E = (M /e)w;i (x -s). Hence, the ion motion is
d2x , --2 = W;i(X -s), dt
where x is the particle pOSItIOn.
s = So + Cds/dOo tin (9) and using (3)
U B < uo• we obtain (9)
Approximating
with s = So and
2927 J. Appl. Ph'ls., Vol. 66, No.7, 1 October 1939 d2x _ 2 2 2 --2 -Wpi ex -so) -,}tuwp;t. dt (10)
Integrating (10), we find
x -So = (xo -so)cosh wpJ -~o sinh wpJ + ~uot, (11)
where we have let x 0:= Xo and x = 0 at t = 0, Choosing xz -UB, consistent with the sheath motion (3), yields a
negligible correction to (11) because U B < u(). Letting x = 0
in ( 11 ), we obtain the ion flight time t from
So = (so -xo)cosh (ilpri + §so sinh wpJ -~uot. (12)
In a time interval between t and t + dt, ions from the
interval between Xo and Xo + dxo are implanted. Differenti
ating (12), we find
dxo wpi(so-xo)sinh(upJ+ijuo(coshwpJ-l)
dt cosh wp;t ( 13)
Using (12) in (13) to eliminate So -xu, we obtain the im
plantation current density j = eno dxo/dt as
J = sinh T -L 2 1 + T sinh T -cosh T
cosh2 T ' 9 cosh2 T ' (14)
where J =j!(enou o) is the normalized current density and
T = wp,t is the normalized time. Equation (14) gives the
implantation current density versus time for those ions in the
initial matrix sheath O<xo<so' Setting Xo = So in (12), we
obtain Tz2.7. At this time, all matrix sheath ions are im
planted; hence (14) is valid for O<T<2.7. Figure 2 gives a
plot of J vs T. The maximum Jmax ;::::;0.55 occurs at
Tmax zO,95. We note that J(2.7) ::::;0.19.
V. QUAS!STATIC CHILD LAW SHEATH IMPLANTATION
Consider the implanted ions having initial positions at
Xo> So. The time t, for the initial sheath edge at So to reach Xl)
is found from (8):
0.4
J
0.2
'-"'"----
-------~----.
T
FIG. 2. Normalized implantation current density J = j/ (enouo) vs normal
ized time T = ()) pi t. The dashed lines show the analytical solution for T < 2,7
[Eq. (14) 1 and T> 3 [Eq.(l9)]; the solid line is the numerical solution
(see Refs. 13 and 14).
M. A. Lieberman 2927
Downloaded 17 Jul 2013 to 131.170.6.51. 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_permissionsOJpJ, = ~(Xb/S6) -~. (is)
At time ts' an ion at Xo begins its flight across the sheath. The
ion flight time is given by8
OJ p' t' = 3xo/so. (16)
Hence, an ion at Xo reaches the target at a time t = t, + t '
given by
T = wpJ = ~(X6/S~) -~ + 3 (xo/so)'
Differentiating (17), we obtain
dxo Uo -=---"---
dt 1(x~/s~) + 3
The normalized implantation current density is thus
J=-----
~(X6/56) + 3 (17)
(18)
(19)
Equations (17) and (19) give J(T) as a parametric
function of xo/so [Although (17) can be solved for Xo and
substituted into ( 18), the result is not illuminating. J For xo/
So = 1, we find T= 3 and J(3) = 2115z0.133. As T ~ 00,
Xo -">Sc >30; hence J( 00) --(2/9)S6/3:. Unnormalizing, we
find j( 00 ) --enou B' which correctly gives the steady-state
Child law current density. However, as noted previously, we
are not interested in this long time scale.
We note that (14) and (19) do not smoothly join at
Xo = so' This is a consequence of the simplifying assumptions
( 4) and (5) that were used to solve for the sheath and ion
motion. Figure 2 shows the analytical results for J vs Tin
both regimes.
The nonlinear partial differential equations for the ion
and electron motion in the planar sheath have been solved
numerically.I3-15 The ion motion is collisionless, the elec
trons are in thermal equilibrium, and Poisson's equation re
lates the densities to the potential. The equations are
ani a -+-(n.u.) =0 at ax I I ,
au au M-'+JlJu.-' at ' ax alP -eax'
ne = no exp( -<PIT"),
az¢ e -;--2 = --(ni -ne)'
oX Eo
Figure 2 shows a numerical solution for Vr/Te = 200. We
see that (14) for T< 2.7 and (19) for T> 3 are in good agree
ment with the numerical results. A numerical solution for
VolTe = 50 also agrees well with the analytical model, and
the predicted scalingjo: V612 is verified numerically.
VI. ENERGY DISTRIBUTION
We assume a voltage pulse of width T> 3, such that all
matrix sheath ions (xo<so) are implanted. For these ions,
since the potential varies quadratically with the distance
from the sheath edge, ions starting at Xo are implanted with
energy
W = vo{1-[(so -xo)2/s6 n. (20)
2928 J. Appl. Phys., Vol. 66, No.7, 1 October 1989 Within the energy interval dW = 2 VoCso -xo)dxo/s~, there
are dN = nodxo ions per unit area implanted. Hence, we find
dN noS6 --= -- Vo(.~o -xo)· dW 2 (21)
Using (20) in (21), we find the energy distribution for the
matrix ions:
dN = nfy."i() CV, _ W)-l12.
dW 2V6/2 0 (22)
For a pulse of width T> 3, all ions from the interval
30<XO<;'XT are implanted at full energy, where XT is deter
mined from (17):
(23)
Hence, the energy distribution contains a delta function,
dN /dW = nO(xT -so)8( W -Vo), for these ions.
Finally, because the sheath edge S T has reached the posi
tion given by (15),
(24)
all ions with XT<XO<ST are in transit when the pulse is
turned off. The density and potential in the Child law sheath
just before turnoff ares
(25)
and
¢(xo) = -Vol (ST -xo)/sd5/3. (26)
Using (26), the ion energy is
W(xo) = Vo{l -[(ST -XO)/ST lS/3}. (27)
Differentiating (27) to obtain dW and using
dN = n(xo)dx o, we find
dN ST --0:---
dW ST -Xo (28)
Using (27) in (28) and normalizing the distribution such
that N = nO(sT -xT), we obtain
dN _ 2 (ST -xT)nO_3/5 (29) dW -5 v2i5 ( Vo -W) .
()
The total energy distribution is the sum of the distribu
tions for ions having O<;,xo<so, so<XO<;,x T and xT<XO<ST'
The total dose implanted is lIoST•
A quantity of interest is the fraction f of ions that hit
the target with W < Wmin < Vo. For example, ions wi.th ener
gies below several kilovolts may produce sputtering of the
target rather than be implanted. Integrating (22) and (29)
from 0 to Wmin, we obtain
f = ~o [1 _ (1 _ W:llin )1/2] + (1 _ XT)
!iT VO 5T
Figure 3 shows f vs T for various values of Wrr:in/VO'
M. A. Lieberman 2928
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~::
FIG. 3. Fraction/of ions hitting the target with energies W < Wm'n vs T,
with Wmin/Vo as a parameter.
ACKNOWLEDGMENTS
This work was supported by a contract from IBM Cor
poration, a grant from Applied Materials Corporation, Na-
2929 J. Appl. Phys., Vol. 66, No.7, 1 October 1989 tional Science Foundation Grant No. ECS-8517363, and
Department of Energy Grant No. DE-FG03-87ER13727.
Helpful discussions with 1. Brown, D. A. Carl, N. W.
Cheung, R Wong, X. Qian, and S. E, Savas are gratefully
acknowledged.
'J. R. Conrad and C. Forest, in IEEE International Conference an Plasma
Science, Saskatoon, Canada, May 19-21, 1986 (IEEE, New York).
2J. R. Conrad and T. Castagna, BuLL Am. Phys. Soc. 31,1479 (1986).
3J. R. Conrad, J. L Radtke, R. A Dodd, F. J. WOfzala, and N. C. Trail, J.
Apr!. Phys. 62, 4591 (1987).
4J. R. Conrad, S. Baumann, R. Fleming, and G. P. Meeker, J. App!' Phys.
65,1707 (1989).
'J. Tendys, t J. Donnelly, M. J. Kenny, and J. T. A. Pollack, App!. Phys.
Lett. 53, 2143 (1988).
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and K. M. Yu (unpublished).
7C. D. Child, Phys. Rev. 32, 492 (l9H).
'C. K. Birdsall and W. B. Bridges, Electron Dynamics of Diode Regions
(Academic, New York, \966).
YJ. R. Conrad, J. Appl. Phys. 62, 777 (1987).
101. J. Donnelly and P. A. Watterson, J. Phys. D 22, 90 (1989).
"K. F. Sander, J. Plasma Phys. J, 353 (1969).
!lA. G. Jack, K. F. Sander, and R. H. Varey, J. Plasma Phys. 5, 211 (1971).
13M. M. Widner, I. Alcxeff, W. D. Jones, and K. E. Lmmgren, Phys. Fluids
13, 2532 (1970).
'4J. R. Conrad, "Plasma Source Ion Implantation," presented at United
Technologies Research Center, Sept. 26,1986 (unpublished).
"J. R. Conrad and T. Castagna, in Proceedings of the 39th Annual Gaseous
Electronics Conference, Madison, WI, October 7-10, 1986.
M. A. Lieberman 2929
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1.457856.pdf | Branching ratios and rate constants for reactions of 16O− and 18O− with N2O and
14N15N16O
Robert A. Morris, A. A. Viggiano, and John F. Paulson
Citation: The Journal of Chemical Physics 92, 3448 (1990); doi: 10.1063/1.457856
View online: http://dx.doi.org/10.1063/1.457856
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/92/6?ver=pdfcov
Published by the AIP Publishing
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141.212.109.170 On: Mon, 22 Dec 2014 14:39:53Branching ratios and rate constants for reactions of 160-and 180-
with N20 and 14N15N160
Robert A. Morris,a) A. A. Viggiano, and John F. Paulson
Ionospheric Physics Division (LID), Geophysics Laboratory (APSe), Hanscom AFB,
Massachusetts OJ 73J-5000
(Received 28 September 1989; accepted 29 November 1989)
Branching ratios for the NO -isotopic products from the gas-phase reactions of 160 -and of
180-with 14NISNI60 have been determined at 143 and 298 K using a variable temperature
selected ion flow drift tube (VT-SIFDT) instl1lment. The reaction of 160-yields the products
14N160- and ISNI60- in approximately equal abundance at both temperatures. The reaction
of 180-produces the four possible NO-isotopes, with the branching ratio being dependent on
temperature. For the latter reaction the rate constant for the 0 -isotope exchange process has
been determined at 143 and 298 K. Rate constants for the reaction of 0-with N20
(unlabeled reagents) have been measured as a function of ion-neutral average center-of-mass
kinetic energy ( (KEc.m. ) ) at several temperatures. The temperature dependence of the rate
constant is expressed as T -0.5. The energy dependences at different temperatures fall on a
single curve and agree well with a previous energy dependence study at 300 K.
INTRODUCTION
There has been interest in the gas-phase reaction of 0-
with N20 for several decades, originally due to interest in
the radiation chemistry and electron scavenging properties
of nitrous oxide and the attendant ion chemistry. The domi
nant reaction pathways are reaction (1)
0-+ N20-+NO-+ NO I:Jl = -0.14 eVI (1)
and exchange of 0 - , which is detectable when isotopically
labeled oxygen is used.2-4 Studies of reaction 1 published
through 1973 are summarized by Tieman.s Subseq~ent in
vestigations including flow-drift tube6 and tandem flowing
afterglow-selected ion flow tube2,3 experiments are dis
cussed by Van Doren.2 Several authors2,7,8 have postulated
that the reaction proceeds via a long-lived complex of the
structure (ONNO) -which would be formed by attack of
0-at the terminal nitrogen in N 2 O. The observation of the
ion N2 O2-in the gas phase resulting from ionization in N20
has been reported in the literature. 9, 10 The aim of the present
work was to obtain information on the site of 0 -attack on
N20 by employing isotopic labels on both N20 and 0 - , as
first proposed by Van Doren2•
In the present study, the reactions of 160 -and 180-
with 14NI5NI60 were investigated at 298 and 143 K, and the
product branching ratios were determined. Additionally,
rate constants for reaction (1) were measured as a function
of ion-neutral average center-of-mass kinetic energy
( (KEc.m. » at several temperatures. This article is comple
mentary to an accompanying paper by Barlow and Bier
baum.11
EXPERIMENTAL
The experiments were performed using a variable tem
perature-selected ion flow drift tube (VT-SIFDT) instru-
a) On contract to GL from Systems Integration Engineering, Inc., Lexing·
ton, MA. ment. The technique has been fully described in the litera
ture.12 Briefly, 160 -ions were generated by electron impact
on N20 in a high pressure (0.1-1 Torr) ion source. 180-
ions were produced similarly from 1802, The ions were then
mass selected in a quadrupole mass spectrometer and inject
ed into a stainless steel flow tube (1 m length) through a
Venturi inlet. Inside the flow tube the ions were entrained in
the fast flow (_104 cm/s at 0.2 to 0.4 Torr) of He or Ar
carrier gas emanating from the Venturi inlet. The neutral
reactant N2 0 was introduced into the flow tube through one
of two ring-shaped inlets and reacted with the ions over dis
tances of50.3 or 35.4 cm depending on which inlet was used.
The reactant and product ions were sampled through a 0.2
mm diameter orifice in a truncated nose cone, mass analyzed
in a second quadrupole mass spectrometer and detected by a
channel particle multiplier. Rate constants were calculated
from the reaction times and the slopes ofleast squares fits of
the natural logarithm of the reactant ion signal plotted ver
sus added reactant neutral gas concentration. The reaction
time was obtained from the reaction distance and from direct
ion time-of-flight measurements in the flow tube. The accu
racy of the measured rate constants is ± 30% and the ex
perimental precision is ± 15%.
The experiments were conducted over the temperature
range 143-515 K by circulating liquid nitrogen through a
copper heat exchanger in contact with the flow tube for cool
ing and by heating the heat exchanger with attached resistive
heaters. The flow tube and heat exchanger are contained
within a vacuum chamber to reduce the conduction of heat
to or from the surroundings. The rate constants were also
measured as a function of (KEc.m.) at each of four tempera
tures by varying a uniform electric drift field in the down
stream half of the flow tube.
The ring-shaped inlets for the neutral reactant gas are of
an improved design compared with the previous inlets used
in this laboratory. The present design, due to Smith and Ad-
3448 J. Chern. Phys. 92 (6), 15 March 1990 0021-9606/90/063448-05$03.00 @ 1990 American Institute of PhySics
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ams,12 is that of a 4 cm diameter ring constructed of 1.5 mm
i.d. stainless steel tubing with eight 0.4 mm diameter holes
pointing in the upstream direction. The previous "finger"
inlets, which were simple tubes with the gas injection occur
ring on the flow tube axis and directed upstream, led to erro
neous rate constants [including those for reaction (1)]
when an electric field was applied to the drift tube. 13-15 The
problem with the finger inlet design is discussed in Ref. 15.
Isotopically labeled reaction product ions were moni
tored as a function of reactant neutral flow rate by scanning
the downstream mass spectrometer over the product ion
peaks and recording the spectra on a strip chart recorder.
The 14N15NI60 used was 99% isotopically pure, and correc
tions to the product distributions were made to account for
the contribution to the 14NI60 -product arising from the
1% 14NI4NI60 present in the reagent.
To facilitate detection of the product ion NO-, the He
carrier gas was replaced with Ar for the experiments involv
ing NO -isotopic branching ratios. The signal levels from
NO -detected in our apparatus at room temperature using
He carrier gas were very low due to collisional detachment
by the helium. At 143 K, the NO-signals observed for He
buffer were substantially larger. The electron affinity of NO
is less than (3/2)kTat room temperature; a recent measure
mentl6 yielded a value of 0.026 ± 0.005 eV. The rate con
stant for detachment from NO -by Ar is substantially
smaller than that for detachment by He,17,18 and conse
quently the NO -signal levels in Ar are greatly enhanced
compared to the He case. Mass discrimination between 0-
and NO -was accounted for by comparing the ratio of the
detector count rate to the ion current measured at the sam
pling nose cone for injection of 0 -into the flow tube (with
no other ions present in the flow tube) to that ratio obtained
for only NO -in the tube.
RESULTS AND DISCUSSION
A. Unlabeled reagents
The experiments using the unlabeled reagents 160 -and
14NI4NI60 were conducted with He as the carrier gas. The
rate constants for reaction (1), kl, were measured as a func
tion of average ion kinetic energy at the four temperatures
143, 194,298 and 515 K and are plotted versus (KEc.m.) in
Fig. 1. At all four experimental temperatures the data on
average kinetic energy dependence are in excellent agree
ment with those from a flow-drift tube study performed at
300 K by Lindinger and co-workers.6 The collision limiting
value of the rate constantl9 is 1.24 X 10 - 9cm3 Is at 300 K,
and the measured rate constants range from 6% to 23% of
that collision value.
The temperature dependence of kl is shown as a solid
line in Fig. 1. The line is a least squares fit to the data points
obtained with no electric drift field at each of the four experi
mental temperatures. The measured temperature depend
ence is negative, as is often the case for exothermic ion-mole
cule reactions with low reaction efficiency, and can be
expressed as T -0.5. The only previous temperature depend
ence measurement of k I , to our knowledge, is due to Marx et
al.18 who obtained a T -0.7 dependence from measurements 10
10 ·10
o 143K o 196K
A 29sK
• 515K
10·n +---------~--------___i
.01 .1
(KEcm) (eV)
FIG. 1. Rate constants for the reaction of 0 -with N2 0 as a function of ion
neutral average center-of-mass kinetic energy at several temperatures.
at two temperatures, 278 and 475 K, in a flowing afterglow
experiment. These workers reported "considerable scatter"
in the data.
In the energy range from thermal to about 0.1 eV the
measured energy dependence of kl is «KEc.m.» -0.5, which
is the same as the observed temperature dependence. Figure
1 shows that, at still higher energies, the rate constant begins
to level off and then to increase with increasing (KEc.m. ), as
is the case for many ion-molecule reactions. The decrease in
ion-molecule rate constants with increasing temperature
(or energy) is generally explained by assuming that the reac
tion proceeds via a long-lived complex. Magnera and Ke
barle20 have pointed out that as collision energy increases,
the higher internal energy of the complex is accompanied by
a larger increase in the density of states of the loose transition
state early in the reaction coordinate than that of the tight
transition state leading to products. This facilitates back de
composition of the complex to reactants compared to the
case of collisions at lower energy. The rise in the rate con
stant at higher energies may be due to another reaction
mechanism, perhaps a direct mechanism, becoming impor
tant. Paulson21 found an increasing preference at increasing
hyperthermal energies for the product NI80 -from the reac
tion of 180 -with N 2160, a result consistent with the partici
pation of direct N -atom abstraction at higher energies.
The (KEc.m.) dependences of kl measured at different
temperatures all lie on a single curve. At a given (KEc.m.),
the rate constants measured at different temperatures ap
pear not to depend on temperature. This suggests that at a
given (KEc.m. ) the rate constant is insensitive to the internal
temperature of the reactant neutral. 13-15 It was noted in Ref.
13 that there is considerable vibrational excitation of the
N 20 at the temperatures employed in the experiment, espe
cially at 515 K. It is interesting that the varying degrees of
vibrational and rotational excitation at different tempera
tures do not affect measurably the rate constant at a given
(KEc.m.). Discussions ofthe use of this technique to investi
gate internal energy effects in certain ion-molecule reactions
can be found in Refs. 13-15 and 22.
J. Chern. Phys., Vol. 92, No.6, 15 March 1990
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141.212.109.170 On: Mon, 22 Dec 2014 14:39:533450 Morris, Viggiano, and Paulson: Reactions of 0 with N20
B. The Reaction of 180-with 14N15N180
The reaction between 160 -and 14N15N160 was investi
gated at 143 and 298 K using Ar as the carrier gas. The
products 14N1~ -and 15N160 -at mass-to-charge ratio
(mle) 30 and 31 daltons, respectively, were monitored as a
function of added 14N15N160 reactant gas flow rate. The ra
tio ofthe count rates for the above product ions C30/c31 was
found to decrease slightly with increasing 14N1SN160 reac
tant flow rate. This small effect was observable above the
scatter in the data only at large extent of reaction and is
probably due to the secondary reaction, reaction (2):
14N160-+ 14NI5NI60-.15NI60- + 14N14NI60
t::.H-OeV (2)
which converts the ion 14N160- to 15NI60-. The observed
trend in C30/c31 with 14N15N160 flow rate indicates that the
reaction between ISNI60 -and 14NI5NI60 to produce
14NI60-, reaction (3):
15N160-+ 14NI5NI60-.14NI60- + 15N15N160
t::.H-OeV (3)
is slower than reaction (2). The rate constants for reaction
(2), determined in separate experiments, are k2 = 9 X lO-12
and 1.25 X lO -11 cm3 Is at 143 and 298 K, respectively.
These values were used in a simple kinetics model of the
c30lc31 product ratio. With the rate constant for reaction
(3) set equal to zero, the model correctly predicted the ob
served trend in c301c31• Investigations of some reactions of
NO-, including reaction (2), will be reported separate
ly.22.23
Since the product ion count ratio c30lc31 was observed
to vary with 14N15N160 flow rate, the branching ratio be
tween the product ions 14N160 -and 15NI60 -was found by
extrapolating c30lc31 to zero 14N15N160 flow. The extrapo
lation yielded relative abundances of 50.5 ± 2% and
49.5 ± 2% for 14N160- and 15NI60-, respectively, at 143
K. At 298 K, the relative abundances are 51 ± 3.5% and
49 ± 3.5% for 14NI60 -and ISN160 - . The data indicate a
very slight preference at both temperatures for 14N160-
over 15N160- which could be due to a small isotope effect.
However, within the experimental uncertainty, the branch
ing ratio is unity at both 143 'and 298 K, in agreement with
the 300 K ratio reported in the companion paper by Barlow
and Bierbaum. I 1 Paulson measured a C30/c31 ratio of 1.25 in
an ion source experiment 7 and, for the reaction of 180 -with
N2 160 studied in a tandem mass spectrometer, found a prod
uct ratio for N180-:NI60-of approximately unity at the
lowest beam energies.21 In a similar experiment, Futrell and
Tieman8 also observed equal production of N180 -and
NI60-.
These results, together with the preponderance of the
0-exchange reaction channel, are consistent with attack by
160 -at the terminal nitrogen in 14N15N160, forming the
complex (16014NI5NI60) _., followed by N=N bond scis
sion. This mechanism, suggested by several authors, 2. 7.8 has
the virtue of simplicity. However, these results are also con
sistent with the formation of a trigonal intermediate com-plex by 0 -attack at the central nitrogen atom in N2 O. Evi
dence is presented in the next section and in the companion
articlell which suggests the possibility of attack by 0-at
the central nitrogen.
C. The reaction of 180-with 14N15N180
The rate constant k4 for the overall reaction of 180-
with 14N15N160, which includes the atom transfer reactions
4(a)~(d):
180 -+ 14NI5NI~-. \4NI~ -+ ISN180
t::.H= -0.14eV,
180-+ 14NI5NI60-.15NI60- + 14N180
t::.H = -0.14 eV,
180-+ 14NI5NI60-.14NI80- + 15N160
t::.H= -0.14eV,
180-+ 14NISNI60-.15NI80- + 14NI60
t::.H= -0.14eV
and the 0-isotope exchange reaction 4(e): (4a)
(4b)
(4c)
(4d)
180-+ 14NISNI60-.160- + 14NI5NI80 t::.H-OeV
(4e)
was measured to be k4 = 3.85 X lO -10 cm3 Is at 143 K. Van
Doren et aU obtained a value for k4 of 4.0 X lO -10 cm3 Is at
300 K. We made no measurement of k4 at 300 K since a
value had been previously reported3 and we had exhausted
the supply ofthe costly 14NISN160. The four different NO
isotopic products of the reactions 4(a)~(d) were moni
tored as a function of added 14N15NI60 reactant gas flow rate
at 143 and 298 K. These experiments again were conducted
with Ar as the carrier gas. The count rate of the 160 -prod
uct of the 0-isotope exchange reaction 4(e) was also fol
lowed as a function of 14N15NI60 flow rate.
At each of the two temperatures, the rate constant for
exchange of 0 - , k4e' was obtained from the difference be
tween the rate constant for the overall reaction k4 and the
rate constant for the reaction of the unlabeled reagents, for
which the 0 -exchange process is undetectable, kl . Since k4
was not measured at 298 K, the k4e for 298 K was found
from the difference between the k1 obtained at 298 K in this
laboratory and the k4 measured at 300 K by VanDoren and
co-workers,3 k4 =4.0XlO-lOcm3/s. The values ofk4e so
obtained are 1.37X lO-10 and 1.7X 10-10 cm3 Is at 143 and
298 K, respectively. The 298 K value is identical to that
obtained by Van Doren et al. These values of k4e are also
consistent with those obtained by modeling the observed
variation in the ion count rates as a function of 14N15N1~
reactant gas flow rate.
The kinetics mOdel was also used to find the branching
ratios between the four NO -isotopic product species result
ingfrom reaction of180-with 14N1SNO. This was done both
to account for the secondary reaction between 160 -and
14N15N160 to produce 14N160- and 15N160- and because of
the presence of a small signal from 160 -at zero 14NI5N160
J. Chern. Phys .• Vol. 92, No.6, 15 March 1990
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141.212.109.170 On: Mon, 22 Dec 2014 14:39:53Morris, Viggiano, and Paulson: Reactions of 0 with N20 3451
flow rate, originating from a minute leak of 14N15N160
through the flow controller valve.
The relative abundances of the NO -isotopic products
determined at 143 and 298 K are presented in Table I. Sever
al observations can be made concerning these data. All of the
four possible NO -isotopic products were observed. At 143
K the sum of the relative abundances of 14N160 -and
14N1S0 -is approximately equal to that sum for 15N160-
and 15N1S0 -. This might be expected in light of the unit
NO -branching ratio for the reaction of 160 -with
14N15N160 which indicated equal probability for the NO -to
contain either 14N or 15N. At 298 K the above sums are not
quite equal, but the uncertainty in the results at 298 K is
substantially larger than that at 143 K.
The dominant NO -product species are 15N160 -and
14N1S0- at both 143 and 298 K. The most simple mecha
nism explaining the roughly equal production of these two
products is attack by ISO -at the terminal nitrogen in
14N15N160, forming the complex ctS014N15N160) -*, fol
lowed by N N bond scission. This is the mechanism postu
lated previously in the literature.2,7,s The other two NO
products, 14N160 -and 15N1S0 - , were observed in signifi
cant abundance and cannot arise from the above mecha
nism. If they are formed following IS0-attack at the termi
nal nitrogen in 14N15N160 (terminal attack), the complex so
formed must then undergo a rearrangement which leads to
the ISO bonded to 15N. If, however, 14N160- and 15N1S0-
arise from IS0-attack at the central 15Nin 14N15N160 (cen
tral attack), then no rearrangement is required of the resul
tant trigonal complex in order to produce any of the four
isotopic NO-products. Posey and 10hnsonlO observed
N 2 O2-produced from low energy electron impact in a super
sonic jet of N 2 O. They present evidence suggesting a cova
lently bound trigonal C2v structure, analogous to the stable
isoelectronic species CO;. In a matrix isolation study, Ha
caloglu et al.24 produced the anion N2 O2-in a discharge of
gaseous N20. They report spectroscopic evidence for a tri
gonal structure NN02-with equivalent oxygen atoms. If the
trigonal complex arising from central attack has C2v symme
try, i.e., symmetric with respect to the oxygen atoms, then all
four NO -products would be expected to occur in equal
abundance. It is assumed here that there is no isotope effect
and that when the complex dissociates into NO and NO - ,
the negative charge has equal probability of remaining on
either of the nascent NO species. If a trigonal complex
formed from central attack is asymmetric with respect to the
oxygen atoms, then unequal abundances of the different
NO -products are expected. The results are consistent with
TABLE I. Percent of total NO -product ion count rate for isotopes of NO -
formed in the reaction of 180 -with 14NI5NI60.
Temperature
143 K
298K 18 ± 1.5
15 ± 2.3 34± 3
34± 5 31 ±2
38 ±4 17 ± 1
13±1.3 the asymmetric trigonal complex mechanism and also with a
combination of terminal attack and either of the two central
attack complexes, but inconsistent with exclusive terminal
attack unless one invokes a rearrangement of either of the
complexes eS014N15N160) -* or IS0-. 14N15N160. The
case for terminal attack may be argued in terms of the
greater accessibility of the terminal nitrogen and by analogy
with reactions of other negative ions with labeled N 2 0.2 Yet
central attack is consistent with the charge distribution of
N20; the central nitrogen has a positive charge of about 0.6
and the terminal N atom is just slightly negative. 25
The measured branching ratio depends on temperature.
Because of the complication introduced by the secondary
reaction of 160 -with 14N15N160 forming 14N160 -and
15NI60-, the most direct indicator ofthe effect of tempera
ture on the branching ratios is the ratio of the count rates of
the products 14N1S0- and 15N1S0-, C32/c33' This fraction
was found to change from 2.9 at 298 K to 1.8 at 143 K. If one
postulates that the 15N1S0 -arises from a trigonal complex,
then the temperature dependence information implies either
that formation of the trigonal complex is favored at lower
temperatures over formation of a different structure produc
ing 14N1S0 -or that the dissociation of the trigonal complex
into the product 15N1S0 -is favored at low temperature. If
the trigonal complex is formed by central attack, one might
expect the formation of that complex to be more favorable at
lower temperature due to the increased time for the ISO -to
interrogate the N20 potential energy surface and to locate
the partial positive charge on the less accessible central ni
trogen. Unfortunately, the results do not establish unambig
uously the mechanism of reaction or the identity of the inter
mediates, but they do strongly suggest the formation of a
trigonal complex for some fraction of collisions. One conclu
sion which can be drawn is that, for certain reactions, em
ploying a different isotopic label on every atom in the react
ing system does not ensure the elucidation of the reaction
mechanism or the identity of the intermediates. A theoreti
cal study of this reaction might provide some insight into the
questions raised here. Further discussion of the possible
mechanisms for reaction (4) is found in the accompanying
paper. 11 In a related study, we have found that solvation of
0-by one and two H20 molecules greatly inhibits the reac
tion with N2 O. Rate constants for the reactions
0-. (H20) n = 0-2 + N2 0 will be presented in a forthcoming
article. 26
To summarize, the reaction of 160 -with 14N15N160
produces approximately equal amounts of 14N160 -and
15N160- independent of temperature from 143 to 298 K.
The reaction of ISO -with 14N15N160 produces all four pos
sible NO -isotopic products, and the branching ratio de
pends on temperature. At both 143 and 298 K the products
15N160- and 14N180- are dominant relative to 14N160-
and 15N1S0-, but this dominance is less at the lower tem
perature. The production of significant quantities of
14N160 -and 15N1S0 -suggests the possibility of attack by
180 -at the central nitrogen atom in nitrous oxide. For the
reaction of the unlabeled reagents, 0-with N20, the rate
constant was found to depend on temperature as T -0.5, and
J. Chern. Phys., Vol. 92, No.6, 15 March 1990
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141.212.109.170 On: Mon, 22 Dec 2014 14:39:533452 Morris, Viggiano, and Paulson: Reactions of 0 with N20
energy dependences measured at different temperatures lie
on a single curve.
ACKNOWLEDGMENTS
The authors thank Fred Dale for technical assistance.
We thank the authors of the companion article, Veronica M.
Bierbaum, and Stephan E. Barlow, for helpful discussions
and for sending us some of their experimental data. We
thank Carol Deakyne for helpful discussions and for making
available to us some unpublished data.
1 S. G. Lias, J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin, and
W. G. Mallard,J. Phys. Chern. Ref. Data 17, I (1988), Suppl. I.
2 J. M. Van Doren, Thesis, University of Colorado, 1987.
3 J. M. Van Doren, S. E. Barlow, C. H. DePuy, and V. M. Bierbaum, J. Am.
Chern. Soc. 109, 4412 (1987).
4R. A. Morris, A. A. Viggiano, and J. F. Paulson in Non-equilibrium Ef
fects in Ion and Electron Transport, edited by J. W. Gallagher, D. F. Hud
son, E. E. Kunhardt, and R. J. Van Brunt (Plenum, New York, 1990).
sT. O. Tieman, in Interactions Between Ions and Molecules, edited by P.
Ausloos (Plenum, New York, 1975), p. 353.
6W. Lindinger, D. L. Albritton, F. C. Fehsenfeld, and E. E. Ferguson, J.
Chern. Phys. 63, 3238 (1975).
7J. F. Paulson, Adv. Chern. Ser. 58, 28 (1966).
8 J. H. Futrell and T. O. Tieman, in Ion-Molecule Reactions, edited by J. L. Franklin (Plenum, New York, 1972), p. 485.
9 J. L. Moruzzi and J. T. Dakin, J. Chern. Phys. 49, 5000 (1968).
1OL. A. Posey and M. A. Johnson, J. Chern. Phys. 88, 5383 (1988).
11 S. E. Barlow and V. M. Bierbaum, J. Chern. Phys. 92, 3442 (1990).
12 D. Smith and N. G. Adams, Adv. At. Mol. Phys. 24, 1 (1988).
13 A. A. Viggiano, R. A. Morris, and J. F. Paulson, J. Chern. Phys. 89, 4848
(1988).
14 A. A. Viggiano, R. A. Morris, and J. F. Paulson, J. Chern. Phys. 90, 6811
(1989).
IS A. A. Viggiano, R. A. Morris, F. Dale, J. F. Paulson, K. Giles, D. Smith,
and T. Su, J. Chern. Phys. (in press).
16M. J. Travers, D. C. Cowles, and G. B. Ellison, Chern. Phys. Lett. 164,
449 (1989).
17 M. McFarland, D. B. Dunkin, F. C. Fehsenfeld, A. L. Schmeltekopf, and
E. E. Ferguson, J. Chern. Phys. 56, 2358 (1972).
18 R. Marx, G. MaucIaire, F. C. Fehsenfeld, D. B. Dunkin, and E. E. Fergu-
son, J. Chern. Phys. 58, 3267 (1973).
19T. Su and W. J. Chesnavich, J. Chern. Phys. 76, 5183 (1982).
2°T. F. Magnera and P. Kebarle, Ionic Proc. Gas Phase xx, 135 (1984).
21 J. F. Paulson, J. Chern. Phys. 52,959 (1970).
22 A. A. Viggiano, R. A. Morris, and J. F. Paulson, J. Phys. Chern. (in
press).
23 R. A. Morris, A. A. Viggiano, and J. F. Paulson, J. Chern. Phys. 92, 2342
(1990).
24 J. Hacaloglu, S. Suzer, and L. Andrews, J. Phys. Chern. (in press).
2SC. A. Deakyne, (personal communication).
26 A. A. Viggiano, R. A. Morris, C. A. Deakyne, F. Dale, and J. F. Paulson
(work in progress).
J. Chern. Phys., Vol. 92, No.6, 15 March 1990
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1.345389.pdf | Phase determination and spatial distribution of an ionbeam mixed internal interface:
Fe/Sn
J. H. Sanders, D. L. Edwards, J. R. Williams, and B. J. Tatarchuk
Citation: Journal of Applied Physics 67, 3121 (1990); doi: 10.1063/1.345389
View online: http://dx.doi.org/10.1063/1.345389
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Published by the AIP Publishing
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34Phase determination and spatial distribution of an ion .. beam mixed internal
interface: FefSn
J. H. Sanders,a) D. L Edwards,b) J. R. Williams,b) and B. J. Tatarchuka),C)
Auburn University, Montgomery, Alabama 36849
(Received 20 September 1989; accepted for publication 8 December 1989)
Ion-beam mixing of tin on iron provides corrosion protection against high-temperature
oxidation. Previous studies have been inconclusive as to the exact composition and distribution
of alloys produced at the Fe/Sn interface. This study provides a detailed diagram of Fe-Sn
specimens after ion-beam mixing with Ar-t at 40 keY and a dose of 5 X 1016 ions!cm2• The
interface was isotopically labeled with 7.5 nrn of 57Pe and Il9Sn so that dual perspective
conversion electron Mossbauer spectroscopy could be performed. Analyses in this manner
allowed comparison of 119Sn conversion electron M6ssbauer spectroscopy (CEMS) and 57Pe
CEMS spectra to accurately assign spectral components which could not be conclusively
assigned using a single CEMS perspective. Information from Rutherford backscattering
spectrometry confirmed the layered nature of specimens prior to implantation and was used
for depth determination of the mixed region after implantation. X-ray photoelectron
spectroscopy, secondary ion mass spectrometry, and scanning electron microscopy also
provided information after implantation. Data indicate the formation of a uniform amorphous
surface during implantation resulting in a heterogeneous mixture of components consisting
mainly of dilute tin in iron (approximately 8~ato % Sn) and FeSnx (x;:::: 1). About 80% of the
37.S-nm tin overlayer was removed by sputtering. The components identified are somewhat
more iron rich than previous assignments and illustrate the difference in surface structures
resulting from various implantation parameters.
I. INTRODUCTION
The application of tin to the surface of iron is known to
retard high-temperature oxidation. The most traditional
method of application is tin plating where FeSn2 alloys have
been identified at the tin/iron interface and serve as a barrier
between the iron substrate and the oxidizing environment. 1
More recent applications of tin include (1) So ~ implanta
tion into the iron surface and (ii) radiation enhanced diffu
sion (RED) of tin overlayers by inert ion implantation
through the tin/iron interface. Both these procedures lead to
formation of an amorphous surface region based on recent
studies. 2-7 Attempts to understand the phases present within
the amorphous regions ofthe Fe-Sn system have been under
taken in an attempt to identify the mechanism(s) responsi
ble for the oxidation resistance of these specimens. S-IO
Dionisio et a/,ll. used Rutherford backscattering spec
trometry (RBS) and 119Sn conversion electron Mossbauer
spectroscopy (CEMS) to analyze iron specimens that were
implanted with Sn t-ions or that had undergone RED. Their
results show the formation of compounds that correspond to
phases expected from the equilibrium phase diagram. RED
produced predominantly FeSn2 after implantation which
decomposed to FeSn near 673 K. Specimens implanted with
Sn -+ yielded ll9Sn CEMS spectra interpreted as amorphous
iron materials containing dilute Sn « 8.5 at. %) which,
similarly to RED, decomposed to FeSn near 700 K. Both
a) Department of Chemical Engineering.
b) Department of Physics.
c) To whom correspondence should be addressed. procedures contained an unidentified component after ther
mal treatments above approximately 773 K which was ex
plained as tin segregation to grain boundaries within the iron
substrate.
Electron microscopy and x-ray diffraction were used by
Gratton and co-workers II to study ion-beam mixing of a
lOO~nm tin tUm on iron. Their results indicated the presence
offine microstructures and FeSnz whiskers that did not pro
vide contiguous surface coverage.
The previous studies on Sn + implantation and RED
provide impetus for further efforts to better clarify the
phases produced after ion-beam mixing. One unique aspect
of this study is the use of multilayer specimens containing an
isotopically labeled tin/i.ron interface so that interfacial mix
ing can be probed. The use of enriched 57Fe and 11<JSn al
lowed CEMS in addition to RBS to be performed on both
aHoy components to assist in data interpretation. Enriched
layers near the interface between iron and tin were kept thin
(i.e., 7.5 nm) to eliminate CEMS spectral contributions
from elsewhere in the film. X-ray photoelectron spectrosco
py (XPS) and static secondary ion mass spectrometry
(SIMS) were also used for measurements of surface proper
ties and to assist in building an overall profile of the specimen
after ion-beam mixing.
Ii. EXPERIMENT
A. Sample preparation
Specimens were prepared in an evaporation chamber
operated at a base pressure of -1 X 10 -1 Pa. Materials were
evaporated onto quartz substrates using two independent 3-
3121 J. Appl. Phys. 67 (6), 15 March 1990 0021-8979/90/063121-11$03.00 @ 1990 American Institute of Physics 3i21
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34k W electron beam guns. Substrates were 3. 81-cm-diam disks
CW. A. Sales Co.) that were precleaned with organic sol
vents and heated to 500 K in vacuum immediateiy prior to
evaporation to remove adsorbed gases. Controlled evapora
tion of materials and measurements of film thicknesses were
obtained using a calibrated Inficon XTC crystal monitor and
a pneumatic shutter assembly.
Figure 1 depicts a specimen after preparation. Table I
lists the treatments each substrate was subjected to and will
be used for a reference throughout the remainder of this pa
per. Iron and tin were obtained from Oak Ridge National
Laboratory in the form of Fe203 and Sn02 and were re
duced in 101-kPa H2 at 1000 and 450 K, respectively, for
·~3h.
Table I shows the preparation steps involved in the fab
rication of each specimen. The first step consisted of succes
sive evaporations of 60 nm of 56Fe (99.9%) at 0.3 nm/s
followed by 7.5 nm of 57Fe (67.9%). These evaporations
were performed using a tungsten boat for 57Fe and carbon
crucibles for 56Fe, 119Sn, and I ;RSn. Following evaporation of
the 57Fe layer, the specimen was transferred to the CEMS
chamber using a high-vacuum sample transporter. Inspec
tion of the 57F e CEMS spectrum confirmed specimen purity.
The specimen was then reduced in 133-Pa H2 at 573 K for 15
min in a UHV compatible quartz reactor vessel [Table I,
treatment (b) 1. The reduction was performed to remove
dissolved oxygen and obtain a well-annealed substrate prior
to tin evaporation. Once again, the specimen was transferred
to the 57Pe CEMS chamber for anaiysis before being re
turned, via the sample transporter. to the evaporation
chamber. Sample preparation was completed by application
of 7.5 nm of 119Sn (84.5%) and 30 um of IIRSn (97.1%,
< 1.0% !lYSn) at an evaporation rate of -0.3 nm/s [Table
I, treatment (c)]. The interface depth of37.5 nm was locat
ed so as to be near the mean depth for the Ar -t implantation
T l1SSn (91.1%) and1H1Sn «1.0%) So.Gnm
f----------------------i +
119Sn (84.5%) and118Sn (15.0%) 15
1----------------1 -f
57Fe (67.9%) and 56Fe (30.2%) 'l'1i run
f----------l t-
SO.Gnm I . , S6Fe (99.9%) and5'rFe «0.1%)
~~""J""7'77~~~..,...,I ~
FIG. 1. Fe-So specimen after completion of sample preparation procedures.
3i22 J. Appl. Phys., Vol. 67, No.6, 15 March 1990 TABLE 1. Fc-Sn specimen treatments.
(al Evaporation u[60.0 mn of "'Fe followed by evaporation of
7.Snffi of'7Fe (67.9%).
(b) Reduction in 133-Pa II, at 573 K for 15 min.
(c) Evaporation of7.5-nm 119S11 (84.5%) followed by evaporation
of 30.0 !lm of 1 "SI1.
(d) Ion beam mixed using 5 X 10'" Ar·' ion~/cm2 at 40 keY, T from
normal.
parameters employed, while remaining sufficiently close to
the surface to obtain high count rate CEMS spectra.
Ion implantation was performed by nco Corporation
[Table I, treatment (d) 1 following initial CEMS and RBS
analyses. The specimen was implanted with 5 X 101b Ar +
ions/cm2 at 40 keY with the beam T from normaL These
conditions yield a calculated mean depth at 31.8 nm with a
standard deviation of 23, 1 nm. The ion current was held at
13 ,uA/cm2 throughout the implantation procedure to keep
the substrate temperature at <;500 ± 25 K.
B. Analysis
CEMS spectra were collected in a UHV chamber oper
ated at ~ 1 X 10 8 Pa. Seven spiraltron electron detectors
aHowed simultaneous collection of independent spectra
which were summed to increase effective counting rates. A
200-mCi '7Co/Pd source was used to collect 57Fe CEMS
data and a lS.2-mCi CaSn02 source was used to collect Il9Sn
CEMS data. A Doppler shift was applied to the source in the
constant acceleration mode with positive velocity defined as
the source approaching the absorber. Zero velocity was re
ferenced to the centroid of a metallic iron spectrum for '7Fe
CEMS or to the center of mass of a Sn02 singlet for 119Sn
CEMS. An spectra were recorded at room temperature and
fit using a Lorentzian curve fitting routine. Further details of
the apparatus and the data fitting procedure are described
elsewhere. 12
R BS spectra were acquired using a 2.0-MeV alpha parti
cle beam from a 3.2-MeV Dynamitron accelerator. The
beam was defined by two O.8-mm-diam apertures located 1.5
m apart. Typical beam currents were 8.0 nA with the beam
normal to the sample surface. Scattered alpha particles were
detected with a surface barrier detector located 5.24 em from
the target and positioned at an angle of 1600 with respect to
the direction of the incident beam. This detector subtended a
solid angle of 5.35 msr at the position of the target. Each
spectrum was normalized to an integrated beam current of
8.0X 10 -bC which corresponds to a total of5.0X 1013 singly
charged alpha particles incident on the sample.
Evaluations of specimens after implantation were also
performed using XPS, static SIMS, and SEM. XPS spectra
were collected using an aluminum anode in a Leybold-Her
aeus LHS-l 0 system operated at -1 X 10 -8 Pa. The analyz
er work function was calibrated against lattice oxygen in the
specimen at a binding energy of 5 31.0 e V. Static SIMS scans
were obtained using a Leybold-Heraeus QMG-511 quadru
pole system over a range from 0 to 250 amu. An Ar -I ion
Sanders et al. 3122
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34FIG. 2. 57Fe CEMS spectra obtained after corresponding treatments listed
in Table 1.
beam with a current of 1 nA/cm2 was rastered over a l-cm2
area on the specimen at a base pressure of ~ 1 X 10 H Pa to
ensure only minimal surface damage. Scanning electron mi
crographs were obtained using an lSI Model 5540 SEM op
erated at a beam energy of 5 kV.
TABLE II. 57Fe CEMS parameters for spectra shown in Fig. 2.
8 ill. RESULTS
A. Before implantation
1. 57Fe conversion electron M(jssbauer spectroscopy
The 57Fe CEMS spectra collected during sample prep
aration [Table I, treatments (a)-(c)] are shown in Fig. 2
and the spectral parameters listed in Table II. Figure 2(a)
shows the spectrum collected immediately after evaporation
of both the 56Fe and 57Fe layers. The sextuplet with a hyper
fine field of 330 kOe and an isomer shift of 0 mmls is repre
sentative of metallic iron. The peak width (FWHM) is 0.72
mm/s and the total resonant spectral area is 29.3 %mrnls.
The spectrum shown in Fig. 2 (b) was collected after
reduction/annealing ofthe above specimen as listed in Table
1. Again a sextuplet is present representative of metallic iron;
however, the FWHM decreased to 0.58 mmls. This reduc
tion indicates the production of a more uniform specimen.
The total resonant spectral area increased to 33.7 %mm/s
indicating that the reductionl annealing procedure solidified
the iron films resulting in an increased recoil free fraction for
57Fe nuclei. 13 Had interditfusion of the two iron layers oc
curred, the decrease in surface s7Fe would have decreased
the total resonant spectral area.
Figure 2 (c) shows the spectrum collected from the
specimen upon completion of the evaporation procedure
l Table I, (c) ]. Once again, a sextuplet representative of me
tallic iron is the only component present in the spectrum.
The total resonant area decreased to 7.2 %mm/s, which is
expected since the tin coating attenuates resonant back
scattered electrons. 12 The FWHM remains constant at 0.57
mm/s, providing evidence against the formation of iron-tin
alloys during evaporation. Any broadening of the peaks
could be explained as interfacial alloying or mixing which
would cause a change in the magnetic moment at the 57Pe
l' f).EQ HF Relative area TRSA"
Spectrum Component (mm/s) (mm/s) (mm/s) (kOc) (%) (%mm/s)
BefOre implantation
2(a) Fe" 0.0 0.72 0 330 100.0 29.3
2(b) Feo 0.0 0.58 0 330 100.0 33.7
2(e) Fe" 0.0 0.57 0 330 100.0 7.2
After implantation
2(d) Fe~, FeSn alloys. 0.44 0.99 0.87 0 26,4 ± 2.5 14.6
FeSn, (l <x<2),
Fet! crystallites
feSn «8.5oat. % Snl
O-nn Sn atoms 0.0 0.79 0 322 35.2 ± 2.5
1,2-nn Sn atoms 0.3 0.95 0 300 38.4 ± 2.5
Trumpy et al., Ref. 14 (data obtained at 77 K)
FeSn (S-aL % Snl
O-nn Sn atoms 0.0 NRh NR 343 51.0' NR
1-nn Sn atoms 0.1 NR NR 321 36.0 NR
2-nn Sn atoms 0.2 NR NR 298 11.0 NR
"TRSA (total resonant spectral area) .
"NR (not reported).
"Reported as percent intensity.
3123 J. AppJ. Phys., Vol. 67, No.6, 15 March 1990 Sanders et al. 3123
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34FlG. 3. \\gSn CEMS spectra of an Fe-Sn specimen after (a) preparation
[see Table I, (ell and (b) implantation [see Tabk, r, Cd)].
nucleus. Even several monolayers of iron interacting with tin
in such a manner would have been detectable as it would
have represented a significant portion of the original 7.5 nm
of 57Fe deposited.
2. 11SSn conversion electron Mossbauer spectroscopy
Figure 3 (a) shows the II "Sn spectrum obtained after
completion of the evaporation procedure [Table 1, (c) J. The
spectra! parameters determined after fitting are listed in Ta
ble Ill. The singie peak has an isomer shift of 2.56 mm/s
indicative ofmetaHic tin. The FWHM is 1.59 mm/s, which
is large with respect to 57Fe peak widths but is in good agree
ment with other I1'JSn Mossbauer studies. '1,14 The increased
peak width for 119Sn Mossbauer resonance is justified by not
ing that the naturallinewidth of mSn is 0.626 mm/s while
TABLE HI. i \"Sn CEMS parameters for spectra shown in Fig. 3.
Spectrum Component
3(a) Sno
3(b) Su01
FeSn( ;::S-a\. % Sn)
FeSn, (l < x < 2, x;:: !)
Reference
23
24
14
14
8
17,18
'TRSA (total resonant area).
bNR (not reported).
"Reported as percent intensity. FeSn
FeSn
FeSn
FeSn,
FeSn2
a-FexSu\ ,
(x;::O.53) D
(mm/s)
2.56
0.0
1.19
1.77
1.76
1.82
L99
2.17
2.24
2.10
",,0.5
3124 J. Appl. Phys., Vol. 67, No.6, 15 March 1990 r
(mm/s)
1.59
1.09
3AO
3.30
NRh
NR
NR
NR
NR
NR
NR that for 57Pe is only 0.192 mm/s.13 The naturallinewidth is
twice the Heisenberg linewidth and represents the minimum
peak width obtainable from a Mossbauer spectrum.
3. Rutherford backscattering spectrometry
Figure 4 displays the RBS spectrum obtained prior to
ion-beam mixing (solid curve). The peak at ~ 1710 keY is
from the tin layer and the peak at -1420 keY is from the
buried iron layer. Table IV lists the calculated front surface
energies of appropriate elements. 15 The front surface energy
is the highest energy obtainable for an element and is defined
as the energy corresponding to an alpha particle that scatters
from the front surface of the sample and therefore experi
ences no energy loss in the matrix. The tin peak extends to
1755 keY which correlates with the value in Table IV, verify
ing its presence at the front surface. The iron peak, at -1490
keY, is about 25 keY less than the front surface energy in
Table IV and is consistent with the energy loss expected from
the 37.5-nm tin overlayer.
Back surface tin and front surface iron peaks have near
vertical sides, consistent with a clean, unmixed interface.
The shoulder visible on top of the tin peak reflects the slight
difference in scattering efficiency for the I1'lSn and 118Sn lay
ers. The difference comes from the slight increase in the kine
matic factor associated with a heavier isotope. The kinemat
ic factor depends on the mass of the incident particle, the
mass of the sample particle, and the scattering angle. 15 This
asymmetry is not apparent for iron because of reduced reso
lution due to energy loss experienced by alpha particles scat
tered from further below the surface.
Other features are arso evident in the spectrum. The
small peak at ~ 1800 keY correlates with tungsten at about 1
at. % within the 57Fe enriched region. This impurity was
likely introduced during evaporation from a tungsten boat;
however, it is not believed to be of sufficient quantity to af
fect the results presented here. The structure below 1200 ke V
is from silicon and oxygen which comprise the quartz sub-
b.EQ HP
(mmlsl (kOc)
0.0 (J.a
O.G 0.0
3.5(J 0.0
1.79 0.0
3.2 0.0
1.7 28
0.0 0.0
0.0 49
O.D 33
0.0 25
NR NR Relative area
(%)
100.0
9.6:1: 4.0
34.8 :L 3.5
55.6 ±_ 3.9
100
100
67"
33"
100"
100
NR TRSA"
(%mm/s)
Sanders et al. 6.4
23.1
NR
NR
NR
NR
NR
NR
NR
3124
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34FIG. 4. RRS spedra of an Fe-So specimen after (i) preparation l Table I,
(c) (solid line) J and (ii) ion-beam mixing [Table I, Cd) (dot-dash line) J.
strate. The small peak at 720 keY is believed to be from
oxygen impurities within the topmost 118Sn layer.
B. After implantation
to 57Fe conversion electron Mossbauer spectroscopy
Ion-beam mixing resulted in the 57Pe CEMS spectrum
shown in Pig. 2 (d). The spectrum was fit using the superpo
sition of two sextuplets and one doublet, which together
yield a total resonant spectral area of 14.6 %mm/s. The two
sextuplets shown below the fit data arise from dilute tin in
iron which was driven into the 57Pe enriched layer during the
mixing procedure. This explanation is suggested by noting
the data of Trumpy et al. 14 for FeSn (Is-at. % Sn), which
were collected at 71 K and are listed in Table II.
Sextuplet (1) in Fig. 2 (d) is a sextuplet corresponding
to O-nn Sn which has a hyperfine field of 322 kOe at 298 K,
an isomer shift of 0 mmls, and a PWHM of 0.79 mm/s.
Sextuplet (1) comprises 48% of the total resonant area COil
tained in sextuplets (1) and (2), which is slightly less than
the 51 % value reported by Trumpy et al. Trumpy et al. re
ported a value of 343 kOe at 77 K which can be adjusted to
337 kOe at 298 K using mean field theory. 16 The reduced
value ofthe hyperfine field is most probably a result of amor
phization caused by ion-beam mixing.
TABLE IV. RES front edge surface energies.
Energy-' Corresponding
Element (keV) channel no. b
Sn 1755 878
'"Pe 1514 758
'"Fe 1522 762
Si 1146 574
0 742 373
C 521 262
Ar 1354 678
W 1838 919
"Reference 15.
"Energy-channel number calibration equation; energy (keV) ~~ 2.0053
X channel no. -5.3476.
3125 J. Appl. Phys., Vol. 67, No, 6.15 March 1990 Sextuplet (2) in Fig. 2 (d) corresponds to both 1-and 2-
nn Sn atoms. Only one sextuplet was used for simplicity in
fitting which is justified since 2-nn Sn accounts for only 11 %
ofthe intensity reported by Trumpy et al. 14 The area of the
sextuplet (2) is 52% of the total sextuplet peak area which is
near that for both 1-and 2-nn Sn atom components reported
by Trumpy et al. (4.7%). The FWHM is broad with a value
0[0.95 mm/s since the single sextuplet represents two differ
ent contributions.
The third component used to fit the spectrum is a quad
rupole doublet with an isomer shift of 0.44 mmis, a quadru
pole splitting of 0.99 mm/s, and a FWHM of 0.87 mm/s.
This component comprises 26.4 ± 2,5% ofthe total spectrai
area. The fitted spectral parameters indicate the presence of
Fe + 3; however, this velocity range is also common to Sn
rich amorphous FeSn aIioys, FeSnx (l < x,;;;; 2 ),17-20 and
magnetically relaxed iron crystallites with mean diameters
less than -4 nm (isomer shift of 0 mm/ s). 2l Clarification of
components within this velocity regime requires insight
from the data of other techniques yet to be presented.
2. 1195n conversion electron !lAossbauer spectroscopy
Figure 3 (b) shows the 119Sn spectrum after ion -beam
mixing. The fitted spectrum was obtained using three com
ponents and has a total resonant spectral area of 23.1
%mm/s. A singlet at 0 mm/s with a FWHM of 1.09 mmls
provides evidence for 5n02 (Figure 3(b), component 1]
which comprises 9.6 ± 4.0% of the total spectral area.
Component (2) is a broad doublet with an isomer shift
of 1.19 mm/s, a quadrupole splitting of 3.5 mm/s, and a
FWHM of 3.4 mm/s. This doublet contains 34.8 ± 3.5% of
the total spectral area and arises from dilute tin in iron. Com
parison to data obtained by Vincze and Aldred22 show that
this component represents dilute tin in iron of nearly 8 at. %,
dose to the maximum solubility limit of 8.5 at. % deter
mined by Trumpy et al. at 1343 K. 14 Typically, a sextuplet is
used to represent this component but the low resolution of
119Sn CEMS compared to 57Fe CEMS, and disorder caused
by implantation, allow adequate representation usi.ng a
broad quadrupole doublet. For such solutions, Vincze and
Aldred's data possess a hyperfine field of 55,7 kOe with an
isomer shift of 1.44 mm/s, consistent with the data of
Trumpy et ai. for FeSn (8-at. % Sn). More dilute solutions
of tin would have larger hyperfine fields of up to 75.0 kOe for
2.1-at, % Sn, which would yield significant spectral area as
low as -4 mm/s and as high as + 6 mm/s. No evidence for
resonant material at these velocities is present in Fig. 3(b).
Component (3) is also a quadrupole doublet and has an
isomer shift of 1.77 mm/s, a quadrupole splitting of 1.79
mm/s, and a FWHM of 3.3 mm/s. This doublet comprises
55.6 ± 3.9% of the total spectral area and indicates the for
mation of Sn-rich FeSn aHoys, e.g., FeSnx ( 1 < x < 2, x:.:::; 1).
Spectral parameters for PeSn and FeSnz have been reported
in the literature, as shown in Table III, and generally indi
cate a narrow hyperfine field between 25 and 49 kOe. The
isomer shift is consistent with FeSn and comparison to spec
tra obtained by Rodmacq et al.17 gives additional evidence
supporting the assignment.
Sanders et at. 3125
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:343. Rutherford backscattering spectrometry
The RES spectrum obtained after ion-beam mixing is
shown in Fig. 4 by the dot-dash line. The most substantial
change occurring as a result of the mixing procedure is a
decrease in yield and area associated with the tin peak. Based
on before and after area ratios, 80 ± 5% of the tin layer is
removed while only about 5% of the iron is lost.
It is noteworthy that the tin peak loses significant yield
without showing evidence for an asymmetric tail at lower
energies. This is interpreted as a reduction in atomic density
throughout a uniform region. The peak width after ion-beam
mixing is roughly 67% of that obtained prior to implanta
tion, indicating that tin now occupies a reduced region ex
tending from the surface. The loss of overlayer tin is also
evident by the + 30-keV shift in the iron peak. Quantifica
tion of the tin depth involves calculations based on iron and
tin density information determined by other analytical tech
niques and is discussed later.
Close examination of the iron peak with reference to
Table IV shows that iron is now present at the surface. The
loss of tin improves resolution so that a small step at ~ 1522
keY is now evident corresponding to the 57Fe front surface
energy. The high-energy side of the iron peak at 1514 ke V
corresponds to the front surface energy for 56Pe. A ratio of
2.6 ± 0.1 for surface iron to surface tin was obtained by
methods discussed elsewhere. 25
Various other features in the spectrum should also be
noted. The peak at 1336 keY is consistent with argon re
tained during implantation. The increase in yield on the low
energy side of the argon peak reflects the skewed depth dis
tribution resulting from implantation with simultaneous tin
removal from the surface. The tungsten and oxygen peaks
seen earlier have been partially removed with the tin layer.
4. X~ray photoelectron spectroscopy
XPS after ion mixing (Fig. 5) shows oxidation of both
tin and iron within the topmost 3 nm of the surface. The Fe
2P3/2 peak is located at a binding energy of 711 e V, charac
teristic of Fe f-3. No metallic iron at 707 eV was detected.
The Sn 3ds/2 peak is asymmetric, and when deconvoluted as
shown in Fig. 4(b) is comprised of two components. The
peak at 486.4 e V indicates Sn02 formation and accounts for
75% of the area. The remaining 25% is contained by the
peak located at 484.7 eV, indicating zero valent tin. Correct
ing for the appropriate cross sections and escape depths pro
vides a Fe ~ 3/811 I-4-ratio of 2.3 ± 0.5.
5. Secondary ion mass spectrometry
Static SIMS was used to determine the amount of mix
ing of the 118Sn layer with the 119Sn enriched layer. Assuming
the same sputtering yields for each isotope, the intensity ra
tio of the 118-and 119-amu peaks provides the relative abun
dances of each. The 119Sn/1l8Sn ratio was found to be
1.4 ± 0.1 revealing that 58% of the tin on the surface is
[19Sn. Investigation of the relative abundances of 56 Fe to 57Fe
could not be performed because this mass range was ob
scured by hydrocarbon fragments typically encountered
after exposure to air.
3126 J. Appl. Phys., Vol. 67, No.6, 15 March 1990 XPS -Mter Mixing
Ife+3
So .....
500 BINDING ENERGY (eV) 430
FIG. 5. Fe 2P11) 1/) and Sa 3d3i2 '12 XI'S scans obtained after Ar + iOIl
beam mixing [Table I, (d) J.
6. Scanning electron microscopy
SEM micrographs of the surface after ion-beam mixing
showed the surface is smooth with no evidence of whisker
formation as seen by Gratton and co-workers. II The texture
of the surface was uniform over the entire area of the speci
men.
IV. DISCUSSION
To obtain an understanding of the ion-beam mixing pro
cess, an overall diagram of the specimen was constructed.
The flow diagram shown in Fig. 6 represents a simplified
procedure by which compound identification and material
balances were used to construct the overal1 profile of the
specimen shown in Fig. 7. The sequence of numbered blocks
dictates the order in which data were analyzed with the most
compelling and informative data being analyzed first.
A. Block 1
The morphology and composition of the specimen be
fore ion-beam mixing were known from sample preparation
procedures (Table O. RBS verified layer thicknesses and
57Fe and 119Sn CEMS verified the metallic, unmixed nature
of the interface. Metallic densities for iron and tin were used,
along with known thicknesses and isotopic abundances, to
determine the total number of 57Fe and 119Sn nuclei within
the specimen.
Sanders et al. 3126
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34start
1. composition
Prior to
Mixing
G,C
2. Specimen Texture, Initial Parameters
and composition
After Mixinq
F,e,E
DfE,C,A,B
5. Amorphous Region:
Components and Information ~
A. 57Fe CEHS E. SIMS
B. 1198n CEHS }' . SEM
C. RES G. Micro-
Balance
D. XPS
Material Balances r------------,
Initialize
for Total
Fe and Sn
After Kixing
Fe and Sn atoms
in each component
volUmetric Percentagesl----p..j
8,1. Subtract Fe and 5n
AtOlilS for Each
Component from the Initial AlIIOWlts
Obtained After Mixing
No "-____________ -1
6. OVerall Profile
(Figure 7)
Is all Fe
and Sn assigned?
Does 57Fe enrichment agree with
CEMS theoretical Bodel for spectral area?
Does mixed layer depth agree with
RES depth determination
method?
stop FIG. 6. Flow diagram demonstrating the procedure used to
obtain an overall profile of an Fe-Sn specimen after ion-beam
mixing.
B. Block 2
To construct a detailed diagram of the specimen after
mixing, certain initial parameters and general concepts of
specimen morphology must be known. SEM micrographs
did not contain evidence of FeSu2 whiskers as seen by Grat
ton et al., where mixing was performed using 1 X 1011 N-t I
cm2 at 100keV and 20f-lA/cm2• SEM micrographs obtained in this study were similar to those of Giacomozzi et al.26 for
ion-beam mixing of tin on nickel at 5 X 1015 Xe + ions/cm2 at
100 keY and 7 pA/cm2• Their samples were characterized
by a smooth appearance with some residual tin grains. The
lack of FeSn1 whisker formation in this study can be attrib
uted to the use of lower implant energies which likely does
not impart sufficient energy to promote surface whisker
growth. Nevertheless, data from the techniques employed in
3127 J. Appl. Phys., Vol. 67. No.6. 15 March 1990 Sanders et at. 3127
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34Surface Region
Fe+3, 65% and Sn+4, 35%
Amorphous Region
FeSn
FeSnx {1<x<2, ""'1)
Feo crystallites 63%
28%
9% -r
1.4nm
~l
23.4nm
50.5nm
~~~~"'77'7Il
FIG. 7. Overall profile of an Fe-Sn specimen after ion-beam mixing as listed
in Table I, (d).
this study reveal a highly amorphous, uniform interfacial
region.
The total number of 57Fe and 119S11 nuclei contained in
the specimen after ion-beam mixing are needed to initialize a
material balance which keeps track of ail assigned atoms.
The total number of 119Sn atoms present after mixing was
determined by assuming an enrichment of58% correspond
ing to the static SIMS ratio of 1.4 and assuming that only
20% of the original 37.5 nm of tin (11 gSn and 119Sn layers)
remained. The total number of 57Fe atoms after mixing was
determined by assuming 5% iron removal (RRS data) from
the 67.9% enriched 57Fe layer.
C. Block 3
1. Conversion electron M08sbauer spectroscopy
After ion-beam mixing, both 57Pe and 119Sn spectra con
tain uniquely identifiable components as well as regions sus
ceptible to overlap with various other species. The 57Fe nu
clei undergoing magnetic hyperfine splitting result from
dilute tin in iron at -8-at. % Sn. The sextuplets used to fit
the data have 0, 1, and 2 nearest-neighbor tin atoms accord
ing to Trumpy et af. 14 The hyperfine fields are ~ 15 kOe less
than the literature values, when corrected for temperature,
and can be explained as a result of amorphization and disor
der. The literature values were obtained from a homoge
neous sample made by diffusion of tin into polycrystailine
iron, whereas our specimen was ion-beam mixed, which is
known to cause severe structural damage.2-7 Rodmacq
et al. IR showed that amorphous compounds of Fe7,Sn25,
Fe60Sn<!i)' and FesoSnso formed by evaporation onto cold
substrates had similar hyperfine fields to crystalline Fe3 Sn,
FeJSnz, and FeSn, respectively, yet amorphization caused
3128 J. Appl. Phys., Vol. 67, No.6, 15 March 1990 by implantation is likely to produce smaller domains, there
by reducing the hypemne field.27 Reference to Rodmacq et
al. suggests that slight broadening of the FWHM for sextu
plet (1) should be expected as a result of disorder. Addi
tional evidence supporting the assignment proposed above
are (i) the broad FWHM of sextuplet (2) due to our mode!
ing of I-and 2-nn Sn atoms with one sextuplet and (ii) the
increase in isomer shift obtained for 1-and 2-nn Sn atoms.
Firm identification of dilute tin in iron provides a means
for deconvoluting part of the 119Sn CEMS spectrum. Since
the enriched layers are adjacent prior to mixing, the mixed
alloys in the interfacial region must contain significant frac
tions of both M6ssbauer isotopes. Therefore, identification
of a component containing tin in a 57Fe CEMS spectrum will
also manifest itself in the corresponding 119Sn CEMS spec
trum. It should be noted that iron-rich alloys contain only
small volumes of tin so that the corresponding spectral com
ponents in the ! 19Sn and 57Fe spectra will not necessarily
reflect the same percentage of their respective resonant spec
tral areas. Based on assignments of the ferromagnetic com
ponents in the 57Fe spectrum, the 1 !<JSn spectrum was fit us
ing a broad quadrupole doublet, component (2), at a
position and quadrupole splitting consistent with literature
values reported for dilute tin ( -8 at. %) in iron. 22
The peak at 0 mm/s in the 119Sn spectrum of Fig. 3(b) is
characteristic of Sn02• This component is also to be expect
ed since the other techniques employed (i.e., RBS, XPS, and
SIMS) indicate that tin from the enriched interface has been
exposed to the surface after ion-beam mixing, Exposure to
air after preparation and mixing produces surface oxides
which typically dominate the topmost 2-3 nm. Using known
hypernne parameters for the SnOz singlet and the dilute
FeSn doubiet, and allowing their intensities to vary, does not
provide a good fit to spectrum 3(b). A third component is
required with the best fit obtained by using the quadrupole
doublet shown by component (3). The parameters in Table
HI, when compared to various literature values for Fe-8n
alloys, allow justification for assigning component (3) to Sn
rich alloys with an approximate composition of FeSnx
(1 < x < 2, x z 1). For this assignment to be valid, corre
sponding evidence in the 57Fe spectrum of Fig. 2(d) should
be visible. This is indeed the case by noting that such
alloys are known to be antiferromagnetic appearing at -O.S
mm/s.17 The limit of x> 1 arises because amorphous FeSn
has been shown to be ferromagnetic, 18 which is not the case
observed here, even though crystalline F eSn is antiferromag
netic at room temperature. Such a component, FeSnx,
would overlap the spectral area associated with Fe + 3 so that
in Fig. 2(d), component (3) must contain contributions
from both components,
2. X~ray photoelectron spectroscopy
XPS reveals chemical state information and allows a
semiquantitative determination ofthe topmost 2-3 nm of the
surface. Since Fe -+ 3 and Sn I-4 were both detected at the
surface, CEMS spectra should also contain these spectral
components. These species are in fact observed so the loca
tion of these features in the CEMS spectra can be assigned to
Sanders at at. 3128
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34the surface region. It is noteworthy that since XPS places
CEMS oxide components at the surface, their CEMS spec
tral areas, when viewed in light of data reported by Zabinski
and Tatarchuk,11 actually represent an inflated portion of
the total area. In other words, the actual amount of Moss
bauerisotopes at the surface (in the form of Fe -+ 3 and Sn +4)
are only about 50% of the areas listed in Tables II and III
because low-energy resonant electrons have a much larger
probability of being detected when they originate dose to the
surface. The quantitative infonnation provided by XPS (i.e.,
Fe + 3/Sn l-4 = 2.3) is used later in the determination of an
overall compositional description of the specimen after mix
ing.
D. Block 4
The surface region was found to be 1.4 nm thick, com
posed of 65% Fe + 3 and 35% Sn + 4 by volume. This was
determined by assuming that 4.8% of the total 119Sn atoms
occupied this region after mixing. This value is half the 9.6%
spectral area found for Sn02 in Table III and was adjusted in
order to correct for surface signal enhancement. The 5n ~ 4
content was determined by correcting the 119Sn concentra
tion for enrichment and then dividing by the density ofSn02•
The Fe -t-3 content was determined using the Fe/Sn ratio of
2.6 found by RES and dividing by the density of Fe20y
Eo Block 5
The ironltin interface, as shown by CEMS, may contain
three distinct components identified as dilute tin in iron
(PeSn), FeSnx (1 <x < 2,x;:::: 1), and PeG microcrystaHites.
In this study RBS reveals that the mixed region contains a
uniform concentration of tin; therefore, a heterogeneous
amorphous layer must result from ion-beam mixing.
1. Dilute tin in iron
The volumetric percentage of FeSn ( -8-at. % Sn) was
determined to be 63% by noting that 34.8% of the ll'lSn
CEMS spectral area originates from dilute tin in iron compo
nents. Using the known atomic concentration of 8% for tin
gives a total iron content which, when reconciled with the
known 57Fe spectral area of73.6%, yields a 37Fe enrichment
of 29.0% after mixing.
2. FeSn x (1 <x <2, x t:::: '1) and iron microcrystal/lies
Of the components in both the ll'lSn and 57Fe CEMS
spectra, the ones comprising the areas designated by FeSnx
are the most difficult to assign. However, in light of previous
depth determinations and overall material balances, a good
evaluation of these components can be made. Performing an
overall material balance, using the total number of iron and
tin atoms in the surface oxide and 57 PeSn layers, yields a
FelSn ratio of2.0 ± 0.2. This ratio suggests that for Sn-rich
alloys to exist, as determined by I! 9Sn CEMS, there may also
exist some small iron crystallites ( < -4 urn). This assign
ment is acceptable based on 57Fe CEMS which has ample
spectral area around 0 mm/s [component (3), Fig. 2(d) 1
and may contain small amounts of Fe203, FeSn~ alloys, and
3129 J. Appl. Phys., Vol. 67, No.6, i 5 March 1990 iron crystallites. Volumetric percentages of28% and 9% of
the amorphous region were determined for FeSux and iron
microcrystallites, respectively.
The amorphous character of the sample portrays effects
of ion-beam mixing that differ from that of Dionisio et ai.8
and Gratton et al.11 Their data show large amounts of initial
FeSnz formation which decompose to FeSn at increasing
temperatures of 673 and 773 K. Both of these studies had
thicker tin films before implantation (80 and 100 nrn, re
spectively), perhaps favoring the formation of tin-rich al
loys.
F. Block: 6
The overaH profile of the specimen is shown in Fig. 7.
The total thickness of the specimen has been reduced from
105.0 to 75.3 nm due primarily to sputtering of the 118Sn
overlayer. An additional material balance revealed a thick
ness of 50.5 nm for the ~6Fe layer below the 23.4-nm amor
phous region.
The atomic densities for both iron and tin in the outer
most 24.S nm were determined by summing the total num
ber of atoms for each component and dividing by the vol
ume. These values are 5.713 X 1022 Fe atoms/cm3 and
1.121 X 1021 tin atoms/em3 and are necessary parameters for
the RBS depth determination that follows.
G. Decision block 1
The questions in the decision block in Fig. 6 must be
satisfied for Fig. 7 to be valid. The foHowing discussion ad
dresses these questions.
1. Material balances
The determination of volumetric percentages for FeSn"
and iron microcrystaHites was based on iron and tin material
balances, requiring all iron and tin atoms to be assigned.
2. Theoretical mode!
The 29.0% 57Fe enrichment obtained after mixing was
confirmed using a depth deconvolution model developed in
our laboratories by Lee and Tatarchuk. 2~-lO This model the
oretically calculates the CEMS spectral area from homoge
neou.s multilayered specimens containing 57Fe nuclei using a
Monte Carlo simulation method.
A correction factor for the 57Fe resonant area obtained
before and after ion-beam mixing was required. This factor
was experimentally determined for our CEMS apparatus, 30
eliminating nonresonant equipment background. This al
lowed a corrected beforc-to-after ratio of 2.5 to be deter
mined.
Modeling of the specimen was performed using 57Fe en
richments, layer depths, layer thicknesses, and atomic densi
ties as input parameters. Physical and nuclear properties of
elemental iron and tin were also necessary. The resonant
calculated areas before and after mixing yielded a ratio of
2.6.
Consistency of the theoretically generated ratio of 2.6
with the experimentally determined value of2.5 confirms an
Sanders et al. 3129
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34enrichment of 29.0% 57Fe throughout the mixed region of
the specimen. It should be noted that a 10% variation in
enrichment used in the model results in approximately a
10% variation in total spectral area. This relationship allows
an estimation of uncertainty for the enrichment of
29.0 ± 1.2% to be determined.
3. RBS depth determination
Estimation of the thickness of a uniform layer at the top
surface of a specimen can be obtained using RBS since the
alpha particle energy before scattering at any depth within
the thin layer is approximately equal to the energy of the
incident beam. This "surface energy approximation" meth
od is discussed in detail by Chu and co-workers15 and is
based on the following formula for a two-component layer
AB:
E = CEo)ABXNAB Xro,
where E is the energy width of a channel (keV Ich), (6()AB
is the alpha particle stopping cross section (ke V X nm2/
atom), NAB is the density (atoms/nmJ), and 'Tn is the thick
ness (nm/ch).
The parameter [Eo lAB for the layer was determined us
ing Bragg's rule and the stopping cross sections, (Eo), of
both iron and tin. Stopping cross sections are based on atom
ic densities determined for the eiements within the specimen
in its final state and are a function of kinematic factors, de
tection angle, and alpha particle energy. An average value of
0.01492 (keVXnm2)/atomfol" (EoV1Bwasdetermined. Sim
ilarly, NAB was determined to be 49.60 atom/nm3• E was
taken to be 2.0053 keY leh as determined from the energy
channel number calibration curve in Table IV.
Solving the above equation for 'To gives a value of 2.71
nm/ch. Multiplication by 10.3 channels, which is the
FWHM of the tin peak after ion-beam mixing, provides Ii
layer thickness of 27.9 nm. This value corresponds well to
the total thickness of 24. IS nm calculated for the surface and
amorphous regions previously and lends credibility to the
assignments shown in Fig. 7.
It should be noted that the procedure discussed in this
study requires considerable calculation based on quantita
tive information obtained from a number of techniques.
Therefore, it is inevitable that uncertainties inherent in these
measurements will propagate throughout the course of the
calculations resulting in some uncertainties in the final
depths assigned to the overall profile shown in Fig. 7. These
errors were minimized, however, by combining results ob
tained using both RBS and CEMS into the depth deconvolu
tion routine.
V. SUMMARY AND CONCLUSIONS
In this study the composition of the amorphous region
obtained after ion-beam mixing of an iron/tin interface was
determined. An isotopically labeled I 19Sn/57Fe interface al
lowed dual perspective CEMS to be used before and after
mixing and provided a vital means for cross-checking spec
tral assignments. RES provided quantitative depth deter
mination as well as quantitative and qualitative front surface
3130 J. Appl. Phys .• Vol. 67. No. 6, ~ 5 March 1990 information. XPS verified the existence of surface iron ox
ides following mixing and helped locate oxide species ob
served by CEMS. Static SIMS provided insight into mixing
of the 118Sn and 119Sn layers while SEM observed no whisker
formation.
The choice of techniques used in this study allowed a
diagram of the specimen to be drawn which is consistent
with 0) independent depth calculations based on RES and
Cii) theoretical models for determining the resonant 57Fe
spectral areas of multilayer specimens. Results suggest the
formation of an amorphous surface region composed of di
lute tin in iron (-8-at. % Sn), amorphous FeSnx (1 < x < 2,
x:;:::;; 1 ), and a sman amount of metallic iron.
This study indicates that ion-beam mixing under var
ious conditions may lead to vastly different structures that
can affect desired properties. For the Fe-Sn system, the
property of interest is oxidation resisiance. The studies cited
in the literature provide a mechanism for ion-beam mixing
beginning with FeSn2 formation when mixed at tempera
tures less than .-700 K. Our results indicate that iron-rich
amorphous alloys may also be precursors for this process.
ACKNOWLEDGMENTS
We gratefully acknowledge support from the Air Force
Office of Scientific Research (AFOSR-84-G-0057) and the
Army Research Office (DoD-DRIP, DAAG28-84-0301).
One of us (I.R.S.) also wishes to acknowledge fellowship
support from the National Aeronautics and Space Adminis
tration Graduate Student Researchers Program (NASA
MSFC) during a portion of this work.
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31, 576 (1988).
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, ..•. ' .••.•.•.• :.:.:.;.; .............. ;.:.:.;.;.;.; •.•.•.•. 0;>....... • .• ~.'.-.-•.•.•.•.•.••••.•••.• :-:.; • .> ••••••••••• :;;:.:.:;;;.;.~.~ ••••• ;.:.:.:.:.:.:.:.:.- ••••••••••••••• ~ ••••• ".~.; •• '., •. ~ .•.•••• > •• ,.,. ••• > •• Reactions (Wiley, New York, 1978), Chap. 6.
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128.59.222.12 On: Thu, 27 Nov 2014 03:41:34 |
1.576925.pdf | Fundamentals of ionbeamassisted deposition. I. Model of process and
reproducibility of film composition
D. Van Vechten, G. K. Hubler, E. P. Donovan, and F. D. Correll
Citation: Journal of Vacuum Science & Technology A 8, 821 (1990); doi: 10.1116/1.576925
View online: http://dx.doi.org/10.1116/1.576925
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Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 137.189.170.231 On: Sat, 20 Dec 2014 18:05:41Fundamentals of ion-beam-assisted deposition. I. Model of process
and reproducibility of film composition
D. Van Vechten, G. K. Hubler, E. P. Donovan, and F. D. Corrella)
Naval Research Laboratory, Washington, D. C. 20375-5000
(Received 10 July 1989; accepted 11 November 1989)
An ion-beam-assisted-deposition (lBAD) system is under development to fabricate Si1_xNx
films for optical devices. Reproducible film composition requires characterization of the
relationship between the incorporated nitrogen atom fraction x and the real time experimental
measurable quantities. In this paper a simple model is presented which relates the film
composition x to the measured beam current density JF, the vapor impingement rate Q, and the
chamber pressure p. Effects included in the model are reflection of energetic particles, sputtering
from the film surface, and charge exchange neutralization of the ions. Each term in the model is
examined as a potential source of both systematic and random deviations of the data from the
model. Data on film composition as a function of the nitrogen ion current to deposition rate ratio
are presented for several sets of ion source voltages and chamber pressures. It is shown that by
modifying the deposition system so as to minimize the identified sources of error, variation in
composition can be reduced below 3 at. % nitrogen. Both the model and the discussion of the
experimental sources of error are applicable to other IBAD systems.
I. INTRODUCTION
Physical vapor deposition (PVD) involves low energy im
pingement of vapor atoms upon the film surface. These
atoms attach to the surface with a sticking coefficient which
is close to unity when the substrate is nominally at room
temperature. At such low substrate temperatures, the atoms
have low mobility and are unable to migrate into the energe
tically most favorable sites. As a result, PVD films are char
acterized by high internal stresses, extended defects such as
voids, and by a columnar grain structure. 1,2
In ion-beam-assisted deposition (lBAD), energetic spe
cies arising from an ion beam are incident upon the growing
film. These may be reflected after a large angle collision with
near-surface atoms or penetrate into the film. The resulting
"knock-on" events and collision cascades collapse voids and
disrupt interfaces3.4 which produce more dense deposits. In
optical films, this results in decreased absorption of water
vapor from the atmosphere and increased stability of the
refractive index.5 The energetic processes also affect film
stress and tend to improve film adhesion to the substrate.5-7
This work arose from an effort to produce "rugate" opti
cal thin film devices by means ofIBAD 10 using substoichio
metric silicon nitride. Control of the film composition to
within 3 at. % was required.8-1O The devices utilize the pre
viously determined9 variation of the index of refraction of
amorphous silicon-nitrogen alloy films from 3.9 to 2.0 as the
nitrogen atom fraction x changed from 0 to 4/7. Figure 1
shows early data of the composition of a number of films,
determined by Rutherford backscattering spectrometry, as a
function of the beam current density at a deposition rate of
10 A/s. There is 15% scatter in the composition at specific
values of ion current which is unacceptable for device fabri
cation. The object of the present work was to better under
stand this behavior, develop methods to reduce the scatter,
and to outline a methodology with which to understand the
IBAD process. A companion paper11 details how parameters in the model
can be experimentally determined. Together, these papers
define the fundamental physical processes which influence
film composition and lay the framework of a model to de
scribe more complicated material systems.
After a description of the initial deposition system, a mod
el for the final film composition is presented. The model
parameters are examined for possible sources of systematic
errors and for insights into the physical causes of random
fluctuations as seen in the scattered composition data of Fig.
1. It is demonstrated that when suitable design changes are
incorporated into the system, the composition of the films
O.B ,----,--,-----,--,-----,---,-----,
0.6
z 0 0
0 ;:: 0 u 0 « 0 0 a: u.. 0.4 oo~o z w
(!) 008 0 a: 0
f-
Z
0.2
350
CURRENT DENSITY (pA/cm2)
FIG. 1. Incorporated nitrogen atom fraction x in Si'_xMx films on Si sub
strates vs Faraday cup current density, scaled to correspond to a silicon
deposition rate of 10 1>../s if done at a different value. Ion source parameters
were VB = 1000 V, VA = 200 V, VD = 50 V and chamber pres
sure = 2.0 X 10 -4 Torr. For this data, the ion source was not aimed in situ
and the ion beam impinged at a 20° angle with respect to the sample normal
which pointed at the e-gun hearth. The curve is model prediction (see the
text).
821 J. Vac. Sci. Technol. A 8 (2), MarlApr 1990 0734-2101/90/020821-10$01.00 © 1990 American Vacuum Society 821
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 137.189.170.231 On: Sat, 20 Dec 2014 18:05:41822 Van Vechten et al.: Fundamentals of IBAD. I
can be controlled to within the required 3 at. % nitrogen.
The model includes a pressure-dependent term for charge
exchange neutralization of the ion beam wherein scattering
with ambient gas molecules neutralizes ions in passage to the
Faraday cups. This model correctly predicts composition
changes with different background pressure. Evidence of im
proved film purity as a result of ion bombardment is also
presented.
II. EXPERIMENTAL
The deposition system which produced the data of Figs. 1
and 4 is schematically shown in Fig. 2. It was housed in a 16-
in.-diameter stainless-steel vacuum chamber enclosed by a
glass bell jar. A base pressure of 2 X 10 -7 Torr was typically
achieved prior to deposition by means of a cryopump config
ured for a pumping speed of ~ 840 cis for air. During film
deposition, a 3-8 sccm gas flow (99.998% pure N2) was
required for stable operation of the ion source. This flow was
controlled by a needle valve and gas-bottle regulator and
resulted in an indicated operating pressure of 2 X 10-4 Torr.
The chamber pressure was measured by an ionization gauge
located on a right-angle port so that there was no line-of
sight path to energetic ions and electrons from the chamber.
A 3 cm Kaufman ion source from Commonwealth Scien
tific Corp. produced the ion beam which impinged on the
substrate at an angle of 20° from the surface normal. The
sample normal was directed at the center of the e-gun hearth.
Collimated graphite dual grids were used as the ion optics.
The ion source power supply was operated in "automatic"
SUBSTRATE
FARADAY
ION
SOURCE
N+
VACUUM
CHAMBER CRYO
PUMP
FIG. 2. Schematic diagram of typical IBAD apparatus which uses a Kauf·
man ion gun.
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 822
mode in which the extracted beam current is held constant.
The beam was space-charge neutralized by means of a fila
ment placed external to the ion source grids. The electron
discharge potential ( V D) was held at 50 V and the potentials
on the inner (VB) and outer (VA) grids were 1000 and
-200 V, respectively. The source housing was rigidly fixed
in the chamber with its symmetry axis aimed at the center of
the sample position. The ion-gun grid to substrate distance
was 21.6 cm. The ion current was monitored by three elec
trostatically suppressed 2.45 mm diameter Faraday cups
that were equally spaced on a 3.14 cm diameter surrounding
the 2.2 cm diameter sample position. The entrance apertures
of the Faraday cup array were in a plane parallel to the sam
ples. The substrates were mounted 0.43 cm closer to the e
gun and ion source than were the Faraday cups to reduce the
likelihood of sputtered particle contamination from Faraday
cup hardware. The extraction grid diameter subtended a
maximum angle of ± T as measured from the substrate and
± 8° as measured from the Faraday cups.
The silicon evaporant was produced by a linear 5 hearth, 4
keV Thermionics electron-gun, with a 1 cc volume per
hearth. Poco graphite hearth liners were used and no scan
ning of the beam spot was possible. A quartz crystal monitor
was rigidly mounted in a plane parallel to that of the sam
ples, 4.3 cm from the sample center and displaced 0.32 cm
further from the e-gun. The quartz crystal was shielded from
the ion beam to prevent heating at high beam currents. An
Inficon XTC rate monitor was used to control the deposition
rate. Values of the film density and "Z ratio" for conversion
of the observed frequency shift to thickness of deposit were
2.33 and 0.712 g/cm3
, respectively. The extrema in the depo
sition rate noise level indicated by the controller at its 4 Hz
update rate was typically ± 0.7 A/s when a 10 A/s rate was
requested.
The samples consisted of one or more layers deposited at a
constant arrival ratio Ra defined as the ratio of the incident
N atom flux/Si atom flux. The layer thickness ranged from
600 to 5000 A. Most were deposited on polished single crys
tal silicon substrates which were sputter cleaned with nitro
gen prior to deposition. The sample to e-gun hearth distance
was 30.5 cm. The sample mounting system was water cooled
and the samples remained below 100°C during the deposits.
Rutherford backscattering spectroscopy (RBS) utilizing
He + at 2 MeV and a scattering angle of 135° was used in
conjunction with the analysis package RUMpl2 to deter
mine the composition of each layer.
III.PHYSICAL PROCESSES
Figure 3 presents the physical processes to be accounted
for by a simple phenomenological model of the IBAD pro
cess. Referring to Fig. 3, the vapor atoms (indicated by a v)
impinge on the growth surface. The N 2+ and N + ions in the
ion beam are either implanted beneath the surface or are
reflected with reflection coefficient r. As a result of ion bom
bardment, some of the deposited atoms are sputter removed
from the surface with sputtering coefficient S, and adsorbed
ambient gas atoms (indicated by G) may be desorbed or
stimulated to chemically react with the surface atoms. Final
ly, in passage through the ambient gas, some of the ions are
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PHYSICAL PROCESSES
20 A
IMPLANTATION 823
IBAD
} GAS REACTION
SURFACE F~ I GAS DESORPTION
®-" c.i", ~ FIG. 3. Schematic representation of
physical processes to be accounted for in
phenomenological models of IBAD.
500 eV N;, N+ 1-t @
I ® If-O.03 eV
I 0.15 eV
Ng. 0--"1 CHARGE
EXCHANGE
N; I ® NEUTRAUZATION
neutralized by charge exchange collisions and therefore are
not counted by the charge collection system. The beam also
contains electrons from space-charge neutralization, but
these are rejected from the beam by the electrostatic suppres
sor in the Faraday cups.
The physical processes described above do not predict the
microstructure of the film. For model purposes it is assumed
that the IBAD process produces an amorphous film with a
density near that of bulk material. It is also assumed that 500
eV N2+ ions break-up upon impact with the surface. There
fore, they are treated theoretically as two, 250 eV N parti
cles.
IV.PHENOMENOLOGICAL MODEL OF IBAD
It is our purpose to relate the film composition and thick
ness to the independently controlled variables of ion current,
deposition rate, and chamber pressure with the processes
mentioned above considered in the model. The model which
follows contains approximations which simplify expressions
that were previously reported.9 Three criteria are necessary
for applicability of the model to experiment. These are: (1)
no incorporation of ambient gas atoms in the film; (2) the
energetic species are ion-implanted into the film which im
plies that their sticking coefficient is (1 -r); (3) no diffu
sion of beam atoms after stopping in the film. Criteria (1),
(2), and (3) imply that the surface is pure Si. All three are
satisfied for 500 e V nitrogen ions incident on silicon at near
normal incidence and for substrate temperatures < 100°C
used in these studies.
The model predicts the composition of synthesized binary
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 alloy films as a function of the impingement ratio Ra. IfF N is
the incident N atom flux and FSi the incident Si atom flux
then,
Ra =FN/F si' (1)
We define the flux of silicon incident on the film as
(2)
where Q is the vapor deposition rate (cm/s) measured by
frequency changes of the quartz crystal rate monitor, NSi is
the atomic density of the film on the quartz (atoms/ cm 3),
and Ye is a dimensionless tooling factor to account for differ
ences in the placement of the substrate and quartz crystal
with respect to the e-gun hearth.
We define the flux of nitrogen incident on the film which
would exist in the absence of charge exchange as
whereJois the current density, eis the electronic charge, Ei is
the number of atoms/ion species (e.g., one for N + and two
for N 2+ , etc. ), n i is the fractional component of each species,
and Yi is a dimensionless tooling factor to account for differ
ences in the beam current density measured at the Faraday
cup and sample positions. The assumption of no significant
charge exchange would be true if ultrahigh vacuum (URV)
conditions prevailed during deposition.
Charge exchange neutralization is a resonant forward
scattering event whose cross section is much larger than for
high angle collisions. It has the effect of transferring an elec-
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trori from a neutral nitrogen in the ambient gas to an energet
ic nitrogen ion without substantially deflecting the path of
the latter. The statistical nature of the process affects the ion
current similarly to absorption oflight, J = Jo exp ( -ad),
where a is an absorption coefficient and d a distance. 13 At an
ambient gas pressure p and temperature T, the measured
current density J F is related to Jo by
(3)
where the absorption coefficient is replaced by (apYp/kB T).
In Eq. (3), a is the charge exchange cross section, I is the ion
source-to-Faraday cup distance, and kB is Boltzmann gas
constant. The parameter YP is a dimensionless pressure tool
ing factor to account for difference in pressure indicated by
the external ion gauge and the actual pressure along the path
between the ion gun and the samples (assumed constant
along the path). For convenience we define the parameter
1 + [3 = exp (alpy / k BT) so that Jo = J F( 1 + [3). Includ
ing all the factors the nitrogen flux becomes
JF FN =-In itiYi(1 +[3i)' e i
where the subscript i has been added to [3 to account for a
possible difference in the charge exchange cross section for
different beam species. The ni are measured at the Faraday
cups.
The experimentally measured film quantities are the aver
age flux: of nitrogen atoms incorporated into the film, F {, ,
and the average flux of silicon atoms incorporated into the
film, F {i' and the atom fraction of nitrogen in the film, x.
The first is obtained by subtracting the fraction ri of the
incident flux that is reflected by the surface for each species,
or
The Si net flux incorporated into the film is obtained by
subtracting the fraction of the deposited flux sputtered by
the ion beam, or
f _ JF FSi -FSi --In itiYi(1 + [3i)Si' e i
where Si is the sputtering coefficient of silicon for each spe
cies in the beam. In the expressions for F {i and F {, it is
assumed that the sticking probability is 1 and 1 -r, respec
tively.
The above expressions are greatly simplified by assuming
that Yi and (1 + [3 i) are equal for all beam species and by
using an average value of the ionic charge per atom given by
(0) = ~initi' Then the nitrogen atomic flux becomes
JF FN = -(o)y(1 + [3). (4) e
It is easily shown that
f JF FN =-(o)y(1 +[3)[1- (r)] =FN(1-(r»), (5)
e
where < r> is the weighted average reflection coefficient,
(r) = ~initiri .
~initi
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 824
Similarly,
f _ JF _ FSi -FSi --(o)y(1 +[3)(S) -FSi -FN(S), (6) e
where (S) is the weighted average sputtering coefficient de
fined by
(S) = ~initiSi .
~iniSi
From Eqs. (1), (5), and (6), the ratio of nitrogen to silicon
in the film R f is given by
F{, FN (1 -(r»)
R f = - = ------,---,-F{ FSi -FN(S) Ra (1 -(r»)
1 -Ra (S) (7)
Finally, the composition of the film expressed as the nitrogen
atom fraction x is
x = --------:---:-----:--
Ra + (1-Ra(S)/I- (r») (8)
It is noted that the parameters [3, (r), and (S) are energy
dependent and (r) and (S) are dependent on the incident
angle of the beam to the substrate. Also, for low ion energies
or for other ion vapor combinations, the variation of (r) and
(S) with changing surface concentration of the ion species
probably cannot be ignored.
V. PARAMETER VALUES
Determination of the absolute value of the parameters
used in Eqs. (1 )-( 8) is described in a separate publication. 1 I
Table I summarizes these parameters for the two deposition
geometries and beam energies used in this work. Table I
shows that the ion beam is composed22 of 11 % N + and 89%
Nt which, for 500 eV beam energy, means that 94% is 250
e V N atoms and 6% is 500 e V N atoms. All of the parameters
in Table I for 500 eV are measured except for (r) and the
constant NSi' Monte Carlo calculations provided the values
of (r). The density of amorphous silicon was used for NSi
which is 98% that of crystalline Si.
The most difficult parameter to determine is [3. Calcula
tion of the charge exchange cross section is complicated by
the deformation of the electronic orbitals during the colli-
TABLE I. Parameter value used to calculate the curves in Fig. 4 (500 eV)
and Fig. 1 (lOOOeV).
VB = 500eV VB = lO00eV
(J 0° 20°
NSi 4.9X 1022 atoms/cm2 s 4.9X 1022 atoms/cm2 s
r, LOS 1.035
r 1.05 1.17
n(N+) 0.11 0.11
n(N,+ ) 0.89 0.89
(8) 1.89 1.89
(r) 0.10 0.08
(S) 0.26 0.38
f3 0.52 0.43
aYp/T 8.17 X 10-22 cm2/K 7.04X 1022 cm2/K
I 0.264m 0.216 m
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sion,14 even for monatomic ions such as argon. For N2+ on
N 2' the transition probability depends on the vibrational and
electronic state of the incident N2+ and the significant
chance of transferring vibrational energy as well as an elec
tron durin~the collision. 14-17 For dischar~e Qotentials below
100 V where many ion sources operate, a significant portion
of the N / beam may be in metastable excited states which
also alter the charge exchange cross section. 18 Also, a decay
of the average ion energy occurs through noncharge ex
change (elastic) scattering events in passage from the ion
source to the substrate at low ion energies « 100 eV).
Harper ef al. 17 quote a cross section for Ar + on Ar gas of
about 10% of the charge exchange cross section for this pro
cess and an energy dependence of E~ 114. At nitrogen ener
gies > 500 eV, the decrease in average ion energy is estimat
ed to be small and we have neglected this effect in our work.
Finally, the charge exchange cross section for N + on N2 has
not been measured and may differ from that ofN2+ on N2.
In order to make modeling of the charge exchange tracta
ble, an effective exchange cross section is used which lumps
all of the above effects into one parameter. This practice
means that Eq. (4) is strictly true only to the extent that the
charge exchange cross sections are equal for N 2+ ground
state and excited state ions and N + ions. We shall see later
that this treatment is consistent with the data. For indicated
pressure of 2 X 10 ~ 4 Torr, /3 = 0.52 which corresponds to
34% of the beam neutralized (for Yp = 1). We also assume
that the energy dependence of the charge exchange cross
section for N2+ and N + on N2 gas is the same as for Ar + on
Ar gas.19
VI. DISCUSSION OF SYSTEMATIC ERRORS
This section will focus on the identification and estimation
of the magnitude of the possible sources of systematic errors
associated with the IBAD process. Each of the factors in
Eqs. (2)-(8) is discussed in turn.
The first term discussed is FSi' Besides sputtering, a sys
tematic way for the ion flux to influence the relationship
between FSi and Q is to change the sticking coefficient on the
sample. However, comparison of the areal densities reported
by the quartz monitor with RBS analysis of low nitrogen
content films indicate that the average silicon sticking coeffi
cients for the sample and the quartz are equal within the
experimental uncertainty. If the prior assumption that the
sticking coefficient of silicon on the quartz crystal (shielded
from the ion beam) equalled one was correct, then the stick
ing coefficient on the sample cannot increase as the beam
current increases. A decrease in the sticking coefficient with
increasing ion flux is unlikely because the surface should
become more pure and more reactive as the energetic flux
increases. Therefore, no systematic deviation is expected
from changes in sticking coefficients for near room tempera
ture deposition of Si.
Deviations caused by systematic error in the deposition
rate Q could arise from several sources. The equation used by
the quartz crystal balance controller software20 to convert
the normalized shift in the resonant frequency F =
(/q -fc )//q to film thickness fl' is
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 -_l_-tan-I(Ztan 1TF).
1TZ( 1 -F) 825
(9)
Equation (9) requires values for the film density PI and the
Z ratio, in addition to the density, thickness., and resonant
frequency of the bare quartz crystal, P q' Tq, and /q, respec
tively, and the resonant frequency fc of the crystal loaded by
the film. It is easily shown that variation in the above param
eters do not lead to error in the thickness measurement of the
quartz crystal mass balance.21 For F(;0.06, Eq. (9) is insen
sitive to the value of Z. This corresponds to about 9 f1 of
silicon having been deposited on a standard 6 MHz crystal.
Our standard practice is to change the quartz crystals after
approximately 3 f1 offilm are deposited, so any inaccuracy in
Z does not influence the accuracy ofthe reported accumulat
ed thickness or deposition rate.
Because of high partial pressures of ion source feed gas
during deposition, the PI used to convert frequency shift to
reported thickness must be chosen with care. It can depend
on the amount of impurity gas incorporated into the film. In
practice, it is useful to calibrate the system by relating an
independent measurement of film thickness and the thick
ness reported on the quartz through the tooling factor Ye and
the density PI'
Next we examine (r), (S), and (0). Both (r) and (S)
depend weakly on angle of the ion beam to the surface nor
mal [approximately (cos 8) ~ 5/3 for S] and on energy (ap
proximately E1I2 G for S). Points of origin of the ion beam
are spread over a 3 cm diameter and the maximum sample
dimension is 2.2 cm. With our ion source to sample dis
tances, the ion beam angular spread is limited to less than 7°.
For our small angles between the geometric axis of the ion
gun and surface normal (0° to 20°), an angular spread of 7°
causes negligible changes in (r) and (S ). In previous work,22
the energy spread of the ion beam was measured to be less
than ± 40 eV for a beam energy of 1000 V. Abrupt plasma
mode shifts in the ion source were found at small values of
beam current extracted from the ion source. These shifts
increased the ion energy distribution, but did not change the
mean energy. Thus, changes in (r) and (S) caused by energy
spread of the ion beam are also negligible. In the same
work,22 the parameter (0) was found to shift from 1.91 to
1.85 as the total extracted beam current I B varied from 10 to
30 mA for a beam energy of 1000 e V and a discharge voltage
VD of 30 V. However, this shift in the species distribution
between N2+ and N + changes the Monte Carlo calculated
weighted average (S) by only 1.2% and (r) by less. The
deviations in (0) can themselves produce a maximum of 1 %
and -2% deviation in composition, and + 0.5% and
- 1 % deviation in film thickness. Thus, changes in the an
gular, energy, and species distributions in the ion beam with
JF are expected to produce less than 2% systematic devia
tions in composition and film thickness.
The next factor in Eq. (7) to be evaluated is /3 as expressed
by Eqs. (3) and (4). The exact value of the charge exchange
parameter /3 is a weighted average of values for N + and N2+ ,
each of which is exponentially dependent on the number
density (partial pressure) of neutral N2 in the volume be
tween the ion source and substrate. A systematic deviation in
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composition or thickness can occur if our effective (J changes
with changes in the species as characterized by (8). To esti
mate the magnitude of this effect, consider the plausible as
sumption that the cross section for charge exchange is three
times smaller for N + than for N2+ . One can calculate that if
(8) varies from 1.91 to 1.85, the effective (J changes from its
average value by + 0.6% to -1.2%, respectively. These
changes in turn cause similar magnitude errors in the pre
dicted composition and thickness. Thus, the assumption
that a single parameter can be used to describe charge ex
change neutralization is justified for error limits on the order
of2%.
A more significant source of systematic deviation is the
focus condition of the ion beam, which depends on J F' While
the effect has not been studied in detail, 10% changes have
been observed in the parameter r as a function of ion beam
current when the ratio VA /VB is 0.2 as was used in fabricat
ing the films described in Fig. 1. This will be discussed in
more detail in the next section.
The measurement of ion current density JF in Eq. (4) at
low ion energies and high background pressure requires
proper Faraday cup design. First, the angular acceptance of
the cups must accommodate the angular spread of the ion
beam at each aperture (about ± 8° in our system). Second,
the cups must suppress23 both the space charge entrained
electrons and the secondary electrons generated when fast
particles stop within the cup. The electrons in the chamber
arise from three sources: ionization of the gas in the
chamber, emission of the neutralizer filament of the ion
source, and secondary electrons from the substrate assembly
and from the e-gun hearth. The design must shield the
suppressor from energetic sputtered neutral atoms genera
ted when the ion beam strikes the cup walls. Otherwise, sec
ondary electrons so generated are accelerated back into the
cup, thereby reducing the measured current. Any error asso
ciated with the Faraday cups is expected to be simply pro
portional to the current reading.
The last factors to be considered are x, the experimentally
measured nitrogen atom fraction and the film thickness. Re
call that the measured value of atoms/cm2 in the film divided
by the total time of deposition is the experiment flux. The
Rutherford backscattering technique for determining x and
the number of atoms/cm2 contains three potential sources
for systematic deviation between the data and the model.
The first is an approximate 15% uncertainty in the stopping
powers used in the analysis package RUMP. This influences
the film thickness results directly but has a minor effect on
the composition analysis. Second, the scattering cross sec
tion for 2 MeV He on Si is known to obey the Rutherford
scattering equation to high accuracy. However, there are
indications that the cross section for 2 Me V He on N may
differ by as much as 5% from the Rutherford value at ener
gies between 1 and 2 MeV, and there are two weak reson
ances in the elastic scattering cross section at 1.53 and 1.61
Me V which is further evidence of non-Rutherford behavior.
Thus, while RBS analysis is capable of producing small rela
tive errors of thickness and composition of the films, at pres
ent it is incapable of providing accurate absolute values to
better than ~ 15 % in thickness and 5 % in composition. This
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 826
problem is under further investigation. The third source of
error is interference associated with the silicon signal from
deep within the substrate and the nitrogen signal from the
film. For low nitrogen concentrations and for multilayer de
posits, the N step edges were sometimes difficult to distin
guish from statistical fluctuations in the Si background. This
problem could lead to a systematic deviation at low ion cur
rents.
VII. COMPOSITIONAL VARIATION
The sources of systematic deviation discussed above may
cause inconvenience when transferring a process from one
laboratory to another, but do not detract from the utility of
the deposition method if a highly reproducible calibration
can be established. Random sources of sample to sample
variations in the composition produced by single reported
values of J F' Q, and p are a more major concern and are now
discussed from the viewpoint of Eqs. (3), (5), and (6).
Several factors can be eliminated as sources of random
fluctuations. For substrates held at a constant temperature,
the Si sticking coefficients should not change between depo
sition runs. The tooling factor re does not vary because the
geometry of the e-gun hearth, the quartz crystal, the sample,
and the ion gun was held constant. Moreover, films took
from 60 to 250 s to deposit. The 4 Hz noise of the quartz
crystal monitor then averages out to ± 0.2 A/s at a request
edrateoflOA/s. The factors (S) and (r) are not expected to
fluctuate because they are also determined by the system
geometry (incident angle) and ion source voltages (beam
energy) which are fixed during a set of deposits.
RBS does produce consistent compositional data. For ex
ample, when the simulated composition is changed ± 2
at. % N from the quoted values, the fit to the RBS data for
carbon substrates becomes obviously poor. Even for silicon
substrates, ± 3 at. % is expected in the probe area except at
the smallest nitrogen fractions.
We believe that much of the scatter in Fig. 1 arises as
follows. First, the position within the sample area where the
RBS analyzing beam was located may not have been chosen
consistently. If r=l= 1, then different positions would experi
ence different arrival ratios. Second, pressure fluctuations in
the chamber due to changes in the effective pumping speed
could have produced scatter through the charge exchange
neutralization factor (J which is exponentially dependent on
the chamber pressure. Third, variation in the temperature of
the gas in the chamber during a deposition run caused by the
thermal load of the e-gun and ion gun could also contribute
to fluctuations in the experimental (J [see Eq. (3)].
Finally, the center of the ion-beam profile was found to
move away from the sample center in an unpredicted and
uncontrolled manner. That is, the beam is not guaranteed to
be properly centered when the dual extraction grids are well
aligned relative to one another prior to pump down and the
source housing is accurately aimed at the sample center.
When the grids change temperature, their relative positions
may shift and redirect the ion beam. A change in filament
current or chamber vibration which causes the filament to
move within the anode assembly may alter the pattern of
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electron injection into the source plasma.24 This could modi
fy the ion intensity distribution across the face of the grids
and alter the beam profile, including the location of the beam
center. With an ion source to sample distance of2I.6 cm and
VA /VB = 0.2, shifts of more than 1 cm have been observed
in the position of the center of the beam profile when J F is
changed by a factor of 10. Clearly, if the ion beam is not
centered on the sample, then collecting the same total cur
rent from the three Faraday cups which surround the sample
does not guarantee the same current density at the sample
center. Thus, attempts must be made to detect and compen
sate for this motion of the beam center if compositional con
trol is to be achieved.
VIII. IMPROVEMENTS TO APPARATUS
The data in Fig. 1 were taken with the system described
earlier in the experimental section. Using the above discus
sion of systematic and random errors as a guide, several
changes in the apparatus and in deposition technique were
implemented to obtain more reproducible film composi
tions.
The most important changes were to improve ion beam
uniformity and stability. A dual-gimbal device was installed
for aiming the ion beam in situ by equalizing the current
collected in each of the three Faraday cups surrounding the
sample. This guaranteed that the beam center lay close to the
sample center. Additionally, the outer (accelerator) grid
voltage of the ion source VA was changed from 20% of the
inner grid (beam) voltage VB to 80% of VB which produces
a substantially more divergent (defocused) beam. This
changed r from 1.17 to 1.05 so that the Faraday cups more
accurately reflect the current at the sample center. It also
reduced the fluctuation in x if the beam center wandered
after the source was aimed.
Installation of new Faraday cups with improved angular
acceptance and electron suppression and a change to normal
incidence for the ions ensured accurate measurements of J F'
The resulting change of incident angle for the evaporant
from 0° to 20° introduced no more than a 5.3% variation of
the silicon flux between the extreme positions of the 2.2 cm
sample area. This variation was reduced to 2% by placing 1
cm samples in the middle of the sample holder. Rotation of
the substrate would also help in this regard, although we
have not implemented it here. A 40 cc hearth volume e-gun
with x-y beam sweep was installed at the same hearth-to
substrate distance as the old one, and no graphite hearth
liner was used. We believe this had no effect on the composi
tional variation of the early data, but did allow the depo
sition of thicker films. For improved pressure control, a
mass flow controller was installed in place of the needle valve
to supply gas to the ion gun. This eliminated a source of
fluctuation in gas pressure. A procedural change was insti
tuted where, to the extent possible, thermal equilibrium in
the chamber was established prior to performing deposits in
order to minimize the variation in fJ caused by changes in the
gas temperature.
Carefully centered carbon substrates were used instead of
silicon wafers and the RBS analysis beam was directed at the
center of the substrate. Moreover, the analyzed films were
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 827
thin enough (2500 A of silicon) to perform, without any
interference from the silicon signal, an explicit background
subtraction in the RBS analysis from above the oxygen edge
to just above the carbon substrate signal. We believe this
background signal is noise associated with pile up from the C
signal and incomplete charge collection in the surface bar
rier detector of scattering events at higher energies. This sub
traction improved the consistency of the composition mea
surements, especially at low nitrogen concentrations.
IX. RESULTS
In Fig. 4, we present data analogous to that in Fig. 1 for
films produced after the substantial modification of the de
position system discussed above. The data demonstrate im
proved compositional control. At the five values of the ion
current where more than one deposit was made, the maxi
mum composition variation was 2 1/2 at. %. The line in Fig.
4 is calculated from Eq. (8) using appropriate parameter
values from Table I. The model curve fits the data for com
position extremely well.
A non negligible and positive value of fJ from Table I
means that more nitrogen is incorporated into the films than
is predicted by the uncorrected Faraday cup current. This is
in disagreement to the dual ion beam sputtering work of
Erler et al. 25 on the silicon-nitrogen system which does not
explicitly mention charge exchange and asserts that only
33% of the N in the beam is incorporated.26 Note also that
the data in Fig. 4 extrapolates to 0% nitrogen at zero ion
current density, indicating that ambient nitrogen is not being
incorporated into the films. This result differs from the dual
ion-beam work of Harper and Cuom027 in the aluminum
nitrogen system where the data extrapolated to 9 at. % N at
zero ion assist current.
ARRIVAL RATIO (R.)
a 0.2 0.4 0.6 0.8 1.0 1.2 0.8.---_--,;--_-; __ -, __ -, __ -;-__ -;--,
0.6
z
0
>= u « a: u. 0.4 z w
c.? 0 0 a: f-
Z
0.2
250 300 350
CURRENT DENSITY (pA/cm2)
FIG. 4. Incorporated nitrogen fraction x in Si, _ x Nx films on C substrates vs
Faraday cup current density. Data shown is from 17 films made in three
separate runs with ion source parameters VB = 500 V, VA = 400 V, VD
= 50 V, and chamber pressure = 2.0x 10-4 Torr. The ion source was
aimed in situ and the ion beam impinged normal to the surface. Except for
the point at the largest current, all films were deposited at 10 A/s. The curve
is a model prediction (see the text).
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0.8 ,-----,-----,---,------,,--------,----,------,
0.6
z 0
0
>= () • ...: a: u. 0.4 0 z
OJ • Cl
0 a:
>-Z
0.2 • • 0
350
CURRENT DENSITY (pA/cm2)
FIG. 5. Incorporated nitrogen atom fraction x vs Faraday cup current den
sity. Two sets of data are shown corresponding to films made for chamber
pressures 4x 10 -4 Torr (e) and 1 X 10 -4 Torr (0). All films were made
with VB = 500 V, V, = 100 V, VD = 50 V, a deposition rate of 10 A/s, the
ion source aimed ill situ and the ion beam impinging along the sample
normal. The curves are model predictions (see the text).
For several samples in Fig. 4 there were 50% fluctuations
in the silicon rate of 10 A/s on time scales of 1/4 s. Despite
poor instantaneous rate control, the average composition
agreed well with deposits performed under normal rate con
trol.
Figure 5 demonstrates the effect of charge exchange
where the composition of films produced at operating pres
sures of I and 4 X 10 -4 Torr are plotted against the mea
sured ion current density. Note that the nitrogen content
ENERGY (MeV)
0.6 0.7 0.8
0.4 828
achieved for a given measured current is uniformly lower at
the lower pressure, consistent with a smaller energetic neu
tral nitrogen flux. These films were produced after the in situ
aiming of the ion beam was instituted and after the rotation
of the sample plane, but before the ion beam was defocused.
The curves in Fig. 5 are calculations from the model after
modification of f3 according to Eq. (3) and no other adjust
ment of the parameters. The agreement is satisfactory indi
cating that the model is capable of predicting the pressure
dependence of the composition.
The curve in Fig. I for 1000 eV was obtained by using the
same functional energy dependence of the charge exchange
cross section as for Ar, scaled to the measured value in Table
I for 500 eV. The agreement of this curve is also satisfactory.
It is apparent from Figs. I, 4, and 5 that the model predicts
quite well the composition of silicon nitride films for beam
energies from 500 to 1000 eV, N2 pressures from I to
4 X 10 -4 Torr, and arrival ratios Ra from 0 to 1.33.
Another advantageous feature of the IBAD process was
evident in the RBS data for samples prepared at low arrival
ratios. Figures 6(a) and 6(b) show RBS data for films with
calculated arrival ratios of 0.02 and 0.04, respectively. Note
that there is approximately 3 at. % oxygen in the lower ar
rival ratio film, whereas there is only about I at. % oxygen in
the higher ratio film. The most likely mechanism for this
improved purity is stimulated desorption of H20 and O2
from the growth surface by the ion beam during the deposit.
Some contributions cannot be ruled out, however, from the
densification of the film by the ion beam which reduces wa
ter absorption upon exposure to air prior to their RBS analy
sis. Silicon films deposited with the ion source fully off but
the chamber backfilled to its operating pressure contain no
0.9
Ra 0.02
0.2
0 -l
W
>-0.0 0 140 w
N
-l 0.4 «
~ a: ++ 0 z 0.2 160 180 200 220
Ra = 0.04 (al
240
(bl 0.0 L......::O------!._.l....- ___ ..L....--=---....:....-_...L..--_......:..---"' __ ........a_
140 160 180 200 220 240
CHANNEL
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 FIG. 6. Rutherford backscattering (RBS)
data ( + ) and multilayer simulation (line) of
two of the films in Fig. 4. The data demon
strates that increasing the nitrogen ion flux
increases the incorporated nitrogen concen
tration and reduces the amount of ambient
oxygen that is incorporated. These currents
correspond to calculated nitrogen to silicon
atom ratios of 0.02 (a) and 0.04 (b). Oxygen
level corresponds to ~ 3 at. % in (a), and ~ 1
at. % in (b).
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nitrogen and -10% oxygen after exposure to air.
The accuracy and consistency of the composition of the
deposits could be further improved by two additional
changes. First, the substitution of a substantially larger ion
source would help to minimize the departure of the param
eter r from a value of one and to stabilize it at that value.
Second, lowering the operating pressure in the chamber
would reduce the charge exchange factor /3 and as a conse
quence minimize errors introduced by uncertainties and
fluctuations in pressure and gas temperature. This perhaps is
most easily achieved by increasing the pumping speed of the
system. In our chamber, for example, a pumping speed of
5000 lis would decrease the operating pressure from
2.0 X 10 -4 to 3.4 X 10 -5 Torr where the charge exchanged
fraction is only 7%. Reduction of the ion source to substrate
distance also reduces /3, but this may not be compatible with
beam uniformity requirements or chamber geometry.
X.SUMMARY
Figure 1 illustrates the limited success of our early at
tempts to control the composition of substoichiometric sili
con nitride films by controlling the ratio of the nitrogen ion
current density to the silicon deposition rate and thereby the
nitrogen to silicon atom arrival ratio. As much as 15 at. %
scatter in the data for the nitrogen composition at a single ion
current was observed. A simple model was presented which
related the fi1m composition to the measured beam current
density, the Si vapor impingement rate, and the chamber
pressure. Effects included in the model were the reflection of
energetic particles, sputtering of the film atoms, geometric
tooling factors for the measurement of ion flux, eva po rant
flux and pressure, and charge exchange neutralization of the
ions. The model was used to help identify the sources of
compositional variation of the data in Fig. 1. Guided by this
analysis, changes to the apparatus and technique were im
plemented to minimize the variation. Figure 4 demonstrates
the degree of improvement of composition control where a
reproducibility of 3 at. % nitrogen was achieved. The most
important factors were the improvement of the uniformity of
the ion flux by defocusing and in situ aiming of the ion gun,
and improvement of the relative precision of the RBS mea
surement of the film composition by the use of graphite sub
strates. Improved control of pressure and ambient gas tem
perature may also have contributed to the reproducibility of
ion flux by reducing fluctuations in the amount of charge
exchange neutralization of the ions. The analysis also
showed that compositional variation could not arise from
changes in the Nt to N + ratio of ions in the beam, from the
angle and energy dependence of reflection and sputtering, or
from artifacts associated with the use of a quartz crystal bal
ance for the measurement of vapor flux.
While the relative reproducibility of the composition is
excellent, uncertainty in the RBS scattering cross sections
for nitrogen and analysis procedures produce as much as 5%
errors in the absolute composition and 15% errors in thick
ness. Systematic errors in the readings of the Faraday cup,
and quartz crystal balance, and the variation of the atoms
lion charge factors as the ion current varies contribute er
rors no greater than 2% each toward inaccuracy in Ra. The
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 829
model of the IBAD process developed herein is able to fit the
data well for plausible values of the parameters for energies
between 500 and 1000 eV, Ra between a and 1.33, and pres
sures between 1 and 4 X 10 -4 Torr. We conclude that the
IBAD technique is indeed capable of producing highly re
producible substoichiometric silicon nitride films when the
ion current and silicon deposition rate are controlled in real
time.
The determination of the values of the parameters incor
porated in the model is the subject of a companion paper. II
The deposition methods described in both papers and the
model presented here should be applicable to the calibration
and improvement of composition control for other IBAD
systems.
,,) U.S. Naval Academy.
'p. A. Thomas, M. H. Brodsky. D. Kaplan, and D. Lepine, Phys. Rev. B
18.3059 (1978).
'J. E. Yehoda, B. Yang. K. Vedam, and R. Messier, J. Vac. Sci. Technol. A
6,1631 (1988).
-'See, e.g., K.·H. Miiller, J. Appl. Phys. 62,1796 (1987).
40. R. Brighton and G. K. Hubler. Nucl. Instrum. Methods Phys. Res. B
28,527 (1987).
'P. J. Martin, J. Mater. Sci. 21. 1 (1986).
"J. J. Cuomo, J. M. E. Harper, C. R. Guarnieri, D. S. Yee. L. J. Attanasio, J.
Angilello, and C. T. Yu, J. Vac. Sci. Technol. 20, 39 (1982), and refer
ences cited therein.
7R. A. Roy reported in talk TF·MoH6 of the 34th A VS national meeting
(Anaheim, 1987) on his ability to minimize simultaneously the tensile
stress and resistivity and to achieve an acceptably large micro-hardness
when depositing copper with argon ion assistance.
'E. P. Donovan, D. R. Brighton, D. Van Vechten, and G. K. Hubler,
Mater. Res. Soc. Symp. Proc. 71, 487 (1986).
"E. P. Donovan, D. R. Brighton, G. K. Hubler. and D. Van Vechten. Nucl.
Instrum. Methods Phys. Res. B 19, 983 ( 1987).
IIIE. P. Donovan. D. Van Vechten, A. D. F. Kahn, C. A. Carosella, and G.
K. Hubler, J. Appl. Opt. 28. 2940 (1989).
"G. K. Hubler, D. Van Vechten, E. P. Donovan, and C. A. Carosella, J.
Vac. Sci. Technol. A 8, 831 (1990).
"L. R. Doolittle, Nucl. Instrum. Methods Phys. Res. B 9. 344 ( 1983).
I 'P. W. Atkins, Physical Chemistry, 2nd ed. (Freeman, San Francisco,
1982). p. 605.
I·See, for example, R. A. Mapleton, Theo~y a/Charge Exchange (Wiley
Interscience. New York. 1972) for a discussion of the different calcula
tional schemes which have been utilized. The work of Friedrich et al.
[Friedrich, Bretislav, S.L. Howard. A.L. Rockwood. W.E. Trafton, Jr.;
Du Wen·Hu. and J. H. Futrell, Int. J. Mass Spectrom. Ion Proc. 59, 203
(1984) 1 quotes the Nt lifetime as "significantly more than 5 X 10 11 s"
with a binding energy of from 0.9 to 1.4 eV. J. Futrell reviews the detailed
modeling of the intermediate state of the N, N,' charge exchange reac
tion in a chapter in Structure Reactivity alld Thermochemistry of /OIlS.
edited by P. Ausloos and S. G. Lias (Reidel, Boston. 1986). pp. 57-80.
"K. J. McCann, H. R. Flannery, J. V. Hornstein. and T. F. Moran. J.
Chem. Phys. 63, 4997 (1975). The differential cross sections plotted in
this reference indicate that for an ion energy of2210 eV. all the amplitUde
is compressed into the forward most 2 deg in the center of mass system.
For energies less than 156 eV the scattering amplitude for angles greater
than 6 deg in the center of mass system becomes appreciable.
IhK. B. McAfee, C. R. Szmanda. R. S. Hozack, and R. E. Johnson, J. Chern.
Phys. 77, 2399 ( 1982).
I7J. M. E. Harper, J. J. Cuomo, and H. R. Kaufman, J. Vac. Sci. Technol.
21,737 (1982).
"M. R. Flannery, P. C. Cosby, and T. F. Horan. J. Chem. Phys. 59, 5494
(1973). This reference quotes lifetimes of the B '~.~ state ofN: as 60 ns
and of the A 'TT,. state as 10 fls. Since the transit time of 500 eV N ,+ ions in
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 137.189.170.231 On: Sat, 20 Dec 2014 18:05:41830 Van Vechten et al.: Fundamentals of IBAD. I
our apparatus is 3.7 f.ls, the A but not the B state will still be populated
when the ions impact the sample.
lOR. S. Robinson, J. Vac. Sci. Technol. 16, 185 (1979). This cross section is
a = c-b[ln(u)'] where c = 1.51 X 10-9, b = 9.53X 10-" for a in m' and
u in m/s.
'°c._S. Lu, "Monitoring and Controlling Techniques for Thin Film Depo
sition Processes," AVS short course notes (1986), p. 45.
"Equation (9) may be recast in the form If = (Po Vq )/(21Tzp;f... )tan-I
(Z tan 1TF) where Vq is the sheer velocity of the particular cut of quartz
used in making the film sensor. In this form it is more obvious that the
customary practice of defining! to be zero at the beginning of each run and
variation in the thickness of sensors as delivered do not represent sources
of error in the values of If derived.
J. Vac. Sci. Technol. A, Vol. 8, No.2, Mar/Apr 1990 830
"D. Van Vechten, G. K. Hubler, and E. P. Donovan, Vacuum 36, 841
( 1986).
"For equations useful in the design of electrostatic suppression systems, see
K. Kanaya, H. Kawakatsu, H. Yamazaki, and S. Sibataa, J. Sci. Instrum.
43,416 (1966).
'4G. Isaacson (personal communication).
"H. J. Erler, G. Reisse, and C. Weissman tel, Thin Solid Films 65, 233
(1980); see also C. Weissmantel, Thin Solid Films 32, II (1976).
'"Reference 25 contains very little discussion of experimental detail, espe
cially of how the ion current was measured. Without this information it is
difficult to reconcile our results with theirs.
27J. M. E. Harper, J. J. Cuomo, and H. T. G. Hentzell, Appl. Phys. Lett. 43,
547 (1983).
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1.1140327.pdf | Variableheatingrate wiremesh pyrolysis apparatus
J. R. Gibbins, R. A. V. King, R. J. Wood, and R. Kandiyoti
Citation: Review of Scientific Instruments 60, 1129 (1989); doi: 10.1063/1.1140327
View online: http://dx.doi.org/10.1063/1.1140327
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51Variablemheating-rate wire-mesh pyrolysis apparatus
J. R. Gibbins, R. A. V. King, R. J. Wood, and R. Kandiyoti
Department a/Chemical Engineering and Chemical Technology, Imperial College a/Science.
Technology and Medicine, London SW7 2AZ, United Kingdom
(Received 31 August 1989; accepted for publication 6 March 1989)
An electrically heated wirecmesh apparatus for pyrolysis studies has been developed which uses
computer-driven feedback control for the heating system and thus can apply virtually any timc
temperature history to the sample. Internal components are water cooled to prevent heat buildup
during long runs. Using this system, coal pyrolysis has been studied at heating rates from 0.1 to
about 5000 K/s and temperatures up to 1000 0c. Alternating current is used for heating; this
allows the thermocouples to be attached directly to the sample holder and also makes power
regulation relatively simple. For atmospheric-pressure experiments, a gas sweep can be forced
through the sample holder to remove products from the heated zone and also to concentrate them
in a trap which can be removed from the apparatus and weighed to establish tar yields directly.
Although the design is optimized for atmospheric-pressure operation, relatively simple
modifications allow operation under vacuum or at pressures of up to 160 bars in inert gas or
hydrogen. The apparatus has been used to investigate a number of phenomena in coal pyrolysis
and, most significantly, has demonstrated thc existence ofa heating-rate effect which is
independent of reactor geometry,
INTRODUCTION
In the wire-mesh apparatus, a small (of the order of 10 mg)
sample of finely ground (typically 100 jLm) substrate is
sandwiched between the layers of a folded wire-mesh sample
holder, which is heated directly by an electric current. The
mesh retains the particles, but offers little resistance to the
passage of volatiles, which can therefore leave the heated
zone around the sample very rapidly and need not undergo
extensive secondary reactions. Total volatile yields are deter
mined by weighing the loaded sample holder before and after
heating, and the volatile products may also be collected for
measurement and analysis. Usually, a fresh sample holder is
used for each run, since cleaning would be impractical. Tem
peratures are measured by one or more thermocouples at
tached to the sample holder.
Because of its low thermal inertia, there is little intrinsic
limitation on the time/temperature profiles that can be ap
plied to the sample holder assembly. The minimum heating
rate can be as close to zero as the control system will anow,
while maximum sample holder heating rates of the order of
104 K/s can be obtained before power requirements become
prohibitive. However, even with the sample dispersed to al
low direct contact between each individual coal particle and
the mesh, heat transfer between the sample holder and the
coal may not be good enough to avoid significant tempera
ture differences under all conditions. Further differences
between the indicated thermocouple temperature and the
actual temperature of the sample will also result if the ther
mocouple junction is not arranged to measure the tempera
ture of the sample holder actually adjacent to the coal sam
ple, or if there is any unresolved interference from the
heating current. While the latter temperature differences
can be minimized by suitable apparatus design (as will be described below), temperature differences between the mesh
and the particles represent an intrinsic problem which must
be taken into account when selecting experimental condi
tions. For example, in this study the effect of heating rate has
deliberately been investigated under conditions (700°C
peak temperature and 30-s isothermal holding period)
where it can be shown experimentally that yields are insensi
tive to variations in temperature.
The wire-mesh apparatus was invented by Loi8in and
Chauvin i in the 19508 and has subsequently seen widespread
use, notably by Howard and various co-workers at MIT2-5
and by other groups in North America,'>-i2 Europe, 13,14 and
Australia. IS 17 The effect of heating rate has, however, large
ly been neglected, and the bulk of data has been obtained at
heating rates around 1000 K/s. While there are fundamental
reasons while higher heating rates have not generally been
used (i.e., increased power consumption and the limitations
of heat transfer to the coal particles), as noted above there
are no similar intrinsic limits on operation at lower heating
rates. However, most experimenters have used simple heat
ing circuits in which either the voltage or current was fixed
for the duration oftlle heating period (the relative merits of
fixed voltage versus fixed-current heating have been de
scribed by NiksaI8), with (usually) a second constant-level
output for an isothermal hold period at peak temperature if
required. At heating rates of the order of 103 K/s and above,
heat losses from the sample holder are relatively insignifi
cant compared to the power required for self-heating, and an
approximately constant rate of heating can therefore be ob
tained with this type of heating system. At the lower power
inputs necessary for slow heating, heat losses are more signif
icant, however, and cause the heating rate to fall off as the
temperature rises. Although the linearity of the heating rate
depends on how dose the desired final temperature is to the
1129 Rev. Sci.lnstrum. 60 (6), June 1989 0034-6748/89/061129-11$01.30 (c) 1989 American Institute of Physics 1129
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51point where the power input and the heat losses will reach
equilibrium, most experimenters have considered that 50-
100 K/s is the lower feasible limit with a constant-level heat
ing circuit.
In addition to the heating circuit, the design of the reac
tor itself may also be a limiting factor at lower heating rates.
If no cooling is provided, reactor internals, particularly the
electrodes which are in contact with the sample holder, can
become overheated in longer experiments. Satisfactory
product collection may also be more difficult at lower heat
ing rates, unless products are kept away from the hot region
around the sample holder, because more time is available for
secondary reactions.
Heating rates below 100 K/s in a wire-mesh reactor
have been achieved by Hamilton and co-workers at
CSIRO, J5-17 who used an analog feedback-control system to
give rates as low as 0.1 K/s. Product yields were not report
ed, however, and the reactor appears to have been used only
to produce chars for optical studies of plasticity phenomena.
A pure feed-forward control system, using a computer to
generate a power/time profile found by trial and error to give
the correct heating rate, has been used to obtain data on
product yields at heating rates of 1 K/s by Freihaut and
Seery,19 who observed a steady increase in volatile yields
under vacuum for heating rates of 1, 100, and 1000 K/s. Tar
yields went through a maximum at 100 K/s, however.
Because of the limitations apparent in previous designs,
it was decided that, in order to allow coal pyrolysis reactions
to be studied at heating rates below 100 K/s, a wire-mesh
reactor would be built incorporating three new features: (i)
a flow of sweep gas forced through the mesh itself to give the
best possible product removal, (iO water cooling to prevent
excessive temperature rises in the parts of the apparatus in
contact with the sample holder, and (iii) a computerized
temperature feedback-control system to allow essentially
any heating rate (or sequence of heating rates) to be applied
to the sample holder. While the first two items required no
special techniques to implement, the design of the last fea
ture drew heavily on recent developments in electronics, and
although it would have undoubtedly been feasible to build an
equivalent system much earlier, it would have required more
effort and expenditure.
I. APPARATUS DESCRIPTION
Figure 1 shows an exploded view of the water-cooled
electrode assembly used in all versions of the new apparatus.
The sample holder is folded from AISI 304, 250 mesh (65-
pm holes, 4O-,um wires) woven wire cloth to give a single
layer above and below the sample with a short flap to close
the open side. This is stretched on top of the cooled support
plate between a fixed and a moving, spring-loaded electrode.
The sweep gas flows through the 30-mm-diam hole in the
support plate and around and through the coal sample,
which is spread in a smaller circle at the center of this work
ing section. The support plate provides a base against which
an offtake tube for the sweep gas can be seated, and also cools
the portions ofthe sample holder which, in the absence of the
cooling effect of the sweep flow, would otherwise reach melt-
1130 Rev. Sci. Instrum., Vol. 60, No.6, June 1989 LI VE ELECTRODE •.
MICA INSULATION -
LIVE TERMINAL ..
'WATER-COOLED SUPPORT PLATE
IIA TER-COOLED
EARTHED ELECTRODE
, -. HOLLOW PI LLAAS
._---- CARRYING COOLING WATER r:§> MICA INSULATION
~ ""-~""'" """
FIG, 1. Water· cooled electrode assembly_
ing point at high power inputs. The sample holder is electri
cally insulated from the support plate by a 0.25-mm layer of
amber mica (phlogobite). A further layer of mica is used
beneath the support plate to prevent the bare thermocouple
wires from making contact.
Cooling water flows up one of the hollow pillars carry
ing the support plate, along a longitudinal hole drilled in the
plate itself, through a U bend of stainless-steel tubing con
nected to the earthed electrode (these tubes also serve as
springs), into a lateral hole through the earthed electrode,
and then continues out by a similar route. The earth line for
the heating current is also via the cooling water tubes.
The live electrode in the heating current circuit is con
nected to a terminal which passes through an insulating
PTFE sealing gland in the base (details not shown). The live
electrode is mounted on the end of the (earthed) support
plate, but is electrically insulated from it, by a layer of mica
at the joint and by insulating bushes on the fixing screws.
The mica sheet, which insulates the sample holder from the
support plate, is also continued over the live electrode. This
avoids any arcing to the sample holder at the joint between
the live electrode and the support plate and also allows the
mica to be held in place by the electrode clamp. Contact
between the live electrode and the sample holder is via the
two studs for the electrode clamp, which pass through the
mica, and then along the electrode clamp itself.
To give an even layer of coal, the sample (nominally
100-150 ,urn) is first placed in a small pile between the folds
of the sample holder, which is then stretched between the
electrodes. A 20-mm-diam depression centered on the sam
ple is formed by scribing a circle in the mesh of the sample
holder (annealed before use) with a blunt point. The circle
contains the sample as it is distributed, by tapping the appa
ratus and by sucking the coal particles into place using a 15-
mm-diam glass nozzle connected to a suction line. This pro-
Pyrolysis apparatus 1130
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51cedure also serves to remove any fine particles that might be
lost during heating and give a spurious extra weight loss.
Temperatures are measured by type-K (Chromel! Alu
mel) thermocouples, formed by inserting 50-pm wires
through the 65-pm apertures in the mesh. The wires are
spaced about 1 mm apart in the direction perpendicular to
the heating voltage gradient and within 0.5 mm apart in the
direction of the voltage gradient. More precise positioning of
the thermocouple wires is not necessary, since (as described
below) interference from voltages induced by the heating
current is avoided by the design of the temperature-control
system, and the thermocouples can easily be located satisfac
torily with the naked eye. After the individual wires have
been threaded through the mesh using tweezers, the upper
ends are flattened using pliers and drawn down into the mesh
to ensure good electrical contact. During heating, the wires
are kept under slight tension by their securing clips to main
tain contact; this also forces the upper and lower folds of the
sample holder together and helps to hold the sample in place.
The thermocouples thus formed have proved extremely reli
able in practice and, compared to more conventional spot
welded thermocouples, have the advantages of causing very
little distortion to the mesh and being extremely easy and
quick to form using only simple hand tools. Because the ther
mocouple wires can only make electrical contact through
the sample holder, there is no possibility of the hot junction
not being in thermal contact with the sample holder and
most thermocouple faults can be identified by an open-cir
cuit pull-up resistor in the thermocouple amplifier.
Two thermocouples are used, placed at the center and 1
mm from the edge of the circular sample area, and their
readings are averaged to give the control value. Typically,
the temperatures measured by the individual thermocouples
are within 20 K of their average, with the difference being
due to variations in local heat capacity and resistance to gas
CERAMIC TIC
INSULATOR
DIFFUSER OVER
HP GAS INLET flow. Since the loading density of the sample tends to dimin
ish unavoidably towards the periphery, measurements from
the two thermocouples represent the approximate maxi
mum and minimum temperatures within the sample area.
Figure 2 shows the base unit, which is common to all
versions of the apparatus, with the electrode assembly in
position. The 50-pm diam thermocouple wires attached to
the sample holder are terminated in spring clips formed in
the end of O.5-mm-diam leads of the same alloy. These in
turn are connected to the thermocouple amplifiers after
passing through a four-way Conax sealing gland (not
shown) in the base. Insulating ceramic guides are used to
hold the thermocouple clips in convenient positions on ei
ther side of the electrode assembly.
Figure 3 shows the reactor in its atmospheric-pressure
configuration. The gas inlet port in the base is unused. and
the sweep gas enters through a connection in the side of the
glass top. A port on the opposite side is connected to a small
rotary vacuum pump. Before heating the apparatus is evacu
ated to less than 1 m bar and refilled with the sweep gas 3
times to remove air (during this process, the open top of the
filter-tube trap is sealed with a rubber bung). A No. 1 poros
ity Pyrex sinter disk is clamped beneath the hole in the sup
port plate by wire clips. This acts as a flow distributor, and
also provides a resistance to counteract the resistance of the
trapping filter and prevent flow reversal when rapid gas ex
pansion occurs during fast heating runs. If there should be a
failure in the gas-flow pattern (usually due to a leak around
the base of the offtake column as a result of misalignment
during assembly), any tar which has not been entrained is
easily visible on the upper surface of the sinter.
A superficial sweep-gas velocity of O. 1 mls is normally
used; this has been found by trial and error to be the lowest
flow velocity to give reliable product entrainment at the
higher heating rates. Tests at flow velocities up to nearly 0.3
FLATTENED 'TAILS' ON
0.05 mm TIC WIRES THREADED
THROUGH THE WIRE-MESH
TAPPED HOLES FOR STUDS
/ TO RETAIN LP GLASS TOP
CERAMIC TIC
TERMINAL BLOCK
EXTERNAL THREAD FDA
HP RETAINING RING
PORT FOR PRESSURE GAUGE
O-RING GROOVE
FIG. 2. General-purpose base assembly.
1131 Rev. Sci. Instrum., Vol. 60, No.6, June 1989 Pyrolysis apparatus 1131
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51#4 GLASS SINTER
FILTER-TUBE TRAP
TO VACUUM PUMP
FOR PURGING
111 SINTER DISC
FLOW DISTRIBUTOR
HP GAS INLET
(NOT USED) COMPRESSION JOINT
WITH O-RING SEAL
(SCREW CAP OMITTED)
---CLAMPING RING
FIG. 3. Wire-me,h reactor configured for atmospheric-pressure operation.
m/s have shown no measurable effect of sweep velocity on
weight loss or tar yield. Helium is commonly used as the
sweep gas, principally because of its high thermal conductiv
ity, but similar weight loss and tar yields have been obtained
with a nitrogen sweep. The greater density of nitrogen meant
that flow was turbulent not laminar, however, and more tar
was deposited on the sides of the offtake tube. The gas f1ow
rate is adjusted by a valve immediately upstream of the reac
tor inlet with a constant inlet pressure of 0.5 bar g, giving a
fairly "stiff" gas supply to overcome the expansion of the gas
in the reactor when heating takes place. A rotameter up
stream of the valve is used to set the desired ftowrate, the
whole system having been calibrated at the operating condi
tiems using a dry gas meter.
Tars yields can be measured at atmospheric pressures
using the single-unit trap shown in Fig. 3, made from a stan
dard No.4 porosity Pyrex sinter filter tube with a nominal
30-mm i.d. During an experiment, liquid nitrogen is poured
on top of the sinter and the sweep gas bubbles up through it
to vent to atmosphere. Tars are caught on the underside of
the sinter and, at heating rates of 100 K/s and above, some
also impinge on the walls lower down due to their almost
explosive release from the coal which can instantaneously
swamp convection by the sweep gas. Even then, except for
the region about 10 mm immediately above the sample hold
er (where little or no tar is collected anyway), the cooling
action of the sweep gas and heat conduction through the
glass keep any tar on the walls cold enough to prevent it
melting and hence significant thermal degradation is unlike
ly. After an experiment, the trap is heated at 50°C in air for
30 min to remove any components with a high enough vola
tility to give significant weight changes during weighing.
The choice ofthis temperature and time was somewhat arbi
trary, but since tests have shown negligible further weight
1132 Rev. Sci. Instrum., Vol. 50, No.6, June 1989 loss from traps on subsequent heating at 70°C for up to sev
eral hours in a vacuum oven, it appears to define a tar frac
tion which is not sensitive to the actual separation conditions
used. Between experiments the traps are heated overnight in
a glassblowers' annealing oven to bum off the tars and re
lieve any heat-induced stresses.
When only weight-loss measurements are required, a
plain (uncooled) quartz-glass offtake column is used. A
sample bag can also be attached in this arrangement to col
lect the gas products for analysis, but this procedure is still
under development, the inevitable dilution from the sweep
gas being a complicating factor.
For vacuum runs the same reactor configuration is used,
but with the filter-tube trap replaced by an unbaffied Pyrex
glass offtake tube of the same bore. A cold finger in the cen
ter of the offtake column, cooled by liquid nitrogen, reaches
down to within about 25 mm of the sample holder. The off
take is connected by approximately 1 m of large-bore glass
tubing to a mercury diffusion pump backed up with a rotary
vacuum pump. An initial vacuum of below 1 m Torr can
easily be achieved, and very little pressure rise is observed
during pyrolysis thanks to the rapid condensation of volatile
species on the cold finger. Only total volatile yields have been
measured, however, since the tars are "sprayed" out onto all
internal surfaces of the reactor within the line of sight of the
sample, and apart from the difficulties of collection, some of
these surfaces (principally the sinter below the sample hold
er and the base of the offtake tube) get hot enough for the
deposited tars to melt, raising the possibility that some ther
mal degradation is also taking place. The peak temperature
for experiments under vacuum has been limited to 800 °C
because of evaporation losses from the present sample holder
material at higher temperatures.
A Sartorius 2024 balance with a reproducibility of
± 0.000 02 g is used to determine the weight loss of the
sample on heating and the amount oftar collected. To mini
mize weighing errors, the balance is placed in a glovebox
kept dry with silica gel and a piezoelectric antistatic gun is
used on glassware prior to weighing. Sample holders, being
light (about 1 g) and electrically conductive, can be weighed
with an estimated repeatability of ± 0.000 04 g, but despite
the precautions described above, static electricity and mois
ture adsorption can reduce the practicable reproducibility to
± 0.0001 g for the 50-g glass tar traps.
Figure 4 gives a schematic of the power supply and tem
perature measurement and control systems. Heating current
is fed from the single-phase 240-V ac mains supply via a
thyristor bridge and a variable and a fixed transformer (or
pair of fixed transformers with the primary windings in par
allel and the secondary windings in series). The thyristors
are interfaced to a microcomputer via an 8-bit (i.e., 2561ev
els of output) digital-to-analog converter which feeds a 0-5-
V de control signal to a proprietary phase-angle trigger mod
ule. During heating, the power is regulated by controlling
the conduction angle of the thyristors, but to minimize the
steps between different power levels (due to the steps in D/ A
output voltage), the variable transformer is used to preset
the heating voltage available at full output, which is varied
from about 12-V rms to 24-V rms, depending on the heating
Pyrolysis apparatus 1132
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51VARIABLE
TRANSFORMER
I..OW VOLTAGE
TRANSFORMER
1115 240 II
A.C.
OIl.
CONVERTER
AD722S
OP-07 9=' 100 HZ MAINS-SYNCHRONISED PULSE GENERATOR
2 STASE ADJUSTABLE
PUL.SE DEI..AY
74L.S1 '23 74L514
PARALLEL
INTERFACE SBooe
I'1CSB230L.B BUS
CONTROL MICROCOMPUTER
SINCLAIR QL RUNNING
FORTH AND S8000 M.COOE
AID CONVERTER
A0574
OP-07 G-80 UIOLATION MULTIPLEXER
MUX 28 SAMPLE So HOLD
L"39SN
U-MICRO U-A/D CAAO JCE/WAfER AMPLIFIER
COLD J'N AD204 ;.1
FIG. 4. Alternating current temperature-control circuit for variable-heating-rate wire-mesh apparatus.
rate and final temperature. Because high power outputs are
only required for short periods, it has proved possible to use
fixed-range transformers with a continuous output rating of
only 60-A rms. Although this is exceeded by about 300% at
peak heating rates, the thermal mass of the transformer is
sufficient to absorb the transient overload.
It was decided at an early stage to heat the sample holder
with alternating current at normal mains frequency (50 Hz
in the U.K.). Using ac is convenient because power can be
regulated at relatively low current levels on the high-voltage
side of the transformers, but it also has the great advantage of
allowing any interference from the heating current that may
be picked up by a thermocouple attached directly to the sam
ple holder to be averaged out. Interference from a dc heating
current cannot be averaged out, nor will a moderate level of
interference necessarily be apparent from a temperature
trace, unless high-speed data acquisition is used, since the
tempeniture will also be changing rapidly when the heating
current is applied.
The magnitUde of possible interference from a dc heat
ing current can be estimated by considering the case with a
typical heating current voltage of 12 V across a 50-mm sam
ple holder. The voltage gradient across the thickness of a 50-
ttm ( 0.002-in.) thermocouple wire would then be 12 mY,
equivalent to about 300 K of input for the commonly used
typc-K (Chromel/ Alumel) thermocouple, which suggests
that effective single-point contact for a thermocouple junc
tion actually touching the mesh cannot be guaranteed with
out special precautions. One solution for a de system is to
place the junction away from the surface, 18 but this gives an'
inevitable risk of a loss in response. More satisfactory alter
natives that have been used are to weld both thermocouple
wires to a single tranverse (i.e., perpendicular to the voltage
gradient) strand ofthe mesh between two longitudinal (i.e.,
in the direction of the voltage gradient) strands, thus avoid-
1133 Rev. SCi.lnstrum., Vol. 60, No.6, June 1989 ing any significant voltage gradient between the thermocou
ple leads when power is applied,20 or to turn the de heating
current off altogether while thermocouple readings are tak
en.ls
With ae supplies any interference should be self-evident
and can, in principle, be removed either by integrating for a
complete cycle or by averaging two readings taken at an
interval of half a cycle. The former is probably the more
convenient option if an integrating analogue-to-digital
(AID) converter is used, the latter ira fast AID converter
coupled to a microprocessor is employed. Experience with
early versions of this apparatus gave poor results when using
an integrating dual-slope-type AID converter with the thyr
istor-regulated supplies, however (probably because of non
linear amplifier responses to the very rapid voltage change
w hen the thyristors were triggered), and a fast AID convert
er (AD574) operated at half-cycle intervals was adopted.
To synchronize the microcomputer to the heating cur
rent frequency, each thermocouple logging sequence is initi
ated by a pulse generated from the same mains supply. The
delay on this pulse can be adjusted (with the aid of a dual
trace oscilloscope) to initiate temperature readings soon
after a thyristor switches off as the supply current reverses
polarity, and by limiting the maximum conduction time for a
thyristor to 7 ms out of the 10 ms half-cycle, a 3-ms period is
available during which no significant heating current flows.
However, because of the phase-angle shift in the transform
ers, the voltage across the sample holder has already passed
through zero when the thyristor switches off and this gives
an alternating offset (of the order of 100 m V) at the start of
each 3-ms period. Because of this offset, two readings at the
same power level have to be averaged to get a true tempera
ture reading, and so only after every complete mains cycle
(i.e., every 20 ms) can a control temperature be obtained
and the appropriate power output be calculated. An oscillo-
Pyrolysis apparatus 1133
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51[--rf--- thyristor triggering point r thermocoup I e I ogg i ng po i nt
~\ J 1 heating voltage
---. ~ 12-24 V r.m.s. at full output
--, n r 0-5 V timing signal L j
10 ms
(for 50 Hz supply)
FIG. 5 Oscilloscope trace showing heating voltage and timing pulse syn
chronization.
scope trace of the heating voltage and the timing pulse is
shown in Fig. 5.
The basic clement of the computer control software is a
conventional PID control algorithm, although preset values
for the integral term can be specified to give more rapid re
sponse at step changes. It is also possible to override feed
back control and specify the power signal; this is usually
done to give zero power for uncontrolled cooling or maxi
mum power for very rapid heating. A SinclairQL (Motorola
68008 cpu) is used to run the software which is implemented
in Forth and machine code. A heating program is defined by
a series of Forth data screens which specify the heating rate,
PID constants or a fixed power, and the end time or tem
perature for each sequence. The complexity of a heating pro
gram is determined only by the limits of internal and exter
nal memory. Integer arithmetic is used for all control
calculations for maximum speed, but scaling factors are em
ployed to allow temperatures to be handled to the nearest 0.1 °C and heating rates to 0.001 K/s. Taking advantage of
the 68008'8 32-bit registers and 16/32-bit arithmetic instruc
tions, it has been possible to implement the whole control
sequence of zero-mains pulse sensing, AID operation, ther
mocouple linearization (interpolated from a look-up table),
PID calculations, and power-signal output in machine code
for maximum speed. The overall time for temperature log
ging and control calculations is about 2 ms, which means
that with the 3-ms delay described above the required signal
can be presented to the thyristor trigger module in advance
of the earliest possible triggering point. This avoids a possi
ble half-cycle's delay before the correct power is set. Forth is
used to set up the control-sequence data tables, to plot results
to the screen in real time, to update the control target tem
perature during heating, and to print out results after a run is
complete. Typically, the average of the two thermocouple
readings can be controlled to be within 10 K of the desired
value (or the temperature rise in 20 ms if greater). Actual
timel temperature traces from a demonstration run showing
different heating/cooling modes are shown in Fig. 6. A more
detailed description of the control software (and other
aspects of the apparatus) is available elsewhere.21
The QL microcomputer is interfaced to the logging
hardware by a simple parallel interface based on the
M68230L8, designed and built in-house. The DI A converter
(ADi224), the zero-pulse circuit, and the signal-condition
ing amplifiers were also assembled in-house, but the AID
converter and associated multiplexer and sample-and-hold
system were purchased as a ready-built board (U-Micro PC
AD card). The input to the AID converter is gated by the
sample and hold integrated circuit, but the rest of the ther-
1500
fJ) ..,1400
J::
"1300 ~~--~---'"->--7-~---r-------r---
;v 900
3;
o
a. 800
'-
o 700
C; 600
U1
W
-0 500
Q) 400 "-
" ... 300 " '-
Q)
"-E 200
'" ... 100
o r
n c
--control temperature
----~ ind:\!idu~1 T/C readings
F
__ -1-__ ----L~o ____ , _____ _'_I ____ _'IL_ ____ ~1 _.--..L __ -'--___ --.J
1 no 200 300 40n 500 GOO 700
data point number
FIG. 6 Actual thermocouple readings from a demonstration heating program showing different modes of heating/cooling (with a sample in place and 0.1-
m/s sweep fiowofhelium at approximately 1.2 bar). A--B. 5-K/s heating to 3(X) 'C; B-C. lO()O-K/s heating to 700 T; C--D, hold at 700 T; D E, controiled
cooling at 5 K/s for 20 s; E-F, uncontrolled cooling for 3 s. One-data-pointl20-ms interval fix B-C, C-D, and E-F. One-data-poilltl200-ms interval lor A
II and D-E.
1134 Rev. SCi.lnstrum., Vol. 60, No.6, June 1989 Pyrolysis apparatus 1134
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51mocouple signal-conditioning circuit fluctuates in response
to the instantaneous sum of the thermocouple output and
any interference. With the large voltage gradients already
noted, if the thermocouple leads arc far enough apart, inter
ference at the peak heating current can exceed the thermal
signal by several times and give a high enough differential
input signal to cause the amplifiers to saturate. Particularly,
if this occurs only in one half-cycle, when the thermal emf
and the interference both have the same polarity, an error
may result if the amplifier takes a relatively long time to
recover and therefore still has a residual offset when the
reading is taken. A jump in recorded temperature when
power is switched on or off will then be observed. In practice,
however, a separation of about 1 mm in the direction of the
voltage gradient is required to cause this effect (at the power
levels required for atmospheric pressure operation at up to
1000 K/s), and thermocouple leads can easily be arranged
to be well within this limit. Common mode interference may
also cause a breakdown in amplifier performance unless iso
lation amplifiers are used, though again only at high power
inputs when peak instantaneous voltage difference across
the sample holder can be above 30 v. This has also not
proved to be a severe problem in practice, and good results
have been obtained with an unisolated AD524 instrumenta
tion amplifier as the first stage at heating rates up to and
including 1000 K/s in atmospheric-pressure helium and un
der vacuum. At 5000 K/s at atmospheric pressure, or under
most conditions at elevated pressure, temperature readings
with this system were found to suffer from excessive "noise,"
however, and this led to the adoption of the system shown in
Fig. 4, with a floating, isolated OP-07 operational amplifier
as the first stage.
To check the accuracy of the temperature-measurement
system, trials were conducted with mineral salts (NaG,
MgClz, LiCl) in the sample holder. A constant power input
was applied, sufficient to heat the sample holder above the
salt's melting point, and the levels at which the temperature
was momentarily arrested during heating and cooling were
noted. The values obtained were 10-15 K lower than the
melting points, but except for MgC12 , which did not wet the
mesh and therefore did not give such good thermal contact,
they were virtually the same during heating and cooling.
This offset, which is quite acceptable, is therefore not due to
interference from the heating current. Corrosion of the ther
mocouple junction and heat conduction down the thermo
couple wires are possible causes, although the uncertainty
for the output of an uncalibrated type-K thermocouple at
these temperatures, about 6 K, must also be taken into ac
count. A time/temperature trace for a calibration run with
NaCl (melting point 801°C) is shown in Fig. 7.
Even with an optimized control system, some inherent
limitations of ac heating must be considered. The first is that
the temperature of the sample holder will not rise uniformly,
but will fluctuate as the instantaneous heating current var
ies. An upper limit for this effect can, however, be estimated
by considering the most extreme (and impossible) case in
which all the heating current is assumed to flow in a pulse of
negligible duration. Just after the heating pulse, the tempera
ture will jump by the sum of the average temperature in-
1135 Rev. SCi.lnstrum., Vol. 60, No.6, June 1989
•••••••••••••••••• ' ••••• y ••••• :.:.:.:.;.:-;.: 1000
C; 900-
0:
~ BOD,·
'" 700
~
:, 600
(U
(arbitrary un:ts)
time (sees)
FIG. 7. Temperature trace for calibration run with NaCI (melting point
801 'C).
crease during half a mains cycle (Le., in 10 ms) plus the
temperature drop that will occur without heating over the
same period. The cooling that would occur over 10 ms can be
estimated from the experimentally observed initial cooling
rates when power is switched off, about 500 K/s at a typical
peak operating temperature of 700 "C. Therefore, at a 1000-
K/s heating rate even for this extreme hypothetical case
there would be a jump of only 15 K during heating, In prac
tice, however, heating current flows for a significant period
in each half cycle (particularly at high heating rates), and so
much smaller differences will occur.
Another consequence of ac heating is that, when using
feedback control in the system described above, control tem
peratures are only available at intervals of one mains cycle
(i.e., 20 ms) and power-inputlevels also can only be adjusted
at the same interval. This limits the precision with which
rapid heating stages can be terminated. Furthermore, the
temperature observed is an historical average and will corre
spond to the value! of a cycle before the second reading in
the pair is taken. Once again, however, these difterences will
only become significant around 1000 K/s and can be al
lowed felr by terminating heating when the indicated tem
perature is slightly lower than the desired peak value; offsets
between ~ (the theoretical optimum) and one mains cycle's
temperature increment have been found adequate in prac
tice.
Potentially, much more serious temperature-control
and measurement problems arise from the inevitable varia
tions in temperature across the sample. While overall the
thermal mass of the coal sample (typically 7 rug) is negligi
ble compared to that ofthe log sample holder, the local effect
ofthe sample is important; even at less than monolayer load
ings the effect of a lOO-,u.m-diam particle resting on a mesh
woven from 40-pm-diam wires clearly cannot be ignored.
This was noted by Freihaut, Zabielski, and Seery,22 who
found that differences of the order of 100-300 K could occur
between a thermocouple placed in the area of the sample and
another deliberately located in a clear area of the mesh. Since
the potential errors from sample loading are thus at least as
great as errors in temperature measurement or control, par
ticular care is taken in our experiments to obtain as uniform
Pyrolysis apparatus 1135
." •• "." •• -............ '.-.'.". • •••••••••••••• -••• -•• -••• _., •• -;O;' ••• ~ ••••••••• ~ ••• :.~.:.:.-:.:.:.;.:.~ •••• , •••••••••••••••••
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51a sample distribution as possible and to allow for the inevita
ble thinning out toward the edge of the sample by placing the
thermocouples at the center and edge of the sample (the
extremes of the temperature) and using the average as the
control value. As already noted, the inherent limitations in
heat transfer to the sample may still be a problem, however,
particularly for nonmeiting coals, with the true particle tem
perature falling below the measured sample holder tempera
ture during very rapid heating ( although many low-and
high-rank bituminous coals which are not classified as cok
ing coals will melt at heating rates of the order of 10J K/s and
above) or in the initial stages of heating in vacuum when
heat transfer is poor in the absence of a conducting gas film.
The high-pressure configuration for the apparatus is
shown in Fig. 8. In this mode a stainless-steel pressure vessel
is attached by a clamping ring which screws onto the outside
of the base itself. The apparatus, with the electrode assembly
in position, has been hydraulically tested to 300 bars, giving
a 50% safety margin over the maximum supply pressure of
200 bars.
When the high-pressure design was conceived, it was
hoped to provide a forced sweep of gas through the sample
holder, as for atmospheric-pressure operation. Even at 20
bars, however, the cooling effect of gas flowing through the
sample holder was found to be so intense that uniform tem
peratures could not be maintained, with slight deviations in
the local cooling intensity giving rise to severe overheating
and melting of the sample holder material. After trials with
various gas-flow arrangements, the best that could be
achieved was to provide a diffuse flow of gas upwards from
the base of the vessel with a volumetric flowrate (at the inter
nal pressure) of 1 11min.
HP GAS
INLET STAINLESS-STEEL r-PRESSURE VESSEL
CLAMPING RING
FIG. 8. Wire-mesh reactor configured for high-pressure operation.
1136 Rev. SCi.lnstrum., Vol. 60, No.6, June 1989 Gas (helium or hydrogen) is fed through a connector in
the base immediately below the working section, with a dif
fuser consisting of approximately 20 layers of wire-mesh in a
brass frame to break up the jet from the small-bore inlet. No
sinter disk is used below the suppport plate in this case. A
simple gas supply system has been found satisfactory, with
the regulator on the gas cylinder controlling the internal
pressure which can be held within 1.5 bars of the desired
value over a range of20--170 bars. The gas flowrate is set by a
pressure letdown/flow control valve on the outlet line from
the reactor and measured, at atmospheric pressure, by a dry
gas meter. Average fiowrates are calculated over lO-s inter
vals by counting (using the timer/counter function ofa Mo
torola 68230 PIA on a second QL microcomputer) the out
put pulses from an optoelectronic shaft encoder added to the
gas meter in-house.
The diffuse flow regime allows the pressure to be held
constant when thermal expansion takes place during an ex
periment and provides some entrainment of the volatile
products. A significant proportion of the condensible vola
tiles are deposited on the internal surfaces of the reactor,
however, and some also appear to recirculate back onto the
sample holder, and so tars cannot be collected and measured
'as was the case at atmospheric pressure with the sweep gas
flow forced through the sample holder.
Apart from the inability to collect representative tar
samples, it was also thought that the absence of a proper
sweep flow might affect mass transfer from the pyrolyzing
sample and depress total volatile yields. To allow this effect
to be estimated. coal was pyrolyzed at atmospheric pressure
with the diffuse gas flow from the high-pressure inlet, but
using a glass top so that the flow patterns could be observed.
The absence of the forced sweep was found to cause only
1 %-3% reduction in total volatile yields,23 however, and
since tars could be seen to be recirculated back onto the sam
ple holder (which was partly discolored) by natural convec
tion, some, if not all, of this reduction can probably be as
signed to redeposition of volatiles rather than to a significant
increase in the surface mass transfer resistance.
Even with the diffuse flow regime, heat losses by convec
tion from the sample holder are very large at elevated pres
sures. At 70 bars the power input must be increased approxi
mately fivefold compared to atmospheric-pressure
operation to hold the temperature steady at the same value
and the ratio between convective and other heat losses,
which can be estimated from vacuum experiments to be
roughly 1: 1 at atmospheric pressure, then rises to about 9: 1.
With convection so dominant, only slight variations in the
gas flow are needed to cause significant (up to about ± 50
K) fiuctations in the local temperature ofthe sample holder,
as the typical high-pressure time/temperature output in Fig.
9 shows. Although the computer feedback-control system
can usually hold the average of the readings from the two
thermocouples within 20 K or less of the desired value, the
instantaneous difference between the individual readings is
determined solely by the unsteady physical conditions inside
the reactor. Similar fluctations in temperature at high pres
sures (measured with a single thermocouple and at a con
stant heating voltage) are reported by Anthony et al.,2 who
Pyrolysis apparatus 1136
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51--contro: temperature
~ ind :vidue I TIC read ings
PIG. 9. Temperature traces from a pyrolysis run in hydrogen at 70 bars, with
diffuse flow regime.
used an insulated baffle below the sample holder to reduce
circulation currents. The temperature fluctuations have a
time scale of the order of 0.2 s, and so to give reasonably
representative time averages for the peak temperature (rath
er than a possibly misleading instantaneous value), a hold
ing period of at least 5 s at peak temperature has been used.
The peak temperature that can be maintained is also restrict
ed by the high power levels required under elevated pressure,
for example, a holding temperature of 800°C is the limit in
70 bars of hydrogen with the present transformers.
II. RESULTS AND DISCUSSION
The effect of peak temperature on total volatile and tar
yields from atmospheric-pressure pyrolysis of a U.K. bitu-
(i)
CIl x
~20
o >
o L----' __ L_--L._---L---::=--=-==--=-='
300 400 500 600 701] 800 900 1000
peak temperature (deg.C)
x • 1000 K/s vol's; + , 1000 K/s tar
t:". 1 K/s vol's; 'V. 1 K/s tar
FIG. 10. Atmospheric-pressure pyrolysis yields from Linby coal for 1-and
lOOO-K/s heating with 30-5 hold at peak temperature (helium sweep at ap
proximately 1.2 bar; flowing at 0.1-0.3 m/s),
1137 Rev. Sci. Instrum., Vol. 60, No.6, June 1989 TABLE I. Summary of tar and total volatile yields from atmospheric-pres
sure pywlysis of Linby coal in helium for heating rates of I and lOOO K/S to
700 "C with a 30-s holding period,
Mean Standard
Number value deviation
ofrulls % w/w, dafcoal % w/w, dafcoal
Tar yield 10 28.9 1.24
1000 K/s
Tar yield 5 22.4 1.67
I K/s
Total volatiles 18 45.9 0,97
1000 K/s
Total volatiles 5 39.7 0.95
I K/s
minous coal (Linby) for 1-and lOOO-K/s heating is shown
in Fig. 10. The averages and standard deviations for the mul
tiple values obtained at 700 °C peak temperature are tabulat
ed in Table 1. The temperature was held at the peak value for
30 s in these experiments; preliminary trials21.24 had shown
that unless sufficient time was allowed at the peak tempera
ture for reactions to run to completion, lower heating rates
could give higher yields in certain circumstances simply be
cause more time was available at reaction temperatures.
