Oxidation studies of hydrogenated amorphous silicon

Oxidation studies of hydrogenated amorphous silicon

488 Surface Science 116 (1982) 488-500 North-Holland Publishing Company OXIDATION STUDIES S.R. KELEMEN, OF ~DRO~ENA~D Y. GOLDSTEIN AMORPHOUS *...

751KB Sizes 0 Downloads 0 Views

488

Surface Science 116 (1982) 488-500 North-Holland Publishing Company

OXIDATION

STUDIES

S.R. KELEMEN,

OF ~DRO~ENA~D

Y. GOLDSTEIN

AMORPHOUS

* and B. ABELES

Exxon Reseurch and Engineering Company, P.O. Box 45, Linden, New Jerse,v 07036.

Received

24 November

198 1; accepted

SILICON

for publication

12 January

USA

1982

Hydrogenated amorphous silicon surfaces, atomically clean and subsequently oxidized to up to 20 A oxide thickness, were studied using AES and UPS. The oxidation was made in 0, in the pressure range 10 W-pTorr to 5 atm and at 23 and 300°C. The oxidation rate at 23’C was found to be the same as that of crystalline silicon while at 300°C it was appreciably faster. Changes in the dN( E)/d E AES Si LVV line shape near 80 eY upon oxidation could be correlated to changes in the silicon-oxygen bonding level observed in UPS. The detailed line shape of the AES Si LVV transition indicates that at 3OO’C a more homogeneous oxide is produced than at 23°C.

1. In~~uetion One of the important new developments in electronic materials research has been the use of hydrogenated amorphous silicon (a-SiH,) as an active element in semiconductor devices. There is lively interest in utilizing a-SiH, films in cheap, large area, solar cells [I], vidicons [2], xerography [3] and large area TFT displays [4]. In these applications, surface and interface properties of the semiconductor often play an important role in determining the performance of the device. For instance, the incorporation of thin oxide layers at the semiconductor-metal interface is found to affect strongly both crystalline silicon [5] and amorphous silicon [&lo] metal-insulator-semiconductor solar cell structures. Thus, oxidation studies, using surface sensitive electron spectroscopic techniques, are of technological as well as of scientific interest. While the oxidation of crystalline silicon at relatively high temperatures was studied fairly extensively [ 111, low temperature studies of well developed oxide structures are lacking. In the case of a-SiH, there are, to our knowledge, no published data on the oxidation kinetics. In this case, only Iow temperature oxidation studies are useful, since above 300°C some of the hydrogen begins to evolve from the material. To this end, we have undertaken a comprehensive study of the a-SiH, surface using Auger Electron Spectroscopy (AES) and UV Photoemission Spectroscopy (UPS). Results are presented for clean a-SiH, surfaces and for controlled low temperature (C 3OO’C) oxidation of a-SiH, at pressures ranging from 10 -9 Torr up to atmospheric pressure. * Permanent

address:

Racah

Institute

0039-6028/82/0000-0000/$02.75

of Physics,

Hebrew

University,

0 1982 North-Holland

Jerusalem

91000. Israel.

S. R. Kelemen et al. / Oxidation of hydrogenuted umorphow

Si

489

2. Experimental The a-SiH, films were deposited on crystalline silicon substrates by glow described previously [ 121. discharge decomposition of SiH,, under conditions The electronic properties of the films were of solar cell quality and have been described elsewhere [9,12]. Using a (Leybold-Heraeus) interlock/preparation chamber, it was possible to introduce the samples into the ultra high vacuum (UHV) system within 10 min. The UHV spectroscopy chamber is of stainless steel and can achieve a base pressure better than 1 X lo-” Torr pumped with ion and titanium sublimation pumps. The chamber is equipped with a double pass cylindrical mirror analyzer. The electron source for AES is a standard Physical Electronics grazing incidence electron gun. For UPS measurements a VUV Associations resonance lamp is used. The UV light enters into the main chamber through a stainless steel capillary tube; the lamp and the tube are differentially pumped, and during operation of the lamp with He the pressure in the main system rises, at most, to 1 X 10 -* Torr. A valve between the main chamber and the differetially pumped system is open only during the taking of a photoemission spectrum. The photon beam is 75’ off the axis of the analyzer. The sample normal can be positioned so as to collect electrons over a wide range of emission angles.

