Adsorption of oxygen on clean gaas surfaces investigated by ultraviolet photoemission

Journal of Electron Spectroscopy and Related Phenomena, 42 (1987) 73-87 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

ADSORPTION OF OXYGEN ON CLEAN GaAs SURFACES INVESTIGATED BY ULTRAVIOLET PHOTOEMISSION

D. FLAMM*

and A. MEISEL

Sektion Chemie der Karl-Marx-Universitiit,

Talstr. 35, Leipzig 7010 (G.D.R.)

E.-H. WEBER Zentralinstitut (G.D.R.)

fiir Elektronenphysik

(First received 21 October

der Ad W der DDR, Hausvogteiplatz

%7, Berlin 1086

1985; in final form 2 July 1986)

ABSTRACT The adsorption of oxygen at exposures of up to 1O’L on differently oriented, ion-bombarded and annealed GaAs surfaces was investigated by UPS. Coverages eAsfor the clean surfaces and oxygen coverages for the oxygen exposed surfaces were estimated by additional SXPS measurements. It was concluded that at small exposures molecular and atomic adsorption are comparable in quantity and that atomic adsorption (oxidation) becomes maximum at OAs% 0.2 for (111)Ga and (001) surfaces. Bonding of oxygen molecules should involve Ga sites. Specific bonding of oxygen atoms (0-Ga or O-As) was not indicated by the two stable UPS peaks as they occurred for arsenic coverages from 0 to 0.5 and did not shift their energy positions. They simply indicate the two states of the adsorbate atoms, single and double bonds, to substrate atoms. For the surfaces prepared here, monolayer coverage by oxygen was obtained at about 1O”L. Likewise adsorption of H,O was investigated. INTRODUCTION

The interaction of oxygen with GaAs surfaces has received much attention in the past. In this paper we will concentrate on the question whether oxygen at the surface adsorbs in molecular and/or in atomic form. In the region of oxygen exposures larger than about lo5 to lo6 L, some years ago, using cleaved (110) surfaces, this question was definitely answered, namely in favour of atomic adsorption (i.e., dissociation of the 0, molecules upon adsorption) by Stiihr et al. [l] from surface EXAFS investigations. Unfortunately, this direct method needs surface coverages above half a monolayer, and thus exposures above 10gL 0, were used. Evidence on dissociative adsorption was also given by Brundle and Seybold [2]. In He(I1) excited photoemission at exposures > 108L 0, they essentially found one oxygen-induced structure at z 5 eV, which is expected for atomic oxygen, and did not find two peaks in 0 1s *Now at Zentralinstitut Leipzig, 7050, G.D.R.

0368-2048/87/$03.50

fiir Isotopen- und Strahlenforschung

0 1987 Elsevier Science Publishers

der AdW der DDR, Permoserstr.

B.V.

16,

74

photoemission at presumably comparable exposures, which again indicates mainly one adsorption form. In the region of small exposures up to about lo5 L 0,) with which we are mainly concerned in this paper, one group of authors [CM] came to the conclusion that the oxygen is dissociated at adsorption and forms chemisorptive As = 0 and bridging Ga-O-As bonds. These results were backed up by exposing evaporated Ga and As films to oxygen [6,7]. These results were also supported by Webb and Lichtensteiger [8] and Bartels et al. [9, lo]. Another group of authors [ll-141 using ultraviolet and X-ray photoemission combined with Auger results obtained information on room temperature oxygen adsorption in two forms (u and /I forms), which were interpreted as arising from chemisorbed oxygen in molecular form (c( form) and as atomically bound oxygen (B form). The results were mostly obtained for polar surfaces prepared by ion bombardment and annealing (IBA) rather than by cleavage as for the (110) surface. Thus, both in the investigated sample surfaces and in the interpretation of the results there are differences, although some features in the He(I) excited ultraviolet photoemission spectra coincided (compare [3] and [12]). Bartels et al. [lo] using Auger electron and electron energy loss spectra also discussed low-exposure results as being partly influenced by molecular adsorption. At small exposures and low temperatures, even physisorptive bonding was observed for clean (110) surfaces at 45 K [15] or for Ga metal surfaces at 20 K [16]. On raising the temperature to 77 K and above, a transfer to chemisorptive molecular bonding was found. In [17] it was shown for (001) surfaces that two UPS peaks at 6.1 and 10.2 eV, which vanish at annealing, resemble the two UPS peaks in Ga metal spectra. In Fig. 1, ionisation spectra of gaseous molecular oxygen [Ml, spectra of physisorbed oxygen at 20 K and of chemisorbed oxygen at 77 K for Ga metal surfaces [16] and of oxygen chemisorbed on Ga rich GaAs (001) surfaces at RT [17] are compared. This led us to the assumption that molecular chemisorption gives rise to the two UPS peaks mentioned above. Furthermore, in ref. 17, evidence was found for another chemisorption form which could be the atomic absorption noted at the higher exposures discussed above. Oxygen adsorption results for exposures up to 105L are presented for the polar (OOl), (lll)Ga and (iii)As surfaces and for the (110) surface prepared by ion bombardment followed by annealing. Surface composition (As/Ga ratio) of the clean surfaces was estimated by measuring SXPS spectra with Zr ll4[ excitation. Oxygen coverage was obtained from SXPS measurements at exposures up to 1013L 0,. From the observed UPS features which were dependent on the As/Ga ratio, suppositions on the mechanism of molecular and atomic adsorption at small exposures were made. EXPERIMENTAL