Similar increases in total volatiles with heating rate have also
been noted for a range of U.S. coals.24 Data obtained wi.th
one of these coals, Pittsburgh No.8, are presented in Fig. 11.
~50 o
u
f20 ~
..., .
~10r I
g D '-'-~~ ___ ..-L ___ -L--.J
1 10 100 1000 heating rate (K/s)
Atm. press.: x, vol's
+ , tar
Vacuum: 'V, vol's
f'Hi. 11. Effect ofheatlllg rates on pyrolysis yields from Pittsburgh No.8 at
atmospheric pressure (helium sweep at approximately 1.2 ba:; flowin~ at
0.1 m/s) and vacuum ( < 1 m Torr); final temperature 7oo"C tor all pomts
with a 30-s holding period.
Pyrolysis apparatus 1137
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51This shows that the yields increase progressively with heat
ing rate between 1 and 1000 K/s, rather than changing more
abruptly around some critical value, and confirms that most
of this increase is due to extra tar production. Similar trends
have also been found for the Linby coa1.25 Total volatile
yields obtained at 1 and 1000 K/s in vacuum are also shown
in Fig. 11 and confirm that the effect of heating rate is, if
anything, enhanced under vacuum, presumably due to bet
ter mass transport. Conversely, as Fig. 12 shows, volatile
yields at elevated pressure in inert gas show little effect of
heating rate, while in high-pressure hydrogen yields have
been found to be significantly greater with l-K/s heating
than lOOO-K/s heating unless a long holding time (of the
order of several hundred seconds) is used to diminish the
relative importance of the greater time available for hydro
gasification reactions at the lower heating rate.23
Because of the observed susceptibility of yield enhance
ments to external pressure, it is likely that physical phenom
ena, probably associated with differing volatile transport
mechanisms under fast and slow heating, are involved. At
higher heating rates, the obvious increase in sample plasti
city and the greater outward fiowrate of volatiles might be
expected to give better tar transport than diffusion through
the pores of a less-fluid sample undergoing slow heating.
Some chemical effects cannot be ruled out, however, since
similar yield enhancements have been observed with a low
rank coal which did not appear to melt at all, while a high
rank bituminous coal which melted showed no effect of heat
ing rate.24
Practical implications of the effect of heating rate on
pyrolysis yields can be seen in Fig. 13, where ASTM proxi
mate volatile matter contents are compared with total vola-
~::~~~.~
!40 Ii to
-0
"'"
"'30 : +
~20 r
~10l
o ~_--.L
1 10 100 1000 heating race (K/s)
x • hydrogen; + • he I I um
FIG. 12. Effect of heating rate on pyrolysis yields from Pittsburgh No.8 coal
in helium a.nd hydrogen at 70 bars with diffuse flow regime; tina.l tempera
ture 600 °C for all points with a lO-s holding period.
1138 Rev. Sci. Instrum., Vol. 60, No.6, June 1989 70
60
(0
850
'I-
LO
'C
~40
III
CI!
-';::;30
(0
0 >
-20
to
+-'
0 ....
10
70 7S 80 85 90
carbon content (% def) ~
95
o . approx. 5000 K/s to 950 deg,C,
with 5 5 hold at peak temperature.
8 , ASTM proximate volati Ie mBtter.
FIG. 13. Comparison between ASTM volatile matter contents and wire
mesh volatile yields for approximately 5000-K/s heating to 950"C with 5-s
hold for Argonne Premium Coals~ helium sweep at approximately 1.2 bar;
flowing at 0.1-0.2 m/s. ( X 2 signifies two data points too close to plot sepa
ratdy.)
tile yields obtained in the wire-mesh reactor from the range
of Argonne Premium Coal Samples26 at approximately
SOOO-K/s heating (without feedback control) to 950°C,
with a 5-s hold at peak temperature. Previous work has
shown that wire-mesh volatile yields obtained under these
conditions appear to be proportional to apparent volatile
production during combustion in explosion chamber tri
als.27 Further investigations to determine whether or not the
apparent rank sensitivity of the differences between proxi
mate volatile matter and the wire-mesh total volatile yields is
supported by trials with other coals and to estimate the ex
tent to which secondary cracking in the ASTM crucible also
contributes to the differences are currently being underta
ken.2g
ACKNOWLEDGMENTS
The authors are grateful to many colleagues in the
Chemical Engineering Department for assistance with this
project, particularly to Dr. G. Saville and T. Meredith for
their contributions to the mechanical design and construc
tion of the apparatus, to S. Roach who built some of the
electrical components, to C. Smith who did the glassblowing
and made many useful suggestions for the design of the glass
components, and to K. Khogali who carried out the 5000-
K/s runs with the Argonne coal samples. The sample of
Linby coal was supplied by British Coal, Coal Research Es
tablishment, and other coals were obtained from the Ar
gonne Premium Coal Sample programme. Financial sup-
Pyrolysis apparatus 1138
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132.203.227.61 On: Mon, 14 Jul 2014 23:22:51port was provided by the UK Science and Engineering
Research Council under Grant Nos. GR/B/58962 and
GR/D/06582.
'R. Loison and R. Chauvin, Chim.lnd. (Paris) 91,269 (1964) (University
of Sheffield, Dept. of Fuel Tech. & Chem. Eng., Trans. DJB/WBD 1,
1964).
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lnstrum. 45, 992 (1974).
'D. B. Anthony, J. B. Howard, H. C. Hottel, and H. P. Meissner, Fuel 55,
121 (1976)
'E. M. Suuberg, W. A. Peters, and J. B. Howard, Fnc/59, 405 (1980).
'w. S. Fong, W. A. Peters, and J. B. Howard, Fuel 65, 25 I (1986).
"P. R. Solomon and M. B. Colket, in Proceedings of the 17th Symposium
(International) on Combustion, 1979, p. 131.
7G. R. Gavalas and K, A. Wilks, AIChE J. 26, 201 (1980).
xJ. D. Frcihaut, M. F. Zabielski, and D. J. Seery, in Proceedings of the 19th
Symposium (International) on Combustion, 1982, p. 1159.
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20th Symposium (International) on Combustion, 1984, p. 1445.
10J. R. Bautista, W. Jl Russel, and D. A. Saville, Ind. Eng. Chern. Fundam.
25,536 (1986).
"E. M. Suuberg, D. Lee, and J. W. Larsen, Fuel 64, 1668 (1985).
1139 Rev. Sci. Instrum., Vol. 60, No.6, June 1989 12p. C. Stangeby and P. L Sears, Fuel 60, [31 (1981).
IJH, JUlltgen and K. H. van Heck, Fuel 47, 103 (1968).
14J. Desypris, P. Murdoch. and A. Williams, Fuel 61, 807 (1982).
"L, H. Hamilton, A. B. Ayling, and M. Shiboaka, Fuel 58, 873 (1979).
16L. H. Hamilton, Fuel 59, 112 (1980).
17L. H. Hamilton, Fuel 60, 909 (1981).
"S. J. Niksa, Ph.D thesis, Princeton, NJ, 1982.
19J. D. Frcihaut and D. J. Seery, Am. Chern. Soc. Div. Fuel Chem. Prepr.
28 ( 4 ), 265 (1983).
20J. D. Freihaut (personal communication).
2IJ. R. Gibbins, Ph.D thesis, Imperial College, London, 1988.
22J. D. Freihaut, M. F. Zabielski, and D. J. Seery, Am. Chern. Soc. Div. Fuel
Chern. Prepr. 27(2), 89 (1982).
OJ}. R. Gibbins-Matharn and R. Kandiyoti, Am. Chern. Soc. Div. Fuel
Chern. Prepr. 33(3), 67 (1988).
'"J. R. Gibbins-Matham and R. Kandiyoti, Energy Fuels 2, 50S (1988).
2sR. J. O'Brien, J. R. Gibbins-Matharn, C. E. Snape, and R. Kandiyoti, in
Proceedings of the 19i57 International Conference on Coal Science, Maas
tricht, Netherlands, 1987, p. 695.
26K. Vorres, Am. Chern. Soc. Div. Fuel Chcm. Prepr. 32(4), 221 (1987).
"K, L. Cashdollar, M. Hertzberg, and I. Zlochower. in Proceedings of the
22nd Symposium (International) on Combustion, 1988.
'xJ. R. Gibbins, K. Khogali, and R. Kandiyoti, to be presented at the 2nd
International Ro!duc Symposium on Coal Science, 1989.
Pyrolysis apparatus 1139
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1.339177.pdf | Negativeresistance characteristics of polycrystalline silicon resistors
K. Ramkumar and M. Satyam
Citation: Journal of Applied Physics 62, 174 (1987); doi: 10.1063/1.339177
View online: http://dx.doi.org/10.1063/1.339177
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/62/1?ver=pdfcov
Published by the AIP Publishing
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Negative resistance switching in nearperfect crystalline silicon film resistors
J. Vac. Sci. Technol. A 2, 1486 (1984); 10.1116/1.572388
A novel parametric negativeresistance effect in Josephson junctions
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ThinMISStructure Si NegativeResistance Diode
Appl. Phys. Lett. 20, 269 (1972); 10.1063/1.1654143
Theory of a NegativeResistance Transmission Line Amplifier with Distributed Noise Generators
J. Appl. Phys. 31, 871 (1960); 10.1063/1.1735710
An Experiment with a Nonlinear Negative-Resistance Oscillator
Am. J. Phys. 18, 208 (1950); 10.1119/1.1932536
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132.174.254.155 On: Tue, 23 Dec 2014 08:30:25Negative .. resistance characteristics of poly crystalline silicon resistors
K. Ramkumar and M. Sat yam
Department of Electrical Communication Engineering, Indian Institute o/Science,
Bangalore-560012, india
(Received 10 October 1986; accepted for publication 10 February 1987)
This paper presents a theoretical analysis to explain the origin of the observed negative
resistance characteristi.cs of poly crystalline silicon resistors. This analysis is based on the effects
of self-heating of the resistor on the transportation of carriers across the grain-boundary
barrier.
INTRODUCTION
Several investigations, mainly experimental, have been
reported in the literature recently on the switching behavior
of poly crystalline silicon films. 1-5 These have been done with
a view to use the films as memory dements. These films are
found, in general, to exhibit four types of characteristi.cs,
viz., (1) nonlinear high-resistance characteristics (2) nega
tive-resistance characteristics (3) very low resistance char
acteristics, and (4) almost open-circuit behavior. The films
exhibit all these characteristics sequentially as the current
through them is increased continuously. The first two types
of characteristics are found to be reversible,4 but once the
film goes through the latter two types of characteristics, per
manent changes in the structure and characteristics are
found to occur:~ The origin of these observed characteristics
is not clearly understood, although qualitatively it is be
lieved that they are caused by thermal effects.I,5 In this pa
per, an attempt has been made to explain theoretically the
origin of the negative-resistance characteristic by incorpor
ating the effects of self-heating on current transport in poly
crystalline silicon films.
L BASIS FOR ANALYSIS
It is wen known that current transport in a polycrystal
line semiconductor is mainly decided by the potential barrier
formed at the grain boundary.6,7 This barrier is formed be
cause of the capture of free carriers from the grain by the
grain-boundary traps. The charge in the grain boundary due
to the trapped carriers and the charge in the grain due to the
depletion of carriers (for compensating the grain-boundary
charge) give rise to a built-in potential barrier for the free
carriers in the grain. The carrier transport across the grain
boundary barrier is by thermionic emission, while in the
grain it is by drift. As current flows through the polycrystal
line film, power is dissipated in the film, leading to self-heat
ing. At high voltages and currents, the temperature of the
film rises significantly. Because of this increase in tempera
ture, the grain-boundary barrier potential decreases.8 This in
turn leads to a further increase in emission current across the
grain boundary. This results in an increase in power dissipa
tion and hence the temperature. This cumulative process of
increase in temperature and increase in current ultimately
leads to a situation wherein a higher current can be main
tained even with a lower voltage because of the reduction of
the barrier. This gives rise to the negative-resistance charac
teristic beyond a certain voltage. The negative resistance continues up to a temperature at which the grain-boundary
barrier becomes zero, i.e., it becomes totally ineffective. Be
yond this temperature, current flow is essentially controlled
by the bulk resistance of the grains ofthe film, and this leads
to a positive-resistance characteristic beyond a certain cur
rent value.
u. ANALYSIS: Vel CHARACTERISTICS
For purposes of analysis, the polycrystalline silicon re
sistor is assumed to consist of a row of N grains, each of
length L with width wand thickness t as shown in Fig. 1. The
structure of a typical grain boundary considered for analysis
and its energy level diagram in the absence of any applied
voltage are shown in Fig. 2. When a voltage V is applied
across this grain boundary, it is distributed between the
grain ( V r) and the two potential barriers on either side of the
grain boundary (VI and Vz). The energy-level diagram of
the grain boundary with applied voltage is shown in Fig. 3.
This indicates that on one side of the grain boundary, the
barrier is reduced by an amount VI while on the other side, it
is enhanced by an amount V2, Under this condition, it is clear
that the emission of carriers from grain 1 to grain 2 is en
hanced, thus leading to an increase in current. The voltage
drops V', VI' and V2 adjust themselves in such a way that the
continuity of current across the grain is maintained.
The potential barrier at the grain boundary can be ex
pressed as7
0)
where nt is the filled density oftmps in the grain boundary,
NA is the doping concentration of the grain, and 2/1 is the
thickness of the grain boundary.
The emi.ssion current across the grain boundary (GB)
from grain 1 to grain 2 can be expressed as
J (KT)112 (-eVB)[ (eVl) ] GBI = sen 2rrm 'exp -rr exp KT -1 ,(2)
where n is the concentration offree carriers in the grain and s
is a correction factor whose value is 0.25.
The drift current in the grain is given by
In = ne,u[ V'/(2L -2t)], (3)
where,u is the carrier mobility, The emission current across
the grain boundary from grain 2 to grain 1 is given by
174 J. Appl. Phys. 62 (1), 1 July 1987 0021-8979!87/130~ 74-03$02.40 @ 1987 American Institute of PhySics 174
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132.174.254.155 On: Tue, 23 Dec 2014 08:30:25~trc Fttll ' ~~~I " i~ ... F?
.. NL '
FIG. 1. Structure of the polycrystalline silicon resistor considered for analy
sis.
JGB2 = sen(2::)1/2exp( -;;B )[ exp( ~e;2) -1]. (4)
If it is assumed that the regions on either side of the grain
boundary behave like to Schottky-barrier diodes connected
back to back, then, for current continuity,
JGBI = -JOE2 =JB•
Substituting for Je I and JB2, one gets
exp(ev1) + exp( -eV2) = 2.
KTJ KT (5)
(6)
This equation indicates that for any applied voltage, VI and
V2 assume such values as to satisfy the above equation.
To compute the V-I characteristics of the polycrysta]
line silicon film, the following procedure may be adopted.
For an assumed value of VI' Vz is calculated from Eq. (6),
and J GBI is calculated from Eq. (2). Using the value ofJ GBl
and realizing that J GBI = JB, V' can be calculated from Eq.
( 3 ). The total voltage across the grain is given by
V=V'+V 1+V2• (7)
The total voltage across the film, which is made up of N
such grains in series, is NVand the current for this voltage is
wtJB (or Jewt). In the same way other values of voltages
and ~urrents can be calculated for different assumed values
of VI and the entire V-I characteristic of the film can be
calculated for any temperature. Using this procedure the V-I
characteristics at any temperature can be computed.
L
FIG. 2. Grain boundary and the energy-level diagram around it.
175 J. Appl. Phys .• Vol. 62. No.1, 1 July 1987 FIG. 3. Energy-level diagram with the voltage applied to the resistor.
III. EFFECT OF SElF~HEATING
As the current through the poly crystalline silicon film
increases with an increase in voltage, the power dissipation
also increases, leading to self-heating. The heat so generated
is conducted away to its surroundings from its bottom sur
face. In equilibrium, the temperature of the film attains such
a value at which the power generated is equal to the power
lost to the surroundings. This condition can be expressed as
VI =AWo(T -TA), (8)
where A is the surface area of the film, Wo is the power con
ducted away from the film per unit area per unit temperature
difference, T is the equilibrium temperature of the film, and
TA is the ambient temperature,
Furthermore, as the temperature of the film increases
due to self~ heating, the grain-boundary barrier decreases due
to the change in the Fermi level of the grain. The variation of
the barrier with temperature is given byR
Ve = Vno -9,7XlO-4(T-To), (9)
where VBO is the barrier at a temperature To.
The V-I characteristic of the polycrystalline silicon film
at different temperatures can be computed by using the bar
rier potential obtained from this equation, From the V-I
characteristics at different temperatures, for a certain value
of voltage, the currents at different temperatures are ob
tained and a curve of I vs T is plotted. For the same value of
voltage, another curve of Ivs Tis plotted from Eq. (8). The
point of intersection of these two curves provides the net
current at that voltage with self~heating taken into account.
These values of voltage and current provide one point on the
V~I characteristic. Along the same lines, for other values of
voltage the corresponding values of current are obtained and
in this way the net V~I characteristic of the film can be com~
puted, This procedure is valid up to a point at which the
temperature is such that the potential barrier becomes zero.
Beyond this point, the value of the bulk resistance of the
grains is used to compute the V-I characteristics.
IV. RESULTS AND DISCUSSION
Using the procedure indicated above, the V-I character
istics of a typical n-type po!ycrystalline silicon resistor have
been computed. The parameters of this resistor are length,
200 .urn; width, 10 pm; thickness, 5,um; and doping con.c~n
tration, 1016 em -3. Wo depends en the thermal conductIVity
K. Ramkumar and M. Sat yam 175
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132.174.254.155 On: Tue, 23 Dec 2014 08:30:25]
.... z
UJ a:: a::
::l u 200
2.0
I
D·t
~
~ VOLTAGE (Volls)
Ib) N = 100
Er = 0·37 eV
NT = 4 ~ 10'Ocm-2
L = 2 "m
W = 1();.rr.
t = 5 ~rn
"110= 1.0.10-4
N = 100
Er. 0·37ell
NT = 4 X 1010 cm-2
L = 2f!m
W ~ 10 I'm
t = 5IJm
Wo: 4>10-4
o.oJ ___ L-._. _ -L. ___ ~,_,_-L_. __ .L._._ o 40 80 120 160 200 240
VOLTAGE (llolls)
FIG. 4. Computed V-J characteristics of the polycrystallirre silicon resistor
with the effect of self-heating taken into account.
of the substrate which has been taken as a varying parameter
with values ranging from 10"4 to 10--3 W /cm2 /K. The com
puted characteristics are shown in Figs. 4 (a) and 4 (b ). The
figures show that the computed characteristic exhibits a neg
ative-resistance region similar to the observed characteristic
176 J. Appl. Phys., Vol. 62, No.1, 1 July 1987 o ...... ="---'-__ -'-__ .-L __ --LI ___ -' __ --11
o 2 3 4 5 6
V (Volts) __
FIG. 5. Observed V-I characteristics of a polycrystalline silicon resistor as
reported by Greve (Ref. 3).
reported by Greve,3 reproduced in Fig. 5 for purposes of
comparison. The calculations also show that Wo decides the
voltage at which the negative resistance sets in. Thus the
model proposed here gives a clue to the processes responsible
for the negative-resistance characteristic of polycrystaUine
silicon films that have been reported in the literature. It also
indicates the parameters that control the negative-resistance
characteristics. It may be realized that in this model, the
entire film is considered to be at one temperature. However,
at higher currents, temperature gradients appear along the
width of the film, the central region being at the highest
temperature. At very high current levels the temperature of
the central zone may reach such values at which localized
melting may take place, leading to the formation of low
resistance paths. A detailed analysis of this process is being
carried out.
'I. E. Mahan, App!. Phys. Lett. 41, 479 (1982).
2H. Kroger, H. A. R. Wegner, and W. M. Shedd, Thin Solid Films 66, 171
(1980).
3D. W. Greve, IEEE Trans. Electron Devices 1<;0-29,719 (1982).
4c. Y. Lu, N. C. C. La, and C. C. Shih, J. Electrochem. Soc. 132, 1193
(1985 ).
5V. Malhotra, J. E. Mahan, and D. L. Ellsworth, IEEE Trans. Electron
Devices ED-32, 2441 (1985).
"J. Y. W. Seto. J. App!. Phys. 46, 1240 (1975).
7K. Ramkumar and M. Sat yam, Appl. Phys. Lett. 40, 898 (1981).
"C. H. S~lger and G. E. Pike, App!. Phys. Lett. 35,709 (1979).
K. Ramkumar and M. Sat yam 176
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1.344471.pdf | Nonlinear optical investigation of the bulk ferroelectric polarization in a vinylidene
fluoride/trifluoroethylene copolymer
A. Wicker, B. Berge, J. Lajzerowicz, and J. F. Legrand
Citation: Journal of Applied Physics 66, 342 (1989); doi: 10.1063/1.344471
View online: http://dx.doi.org/10.1063/1.344471
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/1?ver=pdfcov
Published by the AIP Publishing
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Appl. Phys. Lett. 95, 023303 (2009); 10.1063/1.3176213
Optical sensitization at the phase transition in the ferroelectric vinylidene fluoridetrifluoroethylene copolymer
Appl. Phys. Lett. 57, 2532 (1990); 10.1063/1.103846
Effects of electron irradiation and annealing on ferroelectric vinylidene fluoridetrifluoroethylene copolymers
J. Appl. Phys. 62, 994 (1987); 10.1063/1.339685
Ferroelectric properties of vinylidene fluoridetrifluoroethylene copolymers
J. Appl. Phys. 52, 6859 (1981); 10.1063/1.328679
Nonuniform polarization of vinylidene fluoridetrifluoroethylene copolymer
J. Appl. Phys. 52, 6856 (1981); 10.1063/1.328678
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130.209.6.50 On: Fri, 19 Dec 2014 22:10:25Nonlinear optical investigation of the bulk ferroelectric polarization
in a vinyUdene fluoride/trifluoroethyiene copolymer
A. Wicker, 8. Berge, and J. Lajzerowicz
Laboratoire de Spectrometrie Physique (associe au Centre National de fa Recherche Scientijique),
Universite Joseph Fourier Grenoble, B.P. 87, 38402 Saint lJartin d'Heres Cedex, France
J. F. Legranda)
lnstitut Laue-Langevin, B.P. 156X 38042 Grenoble Cedex, France
(Received 30 September 1988; accepted for publication 14 March 1989)
The ferroelectric polarization ofvinyliden.e fiuoride-trifiuoroethylene copolymers (70/30
mol %) is investigated using complementary measurements of the surface charge and the
second harmonic intensity (at 530 nrn) generated in the polymer from a Nd:YAG laser beam
(at 1060 nrn). Due to the nonzero electrical conductivity of the polymer (especially above
room temperature), the nonlinear optical technique provides better measurement of the bulk
polarization, its changes with time, the applied electric field, and temperature. First, we
present optical results obtained on polarized films after removal of the electrodes: they confirm
the proportionality between the second harmonic intensity and the square of the ferroelectric
polarization, and the centrosymmetric character of the parae1ectric phase; they also show the
temperature dependence of the remanent polarization in the crystal phase. Second, we present
simultaneous measurements of the surface charge and of the second harmonic intensity under
very low-frequency applied voltage through transparent electrodes. The field dependence of the
measured polarization is discussed in terms of a microstructural analysis of the dielectric
properties in the semicrystalline material. It is also shown that unipolar voltages applied
during long periods of time are able to produce charge injection and space charge in the
vicinity afthe electrodes which can result in screening of the applied electric field.
I. INTRODUCTION
Copolymers ofvinylidene fluoride (VDF) with trifluor
oethylene (TrFE) or tetrafluoroethylene (TFE) have n.o
ticeably modified the field of piezoelectric polymers, espe
cially because they are able to crystallize spontaneously into
a ferroelectric phase,I-3 and also because they exhibit a high
degree of crystallinity and strong piezoelectric and pyroelec
tric activities.4-1l Indeed, in these semicrystaUine materials,
the piezoelectric and pyroelectric coefficients are propor
tional to the ferroelectric· polarization of the crystalline
phase and to the degree of crystallinity of the polymer. 7-1 I So
far, several surface charge measurements during polariza
tion switching have been reported, but these have encoun
tered many practical difficulties, especially due to the non
zero conductivity of the materials. 10-16
We present in this paper an optical technique, based on
second harmonic generation (SHG) of light, which allows
nondestructive analyses of the bulk polarizatiollo Specifical
ly, it permits real time investigations to be made under ap
plied electric fields, applied stresses or temperature changes.
It is well known that in noncentrosymmetric solids, one
can observe SHG oflight, 17--18 and this property has already
been used to characterize the polarization of some ferroe!ec
tries 1 9-21 or to design frequency doublers and optical mixing
devices.22,23 Concerning the new ferroelectric copolymers,
only a few preliminary studies using SHG of light have been
reported so far. 10,24.25 We chose to study the VDF-TrFE co-
a) To whom correspondence should be addressed. polymer with composition 70/30 mol %, because of its high
degree of crystallinity and its high remanent polarization
Therefore this composition is one of those most commonly
used in applications, and we have already performed several
structural studies of this copolymer. 4.8,10,26.27
iI.SAMPLES
For optical investigations and poling studies, thin copol
ymer films (3-30 11m) cast from solutions were preferred to
thicker films obtained by hot pressing, for two reasons. First,
they show better transparency and produce less light scatter
ing, Second, they require lower voltage and allow perfor
mance of time-dependent investigations of polarization
switching in a shorter time range (do to the limited ramping
time of the voltage supply). Also, it has been shown that the
coherence length for second harmonic generation in this ma
terial is approximately 351lm.28
The raw material provided by Atochem Company,
France (70/30 VDF-TrFE copolymer, samples of reference:
P 1178 and R2457) was dissolved in methyl-ethyl-ketone.
The solution was cast on a spinning substrate (silicon wa
fer), and then dried in an oven. The desired final thickness of
the polymer film (3-10 pm) was adjusted by changing the
concentration of the solution and the spinning speed. For
greater thicknesses, the spin coating operation was repeated
two or three times.
Two different kinds of specimens were prepared for the
optical studies: (0 For "a posteriori" analyses of the reman
ent polarization (Sec. IV), the polymer films were cast on
silicon wafers coated with aluminum. After annealing the
342 J. Appl. Phys. 66 (1),1 July 1989 0021-8979/89/130342,08$02.40 @ 1 Sa9 American Institute of Physics 342
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130.209.6.50 On: Fri, 19 Dec 2014 22:10:25thin films on the substrate (at 130°C, for 1 h), counterelec
trades were evaporated through circular masks of 4 mm
diam and the electroded zones were polarized using a very
low-frequency ac voltage (0.1 Hz) ofincreasing amplitude,
with continuous control of the surface charge. 10 Afterwards,
the electrodes were removed by chemical attack and the
transparent film detached from the substrate.
(Ii) For real time investigations of the polarization pro
cess using SHG (Sec. V), the cast polymer films were melted
between two glass plates coated with conducting and trans
parent ITO (indium tin oxide), and after a few minutes at
200 °C, the sandwiched specimen was rapidly cooled to
room temperature.
m. EXPERiMENTAL SETUP
We measured the SHG of the light using a Nd:YAG
laser at 1060 nm (Microcontrol, France), with a pulse \vidth
of 150 ns, a maximum instantaneous power of 4 kW, and a
repetition rate of 1 kHz. The laser beam was focused on the
sample with lenses of focal length from 10 to 50 em. The
beam polarization was set horizontal and the copolymer film
could rotate around a vertical axis, alJowing the electric field
of the incident light to have a nonzero component along the
norma! to the film (which is the direction of the applied dc
field and of the resulting ferroelectric polarization). The sec
ond harmonic light at 530 nm was measured with a low
noise photomultiplier either in photon counting or in contin
uousmode.
The planar copolymer films were mounted on a step
motorized translation stage for computer-controlled X-Y
displacements parallel to the plane of the film. The transla
tion stage permitted us to scan through the sample and to
record "polarization maps" of the ferroelectric films. The
spatial resolution of such two-dimensional images of the
specimen was not really limited by the mechanical resoh!
tion, but principally by the size of the laser spot of about 20
pm or greater (depending on the incidence angle and on the
focusing lens). During such measurements, the use of a ref
erence quartz crystal to compensate the fluctuations of the
laser intensity ftnaHy proved ineffective due to the short inte
gration time (0.1 s) necessary for recording images made of
about 5000 pixels.
For investigations above room temperature, the speci
men was positioned in a temperature controlled cell
(Mettler, ref: "FPS"), and for measurements below room
temperature we added a flow of cooled nitrogen circulating
through the furnace. The temperature of the sample was
monitored using a 50-H platinum resistor, but the local tem
perature across the laser beam might have been higher by a
few degrees than that of the resistor. The electric field was
applied to the specimen using a high-voltage ( ± 10 kV)
bipolar amplifier (Trek, ref: "609A") driven by a computer
controHed stabilized generator. The surface charge at the
electrodes was measured by using the Sawyer and Tower
method (with inclusion in the circuit of a 40-,uF series ca
pacitor). Data acquisition and control of the experimental
parameters were done by a microcomputer (Hewlett-Pack
ard, ref: "{PC") ,a digital voltmeter, and a scanner (Keith
ley, refs: "19SDMM" and "705") with IEEE interfaces.
343 J. Appt. Phys., Vol. 66. NO.1. 1 July 1989 IV. OPTICAL iNVESTIGATIONS AFTER POLARIZATION
The optical setup was first used to investi.gate the princi
pal characteristics of the second harmonic light generated in
copolymer films of various thicknesses and different degrees
of polarization.
A. Theoretical background
According to classical analyses ofSHG, l7-20 the coher
ent optical wave at frequency OJ traveling through a homoge
neous noncentrosymmetric medium generates a coherent
optical wave at frequency 2UJ whose amplitude E( 2w) is pro
portional to the square of the optical electric field E «(iJ ),
proportional to the SHG coefficient (element of a third rank
tensor), and proportional to the optical path through the
medium as long as the phase difference due to the optical
dispersion, new) -n(2UJ), can be neglected. For the
P(VDF-TrFE) copolymer we measured an average refrac
tive index n = 1.42, an optical dispersion of about 0.0077,
and a corresponding coherence length ( = 34.5 pm. 2~ From
symmetry analysis, the SHG tensor transforms as the pie
zoelectric tensor according to the representation of the spon
taneous polarization P,. Therefore, it can be expanded in
odd powers of Ps and generally the linear dependence is
enough to describe the observed intensities.
For an incidence angle 0 of the laser beam on a slab of
thickness L made of uniaxial material, and in the presence of
optical dispersion, a general expression for the transmitted
second harmonic intensity has been gi.ven by Jerphagnon
and Kurtzl8:
12,0 un = (51217'2/cw2)d(B)2t", (f)) 4 T2uy (B)p(B)2
x{J;;,/[n(w)2 -n(2wf]2} sin2 W,
wherewis the radius ofthespot, dCB) is a linear combination
of the components d31 and d33 of the second harmonic ten
sor, t", (B) and Tzu, «()) are, respectively, the transmission
factors at frequencies wand 2{iJ, p( e) is a projection factor
depending on the symmetry of the tensor d and
'II = 1rL 121" (e)
= (217'L I A.) [no(lu )cos ();" -ne C2UJ )cos e ;u,]'
sin2 'Ii is an oscillating function which describes the "Maker
fringes."!? We used the above expression to analyze the mea
sured SHG intensities, taking for the copolymer specimens
d33 = OA8dll of quartz, d31 Id}3 = 0,23, and their birefrin
gence ne -no = -0.0083 (for details, see Ref. 28).
B. SHG topography of the polarized zones in polymer
films
The results presented in Fig. 1 show the SHG intensity
transmitted through a copolymer film of 7 pm thickness.
The film had been polarized at about one half of the satura
tion value, using an evaporated circular electrode of 4 mm
diam which was later removed. Figure 1 (a) shows an image
of the brightness of the SHG intensity recorded in a rectan
gular frame made of 183 X 46 pixels, and Fig. 1 (b) shows the
corresponding intensity profile along the X I X line. The ob
served intensity fluctuations through the polarized zone are
due less to polarization or thickness inhomogeneities than to
Wicker et al. 343
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130.209.6.50 On: Fri, 19 Dec 2014 22:10:25n?-ll ~C \.
t!) 0 -' :c. 2~---W
'" DISTANCE (mmJ x ( a )
( b )
( ( )
FIG. l. SHG analysis of the remanent polarization in a thin film of VDF
TrFE copolymer (composition: 70/30 mol %; thickness: 7 p.m; polariza
tion: 40 rnC/m2; incidence angle ufthe laser beam: 45'). (a) SHG image of
the polarized zone (.lX steps: 6 ,urn; D, Y steps: 20 pm; 10 levels of grey
scale). (b) SHG intensity profile along the XX' diameter, (c) Magnified
image of a breakdown region (AX steps: 1.5,um; .:l Y steps: 5 pm ).
fluctuations in the power 10> of the incident laser beam, as
shown by time evolution at a fixed position.
Let us recall here that due to the square dependence in
1(,;, the corresponding relative fluctuations are approximate
ly doubled in the SHG process. Figure 1 (c) shows the mag
nification of an inhomogeneity detected in the polarized
film. The image, made of 80 X 20 pixels, has been obtained
using an X-Y scan with smaller steps, and it shows the high
spatial resolution of about 20 !-tm obtained with this tech
nique. Figure 1 (c) reveals a ring of overintensity around a
dark spot which appeared to be a breakdown channel
through the polarized film. The overintensity around the
hole might be attributed to a higher polarization due to some
local reduction in the film thickness (which could also ex
plain the breakdown in the center); but one must also con
sider the possibility of a local increase of the nonlinear sus
ceptibility which could be due to defects induced by the
breakdown (like trapped charges, free radicals, double
bonds, etc.).
C. Proportionality between 120> and (Pr),z
The proportionality between the second harmonic coef
ficient d«(}) and the remanent ferroelectric polarization P,
was analyzed using the fonowing method. Several circular
electrodes were evaporated on the same polymer film of ho
mogeneous thickness (t = 6.0 pm) and these zones were
polarized at different levels using low-frequency ac fields
(0.1 Hz) of different amplitudes. 10
Over each of these zones the second harmonic intensity
was recorded by X-Y scanning and therefore the average
value of /20} could be compared to the surface charge mea
sured by the Sawyer and Tower method. The results shown
in Fig. 2 confirm that the observed intensity I2w is propor
tional to the square of the remanent polarization Pr with a
344 J. App!. Phys., Vol. 66, No, 1,1 July 1989 POLING
FIELD
(MV
60
80 Pr I (2w)
(mC/m2) (a. \J .l
37 1950
52 3510 I (2",) SHG
Pr"-TOPOGRAPHY
1.4
1.3
FIG. 2. Comparison between the square of the remanent polarization P,
and the second harmonic intensity /2" generated under the same conditions.
Distinct zones in the same sample were poled with fields of different ampli
tudes (thickness: 6,um; incidence angle: 45').
discrepancy smaller than 5% (originating from possible er
rors in the measurement of the surface charge, from thick
ness inhomogeneities, or from nonlinearity of the photomul
tiplier response).
D. Temperature dependence of the ferroelectric
polarization
Reversible and irreversible effects of temperature
changes on the real bulk polarization were analyzed using
SHG measurements and considering the square root of the
observed intensity. Figure 3 shows the temperature depen
dence of the remanent polarization when the specimen is
heated at + 2 K/min. It appears that the ferroelectric polar
ization decreases almost linearly from 300 to 365 K, while it
drops abruptly in the region of the Curie transition between
365 and 385 K. It is also shown in the inset that,up to 360 K,
the temperature dependence of the polarization is almost
reversible upon cooling, while, as soon as the region of the
transition is reached, irreversible depolarization occurs.
This means that the ferroelectric domain pattern ofthe crys
talline phase is stable up to 360 K, while in some crystalline
zones a domain pattern with zero average polarization ap-
30
::::l
~1I"''''''''~t'''''''I'''''''''''liI!IlIli~
I1J 25 t +
>-~~ + 20 + I-30 -~ +
H +
U1 .~w'~ +
z
15 f . """- + w ++ + I- 20 "" TEKP.- + + z .l TEItP., .. + if. H 10 +
+
CD 10 + +
I 280 340 360 380 + U1 5 +
+ + +
°280 300 320 340 360 380
TEMPERATURE (K)
FrG. 3. Square root of the SHG intensity :recorded during a single heating
run at + 2 K/min (composition: 70/30 mol %; thickness: 21 ,urn; initial
polarization: 65 mC/m2; incidence angle: 30'). (Inset) successive heating
and cooling runs below and inside the region of the Curie transition (other
sample with thickness: 6 pm and polarization: 70 mC/m2 ).
Wicker et al. 344
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130.209.6.50 On: Fri, 19 Dec 2014 22:10:25pears after heating in the transition region. In order to evalu~
ate the local heating in the zone across the Laser beam we
used capacitance measurements to determine the Curie tem~
perature on a similar poled specimen and we found Tc = 380
K. Thus we conclude that the local temperature in the region
of the SHG measurement is less than 10 K higher than the
temperature given by the thermometer. From these results,
we can conclude that SHG allows a nondestructive analysis
ofthe remanent polarization and of its thermal stability, we
obtain the temperature dependence of the spontaneous fer
roelectric polarization between 300 and 370 K, we confirm
the strong first-order character of the ferroelectric transition
taking place between 370 and 390 K, and we can derive an
estimate of the pyroelectric coefficient dP, I dT = 45
ftC m-2 K -1, almost constant from room temperature up
to the transition region (for the above estimate we took for
the remanent polarization at room temperature the value
deduced from surface charge measurements: P, = 70
mC/m2).
E. Centrosymmetric character of the paraelectric phase
As can be seen on Fig. 3, the second harmonic intensity
is zero in the paraeIectric phase as expected for the ferroelec
tric polarization. However, in this high-temperature phase
ofhexagonai symmetry this is not a proof of centrosymmetry
due to the random orientation of the crystal axes, especially
if one considers the only two parent groups of the ortho~
rhombic mm2 phase:29 these are 61mmm (centrosymme
tric) and 62m (piezoelectric but nonferroelectric). There
are in the latter case several second harmonic coefficients
(dll, d12, and d26 ) whose resulting effect averages to zero if
the hexagonal axes of the crystallites are not oriented. In
order to resolve this ambiguity we performed SHG measure
ments on polymer films oriented by roUing, and above Tc we
detected no second harmonic intensity regardless of the inci
dence direction or polarization of the laser beam. Therefore,
we conclude that the only possible hexagonal symmetry
group of the paraelectric phase is the centrosymmetric group
6/mmm.
V. SHG MEASUREMENTS UNDER APPLIED ELECTRIC
FIELD
Using semitransparent ITO electrodes, we were able to
perform, for the first time, realtime measurements of the
second harmonic intensity generated by a polymer film dur~
ing the buildup and the switching of the ferroelectric polar~
ization (under relatively slow variations of the applied
field) .
A. Second harmonic hysteresis loops at room
temperature
First, in order to get more accurate measurements of the
intensity, we recorded the SHG under slow variations of the
applied electric field. Figure 4(a) shows the variation ofthe
second harmonic intensity generated in a film of 21 J1ffi
thickness, during a field cycle of very low frequency
if = 10 -3 Hz). It must be mentioned here that such a sym~
metrical loop is obtained only after repeated electrical cy-
345 J_ AppL Phys., Vol. 66, No.1, 1 July 1989
.•.•••.••• -.-................. • -•••• .-e, ••• _.--. ••.•• _ ••••••.••••.••••• "._._ •••.•••••••• y •••.• .-•.• -; •.•••••••••••••• y.:.;.;.;.;o; •••••• -.-••••• ':",.;:.;:.:.:.;.;> ••• ·.v.·.·.v ... -....... -..•... v ... . FIG. 4. (a) Hysteresis loop of the second harmonic intensity after about 30
field cycles at room temperature (composition 70/3() mol %; thickness: 21
flm; frequency; 0.001 Hz). (b) Square root of the SH intensity plotted in
Cal. (c) Hysteresis loop of the surface charge recorded with the same sam
ple.
cling of the specimen. Then, the switching of the local polar
ization (in the area of the focused laser beam) is really
abrupt, and the saturated values of the SHG intensities in +
and -polarizations are almost i.dentical. However, a small
asymmetry remains between the positive and the negative
coercive fields: it could be a memory effect of the sign of the
first polarization of the specimen or of its storage with a
nonzero polarization. In order to get a more classical repre~
sentation of the polarization changes versus electric field, we
plotted in Fig. 4(b) the square root of the SHG intensity
with a change of sign at each zero value: this represents the
ferroelectric hysteresis loop obtained by SHG measure
ments. It is interesting to compare it with the hysteresis loop
obtained by surface charge measurements [Fig. 4 (c) J .
These last measurements were performed at a little higher
frequency if = 10 -2 Hz) in order to eliminate drift due to
nonzero conductivity. Two principal differences can be ob
served with respectto the SHG cycle: (i) the switching of the
polarization is less abrupt and starts at lower fields, and (ii)
the slope dP I dE around zero field is more pronounced than
in the SHG measurements.
The first observation was understood with the use of
SHG topography in the region of the coercive field: after
melting between the ITO electrodes, the polymer film was
slightly wedge shaped and therefore the switching started at
lower applied voltage in the thinner region. Concerning the
second observation, one can compare the observed slopes
with those expected from ac dielectric measurements in po-
Wici<eretal. 345
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130.209.6.50 On: Fri, 19 Dec 2014 22:10:25larized films. Let us recall first that. on conducting elec
trodes normal to the polarization direction, the surface
charge is equal to the electric displacement D inside the ma
terial with D(E) = EoE + P(E).
For ac fields of small ampiitude one can write
peE) = Pr + Eo (E -1 )E, with Pr the remanent polariza
tion (in the crystalline phase), and Eo (E -1) E = 6.P"
+ IlP" the reversible polarization induced by the field in
both the amorphous and crystalline phases (respectively,
6.Pu and APe)' In the above expressions E is the dielectric
constant along the polar axis whose measurements at 1000
Hz give E = 8 (in a poled specimen at room temperature). 10
From the slope of the surface charge hysteresis loop D(E)
around E = 0, one can evaluate the dielectric constant at
0.01 Hz and one gets a higher value (E = 15) reflecting the
role of mobile charges in dielectric measurements at very low
frequencies. In contrast, the SHG hysteresis loop peE) has a
much smaller slope which gives a dielectric constant E of the
order of2.5 (at 0.001 Hz). This means that the proportional
ity between the 8H coefficient and the polarization at zero
field (Pr) does not hold under applied field; and this can be
understood by considering that the nonlinear susceptibility
is likely to be different in the amorpbous regions with respect
to the crystalline ones.
Taking account of the very small size of the mixed
phases one can write the square root of the SH intensity in
the form
fi(E)-= [kallPa + ke (fJ.Pc + Pr)],
which is not proportional to P(E) in the general case where
ka #k('. As shown by other resuits,27.30 the relatively high
dielectric constant at 1000 Hz is attributed to the high (lin
ear) polarizability of the amorphous phase (IlPa > IlPe).
Therefore, the small slope of the SHG hysteresis loop, com
pared with that of the surface charge, confirms that the
amorphous phase has a low nonlinear susceptibility at opti
calfrequencies (ka / kc <{ 1, or possibly ka / kc < 0).
This result supports the view that the SH intensity
comes almost uniquely from the polarization of the crystal
line phase (either remanent or induced), and that the dielec
tric constant along the ferroelectric axis is rather low in the
crystalline phase. It is also remarkable that, after being satu
rated by repeated cyclings, the crystalline polarization in the
copolymer sample is very stable and insensitive to the field.
This is in contrast to the case of the homopolymer PVDF in
which a reversible change of crystallinity under applied field
has been reported,31 leading to a larger slope of the hysteresis
loops around E = O.
B. Absence of second harmonic scattering
Optical inhomogeneities in the almost transparent ma
terial produce some scattering of both the first and second
harmonic light, but polarization inhomogeneities, like fer
roelectric domains for instance, should produce a specific
scattering of the second harmonic generation.17,19 In both
cases however the size of the inhomogeneities must be typi
cally between 0,1 and 100 f.1.m to produce noticeable light
scattering at large or small angles. Taking unpaled or ther
many depolarized samples, the SHG topography revealed a
346 J. Appl. Phys., Vol. 66, No.1, i July 1989 uniform zero polarization even with the higher resolution
achievable. This means that in an area of about 20 f.l-m diam
the average polarization is zero, and there are no ferroelec
tric domains of a size larger than 20 /-lm. The same results
were obtained during electric field cycling in the vicinity of
the coercive field: a uniform weak or zero polarization and
no visible domains. Therefore the question remained: are
there smaller ferroelectric domains able to produce specific
scattering of the second harmonic light?
Precise measurements of the scattered light were per
formed at small angles and wide angles ( up to the back
scattering region ), especially during electric field cyclings,
The result is that we observed a weak scattered SH intensity
but which follows exactly the SH intensity in the forward
direction when the applied field is changed, and also vanish
es in the region of the coercive field. Thus, we conclude that
there is no specific SHG scattering due to nonuniform polar
ization. If ferroelectric domains exist in the material, their
size is much smaller than the optical wavelength
[A(2w) = 373 nrn in the polymer], and likely no larger than
the size of the crystallites (typically 30 nm). Therefore, in
the depolarized material one can conclude that there is no
correlation of the polarization orientation from one individ
ual crystal to its neighbors. This agrees with results of x-ray
scattering which indicate the possibility of a few ferroelectric
domains inside each crystallite, after thermal depolariza
tion. \0
c. Real~time investigation of the first polarization curve
When the applied voltage is slowly increased from zero,
surface charge measurements are perturbed by the nonzero
conductivity of the polymer and reliable first polarization
curves are difficult to obtain in a wide range of time scales.
On the other hand, second harmonic measurements of the
buildup of the ferroelectric polarization can be performed
under slow increases of the applied field. Figure 5 shows the
second harmonic intensity recorded during regular increases
and decreases of the applied voltage at three different rates.
When the field increase is slow (8 V Is), the ferroelectric
polarization appears at lower fields but does not reach the
same level as it does when the field ramp is faster ( 160 V /s).
This observation helps to determine the conditions of poling
FIG. 5. Second harmonic intensity recorded during the first polarization
half-cycle at three different rates: (iI) T!2 = 20 s, ce) T /2 = 100 s, (A)
T /2 = 400 s ( T /2 is the duration of the balf-cyde).
Wicker et at. 346
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130.209.6.50 On: Fri, 19 Dec 2014 22:10:25giving higher remanent polarization: fast application of the
poling field or even a single pulse of high voltage gives higher
ferroelectric polarization of copolymer films.
We interpret this effect as due to the competition
between two processes of different rates: 0) the field-in
duced reorientation of the ferroelectric polarization inside
the small crystallites, which was already proved to be very
fast provided the applied field sufficiently exceeds the coer
cive fieldl:;; and (ii) the buildup of a space charge in the
vicinity of the electrodes, which is a rather slow process due
to the poor conductivity of the material. Such a space charge
is able to screen the applied field inside the material and to
hinder the reorientation of the bulk polarization. In this
case, as already shown by Lang and Das Gupta,32 the ferroe
lectric polarization is no longer saturated in the whole thick
ness of the film.
D. Temperature dependence of the hysteresis loops
Both. SHG and surface charge hysteresis loops were re
corded simultaneously between 233 and 369 K, using low
frequency ac field cyclings at 0.01 Hz and two different sam
ples. Few changes were observed below room temperature,
while important modifications of the shape of the loops ap
peared above room temperature and especially above 330 K:
they are shown in Fig. 6.
In agreement with temperature dependence of the re
manent polarization presented in Sec. IV D, there is only a
weak decrease of the bulk polarization (measured by SHG)
upon heating, while the surface charge loops dramatically
inflate when the temperature is increased. Thi.s is attributed
to the decrease in the resistivity of the copolymer which pro-
FIG. 6. SHG hysteresis loops and surface charge hysteresis loops recorded
simultaneously above room temperature ( composition: 70/30 mo] %;
thickness: 21 pm; frequency: 0.01 Hz ).
347 J. Appl. Phys., Vol. 66, No.1. 1 July 1989 duces charging ofthe series capacitor with an RC time con
stant decreasing as the temperature is increased. The mea
sured charges give a resistivity of the order of 3 X 10\3.0 em
at room temperature dropping to 5 X 1010,0 em at 358 K.
These values are about one order of magnitude smaller than
the resistivities measured with a de field of 5 MV 1m, but one
must consider that the fields applied for the switching of the
polarization were up to 60 MV 1m and that poorly conduct
ing materials generally exhibit nonohmic behavior. Due to
the increase in conductivity, space charges in the vicinity of
the electrodes might build up with a shorter time constant
and in this case there should be inside the polymer film an
inhomogeneous electric field different from the applied field.
This assumption could explain the change observed in the
shape of the SHG hysteresis loops upon heating up to the
transition temperature, and especially the fact that a higher
applied field is required to obtain saturation of the polariza
tion. Nevertheless, bel.ow 320 K the conductivity is low
enough to consider that the SHG hysteresis loops show the
real variation of the bulk polarization versus (homoge
neous) applied electric field. From these loops one can deter
mine the temperature dependence of the coercive field.
which is represented in Fig. 7.
At low temperatures the coercive field increases less
than expected in the vicinity of the glass transition (Tg = 243 K). This could be attributed to a better penetration of
the field lines inside the crystalline phase of the low dielectric
constant due to the decrease of the dielectric constant in the
amorphous phase; such an effect is able to mask a real in~
crease of the effective coercive field inside the crystalline
phase.
Above room temperature, if one still defines the coer
cive field as the applied field for which the average polariza
tion and the SHG intensity vanish, one can observe in Fig. 7
that this field remains almost constant up to the Curie tem
perature. But as mentioned above, the shape of the SHG
hysteresis loops changes noticeably as the Curie temperature
is approached. For instance, at 343 K, the switching of the
50 ..
...
..
E '" '" 40 "'" "- ...... A"* .. """"',,bf>. :> e +* '"
Cl
30r +
....J ++
UJ +r H
lJ.. + u
H 20 I), COeRCIVE FIELD a: I- .. CArTICAL ~lJClEATION F:;:ElD u
lIJ
....J
lW 10
0 200 250 300 350
TEMPERATURE (Kl
FIG. 7, Temperature dependence of the coercive field deduced from SHG
hysteresis loops on two different samples of thicknesses 21 and 18 pm. At
high temperatures the critical nucleation field where the polarization
switehing starts is also represented.
Wicker eta/. 347
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130.209.6.50 On: Fri, 19 Dec 2014 22:10:25polarization starts around 35 MV 1m and finishes only
around 100 MV 1m. To account for this effect, we also plot
ted in Fig. 7 the value of the applied field for which polariza
tion switching starts. If the change in shape of the hysteresis
loops is due to space-charge buildup in the vicinity of the
electrodes this "critical nucleation field" would better repre
sent the actual coercive field inside the bulk. One must, how
ever, consider other possible interpretations of the change
observed in the switching of the polarization close to the
Curie temperature. First we must mention that the hystere
sis loops at high-temperature strikingly resemble those pre
dicted by the model of Wang et al.33 which is based on a
random orientation of the crystal axes, but it is not clear why
such a model would not apply further from the Curie tem
perature. Another interpretation of this change of the hys
teresis loops could be a change in the nucleati.on and growth
processes for the domains of opposite polarization. The re
sults of Naegele and Yoon,34 modeled by Dvey-Aharon et
al.35 have shown, in the homopolymer PVDF, the switching
of the polarization takes place inside each individual crystal
lite by the nucleation of 60" domain walls which move by
kink propagation parallel to the wall. More recently, it was
shown by Guy and Unsworth36 that, after repeated field cy
clings of a copolymer sample, the switching of its polariza
tion at room temperature involves only 180· domain walls.
Their reSult is thus able to explain the difference between the
smooth switching ofPVDF and the rather abrupt switching
ofP(VDF-TrFE) copolymers at room temperature. It also
provides a possible interpretation for the change of shape of
the SHG hysteresis loops upon heating to the vicinity of the
Curie temperature: one can assume that more 60· domain
waHs are nucleated as the temperature is raised, and there
fore consider, from the comparison with PVDF, that these
60° domain walls have a lower mobility, possibly because
they involve a twinning of the orthorhombic lattice (they are
comparable to ferroelastic domain wans). However, before
analyzing further the above interpretations, similar experi
ments above room temperature but at higher frequencies
have to be performed to determine the exact role of the con
ductivity and of the space-charge buildup in the possible
screening of the applied field inside the polymer material.
VI. CONCLUSION
We have shown in this paper that the second harmonic
generation oflight is a very useful and nondestructive meth
od used to analyze the bulk polarization in thin films of fer
roelectric polymers. After confirming that the SHG intensi
ty is proportional to the square of the remanent polarization,
we have determined the weak temperature dependence of
the spontaneous polarization and have confirmed the strong
first-order character of the Curie transition (in the VDF
TrFE copolymer with 70% VDF content). Using SHG to
pography and scattering of the second harmonic light, we
have shown that the size of the ferroelectric domains is much
smaner than the optical wavelength, and that after thermal
or electrical depolarization the domains may appear only
inside the small crystallites. Using oriented copolymer films
we have also shown, from an analysis in terms of broken
symmetry, that the parae1ectric crystal phase belongs to the
348 J. Appl. Phys., Vol. 66, No.1, 1 July 1969 centrosymmetric group 6lmmm. From simultaneous mea
surements of surface charge and SHG hysteresis loops at
0.01 Hz we have shown that the second harmonic generation
mainly originates from the crystal phase and that the field
induced second harmonic coefficient of the amorphous
phase is much smaller than that ofthe crystalline phase (for
the same polarization), and possibly of opposite sign. From
real-time SHG measurements, we have observed that a high
er electric field is necessary to completely switch the polar
ization in the vicinity of the Curie temperature. This may be
pardy due to faster buildup of space charge at high tempera
ture, but we also consider an interpretation in terms of nu
cleation of different kinds of domain walls with lower mobil
ity.
Another result of practical importance for the polariza
tion procedure of piezoelectric and pyroelectric films is the
direct observation of a higher remanent polarization when
the poling field is applied quickly in a few seconds. Moreover
the SHG technique with the possibility of scanning the laser
beam can be used for on-line control of the processing of
ferroelectric polymer films. New results open new questions
or reactivate old questions. The ability of the SHG technique
to give direct information on the ferroelectric polarization
inside the crystalline phase may especially help to clarify the
models describing the composite properties of these semi
crystalline polymers in terms of "primary" and "secondary"
effects.9•11•16
ACKNOWLEDGMENTS
The authors wish to thank the Atochem Company for
kindly providing the raw material, A. Weill from the CNET
laboratories, G. Guilhem and P. Robin from the Thomson
C.S.F. Company for their help in preparing the samples, B.
Daudin and F. Macchi from the CENG for their help in the
resistivity measurements, and P. Palleau for his precious
technical assistance and experimental help. This work has
been partly supported by the Laboratoire d' Analyses Phy
siques of the L.C.R. Thomson-C.S.F.
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Wicker eta!. 348
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130.209.6.50 On: Fri, 19 Dec 2014 22:10:25 |
1.36526.pdf | AIP Conference Proceedings 157, 82 (1987); https://doi.org/10.1063/1.36526 157, 82
© 1987 American Institute of Physics.Photo-induced density-of-
states variation measured by
DLTS method in intrinsic micro-
crystalline silicon (i-μc-Si:H)
films
Cite as: AIP Conference Proceedings 157, 82 (1987); https://
doi.org/10.1063/1.36526
Published Online: 04 June 2008
J. Wang , Q. S. Sun , H. N. Liu , and Y. L. He
82
PHOTO-INDUCED DENSITY-OF-STATES VARIATION MEASURED BY DLTS METHOD
IN INTRINSIC MICRO-CRYSTALLINE SILICON (i-~c-Si:H) FILMS
J. Wang; Q.S. Sun; H.N. Liu and Y.L. He
Department of Physics, Nanjing University, Nanjing, Jiangsu, P.R.C.
ABSTRACT
This paper advances a measurement and two calculations of a
high-frequency DLTS method for the density-of-states g(E) of in-
trinsic micro-crystalline and amorphous silicon film. The method
surmounts the difficulties of DLTS measurement of i-a-Si:H or i-
~c-Si:H samples and applys the common high-frequency DLTS to it,
while the temperature of measurement is extended below 77K. Fol-
lowing the method, we successfully observed the obvious increase
of density-of-states produced by illumination.
PRINCIPLE OF MEASUREMENT
It is an important problem in study of amorphous silicon to
measure the density of localized states in the gap. There have
been DLTS measurements 1,2 of g(E) on heavy doped a-Si:H obtain-
ing a distribution of n+-a-Si only. Because of some difficulties
on sample preparation and high-frequency DLTS itself, the intrinsic
sample can hardly be measured. Besides preparing a satisfactory
i-~c-Si:H Schottky-barrier sample, we achieved several effective
measurement solutions to get a believable DLTS signal:
I. a-Si:H or ~c-Si:H film is deposited on a medium resistance
C-Si substrate. Undoped a-Si:H (or ~c-Si:H) appears in intrinsic
state with semi-insulation. In the DLTS measure process, the
depleted layer can go through the a-Si:H (or ~c-Si:H) film at a
reverse bias Vr, and its boundary is set within C-Si substrate
which has high-mobility, so that high-frequency capacity response
is possible. The current DLTS with IMHZ or 2MHz capacitance
signal measuring frequency could be used and the low-frequency
DLTS (10KHz) is unnecessary.
2. Enough high and wide injection pulses, VD, make the sample
have avalance breakdown. Normally, carriers can not be injected
in amorphous silicon because of high-resistance. If we use enough
high and wide injection pulses the gap states will be filled by
the current of the large number of carriers flowing through the
sample produced by the breakdown. Then, the DLTS signal forms.
In the meantime, we put forward two formulae for deducing
the g(E) from the original DLTS signal AC12:
I. Using the formula similar to that dealing with the interface
states
The transient charge equation when V is applied is r
0094-243X/87/1570082-5 Copyright 1987 American Institute of Physics
83
dQ D dQas C dV r
--+ - (I) dt dt A dt
C is the capacity of sample; A, the area; QD' Qas' the charge den-
sity in C-Si substrate and pc-Si within the depleted layer res-
pectively. If N D stands for the doped concentration in C-Si sub-
strate:
dQ D
d--t-- = qNDdXd (2)
where x d is the depleted layer thickness in the substrate.
Assuming that all pc-Si electronic states are filled down to
an energy below E determined by the thermal emission time q: we
C have:
dOas t 1
d~ - dt q g(E').[1 - e q~]dE' x (3) a
E
V
where x is the pc-Si thickness. a
Since
dV / dt = 0 (4) r
we may integrate Eq.(1) using (2) (3) & (4) within the sampling
range t I to t 2 (the DLTS measurement condition) to get:
t 2 C(tl)'C(t 2)
AC12 : C(tl ) _ C(t2 ) = _ kT.g(E) in (5)
N D t 1 C a
C is the geometric capacity of the ~c-Si layer; approximately a
C(t I) = C(t 2) for calculating.
To obtain this result we have used the rapid variation of
with energy; namely
I T(E,T) = ~exp(E/kT) (6)
~o
with~o, emission factor in the range 1011-1013 sec -I Note that
the value of E which appears on the r.h.s, of Eq.(5) is that de-
termined by the time sampling range and teraperature (see Eq.(11)).
2. Using the relation between broad distribution g(E) and AC12
from equivalent sharp energy level method. The DLTS signal of a
single level is
t I t 2
AC12 : - C o (e T _ e T ) (7)
84
here T is emission time too.
We assume that the broad energy level is formed by a series
sharp levels put in order closely, and introduce the concept of
effective width of DLTS signal of a single level and its corres-
ponding effective enery space. The result would be
: nt2 in [n.(l+n).l~ ]-I kT.~(E) C(tl)'C(t2) (8)
AC12 - [ (1+n)t I N D C a
with n : tl/(t 2 - tl) , if sampling time t 2 : t I
C(t I ).C(t 2 )
AC12 = _ 0.721. kT'~ (E)
N D C a : 2, n : I, thus:
(9
compared to formula (5) in the previous case:
C(tl).C(t 2) AC12 = - 0.693.kT'~ (E)
N D C a (I0
These two equations are very close -although derived differently.
The energy level E of g(E) is determined by
t2 - tl I
E = kT-in [~Oln(t2/tl) ] = E - E c ACI2 < 0
E v - E ACI2> 0 (11
also when t 2 : t I = 2,
we use ~o= 1012 sec-1 E : kT.in (l.44Vot 1
for calculating. (12
SAMPLE PREPARATION AND MEASUREMENT
The structure and size of the sample is shown in Fig. I.
Protective
~~~ Point (For Ultrasonic-ccr@ressicn)
k~
"__ n I--II --
9 . 9 " 1 o
t / " Au Back Electrode
Fig. I. Section Structure of Sample
The sample is cleaved into a square with the size
of 2 X 2 mm2and fixed in a transistor header.
RESULT By glow-discharge method the ~c-Si:H film was grown on the
mirror surface of C-Si wafer with resistance of 3Q.cm. The
grain size of the film is larger than I00~ measured by X-ray de-
fraction. The photo-conductivity and other measurements indicate
the sample is typical pc-Si:H. The Schottky-barrier is formed by
evaporation of Ti then covered by Mo (total thickness 340~). This
semitransparent top electrode allows repeated illumination. There
is a comparatively small Au point for connecting lead. The DLTS
measurement is applied to the annealed State A and exposure State
B with the proper Vr, Vp and other factors. It is interesting
that the DLTS signal is largest when reverse bias V ~ 0 and in- r
jection pulse V > 30V. Because of strong field in pc-Si film P
when higher bias is applied, the carriers could be swept out before
they contribute to the signal. After 60min. at 160~ annealing or
3hr. and more than 150mW/cm 2 tungten halogen lamp illumination,
the sample will be in the State A or State B respectively.
I .39 The DLTS curves and corresponding g(E) of State A and B are
shown in Fig. 2 and Fig. 3.
I i i ! 1017
0.00
c~ - 1.39
0
e~ _ 2.78
- 4.17 85
II
II. State B
- 5.56 I I I I
77 185 259 321 377 428
T (K)
Fig. 2. The DLTS signal AC12 T
Both curves were obtained with the
lowest sensitivity of the equip-
ment to prevent X-Y recorder from
overload in the case of State B. 1016
T8 I~
v
1016
1013
0.0 I I I I I I I
I. State A
II. State B
I I I I I I I
0.2 0.4 0.6
E - E (ev)
C
Fig. 3. g(E) of State A
and State B 0.8
There is a distinct peak at E -E c = 0.6ev in any of the g(E)
curves. The value of g(E) at this max. point ranges from 2.83X
1015 to 1.21X 1016cm-3during illumination. These two curves in
86
Fig. 3 are almost exactly reversible if the sample is annealed or
illuminated. From the curves we also can get some conclusions:
I. The photo-induced enhancement of density of states is clear
(near I016cm-3), but not as dramatic as the variation occurring in
a-Si (1017 to I018cm -3) shown by Lang etc. 3 We attempt to in-
terpret this phenomenon easily with two-phase model of pc-Si which
assumes that pc-Si grains are surrounded by amorphous matrix as
a-Si:H. The composition of amorphous which has chief feature of
SWE decreases relatively in pc-Si 4. So that the effect of light
will be much weaker.
2. The peak position has no distinguished shift when the state of
sample changes. It seems to indicate that the properties of
photo-induced defects are similar to the instinct defects which
were already existing in the gap, at least, on the hand of the
energy state within the range studied.
Having above initial results, we conclude that this effective
high-frequency DLTS measurement is very significant on the density
of states study of a-Si and pc-Si. Plentiful and interesting phe-
nomena have been observing during our experiment process. The
detailed theoretical explanations of properties of these metastable
defects in pc-Si:H and other support measurements for revealing
the deep-level state and its changes such as SWE method, ESR, PL
etc. are also carried on by us now.
REFERENCES
I. J.D. Cohen, D.V. Lang and J.P. Harbison, Phys. Rev. Lett. 45,
197 (1980)
2. C.H. Hyun, M.S. Shur and A. Madan, Appl. Phys. Lett. 40, 178
( 198O )
3. D.V. Lang, J.D. Cohen, J.P. Harbison and A.M. Sergent, Appl.
Phys. Lett. 40, 474 (1982)
4. Hsiangna Liu and Ming-de Xu, Solid State Commun. 58, 601 (1986)
|
1.98653.pdf | Co/Si(111) interface: Formation of an initial CoSi2 phase at room
temperature
J. Y. Veuillen, J. Derrien, P. A. Badoz, E. Rosencher, and C. d’Anterroches
Citation: Appl. Phys. Lett. 51, 1448 (1987); doi: 10.1063/1.98653
View online: http://dx.doi.org/10.1063/1.98653
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v51/i18
Published by the American Institute of Physics.
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Downloaded 20 Jun 2013 to 128.112.200.107. 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_permissionsCo/Si(111) interface: Formation of an initial CoSi2 phase at room temperature
J. Y. Veuillen and J. Derrien
Centre National de la Recherche Scienlijique, Laboratoire d'Etudes des Proprietes Electroniques des
Solides~ associated with Universite Scientijique, Technologique et Medicale de Grenoble, B.P. 166, 38042
Grenoble Cedex. France
P. A. Badoz, E. Rosencher, and C. d'Anterroches
Centre National d'Etudes des Telecommunications, Chemin du Vieux Chene, B.P. 98, 38243 Meylan
Cedex, France
(Received 2 July 1987; accepted for publication 9 September 1987)
Ultrathin films ( S 50 monolayers) of Co have been deposited on atomically clean 7 X 7
Si( 111) surfaces at room temperature and characterized by in situ surface techniques such as
Auger electron spectroscopy and low-energy electron diffraction. Formation of a boundary
CoSi2-like phase is surprisingly found at a very low coverage range ( S 4 monolayers) as
evidenced by low-temperature transport measurements (resistivity and Hall effect) and also by
cross-sectional high-resolution transmission electron microscopy.
Recently, it has been demonstrated that quasi-perfect
epitaxial CoSi2 layers may be formed by solid phase epitaxy
technique, i.e., simply annealing to higher temperature
( -600°C) Co thin films previously deposited at room tem
perature (RT) on Si substrates. 1-6 Further, new transistor
devices have been achieved based on the re-epitaxy of Si on
CoSi2 leading to SilCoSi2/Si heterostructure.7-9 In spite of
these advances, the physical mechanisms of CoSi2 growth
are still not fuHy assessed in their details. Knowledge of the
stoichiometry, morphology, and growth mechanisms of the
initial stages of the interface formation is still required in
understanding silicide growth and Schottky barrier forma
tion. According to Walser and Bene,1O a basically "glassy
interphase" region is thought to form prior to the first com
pound phase nucleation and this region acts as a membrane
in controlling subsequent first-phase nucleation. The com
position of this glassy membrane is dose to that of the deep
est eutectic in the binary phase diagram. For Co and Si, this
would be ~C03.3 Si.1l
However, so far an measurements performed on the
Co/5i ( 111) prepared under ultrahigh vacuum conditions
(UHY) plead for an initial CoSiz -like phase even at RT
within a very low Co coverage range [e < 4 monolayers
where a monolayer is equal to the SiC Ill) surface atomic
density, Le., ~ 7.8 X 1014 atomsicm2 and equivalent to
~0.87 A in mean average thickness]. Indeed, in the past, we
have used low-energy electron diffraction (LEED) to study
ordering, ultraviolet photoemission spectroscopy (UPS) to
investigate the valence bands, and x-ray photoemission
(XPS) to observe changes in the core level signatures.4 Our
results suggest that, at very low coverages (8 -4 ml), a
CoSiz -like phase is surprisingly formed at the interface at
room temperature. Later12 a surface electron energy loss fine
structure spectroscopy (SEELFS) determines the Co-5i
bond length ( -2.32 ± 0.05 A) at the evolving interface and
also confirms the silicide boundary layer formation. Recent
ly, Boscherini and co-workers13 were able to model the de
velopment of the interface, confirming the CoSi2 formation
at very low coverages followed with a Si solid solution in a
thin Co film. These latter results were obtained with high
resolution core-level spectroscopy dealing with the Si sub
strate core level and using synchrotron radiation facilities. The RT Co/Si (111) interface has been revisited by us re
cently14 using the Auger line shape of the Co adsorbate to
describe the local density of states around the Co adsorbed
atom and to identify the formed phase. However, aU these
described. results4.6,'J,i2-14 have been obtained with in situ
techniques which are sensitive to smaH amounts of materials
and reflect mainly the local characteristics of the interface.
In order to assess the CoSi2 initial phase formation at R T in
a long range scale, we report, in this letter, electrical trans
port measurements ofa Co ultrathin film (8-4 ml) deposit
ed at R T on a very resistive Si (111) substrate. The results
unambiguously demonstrate that Co atoms intermix with Si
surface atoms to form a very thin CoSiz -like layer displaying
electrical transport properties identical to those of genuine
single-crystal thin films of CoSi2 as reported in the pioneer
ing work of Hensel et a1Y·16 and later by some of us.n.is
Moreover, with high-resolution transmission electron mi
croscopy (HRTEM), we confirm in a direct way the pres
ence of a CoSiz -like phase at the interface.
Resistive Si (111) wafers (p-type W' n em) were cut in
squares of 10 X 10 mm2 and loaded into a DRY chamber
(base pressure ~ 10 -10 Torr) equipped with LEED and
various electron spectroscopy facilities. The Si substrates
were cleaned with conventional ion etching and annealing
cycles to obtain the well known 7 X 7 Si ( 111) reconstructed
surfaces. Co atoms were then evaporated with a miniature
home-made electron gun evaporator especiaUy designed to
work under UHY conditions with very low power ( -20 W)
in order to avoid the base pressure increasing and the 5i
surface heating by irradiation; the temperature of which was
controlled to be maintained at R T thanks to a thermocouple
attached to the sample. The Co rate evaporation (ranging
from 1 to 20 A per minute) was calibrated by in situ quartz
microbalance. Sequential deposits of Co were performed on
Si and characterized with Auger, LEED, and SEELFS tech
niques in order to recover aU previously mentioned re
sultS.4.6.9.12-14 They will not be reproduced here.
In order to assert the initial CoSi2 -like phase formation
at R T, severa! Co ultrathin films ([J ranging from 4 to 50 ml)
deposited on Si ( 111 ) were removed from the UHY chamber
and characterized by electrical transport measurements and
HR TEM. Resistivity and HaH constant measurements were
1448 AppL Phys. Lett 51 (18),2 November 1 S87 0003·6951/87/441448-03$01.00 ® 1987 American InstiMe of PhYSics 1448
Downloaded 20 Jun 2013 to 128.112.200.107. 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_permissionsperformed with the standard Van def Pauw method'" from
300 K down to ~ 20 K. Figure 1 shows the temperature
dependent resistivity curve obtained on a 4-ml Co/SIC 111)
interface. The high-temperature region (T> 220 K) corre
sponds to the Si substrate contribution (and possibly related
contact artifacts), the carriers of which are frozen out at
T -220 K. Below the saturation region (T < 20 K) the onset
of a resistive increase is observed. It might be explained ei
ther by a localizationHke effect in such an ultrathin film20•21
or a kind of Kondo elfect22 due to a few Co atoms not fully
reacted with Si to form CoSi2 and therefore acting as mag
netic impurity. A more detailed study of the resIstivity in
crease at low temperatui'e is in progress in order to assess the
scattering mechanisms in this ultrathin film. Between 220
and -20 K, the measured resistivity behaves conventionally
as a normal metal displaying a linear decrease with decreas
ing temperature and a saturation at its residual resistivity
value Po (around 118,uH em in Fig. 1), The resistivity may
be expressed as the sum:
peT) =Pn+PL(T)
of additive contributions according to Matthiessen's rule of
Po the residua! resistivity (due to carrier scattering by struc
tural defects, impurities, surfaces, ... ) andpl. (T) the phonon
(BIoch-Gruneisen) resistivity. Our measurements in Fig. 1
faithfully reproduce aU the characteristics measured on gen
uine ultrathin films of CoSi2 0 It is generally admitted that
the resistivity in metallic films, as they become thinner than
the electrons' bulk scattering length i", will be dominated by
surface scattering and there will be a "size effect," especially
in the case of a diffuse scattering. As regards the CoSiz films,
Hensel et al.ls.ll> have demonstrated that down to very low
thicknesses (-100 A.), the CoSi2 film resistivity exhibits
little dependence with its thickness d. If the film size effect is
negligible and since this dimension d is much less than Ie
(-1000 A.), 16 boundary scattering of the carriers is there
fore essentially specular. The only sensitive measure of the
"size effect" is the residual resistivity increasing with de
creasing thickness. l6 This effect is clearly emphasized by Sa-
'''T?l 1 E
124 '~~j u
c: 0..0
:L iO &
'-' 122
>-,
f- o 0.2 0.4 0.6 0.8
> 120 d-' (om-' ]
i-ll) .</ iJ5 118 w "' Ii <I< -It ."...)1.****
0::
o 50 100 150 200 250 300
TEMPERATURE [K]
FIG 1. Temperature dependence of the resistivity of a four monolayer Co
film 011 top of a resistive Si( J 11) substrate. The high-temperature region
T> 220 K shows lhe 5i carriers effect. From T ~ 200 K lllltil20 K, the resis
tivity foHows the standard metal resistivity behavior according to Matthies
sen's ruJe:p ~= Po + PI-(n. The residual resistivity Po is ~ 118.uH em. The
inset shows the dependence of Po with thickness d of several genuine COS!2
thin films (.) (Ref. 18). The value measured on the 4-ml eo/Si interface is
also reported (,,).
1449 AppL Phys. Lett. Vol. 51 , No. 18,2 November 1987 doz et al.18 with CoSi2 films thinner than ~ 100 A. These
authors observe a strong Po increase with thickness d which
could not be explained within the framework of usual mod
els dealing with low-temperature t!-ansport in thin metal
films including the Fuchs-Sondheimer theory,23 localiza
tion effects20-22 Cooper,24 or McMiHan25 model. The inset in
Fig, 1 reproduces the results of Badoz et aL 18 obtained on
several genuine CoSiz epitaxially grown on Si. The data
might foHow a phenomenological law:
Po(d) ~Poc exp(A Id),
where Po Cd) is the residua! resistivity of a CoSi2 film with a
given thickness d, p"" is the CoSiz bulk residual resistivity
( ,-2.6 p.H cm), and A is found to be-66 A.
Our Po value measured on a 4-ml Co fUm deposited on Si
at RT nicely complements this curve if one assumes that 4 ml
of Co (_. 3.5 A) have expanded the Si top layers to form
around four unit cells of CoSiz which are, according to the
CoSi2 structure6 nearly equal to ~ 13 A of mean average
thickness (see also the HRTEM results below).
Figure 2 shows the results deduced from Hall constant
measurements in the standard Van der Pauw configuration
with two magnetic field orientations. At high temperature
( T> 220 K), the Si substrate influence is again clearly seen
in good correlation with the resistivity results (Fig. 1). Once
the s1' carriers are frozen cut (T < 220 K), the main contri
bution to the HaH constant is provided by the thin metallic
film on top of the Si resistive substrate.
We measured a positive Hall coefficient around-2.4
X 10-4 cm3/C which remains nearly constant over the
whole range oftemperature investigated (~20--2oo K). Us
ing the one-band model, we deduce a free-carrier density of
~ 2.6 X 1022 em -3 type p very close to that already reported
for bulk CoSiz, i.e., -3 X 1022 cm-3 type p and other thin
films of CoSiz epitaxiaHy grown at high temperature
( -600 ·C) on SiC 111).15-18 This result may be taken as evi
dence that 4 ml ofCa atoms have reacted with the Si surface
atoms and formed a COSAz -like phase. The RT initial CoSi2
phase formation is unambiguously demonstrated because
the (2.6X 1022 em --3 typep) CoSiz free-carrier value, with-
1024
....., 10"r-~-~~ '" ,
E
u '-'
>-I i-
10"[ tf5 ~ " ~ e ,,". f).~~"fiI~~~",.,~ .. ?t " Z
W ""-,,, a
10,,1 £k: w
~
0:: « u 0 50 100 150 200 250 300
TEMPERATURE (K J
FIG. 2. Han measurements on the 4-ml Co/Si interface for two signs of
magnetic fIeld. The high-temperature region T> 220 K shows the Si carri
ers contribution. The Hall constant measurement is converted into the free
carrier density on the ordinate axis using a one-band model.
Veui!!en eta/. 1449
Downloaded 20 Jun 2013 to 128.112.200.107. 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_permissionsFIG. 3(a) shows a lattice imaging micrograph of the boundary CoSi1-1ike
phase at the interface between the Si substrate. and the almost pure Co film.
Cross-sectional TEM diffraction of the whole interface also displays spots
of COl Si and CoSio Initial coverage ~ 50 A of Co. (b) i;; an image of the
whole interface at lower magnificatioTI. Note the lateral uniformity of the
film.
in our experimental accuracy, cannot be confused in any
case, either with those of CoSi (0.1 X 1022 em -3 type n) and
C02Si (O.2X 1022 em -3 type p) or with that of Co
(4.7x 1022 cm-3 type n).26
FinaHy, to illustrate this interface formation in a direct
way, Fig. 3 (a) shows a lattice image with HRTEM of a-50
A Co thin film deposited on a (7 x: 7) Si ( 111) surface at R T,
We observe a boundary layer ( < 13 A of thickness) display
ing the same Si (or CoSi2) structure but with a different
contrast (CoSi2 -like phase). This transition layer is laterally
uniform as seen at lower magnification [Fig. 3 (b) J and has
always nearly the same thickness [arou.nd -4 monolayers
of Co (-3.5 A) react with 8i to form ~4 unit cells ofCoSi2
(~13 A)] whatever the initial Co deposit thicknesses were
(() ranging from ~ 4 to 50 "&"), provided that they were de
posited monolayer by monolayer under the same experimen
tal conditions. A cross-section TEM diffraction of the inter
face also shows spots identified as CoSi and CO2 8i along with
the Si substrate ones if the film thickness is larger than -15
A. This latter result may be taken as evidence of some grains
of more metal-rich silicides in the almost pure Co top layer,
It is worth mentioning that silicide formation is highly kine
tically limited at RT. As a consequence, metal-semiconduc
tor interfaces usually behave as rather metastable systems,
the morphology and chemical composition of which depend
drasticaHy upon the so-called "R T conditions" (substrate
surface crystallography and cleanliness, substrate surface
rea! temperature under metal evaporation, evaporation rate,
thickness of the deposited film, ... ). Our results, therefore,
are not in contradiction with the findings of other groups
who observe only CoSi2 phase formation on (2 Xl) Si ( 11 ! )
surface at RT13 or further reaction under similar experimen
tal conditions on (7 X 7) Si ( 111) surface. 27
1450 Appl. Phys. Lett., Vol. 51, No. 18,2 November 1987 In summary, we are able to measure the electrical trans
port properties of an ultrathin layer of Co atoms ( ~ 4 ml)
deposited at RT on a resistive Si substrate. The results lead to
the conclusion that Co atoms react with Si to form an inter
facial CoSi2 layer phase, which, in turn, acts as a diffusion
barrier at RT to additional Co atoms arriving on the surface.
These latter atoms then form an almost pure Co thin mm
spreading on top of the boundary CoSi2 -like layer. The elec
trical measurements confirm previous in situ spectroscopic
results.4.h,12-1~ The RT CoSiz formation does not agree with
the prediction of Walser and Bene, if) probably due to the
peculiar role of the limited intermixing between a few adsor
bate atoms and the substrate. Mild annealing of the RT in
terface enhances the intermixing once the heating tempera
ture is sufficient to overcome the activation energy of the
diffusion barrier and then leads to high-temperature sequen
tial silicide formation (Co2 Si, CoSi, CoSi2 ).5.1'>,9
's. Saitoh, H. Ishiwara, and S. Furukawa, App!. Phys. Lett. 37, 203
(1980).
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(1983).
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3391 (i 984).
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Techno!. B 3,770 (1985).
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7E. Rosencher, S. Delage, Y. Campidelli, and F. Arnaud d'Avitaya, Elec
tron. Lett. ::W, 762 (1984).
xJ. C. Hensel, R. T. Tung, J. M. Poate, and F. C. Unterwald, App!. I'hys.
Leu. 47, 151 (l984-).
"See a review paper by J. Derrien and F. Arnaud d'Avi!aya, J. Vac. Sci.
Techno!. A 5,2111 (1987),
1OR. M. Walser and R. W. Bene, App!. Phys. Lett. 28,624 (1976); also J.
Vac. Sci. Techno!. 17, 911 (1980).
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1958 ).
12£. Chalnet, M. Dc Crescenzi, J. Derrien, T. T. A. Nguyen, and R. C. Cinti.
Surf. Sci. 168 309 (1986).
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B35, 4216 (1987).
14J. Dcrrien, M. De Crescenzi, E. Chalnet, C. d'Anterroches, e. Firri, G.
Gewinner, and J. C. Peruchetti, Phys. Rev. B (to be published).
"J. C. Hensel, R. T. Tung, J. M. Poate, and F. C. Untcrwald, Appl. Phys.
Lett. 44, 913 (1984).
16J. C. Hensel, K T. Tung, J. M. Poate, and F. C. Unterwald, J'hys. Rev.
Lett. 54, 1840 ( 1985).
l1p. A, Badoz, A. Briggs, E. Rosencher, and F. Arnaud d' Avitaya, 1. Phys,
Lett. 46 L979 (1985).
ISp. A. Badoz, A. Briggs. E. Rosencher, F. Amaudd'Avitaya, Ilnd C. d'An
terroches, Appl. Phys. Lett. 5t, 169 (1987).
;YL. J. Van cler Pauw, I'hillips Res. Rep. 13, ! (1958).
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nan, Phys. Rev. Lett. 42, 673 (1979).
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Nagaoka, Progr. Theor. Phys. 63,707 (!980).
121. Kondo, Progr. Theor. Phys. 32, 37 (1964).
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heimer, Phys. Rev. 86, 401 (1950).
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(1966).
25W. L. McMillan, Phys. Rev. 167, 331 (1968).
20e. D. Lien, M. Finetti, M. A. Nicolet, and S. S. Lau, J. Electron. Mater.
13,95 (1984).
DF, Arnaud d' A vitaya, J. A. Chroboczek, C. d' Anterroches, G. Glastre, Y.
Campidelli, and E. Rosencher, J. Crystallogr. Growth 81, 463 (1987).
VeuHlen et al. 1450
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1.101961.pdf | Shubnikov–de Haas effect in thin epitaxial films of gray tin
L. W. Tu, G. K. Wong, S. N. Song, Z. Zhao, and J. B. Ketterson
Citation: Applied Physics Letters 55, 2643 (1989); doi: 10.1063/1.101961
View online: http://dx.doi.org/10.1063/1.101961
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Appl. Phys. Lett. 43, 77 (1983); 10.1063/1.94127
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129.22.67.107 On: Thu, 27 Nov 2014 07:38:50Shubnikov-de Haas effect in thin epitaxial fUms of gray tin
L. w. Tu, G. K. Wong, S. N. Song, Z. Zhao, and J. 8. Ketterson
Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208
(Received 3 July 1989; accepted for publication 11 October 1(89)
The transverse magnetoresistance and Hall effect have been studied for n-type gray tin
epilayers grown on (001 )CdTe substrates by the molecular beam epitaxy technique.
Shubnikov-de Haas oscillations were observed in samples having Hall mobilities"> 104 cm2/V s
at low temperatures. Measurements were carried out using both the dc method and field
modulation techniques in the temperature range from 1.2 to 10K and in magnetic fields up to
10 T. Beat patternR were observed in the Shubnikov-de Haas spectra which we ascribe either to
inhomogeneous doping, arising from the diffusion of Cd and Te from the substrate, or to
quantization of the motion in the direction parallel to the film normal. The Shubnikov-de
Haas carrier concentration ofa 1210 A film was determined to be nSdH = 2.3 X 1017 cm-3, in
good agreement with the Hall density.
Gray tin (a-Sn) is unique among the group-four semi
conductors in that it has a zero band gap. Although this
material is unstable above 12.3 °C it can be stabilized by he
teroepitaxy on an appropriate Rubstrate, such as edTe. We
have described the preparation and growth techniques ear
lier.! Extensive transport measurements have been per
formed: Hall measurements showed that films thinner than
400 A are p type, with low mobilities, and thicker films are n
type, with much higher mobilities.! Observation of a thick
ness-dependent band gap for films with thicknesses under
400 A was reported recently.2 Films with thicknesses of
~ 1000-3000 A have Hall mobilities PH = (1-2) X 104
cml/ Vs, and Han carrier densities nH = (2-3)XlO!7
cm -3 at low temperatures. We report in this letter the obser
vation of the Shubnikov-de Haas (SdH) oscillations in these
films.
Our investigations of the SdH effect involved both the
de method and a field modulation technique. Magnetic fields
up to 10 T and temperatures from 1.2 to 10 K were em
ployed. The van der Pauw method was used, and the sample
geometry was a 5 mm X 5 mm sqmlre. The field direction was
perpendicular to the (001) plane of the films for the mea
surements reported here.
Figure 1 shows an SdH trace ofa 121O-A.-thick sample
with flH = 1.3 X 104 cm2/V sand lIu = 2.1 X 1017 cm3 at
4.2 K. The oscillations were detected at the first harmonic of
the modulation frequency versus the reciprocal magnetic
field liB. In addition to the periodic oscillations, note the
beating effect which we neglect in the first approximation
(but will include later).
The energy of electrons in a parabolic band with mag
netic field B in the z direction is3
( 1) fz2k2 f3 E= n +-IktJ+-_z ±2.gB, 22m'; 2 (1)
where (tJ = eB /m*c, m* is the cyclotron effective mass, kz is
the wave vector parallel to B, /30 = efz/2m,c (the Bohr mag
neton), g is the effective spin-splitting factor, and n = 0, 1, 2,
... denote the Landau levels.
As successive harmonic oscillator levels pass through the extremal cross section of the Fermi surface, properties
such as the resistivity oscillate with a period
p= a(l/B) = l/F= efz/m*cEF' (2)
where Fis the frequency. A plot of the maxima and minima
in terms of the reciprocal magnetic field 1/ B versus the
quantum number n, yields the period of the oscillations, p,
from the slope of the line, as in Fig. 2; we obtain a value
p= 0.084 T-!. Using a spherical Fermi surface-parabolic
band model, we have
EF = (1l2/2m*)(3-zrnSdH )2!3. (3)
Combining Eqs. (2) and (3) gives flSdH = 2.3 X 1017 cm-3,
and agrees well with the Han density flH = 2.1 X 1017 cm-3.
The temperature dependence of the amplitUde of the
first harmonic in the Adams-Holstein expression4 for the
Shubnikov-de Haas effect is given by
f Te /3Tvrn'/B )
A~ -CI -\B t!2 sinhC/3Tm'/B) , (4)
where m' = m*/m" is the reduced mass, /3= 21T2kBm<c/
eli = 1.468 X 105 G/K, and C = 5>/2rrkR (m*c/EFeft) 1!2 is
30 , , I ' , , , I ' , , , I ' . , , I' , , , I ' , '-l
20
,-..
~I ;:s 10
(tj
I~~--'-"
0 ro
"d
'"'-
~ -tQ
-20
-:10
0.5 1.(1 1.5 2.0 2.5
liB -1 (Tesla )
FIG. I. SdH oscillations at 1.4 K plotted vs the reciprocal of the magnetic
field to reveal the periodicity.
2643 Appi. Phys. Lett. 55 (25), 18 December 1989 0003-6951/89/512643-03$01.00 @ 1989 American Institute of Physics 2643
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129.22.67.107 On: Thu, 27 Nov 2014 07:38:502.0
1.5 -
1.0
0.5 ro-'--r"--'-'-~o-T~I-'-T' ~"'-!~
.~
1
. ~
~
0.0 ~-'--LJ-l.L~-LL_LJ-L_JL-L_.L.LL ~
Q 5 10 15 20 25
n-QUANTUM NUMBER
FIG. 2. Positions of the extrema of the SdH oscillations vs the quantum
number /1. ( + ) maxima; (0) minima.
a constant for a particular sample and magnetic field direc
tion. Equation (4) is valid when the oscillatory part is small
compared with the nonoseillatory part of the magnetoresis
tivity, and when Er is not less than a few cyclotron level
spacings flw. These criteria are satisfied in the lower field
region. The electron effective mass can be derived from fit
ting the amplitudes at different temperatures for fixed B us
ing
T A 0:.------
Sillh((3 I'm' I B) (5)
Figure 3 shows a fit at B = 0.88 T for the 121O-A-thick sam
ple. An average value of m' = 0.029 was obtained in the low
field region (below ~ 1.5 T). This value is ~ 10% higher
than that of Booth and Ewald'; this may be due to the in
homogeneity and/or strain in our films.
induding the Bessel function factor associated with the
field modulation technique,6-3 the Dingle temperature T~
can be derived by fitting the field dependence of the ampli
tude at a fixed temperature with the equation
AT -fl(l+ TJ))m'lll
a:~e 9 B ,-(6)
where the sinh term in Eg. (4) has been approximated by its
2 4 s 3
T (K)
.FIG. 3. Plot of SdH amplitudes vs temperatures at B = 0.88 1'. The solid
curve is the best fit obtailled using Eq. (5).
2644 Appl. Phys. Lett., Vol. 55, No. 25.18 December 1989 O.B 0.8 1.0
-1 liB (Tesia ) 1.2
F1G. 4. Plot of In(A I BSI» vs liB at 1.4 K. The solid curve is the hest ill
obtained using Eq. (6).
exponential form, which is valid under the condition
(/3Tm')/ B"Jp 1. Figure 4 is a fit to the data at 1.4 K. From fits
at five different temperatures between 1.4 and 7.2 K, the
average TD was 8.8 K, where m' = 0.03 has been used.
We now discuss the beat structure which is evident in
Fig. 1. From the positions of the nodes, we can obtain two
frequencies FJ = 11.8 T and F] = 13.0 T. Previous studies
showed that our samples were somewhat inhomogeneous
along the growth direction z. Therefore, we will employ a
simple "two-region model" to describe the inhomogeneity of
our samples. We visualize the sample as consisting of two
layers, a purer (upper) region (adjacent to the free surface)
having a thickness d1 and a more highly doped (lower) re
gion (adjacent to the CdTe substrate) having a thickness d2,
each with slightly different carrier densities. This can ex
plain the heating effect. A model similar to this was used by
Booth and Ewald.5
Figure 5 (b) shows a computer-generated curve using an
expression for the SdH oscillations with two frequencies, rc-
I-'~~·~ iro-'-'T '-~-'T ,-,-
1-4~N~\NV~\ I
~H+->+H' 'I ' H~+I
"--L.L..'---~----'---'--'.-l....~ --'--,~L-l._
() 2 3
B (Tesla)
FIG. 5. (a) SdH oscillations as a function of B for1hcsamesampleas thai in
Fig. 1. (b) A computer·generated curve using Adams-Holstein expression.
Tu et a/. 2644
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129.22.67.107 On: Thu, 27 Nov 2014 07:38:50taining the fundamental and the first four harmonics of the
Adams-Holstein expression; the spin-splitting factor is also
induded.9 The parameters used were dj = 423 A, d2 = 787
A, m; = m; = 0.031, TJ)[ = 8.5 K, 1'D2 = 9.5 K, FI
= 12.0T,}~ = 13.2 T,g, = ~ 29,g2 = -28,5 where sub
script 1 refers to the upper region and subscript 2 refers to
the lower region. The overall factors for both frequencies
used were the same. Nate that Fig. 5 (b) agrees wen with Fig.
5(a), and reproduces the beating features as well as spin
splittings at high fields.
Another possible explanation for the beat structure is
that it arises from quantization of the motion of the electrons
parallel to the film normal, i.e., from the quantum size effect.
The Fermi surface is then replaced by a set of disks with kz
values given by kz = -uN Id; here d is the film thickness, and
N is a quantum number. Although approximately six pairs
of kz values would be expected in our case, not all of them
may be observable due to (i) a very close spacing (the N = 1
and 2 levels near the extreme section ofthe three-dimension
al Fermi surface) or (ii) a shorter scattering time (for disks
with higher N values). Our measured beat frequency is close
to that expected for the interference between the N = 1 (or
2) and the N = 3 levels.
In conclusion, we have observed Shubnikov-de Haas
oscillations for the first time in thin gray tin mms. A beat
2645 Appl. Phys.lett, Vol. 55, No. 25, 18 December 1989 structure in the SdH spectra was interpreted as evidence for
inhomogeneous doping via diffusion from the substrate or
size effect quantization.
This work was supported by the NSF-MRL program
through the Materials Research Center of Northwestern
University under grant DMR-85-20280, and by the Nation
al Science Foundation under grant DMR-86-02857. We
... vould like to thank Professor Lin Liu for valuable theoreti
cal discussions and W. Nieveen, who designed and built the
molecular beam epitaxy system and provided generous tech
nical assistance.
'J-. w. Tu, G. K. Wong, and J. B. Kettersoll, Appl. Phys. Lett 54, 1010
(1989).
"L. W. Tu, G. K. Wong, and J. B. KeUcrson, App!. Phys. Lett., 55, 1327
(1989).
'R. A. Smith, Semiconductors, 2nd cd. (Cambridge University, Cambridge,
1978), p. 406.
4E. N. Adams and T. D. Holstein, 1. Phys. Chern. Solids 10, 254 (1959).
'8. L. Booth and A. W. Ewald, Phys. Rev. Lett. 18,491 (1967); B. L.
Booth, Ph.D. thesis, Northwestern University, Evanston, XL, 1967.
('A. Goldstein, S. J. Williamson, ami S. Foner, Rev. Sci. Instrum. 36. 1356
( 1(65).
7L. R. Windmiller and 1. B. Ketter<;on. Rev. Sci. lustrum. 39,1672 (1968).
'R W. Stark and L R WindmiHcr. Cryogenics 8,272 (1968).
"M. H. Cohen and E. L Blount, l'hiins. Mag. 5, 115 (i 960).
Tu et al. 2645
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1.101708.pdf | Selective nucleation and growth of diamond particles by plasmaassisted chemical
vapor deposition
Jing Sheng Ma, Hiroshi Kawarada, Takao Yonehara, JunIchi Suzuki, Jin Wei, Yoshihiro Yokota, and Akio Hiraki
Citation: Applied Physics Letters 55, 1071 (1989); doi: 10.1063/1.101708
View online: http://dx.doi.org/10.1063/1.101708
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/55/11?ver=pdfcov
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J. Appl. Phys. 66, 4676 (1989); 10.1063/1.343824
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128.114.34.22 On: Sat, 22 Nov 2014 04:38:47Se~ective nucleaUon and growth of diamond particles by p~asmagassisted
chemical vapor deposition
Jing Sheng Ma, Hiroshi Kawarada, Takao Yonehara,a) Jun-Ichi Suzuki, Jin Wei,
Yoshihiro Yokota, and Aklo Hiraki
Department o.lElectrical Engineering, Osaka University, Suita-shl; Osaka 565, Japan
(Received 6 March 1989; accepted for publication 30 June 1989)
Diamond particles have been selectively synthesized on a Si02 dot-patterned Si substrate using
plasma-assisted chemical vapor deposition (plasma CVD). Nucleation densities on both Si and
Si02 have been increased, first by pretreatment using abrasive powders; then, to eliminate the
pretreatment effect from almost all of the substrate and to retain the effect only at designed
sites, an Ar beam is used to obliquely irradiate the pretreated substrate. After deposition using
plasma CVD, diamond particles have selectively formed on one edge of the Sial dots
according to the pattern and have grown large to adjoin with adjacent particles. Polycrystals
with equal grain sizes have been synthesized.
Synthetic diamond thin films I 5 have potential for fabri
cating high-temperature semiconducting and optical devices
because diamond has many extraordinary properties such as
high thermal conductivity and a wide band gap (5.4 eV).
Studies on semiconductivi ty6.7 and luminescenceH,9 of syn
thetic diamond thin films have shown the first step for the
realization of wide applications to semiconducting devices.
But until now, only homoepitaxial growth has been success
ful. Diamond thin films formed on nondiamond substrates
such as Si, Si02, etc., are polycrystaliine with random nu
cleation sites and different grain sizes. These kinds of ran
dom polycrystalline films seriously limit the wide applica
tions of diamond. For semiconducting device applications,
well-controlled positions and sizes of grains are essential.
Selective nucleation based epitaxy (sentaxy) of silicon
has been proposed by one of the authors to manipulate the
nucleation site and period artificially in order to obtain crys
talline films with controlled location of grain boundar
ies, 10.11 Small portions of a material of high nucleation den
sity are surrounded by another material of low nucleation
density. Single nuclei of silicon preferentially form on each
of the artificial nucleation sites and eventually grow large to
adjoin with adjacent crystals. The application of this tech
nique to diamond may be the nearest approach for fabricat-
Ca) pretreatment:
diamond powders
'11'11
\ \ ,,\
Si (h) oblique irradiation:
Ar beam
FIG, L Schematics of the preparation processes of the substrates for selec
tive growth. (a) The cleaned substrate was pretreated firstly by abrasive
powders (diamond powders of about 30 pm) using an ultrasonic cleaner
(b) The pretreated Substrate was irradiated by all Ar beam with an incident
angk of a 00").
a'Canon Inc. RID Headquarters, 6770 Tamura. Hiratsuka City. Kanagawa
254. Japan, iug diamond semiconducting devices, since heteroepitaxial
growth of diamond has not been realized.
Unlike Si, however, high nucleation density of diamond
on ncndiamond substrates has only been obtained after pre
treatment by abrasive powdcrs.~ Selective nucleation of dia
mond has becn reported by Hirabayashi et at., by using Ar +
ion beam vertical incidence on a pretreated substrate which
is patterned with resist. 12 The role of the Arl ion beam was
explained as etching the pretreated surface layer which leads
to the formation of a densely packed nuclei. Here we devel
oped a technique to realize diamond selective growth by in
troducing oblique irradiation of an Ar beam-the Ar beam
irradiates the pretreated surface with an angle of a . We have
achieved much more accurate positioning of I he nucleatiDn
sites than the previous works. 10-12
FIG. 2, Typical scanning electron microscope image of selectively grown
diamond particles. The substrate is a SiO, dot-patterned Si wafer. The size
of the SiO, d()t~ is 1.2 X 1,2 tim', with a height 0[0,2,urn and an interval of
10 pm between the dots, Diamond particles have similar grain sizes of about
10 pm and they have adjoined adjacent partil·ies.
1071 Appl. Phys. Lett. 55 (11). 11 September 1989 0003-6951/89/371071-03$01.00 (.0) i 989 American Institute of Physics 1071
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128.114.34.22 On: Sat, 22 Nov 2014 04:38:47In the present research, both SiOz dGt~ and strip-pat
terned Si wafers were used as the substrates, and several
kinds of patterns with different dot sizes were fabricated,
Figures 1 (a) and 1 (b) are schematics of two main steps
of preparation processes of the substrate for selective dia~
mond growth:
(a) Pretreatment. The cleaned substrate was pretreated
first by abrasive powders (diamond powders of about 30
pm) using an ultrasonic generator for 3 min, followed by
cleaning, This was a conventional treatment process for dia
mond synthesis using chemical vapor deposition (CVD),
The nucleation densities on this pretreated substrate would
1072 Appl. Phys. Lett., Vol. 55, No. 11. i 1 September 1989 ]'<'1G, 3. After oblique irradiation with
the AI' beam, diamond particles grow
only on the opposi Ie euges of either the
strips or the dots. The lower images are
enlarged ones from the uppers. The
heights and intervals for all the Si02
strips and dots arc the same, D.2 amI 10
pm, respectively. The width of the
strips of (a) is 2.0 pm, and the dot
sizes are 6.0,2.0, and 1.2 !lm for (b),
(e), and (d). The arrows indicate the
directions of the incident Ar beam,
and the dotted lines ,ire the SiO 2 strips
and dots. Single particles have been
obtained on 1.2.um dots.
be (2-5) X lOR particles/cml for the Si surface and 7-8 X 107
partides/cm2 for the SiO; surface. However, this difference
is not enough to achieve the selective growth as in Si. 10.1 !
For the purpose of selective nucleation of diamond, an
other process was absolutely necessary:
(b) Oblique irradiation. The pretreated substrate was
then irradiated by an AI' beam with an incident angle of 300
for 10 min. The Ar beam was produced by a de ion source.
The pressure of AI' gas was about 10 4 Torr and the accel
eration voltage was 5 kV with a beam current of 50 /lA.
Because ofthe divergence of the beam, the 10 min irradiation
was appropriate to obtain a good selectivity of nucleation at
Maetal. 1072
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128.114.34.22 On: Sat, 22 Nov 2014 04:38:47the large area of the substate, but longer irradiation would
result in no nucleation even on the SiOL dots,
Conventional microwave plasma CVD' or magneto
microwave plasma CVD5 was used to synthesize diamond.
A C0(15% )/H2 mixture and a CH4 (2% )/H2 mixture
were used as reaction gases. The substrate temperatures
were between 850 and 900 °C .
Figure 2 is a scanning electron microscope (SEM) im
age of selectively grown diamond particles. The substrate
used here was a Si02 dot-patterned Si wafer. The dots were
formed 1.2 X 1.2 ,urn) in size, O.2,um in height, and 10 pm in
intervaL 5 h deposition was carried out using plasma CVD
with a reaction gas of CD( 15% )/H2• Diamond particles
have grown to as large as 10 pm according to the designed
pattern. Single particles form on each dot, indicating that
location selectivity of nucleation is well controlled, The par
ticles have almost similar grain sizes and they have adjoined
with adjacent particles in the middle of the Si02 dots.
Some features concerning selective nucleation have
been investigated. Figures 3(a), 3(b), 3(c), and 3(d) are
images of the selective growth of diamond on the substrates
with different sizes of the Si02 patterns. In order to achieve
the information of the initial stage, the particles have not
been synthesized so large, The lower images correspond to
the enlarged upper images. The arrows indicate the direction
of the incident Ar beam. As shown in Fig. 3 (a), diamond
particles grow in line on the Si02 strips. They never grow on
the irradiated Si surface. AU of the particles grow just on the
edges of the strips; there is no particle on the nonedge area.
These edges are the opposite sides to the direction of the
incident beam. We can these edges "the opposite edges" for
convenience. In the cases of dots, the particles are also found
to grow on the opposite edges ofSi02 dots only. There is no
particle on the three other edges. As a result of the oblique
irradiation of the beam, one-dimensional control of diamond
nucleation is achieved. We can therefore control the
numbers of nuclei on each dot by changing the sizes of the
dot fitting with nucleation density on Si02• The line density
of diamond nucleation is 1 partide/1.2 fim on the Si02 sur
face under the pretreatment conditions described above, if
we take the simplest calculation on the assumption that the
particles nucleate in a periodic array. Reducing the size of
the Si02 dots from 6.0 to 2.0, to 1.2 ,urn, the numbers of
diamond particles on one dot are reduced from five or six
particles to two particles, and to one particle, respectively, in
agreement with the calculation. This fact suggests that there
is almost no effect of the irradiation on the opposite edges of
the dots or strips, or at least the effect is so weak that dia
mond stiH nucleates there.
Ar beam oblique irradiation plays an important role in
the selective nucleation of diamond. It reduces the nuclea
tion on the Si surface and on the facing parts of the Si02
1073 Appl. Phys. Lett.. Vol. 55, No. 11, 1 i September 1989 patterns but only retains the opposite edges of the Si02 pat
terns as nucleation sites. This irradiation can etch away a
very thin Si surface layer, but it is not strong enough to etch
away all the pretreated Si surface layer because a high nu
cleation density on the Si surface is still obtained which has
been chemically etched to as deep as several pm. Therefore,
the structure change associated with the Ar beam should be
considered. Reflection electron diffraction (RED) using a
75 keY electron showed that the Si surface changed to amor
phous after 10 min of Ar beam irradiation under the condi
tions listed above, We believe that it is this amorphous layer
that obstructs the pretreatment effect which is the cause of
diamond nucleation. As a result, diamond no longer nu
cleates at the irradiated surface. Although the Ar beam is
able to play the same role on the Si02 patterns as the beam
irradiates with an angle of 30°, it affects the facing parts and
the opposite parts of the Si02 patterns in different ways. The
facing part may take place as a structural change corre
sponding to what happens on the Si surface. On the other
hand, diamond nucleation on the opposite edge of the SiOz
pattern indicates an important edge effect. The reason of the
edge effect has not been revealed, but two essential factors
are considered. The first is the heavier pretreatment effect on
the edges of the Si02 patterns compared with that on the
surface. The second is the scarce irradiation effect on the
opposite edges because of the oblique incident beam. It is the
edge effect that makes the nucleation control more accurate
than previous works, ID.I]
The authors wish to thank the Ministry of Education,
Science and Culture of Japan for the support by a Grant-in
Aid for Developing Research (63850008).
• It V. Derjaguin, D. V. Fcdoseev; V. M. Lykuanovkh. B. V. Spitsyn, Vo A.
Ryunov, and A. V. Lavrentyev. J. Cryst. Growth 2. 380 ( 1981).
's. Matsumoto, Y. Sato, M. Kamo, and N. Setaka, Jpn. J. Appl. Phys. 21,
Ll83 (l9g2).
'M. Kamo. Y. SatG, S. Matsumoto, and N. Setaka, J. Cry>!. Growth 62,642
( 1993).
"A. Saw abe and T. Inuzuka. J. Cryst. Growth 137, 89 (\98b).
'H. Kawarada, K. S. Mar. and A. Hiraki, JplI. 1. Appl. Phys. 26, Ll032
( 1987).
"N. Fujimori, To lmai. and A. Doi, Vacumn36, 99 (1986)
'G. Sh. Gildellblat. S. A. Grot, C. R. Wronski, A. R. Hadzian, T. Badzian,
and R. Messier, Appl. Phys. Lett. 53. 586 (1988 l.
'V. S. Vavilov. A. A. Gippius, Ao M. Zaitsev, B. V. Dcr~aguin, B. V. Spit
syn, and A. E. Aleksenko. Sov. Phys. Semicond. 14, 1078 (1980).
"H. Kawarada, K. Nishimura, T. Ito, J. Suzuki. K. S. Mar, Y. Yokota, and
A. Himki.Jpn. J. Appl.l'hys. 27, 1,683 (198H).
,oT. Yonehara, Y. "'ishigaki, H. Mizutani, K. Yamagata. and T. khigawa,
Extended Abstract of 19th Conference 011 Solid-State Devices and Materi
als (Japan Society Gf Applied Physic,s, Tokyo 1987). p. I'll.
"I'. Yonenara, Y. Nishigaki, H. Mizutani, S. Kondoh, K. Yamagata, T.
Noma, and T. Ichigawa. Appl. Phys. Lett. 52, 1231 (1988).
"K. Hirabayashi, Y. Taniguchi, O. Takamatsu, T. Ikeda, K. lnoma, and N.
Iwasaki-Kurihara. Appl. Phys. Lett. 53. 1815 (1988).
Ma etal. 1073
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1.344218.pdf | Modeling of luminescence phase delay for nondestructive characterization of Si wafers
D. Guidotti, J. S. Batchelder, A. Finkel, P. D. Gerber, and J. A. Van Vechten
Citation: Journal of Applied Physics 66, 2542 (1989); doi: 10.1063/1.344218
View online: http://dx.doi.org/10.1063/1.344218
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/6?ver=pdfcov
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39Modeling of luminescence phase delay for nondestructive characterization
of SI wafers
D. Guidott!, J. S. Batchelder, A. Finkel, and P. D. Gerber
iBM Thomas J. Watson Research Center, P. O. Box 218, Yorktown Heights. New York 10598
J. A. Van Vechten
Department of Electrical and Computer Engineering, Oregon State University. Corvallis, Oregon 97331
(Received 17 April 1989; accepted for publication 30 May 1989)
We have modeled the generation, diffusion, and recombination of photoexcited electrons and
holes for the case of Czochralski Si wafers having a defect-free-zone (DFZ) device layer of
thickness d above a highly precipitated wafer core and having a finite surface recombination
velocity, S. The incident photoexcitation source has a Gaussian power distribution and is
focused to a small spot on the sample surface. When the source is sinusoidally modulated at
frequency v, the intrinsic band-edge photoluminescence (PL) emission displays modulations
at the fundamental and first overtone of the modulation frequency. The PL signals at
frequencies v and 2'11 are delayed in phase, with respect to the source modulation by angles
£/;2 ( v) and ¢2 (2 v). We relate these phase angles to material properties such as d, S, the optical
absorption coefficient a at the incident wavelength, and to the effective carrier lifetimes 1'1 and
72 in the DFZ and precipitated wafer core, respectively. We show that when 1'] and 1'2 are
independently measured and S-s. 100 em/s, as is common for a Si surface passivated with a
thermally grown oxide layer, it is possible to deduce d from a measurement of ¢2( v) or ¢2(2v).
t INTRODUCTION
We report a nondestructive and noninvasive technique
for inspecting Si wafers at various stages of processing using
the method of photoluminescence (PL) phase delay, or
PPD. The goal of our modeling is to guide the optimization
of the PL technique although many of our conclusions also
apply to other methods of monitoring photoexcited carrier
transport properties. These include recombination time
measurements by microwave absorption, t plasma reftec
tance,2-5 plasma absorption,6.7 eddy currents,8 or diffusion
length measurements by surface pnotovoltage.')
Although our discussion of the PPD method appears to
be applicable to any semiconductor, our modeling has so far
been restricted to Si. In addition, our model is restricted to
the case of a sample whose transport properties are homoge
neous in any plane parallel to its surface. The effective car
rier lifetime is allowed to vary in layers in the z direction.
This is because processed Czochralski (CZ) 5i wafers can
contain SiOx precipitates in the bulk and a defect-free zone
(DFZ) which extends some distance d, nominally about 30
pm, below the surface. Precipitate formation is important in
commercial Si in that this provides intrinsic gettering,
O--12
sites for impurities in the bulk and away from the surface
layer where devices are to be fabricated. However, the device
layer itself, the DFZ, must be free of precipitates as these
would constitute a fault if they protruded too close to a de
vice structure. The DFZ forms because at elevated tempera
tures oxygen diffuses out from the free surfaces afthe wafer,
producing an oxygen denuded layer of nominal thickness, d.
During high temperature processing, SiD" complexes pre
cipitate out of the dissolved oxygen in the interior of the
wafer and form inclusions with typical dimensions of less
than 100 nm. A delicate balance must be maintained during the precipitation process to ensure that the precipitates are
formed near enough to the surface that their strain fields can
affect the gettering action but not so near that they disrupt
the device structure. Thus, the depth d of the denuded layer
is a critical parameter in the early stages of wafer processing
and one that we will show can be nondestructively moni
tored by the PPD method.
Other nondestructive techniques for determining d have
been reported. Many use photoexcited carrier decay signa
tures and modeling, as we also do, in order to deduce this
parameter. Microwave reflection,l eddy currents,8 optical
plasma refiectance,2-5 and optical plasma absorptionli•7 mea
sure carrier recombination times, while surface photovol.
tage9 measures carrier diffusion length. In each of these
cases, with the possible exception of plasma absorption, one
needs to know II' the spatially averaged excess carrier life
time in the DFZ as well as 1'2' the effective carrier lifetime in
the bulk, in order to deduce d unambiguously. The spatial
resolution in microwave reflection and eddy currents is gen
erally much less than that of plasma reflectance/absorption
and photovoltage techniques. Because precipitates are gen
erally decorated with chemical. impurities intrinsically get
tered from the surrounding crystal lattice, the nonradiative
recombination rate for photoexcited carriers can be very
high in a volume containing a large number of precipitates.
As a result, when carriers are generated on or near precipi
tates, the local lattice temperature can rise measurably above
recombination heating in precipitate-free regions, Because
enhanced nonradiative recombination at or near precipitate
sites results in enhanced local lattice heating, and because
the optical reflectivity of Si is a function of surface tempera
ture, photoinduced thermorefiectance measurements2,13 can
also have some sensitivity to the depth of the precipitate lay
er. However, in this case, carrier diffusion, heat generation,
2542 J. AppL Phys. 66 (6),15 September i989 0021-8979/89/182542-12$02.40 @ 1989 American Institute of PhySics 2542
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39and heat di.ffusion must be modeled with care and the prob
lem increases in complexity. In addition, at high modulation
frequencies the complex dielectric constant of Si becomes
less modulated by temperature excursions, whose amplitude
decreases with increasing modulation frequency. At the
same time, the Drude modulation of the dielectric constant
increases because at high modulation frequencies the probe
beam samples a higher average free carrier density during
each modulation cycle. Therefore, a transition from ther
moref'iectance to plasma-reflectance generaHy occurs as the
modulation frequency approaches 1/1"1 (or 1/'12) even at
visible probe beam wavelengths. This effect is graphically
demonstrated in Ref. 2.
The generally accepted method for determining d is to
angle lap and etch a wafer sample. 14 This procedure is de
structive, requires considerable sample preparation and is
often dependent on the skill of the operator. X-ray section
topograph yl5 can also be used to measure d, and while this
method does not require sectioning the sample, several hours
of film exposure are required with conventional x-ray
sources. Synchrotron radiation 16 can substantially shorten
exposure time. Finally, metal-oxide-semiconductor (MOS)
structuresl7 can be used to make an electrical determination
of the DFZ based on the Zerbst 18 transient capacitance anal
ysi.s. The advantage of the MOS measurement is its sensitiv
ity to pre-precipitation nuclei and other atomic imperfec
tions of the crysta.l within the DFZ because of the effect
which these "trapping centers" have on carrier capture and
carrier re-emissionY·18 Therefore, it is not necessary to
cause fun precipitation of oxygen in the silicon in order to
obtain a. measure of the DFZ by the MOS method. Unfortu
nately, because of oxide breakdown under high electric field
strength, this method is limited to a depth sensitivity of less
than 10 pm. In addition to being sensitive to precipitates and
point defects in Si, the temporal behavior of the MOS capaci
tance is also sensitive to impurities in the oxide, and sample
preparation is a critical factor. A contaminated oxide film
cannot be differentiated from a defective substrate. Finally,
the MOS method is not usable when the sample is highly
conductive.
The principle ofPL phase delay is to periodically modu
late the intensity of a light source which is focused on the
sample to a spot size of a few microns. The absorbed light
creates a periodically modulated plasma wave of electron
hole pairs which propagates away from the excitation spot
due to density gradient diffusion and is damped by both dif
fusion and carrier recombination. A fraction of the photo
generated carriers recombine by emitting light in a wave
length band that is characteristic of the 5i band gap and of
the phonon-assisted radiative processes. 19 A fraction of this
light refracts out of the sample and is detected quite effec
tively even at room temperature. At a sufficiently high pho
toexcitation level, pair recombination among generated car
riers dominates and the amplitUde of the luminescence
becomes modulated with components at both the fundamen
tal and first overtone harmonic20 of the modulation frequen
cy. When the source intensity is modulated sinusoidally,
only the fundamental and first harmonic overtone are pres
ent in the modulation of the luminescence.2() The amplitudes
2543 J. Appl. Phys., Vol. 66, No.6, 15 September 1989 and phases (relative to the modulation waveform of the
source) are functions of the parameters that affect the gener
ation, diffusion and recombination of photoexcited electrons
and holes. Experimentally, the phase and amplitUde of the
modulated photoluminesence signal at both the fundamen
tal and first overtone frequencies can be monitored with
standard lock-in amplifiers or phase meterso20•21 Figure 1 of
Ref. 21 shows a map of the relative variations of the phase
delay 4;2 (v) at the modulation frequency v mapped across Ii
wafer and showing the typical swirl pattern of precipitate
distribution that is commonly observed by x-ray topography
or scanned surface photovoltage.22•23 Similar detail is ob
served in a map of the phase delay epz (2v) at the first over
tone of the modulation frequency as well as in a map of the
relative variations in the integrated spectral intensity.
We have concentrated this modeling effort on the effect
which the parameters r l' 1'2' d, and S, have on the calculated
PPD and on how well these predict the measured phase de
lays 4;z(v) and ¢2(2v) of the PL signal at the fundamental
and first overtone frequencies. We also examine the effect of
varying parameters of the experiment that we can controL
These are the source modulation frequency (v), the wave
length ofthe incident light and the corresponding absorption
coefficient (a), and the spot size (w) to which the incident
light is focused on the sample surface.
Our experimental arrangement for measuring the PL
phase shift is described in Sec. II. Carrier diffusion is dis
cussed in Sec. III where we also justify use of the linear form
ofthe diffusion equation for photoexcited carriers. A Hankel
transform method for obtaining the three-dimensional solu
tions of this equation in cylindrical symmetry is described in
Sec. IV. Model predictions and fitting of experimental data
are left to Sec. V. Finally, in the Appendix we summarize
results for one-dimensional diffusion.
II. EXPERiMENTAL DETAilS
Photoexcitation was provided by a Kr-Ar laser operat
ing at 647 11m in the TEMoo mode and capable of a maximum
output of 300 mW. The incident beam was fully modulated
sinusoidally at frequency v up to 20 MHz by an acousto
optic modulator operating at 200 MHz and having approxi
mately 50% transmission efficiency in the first-order dif
fracted beam which was used to illuminate the sample at
normal incidence. Sinusoidal modulation is important when
detecting the first overtone response of the PL signa120 as
non sinusoidal waveforms generally have Fourier compo
nents at 2v and higher frequencies. A lens assembly with a
numerical aperture ofO. 8 and a working distance of approxi
mately 1/2 mm served the dual purpose offocusing the inci
dent light on the sample surface and of efficiently collecting
and collimating the emitted PL light into a beam which,
upon a reflection from a dichroic beamsplitter at 45° to the
incident direction, could be conveniently directed either into
a monochromator for wavelength analysis or into a detector
for measuring phase retardati.on of the modulated PL signal
with respect to the incident photoexcitation waveform. The
incident beam underfiHed the entrance pupil of the lens as
sembly and was focused to a spot size of about 10 jlm. The
Guidotti et al. 2543
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39spot size is defined as the full width at 1/ e of the cylindrically
symmetric Gaussian power distribution of the TEMoo mode
of the excitation laser and was measured using a moving slit
commercial instrument designed for this purpose ("Beams
can" model 1 080, sold by Photon, Inc.). PL phase delay data
were obtained with an infrared~extended Si photodiode
(EG&G Y AG-444) up to 1 MHz, or up to 20 MHz with a
cooled photomultiplier tube (Hamamatsu R 632) having an
S~ 1 photoresponse. Standard Schott color glass filters were
used to reject coincident low level laser light and pass the PL
light. Wideband amplifiers EG&G/Ortec models 9305
preamplifier and 535 quad amplifiers were used with the
photomultiplier tube, while high gain amplifiers EG&GI
Princeton Applied Research models 184 and 114 were used
with the photodiode. Lock-in amplifiers, EG&G/Princeton
Applied Research models 124A (up to 200 kHz) and 5202
(l00kHzto 50MHz) were used for PL phase and amplitude
measuremen ts.
The spectral distribution ofPL emission from Si at room
temperature as well as its power dependence, in particular,
the transition from a linear to a quadratic power depend
ence, have been reported elsewhere.20 In this article we con~
sider information contained in the phase of the modulated
PL signal, but only in the regime in which carriers recombine
via photoexcited electron and hole density of states; that is,
in the regime of quadratic power dependence of the PL sig
na1.20 In this regime recombination via near~band-edge den
sity of states, corresponding to the linear portion ofthe pow
er dependence, is assumed to make an insignificant
contribution to the observed phase shift at v. The PL phase
shift ¢2 ( v) is recorded as a function of mod ulation frequency
11 as the difference ¢2( v) = ¢r (v) -¢s (v), where ¢r (v) is
the instrumental phase shift measured on the lock-in ampli
fier when a small fraction of the modulated laser light is
made incident on the detector and its power adjusted so as to
give nearly the same magnitude on the lock-in as the PL
signal, and ¢s (v) is the phase shift which is measured when
only PL light is incident on the detector. Because of the large
phase shifts introduced by the acousto-optic modulator, it
was not possible to measure the phase shift ¢2 (2v) at the first
overtone frequency in the same way. The acousto-optic
modulator introduces a phase shift in the modulation wave
form which depends strongly on the modulation frequency,
making it difficult to reliably obtain the instrumental phase
shift at 2v. Electro-optic modulators may suffer from the
same frequency-dependent phase shifts, although we have
not examined this behavior. Mechanical choppers, on the
other hand, have to be carefully designed to give a sinusoidal
waveform, but much more seriously, these can only be oper
ated practically at v < 30 kHz, much too low for useful data
analysis.
The data reported here were taken at an incident power
of 10-20 mW up to 200 kHz, and 30-40 mW between 100
kHz and 10 MHz.
III. GENERAL DESCRIPTION OF CARRIER DIFFUSION
AND Pl EMISSION
At any point r in a semiconductor, the time rate of
change in the concentration N(r,t) of excess electron-hole
2544 J. Appl. Phys., Vol. 66, No.6. i 5 September i 98S pairs is governed by both diffusion and recombination, and
can be described by an ambipolar diffusion equation of the
form24,25
aN D .... ~N N Rle2 II.T3 -= ,,~ --- 1'>/ -yl'll' +g(r,t), at 7' (1)
where g(r,t) is the carrier generation rate per unit volume
from an external source of excitation and D is the ambipolar
diffusion coefficient. 24,25 In the case of Si at 300 K, D = 18
cm2 s -\ and is not significantly altered26 by excess carrier
concentrations up to 1 X 1017 cm'}. The effective excess car
rier lifetime is given by 1/1' = 1/1'R + 1/1'NR, where 1'NR is
the carrier lifetime against nonradiative recombination
through impurity states, and 1'R is the lifetime against radia
tive recombination via shallow donor or acceptor states and
can be approximated by27 7'R = lIB(no + Po + N), where
no and Po denote the equilibrium carrier concentrations and
N is the excess carrier density. The probability for radiative
recombination is reported28 to be B = 1.1 X 10-,4 em3 s -I
for nominally (unintentionally) doped Si at 300 K, in rea
sonable agreement with values calculated27 in the presence
of shallow acceptor states. Finally, the free carrier Auger
coefficient at 300 K has a generally accepted value29 of
y = 4X 10-31 cm6 s-'.
The solution ofEq. (1) must satisfy appropriate bound
ary conditions. For a semi-infinite semiconductor which oc
cupies the half-space z > 0 and whose surface coincides with
the x-y plane at z = 0, the appropriate boundary conditions,
when photoexcitation occurs over the entire surface of the
sample, are30
N-O as z-00
and
DaNj =SN(z=O). az Z~O (2)
The initial condition: N = 0 for 1<;:;0, when the excitation is
turned on at t = 0, and the relaxation condition: N -+ 0 for
(t -to) >1', when the excitation is turned off at t = to, are
also implicit requirements for the solution of the diffusion
equation for excess carriers. The density of defect states at
the surface is characterized by the parameter S, the surface
recombination velocity. When S = 0, the surface retains
bulk properties. When S> 0 the surface acts as a sink for
photogenerated carriers. Values of 8 in the range 0<;:;8 < 100
cm/s generally indicate good passivation for a silicon sur~
face.
Equation (1) can be linearized when the excess carrier
density is sufficiently small. In particular, when
N = 1 X 1017 cm-3, we find BN2~rN3..(N /T and Eq. (1)
can be written as
aN -D""'2Iv N --"H--+g. at l' (3 )
The characteristic diffusion length for excess carriers in the
absence of modulation is given by Ao = .,fliT. The average
carrier density (N) over a hemispherical volume of radius
;\0' assuming unity carrier generation efficiency, is given by
(N) = !CP( rlj 17-A~),
where Pis the incident power, and C = 1I( 1.6 X 10-19 hf!)
Guidotti et al. 2544
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39converts the incident power (in Watts) into photon current
(when 11ft, the incident photon energy, is expressed in eV).
If we use r = 10 ps, which is typical3l for p-type Si with
resistivity 10-15 n cm, we find (N) = L6x 1016 cm-3
when P = 10 mW. The data which we discuss in Sec. V was
taken at an incident power between 10 and 40 m W. Equation
(2) should, therefore, remain valid even when the excitation
radius is much smaller than Ao. This is because carrier diffu
sion rapidly depletes the excitation volume, defined by the
absorption depth and spot size for the incident light. Equa
tion (2) has been solved for one-dimensional geometry (cor
responding to a photoexcitation area whose linear dimen
sions are much greater than AoJ by many authors and our
results, in particular, are summarized in the Appendix. We
have previously solved Eq, (2) in three-dimensional spheri
cal geometry for a point photoexcitation in an infinite semi
conductor.2o It is shown in Ref. 20 that the phase shift and its
frequency dependence predicted by the one-dimensional and
spherical three-dimensional models are quite different It is
important, therefore, that the mathematical model represent
as closely as possible the physical conditions of photoexcita
tion, PL generation and PL reabsorption in the semiconduc
tor. Since our photoexcitation geometry is cylindrically sym
metric about an axis normal to the surface, we solve Eq, (2)
in cylindrical polar coordinates in the presence of a photoex
citation whose power distribution varies radially as a Gaus
sian function. In this geometry Eq, (2) is conveniently
soived by use of Hankel transformationJ2 and application of
fast Fourier transfonns33 for the inverse transformation.
IV. THREEBDIMENSIONAl SOLUTION FOR FINITE
SPOT EXCITATION
We will briefly summarize some of the properties of
Hankel transforms which are particularly useful in the nu
merical solution of Eq. (2). An excellent treatment may be
found in Courant and Hilbert.34 The Hankel transformation
of the function NCr) is defined34 as
H" [NCr) J =Nl' (0) = i'" rJ" (or)N(r)dr
()
for v> -!. (4)
When v = ±~, H" reduces to the Fourier sine and cosine
transform. The inverse transform is given by
N(r) =J~oo oJv(or)Nv((7)d(7.
o (5)
Equation (5) demonstrates the autoreciprocity relation be
tween N(r) and N" (0"). Of course, J" (x) is the Bessel func
tion of order l' and satisfies the equation~4
d2y 1 dy ( v\ -. +-~+ l--:;-)y=O. dx2 x dx x-f (6)
Making use of Eq. (6), it can easi.ly be shown that
H --N(r) + --j\ (r) --N(r) 1= -a2N ((7). (d2 1 d r v, \
v dr r dr· i2 I v
(7)
This is a useful property which we will use later.
In cylindrical polar coordinates, Eq. (2) becomes
2545 J, Appl. Phys,. VOi. 66, No.6. 15 September 1989 The source term, or the rate of carrier generation, can be
written to represent absorption of an incident beam with a
Gaussian power distribution having a 1/ e radius at the sam
ple surface (at z = 0) given by wand propagating in the z
direction;
g = (go/2)e- az( 1 + a cos wt)e- (rlw)',
where a is the optical absorption coefficient at the wave
length of the incident light. This expression can also be writ
ten as
g = (go/4 )e-aZ(l + aeiwt)e -(,Iw)' + c.c.;
where (11 = 21TV, a is the modulation amplitUde and, as usual,
c.c. signifies complex conjugate. The peak carrier generation
rate go is related to the peak incident power Po by
go = sa[( 1 -R)/mv2hH]Po'
where g is the efficiency for carrier generation and R is the
sample reflectivity at the incident wavelength.
We are interested only in the long time U>r) behavior
of the solutions of Eq. (8) and therefore neglect initial tran
sients which decay in times of order r and occur when the
sample is first illuminated. We assume that the polar angular
dependence of the solution can be written as
N(r,</J,z,t) = N(r,z,t)eim,p. Then Eq, (8) becomes
eim",[a2N + l.. aN _ (~+ _1_. ~) + a2N _ ~ aN]
ar r dr \ r /\.~ a:i2 D at
g == -~
D (9)
Taking the Hankel transform ofEq. (9) and making use of
Eq. (7) we obtain
eitn¢(02Nm (0) _._ Nm <.0) _ 8Nm (~)
a:i2 /\.2 at
= -~; e-aZ(1 +acosoJt)Hm (e-(rlw)'), (10)
where lim [N(r,z,t)] :=:N", (a,z,t) and 1/1\2= (a2 + 1/ /\.6),
We assume cylindrical symmetry and set m = 0, Then Eq.
(10) becomes
;PNo(O") _ No((7) _l.. aNo(a)
a:i2 /\.2 D at
= __ (:~ e·~ az( 1 + a cos (ut) ) ~2 e . (.m>/2)' (11)
The zero-order Hankel transform of the Gaussian term in
Eq. (10) has been evaluated using standard table of integrals
of Bessel functions.35 Equation (11) is a one-dimensional
diffusion equation for the Hankel transform of the excess
carrier density in the presence of an oscillatory source term
and can be written more compactly as
a2No(0") _ No(O") _ ~ 8No(u)
8z2 1\.2 D at
= A (0) (1 + ae'"")e -?a + C,C., (12)
where
Guidotti et a/. 2545
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39A(u) == -(w2gol8D)e- (<,w/2)'.
Therefore, the problem of solving a three-dimensional diffu
sion equation for N(r,z,t) has been reduced to one which
requires solving a one-dimensional diffusion equation for
No (u,z,t), plus the Hankel transform operation
N = HoUVo(a,z,t)] toobtainN(r,z,t). In form, the solutions
ofEq. ( 12) are just those for the one-dimensional solution of
Eq. (3) in the presence of finite absorption and a sinusoidal
ly varying excitation term. These solutions are described in
the Appendix. A particular solution ofEq. (12) is given by
NT ( t) ~ A(a)A2
-az o U,Z, -2 1 e
P a A--1
+ ( Ce-bez + ;~~2 e-az)ej
'''' + C.c., (13)
where{32 = 1/ A 2 + i6J1 D. The homogeneous solution ofEg.
(12) is given by
No. (o-,z,t) = A Ie -z/A + A2e -their", + C.c. ( 14)
The constants C, A I. and A 2 are determined from the bound
ary condition [Eq. (2)] whose Hankel transform is
(15)
where the + z direction is into the sample and the boundary
condition requiring that No-+O as Z-l> 00 is already satisfied
by Eqs. (13) and (14). ThegeneralsolutionofEq. (12) is
given by the sum of the homogeneous and particUlar solu
tions, and is
N.( 0= A(a)A2
( -az+ Da-S -z/I\)
o if,Z, a2A 2 _ 1 e S _ D I A e
+(e-az+ Da-S e~~l'z) Q?A(a) ej,ut+c.c.
S -f3D I a--(32
(16)
The three-dimensional solution of Eq. (3) for t> r, or the
steady-state excess photo excited carrier density, is given by
the Hankel transform ofEq. (16):
N(r,z,t) = L'" o-Jo(ar)NIj(if,z,t)da. (17)
Equation (17) can be solved conveniently by using a quasi
fast Hankel transform algorithm.32,33 The complex constant
f3 gives rise to a phase delay (with respect to the excitation
source) in the calculated periodic PL signal and this is then
compared with the measured photoluminescence phase de
lay.
The total rate of PL emission from the entire sample is
given by
R(t) = roo roc (NCr,Z,t) + BN2(r,Z,O)e- 11z21Trdrdz,
Jo Jo 7R
(18)
where the first term represents recombination of photoexcit
ed carriers via donor (or acceptor) density of states which
may be present in the semiconductor from doping, while the
second term describes bipolar recombination via photogen
erated electron and hole density of states. The optical ab
sorption coefficient 1] for the PL can affect the PL phase
shift, however 17 is small for Si and its effect on 4;2 ( v) and
2546 J. Appl. Phys., Vol. 66. No, 6,15 September 1989 ¢iz(2-v) Cal') be neglected to first order. It is clear from Eq.
(18) that, as in the one-dimensional case (see Appendix),
PL emission will be modulated in amplitude at both the fun
damental and first harmonic overtone of the modulation fre
quency.
As discussed in the introduction, we are interested in
modeling an inhomogeneous sample whose geometric sur
face is located at z = 0, A region of the sample (the DFZ)
extends from z = 0 to z = d and is characterized by an effec
tive carrier lifetime r l' The remainder of the sample has an
effective carrier lifetime 1'2' The condition 7R ~1'l >72, while
not a restriction on our model, is generally true. The effective
carrier lifetime 72 in the bulk has a strong contribution from
impurities and nonradiative defects gettered near oxide pre
cipitates. On the other hand, 1'1 ~1'R even for the highest
purity Si because residual impurity states which do not affect
device performance wiH, nevertheless, enhance the rate of
radiative recombination and decrease2X,36 rR• The recombi
nation properties of the sample surface are described by the
surface recombination velocity parameter, S, through the
boundary conditions expressed in Eq. (2). In addition, in the
stratified model of a Si wafer with a DFZ, those three-dimen
sional solutions of Eg, (3) which propagate both in the -z
and + z directions must be included within the DFZ layer.
Finally, the general solutions for carrier densities in the bulk
(Nz) and DFZ (N,) are subject to boundary conditions
which assure continuity of both particle density and current
at z = d. Such boundary conditions require that NJ = Nz
and Dj(aN,/oz) = D2CJN2/Jz) atz = d.
Due to the complexity of the numerical portion of this
modeling, and to ensure correct results, two independent
computer codes were developed, one in APL2 (by AF) and
the other in FORTRAN (by FDG). The results were com
pared and not accepted until they agreed within the round
ing errors. In addition, in the limit w> Ao. the three-dimen
sional solutions for tPz ( v) and ¢i2 (2v) must coincide with the
one-dimensional phase shifts (see Appendix). This asymp
totic convergence is verified in Figs, 1 and 9 for the case of a
uniform sample.
v. GENERAL RESUI.. TS OF THREEsDiMENSIONAL
CARRIER DIFFUSION AND COMPARISON WITH
EXPERIMENTAL DATA
The PL signal in the linear range of the power depend
ence20 is very weak and it is difficult to obtain good data at
high modulation frequencies. We therefore report only data
which were taken when PL emission varies quadratically
with incident power20 and therefore may only consider the
second term in Eq, (18) when calculating the phase shifts
¢2 ( v) and tP2 (2 v). There is no inconsistency in assuming
that the linear term in Eq. ( 18) is small, while that in Eq. (3)
is dominant. Note that for a typical wafer the ratio rR 11' is of
the order 10-100 in the PFZ and much greater in the bulk.
Figure 1 shows the calculated phase shift ¢2 ( v) as a
function of the modulation frequency v in the case of a uni
form Si sample that is thick compared to the dc diffusion
length AQ• The parameters used are: l' = 100 fLs, S = 500
cmls, and D = 18 cm2/s, The calculated phase shift is seen
to be strongly dependent on excitation spot size w, In curve
Guidotti at aI, 2546
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39100
80
'" 60, ..
I ~
.=.
'" il-,or
20
0 !0'1 10 106
1/ (kHz)
FIG.!. Dependence of "'2 (v) on the excitation spot radius w ill the case of a
uniform, thick Si sample. Transport parameters are assumed to have the
following values: S = 500 em/s, D= 18 crnl/s, and r = 100 J.1.S, The spot
radius has assigned values w2 = 4 X 10 -q em', where q = 8,7,6,5,4, I and
its effect is seen in curves (a)-(O, respectively. Curve (g) is obtained from
the one-dimensional model [Eq8. (A3) alld (A4) J and is the asymptotic
limit for the three-dimensional model when w becomes large. Good asymp
totic behavior is achieved in curve (n where q~" 1, Of w-6.3 mm.
(a) w = 2 pm. The one-dimensional asymptotic limit, curve
(g), is essentially reached by curve (f) for which w is about
6.3 mm. The only significant deviation between curves (f)
and (g) occurs for v> 7 MHz. It is clear that the one-dimen
sional model can be used to adequately predict PL phase
shifts only when w>Ao. Curve (e), for which w = 200;umis
iOO
80
'" 60
:!
~
N
-@o 40
I
20 l
0 10.1 10 103 104 105 106
II (kHz)
FIG. 2. Effect of r on ¢2( v) for two spot radii and a thick Si sample with
uniform transport properties. D and S are the same as in Fig. 1. In curves
(a)-(c) or = I, 10, 100 J.1.S and w2 = 4X 10-8 crn:. In curves (d)-(f), 7
takes OIl the same values, but w2 = 4X 1O~ I em2,
2547 J. Appl. Phys., Vol. 66, No.6, 15 September 1989 comparable to Ao, still shows significant deviation from
curve (g). Unfortunately, in order to obtain a measurable
PL signal from Si at the moderate incident power level used
here, it is necessary to focus the incident light to a spot size of
about 10 pm. In this way a sufficiently high density of inject
ed carriers is achieved to be in the regime of quadratic power
dependence. Note that the asymptotic limit for ¢2 (11) is 90·
in all cases .
Figure 2 shows the effect ofthe effective carrier lifetime
7 on ¢z (11) for a uniform, thick Si sample represented by the
same values forSandDasin Fig, L For curves (a), (b), and
(c), 7 is 1, 10, and 100 ;Us, respectively, and the w = 2 f.1m.
Note that curves (b) and (c) are nearly superimposed. For
curves (d), (e), and (0, 7 has the same variation but
w2 = 4 X 10 -1 em 2, corresponding to a 1/ e radius of 6. 3 mm.
Clearly, sensitivity to effective carner lifetime 1'diminishes
with decreasing excitation spot size.
Figure 3 shows the effect of surface recombination ve
locity, S, on 0/2 (11) for a uniform, thick Si sample represented
by the same l' and D parameter values as in Fig. 1. Curves
(a), (b), (c), and (d) have values S = 105, 104, 103, and 0
em/s, respectively, and w = 2p,m. Curves (e), (f), (g), and
(h) range over the same values of S, but for this family of
curves w = 6.3 mm. Sensitivity to S decreases somewhat
with decreasing spot size.
We now consider an inhomogeneous semiconductor
which consists of a homogeneous layer extending from z = 0
to z = d. The effective carrier lifetime in this layer is 71 and is
the only transport parameter which distinguishes between
the surface layer and the underlying bulk which extends
from z = d to z = co. Both the surface layer and bulk extend
100
80
-; 60 I '" -0
~
",40 r -S-
20
10 105 lOG
11 (kHz)
FIG, 3. Effect of S on ¢2(V) for two spot radii and a thick Si sample with
uniform transport properties, D and r a.e the same as in Fig. 1. S takes on
assigned vulues S= 1O~~ "ern/so Ineurves (a}-(d) W2,~ 4x 1O-"cm2 and
q = 5,4,3,0, respectively, Incurvcs (e)-fh), tV' = 4X 10~ I cm2whileqvar.
ics thfough the same range.
Guidotti et at. 2547
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:3911 (kHz)
10 102 103 104 105
100 100l
80~ 80
g; 601- 60 0.
Q) -8 "0
.ol ~ 140 ~
N N
-&- r--&-
I
\20 20r
° 10 IOZ 103 !04 I --lO
105
:v (kHz)
FIG, 4. Inhomogeneous sample composed of a homogeneous surface layer
of thickness d in which 1'1 =, 100 jis, over a homogeneous, semi-infinite sub
strate in which 1'2 = 1 ft •. Sand D are the same throughout the sample and
have the same values as in Fig. I. The effects of various values of d and ware
apparent. In curves (a)-(c) w2=4XIO-1 and d=O, lxlO-", and
9X 10-' em, respectively. In curves (d),-(f), u?~' 4X 10 I cm2 anddas
sumes the same variation.
to infinity in the x and y directions. The effective carrier
lifetime in the bulk is 72 < 7,. In Fig. 4 we show the depend
ence of cP2 (v) on DFZ layer thickness d. The ambipolar dif
fusion coefficient, D is assumed to be independent of z and
has a nominal value of 18 cm2 Is. Other material parameters
have nominal values: S = 500 cmls, 7\ = 100 jls, and 72 = 1
C. 60 .,
."
;:::::
.=.
N 40 -e-
FIG. S. Experimentally obtained PL phase shift (circles) at various modu
lation frequencies, v, for a uniform (float-zone grown) N-type Si sample
with 10 n em resistivity. The dashed curve is a best fit of the three-dimen
sional (uniform sample) mode! to the data. Best fit parameters are: l' = 100
ItS, S= 500cm/s,D= 18 cm2/s, and 11/.= 6X 10-'1 em), consistent with
the measured 11 e spot size of about to {lm,
2546 J. Appl. Phys, , Vol. 66, No.6, 15 September 1S89 jlS. The layer thickness, d, takes values, 0, 10, and 90 {lm,
respectively, for curves (a)-(c) for which w2 = 4x 10-7
cm1 (-6 pm), In curves (d)-(f) d has the S8.me variation
but w2 = 4x 10-1 cm2 (-6.3 mm). The results in curves
(d)-(f) are essentially equivalent to one-dimensional diffu
sion in a layered semiconductor. Of course, when d = 0, we
retrieve the case of a homogeneous sample with 7 = 72,
whereas when d is very large, we approach the case of a
homogeneous sample with 7 = r ,. Note that greater sensitiv
ity to d is obtained in the limit of one-dimensional diffusion.
Figure 5 shows the experimentally measured PL phase
delay for a sample of N-type, float-zone Si with resistivity of
10 n em, a chemical-mechanical surface polish and a native
oxide coverage. The sample can be considered homogeneous
in the context of this measurement. We measured the phase
delay (circles) at the modulation frequency v and in the
presence of quadratic recombination up to 9 MHz as de
scribed in Sec. n. The dashed Hne is a best fit to the data and
is the calculated phase delay for a uniform sample with
S = 500 emls, T = lOOps, D = 18 cm2/s, and w = 7.7 pm,
dose to the experimentally determined lie spot size of ap
proximateiy 10 p,m.
In Fig. 6, the frequency dependence of the PL phase
delay (circles) is measured for a P-type, float-zone Si wafer
of resistivity 10-15 n cm having a chemical-mechanical sur
face polish and a native oxide coverage. Again, this sample is
considered homogeneous. The dashed line is a best fit to the
data and is obtained from the uniform sample model using
the parameters S = 80 em/s, 7 = 125 fis, and the same val
ues of wand D as in Fig. 5,
100r
I eor
S' 60~
.3
I ..=
(\J -e-40r
20
O~----~-------L------J-------~----~
10-1 10 102 103 104
:v (kHz)
EG. 6. Experimentally obtained PL phase shift (circles) at variolls modu
lation frequencies, v, for a uniform (float-zone grown) P-type Si sample
with 10-15 n em resistivity. The dashed curve is a best fit of the three
dimensional (uniform sample) model to the data. Best til parameters are:
1'~ 125,18, S= 80cm/s, w" == 6X [0-'1 em' andD= 18 cmc/s,
Guidotti et ai 2548
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39The discrepancy between observed and calculated be
havior for the PL phase delay may, in part, be due to contri
butions from recombination via shallow impurity states in
both N-and P-type material. These states contribute to the
first term in Eq. (18) and give rise to a PL phase shift in the
regime of linear recombination. We have consistently ne
glected this contribution in our calculations and have as
sumed instead that the excitation level is sufficiently high so
that only quadratic recombination occurs. In addition we
have also neglected, with justification, the nonlinear terms in
the diffusion equation. It is quite possible that at the excita
tion levels used in the experiment, the linear approximation
to Eq. (1) begins to break down. Nevertheless, the qualita
tive behavior of the data is very well described.
In the case of the two-layer model, the question of
uniqueness of the parameter fit to experimental data should
be addressed. For example, by suitably increasing d and de
creasing 71 while keeping 72 fixed, one can retrieve very near
ly the same frequency dependence of ¢2 ( v), as is demon
strated in a fit to the experimental data in Figs. 7 and 8. This
lack of uniqueness negates the ability to unambiguously de
termine the DFZ depth in a measurement of the PL phase
delay unless 7( and 7'2 are independently determined. This
ambiguity in the detennination of d is common to all layer
modeling of ambipolar diffusion which attempt to obtain the
DFZ depth from the measurement of a single transport pa
rameter such as diffusion length <.) or carrier lifetime. I
The PL phase delay data in Fig. 7 are measured on a CZ
Si sample which has undergone thermal cycling designed to
produce a DFZ which is 20-30 pm deep and a wafer core
with substantial oxide precipitation. A thick, thermally
grown oxide on the wafer surface assures good surface passi-
10°1 J
80 -c}
0
(Q
o / 0/
9:f O{
:#
~ c;. 60 ..
~
;,.
'" -e-40
t
-1 /
I~ " I 6-<J2 ... ~&r60~ I 20
I 10 102 103 104
II (kHz)
FIG. 7. Experimentally obtained PL phase shift (circles) at various modu
lation frequencies, v, for an inhomogeneous, CZ Si sample with a known
DFZ in the range 2~30 {tm. The dashed curve is a best fit of the three
dimensional, two-layer model to the data. Best fit parameters are: T, = 500
liS, '2 = 2 {ts, S= 50 em/s, w? = 6 X 10 -7 cm), D =, 18 cmz/s, and d = 20
,urn.
2549 J. Appl. Phys., Vol. 66, No.6, 15 September 1989 vation. The two-layer model is appropriate in this case. The
dashed curve is a fit to the data using the fonowing set of
parameters. D and ware the same as in Fi.gs. 5 and 6, S = 50
em/s, d = 20 jim, 7'] = 200 ,us, and 72 = 1 flS. Except at fre
quencies above 5 MHz, the two-layer model seems to repro
duce the experimental data fairly weIL However, to demon
strate the inability of a two-layer model of carrier transport
to uniquely determin.e d from the measurement of a single
transport-related parameter, in this case the PL phase shift,
we show in Fig. 8 another possible fit to the same data in
which we purposely increase d by a factor of 10 and then
adjust 7, and 72 in order to obtain a good fit. In Fig. 8, the
dashed line represents such a fit using the same parameter
values as in Fig. 7, except that now d = 200 p.m, 71 = 500 /is,
and 72 = 2 J..ts. Clearly this fit is equally acceptable within the
uncertainty in the data which is as large as the circle diame
ter at high frequencies.
Finally, while the PL phase delay at the first overtone of
the modulation frequency, ¢2(2v), is not measurable in the
present experimental arrangement, as discussed in Sec. III,
we may, nevertheless, look at the expected behavior of
¢2(2v) and its dependence on incident excitation spot size.
This is shown in Fig. 9 for a uniform sample. The qualitative
behavior of ¢2 (2 v) on 7, S, W, and d follows that of ¢2 ( v)
except that the asymptotic limit of ¢z (2v) for large v is Jr, As
in Fig. 1, ¢2(2v} reaches the one-dimensional asymptotic
limit (curve g) when w~A().
VI. CONCLUSION
We have shown that three-dimensional modeling of car
rier diffusion is very important in order to obtain reliable
10°1 J 80~ c}
I ° I c»
a-GO .,
~
~ ... -e-40
20
0 6
10-1 I !O 103 104
if (kHz)
FIG. 8. Same datu as in Fig. 7. In this case d is made unreasonably large in
order to demonstrate that a reasonable fit can also be obtained by adjusting
only 1", and ro in the three-dimensional, two-layer model. In this case we set
d = 200 p,m, and a best fit to the data is then obtained when 1", = 200 f.1S,
72 ~= t IlS, with Sand was in Fig. 7. This figure and Fig. 7 demonstrate the
lack of uniqueness inherent in the two-layer model when it is used to fit one
set of transport data. If 1", and 1"1 are independently known and S< 100 em/s,
then d can be deduced.
Guidotti et af. 2549
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39150
Ii
1 '" ~ ;; 100
N I -+f
50 I-I 1
0 I
10.1 !O 102 103 104 105 106
II (kHI)
FIG. 9. Dependence of <P2(2v) on the excitation "pot radius win the case of
a uniform, thick Si sample. Transport parameters are assumed to have the
following values: S = lOOcm/s, D = 18 cm2/s, and 7 =' lOO,US. Asin Fig. 1.
the spot radius has assigned values w2 O~ 4 X 10 -q cm' where
q = 8,7,6,5,4,1 and its effect is seen in curves (a)-CO, respectively. Curve
(g) is obtained from theone-dimensionalmodellEqs. (A3) and (A4) 1 and
is the asymptotic limit for the three-dimensional model when w becomes
large.
transport parameters from the measured PL phase shift. The
asymptotic limit of one-dimensional diffusion is obtained
only when the characteristic Hnear dimensions ofthe volume
in which carriers are generated is much greater than A(). In
general, three-dimensional modeling of carrier diffusion is
expected to be important in other transport measurements,
such as diffusion length or carrier decay rate, unless the car
rier generation spot size is abcut an order of magnitude
greater than Ao as shown in Figs. 1 and 9,
We have also shown the lack of uniqueness, at least
within experimental uncertainty, in determining the PFZ
from a two-layer model of carrier transport. The PFZ layer
depth can be obtained unambiguously only if the carrier life
time in both the PFZ and substrate are independently
known. Determination of the PFZ depth from other trans
port measurement, such as diffusion length or decay rate of
the carrier plasma, is similarly limited.
The main advantage of measuring carrier transport
properties at high modulation frequencies, namely increased
sensitivity to conditions near the surface, is dearly demon
strated in Figs. A2(a) and A2(b). Comparison of these two
figures clearly demonstrates the effect of localization of the
time varying (in steady-state) component of the carrier den
sity near the surface as the modulation frequency increases.
APPENDIX: SUMMARY OF ONE-DIMENSIONAL
RESULTS
In the one-dimensional model, the sample occupies the
half-space O<z< 00 and is uniformly illuminated with light
of wavelength A over its entire surface which lies in the x-y
plane at z = O. When the incident intensity varies sinusoidaI-
2550 J. Appl. Phys., Vol. 66, No.6, 15 September 1989 ly with time at an angular frequency w = 21TV, the generation
rate for free electron-hole pairs can be written as
g = ~ go( 1 + a cos wOe -az,
where
go = [.saO -R)/hnlI o'
The parameters S, a, a, R, and hH are defined in Sec. IV, and
10 is the peak incident intensity. Then a particular solution of
Eq. (3), which can be obtained by Laplace transform meth
ods, is given by
where A6 = Dr and Q 2 = (1 + iwr)1 AG. The solution of
the homogeneous form of Eq. (3) is
Nh(z,t,w) =A1e- zlA" +Aze-Qzeiw, + c.c., (A2)
where A I' A2, and K are constants to be determined from the
boundary condition
D dNI = SN(z = 0).
dz Z~O
The boundary condition N(z-> oo,t,(1)) ...... O is already satis
fied by Eqs. CAl) and (A2). The steady-state density of
photogenerated carriers is then given by
N(z,t,(;)) = (e -{<Z+ Cle-Z/A")C2
+ (e-az+Eje -QZ)E2e'w,+c.c., (A3)
where
C2 = gQTI( 1 -a2A6),
Ez = ago7/(Q2 -a2)A6,
C1 = -(SID+a)/(SID+ lIAo),
and
E1= ~(SID-a)/(S/D+Q).
The terms in C constitute a background distribution of ex
cess carriers that is independent of time, while the terms in E
represent a periodic temporal variation in the photoexcited
plasma density having a phase delay which is calculated
from Q and the complex coefficients El and E2• This phase
delay is not measurable with PL but could be measured near
the surface using modulated plasma reftectanceZ-s at visible
wavelengths? The PL phase delay for one-dimensional dif
fusion is obtained from Eq. (18) with N(z,t,(;)) given by Eq.
(A3). When the photoexcited carrier density is much
greater than the donor (or acceptor) concentration then the
quadratic term in Eq. (18) dominates and the PL yield de
pends quadratically on incident power20 and is proportional
to
R2(t) = roc BN2(z,t,w)e''1Z dz. (A4) Jo
In this case only the phase shifts qJ2( v) and qJ2(2v) obtained
from R2 need be considered. Note that in the one-dimension-
Guidotti et al. 2550
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39a1 case R2 is the rate of radiative recombination per unit
(illuminated) area. In the simplified one-dimensional diffu
sion case in which aU carriers are assumed to be generated at
the surface, analytical expressions for these phase shifts are
easily obtained in closed form.20•21 However, in the presence
of finite absorption and nonzero S, closed form expressions
for these phase shifts are lengthy and cumbersome.37 It is
more convenient to integrate Eq. (A4) and compute 'Pz (v)
and 'P2(2v) directly for each frequency.
In Eq. (A3) there are three characteristic lengths. (1)
Ao, which is the ambipolar diffusion length for photoexcited
carriers and is typically of the order of 100-300 ,urn for Si,
depending on 7. (2) The absorption depth (Va) for the
incident light which is 2 pm at 647 nrn, and (3) the frequen
cy-dependent diffusion length
A(w) = lIRe(Q) = A(j/.J(r-t-l)/2,
where r = ff+07r". Because of its dependence on the
modulation frequency, A (w ) can become smalI when a)7~ L
As pointed out previously20.21 this means that the spatial
envelope in which Rz varies with time (and is detected as
time varying PL emission) becomes increasingly confined
near z = 0 as v increases. This dynamic confinement of the
time varying part of the carrier concentration with increas
ing modulation frequency makes it possible, in principle, to
use modulated PL as a monitor of the carrier lifetime in
regions near the surface, and to obtain a measurement of the
DFZ. It should be noted that the amplitude of the PL signal
also decreases with increasing (ur imposing a practical upper
limit on the frequency for which the PL signal can be detect
ed. This upper frequency limit corresponds to the narrowest
proximity for which PL is able to sample the surface region.
These characteristics are common to diffusion processes and
can also be applied to thermoreflectance2 measurements as
discussed in the introduction.
In Figs. Ai (a)-A! (c) is shown the dependence of the
steady-state concentration N(z,t,o)) on the absorption coef
ficient a as obtained from Eq. (A3). N(z,t,w) is plotted as a
function of depth z into the sample and for various times (1/
41T(i}) m (m = 0,1,2, ... ,7) during one period of modulation
and in the sequence described in the respective figure cap
tions. In each of these figures r = 100 ps, S = 0 em/s,
D = 18 cm2/s, and v = 18 kHz; while the absorption depth
is allowed to vary and is 200 /-Lm in Fig. Al (a), 2 ttm in Fig.
A1(b), and O.2/lm in Fig. Al(c). Clearly, a has an influ
ence on the effective, frequency-dependent sampling depth
A «(;) only when it is comparable to it in magnitude. For the
present set of parameter values, A(ro) -400 /-Lm. A«(i)) can
be defined for convenience as the depth at which the ampli
tude variation in N(z,t,{i) is reduced to lie of the peak vari
ation near the surface. The reason for specifying near the
surface rather than at the surface (z = 0) is that when Sis
large, the peak of N occurs at some point below the surface
(z> 0), as is apparent in Fig. A3 below. As discussed above
and in Refs. 20 and 21, A (w) decreases as the product (v1"
increases. This effect forms the basis for enhanced sensitivity
of PL phase delays to transport parameters near the surface
at high modulation frequencies. This is shown graphically
on comparing Figs. A2(a) and A2(b). In these figures all
2551 J. Appl. Phys., Vol. 66, No.6, 15 September 1989 !J g:
::I
£i ...
.!J,
-:.
N 2.4 z
6
'" .t <:
"" .ci
0 4 -N :3 z
2
0
:t2 ";::
:>
..::i ... 4-10 -N 3 z
2
o "''''' ...
II" 18kHz
0.01 z (em)
(b)
a-I,. 2 fLm
::: w/4w )( (I,O,2,1,3,6,4,5)
z (em)
(c) -l,
a-I" O.2fLm ~
t "w/4w )( (1,0,2,7,3,6,4,5) 1
0.02
z (em) 0.03 1 1
FIG. AI. Influence of the optical absorption coefficient a on the depth of
the plasma density modulation. N(z,t,w) is plotted as a function of depth z
into the sample and at time intervals of r./4t1J during one modulation cycle.
Curves are in descending order from the top at times noted in each graph. In
each of the three figures l' = 18 kHz, 7' = lOOps, and S = Ocm/s, however,
alpha assumes the values 50,5000, and 50000 cm-I in parts (a), (b), and
(c), respectively.
model parameters are identical to those used in Figs. Al (a)
A 1 (c) except for the modulation frequency which is two
orders of magnitude greater in Fig. A2(b), making A(w)
smaller by approximately a factor of 10. Note the difference
in scale for z between these two figures. Also note the reduc
tion in the excursion for N(z,t,(}) at the higher frequency by
Guidotti et al. 2551
... ---.-...•.•• , •....•...• ;--.~... .. ..• -•.• -;-.-................. ;' •••.• :.:.:.~.:.:.:.:.;.-;:.;.;.; ••• ; ••••••••••••.•••.• -•.• :.:.:.;.:-.:.; •.•.•.•••• ······:·:·~·7·:·:·:·:·:·:·;·:·.·;·~·.·,··:·~·:·:,:·:;;:·:;:.:':.:.:.:.:.:' ••••••••• :.'.:.:.:.:.;-.:.:.:.:.:.:.:.:.;.:.:.; •.••••••••• ,:.~;:.;.:.:,:;:.:.:.;o:';o:.: •.•... ' .•.•. '; .••• ' •.•. <.:.:.;.;.;.:-;.; •••• 0; ••••• ,.; •••••••• :':.:;;:.:.:.:':0:.:.:.;.;' •• ; •••••••••••••••••• ; •.•.•.• ;.; •.•.• ' ••.••• 7' •• ; •••• -••••••••••• " .. , •• ;.:.; .................... <;" ••••••••••••• -••••••••••• ,.~ ••
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
128.240.225.44 On: Mon, 22 Dec 2014 10:02:39~
C " .ci ...
10
.....
z
.It! ·c
" .Q ...
10
-N
z 2
I ° 0.01
3.9
3.7
3.6
3.5
3.4 (Q)
II" 18kHz
t = 1f14w x ( 1,0,2,7,3,6,4,5)
0.02
z (em)
0.003
z (em) 0.03
0.004 0.04
0.005
FIG. A2. Influence of the modulation frequency von the plasma density
modulation depth. In part (a), the modulation depth is about 400 Itm, in
agreement with the calculated value for A(). In part (b), the modulation
depth is about 30 flm. N(z,t,6» is plotted as a function of depth z into the
sample and at time intervals of 1/"/46) during one modulation cycle. Curves
are in descending order from the top at times noted in each graph. In both
Ca) and (b), S= 100cm/s, 1"= lOOftS, and a = 5000cm '
J!! .<:
:>
.0 5
~ -,.;
:z 5
4
:3
0.01 t" 1f/4w x (0,1,7,2,6,3,5,4)
1/ " 18 kHz
S = IxlOS emls
0.02
l (em) 0.04
FIG. A3. Influence ofa large surface recombination velocity on the plasma
density at the surface (z = 0). Note that N{z,t,OJ) nearly vanishes at the
surface which acts as an efficient sink for carriers when S is large. N(z,I,{IY) is
plotted as a function of depth zinto the sample and at time intervals of 1l"/4w
during one modulation cycle. Curves are in descending order from the top
at times noted in each graph. Here, a = 5000 cm -I, r = laO ItS, and
S= lO"cm/s.
2552 J. Appl. Phys., Vol. 66, No.6, 15 September 1989 100
co 60
III
~
-= N
-G-40
20
" (kHz)
FI G. A4. Influence of various parameters on <P2 ( v) . Comparing curves (b)
and (e) (8 = 0 em/s and a ~ 4()OO cm -I), but 1" = 100 itS for (b) while
1"'~ 10 j.lS for (c). Comparing curves (c) and (d) (r ~~ 10 Its and a = 4000
em-I), but S = 0 em/s for (e) while S =, 105 cm/s for Cd). In curves (a)
and (b), (8 = 0 cm/s and r = 100 ,its), but a"" 40 em -I fOJ" curve (a),
while a =, 4000 em 1 for curve (b).
about a factor of 10, Finally, in Fig. A3 is shown the effects of
S on the steady state distribution, N(z,t,w). Note that for
large values of S, the surface acts as an efficient sink for
carriers and the peak in the carrier distribution always oc
curs for z> o.
150
a. ., ..,
~
~ 100
'" -G-
50
FIG. AS. Influence of various parameters on Ih(2v). Comparing curves
(b) and (e) (S = o cm/s and a O~ 4000 em ') but 1" '" 100fl8 for(b) while
T = 10 fls for (c). Comparing curves (c) and (d) (r =" 10 liS and a = 4000
cm-I
) but S = 0 cmls for (e) while 8 = 10' em/s for (d). In curves (a)
and (b), (S = 0 cm/s and 1" = 100 fis) but a = 40 cm-I for curve (al,
'while a ~= 4000 cm -, for curve (b).
Guidotti et at. 2552
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128.240.225.44 On: Mon, 22 Dec 2014 10:02:39The variation in PL phase shift <fJ2 ( v) as a function of v
is shown in Fig. A4 for nominal parameters relevant to Si.
Note that the asymptotic limit when (ll1"~ 1 is 90°. Note also
the variation with r, a, and S. In the range 0 < S < 100 cm/s,
S has Htde effect on i{J2 (v). Clearly, if the Si surface is well
passivated, one can use C{J2 ( v) to obtain the bulk lifetime 1" for
a uniform sample since a is known. In Fig. AS a similar plot
is made of the phase lag '1'2 (2 y) at the first overtone re
sponse. The behavior with a, 1", and S is similar to that for
(jJ2 (v), however, the sensitivity to these parameters is greater
for f{J2 (2 v) and the asymptotic limit is now 180·. Also A (2(U )
is somewhat smaner than A(w). Figures A4 and AS show
the effects of varying a, r, andS on ({Jz (v) and (jJ2( v), respec
tively. In both figures, for curves (a) and (b), Sand rare
fixed but a = 40 em -1 in curve (a), while a = 4000 em --J in
curve (b). For curves (b) and (c),Sandaarefixed, where
as r = 100 f-ls for curve (b) and 10 f-ls in curve (c). Finally, 7
and a are held fixed in curves (c) and (d), but S = 0 in curve
(c) while S = 105 em/sin curve (d). As in the three-dimen
sional case, if the sample is uniform and has a well-passivat
ed surface (S.;;;100 cm/s), either f{J2(V) or 'P2(2v) can be
used to obtain 1".
IJ. M. Borrego, R. J. Gutmann, N. Jensen, and O. Paz, Solid State Electron.
30, 195 (1988).
"D. Guidotti and H. M. van Driel, Appl. Phys. Lett. 49,301 (1986).
3M. 1. Gallant and H. M. van Driei, Phys. Rev. B 26,2133 (1982).
4A. Skumanich, D. Fournier, A. C. Boccara, and N. M. Amer, App!. Phys.
Lett. 47, 402 (1985).
5J. R. Meyer, E. J. Bartoli, and M. R. Kruer, Phys. Rev. B 21,1559 (l9B0).
"K. Nauka, H. C. Gatos, and J. Logowski, App]. Phys. Lett. 43, 241
(1983).
7M. A. Briere, J. Phys. Coli. C449, C4-141 (1988).
"E. Yablonovitch, R. M. Swanson, W. O. Eades, and B. R. Weinberger,
App!. Phys. Lett. 48, 245 (1986),
"T. J. Chappell, P. W. Chyc, and M. A. Tavel, Solid State Electron. 36,33
(1983).
lOS. M. Hu, J. Vac. Sci. Technol. 14, 17 (1977).
2553 J. Appl. Phys., Vol. 66, No.6, 15 September 1989 "1'. Y. Tan, E. E. Gardner, and W. K. Tice, App!. Phys. Lett. 30. 175
(1977).