3. Clean a-SiH, surface Two different techniques were used to clean the a-SiH, surface - wet chemical etching and sputtering. The chemical cleaning.was done just prior to insertion into the UHV system by etching in buffered HF for 1 min and rinsing with deionized water. Sputter cleaning was done in the UHV system by sputtering with ultra pure neon gas while simultaneously pumping with a Ti sublimation pump cooled with liquid nitrogen. The samples were annealed below 3OO”C, since hydrogen begins to evolve from the a-SiH, samples above this temperature [ 131. The effect of the cleaning procedures on the AES d N/d E spectra is shown in fig. 1. The upper curve was obtained on a sputter cleaned sample after repeated cleaning cycles, and we believe is representative of a clean a-SiH, surface. The amplitude of the oxygen Auger transition at 510 eV [0(510)] on this sample was 0.002 of that of the Si LW transition at 90 eV [Si(90)] and is too small to be seen in the figure. Apart from the silicon line, there also appears to be a slight trace of carbon at 270 eV. The second curve in the figure was measured on a sample which was etched just prior to introduction into the UHV system. Here the oxygen signal is already noticeable [0(51O)/Si(90) = 0.051, and we also see more of a carbon signal. The details of the silicon line shape are also somewhat different from the one shown above. For comparison we include the bottom curve which shows an AES spectrum obtained on an

490

S. R. Kelemen et al. / Oxidation of hydrogenuted umorphow

I

---r-----

I

SPUTTER

I

CLEANED

I 300

100 ELECTRON

Si

ENERGY

1 500 E (a’/)

Fig. I. Auger derivative spectra of three sputter cleaned, etched, and oxidized.

a-SiH,

samples

with different

surface

preparations:

oxidized a-SiH, sample. Here we see a large oxygen signal, comparable to that of silicon. In addition, the detailed line shape of the silicon transition is quite different from those above. This can be seen much better in fig. 2 where we show the details of the Si LW Auger transition for a sputter cleaned sample (A), and two progressively more oxidized samples (B and C). SampleA shows a main peak at 90 eV and not much structure at lower energies. This general line shape was reproducible both on sputter cleaned a-SiH, and on sputter cleaned crystalline silicon and agrees with published data on crystalline Si [ 141. Accordingly, we identify this shape with the clean a-SiH, surface. Upon oxidation, the silicon Auger transition develops much more of a structure at lower energies. In particular, a dominant peak develops just below - 80 eV

S. R. Kelemen et ui. / Oxidurion of hydrogenated amorphous Si

491

C

t

I

I

I

50

M)

70

80

ELECTRON

ENERGY

I

90

I

loo

E (eV)

Fig. 2. Expanded Auger derivative spectra for a sputter cleaned a-SiH, sample (A) and two progressively more oxidized samples, (B) and (C). The definitions of the SiO,(75) and Si(90) lines are shown by the arrows in (C).

(marked by the larger arrow in curveC). This structure was found to be very sensitive to the oxidation conditions and the peak, labeled SiO,(75), was utilized to characterize the oxide layers; we will later discuss the method of analysis of the Si LW Auger transition and the oxidation kinetics. The state of cleanliness of the surface is revealed more dramatically by UPS. This is demonstrated in fig. 3 by the UPS spectra obtained with the He11 line (hv = 40.8 eV) for three different oxygen coverages. (For the sake of clarity, the curves are shifted vertically.) The bottom curve was obtained on an etched sample while the two upper curves were obtained on sputter cleaned samples. The ratio of the O(SlO)/Si(90) Auger signals for each surface is marked in the

492

S. R. Kelemen et al. / Oxidation of hydrogenated amorphous Si

I

I

I

0(51O)/si(W) = 0.002

hv = 40.8 eV

Fig. 3. UPS density of states spectra for three a-SiH, samples obtained with the He11 line. The ratio of the oxygen O(510) to the Si(90) line intensity, obtained from Auger measurements, is marked on each curve.

figure. The bottom curve for which the ratio 0(510)/%(90) is 0.05 exhibits marked oxide characteristics [ 151. This can be seen both by the positions of the main peak at - 8 eV and weaker peaks near - 11 and - 13.5 eV, as well as by the distance of the trailing valence emission edge from the Fermi level. The next curve is characterized by an 0(51O)/Si(90) ratio of 0.014 and is markedly different from the bottom one. The valence band edge is moved by - 3 eV closer to the Fermi level, the main peak is shifted to - 7 eV, a new peak around -2 eV is developing and the details of the lower energy peaks are all but lost. Upon additional cleaning, these changes in the UPS spectra saturate and we observe only a sharpening of the spectrum as shown by the topmost curve in the figure. The latter curve is similar to those observed by von Roedern et al. [16] on a-SiH, surfaces prepared in situ. An estimate based on standard Auger sensitivity factors sets the surface oxygen concentration at less than 0.2% of a monolayer.

