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Growth of homoepitaxial Ge(001)2 × 1 by ultrahigh vacuum ion beam sputter deposition
212
Thm Solid Films, 223 (1993) 212 217
Growth of homoepitaxial Ge(001)2 x 1 by ultrahigh vacuum ion beam sputter deposition G. A. Tomasch, Y.-W. Kim, L. C. Markert, N.-E. Lee and J. E. Greene Materials' Science Department, The Coordinated Science Laboratory, and The Materials" Research Laborato~T, U)m ersi O' qf Illinois', 1101 Springfield At,enue, Urbana, IL 61801 (USA)
(Received June 1, 1992: accepted August 25. 1992)
Abstract High quality epitaxial Ge(001)2 x 1 films have been grown on Ge(001) substrates by ultrahigh vacuum (UHV) ion beam sputter deposition. The load-locked multichamber growth system is equipped with in situ reflection high-energy electron diffraction. Sputter deposition was carried out using a 1 keV Kr + ion beam generated by a modified UHV Kaufman-type ion source with post-extraction electrostatic ion optics. All films were 1 gm thick and deposited at a rate of 0.5 iam h ~. Results of plan-view, cross-sectional, and convergent-beam transmission electron microscopy analyses showed that films deposited at temperatures 7~, between 300 and 650 C had a high degree of perfection with no observable defects. Temperature-dependent carrier mobilities were found to be equal to or to exceed the best reported values for bulk Ge. Al-doped p-type films grown at TS= 400 600 c'C exhibited 77 K hole mobilities ranging from 3890 to 1200 cm2V ~s ~ for corresponding hole concentrations between 2.8 × 10 ~6 and 1.5 x 1017cm ~.
1. Introduction Sputter deposition offers potential advantages for the growth o f epitaxial overlayers including uniform deposition over large areas and the inherent ability to exploit the use of low-energy ion-surface interactions during film growth. Low-energy ion irradiation during film growth has been shown to provide enhanced a d a t o m surface mobilities [1-3], lower epitaxial temperatures [4-6], increased d o p a n t incorporation probabilities [7, 8], and better control over d o p a n t depth distributions (including 6-doped layers) [8, 9]. Nevertheless, there have been relatively few reports o f sputter-deposited group IV semiconductors with g o o d electrical properties [10], In this paper, we report results from an investigation o f ultrahigh v a c u u m ( U H V ) ion beam sputter deposition (IBSD) for the growth o f high quality semiconducting layers. These initial experiments were carried out under conditions such that the sputtered particles incident at the substrate had average energies o f ± 12 eV [ 11] while irradiation by other fast particles such as ions or backscattered neutrals was either suppressed or greatly diminished. The deposition system, however, is easily modified to a c c o m m o d a t e ion-surface interaction studies. Homoepitaxial Ge(001)2 × 1 films, with no indication of extended defects observable in plan-view or cross-sectional transmission electron microscopy ( T E M or X T E M ) , were obtained at growth temperatures T~ between 250 and 650 C with deposition rates R = 0.5 p,tm h-~. Temperature-dependent carrier mobilities
0040-6090/93/$6.00
t t p ( T ) were f o u n d to equal or exceed the best reported
results for bulk Ge. F o r example/~p for Al-doped films withp=l × 10~Tcm 3 ranged from 1460cm 2V ~s at 200 K to 341 c m 2 V ~s -t at 15 K reaching a maxim u m of 2 8 0 0 c m 2 V Is ] at 80 K. Hole mobilities at 7 7 K varied from 1200cm2V-~ s z with p = l . 8 x 10]7cm 3 t o 3 8 9 0 c m 2 V Ls ~ w i t h p = 2 . 8 x 101~'cm -~.
2. Ion gun and system design The stainless steel growth system, illustrated in Fig. 1, consists of a c r y o p u m p e d deposition c h a m b e r with a base pressure o f 1 x 10-~0 Torr and a liquid-nitrogentrapped diffusion-pumped load-locked sample introduction c h a m b e r with a base pressure o f 2 x 10 ~ Tort. Samples are exchanged using a magnetically coupled transfer arm. The deposition c h a m b e r contains two modified U H V K a u f m a n - t y p e b r o a d - b e a m ion sources which are m o u n t e d on dual-axis angular manipulators to allow in situ spatial adjustment o f the ion beams. The targets are suspended on an insulating cantilever and can be rotated with respect to the substrate to exploit the angular emission distribution o f reflected neutral particles a n d / o r to optimize film thickness uniformity. Deposition rates are controlled by monitoring the ion neutralization current flowing in the ground-return circuit of each target. Beam focus and alignment are optimized by minimizing the ion current collected at apertures extending from the perimeter o f each target.
.c 1993
ElsevierSequoia. All rights reserved
G. A. Tomasch et al. / IBSD o f homoepitaxial Ge(O01)2 x 1 Gas Inlet--.--~
.............
213
Neutralizer
•
Plasn'~./Beam
To )ad-Lock
Water
i'
Cos In
(a)
Cooling Ge or Si
Fig. 1. A schematic cross-sectional drawing of the U H V IBSD system used in these experiments. Neutralizer Filament Transformer
The ion beams are provided by double-grid, multiaperture, broad-beam ion sources employing hot cathode, magnetically supported d.c. discharges. A cross-sectional drawing of the ion source and a schematic circuit diagram are shown in Figs. 2(a) and 2(b), respectively. The ion source design is similar in concept to the broad-beam ion thruster originally introduced by Kaufman and Reader [12], but with several key improvements and engineering refinements. The beam is focused by a post-extraction unipotential electrostatic ion lens [13] and thus does not require the use of additional or modified grids in the extraction system [14]. The negatively biased center diaphragm of the ion lens also acts as a mirror to suppress electron backstreaming from the downstream beam neutralization device (described below), instead of relying on the accelerator grid potential as in conventional designs [ 15]. The large negative accelerator grid voltages (typically 200 V or more for 1 keV ions) required to suppress electron backstreaming through the apertures of a Kaufman ion gun cause overfocus of individual beamlets, resulting in diverging composite beam profiles. The use of the ion lens, on the contrary, imposes no constraints on the magnitude of the negative accelerator grid voltage, which can now be adjusted for optimal para-axial ion extraction.