12G. A. Rozgonyi and C. W. Pearce, App!. Phys. Lett. 32, 747 (1978).
uD. Guidotti and H. M. van Driel, Appl. Phys. Lett. 47,1336 (1985).
14K. H. Yang, J. Electrochem. Soc. 131,1140 (l984).
ISS. Uda, H. Sllgyiama, Y. Sugita, and H. Kawata., Jpn. J. App!. Phys. 27,
1081 (1988).
'61'. Tuomi, M. Tilli, and O. Anttila, Mater. Res. Soc. Symp. Proe. 41, 101
( 1985).
nO. Paz and C. Schneider. IEEE Trans. Electron, Dev. ED-32, 2830
(1985).
'8M. Zerbst, Z. Angew. Phys. 22" 30 (1966).
19D. Guidotti and F. Dassani, in Proc. of the XVI lntl. Conf. on the Physics
of Semiconductors, Montpellier, France, 1982, Phys. 117 and 118B+C,
Part I, 549 (1983),
2°D. Guidotti, J. S. Batcheldcr, A. Finkel. and J. A. Van Veehten, Phys.
Rev. B 38, 1569 (1988).
210. Guidotti, J. S. Batchelder, J. A. Van Vechten, and A. Finkel, AppL
Phy~. Lett. 48, 68 (1986).
22D. E. Hill, J. Appl. Phys. 51,4115 (1980).
23R. S. Olyha, Jr., Master'S thesis, Dcpt. of Eleetr. Eng. and Compo Sci.
MIT, May 1984.
24W. van Roosbroeck, Phys. Rev. 91, 282 (1953).
25e, Kittel, Introduction ro Solid State Physics, 3rd ed. (Wiley, New Yark,
1966), p. 323.
J"J. F. Young and H. M. van Driel, Phys. Rev. B 26,2147 (1982).
27K N. Hall, Proc. lnst. Elect. Eng. Part B 106, SuppJ. 17,923 (1959).
"G. H. Schlangenotto and H. Maeder, Phys. Status Solidi A 13, 277
(1972).
29M. S. Tyagi and A. Van Overstraeten, Solid State Electron. 26, 57i
(1983).
30J. p, Mckelvey, Solid State and Semiconductor Physics (Harper and Row,
New York, 1966), p. 346.
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501.
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( 1985).
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ence, New York, 1953), Vol. I. p. 468.
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GPO, Washington, DC, 1972), p. 443.
-'OR. N. Hall, Proc. lnst. Electr. Eng. 106, Part B, Suppl. 17,923 (1959).
37Closed form expressions for the PL phase delays at v and 2v have been
obtained by A. Finkel for the case of one-dimensional diffusion in a uni
form, semi-infinite sample (unpublished results).
Guidotti et a/. 2553
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1.101484.pdf | Ion beam induced damage and superlattice formation in epitaxial YBa2Cu3O7−δ thin
films
C. H. Chen, A. E. White, K. T. Short, R. C. Dynes, J. M. Poate, D. C. Jacobson, P. M. Mankiewich, W. J. Skocpol
, and R. E. Howard
Citation: Applied Physics Letters 54, 1178 (1989); doi: 10.1063/1.101484
View online: http://dx.doi.org/10.1063/1.101484
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132.174.254.155 On: Tue, 23 Dec 2014 05:30:33Ion beam induced damage and superlattice formation in epitaxial
YBa2Cua07_5 thin films
c. H. Chen, A. E. White, K T. Short, R. C. Dynes, J. M. Poate, and D. C. Jacobson
AT&T Bel! Laboratorie~~ Murray Hill, New Jersey 07974
P. M. Mankiewich, W. J. Skocpol, and R. E. Howard
AT&T Bell Laboratories, Holmdel, New Jersey 07733
(Received 23 November 1988; accepted for publication 26 January 1989)
We have studied the effect of ion beam irradiation on the microstructure of epitaxial
YBa2Cu,07. 8 thin films. The ion beam induced defects are found to cluster in small ( < ! 00
A) disordered areas. The size and density of the disordered areas are found to increase with
the ion fluence. The presence of these small disordered areas can lead to the reduction of phase
coherence of the electron pair wave function. Ion beam irradiation is also found to reduce the
orthorhombicity of the lattice structure. A new incommensurate superlattice phase due to ion
beam induced defect ordering has also been observed.
The superconducting state of the high Tc oxide super
conductors has been shown to be very sensitive to defects
created by ion beam irradiation. 1-5 Earlier electron micros
copy studies 1 of ion beam damaged polycrystalline
YBa2Cu307 _ 8 thin films have noted the growth of an amor
phous layer at the grain boundaries. The presence of such an
amorphous layer at the grain boundaries can reduce the cou
pling of the electron pair wave function between grains,
breaking the phase coherence, and resulting in the complete
disappearance of superconductivity at higher ion fluences.
Recently, we studied the effects of ion beam irradiation
on the electrical properties of epitaxial superccnducting
YBaZCu307 _ 8 thin films.4 It was found that the onset tem
perature did not vary significantly with the ion fluence; how
ever, the width of the superconducting transition broadened
until the resistance no longer reached zero. This behavior is
very similar to that observed in many "granular" supercon
ducting thin-film systems." In this study, we report the re
sults of an electron microscope investigation of the same
samples. OUf observation of disordered regions supports the
general idea of a reduction of phase coherence. Other ion
beam induced effects such as the reduction of crystal ortho
rhombicity and the formation of a new incommensurate su
per lattice are also reported.
The epitaxial thin-film YBa2CuJ07 _ {j sample ( -1500
A thick) was grown on a single-crystal [100) SrTi03 sub
strate using a technique described previously.7 The film ex
hibits a Tc (R = 0) of 91 K with a superconducting transi
tion which is less than 1 K wide. It is highly oriented with the
c axis perpendicular to the film surface. Pieces of the sample
were irradiated by 3.5 MeV Be + , 1 MeV Ne + , and 2 MeV
Ar t· ions at various fiuences. These ion energies were cho
sen such that the rate of nuclear energy loss was approxi
mately constant throughout the films and the ion ranges far
exceeded the film thickness, Samples for transmission dec
tron microscope studies were prepared by mechanical thin
ning of the substrate followed by Ar + ion milling at 6 ke V.
To reduce the damage due to Ar + ion milling, 4 keV AI' +
ions were used during the final stage of the sample thinning.
We do not expect the sample thinning procedure to have any
significant effect on our study of defects induced by ion beam
irradiation, since our examination shows that the virgin sample thinned by the same procedure is largely defect free.
A sequence of resistive transitions after irradiation with
1 MeV Ne+ ions is shown in Fig. 1. We note that these
resistive transitions broaden with increasing ion influence
and a hump develops in the transition region. The hump was
observed in samples irradiated with Be + and Ne I-ions, but
not in one irradiated with Ar + ions. The appearance of the
hump is indicative of the presence of other phases of
YBa2Cu307 _ 8 with slightly varying oxygen concentra
tions.8
We have examined samples irradiated with Ar + at total
fiuences of 2.5 X 1013 and 6 X 1014 ions/cm\ Nel-at total
fiuences of 1 X 1014, 1.5 X 1014, and 3 X 1014 ions/cmz, and
Be I-at a total ftuence of 4 X 1015 ions/cm2• In Fig. 2(a) we
show a bright field image obtained from a sample prior to ion
irradiation. Besides the presence of Y 203 particles, common
to films grown by molecular beam epitaxy, the contrast of
the sample is quite homogeneous and twin boundaries along
the [1 10 1 direction are clearly visible. The typical width of
the twin domains is -250 A in this case. After a light dose of
1 X 1014 Ner ions/cm2, bright field images of the twin do
mains become heavily speckled as shown in Fig. 2(b). The
contrast of the speckles arises from the local strain field asso
ciated with the defects created by the ion irradiation.
FIG. L R vs Tcharac!eristics for sample after bombardment with 1.0 MeV
Nt: ions at fiueilceof (a) 0 (undamaged). (b) 0.1 X 10'·, (c) OA X 10'4, Cd)
O,7XlO14, (e) l.OXlO14, (f) I.4XlO'4, (g) 1.8XI014, (h) 2.2XlO'4,and
(i) 3.0X 10'4 ions/em?
1178 Appl. Phys. Lett. 54 (12), 20 March i 9139 0003-6951/89/121178-03$01,00 @ 1 9S9 American Institute of Physics 1178
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132.174.254.155 On: Tue, 23 Dec 2014 05:30:33FIG. 2. Bright field images of (a) an undamaged sample and (b) a sample
after a dose of 1 X 10" Ne + ions/cm'. Note the presence of speckles in (b).
The contrast of twin boundaries which are clearly visible in (a) is reduced ill
(b). The presence of Y 20, particles is indicated by the arrows in (h).
Similar bright field images were also seen in samples
irradiated with Arl at a fiuence of 2.5 X lOn ions/cm2•
Twin boundaries are still quite visible at this level of ion dose;
however, electron diffraction studies show that the twin
spots splitting along the [110] direction has now been re
duced. The reduced splitting indicates that the crystal struc
ture has become more tetragonal after the ion irradiation.
Since oxygen atoms in the Cu-O linear chains are the most
loosely bound species in the crystal lattice, q it is reasonable to
assume that ion irradiation creates defects that lead to re
arrangement of the oxygen atoms. Diffusion of some of the
oxygen atoms from the b-axis chains to vacant sites along the
a-axis direction could make the crystal structure more tetra
gonal.
The speckled contrast of the defects induced by ion irra
diation shown in Fig. 2 (b) could not reveal details of the
defects. Furthermore, the dimension of the speckled con
trast does not reflect the actual size of the individual defects
since the strain fieid associated with these defects is long
range in nature, Therefore, we resort to high-resolution lat
tice imaging for the details of the defect structure. Figure 3 is
a high-resolution lattice image obtained along the [001]
zone axis from the sample irradiated with 1 X 1014 Ne t
ions!cm2• Small disordered regions ( -30 A), where the lat
tice image has become either very weak or absent, are visible.
The disordered regions still retain variable degrees of crys
tallinity and have not become amorphous in the usual sense.
Some of these disordered regions are indicated by arrows in
Fig. 3. A typical spacing between the disordered regions is
-100 A. This type of disorder defect was not observed in the
sample without ion beam irradiation. In general, we find a
higher disorder-defect density at the twin boundaries. The
1179 Appl. Phys. Lett., Vol. 54, No. 12,20 March 1989 FIG. 3. High-resolution lattice image obtained along the rOO I J zone axis
from a sample irradiated with 1 >< 1014 Ne" ions/em'. Some damaged dis
order regions are indicated by arrows.
presence of these small disordered regions, which most likely
are insulating, could lead to the gradual decoupling of the
superconducting regions with increasing ion fiuence. The
decoupling of the superconducting regions would result in
the reduction of the phase coherence of the pair wave func
tion giving the observed resistive transitions (Fig. 1) and
eventually destroying the superconducti.vity.
In samples irradiated with a slightly higher Ne +
fluence of 1.5 X lOI4 ions/cm2, similar disorder defects were
also found. However, the distribution ofthe ion-induced dis
order defects was found to be inhomogeneous. There are
areas in the sample that show no signs of damage. In general,
we found this sample to be slightly less defective than the one
irradiated with a slightly lower ion fluence of 1 X 1014
ions/cm2• Although the starting samples came from the
same substrate, they were cut from opposite halves and the
quality of the thin-film sample (as measured by ion channel
ing) did vary across the substrate. Channeling data obtained
from these two samples also indicated that the sample which
received a dose of 1.5 X 10;4 ions/cm2 was slightly less defec
tive before irradiation and showed less dechanneling after
irradiation, even though the dose was higher, consistent with
our electron microscope observations.
With Nc -t fl.uence increased to 3 X 1014 ions/cmz, elec
trical measurements indicated that the sample is no longer
superconducting with a rising resistivity at low tempera
tures. Our studies show that the disordered regions in this
sample have now grown to ;;e 75 A in size and they cover
more than 70% of the area. In addition, the contrast of the
twin boundaries has weakened considerably and is barely
visible in the image. Diffraction spot splittings along the
[ 11 0 1 direction due to twinning have now become vanish
ingly small, indicative of a tetragonal phase in which no su
perconductivity is expected.
Samples irradiated with 2 MeV Ar -! to a fiuence of
6 X 1014 ions/cm2, which have completely lost superconduc
tivity and show no evidence of ion channeling, are found to
be polycrystaUine predominated by a fcc phase with heavily
Chen etal. 1179
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132.174.254.155 On: Tue, 23 Dec 2014 05:30:33defective grains ~ 500 A in size. Radiation damage pro
duced by the 3.5 MeV Be-+ ions at a fluence of 4X 1015
ions/cmz is found to be similar to that shown in Fig. 3. The
deposited nuclear energy (-1 eV / A) in this case is similar
to that for 1 MeV Ne -t ions with a fluence of 1 X 1014/cm2.
The deposited energy for 6 X 1014 Art ions/cm2 is roughly
25 times higher. These results show that the level of damage
scales with the deposited nuclear energy ofthe ions, in agree
ment with earlier electrical measurements.4
One of the most interesting findings during our studies
of the ion irradiation effects on YBa2Cu307 .. fj epitaxiai thin
films is the formation of a new incommensurate superlatti.ce
modulation. Satellite reflection spots due to the superlattice
modulation were found to lie in the a*-b '" plane. Figure 4( a)
is an electron diffraction pattern along the (0011 zone axis
which shows the existence of these supedattice reflections.
For darity, a schematic, in which the diffraction spots due to
multiple reflections present in Fig. 4(a) have been removed,
is shown in Fig. 4(b). Among aU the samples we have stud
ied, the superlattice modulations were only observed in the
two samples irradiated with Ne + fluences of 1 X 1014 and
1.5 X 1014 ions/cm2• We note that the supedattice phase oc
cupies less than 10% of the sample area we have examined.
We speculate that the formation of the incommensurate su
pedattice could be facilitated by some defects that exist prior
to the ion irradiation. The formation of superlattice by Ion
beam bombardment is quite surprising. In most cases, such
as NbO and VC, ion beam irradiation would conversely
cause the disappearance of the superIattice.1O On the other
hand, it is interesting to note that amorphous Zr02 crystal
lizes under low-energy ion irradiation. 10 However, the phe
nomenon occurs under a much higher deposited energy con
dition (~103
) than the high-energy irradiation of our
experiment.
From Fig. 4 it is clear that the superlatticc is modulated
only in one direction (either along the a axis or the b axis) in
a given area. Since the distinction of the smaH difference
between the a and b axes is beyond the accuracy of our dif
fraction study, we shall arbitrarily assume that the modula
tion is along the b axis. With this assumption, we :find that
the superlattice reflections shown in Fig. 4 can be indexed by
a c-face-centercd orthorhombic supercell with cell dimen
sions of3.8Ax 33.6Ax 11.7 A. Note that thea andclattice
parameters remain unchanged and the superlattice modula
tion periodicity of 33.6 A along the b axis is approximateiy
8.6 times the sublattice b lattice parameter. The supcrlauice
lattice modulation, therefore, appears to be incommensurate
with the sublattice.
In YBa2Cu307. /j superconductors several types of su
perlattice reflections have been reported, and they have an
been attributed to the ordering of oxygen vacancies. ] I Super
lattice reflections due to oxygen vacancy ordering have been
found to be commensurate and somewhat diffuse with rela
tively low intensity. The supedaUice reflections shown in
Fig. 4, on the other hand, are incommensurate with relative
ly high intensity. Therefore, we believe that the incommen
surate superlattice reflecti.ons observed in the present case
1180 Appl. Phys. Lett. Vol. 54, No. 12. 20 March 1989 (b)
8(100)
FIG. 4. Ca) Selected area electron diffraction pattern obtained along the
[001 J zone axis from a sample irradiated with 1.5 X 1014 Ne" ions/em'
which shows the existence of superlattice modulations. (b) is a schematic
illustration of (a) in which diffraction spots due to the sublattice and the
superlattice are llelloted by big dots and small dots, respectively. The dif
fraction spots due to multiple reflections are omitted in (b).
are not derived from ordering of oxygen vacancies. In fact,
these incommensurate superlattice reflections bear some re
semblance to the incommensurate phase observed in the Bi
based superconductors. 12 We speCUlate that the incommen
surate modulation arises from periodic cation distortions
induced by the ion beam irradiation. To the best of our
knowledge, this is the first observation of a periodic distor
tion in YBa2Cu307 /j due to cations.
In conclusion, ion beam irradiation creates sman disor
der defects ( < 100 A in size) in the epitaxial YBa2CuJ07 _ /j
thin films. The growth in size and density of these disorder
defects with increasing ion fluence can explain the degrada
tion ofthe superconducting transitions. A new incommensu
rate super lattice induced by ion beam irradiation is reported.
In addition, ion beam irradiation reduces the twin spots
splitting in diffraction, suggesting a more tetragonal crystal
structure.
'G. J. Clark, A. D. Marwick, R. H. Koch, and R. R Laibowitz, App\. Phys.
Lett. 51, 139 (1987).
2B. Egner, J. Geck. H. C. Li, G. Linker, O. Meyer, and R Strchlan, lpn. 1.
App!. Phys. 26, 2141 (1987).
lG. J. Clark, F. K. LcGoues, A. D. Marwick, R. B. Laibowitz. and R.
Koch, App!. Phys. Lett. 51, 1462 (I98i). .
4A. E. \Vhitc, K. T. Short, D. C. Jacobson. 1. M. Poate, R. C. Dynes, P. M.
Mankiewich, w. J. Sk()cpol, R. E. Howard. M. Anz]()war, K. W. Ila!dwin,
A. F. J. Levi, J. R. 1(wo, T. Hsieh, and M. Hong, Phys. Rev. B 37,3755
(19g8) .
SA. E. White, K. T. Short, R. C. Dynes, A. F. J. Levi, M. Anzlowar, K. W.
Baldwin, P. A. Po!akos, T A. Fulton, and L DUKlkleherger, App!. I'hys.
Lett. 53, 1010 (1988).
"R. C. Dynes, 1. P. Garno, and J. M. Rowell, Phys. Rev. Lett .. G. 479
(1978); A. E. White, R. C. Dynes, and J. P. Garno, Phys. Rev. B33, 3549
( 1986).
7p. M. Mankiewich, J. H. Scofield, W. J. Skocpol, R. E. Howard, A. H.
Dllyem, and E. Good, Appl.l'hys. Lett. 51,1753 (1987).
"R. J. Cava, R Batlogg, C. H. Chen. E. A. Reitman, S. M. Zahurak, and D.
Werder, Pltys. Rev. B 36,5719 (1987).
"J. S. Swinnea and H. Sleinfink. J. Mater. Res. 2, 424 (1987); A. Santoro.
S. Miraglia, F. Beech, S. A. Sunshine, D. W. Murphy, L r. Schneemeycr,
and J. V. Waszczak, Mater. Res. Hull. 22, 1007 (1987); D. S. Ginley, 1'. J.
Nigrey, E. L Venturi, B. MOrQsin, and J. F. Kirak, J. Mater. Res.·2, i33
( 1987).
'''Por example, see R. Kelly, in lOll Bombardment Modificatioll oj.)'Uljaces,
edited by O. AucielJo and R. Kelly (Elsevier, New York, 1984). p. 79.
'D. J. Werder, C. H. Chen, R. J. Cava, and H. Batlogg, Phys. ReI!. B 38,
5130 (l9Sll).
!le. H. Chen, D. J. Werder, S. H. Liou, H. S. Chen, and M. Hong, Phys.
Rev. B 37, 9834 (1988).
Chen eta/. 1180
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1.866437.pdf | Poloidal flux loss and axial dynamics during the formation of a fieldreversed
configuration
R. D. Milroy and J. T. Slough
Citation: Physics of Fluids (1958-1988) 30, 3566 (1987); doi: 10.1063/1.866437
View online: http://dx.doi.org/10.1063/1.866437
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Published by the AIP Publishing
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Fluxtrapping during the formation of fieldreversed configurations
Phys. Fluids 25, 2121 (1982); 10.1063/1.863671
Flux loss during the equilibrium phase of fieldreversed configurations
Phys. Fluids 25, 1696 (1982); 10.1063/1.863645
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Appl. Phys. Lett. 41, 31 (1982); 10.1063/1.93311
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1.583677.pdf | Ultimate resolution and contrast in ion‐beam lithography
M. D. Giles, R. K. Watts, and E. Labate
Citation: Journal of Vacuum Science & Technology B 5, 1588 (1987); doi: 10.1116/1.583677
View online: http://dx.doi.org/10.1116/1.583677
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Ion‐beam lithography for IC fabrication with submicrometer features
J. Vac. Sci. Technol. 16, 1897 (1979); 10.1116/1.570323
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.237.29.138 On: Sat, 15 Aug 2015 13:27:20Ultimate resolution and contrast in ion-beam lithography
M. D. Giles, R. K. Watts, and E. Labate
AT&T Bell Laboratories, Murray Hill, New Jersey 07974
(Received 11 May 1987; accepted 12 August 1987)
Ion-beam lithography with a stencil mask is capable of very high resolution. We quantify the
resolution by calculating the modulation transfer function for protons of energies suitable for
patterning resist of useful thickness.
I. INTRODUCTION
Among lithographic techniques there are several candidates
for high-throughput imaging with higher resolution than is
available with advanced optical wafer steppers: X-ray lith
ography with storage ring source, electron projection, elec
tron proximity printing, ion projection, and ion proximity
printing with a stencil mask. The ion-lithography methods
appear to have highest resolution, although electron printing
with very high electron energy has not been adequately ex
plored. In this paper we evaluate the modulation transfer
function for proton-beam lithography with a stencil mask.
For resist exposure with ions the two parameters of immedi
ate interest are R p and ax, the projected range of the ion in
the resist and the perpendicular straggle, respectively. Rp
must be larger than the resist thickness, and ax is a measure
of the resolution. Light ions have the smallest ratios axlRp
in resist for energies of interest because they lose energy
largely through electronic inelastic scattering rather than
nuclear. Of the light ions H+, the lightest, promises highest
resolution because it has the highest ratio of nuclear charge
to mass.
II. PROTON INTERACTION WITH RESIST
When an energetic light ion enters resist, energy is depos
ited along the ion track primarily through electronic scatter
ing mechanisms. This energy causes either scission or cross
linking of resist molecules which alters the resist solubility in
a suitable developer and so allows a pattern to be delineated.
The number of chemical events per 100 e V of deposited ener
gy is known as the radiation yield Gs and can be related to the
molecular weight of the resulting polymer fragments. 1 This
paper considers the exposure of polymethylmethacrylate
(PMMA), which occurs by bond scission. It has been
shownl that the radiation yield for protons is 70% higher
than the yield for heavier ions such as helium, so more of the
energy deposited into electronic processes is effective in
breaking bonds. However, the energy deposited per unit
path length at a particular energy is at least a factor of 2
lower for protons so the overall sensitivity of the resist is a
little smaller. Sensitivity of resist to ion beams is much
greater than to electron beams. The primary advantage of
proton-beam exposure compared to heavier ions and to elec
tron beams is that lateral scattering is minimized, so we ob
tain the maximum resolution possible. This makes proton
beam lithography an attractive choice for linewidths below
0.3 pm. The main difficulty for ultrafine line lithography is the
choice of patterning method. Direct-write ion-beam lithog
raphy is possible, but offers only low throughput. Various
forms of proximity printing have been proposed, trading
resolution for ease of masking. For example, Economou et
al.2 used a thin polyimide membrane in the transmission
areas of the mask. The membrane must be < O. 51-'m thick to
minimize scattering, and this raises questions of mask stabil
ity and durability. Bartelt et at.3 used a silicon membrane
aligned along a channeling direction to minimize scattering.
The beam divergence from a 0.7 /-tm membrane was found to
be 0.4°, requiring a separation between mask and wafer of
< 14/-tm to maintain 0.1 pm resolution. Ideally, the open
areas of a mask would be completely unobstructed but such
stencil masks can only print patterns without isolated fea
tures so two masking steps are necessary to expose general
structures. Randall et al.4 used a fine support grid to allow
general stencil patterns, but the resolution is then degraded
by the beam rocking used to wash out the support grid im
age. Since several patterning methods are possible, the fol
lowing discussion will consider the ideal case where beam
divergence from the masks can be neglected and the mask is
thick enough to absorb ions scattered near the edge of an
opening.
III. SIMULATION
Resist exposure was simulated using numerical integra
tion of the Boltzmann transport equation to follow the mo
tion of the ions through resist.s Electronic stopping coeffi
cients were taken from the work of Anderson and Ziegler, 6
and have previously been verified by comparison with ex
perimental measurements of proton ranges in resist.7 Nu
clear stopping was based on the universal potentia! of Bier
sack and Ziegler,8 but since nuclear stopping is much smaller
than electronic stopping, this did not have a significant influ
ence on the profile shapes. The calculation yields the two
dimensional ion distribution and the two-dimensional total
deposited energy profile resulting from a line ion source.
The deposited energy profile can then be used to predict
the resist exposure characteristics, as described above. Sec
tions through the energy profile for various depths and ener
gies are shown in Figs. 1,2, and 3.
IV. MODULATION TRANSFER FUNTION
Over several orders of magnitude the point spread func
tions U(x, z) of Figs. 1,2, and 3 are well-approximated by an
exponential function,
1588 J. Vac. Sci. Technol. B 5 (6), NovlDec 1987 0734-211 X/871061588-o3$01.00 @ 1987 American Vacuum Society 1588
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120 key 1 -e
~ 8
>-
<!> a:
IJ.I z
W
Cl
W
f-
en a: 10-2
w
0
FIG.!. Calculated point spread functions for 120 keY protons in PMMA
resist at two different depths. The dashed curves are exponential fits.
UE(x,z) =A exp[ -xly(z)]. (1)
The fits to exponentials are indicated by dashed curves in
the figures, and the width parameters y(z) are collected in
Table 1. The point spread function is analogous to the Airy
pattern of light optics and to the point spread function of
electron-beam lithography, called E(r) by Greeneich.9 Once
the point spread function is known, it is a simple matter to
calculate the modulation transfer function.
The modulation is defined for a grating pattern of equal
lines and spaces of width I, period 21, and spatial frequency
u = (21) -1 by the expression,
.0 ... "
>(!)
Il:
W
Z
W
Q
lLI
~
en o a. w o z = 0.3;Urn
-20 -10 (2)
70kev
X!fLm)
FIG. 2. Calculated point spread functions and exponential fits for 70 keY
protons.
J. Vac. Sci. Techno!. S, Vol. 5, No.6, Nov/Dec 1987 30 "vi D
is
>-(!) a:
IJ.I
Z
W
Cl
LU f-
~ 10-2
a.. w 0
x(nm)
FIG. 3. Calculated point spread functions and exponential fits for 30 keY
protons.
1M and 1m are the maximum and minimum values, respec
tively, of the intensity in the pattern. The modulation trans
fer function (MTF) is given by
(3)
where i and 0 signify image and object.
In the proximity printing arrangement of ion lithography
the object is the mask and the (latent) image is the pattern of
deposited energy in the resist. Figure 4 shows the square
wave pattern on the mask. The opague portions of the mask
are assumed to block the ion beam completely, so that
Ie;" = O. Thus, from Eq. (2), the modulation of the object
MO = I'JwII'Jw = 1, and MTF = Mi. Mi is calculated by
evaluating in turn I:w (at x = 0) and I;" (at x = I). This is
done by placing a point spread function at each point where
the mask is transmissive and evaluating its contribution at
x = 0 and x = I. That is, U(x, z) is integrated over the
square-wave pattern. The integration is simplified by mak
ing use of the substitution theorem of Chang. lO For a Gaus
sion point spread function M i has been calculated by
Broers.l1 Many point spread functions encountered in ion
beam lithography may be better approximated by an expo-
TABLE I. Gaussian fits to point spread functions for several proton energies
at several depths z in PMMA resist. The top surface of the resist is at z = O.
Proton energy z r
(keY) (jlm) (nm)
30 0.2 2.6
30 0.3 3.4
70 0.3 3.4
70 0.4 3.5
120 0.3 3.1
120 0.4 3.4
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. . . I(X)
t rO -r--....M, :r. r;r,;-0-
I \ , \ , , I \ , \ , I , \ , \ , \ , \ I \ I \ , \ I \ '0 ,I
-7.f. -5L -34 -t O.e ~ 5t .71. 22222222 . . .
x
FIG. 4. Illustration of method of calculating MTF. The soiid curve repre
sents the mask image. The dashed curves represent point spread functions,
which must be integrated over the square-wave pattern.
nential exp( -xly); we give the formula for this case in Eq.
(4) :
Mi=NID,
N = 2 f exp ( -(4n -1) I] _ f exp [ -(4n + 1) 1]
n = 1 2y n -~ 1 2y
~ [ -(4n -3) I) -£.. exp ,
",~2 2y (4)
D 1 ~ [ -(4n -3) I] ~ [ -(4n + 1) I] = + £.. exp -£.. exp .
n~2 2y n=! 2y
Figure 5 shows the MTF (or M i) plotted as a function of
linewidth I for the exponential functions UE (x, z) of width
y = 2.6 and 3.5 nm. All the other entries in Table I lie
between these two extremes. Also included for comparison is
the MTF of the highest resolution optical wafer stepper yet
reported.12 The value MTF = 0.6, a commonly accepted
minimum value of the modulation necessary for exposure of
photoresist, occurs at linewidths 0.017 and 0.022 /.lm for the
two curves. From Fig. 5, the MTF rolls off at feature sizes
one order of magnitude smaller than for the deep ultraviolet
wafer stepper.
V. CONCLUSIONS
Proton-beam exposure offers many attractive features for
exposure of lines below 0.5 /-tm. Resist is very sensitive to
proton beams, so exposure times are small, and lateral scat
tering and reflection are orders of magnitude better than for
other approaches. For small linewidths, there are several
candidates for a masking technology. Proton-beam exposure
is intrinsically capable of much higher resolution, but that
resolution can only be obtained with slower approaches of
double stencil mask (to pattern re-entrant geometries, such
J. Vac. Sci. Techno\. B, Vol. 5, No.6, Nov/Dec 1987
. . . . . . ! . . I
I /
I
I
IDUV
1)..= 248nm
I NA= 0.38
J S= 0.1
I
I
I
I
I
I
f
I
I
i
O~~~~ __ ~-L~~-k~~ __ ~~~~~~~
0.0\ 0.1
j,(fLml
FIG. 5. MTF curves plotted vs linewidth for proton-beam lithography (see
Table I) and deep UV photolithography at 248 nrn with a lens of numerical
aperture NA = 0.38 and partial coherence S = 0.7.
as a donut shape) or direct write. By means of the modula
tion transfer function the resolution and contrast of different
lithographic techniques may be compared. Proton-beam
lithography offers highest resolution in resist of useful thick
ness (0.3 /.lm or thicker).
'1. Adesida, C. Anderson, and E. B. Wolf, J. Vac. Sci. Techno!' B 1, 1182
(1983).
IN. P. Economou, D. C. Flanders, and J. P. Donnelly, J. Vac. Sci. Techno!.
19,1172 (1981).
3J. L. Bartelt, C. W. Siayrnan, J. E. Wood, J. Y. Chen, and C. M. McKenna,
J. Vac. Sci. Techno!. 19, 1166 (1981).
4J. N. Randall, D. C. Flanders, N. P. Economou, J. P. Donnelly, and E. 1.
Bromley, J. Vac. Sci. Technol. B 3,58 (1985).
sM. D. Giles, IEEE Trans. Comput.-Aid. Des. 5, 679 (1986).
"H. H. Anderson and J. F. Ziegler, Hydrogen Stopping Powers and Ranges
in all Elements (Pergamon, New York, 1977).
7L. Karapiperis, I. Adesida, C. A. Lee, and E. D. Wolf, J. Vac. Sci. Techno!.
19,1259 (1981).
8J. P. BiersackandJ. F. Ziegler, "The stopping and rangeofions in solids,"
Ion implantation technique., Vol. 10 in Springer series in Electrophysics,
edited by H. Ryssel and H. Glaswischnig (Springer, Berlin, 1982).
9J. S. Greeneich, Electron Beam Technology in Microelectronic Fabrication
(Academic, New York, 1980), Chap. 2.
lor. H. P. Chang, J. Vac. Sci. Techno!. 12,1271 (1975).
!lA. N. Broers, J. Electrochem. Soc. US, 166 (1981).
!2V. Pol, J. H. Bennewitz, G. C. Escher, M. Feldman, V. A. Firtion, T. E.
Jewell, B. E. Wilcomb, and J. T. Clemens, Proc. SPIE 663,6 (1986).
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1.345657.pdf | Effect of Mn concentration on the cathodo and photoluminescence of ZnS:Mn
Milind D. Bhise, Monica Katiyar, and Adrian H. Kitai
Citation: Journal of Applied Physics 67, 1492 (1990); doi: 10.1063/1.345657
View online: http://dx.doi.org/10.1063/1.345657
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128.189.203.83 On: Thu, 11 Dec 2014 21:32:39Effect of Mn concentration on the cathodo- and photoluminescence
of ZnS:Mn
Milind D. Bhise and Monica Katiyar
Department afMaterials Science and Engineering. Afcl'vfaster University. Hamilton, Ontario,
Canada L8S 4Ml
Adrian H. Kitai
Departments of Engineering Physics alld 1!1aterials Science and Engineering. l'vfcJ{aster University.
Hamilton, Ontario, Canada L8S 4Ml
(Received 5 June 1989; accepted for publication 12 October 1989)
A novel method of calibrated doping of Mn in ZnS thin films has been used to study room
temperature cathodo- and photoluminescence characteristics of ZnS:Mn films for a Mn
concentration range of 0.07-26.4 wt. %. It was observed that the luminescent intensity
increases with Mn concentration up to -2 wt. %, beyond which intensity decreases. Emission
spectra revealed a lower energy peak in addition to the 5S0-nm yellow peak for higher dopant
concentration. The occurrence of this red peak and quenching of yellow emission is probably at
approximately the same activator concentration ( ~ 2 wt. %). We attribute these to the
phenomenon of energy transfer to energy sinks via unexcited Mn.
I. INTRODUCTION
The possibility of a cheap, flat cathode ray tube (CRT)
and flat panel television has motivated a systematic study of
the luminescence ofZnS:Mn.l The wide-band-gap semicon
ductor has also been thoroughly investigated for the phe
nomenon of electroluminescence. 7.-5
Mn2 -can be incorporated in the ZnS lattice up to very
high concentrations, in fact up to complete miscibility." As
the yellow luminescence (585 nm) in this material is due to
the excitation and decay of the Mnz I ion, one would expect
the intensity ofluminescence to increase with Mn concentra
tion. Such is not the case in practice.2-5.7 It is well known
that the intensity of luminescence of ZnS:Mn increases ini
tially until a certain optimum concentration of the activator,
after which there is a drastic reduction in the intensity (con
centration quenching). This typical behavior is characteris
tic of the luminescent ZnS:Mn irrespective of the exciting
mechanism. The physical basis of concentration quenching
is, as yet, poorly understood. In the present paper, we pres
ent the results of the investigations carried out in our labora
tory on the effect of Mn concentration quenching on the
cathodoluminescent (CL) characteristics of ZnS:Mn. To
our knowledge, a careful study ofthis has never been report
ed in the literature. Leveren/ has reported that adding an
increasing amount of Mn in self-activated ZnS, the original
(Zn) emission band remains fixcd in position but decreases
in efficiency as a new (Mu) band appears. The luminous
efficiency of the Mn band increases up to about 1-2 wt. %
Mn.
Photoluminescence (PL) measurements have also been
carried out on the same samples. We observed a qualitatively
similar behavior for the changc in luminous intensity with
the activator concentration when the phosphor was excited
by photons and by electrons.
Spectral emission characteristics (P Land CL) were
also studied. In addition to the conventional yellow peak due to 4 T,_6 A J transition in Mn2 -t-, an additional peak is ob
served in the red region.
II. EXPERIMENT
The ZnS thin films (190 nm thick) were deposited by
the conventional vacuum deposition (resistance-heated)
met hod. Substratcs used were ( III ) Si wafers obtained from
Monsanto Co. A specially designed deposition unit was em
ployed. Figure 1 shows a vertical section of the top plate of
the deposition chamber. The assembly consists of a Cli block
attached to a hollow stainless-steel cylinder and a solid Cu
cylinder housing the cartridge heater and a thermocouple. A
mechanical mask was attached to a rotary feedthrough. The
geometrical alignment of thc feedthrough, mask, substrate,
and the evaporation source is coaxial to ensure a uniform
!Hm thickness (concentrically) on the 3-in. Si wafers.
First, without the mask, a ZnS thin film was evaporated.
The substrate temperature during the ZnS deposition was
maintained at 200 ± 5 dc. Film thickness was measured on
an Alphastep instrument. The mask was then installed and
Mn evaporated: The wafer, with a ZnS film on it, was
cleaned and placed in proper position so as to achieve coaxial
alignment (see Fig. 1). By changing the position of the mask
(using the rotary switch), a selected portion of the film
could be exposed to the Mn vapor flux. Hence it was possible
to prepare as many as 16 samples, with a different level of
Mn doping in each, from a single ZnS film. Knowing the
thickness of the parent ZnS film and that of Mn, as deter
mined from the crystal thickness monitor, the specific dop
ing level that results from diffusion could be calculated.
Rutherford back scattering was later used to confirm the
amount of Mn present in the ZnS films.
Mn, thus deposited, was thermally diffused in the ZnS
host by a two-stage heat treatment. First, a vacuum anneal
was carried out in the deposition chamber (pressure
< 2 X 10 5 Torr) at 345 ± 2 °C for 4 h. After cooling the
1492 J. Appl. Phys. 67 (3), 1 February 1990 0021-8979/90/031492-05$03.00 (C) 1990 American institute of Physics 1492
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128.189.203.83 On: Thu, 11 Dec 2014 21:32:39~J-Rotary
FeedUu"ugh
ROlary
Heater ~ I ~/SWiiCh
~ A
~ Quartz
~ ~ Crystal %
1111
, , Mechenlcal , Mask , ,
To Pump , ,
! , , , , i, , , , , , , , , , ,
ZnS Source ' , , ,
\ " " \ " ~
FIG.!. Vertical section of the top plate of the deposition chamber.
film to ambient temperature, the individual samples were
cut, and the second anneal was carried out in an N2 tube
furnace at 400 ± 2 cC for 2 h. The samples were then allowed
to cool down to room temperature, still in a N2 atmosphere.
Despite the extensive precautions, Rutherford buckscatter
ing and Auger electron spectroscopy showed the presence of
oxygen in the samples with higher Mn content (see next
section) .
III. MEASUREMENTS
CL measurements were made with a Perkin-Elmer
grazing-incidence (04-015) electron gun. The samples were
mounted on a speciaUy fabricated sample holder that al
lowed the analysis of more than one sample, without break-
Zn ;:
800 ~
600 i:
600 600
c Q -' -' III W ): 51 Edge >= 400
S
'.':. 200 ing the vacuum. All the measurements were carried out at a
chamber pressure less than 10 6 Torr. To eliminate noise,
the beam current of the electron gun was modulated and a
lock-in amplifier wa<; used. The light was collected through a
collimating lens on a Si photodiode (Oriel) and the signal
was later amplified. One of the samples in every set (two
independent sets were analyzed) was taken as an internal
standard. Observations were recorded for the standard and
the sample(s), simultaneously. A comparison of normalized
CL intensity was made, rather than that of the absolute in
tensities. Elimination of an error due to the slight changes in
the experimental conditions was thus possible. To justify the
use of the internal standard, its absolute CL intensity was
recorded intermittently. No appreciable deviation was de
tected. The CL signal intensities were taken at spatially dif
ferent points, on the same sample. Although in some cases a
large discrepancy was observed, the majority of the samples
showed less than 10% deviation. All the measurements were
taken at a constant beam current density, at room tempera
ture.
The spectra were recorded using a photomultiplier tube
(PMT) and an Oriel monochromator with a resolution of
~ 3 nm. Experimental points were taken at regular intervals
and a smooth curve drawn to yield the final spectra.
The PL measurements were made at room temperature.
Excitation was by the 488-nm line of a 30-m W air-cooled Ar
ion laser with a full width at half maximum (FWHM) spot
size of 0.25 mm. Emission spectra were taken using an ISA
HR640 spectrograph that was calibrated for the region of
500-900 nm. For slits completely open, Le., 1.5 mm, resolu
tion of the spectrograph was 2 nrn at 500 nm. Because of the
low absorption coeff1cient at 488 nm in ZnS:Mn, the emitted
signal was very weak. Therefore the standard lock-in tech
nique was used to measure the signals from a thermoelectri
cally cooled PMT. Data were recorded using a computer and
a current-to-frequency counter. It was normalized for the
radiant sensitivity of the PMT in the spectral range 500-900
11m. Emission spectra were generated using data taken every
lOnm.
To observe concentration quenching, the emission in
tensity at 580 nm for different samples with varying concen-
s
:"', Zn
FiG. 2. Backscattering spectrum ofa ZnS:Mn
thin film (sample 14,0.91 WI. % Mn): left
panel, central region; right pand, upper re
gion. The figure show5 ul!ifonnity and homo
gcneity of the ZnS:Mn film on the large-area
substrate.
o~~~~~~~~~~~~
50 100 f 50 200 250 300 350 401l
CHANNEL NUMBER 50 100 150 200 250 300 350 400
CHANNEL NUMBER
1493 J. Appl. Phys., Vol. 67, No.3, 1 February 1990 Shise, Katiyar, and Kitai 1493
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128.189.203.83 On: Thu, 11 Dec 2014 21:32:39trations of Mn was measured without changing the geome
try and other experimental parameters. The error bars (Fig.
7) show that intensity measurements were not quite uniform
at different points (spatially) on the same sample.
IV. RESULTS AND DISCUSSION
A. Compositional analysis
X-ray diffraction (XRD), Auger electron spectroscopy
(AES), and Rutherford backscattering (RBS) confirmed
that the films were ZnS with uniform Mn concentration as a
function of depth ± 10% of the intended concentration.
Even though they were deposited on a large-area substrate,
the analysis indicates uniformity and homogeneity through
out the film. Figure 2 supports this. This figure shows the
backscattering spectra obtained from different regions of the
same sample. Although no O2 was detected in the samples
containing a lower amount of the dopant (Fig. 2), it was
detected in the samples with a higher level of Mn (Fig. 3).
Based on AES analysis, we believe that most of the O2 is
present as Mn oxide on the surface. The oxygen could have
been introduced during either the first or second heat treat
ment. The surface layer does not contain a significant
amount of Mn, since its thickness is much smaller than the
ZnS film thickness. Sands et al.9 have also reported detect
ing O2 in their films. No metallic impurities were present in
detectable quantities.
The Mn concentration was determined by RBS J() and
confirmed the values predicted from growth. Backscattering
spectra of a sample with lower Mn content are shown in Fig.
2; those obtained from a sample with a higher Mn content
are shown in Fig. 3. The presence of oxygen may be noted in
the latter. One can calculate the number of atoms of individ
ual elements per unit volume of the film from RBS study. Mn
and Zn, being very close in atomic mass, have closely spaced
high-energy edges when analyzed by a 1-2-MeV 4Hef
2000
1500
Cl
iii 1000
>=
500 '
100 o
150 Zn
Mil
SiEdge
s
200 250 300 350
CHANNEL NUMBER
FIG. 3. Backscattering spectrum of a ZnS:Mn thin film (sample 27, 9.2
wI. % Mnl with a higher Mn content. Note the presence of oxygen.
1494 J. Appl. Phys., Vol. 67, No.3, 1 February 1990 beam. As a result, deconvolution of this peak into the two
individual peaks was carried out to determine Mn concen
tration. Agreement between the desired Mn concentration
and the RBS value was close: e.g., for sample 14, RBS data
gave 1.2 ± 0.2 wt. % Mn and the mask system 0.91 wt. %
Mn; for sample 27, RES gave 10.2 ± 1.02 wt. % Mn and the
mask system 9.4 wt. % Mn. Data reported here correspond
to those obtained from the mask system.
B. Cathodoluminescence
The films were systematically investigated for the
change in CL characteristics due to a change in the dopant
level. Results thus obtained are shown in Fig. 4. The acceler
ating potential for all the measurements was 5.0 kV. The
relative CL emission intensity did not change appreciably
for different accelerating potentials within the narrow range
3.0-5.0 kY. This suggests a complete penetration of elec
trons in the thin films (-190 nm thick), even at 3.0 kY.
These results agree well with those of Theis and Wengert. ] i
It may be seen that initially the CL intensity increases with
Mn content, up to ~ 2 wt. %, beyond which there is a drop
in the intensity (quenching). This well-known phenomenon
has been observed in the electroluminescence and PL of
ZnS:Mn (Refs. 2, 3, and 7) but to our knowledge has never
been reported for CL.
Spectral characteristks of emission of ZnS:Mn thin
films under the influence of cathode rays were also studied.
Figure 5 shows the CL emission spectra of four samples,
taken under identical conditions. The Mn concentration in
creases as one goes from the bottom to the top of the figure. It
may be seen that although the position of the yellow peak
. ,....--.,. ---" ...... 0 (f)1 '/ a "--+-' 0
C / ,
::J . / . \ / . . 0
D / \
.D / \ L / 0 I " \ '---" I a
I • \
>, \
-+-'
(f)
C
Q)
-+-'
C
0.1 (wt %) 10 Mn
FIG. 4. Variation ofrdative CL intensity with Mn concentration: D, set 1:
., set 2.
Bhise, Katiyar, and Kitai 1494
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128.189.203.83 On: Thu, 11 Dec 2014 21:32:39(s)
/::'-~ '--0 (b)
/ "----_/-/~ -j/\
/ '" (1:) j/\,-
I (d)
450 500 550 600 SSC 700
WAVELENGTH (rim)
FIG. 5. CL emission spectra ofZnS:Mn thin films: (a) 26.4 wt. % Mn; (b)
11.4 wt. % Mn; (e) 1.64 wt. % Mn; (d) 0.43 wt. % Mn. The spectra are
shifted for clarity; the y axis is not an ldentical scale for each.
(4 T 1.6 A I ) is not altered, an additional peak in the red re
gion appears at higher Mn concentrations and that the oc
currence of the red peak and quenching of yellow lumines
cence is approximately at the same Mn concentration.
These results are in agreement with the PL results of
Zn5:Mn, as reported by Go'ede, Benecke, and co-work
ers12-14 except for the threshold of the onset of the red peak.
While Refs. 12-14 reported the appearance of a lower-ener
gy peak in samples having ~ 1 mol % Mn, it was not detect
ed in the samples containing < 1.64 wt. % «2.3 mol %)
Mn during the present investigation. Emission spectra for
samples with Mn concentration between 1.64 and 11.4
wt. % were not recorded; hence it is difficult to predict the
exact threshold concentration afMn in ZnS thin films far the
occurrence of the red emission. The ratio of the intensity of
the yellow peak (580 nm) to that of the red peak (700 urn)
changes with Mn concentration as shown in Fig. 6, The re
sults do not follow the relationship
1,./ Iy = (eMn ) 1.4
as reported in Ref. 13. The actual drop in the CL intensity
beyond C Mn = 2 wt. % is more than that interpreted from
Fig. 4 due to the appearance of the red peak at those concen
trations. Thus the total light intensity as indicated by the
diode is the integrated intensity under the yellow and the red
peaks. Moreover, the response of the 5i photodiode, used to
detect the light, is stronger to the red light than to the yellow.
C. Photoluminescence
Concentration quenching results ofPL ofZnS:Mn pub
lished so far do not entirely agree. While Refs. 3 and 7 report
an optimum at -1 mol %, others 15 have reported an opti
mum of 0.5-1.2 mol % depending on the method of film
preparation. The results obtained during the current investi
gation (Fig. 7) yielded a maximum yellow intensity at ~ 2
1495 J. Appl. Phys., Vol. 67, No.3, 1 February 1990 10
" o
0.1 e..:CL
o:PL
o
10 o
FIG. 6. 1,.11,. as a function ofMn concentration.f" is the peak yellow inten
sity, while I, is the intensity at 700 nm. 0, photoluminescence; 0" <.:athodo
lumincsccn>.:c.
wt. % Mn; although given the experimental error in the de
termination of Mn content, it is difficult to predict an exact
value (see Fig. 7). Remembering that the PL and CL mea
surements were made on the same samples, it may be seen
that the scatter in the PL data is much more pronounced
than that in the CL measurements. This may be due to the
fact that the PL signals are extremely weak, resulting in
alignment difficulties since the PL cannot be seen with the
eye. Also, long (60-8) integration times are necessary to ob
tain data. Nonuniformities in the samples themselves are not
seen as the source of the uncertainty.
O. 1 o·J.:---1 --'---'-.!......I..-L..l..J'"+-1 --'--'--'--'-I-!..w1~O;---'---'--"--'
CMn( wt%)
FIG. 7. Variation of the relative PL intensity with Mn concentration.
Bhise, Katiyar, and Kitai 1495
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128.189.203.83 On: Thu, 11 Dec 2014 21:32:39Figure 6 supports the theory that until a certain thresh
old Mn concentration, the red emission is negligible, irre
spective of the exciting mechanism. It was not possible to
determine this threshold exactly, as the samples with Mn
content between 1.64 and 11.4 wt. % were not investigated
for this effect. The fact that the quenching of luminescence
and the appearance of the red emission occur at approxi
mately the same level ofMn leads us to believe that these two
phenomena are related. The relative intensity of the red peak
with respect to the yellow is different for the same sample,
depending on the exciting mechanism. The results indicate
that the red centers have a greater absorption cross section
for photons than that for electrons, compared to the Mn
yellow cross section.
It was proposed initially that formation ofMn pairs and
clusters leads to the quenching ofluminescence. If this were
entirely true, one should observe an exponential decay in Mn
emission with time that is not influenced by the activator
concentration. As Ref. 16 points out, the fraction of Mn
pairs (assuming a random distribution) is not enough to
explain the extent of quenching at -1-2 mol % Mn.
The explanation based on radiationless energy transfer
to an energy sink via Mn centers seems to fit the observed
data. These sinks may be red, infrared, or nonradiative
centers. This idea, first proposed by Yang, Owen, and
Smith,17 was further explored in Ref. 14. A more thorough
model, based on a relaxation law limited by diffusion,IH
could successfully predict the nonexponential decay behav
ior. Unfortunately, this extended model is unable to explain
the change in the decay time at higher concentrations of Mn
(>0.5 mol %).
The phenomenon of concentration quenching can be ex
plained quantitatively using a mathematical model that
treats the energy transfer as percolation. This model is ad
vantageous in that it involves less restricting assumptions
than those in the earlier models. Here, energy loss to the sink
is proportional to the probability of finding a sink near the
excitation path. A detailed description of this model has
been undertaken. 19 One can calculate the number of excited
Mn ions in the luminescing sample, e.g., for sample 31 (1.64
wt. % Mn) the intensity ofCL emission was 2.8 fL. Assum
ing that one Mn atom may be excited every 2 ms, the fraction
of excited Mn atoms may be estimated to be 1. 71 X 10 5,
which means that the mean distance between nearest-neigh
bor excited Mn is of the order of tens of nanometers. This
immediately rejects any possibility of interaction among the
excited Mn ions. Hence energy transfer via unexcited Mn is
more probable.
The chemical identity of the red centers could not be
determined, due to the limited extent of the available infor
mation. These centers cannot be transition-metal impurities,
as none of the red-emitting species, viz., Fe} + and eu + (at
higher concentration), was detected. The possibility of an
octahedrally coordinated Mn2+ (Ref. 13) being responsi
ble for the red emission needs further investigation.
V. CONCLUSION
Concentration quenching in ZnS:Mn thin films under
the action of cathode rays (3.0-5.0 kV) and photons (488-
1496 J. Appl. Phys., Vol. 67. NO.3. 1 February 1990 nm Ar + ion) was studied. It was found that CL and PL
emission intensity was at a maximum at a Mn content of ~ 2
wt. %. Besides the conventional yellow peak centered at 580
nm, an additional peak in the red region was observed for a
higher dopant concentration. The relative intensity of this
lower-energy peak was seen to increase with an increasing
activator concentration. The level of Mn at the onset of this
peak was found to be different than that reported by Refs.
12-14. We believe that the quenching of luminescence and
the appearance of the lower-energy peak are related. Energy
transfer to the sinks via unexcited Mn ions is the probable
cause of quenching. This idea is further strengthened by the
fact that there exist very few excited Mn centers at a given
time, leading to a negligible interaction among the excited
centers. Treatment of this phenomenon as an energy perco
lation problem seems to explain, among other things, the
nonexponentiality of the decay time measurements. It was
not possible to determine the nature and chemical identity of
the red centers. Carefully controlled and systematically
planned experiments are necessary to obtain information
concerning these low-energy centers.
ACKNOWLEDGMENTS
We would like to thank the Department of Energy,
Mines and Resources for financial support. One of us (M.
B.) would like to acknowledge the scholarship support by
the Department of Materials Science and Engineering,
McMaster University.
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K159 (1984).
I"M. Katiyar and A. H. Kitai (unpublished).
Shise, Kaliyar, and Kitai 1496
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1.1140735.pdf | Diffractometer for synchrotron radiation structural studies of high temperature melts
F. Marumo, H. Morikawa, Y. Shimizugawa, M. Tokonami, M. Miyake, K. Ohsumi, and S. Sasaki
Citation: Review of Scientific Instruments 60, 2421 (1989); doi: 10.1063/1.1140735
View online: http://dx.doi.org/10.1063/1.1140735
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/7?ver=pdfcov
Published by the AIP Publishing
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146.189.194.69 On: Mon, 22 Dec 2014 20:05:38Diffractometer for synchrotron radiation structural studies of high
temperature melts
F. Marumo, H. Morikawa, and Y, Shimizugawa
Research Laboratory of Engineering Materials, Tokyo Institute of Technology. Nagatsuta, Midori-ku,
Yokohama227, Japan
M. Tokonami
Mineralogical Institute. UniversityafTokyo, Hongo, Bunkyo-ku. Tokyo J J 3, Japan
M, Miyake
Departmentaf Applied Chemistry, Yamanashi University, Takeda, Kofu 400. Japan
K. Ohsumi and S. Sasaki
Photon Factory. National Laboratory for High Energy Physics. Oho, Tsukuba 305. Japan
(Presented on 31 August 1988)
A diffractometer has been constructed for structural studies of high-temperature melt with
synchrotron radiation. It was designed to measure diffracted intensities from the free surface of a
molten sample by scanning a scintillation counter with a fixed glancing angle of the incident
beam. In order to heat samples up to 1500 ·C, a small electric furnace is attached to the
diffractometer. It carries a hemicircular ( 100 mm in diameter) cover, which has a window for the
passage ot x rays. The window is covered with a Kapton film. The sample container made of 30
Rh-Pt is mounted at the center of the furnace. A test measurement was performed on Ge02'
Monochromatic beams with A = 1.32A and its second harmonics were taken out of synchrotron
radiation by a,8-aluminacrystal (d002 = 11.3 A) and used as incident beams. To partial
scattering curves obtained with A and A /2 were combined to a single curve after correction for
absorption. The radial distribution function obtained from these data is in good agreement with
that previously reported which was derived from diffraction data collected on a conventional
diffractometer.
iNTRODUCTION
An x-ray diffractometer designed especially for studies of
liquid structures was described by Levy et a/.f The instru
ment, a so-called 0-8 type goniometer, provides for simulta
neous angular motion of the x-ray tube and the detector
around a horizontal axis lying in the liquid surface. In the
case of diffraction experiments with synchrotron radiation
(SR), it is practically impossible to rotate the incident beam
around the horizontal axis. Therefore, we cannot use the
conventional, B-() type diffractometer for the experiments
with SR. The diffraction intensities from amorphous materi
als, however, are quite weak and require hundreds of hours
to collect a set of reliable diffraction data with an ordinary x
ray generator. Therefore, it is desirable to use intense pri
mary beams for diffraction studies of amorphous materials.
Further, liquid materials at high temperatures usually have
high vapor pressures. They easily evaporate from the sample
container and coat the window of the high-temperature
chamber, which cause severe absorption of incident and dif
fracted x rays. The evaporation also lowers the free surface
of the liquid sample due to a decrease of the materiaL Thus,
these phenomena make measurements of weak diffracted in
tensities unreliable. For these reasons, it is important to col
lect diffraction data of molten samples quickly, and SR is a
requirement for accurate structural studies of high-temper a
ture melts. This article describes a diffractometer which is available
to measure scattering intensities from high-temperature
melts by using SR. In order to select a particular wavelength
of incident beam, a crystal of,8-alumina was used as a mono
chromator.2 We also constructed a small furnace for high
temperature works up to 1650 ·C.3
I. DIFFRACTOMETER
Figure 1 shows a schematic diagram of the diffractome
ter constructed to coHect intensity data with SR for radial
distribution analyses of noncrystalline materials such as gas
es, liquids, high-temperature melts, and glasses. The instru
ment has three arms [a detector arm (D), a sample arm
(A), and a slit arm (8)], which can be individually rotated
around the same axis. On the detector arm, a scintillation
counter (D 1) is set in the usual angle-dispersive measure
ments. However, a handy-type solid-state detector (intrinsic
germanium detector, Princeton Gamma Tech.) can be at
tached in place of the scintillation counter for the purpose of
energy-dispersive measurements. To the sample arm, a fiat
sample holder is attachable for power diffractometry. It is
also used for alignment of the diffractometer. In power data
collection, the diffractometer is operated in the symmetrical
B-2() mode. To carry out diffraction experiments on melts,
an electric furnace is set at the prescribed position together
2421 Rev. Sci. Instrum. 60 (7), July 1989 0034-6748/89/072421-04$01.30 ® 1989 American Institute of Physics 2421
•••••• -••• -.-•• -•••••••••••••••••••• ~.:.:.:.: •••••••••••••• :.:.:.:.:.-:-.:.:.;.:.:.~ ....... -: ••• :.:.~.:.:.:.:-:.:.: •••••••••• ~.:.:.:.:.;.:.:.:.:.:.;.; ••••••••• ,'.'.:.:.:.:.:.:.:.;.:.;' •••••••••• ; •••••••• :.:.:.;;:;;.;.;.; ••••• ;>; •••••••••••• :.:.~.:.:.: •• , ••••••••• '.' ••••••••• :.:.:.:.:.:.: ............................ :.:.:.:.: ••••• ; •••••• ' •••••••••••••
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146.189.194.69 On: Mon, 22 Dec 2014 20:05:38FIG. I. Schematic diagram of the x-ray diffractometer designed for mea
surements on molten samples with synchrotron radiation.
with a sample container after removing the fiat sample hold~
er from the sample arm.
When monochromated x rays are used, the diffractome
ter is lowered from the level of SR beam (by 85 em in the case
of the present diffractometer) by adjusting the height of the
legs (L) of the base. TheSRbeamismonochromatedbyap
alumina crystal. The glancing angle of the monochromator
can be changed to select radiation of the desired wavelength.
The monochromator can slide along the SR beam to make
the diffracted beam fall on the center of the goniometer. The
slit arm (S) must be rotated to receive the monochromated
beam properly. A SORD 223 MARK V computer controls
the stepping motors driving the detector and slit arms, and
the single channel analyzer. The block diagram of the data
collection system is shown in Fig. 2.
II. J3-ALUMINA MONOCHROMATOR
Continuous spectra of synchrotron radiation make it
possible to select a particular wavelength by using a crystal
monochromator. The monochromator crystal is required to
be nearly perfect and resistable for radiation damage, and to
have a large d spacing in the diffractometry of liquids. For
these reasons {3-alumina (N aAI 11017) was selected, and the
002(d = 11.3 A) and 004(d = 5.7 A) reflections were used
to monochromate the SR beam.
A transparent crystal of p-alumina (space group-P63/
mmc, a = 5.592 A, c = 22.61 A, Z = 2) was supplied from
2422 Rev. Sci. Instrum., Vol. 60, No.7, July 1989 SYnchrotron
radiaticns
)
monochromator
(dc..::l.13nm}
melt sample
counter S. C. A,
Timer t--(A)
Printer II
Floppy
Disk I\--~
Counter i--S.C.A.
Timer P.,i2)
FrG. 2. Block diagram of the data collection system. i '\,
Li near
/ Amp
Toshiba Ceramics Co. Ltd.4 It was grown from Bayer alu~
mina at a temperature of more than 2000 °C, and has dimen
sions of20X30X2 mm3. A small amount of soda lime was
added to promote crystal growth.
The rocking curve of the 002 reflection was measured
with monochromatic x rays (1.517 A) in the ( +, +, -)
setting on a triple-axis diffractometer at the BL-4A station of
Photon Factory in the National Laboratory for High Energy
Physics, Tsukuba to check the quality of the p~alumina crys
taL The fore crystal is a SiC ill) plate and the monochrocol
Iimator is a fine-fold 8i(220) channel cut. The beam size is 3
mmHXO.02 mmV. The e-2B technique was used to collect
the intensity data with intervals of 0.0010 in e. The diffrac
tion peak profile shows a slightly split peak with
FWHM = 0.018° (Fig. 3). The reflectivity was about 70%.
III. FURNACE
Figure 4 shows a schematic illustration of the electric
furnace constructed for the diffractometer. Four lanthanum
chromate heating elements (Keramax) of 8 mm in diameter
and 140 mm in length were mounted in alumina protective
tubes of lO-mm Ld., 80 mm length, and 1 mm thickness.
They were arranged horizontally and in parallel to the SR
beam in two levels. Two were in the upper and the remaining
two in the lower level to yield a square-prismatic space in the
furnace. The horizontal and vertical distances between the
centers ofthe heating elements are 40 and 20 mm, respective
ly.
The sample container made of 30Rh-Pt is 10 mm wide,
20 mm long, and 3 mm deep, and placed at the center of the
furnace. The whole thing is set in a hemicircular cover which
Structural studies 2422
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146.189.194.69 On: Mon, 22 Dec 2014 20:05:38IJ)
0
.-l "-
~
;><
I'-< H
Ul
7: fil 8
:<: H on
'" ... " '" ...
'" '" ... X
"' '"
J
-0.0) -0.02 -0.01 O.Ql 0:02 0.03
~e (deg)
FIG. 3. Diffraction peak profile of the {J-alumina crystal used as monochro
mator.
has a window of 10 mm in width for the passage of x rays.
Kapton film of75 pm thickness is used for the window mate
rials. The frame of the furnace is cooled by water (1 llmin)
to protect the Kapton film from thermal decomposition.
Temperature is monitored by a Pt6-30Rh thermocouple lo
cated at the middle of the upper and lower heating elements.
The maximum temperature recorded with this system was
1750 "C.
FIG. 4. Small Keramax furnace attachable to the x-ray goniometer. 1, pro
tective tube, 2: Keramax heating element, 3: connector for power supply, 4:
connector for cooling water, 5: bolt for fix the furnace, and 6: guide for the
furnace.
2423 Rev, Scl.lnstrum., Vol. 60, No.7, July 1989 IV. PERFORMANCE
The use of f:1-alumina and the construction of the dif
fractometer allow us to perform simultaneous measure
ments of diffraction intensities from a melt with two wave
lengths A( = 1.4 A) and A 12( = 0.7 A), giving data over a
wide range of S(0.7";:S,.;:18 A-I). Here,
S = 41T sin [(a + /3)/2]/ A, with a = 5.60 in the present case
and /3 = diffraction angle. This range of observable S is ade
quate for structure analysis based on the radial distribution
functions.
As a test run, we performed a structure analysis of mol
ten Ge02' The Ge02 sample was placed in the 30Rh-Pt con
tainer and heated in the Keramax furnace. The temperature
was controlled to 1200 ± 5 ·C during the measurement. The
power supply for the furnace was about 1.5 k W. The dichro
matic beam with A = 1.32 A and its second harmonic were
taken out from SR by the f:1-alumina monchromator and
used for the measurement in a step scanning mode at inter
vals of OS in f3 from 7° to 140°. After correction for absorp
tion, two partial scattering curves obtained with A and A 12
were combined to a single scattering curve. The absorption
correction factor A was calculated with the formula,
A _ s1n(2/3 -a)
-Ii [sin a + 5in(2/1 --a) 1 '
where p is the linear absorption coefficient of the sample.
The intensity data were normalized by the Krough-Moe and
Noman method.5•6 The radial distribution function DCr) is
shown in Fig. 5. The first peak in the D(r) curve is observed
at r = 1.75 A. The number of oxygen atoms around the Ge
atom was calculated from the peak area to be 4.0. These
results are in good agreement with those in the previous re
port by Kamiya et al.7
,0_0 ______________________ 0 __ _
I
7.15r
(~ I
-'::-10-
[)
o -
o 2 3 5
FIG. 5. Radial distribution function D(r) of molten GeO,_ The DC!') curve
obtained in the present study is compared with that reported by Kamiya
eta!' (1986).
Structural studies 2423
.·.·.-·x·:·;·;·.·.·.·.'.·.·.·.·.:.:.:.:.;·:·;·.·.·.·.~ .~.~.:.:.:.:.:.:;;.: •.. 7" ••••••• :.:.:.:.:.:.;.;.;.:.; ••••••••• ,' ................ -••• -.~.,. .., •.••.•.•.•.•••. '? .................... ;.;.;.;.; ••••••• ; ••••••• .',.: .. , •. ,.:.;.;.;.;.; ............ , ...... "',.;.;.;.:.;.:.,.; •• ' .. , •• ' ••• ~ •. ,.:.:.;.:.:.;' •••••• .t.~ ...... ;:' •. ,··.":··.·:-;·; •. ·,..·;~.·.· ... ·.,~.·.·:,,·· .• :.z·.' ..... ~.·.·.v;t •••• ~.: ••••• -••••• , •••
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146.189.194.69 On: Mon, 22 Dec 2014 20:05:38In conclusion, the present system can supply sufficient
x-ray scattering data of molten samples with SR in time
about one third of that required to collect data with an ordi
nary (J-(J type diffractometer. Ifwe use a curved-crystal mon
ochromator in place of the flat-crystal monochromator of
the present system, the required time is expected to be short
ened to one tenth of the ordinary measurements.
'H. A. Levy, M. D. Danford, and A. R. Narten, Rep. ORNL·3960, Oak
Ridge National Laboratory Contract No. W-7405-eng-26 (1966).
2424 Rev. SCi.lnstrum., Vol. 60, No.7, July 1989 2S. Sasaki, H. Morikawa, T. Ishikawa, and Y. Shigeto, Photon Factory Ac
tivity Rep. 1983/84, VI-IO (1984).
JR. Morikawa, F. Marumo, M. Miyake, T. Suzuki, T. Fukamachi, M. Yo
shizawa, and S. Sasaki, Photon Factory Activity Rep. 1983/84, VI-II
(1984).
'A. Itoh, Yogyo Kyokai-shi 78,449 (1970).
'J. Krough-Moe, Acta Cryst. 9, 951 (1956).
"N. Norman, Acta Cryst. 10, 370 (1957).
7K. Kamiya, T. Yoko, Y. Itoh, and S. Sakka, J. Non-Cryst. Solids 79, 285
(1986).
Structural studies 2424
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1.576262.pdf | Highrate reactive sputter deposition of aluminum oxide
Fletcher Jones and Joseph Logan
Citation: Journal of Vacuum Science & Technology A 7, 1240 (1989); doi: 10.1116/1.576262
View online: http://dx.doi.org/10.1116/1.576262
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
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.126.253 On: Tue, 25 Nov 2014 21:40:16High-rate reactive sputter deposition of aluminum oxide
Fletcher Jones and Joseph Logan
IBM Research Division, Thomas J. Watson Research Center. Yorktown Heights. New York 10598
(Received 21 November 1988; accepted 30 January 1989)
Using a new reactive sputter deposition approach, we are abie to consistently deposit
stoichiometric Alz03 films at a rate of 220 nm/min with ± 6% thickness variations across 82-
mm-diam substrates. Thus a lO-pm-thick aluminum film could be deposited in 40 min.
Deposition rates as high as 500 nm/min have been demonstrated. However, at these deposition
rates, the voltage levels were high and the system was prone to arc during long runs. This paper
describes the system and some of the properties of the films deposited at rates of ~ 220 nm/min. It
is shown that there is a range of experimental parameters over which the properties of films
deposited at 220 nm/min show small variations.
I. INTRODUCTION
Depositing thick films of aluminum oxide onto substrates by
conventional sputtering (where an Al203 target is sputtered
in an argon plasma) is a very slow process. A 10-,um-thick
film deposited at a typical rate of25 nm/min requires -6.6 h
of sputtering time. Because of the slow sputtering rates of
aluminum oxide targets, reactive sputter deposition was in
vestigated.
In reactive sputter deposition of aluminum oxide, alumi
num is sputtered from an aluminum target onto a substrate.
The sputtering gas mixture is usually argon and oxygen. At
the substrate, the aluminum and oxygen react to form stoi
chiometric aluminum oxide if there is sufficient oxygen.
Aluminum can be sputtered at rates which are at least a
factor of 10 higher than alumina. Hence, the possibility ex
ists for depositing aluminum oxide at very high rates. It
should be noted that Grantham, Paradis, and Quinn
ac):lieved an aluminum oxide sputter deposition rate around
6000 A/min by sputtering an alumina target (see Ref. 5) at
high power densities. However, the target consistently
cracked after 15 min of sputtering time.
The oxygen introduced into the system also reacts with
the aluminum target. At sufficiently high oxygen concentra
tions, the surface of the aluminum target oxidizes. When this
happens, the sputtering rate of the target in this "compound
state" is at least an order of magnitude smaller than that of a
pure aluminum target. In practice, it is difficult to obtain
high deposition rates of stoichiometric aluminum oxide on
large-diameter substrates without designing special features
into the vacuum system. This is because the concentration of
oxygen needed to ensure that the aluminum oxide film de
posited on the sub!'.trate is stoichiometric, is usually large
enough to convert the aluminum target from the metallic
state, in which aluminum can be sputtered at very high rates,
to the compound state in which the surface oxide is sputtered
at a very low rate. Other investigators (see Refs. 1 and 2)
have shown that with certain arrangements of baffles and
apertures and with a dc or rf discharge to excite the oxygen in
the vicinity of the substrate, aluminum oxide can be reactive
ly deposited at high rates. The primary aim of the baffle
arrangement is to produce an oxygen concentration which is
high in the vicinity of the substrate and low in the vicinity of
the aluminum target. This of course make it easier to oxidize the film deposited on the substrate. The result is that high
deposition rates of aluminum oxide have been reported with
the target sputtering in the metallic mode (see Ref. 2). How
ever, the high deposition rate is achieved at a great penalty.
The baffle arrangements reported in the literature result in a
great waste of target material. It has been reported that from
60% to 90% of the sputtered aluminum material is deposit
ed on the baffles instead of on the substrate. Also the aper
ture arrangement results in a great degree of nonuniformity
in the thickness of the deposited film. Consequently, sub
strate motion must be employed in order to make the depos
ited film more uniform for large substrates. Substrate mo
tion for the purpose of achieving better uniformity usually
results in a drop in the net deposition rate when the substrate
is large. The system described in the present work gives high
er deposition rates across large areas because there are no
obstructions between the target and the substrate. In this
system deposition on 82-mm substrates is achieved without
moving the substrate. The uniformity is ± 6%. However, in
a scaled up version of the same system, zirconium dioxide is
deposited at high rates on 125-mm-diam substrates.6 The
corresponding uniformity of the films is ± 4%. Recent
modifications to the scaled up system give film thickness
variations on the order of ± 1 % across 125-mm substrates.
II. EXPERIMENTAL PROCEDURE
A schematic of the reactive sputter deposition system used
to deposit films is shown in Fig. 1. The sputtering source is a
2oo-mm-diam magnetron with an aluminum target bonded
to it. A mass spectrometer is used to measure partial pres
sures of gases in the system and an emission spectrometer is
used to measure the optical spectra of excited argon, oxygen,
and aluminum atoms in the plasma. The substrate rests in
side a hollow cathode electrode which is covered with an
aperture plate. The hole in the aperture plate allows the alu
minum sputtered from the target to be deposited onto the
substrate. The inner diameter of the hollow cathode elec
trode is 200 mm and its height, i.e., the distance from the
substrate plane to the plane of the circular aperture is 25 mm.
Using spacer rings in the top and bottom plates of the vacu
um system, the separation between the target and the sub
strate is easily varied from 44 to 200 mm. Argon and oxygen
are introduced into the system through a stainless-steel gas
1240 J. Vac. ScI. Technol. A 7 (3), May/Jun 1989 0734-2101/89/031240-08$01.00 @ 1989 American Vacuum SOCiety 1240
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FIG. L Reactive sputter deposition system configuration.
inlet ring which is physically in contact with the hollow cath
ode (He) electrode but electrically isolated from the vacu
um chamber which is at ground potential. Hollow cathodes
have been used in ion beam sources and tokamaks to create
high-density plasmas. In our experiments the He electrode
is used to facilitate the oxygen-aluminum reaction at the
substrate. The pumping package consists of an oil diffusion
pump backed by a mechanical pump.
Radio-frequency power is delivered to the target and hol
low cathode substrate holder by separate rf power supplies.
A common exciteris used to supply a 13.56-MHz rfsignal to
the excitation stage of both rf power supplies. The common
exciter also allows adjustment of the relative phase of the
power supplied to the magnetron and hollow cathode. With
out the common exciter, it was difficult to eliminate large
oscillations that were frequently observed in the dc bias vol
tages of the target and substrate electrodes. The common
exciter eliminated these oscillations. Matching networks are
used to match the impedance of the plasma in the sputter
deposition chamber to the impedance of the rf generators.
Although de power supplies are more efficient and give
higher metal sputtering rates, [() rf power supplies are prob
ably better suited for exploring the region to the right of the
reactive sputter deposition transition. The oxide layer which
forms on the target's surface does not readily transmit direct
current and often gives rise to a profusion of arcs in the sput
tering system. These arcs give rise to particulates which ruin
the deposited film. However, a dc power supply can prob
ably be used if the oxygen flow can be maintained at a low
enough level to prevent significant target oxidation while at
the same time depositing a stoichiometric film at the sub
strate.
J. \lac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989 The properties of Alz03 films deposited at high rates were
measured as a function of system parameters such as argon
pressure, oxygen flow rate, substrate bias, and deposition
rate. The film properties measured were stress, hardness,
and refractive index. An examination of opaque films with a
four-point resistivity probe showed them to be conducting
films. The chantcterization of such films was not an objec
tive of this study and therefore will not be discussed further,.
The stress, hardness, and refractive index were only mea
sured for transparent films. The resistivity of transparent
films was too high to be measured. The refractive index and
hardness were measured using an ellipsometer and a Knoop
microhardness tester. The thickness of the mms were typi
canyon the order of 2.2 f..lm. The deposition rate, unless
otherwise specified was 220 nm/min. Film stress was deter
mined from the curvature induced in the silicon substrate
after the film was deposited. In all cases, the films exhibited a
compressive stress. The error in the refractive index, stress,
and Knoop microhardness measurements, were 1.2%, 2%,
and 8%, respectively. The composition of the films was de
termined by electron microprobe analysis. The atomic per
cent of the clear A120:, films, which are called stoichiometric
in this paper, had typical 0, AI, and Ar values of38%, 58%,
and 4%, respectively. The aluminum/oxygen ratio is 0.66
which is in good agreement with the ideal value of 2/3.
iii. RESULTS AND DISCUSSIONS
A. Effects of the hollow cathode substrate holder
There are several aspects of the hollow cathode substrate
holder which make it very useful for reactive sputter depo
sition. The small height, 25.4 mm, of the He substrate hold
er and the close proximity of the substrate to the gas feed ring
ensures that an oxygen molecules and atoms must come
close to the substrate before traveling to other parts of the
vacuum vessel. Hence the concentration of oxygen in the
vicinity of the substrate is increased for this configuration as
opposed to a more open configuration with the gas feed ring
outside the He substrate holder and/or closer to the vacuum
chamber wall. At a target-substrate separation of 120 mm
and a 1 OO-mm aperture diameter in the HC substrate holder,
line of sight conditions exists between every point on the
target and every point on the 82-mm-diam substrate. Hence,
if there were no gas phase scattering, none of the material
sputtered in the direction of the substrate would be inter
cepted by the aperture of the hollow cathode substrate hold
er. This is a major improvement since as mentioned earlier,
other investigators lost 60%-90% of the material sputtered
in the direction of the substrate to intervening baffles and
apertures.
The size of the opening in the hollow cathode substrate
holder affects the critical oxygen flow rate. It was observed
that at an operating pressure of 33 Itm of argon, a target
sputtering power of 4.5 kW, and a target-substrate separa
tion of 145 mm, the target transition occurred at 20.2 std.
cm.l/min (seem) when the He substrate opening was 100
mm in diameter. When the HC substrate cover was re
moved, the oxygen flow rate needed to convert the surface of
the target to aluminum oxide was 23.4 seem. These results
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were found to be true whether or not power was supplied to
the substrate. It should be noted that the dependence of tar
get transition point on oxygen flow rate for the hollow cath
ode design is opposite to that produced by the configurations
of other investigators, i.e., when baffles are inserted, more
oxygen can be introduced into the system without poisoning
the target.
The reason for this is as follows: When the opening in the
aperture is reduced, at constant flow rate, the oxygen con
centration within the opening increases. A simple diffusion
analysis shows that this results in an increase in the oxygen
concentration in the center of the target. If the sputtering
tracks are located close to the center oftarget, the increase in
oxygen concentration will result in a decrease in the critical
oxygen flow rate. The effect should be more pronounced for
tracks having smaller diameter than for those having a larger
diameter. In the 200-mm-diam magnetron, the radius ofthe
groove in the sputtering track is 64 mm. An attempt was
made to examine aperture size effect in the 305-mm-diam
magnetron used in Ref. 6. For this magnetron the radius of
the racetrack is 114 mm. Unfortunately, the location of
shields in this system prevented target -substrate separations
of < 197 mm. While the effect is clearly visible for the
smaller magnetron, it was not observed at all for the larger
magnetron when the target-substrate separation was 197
mm. This behavior indicates that the effect depends on the
geometry of the reactive sputter deposition system.
Films obtained with the HC substrate cover removed were
opaque and conducting. On the other hand, the films ob
tained with HC substrate cover in place were transparent.
B. Reactive sputter deposition transition
Experimentally, the transition of the target from the me
tallic state to the compound state is easily observed by moni
toring the changes in system parameters as the flow rate of
oxygen is increased. The position of the exhaust throttle
valve separating the diffusion pump from the process
chamber is fixed. Figure 2 shows how the magnetron's vol
tage, argon partial pressure, and oxygen partial pressure
vary as the oxygen flow rate is increased from 0 to 30 sccm.
The power supplied to the target is fixed at 3.5 kW. The
substrate power supply is turned off. The argon pressure in
the process chamber is 40 pm. The partial pressure readings
in Fig. 2 are measured using the mass spectrometer. The
quadrupole detector of the mass spectrometer is too sensitive
to measure mass concentrations at high pressures and hence
the pressure in the vicinity of the quadrupole is significantly
reduced by differential pumping. The partial pressures
shown in Fig. 2 are those measured in the vicinity of the
quadrupole detector. We assume that the partial presssures
at the quadrupole detector is proportional to the partial pres
sure in the process chamber.
At oxygen flow rates between 0.0 and 16.7 sccm, little
change is observed in the magnetron's dc bias voltage as wen
as the argon and oxygen partial pressures. In this range of
oxygen flow rates, the oxygen partial pressure signal (mass
32) is buried under the noise created by trace amounts of
hydrocarbons in the system. However, the full amplitUde of
this signal at mass 32 is plotted as the "oxygen partial pres-
J. Vac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989 W tr ::J
VI VI w 00 ":z a
~ 10r-----~----_.-----.------r-----._----~
B --.-.--.-0:......
6
_-w-. _____ • __ _
OL-----L--- __ L-____ ~ ____ ~ ____ ~ ____ ~
10r-----.------.-----.------r-----.-----~
B ----.----.---- ..... ~ III" ..... -.-.----.---.: 6
4
2
OL-----L-----~ ____ ~ ____ ~ ____ _L ____ ~
~ 10
=> 8
6
: ----.--.--.-.. .J o o 5 10 15 o~ 0---.. 0-6
20 25 30
OXYGEN FLOW RATE (SCCM)
FIG. 2. Oxygen flow rate dependence of magnetron de bias voltage (D),
argon pressure at quadrapole of mass spectrometer (e), and oxygen pres
sure before the transition (. ) and after the transition (0). The scale factors
for the diamonds and circles are 1.0-9 and 1.0-7, respectively.
sure." It is clear that even though the amount of oxygen
introduced into the process chamber increases by significant
amounts, the mass spectrometer does not detect any in
crease. In this flow rate range, the aluminum target is in the
metallic state and aluminum is being sputtered at high rates
onto surfaces in the process chamber. The oxygen arriving at
the aluminum coated surfaces quickly reacts with the alumi
num and therefore is efficiently removed from the process
chamber. Consequently, the oxygen partial pressure does
not change over a wide range of oxygen flow rates. Above an
oxygen flow rate of 16.8 seem, the magnitUde of the magne
tron's voltage drops dramatically and the oxygen partial
pressure increases by two orders of magnitude. The argon
partial pressure does not change significantly.
The reasons for the changes are as follows: Above the 16.8
sccm oxygen flow rate, the surface of the target oxidizes and
the rate at which aluminum is sputtered into the process
chamber drops far below that required to absorb much of the
available oxygen. Hence, the oxygen pressure increases dra
matically. The mass spectrometer shows that it increases by
at least two orders of magnitUde.
The dramatic change in the magnetron's voltage is also
related to the formation of an oxide film on the target's sur
face. Argon ions bombarding the target cause electrons to be
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ejected from the target's surface. These secondary electrons
are accelerated away from the target and into the plasma by
the large negative de bias voltage existing between the target
and the plasma. The highly energetic secondary electrons
are responsible for the production of the ion-electron pairs
that comprise the plasma. When the target oxidizes, the as
sociated increase in the level of secondary electron yield
causes an increase in the plasma density. At constant power,
an increase in the plasma density results in a drop in target
voltage. The transition has been modeled by several investi
gators and the interested reader is referred to the papers of
Shinoki and Itoh3 and of Affinito and Parsons,4
C. Characterization of the system
Figure 3 shows a plot of the critical oxygen flow rate ver
sus power. The points on the curve were taken while the
target was being sputtered in the metallic mode. At any point
on the curve, an increase in oxygen flow rate by more than
0.5 sccm causes the surface of the target to oxidize.
Figure 4 shows the dependence of critical oxygen flow rate
on target-substrate separation. The power is fixed at 5 kW.
Increasing the target-substrate separation has two conse
quences. First of all, the surface area of the vacuum chamber
increases. The aluminum is then spread out over more sur
face area and consequently the oxygen is gettered more effi
ciently. Second, an increase in target-substrate separation
moves the gas feed ring farther away from the sputtering
target. Both effects cause a decrease in the oxygen concen
tration at the target. Hence, at fixed power, more oxygen can
be introduced into the system if the target-substrate separa
tion is increased.
Figure 5 shows the deposition rate of stoichiometric alu
minum oxide as a function of target power. Substrate power
35 I I I I
,............,.
30 I- /. -
~ 25 I- / -
u / u
.t£!,
w
~ 20 I-- -
S: q
u: 15 I--;--:z w
<.:>
S< / 0 10 I-- -
51--/ -
•
0 I I I I
0 2 4- S B 10
POWER (KILOWATTS)
FIG. 3. Critical oxygen flow rate vs rf magnetron power. The points on the
curve are points of stable operation of the system. At any point on the curve,
an increase in oxygen flow rate more than 0.5 seem causes the target to
oxidize.
J, Vac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989 34
~ 32 ~.
~ :so / u
M.
w
~ 2B
:S: 9 .... 26 :z w
<.:> >-x j 0 24
22
20 ~ I I ~
10 12 14- 16 18 20 22 24
TARGET -SUBSTRATE SEPARATION (eM)
FIG. 4. Critical oxygen flow rate vs target-substrate separation. Target pow
er is fixed at 5 kW.
and bias were approximately 240 Wand -160 V respec
tively. Cooled (see below) substrates were used. The target
substrate separation was 120 mm and the argon pressure was
60 mTorr. The maximum rate plotted is 3200 A/min. On
uncooled substrates, rates as high as 5200 A/min were ob
tained. However, the system was prone to arcing.
Above 500 V, the slope of the voltage versus sputter yield
curve starts to drop. In this region, the target sputtering rate
will not be a linear function of power. The target power for
the three points as shown in Fig. 5 are 1, 3, and 5 kW, respec
tively. The corresponding target voltages are -366, -752,
3500
3000
Z :i '" Vl ::;: 2500 0
~ t:I
Z
~ 2000
~
z 0 E 1500 Vl g
CI
1000
500
0 2 3 4 5 6
POWER (KILOWATTS)
FIG. 5. AI20" deposition rate vs rf magnetron power. Target-subsirate sepa
ration is 12 cm. Hollow cathode power and bias voltage are approximately
240 Wand .-160 V, respectively.
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and -1024 V respectively. Hence, in the 3-5 kW range, the
sputtering rate and consequently the deposition rate is not
expected to increase linearly with target power. Therefore a
eurve is drawn through the three points in Fig. 5.
D. General effects of process conditions
After running almost 80 samples in the hollow cathode
reactive sputter deposition system and observing those con
ditions needed to produce transparent films, the effects of
various system parameters can be stated. These parameters
are system pressure, substrate bias voltage, substrate tem
perature, and target-substrate separation. Unless stated oth
erwise, the target power was held fixed at 3.5 kW, the sub
strate dc bias voltage was around -220 V, and the target
substrate separation was 120 mm. The oxygen flow rate was
fixed just below the flow rate needed to oxidize the target.
This was typically around 17.6 seem of oxygen.
t. Effect of system pressure
The system pressure was increased by increasing the flow
rate of argon into the system. The position of the manually
controlled exhaust throttle valve was fixed throughout the
experimental run. At low system pressures, e.g. 20 jim, the
aluminum oxide films were opaque and conducting. As the
system pressure was increased, the conductivity of the film
decreased until it could not be measured. At pressures ex
ceeding 50 f-lm the films were transparent. The transparent
films are of course nonconducting. At high gas pressures, the
arrival rate of aluminum at the substrate decreases because
of an increase in the number of gas phase collisions between
aluminum and argon atoms. This results in an increase in the
amount of aluminum deposited on the walls and other sur
faces in the vacuum chamber. Since the oxygen is introduced
very close to the substrate, the increase in pressure should
not cause the flux of oxygen at the substrate to decrease as
fast as that of the aluminum. The net result is an increase in
the arrival rate of oxygen relative to that of aluminum and
hence films that become more transparent with increasing
argon pressure.
2. Effects of substrate dc self-bias voltage
By varying the bias voltage on the He substrate electrode,
opaque conducting and nonconducting films as well as films
that were transparent could be made under otherwise similar
conditions. The self-bias voltage on the substrate is increased
indirectly by increasing the power supplied to the substrate
electrode. At a pressure of 60 mTorr, films made using low
substrate voltages, e.g., -80 V, were opaque and conduct
ing. At higher dc bias voltages, e.g., -200 V the films ob
tained were transparent and nonconducting.
The magnitUde of the substrate bias increases with in
creasing power. This has several consequences. The in
creased bias voltage leads to more energetic argon-ion bom
bardment of the substrate. The ion bombardment activates
the surface of the film and facilitates the reaction between
aluminum and oxygen species on the surface. Increasing the
power also increased the level of aluminum and oxygen re-
J. Vac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989 sputtering. The aluminum-aluminum bond is weak in com
parison to the aluminum-oxygen bond. Hence, aluminum
should be preferentially sputtered from a substrate where the
aluminum condensation rate exceeds the value required to
produce a stoichiometric oxide and consequently produces
an aluminum rich film. It follows that if the excess alumi
num condensation rate is not too high, a transparent stoi
chiometric film can be produced under the same conditions
as those used to produce an opaque and conducting films by
simply using substrate power and voltage levels large enough
to sputter away the excess aluminum.
Two experiments were carried out to measure the alumi
num deposition rate in an oxygen-free environment with and
without turning on the hollow cathode power supply. The
target-substrate separation was 19.6 cm. The argon pressure
was 40 mTorr. The target power was 5 kW. With the sub
strate power turned off, the target voltage was -1109 V and
the aluminum deposition rate was 1699A/min. In the sec
ond experiment, the target power was set to 5 kW and the
substrate power was adjusted to obtain a substrate dc bias
voltage of -160 V. Under these conditions, the substrate
power was 150 Wand the target de bias voltage was -1050
V. The aluminum deposition rate was 1551 A/min. Both
depositions were carried out on cooled substrate. Therefore,
the rate at which aluminum was resputtered from the sub
strate was 148 A/min and one can conclude that ~9% of
the aluminum arriving at the substrate is resputtered under
these conditions. When oxygen was added to the system at a
flow rate of 28.5 sccm, a stoichiometric film was produced.
The target power and voltage were 5 kW and -1069 V,
respectively. The substrate power and voltage were 150 W
and -160 V, respectively. The thickness, deposition rate,
and density of the deposited aluminum oxide film were,
20254 A, 2025 A/min, and 3.6 g/cm3. The density of the
film was determined by measuring the change in weight of a
l-in. -diam silicon before and after the Alz03 film was depos
ited. The mass of the deposited oxide film was 3.9 mg. The
thickness was measured using an ellipsometer. The theoreti
cal density8 of y-Alz03 is 3.7 g/cm3 (which is close to the
measured density given above), The density of aluminun is
2.7 g/cm3. Using these density values and the molecular
weights of oxygen and aluminum, it is easy to show that the
theoretical ratio of the aluminum oxide deposition rate to the
aluminum deposition rate should be 1.4. Using the experi
mental oxide and metal deposition rates of2025 and 1551 A/
min, respectively, the rate ratio is found to be 1.3, This indi
cates that the net aluminum deposition rate is smaller than
1551 A/min when substrate power is turned on and the oxy
gen flow rate is at 28 secm. The actual aluminum deposition
rate is found by dividing the oxide deposition rate by 1.4. The
aluminum deposition rate is therefore 2025/1.4 or 1446 A/
min. Hence, when oxygen is added, the net metal deposition
rate drops from 1699 to 1446 A/min or by -15%. About
9% of the change in metal deposition rate is due to substrate
resputtering. The other 6% is probably caused by the oxida
tion of the center of the target. Hence, if res puttering was not
employed in this case, the amount of excess aluminum in the
film could be as high as 9%. Films produced without sub
strate power were dark. However, the resistivity was stm
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very high. Characterization of the dark films was beyond the
scope of this work.
3. Effects of substrate temperature
The HC substrate holder is water cooled. At substrate bias
voltages between -200 and -300 V, the rf power deliv
ered to the substrate holder is around 0.8-1.5 kW. In this
ease the target-substrate separation is 12 cm. The flow rate
of the water is such that the temperature rise of the substrate
holder at these power levels is < 10 °C. Experiments were
conducted (i) with the silicon substrates simply resting on
the substrate holder (uncooled or hot substrates) and (ii)
with the silicon substrates attached to the substrate holder
using a thermally conducting vacuum grease (cooled sub
strates). Small-diameter silicon substrates, 25 mm, were
chosen so that both cooled and uncooled substrates could be
placed on the substrate holder at the same time. It was ob
served that the minimum pressure required to produce
transparent films was different for the hot and cold sub
strate. At a pressure and substrate voltage of 50 mTorr and
-200 V, the film produced 011 the uncooled substrate was
transparent and the film on the cooled susbstrate was opaque
and conducting. When the pressure was increased to 70
mTorr, clear and transparent films could be produced on
both cooled and uncooled substrates. The higher tempera
ture of the hot substrate could result in an increased reaction
probability for the oxygen and aluminum species arriving at
the substrate. The oxygen density above the hot substrate is
probably lower than the oxygen density above the cold sub
strate. The proposed increase in reaction probability would
have to be large enough to produce a stoichiometric film in
spite of the drop in the flux of oxygen molecules associated
with the higher substrate temperature. The pressure effect
for the cooled film is probably due to a drop in aluminum
arrival rate relative to the oxygen arrival rate as discussed
earlier.
4. Effects of target-substrate separation
The separation between target and substrate also affects
film formation. It is more difficult to obtain transparent alu
minum oxide films at sman target-substrate separations than
at large target-substrate separations. In general, the dc bias
voltage and operating pressure needed to obtain transparent
films were higher for the 120-mm target-substrate separa
tion when compared to larger target-substrate separation
(e.g., 197 mm). The arrival rate of aluminum at the sub
strate falls rapidly as the target-substrate separation in
creases. From the above discussions, it follows that one is
more likely to obtain transparent films at larger target-sub
strate separations because the concentration of oxygen in the
vicinity of the substrate does not drop on increasing the tar
get-substrate separation. However, the net deposition rate
decreases as the target-substrate separation increases.
IV. PROCESS WINDOW FOR REACTIVELY
SPUTTERED AI203 FILMS
The films discussed below were made using the following
system configuration. The target-substrate separation was
J. Vac. Sci. Technol. A, Vol. 7, No.3, May/Jun 1989 197 mm. Increasing the target-substrate separation allowed
us to increase the oxygen flow rate as well as reduce the
pressure for a given target power. Hence at a target power of
5 kW, the critical oxygen flow rate was -30 scem. The ar
gon pressure for clear stoichiometric aluminum oxide depo
sition on cooled silicon substrates was as low as 20 roTorr.
However, for most of the films discussed in this section, the
operating pressure was-40 mTorr. The diameter of the
hollow cathode aperture was 100 mm. The hollow cathode
power and de bias voltage were 150 Wand -160 V, respec
tively. The results are summarized in Tables I-IV. In Run
186 of Table II, the substrate power and dc bias voltage were
255 Wand -227 V, respectively.
Table I shows how variations in oxygen flow rates affect
film properties at target powers of 5 and 3 kW. At 5 kW,
variations in oxygen flow rates from 27.4 to 29.4 seem result
ed in hardness and refractive index variations on the order of
2.5%. Film stress varied by 6%. In all cases, the stress was
found to be compressive. At 3 kW, target oxidation occurs
around 22.5 secm. Decreasing the oxygen flow rate from
22.0 to 19.2 secm produced variations in the refractive index
and hardness on the order of 0.5% and 0.4%, respectively.
The stress varied by 4%. On comparing the 3-and 5-kW
data, it is easy to see that the film property that changes the
most is the film stress which changes from 2.8 X 109 to
1.5 X 109 dyn/cm2. This could be a consequence of the
change in deposition rate from 140 to 220 nrn/min, respec
tively. It could also be caused by the resputtering processes
driven by the hollow cathode substrate holder. The ratio of
the resputtering rate to the deposition rate is higher for the
low deposition rate. Increasing the deposition rate should
therefore reduce the effects of res puttering. At 7kW, the de
position rate was -250 nm/min. The corresponding stress
was 1.9 X 109 dyn/cm2. Although this is a higher stress than
what is found at 220 nrn/min it is still much lower than the
stress found at the 140 nm/min deposition rate.
The effects of substrate bias are shown in Table II. At a
substrate bias voltage of 227 V, the substrate power is ~ 255
W. The change in film parameters when the bias voltage is
varied from -160 to -227 V are obviously small and
within the error of the measurements. (However, at lower
substrate voltages, e.g., -80 V, the deposited films were
dark.)
The pressure dependence is shown in Table III. When the
pressure was increased from 40 to 90 mTorr, the deposition
rate dropped from 218 to 159 nm/min. Associated with this
TABLE I. Dependence of stress, refractive index, and hardness on oxygen
flow rate Qo, and magnetron power, Pm •• , for several experimental runs.
v"Ub is the substrate dc dias voltage. Film stress is in units of dyn/cm2•
Q", Stre.<;.~ Hardness Pmag V:"ub Rate
Run (seem) (l.OX 10") N (kg/mm2) (kW) (V) (A/min)
185 29.4 1.5 1.70 808.2 5 -l60 2180
187 28.4 1.6 1.66 804.2 5 ·-160 2237
188 27.4 1.6 1.67 778.2 5 -160 2245
192 22.0 2.8 1.66 782.8 3 -160 l431
190 19.2 2.7 1.67 786.2 3 --160 1825
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TABLE It Dependence of stress, refractive index, and hardness on substrate
de bias voltage. v"ub' for several experimental runs. Film stress is in units of
dy/cm2.
V,uh Stress Hardness Pmag Rate
Run (V) (l.OX 10") N (kg/mm') (kW) CJl..!min)
185 ··160 1.5 1.70 808.2 5 2180
186 -227 1.5 1.70 821.1 5 2220
was a drop in film stress and an increase in the hardness. It is
well known for metals that the stress of sputtered films de
pends on pressure. At low pressures, the stress is usually
compressive. At high pressures, the film exhibits tensile
stress. The dependence is believed to be related to the kinetic
energy of the sputtered atoms and reflected neutrals that
arrive at the substrate. In our experiments, the large target
substrate separation and high pressures (40 mTorr or
greater) almost certainly guarantee the thermalization of
the sputtered aluminum atoms and reflected argon neutrals
before they reach the substrate.7 However, the hollow cath
ode substrate holder produces ions that are accelerated
towards the substrate with an energy of 160 eV. Increasing
the pressure from 40 to 90 mTorr should produce more gas
phase collisions between the argon ions and the neutral ar
gon atoms. Consequently, the argon ions probably arrive at
the substrate with reduced kinetic energy. The mean free
paths of an argon atom at 40 and 90 mTorr are about 1.25
and 0.5 mm, respectively. If the thickness of the plasma
sheath above the substrate is ~ 1 cm, then at 40 and 90
mTorr an argon ion experiences 8 and 20 collisions, respec
tively, as it is accelerated from the plasma to the substrate by
the 160-V potential difference. The argon ions arriving at the
substrate would therefore have a much lower kinetic energy.
This could be the cause of the lower stress in the alumina film
at low high pressures.
Finally, Table IV shows how film properties change for
cooled and uncoaled substrates. The temperature of the
cooled substrate is on the order of 23°C. The uncooled sub
strate is labelerd hot. It is estimated that the uncooled sub
strate may reach temperatures as high as 200 cC during the
deposition. The stress and hardness of the hot substrate are
clearly quite different from those of the cooled substrate for
otherwise similar operating parameters. The hot substrate,
in combination with substrate resputtering, could give rise to
a denser and consequently a harder film. The refractive in
dex and density of sapphire or a-Al20) are 1.765 and 3.97 g/
cm3• Microcrystalline y-AI20J has a density which varies
between 3.5 and 3.9 g/cm3 and a refractive index of 1.7 (see
Ref. 9). Hence, the higher refractive index and hardness of TABl.E IV. Dependence of stress, refractive index, and hardness OIl sub
strate temperature T, for several experimental runs. Film stress is in units of
dyn/cm2.
T Stress Hardness Pmag V<;:.:o Rate
Run eel (l.Ox 109) N (kg/mm2) (kW) (V) (A/min)
185 23 1.5 1.70 808.2 5 --160 2180
193 Hot 2.6 1.74 937.1 5 -160 2220
the uncooled film may indicate that it may contain a signifi
cant amount of the a-A1203 phase. The crystallinity of the
deposited films was not examined. The high stress might also
be due to the difference in thermal expansion between the
alumina film and the silicon substrate.
The above results can be summaraized as follows: Clear
stoichiometric alumina films can be deposited at a depo
sition rate of220 nm/min (13.2,um/h) on cooled substrate.
The nominal operating parameters of the deposition are car
ried out at a magnetron power of 5k W, an argon pressure of
40 mTorr, an oxygen flow rate of28.9 sccm, and a substrate
temperature of 23 "C. The target-substrate separation is 197
mm and the hollow cathode aperture size is 100 mm. The
power and voltage of the hollow cathode electrode are 240 W
and -160V, respectively. Allowing the oxygen flow rate to
vary by ± 0.5 sccm and or the substrate bias vary between
-160 and -227 V produced the following results for a
group of 12 substrates. The nominal values of the refractive
index, stress, and Knoop microhardness were 1.68, 1.5 X 109
dyn/cm2, and 813 kg/mm2. The standard deviations were
0.017, 0.135 dyn/cm2, and 25 kg/mm", respectively. The
oxygen flow rate is controlled by a mass flow controller. The
mass flow controller does not let the flow rate vary by more
than 0.1 secm about the operating point (i.e., 18.9 sccm).
Thus a range of ± 0.5 sccm is very wide with respect to the
precision to which the oxygen mass flow controller can con
trol the oxygen flow rate. Increasing the pressure to 90
mTorr causes a decrease in the deposition rate. It also caused
an increase in the micro hardness of the film. Deposition on
uncooled substrates causes large increases in film stress and
microhardness.
V.SUMMARY
We have shown that transparent aluminum oxide films
can be deposited (with good uniformity ± 6%) over large
areas (82-mm-diam substrates) at rates exceeding 2200 AI
min using a new reactive sputter deposition technique. In
deed, deposition rates as high as 5200 A/min were achieved.
The depositions were made without moving the substrate. In
TABLE III. Dependence of stress, refractive index, and hardness on argon pressure for several experimental runs. Film stress is in units of dyn/cm2.
Pressure Stress Hardness Pmag ~<;:.iO Rate
Run (mTorr) (I.OX \09) N (kg/mm2) (kW) (V) (A/min)
185 40.4 1.5 1.70 808.2 5 .-160 2180
189 90.l 1.1 1.66 856.1 5 -160 1591
J. Vac. Sci. Techno!. A, Vol. 7, No.3, May/Jun 1989
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.113.126.253 On: Tue, 25 Nov 2014 21:40:161247 F. Jones and J. logan: High-rate reactive sputter deposition of aluminum oxide 1247
order to do this, a new hollow cathode substrate holder was
designed and built into the sputter deposition system. The
main advantages of the hollow cathode substrate holder are
that 0) it produces an intense localized plasma which ex
cites the oxygen and hence promotes the reaction between
the aluminum and oxygen atoms at the substrate and Oi) it
concentrates the oxygen entering the system in the vicinity of
the substrate holder. The general effects of system pressure,
substrate dc bias voltage, substrate temperature, and target
substrate voltage were described. Materials properties such
as refractive index, stoichiometry, microhardness, and stress
were measured as a function of system parameters. A pro
cess window was found over which clear stoichiometric
A1203 films could be made. These results were achieved
while sputtering the target in its metallic state and using an
oxygen flow rate which was at least 1 scem below the target
transition point.
ACKNOWLEDGMENTS
We would like to acknowledge Hollavanhall S. Nagaraj
and Benal Owens for measuring the hardness of the films
J. Vac. Sci. Technol. A, Vol. 7, No.3, May/Jun 1989 shown in this paper and for useful discussions. We also ac
knowledge Henry Grabarz. John Costable, and Jim Lucy for
technical support during the setup of the vacuum system.
We thank Frank Cardone for the electron microprobe me
surements.
'G. Este and W. D. Westwood, J. Vac. Sci. Techno!. A 2,1238 (1984).
2M. Scherer and P. Wirz, Thin Solid Films 119, 203 (1984).
3p. Shinoki and A. Itoh, J. App!. Phys. 46, 3381 (1975).
4J. Affinito and R. R. Parsons, J. Vac. Sci. Techno!. A 2, 1275 (1984 l.
5D. H. Grantham, E. L. Paradis, and D. J. Quinn, J. Vac. Sci. Technol. 7,
343 (1970).
OF. Jones, J. V&;. Sci. Techno!. A 6,3088 (1988).
7J. Thornton and A. Penfold, in Thin Film Processes, edited by J. Vossen
and W. Kern (Academic, New York, 1978), p. 106.
g Engineering Properties of Selected Ceramic Materials, edited by J. Lynch,
C. Ruderer, and W. Duckworth (American Ceramic Society, Columbus,
OH,1986).
9CR C Handbook of Physics and Chemistry (CRC, Boca Raton, PL, 1986).
lOA. Nyaiesh and L. Holland, Vacuum 31,315 (1981).
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1.100817.pdf | Lownoise thinfilm TlBaCaCuO dc SQUIDs operated at 77 K
R. H. Koch, W. J. Gallagher, B. Bumble, and W. Y. Lee
Citation: Applied Physics Letters 54, 951 (1989); doi: 10.1063/1.100817
View online: http://dx.doi.org/10.1063/1.100817
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/10?ver=pdfcov
Published by the AIP Publishing
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On: Sat, 20 Dec 2014 14:15:27Lowanoise thinwfilm TIBaCaCuO de SQUIDs operated at 77 K
R. H. Koch, W, J. Gallagher, and 8. Bumble
IBM Research Division, T. 1. Watson Research Center, P. O. Box li8, Yorktown Heights, New York 10598
W, y, lee
IBM Research Division. Almaden Research Center, 560 Harry Road, San Jose, California 95120
(Received 4 November 1988; accepted for pUblication 4 January 1989)
We have made a series of single-level de superconducting quantum interference devices
(SQUIDs) from 4-j.lm~thick TIBaCaCuO tUrns with large grain sizes and operated them in
liquid nitrogen. Although device characteristics could not be precisely controlled, some devices
had white~noise levels that approached thermally limited noise above ~ 1000 Hz. In addition,
devices with 5 and 80 pH loop inductances had 1/ inoise levels at 10 Hz of2X 10-2'1 and
5 X 10-29 J/Hz, respectively. The noise levels at these frequencies are comparable to
commercial rfSQUIDs operating in liquid helium, but the hysteresis of the voltage-flux
characteristic of the high T, SQUIDs remains large.
dc superconducting quantum interference devices
(SQUIDs) have now been fabricated by a number of groups
from thin films of the high-temperature superconductors,
and several groups have reported devices that operate at 77K
and a few have also reported noise measurements. [-4 To
date, none of these devices has incorporated deliberately
made Josephson elements, but instead has relied on the
weak-Hnk~like nature of grain boundaries. When films with
sufficiently low critical current grain boundaries are used,
these same boundaries unfortunateiy offer very little resis
tance to vortex motion in the film, and this motion leads to
large amounts of hysteresis and low~frequency noise in the
device characteristics. While the problem is becoming ap
parent,S only a few systematic studies6 of vortex motion di
rectly related to SQUID performance are available.
In this letter we report on SQUIDs made from large
grained polycrystaHine TIBuCaCuO films.7 The devices op
erated wen at 77 K yet displayed large amounts of hysteresis
in the flux-to-voltage transfer function. The 1/ inoise was
appreciably less than any other previously measured high T"
SQUIDs.",9 We have in favorable cases obtained devices
comprised of just a few grain boundaries and have observed a
correlation between large grains with sharp "junction-like"
current~voltage curveslO•ll and good SQUID performance,
The TlBaCaCuO films used in this work 7 were fabricated in
a symmetrical rf diode sputtering system, The films used
were nominally 4pm thick on Y -stabilized ZrOz substrates,
and th.: best results were obtained from a predominantly
T12Ba2CazCu30y film with large terraced grains 10-40 pm
in size. Patterning was done with an argon ion mill through a
5-,um-thick coating of Shipley AZ4620 resist.s The dc
SQUID patterns used consisted of parallel lines connecting
two large superconducting regions forming the pads. The
TABLE L TlBaCuCuO SQUID parameters at 77 K.
Loop Link
size width L In
Device (lim)l (pm) (pH) (rnA)
A 47X47 11.5 -80 3.54
B 47X47 6.7 ,-80 0.57
C ~5X5 12.4 ~,5 1.70 film's resistive transition was predominantly at 120 K with a
small ( 10% ) tail extending down to 117 K before patterning
and down to -105 K after patterning.
We will discuss the results of three SQUIDs whose di
mensions and electrical parameters-se1f-inductance L,
maximum critical current 1o• dynamic resistance just above
the critical current R D, and peak values of the transfer func
tion av I ~-are given in Table I. Unlike SQUIDs A and B,
SQUID C was not fonned by the lithographically patterned
loop, but operated using a naturally occurring hole that was
optically visible in one link of the SQUID. The current-vol
tage (l-V) curves at 77 K for the SQUIDs are shown on Fig,
]. Comparing aU the SQUIDs we have tested, both
YBa2CU30y (YBCO) and TlBaCaCuO, we have found
what we call a "junction-like" to.ll 1-V curve, similar to B or
C with a sharp break at the maximum critical current, usual
ly results in better SQUID performance than a rounded
power~law type of /-V curve as SQUID A has and as is char
acteristic of high quality YBCO films. 12
Figure 2 plots the field-voltage curve SQUID B for two
ranges of field sweep. A field of about 3 X 10-3 T I A was
applied with a 300-tum copper coil of mean radius 7.5 mm
whose center plane was located 13-15 mm from the
SQUIDs. The measured periodicities for SQUIDs A and B,
500 !-lA, are consistent to -30% with an estimate based on
the size of their loopso The period of SQUID C, 5 rnA, is too
large to be from the lithographically defined loop, but is con
sistent with the hole in one of the links that was approxi
mately 5 pm in size.
Most high Tc SQUIDs that we have studied have flux
voltage curves that display a relatively large amount ofhys
teresis and many show a pronounced local minimum in the
voltage near zero field when large field sweeps are applied,
RD aVIiJ<t> S<:p (l kHz) E( 1 kHz) 9ksTLIR
(H) (,;;:VI<P"l «P"I,jHz) (J/Hz) (J/Hz)
(J.on 5 IX 10 24
(UB 40 2X 10-5 ! X 10>9 6XlO-lG
(U99 > 100 2X 10-6 2X 10 1() 2><10-31
951 AppL Phys, Lett. 54 (10), 6 March: 989 0003-6951/89/100951-03$01.00 @ 1989 American Institute of Physics 951
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On: Sat, 20 Dec 2014 14:15:27800
-----. 400 ~
~ .........
<I) 0 O'l
0 ->-' (5 >
-400
-800 '-------'-----'------'--------'
-8 -4 a 4 8
Bias Current (mA)
FIG. t. Current-voltage characteristics lor SQUIDs A, lJ, and Cat 77 K.
such a..<; the 3% dip evident in Fig. 2(b). We tentatively
ascribe this local minimum as due either to the fiux flow
voltage from additional vortices that penetrate the films
away from zero field or to Josephson effects from small loops
elsewhere in the link structures. We have observed a general
trend in the flux-voltage curves of many SQUIDs that as the
temperature is increased, the amount of hysteresis increases.
This can be explained by examining the effects of the lack of
perfect shielding in the film areas making up the SQUID and
the contact pads. Reduced shielding indicates some vortices
are entering the films, most likely along the grain boundar
ies. For example, as the applied field is reduced to zero from
H> 0, the sign of trapped vortices or magnetization will re
flect the direction of the previously applied field and these
vortices will apply a net field through the SQUID loop and
links that is opposite in sign to the previously applied field.
Hence in this example, the voltage at H = 0 will reflect the
SQUID voltage with a net negative fiux linking the SQUID,
as is clearly seen in Fig. 2(b). This effect, figuratively caned
"magnetic antibacklash," depends on the size and frequency
of the applied fiux sweep.
Figure 3 plots (a) the transfer function av la<l> mea
sured at 29 Hz, (b) the voltage noise power S" at 100 Hz, (c)
the slope of the 1/ f-like noise a log S"IJ log/at 100 Hz, and
(d) the flux noise powerSq, at lOOHzforSQUIDBat77K.
SQUIDs A and C displayed very similar behavior. In some
aspects the shapes of these curves are very similar to {ow Tc
SQUIDs, in that the transfer function has a maximum where
the flux noise power shows a minimum. On the other hand,
the 1/ / voltage noise power is almost independent of the
applied flux, which is rather unusual. The 1// flux power
spectrum S4;> of all the TlBaCaCuO SQUIDs measured
scaled as (aV IJc'J)) -2 over the entire range of measurements
in the case where the flux was swept as in Fig. 3, when the
bias current was swept, and when each SQUID was modula
ted. The 1/ /voltage noise power as a function of increasing
bias current shows a rapid rise as the critical current is
reached and then continues to slowly increase at higher cur
rents.
We have compared in Fig. 4 the uncoupled energy reso
lution E( j) -=S4;> 12L of these three TlBaCaCuO SQUIDs
952 Appl. Phys. Lett., Vol. 54, No. 10,6 March 1989 24 t'
I \
20 L'~{, /_;----. 'V\(J , 16 I ' I \
, I \ I
\1 1'\_ /'
~ 12 (0) SQ;;;-0J
Q.) 8 0>
0 -1.2 -0.6 0.0 0.6 1.2 .... g
Q.) 125.0
.~ /~ ....,
" ~ 122.5 ---->/_~J I
et:: r ,
\ 1\/ r
120.0 fo~'A,/\{''-~ I
\ I /\;/ \ t
117.5
(b) SQUID 8\/ . 1 <c------
115.0
-4 -2 0 2 4
Applied Field (mA)
FIG. 2. Field-voltage characteristic for SQUID B at 77 K measured during
two diftcrent cooldowns. The field was sinusoidally swept with the arrows
indicating the direction of the field sweep. (a} The bias current at 0.60 rnA.
(h) The bias current at 1.03 mA,
operating at 77 K with an YBCO SQUID operating at 77
K8,'} and three readily available types of niobium-based
SQUIDs operating at 4.2 K. 13-·15 For the high T< SQUIDs in
this figure we only used data from regions where the transfer
~ : I <J'I . . ~'- ~UID 'J
~ /" -----~-
-10 -.-"~------'----~--
r~I~~~-~1
~·~:[~~~~~l
~ ::~r----'--\-, ~'::7l }
~ ,;:~:~ ~ ~-j
3.00 3.25 3.50 3.75 4.00
Applied Flux (rnA)
FIG. 3, (a) Unmodulated transfer function, (b) the voltage noise power at
100Hz, (c) the slope of the noise at 100Hz, and (d) the flux noise at lOO Hz
of SQUID B at 77 K. The period for these measurements was 770 /J,A be
cause the field coil was moved relative to the SQUID. While two peaks in
the transfer function can be seen, the SQUID was obviously very asymmet
rical.
Koch eta!. 952
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On: Sat, 20 Dec 2014 14:15:27~
N :::c 2: 10-28
10-29
10-30
10-31 YBCO DC, 77 K
,
.~ .. -.-'-.-- .. ---------"-
" IBM DC, 4.2 K '_ ... --.---~-------,._-----
10-32 -~--""~"-"~"-"~--
10-1 100 101 102 103 104
Frequency (Hz)
FIG. 4. Measured energy resolution of the three SQUIDs operating 77 K
compared to that of commercial rf and dc SQUIDs and to published results
for two IBM de SQUIDs all operating at 4.2 K. Data shown as dashed were
obtained using modulation schemes that reduce low-frequency noise from
critical current tluctuations. Also plotted is the energy resolution of a mod
ulakd YBCO SQUID at 77 K; the un modulated energy resolution was con
siderably higher.
characteristics were essentially periodic and symmetric,
such as in Fig. 2(a). The two upper most curves plot the
noise of SQUID A when operated in the samll signal or "di
rect" mode and in the "modulated" mode measured during
the same cooldown. In the modulated mode a 3 kHz sine
wave was used to modulate the flux in the SQUID through
± 1/4$0 or slightly less. The relative voltage noise powers
of aU the high Tc SQUIDs differ by far less than the relative
energy resolutions. The better performance of SQUIDs B
and C is mainly because these devices have larger transfer
functions than SQUID A. On the other hand, the ill-defined
quality of a grain boundary tha.t makes it suitable or not
suitable for SQUID operation is probably more relevant in
predicting the performance of a set of high Tc SQUIDs.
We can use the above data to ascertain what we can
about the source of the 1/ fnoise. Since Sq, is not indepen
dent of av liN> for large values of av la<I>, conventional flux
noise,<>·16.17 while probably present, cannot be the dominate
1/ fnoise source. The voltage noise power from critical cur
rent fluctuations would decrease for higher currents. This
was not observed as mentioned above. Secondly, since the
fiux power spectrum from critical current fluctuations from
any source is oniy weakly dependent on (JV la.:!> when the
transfer function is large,18 "conventional" critical current
fluctuations can also be ruled out. The order of magnitude
reduction in the noise ml shown on Fig. 4 for SQUID A, when
comparing direct to modulated results, has also been seen on
several YBa2CujOv SQUIDs. In our case, where flux noise is
953 Appl. Physo Lett., Vol. 54, No.1 0,6 March 1989 not a dominate source, an approximate factor of two reduc
tion is expected. The larger reduction seen can be attributed
to additive 1/ f voltage noise in the presence of a current
from thermally activated vortex motionl2 in the pads, links,
and other parts ofthe structure of the SQUID.
In this letter we have demonstrated low-noise operation
of high Tc SQUIDs in liquid nitrogen. For many applica
tions, an energy resolution of 1 X 10-29 J/Hz is adequate and
operation in liquid nitrogen would be a great advantage.19
This would, of course, mean fabricating a coupling structure
to these SQUIDs which has not been done yet. The hystere
sis in the flux-voltage curve represents a problem, but the
amount seen in Fig. 2 is greater than what would be present
in the usual modulation scheme where only a flux modula
tion of ± £4>0 is used. Modulation methods that work
around relatively large amounts of hysteresis can also be
imagined, Reducing film area and increasing film quality
should also reduce the problem.
We thank D. Bullock, V. Foglietti, R. B. Laibowitz,
V. Y. Lee, and J. R. Salem for assistance. This work was
partially supported by U. S. Office of Naval Research con
tract No. N00014-88-C-0439.
'R. H. Koch. C. P. Umbach, G. J. Clark, P. Chaudhari, and R. B.
Laibowitz, App\. Physo Lett. 51, 200 (1987).
2H. Nakane, Y. Tarutani, T. Nishinio, H. Yamada, and U. Kawabe, Jpn. Jo
Apr!. Phys. 26, Ll925 (1987).
'B. Hauser, M. Diegel, and H. Rogalla, App!. l'hys. Lett. 52, 844 (1988).
4R. Yusa, M. Nakao, S. Fujiwara. K. Kaneda, S. Suziki, and Ao Mizukami,
in Proceedings 5th International Workshop on Future Electron Devices-
High Temperature Supercollducting Devices (R. and D. Assoc. for 'Future
Electronic Devices, Tokyo, 1988), p. 225.
5R. H. Koch, C. P. Umbach, M. M. Opryski, J. D. Mallnhart, B. Bumble,
G. J. Clark, W. J. Gallagher, A. Gupta, A. Kleinsasser, R. B. Laibowitz,
R. B. Sandstrom, and M. R. Scheuermann, Physica C 153-155, 1685
( 1988).
OM. Jo Ferrari, M. Johnson, F. C. Welistood, J. Clarke, 1'. A. Rosenthal, R.
H. Hammond, and M. R. Beasley, App!. Phys. Lett. 53, 695 (1988).
7W. Y. Lee, V. Y. Lee, J. Salem, T. c:. Huang, R. Savoy, D. C. Bullock, and
S. S. 1'. Parkin, Apply. Phys. Lett. 53, 329 (1988).
gR. L. Sandstrom, W. J. Gallagher, To K Dinger, R. H. Koch, R. B.
Laibowitz, A. W. Kleinsasser, R. J. Gambino, B. Bumble, and M. F. Chis
holm, App!. Phys. Lett. 53,444 ( 199B).
"W. I. Gallagher. R. flo Koch, R. L Sandstrom, R. B. Laibowitz, A. Wo
Kleinsasser, R Bumble, and M. F. Chisholm, in Proceedings afthe First
International Symposium on Supercanductiuity-ISS'88 (Nagoya, Japan,
1988).
"'w. Co Stewart, Appl.l'hys. Lett. 12,277 (1968).
"D. E. McCumber, J. App\. Phys. 39, 3tl3 (1968).
'-'R. R Koch and W. J. Gallagher (unpublished).
I.lniomagnetic Technologies, Incorporated, San Diego, CA 9212L
,-IV. Foglietti, W. J. Gallagher, M. R Ketchen, A. W. Klein&asser, R. H.
Koch, So I. Raider, and R. L Sandstrom, App!. Phys. Lea. 49, 1393
( 1986).
"C. D. Teschc, K. H. Brown, A. C. Callegari, M. M. Chen, J. H. Greiner,
H. C. Jones, M. B. Ketchen. K. Ko Kim, A. W. Kleinsasser, H. A. No
tary" G. Proto, R. H. Wang, and T. Yogi, IEEE Trans. Magn. MAG-21,
lO32 (1985).
If'R. H. Koch and A. P. Malozemotr, in Proceedings aftheJirst International
Symposium on SuperconductilJity-ISS'88 (Nagoya, Japan, 1988)0
17R. H. Koch, J. Clarke, W. M. Gauball, J. M. Martinis. C. M.l'egrum, and
D. J. Van Harlingen, J.I,ow Temp. PhYR. 51,207 (1983).
I"R. H. Koch (unpublished) 0
I"J. Clarke and R. H. Koch, Science 242,217 (1988).
Koch et al. 953
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1.1140830.pdf | Development of a VUV/soft xray monochromator for undulator radiation at the Photon
Factory
Yasuji Muramatsu and Hideki Maezawa
Citation: Review of Scientific Instruments 60, 2078 (1989); doi: 10.1063/1.1140830
View online: http://dx.doi.org/10.1063/1.1140830
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/7?ver=pdfcov
Published by the AIP Publishing
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Construction of a New VUV/Soft Xray Undulator Beamline BL13A in the Photon Factory for Study of Organic
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AIP Conf. Proc. 1234, 709 (2010); 10.1063/1.3463308
Evaluation of a new VUV/soft xray toroidal grating monochromator with a movable exit slit
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A soft xray beam line (BL13C) at the Photon Factory with a CEM using undulator radiation
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Soft xray microscope at the undulator beamline of the Photon Factory
Rev. Sci. Instrum. 60, 2448 (1989); 10.1063/1.1140695
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130.102.42.98 On: Mon, 24 Nov 2014 04:42:31Development of a VUV I soft x-ray monochromator for undulator radiation
at the Photon Factory
Yasuji Muramatsu
NTT Applied Electronics Laboratories, Musashino, Tokyo 180, Japan
Hideki Maezawa
Photon Factory, National Laboratory jar High Energy Physics, Tsukuba, lbaraki 305, Japan
(Presented on 29 August 1988)
A VUV Is oft x-ray monochromator was developed for utilizing undulator radiation from a 26-
period multipole wiggler/undulator at the Photon Factory. An entrance slitless quasi-Rowland
circle mounting was adopted to the monochromator optics, aimed at achieving the compatibility
of high resolution with high-output flux. The optics was realized by a decoupling of horizontal
focusing with a deflection mirror from vertical focusing with an aberration-free vertical
dispersion system which was composed of a cylindrical mirror and a concave grating. The optics
and mechanism of the monochromator as well as its test operation made after installation are
described.
INTRODUCTION
The high brilliance ofundulator radiation has paved the way
for achieving the compatibility of high resolution with high
output flux in a grazing incidence optics of a synchrotron
radiation monochromator. I The narrow divergence proper
to the radiation from a many-period undulator makes it pos
sible to fully accept the radiation with small optical elements
and makes it easy to form an aberration-free optical system
even in the grazing incidence configuration. With a low
emittance beam of a storage ring operation, it is also possible
to achieve the high resolution even in an entrance sUtless
system which leads to the high-output flux of the monochro
mator.
However, a requirement for avoiding strong r rays
properly accompanied by the undulator radiation imposes a
boundary condition on the beamline optics.2 A branching of
the beamline with a horizontal deflection mirror is inevitable
for use of the VUV and soft x-ray radiation from the undula
tor. In addition, high-power density of the radiation also
imposes another constraint on the optical design of the
branch beamIine. The first deflection mirror should not take
part of vertical focusing because such an arrangement is a
most severe usage of a first mirror in view of the thermal
deformation effects. 3
The characteristic coherent property of the undulator
radiation leads us to adopt the vertical dispersion system in a
grating monochromator.4 A radiation cone in which pho
tons are coherent is as narrow as /;:7[, where A is the wave
length of interest and L is the length of the undulator.5 Tak
ing into account the profile ofthe stored electron or positron
beam, the vertical dispersion has an advantage in achieving
the high-output flux.
This work was an attempt to develop a high-resolution
high-output flux VUV 150ft x-ray monochromator for undu
lator radiation, clearing all the above constraints with a so
phisticatedly devised optics. This monochromator covers
the photon energy range of 40-600 eV, which includes the tunable range of the first harmonic peak of the undulator
radiation from a 26-period multipole wiggler/undulator at
the Photon Factory.6-9 In this paper, the optical and me
chanical design of the monochromator is described in detail
as wen as a test operation of the monochromator.
I. OPTICAL DESIGN
A. Optical arrangement
The design concept of the VUV Isoft x-ray monochro
mator is summarized as follows: (1 ) an entrance sUtless op
tics to achieve high total output flux, (2) decoupling ofhori
zontal focusing from vertical focusing to avoid undesired
high-order aberration, (3) horizontal focusing on an exit slit
by a deflection cylindrical mirror, (4) vertical dispersion
optics to obtain the high-output flux, (5) an aberration-free
dispersion optics to achieve high resolution, even in an asym
metric arrangement, and (6) a bent cylindrical mirror
placed in the 1: 1 configuration for refocusing of monochro
matized rays.
The optical arrangement of the monochromator is
shown in Fig. 1. The front mirror MO, placed 15 m distant
from the source center, is a cylindrical mirror made of SiC
for horizontal deflection and collection which focuses the
beam onto the exit slit S placed 24 m distant from the source
point. Its incidence angle is fixed at 87.0°. The plane mirror
Ml is to constantly deflect rays to the M2 mirror. The inci
denceangleofMl varies from 81.3° to 87.8°. The cylindrical
mirror M2 and the concave grating G constitute an aberra
tion-free vertical dispersion system which focuses the dif
fracted rays onto the exit slit. The bent cylindrical mirror
M3 placed in the 1; 1 configuration refocuses the monochro
matized rays to the sample position both horizontally and
vertically. Its incidence angle is fixed at 86.25°. The exit an
gle of diffracted rays from S was fixed at 7.50°, which was
determined to suppress higher-order harmonics of undula
tor radiation most effectively by considering the reflectivity
ofM! and M3.
2078 Rev. Sci. Instrum. 60 (7), July 1989 0034-6748/89/072078-03$01.30 @ 1989 American Institute of Physics 2078
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130.102.42.98 On: Mon, 24 Nov 2014 04:42:31UR
1111118 metor
FIG. 1. Schematic drawing of the monochromator. MO, a cylindrical deflec
tion mirror for horizontal collection and focusing onto the exit slit; MI, a
plane mirror for dellection to M2; M2, a cylindrical mirror; G, a concave
grating; S, the exit slit; and M3, a bent cylindrical refocusing mirror.
B. Aberration~free dispersion optics
In order to achieve high resolution, the diffracted rays
must be weH focused on the exit slit. To satisfy the require
ment for the entrance sUtless system of the asymmetric con
figuration, an aberration-free dispersion optics shown in Fig.
2 was devised. The system composed of a cylindrical mirror
and a concave grating corrects vertical coma aberration by
satisfying second-order focusing conditions at the exit slit.
The idea of this aberration-free optics is basically as same as
that of the optical system consisting of two concave mir
rors. to
The conditions were derived in the following way. As is
defined in Fig. 2, we suppose a forward ray emitted from the
source point to the cylindrical mirror with small devergence
angle (j and a backward ray emitted from the focal point to
the concave grating with small divergence angle €. The fo
cusing conditions are derived from the matching of the for
ward ray and the backward ray to the degree of second order
for a and € both in their positions and directional cosines at
the matching point between the mirror and grating. When
the relative position of the mirror and grating is fixed, there
is only one solution of the conditions corresponding to one
diffraction angle. Therefore, it is necessary to optimize at
one arbitrary diffraction angle or wavelength.
The best optical parameters were determined from the
focusing conditions, by optimizing the system at 530 eV to
realize the high-energy resolution (E / AE) of 2000 at the
oxygen K-absorption edge. The initial parameters imposed
FIG. 2. Optical system composed of a cylindrical mirror and a concave grat
ing. Rm and R", radii of curvature of the mirror and the grating; (I., iJ}ci
dence angle of both the mirror and the grating; (3, diffractioll angle; and y"" ,
r'nZ' rgl, and rg2, lengths from a source point to the mirror, from the mirror
to a matching point, from the matching point to the grating, and the grating
to a focal poillt.
2079 Rev. Sci. Instrum., Vol. 60, No.7, July 1989
•.•••••.•.•.• -•.••••• ,' ......... -••• -•••• ' ••••••• :.:.:-:.;.; ••••• ,' •.• :.:.:.:.; •••••••• ~.' •.• :.:.:.:.:.;.~ ...... :.:.:.:.:.:.:.; ••••• ;>".~.:.:.;.:-: ••• ; •••• ~ ••• :.:.:.:.:.: •••••• ';' :.:.;.;.: ••••••• ,'.: •.• :.:.:.: •••••••• ~ ••••••• ' •••.• ~.-~.,. ,_ •• FIG. 3. Spot diagrams of ray tracing
simulations for diffracted rays from
80 to 600 cV with a 2400-
Iin.:s mm -I grating.
by spatial constraints were a = 87.0°, Rg == 2000 mm, and
r m; = 22.0 ffi_ The other optical parameters were deter
mined automatically with the initial parameters. These pa
rameters were Rm = 1795.8 mm, 'm2 =,47.092 mm, and
r 1 = 78.762 mm. The diffraction angle {J varies from 74.0°
t~ 87.0·, and 'g2 varies from 585 to 156 mm, during the scan
ning from 40 eV to the zeroth-order position using a 1200-
lines mm -1 grating.
Co Ray tracing simulations
The realistic optical arrangement of the monochroma
tor was checked by ray tracing simulations. The ray tracing
software developed at the Photon Factory was used.l' Fig
ure 3 shows the spot diagrams of the simulations for diffract
ed rays from 80 to 600 e V when a 2400-lines mm -I grating
was supposed. The source size of ax = 0.655 mm and
O'y = 0.125 mm was also supposed. The image plan~ was set
on the exit slit. The spot diagrams show that the dIffracted
rays can be well focused vertically (along the z axis) satisfy
ing quasi-second-order focusing conditions and the verti~al
size of all the focal spots are less than 25 f-llU over the entlre
photon energy range. This result indicates that an energy
resolution of 2000 will be achieved at 500 eV when a 2400-
lines mm-j grating and a lO-f-lm slit are used.
II. MECHANICAL DESIGN
Ao Scanning mechanism
A mechanically linked system of MI, M2, and G was
designed to achieve the required movement for the optical
elements with the desired accuracy. In Fig. 1, MI, M2, and
G are mounted on a table and their relative positions are
fixed. Centers ofMl and G move along a horizontal straight
Ml rail and an inclined straight G rail, respectively. The
grating is automatically rotated through the translation, in
such a way that the relation between the translation length
and the diffraction angle satisfies almost all the focusing con
ditions. In addition, Ml is automatically rotated to deflect
rays onto M2 by a half-angle rotation mechanism which re
sembles that of the Grasshopper. 12 The wavelength scanning
can be simply carried out only by the translation of G. The
mechanically linked scanning mechanism of this monochro-
VUVand x-ray optics 2019
, .... -,-.-., •.•. '.' ... ~ ..... ;.; ... ; ....•.. , .. ~.:.:.:-;.: ....•.•.•. :.-.:.:.:.:.:.,. ...... ;-..... ~.: •.•...•..... ~.-..... -.-....•.•.. ,~ .. , ... -.•.. '.' ...•.
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130.102.42.98 On: Mon, 24 Nov 2014 04:42:31VACUUM' • ATMOSPHERE
UR outside wall
.• _() of a chamber // . --fD /
flexible-shaft micrometers
FIG. 4. Optical aligning system composed of a knife edge and a photocath
ode plate. The knife edge and the photocathode plate can be controlled from
the outside of the monochromator chamber by flexible-shaft micrometers.
mator is basically the same as that of the lO-m grazing inci
dence monochromator at the Photon Factory_ 13
B. Optical aligning system
The optical aligning system shown in Fig. 4 was devised
to easily adjust the geometric positions of the optical ele
ments without breaking vacuum of the monochromator.
The system consists of a knife edge and a photocathode
plate. The knife edge can be moved up and down against the
optical element surface with the minimum space of 100 pm
between them. The knife edge also takes a role of a dia
phragm. The photocathode plate placed just after the optical
element can also be moved up and down. When the photo
cathode plate is moved down, the plate is across an optical
axis. The systems were equipped to MI, M2, and G holders,
respectively.
The optical elements can be aligned in the following
way. The knife edge and the photocathode plate are moved
down. Then, the optical element is aligned by using the flexi
ble-shaft micrometers to make photocurrent detected by the
photocathode plate maximum. After the optical element is
precisely aligned, the knife edge and the photocathode plate
are moved up.
C. Test operation
Undulator radiation was measured by the monochro
mator, and its preliminary performance was evaluated, Dur
ing the measurement, the stored current of positron beam
was about 250 rnA and the magnetic gap of the multipole
wiggler lundulator was set to about 70 mm. A gold mesh
mounted after the exit slit was used as a photocathode, and
its photocurrent was monitored. Figure 5 shows the mea
sured spectral response of the monochromator over the pho
ton energy range from 40 to 300 eV with a replicated 1200-
lines mm---1 grating and a 50-{lm exit slit. The higher-order
harmonics up to the fourth were observed.
Although a large amount of stray component by scatter
ing was observed in the higher photon energy region due to
the radiation damage of the grating surface, 14 these mea.<;ure
ments confirmed that the entrance slitless quasi-Rowland
2080 Rev. Sci. lnstrum., Vol. 60, No.7, July 1989 :. / :./ ;' .
o '50 2nd
~
:.
. 3rd 4th . , ~ .~~
250 300
FIG. 5. Spectral response of the monochromator for the undulator radiation
over the photon energy range from 40 to 300 eV. A replicated 1200-
lines mm -1 grating and a 50-flm slit were used.
circle mounting monochromator had a possibility of achiev
ing both high-resolution and high-output flux.
m. CONCLUDING REMARKS
Further improvements are necessary to make full use of
the monochromator. Thermal resistant gratings such as
master gratings made of SiC should be developed, The repli
cated gratings used could not withstand the high-power den
sity of the undulator radiation. In addition, the devised op
tics will be much more improved by adoption of an aspheric
grating or a variable-pitch grating in order to make the focal
spot of diffracted rays smaller over the entire photon energy
range.
ACKNOWLEDGMENTS
The authors wish to express their thanks to Professor
Takeshi Namioka of Tohoku University for his helpful dis
cussions and to Professor lunichi Chikawa of the Photon
Factory for his encouragement.
'll. Maezawa, A. Toyoshima, Y. Kagoshima, K. Mori, and T. Ishikawa,
these proceedings.
'H. Maezawa, M. Ando, T. Ishikawa, M. Nomura, H. Kitamura, A. Mi
kuni, and T. Sasaki, Proc. SPIE 733,96 (1986).
'R. Maezawa, S. Sato, and A. Iijima, these proceedings.
4T. Miyahara, Jpn. J. App!. Phys. 25,1672 (1986).
3K. J. Kim, Nue!. lnstrum. Methods A 246,71 (1986).
6T. Shioya, S. Yamamoto, S. Sasaki. M. Katoh, Y. Kamiya, and H. Kita
mura, these proceedings.
7T. Koide, S. Sato, N. Kanaya, and S. Asaoka, these proceedings.
"T. Matsushita, H. Maezawa, T. Ishikawa, M. Nomura, A. Nakagawa, A.
Mikuni, Y. Muramatsu, Y. Salow, T. Kosuge, S. Sata, T. Koide, N. Ka
naya, S. Asaoka, and I. Nagakura, these proceedings.
"H. Maezawa, Y. Muramatsu, T. Shioya, S. Yamamoto, and n. Kitamura,
these proceedings.
\lIT. Namioka, H. Noda, K. Goto, and T. Katayama, Nuc!. lustrum. Meth
ods 208,215 (1983).
1 Iy' Muramatsu, Y. Ohishi, and ll. Maezawa. Jpn. J. App!. Phys. 27, Ll539
(1988) .
12F. C. Brown, R. Z. Bachrach, S. B. M. Hagstrom, N. Lien, and C. H.
Pruett; Vacuum Ultraviolet Radiation Physics (Pcrgamon-Vieweg,
Braunschweig, 1974), p. 785.
"n. Maezawa, S. Nakai, S. Mitani, H. Noda, T. Namioka, and T. Sasaki,
Nuc!. lustrum. Methods A 246,310 (1986).
145. Mitani, T. Namioka, M. Yanagihara, K. Yamashita, J. Fujita, S. Mo
rita, T. Harada, T. Sasaki, S. Sato, T. Miyahara, T. Koide, A. Mikuni, W.
Okamoto, and H. Maezawa, these proceedings.
VUV and x-ray optics 2080
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1.342929.pdf | Resonant tunneling bipolar transistors using InAlAs/InGaAs heterostructures
T. Futatsugi, Y. Yamaguchi, S. Muto, N. Yokoyama, and A. Shibatomi
Citation: Journal of Applied Physics 65, 1771 (1989); doi: 10.1063/1.342929
View online: http://dx.doi.org/10.1063/1.342929
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/4?ver=pdfcov
Published by the AIP Publishing
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160.36.178.25 On: Fri, 19 Dec 2014 22:47:42Resonant tunneling bipolar transistors using InAIAs/lnGaAs
heterostructures
T. Futatsugi, Y. Yamaguchi, S, Muto, N. YOKoyama, and A Shibatomi
Fujitsu Laboratories Ltd., 10-1 Morinosato- Wakamiya, Atsugi 243-01, Japan
(Received 3 August 1988; accepted for pUblication 13 October 1988)
Resonant tunneling bipolar transistors (RBTs) using InAIAs/lnGaAs heterostructures were
fabricated. These devices are bipolar transistors which use a resonant tunneling barrier as a
minority-carrier injector. The RBT exhibits a collector current peak as a function of the base
emitter voltage at room temperature. The peak-to-vaney ratio of the collector current is 3.5,
and the peak collector CUI'rent density is 'j,7X 104 A/cm2, The common-emitter current gain
reaches a value of 24. These InAIAs/InGaAs RBTs characteristics are much better than those
of AIGaAs/GaAs RBTs. We measured the microwave characteristics of the InAIAs/InGaAs
RBT at room temperature, and obtained a cutoff frequency of 12.4 GHz. An equivalent circuit
analysis and device simulation yielded an estimated resonant tunneling barrier response time of
1.4 ps.
I. INTRODUCTION
In recent years, resonant tunneling structures, such as
InAlAs/lnGaAs (Ref. 1) and AIAs/lnGaAs,2 are being
studied along with AIGaAs/GaAs (Refs. 3-5) and AIAsl
GaAs (Ref. 6) heterostructures. InAIAs/lnGaAs and
AIAs/lnGaAs resonant tunneling diodes exhibit a pro
nounced negative differential resistance (NDR) at room
temperature. Several three-terminal resonant tunneling de
vices,7-10 such as the resonant tunneling hot electron transis
tor (RHET), i I, i2 have been proposed and fabricated. These
devices are attracting much interest as new functional de
vices.
In 1986, we proposed a resonant tunneling bipolar tran
sistor (RBT),13 which has a resonant tunneling barrier in
the base-emitter junction. This structure overcomes the
RHET drawbacks, which are poor current gains and large
base-collector leakage current at room temperature. These
drawbacks originate from the RHET's collector potential
barrier in the base-collector junction. Since the RET has a
p-n junction instead of a collector potential barrier as a base
collector junction, we expect high current gains at room tem
perature operation. However, the AIGaAs/GaAs RBTs
that we fabricated exhibited no NDR at room temperature
and exhibited only a small NDR at 77 K. i3
To improve this, we fabricated RET!! using InAIAsl
InGaAs heterostructures. Such heterostructures have ad
vantages over the AIGaAs/GaAs heterostructure. First, the
electron-effective mass of the barrier layer InAIAs is as low
as 0.075. We can thus expect an extremely high tunneling
current density. Second, InAIAs is a direct gap material
Therefore, we do not have to worry about band mixing
which occurs for indirect gap AIGaAs barriers. 14 Third, the
InGaAs layer can be heavily doped with Si. This decreases
the RBT's parasitic resistance.
In this paper, we describe RBT fabrication using
InAlAs/InGaAs heterostructures, and report on its electri
cal characteristics. We discuss the response time of the reso
nant tunneling barrier by using analysis of an equivalent cir
cuit and device simulation. II. EXPERIMENT
Figure 1 diagrams the schematic cross section of our
InAlAs/lnGaAs RET. The Ino.s2 Alo.4s As and
lIlo.s3Gllo.47As epitaxial1ayers were grown on a (100) ori
ented semi-insulating loP substrate by molecular-beam epi
taxy (MBE) at a temperature of 470"C. The emitter layer
was doped with Si to a concentration of 1 X 10ill cm-3• The
base layer is 150 nm thick, and is doped with Be to 5 X 1018
cm O~ 3. The collector layer is 300 nm thick, and is doped with
Si to 1 X 1017 cm -3. The resonant tunneling barrier consists
of a 3.8-nm InGaAs layer sandwiched between 4.4~nm
lnAlAs barriers. Undoped 1.5-nm-thick InGaAs spacer lay
ers were formed on both sides of the resonant tunneling bar
rier. Assuming that the conduction-band discontinuity
between InAIAs and InGaAs is 0.53 eV,15 the first resonant
level of electrons is estimated to be at 175 meV. A 4O-nm 11.
type InGaAs layer was introduced between the resonant
tunneling barrier and the base layer to reduce hole injection
from the base to the emitter and to reduce the excess current
caused by Be diffusion.
RBTs were fabricated by the following procedure. First,
wet etching was used to form the emitter, base, and collector
mesa areas. Next, AuGel Au collector contact metals were
Inl' substrate c::::=I InGsAs
" ... ·,""M. InAIAs
FIG.!. Schematic cross section of the InAIAs/InGaAs RET.
1771 J. Appl. Phys. 65 (4), 15 February 1989 0021-8979/89/041771-05$02.40 © 1989 American Institute of Physics 1771
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160.36.178.25 On: Fri, 19 Dec 2014 22:47:42(a)
(b)
(el FIG. 2. Schematic band dia
grams of the RET. (a) VBE = 0
V. (b) VBE is increased but is less
than the built-in voltage of the p
n junction. (c) VSE is increased
further,
evaporated and alloyed at 400 ·C. After that, the AuZnl Au
base contact metals were evaporated and alloyed at 350°C.
Finally, Crl Au nonaHoyed ohmic contacts were formed for
the emitter electrodes.
Figure 2 gives the schematic band diagrams, and reflects
the operational principle of the device. (a) When the base
emitter voltage is zero, the resonant tunneling barrier is in
the neutral region adjacent to the p-n junction. (b) When the
base-emitter voltage increases, the potential difference
across the p-n junction also increases, and near-fiat-band
conditions are achieved. (c) When the base-emitter voltage
is increased further, a potential difference develops across
the InAIAs barriers, and hot electrons are injected due to
resonant tunneling. However, when the base-emitter voltage
exceeds a certain value, the resonant tunneling condition is
not satisfied, and electrons cannot be injected into the base
layer. We thus expect a collector current peak as a function
of the base-emitter voltage.
lit RESULTS AND DISCUSSION
A. Static characteristics
The InAIAs/lnGaAs RBT exhibited a collector current
peak at room temperature. Figure 3 graphs the collector and
base currents as functions of the base~emitter voltage with a
16 Emitter: 2.31Jm x 8.611m
VeE == 2.0 V <" 12 .§ -c S
~ ".
(,) 4
Base
0
0 0.4 0.8 1.2 1.6
Base-emitter voltage (V)
FIG. 3. Collector and base currents as functions of the base-emitter voltage
with a constant collector-emitter voltage of 2.0 V measured at room tem
perature.
1772 J. Appl. Phys., Vo!. 65, No.4, 15 February 1989 30 VeE == 2.0 V
Ii: 'iii m 20 C
0\1 ... '-::. 10 (,)
o L-__ -L ____ J-____ L-__ ~
(I 0.4 0.8 1.2 1.6
Base-emitter voltage (\I)
FIG. 4. Common-emitter small signal current gain measured as a function
of the base-emitter voltage.
constant collector-emitter voltage of 2.0 V measured at
room temperature. The base-emitter junction is 2.3 X 8.6
f.tm2• There is a collector current peak. at around 1.2 V due to
electron resonant tunneling. The peak-to-valley ratio is 3,5,
and the peak current density is S.7x 104 A/cm2• These char
acteristics of the InAIAs/lnGaAs RBT at room tempera
ture are much better than those of the AIGaAs/GaAs RBT
at 77 K,13
Figure 4 shows the common-emitter small signal cur
rent gain as a function of the base-emitter voltage. The cur
rent gain reached 24 at around 0.8 V. The base~emitter junc
tion is a homojunction. However, a high current gain is
obtained for several reasons. One is the small tunneling
probability of holes through the resonant tunneling barrier.
Another is that the injection current of holes from the base to
the emitter and the excess current of resonant tunneling are
reduced by introducing an n-type InGaAs layer between the
resonant tunneling barrier and the base layer.
Figure 5 shows the collector current-voltage character
istics of the RBT for the common-emitter configuration with
the base current as a parameter. When the base current is less
than 0.8 mA, the RBT operates like a conventional bipolar
transistor. However, when the base current is more than 1.0
rnA, the RBT exhibits unique characteristics. For example,
for a base current of 1.0 rnA, the collector current changes
the path from A to D as a function of the collector voltage,
indicating that the collector current decreases abruptly at
<' 16
E .... .... 12 c: e ... ..
::!
I,) 8 ..
0 ... 0.2 u .! 4
'0
U III = 0.0 mA
1 2 3 4 5
Collector-emitter voltage (V)
FIG. 5. Collector 1-V characteristics of the RET for the common-emitter
configuration with base current III as a parameter. When IBis more than 1.0
rnA, Ie is decreased abruptly at around VeE of 0.6 V.
Futatsugi at a/. 1772
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160.36.178.25 On: Fri, 19 Dec 2014 22:47:42FIG, 6. Collector current and base current measured as functions of the
base-emitter voltage oftne RBT used for microwave measurement. The de
vice has a4X 14.5 pm2 base-emitter junction and a 16X i7.5j.tm2 base-col
lector junction.
around VeE of 0.6 V due to resonant tunneling. This is be
cause the base-emitter voltage changes markedly in the satu
ration region, even if the base current is constant. We mea
sured the static characteristics of the RBT at 77 K. The
collector current exhibited a pronounced peak as a function
of the base-emitter voltage, with a peak-to-valley ratio of
11.4 and a peak current density of 5.8 X Ht A/cm2• The
maximum current gain reached 42.
B. Microwave characteristics
We measured the room-temperature microwave char
acteristics ofthe RBT. The device has a 4X 14.5 p.m'J. base
emitter junction and a 16 X 17.5 p.m'J. base-col1ector junction.
Figure 6 shows one of the static characteristics of the device.
The collector current exhibited a peak with a peak-to-valley
ratio of 2.3. The peak current was 26.6 rnA. The small signal
current gain reached 26 at a base~emitter voltage of 0.8 V.
Microwave measurements were performed using an HP851 0
network analyzer with a Cascade Microtech probe station.
The common-emitter current gain h21 was calculated from
the measured S-parameter data. The dependence of IT on the
Ie for a collector-emitter voltage VeE of 1.8 V is plotted in
Fig. 7. Initially, IT continues to increase with Ie. This is
because the differential resistance of the base-emitter p-n
100
VCE == 1.80 V
N
::I: 10 £!
.!:
10.1 100
Collector current (rnA)
FIG. 7. Cutolffrequency IT against collector current with a constant collec
tor-emitter voltage of 1.8 V. Lines indicated by the solid circles correspond
to before NDR. Lines indicated by the circles correspond to after NDR.
1773 J. AppL Phys .• Vol, 65, No.4. 15 February 1989 VCE = 1.80V
VIIE:= 1.02 V
Jc = 2.6 X 104 A/cm2
10 100
Frequency ( GHz )
FIG. 8. Frequency dependence of h2J at a collector current of 15 rnA before
NDR. The collector current de!l§ity, Ie. is 2.6X itt A/cm2•
junction and of the resonant tunneling barrier decrease.
However,IT decreases just before NDR occurs, because the
differential resistance of the resonant tunneling barrier in
creases again. Under the conventional bipolar transistor bias
condition, IT reaches 12.4 GHz at an Ie of 15 rnA. The
frequency dependence of ;'21 at the operating point VBE = 1.02 V,Ie = 15mA (Jc = 2.6X Ht A/cm2) is shown in
Fig. 8. 12.4 GHz is extrapolated from the data with the -6
dB/octave sloped line.
We analyzed the delayed times, which make up thefr of
12.4 GHz, using the simpie equivalent circuit model dia
grammed in Fig. 9. The resonant tunneling barrier is repre
sented by the resistance RT and the capacitance CT' The
base-emitter p-n junction is represented by the resistance R E
and the capacitance CEO These element combinations are
connected in series. We assumed the common-base current
gain a can be expressed by the following equation:
a=aoexp( -j(j)Tc)/(1 +j(;)TlJ). (1)
where T B is the base-layer transit time and T e is the collector
depletion-layer transit time. The device model parameters
were determined from curve fitting to the measured S-pa
rameter data. The device model parameters are listed in Ta
ble I. The S-parameter data of the circuit model agree wen
with the measured S-parameter data, as shown in Fig. 10.
Considering the first order of frequency, the current
gain h21 is expressed by the foHowing equation:
Ree FIG. 9. Equivalent circuit
B C model for the RBT.
CE
CT
Futatsugi et al. 1773
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160.36.178.25 On: Fri, 19 Dec 2014 22:47:42TABLE I. Model parameters of the RBT equivalent circuit.
ao = 0.94
Tn = 1.91 ps
1"c = 0.73 ps
CE =O.40pF
CT =0.47pF
Cc =O.17pF
Ccc =0.16pF Rz.;= 1.73 H
R,.= 2.99H
RB =275 n
Rc= ll.lkn
REE= 6.79H
RBB = 6.61 n
Rcc= 6.460
(2)
The emitter-to-collector delay time TEe represents the sum
of five delays
(3)
where 7 E is the emitter p-n junction charging time, 7(: is the
collector charging time, and 7 T is the resonant tunneling
barrier response time. Therefore, for the RET, the total de
lay 7Ee is composed of 1"T and the four delay times of a
conventional bipolar transistor.16 Components 1"11 and 'i e
are used in the circuit model to express the common-base
current gain a. Using the circuit model parameters,
'T£, Te, and 'iT are expressed by the following equation:
7£ =REC E,
Tc = [(RE + RT + REE)/a O + Rec] (ec + Cec)
+ RlICcc(l -ao)/ao,
71' =RTC p (4)
(5)
(6)
The delay times estimated from the circuit model are
listed in Table II. The'TEc value is estimated to be 13.6 ps
corresponding to iT ( = 1I21T1"Ec) of 11.7 GHz and agrees
wen with the experimentalj~ of 12.4 GHz. For our RET, the
collector charging time T~ is large, thus decreasing/To This
is because the pattern design of the RBT has not been opti
mized for high-frequency operation. The base transit time is
estimated to be 1.91 ps. This value is too large if electrons
transit the base region ballistically, indicating that hot elec
trons injected into the base lose their kinetic energy due to
scattering in the p-type InGaAs region.
--MeaslIred
-----. Modeled VCE = 1.S0 V
VeE = 1.02 V
Ic = 15 mA
FIG. to. Measured and modeled S-parameter data of the RBT.
1774 J. Appl. Phys., Vol. 65, No.4. 15 February 1989 TABLE II. Delay times in ps of the RBT estimated from the equivalent
circuit analysis.
TEe "E Tn "c T' C 1",.
13.6 0.69 1.91 0.73 8.92 1.39
C. Response time of the resonant tunneling barrier
The response time of a resonant tunneling barrier has
been analyzed,17-19 but it is still not clearly defined. As a
result of an analysis of the equivalent circuit, we obtained a
T T of 1.39 ps, as listed in Table n. To evaluate the validity of
this value, we estimated the resonant tunneling barrier re
sponse time using a device simulation.
Figure 11 shows the model energy-band diagram used
for our simulation. Both sides ofthe resonant tunneling bar
rier are doped with donors to a concentration of 1 X 1018
em -3• By solving the Poisson and Schrodinger equations, 13
we calculated the tunneling current I, the electron charge
accumulated in the accumulation region QA' and the charge
built in the quantum-wen region Qw' We assumed that 7T is
the sum of R TC A and the delay related to the dwell time T TT'
where CA ( = dQA/dV) is the capacitance of the accumula
tion and depletion layers and 'Trr = dQw/dI.
At a collector current of 15 rnA, we obtained a TTT of
0.71 ps and a CA of 0.23 pF from the device simulation.
Using an equivalent circuit model parameter of the resonant
tunneling barrier resistance R T = 2.99 n, r T is
(7)
Therefore, 'iT obtained from the simulation agrees with that
of the equivalent circuit analysis. The capacitance of the res
onant tunneling barrier in the equivalent circuit model Cr is
considered to correspond to CA + T TT/ RT in the device sim
ulation.
IV. CONCLUSiONS
Resonant tunneling bipolar transistors (RBTs) were
fabricated using InAIAs/lnGaAs heterostruetures. These
RBTs operate at room temperature. We obtained a collector
current peak-to-valley ratio of 3.5, a peak current density of
5.7X 104 A/cmz, and a common-emitter small signal cur
rent gain of24. We also measured the microwave character
istics of RBTs, obtaining a cutoff frequency of 12.4 GHz.
'r'" 'trr+RTCA
'tn::: ~Qw
~I
CA= ~QA
~v
Tn: 0.71 ps
RTe ... : 0.69 ps
FIG. 11. Model energy-band diagram ofthe resonant tunneling barrier used
for the device simulation.
Futatsugi et al. 1774
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160.36.178.25 On: Fri, 19 Dec 2014 22:47:42The response time of the resonant tunneling barrier is esti~
mated to be L4 ps from the results of the equivalent circuit
analysis and the device simulation.
The characteristics of RBTs are greatly improved by
using InAIAs/lnGaAs heterostructures. These results indi~
cate that RBTs are promising for practical room tempera~
tun; applications.
ACKNOWLEDGMENTS
We thank T. Misugi, M. Kobayashi, Y. Yamaoka, and
E. Miyauchi for their continuous encouragement, T. Fujii
and T. Inata for the crystal growth, and K. Jhoshin for assis~
tance with the microwave measurement.
'1'. Inata, S. Muto, Y. Nakata, T. Fujii, H. Ohnishi, and S. Hiyamizu, Jpn.
J. AppL Phys. 25, L983 (1986).
21'. Inata, S. Muto, Y. Nakata, S. Sasa, T. Fujii, and S. Hiyamizu, Jpn. J.
App\. Phys. 26, L1332 (1987).
JR. Tsu and L. Esaki, App!. Phys. Lett. 22, 562 (1973).
1775 J. Appl. Phys., Vol. 65, No.4, 15 February 1969 4L, L Chang, L. Esaki, and R. 1'su, AppL Phys. Lett. 24, 593 (1974).
sT. C. L. G. Sollner, W. D. Goodhue, P. E. Tannenwa!d, C. D. Parker, and
D. D. Peck, Appl. Phys. Lett. 43, 588 (1983).
oM. Tsuchiya, H. Sakaki, and J. Yoshino, Jpn. J. AppL Phys. 25, L185
( 1986).
7p. Capasso and R. A. Kiehl. J. App!. Phys. 58,1366 (1985).
'S. Luryi and F. Capasso, App\. Phys. Lett. 47, 1347 (1985).
"Po Capasso, S. Sen, A. C. Gossard. A. L. Hutchinson, and J. H. English.
IEEE Electron Device Lett. EDL-7, 573 ( 1986).
'OF. Capasso, S. Sen, A. Y. Cho, and D. Sivco, IEEE Electron Device Lett.
EDIAi,297 (1987).
• 'N. Yokoyama, K. Imamura, S. Muto, S. Hiyamizu, and H. Nishi, Jpn. J.
App!. Phys. 24, L853 (1985).
'2K. Imamura, S. Muto, H. Ohl1ishi, T. Fu.jii, and N. Yokoyama, Electron.
Lett. 23, 870 (1987).
"T. Futatsugi, Y. Yamaguchi, K. Ishii, K. Imamura, S. Muto, N. Yo
koyama, and A. Shibatomi, IEDM Tech. Dig., 286 ( 1986); lpn. J. App!.
Phys. 26, Ll3! (1987).
'4E. E. Mendez, E. Calleja, and W. 1. Wang, Phys. Rev. B 34, 6026 (1986).
15y' Sugiyama, T. Inata, T. Fujii, Y. Nakata, S. Muto, and S. Hiyamizu,
Jpn. J. App!. Phys. 25, L648 (1986).
1"S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981), p.
158.
l7B. Ricco and M. Y. Azbel, Phys. Rev. B 29, 1970 ( 1984).
J"D. D. Coon and H. C. Liu, App!. Phys. Lett 49, 94 (1986).
J9N. Harada and S. Kuroda, Jpn. J. AppJ. Phys. 25, L871 (1986).
Futatsugi et al. 1775
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160.36.178.25 On: Fri, 19 Dec 2014 22:47:42 |
1.1140547.pdf | Design considerations and performance characteristics of a dual mode timeofflight
mass spectrometer system for surface reactivity studies
Benjamin N. Eldridge
Citation: Review of Scientific Instruments 60, 3160 (1989); doi: 10.1063/1.1140547
View online: http://dx.doi.org/10.1063/1.1140547
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/10?ver=pdfcov
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:42Design considerations and performance characteristics of a dual mode
time-of-flight mass spectrometer system for surface reactivity studies
Benjamin N. Eldridge
IBM T. J. Watson Research Center, Yorktown Heights, New York 10598
(Received 9 March 1989; accepted for publication 9 June 1989)
We have designed a retlectron type time-of-tlight mass spectrometer (TOFMS) system with both
SIMS and electron impact (EI) ion optics. The system was designed to study the interaction of
solid surfaces with reactive species initially in the gas phase. In the El mode, the main TOFMS
assembly, which consists of the flight tubes, refiectron, and a channel plate detector, is fitted with
the El ion source. The entire TOFMS is mounted in a rotatable cage permitting 2700 of rotation
about the sample center. Reactive species are supplied by a pulsed molecular beam source. The EI
source has an unobstructed view of the target throughout most of the 217' solid angle defined by the
plane of the sample. Angular and time-resolved detection of scattered beam species and desorbed
reaction products to a limiting partial pressure of -10- 13 Pa is possible in this configuration. In
the SIMS detection configuration, the main TOFMS assembly is refitted with SIMS extraction
optics and repositioned on the sample axis. The pulsed molecular beam source is again used to
supply reactive species. By controlling the timing relationship between the arrival of the pulsed
molecular beam and the pulsed ion beam used to sputter the target, time-resolved detection of
transient surface species is possible. A 50% mass resolution of -1400 has been demonstrated in
both operating modes. The feasibility oftime-resolved detection in both operating modes has also
been demonstrated.
INTRODUCTION
The apparatus described in this article is the result of our
desire to pursue the dynamic character of the reaction of a
surface with gas phase species to a level of detail not revealed
by previous molecular beam studies of these phenomena.
The time-of-flight mass spectrometer (TOFMS) is in many
ways the ideal choice as a detector for studying the real time
result of striking a surface under vacuum with a sharp pulse
of reactive gas. There are a number of reasons for this, the
first of which is that operation of the TOFMS is inherently
time resolved. The time sampling of the detection volume for
an electron impact (EI) ion source, or the equivalent sam
pling of the sample surfaace with a pulsed primary ion beam
for SIMS produces a mass spectrum whose time relationship
with respect to a neutral molecular beam pulse is well
known. The transmission of the TOFMS is also superior by a
factor of 10-100 over typical quadrupole mass filters operat
ing at equivalent resolution. Signal levels for molecular beam
experiments are quite low, so this improved transmission is
of considerable importance. Finally, the TOFMS offers truly
parallel mass detection. For monitoring simple reactions
this represents a small advantage, however for complicated
reactions where branching and other phenomena are taking
place, the ability to monitor simultaneously many mass
peaks can more than compensate for the small duty cycle
that is the main drawback of the TOFMS.
In order to study surface reaction processes in as much
detail as possible, we have designed a system whose nucleus
is a retlectron 1.2 type TOFMS capable of two different modes
of detection. The first operating mode permits time and an-
, gular resolved detection of scattered and desorbed species
leaving the sample throughout most of the 217' solid angle
defined by the sample surface. In this mode the angular dis-tributions for scattered reactive species, as well as angular
distributions for desorbed reaction products are discernible
as a function of time to a limiting resolution of 10 f-ls. This
mode of operation represents an extension of the modulated
molecular beam scattering technique described in the litera
ture.3-IO Angular resolved detection of desorbed and scat
tered species should provide insight into the local environ
ment in which the desorbed products are situated just prior
to the desorption step. Some insight into the reaction kinet
ics and reaction pathways may be derived from the time de
pendence of the product waveform by determination of the
surface transfer function for the reaction.7,8.10
The transfer function method for data reduction is use
ful, and in fact has been used by us in previous work. 11.12
However, our experience lead us to conclude that while this
information was very much of interest, it does not present
enough of the picture. The "black box" which is made of the
surface/adsorbate complex can be frustrating. While intu
ition may suggest the possible nature of surface intermediate
species leading to volatile products, no access to these species
is permitted by the experiment. Further, for reactions which
produce a nonvolatile final product no direct observation of
the product population time dependence is possible. It is be
cause of this measure of inaccessibility for conventional
modulated molecular beam experiments that the system was
designed for the second mode of operation.
In mode two, the TOFMS is fitted with a set of SIMS
extraction optics. Operation of the time-of-flight in the
SIMS mode requires a sharp ( -IOns) pulse of primary ions
as the stimulus for production of ions from the surface. In
this mode the pulse of neutral reactive species supplied by
the molecular beam may easily reach the target through the
large extraction field required for successful secondary ion
extraction. Once adsorption takes place, firing the primary
3160 Rev. Sci.lnstrum. 60 (10), October 1989 0034-6748/89/103160-11 $01.30 @) 1989 American Institute of Physics 3160
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:42ion beam should permit the observation of "snapshots" of
the species present on the surface during the reaction and
allow us to observe the time evolution of their populations
through their respective secondary ion yields,
The two TOFMS operating modes are complimentary,
and in the best case should permit us to observe the full time
dependent behavior throughout the course of the reaction
process. When this information is used in conjunction with
the angular dependence information for desorbed species, a
more complete picture of the surface reaction should
emerge.
I. DESIGN
The design process involved the resolution of conflicting
objectives; however, many of the general design principles
can be expressed in a straightforward fashion. We will start
with the design considerations for the refiectron.
A. Reflectron
The reflectron electrostatic mirror was first proposed by
Karataev et al.1,2 as a means to compensate for the flight
time dispersion of a compact ion packet with finite energy
spread. A number of mass spectrometers based on this prin
ciple have been constructed. 2,13-16 Mass resolution figures in
excess of 10 000 have been achieved.15•16 While high mass
resolution is desirable, it is the high transmission of the
TOFMS that is most useful for our molecular beam studies.
