J. CHEM. SOC. FARADAY TRANS., 1995, 91(18), 3021-3026 302 1

Kinetic Study of the Effects of H, 0, N, S, NO, NO, and 0, on the Surface States of InGaAs and GaAs George Gu, Hongjun Li and Elmer Ogryzlof Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada V6T 121

The photoluminescence intensities (PLls) from 300 nm layers of lno.53Gao.47As and GaAs have been used to monitor carrier lifetimes, in situ, during exposure of their surfaces to a number of gaseous species at 25°C. PLls have been observed to increase by a factor of 30-50 following a few seconds exposure to atomic hydrogen at gas-phase atom densities of ca. 1013 ~ m - ~ . For both semiconductors the PLI was found to be ca. 40% lower while the surface was being exposed to these atoms. Additional exposure of the InGaAs to H atoms was found to reduce the PLI from this maximum by a factor of 2-3. Both the InGaAs and GaAs surfaces that had been passivated by hydrogen atoms could be de-passivated in less than a second by exposure to 0, N, NO or NOz at comparable gas-phase densities or, at a much slower rate, by exposure to 02. The surfaces thus formed could be re-passivated by exposure to atomic hydrogen, but in the case of oxidizing species, the original behaviour of the InGaAs surface could only be restored with an HF wash. X-Ray photoelectron (XP) spectra of the surfaces under all these conditions were found to be indistinguishable. Exposure of the surface to S atoms was found to improve the passivation level of an HCI-washed surface by almost four orders of magnitude.

Unlike silicon, 111-V semiconductors form oxides that are neither useful as insulators nor effective in eliminating surface states, which adversely affects their performance in electronic circuits. As a consequence, the development of MOSFETs, MISFETs and a variety of photonic devices are hampered by shortened carrier lifetimes. Until recently, the only satisfac- tory solution to this problem has been the deposition of an epitaxial layer of a second semiconductor that is lattice matched, and has a larger band gap than the first semicon- ductor.’ However, this technique cannot be used under all circumstances, and there remains a need to identify these surface states, so that alternative methods of passivating the surfaces of 111-V semiconductors can be developed.

The’exact nature of the surface states that trap carriers on 111-V semiconductors is poorly understood. Most workers have begun with the assumption that the surface states result from the presence of point defects (such as vacancies or antisites,) or dangling bonds3-’ (i.e. unsaturated valencies of surface atoms).

Photoluminescence intensities (PLIs) from 111-V semicon- ductors have been used to follow changes in the surfaces of semiconductors for a number of years.’-’ The technique relies principally on the fact that when carriers are created near the surface of a semiconductor, surface states that lie within the band gap can act as recombination centres that catalytically recombine the carriers. If a thick sample of the semiconductor is studied, the sensitivity of the technique to the passivation level of the surface depends on how close to the surface the carriers are created. This is a function of the wavelength of the exciting light because it affects its penetrat- ion depth. Using visible light to excite the photolumine- scence, two groups have studied the effects of 1 atm pressure of gases like N,, 0, , H, and Ar on the PLI from GaAs and InP.’.’ There is little agreement about the magnitude or direction of the changes in the PLI with these gases. The rela- tively small changes were tentatively attributed to the effect of chemisorbed 0, on the positions of the surface atoms. More recently, Gottscho and c o - w o r k e r ~ ~ ~ - ~ ~ described the effect of a hydrogen plasma on the PLI from GaAs wafers. The novel behaviour which they report, and which we also observe, is an initial small increase in the PLI from the semi- conductor when it is being exposed to a hydrogen plasma,

t e-mail: ogryzlo@Chem.ubc.ca

and then, quite remarkably, a second, sometimes larger, increase in the PLI when the plasma is extinguished. Gottscho et al. originally suggested” that this could be simply due to discharging of the accumulated electrons from the GaAs surface. However, they subsequently found, as we have, that this effect can be observed on a sample placed downstream from the plasma.” This led them to conclude that this explanation was unsatisfactory. They then proposed that hydrogen atoms absorbed on the surface actually create traps for carriers, so that when the discharge is turned off, hydrogen leaves the surface as H,, creating a surface with fewer carrier traps.