Ion L~ Power S Accelerator Grid Power Supply4J
ScreenGrid
Beam I ~ Acceleration Power Supply+
~
Discharge PowerSupply
Filamenl
~1- Gas In
Calhode Filament Transformer J Power Solenoid Supp y +-
II
" fMagnetic Field Solenoid~
(b) Fig. 2. A schematic (a) cross-sectional drawing and (b) circuit diagram of an ion source used in the U H V [BSD system.
Space charge spreading of the composite ion beam is suppressed immediately downstream from the final lens aperture by electron injection from a circular thermionic W filament (typically run at 6 A and 25 V) located outside of the beam. An a.c. transformer with a
214
G. A. Tomasch et al. / IBSD q[" homoepitaxial Ge(O01)2 x 1
negatively biased (15 V d.c.) secondary winding is used to resistively heat the neutralizer and to enhance electron emission. This design was employed instead of the K a u f m a n immersed-wire neutralizer [14] in order to eliminate W contamination of the films. Magnetic field support to increase plasma ionization is provided by solenoid coils, rather than permanent magnets, to facilitate the implementation of in situ reflection high-energy electron diffraction ( R H E E D ) without beam deflection by stray magnetic fields. The solenoid is powered by a current-regulated d.c. supply to maintain constant field strength during operation. The ion source provides narrow beam profiles, high temporal beam stability, high reliability, and low chemical contamination as a result of extremely careful design and materials selection. The extraction grids and their alignment jigs were drilled simultaneously to ensure accurate hole registry between the screen and accelerator. High purity A1203 and Kovar, which are thermal expansion coefficient matched with respect to the Mo grids, were used to fabricate the grid-mounting hardware in order to ensure that the screen and accelerator remain accurately located during the thermal cycling resulting from operation. The discharge chamber is constructed from modified high purity Mo crucibles to minimize metallic contamination of the ion beams resulting from sublimation and sputter erosion. Sputter contamination from the ion lens is eliminated by fabricating the beam-stopping aperture from the same material as the target. The solenoid, which consists of Cu wire hermetically sealed inside 304 stainless steel capillary tubing and insulated with compacted MgO powder, is completely U H V compatible and possesses a high linear turn density (7.3cm t), resulting in low power dissipation. Excessive heating of the ion source is prevented by a cylindrical water cooling jacket incorporated into the source body.
Substrate cleaning consisted of degreasing using successive rinses in trichloroethane, acetone, methanol, and 1.8 × 107Dcm deionized water followed by a UV ozone treatment similar to that described in ref. 16. The passivated wafers were bonded to the electron-bombardment-heated substrate platen using In and immediately inserted into the vacuum system. Final substrate preparation consisted of thermal oxide desorption at T~ > 500 C for 10 min. In situ R H E E D patterns obtained using a 15 keV electron beam incident at a grazing angle of about 1"~' were 2 x 1, with approximately equal intensities in fundamental and half-order diffraction rods, and exhibit sharp Kikuchi lines. Film doping was accomplished by placing small amounts of 99.999% pure A1 on the target surface. The 99.999% pure Kr sputtering gas was further purified by passing it through a Z r - V Fe sponge getter maintained at 400 C [ 17]. During deposition, the pressure in the growth chamber rose to 1 x l0 4 Torr. TEM and X T E M analyses were carried out using a Philips CM-12 operated at 120 kV. Specimen preparation, including grinding and A r - ion milling, followed the general procedure described previously [ 18]. Chemical analyses of as-deposited films were carried out using a Cameca IMS-3 secondary ion mass spectrometer operated with a 16 keV 0 2 primary ion beam when analyzing for both metallic impurities and ~4Kr. A 16 keV Cs- primary ion beam was used to analyze for 217CsKr~ metastable molecular ions [19], but the detection limit was found to be higher than for 84Kr + owing to interference. Resistivity and Hall effect measurements were conducted as a function of temperature between 15 and 200 K using the van der Pauw technique [20]. For this purpose, In contacts were pressure bonded symmetrically to the sample perimeter and Inclad Pt electrical leads were attached to the contacts. The magnetic field strength for the Hall measurements was 10 kG.
3. Experimental procedure 4. Experimental results A single 1 keV Kr+ ion beam was used to sputter a 10 × 7.6 cm ~ 99.9999% pure 43 if2 cm single-crystal undoped Ge target. The ions were extracted from the discharge at a current density of 3 mA cm 2 and focused into an 8.8 mA beam. The optimal beam profile, as judged by the ion current collected on the target aperture, was obtained with the accelerator grid and center lens diaphragm biased to - 1 2 5 V and - 2 0 0 V, respectively. The beam was incident at a polar angle of 60 ~, with the sample centered over the erosion crater such that the target and substrate normals formed an included angle of 48 ~. The substrates used in these experiments were 1 x 10 ~5 cm -3 As-doped Ge(001) single-crystal wafers.
4.1. Characterization oJ" the ultrahigh vacuum s p u t t e r & g ,~;vstem
In these experiments, the ion beam intercepted the target at a polar angle 4~on = 6 0 in order to enhance the target sputtering yield S [21]. T R I M 9 0 Monte Carlo computer simulations [22] showed that, for Ge bombarded by 1 keV Kr + ions, S~-60/S~_o = 4.5 with the sputtered atom distribution being overcosine and skewed ~-30" forward. With this geometry, placing the substrate normal at an angle of 48 ° with respect to the target normal was found to optimize film thickness uniformity consistent with geometric constraints in system construction. Further T R I M 9 0 simulations re-
G. A. Tomasch et al. / IBSD of homoepitaxial Ge(O01)2 ×
vealed that, with ~bio. = 60 °, there is a prominent maximum in the reflected-neutral distribution at a forward polar angle of -~50 ° (near specular reflection) with a full peak width at half-maximum of ~-40 °. Thus, locating the substrate over the center of the erosion profile (~b = 0 °) minimized irradiation by energetic backscattered Kr. The use of a sputtering gas with a heavier mass than the target ( m K r = 8 3 . 8 a m u and rnGe = 72.6 amu) further reduced bombardment of the substrate by backscattered particles. Using the target geometry and ion beam parameters (1 keV Kr ÷, 8.8 mA total beam current) described above, the film deposition rate from a single-crystal 10 × 7.6 cm 2 Ge target was 0.5 gm h -~. Microstylus profilometer measurements showed that the thickness uniformity was greater than 90% over the 1.5 × 1.5 c m 2 substrate. Examination of the eroded Ge target and target aperture revealed that the full width half-maximum convergence angle of the beam was -~4 ° over the 10 cm drift length and that beam fringes were sufficiently narrow that they were intercepted by the target. This result was further confirmed by secondary ion mass spectrometry (SIMS) analyses of IBSD Ge films which showed that the concentrations of system construction materials (Fe, Cr, Ni, Mo and W) were below detection limits (5 x 1015-1 × 1 0 1 6 c m - 3 ) . Plasma-substrate interactions are negligible in these experiments since the d.c. discharge is confined inside the ion gun and resonant charge exchange is minimized by the low chamber pressure (the charge exchange mean free path for Kr at 1 × 1 0 - 4 T o r r is -~1 m) [23]. Figure 3 shows the energy distribution of Kr atoms backscattered from the Ge target within the solid angle subtended by the substrate, calculated using the TRIM90 Monte Carlo computer program. The average energy of reflected Kr atoms bombarding the growing film is 47.5 eV and the energy distribution fr(Er) is highly skewed towards lower energies. The total fractionf~ of Kr ions incident at the target which recoil and
l
215
strike the substrate is only 1.2 × 10 - 3 . On the basis of these results, together with the measured film deposition rate, backscattered Kr provides an average energy to the growing film of 2.2 eV per incident Ge atom. This is considerably less than the average ejection energy of the sputtered particles, ---12 eV atom -1, and the 4.0 eV a t o m - 1 heat of condensation. Thus, the majority of excess energy supplied to the growing film stems from thermal accommodation of translationally hot Ge atoms. No Kr was detected (detection limit ~ 5 × 1017 c m - 3 ) in the films during SIMS analyses.