4. Oxidation studies Oxidations of the samples were carried out in the UHV preparation chamber or in the UHV system itself. In fig. 4, we show UPS spectra of a-SiH, surfaces at various stages of oxidation, taken with the He11 line (hv = 40.8 eV). Curve A corresponds to the sputtered clean surface, curve B to an etched

S. R. Kelemen et ui. / Oxidation

-20

-18

-16

-14

-12

-10

-8

E-EF

(eV)

-6

-4

-2

of hydrogenuted omorphour Si

493

$=a

Fig. 4. UPS density of states spectra for several a-SiH, samples obtained with the He11 line. (A) Atomically clean surface; (B) Etched surface; (C)-(F) Surfaces exposed to atmospheric pressure of oxygen at 300°C for progressively increasing times. The ratio of the AES SiO,(75)/Si(90) lines was: (C) 0.25, (D) 0.74, (E) 2.44, (F) 9.33.

surface, and curves C through F correspond to surfaes oxidized at 1 atm pressure and 300°C for progressively longer times. The degree of oxidation is characterized by the AES SiOZ(75)/Si(90) ratio. (The AES Si LW line shape for the oxidized surface, corresponding to line F, is similar to that of curve C in fig. 2.) We note that with increasing oxidation, there is a gradual recession of the valence band edge away from the Fermi level and the development of peaks near -8.0, - 12.0 and - 15.0 eV. This trend is consistent with that observed in crystalline Si on going from monolayer oxygen coverage to a developed SiO, structure [ 151. In fig. 5, we compare the UPS N(E) spectra taken with the He1 line (hv = 21.2 ev), for a well developed oxide layer (like the one represented by curve F in fig. 4) with a UPS spectrum obtained on an etched sample. The major changes in valence band emission which occur upon oxidation can be seen clearly in the arithmetic difference of the spectra, AN(E), for the oxidized and etched surface, shown in the upper part of the figure. The AN(E) spectrum shows a loss of emission beginning at - -6 eV and trailing toward the Fermi level, and two pronounced emission maxima at - 8.0 and - 12.0 eV. Similar peaks have also been observed by Ibach and Rowe [ 151 on oxidized crystalline Si; the magnitude of the splitting of the peaks changed with the degree of oxidation. At monolayer coverage, a 3.2 eV splittig was observed which increased to 3.9 eV for the silicon dioxide phase [ 151. Our observation of a 4.0 eV splitting for an oxidized surface provides evidence that there is a major SiO, component to this oxide.

494

S. R. Kelemen et al. / Oxidation

-16

-I 4

-12

-IO

-8 E-EF

-6

hv

= 21.2

-4

-2

of hydrogenated amorphous Si

eV

EF = 0

(A’)

Fig. 5. UPS density of states spectra for two a-SiH, samples obtained with the He1 line. The lower curve corresponds to an etched surface while the upper curve corresponds to a surface exposed to atmospheric pressure of oxygen at 3OO’C for 1 h; the difference AN(E) of the two spectra is shown in the upper part of the figure.

Fig. 6 shows the effect of progressive oxidation on the AES Si LVV line shape. The curves in the figure, labeled alphabetically, correspond to different oxygen exposures; the exposure increases from A to H. Curve A was measured on an atomically clean surface, while H after an exposure of 10” L (10e6 Torrs). The oxygen pressure used was increased with progressive exposure. Curves B and C correspond to oxidation near 10 -’ Tort-, D, E, and F near 10 Torr, G and H near 1 atm. The signal decreases upon oxidation, and to compensate for this we increased the gain of the instruments as marked in the figure (by 2.5 for curves B and C and by 5 for curves D through H). There is a continuous decrease in the relative magnitude of the peak near 90 eV, while the oxide structures below 80 eV show a continuous increase in amplitude and a shift to lower kinetic energies with oxidation. Curve H, corresponding to the higest oxygen exposure, resembles closely the lineshape of 50, [ 171. Fig. 7 shows a comparable oxidation sequence performed at 23°C. Curve A

S. R. Kelemen ef al. / Oxidation of hydrogenated umorphow Si

50

M) ELECTRON

ENERGY

E (e”)

70

ELECTRON

80 ENERGY

90

495

,ca E(a)