The machine was designed with transmission and the geo
metrical constraints of our experimental geometry as the pri
mary design concerns. The mass resolution, while impor
tant, was a secondary consideration. The reflectron
deceleration and turnaround distances are 10.5 mm and 80
mm, respectively, These distances were chosen in order to
accommodate the behavior of the two different ion sources
used by the mass spectrometer. This behavior will be dis
cussed in detail later in this article. The grids in the reflec
tron are 90% transparency stainless-steel mesh from Unique
Wire Inc. 17 The mesh was stretched and lightly chrome plat
ed, then transferred by spot welding to mounting rings
which are in turn screwed to the reflectron elements.
Chrome plating increases the stiffness of the mesh, making it
somewhat more manageable during the transfer operation to
the mounting rings. Eight ring-shaped electrodes are used to
terminate the linear field in the turnaround section of the
reflectron. The reflectron elements are dish shaped in order
to conceal the insulators used in assembling the element
stack from the ions during reflection. The resistor divider
used to set the potentials on the refiectron termination plates
has a total resistance of 11 MO,. The resistor elements were
purchased from Vishay-Anghstrom. IR They are hermetical
ly sealed and are mounted in vacuum,
B. SIMS extraction optics
The primary SIMS extraction optics are critical to the
performance of the TOFMS in the SIMS mode so some de
tail will be presented concerning the behavior of these optics.
Figure 1 shows the somewhat simplified picture of the ex
traction optics used in the analysis. The two quantities of
3161 Rev. Sci. Instrum., Vol. 60, No. 10, October 1989 AP[RTURE.
¢=¢(j
FIG. 1. Extraction geometry for the SIMS extraction optics. ¢> a is the ex
traction potentiaL '0 is the radius of the extraction aperture. d" is the <;lis
tance from the sample plane to the extraction aperture. An ion of mass In
leaves the sample surface with starting energy Eo at an angle e and dis·
placed from the axis by a distance x".
interest are the transmission and mass resolution implied by
the ion extraction potential <I> a' the extraction aperture radi
us ro, the initial displacement of the ion from the optical axis
x, the distance from the sample to the ion extraction aperture
da and the effective drift length L which the ions traverse
after leaving the extraction region. To evaluate the transmis
sion for the optics, we assume that the ion energy distribu
tion P(Eo) is Thompson t9Hke,
P(Eo)dE o r:t: [Eo/(Eb + EO)3]dEo• (1)
and that the polar angle distribution of the ions is cosine. The
transmission is evaluated by integrating over solid angle and
Eo subject to the constraint that the final radial position of an
ion at z = da cannot exceed roo The integrals were evaluated
numerically using a PC program called MathcAD.20 The re
sults are presented in terms of the dimensionless parameters
o-=ro/du, a=(Eb/<Pa). (2)
It was established that for x less than 0.5 ro the transmis
sion is approximately that for ions starting on the axis. We
have used this information to restrict the analysis used to
produce Fig. 2 to the on-axis condition where integration is
simpler. Figure 2 shows the extraction efficiency through the
first aperture for ions starting on the optical axis (x = 0), as
a function of the parameter a for 0-= 0.1, 0,2, and 0.5. It is
important to note that Fig. 2 reflects only the efficiency with
which ions are transmitted through the first aperture. The
ion beam produced by extraction when both 0-and a are
large, would prove difficult to transport to a detector over
the typical flight distance of a TOFMS, Values for these pa
rameters for a viable TOFMS, and those used for our design,
are well removed from this regime. Figure 2 may be used to
assess the transmission and energy discrimination effects of
the extraction optics. The figure indicates that some caution
will be required in interpretation of relative intensities dur
ing simultaneous detection of ions whose starting energy dis
tributions differ by a significant extent.
In order to choose an operating point we must also know
the effect of the above parameters on the mass resolution of
TOF mass spectrometer 3161
••••• -,-,. .-' ••••••• , ••• ~ ••••• ..' •• , •••••• -.-.-•••• -. ••••••••• T ••••••••••••••••••••••• 7"":.~."';.~.:.; •••.• ; •••• " •••• ; ••. ,·.·.· ......... v.y."'!-:.:o;.: •• o;.~o; ••••••• ;o-•••••••••• ,.;-•••••••••••••••
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 01:23:42FIG. 2. Total percent transmission through the extraction aperture of Fig. I
as a function of the dimensionless parameter a for a ,---, 0.1,0.2, and 0.5 and
x=O.
the TOF. An estimate of the mass resolution may be made in
the foHowing way. The ideal action of the reflectron electro
static mirror is to reproduce at an effective distance L from
its source the temporal distribution of an initial ion pulse
with some nonzero spread in ion energy. The mass resolu
tion of a TOFMS is given by
M / f:.M = t /2ll.t. (3)
For the situation of Fig. 1, when Eo = O. The time tin Eq.
(3) is given by
(4)
Ifwe neglect the ion angular distribution, we may write the
time t( k,d" ), required for traversal of the acceleration gap
do as
t(k,da) = (mI2q<pa)-i/22da[kI/2-(k_1)1/2], (5)
where
(6)
Examination of Eq. (5) will show that the effect of the ex
traction process for an ion bunch with some initial energy
spread is twofold. First, the pulse time duration at the posi
tion of the extraction aperture is broadened from its starting
value at the surface. Second, the apparent temporal mini
mum source point, that is the point at which the pulse dura
tion is a minimum looking back through the aperture, is
displaced in the -z direction of Fig. 1 to a point which
coincides with the sample surface only in the limit of large
ion energy spread. We may assess the magnitude of this ef
fect and calculate the z position at which the pulse is a mini
mum by defining t ' (k,z) as
t'(k,z) =tCk,da) +z(m!2kq<pa)I/2. (7)
The time spread At may be estimated as twice the RMS value
implied by Eq. (7), i.e.,
2 lkmax
L\t(kmax) = It'(1,z) -t'(k,z)idk,
kmax -1 I (8)
Substitution of Eqs. (8) and (4) into Eq. (3) allows the
determination of the projected resolution and the position
3162 Rev. Sci.lnstrum., Vol. 50, No. 10, October 1989 g
o
'"
g
o MASS RESOLUTION
lMIN
(KMAX -1)x100 ~
I z " N
FIG. 3. Spectrometer resolution and virtual source position z . as a func
tion of percent relative energy spread for a gap width da of Ie mm and a
ratio of effective drift length L to extraction gap width da = 170.
Z = zmin at which the pulse duration is a minimum. For this
calculation the ions are assumed to be evenly distributed in
energy between k = 1 and k = kmax. The results are plotted
in Fig. 3 for a ratio of effective drift length L to extraction
distance da of 170. The resolution is plotted as a function of
100(kmax -1) or percent relative energy spread. The re
sults for this value of L Ida are shown because it is this value
which is used by our final design. Other operating points
may be assessed by noting that for fixed kmax the resolution
scales as L Ida while the source position Zmin scales as da.
Figures 2 and 3 may now be used to assess the available
performance in conjunction with two other experimental
limitations. The pulsed molecular beam used as the source of
reactive species in the experiment has a diameter of roughly
4 mm. 10 For reasonable dimensions of the extraction optics a
gap width da of I em is required to assure access of the
molecular beam to the sample region of interest. The pri
mary ion beamline, which will be discussed in more detail
later, delivers a pulse of primary ions with a spot diameter at
the target of 1 mm. Each pulse contains -104 ions. The
relatively large primary ion beam spot diameter is necessary
to sample a reasonable portion ofthe-lO --2 monolayer per
pulse coverage of molecules delivered by the pulsed molecu
lar beam while at the same time maintaining essentially stat
ic surface conditions with a minimum of ion beam induced
damage and surface chemistry. If we assume that all sput
tered particles originate from the first monolayer, and that
both the sputter yield and ionization probability of the mo
lecular beam related surface species are unity, then 102 ions
related to the surface reaction should be generated per ion
beam pulse. Under these conditions _10-9 monolayers are
removed per beam shot.
Our examination of the transmission of the extraction
optics suggests that the extraction aperture should be at least
twice the primary beam spot diameter for uniform accep
tance of sputtered ions across the ion beam spot so we set
Yo = 1.0 mm. The parameter a is set by this choice of Yo to a
value of 0.1. If we assume a value of Eb of 15 eV, and an
extraction potential of 3 keY, the extraction behavior is de
termined. The parameter a = 0,005. Figure 2 predicts a
rOF mass spectrometer 3162
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:42transmission of -38%. The resolution may be assessed if we
assume an effective drift length L of 1.7 m. L / da is now equal
to 170. The 50% width ofEq. (1) is approximately twice Eb
giving krnax = 1.01 and a projected mass resolution of 1760.
A better estimate of the obtainable mass resolution, the
total transmission to the detector, and the design of an Einzel
lens to compensate for the divergent lens formed by the ex
traction aperture was accomplished by ray tracing. The
Herrmannsfe1dt electron optics code developed at SLAC
was used to simulate the behavior of the extraction optics
using randomly generated initial conditions for the ions
which conformed to the assumptions used in calculating Fig.
2. For the simulation the extraction potential was set to 3
keY and the characteristic emission energy E" to 15 eV. The
simulation confirmed the analytical results of Fig. 3 for the
fraction of ions successfully extracted through the extraction
aperture.
The number ofions to reach the detector will be smaller
than the number sllccesfully extracted from the target since
some of these will lie outside the acceptable phase volume
defined for the TOF at the entrance to the flight tubes. Under
the initial conditions described above 50% of the ions ex
tracted from the target lie within this accepted phase vol
ume. This percent transmission figure is essentially indepen
dent of the starting radial position for ions starting from a
distance less than 0.5 ro from the optical axis. This result, in
conjunction with an assumed 90% mesh transparency in the
refiectron, gives an overall transmission to the detector of
12 %, The virtual source position Zmin was found to be 34 em
upstream of the aperture position. The obtainable mass reso
lution was found to be 1600.
The design for the SIMS extraction optics is shown in
Fig. 4. The conical ground shield surrounding the extraction
electrode serves to shield the primary ion beam from the
extraction field until it is close to the target, It also increases
by about 7% the extraction field strength at the sample sur
face when compared to a simple truncated conical extractor
of similar dimensions. Simulation of both geometries indi
cates that use of the ground shield produces a few percent
increase in the extraction efficiency. The slight recess of the
extraction aperture into the electrode structure decreases the
strength of the single aperture divergent lens formed by the
extraction aperture. A cylindrical Einzellens is used to com
pensate for the divergence of the extracted ions produced by
the extraction aperture and refocuses the ions onto the detec
tor at the end of the flight tubes. For positive ion detection
the extraction voltage is typicaUy set to -3 kV. The Einzel
lens voltage is typically 0.26 of the extraction voltage. The
system insulators will support extraction potentials of up to
±5kV.
c. EI ion source
The design of the EI ion source is the dual field type of
Wiley and McLaren.21 For EI detection of scattered and
desorbed molecules we have chosen a geometry, shown sche
matically in Fig, 5, which allows for extraction of ions per
pendicular to the direction of their entrance velocity into the
source. This enhances the mass resolution by eliminating to
some extent the effect of the thermal velocity spread of the
3Hli3 Rev. Scl.lnstrum., Vol. SO. No. 10, October 1989 --1
FIG. 4. Layout of SIMS extraction optics. The extraction potential <fI" is set
to between 3 and 5 keY. The Einzellens potential <file", is set to 0.26 of the
extraction potential. The jJolarity of these potentials depends on whether
positive or negative secondary ions arc of interest The extracted ion beam
envelope is represented schematically by the hatched area of the figure. The
figure is to scale.
detected species on their time-of-flight to the detector. The
geometry chosen, along with the small ion source dimen
sions permits observation of a large solid angle above the
surface. Rotation of the sample about the axis ofthe molecu
lar beam in conjunction with rotation of the TOFMS allows
us to accomplish this while at the same time maintaining a
constant angle of incidence for the molecular beam with re
spect to the sample surface normal. This feature is important
because it prevents reactivity variation with incident angle
from being folded into our angular dependence measure-
»(C
SAMPl ~
Ot-GRf::.f:S :::w
FRf i-.DOI<\ IClN i ZA r ID,\
R[~GION
FIG. 5. Schematic representation of the geometry used for EI detectioJl, The
extraction direction for ions is normal to the plane of the figure. The pri"
mary molecular beam may be sampled at the points labeled A and B. Ex"
traction of ions is normal to the plane of the figure,
TOF mass spectrometer 3163
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:42,
J
(L1) './ I r-SOn
f -Ij' --,
. I "' J,
/! /
de
FIG. 6. Layout of the electron impact ion source. The pulsed extraction vol
tage V2 has a maximum amplitude of + 200 V. The acceleration voltage
VI = -300 to -1000 V. The pulsed electron injection voltage V4 has a
magnitude of -200 V and a duration from 100 ns to 10 J1s. The maximum
emission current into the ionization region is 10 rnA. The filament current
11 is supplied through the inductance shown which consists of 40 turns each
of No. 18 magnet wire wound in the same direction of the ferrite pot core.
This inductance prevents the electron injection pulse voltage V 4 from seeing
the low impedance to ground of the filament bias supply V3. de .. ~ 1.05 em,
da = 5.12 em, CPr = 4 mm, R = 1 MD. The shaded rectangle is the ioniza
tion region. The electrode portion of the figure is drawn to scale.
ments. There are also two points, labeled A and B in Fig, 5,
to which the TOFMS may be rotated to determine the pri
mary molecular beam flow velocity and beam temperature.
For the EI experiments the scattered and desorbed product
signals are quite small. The primary consideration for the EI
source was, thus, not mass resolution but transmission of the
extracted ions to the detector. The projected flight length of
~ 1. 7 m implies a highly parallel beam must be produced.
The source is shown schematically in Fig. 6.
In order to reduce scattering of the ions, no grids are
used in the source. The source dimensions were arrived at by
using the flight time dependence for ions extracted from the
ionization volume, in conjunction with ray tracing simula
tion, to minimize the divergence of ions leaving the accelera
tion region while at the same time creating a minimum tem
poral width ion packet just downstream from the exit of the
source. For an acceleration potential of -1000 V, ray trac
ing gives a figure of 80% for the number of ions initially in
the ionization volume which leave the source with trajector
ies within the acceptance of the TOF. Assuming 90% trans
parency mesh in the refiectron, 53% of these should reach
the detector.
The location of the minimum width packet is from 0 to
14 cm downstream from the source exit for acceleration po
tentials in the range of -300 and -1000 V. While a mass
resolution calculation similar to that for the SIMS source is
possible, the strong effect of the thermal velocities of the
detected particles on the mass resolution makes this tricky.
The cross flow design of the ionizer limits, but does not elimi
nate this effect. If the effect of the thermal velocities is ne
glected, we estimate the achievable resolution of the
spectrometer by ray tracing at about 2000 for an acceleration
potential of -1000 V and an effective drift length of 2 m.
This resolution figure is limited by a nonvanishing third or-
3164 Rev. Sci.lnstrum., Vol. 60, No. 10, October 1989 der spatial derivative of the electric field in the extraction
region due to the rather tight source geometry. The gap in
between the ionization region and the acceleration region is
at zero field during the time when the electron beam is
switched on. This prevents ions formed during the early
phase of a relatively long ionization pulse from being extract
ed and creating spurious noise. The gap is also favorable in
terms of creating a minimum width ion packet at the source
exit.
The ionization portion of the ion source was constructed
by modifying a commercially available Balzers quadrupole
mass spectrometer ion source. This has the advantage that
replacement parts and filament assemblies may be obtained
off the shelf. The resistors shown are hermetically sealed
Helium-filled resistors obtained from Vishay-Angstrohm. 18
They are mounted directly to the source. The resistors were
purchased unmarked and uncoated to maintain compatibili
ty with the URV system. The electrodes for the acceleration
region da are dish shaped in order to conceal the insulators
used in assembling the electrode stack from the ions during
acceleration.
The EI source is operated as follows: The filament cir
cuit is first switched on by driving the filament circuit nega
tive with respect to ground using V4 of Fig. 6. The duration
of this pulse may be from IOns to 10 f1s, the latter being the
approximate time required for thermal particles to traverse
the 4-mm detection volume diameter. The width of this
pulse determines the time resolution for the TOF, as well as
the number of particles per shot to reach the detector. The
filament bias potential V3, in conjunction with the magni
tude of the electron injection pulse V 4, determine the elec
tron injection energy, This feature makes it possible to utilize
the variation of appearance potential to discriminate
between ions at the same nominal mass and can be used to
reduce background signals in certain experiments. In prac
tice the electron injection pulse V 4 is fixed at -200 V and
the filament bias voltage V3 is used to determine the electron
energy. This is because the Avtech22 pulse units used to gen
erate the pulsed electron injection voltage V3 and ion extrac
tion voltage V2 display their best risetime behavior when set
to the maximum output voltage. The filament bias voltage
V3 must be set to at least + 110 V in order to prevent injec
tion of electrons into the ionization volume when the + 200
V extraction pulse V2 is applied to the extraction electrode.
This pulse is applied immediately after the electron injection
pulse terminates, and must remain on until all the ions of
interest have left the extraction region de and entered the
acceleration region d". The pulse units have rise and fall
times of approximately 1 0 TIS.
D. Additional considerations
The SIMS and E1 ion sources mount interchangeably on
the deflection plate housing used for ion beam steering. The
total drift distance from the entrance to the deflection plate
housing to the detector is 1.2 m. The total effective drift
length L is 1.7 m. The flight tubes are 5 cm in diameter and
are electrically isolated from the rotatable mounting cage.
Complete isolation of all the TOFMS components from
ground allows us to detect both positive and negative sec-
TOF mass spectrometer 3164
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3
20
FIG. 7. Cutaway view of the TOFMS in the SIMS mode configuration. I: isolation valve to LEED and XI'S, 2: Huntington manipulator, 3: pulsed molecular
beam source, 4: reflection, 5: final primary ion beam (PIn) blanking aperture, 6: second stage ofPIB double deflection, 7: first stage ofPIB 'double deflection
and second stage ofPIH beam gating. 8: channelplatt: detector, 9: Einzellclls, 10: first beam gating aperture, 11: isolation valve, 12: tilt axis of PIll source, 13:
tilt compcnsation and first-stage beam blanking plates, 14: PIB source, 15: mirror, 16: sample chamber, 17: TOF chamber, 18: electrical feedthroughs, 19:
. offset nipple, 20: sample, 2 I: flight tubes. Inset is a close up view of the experiment geometry at the sample position.
ondary ions in the SIMS mode by reversing the polarity of
the system voltages, The reflectron deceleration and reflec
tion region distances are chosen to permit the reflectron ac
tion distance, that is the drift distance from Zmin to the detec
tor, to allow for a Zmin position 15 em downstream of the EI
source exit to a position 40 em upstream of the sample posi
tion for SIMS by a suitable choice of the reflectron poten
tials.
Figures 7 and 8 are cutaway views of the system config
ured for the SIMS and EI modes of operation, respectively,
Both views are from above looking down on the apparatus,
The spectrometer is shown in the EI configuration in one of
the two positions at which detection of the primary molecu
lar beam pulse is possible. The rotation of the system is ac
complished by mounting the entire TOP inside a rotatable
cage in vacuum. No rotating seals are used to accomplish
rotation of the TOF assembly. The cage housing the TOF is
driven via gears and a rotatable UHV feed through. The cage
runs on stainless-steel bearings which have been dichronit
ed23 to prevent pitting and sticking after system bakeout. A
liquid nitrogen cooled OFHC copper shroud surrounds the
E1 source, Our previous experience suggests that this type of
a cryopump in the vicinity of the ionizer has a significant
impact on background noise levels. Both the liquid nitrogen
lines and the TOF potentials with the exception of the chan
nelplate signal are supplied by a watch spring arrangement
3165 Rev. Sci. instrum., Vol. 60, No. 10, October 1989
••• -••••••• -.-.-.; ••• < •••••••••••••••••• : ••• :.:.:.:.:.:.:.:.:.:.~.:.: •• ':'.'.' ••••••• ;<.......... .." --".-.-.-,., .•... ,. _..... . at the rear of the spectrometer. The rather large half angle of
4° was chosen to permit a stationary channelplate detector to
be mounted on the system axis just aft of the reflectron. This
should allow us to detect neutral species produced by the
decay of metastable ions without hindering rotation of the
spectrometer. The dispersive contribution of the deflection
plates for ions of different energies is largely compensated by
their trajectories in the reflectIOn. The signal from the chan
nelplate detector is brought out via a BNe feedthrough in
the wall of the sample chamber. The sample chamber is con
nected by an isolation valve to a system equipped with
LEED, XPS, etc., for cleaning and characterization ofsam
pIes used in the experiments. All of the rotatable degrees of
freedom are equipped with stepping motors for computer
control of the scattering experiments. The sample chamber
is equipped with a 300 tIs turbomolecular pump. The TOF
chamber is pumped by a liquid nitrogen cooled titanium sub
limation pump and a 50 tIs ion pump.
The detector is a microchanne1plate with an active sur
face diameter of 4.5 cm. The channel plate is mounted in a
shielded enclosure. A grid at the entrance of the enclosure
allows a post-acceleration potential to be appiied to the
channel plate entrance plane without significant extension of
the field from that potential into the flight tube. The postac
celeration potential increases the detection sensitivity for
more massive ions. The electrical signal from the channe!-
TOF mass spectrometer 3165
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:42FIG. 8. Cutaway view of the TOFMS in the EI mode configuration. For
unlabeled items sec Fig. 7.1: E1 ion source, 2: LN2 reservoir, 3: OFHC cop
per shroud, 4: sample manipulator, 5: pulsed molecular beam source, 6:
deflection plates, 7: TOF positioning mechanism, 8: forward radial bearing
location, 9: inner rotating cage, 10: rear radial and axial bearing location,
11: drive pinion, 12: rotary fcedthrough, 13: LN2 feedthru, 14: metastable
detection point.
plate is picked up from a phosphor screen in back of the
second plate and brought out along a 50-fl line through a
BNe UHV feedthrough.
Figure 9 is a cutaway side view of the primary ion beam
line used to produce the primary ion pulse in the SIMS oper-
1 • S° ating mode. Some supporting hardware has been eliminated
for clarity. An Atomika ion source is used as the source of
the Ar+ primary ions. The beamline uses a double gating
scheme with fast (10 ns) rise time pulsers to produce a mass
separated 40 Ar+ pulse of 6-ns duration FWHM. The Avtech
pulsers used to gate the ion source for the El mode of oper
ation are used in the SIMS mode to produce the ion beam
gating potentials. By adjusting the delay between the pri
mary and secondary ion beam gating pulses a mass separated
ion beam is obtained. The second pulse produced by the re
trace of the primary gate pulse is also eliminated. The mass
resolution of the beam line is about 40 at 40 amu. Removal of
the less abundant Argon isotopes prevents the generation of
spurious mass peaks.
The l.so y tilt of the ion source with respect to the final
ion beam axis is used to remove fast neutrals from the ion
beam which would otherwise result in spurious noise during
operation of the spectrometer. The jacking screw shown on
the figure is used to introduce the tilt shown after a good dc
beam condition is established for the ion source coaxial with
the lens/double deflection section. The lens/double deflec
tion section is rigid and is aligned with a telescope during
assembly. Thus, the only alignment necessary during the set
up of the beamline is between the ion source axis and the
lens/double deflection section axis. In practice this is
straightforward and a good dc and pulsed beam condition
can be obtained in about two hours.
The continuous duty ion current available from the
beamline is 80 nA for a 6-keV primary beam energy. The x
double-deflection beam steering plates built into the final leg
of the beamline permit steering of the ion beam to compen
sate for the lateral displacement of the ion beam spot by the
extraction field of the SIMS optics. The slight rotation of the
TOFMS which is still possible in the SIMS configuration
allows us to compensate for any small mechanical misalign
ment between the axes of the SIMS optics and the primary
ion beamline.
FIG. 9. Cutaway side view of the mass separated pulsed primary ion beam used for SIMS. 1: jacking screw, 2: Atomika ion source, 3: location of pivot for ion
source mounting cradle and first y beam blanking plates, 4: bellows, 5: isolation valve, 6: Einze11ens, 7: second y beam blanking plates, and first stage of x
double deflection, 8: second stage of x double deflection, 9: I-mm-diam final beam blanking aperture, 10: sample position, 11: 2-mm first beam blanking
aperture, 12: support frame for ion source pivot cradle, 13: pumping port, 14: ion source mounting cradle.
3166 Rev. SCi.lnstrum., Vol. 60, No. 10, October 1989 TOF mass spectrometer 3166
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:42100 MHZ
TRANSIENT
RECORDER RES ~ 10l1s DIVIDE-BY-N
COU~TER SRS DG-535 ::OUR CHA~NEL DIGITAL DELAY
!GENERATOR
TRIG elK iii
IBM PC GPIB IN OUT
EXPERIMENT TRIGGERS.
'---.".,.- SIMS MODE,
ANALOG PULSED MOLECULAR BEAM ION BEAM GATING
DISC AMP EI MODE.
PULSED MOLECULAR BEAM
ELECTRON INJECTION
ION EXTRACTION
f-----__a----e CHANNELPLATE SIGNAL
FIG. 10. Schematic representation of the data-acquisition electronics. The TDC shown was not available during the EI mode testing.
E. Data acquisition and experiment timing
Figure 10 is a schematic representation of the data ac
quisition and experiment control electronics. All of the elec
tronics used are off-the-shelf units from commercial ven
dors. The 100 MHz signal averaging transient recorder
clock serves as the master clock for the experiment trigger.
This clock is divided to produce a clock frequency which is
compatible with the repetition frequency of the experiment
and is fed to the Stanford Research Systems DG-535 digital
delay generator. This four channel unit generates all of the
time delays necessary for both modes of operation. In the
SIMS mode two of the channels are used to trigger the high
voltage pulsers used to gate the primary ion pulse. A third
channel is used to trigger the pulsed molecular beam. In the
El mode two channels are used to trigger the electron injec
tion and ion extraction pulses required by the EI source,
while the third may be used to trigger the molecular beam.
The SRS DG-535 is fully programmable and GPIB compati
ble. In both operating modes the SRS DG-535 also triggers
the lOO-MHz transient recorder. This eliminates the effect
that dither of the transient recorder clock with respect to an
asynchronous trigger would have on the observed pulse
width. A fourth channel may be used to trigger a single stop
TDC to examine a single mass peak at much higher resolu
tion than can be obtained by the transient recorder. This was
added after the testing of the TOF in the EI mode, so only
transient recorder data are available to demonstrate the EI
source capabilities.
The signal from the channelplate detector is amplified
and fed to a discriminator to produce pulse data for the tran
sient recorder or the TDC. The net effect of recording the
pulse data with the transient recorder instead of the TDC is
to add to the true mass peak width the fixed time width of the
discriminator output pulse, This is 7 ns for the discriminator
used in these studies. In conjunction with the minimum tran
sient recorder channel width of 10 ns this places an upper
3157 Rev. SCi.lnstrum., Vol. 60, No. 10, October 1989 limit on the obtainable mass resolution. This effect degrades
the apparent obtainable mass resolution and has been cor
rected for in our determination of the mass resolution from
the data for the EI source. The advantage of the transient
recorder lies in the ability to record a large mass spectrum
with essentially zero dead time between adjacent channels.
The 8 kbyte transient recorder memory represents a mass
range of 0--1000 amu for a 3 ke V acceleration potential for
SIMS. Very large signals created by pulse pile up at the de
tector may be recorded as analog data by connecting the
output of the channel plate directly to the transient recorder.
Stepping motor controllers mounted in the CAMAC crate
permit access by the computer to the angular degrees of free
dom for scattering experiments.
II. PERFORMANCE
The system has been set up and tested in both modes of
operation. The time required to convert from one operating
mode to the other is only one day, excluding the time re
quired to bakeout the system. The system has been baked
mildly and a base pressure of I X 10-7 Pa has been obtained.
We feel that pressures of 1 X 10-8 Pa are obtainable after a
full bakeout.
A. EI mode
The repetition frequency of the experiment in the E1
mode is limited to either the maximum pulsing frequency of
100 Hz at which the pulsed molecular beam valve may be
operated or by the time scale of the reaction under study,
whichever is less. The aperture in the liquid nitrogen shroud
through which the molecules pass when entering the ion
source has a maximum diameter of 8 mm, for an angular
resolution of 4.5" and a solid angle of acceptance of 2 X 10-2
Sr. This diameter may be reduced, at the expense of signal to
improve the angUlar resolution if necessary.
TOF mass spectrometer 3167
••••••••••••••••••••••••••••• -;.-.;.;.............. .' •••• ;" ••••• r ••••••••• ;.~.:.:.:-; ••••• ; ............... ";~ ....... ~ •• ;".>.> ••••••• -••• :.-•••• ; •••••••••••••••••••••• -;".';.-••••• :.:.;0;-.:0;.:.; •• ' •••••••••••••.
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:42o 8 -
128
124 126
64000 65000
TiME IN nSEC
FIG. 11. Mass spectrum of pulsed xenon beam. The spectrum required
30000 molecular beam pulses. The acceleration potential VI of Fig. 6 was
-300 V. The 50% mass resolutoll is 1426 at mass 129.
Figure It is a mass spectrum of xenon taken by allowing
a xenon molecular beam pulse to go directly through the
TOF ionizer and synchronizing the EI source trigger with
the passage of the molecular beam. The xenon molecular
beam pulse had a duration of 300 j.1s FWHM. This was mea
sured by varying the delay between the pulsed valve and EI
source triggers. In future operation the detailed primary
pulse shape will be measured by making a series of these
measurements using the programmable delay in conjunction
with the transient recorder to assemble a sequential data set.
In this way the primary beam energy and temperature may
be deduced. Currently the software to make this a routine
operation is under development. Direct detection is accom
plished by rotating the TOF to one of the two direct beam
detection points. By detecting directly the high Mach num
ber pulsed molecular beam the thermal velocity effect on the
observed mass resolution is greatly reduced.
The EI ion source produces a peak ionization current of
lOrnA. The direct molecular beam signal under these condi
tions is intense. The spectrum was acquired using the tran
sient recorder in the pulse counting configuration of Fig. 10.
In order to prevent nonlinearity in the peak heights due to
pulse pile-up, the emission current was backed off to 1/100
of this peak value. The electron injection pulse width was
100 ns. The spectrum was taken using the discriminator
pulses as input to the transient recorder. Acquiring the spec
trum required 30 000 molecular beam pulses, or about 300 s.
Considerably fewer pulses would be required if only the ma
jor isotopes were of interest. The abundance values are quite
good for all isotopes including 124Xe -I and !2('Xe + , indicat
ing that nonlinearities due to pulse pileup at the detector
were not present. The i29Xe+ mass peak width is 23 ns
FWHM for a mass resolution of 1426 at 129 amu.
Figure 12 is a series of mass spectra taken by scattering a
pulsed molecular oxygen beam off of the sample holder and
synchronizing the EI source operation to detect the scat
tered signal. This signal was acquired as a function of the
polar angle of the ionizer head relative to the surface normal.
The molecular beam strikes the sample holder normal to its
3168 Rev. SCi.lnstrum., Vol. 60, No. 10, October 1989 0.04
0.03
I
~ 0.02
::oJ ;t::
0..
E <:
0.01
'b0
IZJ"> roO ~f/j
\)<c <::;)eOJ
o -2Z;-'~~~~-:-'-~-L,.) c . .s-27.4 27.6 27 . 'l,; ~
Time In M· .8 2B.0 28.2 c .... ~0 Jcroseconds
FIG. 12. Angular resolved spectrum of molecular oxygen beam scattered
from the sample holder. The oxygen signal is at t = 28 f..ts. The extinction of
the signal near 0 = () is due to the ionizer head eclipsing the primary beam
and preventing it from reaching the target. The angle e is defined as in Fig.
5. The angle 'P of Fig. 5 = o. The mass 31 signal at! = 27.5,us is probably a
CH20H+ ion signal produced by cracking of the ethyl alcohol used to clean
the ionizer.
surface. The oxygen pulse had a duration of 250 JLs FWHM.
This was measured by varying the delay between the pulsed
valve and EI source triggers. The electron injection pulse
width for this data set was set to 1 f.1s, and the emission
current was set to 10 mAo 1000 molecular beam pulses were
required for each trace in the figure, The resulting variation
in the mass 32 signal with polar angle clearly demonstrates
both the angular and time-resolved capabilities of the sys
tem. The scattered signal is easily visible on an oscilloscope
running at the experiment repetition frequency. The invar
iant mass 31 peak seen in the figure is most likely a cracking
product of the alcohol used to clean the ionizer assembly
prior to the measurement. In operation, these constant back
ground peaks should serve as a convenient indicator of sys
tem stability when running experiments.
The total sensitivity of the TOFMS in the El mode was
measured by observing the detected ion current for a mea
sured background of oxygen admitted to the system. The
total sensitivity for oxygen was 0.001 Pa--I (0.1 Torr-i).
The minimum detectable partial pressure of oxygen is esti
mated to be 1 X 10-13 Pa (1 X 10 i5 Torr). This sensitivity
figure and the observed magnitude of the scattered signal of
Fig. 12 makes us optimistic for detecting products of low
abundance in surface reactions.
B. SIMS mode
In the SIMS operating mode, the mass resolution of the
TOFMS in the low mass end is limited by the primary 40 Ar +
ion beam pulse width. The low mass peaks suffer the smallest
broadening in their time-of-flight distribution and therefore
are representative of tile primary ion beam pulse width. This
pulse width may be measured as the width of the H+ ion
peak which is produced in abundance from aU the samples
rOF mass spectrometer 3168
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:428 a ---, H +, FWHM = 5.8 "SEC
\
\
~2,L50--.L---::C2J:!::OO:---J<..--~2J'!'.:5:-0 -....l.---:24::0'::"0 ---'---::-:2450
TIME IN nSEC
FIG, 13, He-secondary ion peak from Csi on Ta sample. The peak width
indicates a primary ion beam pulse width of 5, i ns FWHl'vL The spectrum
was taken with an acceleration potential ¢" of -3 k V.
examined so far. This peak is thought to be a product of
surface contaminants and is possibly due to hydrocarbons
present at low levels in the vacuum system. Figure 13 is an
Ht secondary ion peak produced from a sample of cesium
iodide on tantalum. The primary ion beam pulse width is
given by the width of this peak at 5.7 ns FWHM. Measure
ment of the mass resolution requires observation of a fairly
intense peak at high mass. The Csl on Ta sample used to
produce the H+ secondary ion peak for measurement of the
primary beam pulse width also produces cluster ions of the
Cs [ Csl) It type with the n = 1 peak as the most abundant.
This peak taken with an extraction potential of 3 keY is
shown in Fig. 14. The peak width is 16.8 ns FWHM yielding
a mass resolution of 1375 at mass 393. The observed mass
peak width is the sum in quadrature of the primary ion pulse
width and the intrinsic width or mass resolution of the
spectrometer. The intrinsic width is then given by
Cs[Csll +, FWHM = 16.8 nSEC
\.
46100 46200 46300 46400 46500
TIMe IN oSEC
FIG. 14. Cs[Csl] + secondary ion peak from Csl on Ta sample. The peak
width indicates a mass resolution of 1375 at 393 amu. The spectrum was
taken with an acceleration potential ¢" of -3 kV.
3169 Rev. Sci.instrum., Vol. 60, No. 10, October 1989 ! POL vSIYRENE -1000 ON Ag
I
JJJ. I. Il j
iI.i.L ,l I
BOO ,200 1600 20~0
MASS IN AMU
FIG. 15. Positive iOIl spectrum of polystyrene with an average molecular
weight of 1000 amu on nitric acid etched Ag taken under static SIMS condi
tions. The primary 4°Ar' ion beam energy was 6 keY. The integrated ion
dose was 6.3X 10" cm-2 The extraction potcntial¢a was -3 kV.
il.t fntrinsic = 6.t ~bscrvCd ~ Ilt;"r I • (9)
The intrinsic pulse width is 15.7 I1S for an intrinsic mass
resolution of 1472. This is in good agreement with the ex
pected value, and is acceptable for our purposes. The total
TOF transmission in the SIMS mode was estimated in a
rather crude fashion by multiplying the 80 nA dc beam ob
tainable from the ion source by the beam pulse width to
obtain an estimate of 9000 ions per pulse. The Csl used for
the mass resolution calibration was found by wet chemit,try
to contain 0.39 ± 0.02 At. % rubidium. This rubidium signal
is easily visible in the secondary ion spectrum. If we assume
that the sputtered Rb is fully ionized, that the sputter yield is
unity, and that the surface concentration of rubidium is
equivalent to that of the bulk, we may count ions in versus
ions out to determine total transmission. Measured in this
way the total transmission is 0.7 ± 0.03%, a factor of 17 less
than our estimated value but still rather close considering
the many assumptions involved. Assuming that the surface
concentration of rubidium is equivalent to that of the bulk, it
represents a coverage of 4 X 10-:1 monolayer, This is on the
order of the instantaneous coverage of reactive species deliv
ered by the molecular beam. The rubidium is easily detect
able, leading us to be optimistic for our prospects of detect
ing transient and nonvolatile surface species produced
during molecular beam bombardment of the surface.
As a prelude to the molecular beam experiments, we are
currently using the TOF in a static SIMS mode to evaluate
the utility ofthe TOF for the study of organic molecules used
in semiconductor processing. Figure 15 is a spectrum of
polystyrene with an average molecular weight of 1000 on a
silver sample taken under static SIMS conditions. The use
fulness of acquiring a full mass spectrum in parallel is clearly
demonstrated by the rich character of the mass spectrum.
The sensitivity of the organic molecules to radiation damage
gives the TOF with its low ion beam duty cycle a significant
advantage over serial acquisition mass filters.
We have demonstrated the viability of the dual mode
TOFMS for the study of both nonvolatile and volatile spe
cies at levels of concentration typical for molecular beam
TOF mass spectrometer 3169
.~.-••• , •••• -. ',' '."._ "." _. '_.~._.'.'._.~._ •• -., •• ~n ' ••••••• ~.;" ••• ' ••••••• _.'._._ ••• ;._ ••••••••••••••
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130.216.129.208 On: Mon, 01 Dec 2014 01:23:42scattering studies. The main hurdle yet to be overcome with
this system is the creation of software capable of controlling
the angular and temporal degrees of freedom which charac
terize the experiment. This software is under development,
and we hope very soon to have the first results of actual
experiments with this system for presentation.
ACKNOWLEDGMENTS
The author wishes to thank B. Olson for the wet chemis
try results and M. L. Yu and W. Reuter for a critical review
of the manuscript.
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22Avtech Electrosystems Ltd., P. O. Box 265, Ogdensburg, New York
13669.
2'Northwest Bearing, 1954·H Old Middlefield Way, Mountainview, CA.
TOF mass spectrometer 3110
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1.1141888.pdf | A model of charge collection in a silicon surface barrier detector
Ikuo Kanno
Citation: Review of Scientific Instruments 61, 129 (1990); doi: 10.1063/1.1141888
View online: http://dx.doi.org/10.1063/1.1141888
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52A model of charge collection in a silicon surface barrier detector
Ikuo Kanno
Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-11, Japan
(Received 26 June 1989; accepted for publication 25 September 1989)
Charge collection process in a silicon surface barrier detector (SSE) was investigated as a
following phenomenon of the formation and erosion of a plasma column the author reported
elsewhere [Rev. Sci. lnstrum, 58,1926 (1986)). As an application of Ramo's theory, a model of
charge collection process was presented. With this model, the top and bottom position and
disappearance time of a plasma column were determined analytically. The induced currents and
charges were calculated for alpha particle and 40 Ar ion whose plasma delays were determined as a
function offield strength by other authors' experiments. The contributions of electrons and holes
to the induced currents and charges were determined separately. The times of the plasma column
disappearance and the last hole arrival to the negative electrode, and the maximum induced
currents were tabulated. The peak time of the induced currents became slightly longer as the bias
voltage of the SSB increased from 50 to 200 V.
INTRODUCTION
The silicon surface barrier detector (SSB) is widely used for
the detection of charged particles" However, the SSB has two
unfavorable characteristics due to the formation of a plasma
column by an incident particle: pulse height defect 1 and plas
ma delay. 2 The author reported models of the formation and
erosion of a plasma column, which illustrated the pulse
height defect and plasma delay fairly wel1.3
The SSB has another well-known feature: "plasma
time," which relates to the pulse rise time of the current
pulse of the 8SB.4 The plasma time does not affect in the
determination of incident particle energy; however, it
changes the trigger time of electronic circuits and causes
time retardation and time jitter.
The definition of the plasma time depends on the au
thors. Quaranta, Taroni, and Zanarini assumed the squared
plasma time is obtained by subtracting a squared calculated
rise time from a squared observed rise time.4 Seibt, Sund
stroem, and Tove determined the plasma time by subtracting
a calculated rise time from an observed rise time.5 For the
simplicity of the definition, the author would like to employ
the term "peak time" instead of the plasma time, which
means the time when the calculated current pulse has its
peak value refered the beginning time of the plasma column
erosion which is regarded as the zero time. The peak time
reflects the charge collection process after the erosion of the
plasma column.
In this article, a model of the charge collection process
in a SSB is described. The charge collection process of a
single carrier as a function of time was first described by
Ramo.6 Many textbooks show this Ramo's theory with the
illustrations of induced charge and current as a function of
time.7 No application of this theory to the large number of
carriers as in the plasma column erosion has been proposed.
The model of this part of the charge collection process is
closely related to models of formation and erosion of a plas
ma column. ~ It requires information of field strength-depen
dent plasma delay to determine constants associated with
the plasma column" In this article, models of the formation and erosion of a
plasma column are described first. A detailed description is
given in Ref. 3. Next, a model of charge collection is present
ed and is applied to the alpha particle and 40 Ar which plasma
delays as a function of field strength have been experimental
ly measured by Bohne et al. g
I. MODELS OF PLASMA COLUMN FORMATION AND
EROSION
A. Model of plasma column formation
The plasma column is composed of electron-hole pairs
which are created by an incident ion. The plasma column
formation is considered by the two steps as electrons in the
silicon are recoiled by the incident ions, and each recoil elec
tron creates electron-hole pairs as it loses energy along its
path.
The incident ion recoils electrons according to the
Rutherford scattering cross section u:
-1-( kMe2ZCff)2 {}'- ,
4 2mE (1)
where m and Mare the masses of electron and incident ion, e
is the electronic charge, E and Zeit" are the energy and effec
tive charge of incident ion, and k is the conversion constant
1.0365 from MKSA unit to em, amu, ns, and MeV in energy.
The effective charge Zdf is given as9
Zctf = Z [1-exp( -125,8 /Z2i3) J, (2)
where,Bis the velocity of the incident particle divided by the
velocity of light.
The electron energy Ee recoiled at an angle (J with re
spect to the incident ion track is given asIO
E" = 2m V cos e = --cos e. ( M )2 4mE 2
. M+m kM (3)
The range of the primary electron R can be given by
R = gEe = Ro cos2 e, (4)
In Eg. (4},gis 9.93 X 10-3 g/cm3 MeV (Ref. 11), andRo is
the recoil electron range for the case of () = O. The region in
129 Rev. Sci.lnstrum. 61 (1), January 1990 0034-6748/90/010129-09$02.00 ® 1990 American Ilistitute of Physics 129
...•• -••••• ',~.' ••.. ,'.',";-.'.'.-.:.:.;.?'.'.'.'.'.'.'.'.'.'.'."",' •.•.• -.;-:.: ••• :., ••••••••• , •••••••••••• : •.• ;.:.;-:.:-;.:.: •••••••••••• >; •• ' ••• :.:.:':.:.:-;.;.; •• 0;0;., •••• <; ................. ;:.:.:.:.:.: •••••••• ' ••• ' ••••• :.:.~.:.:.:o:.:.:-; •.• , •...•••. T •• ,. ...... v,~ •.•.• '7.:.-•.•.•.• , ....•.. ,> ••• '7.-.-.
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52which the recoil electrons move is shown in Fig. 1. We define
this three-dimensional region as p(Roex)], where x is the
position of the incident ion in the 5SB. Ro depends on x due
to the energy loss of the incident particle in the silicon along
its track.
The recoil electron is assumed on the average to create
an electron-hole pair along its path as it loses energy of 3.6
eV. The number of electron-hole pairs is given as a product
of (J and the volume of the region P[Ro(x)]. The volume of
the region P[Ro(x)] is given as12
P [Ro(x)] = -}j1TRo(X)3. (5)
The plasma column is obtained by superimposing the num
ber of electron-hole pairs which is given as a function of the
mass, charge, and energy of the incident ion and the position
in the SSB.
B. Model of plasma column erosion
In the plasma column formed by an incident charged
particle, there exist a high density of electron-hole pairs al
most like a conductor. Electrons and holes inside the plasma
column are not affected by the external electric field. The
charge collection does not start until the electric field pene
trates inside the plasma column. The plasma delay is the
time interval from the plasma column formation to the be
ginning of plasma column erosion.
The density of the electron-hole pairs is reduced initially
mainly by the enlargement of the plasma column radius due
to the diffusion of electrons and holes, because the number of
the recombinations can be estimated to be only a few percent
of the total number of electron-hole pairs by Finch, Asghar,
and Forte. 13
Regarding the plasma column as an infinite cylinder,
the electric field strength F inside the plasma column in the
external field strength Fe is obtained as
F= (tlE')Fe, (6)
where IE is the dielectric constant of silicon, E = 1215'0 (eo is
the permittivity of free space), and E' is the dielectric con
stant inside the plasma column. We assume that the dielec
tric constant inside the plasma column, E', is proportional to
the electron-hole pair density, employing the mean-squared
radius of the plasma column, r, as14
/1
"-121 I
r:-'iJc, I
li'0 I e Ro _-1 __ _
I
\
I
\
\
\
\ (7)
FIG. 1. The region in which primary electron recoiled at position x by a
heavy ion can move. Ro is the maximum range of the primary electron and is
a function of mass and energy of the heavy ion.
130 Rev. Sci.lnstrum., Vol. 61, No.1, January 1990 (8)
where a is a constant, Dais the ambipolar diffusion constant,
"f6 is the initial mean-square radius of the plasma column,
and Po is the initial electron -hole pairs density.
The field strength inside the plasma column is written as
F= 4Dat + "f6 Fe. (9)
aporo
At t = 0, the denominator of Eq. (9) is very large, and the
electric field strength inside the plasma column is nearly
zero.
The author assumes here that the electron and hole col
lection starts when the internal electric field strength reaches
a certain value Fi:
4Dat + Po -..::--_-'- Fe = Fj•
apo1''o (10)
Solving on t, the plasma delay is obtained as
"f6 ( Fi ) t=--apo--l .
4Da Fe (11 )
The author assumes here that the ambipolar diffusion
constant depends on the electric field strength and the plas
ma column volume Da (Fe. V). The plasma column cannot
enlarge itself freely because of the Maxwell's stress which
depends on the electric field strength as F;. The electrons
and holes diffuse more when the volume of the plasma col
umn is larger. Then we write the ambipolar diffusion con
stant as
(12)
where c is a normalization factor, and D aO is the ambipolar
diffusion constant which is independent of the electric field
strength and the volume of the plasma column, 16 cm2/s
(Ref. 5). With Eqs. (11) and (12), we obtain the plasma
delay as
(13)
The differential of the plasma delay against the external elec
tric field strength is
dt cr6 --=-- (apoF -2F). dFe 4DaO V I C (14)
The plasma delay has a maximum value at the external field
strength, J<~nax' of
apOFi
Fmax =--2-
II. A MODEL OF CHARGE COLLECTION
A. Ramo's theory (15)
Ramo's theory has been employed for the calculation of
the induced charge and current as a function oftime.7 When
a charge is moving a distance .6.x between two parallel elec
trodes separated by a distance w, a charge AQ is induced on
the electrode as
Surface barrier detector 130
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52aQ = e(..:1x/w). (16)
The induced current I is given as
1= dQ.
dt (17)
This theory was proposed by Ramo for the case of no fixed
space charge between the electrodes. Jen15 and Cavalleri et
al. 16 showed the validity ofEq. (16) for the case when space
charges were present.
B. Tearing the jammed carriers
The electrons and holes inside a plasma column are un
der the influence of the electric field strength with the magni
tude indicated in Eq. (9). Regarding the plasma column as a
cylinder with constant carrier density between the two elec
trodes as is shown in Fig. 2, the electrons and holes move
toward the positive and negative electrodes with the veloc
ities ve and v" of
Ve =I-te F,
Uk = -/Lk F, (18)
(19)
where /Le and Ph are the mobilities of electrons and holes.
The movements of electrons and holes at the top and bottom
positions of the plasma column are illustrated in Fig. 3. The
x coordinate of the plasma column top It (t) decreases as
dl,(t) --= -!-lh F. (20) dt
The field strength in a SSB is given as17-19
F ( ) = (d -x) eX, (21)
!-le T
where T is the electron charge collection time, l' = EEoA
= A X 10 -12 s, with A being detector resistivity, and d is the
depletion layer length,
d = (2711" U) 1/2, (22)
with a given bias voltage U. Employing Eqs. (9) and (21),
the differentia! equation on the top position of the plasma
column is obtained as
dl, (t) !-lh [d -It (I) J .,.2 (23) --= - (4Dat+'l),
dt aplPi Ite T
where r; and PI are the mean-squared radius and the elec-
"O=:
-0'
'" <=
~ ~ '-----------------'==-- Sx
,6" Au wi~dow
~---------------------~
Fe I Si licon £ r l: Plasma Column E' Ioo------------i ~ :----10 ",_ __ "'--------1
-V i------------d
FIG. 2. A schematic drawing oran SSB and a plasma column.
131 Rev. SCi.instrum., Vol. 61, No.1, January 1990 -v Escaped holes
(!le+ ,uh) .1, Fe Ilb1tlJS(t J Phft J E Escaped eleclrons
lile + Ilhl {; Fe (I,It) lS(t i Pel t )
FIG. 3. The top and bottom positions of a plasma column and the numbers
of escaped electrons and holes.
tron-hole density of the plasma column at the time the col
umn begins to erode. In the fonowing, the time t indicates the
time after the beginning of the plasma column erosion. By
solving Eq. (23), the top position of the plasma column at
time t after the column erosion, It (t), is determined as
I ( 2Dat2+~t)
t (t) = d -Cd -lo)exp f..lh _, ,
apllJ f..leT (24)
where 10 is the top position of the plasma column.
In the same way, the differential equation on the bottom
position of the plasma column, I h (t), is obtained as
dl" (t) _!-le [d -Ii> U)] -:2 --- (4Dat + TI)' (25)
dt aplG /Le T
With the initial condition h (0) = 0, the bottom position is
obtained as
( 2Dat2+Pit) lb (£) = d -d exp -/Lc _. .
aplr:. f..leT (26)
The number of electrons leaving the plasma column at
time t is given as
cd-It (t)
Ne (t) = (Pe +!-llz) -; S(t) Pe (t). (27)
(; Pe l'
Here, S(t) is the top area of the plasma column, andpe (t) is
the electron density inside the column. Corresponding to Eq.
(27), the number of holes leaving the plasma column is ob
tained defining Ph (t) as hole density as
to d -lb (t) Nh (t) = ( !-le +!-lh) -; S(t) Ph (t). (28)
(; /L" l'
C, Characteristic quantities in the charge coilection
process
In this section, the time and position of the plasma col
umn disappearance and the time the last hole arrives at the
negative electrode are determined.
Equating the Eqs. (24) and (26), following quadratic
equation is obtained to calculate the time of plasma column
disappearance td:
2Dat 2 + Pi t _ aplPi l1eT In _d_ = 0,
!-le + !-liz d -10 (29)
and the time of plasma column disappearance td is deter
mined as
Surface barrier detector 131
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52td =_1_ [_~
4Da
+ {Pi + SDaap/f Pe riC Pe + Ph )In d I(d -10') ],
(30)
The position at which the plasma column disappears, Xd is
calculated with Eqs, (24) and (30) as
Xd -d 1--- , - r (d_lo)IL/(i£erlLhl]
c d (31)
The positions of electrons xe (t) and holes Xh (t) which
start to move at the position Xo are determined by the follow
ing equations:
(32)
(33)
From Eqs, (30), (31), and (33), the time last hole arrives at
the negative electrode th is determined as
(34)
D. Induced current and charge
Only one electron-hole pair was considered in the Ra
mo's theory, In this section, we apply the Ramo's theory to
the plasma column which consists oflarge numbers of elec
trons and holes, Induced currents by electrons and holes are
determined first and induced charges are calculated as time
integrals of these currents,
The induced currents are separately determined as the
current induced by the carriers outside the plasma column
and the current induced by the carriers inside the plasma
column,
1. Currents due to the carriers outside the plasma
column
The position of the electron at the time t, x e (t) , moving
from the initial position Xo is determined by Eg, (32), The
electrons which are observed outside the plasma column at
the time t left the column between the time 0 and t, The initial
position of the electrons left at the time {; (0<;< t) is given as
the top position of the plasma column, 1, (;), as described in
Eq, (24), The position of electrons at the time t, which left
the plasma column at the time (;, Xe (t,t;), is determined as
Xe (t,!;) = d -(d -It (;) ] exp (_ t ~ (;)
( t-;) Xexp --'T-' (35)
The number of electrons left the plasma column at the time t;
is given in Eq, (27), The current induced by electrons out
side the plasma column Ie,out (t) is calculated as
132 Rev, Sci.rnstrum" Vol. 61, No.1, January 1990 (36)
The current induced by holes outside the plasma column
Ih,out (t) is calculated in the same way except the lower inte
grallimit due to the arrival of the earlier holes to the negative
electrode, Among the moving holes outside the plasma col
umn at the time t, the hole left the column at the time!; is in
the position x h (t,(;) of
( 2DaC + "Fi!;) Xh (t,!;) = d -d exp -flo --_--
ap,Yi Per
( !-th (t -(;) ) Xexp ,
fl'e r (37)
Equating the Eq, (37) to zero, we obtain the time (;,. (t), the
time the hole, which arrives at the negative electrode at the
time t, left the plasma column, The lower integral time (; h (t)
is the solution of the next quadratic equation:
2D,,(;2+"Fi (1 +ap,~)s-ap,~t~=O, (38)
f.L e !-te
and is determined as
(;/1 (1) = _1_ [ -"Fi ( 1 + apt ~)
4Da Pe
+ -J?fTI + a;;;PhIPe)2 + 8Daapl~ tPillPe ] ,
(39)
The holes which left the plasma column before the time
(;h (t) already arrived at the negative electrode by the time t
and do not contribute to the current. The current induced by
holes outside the plasma column is calculated as
e it dXh (t,s) .
IiI,out (t) = -d Nh (~') ds,
,,,(I) dt (40)
2. Currents due to the carriers inside the plasma
column
Inside the plasma column, the positions of electrons
x; (t) and holes x~ (t) moving from the initial position x are
determined by the following equations,
( €t \ x;(t) = d -Cd -x)exp -c'r) , (41)
Xh (t) = d -(d -x)exp (Plitt) .
Pet'r (42)
The number of carriers between x and x + !l.x inside the
plasma column are given as
• N; (t) = S(t) Pc (t)!u
for electrons, and
N~ (t) = S(t) Ph (t)!l.x (43)
(44)
for holes, assuming the densities constant inside the column,
With the Ramo's theory, Eq, (17), the currents induced by
the electrons Ie,in (t) and holes I h.in (t) inside the plasma col
umn are calculated as
Surface barrier detector 132
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52Ie,in (t) = -!!-.- N; (t) --<' -dx, 1/,«) dx'(t)
d 1,,(1) dt
e JIM dx' (t) I",in (t) = -:-N h (I) _h __ dx,
d ',,(t) dt
m. RESULTS AND DISCUSSION
A. Plasma columns (45)
(46)
For the calculation of the currents and charges induced
by electrons and holes after the plasma column erosion, plas
ma columns were calculated for a 8.78 MeV alpha particle,
and 268-and 476-MeV 4°Ar ions.
An example of a plasma column is shown in Fig. 4. Oth
er plasma columns are illustrated in Ref. 3. The calculated
plasma delays are plotted in Fig. 5 compared with the experi
mental results of Bohne et al.8 The densities of electron-hole
pairs, the volumes, lengths, and mean-squared radii of the
plasma columns created by those charged particles are listed
in Table I. The normalization constant c and the constant
correspond to the maximum plasma delay aFt are also tabu
lated.
B. Induced currents and charges
In the calculation of the induced currents, effective field
strength was taken as that} along the initial plasma column
length.8 The resistivity of the detector employed in Ref. 8,4,
was 4700 n em, and the mobilities ofelectronpe and hole/iii
were 1481 and 480 cm2/V s, respectively. The densities of
electrons and holes vary during the erosion because of the
difference of the velocities of the top and bottom positions of
the plasma column. This change of carrier density causes the
change of die1ectricity of the plasma column and affects the
rapidity of the column erosion. However, the changes of car
rier densities were as small as some tens of percent of the
initial densities, except for the case of 476-MeV 40 Ar with
E
"'co o
o
1 _----Au window
~~----58,m ------
I
FIG. 4. An example of the plasma column formed by alpha particle with 8.78
MeV. The numbers on the contour lines show the density of electron-hole
pairs. For other plasma columns, see Ref. 3.
133 Rev. SCi.lnstrum., Vol. 61, No.1, January "1990 2.5
'" 20 <:
.t~268Mev >. 15 ,," II. c .., ..........
C>
e~ev <:I 1.0
E
'" <:> e~ 0-0.5
40Ar 476 MeV
0 0.1 0.2 0.3 0.4 05 0.6
1 ( F!!f (em IkV I
FIG. 5. The plasma delay of 8.78-MeV alpha particles, and 268-and 476-
MeV 4"Ar. The symbols are the experimental data of Bohne et al. (Ref. 8).
Solid lines are the results obtained by the present work.
TABLE 1. The density of electron-hole pairs, volume, length, and mean
squared radius G of the plasma column at the time ofits creation. The maxi
mum plasma delay t",,, and the Held strength F.n., are taken from Bohne et
al. (Ref. 8). The constants c and aFi are calculated and listed in the bottom
two columns.
Particle alpha <OAr
energy (MeV) 8.78 268 476
Density (n/em') 1.71 X 101" 4.34X 10'0 1.07 X 1{)19
Volume (em') 1.45 X 10.13 1.69x.1O-12 1.23 X 10-II
Length (em) 5.76X lO-:< 7.28X 10-3 1.57 X 10-2
Radius (em') 8.01 X 10 12 7.39Xl0-!1 2.49X 10-10
fn:.;)), (8)· 1.25 X 10-9 1.85XHy9 O.60X 10 9
l:nax (V /em)· 3.77X 103 4.44 X 10' 5.26X 10'
c 1.02 X 10 16 1.37x 10-16 6.86 X W-17
aJ;: 4.41 X 10-16 2.05X 10-'" 9.83X 10 16
a Reference 8.
(X 10-5)
7r-~~----~--~-----r--~
6
o 5 10 15 20 25
Time (Second) (x 10-9.1
FIG. 6. Current induced by 8.78-MeV alpha particle with bias voltages of
100 V with Fi = 50, 100.300,500, and 1000 V /em.
Surface barrier detector 133
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52bias voltage of SO V (three times the initial density), and
were not taken into consideration.
In the model of the charge collection process, the con
stant a plays an important role as shown in Eqs. (23), (25),
and ( 30) . The smaller the constant a, the quicker the plasma
column disappearance. The determination of the constant a
has another interesting aspect: the determination of F,. By
fixing the constant a, the internal field strength at which the
(xl0-5)
6
~ ( 0) .., 5 0. E <! 4 -c:
~ 3
=> u
'0
'" '" :> '0 c:
10 15 20 25
Time (Second) f x 10-9)
( )(10-5)
6
(b)
~ 5 8-E <( 4
c 3
~
:::> u 2
'0 Q)
<.>
::> = c: ......
0 5 10 15 20 25
Time ( Second) (x 10-9)
(xfo-5j
6
~ Ie)
& 5
E <! 4
C
~ 3
::I
U
= ..,
'" => 'e c:: ......
Time ( Second)
FIG. 7. Current induced by 8.78-MeV alpha particle with bias voltage of (a)
so V, (b) 100 V, and (e) 200V.
134 Rev. Sci. (nstrum., Vol. 61, No.1, January 1990 plasma column begins to erode can be estimated. Before
hand, the calculation of the induced currents and charges by
the three kinds of charged particles, the author examined the
Fi dependence of the peak time of alpha particle to have
several nanoseconds with the bias voltage of 100 V. Some
results of induced currents obtained by changing Fi are
shown in Fig. 6. The greater the Fi, the smaner the constant
a and the peak time. If the plasma column begins to erode at
(x 1O-!4)
40
J::> 35 (0)
E .E
::> 30 0 u
25
GJ e' 20 c .c
U 15
'2 10 <.> ::> "0 5 c::: ..... Hole
0 5 10 15 20 25
Time (Second) I x 10-9)
I xlO-14)
40.
.0
E 35 .2
::> 30 <:> u
25
'" :=' 20 <) .c:
U 15
'0 Q) 10 <.> :>
't:> 5 c
0 5 10 15 20 25
Time ( Second) [x 10-9)
(x 10-14)
40
.0 35 E .2
=> 30 0 u 25
a.>
~ 20
0 = u 15
"0 10 ..,
u
'" 5 "0
oS
0 5 10 15 20 25
Time ( Second) ( x 10-9)
FIG. 8. Charge induced by 8.78-MeV alpha particle with bias voltage of (a)
50 V, (b) 100 V, and (e) 200 V.
Surface barrier detector 134
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52small internal electric fieid strength, the carriers move very
slowly and the current pulse becomes very broad. On the
contrary, rapid charge collection occurs with the higher in
ternal field strength. The constant Fi must be determined by
fitting the current peak time to experiments. In this article,
the author chooses Fi as 300 V /em.
For the 8.78-MeV alpha particle, the induced currents
are shown in Figs. 7 (a) -7 (c) for the bias voltages of 50, 100,
and 200 V. The currents induced by electrons and holes are
(x \0-4)
14.---~-.--.----'~---r---'
i 12 (a)
E
<l: to
8
6
o 5 10 15
Time (Second)
(x 10-4)
14~--~----~--~r----r---'
~ 12 K
~IO
c 8
~
.3 6
o
( xlO-4j
14
~ 12 ." 0.
E to <l: -c:
~
::l 6 u
'0
'" U
'" '0 c:
0-1
0 (bl
5 10 15
Time (Secondl
(c 1
5 10 15
Time (Second l
FIG. 9. Current induced by 268-MeY 4CAr with bias voltage of (a) 50 V, (b)
100 Y, and (el 200 Y.
135 Rev. SCi.instrum., Vol. 61, No.1, January 1990 indicated in the figures and the total currents are also plot
ted. The induced charges are shown in Figs. 8(a)-8(c) in
the same manner as in the Figs. 7(a)-7(c). The peak time
becomes a little greater from 5.5 to 6.1 ns as the bias changes
from 50 to 200 V.
Induced currents and charges are shown in Figs. 9(a)
g(e) and Figs. lO(a)-lO(c) for the 268-MeV 4°Arion and in
Figs. 11 (a)-ll (c) and Figs. 12(a)-12(c) for the 476-MeV
4°Ar ion.
(XW-12)
12r----r----r----r----~---,
(0)
o 5 fO 15
Time (Second)
( x 10-12)
12~----r_----r_----._---,r---~
..0
.§ 10
'" o
U 8
4
2
o
(x 10-12)
12
.Q
E 10 .2
::>
0
U 8
....
~ 6 "" .c
U
-0 4
G> u
=> 2 -0
<=
0 ( bl
Hole
10 15
Time (Second)
(el
5 10 15 20 25
Time (Second) (x 1O-9)
FIG. 10. Charge induced by 268-MeV 4°Ar with bias voltage of (al 50V, (b)
100 Y, and (e) 200 V.
Surface barrier detector 135
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52(x 10-4)
30
(0)
~ 25 .... Q.
E « 20 -c:: 15
~ :I u 10
"0
'" u 5 ::> "0 c:
20 25
Time (Second) (xlO-9 )
(x 10-4)
30
!bl
~ .... c.. E « -<= 15 ~ ::> u 10
-0 .... u 5 ::> "0 c:
0 5 10 15 20 25
Time (Second) (x 10-9)
( xl0-4)
30
(e) .... :u 25 c. E « 20 -<=
~
::l u
"0 10
<I.>
U
::::> 5 -0 c:: >-<
5 10 15 20 25
Time ( Second) (x 10-9)
FIG. 11. Current induced by 476-MeV 40Ar with bias voltage of (al 50 V,
(b) 100 V, and (c) 200 V.
The depletion depth, effective field strength, time of
plasma column disappearance, time of hole disappearance,
and peak current are shown in Table II for the 8.78-MeV
alpha particle, for the 268-MeV 40 Ar ion, and for the 476-
Me V 40 Ar ion. The peak time corresponds to the time of
plasma column disappearance for all cases.
The time of plasma column disappearance is greater
with the greater bias voltage. This stems from the Maxwell's
stress which strongly disturbs the plasma column diffusion
with greater bias voltage. The time the last hole arrives at the
136 Rev. Sci.lnstrum., Vol. 61, No.1, January 1990 (X 10-12)
25
.Q (0 )
E 20 0
S 8
15
'" ~ Electron <:> 10 .c
U
"0 w 5 Hole u
=> -0
C
0 15 20 25
Time ( Second) (x 10-9)
(x 10-12)
25
-c (bl E .9 20 ::J a u
.... 15 E'
<:>
.J::
U 10
-0
'" <.> ::> 5 "0 c: .....
0 10 20 25
Time ( Second) (x 10-9)
(x 10-12)
25
.0 tel
E 20 S?
:::I a u
15 ..,
E'
'" -'= 10 U
-0
'" u 5 '" "0 c:: Hole .....
0 5 10 15 20 25
Time ( Second 1 (xlO-9)
FIG. 12. Charged induced by 476-MeV .oAr with bias voltage of (a) 50 V,
(b) 100 V, and (c) 200 V.
negative electrode, as given in Eq. (34), is shorter with
greater bias voltage. With greater bias voltage, the position
of the plasma column disappearance is closer to the negative
electrode, as indicated in Eq. (31), and it takes less time for
the hole to arrive at the electrode.
The shape of the currents do not change very much ex
cept for the one of the 476-MeV 4°Ar with 50-V bias. In this
case, the carriers inside the plasma column moved very rap
idly with the weak electric field and sparse carrier density,
and as a result, carriers induced large current before most of
Surface barrier detector 136
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131.94.16.10 On: Sun, 21 Dec 2014 07:25:52TABLE II. The depletion depth d, etfective field strength Fe." plasma delay,
time of plasma column disappeara11ces t d' time oflast hole disappearance t h'
and peak current 1m" are listed for tile bias voltages of SO, 100, and 200 V.
Bias (V) 50 100 200
d (em) 2.64X 10-2 3.73xlO-2 5.28X 10 2
8.78-MeValpha
Fer; (V/em) 3.24X 103 4.81 X 103 7.03XIO'
Delay' (s) 1.25 X 10-9 l.10x 10-9 0.70X 10-9
td (8) S.S2X 10 -9 6.03 X 10 9 6.07 X 10-9
th (s) 8.21 X 10-9 7.87x 10-9 7.34X 10-9
Ima"A (Al S.69X 10-9 5.45 X 10-5 5.26X 10-5
268-MeV 4°Ar
Felf (V/cm) 3.09X 10' 4.66X 10' 6.88X 10'
Delay' (8) 1.60 X 10 -9 1.80 X 10-9 1.55 X 10-9
td (s) 6.96X 10-9 7.82X 10 9 8.1SX 10-9
tli (8) 1.05 X 10-" L02X 10 -8 9.77XlO-9
[max CAl 1.39 X 10-j 1.33 X 10-3 1.26 X 103
476-MeV 4°Ar
Ferf (V/cm) 2.29XlO' 3.86X103 6.08Xl01
Delay' (8) 0.30X 10-9 O.5SX 10-9 O.SOX 10-9
td (8) 4.94X 10-9 6.31 X 109 7.64X 10-9
th (s) L49X 10-8 1.23 X 10-" US X 10-9
Imax (A) 2.31 X 10-3 2.58 X 10-3 2.58X 10-3
• Reference 8.
them left the column. However, the carrier density became
as large as three times the initial one, and the density change
must be taken into account into the increase of the dielectri
city which reduces the rapidity of erosion.
In the model of plasma column erosion, the plasma col
umn was treated as a dielectricity. Inside the plasma column
of pure dieiectricity, no carrier moves and no current can be
induced. Treating the plasma column as a diclectric body,
the contributions of the electrons and holes inside the plasma
column expressed in the Eqs. (45) and (46) should be ne
glected and the total charge could not have the same value
with different bias voltage. For the charge conservation, car
riers must move inside the plasma column and induce cur
rents both at positive and negative electrodes. With regard
ing the plasma column after the erosion as a conductor like
body, the value of the currents at the time zero became non
zero. This stems from the simple assumption that the plasma
column changed its electric character from dielectric to COll
ductorlike immediately at the time the erosion began. For
137 Rev. SCi.lnstrllm., Vol. 61, No.1, January 1990 future improvement of the charge collection model, the
changing process of electric character of the plasma column
should be taken into account.
Experiments on the plasma delay being not sufficient,
only three kinds of charged particles are applicable to this
model. More experiments on the plasma delay must be car
ried out in the future to investigate the charge collection
process.
ACKNOWLEDGMENTS
The author would like to thank Professor 1. Kimura of
Kyoto University for his fruitful discussion on this study. He
is also grateful to Dr. Y. Kaneko of Japan Atomic Energy
Research Institute for his encouragement.
'H. W. Schmitt, W. M. Gibson, J. H. NeHer. F. J. Walter, and T. Do Thom
as, in Proceedings of the IAEA Conference on the Physics and Chemistry of
Fission. Saltzburg (International Atomic Energy Agency, Vienna, 1965),
p. 53!.
2 A. A. Quaranta. A. Taroni, and G. Zanarini, Nuc!. lnstrum. Methods 72,
72 (1969).
31. Kanno, Rev. Sci. lnstrum. 58,1926 (1987).
4A. A. Quaranta, A. Tarom, and G. Zanarini. IEEE Trans. Nuc!. Sci. NS-
15,373 (1968).
'W. Seibt, K. E. Sundstroem. and P. A. Tove, N ucl. lnstrum. Methods 113.
317 (1973).
6S. Ramo, Peoc. IRE 27, 584 (1939).
7G. Bertolini and A. Coche, Semiconductor Detectors (North-Holland,
Amsterdam, 1968).
"W. Bohne, W. Galster, K. Grabisch, and H. Morgenstern, NucJ. lnstrum.
Methods A 240, 145 (1985).
9W. H. Barkas, Nuclear Research Emulsions (Academic, New York,
1963). Vo!' 1, p. 371.
10K. H. Weber, Nuc!. lllstrum. Methods 25, 261 (1964).
HE. J. Kobetich and R. Katz, Phys. Rev. 170,391 (1968).
'2r. Kanno and Y. Nakagome, Nue!. lnstrum. Methods A 244, 511 (1986).
BE. C. Finch, M. Asghar, and M. Forte. Nuc!. lnstrum. Methods 163, 467
( 1979).
14p. A. Tove and W. Seibi, Nucl. lnstrum. Methods 51,261 (1967).
ISC. K. Jen, Proe. IRE 29, 345 (1941).
16G. Cavallevi, G. Fabri. E. Gatti, and V. Svelto, Nucl. lnstrum. Methods
21,177 (1963).
pP. A. Tove and K. Falk, Nucl.lnstrurn. Methods 12, 278 (1961).
'·P. A. Tove and K. Falk, Nllc\. lnstrum. Methods 29.66 (1964).
19N. J. Hansen, Progress ill Nuclear Energy (Pergamon, Oxford, 1964),
Vol. 4.
Surface barrier detector 137
.' ••••••••••••••••••••••••• -.;.: ••• ~ •••••••••••••• O;' ••• ·.·.:.-·:·:·:·: •• ·7 ••• ·.·.·.·;>.>.·.-.':".·.~.:O:.:.:·:·:·.·.· .... ;o;·.!,;·.·.·.·.·.-.·.:·x·:·:·: •. ·;·.· .• ;~.·;>.·.· ·"n' ••• ' ••• ~.:.;.; ••••••••• ,. ••••.
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1.338203.pdf | Electrical transport properties of transitionmetal disilicide films
F. Nava, K. N. Tu, E. Mazzega, M. Michelini, and G. Queirolo
Citation: Journal of Applied Physics 61, 1085 (1987); doi: 10.1063/1.338203
View online: http://dx.doi.org/10.1063/1.338203
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/61/3?ver=pdfcov
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128.114.34.22 On: Fri, 28 Nov 2014 22:25:04Electrical transport properties of transition-metal disilicide films
F. Navaa) and K. N. Tu
IBM Thomas J. Watson Research Center, Yorktown Heights. New York 10598
E. Mazzega and M. Michelini
Dipartimento di Fisica, Universita'di Modena. 1-41100 Modena, Italy
G. Queirolo
SGS-Microelectronica, 1-20041 Agrate MJ, Italy
(Received 23 June 1986; accepted for publication 22 September 1986)
Electrical resistivity in the temperature range of 2-1100 K and Hall-effect measurements from
10 to 300 K of CoSi2, MoSi2, TaSi2, TiSi2, and WSi2 polycrystalline thin films were studied.
Structure, composition, and impurities in these films were investigated by a combination of
techniques of Rutherford backscattering spectroscopy, x-ray diffraction, transmission electron
microscopy, and Auger electron spectroscopy. These silicides are metallic, yet there is a
remarkable difference in their residual resistivity values and in their temperature dependence
of the intrinsic resistivities. For CoSi2, MoSi2, and TiSi2, the phonon contribution to the
resistivity was found to be linear in temperature above 300 K. At high temperatures, while a
negative deviation from the linearity followed by a quasisaturation was observed for TaSi2, the
resistivity data ofWSi2 showed a positive deviation from linearity. It is unique that the residual
resistivity, p(2 K), of the WSi2 films is quite high, yet the temperature dependent part, i.e.,
p(293 K) -p(2 K), is the smallest among the five silicides investigated. This suggests that
the room-temperature resistivity of WSi2 can be greatly reduced by improving the quality of
the film, and we have achieved this by using rapid thermal annealing.
I. INTRODUCTION
As the complexity of device integration in very large
scale integration (VLSI) technology increases, transition
metal silicides have found a new application as gate intercon
nections because of their high conductivity and the ability of
surviving oxidation.t-5 Currently, the disilicides of Co, Mo,
Ta, Ti, and Ware considered for the application; indeed a
couple of them are actually in use. Nevertheless electrical
transport properties of these silicides have not been well
characterized, and none of them has been demonstrated to
be superior to the rest since no decisive comparison of their
conduction behavior has been made.
We report in this paper the temperature dependence
from 2 to 1100 K of the resistivity behavior of the five disili
cides and their residue resistivities. We show that it is more
fundamental to compare these quantities than the room
temperature resistivity although the latter is the application.
The formation of these silicides was by coevaporation since
the technique offers very reproducible films. Disilicides were
chosen because they are thermodynamically stable on sili
con.
II. EXPERIMENT
Amorphous thin alloy films of CoSi2, MoSi2, TaSi2,
TiSi2, and WSi2 were prepared by simultaneous evaporation
of high-purity metal and silicon in a dual electron-beam
evaporation system. Typical rates were 5 A/s for the metal
.j Present Address: Department of Physics, University of Modena, Mo
dena. Italy. and 12 A/s for the silicon. The pressure of the evaporation
chamber was typically 1 X 10-8 Torr during the deposition.
The thin films were deposited on oxidized silicon substrates
or on undoped polycrystalline Si films held at room tempera
ture through metallic masks to obtain a van der Pauw pat
tern. Specifically, details of the deposition of tungsten disili
cide films on oxidized silicon wafers and on chemical vapor
deposition (CVD) polycrystalline Si films have been report
ed by Ahn et al.6
Particular care has been taken in the specimen prepara
tion in order to obtain a stoichiometric ratio as correct as
possible in the as-deposited state in order to avoid Si or metal
segregation in the grain boundaries and/or at the surfaces
and interfaces during subsequent heat treatments. It is
known that the Si segregation towards the surface can
change the state of the electrical contact and induce electri
cal noise in the measurement particularly at high tempera
tures, while Si or metal segregation at the grain boundaries
can enhance the scattering processes, causing deviation from
the Matthiessen's rule.7 Furthermore, in order to reduce the
intake of impurities upon heat treatment, attention has also
been taken during the furnace annealing by covering the sur
face of the thin films with another wafer, or a short-time
annealing is used. For all the silicides studied, the impurity
content is at about the detection limit of the Auger electron
spectroscopy (AES) technique of analysis. Although the
five disilicides were prepared by the same procedure, a
slightly higher quantity of oxygen and carbon have been ob
served in WSi2, TiSi2, and MoSi2 than in TaSi1. and CoSk
thin films.
To crystallize the as-deposited films, the standard fur
nace heat treatments at 1000 or 900 ·C for 30 min were car
ried out in a flowing helium tube furnace where the helium
1085 J. Appl. Phys. 61 (3). 1 February 1987 0021-8979/87/031085-09$02.40 © 1987 American Institute of Physics 1085
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128.114.34.22 On: Fri, 28 Nov 2014 22:25:04was purified by passing through a bed of titanium held at
1000·C. On the other hand, the isothermal short-time an
nealing (ST A) of the tungsten disilicide films has been em
ployed with two different heat sources: one made by an array
of tungsten halogen lamps and the other by a resistant-heat
ed graphite. All measurements of the electrical properties
have been carried out on samples after either one of the
above heat treatments.
The low-temperature measurements of resistivity were
performed in a liquid-helium cryostat. The specimens were
mounted on a copper block located in a double vacuum
chamber which enabled the specimen temperature to be var
ied from 2 to 300 K. The temperature was measured with a
calibrated germanium resistance thermometer to within
± O. I K in the temperature range of 2-97 K and with a
calibrated iron-Constantan thermocouple to within ± 1 K
in the temperature range of 97-300 K.
The measurements of the resistivity at T> 300 K were
carried out with the sample placed in the above-described He
furnace, whose temperature was now increased with a con
stant heating rate of I ·C/min. The temperature was mea
sured by a calibrated Chromel-Alumel thermocouple at
tached to the sample holder and in direct contact with the
specimen. Four spring-loaded tantalum wires were contact
ed to the pads of the comers of the van der Pauw pattern.
Hall voltage measurements below room temperature
were performed with a variable-temperature cold-end sys
tem (Air Products model CS-202). The temperature was
measured with a calibrated thermocouple (Scientific Instru
ments Inc. model C907F Au-O.07 at. % Fe/Chrome!) to
within ± 0.5 K in the temperature range of 10-350 K. For
low-temperature measurements, dc technique was used to
measure the Hall voltages. The magnetic field was 8 kG and
the Hall voltage was linear with the magnetic field up to this
> I-
>
Ien
en w a: 100 I-
50 I-
o
1086 o Mo SI 2
o T a SI 2
W SI2 I
950 ·C. 30 min
900 ·C .
1000 ·C.
• CoSi 2
I
500
TEMPERATURE (K)
J. Appl. Phys .. Vol. 61, No.3, 1 February 1987 value. Currents of 1-15 rnA were injected, and the measure
ments showed that the Hall voltage depended linearly on the
current in the range utilized. The contacts were formed by
indium soldering thin copper wires to the four contact pads
of the van der Pauw pattern. Furthermore the power dissi
pated in the specimen during the measurements was less
than 1 mW, which reduced the resistive self-heating effects.
Depth composition analysis and the thickness measure
ment of the silicides were performed using Rutherford back
scattering spectrometry (RBS) with a 2-MeV 4He+ -ion
beam. Two different x-ray diffractometry (XRD) tech
niques (a wide-angle Bragg-Brentano reflection goniometer
with scintillation counter and a Wallace-Ward x-ray cylin
drical texture camera with photographic recorder), both
employing Cu Ka radiation were used to identify the silicide
phases present in the films. Auger electron spectrometry
(AES) was used to obtain information on the presence of
impurities (mainly C and 0) and on the depth distribution
of the various atomic species in the annealed samples. The
measurements were performed on a Varian Auger micro
probe, with a 5-keV, w-pA primary-beam energy and cur
rent; the instrument was equipped with a cylindrical mirror
analyzer with a 0.6% energy resolution. The base pressure in
the analysis chamber was typically less than 4 X 10-10 Torr.
For depth profiling, all the derivative Auger spectra in the
energy range of interest (200-1800 eV) were acquired with a
computer-controlled interface and an ion-etching step was
performed at each profiling. A preferentially pumped ion
gun was used, with a 2-keV argon-ion beam at about
2 X w-4-A/cm2 current density. The argon pressure in the
chamber was fixed at 1 X 10--7 Torr during the measure
ment. The relative sensitivity factors for the quantitative Au
ger analysis were obtained on a single compound film identi
fied with XRD and used as an internal standard. In the case
I
900 ·C , 30 min
900 ·C
I
1000 FIG. I. Temperature dependence of the resis
tivity of MoSi2• TaSi2• WSi2• TiSi" and CoSi2
thin alloy films after the heat treatment indi
cated.
Nava et al. 1086
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128.114.34.22 On: Fri, 28 Nov 2014 22:25:04TABLE I. Summary of the resistivity results for the specimens of Fig. I and monocrystalline CoSi, and MoSi, samples.
Compound p(293 K)
(lin cm) p(2K)
(lin em) p(293 K) p(2 K)
(lin em) p(293 K)
p(2K)
Polycrystalline
TiSi,
CoSi,
TaSi,
MoSi,
WSi,
Monocrystalline
CoSi, 23.50
21.02
49.80
58.70
33.48 3.40
6.30
12.02
39.89
22.01 20.10 6.91
14.72 3.33
37.78 4.14
18.81 1.47
11.47 1.52
thin films'
thick materialsb 15-15.68
14-16.00 2.6-3.53
3.0-3.50 12.4-12.15
11.0-12.50 5.76-4.48
4.66-4.57
MoSi,'
thick materials 12.7-17 0.101-0.097 12.61-17 126-176
• Reference 8.
b Reference 9.
c Reference 10.
of WSi" specimens (x ranges from 1.4 to 2.2), the Auger
transitionsofSi(KLL) and W(MNN) at 1619 and 1736 eV,
respectively, was monitored at the same conditions.
III. RESULTS
A. Electrical measurements
Figure 1 shows for each compound the resistivity (p)
data as a function of the absolute temperature (T) from 2 to
1100 K. The most striking feature in this figure is the differ
ent behavior of p( T): While for CoSi2, MoSi2, and TiSi2
compounds the linear behavior is what is expected for the
temperature dependence of the resistivity of a normal met
af· for TaSi2 thep( T) curve flattens out at high temperature
wi~h a negative deviation (d 2p/dT2 < 0) from the linearity;
and for WSi2 the p( T) curve shows a positive deviation
(d 2p/dT2 > 0) from the linearity.
I
W Sll.o/polv -51
* IFURNACE HEAT TREATMENT E
---- () '1000 DC • 30 min
--.. ' C; 50 .... /' :::L
---- j(293 KI = 1 53 " 00 > (2 KJ . .- 00 ..... 00 t-0° S; .' 00 .' 0° .' 00 t- .-00 en .' 00 .' 00
00 .... 00
0° .. ' 00 W ...... 00 \ s, T. A . Il: • 0
00
°11200·C, ..,-" 6 s
COSMIO •• •••
.S(293 K) = 199
S (2 K) ,
OL-~ __ -L __ ~~L-~~J~L--L~~~
500 o
TEMPERATURE (K)
FIG. 2. Temperature dependence of the resistivity of WSi, 6/polycrystal
line Si specimens after the heat treatment at 1000 DC for 30 min (.) or at
1200 DC for 6 s (e).
1087 J. Appl. Phys., Vol. 61, No.3, 1 February 1987 In Table I the resistivity values at 293 and 2 K, their
differences, and their ratios are reported for the five disili
cides examined, and for the purpose of comparison we also
list the same data for bulk and thin-film monocrystaIIine
CoSio (Refs. 8 and 9) and bulkmonocrystaIIine MoSi2 (Ref.
10). As can be seen from Fig. 1 and Table I the polycrystal
line WSi, thin films are characterized by having the lowest
RRR (r~sidual resistivity ratio, the last column) and the
lowest temperature-dependent contribution to the resistivity
(intrinsic resistivity, the fourth column). We note that the
TaSiz film has the highest value of intrinsic resistivity. The
polycrystalline MoSi~ thin films show a value of RRR simi
lar to that of WSiz but has the disadvantage of having a
higher intrinsic resistivity as compared to WSi2• Conse
quently, while MoSi2 single crystals exhibit a very low value
of residual resistivity at 2 K, they show a very high value of
RRR because of the high room-temperature resistivity. On
the other hand, if we could reduce the residual resistivity of
the WSi~ film, it would become the best candidate among the
five silicides to achieve the lowest room-temperature resis
tivity.
Following these considerations, several WSi2 samples
have been pre-heat-treated by the short-time annealing tech
nique at 1200 °C for 6 s in order to reduce the time exposure
and in turn the amount of contamination introduced during
the annealing. The purpose was to obtain purer materials
characterized by a lower residual resistivity value at 2 K. For
this study WSi, thin films deposited on undoped polycrys
talline Si on Si~: with x ranging from 1.6 to 1.9 have been
used to avoid the reaction between WSi2 and Si02 at the high
annealing temperature.
Figure 2 shows the p-vs-Tcurves for WSil.6 films depos
ited on un doped polycrystalline Si layers, pre-heat treated ~~
1000 °C for 30 min in a standard furnace (upper curve) and
at 1200 °C for 6 s by the short-time annealing technique
(lower curve). The most striking features are the low value
of the room-temperature resistivity (23 fin cm), and the
Nava etal. 1087
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128.114.34.22 On: Fri, 28 Nov 2014 22:25:04E u
'" '0
t
Z
LU
()
U.
U.
LU o
()
...J
...J
<l:
J: 0 Mo S" allo y 950°C 30 min
60 0 T a SI;! 900°C
[; w S" 1000 °c
50
* T lSI 2 900 °c
40 • CoS" 900 °c
lIoo.o.6 ,,0"
30 r 0 0
0
00"
, .... • • • • • .0 • • • • • • r • 20 0
0
00
I- ..
0
10 .. 0
M'l~~o 0
*~OODO 000000
0." 0 0 0 0
t:Jl,Ao, •• oOoSooo
o:~o*~oo 000000
00 0 0 0 0 0 .......................... * .. .. o
o 100 200 300
TEMPERATURE (K)
FIG. 3. Temperature dependence of the Hall coefficient (RII) of the same
specimens as Fig. I. Note the almost constant value of (RII) for CoSi, and
the change in amplitude and in sign for all the other disilicides.
increment (-30%) of the residual resistivity ratio obtained
after the short-time annealing.
We note that (a) the same super linear behavior was
observed in both specimens, which can be considered as an
intrinsic property of WSi I 6 thin films, independent of the
sample preparation and the heat-treatment methods, (b) the
intrinsic resistivity value measured at room temperature,
p (293 K) -p (2 K), is almost the same for samples under
going the two different heat treatments, and (c) resistivity
data for T> 600 K of the WSi2 film on polycrystalline Si
were unreliable due to the parallel conduction contribution
of the poly crystalline Si and therefore are not reported.
B. Hall coefficient measurements
The results of Hall effect measurements (RH) in the
temperature range of 10-300 K are shown in Fig. 3, The
experimental data were obtained with an injecting current of
10 rnA and a magnetic field of 8 kG. For several other non
destructive values of the current and for values of the mag
netic field between 3 and 8 kG, we observed that the trends
and magnitudes remained the same within the measurement
errors. Samples of CoSi2 show Hall coefficient values of the
order oflO-4 cm'/C, positive and almost constant with tem
perature, which give an apparent charge density of about
3 X 1022 cm -'. For the other four silicides, their Hall coeffi-16
0')
0 12
>< '--
(J)
f-
Z 8
:J
0
U
4
o r ,
0.4 0,8 1,2
BACKSCATTERING 4 + 2.3MeV He
1,6 2,0
ENERGY (MeV)
FIG. 4. Rutherford backscattering spectra of WSi, 6/polycrystalline Si
specimens as deposited and after heat treatment at 1200 °C for 6 s. The hori·
zontal arrows indicate the calculated silicon and tungsten signal heights at
the surface of the silicide.
cients become temperature dependent and there is a change
of sign for TiSi2, MoSi2, and TaSi2• The general behavior of
the Han measurement suggests the presence of complex
multicarrier effects, the detailed study of which lies beyond
the scope of this work.
C. Structural and compositional analysis
All the thin films have been analyzed by RBS, XRD,
and AES techniques both in the as-deposited state and after
heat treatment. The structural results are summarized in Ta
ble II for those specimens which have undergone the heat
treatment at temperatures and times reported in Fig. 1. The
last column refers to the impurity (oxygen and carbon) con
tent observed inside the thin films; the values measured on
the surface have been omitted. A more detailed structural
analysis by TEM has been performed on the specimens of
WSi'6/polycrystalline Si to relate defect density to the low
er residual resistivity observed in these samples after the
short-time annealing.
Figure 4 shows the RBS spectra of a specimen of WSi 1.6
on polycrystalline Si before and after heat treatment at
1200 °C for 6 s; a RBS spectrum very similar to this one has
also been observed after a heat treatment in a standard fur
nace at 1000 °C for 30 min. In both cases the excess W in the
alloy reacts with the polycrystalline Si to form WSi2 and the
thickness and composition of the disilicide are quite uniform
and of the same value (-1550 ft.). In the figures, the two
T ABLE II. Summary of the structural and compositional analysi, on the specimens of Fig. I. Si/M represents the atomic ratio by RBS after heat treatment.
Compound Si/M Thickness (A) Phase Impurity content (at. %)
TiSi2 2.0 'ISO Orthorombic-C 54 051%, C<I%
CoSi, 2.0 \060 Cubic-CI 0<1%, C<I%
TaSi, 2.0 1100 Hexagonal-C 40 0<1%. C<I%
MoSi, 2.0 \080 Tetragonal-C II 051%. C<I%
WSi, 2.0 980 Tetragonal-C II 051%, C<I%
1088 J. Appl. Phys., Vol. 61. No.3, 1 February 1987 Nava etal. 1088
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128.114.34.22 On: Fri, 28 Nov 2014 22:25:04W Si 1.6 lSi -poly: 1000 ·C. 30 min
18
W Si2 TETRAGONAL
Si-poly <;
14 '6 e
0 '" 10 ~ 0
~
0 I 'N'
'" ,... 6 0 0
:::; -;;; ~ ,
-;-5 '" ro 1:: 0 0 c-
I ,
> 2
~
en z 18
w 0 W S'2 TETRAGONAL
~ V S i-poly e
Z "0
14 M' ..
~ 0
"0 0
0 e
10 0
'"
6 0 ,
~ -t) .., :::-
2 '" .. v ~'- / I
80 70 60 50 40 30 20
29 (DEGREES)
FIG. 5. Bragg-Brentano x-ray diffraction spectra ofWSil../polycrystalline
Si specimens after the heat treatment (a) at 1000 'C for 30 min and (b) at
1200'C for 6 s.
horizontal arrows indicate, respectively, the calculated posi
tions of surface Wand Si signals ofWSi2, and they agree well
with the measurements.
The x-ray diffraction spectra obtained in the same ex
perimental condition, Figs. 5 (a) and 5 (b), indicate that
after the reaction no metal-rich silicide or hexagonal WSi2 is
present but only tetragonal WSi2 and unreacted polycrystal
line Si are observed. The insets, which show the < 101) dif
fraction peak in an expanded scale, can be used to illustrate
that the average grain size of the crystallites is almost the
same in both the thin films. The same is also true by analyz
ing the (110 > diffraction peak. Figures 6 (a) and 6 (b) show
the bright field transmission electron micrographs of the sili
cide obtained with 200-keV electrons on a WSiI.6/polycrys
talline Si specimens after the heat treatment (a) at 1000·C
for 30 min and (b) at 1200·C for 6 s, respectively. The im
ages of the two microstructures are similar and electron dif
fraction has verified that the phase is the tetragonal WSi2•
By using AES, the as-deposited alloy has been found, in
agreement with RBS data, to be uniform in composition in
depth and with a concentration of oxygen estimated in the
order of 1 % mainly located on the surface and at the inner
interface as shown in Fig. 7 (a). No oxygen has been detected
in polycrysta1line Si. After heat treatments both the in-depth
profiling spectra in Figs. 7 (b) and 7 (c) show that a uniform
reaction between the polycrystalline Si and the excess W in
the alloy has occurred and a correct stoichiometric ratio of
1:2 for W:Si is measured. The main difference between the
1089 J. Appl. Phys .. Vol. 61, No.3, 1 February 1987 two heat-treated samples is the amount of oxygen in the
tungsten disilicide and in the polycrystalline Si. While in the
specimens which were heat treated at 1000"C for 30 min
(standard furnace) the contamination by oxygen is measur
able in the disilicide layer even ifit is at a very low concentra
tion [see Fig. 7(b)], in the specimens which experienced the
short-time annealing (1200 ·C, 6 s) the oxygen is absent in
the WSi2 or at least it is below the detection limit (-0.5%).
Furthermore, the latter shows a lower oxygen content in the
polycrystalline Si and a buildup of oxygen at the silicide/
polycrystalline Si interface, yet in the former the oxygen con
tent in the polycrystalline Si layer is much higher. In either
case, a higher oxygen content in the polycrystalline Si than in
the WSi2 is found which substantiates the observation that
the disilicide is stable against oxidation in the presence ofSi.
IV. DISCUSSION
A. Residual resistivity-The temperature-independent
part of resistivity of the sUicides
Residual resistivity depends on impUrIties, defects,
stress, and microstructure of the materials. Although we
have prepared the five disilicides with the same procedure,
their microstructures and defects such as dislocations and
stacking faults are found to be comparable, yet their impuri
ties and stress may be different since they were not prepared
in UHV conditions and since they were prepared on a sub
strate so the thermal stress is different due to different ther
mal expansion coefficients. The residual resistivity data
shown in Fig. 1 tend to suggest that MoSi2 and WSi2 contain
more impurities and/or were stressed more than the others.
The study of residual resistivity can no doubt be best per
formed with a pure single crystal, and this is a case for WSi2•
Nevertheless, the curves in Fig. 1 and the data in Table I are
sufficient to show that the goal of achieving a low room
temperature resistivity can be satisfied better using WSi2
than, for example, TaSi2• In Figs. 2 and 7 we showed the
beneficial effect of a rapid annealing in reducing the oxygen
content in WSi2 and in turn its residual resistivity. However,
we note that most silicides are formed on heavily doped poly
crystalline Si rather than un doped polycrystalline Si, so the
dopant effect during a short-time annealing on the residual
resistivity of silicides must also be considered. II
B. Intrinsic resistivity-The temperature-dependent
part of resistivity of the silicides
We discuss only the high-temperature part, i.e., the lin
ear dependence and its positive and negative deviations, and
we assume that this part is intrinsic to the materials. We
make no attempt to try to explain why a silicide should be
have linearly or not. We just analyze the behaviors according
to the known models.
Concerning the linear curves of Fig. I, it appears evident
that CoSi2, MoSi2, and TiSi2 compounds show a similar be
havior, which is usually expected for the resistivity of a nor
mal meta1.7 This classic behavior has been described by the
following expression:
Pid (T) = Po + Pe -ph (T), (I)
where Po is the temperature-independent residual resistivity
Nava eta/. 1089
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128.114.34.22 On: Fri, 28 Nov 2014 22:25:04due to the scattering processes with impurities and defects
andpe_ph is the phonon-scattering contribution, which is lin
ear in the high-temperature limit The temperature-depen
dent contribution of the ideal resistivity (P,d) can be ap
proximated by the Bloch-Griineisen expression 12;
Pe-ph(T) =pITG(S/T),
G(S) _ 4(I..)4 r81Tdx X5
T -S Jo (e' -1)( I -e X)' (2)
where S is the Debye temperature and pi is the high-tem
perature limit of Pe-ph (T) IT
The experimental data for CoSi2, MoSi2, and TiSi2 were
fitted by minimizing the root-mean-square (rms) deviation,
1090 J. Appl. Phys., Vol. 61, No.3, 1 February 1987 o
50QO ~ FIG. 6. Bright field transmission elec
tron micrographs of the WSil 6/poly
crystalline Si specimens after the heat
treatment (al at 1000 T for 30 min
and (b) at 1200 °C for 6 s. The diffrac
tion patterns reveal no reflections be
longing to silicon. In both cases an
average grain size of 2700 A has been
estimated.
allowing the three parameters, Po, pi, and S to float The
values of the parameters which minimize the rms errors are
reported in Table III.
The resistivity behavior of TaSi2 is quite different and
cannot be described by Eqs. (1) and (2). The P (T) curve
shows a negative deviation from linearity and a quasisatura
tion phenomenon at high temperatures. This behavior, simi
lar to that previously reported for several A 15 compounds, 13
has been explained by assuming that the conduction-elec
tron mean free path approaches a lower limit of interatomic
distance with the consequent breakdown of the classical
Boltzmann theory. 14 To describe this effect the phenomeno
logical shunt resistor model has been proposed,ls
Nava et al. 1090
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128.114.34.22 On: Fri, 28 Nov 2014 22:25:04WSi1.6/pOly-Si.AS deposited
80 w
60
40 l .................... __ ... -.............. ,,-
~
::c
(!) 20
80
LU 60 :c
~ 40 «
LU
a..
e::: w
(!)
=> « 20
0
80
60
40
20 Si
w
Si
C
'\ --,
'.
Si o
,"'oO ...... _ ....
, \ . , '-.. , .. ..,--... ,
(c) o U-____ ~ ____ _L~~ ____ ~~
o 5 10 15
SPUTTERING TIME (min)
FIG. 7. Depth profiles obtained by AES analysis from the WSiI.6/polycrys·
talline Si specimens (a) as deposited, (b) after 1000·C for 30 min, and (cl
after 1200 ·C for 6 s.
1 1 1 --=---+-,
p( T) Pid (T) Psat (3)
where Psat appears to be independent of temperature and
defect concentrations and represents a limiting value in the
resistivity saturation phenomenon. The ideal resistivity Pid is
given by Eq. (1). The experimental data of TaSi2 have been
TABLE III. Parameters used in Eqs. (1 l, (2). and (3) to fit the electrical
resistivity curves of CoSi2• MoSi2• TaSi2• TiSi2• and WSi2 shown in Fig. I.
For the WSi2 specimens the best fit has been done in the temperature range
(2-300 K) (see text).
Compound
TiSi2
CoSi2
TaSi,
MoSi2
WSi2
1091 Po p'
(,un cm) (,un cm/K)
3.26 0.0770
5.90 0.0620
13.12 0.1741
40.00 0.0783
23.15 0.0458 e
(K)
423.94
491.13
389.85
623.79
490.03 Put
(,un cm) X~
294.84 0.064
0.406
0.888
0.086
0.037
J. Appl. Phys., Vol. 61, No.3, 1 February 1987 ,--...
E
()
q
::l.
;>
~
>
I-rn
rn
U.J
0:: 130
110
90
70
50
30
160
120
80
40 MOSi2 alloy
950 ·C, 30 min
TaSi2 alloy
900 ·C, 30 min
o +----r--~---r---~--~--~_+
200 400 600 800 1000 1200
TEMPERATURE (K)
FIG. 8. Comparison between the best·fit curves based on the parameters
shown in Table III and the experimental resistivity data for (a) MoSi, and
(b) TaSi,. The experimental values are the same as Fig. I. The open dots
represent the data and the solid lines are the calculated curves.
fitted by a curve using Eq. (3) with the temperature-depen
dent contribution of the ideal resistivity described by Eq.
(2). The best fit was achieved by minimizing the rms devi
ation, allowing the four parameters Po, pi, e, and Psat to
float. In Table III the set of the parameters which minimize
the rms error is reported. It should be mentioned that the
saturation resistivity is of the order of 300 p.O em, which is
much higher than those (150 p.O cm) reported for the A 15
structure compounds,15.16 and very similar to those found
for VSi2 (Ref. 17) and NbSi2 (Ref. 18). For illustration, we
show in Fig. 8 the agreement between the experimental data
and the calculated curves for the cases of MoSi2 and TaSi2•
Regarding the resistivity data ofWSi2 shown in Fig. 1,
we have restricted its analysis in the temperature range 2-
300 K, where the superlinearity is not observed (see Fig. 2).
Therefore, in a first approximation we can assume that it is
valid to take the Bloch-Griineisen formula to fit the curve.
The best-fit parameters are reported in Table III. The ob
served superlinear behavior is quite unusual for a metallic
compound, but it has already been observed for WSi2 in a
shorter temperature rangel9 and for good conductors like W,
Au, Cu, and Ag.20 In a first approximation this effect was
assumed to be a consequence of the thermal expansion with
an associated decrease of the Oebye temperature. Following
Mott and Jones,21 it was shown that for T> To> e,
[p(T) -Po]!To: (1 + 2jT)
and
. d(lnS) J - a~-..-:.-...:..... dOnV)
where a is the thermal expansion coefficient, C v the specific
heat, do the density and xo the compressibility at zero tem
perature. We have calculated the quantity
Nava eta/. 1091
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128.114.34.22 On: Fri, 28 Nov 2014 22:25:041.3
u W SI,:? our data
o W Grunelsen ref 2U
o
400
FIG. 9. The figure shows the use of Eg. (5) in describing the superlinearity
behavior of p( T) for W (open dots) and WSi, (open triangle,) at high
temperatures.
Q(T) = {[peT) -Po]~J/{[p(To) -Po]T}
= 1 + [2j/(l + 2j~»](T- ~» (5)
for both WSi2 (our sample) and W (data from Ref. 20). For
To = 870 K, we have obtained the results of Q( T) vs
(T -To) shown in Fig. 9.
The two linear behaviors in this figure indicate that Eq.
(5) describes very well the resistivity of the two materials,
but for W,j = 3 X 104 K I as calculated from the slope is
15 times greater than the tabulated value based on Eq. (4).
This fact is in accord with the remarks given in Ref. 21 and is
still not understood. Finally we note that the twoj values for
Wand WSi2 are surprisingly close to each other.
We now turn to the data listed in Tables I and III for a
further discussion of the temperature-dependent behavior of
the resistivities. The total temperature-dependent part of the
resistivity at room temperature is about 18 f1f'! cm in MoSi2,
which is very similar to that observed in other thin films22
and in bulk single crystals. 10 This means that in spite of the
high density of grain boundaries in thin films, deviations
from the Mattiessen's rule with consequent increments in
the electron-phonon and/or electron-electron scattering by
the defects are almost negligible, so the calculated param
eters on the basis of the temperature-dependent resistivity
measured from our MoSi2 films is accurate. We found that
MoSi2 has a very high Debye temperature as shown in Table
III, which is very close to that (630 K) calculated for a single
crystal of MoSi2. 23
Concerning TaSi2, the observed intrinsic resistivity val
ue, which is very high and similar to that (36 f1f'! cm) ob
tained by Huang et al.24 on TaSi2 thin films prepared by
furnace reaction of sputtered tantalum films with silicon, is
twice that of MoSi2. This fact reflects itself on the highest
value of the parameter pi obtained for TaSi2 as shown in
Table III, which is similar to the consideration on the satura
tion effects outlined recently by Gurvich.25 The very high
value of pi would suggest an enhanced electron-phonon in
teraction. Functionally, pi depends on the dimensionless
electron-phonon coupling constant, A'r' on the density of
states at the Fermi level, 2N(0), and on the mean-square
electron velocity at the Fermi surface < V~.), and the explicit
expression is20
1092 J. Appl. Phys., Vol. 61, NO.3, 1 February 1987 (6)
where kB and h are the Boltzmann and Plank constants,
respectively, and q is the electronic charge. In the case of
TaSi2, a high value of p I may be due to a large A Ir or a small
2N(0) and < V;.); thus no conclusion can be drawn until the
relevant electronic properties such as the density of states,
Fermi velocity, and phonon dispersion relations are known.
An interesting finding of our analysis is that the satura
tion resistivity found for TaSi2 (p ~ 300 f1f'! cm) is almost
twice of those ( ~ 150f1f'! cm) reported fortheA 15 structure
compounds such as V 3Si. 17 Although the saturation resistiv
ity model gives reasonable physical parameters ofTaSi2, the
model may not necessarily be unique in describing its resis
tivity behavior. It has been pointed out that such a model has
failed to explain the negative temperature coefficient of resis
tivity observed in many disordered materials of very short
mean free paths.27 However, the Mooij correlation,28 which
relates the sign of temperature coefficient of resistivity to the
magnitude of resistivity for disordered materials, would re
quire a change of sign beyond 150 !-if'! cm, thus making the
case of a high saturation value ( ~ 300 f1f'! cm) and a positive
coefficient ofTaSi2 an ambiguity. Clearly the saturation phe
nomenon is still a subject worth studying and the data pre
sented here will be valuable in examining other models.29
V. CONCLUSIONS
Resistivity measurements in a wide temperature range
(2-1100 K) show the linear behavior, characteristic of an
ideal metal, for CoSi2, TiSi2, and MoSi2, while a negative
deviation from the linearity and a quasisaturation at higher
temperature have been observed for TaSi2. WSi2, on the con
trary, shows an intrinsically superlinear behavior.
The experimental data of CoSi2, TiSi2, and MoSi2 have
been interpreted in terms of a model consisting of an ideal,
temperature-dependent, metal-like conductivity, while
those of TaSi2 have been interpreted by adding to the pre
vious model a constant conductivity which is essentially
"shunting" the ideal one and prevails at high temperatures.
One of the most striking features in the saturation phenome
non is the high value of Psat (300 !-if'! cm) with a positive
temperature coefficient of resistivity ofTaSi2, which is clear
ly in contradiction to what is suggested by the Mooij correla
tion.
The lowest values of the residual resistivity ratio [p(293
K)lp(2 K)] and of the temperature-dependent part of the
resistivity [p (293 K) -p (2 K)] have been observed for
WSi2 among the five silicides. They indicate that by reducing
the residual resistivity ofWSi2, it may offer the lowest room
temperature resistivity among the five silicides. We have
demonstrated the feasibility of this conclusion by using a
short-time annealing to lower the room-temperature resis
tivity of WSi2 from that of standard furnace annealing.
ACKNOWLEDGMENTS
The authors gratefully acknowledge K. Ahn and the
staff in the Central Scientific Service Materials Laboratory
at Yorktown for specimen preparations, A. Armigliato of
LAMEL-CNR (Italy) for TEM analysis, and T. Sedgwick
(Yorktown) for the use of short-time annealing equipment.
Nava eta!. 1092
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Navaetal. 1093
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1.344325.pdf | Parametric studies of xray preionized discharge XeCl laser at single shot and at high
pulse rate frequency (1 kHz)
Marc L. Sentis, Philipe Delaporte, Bernard M. Forestier, and Bernard L. Fontaine
Citation: Journal of Applied Physics 66, 1925 (1989); doi: 10.1063/1.344325
View online: http://dx.doi.org/10.1063/1.344325
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130.102.42.98 On: Mon, 24 Nov 2014 19:34:22Parametric studies of x .. ray preionlzed discharge XeCllaser at single shot
and at high pulse rate frequency (1 kHz)
Marc L. Sentis, Philipe Delaporte, Bernard M. Forestier, and Bernard L. Fontaine
Institut de Mecanique des Fluides de Marseille, UM 34 CR.N.S, Universite d'Aix-Marseille,
1 rue Honnorat, 13003 1t1arseille, France
(Received 30 January 1989; accepted for publication 12 May 1989)
The design and performance of a high repetition rate (1 kHz) and high average power (200
W) XeCl discharge pumped laser (ti = 308 om) using a cold cathode x-ray gun or a wire ion
plasma gun for preionization are presented. The dependence of the output energy and of the
average output power at low and high pulsed repetition frequency (PRF) on xenon partial
pressure is studied. The discharge stability at high PRF is better with lower xenon partial
pressure. The influence of preionization level and temporal delay between x-ray pulse and laser
discharge is discussed, as well as the required preionization level at higher PRF.
I. INTRODUCTION
It is well known that the output laser characteristics
(i.e., pulse energy, pulse duration, beam profile) of gas dis
charge lasers are very sensitive to discharge stability and
discharge quality. To obtain good discharge stability and to
prevent the formation of spark channels, minimum electron
densities and homogeneities are required at the moment of
voltage breakdown. These minimum requirements are asso
ciated with a number of parameters, including electrode pro
files, electrode types (screen or solid), gas pressure, and the
voltage rise time at the laser head before the voltage break
down. !.2 The minimum necessary electron densities were ini
tially estimated to be between 104 (Ref. 3) and 109 cm--3
(Ref, 4); now it is generally accepted that under typical rare
gas-halide laser conditions, this minimum is 107_108 em -J
for optimum laser output.5•6 Although preionization is com
monly performed by ultraviolet (UV) radiation, the use of
x ray or e-beam has become the focus of more interest in the
last few years.2•7•8 X-ray or e-beam preionization has much
higher penetration power compared with UV radiation;
therefore, x ray and e-beam should have advantages for wide
aperture and high-pressure discharge laser devices. The ad
vantages of using x-ray preionization rather than e-beam
preionization are summed up in Table 1.'),10
In this paper we investigate the dependence oflaser out
put energy and pulse duration on the composition of the gas
mixture and especially on xenon partial pressure. The depen
dence of this energy and pulse duration on the composition
of the mixture is studied in the case w here He! is the chlorine
donor. The effect of x-ray preionization in the XeCI laser will
then be discussed. Our interest will focus on the dependence
of laser output on preionization parameters such as x-ray
dose and the timing between the x-ray pulse and the laser
discharge. Experiments of single pulse, and high pulsed re
petition frequency (PRF) (i-kHz) operation from these ex
periments shed light on the stability of discharge and on
attachment kinetics.
II. EXPERIMENT
A laser test bed called LUX (laser ultraviolet preionise
par rayons-X) has been constructed to determine the param
eters necessary for the power scale up of the XeCl laser (ti = 308 nm) to a very high average power and PRF (pulse
rate frequency) and has been used for these studies. The
LUX test bed is mainly composed of a fast flow subsonic
closed cycle wind tunnel and a high average power electrical
excitation system.
Ao Wind tunnel
The 170-t nickel-plated stainless-steel1oop, already de
scribed in part ehew here I I.] 2 is designed for achieving a long
"life time" operation in the working mixture. It has very low
baseline flow turbulence level and flow pressure drop. Provi
sions have been made for fast damping of acoustic waves. A
centrifugal compressor powered by a 4-kW continuous cur
rent motor allows an average flow velocity up to 65 m/s in
the laser discharge head (2.5 X 30 cm2 cross section). The
maximum working pressure is 2.5 atm and is limited by the
motorcompressor ferrofluidic seal capabilities. A turbo-mo
lecular vacuum pump allows us to pump down to a residual
pressure as low as 10 -5 Torr to achieve good initial purity of
gas mixtures.
B. Electrical excitation
The electrical excitation system consists essentially of
an x-ray preionizer and a main discharge pulse.
1. The x~ray preionizafion generator
Two different x-ray generators have been used, a cold
cathode electron gun and a wire ion plasma electron gun
(WIP gun).
TABLE 1. Comparison between Ji.-ray and e-beam preionization for an
avalanche discharge."
Parameters
voltage (kV)
lifetime
(shots)
initial electron
density (cm-')
"See Reference 9_ x ray e-beam
50 100
10'_10' 1-3.><10-'
10" __ 10" 10"'·10'4
1925 J. Appl. Phys. 66 (5), 1 September 1989 0021-8979/89/171925-06$02.40 (.c) 1989 American Institute of Physics 1925
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130.102.42.98 On: Mon, 24 Nov 2014 19:34:22Thermionic f\ ~H---rires -1 l ---SO--==:::O--:--:---+7L.c::::::::::::=>
Trigqer !lIilse
ct 15 kV
,--++-.c:.High voltage
cathode
ContinllOtis negative high voltage
(-40 kV to -150 kV)
FIG.!. Schemaiic diagram of the WIP gun.
The cold cathode electron gun consists of a carbon felt
cathode and a metal screen anode. It is excited by a pulse
through a fast transformer triggered by a hydrogen-fined
spark gap. Energy is stored in a 50-kJ capacitor bank which
allows fast charging oftheO.5-nF-15- kV primary capacitor.
A triggered spark gap connects this capacitor to the trans
former. At the other end of the transformer a peaking ca
pacitor of 1 nF reaches -200 k V and self-triggers a series
spark gap that generates a fast rise time high voltage pulse
( ;:::; 20 J, -100 to -180 k V, 300 ns) which is applied to the
cathode gun. Repetition rates up to 1 kHz are possible with
this device. The electrons are accelerated in the 5 X 10-0
Torr vacuum towards the grounded anode and produce
bremsstrahlung radiation mainly in the forward direction
when they hit a 12.5-flm-thick tantalum (Z = 73) foil
placed directly behind the screen anode. Beyond the tanta
lum foil is a O.6-mm-thick aluminum foil which serves as a
high-pressure window for the x ray. The performance of the
carbon felt cathode was found to be very reliable after 10 000
shots.
The WIP gun was developed at O.N.R.A 13 (Office Na
tional de la Recherche Aeronautique). A schematic of the
WIP gun is shown in Fig. 1. This type of electron gun allows
the control of the high voltage pulse by the low voltage ( 1 S
kV) control pulse on four thermionic wires. When a dis
charge puise of 15 kV is applied to the anode, in the ioniza
tion cavity filled to 1.5 X 10-2 bars of helium, a plasma is
created. The ions are accelerated to a ground extraction grid.
Beyond this grid, ions are accelerated up to the constant
negative high voltage cathode ( -40 to -150 kV) where
these generate secondary electrons by shock. Secondary
electrons are accelerated to the grid and beyond up to the
ground window. The window is the same as the window of
the other gun.
The x-ray dose is measured with a pocket dosimeter
(Seq. 6 Physiotechnie). X-ray homogeneity measurements
are realized with films (Kodak X-OMAT-MA). The time
history of the x-ray pulse is measured with a wavelength
converter (102 A) placed above the window and with a pho
tomultiplier.
2. The main discharge circuit
A classical thyratron switched C-L-C transfer circuit
allows an average laser power of 200 W at 1 kHz operating in
1926 J. Appl. Phys., Vol. 66, No.5, 1 September 1989 FIG. 2. Electrical circuit diagram.
the burst mode. An electrical circuit diagram of the com
plete system, including the WIP gun, is shown in Fig. 2. The
charging voltage from the power supply, Vmax = 15 kV, is
doubled through the resonance charging circuit to charge
the main storage capacitors Cpo The thyratron (EVV CX
1572) is triggered by a synchronized pulse via a transformer
connected to the (lS-kV) input trigger pulse of the x-ray
gun, which slowly charges the transfer capacitor bank (C T)
with a rise time of 150 ns. The time delay between the x-ray
pulse and the laser discharge is controlled by a synchroniza
tion unit. The transfer capacitor array approximates a rela
tively fast transmission line with time constant fast com
pared with the lSO-ns voltage rise time of the primary
circuit. When the electrode voltage reaches the breakdown
voltage, the energy stored in the transfer capacitor array is
quickly deposited into the gas mixture. The optical resonator
is of the stable type, with long radius mirrors. The dielectric
coated fused silica mirrors are set directly in the laser head
side walls in contact with the working medium. The curva
ture radius of the mirrors are r = 2 m. The reflection coeffi
cient at /l. = 308 nm is RJ = 0.98 (1'1 ;::;;0.01) for the rear
mirror and R2 = 0.48 (T2;:::;0.5) for the extraction mirror.
The laser output power waveform is recorded by a high
current fast vacuum photodiode (TF 1850 S20 from ITL)
filtered with narrow band interference and neutral density
filters. The laser energy per pulse is measured by a pyroelec
tric detector (ED sao, Gen Tec). The total energy in a burst
and the average power are measured by a surface absorption
calorimeter (360001 Scientech). Shot-to-shot variations in
the beam energy profile in the near field are obtained using a
speciaUy designed home detector.14 Most of the electrical
signals are recorded on a Lecroy 9400 oscilloscope and
stored on an IBM. PC XT microcomputer for processing. It
is noteworthy that the Lecroy oscilloscope allows the re
cording and storing of signals from 250 successive shots at 1-
kHz PRF with lOons time resolution by use of its segmented
memory (32 kwords) capability.
III. RESULTS AND DISCUSSION
The cold cathode electron gun delivers a 30-mR x-ray
dose inside the laser cavity in the best gun configuration (felt
cathode, anode-cathode distance d = 68 mm, diode resis
tance R = 220 n). The emission is highly uniform and shot
to-shot variations at high PRF (1 kHz) were not noticeable
over the whole area of the x-ray window (I X 27 cm2) at a
Sentis et al. 1926
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130.102.42.98 On: Mon, 24 Nov 2014 19:34:22o
i30 o
~
25 20 i
*~~o _-"
o L t:------~
80 100 130
X-FlAY DIOOE VOLTAGE (kV) G----Q at the 9IJI1 il'ldol
A---8 at tile anOOe
0-__ '" at the tothodll
FIG. 3. X-ray dosage at the gun window, the anode, and the cathode as a
function of the voltage of the x-ray diode.
distance of 2 cm from the tantalum target. The x-ray pulse
duration is 250 ns under these same conditions.
In Fig. 3 the x-ray output dose at the gun window, the
anode, and the cathode of the laser cavity, is displayed as a
function of the negative high voltage on the cathode of the
WIP gun for typical conditions (loS X 10--2 bars of helium
and 15-kV trigger pulse). Figure 4(a) shows the longitudi
nal distribution of the x-ray emission at low PRF ( 1-50 Hz)
and at high PRF ( 1 kHz); and Fig. 4(b) shows oscillograms
of the x-ray emission at different points of the longitudinal
axis of the laser cavity at low PRF. At low PRF the x-ray
emission (l) is not uniform [DIll:::;: 100%, Fig. 4(a) 1 along
the longitudinal axis of the cavity, and the x-ray emission
timing among different points located along the axis is very
different (the delay between point I and point 5 is about 400
ns, Fig. 4(b) J. However, at 100 Hz timing and uniformity
are much improved and at high PRF (1 kHz) unifonnity is
DIll:::;: 10% with the x-ray emission delay among the differ
ent points being very low ( < 5 ns). This WIP gun is opti
mized for high PRF ( 100-1000Hz) but can be optimized to
work at low PRF by using only one thermionic wire instead
of the four thermionic wires used at high PRF.
olk------.:l 'OOIJiz
':"~~.~~~_"':I \) tc 50 HI
CAVllY AXIS (em)
(0) o 500 1000
(b)
FIG.4. (&) Longitudinal distribution of the x-ray emission at low PRF ( I
so Hz) and at high PRF (1 kHz); (b) x-ray emission at different points of
the longitudinal axis of the laser cavity at low PRF as a function of time.
1927 J. Appl. Phys., Vol. 66, No.5, 1 September 1989 110
100 -. ........ I en r-.. c;
~90
~ ----
~ 80 ---G, 70 ~
~ 70 " 60 : r ""G._._. ____ ~ .... _
~ 50 ~
:5 I 40 g;j
:5
8 16 32 48 60
XENON PARTIAL PRESSURE (torrs)
FIG. 5. Dependence ofXeCllaser energy and pulse width on xenon partial
pressure.
A. Effects of xenon partial pressure
t. Results for one shot
Figure 5 shows the dependence of the laser output ener
gy and the laser pulse width on the xenon. partial pressure
when the partial pressure of HeI is 3 Torr with neon as the
buffer gas up to a total pressure of 1125 Torr. Figure 6 shows
the dependence of the laser output energy on the charging
voltage of the main storage capacitors (Cp ::::40.8 nF) for
different xenon partial pressures. When the xenon partial
pressure is increased, we observe the discharge becomes fila
mentary. For low xenon partial pressures (8 Torr) the laser
pulse width has a duration substantially larger than,that for
high xenon partial pressure (;;.,32 Torr) (Fig. 5). The dis
charge stability depends widely on xenon partial pressure.
An important observation is that the evolution of the laser
output energy as a function of the xenon partial pressure is
dependent on the input energy (Fig. 6). For low charging
voltage (:::;: 18 kV), laser output energy is greater at low xe
non partial pressure than at high xenon partial pressure. For
100
15 a torrs XENON
16 torrs XEIDN
32 torrs XEli'lN
",,---..0 60 torrs XOON
1fl 23
CHARG INC VOLTAGE (kV) 28
FIG. 6. Laser energy as a function of charging voltage for various xenon
partial pressures.
Sentisetal. 1927
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130.102.42.98 On: Mon, 24 Nov 2014 19:34:22sOOt 1 250
moo It! shot 1 250
• 750 It!
SOO liz
tOO Hz
3 torrs wel/l. torrs h/ 1100 10m He
(el l j~fS Hal 60 fom Xe/VOG jl)ffS He
(b)
FIG. 7. Pulse-to-pulse time resolved laser power histograms at different
PRF for (a) low and (b) high xenOJ1 partial pressures.
high charging voltage ( :::; 25 k V), the opposite is true.
This behavior seems to be dependent on the kinetics of
the exited gas mixture. When the xenon partial pressure
is increased, E IN decreases due mainly to the high
ionization of Xe through the fonowing reactions:
Xe + e-...... Xe* + e--, and Xe'" + e----Xe-+-+ 2e-.
The electronic temperature, proportional to E IN,
also decreases. Finally the formation of CI-through the
following reactions HCl + e--> HCI (v) + e-,
HCI (v) + e--H + Cl--, and the formation of Xe+ are
decreasing.
2. Results for high PRF
Figures 7(a) and 7(b) show a typical histogram of the
time evolution of the power of 250 shots at various pulse
repetition rates from 100 to 1000 Hz. Figure 7 (a) is for a gas
mixture oO-Torr Hel, 16-Torr xenon, and 1700-Torr neon,
and Fig. 7(b) for a gas mixture of 3-Torr HCI, 60-Torr xe
non, and nOD-Torr neon. Some of the drop in energy as a
function of the number of pulses is due to the drop in charg
ing voltage, but most of this drop at high repetition rates is
due to the degradation of the discharge. It can be seen from a
comparison of Figs. 7 (a) and 7 (b) that this drop as well as
-the shot-to-shot fluctuations are sharply dependent on the
xenon partial pressure for the same discharge conditions
(V4 = 18 kV, Cp = 78 nF, CT = 75 nF, V = 50 m/s). For
high repetition rate operation the discharge stability is de
graded by transversal and longitudinal acoustic waves, 15 this
degradation is made especially severe at higher xenon con
centrationsl'" [Fig. 6(b)].
B. Effects of xaray preionization
1. Time delay
The time delay 151 is defined as the time between the
beginning of the x-ray pulse and the beginning of the laser
pulse. The dependence of laser pulse energy on I5t for an x
ray dose of 30 mR is shown in Fig. g for a gas mixture of 2-
Torr HCI, 35-Torr xenon, and 975-Torr neon. The cold
cathode x-ray gun is used in these studies. It is apparent that
the laser action is present not only for the duration of the x
ray pulse. but indeed is degraded only some 45 % 700 ns after
the end of the x-ray pulse. An energy plateau is reached for
dt:::; 150 ns, this corresponds to the rise time of the x-ray
pulse. This energy plateau has a duration of about 450 ns.
1928 J. Appl. Phys., Vol. 66, No, 5, 1 September 1989 -500 0 500 " o
-----X-RAY EMISSION
I I
1000 1500 (0. u.)
t (ns)
FIG. 8. Dependence of the laser output for an x-ray dose of 30 mR delivered
by the cold cathode gun on lime delay Of.
This duration corresponds to the duration of the x-ray pulse
(FWHM:::; 250 ns) and to the duration of the HCI attach
ment. Indeed, without an electric field between the elec
trodes, the preionization electron density (ne) with initial
value of So is mainly controlled in the XeCllaser mixture by
the attachment rate b of the halogen donor n H according to
the foHowing equation:
dne . --= So -bnenJl' dt
when electron losses due to diffusion and recombination are
neglected. If So is constant we have:
ne = (S(/bn H) (1 -exp( -bnlJt)].
The attachment rate b is dependent on the ratio E IN. For
E IN:::; 1.5 X 10--16 V Icm2, b:::; 10-10 cm3 S--I.l7.18 The lie
time of the preionization electronic density is with nil
:::; 6 X 1016 em --3 equal to 200 ns. This is in agreement with
the beginning of the laser energy decay (Fig. 8).
2. Preionization level
a. Results for one shot: In Fig. 9 the output laser energy
and the laser pulse width of a standard gas mixture for XeCl
~,J ~
'" jIs-.----_.&>------:~~~! ... c: "--'
~ I " 60 = " " 9 i::; /'~ 30:
0:: I 50
~ ~ w I cr::
~50 4ll ffi en ::i
30
20
0
50 70 90 110 130 150
HIGH VOLTAGE CATHODE (K\I)
FIG. 9. Laser energy and pulse width as a function of the x-ray diode voltage
for a standard gas mixture.
Sentis et al. 1928
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130.102.42.98 On: Mon, 24 Nov 2014 19:34:22shot 1 125
FIG. 10. Pulse-to-pulse time resolved laser power histograms at (a) 100 Hz,
(b) 500 Hz, and (e) 750 Hz for different x-ray diode voltages,
are shown as a function of the negative high voltage of the
cathode of the WIP gun. For this experiment and the next
experiment, x-ray emission has to go through the laser cath
ode which is a OA-mm stainless plate. Over a long range the
output laser energy and the laser pulse width depend logar
ithmically on the x-ray dose (high voltage cathode) until
approximate saturation is reached at about -100 k V. The
dependence of output energy and pulse width on initial elec
tron density has already been observed by severa] au
thors.19-11 Beyond -100 kV the output energy increases
much more slowly and the pulse width reaches a plateau.
This saturation has been observed by Taylor' when the ini
tial electron density has reached the value of lOR elec
trons/em3,
b. Results for high PRF: Figures lO(a), 10(b), and
1O( c) show three series of 125 laser output pulses at repeti
tion frequencies of 100,500, and 750 Hz, respectively, each
obtained at three different preionization levels ( -80 kV,
-100 kV, and -130 kV). These figures have been ob-
tained for the same experimental conditions (gas mixture,
power deposition, x-ray WIP gun configuration) as those for
Fig. 9. At 100 Hz [Fig. lO(a)] the drop in energy as a func
tion of the number of pulses is due only to the drop in charg
ing voltage. The output pulse evolution and shot-to-shot
fluctuations are very similar for all different preionization
levels. At 500 Hz [Fig. IO(b)] and at a high preionization
level ( --130 kV), some output pulse fluctuations are ob
served which are due to the degradation of the discharge.
But for lower preionization levels, -100 and especially
-80 kV, shot-to-shot fluctuations are very important and
after some 70 shots, the discharge begins to be so poor that
no laser output is observed every three or four shots. At 750
Hz [Fig. 1O(c)], for low preionization level ( -80 kV) this
effect is even worse. Indeed, after about 50 shots no lasing is
observed due to the degradation of the discharge, In Fig. 11
the normalized average laser output power (Px IPJ}OkV) is
shown as an function of the preionization level. We observe
the sharp dependence of the required preionization level
needed to obtain stable discharge with respect to the PRF
1929 j. App\. Phys., Vol. 66, No.5, i September 1989 i
~ O.5r ! ! I 100 HZ
500 HZ
750 HZ
oiL ~ ______ Li ______ ~! __ ~
80 100 130
HIGH VOLTAGE CATHODE (kV)
FIG. II. Normalized average output power (P, / P1JOk'J ) as a function of
the voltage of the x-ray diode at differe-nt PRF (100, SOO, and 750 Hz),
value. At 750 Hz, high voltage cathode diminution from
-130 to -80 kVinvolves an average power drop of75%,
at 500 Hz this average power drop is 50% and at 100Hz it is
only 20% (the same value as that for the one shot experi
ment). All of the above studies are done without acoustic
dampers. Induced aerodynamic phenomena (mainly trans
versal and longitudinal acoustic waves) which degrade the
density homogeneity of the active gas mixture, are all the
more important for discharges operating at an increasing
PRF. 15 The initial electron density level required for obtain
ing a good discharge is therefore sharply dependent on the
density homogeneity of the gas mixture, At high PRF with
degradations due to aerodynamic phenomena the initial
electronic density value of 108 electrons/em3 (Ref. 5) seems
to be too low to obtain a stable discharge.
IV. CONCLUSION
An advanced XeCl laser system called LUX which is
composed of 11 subsonic wind tunnel, a WIP x-ray gUll, and a
classical thyratron switched C-L-C transfer circuit, allows
an average laser output power of 200 W at I-kHz operating
in burst mode.
The advantage of using low xenon partial pressure rath
er than high xenon partial pressure regarding the stability of
the discharge has been shown at high pulse repetition rate,
Up to a saturation value, laser output and laser pulse
width have logarithmically linear dependence on the x-ray
dose. At high PRF with degradations due to aerodynamic
phenomena the initial electron density value has to be higher
than for single shot operation. The initial density level re
quired for obtaining a good discharge has been shown depen
dent on the density homogeneity of the gas mixture.
ACKNOWLEDGMENTS
The research work leading to this paper was performed
under n.R.E.T. and C.E.A. contracts.
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QE-20, 198 (1984).
'V. M, Borisov, Yu. B. Kiryukhin, L V. Kochetov, and V. P. Novikov, Sav,
Sentis et I'll. 1929
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130.102.42.98 On: Mon, 24 Nov 2014 19:34:22 |
1.1140302.pdf | Gating circuit for linearfocused photomultiplier
M. Bruce Schulman
Citation: Review of Scientific Instruments 60, 1264 (1989); doi: 10.1063/1.1140302
View online: http://dx.doi.org/10.1063/1.1140302
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Normally on photomultiplier gating circuit with reduced postgate artifacts for use in transient luminescence
measurements
Rev. Sci. Instrum. 63, 5454 (1992); 10.1063/1.1143417
An ‘‘on’’gated photomultiplier circuit for the determination of phosphorescence lifetimes
Rev. Sci. Instrum. 61, 3726 (1990); 10.1063/1.1141543
A highspeed photomultiplier gating circuit for luminescence measurements
Rev. Sci. Instrum. 60, 2924 (1989); 10.1063/1.1140628
A Linear Gate for Photomultiplier Signals
Rev. Sci. Instrum. 44, 615 (1973); 10.1063/1.1686196
Linear Gate and Stretcher for Photomultiplier Dynode Pulses
Rev. Sci. Instrum. 35, 1360 (1964); 10.1063/1.1718748
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132.174.255.116 On: Fri, 28 Nov 2014 20:00:23Gating circuit for linear-focused photomultiplier
M. Bruce Schulmana)
DepartmentojPhysics, UniversiiyojWisconsin-Madison, Madison, Wisconsin 53706
(Received 30 January 1989; accepted for publication 2 April 1989)
A gating circuit is described which provides a cutoff efficiency of approximately 99% by pulsing
the first dynode of an RCA C31 034 type linear-focused photomultiplier by only 18 V. This is
accomplished by utilizing a local minimum in the photomultiplier gain which occurs immediately
before the sharp turn-on point. The circuit achieves the requirements offast turn-on, variable
gating duration, and minimal switching transients. The technique is especially suited to photon
counting applications in which stability of the anode baseline voltage is criticaL
INTRODUCTION
Photomultiplier-gating schemes which achieve gain reduc
tion by switching the first dynode CDl ) from its usual bias in
the voltage-divider chain to a potential negative with respect
to the photocathode have been described. 1,2 When this tech
nique was considered for application to an RCA C31034A02
linear-focused, reflective-cathode type photomultipler tube
(PMT), it was found that for a moderate supply voltage of
-1300 V, the voltage between 0 i and O2 would exceed its
absolute-maximum rating of 250 V when the PMT is gated
off.3 Tests also revealed that at the transition to full gain, a
switching transient shifted the anode baseline voltage for
several microseconds, thereby displacing the photoelectron
pulses relative to a fixed discriminator level and altering the
count rate. = 30 V before increasing sharply. If the magnitude of the
high-voltage input is reduced, the gain curve is shifted down
ward, and the sharp onset moves to slightly smaner values of
cathode-to-DJ voltage. In the optimized gating circuit, the
turn-off voltage on D] is to be fixed at a value of Vcd just
This article describes a gating technique in which the
PMT operates at somewhat reduced gain but retains its good
photon-counting characteristics. The concept originated
from a communication with RCA applications personnel,4
who suggested that the PMT should be gated on with a re
duced cathode-to-O] voltage (VCd. ). Our follow-up investi
gation showed that the smaller first-dynode pulse which is
then required to achieve the necessary cutoff allows faster
switching and minimizes transients in the output. In the final
circuit, particular attention has been paid to steady-state
noise reduction. We have applied the technique to the detec
tion of a weak fluorescence signal by photon counting after
the PMT has been gated off during strong electron-impact
excitation of a gas sample 5
First the test circuit shown in Fig. 1 was employed to
determine the proper range for V cd' It alJows measurement
of the anode current as Vcd is varied from 0-47 V with the
potentials of the cathode and other dynodes held fixed. With
a supply voltage of -1300 V, the chain of resistors labeled
R\ carries 1.25 mA, and 0.21 rnA passes through the dyn
ode-resistor chain. The resistance ratio Rz/R3 and the ca
pacitor values in the dynode chain are those recommended
by the PMT manufacturer. 3 A resistor-capacitor noise filter
protects the high-voltage input.
The change in gain which this voltage shift on OJ pro
duces can be seen in Fig. 2, which shows the resulting vari
ation in the response of the cooled PMT to a steady light
source. The gain of the PMT exhibits a local minimum at V cd -1300V
CATHODE
S80K
52 OK O2
Os
04
05
TO ELECTROMETER
FIG. 1. Test circuit employed to obtain the gain curve shown in Fig. 2. In
this work the Zener diode shown isa IN4756A, which allows Vcd to vary up
to 47 V.
1264 Rev. Sci.lnstrum. 60 (7), July 1989 0034-6748/89/071264-03$01.30 © 1989 American Institute of Physics 1264
.................................................................... , ....... .
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132.174.255.116 On: Fri, 28 Nov 2014 20:00:23GATED GATED
OFF ON t ~
10 20 30 40 50 1
O~~--~--~~~~---L--J---L-~--~
VCD
FIG. 2. Signal response of the photomultiplier as a fUllction of cathode-ta
D, voltage Vod' The optimum turn-off voltage is indicated at Vc<1 ","c 27 V.
The chosen operating level is indicated at V"d = 45 V.
below the steep increase in gain. The turn-off voltage (Ved
= 27 V) and tum-on voltage (Ved = 45 V) chosen for this
work are indicated by the arrows in Fig. 2. For the signal
levels shown, a 98.7% reduction in the average anode cur
rent is achieved when the illuminated PMT is gated off.
t GATING CIRCUIT
A diagram of the gating circuit applied to the
C31034A02 dynode chain is shown in Fig. 3. The pulse on
DJ is an amplified compiement of the TTL-generated gate
input pulse, which arrives at the 6N136 optocoupler after
passing through a TTL output driver and a 50-n cable. With
the gate input low, QJ is nonconducting and Q2 is conduct
ing. Thus DJ is set to the potentia! V cd = 45 V, and the PMT
is biased on. When the gate input is high, QJ is conducting
and Q2 is nonconducting; therefore, DJ goes to the potential
V~-d = 27 V, and the PMT is biased off by the optimum
amount.
To switch the photomultiplier to its operating condition
(Vc<l = 45 V), QI must be turned off. To reduce the delay
caused by this process, R4 limits the turn-on current for QJ
so it will have a faster switchoff. Also, Rs shortens the
switch off time of Q! by aHowing current to drain from its
base. The best values for R4 and Rs vary slightly from one
6N136 optocoupler to another. It is assumed that the opti
mum value of V cd for gating the PMT off will vary slightly
from one RCA C31034 type unit to another. The I-Mfl re
sistors in parallel with the Zener diodes serve to reduce noise
and stabilize voltage drops. Several high-voltage ceramic
disk capacitors are added at strategic points to reduce
switching transients. The low-leakage diodes at the anode
serve as protection if it is accidentally left unterminated.
Figure 4 shows simultaneous oscilloscope traces of an
1265 Rev. Sci.lnstrum., Vol. 60, No.7, July 1989 A
CATHODE
~~---r-+--~----------~D,
~N--f--L
+15 10.005
~ lOOK R, c
o
"' II
N
"5
TTL OU'TPUT BUFFER
PHOTOELECTRON
PULSE COUNTING
SYSTEM Rt
R,
R,
R,
R,
A,
A,
Rt
R, 0.02
<:=500 SSOK R2 D,
520K R3
R3 D3
R3 D.
R. Ds
R. D.
Rs D7
O.OOS
.r1 Ds
~ 0.0 1 Rs
~ Rs D.
~ 0.02
.r1 R. D'0
~ 0.05
Dt 1
R. ANODE
1N483B
FIG. 3. Schematic diagram of the photomultiplier gating circllit. The nomi
nal Zener voltages of the IN5242, IN965A, and IN967A components are
12,15, and 18 V, respectively.
input gating pulse, the first dynode pulse, and the switching
transient at the anode. The dynode pulse begins to rise after a
delay of2,.us, which is related to the tum-off time for QJ. For
the optocouplers tested, this is an approximately average
value; it also varies with the size of the input pulse. The
100% rise and fall times of the dynode pulse are 0.4 and 3,.us,
respectively, which are the inherent on and off times for Q2
in this circuit. As desired, the anode transient at the tum-on
point ends before the pulse on DJ has reached its maximum.
For a more sensitive check of the anode baseline stability. the
FIG. 4. Oscilloscope traces of the TTL gate input (upper, 2 V Idiv), the first
dynode pulse (middle, 10 V /div), and the transient anode spike across 50
f!(Iower, 0.5 mV Idiv). The horizontal scale is l,us/div.
Gating circuit 1265
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132.174.255.116 On: Fri, 28 Nov 2014 20:00:23Q
Z o
(J w
00 1 O'
a: w
C-
OO I-
Z
::J r
f
IOl (a) -1260V
1 O·'LL---L-.-l...:1-::!0'-:::O~-I:>....L-L.2:;:OO::-' O::o-l----'----'---'-::;3c:!OC:;;O-L-L-L.-'-:4":;;O 0
DISCRIMINATOR LEVEL (mV)
FIG. 5. Dark-count rate vs discriminator level when the cooled PMT is gat
edon.
cooled PMT was exposed to a steady light source, and the
TTL input was set to a 20-ps-period square wave. In each
period two photon counter gates were enabled for 4 fls begin
ning at 2.4 and 6.4ps, respectively, after the fall of the TTL
input. With the discriminator level set just beyond the dark-
1266 Rev. ScLlnstrumo, Vol. 60, No.7, July 1989 noise level of the system, no difference in the two counting
rates could be detected.
To verify that the PMT retains its good photon-count
ing characteristics under these conditions, the dark-noise
rate was measured as a function of discriminator level with
the PMT gated on and cooled to -20 "C. The PMT output
pulses were amplified by a factor of 10 before entering the
discriminator. Curves were obtained for supply voltages of
-1260 V and -1600 V. As illustrated in Fig. 5, in both
cases the photoelectron pulse-height distribution is well sep
arated from the inherent electrical noise.
To our knowledge, this is the first description of a gating
circuit designed specifically for this type of photomultiplier.
Aside from the inherent rise and fall times, there are no con
straints on the lengths of the on and off portions of the gating
cycle, and they are fully independent. The achievement of
very low switching noise at the anode allows the PMT to be
used for photon counting applications which require a short
and precise switching time without distortion in the count
rate following the turn-on transition. Finally. the circuit can
be incorporated into a standard photomultiplier housing,
and it does not require special high-voltage components.
ACKNOWLEDGMENT
The author wishes to thank Mike Murray of the Univer
sity of Wisconsin-Madison Physics Electronics Shop for his
assistance in the development of the gating circuit.
a) Present address: Advanced Development, Philips Lighting Company,
Lynll, MA Ol901.
IF. de Martini and K. P. Wacks, Rev. Sci.lnstrum. 38, 866 (1967).
2M. Yamashita, Rev. Sci. lnstrum. 45,956 (1974).
'See RCA Technical Data Sheet, C31034 Series 10-85. RCA Corp., New
Products Division, New Holland Ave., Lancaster, P A 17604.
'RCA Corp, New Products Division-Tube Operations, MS-058, New
Holland Avt., Lancaster, PA 17604.
sM. B. Schulman, F. A. Sharpton, L. W. Anderson, and C. C. Lin, Bull.
Am. Phys. Soc. 34, 294 (1989).
Gating circuit 1266
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132.174.255.116 On: Fri, 28 Nov 2014 20:00:23 |
1.1140931.pdf | HESYRL: Recent status
ZhongMou Bao
Citation: Review of Scientific Instruments 60, 1698 (1989); doi: 10.1063/1.1140931
View online: http://dx.doi.org/10.1063/1.1140931
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Rev. Sci. Instrum. 63, 1578 (1992); 10.1063/1.1142979
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129.49.23.145 On: Thu, 18 Dec 2014 21:45:34Storage Ringe and its Injector
HESVRl: Recent status
Zhong-Mou Bao
University a/Science and Technology 0/ China, Heifei, Anhui, People's Republic a/China
(Presented on 31 August 1988)
The 2oo-Me V linac injector was completed in October 1987 and commissioned to the designed
targets. The 800-Me V storage ring is expected to complete the installation by the end of 1988. Five
beamlines and experimental stations, which are under constmction, are described.
INTRODUCTION
The purpose for establishing Hefei National Synchrotron
Radiation Laboratory (HESYRL) are: (1) to develop
synchrotron radiation (SR) scientific research and techni
cal application; (2) machine study for developing more
powerful SR machines to meet the needs of users; (3) to
provide an education base combined with scientific research.
Started from research and development March 1978 to
July 1981, through a series of reviews, the proposal to found
HESYRL was finally authorized as a key project of China by
China's State Planning Committee in October 1984. The
ground breaking took place November 20, 1984.
The HESYRL project consists of three major parts: (1)
light source, (2) experimental area; (3) auxiliaries. The
present status is as follows.
I. LIGHT SOURCE (Refs, 1 and 2)
An SOO-MeV ring with a 200~MeV linac as its injector
(Fig. 1). The main design figures are: Storage ring 800 MeV,
100-300 rnA, 66.13 m in circumference, 22 m in average
diameter, 12 dipoles with field strength of 1.2 T, and critical
wave length 24 A. Linac 200-240 MeV, 50 mA, energy
spread AE IE = ± 1 %, beam pulse width 2-4 ns, or 0.2-1
ps, and repetition rate 50 pps.
By the end of October 1987, the !inac was installed in the
underground tunnel, and commissioned to the designed tar-
FiG.!' Plan layout of Hefei machine. gets of energy and current on November 24, 1987 (220 Me V
and 58 rnA, respectively); In June 1988, the energy spread
was measured to be AE IE = ± 0.8%. By the beginning of
February 1988, all the transport lines, to the ring, to the
nuclear experimental hall, and to the dump, 126 m in total
had been set up, and received the electrons with full energy at
the terminals.
All the components of the ring have been made and test~
ed or are under test (except for the rf cavity, which will be
delivered by the end of September). The whole ring has be
gun installation recently. It is expected to complete the in
stallation by the end of 1988, after which the commissioning
will take place.
II. EXPERIMENTAL AREA (Ref. 3)
!he S~ ex~erimental hall is in the light source building
and IS 50 m m dIameter. From the ring, each bending magnet
vacuum chamber has been designed to provide two ports for
extracting light; 27 beamlines in total (include three from
insertions) are available (Figs, 2 and 3). The five beamlines
and experimental stations which are proposed to be setup at
the first stage are listed in Table I.
The fast closing valve of the front end was designed by
us and manufactured by Shenyang Scientific Instrument
Factory. It has given good results: leak rate, 0.33 Torr £"/s;
FIG. 2. Experimentalarea (a) Preparation lab; (b) clean rooms, (c) lithog
raphy. (d) basic research, (c) metrology.
1698 Rev. Sci.lnstrum. 60 (7), July 1989 0034·67 48f89f071698-03$01 ,30 © 1989 American Institute of Physics 1698
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129.49.23.145 On: Thu, 18 Dec 2014 21:45:34Vf Vp \'1M Vm
---SR
FIG. 3. Side view of front end: Vm; manual valve, WM; water-cooled mask,
Vp; pneumatic valve, Vf; fast closing valve, BS; beam shutter.
closing time, 6 ms (including the blade closing time and the
response time of the trigger magnet), and after 800 opera
tions the leak rate kept the same figure of 0.33 Torr tIs.
The status of the beamlines and the corresponding ex
perimental stations are as follows.
A. Beamline U1• for lithography
The optical system of beamline U I is shown schemati
cally in Fig. 4. U I has been designed, the range of wavelength
is 5-20 A. The main goals of the lithography station are to
use SR single exposure for developing practical devices such
as metrology gratings and zone plates for studying masks,
resists, large area uniform exposure, alignment methods, etc.
The station is simply a single-exposure machine which has
been designed and will be manufactured; it is expected to
have the machine by the end of March. According to the
schedule, the whole beamline Uland the station will be con
nected to the ring in July 1989.
S. BeamUne U10A ~ for photochemistry
Beamline U lOA is sketched as Fig. 5 which contains a
I-m seya grazing incidence monochromator with two grat
ings of 1200 and 600 lImm; the wavelength covered range is
200-4000 A. This station is used for gas phase experiments,
for studying ( 1) VUV absorption spectra, fluorescence spec
tra, and ionization spectra of atoms and molecules; (2)
chemical reactions of highly excited state molecules; and (3)
chemical reactions of ions with molecules.
The station consists of an analyzer chamber, a differen-
TABLE I. Beamlines and experimental stations.
Beamline
V,
UlOA
DWll
U'2A
U2D Wavelenlfth range
(A)
5-20
200-4000
500-6000
20-50
10-1200 Station
Lithography
Photochemistry
Time resolve spectroscopy
Soft x-ray microscopy
Photoelectron spectroscopy
1699 Rev. Sci. lostrum., Vol. 60, No.7, July 1989 FIG. 4. Optical system of beam line U,.
tial pumping system and a data-acquisition system. The ana
lyzer chamber contains a pulsed molecular beam source, two
quadrupole mass spectrometers used for cation and anion
analysis, respectively, and a port for photomultiplier and UV
lasers. The differential pumping system inserted between the
post mirror box on the beamIine and the analyzer chamber
can produce about three orders of magnitude pressure drop.
The data-acquisition system was designed to accommodate
coincidence experiments such as photoelectron-photoion,
photocation-photoanion, and photon-photoion coincidence
measurements.
C. Beamline U1oa, for tlmeMresolved spectroscopy
The U lOB is sketched as Fig. 6 which contains a l-m seya
monochromator with gratings of 2400, 1200, and 600 1/mm,
the covered wavelength range is 500-6000 A. The station
was designed for studying the properties of spectra of solid,
liquid (including bio-sample), and gas phase samples and
fluorescence life time. The spectra include emission, absorp
tion, excitation, transmission and reflection, and time-re
solved spectra and the related dynamic changes caused by
thermal effects, and magnetic effects.
The station consists oftwo sample chambers, four mon
ochromators with high luminosities and high resolution, and
a fast data~acquisition system. Combined with two analysis
monochromators ofj= 2.7 forUV-VIS and! = 3 forVUV,
one chamber (if> 150 mm) equipped with an electron gun and
a sample holder at 77 K are used for experiments requiring
high sensitivity. The other if; 350~mm chamber equipped
with a sample holder at 4.2 K and a superconducting magnet
of 6 T together with two high resolution monochromators
(0.2 A for VUV, 0.15 A forUV-VIS) is used for high resolu~
tion.
D. Beamllne U12A• for soft x~ray microscopy
The U J2A with its microscope optics is sketched as Fig.
7. The aims of the soft x-ray microscopy station are (1) to
develop soft x-ray contact and scanning microscopy tech
niques at present and x-ray microholography in future and
(2) to provide a powerful instrument to the biological and
medical community for studying biological specimens.
UIlA was designed mainly for scanning x-ray micros
copy. A condenser zone plate (CZP) combined with a dia
phragm to demagnify the SR source acts as a linear mono-
3200 Ml
SR --,"""""""",,~.::;!,~~.;;. 1885
M2
FIG. 5. Optical system ofbeamIine U IDA'
Storage ring and injector 1699
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129.49.23.145 On: Thu, 18 Dec 2014 21:45:34SR 3000 2656 1400
Ml
FIGo 6. Optical system of beam line UIOB'
chrornator. For covering the wavelength range 20-50 A,
three CZPs can be slid to the prepared site. The microzone
plate (MZP) focuses the x ray to a small spot. The specimen
in the air environment can be scanned through the focal spot
mechanically under computer control. The x-ray micro
graph can be displayed on a screen or be stored.
The components of U 12A are being manufactured.
Zoneplates, flow gas proportional counter, and contraelec
tronies are being developed in our labs, which have obtained
hopeful results. We expect to check out our prototype scan
ning soft x-ray microscope on-line after our SR source be
comes operational.
E. Beamline U20, for photoelectron spectroscopy
U20 contains a spherical grating monochromator, the
covered wavelength range is 10-1200 A. V2G is sketched as
in Fig. 8. The main goal of setting up this station is for sur
face and interface science research. It can perform ordinary
photoemission research such as angle-resolved ultraviolet
photoemission spectroscopy (ARUPS), ultraviolet and x
ray photoemission spectroscopy (UPS,XPS) as wen as near
edge x-ray absorption fine structure (NEXAFS), and sur
face extended x-ray absorption fine structure (SEXAFS).
This station comprises four parts: analysis, preparation
and sample production chambers, and a data-acquisition
system.
In the analysis chamber, conventional UV and x-ray
sources and an electron beam source are provided, and it is
equipped with two analyzers for angle-resolved and angle
integrated measurement, respectively.
The sample production chamber is composed of five
beam sources and a thickness monitor. Thin films of single
DIAPIlRACM ; c~p '~.zp
SR . 0.· . .. +>! ::$-L---------1QQ.Q--------- ----T~02.+!~_4
FIG. 70 Microscope optics of beamline U 12A •
1700 Rev. Sci.lnstrum., Vol. 60, No.7, July 1989 F.XPERIMENl'AL l'i)VlIBLE SPHERICAL GRATING
sTATION EXIT SLIT IO'lCCHRCMA'IOR
FIG. 8. Optical system of beam line U20• SPHERICAL
f1IRR'0R.'; PAIR
.~ SR SOURCE
and multiple layers can be made on metal and semiconduc
tor substrates.
The sample preparation chamber is equipped with a fast
sample introduction system, a specimen transfer system, an
argon ion gun, two evaporation sources, and thickness moni
tor etc.
The data-acquisition system is an IBM-based multitask
ing system. It can be operated for SR with EDe, CIS, CFS,
and PED modes.
Most parts of this station will be imported from VSW
Company, England, except the sample production chamber
which will be made by Shenyang Scientific Instrument Fac
tory.
III. AUXILIARY FACILITIES
HESYRL is open to users both domestic and from
abroad. Auxiliary facilities have been planned and con
structed for supporting users.
The main building-light source building of95oo m was
completed, in which an experimental hall of 2000 m2 is sur
rounded by rooms of 831.5 m2 for experiment preparation,
clean rooms for lithography, rooms for installing computer
for data processing etc.
In addition to the light source building, a research build
ing of 3800 m2 which was completed in January 1986 is pro
vided for users and in-house staff.
A guest house of 2200 m2 with 70 rooms, will be com
pleted in the coming fall for users.
ACKNOWLEDGMENTS
The author would like to thank the staff ofHESYRL; Y.
W. Zhang, X. S. Xie, Y. J. Pei, P. S. Xu, S. N. Qian, C. S. Shi,
and D. M. Su of BEPC for the materials provided for this
research.
"R Zhongmou et ai., NucJ. lnstrum. Methods 208, 19 (1983).
2H. Tohui, Proceedings of the Workshop on Construction and Commis
sioning of Dedicated Synchrotron Radiation Facilities (1985), pp. 155-
166.
3Z. Yanwu et 01., Internal Report, 1988 Present Status of Beam Lines and
Experimental stations at HESYRL
Storage ring and injector 1700
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129.49.23.145 On: Thu, 18 Dec 2014 21:45:34 |
1.102390.pdf | Dielectric properties of glasses prepared by quenching melts of superconducting BiCa
SrCuO cuprates
K. B. R. Varma, G. N. Subbanna, T. V. Ramakrishnan, and C. N. R. Rao
Citation: Applied Physics Letters 55, 75 (1989); doi: 10.1063/1.102390
View online: http://dx.doi.org/10.1063/1.102390
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141.210.2.78 On: Wed, 26 Nov 2014 03:21:04Dielectric properties of glasses prepared by quenching melts of
superconducting EU~Ca~Sr .. Cu~O cuprates
K. B. R. Varma, G. N. Subbanna, T. V. Ramakrishnan, and C. N. R. Raoa)
Solid State and Structural Chemistry Unit. 1Vlaterials Research Centre and Departmelll afPhysics. Indian
Insritute of Science. Bangalore 560012. India
(Received 8 March 1989; accepted for publication 2 May 1989)
Glasses obtained from quenching melts of superconducting bismuth cuprates of the formula
Bi2 ( Ca,Sr) n t 1 CUll 02n t 4 with fl = 1 and 3 exhibit novel dielectric properties. They possess
relatively high dielectric constants as well as high electrical conductivity. The novel dielectric
properties of these cup rate glasses are likely to be of electronic origin. They exhibit a weak
microwave absorption due to the presence of microcrystallites.
Relation between ferroelectricity and superconductivi
ty in perovskite oxide structures has been a subject of interest
for the past several years. 1-7 Recently, a possible relationship
between ferroelectricity and high Tc superconductivity in
YBa2Cu307 .. 8 and Bi2 (Ca,Sr) 3CUZOg t-8 has been pointed
out.8-lO It has been suggested that a phase transition to a
relaxor type ferroelectric state could occur above the super
conducting transition. Dielectric constant measurements in
the high-temperature phase of YBaZCuj07. Ii suggest the
presence of ferroelectric-like polarizability in the phase pre
ceding the metallic or the superconducting state. g The series
of Bi cuprates of the general formula
Bi2(Ca,Sr)n f J Cun02n + 4-1 b reported in recent months II
has afforded the possibility of examining the dielectric prop
erties of glasses of these cup rates since melts of these
cuprates can be readily quenched to the glassy stateY We
have found that these glasses exhibit rather high dielectric
constants and related properties probably associated with
the large electronic polarizability of clusters containing CuO
sheets.
Polycrystalline samples ofBi~Ca~Sr-Cu-O cuprates cor
responding to the nominal compositions Bi2CaSrCu06 + 8
(n = 1) and Bi 1./, PbO.4 Ca2Sr 2CU30 10 + 8 (n = 3) were pre
pared by heating a mixture of Bi20~1> CaC03• SrCO 3' CuO,
and PbO (added only to the latter composition). The first
composition was obtained by heating the mixture without
PbO around 1070 K for about 8 h with intermediate grinding
steps. The latter composition was prepared by heating the
mixture around 1090 K for several days with intermediate
grinding steps and at 1100 K for about 8 h. X-ray powder
diffraction patterns of the resulting compounds confirmed
them to be the n = 1 and n = 3 members of the homologous
series. 11 Homogeneous glasses of the two cuprates were ob
tained by melting them in a platinum crucible followed by
splat quenching. It may be remarked here that the n = 2
cup rate gives a heterogeneous glass due to disproportiona
tion 12 and we have therefore restricted our studies to the
n = 1 and 3 members. The amorphous nature of the glasses
prepared by us from the melts of the n = 1 and 3 members
was confirmed by x-ray diffraction, electron microscopy,
and differential scanning calorimetry (Fig. 1) measure-
,,' To whom correspondence should be addressed at the Solid State and
Structural Chemistry Unit. ments. Capacitance measurements were carried out on the
glassy samples as a function of both frequency (1-100 kHz)
and temperature (300-700 K) with a signal strength ofO. 5 V
rms. de magnetic susceptibility measurements on the crys
talline samples were carried out by the Faraday method and
resistivity measurements by the four-probe method. Dielec
tric hysteresis of the glasses was examined at 50 Hz by means
of a Sawyer-Tower circuit. ac conductivity was measured on
O.5-mm-thick glass disks using a HP, LCR bridge model
4274A.
de magnetic susceptibility studies on the crystalline
samples showed the onset of superconductivity in the n = 1
and n = 3 members to be around 80 and 110 K, respectively.
Glasses of these materials show a single glass transition tem
perature ( 1~ ) around 650 K. This is followed by exothermic
crystallization transition (T~r ) around 720 K (Fig. 1). Spe
cific heat (Cp) measurements show these transitions around
o a'
"0 c:
W
r
o x
W Tg
I
/ I :
U :
"--L .. _:._._~ ~.-.L .. ~ .. _J
380 500 620 740
T(K)
FIG. t. DSC curves of splat-c.jucnched samples of Bi,CaSrCuOb c h (a')
and Bi",Pb" .. Sr,Ca,Cu,O\() , ,\ (b '). Heat capacity data are shown in the
inset.
75 Appl. Phys. Lett. 55 (1).3 July 1989 0003-6951/89/270075-03$01.00 @ 1989 American Institute of Physics 75
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141.210.2.78 On: Wed, 26 Nov 2014 03:21:04the same temperatures (see inset of Fig. 1). Annealing these
glasses above Ter showed the formation of crystalline super
conducting phases.
In Fig. 2 we show the temperature variation of the di
electric constants of the two glasses at 1 kHz. The dielectric
constants t:r of the n = 1 and 3 glasses at room temperature
are around 6 and 490, respectively, showing that the dielec
tric constant increases with the number of CuO sheets. The
dielectric constant increases gradually with temperature and
reaches high values up to 10 000 above 600 K. The ac con
ductivity of the glasses increases markedly with increase in
temperature, the n = 3 member showing considerably high
er conductivity than the n = 1 member. The n = 3 sample
shows ajump in conductivity around 400 Kat 1 kHz (Fig.
3) and it shifts towards higher temperatures as the frequency
increases (up to 100 kHz), possibly due to a local structural
relaxation with a relaxation time of ~ 10-13 s. The value of
tan .5 also shows an increase with temperature just as the
electrical conductivity (Fig. 2). The dielectric constants of
the glasses do not vary significantly with the frequency in the
I-100kHz range. The n = 1 glass exhibits dielectric hystere
sis, but shows no saturation; the absence of the saturation is
probably due to the relatively high electrical conductivity of
the samples. The n = 3 glass, however, does not show a hys
teresis loop because ofits high electrical conductivity. Such a
behavior is encountered in ferroelectric semiconductors.
Both the n = 1 and 3 glasses show pyroelectric behavior
with a pyroelectric coefficient of the order of 3 X 10-6 C/
cm2K.
The relatively high dielectric constant exhibited by Bi
cuprate glasses is of interest. It could be due to the space
charge polarization caused hy heterojunctions arising from
the presence of small clusters or ultra microcrystallites in the
glassy state.13 Evidence for the presence of such clusters
( ~ 30 A. diameter) has been found by us in electron micro
graphs of the glasses. Furthermore, we find weak microwave
absorption (9.1 GHz) in the glasses at 77 K. It is to be noted
that crystalline superconducting cuprates show intense mi
crowave absorption below the superconducting transition
temperature and this property can be used to characterize
superconductivity in these materials. Il.I4.15
/
;". "~h "'"' -----------}. /
~-~ ~ ~~
l~.>~/~, '~
290 370 450 530 610 590
TlKi -7
-3
770
FIG. 2. Dielectric constant E, a.nd loss tangent data of bismuth cuprate
gla.sses at 1 kHz. Designation of the samples is same as in Fig. I.
76 Appl. Phys. Lett., Vol. 55, No.1, 3 July i 989 -2.6[' __ ~:--_~
_ 1 .L-~.~.-- __ ,*_-'
2.0 2·~ 2.8 3.2
1000 (K)
T
FIG. 3. ac electrical conductivity of (n = 3) glassy bismuth cuprate. de
conductivity measurements show the intrinsic semiconducting behavior
above room temperature.
The rather large dielectric constants of the cuprate
glasses are likely to be mainly of electronic origin, in contrast
to its lattice origin in conventional glassy and other ferro
electrics. Experimental evidence in support of this idea in
cludes our finding (e.g., Figs. 2 and 3) that the temperature
variation of Er parallels that of the electrical conductivity,
which is electronic. The weak frequency dependence of Er as
well as the near absence of nonlinearity and genuine hystere
sis in polarization versus electric field, are also indicative of
an electronic origin for the high Cr' Furthermore, the ob
served dependence of Er on temperature is consistent with a
lattice picture only if one assumes that the putative ferroelec
tric transition temperature is above the highest temperature
of measurement.
The oxygen ions in the cuprates are highly polarizable,
since the ionization level (i.e., the state with a hole, namely,
01-) is only 1 eV or so away. Thus, even in the absence of
holes, there should be a considerable local field enhanced
Clausius-Mossod polarizability, for the square oxygen or
02 -lattice. The material actually consists of clusters or mi
crocrystalIites of the size ~ 30 A.-At about 0.1 holes per unit
cell (the estimated density in the bismuth cuprates) the
number of holes in a cluster of diameter 30 A is small, being
about 10. The quantum polarizability of a hole moving in a
cluster of size Rc is proportional to R ~ a~ff and can thus lead
to a large increase in Cr' Here (lelf lies close to the enhanced
polarizability of the lattice without holes, since the local field
is poorly screened on account of the small number of holes in
a cluster. As the temperature increases, there is greater inter
cluster hopping of holes which effectively increases Rc and
hence the dielectric constant.
A possible implication of the above model is that the
large dielectric constants in many other perovskites could
have an electronic origin also, involving ubiquitous oxygen
holes moving in a small region or forming shallow traps.
The authors thank the Department of Science and Tech
nology and the University Grants Commission for support
of this research.
'R. A. Hein, J. W. Gibson, R. Mazelsky, R. C. Miller, and J. K. Hulm,
Phys. Rev. Let.t. 12, 320 (\964).
Varma eta!. 76
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141.210.2.78 On: Wed, 26 Nov 2014 03:21:04213. T. Mathias, Mater. Res. BulL 5, 665 (1976).
'J. Birman, Ferroelectric!; 16,171 (1977).
·P. B. Allen and M. L. Cohen, Phys. Rev. 177, 704 (1969).
5G. S. Pan'ley, W. Cochran, R. A. Cowley, and G. Dolling, Phys. Rev.
Lett. 17,753 (1966).
0p. W. Anderson and E. L Blount, Phys. Rev. Lett. 14, 217 (1965).
7L, R. Testardi, Phys. Rev. Lett. 31, 37 (1973).
"L, R. Testardi, W. G. Moulton, H. Mathias, H. K. Ng, and C. M. Rey,
Phys. Rev. B 37,2324 (1988).
oS. K. Kurtz, J. R. Hardy, andJ. W. Flocken, Ferroelectrics87, 29 (1988).
"'S. K. Kurtz, L E. Cross, N. Setter, D. Knight, A. Bhalla, W. W. Cao, and
77 Appl. Phys. Lett., Vol. 55, No.1, 3 July 1989 W. N. Lawless, Mater. Lett. 6, 317 (1988).
"e. N. R. Rae, L Ganapathi, R. Vijayaraghavan, G. R. Rae, K. Murthy,
and R. A. Mohan Ram, Physica C 156, 827 (1988) and the references
cited therein.