The GaAs crystals used were both Te doped (n z 5 x 1017cm-3) and undoped (n x 3 x 1015cm-3) with orientations (llO), (OOl), (lll)Ga, and (iii)As.

75

02 (gas) I (eV1 16 16 11 11 11

15 -

I 14 ’

“1

a

I

I 12

1”

‘1’

0 10 5 BINDING ENERGY (eV)

Fig. 1. Compilation of spectra from emission for molecular oxygen. (a) Ionisation spectrum of gaseous molecular oxygen [18], (b) UPS difference spectrum for physisorbed oxygen at Ga metal surface [16], (c) UPS difference spectra for molecular oxygen adsorbed at GaAs (001) surfaces taken from [17] (102 and lO’L), (d) UPS d’ff 1 erence spectrum for chemisorbed oxygen at a Ga metal surface [16]. All UPS spectra were He(I) excited.

Before introducing them into the measurement chamber the crystals were etched in hydrogen, 0.07% HCl mixtures at about 870°C (gas phase etching [19]). About 10pm of material was removed and crystals, free of polishing and etching contaminants, remained. In the preparation chamber of the electron spectrometer the samples were cleaned by 400 eV Ar+ bombardment (6 PA cm-“) followed by 670 or 770 K annealing. Bombardment-annealing cycles were repeated as long as C and 0 XPS

76

peaks were visible and were then continued until the UPS oxygen derived features vanished. Part of the (001) surfaces after cleaning were investigated by LEED. A (4 x 1) structure was observed, which according to Bachrach et al. [20] should be a distorted c (8 x 2) structure. It was also found by Ludeke and Koma [21]. According to refs. 22-24, the surface in this state should be Ga-rich with an As coverage of about 0.2. For the photoemission investigations a VG ESCA III spectrometer was used. Ultraviolet photoemission was excited by He(I) and the spectrometer resolution was about 300meV. XPS investigations were performed with unmonochromised Al Ku radiation, mainly for measuring the 0 1s peak height and structure, and with surface sensitive Zr Ml radiation (hv = 151.4eV) [25] to estimate the As/Ga ratio for the clean surfaces. For oxygen exposition the crystal was shifted to the preparation chamber where dry oxygen was admitted using a gas inlet valve. The pressure was measured using a cold cathode head. RESULTS

UPS with oxygen adsorption

and post-annealing

Figures 2 to 5 present the results of oxygen adsorption and post-annealing for (OOl), (lll)Ga, (iii)As, and (110) surfaces and appropriate difference spectra. The intensity scales in all figures are proportional to the abscissa scale (b)

(01 DIFFERENCE GaAs(001l hv

~212

SPECTRA 1 50eV

GaAs(0011 eV

’

1

2

61eV

I

3

76eV

-.. I

-

/

,

I

10

I

BINDING

I

I,

I1

I

ENERGY

!,

I,

5

0 IeV)

10 -BINDING

I

I

I

I

,I

5 ENERGY

I

I

I,

0 Id’)

Fig. 2. (a) Ultraviolet photoemission spectra of a GaAs (001) surface after ion bombardment and annealing (clean surface, fl,, z 0.3), after exposures of lb and 104L oxygen and after vacuum annealing at 770K. (b) Difference spectra for oxygen exposures of lo’, lo*, and 1O’L and after the subsequent vacuum annealing. Four peaks at 5.0,6.1, 7.6, and 10.2 eV can be observed.