Fig. 6. Si LW line shape following exposure to oxygen at 300°C of initially clean surfaces. (A) Atomically clean surface. Oxygen exposures (in L) are: (B) 105, (C) IO”, (D) IO’. (E) 109, (F) 10”. (G) 2X IO”, (H) IO’*. Oxygen p ressures used: (B), (C) 10P4 Torr; (D)-(F) -10 Torr; (G), (H) atmospheric pressure. Note also that the instrument gain was increased by 2.5 for (B) and (C), and by 5 for (D)-(H). Fig. 7. Si LW line shape following exposure to oxygen at 23°C of initially clean surfaces. (A) Atomically clean surface. Oxygen exposures (in L) are: (B) IOs, (C) 6X IO’, (D) 2X 104, (E) 2X 10s. (F) 2X IO*, (G) 3X 1013, (H) 6X IO”. Oxygen pressures used: (B), (C) -IO-’ Torr; (D), (E) - 10 -’ Torr; (F) -5 Torr; (G) - I atm; (H) -5 atm. Note also that the instrument gain was increased by 2.5 for (E) and (F), and by 5 for (G) and (H).

was again measured on an atomically clean surface and the alphabetic order corresponds to increasing exposure to oxygen, curve H to 6 X lOI L. Here oxide development is much slower than at 3OO”C, yet the Si LW fine structure shows a similar development. Greater degrees of oxidation than that shown in

S. R. Keiemen et al. /

496

Oxidution of hydrogenated umorphous Si

curveH could be achieved at longer oxygen exposures at 23’C but were not grown in situ in a continuous run, since this would have required dedicating the apparatus for prohibitive lengths of time to this endeavor. Fig. 8 summarizes graphically the kinetic results of the oxidation process of a-SiH, samples at both 300 and 23°C (full symbols). We included, for comparison, the results of identical experiments which were performed with standard crystalline Si materials (open symbols). The oxidation rates at 23°C (triangles) of amorphous and crystalline silicon are similar. At 300°C both materials show higher oxidation rates than at the lower temperature, as expected, however, the oxidation rate of a-SiH, is seen to be appreciably higher than that of crystalline Si. For a-SiH, surfaces that were oxidized in the UHV system under clean conditions, we find that the amplitudes of the AES O(510) line and that of the SiO,(75) line are proportional to each other. However, this proportionality

i-6 I”““““““I

6 6 5

1.4

-

1.2

-

1.0

-

0.8

-

m 3OiPc A 23OC

0 a-5 H,

0

0

z 0.6

-

0

.

0 0.4

0

-

.

_

A

l 0 0.2

0. .A4 .oA

a0.0

A

.A

-

t=t”, IO0

1

1

0

8

3



OXYGEN

EXPOSURE



*

10'0

IO5 (lOA

TORR



*



1

lOI SEC)

Fig. 8. The AES oxygen signal normalized to the Si(90) line as a function of oxygen exposure in langmuirs. The full symbols correspond to a-SiH,, the open symbols to crystalline silicon. The triangles represent measurements at 23°C while the circles at 300°C.

S. R. Kelemen et al. / Oxidation of hydrogenated omorphour Si

497

does not always hold for oxidations performed under “dirty” conditions outside the UHV system (such as for the fabrication of solar cell structures). The deviations from the linear dependence between O(510) and Si4(75) can be correlated with the presence of surface contaminants other than oxygen, usually carbon. This is illustrated in fig. 9 where we plot the amplitude of the O(510) line as a function of the Siq(75) line for “dirty” oxidations. Both lines are normalized to the Si(90) line. (The amplitudes of the SiO,(75) and Si(90) peaks are indicated in fig. 2, curve C.) The full dots represent data obtained on samples with low carbon contamination,and show a linear relationship down to SiO,(75)/Si(90) = 1.5. The open circles in the figure represent data taken.on samples which showed appreciable carbon contamination and are seen to deviate from the straight line. We believe that this is due to the fact that the oxygen and silicon lines are attenuated differently by the presence of surface contaminants. Thus, for “dirty” oxidations the ratio SiOZ(75)/Si(90) is a more accurate quantitative measure of oxidation than the 0(51O)/Si(90) ratio. The SiO,(75)/Si(90) ratio can be used also to obtain an estimate of the oxide thicknesses. We use a model that was applied [ll] to crystalline Si to correlate Auger signals to oxide thicknesses. This model assumes a layer-bylayer oxide growth and an exponential decay of the substrate signal [Si(90)] with oxide thickness. We furthermore assume that the electron attenuation

AU

Si02(75)/S(50)

Fig. 9. The ratio of the O(510) to the Si(90) line as a function of the SiO,(75)/Si(90) ratio. Full dots represent data obtained on samples with low carbon contamination, open circles data on samples with appreciable carbon contamination.