12K. B. R. Varma, K. 1. Rao, and C. N. R. Rao, App!. Phys. Lett. 54, 69
( 1989).
l3J. O. IS<'lrd, Proc. Inst. Elec. Engrs. B 109 (Stipp!. 22), 440 (1962).
"S. V. Bhat, P. Ganguly, T. V. Ramakrishnan, and C. N. R. Rao, J. Phys. C
20, L559 (1987).
"K. Murthy, K. B. R. Varma, S. V. Ehat, and C. N. R Rao, Mod. Phys.
Lett. B 2, 1259 (1988).
Varma etal. 77
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141.210.2.78 On: Wed, 26 Nov 2014 03:21:04 |
1.2811101.pdf | A. Vibert Douglas
Helen Sawyer Hogg
Citation: Physics Today 42, 7, 88 (1989); doi: 10.1063/1.2811101
View online: http://dx.doi.org/10.1063/1.2811101
View Table of Contents: http://physicstoday.scitation.org/toc/pto/42/7
Published by the American Institute of PhysicsJulian H. Webb
neering at Clemson College. He con-
tinued his studies at the University of
Wisconsin, receiving a master's de-
gree in electrical engineering in 1925
and a PhD in physics in 1929. His
thesis research with Warren Weaver
was in mathematical physics. Webb's
graduate student contemporaries at
Wisconsin included Lee DuBridge and
Guy Suits.
In 1931, after two years as an
instructor in physics at Williams Col-
lege, Webb joined the research labora-
tories of Eastman Kodak. At Kodak
Webb became interested in the phys-
ics of latent-image formation—the
fundamental basis of the photograph -
ic process. He was a pioneer in the
application of the quantum mechan-
ics of crystalline solids to silver ha-
lides and to the photographic process.
Webb's experimental work was
characterized by a strong analytical
foundation. His early publications
discussed an experimental study of
the photographic intermittency and
reciprocity-failur e effects. These ef-
fects, which are responsible for the
dependence of photographic speed on
the individual values of the intensity
and time of exposure instead of just
the product of the two, greatly compli-
cated the practical application of pho-
tography. These effects are now
largely under control in commercial
photographic emulsions . Webb was
able to use insight gained from these
studies to separate the effects of the
electronic and ionic processes in-
volved in latent-image formation.
His subsequent experiments support-
ed the Gurney-Mott quantum me-
chanical theory of latent-image for-
mation and permitted the under-
standing of many important
photographic phenomena, including
reciprocity failure, intermittency, theHerschel effect, solarization, dye sen-
sitization and, above all, latent-image
formation, in terms of concepts that
pointed the way to improved photo-
graphic films.
During World War II Webb worked
on the electromagnetic separation of
uranium isotopes in the Manhattan
Project, spending time in Berkeley
and Oak Ridge. During this time he
also contributed to the development
of a process to mold high-precision
optical elements, which has become
important in the large volume manu-
facture of high-quality glass lenses.
With the end of the war Webb again
turned his attention to studying the
formation of the photographic latent
image. He concentrated on experi-
mental studies of photographic effects
in order to develop a statistical model
for latent-image formation. From
this model and the known size-distri-
bution of grains in an emulsion, he
confirmed that one to ten absorbed
photons can render a photographic
grain developable. In 1949 he con-
cluded that two silver atoms can form
a stable sub-latent-imag e site. This
led to the useful suggestion that pre-
exposure of astronomical plates to
low-intensity light, to form stable sub-
latent-image specks, can greatly in-
crease the sensitivity of the plates.
Webb also became interested in the
formation of image tracks by ener-
getic particles and in nuclear track
emulsions, and these interests led to
his solving several serious fallout-
related problems that occurred in the
manufacture of film. In one such
instance, during late 1945, spots be-
gan to appear mysteriousl y in x-ray
film. An affected film would typically
show from 10 to 100 small, black spots
after processing. By a careful set of
experiments, which had to be carried
out quickly because of the urgency of
the problem, Webb showed that the
spots were caused by the presence of a
radioisotope in cardboar d packaging
for the x-ray film made by a particu-
lar paper mill in Indiana. He deduced
that the isotope (probably Ce141) had
been produced in the first atomic
bomb test in July 1945. It was subse-
quently washed as fallout into the
Wabash River, from which process
water was taken by the paper mill.
This discovery permitted solutio n of
the fogging problem and minimized
its impact on medical diagnostics.
During the 1950s Webb assumed
increasing responsibility for the man-
agement of the physics division of the
Kodak Research Laboratories. He
played a central role in strengthening
the solid-state physics and analytical
bases for photographic science, to
complement an already strong photo-graphic chemistry effort at Kodak.
He established the solid-state physics
laboratory and, with George Higgins,
built a strong program in what is now
known as image science, the informa-
tion theoretic approach to image
structure and the analysis of imaging
system performance.
The work of Julian Webb was
instrumental in making practical
photography available to us for our
profession and our pleasure.
BENJAMIN B. SNAVELY
Eastman Kodak Company
Rochester, New York
A. Vibert Douglas
A. Vibert Douglas, an astrophysicist
and university educator, died in
Kingston, Ontario, on 2 July 1988 at
age 93. One of Canada's most distin-
guished citizens, she was made a
member of the Order of the British
Empire by King George V in 1918 for
her work in the War Office, and an
Officer of the Order of Canada in
1967. Her great zeal for astronomy,
keen interest in her students and
involvement in fostering interna-
tional relations made her widely ad-
mired and loved.
Born in Montreal, Douglas started
her university education at McGill
University, interrupted it for war
work and then returned to receive her
bachelor's degree in 1919 and her MS
a year later. Her postgraduate work
at the Cavendish Lab of Cambridge
University with Rutherfor d and her
work with Arthur Eddington, also at
Cambridge, whetted her interest in
astronomy, and in 1926 she received
her doctorate in that field from
McGill. She remained on the McGill
staff for 17 years. She and John
A. Vibert Douglas
88 PHYSICS TODAY JULY 1989WE HFAB TUAT
Stuart Foster investigated the spectra
of A- and B-type stars and the Stark
effect with the 72-inch telescope of the
Dominion Astrophysical Observatory.
In 1939 Douglas was appointed dean
of women at Queen's University,
Kingston, where she continued to
work until her retirement.
Despite her heavy university du-
ties, she found energy for remarkable
international achievements. A cita-
tion accompanying her receipt of an
honorary degree from Queen's Uni-
versity described her as an "inveter-
ate internationalist." She became the
first Canadian presiden t of the Inter-
national Federation of University
Women in 1947, and she represented
Canada at the UNESCO conference in
Montevideo in 1954.
A member of the International
Astronomical Union, she held the
Canadian record for attendance at its
triennial General Assemblies. When
the IAU met in Germany in 1964, a
special bus trip was arranged to take
some members to East Berlin for a
few hours. The fear of the Berlin
Wall was then near its height. At
dinner that night we bus travelers
learned in astonishment that Allie
Douglas (a nickname she preferred to
the more formal "Alice") had walked
through Checkpoint Charlie all alone
and had spent the day crisscrossing
the city on public conveyances, be-
cause, as she said, "I think that's the
best way to see a city."
Douglas's many writings will con-
tinue to spread her knowledge. Her
most outstanding literary contribu-
tion was The Life of Arthur Stanley
Eddington (1956), a project started at
the request of Eddington's sister,
Winifred. Probably Douglas's Quak-
er background, which she had in
common with the Eddingtons, made
her specially attuned to this task, and
her erudition shines through it, as in
her use of the first four bars of
Schubert's Unfinished Symphony to
introduce the chapter on Fundamen -
tal Theory. Her other publications
appeared in the Journal of the Royal
Astronomical Society of Canada, Hib-
bert Journal, Atlantic Monthly, Dis-
covery and several university quarter-
lies, among other places.
In 1984 the Canadian Astronomical
Society held a special session at the
Herzberg Institute of Astrophysics in
Ottawa in honor of Douglas's forth-
coming 90th birthday on 15 Decem-
ber. To attend, she traveled alone by
bus from Kingston to Ottawa. For
many of us, this was our fond farewell
to her.
HELEN SAWYER HOGG
David Dunlap Observatory
University of Toronto •. THE COMPLETE
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PHYSICS TODAY JULY 1989 89 |
1.343149.pdf | Instrumentation of a resonant gravitational radiation detector with a planar thinfilm dc
SQUID
W. M. Folkner, M. V. Moody, J.P. Richard, K. R. Carroll, and C. D. Tesche
Citation: Journal of Applied Physics 65, 5190 (1989); doi: 10.1063/1.343149
View online: http://dx.doi.org/10.1063/1.343149
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/12?ver=pdfcov
Published by the AIP Publishing
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130.113.111.210 On: Fri, 19 Dec 2014 14:11:29Instrumentation of a resonant gravitational radiation detector with a planar
thin-fUm de SQUID
w. M. Folkner, M. V. Moody, J.-P. Richard, and K R. Carroll
University of Maryland, Department of Physics and Astronomy, College Park, Maryland 20742
C. D. Tesche
IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598
(Received 21 October 1988; accepted for publication 24 February 1989)
The instrumentation of a low-temperature three-mode gravitational radiation antenna
incorporating a low-noise dc SQUID provided by IBM is described, The feedback circuitry
necessary to maintain the linearity and dynamic range of the SQUID was found to drive the
resonant system due to high coupling between the input coil and the feedback coil of the
SQUID. In order for this type of planar thin-film dc SQUID to be useful for gravitational
radiation detectors and other applications requiring high Q input circuits, a solution to this
feedback problem is needed. To this end, the nonlinear equations describing the dc SQUID
with linear feedback are solved in terms of an isolated SQUID. The important feedback
parameters for a high Q resonant system are found to be the slew rate of the electronics and the
coupling constant ratio afr1aJ, where aif is the energy coupling efficiency between the
feedback coil and input coil and a; is the energy coupling efficiency between the feedback coil
arid the SQUID loop. Methods to reduce the effect of the feedback on the input circuit are also
discussed.
i. INTRODUCTION
A significant effort is underway in many countries to
develop massive resonant gravitational radiation antennas
of the type originated by Weber.l It is hoped that such anten
nas, operated at a sensitivity near the single-phonon level,
will permit observation of catastrophic astrophysical events
within the Virgo cluster of galaxies. Resonant gravitational
radiation detectors operate by measuring changes in the en
ergy of the antenna, which usually takes the form of a right
circular cylinder of aluminum. The energy sensitivity of the
Weber cylindrical antenna is limited by its Brownian noise
and by the noise introduced by the transducer and amplifier
used to measure the energy of the antenna. By cooling the
antenna to low temperature, the noise originating in the an
tenna and transducer can, in principle, be reduced to any
desirable level. Then, in the absence of a back-action evasion
procedure, the sensitivity of the detector is limited by the
noise of the amplifier. For an antenna employing a resonant
transducer and a SQUID (superconducting quantum inter
ference device) amplifier, the sensitivity is approximately
given by2
+(SV+ST) f3r (1+ 2 2)' 4L1' (fJT{;) A ) (1)
where TA is the equilibrium temperature of the antenna, W A
is the angular frequency of the antenna, r is a time constant
characteristic of the filtering process, /3 is the ratio of the
energy available at the amplifier to the total energy of the
detector, 7,H is the damping time ofthe antenna with /3 = 0,
LT is the output inductance of the transducer, Si is the
SQUID current noise power spectral density, Sv is the
SQUID voltage noise power spectral density, and S1' is an effective voltage noise power spectral density resulting from
losses in the transducer. For present systems, the SQUID
voltage noise is negligible compared to the transducer volt
age noise.
A useful figure of merit for comparing SQUIDs is the
energy resolution Er which is related to the current noise by
the expression
(2)
where a7 is the inductive energy coupling constant between
the SQUID input coil inductance L; and the SQUID loop.
The energy resolution per unit bandwidth is commonly ex
pressed in units of 11, where Ii is Planck's constant divided by
211'. A "quantum-limited" SQUID would have an energy res
olution of order Ii. Generally, the sensitivity of a gravitation
al radiation detector can be improved by use of a SQUID
with better energy resolution by choosing the parameter /37
ofEq. (1) to maximize the detector sensitivity.
Because of the improvements that have been made in the
energy resolution of planar thin-film SQUIDs, an antenna
incorporating these devices is highly desirable. However, as
was discovered in the experiment, strong inductive coupling
between the input coil and feedback coil allows the SQUID
electronics to drive the resonant system. SQUIDs that have
the input and feedback coils in separate toroidal cavities3
exhibit reduced stray coupling and have been successfully
used on cryogenic antennas. Even with this reduced cou
pling, however, the resonant system can still be driven when
the slew rate of the electronics is not large enough (see Sec,
V).
The two-mode inductance modulation transducer in use
at the University of Maryland is described in the next sec
tion. In Sec. HI the parameters of the IBM de SQUID are
given. In Sec. IV, the antenna parameters are given along
with the other experimental details and results. Section V
5190 J. Appt. Phys. 65 (12), 15 June i 989 0021-8979/89/125190-07$02.40 © i 989 American Institute of Physics 5i90
[This article is 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 14:11:29presents the feedback analysis from which the effective
SQUID input impedance is computed for the resonant sys~
tern. Based on the feedback analysis, possible solutions are
discussed in Sec. VI that can make the low~noise planar thin
film SQUIDs useful for applications involving high Q reso~
nant circuits.
II. INDUCTANCE MODULATION TRANSDUCER
The two-mode transducer developed by Richard4 for
the Maryland three-mode gravitational wave antenna is
shown in Fig. 1, The two mechanical resonators with masses
1.476 and 0.004 kg amplify the motion of the 1200~kg cylin
drical bar by the square root of the ratio of the bar dynamic
mass of 600 kg to the final test mass of 0.004 kg. The motion
amplification allows for a better impedance match between
the mechanical and electrical parts of the detector. The final
test mass is in the form of a thin conical surface to provide a
large effective area to mass ratio for high electrical coupling.
The three-mode structure of the detector response also al·
lows for a wider detection bandwidth. 5
The inductance modulation mechanism developed by
Paik6 is used to convert the motion of the detector into an
electrical signaL Figure 2 shows schematically the final test
mass suspended between two "pancake" coil inductors L\
o 2 4 6
I
Co}
,
~PANCI\KE COUPLING COILS
, INAL RESONATOR m3
iNTERMEHATE RESONATDR m2
(b)
FIG, I, Two-mode indllctance modulation transducer. (a) Top view of the
transducer resonator assembly showing the annular second resonator and
the cantilever spring suspension of the thin final resonator. (ll) Cross sec
tion of the assembled transducer showing the position of the coupling coils.
5i91 J. Appl. Phys., Vol. 65, No. 12, 15 June 1989 r--------------,
I Li Sy i
I I
I ! I I
I I I , I L ____________ ....J
SQUID
FIG, 2. Schematic of an inductance modulation transducer connected to II
dc SQUID. The SQUID is modeled as an ideal current amplifier with two
conjugate noise sources and an input impedance.
and L2• The SQUID is represented by a current amplifier
with input inductance Li and voltage and current noise
sources which have power spectral densities S" and Si' The
noise source STand resistor R T model the losses in the trans
ducer.
A persistent current 10 is stored in the loop containing
the superconducting coils L I and L2• With the test mass cen
tered so that So = SI = S2' the inductances L[ and L2 take on
the same value:
Lo = Jion2 Aso, ( 3 )
where A is the surface area of the coils and n is the number of
turns per unit radial distance. A change of the test mass
position x produces the voltage across the output of the
transducer
(4)
where x denotes the time derivative of the position. This
voltage drives the current Ii through the SQUID input in
ductor Li•
The effect of the electrical losses on the system quality
factor QA can be shown to be7•s
1 (l-f3 )1/2 R _= e +{3e T (5)
QA QM WA (Li + L]')
where QM is the mechanical quality factor of the antenna, P e
is the fraction of the energy of motion appearing in the elec
trical circuit, and L1' is the output inductance of the trans
ducer determined by the parallel combination of Ll and L2•
The electrical quality factor is
Q. = aJA (Li +LT)IR r
(6)
where WM is the antenna frequency with no electrical cou
pling. Generally Pe -< 1, therefore, to first orderin,Be, Qe is a
constant. The value of Pc can be adjusted by changing the
value of the persistent current If). The energy fraction fl
available at the amplifier is smaller than f3e by the ratio of
the load inductance L; to the total inductance LT + L;.
m. IBM de SQUID
The de SQUIDs used in this experiment were produced
at the IBM Thomas J. Watson Research Center using tech
niques developed for the fabrication of superconducting log-
Folkner et al. 511:11
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130.113.111.210 On: Fri, 19 Dec 2014 14:11:29ic circuits.9 These devices are particularly attractive for the
instrumentation of resonant gravitational radiation anten
nas because oHheir good coupling to a high impedance input
coil (Li::::: 1 ,uH) and their state-of-the-art noise perfor
mance at the operating frequencies of the antennas (800-
1600 Hz). The IBM SQUIDs also exhibit very low
1/ f noise levels.
The IBM SQUID uses a planar design in which the
SQUID loop is formed from a large planar washer which is
in series with two Josephson junctions. The input coil con
sists of two 39-turn spiral coils connected in series. One of
these coils is located on the top surface of the planar washer
while the other is located on the bottom surface of the wash
er. The modulation coil consists of a single turn at the out~
side edge of the washer. The SQUID loop inductance has
been measured to be 96 pH. The 78-turn input coil induc
tance is 0.7 ,uH. The mutual inductance between these two
coils is 7.3 nR.
The best energy resolution that has been obtained9 from
carefully selected devices is 315 ft in the white noise region.
The best 1/ f noise exhibited is 770 ft at 0.1 Hz. This value
translates to a l/fcolltribution ofless than 1 ft at 100 Hz. For
the particular IBM SQUID used in the experiment, the ener
gy resolution was 6700 ft when tested in a storage dewar. On
the antenna, the noise performance deteriorated to 20 000 ft.
Better shielding and filtering of the leads used to inject the
persistent current 10 would probably improve the measured
energy resolution.
IV. EXPERIMENTAL THREE-MODE SYSTEM
The small three-mode system consists of the two-mode
transducer mounted on a 50-kg aluminum bar, which is
dumbbell shaped to fit in our I-m test cryostat while resonat
ing near the! 600 Hz frequency of a larger I200-kg antenna.
Because of the small dynamic mass of this bar, compared to
the 1200-kg bar for which the transducer was designed, the
three modes of the system do not have the same properties as
Isolation mass
Aluminum springs
Brass I
rubber stacks
I i ,,,,,,,u,,u,,,,'»,,»U»»>)»">))":»»>>>'"'' I
o
I Helium reservoir (bottom)'
0.25
! 0.5
I 0.75
I 1m
I
FIG. 3. Small-scale three-modo: system, showing the dumbbell bar suspend
ed in the cryostat by vibration isolation filters.
5192 J. Appl. Phys., VoL 65. No. 12, 15 June 1989 the design system. Figure 3 schematically shows the ar
rangement of the prototype three-mode system. The nio
bium transducer assembly is inserted in the hole bored in one
end of the bar. The clearance between the transducer and the
wall of the hole is 150 Itm at room temperature. When cooled
to 4 K, the differential thermal contraction between the alu
minum bar and the niobium transducer produces a rigid
mounting arrangement. The bar is supported on a four-point
aluminum suspensionlO bolted to a lO-kg brass base block.
The base block rests on a cart which is suspended by two 3-
mm~diam fiberglass rods extending to vibration isolation
filters at room temperature.
The SQUID assembly was damped to the bottom of the
cryogenic vacuum chamber. The leads to the SQUID were
thermally connected to the vacuum chamber to cool the
SQUID to its operating temperature. The output leads from
the transducer to the SQUID and the current input leads
were shielded by lead-indium tubing. Simple mechanical
filters for the shielded connections were installed after initial
tests showed excess vibrational noise and erratic Q 's for the
three modes. After the filters were installed, the Q'8 im
proved, although the system noise was still above the ther
mal level.
Because the small-scale system is not specifically
matched for transmission of energy from the bar to the final
test mass, the frequency and Q for the central mode reflect
the properties of the last resonator. The Q val ue of 3 X 106 is
consistent with previous measurements for heat-treated nio
bium. II The transducer leads probably still limited the Q of
the other two modes. Multistage mechanical filters, which
are used on the 1200-kg detector, would possibly improve
the situation.
For each mode, the ratio of energy stored in the electri
cal spring to the total energy of the mode is approximately
(7)
wherej;n is the mode frequency with current stored andfrno
is the mode mechanical frequency. The data show that the
central mode is much more strongly coupled to the electrical
spring than the other two modes. An estimate of the electri
cal Q of the transducer was made by measuring the change in
the mode Q for different electrical coupling 13m using a pie
zoelectric crystal glued to the antenna. The value of the elec
trical Q obtained was Qe = 3.6X 104, consistent with the
values reported by the Stanford group. 12 Because the electri
cal spring term couples the diaphragm to the relatively dissi
pative bar, the measurement gave only a lower bound to the
electrical Q.
Ground vibrations coupling to the antenna at low fre
quencies through the vibration isolation generates noise cur
rents at the SQUID input. These noise currents can be sever
al flux quanta in amplitude at the resonance frequencies of
the vibration isolation. Consequently, to keep the SQUID at
its optimum operating point, a feedback system must be em
ployed. However, at values of the electrical coupling 13m
above 1 X 10-4, closing the feedback loop of the SQUID
causes positive feedback to drive the antenna to very-high
energy levels and saturate the SQUID electronics. This posi
tive feedback problem is related to the slew rate of the
Fo!kner et al. 5192
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130.113.111.210 On: Fri, 19 Dec 2014 14:11:29SQUID system and is discussed below. As we will show, the
design of the SQUID and the characteristics of the control
electronics contribute to the positive feedback problem.
V, FEEDBACK ANALYSIS
Figure 4 shows schematically the mutual inductances
between the SQUID inductors. The input inductance Li
couples the input current Ii to the SQUID loop inductance L
through the mutual inductance Mi' The feedback current If
couples from the feedback inductor Lf to L through the mu-'
tual inductance Mf. Because the thin-film de SQUIDs used
for these experiments have both the input coil and feedback
coil formed around a common ground-plaue hole, the cou
pling a~. between the input coil and the feedback coil is close
to unity. The coupling constants are defined through the
mutual inductances by the equations
Mij.=aif(L;Lf) 1/2, M{ =ai (L;L) 1/2
and
Mf=af(LfL)1!2.
The voltage equation for the input circuit of Fig. 4 is
Vi = [jw(L i + L1') + RT + ZA]I; + jwM;J + jwMfiIf,
(8)
where {o is the signal angular frequency, L1' is the output
inductance ofthe transducer, RT was defined in Sec. II, ZA is
the effective bar impedance that contributes to the input im
pedance at the resonance frequency, and J is the SQUID
circulating current. The feedback fiux to the SQUID loop is
given by
Mflf = {-G(w)/[ 1 + G(w)]}( V + Vn )IV~,
MJ1f = {-G(w)/[ 1 + G(w) ]}(MJ + VnIV¢), (9a)
(9b)
where G(w) is the open loop gain of the feedback, Vis the
SQUID output voltage, V" is the voltage noise at the output
of the SQUID, and V~ = (BV /Jrj;)' is the reduced flux-too
voltage transfer function of the SQUID with an input circuit.
The slew rate ¢.f,max is the maximum rate at which the elec
tronics can feed back flux to the SQUID and is related to the
open loop gain by the relation 13
(10)
where ¢o is the flux quantum. Eliminating the feedback cur-
Lr
v
FIG. 4. Schematic for a de SQUID connected to a resonant input circuit.
5193 J. Appl. Phys" Vol. 65, No. 12, 15 June 1989 rent from Eq. (8) we have
ZI . MJ G(w)
Vi = i i + jill i-I + G(w) wllil). (vr )--lV __ '
1> n M '
f (11)
where the input circuit impedance Zi is defined by
Z; =iw[Li(l -G(w) a(Pi) + LT] + ZA + RT.
1 +G(w) at
(12)
Assuming the resistively shunted junction model [4 for
the weak links, the nonlinear equations describing the
SQUID in Fig. 4 are
~ db, =1. _ J -Ie sin 8j + IN[,
2rrR dt 2 (13)
¢o d02 I, J 1 . >: + I ----=-+ -c slnu2 N2' 21TR dt 2 (14)
V = .!l!!L (dBl + d02) ,
2rr dt dt (15)
.!l!!L (8j -82) = LJ + M;l; + Mflf, 2rr . (16)
where 01 and O2 are the phase differences of the wave func
tion across the Josephson junctions, I is the bias current, Ie is
the critical current of the junctions, R is the shunting resis
tance of the junctions, and IN I and I NZ represent the intrin
sic current noise sources of the junctions. Equations (13)
and (14) represent the current flowing through the junc
tions. Equation (15) is the Josephson relation for the weak
links, and Eq. (16) relates the phase drops to the total mag
netic flux in the SQUID loop.
One method of solving the above system of equations is
to rewrite the nonlinear SQUID equations in the same form
as those of an isolated SQUID for which the solutions are
wen known.15 This technique has been demonstrated by
Teschel6 and Martinez and Clarke17 for a SQUID connected
to an input circuit without any applied feedback. The proce
dure is slightly complicated by the dependence of the input
and feedback currents on the circulating current as can be
seen from Eqs. (9) and ( 11) . Once this implicit dependance
is removed from Eq, (16), Eqs. (13 )-( 16) have the desired
form. Using Eq. (8), thetermMJi + Mflf in Eq. (16) can
be written as
G(w) V(V,)-.l
n '" ' l+G((())
(17)
which implies that the feedback effectively reduces the mu
tual inductance M; by a factor of [1 + G(w)]. Ii can be
eliminated from the right-hand side ofEq. (17) by the use of
Eq. ( 11), In order to substitute this result into Eq. (16), Eq.
(16) must be converted to the frequency domain. This con
version has been previously done in Refs. 15 and 16 with
slightly different techniques. Provided that G(wJ) = 0 and
jWJ (Li + LT) >ZA' where {t)J is the Josephson frequency,
the result in the frequency domain is
(18)
Follmer 91 al. 5193
····-···-·-············-·-········ ... ·-·-·····i,·.··-··· :> ••••••••••• -' •••• ' ••• -••••• ~.; ............ :.:.: •• , ••• '.~.y;:.:.;.; •.•••••• < •• «.; ..... ~ ..•...•. :.~ .. , ......... --:.:.:.:· .•. '.·.·.:.· .. z.;.:·.· ••• ·<.--:.:.:.;.-•• .::.~.:.:.:.:-;.;.; •••• ,:.~.:.:.; ••••• : ••• :.: .• :.;.; • .' •••••• ~ •.. o:.:.; ••••• ; •• :.::..;.:.:; •••••• '<.;.;.:': •. " •••••••••• ".:.: •••.••• ; ...... _~._.>, ..•. c ••••• ~ ••• " •••••••• :.:.:.:-:.~ •• ,.<.:.~.:.~.~.~ ..... ; .... :.~.:.:, .•.•.•....... :.: ....•.•..... _ ..... ,. •• ,.~ .. _
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130.113.111.210 On: Fri, 19 Dec 2014 14:11:29where
Le = (1-a;)L,
a; = a;LJ(L i + L1')'
MV <t> = I I
e ZJl+G(w)]
and (19)
(20)
(21)
<I> = UJ V (V') -\ 1--' If-I G('·') (J'W£' a·A'!· )
n 1 + G(w) n 1> Zi af[l + G(w)] ,
(22)
Thus the solution to the SQUID Eqs, (3)-(16) is that of an
isolated SQUID with a reduced inductance given by Le. <I> e
and <1> n are the effective applied flux and feedback finK noise
which only contain frequency components much less than
the Josephson frequency. While in the above analysis the
transducer output impedance has been assumed to be purely
inductive, the physical construction of this inductor may
have capacitive shunts between the windings of the coil
which short out the transducer inductance at the Josephson
frequency. In which case, one sets LT = 0 in Eq. (20).
To determine the net effective input circuit impedance,
the circulating current J must be removed from Eq. (11).
The circulating current is given by
(23)
whereJ;P E (oj /8¢)ris the reduced fiux-to-current transfer
function which can be a complex number in the frequency
domain. Using Eqs. ( 11 ), (21), and (23), the effective input
circuit impedance Z; can be shown to be
V jOJM;J~ Z ~ = ....-!... = Z + (24)
I Ii I [1 + G(w)J(1-J:;a;L) '
where Z; was defined previously. The second term in this
expression represents the effective SQUID impedance re
flected through the mutual inductance Mi'
In Eq. (24), only the inductive contributions to the in
put circuit are modified by the presence ofthe SQUID. This
result is different from that found previously by Tesche16
where both the resistive and inductive parts were found to be
modified by the presence of the SQUID. In that paper, the
term containing the circulating current J in the definition of
the effective flux <P e was not retained when the effective in
put circuit impedance was computed. Whether this term
should be induded in the effective flux is not clear. However,
the impedance of the additional resistive term, which results
from Tesche's method, is several orders of magnitude
smaller than the damping contributions of the feedback elec
tronics. Thus, this term does not need to be considered for
the high Q system discussed in this paper.
For G( (IJ) = 0, the impedance given by Eg. (24) is also
different from that computed by Martinez and Clarke. 17
They inferred the effective input impedance Zcff from the
voltage gain relation of the SQUID given by
v = M; V¢ V;lZelI'
which results in
Zctf = Z; -J;a;L [RT + (l/jmC i)]·
5194 J. Appi. Phys .• Vol. 65, No. 12. 15 June 1989 (25)
(26) However, a better interpretation of the solution to the equa
tions is to define an effective input voltage V; given by
V; = V,/O-a;LJ'q,). (27)
The voltage that appears at the output of the SQUID is then
V' =M;V;(V;/Z;), (28)
which gives the same output voltage as Eq. (25).
The effect of the feedback on the mechanical system can
be determined from the modified input circuit impedance
Z ;. This impedance includes damping terms, determined by
the electrical and mechanical losses of the system, and driv
ing terms, which involve the open loop gain G(w). Negative
Q 's which are computed by taking the ratio of the real part to
the imaginary part of the driving terms can be compared to
the Q of the system.
The transducer inductance of 5 pH and electrical Q of
3.6>< 104 implies a value for RT:::.::-10--6 fl. The primary ef
fect of the feedback results from the modified SQUID input
inductance, given in Eq. (12). For the IBM SQUID and
Clarke SQUID electronics, 18 a,!:::.::-ai :::.::-af:::.::-l and
G(w )/[ 1 + G(w) 1 ~ -(0.91 + 0.54j) at {t} = 104 rad/s,
which gives a real part of the input impedance of about
-10.-2 n. This driving term is four orders of magnitude
larger than the damping term and the effect is to drive the
bar until saturation of the electronics occurs.
In order to determine the effects of the reflected SQUID
impedance, an estimate of J; is needed. This estimate can be
obtained by extrapolation from measurements obtained for a
similar thin-film SQUID. 19 Using L = 100 pH and R = 2 n
for the IBM SQUID,9 the result is J; = (109 ± 2>< 10,})
H -I. In the absence of feedback, the real part of the reflected
SQUID impedance is approximately ± 10-8 fl., which is at
least two orders of magnitude smaller than the other terms
discussed above. The quality factor of an isolated SQUID
can be shown from Eq. (24) to be approximately 106• The
feedback reduces these losses by the factor of [1 + G( w) J.
VI. MUTUAL INDUCTANCE DECOUPllNG SCHEMES
The amount of driving that the feedback electronics has
on a resonant system can be reduced by two methods. One is
by improving the electronics, and the other is by decoupling
the feedback coil from the input coil. If the absolute value of
the driving term is no larger than a tenth of the dominant
damping term, the effect of the feedback is negligible. From
Eqs. (5), (6), and (24), the following condition results (to
first order in /3. ):
i.11m( G({j) )1 ail S_1 [_1 +/3e(-1 ._~)],
2 1 + G(O) J af lOPe QM Qe 2QM
(29)
where we have assumed LT :::.::-L; and a, :::.::-1. For our system,
the left-hand side of this equation is approximately 0.1,
which implies a need for five orders of magnitUde improve
ment for Q's on the order of 106 and f3e :::.::-0.1. A feedback
system has been described by WeUstood, Heiden, and
Clarke 13 with a slew rate three orders of magnitUde larger
than the present system. While this improvement allows bet-
Falkner et al. 5194
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130.113.111.210 On: Fri, 19 Dec 2014 14:11:29Feed back
II II
SQUIO EJ klcp
oree
I Ii Input
Ground Plane
FIG. 5. Layout for II feedback flux cancellation scheme.
ter flux cancellation in the SQUID loop, an improvement of
two orders is still needed.
For the IBM de SQUID and other SQUIDs having
planar designs, the ratio aif/ar could be reduced by rede
signing the SQUID loop. This would involve physically se
parating the input and feedback coils from one another while
maintaining a;;;;'O.8. This solution wouid most likely intro
duce the undesired effects of additional resonances in the V
if> characteristics. 20
A simpler solution is to keep the present coil arrange
ment but provide a flux cancellation region for the feedback
and input coils. Fig. 5 shows how the N-turn input coil can be
extended one more turn to another hole in the ground plane
while the one-tum feedback coil is extended by IV turns op
positely wound over the extension of the input coil to pro
duce a zero net flux coupling, The circuit mode! that this
arrangement can be modeled as is shown in Fig. 6. The input
equation becomes
Vi = jw(L,! + Lf2 )1i + jw(Mfi 1 -Mfi2) If
(30)
where the subscripts 1 and 2 refer to the inductances wound
over the SQUID and the hole, respectively, and
V' I
FIG. 6. Circuit diagram for the flux cancellation layout.
5195 J. Appl. Phys., Vol. 65, No. 12, 15 June 1989
••••••.•.• ~.-•• , .. ' ••. ? .• -.-•••••.••• v ••• _." .................... _._.".; •••••••••••••••••• ·.~.; •• '.· ••• v .•. '.·.:.-;-.:.; ..•.••• ;>.;.:.:.:.;.:-; •.•.•••••• :.~.-:-.:.:.;.: •••••••••••• :<.~.:.:.:.;.: •.••••.••• :.~.:.:.:O;';';O;""~':':;;:':':':':;;;'.'.O;'."""""""" • M7 = a;LItL. In order for Mfi2' ta cancel Mfil' the hole
must have a break in it to prevent image currents from reduc
ing the inductances involved. The coupling ratio is approxi
mately given by
ail _ (LI2 Li2 )112 --1--- ,
af LIt Lit (31)
where all the coupling constants have been assumed to be
close to unity. If the inductances of Lf1 and Lf2 can be
matched to within 10%, thenaif/al=O.05, whi.ch is close to
satisfying the additional two orders of magnitude needed to
satisfy Eq. (29). This value of aif/af is approximately the
value that we have measured far a BTi (Biomagnetic Tech
nologies, Inc" San Diego, California) de SQUID which has
the input and feedback coils in separate toroidal cavities.
Vlt CONCLUSION
The feedback system necessary for the operation of thin
film de SQUIDs with resonant gravitational radiation detec~
tors presents difficulties because of the high-quality factor of
the resonators. A combination of better feedback electronics
and reduced stray mutual inductance will make thin-film
SQUIDs usable on the present gravitational radiation detec
tors. For future systems with higher-quality factors, much
more attention will be needed to avoid the SQUID feedback
problem.
ACKNOWLEDGMENTS
We would like to thank Professor J. Clarke and Dr. J.
Martinez of the University of California at Berkeley for pro
viding the SQUID electronic circuits used in these experi
ments. We are also indebted to the foHowing scientists of the
IBM Thomas J. Watson Research Center who are responsi
ble for the fabrication of the IBM SQUID: K. H. Brown, A,
C. Callegari, M. M. Chen, J. H. Greiner, H. C. Jones, M. B.
Ketchen, K. K. Kim, A. W. Kleinsasser, H. A. Notarys, G,
Proto, R. H, Wang, and T. Yogi. Finally, we thank Dr. R. E.
Sager of Quantum Design in San Diego, California for useful
conversations about how to decouple the input and feedback
coils, This work was supported in part by the National
Science Foundation under grant No. PHY-82-15218.
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Savelli, G. Lecoy, and J.-P. Nougier (North-Holland, Amsterdam,
1983). p. 137.
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"J. Clarke, W. Mo Goubau, and M. B. Ketchen, J. Low Temp. Phys. 25, 99
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'"C. Hilbert and J. Clarke, Appl. Phys. Lett. 45, 799 (1984).
zoe. D. Tcsche, J. Low Temp. Phys. 47, 385 (1982)0
Folkner et al. 5196
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1.101842.pdf | 130 GHz GaAs monolithic integrated circuit sampling head
R. A. Marsland, V. Valdivia, C. J. Madden, M. J. W. Rodwell, and D. M. Bloom
Citation: Appl. Phys. Lett. 55, 592 (1989); doi: 10.1063/1.101842
View online: http://dx.doi.org/10.1063/1.101842
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Downloaded 30 Jun 2013 to 18.7.29.240. 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_permissions130 GHz GaAs monolithic integrated circuit sampling hea.d
R. A. Marsland, V. Valdivia, C. J. Madden, M. J. W. Rodwell,a) and D. M. Bloom
Edward L. Ginzton Laboratory, Stanford University, Stanford, CaiijiJrnia 94305
(Received 31 October 1988; accepted for publication 31 March 1989)
We have fabricated a GaAs diode sampling head which has a bandwidth of 130 GHz, which is
a five times improvement over previous rcom-temperature designs. This speed is attained with
a monolithic sampling head design integrated with tWG nonlinear transmission lines which
serve as the strobe pulse and test signal generators. A 4 ps transition time has been measured
with the sampler, We have also measured sinusoidal waveforms to 120 GHz.
Diode sampling circuits, which form the heart of most
microwave network analysis and time-waveform instrumen
tation, are limited to ~ 30 GHz bandwidth by hybrid circuit
layout parasitics, device parasitics, and by slow pulse genera
tors used for diode gating. We have fabricated a monolithic
diode sampling circuit with a bandwidth of 130 GHz, which
is a factor of 5 better than previous room-temperature elec
trical sampling circuits and comparable with recent Joseph
son junction sampling circuits I wh:ich require cryogenic
cooling. In this circuit, monolithic design minimizes circuit
layout parasitics, a 300 GRz epitaxial Schottky diode design
minimizes device parasitics, and a 3.5 ps nonlinear transmis
slonline (NLTL) strobe pulse generator2 minimizes the di
ode gating period. The sampling circuit will permit 100 GHz
sampling oscilloscopes and dc-lOO G Hz network analyzers.
The high-speed sampling circuit is shown in Fig. 1 (a)
and its high-frequency equivalent circuit in Fig. 1 (b). A vol
tage strobe pulse from the local oscillator (LO) turns on
diodes D 1 and D 2 which sample the input signal (rf). The
diodes are connected across the split in the rf ground which
is a balanced transmission line shorted at both ends. This
shorted transmission line with characteristic impedance Ze
differentiates the sawtooth LO waveform to produce a vol
tage pulse across the diodes. The sampled output (IF) is
filtered by two 1 kn resistors. For equivalent time sampling,
the strobe frequency is offset by AJ from a subharmonic hr / n
of the input signal's fundamental frequency hI"> resulting in a
sampled signal mapped out in equivalent time at a frequency
b.p
Parasitics in the sampling loop include diode series re
sistance and junction capacitance and the inductance of the
diode connection.4 Diode parasitics arc minimized through
use of devices with the highest attainable cutoff frequency. A
transmission line which has both even and odd modes of
propagation is used to connect the sampling diodes to the rf
and LO ports. The diodes arc presented in series to the LO
pulse propagating in the balanced mode and in parallel to the
rf signal in the unbalanced mode. Because the diodes are
directly connected across transmission lines, the inductance
of the diode connection is reduced to the inductance of the
diode package and any wire bonds or beam leads. This
scheme also provides natural isolation if the balanced line is
loaded symmetrically. Transmission lines used for this ap
proach include coax and radiaLS multilayer microstrip and
,,) Now at the Department of Electrical and Comput<:r Engineering, Univer
sity of California at Santa Barbara, Santa Barbara, CA 93106. sIotiine,4 and microstrip, slotline, and coplanar waveguide
(CPW).3
Our design in Fig. 2 reduces the structure to one plane
by using the even and odd modes of CPW,6 a!lowing mono
lithic integration of the entire sampling head. The rfsignal to
be sampled is applied to the external signal input and travels
in the normal (odd) mode on the vertical CPW. Two
NLTLs (not shown in the figure) provide the strobe pulse
and internal test signal. The NLTL design we used is identi
cal to that reported in Ref. 7. The sawtooth wave applied to
the strobe pulse input of Fig. 2 travels in the odd mode on the
horizontal CPW until it is applied to the sampling diodes
and the sIodine (even) mode of the rf CPW. The even mode
is shorted by airbridge connections 180 p.m from the sam
pling diodes. The reflected wave turns the diodes off after a
round trip time of ~ 4 ps. An additional NL TL is located
below the sampling head to provide a high-speed test signaL
If the sampling loop has sufficient bandwidth, the
sampler aperture time will be limited by the time the sam
pling diodes are 011. Due to the exponential [-V relation of
the diode, and dependent upon the diode bias, the aperture
(a)
Input
Signal
(RF)
(b) rj
fj Sampled
Output (IF)
Strobe
Pulse
(LO)
~
FrG. I. (a) High-speed sampling circuit schematic diagram and (b) its
high-frequency equivalent circuit.
592 Appl. Phys. Lett. 55 (6), 7 August 1989 0003-6951/89/320592-03$01.00 @ 1989 American Institute of PhysiCS 592
Downloaded 30 Jun 2013 to 18.7.29.240. 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_permissionsStrobe
pulse
input 1 External signal input
Interconnect
Metal
~
MIM Capacitor
Ed
Air bridge -N+ Resistor -
f Internal test signal input
FIG. 2. Sampling head layout. The strobe pulse is applied from a NLTL
(not shown) and is coupled to the sampling diodes through the MIM capa
citors. The rf input signal is applied to the vertical CPW at the top of the
figure, or a test signal can be applied from another NL TL (not shown) at
the bottom of the figure.
time is typically one-half to one-quarter the width of the
applied voltage strobe pulse. Silicon step recovery diodes
(SRDs) having -35 ps rise time are currently used for gat
ing diode sampling bridges, and provide -·18 ps aperture
times. Vlith nonlinear transmission lines, much shorter
strobe pulses can be generated.7•8
The NLTL is a relatively high impedance transmission
line loaded with reverse-biased Schottky diodes at regular
intervals along the line which serve as voltage-dependent
shunt capacitances. The nonlinear shunt capacitance intro
duces a variation in the propagation velocity with voltage.
which results in steepening of negative-going ",,"ave fronts of
signals propagating on the line. As the signal faU time de
creases, wave front dispersion arising from the diode cutoff
frequency and the line periodicity competes with the wave
front compression arising from the capacitance variation. A
final, limited faU time is reached at which these two pro
cesses are balanced.
The NL TL we use as a strobe pulse generator (also
caned the local oscillator or LO) produces a 2.5 V, 3.5 ps faU
time sawtooth wave when driven with an 8 GHz, 23 dBm
sine wave. Since the slope of the rising edge is negligible in
comparison with that of the falling edge, the derivative of the
waveform is a 3.5 ps impulse, full width at half maximum
(FWHM). Used as a strobe pulse generator, -1.8 ps sam~
pIing bridge aperture times could be attained if the diode bias
is set at -75% of the impulse magnitude.
Diode capacitance and series resistance prevent the
sampler from attaining the bandwidth made possible by the
strobe pulse in two ways. First, the capacitive loading of the
shunt diodes on the rfline causes the rfvoltage at the diodes
to have a pole in its frequency response at
(iJ = 1/[2CjO X (25 n + rJ2)], where the 25 n is the par
anel resistance of the source and termination resistances of
593 Appl. Phys. Lett., Vol. 55, No.6, 7 August 1989 the rfline. Second, the diode capacitance broadens the strobe
pulse width applied to the diodes by introducing a pole in the
LO transfer function at (u=2IR,eCfij' From Fig. l(b), the
equivalent resistance in series with the diodes is
Rsc = 21', + (Z,Ze )/(Z, + Z,,), whereZe is the impedance
of the even mode on the rf CPW, and Z, is the LO source
impedance.
In our design, Ze = 75 n/2, Zs = 100 n. r, = 60 n,
and CJ.l = 8 iF, giving the sampling loop a time constant of
0.8 ps. Using root-sum-squares (r5s) convolution, the result
ing strobe pulse width win be approximately
J3.¥+O.82 = 3.6 ps, giving an aperture time of 1.8 ps. The
pole in the rf circuit will contribute 1,9 ps to the system rise
time, so the total system will have a rise time of
il.K! +l§' = 2.6 ps and a corresponding 3 dB bandwidth
of 130 GHz. -
The Schottky diodes were fabricated on GaAs MBE
material with a 0.6 p.rn N active layer (3 X 1Olb/cm3 dop
ing). A buried O.8l-lm N I active layer (3x 101S/cm3 dop
ing) provided the diode cathode connection. Proton implan
tation outside the diode active regions provides> 40 MH pet
square isolation. First-level interconnecti.ons and Schottky
contacts are formed withaO. 1,um Ti/O.75,um Pt/1.4,um Au
lift-off. 1000 A of Si3N4 deposited by plasma-enhanced
chemical vapor deposition is used ror the dielectric of the
metal/insulator/metal (MIM) capacitors. Plated air
bridges provide second-level interconnections.
The sampler bandwidth was evaluated by probing inter
nal nodes of the circuit using direct electro-optic sampling/,)
and by using the sampling circuit to measure the output of
both the NLTL internal test signal generator and an external
60-100 GHz frequency multiplier. A strobe pulse width of
4.0 ps was measured using direct electro-optic sampling. Us
ing rs§ deconvolution of the 1.9 ps electro-optic measure
ment system impulse response, the strobe pulse width is
-3.5 ps FWHM, Depending on the diode bias, the corre
sponding diode sampler aperture time is between 1.8 and 3.5
ps.
The test signal generator, identical to the LO strobe
pulse generator, is a NLTL whose output is attenuated 50: 1.
Measured by electro-optic sampling, the test signal has a fan
~
,& 0-30
11 -a s -40
'" v:
-50
0.0 \
10 20 30 40
Time (Picoseconds)
FIG. 3. 4 ps fall time of the 50: I attenlUlted output of the test signal NLTL
measured with the diode sampling bridge.
Marsland et at. 593
Downloaded 30 Jun 2013 to 18.7.29.240. 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_permissionstime of approximately 3.5 ps. The test signal 10%-90% fall
time measured by the diode sampling head was 4.0 ps (Fig.
3). Using rss deconvolution, we estimate that the sampling
circuit has a 2.7 ps 10%-90% rise time, with 11 correspond
ing bandwidth of roughly 130 GHz. When measuring the
output of the external frequency multiplier probe,lo the
sampler was able to measure 11 180 mY, 95 GHz sine wave,
and harmonics were measured to 120 GHz. The sampler was
within 0.5% of linearity to 400 mY. The equivalent input
noise voltage was 90 n V /.JHz. At 5 GHz, rfto IF isolation
was 55 dB, LO to IF isolation 63 dB, and the LO to rfisola
tion was 68 dB.
In conclusion, we have fabricated a room-temperature
monolithic GaAs diode sampling circuit having a band
width of 130 GHz. When packaged in a coplanar probe, the
sampling circuit will allow on-wafer measurements in excess
of 100 GHz. With reduction in diode parasitics and NLTL
transition times, a 300 GHz sampling circuit bandwidth
should be achievableo The sampling circuits are simple and
compact and are suitable for application in sampling instru
mentation for millimeter wave and picosecond electronic de
vices.
594 Appl. Phys. Lett., Vol. 55, No.6, 7 August 1989 The authors would like to thank Y. C. Pao for providing
the MBE material, Gerald Li for performing the nitride de
position, and Lance Goddard and Tom Carver for help in
processing. This work was supported by Office of Naval Re
search (ONR) contract NOO014-85-K-0381. R. A. Mars
land acknowledges an ONR Fellowship.
'P. Wolf, in Picoserolld Optics and Optoelectronics, edited by G. A.
Monfoll, D. M. Bloom, and C. -H. Lee (Springer, Berlin, 1985), p. 236.
'M. J. Rodwell, D. M. Bloom, and B. A. Auld, Electron. Lett. 23, 109
( 1987).
is. R. Gibson, Hewlett-Packard Journal. February \986, p. 4.
4J. Merkclo and R. D. Hall, mEE J. Solid-State Circuits SC·7, 50 (1972).
'w. M. Grove, IEEE Trans. Microwave Theory Tech. MTT-14, 629
(1966).
"K. C. Gupta, R. Garg. and I. J. Bahl, Microstrip Lines alld Stotlines (Ar
tech House, Norwood, 1979), p. 356.
'c. J. Madden, M. J. Rodwell, R. A. Marsland, D. M. Bloom, and Y. C.
Pao, IEEE Electron Device Lett. 9, 303 (19R8).
"M. J. W. Rodwell, C. J. Madden, B. T. Khuri-Yakub, D. M. Bloom, Y. C.
Pao, N. S. Gabrid, and S. P. Swierkowski, Electron. Lett. 24, 100 (1988).
0K.J. Weingarten, M. J. W. Rodwell, and D. M. Bloom, IEEEJ. Quantum
Electron. QE·24, I yg (1988).
lOR. Majidi-Ahy and D. M. Bloom, Eke!ron. Lett. 25, 6 (1989).
Marsland et al. 594
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1.344012.pdf | The effect of fluorine implantation on the interface radiation hardness of Sigate metal
oxidesemiconductor transistors
Yasushiro Nishioka, Kiyonori Ohyu, Yuzuru Ohji, Nobuyoshi Natsuaki, Kiichiro Mukai, and T. P. Ma
Citation: Journal of Applied Physics 66, 3909 (1989); doi: 10.1063/1.344012
View online: http://dx.doi.org/10.1063/1.344012
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/8?ver=pdfcov
Published by the AIP Publishing
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128.143.199.160 On: Sun, 14 Dec 2014 11:05:29In summary, we have fabricated as-grown Y-Ba-Cu-O
thin films using reactive coevaporation followed by the rf
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by this O2 rf-plasma cooling process, indicating that activat
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The authors would like to acknowledge helpful discus
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members of the superconductivity section in the Electro
technical Laboratory. They also would like to thank K. Ya
mano of San yo Electric Co. and T. Kasahara ofChiba Insti
tute of Technology for sample measurements. Dr. T.
Tsurushima and Dr. K. Kajimura are greatly appreciated
for continuous support and encouragement.
'M. K. wu, R. J. Ashbllrn, C. J. Torng, D. H. HOT, R. L. Meng. 1. Gan, Z.
J. Huang, Y. Q. Wang, and C. W. Chll, Phys. Rev. Lett. 58, 908 (1987). "P. Chandhari, R. H. Koch, R. B. Laihowitz, T. R. MacGuire, and R. 1.
Gambino, Phys. Rev. Lett. 58, 2684 (1987).
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Phys. Lett. 51, 2263 (1987).
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The effect of fluorine implantation on the interface radiation hardness
of Si ... gate metalaoxide-semiconductor transistors
Yasushiro Nishioka, Kiyonori Ohyu, Yuzuru Ohji, Nobuyoshi Natsuaki,
and Kiichiro Mukai
Central Research Laboratory, Hitaclll: Ltd., Kokubunji, Tokyo 185, Japan
T. P. Ma
Center for Microelectronic Materials and Structures and Department of Electrical Engineering,
Yale University, New Haven, Connecticut 06520-2157
(Received 19 December 1988; accepted for publication 19 June 1989)
The radiation hardness offtuorinated Si02/Si interface in metal-oxIde-semiconductor field
effect transistors has been found to depend strongly on the amount offtuorine introduced. In
this study, the fluorine was introduced by low-energy F implantation onto the surface of the
polycrystaIline silicon. gate electrode, followed by annealing at 950°C to diffuse F into the gate
Si02 toward the SiOz/Si interface. The improved radiation hardness is attributed to the strain
relaxation near the Si02/Si interface by fluorine incorporation.
It has recently been reported that, by introducing min
ute amounts of fluorine or chlorine in thermal Si02, the reli
ability of metal-oxIde-semiconductor (MOS) capacitors can
be significantly improved.l-4 In a recent study, we intro
duced fluorine by the use of a new technique involving flu
orine implantation and subsequent diffusion, and achieved a
significant improvement in the channel-hot-electron hard
ness of MOSFETs (metal-oxide-semiconductor field-effect
transistors) .5 In this communication, we will show that the
same fluorine implantation technique also gives rise to a sig
nificant improvement in the radiation hardness of MOS de
vices due to the reduced generation rate of interface traps.
The MOS capacitors (area = 9.36X 10-4 cm2) and n
channel MOS transistors (channel length = 2 p.m, channel
width = 10 flm) used in this study were fabricated on (100) oriented p-type Si wafers with a resistivity of 10 n cm. Two
types of gate oxides were investigated: (A) wet oxide (con
trol), and (B) fluorinated wet oxides. The processing details
of these oxides are described below.
(A) Wet oxide (control): The wet oxide was formed
pyrogenically at 850 °e, and the oxidation time was adjusted
to yield an oxide thickness of 18 nrn. After the gate oxide was
formed, a polycrystalline silicon film was deposited at
650°C, foHowed by phosphorous diffusion at 875°C for 10
min. The thickness of the polycrystaliine film was 350 nm,
and the poly-Si gate area was defined photolithographically.
The devices were then annealed in nitrogen at 950°C for 10
min in order to make the heat cycles exactly the same as the
fluorinated ones.
(B) Fluorinated oxides: Fluorine was introduced into
3909 J. Appl. Phys. 66 (8), 15 October 1989 0021-8979/89/203909-04$02.40 @ 1989 American Institute of Physics 3909
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128.143.199.160 On: Sun, 14 Dec 2014 11:05:29the wet oxide after the aforementioned process steps as fol
lows: (1) low-energy (25 keY) F ion implantation into the
surface region of the polycrystaHine silicon gate and the
source-drain regions, and (2) subsequent heating (950°C,
10 min) to anneal out the implant damage and to drive some
of the F atoms into the gate SiOz toward the Si02/Si inter
face. The depth of the implant is estimated to be around 25
nm, which is less than 10% of the thickness of the polyerys
talline silicon. Such a shallow implant is desirable to mini
mize the implant damage to the gate oxide. The dose of the
implanted fluorine ranges from 0 to 1016 em 2, SIMS (sec
ondary ion mass spectrometry) analysis indicates that the
implanted fluorine will diffuse into the gate oxide from the
polycrystalline silicon surface after the subsequent heating
step,
After these processing steps, all wafers underwent a full
NMOS (N-channel MOS) fabrication process (the maxi
mum wafer temperature was 900°C), and the control de
vices received the same heating cycles as the fluorinated de
vices.
The radiation source used in this study was an x-ray
beam generated from a W target bombarded by SO-keY elec
trons, The dose rate was maintained at approximately 22
krad(Si)/min. Total dose ranged from 0 to 500 krad(Si).
Note that a conversion factor 1 rad(Si) = 0.56 rad(SiOz)
(Ref. 6) may be used to compare with other reported results
in which the radiation dose is expressed in terms of rad
(Si02)·
The density and energy distribution of the interface
traps of the MaS capacitors before and after x-ray irradia
tion (gate floating) were analyzed by measuring the high
frequency and quasi-static capacitance-voltage ( C-V)
curves.
Radiation experiments were also performed on MOS
FETs with a gate voltage of + 1.8 V ( 1 MV / cm) or floating
during x-ray irradiation. After irradiation, the subthreshold
characteristics of these MOSFETs were measured. The in
crease of interface trap density, flit' the shift of threshold
voltage, Vtll, and the shift of mid gap voltage due to oxide
trapped charge, VOl' were obtained by analyzing the sub
threshold characteristics using the previously reported
methods,7.8 The maximum transconductance in the linear
region, gm , was also measured, The voltage parameters used
in the measurements were: drain voltage: 0.1 V, source vol
tage: 0 V, substrate bias: -3 V, and the gate voltage was
swept from -1 to 5 V.
Figure 1 shows the midgap interface trap density in
MOS capacitors as a function of fluorine implant dose mea
sured (a) before irradiation, (b) after 200 krad(Si) of x-ray
radiation, and (c) after 500 krad(Si) of x-ray radiation.
Note that a minimum of the generated mid gap interface trap
density occurs at a fluorine dose level of about 2X 1015
em -2. This result is in good agreement with the data pre
sented in Ref. 4, in which the channel-hat-electron injection
time required to cause a predetermined amount oftranscon
ductance and threshold voltage degradation is plotted as a
function of the fluorine dose. These data are also consistent
with the fluorine dependence observed in oxides grown in
NF 3.1 In addition, the radiation-induced positive charge is
3910 J. Appl. Phys., Vol. 66, No. 8,15 October 1989 :;: E u
> <II
o ...
-c 2.0 ..----------------, •
1,6
1.2
0.8
(b) 200 krad(Sil
0.4
(a) aerore ifr.
o I.±I~I ±I ====I!I:=:· de==**==*==d
14 o 10 1015
F implant dose (em -~ )
FIG. I. X-ray radiation-induced midgap interface trap density as a function
of fluorine implant dose. Gate bias was kept floating during irradiation. Cal
Before irradiation; (b) after 200 krad(Si) radiation; and (e) after 500
krad(Si) radiation, Note that 1 rad(Si) = 0.56 radC SiO,) (see Ref. 6).
also reduced for fluorinated MOS capacitors in the dose
range 5 X 1014_5 X 1015 cm 2 as compared to the control.
In addition to MOS capacitors, the interface radiation
hardness of the MOSFETs has also been greatly improved
by the fluorine introduction. Figure 2 shows the drain-cur
rent versus gate-voltage characteristics for a set of control
samples before x-ray irradiation (curve a) and after 200
krad(Si) irradiation (curve .b) during irradiation and a
fluorinated MOSFET (fluorine implant dose = 2x 1015
cm-2) after 500 krad (Si) irradiation (curve c), with gate
bias floating during irradiation for both samples. The char-
-4 10
-6 10
-8 10
-12 10
-13 10
-0,5 Bomb. bias
floating
o .... "
.., /' "," ----
L (a) Controi
I aefor. X -ray
~ I
~
I
I
f
I
I
I {b} Contro!
200krad(Sil
0.5
GATE VOLTAGE (V)
FIG. 2. Subthreshold characteristics for (a) control MOSFET before x-ray
irradiation, (b) control MOSFETafter 200 krad(Si) x-ray irradiation, (c)
fluorinated MOSFET (2X 10" F/cm') after 500 krad(Si), with gate float
ing during irradiation. (Gate length: 2 p.m, width: 10 pm.) Note that I
rad(Si) = 0.56 rad(SiO,) (see Ref. 6).
Nishioka et al. 3910
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128.143.199.160 On: Sun, 14 Dec 2014 11:05:29-4
10
Bomb. biaa
-6 +uw 7 3 10 Vth
I-z -8 w "J..-----:(al Control III: 10 (c) Fhlorillated &it /;1 aefor. X-ray ::I 500krad(SIJ (.)
iii " I -10 ~ (b) Control «
l1li: 10 a :' / :lOOlu.dISIl
-t2 ~ I . ,
10
/ j,Vot -13 " , 10
-0.5 0 0.5
GATE VOLTAGE IV)
FIG. 3. Subthreshold characteristics for (a) control MOSFETbeforc x-ray
irradiation, (b) control MOSFET after 200 krad(Si) of x-ray irradiation,
(e) fluorinated MOSFET (2X 1015 F/cm2) after 500 krad(Si), with gate
biased + 1.8 V during irradiation. (Gate length: 2 f..lm, width: W f..lm.)
Note that 1 rad(Si) ~~ 0.56 rad(Si02) (see Ref. 6).
acteristic of the fluorinated MOSFET before damage is not
shown here because there is little difference in the sub
threshold characteristics between the control and fluorinat
ed MOSFETs. The slope change of the fluorinated MOS
FET after irradiation is very small compared to the
irradiated control MOSFET, despite the fact that a higher
dose is used for the fluorinated device. This indicates that the
interface generation in the fluorinated MOSFET is much
smaller compared to its control. In addition, the shift of mid
gap voltage due to oxide trapped charge, Vot' obtained by the
method described in Ref. 8 is also smaller in the fluorinated
MOSET.
Even stronger evidence for the reduction in the genera
tion of interface traps is presented in Fig. 3, where the results
are shown before and after biased radiation with a gate vol
tage of + 1.8 V (1 MV Icrn). Note that the post-radiation
characteristic of the fluorinated device in Fig. 3 is very simi
lar to the one in Fig. 2, suggesting that biased radiation
causes little additional interface trap generation. In contrast,
the subthreshold slope of the control device degrades much
more significantly after bias-radiation due the higher density
of radiation-induced interface traps. The shallower slope of
the control device actually gives rise to a smaller threshold
voltage shift. @)
II) ,
<) ...
au
CJ
:II: « I-g
I'll z 0 (.)
1/1
~
l1li: i-80
!l0
40
30
20
10
0
0 Bomb. biaa
+1.8V
(II) Contfol
200Ilr.«81)
GATE VOLTAGE (V)
FiG. 4. Gate voltage dependence of MOSFET transconductance for (a)
control MOSFET before x-ray irradiation, (b) control MOSFET after 200
krad(Si) of x-ray irradiation with gate biased + 1.8 V during irradiation,
(e) fluorinated MOSFET (2X 10" F/cm2) before x-ray irradiation, (d)
fluorinated MOSFET (2X 1015 F/cm') after 500 krad(Si) with gate biased
+ 1.8 V during irradiation, (Gate length: 211ffi, width: lO lIm.) Note that I
rad(Si) = 0.56 rad(SiO,) (sec Ref. 6).
The interface hardness of the fluorinated MOSFET is
more clearly exhibited in Fig. 4, where the transconduc
tances in the linear region for the two devices before and
after biased radiation are compared. A substantial reduction
of transconductance, gm' is observed in the control MOS
FET. In contrast, little reduction in the peak height of gm is
observed in the fluorinated MOSFET.
A comparison of the characteristics of the control and
fluorinated MOSFETs after either biased or floating gate
irradiation is summarized in Table I. The relationship
between Agm/g", and the density of interface traps after ra
diation damage has been reported, 9.10 and it is consistent
with the present result.
The improvement of the interface hardness against hot
electron injection or radiation damage by incorporating
small amounts of F or Cl was first reported by the Yale
group.I-3 They proposed that the bond strain distribution
near the SiOz lSi interface may be altered by the presence of
For Cl. A strong piece of evidence suggesting that the inter
facial bond strain gradient is significantly reduced by incor-
TABLE I. Comparison of x-ray irradiated control and fluorinated MOSFETs.
x-ray dose Bomb. bias boD" t. V;" b,V,h /::;.g",/g",
Oxide [rad(Si) I" (V) (1010 eV-l cm-") (mV) (mV) (%)
Control 200k + 1.8 56 ±5 -160±20 -25.2 ± 0.3 --I() ±0.2
Control 200k floating 16 ±5 -157 ± 20 -106 ± 0.4 -2.4 1: 0.2
Fluorinated 500k + 1.8 4.7 ±2 -150± 4 - 133 ± 0.5 undetected
Fluorinated SOOk floating 1.6 ± 0,5 -140±4 -131 ::!:0.5 undetected
"Note that 1 rad(Si) = 0.56 rad(SiO» (see Ref. 6).
3911 J. Appl. Phys., Vol. 66, No.8, 15 October 1989 Nishioka et al. 3911
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128.143.199.160 On: Sun, 14 Dec 2014 11:05:29poration of F or CI is the lack of gate size dependence of
interface trap generation in the fluorinated or chlorinated
samples.I-3 A possible mechanism leading to the reduced
strain is the interaction of a F (or Cl) atom with a strained
Si-O bond, forming a Si~-F bond and a nonbridging oxy
gen bond, resulting in a local strain relaxation. When exces
sive amounts of F are incorporated, too many non bridging
oxygen centers are produced, which negate the beneficial
effect of strain relaxation. Thus, an optimum F concentra
tion exists, at which the beneficial effect of strain relaxation
exceeds the negative effect of the nonbridging oxygen
centers, and our results indicate that such an optimum F
concentration corresponds to an implant dose in the range
5 X 1014-2 X 1015 em .2.
In summary, we have presented a technique based on
ion implantation and diffusion to introduce F into the gate
oxide. The interfaces of the fluorinated oxides over a wide
range of F doses are more resistant to radiation damage. These results may be attributed to the formation of Si-F
bonds and the resulting local strain relaxation.
'E. F. da Silva. Jr., Y. Nishioka, and To P. Ma. IEEE Trans. Nuc!. Sci. NS-
34. 1190 (1987).
?Y. Wang, Y. Nishioka, T. P. Ma, and R. C. Barker, Appl. Phys. Lett. 52,
573, (1988).
'Y. Nishioka, E. F. da Silva, Jr., Y. Wang, and T. P. Ma, IEEE Electron
Device Lett. EDL-9, 38 (1988).
4W. Long, Y. S. Xu, and Y. S. Zheng. J. Electrochem. Soc. 135, l3SC
(1988) .
'Yo Nishioka, K. Ohyu, Y. Ohji, N. Natllaki, K. Mukai, and T. P. Ma,
IEEE Electron Device Lett. EDL-I0, 141 (1989).
"D. M. Fleetwood, P. S. Winokur, R. W. Beegle, P. V. Dressendorfer, and
B. L. Draper, IEEE Trans. Mucl. Sci. NS-32, 4369 (1985).
7M. Gaitan and T. 1. Russell, IEEE Trans. Nuc!. Sci. NS·31, 1256 (1984).
'I'. S. Winokur, J. R. Shwank, P. J. McWhorter, P. V. Dresscndorfer, and
D. C. Turpill, IEEE Trans. Nucl. Sci. NS-31, 1453 (1984).
"K. F. Galloway. M. Gaitan, and T. J. Russel. IEEE Trans. Nuc!. Sci. NS-
31,1487 (1984).
JOF. W. Sexton and J. R. Schwank, IEEE Trans. Nucl. Sci. NS·31, 3975,
(1984 ).
Magnetogoptical properties of transparent plastic material
Shinzo Muto, Shin-ichiro Ichikawa, Takashi Nagata, Akihisa Matsuzaki, and Hiroshi Ito
Faculty of Engineering, Yamanashi University, Kofu 400, Japan
(Received 20 March 1989; accepted for publication 13 June 1989)
The measured values of the Verdet constant of transparent plastic fiber materials such as the
poly-a-methylstyrene and polystyrene are comparable to that of a NaCI crystal, In these
magneto-optical plastics, the wavelength giving the maximum value of the figure of merit for a
fiber-type optical isolator is about 500 nm. They are also used for constructing plastic fiber
sensors for higher magnetic field or higher current.
and
e = V H(t)l, (1)
(2) An optical isolator is an indispensable device in optics
because it can prevent optical feedback which often seriously
impairs optical systems. 1-41n particular, a fiber-type isolator
is strongly required in fiber optics for constructing a variety
of high-sensitive optical devices such as fiber gyroscopes and
fiber sensors. Therefore, the search for magneto-optical plas
tics which can be used for an inexpensive fiber-type isolator
or for a magnetic field sensor is an interesting matter. How
ever, no data of the Verdet constant in plastic materials has
been published. Therefore, we attempted to measure these
properties. where e is the Faraday rotation angle of the polarization of
light, V is the Verdet constant, and I is the sample length.
Coincident with this light signal, the intensity of the applied
As the test materials, poly-a-methylstyrene (PaMS)
and polystyrene (PS) were chosen since they have a wide
transparent range in the visible region and a diamagnetic
moment due to the phenol resin in the polymer unit. The
samples were prepared in the form of rod of about 5
mmcp X 7 cm and were mounted between two parallel polar
izers as shown in Fig. 1. We used various light sources and a
pulsed magnetic field for the Faraday rotation measurement
of these plastic rods. When a pulsed magnetic field H (t) is
applied to the sample, the detected light intensity let)
changes from the initial value Io by the following relations: p1ckup coil
potarizer \ analyzer
J l J
i f000000009F~' PM
, V?ooooo~oo I::
plastic rod : solenOId f 0
integrator C.R.O
FIG. J. Experimental setup for the measurement of the Verdet constant of
transparent plastic rod.
3912 J. Appl. Phys. 66 (8). 15 October 1989 0021-6979/89/203912-02$02.40 @ 1989 American Institute of Physics 3912
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1.1140805.pdf | Lithography beamline design and exposure uniformity controlling and measuring
Shinan Qian, Dikui Jiang, Zewen Liu, Qianhong Chen, Ya Kan, and Wanpo Liu
Citation: Review of Scientific Instruments 60, 2148 (1989); doi: 10.1063/1.1140805
View online: http://dx.doi.org/10.1063/1.1140805
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/7?ver=pdfcov
Published by the AIP Publishing
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Control system design and alignment methods for electron lithography
J. Vac. Sci. Technol. 12, 1252 (1975); 10.1116/1.568510
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128.114.34.22 On: Sun, 30 Nov 2014 00:46:18Lithography beamline design and exposure uniformity controlling
and measuring
Shinan Qian,Dikui Jiang, Zewen liu, Qianhong Chen, Ya Kan, and Wanpo Liu
Hefei National Synchrotron Radiation Laboratory, University of Science and Technology afChina, Hefei,
Anhui, The People's Republic of China
(Presented on 29 August 1988)
The lithography beamline design ofHefei National Synchrotron Radiation Laboratory is
presented. A scanning mirror is used to cut off short wavelength radiation and to expand the
vertical exposure dimension to 50 mm. A thin beryllium window is installed before the scanning
mirror to prevent the longer wavelength radiation from going through. An exposure chamber
with a vacuum of 5 X 10E - 7 Torr is located at 7 m downstream from the source point. Because
there is no window at the entrance of the chamber, a differential pumping system is used. The
scanning mirror is driven by a stepping motor which oscillates through a 10 angle. The required
driving speed curve is determined by a computer in order to obtain a uniform exposure area. An in
situ moire fringe grating system is used to measure the uniformity of the motor speed.
INTRODUCTION
Synchrotron radiation x-ray lithography has clear advan
tages for submicron semiconductor fabrication: high colli
mation, strong intensity, and a broadband spectrum. Fur
thermore, there seems to be a worldwide agreement that
only synchrotron radiation allows the full advantages of x
ray lithography to be used in semiconductor production. 1,2
Submicron VLSI chips fabricated with synchrotron radi
ation x-ray lithography will hopefully be available commer
cially by the 19908. Realizing the importance of synchrotron radiation lith
ography, Hefei National Synchrotron Radiation Lab pays
strong attention to it. A quarter of the storage ring with a
large area for clean rooms is dedicated for lithography use,
At least six ports are available for six lithography beamlines.
400 LIS
IDN PUMP
FIG. 1. Drawing oflithography beamline.
2148 Rev. Sci. Instrum. 60 (7), July 1989 The first step in our program is to install one lithog
raphy beamline and put it into use in the middle of 1989. The
final design has been finished and manufacture has started.
Some important studies and simulation experiments such as
the control of exposure uniformity have already been done,
DIFfERENTIAL PUMP SYSTEM
50 LIS
ION PUMP VATDN63
VALVE
TURBO
0034-6748/89/072148-02$01.30 @ 1989 American institute of Physics 2148
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128.114.34.22 On: Sun, 30 Nov 2014 00:46:18FIG. 2. Beamline optics.
I. BEAM liNE DESIGN
The main goals of the first lithography beamline are as
follows:
( 1) To obtain good quality submicron lines and to study
the exposure properties of aU possible resists as well as dis
tortion and radiation damage at mask and wafer.
(2) To measure and improve the uniformity of the expo
sure area in order to meet the practical requirements of chip
fabrication.
(3) To make some usable optical elements such as zone
plates and metrology gratings as weB as some simple devices
with a simple exposure; Put the lithography beamline into
practical use as early as possible.
The beamline is shown in Fig. 1. A scanning mirror with
a 20 mrad grazing angle for cutting off short wavelength
light is located at 3 m downstream from the source point (see
Fig. 2). There is a 7.6~Jlm-thick Be window behind the front
end to absorb the longer wavelength light.3 The horizontal
acceptance angle is 5.5 mrad for a simple exposure chamber
located at 7 m from the source point. The exposure area of
the chamber is 40 X 40 mm2 with 10% uniformity. The mir
ror is oscillating with ± 4 mrad angle around its center line
at 25 Hz. In front of the mirror box there is a special expo
sure shutter with a response time of 10--100 ms. The shutter
is operated by a computer-controlled stepping motor.
The whole instrumentation consists of three main de
vices:
(1) A slit, which consists of four feed-through plates,
can be changed from 0 to a 50 X 50 mm2 hole.
(2) A 5X 10E -7 Torr high vacuum exposure
chamber with sensors and vacuum parts in it.
FIG. 3. Simulation equipment.
2149 Rev. Set Instrum., Vol. 60, No.7, July 1989 FIG. 4. Scanning moire fringe signal.
(3) A mask-wafer system, which is fixed on a ball bear
ing stage, is driven by a stepping motor. The moving distance
is 50 mm and the minimum resolution is 0.01 mm.
The vacuum in the mirror box is 5 X lOE -10 Torr
when there is no beam in it. Since there is no window in front
of the chamber, a three-section differential pumping system
is adopted.
II. UNIFORMITY CONTROLLING AND MEASURING
A scanning mirror is driven by a stepping motor, which
is turning back and forth within a 10 angle. The required
driving speed variation is programmed by a computer to ob
tain a uniform exposure area which is as large as possible.
An in situ moire fringe grating system, which consists of
two gratings, is used to measure the motor speed uniformity
during exposure. The first grating is attached to the scanned
mirror, while the other grating is fixed. A He-Ne laser is
used as the light source. The mainS fringe signal is displayed
on an oscilloscope, and processed by a computer.
A simulation experiment is shown in Fig. 3. A cylindri
callens is used to expand the light beam of the He-Ne laser
horizontally to simulate the synchrotron radiation. The uni
formity of the moire signals intervals represents the motor
speed uniformity which corresponds to the exposure unifor
mity. Figure 4 shows that a nonuniform phenomenon ap
peared at both ends of the scanning period, where the speed
slows down. Since the grating is 100 lines/mm, one moire
signals interval indicates a displacement ofO.m mm. In this
way it is easy to know the in situ uniformity and to adjust it
by computer programming from time to time. The time-con
suming, frequently interrupting measurement using a photo
densitometer is only seldom needed.
The whole beamline and its equipment are controlled by
an IBM PC/ A T computer.
I A. Heuberger, Microelcctron. Eng. 5 (5), 1 (1986).
2R. P. Haelbich, J. P. Silverman, W. D. Grobman, J. R. Maldonado, and J.
M. Wariaumollt, J. Vac. Sci. Techno!. B 1, 1262 (1983).
"F. Cerrina, H. Gllckel, and J. D. Wiley, J. Vac. Sci. Techno!. B 3, 227
(1985).
lithography 2149
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1.342457.pdf | Wigner function modeling of resonant tunneling diodes with high peaktovalley ratios
R. K. Mains and G. I. Haddad
Citation: Journal of Applied Physics 64, 5041 (1988); doi: 10.1063/1.342457
View online: http://dx.doi.org/10.1063/1.342457
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/10?ver=pdfcov
Published by the AIP Publishing
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Appl. Phys. Lett. 91, 032104 (2007); 10.1063/1.2756363
Peaktovalley ratio of small resonanttunneling diodes with various barrierthickness asymmetries
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and peaktovalley ratios at room temperature
Appl. Phys. Lett. 58, 2255 (1991); 10.1063/1.104943
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69.166.47.134 On: Wed, 17 Dec 2014 09:43:02Wlgner function modeUng of resonant tunneling diodes with high
peakMto~vaney ratios
R. K. Mains and G. I. Haddad
Center for High-Frequency Microelectronics. Department of Electrical Engineering and Computer Science.
The University of Michigan. Ann Arbor, Michigan 48109
(Received 8 January 1988; accepted for publication 2 August 1988)
Wigner function simulations of structures with experimentally observed high peak-to-vaney
ratios are carried out. It is shown that if care is taken with the numerical method used, the
simulations reproduce these sharp resonances. When scattering is ignored, peak-to-valley
ratios of 33.7 are obtained for a pseudomorphic InGaAs-AIAs structure. The effects of phonon
scattering are included to first order. Also, a small-signal analysis is carried out and the results
are used to predict the rf power generation capability of these devices.
I. INTRODUCTION
Estimates of the upper frequency limit for resonant-tun
neling diodes'-s have indicated that these devices are useful
into the THz range. Experimentally observed oscillation at
56 GHz6 and detection of 2.5-THz signals7 has generated
considerable interest in the potential of these devices. The
basic mechanism responsible for negative differential resis
tance and fast response times has recently been questioned in
the literature.5•s It is therefore desirable to develop methods
from quantum transport theory to model the transient be
havior of resonant-tunneling devices.
The Wigner function method has been successful in
modeling the general features of resonant tunneling di
odes.9•lo However, it has been found that this method under
estimates the peak-to-valley ratios observed experimentally
at low temperatures.9 Also, simulations showing the high
peak-to-valley ratios observed recently in InGaAs-InAIAs
structures II have not yet been presented.
In this paper, a modified numerical method is used12
which does predict high peak-to-valley ratios for these struc
tures. Both the simulated peak-to-valley ratio and the peak
current density are in the range of experimental results.
Since agreement was not possible using the original numeri
cal formulation of this method,9.12 it is concluded that care is
required in the numerical implementation of the problem.
The effects of phonon scattering have been included to
first order in the modeling of GaAs-A1GaAs devices. 13 This
article shows the effect of including InGaAs phonon scatter
ing rates on device performance. Inclusion of phonon scat
tering at room temperature reduces the peak-to-vaHey ratio
from 33.7 to 6.81.
An advantage of the Wigner function simulation meth
od is that it readily allows modeling of transient and smaU
signal effects.9.10 In this article, a small-signal analysis of an
InGaAs-AIAs device is carried out. By assuming that the
small-signal equivalent circuit of the device is constant over
a given rf voltage magnitude range, an estimate of the rf
power generation capability of the device is obtained.
The Wigner function method used is a single particle
approach, i.e., many-body effects such as wave-function
anti symmetrization and carrier-carrier scattering have not
been included. The analysis is one dimensional with the as
sumption of a thermal equilibrium distribution for states in the transverse direction. Although the self-consistent field
has been included in Wigner function simulations,12 self
consistency has not been included in the results presented
here.
II. BASIC METHOD AND EQUATIONS SOLVED
The equation for the time evolution of the Wigner func
tionRx, k) iSl4
Jj(x,k) _ flk ajtx,k) + (aRx,k))
m* ax \ at c at
1 ,.,~ {1°O ---J dk' 2 dysin([k-k']y) 2trli oc 0
x[v(x+ ~)-v(x- ~)]}f(X,kl), (1)
wheref is the one-dimensional Wigner function in m-2, m*
is the effective mass in kg, and Vex) is the potential energy
for electrons in joules. In this equation, (Jf / at) c represents
the time evolution off due to scattering processes. To first
order, scattering may be included in a manner similar to the
scattering term appearing in the Boltzmann transport equa
tion. 15 In the simulations including phonon scattering in this
paper, a scattering term of the following form was included
in the equations:
(Jfex,k) )
at c
+ ,,£Sin(k',k)f(x,k') ,
K' (2)
where SOU! (k) is the scattering rate from the state with wave
vector k to aU other states, and Sin (k ',k) is the scattering
rate into state k from another state k '. These scattering rates
are calculated using the expressions from first-order pertur
bation theory,16 considering bulk phonon modes only for
acoustic and polar optic scattering. After proper normaliza
tion consistent with the one-dimensional Eq. (1), these ex
pressions were evaluated numerically by summing over all k
space intervals included in the simulation to obtain the
scattering rates ofEq. (2). The material constants appearing
in the scattering expressions for InGaAs were obtained from
Ref. 17.
5041 J. Appl. Phys. 64 (10). 15 November 1988 0021-8979/88/22504 i -04$02.40 @ 1988 American Institute of Physics 5041
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69.166.47.134 On: Wed, 17 Dec 2014 09:43:02The discretized form used to solve Eq. (1) is (for k > 0)
/ f(x,k",) -f(x,km) = _ flkm /f(x,km) - / lex -l1x,km) + (afl(x,k m ») + _2_
M m" I1x at c fzNk
sin{21T[(p-m)/N k]} Nmax [f p-m)] XL L sin i 2nrr -- [V(x + nl1x) -Vex -nl1x)l[f(x,kp) ,
p 2rr[Cp-m)/Nd n~! \ Nk
where m and p are indices indicating k and k " respectively,
and where }\h is the number of k values included in the
simulation. Equation (3) differs from previous discretiza
tions9,J2 ofEq. (1) in the sin (k)/k weighting, which effec
tively is a window that deemphasizes the high-frequency
components in the discrete Fourier transform of the poten
tial energy function. As is shown in Ref. 12, the effect of this
weighting is to provide a more consistent approximation to
the moment equations that are derived by multiplying Eq.
(l) by kn and integrating over all k space. In Eq. (3), the
superscript/indicates that a quantity is considered to be at
the future time, t + at; therefore, this is a fully implicit
method. The upper limit in the Fourier transform of the
potential energy, Nmax' is chosen so that Eq. (3) is numeri
cally consistent with Eq. (1) for the linear potential casel2;
the value Nmax = (2/3 )Nk was used. For the simulations in
this paper, Nk = 60, Nx = 80, and Nmax = 40, where N.~ is
the number of mesh points in real space.
To obtain steady-state operating points on the /-V
curve, Eq. (1) is solved for a/ fat = 0, given a particular
value of applied bias, which is assumed to be dropped uni
formly across the double barriers and well. Starting from a
particular dc solution, small-signal solutions are obtained by
assuming a Wigner function of the form:
/(x,k) =hc (x,k) + Isms (x,k)eic.)t.
Also, the potential energy becomes
Vex) = Vdc (x) + Vsms (x)eiw, , (4)
(5)
where Vsms (x) is applied across the double barriers and well
only. Substituting Eqs. (4) and (5) into Eq. (3) and retain-
0.20
-'" 'e
~ 0.15
Q
z o
~ 0.10
1'.11: f-
Z
~oms 8 \ 0-f 1.50
1.00 ~
:>
<!I
~
0.50 !i
w
S Q.
-0.50 o 0.01 0.02 0.03 0.04 0.05
DISTANCE (MiCRONS)
FIG. 1. Zero-bias solution for Inn.53 Ga0.47 As-AlAs structure showing elec
tron density and potential energy profile. Barrier width = 22.6 A, well
width = 45.2 A.
5042 J. Appl. Phys., Vol. 64, No. 10, 15 November 1988 (3)
ing first-order terms only yields a solution for the small
signal current density in the device. When the small-signal
current density over the barrier and wen regions is integrat
ed and the ratio of the complex small-signal current and
voltage phasors is taken, the admittance per unit area of the
device is calculated. Since this admittance is due to the con
duction current alone, the term iwC is added to the admit
tance to account for the displacement current. The capaci
tance per unit area is determined from
C=E/W, (6)
where Wis the distance between the outer edges of the dou
ble barriers. For the structure analyzed in this paper,
W= 90.4A.
iii. DEVICE STRUCTURE AND de SIMULATION
RESULTS
For the simulations, a pseudomorphic Ino.53 GaO.47 As
AlAs structure was chosen for which excellent experimental
results recently have been reported.!! Figure 1 shows the
electron concentration and assumed potential energy for the
dc, zero-bias solution of this structure. For this device, peak
to-vaHey ratios of 14 at room temperature and 35 at 77 K
have been obtained experimentally, with peak current densi
ties in the range of 2-4 X 104 A/cm2. The conduction band
discontinuity in Fig. 1 is assumed to be 1.2 eV, and the effec
tive mass is assumed constant throughout the device and
equal to 0.042 mo, the InGaAs value.
Figure 2 shows the static J-V curve calculated at room
0.30
N 0.25 'e
<I{
"'g 0.20 \ >-f-0.15 iii z
I>J 0 ,'.
!- 0.10 z
IfJ a::
0:: 0.05 ::.l u
0 0.20 0.40 0.60 O.SO
VOLTAGE (VOLTS)
FIG. 2. Static 1-V curve for the structure of Fig. 1 at room temperature both
with (dotted curve) and without (solid curve) phonon scattering. Peak-to
valley ratio is 33.7 without scattering, 6.81 with scattering illcluded. Nu
merical method of Eq. (3) was used.
R. K. Mains and G. I. Haddad 5042
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69.166.47.134 On: Wed, 17 Dec 2014 09:43:02temperature for this device, both with and without phonon
scattering included. For the case without phonon scattering,
a peak-to-valley ratio of 33.7 was calculated. When InGaAs
scattering rates were used throughout the device, the calcu
lated peak-to-valley ratio was 6.81. It is believed that this
calculation underestimates the experimental peak-to-valley
ratio due to uncertainties in the scattering rates as wen as to
numerical problems inherent to the solution of the Wigner
function equations. 12 The calculated peak current densities
with and without scattering were2.31 and 2.43 X 104 A/cru2,
respectively, on the low end of the experimental current
range.
In the experimental J-V curves, the peak and valley cur
rents occur at higher voltages than the calculated values, at
0.7 and 1.0 V, approximately. It is believed that this diiscrep
aney is due to the fact that self-consistency has not been
included in these calculations. Preliminary work on Wigner
function simulations including self-consistency)Z shows that
approximately half the applied voltage can be dropped
across accumulation and depletion regions adjacent to the
device barrier regions.
For comparison, Fig. 3 shows the J-V curve for the case
without scattering using the original numerical method. 9, [2
This numerical method is obtained from the discretization
given in Eq. (3) leaving out the sin(k)/k weighting. The
peak-to-valley ratio obtained using this method was 2.57.
IVo SMALL-SIGNAL ANALYSIS AND ESTIMATED
POWER GENERATION
To carry out the small-signal analysis, the device dc so
lution at V de = 0.44 V, J de = 1.28 X 104 A/cmz was calcu
lated, in the middle of the negative conductance region of
Fig. 2. Figure 4 shows the electron concentration and poten
tial energy profile for this solution, which corresponds tolde
and V de in Eqs. (4) and ( 5). A small-signal voltage, V,ms, of
different frequencies is superimposed on V de and is assumed
to exist entirely across the barrier and well regions, All the
small-signal calculations were carried out at room tempera
ture and without phonon scattering.
0.40
~je
<
'" 52 0.30
>-!-w z 0.20
l.!J 0
i-z w 0.10 Q:
Q:
::> u
0 0.20 0.40 0.60 0.80
VOLTAGE (VOLTS)
FIG. 3. Static J-V curve for the structure of Fig, I at room temperature and
without scattering using the old Ilumerical method [no weighting in Eq.
(3)J.
5043 J. Appl. Phys., Vol. 64, No.1 0, 15 November 1988 0,20 1.50
'" r 'E
'" 0.15 1.00 OJ
Q :> f'. ~
Z ...l 0 j:; 0.10 " 0.50 :5
"I:
~J I-
0:: ( Z I- W Z !-
~ 0.05 0 0
z \\J a..
0 u
\ -0.50
0 0.01 0.02 0.03 0.04 0.05
DISTANCE (MICRONS)
FIG. 4. de solution at room temperature with Vac = 0.44 V,
Jdc = L28 X 104 A/cm2 for the case with no phonon scattering.
Figure 5 shows the real and maginary parts of the admit
tance calculated from the small-signal conduction current,
as well as the wC displacement current component. At low
frequencies, G is just equal to the negative slope of the J-V
curve in Fig. 2. From Figure 5, it is seen that the negative
conductance of the device remains essentially constant up to
a frequency of 3 X 1012 Hz. However, it is also seen that the
wC term in the admittance dominates above lOlD Hz so that
the device capacitance limits the power generation capabili
ty at these frequencies.
The available rf power from this device as a function of
frequency was estimated as follows. The small-signal admit
tance of Fig. 5 was assumed to remain constant over a large
signal voltage range of (V rf ) peak = 0.1 V. The area of the
device was chosen so that it is matched to I-H circuit resis
tance, which requires that
A= _G/(G2+B2), (7)
where B is the total susceptance, Le., the conduction plus
displacement current parts. Again, since from Fig. 5 the we
term dominates at high frequencies, the device area and rf
power are limited by the device capacitance. The available rf
power is given by
0.3
I
I
'" 0.2 I , / E " f.> / , .. 0.1 I
"' I ,~,'\ Q / , ,/ ,
-"" ,-' ,
llJ 0 U
Z
~ I--0.1
~ 0
<:( -0.2
-0.3
109 1010 lO" 1012 10:5 1014
FREQUENCY (Hz)
FIG. 5. Small-signal admittance calculated for the dc operating point at
Vdc = 0.44 V in Fig. 4. Solid curve is Re( y), small dashed curve is 1m ( Y),
large dashed curve is UJc.
R. K. Mains and G. I. Haddad 5043
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69.166.47.134 On: Wed, 17 Dec 2014 09:43:026.0 O.SO
5.0
0.60
~ '" .§. 4.0 E .,u
a:;
0.40 'g w 3.0 :t 0 .,g; a. w
LL 2.0 II:
II: 0.20.,g;
1.0
0 0
109 ICIO 10" 1012 1013
FREQUENCY (Hz)
FI G. 6. Estimated power generation (solid) and device area ( dashed) for 1-
n matching based on the small-signal data of Fig. 5 and using
(V,,jpeak = 0.1 V.
(8)
Figure 6 shows the estimated power generation capabili
ty of this device and the area required for I-n matching.
Note that these results assume no parasitic series resistance
in the circuit, i.e., the circuit resistance is entirely made up of
the I-n load resistance R [. , that is absorbing power. If non
zero series resistance, R" exists in the circuit such that
Rs + RL = In, the Prr values in Fig. 6 would be scaled by
the actual value of Rr..
The efficiency of the device may also be estimated as
follows:
Prr ( -G) ( Vrf ) ~eak /2 7
1} = - = = 8.88 X 10·· (-G) .
Pdc V;kJdc
(9)
From Fig. 5, the negative conductance is essentially
constant at the value 2.32 X 105 S/cm2 up to 3 X 1012 Hz;
putting this value in Eq. (9) yields a maximum efficiency of
17 = 20.4%. At higher frequencies, 71 decreases as the nega
tive conductance.
v. CONCLUSIONS
It has been shown that, if care is taken with the discreti
zation ofEq. (1), it is possible to resolve sharp resonances in
Wigner function modeling of resonant tunneling diodes.
Both the calculated peak current density and peak-to-valley
5044 J. Appl. Phys., Vol. 64, No.1 0, 15 November 1988 ratio are within experimental ranges, although inclusion of
phonon scattering underestimates the experimental peak-to
valley ratio at room temperature. A small-signal analysis
predicts that these devices should exhibit negative differen
tial conductance up to frequencies of several THz, however
device capacitance places a practical limit on device perfor
mance at several hundred GHz.
Further work needs to be done on refining the numerical
method used to discretize Eq. (1). Also, self-consistency
should be included in the simulation to bring the applied
voltage values more in agreement with experimental results.
ACKNOWLEDGMENTS
This work was supported by the U.S. Army Research
Office under the URI program, Contract No. DAAL03-87-
K-0007. The authors wish to thank Dr. William Frensley for
insightful discussions on quantum transport theory.
lB. Jogai, K. L. Wang, and K. W. Brown, App!. Phys. Lett. 48, 1003
(1986).
"D. D. Coon and H. C. Liu, App!. Phys. Lett. 49, 94 (1986).
'T. C. L. G. Sollner, E. R. Brown, W. D. Goodhue, and H. Q. Le, App!.
Phys. Lett. 50, 332 (1987).
"D. S. Pan and C. C. Meng, J. App!. Phys. 61, 2081 (1987).
'T. Weil and B. Vinter, App\. Phys. Lett. 50, 1281 (1987).
"E. R. Brown, T. C. L. G. Sollner, W. D. Goodhue, and C. D. Parker, App!.
Phys. Lett. 50, 83 (1987).
7T. C. L. G. Sollner, W. D. Goodhue, P. E. Tannenwald, C. D. Parker, and
D. D. Peck, App!. Phys. Lett. 43, 588 (1983).
"S. Luryi, App!. Phys. Lett. 47, 490 ( 1985).
9W. R. Frens!ey, Phys. Rev. B 36,1570 (1987).
lOW. R. Frens!ey, App!. Phys. Lett. 51, 448 (1987).
lIT. Inata, S. Muto, Y. Nakata, S. Sasa, T. Fujii, and S. Hiyamizu, Jpn. J.
App!. Phys. 26, L1332 (1987).
12R. K. Mains and G. 1. Haddad (unpublished).
"w. R. Frens]ey, Solid-State Electron. 31, 739 (1988).
l4S. R. deGroot and L. G. Suttorp, Foundations of Electrodynamics (North
Holland, Amsterdam, 1972).
ISJ. Lin and L. C. Chill, J. App!. Phys. 57, 1373 (1985).
IhW. Fawcett, in Electrons in Crystalline Solids [International Atomic En
ergy Agency (lAEA), Vienna, 1973].
17S. R. Ahmed and B. R. Nag, Solid-State Electron. 28, 1193 (1985).
R. K. Mains and G. I. Haddad 5044
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1.101323.pdf | Broadband (6 GHz) GaAs/AlGaAs electrooptic modulator with low drive power
R. G. Walker
Citation: Applied Physics Letters 54, 1613 (1989); doi: 10.1063/1.101323
View online: http://dx.doi.org/10.1063/1.101323
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/54/17?ver=pdfcov
Published by the AIP Publishing
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Appl. Phys. Lett. 50, 798 (1987); 10.1063/1.98048
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gap
Appl. Phys. Lett. 48, 989 (1986); 10.1063/1.96633
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129.105.215.146 On: Fri, 19 Dec 2014 21:19:59Broadband (6 GHz) GaAsl AIGaAs electroaoptlc modulator with low drive
power
R. G, Walker
Plessey Research and Technology, Allen Clark Research Centre, Caswell, Towcester, Northants;
United Kingdom
(Received 28 November 1988; accepted for publication 8 February 1989)
A GaAsl AIGaAs Mach-Zehnder modulator using a push-pull drive configuration is reported.
The bandwidth/drive-voltage figure of merit is approximately double that alan equivalent
single-sided device and is the highest reported for any non-traveling-wave structure, V" is 9 V
at 1150 nm. Using unterminated drive a bandwidth of 6,25 GHz is achieved,
Techniques for optical modulation and switching at mi
crowave frequencies () 1 GHz) are of significance for future
fiber optic systems in such areas as telecommunications, ra
dar, and signal processing. Various device results have been
published using both lithium niobate and compound IH-V
semiconductor technologies, with the former predominat
ing,
We have previously defined 1 a figure of merit for elec
tro-optic modulators which compares two external "black
box" parameters, i.e., bandwidth if 0) and the voltage swing
which the generator must provide to achieve full modula
tion. The common practice of quoting only Jr.) and V" (the
internal device drive voltage) may be misleading if load re
sistance (RL) has been used, reducing utilization of the gen
erator power. Thus we define
figure of merit (F) = [2RLI(SO+RL)](hIV1T)/L
(1)
Voltage, rather than power, is used so that the equiv
alence of different lengths of the same structure is apparent.
Wavelength ()., in microns) is included to extract the linear
dependence of V" on A, thus facilitating the comparison of
various results, Bandwidth (/0) is here defined as the fre
quency for which the relative intensity modulation depth has
fallen to 0.707 (not 0.5), This ensures consistency with elec
trical systems parameters in an eventual receiver.
A recent review of broadband modulators in terms of
this figure of merit 1 revealed a trend for lumped modulators,
in both lithium niobate and compound semiconductors, to
have Fvalues fromO,S to 1.0 GHz V 1 11m, whereas lithium
niobate traveling-wave devices (e.g., Ref. 2) were usually in
the range 1-2 GHz V 1 !-lm.
The Schottky ion, GaAsl AIGaAs double-heterostruc
ture waveguides used in this work have produced lumped,
Mach-Zehnder modulators with Fvalues towards the upper
limit of the lumped range (fa = 6.5 GHz, V1T = 17.S V, RL
= 00, F = 0,86 GHz V ; fim) 1 using single-sided drive.
This was achieved by the use of an air-bridge/trench tech
nique to eliminate the depletion capacitance of the contact
pads. In this letter we report an extension of the performance
of such lumped modulators by a factor of 2 using a novel
push-pull drive arrangement. The new device figure of merit
is 1.6 G Hz V -I ,urn-wen into the domain of existing travel
ing-wave devices,
The concept of push-pull drive applies to two-path (or
two-mode) interferometric devices in which a common field or voltage is used to phase modulate the two interfering
waves in antiphase, thus doubling the effect and virtually
eliminating phase chirp at the output. In Mach-Zehnder in
terferometers, the two arms may be considered as indepen
dent capacitive devices; paranel cross-connection achieves
push-pun but also doubles the capacitance thus leaving the
figure of merit unchanged, although a shorter device may
result. Series connection divides the modulating voltage
between the arms (thus doubling the voltage tolerance) but
the effects sum at the output; thus, the overall drive voltage is
unchanged. However, because the external capacitance is
halved, the bandwidth-and consequently the figure of mer
it-is doubled.
This series push pull is the most natural to implement in
semiconductor devices because of the built-in back-to-back
connection of the Schottky diodes which constitute the
Mach-Zehnder electrodes.
The main difficulty with the implementation of push
pull in III -V materials arises from the need for substantial de
reverse bias applied to both diodes in paralleL We achieve
series connection at rfwith simultaneous parallel connection
at de by use of monolithic RLC decoupling, to permit the
diode back connection to float at rf while being held at dc
positive, The decoupling components are realized on-chip by
making use of Schottky metallization and the material sheet
resistivity (50--100 !l/D). The device would be expected to
revert to a single-sided mode at low frequencies where the
decoupling becomes ineffective, but this will not affect the
frequency response since the phase/voltage curve is linear at
the bias voltages used. 1
The Mach-Zehnder interferometer design uses triple
guide couplers to split and recombine the light. Electrodes
were 3 mm long and a 6.4 GHz bandwidth was expected,
assisted by series inductance of around 1 nH from the coaxial
drive probe. The expected half-wave voltage ( V.".) was 8.5 V.
The symmetrical waveguide structure (GaAs core,
Ala.l G~)'l As claddings) was grown by metalorganic vapor
phase epitaxy on 2 in. semi-insulating (S1) GaAs wafers.
The strip-loaded waveguides were self-aligned to the elec
trodes by using the l-,um-thick aluminum metallization pat
tern as the etch mask(see Fig. 1, upper inset). Unwanted
metal was removed in a final processing step. The waveguide
width in the device reported here was 3 !lm. Unmetallized
waveguides of this type have achieved losses as low as 0.5
dB em J.:\ Figure 1 illustrates the device configuration. Iso
lation trenching, with air-bridge connections to the interfer-
'1613 Appl. Phys. Lett. 54 (17), 24 AprH 1989 0003-6951/89/171613-03$01.00 1613
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129.105.215.146 On: Fri, 19 Dec 2014 21:19:59AI Electrode Regions Etched Through
to 81 Substrate
\1"-""''-l..J'-~~~"",",!
~ 81 SUbstra~te.--::;:;;~;;;r.~;:::~\
'''-' .....
ometer electrodes, was used to eliminate the contact-pad de
pletion capacitance and to create the long, narrow, resistive
paths for decoupling-here identified with the 2 mm triple
guide coupler sections at each end. The ground pad served as
decoupling capacitor and separate pads for the de bias were
nearby.
Figure 2 combines the experimental biasing circuit with
an equivalent circuit of the chip itself, illustrating the on
chip decoupling.
The modulators achieved -20 dB optical extinction
and V IT was 9 Vat 1150 nm. The phase/voltage slope of 6.7°
V -I mm . I is typical for the structure used. The frequency
response was measured by an indirect swept-frequency tech
nique used by several workers. 1.2
Figure 3 shows the recorded frequency response (using
internal leveling with slope correction of the sweep oscilla
tor) from 100 MHz to 804 GHz: both push pull at 20 ± 3.3 V
bias (the ± 3.3 V is the unbalancing voltage, required to
achieve a null)and single sided at 15 V bias. The precise
bandwidth is slightly obscured by the fine structure, due to
reflections between device and bias tee, but is about 6.25
GHz in push pun and 3.5 GHz in the single-sided mode. The
latter result was obtained by setting the main bias supply ( VI
Bias tee r----------~ ; ! DC levels 11
L ( probe )1 [ V 21 I
I
I MZ 1
arm 1 I
lV1+~2]:
I V 2b ~-4.-~~__<>___+~ ....... ~~--' a~mz2 : ~_ [OJ
:.-~h'p ~q~v~l~t.:i~~~ _~
FIG. 2. Equivalent circuit ofmodulatnr chip with external drive and bias
ing. VI is the common bias while V] unbalances the interferometer to set the
operating point.
1614 Appl. Phys. Lett., Vol. 54, No. 17,24 April 1989 + d.c. FIG. 1. Schematic of push-pull Mach·-Zehnder modula
tor using triple-guide directional couplers. Insets show
the GaAs/ AIGaAs electro-optic waveguide cro~s sectioll
and a SEM micrograph of an air-bridge connection from
pad to electrode.
in Fig. 2) to zero and increasing the unbalancing voltage
(V2 ), thus forcing the ground-side diode to slight forward
bias. This operation caused a general output drop of -O.S
dB, probably because the double connection to the ground
side electrode (see Fig. 1) gives the latter a greater effective
length, the advantage of which is only gained in push-pull
mode. By the same argument, the slight low-frequency fall in
the push-pull response probably illustrates the predicted re
version to the single-sided mode.
The theoretical curves in Fig. 3, which match the experi
mental very closely, were obtained using a detailed computer
model I which provides a full traveling wave solution of the
electrode voltage, including all transit time effects, conduc
tor skin effect, and the effect of finite semiconductor sheet
conductivity.
Our high-efficiency modulator structures have an inher
ent slow-wave characteristic at microwave frequencies and
are thus unsuitable for use in direct traveling-wave mode.
Wang et al.4 attempt to harness the electrical/optical veloc
ity matching possible in GaAs by the use of low-efficiency
undoped structures between coplanar electrodes designed as
50 n stripline. Their TE~-TM polarization transformer de
vice (E II (011») has an inherent push-pull action giving vol-
Input power
o.-~~~~,
·1 ~~ ~\1. _Push-pul
·2 -...: ~~~ Computed response
rn -3 '\ hl11
"-4 \ 1,,\
\ Single sided 10~
·5 \ \
~ ~\
-6 Computed \ ~I~\ \ 'Vl\
-7 \ ~\
~8 _h_J~~_~ ! Ie \! o 23456789
Frequency ( GHz )
FIG. 3. Experimental and computed frequency responses.
R. G. Walker 1614
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129.105.215.146 On: Fri, 19 Dec 2014 21:19:59tuge harfing compared with a similar phase modulator
(E II (100) as in our devices) and is thus directly comparable
with the device reported herein. The quoted performance of
the device of Wang et aI." (10 = 16 GHz, V1T = 11 V, RL
= 50 .0, A = 1.3 /-tm) gives a figure of merit of 1.89
GHz V I J1m. However, it is clear from their derived pa
rameters and quoted bandwidths that optical, not electrical
dB have been used; their - 3 dB levels are thus -6 dB
according to our definitions (see introduction). Thus, for
consistency with our results the bandwidth oftlle Wang de
vice4 should be taken as 7-8 GHz (their Fig. 4) giving a
figure of merit of 0.83-0.95 GHz V-111m.
This suggests that the sacrifice of electro-optic overlap
efficiency to utilize the velocity matching effect may not be
worthwhile unless the match can be made very close and
conductor losses very low. We have shown I that open circuit
drive makes a lumped device fairly insensitive to electrode
resistance.
In conclusion, we have demonstrated the first imple~
1615 Appl. Phys.lett, Vol. 54, No. 17,24 April 1989 mentation of series push-pull drive in a Mach-Zehnder
modulator in I1I-V materials, Even though the device is
"lumped," the figure of merit is at a level normally associat
ed with traveling~wave modulators. The bandwidth is 6.25
GHz and V". = 9 V, although only 4.5 V need be generated
for 100% modulation due to the open circuit drive. In terms
of rf power, 17 dBm (50 ill W) would achieve 100% modula
tion while for greater linearity, 7.5 dB m (5.5 mW) or 11
dB m (12.6mW) would give 50% modulation or 70% mod~
ulation, respectively.
This work was carried out with the support of the U. K.
Ministry of Defence Components Procurement Executive
(DCVD).
'R. G. Walker, J. Lightwave Technol. 5, 1444 (1987).
'CO M. Gee. G. D. Thurmont!, and H. W. Yen, App!. Phys. Lett. 43, 998
(1983).
'R. G. Walker, H. E. Shephard, and R. R. Bradley, l~lectron. Ldt. 23, 362
( 1(87).
,'s. Y. Wang, S. H. Lin, and Y. M. Houng, AppL Phys. Lett. 51, 83 (1987).
R. G. Walker 1615
.................. -..:.:.:.:.:.:.:.:.:.;.~.: ....•..... ; .... -.-.. ; ......••.... -.............. ~ ... ,-.. .
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1.100499.pdf | Radiation damage effects in ionimplanted BiSrCaCuO superconducting thin films
S. Matsui, H. Matsutera, T. Yoshitake, and T. Satoh
Citation: Applied Physics Letters 53, 2096 (1988); doi: 10.1063/1.100499
View online: http://dx.doi.org/10.1063/1.100499
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/53/21?ver=pdfcov
Published by the AIP Publishing
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Superconducting thin films of BiSrCaCuO obtained by laser ablation processing
Appl. Phys. Lett. 53, 321 (1988); 10.1063/1.100597
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128.114.34.22 On: Mon, 01 Dec 2014 01:59:31Radiation damage effects in ionalmpianted BigSruCa .. Cu .. Q superconducting
thin fUms
S. Matsui, H. Matsutera, T. Yoshitake, and T. Sateh
NEe Corporation, Miyazaki, Kawasaki, 213 Japan
(Received 16 August 1988; accepted for publication 23 September 1988)
Transition temperature (Tc ) control and annealing effects of Bi20 Sr 14 Cau CUZ2 Oy
superconducting thin films implanted by 200 keY Ne I have been investigated. Tc end points
for O.4-1I.m-thick BiZOSrl4 Ca18 CUL2 Oy films for 0, 1 X 1012, and 1 X 1013 ions/cm2 doses are
78, 76, and 54 K, respectively. The ion dose, to achieve a nonsuperconductor for
Bizo Sr 14 Ca]8 CU22 Oy films, is two or more orders of magnitude lower than that for
YBa2Cu307 ~ x films. The c-Iattice constant increases were observed for the implanted films.
Moreover, it was confirmed that the superconducting characteristics for the implanted films
are recovered by annealing in O2 atmosphere.
Recently, Bednorz and Muller reported compound ox
ides with very high superconducting transition temperature
1:0'] The Tc value was raised to 90 K, using the oxide
Y-Ba-Cn-O.2 Subsequently, Maeda etal. reported new high
Tc superconducting materials for the Bi-Sr-Ca-Cu-O sys
tem.3 This Bi system was found to have two superconducting
phases with different 1'" values, 85 and 110 K. These results
offered the possibility of operating superconducting devices
at liquid-nitrogen temperature. Super conducting thin films
are necessary for application to microelectronic devices.
Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O thin films, with complete
transition temperatures above 85 K, have been reported by
Laibowitz et al.4 and Hong et af.5 respectively. It has been
reported that Tc control can be achieved by using radiation
damage by ion implantion.6 Superconducting quantum in
terference devices were fabricated on an Y-Ba-Cu-O thin
film, using radiation damages for As and ° in implantation.7
Ion implantation techniques can also be used as 8. method for
impurity doping and synthesizing by component ion dop
ing.8 To achieve the above-mentioned processes, it is neces
sary to study annealing characteristics for the implanted
high 1:, superconducting thin films. This letter describes Tc
controi, utilization of radiation damages, and annealing ef
fects for the implanted Bi-Sr-Ca-Cu-O superconducting thin
films.
200 keY Ne + ion implantation was applied for this ex
periment. The chemical compositions of the films were de
termined by electron probe microanalysis. Structures of the
annealed films were studied by x-ray diffraction method
with Cu Ka radiation. The dc resistivity was measured by a
conventional four-probe method, using gold electrodes sput-tered on the sample surface and silver paste for outer lead
connection. The O.4-,um-thick Bi2.0 Sr 1.4 Cau CU2.2 Oy films
were prepared on the (100) MgO substrates, employing the
multiple source evaporation system. 9 Bi203, ST-Ca alloy, and
eu were evaporated from three separate electron beam heat
ing sources. The three individual deposition rates were con
troned by three separate quartz crystal monitors. During
deposition, the substrate temperature was kept at 400 0c.
The oxygen pressure was controlled to maintain a constant
1 X 10 4 Torr pressure. The films were annealed in the O2
atomosphere at 890 ·C for 5 h.
Table I shows experimental conditions regarding 200
keV Ne + ion implantation. Three stage ion implantations
were carried out to obtain uniform dose depth profiles in the
films. The O.4-,um-thick BiZ.OSr1.4 Cau CU2l Oy thin films
were irradiated at room temperature with 200 keY Ne + in
the 1 X 1012 to 1 X lOt5 ions/cm2 dose range. The 200 keY
Ne ; average projected ion range was calculated to be about
0.2 !-lm, in the middle of the film. The ion range is nearly the
same as that for the YBaZCu07 ." fllm.
Superconducting transition temperatures for the films
were measured as a function of dosage. Figure 1 shows the
resistivity temperature dependence for
Bi2.0SrI.4CaLRCuZ.20y films, implanted at 0, 1 X 1012
,
1 X 1013, and 1 X 1014 ions/cm2 doses. The results indicate
that several changes in the film properties took place, pro
gressively with increasing dosage. The Tc end point was
most affected by the irradiation, decreasing at greater ion
dose. The Tc onset was changed more slowly.
Biz.oSr1.4 Ca1.8 CUZ.2 Or Tc end points for 0, 1 X 1012, and
1 X 1013 ions/cm2 do~es are 78, 76, and 54 K, respectively.
TABLE L Experimental T" control conditions for 200 keY Ne f ion implantation.
2096 Sample 1
200 keY Ne+
(lxlO'2/cm')
200kV IX 10 "/cm2
120 kV 5 X lO"/em!
50 kV 2X 101 '/ern' Sample 2
200 keV Ne+
(I X 10 Ll/cm , )
200 kV 1 X lOu/em2
120 kV 5)( lO'2/crn?
50 kV 2X lOl? /ern' Sample 3
200 keY Ne+
(I X 1O!·/cm2)
200 kV I X lO'4/crn2
120 kV 5 X IOI3/em'
50 kV 2 X IO"/cm" Sample 4
2()OkeV Ne+
(I X IOIS/cm2)
200 kV 1 X lOIS/em!
120 kV 5;< lO'4/cm2
50 kV 2X lO"'/cm2
Appl. Phys. Lett. 53 (21),21 November i 988 0003-6951 i88/472096-03$01.00 @ 1988 American Institute of Physics 2096
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128.114.34.22 On: Mon, 01 Dec 2014 01:59:311.5,.---------------..:-------,
200~eV Ne
1.0
0.5
o 250 300
T (k)
FIG. L Resistivity temperature dependence, normalized by resistivity at
300 K (Ro), for OA-ILm-thick Bi,o Sr'4 Calf; CU'2 0., films implanted by
200 keY Ne I. Doses: unimplanted. 1 X 101.', l:x; 10", and! 1014
ions/em'.
The film implanted at 1 X 1014 ions/cm2 dose indicates non
superconductor characteristics. The superconductivity was
destroyed by radiation damage.
The buildup of structural disorder in the film was moni~
tored by measuring the x-ray diffraction pattern. Figures
2(a), 2(b), 2(c), and 2(d) show x-ray diffraction patterns
for Bi2.oSrI.4CaUCul,,20y films implanted at 0, IX 1013,
1 X 1014, and 1 X 1015 ions/cm2, respectively. The film has
two phases: 85 and 110 K. Both of them have a strong pre
ferred orientation, with the c axis perpendicular to the mm
plane. Diffraction lines for the 85 and 110 K phases are indi
cated by dots and arrows, respectively. The c-iattice con-
200 keV Ne
I x IOI:5/cm2
200!(cV Na
j)( IOl3/cm2
10 20 :SO 40 50 60 70 60
28 ideg}
FIG. 2. X-ray di!Traction patterns for O.4-flm-thick BizoSr'4 Ca'8 CU'-2 OJ'
films implanted by 200 keY Ne+. (a) Dose: llnimplanted. (b) Dose:
I X 10" ions/em'. (c) Dose: 1 X 1014 ions/cm". (d) Dose: 1 X 1015
ions/em'.
2097 Appl. Phys. Lett., Vol. 53. No. 21. 21 November 1988 stants for unimplanted film, calculated from Fig. 2(a), are
30.71 and 37,05 A for the 85 and the 110 K phases, respec
tively. The diffraction lines for the 110 K phase disappeared
due to 1 X 10'" and I X 10]4 ions/em2 dose ion implantation,
as shown in Figs. 2(b) and 2(e). These x~ray diffraction
results are in accord with the resistivity measurement results
shown in Fig. 1. The diffraction patterns, shown in Fig.
2 (d), indicate that the film had become amorphous by ion
implantation at the 1 X 1015 ions/em2 dose.
The authors previously reported ion dose dependences
for Tc and c-Iattice constant for O.4-fL.m~thick
YBu2CU307_x films by 200 keY Ne ion implantation. 10 Ion
implantation conditions are the same as those for
Biz.oSrI.4CaLSCu2.20y thin films. Ion dose dependences for
T" end point and c-lattice constant for
Bi2.0SrL4CaLgCU2.20y and YBa2Cu307_x films are shown
in Fig. 3. It became clear that the ion dose, to achieve a
nommperconductor for Biz.o Sr 1.4 Cal.8 CU2.2 Oy films, is two
or more orders of magnitude lower than that for
YBa2Cu307 _ x films. It also became clear that c-!attice con
stant variation for Bi2.0 Sr J.4 Cal.8 CUZ.2 Oy thin films is larger
than that for YBa2Cu307 __ x thin films. These results indicate
that the Biz.o Sr 1.4 Ca1.8 CU2.2 ° y thin-film crystal structure is
destroyed with a lower ion dose, compared with
YBa2Cu207 _ x thin films. It is considered that these differ
ent characteristics for ion beam irradiation between
Bi2.0Sr!.4 Ca1.SCuZ.z Oy and YBa2Cu~07 -x thin films are at
tributed to the concept that the crystal structure for
Bi20Srl.4 Ca1.8 CUZ.2 Oy is marked by a layer structure, com
pared with that of YBa2Cu307 _ x' Furthermore, it was ob
served that the Bi2.0SrI.4CaI.8CuZ.20y thin-film color
changes with ion implantation. As the ion dose increases, the
implanted Biz.oSf\.4 Cal.8 Cllz.2 Oy thin-film transparency in~
creases. Bi2.0 Sr 1.4 Ca!.8 CU2.2 0 y thin films, implanted at
1 X 1015 ions/cm2 dose, became completely transparent. On
the other hand, no film color changes were observed for
200koV Ne
30.90 III! 100
Y-Sa-Cu-O
-<l: :')0.80 Bi-Sr-Co-Cu-O 80
I- ~-. ~
Z lIS <l: 30.70 l-'ll! !o-
III 60 Z
Z (5 0 • <..l \ IL 30.60 I
W \ 0
\ Z <.) \ 40 W ..... \ l- I !o-II.SO \ u « \ I-
..J \
\ _0 20 11.70 ~D-D-EI o t " ! ... ""nf"up.r nO"jSLiPsr
11.60 0
0 10'2 10" 1O'~ 10'" 10'· JOlT
DOSE lions tern'll
FIG. 3. Ion dose dependences of 1~ and c-Iattice constant for 0.4-,um-thiek
YBa2Cu,O, x and Bi2(;Sr'4Ca"CU2,O, films implanted by 200 keY
Ne+ ion.
Matsui et al. 2097
-•• --• -,', .-.-.-.-.-.-•• .....-., •• -. -•••• : •.••• ~ ••• -••••••••••••• ;<; •• ~;> ••• 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:
128.114.34.22 On: Mon, 01 Dec 2014 01:59:31!.o
0.75
o
!?:: 0.5
c:::
0.25
>I-
iii z
W I
Z o
(a)
~§ r
(b) 200 KeV Ne I xl O"/cm"
A:1nea! 890 cC 5H
50 100 150
T iKJ
21
0 " !e
~ 0; .
0 g !e ~l ,.
0 " !
21 " 8 " ro II £
0 I s I . \ 200 250 300
200keV N. I x Io''}cm 2
Anneal 690·C 5H
£1
0 2
~I
0
!e
'"'
It . is' ~I 2 0 t . s
i 0 20 30 40 50 60 70 80
28 (deg)
FIG. 4. Annealing characteristics for O.4-,um-thick Bi,.D Sr].4 Ca 1H CU'2 0 y
films implanted at 1 X 10'4 ions/cm2 by 200 keY Nc .. Anneal conditions:
890 ·C, 5 h. (a) Resistivity temperature dependence, Normalized by Ro
resistivity at 300 K. (b} X-ray diffraction patterns.
YBa1CU307 _ x thin films even implanted at a 1 X 1017
ions/cm2 dose.
Bi20Sr1.4 Ca1.8 CU22 Oy films, implanted at a 1 X 1014
ions/cruz dose, were annealed in O2 atomosphere at 890°C
for 5 h. Figures 4(a) and 4(b) show resistivity temperature
dependence and x-ray diffraction patterns, respectively, for
annealed film. The resistivity temperature dependence is the
same as that for the un implanted film, as shown in Fig, 2.
2098 AppL Phys. Lett., Vol. 53, No. 21,21 November i 9S6 The diffraction lines for the 110 K phase appeared by anneal
ing at 890°C for 5 h, as shown in Fig. 4(b). It became clear
that the structure for Bi2.n Sr 1.4 Cal.8 CU2.2 Oy films, implant
ed at 1 X 1014 ions/cm2 dose, can be recovered by annealing
at 890°C for 5 h.
In conclusion, Fe control and annealing effects for
Bi2.o Sr 14 Cau CU2.2 Oy superconducting thin films, implant
ed by 200 keY Ne t , have been investigated.
Bi2oSrl4CauCu220v T,. end points for 0, lXl012, and
1 X lOu ions/cm2 do~es are 78, 76, and 54 K, respectively,
Film implanted at a 1 X 1014 ions/cm2 dose indicated that it
was a nonsuperconductor, The result indicates that the ion
dose, which would result in a nonsuperconductor for
Bi2.0 Sr 1.4 Cal.8 CUll Oy films, is two or more orders of mag
nitude lower than that for YBa2Cu307 __ x films. The c-lattice
constant increases were observed for implanted films, More
over, it was confirmed that the superconducting characteris
tics for implanted films were recovered by annealing in an
O2 atmosphere .
'I. G. Bednorzand K. A. Miiller, Z.l'hys. 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).
'H. Maeda, Y. Tanaka, M. Fukutomi, and T. Asano,JplI. J. App!. Phys. 27,
1.209 (1988).
4R, B. Laibmvitz, R. H. Koch, P. Chaudhari, and R. J. Gambino, Phys.
Rev. IDS, 8821 (1987).
SM. Hong, J. Kwo, and J. J. Yeh, J. Cryst. Growth 91,382 (1988).
"G. J. Clark, A. D. Marwick, R. H. Koch, and R. B. Laibowitz, Appl. Phys.
Lett. 51,139 (1987).
7R. H. Koch, C. P. Urnback, G. J. Clark, P. Chaudhari, and R. B.
Laioowitz, AppJ. I'hys. Lett. 51, 200 (1987).
8M. Nastasi, J. R. Tesmer, M. G, Hollander, J. F. Smith. and C. J. Maggio
re, AppJ. Phys. Lett. 52, 1729 (1988),
"T. Yoshitake, T. Saloh, Y. Kubo, and H. Igarashi, Jpn. J, Appl. Phys. 27,
Ll089 (1988).
lOS. Matsui, Y. Ochiai, a Matsutera, J. Fujita, T. Yoshitake, and Y. Kubo,
J. App!. Phys. 64, 936 (1988).
Matsui et al. 2098
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1.101169.pdf | Lowpressure hollow cathode switch triggered by a pulsed electron beam
emitted from ferroelectrics
H. Gundel, H. Riege, J. Handerek, and K. Zioutas
Citation: Appl. Phys. Lett. 54, 2071 (1989); doi: 10.1063/1.101169
View online: http://dx.doi.org/10.1063/1.101169
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v54/i21
Published by the American Institute of Physics.
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Downloaded 20 Jan 2013 to 128.148.252.35. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionslowg!=nessure hollow cathode switch triggered by a pulsed electron beam
emitted from ferroe~ectrics
H. Gundel and H. Riege
CERN, PS Diuisioll, CH-1211 Genelia 23, Switzerland
J. Handerek
University of Silesia, Institute o.!,Physics, P-40007 Katowice, Poland
K. Zioutas
Uniuersity o.fThessaloniki, Nuclear and Elementary Particle Physics Section, GR-54006 Thessaloniki,
Greece
(Received 7 December 1988; accepted for publication 10 March 1989)
A new type of low-pressure gas switch is described. The switch is triggered by an electron
beam that is emitted from the surface of a ferroelectric samplc. The electron be<lm is gcnerated
within the hollow cathode and ejected through a hole of arbitrary shape into the main gap of
the switch. The beam current and the electron energy C<l11 be chosen such that breakdown is
achieved with small jitter. The switch with its ferroelectric trigger requires neither heating nor
an auxiliary gas discharge. The fast spontaneous polarization change tiP" which is the cause
of electron emission, is induced by a high-voltage pulse from an electronic switching circuit.
High-power gas switches are widely used in pulsed pow
er devices, such as in modulators for radar, in laser systems,
or in pulsed magnet systems of accelerators. Conventional
switches, such as thyratrons or high-pressure spark gaps, are
sometimes the limiting elements in these systems owing to
factors such as maximum current density, precision, voltage
hold-off capability, or erosion. Recently, a new class oflow
pressure gas switches:'~ was introduced, which significantly
improves on current density compared to thyratrons, on pre
cision compared to ignitrons, and on erosion compared to
high-pressure spark gaps.
In this letter the authors report on a new type of low
pressure gas switch which is characterized by the direct fir
ing of a high-voltage gap with a high-density, low-energy
electron beam of short duration. This switch need~ neither a
heated cathode nor a permanent glow discharge for trigger
ing. The breakdown initialization is direct in the sense that
the initial number density of electrons is greater than 1013
cm -2 and no additional charge carrier multiplication pro
cess is required to commute the switch resistance from an
infinite to a small v<llue in a short time.
The electron beam is emitted from the surface of a fer
roelectric material. In order to obtain such an emission, the
macroscopic spontaneous polarization P, of the sample
must be changed in a short time interval (nanosecond
range)3A by a fast, high-voltage (HV) pulse applied to the
sample via partially perforated electrodes. The switching
time ofP, is determined by the current amplitUde of the HV
pulse.
In Fig. 1 the schematic of an experimental switch is
shown, which is used to study the electron beam production
from PLZT samples such as (Pbo93 LaO.07 ) (Zr06, Ti,us )0.1
or (Pb09H L3.rW2) (Zr095 Tio.os )03, and the breakdO\vn ini
tialization of the main gap. The PLZT sample is placed in
side the hollow cathode C, of the switch between two plane
electrodes GE and RE. The whole assembly is filled with a
low-pressure gas such that the breakdown behavior is char
acterized by the left-hand branch of the Paschen curve. The sample electrode GE facing the main switch cathode C, is
partially (about 50%) perforated (grid or sieve). Between
the main cathode anel the sample another metallic grid AG
with a transparency of approximately 25% can be charged
with a modest dc potential ( ± 10 to ± 200 V) to accelerate
or decelerate thc emitted electrons and to raise or to lower
the main breakdown voltage. 1 In the experimental switch of
Fig. 1, the auxiliary grid serves also as the main diagnostic
tool for determining current and charge of the emitted elec
tron beam.
Pulse length, current density, and energy of the electron
beam are controlled by the current amplitude, by the polar
ity and the rise time ofthe high-voltage pulse, and by the dc
Z= 50 Q
HVs
R:: Z
,/ / ./ ..- / .... /'
A.,.
-e
+
[$
AG I Integrator
444 GE I " , • " ,
HVf-pl.lIse
FE LJ
FIG. L Schematic of electron-beam-triggered hollow cathode switch.
A, •. 7 switch anode; C, switch cathode (gnp distance 10 mm):
BV,' switch charging voltage; L,R charging cablc impedance and
matching resistor; ! c insulators, AG = auxiliary grid (25% transparen
cy); GE = grid electrode (50% transparency); RE = rear sample elec
trode; FE = ferroelectrk sample; HV} -, HV pulse for polarization rever
sal of FE; e = electron beam.
2071 Appl. Phys. Lett 54 (21), 22 May 1989 0003-6951/89/212071 -03$01.00 (c) 1989 American Institute of Physics 2071
Downloaded 20 Jan 2013 to 128.148.252.35. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsO.25A1div
100 ns/div
500 V/a!v
200 V/div
FIG. 2. Waveforms of emitted current measured on AG (top), voltage on
A, (middle), and voltage on RE (bottom) measured with a negative HV v
pulse of I kV amplitude on RE of the switch shown in Fig. 1. The hias
potcntial Oil AG is zero. The total emitting area ofthc sample is SO mm', but
less than 10% of the electron beam is injected into the main switch.
potential applied to the auxiliary grid AG. It is also impor
tant to which electrode of the sample the HV pulse is applied.
Figure 2 demonstrates emission lit the start of a negative
pulse applicd to the rear electrode RE of a PLZT 7/65/35
sample. Whenever the potential difference betwccn GE and
RE reaches a threshold value, P, is changed and emission
takes place. The electron beam emission can only start from
free parts of the ferroelectric surface. When the HV pulse is
over, P, in part of the domains and grains reverses back to
the original state induced by depolarization fields arrd me
chanical stresses inside the sample. The next HV pulse can
be repeated shortly afterwards. The maximum charge
!::.q, = !::.P,Fo that can bc emitted during one emission cycle,
is limited by the possible value of spontaneous polarization
change !::.P \ and by the nonelectroded surface area F;). In
order to enable fast polarization changes, the HV F pulse
must have a sufficiently high electric field amplitUde
(threshold). The HV F pulse must also be generated in a low
impedance circuit to provide a high current amplitude, pre
ferably of the order of 100 A or more, in order to remove or
restore compensation charges via the electrodes. The current
amplitude iF determines the rise time tr o[the voltage HVF
on the sample during P, change, so that iFt,:z !::.P\F(l",
where te is the emission time; t" will tin ally be limited by
material properties. but at present this limit has not been
reached yet.
Figure 2 demonstrates that the main discharge of the
switch coincides with the electron emission. The results of
the experimental switch were obtained with a thyristor pulse
generator of 1.0 k V amplitude, 100 ns rise time on 50 fl, and
20 A current amplitude. The whole assembly was operated
in air at atmospheric pressure, whereas the discharge volume
and the ferroelectric sample were under nitrogen pressure
ranging from 0.1 to 100 Fa. The main switch voltage HV, of
the 50 n discharge circuit was limited to a few kilovolts.
There is evidence that the switch is fired directly by the
electron beam. The maximum energy of the emitted elec
trons depends on the sample thickness d and is of the order of
b.Psd IE, where IE = ErEn is the permittivity of the ferroelec
tric material at the moment of reversal. The average energy
ofthe emitted electrons should be chosen such that the maxi
mum ionization cross section for the given low-pressure gas
atmosphere (typically arourrd 100 eV for most gases) is
achieved. If every electron has to perform several ionization
2072 Appl. Phys. Lett., Vol. 54, No. 21,22 May 19S9 5kV
OkV
5ns/d!v
(a)
15kV
OkV
5ns/div
(b)
FIG< 3. Jitter orthe main discharge pulse on A, connected to a storage ca
pacilorof2 nF charged to (a) 5 kV (voltagconAG = + 100 V) and (b) 15
kV (vohilge on AG C~ + 400 V). In both cases the pressure was 150 Pa.
The PZLT 2/95/5 sample was pulsed with -' kV amplitude from 50 n. The
discharge starts about 100 ns after the start of the HV F pulse.
acts on its way to the main anode of the switch, then starting
energies of less than 1 ke V arc needed.
Figures 3 (a) and 3 (b) show the Jitter ofthe discharge of
a 2 nF capacitor through the main switch for two different
charging voltages. A PLZT 2/95/5 sample served as emitter
which was pulsed from a 50 n generator with an amplitUde
of 3 kV, 15 ns rise time, and a pulse length of 200 ns. Al
though the trigger geometry and the emission current ( < 10
A) injected into the main switch were far from being opti
mized, jitter values below ± 5 ns could be reached. The
same is true for the bias voltage on AG which was chosen in
the range of 100--400 V. The ferroelectric sample was placed
31 mm away from the main cathode C, .
Compared to vacuum the low-pressure gas leads to
some gas amplification (by a factor of less than 3) of the
electron current. The new method of large-volume ioniza
tion by electron emission from a ferroelectric medium is
ideally suited for precisely switching high voltages and high
currents at high-power levels. High-volt.<ge closing switches
profit from the absence of heated electrodes, auxiliary glow
discharges, or the intermediate stage oflaser illumination to
produce enough charge carriers for breakdown. The high
density electron beam allows arbitrary discharge cross sec
tions to be chosen, such as multiple channels or ring-shaped
discharges. Very low inductance switches can be built with
hollow beams. Also the erosion of the main electrodes will be
reduced accordingly. The beam energy, the gas pressure, and
the discharge geometry can be chosen such that optimum
ionization rates are obtained. One can envisage using this
effect also for closing-opening switches where a large volume
discharge is controlled by the presence of an electron beam.
Gas amplification has to be avoided by choosing a mixture of
attaching and nonattaching gases.5
Adaptation to a large range of power-switching situa-
Gundel eta/. 2072
Downloaded 20 Jan 2013 to 128.148.252.35. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionstions can be achieved through the control of the electron
beam by such factors as material, geometry, and electrodes
of the sample, HV pulse characteristics and circuit imped
ance, auxiliary grid potential, and gas pressure.
The authors thank Daniel Boimond for his help during
the experiments and for the construction of the electronic
switching circuit.
'D. Bloess, L Kamber, H. Riege, G. Bittner, V. Bruckner, J. Christiansen,
K. Frank, W. Hartmann, N. Lieser, C Schultheiss, R. Seebikk, and W.
2073 Appl. Phys. Lett.. Vol. 54, No. 21.22 May 1989
........................... •••••• ............. ~ ••••• o;.7'NN ••••••••• q: •• ~.7'7' •••• ~"7' •••••••••••••••••••• ~ ••••••••••••••••• •••••••• Steudtner, Nucl. Instrum. Methods 205, 173 (1983).
'P. Billault, H. Riege. M. van Gulik, E. Boggasch, K. Frank, and R. See
bock. CERN 87-13,1987.
'n. Gundel, H. Riege, E. J. N. WilSall, J. Handerek, and K. Zioutas, "Fast
Polarization Changes in FC'!Toelectrics and their Applicatiolls in Accelera
tors," CERN PS/88-53 (AR) 1988, to be published in Nuel. lustrum.
Methods.
4H. Gundel, H. Riege, E. J. N. Wilson, J. Handen.,k, and K. Zioutas, "Fast
Polarization Changes in PZT Ceramics by High-Voltage Pulses," CERN
PS/88-54 (AR) 1988, presented at the 1st European Conference on Appli
cations of Polar Dielectrics and International Symposium on Applications
of Perroelectrics, Ziirich. August 1988.
'I:. Vitkowitsky, High Power Switching (Van Nostrand Reinhold, New
York, 1987), p. 213.
Gundel et a/. 2073
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1.343031.pdf | Phthalocyanine semiconductor sensors for roomtemperature ppb level detection of
toxic gases
T. A. Temofonte and K. F. Schoch
Citation: Journal of Applied Physics 65, 1350 (1989); doi: 10.1063/1.343031
View online: http://dx.doi.org/10.1063/1.343031
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129.105.215.146 On: Fri, 19 Dec 2014 01:55:47Phthalocyanine semiconductor sensors for roomatemperature ppb
level detection of toxic gases
T. A Temofonte and K. F. Schoch
Westinghouse Research and Development Center, Pittsburgh, Pennsylvania 15235
(Received 31 May 1988; accepted for publication 4 October 1988)
Nickel and lead phthalocyanine (NiPc and PbPc) thin films prepared by vacuum sublimation
have been investigated for use as gas sensors. High sensitivity (25 ppb), reversibility, and very
fast response time (~ seconds) have been demonstrated for detection of N02 at room
temperature. The sensor output increases by five to seven orders of magnitude as the gas
concentration is increased from ~ 2 X 101 to 6 X 104 ppb. Measurements of transient response
characteristics versus gas concentration exhibited a simple logarithmic dependence.
Application of this approach to the detection of other agents is discussed. It appears that the
unique electrical properties of these films are a result of the film morphology associated with
our specific deposition approach. Detection based on optical sensing ofN02 has also been
demonstrated.
I. INTRODUCTION
Phthalocyanine compounds have been known for some
time to display substantial changes in electrical, optical, and
magnetic properties in reaction with various oxidizing and
reducing agents. 1-8 These properties can be exploited for a
number of chemical sensor applications. A generalized
phthalocyanine molecule is shown in Fig. 1, Metal phthalo
cyanine (MPc) is distinguished from the free ligand by the
presence of a metal atom (e.g., Ni, Pb, Zn, Cu, etc.) which
replaces two hydrogen atoms in the center of the phthalo
cyanine ring. The crystal structure of these compounds is
such that they can easily accommodate dopant molecules in
channels adjacent to the phthalocyanine stacks. When a
molecule such as N02 is chemisorbed onto MPc or H2Pc,
surface charge-transfer interactions occur, resulting in a
very large increase in surface conductivity. The process is
somewhat analogous to doping intrinsic silicon to produce a
p-type semiconductor. The charge transfer increases the
conductivity by greatly increasing the number of charge car
riers, which for phthalocyanines are holes. The highest occu
pied molecular orbital (HOMO) in MPc compounds is the
ligand centered, so the metal atom has little influence on the
oxidation reaction. Moreover, the energy level of HOMO is
such that a reaction with oxidizing agents readily occurs,
removing an electron from the ring system. The result is a p
type material which has been confirmed by thermopower
measurements of iodine-doped compounds.9 locyanine (MPc) films by gas species. Wright and co
workers have investigated the effects of static pressures of
O2, N02, and BF3 on needlelike single crystals ofperylene,
tetracyanoquinodimethane, and various phthalocyanines
(Zn, Co, Mn, Pb, Cu, Ni, and HZ).12 They have shown that
pressures as low as 7.6 mTorr ofN02 will lower the surface
For example, after deliberate chemical doping with io
dine,1 the resistivity of NiPc powder increases from 1012 to
0.7 n cm. Similar conductivity enhancements are observed
in other phthalocyanine compounds, such as CuPe, FePc,
PbPc, and even in the free ligand, HzPc, and a class of phtha
locyanine polymers, [Si (Pc) 0 1" . 1.10 MPc compounds have
several other extremely attractive properties, including their
impressive thermal stability, simple processing characteris
tics, and low cost. It
In recent years, three groups in England have investigat
ed the electrical resistance changes produced in metal phtha-FIG. !. Metal phthalocyanine molecular stacking arrangement after dop
ing
1350 J. Appl. Phys. 65 (3), 1 February 1989 0021-5979/59/031350-06$02.40 @ 1959 American Institute of Physics 1350
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129.105.215.146 On: Fri, 19 Dec 2014 01:55:47resistivity ofMPc crystals, but a sheet resistance decrease of
approximately seven orders of magnitude is observed when
the pressure is increased to 7.6 Torr. They report that the
effect can be reversed by exposure to NH 3 gas or by heating
at 150·C under vacuum but observed no effect on exposure
to C2H4, CO, or alkylated pyridines. They have also investi
gated the effects of N02 on thin films of PbPc measured at
155 ·C,13 which will be discussed in Sec. HI.
Batt and Jones have reported the use of thin films of
PbPc and H2Pc as detectors of N02 in air mixtures at con
centrations of 1 ppb-20 ppm. 14 They made all their measure
ments at 100-170·C in a flow apparatus. Films of 0.3-1.0
p,m thickness were deposited on alumina substrates and they
were stable at 150 ·C for 4-6 months of continuous operation
in still air. They report that S.O-ppm HzS, 40-ppm NH3, 10-
ppm S02' lOO-ppm H2, 1 % CH, and H20 vapor at 50%
relative humidity caused less than 20% change in the re~lis
tance of either type of Pc film. They contend that this resis
tance change is essentially a surface phenomenon as the ef
fect is the same in films throughout the thickness range.
Copper phthalocyanine has been incorporated into a
Langmuir-Blodgett (LB) film,15 Films consisting of eight
layers were transferred onto metallized substrates for resis
tivity measurements. The effect ofN02 in N2 was studied at
concentrations 00-120 ppm. LB films, however, are expect
ed to be quite fragile and the sensitivity reported here is rath
er low compared to vapor-deposited films.
On the basis of these developments, we have begun an
investigation of the MPc family of organic semiconductors
using NiPc and PbPc as the active sensing materials. These
were initially deposited onto a variety of substrates, some
containing interdigitated electrodes to permit preliminary
analysis and evaluation of resistivity changes of various sens
ing layers on exposure to NOz. We have determined the
structure of these films by transmission electron microscopy
(TEM},16 The substrate temperature during deposition is
crucial for determining the morphology ofthe film. Because
the electrical properties MPc single crystals are highly aniso,
tropic, the crystallite orientation in the film is expected to be
important in determining the electrical properties of the
film. This point will be discussed further in Sec. III C.
Work is also currently underway using field-effect tran
sistor (FET) sensor structures to permit measurement of
gas species with comparable sensitivity but lower noise than
current observed, 17 This device structure is similar to that
reported by workers at MIT Lincoln Laboratory. 18 Since the
deposition conditions for our thin films are completely com
patible with such structures, smart multisensor arrays are
feasible. Other work in progress includes the response of
CuPc and H2Pc to N02 and CO, as well as the temperature
dependence of the transient and equilibrium sensor re
sponse. The work reported here, however, focuses on the
response of NiPc and PbPc to N02 using an interdigitated
electrode structure.
H. EXPERIMENT
A. Sample preparation
Nickel phthalocyanine (Eastman) was purified by mul
tiple vacuum sublimations (430 ·C, 1-pm Hg). Lead phtha-
1351 J, Appl. Phys., Vol, 65, No.3, 1 February 1989 locyanine was prepared by the literature procedure from
phthalonitrile and PbO. 11 This compound was subsequently
purified by sublimation under identical condi.tions. Compo
sition was confirmed by infrared spectrum and elemental
analysis.
Typically, a thin film of NiPc was deposited by vacuum
sublimation in a tube furnace at 430 .c. The source material
wasO.14 g of multiply sublimed NiPc. Deposition took place
over 25-30 min in a dynamic vacuum of < 1 mTorr. Films of
PbPc were deposited in a similar manner, in that case using
0.12 g of sublimed material and depositing for 40-60 min at
160'C. The substrate with an interdigitated electrode pat
tern was positioned such that its temperature would be ap
proximately 160°C.
Films of NiPc and PbPc were deposited onto bare and
e1ectroded ceramic and glass substrates as well as NaCl
plates for IR characterization (see Sec. II D), Bare sub
strates were used for surface profilometry measurements us
ing a Dektak H. Electroded substrates were used to monitor
surface sheet resistance (Ps) changes of the films on expo
sure to various concentrations of N02•
B. Sensor studies
After film deposition, samples were transferred from
the deposition tube to the gas exposure system. Each speci
men was baked out at 190°C for 2 h under dynamic vacuum
prior to gas exposure. Initial values of sheet resistance [typi
cally (1-4) X 1016 OlD] were determined prior to exposing
the sensors to mixtures of N02 in air. Concentrations of 25
ppb, 1.2 ppm, and 60 ppm were used. These mixtures were
prepared by successive dilution from either neat N02
(Matheson, > 99.5% purity) or an anlayzed NOz/air mix
ture. Matheson Ultra-Zero air ( < 0.1 ppm total hydrocar
bon) was used for these dilutions. The total gas pressure
above the samples during the measurement was 560-580
Torr. Measurements of current were made at various inter
vals after initial exposure for typical electrode potentials of 1
v.
C. Infrared studies
Infrared spectra were recorded on a Perkin-Elmer 1310
grating spectrometer using films deposited on salt plates un
der the same conditions as those to be used in preparing the
samples for electrical measurements. This technique was
used to establish the need for multiple sublimation of the
starting material in order to permit deposition of thin films
ofpUTe MPc.
Because of previous work with phthalocyanines, which
showed a dramatic change in IR transmission due to chemi
cal doping, 10 similar measurements were made with these
films. The results will be discussed in detail elsewhere, it} but
the IR transmission diminishes dramatically due to the cre
ation of free carriers during reaction with NOl, The IR re
sponse to variously treated NiPc films deposited on a salt
plate is depicted in Fig. 2. This effect is reversible, corre
sponding to the change in electrical conductivity. The broad,
free-carrier absorption can be related to gas concentration
qualitatively.19 This method of detection, however, was
T. Ao Temofonte and K. F. Schoch 1351
··.·.·.·.·'".· .•••• __ ......... , •••••••••••••.•.••• _r ••• ~r."-u •• -__ •.•• -~ -r -• -•• " ••••••••••••• ~ •• ;O;' •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• :.:.:.;.:.:.;.:.:.:.:.:.;.:.:.:.:.:.,.:.:.:.:.: ••• :.:.: ••• : ••• v .•...•.•.•. , .............. 7 ... __ ...... _ .......... --; •• _.~;- ........... ~ .. ..
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129.105.215.146 On: Fri, 19 Dec 2014 01:55:47C60~!I'~/J .~
~
~
~ 40 NiPc + NOZ '
2. 20 I
.~ o[ (bl , I I .. ..J.. __ .L I I 1 1 . J
so
60 -NiPc + N02 + Heat + Vac
-1 Wavenumbers{cm I
FIG. 2. IR spectra of NiPc films: (a) as-deposited, (b) after exposure to
677-Torr N02 for 1 h, (e) after subsequent heating at 150"C for 16 h under
dynamic vacuum.
found to be less sensitive than the electrical effect. The figure
also shows evidence of trapped NOz in the film after bake
out, with absorptions near 1700 em -I and interaction of
NOz with the NaG substrate at 1360 em -I. The response to
ppm concentrations and a more detailed treatment of this
result are given in Ref. 19.
III. RESULTS AND DISCUSSION
Films of NiPc and PbPc were deposited from multiply
sublimed source material on Pyrex or alumina substrates.
Depositions onto alumina substrates gave coarse grainy lay
ers, evident in the SEM photograph shown in Fig. 3. Films
FlG. 3. SEM micrograph taken at 5000X of NiPc film deposited Ollto alu
mina.
1352 J. Appl. Phys., Vol. 65, No.3, 1 February 1989 deposited simultaneously onto Pyrex were much more uni
form, as shown in Fig. 4, Other workers 14 have found that
films vacuum deposited onto alumina substrates were dis~
continuous at thicknesses below 0.3 11m. Based on these two
observations and because we thought that very thin films
should be crucial to obtaining fast, sensitive detector re
sponse, work on alumina substrates was not pursued further.
Films of NiPc and PbPc used for gas sensor measurements
were deposited over the thickness range 0.02-0.3 11m, but
typical thicknesses were 0.15 pm. The layers were fragile and
easily penetrated by the profilometer stylus, but a thin metal
overlayer was sufficient to overcome this problem.
Immediately after deposition, in situ measurements of
thin-film surface sheet resistance prior to gas or ambient ex
posure are typically (1-4) X 1016 HID. The desired concen
tration ofN02 was obtained by successive dilution with air.
Several specimens were initially exposed to pure air and
showed no response.
The dependence of surface sheet resistance on gas con
centration for one NiPc sensor exposed to from 25 to 63 000
ppb N02 is given in Fig. 5. Similar results were obtained for
PbPc sensors, as shown in Fig. 6, but with greater response at
low concentrations. In this concentration range, the sensor
conductivity is easily returned to its initial value prior to gas
exposure by heating to 160"C under dynamic vacuum.
If one assumes that the conductivity change is linearly
related to the number of absorbed gas molecules on the MPc
surface, i.e.,
[u(P) -0'(0)] = k/J, (0
then the behavior shown in Figs. 5 and 6 can be described by
the Fruendlich adsorption isotherm, 12
e=k1pr, (2)
where e is the fraction of surface covered at some pressure P,
and kJ and yare constants. Also, (J(P) is the film conductiv
ity at some gas partial pressure P, 0'( 0) is the film conductiv
ity prior to gas exposure and 0' = P 1 = Up, ) -I, and p, p"
and t are the bulk resistivity, surface sheet resistance, and
thickness, respectively, of the film.
FIG. 4. SEM micrograph taken at 5000 X of NiPc film deposited onto glass.
T. A. Temofonte and K. F. Schoch 1352
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129.105.215.146 On: Fri, 19 Dec 2014 01:55:471016
1015
1014
1013 -
0
C
1012
'" "'-
lOll
l--
1010'
109
108
10° 106
FlG. 5. Variation of surface sheet resistance of NiPc sensors with NOz con
centration at room temperature.
1016
1015
1014
1013
1012
0 a
in
lOll 0.
1010
~
>-
1Q9 i
108
10° 105 106
FIG. 6. Variation of surface sheet resistance ofPbPc sensors with NOz COll
centration at room temperature.
1353 J. Appl. Phys .• Vol. 65, No.3, 1 February 1989 At fixed volume and temperature, the gas partial pres
sure is proportional to the gas concentration. The depen
dence of surface sheet resistance on gas concentration can
thus be written
(3)
where" is a constant, c is the gas concentration, and Ps (0) is
the value of surface sheet resistance prior to gas exposure,
i.e., Ps = Ps (0) at e = O. When !3er ~ 1, it follows that
logps = log[ps (O)//3l -r log c. (4)
Therefore, r is the slope of a plot of log Ps vs log c.
The equilibrium
N204~2 NOz
is well known to be extremely dependent on temperature and
concentration.2o,2J The equilibrium constant, however, un
der the conditions of these experiments shows that the gas
phase consists of approximately 98%-99% NOzo If a gas
molecule dissociates upon adsorption to n atoms, each occu
pying a separate site, and the adsorbed layer is immobile,
then ideally y = lIn.22 For nondissociative adsorption of a
single species, r = 1. However, the value of rfor NiPcis 1.9
based on the data shown in Fig. 5. The value ofrforthe PbPc
sensor response shown in Fig. 6 is 1.7. Data characterized by
such large slopes have also been reported12 for room-tem
perature measurements of needlelike single crystals of other
metal phthalocyanines, The physical significance of r> 1 is
not currently understood. However, two possible mecha
nisms which we speculate might give rise to r> 1 are (])
association of N02 molecules on the surface forming N204
(also an electron acceptor), and (2) reaction of N02 with
MPc sites in the bulk. Verification of the latter mechanisms
would require measurements of Ps at much smaller incre
ments of gas concentration than was done in the current
study.
The transient response characteristics for a PbPc sensor
measured at room temperature are shown in Fig. 7, where
the log of normalized sensor current versus log of time is
plotted for various concentrations. At 25-ppb NOz, the rise
time to 90% change is -90 s. At the same concentration, the
NiPc sensors gave a rise time of less than 60 s. Note that
these values include the time required for establishing equi
librium gas concentration within the exposure chamber.
Bott and Jones 14 have reported a 90% rise time of -90 s for
PbPc films measured at 170°C on exposure to 50-ppb NOz'
They also measured response time as a function of tempera
ture. If we assume that their data can be extrapolated to
room temperature, the response time of their sensor would
be expected to be ;;;, 3 X 104 s. At 1.2 ppm, the behavior shown
in Fig. 7 is similar to that reported by Wright and co
workers \3 for thin films of PbPc-exposed to 4-ppm NOz at
155°c' At the highest concentration sensed (60 ppm), there
is an initial fast rise « 60 s) followed by a slower approach
to equilibrium. Workers at Durham Englandl5 have report
ed similar results for the transient decay characteristics ofL
B films (eight monolayers) incorporating copper phthalo
cyanine exposed to 120-ppm N02 in N2•
The transient response of the PbPc detector shown in
Fig. 7 can be described by the Elovich equation 12
T. A. Temofonte and K. F. Schoch 1353
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129.105.215.146 On: Fri, 19 Dec 2014 01:55:47" -:: r_--r--,-,-.,.,--.--r-r,-,--,--,-",,--r'-60'pp'm-''''-' Tll
104
/
!O3 )'
/
/
/----1,2ppm
/ •
,/
/
,// ... ~--------25PPD
Hf .;'" .;
lOG L--'---'--L.W_'---'-.!-U._'--..I-1.--W.---'_L..J...,w
~ 0 ~
t. soc
FIG, 7, Transient response of a PbPc sensor at 2S-ppb, L2-ppm, and 60-
ppm NO, at room temperature,
de - = a exp( -be),
dt (5)
where a and b are time invariant. It is commonly observed
that the rate of adsorption of gases onto a variety of solid
surfaces can be described by this equation, 23
Integrating Eq, (5) once gives
8= (l/b)lnCt/to+ 1), (6)
where to = Cab) -1, Recall that () is assumed proportional to
the conductivity change, which in turn is proportional to the
change in sensor current, Figures 8 and 9 show the data from
700 , "" '1
1 """'7" 1 i II
600
500 --
o 400
200
100
o~~~~~~-'--~~~~~~~~~~~~
101 102 103 :05
t, sec
FIG, 8, Elovich plot a PbPc sensor for exposure to 1.2-ppm N02 at room
temperature,
1354 J, Appl. Phys" Vol. 65, No, 3,1 February 1989 700,000 -
600,000
500,000
~::~
2OO,ooor I
100 ~ LI_L--'...LJ-LU 'l:---'--~L.w!..LLL"L,-...J..........L.JI--'I.J.I.L1I1.i.l"-:----'--.LL1 --'-I ,WI !..uJ]
101 102 103 104 10'
t, sec
FIG, 9, Elovich plot a PbPc sensor for exposure to tiD-ppm N02 at room
temperature,
Fig, 7 (for response of PbPc to 1,2-and 60-ppm N02 in air,
respectively) replotted in terms of normalized sensor cur
rent versus log time, As expected from Eq, (6), the current
increases slowly at first for t small compared to to, then in
creases linearly with log t for t ~ to'
There is also considerable interest in the detection of
chemical warfare (CW) agents, Since the organic com
pound dimethyl methylphosphonate (DMMP) is chemical
ly similar to and exhibits similar adsorption onto charcoal as
organophosphorus CW agents, it has been considered to be a
simulant for these agents. A simple experiment was thus
done to test the response of NiPc to DMMP. Detection of 6, 7
ppb at room temperature was observed, Subsequent heat
treatment (as was done for N02 exposure) returned the sen
sor sheet resistance to approximately the value measured
prior to exposure. Further experiments are clearly indicated,
The reason for the greater sensitivity of the present films
to NOz at room temperature can be understood by examina
tion of the crystallite structure of the films.16,24 If the sub
strate temperature is between 5 and 15°C during film depo
sition, the crystallites form with the stacking axis directed
away from the substrate surface, the subsequent heating will
not change the structure,24 Other workers have studied the
electrical properties of films having this structure,13 If the
substrate temperature is higher during deposition, as with
the present films, the stacking axis is parallel to the substrate
surface,16,24 Figure 10 shows TEM and electron diffraction
patterns of one of our NiPc thin films deposited at 16O·C
under the same conditions as the optical and electrical sensor
samples, The stacking axis (coincident with the needle axis
in this case) is parallel to the growth surface, and the spacing
between lattice lines is 12 A, as shown in the figure. A more
detailed treatment of these results is given in Ref, 16, It is
well known that the conductivity in iodine-doped MPc com
pounds is approximately 100 times higher parallel to the
stacking direction than perpendicular to the stacking direc
tion, I Thus, measuring the sheet resistance of a film the
stacking axes of which are parallel to the substrate will pro
duce a much more sensitive N02 detector than measuring
T, A, Temofonte and K, F, Schoch 1354
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129.105.215.146 On: Fri, 19 Dec 2014 01:55:47{i,OOOX 2,OOO.OOOX
FIG. 10. TEM and electron diffraction pattern of a NiPc thin film deposited
at 160 'c.
across the stacking axes, which occurs in Iilms deposited on
cooled substrates.
IV. CONCLUSION
Organic semiconductor thin-film (MPc) gas sensor
having ultrahigh sensitivity (0;,;;25 ppb) and very fast re
sponse time (0;,;;60 s) have been demonstrated. The vacuum
deposited films used in this study were significantly thinner
than those reported by other workers. Results presented ill
this report were obtained at room temperature, whereas all
other results regarding sensitivity and transient characteris
tics for vacuum-deposited thin films have been obtained at
elevated temperatures (150-170 °C). Also, detection of
DMMP at room temperature is a result which should be
studied further. Note that the active layers used in our sen
sors have not been optimized. The device structures onto
which the films were deposited were passive structures simi
lar to those reported by other workers.
Future work to optimize sensor characteristics is needed
and low-noise, Ie-compatible FET device structures have
been designed and fabricated for this purpose. Since the de
position conditions for our thin films arc completely compa
tible with such structures, smart multisensor arrays for toxic
gas detection are feasible.
1355 J. Appl. Phys., Vol. 65, No.3, 1 February 1989
--.-.-••• '.~ ." n ••• ",""n ••• " .~>........ . " ... -.--;-;-.-.· .... v.· •• -.;.; •• o;o;O;'~'.'-••• '.'.'.'.'.';~.";,,.~.'" .w>, ._ •• y •••••• Sensors based on optical detection of N02 are currently
being evaluated, and detection in the hundreds of parts per
million has already been demonstrated.
Finally, we believe that the unique electrical properties
of these films are a result of the film morphology associated
with our specific deposition approach.
ACKNOWLEDGMENTS
The authors are pleased to acknowledge G. Kostyak, K.
Pfeiffer, M. Testa, and J. Bronner for substrate preparation;
J. Greggi and W. Hughes for TEM and electron diffraction
characterization; J. Szedon for useful discussions; and D.
Smoody, G. Law, K. Mercalde, and K. Haun for assistance
in manuscript preparation.
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19K. 1'. Schoch, JT. and T. A. Temofonte, Thin Solid Films (in press).
2°F. II. Vcrhoek and F. Daniels, J. Am. Chern. Soc. 53,1250 (1931).
2tW. F. Giauque and J. D. Kemp, 1. Chern. Phys. 6, 40 (1938).
"J. Szekely. J. W. Evans, and H. y'. Sohll, Gas-Solid Reactions (Academic,
New York, 1976), p. 39.
Be. Aharoni and P. e. Tompkins, Adv. Catal. 21, 1 (1970).
2.p. Vincctt, Z. D. Popovic, and L. McIntyre, Thin Solid Films 82, 357
(1981).
T. A. Temofonte and K. F. Schoch 1355
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1.341038.pdf | Fieldeffect transistor using a solid electrolyte as a new oxygen sensor
Yuji Miyahara, Keiji Tsukada, and Hiroyuki Miyagi
Citation: Journal of Applied Physics 63, 2431 (1988); doi: 10.1063/1.341038
View online: http://dx.doi.org/10.1063/1.341038
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/63/7?ver=pdfcov
Published by the AIP Publishing
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147.188.128.74 On: Mon, 13 Oct 2014 20:57:16Field .. effect transistor using a solid electrolyte as a new oxygen sensor
Yuji Miyahara, Keiji Tsukada, and Hiroyuki Miyagi
Central Research Laboratory, Hitachi Ltd., 1-280, Higashikoigakubo, Kokubunji. Tokyo 185, Japan
(Received 27 April 1987; accepted for publication 13 November 1987)
A field-effect transistor (FET) using a solid electrolyte is proposed in the present study as a
new oxygen sensor. The sensor is fabricated by depositing a thin layer ofyttria-stabilized
zirconia (YSZ) on a gate insulator of an insulated gate field-effect transistor (lGFET). As an
IGFET has an ability to transform impedance, the potential change produced at the interface
between the YSZ layer and a platinum gate electrode can be detected stably, even if the
impedance of the YSZ is very high. The response of the fabricated sensor showed good
reproducibility at 20 "c. A linear relationship between output voltage and logarithmic partial
pressure of oxygen was obtained in the range from 0.01 to 1 atm. Sensitivity of the sensor was
found to depend on the thickness of the Pt-gate electrode and sputtering conditions of the YSZ
layer. Although selectivity to hydrogen and carbon monoxide was not good at room
temperature, it could be improved by increasing the operating temperature to 100"C. The
developed sensor has several advantages induding small size, low output impedance, and solid
state construction. It is potentially applicable to medical uses, process control, and
automobiles.
I. INTRODUCTION
There is increasing interest in miniaturized and multi
functional chemical sensors for rapid analyses using sman
samples and/or on-line measurements. For example, minia
turized Clark-type oxygen sensors have been fabricated us
ing integrated circuit technologies, especially for use in the
medica! field.l--4 However, these sensors have complicated
fabrication processes, because they generally need electro
lyte solutions between the cathode and anode to operate.
On the other hand, zirconia oxygen sensors have been
utilized in automobiles and the steel manufacturing indus
try. These sensors can be operated only at elevated tempera
tures, i.e., higher than 500 ·C, because the resistivity of zir
conia is too high for stable operation at lower temperatures.
Although a thin-film oxygen sensor using sputtered calcia~
stabilized zirconia has been reported operable at a lower
temperature than the conventional zirconia oxygen sensor,
Le., at about 300 "C,5 there are no reports of zirconia oxygen
sensors which can be operated at around room temperature.
This study proposes a new solid-state oxygen sensor,
operable at room temperature. It is fabricated by depositing
a thin layer of solid electrolyte on a gate insulator of an insu
lated gate field-effect transistor (I G FET). In this paper, this
sensor is referred as a FET~type oxygen senser. Since an
I GFET transforms an input signal with high impedance into
an output signal with low impedance, the FET -type oxygen
sensor is operable at room temperature, even if solid electro
lytes with high resistivity, like zirconia, are used. Funda
mental characteristics of the PET -type oxygen sensor with a
zirconia solid electrolyte are described in this paper.
iI. EXPERIMENT
The cross section and the layout of the FET -type oxygen
sensor are shown in Figs. 1 (a) and 1 (b), respectively. Eight
PETs are integrated in a 5 X 5 mm2 square chip. The sensor is
an n-channel depletion-mode PET, having a channel 40 pm long and 1600 pill wide. The gate insulator consists of a 60-
nrn-thick layer of Si02 covered by a 94-nm-thick layer of
Si3N4• On the SiJN4 layer, a 200-um-thick layer of yttria
stabilized zirconia (YSZ) was deposited. The gate electrode
is about a lO~nm-thick layer of platinum which shows cata
lytic activity towards oxygen dissociation.
The starting material was p-type silicon having a (100)
orientation with 9-12 .n em resistivity. The gate oxide was
thermally grown to a thickness of 60 nm in dry oxygen at
1000 ·C. The source and drain were formed by an arsenic
implantation at 80 keY with a dose ofS X 1015 em -2. Silicon
nitride was deposited on the gate oxide by low-pressure
chemical vapor deposition to a thickness of 94 urn. After
etching of the source and drain contact hoies, the source and
drain contacts were made by evaporating aluminum silicon
Pt 10 nm
'.:.;<:::.:':.'?:'~:/;:;:':'~'.~.'\::':":';~'::'?'{'.:::: YSZ 200 nm
1-----------1 SitN4 94 nm
1------.,---- .... --.... SiOz GO nm ~ .~
p-51
Is)
AI pad
{bl --1tnm FIG. 1. Structure of the
FET-type oxygen sensor
incorporating an yttria-sta
biIized zirconia (YSZ) thin
layer: (a) cross section; (b)
layout.
2431 J. AppL Phys. 63 (7), 1 April 1968 0021-8979/88/072431-04$02.40 © 1988 American Institute of PhYSics 2431
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147.188.128.74 On: Mon, 13 Oct 2014 20:57:16(1 %) to a thickness of 900 nm.The wafer was then annealed
at 450 DC in hydrogen atmosphere for 30 min.
YSZ was deposited on the silicon nitride layer by rf sput
tering through a mask in a plasma atmosphere of 50% 02-Ar
mixture at a total pressure of 6.6 Pa using the sintered YSZ
target whose composition was 8 mol % Y 203 and 92 mol %
Zr02• The substrate temperature was about 70 DC during
sputtering. The structure of the sputtered YSZ layer was
studied by reflective high-energy electron diffraction. From
the pattern of reflective high-energy electron diffraction, the
YSZ layer deposited on the Si3N4 layer was found to be a
cubic Zr02 structure with (110) orientation.
The platinum electrode was deposited by rf sputtering
through a mask in a plasma atmosphere of pure argon at a
pressure of 6.6 Pa and at ambient temperature.
Conceptually, the potential at the Pt-YSZ interface
changes foHowing a change in partial pressure of oxygen and
it results in an equivalent gate voltage change of the FET.
This in turn causes a change in the drain current of the FET
which is to be measured. But it is better to detect a potential
change at the Pt-YSZ interface directly in order to analyze
the response mechanism. In this study, the responses of the
FET-type oxygen sensor were measured using the circuit
shown in Fig. 2, which has been used to measure sensitivity
of an ion-sensitive field-effect transistor (ISFET).6 The po
tential change at the Pt-YSZ interface, that is, the change of
the threshold voltage, could be read out directly at a con
stant drain current. Selectivity of the FET -type oxygen sen
sor was measured using the system shown in Fig. 3. The FET
sensor was mounted in a flow cell having a dead volume of
approximately 50 pI. The flow ceU was set in an air oven for
which temperature could be controlled in the range from
room temperature to about 400 0c. The carrier gas was ni
trogen, which contained 1-ppm oxygen as an impurity, flow
ing at 70 ml/min. One ml of various kinds of gases was inject
ed into the carrier gas by a syringe and carried through a
stainless tube to the flow celio All the gases used in the selec
tivity experiment were gases (99.9% purity) commonly
used for gas chromatography. Oxygen content in these gases
was less than 0.02%.
iii. RESULTS AND DISCUSSION
The FET -type oxygen sensor was exposed to ] -atm oxy
gen and nitrogen in turn. A typical time response curve of
the sensor is shown in Fig. 4. This is good reproducibility in
the repeated stepwise change in a partial pressure of oxygen.
The time course is composed of rapid response with a time
2432 FIG. 2. Schematic diagram
of the measuring circuit.
J. Appl. Phys., Vol. 63, No.7, 1 April 1988 Syringe
Sample
Carrier
Gas ____ ~ __ _rT-~~~
Nz
Air Oven Pt Resistor
--1
: Exhaust
Sensor
FIG. 3. Experimental setup used for the selectivity measurement.
constant of about 30 s and a subsequent slow drift. The out
put voltage reaches a steady state 40-60 min after the partial
pressure of oxygen is changed. The slow response might be
due to diffusion and/or drift of the charged species, i.e., 02-
ions or electrons in the YSZ layer. The threshold voltage of
the FET increases in the presence of oxygen and when oxy
gen is replaced by nitrogen, it decreases to the original level.