4.2. Growth of epitaxial Ge A series of 1 gm thick Ge films, together with several multilayer films, were grown on Ge(001)2 x 1 substrates at Ts between 300 and 600 °C. All films were found by a combination of R H E E D , T E M and X T E M to be high quality single crystals with no observable differences. Figure 4 shows a typical 15 keV R H E E D pattern (incidence angle, ~-1 °) along the [110] azimuth from a Ge film grown at Ts = 500 °C. The 90°-rotated two-domain (2 x 1) streak pattern is characteristic of a surface topography that is flat over dimensions larger than the electron beam coherence length ( " 2 0 0 nm) and indicative of two-dimensional growth [24]. Sharp Kikuchi lines were observed at higher electron beam incidence angles. Typical plan-view T E M and high resolution X T E M microgaphs, together with corresponding diffraction patterns, from a Ge film grown at Ts = 500 °C are shown in Fig. 5. The (004) bright field plan-view micrograph is featureless with no evidence of extended defects and the diffraction pattern is consistent with a high quality single crystal. The {111} lattice fringes in
Backscattered Kr . ~ , Ge(O01l (E,) = 47.5 eV ~, f,(EJ = 1.2x10 3
~2
0
50
100
150 E, leVI
200
250
300
Fig. 3. The fraction fr of Kr ions backscattered from the target colliding with the substrate as a function of reflected particle energy
Fig. 4. R H E E D pattern, along a [110] azimuth, from a 1 lam thick homoepitaxial Gel001) film grown by IBSD at T s = 500 °C
216
G. A. Tomasch et al. / I B S D q/'homoepitaxial Ge(O01)2 × 1
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S
'
'
'
' ~ ~
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'
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'
'
'
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IBSDAI-DopedGe T,= 400-600 °C t= 1tam
= presenl results
• ~, =t:irowneTol 10 3
i 1 0 ~6
2
j i
L
= 5
i
~ ~ J l 1 0 ~7
2 X 1 0 ~7
Ionized ImpurityConcentration(cm~)
Fig. 6. Hole mobilities #77K at 77 K as a function of ionized acceptor concentration p for Al-doped l~m thick Ge(001) films grown by IBSD at T , - 4 0 0 600'C. Data from Ga-doped bulk Ge [23, 25] are also shown for comparison.
Fig. 5. (a) Bright field (004) plan-view TEM micrograph with a selected area electron diffraction pattern and (b) high resolution (110) XTEM micrograph and a corresponding convergent beam diffraction pattern from an IBSD Ge(001) film grown at Ts = 500 °C.
the ( l l 0 ) high r e s o l u t i o n X T E M m i c r o g r a p h are cont i n u o u s across the s u b s t r a t e - f i l m interface with no i n d i c a t i o n o f disorder. The p o s i t i o n s o f the higher o r d e r L a u e zone lines in the c o n v e r g e n t b e a m diffraction p a t t e r n s were identical in the film (Fig. 5(b)) a n d substrate, signifying the absence o f m e a s u r a b l e strain.
p - T y p e epitaxial I B S D G e layers, 2 1 lam thick, were g r o w n at T, = 4 0 0 - 6 0 0 °C with A1 c o n c e n t r a t i o n s ranging from l x 1017 to 4 X 101Scm 3 F o r a relatively n a r r o w b a n d g a p s e m i c o n d u c t o r such as G e (Eg at 300 K is 0.68 eV), p - n j u n c t i o n isolation o f h o m o e p i taxial films is not sufficient to prevent significant leakage current due to t h e r m a l excitation o f carriers across the j u n c t i o n d u r i n g r o o m t e m p e r a t u r e H a l l m e a s u r e ments. This leads to artificially low resistivities and Hall voltages. L e a k a g e current m e a s u r e m e n t s carried out as a function o f t e m p e r a t u r e s h o w e d that, for the carrier c o n c e n t r a t i o n s used in the present experiments, adequate carrier isolation (defined here by requiring that the drive current be m o r e t h a n three orders o f magnitude larger than the leakage current) c o n s t r a i n e d Hall m e a s u r e m e n t t e m p e r a t u r e s to less t h a n 200 K. F i g u r e 6 shows hole mobilities ~v77K o f I B S D Ge films m e a s u r e d at 77 K as a function o f the carrier c o n c e n t r a tion p . ,/L77K increased c o n t i n u o u s l y with increasing p and r a n g e d f r o m 1 2 0 0 c m 2 V ~ s -~ with p = 1.8 x 1017 cm s to 3 8 9 0 c m 2 V - ~ s -1 with p = 2 . S x 10~6cm s. These results are, within e x p e r i m e n t a l error, equal to the best r e p o r t e d values for bulk p - t y p e Ge [25] (also shown in Fig. 6 for c o m p a r i s o n ) . A typical plot o f carrier m o b i l i t y v s . t e m p e r a t u r e for I B S D G e is shown in Fig. 7 for an A l - d o p e d film with p = 1 x l0 w cm 3. ~Lp was f o u n d to increase from 1 4 6 0 c m 2 V ~s ~ at 200 K to reach a m a x i m u m o f 2800 cm 2 V ~s t at 8 0 K with a t e m p e r a t u r e d e p e n d e n c e given a p p r o x i m a t e l y by T ~5. The p o w e r law d e p e n d e n c e is in a c c o r d a n c e with that expected for d e f o r m a t i o n potential scattering [25, 26]. D e c r e a s i n g the d o p i n g c o n c e n t r a t i o n results in the p o w e r law e x p o n e n t decreasing c o n t i n u o u s l y to 2.3 because o f an increasing c o n t r i b u t i o n from intervalley optical p h o n o n scattering [25, 27]. F u r t h e r decreases in T below 80 K resulted in ~p initially decreasing with the classical w e a k l y screened ionized i m p u r i t y scattering
G. A. Tomasch et al. / IBSD of homoepitaxial Ge(O01)2 × 1 5x103
5.