S. R. Kelemen et al. / Oxidution

498

of hvdrogenoted amorphous Si

length, h, in silicon and in the oxide is the same. For X we use the value corresponding to [ 1 l] SiO, on crystalline Si (X = 6.5 A), and for the relative Auger sensitivity of SiO, to Si we take [14] 0.2. With these assumptions, we obtain SiO,(75)/Si(90)

= 0.2( 1 - A)/A,

(1)

where A is the ratio of the Si(90) peak to its value for clean surface conditions. The thicknessd of the oxide layer is then given by d=Xln(l/A). Using

eq. (2), we obtain

(2) an estimate

of 20A for our thickest

oxide at 3OO’C.

5. Discussion To interpret the UPS results on the a-SiH, films, we are guided by what is known about oxidation of crystalline silicon [ 18,191. Upon oxidation the electronic structure of the valence band consists of two groups of states. One group is located at - 6 eV below the valence band edge and usually gives rise to a double peak in the UPS N(E) spectra. This group of states stems from the oxygen 2p and silicon 3s and 3p orbitals and is primarily bonding-like. The other group stems primarily from the oxygen non-bonding states and is located just below the valence band edge [19]. Features in the emission spectra of oxidized a-SiH,, which can be interpreted as due to these two groups, can be observed in fig. 4, curve F. Measured from the Fermi level, the oxygen-silicon bonding levels are seen at - 12 and - 15 eV, while the oxygen non-bonding eve1 occurs near -8 eV. At earlier stages of oxidation we find these features somewhat closer to the Fermi level. For instance, the - 12 eV peak in curve F “moves” - 1.5 to - 10.5 eV in curve C. The above “movement” of the valence band features also manifests itself in the AES data. However, because the AES spectra reflect the self-convolution of the (p-like) states in the valence band [20,21], this “movement” is 1.5 eV is expected to result in a - 3 eV increase of the kinetic energy in the Si LVV Auger spectrum. Such a trend can be indeed observed in curves H through D in fig. 6, where we see a systematic shift of - 3 eV toward higher energies of the minimum in the AES dN/dE spectra around 80 eV. Thus at the early oxidation stages, the minimum is at - 83 eV and shifts to - 80 eV upon further oxidation. Based on data on crystalline silicon [19], we associate the valence band level that dominates ‘the AES spectrum around 80 eV with the - 12 eV peak in the UPS spectrum (curve F, fig. 4). At early oxidation stages, the oxidation is expected to favor the occurrence of non-stoichiometric oxidation states. This is most probably [17] the reason for the spectral distortion (relative to the pure SiO* line shape) apparent, for instance, in curves D and E of fig. 6 (300°C oxidation) in the range of - 70-85

S.R. Kelemen et al. / Oxidation of hydrogenated amorphous Si

499

eV. An even more pronounced distortion is observed in the AES spectra obtained upon room temperature oxidation - curvesG and H in fig. 7 exhibit anomalous Si LW line shapes near 80 eV in that a double. minimum is evident. We interpret this feature as being due to inhomogeneities in the degree of oxidation. At 300°C this feature is not as pronounced, implying that the oxidation is more homogeneous.

6. Summary To our knowledge, the present study is the first comprehensive investigation of the oxidation kinetics and oxide composition on a-SiH, surfaces. At room temperature the rate of oxidation of a-SiH, is very similar to that of crystalline Si. At 300°C however, the oxidation kinetics of a-SiH, is appreciably faster than that of crystalline Si. We have developed a quantitative measure of oxidation which can be used also for “dirty” a-SiH, surfaces. We find that the detailed Si LW line shape is particularly sensitive to oxidation and the peak near 75 eV can be used to characterize the oxide layer even in the presence of carbon contamination. The peaks in the UPS data show a systematic “movement” upon oxidation. This movement, as well as the corresponding shift in the AES minimum (at - 80 eV) are probably due to the lowering in the energy of the silicon oxygen bonding level. Finally, the results of AES and UPS presented here show that it is possible to obtain atomically clean surfaces on a-SiH, not prepared in situ in the UHV system. We consider this as an extremely important result because it opens up a completely new research direction, namely the application of the various surface research techniques to a-SiH, prepared in conventional reactors. Using these methods, one can investigate the electronic band structure and in particular the localized gap states near the band edges. These localized states determine the upper limit of a-SiH, solar cell efficiency, and until now only indirect information on them is available [22]. UPS measurements from such localized states on a-SiH, prepared in situ were performed by von Roedern et al. [16], however, the density of gap states derived from their measurements was orders of magnitudes higher than that derived from transport measurements on good, solar-cell quality, material [22].