The present report describes a preliminary study of the effects of a variety of atoms and molecules on the surface states of InGaAs and GaAs. The objective was to determine the nature of the surface reactions that can be detected by this technique, so that detailed kinetic studies can be carried out on the processes. The In/Ga/As system is particularly interesting because it appears that unlike other 111-V semi- conductors, H atoms have the effect of eliminating carrier traps in the bulk.16 Therefore, if surface passivation also improves with H-atom treatment, this passivation technique could prove very useful for InGaAs.

Experimental A 0.3 pm layer of undoped Ino.53Ga0.4,As was grown epi- taxially on an InP wafer from Bell Northern Research by molecular beam epitaxy. The 0.3 pm layer of undoped GaAs was grown on a 0.1 pm layer of AlGaAs at UBC. The thin layers of InGaAs and GaAs were grown on wider band-gap materials (on InP and AlGaAs, respectively) which confined the carriers to the InGaAs or GaAs layers, so that bulk recombination was minimized, and there was an enhanced sensitivity of the PLI to changes on the exposed surface.

The samples were cleared of native oxides by washing with 10% H F for 30 s. They were quickly inserted into the reactor, and dried in a flow of pre-purified helium

The PLI-monitoring system is shown in Fig. 1. A 5 mW 633 nm He-Ne laser beam was chopped at 200 Hz, and directed at the sample surface. The photoluminescence at 860 nm (from GaAs) or 1650 nm (from InGaAs) was focused on the slits of a monochromator and then onto a liquid- nitrogen-cooled intrinsic germanium detector (for the

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3022 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

- --, semole holder thermocouple

atom sensor

Fig. 1 Schematic of the downstream passivation system. The H,-He mixture is dissociated in the discharge and is drawn over the sample and the calorimetric atom sensor by a mechanical pump.

InGaAs) or a red-sensitive photomultiplier tube (PMT) (for the GaAs). The detector signal was then filtered through a lock-in amplifier tuned to 200 Hz, and fed into a chart recorder. With this arrangement the PLI from the semicon- ductor film could be monitored in situ as a function of real time during the entire treatment process. Note that any charge carriers (electrons or holes) which might be created directly by, for example, the recombination of gaseous atoms on the semiconductor surface cannot contribute to the mea- sured signal, because only carriers whose concentrations are modulated at 200 Hz are detected by the lock-in amplifier.

A stream of H atoms at a partial pressure of ca. 1 mTorr was created by passing a 10% H, in He mixture through a microwave discharge at a pressure of 0.3 Torr. When the mixture was changed to ca. 0.2% H2 in He the partial pres- sure of H was ca. 0.1 mTorr. A coil of platinum wire was employed as an isothermal calorimetric sensor to determine the hydrogen atom concentration by measuring the heat of recombination of atoms on the coil surface.17

Nitrogen atoms were prepared by discharging a 5% N,-He mixture. N-atom concentrations were estimated by titration with NO.

Atomic oxygen was obtained under three conditions: (a) by discharging 300 mTorr of molecular oxygen, which provided a partial pressure of 0 of Po = 10 mTorr, (b) by discharging a 1 : 50 mixture of O2 : He yielding P, = 1 mTorr and (c) by discharging He (which has a small partial pressure of 0, present as an impurity), which provided a Po 4 0.1 mTorr.

Sulfur atoms were prepared by a technique described by Fair and Thrush'* in which H,S is added to an H-atom stream. S atoms are then formed by the following sequence of rapid reactions :

(1)

(2)

H + H,S -+ H, + HS

H + HS --* H, + S

which yields one S atom for every two H atoms consumed.

Resuits Exposure of GaAs and InGaAs to Atomic Hydrogen

Fig. 2 illustrates the changes in the 860 nm PLI from GaAs when it was exposed to a sequence of 5 and 10 s bursts of

3 0

3 a

3

GaAs H-on H-on H-Qn]

I

0 5 120 timds

Fig. 2 PLI from GaAs after exposure to H atoms at partial pres- sures of 1 mTorr for 5 s and then for 10 s several times

atomic hydrogen at a partial pressure of ca. 1 mTorr. The PLI increases while the discharge is on and then rises to a higher plateau after the discharge is shut off. Furthermore, it is possible to cycle between these two passivation levels (10 x and 25x the original) almost indefinitely, as long as the exposure to H atoms is kept short.