77

G~AslllllG~ /,I

-6lNDlNG

\

h”

ENERGY

:212

e”

[eV)

DIFFERENCE GaAs(llllGa

-BINDING

(b

SPECTRA

1

ENERGY

(eV)

(C

DIFFERENCE GoAsIllllGa

SPECTRA

1 1

2

I,,

I

10 -BINDING

,

I

5 ENERGY

,

,

,

,

0 (eVl

Fig. 3. (a) Ultraviolet photoemission spectra for a GaAs (111)Ga surface cleaned by ion bombardment and vacuum annealing at 670 K (---) or at 770 K (-) with As coverages in the clean state of8, x 0.2 or 0.1, respectively. (b) Difference spectra of the sample annealed at 670K (--- in a), intensity scale reduced by a factor of two compared to the other difference spectra (sample more sensitive). (c) Difference spectra for the sample annealed at 770 K (in a).

with the same factor of proportionality. For the (001) surface four peaks could be clearly resolved (Fig. 2b). Peaks 2 and 4 are unstable and vanished at 770 K annealing. They should correspond to loosely bound oxygen. Peaks 1 and 3 were stable. It can be seen that at small exposures (102L), first peaks 2 and 4 develop, and then at larger exposures (105L) they become overwhelmed by peaks 1 and 3. In Fig. 3 the results for (lll)Ga are presented for two pre-treat-

(al GoAsl7WAs hv

=21 2 eV

(b) DIFFERENCE GaAs(iti)As

SPECTRA

II’

4

ANNEALE

CLEAN L_ L

I, I 10 --BINDING

I

I

I,

I,

5 ENERGY

I

I,

0 l&I

,,

10 -BINDING

I 41 I ( I II 5 ENERGY

1, 0

(eV)

Fig. 4. (a) Ultraviolet photoemission spectra for a GaAs (iii)As surface cleaned by ion bombardment and annealing at 670 K (---) or at 770K (-) with As coverages in the clean state of lI,,, x 0 or 0.5, respectively. (b) Difference spectra with foregoing annealing at 670 K (---) or at 770K (-) according to (a). i.e., following the last ion-bombardment, annealing at 670 and 770K, respectively. In Fig. 3(a) it can be seen that the lower-annealed sample was far more sensitive to oxygen exposition. Figs. 3(b and c) represent the difference spectra for the samples pre-annealed at 670 and 770 K. Peaks 2 and 4 cannot be seen as clearly as for the (001) surface. Peak 4 is seen in Fig. 3(a) for the 770 K annealed sample but cannot be resolved in the difference spectra of Fig. 3(c) due to an emission change from valence band states. Peaks 1 and 3 are present for both surfaces in Figs. 3(b and c) but with different. ratios. Fig. 4 shows results for 670 and 770 K pre-annealed (iii)As surfaces. Distinct features cannot be resolved for the 670K pre-annealed sample. For the 770K annealed sample, peaks 1 and 3 are approximately equal in height, but peaks 2 and 4 are scarcely visible. Similar results for the (110) surf&e are presented in Fig. 5. men&,

UPS and SXPS

with H,O adsorption

and post-annealing

As it may be that a small unknown H,O content in the oxygen could influence the UPS spectra, separate measurements with H,O exposition were undertaken. In Fig. 6, difference spectra are presented for a (001) surface exposed to 10’ L H,O (a), to 1O’L H,O (0.1 Torr, 1000 s) (b), and annealing to 570 and 770 K for 5 min, (c) and (d), respectively. The peak positions are similar to those obtained for dry oxygen exposition, the peak heights being slightly smaller. Only three peaks were observed, one of them coinciding with the peaks obtained for 0, adsorption. Biichel and Liith [26] also found for cleaved (110) surfaces three H,O induced peaks at binding energies of 3.2, 5, and 9.4eV. At

