Electron-beam assisted adsorption on the Si(111) surface

S U R F A C E SCIENCE 21 (1970) 253-264 © North-Holland Publishing Co.

ELECTRON-BEAM ON

ASSISTED

THE

Si(lll)

ADSORPTION

SURFACE

J. P. COAD

Metallurgy Department, University of Oxford, Oxford. England and H. E. BISHOP and J. C. RIVI~RE

Solid State Division, AERE, Harwell, England* Received 28 February 1970 Observations by Auger emission spectroscopy on silicon surfaces have shown that considerable enhancement of the oxygen Auger peak occurs over the area irradiated continuously for many hours by a 2000 eV electron beam, while the silicon crystal is standing in a vacuum of 5 x 10 to torr at room temperature. On dirty silicon, i.e. on a surface partly covered with carbide, the oxygen saturation level inside the beam was at least an order of magnitude greater than that outside, while on clean silicon there was no accumulation of oxygen outside the beam. In both cases the rate of accumulation of carbon was the same inside as outside the beam, but on clean silicon the amount of carbon that built up was much less than that on the dirty silicon. The gas taking part in the beam-assisted adsorption was almost certainly carbon monoxide; its initial sticking coefficient was calculated approximately, and found to be very similar to that observed experimentally by other workers for molecular oxygen. The minimum detectable surface concentrations of oxygen and of carbon, at the beam currents used, were estimated to be about 4 ~ and I ~,:,, respectively, of a monolayer. The results are interpreted in terms of electron dissociation of physisorbed carbon monoxide, followed by surface diffusion of carbon but not of oxygen. Chemisorption of carbon monoxide on silicon carbide particles is suggested to account for part of the greater accumulation of carbon on dirty silicon compared with that on clean silicon.

I. Introduction Oxygen build-up on silicon surfaces has been the subject of study by many workers1-11), using a wide variety of techniques. In only three of these studies, however, has a technique been used, namely LEED, where the surface has been subjected to electron irradiation during at least part of the experim e n t . S c h l i e r a n d F a r n s w o r t h z) s t a t e d d e f i n i t e l y t h a t t h e L E E D g u n f i l a m e n t w a s o f f d u r i n g o x y g e n e x p o s u r e ; L a n d e r a n d M o r r i s o n 8) m a k e n o m e n t i o n of whether the LEED

optics were or were not operated during exposure;

* Address for correspondence. 253

254

J . P . COAD, H. E. BISHOP AND J. C. RIVIERE

Rovida et al. 1°) examined the possible effect of electron irradiation during exposure, and came to the conclusion that there was no significant effect, at least at the level of beam intensity ( ~ 1 pA) normally used in LEED. All the experimental studies quoted above used oxygen pressures above 10 -8 torr, and several were carried out at very much higher pressures. The presence of carbon on silicon surfaces has been suspected very frequently, but, apart from observation of the formation of SiC itself, any systematic experiments on the build-up of carbon on silicon during reaction have had to await the revival of Auger emission spectroscopy (AES). The first of such experiments has been reported by Charig and Skinner12), in which the treatment required to produce a clean Si(111) surface, and the reaction of that surface with ethylene, were followed by AES. At room temperature very long exposures, of the order of 3 × 10 -4 torr-sec, were necessary to produce a carbon Auger peak of significant size. Although the attthors did not say so, it is presumed from other evidence in their paper that the LEED gun tilament was not in operation during ethylene exposure. The interaction of carbon monoxide itself with an atomically clean silicon surface does not appear to have been studied at all, certainly not systematically. In their multi-technique study of the production of clean silicon surfaces, Allen et al. 5) stated that carbon monoxide adsorption from the ambient was not affecting the oxygen adsorption process. Schlier and Farnsworth e) could find no contamination effects as a result of annealing silicon in a vacuum poorer than their normal 10 - l ° torr, although their criterion was the quality of the LEED pattern, a somewhat insensitive test. Carbon monoxide is of course one of the main residual gases left in a vacuum system at low pressures after water vapour has been largely removed by baking. The present observations of increased or induced reactivity towards the residual gases as a result of electron irradiation of a silicon (111) surface were made at first accidentally, but then continued more systematically once the effect had been established. They are based entirely on surface analysis measurements using AES.