The 1520 nm PLI from InGaAs showed a very similar response to H atoms (ca. 10 x initial PLI with H on, and ca. 25 x initial PLI with H of€). The only significant difference between the responses of GaAs and InGaAs lies in the effect of continuous exposure to H atoms. Although the GaAs PLI maximum increases, and then plateaus with continuing expo- sure to pulses of H atoms, the InGaAs PLI passes through a maximum after only a relatively short exposure to H atoms. In this regard it is probably significant that exposure of GaAs to this partial pressure of H atoms, for ca. 1 h, resulted in some etching of the layer. On the other hand, exposure of the InGaAs to similar partial pressures of H atoms resulted in no etching, although some surface roughening appeared to occur when the recombination of hydrogen atoms heated the surface to temperatures in excess of the melting point of indium (135 "C). Subsequent experiments with InGaAs were therefore conducted in only two modes in order to eliminate heating the surface: (a) 500 ms pulses of 1 mTorr H atoms and (b) continuous exposure to pressures of <0.1 mTorr of H atoms.

Fig. 3 illustrates the effect of exposure of an InGaAs surface to a long series of 500 ms pulses of H atoms. The InGaAs PLI rises after each of the first seven pulses (recall that part of each rise occurs during the H-atom pulse and part of it occurs immediately after the pulse). After the first seven pulses, further exposure to H atoms causes a decrease in the PLI. There was no visible etching of the surface with this treatment. When this sample was removed and washed with dilute HF the same PLI response to H-atom pulses could be repeated.

A continuous exposure of InGaAs to 0.1 mTorr of H atoms caused the PLI to pass through the same maximum after a few seconds, and then to decrease to the same level as that which is recorded at the end of the experiment shown in Fig. 3.

Exposure of InGaAs to N atoms

An HF-washed InGaAs surface was subjected to the sequence of exposures shown in Fig. 4. It was first passivated by two 5 s bursts of H atoms. The first exposure took the PLI to near the maximum. The next burst simply reduced the PLI by a few per cent. Following this, the surface was exposed to a series of 300 ms pulses of 3 mTorr of N atoms.

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N

3023

N(3 s

i f

H-off

LnGaAs H

id etc.

k

LV time/min Fig. 3 PLI from InGaAs resulting from a series of 500 ms pulses of H atoms at a partial pressure of 1 mTorr

After each N-atom pulse there is a slow recovery to a PLI level that became progressively lower with each pulse. When the PLI was reduced to about 8% of the maximum, the surface was then exposed to 0.1 mTorr of H atoms for 5 min. The effect of this exposure was to take the PLI through the maximum observed earlier. Finally the passivation level was again destroyed by N atoms, and recovered by exposure to H atoms, showing that the entire process could be repeated indefinitely (see changes at 27 and 34 s). Using XPS we could find no evidence for the incorporation of N into the InGaAs surface after exposure to N atoms at room temperature. As long as the discharges were not kept on for too long, and the atom concentrations were not made too large, the effects described above could be repeated indefinitely.

Exposure of InGaAs and GaAs to Atomic Oxygen

After a 3 s exposure of InGaAs to 10 mTorr of atomic oxygen, the effect of 0.5 s pulses of atomic hydrogen on the PLI is shown in Fig. 5. The 0 atoms quickly reduced the PLI to a value lower than that of the original sample. The sub- sequent exposure of this ‘ 0-atom oxidized InGaAs surface’ to pulses of H atoms resulted in behaviour that differs qualit-

r-(

c3 a

10 30 timelmin

Fig. 4 PLI from InGaAs during a series of 300 ms pulses ot 3 mTorr of N atoms after two 5 s pulses of 0.1 mTorr of H atoms

2

o( ulse) P Htoulses)

I 20 time/min

Fig. 5 Effect on PLI of the sequence of: 1 mTorr of H for 2 s, 10 mTorr of 0 for 3 s, followed by a series of 300 ms pulses of H at 1 mTorr. The letters A-E indicate the points at which X P spectra were taken.

atively from that seen before exposure to 0 (Fig. 3). Whereas the un-oxidized sample PLI responds to the initial H-atom pulses by slowly rising to a plateau when the gaseous H atoms are removed, the oxidized sample responds to an H-atom pulse by rising instantly to a peak PLI, and then slowly falling to a plateau when the H atoms are gated off. The only treatment which would return this ‘0-atom oxi- dized InGaAs surface’ to the behaviour shown in Fig. 3, was an H F wash.