79 T---

(b)

GaAslllOl hv =21(1

DIFFERENCE GoAsillOl

eV

\

SPECTRA

3 2’ I,1

4

ANNEALEI

\

i

J \

L_

CLEAN

I _

(,

( I I 10 BINDING

I,

I I I,, 5 0 ENERGY I@,)

b

5

i0

--BINDING

ENERGY

kV)

Fig. 5. (a) Ultraviolet photoemission spectra of a GaAs (110) surface cleaned by ion bombardment and annealing, BAs 2: 0.4. (b) Difference spectra.

I

I

I

I

1

I

10

-

BINDING

I

I

1

I

1

I

I

5 0 ENERGY (eV)

Fig. 6. UPS difference spectra for H,O adsorbed on GaAs (001). (a) 16L, subsequent vacuum annealing at 570 and 770 K, respectively.

(b) lO*L, (c) and (d) for

80

very large H,O exposures Webb and Lichtensteiger [8] observed a main UPS peak at 6.1 eV which seems to correspond to our main one at 5.8eV. After annealing at 770K, stable peaks at 5.8 and 7.6eV remained, whereas for dry oxygen stable peaks at 5.1 and 7.6eV were found. Only one unstable peak at 9.9eV was observed but for oxygen exposition two unstable peaks at 6.1 and 10.2 eV were obtained. SXPS spectra with Zr MC excitation have been measured. In the As 3d and Ga3d parts of the spectra neither chemically shifted components nor an increase in linewidth were observed. But, on increasing the H,O exposure from 0 to lO*L, the emission intensity (peak area) of the As3d and Ga3d peaks likewise decreased to 0.67 which indicates the presence of physically adsorbed species. SXPS for clean and oxygen exposed surfaces ZrM[ excited SXPS was mainly undertaken to obtain the As/Ga ratios of peak area for the As3d and Ga3d levels of the clean surfaces. The obtained ratios for the surfaces discussed previously are presented in Table 1 together with As coverages OAsestimated as described in Appendix 1. SXPS spectra were also measured with oxygen expositions up to 1013L. Fig. 7(a) shows a series of spectra for the (001) surface, ion bombarded and annealed at 770 K. The results are similar to those for (110) surfaces reported in ref. 3. The SXPS spectra were split into unshifted and chemically shifted components using a trial and error procedure where peak heights, energy positions, and half widths were fitted for minimum standard deviation between the measured and calculated lines [27]. The linewidths of the unshifted As 3d and Ga 3d components amounted to about 1.4 eV mainly resulting from the natural linewidth of the Zr MC line (x 1 eV [28]) and the spectrometer broadening. For the chemically shifted components (chemical shift 3.2-3.3 eV for As3d and 0.95l.OeV for Ga3d) larger linewidths were observed. Fig. 7(b) shows in the lowest curve the ratio of the sum of the shifted to the sum of the unshifted TABLE

1

Measured As 3d and Ga 3d peak areas (rel. units), the As/Ga peak area ratios and the estimated As coverages 9, for clean surfaces from SXPS measurements and according to Appendix 1 Surface

W) (11l)Ga

(iii)As (110)

Annealing

non-annealed 770 K, 15 min 670 K, 15 min 770 K, 15 min 770 K, 45 min 670 K, 15 min 770 K, 15 min 770 K, 10 min

As 3d + Ga 3d peak area sum

77 116 72 63 80 75 86 79

Peak ratio As/Ga

0.8 0.75 0.85 0.75 0.74 0.67 1.03 0.8

6As (rl = 1)

(tl = 1.07)

0.36 0.32 0.22 0.11 0.10 0.00 0.54 0.39

0.30 0.27 0.16 0.05 0.04 - 0.06 0.51 0.35

81

(a) 110-

KIN. ENERGY (eV)-150 130 I

I

GaAs (001) hu = 151.4 eV

60

40

-

BINDING

1

Go 3d

20 ENERGY (eV)

0

’

4

8 12 lg (EXPOSURE)

14

Fig. 7. (a) Zr Ml excited SXPS photoemission spectra for a (001) surface in the clean and oxygen exposed states. (b) Lower curve: ratio of the sum of the chemically shifted components to the sum of the unshifted components taken from (a). Using this ratio and the attenuation lengths of photoelectrons, oxide coverage was estimated according to Appendix 2 (central curve). Oxygen coverage was assumed as 1.5 times oxide coverage (upper curve).