2. Experimental A (1 l l) oriented wafer of 10 ohm-cm p-type silicon was mechanically polished and cut to size approximately 15 × 5 x0.25 mm, then dipped in buffered HF until the oxide layer had been removed. It was mounted across flats cut in the ends of two 3 mm diameter molybdenum rods by crimping between strips of molybdenum foil spot-welded to the rods. The crystal could be heated directly to any desired temperature up to its melting-point

ELECTRON-BEAM ASSISTED ADSORPTION

255

by passage of current through the rods. Measurement of temperature was by means of an optical pyrometer; appropriate corrections were made to the observed temperatures. Energy analysis of back-scattered electrons was carried out using the 3-grid LEED optics supplied by Vacuum Generators Ltd., in the configuration used by Palmberg and Rhodin 13), that is, with grids l and 3 earthed, and the retarding potential applied to grid 2. Generation of the modulating voltage applied on top of the retarding potential, pre-amplification of the current to the screen (acting as collector), and selective amplification by phase sensitive detection of the first or second harmonic of the collected current, were provided by the purpose-built unit from the same manufacturers. Neutralization of the capacitive pick-up between the collector and the scanning grid was achieved with an inductance ratio arm bridge neutralizing module. The LEED gun itself was used as the source of primary excitation in these measurements, although a more powerful auxiliary gun was available on the system, in order to minimize any additional effects due to beam heating of the surface. At 2000 eV and 1 #A, the energy dissipated at the surface was only about 0.2 W/cm 2. After routine baking and processing the base pressure was about 5 x 10- lo tore Although the 20th Century Q806 quadrupole mass spectrometer normally on the system was out of action at the time of these experiments, frequent previous operation at the same pressure level confirmed that the the two principal residual gases were always hydrogen and carbon monoxide. Oxygen itself was never detected, while methane and carbon dioxide, although detectable, were invariably at much lower partial pressures than the main constituents. 3. Results

Since the observations made on incompletely clean surfaces were rather different from those on atomically clean (as determined by AES) surfaces, the two will be described separately. 3.1. C A R B O N - C O N T A M I N A T E D S I L I C O N S U R F A C E

To remove all traces of carbon from a silicon (i11) surface, according to AES measurements, involves heating to over 1200 ~C for a few seconds 12). Heating at temperatures in the range 8(~-900 °C does not remove the carbon. Immediately after the silicon crystal used here had been heated to about 850°C, the differential distribution revealed no oxygen, a trace of nitrogen, and quite a lot of carbon on the surface, as can be seen in the top trace of fig. l. Associated with the carbon peak at 272 eV was a well-developed

256

J . P . COAD, H. E. BISHOP AND J. C. RIVIERE

p l a s m a loss peak at 245 eV. On being allowed to stand in the system without further heat treatment, the a r e a o f the surface under continuous irradiation by the electron beam a c c u m u l a t e d both oxygen and c a r b o n over a period o f m a n y hours, as shown by the successive traces o f fig. 1. A t the same time the p l a s m a loss peak from the c a r b o n A u g e r peak weakened and d i s a p p e a r e d , and the silicon A u g e r peak was d e g r a d e d generally by being reduced in a m p l i t u d e , b r o a d e n e d in width, and shifted in a p p a r e n t position. There also seemed to be a slight reduction in the nitrogen peak, but that m a y not have been signiticant. W h e n the electron beam was deflected at times within the tirst few hours to areas o f the crystal not previously irradiated, no oxygen

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Fig. 1. Differential energy distributions from a silicon (111) surface, after heating to about 850°C, at intervals of a few hours during continuous electron irradiation in a vacuum of 5 × 10-l° tort. The initial heat treatment was insufficient to clean the crystal, leaving carbon (probably in the form of carbides) and some nitrogen, on its surface. Both carbon and oxygen accumulate under the beam.