lO s
,p a ~ -a
_~ 5
p=lxlO,~cm -~ T~=500°C t'= lp.m
,/ ~
2
• = present resulls o = Tyler et al., bulk C,a-dope0 Ge
102
, 10
20
,
,
,
,
~ , ,I
50
100
, 200
, 400
T (K) Fig. 7. Hole mobilities ,Up as a function of temperature T for an Al-doped (p = 1 x 1017cm -3) 1 gm thick Ge(001) film grown by IBSD at T s = 500 cC. Data from p-type bulk Ge [25] are also shown for comparison.
temperature dependence of T ~5 [26, 28] followed by a more rapid decrease associated with impurity band conduction at temperatures ~<30 K [29-31]. In conclusion, we have shown, for the first time, using a combination of in situ RHEED with post-deposition analyses by TEM, XTEM, and temperature-dependent Hall measurements, that UHV IBSD can be used to grow high quality epitaxial Ge(001)2 x 1 films.
Acknowledgments The authors gratefully acknowledge the financial support of the Joint Services Electronics Program and the Materials Science Division of the US Department of Energy (Contract DE-FG02-91ER45439) during the course of this research.
References 1 L. Hultman, U. Helmersson, S. A. Barnett, J.-E. Sundgren and J. E. Greene, J. Appl. Phys., 61 (1987) 552. 2 M. A. Hasan, J. Knall, S. A. Barnett, A. Rockett, J.-E. Sundgren and J. E. Greene, J. Vac. Sci. Technol. A, 5(1987) 1883. 3 L. Hultman, S. A. Barnett, J.-E. Sundgren and J. E. Greene, J. Cryst. Growth, 92 (1988) 639.
217
4 T. Narusawa, S. Shimizu and S. Komiya, J. Vac. Sci. Technol. A, 5 (1979) 366. 5 G. E. Thomas, L. J. Beckers, J. J. Vrakking and B. R. DeKoning, J. Cryst. Growth, 5 6 ( 1 9 8 2 ) 5 5 7 . 6 C. H. Choi, L. Hultman and S. A. Barnett, J. Vac. Sci. Technol. A, 8 (1990) 1587. 7 M. A. Hasan, J. Knall, S. A. Barnett, J.-E. Sundgren, L. C. Markert, A. Rockett and J. E. Greene, J. Appl. Phys., 65 (1989) 172. 8 W.-X. Ni, J. Knall, M. A. Hasan, G. V. Hansson, J.-E. Sundgren, S. A. Barnett, L. C. Markert and J. E. Greene, Phys. Rev. B, 40 (1989) 10449. 9 L. C. Markert, J. E. Greene, W.-X. Ni, G. V. Hansson and J.-E. Sundgren, Thin Solid Films, 206 (1991) 59. 10 T. Ohmi, H. Iwabuchi, T. Shibata and T. Ichikawa. Appl. Phys. Lett., 54 (1989) 253. 11 L. I. Maissel, in L. 1. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970, chap. 4. 12 H. R. K a u f m a n and P. D. Reader, Am. Rocket. Soc. Paper 1374-60, 1960. 13 B. Paszkowski, Electron Optics, Elsevier, New York, 1968, pp. 210-218. 14 H. R. K a u f m a n in L. Marton (ed.), Advances in Electronics and Electron Physics, Vol. 36, Academic Press, New York, 1974, pp. 265 373. 15 H. R. Kaufman, J. Vac. Sci. Technol. A, 4 (1986) 764. 16 J.-P. No~l, J. E. Greene, N. L. Rowell, S. Kechang and D. C. Houghton, Appl. Phys. Lett., 55 (1989) 1525. 17 D. Lorimer, E. J. Baker, M. Succi and D. K. Weber, Solid State Technol., 33 (1990) 77. 18 J.-P. No61, N. Hirashita, L. C. Markert, Y.-W. Kim, J. E. Greene, J. Knall, W.-X. Ni, M. A. Hasan and J.-E. Sundgren, J. Appl. Phys., 65 (1989) 1189. 19 M. A. Ray, J. E. Baker, C. M. Loxton and J. E. Greene, J. Vae. Sci. Technol. A, 6 (1988) 44. 20 L. J. van der Pauw, Philips Res. Rep., 13 (1958) 1. 21 H. Oechsner, Appl. Phys., 19 (1979) 421. 22 J. Ziegler, J. A. Biersack and U. Litmark, The Stopping and Ranges of Ions in Matter, Pergamon, New York, 1985. 23 B. C h a p m a n , Glow Discharge Processes, Wiley, New York, 1980, pp. 9 11. 24 M. Prutton, Surface Physics, Clarendon, Oxford, 1983, p. 53. 25 D. M. Brown and R. Bray, Phys. Rev., 127(1962) 1593. 26 H. Brooks, in L. Marton (ed.), Advances in Electronics and Electron Physics, Vol. 7, Academic Press, New York, 1955. 27 R. Newman and W. W. Tyler, in F. Seitz and D. Turnbull (eds.), Solid State Physics, Vol. 8, Academic Press, New York, 1959. 28 E. Conwell and V. F. Weisskopf, Phys. Rev., 77 (1950) 388. 29 E. H. Putley, The Hall Effect and Related Phenomena, Butterworths, London, 1960, pp. 161 i65. 30 C. M. Wolfe, N. Holonyak, Jr., and G. E. Stillman, Physical Properties of Semiconductors, Prentice-Hall, Engelwood Cliffs, N J, 1989, pp. 87 88. 31 G. E. Stillman and C. M. Wolfe, Thin Solid Films, 31 (1976) 69.