As 3d + Ga 3d components from which, using a procedure described in Appendix 2, the oxide coverage could be deduced (central curve). It was found that for As 3d a remarkable amount of shifted components only appears above lo6 L, but for Ga 3d, shifted components could be observed even at lo4 L 0,. Thus, in Fig. 7(b) the contribution in the ratio of components at 106L comes exclusively from shifted Ga 3d, and at 10’ L about one fourth is due to shifted As 3d. Finally, at 1013L, slightly above one third of the shifted components is shared by As 3d. Further, in Fig. 8, the area of the sum of the shifted components for different exposures is compared to the area of the 0 2p peak also shown in Fig. 7(a). A linear increase was found which suggests a uniform oxidation mechanism for all exposures. From the observed chemical shifts, according to [2], oxidation to Ga,O, and Asz03 can be assumed on average. That is, oxide coverage must be multiplied by 1.5 to obtain the coverage by dissociated (atomic) oxygen. This is shown as the upper curve in Fig. 7(b). Monolayer coverage is attained slightly above 101’L 0,, i.e., at least three orders of magnitude smaller exposures are required for this coverage as compared to cleaved (110) surfaces [3]. At lo4 L 0, more than one tenth of the surface seems to be covered by dissociated oxygen. This explains why even at lo5 L 0,) well-defined 0 1s spectra could be measured, as described below.

82

20

10

0

0 2p INCREASE ( REL. UNITS) Fig. 8. Peak area sum of chemically shifted components oxygen exposures of Fig. 7(a). Linear increase is noted.

against

02p peak area for the various

0 1s spectra Al Ku excited 0 1s emission spectra of an oxygen-exposed (001) surface are shown in Fig. 9. For a surface exposed to 105L 0, two peaks with an energy separation of about 1.5eV were observed (right). The peak with the larger binding energy has a half width x 0.5eV larger than that of the lower-energy peak. The high binding energy peak disappears on annealing. Simultaneously, it was found that peaks 2 and 4 in He(I) UPS vanished (Fig. 2b). DISCUSSION

On account of the differences observed both in the UPS and SXPS spectra for oxygen and water exposure we conclude that H,O does not appreciably influence the results obtained for dry oxygen exposition. Using the eA8values of clean and the 19,,(oxygen coverage) values of oxygen

532 -BINDING

530 ENERGY (eV)

528 534 -BINDING

532

530

ENERGY (eV1

Fig. 9. Al Ka excited 0 1s spectra for a GaAs (001) surface after exposure to 106L molecular oxygen (right) and after subsequent vacuum annealing at 770 K (left).