Threshold voltage of an n-channel metal-insulator-sem
iconductor field-effect transistor (MISFET) is written as?
VT = ¢1ms -(Q,IC 1) + 2t/;B + [~4€sqN---;'-(2lP~) leI] ,
(1)
where V T is the threshold voltage, ¢ms is the work function
difference between the gate metal and silicon substrate, QT is
the sum of the effective net oxide charge per unit area, C[ is
the gate capacitance per unit area, tPo is the potential differ
ence between the Fermi level and the intrinsic Fermi level, Es
is the permittivity of silicon, q is the elementary charge, and
NA is the density of the acceptors. The values of the third and
the last terms in Eq. (1) are not changed by a change in
oxygen partial pressure. Accordingly, the change in the
threshold voltage of the FET-type oxygen sensor is caused
by a change in the first or the second term in Eq. (1). A
hydrogen-sensitive Pd-or Pt-gate metai-oxide-semiconduc
tor field-effect transistor (MOSFET) whose threshold vol
tage decreases in the presence of hydrogen has been reported
to be sensitive to oxygen in the presence ofhydrogen.8•1! This
phenomenon is explained by a change in the work function
difference as a result of production of water by the reaction
between adsorbed hydrogen atoms and oxygen molecules. In
the case of the FET -type oxygen sensor, sensitivity to oxygen
5 10 15 20 25
TIME [mill]
FIG. 4. Time response of the FET -type oxygen sensor measured at 20 "C.
Miyahara, Tsukada, and Miyagi 2432
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147.188.128.74 On: Mon, 13 Oct 2014 20:57:1614
! 12 THEORETICAL VALUE
\!5 z 10 14,5 mY/decade « :;;: u
~t Lt.I
<!) FIG, 5. Calibration curve
;:! of the PET -type oxygen -' g 6,2 mY/decade sensor at 20 "C.
>-::J 20 "C f!: B
o 0.01 0.1
PARTIAL PRESSURE OF OXYGEN [atm]
can be obtained in the absence of hydrogen. The response
mechanism of the Pt/YSZ~MIS system seems to differ from
that of the Pd-or Pt-MOS system. According to the model
proposed for the conventional zirconia oxygen sensor, the
following reaction occurs at the gas-Pt-YSZ three-phase in
terface12:
(2)
Based on this reacti.on, the potential at the Pt-YSZ inter
face changes according to the Nernst equation below:
<Pi = const -(RT/4F)ln P02 ' (3)
where ¢lj is the interface potential, Fis Faraday's constant, R
is the gas constant, T is absolute temperature, and Po, is
partial pressure of oxygen.
The potential change serves as an equivalent threshold
voltage change cfthe FET. As a result, the threshold voltage
V T of an n-channei FET -type oxygen sensor is written as
V1,=Vr-¢;" (4)
where VT is the threshold voltage in the nitrogen atmo
sphere.
A calibration curve of the FET-type oxygen sensor is
shown in Fig. 5. The linear relationship between the poten
tial change and the logarithmic partial pressure of oxygen is
obtained in the range from 0.01 to 1 atm. The slope of the
experimental curve is 6.2 m V /decade, which i.s smaller than
the theoretical value calculated from the Nernst equation.
The results of our preliminary experiment show that the sen
sitivity of the sensor can be increased by changing the sput
tering condition ofYSZ. Accordingly, the reason for the low
sensitivity of the sensor might be due to non optimized phys
ical and chemical characteristics of the YSZ layer.
The sensitivity of the FET -type oxygen sensor depends
on the thickness of the Pt-gate electrode as shown in Fig. 6.
The sensitivity becomes very poor as the Pt-gate electrode
becomes thicker. The sensor with a Pt gate thicker than 50
nm shows hardly any response to changes in partial pressure
of oxygen. This may be because the gas-Pt-YSZ three-phase
interface becomes too small in a thick Pt layer to bring about
efficient oxygen dissociation as shown in Eq. (2), On the
other hand, the output voltage of the sensor having about a
5-nm thick Pt layer is unstable. It appears that the 5-nm
thick Pt layer is too thin to make electrical contact as a gate
electrode of the FET sensor. From the above results, the gas
Pt-YSZ three-phase interface is essential for opera.tion of the
2433 J. Appl. Phys., Vol. 63, No, 7,1 April 1988 ., 7,
'8 61 0 .,
VSZ 200nm ~
:r. 'E
>->-3t 20 ·C :: ;-
iii :r z w
!Jj L
0 20 lJ:) 60 80 100
THICKNESS OF PI [nmJ
FIG, 6 Effect of the thickness of the Pt-gate electrode on the sensitivity of
the FET -type oxygen sensor.
FET -type oxygen sensor and the optimum thickness of the
Pt-gate electrode is found to be about 10 nm.
Selectivity tests of the FET-type oxygen sensor were
carried out while changing the temperature of the device
from 22 to 100 ·C. Using the system and the method as de
scribed before, the obtained responses of the sensor are not
steady state, but transient. The temperature dependence of
peak h~~ights for various kinds of gases tested in the present
study is shown in Fig. 7, The sensor responds not only to
oxygen but also to hydrogen and carbon monoxide, and
slightly to nitrous oxide at 22 "c. The threshold voltage of
the FET decreases on exposing the Pt gate to hydrogen,
while it shifts in the positive direction in the presence of
carbon monoxide or nitrous oxide. The peak height for oxy
gen is about half as high as that for hydrogen and about three
times as high as that for carbon monoxide at 22 ·C. The peak
height f.ar nitrous oxide is so small that it cannot be distin
guished from the fined circles in Fig. 7. The peak heights for
those gases increase with operating temperature. Besides,
the response to ethylene in the negative direction is observed
at 50 ·C and increases with temperature.
As JJd-MIS hydrogen sensors with Si3N4, A1203, Ta20S
between Pd and SiOz layers are known to be sensitive to
He 300 Ar
CO2
> +--+ N20 O2 E 200 eM,
I-C2H6
:r: C3HS
I.!) ~ w 100
I
::.:: TEMPERATURE CoC J
<I: 0 W n. 100
-100 ~
-200~
PI G. 7. Temperature dependence of peak heights for various kinds of gases,
Miyahara, Tsukada. and Miyagi 2433
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147.188.128.74 On: Mon, 13 Oct 2014 20:57:16hydrogen,13 it is considered that the process occurring in the
PtIYSZ- MIS system in response to hydrogen may be similar
to that in the Pd-MIS system, that is, formation of a dipole
layer at the Pt-YSZ interface giving a negative shift of the
threshold voltage. The positive shift of the threshold voltage
in response to carbon monoxide is in accordance with the
observations by Krey, Dobos, and Zimmerl4 and Dobos,
Strotman, and Zimmer, 15 who used the Pd-MOSFET with a
hole-structure gate. Although they explained the response in
terms of a change in the work function difference, details of
the mechanism are unclear and investigation is continuing.
It is known that chemisorbed ethylene on the Pt surface is
decomposed in the temperature range from 290 to 500 K,
resulting in production of both ethylidyne and hydrogen,
while only chemisorption of ethylene on the Pt surface oc
curs in the temperature range from 100 to 290 K. 16---18 It is
the hydrogen produced that shifts the threshold voltage in
the negative direction above 50 0c, The sensor shows no re
sponse to any other gases tested in this study.
IV. CONCLUSION
An FET -type oxygen sensor incorporating a zirconia
solid electrolyte has been proposed in this paper. By using a
FET structure, the potential change produced at the Pt-YSZ
interface could be measured at room temperature. At pres
ent, the sensor shows a slow drift at room temperature when
partial pressure of oxygen is changed. Further investigations
on the mechanism occurring at the Pt-YSZ interface and in
the YSZ layer are necessary to improve the characteristics of
the FET-type oxygen sensor.
The proposed FET -type oxygen sensor has potential ad
vantages over conventional oxygen sensors due to its small
size, low output impedance, and solid-state construction.
Additionally, it can be integrated with other semiconductor
sensors such as ISFETs and signal processing circuits on one
chip. The FET -type oxygen sensor might be useful as a trans
ducer in biosensors instead of the conventional Clark-type
2434 J. Appl. Phys., Vol. 63, No.7, 1 April j 988 oxygen electrode. The proposed sensor is expected to be ap
plicable in several fields including medicine, process control,
and automobiles.
ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid from the
Ministry of International Trade and Industry of Japan.
'G. Eden, G. I Inbar, 1. Timor-Tritch, and H. 1. Bicher, IEEE Trans.
Biomed. Eng. BME-22, 275 (l975).
2W. Siu and R. S. C. Cobbold, Med. BioI. Eng. March, 109 (1976).
3M. Esashi, J. Kouzu, and T. Matsuo, Jpn. J. Med. Electron. BioI. Eng. 18,
966 (1980).
'Y. Miyahara, F. Matsu, S. Shiokawa, T. Moriizumi, H. Matsuoka, I. Kar
ube, and S. Suzuki, in Proceedings of the 3rd Sensor Symposium (The Insti
tute of Electrical Engineers of Japan, 1983), p. 21.
5M. Croset, P. Sch.'lell, G. Velasco, and J. Siejka, J. Vac. Sci. Techno!. 14,
777 (977).
6H. Nakajima, M. Esashi, and T. Matsuo, Nippon Kagaku Kaishi No. 10,
1499 (1980).
7S, M. Sze, Physics of Semiconductor Devices, 2nd ed. (Wiley, New York,
1981).
BI. Lundstrom, S. Shivaraman, C. Sevensson, and L. Lundkvist, Appl.
Phys. Lett. 26,55 (1975).
"K. L Lundstrom, S. Shivaraman, and Co M. Sevensson, J. AppL Phys. 46,
3876 (1975).
101. Lundstrom, Sensors and Actuators 1, 403 (1981).
I'M. Armgarth, D. Soderberg, and 1. Lundstrom, App!. Phys. Lett. 41, 654
(1982).
12J. Fouletier, P. Fabry, and M. Kleitz, J. Electrochem. Soc. 123, 204
( 1976),
13K. Dobos, M. Anngarth, G. Zimmer, and I. Lundstrom, IEEE Trans.
Electron Devices ED·31, 508 (1984).
14D. Krey, K. Dobos, andG. Zimmer, SensorsandActuators3,169 (1982/
83).
15K. Dobos, R. Stratman, and G. Zimmer, Sensors and Actuators 4, 593
(1983).
!6H. Ibach and S. Lehwald, J. Vae. Sci. Techno!. 15,407 (1978).
17L. L Kesrnodel, L. H. Dubois, and G. A. Somorjai, J. Chern. Phys. 70,
2180 (1979).
lSA. M. Baro and H. Ibach, J. Chern. Phys. 74,4194 (l98!).
Miyahara, Tsukada, and Miyagi 2434
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1.344205.pdf | Development and quality measurements of cold relativistic electron beam for lowγ free
electron lasers
M. Kawai, Y. Kawamura, and K. Toyoda
Citation: Journal of Applied Physics 66, 2789 (1989); doi: 10.1063/1.344205
View online: http://dx.doi.org/10.1063/1.344205
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/7?ver=pdfcov
Published by the AIP Publishing
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Beam quality and emittance in freeelectron lasers
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J. Appl. Phys. 70, 517 (1991); 10.1063/1.350265
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130.239.20.174 On: Mon, 24 Nov 2014 11:43:35Development and quaUty measurements of cold relativistic eiectron beam for
low~'Y free~electron lasers
M. Kawai
Department of Physics, Faculty 0/ Science, Takai University, Hiratsuka-shi, Kanagawa 259-12, Japan
Y. Kawamura and K. Toyoda
Riken, The lnstiiute a/Physical and Chemical Research, Wako-s/ll; Saitama 351, Japan
(Received 7 March 1988; accepted for publication 8 June 1989)
A relativistic electron beam source with low temperature (cold) using a field emission cathode
and uniform electrostatic acceleration has been developed for use i.n low-y free-electron lasers.
An energy of 0.51 MeV and a current of 60 A (200 A/cmz) were obtained. The energy spread
and the angular velocity spread were measured to be IlE / E = 0.14% and (3i/PII = 4 X 10-2,
respectively.
I. INTRODUCTION
An intense relativistic electron beam (IREB) which is
energized by a Marx generator provides an efficient laser
gain in Iow-y free-electron lasers (low-y FELs). 1-4 For
further improvement of the low-y FEL operation, the devel
opment of the relativistic electron beams with small energy
spread and high current density is necessary. The FEL oscil
lation also requires a beam duration with a many times long
er than round trip time oflight in a cavity. Previous attempts
to generate the relativistic electron beam and measure the
beam quality have been reported,S-lo but these electron
beam durations were short (10-100 ns) and not long enough
for the cavity oscillation in the FEL.
Intense relativistic electron beam (IREB) can produce
a very high current electron beam. However, the energy
spread of a conventional IREB is very large, which is mea
sured to be 6%.11 Recently, low emittance IREBs with im
proved diode structures are reported. Single anode diodes
with thermionic cathodes have generated long pulse electron
beams (;;;.1 Its) and beam currents of up to :::::: 10 A at a
voltage approaching 0.5 MV, but at higher voltages oper
ation of the single anode diode is difficult due to the break
down between the cathode and the anode. Practically speak
ing, the thermionic cathode requires a heater power supply
insulated from the earth potential, and operation in high
vacuum « 10--0 Torr) to protect from damage by the resid
ual gas. Relativistic photoelectrons (RPE) have a very small
energy spread, which is of the order of the work function of
the photocathode materials. 12 (The work function of several
common pure metals is 2-5 e V.) The current density of the
photoemission was 0.5 A/cm2, which was limited by the
quantum efficiencies of photoemission and also the photon
flux densities on the photocathode surface. 13 Furthermore,
in case of photoemission, the pulse width of the electron
beam is determined by a pulse duration of the irradiated
laser beam ( < 30 ns) .
In this paper, we show the generation of a cold relativis
tic electron beam (CREB) having a relatively small energy
spread and a small angular velocity spread using a field emis
sion cathode and uniform electrostatic acceleration. This
type of electron beam source has several advantages. The
energy spread and the angular velocity spread at high cur-rents are extremely small compared with the conventional
IREB diodes. Long duration electron beam currents are ob
tained. The high-voltage components of the cathode may be
extremely simple (even dc) compared with the thermionic
cathode components.
Moreover, the method to measure the energy spread
and the angular velocity spread of the CREB are presented.
The measurements of the electron beam energy spread l:!.E /
E were carried out using a magnetic-deflection-type energy
analyzer, II where E = (y -1 )moc2, y is the relativistic fac
tor, and moc2 is the rest mass energy of an electron. The
angular velocity spread/.11/f3
11 was measured using an angu
lar velocity probe, 12.14 where f3 c and (3!1 are the transverse
and parallel component of the electron velocity, respective
ly. The method of measurement is based on measuring the
Larmor radius, which corresponds to the transverse compo
nent of the electron velocity /.1 ! .
II. DIODE STRUCTURE AND ACCELERATING TUBE
FOR CREB GENERATION
The experimental apparatus for the generation of the
CREB is shown in Fig. 1. A Marx generator is used to apply
a high voltage to a diode as well as an accelerating tube. The
electron beam is extracted from the diode of the graphite-tip
cathode. The cathode-anode spacing, the cathode tip diame
ter, and the inner diameter of the anode aperture were 7, 6,
and 6 mm, respectively. The accelerating tube 32 cm long
consists of eight gradient rings with an aperture of 20 mm in
diameter. These rings are applied equally, dividing poten
tials through the discharge resistors in order to obtain a uni
form electric field. A magnetic field of 1.6 kG is applied in
the accelerating tube by a pulsed solenoid coil to guide the
electron beam along its axis. The spatial distribution of the
guide magnetic' field is shown in the lower portion of Fig. 1.
Details of the diode and the accelerating tube are shown in
Fig. 2. The total value of the discharge resistors was 5 kH and
the total capacity of the Marx generator was 2.3 nF. The
calculated time constant of the discharge voltage decay was
about 11.5 f1s. Hence, a relatively long pulse duration 0[200
ns was obtained within a 1 % decrease in the maximum vol
tage. The dumping resistors were connected in series
2789 J. AppL Phys. 66 (7), 1 October 1989 0021-8979/89/192789-05$02.40 @) 1989 American Institute of Physics 2789
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130.239.20.174 On: Mon, 24 Nov 2014 11:43:35Anguiar Votloci!y
Probot
~ 1.0l
.d
"-III
!ON O.S !;... ...... --'I...---_--~
Z(em)
between each capacitor module in the Marx generator to
minimize the ripple of the discharge voltage waveform. A
self-integrating magnetic probe mounted at the beam exit
was used to measure the electron beam current. The acceler
ating voltage was measured by a copper-sulfate resistive vol
tage divider. Typical output waveforms of the CREB cur
rent and the accelerating voltage are shown in Fig. 3. The
maximum accelerating voltage and the maximum CREE
current were 0.51 MV and 60 A, respectively. Damage pat
terns by RADCOLOR film 15 indicated that the profile of the
electron beam was a solid core of approximately 6 mm in
diameter. Hence, the electron beam current density is calcu
lated to be approximately 200 A/cm2•
III. BEAM QUALITY MEASUREMENTS
The electron beam was qualified by measuring the ener
gy spread and angular velocity spread. The energy spread
b.E / E using a magnetic-deflection-type energy analyzer and
the angular velocity spread f31 IfJ
11 using an angular velocity
probe are described in detail below. The location ofthe mag
netic-deflection-type energy analyzer and the angular veloc
ity probe are shown in Fig. 1. These measurements are per
turbing diagnostic, but a high degree of accuracy can be
expected.
Gradient ring
\
.--._.- _._._--- -.-----_._. _._.-
! l' ! o 1 2 3 4 S(em)
FIG. 2. A detail of the diode region and the reaccelerating tube.
2790 J. Appl. Phys .• Vol. 66, No.7, 1 October 1989 MAgnetic [)ofl..: lion TyplP
Enotrgy An .. ll'nr
A. Energy spread FIG. L Experimental arrangement showing
the CREB source and the location of the mag
netic-deflection-type energy analyzer and the
angular velocity probe .
The magnetic-deflection-type energy analyzer used in
this experiment had a 1800 deflection with a Larmor radius
Ro of 10 cm in a homogeneous transverse magnetic field. A
schematic of the measurement system is shown in Fig. 4. The
electron beam was allowed to pass through the entrance slit
having a width of S = 0.36 mm. A thin tungsten wire with a
diameter of d; = 0.1 mm was used as an electron collector.
After deflection by the magnetic field, the beam was focused
on an electron collector wire. In order to minimize the effect
of secondary electrons with the scatter from the back of the
electron collector by an incidence electron beam, a graphite
beam dump was placed behind the electron collector wire.
The resolving power ofthe magnetic-deflection-type en
ergy analyzer is given by f:.So/2Ro• where 6So is the full
image width of the electron beam after half a revolution. !::..So
is expressed by f:.So = (S + ¢) + 2Ro (1 -cos 8), where 8
is the divergence angle at the entrance slit and S and if; are the
width of the entrance slit and the diameter of the collector
wire, respectively. Hence, the energy resolution !::..Ep I E() is
given by
/:;,Ep = YI-Yo =(1 +.l)[(b.S o)+..!.(b.SO)2J', (1)
Eo Yo -1 Yo 2Ro 2 2Ro
where !::..Ep is the energy spread due to the finite energy reso
lution, Yo and Eo are the relativistic factor of electron and the
a)
b)
FIG. 3. Typical oscilloscope traces of (a) CREB current and (b) accelerat
ing voltage.
Kawai, Kawamura. and Toyoda 2790
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130.239.20.174 On: Mon, 24 Nov 2014 11:43:35Guid Coil Electron Collector
(0.1" W wire)
~-Beam
(graphite)
Slit(widlh O.4mm
FIG, 4. Schematic of the magnetic-deflection-type energy analyzer.
electron beam energy which correspond to the Larmor radi
us Ro. The factor YI is the relativistic factor of the electron
which corresponds to the Larmor radius Ro ~-t:.Sj2. In
case of b.Sol2Ro4!..1 (in this experiment, b.Sol
2Rn = 2.3 X 10-3), the first-order approximation is given:
t:.b~ Y! -Yn --=-'--"--":""="
En Yo -1 (1 + 1.) !.J.So
Yo 2Ro' (2)
In this experiment, the divergence angle of the electron beam
at the entrance slit is so small that !.J.So is considered to be
6.So;::::; (S + ¢). Because in the region of no guide magnetic
field at the end of the guide coil, the electrons having large
transverse velocity components are not allowed to pass
through the entrance slit. For the observed energy Eo = 500
keY and the effective slits width b.So = 0.46 mm, the energy
resolution is calculated to be (tiEpl Eo) = 0.35%. The reso
lution of the energy analyzer was tested with a monochro
matic dc electron beam having an accelerating voltage of 10
kV and a beam current of 0.3 rnA. The measured energy
resolution was in good agreement with the theoretical value
from Eq. (2); the absolute value of the measured electron
beam energy also corresponded to the applied accelerating
voltage. In the low-energy region, the energy analyzer was
tested but the result will be applicable to the relativistic ener
gy region. Figures 5(a), 5(b), and 5(c) show the time his
tory of the electron beam current, the accelerating voltage,
and the electron collector signal obtained for the observation
energy Eo = 499.5 keY, respectively. The electron collector
signal shown in Fig. 5 (c) has two pulses which correspond
to rising and falling periods of the voltage. The first pulse
was ignored since the accelerating voltage change is very fast
in this period. The center of the second pulse coincides exact
ly with the instantaneous voltage corresponding to Eo.
The observed pulse width is proportional to the energy
spread I1E and inversely proportional to the change of the
accelerating voltage dV /dt. If the accelerating voltage
changes monotonously, the energy spread !.J.E is given by
dV b.E = I:o --I1E dt p' (3)
where Tr.) and dV Idt are the observed pulse width and the
2791 J. Appl. Phys., Vol. 66, No.7, 1 October 1989 a)
b)
c)
-000I---I-
200 nsec
FIG, 5. Typical oscilloscope traces of (a) CREB current, (b) accelerating
voltage, and (c) the electron beam collector signal obtained for observation
energy.
time change in the accelerating voltage waveform. In this
case, the actual energy spread is given by subtracting the
instrumental resolution t:.Ep. The analysis was done for the
second pulses. The observed pulse width of an average of
eight shots was To = 72 ns. The time change in the accelerat
ing voltages at the second pulses is 34 V Ins. Using these
values, the energy spread was calculated to be b.E I
E=O.14 ± 0.04%. In this experiment, the slit structure of
the entrance terminating the drift tube provides a boundary
condition which will act to counter most of the beam space
charge effects. Hence, the experimental value will not in
clude the energy spread due to the potential depression of the
space charge, as discussed below.
B. Angular velocity spread
The schematic of the angular velocity distribution probe
is shown in Fig. 6. The method of measurement is based on
B -
,V,
L __ ~_'~
FIG. 6, Schematic of the angular velocity distribution probe.
Kawai, Kawamura, and Toyoda 2791
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130.239.20.174 On: Mon, 24 Nov 2014 11:43:35measuring the Larmor radius which corresponds to the
transverse component of the electron velocity fJ l' The angu
lar velocity spread is given by the ratio ofthe drain current to
the collector current. The drain current is proportional to
the Larmor radius of electrons gyrating in a uniform axial
magnetic field and the collector current is the remainder of
the electron beam which is not caught by the drain electrode.
These are assumed that (1) the electron beam density is
approximately uniform, (2) the alignment of the magnetic
field axis and the probe axis are concentric, (3) the Larmor
radius is much less than the radius of the drain electrode, and
( 4) the phase of the electron cyclotron orbits is mixed. The
relation between the Larmor radius of the electrons and the
ratio of the currents is given by
IclUe + In) = (rD -rlYlrt, rD~rU (4)
where Ie, In, and rD are the collector current, the drain
current, and the radius of the drain electrode, respectively.
rL is the Larmor radius due to the electron gyrating in the
axial magnetic field with the transverse velocity component
{3 J. which is given by
(5)
where B, elmo and yare the magnetic flux density on the
axis, the charge mass ratio of electrons, and the relativistic
factor, respectively. In order to collect all the drain current
by the drain electrode, the electron cyclotron pitch Pmust be
smaller than the length ofthe drain electrode (L = 4.0 em).
In the case of P<,L, IJ)' and Ie are given by
If) = iD,
Ie = ie, (6a)
(6b)
where ic and in are the measured currents by the collector
and the drain electrode, respectively. In case of P>L, it is
necessary to make a correction by using a factor of K that is
the ratio of the electron cyclotron pitch to the length of the
drain electrode. If) and Ie are given by
Ie = ( -(ID -iv)·
Here, K is given by (7a)
(7b)
K=PIL={3II/(LfJc), (8)
where Ic and /3:
1 are the cyclotron frequency
(1;: = eB 121Tymo) and the paranel velocity component, re
spectively. In this calculation, it is assumed that the elec
trons are given a parallel velocity component which just co
incides with the applied diode voltage, because [J I is very
small compared with (31! .
Figure 7 shows the calculated curves obtained from Eq.
(3) as a function of BrD, where Band rD are the magnetic
flux density and the radius of drain electrode, respectively.
Each curve is corresponded to the electron energy of perpen
dicular component. The experimental data are plotted as a
function of Br D' The dotted lines are the best fit curves to the
data. For the observed points of Z = 0 and 70 em the mag
netic rigidity values of the electron Byf, were 1.2 and
2.2 X 102 G cm, which correspond to 0.6 and 2 keY, respec
tively, The angular velocity spreads were calculated to be
Pll{31! = 4x 10-2 and 7X 10-2, respectively. As discussed
2792 J. Appl. Phys., Vol. 66, No.7, 1 October 1989 Cl
"; 0.5
..::: -
o o 5 10
Bro' 102 (Gem)
FIG. 7. Current ratio IclUe + ID) as a function of the BrD for various
magnetic rigidity BrL (G em). Dotted lines show the best lit curves to the
experimental results for Z = 0 and 70 em.
below, the magnetic moments of the electron beam are cal
culated to bep = 0.38 eV IG at Z = 0 cm andp = 0.57 eV I
Gat Z = 70 cm. The increase in the magnetic moment is due
to the nonadiabatic conditions of the guiding magnetic field.
IV. DISCUSSION
In application to FELs, it is desirable to transport the
electron beam into the interaction region of the magnetic
undulatcr by conserving the magnetic moment of the elec
tron. Moreover, confinement of electron beam to a small
diameter is of practical importance to obtain a high laser
gain.
We discuss the adiabatic conditions for guiding the elec
tron beam through the guiding solenoid coil which will be
necessary for designing the guiding system of the electron
beam for the free-electron laser. To obtain the conservation
of the magnetic moment, the electron cyclotron pitch Pmust
be much smaller than the longitudinal distance in which the
strength of the magnetic field changes I(P If ~ 1). In this sys
tem, a cyclotron pitch of the electron for Z = 25 em, at
which the strength of the guiding magnetic field changes
gradually, is calculated to be typically P= 10 cm. The length
I is estimated to be 1=20 cm (see in Fig. 1). The factor P /1
was calculated to be P 1/,,,,,,.0.5, which shows that the condi
tions for the adiabatic transportation were not fully satisfied.
The magnetic moment p at Z = ° cm is calculated to be
f.1 = 0.38 eV IG, using ymo(f3l C)2 12 = 0.6 keY and Bz
= 1.6kG. The magnetic moment atZ = 70 em is calculated
to bep = 0.57 eV IG. The increase of about 50% in the mag
netic moment is considered to be due to the imperfection of
the condition for the adiabatic transportation under the as
sumption of the phase mixed cyclotron orbits.
In Sec. III, the energy spread was measured to be about
0.14% for well-collimated electrons. For the electrons
which are guided with a uniform magnetic field at Z = 70
cm, the angular velocity spread was measured to be /3 1 I {3lj
= 7 X 10-2• The transverse energy spread and the parall~l
Kawai, Kawamura, and Toyoda 2792
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130.239.20.174 On: Mon, 24 Nov 2014 11:43:35energy spread are equivalent values under the assumption of
the energy conservation. For these electrons, the energy
spread of the paranel component is estimated to be about
AEII/EI =0.4%, where Ell =(y-1)moc2 and .6.EII =E1 = rmO({Jl C)2/2. Relativistically the energy spread due to
the potential depression of the space~charge effect in the
electron beam is 30XI V, where I is the electron beam cur
rent. In this experiment, it is calculated to be about 1.8 kV,
which corresponds to 0.36% of the total energy. These three
kinds of energy spreads have different origins, therefore, the
energy spread of the parallel component in the total elec
trons will be estimated to be about 1 % of the total energy by
summing up these values.
The cold relativistic electron beam (CREE) is extracted
from the cold cathode in the same process as an intense rela
tivistic electron bean (IREB), but IREB is accelerated with
a very large electric field in the small acceleration gap. The
pulse duration is limited by the shortening of the diode due
to expansion of the cathode flare plasma. On the other hand,
the CREB is reaccelerated with a uniform electric field
through a very long acceleration gap, Hence, the energy
spread of CREE is low as compared with the IREE. Further
more, we consider that the generation of the CREE having a
microsecond pulse duration will be possible by using these
techniques.
v. CONCLUSIONS
The cold relativistic electron beam source using reac~
celerated field emission electrons has been developed for use
in the low-r free-electron laser, The energy, the beam CUf-
2793 J. AppL Phys., Vol. 66, No.7, 1 October 1989 rent, and the pulse duration of the accelerated electrons were
O. 51 MeV, 60 A (200 AI em 2), and 200 ns, respectively. The
energy spread IlE / E and the angular velocity spread{J 11/3\1
were measured to be 0.14% and 4 X 10-2, respectively. It
has been shown that the reaccelerated field emission electron
beam has adequate coldness for application to the Iaw-r free
electron laser as compared with conventional IREBs.
1M. Friedman and M. Herndon, Appl. Phys. Lett. 22, 658 (1973).
2T. C. Marshall, S. Talmadge, and P. Efthimion, App!. Phys. Lett. 31, 320
(1977).
's. H. Gold, W. M. Black, H. P. Freund, V. L Granatstein, and A. K.
Kinkead, Phys, Fluids 27, 746 (1984).
'Y. Kawamura, K. Toyoda, and M. Kawai, Appl. Phys. Lett. 51. 795
(1987).
5D. A. Kirkpatrick, R. E. Shefer, and G. Bekell, 1. App!. Phys. 57, 5011
(1985).
"P. Avivi, C. Cohem, and L Friedland, Appl. Phys. Lett. 42, 948 (1983).
'S. C. Chen and T. C. Marshall, Phys. Rev. Lett. 52, 425 (1984).
"R. E. Shefer, Y. Z. Yin, and G. Bekeli, J. App!. Phys. 54, 6154 (1983).
9p. Hartemann and G. Bekcfi, App!. Phys. Lett. 49. 1680 (1986).
lOG. Bekefi, R. E. Shefer, and S. C. Tasker NucL Instrum. Methods A 250.
91 (1986).
"M. Kawai. Y. Kawamura, and K. Toyoda, Jpn. J. Appl. Phys. 24, 1347
(1985).
12M. Kawai, Y. Kawarnara, and K. Toyoda, App!. Phys. Lett. 45, 1287
(1984).
l3y. Kawamura, K. Toyoda, and M. Kawai, App!. Phys. Lett. 45, 307
(1984).
:4R. H. Jacson, S. H. Gold, R. K. Parker, H. P. Freund, P. C. Eft.himion, V.
L. Granatstein, H. Herndon, A. K. Kinkead, J. E. Kosakowski, and T. J.
T. Kwan, IEEEJ. Quantum Electron. QE-19, 364 (1983).
15RADCOLOR film No. 381 is manufactured by Nitte Electric Industrial
Co., Ltd .. Japan.
Kawai, Kawamura, and Toyoda 2793
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130.239.20.174 On: Mon, 24 Nov 2014 11:43:35 |
1.343010.pdf | Electrical properties of oxygen thermal donors in silicon films synthesized by oxygen
implantation
F. Vettese, J. Sicart, J. L. Robert, S. Cristoloveanu, and M. Bruel
Citation: Journal of Applied Physics 65, 1208 (1989); doi: 10.1063/1.343010
View online: http://dx.doi.org/10.1063/1.343010
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/3?ver=pdfcov
Published by the AIP Publishing
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129.105.215.146 On: Tue, 23 Dec 2014 00:17:06Electrical properties of oxygen thermal donors in silicon fUms synthesized
by oxygen implantation
F. Vettese, J. Sicart, and J. l. Robert
Groupe d'Etudes des Semiconducteurs, VA 357, u.s. T.L., F. 34060 Montpellier Cedex, France
S. Cristoloveanu
Laboratoire de Physique des Composants a Semiconducteurs, VA 840, INPG-ENSERG, F. 38031 Grenoble
Cedex, France
M. Bruel
Laboratoire d'Etudes et de Technologie de /'lnformatique, CENG, F. 38041 Grenoble Cedex, France
(Received 2 May 1988; accepted for publication 29 September 1988)
Conductivity and Hall measurements have been carried out on thin silicon films formed by
oxygen implantation (SIMOX) and high-temperature annealing. These layers have then been
annealed between 450 and 850°C for 1 h in order to study the electrical behavior of oxygen
thermal donors (TD). The maximum donor concentration occurs at 550°C for TD-I and
750°C for TD-II. The concentration ofTD-II is higher than that of TO-I and the distribution
ofTD-II can be nonuniform. Thermal ionization energies of these donor states are also
derived. A TD level (220 mcY) deeper than the typical one (150 meV) is responsible for the
electrical properties cfthe SIMOX layers. Subsequent annealing activates shallow TD states
and compensation centers. Thus the ionization energy of the deep TD level decreases greatly,
when TDs are generated. High carrier mobilities have been measured which have been limited
only at low temperatures by interface scattering.
I. INTRODUCTION
Silicon-on-insulator (SOl) structures are the subject of
current research as substrates for radiation hardened, ad
vanced very-large-scale integrated (VLSI) circuits. An
nealed or recrystallized layers of chemical vapor deposited
silicon on SiOz substrates have been tested in integrated cir
cuits technology but these amorphous or polycrystalline SOl
structures have shown electrical properties inferior to those
of single-crystal silicon. However, the formation of buried
dielectric layers of silicon dioxide by the implantation of
oxygen ions (SIMOX) has indeed been demonstrated and
provided a very promising alternative to the silicon-on-sap
phire (SOS) technology. 1.2
The oxygen content in bulk silicon is shown to be re
sponsible for the generation of thermal donors around
450°C (TD-I) and new donors around 750"C (TD-II). As
only a few papers have been published on the electrical prop
erties of these thermal donors in SIMOX films,3-0 we present
here additional data concerning their electrical activity.
A significant change has been observed in the donor
densities and ionization energies of such heat-treatment in
duced donor states, suggesting the formation of several spe
cies of oxygen clusters distributed in the bulk and near the
buried SiOz interface. This problem is not only offundamen
tal interest, but has important implications in SIMOX tech
nology.
Due to the high dose of implanted oxygen ions, a high
density of thermal donors can be generated affecting the in
tentional doping and thus the performance of integrated cir
cuits. The electrical activity of oxygen thermal donors is
studied as a function of the anneal temperature from 450 to
850 0c. The experiment is described in Sec. II while a theo
retical interpretation of these data is proposed in Sec. III. It EXPERIMENT
Buried silicon dioxide was synthesized by implantation
of 1.5 X 1018 0+ cm-2 at 200 keY into ap-type Si( 100) sub
strate. A post-implantation anneal was performed in an ar
gon ambient for 6 h at 1345 °C resulting in a silicon film
(t = 250 nrn) being converted into n type.
The samples were cut in a symmetrical square Han-van
der Pauw pattern (5X5 mm2) with four ohmic contacts
formed by phosphorus implantation (3 X 1015 cm-2, 60
keY) and subsequent annealing (950°C, 30 min) in a N2
ambient. In order to generate oxygen thermal donors in the
silicon film, several additional anneals were performed
between 450 and 850 ·C. The anneal lasted I h for an the
samples. Contact metallization was made without further
annealing. van der Pauw and Hall measurements were car
ried out on these samples between 4 and 380 K by using a
regulated helium flow cryostat and reversing both the cur
rent I and the magnetic field E (1 T).
The Hall factor was taken to be unity and the Hall car
rier density was derived from the Hall voltage V H using the
classical expression
fl = IE IqVHt.
Figures 1-6 show the experimental behavior of the sheet re
sistance, carrier concentration, and Han mobility for TO-I
and TO-II, respectively. The thermal ionization energies of
the donor states are derived from the In(nT -3/2) vs 103 IT
curves7:
E= d [In(nT-3!2)]. d(kT) -J
The In(nT ---3/2) vs 1031Tpiot shows two Iinearparts, corre-
1208 J. Appl. Phys, 65 (3), 1 February 1989 0021-8979/89/031208-05$02,40 © 1989 American Institute of Physics 1208
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129.105.215.146 On: Tue, 23 Dec 2014 00:17:06UJ u :z « r
<Jl
Vi w
'" 10"
r-w
I.IJ
:I:
;f)
o o ...
+ •
o
0 ... III
0+ 1&
+8
<:) '" + 0", +
~. <1
~ V"
tf:!;<1V"
4 6 8
TEMPERATURE 1000fT 1 K-~) 10
FIG. l. Resistivity vs 103/T for the reference and TD-I thennal donors in
SIMOX (crosses: unanneliled; closed circles: 450 'C; inverted open trian
gles: 550 'C; open circles: 650 "C).
span ding to an activation energy c' at high temperatures and
an activation energy E" at low temperatures.
Table I gives the electrical data at T = 300 K and the
two activation energies 1:' and E" • The data for an unannealed
control sample containing residual donors were also collect
ed for comparison. The maximum densities ofTD-I and TO
n were measured at 550 and 750 'C, respectively. Moreover,
for a l-h anneal TD-II is in greater density than TD-I. The
Hali mobility was very high and dose to the theoretical value
in bulk silicon at 300 K for similar doping levels.
m. DISCUSSION
The role of oxygen in single-crystals CZ silicon has been
studied intensively in the past few years in order to try to
stabilize the wafer resistivity." It is well known that TD-I's
are generated between 300 and 600°C and TD-II's between
600 and 900 DC. However, the exact origin of these TDs in
oxygen-rich silicon (Si:O), first interpreted by the Si04
model,9 is the subject of several works which are based on
;) 101..6
'~
z o
i= «
"" ; '014
w
U
Z o
U
IX
oJ
IX
"" <C
U 1012
o ,~v
o v
~ ... ",
011 V
+ II V
0+ II
0+ It o. III
0+ III
0+
0 + ,.
II 6 8 10
TEMPERA TUllE 1000/T (I~-11
FIG. 2. Hall carrier density vs 103/T for the same samples as in Fig. 1.
1209 J. Appl. Phys., VoL 65, No. 3, 1 February i 989 3000
A
2000
A
0 <:) 0 •• A 0 .!lI • 8 + II 0 ... . • • +0
~o 1000
III +·0
+
• B ... 700
500,-. __ ......b_--' __ --' __ '----'
100 150 200 300 400
TEMPERATUIU T 11\")
FIG. 3. Hall mobility vs temperature for the same samples as in Fig. l.
electrical and spectroscopic investigationsIO-.13 or structural
and kinetic models,l4-17
Our results show some differences with the previous
ones, because SIMOX technology requires heavy oxygen ion
implantation and very high temperatures for the post-im
plantation annealing.
A. Thermal donors TD~I
The maximum TO-l generation rate is generally ob
tained around 450°C in bulk Si:O while in our SIMOX lay
ers it occurred at about 550 "C. Two TD-I states have been
revealed in Si:O by Hall measurements with ionization ener
gies E, for the shallow and E2 for the deep level. Most auth
ors found a shallow donor state, with E{ in the range of 20-
60 meV, and a deep donor state ",rith E2 in the range of 1 10-
160 meV, below the conduction-band edge, W,ll, 13 The model
of a divalent oxygen donor has usually been proposed as an
explanation having been supported by Hall measure
ments,lO.IZ However, an acceptor level, acting as compensa
tion center, exists in oxygen-rich silicon and makes a quanti-
10"
a
C
!J.J
U
Z « I-10"
!:2
'" '" IX
r-
ILl
'" :l:
(/)
o 10 20 30 40
FIG. 4. Resistivity vs 10'/1' for SIMOX samples annealed at the 750·C
(crosses) and 850 'c (closed circles). (TD-II thermal donors.)
Vettese et a/. 1209
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129.105.215.146 On: Tue, 23 Dec 2014 00:17:06z
Q r«
'" r-z
'" U z o u
'" w
'" '" « u 1015
10"" "-__ -'--__ -'-__ ---' __ ---'
13 20 30 40
TEMPERATURE 1()OO/T (K-~l
FIG. 5. Hall carrier density vs lOilT fur the same films as in Fig. 4.
tativc analysis of Hall data [0 rather difficult. Indeed,
infrared spectra 12 analysis show that there are several shal
low TD-I levels lying between 40 and 80 meV in Si:O. The
kinetic of the generation of the different TD_I's16 can explain
a shift of the maximum donor generation peak from 450 to
550°C because the species and densitie..<; ofTD-I can be dif
ferent in our SIMOX layers (isochronal anneal). Table I
gives a density of2 X 1015 cm-3 at 300 K for the unannealed
reference sample, indicating that a small amount of residual
TDs are still present, probably generated during the 950 "C
post-implantation anneal of the contacts. Table I also shows
that TDs are almost completely annihilated at 650 0e.
Hall data from at the highest temperature reached in the
experiments seem to confirm the existence of a compensa
tion center which was generated with TDs during the heat
treatment. Indeed the reference sample has the highest den
sity at high temperature.
B. Thermal donors TIHI
Although TD-l's may have been investigated extensive
ly, this is not the case for TD-II. One striking result is the
C1112/V.s!;!c
10000 I
,
1000
o 0000
lOG T [iO
10 lOO woo
FIG. 6. Hall mobility vs temperature for the 750·C (open circles) and
850 'C (closed circles) annealed sampleso
1210 J. Appl. Phys., Vol. 65, No.3, 1 February 1989 TABLE I. Electrical data at T = 300 K and activation energies E' &1d E" for
unannea1ed and l-h annealed SIMOX films.
Anneal
temperature
eel Unannealed 450 550 650 750 850
p [(kS1)/O] 120 90 58 182 62 20
n [(em-oJ) X 1015] 2 2,6 3,84 1,2 76 15,6
!l (em2/V s) 1000 1100 lI20 1150 530 810
E' (meV) 105 58,5 41 111 0 8
£" (meV) 204 106 80 227 0 42
opposite behavior of TD~n in SIMOX layers compared to
Si:O in terms of generation rates.
For our samples the maximum density ofTD-II is high
er than that ofTD-L Table I gives 7.6 X 1016 em -3 for TD-II
at 750°C and 3.84X 1015 cm-3 for TD-I at 550·C for a I-h
anneal whereas in Czochralski oxygen-rich silicon the maxi
mum donor generation per hour is around 9X 1014 cm-3 at
450°C and 1.5 X 1014 cm-3 at 750 0e.g This very high gener
ation rate for TD-JI in SIMOX layers is not well explained
because the origin and kinetics ofTD-II still remain obscure.
Figure 5 shows that in the sample annealed at 750 ·C,
the carrier concentration is almost independent of tempera
ture below 40 K. This is typical for a degenerated semicon
ductor. As the average carrier density (lOt? cm-3) is too
small for degeneracy to occur at this temperature, we can
therefore deduce that in this sample only a part of the layer
(10-15 nm thick) with a higher density (> 1018 cm-3) is
responsible for conduction. We conclude that TD-II's are
generated inhomogeneousiy, in high densities probably near
one Si/Si02 interface.
As for TD-I, the ionization energies are functions of the
carrier densities and are vanishing for the highest densities
(sample 750 °C). Photoluminescence analysis II however re
veals several levels with different kinetics, then modifying
the activation energies with the heat treatment.
C, TD states ionization energies
In bulk silicon containing lower doses of oxygen, TDs
have been activated after very long anneals such that several
shallow TD states are involved in the electrical behavior and
make the Hall analysis inadequate.
In contrast, our experiments are based on a short anneal
time involving only the deepest TD level and the 650°C an
nealed sample gives the location of this level at about 220
me V below the conduction band in accordance with the Fer
mi-level position. This sample indeed shows the lowest TD
density. The activation energy EN gives then the ground-state
location, the Fermi level lying around the donor level (Fer
mi level lying at about 220 meV for TD density 2X 1013
cm-3 at 200 K). The plot In(nT-312) vs 103/T clearly
shows two parts giving an activation energy Elf = Ed at low
temperature and an energy fo' around Ed/2 at high tempera
ture. It is now clear that the SIMOX layer behaves as a com
pensated semiconductor. 7
In order to fit the experimental curves with the assump
tion of one deep TD level Nd at energy Ed and a compensator
level Na, we use the classical expression of the carrier den
sity7;
Vettese et al. 1210
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
129.105.215.146 On: Tue, 23 Dec 2014 00:17:06with A = 1+ (N(jl3Nc) exp(E d) where Nc is the conduc
tion-band density of states, f3 the degeneracy factor, and Ed
= Ed/kT.
The best fit gives Nd = 1 X 1016 cm -3 with Ed = 250
meV and Na = 2x 1015 cm-3 for the 650°C annealed sam
ple. For the other samples, the fit can be obtained only if we
assume a concentration of the acceptor level which increases
with the anneal. This result differs from that of Gawor
zewski and Schmalz10 who assumed N" = const in oxygen
rich silicon. The fit does not agree exactly with experiment
when the TD concentration increases and the ionization en
ergy strongly decreases as other TD species are formed and
the assumption of only one deep level thus fails.
It must be pointed out that the TD level in SIMOX
layers is deeper than the classical one ( 150 me V). This result
confirms that TD centers in SIMOX are not the same as the
typical in 8i:0 silicon. Several species of oxygen clusters,
oxygen-silicon, or oxygen-carbon complexes can be respon
sible for the multiple behaviors exhibited by SIMOX films.
However, the bistable character of deep TD levels in silicon
has recently been demonstrated. 18-20 This bistable leve! cor
responds to the transition £(0, + + ) between the neutral
state and doubly ionized state, the TD behaving as a system
with a negative correlation energy. 19.20
D. Mobility in SIMOX films
Let us now discuss the Hall mobility which is a funda
mental electrical parameter. Table I gives the Hall mobility
at T = 300 K deduced from the relation f.1-H = (nq p ) -I.
These values are very good for the integrated circuit technol
ogy on SIMOX substrates and are around the expected val
ues for n-type bulk silicon. The variation of the Hall mobility
with temperature is usual for the high-temperature range
where acoustic phonon scattering prevails. When the tem
perature is lowered below 150 K, the mobility greatly de
creases, except for the 750°C annealed sample which be
haves as a degenerated layer at low temperature (T < 50 K).
After partial annihilation of TD-Irs at 850 ·e, the
SIMOX material recovers a more normal temperature de
pendence with the mobility (Fig. 6). Figure 7 shows the Hall
mobility for the 650°C annealed sample (n = 1015 cm--3)
and for the sake of comparison we have also plotted the mo
bility of Si:O with similar concentration 10 and the mobility
of an As-implanted SIMOX film (Nd = 3 X 1017 cm-3) <
The large decrease of the mobility in SIMOX layers at
low temperatures can be attributed to the thinness of the
film. The scattering of both the upper and lower Si02/Si
interfaces becomes the main process. The mean free path
increases and is close to half the film thickness (125 urn). In
this low-temperature range, the mean free path is about the
separation between ionized impurities, which is large ( > 100
nm) because the TD concentration is jow (~lOIS em -3).
Thus the mobility at 150 K is only 1600 cm2/V s whereas it
reaches 4000 cm2 IV s in Si:O samples.
1211 J, AppL Phys., Vol. 65, No.3, i February 1989 FIG. 7, Hall mobility vs temperature for th!? 650 <C annealed SIMOX films
(inverted closed triangles) and SIMOX As films (closed circles), Full lines
show the Hall mobility in Si:O (Ref. 10) and Si:As (Ref. 21 ).
IV. CONCLUSION
For the first time complete Hall data on SIMOX materi
al are given. The electrical activity of thelmal donors has
been studied as a function of temperature for a I-h anneal.
SIMOX thin layers show many particularities com
pared with oxygen-rich silicon: (i) the maximum generation
rates ofTDs are not the same: the maximum TD-I density is
obtained at 550 "e (not at 450 "C) and is lower (not higher)
than the maximum TD-II density at 750"C. (ii) The donor
levels are found to be deeper than in Si:O. (iii) The kinetics
of TD generation seems to be more complex.
The lowest TD-I concentration is obtained for the
650·e annealed sample. However, the control of the residu
al donor concentration still remains to be resolved for TD-U.
ACKNOWLEDGMENTS
We would like to thank the implantation group of
LEn-Grenoble (Dr. J. MargaiI and C. Jaussaud) for sup
plying the SIMOX layers. This work was supported by the
organization "Groupement Circuits Integres Silicium."
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and Devices, edited by G. G, Benlini (MRS, France, 1986), p, 379.
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129.105.215.146 On: Tue, 23 Dec 2014 00:17:06 |
1.37959.pdf | AES and EELS analysis of TlBaCaCuO x thin films at 300 K and
at 100 K
A. J. Nelson , A. Swartzlander , L. L. Kazmerski , J. H. Kang , R. T. Kampwirth , and
K. E. Gray
Citation: AIP Conference Proceedings 182, 269 (1989); doi: 10.1063/1.37959
View online: https://doi.org/10.1063/1.37959
View Table of Contents: http://aip.scitation.org/toc/apc/182/1
Published by the American Institute of Physics269
AES AND EELS ANALYSIS OF TIBaCaCuOx THIN FILMS AT 300K
AND AT 100K
A.J. Nelson, A. Swartzlander and L.L. Kazmerski
Solar Energy Research Institute, Golden, CO 80401
J.H. Kang, R.T. Kampwirth and K.E. Gray
Argonne National Laboratory, Argonne, Illinois 60439
ABSTRACT
Auger electron spectroscopy line-shape analysis of the Tl(NOO), Ba(MNN),
Ca(LMM), Cu(LMM) and O(KLL) peaks has been performed in conjunction with
electron energy loss spectroscopy (EELS) on magnetron sputter deposited
T1BaCaCuOx thin films exhibiting a superconducting onset at ll0K with zero
resistance at 96K. AES and EELS analyses were performed at 300K and at 100K.
Changes in the Auger line shapes and in the EELS spectra as the temperature is lowered
below the critical point are related to changes in the electronic structure of states in the
valence band (VB). Bulk and surface plasmon peaks are identified in the EELS spectra
along with features due to core level transitions. Electron beam and ion beam induced
effects are also addressed.
INTRODUCTION
The recent empirical discovery of superconductivity above 100K in the T1-Ba-Ca-
Cu-O system 1 has once again stimulated the synapses of the high-Tc superconductor
community. The fact that all of the recent high-Tc materials research has been empirical
in nature points to a clear need for experimental results which may help define the
superconducting mechanism relevent to these new materials. Since Auger electron
spectroscopy (AES) is sensitive to the variation of the local atomic charge density
across the VB the technique is useful in characterizing states found near the VB
maxima. Similarly, electron energy loss spectroscopy (EELS) stimulates transitions
from core levels to empty states above or near the VB maxima.
In this paper, we report observed changes in the TI(NOO), Ba(MNN), Ca(LMM),
Cu(LMM) and O(KLL) Auger line shapes as well as observed changes in the EELS
spectra for a T1BaCaCuOx film on yttrium-stabilized ZrO2 after it was cooled to 100K.
The observed changes are related to changes in the electronic structure of states in the
VB as the material passes through its critical transition temperature.
EXPERIMENTAL
The T1BaCaCuOx films were prepared 2 by using a three-gun dc magnetron
sputtering system equipped with a turbomolecular pump which provided a typical base
pressure in the low 10 -8 torr range. The three dc magnetron sputtering guns are aimed
at a common point about 15 cm above the sources providing compositional uniformity
to +1% over a 2 cm 2 substrate area. Targets of T1, Cu and a 1:1 BaCa mixture were
simultaneously sputtered in a 20 mtorr argon atmosphere with an oxygen partial
pressure of -~0.1 mtorr being introduced directly adjacent to the substrate. A quartz
crystal monitor is placed next to the substrate to determine the sputtering rates of each
source prior to starting a deposition. The best films were deposited onto (100) oriented
© 1989 American Institute of Physics
270
single crystal or polycrystalline ZrO2-9%Y203 substrates maintained at ambient
temperature during deposition. Ex-situ post-annealing treatment was performed in a
flowing oxygen atmosphere. In order to avoid the loss of the highly volatile TI during
the annealing process, the films were placed in a closed Au crucible, then placed in a
flowing oxygen tube furnance and annealed at 850C for about 5 minutes.
Auger and EELS analysis were performed on a Perkin-Elmer/Physical Electronics
Model 600 Scanning Auger Microprobe (SAM) system having a base pressure of
lxl0 -10 Torr. AES data was obtained with a primary electron beam energy of 5 keV
and a current of 100 nA at an energy resolution of 0.2%. Primary electron beam
energies of 300 eV and 600 eV were used for the EELS analysis (0.2% energy
resolution) in order to distinguish between surface and bulk effects. Ion beam sputter
etching was performed with a differentially pumped ion gun operating with a 3 kV Ar +
ion beam (10 -2 Pa Ar pressure) rastered over a 1.0xl.0 mm 2 area. Samples were
cooled in vacuum to 100K using a LN2 dewar equipped with a copper cold finger. The
AES and EELS data were both recorded in N(E) (i.e. counts vs. energy) mode with the
EELS data being displayed as -d2N/dE 2. Quantitative compositional analysis was
performed in a scanning electron microscope (SEM) using energy dispersive x-ray
(EDX) analysis.
RESULTS AND DISCUSSION
Fig. 1 shows the variation of resistivity versus temperature, as measured by the
standard four-probe technique, for a film on yttrium-stabilized ZrO2. Result of the
quantitative EDX compositional analysis indicated a film composition of
TI2Ba2Cal.3Cu2Ox which is in reasonable agreement with the 2212 structure. A single
superconducting transition is observed which begins at about 110K and shows zero
' ' ' ' I ' ' ' ' I ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' '
E
? 3
E
2 ._>
o o
Or) o
1 o _
o 4
' I I
0 o
50 100 150 200 250 300
T (K)
Fig. 1 Resistivity vs. temperature for Tl2Ba2Cal.3Cu2Ox thin film on ZrO2:Y.
z
O3
<
Kinetic energy (eV) i I I I I I i v ~t.l-' I /
O0°K
69 75 81 87 93
c(KLL) R.T. Fig. 2. N(E) Auger spectra obtained at
300K and 100K for a T12Ba2CaCu2Ox
film (a) TI NOO, (b) Cu LMM and (c)
O(KLL).
z
E
O3
< l t l ~ I t 1 I I
[lll llllll
900 906 912 918
Kinetic energy (eV) 271
0 (KLL)
924 930 500 505 510 515
Kinetic energy (eV)
272
resistance at Tc(0)=96K. The low temperature AES and EELS measurements were
obtained at 100K (the limit of the apparatus) which is between the superconducting
onset at 110K and Tc(0) for this film.
Auger results for the Tl(NOO), Cu(LMM) and O(KLL) line shapes obtained at
300K and 100K are presented in Fig. 2a, b and c, respectively. The line shape and
peak energy of these lines are strongly influenced by the chemical environment since
the Auger electron emission involves valence electrons and the core level binding
energy. Line shape analysis of the Ba(MNN) and Ca(LMM) Auger lines revealed no
pertinent chemical or electron beam induced effects and thus are not included in this
presentation. However, the change in the TI(NOO) Auger line pictured in Fig. 2a as the
film is cooled should be noted. Specifically, the 300K spectra centered at 81.0 eV with
a small shoulder at 78.3 eV broadens and develops an additional peak at 79.3 eV as the
temperature is lowered to 100K. This additional feature in the Tl(NOO) peak is
possibly due to preferential electron beam assisted hydration/carbonation of T1 at the
surface of the superconductor since the sample will act as a "cryogenic pump" for H20
and CO at these lower temperatures or may be due to reordering of the T1-O layer 3 as
determined by neutron scattering. Additional evidence for one of these processes is
seen in the O(KLL) spectra pictured in Fig. 2c. The 3OOK O K1L2,3L2,3 spectra is a
broad peak centered at 507.3 eV. The width of this peak indicates multiple states
probably due to a continuum of holes in the K band and crystal-field effects. Upon
cooling, the O K1L2,3L2,3 peak broadens and develops two distinct features at 506.0
eV and at 508.6 eV. The additional feature at lower kinetic energy may again be due to
the presence of an OH molecule on the surface or to the aforementioned reordering of
the T1-O layer which correlates with the probable causes of the change observed in the
T1 spectra. Features representative of intrinsic physical effects (e.g., structural
modifications of the square-planar CuO4 clusters or of the T1-O layers) as the material is
cooled cannot presently be separated from extrinsic chemical effects and thus no
definitive conclusions can be drawn from the T1 or O data concerning the occurance of
this reordering phenomena.
The Cu L3M4,5M4,5 measured at 3OOK and at looK is presented in Fig. 2b. In the
L3VV Auger transition, a valence band (VB) electron fills a previously created core (L3)
hole. The excess energy causes ionization of a second valence band electron which is
the measured Auger electron. The energy distribution of this Auger electron yields
information about the VB density-of-states (DOS) spatially localized around the atom
containing the core hole. The 3OOK spectra is rather broad and is composed of two
main features evident at 911.4 eV and at 914.3 eV. The initial states of the Cu L3VV
Auger process are formed from the 3d 9 and 3d 10 L states IL designates a ligand (O 2p)
hole] of divalent copper with the final states formed by the 3d 7 and the 3d 8 L multiplets
with the 3d 8 L states dominate. 4-9 The width of the Cu L3VV line is probably the result
of the continuum of holes in the L band and crystal-field effects. As the sample is
cooled to 100K, the Cu L3VV line narrows and is centered at 912.2 eV. This change
(narrowing) in the Auger line shape with decreasing temperature is distinct and opposite
to the results for the T1 and O Auger lines and consequently is interpreted as possibly
being due to a structural modification of the square-planar CuO4 clusters 10 associated
with a change in oxidation state (Cu +2, Cu ÷1) leading to a different hole-hole
correlation energy.
Fig. 3a and b presents the EELS results obtained at 3OOK and 100K with primary
beam energies of 3OO eV and 6OO eV, respectively. The 3OO eV EELS spectra is more
representative of the surface states while the 600 eV EELS spectra is more
representative of bulk states. Also included in this figure is the EELS spectra at 100K
after argon sputtering to remove adsorbed surface molecules due to the low tempera-
273
600V EELS
I I I I
ooo
I I I I
A D
-5 -10 -15 -20 -25 -30 -5 -10 -15 -20 -25 -30
Energy loss (eV) Energy loss (eV)
Fig. 3 Electron energy loss spectra for T]2Ba2Cal.3Cu2Ox film on ZrO2:Y obtained at
300K and looK (a) 3OO eV primary beam energy, (b) 600 eV primary beam energy.
Table I Electron energy loss features for T12Ba2Cal.3Cu2Ox
E~(eV)
3OO
6OO Loss Energies (eV)
T A B C D E F G
3OOK 5.1 9,1 - 16,7 22.6 24.5 27.9
100K - 8,7 11.9 16,6 22,3 24.2 27.8
looK - 8,6 12.5 16.5 22,5 24,1 28.0
(3 min. sputter)
3OOK - 9.1 12.2 (18.1) - 23,6 -
looK 5.3 8.8 12.2 16.9 - 23.6 -
100K 5.3 8.9 11.9 16.8 - 23.6 -
(3 min. sputter)
274
tures. AES results on a sputtered sample of T1BaCaCuOx showed no significant change
in the surface composition as long as the material was maintained at looK. However,
loss of TI was evident if the sample was sputtered at 300K.
Table I summaries the measured loss energies, with respect to the elastic peak, of
the seven observed features as labelled on the EELS spectra of Fig. 3. The low
intensity of the 600 eV EELS spectra taken at 300K is due to instrumental effects and is
not representative of any intrinsic physical phenomena. Interpretation of the peaks in
the EELS spectra for T12Ba2CaCu2Ox is partially based on photoemission
spectroscopy 4-9 and energy loss 11-13 results for YBa2Cu307-x superconductors.
Utilizing these previous results, one may also infer the presence of two maxima in the
density of states above the Fermi level (EF) for the TI-Ba-Ca-Cu-O system. The
previously described empty-state level 2.3-2.5 eV above EF, attributed to antibonding
Cu 3d electrons, along with the other unoccupied state 4.3-4.6 eV above EF determine
the allowed transitions to be used for the interpretation of the observed spectral
features. Based on these suppositions and the assignments found in the literature,
feature A is interpreted as being due to a transition from the O 2pxy eigenstate to the
unoccupied Cu 3d antibonding state. This feature is not visible in the 300 eV spectra
taken at 100K. Since the 300 eV spectra is more sensitive to surface overlayers, one
concludes that the Cu 3d antibonding state is smeared out by the aforementioned
surface contamination accumulated during cryogenic cooling. The fact that this feature
is not observed in the 600 eV spectra taken at 3OOK can be accounted for by assuming
that this unoccupied state for the bulk material is closer to the Fermi level at this
temperature and/or has not fully developed. Therefore, the shift in energy and/or
development of states in this band upon cooling could also be indicative of a structural
modification of the square-planar CuO4 clusters.
Further support of this interpretation may be evident from the energy shift upon
cooling of feature B, identified as a transition from the occupied bonding Cu 3d level to
an unoccupied state 4.3-4.6 eV above EF. The peak intensity of this feature in the 600
eV spectra greatly increases as well upon cooling. Rearrangements in the steric
configuration of the CuO4 clusters would induce small observable energy shifts of
states comprising the VB and thus would offer one explaination of the observed energy
shifts in the EELS spectra.
Feature C is assigned to a transition between the O 2pz eigenstate and the
unoccupied Cu 3d antibonding state. The peak intensity of this feature in both the 300
eV and 600 eV spectra increases upon cooling, showing that more electrons are allowed
to make this transition to the more developed Cu 3d antibonding band above the Fermi
level. Feature D has previously been interpreted as a surface plasmon state, but is
probably due to a transition from a Ba 5p level associated with Ba(OH) 3 since it is
greatly diminished when the material is sputtered. Feature F is interpreted as a bulk
plasmon state and exhibits no energy shift upon cooling. Features E and G are only
resolvable in the 300 eV spectra with their intensities decreasing upon sputtering and
thus are also believed to be associated with surface overlayers.
CONCLUSIONS
Auger line shape analysis and EELS analysis have been used to characterize the VB
DOS of a TI2Ba2Cal.3Cu2Ox thin film superconductor at 300K and at 100K. Changes
in the TI NOO Auger line shape as the film is cooled to looK have been interpreted as
being due to either hydration/carbonation of T1 at the surface or to reordering of the T1-
O layer. Changes in the Cu L3VV Auger line shape as the film is cooled to looK have
been interpreted as being due to a structural modification of the square-planar CuO4
275
clusters associated with a change in Cu oxidation state (Cu+2,Cu+l). EELS results give
evidence which tends to support this interpretation.
Future work will include the construction of an apparatus to provide continuous
cooling of a sample through its critical transition temperature and temperature cycling
during AES and EELS analysis. Also, this study will be performed on T1
superconductor films exhibiting higher Tc's.
ACKNOWLEDGEMENTS
The work at SERI was supported by the US Department of Energy under Contract
No. DE-AC02-83CH10093 and the work at Argonne was supported by the Department
of Energy, BES-Materials Sciences, under Contract No. W-31-109-ENG-38.
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J.M. Tarascon, AIP Conference Proceedings No. 165, 240 (1988)
13. K. Jacobi, D.D. Sarma, P. Geng, C.T. Simmons and G. Kaindl, Phys. Rev.
B38, 863 (1988)
|
1.37940.pdf | AIP Conference Proceedings 182, 74 (1989); https://doi.org/10.1063/1.37940 182, 74
© 1989 American Institute of Physics.Superconducting Tl-Ca-Ba-Cu-O
thin films by reactive magnetron
sputtering
Cite as: AIP Conference Proceedings 182, 74 (1989); https://
doi.org/10.1063/1.37940
Published Online: 04 June 2008
D. H. Chen , R. L. Sabatini , S. L. Qiu , D. Di Marzio , S. M. Heald , and H.
Wiesmann
74
SUPERCONDUCTING TI-Ca-Ba-Cu-O THIN FILMS
BY REACTIVE MAGNETRON SPUTTERING
D. H. Chen, R. L. Sabatlnl, S. L. Qiu, D. Di Marzio,
S. M. Heald, and H. Wiesmann
Brookhaven National Laboratory, Upton, NY 11973
ABSTRACT
Superconductin E Ti-Ca-Ba-Cu-O thin films with T c onsets of
115 K and T c (R-0) of 95 K have been prepared by reactive maEne-
tron sputtering usin E TI, Cu and Ca/Ba metal targets. It was found
that proper thallium content is crucial for obtalnin E a hiEh tran-
sition temperature. Wet oxyEen and a sealed 8old tube with addl-
tional thallium compounds were used to reduce the loss of thallium
during annealin E. X-ray diffraction spectra show that films with
the sharpest transition at 95 K are predominantly c-axis oriented.
XANES also shows a preferred c-axis orientation for the supercon-
ducting film, while for a nonsuperconductin E film the near edEe
structure suEEests Ereater disorder. X-ray microprobe fluorescence
measurements indicate that these films are close to the 2122
stoichiometry. Scanning electron microscopy on these films is also
presented.
INTRODUCTION
The discovery I of superconductivity in the TI-Ca-Ba-Cu-O
systems has resulted in the hiEhest superconductin E transition tem-
peratures reported to date. 2 This class of superconductors con-
tains no rare earth elements makin 8 applications of the supercon-
ductor more practical and cost effective. In contrast to the
yttrium based superconductors there have been only a few reports in
the literature describin E the fabrication of the thallium based
thin films. This is partly due to the toxicity of thallium and its
compounds. Epltaxlsl and polycrystalline films have potential
applications in intesrated circuits, SQUIDS and IR detectors. Some
of the deposition techniques which have been employed for fabrica-
tion of thallium based thin films are RF maEnetron sputterln E of a
sinEle composite bulk tarset, 3 sequentlal electron beam evapora-
tion, 4 off axis RF diode sputterinE of bulk tarEets 5 and
simultaneous reactive metal maEnetron sputterinE. 6 In this paper
we discuss the preparation of Ti-Ca-Ba-Cu-O thin films in a manner
similar to that employed in Ref. 6. X-ray diffraction data are
presented showin 8 that the films with the sharpest superconductin E
transitions have the Ereatest deEree of preferred orientation. In
addition to resistance versus temperature measurements, SEM photo-
microEraphs , x-ray fluourescence microprobe results and x-ray
absorption near edge structure (XANES) are also included.
EXPERIMENTAL
The Ti-Ca-Ba-Cu-O films were fabricated in a conventional
(~) 1989 American Institute of Physics
75
commercial sputter deposition system which has been described in
our previous report. 7 All the depositions were performed under
identical conditions. The argon gas flow was fixed at 13.0 sccm
and oxygen flow rate at 0.20 sccm with the gas flow rates con-
trolled via electronic mass flow controllers. The total gas pres-
sure during sputtering was 5 microns and the base pressure of the
vacuum system prior to deposition was in the range I-5xi0 -6. The
substrate temperature during deposition was 300"C.
All the targets were presputtered for approximately 1 hour
prior to deposition. A quartz crystal rate monitor was fixed next
to the substrate holder and used to calibrate the deposition rates
of the individual targets prior to deposition. The thallium and
copper targets were sputtered with dc power supplies while the
Ba/Ca (1:1) target was sputtered using a 13.56 MHz power supply.
It was found that the proper thallium content is crucial for
obtaining films with high superconducting transition temperatures.
In order to reduce the loss of thallium during annealing the as-
deposited films were placed in a sealed gold tube with additional
T1203. Water vapor was introduced into the furnace during the
annealing cycle in combination with oxygen. The presence of water
vapor resulted in films with higher transition temperatures than
films annealed in dry oxygen. Two annealing steps were employed.
The gold tube (containing the films) was inserted into the furnace
for 2-3 minutes at 850"C, removed quickly and allowed to cool to
room temperature. The film was reinserted into the furnace for 2-5
minutes at 800-820"C and furnace cooled. After heat treatment the
films were 0.5-1.0 micrometers thick.
Two different substrates were used, single crystal sapphire
and single crystal yttrium stabilized cubic zlrconia with (100)
orientation. The zirconia substrates gave superior results and all
of the data shown here were for films grown on this substrate. The
superconducting transition was measured using the recommended four
probe resistance technique. Four silver strips were painted onto
the surface of the film and copper wires were embedded in the
silver strips prior to drying and hardening. The area encompassed
by the voltage sensing strips was approximately 3 mm x 4 mm for all
samples.The current density during measurement was approximately I
Amp/cm 2 .
RESULTS AND DISCUSSION
Figure 1 shows resistance versus temperature for three samples
which were annealed under different conditions (see Fig. I cap-
tion). For sample A the onset occurs at 115"K and the transition
is complete at 95"K. The remaining films, B and C, show deteriora-
tion in the slope of the resistance versus temperature, T c onset,
and the temperature at which the transition is complete. X-ray
diffraction measurements were performed on a Philips powder dlf-
fractometer using CuKu radiation and the results are shown in Fig.
2. The curves are labelled A, B, and C and correspond to the
sample labels in Fig. I. Indexing of the 28 scans identifies all
the films as belonging to the 2122 phase. All three 20 scans are
dominated by (00~) reflections consistent with a preferred
76
XD
O
~D
Z
r~
2'o ' ' 8'5 i I I i I I i i I
A
TEMPERATU,E (K) 310
Fig. I. Resistance vs temperature for Tl-Ca-Ba-Cu-
0 films on yttrium stabilized ZrO 2 (100). The
films were annealed (A) at 850°C for 2 minutes fol-
lowed by rapid cooling and subsequently annealed at
820°C for 2 minutes then furnace cooled to room
temperature, (B) at 850°C for 3 minutes and rapid
cooled to room temperature, (C) same as (A) but the
subsequent annealing was followed by rapid cooling
instead of furnace cooling.
orientation wherein the c-axis of the films is perpendicular to the
surface of the substrate. Located at the bottom of Fi E . 2 is a
computer 8enerated 28 scan showin E the location and intensity of
the (00~) reflections for a film with a c-axis orientation
perpendicular to the surface of the sampe. There is excellent
agreement between the computer Eenerated scan and the experimental
data. There is also a correspondence between the degree of orien-
tation and the sharpness of the superconductin E transition.
Referrin 8 to Fi E . I we observe that the film with the sharpest
superconducting transition also exhibits the greatest degree of
preferred orientation. As the quality of the superconducting
transition deteriorates so does the degree of preferred orientation
as evidenced by samples B and C. Examination of the 28 diffraction
scans reveals only a few impurity reflections of small intensity.
The dominant reflection for polycrystalline 2122 phase material is
located at 31.5 degrees. A decrease in the degree of preferred
orientation is juxtaposed by an increase in the intensity of this
reflection. This is consistent with the small quantity of impurity
phases present. X-ray fluorescence microprobe measurements were
used to determine the elemental composition of each of the films.
Wavelength dispersive spectroscopy was employed and separate
standards for each of the individual elements were used to
77
b ,~ 2b 25 3b 3~ ~ is ~ 5'5 60
2e(deO)
io 2"5 3"o 3~ 4b W~ ~o ~
2 e M~)
lO ]5 20 25 30 35 40 45 50 55 60 2e(~)
28(~)
Fig. 2. X-ray diffraction pattern from three
samples labeled A, B, and C which correspond to
the sample labels in Fig. I. For comparison, a
computer generated 28 scan is shown at the
bottom of this figure which shows the location
and intensity of the (00t) reflections.
78
Table I. The Atomic Composition for TI-Ca-Ba-Cu-O Thin Films.
Sample T1 Ca Ba Cu
A 2.33 1.89 2.89 3.17
B 2.29 1.45 2.27 2.89
C 4.03 1.92 3.37 4.08
calibrate the spectrometer
crystals. The results are shown
in Table I. There is substantial
deviation from the atomic composi-
tion expected of a film which con-
sists of the stoichiometric 2122
phase. We are investigating this
further.
SEM photomicrographs of
samples A, B, and C are shown in
Fig. 3. The maEnification is
5000x. The films appear to be
extremely porous. The reason for
this porosity is not understood
but may be related to the addition
of water vapor to the flowing
oxyKen used in the annealing of
the films.
X-ray absorption near edge
structure (XANES) displays large
modulations of the atomic
absorption coefficient and
therefore is sensitive to local
atomic structure. An energy range
from -20 eV to 40 eV (with the
-- edge defined as 0 eV) is typical
---- and it encompasses both pre-edge
and post-edge features as well as
the edge and main peak. For the
case of the TI-Ca-Ba-Cu-O thin
films considered here, the XANES
from the Cu k-edse (Is44p
transition) was measured. This
was done at beam line X-IIA at the
National Synchrotron Light Source
(NSLS) at Brookhaven National
Laboratory. A Si(lll) double-
crystal monochromator with a
nominal energy resolution of
Fig. 3. Scanning electron -2.0 eV was used. An advantage
micrograph of the same films of synchrotron radiation is its
as in Figs. 1 and 2. polarization (in the horizontal
plane), which can be used to
determine the orientation of anisotropic materials and to probe
79
8
Q.
o
-6'0 -4'0 C ~
b~
-zG 6 z'o ,~o 6'o
ENERGY (eV)
Fig. 4. The absorption edge of
superconducting
Tl2CalBa2Cu20x thin
film with the x-ray electric
field vector parallel to the
substrate surface. 8
o b~
-G0 -40 -20 0 20 40 G0
ENERGY (eV)
Fi E . 5. The absorption edge
of a T1 deficient nonsuper-
conducting film with
the x-ray electric field
vector parallel to the
substrate surface.
electronic structures along a particular direction. The T1 thin
film was positioned so that the substrate normal was 30" from hori-
zontal and the x-ray electric field vector was parallel to the film
surface. X-ray fluorescence was measured with a detector placed
above the film.
Figure 4 shows the absorption edge for sample B. The pre-edge
feature marked a, is characteristic of the Is*3dx2_y2
(antibonding) transitlon. 8 This transition, which is dipole
forbidden but quadrupole allowed, is weak. The shoulder marked b,
represents the dipole allowed Is*4p transition with a shakedown of
charge from the occupied 03p u state to the empty Cu 3dx2_y2
s 9 tare. This charge transfer screens the Is hole and lowers the
Is~4p transition energy. The main peak marked c is the unscreened
Is~4p transition. If the x-ray electric field vector is parallel
(61c-axis) to the CuO 2 plane, then the main transition is
Is44pa, while for the electric vector perpendicular (611c-axis) to
the CuO 2 plane, the transition is Is~4p~. ° It has been
observed for oriented TI2CalBa2Cu?O x powder that a weak
shakedown shoulder appears when ~Ic-axis, while for ~llc-axis a
strong shoulder appears. I0 This is consistent with near edge
data on CUC14-2 compl~xes. 8 For the oriented powder the
shoulder height for ~Ic-axis is ~29% of the total c peak
height, while for ~llc-axis it is ~53X of the c peak height. In
Fig. 4, the shoulder marked b is -32% oflthe total c peak height,
which is close to the value of 29~ for ~Ic-axis for the oriented
powder. This suggests that the c-axis is perpendicular to the
substrate. For comparison, the absorption edge of a T1 deficient
nonsuperconducting film is shown in Fig. 5. Here the height of
shoulder h is ~51% of the total peak height and the weaker struc-
ture above the edge suggests greater disorder. In addition, the
~Ic-axis polarization for the oriented powders show a strong and
relatively narrow main peak c, as is the case in Fig. 4. The pre-
edge Is43dx2_y2 feature a in Fig. 4 is also stronger in the
80
~Ic-axis oriented powder than in ~llc-axis. 8
CONCLUSIONS
We have fabricated thin films of Tl2CalBa2Cu2Ox on
yttrium stabilized cubic zirconia by the technique of simultaneous
reactive metal magnetron sputtering using three metal targets.
Superconducting onsets of II5°K with Tc(R=0) of 95°K have been
achieved. A correlation has been observed between the quality of
the superconducting transition and the degree of preferred orienta-
tion in the films. The sharpest transition is exhibited by films
having the greatest degree of preferred orientation. XANES show
structure consistent with a preferred orientation of the c-axis
perpendicular to the substrate plane. X-ray microprobe fluores-
cence measurements of the film compositions show that the films are
close to the 2122 stoichiometry except for the presence of excess
copper. The films exhibit a rather porous microstructure at 5000x
magnification. This microstructure is believed to be an artifact
of the annealing procedure. Future work will be concetrated on
achieving films with improved superconducting transition and a more
homogenous microstructure.
ACKNOWLEDGEMENTS
We wish to thank the staff of the National Synchrotron Light
Source at Brookhaven National Laboratory, where the XANES measure-
ments were performed. This work was performed under the auspices
of the U.S. Department of Energy, Division of Materials Science,
Office of Basic Energy Sciences under Contract No. DE-AC02-
76CH00016.
REFERENCES
i. Z. Z. Sheng, A. M. Hermann, A. E1 All, C. Almasan, J. Estrada,
T. Datta, and R. J. Matson, Phys. Rev. Lett. 60, 937 (1988).
2. S. S. P. Parkin, V. Y. Lee, E. M. Engler, A. I. Nazzal, T. C.
Huang, G. Gorman, R. Savoy, and R. Beyers, Phys. Rev. Lett.
60, 2539 (1988).
3. M. Nakao, R. Yuasa, M. Nemoto, H. Kuwahara, H. Mukaida, and
A. Mizukami, Jpn. J. Appl. Phys. 27, L849 (1988).
4. D. S. Ginley, J. F. Kwak, R. P. Hellmer, R. J. Baughman, E.
L. Venturini, and B. Morosin, Appl. Phys. Lett. 53, 406
(1988).
5. W. Y. Lee, V. Y. Lee, J. Salem, T. C. Huang, R. Savoy, D. C.
Bullock, and S. S. P. Parkin, Appl. Phys. Lett. 53, 329
(1988).
6. J. H. Kang, R. T. Kampwirth, and K. E. Gray, Phys. Lett. A
131, 208 (1988).
7. H. Wiesmann, De Huai Chen, R. L. Sabatini, J. Hurst, J. Ochab,
and M. W. Ruckman, J. Appl. Phys., to be published.
8. N. Kosugi, T. Yokoyama, K. Asakura, and H. Kuroda, Chem.
Phys. 91, 249 (1984).
81
9. R. A. Bair and W. A. Goddard III, Phys. Rev. B 22, 2767
(1980).
10. S.M. Heald, J. M. Tranquada~ C. Y. Yang~ Y. Xu, A. R.
Moodenbaugh, M. A. Subramanian, and A. W. Sleisht~ Proc.
Intern. Conf. EXAFS, Seattle, WA, Au K . 1988.
|
1.343497.pdf | Prich Si particles in separation by implanted oxygen structures revealed by low
temperature electronspin resonance
G. Van Gorp and A. Stesmans
Citation: Journal of Applied Physics 66, 780 (1989); doi: 10.1063/1.343497
View online: http://dx.doi.org/10.1063/1.343497
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/2?ver=pdfcov
Published by the AIP Publishing
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[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
130.63.180.147 On: Sat, 22 Nov 2014 08:20:11P-rich 51 particles in separation by implanted oxygen structures revealed
by lowwtemperature electron-spin resonance
G. Van Gorp and A Stesmans
Departement Natuurkunde, Katholieke Universiteit Leuven, 3030 Leuuen, Belgium
(Received 24 January 1989; accepted for publication 13 March 1989)
Low-temperature X-and K-band electron-spin-resonance measurements on separation by
implanted oxygen structures formed by implanting oxygen to a dose::::; 1.7X 1018 cm-2 on
[001] c-Si wafers-both n andp type [dopant concentration;:::::: (9-28)xl014 cm -3]-reveal
the presence of a signal due to submetallie Si:P effectively doped to [Pi ;::::::2.0 X 1018 em -3. The
signal is identified as originating from the polyhedron-shaped c-Si precipitates known to
remain in the buried SiOz layer near the bulkside 5i1Si02 interface, even after high
temperature annealing. The capstone in this identification stems from the faceted structure of
these Si islands, which, combined with the concomitant misfit-induced and plane-index related
strain, accounts for the anisotropic g and linewidth-not observed as such in bulk Si:P. This
result indicates an impurity effect as contributing to the persistence of these Si microcrystaHites
upon annealing.
I. INTRODUCTION
During the last few years considerable progress has been
made on the fabrication of high-quality silicon~on~insulator
(SO!) material. In a particularly promising technique caned
SIMOX1 (separation by implanted oxygen) an oxide layer
buried in a silicon substrate is formed by high-energy oxygen
implantation and subsequent high-temperature annealing.
Until now the SIMOX structure and composition has been
analyzed mainly by high-resolution transmission electron
microscopy (HRTEM},2 Rutherford backscattering
(RES),3 secondary ion mass spectroscopy (SIMS),4 and
electrical measurements.5 While the latter technique is used
to investigate the electrical quality of the top Si layer, the
former techniques studied the formation and the structure of
the underlying buried oxide.
This has led to a significant optimization of the struc
ture:initiaHy, the interfaces of the SIMOX were rough and
the top and bottom silicon layers in the neighborhood of the
oxide were decorated with Si02 platelets and 8i02 precipi
tates. In the oxide many Si inclusions were embedded, while
the electrical quality ofthe top silicon iayer was poor, mainly
due to a high density of threading dislocations and c-Si pre
cipitates.6,7 Subsequent optimization of processing param
eters such as implant energy, dose, and implant temperature
and high-temperature annealing (;:::::: 1405 ec)8 completely
annihilates the 5i02 clusters at both interfaces and produces
a good Si top layer with a strongly reduced dislocation den
sity. The S1 inclusions in the buried oxide disappear except
near the bulkside SilSi02 interface where c-Si polyhedra,
bordered by low-index crystal planes and aligned to the un
derlying c-Si matrix,9 are formed. Whereas a higher concen
tration ofSi inclusions at the backside of the buried oxide can
be understood in terms of the skewed O-implantation pro~
file, to. I I which is much less abrupt at the backside, and the
low diffusivity9.10 of Si in 8i02, the resistance of these inclu
sions to anneal out even at temperatures near the Si melting
point ( = 1412 eC) is not well understood.
In light of its intrinsically high sensitivity and outstand-ing ability to discriminate between spin-active centers-and
indeed, point defects and contaminants often are spin ac~
tive-electron-spin resonance (ESR) is likely to add infor
mation not readily accessible by the previous methods.
While many ESR signals are detected, the present report will
mainly focus on a signal observed in an O-implanted wafers,
which is believed to be particularly inherent to the SIMOX
technique; it is related to the existence and annealing resis
tance of the faceted 8i islands in the backside of the buried
oxide. The present ESR data indicate that these 8i crystal
lites have a high P concentration (::::::2X 1018 cm-3), even if
starting fromp-type (B-doped) Si wafers.
II. EXPERIMENTAL DETAILS
Ao Sample preparation
Czochralski(Cz)-grown P-and E-doped [001] 8i wa
fers (room-temperature resistivity PRy;::::::5 n cm) measur
ing 547 Jlm thick were ion implanted with 150 keY oxygen
ions to a dose of ;::: 1.7X 1018 cm -2. During implantation the
wafer temperature was maintained at ;::::::600 "C. Using low
pressure chemical vapor deposition the wafers were capped
with;:::; 100 nm ofSi02 to screen oft'ambient contaminations
and subsequently annealed at temperatures ranging from
1000 to 1250 DC in N2 ambient for 8 h. After annealing, the
capping layer was stripped in buffered HF. This resulted in a
buried oxide layer;:::::: 350 nm thick covered with a 8i over
layer;:::::: 120 nrn thick. In some preparations, before deposit
ing the capping layer, samples had grown on a 5-.um-thick
epitaxial 5i layer in a reduced-pressure reactor. Prior to this
deposition, a HF dip was applied to remove the native oxide.
After the removal of the cap, the wafers were cut to platelets
of 2X9xO.547 mm3 size having their long edge along a
[110] direction. To enlarge the surface-to-volume ratio, the
samples were thinned down at the backside in planar etch
(HN0 3:CH3COOH:HF;7.5:2.5:1) to a final thickness of
176 ± 10 .urn. The experimental results presented here will
mainly concern P-doped Si wafers.
780 J. Appl. Phys. 66 (2), 15 July 1989 0021-8979/89/140780-07$02.40 @ 1989 American Institute of PhYSics 780
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
130.63.180.147 On: Sat, 22 Nov 2014 08:20:11B. ESRmtechnique
ESR measurements were carried out at K (20.9 GHz)
and X (9 GHz) bands in the temperature range 2.4< T,34
K. Because ofthe better signal-to-noise ratio, K band experi
ments were generally preferred. Modulation of the externa!
ly applied magnetic induction B and phase-sensitive detec
tion resulted in recording microwave absorption-derivative
dP!, I dB spectra. During the measurements the direction of
B was varied eitherin the (001) or (110) plane; the direction
of B in these is specified by the angle ~ relative to the [1101
and [001] direction, respectively.
R.elying on careful line-shape analysis, the signal inten
sity I (area under the absorption curve) oc X, the static mag
netic susceptibility, was determined using the 1 = kAppAB;p
method. In this, k is the line-shape factor while tlBpp and
2App represent the peak-to-peak linewidth and height of the
dP1, I dB response, respectively; microwave saturation effects
were well taken care of. Values of g were measured relative to
the LiF:Li g marker of gLiF:Li = 2.002 29 ± 0.000 01.
UI. EXPERiMENTAL RESULTS
Initially, the basic 8i material was checked for ESR sig
nals. Apart from the strong hyperfine doublet in P-doped
material, 12 none were found. However, as expected, the low
T ESR spectrum of the implanted samples shows various
signals, depending on the dopant and the annealing tempera
ture. A first defect, the PbO center, 13-15 has been well studied
before. Secondly, measurements on a nonannealed sample
show an intense signal of the amorphous defect center (dan
gling Si bond; g = 2.0055) which also has been investigated
before.14,15 Thirdly. somewhat unexpected, in all the im
planted samples there are traces of iron. In the n-type sam
ples interstitial neutral iron16 [FeO; g( 4,2
K) = 2.0070 ± 0.0001] is detected, while, additionally,
neutral (FeB)opairs17,18 (effective spinS = 3/2; 4.2 K prop
erties:gn = 2<0676 ± 0.0002 andgl = 2,0452 ± 0.0004, fine
structure constant ID 1= 2.7 ± 0.5 cm -I) show up in thep
type samples. Due to strong saturation effects it was not
possible to reliably determine the concentration of these
centers. However, by taking into account the signal-to-noise
ratio we could estimate a lower bound of :::::: 25 for the con
centration enhancement relative to the unimplanted sample,
in which no traces of iron have been detected. Whether this
iron enters the sample by implantation andlor heat treat
ment or whether it already resides in the sample in a nonpar
amagnetic state before is crucial to the inherent quality of
SiiSiOz structures achievable by the SIMOX method. A
fourth signal with g = 1.998 33 ± 0.000 05 and !1Bpp
= 2< 12 ± 0.08 G, both being <P and T independent within
experimental error, is only observed in n-type samples for
T> 12 K. Below;::: 16 K the signal strongly decreases to
become unobservableo A log (l1)-vs-Tplot indicates an acti
vated behavior giving EA = 5.8 ± 005 rneV; at 18 K, a spin
concentration Ns = (3.5 ± 0.7) X 1011 em" 2 correspond
ing to 2.1 X 1013 cm -3 if referred to the total sample volume
is measured. This signal has been observed beforel9,2o and is
ascribed to a thermal donor,21
All these signals have previously been analyzed and sev-
781 J. Appl. Physo. Vol. 66, No.2, 15 July 1969 1.9997 r-(a-:-r~i--r6--A""'A----rt-A-r--"""-"""1
.. &i A A
1.1 '--_-'--_-L __ '--_~--"--__::":-_~
o 10 20 HKI 30 3S
FIG. 10 Temperature dependence of the gvalue (a) and linewidth (b) of the
central pair P signal ascribed to P-rich faceted c-Si precipitates residing in
the buried oxide layer near the bulkside Si02/Si interface. Data are taken at
K band using.;; -0.9 dB m of microwave power.
eral of them are relatively well known; they are not further
addressed here. Of main concern, presently, is an additional
signal which shows up in all the implanted samples (n and p
type). The signal is observed at the K band at
{jg = 1.999 63 ± 0.000 03 with !1Bpp = 1.36 ± 0.04 G for
B!I[OOl] and T=4.3 K As displayed in Fig. 1(a) for a
SIMOX structure fabricated by implanting (001) n-Si with
1.7 X 1018 em 20+ ions at an energy of 150 keY, thegvalue
remains almost constant with an increasing temperature up
to 20 K. From there on it declines to reach the value
g = 1.999 46 ± 0.000 05 at 34 K As shown in Fig. 1 (b), for
Ell [001] the linewidth decreases from ABpp = 1.39 ± 0.02
G at T = 2.4 K to ABpl' = 1.26 ± 0.03 G at T = 10 K, to
increase again up to ABpp = 2.12 ± 0,02 G at 34 K. For
Ell (0011 the line shape is almost Lorentzian and the line
broadening homogeneous at all temperatures, as concluded
from comparative X-band measurements. As can be seen in
Fig. 2, I( n measurements reveal a Curie-Weiss behavior,
that is,!:::::: (T + e a ) -I, where 9" is the asymptotic Curie
Weiss temperature given as 9a = 2.7 ± 0.3 K. The spin den
sity Ns has been determined for BII[OOl]. Relying on the
observed Lorentzian line shape and the Curie-Weiss behav
ior, a value of Ns = 3.4X lOll em -2 ( ± 20%; S = 112) (or
2,0 X 1013 cm-3) has been found.
At 4.3 K, no saturation effects are observed for micro
wave powers P" incident on the cavity up to 0.9 mW. This
corresponds to the value B1 = 0.09 G for the amplitUde of
the in-phase rotating part of the microwave field at the cen
ter of the TEo!! cavity (loaded quality factor Q;:::30(0),
which means that this signal is fairly insensitive to satura
tion-rather unexpected for isolated defects in the Si/Si01
structure.22,23
Measurements for B varying in the ( 110) plane show a
clear anisotropy both ing and ABpp, as may be seen in Fig. 3.
G. Van Gorp and A. Stesmans 781
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130.63.180.147 On: Sat, 22 Nov 2014 08:20:11°O~----~------~10~----L-----~2~O----~~--~30
T(K)
FIG. 2. Plot of the inverse K-band ESR intensity (area under absorption
curve a:: X) of the P donor central signal observed on II (001) SIMOX struc
ture, showing a Curie-Weiss behavior with the asymptotic Curie-Weiss
temperature e. = 2.7 ± 0.3 K. The signal is ascribed to P donor electrons
delocalized over P clusters in Si islands of high P concentration :::;;2 X 10"
cm -2 (submetallicregion). Incident microwave power.;; -0.9 dB m. The
SIMOX sample results from implanting an n-Si wafer with 1.7 X 10'8 0 ~
ions at 150 keY followed by an S-h anneal at 1250 'CO
For HII [001 J one observes g = 1.999 63 ± 0.000 03, tlE""
= 1.36 ± 0.04 G both at the X and K band, which indicates
a homogeneously broadened signal: the line shape is closely
Lorentzian. For HII [110], g has decreased down to
1.9997
, 1 (a)
1.9996
" ""
1.9994
1.9992
5
(b)
'" C. Q,
"" <I
3
1~-9~O~--~--~--~Q----~--~--~9~O
1110l [ifl] (001) 4:>8(0) mOl
FIG. 3. g and linewidth anisotropy of the donor electron (P) central signa!
observed 011 a (001) SIMOX structure at T = 4.3 K for Ii varying in the
(I1O) plane. The sample has been fabricated by implanting n-Si ,,'lith
1.7X 10'80 + ions cm --2 at an energy of 150 keV and subsequent annealing
at 1250 'C fo:r 8 h in N2 ambient. Open and full symbols represent X (8.982
GHz) and K (20.95 GHz) band data, respectively. Incident microwave
power.;; -0.9 dB m.
7132 J. AppL Phys., Vol. 66, No.2, 15 July 1989 1~-~90~--~~~--~O~--~--~--~9~O~
i110J [100J [110] ¢B1') [110J
FIG. 4. A similar plot to Fig. 3, but now for Ii varying in the (001) plane.
1.999 32 ± 0.000 04, while 6.Bpp has increased to 3.6 ± 0.2
G due to inhomogeneous broadening; accordingly, the line
shape now looks more Gaussian. This inhomogeneous
broadening is confirmed by measurements at the X band [see
Fig. 3(b)]; indeed, for HII [110J the linewidth atthe K band
is substantially larger than at the X band. When H varies in
the (OOt) plane (see Fig. 4) the g anisotropy is less substan
tial, but the linewidth anisotropy is of about equal size as for
BE( 110).
The removal of the top Si layer by etching produced no
significant change in ESR intensity. However, etching off
the buried oxide totany eliminated the signal, indicating that
the defect centers are located in the the buried oxide or near
the bulkside SilSi02 interface.
IV. INTERPRETATION AND DISCUSSION
A. Signal Identification
Regarding its microscopic identification, the ESR data
provide clear evidence that the signal originates from phos
phorus-rich 8i inclusions in the backside ofthe buried oxide.
As mentioned, these c-Si inclusions are formed during the
buried Si02 layer growth as a result of the nonabruptness of
the buried Si/SiOz interfaces-where a mesh of SiOx re
gions is intimately admixed with Si-rich parts9 -and the low
diffusivity ofSi in Si02 0 Most likely, because of the skewness
of the implantation profile6,IO-the backside wing's slope is
much smoother than the front wing-the Si inclusions are
predominantly formed and persist longer during high-Tan
nealing near the bulkside interface. As a result ofthe minimi
zation of the interfacial energy (both strain-and surface-free
energy) and the orientational dependence of the oxidation
rate9 the Si inclusions are of polyhedral shape, the facets
consisting mainly of{lOO} and {11I} planes.24 P atoms lo
cated in the region implanted with oxygen, either as a dopant
in n-type material or as an impurity in p-type material, are
driven to these Si islands during formation for two reasons:
firstly, because of their higher solubility in Si than in SiOz
G. Van Gorp and A. Stesmans 782
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130.63.180.147 On: Sat, 22 Nov 2014 08:20:11(natural segregation effect) and secondly, because of the
lower chemical potential (strain-relieving properties) ofP at
the SVSiOz interface. Such P-piling up effect is weU docu~
mented in literature (see, e.g., Ref. 25 and references there
in). This results in c-Si i.nclusions of high effective P concen
trations, i.e., n-type c-Si islands. There is plenty of
experimental evidence for attributing the present ESR signal
to P atoms in these c-Si regions in which P has piled up to net
PdonorconcentrationsN p == [Pl :::;2 X 10lBcm 3, that is, Si
doped to the submetallic regime. This is much larger than
the homogeneous P and B dopant concentrations
[ :::; ( 1-3) X 1015 em -31 of the starting n~ and p-type Si sub
strates, respectively. More specifically, the signal appears
identical to the central pair line25•26 ascribed to donor elec
trons ddocalized over small P clusters.
Indeed, it is well documented that donor electrons in c
Si:P with N D > 1018 em -l are delocalized. Thus, the strong
31 P (nuclear spin I = 1/2) induced hyperfine splitting of the
ESR spectrum, which is typical for Si:P onow P concentra
tions, is averaged out. 27,28 This results in an ESR spectrum
consisting of a single Lorentzian line,29 showing the same
~pp~vs-Tbehavior as depicted26 in Fig. 1 (b); that is, a de
crease in linewidth with the temperature increasing from 2 to
8-12 K followed by a continuous increase from there on. The
temperature at which the linewidth is minimal is a measure
of the P concentration26 resulting for the present case in Np
= (1-2) X 1018 em --3.
This estimated concentration accounts quite well for the
ABpp values measured: at 4.2 K the linewidth26 for c-Si:P
dopedtoN p = 2X 1018 em -3 is ABpp = 0.90 ± 0.15 G. This
is not the case for other donors in 8i, such as As or Sb, which
exhibit a similar ~pp-vs-Tbehavior as P : the linewidth of
these impurities is always much larger. 30
The asymptotic Curie-Weiss temperature provides an
other way to determine Np• This Curie-Weiss behavior of X
appears as a typical fingerprint for high-concentration c
Si:P. Using ea = 2.7 ± 0.3 K. this again leads t026 Np
= (2± 1)x1013cm-3•
Also the observed g value (::::: 1.9996 for HI! (0011)
closely matches the Si:P signal.26 For HII [001] a Lorentzian
and homogeneously broadened line, a result of exchange
narrowing, is observed, which also agrees with observa
tions31 on c-Si:P, This, however, is not the case for the other
directions of:8, which will be addressed together with the
observed g anisotropies in the next section. The signal is fair
ly insensitive to saturation at 4.2 K. which almost precludes it
to originate from an isolated defect in Si or Si02• 22,23 The
response almost behaves as a metallic (conduction electron)
resonance. It is wen known that the delocalized donor elec
trons in Si:P of Np:::;2X101& cm-3 exhibit such behav
ior.23,25
Additional support for the proposed identification
stems from the observed density and size of the silicon pre
cipitates. SIMOX samples doped to doses comparable to the
one presently used and annealed at T",n:::; 1150-1250 ·C
show the Si polyhedra near the bulkside SilSi02 interface to
have an average mean size Da :::::40 nrn. Hence, the average
vol.ume of such a grain is V AV ~33 X 10--18 em3• As report-
783 J. Appl. Physo, Vol. 66, No. 2.15 July 1989 ed, we have measured Ns = 3.4 X lOll ( ± 20%) spins per
cm2 ofthe sample. In light of the susceptibility enhancement
eifeee2,33 this value rather represents the effective P concen~
trationNejf• ForSi doped to ~2x 10'8 P atoms em -3, the P
donor concentration is given as Np = (0.8 ± 0.06)Neffo, re
sulting in Np = 2.7 X 1011 em -2 for the present case. Sup
pose then there is a homogeneous P distribution in the Si
grains,34 we find such a grain to contain 2 X 1018 cm -3
X 33 X 10 -18 ern3 = 66 P atoms. Thus we find as the average
numberofSipolyhedraN ph = 207X lOll cm-2/66:::;4X 109
cm-2, which gives an average interpolyhedra distance dph
~ 156 nm if we suppose that their centers are homogeneous
ly distributed in one plane near the back Si/Si02 interface.
This NPh number agrees with the density derived from direct
HRTEM observations. 1,6
The high net N p value just derived is much higher than
the doping concentration in our p-type samples
([B] :::;2.6 X 1015 cm -3), thereby inverting the 8i crystal~
lites into n type, This is why the signal is also observed in the
SIMOX structure formed on p-Si substrates. Crucial, of
course, is the question where the necessary amount of P
atoms needed to cause such an effect comes from. As out
lined previously, the answer lies in the non-negligible com
pensation ratio Rc=ND INA (where ND and N.4 represent
the total donor and acceptor concentration) of commercial
Si wafers in general, 25 For the present case and items dis
cussed we may identify ND = NI> = [PI. NA = [HI. SO, P
impurities are inherent to the starting Si material, e.g., for
Rc = 0.5 the startingp-type Si would contain :::::6X 1014 P
atoms em --3. It has been wen pointed oues that thermal
treatments (typical as used here) will anow these P impuri~
ties to pile up in small Si layers (regions) near the Si/Si0 2
interfaces, herewith denuding the surrounding zones of di
mension;::; l-lOp;mofP. LocalconcentrationsN D>6X 1018
cm-3 may be easily reached in this way.
Hence, the model of the formation of P-rich Si precipi~
tates in the buried Si02 layer readily explains the majority of
the experimental results. Regarding this identification, how
ever, the g and MI'P anisotropy results remain to be ad
dressed. These merit special attention, partly because these
cannot directly be observed in bulk c-Si:P and partly because
these data are felt to provide conclusive evidence for the P
in-Si model.
B. 9 and .&Bp" anisotropy: Stress model
As outlined, the P-rich Si regions observed are identified
with the Sf microcrystallites near the bulkside Si02/Si inter
face. So far, the hint for this has been threefold: Firstly, etch
ing experiments have shown the phosphorus to be located in
or somewhere in the vicinity of the oxide. Secondly, the ex
planation for the existence of regions of high P doping needs
effects such as diffusion to Si/Si02 interfaces or pile up in Si
inclusions. Thirdly, the number of spins measured by ESR
can be explained in terms of the estimated local P concentra
tion and the density and size of the 8i polyhedra at the back
of the buried oxide, However, these facts still are not a direct
prooHor identifying the observed P-rich Si regions with the
Si islands. They cannot, for example, exclude the possibility
G. Van Gorp and A. Stesmans 783
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130.63.180.147 On: Sat, 22 Nov 2014 08:20:11that aU the phosphorus has piled up at the two "buried"
SilSi02 interfaces. The direct evidence for the P location
comes from the g and ABpp anisotropy, which, as will be
demonstrated, can only be explained in terms of polyhedron
shaped Si globules.
It is known that P impurities in c-Si may exhibit a
(weak) anisotropy.35-37 This g anisotropy ofP as wen as of
any other shallow donor in silicon is caused by strain. The
strain of the lattice, in which the P impurity is embedded, has
some implications on the electronic ground state of the do
nor electron resulting in g anisotropy. The theory of shallow
donors, as developed by Kahn and Luttinger,35 states that
the ground state of a donor electron comprises equal admix
tures from the six conduction-band valleys; this is modula
ted by an envelope function which is the solution of the asso
ciated hydrogenlike Schrodinger equation, thus giving rise
to an isotropic g value. Thus, in bulk unstrained c-Si:P, gener
ally no g anisotropy can be observed. This wave function can
be written as
6
W(r) = 2: a(j)F(j)(r)u(j)(r)exp[ik~j)rJ '
j= 1
where u(j)(r)exp[ik~j)rJ is the Bloch function of thejth
vaHey minimum, P< j) (r) is the solution of the hydrogenlike
problem, and aU) is the coefficient which describes the ad
mixture of the corresponding vaHey minimum. However,
when the crystal is strained, its symmetry is altered and the
di~erent valleys are no longer equivalent; some rise in energy
while others decrease. This also changes the admixture of the
different valleys in the ground-state wave function of the
donor electron: valleys where the energy is decreased will
have an increased population and thus an increased a( j), and
~ice versa. Via the spin-orbit coupling this valley repopula
tIon also changes the average g value which will be no longer
isotropic. A second effect of the strain is the deformation of
the valleys themselves. This influences the effective mass of
the electron in the valley and thus the g value, too.
. This indicates that the presently observed anisotropies
m g and .t.Bpp result from stressed P-rich Si regions. Hence
as an ansatz to account for these data, we suppose that the P
atoms have piled up in the Si inclusions in the buried oxide,
we determine the stress axes prevailing, and from there on
try to simulate the measured g-vs-$ and f1B -vs-<I> behav-. w lOr.
To determine the stress axes it is necessary to look at the
shape of the Si inclusions in the oxide. As mentioned before
HR TEM has shown that these inclusions are of polyhedral
shape bordered mainly by {Ill} and {1 OO} planes.24 Along
these facets there exists a large tensile stress due to the large
misfit in bond density between Si and Si02• As it may be
assumed that the P atoms in the Si globules are preferentially
situated at the SilSi02 interfaces of these globules25-even
when the P distribution is homogeneous, the assumption for
the strain remains valid for the main part of the P atoms
the kind of strain experienced by the P atoms (Si lattice)
depends on the kind offacet plane to which they are nearest
i.e., {lll} or {lOO}. For these cases the g anisotropy ha~
been calculated. For a tensile stress38 along a {lOO} plane,
the anisotropy is caused by vaHey repopUlation only and is
784 J. Appl. Phys., Vol. 66, No.2, 15 July 1989 described by
gee) -go = U( 1 -1.5 sin2 f)) ,
where f) is the angle between B and the stress axis (perpen
dicular to the plane in which the tensile stress is experi
enced), Uis the compilation of some physical constants and
parameters, such as the maximum g anisotropy and the val
ley strain; go is the g value in the absence of strain. For a
tensile stress along {Ill}, the formula for the g anisotropy is
formally identical except that U has been replaced by an
other constant Vand that the g anisotropy instead is caused
by the deformation of the valleys. Since the g value of the
piled up P donors has apparently not shifted much from the g
value [cf. g(T<4.2 K)::::;1.99875+0.00011 for
[P] ::::;8 X 1011> cm-3] oflow concentratio; P donors,23 we
may assume that the donor electrons spend most of the time
localized at donor sites. This permits the use of the above
formulas calculated for isolated donors.
So, the supposition that P atoms are piled up in strained
Si inclusions leads to the conclusion that instead of observing
an isotropic g value and a single homogeneously broadened
Lorentzian ESR signal as would be the case for highly doped
bulk c-Si:P ([Np] > 1018 cm -3) in the absence of strain, a
superposition of homogeneously broadened Lorentzian lines
is observed. The "various" P lines originate from P atoms
embedded in various Si layers adjacent to internal Si/SiO
interfaces which are at different angles with respect to B,
each "plane" exhibiting an anisotropic g value as a result of
strain. Because the constituent lines cannot be resolved, a
convoluting single line with an anisotropic g value and an
attendant "breathing" linewidth behavior is observed. In
case the Si-precipitate facets would comprise all kinds of
orientations (various Miller indices), or if the various 3i mi
crocrystals would not be aligned along one Si matrix (e.g.,
the substrate matrix), the averaging effect would result in an
isotropic signal (cf. powder effect).39 However, it is exactly
due to the common orientation of these precipitates and the
limited number of border plane indices prevailing (i.e.,
mainly {Ill} and {I OO} ) that some strain-induced anisotro
py ing (and consequently in flBpp, too) results, thus strong
ly evidencing the proposed identification. For certain B di
rections the g value of many of the lines coincides and a
single homogeneously broadened Lorentzian line with a
smalllinewidth is observed; for many other directions, how
ever, the signal is inhomogeneously broadened; it gets more
Gaussian character and exhibits a larger linewidth.
Co Application of the stress model
The model has been tested quantitatively by fitting to
the measured g-vs-<P and I:l.Bpp -vs-<I> relations along the fol
lowing guides. Firstly, for each spectrum (1) value) the
mean g value, as calculated from the resonance fields of the
constituent facet-related lines and weighed in proportion to
their relative abundance, is equaled to the measured g. Sec
ondly, the standard deviation of the distribution of reso
nance fields is a measure for the linewidth of the observed
inhomogeneous broadening, which is assumed to be Gaus
sian. Since the homogeneously broadened components of the
signal are Lorentzian, an overall Voigt line shape may be
G. Van Gorp and A. Stesmans 784
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130.63.180.147 On: Sat, 22 Nov 2014 08:20:11assumed. Its linewidth is calculated using an empirical relationship established by Stoneham40:
(AB ~p}2 + O.9085(AB;p }(.6.B~p) + 0.4621(6.B ~p )2
Mlpp= ilB~p+O.4621(LlB~p) ,
where ilB ~p is the width of the isotropic Lorentzian part
which is determined experimentally and aB ~p is the
linewidth of the observed inhomogeneous broadening; the
latter is anisotropic and is calculated from the distribution of
the different constituent resonance fields. Third, the magni
tude of the tensile stress along border planes with equivalent
Miller indices, e.g., aU {IOO} planes, is assumed equal.
Fourth, every P atom is subjected to the tensile stress of only
one border plane of the precipitate. So, the stress is assumed
to be perfectly tensile with the stress axes oriented exactly
along the (100) or < 11 1> directions. This implies that an
precipitates are perfectly aligned with respect to the underly
ing Si substrate. Fifth, because of the symmetry of the pre
cipitates and their location, the total surface of the four
{Ill} facets has been taken equal in the calculationso For the
same reason, the total surface of the (100) and (010) planes
is taken to be the same while a total (001) surface-this is
the surface parallel to the buried layer-larger than the
(100) and (010) ones is admitted. 1.9 The fitting results are
shown in Figs. 3 and 4 by the full curves. AlI four curves are
obtained with one set of six parameters: two parameters de
scribe the relative ratio of the total surfaces of the different
border planes of the precipitates and one parameter each
defines go (the g value in absence of strain), M ~p (the ho
mogeneous linewidth), U (comprising the maximum g ani
sotropy for stress along a {lOO} plane), and V (comprising
the maximum g anisotropy for stress along a {Ill} plane) 0
'While the general features are wen described, the fitting
is not quite perfect at all angles. This has to be partly ascribed
to the small signal-to-noise ratio and partly to the simplicity
of the model. For example, the smail g anisotropy which is
measured for the BE(OOl) plane and which, as such is not
predicted by the model, may be caused by the varying shape
of the signal; this does not need to be perfectly symmetric for
all angles, a fact which has not been included in the model.
Neither did we include a distribution in stress nor stress an
gle 8, which eventually occurs. Nor did we take into account
the effect upon Hnewidth of the change in wave-function
overlap between the donor electrons as stress is applied.
Also, the exact distribution of P in the Si globules is un
known. Taking into account these effects would certainly
improve the model but would at the same time drastically
increase the computation time without adding much to the
physical significance. Indeed, one can hardly expect to have
a correct knowledge of microscopic properties as, e.g., the
stress distribution present and the ratio of the surface of the
{lOO} and {I I I} facet planes.
In fact, the mode! satisfactorily accounts for the g ani
sotropy and the attendant "breathing" linewidth behavior.
A further test of the model consists in probing the physical
relevance of the numbers obtained for the fit variables U and
V. Using these numbers it is possible to get an order of mag
nitude estimate of the stress Tat the internal Si/Si02 inter-
785 J. Appl. Phys., Vol. 66, No.2, 15 July 1989 faces of the globules. For example, the fit variable V can be
expressed35,36 as V = 0.44T I(3C44), where T is the stress
along < 111 }-of the same order of magnitude a~ the equiv~
lent tensile stress in the {lIt} planes-and C44 IS the eiastlc
constant, which for silicon4! is equal to 8 X lOll dyn/cm2.
This results in T= (1O±5)X109 dyn/cm2, which com
pares reasonably with the stress measured in Si/Si0 2 wa
ferso42
'110 CONCLUDING REMARKS
It has been shown that the faceted Si microcrystals re
maining in the buried Si02 near the bulkside SilSi0 2 inter
face after high-T (:::: 1000--1250 ·C)annealing of a SIMOX
structure have accumulated a high local P density. While the
general characteristics of the ESR signal observed match the
central (pair) line due to donor electrons in submetallic Si:P
doped to ::::2 X lOIS em -3, the key evidence for the P signal
to originate from the embedded 8i islands stems from the
observed anisotropy in g and .6.Bpp. This finding may closely
relate to the elimination behavior of the Si islands, i.e., their
migration from the Si02 layer towards the Si matrixo It has
been well observed that contrary to the Si precipitates near
the front Eli/SiD, interface, the Si islands near the back inter
face are extremely difficult to eliminate even after prolonged
high-T annealing at T-z 1405 "c. This has previously been
ascribed9 to the very low ditfusivity of Si in 8i02, cf. at
1300 "C, DSi = 3.25 X 10 17 cm2/s. The present result, how
ever, indicates, that this may be only part ofthe reason. Like
ly. there is a significant effect of impurity pinning; the P
impurities have a much higher solubility in 8i than in Si02
f cf. the P segregation coefficient43 ml' == C" (SO/ Cp (Si02) -z 10, where Cj (X) represents the equi1~briurn
concentration of impurity i in solid Xl, thus accountmg for
the Si islands' persistency. A similar statement may apply for
As and Sb impurities.43
Hence, in that case future SIMOX work should aim for
a starting material of higher purity. One possibility is to start
from p-type (B-doped) material-B does not pUe up near
the SilSi02 interface during Si02 growing43 (cf. m II
<0.3 )-with, however, the strict demand of a negligible
compensation ratio, i.e., [PJ, [As], [Sb] all very low. Also
the dean ness ofthe implantation process and the heat treat
ment are of utmost importance because of possible conta
mination by transition metals. In this way it might become
possible to fabricate homogeneous buried oxides with wen
defined SilSi02 interfaces.
ACKNOWLEDGMENT
One of us (A.S.) was supported by the Belgian National
Fund for Scientific Research.
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therein.
G. Van Gorp and A. Stesmans 78S
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22See, e.g., D. L. Griscom, Phys. Rev. B 20, 1823 (1979).
23G. Feher, Phys. Rev. 114, 1219 (1959).
786 J. Appl. Phys., Vol. 66, No.2, i 5 July 1989 20c. Jaussaud, J. Stoemenos, J. Margail, M. Dupuy, B. Blanchard, and M.
Bruel, App!. Phys. Lett. 48, 1470 (1986).
25 A. Stesrnans and 1. Braet, Surf. Sci. 172, 398 (1986).
26J.D. Quirt a,~d l.R. Marko, Phys. Rev. B 7, 3842 (1973).
27G. Feher, R. C. Fletcher, and E. A. Gere, Phys. Rev. 100, 1784 (1955).
2BC. P. Slichter, Phvs. Rev. 99, 479 (1955).
29S. Maekawa and N. Kinoshita, l. Phys. Soc. Jpn. 20,1447 (1965).
30J. H. Pifer, Phys. Rev. B 12,4391 (1975).
J'H. Kodera, 1. Phys. Soc. Jpn. 27,1197 (1969).
'2A. Stesmans,J. Magn. Reson. 76,14 (1988).
3:1H. Ue and S. Maekawa, Phys. Rev. 3, 4232 (1971).
34Por Si particles of Da;:::4O nm, a discussion about whether the P donors
are either homogeneously distributed throughout the grain or piled up at
the Si/Si02 grain interfaces would appear rather superficial. In the latter
case the pertinent Si/Si02 region would readily extend several tens of nrn,
thus enclosing the whole grain.
35W. Kahn and M. Luttinger, Phys. Rev. 97,1721 (1955).
36D. K. Wilson and G. Feher, Phys. Rev. 124, 1068 (1961).
37y Yafet, in Solid State Physics, edited by F. Seitz and D. Turnbull (Aca
demic, New York, 1963), Vo!. 14, p. 1.
3'The formulas used actually describe the anisotropy of g for a uniaxial
stress along (100) and < 111) axes. Since the alteration of the Si crystal's
symmetry is the same whether a uniaxial compressive stress is applied,
e.g., along (100), or a tensile stress in a plane perpendicular to this axis,
the use of these formulas to describe the angular variations of g is justified
(see Ref. 36).
39See, e.g., P. C. Taylor and P. J. Bray, J. Magn. Resoll. 2, 305 (1970).
'GA. M. Stoneham, J. Phys. D 5, 670 (1972).
"H. J. McSkimin. J. App!. Phys. 24, 988 (1953).
42See, e.g., Eo Kobedaand E. A. Irene,I. Vac. Sci. Techno!. B4, 720 (1986).
43A. S. Grove, O. Leistiko, Jr., and C. T. Sah, J. App!. Phys. 35, 2695
(1964).
G. Van Gorp and A. Stesmans 786
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1.343142.pdf | Substrate effect on the deposition of Zn3P2 thin films prepared by a hotwall method
Shunro Fuke, Tetsuji Imai, Kazushige Kawasaki, and Kazuhiro Kuwahara
Citation: Journal of Applied Physics 65, 564 (1989); doi: 10.1063/1.343142
View online: http://dx.doi.org/10.1063/1.343142
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/2?ver=pdfcov
Published by the AIP Publishing
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130.70.241.163 On: Tue, 23 Dec 2014 03:05:15Substrate effect on the deposition of Zn3Pa thin fUms prepared
by a hot .. wan method
Shunro Fuke. Tetsuji !mai, Kazushige Kawasaki, and Kazuhiro Kuwahara
Department of Electronics, Faculty of Engineering, Shizuoka University, lohoku, Hamatsu 432, Japan
(Received 10 August 1988; accepted for publication 6 October 1988)
Z113PZ thin films have been deposited by a hot-wall method on Pyrex glass and (lOO)GuAs
substrates. For GaAs crystalline substrates, higher deposition rates were obtained at the same
source and substrate temperatures than those for glass substrates. Layers deposited on ( 100)
GaAs substrates showed stronger preferential (004) orientation of the tetragonal structure, and
hence an improved columnar structure is obtained as compared with those on glass substrates.
The films having higher crystalline quality deposited on GaAs substrates have room
temperature resistivities as low as ~ 10 n cm.
l. INTRODUCTION
Zinc phosphide (Zn3P2), the II3-V2 p-type compound
semiconductor, has attractive properties as a material for
optoelectronic applications. It has a direct band gap near 1.5
eV (Ref. 1) and long minority-carrier diffusion lengtns2
which are required to obtain optimum optoelectronic con
version efficiencies for photovoltaic devices.3-5 Because of its
large optical absorption coefficient, thin films of ZU3P2 may
be suitable for solar cell. Polycrystalline Zn~P? thin films
have been deposited by hot -waH deposition, 6-8 ~a~uum evap
oration,'} close-spaced vapor transport technique, 10 and ion
ized cluster-beam deposition II so far. However, the electri
cal resistivity of these films is often too high to be used for
solar-cell application.
The deposition method and some structural and electri
cal properties of Zn3P 2 thin films prepared by using the hot
wall deposition technique have been presented in some detail
in previous papers.6•7 In this paper we report the deposition
characteristics of uudoped Zn3P2 thin films on both Pyrex
glass and GaAse 100) substrates. The effects of growth con
ditions and of the substrate materials on the structural and
electrical properties of these films are discussed.
It APPARATUS AND EXPERIMENTAL CONDITIONS
The hot-waH deposition system used in this experiment
for the preparation of Zn3P 2 thin films was described in de
tail in previous papers.6•7 The temperatures of the source,
quartz wall, and substrate were controlled independently.
The polycrystaHine Zn3P 2 source material was synthesized
by direct combination of high-purity zinc and phosphorus in
a quartz tube. The substrates used in this experiment are
Pyrex glass and Cr-doped semi-insulating (lOO)GaAs sub
strates. Zn3P2 has a tetragonal crystal structure with an a
axis lattice constant of 8.095 A, while GaA::; is a cubic crystal
with an a-axis lattice spacing of 5.653 A (diagonal dis-
tance = 5.653X~2 = 7.995 A). GaAs, therefore, seems to
be an adequate substrate material for depositing Zn3P 2 films
from the viewpoint oflattice matching. The Pyrex glass sub
strates are ultrasonically cleaned prior to deposition in tri
chlorethylene, acetone, and methanol, sequentially. The
GaAs wafers are etched in a solution of H2S04 :HZ02:H20
( 4: 1: 1) at room temperature. The deposition conditions are
summarized in Table 1. The crystallinity of the thin films obtained at various
deposition conditions has been characterized by x-ray dif
fraction. The diffraction patterns indicate strong peaks cor
responding to (004) and (008) planes of ZnJP2 having a
tetragonal crystal structure. The room-temperature resistiv
ity of the films is measured by a two-terminal method in the
dark. The ohmic Ag contacts to the films are formed by
vacuum deposition, and copper lead wires are attached using
a conducting silver paste.
iii. DEPOSITION BEHAVIOR AND CHARACTERIZATION
OF DEPOSITED FILMS
A. Deposition rate
Figure 1 shows the source temperature dependence of
the deposition rate of Zn3P2 films on both Pyrex glass and
( 100) GaAs wafer substrates. The substrate temperature is
maintained at 280 or 360°C. As the source temperature be
comes higher, the deposition rate increases exponentially,
reflecting the dissociation pressure of the Zn3P2 source. For
the condition of smaller temperature differences between the
source and the substrate, the deposition rates decrease rapid
ly.6 For the GaAs substrate, the deposition rate is much larg
er than that on Pyrex glass. Furthermore, the deposition
takes place at a lower temperature difference between the
source and substrate (about 120°C) than for the Pyrex glass
substrate (about 160 °C).
Figure 2 is a plot of the deposition rate of Zn3P 2 films as
a function of the substrate temperature for different source
temperatures. The source temperature is maintained at 480
or 540°C. For both substrate materials, the deposition rates
tend to saturate at larger substrate-source temperature dif
ferences. And the deposition rates decrease rapidly at higher
substrate temperatures, depending on the combination of
substrate materia! and source temperature. When the sub-
TABLE L Experimental conditions for deposition of undoped Zn3 P 2 films.
Source temperature
Wall temperature
Substrate temperature
Substrate
Vacuum pressure
Deposition time (1:ou ) 420-540 ·C
(Tw) 420-54O·C
( 'T"ub ) 240-380 ·C
GaAs( leoO), Pyrex gla.~s
<5X 10-' Torr
Ih
564 J. Appl. Phys. 65 (2), 15 January 1989 0021-8979/89/020564-03$02.40 @ 1988 American Institute of Physics 564
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130.70.241.163 On: Tue, 23 Dec 2014 03:05:15• "-on glass
° {;. on GaAs /' / 1>./ o e/ / ./1>, /'
/0 1:(r../"-
o /- I>, / 0/ I
/ I 360 I
Tsub~280'C / A I
,,280 I '" 360
420 450 500
SOURCE TEMPERATURE
FIG, 1, Deposition rate of Zn,P 2 films deposited on glass and GaAs sub
strates vs source temperatures, Depositions were made by a hot-wall tech
nique,
strate temperature is low enough, the reevaporation rate of
Zn adsorbed on the growing surface is much smaller than the
impingement rate of Zn from the source. Hence, both the
substrate temperature and the substrate material have rela
tively little effect on the deposition rates for a constant
source temperature.
As mentioned above, despite the same source tempera
ture and the same impingement rate of Zn. the deposition
rate on crystalline GaAs substrates is larger than that on
Pyrex glass substrates except for the lower substrate tem
perature range,
Furthermore, for the GaAs substrate, deposited Zn3P 2
films with better crystalline quality and better surface ap
pearance can be obtained as discussed below. These favor
able structural features of Zn3P2 films deposited on
(lOO)GaAs substrates promote the deposition rate. In addi
tion, the smaner source-substrate temperature difference
which is required for film deposition is also obtained.
B. X~ray diffraction
Figure 3 (a) shows the x-ray diffraction patterns for the
Zn3PZ films deposited on the Pyrex glass substrate. No dif
fraction peaks were detected within the measured diffrac
tion -angle range less than 70·, except for the ( (04) and
(008) diffraction peaks from the tetragonal structure. This
indicates that the deposited layers have a structure with col-
,......
.s::.
E
:J.. 30 It .. on glass
o A on GaAs
w 20 I-4:
0: ,0
Z e 5
lo-
U}
0 2 a. w 0 250 300 350 380
SUBSTRATE TEMPERATURE ( ·C)
FIG. 2. Deposition rate of Zn'P2 films deposited on glass and GaAs sub
strates vs substrate temperatures.
565 J, Appl. Phys., Vol. 65, No, 2, 15 January 1989 I J 10 ~ A (004) (OOS)
1\ Koil
r!K_Z
5
1 r I \ j \
III ) \ / \
0 / " o --_/, ' -+ a.
u 31 32 65 66 x: (a) on Pyrex glass
>-60 (008) 10-
~:OO41 ~ 100 K"'f
(\
GaAs sub, I'
lU II
I- 40 I II Koc2 Ko\! (400)
Z I \ ri
50 I V,
I I 1\ ~z I ' 20 I \ I ~I
I \ \ ) \) \ / ~.-0 32 0 / ,
31 65 66
DiFFRACTION ANGLE 26 (deg)
( b) on GaAs
FIG, 3, X-ray diffraction patterns of Zn'P2 films deposited on (a) Pyrex
glass substrate at T,,,u = 520 ·C, l~ub = 280 'C, and (b) (IOO)GaAs sub
strate at T,,,u = 540 "C, T",b = 360 "C,
umnar growth perpendicular to the substrates. This has been
confirmed by scanning electron microscope observations. 7
The diffraction peak intensity increases significantly and the
surface appearance also indicates larger grain sizes as the
film thickness becomes large. This means that the crystallin
ity of Zn3P2 films deposited on Pyrex glass substrates im
proves gradually to give the distinct columnar structure with
larger grain sizes,
The diffraction pattern for the Zn.lP 2 layer deposited on
a GaAs(100) substrate [Fig. 3(b)) showstheobvioussepa
ration of the (008) diffraction peaks corresponding to
eu Ka, and Ka2 x rays. Such peak separation has not been
observed in Fig. 3(a). The values offull width at half maxi
mum (FWHM) of diffraction peaks for the layer are smaller
than those for the layer deposited on glass substrates shown
in Fig. 3 (a). In Fig, 3 (b) the diffraction pattern for the used
GaAs substrate is also given, This shows that even the layer
deposited on the GaAs substrate has much larger values of
FWHM than that for the GaAs substrate itself. This means
that the crystallinity ofZn3P 2 films deposited on GaAs wafer
substrates is much superior to that deposited on Pyrex glass
substrates, probably because the crystalline substrate assists
the growth of highly oriented layers from the initial stages of
growth. These results show the possibility of epitaxial
growth of Zn3P2 layers on GaAs wafer substrates by opti
mizing the deposition conditions.
Figure 4 shows the rocking curves of the Zu3PZ films
deposited on Pyrex glass and GaAs substrates. The diffrac
tion angle 2e was fixed at the peak position of (004) diffrac
tion pattern. The coordinate is the x-ray incident angle e,
and the abcissa is the diffraction intensity. The film thick-
Fuke eta/, 565
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130.70.241.163 On: Tue, 23 Dec 2014 03:05:15on Pyrex glass
THICKNESS
RESISTr"ITY
:> l-
t!)
Z
W I
Z 2.0 6.2 (jJm)
I.Sxl04 (ncm)
20
10
DIFFRAC TION ANGLE on GaAs
5.4 (}1m)
6.4 (.fUm)
FWHM:1.2
15 25
e (deg)
FIG. 4. X-ray rocking curves of Zn,P? films deposited on Pyrex glass ( T"",
= 480°C, and T.,uh =-280 ·C) and GaAs substrates (1;",,= 500·C and
I~ub = 360°C).
ness of two specimens is chosen to be nearly the same in
order to compare the (004) or c-axis preferential orienta
tion. The FWHM values of the diffraction peaks obtained
from the films on GaAs and Pyrex glass substrates are 1.20
and 7.6°, respectively. These values indicate that films on
GaAs have a better columnar structure7 with a superior
crystalline quality.
c. Resistivity
Figure 5 shows the relation between the reciprocal of the
deposition temperature and the room-temperature resistiv
ity of the films deposited at the source temperatures of 480
and 540 °e. All films show p-type conduction. Films on
GaAs substrates have much lower resistivity than those on
Pyrex glass substrates. This is probably due to the better
crystallinity and the lower density of grain boundaries. The
lowest value of the resistivity is about 10 n em, which is
comparable to that obtained for single-crystal ingots.12 If
deposition conditions are optimized, epitaxially grown lay
ers having low resistivity values of several n cm can be ob
tained.
The room-temperature resistivity tends to a somewhat
larger value for lower substrate temperatures, and it de
creases exponentially as the substrate temperature is in
creased to higher values. The activation energy for the sub
strate temperature dependence of the resistivity is about 2 e V
for layers on both substrates. Though the values of resistivi
ties are different, depending on the substrate, crystallinity,
and growth rate, the mechanism deciding the carrier con
centration is believed to be the same for both substrate mate
rials. The reciprocal of the film resistivity (hole concentra
tion), for an assumed constant mobility, becomes larger
when depositions are made at lower source temperatures
and higher substrate temperatures. These facts suggest that
the hole concentration is qualitatively controlled by the dif
ference between the inpingement rate and the reevaporation
rate of Zn atoms.
566 J. Appl. Phys., Vol. 65, NO.2, 15 January 1989 ot:. on glass
on GaAs
1.5 1.6 1.7 1.8 1.9
103/Tsub (11K)
FIG. 5. Roomotemperature resistivity of films deposited on glass and GaAs
substrates vs reciprocal of the deposition lemperatur<:!.
IV. CONCLUSION
ZR,P 2 layers are deposited on both Pyrex glass and
( 100) GaAs substrates by a hot-waH method, and the growth
rate, structural properties, and electrical resistivity of the
grown films were compared. The deposition rate of the lay
ers deposited on GaAs substrates is much larger than that on
Pyrex glass substrates under the same deposition conditions.
The strong preferential (004) orientation of the tetragonal
structure was confirmed for films deposited on GaAs sub
strates using x-ray diffraction analyses. The resistivity of the
layers decrease for the lower source temperatures and higher
substrate temperatures. Highly oriented layers having low
room-temperature resistivities (as low as -10 n cm) were
obtained on crystalline GaAs( 100) substrates.
ACKNOWLEDGMENT
This work was partially supported by the Yazaki Foun
dation.
'E. A. Fagen, J. App!. Phys. 50. 6505 (1979).
2N. C. Wyeth and A. Catalano, J. Appl. Phys. 50, 1403 (1979).
'P. S. Nayer and A. Catalano, App!. Phys. Lett. 39, 105 (1981).
4M. Bhushan and A. Catalano, Appl. Phys. Lett. 38, 39 (1981).
'T. Sllda, M. Suzuki, and S. Kurita, Jpn. J. Appl. Phys. 22, L656 (1983).
6S. Puke, S. Kawarabayashi, K. Kuwahara, and T. Imai, J. App!. Phys. 60,
2368 (1986).
7T. Imai, S. Puke, S. Kawarabayashi, and K. Kuwahara, App!. Surf. Sci.
33/34,594 (l988).
"K. R. Murali, P. R. Vaya, and J. Sobhanadri, J. Cryst. Growth 73, 196
(1985).
9 A. Catalano, V. Dalal, E. A. Fagen, R. B. Hall, J. V. Masi, J. D. Meakin,
G. Warfield, and A. M. Barnett, in Photovoltaic Solar Energy Conference,
1977 (Reidel, Dordrecht, Holland, 1978), p. 644,
10M. Bhushan, Appl. Phys. Lett. 40, 51 (1981).
liT. Suda, T. Kanno, and S. Kurita, Jpn. J. App!. Phys. 22, L777 (1983).
12S. Fuke, Y. Takatsuka, K. Kuwahara, and T. Imai, J. Cryst. Growth 87,
567 (1988).
Fuks etaJ. 566
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1.2810398.pdf | SSC Design Revisions Call for Thinner Beams and Fatter Magnets
Bertram Schwarzschild
Citation: Physics Today 43, 1, 47 (1990); doi: 10.1063/1.2810398
View online: http://dx.doi.org/10.1063/1.2810398
View Table of Contents: http://physicstoday.scitation.org/toc/pto/43/1
Published by the American Institute of PhysicsWASHINGTON REPORTS
DOE should move forward on the
"footprint" for building the machine
at the site. Approval is necessary
before the state begins buying land
with some of the $1 billion in general
obligation bonds made available last
year by voters. Six counties, includ-
ing Ellis, have agreed to raise vehicle
license taxes to pay for roads to the
SSC laboratory around the turn-of-
the-century town of Waxahachie.
Barton believes that DOE's prudent
approach to the SSC was correct. He
insists that Hunter had the best
interests of the country in mind in
being cautious. But sources in the
Administration insist Hunter was
really trying to micromanage the
project. Most criticism centers on
how Hunter sought to slow the mo-
mentum for the SSC. In January
1989, when Schwitters became SSC
director, Hunter organized a separate
unit within DOE's high-energy re-
search office to keep tight control on
the project.
Hunter, for his part, claimed in an
interview that he hadn't been able to
get a grip on SSC expenditures or get
a schedule of project milestones and
deliverables. "Whenever I asked for
these," Hunter said, "I would get a
runaround: 'We're working on those.'
I wasn't getting answers about spend-
ing rates or magnet progress."
Schwitters characterizes Hunter'sgrievances as "nonsense." He argues
that he kept Hunter completely in-
formed but wasn't receiving much
communication from him in return.
Meanwhile, Clements and Luce
complained that the project appeared
to be going from bad to worse and that
Hunter was causing many of the
management and morale problems.
On 7 September, Watkins issued
DOE's management plan for the SSC,
overriding Hunter, by limiting the
number of officials in Washington to
provide oversight to 30 and authoriz-
ing no more than 60 at the site. He
restored fiscal supervision to DOE's
Chicago Projects Office, which Hunt-
er had removed from the loop.
Competing to build the SSC
Moreover, when members of Con-
gress expressed worry that DOE
would abuse its power to choose sub-
contractors, lawmakers clamped fet-
ters on the department with specific
language in a report by the Senate
Committee on Energy and Water
Development. Watkins made it clear
in his memorandum that he would
not tolerate the department's inter-
ference with the choice of subcontrac-
tors by SSC and Universities Re-
search Associates, the organization of
72 US and Canadian universities that
directs both the SSC and Fermilab.
Somewhat ironically, it was neitherDOE nor URA that made public the
names of the three teams of industrial
firms that will compete for the $1
billion contract to manage engineer-
ing and construction for the SSC
tunnel. On 6 December the news was
released by Barton's office, because,
says the Congressman, "the winners
and losers were all calling me and so I
thought the information should be
made public." The finalists, from
among 14 contenders for the contract,
are Fluor-Daniel, the construction
arm of Fluor Corporation and ICF
Kaiser Engineers; Parson, Brincker-
hoff, Quade and Douglas, MK Fergu-
son and CRSS of Houston; and a joint
venture of Daniel, Mann, Johnson
and Mendenhall of Los Angeles and
Bechtel National Inc.
It is likely that the engineering and
construction contract will be awarded
this year, though there is little money
in the SSC budget to begin work.
Much will depend, obviously, on the
fiscal 1991 budget, which Presiden t
Bush will deliver to Congress on 22
January. Members of Congress from
Texas say the DOE budget will contain
$393 million for the SSC. But at the
White House Office of Management
and Budget they speak about $310
million—scarcely enough to get on
with producing the remodeled ma-
chine at its higher new price.
—IRWIN GOODWIN
SSC DESIGN REVISIONS CALL FOR
THINNER BEAMS AND FATTER MAGNETS
The Central Design Group for the
Superconducting Super Collider pro-
duced its conceptual design for the
proposed 20x20 TeV proton-proton
collider in 1986. Since then, a specific
site for the SSC has been selected in
Ellis County, Texas; experimental
models of the 6.6-tesla bending mag-
nets required for the collider ring
have been extensively tested; and
powerful new computer codes have
now made it possible to simulate the
trajectories of individual protons over
millions of circumnavigations of the
54-mile storage ring.
Armed with this new knowledge of
how the protons stored in the ring will
behave during the crucial beam-injec-
tion phase and how the unpredecen-
tedly long and powerful superconduct-
ing bending magnets perform at oper-
ating temperature and currents, the
SSC Laboratory in Dallas, which has
taken over the responsibilities of the
CDG, has produced a supplemental
design for the accelerator. The princi-
pal changes in the revised SSC designare a doubling of the injection energy,
more focusing magnets in the ring and
a 25% increase in the width of the
vacuum beam pipe.
The 1986 conceptual design called
for the countercirculating protons to
be injected into the final ring at an
energy of 1 TeV after preacceleration
in a sequence of linacs and booster
rings. Filling the ring with its full
complement of protons will take a
half hour, after which the rf cavities
spaced around the ring begin acceler-
ating the protons up to their final
energy of 20 TeV. During the filling
phase the protons will have to survive
107 trips around the ring without
being lost in collisions with the walls
of the vacuum beam pipe.
Beam-pipe aperture
In the original design, the aperture
diameter of the vacuum beam pipe
that threads its way through the
thousands of bending and focusing
magnets is specified as 4 cm. Was
that wide enough? The larger theaperture, the smaller is the likelihood
of wayward beam protons striking the
wall. But bigger apertures are also
more expensive. They move the mag-
net coils farther away from the beam
axis, making it necessary to build
magnets with more superconducting
cable. The unprecedently narrow 4-
cm original design was described by
the Central Design Group's director,
Maury Tigner, as one of the "aggres-
sive" specifications chosen for reasons
of economy. (See PHYSICS TODAY,
April 1988, page 17.)
It's not just a matter of the beam
scraping the walls. Ideally the bend-
ing magnets would have perfect dipole
fields. But real bending magnets are
inevitably plagued with higher-multi-
pole field components, whose adverse
effects on beam quality become worse
as the protons find themselves farther
from the magnet axis. The beams
must also be kept narrow so that the
experimenters will have adequat e col-
lision rates where the beams intersect.
The question is, how good can and
PHYSICS TODAY JANUARY 1990 47must the field quality be?
Traditionally, choosing a beam-pipe
aperture has been something of a
black art. Lacking the powerful sim-
ulation codes that have been devel-
oped for supercomputers in the last
year at the SSC Lab by Yton Yan,
David Ritson (SLAC) and their col-
leagues, accelerator designers had to
rely heavily on intuition. The beam is
most likely to stray beyond acceptable
limits in the horizontal plane, as a
result of betatron oscillation and
chromatic aberration. The latter is
due to the spread of particle mo-
menta: Particles of different momen-
tum experience different curvatures
in the bending magnets. Both effects
scale with beam energy like 1/\[E.
The lower the beam energy, the
greater are the excursions from the
beam axis. That's one of the reasons
why the half-hour injection and fill-
ing phase is the most precarious.
The simulation code
The new computer code lets the accel-
erator designers follow 64 individual
protons on a Cray at about one or two
percent of the real-time rate. That is
to say, it takes a day or two of
supercomputer running time to simu-
late 64 protons with different initial
conditions making 107 circuits of the
SSC durin g the half-hour filling
phase. These simulations seek to
determine how many of the injected
protons will survive this billion-kilo-
meter initial journe y under a variety
of machine parameters. The code can
also simulate the acceleration phase
that follows filling. But in the at-
tempt to optimize machine param-
eters within cost constraints, the em-
phasis has been on the filling phase.
This lowest-energy phase of the SSC
ring cycle is also the time at which
"persistent current" magnet prob-
lems are the most severe. All cycled
accelerator magnets have hysteretic
problems at the low-field beginnings
of their cycles. But such problems are
particularly acute for superconduct-
ing bending magnets. Experience at
Fermilab with the superconducting
Tevatron magnets since 1986 has
shown that flux creep produces persis-
tent currents that are very hard to
compensate for because they grow
with time and depend unpredictably
on the details of superconductor fabri-
cation. These persistent currents in-
troduce an unwanted parabolic (sextu-
pole) field component whose adverse
effect is worst when beam energy and
field intensity are at their lowest.
In recent months the accelerator
physicists at the SSC Laboratory have
been running the codes assiduously to
determine whether the original de-sign parameters offered sufficient op-
erating margin. It wasn't just a
question of whether the protons sur-
vive when the machine is perfectly
tuned and aligned. The machine
must also be "operable"—one must
allow for reasonable errors of tuning
and alignment. Cost considerations
and the relation of revised machine
parameters to concerns about the
bending magnets have also been very
much on the mind of SSC Director
Roy Schwitters and his colleagues.
Skinnier beams, fatter magnets
The cheapest and simplest measure
that offers a greater margin of injec-
tion latitude is simply to introduce
more focusing quadrupole magnets
into the line. The original design
called for one quadrupole after every
six 17-meter bending magnets. The
plan now is to reduce the spacing
between quadrupoles from the 114
meters orginally called for down to 90
meters, with only five bending mag-
nets between consecutive focusing
magnets. This greater degree of fo-
cusing would reduce the beam width
by about 40%. Incidental conse-
quences of the revised beam optics are
a reduction of the bending-magnet
lengths from 17.35 to 15.85 meters
and an increase of the ring circumfer-
ence from 53 to 54 miles.
In addition to making the beam
thinner, one can also make the mag-
nets fatter, with similar benefits.
That is to say, if one increases the
beam-aperture bore that threads the
magnet, a beam of given width be-
comes less sensitive to the undesira-
ble higher-multipole field compo-
nents of the bending magnet, because
the field quality at any point depends
only on its fractional distance from
the magnet axis to the coils. The
supplemental design increases the
aperture from the original 4 cm to 5
cm, thus increasing the effective
phase-space window for the injection
of protons by about 60%. This will of
course necessitate more superconduc-
tor in the fatter magnets, with a
corresponding cost increase.
If one scales up the thickness of the
cable itself, it should become easier to
meet the Dipole Review Panel's call
for magnets that can operate with a
safety margin of 10% above the 6.6-
tesla bending field required to hold a
20-TeV proton in the ring. This
recommendation was one of several
contained in the June 1989 report of
the panel, convened by Schwitters
last April to examine the progress of
the SSC bending-magnet program.
The panel, whose cochairmen were
Tom Kirk from Fermilab and Gus
Voss from DESY, concluded that themagnet program had not yet devel-
oped a prototype bending magnet
with adequate operating margin.
The SSC magnet development pro-
gram, operating at Brookhaven, Fer-
milab and the Lawrence Berkeley
Laboratory, has acquired consider-
able experience with short and full-
length magnets of 4-cm aperture.
Much of this experience will still be
relevant to the new 5-cm design. But
this change, if approved, will entail
some disruption of the schedule envi-
sioned for preparing a final magnet
design for industrial mass production.
Looking much further ahead, the 5-
cm aperture should make it easier
eventually to increase the luminosity
of the SSC well above its design goal of
1033 events per second per cm2.
The third principal revision called
for in the supplemental SSC design is
the injection of the protons into the
main ring at 2 TeV instead of 1 TeV.
This would require a final booster ring
twice as energetic as the Tevatron, the
world's largest existing proton accel-
erator. But it would mean a thinner,
better-behaved beam at injection,
with higher initial magnetic fields,
less plagued by persistent currents.
The computer simulations have
convinced the SSC designers that all
three of these changes—more quadru-
poles, a larger beam aperture and
higher injection energy—should be
adopted. This conclusion, Schwitters
told us, has been strongly endorsed by
the SSC Laboratory's Machine Advi-
sory Committee, headed by Roy Bill-
ing of CERN. Among the economies
that are being undertaken to offset
these expensive revisions is a reduc-
tion and postponement of the bypass
scheme of beam shunts that was
recently introduced into the machine
design to make it possible for some of
the accelerator's four detectors to take
beam while others are being worked
on in a beam-free environment .
The bending magnets
The 8000 bending magnets required
by the SSC ring constitute the most
expensive component of the accelera-
tor. Hence the great attention paid to
the magnet program. Five 17-meter
bending magnets have been complet-
ed since the Dipole Review Panel's
examination of the program last
spring. These new magnets have all
reached the nominal operating field
at 4.35 K with very little "training."
Apparently the design changes intro-
duced to constrain the magnet coils
against quench-causing movements
have been successful.
But the magnets still have not
achieved the 10% operating margin
recommended by the review panel.
48 PHYSICS TODAY JANUARY 1990WASHINGTON REPORTS
One option would be to operate the
magnets at 3.5 K rather than the
nominal 4.35 K. At lower tempera-
ture, the superconductor can take
more current before quenching, and
3.5 K is thought to be no great problem
for the SSC's cryogenic system.
One reason for the Dipole Review
Panel's recommendation of a 10%
operating margin was batch-to-batch
variation of the superconducting nio-
bium-titanium wire fabricated for
the experimental magnets. The SSC
Lab could ill afford to have a goodly
fraction of the ring's 8000 magnets
quench during operation because of
such a spread in wire quality. But in
recent months, Schwitters told us, the
industrial suppliers of the supercon-
ducting wire have achieved a signifi-cant improvement in quality control,
so that one could probably make do
with a lesser margin. "In any case,"
Schwitters went on, "we could cer-
tainly run in the first year at 90% of
the nominal SSC energy without any
loss to the physics. The Tevatron,
after all, is considered a great success,
even though it runs at only 90% of its
nominal 1000-GeV beam energy."
In recent months the magnet pro-
gram has been concentrating on the
achievement of adequate dipole-field
quality. This problem is of course
closely linked to the changes that
have now been made in the overall
SSC design. Adequate field unifor-
mity should be easier to achieve with
wider magnets and narrower beams.
—BERTRAM SCHWARZSCHILD
HUNTER DEPARTS DOE AFTER RILING
KEY LAWMAKERS AND TOP TEXANS
Rumors had circulated almost every
month since last April that Robert O.
Hunter Jr would soon be out on his
ear as the Department of Energy's
director of energy research. After all,
he had angered influential members
of Congress in his efforts to realign
DOE's fusion program. He had pro-
posed to reduce the funds available
for magnetic fusion research and to
fatten the budget for inertial confine-
ment fusion at the expense of magnet-
ic fusion.
When Hunter's strategy was made
known, many plasma physicists ex-
ploded. Hunter had argued that ICF
research with lasers, as practiced at
Lawrence Livermore and Los Alamos,
needed far greater support from DOE
and Congress if it was ever going to
show any commercial feasibility. It
didn't escape the notice of fusion
researchers and members of Congress
that Hunter's former company, Wes-
tern Research in San Diego, did ICF
work under contracts with the De-
fense Department. Nor did they ig-
nore Hunter's ambitious plans to
make both fusion technologies com-
pete for funds in DOE's constrained
R&D budget.
Among those scrutinizing the plans
was Representative Robert A. Roe, a
New Jersey Democrat who heads the
House Science, Space and Technology
Committee. At hearings and in pri-
vate, Roe fumed at Hunter's proposal,
which would have the effect of curtail-
ing work at the Princeton Plasma
Physics Laboratory in New Jersey.
Roe took his complaint directly to
Hunter's boss, Energy Secretary
James D. Watkins. Other antagonists
included Senators Bill Bradley andHunter: Cone but not forgotten.
Frank Lautenberg, both New Jersey
Democrats. During one call Bradley
demanded that DOE officials "stop
messing with Princeton."
In the meantime, Capitol Hill was
rife with tales about the sale of
Hunter's company, which took place
before he was confirmed by the Sen-
ate last year for the DOE job. The
stories led Roe to unleash the staff
watchdogs on his House Subcommit-
tee on Investigations and Oversight to
determine their accuracy. Staff
lawyers and outside experts scoured
the financial accounts of Hunter's old
firm, interviewed former employees
about Pentagon contracts dealing
with large excimer lasers such as
those used by Los Alamos for ICF
research and reviewed patents held
by Hunter that might suggest a con-
flict of interest. For all their efforts,though, Roe's investigators have
come up with few leads and even less
evidence, say subcommittee sources.
Tripped on the SSC
Neither the problems over fusion nor
the congressional investigation was
the main reason for Hunter's sudden
departure, however. He was tripped
up by something altogether differ-
ent—the Superconducting Super Col-
lider. It seems that Texans in Con-
gress and back home had made no
secret to DOE and the White House
that they wanted Hunter to cease his
resistance to hiring certain scientists
for the laboratory and to desist inter-
fering with decisions by SSC manag-
ers. One particular irritant was
Hunter's opposition to approving a
"footprint" (see page 45) produced by
the SSC team for locating the collider
ring around the town of Waxahachie.
Until DOE approves the precise loca-
tion of the 54-mile racetrack-shaped
ring and other components and build-
ings, the state is unable to purchase
the 16 000 acres on which to construct
the giant project.
Informed of Hunter's disagree-
ments with SSC scientists, some of
Texas's most prominent figures began
bashing Hunter in front of Presiden t
Bush, Secretary Watkins and others.
As the Administration grew more
exasperated and embarrassed, it be-
came clear that Hunter's days at DOE
were numbered.
Finally, in early October, John C.
Tuck, DOE's under secretary, who
maintains strong connections to influ-
ential Republicans in Congress and to
important White House officials, re-
portedly ordered Hunter to leave the
agency. On 16 October, Hunter sent a
hand-penned letter of resignation to
Watkins. "As we have discussed,"
Hunter wrote in his characteristically
cramped hand, "it is now time for you
to pick a person for the Bush Adminis-
tration. Several weeks ago I took
steps to ensure that the work of the
office would be smoothly conducted,
and my presence is not now required.
Therefore, I would like to resign,
effective immediately."
Ironically, though Hunter is gone
from DOE, his ideas have not been
forgotten. In the next weeks Watkins
intends to name a blue-ribbon panel
to examine the country's entire pro-
gram of controlled fusion. He also is
maintaining a vigil on the SSC.
With Hunter's departure, James F.
Decker is once again acting director of
DOE's research office. He filled in for
a year and a half after the departure
of Hunter's predecessor, Alvin W.
Trivelpiece, in 1987.
—IRWIN GOODWIN •
PHYSICS TODAY JANUARY 1990 49 |
1.584668.pdf | Nonradiative damage measured by cathodoluminescence in etched multiple quantum
well GaAs/AlGaAs quantum dots
E. M. Clausen Jr., H. G. Craighead, J. P. Harbison, A. Scherer, L. M. Schiavone, B. Van der Gaag, and L. T.
Florez
Citation: Journal of Vacuum Science & Technology B 7, 2011 (1989); doi: 10.1116/1.584668
View online: http://dx.doi.org/10.1116/1.584668
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
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 137.189.170.231 On: Fri, 19 Dec 2014 23:20:11Nonradiatlve damage measured by cathodoluminescence in etched multiple
quantum wen GaAsl AIGaAs quantum dots
E. M. Clausen, Jr., H. G. Craighead,a) J. P. Harbison, A. Scherer, L. M. Schiavone,
8. Van derGaag, and l. T. Florez
Bellcore, Red Bank, New Jersey 07701
(Received 14June 1989; accepted 11 July 1989)
We report a study of nonradiative surface recombination in etched GaAs quantum wen
structures. Low-temperature cathodoluminescence was used to measure the relative
luminescence efficiencies of etched quantum dots as a function of size, etch depth and etching
conditions, and quantum well width. The relationship between etching damage and quantum well
width was determined by using three samples, each consisting of three quantum wens of2, 4, and 9
nm thickness, with the placement of the wells relative to the surface varied systematically. Arrays
of quantum dots which ranged in size from 5 pm down to 40 nrn were produced by electron beam
lithography and reactive ion etching or ion beam assisted etching. The nonradiative surface
damage produced by the etching process degrades the luminescence efficiency in quantum dots
smaller than 1 pm in diameter. We have determined that etching processes which use argon gas
increase the nonradiative surface layer thickness compared to etching processes which use xenon.
We have also found that the lowest confinement energy quantum well is most strongly affected by
the sidewall damage and the highest confinement energy quantum welt is affected the least by the
damage.
I. INTRODUCTION
There is great interest in developing reduced dimensionality
semiconductor structures to exploit quantum confinement
effects for optical and optoelectronic applications. Because
molecular-beam epitaxy (MBE) and other growth tech
niques are capable of producing high-quality planar quan
tum wens, much information has been established on the
quantum-confined state in III-V materials. It is believed
that greater enhancement of electrical and optical properties
can be achieved by producing structures of higher dimension
quantum confinement. Growth techniques have just recent
ly been used to produce one-dimensional quantum wires by
growth on vicinal surfaces. I To date, however, all structures
produced in quantum well materials which are small enough
to achieve confinement in the third dimension have required
some type of ion beam fabrication process. Structures small
enough to provide quantum confinement for carriers in two
or more dimensions can be produced by electron beam lith
ography and reactive ion etching (RIE) or ion beam assisted
etching (IBAE). 2.3 The damage that occurs during this type
of fabrication process and the free surfaces created can lead
to severe carrier depletion in the smallest size structures that
would exhibit quantum confinement effects. To circumvent
this problem the damage must be removed and the free sur
faces must be passivated by some type of post-etching tech
nique.4•5 Regrowth of a semiconductor cladding layer is the
ideal technique for passivation since high vacuum ion etch
ing processes can be made compatible with the growth pro
cess. Only a few studies have been reported on in situ pro
cessing in which an etched microstructure is maintained
under ultrahigh vacuum and transferred directly to a growth
chamber.5,6,1(-) This type of processing has not yet been ap
plied to the fabrication of ultrasmall structures. Aside from
the difficulties associated with in situ processing. the exact
nature of the damage induced in nanostructures from ion beam processing is not well understood at this time. Before
successful passivation by regrowth can be achieved, the rela
tionship between the damage and processing parameters
must be understood. The current challenge therefore is to
identify the nature of the damage so that surface and defect
recombination can be minimized in etched nanostructures.
Our recently completed study of etching sidewall damage
in single quantum wen GaAs quantum dots concluded that
the efficiency is strongly dependent on the etching condi
tions.7 A damage layer width 5 was used to describe the
luminescence degradation in the smallest size dots. This lay
er was defined as the radius of the largest size dot from which
no luminescence could be measured. For a given set of condi
tions the smallest size quantum dot which still emitted light
increased in size with increasing etch depth.
In this paper we present a further study of the damage
layer width as a function of etching conditions. We make a
comparison between argon and xenon as the inert gas for ion
beam assisted etching. The damage layer width S is found to
be greater and increased more rapidly with ion energy when
argon was used compared to xenon. We also have studied the
effect of etch damage on the luminescence efficiency of dif
ferent width quantum wells in a mUltiple quantum wen
(MQW) structure. The lowest confinement energy quan
tum well is found to be most strongly affected by the etching
process for all etching conditions. These results indicate that
slight differences in mobility and radiative recombination
lifetime associated with quantum wen width can contribute
significantly to the luminescence efficiency of etched struc
tures.
II. EXPERIMENTAL
Samples used for this study were prepared from films
grown by MBE on CrO-doped GaAs substrates. Each struc
ture consisted of a 0,5 pm thick buffer layer, three quantum
wells 2, 4, and 9 nm thick of undoped GaAs, clad on either
2011 J. Vac. Sci. Techno!. B 7 (6), Nov/Dec 1989 0734-211X189/062()11-04$Oi.OO ~~) 1989 American Vacuum SOCiety 2011
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 137.189.170.231 On: Fri, 19 Dec 2014 23:20:112012 Clausen, Jr. et sl.: Nonradiative damage measured by cathodoluminescence 2012
side with harrier layers of 20 urn thick AIo3 G~J.7 As, and a
30 urn capping layer of un doped GaAs. Three separate sam
ples were prepared with the order of the quantum wells var
ied systematically. A schematic of the quantum well struc
ture is shown in Fig. 1. The designation of the three
structures is x-y-z, where x is the well closest to the surface.
For example, sample 2-4-9 is the leftmost structure shown in
Fig. 1, with the 2 nm well closest to the surface.
Microstructure fabrication was accomplished by electron
beam lithography and reactive ion etching or ion beam as
sisted etching. Arrays of quantum dots with diameters from
5 pm down to 40 urn were patterned in a poly ( methylmetha
crylate) (PMMA) resist using a JEOL JBX-SD electron
beam writing instrument. The array of the smallest dots were
produced by variation of the electron beam dose in single
spot exposures, for all the dots in a given array. This pro
vided a range of arrays with dot sizes that varied in steps of
::::: 10 nm between 80 nm and the smallest size produced. The
square arrays measured 20 11m on a side with dot coverage
from 6 to 15%. A single dot of200l1m diameter was written
next to each set of arrays to measure the quantum wen lumi
nescence from an "unetched" region to show any variation
in wen thickness with position on the wafer, and to test for
ion damage through the etch mask. No lateral variations or
ion implant damage effects were observed. Etch masks were
made by evaporating 50 nm of SrF2 onto written patterns
and then applying standard liftoff techniques.
Two different etching systems were used to prepare sam
ples for this study. Reactive ion etching (RIE) was carried
out in a commercially available parallel-plate system. Typi
cal etching conditions employed 15% BCIl in argon, with a
rfpower density of 0.27 W /cm" and dc bias voltages of ::::;200
V. These conditions provided etch rates of25 nm/min which
produced smooth surfaces and straight sidewalls. Greater
details for obtaining optimum etching conditions arc de
scribed elsewhere. x A custom built ion beam assisted etching
(IBAE) system was used to etch other samples with either
argon or xenon as the inert gas and Cl2 as the reactive gaso A
Kautman ion source was used to generate ions with energies
ranging from 300 to 1500 eV. Typical ion flux measured at
the sample was varied from S to 50pA/cm2. The reactive gas
was introduced near the sample surface through two gas jet:.;,
SAMPLE
#2->,
,""mS,F, ~ Etch Mask
lO nm GIIAltCIIj) ___ ~
~O om Al.3Gil' 7AI !
,"mG ••• ~
20 nm AbGII.7Aa.
4nmGIIAs
20nmAI3G.a.1As 1
9nmGaAa i
20 nm AbGiI,1A~ j
SOCInm GaAs
BuffE!rlaYfM"
Semi-in5ul~ting G~As
Subslrate SAMPLE
#9-2-4 #409-2
Dot Diameter
~z;;y
,"ma.ME! .-1
91'lmGaAs
2nmGIIAs '
1 E'eli
Depth
FIG. 1. Schematic drawing of (he three different multiple quantum struc
tun~s used for this study. Indicated on drawing arc the well and barrier
widths.
J. Vac. Sci. Technol. 8, Vol. 7, No.6, Nov/Dec 1989 and total chamber pressure was typically maintained at
I X 10 -4 Torr. A further description ofthis particular etch
ing system is published elsewhere. <)
Several sets of samples were prepared in both systems with
etch depths of either 140 or 300 nm. After calibration of the
etch rates for the various conditions, the actual etch depths
were determined by profilometry and by examination in a
scanning electron microscope (SEM). Table I lists the var
ious combinations of etching conditions used for IBAE and
the typical etch rates obtained. Cathodoluminescence spec
troscopy was used to measure the luminescence efficiency of
the etched structures. Reference 10 gives a complete descrip
tion of the cathodoluminescence setup employed for this
study. Samples were cooled to::::: 20 K for spectra acquistion.
Typically a IS X IS/Lm area in each array was irradiated by
scanning a focused electron beam operating at an accelerat
ing voltage of 15 ke V and at a beam current of25 nA. Spectra
acquired over two and half orders of magnitUde beam cur
rent showed a linear increase in luminescence intensity with
beam current and no shift in peak position due to band filing.
Areas slightly smaller than that of the array of dots were
irradiated to reduce problems with specimen drift.
III. RESULTS AND DISCUSSION
A. Crystal structure
The primary concern in determining the effect of etch
damage as a function of quantum well width is that when
!'tching through a MQW structure, the wells near the surface
will be exposed to the etching plasma longer, and will be
damaged to a greater extent, To differentiate the two differ
ent effects of etch depth and well width, it was necessary to
grow three different structures so that a given quantum well
was placed either at the top or bottom of the growth order.
Figure 1 shows how this variation was achieved. This strate
gy, however, adds a complicating factor because of the possi
ble variability in quantum well width that can occur in MBE
growth. Figure 2 displays the low-temperature cathodolu
minescence spectra of the three wells from the three separate
samples. By the position ofthe luminescence peaks the weBs
appear to be very close in thickness. The greatest difference
between samples is seen in the 2 nm well. However, less than
one monolayer difference can be attributed to the spectra
shifts observed.
TABLE I. Etching ratcs obtained with different ion species for ion beam
assisted etching with ell Ion flux 50 flA/cm2•
Ion energy Etch rate with argon Etch rate with xenon
300eV
600eV 200nm/min
1500eV 202 Jlm/min
a Etch rate not determined. 750nmlmin
3.llun/min
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 137.189.170.231 On: Fri, 19 Dec 2014 23:20:112013 Clausen, Jr. et al.: Nonrad!ative damage measured by cathodoluminescence 2013
B. Reactive ion etching
As previously reported, the etching conditions described
above lead to sman damage layer widths for shallow etch
depths. When etching through the MQW structure it was
expected that the top well would sustain greater etch dam
age. The results for the sample with the series 2-4-9 therefore
were unexpected. Shown in Fig. 3 is a plot of the normalized
cathodoluminescence intensity as a function of quantum dot
size, for the three different quantum wells. Luminescence
could be measured from all three wells in quantum dots
down to 70 nm in diameter. The intensity drop-off with dot
size is observed to be much stronger in the 9 nm quantum
well. The normalized intensity of this well is reduced over
two orders of magnitude in the smallest size dot. This trend
was duplicated in the other two MQW samples, in which the
9 nm well was even closer to the surface and therefore ex
posed to the plasma longer during the etching process. The
sample with the 9 nm wen nearest to the surface was found to
cut off at the larger size of 100 nm when the sample was
etched to 140 nm. This is further evidence that the damage
layer width increased with increased etching time, but also
indicates that the definition of the damage layer 15 is compli
cated by the quantum well width. The low-energy well ap
pears to be more susceptible to the etching damage and
therefore the damage layer 15, by definition, is greater in these
wens. The difference in exciton mobility with quantum well
energy may explain the differences observed in the intensity
levels of the various wens, The mobility difference, however,
does not differ by the two orders of magnitude which the
intensity levels suggest. Additional factors that may contrib
ute to the observed variation are the differences in surface
area and in the radiative lifetime.
C. ion beam assisted etching
Further investigations of sidewall damage were made with
ion beam assisted etching, since it has been shown that much
higher etch rates with greater anisotropy can be obtained.
Additionally, this technique is more compatible with in situ
processing. Since the 9 nm wen is the most sensitive to etch
3E4r---
3E41 20 K Temperature I 15 keY 1 nA Excitation
-; 2E4t
~ f 2E4T.1
2 ~ 1E4
5000 Solid -Sompl6 2-4-9
Oct -Sampl@ 9-2-4
Da.h -Sample 4-9-2 :J :.,1
-:'1 :11
· I
I
I
I
I
· I
· I '
o+----+~--~--~--~~,~~----~; ~
670 SilO 710 7JO 750 77Q 790 810
Wavelength (nm)
FIG. 2. Low-temperature cathodoluminescence sPectra of the three differ
ent structures shown in Fig. 2. The nominal pcak positions of 695,750, and
792 nm correspond respectively to the 2, 4, and" 11m quantum well widths.
J. \lac. Sci. Technol. B, Vol. 7, No. 6, NOli/Dec 1989 Quantum Dat Radlus Cum)
FIG. 3. Normahzed ljuantliITl well luminescence measured from different
size dots, from the three ditrerent quantum wells in the sample desigtutted 2-
4-9. Th" intensity is llormalized to the fractional volume of quantum well
material irradiakd hy the electron beam.
damage, we examined the luminescence efficiency versus
size for the sample with the 9 nm well near the surface as a
function of etching conditions. Figure 4 displays the results
in which both the ion energy and ion species were varied.
The damage layer g is plotted as a function of these two etch
parameters, As shown, argon produces the greatest amount
of damage and this damage increases more rapidly with in
creasing energy, compared to xenon. Etching with Xe below
600 e V was not investigated, as it was found that the ion
beam flux would drop severely below this energy. The ion
beam flux was maintained at 50 p.A/cm2; however, the etch
time was varied so that a constant etch depth was obtained in
each sample. As shown in Table I, between the extremes of
the stated conditions, there was a factor of ten difference in
etch rate. We believe that a constant etch depth is the proper
normalization since it is the depth of etch which defines the
quantum dot structure.
E
L
tll >, a -'
tll en o
E o o 40
20
o a I I +---1-1 I I
200 400 600 800 1000 1200 1400 1600 1800
Ion Energy (eV)
FIG. 4. Damage layer width as a function of ion energy for argon and xenon
ion beam etching with C12• The damage layer determined for the I) urn well
in the Even at lower ion beam energies argon produces more damage during
the etch.