References [I] D.E. Carlson, Solar Energy Mater. 3 (I 980) 503; Y. Hamakawa, in: Proc. 9th Intern. Conf. on Amorphous and Liquid Semiconductors, Grenoble, 1981 (J. Physique, to be published). [2] Y. Imamura, S.A. Ataka, Y. Takasaki, C. Kusano, T. Hirai and E. Mariyama, Appl. Phys. Letters 35 (1979) 349.

SO0

S.R Kelemen ei al. / Oxidaiion 01 hydrogenated amorphotu Si

[3] I. Shim& T. Komatsu and E. Inoue, Phot. Sci. Eng. 24 (1980) 251. [4] P.G. LeComber, in: Proc. 9th Intern. Conf. on Amorphous and Liquid Semiconductors, Grenoble, 1981 (J. Physique, to be published). [S] R. Singh and M.A. Green, Solar Cells 3 (1981) 95, and references therein. [6] CR. Wronski, in: Proc. 13th IEEE Photovoltaic Specialists Conf., Washington, DC, 1978 (IEEE, New York, 1978) p. 744. [7] J.I.B. Wilson, J. McGill and S. Kimond, Nature 272 (I 978) 152. [8] A. Madan, J. McGill, W. Czubatyj, J. Yang and S.R. Ovshinsky, Appl. Phys. Letters 37 (I 980) 826. [9] B. Abeles, C.R. Wronski, Y. Golstein, H.E. Stasiewski, D. Gutkowin-Krusin. T. Tiedje and G.D. Cody, in: Tetrahedrally Bonded Amorphous Semiconductors, AIP Conf. Proc. No. 73, Eds. R.A. Street, D.K. Biegelsen and J.C. Knights (AIP, New York, 1981) p. 298. [IO] CR. Wronski, Y. Goldstein, S.R. Kelemen, B. Abeles and H. Witzke, in: Proc. 9th Intern. Conf. on Amorphous and Liquid Semiconductors, Grenoble, 1981 (J. Physique, to be published). [l I] B. Lang, P. Scholler and B. Carriere, Surface Sci. 99 (1980) 103, and references therein. [12] G.D. Cody, B. Abeles, C.R. Wronski, B. Brooks and W.A. Lanford, J. Non-Crystalline Solids 3S/36 (1980) 463. [13] H. Fritzsche, M. Tanielian, CC. Tasai and P.J. Gani, J. Appl. Phys. SO (1979) 3366. [ 141 L.E. Davis, N.C. McDonald, P.W. Palmber, G.E. Riach and E. Weber, Handbook of Auger Electron Spectroscopy (Physical Electronic Industries, Edina, MN, 1976). [IS] H. Ibach and J.E. Rowe, Phys. Rev. BlO (1974) 710. [16] B. von Roedem, L. Ley, M. Cardona and F.W. Smith, Phil. Mag. B40 (1979) 433. [17] CR. Helms, Y.E. Strausser and W.E. Spicer, Appl. Phys. Letters 33 (1978) 767; C.R. Helms, W.E. Spicer and N.M. Johnson, Solid State Commun. 25 (1978) 673. [ 181 B. Fischer, R.A. Pollak, T.H. De Stefano and W.D. Grobman, Phys. Rev. Bl S (I 977) 3 193. [ 191 D.E. Ramaker, J.S. Murday, N.H. Turner, G. Moore, M.G. Lagally and J.E. Houston, Phys. Rev. B19 (1979) 5375, and references therein. [20] P.J. Feibelman, E.J. McGuire and K.C. Pandey, Phys. Rev. BIS (1977) 2202; P.J. Feibelman and E.J. McGuire, Phys. Rev. B17 (1978) 690. [21] J.E. Houston, G. Moore and M.G. Lagally, Solid State Commun. 21 (1977) 879. [22] See, for example, review article by B. Abeles, G.D.Cody, Y. Goldstein, T. Tiedje and C.R. Wronski, presented at 5th Intern. Thin Film Congress, Herzlia-on-Sea, Israel, 1981 (Thin Solid Films, to be published).