Thm Solid Films, 223 (1993) 212 217
Growth of homoepitaxial Ge(001)2 x 1 by ultrahigh vacuum ion beam sputter deposition G. A. Tomasch, Y.-W. Kim, L. C. Markert, N.-E. Lee and J. E. Greene Materials' Science Department, The Coordinated Science Laboratory, and The Materials" Research Laborato~T, U)m ersi O' qf Illinois', 1101 Springfield At,enue, Urbana, IL 61801 (USA)
(Received June 1, 1992: accepted August 25. 1992)
Abstract High quality epitaxial Ge(001)2 x 1 films have been grown on Ge(001) substrates by ultrahigh vacuum (UHV) ion beam sputter deposition. The load-locked multichamber growth system is equipped with in situ reflection high-energy electron diffraction. Sputter deposition was carried out using a 1 keV Kr + ion beam generated by a modified UHV Kaufman-type ion source with post-extraction electrostatic ion optics. All films were 1 gm thick and deposited at a rate of 0.5 iam h ~. Results of plan-view, cross-sectional, and convergent-beam transmission electron microscopy analyses showed that films deposited at temperatures 7~, between 300 and 650 C had a high degree of perfection with no observable defects. Temperature-dependent carrier mobilities were found to be equal to or to exceed the best reported values for bulk Ge. Al-doped p-type films grown at TS= 400 600 c'C exhibited 77 K hole mobilities ranging from 3890 to 1200 cm2V ~s ~ for corresponding hole concentrations between 2.8 × 10 ~6 and 1.5 x 1017cm ~.
1. Introduction Sputter deposition offers potential advantages for the growth o f epitaxial overlayers including uniform deposition over large areas and the inherent ability to exploit the use of low-energy ion-surface interactions during film growth. Low-energy ion irradiation during film growth has been shown to provide enhanced a d a t o m surface mobilities [1-3], lower epitaxial temperatures [4-6], increased d o p a n t incorporation probabilities [7, 8], and better control over d o p a n t depth distributions (including 6-doped layers) [8, 9]. Nevertheless, there have been relatively few reports o f sputter-deposited group IV semiconductors with g o o d electrical properties [10], In this paper, we report results from an investigation o f ultrahigh v a c u u m ( U H V ) ion beam sputter deposition (IBSD) for the growth o f high quality semiconducting layers. These initial experiments were carried out under conditions such that the sputtered particles incident at the substrate had average energies o f ± 12 eV [ 11] while irradiation by other fast particles such as ions or backscattered neutrals was either suppressed or greatly diminished. The deposition system, however, is easily modified to a c c o m m o d a t e ion-surface interaction studies. Homoepitaxial Ge(001)2 × 1 films, with no indication of extended defects observable in plan-view or cross-sectional transmission electron microscopy ( T E M or X T E M ) , were obtained at growth temperatures T~ between 250 and 650 C with deposition rates R = 0.5 p,tm h-~. Temperature-dependent carrier mobilities
0040-6090/93/$6.00
t t p ( T ) were f o u n d to equal or exceed the best reported
results for bulk Ge. F o r example/~p for Al-doped films withp=l × 10~Tcm 3 ranged from 1460cm 2V ~s at 200 K to 341 c m 2 V ~s -t at 15 K reaching a maxim u m of 2 8 0 0 c m 2 V Is ] at 80 K. Hole mobilities at 7 7 K varied from 1200cm2V-~ s z with p = l . 8 x 10]7cm 3 t o 3 8 9 0 c m 2 V Ls ~ w i t h p = 2 . 8 x 101~'cm -~.
2. Ion gun and system design The stainless steel growth system, illustrated in Fig. 1, consists of a c r y o p u m p e d deposition c h a m b e r with a base pressure o f 1 x 10-~0 Torr and a liquid-nitrogentrapped diffusion-pumped load-locked sample introduction c h a m b e r with a base pressure o f 2 x 10 ~ Tort. Samples are exchanged using a magnetically coupled transfer arm. The deposition c h a m b e r contains two modified U H V K a u f m a n - t y p e b r o a d - b e a m ion sources which are m o u n t e d on dual-axis angular manipulators to allow in situ spatial adjustment o f the ion beams. The targets are suspended on an insulating cantilever and can be rotated with respect to the substrate to exploit the angular emission distribution o f reflected neutral particles a n d / o r to optimize film thickness uniformity. Deposition rates are controlled by monitoring the ion neutralization current flowing in the ground-return circuit of each target. Beam focus and alignment are optimized by minimizing the ion current collected at apertures extending from the perimeter o f each target.
.c 1993
ElsevierSequoia. All rights reserved
G. A. Tomasch et al. / IBSD o f homoepitaxial Ge(O01)2 x 1 Gas Inlet--.--~
.............
213
Neutralizer
•
Plasn'~./Beam
To )ad-Lock
Water
i'
Cos In
(a)
Cooling Ge or Si
Fig. 1. A schematic cross-sectional drawing of the U H V IBSD system used in these experiments. Neutralizer Filament Transformer
The ion beams are provided by double-grid, multiaperture, broad-beam ion sources employing hot cathode, magnetically supported d.c. discharges. A cross-sectional drawing of the ion source and a schematic circuit diagram are shown in Figs. 2(a) and 2(b), respectively. The ion source design is similar in concept to the broad-beam ion thruster originally introduced by Kaufman and Reader [12], but with several key improvements and engineering refinements. The beam is focused by a post-extraction unipotential electrostatic ion lens [13] and thus does not require the use of additional or modified grids in the extraction system [14]. The negatively biased center diaphragm of the ion lens also acts as a mirror to suppress electron backstreaming from the downstream beam neutralization device (described below), instead of relying on the accelerator grid potential as in conventional designs [ 15]. The large negative accelerator grid voltages (typically 200 V or more for 1 keV ions) required to suppress electron backstreaming through the apertures of a Kaufman ion gun cause overfocus of individual beamlets, resulting in diverging composite beam profiles. The use of the ion lens, on the contrary, imposes no constraints on the magnitude of the negative accelerator grid voltage, which can now be adjusted for optimal para-axial ion extraction.