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 137.189.170.231 On: Fri, 19 Dec 2014 23:20:112014 Clausen, Jr. et af.: Nonradiative damage measured by cathodoluminescence 2014
The above variation of damage with ion species may be
attibuted to the larger momentum and size associated with
the xenon ion. Because of its size, most of the xenon ion
momentum is transferred on the initial impact with the sur
face atoms. This would produce two effects, First, fewer ions
would channel deeply into the crystal structure, thus pro
ducing Jess damage, The second effect would be due to in
creased lattice temperature, which could produce enhanced
reaction with the e12• This may also explain the faster etch
rates observed with xenon. The large size of the xenon ion
would limit any diffusion into the crystal structure, Several
studies have shown that even low-energy argon channels
deeply into the crystal structure, 11,12 which may explain the
difference in the damage observed here
These results indicate that xenon at moderate ion energies
is the optimum system for ion beam assisted etching with
Cl2• Since a high confinement energy quantum wen is less
susceptible to sidewall damage, this combination plus etch
depths to just below the bottom barrier should have the
greatest chance of producing lateral confined structures.
Figure 5 displays the luminescence efficiency as a function of
size for the 2 nm well in the 2-4-9 sample etched with 600 e V
Xe to a depth of 60 nm. Compared to Fig. 2 the intensity does
not appear to drop off as quickly, and the damage layer
width 5 described above is reduced to 25 urn. This means,
however, no luminescence could be measured from dots of
50 nm diameter or smaller, which is approximately the criti
cal size for the measurement of lateral confinement effects.
The absence of any luminescence from dots smaller than
50 nm may be an indication ofthe ultimate limiting factor for
producing laterally confined quantum dot structures. The
surface of undamaged high-quality GaAs epilayers is known
to have a large number of defect surface states which lead to
severe depletion in the nearest surface layers.13-15 To de
scribe the large nonradiative surface recombination velocity
that occurs in GaAs, a "dead layer" is usually ascribed to a
portion of the nearest surface layers of the depletion region.
Even if it were possible to produce dot structures with no
etching damage, the so-called "dead layer"· of the native
GaAs surface would extend through the entire volume of the
smanest dots whi.ch would exhibit lateral confinement ef
fects. To circumvent this problem some type of surface passi
vation or regrowth is necessary.
IV. CONCLUSIONS
The results presented here indicate that the differences in
exiton mobility and radiative lifetime in quantum wells of
different widths influence the nonradiative surface recombi
nation. High confinement energy, narrow quantum wells are
less sensitive to the damage induced during the etching pro
cess. The intensity difference of over two orders of magni
tude, however, indicates that there must be other contribut
ing factors which are not understood at this time. We have
shown that ion beam assisted etching with Xc plus el2 pro-
J. \lac. Sci. Technol. e, Vel. 7, No.6, Nov/Dec 1989 6.51-------------.----------------,
J;-s.o t _0 ---t
'iii 5.5 T 0/0- i 2 5.0 + / 60 nm Etch Depth I d 4.5 + r 600 eV Xe + el2 i
-g 4.0t ~ I
~ :5.5 r8 2 nm Quantum Well E 3.0 tl Cathodoluminescence I
.fE: 2.5 ~ 10 keY Excitation 20 K Temperature
'" 2.0.10
.3 1.51 I
1.0~---+----+-----+-----t-----1 o 500 1000 1500 2000 2500
Dot Radius (nm)
FIG. 5. Normalized luminescence measured from the 2 nm quantum well as
a function of dot size. These data were measured from the 2 nm quantum
well in the sample designated 2-4-9.
duces the least amount of damage in etched structures. The
failure to observe any luminescence from the smallest dots,
even when etched under the most optimized conditions, in
dicates that there is a limit to producing lateral quantum
confined free-standing structures in GaAs materials. These
results indicate the need for the developement of in situ pro
cessing for the surface passivation of etched nanostructures.
aj Cornell University, Ithaca, NY 14853.
lC. W. Tu, R. C. Miller, P. M. Petroff, R. F. Kopf, B. Deveaud.
T. C Darnen, and J. Shah, J. Vac. Sci. Techno!. B6, 2 610 (1988).
2M. B. Stern, H. G. Craighead, P. F. Liao, and P. M. Mankiewich, App!.
Phys. Lett. 45, 410 (1984).
3B, E. Maile, A. Froehel, R. Gremann, A. Menshig, K. Struebel, F. Scholz,
G. Weimann, and W. Schlapp, Microelectron. Eng. 6,163 (1987)
'c. J. Sandroff, R. N. Nottenburg, J. C. Bishoff, and R. Bhat, App!. Phys.
Lett. 51, 33 (1987).
sH. Tempkin, L. R. Harriott, R. A. Hamm, J. Weiner, and M. P. Panish,
App\. Phys. Lett. 54, 1463 (1989).
6H. Miyamoto, N. Furuhata, H, Hoshino, A. Okamoto, and K. Ohata,
Inst. Phys. Conf. Ser. No. 96, Chap. 2, 47 (1988).
7E. M. Clausen, Jr., H. G. Craighead, J. M. Warlock, J. P. Harbison,
L. M. Schiavone, L. Florez, and B. Vall der Gaag, App!. Phys. Lett. 55,
1427 (1989). .
SA. Scherer, H. G. Craighead, and E. D. Beebe, J. Vae. Sci. Techno!. D 5,
1599 (1987).
"A. Scherer, M. L. Roukes, B. P. Van der Gaag, T. L. Cheeks, and
E. M. Clausen, J r. (to be published) .
IOE. M. Clausen, Jr., H. G. Craighead, M. C. Tamargo, J. L. deMiguel, and
L. M. Schiavone, Appl. Phys. Lett. 53, 690 (1988).
)lH. F. Wong, D. L. Green, T. Y. Lin, D. G. Lishall, M. Denis, E. L. Hy, P.
M. Petroff, P. O. Holtz, and J. L. Mertz, J. Vae. Sci. Techno!. 6, 1906
(1988).
12R. Gennallll, A. Proehel, H. Y. Meyer, and D. Grlitzmaeher. 7, 1475
( 1989) (these proceedings).
I3D. B. Wittry and D. F. Kyser, J. App\. Phys. 38, 375 (1967).
14L. lastrzebski, J. Lagowski, and H. C. Gatos, App\. Phys. Lett. 27, 537
(1975).
15W. Hergert, P. Reck, L. Pasemalln, andJ. Schreiber, Phys. Status Solidi A
101611 (1987).
16J. P. Harbison, A. Scherer, D. M. Hwang, L. Nazar, and E. D. Beebe, Mat.
Res. Soc. Symp. Pmc. 26, 11 (1988).
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1.584659.pdf | An opticalheterodyne alignment technique for quartermicron xray lithography
Masanori Suzuki and Atsunobu Une
Citation: Journal of Vacuum Science & Technology B 7, 1971 (1989); doi: 10.1116/1.584659
View online: http://dx.doi.org/10.1116/1.584659
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 141.212.109.170 On: Mon, 15 Dec 2014 16:14:07An optical ... heterodyne alignment technique for quarterµn xMray
lithography
Masanori Suzuki and Atsunobu Une
NTT LSI Laboratories, 3-1. Morinosato Wakamiya, Atsugi-shi, Kanagawa Pre/. 243-01, Japan
(Received 30 May 1989; accepted 7 July 1989)
A new optical-heterodyne interferometry alignment technique with diffraction gratings is
developed for quarter-micron x-ray lithography. To obtain detection accuracy as good as a few
tens of nanometers, a phase signal is utilized instead of a conventional intensity signal. The
relative lateral displacement between mask and wafer is detected by measuring the phase
difference between heterodyne beat signals generated by projecting two laser beams from + first
order and -first-order diffraction directions on the mask and wafer grating marks. The
displacement signal is only slightly influenced by gap variation using symmetric optics. A lateral
displacement detection resolution better than 10 nm is obtained by the experimental alignment
setup. A nonsymmetric beam from the -third-order diffraction direction is added to the
symmetric beams to detect the gap, The phase difference between two beat signals emitted to the
second-order diffraction direction from the same mask and wafer marks is used as the gap
detection signal. The cyclic gap signal makes it possible to set an arbitrary gap. A gap detection
resolution of < 20 nrn is realized. Using this optical-heterodyne interferometry alignment
method, a four-channel alignment system is developed for synchrotron x-ray lithography. Six
axis alignment servo control is established by combining this system with highly accurate stages.
!. INTRODUCTION
Synchrotron radiation (SR) lithography is a most promis
ing technique for replicating quarter micro and high
throughput lithography. ! Quarter-micron x-ray lithography
requires alignment accuracy of the order of ± 0.05 pm to
replicate minimum feature size patterns between 0.2 and 0.3
pm. In order to meet this requirement, both relative dis
placement and detection methods must be found. Their de
tections should also be set during exposure to decrease align
ment errors. Many methods for solving this problem have
been proposed.2-8 Some of the methods are visual observa
tion with microscopes commonly used in commercial prox
imity mask aligners.2-4 They are limited chiefly by signal-to
noise ratio and by wafer mark technology. Another group of
optical methods uses interference effects of diffracted light
generated by gratings formed on the mask and wafer.5-7
These methods use diffracted light intensity as an alignment
signal. Thus, the intensity decreases wich are caused by var
iations oflaser light source intensity and wafer mark diffrac
tion efficiency due to semiconductor processing brings about
the poor alignment accuracy. Furthermore, the relative lat
eral displacement signal cannot be detected independently
from gap and alignment control must be performed within
very narrow gap ranges. An optical-heterodyne interferome
try lateral displacement detection method with gratings was
proposed. ~ However, this method has some difficulties in
mark fabrication and preaIignment technique caused by an
extremely narrow diffraction pitch.
A new optical-heterodyne interferometry alignment
method with diffraction gratings is developed for detecting
and controlling relative lateral displacement and gap. To
obtain detection accuracy as high as a few tens of nano
meters, a phase signal is utilized instead of a conventional
diffraction light intensity signal.
This paper describes a new optical-heterodyne interfero-metry method, the theoretical analysis of the signal and the
comparison with experimental results. Furthermore, it de
scribes alignment accuracies achieved by combining the
four-channel alignment system and highly precise vertical
stages.
II. OPTICALaHETERODYNE INTERFEROMETRY
PRINCIPLE
A. Fundamental
The fundamental principle of the optical-heterodyne in
terferometry method with a diffraction grating is illustrated
in Fig. 1(a). As shown with solid lines, two linearly polar
ized coherent beams of the optical frequencies./;, andlz are
projected to a grating with plus and minus first-order dif
fraction angles 8 II> and 812, respectively. The incident beams
are symmetrical about the z axis vertical to a grating plane.
Then, -1st-order diffraction directions of these beams cor
respond to the z axis. The diffraction angles 811, and 812 are
expressed as follows, where A, and 22 are wavelengths of
beams of optical frequencies!, and!z, respectively. P is dif
fraction grating pitch. The wavelengths are nearly equal,
(AI ,*,A.2) , since the frequency difference};, ( = 1fI -j;l) in
the two beams is much smaller than that of an optical fre
quency (_1014 Hz);
(1)
Electric displacements of the two beams are expressed by
Eq. (2), where Al and Az are amplitudes, ¢l and tP2 initial
phases,
E, (t) = A I exp(2rrJ;t+¢;j )j,
E2(t) = Az exp(21T!2 t + <P2)j
= A2 exp(21T(f~t + ibt) + ¢2]J· (2)
1971 J. Vac. Sci. Techno!. B 7 (6), Nov/Dec 1989 0734-211X/89/061971-06$01.00 (0) 1989 American Vacuum SOCiety 1971
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Beat signal
/--/_.,
(~ 1M
I'll f---.. ~ Diffraction light
\FreQuency : f I 1'\ /
\ I \. I Beat signal
(a) (bl \
\ \
The optical-heterodyne interferometry beat signal1r (t)
emitted in the z-axis direction is given by Eq. (3), where
/).rpo = tPl -rp2 and
Ir (t) = lEI (t) + E2UW
= Ai + A ~ + 2AIA2 COS(27T!bt + Arpo) , (3)
As shown in Fig. 1 (a) with dotted lines, when the grating
moves by Ax distance from point C to point C', the light
path-length variations of the two beams are Ax sinOI1, and
-tlx sinew respectively. Therefore, a beat signal h (t)
after tlx movement is given by Eqs, (4) and (5),
Ib(t) =Ai +A~ + 2A1A2 cos (27T};,t + Arpo+ Arp), (4)
At/; = 21TAx/(P /2). (5)
It should be noted that a phase shift !:!..rp varies linearly
with the grating movement tJ.x and that the period Arp is
equal to half the diffraction pitch.
B. lateral displacement detection principle
A relative lateral displacement detection technique to
align wafer to x-ray mask with high accuracy using the opti
cal-heterodyne interferometry method is developed. In Fig,
1 (b), a grating mark and a rectangular membrane window
are formed on the mask, and a grating mark is just under to
mask window on the wafer. The mask grating pitch is the
same as the wafer grating pitch. The mask grating line direc
tion (y axis) corresponds to that of the wafer.
Two linearly polarized coherent beams of optical frequen
cies!1 andh illuminate the mask and wafer gratings from the
plus and minus first-order diffraction angle directions. As
these beams are symmetrical about the z axis, diffracted
beams emitted from the mask and wafer gratings overlap
and two beat signals 1M and I ware obtained in the z axis
direction. Phase shifts tJ.<f;M and Arpw of IM andl ware given
by the following equations from Eq. (5), where tJ.M and tJ. W
are position errors of mask and wafer, respectively,
J, Vac. Sci. Techno!. B, Vol. 7, No.6, Nov/Dec 1989 Water grating FIG. 1. Determination of relative lateral dis
placemcnt. (a) Principle of optical-hetero
dyne method. Beat frcquency:.r. =, j; -f,.
(b) Optical-heterodyne relative lateral dis
placement detection method.
D.dJM = 21T' AM I(P /2),
D.rpw = 27T' tJ. W I(P /2). (6)
(7)
The relative lateral displacement tJ.d between mask and
wafer can be detected by measuring the phase difference
tJ.rpd:D.rpd is expressed by Eq. (8),
!:!..r/Jd = D.r/J"v -D.r/JM = 2rr'tJ.d /(P 12). (8)
It should be noted that the phase difference D.r/Jd varies
linearly and has a cycle ofhalfthc grating pitch as a function
of the lateral displacement !:!..d.
c. Gap detection principle
A gap detection technique using optical-heterodyne inter
ferometry is developed. Optical illustrations when grating
moves by ~ W distance from point C to point C' and moves
by A WZ distance from point C to point C' is shown in Figs.
2(a) and (b). 01, O2, and 613 are the first-order, second-order,
and third-order diffraction angles, respectively. Nonsymme
tric incident beamh optics in the -third-order diffraction
direction and diffraction beam detection optics in the +
second-order diffraction direction are added to the symmet
ric optics shown in Fig. 1 to detect mask-to-wafer gap. The
first-order diffracted beam due to incident beam!l projected
from the -first-order diffraction direction and the -
first-order diffracted beam due to incident beam}; projected
from the -3rd-order diffraction direction are combined
and two optical-heterodyne diffraction beams are emitted in
the second-order diffraction direction from these gratings.
As shown in Fig. 2(a), the gap signal varies with the dis
placement!:!.. W. The phase shift rp Iva of wafer signal I wz is
given as follows:
r/Jwzx = 27T' ( -tJ.W)/(P /2). (9)
The phase difference between two gap signals is expressed
by Eq. (10),
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 141.212.109.170 On: Mon, 15 Dec 2014 16:14:071973 M, Suzuki and A. Une: An optical-heterodyne alignment technique 1973
p
Z h~=n~~~~~~==
(a)L'~ e
~
'" \
\ \93
P \ \
FIG, 2. Optical-heterodyne gap detection method using wafer diffraction
grating mark. (a) Lateral displacement variation 1l. Wand (b) gap variation
AZW.
ll¢;zx = 217' (11M -11 W)/(P /2)
= 21T'!1d /(P /2) = -ll¢;d' (10)
On the other hand, as shown in Fig. 2 (b), the gap signal
varies also with the gap variation !1 Wz. The phase shift
¢; wzz of wafer signal 1 wz is given as follows:
(11 )
The phase diffreence between mask and wafer gap signals
is written by Eq. (12), where !1.G( = !1WZ -!1MZ) is the
distance between mask and wafer, that is the gap,
(12)
Therefore, the total phase measured by the gap detection
technique is (!1¢;zx + !1¢;wMZ) which actually consists of a
displacement term (ll¢;zx) plus the gap ternl !1¢ WMZ' Since
/:}.¢zx = -!1rPd' the displacement term may be eliminated
from the gap measurement by adding the phases measured in
the lateral displacement and gap measurement technique to
give
IIi. SIMULATION AND EXPERIMENTAL RESULTS
A. latera! displacement detection
Alignment signals are discussed theoretically and experi
mentally. In an experiment, the one-channel two-axis setup
J, Vac. Sci. Technol. B, Vol. 7, No.6, Nov/Dec 1989 PO,
Devlc'e f6gion Beom spot 'Mask qrotlng
FIG. 3. Optical-heterodyne experimental alignment setup.
shown in Fig. 3 is used. Mask grating patterns consist of 51
Ta absorberlines formed on 2.um thick silicon nitride (SiN)
membranes. These patterns have dimensions of 0.6 11m
thickness, 211m width, and 200 11m length, and are fabri
cated using x-ray mask fabrication technology.9 Wafer grat
ing patterns consist of 5 I grooves formed on a Si wafer using
conventional etching processes and have dimensions of 0.5
p.m depth, 2.um width, and 200 /till length. Grating pitches
are 4 pm. A frequency Stabilized 633 nm He-Ne transverse
Zeeman laser (STZL) emitting two linearly polarized beams
that cross each other perpendicularly is used as a light
source. 10 A horizontal linearly polarized coherent beam!1
and a vertical linearly polarized coherent beamlz are sepa
rated into two laser beams by polarized beam splitter (PBS)
and are projected on the mask and w.:!fer gratings from +
first-order and -first-order diffraction directions using
mirrors M2 and My Then, the two beams are diffracted in
the z-axis direction by the mask and wafer grating, respec
tively, and are combined. Two optical-heterodyne interfero
metry beams are generated and are reflected by mirror M4,
and then are detected with two separated photodetectors,
PDj and PD2, as two heterodyne beat signals 1M and I w'
As shown in Fig. 4(a), the beat signal 1M and 1w are
calculated by Eq. (8). The beat frequency is 128 kHz. The
solid line is mask signal 1M, and the dotted line is wafer signal
I w' The phase of the wafer signal varies with the wafer dis
placement /:}. W, but the signal amplitude does not change.
This is one of the advantages of the proposed method. The
phase difference ArPd examined experimentally is shown in
Fig. 4(b). The phase difference !1¢Jd changes linearly with
the displacement !1d, and is repeated cyclically with half the
diffraction grating pitch.
The mask beat signal intensity variation caused by chang
ing the mask-to-wafer gap from 50 to 55 pm is shown in Fig.
5. The experimental result shown in Fig. 5(a) agrees with
the calculated result shown in Fig. 5 (b). The beat signal
phase is kept constant in spite of mask-to-wafer gap varia
tions, because the incident!] andj; beam optics are symmet
rical in the direction perpendicular to the grating plane. An
other advantage of the proposed method is that the
displacement beat signal phase is only slightly influenced by
mask-ta-wafer gap variation.
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 141.212.109.170 On: Mon, 15 Dec 2014 16:14:071974 M. Suzuki and A. Une: An optical-heterodyne alignment technique 1974
fb" 126 kHz
P = 4 jim tlW = -OA ....... tO.4\.1m
11
4. 0 0 ""1':"'Ki~;''''''' r-.. ,. "'--,·"'F""··V""""'; \V..,....'~.,. --'--"""/1'1""""'\"'-"-1\'
3 • 50
+-+-'1 \ '.+--~, \ .f-,'-I/i-f:-i+. Ii U++, \ \ ... -+. -;-+-',' ;1 !~\i \ "
3. a a -t':-'\+-':-+-';"'i:'hHf--i~':f,,~;..J.\H·H--H-4. (hi "-f-l
2, 5 0 +:-\i-:--'-+~iH+';"'++-'4~+-!:....;..j-,-360 P =4~m
i=
H
~ 2. aa
H
lal : : '. '. , " I , '
b 8 1i 12 14 ib
TIME (MICRO-SEC)
B. Gap detection (b)
Gap detection optics are basically the same as lateral dis
placement optics. As shown by dotted lines fn Fig. 3, non
symmetric incident beam optics, mentioned before, are add
ed to symmetric optics for lateral displacement detection.
The bcam!2 is split by beam splitter (BS) and projected by
mirror Ms onto the mask and wafer gratings. The beam!,
reflected by mirror M 2 and the beam h reflected by mirror
tal
2.5e
::IE .....
1. 5i
l.je
0.50
I e.ee i , I
(bl Q b e 11 12 14 1,
TIME !HICRO-SE':)
FIG. 5. Waveform variation. (a) Beat signals detected experimentally by
gap variation. 2 V /div, 5 ,us/div, (b) Mask beat signals calcnlated as a
function of mask-to-wafer gap. b.G = 50~55 ,urn.
J. Vac. Sci. Techno!. B, Vol. 7, No.6, Nov/Dec 1989 3
Ad ! J,lm ) 4 5 FIG. 4. Lateral displacement signals.
(a) Waveform variation of the beat
signals calculated as a function of wa
fer displacement. Beat frequency is
128 kHz. (b) Detection signal mea
sured experimentally.
Ms are combined and the two optical-heterodyne diffraction
beams are emitted from these gratings as mentioned before.
These two beams are detected by two separated photodetec
tors PD3 and PD4 as two heterodyne beat signals IMz and
lwz·
The phase ¢w and <Pwzx measured experimentally as a
function of the displacement d Wis shown in Fig. 6(a). The
phase rPwzx exactly equals the phase -¢w' Figure 6(b)
400r-----------~
300 .
'" 200 e
"" '" -0
3t -200
"6-
-300 . ¢w ¢"" tiff
f2\WJ2
Z
-400 ~t".x
,.~ J <l>wzx
(al Displacement !.W (jim) P = 8 )1m
Water grating
-;;; 400
'" '" ~ 300
"C
N 200
N
~
"6-100 ~ I
0
(bl Displacement !. WZ (jim)
FIG. 6. Phase difference. Ca) tPw and tPwz measured as a function of wafer x
axis displacement and (b) if> WLZ measured as a function of z-axis displace
ment.
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 141.212.109.170 On: Mon, 15 Dec 2014 16:14:071975 M, Suzuki and A. Una: An optical-heterodyne alignment technique 1975
shows the phase <Pwzz measured experimentally as a func
tion of the z-axis displacement!l Wz. The diffraction grating
pitch is 8 !lm. The phase shift cycle for z-axis displacement is
about 24.6!lm from Eq. (11).
IV. SERVO CONTROL
An example of the relative lateral displacement alignment
experiment is shown in Fig. 7 (a). The wafer-on-wafer stage
is automatically aligned with the mask in the horizontal x
axis by the feedback control to keep the phase difference !l<pd
zero. As seen in Fig. 7(a), alignment accuracy is better than
± 1". The phase difference ± 1° corresponds to the relative
lateral displacement ± 6 nm [ = 4000/ (2 X 360) ] .
An example of the gap control experiment is shown in Fig.
7(b). The mask-on-mask stage is automatically controlled
with the wafer in the vertical z axis by a feedback control to
keep the phase difference f:..t/>G zeroo As seen in Fig. 7 (b), the
gap control accuracy is better than ± 10. In the case of grat
ing pitch 4/-lm, the phase difference ± 10 corresponds to the
mask-to-wafer gap ± 16 nm.
V. APPLICATION TO SIX~AXIS ALIGNMENT
Using an optical-heterodyne interferometry alignment
method, a four-channel alignment system with highly pre
cise vertical stages is constructed fot synchrotron x-ray lith
ography. The alignment marks sizes on the mask and wafer
are 200 X 299 !lm, and the grating pitches are 6 ,urn. Four
pairs of alignment marks arc formed at points A(x j, Zj),
B(X2,Z2)' C(Yl,Z,,) , and C(Y\>Z4) on the mask and wafer chip
edges (Fig. 3). The lateral displacement signals Xj' X2, Yj,
andYl are used for X, Y, and 8 alignment, and the gap signals
Z" Z2' Z3' and Z4' are used for a, {3, and Z control between
mask and wafer. For fine adjustment, six-axis freedom is
utilized for X and Yaxis motions of the wafer stage, and (X, (3,
on ... ... +\00 ... .. '"
'" 0
~
'" -\00
-360 P'4~m
uncontrolled - ~_. ,controlled _ ~-.. -.~
. -
1_~±6nm
Time (0)
Time (b) , 2sec/div
2 sec/div
50·/ div
2sec/div
100°/ div 2°/ div
FIG. 7. Alignment experiment, (a) Relative lateral displacement signal and
(b) mask-to-wafer gap signal.
J. Vac. Sci. Techno!. B, Vol. 7, No.6, Nov/Dec 1989 ~
~ 20
<:r
'" LL:
t,-,~
-0.3-0.2-0.\ 0 O.! 0,2 0,3
lal Overlay error in X (pm I
40 ~~-.--------,
: N"99
I, tTy" 0.043~",
,... 30: j "-0.017 .urn
~ .
! ::~ r I
oLI --O~.3--~O.L2~-O~,I~O~~~,I~O.-2-0~,3~
(bj Overlay error in Y (pm) FIG. 8. Overlay accuracy mea
sured by Nikon-21 using signal
mask and multiply exposure. (a)
Errors in x and (b) errors in y.
(I, and Z axis motions ofthe mask stage. Six-axis servo con
trols are performed during exposure.
Overlay accuracy is investigated using FBM-G positive
resist. To eliminate mask-t~-wafer distortion and process in
fluences, the double exposure technique is used with one x
ray mask. Figure 8 shows overlay accuracies measured with
laser interferometry coordinate measuring equipment
(Nikon-21) 0 The total overlay accuracies for x and y axes
within a 10 mm field are about O.l3/1m (3(7).
VI. CONCLUSION
An optical-heterodyne alignment technique for quarter
micron x-ray lithography is developed. It is shown that later
al displacement is detected independently on the gap varia
tions using optical-heterodyne symmetric optics and that
gap can be detected using nonsymmetric optics. In the align
ment experiment, lateral displacement detection resolution
better than 10 nm and gap detection resolution < 20 nm are
realized. Using a four-channel alignment system with highly
precise vertical stages, six-axis alignment servo control is
achieved with high overlay accuracy within 0.13 !lm (3(7)
for a 10 mm field.
ACKNOWLEDGMENTS
The authors would like to express their appreciation to
Dr. 1'. Kitayama, T. Hayasaka, and H. Yoshihara. They
would also like to thank the x-ray mask fabrication group,
synchrotron light source group, and beamline group for con
tinual guidance and excellent support.
1M. Suzuki, T. Kaneko, and Y. Saitoh. J. Vac. Sci. TechnoL B 7, 47 (1989).
'T. Hayasaka, S. Ishihara, H. Kinoshita, and N, Takeuchi, J. Vac. Sci.
TechnoL B 3,1581 (1985).
'E Cullman, K. A, Cooper, and W, Vael!, SPIE 773,2 (1()87).
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 141.212.109.170 On: Mon, 15 Dec 2014 16:14:071976 M. Suzuki and A. Une: An optical-heterodyne alignment technique 1976
4No Bobrolf, R Tibbetts, J. Wilczynski, and A. Wilson,1. Vac. Sci. Tech
nol. B 4,285 (1986).
5 A. Une, NM. Suzuki, I. Okada, Y. Saitoh, and H. Y oshihara, SPlE 773, 4S
(1987).
"n. Fay, J. Trotel, and A. Frichet, J. Vaco Sci. Techno!. 16, 1954 (1979).
7E. Kouno, Y. Tanaka, J. Iwata, Y. Tasaki, E. Kakimoto, K. Okada, K.
J. Vac. Sci. Technol. e, Vol. 7, No.6, Nov/Dec 1989 Suzuki, K. Fujii, and E. Nomura, J. Vac. Sci. Techno!. B 6,2135 (1988).
RJ. Ito, T. Kanayama, J. Atoda, and K Hoh, SPIE 773,7 (1987).
"M. Sekimoto, Ao Ozawa, T. Ohkubo, and H. Yoshihara, ill Extended Ab
stracts of the 16th Conference on SSDM, Kobe, Japan 1984 (unpub
lished).
'''H. Takasaki, N. Umeda, and M. Tsukiji, App. Opt. 19, 435 (1980).
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1.584496.pdf | Characterization of thin borondoped silicon membranes by doublecrystal xray
topography
David I. Ma, Syed B. Qadri, Martin C. Peckerar, and Mark E. Twigg
Citation: Journal of Vacuum Science & Technology B 7, 1594 (1989); doi: 10.1116/1.584496
View online: http://dx.doi.org/10.1116/1.584496
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
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 132.236.27.111 On: Mon, 15 Dec 2014 18:46:23Characterization of thin boron-doped silicon membranes by double-crystal
x-ray topography
David I. Ma,a) Syed B. Qadri, Martin C. Peckerar,a) and Mark E. Twigg b)
Naval Research Laboratory, Washington. D. C. 20375
(Received 30 May 1989; accepted 7 July 1989)
Heavily boron-doped silicon transparent membranes and x-ray masks were examined using
double-crystal x-ray topography. The topographs revealed the strain distribution in two
dimensional (20) pattern. Slip bands and Frank-Read sources were observed. The stress
variation across the membrane was confirmed using the Stoney's stress analysis by laser beam
reflecting technique. Surface strain versus bulk strain were also analyzed by choosing (224) and
( 115) reflecting planes. Deposition of patterns on a bare membrane did not alter the overall strain
pattern of the membrane, which was apparently determined by the warpage of the silicon
supporting ring. Local deformations associated with gold absorber features were visible and their
associated strain fields were measured.
I. INTRODUCTION
Con tactless replication of submicrometer linewidth patterns
with soft x-ray lithography was demonstrated by Spears and
Smith in 1972.5 Since then, to improve lithographic through
put and the linewidth control, many modifications were pur
sued. These improvements include: the examination of var
ious high brightness sources; the development of the
interference alignment concept; the testing of new x-ray sen
sitive resists; and the fabrication of thin, transparent x-ray
masks. The last of these items is now considered the final
critical element necessary for production line adoption of
this technology. This study documents a new, nondestruc
tive technique useful for evaluation of x-ray masks.
Presently, boron-doped epitaxial silicon is one of the best
starting materials for thin, x-ray transparent, free-standing
membranes used as x-ray masks. This is because such mem
branes have high x-ray transmission, elastic modulus, and
optical transparency. In addition, well established fabrica
tion techniques can be used to separate the lightly doped
bulk from the heavily doped membrane. The fabrication of
the thin, transparent membranes begins with growing a 1-
11m thick boron (B) doped epitaxial (epi) silicon layer on a
conventional 3 in. (100) p-type silicon wafer. The epi layer is
doped to a density of 1 X 1020 B atoms/cm3 to provide an
etch stop for subsequent ethylenediamine-pyrocatechol
water selective etching. 6 This doping level generates a tensile
stress on the silicon surface which is approximately 1 X 109
dyn/cm2•7
There are several techniques to analyze the surface stress,
such as: topographic image intensity analysis,8,9 laser beam
reflecting techniques, 10,12 and eHipsometric multiangle mea
surements.13,14 Among these, the Bonse's double crystal x
ray topographic image intensity analysis was chosen for this
study, due to its extremely high sensitivity to lattice distor
tion and misorientation. The sensitivity is estimated to be
approximately one part in 107 of a lattice constant. 15,16 The
resolution is limited by the wavelength of the x-ray source,
and by the quality of the receiving nuclear plate. Further
more, the strain distribution can be clearly observed with
spatial resolution approaching 111m.
Bonse's theory provides a simple description of intensity contrast versus surface strain.8 According to this theory, the
stress variation can be expressed as
6oI=M6oS,
60S = tan 0B [ (6.d /d) + 8&], (1)
(2)
where I is the intensity, M is the slope of the rocking curve at
half of its peak height, S is the corresponding stress on the
membrane, e B is the Bragg angle, d is the Bragg plane spac
ing, and 80 is the component of local lattice rotation with
respect to the goniometer axis. From these equations, the
relative stress variation can be derived from the correspond
ing changes in image intensity recorded on the topograph.
But the absolute value of the stress cannot be obtained di
rectly from this technique.
The parallel beam, single crystal reflecting technique4 was
also used to confirm the stress results obtained from topo
graphs. This technique has the benefits of easier setup,
simpler maintenance, and faster data collection. The draw
backs are lower resolution and one-dimensional mapping.
The data pairs are collected on a parallel beam curvature
measurement system, and the average surface stress is de
rived from Stoney's equation3,12
E t; 1 ae == ~~,
6(1 -v} te R (3)
where te and t, are the thickness of the epi layer and the
substrate, respectively, E /O-v) is taken as 1.805 X 1012
dyn/cm2 for (100) silicon,17 where E and v are Young's
modulus and Poisson's ratio, respectively, and R is the mea
sured radius from the sample. This curvature measurement
configuration can be found in previous reports by Sinha's
group and by Rossnagel's group. 18,19
II. EXPERIMENTAL SETUP
The double-crystal x-ray topography setup includes a first
silicon (111) crystal asymmetically cut at 13S [1° off the
Bragg angle of ( 111) planes 1 and aligned to diffract from
( 111) planes. This gives a horizontal magnification about
26. Copper Ka radiation from a conventional x-ray tube is
used. The second crystal (in this case, the thin silicon mem
brane) is aligned to satisfy the desired refiecting plane, the
1594 J. Vac. Sci. Technol. B 7 (6), Nov/Dec 1989 0734-211X/89/061594-06$01.00 © 1989 American Vacuum SOCiety 1594
························1···
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TABLE I. Reflection parameter.
Item (224) (115)
A, 1.54178 A 1.541 78 A
N 5.0X 1022 5.0X 1022
F 6.72 5.11
r 2.295X 10' 1.746 X 103
°B 44.03' 47.45'
¢ 35.27' 15.79'
BB -rp 8.76' 31.66'
ZeIT 0.664 pm 3.011l·m
image is recorded on an llford nuclear emulsion plate. There
are a few requirements to determine a proper reflecting
plane. These include: a high reflected x-ray intensity to re
duce the exposure time, near-90°-reflecting angle to avoid
image distortion, small incoming angle to guarantee the uni
formity of the exposure field, and proper effective penetra
tion depth (Zelf) to provide depth profile of the membrane
defect structure. Since the membrane is on a (100) silicon
wafer, from the stereographic projection20 and the lattice
parameters for silicon,21 the possible reflecting planes are
(224), (115), (113), (404), and (135). Among these, only
(224) and (115) reflecting planes are suitable for surface
and bulk stress analysis.
The effective penetration depth (Zoft) can be calculated
from dynamical scattering theory.22 The assumption of dy
namic theory presupposes the silicon membrane and sub
strate to be near perfect crystals. The average attenuation
coefficient is given by the following equation:
17" ( e2
\ (r) =--INJ...IFI, 2 mc2 ) (4)
where N is the number of atoms per unit volume, Ii is the
wavelength of the x-rays, and IF I is the structure factor per
unit cell. The primary beam decreases in intensity by a factor
of exp [ -rt I sin ( e B -¢) ] , after penetrating a perpendicu
lar distance t from the surface; where e B is the Bragg angle,
and ¢J is the angle between the surface normal and the chosen
Bragg plane. Thus the effective penetration depth is given by
sin(On -¢)
Zo!r =. (5)
or
Cal (bl
~
3 mm
J. Vac. Sci. Techno!. S, Vol. 7, No.6, Nov/Dec 1989 Ie} 1595
The parameters for both (224) and (115) reflecting planes
are shown in Table I. Note that Z"jf is 0.664 pm for the (224)
plane, 3.01 j.tm for the (115) plane.
III. RESULTS AND DISCUSSION
Two transparent membranes were examined. (Mem
branes provided by Hampshire Instuments Inc., Marlbor
ough, Massachusetts.) Membrane A was measured after
etching and after depositing 2 to 5 j.tm patterns on it. Mem
brane B was a completed x-ray mask with 0.37 j.tm gold
(Au) patterns already defined on it. On this x-ray mask, the
local strain distribution was studied.
Concerning the results obtained in analysis of membrane
A, the blank epi layer was examined with double crystal x
ray topography using (224) reflecting. The effective pene
tration depth as given in Table I is 0.664 j.tm, which is about
half the epi layer thickness. Thus, the intensity variations on
the topographs can be attributed to the presence of surface
strain. These topographs are shown in Fig. 1, and the detail
of each one is described as following:
In Fig. 1 (a), the x-ray source was aligned to get the mini
mum intensity of the thin membrane portion, but the sup
porting ring satisfied the Bragg's condition. There were
planar defect lines continuing through both membrane and
supporting substrate in < 110) directions. These continuous
lines provi.ded evidence that the defects are formed before
the etching process of the thin membrane. Furthermore, a
few loops could be found on the thin membrane area, but not
on the supporting ring. Examining the shape of these loops
revealed the charateristic features of Frank-Read sources?
In Fig. 1 (b), after changing the incoming angle by + 340
arcsecond, the central membrane portion was in the maxi
mum diffracting condition. The image of the supporting ring
was still present. The vertical defect lines could be seen clear
ly. The horizontal defect lines were very faint.
In Fig. 1 (c), the topograph was taken with the sample
rotated by 90·. Now the horizontal planar defect lines could
be seen clearly. The strain patterns on both membrane and
rings, which can be derived from the dark bands, were also
rotated 90°.
It is clear from Fig. 1 that the dark bands in the membrane
and in the supporting ring are nearly orthogonal to each
other. The dark bands are caused by the warpage of the sili-
FlO. L Double-crystal topographs for
blank membrane A using (224) reflection.
(a) Epi layer is not diffracting. (b) Epi
layer is diffracting. (c) 90" rotation from
(b).
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20,-------------------------------------------------------,
( "\ -s Parallel "" )'
".-Perpendicular (I) ~
E
~ ...
~o
0 ,...
~
II)
'"' ... .,
::J > 1ii c:
> 0 ... ~ :l
U
0
a.
.,
> " u
'" 0
~
-10 i
\
~ \
0 -... -~
.,
20 •
Membrane
Area
40
Spacing (mm)
con supporting ring. The pattern of the warpage changes
between the ring and the membrane, which creates a discon
tinuous stress band between those two areas. Also, the war
page is not symmetrical along the radial axis, so the strong
stress bands have two sections on opposite sides of the mem
brane. The pattern of the warpage was verified by the paral
lel beam curvature measurement, shown in Fig. 2. The sam
ple had different curvatures on vertical and horizontal
directions. One changed from convex toward concave; and
the other one changed from almost flat to concave. This indi
cates a "saddle" shape to the supporting ring. A similar re
sult was reported by using laser beam curvature measure
ment.23 According to Eq. (3), te and ts are 1 and 350 pm,
respectively, the stress on the supporting ring area is between
4220X 108 and 1.953X 109 dyn/cm2.
An interesting aspect is the variation in lattice spacing
between the membrane and the supporting ring. The relative
change in lattice spacing can be calculated from the follow
ing equation:
f1d
d (6)
For Figs. 1 (a) and 1 (b), ll.() and On equal + 340 arcsecond
and 44.03", respectively. Theratioofl ad /d I is 1.708 X 101.
Since f18 > 0 in changing diffracting condition from support
ing ring to membrane, this would imply that the lattice spac
ing of the membrane is smaller than it in the supporting ring.
This, of course, is a manifestation of the fact that the mem
brane is in tension.
A few broken membrane pieces were examined by trans
mission electron microscope (TEM), as shown in Fig. 3.
J. Vac. Sci. Technol. S, Vol. 7, No.6, Nov/Dec 1989 60 )
J
80 FIG. 2. Curvature data obtained by
parallel beam technique at diftCrent
portions ofmembrallc A,
The planar defect appears as discontinuous islands when ex
amined with a magnification of 17 500 X. Since each indi
vidual island has its own particular shape and size, the actual
defect structure is extremely complicated. The only point of
commonality among these defects is that all defect islands
are aligned to the < 110) cleavage planes of the silicon. A
similar planar defect structures after heavily boron doping
was reported by Resener's groupo7 One possible explanation
for these structures is that the defects are probably induced
by boron implantation. After a temperature annealing cycle
to release the excess stress, all defects accumulate on the
natural cleavage plane, and form slip bands 1 on the sample.
In order to get depth profile of the defect distribution, a
topograph was taken with (115) reflecting planes. This is
FIG. 3. TEM micrograph of a piece of the membrane.
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FIG. 4. Double-crystal topograph for blank membrane A using ( 115) reflec
tion.
shown in Fig. 4. The defect structures are the same as ob
tained from (224) results. No additional defect features
have been detected in the supporting ring area, Thus, these
defects extend fairly deep into the bulk silicon.
After the analysis of the initial stress distribution of mem
brane A, this membrane was patterned with electron-beam
(e-beam) alignment marks ranging from 2 to 5 ,lID in size. 1597
up. The topograph of this patterned mask is shown in Fig. 5.
The sample was orientated in the same configuration as for
the topographs shown in Fig. 1. Comparing Figs. 1 (b) and
5, shows that the gross strain distribution in the membrane
area is not affected by the pattern. This indicates that the
mask-making process does not generate any new defects, nor
does the process result in additional strain on the membrane.
In the vicinity of the patterned gold membrane area, the
change of the incoming angle aB from minimum diffraction
to maximum diffraction is + 130 arcseconds. Based upon
Eq. (6), the ratio of the lattice spacing !ad /d I is 6.5 X 10-4
which is less than the orginal 1. 708 X 10 3 bare membrane
case. Since the Young's modulus (E) is 1.689 X HjIl dynl
cm2,17 the stress of the membrane before and after the pat-'
terning equals 2,886 X 10'1 and 1.098 X 109 dyn/cm2, respec-
The mask making steps for this blank membrane involved (al
the following: deposition of 50 A chromium (Cr), 150 A
gold (Au), and 7000 A polymethylmethacrylate (PMMA)
which prepared the membrane for patterning. A Cambridge
EBMF 6.5 e beam wrote the pattern directly on PMMA.
After the development of PMMA, 5000 A gold was plated
FIG, 5, Double-crystal topograph, obtained after depositing a pattern on
membrane A, using (224) reflection,
J. \lac. Sci. Technol. S, Voi. 7, No.6, Nov/Dec 1989 (b)
5mm
FIG. 6. Double-crystal topograph of thc x-ray mask on membrane Busing
(224) reflection. (a) Stress pattern can be seen on the supporting ring. (b)
Pattern can be observed by adjusting the contrast.
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lal
1 mm
FIG. 7. Comparison between the optical micrograph and x-ray topograph at
the same location on membrane B. (a) Optical micrograph shows the de
tails of the absorber pattern. (b) X-ray topograph shows the stress distribu
tion in the vicinity of the pattern.
tive1y. This would imply that the patterned gold mask intro
duces a compressive stress to reduce the initial tensile stress.
N ow consider the results of the analysis of membrane B,
the completed mask. This mask was examined using a topo
graph from (224) reflecting planes. The topographs are
shown in Fig. 6. Both of these were developed from the same
Ilford nuclear plate with different degrees of contrast. In Fig.
6 (a), the stress pattern is clear all over the ring area, but the
Au patterns are hardly distinguishable. In Fig. 6 (b), the Au
patterns are observable by changing the contrast in the mem
brane area. These topographs show the slip bands clearly
along < 110) direction and some square patterns. However it
does not show the Frank-Read source pattern.
A portion of the Au patterns from the Ilford nuclear plate
was enlarged for a study of local structure as shown in Fig.
7 (b). The corresponding optical micrograph using back illu
mination technique is shown in Fig. 7(a). The magnification
of these two pictures is 50. In Fig. 7 (b), the intensity varies
slightly from one area to another, indicating a small stress
variation in the membrane area. This is consistent with the
hypothesis of a freestanding film containing no microscopic
J. Vac. Sci. Technol. B, Vol. 7, No.6, Nov/Dec 1989 1598
stress variation. Around the Au patterns, though, stress re
lated contrast is locally evident. Brighter bands correspond
to the location of the Au patterns, and darker bands sit
around interdigitated fingers areas. The difference in the
stress distribution in the region of interdigitation is below the
detection limits of this technique. But, there is an apparent
200% increase in stress in the regions surrounding the finger
patterns. Based upon the average stress of 1.098 X 109 dynl
cm2 on the membrane, the stress around the fingers areas is
around 2.196 X 109 dyn/cm2. Other darker areas also exist
on the topograph, but can not be correlated with features in
the optical micrograph.
IV. CONCLUSION
The double crystal topographs of silicon membrane and x
ray mask provide a quick analysis of the strain distribution
and their defect characteristics. The following important
conclusion can be drawn fron this study:
0) The heavy boron-doping step creates defects, which
may eventually form slip bands along the (110) cleavage
plane.
(ii) The Bragg plane spacing in the membrane area is
smaller than it in the supporting ring area, which is a mani
festation of the fact that the membrane is in tension.
(iii) Lithography involving the deposition of gold pat
terns does ~ot alter the overall strain distribution on the
membrane, but it does reduce total stress on the mask due to
the stress compensation by the gold patterns.
(iv) The deposition of gold patterns may vary the local
strain structure. Double-crystal topography provides a
means of examining such variation with a few pm resolution.
(v) Double-crystal topography provides a reliable, non
destructive, and rapid spatial surface strain evaluation on
both blank silicon membrane and on the patterned x-ray
mask.
ACKNOWLEDGMENTS
I would like to express my gratitude to Dr. Daniel McCar
thy who gave valuable suggestions and insights to this work.
Special thanks to Mr. Christopher R. Morrow for handling
our endless enlargment of the Ilford nuclear plates.
a) Also, University of Maryland, College Park, MD 20742.
b) Geo-Centers Inc. Fort Washingtoll, MD 20744.
'R. E. Reed-Hill, Physical I14etallulXY Principles (Brooks/Cole Engineer
ing Division. City, 1973), p. 192.
'c. Kittel, Introduction to Solid State Physics, 6th ed. (Wiley, New York,
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UM. E. Pcdinotf, M. Braunstein, and O. M. Stafsudd, App!. Opt. 18,201
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1.584691.pdf | Differential metrology of very large scale integration, oxide isolation structures using a
confocal scanning laser microscope
K. M. Monahan, R. H. Fastenau, and T. Tien
Citation: Journal of Vacuum Science & Technology B 7, 1913 (1989); doi: 10.1116/1.584691
View online: http://dx.doi.org/10.1116/1.584691
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
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structures using a confocal scanning laser microscope
K. M. Monahan, R. H. Fastenau, andT. Tiena)
Philips Research Laboratories/Signetics Company, Sunnyvale, California 94088-3409
(Received lJune 1989; accepted 12 July 1989)
A confocal scanning laser microscope (CSLM) has been used to study a variety of oxide isolation
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definition and local thermal oxidation), between individual wafers (levels of nitride mask
thickness), and between individual features on a wafer (levels of nitride mask linewidth). Unique
metrological evidence is presented for a stress-related deformation mechanism in both LOCOS
and SLOCOS structures at nitride mask thicknesses greater than about 250 nm.
I. INTRODUCTION
The confocal microscope distinguishes itself from a conven
tional microscope by its high resolution (27% narrower
point spread function), reduced diffraction, I high phase
edge contrast,2 and excellent focal plane sensitivity.3 The
signal due to a defocused point object is
leu) = [4sin(uI4)/uj4,
where u is the normalized distance from the focal plane of
the lens. This response is ideal for the observation of the so
called Fizeau fringes which emanate from monochromati·
cally illuminated wedge films such as those seen in the oxide
isolation structures used for the fabrication of integrated cir
cuits. The Fizeau fringes prod,liced by a source which is large
relative to the fringe separation are localized in the plane of
the film.4 The first dark fringe appears when the film thick
ness is equal to a quarter of the illumination wavelength in
the film. Other dark fringes appear for successive odd multi
ples of the thickness at which the first fringe appears.
Experience has demonstrated that one ofthe most reliable
measurement criteria for oxide isolation structures is the
CSLM flat zone width,5 defined as the lateral distance
between the first dark Fizeau fringes on either side of the
center of the nitride mask. These fringes correspond to a
thickness of one-quarter wavelength in oxide (84 nm for
n = 1.46) and, unlike SEM images, are intrinsically calibra
ted to a wavelength oflight. Due to the very high slope of the
intensity profiles 00%-90%, rise length about 0.15 ,urn),
precision of the measurement is greatly enhanced by using a
threshold (25%) rather than a minimum detection algo
rithm. Threshold detection introduces a nearly constant off
set (b), so that true linewidths (LI and L2) are given to a
good approximation by
Ll =mX 1 +b,
L2 = mXz + b,
where Xl and X2 are the respective measured values and m is
the magnification correction. Since In can always be set to unity by calibrating to the pitch of a reference structure, we
have the simple relation
Ll -L2 =X\ -X2'
for the difference measurement. Using this method we can
obtain calibrated measurements of mask encroachment, de
fined as the difference between the flat zone width of the
mask prior to oxidation and the fiat zone width of the grown
structure after oxidation.
II. EXPERIMENTAL
LOCOS and SLOCOS oxide isolation structures have
been described previously.s The process for fabrication of
these structures has two principal steps: nitride mask defini
tion and local thermal oxidation. In the case of our LOCOS
process, a IOO-um-thick silicon nitride film is deposited over
a much thinner silicon oxide film grown on a crystalline sili
con wafer. The nitride/oxide sandwich is lithographically
patterned and etched to provide a resistant mask for subse
quent thick oxide growth. SLOCOS differs from the homo
logous LOCOS structure by the substitution of an oxynitride
film for the pad oxide. The oxynitride film serves to further
inhibit mask encroachment during the oxidation stage. In
this work, we study silicon wafer samples with LOCOS and
SLOCOS structures fabricated at six levels of nitride mask
thickness: 100,200,250,300,350, and 400 nm, respectively.
Thicker nitride films are an attractive approach to reducing
mask encroachment since they require only minor process
modification. The disadvantage of thicker nitride films is
that much higher levels of stress are created in the silicon
and, at some point, deformation will occur.
Figure 1 (top, left) shows a 25 000 X SEM cross-section
micrograph (Hitachi S-570, 25kV) of a post-oxidation
LOCOS structure as an example. Nitride (N), oxide (0),
and silicon (S) structures are labeled along with a polysili
con layer (P) which has been added to delineate the oxide
contour. Oxide encroachment under the mask is easily ob-
1913 J. Vac. Sci. Techno!. B 7 (6), Nov/Dec 1989 0734-211)(/69/061913-05$01.00 @ 1989 American Vacuum Society 1913
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FIG. I. SEM cross-section micrograph (top, left) of LOCOS structure
(25 000 X ) shows silicon substrate (S), deformed nitride mask (N), and
grown oxide (0). A layer of polysilicon (P) has been added to delineate the
boundaries of the oxide, an extreme case of oxide encroachment under the
mask (bar 2 .urn). SEM normal-incidence micrographs (top, right)
show SLOCOS ~tructures (10000 X ) grown with a 2.25 .um nitride mask
Iinewidth and too and 400 nm mask thicknesses. Confocal micrograph
(bottom, left) shows LOCOS structures (8000X) grown using 100-nm
thick nitride and (A) 1.00, (B) 1.25, (C) 1.50, and (D) 1.75 micrometer
mask linewidths, respectively. Confocal micrograph (bottom, right) shows
LOCOS oxidation mask (8000 X ) prior to oxide growth.
served for the LOCOS structure. The nitride mask linewidth
in Fig. 1 (top, left) is nominally 1,00 11m. Each silicon wafer
sample has constant mask thickness and six levels of mask
linewidth, ranging from 1.00 to 2.25 f.1m in 0.25 f.1m incre
ments and labeled A, B, C, D, E, and F, respectively. Space
widths between mask lines are constant at 2.00 pm. Normal
incidence SEM micrographs (10 000 X ) of the 2.25 11m
SLOCOS F structure are shown in Fig. 1 (top, right). These
micrographs were acquired after field oxide growth and
stripping of 100-and 400-nm-thick nitride masks, respec
tively. Note the silicon dark zone at the center of each feature
surrounded by a bright fringe delineating the silicon/oxide
interface. For the purposes of this work, we define the SEM
fiat zone width as the distance between the two bright sec
ondary electron fringes that appear at the oxide transition on
either side of the silicon dark zone. The second set of fringes
outside the transition zone correspond to a shallow groove
that forms in the oxide near the edge of the nitride mask.
Note that the SEM micrograph of the sample processed with
the 400-nm-thick nitride mask shows distinct deformation at
the nitride/oxide interface.
An 8000 X CSLM micrograph (Siscan Systems, 488 nm)
of the ABCD LOCOS structures is shown in Fig. 1 (bottom,
left). Note that we observe bright, high-contrast fringes at
nearly the same magnification used for the SEM images. The
FWHM measurements for the fringes observed on this sam
ple are typically less than 0.2 11m and well below the Ray
leigh limit (about 0.3 11m). That these are indeed Fizeau
fringes is corroborated by their absence in the CSLM micro-
J. Vac. Sci. Techno!. B, Vol. 7, No.6, Nov/Dec 1989 graph of Fig. 1 (bottom, right), which shows the planar
ABCD LOCOS mask lines prior to the oxidation step. The
intensity contrast of the Fizeau fringes is greater than 50%
because the focal plane of the CSLM has been optimized for
Fizeau contrast.6 Reproducible images of fringe patterns
with 0.3 11m average pitch are obtained with measured edge
sharpness better than 0.15 fim ( 10%-90% ). This resolution
rivals that obtained in earlier attempts to circumvent the
diffraction limits of optical imaging by near-field optical mi
croscopy at working distances on the order of20 nm.7•8 Un
like near-field optical scanning (NFOS) microscopes, how
ever, confocal microscopes typically operate at working dis
tances of 200 11m. Thus the range of application of the
confocal microscope to topographic structures is consider
ably greater.
III. RESULTS AND DISCUSSION
A. intertool differential metrology: SEM vs CSLM
Intertool differential metrology serves the purpose of pro
viding a comparison between the responses of two different
measurement tools (e.g., SEM and CSLM) to identical spa
tial fiducials. It can be used to establish both constant and
parametrically dependent offsets between the tools, so that
one may be used as a substitute for the other. Further, it can
reveal areas in which the responses of the systems are funda
mentally different and in which they can yield complemen
tary or unique information.
An elegant example of intertool metrology is the compari
son of SEM and CSLM fiat zone widths for the SLOCOS
process. The CSLM fiat zone widths and the CSLM-SEM
difference values are shown in Fig. 2 at six levels of nitride
mask thickness and four levels of mask linewidth. In general,
the behavior of the SEM and CSLM measurements are simi
lar: Flat zone widths increase rapidly from 100 to 200 nm
nitride thickness and become asymptotic with the mask
linewidth at higher values. The first observation is a nearly
constant off<;et between the SEM and CSLM measurements
of about 0.35-0.45 11m for the larger structures. This is an
estimate of the b parameter discussed in the preceding sec
tion. The significant differences between the two sets of mea
surements become apparent only when they are compared
differentially to show substantial defonnation of the smaller
SLOCOS structures for nitride thicknesses around 300 nm.
The differences are clearly not optical artifacts since they
replicate well for the 250 and 350 nm nitride thicknesses, and
since cross-section SEM and TEM data show a relatively fiat
oxide encroachment structure (wedge angle less than 200
) in
the vicinity of the first quarter-wavelength fringe. To date,
we have observed significant optical artifacts only when
measuring higher-order fringes on more steeply sloped oxide
structures.
Examination of the data for the smaller structures pro
cessed with 300-nm-thick nitride reveals one of the weak
nesses of intertool calibration; that is, the b parameter itself
becomes a parametrically dependent offset, and neither the
SEM nor the CSLM can be considered "sacred." In this
case, a classical linear calibration against the SEM would
fail, leading to errors as great as 0.3 f.1m. The root of the
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problem lies in the different physical contrast mechanisms
used to form a secondary electron image on the SEM and an
optical image on the CSLM. The SEM responds to material
and topographic contrast, while the CSLM responds to dif
ferences in optical pha..~e and relative reflectance. The SEM
image of the structure grown with a 400-nm-thick nitride
mask shown in Fig. 1 (top, right) exhibits material contrast
(including charging effects) from the oxide/silicon interface
at the edge of the fiat zone and topographic contrast from the
roughness of the groove that still delineates the edge of the
nitride mask after its removal. The CSLM image of the D
structure in Fig. 1 (bottom, left) exhibits a form of phase
contrast (Fizeau fringes) as the thickness of the oxide isola
tion varies and reflectance contrast from bare silicon in the
fiat zone itself. The CSLM has a clear advantage in that the
dark Fizeau fringes occur precisely at integral odd multiples
of the quarter-wave thickness in the oxide and are therefore
directly referenced to the wavelength of the laser (488 nm).
B. intratooi differential metrology: External
reference
Intratool differential metrology serves the purpose of
eliminating artifacts (and potentially information) due to
differences in the contrast mechanisms used for imaging.
Typical integrated circuit applications require an external
reference so that the tool dependent offsets are canceled in
la!
(bl 0.4 j -
lao e-
140 lSa 220 260 300 340 380
NITRIDE THICKNESS (111M)
NITRIDE THICKNESS (NM)
FIG. 2. Ca) SLOCOS flat zone widths. (b) SEM-CSLM difference values
are plotted vs nitride mask thickness. + 1.25, 0 1.5, I:!J. 1.75, X 2.0 Elm
mask linewidth.
J. Vac. Sci. TechnoL S, Vol. 7, No.6, Nov/Dec 1989 the differential measurement. Measurements requiring an
external reference are those which compare values to a fixed
reference or standard, such as comparisons between two dif
ferent processes (e.g., LOCOS versus SLOCOS) or between
steps in a given process (e.g., LOCOS oxide versus LOCOS
mask). In the two cases discussed below, "SLOCOS" and
"LOCOS mask" wafers are being used as external refer
ences. All of the "measured values" are means computed
from sets of 49 line-scan measurements with standard devia
tions typically less than 0.007 Jim. The standard error of the
mean values is about 1 nm.
1. Interprocess measurement: LOCOS vs SLOCOS
Flat zone widths (CSLM) for SLOCOS structures are
shown in Fig. 2 (top) for the six nominal nitride mask thick
nesses and several nominallinewidths. The main systematic
effect is the apparent increase in flat zone width with increas
ing mask thickness, indicating that oxide encroachment un
der the mask is minimized by the use of thicker nitride
masks. The decreasing slopes suggest a diminishing effect
above 200 nm for SLOCOS structures. Note the departure
from monotonicity (deformation) near 300 nm. SLOCOS
and LOCOS processes are compared differentially in Fig, 3
(top) at two different nominal mask linewidths. Since all of
the component measurements were taken on a pitch calibra
ted CSLM, the optical offsets are compensated and the
line width difference scale is in absolute micrometers. Thus,
we may say that SLOCOS affords a 0.6 fim (0.3 Jim/edge)
reduction in relative encroachment for mask thicknesses up
to 300 nm.
2<lntraprocess measurement: LOCOS IfS LOCOS
mask
Flat zone widths for the LOCOS structures can also be
compared to flat zone widths measured on the nitride mask
prior to oxidation as in Fig. 3 (bottom). The LOCOS oxida
tion and LOCOS mask steps are compared differentially at
two different nominal linewidths. As in the LOCOS
SLOCOS comparison, all of the component measurements
are taken on the CSLM so that the linewidth difterence scale
is in absolute micrometers. Thus we may say that increasing
the LOCOS nitride mask thickness from 100 to 200 nm af
fords a 0.5 11m (0.25 ,um/edge) reduction in relative en
croachment, with diminishing returns at greater thickness.
C.lntratool differential metrology: Internal reference
Some types of intmtool differential metrology do not re
quire the maintenance of an external standard; that is, mea
surements are calibrated using other measurements within
the same data set (e.g., using an "internal reference"). As an
example, consider a full matrix of LOCOS or SLOCOS data
(flat zone widths versus mask linewidth and mask thick
ness). A point-to-point difference between any two mea
sured values in the matrix results in a calibrated absolute
measurement Two special cases are discussed below for
which only nearest-neighbor values in the matrix are com
pared.
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0:: Ii
0.46
0.44
0.42
0.4 I
(al 100 140 380 180 220 260 300 340
NITRIDE THICKNESS (!>1M)
1.61
1.5
I
1.4 J
i? .5 .. 1.3 -
0 z
W 0: 1.2 w "-Ie
15 1.1 :r: l-e
~ 1 -
~ :; 0.9
0.&
0.7
100 14"
(b)
FIG. 3. (a) LOCOS-SLOCOS<) 1.5, V' 2.25 and (b) MASK-LOCOSdifter
ence values are plotted versus nitride mask thickness. X 2.0 pm, \7 2.25 pm.
1. Interwafer measurement: Linewidth change vs
thickness
The change in the LOCOS flat zone width versus nitride
mask thickness, shown in Fig. 4 (top), is derived by sub
tracting the measured values at the next smaller thickness
from the values at the current thickness. Note that the deriv
ative data at all four nominal linewidths overlay to within
0.04 !-lm, a clear example of the power of differential metro
logy to eliminate systematic offsets. A significant enhance
ment of the erratic behavior of flat zone widths for nitride
thicknesses greater than about 250 nm is also observed. Con
comitant SEM analysis, as well as previous work by one of
our colleagues,'! supports the hypothesis that these lateral
deformations are correlated with the buildup and release of
stress in the nitride/oxide/silicon layers (Fig. 1: top, right).
Assuming that this hypothesis is correct, we recommend
that LOCOS nitride mask thicknesses be limited to the re
gion in which the derivative in Fig. 4 (top) is still positive
and monotonic; that is, to values below 250 nm where defor
mation and destructive relief of stress have not been ob
served.
2. Intrawafer measurement: Linewidth increment vs
thickness
The LOCOS linewidth increment (nominally 0.25 11m)
versus nitride mask thickness is derived by subtracting the
J. Vac. Sci. Techno!. S, Vol. 7, No.6, Nov/Dec 1989 ~
51 i .5
(/)
III
W z " 0
'i: ... " :< ... c ii
"
lal 0.3 -
0.25 ,
0.2
0.15 -
0.' i
0.05
0.05 -I
o.,L
140
0.227 ~ -\ \
~ ----r----~ ._.--
180 220 260 300 340 380
NITRIDE THICKNESS (NM)
-......... Ii
.:: /\ /:
.5 0.224 / \ '
1::~T'T\1l
0.221 ~-----~-.~J !
0.22...,- r 1 1 -,-- ---~- ------;---T- .,~- r 1 _..,._~
~ m ~ ~ ~ ~ = _ ~ ~ _
(bl NITRIDE THICKNESS (NM)
FIG. 4. (a) LOCOS linewidth derivative <) 1.5,um, L',. 1.75 ,llm, X 2.0 lIm, \7
2.25 I'm and (b) linewidth increment are plotted vs nitride mask thickness.
o local average, + global average.
measured values at the next smaller linewidth from the val
ues at the current linewidth. To obtain the data displayed in
Fig. 4 (bottom), the three differential measurements ob
tained at each nitride thickness were averaged locally and
compared to the global average over all thicknesses. The
total included range of the local averages is only 7 nm, indi
cating that optical offsets can be compensated to at least this
level for intrawafer measurements. The global average incre
ment appears to deviate below the nominal by 0.03 !-lm. This
result indicates that oxide encroachment is a very weakly
increasing function of mask linewidth, and again illustrates
the remarkable sensitivity of differential techniques in
metrology.
IV. CONCLUSIONS
From a practical point of view, we have found that the
methodologies utilized in applying confocal differential Fi
zeau metrology (CDFM) to the measurement of oxide isola
tion structures can be critical. The establishment of the "flat
zone width" as a stable measurement criterion was especial
ly useful since it allowed a rigorous definition oflinewidth at
a specified thickness of the oxide structure. Using this crite
rion, differential comparisons could be made between mea
surement tools (CSLM and SEM), between processes (LO
COS and SLOCOS), between process steps (nitride mask
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 142.244.5.197 On: Thu, 11 Dec 2014 15:02:511917 Monahan, Fastenau, and Tien: Differential metrology of VLSi oxide isolation structures 1917
definition and local thermal oxidation), between individual
wafers (levels of nitride mask thickness), and between indi
vidual features on a wafer (levels of nitride mask linewidth).
Unique metrological evidence for a stress-induced deforma
tion in both LOCOS and SLOCOS structures at nitride mask
thicknesses greater than 250 nm was discovered. Since most
of the reduction in mask encroachment occurs below 250
nm, we were able to recommend a 200 nm-thick nitride mask
for both oxide isolation processes as a safe compromise.
ACKNOWLEDGMENTS
The authors wish to thank E. Kooi and D. Kyser for their
support of this work. In addition, we acknowledge S. Ooka
for performing the measurements and J. Chen for stimulat
ing discussions of the problems encountered when inspect
ing LOCOS structures on the SEM. A special acknowledg-
J. Vac. Sci. Technol. S, Vol. 7, No.6, Nov/Dec 1989 ment is due to E. Kooi, who originally suggested the
application of the CSLM to metrology of LOCOS struc
tures.
"J T. Tien is currently an employee of Applied Materials, Inc.
'1'. Wilson and C. Sheppard, Theory and Practice of Scanning Opticai Mi
croscopy (Aeadcmic, London, 1984), p. 48.
2Ibid., p. 70.
'Ibid., p. 71.
4R. S. Longhurst, Geometric and Physical Optics (Wiley, New York, 1967),
p. 140, and references therein.
'1. A. Appels, E. Kooi, M. M. PalTen, 1. J. H. Schatorjc, and W. H. C. G.
Verkuylcn, Philips Res. Rep. 25, 118 (1970); P. A. van der Plaas, W. C. E.
Sneis, A. Stolmcijer. H. J. den Blanken, and R. de Werdt, Proc. 1987
Symposium on VLSI Technology, Kuruizawa, 18-21 May 1987; U.S. Pat
ent 3 886000; European Patent 71.203; and Japancse Patent 56-93344.
"K. M. Monahan amI j. T. Chen, Proc. SPIE 921, ! 70 (1988).
7E. Betzig, M. Isaacson, and A. Lewis, Apr\. Phys. Lett. 51, 2088 (1987).
'u. eh. Fischer. U. T. Durig, and D. W. Poh], App!. Phys. Lett. 52. 249
(1988).
oK. N. Ritz, Masters thesis, Polytechnic Institute of New York, June 1982.
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1.342574.pdf | Investigation of the negative peak in photoinduced transient spectra of semiinsulating
gallium arsenide
S. R. Blight and H. Thomas
Citation: Journal of Applied Physics 65, 215 (1989); doi: 10.1063/1.342574
View online: http://dx.doi.org/10.1063/1.342574
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/65/1?ver=pdfcov
Published by the AIP Publishing
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19Investigation of the negative peak in photoinduced transient spectra
of semi .. insulaiing gallium arsenide
s, R. Blighta)
GEC Hirst Research Centre, East Lane, Wembley, Middlesex HA9 7PP, United Kingdom
H, Thomas
Department of Physics. Electronics and Electrical Engineering, University of Wales Institute of Science and
Technology, P.D. Box 25, CardijfCF13XE, United Kingdom
(Received 4 January 1988; accepted for publication 25 August 1988)
A common observation in the photoinduced transient (PITS) spectra of semi-insulating GaAs
is the appearance of a negative peak, which is anomalous in that both electron and hole traps
should give rise to positive peaks, In this paper, it is shown that the negative peak can be
explained in terms of charge exchange with the GaAs surface and only occurs in material
which displays particular types of current-voltage and current-temperature characteristics. The
dependence of this peak on the processing effects of surface passivation, etching, and baking
and polishing has been investigated and its sensitivity to variations in incident light intensity is
demonstrated. A new variation of PITS, namely gated-PITS has been employed. This
technique suppresses the negative peak in the spectrum, allowing transients corresponding to
emission from EL2 to be detected in particular undoped liquid encapsulated Czochralski GaAs
substrates for the first time.
I. INTRODUCTION
The problems associated with the characterization of
deep levels in semi-insulating GaAs are appreciable. Because
of the high resistivity of the material and hence the difficulty
of injecting free carriers by electrical stimulation, conven
tional capacitance-mode deep level transient spectroscopy
(DLTS) techniques' are not possible. Fortunately, how
ever, a variation of DL TS known as photoinduced transient
spectroscopy (PITS)2-12 was developed and this has found
application in the electrical assessment of both bulk sub
strates and high-resistivity epitaxial buffer layers. This tech
nique monitors current transients induced in the material
due to thermal emission of carriers from traps filled with
photogenerated carriers during a light pulse and a trap spec
trum is obtained as in a standard DLTS experiment.
The principles, theory, and experimental implementa
tion of PITS are wen established2-1' and, consequently, only
a brief outline of the technique will be given here. The experi
mental arrangement used is shown in Fig. 1. A voltage was
applied between two coplanar AuGe/Ni ohmic contacts sit
uated on the surface of the semi-insulating GaAs corre
sponding to a constant average de field of typically 200
V em" I. Electron-hole pairs were generated in the sample
by a light pulse. These photogenerated carriers were then,
depending on the temperature of the sample, available for
trapping by electron or hole trapso After removal of the opti
cal pulse at t = 0 in Fig. 1, a rapid decrease in the current
flowing through the sample owing to the recombination of
free photocarriers is observed followed by a slower current
transient owing to the thermally stimulated release of carri
ers from the traps. This current transient was sampled at two
points with time delays of II and t2 and the difference
iUI) -i(t2) plotted, as described by Lang,l as the sample
a) Present address: Keithley Instruments Ltd., 1-3 Boulton Road, Reading,
Berkshire RG2 ONL. UK. temperature was slowly swept from 77 to 450 K. Peaks occur
in the spectrum when the emission rate of carriers from the
trap corresponds to the "rate window" set by the chosen
values of t, and t2• As shown by Hurtes et al.,2 for t2>tl' a
peak is produced when the emission rate is 1/t [. As the tran
sients observed in this study were usually nonexponential,
low f21tl ratios where chosen in order to resolve closely cou
pled level.s and minimize the effects of non exponential tran
sients.12.!3 The corresponding peak emission rates for var
ious f2lt] ratios were determined by Itoh and YanaiI4 and
these were utilized here.
Assuming that the densities of photogene rated electrons
and holes, !ln and Ap respectively, are large compared to the
free-carrier densities nand p, it has been shown by Hurtes et
al.2 that the current transient observed after the termination
of the light pulse is given by
lSiU) = NTK(e" -ep) [(1 + cpflplcn!ln)-1
(1)
where NT is the trap concentration, K is a constant which
depends on the penetration depth of the light and the geome
try of the sample, en•p and cn,p are the emission and capture
rates of electrons (holes), respectively.
Foranelectrontrap,e,,>ep andc" >cp' soEqo (1) sim
plifies t02
DiU) = KNTe"e -ent, (2)
which is the simple classical current transient equation for a
trap completely fined at t = 0 and empty at t = 00.
Despite the simplicity ofEqo (2) limitations exist in the
application ofEq. (1). Both electron and hole traps produce
current transients which result in positive-going PITS peaks,
creating difficulties in determining the nature of the trap.
Concentrations of traps are unable to be determined from
PITS data, as the uncertainties in the constant K make the
amplitUde of the current transient a function of several un-
215 J. Appl. Phys. 65 (1), 1 January 1989 0021-8979/89/010215-12$02.40 @ 1988 American institute of PhySiCS 215
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19Cryostat
iph
'-P_U_I_S_ed--.Jr----V--light Heater
power
supply io> ==-=-='='==----
source PAR 162 FIG. 1. Schematic of experimental arrange
ment used for PITS experiment. Inset shows
the light pulse and transient response of the
semi-insulating sample.
Thermocouple
amplifier and
DVM. x-v
r~-\e-m-p-OOe-ra-tu-r-e-i X Recorder Y \----....".=--' PITS signal
signal
known parameters and not only a function of the trap con
centration IV T' Care must also be taken when using this tech
nique since the light pulse may not be sufficiently intense to
fill the traps. If the levels are not saturated during the pulse,
then at temperatures below the PITS emission peak, the level
could steadily fiU because the emission rate falls below the
filling pulse repetition rate. This would result in an increase
in the amplitude of current transients with each light pulse,
increasing the PITS signals and distorted spectra may result.
To minimize this possibility long, high intensity filling pulses
( > 30 ms) were used so that saturation was achieved.2 In
addition, pulse repetition rates were chosen so that they were
much lower than the emission rate under analysis, and the
rate of change of temperature of the sample was slow enough
so that it did not change appreciably over successive light
pulses. Equations (l) and (2) implicitly assume that the
dark current level is zero, i.e., there are no thermally genera
ted free carriers present in the material. At temperatures in
excess of 300 K, the dark current becomes appreciable con
tradicting this assumption. In this temperature range, a
common observation is the appearance of a negative-going
PITS peak, which is anomalous in that both electron and
hole traps should, according to Eq. (1), give rise to positive
going peaks. This paper aims to investigate the appearance of
this peak and to attempt to identify its origin.
II. EXPERIMENTAL RESULTS
A. PITS results
Figures 2 and 3 show PITS spectra and activation ener
gy plots obtained from two undoped liquid encapsulated
Czochralski (LEC) wafers. Figure 2 shows a peak at
(0.82 ± 0.02) eV, corresponding to the well-known EL2
center. While the characterization ofEL2 is highly desirable
as this deep donor level is recognized to be the main compen
sating center in undoped LEC GaAs,15 the observation of
this trap is not a common occurrence using the PITS tech
nique. The usual type of PITS response for undoped LEC
material is shown in Fig. 3. This spectrum does not have a
peak corresponding to EL2, but a negative peak appears in-
216 J. Appl. Phys .. Vol. 65, No.1. 1 January 1989 stead. In fact, EL2 is seen as an exception rather than as a
rule in the PITS spectra of un doped LEe GaAs, a phenome
non also noted by Young et al.5
A consistent observation during the course of this work
was that EL2 was only detected in samples not exhibiting a
negative peak in its PITS spectrum. The problem is, there
fore, to account for the appearance of the negative peak and
to attempt to eliminate it so that the true ELl emission can
j! Ii i'Oi iii tj
f
10J~
;::; O'38~V ." ii, .. !' ELl
(1)
400
Temper!lture I K) ---
1 10)~ (11 + \ i
-1"-t \ \ '\. 1 ,
""
"-'I \ \ \ '00"' "'I j
,o-~I ................ , "',,. "," .01 .. ",., ' .. ",,,.01,,,,,, .... ,." ... .1
1 2 S 5
FIG. 2. PITS spectrum for undoped LEe wafer not exhibiting a negative
peak.
S. R. Blight and H. Thomas 216
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19:;;
~
a c: on
Vi
V1 .... e;: .1
100
10J
N ,
'" , 10-2 '" ,Ne-
10-3
w4
j 111
\ 12l ~
200
o 63..eV
O·98.aV (3)
m ...
\ \ \ ..
\ \ \
4 300
O·lB.9,v
11 ) .. Temperature (I( I -
1 o 36..&lV
~ {2)
\ \ <-'\ " \
1000lT (K-11
5 7 8
FIG. 3. A more typical response for undoped LEe material showing nega
tive peak at temperatures in excess of 300 K.
be observed and the trap signature obtained. Severa] expla
nations for the negative peak have been proposed,2.5-7 but
the origin of the peak has not been isolated and its relevance,
if any, to the compensation mechanism of semi-insulating
GaAs has not been identified although Fairman et ai.8 found
it particularly prevalent in LEe material grown by melts
encapsulated by wet B203. The fact that the peak is negative
does not produce any information as to the electron or hole
trapping nature of the trap, although Rheel6 proposed that
negative transients were due to electron traps. As shown by
Hurtes et al? and Look,9 Eq. (1) can only adopt a negative
form if
O'pvplO'nvn = enlep. (3)
Physically, this would correspond to a trap located in the
lower half of the band gap with a greater capture cross sec
tion for electrons than for holes (or a trap located in the
upper half of the band gap with a greater capture cross sec
tion for holes than for electrons). However, the approxima
tions used by Hurtes et al.2 for the analysis of the current
transient equation do not necessarily apply for mid-gap lev
els and this can lead to uncertainty in deconvoluting the
signals obtained for these centers. Deveaud and Toulouse'
adopted a fitting procedure which enabled them to separate
the transients into several exponential components. How
ever, their fitting parameters were not unique and this result
ed in quoting two sets of trap signatures, one corresponding
217 J. Appl. Phys., Vol. 65, No. i, 1 January i 989 100 1=0
-~-Light
elf
_ iD~~~.\urr.nt -Time
,
200 300 '.mpe,4t"". IKI ,
400 Dark
current
FIG. 4. Schematic representation of current transient shapes over tempera
ture range of PITS experiment, superimposed on the thermaliy generated
dark current. Inset shows a typical response of material to an optical pulse
at T= 350K.
to an electron trap, the other a hole trap, for each level de
tected. Abele, Kremer, and BIakemore17 have also recently
highlighted the considerable ambiguity possible when
adopting a digital fitting technique to the highly complex
transients observed.
The negative peak results from a current transient
which increases form below the dark current to the dark
current level after the cessation of the light pulse in a PITS
experiment. Figure 4 shows, schematically, the shape of the
current transients over the full temperature range of a PITS
scan superimposed on the thermally generated dark current
level flowing through the sample. At temperatures in excess
of -300 K, the dark current becomes appreciable, contra
dicting the assumptions made by Hurtes et al. 2 and Deveaud
and Toulouse.? A complicating factor immediately arises i.n
that thermally generated free carriers are available for trap
ping and detrapping, even in the absence of photoexcited
carriers. Recent studies by Kremer et al. 18 have shown that
negative peaks are induced in samples at temperatures below
300 K, when a constant background illumination is present
on the sample, so that the sample does not return to dark
conditions after the light pulse. The background of photo
generated carriers may be considered equivalent to the ther
mally generated case. However the latter aspect will be con
centrated upon in this paper, as it is under these conditions
that PITS is normally performed.
The negative transients observed were highly nonex
ponential. Attempts to digitize the signal and separate it into
its exponential components in the manner of Deveaud and
Toulouse7 and Abele, Kremer, and Blakemore17 only served
to emphasize its nonexponential, as opposed to multiexpon
ential, form. Thus, while the process giving rise to this dis
tortion is thermally activated, it need not necessarily be the
usual well-behaved thermal activation of a deep level. The
presence of EL2 in these samples was confirmed by its low
temperature photoquenching properties,19 and although
ELl was not observed by PITS at the appropriate elevated
temperature, the possibility that the generation of the nega
tive peak involves EL2 cannot be discounted.
Table I lists the activation energies obtained by double
S. R. Blight and H. Thomas 2'17
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19TABLE I. Measured activation energies for the negative peak.
Vendor Ingot Eu ( ± 0.02 eV)
A a 0.90
b (seed) 0.57
b (tail) 1.17
B a 0.98
b 0.87
c 0.97
d 0.73
C a (seed) 1.18
a (taii) 0.76
b 0.78
D a (seed) 0.98
a (tail) 0.89
b (seed) 1.12
b (tail) 0.77
c 1.l7
E 0.80
F a 0.82
b 0.71
boxcar analysis of the transients for a variety of wafers exhi
biting negative peak behavior and little consistency is ob
served between the energies obtained from sample to sample.
Examination of the shape of the transients, however, reveals
the nonexponential behavior and Fig. 4 shows the transient
observed at 350 K for a particular undoped LEe sample.
Immediately the light is removed a rapid decrease to a level
below the dark current is observed foHowed by a recovery to
the dark current level. As the time constants of both compo
nents can fall within the rate window of the DLTS system,
and both components can be nonexponential, the accuracy
of activation energies reported in Table I should be treated
with caution. In addition, the shape of the response indicates
the presence of more than one component to the overall
mechanism, rendering a simple physical interpretation of
the effect far from straightforward.
B. Correlation of the negative peak with the current~
temperature and current~voltage characteristics
A classification will now be introduced to distinguish
those semi-insulating GaAs wafers which produce negative
peaks from those which do not. Those samples which pro
duce a negative peak will be called type I and those which do
not produce a negative peak will be called type II.
Since the thermally generated dark current becomes ap
preciable over the temperature range of interest, monitoring
of the dark current variation of the PITS samples as a func
tion of temperature under the normal bias conditions of the
PITS experiment (-200 V fcrn) was undertaken and a large
variation in the magnitudes of dark current for the various
samples obtained. Arrhenius plots of conductivity and reci-
218 J. Appl. Phys., Vo!. 65, No.1, 1 January i 989 Vendor A
~ngot a
Cb
Do
De
Ac
Cc
iDOO
Ii (volls I
FIG. 5. Room temperature, dark current-voltage characteristics for a var
iety of wafen; from different vendors. Two distinct forms of characteristics
are noted over the voltage range of interest for type I and type n materia!'
procal temperature were produced for the various substrates
which had previously undergone PITS analysis and these
produced straight line grapbs of two distinct types. Whereas
type II material produced activation energies of approxi
mately half-band gap for GaAs, type I samples exhibited
activation energies between 8% and 15% higher. This was
also observed by Fairman et al.8 Carriers in type I material
are, therefore, generated and collected at the contacts in ex~
cess of the intrinsic thermal generation rate of electron-hole
pairs, which may be attributed to surface conductivity as
discussed by Sriram and Das.20 The total curren t collected at
the contacts therefore comprises two temperature depen
dent components, Ibu1k and I,urface • Considerable experimen
tal evidence for the existence of surface and near~surface
conductivity in semi-insulating GaAs exists in the litera
ture20-31 and in MESFETs surface conductivity has been
shown to be thermally activatedo 32 A positive correlation
exists, however, between the occurrence of a negative peak
and an activation energy greater than half-band gap for cur
rent increase with temperature in semi-insulating GaAs.
The room temperature dark current-voltage character
istics of all PITS samples were determined over a voltage
range of 1-150 Vo Figure 5 shows the I~ V characteristics for a
number of wafers. They can be seen to fall into two separate
categories: (i) samples showing a sudden increase in current
of up to several orders of magnitude after reaching a thresh
old voltage VT, and (ii) those exhibiting ohmic behavior
throughout the entire voltage range. Of particular signifi-
S. R. Blight and H. Thomas 218
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19O.1SeV emax :::: 41C5-1
Before etch
After etch
O.37eV O.B1.V
I04.V
FIG, 6. Typical PITS spectra for samples which made the transition fmm
type II to type 1 after etching.
cance is that in all cases, samples which produced a negative
peak in the PITS spectrum, i.e., type I behavior, displayed
the rapid increase in current at the threshold voltage, where
as type II samples displayed ohmic behavior.
It is seen, therefore, that type I substrates show distinct
ly different J-T and J-V characteristics from type II sub
strates, as weE as differences in PITS spectra. (This correla
tion also holds for both In-doped and Cr-doped material.) A
common physical interpretation to link all three different
experimental observations is therefore suggested.
Already proposed is the existence of a temperature-de
pendent surface or near-surface conductivity in type I sub
strates. To test if the near-surface region played a role in the
occurrence of the negative peak, various samples were
etched in order to remove the top 1.5 .um. In general, it was
found that type I samples produced a negative peak both
before and after etching, the J-V and J-T characteristics re
maining the same and similarly, type II samples did not
change in these respects before and after etching. The excep
tion, however, was a batch of samples which made the transi
tion from type II to type I. Before etching, the sample did not
have a negative peak in its PITS spectrum, instead showing a
very strong EL2 peak. Both the J-V and 1-T characteristics
were typical of type II material.
After etching, however, the sample made the transition
to type I semi-insulating GaAs in every respect and pro
duced a negative PITS peak in place of the EL2 peak. The I
V characteristic showed a large increase in current at a
threshold voltage VT and the activation energy of the loT
characteristic increased. Figure 6 shows the PITS spectra
before and after etching. These changes would not appear to
be caused by EL2 exodiffusion from the surface as postulat
ed by Chang et al.27-28 as a transition from type II to type I
behavior was seen after etching and the sample underwent
no thermal treatment. Emphasized, however, is the correla
tion between the negative PITS peak, the current-voltage
and current-temperature characteristics and, as a result, we
propose a possible mechanism for the appearance of negative
transients in the PITS experiment. This is shown, schemati
cally, by Fig. 7.
At a fixed temperature, T1, assume a type I sample is
excited with a single light pulse. During the i.llumination
period a photogenerated current is produced. However, on
219 J. Appl. Phys .• Vol. 65, No.1, 1 January 1989 ~JOOK
FIG. 7. Possible mechanism for the appearance of the negative PITS peak.
cessation of the light pulse, the current decay does not return
to the type I curve, but to a current level corresponding to
the type II curve. The return of the current to the type II
level is thought to be due to charge trapped at the surface
during illumination increasing surface band bending. The
corresponding increase in surface depletion depth lowers the
bulk contribution to the overall current. As charge is ther
mally emitted from the surface states the current recovers to
the type I level with a time constant which falls within the
rate window of a typical DLTS experiment. The time con
stant of this negative transient becomes faster as the tem
perature is increased, producing a negative peak in the PITS
spectrum until the time constant of the transient is so short
that it falls outside the rate window and is no longer detect
ed. It would be expected that as the J-T curves for the type I
and type II materials diverge with increasing temperature,
so the amplitude of the negative PITS transient should also
increase with increasing temperature, its magnitUde corre·
sponding to the difference between the two J-T curves. This
was consistently observed for the samples which made the
type II to type I transition, in support of the argument pre
sented here.
C. The roie of the surface in the I~Vcharacteristics
In view of the strong link between the appearance of a
negative PITS peak and the type of J-V characteristic ob
served in semi-insulating GaAs, an understanding of the J-V
characteristics is of fundamental interest. The type of J-V
characteristics observed in typc I samples (Fig. 5) appears
to foHow the "space-charge-limited current" (SCLC) mod
el, after Lampert and Mark. 33 What has previously been re
ferred to as the threshold voltage V T' corresponds to the
trap-fin limit voltage VrFL in Lampert and Mark's model.
Despite the resemblance of the J-V characteristics of type I
semi-insulating GaAs to those of Lampert and Mark, many
features of the characteristics observed during this study do
not agree with their model. The first is that, on measuring
the /-V characteristics on wafers with different contact spac
ings, VI' showed a linear dependence on the contact separa
tion, L, in contrast to their L 2 dependence although Guerst34
predicted a linear dependence of VI upon L for ideal thin
semiconductor layers. Second, "breakdown" in the samples
examined in this study always fell (assuming a linear field
S, R. Blight and H. Thomas 219
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19between the contacts) between 1 and 3 k V / em, in agreement
with the observations of Hasegawa et al. 24 so that V T appears
to occur at far too Iowa voltage for typical trap densities in
semi-insulating GaAs which can vary from lOIS to 1017
cm -3.23.27 Sriram and Das20 interpreted the proportional de
pendence of VT on L as being due to an effective thin-film
behavior near the surface of the material, as described by
Guerst. 34 The low V T was then explained in terms of an
effective empty trap concentration at the surface which was
lower than in the bulk due to surface band bending.
Deviations from SCLC-type behavior have been investi
gated by many workers, and many interpretations of these
differences have been forthcoming, especially in terms of the
relevance of substrate conduction to sidegating and backgat
ing mechanisms in GaAs integrated circuits. VT has been
shown by Lee35 to be the voltage at which the onset of side
gating in GaAs MESFETs occurs and, therefore, the litera
ture devoted to sidegating and backgating is of particular
relevance to this study. Early studies on Cr-doped material
by Kitahara et al.36,37 showed two distinct types of 1-V char
acteristic which they explained by a trap-fill limit model in
volving centers of different depths. Jiminez-Lopez, Bonnafe,
and FiIlard25 stressed the importance of the surface and ex
pressed scepticism of a single-carrier injection interpreta
tion.33 They showed, by use of a guard ring contact configu
ration that the surface played a dramatic role in the
triggering of breakdown. Hasegawa, Sawada, and Ki
tagawa22 investigated the role of surface passivation on the I
V characteristics of planar contact semi-insulating samples
and came to the conclusion that a high density of surface
states formed a surface conduction channel which actually
dominated the conduction mechanism of the material. Sub
sequently23-24 they demonstrated white light emission from
the anode edge at voltages greater VI' and attributed this to
avalanche microplasma breakdown of the surface. Chang et
ai.27 proposed a mechanism for surface conduction in semi
insulating GaAs and observed that the threshold voltage,
V To was much lower than would be expected from the
known trap concentration in the bulk semi-insulating GaAs,
as observed in this study. They demonstrated, by etching the
surface, that VT increased as xu, where x was the etch
depth. This was attributed to a low trap density surface lay
er, which lowers V T for surface conduction, based on a sur
face trap-fill limit law. They argued that the main reason for
the low surface layer trap concentration is the exodiffusion
of EL2 after thermal treatment.27•JM The same group mod
eled the resu1t28 and achieved excellent agreement with ex
perimental observations. Unfortunately, the samples which
have been the focus of this study have not undergone the
post-implantation thermal treatment at the temperatures
(~850 °C) required to produce their sort of result. This of
course highlights the relevance of the theory for sidegating
problems in ion-implanted integrated circuits but does not
really apply to this study. Makram-Ebeid and Minond021
suggested that a near-surface defect-related conduction
component is responsible for leakage currents with low acti
vation energies, a view shared by Hasegawa, Sawada, and
Kitagawa.22
In order to investigate the effect of the surface treat-
220 J. Appl. Phys., Vol. 65, No.1, 1 January 1989 ..
0. e
'" -Unpcssivated
after preclea.n
T 0 300K
Volts If}
: , : , : ,
: i : ,
• I
100
FIG. 8. Current-voltage characteristics of type I material before and after
surface treatments.
ments on the J-V characteristics of semi-insulating GaAs,
the following treatments were carried out: (i) 2000 A. of
Si3N4 was deposited on the free GaAs surface by plasma
enhanced chemical vapor deposition (PECVD). The sam
ples had only undergone a solvent clean in order to degrease
the surface. (ii) 2000 A. ofSi3 N4 was deposited by PECVD
after an "in situ" gas etch. The etch used was a 1:2 ammo
nia:nitrogen plasma etch, which acted only as an "ultra pre
clean." These treatments were intended to act as a modifica
tion ofthe surface only and no attempt was made to control
surface conditions in this set of experiments.
A typical set of J-V characteristics before and after de
position ofSi3 N4 for a type I wafer is shown in Fig. 8. In this
particular case V T increases from 50 V (corresponding to an
average field of 2 kV!em) before passivation to 80 V (3.2
kV fcm) after deposition of Si3N4• After an in situ preclean
prior to deposition of Si3 N4, the low-field current is seen to
increase and V T increases to approximately 70 V. Regardless
of the physical mechanisms responsible for conduction
along, or near, the surface, these results show that modifica
tion of surface conditions produces a perturbation in the J-V
characteristics of these planar geometry samples. In agree
ment with the findings of Hasegawa and co-workers22 the
surface conductivity contribution plays a significant role in
the total current collected at the contacts. In general, type I
:;amples maintained the same profile of 1-V characteristics
S. R. Blight and H. Thomas 220
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19before and after passivation. Similarly, type II wafers
showed no change in the ohmic characteristics although
small variations in the magnitude of observed currents were
noted.
In view of the surface conduction component of current,
samples having guard rings were prepared to eliminate this
component, and in addition to compare coplanar and sand
wichlike contacts. Figure 9 shows the room temperature l-V
characteristics obtained under various bias configurations.
Figure 9(a) shows the characteristic between two AuGe/Ni
coplanar contacts, the contact separation being of the order
of the thickness of the sample (300 pm). Ohmic behavior is
observed until approximately 40 V bias is present between
the contacts. A sharp rise in current is observed for voltages
greater than 40 V. The current measured under this configu
ration is thought to be mainly due to surface conduction with
a smaner contribution due to the bulk. Figure 9(b) shows
the characteristic obtained for a sandwich structure. As can
be seen, the current in the low-voltage regime is larger than
for the coplanar contacts as surface conduction is not pre
vented by this contact configuration. Figure 9 (c) shows the
effect of surrounding the top electrode with a guard ring. A
large decrease in low-field current is observed due to the
prevention of surface current and accompanied by an in
crease in VT> from typically -40 to -60V.
D. The effects of light intenSity and temperature on tile
/s V characteristics and the PITS negative transient
The effect of varying light intensity on J-V characteris
tics and PITS transient response was investigated at various
10-' /11 T = 3001( If
I I ./
~O" !I
II
if ./
Illl Ib) ~
-;;; 10-' ~ ~ 1 c.. If E " I' -I! ~ c: t 1,/,/--I I
::I
'J i u
1t)"'
..... ,/
//" I /" .-._.1 /" ,/ /rlii ,/
10-8 ",,-
.,/.,/,,,,,-."" ttl /"
,/ T ." ,/'
,/
10-9
1 10 100
Volts
FIG. 9. Typical current-voltage characteristics for type I material using
guard-ring sample under various contact configurations. (c) shows drop in
current observed when guard ring is present.
221 J, Appl. Phys., Vol. 65, No.1, 1 January 1989 temperatures in the range of interest. The light source used
was a 100 mW GaAsl AIGaAs laser operating at a wave
length of 850 nm, the output of which was focused onto the
sample placed in a continuous-flow liquid-nitrogen cryostat.
Figure 10 shows the l-V characteristics of a type I sample at
various light intensities at a temperature of250 K. The effect
of increasing light intensity is to increase the current in the
sample, but the overalll- V profile is preserved. For all light
intensities at this temperature, PITS transients were posi
tive. However, at temperatures where the thermally genera
ted dark current starts to become appreciable, a very differ
ent type of behavior occurs. At 300 K [Fig. 11 (a)], the J-V
"breakdown" threshold V T becomes less abrupt and "sof
tens" for a laser intensity exceeding 40 mW. This behavior
coincides with the observation of changes in the PITS tran
sients. Below 40 mW laser power, the negative transient ap
pears, but for laser outputs above this value the transient
becomes positive.
Figure 11 (b) shows the corresponding 1-V characteris
tics at a temperature of 350 K. This time, type I behavior is
preserved to laser powers of typically 50 m W. Correspond
ingly, a negative PITS transient is produced by laser outputs
up to this intensity, but not for intensities in excess of 50
m W. The characteristic at 400 K [Fig. 11 (c)] shows that
type I behavior is obtained even for 100 mW laser power so
that a negative PITS transient was observed even for 100
m W laser power at this temperature. A correlation holds
therefore between the appearance of a negative PITS tran
sient and the shape of the l-V characteristics measured both
in the dark and under illumination.
The above results clearly demonstrate that at tempera
tures in excess of 300 K, the process giving rise to the nega-
001
T~250K
0.001 Vendor 8
Ingot c
0.0001
10-5
'" 10-6
0-
E
~
C
~ 10-7
5 u
10-8
10-9
10-10
10-11
10-12
1 10 'DO
Volts
FIG. 10. Current-voltage characteristics fOT a type I sample at various light
intensities for T = 250 K.
S. R. Blight and H. Thomas 221
•• ;.:.:.;.; ••••• ;.: ••••••••••••••••• ;> .............. :;;~.~.~;:.:.:.:O;':.:.:.:.:.:'; •.• : ............................. -;r ••• ·.·;".·.v.·.·.- ..... ~.~.;.; •••••••••. -•••••• <; ••••••••••• ,. ............... ," •••••• Tr .•••.•.•.• ,..";"'" •••••• _._ ................ _ •.• _ ••• _ •• _ •••••••.•.•.•• ,.;",.-••• ~.:.~.:.:.:.:.:-.:.:.:.: ••• ;.-••••••••••••• > ••••••••• y.""; ••.•••••
• ••••••••••••••• -.--. ••••••••••• ; •• ';O ••••••••••• ~ ••••• , ............... -.-.-.-.-••• '.-. •• ; ............. <; ......... , •• --;" ••••••• ~."".f':.; •. O; •• O;; ••••••••••••••••••••••
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:190,01 omf 0,01
T=300K T=350K T=400K
0,001 0,001 0,001
0,0001 0.0001
O,QOOI II) 10-5 II) 10-5
II)
~1O-6 ~ 10-6 ~ 10-5 0 Cl 0 :;: 10-7 -E 10-7
c: 10-6 c: Dark
~ ~ 10-8 t 10-8
8 10-7 810-9 '" u 10-9
10-a 10-10 10-10
10-11 10-11
10-9 10-12 10-12
1 10 100 1 10 100 1 10 100
Volts Volts Volts
(a) ! b) ( c)
FIG, 11. 1-V characteristics for same sample as Fig. 10 at (a) T = 300 K, (b) T = 350 K, and (c) T = 400 K.
tive PITS transient and the type of /-V characteristic ob
served is the same. The results also show that this process
can be saturated by using incident light at intensities above a
particular threshold. The higher the sample temperature,
the greater is the light level required to observe these effects
which suggests a thermal contribution to the mechanism.
Figure to, however, shows that for temperatures less than
300 K, where the thermally generated dark current was low,
it was not possible to change from type I to type II behavior
through optical excitation. This will be discussed later. At
300 K, the threshold intensity required to change the sam
ple's behavior from type I to type II was 3.2 X 1016 photons
cm-2 8-1 (corresponding to 9.6x 1014 photons cm-2 illu
minating the sample during a 30 ms pulse as would be typical
in a PITS experiment).
E. PITS response of baked and pOlished substrates
Annealing undoped LEe ingots has been shown to im
prove their homogeneity in terms of resistivity, mobility, and
EL2 concentration.39-42 It has also been shown that wafer
annealing at temperatures of 750-850 °C for extended per
iods (baking) followed by a repolish of the wafer has pro
duced improvements in the quality of epitaxial layers grown
on the substrate as manganese and other impurities outdif
fuse or getter towards the surface during the heat treatment
and are then removed by repoiishing.43.44 This has the effect
of reducing subsequent out-diffusion of impurities during
growth improving both the quality of the epilayer and the
epilayer/substrate interface. Similar observations have been
noted for ion-implanted layers.45
PITS spectra were obtained for wafers which had under
gone this baking and repolishing treatment. Adjacent wafers
were selected from several bouIes and one wafer from each
was subjected to a ISh anneal at 750°C under H2, similar to
the procedure outlined by Maki43 and Palmateer44 and were
then repolished. PITS samples were cut from the same posi
tions on both the baked and unbaked wafers from each boule
and a comparison was made between them. Figure 12 shows
the responses for two adjacent wafers from a 3-in.-diam
222 J, Appl. Phys., Vol. 65, No.1, 1 January 1989 ingot having type I behavior. Whereas the unbaked wafer
shows a PITS spectrum typical of undoped LEe material,
complete with a negative peak, the baked and polished wafer
shows a decrease in the number of peaks observed, no nega
tive peak, and very little detail in the PITS spectrum. The
unbaked wafer produced type I 1-V characteristics, typical
of negative peak samples whereas the baked and polished
wafer produced a characteristic which exhibited higher leak
age currents and no well-defined threshold V T in agreement
with the previous correlations.
o 25eV
(a)
100
u 0 neV c
C~
!ii
00
f-
0: I (.bl
100 037eV O.1.8eV Unbaked
ernex. = 176s"
\ ."
200 --30-0 --~;,oo~
Temperature (K) 12eV
O.1.8eV
I
200 300
Temperature (f<) Baked i polisr-ed
emcx;: 1765-1
400
FIG. 12. Comparison of PITS spectra (a) unbaked and (b) baked and pol
ished wafers from the same ingot.
S. R. Blight and H. Thomas 222
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19As the polishing mechanisms were identical for both the
baked and unbaked wafers, the differences in the spectra can
be attributed to the baking process. This would suggest that
subsurface crystal damage5,46 caused by the polishing alone
is not responsible for the negative peak unless it is impurity
related, possibly enhanced by the aggregation of impurities
andlor Doint defects around damage sites caused by the po
lishing process. The increased leakage current observed after
baking, however, means that the overall compensation
mechanism or the stoichiometry of the material has
changed. Ogawa47 explained the decrease in resistivity after
heat treatment in terms of an increase in EL3 and EL6 con
centrations. Unfortunately, the smearing of the PITS spec
tra after annealing made this difficult to confirm and, as
discussed previously, quantitative analysis of trap concen
trations by PITS is not possible. Outdiffusion of EL2 as de
scribed by Makram-Ebeid38 could also be responsible for the
lowering of the resistivity after baking, as may the passiva
tion of EL2 by hydrogen diffusing in from the surface.48
These seem less likely as typically 75 /-tm is removed from the
surface during repolishing, requiring the change in the com
pensation mechanism to be a bulk effect and not confined to
a layer near the surface.
More probable is a change in the stoichiometry of the
GaAs due to the loss of arsenic during the long duration
anneal. This would result in lower resistivity material and
different surface conditions, despite identical polishing pro
cesses, for the unbaked and baked samples and could be the
reason why the baked and polished wafers did not produce a
negative PITS peak.
OJ5eV
~ \ O.l3eV
« :z
'" Vi
V1 0-e:
100
100 200
Temp.r~ture. K
O.71.V
200
T emperuture, K Ungated
ema.=110s-1
C,73eV
(luted 400
400
FIG. 13. Comparison of PITS spectra for ungated and gated samples oftype
I materiaL The negative peak disappears for gated structures.
223 J. Appl. Phys., Vol. 65, No.1, 1 January 1989
•••••••••••••••••• -;r;. ••••••••••••• ;-••• -.: ••• ; •• -.7'.-..-.; •••• ~.;< ........................ -; .......... - -•••••••• " ••••••••••• -. •••• -.-.-.-••••• ~~, ••••• ,.. '~.".'.·N ••••• •••• ~.' ••••• F. Gated PITS experiments
The coplanar geometry of the PITS samples used leaves
a large area offree GaAs surface between the contacts which
has been shown, for type I samples, to have a temperature
dependent surface conductivity, implying that the free-sur
face potential varies between the contacts. The following ex
periment was therefore devised to avoid this effect.
A long gate field-effect transistor (FATFET) structure
was fabricated on semi~insulating GaAs. The two ohmic
contacts were the source and drain contacts of the F ATFET
but a 70-A.-thick layer of piatinum, thin enough to allow a
significant proportion of the incident light to pass through,
was deposited as the gate of the structure. The gate had di
mensions of 300 X 3oo;.tm and acted as an equipotential sur
face, floating at some potential between the two ohmic con
tact potentials, to within 2 f1.m of either ohmic contact.
Immediately adjacent to this device, normal l,mgated struc
tures were included for comparison. Figure 13 shows a com
parison of the PITS spectra for gated and ungated structures
on a type I wafer. Whereas the ungated samples show a spec
trum typical of undoped LEe material, including the nega
tive peak, the gated structure shows no negative peak behav
ior, instead an entirely positive spectrum was obtained. The
lower temperature peaks remain similar for both structures,
emphasizing the bulk nature of these levels but, as can be
seen, the high temperature end of the spectrum is modified.
The experiment was repeated on another portion of the
same wafer, this time with ~ 20 f..lm etched from the surface
prior to contact evaporation. Figure 14 shows that similar
differences between gated and un gated structures were ob-
Vl ....
a:
O.1ZeV
100 200
Temperature, K
Temp@ralllre,K 300 Etched
Ungated
ellill1;(lE 1'Os-1.
C.67eV
Etched
Gnted
Ie mer: J;:: 110$:-1
400
FIG. 14. Comparison of PITS spectra for un gated and gated samples of the
same wafer as Fig. 13, after the removal of ~ 20 p.m from the surface by
etching,
S. R. Blight and H, Thomas 223
•••••• n •••• ';-.~.~.~.",' ••• ' •• '-'-07 •••••••••••••••••••••••• ,', .'7. •••• -07 •••••••••• .-.-, ••• '.' 0;0-' •••••••••••••••••••• ~ H-..... ~ __ _
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19served to those obtained for the unetched wafer. It has been
shown, therefore, that by covering the ungated free surface
with a thin layer of metal and thereby maintaining an equi
potential surface between the contacts that the negative peak
can be eliminated, and hence its origin is not due to bulk
traps. Gated structures, by eliminating the appearance of the
negative peak, can therefore be employed to characterize
EL2 which, as described earlier, is seldom detected by the
normal PITS configuration in type I material.
III. DISCUSSION
The results presented illustrate that perturbing surface
conditions invariably result in modification or elimination of
the PITS negative peak as well as changing the current-vol
tage characteristics of the material. This highlights the need
to consider surface related effects as wen as purely bulk phe
nomena when interpreting electrically or optically based
measurements involving semi-insulating GaAs. It is well es
tablished that remnant subsurface damage can be detected in
semi-insulating GaAs wafers, regardless of the polishing
technology.46 Cross-sectional transmission electron micros
copy has shown that the damage occurs in a surface layer of
the order of 1000 A deep, and consists of dense dislocation
networks.46 In view of the PITS response of etched wafers
and baked wafers, it is thought unlikely that subsurface dam
age is the sole cause of the negative peak. It is possible that
impurity gettering around damage sites may contribute but
the gated-PITS results indicate that the free GaAs surface is
more likely to be the origin of the negative peak and that
possibility will be considered here. Experimental evidence in
favor of this approach was provided by Young et al.5 who
discovered that the magnitude of the negative peak was ac
centuated when the surface was abraded. They concluded
that crystal damage due to the abrasion was the source ofthe
effect but then proceeded to show that the negative peak
could be explained in terms of several bulk-trapping models,
without directly attributing the surface itself as being the
physical source of the phenomenon. In fact, all previous in
terpretations of the negative peak2•6-8,16.49 have invoked bulk
trapping, the mechanisms of which have been outlined by
Young et aU The mechanism proposed by Hurtes and co
workers2 has already been discussed. Oliver et 01.49 found a
strong negative peak in undoped LEC GaAs grown with wet
B203 but not in samples produced with dry H203• They sug
gested that this level was related to oxygen and was partly
responsible for the semi-insulating condition of the material.
Ogawa, Kamiya, and Yanai6 reported a negative peak in
undoped LEe material and again attributed it to a bulk trap
ping mechanism. Rheel6 assigned the negative peak to an
electron trap. This was explained by assuming the presence
of centers with large capture cross sections which contain
photoexcited holes. These holes then recombine with free
and very shallow trapped electrons giving rise to a negative
transient and hence negative PITS peaks. In no instance,
however, has the possibility of the involvement of charge
exchange with the surface been considered. Uncontrolled
surface conditions have an influence on PITS spectra and
also on circuit performance and could account for many of
the unpredictable effects encountered in MMICs.50 Addi-
224 J. App!. Phys., Vol. 65, No.1. 1 January 1989 tionally, it has been shown elsewhere51.52 that type I sub
strates have a different conductance DL TS response after
ion implantation and subsequent MESFET fabrication than
type II materials, the differences being traced to the ungated
surfaces of the MESFET.5O-5~
The results presented in Sec. II D showed that at
T= 300 K, 3.2X 1016 photons cm-2 S--·I incident on the
sample under examination was the threshold required to
cause a transition from type I to type II behavior, both in
terms of the PITS negative peak and current-voltage re
sponses of the material. It can be inferred therefore, that this
number of photons saturates the process giving rise to type I
behavior and hence negative PITS peaks. Any number of
photons in excess of this at temperatures greater than 300 K
stimulates type II processes, producing positive PITS peaks,
usually reSUlting in trap signatures corresponding to bulk
EL2 centers.
If the V T threshold in the J-V characteristics of type I
material is due to a surface-state filling mechanism, then this
would be achieved at a lower voltage when states are fined by
sample illumination. This is exactly what was seen in Fig. 11.
The form of the transient during illumi.nation as shown in
Fig. 7 is also indicative of a net reduction in the number of
carriers reaching the contacts. This type of behavior was also
reported by Young et aJ. 5 and shows that the photocurrent
actually decreases with time, although it exhibits a different
time constant to the post-illumination negative PITS tran
sient. This may be due to carriers being captured by surface
states as the observed capture transients are much slower
than would be expected for bulk levels.55 Surface-state
charging would then deplete the bulk of free carriers as dis
cussed in Fig. 7. This mechanism is considered below.
During the illumination period, optically generated
electron-hole pairs flood the top 1 f.1.m of material, the num
ber of which, n, depends on the illumination intensity. As
sume a number of uniformly distributed electron traplike
surface states exist between the contacts, the available den
sity of which is, say, Ds' If the thickness of the substrate is w"
then Ds occupied surface states at equilibrium during illumi
nation could fully deplete the substrate of D,./w, carriers.
Now consider a population change in Ds due to thermal
emission from surface states after cessation of the light pulse
of !.lD" where
AD,. = Dlfje- 1 h', (4}
and D<fl depends on n, and the capture and emission cross
sections of the surface states, From charge balance consider
ations,
b.Ds = Nbu1ki:lW, (5)
where Nhu1k is the carrier concentration in the bulk and i:lw is
the change in depletion depth, If the current flow through
the sample, per unit width is J, the voltage applied between
the contacts V, and the conductivity of the material a, then
la:. V[u(w, -Dsoe~t/TINlmlk)]'
that is,
la:. Vow,. -VuDlOe- tl-rINbu1k, (6)
This would produce a current transient which increases with
time, i.e., a negative PITS transient, to a steady-state value
S. R. Blight and H. Thomas 224
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160.36.178.25 On: Mon, 22 Dec 2014 18:06:19determined by the dark current level,
Physically, this would correspond to the recovery of the
bulk current flow as charge is thermally emitted from sur
face states and either recombines or is collected at the con
tacts. An exponential dependence for the change in popula
tion of surface states due to thermal emission has been
assumed. This need not necessarily be true, however, experi
mental evidence suggesting that some other form of behavior
actually occurs in practice, e.g., emission from a band of
levels so that the transients superimpose on one another re
sulting in nonexponential behavior. A further complication
is that thermally emitted carriers from the deeper surface
states may produce a surface leakage current via shallower
surface states, due to a hopping process.32 Evidence that
thermal emission from surface states plays a part in this
mechanism can be obtained from Fig. 11. At elevated tem
peratures, the net population of these states is reduced by the
increased thermal emission rate, hence a greater number of
photons is required to effect the transition from type I to type
II behavior.
This mechanism would explain the results obtained in
Fig. 7, since the current observed after etching was actually
greater than before etching. This is difficult to understand in
terms of purely bulk conduction as the substrate thickness is
actually decreased after etching. An increase in surface con
ductivity would account for this, however. This means that
although the negative transient is obtained by a decrease in
the depletion of bulk carriers it is also accompanied by an
increase in surface conductivity, for example, through emis
sion from surface states.
Above the critical illumination intensity, photogenerat
ed carriers become available for bulk trapping and detrap
ping and hence a positive transient contribution to the over
all PITS transient is obtained, first reducing the amplitUde of
the negative transient and eventually overwhelming it to
produce positive peaks in the PITS spectrum.
Recent publications have demonstrated50-52 that semi
insulating GaAs which shows the negative peak in its PITS
spectrum is more susceptible to effects such as transconduc
tance dispersion, "hole-trap" conductance DLTS spectra
and backgating in an ion-implanted MESFET than type II
materiaL The negative PITS peak can be utilized as an indi
cator of these effects and as such provides a preprocessing
characterization tool which is simple to implement in prac
tice. It can also be used as qualification indicator as material
which gives rise to negative peaks invariably produces good
implant grade material whereas type II material seldom
passes all acceptance procedures. ~6
IV. CONCLUSIONS
Whereas previous interpretations of the negative PITS
peak have invoked bulk-trapping phenomena, evidence has
been accumulated during this study which points towards a
surface contribution. This paper has attempted to propose a
mechanism for its appearance without making any physical
assumptions with regard to either the bulk or the surface of
the semi-insulating GaAs under examination. It merely
points out that the negative peak occurs as a consequence of
charge exchange with surface states, the capture and em is-
225 J. Appl. Phys., VoL 65, No.1, 1 January i 989 sion properties of which remain largely unknown as present.
Clear correlations between the appearance of the nega
tive PITS peak and the steady-state current-voltage charac
teristics of the material both in the dark and under a variety
of illumination and temperature conditions have been dem
onstrated. Gated-PITS has been introduced as a method for
eliminating the appearance of negative peaks and, as such,
can allow the detection of EL2 in undoped LEe GaAs, sel
dom previously observed by PITS.
ACKNOWLEDGMENTS
Discussions with R. H. Wallis and P. H. Ladbrooke are
gratefully acknowledged. We are also grateful to D. C. Bar
tle, A. D. Page, J. P. Nagle, B. P. Davies, M. S. Frost, and D.
Wannan for valuable experimental help.
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1.343623.pdf | Surface stoichiometry and valence electronic structure of YBa2Cu3O7−x
F. Parmigiani, G. Samoggia, C. Calandra, and F. Manghi
Citation: Journal of Applied Physics 66, 5958 (1989); doi: 10.1063/1.343623
View online: http://dx.doi.org/10.1063/1.343623
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/12?ver=pdfcov
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129.22.67.107 On: Sat, 22 Nov 2014 15:40:25Surface stoichiometry and valence electronic structure of YBa2CU307_X
F. Parmigiani
CISE S. p. A. Materials Div .• P. O. Box 12081. 20134 Milano, Italy
G. Samoggia
Dipartimento Fisica Generale Universita di Pavia Via Bassi No.6. Pavia. Italy
C. Calandra and F. Manghi
Dipartimento FI:~ica Universita di Modena. Via G. Campi No. 2131A, 41100 Modena. Italy
(Received 9 February 1989; accepted for publication 21 August 1989)
We report x-ray photoemission data from YBa2Cu307 x showing that the measured
composition and the electronic structure changes significantly upon the photoelectron escape
depth. For large take-off angles the valence band spectrum near the Fermi edge is well
structured and shows a number of features arising from Cu-o hybrid states. Their intensities
are drastically reduced in the energy distribution curves measured at grazing angles. These
results are interpreted assuming that the surface is mainly composed by Ba-O planes. This
interpretation is shown to be consistent with electronic structure calculations for the bulk and
for a YBa2Cu307 _ x surface terminated with a Sa plane and produced by cutting the
CU2 -04 bond normal to the c axis. The local density of states calculated on this basis and
taking into account electron correlation effects is shown to be consistent with many significant
behaviors experimentally observed in the YBa2Cu307 _ x valence band.
I. INTRODUCTION
After the discovery of high Tc superconductivity in cop
per oxide materials (HTSCs), a large number of papers have
appeared dealing with their electronic structure, since, as it
is widely recognized, an accurate knowledge of the valence
band states is fundamental in order to understand the basic
mechanism of high Tc superconductivity.
In spite of the great efforts on this subject, 1-10 some dis
agreements still exist in the experimental data. In particular,
resonant photoemission (RESPES), x-ray photoemission
(XPS), and ultraviolet photoemission (UPS) experiments
performed on sintered YBa2Cu307 (YBCO) systems give
different results :in the binding energy region between E F and
2 e V. The RESPES spectra do not show any feature, 1,2 while
the XPS and wen-resolved UPS data indicate that at least
one structure is present.4-10 Also, RES PES measurements
performed on HTSC single crystals) show that the region
between Ep and 2 eV is more structured than in sintered
samples, therefore indicating that the valence band structure
of these systems is still far from a complete description.
The aim of this paper is to demonstrate that the dis
agreement between the experimental data is mainly due to
the difference between the surface and the bulk stoichiome
try. In particular, the surface appears to be richer in Ba than
the bulk, while no significant change in the oxygen content is
observed. We also show that XPS valence band spectra are
sensitive to the take-off angle: in particular the energy distri
bution curve (EDC) measured at 90° is more indicative of
bulk properties than RESPES or UPS and exhibits features
in the energy region between E F and 2 e V.
To understand these observations we have performed a
theoretical calculation of the surface electronic structure of
YBaZCu307 assuming a Ba-04 termination of the crystal. To
make the results comparable with the experimental data we
evaluate self-energy effects using an approximate approach to treat the electron correlation. To our knowledge this is the
first calculation of the one-hole spectrum with surface and
correlation effects included. Moreover, it indicates that the
disagreement already emphasized between the experimental
and theoretical data can be largely reduced when the surface
effects and the electron correlation are properly taken into
account.
II. EXPERIMENTAL RESULTS
Details of the YBCO sample preparation are given in
Ref. 11. XRD measurements showed that only the typical
perovskite orthorombic structure is present. The sample
conductivity was characterized by an onset at about 98 K
and zero resistance at 95 K. Magnetic measurements
showed, below Tc, the typical diamagnetic behavior of sin
tered YBCO systems. Beside the YBCO sample with an oxy
gen stoichiometry of 6.9, a sample with an oxygen stoichi
ometry of 6.4 has been prepared by reducing with a
thermogravimetric method the oxygen content of a wen oxy
genated sample. The oxygen reduced sample showed at the
XRD analysis a tetragonal structure and, as expected, no
superconducting behavior. The XPS apparatus and the ex
perimental procedures have been discussed in previous
works. 12
Since CuO based superconductors are known to be very
sensitive to radiation induced oxygen losses, we found it im
portant to combine the low x-ray intensity, i.e., 1/10 ofa 300
W x-ray AlKa source, of a quartz single crystal monochro
mator with the high sensitivity of a multichannel detector.
The surface stoichiometry as measured from the XPS Cu,
2p, Sa 3d, Y 4d, and 0 ls lines before and after the valence
band measurements showed a stable oxygen content within
the sensitivity of this technique. Since in other experiments
performed using a synchrotron radiation source an oxygen
depletion of the source has been observed,13 it is possible to
5958 J. Appl. Phys. 66 (12). 15 December 1989 0021-8979/89/245958-04$02.40 © 1989 American Institute of Physics 5958
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129.22.67.107 On: Sat, 22 Nov 2014 15:40:25TABLE l. The Cu 2p and Ba 3d at. % as measured with a take-olf angle of 15' and 90' and as computed from the stoichiometric formula.
Element
Cu2p
Ba3d Sensitivity
factor
4.798
6.361 Concentration (%)
(theoretical)
59.92
40.08
conclude that the oxygen losses in these systems are strongly
influenced by the intensity of the light beam probe.
In order to avoid any contamination after sintering, the
samples were directly transferred into a desiccator and then
introduced through a vacuum interlock into the spectrom
eter. Clean surfaces were prepared by fracturing or scraping
the sample in a vacuum chamber having a base pressure of
8 X 10 -10 Torr, Nevertheless, it was not possible to avoid the
presence of contaminants such as carbon, as already report
ed in a previous study. 12 The presence of carbon could be a
source of uncertainty in the interpretation of the experimen
tal data; however, the spectral region between E F and 2 e Y of
binding energy is free from this problem. 14
The sample stoichiometry, at different take-off angles,
was measured from the Cu 2p, 0 Is, Ba 3d, and Y 4d core
level lines. Table I summarizes the Ba versus Cu content as
measured at 10° and 90· using the sensitivity factors reported
in the literature. 15 It is evident that the copper content near
the surface is quite different from the one expected on the
basis of the chemical formula. This behavior was also con
firmed by the stoichiometry data obtained through the com
parison oflow and high binding energy core level lines, 15 The
observed gradient of the Ba/Cu ratio has been reported also
by other authors and interpreted in terms of Ba segregation
towards the grain boundaries. 16
Here we give another interpretation of this effect based
on the fact that the Ba-O plane is the most stable basal plane
of the YBCO unit cell, this plane being electrostatically neu
tral. A possible experimental demonstration of this interpre
tation arises from the comparison of the Ba content mea
sured on the following surfaces: (i) unmodified sintered
surface (about 100% grain boundary); (ii) fractured sur
face (grain boundary reduced at about 60%) 12; (iii) scraped
surface (intragrain surface dominant), Since in all these
cases a constant Ba content was found, it is possible to argue
that the Ba-O plane is always the topmost surface layer. That
is clearly in disagreement with the hypothesis of segregation
process, but it is in good agreement with the fact that the Ba
o is the most stable plane. Indeed in such a case a XPS
analysis performed at grazing angles, i.e" with an escape
depth of about 4 A, is expected to give a Ba/Cu ratio about
near one, w hUe with a 90· take-off angle, corresponding to an
escape depth of about 30 A, the BalCu ratio should have
approximately the stoichiometric value of 1.5, since more
than two cells can be reached. These predictions are wen
verified, as shown by the measured ratios reported in Table I.
In the light of these observations, the valence band mea
surements were also performed at a take-off angle of 90° and
15° using the monochromatized AlKa line and a final resolu
tion of the analyzer of ;:::0.35 eyI2 as measured on the Si 2p
core line of a 100 Si single crystal. Figure 1 (a) reports the
5959 J. Appl. Phys., Vol. 66, No. 12, 15 December 1989
• -.-.-•••.••.•••.•.•••••••••••••••••••••••••••••••••• -. ••••• : ••••••••••••••••••••••••••••• ~ •.• ~ ........... "' •••• '.:.:.:.:.-;.:o:.:.x-:.:.:.;-.;.:.:.;.;.:.:o:.;.;.:.;.;.:o;.:o:o;.:.:.:o;.:.:.:.;.:;o;.:.; ••••• ~ ••••••••••••••••• ; •••••••••••• ' ••• Concentration (%)
15'
54.2
45.8 Concentration (%)
90'
60.4
39.6
valence band spectrum between 0 and 18 e Y of a sintered
YBa2Cu307 _ x sample measured at 90·. Beside the struc
tures P4-Pg, already observed and widely discussed in the
literature, l-lO the salient features, which are intrinsic of the
YBCO system, are a first band ( PI) centered at about 0.7 e Y
followed by two other structures at about 1.6 ( P2) and 2.3
( P3)' respectively.
The intensity of these lines decreases significantly when
the valence band measurements are performed at 15°, as
shown in Fig. 1 (b). Since grazing angle XPS measurements
detect a surface region, which is copper deficient, it is possi
ble to infer that the PI' P2' and P3 structures arise from states
having a significant Cu contribution. On the other hand, the
EDC obtained from a surface where the oxygen content was
.I!l " 1\
I \ /1 -C I \ I ;;j 'r\/ (b) I , .
.ri ,
... / I
~ I
>. IEF
'iii c -.-I OJ
C Pe I
+
2 -2
Binding Energy (eV)
FIG. 1. Valence band XPS spectra ofsintered YBa2Cu307 x superconduc
tor. Measurements have been performed using the monochromatized AIKa
line and a multichannel detector and scraping the samples in ultrahigh
vacuum. Curve (a) shows the valence band spectra measured using a take
off angle of 90·. The p,.p, bands are correlated with the Cu-o orbitals.
Curve (b) reports the XPS valence band spectrum measured at 15°. As it is
possible to notice the spectral intensity in the E ",-2 eV region is significantly
reduced. Curve (c) reports the XI'S valence band measured at 90° on a
surface with reduced oxygen stoichiometry. Also in this case the spectral
intensity between Er and 2 eV is reduced.
Parmigiani at al . 5959
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129.22.67.107 On: Sat, 22 Nov 2014 15:40:25(a) 8
Pc +4 ~
J5 ~ P !2
.......
Ul
-'-'
c
::J
..d
L
3
>. -'-'
Ul c:
(l)
-'-' c ..-
(a-b)
18 16 14 12 10 8 6 4 2 0
Binding Energy (eV)
FIG. 2. Valence band XPS spectra ofsintered YBa"Cu,O, " superconduc
tor (a) and YBa2Cu,06.4 semiconductor (b) are reported. Also the differ
ence between the spectra (al and (b) is reported. The PI-P, bands correlat
ed with the Cu-O orbitals are strongly reduced in the oxygen deficient
sample [curve (b) J. On the other hand, as expected, the band P" is en
hanced, whereas the band P7 assigned to the IG term of the d 0 multiplet, i.e.,
to the CuH state, is significantly reduced.
reduced of about 10% by a mild sputtering, shows a similar
decrease in intensity [see Fig. 1 (c) J. This suggests that hy
bridized Cu-O nonlocalized states significantly contribute to
the spectrum in the 0-2 eV binding energy region, whereas
the persistence of the P4-PS structures indicates that they are
mainly due to Cu 3d and ° 2p localized orbitals.
The behavior of structure P6 is rather interesting. This
structure has been originally imputed to a two-hole oxygen
satellite.17 However, recent experimental work on mono
crystals 18 has established that it is better seen in oxygen poor
samples where its intensity relative to other peaks is consid
erably enhanced. We have performed similar measurements
on sintered samples with different stoichiometry and found
that indeed the structure is more intense when the oxygen
content is reduced, as shown in Fig. 2.
The fact that in curve (b) of Fig. 1 the structure p() does
not seem to be enhanced compared to the curve (a) suggests
that the oxygen content in the region sampled by the experi
ment at 15° is the same as in the 90° spectrum. This linding is
supported by the spectrum (c), corresponding to the oxygen
poor surface phase, obtained by mild sputtering, where the
peak is enhanced significantly with respect to the bulk spec
trum. Also the position of the P7 band, which is found at 12.4
5960 J. Appl. Phys., Vol. 66, No. i 2, 15 December 1989 eV,13 is consistent with the position found in well-oxygenat
ed samples, i.e., x = 6.9.
As to the high binding energy region, we notice that the
structures P7 and Pg undergo a substantial intensity reduc
tion on passing from the 90° to the 15° curve. It is well known
that these structures result from the overlap between the Cu
d R satellite and the Ba 5p core levels: P7 is conventionally
assigned tothe tG termofthed 8 multiplet, whilePgis mainly
due to emission from Ba 5P3!2 core level with some contribu
tion from the lower terms of the multiplet, to In view of the
fact that the surface is Ba rich, we can understand the modi
fications in intensity only by assuming that a substantial
weakening of the satellite occurs at the surface. Of course
since the P7 band is assigned to the d8 multiplet (Cu2+
state), oxygen depletion also, that favors the Cu (·1 state, will
reduce the P7 band, as confirmed by the data shown in Fig. 2 .
m. THEORETICAL RESULTS
Further support to this interpretation concerning the
role of Ba-O planes in determining the photoemission spec
tra is obtained by comparing our data with theoretical elec
tronic structure calculations.
In a previous theoretical study two of the authors have
shown that the absence of a surface may modify in several
different ways the electronic distribution in the outer layers:
in particular, the states lying near the Fenni energy have
been shown to be extremely sensitive to the surface condi
tions.19 Therefore, it is interesting to see if the changes ob
served in the experimental energy distribution curves are
consistent with the modifications of the local density of
states (LDOS) that take place on passing from the bulk to
the surface. To this end we have performed a calculation of
the LDOS for a YBa2Cu307 crystal terminated with a Ba
plane. This surface can be produced by cutting the CU2 -04
bond normal to the c axis.20 Among the basal plane surfaces,
this is expected to be the most stable, since it is electrostati
cally nonpolar.
We performed the calculation assuming an ideal surface
geometry, i.e., a truncation of the crystal without any change
in the atomic positions, and using a tight-binding Hamilto
nian derived by parametrization of the band structureY To
determine the surface features we calculated the electronic
structure of a slab obtained by stacking four cells along the c
axis, corresponding to approximately 30 atomic planes, the
surface boundary conditions being accounted for by remov
ing the interactions between the atoms of the external cell
and their missing neighbors.
Due to the importance of correlation in YBa2Cu307,
theoretical spectra derived from single particle calculations
are not expected to reproduce accurately the location of the
main structures in the experimental EDCs. To make a mean
ingful comparison with the experiments a one-hole spec
trum, that includes correlation effects, has to be derived
from the LDOS. To this end we have used a Hubbard model
Hamiltonian with intra-atomic Coulomb integrals Uel and
Up giving, respectively, the repulsion between two d holes
sitting on a Cu ion or two p oxygen holes. We evaluated the
hole self-energy using the t-matrix approximation, which is
correct in the low density limit, since it treats the correlation
Parmigiani et al. 5960
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129.22.67.107 On: Sat, 22 Nov 2014 15:40:25to all orders. 22-24 This same approach has been recently used
to determine the correlated one-hole spectrum of
La2Cu04•24 Once the self-energy for a hole sitting on a given
atom is determined, the spectral function can be obtained by
summing the contributions of the unequivalent atoms, For
the Cu case we introduced the Coulomb interactions appro
priate to the d 8 multiplet structure23 by fixing the Ud value
for the lG term so as to reproduce the Cu satellite position
around 12-13 e V, The energy location of the other terms was
calculated using the F2 and F4 Slater integrals derived from
L2,3M4•sM4•5 Auger spectra.25 The theoretical EDCs were
derived by summing the contributions of every atomic plane
to the spectral function, weighting each contribution with an
exponential factor to account for the escape depth.
Figure 3 displays our calculated photoemission curves
for escape depth A = 30 A and It = 4 A, It is immediately
obvious that the surface affects the spectrum in two ways:
First, it decreases the emission in a 2 e V energy range below
E F compared to the bulk, enhancing the intensity of the peak
around 4 eV; second, it significantly reduces the intensity of
the Cu satellite, located between 12 and 13 eV. Both effects
are a consequence of the drop in the density of 04 and Cu
states near E F caused by the removal of CU2 d states and by
the narrowing of the 04 derived bands. The surface peak
around 4 e V is mainly due to oxygens belonging to the chains
of the outermost cell and corresponds to bands of surface
states and resonances. A full discussion of the theoretical
details, including the dependence of the theoretical spec
trum upon the Coulomb parameters, is given elsewhere,26
Comparison with the modifications observed in the ex
periments shows that the theory correctly predicts the sub
stantial decrease of emission found below E F and the conse
quent narrowing of the main structure. Moreover, it
provides an explanation of the intensity decrease in the 12-
14 eV binding energy region, showing that it is due to the
reduction in the satellite weight.
It should be noticed that these are the first theoretical
results which include both correlation and surface effectso
Previous comparisons between band theory and experiments
have been performed using the eigenvalues appropriate to
r-~, -_r-__ -_-f..,-=-.3T~iT·
_A= 41
/,
I,
I,
"
18 16 14 12 10 ,
I I
/
-'
8 6 4
Binding Energy (eV)
FIG, 3, Theoretical photoemission spectrum calculated with U" ('G) = 6.5
eVand Uo = 2.0eV. Spectrum (a) is obtained with escape depth A = 30 A;
speetrum (b) with A""' 4 A.
5961 J. Appl. Phys., Vol. 66, No. 12, 15 December 1989 the ground state, Leo, neglecting hole self-energy effects, and
without including the modifications in the electronic distri
bution due to the surface.27 As a consequence, these theories
do not predict the existence of satellites and their modifica
tions at the surface. They also show significant discrepancies
in the location of the main peaks. With the present model the
Cu satellite behavior is correctly predicted and the discrep
ancy in the main peak location is reduced, although we are
still far from a complete description of the spectrum. It is
interesting to notice that the oxygen satellite is not predicted,
although correlation in oxygen bands is explicitly intro
duced. This confirms the conclusion thatthe peak at 9-10 e V
has a different source.
In conclusion we have shown that the observed photoe
mission spectra are very sensitive to the photoelectron es
cape depth and that the surface induced modifications are
consistent with those expected for Ba-04 surface planes.
'Y. Chang, M. Onellion, D, W. Niles, R. Joynt, and G. Margaritondo,
Phys. Rev, B 36.819 (1987).
2J. A. Yarmolf, D. R. Clarke, W. Drube, U. O. Karlsson, A, Taleb-Ibra
himi, and F. J. Himpsel, Phys. Rev. B 36, 3967 (1987).
-'N. G. Stoffel, y, Chang, M. K, Kelly, L Dotti, M. Onellion, p, A. Morris,
W. A, Bonner, and G. Margaritondo, Phys, Rev. B 37, 7952 (1988).
4D. D. Sarma, K, Sreedhar, P. Ganguly, and C. N. R. Rao, Phys. Rev. B 36,
2371 (1987).
'z, Shen,], W. Allen, J. J. Yeh,], S. Kang, W. Ellis, W. Spicer, L Lindau,
M, E, Maple, Y. D. Dalichallch, M. S. Torikachvili, J, Z. Sun, and T. H,
Geballe, Phys. Rev. B 36,8414 (1987).
"G. Wendin, J. Phys. (Paris) Colloq. (,'9, US7 (1987).
7M. H. Frommer, Phys, Rev. B 37.2444 (1988).
"AI. Viescas, J, M. Tranquada, A. R. Maadenbaugh, and P. D. Johnson,
Phys. Rev, B 37,3738 (1988).
°Z, Iqbal, E. Leone, K Chin, A. ], Signorelli, A. Bose, and H. Eckhardt, J,
Mater. Res. 2, 768 (1987).
Illp. Thiry, G. Rossi, Y. Petroff, A. Revcolevschi. and J. Jegoudez, Eur
ophys. Lett. 5, 55 (1988),
I 'F, Parmigiani, G. Chiarello, N< Ripamonti, II, Goretzki, and U. Roll,
Phys. Rev.1l36. 7148 (1987).
'2F, Parmigiani and G. Samoggia, Europhys. Lett. 7, 543 (1988).
"'R. S, List, A. J. Arko, Z, Fisk, S. W. Cheong, S. D. Conradson, J, D.
Thompson, C. B. Piece, D. E. Peterson, R. J. Bartlett, N. D. Shinn, ], E.
Schirber, B. W. Veal, A. P. Paulikas, and J. C. Campuzano, Phys. Rev. B
38,11966 (1988).
'''The spectral region more affected by C eontamination lies more than 4 e V
below Ep, Moreover, since the carbon content in the samples studied in
this work was less than 10 at. % on the surface, none of the structures
reported in Fig. 1 (a) are due to this element.
"D. Briggs and M, p, Seah, Ed;;" Practical Surface Analysis by Auger and x
ray Photoelectron Spectroscopy (Wiley, Chichester. 1983), pp, 133-134
and 511-514.
H,p. Stucki, P. Aumesh, and T, Baumann, Physica C 158, 481 (1988),
17M. Tang, N, G. Stoffel, Q. B. Chen, D. La Graffe, P. A. Morris, W. A.
Bonner, G, Margaritondo, and M. Onellion, Phys. Revo B 38,897 (1988).
IKC. Calandra, F. Manghi, T. Minerva, and G. Goldoni, Europhys, Lett. 8,
791 (1989).
'"Here and in the following we will use the notation ofM. A. Beno, L. Soder
holm, D, W, Capone II, D, G, Hinks, J. D. Jorgensen, 1. D. Grace, Ivan K,
Schuller, C. U. Segre, and K. Zhan. Appl. Phys, Lett. 51, 57 (1987),
2uM. J. DeWeert, D, A. Papaconstantopouios, and W. E. Picket (unpub-
lished).
2ID. Penn, Phys. Rev. Lett. 42, 921 (1979).
nA. Liebsch, Phys, Rev, B 23,5203 (1981).
23K, J. Chang, M. L. Cohell, and D. R. Penn, Phys. Rev, B38, 8691 (1988).
24E. Antonides, E. C. Janse, and G. A. Sawatzky, Phys. Rev, B 15, 1699
(1977).
2Sc. Calandra and T. Minerva (unpublished).
2hFor a review see W. E. Piekett, Rev. Mod, Phys. 61,433 (1989).
Parmigiani et al. 5961
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1.458544.pdf | Diffusion, adsorption, and reaction in pillared clays. I. Rodlike molecules in a regular
pore space
Muhammad Sahimi
Citation: The Journal of Chemical Physics 92, 5107 (1990); doi: 10.1063/1.458544
View online: http://dx.doi.org/10.1063/1.458544
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/92/8?ver=pdfcov
Published by the AIP Publishing
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129.22.67.107 On: Mon, 24 Nov 2014 13:42:31Diffusion, adsorption, and reaction in pillared clays. I. Rod-like molecules
in a regular pore space
Muhammad Sahimi
Department of Chemical Engineering, University of Southern California,
Los Angeles, California 90089-1211
(Received 21 September 1989; accepted 10 January 1990)
We report the results ofthe first computer simulation oftransport, adsorption, and reaction
processes in pillared clays, which are a class of catalytic materials with high catalytic activities.
These materials have a very restricted pore structure which gives rise to the phenomenon of
hindered diffusion in their pore space. We develop a dynamic Monte Carlo method and study
diffusion, adsorption, and reaction phenomena in such systems. The pore space of the pillared
clays is represented by parallel silicate layers connected to one another by pillars of various
sizes, and the molecules are in the form of long, needlelike objects. Diffusion is represented by
a random walk process, the adsorption of the molecules on the surface of the pillars takes place
with a probability proportional to a Boltzmann factor, and the efficiency of the reaction
properties of the pillared clays is investigated by measuring the average distance that a
molecule has to travel in the pore space in order to reach a reactive site. Our results indicate
that the structure of the pore space of the clays, the intermolecular interaction, the size of the
molecules, and their adsorption on the surface of the pillars strongly affect their effective
diffusivity and, even if the pore space of the clays is very regular and homogeneous, they can
give rise to anomalous diffusion in which the effective diffusivity of the molecules varies slowly
with time. Moreover, if the size of the molecules is comparable to the effective size of the pores,
the irreversible adsorption of the molecules gives rise to a percolationlike phenomenon, in
which the effective diffusivity decreases as the number of adsorbed molecules increases. The
effective diffusivity would ultimately vanish if enough molecules are adsorbed so that a sample
spanning path of open pores would no longer exist.
I. INTRODUCTION
Diffusion and reaction in porous catalysts have been the
subject of considerable research activity in the last few
years. 1-6 These systems, in addition to their great industrial
importance, also represent ideal model porous systems well
suited for theoretical and experimental studies of hindered
diffusion and reaction phenomena. Such phenomena, which
involve the transport and reaction of large molecules in
small pores, occur also in many processes of current scientif
ic and industrial interest, such as separation processes, sol
vent swelling rubbers, polyelectrolyte gels, enzyme im
mobilization in porous solids, and size exclusion
chromatography. Numerous experimental and theoretical
studies 7-20 have found hindered transport and reaction pro
cesses in porous media to be less efficient than unhindered
transport in an unbounded solution. This reduced efficiency
is generally caused by the molecules being excluded from a
fraction of the pore volume and by the hydrodynamic resis
tance hindering the transport of the molecules through the
porous medium.
Among all catalytic systems, which are of prime interest
in this paper, zeolites have received the greatest attention,21
but considerably less attention and research effort have been
focused on studying diffusion and reaction phenomena in
another class of catalytic materials, namely, pillared clays, 22
although they have recently received considerable atten
tion.23-25 The original idea for producing pillared clays, due
to Barrer and MacLeod,36 was to insert molecules into clay minerals to prop apart the aluminosilicate sheets, thereby
producing larger pores than in"native clays, or even in zeo
lites. However, such materials did not have the thermal sta
bility that zeolites usually possess, but pillars ofhydroxyalu
minum and other cations, which are capable of being
dehydrated to oxide pillars and to support temperatures of
up to 5OO·C without structural collapse under catalytic
cracking, are new and were first reported by Brindley and
co-workers37,38 and independently by Lahav et al.39 and
Vaughan and Lussier.40
In general, pillared montmorillomites are 2: 1 dioctahe
dral clay minerals consisting of layers of silica in tetrahedral
coordination, holding in between them a layer of alumina in
octahedral coordination. Substituting Si4+ with AI3+, or
AI3+ with Mg2+ gives the silicate layers a negative net
charge, which is normally compensated by Na+, Ca2+, and
Mg2+ ions.41 By exchanging the charge compensating ca
tions with large cationic oxyaluminum polymers, one can
synthesize molecular sieve-type materials.39,40 These inor
ganic polymers, when heated, form pillars which prop open
the clay layer structure and form permanent pillared clays.
The location and size of the pillars can, in general, vary, such
that they may give rise to an irregular pore space, but at least
in some cases, pillared clays have a very regular morphology,
as shown in Fig. 1. Pillared clays can also be in the form of a
tactoid, i.e., a broad range of pore radii (and thus pillar
sizes) between stacks of silicate layers, which is not as regu
lar as that shown in Fig. 1.
The structure of pillared clays is such that they behave
J. Chern. Phys. 92 (8),15 April 1990 0021-9606/901085107-12$03.00 @ 1990 American Institute of Physics 5107
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A
~ I I ~
~ ~ tI -
~ ~ tI
B
~ ~ ~ ~
I I ~
tI tI ~ ~
FIG. 1. A schematic representation of a pillared clay catalyst. (A) Irregular
and (B) regular distribution of the pillars.
as systems with a dimensionality between two and three,
since molecules are forced to move in a very restricted pore
space between silicate layers. The molecules might also be
able to move from one layer to another, although this may be
difficult, especially if the molecular sizes are large. There
have been some speculations42.43 that pillared clays may
have a fractal structure with a surface fractal dimensionality
df which is slightly less than two. If this is the case, it can
have important implications for diffusion and reaction in
pillared clays, since the behavior of such phenomena in frac
tal systems is totally different from that in regular and Eu
clidean ones. We shall discuss this later in this paper.
Pillared clays have shown high catalytic activities for
gas oil cracking, similar to zeolite-based catalysts. They have
also shown large initial activities towards methanol conver
sion to olefins and toluene ethylation, but they are substan
tially deactivated by coke deposition.35 One reason for the
interest in pillared clays is that their pore sizes can be made
larger than those offaujastic zeolites. Moreover, as access to
the interior pore volume of pillared clays is controlled by the
distance between the silicate layers and the distance between
the pillars, one or both distances may be adjusted to suit a
particular application.
Despite their industrial importance and in spite of the
fact that pillared clays can provide a testing ground for var
ious theories of transport and reaction in catalytic systems
and other porous media, no fundamental theoretical effort
has been taken so far to model transport and reaction pro
cesses in pillared clays. In this paper, we report the results of the first computer simulation of diffusion, adsorption, and
reaction processes in idealized models of pillared clays using
a dynamic Monte Carlo method (DMCM). Our results rep
resent the first step towards a comprehensive theory of trans
port and reaction of large molecules in pillared clays. In the
present paper, which is part of our fundamental study of
hindered transport processes in restricted environ
ments,20.44-47 we only use small, needlelike molecules in or
der to understand the role of molecular sizes on transport
processes. In a future sequel to this paper, we will employ
more realistic molecular shapes and sizes, e.g., in the form of
parallelepipeds of given effective dimensions, or spherical
and ellipsoidal particles.
This paper is organized as follows: In Sec. II, we develop
a DMCM to study diffusion, reaction, and adsorption in
pillared clays. We pay particular attention to the roles of
molecular sizes, the spacing between the pillars and between
the silicate layers, and irreversible adsorption of molecules
on the surface of the pillars. We also study the asymptotic
behavior of the diffusivity of the molecules (Le., in the limit
of long times) and the conditions under which it may
achieve a time-independent value. Our results are presented
and discussed in Sec. III. The paper is summarized in Sec.
IV, where we also discuss transport and reaction of mole
cules which may have more complex structures than those
considered in this paper.
II. DYNAMIC MONTE CARLO METHOD
We first represent the pillared clays with the structure
shown in Fig. 1. One can also represent the system by a
tactoid which has a much less-ordered structure than that
shown in Fig. 1, but we do not pursue this in the present
paper. This is justified to some extent by the realization
that42.43 the surface fractal dimension of pillared clays is
close to two, which implies a nearly homogeneous distribu
tion of the pillars. We shall, however, study this issue in a
future paper. The distance h that separates two silicate layers
is one parameter of our simulations. The pillars are repre
sented by b X b X h orthogonal parallelepipeds, where b is
also varied in order to assess the effect of the pillars' size on
the results. In principle, b can be a distributed quantity.
However, there is practically no experimental information
on the statistical distribution of b, if any and therefore we
assume that b is the same for all the pillars. In reality, ifthe
distribution of b is not very broad, it should have little elfect
on the qualitative features of our results discussed below. As
we obtain more experimental information about the struc
ture of pillared clays, we will also refine our model. While we
admit that our model of these catalytic materials may not be
exact, we do believe that our simulations will contribute to a
better understanding of transport and reaction in such sys
tems. This is particularly important in the light of the fact
that our simulations represent the first of its kind in the stud
ies of such systems. The pillars cause an excluded space in
the pore space and give rise to tortuous diffusion paths for
the molecules. The pore space (the space between the pillars
and the silicate layers) is assumed to have cubic symmetry
with a lattice spacing (the distance between two neighboring
J. Chem. Phys., Vol. 92, No.8, 15 April 1990
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sites) of I. That is, the molecules move along the principal
directions of a simple-cubic lattice. This assumption is one of
convenience and can be easily relaxed if necessary. The over
all effective dimensions ofthe system are L X L X h, where L
is the effective length of the silicate layers (all dimensions are
measured in units of the cubic lattice spacing). Thus, if there
are np pillars between two silicate layers, the porosity tP of
the system, i.e., the volume fraction of pore space available
for transport, is tP = 1 -npb 2/L 2.
After generating the pore structure of the clay, N mole
cules are injected into the system to initiate the computer
simulations; this is time t = O. The molecules are represented
as straight lines, needlelike objects, each occupying n nodes
of the pore space. Thus, the length of each molecule is
(n -1) I. In reality, of course, the molecules are not one
dimensional objects. However, in the present paper, we are
mainly interested in the qualitative features of mechanisms
of transport and reaction of finite-size molecules in the re
stricted pore space of pillared clays and, as mentioned in the
Introduction, in a future paper we shall report our results
with molecules that are represented by three-dimensional
objects of given shapes and sizes. Each molecule (or its cen
ter-of-mass) performs a random walk in the pore space of
the pillared clay; this random walk represents the diffusion
process. The random walk in the pore space is executed by
steps along the principal directions of the simple-cubic lat
tice. At every step of the simulation, a molecule selects one of
the available directions with an equal probability and makes
a transition to another part of the pore space. The random
walk is assumed to be of P6lya type, i.e., a nearest-neighbor
random walk. Each time a direction is selected, the center of
the molecule is moved one lattice unit to the new node and
the rest of the molecule is also displaced accordingly. Two
types of simulations have been carried out. In the first one, a
molecule ignores the presence of all other molecules and
moves to the selected new nodes. Hence, because of the ab
sence of any interaction between the molecules, one can in
ject into the system one molecule at a time, follow its motion,
and compile the statistics of interest. This simulation corre
sponds to a "tracer experiment" in which only a few test
molecules are sent into the pore space to probe the structure
of the catalyst. In the second type of simulations, two mole
cules are not allowed to occupy the same points in the pore
space, i.e., there is an effective hard-core repulsion at play
between the molecules. Thus, if a molecule attempts to move
to new nodes which are already occupied by another mole
cule, the move is rejected and the molecule stays at its pres
ent location. Because of this intermolecular interaction, one
has to inject simultaneously many molecules into the system
and follow their motions. In the first method, if N is large
enough and if there is no irreversible adsorption of the mole
cules on the surface of the pillars, the effective diffusivity De
and other properties of the system become independent of N,
whereas in the second method the density of the molecules
may affect the properties of the system, at least at the initial
stages (short times) of the simulations. Although other
forms of intermolecular interactions might be important,
most of the available experimental data have been reported
at low temperatures and pressures. Therefore, it is reasona-ble to neglect all other forms of interaction between the mol
ecules, or between the solid surface of the catalyst and the
molecules. However, if need be one can use molecular dy
namics simulation to include other forms of intermolecular
interaction. Work along these lines is currently in progress.
Once all molecules have made one attempt to move, the pro
cess time is increased by one unit and the process of moving
the molecules is repeated.
When a molecule comes in contact with the walls of the
system (i.e., the silicate layers), it is reflected back into the
allowed region of the pore space. This model, therefore, does
not allow for the adsorption of the molecules on the walls, in
agreement with the current understanding of sorption phe
nomenon in pillared clays. Allowing for finite adsorption
rates on the walls represents a rather minor extension ofthe
model, as discussed below. When a molecule comes in con
tact with a pillar, it is either adsorbed on it with probability p,
or is reflected back into the pore space, where p is propor
tional to a Boltzmann factor exp( -CEj), where C is a con
stant and Ej is the binding energy of a molecule on a site i of
the pillar. Therefore, fixing p is equivalent to specifying a
particular molecule and its binding energy to the surface.
The limit of very small p represents a system under sorption
kinetic control, whereas the limit p -1 represents a diffu
sion-controlled process. In our simulations, we do not allow
for desorption, an assumption which can, however, be very
easily relaxed. An adsorbed molecule causes an excluded
surface and volume in the pore space, where other molecules
can neither adsorb on nor diffuse through. The simulations
are terminated when a large number of molecules have been
adsorbed on the pillars, so that there is no significant further
adsorption, or the adsorbed molecules, due to their finite
volumes, have effectively blocked the pore space and no
further macroscopic molecular motion occurs. This simula
tion procedure is a DMCM, by which one can calculate all
the dynamic, as well as static properties of interest for the
system. The dynamics of this model are of course governed
by a master equation, i.e., a discretized diffusion equation
(represented by the discrete random walk described above) .
To calculate the effective diffusivity of the molecules, one
determines the time dependence of the mean-squared displa
cements of the centers-of-mass of the molecules R 2(t}:
(1)
where < ... ) denotes an average over all molecules and their
initial positions in the system. In the limit of long times and
for large systems, the ratio R 2(1)/t is proportional to the
effective diffusivity De of the molecules. The above calcula
tions are, of course, repeated for a large number of time steps
and many randomly selected initial positions of the mole
cules, and the averages of the quantities of interest are com
puted in order to obtain representative values of De. How
ever, if pillared clays do have a fractal structure, De will
never become independent of time. We shall return to this
point later in this paper.
This model should, in principle, without resorting to
any adjustable parameter, predict the experimentally mea
sured diffusivities. However, when comparing the results
with the experimental data, there are several factors that
J. Chem. Phys., Vol. 92, No.8, 15 April 1990
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have to be considered. The most serious problem with the
available experimental data is a direct consequence of the
nature of diffusion and adsorption phenomena. Since the
size of most molecules (such as straight chain alkanes) with
which pillared clays have been used is comparable to the size
ofthe pore openings, sorption of such molecules on the pil
lars and/or the walls results in a reduced pore space avail
able for further diffusion and, therefore, in diffusivities
which are decreasing functions of experimental time. In the
case of the diffusion-limited regime p -1, sorption will pri
marily occur at the outside external perimeter of the catalyst
particle and the phenomenon of pore-mouth blocking will
occur. The effect of molecular sorption on diffusivity is then
a strong function of the adsorption/desorption rates relative
to the diffusive fluxes. In any event, the DMCM developed
here can be used to systematically study the effect of all pa
rameters of the system on the transport process.
In the present paper, we have used a simple-cubic struc
ture with L = 200 (i.e., the length of the system is 199 lattice
units). Our simulations indicated that larger values of L do
not have a significant effect on the results. Various values of
p the adsorption probability, h the height of the system (the
distance between the silicate layers), and b the size (width)
of the pillars have been used. We have also used 20 different
realizations, i.e., 20 different initial positions for the mole
cules on the external surface of the system (excluding the
surfaces of the pillars and the silicate layers) and averaged
the results over all realizations. Our computations were car
ried out with a VAX 11/750 and a Cray X-MP supercom
puter. In what follows we describe and discuss our results.
0.07
0.06
~
>
.~ 0.04 -~ o
.~ 0.03 .... u
Q) -W 0.02
0.01 n=3 III. RESULTS AND DISCUSSION
The first issue that one has to investigate is whether, in
the limit of long times and for a fixed and large value of N,
the effective diffusivity of the molecules will approach a con
stant value. This can also shed light on the structure of the
system as well. For example, if the system does have a fractal
structure, then even in the limit oflong times one has48
R2(t)_t6, (2)
where 8 < 1 for most fractal systems, whereas 8 = 1 for Eu
clidean and macroscopically homogeneous media (which is
the usual Fick's law of diffusion). Since De -R 2(t)/t, Eq.
(2) would predict that
(3)
so that De -0 as t -00 • If Eqs. (2) and (3) are applicable to
a system, then the diffusion process cannot be described by
the classical continuum equation of diffusion (with constant
diffusivity). Instead, the concentration C of the molecules,
at time t at a distance r from the origin, is given by49-51
(4)
where a = 8drl2, v = 2(2 -8) -1, and {3 is a constant of
order of unity. Note that equation (4), which can also be
interpreted as the probability of finding a molecule at point r
at time t, reduces to the well-known Gaussian distribution in
the limits of 8 = I and df = d for a d-dimensional system.
Therefore, deviations from Eq. (4) can be interpreted as an
indication that a given system may have a fractal structure.
Figure 2 represents the calculated effective diffusivities
p= 0
o 100 200 300 400 500 600 700 800 900 1000
Time, t
FIG. 2. Variations of the effective diffusivity De with the process time for various molecular sizes n. There is no adsorption (p = 0) or intermolecular
interaction.
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vs time with h = to, b = 3, p = 0, and N = 3000 for various
values of n (the number of monomers in the molecules). In
this simulation, the hard-core repulsion between the mole
cules has been ignored. As can be seen after about 800 time
steps, the effective diffusivities are essentially independent of
time, although we cannot rule out the possibility of having a
value of 8 very close to 1 (i.e., a very slow variation of De
with t). Figure 3 represents the results of the same simula
tion, except that p = 0.01, i.e., a few molecules are adsorbed
on the surface of the pillars. Aside from the expected differ
ence between the numerical values of De' Figs. 2 and 3 are
essentially similar. Note, however, that the effective diffusi
vities shown in Fig. 3 appear to have a weak dependence on t,
even after 1000 time steps. This is presumably caused by the
adsorption of the molecules on the surface of the pillars. As
mentioned above, the adsorbed molecules create excluded
surface and volume in the pore space, so that the diffusion
paths of the unadsorbed molecules become increasingly tor
tuous. This makes the diffusion process, and its approach to
the asymptotic regime, slower which is reflected in the slow
variations of De with the time t.
Figure 4 represents the results with the same simulation
parameters as those of Fig. 3, except that there is a hard-core
repUlsion between the molecules. Now, for n > 1, one can see
a clear dependence of De on t even for relatively long times.
This is of course caused by the fact that the hard-core repul
sion makes the motion of the molecules in the pore space
somewhat more difficult. As a result, the molecules need a
longer time to probe the structure of the system. However, if
we repeat the simulation for a much larger number of steps
for molecules with n> 1, we observe that De still changes
slowly with time. As an example, we show in Fig. 5 the vari-
0.14
CII
o O.lD
,., ->
.~ 0.08 --o
~ 0.06
(J
CII --w 0.04
0.02 ation of De with the time 1 for molecules with n = 7. All
other parameters of the simulation are the same as in Fig. 4.
As can be seen, even after 1 = to4, De' still varies slowly with
time. In fact, one may speculate that De -In I, so that
R 2(t} -I In I. Ifwe attemptto present the results of Fig. 5 by
this logarithmic law, we find that such an equation would
provide a very good representation of the data. If we fit De to
an equation such as Eq. (3), we find 8~0.95, which indi
cates a weak dependence of De on I. Since the height h of the
system is much smaller than its length L and because the
length of the molecules is comparable to h, pillared clays
behave essentially as two-dimensional systems. Thus, the
hard-core repulsion (which is surely at work in a real sys
tem) and the restricted pore space of the catalyst can cause
deviations from the classical diffusion equation, and make
the system to behave like a fractal, even if the system is not
geometrically a fractal object. Similar behavior has been ob
served in the diffusion of molecules in a one-dimensional
system in which there is a hard-core repulsion between the
molecules.2•52 For such a system, one can show that
R 2(t) _1112. This dependence of De on time has been ob
served in many diffusion experiments43 in pillared clays and
our simulation appears to provide an explanation for this
phenomenon.
We note from Fig. 4 that the long time effective diffusi
vities of the molecules with n>5 are close to one another.
This is explained by the fact that the size of the molecules
with n>5 is comparable to the spacing between the pillars.
Therefore, all such molecules "see" effectively the same en
vironment and, as a result, their long time diffusivities are
essentially the same, although their effective diffusivities for
short and intermediate times are not necessarily equal be-
p=0.01
0.0 100 200 300 400 500 600 700 800 900 1000
Time, t
FIG. 3. The same as in Fig. 2, but for p = 0.01.
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0.14 '
cO> 0.10
.~
.~ 0.08
:> --C
0> 0.06
> -o
0> -W 0.04
0.02
o 10 100 2,00 300 400 500 600
Time, p = 0.01
700 800 900 1000
FIG. 4. The same as in Fig. 3, except that hard-core repulsion between the molecules has been taken into account.
cause the molecules have not yet probed the pore space in
sufficient details.
Figure 6 shows the results with the adsorption probabil
ity p = 0.4. In this simulation, the hard-core repulsion has
been neglected, but all other parameters of the system are as
before. Since a larger fraction of the molecules can be ad
sorbed on the pillars and because the adsorbed molecules
occupy a finite volume fraction of the pore space, the motion
of the molecules becomes increasingly more difficult and, as
a result, the asymptotic regime will be reached after a much
longer time, if at all. Figure 7 represents the results with the
same simulation parameters as those of Fig. 6, except that
the hard-core repulsion between the molecules has not been
ignored. The results are qualitatively similar to those of Fig.
6, except that the time dependence of De at longer times
seems to be weaker. The numerical values of De in Fig. 7 are
0.04
0"0.03
,..
'> .;;;
::J P =0.01
n = 7
~ o.02:--------
U ------------ __ J .2! w o.Ot
0L-~2~0~00~~-4~0~0~0~L-~6~OOO~~~8~0~0~0~L-~I00~0~0~
Time, I
FIG. 5. Long time variations of De with the process time t. somewhat smaller than those in Fig. 6, which is expected
since the hard-core repulsion reduces the available pore
space for the diffusion of the molecules and introduces an
effective tortuosity into the system.
Next, we investigated whether the number of molecules
N used in our simulations can affect De. We fixed all param
eters of the simulation as before, except the number of mole
cules N and the adsorption probability p. In the first series of
simulations, we ignored the hard-core repulsion between the
molecules. In Fig. 8 we present the dependence of De on N
for p = O. As can be seen, for n.;;;5 there is practically no
dependence of De on N, as expected, but for n>5, one ob
serves a weak dependence of De on N. However, for finite
values of the probability of adsorption p, one does observe a
relatively weak dependence of De on N for 1000 .;;;N.;;;4000,
but the dependence appears to be much weaker for N>4000.
The results are shown in Figs. 9 and 10, which show the
dependence of De on N for various values of the probability
of adsorption p. This dependence of De on N arises because
of the fact that the adsorbed molecules cause an excluded
region of pore space into which the molecules cannot diffuse.
Higher values ofp can particularly affect De since in such a
case a large number of molecules are adsorbed on the surface
of the pillars, reducing the available pore space for diffusion
and, thereby, affecting De. However, after some time, the
surface of the pillars is covered by the molecules and, as more
molecules are injected into the system, few of them can be
adsorbed on the pillars and therefore injection of more mole
cules has little effect on De.
However, the variation of De with N is somewhat differ
ent when the hard-core repulsion between the molecules is
not ignored. Figure 11 represents our results for p = O. All
other parameters of the system are the same as in the pre
vious case (Figs. 8-11). Initially, De appears to be strongly
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0.14
0.12
Q)
o 0.10
>
·iii 0.08
:J --o
CII 0.06
. !! -u
CII
50.04
0.02 FIG. 6. The same as in Fig. 3, but for
p=O.4 .
o 100 200 300 400 500 600 700 800 900 1000
Time, t
dependent on N, which is surely because of the excluded
volume effect that arises as a result of the hard-core repul
sion. However, as more molecules are injected into the sys
tem, De decreases and, for N> 3500, appears to only have a
weak dependence on N. Note also that De decreases as N
increases (because the available pore space for anyone mole
cule decreases), whereas when hard-core repulsion is not at
work, De increases before reaching its asymptotic value. The
reason is that in the simulation in which the hard-core repul
sion is at work, the first few molecules are free to diffuse
everywhere, but as more molecules are injected into the sys-
0.14
0.12
->. -.;;
·in 0.08
" --o
. ~ 0.06
"0
Q) -W 0.04 tern, the available pore space decreases and therefore De de
creases. Similar results are obtained whenp = 0.1, 0.4, and
0.8, the results of which are shown in Figs. 12-14. Figures
10, 13, and 14 indicate that as p increases, De attains a maxi
mum before decreasing with increasing N. This is caused by
the fact that for larger values of p, a relatively large number
of molecules are adsorbed on the pillars that are close to the
external surface of the catalyst. As a result, the incoming
molecules have to take straight paths (with no tortuosity) to
the interior of the pore space, since the adsorbed molecules
and the hard-core repulsion (in the case of Figs. 11-14) pre-
P =0.4
n= FIG. 7. The same as in Fig. 6, but with
the hard-core repulsion included .
0.02 ~::::::::::::::::! ~
-.;::::::::::::::::::::~~;; 5
7
010~--'0~0~--2~0~0--~3~0~0---4~0~0--~5~0~0---6~0~0--~7~0~0--~8~0~0--~9~00~-'~OOO
Time,t
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~ .;;
'iii
" -0.09
0.08 p = 0 _-_o__-_--_-- ....... --... n = 1
~.--....... --_4_--+_---_---4. 3
_-_o__-~.~--4.--~--... 5
i5 0.07
-----~----~----~----o__ __ __ ... 7
0.06 .... ----....... ----~----~----o__ __ __ .... 9
0.05 L---"'5:-4oo=--.l.--1;;::5-!::0:;::0---1...--;;2~50?\.0n----L--:3z;;j5oo
Number of Molecules, N
FIG. 8. Variations of De with the number of molecules N. There is no ad
sorption or intermolecular interaction.
vent the incoming molecules from moving freely in the pore
space. Thus, De initially increases with N. However, as more
molecules are injected into the system, some of them will
penetrate deep into the pore space and are adsorbed on the
pillars that are closer to the center of the catalyst. As a result,
the diffusion paths become increasingly more tortuous and,
therefore, De decreases for large values of N. This again dem-
-~
:~
<to
" -0.08 p = 0.1
•• --...... ---------.------.. n = 1
~0.04 .. >
~ .!
W 3
0.02
5
7
o L---~50~0~--L---l~5~0~0---L--~2~5~0~0--~~3~5oo
Number of Molecules, N
FIG. 9. The same as in Fig. 8, but for p = 0.1. .. o
o .. > 0.07
~ 0.03 .. -~ n=3
0.01 5
7
1000 p :. 0.4
3000 5000
Number of Molecules, N
FIG. 10. The same as in Fig. 8, but for p = 0.4. 7000
onstrates the strong effect of adsorption and intermolecular
interaction on De.
The above explanation of the variations of De with N
was based on the intuitive picture of diffusion of large mole
cules in the pore space of the catalyst. However, there is an
alternative way of explaining the variations of De with N that
is more rigorous and is based on the concepts of percolation
theory. 53 To begin with, we note that an issue of great inter-
O.IOr-----.::::===---------,
0.08
> .;;; 0.06
" --o .. > -u = 0.04
UJ
0.02 p = 0 n = 1
7
3500
FIG. 11. The same as in Fig. 8, but with the hard-core repulsion included.