Fig. 6 shows the effect of an 0 atom pulse on the PLI of an H atom passivated GaAs surface, and the effect of this oxida- tion on the response of the PLI to subsequent H-atom expo- sure. In contrast to InGaAs, the response of an oxidized GaAs surface to H atoms is qualitatively similar to the response of a clean GaAs surface, although it does take a longer exposure to H atoms to raise the PLI to the maximum after oxidation. This response is consistent with evidence cited earlier indicating that H atoms can remove (or at least undercut) the oxide layer from GaAs, but not from InGaAs at room temperature.

XPS Experiments

To determine whether it is possible to identify changes in surface composition with the changes in surface passivation recorded above, X P spectra of the sample were taken at points A to E on Fig. 5. This was accomplished by exposing

50

2

J/,H-on V I : I

0 5 100 timels

Fig. 6 Effect of oxidation of GaAs on the PLI response to 0.1 mTorr of H atoms

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3024 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

several samples to the same sequence of H atoms and 0 atoms displayed in Fig. 5, and withdrawing individual samples into the XPS chamber at points B, C, D, and E.

Although, under other circumstances, we have been able to produce oxidized surfaces in which the XP spectra indicate that almost all the surface Ga and As is bound to oxygen, there was no detectable difference between the XP spectra in the vicinity of the As 2p3,, and Ga 2p3,, core levels for all five samples labelled A-E above. The relatively small amount of surface oxide that was present on all five of these samples was identical, within the experimental error of our measure- ments (5%).

Exposure to NO, NO2 and O2

These experiments were performed to see whether NO would act like N atoms by stripping off H atoms, and whether NO, and O2 would act like 0 atoms to yield an oxidized surface.

The effect of 1 Torr of NO on the PLI of InGaAs is illus- trated in Fig. 7. The effect of NO is very similar to that of N atoms.

The effect of exposure to NO, is shown in Fig. 8. Although the effect of NOz is less rapid, it resembles the effect of atomic oxygen seen in Fig. 5, i.e. once the surface has been oxidized, exposure to H atoms results in an instantaneous rise in PLI, followed by a slow fall to a plateau.

iGaAs NC

H f off

c m

P U p i f / I

i t H(5

30 timdmin

he

Fie. 7 Effect of 100 mTorr of NO on the PLI after H atom passiva- ti& and its effect on later H-atom passivation.

:nGaAs 9 1

1

ise

timdmin 0

Fig. 8 Effect of 10 mTorr of NO, on H-atom-passivated InGaAs PLI.

Although the reaction of 0, with the InGaAs surface is very much slower than the reaction of either 0 atoms or NO,, after an exposure of ca. 15 min, the PLI behaviour begins to resemble that for an 0-atom oxidized surface.

Passivation with S Atoms When an InGaAs surface, whose passivation level had been optimized with atomic hydrogen, was exposed to a stream of S atoms, generated from H,S by reactions (1) and (2), the PLI and hence the surface passivation level, was increased, within a few seconds, to a value that was 9000 times higher than that of an HF-washed sample when it is first introduced into the flow system.

Discussion GaAs + H Atoms

Our work with GaAs confirms the qualitative observations of Gottscho and c o - ~ o r k e r s ' ~ - ' ~ which showed that the PLI of this material increases very rapidly, by a factor of ca. 10, when the surface is exposed to H atoms, and then increases more slowly by an additional factor of ca. 2.5, when the H atoms are turned off. In their more recent work, Gottscho and co-w~rkers '~ attributed the initial surface passivation by hydrogen atoms to the removal of As and As,O, from the surface. These workers also observed the removal of arsenic oxides directly by attenuated total reflectance spectroscopy (ATR), but found that this occurs before any improvement in the PLI is detected. This means that either the As,O, is such an effective surface trap that its concentration must be reduced to undetectable levels before any PLI increase can occur, or arsenic oxides are not the principal carrier traps on GaAs.

Gottscho and co-workers also observed a correlation between the passivation level of GaAs and the appearance of H-As bonds (Ga-H bonds could not be seen). This observ- ation is somewhat surprising because a number of workers have noted that H atoms selectively remove arsenic from the surface of GaAs [by forming AsH,(g)]. Gallium hydrides are not detected in the gas phase2* during or after exposure to H atoms, and it has been suggested that they readily decompose on the surface to yield H,, thus leaving the surface Ga- rich., l-'* This is consistent with recent calculation^^^ that show that H atoms are bound 2 eV more strongly to As than to Ga. It would seem that although the As can be etched more easily than Ga from the GaAs surface, both can be removed and the competitive reaction dynamics simply result in more Ga on the surface than As. Nevertheless, it is quite possible that the H is firmly bound to the As, and less so to the Ga, yielding a surface that can be better passivated by species such as S atoms, which readily attach themselves to the Ga.