ELECTRON-BEAM

257

ASSISTED ADSORPTION

was ever detected, a l t h o u g h the increase a n d changes in the c a r b o n p e a k were much the same as those a l r e a d y described. Otherwise the trace was identical to that at zero time in fig. I. W h e n the size o f the oxygen A u g e r p e a k had s t o p p e d increasing, a series o f scans were r e c o r d e d as the incident b e a m was m o v e d to left and right o f its original position, by k n o w n fractions o f a millimetre. The i m p o r t a n t results from these scans are shown in fig. 2. A l t h o u g h after a very long time some oxygen did eventually a p p e a r on the u n i r r a d i a t e d parts o f the crystal, the greatly enhanced a c c u m u l a t i o n o f oxygen in the a r e a struck by the b e a m is strikingly evident. T h e highest c o n c e n t r a t i o n o f oxygen in fig. 2 is an o r d e r o f m a g n i t u d e greater than that o f the b a c k g r o u n d c o n c e n t r a t i o n

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Fig. 2. Results of scanning across the original position of the electron beam, after reaching saturation in both carbon and oxygen. Whereas the level of carbon is much the same inside as outside the beam, the peak of oxygen concentration inside the beam is at least an order of magnitude greater than that outside.

258

J. P. C O A D ,

H . E. B I S I ' I O P A N D

J. C .

RIVIERE

on u n i r r a d i a t e d areas. There was some indication that the c a r b o n concent r a t i o n was higher at the edges o f the beam position than elsewhere, b u t the scatter o f experimental points was rather too large to place any reliance on it. Figs. 1 and 2 refer to an emission current in the L E E D gun o f 0.9 m A , which p r o d u c e d a current at the specimen o f 1.60/~A under a bias o f + 18 V. The rates o f increase o f the oxygen and c a r b o n c o n c e n t r a t i o n s at the same emission current are shown in fig. 3; the curve for c a r b o n a c c u m u l a t i o n 5 ........"'"

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Fig. 3. Increases in the normalised heights of the A u g e r peaks o f carbon a n d o f oxygen on the dirty silicon surface as a function of time of exposure to the electron beam. Electron g u n emission current was 0.9 m A .

u n d e r the b e a m was similar to that for areas outside the beam. The rate o f oxygen a c c u m u l a t i o n seemed to be close to linear for the tirst four hours or so before d r o p p i n g off to reach s a t u r a t i o n in the way presumed by the d o t t e d line, whereas the rate for c a r b o n did not a p p e a r to be linear at any stage. T o examine the possible effect o f different beam currents, the specimen was flashed to a b o u t 900°C, which always returned it r e p r o d u c i b l y to the c o n d i t i o n at zero time in fig. 3, and the e x p e r i m e n t repeated at an emission current o f 0.6 m A , c o r r e s p o n d i n g to a specimen current o f 0.65 j~A at the same bias as before. W i t h i n experimental error, there a p p e a r e d to be no d e p e n d e n c e on beam current at the relatively low levels o f current used here. It was e n c o u r a g i n g that the s a t u r a t i o n levels o f c a r b o n and o f oxygen respectively in the two cases, were identical, allowing more confidence than h a d been suspected to be placed in the n o r m a l i s a t i o n procedure, which

259

ELECTRON-BEAMASSISTED ADSORPTION

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Differential energy distributions from the silicon surface, after heating to over

1200°C for several minutes, u n d e r the s a m e conditions as for fig. I. In this case the heat t r e a t m e n t produced an initially clean surface, as s h o w n by the u p p e r - m o s t trace in the lower half o f the figure. Oxygen a c c u m u l a t e s as before u n d e r the influence o f the beam, but the a c c u m u l a t i o n o f c a r b o n is negligible c o m p a r e d with that s h o w n in fig. 1.