J. Chem. Phys., Vol. 92, No.8, 15 April 1990
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0.05 p = 0.1
·.----+---~·----~.r---~--__4.n=1
Q/0.04 o
>
III
:::J
;: 0.03 o
Q/
> -(.) Q/ --w
0.02 3
5
7
0.01 L-5...J0L:0:----1---:-::15~0:-::;0:----L--;:2;;;:-5~00;:;---''--uj3500
Number of Molecules, N
FIG. 12. The same as in Fig. 11, but for p = 0.1.
est, closely related to the phenomenon of pore blocking by
the adsorption of the molecules on the surface of the pillars,
is the existence of a critical volume fraction Vc above which
no appreciable amount of adsorption takes place, and no
macroscopic diffusion within the pore space of the clay
is possible. For example, for the straight chain alkanes
(C5-e1O), Vc is well-defined and its value has been deter
mined experimentally.33-3s For these alkanes, the values of
V are all less than 66%. Of course, Vc is a sort of percolation c
threshold,53 i.e., the maximum volume fraction of adsorbed
molecules that is allowed for macroscopic transport to take
p =.4
0.03
>
.;;; 0.02
:::J
0=1
0
Q/
> 3 -(.) Q/ 0.01 --w
o 1000 3000 5000 7000
Number of Molecules, N
FIG. 13. The same as in Fig. II, but for p = 0.4. 0.015
oQ/
->--
.~ 0.010
:::J -~ o
Q/
.~
(.) Q/
~0.005 w p = 0.8
•
1000 3000 5000
Number of Molecules, N
FIG. 14. The same as in Fig. II, but for p = 0.8. 0=3
5
7000
place, if blocking of the pore space due to adsorption occurs
randomly. Thus, if liJ is the volume fraction of pore space not
occupied by the adsorbed molecules, for liJ > 1-Vc' macro
scopic transport can take place, whereas for liJ,1 -Vc'
there would be no sample-spanning of open pores and there
fore the molecules cannot have a sample-spanning diffusion
path in the pore space of the catalyst. We expect Vc to de
pend on the molecular sizes. If the molecules are not very
large, they cannot effectively block the pore space, even if the
adsorption probability is very large. However, as the size of
the molecules increases and becomes comparable to the
spacing between the pillars, and as more molecules are ad
sorbed, blocking of the pore space becomes more effective
and the available open pore space decreases. This is why De
appears to depend on N, the number of injected molecules
since, similar to the percolation theory in which all quanti
ties of interest are functions of the volume fraction of
open pore space, De is also a function of the variable
x = 1 -pNn/(L 2hl/J) , which is the volume fraction of open
pore space after N molecules of size n have been injected into
the pore space and have been adsorbed on the pillars with the
probability p. Hard-core repulsion can, of course, make this
effect stronger. This explains the behavior of De shown in
Figs. 9-14. If instead of a needlelike shape the molecules
were two-or three-dimensional objects, the dependence of
D on N (for Pi=O) would be much stronger and, in fact, De
c;uld vanish for large values of N, since no macroscopic
transport would be possible. For this reason, simulations
with two-or three-dimensional molecules are essential for
obtaining a full understanding of transport and reaction in
pillared clays. Work along these lines is currently in prog
ress.
Next, the effect of the distribution of the pillars was
investigated. In Fig. 15, we present the results of our simula
tions in which the width b of the pillars has been varied, but
the total number of pillars has been fixed as before (the
height of the pillars is, of course, fixed by the distance
J. Chem. Phys., Vol. 92, No.8, 15 April 1990
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0.10
.,
00.09
.,
::J --o
. ~0.08
u
.! -w
0.07 n=1
3
5
7
0.06L---~2~--~--~4~--~--~6~--~--~8
Pillar width, b
FIG. IS. Variations of D, with the pillar width b. There is no adsorption or
intermolecular interaction.
between the silicate layers). In these simulations,p = 0 and
the hard-core repulsion between the molecules has been ig
nored. All other parameters of the simulations are as before.
As can be seen, De is essentially a linear function ofthe width
of the pillars, since increasing b decreases the available pore
space for diffusion. Moreover, as b increases, the effective
diffusivities for molecules of various sizes appears to ap
proach one another for molecular sizes n;;;.5. This is because
for n;;;.5, the distance between the pillars becomes compara
ble to the size of the molecules and therefore all molecules
essentially "see" and probe the same regions of the pore
space, and their mobilities are comparable to one another.
Figure 16 shows the results of the same simulation as in Fig.
15, except that the effect of hard-core repulsion has been
taken into account. While the numerical values of De in this
case are smaller than those in Fig. 15 (which is expected),
the qualitative trends of the results are very similar. Thus,
size and distribution of the pillars can strongly affect the
values of the diffusivities.
The anomalous variation of De with some of the system
parameters that has been discussed so far is surely caused by
the structure of the pore space which is very restricted and
nearly two dimensional. In all of the simulations discussed
so far, the height of the system has been fixed at h = 10. This
particular value of h was used because for most pillared clays
of the type studied here the ratio L I h ranges33-35,43 between
20 and 25. Thus, the size ofa molecule with n;;;.5 is compara
ble to the height of the system and this, together with the fact
that the pillars, the adsorption of the molecules on the pillars
(and the resulting excluded volume effect), and the hard-0.10
0"0.08
> .,
::J
00.06 .. >
u ., --w
0.04 p:O
n=1
3 5 7
0.02'------:2!:------'-----4~----'------:6!:------'---~8
Pillar width. b
FIG. 16. The same as in Fig. IS, but with intermolecular interaction includ
ed.
core repulsion beween the molecules severely restrict the
motion of the molecules, gives rise to some of the anomalous
behavior of De. If we increase h (creating a larger pore
space), some of this anomalous behavior may disappear al
together. To make this point clearer, we present in Fig. 17
the dependence of De on the height of the system for molecu
lar size n = 5. In these simulations, there is no adsorption,
hard-core repulsion between the molecules has been taken
into account, and all other parameters of the model are as
before. It is seen that only if h In;;;. 10, does the diffusivity
appear to be independent of h, but for smaller values of h In,
the diffusivity is a strong function of h. Thus, it is the restrict
ed structure of the pore space of pillared clays that is partly
responsible for the observed anomalous behavior of De' both
in experimental studies33-35,43 and in simulations reported
here, Therefore, even if these catalytic materials are not frac
tal objects from a geometrical point of view, they may behave
as such because of their very restricted pore space and the
nature of the diffusion-adsorption process that takes place
in the pore space,
Finally, we looked at the efficiency of pillared clays for a
reaction process. This can be studied by looking at the dis
tance that a molecule has to travel in the pore space in order
to reach a reactive site. Thus, the simplest measure of the
reaction efficiency of pillared clays is the number of steps Sw
that a molecule takes in order to reach a reactive site.24,54
Some of the molecules may not even reach the reactive site
and be adsorbed on the surface of the pillars. The efficiency is
thus a function of p, the adsorption probability. It is also a
function of molecular sizes, since the restricted structure of
the pore space forces a large molecule to take a larger num
ber of steps to reach the reactive site. It may also depend on
J. Chem. Phys., Vol. 92, No.8, 15 April 1990
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129.22.67.107 On: Mon, 24 Nov 2014 13:42:31Muhammad Sahimi: Pillared clays. I 5117
0.15
>
UI 0.10
::l --o
IV > -U
IV -W 0.05 n=5
p=o
90
Distance Between Silicate Layers, h
FIG. 17. Variations of D, with the distance between the silicate layers h.
the size of the system since, if Land h are large, the molecules
have to take a large number of steps to reach a given reactive
site. If each molecule occupied only one site of the cubic
lattice used in this work and if the number of sites n s of the
lattice were very large, then Montro1l54 has shown that
Sw -1.5164n s• (5)
However, in the present study h 4,L, so that our system can
not really be considered a three-dimensional lattice. Its
properties are mostly between those of two-and three-di
mensional systems. For a two-dimensional lattice, Mon
tro1154 has shown that
(6)
where the prefactor Yl is of order unity and its numerical
value depends on the structure (coordination number) of
the lattice. Moreover, in our simulations, each molecule oc
cupies more than one site of the lattice and the size of a
molecule is comparable to that of the spacing between the
pillars, as a result of which the motion of the molecules is
slow and inefficient. We may expect that for our system Sw
would follow a scaling law in which the prefactor is depen
dent on the molecular as well as pillar sizes. If hard-core
repulsion between the molecules is also at work, the prefac
tor may also depend on this effect as well.
We studied this issue by calculating Sw for molecules of
various sizes. The reactive site was assumed-to be the center
of the lattice. We used h = 10 and p = 0, and ignored hard
core repulsion between the molecules. We found that an
equation similar to Eq. (6) would fit our data relatively well.
As expected, the prefactor in Eq. (6) did depend on the size
of the molecules. This is shown in Fig. 18, where we plot the
variations ofy] with the molecular size n. Note that even for n = 1, which is the case studied by Montroll,54 our value of
Yl is neither close to YIg;;1.5164 for the simple-cubic
network, nor is it close to YI = 1T-1 which is the correspond
ing value for the square network. Instead we find
Yl (n = 1) g;;0.8. However, we find that a much better fit to
the simulation results is provided by the following equation:
f.7r-----------------,
f.5
1.3
y.
t 1.1
0.9
0.7
0.5 '------'--...,2±----'--4~--L-+----.L--±----.L----'.fO
Moleculor Size, n (7)
FIG. 18. Dependents of the prefactor y. [Eq. (6) 1 on the molecular size n.
J. Chem. Phys., Vol. 92, No. 8,15 April 1990
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where 11 ~ 1.2, regardless of the size of the molecules. Note
that this value of 11 is larger than 11 = 1 for three-dimensional
systems. A fractional value of 11 is an indication that the
system behaves as a fractal object. Note also that 11 = 2 for a
linear chain. 54
IV. SUMMARY AND CONCLUSIONS
We have investigated diffusion, reaction, and adsorp
tion of needlelike molecules in pillared clays which are a
class of catalytic materials with restricted pore space. We
have found that the finite size of the molecules, adsorption,
intermolecular interaction, and the restricted morphology of
the pore space all affect strongly diffusion and reaction phe
nomena in pillared clays. Even if pillared clays have perfect
ly homogeneous surface and pore space, inefficient and hin
dered transport and reaction processes in their pore space
can make pillared clays behave effectively as fractal objects.
This paper represents only the first computational step
towards a comprehensive understanding of transport and
reaction processes in pillared clays. Many important issues
remain for future studies. For a more realistic representation
of the molecules, one has to use three-dimensional objects.
This is particularly important for checking the applicability
of percolation theory to the interpretation of experimental
data of diffusion and irreversible adsorption of large mole
cules in pillared clays. If the molecules are indeed three
dimensional entities, their irreversible adsorption on the sur
face of the pillars can give rise to a percolation phenomenon,
in which the pore space becomes increasingly more inacces
sible to the outside molecules and, in effect, becomes deacti
vated, a very common and severe problem in catalysis. If the
reactive sites are in the form of connected clusters of finite
sizes, then an equation similar to Eq. (6) or Eq. (7) may not
hold at all. Multipolar correlations between the reactants
can also influence transport and reaction in pillared clays.
The location of the pillars and their sizes can, in general, be
distributed quantities which can also affect transport and
reaction in the pore space. Molecular dynamics simulations
are necessary to assess the effect of interaction between the
solid surface of the catalyst and the molecules, and the inter
molecular interaction. These are but a few of the issues that
still remain to be pursued in the areas of diffusion, reaction,
and adsorption oflarge molecules in pillared clays. Work in
these directions is currently in progress and the results will
be reported in the future.
ACKNOWLEDGMENTS
I would like to thank Mario L. Occelli for introducing
me to this problem, and Theodore T. Tsotsis for useful con
versations at early stages of the work reported here. I am also
grateful to Valerie L. Jue for her computational help at the
beginning of this work. Partial support of this work by the
University of Southern California Faculty Research and In
novation Fund, the Petroleum Research Fund administered
by the American Chemical Society, United States Depart
ment of Energy, and the San Diego Supercomputer Center is
gratefully acknowledged. I would like to thank Karen Woo for her expert typing of this paper and her endless patience. I
dedicate this work to my dear friend Faezeh Golboo, with
out whose friendship, encouragement, and support this pa
per would have never been completed.
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47M. Sahimi, Macromolecules (to be published).
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49R. A. Guyer, Phys. Rev. A 32,2324 (1985).
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J. Chem. Phys., Vol. 92, No.8, 15 April 1990
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1.1140504.pdf | A pressure modulator radiometer for measuring stratospheric trace gases
J. R. Drummond, D. Turner, and A. Ashton
Citation: Review of Scientific Instruments 60, 3522 (1989); doi: 10.1063/1.1140504
View online: http://dx.doi.org/10.1063/1.1140504
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/60/11?ver=pdfcov
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06A pressure modulator radiometer for measuring stratospheric trace gases
J. R. Drummond, D. Turner, and A. Ashton
Department a/Physics, Universityo/Toronto, Toronto, Ontario, Canada M5S lA 7
(Received 30 December 1989; accepted for publication 23 June 1989)
This article describes a pressure-modulator instrument which is designed to measure trace
constituents of the stratosphere from a balloon platform at an altitude of about 40 km. Double
sided limb-scanning allows profiling below the instrument and a direct determination ofthe
instrument attitude from the radiance data. The instrument is described in detail and the methods
of radiance measurement and calibration are discussed. Some of the supporting laboratory
measurements are described. A concentration profile of carbon monoxide from 20 to 45 km is
presented as an example of the results of the first series of flights.
INTRODUCTION
Chemically active trace constituents of the upper atmo
sphere are important because the chemical balance of the
region affects the radiation balance, and hence the dynamics.
Therefore these compounds, although present in very small
amounts, can have a significant overall effect upon the state
of the stratosphere. However measurements of these com
pounds are hampered by their low concentrations and the
small signals which they produce.
Pressure modulator radiometry is a technique by which
small signals from the thermal emissions of constituents can
be detected and analyzed to determine a con,centration pro
file. Previous applications of this technique have been to
temperature sounding and to constituent measurements. 1
The temperature sounding instruments, using CO2 as the
pressure modulating gas, have been flown on balloons and
satellites, notably the pressure modulator radiometer
(PMR) instrument on the Nimbus 6 satellite and as part of
the stratospheric and mesospheric sounder (SArvIS) instru
ment on Nimbus 7. In the area of composition measure
ments, previous balloon-borne instruments have measured
nitrogen oxides2,3 and a variety of measurements have been
made using the SAMS instrument mentioned above.4 The
principal advantages of the technique are the large energy
grasp, which serves to increase the available signal, and the
high effective spectral resolution, which enables the instru
ment to detect signals from specific gases while rejecting
those from other species. This is achieved without sensitive
mechanical or optical systems, allowing the instrument to be
readily used in banoon-borne and satellite systems.
This article discusses a new balloon-borne instrument
which is designed as a general-purpose instrument for stra
tospheric composition measurements. For the first experi
ments the instrument, which has three independent chan
nels, was equipped to measure carbon monoxide at 4.7 pm,
methane at 7.6 ,um, and formaldehyde at 5.7 ,urn.
I. PRESSURE MODULATION TECHNIQUES
A. Overall description
The schematic of a pressure modulator radiometer is
shown in Fig. 1. It consists of a conventional filter radio-meter with a fast chopper at the input to modulate the radi
ation and an additional cell, the pressure modulator cell or
PMC, filled with the gas being studied. The density of the gas
is cycled mechanically at a rate which is much less than the
modulation frequency of the chopper. The fast chopper al
ternately selects the atmospheric signal or an internal refer
ence signal. A discussion of the spectral, mechanical, and
signal characteristics of a modulator is given below with par
ticular emphasis on features which are used in the instru
mentation to be described.
B. Spectral characteristics
The essential feature of the pressure modulation tech
nique is the use of the emission lines of the gas itself as a
precise optical filter for incoming signals. Variations in the
density of the gas in the modulator cell cause variations in
the absorption of incoming radiation only in spectral regions
near spectral lines of that gas (Fig. 2). Electronic systems
can extract this signal which, being a function of radiation
near spectral lines of the gas in the cell, is likely to be indica
tive of emission from the same gas. There is a precise correla
tion between the radiation coming in from the gas in the
Incoming
Rodiolion COld block body
CVWIMMI\) field slop filter
\, I '\ (J11
5OOHzc~r ;
t
pressure
modulolor delector
FIG. 1. Schematic of a pressure modulator radiometer. Radiation enters the
system from the left-hand side and is interrupted by a fast rotating choppeL
The rear face of the chopper is reflecting and therefore the ongoing radi
ation in the system is alternately the input radiation and the radiation from
the reference blackbody. The field of view is delincated by the field stop. The
radiation thcn passes through the pressure modulator cell containing a sam
ple of the gas being measured. The gas density is varied cyclically at about 11
Hzo The radiation is spectrally limited by a conventional multilayer filter
and then falls on a detector. The signal produced is processed by the elec
tronics to produce "wideband" and "sideband" signals.
3522 Rev. Sci.lnstrum. 60 (11), November 1989 0034-6748/89/113522-11$01.30 @ 1989 American Institute of Physics 3522
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06.~
"> .">
~ §
~
'" .~
.~
~
~
'" " .6
~
I:i
"I:l
.11; .;::
<:i
~
~ i.OO
0.75 (0 )
0.50
0.25 TRANSMISSIONS
0 '---___ ..-L--___ ~ • .L! ____ ._~ ____ -'
1.00
!
0.75
0.50
0.25
0.50
0.25
0
-0.8 EFFt.crIVE WIDEB,ANO
TRANSMISSION FUNCTION
EFFECTIVE SIDEBANO _
TRANSMISSION fUNCTION
-0.4 o 0.4 0.8
FiG. 2. The operation of the pressure modulator cell in spectral space, Fig
ure (a) shows the transmission function of the modulator gas ill spectral
space at its two pressure extremes. The effect of the PMC in a PMR, after
electronic processing, is the replacement of a single gas cell/detector chan
nel by two independent channels, each containing an effective transmission
filter (b) instead of a gas cell. One of these filters has an effective transmis
sion function equal to the average transmission function of the two trans
mission extremes, the other to the transmission differences. The resulting
detected radiances (c), are known as the "wideband" and the "sideband"
response, respcctivdy. The PMR output is the total dctected radiant energy
integrated over wave-number space from each channel.
atmosphere and the absorption features of the same gas in
the cell. Since this perfect spectral alignment is achieved
without any dispersing components and their attendant ad
justments, the instrument is inherently robust.
The selection of the spectral region to be used is deter
mined by the constraints of maximum signal and minimum
contamination of that signal by emission from other gases
resulting from chance coincidences between the spectral
lines of the required and contaminant gas. Since any specific
gas being studied has only a few infrared vibration-rotation
bands, the choice is often not perfect and it will be necessary
to account for some contaminant signal.
Chance co-incidences between spectral lines of the de
sired gas and other gases, particularly of plentiful constitu
ents such as water, do occur as shown in Fig. 2( c). These
must be allowed for by exact calculation requiring knowl
edge of the concentration profile of the contaminant. Since it
is desirable to obtain this profile as accurately as possible, the
"wideband" signal is used as an additional source of infor
mation. This signal is the total signal received, attenuated by
a cell transmission function appropriate to an average cell
pressure. It may be obtained simultaneously with the pres
sure modulator signal by methods discussed below. This
overall signal is usually influenced more by the contaminant
gas (es) than the gas under observation. Thus, for a CO mod
ulator the wideband signal is almost entirely due to ozone
whereas the pressure modulator signal is 70% due to 0],
3523 Rev. ScLlnstrum., Vol. 60, No.11, November 1989 30% due to CO. In this case an ozone profile can be deter
mined first using the wideband signal and then that profile
used to calculate the effect on the pressure modulator signal.
C. Signal calibration
The reference signal (Fig. 1), which is viewed alternate
ly with the atmosphere, must be very stable. If it is not stable
then it is very difficult to separate out the effects of a variable
reference from those of the atmosphere. In order to achieve
such stability, a separate blackbody source and a reflecting
chopper is used, rather than the emission of the chopper
blade alone. The two choices for the reference blackbody
temperature are room temperature, which is easy to manu
facture, or a low-temperature source, which produces al
most zero radiation. The choice of a cold blackbody is appro
priate for an atmospheric emission sensor as this signal is
comparable with the small atmospheric signal. The use of a
room temperature source would mean that the two signals
were always significantly different and comparatively small
relative variations in the large reference signal would be
come important.
D. Mechanical characteristics
Since the oscillation in gas pressure is essential to the
operation of the instrument, care must be taken to maximize
this without compromising other parameters. Typically
compression ratios of 3: 1 are used in the cells at oscillation
frequencies in the range 10-20 Hz. The frequency used is a
compromise between a low frequency which simplifies the
mechanics, and a high frequency which is easier to deal with
in the electronics and leads to fewer problems with gas leak
age. Previous instruments have used various designs for the
cell, most of which have required extreme care in fabrication
and assembly.; Our new design, described in Sec. HI B, is
simple to assemble but requires more drive power.
It is evident that a modulator running at a sufficiently
low frequency will have isothermal compression cycles,
whereas one running at a high frequency will have adiabatic
compression cycles. In general, modulators run somewhere
between these two extremes and although we have some evi
dence for temperature cycling in our modulators, it appears
from measurements that the cycle is predominantly isother
ma1.s
E, Signal characteristics
Incoming radiation is modulated first by the chopper
and then by the modulator. The pressure modulator signal
therefore appears both as a "baseband" signal at the modula
tor frequency and as sidebands around the chopper frequen
cy ( ~ 500 Hz). The sideband signal is the one processed by
the electronics for two reasons: First, the increase in frequen
cy from the basic modulator rate to almost the chopper fre
quency, improves the signal-to-noise ratio obtainable with
many detectors as it raises the frequency above that of signif
icant l/fnoise. Second, thermal emissions from the modula
tor, modulator gas and the associated optics, which are ei
ther approximately constant or vary at the modulator
frequency, are easily filtered out in the electronics.
Pressure modulator 3523
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06If the volume of an isothermal pressure modulator is
varied sinusoidally at a frequency J, then the density vari
ation is given by
11[1 +A sin (21Tft)],
where the constant A is related to the compression ratio, U,
by A = (U -1) / ( U + 1). Since the density modulation is
not sinusoidal, the signal modulation will not be sinusoidal
and there will be sidebands at harmonics of the PMC fre
quency about the chopper frequency. In the case of a rotat
ing high-frequency chopper, there will also be sidebands due
to the imperfections of the blade which occur at harmonics
of the chopper rotation rate. All these effects can clearly be
seen in Fig. 3 which illustrates the complexity ofthe modula
ted signal after detection. The electronics necessary to ex
tract the sideband signal are described in Sec. III C.
II. LIMB-SCANNING
The technique of "limb-scanning" is used to maximize
the amount of material within the field-of-view or the bal
loon-borne instrument. The atmosphere is viewed almost
horizontally as shown in Fig. 4. The amount of material in a
limb path is up to 70X that in a corresponding vertical path
with the same lowest height, known as the "tangent height."
In the case of an absorber with a constant mixing ratio at all
altitudes, the signal from a limb path is strongly weighted by
the atmospheric pressure profile to the tangent height pro
ducing a narrow weighting function 6 and therefore good ver
tical resolution. Different regions of the atmosphere are sam
pled by varying the view angle in a step-wise manner during
an instrument "scan." An additional advantage is that the
background to any limb path is space which is effectively a
zero signal source for an infrared instrument.
{l 1.0 C2
~ :::::
~
~ 0.1
t;
.~
V5 0.01 CI
P2
P3
450 c
CI
PI PI
~
500 550 600
Frequency (Hz)
FIG. 3. A spectral analysis of the electronic signal produced from the detec
tor in Fig. 1 for the particular case of a CO pressure modulator. The region
of the spectrum around the chopper frequency is shown. The central peak
(C) is the chopper fundamental. The average value of this signal is the
"wideband" component. The pressure modulator sidebands (marked PI,
P2, and P3) can be seen and the average magnitude of these represents the
"sideband" signal. The peaks marked Cl and C2, are the subharnlOnics of
the chopper due to the imperfections of the 12-bladed system. These occur
at ahout 41-Hz intervals.
3524 Rev. SCi.instrum., Vol. 60, No. 11, November 1989 The major disadvantages of limb-scanning are the poor
horizontal resolution, the complexity of the path and the
necessity of knowing the view angle very precisely.
This last problem of determining the tangent height of
the view is critical to correct retrieval of the concentration
profile. In order to determine thc tangent height to an accu
racy of ± 0.5 km at 15 km from a balloon at 40 km, the angle
must be known to within ± 0.05°. This angle is measured
relative to the horizontal, which is taken as the tangent to the
average isobaric surfaces in the stratosphere.
We measure the angle of the optical beam relative to the
horizontal by using the information contained in double
sided scanning data which is shown in Fig. 4. The direction
of view of the instrument is steered by a single mirror (see
below) which is capable of rotation through nearly a full
circle.
Since a scan position obtained using the shaft encoder is
measured relative to the package, the package must be stable
during a complete scan sequence (~1O min) in order to be
able to compare the two sets of data. Within this constraint,
any differences between left and right scans are attributed to
instrumental effects, which may be the tilt of the package or
asymmetry of the beam under rotation. The latter occurs if
the optics are not truly axial and can be successfully elimin
ated by careful alignment.
We define a as the dip angle of the beam between the
nominal horizontal and the true horizontal, and the beam
asymmetry angle, /3, as the excess angle by which the beam
rotates when the mirror is rotated by exactly 180°.
If we select a case when the left and right signals are
identical, then if the atmosphere is horizontally homoge
neous, the true depression angles for the two cases are identi
cal. If the true angle is () and the measured angles for these
cases are Land R, we find (Fig. 5) that
L1MBSCAN
EARTH
(not 10 scolol LlMBSCAN
FIG. 4. Schematic of the TORBAR implementation oflimb scanning. The
atmosphere is viewed from two sides of the instrument by scanning at angles
between -2' and 6' from the local horizontal. A radiation "zero" is estab
lished by views at -25' and a known radiance by rotating to -90' to view
an internal calibration target of known emissivity and temperature.
Pressure modulator 3524
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06H-_A_~_a:--I.L~~""""''''''-''''''-a:-+fl--H
/ ~B
S S
FIG. 5. Scanning geometry. H-H is the tru~ horizontal and the lines A and B
are the beam orientations when the shaft encoder indicates horizontal. The
error comprises two parts: The dip angle a which is a measure of the pack
age tilt, and the beam asymmetry angle fJ which is the angular error in the
beam between the two "horizontal" positions. The apparent angles Land R
are the results of measurements at the true angle () at which the 1cft and right
signals are equal. These measurements are afft'cted by the dip angle and
beam asymmetry angle, resulting in the equations for the apparent angles
L.~ () + a and R C~ () -a -fJ.
and
R={}-a-/3
or
L -R = 2a + {3 = y,
where r is defined as the tilt angle.
By comparing left and right scans using a least squares
technique for each scan we can determine a and (3, and then
using estimates from scan pairs throughout the flight, we can
then estimate y as a function of time.
The ideal signal source for this measurement is one
which is large, shows a strong dependence on scan angle and
is uniform in space and time. The wideband signals fulfill
these requirements, being predominantly due to the more
plentiful minor constituents such as water and carbon diox
ide. We use the wideband signal from the formaldehyde
channel centered at 1746 cm-1 for our attitude sensing,
whose signal is predominantly due to water.
Using scan pairs throughout the flight, we can estimate
y as a function of time. Data from the area of the tropopause
are eliminated and the various estimates in the scan weighted
appropriately. The function
is minimized, where S is the signal, k the nominal scan augle,
sUbscripts Land R refer to left and right scans, respectively.
The sum is done over a restricted range of scan positions,
interpolating data between real data points using a linear
formula. The sum of squared residuals is shown in Fig. 6 as a
function of the angle y and a definite minimum is found. A
more complete discussion of this technique has been given in
a previous paper. 7
A limb~scanning instrument is generally only able to
derive concentration profiles from flight altitude down. For
view angles above the balloon the material in the path is
weighted towards the balloon level for an scan angles and
therefore vertical resolution is poor. For view angles below
the balloon the path between the tangent height and the bal
loon becomes less transparent at wavelengths where the in
strument is sensitive and eventually becomes opaque as the
3525 Rev. Sci.lnstrum., Vol. 60, No. 11, November 1989 FIG. 6. Sum of squared residuals for some flight data (from the flight of
August 1(83) as a function of y. The vertical scale is in units of [nW m 2
Sf 1 (em 1) J 2. A detinite minimum can be seen at 0.68". Data from vieWS
near the tropopause are excluded from the summation as these are not ex
pected to be similar for the two scans.
tangent height moves down through the atmosphere. When
the path becomes opaque before the tangent height is
reached, the instrument does not detect emission from the
tangent level and the concentration profile is again indeter
minate. The level at which the instrument loses sensitivity
can be adjusted to a certain extent by adjusting the pressure
in the pressure modulator cell, but as sensitivity to lower
levels is increased, sensitivity to upper levels is decreased.
iii. THE TORONTO BALLOON RADIOMETER
(TORBAR)
Ao Optical design
The Toronto Balloon Radiometer is a three-channel in
strument designed to sense thermal emission using double
sided limb scanning. The optical design is shown in Fig. 7. It
consists of a scanning mirror, M 1, a simple 0.1 ~m-diam tele
scope and folding optics, M2/3/4, a field lens and field split
ter assembly, U/M6a/M6b, followed by an individual PMC
for each channel and detector optics discussed below, Since
the energy available is small, care must be taken to maximize
the energy usage of the instrument consistent with its size
and other constraints. This can be at the expense of image
quality if the field-of-view can be maintained. A ray-trace
computer program was used to aid the design and the final
result has efficient energy collection but relatively poor im
age quality. Compromises had to be made, particularly in
the area of the field lens, Ll, as many components need to be
situated at the field stop position for maximum efficiency.
The optics is not spectrally selective as gold coatings are used
for all the mirrors and calcium fluoride for the field lens and
all windows in the individual channels,
An important feature of the instrument is the facility for
two-sided limb scanning discussed above. This is realized by
the 45° scanning mirror Ml which is pivoted on an axis
aligned with the optical axis of the system. The mirror can
rotate through almost 3600 and therefore input radiation can
come from either side ofthe instrument. The external instru
ment field of view is clear to a scan angle of --25° to allow
for a "space view" (see below) and at -90° (vertically up
wards) a reentrant cone in the instrument allows the system
to view a known radiance. These last two facilities form a
Pressure modulator 3525
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06ATMOSPHERIC
RADIATION
PIG. 7. Schematic (not to scale) of the main optical system of the Toronto
Balloon Radiometer. Input radiation falls on Ml, the scanning mirror and
is then focused by M2 onto the field assembly Ll/MS. Folding mirrors,
M3/M4, rotate and steer the beam through the reflecting chopper C. From
the field-splitting assembly, radiation passes into one of the three indepen
dent pressure modulator/detector channels (1'1-P 3, D 1-D 3) lor measure
ment.
major part of the radiometric calibration of the system and,
since they are situated at the input of the instrument, allow
the emissions of the individual optical components to be eli
minated from the signals. The mirror is driven by a stepper
motor geared down to give a rotation of O.163°/step. The
position of the mirror is monitored independently by a shaft
encoder with 0.044° resolution.
The fast rotating chopper described in Sec. In B is situ
ated in front of Ll. It is a 6-mm-thkk disk of stainless steel
0.2 m in diameter with 12 blades and rotates at about 4000
r.p.m. One face of the chopper is polished and gold-plated
and the instrument alternately views the atmosphere in the
clear sections and a cold blackbody in the reflecting sections.
The cold blackbody is cooled with liquid nitrogen and is
contained within a dewar assembly.
The three fields-of-view of the instrument are spatially
separated by the mirror pair MSa/b, the central field being
undeflected. Each of the mirrors MSa, M5b can be indepen
dently adjusted for alignment purposes. The fields are delin
eated with a field stop on the back of Ll. This is a grid or
three 22 mm X 5 mm slots corresponding to the three fields
of 2.04° hOlizontal X OAY vertical.
The pressure modulator optics for each channel simply
consists of a pair of calcium fluoride windows for entrance
and exit to the cell.
The filter, detector optics and detector for each channel
are all contained within a dewar and are cooled to liquid
nitrogen temperature. This has the dual advantage of de
creasing the total radiation on the detector, which improves
its performance in some cases, and of providing mechanical
protection for these components which are the most delicate
3526 Rev. SCi.instrum., Vol. 60, No.1t, November 1989 in the system. The filters are coated germanium with specific
passbands for each gas. They were designed and supplied by
Dr. J. Seeley of Reading University and are temperature in
variant between room temperature and the cryogenic tem
perature of operation. The detector lens consists of an antire
flection coated germanium doublet. 8 The use of a high
refractive index material allows the construction of a fast,
aberration-free condensing lens. The detectors used are
photo-conductive, mercury-cadmium-telluride for the
methane and formaldehyde channels and a photo-voltaic in
dium antimonide for the carbon monoxide channel.
There are a large number of sources of mechanical vi
bration in the instrument and therefore the detector dewars
are designed to be mechanically stable with respect to the
main optical plate, which is the optical reference for all the
other optical components. This is achieved by anchoring the
base of the cryogen vessel to the dewar base, which is in turn
bolted directly on to the plate, with three short (80 mm)
glass/epoxy pillars as shown in Fig. 8. The use of glass/
epoxy maintains the necessary thermal isolation whilst pro
viding the required mechanical rigidity. The fill and vent
tube for the cryogen also pass through the baseplate and
therefore it is necessary to fill the dewars by means of a pres
surized system. Many layers of aluminized mylar are used as
radiation shields and a container of molecular sieve material
is attached to the cryogen vessel to adsorb as much residual
TO VACUUM SYSTEM --
AND SEALING w:lLVE
LIQUID /\12 FILL -~t~~==~:=::::0.
rUBES
CENTRAL SUPPORT
POSTS
DEWAR OUTER
WALL
DEWAR INNER ----f-TI-l
WALL
LlOUlD /\/2 CRYOGEN --Hct+--- ....
SUPPORT POSTS -_.
DETECTOR FILTER
~-PLANOCONVEX LENS
MENISCu..5 LENS
FIG. 8. Liquid nitrogen dewar (capacity of I t) used to cool the detector
optics attached to the base of the cryogen vessel. Efficient evacuation of the
dewar along with multiple layers of aluminized mylar wrapped around the
cryogen vessel give a cryogen lifetime of up to 40 hours. The support posts
provide thermal insulation and mechanical rigidity with respect to the rest
of the optical system.
Pressure modulator 3526
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06gas as possible. Together with efficient evacuation of the sys
tem, these precautions reduce the heat leakage to such an
extent that a duration of 24-40 h is obtained from the 1-1'
charge of liquid nitrogen. The nitrogen vapour over the liq
uid is pressurized to about 105 Pa using an absolute pressure
relief valve. This prevents excessive boil-off during balloon
ascent and freezing of the liquid, with the associated loss of
thermal contact between the dewar body and the cryogen, at
ceiling.
Since all the frequency-selective optics are contained in
the detector dewar, a change of instrument operation from
one gas to another requires only the substitution of an appro
priate set of detector optics and detector. This design flexibil
ity should enable the instrument to be used for monitoring of
many trace constituents in the future.
B. Pressure modulator cells
Previous designs of pressure modulator cells have used a
freely suspended piston/spring arrangement driven magne
tically at the resonant frequency. The gap between the piston
and the bore is not sealed and gas leakage is minimized by the
small size of the gap. 1 However this makes the cells difficult
to manufacture and assemble. We have used a more conven
tional arrangement of piston and cylinder with sliding bear
ings and a PTFE sealing ring (Fig. 9). The drive power must
be increased to compensate for the frictional losses in this
arrangement but any oscillation frequency may be used as
the system is nonresonant. A Ferrofluidic seal (trademark of
the ferrofluidic Corporation Inc.) transmits the motor drive
power through the wall of the pressure modulator cell and a
flywheel/con-rod system transmits the drive to the piston.
The d.c. motor consumes about six watts of power whilst
running the modulator at about 11 Hz.
The pressure cycling within the modulator is monitored
by a variable-reluctance diaphragm pressure sensor attached
PRESSURE MODULATOR CELL (PMC)
To pms51J1l1
Transducer directly to the cell. The upper-and lower-pressure values are
logged by the data system and the pressure cycle is used to
derive a square· wave reference signal for the signal process
ing electronics (see below) .
The formaldehyde modulator is unique in this instru
ment in that the gas is unstable and polymerizes in the cell.
In order to maintain a constant pressure of formaldehyde, it
is necessary to generate fresh gas continuaHy. This is
achieved by heating paraformaldehyde powder in a small
container attached to the modulator. l>araformaldehyde de
composes into formaldehyde and water, the latter being re
moved by a calcium sulphate dryer. The heater is controlled
by the on-board computer which maintains a constant pres
sure in the modulator cell using the pressure sensor de
scribed above.
C. Signal channels
As can be seen from Fig. 3, the electronic signal from the
detector is very complex in nature. It is also very small. A
battery-powered pre-amplifier attached to each detector
raises the signal level and the signal is passed to the signal
processing electronics, which are also powered by separate
batteries. The use of separate power supplies minimizes
ground loops and noise coupling in the system. The signal
processor is coupled to the on-board computer through
opto-isolators to maintain the electrical isolation of each sec
tion.
The function of the signal processor electronics is to
determine the pressure modulator signal and the overall
"wideband" signal. After amplification and filtering to sup
press d.c. offsets and low-frequency signals from the modu
lator, the signals are extracted in two stages. First a phase
sensitive detector (PSD) using a reference signal derived
from the rotating chopper shifts the wideband component at
the chopper frequency to d.c. and the sidebands from the
Optical cell ___ _
FIG. 9. Schematic of the pressure modu.
lating celt. The optical thickness of the
gas varies with the cell pressure. The
pressure is varied by a piston oscillating
in a cylindrical bore beneath the optical
eelL The piston is driven by an internal
flywheel crank and con-rod mechanism,
whieh is connected to an external drive
via a rotary vacuum seal. The system
may be driven at any frequency. Pres
sure extremes and a square wave ref~r
ence are obtained from a transducer on
top oflhe cell. The base of the PMC may
be easily modified for the inclusion of a
gas source, if required (e.g., a CH)O
generator) . Piston
bore Optical cell
5cm
3527 Rev. SCi.lnstrum., Vol. 60, 1'110.11, November 1989 Compression volume·~
Ferrofluidic
seal
Drive bel!
Pressure modulator 3527
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06pressure modulator to the modulator frequency and its har
monics. The average value of this signal is the wideband
output. The average signal is derived by averaging the fre
quency of a voltage-to-frequency converter for one second.
The signal from the PSD is also applied to a filter/amplifier
system tuned to the pressure modulator frequency. The pass
band is narrow enough to exclude the chopper sidebands
( C 1, C2 in Fig, 3). The resulting signal is then detected by a
second phase-sensitive system using a reference derived
from the modulator pressure transducer. The average value
of the resulting d.c. level is obtained with a second voltage
to-frequency converter.
The voltage-to-frequency converters use a double
counting system as shown in Fig. 10 to improve the overall
resolution. A fast clock is counted for the nearest integer
number of converter cycles that occur in one second. The
average frequency is then obtained from the ratio of counts
and the clock frequency. The advantage of this technique is
that the average is accurate to within one cycle of the fast
clock (~1 MHz), giving a theoretical resolution of 1Q-1> in
one second, whereas a simple counting scheme would only
be accurate to one cycle of the converter (-2 kHz) .In order
to ensure that the true measurement period remains approxi
mately synchronous with the required measurement period,
the converter frequency is offset from zero and is never al
lowed to drop to a low value. Two setsof counters are used so
that measurement cycles are continuous with one set of
counters being read and cleared whilst the other set is in use.
The measurement cycle for each channel is also synchro
nized to the reference frequency of the corresponding phase
sensitive detector in order to suppress the variations in the
output due to a noninteger number of reference cycles occur
ring in one measurement period. Synchronization ensures
that there is an integer number of cycles in one measurement
period and therefore cyclic variations average out exactly.
This is particularly useful for the pressure modulator signal
whose reference goes through only about 11 cycles in one
second.
D. Overall instrument control
It was decided very early in the instrument design that
the instrument should be completely computer controlled.
I
I
___ ~---L __ _
---:~-- ----1.';-: ---!\....----
I I· I I : I
__ I II 2 3 __ rtF-\N~N~1 n n __ 1iflJ1.PJlJlSL j U U U IU U L I I I I j--ACTUAL MEASUREMENT TIME---! --I
I '
\" NOMINAL MEASUREMENT TIME --I
I I
3528 Rev. Sci. Instrum., Vol. 60, No. 11, November 1989 Thus a full microcomputer system is used to control all
mechanisms and measurement systems. The system also
handles formatting the data frames (see Sec. V A) for trans
mission to the ground, producing a split-phase data stream
for the data transmitters.
The command system for the instrument consists of a
16-bit word sent one bit at a time using a total of 18 com
mands. Sixteen of these commands set single bits in a 16-bit
command word, one causes the command to be executed and
one is used to reset all the bits of the command word to zero
in the case of an error. The partial command word is re
turned in the transmitted data stream so that it can be
checked before the "execute" command is sent. The com
mands alter the contents of the computer's operating tables
which in turn determine the mode of operation of the various
mechanisms. Commands are not essential for instrument op
eration, but are used to adjust the instrument to varying con
ditions, e.g., compensating for an excessive tiit by adjusting
the scanning system.
There are also many temperature and other sensors in
the instrument which allow the overall instrument condition
to be monitored continuously. This information has been
invaluable in the development phase for determining the
cause of instrument anomalies and generally adjusting the
performance, as well as in helping prepare the instrument for
flight.
Complementary to the on-board computer system is the
ground computer for real-time analysis. This takes the data
stream from the instrument and decodes it in real time to
produce engineering displays of the instrument status and
plots oftime variations. This system is also programmable to
cope with various instrument conditions. The engineering
display allows the continuous monitoring of over 100 pa
rameters. The data are also recorded in several different
forms for later retrieval and scientific processing.
IV. FLIGHT SUMMARY
The TORBAR instrument has been flown four times
from the National Research Council of Canada facility at
Gimli, Manitoba over a period of three years. The first flight
in August 1983 wa'> an engineering flight and was successful
REFERE.VCE OUTPUT
VOLTAGE- FREQUENCY
CONVERTER OUTPUT
CLOCK
Pressure modulator FIG. 10. The synchronization technique
used for the voltage to frequency converters.
A measurement period starts 011 the first
positive edge of the reference square wave
after the nominal start of the measurement
period. and ends on the first positive edge
after the nominal end of the period. Cyclic
variations at the reference frequency there
fore average out because the measurement
period is an exact number of cycles in
length. The measurement period is also syn
chronized to the output ofthe voltage to fre
quency converter. The final average fre
quency is (MIN)/, where f is the clock
frequency. The resolution is liN which for a
I-MHz clock and a 1-8 measurement is
~ 10-".
3528
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06apart from landing in Lake Winnipeg which severely da
maged the instrument. In 1984 the second flight was moder
ately successful and many of the instrument faults were cor
rected, In 1985 two launches were made on July 31st and
August 8th, The last flight was the most successful of all,
Launched on a 640 OOO-m3 balloon, the instrument reached
an altitude of 43.3 km with a maximum variation of 1.5 km,
Data were collected for 20 h during which time all mecha
nisms functioned welL The instrument was successfully re
covered and the post-flight checks were made without diffi
culty,
v. DATA PROCESSING
The data processing for the TORBAR instrument is di
vided into three sections. First is the reduction of the house
keeping data comprising temperature, voltage, pressure, and
status readings, Second is the processing of the radiance data
from the various channels to allow for instrument attitude
and altitude variations and the conversion from frequency to
radiance units, a process referred to as calibration, Third is
the analysis of the radiance data in conjunction with other
data and models to deduce concentration profiles of various
gases in the atmosphere.
A. Raw data format
The data from the instrument are transmitted and
stored as data frames, These occur at the rate of 1 Hz and
consist of 128 16-bit words. Important quantities and/or
those whose values change rapidly, e.g., radiance signals, are
sampled and encoded into every frame. Quantities which
generally vary more slowly are multiplexed into frame
words at a slower rate. This is accomplished by three multi
plexers. A fourth multiplexer slowly reads out the contents
of the computer control tables for confirmation purposes.
AU data analysis starts from these data frames.
B. Housekeeping data
The physical quantities monitored in the instrument are
easily transformed into voltages using appropriate trans
ducers and electronics. Two instrument multiplexers mea
sure these voltages using solid-state switches to sample the
channels, The analogue-to-digital conversion scheme is a
simpler version of the "double-counting" voltage-to-fre
quency technique used for the signal channels, Calibration
voltages on some channels allow the whole multiplexer to be
calibrated using a linear calibration scheme, Each individual
channel can then be related to the corresponding physical
quantity using a second calibration. In most cases the cali
bration is linear and is simply accomplished. This double
calibration is performed in real time by the ground station
computer so that the engineering data appears in appropri
ate units.
Temperatures are measured using two different tech
niques. Linear current sensors (National Semiconductors
LM 134) are used for most temperatures as the current
source system is ideally suited for a large switching matrix
and lead lengths are not relevant. The current mUltiplexer
uses reed relays and diode isolation to achieve a low-leakage
3529 Rev, SCi.lnstrum., Vol. 60, No. 11, November 1989 matrix. However the precise calibration of the sensors is dif
ficult and it has been found that for precise, consistent tem
perature measurements of the blackbody and other critical
components, precision thermistors (YSI Precision Thermis
tors 44006) whose resistance vs temperature characteristics
are known to ± 0.1 °C are preferred. These are also accessed
using a reed-relay switched matrix. The resistance versus
temperature relationship for the thermistors is known but
nonlinear and therefore a set of straight-line segments is used
in the ground computer to allow the high-speed computa
tion of the temperature. A more accurate, but more time
consuming, curve-fit scheme is employed in the final data
reduction programs,
c. Signal channel data
The output from the signal channels consists of six fre
quencies from the corresponding voltage-frequency con
verters, Since the voltage-frequency and radiance-voltage re
lationships are both linear, a conversion directly from
frequency to radiance is made. The equivalent instrument
spectral passband varies somewhat as the gas pressure in the
modulator drifts, and therefore it is removed from the analy
sis at this stage by calibrating in terms of the monochromatic
blackbody signal at the center of the passband (2140, 1746,
and 1308 em -1 for the CO, eHzO, and CH4 channels, re
spectively) using a calibration sequence at the end of each
atmosphere scan,
The instrument scans the atmosphere on each side alter
nately, Between the scans a view is taken at -25° where the
atmospheric content of the path is sufficiently small that the
radiance may be taken as zero to first order. At 0_ 90°, as the
view passes through the vertical, it is intercepted in the in
strument by an ambient-temperature retro-cone blackbody
oflarge thermal inertia. These two readings, of a zero radi
ance and a known Planck radiation suffice for a two-point
calibration of the system, Since the accuracy of the calibra
tion' particularly the knowledge of the zero value, is critical
to the accuracy of the experiment, these views are longer
than the individual atmosphere views in order to improve
the signal-to-noise ratio and the zero measurement is repeat
ed at the beginning and the end of the scan. Thus the se
quence is as follows: left atmosphere-zero-blackbody-zero
right atmosphere-zero--blackbody-zero and repeat.
The calibrated measured radiances are calculated as
[C'/«()) -Sri] S'/(())=BCi',T) 'a €X ,
a D b (fa -SPZ)
where B( v,Tb) is the retro-cone blackbody Planck function
evaluated at its temperature and the band center of the delin
eating filter profile, C Z (B) is the frequency from the atmo
sphere view at angle e, I" is the frequency from the black
body view, SP Z is the frequency from the -25° zero view, a
denotes a wideband or sideband response, and 1] denotes a
left or right view. The values of SPZ, la' and Tb vary slowly
with time and are linearly interpolated from their measure
ment time to the time of the atmosphere view.
Signal data are carefully checked for anomalies and
noise spikes. In particular, readings taken when the mirrors
are moving or stabilizing are rejected.
Pressure modulator 3529
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06D. Height and attitude corrections
The initial calibrated data are not in a form that can be
compared to the models. For this purpose the instrument
views must be related to the atmosphere in terms of the in
strument height and the view angle of the ray path or, alter
natively, the tangent height. The balloon height is provided
by the tracking data and the attitude of the instrument can he
corrected for using the technique outlined in Sec. n. These
processes result in plots of signal versus view angle which
allow comparison between the experimental results and cal
culations using atmospheric models. A typical pair of signals
(wideband and sideband) from the CO channel are shown in
Fig. 11.
VI. LABORATORY TESTING
In order to verify instrument performance and the
agreement of instrumental results with theoretical calcula
tions, it is desirable to use the instrument to measure known
gas concentrations in the laboratory. It is not possible to
completely reproduce the atmospheric path in the laborato
ry since this varies in pressure, temperature, and composi
tion along its length of up to 400 km. Laboratory experi-
Tangent height (km)
200r-_________________ 4T4~ _____ 4~0~--~30T_--~20
150
50
o 5
Scan angle (degrees)
F!l'. 11. Signals from the CO channel of the TORBAR instrument at an
altitude of 45 km. The signals have been sorted into scan angle bins
(ilB = 0.25') and time averaged over the late morning of August 8th 1985
(3-h time period). The sideband signal (upper) is noisier than the wideband
signal (lower) becausethe spectral bandwidth is narrower. The PMC (path
length of lO-mm) pressure varied between 2000 and ROO I'a at a tempera
ture of 273 K.
3530 Rev. Sci.lnstrum., Vol. 60, 1110.11, November 1989 ments are restricted to measurements using a single cell of
gas and a blackbody as the radiation source.
The equations for the wideband and sideband signals Sw
and Ss' respectively, are
Sw = G f: oc 7/fpB [v,T(x) ]7cdv,
S, =G' f""oo 7f{l"Tp -l'p!)B[v,T(x)]"T cdv.
G and G I are the combined radiometric and electronic gains
of the wideband and sideband signal channels, 'f is the filter
transmission, Ip is the instantaneous pressure modulator
cell transmission, and "Tc is the transmission of the test cell.
An overbar indicates a time average. Wave number depend
encies in all terms have been dropped for clarity.
In order to concentrate on the gas properties at the ex
pense of the instrumental gains, which are only required to
be stable, the ratio SslSw is used, normalized to the empty
test cell. This process results in a determination of modula
tor "transmissions" which can be compared to calculations.
In several cases the value of Sw is only slightly influenced by
the term involving the test cell, 7 c' and therefore this term
can be considered as being a monitor of the source intensity
and the ratio as a method of eliminating the effect of source
intensity variations from the experiment.
Although for mechanical reasons the test cell used is
short, the average pressure and total amount can be made
comparable with that of the stratospheric limb path by rais
ing the mixing ratio from a typical stratospheric value of
10 -8 to about 0.01-0.1. At concentrations higher than this
value the correction to the "infinite dilution" solution for the
gas becomes appreciable and requires a detailed knowledge
of the "self-broadening coefficient" or equivalently the mag
nitudes of the collision-broadened half widths for both self
collisions and collisions with the other gases in the mixture,
notably N2 and O2, These data are available for some gases,
e.g., CO (Ref. 9), but not for all, and may vary considerably
from line to line and gas to gas. Thus measurements with
pure gas and gas mixtures at various dilutions are required.
The technique we have adopted is to inject a sample of
pure gas into the transmission cell and then add inert broad
ening gas (N2) to increase the total pressure. We have found
that extreme care is required to ensure that the gases are
uniformly mixed and that the mixture concentration is
known and can be duplicated. In the case ofCH20, particu
lar care must be taken as the CH20 in the transmission cell
must be generated, thus one must be certain that the CH20
gas level is stable prior to mixing in N 2 .10.11 Since all the
experimental parameters are known, the instrumental re
sults can be compared with the theoretical calculations. Wc
have performed this experiment for both CO and CH20 us
ing both pure gas and gas/Nz mixtures. 10.11,12 The results of
a single CH20/N2 experiment are shown in Fig. 12.
VII. ATMOSPHERIC MODELLING
The equations that describe the pressure modulator sig
nals from an atmospheric path are
Pressure modulator 3530
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06T
T I I T II rTf -----r---------L-___ 11 I T --.----------.i.. ____ -__ 1._ FIG. 12. Comparison of experimen
tal results with GENLEN predic
tions for a series of CH20 transmis
sion experiments. The solid line
represents pure CH20 in the trans
mission cell and the dashed lines rep
resent cases with fixed amounts of
CHoO ( 190, 780, 1620, and 4220 Pa)
mixed with N2 ill the transmission
cell. Deviation from the calculations
can be seen at high amounts, but
these are unrealistic for atmospheric
soundings. The PMC operated
between 2600 Pa and 500 Pa at a
temperature of 298 K. The test cell
length is 101 mm and the I'MC
length is \ 0 mm. 0.8
0.4
0.2
O.OL---~--~~~~~~---L--~~~~~3~--~~~~~~~
101 102 10 104
Total pressure (Po)
S GIW i"" -B[-T( )]dr(O,x)d d-w = 7/Tp v, X X 11
-00 0 dx
for the wideband signal and
S, = G' f'" r~ 7(( \7 ;-"':'-1'~)
-0: Jo
XB l v,T(x)] dr~;X) dx dv
for the sideband signal. B is the Planck function and T the
local temperature along the ray path, x, which goes from
zero at the instrument to infinity. r(O,x) is the atmospheric
transmission from the instrument to the point x on the path.
Because of the complexity of the atmospheric situation
the models used to evaluate these integrals are also complex,
consisting of a multilayer generalline-by-line emission mod
el supplemented by simpler models for rough calculations at
higher speed. A discussion of the models used is given else
where.D Each of the spectral regions used has a different set
of emitting gases and these influence the relationship
between the wideband and sideband signal considerably. In
particular, both the CH20 wideband and sideband channels
appear to be strongly inft.uenced by H20 and 03 and HN03•
Their influence is so strong that the CH20 emissions may be
masked by them. The carbon monoxide channels are in
fluenced by ozone. Ozone accounts for all the wideband sig
nal and about 70% of the sideband signal, implying a strong
overlap between the ozone and carbon monoxide spectra. In
fact, an examination of the spectra reveals an almost exact
overlap between the P-branch of the CO spectrum and the
3531 Rev. SCi.lnstrum., Vol. 60, No.11, November 1989 4.8-f-lm ozone band which consists of a very large number of
weak lines. On the other hand, the methane sideband chan
nel appears to be insensitive to the presence ofN20, whereas
the wideband is strongly influenced by NzO. I3
VIII. CONCENTRATION PROFILES
The concentration profile of carbon monoxide has been
determined for the final phase of the Hight of August 8th
1985 which corresponds to late morning. This is shown in
Fig. 13. The profile was fitted by first fitting an ozone profile
to the wideband signal (the contribution of CO to this signal
is negligible), followed by the use of that ozone profile with a
CO profile to fit the sideband signal. In both cases a manual
iterative scheme was used to match the actual radiance with
a computed radiance starting at the top of the atmosphere
and working down. The comparison between the final com
puted radiance and the actual measurements is shown in Fig.
11. Perfect matching is not obtained and a mismatch around
the balloon level is exaggerated by the scaling of Fig. 11 in
angle. Profiles with a doser match show unreasonable
"noise" as large fluctuations in the concentrations with alti
tude, which is physically difficult to reconcile with the long
path length used in limb scanning which has a smoothing
effect,
The errors on this profile are difficult to represent be
cause of the interdependence of the radiances from various
tangent levels. In order to obtain an estimate of the sensitiv
ity of the profile to errors in the radiance, a "truncated
spike" of increased concentration, 3 km thick, was intro-
Pressure modulator 3531
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06Mixing ratio (v/v)
FIG. 13. Carbon monoxide profile for late morning over the southern Cana
dian prairies on August 8th i 985. The vertical error bar is the mean thick
ness of a truncated triangular spike used to calculate the horizontal error
bars. The horizontal error hars represent the magnitude of the spike re
quired to change the calculated radiance by one standard deviation. Each
error bar was derived independently.
duced into the concentration profile. The magnitude of the
spike was adjusted so that the calculated radiance chaI1ged
by approximately one standard deviation of the measured
radiances over the relevant portion of the radiance path. The
error is substantially influenced by the magnitude of the un
derlying ozone signal and if this were eliminated. the errors
would be considerably less. In future flights, it would be
advantageous to selectonly the CO R branch as the elimina
tion of the ozone signal would more than compensate for the
loss of 50% of the signal.
Further discussion of the above results and the results
from the other channels wiil be presented elsewhere. 13
IX. DISCUSSION
The first series of flights of the Toronto Balloon radio
meter instrument have provided valuable additional data on
3532 Rev. Sci.lnstrum., Vol. 60, No.11, November 1989 the concentration of carbon monoxide and other carbon
components in the stratosphere. It is expected that refine
ments of the instrument will allow the precision of the mea
surements to be increased and the flexibility of the instru
ment will allow it to be rapidly adapted to measure other
gases. One such possibility is to equip the instrument to
study the time variations of the nitrogen oxides as well as
provide a confidence check on the instruments for global
measurements of the same gases on the Upper Atmosphere
Research Satellite, due for launch in the early 19908.
ACKNOWLEDGMENTS
The TORBAR project is supported by grants from the
National Science and Engineering Research Council of Can
ada, Atmospheric Environment Service and the Physics De
partment, University of Toronto . We would like to thank the
workshop staff of the Department of Physics for their assis
tance wiih the instrument fabrication, the staff of ADGA
systems for their support during the flight preparations of
this instrument and the staff of the Canada Center for Space
Science for their assistance with instrument preparation and
funding of the flight itself. The data presented here were
gathered during the 1985 balloon campaign with the assis
tance of S. Heggie and R. Cameron.
IF. W. Taylor, Pressure Modulator Radiometry, in Vol. III of Spectroscopic
Techniques, edited G. A. Vanasse (Academic, New York, 1983).
"e. P. Chaloner,J. R. Drummond,J. T. Houghton, R. F. Jamot,and H. K.
Roscoe, l'roc. R. Soc. London A 364,145 (1978).
'J. R. Drummond and R. F. Jarnot, Proeo R. Soc. London A 364, 237
(\978).
4J. R. Drummond. J. T. Houghton, G. D. Peskett. e. D. Rodgers. M. J.
Wale, J. Whitney, and E. J. Williamson, Phil. Trans. R. Soc. London A
296, 19 (1980).
'J. R. DrummondandA. Ashton, J. Alrnos. and Oceanic Tech. (in press).
oJ. e. Gille and F. B. House, J. Atmos. Sci. 28, 1427 (1971).
'I. R. Drummond, D. Turner, and A. Ashtoll, J. Atmos. and Oceanic Tech.
3,9(1986).
'A. E. Murray, Infrared Physics 2,37 (1962).
9'1'. Nakazawa and M. Tanaka, J. Quant. Spectrosc. Radiat. Transfer 28,
471 (1982).
;<lA. G. Ashton. Stratospheric Measurements of CO Concentration, M.Sc.
Thesis (Department of Physics, University of Toro11lo, 1985).
liD. Turner. Radiometric Measurements of CH,O Concentration, M.Sc.
Thesis (Department of Physics, Univel'sity of TO[Qnto. 1983).
"'D. Turner, Radiometric Measurements of Stratospheric Trace Gases,
Ph.D. Thesis (Department of Physics. University of TOfOnto. 1987).
I'D. Turner and J. R. Drummond (unpublished).
Pressure modulator 3532
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147.143.2.5 On: Sun, 21 Dec 2014 08:23:06 |
1.339645.pdf | Fluorinated chemistry for highquality, low hydrogen plasmadeposited silicon nitride
films
ChorngPing Chang, Daniel L. Flamm, Dale E. Ibbotson, and John A. Mucha
Citation: Journal of Applied Physics 62, 1406 (1987); doi: 10.1063/1.339645
View online: http://dx.doi.org/10.1063/1.339645
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Published by the AIP Publishing
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28Fluorinated chemistry for highMquaUty, low hydrogen plasma-deposited
silicon nitride films
Chorng-Ping Chang, Daniel L. Flamm, Dale E. Ibbotson, and John A Mucha
AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974
(Received 3 February 1987; accepted for publication 10 April 1987)
We have developed a low-temperature (S 300 ·C) plasma deposition process to prepare novel
fluorine-containing silicon nitride films (p-SiN:F) using SiHcNF3-N2 discharge mix!ure~ at
14 MHz rf applied frequency, The deposition rate can be extremely high, up to 1600 A/mm.
Data indicate p-SiN:F has electrical properties (dielectric constant, breakdown strength,
resistivity, etc,) which compare favorably to high-temperature, chemieal-vapor-deposited
silicon nitride, By controlling the feed chemistry and physical variables of the discharge, a wide
variety of film compositions are achieved. Moreover, this chemistry is superior to the only
other p-SiN:F which was prepared from a SiF2/SiF4-Hz-N2 feed. Two classes of films were
identified as stable or unstable to air exposure and the instability of the films correlated with
the atom fraction of fluorine initially incorporated, Infrared, Auger electron, and Rutherford
backscattering spectroscopy measurements show that low hydrogen concentrations are
produced by the introduction offiuorine in the silicon nitride films, More importantly, the
concentration of Si-H is extremely low because strong Si-F bonding replaces the weak Si-H
bonds that satisfy free Si orbitals found in conventional plasma nitride, and the hydrogen
remaining in the film is present as stable N-H bonds. We believe this substitution of silicon
bound hydrogen, caused by the gas phase and surface-driven reactions, is a source of superior
film properties. The mechanism for this novel discharge chemistry is discussed.
I. INTRODUCTION
Silicon nitride is the most important plasma-deposited
dielectric in integrated circuit technology. While it is mainly
used as a passivation coating over silicon integrated circuits,
other applications include its use as a gate dielectric for tran
sistors, as an insulator between metal levels, as coatings for
gallium arsenide circuit technology, and dielectric and anti
reflection coatings for solar cells and photoconductors. The
main advantage of plasma-deposited silicon nitride over
thermal chemical-vapor-deposited nitride is that it can be
formed at much lower substrate temperatures, 200-300°C
vs 700-900 0c.
Plasma-deposited silicon nitride is commonly prepared
from SiH4/NH3 or SiH~N2 feed gas mixtures. Although
these deposited films are called "silicon nitride," in reality
they are amorphous silicon-nitrogen-hydrogen alloys (p
SiN:H), not the stoichiometric compound Si3N4• As a con
sequence, these p-SiN:H alloys exhibit inferior electrical
properties and resistance to chemical attack.
It is understood that hydrogen in p-SiN:H can be a
source of instability in MOS devices, I Hydrogen atoms in a
p-SiN:H capping layer may slowly diffuse into gate or field
oxide below and create traps. It has also been suggested that
Si-H bonds act as traps in the silicon nitride; this would be a
serious problem for its use as a gate dielectric. Hence there is
great interest in minimizing the hydrogen content of p
SiN:H, or at least avoiding the harmful effects ofSi-H.
Fujita et al. 2-4 attempted to minimize the hydrogen con
tent by substituting SiF4/N2 for SiH4/NZ in the plasma de
position of p-SiN (p-SiN:F). However, they found negligible
deposition for SiF 4/N 2 unless H2 was added to the feed, and
even then the deposition rate was very low (100-150 A/ min). These films had less nitrogen than stoichiometric ni
tride, and there was always an appreciable amount of oxygen
in the film (the source of oxygen was unclear). Nevertheless,
in many ways the quality of these films appeared to be supe
rior to conventional p-SiN :H.
In an attempt to increase the deposition rate and im
prove film characteristics, Fujita et aI.3,4 also prepared films
by using SiF4 that was prereacted with Si at 1165 °C to form
SiF2• It was reasoned that since SiF2 is more reactive than
SiF4, it would give a higher deposition rate. Films prepared
with SiF2 rather than SiF4 did in fact have higher deposition
rates; deposition was possible without added hydrogen, and
the films had less hydrogen and oxygen. However, a process
based on this technique would be difficult to control. When
prereactor temperature is increased (> 1000 ·C) to obtain
acceptable SiF2 production rates, the effluent composition
becomes a complex function of residence time, temperature,
and dilution. Moreover, the SiF 4/Si disproportionation re
action is known to deactivate and SiF2 concentration de
clines as polymeric SiFz deposits accumulate on the inlet
tubing walls and on all reactor surfaces.
Independently, we realized that one way to minimize or
avoid the problems associated with hydrogen in p-SiN might
be to minimize the use of hydrogen-bearing reagents as reac
tants. Furthermore, the voluminous literature on fluorinat
ed amorphous smeons suggests that incorporation of flu
orine into such films might lend stability and reduce free
dangling bonds (traps). For these reasons we decided to use
NF 3 as a nitrogen carrier in place of ammonia. We are aware
of only one other attempt to use NF3 for deposition6 (by
thermal chemical vapor deposition) and these researchers
failed in an attempt to deposit silicon nitride. Of course NF3
will liberate large quantities of fluorine in a plasma, which is
1406 J. Appl. Phys. 62 (4). 15 August 1987 0021-8979/87/161406-10$02.40 © 1987 American Institute of Physics 1406
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28an etchant for silicon and both thermal and plasma-deposit
ed silicon nitride. The gas-phase concentration of free flu
orine can, however, be controlled either by the addition of
unsaturated compounds, by adding a source of hydrogen or
by dilution. Following this theme, avoiding hydrogen-con
taining reagents entirely would be one way to eliminate hy
drogen from the p-SiN films.
In preliminary experiments we attempted to deposit p
SiN from NF3/SiC!4/N2/He mixtures. This experiment
turned up an unexpected difficulty. While it has been com
monly believed that Cl and Cl2 at moderate temperatures do
not etch undoped silicon at appreciable rates, we found that
at ;(; 200 ·C the thermal reaction with CI/Clz from SiCl4 de
composition would sometimes etch our silicon substrates.
The reaction between Cl atoms and undoped Si at ;(; 200 °C
without ion bombardment, has recently been verified down
stream of a Cl2 discharge.? Conceivably this might be over
come by using unsaturates or hydrogen to reduce free Cl in
the discharge,8 or by using feed bearing a lower atom frac
tion of Cl; we elected instead to study the NF,/SiH4/N2
system first.
It is well known that a variety of plasma parameters can
drastically alter the nature of plasma-deposited films. Be
sides feed composition, temperature, discharge frequency
and power are primary variables, High temperature ordinar
ily favors denser, more stable films with lower hydrogen con
tent,9 but post-growth annealing of low-temperature films
brings about densification and some hydrogen removal.
Films deposited at low rf excitation frequency ( ~ 1 MHz)
are usually under compressive stress while films deposited at
high frequency (10-20 MHz) are tensile. Film stress is gen
erally not altered by post-growth annealing. At low frequen
cy, sheath potentials are high and the growing films are sub
jected to high-energy ion bombardment. 10 Ion
bombardment can enhance surface diffusion and induce
cross linking. Hence these parameters must be varied to de
termine the potential value of a feed stoichiometry.
We report on films deposited from NF3/SiH4/N2 mix
tures at 14 MHz and briefly mention deposition results using
this feed mixture in low-frequency plasmas, and other low
hydrogen feed stoichiometries that we have explored. These
films exhibit electrical and optical properties that compare
favorably to thermal chemical-vapor-deposition silicon ni
tride. Test devices capped with this material show minimal
threshold shifts after accelerated aging in the presence of
moisture. The mechanism for film formation is discussed.
II. EXPERIMENT
A. Deposition system
The deposition flow system is shown in Fig. 1. The reac
tor was a modified Plasma Technology, Ltd. parallel plate
unit. A 28.S-em Ld. X II-em-tall glass ring separated the two
electrodes, The electrode spacing, set for these experiments
at 3.7 em, is adjustable by changing the height of the support
legs of the lower electrode that are exposed to the reactor
through O-ring seals. Its 24.0-cm-diam aluminum lower
electrode had an embedded resistance heater and could be
operated as high as 370°C. However, the typical operating
temperature in most experiments was 300 ·C. Gas was fed
1407 J. Appl. Phys., Vol. 62, No.4, 15 August 1987 OM.t\ I---
II
+ T,C.lHEATER
EXHAUST
MASS SPEC
FIG. L Schematic ofthe modified Pla.~ma Technology, Ltd, parallel plate
reactor used inp-SiN:F experiments,
from a 6.6-cm-diam "showerhead" centered on the water
cooled 26.6-cm-diam upper electrode.
Nz and Ar used in these experiments had a stated purity
better than 99.998%, while the NF3 purity (Air Products)
was 98.0%, and SiH4 (Air Products) was "semiconductor
grade" with a purity of about 99.8%. Typically, deposition
was done at a high dilution, with very low [SiH4]1rN2J and
[NF311[N21 ratios in the feed (-0.01-0.05). The [NF311
[SiH4] ratio ranged from ~O.O to 3.0. Small amounts of
argon were added as a reference for optical emission actino
metry. 11 The total ftow rate of feed gas was commonly about
220 seem, and total pressures between 0.2 and 1.2 Torr were
studied.
rf power at 14 MHz was provided by a Heathkit
DXlOOB transmitter or a Plasmatherm HFS SOOE. Output
power was between 50 and 2S0 W, corresponding to -0.11-
0.55 W Icm2 on the lower electrode, A PI matching network
was used to drive the reactor and a Bird model 4382 watt
meter with 50-and 250-W sensors monitored power input to
the network. rf power at 200 kHz was supplied by an HP
65 IE oscillator driving an EN! 1140LA amplifier, At low
frequency matching was done with a multiple-tap balun
transformer.
The voltage-current-dynamic power characteristics of
the discharge were also directly monitored. rf voltage was
measured with a 100 X oscilloscope probe connected to the
upper electrode. The lower electrode was insulated from the
reactor chassis, and grounded through a copper braid which
passed through a Pearson current probe to monitor the rf
current. A wideband multiplying oscilloscope (Phillips PM-
3265, 100 MHz bandwidth) was used to display both signals
and the instantaneous power (the product of the two sig
nals).
In a typical experiment using 5 sccm SiH4, 2 sccm NF3,
10 sccm Ar, and 200 scem N2, a 120-W, 14-MHz discharge
at 0.5 Torr total pressure sustained an applied rfvoltage of
250 V (peak-to-peak) and 3.0 A (peak-to-peak) discharge
current. The current led the voltage by 70°, hence the dis
charge was mainly capacitive. NF 3 added to an SiH4-N 2 dis
charge at constant power and pressure lowers the rf voltage
and current~ 10% and makes the discharge somewhat more
resistive. In appearance, the discharge with NF3 is better
confined between the upper and lower electrodes, and opti
cal emission is more intense,
Changetaf. 1407
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28Emission spectra from the discharge were taken with an
optical multichannel analyzer (Princeton Applied Research
model 1205 with Jarrell-Ash O.3-m polychromator) inter
faced to an Integrated Solutions 68010 computer system
running Unix™. Spectra were collected, transmitted, and
stored under computer control. A mass spectrometer (EAI
model 1200 quadrupole) was used to sample the discharge
effluent from a downstream "flow-by" loop.
B. Film properties determination
Substrates used in these experiments were 3-in.-diam p
type 8i (100) wafers cleaved into smaller pieces. In addition,
films were deposited on Ti-coated Si wafers for electrical
properties determination. A typical sample was a quarter
wafer and was masked by a smaller piece of substrate. The
film thickness and the deposition rate were determined by
measuring the step change at the masked-unmasked inter
face. The step height was measured with a Sloan Dektak II
stylus. In addition, the film thickness was also measured
with a Nanometrics Nanospec/AFT microarea gauge using
optical interferometry, By matching thickness between these
two measurements, the refractive index of the film can be
estimated.
Transmission infrared spectra of the samples were taken
with a Nicolet lODX Fourier transform infrared spectrom
eter (FTIR). For each sample, two spectra were taken
through the (unmasked area) deposited film plus Si and
through the bare 8i (masked area). The difference between
the two gave the spectrum of the film alone.
The presence of Si-H and N-H bonds in a p-SiN film
can be detected from infrared OR) absorption peaks corre
sponding to the Si-H bond at 2170 cm-1 and N-H peak at
3350 cm-I. The number of bonds per unit volume can be
determined by
n = (108IK)(A It) cm-J, (1)
where A is the area under the absorbance peak, t is the film
thickness in A, and K is the absorption coefficient, equal to
7.8X 10-18 cm2 for the Si-H bond and 5.3X 10-18 cm2 for
the N-H bond. 12 In conventional p-SiN films, the total hy
drogen content would be the sum of the Si-H and the N-H
concentrations. Applying IR to the determination of hydro
gen in some films is tenuous and may tend to overestimate
the H content. For example, in some of these experiments
absorption bands near 1430 and 725 cm-I were evident in
the IR spectrum, suggesting that -NH2 and NH/ may con
tribute to the N-H stretching regionl3 [forming a broad
peak Llv> 200 cm-1 J at about 3300 em-I. In addition, sur
face hydrolysis ofSi-F bonds (vida infra) to form -OH may
contribute to the NH absorption peaks, While it is difficult
to determine the hydrogen content corresponding to these
overlapping peaks, we have calculated the N-H bond con
centration and total hydrogen concentration for those sam
ples that did not exhibit absorptions at 1430 and 725 cm -!.
The proportions ofSi, N, and F in selected films were mea
sured by Rutherford backscattering (RBS), and the depth
profiles of some films were obtained from sputter-Auger
electron spectroscopy (AES).
Ti dots were evaporated through a shadow mask over
1408 J. Appl. Phys., Vol. 62. No.4, 15 August 1987 the film deposited on Ti-coated Si wafers. Using the mea
sured capacitance between two dots, the capacitance and
dielectric constant of the film can be obtained, These mea
surements were done on a Hewlett-Packard 4192A low-fre
quency (5 Hz-13 MHz) impedance analyzer. Current-vol
tage characteristics of the film were taken with a
Hewlett-Packard 4145A semiconductor parameter analyzer
to determine the breakdown strength, resistivity, and pres
ence of deep traps.
The optical absorption edge was measured by spectro
photometry for selected films deposited on sapphire sub
strates. In these measurements the photon energy at which
the absorption coefficient a = 104 em -I was equated to the
absorption edge.
Finally, we measured the etch rate of the film in 7:1
buffered HF [7 parts NH4F(40%):1 part liF (49%) solu
tion] which etches thermal Si02 at 800 A/min.
III. RE5UL 15 AND DISCUSSION
A. Deposition rate and refractive index
Depending on operating conditions, the deposition rate
of the p-SiN:F was typically 400-1000 A/min with some
rates as high as 1600 Almin (2.7 nm/s). We will refer to
nominal deposition conditions as 120-W applied rf power at
14 MHz, 0.2-0.5 Torr, 300°C electrode temperature, 200
scem N2• and small additions ofSiH4 and NF3• We observed
that the rate increases with increasing [NF)V[SiH41 ratio
in the feed. Film growth using 1: 1 [NF 3] / [SiH4] is a factor
of -2--4 faster than deposition without NF 3 in the feed gas.
Increasing the ratio of [SiH4]1[Nz] in the feed also leads to
higher deposition rates. For example, at [NF3]1[SiH41 of
1 :2.5 the deposition rate increases from 400 to 850 A/min by
increasing [SiH4]![N2] from 1 % to 2.5%. The deposition
rate increases 20%-50% as total pressure increases from 0.2
to 1.2 Torr, diminishes very slightly as substrate tempera
ture is raised (200 to 300 ·C), and increases with applied rf
power. For example, the deposition rate roughly doubles
from 500 to 1100 Almin as applied rf power is increased
from 50 to 250 W, but it is nearly constant with substrate
temperature, as shown in Fig. 2. Deposition conditions for
the data presented in Fig. 2 are P = 0.5 Torr and reactant
feed rates SiH4/NF3/N2 = 5:2:200 seem,
The current deposition rates are up to 3-10 times higher
than literature9•14 values for the deposition of conventional
p-SiN:H using similar reactant ratios. Moreover, the current
deposition rates are 6-10 times faster than the p-SiN:F de
posited by Fujita et al.2" from SiF2/SiF4-H2-N2 discharges,
even though we used only about half as much power and less
than half the power density (W I cm 2).
The refractive index (n) of near-stoichiometric (Sil
N ~ 1) p-SiN:F films in these experiments, ranged from 1.8
to 2,2. n decreased with total pressure at constant feed com
position and flow rate, and with increasing [NF3]1[SiH4J
feed gas ratios, or with decreasing [SiH4]1[N2] ratios at
constant total pressure and flow rate. This latter trend has
been widely reported. Film densities ranged from 2.6 to 2.8
g/cm3 which compare with 1.8-3.2 g/cm3 for p-SiN:H. For
a given SiiN ratio (as determined by RBS analyses), our p
SiN:F films consistently have a lower refractive index than
conventional p-SiN:H, which is probably an effect of flu-
Chang eta!. 1408
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28f500r-------------------------------~
'" ·S
"-• ..: 1000 A '" 250W
w
!;:[
ll: 0
Z Cl 120W 0
1=
(j) 500 0 0 0 0 0 50W a.
III
0
0 200 250 300 350 400
T. ·C
FIG. 2. Deposition rate of p-SiN:F as a function of electrode temperature at
50,120, and 250 W applied rfpower (14 MHz), P ~o 0.5 Torr, and reactant
feed rates SiH./NF 3/N2 = 5:2:200 secm.
orine incorporation in the film. Fujita et al.4 reported even
lower refractive indices, 1.6-1.8, consistent with larger
amounts of fluorine in their class II films (see Sec. HI B).
B. Film composition
Equilibria between a SiHcN2-NF3 feed, solid Si3N4,
and gas-phase products including fiuorosilanes, HF, H2, N2,
and NH3, strongly favor Hz and SiF4 when [SiH4]!
[NFd S 1 in the feed. As [SiH4V[NF3J is increased, the
product distribution shifts to H2, SiH4, and fluorosi.lanes
(SiH.~Fy, x + y = 4). Feed composition changes have a
roughly parallel effect on gaseous plasma products and the
p-SiN:F film composition-when fluorine content of the
feed is high enough, Si-N, Si-F, and N-H bonds are detect
ed in the film with SiE~ and Hz present in the gas phase. Si-H
bonding appears in the deposited films when NF3 additions
are small. Thermodynamically, at least, fluorine diminishes
Si-H bonding by substituting Si-F bonds, but does not re
duce the concentration of H2 in the gas phase. Significantly,
the effect ofN2 additions to the feed does not conform to this
analogy between thermodynamics and plasma kinetics.
Modest nitrogen additions drastically alter the thermody
namic equilibria, while plasma-enhanced film growth is in
sensitive to Nz. We believe the reason is that Nz decomposi
tion reactions are kinetically slow in the plasma, hence
mole-for-mole N2 is an ineffective source of nitrogen for film
growth.
The AES and the RES analyses show two general classes
of film composition. Class I films have a NISi ratio in the
range of -0.8-1.2 and a F/Si ratio ~0.2-O.5. In class I films
some oxygen is observed on the surface, but oxygen concen
tration drops below the detectable limit within a few
hundred A of the surface. The surface OISi ratio varies from
0.2 to 0.6. Typically, class I films were deposited with a lower
[NF3]1 [SiH4] ratio (S: 1) at short residence time
( 'TEL = 50 ms based on the volume subtended by the elec
trodes) and low total pressure (PSO.2 Torr).
Films that were deposited from high [NF3]1(SiH 4] ra
tio (;;;.2) feeds andlor at long residence time ('TEL> 0.3 s,
P> 1.2 Torr) were unstable in air "as deposited." This film
1409 J. Appl. Phys., Vol. 62. No.4, 15 August 1987 composition type, which we call "class II" showed, for ex
ample, NISi s: 0.7 and a high F/Si ratio ( ~ 0.5) when ana
lyzed by RBS immediately after deposition. RBS analysis
after exposure to the ambient showed Si, substantial 0, and
reduced F and N. Moreover, when class II films were ex
posed to the ambient atmosphere continuously, IR spectra
showed a gradual composition change in which fluorine and
nitrogen in the film were replaced by oxygen. This change
continued for several days until. oxygen was incorporated
throughout the bulk of the film. In some extreme cases, the
final film stoichiometry was close to SiOl with little F and N.
Sometimes, on the ambient-exposed samples, there was 11
frosty, water soluble film visible which we believe is
(NH4hSiF6•15
Both composition changes could be accelerated by ex
posure to water or moist air. The silicon in the nitride con
tains too high a degree of -Si-F bonding to be stable to oxida
tion or hydrolysis. These class n films probably contain
frequent sequences of -SiFz-(SiFz)x groups in the nitride
polymer, and it is well known that SiF2 oligimers and poly
mers are converted to Si02 by oxygen and moisture.lo•l7 In
fact, fluorine additions have been used to increase the rate of
high-temperature thermal oxidationl8 (added as NF,) and
low-temperature plasma oxidation 19 of silicon. Presumably
these latter processes proceed by first forming a chemisorbed
SiF2-like layer which is more readily oxidized. Although the
detailed replacement reactions are uncertain at this point,
the overall reactions may proceed as
O2 + -SiF2-SiF2 -(s) --SiFz -O-SiF2 -(8) ( + F2SiO),
(2a)
(2b)
which will cause nitrogen to be removed via
-Si-NH, + nHF -> {-SiF-+ NH4F (2c)
- -SiF2-+ NH3 '
and form (NH4) 2SiF 6 by reaction with the ambient
-[SiFz-SiF2]-+ 2NH3--Si-+ SiF4'2NH3, (2d)
H,O
SiF4'NH3 -> (NH4)2SiF6 + :5102 + NH4F. (2e)
Possibly class II films initially contain groups such as
~Si-NFH ~Si-NF (3)
/ / 2
which are unstable. These NF groups, if present, cleave and
migrate to the surface where NH4F and (NH4}zSiF6 are
formed in the presence of moisture.
We believe that film stability is mainly determined by
the initial bonding and microstructure of the film, rather
than F content per se. p-SiN:H films deposited at 300°C with
a FIN ratio below 0.5 appeared to be stable to air and water
exposure--only surface oxidation was observed. Figure 3
shows IR spectra of a stable film immediately following de
position and 12 days later. For comparison, the IR spectra of
an unstable film, shown in Fig. 4, reflect a drastic composi
tion change with time. Figure 4 also shows that water rinsing
an exposed unstable film causes a large decrease in the 3330,
1430, and 725 cm-1 absorbance peaks, which intensify dur-
Chang etal. 1409
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28CLASS I FILM
I
(b I:
(01
3800 3400 3000 2600 2200 1800 1400 1000 600
WAVENUMBER (em-I)
FIG. 3. Infrared absorbance spectra of a class I stable p-SiN:P film (a)
immediately after deposition and (b) after 12 days air exposure. The mm
was deposited from a feed ofSiH 4/NF,/N2 = 2:1:200 seem at 0.2 Torr us
ing 120 W of 14 MHz power and 300"C substrate temperature.
ing air exposure, and are attributed to (NH4)2SiF6. 15
Fluoro-nitride films reported by Fujita et al.2.-4 consis
tently contained appreciable concentrations of oxygen (01
Si~O.1-0.3), generally had low NISi ratios (0.5-0.8), and a
high initial F/Si ratio (0.7-0.9); that is, they were all class
II. These investigators believed that oxygen in the films
came from the walls of their reactor and was incorporated
during growth.2•3 Although their feed chemistry and depo
sition conditions were different from the present study, the
instability of class II films provides strong evidence that the
oxygen in their films was actually incorporated after depo~
sition.
Fluorine is incorporated into class I films even when
only 0.5% ofNF3 is added to the feed gas. Figure 5 summar
izes the dependence of the NISi and the F lSi ratios in the p-
::j
<i.
IoU
U z <t CLASS II FILM
~ RINSED WITH WATER
o if)
'" <f
AFTER DEPOSITION t < 1 h'
, Si-K
2400 2000 1600 1200 800 400
WAVENIJM8ERS (em -1)
FIG. 4. Infrared absorbance spectra of a class II unstable film. From bottom
to top in the figure: (1) immediately after deposition, (2) after 18 h expo
sure to moisture, and (3) after rinsing in Dl water. The film wa, deposited
from a feed ofSiH./NF/N2 = 4:5:200 seem at 0.5 Torr using 120 W of 14
MHz power and 200°C substrate temperature.
1410 J. AppL Phys" Vol. 62, No.4, 15 August 1967 \.0
0
~
'" u
~ !i
(ij
" 0.5 .....
Ui "-:z
~
0.0 0
O,Q o
NISi
F/S; o
It
1.0
FIG. 5. F lSi and NISi atomic ratios in p-SiN:F films deposited as a function
of NF3/SiH. feed ratio. Flowrates of SiR. and N2 were 25 and 200 seem,
respectively, with depositions doneatO.2 Torr, 120W, 14 MHz, and 300 'c.
SiN:F films on the [NF3]1[SiH4] ratio in the feed gas for
NISi -1. The NISi ratio decreases and the F lSi ratio in
creases with increasing [NF3)1[SiH4] ratio. Since depo
sition was done at high dilution ([SiH4]![N2] and [NF3]/
[Nz] < 0.05), this is attributable to fluorine from NFx radi
cal species displacing Si-N bonds in favor of Si-F bonding
(see Sec. III E). When [NF 31/ [ SiR.] is held fixed and the
[NF 3] / [N 2] ratio is raised (that is, going into a less diluted
regime), both NISi and F/Si increase slightly. Thus, N in
corporation may be primarily due to NF3, not N2 in the
discharge, but the F lSi ratio must be controlled to minimize
competition with and displacement ofSi-N bonding.
The most "stoichiometric" p-SiN film reported by Fu
jita et a/,3 had F/Si-O.3, N/Si-I.3, and 0/5i-0.l, with a
deposition rate of only 80 A/min. By contrast, our class I
oxygen-free p-SiN:F film was routinely deposited at better
than 500 .A/min. Moreover, by adjusting the deposition pa
rameters, the SiH4/NFJ/N2 chemistry can be manipulated
to select from a wide range of oxygen contents (O/Si-O.O-
1.9). We note that our class II oxygen-containing films ex
hibited current-voltage response instability, although the
electrical properties reported by Fujita et al. were excellent.4
The Si-H bond concentration, calculated from the ab
sorbance peak area using Eq. (1), decreases with increasing
[NF3]1[SiH41 ratio in the feed gas. In some cases, the Si-H
concentration was below the limit of detectability
(~1 X 1021 cm-3). For example, with high substrate tem
peratures (320°C), nearly equal flows of NF3 and SiH4
([NF3]1[SiH4] ~ 1) and low [SiH4]1[N2](-0.0l), the
deposited film showed no detectable Si-H bonding. Si-H is
easily suppressed by a factor of 2 (compared to our p-SiN:H
films) using.an [NF3]1[SiH4] ratio of 0.6. Fujita et al.
achieved a similar Si-H bond diminution, which they felt
was one of the main reasons for superior electrical proper
ties.4 By comparison, a typical plasma-deposited p-SiN:H
nitride film has an Si-H concentration of about 1 X 1022
cm-:'. Hydrogen present as N-H in the films was estimated
Chang etal. 1410
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28to range from 1.2 X 1021 to l. 3 X 1022 cm -.; based on the IR
N-H absorbance peaks. The N-H concentration tends to
increase with the [NF,]I(SiH4] ratio and total pressure.
We observed the Si-N absorbance peak near 840 cm--1
for p-SiN:H films deposited without NF 3 additions. How
ever, as NF3 was added, the Si-N peak broadened and shift
ed to higher wave numbers, 870-920 em-I. The Si-N peak
shift is probably caused by bound fluorine and we expect the
broad peak to include contributions from Si-N and Si-F
(and even Si-O in some cases). Recalling Fig. 4, we can
identify the Si-F absorption peak around 930 crn-I and Si
o at 1080 em -l. The Si-H peak is also shifted to a higher
wave number by fluorine, whereas the N-H peak merely
becomes broader. Figure 6 compares IR spectra of films with
and without fluorine incorporation and Table I shows how
these peaks shift and broaden with increasing [NFl] /
[SiR,] .
The infrared data confirm the effectiveness offiuorine in
reducing the hydrogen content of the nitride films. Optical
emission spectra in the 600-800 nrn region exhibit promi
nent bands from Nz (first positive) and atomic hydrogen
(656.2 nm); however, within our sensitivity limit, no emis
sion from atomic fluorine (703.7 nm) was detectable. The
intensity of the hydrogen line relative to an Ar emission line
at 696.5 nm showed a maximum at an [NF3]1[SiH4] ratio
of 1. Mass spectra of the effluent showed that more than
97% ofthe SiH4 and NF3 in the feed were converted to prod
ucts.
Although the detailed mechanism and identity of active
species are still in doubt, the overall results suggest that film
formation involves deposition of SiFx and SiHx species
which undergo further reaction with NFx and NH". In the
case of class I films, the hydrogenated and fl.uorinated spe
cies are available in comparable amounts and can react to
CLASS I FILMS
3800 3400 3000 2600 2200 1800
';AVENUIilBERS \cm-1)
FI Q. 6. Infrared absorbance spectra of class r p-SiN:F film (top curve) film
deposited from SiH4/N2 (lower curve). Note the Si-N peak shift caused by
the presence of bound fluorine. Deposition conditions were 200 W of 14
MHz power, 320 'c substrate temperature, and a feed composition SiRt/
N2 = 2.5:200 seem at 0.25 Torr (lower curve), with 2.5 seem NF, added to
grow p-SiN:F (top curve).
1411 J. Appl. Phys., Vol. 62, No.4, 15 August 1987 TABLE I. IR peak positions and peak widths.
[NF,/lSiH 41
feed ratio
o
0.2
1.0 SiN
Vp(ll.v1I2)crn-'
832(202}
881 (213)
901(221 ) SiR
vp(ll.vl/?)cm-· ,
2184(108)
2239(108)
2256(105) NH
vp(ll.vllolcm-'
3367(76)
3373(86)
3373(l04 )
produce nearly Si-H free, low-fluorine films. For class II
films, the system is fluorine rich and extensive SiF2 networks
are probable. These films continue to be reactive after depo
sition. Similar behavior has been observed for SiF2 films de
posited downstream from a high-temperature, SiF 4/Si reac
tor. 16 The RBS and AES analyses also tend to support the
formation ofSiFx from NF 3 additions, as might be anticipat
ed from equilibrium calculations (see above).
c. Electrical and optical properties
p-SiN:H films have superior electrical properties. Fujita
et al.4 reported that typical p-SiN:F films had breakdown
strengths of (5-10 MV/cm) and resistivity (1014_1016
n em) similar to high-temperature, chemical-vapor-depos
ited Si3N4, while the dielectric constant was somewhat lower
(4--6). By comparison, good p-SiN:H films have a break
down strength ranging from 1 to 6 MV /em, a resistivity
from _106 to 1015 n em, and dielectric constants from 6 to
8.9 More importantly, the reported deep trap density and
optical absorption edge of the p-SiN:F films were also com
parable to high-temperature Si3N4.4 Both Si dangling bonds,
which increase in number when the N/Si ratio is less than
the stoichiometric value of 1.33, and the high volumetric
density of Si-H bonds are commonly associated with the
inferior properties ofp-SiN:H.2.4 The superior properties of
p-SiN:F have been attributed to fluorine passivation of Si
dangling bonds and a lower concentration of Si-H."
We have performed extensive electrical characteriza
tions of the p-SiN:F films deposited from SiH4-NF3-N2 us
ing a metal-nitride-silicon structure for capacitance-voltage
measurements, and a metal-nitride-metal-silicon structure
for capacitance -frequency and dc current voltage character
istics, Film thicknesses ranged from 1000 to 2500 A. The
maximum de voltage of the analyzer was 100 V, which limit
ed the maximum electrical field strength. We plan to study
the electrical film properties in more detail after further ex
ploring the effect of discharge variables and chemistry.
Tests of selected samples showed a typical breakdown
strength around 4 MV /em, and resistivity between 1014 and
1016 !l cm. The dielectric constant of typical class I films is
about 4-6, which compares with 6-7 for thermal Si)N4, and
it decreased 5%-15% in going from 100 Hz to 10 MHz. By
contrast, p-SiN:H usually exhibits a -40% decrease in the
dielectric constant between ! kHz and 1 MHz. The films we
tested were almost totally ohmic up to breakdown in I-V
scans. The absence of Frenkel-Poole emission seems to sug
gest much lower trap center densities than p-SiN:F films
reported elsewhere.4 Class II films (with a significant oxy~
gen content, O/Si > 0.6) had lower breakdown strength and
lower resisitivity ( < 1013 n cm).
Chang etal. 1411
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28The optical absorption edges of selected films were mea
sured by spectrophotometry. OUf p-SiN:H films nominally
give an absorption edge of3 eV, which compares with ~ 3-4
e V reported for conventional p-SiN :H.9 The absorption edge
measured depends on the amount offiuorine incorporated in
the film. For [NF3]1[SiH41-1, for example, the absorp
tion edge increases to > 5 e V, as high as 5.6 e V. This is better
than absorption edges reported for high-temperature chemi
cal vapordepositedSi}N4 (-S.2eV). Fujita4 reported lower
absorption edges for p-SiN:F fi1ms (4.9-5.2 eV), however,
these values are not directly comparable because they esti
mated absorption edges using an extrapolation procedure.
D. Buffered HF etch rate (BHFER)
The buffered HF etch rate (BHFER) of p-SiN:F was
much higher than p-SiN:H. Other workers report a BHFER
of p-SiN:H between 20 and 400 A/min, increasing with hy
drogen content9 The BHF etch rate for our class I films
varied between 300 and 2000 A/min, while the class II films
etched at more than 2000 A/min. By contrast, film deposit
ed without NF 3 in the feed etched only about 30-100 A/min.
Figure 7 shows how the BHFER increases with [NF 3]/
[SiH4). Conversely, the BHFER decreases with Si-H con
centration in contrast to the general behavior of p-SiN:H
films. II We believe that the densities ofN-H and Si-H groups
in conventional p-SiN:H increase hand in hand, and that the
correlation of BHFER with Si-H is a consequence of reac
tions similar to Eq. (2c).
It is obvious that the high BHFER is related to fluorine
in the film. This might be anticipated since the substitution
ofSi-X bonds (X = halogen) for Si-H bonds in polysilanes
lowers the intrinsic Si-Si backbone bond energy, which is
related to reactivity.20 Hence the decrease in BHFER with
[Si-H] is the result of an inverse relationship behveen
[Si-H] and [5i-F] in the films. We believe there is a strong
possibility that a halogen discharge would etchp-SiN:F se
lectively over silicon dioxide. Coupled with other superior
properties, this could make p-SiN:F a desirable passivation
layer for gate oxide.
"oo~ 0
0 0
0
.S
f ~ 0<
'" '" "-::
'" 100
0
0
10L--J----------~ ________ L_~
0.0 1.0
FIG. 7. Buffered HF etch rate ofp-SiN:F as a function ofNF,/SiH4 ratio.
1412 J. Appl. Phys., Vol. 62, No.4, i 5 August 1987 E. Mechanisms
At first glance, one might expect free fluorine atoms
from NF3 dissociation to abstract hydrogen from SiHx spe
cies, and combine with Hand H2 to form highly stable HF.
This would reduce the hydrogen available for film incorpo
ration. However, as pointed out, the data and analyses indi
cate a more complex interaction in which H2 is one of the
stable reaction products, and Si-H in the deposited nitride
arises from incomplete removal of hydrogen from trapped
-SiHx moieties. In Sec. III B we pointed out a loose analogy
between gas phase equilibria and the effect offtuorine on film
composition. However, while modest Nz additions drastical
ly alter the equilibria, plasma-enhanced film growth is insen
sitive to changes in [N2]. We believe the reason is that N2
decomposition reactions are kinetically slow in the plasma,
hence mole-for-mole N 2 is an ineffective source of nitrogen
for film growth. While there is not enough information to
propose detailed elementary reactions, we have been able to
formulate a general scheme which explains the effects offeed
composition on the nitride film stoichiometry and proper
ties.
A variety of information is taken into account. First,
studies ofconventionalp-SiN:H deposition (e.g., from SiH4
and NH3) show that SiH4 is rapidly decomposed into hydro
gen (H,H2) and Si:Hx radicals, and that the degree ofSi:H4
decomposition is insensitive to other species (e.g., NH" ).21
Second, no emission from atomic fluorine was observed in
the present studies, which suggests the steady-state fluorine
atom concentration is very low. Third, adding SiH4 to a N2/
NF 3 discharge decreased the HF peak intensity observed in
our downstream mass spectrometer. By contrast, the con
centration of SiF4 in the effluent increased with SiH4 addi
tions. Fourth, raising the proportion of [NF3]![SiH41 be
yond about 1 caused too highly fluorinated, unstable, class II
films to form. Long residence times favored this trend too.
Fifth, for many of our films, particularly for the most stoi
chiometrically stable films (closest to Si3N 4)' hydrogen is
mainly or entirely bonded to nitrogen, while fluorine is
mainly or entirely bonded to silicon. Finally, NF3 greatly
enhances the nitride deposition rate over SiH4/NZ mixtures
and gives favorable deposition rates compared with conven
tional SiH4/NH3/N2 deposition reported in the literature,
These facts and consideration of the relative bond ener
gies and thermodynamics (Table II) lead to the proposed
mechanism. The initial reaction steps involve the dissocia
tion of silane by electron impact,
(4)
and H in turn causes rapid decomposition of NF3:
H + NF3-+NF2 + HF. (5)
Either by elementary reactions or overall, NF2 and NF3
react with SiHx forming fluorosilyJ and N-H groups. Pre
cursors may form by thermodynamically favorable direct
reactions and rearrangement [which appear to occur during
thermal chemical vapor deposition from NH3/SiH4 (Ref.
22) ],
Chang etai. 1412
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28TABLE II. Selected bond energies.
Energy
Bond (kcallmole) Reference
H-F 136 23
H-H 104 23
N"",N 226 23
NH2-H 104 24
NH-H 95 24
NF2NF-F 71 24
NFrF 57 24
H2N-NH2 38 24
H2N-NOz 67 24
SiF3-F 144 25
SiFz-F 136 25
SiF-F 153 25
Si-N (a·Si3N4) 79 23
Si-O(quartz) III 23
SiH,-H 92 26
SiHz-H 72 26
SiH-H 76 26
SizH,-H 89 27
H,Si-SiH3 77 27
F-P 38 23
{HX _ n F m Si-NF y._ m Hn
SiHx + NFy -> or
Hx. n 2FmSi-NFy_mH" + H2
for instance,
{SiFH+ NFH
NF2 + SiR2-" or
SiFH2NF
SiH4 + NFr-.F2SiNH 2 + H2, , (6)
(7)
(8)
or by reaction of HF, formed by abstraction, with silylene.
For example,
fSiH3F
8iRz + HP ..... t· or
SiFH + Hz. (9)
The key point to note is that while H-F and Si-F bonds have
about the same enthalpy of formation, the H-H bond is fa
vored over Si-H (see Table II). Hence hydrogen originally
bound to SiH4 can be removed from the system as Hz, and
excess fluorine can be eliminated as SiF4, the most stable
silicon-and fl.uorine-containing molecule here. Similarly,
N-H bond formation is much more exothermic than N-F,
so exchange reactions in which hydrogen replaces fluorine
on a nitrogen molecule are favored. By contrast to silicon,
the N-Si and N-H bond energies are stronger than N-F.
Hence available free hydrogen and sHyl radicals in the dis
charge can convert N-F to N-H.
At present we have no way to distinguish between gas
phase and surface driven reactions involving these bonds.
That is, conversion ofSi-H to Si-F and N-F to N-H could
also proceed on the surface. For example, hydrogen abstrac
tion by NF followed by recombination can effectively trans
fer hydrogen to nitrogen.
1413 J, Appl. Phys" Vol. 62, No.4, 15 August 1987 NF + H H " ./ Si
/ " N N --H F " ./ N H " / 51
./ " N N ( lOa)
Similarly, silylene insertion into N-F bonds can transfer flu
orine to silicon:
+ --
(lOb)
while thermal or ion-induced migration may account for si
multaneous transfer
--
( We)
and cross linking
--
( lOd)
The specifics of elementary reactions, the radicals involved,
and whether surface or volumetric atom exchange domi
nates remain open questions.
Compared to SiH4, strongly bonded N 2 is difficult to
dissociate by electron impact, and we expect ground state N2
to be unreactive toward silanes, Hand NF x radicals. The
role of ammonia in conventional SiH4/NH3 p-SiN:H depo
sition is to supply "reactive nitrogen" for film growth and
NFJ serves the same purpose in the present chemistry.
Hence it is likely that a large proportion of the nitrogen in
our films originates from the NF3, rather than the Nz bath
gas. Smaller amounts of N from Nz probably supply "make
up" nitrogen in the most stoichiometric films.
There is a crucial distinction between NF3 in this chem
istry and NH3 used with SiH4 to make conventional p
SiN:H. Besides being a source of reactive nitrogen, NF3 also
carries the fluorine that replaces Si-H bonds with Si-F
bonds in the growing nitride film. This presents a limitation,
since the ratio of reactive nitrogen to fluorine is fixed (at 1: 3 )
by the atomic composition ofNF3: for a given SiH~ flow, the
proportion of available nitrogen cannot be increased without
also increasing gas-phase fluorine. When there is a high con
centration of gas-phase fluorine, silyl and siIylene radicals
become more fluorinated (either homogeneously or perhaps
by fluorination of dangling bonds as the film grows),
(SiF2)x segments are incorporated into the film, and fiu
orine competes excessively with nitrogen-silicon bond for
mation. This results in highly fluorinated, class II films.
These concepts account for the formation of class I or
class II films, and can be extended to other feed gases and
Chang eta!. 1413
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28their effects on bonding and film stoichiometry.
F. Other parameters
In low-temperature film deposition, activation barriers
prevent deposited species from reconstructing to physically
and thermochemicaHy stable states. Rearrangements during
high-temperature chemical vapor deposition lend stability
and inertness to the films. Structural constraints during
growth oflow-temperature plasma nitride films can prevent
silicon and nitrogen from moving and thus satisfying all of
their bonding orbitals (hence there are "dangling bonds").
However, the plasma environment can be engineered to
compensate for the absence of thermal energy-first, the
plasma chemistry can be staged so that mobile radical spe
cies combine with and passivate dangling bonds; second, dis
charge conditions can be selected where energetic ion bom
bardment facilitates surface mobility and chemical
rearrangement. Hydrogen and fluorine passivate dangling
bonds inp-SiN:F: hydrogen provides a stable termination for
free N-groups and fluorine forms a strong bond with silicon.
Thus hydrogen is a necessary element for high-quality plas
ma deposited films and our results make it clear that the
presence of hydrogen in the plasma does not necessarily pro
duce Si-H bonding in the film. As long as there is enough
fluorine in the plasma, Si-H bonding is unfavorable. More
over, the feed gas reagent used to supply fluorine is unimpor
tant, as long as it is reactive in the plasma.
We have tested these two concepts as follows. Ammonia
additions to a feed mixture could supply additional "reac
tive" nitrogen to increase the N/Si ratio and the growth rate
of p-SiN:F films without undue Si-H incorporation. The
data confirm this. Since we conclude that the source of flu
orine is secondary, we substituted HF(g) for NF3 at low
[SiH4J1[N2J and obtained p-SiN:F films with properties
comparable to the class I films noted earlier.
Low-frequency plasmas supply higher ion bombard
ment energylO than 14 MHz discharges and hence can stimu
late surface diffusion and rearrangement. This probably ac
counts for the low, more favorable compressive stress
(versus tensile stress in high-frequency deposition) found in
p-SiN:H (Ref. 9) deposited at low frequency. Here again, we
have deposited p-SiN:F films in low-frequency (200-kHz)
plasmas at the same feed and substrate temperatures used
here, which show lower Si-H content and eve; better electri
cal properties. These low-frequency p-SiN:F films exhibit a
virtually constant dielectric constant from 100 Hz to 10
MHz, large absorption edges (> 5.5 eV), and no measurable
Si-H bonding. These will be discussed in detail elsewhere.
IV. CONCLUSIONS
We have prepared low-temperature (:S 300°C) p
SiN:F, films using a SiHcNF3-N2 discharge. This material
compares very favorably with other plasma-deposited sili
con nitrides and our chemistry is superior to the only other p
SiN:F reported, prepared from a SiFz/SiFcH2-N2 feed.2--4
The film properties are summarized as follows.
(1) In fluorinated p-SiN:F films prepared from NF3!
SiH4/N2, streng Si-F bonding replaces the weak Si-H bonds
found in conventional p-SiN:H. Free nitrogen orbitals are
1414 J. Appl. Phys., Vol. 62, No.4, 15 August 1987 satisfied with stable N-H bonds. The presence of hydrogen is
necesswy for the formation of these high-quality, plasma
deposited silicon nitride films.
e 2) The deposition rate can be extremely high, up to
1600 A/min. This is a factor of 6-10 faster than uncontrolled
p-SiN:F films deposited from SiFz/SiFcHz-N z discharges.
(3) Films with F/Si less than-O.S (class I) were sta
ble, while those with more than -0.5 (class II) were oxy
genated on exposure to air or moisture. These limits are
probably dependent on deposition temperature.
(4) p-SiN:F has electrical and optical properties (di
electric constant, breakdown strength, resistivity, optical ab
sorption edge, etc.) which compare favorably to high-tem
perature, chemical-vapor-deposited silicon nitride.
(5) The films have a refractive index between 1.8 and
2.2, which is slightly lower thanp-SiN:H films with the same
Si/N ratio. Film density ranges from -2.6 to 2.8 g/cm3.
(6) Stable, oxygen-free class I films can be prepared
with less fluorine than the class IIp-SiN:F films reported by
Fujita et 01.2-4 Class I films are made using a low [NF3]/
[SiH41 in the feed. Thep-SiN:F films can be prepared with a
Si-H content below 1 X 1021 cm --3.
(7) Air exposure or water rinsing leads to oxygen incor
poration in class II p-SiN:F films. During stabilization of
class II films, fluorine and nitrogen are replaced by oxygen.
e 8) p-SiN:F film has a high buffered HF etch rate (300-
2000 A/min), and this rate increases rapidly with fluorine
content.
ACKNOWLEDGMENTS
We wish to thank to F. A. Baiocchi and H. S. Luftman
for the RBS and the AES analyses, R. C. Frye for his kind
help with electrical properties determination, T. M. Dun
can, T. W. Root, and J. M. Cook for their assistance with IR
spectroscopy, and D. D. Lisi for her contributions to the
mass spectrometric studies.
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132.174.255.116 On: Wed, 24 Dec 2014 02:27:28 |
1.574517.pdf | Insulator interface effects in sputterdeposited NbN/MgO/NbN
(superconductor–insulator–superconductor) tunnel junctions
S. Thakoor, H. G. Leduc, J. A. Stern, A. P. Thakoor, and S. K. Khanna
Citation: Journal of Vacuum Science & Technology A 5, 1721 (1987); doi: 10.1116/1.574517
View online: http://dx.doi.org/10.1116/1.574517
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Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 129.174.21.5 On: Tue, 23 Dec 2014 14:45:15Insulator interface effects in sputter .. deposited NbN/MgO/NbN
(superconductor-insulator-superconductor) tunnel junctions
s. Thakoor and H. G. Leduc
Jet Propulsion Laboratory, California Institute o/Technology, Pasadena, California 91109
J. A. Stern
California Institute o/Teclmology, Pasadena, California 91125
A. P. Thakoor and S. K. Khanna
Jet Propulsion Laboratory, Cal(fornia Institute a/Technology, Pasadena, California 91109
(Received 5 November 1986; accepted 22 December 1986)
All refractory, NbN/MgO/NbN (superconductor-insulator-superconductor) tunnel junctions
have been fabricated by in situ sputter deposition. The influence ofMgO thickness (0.8-6.0 nm)
deposited under different sputtering ambients at various deposition rates on current-voltage (I
V) characteristics of small-area (30 X 30 pm) tunnel junctions is studied. The NbN/MgO/NbN
trilayer is deposited in situ by dc reactive magnetron (NbN), and rf magnetron (MgO)
sputtering, fonowed by thermal evaporation of a protective Au cap. Subsequent
photolithography, reactive ion etching, planarization, and top contact (Pbl Ag) deposition
completes the junction structure. Normal resistance of the junctions with MgO deposited in Ar or
Ar and N2 mixture shows good exponential dependence on the MgO thickness indicating
formation of a pin-hole-free uniform barrier layer. Further, a postdeposition in situ oxygen
plasma treatment of the MgO layer increases the junction resistance sharply, and reduces the
subgap leakage. A possible enrichment of the MgO layer stoichiometry by the oxygen plasma
treatment is suggested. A sum gap as high as 5.7 mV is observed for such a junction.
f. INTRODUCTION
All refractory, high Tc material based, superconductor-in
sulator-superconductor (SIS) tunnel junctions (e.g., NbNI
MgO/NbN) are ideally suited for use as quantum mixers in
submillimeter wave heterodyne receivers. 1.2 The robustness
of all such refractory devices makes them extremely stable
for repeated thermal cycling and long-term use. For an SIS
tunnel junction to be used as a sensitive low-noise mixer for
high frequencies (up to 1500 GHz), it should have a high
superconducting sumgap (D,::;: -6 mY), a high-subgap leak
age resistance R,g computed at one-half the sumgap value
(-3 mY), and a sharp nonlinearity, i.e., a small flV, the
width of the quasiparticle tunneling onset. The quality of the
junction is usually expressed as the quality factor V m
= leR,!" where Ie is the Josephson critical current.
Recently3-5 sumgap values in excess of 5 m V have been
demonstrated for NbN/MgO/NbN junctions proving the
advantage of using a thermodynamically stable artificial
barrier like MgO over the native oxide barrier. The native
oxide barrier grown on base NoN is known to cause reduc
tion of the energy gap of the countere1ectrode through a re
action ofNbN with oxygen atoms from the barrier (Nb20s)
at the interface, a critical region in the junction. In addition
to the overall quality of the bulk of NbN electrodes, it is
crucial to have the high Tc' B 1 phase ofNbN at the NbN/
MgO interfaces. Further, the interfaces should also be physi
cally smooth and contamination free. To achieve this, in situ
deposition of the junction trilayer, NbN/MgO/NbN is the
usual choice. Reactive dc magnetron sputtering has been
successfully used2-6 to deposit the NbN base and counter
electrodes. A variety oftechniquesl•3,7 have been explored to obtain uniform, homogeneous, mechanically and chemically
stable, thin MgO layers. Thermal oxidation or ion beam oxi
dation of thin magnesium overlayers has been used by Tal
vacchio et al. 7; the resulting junctions were quite leaky, prob
ably due to the tendency of un oxidized Mg to diffuse into the
base electrode. It has been further suggested8,9 that polyepi
taxy or single-crystal epitaxy (during the junction trilayer
deposition) may be useful in realizing the full sumgap in an
all NbN junction. Normally high substrate temperature
and/or single-crystal substrates are used to induce such epi
taxial growth. In a recent study! in our laboratory, an anom
alous dependence of the tunneling resistance on barrier
thickness is observed for junctions with electron beam (e
beam) deposited MgO barriers. The Stranski-Krastinov
mechanism for epitaxial growth has been proposed to ex
plain this data. In this growth mode, the first monolayer of
MgO grows extremely coherently, essentially to minimize
the free energy at the surface. However, later growth ofMgO
occurs by nucleation, resulting in a barrier of somewhat non
uniform thickness. Tunneling through the thinner barrier
regions then dominates the junction's 1-V characteristics.
Alternatively, rf sputtering of MgO has been successfully
used by Shoji et aU-6 yielding high-quality junctions (D,};
= 5.4 mY) with barrier thickness as low as 0.5 nm without
any special parameters to induce MgO epitaxy. Moreover, rf
sputtering of MgO does not require ultrahigh vaccum; such
an "all-sputter-deposition" sequence thus allows quick
changes between NbN deposition and MgO deposition by
simple repositioning of the substrates with respect to the
sputter targets. This may have some effect in minimizing
contamination of the junction interfaces from the chamber
ambient. Futhermore, vapor flux in sputter deposition
1721 J. Vac. ScI. Techno!. A 5 (4), JullAug 1987 0734-2101/871041721-05$01,00 © 1987 American Vacuum Society 1721
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reaches the substrate over a wide range of incidence angles
due to the multiple scattering and small mean free path in the
sputtering ambient. This is in contrast with the line of sight
deposition by a technique such as e-beam evaporation in
UHV. Sputter deposition is therefore expected to yield a bet
ter coverage and thickness uniformity over the substrate,
which may be further enhanced by substrate rotation.
In this paper, we report on the influence of deposition
conditions on the performance of sputter deposited NbN/
MgO/NbN tunnel junctions. MgO films are deposited with
substrate rotation in ambients of high-purity Ar and
Ar + N2 mixture. The junction resistance as a function of
effective MgO thickness ranging from 0.8 to 6.0 nm is stud
ied.
II. EXPERIMENTAL DETAILS
A. Deposition
A four-layer structure composed of a dc reactively sput
tered NbN base (-360 nm), rfmagnetron sputtered MgO
barrier (~O.8 to 6.0 nm), a counter NbN electrode (-120
nm), and finally a thermally evaporated protective gold
coating (-50 nm) was deposited in situ on sapphire sub
strates. An ultrahigh-vacuum system described elsewhere6
was modified to include an rf magnetron sputtering gun for a
2-in.-diam MgO target, in addition to a dc magnetron sput
tering gun for a 2-in.-diam. Nb target, a substrate holder
which can be rotated axially, and a source for thermal evapo
ration of gold. A bottom-up deposition geometry was used
with the substrates ~ 6.25 cm away from the Nb target and
-10 cm away from the MgO target. No intentional sub
strate heating or cooling was utilized. The superconducting
NbN films for the base as well as the counterelectrode were
deposited by dc reactive magnetron sputtering of the Nb
target (99.99% pure) in a mixture of Ar and N2 gases
(99.999% pure). The nitrogen consumption injection char
acteristics for the reactive sputter deposition of NbN estab
lished earlier6 were used as guidelines for fine tuning the
deposition parameters to yield high Tc NbN in the modified
configuration of the system. Typical characteristics of the
NbN films utilized for the junction fabrication are as fol
lows: Superconducting transition temperature Tc: -16 K,
transition width: 0.2 K, resistivity: 175 pO cm, and resis
tance ratio R300 K/R29 K: 0;95. The MgO barrier layer was
deposited by rf magnetron sputtering in pure argon (-13
mTorr). However, barrier layers for some junctions were
deposited by sputtering of MgO in a mixture of Ar (-13
mTorr) and N2 (~3.6mTorr), the same gas composition as
selected for a deposition of high-Tc NbN films. In addition
to a study of the effect of nitrogen presence during sputtering
ofMgO on its film quality, such a sequence also reduced the
time between the deposition of the subsequent layers since
the sputtering ambient did not require a readjustment for
each layer.
To obtain a uniform coverage of MgO film over the NbN
base layer, and a better control over the film thickness/depo
sition rate, particularly for thin ( < 2.5 nm) MgO layers,
substrate rotation (-30 rpm) was used. Thicker (2.5-6.0
nm) MgO films, however, were obtained by deposition on
stationary substrates. The MgO thickness was varied by
J. Vac. Sci. Technol. A, Vol. 5, No.4, Jul/Aug 1987 varying the product of the power applied to the target and
the total deposition time. At 400 W of power, a direct depo
sition rate of -2 nm/min was obtained, as measured on a
precalibrated quartz-crystal oscillator, whereas, with rota
tion, the effective deposition rate became -0.4 nm/min.
The values of barrier thickness so controlled are accurate to
-0.0 15 nm. The MgO thickness was systematically varied
in the range of -0.8 to 6.0 nm. The effect of an additional
post-MgO-deposition, in situ plasma oxidation treatment
(in 75 mT pressure of99.999% pure oxygen) at 500 V for 3
min was studied in some junctions particularly with thin
«2.5 nm) MgO layers.
B. Junction patterning
The gold cap over the junction trilayer prevented oxida
tion of the NbN counterelectrode top surface on exposure of
the deposited quadlayer to atmosphere. The fabrication
steps are shown schematically in Fig. 1. Standard photolith
ography (photoresist AZ4330), was used to mask the area of
the junction (30 X 30 pm). Selective reactive ion etching of
the top gold layer by CCIF3, followed by etching through the
NbN counterelectrode with CF4, defined the junction area in
the form of a mesa structure. Next the base electrode was
electrically isolated and the mesa structure was planarized
by thermally evaporating an SiO layer (-300-500 nm).
1. DE POS mON
Ii' < J ~ ( J < ( 2 ? ( U ( ~~~N(~~2~~~)
, _:"" -,- -- A ,_ _ ;:::-1'.190 (0.8-6 nm)
L SUBSTRATE 7 ---NbN (360 nm) 7 7--SAPPH IRE
2. JUNCTION DELI NEATION (PHOTOLITHOGRAPHY & R IE)
3. PLANARIZATION
4. LIFT OFF AND CONTACT
~~~ "~_---Jr -----CONTACT (Pb/Ag)
~,?(?«~
FIG. 1. Schematic representation of the junction patterning process: (a)
deposition, (b) junction delineation (photolithography and RIEl, (cl
planarization. and (d) liftoff and contact.
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Finally the photoresist mask is lifted off and a contact elec
trode (Pbl Ag) is thermally evaporated onto the gold cap to
complete the structure. In essence, other than the gold etch
step the patterning process is similar to that used by Shoji et
aI.1O The current-voltage (1-V) characteristics of these SIS
junctions were measured at 4,2 K to study their electron
tunneling properties and thereby the junction quality.
III. RESULTS AND DISCUSSION
The deposition rate of MgO was found to be the most
important parameter in obtaining good junctions. For lower
sputtering power « 250 W) the junction yield was very
poor. Junction shorts indicated that the MgO layer mostly
suffered from pin holes. A 400-W power level for deposition
was found to be optimum and was used for the following
study. Figure 2 shows a typical I-V characteristic of a junc
tion with -l.O-nm-thick MgO. It has a sumgap of -4.8
mY, normalresistanceRN (at 8 mY) -1 n, and the Joseph
son current Ie -23 mA (-2/3 of the theoretical value as
calculated using the Ambegaokar-Baratoff relation 1I).
The presence of Nz in the sputtering ambient during the
deposition of MgO had little effect on its deposition rate as
well as the barrier quality. Junctions prepared with MgO
deposited with or without Nz showed comparable junction
quality. This suggests that Nz does not interfere, physically
or chemically, with the growth kinetics of MgO; and that the
reduced time gap between the deposition of successive layers
of the tri-structure, expected to reduce the "interface-conta
mination" effects, had undetectable effect on the overall in
terface quality. Sumgaps of the junctions made with or with
out N2 during the MgO deposition, ranged mostly from
-4.5 to ~5.2 mY, however, a sumgap of as high as -5.7
m V (Fig. 3) has been observed. Although large sumgap val
ues are achieved in these junctions, the large tJ. V (-1 m V)
~ z w a:
ll:
::> u
VOLTAGE (2 mV/div)
FIG. 2. Typical current-voltage (I-V) characteristics for a junction with
MgO thickness ~ 1.0 nrn.
J. Vac. Sci. Techno!. A, Vol. 5, No.4, Jul! Aug 1981 VOLTAGE (2 mV/divj
FIG. 3. Current-voltage (I-V) characteristics ofajunctioll with as-deposit
ed MgO thickness ~ 1.8 nrn, followed by plasma oxidation treatment, exhi
biting a sumgap ~" ~ 5. 7 mV and quality factor v,,, -27 m V.
should be primarily attributed to the spatial variation in the
NbN quality over the active area ( -900 pm2) at the junc
tion interface.
Figure 4 shows a plot of the normal resistance (R.II,' at 8
m V) of junctions of varying MgO thickness, deposited with
or without N2, as well as some with a post-plasma-oxidation
treatment. Clearly, the junctions made with MgO
(Ar + Nz) are indistinguishable from those with MgO
(Ar). The linear dependence of log R N on MgO thickness,
down to -0. 8 nm, indicates formation of a coherent, contin
uous layer ofMgO in these junctions. This is unlike the MgO
10000
MgO DEPOSITION PARAMETERS: 0
'" 0 PURE Ar AMBIENT ""
> 1000 • Ar + NZ AME'IENT
E c,. OXYGEN TREATMENT 00
>-« . 100 0 z a::: c,.
u.j u c,. z c,. ;:: 10 en c,.
V'l 0 w a:::
z 0 It
>-1.0 u
Z
::::l
0.1
0 1.0 2.0 3.0 4.0 6. 0
BARRIER THICKNESS (nm)
FIG. 4. Junction resistanceRn (at 8 mY) as a function of as-deposited MgO
thickness.
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barrier recently grown bye-beam evaporation I in our labo
ratory, where the growth is believed to follow the Stranski
Krastinov model. The growth of coherent MgO with uni
form thickness in the present case should be primarily attri
buted to the wide range of incidence angles due to the multi
ple scattering of vapor flux in sputtering, further enhanced
by a fast substrate rotation.
If, however, the MgO barrier layer is truly uniform and
coherent, then the rather low subgap resistance (Rsg -7.5 fl
at 3 mY, Fig. 2) suggests the possibility of an inherently
leaky MgO. It is known 12 that MgO films prepared by phys
ical vapor deposition can be off stoichiometric as MgO can
decompose in the vapor phase. On the other hand, smooth,
superstoichiometric MgOx films have been deposited by ion
beam sputtering13 ofMg (using Ar ions in a reactive oxygen
ambient) with improved mechanical and dielectric proper
ties. It is also observed that surface quality of MgO films
could be substantially improved at high temperatures by an
"oxygen treatment." 14
Deposition of MgO in a mixture of Ar and O2 was not
desirable in the present case, since it could cause a degrada-
<' E -
-«
E -8
4
o
-4
2
1
o
-1
I
4
V (mV)
FIG. 5. Current-voltage (1-V) characteristics of junctions, (a) without and
(b) with post-MgO-deposition, in situ plasma oxidation treatment.
J_ Vac_ Sci. Technot A, VoL 5, No.4, Jull Aug 1987 tion of the surface of base NbN by its partial oxidation. As an
alternative, the NbN base was first "sealed" with MgO layer
deposited in pure Ar and then it was subjected immediately
afterwards to an in situ plasma oxidation treatment. It has
been recently reported 15 that "wet" plasma oxidation of thin
Mg films had better success than a dry plasma oxidation
treatment in obtaining good-quality thin, continuous MgO
barrier layers for Mg-MgO-Pb tunnel junctions. In the pres
ent case, however, water vapor was not intentionally added
during the oxidation treatment. Figures 5 (a) and 5 (b) show
I-V characteristics of two junctions, with 0.8-nm-thick
MgO, without and with oxygen plasma treatment, respec
tively. The parameters for the two junctions are llx = 5.1
mY, 5.2 mY, R" = 0.95 n, 6.9 D; R,g = 5 n, 75 fl; and
quality factor Vm = 12, 25, respectively. A substantial im
provement in the subgap leakage resistance is clearly evi
dent. The normal resistance (RN at 8 mY) for a set ofjunc
tions with oxygen treated MgO is plotted in Fig. 4, for
comparison. The increased normal resistance for a given
thickness of MgO is probably an indication of significantly
changed MgO. If the observed R N value of the junction in
Fig. 5(b) is attributed primarily to the change in physical
thickness of as-deposited MgO, it should have changed from
~ 1 to ~ 2 nm. Such a change in MgO film thickness is unex
pected. On the other hand, this treatment may have caused
an oxygen enrichment of the as-deposited MgO giving rise to
a superstoichiometric phase as obtained by Hebard et ai., 13
with improved dielectric properties. Thus, the observed
change in resistance is possibly a cumulative effect of a sig
nificant change in the dielectric properties accompanied
with an associated minor change in the physical thickness of
MgO. Although the junction sumgap and oxygen treatment
of MgO were not directly correlated, the high sumgap
(ll:::; ~5.7 mY, Fig. 3) was realized in a junction with oxy
gen treatment MgO.
IV. CONCLUSIONS
All refractory, sputter-deposited NbN/MgO/NbN junc
tions with sumgap llx as high as 5.7 mY, and quality factor
Vm -27 have been fabricated. In these junctions the forma
tion of coherent, pin-hole-free MgO barrier layers, as thin as
0.8 nm, is confirmed by the exponential dependence of RN
on MgO thickness. Junctions made from trilayers with MgO
deposited in pure argon or argon and nitrogen mixture
showed comparable junction quality. An in situ, post-MgO
deposition, oxygen treatment improved the subgap leakage
considerably, thus improving the quality of the junction.
This improvement is attributed to the oxygen enrichment of
the MgO, enhancing its stoichiometry and thereby its dielec
tric properties.
ACKNOWLEDGMENTS
This work was carried out by the Jet Propulsion Laborato
ry, California Institute of Technology, and was supported by
the National Aeronautics and Space Administration
(NASA) and Strategic Defense Initiative Organization
(SD 10) through an interagency agreement with NASA. We
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benefited greatly from discussions with Dr. John Lambe and
Professor Totn Phillips.
JH. O. Leduc, I.A. Stern, S. Thakoor, and S. K. Khanna, Applied Super·
conductivity Conference, 1986.
2S. Thakoor, H. G. Leduc, A. P. Thakoor, J. Lambe, and S. K. Khanna, J.
Vac, Sci. Technol. A 4, 528 (1986).
3 A. Shoji, M. Aoyago, S. Kosaka, F. Shinoki, and H. Hayakawa, Appl.
Phys. Lett. 46,1098 (1985).
"T. Yamashita, K. Hamasaki, and T. Komata, in Advances in Cryogenic
Engineering-Materials, edited by A. F. Clark and R. P. Reed (Plenum,
New York, 1986), Vol. 32, pp. 617-626.
5 A. Shoji, M. Aoyagi, S. Kosaka, and F. Shinoki, Applied Superconductivi
ty Conference, 1986.
J. Vac. Sci_ Technol. A, Vol. 5, No.4, JullAug 1987 OS. Thakoor, J. L. Lamb, A. P. Thakoor, and S. K, Khanna, J. Appl. Phys.
58,4643 (1985).
'J. Talvacchio, J. R. Gavaler. A. L Braginski, and M. A. Janocko, J. Appl.
Phys. 58, 4638 (1985).
8J. Talvacchio and A. I. Braginski, Applied Superconductivity Conference,
1986.
90.·1. Oya, M. Koishi, and Y. Sawada, J. Appl. Phys. 60, 1440 (1986).
to A. Shoji, F. Shinoki, S. Kosaka, M. Aoyagi, and H. Hayakawa, App!.
Phys. Lett. 41, 1097 (1982).
"V. Ambegaokar and A. Baratoff, Phys. Rev. Lett. 10,485 (1963); 11,
104(E) (1963).
i2Handbook a/Thin Film Technology, edited by L. I. Maissel and R. Glang
(McGraw-Hill, New York, 1970), pp. 1-70.
BA. F. Hebard, A. T. Fiory, S. Nakahara, and R. H. Eick, App!. Phys. Lett.
48,520 (1986).
lOR. Dale Moorhead and H. Poppa, Thin Solid Films 58,169 (1979).
15W. Plesiewicz and J. O. Adler, Phys. Rev. B 34, 4583 (1986).
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1.583687.pdf | Secondary ion mass spectrometry study of Pd‐based ohmic contacts to GaAs and
AlGaAs/GaAs
C. L. Chen, M. A. Hollis, L. J. Mahoney, W. D. Goodhue, M. J. Manfra, and R. A. Murphy
Citation: Journal of Vacuum Science & Technology B 5, 902 (1987); doi: 10.1116/1.583687
View online: http://dx.doi.org/10.1116/1.583687
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/5/4?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 152.14.136.77 On: Wed, 12 Aug 2015 07:24:15Secondary ion mass spectrometry study of Pd-based ohmic contacts
to GaAs and AIGaAs/GaAs
c. L. Chen, M. A. Hollis, L. J. Mahoney, W. D. Goodhue, M. J. Manfra, and R. A. Murphy
Lincoln Laboratory, 1Ylassachusetts Institute a/Technology, Lexington, Massachusetts 02173
(Received 24 November 1986; accepted 16 April 1987)
Secondary ion mass spectrometry (SIMS) has been used to study nonalloyed Zn/Pd!Au (p
type) and Ge!Pd/Au (n-type) ohmic contacts to GaAs. Both contacts have very low contact
resistances and smooth surface morphologies. It appears that Pd, a fast diffuser in GaAs, helps the
diffusion of Zn into GaAs in the Zn!Pd! Au contact. On the other hand, it was found that Pd and
Ge diffused together during the heat treatment of the Pd!Ge! Au contact. SIMS profiles of a Pdf
Ge! Au ohmic contact fabricated upon a high-electron mobility transistor (HEMT) structure
provide two possible explanations for its significantly higher contact resistance than a
conventional NilGe! Au alloyed contact. The SIMS data indicates that Pd may have caused Al
and Ga interdiffusion at the AIGaAs/GaAs interface and that the metallization failed to make
contact to the electron gas because of the interdiffusion.
I. INTRODUCTION
Recently we reported a Zn!Pd/ Au ohmic contact to p-type
GaAs with a contact resistance approximately one order of
magnitude smaller than a Zn! Au contact I and demonstrat
ed a sintered Ge/Pd! Au contact to n-type GaAs2 with a
smoother surface and a contact resistance comparable to al
loyed Ni/Ge! Au contacts. It has been suggested that Pd
accelerates the diffusion of dopants (such as Ge for the n
type dopant) into the GaAs and facilitates the doping of the
surface layer in an ohmic contact.3 Because of the reaction of
Pd and GaAs, it also has been proposed that Pd creates Ga
vacancies to further aid the doping action.3-5 This theory is
supported by our observations that Pd improved the ohmic
contact resistivities to both n-and p-type GaAs.
However, there is very little information available to date
regarding the distribution of different constituents in the me
tallization. The objective of this work was to investigate the
concentration profiles of individual elements in Zn!Pd! Au
and Ge/Pd! Au ohmic contacts to better understand Pd
based nonalloyed ohmic contacts. Secondary ion mass spec
trometry (SIMS) was used to study the diffusion of various
elements in an ohmic contact because of its high sensitivity
for trace concentrations and good resolution for depth pro
filing. In addition, we also examined ohmic contacts to an
AIGaAs!GaAs heterojunction structure used for the high
electron mobility transistor (HEMT).
II. EXPERIMENTAL PROCEDURES
Contacts were made on n-or p-type GaAs epitaxial layers
grown on semi-insulating substrates by molecular-beam epi
taxy. Silicon was used as the n-type dopant at a concentra
tion of 1 X 1018 cm-3• Beryllium was used as the p-type do
pant at a concentration of 1 X 1019 cm-3• The thickness of
both the n-type and p-type epitaxial layers was approximate
ly 3000 A. The HEMT structure consisted of a l-,um-un
doped GaAs buffer layer, a 30-A-undoped AIGaAs spacer
layer, a 500-A n-type AleJ3 Gao.7As layer doped to 1 X 1018
cm -3, and a 300-A n + GaAs cap layer doped to approxi
mately 2X lOIS cm-3• The GaAs surface was cleaned and etched in a dilute am
monium hydroxide-hydrogen peroxide solution prior to the
metallization. All the metal layers were deposited by elec
tron beam evaporation after a single pumpdown. To mea
sure the specific contact resistance, transmission line pat
terns were defined by photoresist liftoff. After liftoff the
contacts were encapsulated wth 3500 A of phosphosilicate
glass (PSG) at 250·C on a graphite heater strip. Then the
temperature was gradually increased to 450°C and held at
this temperature for 30 s while the deposition of PSG was
. continued and the contact was annealed.
A Cameca IMS 3/ SIMS instrument using an ot primary
sputtering beam was used in this study. In the SIMS profil
ing, 02+-beam was rastered to produce a 500 X 500-.um
crater, and the secondary ion optics were adjusted to sample
the secondary ions from a central area only 80.um in diame
ter. This technique not only suppresses collection of second
ary ions from the crater edge, but also enhances depth reso
lution by minimizing the curvature of the crater floor in the
sampling area.
In order to determine the diffusion depths of various con
stituents of the metallization into GaAs, the sputtering rate
of GaAs was used for depth calibration. The concentration
(atoms!cm3) profiles for AI, Be, Ge, Ni, Zn, and Pd were
calibrated by adjusting each profile so that the total area
under the curve (atoms/cm2) matched the areal density of
that element known to be in the sample. On the other hand,
the concentration profiles for Au, Ga, and As were calibra
ted by setting the fiat portion of the profiles away from the
interface to the known concentrations in the solid. It is
known that in a multielement matrix the ion yield of one
element can be affected by the presence of the others. 6. 7 As a
result, in complex systems such as the ohmic contacts in this
work, the measured concentration values for each element at
a given depth can be in error by a factor of 2 to 3. Therefore,
throughout this work we tried nbt to draw any conclusions
from the absolute value of concentration. Instead, we used
the relative concentration as a function of depth for each
element to obtain semiquantitative measurements of the dif
fusion depths.
902 J. Vac. Sci. Technol. B 5 (4), Jull Aug 1987 0734-211 X/871040902-oS$01.00 © 1987 American Vacuum Society 902
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III. RESULTS
A. Contacts to pmtype GaAs
The Zn/Pd/ Au ohmic contact we have successfully fabri
cated to p-type GaAs has been described elsewhere. I The
contact has low specific resistance, a smooth surface, and
good adhesion,and is thermally stable. In the current study
we also fabricated Zn/ Au and Pdl Au contacts using the
same annealing procedure as that for the Zn/Pd/ Au con
tact. The SIMS results for these contacts will be compared
and discussed in this section.
0.11 1.0
{a}
t 016 '--_--l __ --' __ --' __ LJ....!!CBe!L----l
o 0.2 0.4 O.S O.B 1.0
(bl DEPTH (/lm)
FIG. 1. SIMS depth profiles ofa Pd (500 All Au (3000A) contact top-type
Ga.l\s. (a) As deposited. (b) After 30 s of heat treatment at 450'C. The
sputtering rate of Au is higher than that of the GaAs which was used for
calibration. 'TIlerefore, the apparent thickness of the Au in this profile is
thinner than that of the actual layer. In (b) the area under the Be curve is
the same as that in (a) to satisfy the conservation ofthe total amount of Be.
J. Vac. Sci. Techno!. S, Vol. 5, No.4, Jul/ Aug 1987 1.Pd/Au
Figures lea) and l(b) show SIMS depth profiles ofa Pd
(500 A)/Au (3000 A.) contact top-type GaAs before and
after annealing, respectively. The specific contact resistance
of the Pdf Au contact is in the low 10-4 n cm2 range which
is much higher than that of the Znl Au or Zn/Pdl Au con
tact. In Fig. 1, we calibrated the sputtering rate by assuming
that the full width at half-magnitude (FWHM) in the Be
profile is equal to the p-type epitaxial-layer thickness. Since
the sputtering rate of Au is much faster than the other metals
and the GaAs, the apparent thickness of the Au layer in the
SIMS profiles is much less than its actual thickness.
A comparison of Figs. 1 (a) and 1 (b) shows that Be piles
up near the GaAs surface as a result of a 450 ·C heat treat
ment for 30 s. We speculate that the Pd reacts with the
GaAs, allowing Ga to diffuse into the Pd and Au layers,
thereby creating Ga vacancies which help the outdiffusion of
Be. It is likely that this diffusion can take place even at the
moderate temperatures used for annealing because Be has a
high diffusion coefficient. However, in view of the compara
tively high contact resistance, this diffusion is not sufficient
to ensure a good ohmic contact. We have observed this pile
up of Be only when Pd is present in the metallization. For
example, as discussed in later sections, the accumulation of
Be near the GaAs surface was also observed in the Zn/Pd/
Au contact but not in the Znl Au contact to the same materi
al. The same Be-outdiffusion phenomenon has also been ob
served in the SIMS depth profile for a Mn/ Au contact to Be
doped GaAs. Ii
A significant amount of Ga outdiffused into the Au layer
during annealing, while the outdiffusion of As was negligi
ble. Therefore, the As profile can be used as a reference for
M
E
'" "-E 1
<:> -
~
z o
t= <t a: ;.... z w
U
Z o u I
I
I
I
I
I
I
I
I -\,_-------- A.
" 1 '1'\-e N2
,j \,
\
\ AFTER HEAT
\ TREATMENT
\
AS
DEPOSITED \
\
\
\
\
L-_____ ~ ____________________ ~
DEPTH
FIG. 2. Calculation of the diffusion distance of II constituent of the metalliza
tion. The position xo• where the concentration of As becomes one-half of its
bulk value, is defined as the metal-GaAs interface.
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T ABI,E r. Measured diffusion distances of Pd, Zn, and Ge in different metal-
lizations.
Metallization Element Diffusion distance
Pd/Au Pd 1770 A
Zn/Au Zn 263 A
ZnIPd/Au Zn 682A
Pd 1390 A
Pd/Ge/Au Pd,Ge 210A
the metaI-GaAs interface in the annealed profile of Fig.
1 (b). The method we used for estimating the depth of Pd
diffusion is illustrated in Fig. 2. In the SIMS depth-profile
plots, we define the metal-GaAs interface Xo as the depth
where the As concentration is one-half of its bulk value. Dis
tances d I and dz are measured from Xo to the positions where
the Pd concentrations equal lie of their peak concentrations
for as-deposited and annealed samples, respectively. The dif
ference between dj (as deposited) and d2 (annealed) was
used as the distance which Pd diffuses during heat treat
ment. This method minimizes the errors due to the spread
ing of the concentration profile caused by sputtering arti
facts and to the uncertainty in the absolute value of the
element concentration. Comparing Figs. 1 (a) and 1 (b), we
measured the diffusion distance ofPd to be 1770 A as a result
of30's of heat treatment at 450"c' The diffusion distances of
this and other metallizations after the heat treatment are
summarized in Table I.
As stated in the work by Olowolafe et al., 5 the penetration
of Pd into GaAs is diffusion controlled. Under the assump
tion that the majority of Pd diffuses into the GaAs during
annealing, the doping profile in the GaAs can be approxi
mated by a Gaussian distribution, that is C(x) = (Qo/
jiTDt)exp( -x2/4Dt). In this expression, C(x) is the con
centration at depth x, Qo is the amount of impurity, D is the
diffusion constant, and t is the diffusion time. A plot ofloga
rithm of the impurity concentration C(x) vs x2 should yield
a straight line with a slope of 1/4Dt. Figure 3 shows such a
plot in which the Pd concentration is plotted against the
square of the depth measured from the peak of Pd. The plot
is a straight line and the slope yields a diffusion coefficient of
2.36X 10-12 cm2/s. This diffusion coefficient is slightly
higher than the 8.55X 10-13 cm2/s measured in Ref. 5.
However, the apparent diffusion coefficient inferred from a
similar plot for an as-deposited sample is approximately
30% that of the annealed sample. Consequently, this meth
od cannot be used to measure any diffusion coefficient which
is three times smaller than the one we obtained for Pd, and
the measured diffusion coefficient of Pd could be overesti
mated by 30%. From this result we believe that the drive-in
of Pd is dominated by the diffusion process and that the
diffusion coefficient is close to that measured by Olowolafe
et al.s
2. Znl Au and ZnlPd/ Au
Figure 4 shows the SIMS profile of an annealed Zn (300
A)/ Au (900 A.) contact to p-type GaAs. This metallization
J. Vac. Sci. Techno!. B, Vol. 5, No. 4, JullAug 1987 M
E u
".,
E o
~
Z o
1= « a: Iz w
(,)
Z o u
1018 L:-----'-----:-'----~ o 0.01 0.02 0.03
FIG. 3. The Pd concentration after heat treatment in a semilog plot against
the square of the depth measured from the peak of Pd.
had a specific contact resistance of low 10-.5 n cm2 after
annealing. The SIMS data shows that, in contrast to the Pdf
Au contact, the concentration of Be in the epitaxial layer was
unchanged by the heat treatment. However, a large amount
of Zn was found in the Au overlay in the annealed sample.
-......... .
..•. E >;-:----------AS
~ 1 022 £~-/(. .... ..
III I \ ......
o I \ ···.· ....... · ....... , ... ·./ ..... ·.····Au
::t I \
-1021 -I ,
Z -. I \
52 -I " I- I , « I I a: 1020 .' I \ I- _1',1 I
~ I U I
~ /'--"+'\
(,) . \
I \·'Vl\A'..~
1018 --I ~\ "Zn
. \ Zn/Au
ANNEALED
1017 ':--'----' __ -':---"" ...... ""'Be"----:~_--J o 0.2 0.4 0.6 O.B 1.0
DEPTH I/-!m\
FIG. 4. The SIMS depth profile of an annealed Zn (300 A)/Pd (900 A)
contact to p-type GaAs.
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We believe that Zn has a tendency to move toward the sur
face of the metallization because it has a high vapor pressure
and can form various compounds with Au at relatively low
temperatures. The diffusion distance ofZn into GaAs. calcu
lated the same way as for Pd, was 263 A.. Clearly, the diffu
sion of Zn is very slow and this could be the reason for the
comparatively high contact resistance of the Znl Au contact.
It is not possible to calculate the diffusion coefficient of Zn
because its depth profile is comparable to the SIMS profile
broadening.
The SIMS profile of an annealed Zn (300 A.)/Pd (400
A)/Au (3000 A) contact is shown in Fig. 5. The average
specific contact resistance of this metallization is in the low
10-6 n cm2 range after heat treatment. We once again ob
served the apparent redistribution of the Be dopant atoms,
similar to the Pdl Au contact shown in Fig. 1 (b). The diffu
sion distances for Pd and Zn are 1390 and 682 A, respective
ly, as listed in Table 1. Although a significant amount of Zn
was also found in the Au layer, the diffusion of Zn into the
GaAs was considerably deeper than for the Znl Au contact.
This is consistent with the premise that the diffusion of the
Zn in the GaAs is enhanced by its coexistence with Pd in the
metallization. The diffusion distance of Pd is only slightly
smaller than that in the Pdl Au contact, indicating that the
presence ofZn did not interfere with the fast diffusion of Pd.
It appears that Pd and Zn diffuse in the GaAs separately
because the ratio of Pd to Zn concentrations changes with
depth in the SIMS profile.
The SIMS data suggest the following explanation for the
fact that the Zn/Pdl Au contact has a lower contact resis
tance than the Znl Au contact to p-type GaAs. The diffusion
of Pd into GaAs speeds up the diffusion of Zn and causes a
thicker layer of GaAs to contain significant concentrations
In/Pd/Au
ANNEALED
10"'0':---~--,,-L----'----'------Il a
DEPTH {f.LmJ
FIG. 5. The SIMS depth profile of an annealed Zn (3ooA)/Pd (400 Al/Au
(3000 A) contact to p-type GaAs.
J. '!lac. Sci. Technol. EI, Vol. 5, No.4, Jull Aug 1987
." ....... ' ................ ;0... . ......... :.;.~.~.:.:-:;" ............... -;.;....... . ...... . ofZn. In addition, the reaction ofPd with the GaAs causes a
large number of Ga vacancies to becomes available. There
fore, more Zn atoms can occupy Ga sites and become electri
cally active. Consequently, a thicker layer of more heavily
doped GaAs is formed in the presence of Pd, which reduces
the contact resistance.
Et Contacts to n~type GaAs
1.PdIGe/Au
As mentioned earlier, we have developed a nonaUoyed
Pd/Gel Au ohmic contact to n-type GaAs.2 Thismetalliza
tion has a specific contact resistance of low 10-6 n cmz,
which is comparable to that of the alloyed Ni/Gel Au con
tact, and the surface is much smoother. The SIMS depth
profile of the heat-treated Pd (300 A)/Ge (400 A.)/Au
(2000 A) contact is shown in Fig. 6. A Si profile is not shown
here because the Si concentration is below the detection limit
of SIMS. Note that the Pd and Ge profiles coincide every
where after heat treatment, indicating that the Pd and Ge
have reacted completely to form germanides. This is in con
trast to the Zn/Pdl Au contacts in which Zn does not com
pletely react with Pd. Therefore, the current SIMS results
support our speculation in Ref. 2 that the alloying of Ge and
Au is inhibited by the formation of stable germanides in this
contact even when it is heated above the Au-Ge eutectic
temperature.
High resolution transmission electron microscopy
(TEM) was also used to examine this nonalloyed ohmic
contact. Figure 7 is the cross-sectional transmission images
ofaPd (300A)/Ge (400A.)/Au (2000 A) contact after
heat treatment. The total metallization thickness measured
with the TEM is 2600 A. The metaUization-GaAs interface
06 1.0
FIG. 6. The SIMS depth profile of the heat-treated Pd (300 A)/Ge (400
A)!Au (2000 Al contact to n-type GaAs. The n-type Si dopant is not
shown here because it is below the detection limit of the SIMS.
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FIG. 7. TheTEM cross section ofaheat-treatedPd (300A)/Ge (400A)/
Au (2000 A) contact to GaAs. The roughness at the metallization-GaAs
interface is approximately 120 A.
is very smooth and the roughness is approximately 120 A.
The diffusion distance ofPd, measured from the SIMS depth
profile, was approximately 210 A which is comparable to the
uniformity at the interface shown in Fig. 7. Consequently, it
can be concluded that the diffusion distance obtained from
the SIMS depth profile may not be accurate, but it is definite
ly much smaller than that for the Pdf Au or Zn/Pd/ Au con
tact.
The concentration profile of Pd in Fig. 6 cannot be ap
proximated by a simple constant-source diffusion theory.
We believe that the Pd and Ge diffuse together as a pair in
the Pd/Ge/ Au contact, so that the diffusion process is much
slower and more co~plicated. Because of the low contact
resistance measured, it appears that Pd can still effectively
create Ga vacancies to help the doping by Ge. However,
from currently available data we cannot conclude to what
degree Pd has helped the diffusion of Ge into the GaAs.
Because of the smooth interface and the small diffusion
depth of the metallizations, the Pd/Ge/ Au ohmic contact is
ideal for a device in which a shallow junction is required.
C. Contacts to a HEMT structure
1. Pd/Ge/ Au and Ni/Ge/ Au
BothPd (300A.)/Ge (400A)/Au (3000 A.) andNi (300
A)/Ge (400 A.)/Au (3000 A.) ohmic contacts were fabri
cated on the HEMT structure described in Sec. n. After 30 s
of heat treatment at 450°C, the average transfer resistances
are 0.84 n mm for Ni/Ge/ Au and 3.55 n mm for Pd/Gel
Au, respectively. Increasing the alloying time to 5 min re
duced the average transfer resistance to 0.198 n mm for the
Ni/Ge/ Au contact, which is low enough to fabricate a good
HEMT.9,10 However, the transfer resistance of the Pd/Ge/
Au contact increased slightly to 4.45 n mm after 5 min of
annealing, remaining much higher than that of the NilGel
Au contact.
The SIMS depth profiles of the Pd/Ge/ Au contact after
J. Vac. ScI. Technol. B, Vol. 5, No.4, JullAug 1981 .... Pd/Ge/Au
5 min ANNEALING
Ga
i
\i
'WX'Pd
\Ge
1017 '--~:----:c'-:---::'-::------,c'-=----l o 0.2 0.4 0.6 0.8
DEPTH (j.im)
FIG. 8. The SIMS depth profile of the Pd (300 A)/Ge (400 A)/Au (3000
A) contact to a HEMT structure. The contact was heat treated at 450 'c for
5 min.
30 s and after 5 min of annealing are very similar, The depth
profile after 5 min of annealing is shown in Fig. 8. In both
sets of profiles the Pd and Ge concentrations coincide every
where and the Al concentration coincides with that ofthe Pd
and Ge at the diffusion front near the AIGaAs/GaAs inter
face where the eleciron gas resides. It appears that the diffu
sion of Al is appreciable only in the presence of the Pd-Ge
compound. Because Pd forms compounds with both Ga and
AI, it can create both Ga and Al vacancies in AIGaAs. These
vacancies could be responsible for promoting the interdiffu
sion ofGa and AI at the AIGaAs/GaAs interface just as Zn
vacancy pairs cause disorder in AIAs-GaAs superlattices. II
Within the accuracy of the measurement, the diffusion dis
tance ofPd-Ge after the 5-min anneal is the same as that for
the 30-8 anneal. In both cases the diffusion stops near the
AIGaAs/GaAs interface. This suggests that the Pd-Ge
compound which diffuses is negatively charged and is im
peded by the potential barrier which exists near the
AIGaAs/GaAs interface.
The SIMS results provide possible explanations for why
the contact resistance of Pd/Ge/ Au is higher than that of
NilGe/ Au on the HEMT structure. First, enhanced inter
diffusion at the AIGaAs/GaAs interface appears to be
caused by the Pd, and no doubt creates many vacancies and
crystal defects. This may destroy the quality of the two-di
mensional electron gas under the ohmic contact and degrade
the ohmic contact resistance. Secondly, because Pd/Ge can
not easily diffuse beyond the AIGaAs/GaAs interface, there
may be a small barrier between the ohmic contact and the
two-dimensional electron gas residing in the GaAs. For
comparison, a SIMS depth profile of a NilGe/ Au contact
after 5 min alloying is shown in Fig. 9. Significant amounts
of the constituents of the metallization were found in the
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M
E " "..
E o
~
Z o
i= « a: Iz
IIJ
U
Z o () Ii min ALLOYING
1 017 '--~-'-----''-----'~----'"--'"'' o 0.2 0.4 0.6 O.B 1.0
DEPTH (I.m)
FIG. 9. The SIMS depth profile of an alloyed Ni (300A)/Ge (400A)/Au
(3000 A) contact to a HEMT structure. The alloying was performed at
450 ·C for 5 min.
GaAs layer under the AIGaAs. In fact, the GaAs layer in
corporates more Ge and Ni than the AIGaAs layer above it
does. We believe that this plentiful distribution of Ge in the
GaAs below the AIGaAs is needed to provide very low con
tact resistance to the electron gas.
IV, SUMMARY AND CONCLUSIONS
We have studied the SIMS depth profiles ofPd-based oh
mic contacts to n-and p-type GaAs as well as to a HEMT
structure. The results can be summarized as follows:
( 1) Pd has proved to be a fast diffuser in GaAs with a
diffusion coefficient <2.36x 10--12 cm2/s at 450°C.
(2) In Zn/Pdl Au contacts, Pd and Zn diffused separately
and Pd was the faster diffuser. It appears that the diffusion of
Pd was not appreciably impeded by Zn while Zn diffusion
was enhanced by Pd.
(3) Pd and Ge formed germanides upon heat treatment,
and the diffusion of these germanides is significantly slower
than that of Pd alone.
(4) The transfer resistance of a Pd/Gel Au contact to a
HEMT structure was significantly higher than that of a Nil
Gel Au contact. We speculate that the Pd/Gel Au metalli
zation failed to make a good con tact to the electron gas result
J. Vac. Sci. Techno!. B, Vol. 5, No.4, Jul/Aug 1987 of either the inability of the Pd/Ge to diffuse beyond the
AIGaAs/GaAs interface or the interdiffusion of Al and Ga.
Because both Pd/Gel Au and Zn/Pdl Au metallizations
formed good ohmic contacts to GaAs, we believe that the
primary role of Pd is to accelerate the creation of Ga vacan~
des. The interdiffusion of Al and Ga at the AIGaAs/GaAs
interface can also be explained by these vacancies generated
by the presence of Pd.
In conclusion, our SIMS depth profile studies support the
model that Pd can sweep the dopants into the semiconductor
as well as create Ga vacancies to facilitate doping. 3 However,
the sweeping effect is less obvious in n-type contacts due to
the formation of germanides. Also, as seen by the TEM
study, the Pd/Gel Au ohmic contact has a very shallow and
uniform interface with the GaAs. From our preliminary re
sults, it appears that Pd/Gel Au is not a good contact to a
HEMT structure. On the other hand, the Pd/Gel Au con
tact could be useful in some heterojunction devices, such as
quantum well structures, in which the apparent inability of
the Pd-Ge to diffuse beyond the AIGaAs/GaAs interface
could be used to advantage.
ACKNOWLEDGMENTS
We are grateful to the SIMS staff of Charles Evans and
Associates, Inc., for their extensive work in this project. We
also sincerely thank R. C. Brooks, K. M. Molvar, and N. J.
Bergeron for their technical support, and G. D. Johnson for
preparing the TEM samples. This work was supported by
the Department of the Air Force and the Department of the
Army.
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1.341624.pdf | Highpower (2.2 W) cw operation of (111)oriented GaAs/AlGaAs singlequantumwell
lasers prepared by molecularbeam epitaxy
T. Hayakawa, T. Suyama, M. Kondo, M. Hosoda, S. Yamamoto, and T. Hijikata
Citation: Journal of Applied Physics 64, 2764 (1988); doi: 10.1063/1.341624
View online: http://dx.doi.org/10.1063/1.341624
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/5?ver=pdfcov
Published by the AIP Publishing
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132.174.255.116 On: Tue, 23 Dec 2014 14:55:48'Y. Hornma, Y. Ishii, T. Kobayashi, and J. Osaka, J. Appl. Phys. 57, 2931
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High-power (2.2 W) cw operation of {111 )-oriented GaAsl AIGaAs
single .. quantum-weU lasers prepared by molecular-beam epitaxy
T. Hayakawa, T. Suyama, M. Kondo, M. Hosoda, S. Yamamoto, and T. Hijikata
Central Research Laboratories, Sharp Corporation, Tenri, Nara 632, Japan
(Received 15 January 1988; accepted for publication 26 April 1988)
High-power (2.2 W) cw operation has been achieved in a (111 )-oriented GaAs/ AIGaAs
graded-index separate-confinement-heterostructure single-quantum-welliaser with the lOO-pm
wide stripe geometry. High differenti.a! quantum efficiency of 81 % has been obtained up to
-1.2 W, and high total power-conversion efficiency of 46% has been achieved at 1.5 W.
Phase-locked laser-diode arrays have been extensively
investigated for achieving the high cw optical-power exceed
ing 1 W. 1-6 In contrast to the phase-locked arrays, the broad
stripe lasers with a wide aperture of 50-100 ,.tm prepared by
molecular-beam epitaxy (MBE) and metalorganic vapor
phase epitaxy have shown a fairly uniform near-field pattern
although no mode-stabilization scheme is employed.7-10
This is due to the excellent uniformity of epitaxial layers
grown by these methods. The very simple fabrication proce
dure for the broad-stripe geometry is attractive for the pro
duction of these lasers for practical applications when it is
compared with the complicated phase-locked array struc
tures. The quantum-well (QW) structure is usually em
ployed for high-power lasers since the low threshold current
and the high differential quantum efficiency in QW lasers are
suitable to reduce the operating current,I-Y,!1 Recently, we
have found that the threshold current density and the
threshold-temperature stability are improved in (111 )-ori
ented QW lasers in comparison with the conventional
(l00)-oriented ones. 12.13 In addition to these improvements
in fundamental properties, the slip line defects in the MBE
grown wafers are eliminated by using the (111 )-oriented
substrate, which results in the high yield of reliable lasers
grown by MBE on (Ill) -oriented substrates. 14
In this communication, the high-power (2.2 W) cwop
eration and the high differential and total power-conversion
efficiencies in a (111 )-oriented GaAsl AIGaAs graded-in
dex separate-confinement-heterostructure (GRIN SCH)
single-quantum-well (SQW) laser with the lOO-,um-wide
stripe geometry are repored.
GRIN SCH SQW laser diodes in the present study were
grown by MBE on Si-doped (111 )B-GaAs substrates with the misorientation of OS toward (100). The substrate tem
perature was 720 cC and the group V IIII flux ratio was 2-3.
Details of the crystal growth have been reported elsewhere. 12
The layer sequence of laser diodes is as follows: (1) an n
GaAs buffer layer (1 pm, Si = 5 X 1017 cm -.3), (2) an n
Alo.! Gao9Asbufferlayer (0.2,um, Si = 5X 1017 cm-3), (3)
an n-AlvGal_ vAs compositionally graded buffer layer
(v=O.1 to x, 0.2 p,rn, Si,=5X1017 cm-3), (4) an n
AlxGa1xAs cladding layer (1.4p,m, 8i = 5 X 1017 cm-3),
(5) an undoped A1w Gal ._ wAs GRIN layer (w was parabo
lically graded from x to y, 0.15 pID), (6) an undoped GaAs
QW (60 A ), (7) an undoped AlwGa1_ wAs GRIN layer
(w = y to x, 0.15 ,urn), (8) a p-Alx Gal __ xAs cladding layer
(l/im, Be = 5X 1017 cm-3), (9) ap-GaAs cap layer (0.2
p,m, Be = 5 X 1017 cm-3), and (10) ap-GaAs contact layer
(0.2,um, Be = 5 X 1018 cm-3). The SiNx film was deposited
by plasma assisted chemical vapor deposition and a lOO-,um
wide stripe was opened by chemical etching. n-and p-side
ohmic contacts were formed with AuGe/Ni/ Au and
AuZnl Au, respectively. The cavity length was 375,urn. The
top p-GaAs contact layer was heavily doped with Be in order
to reduce the contact resistance, which is very important to
reduce the heat dissipation and to increase the power-con
version efficiency. 11 The front and the rear facets were coat
ed with the quarter-wavelength-thick A1203 and and the
combination of the quarter-wavelength-thick A120J and
amorphous Si films so as to give the refiectivity of 4% and
90%, respectively. Laser chips were mounted on the
Mol Au-coated Cu heat sinks with the In solder with the
junction-down configuration. 15
In order to increase the maximum output power or the
catastrophic optical damage, it is important to reduce the
2764 J. Appl. Phys. 64 (5),1 September 1988 0021-8979/88!172764-03$()2.40 @ 1988 American Institute of Physics 2764
..................... -..........................•.•.....•.•.......• ) ...... -.-... .
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132.174.255.116 On: Tue, 23 Dec 2014 14:55:48optical power density at the mirror facet. In the case of the
GRIN SCH SQW structure, the optical intensity profile per
pendicular to the junction plane is determined by the vari
ation of the AlAs mole fraction in the GRIN layer. We pre
pared GRIN SCH SQW lasers with various AlAs mole
fractions x and y. In Fig. 1 is plotted the full angle at half
maximum Bl of the far-field pattern perpendicular to the
junction plane as a function of the change in the AlAs mole
fraction in the GRIN layer b.x ( = x -y). As I1x is de
creased from 0.6 to 0.2, (}l decreases from 6rto 43°, and thus
the near-field pattern perpendicular to the junction plane
becomes broader. Therefore, the increase in the maximum
output power is expected by decreasing Ax.
The performance characteristics at high output powers
were compared for lasers with x = 0.5 and 0.7, and y = 0.2;
thus b.x is 0.3 and 0,5. Light output-current curves under the
cw operation at room temperature are shown for these two
devices in Fig. 2. The important device parameters and char
acteristics in these devices are summarized in Table I. The
maximum output power increases from 1.6 to 2.2 W by de
creasing x from 0.7 to 0.5 to reduce the optical power density
at the mirror facet. In the case of x = 0.5, the series resis
tance is as low as 0.38 n and the differential quantum effi
ciency is as high as 81 % up to -1.2 W. As a result, the total
power-conversion efficiency is as high as 46% at 1.5 W. In
the present Si-doped AIGaAs grown by MBE, the resistivity
becomes slightly lower by reducing the AlAs mole fraction,
which improves the power conversion efficiency. The series
resistance of 0.38 n is the lowest ever reported for lasers with
a l00-.um-wide aperture.3•11 The further reduction in the se
ries resistance will be possible by increasing the Si and Be
concentrations in the cladding layers and by doping Si and
Be in the GRIN layers. In addition to the low series resis
tance, the lower threshold~temperature sensitivity in (111)
oriented QW lasers compared with (lOO)-oriented ones re
duces the junction temperature, which is advantageous for
achieving the high-power cw operation.
70
GRIN SCH saw • . /'
:;0-
60 / /S
/
I • --I dI
Q,j ,
'U
~ 50 I.
I • cE I • ,
4 I
300 0.5
L1X (Xdad -;(barril.'r)
FIG, 1. Dependence of the full angle at half maximum of the far-field pat
tern perpendicular to the junction plane 81 on the change in the AlAs mole
fraction t:.x in the graded-index layer from the cladding layer side to the
quantum-well side.
2765 J, Appl. Phys., Vol. 64, No.5, 1 September i 988 cw
2
1 2 3
CURRENT (A)
FIG. 2. cw light output-current characteristics of the devices with the AlAs
mole fraction in the cladding layer x of 0.5 and 0.7.
The typical far-field patterns and the corresponding
near-field patterns of the devices with x = 0.5 (left side) and
x = 0.7 (right side) are displayed in Figs. 3(a) and 3(b),
respectively. The near-field pattern is fairly uniform al
though no mode-control scheme is employed. This is due to
the extremely uniform epitaxial layers grown by MBE. In
addition to the thickness, composition, and doping unifor
mity, the uniformity of the quality of epitaxial layers grown
by MBE on (111 )-oriented substrates is considered to be
excellent since the yield of reliable lasers is extremely high
due to the elimination of the slip line defects. 14 The shape of
the far-field pattern can be divided into two categories; that
is, the single-lobe-like far-field pattern shown for x = 0.5 in
Fig. 3 and the double-lobe-like one shown for x = 0.7 in Fig.
3. As a result of observing these properties for several differ
ent lots, we found a correlation between the near-field and
the far-field patterns as shown in Fig. 3. In the case of devices
with a singJe-lobe-like far-field pattern, the optical intensity
in the near-field pattern decreases as the position is moved
from the center to the side in the stripe. By contrast, devices
with a double-Iobe-like far-field pattern show the near-field
pattern where the optical intensity increases as the position
TABLE I. Series resistance (R,), maximum differential quantum efficiency
(1Jdmax ), maximum power conversion efficiency ('Y}pmax)' and maximum
light output power (Pmax) for lasers with the AlAs mole fraction in the
cladding layer x = 0,5 and 0,7.
x R, (n) 1(dmax (%) llpma"K (%) Pm .... .,. (W)
0.5 0.38 81 46 2.2
0.7 0.67 73 34 1.6
Hayakawa et al. 2765
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132.174.255.116 On: Tue, 23 Dec 2014 14:55:48IH9728
(a)
(b) 100}Jm
DISTANCE 11'64112
, ! ,
-20 0 20
ANGLE (deg.)
100 }Jm
DIS T ANCE
FIG. 3. (a) Typical far-field patterns and (b) corresponding near-field pat
terns oftne devices with the AlAs mole fraction in the cladding layer of 0.5
(left side, No.79728) and 0.7 (right side No. 64112), Measurements were
carried out at an output power of 200 m W cw.
is moved from the center to the side. This result is inconsis
tent with the previous result. 8 The difference in the stripe
structure or the current injection scheme possibly results in
the different modal properties since the transverse mode is
strongly affected by the spatial gain-loss distribution. In the
present devices with the SiNx-delineated stripe geometry,
the lateral current spreading is very large and it is larger for
the devices with the lower AlAs mole fraction in the dad
ding layer due to the lower resistivity as listed in Table I.
Thus, the near-field patterns shown in Fig. 3 are mainly de
termined by the lateral carrier distribution in the QW. The
single wide-stripe lasers in this study operate with the multi
ple transverse modes although the single-Iobe-like narrow
far-field pattern corresponds to the relatively large coherent
width. Further experimental and theoretical analyses are
necessary to understand the modal characteristics in the
wide stripe-geometry lasers.
2766 J. Appl. Phys., Vol. 64, No.5, 1 September 1988 In summary, the high-power (2.2 W) cw operation has
been achieved in a (111 )-oriented GaAsl AIGaAs GRIN
SeH SQW laser with the lOO-,um aperture prepared by
MBE. The differential quantum efficiency is as high as 81 %
up to ~ 1.2 Wand the total powerconversion efficiency is as
high as 46% at 1.5 W. The fairly uniform near-field and the
single-lobe far-field patterns have been realized, The present
high-power lasers are very promising as efficient excitation
sources in a variety of applications.
Note added in proof. After the submission of this com
munication, 3.7-W cw operation was achieved in a laser with
ax = 0.15 (x = O.S andy = 0.35), the well width of 70 A,
the cavity length of750 pm, and the threshold current of260
rnA.
We would like to thank K Hayashi, I. Fujimoto, and S.
Kataoka for continuous encouragement throughout this
work.
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