Ion L~ Power S Accelerator Grid Power Supply4J
ScreenGrid
Beam I ~ Acceleration Power Supply+
~
Discharge PowerSupply
Filamenl
~1- Gas In
Calhode Filament Transformer J Power Solenoid Supp y +-
II
" fMagnetic Field Solenoid~
(b) Fig. 2. A schematic (a) cross-sectional drawing and (b) circuit diagram of an ion source used in the U H V [BSD system.
Space charge spreading of the composite ion beam is suppressed immediately downstream from the final lens aperture by electron injection from a circular thermionic W filament (typically run at 6 A and 25 V) located outside of the beam. An a.c. transformer with a
214
G. A. Tomasch et al. / IBSD q[" homoepitaxial Ge(O01)2 x 1
negatively biased (15 V d.c.) secondary winding is used to resistively heat the neutralizer and to enhance electron emission. This design was employed instead of the K a u f m a n immersed-wire neutralizer [14] in order to eliminate W contamination of the films. Magnetic field support to increase plasma ionization is provided by solenoid coils, rather than permanent magnets, to facilitate the implementation of in situ reflection high-energy electron diffraction ( R H E E D ) without beam deflection by stray magnetic fields. The solenoid is powered by a current-regulated d.c. supply to maintain constant field strength during operation. The ion source provides narrow beam profiles, high temporal beam stability, high reliability, and low chemical contamination as a result of extremely careful design and materials selection. The extraction grids and their alignment jigs were drilled simultaneously to ensure accurate hole registry between the screen and accelerator. High purity A1203 and Kovar, which are thermal expansion coefficient matched with respect to the Mo grids, were used to fabricate the grid-mounting hardware in order to ensure that the screen and accelerator remain accurately located during the thermal cycling resulting from operation. The discharge chamber is constructed from modified high purity Mo crucibles to minimize metallic contamination of the ion beams resulting from sublimation and sputter erosion. Sputter contamination from the ion lens is eliminated by fabricating the beam-stopping aperture from the same material as the target. The solenoid, which consists of Cu wire hermetically sealed inside 304 stainless steel capillary tubing and insulated with compacted MgO powder, is completely U H V compatible and possesses a high linear turn density (7.3cm t), resulting in low power dissipation. Excessive heating of the ion source is prevented by a cylindrical water cooling jacket incorporated into the source body.
Substrate cleaning consisted of degreasing using successive rinses in trichloroethane, acetone, methanol, and 1.8 × 107Dcm deionized water followed by a UV ozone treatment similar to that described in ref. 16. The passivated wafers were bonded to the electron-bombardment-heated substrate platen using In and immediately inserted into the vacuum system. Final substrate preparation consisted of thermal oxide desorption at T~ > 500 C for 10 min. In situ R H E E D patterns obtained using a 15 keV electron beam incident at a grazing angle of about 1"~' were 2 x 1, with approximately equal intensities in fundamental and half-order diffraction rods, and exhibit sharp Kikuchi lines. Film doping was accomplished by placing small amounts of 99.999% pure A1 on the target surface. The 99.999% pure Kr sputtering gas was further purified by passing it through a Z r - V Fe sponge getter maintained at 400 C [ 17]. During deposition, the pressure in the growth chamber rose to 1 x l0 4 Torr. TEM and X T E M analyses were carried out using a Philips CM-12 operated at 120 kV. Specimen preparation, including grinding and A r - ion milling, followed the general procedure described previously [ 18]. Chemical analyses of as-deposited films were carried out using a Cameca IMS-3 secondary ion mass spectrometer operated with a 16 keV 0 2 primary ion beam when analyzing for both metallic impurities and ~4Kr. A 16 keV Cs- primary ion beam was used to analyze for 217CsKr~ metastable molecular ions [19], but the detection limit was found to be higher than for 84Kr + owing to interference. Resistivity and Hall effect measurements were conducted as a function of temperature between 15 and 200 K using the van der Pauw technique [20]. For this purpose, In contacts were pressure bonded symmetrically to the sample perimeter and Inclad Pt electrical leads were attached to the contacts. The magnetic field strength for the Hall measurements was 10 kG.
3. Experimental procedure 4. Experimental results A single 1 keV Kr+ ion beam was used to sputter a 10 × 7.6 cm ~ 99.9999% pure 43 if2 cm single-crystal undoped Ge target. The ions were extracted from the discharge at a current density of 3 mA cm 2 and focused into an 8.8 mA beam. The optimal beam profile, as judged by the ion current collected on the target aperture, was obtained with the accelerator grid and center lens diaphragm biased to - 1 2 5 V and - 2 0 0 V, respectively. The beam was incident at a polar angle of 60 ~, with the sample centered over the erosion crater such that the target and substrate normals formed an included angle of 48 ~. The substrates used in these experiments were 1 x 10 ~5 cm -3 As-doped Ge(001) single-crystal wafers.