260

J.P. COAD, El. E. BISHOP AND J. C. RIVII~RE

c o n s i s t e d o f dividing each peak height by a measure o f the slope o f the b a c k g r o u n d current (being p r o p o r t i o n a l to the actual b e a m current) taken between two fixed energies, in this case 200 and 600 eV. A c c o u n t was also t a k e n o f the decrease in i o n i z a t i o n cross-section, at a fixed p r i m a r y energy, with increase

in c r i t i c a l i o n i z a t i o n

potential

[see B i s h o p

and

Rivi~re14)],

by multiplying by the square o f the critical ionization potential o f the p a r t i c u l a r A u g e r transition involved. 3.2. CLEAN SILICON SURFACE A f t e r heating the silicon crystal to over 1200°C for several minutes, the differential energy d i s t r i b u t i o n showed no indications whatsoever o f p e a k s or even inflections in the oxygen, nitrogen, and c a r b o n positions. A t the level o f sensitivity o f A E S the surface was clean (although two o f the very small peaks near the large silicon 93 eV p e a k may be associated with traces at the surface o f a metallic i m p u r i t y , as discussed by Bishop and Rivi~re 13). On s t a n d i n g in v a c u u m under c o n t i n u o u s i r r a d i a t i o n by the electron beam, at an emission current o f 0.9 m A as before, no change in the A u g e r spectrum was observed for the first h o u r or so. At an exposure o f 242 hr, there was a definite indication in the oxygen position, and, as seen in fig. 4, the indication became a recognizable peak which c o n t i n u e d to increase with time until it reached saturation. At the same time there was something h a p p e n i n g in the

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Fig. 5. Increases in the normalised heights of the Augcr peaks of carbon and of oxygen on the clean silicon surface as a function of time of exposure to the electron beam. The saturation level of oxygen is identical with that of oxygen in fig. 3.

ELECTRON-BEAM ASSISTED ADSORPTION

261

carbon position, but the ill-defined feature that appeared there never became large enough to be resolved into an Auger peak ofthe usual shape. Deflections of the beam to unirradiated parts of the crystal during the exposure, for just long enough to record an Auger spectrum, showed that at no stage did any oxygen appear there, although the amount of carbon was as far as could be judged the same as in the irradiated area. The rates of increase of oxygen and of carbon concentrations on the initially clean silicon surface are shown in fig. 5. After the apparent induction period mentioned above, the oxygen concentration again rose linearly for the first few hours, but with a steeper slope, i.e. faster rate, than on the dirty surface at the same emission current of 0.9 mA. Eventually it reached saturation, and again the saturation level turned out to be exactly the same as in fig. 3. On the other hand, the carbon concentration, in so far as it could be estimated from what there was of an Auger peak, appeared to reach saturation relatively quickly at a level much below that observed on the dirty surface. 4. Discussion Although the rate of build-up of carbon appeared to be the same inside as outside the beam, there was no doubt that the beam was assisting markedly the adsorption of oxygen atoms on the silicon surface. Brief consideration of the sticking coefficients involved in the linear portions of figs. 3 and 5 shows at once that the oxygen-containing gas taking part in the activated adsorption must have been a major constituent of the residual atmosphere which means that it could hardly have been oxygen itself. For instance, making the assumptions that the oxygen saturation levels represented monolayer coverage of oxygen atoms, and that the oxygen-containing gas was carbon monoxide at a partial pressure of about 3 x l0 -1° tort, leads to an approximate sticking probability for carbon monoxide of 0.1 during the linear rate of rise. Although as stated in the Introduction there do not appear to be any measurements of the sticking probability of carbon monoxide on silicon, the above rough figure may be compared with the recent work by Rovida et al.l°), and by Carosella and Comas 11) on the sticking probability of molecular oxygen on silicon, when values of 0.14 (average) and 0.15, respectively, were obtained. Despite the fact that the comparison is for different gases under different conditions of electron irradiation, the figures are of the same order of magnitude. It can be concluded that the gas involved was indeed carbon monoxide; the partial pressure of carbon dioxide would have been at least an order of magnitude lower, leading to an absurdly high sticking coefficient. If the assumption of monolayer coverage at the Auger peak saturation