The subsequent rise in the PLI that occurs when the H-atom exposure is discontinued, is attributed by Gottscho et al. to the desorption of hydrogen atoms from the surface. Without trying to identify the nature of this adsorbed hydro- gen they suggested that these adsorbed atoms could be cre- ating surface states near the mid gap, and catalysing the recombination of charge carriers. H atoms could be absorbed in interstitial sites in the crystal, near the surface. Hydrogen atoms are known to diffuse readily into GaAs at tem- peratures above 200°C26 and can be introduced into the lattice by mere exposure. Although diffusion into the bulk is slow at room temperat~re,,~ sites in the immediate vicinity of the surface could be more easily occupied.28 E~perimental,~ and theoretical studies30 indicate that the most energetically

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favourable position for an absorbed hydrogen atom is in a tetrahedrally coordinated interstitial site (a ‘T-site’). H atoms are also known to assume a bond centre position in GaAs29*30 (a BC site). Any such free radical species could serve as carrier traps. Such interstitially adsorbed atoms could temporarily reduce the PLI. If these atoms are then desorbed (presumably as H2) the PLI could return to its pre- vious value.

There is a second possible explanation for the differing passivation levels in the presence and absence of H atoms: surface reconstruction. The unreconstructed (100) surface of GaAs consists of atoms with two dangling bonds sticking out. Two adjacent bonds can link together to form a recon- structed surface in which there are dimerized atoms and only one dangling bond per surface a t ~ m . ~ ’ . ~ ~ The formation of such as ‘2 x 1’ unit cell on the surface distorts that layer, making the remaining single dangling bond on each atom point in a direction that is more perpendicular to the surface. This leads to less steric hindrance between hydrogen atoms that become attached to these dangling bonds. The partial destruction of such a reconstructed surface by an influx of gaseous H atoms (which form additional bonds by breaking surface dimers) could account for a drop in PLI. Desorption of H in the form of H, when the gaseous atoms are removed could then account for the improvement in the passivation level.

InGaAs Experiments There are seven observations that we would like to draw attention to.

(1) Since a freshly cleaned surface of InGaAs shows a rise in PLI during exposure to H atoms, and an additional increase when the H atoms are shut off, both of which are of very comparable magnitude to the values recorded in identi- cal experiments with GaAs, there is no reason to believe that we are not observing essentially the same phenomenon with both semiconductors.

(2) From our XPS measurements it would appear that the changes that are responsible for the increase in the PLI of InGaAs by a factor of 25 with exposure to H atoms, for the drop in PLI by a factor of 40 with exposure to 0 atoms, and for the re-passivation of an oxidized surface by further expo- sure to H atoms are due to changes which cannot be detected by XPS.

(3) Exposure of the H-atom passivated InGaAs surface to N atoms or NO leads to depassivation. This depassivation is understandable only if such species can remove ‘surface- passivating H atoms’. We can conceive of no other explana- tion which can account for the recovery of the optimum passivation level by a re-exposure of the surface to H atoms, and explain how the process of depassivation by NO and by N atoms and repassivation by H atoms can be repeated indefinitely. If the removal of arsenic (or As,03) by H atoms has anything to do with the initial improvement in the PLI of these 111-V semiconductors, it is clear that the later rise and fall in the passivation level can have nothing to do with the removal of arsenic, since this would require the removal of In, which does not occur with H atoms.

(4) The lower PLI which occurs in the presence of gaseous H atoms is also observed in the presence of gaseous N atoms, and NO. This is recorded as a ‘transient’ effect in Fig. 4 and 7, i.e. when the H-passivated surface is exposed to N atoms or NO, the PLI drops dramatically, but then slowly recovers to a level that depends on how long an exposure the surface has had to N atoms or NO. Since such transient behaviour is seen for H atoms, N atoms and NO, it appears reasonable to attribute all three effects to the relatively weak physisorption

of H, N and NO on the surface. Such absorbed species could form effective recombination centres for charge carriers.