4.1. Characterization oJ" the ultrahigh vacuum s p u t t e r & g ,~;vstem
In these experiments, the ion beam intercepted the target at a polar angle 4~on = 6 0 in order to enhance the target sputtering yield S [21]. T R I M 9 0 Monte Carlo computer simulations [22] showed that, for Ge bombarded by 1 keV Kr + ions, S~-60/S~_o = 4.5 with the sputtered atom distribution being overcosine and skewed ~-30" forward. With this geometry, placing the substrate normal at an angle of 48 ° with respect to the target normal was found to optimize film thickness uniformity consistent with geometric constraints in system construction. Further T R I M 9 0 simulations re-
G. A. Tomasch et al. / IBSD of homoepitaxial Ge(O01)2 ×
vealed that, with ~bio. = 60 °, there is a prominent maximum in the reflected-neutral distribution at a forward polar angle of -~50 ° (near specular reflection) with a full peak width at half-maximum of ~-40 °. Thus, locating the substrate over the center of the erosion profile (~b = 0 °) minimized irradiation by energetic backscattered Kr. The use of a sputtering gas with a heavier mass than the target ( m K r = 8 3 . 8 a m u and rnGe = 72.6 amu) further reduced bombardment of the substrate by backscattered particles. Using the target geometry and ion beam parameters (1 keV Kr ÷, 8.8 mA total beam current) described above, the film deposition rate from a single-crystal 10 × 7.6 cm 2 Ge target was 0.5 gm h -~. Microstylus profilometer measurements showed that the thickness uniformity was greater than 90% over the 1.5 × 1.5 c m 2 substrate. Examination of the eroded Ge target and target aperture revealed that the full width half-maximum convergence angle of the beam was -~4 ° over the 10 cm drift length and that beam fringes were sufficiently narrow that they were intercepted by the target. This result was further confirmed by secondary ion mass spectrometry (SIMS) analyses of IBSD Ge films which showed that the concentrations of system construction materials (Fe, Cr, Ni, Mo and W) were below detection limits (5 x 1015-1 × 1 0 1 6 c m - 3 ) . Plasma-substrate interactions are negligible in these experiments since the d.c. discharge is confined inside the ion gun and resonant charge exchange is minimized by the low chamber pressure (the charge exchange mean free path for Kr at 1 × 1 0 - 4 T o r r is -~1 m) [23]. Figure 3 shows the energy distribution of Kr atoms backscattered from the Ge target within the solid angle subtended by the substrate, calculated using the TRIM90 Monte Carlo computer program. The average energy of reflected Kr atoms bombarding the growing film is 47.5 eV and the energy distribution fr(Er) is highly skewed towards lower energies. The total fractionf~ of Kr ions incident at the target which recoil and
l
215
strike the substrate is only 1.2 × 10 - 3 . On the basis of these results, together with the measured film deposition rate, backscattered Kr provides an average energy to the growing film of 2.2 eV per incident Ge atom. This is considerably less than the average ejection energy of the sputtered particles, ---12 eV atom -1, and the 4.0 eV a t o m - 1 heat of condensation. Thus, the majority of excess energy supplied to the growing film stems from thermal accommodation of translationally hot Ge atoms. No Kr was detected (detection limit ~ 5 × 1017 c m - 3 ) in the films during SIMS analyses.
4.2. Growth of epitaxial Ge A series of 1 gm thick Ge films, together with several multilayer films, were grown on Ge(001)2 x 1 substrates at Ts between 300 and 600 °C. All films were found by a combination of R H E E D , T E M and X T E M to be high quality single crystals with no observable differences. Figure 4 shows a typical 15 keV R H E E D pattern (incidence angle, ~-1 °) along the [110] azimuth from a Ge film grown at Ts = 500 °C. The 90°-rotated two-domain (2 x 1) streak pattern is characteristic of a surface topography that is flat over dimensions larger than the electron beam coherence length ( " 2 0 0 nm) and indicative of two-dimensional growth [24]. Sharp Kikuchi lines were observed at higher electron beam incidence angles. Typical plan-view T E M and high resolution X T E M microgaphs, together with corresponding diffraction patterns, from a Ge film grown at Ts = 500 °C are shown in Fig. 5. The (004) bright field plan-view micrograph is featureless with no evidence of extended defects and the diffraction pattern is consistent with a high quality single crystal. The {111} lattice fringes in
Backscattered Kr . ~ , Ge(O01l (E,) = 47.5 eV ~, f,(EJ = 1.2x10 3
~2
0
50
100
150 E, leVI
200
250
300
Fig. 3. The fraction fr of Kr ions backscattered from the target colliding with the substrate as a function of reflected particle energy
Fig. 4. R H E E D pattern, along a [110] azimuth, from a 1 lam thick homoepitaxial Gel001) film grown by IBSD at T s = 500 °C
216
G. A. Tomasch et al. / I B S D q/'homoepitaxial Ge(O01)2 × 1
10~
S
'
'
'
' ~ ~
•
'
'
'
'
'
I
IBSDAI-DopedGe T,= 400-600 °C t= 1tam
= presenl results
• ~, =t:irowneTol 10 3
i 1 0 ~6
2
j i
L
= 5
i
~ ~ J l 1 0 ~7
2 X 1 0 ~7
Ionized ImpurityConcentration(cm~)
Fig. 6. Hole mobilities #77K at 77 K as a function of ionized acceptor concentration p for Al-doped l~m thick Ge(001) films grown by IBSD at T , - 4 0 0 600'C. Data from Ga-doped bulk Ge [23, 25] are also shown for comparison.
Fig. 5. (a) Bright field (004) plan-view TEM micrograph with a selected area electron diffraction pattern and (b) high resolution (110) XTEM micrograph and a corresponding convergent beam diffraction pattern from an IBSD Ge(001) film grown at Ts = 500 °C.
the ( l l 0 ) high r e s o l u t i o n X T E M m i c r o g r a p h are cont i n u o u s across the s u b s t r a t e - f i l m interface with no i n d i c a t i o n o f disorder. The p o s i t i o n s o f the higher o r d e r L a u e zone lines in the c o n v e r g e n t b e a m diffraction p a t t e r n s were identical in the film (Fig. 5(b)) a n d substrate, signifying the absence o f m e a s u r a b l e strain.
p - T y p e epitaxial I B S D G e layers, 2 1 lam thick, were g r o w n at T, = 4 0 0 - 6 0 0 °C with A1 c o n c e n t r a t i o n s ranging from l x 1017 to 4 X 101Scm 3 F o r a relatively n a r r o w b a n d g a p s e m i c o n d u c t o r such as G e (Eg at 300 K is 0.68 eV), p - n j u n c t i o n isolation o f h o m o e p i taxial films is not sufficient to prevent significant leakage current due to t h e r m a l excitation o f carriers across the j u n c t i o n d u r i n g r o o m t e m p e r a t u r e H a l l m e a s u r e ments. This leads to artificially low resistivities and Hall voltages. L e a k a g e current m e a s u r e m e n t s carried out as a function o f t e m p e r a t u r e s h o w e d that, for the carrier c o n c e n t r a t i o n s used in the present experiments, adequate carrier isolation (defined here by requiring that the drive current be m o r e t h a n three orders o f magnitude larger than the leakage current) c o n s t r a i n e d Hall m e a s u r e m e n t t e m p e r a t u r e s to less t h a n 200 K. F i g u r e 6 shows hole mobilities ~v77K o f I B S D Ge films m e a s u r e d at 77 K as a function o f the carrier c o n c e n t r a tion p . ,/L77K increased c o n t i n u o u s l y with increasing p and r a n g e d f r o m 1 2 0 0 c m 2 V ~ s -~ with p = 1.8 x 1017 cm s to 3 8 9 0 c m 2 V - ~ s -1 with p = 2 . S x 10~6cm s. These results are, within e x p e r i m e n t a l error, equal to the best r e p o r t e d values for bulk p - t y p e Ge [25] (also shown in Fig. 6 for c o m p a r i s o n ) . A typical plot o f carrier m o b i l i t y v s . t e m p e r a t u r e for I B S D G e is shown in Fig. 7 for an A l - d o p e d film with p = 1 x l0 w cm 3. ~Lp was f o u n d to increase from 1 4 6 0 c m 2 V ~s ~ at 200 K to reach a m a x i m u m o f 2800 cm 2 V ~s t at 8 0 K with a t e m p e r a t u r e d e p e n d e n c e given a p p r o x i m a t e l y by T ~5. The p o w e r law d e p e n d e n c e is in a c c o r d a n c e with that expected for d e f o r m a t i o n potential scattering [25, 26]. D e c r e a s i n g the d o p i n g c o n c e n t r a t i o n results in the p o w e r law e x p o n e n t decreasing c o n t i n u o u s l y to 2.3 because o f an increasing c o n t r i b u t i o n from intervalley optical p h o n o n scattering [25, 27]. F u r t h e r decreases in T below 80 K resulted in ~p initially decreasing with the classical w e a k l y screened ionized i m p u r i t y scattering
G. A. Tomasch et al. / IBSD of homoepitaxial Ge(O01)2 × 1 5x103
5.