262

J . P . COAD, H. E. BISHOP AND J. (7. RIVI[RE

level is correct, the m i n i m u m detectable surface c o n c e n t r a t i o n o f oxygen on silicon for the low b e a m currents used here was a b o u t 49~, o f a m o n o l a y e r . By the same token, that for c a r b o n was a b o u t 1% o f a m o n o l a y e r . R o u g h calculation based on likely signal-to-noise ratios at different beam currents shows that at a beam current o f 26 :tA the m i n i m u m detectable levels would be reduced to 1% and 0.2595 o f a m o n o l a y e r , for oxygen and c a r b o n respectively. Such a current is o f an o r d e r o f m a g n i t u d e m o r e n o r m a l l y e m p l o y e d in A E S a p p l i c a t i o n s at the time o f writing. The absence o f oxygen (within the sensitivity o f A ES) from the u n i r r a d i a t e d areas o f the clean silicon for as long as the o b s e r v a t i o n s were c o n t i n u e d (22½ hr), leads to a m a x i m u m value for the sticking p r o b a b i l i t y o f chemisorbed c a r b o n m o n o x i d e without the assistance o f an electron beam, o f 0.002. However, the inability to detect oxygen does not mean that c a r b o n m o n o x i d e was not present, and indeed therc must have been a certain coverage o f p h y s i s o r b e d c a r b o n m o n o x i d e on the surface in equilibrium with the partial pressure o f the gas. It is easy to show that an equilibrium coverage o f a b o u t 3 × 10- 5 would have been sufficient to account tbr the estimated m a x i m u m rate o f growth ot oxygen o f 3 x 10 l° a t o m s cm -2 sec-1. Since the electron flux in a current o f 1.60 llA m m - 2 is a p p r o x i m a t e l y l0 t 5 electrons c m - 2 s e c - 1, it is not necessary to invoke dissociative efficiencies near unity, and for the same reason the lack o f d e p e n d e n c e o f the rate o f a c c u m u l a t i o n o f oxygen on beam current when the latter was reduced from 1.60 F~A to 0.65 I~A is explained. The a f o r e m e n t i o n e d steady state coverage is well below the detection limit for oxygen under these analysing conditions, and in fact the coverage could well be greater by one or two orders o f m a g n i t u d e and still not be detected on the u n i r r a d i a t e d areas. I n t e r p r e t a t i o n o f the o b s e r v a t i o n s on clean silicon in terms o f dissociation by the electron beam o f p h y s i s o r b e d c a r b o n m o n o x i d e thus seems feasible. On the c a r b o n - c o n t a m i n a t e d silicon surface some oxygen was observed to a c c u m u l a t e after a long time outside the area c o n t i n u o u s l y irradiated by the electron beam. Such a surface has been shown by, a m o n g s t others. Krausel~), using R H E E D , to consist o f particles of/3-SIC in a matrix o f silicon. A l t h o u g h c h e m i s o r p t i o n o f c a r b o n m o n o x i d e on the areas o f silicon between the particles has been shown by the observations on clean silicon to be either very slow or non-existent, there is always the possibility that it could take place on the surface o f the c a r b i d e itself. In fact, since o u t w a r d diffusion o f oxygen from the i r r a d i a t e d area on clean silicon did not seem to occur, the observed presence o f oxygen outside the beam on dirty silicon makes dissociative c h e m i s o r p t i o n o f c a r b o n m o n o x i d e on silicon c a r b i d e very likely. There do not seem to be any published experimental m e a s u r e m e n t s to s u p p o r t the hypothesis, but Dillon et al. aT) have shown that oxygen