(5) Since we can increase the PLI of both semiconductors by at least two orders of magnitude by treatment with S atoms, the reconstruction that occurs with exposure to H-atoms must not eliminate a certain fraction of the dangling bonds. As we have s ~ o w ~ , ~ ~ * ~ ~ passivation by S atoms appears to be associated with the removal of surface arsenic, followed by bonding of S to the surface metal atoms (principally indium in this case).

(6) For an oxidized surface (see Fig. 5) the transient effect after a pulse of H atoms is in the opposite direction, i.e. the PLI is higher in the presence of atoms and falls when the atoms are gated off. An explanation of this could lie in chemi- sorption (rather than physisorption) of the H atom on the oxidized surface. This would require that ‘passivated sights’ (such as In-0-H for example) formed in the presence of gaseous H atoms, readily decompose (to In-0 + 3H2 for example) when the gaseous atoms are removed.

(7) The drop in the InGaAs PLI to ca. one-third of its maximum, as a result of extended exposure to H atoms, is more difficult to explain. It occurs for both the freshly washed (Fig. 3) and oxidized (Fig. 5) surfaces, for pulses of H atoms and for continuous exposure to H atoms (see the latter part of Fig. 4). The change is therefore not associated with any ‘surface heating’ that might occur with long exposure times. Furthermore, the data in Fig. 4 show that the entire pheno- menon is repeatable for a given surface when the H atoms are stripped off the surface by N atoms.

First it is important to note that the drop in passivation level with continuing exposure to H-atoms does not occur with GaAs, and therefore it must be associated with the indium component of InGaAs. The different behaviour of InGaAs and GaAs could lie in the fact that H atoms are constantly renewing the GaAs surface, because both the As and Ga components can be etched by H atoms.33 Hence, for GaAs, an excess of surface-bound H atoms is prevented by the volatilization of highly hydrogenated species. This is not true of InGaAs, because the indium cannot be removed by H

It is tempting to suggest that the maximum in the InGaAs PLI (see Fig. 3) occurs when the addition of H atoms results in the formation of a particularly effective surface reconstruction. The drop in the PLI from that maximum with the addition of more hydrogen to the surface would then be the result of a new surface reconstruction which has more surface states per cm2. This explanation does not account for why an 0-atom or NO,-oxidized InGaAs surface passes through the same kind of PLI maximum when exposed to H atoms (see Fig. 5). However, note from the XPS data that we may be observing changes that do not involve the dominant surface species.

Conclusion The PLIs from 300 nm layers of In,~,,Ga,~4,As and GaAs increased by a factor of 30-50 following a few seconds expo- sure to atomic hydrogen at gas-phase atom densities of ca. loi3 cmV3.

Re-exposing the GaAs to H atoms caused a reduction in the PLI for the period that the atoms were present above the surface. When the gaseous atoms were removed, the PLI resumed its original value. Although the InGaAs initially responded similarly, longer exposures of the InGaAs to H atoms were found to reduce the PLI to ~ 4 0 % of this maximum. We associate this difference between GaAs and InGaAs with the fact that H atoms can etch away both gallium and arsenic, but not indium. To explain how the higher PLI can be recovered by treatment with N atoms, a

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surface reconstruction unique to an indium-covered surface had to be invoked.

Exposure of H-atom-passivated surfaces of InGaAs to N or NO reduced the passivation to the original level, from which the repassivation by H-atoms could be repeated. The same was found to be true when the InGaAs was exposed to 0 or NO,, although its response changed irreversibly. We attribute this to an oxidation of the indium which cannot be reversed by exposure to H atoms at room temperature. Since XP spectra of the surfaces under all these conditions were found to be indistinguishable, the surface changes that occur must involve a small fraction of the total surface sites.

We have also shown that treatment with atomic sulfur (after H-atom exposure) provides the most effective passiva- tion of an InGaAs surface.

If our interpretation of the PLI changes is correct, it is quite possible that this technique can be used to follow the kinetics of addition and abstraction reactions on the surfaces of semiconductors, in situ, in real time. What we require now is greater certainty about the nature of the sites that undergo these reactions. We hope that our proposed ATR-FTIR and HREELS studies coupled with the temperature dependences of these processes, and temperature-programmed desorp- tions, will provide better evidence for the nature of these surface sites.

The authors are grateful to Bell Northern Research for pro- viding the semiconductor films used in this work and the Natural Sciences and Engineering Research Council of Canada for financial support. We also wish to thank Phil Wong for conducting the XPS measurements.

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