lO s
,p a ~ -a
_~ 5
p=lxlO,~cm -~ T~=500°C t'= lp.m
,/ ~
2
• = present resulls o = Tyler et al., bulk C,a-dope0 Ge
102
, 10
20
,
,
,
,
~ , ,I
50
100
, 200
, 400
T (K) Fig. 7. Hole mobilities ,Up as a function of temperature T for an Al-doped (p = 1 x 1017cm -3) 1 gm thick Ge(001) film grown by IBSD at T s = 500 cC. Data from p-type bulk Ge [25] are also shown for comparison.
temperature dependence of T ~5 [26, 28] followed by a more rapid decrease associated with impurity band conduction at temperatures ~<30 K [29-31]. In conclusion, we have shown, for the first time, using a combination of in situ RHEED with post-deposition analyses by TEM, XTEM, and temperature-dependent Hall measurements, that UHV IBSD can be used to grow high quality epitaxial Ge(001)2 x 1 films.
Acknowledgments The authors gratefully acknowledge the financial support of the Joint Services Electronics Program and the Materials Science Division of the US Department of Energy (Contract DE-FG02-91ER45439) during the course of this research.
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217
4 T. Narusawa, S. Shimizu and S. Komiya, J. Vac. Sci. Technol. A, 5 (1979) 366. 5 G. E. Thomas, L. J. Beckers, J. J. Vrakking and B. R. DeKoning, J. Cryst. Growth, 5 6 ( 1 9 8 2 ) 5 5 7 . 6 C. H. Choi, L. Hultman and S. A. Barnett, J. Vac. Sci. Technol. A, 8 (1990) 1587. 7 M. A. Hasan, J. Knall, S. A. Barnett, J.-E. Sundgren, L. C. Markert, A. Rockett and J. E. Greene, J. Appl. Phys., 65 (1989) 172. 8 W.-X. Ni, J. Knall, M. A. Hasan, G. V. Hansson, J.-E. Sundgren, S. A. Barnett, L. C. Markert and J. E. Greene, Phys. Rev. B, 40 (1989) 10449. 9 L. C. Markert, J. E. Greene, W.-X. Ni, G. V. Hansson and J.-E. Sundgren, Thin Solid Films, 206 (1991) 59. 10 T. Ohmi, H. Iwabuchi, T. Shibata and T. Ichikawa. Appl. Phys. Lett., 54 (1989) 253. 11 L. I. Maissel, in L. 1. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970, chap. 4. 12 H. R. K a u f m a n and P. D. Reader, Am. Rocket. Soc. Paper 1374-60, 1960. 13 B. Paszkowski, Electron Optics, Elsevier, New York, 1968, pp. 210-218. 14 H. R. K a u f m a n in L. Marton (ed.), Advances in Electronics and Electron Physics, Vol. 36, Academic Press, New York, 1974, pp. 265 373. 15 H. R. Kaufman, J. Vac. Sci. Technol. A, 4 (1986) 764. 16 J.-P. No~l, J. E. Greene, N. L. Rowell, S. Kechang and D. C. Houghton, Appl. Phys. Lett., 55 (1989) 1525. 17 D. Lorimer, E. J. Baker, M. Succi and D. K. Weber, Solid State Technol., 33 (1990) 77. 18 J.-P. No61, N. Hirashita, L. C. Markert, Y.-W. Kim, J. E. Greene, J. Knall, W.-X. Ni, M. A. Hasan and J.-E. Sundgren, J. Appl. Phys., 65 (1989) 1189. 19 M. A. Ray, J. E. Baker, C. M. Loxton and J. E. Greene, J. Vae. Sci. Technol. A, 6 (1988) 44. 20 L. J. van der Pauw, Philips Res. Rep., 13 (1958) 1. 21 H. Oechsner, Appl. Phys., 19 (1979) 421. 22 J. Ziegler, J. A. Biersack and U. Litmark, The Stopping and Ranges of Ions in Matter, Pergamon, New York, 1985. 23 B. C h a p m a n , Glow Discharge Processes, Wiley, New York, 1980, pp. 9 11. 24 M. Prutton, Surface Physics, Clarendon, Oxford, 1983, p. 53. 25 D. M. Brown and R. Bray, Phys. Rev., 127(1962) 1593. 26 H. Brooks, in L. Marton (ed.), Advances in Electronics and Electron Physics, Vol. 7, Academic Press, New York, 1955. 27 R. Newman and W. W. Tyler, in F. Seitz and D. Turnbull (eds.), Solid State Physics, Vol. 8, Academic Press, New York, 1959. 28 E. Conwell and V. F. Weisskopf, Phys. Rev., 77 (1950) 388. 29 E. H. Putley, The Hall Effect and Related Phenomena, Butterworths, London, 1960, pp. 161 i65. 30 C. M. Wolfe, N. Holonyak, Jr., and G. E. Stillman, Physical Properties of Semiconductors, Prentice-Hall, Engelwood Cliffs, N J, 1989, pp. 87 88. 31 G. E. Stillman and C. M. Wolfe, Thin Solid Films, 31 (1976) 69.