ELECTRON-BEAM ASSISTED ADSORPTION

263

chemisorbs on silicon carbide with an initial sticking coefficient of 0.01. The increases in carbon concentration were not confined to the irradiated areas, but on both the clean and dirty surfaces were the same outside as inside the beam, leading to the conclusion that surface diffusion of carbon was possible under the experimental conditions. The actual surface temperature was probably only 20°C to 80°C; at 30/aA and the same primary energy temperature rises of 150-200°C have been measured in thin metal foils. However, the total increase in carbon accumulation on the dirty surface was a factor of at least 6 greater than the corresponding increase on the clean surface (compare figs. 3 and 5), even though the increases in oxygen accumulation were identical in the two cases. The difference can be explained easily in terms of what has already been postulated above; whereas carbon appears on the unirradiated clean silicon only by surface diffusion from the irradiated area, on the dirty silicon there is the additional contribution from carbon monoxide chemisorbed on silicon carbide particles. Surface diffusion can also account for the low saturation level of carbon on clean silicon compared with that of oxygen (fig. 5). The area of clean crystal surrounding the beam position was of course much greater than that under the beam, and would have formed an effective sink for the outward diffusing carbon. On the other hand, the oxygen produced by dissociation was apparently immobile, and eventually sufficient surface sites would have been occupied by oxygen to have affected the steady-state coverage of physisorbed carbon monoxide, and hence the overall efficiency of the dissociation process. When the process finally came to a halt, the carbon, which obviously must have been mobile even in the presence of near unity coverage of chemisorbed oxygen, had redistributed itself over an area at least twenty times that of the beam. 5. Conclusions Carbon monoxide from the residual atmosphere in an ultra-high vacuum system can be dissociated by a 2000 eV electron beam while physisorbed on a silicon (11 I) surface. The apparent initial sticking probability for chemisorption following the beam-assisted dissociation is of the order of 0. I. Carbon atoms formed by the dissociation are mobile at the temperature (20-80°C) of the electron-irradiated area and diffuse across the surface to produce the same carbon coverage outside and inside the beam. Oxygen atoms, on the other hand, appear to be immobile and with continued irradiation their concentration on the surface under the beam builds up to a saturation level identified with a monolayer. On clean silicon no oxygen is found outside the beam, but on a silicon surface containingcarbide inclusions

264

J.P. COAD. H. E. BISHOP AND J. C. RIVI/:RE

some oxygen a c c u m u l a t i o n occurs and is suggested as being due to chemisorption of c a r b o n m o n o x i d e on silicon carbide. The same e x p l a n a t i o n is put forward to a c c o u n t for the greater total increase of c a r b o n c o n c e n t r a t i o n on a c a r b i d e - c o n t a i n i n g surface compared with that on a clean silicon surface. If the s a t u r a t i o n levels of oxygen and of c a r b o n , were each assumed equal to a monolayer, then the m i n i m u m detectable c o n c e n t r a t i o n s of the two elements were 4 ~ and l ~ of a monolayer, respectively, for the low beam current (1.60 #A) used in the experiments.

Acknowledgements One of the authors (J.P.C.) would like to acknowledge gratefully the help and supervision of Dr. M. J. Whelan of the D e p a r t m e n t of Metallurgy, University of Oxford, and to t h a n k Professor P. B. Hirsch for the provision of facilities within the D e p a r t m e n t .

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) ll) 12) 13) 14) 15) 16) 17)

J. Eisinger and J. T. Law, J. Chem. Phys. 30 (1959) 410. R. E. Schlier and H. E. Farnsworth, J. Chem. Phys. 30 (1959) 917. S. P. Wolsky, J. Phys. Chem. Solids 8 (1959) 114. F. G. Allen, J. Phys. Chem. Solids 8 (1959) 119. F. G. Allen, J. Eisinger, H. D. Hagstrum and J. T. Law, J. Appl. Phys. 30(1959) 1563. M. Green and K. H. Maxwell, J. Phys. Chem. Solids t3 (1960) 145. H. D. Hagstrum, J. Appl. Phys. 32 (1961) 1020. J. J. Lander and J. Morrison, J. Appl. Phys. 33 (1962) 2089. R. J. Archer and G. W. Gobeli, J. Phys. Chem. Solids 26 (1965) 343. G. Rovida, E. Zanazzi, and E. Ferroni, Surface Sci. 14 (1969) 93. C. A. Carosella and J. Comas, Surface Sci. 15 0969) 303. J. M. Charig and D. K. Skinner, Surface Sci. 15 (1969) 277. P. W. Palmberg and T. N. Rhodin, J. Appl. Phys. 39 (1969) 2425. H. E. Bishop and J. C. Rivi6re, J. Appl. Phys. 40 (1969) 1740. H. E. Bishop and J. C. Rivi~re, Surface Sci. 17 (1969) 462. G. O. Krause, private communication 0970). J. A. Dillon, Jr., R. E. Schlier and H. E. Farnsworth, J. Appl. Phys. 30 (1959) 675.