83

covered surfaces, obtained in Appendices 1 and 2, as guides, the results of the UPS measurements can be discussed in a semiquantitative way. For the (001) surface the most detailed results were obtained. From Fig. 7(b), it can be seen that at lo4 L the oxygen coverage 8, z 0.15, at lo5 L 8, z 0.25, i.e., one quarter of the surface sites should be covered by dissociated oxygen. For the other surfaces the coverages can be estimated from the combined heights of peaks 1 and 3, as all intensity scales in the difference spectra are the same (see above). At 104L, peaks 1 and 2 (Fig. 2b) seem to be equal in height, so that for this surface, after evacuation, the loosely bound oxygen should equally amount to about 0.15 monolayers. Formerly, the loosely bound oxygen was discussed as dissociated oxygen adsorbed at defect sites [4] (on account of the saturation observed at about lo2 L 0,, compare the three curves of Fig. 2b for the heights of peak 4) or as adsorbed molecular oxygen [12]. In [17] we supposed it to be chemisorbed molecular oxygen, This is further confirmed by Fig. 9, where after exposition two binding forms of oxygen were found and after annealing one binding form is lost. Likewise in Fig. 2b peaks 2 and 4 vanished after annealing. As a rule, at exposures up to 104L the molecular chemisorbed oxygen (after evacuation) seems to be present in amounts which can equal the amount of dissociated oxygen, but because of the adsorption saturation which occurs, at increasing exposures the portion of molecular oxygen becomes more and more negligible. Due to the similarity of the relative heights and separation of peaks 2 and 4 to that for chemisorbed molecular oxygen at Ga surfaces [16], molecules bridging adjacent Ga atoms can be imagined, or oxygen molecules may be bound as peroxy radicals [29]. As peaks 2 and 4 were largest for (001) surfaces, Ga surface atoms in a (001) surface bonding configuration (two unsaturated surface bonds) could be preferably envisaged as sites for molecular adsorption. As to the adsorption of oxygen as dissociated species, or to the oxidation at small exposures, the highest response was observed for eAs x 0.2 for (lll)Ga after annealing at 670 K followed by O,, z 0.3 for (OOl), i.e., for surfaces which exhibit a Ga excess but still retain an appreciable amount of As. Thus oxygen bonding to Ga and As or to both should be envisaged. All other surfaces, i.e., those with a oAs smaller or larger than this optimum value, showed a reduced sensitivity to oxygen. For eAs x 0.25, for a (lll)Ga surface each fourth surface Ga atom should be missing, and Tong et al. [30] observed a (2 x 2) reconstructed surface structure with one unsaturated surface bond per Ga atom and one unsaturated surface bond per As atom of the second layer. In our case, the surface structure was certainly distorted. Nevertheless, oxygen could be assumed to bridge adjacent Ga and As atoms as an underlying mechanism for peak 1. This assumption was also made for cleaved (110) surfaces [3]. But, on account of the negligible amount of shifted As 3d observed in the SXPS spectra at small exposures below 106L 02, bonding to surface Ga atoms alone should be considered as a further mechanism. At 770 K annealing (Fig. 2b) a loss in height of peaks 1 and 3 resulted. Annealing of oxygen exposed (110) surfaces [3] even at 37O’C yielded a nearly complete loss of shifted As 3d, i.e., at our surfaces after

iowest

annealing at 770 K. As oxides should be no longer present. Nevertheless, peaks 1 and 3 remained at the same energy positions. Peak 3, also stable against annealing, was present for all investigated surfaces (f?,, z 0 to 0.5). It could correspond to Ga-oxygen bonding, as it was also present for O,, x 0 ((iii)As, Fig. 4b). Considering that as well as peak 1, peak 3 could be due to oxygen bonding to surface Ga or As atoms or to both, these peaks could simply represent the two possible binding forms of atomic oxygen to GaAs surface atoms, namely by double bonds to one, or by single bonds to two surface atoms. Ranke et al. [13,14] at small exposures observed a strong orientation dependence of oxygen adsorption for structurally perfect surfaces. Arsenic coverage is also dependent upon surface orientation and surface reconstruction, which are probably closely related. For slightly imperfect surfaces as ours were, a definite As coverage certainly implies definite average bonding configurations of the surface Ga and As atoms, and at distinct As coverages optimum configurations for a reaction with oxygen should be obtained. These optimum oxidation conditions for our sputtered and annealed surfaces were apparently attained at 8,, x 0.25 for (111)Ga and (001) surfaces. CONCLUSIONS

For small oxygen exposures up to 104L it was found that chemisorbed molecular oxygen and atomic oxygen should be present in comparable quantities. Molecular adsorption saturates at this or smaller exposures and, therefore, at higher exposures becomes negligible compared to atomic adsorption. In ultraviolet photoemission, peaks at 6.1 and 10.2 eV should correspond to adsorption of molecular oxygen and peaks at 5.0 and 7.6eV to bonding of atomic oxygen. This was confirmed by the splitting of the 0 1s level at small exposures and by annealing experiments. As to the mechanism of molecular chemisorption, oxygen molecules bridging adjacent surface atoms or bound as peroxy radicals may be assumed. For atomic adsorption at low exposures it was found that optimum sensitivity was reached for surfaces with eAs z 0.2 to 0.3, for conditions where Ga as well as As atoms were present, i.e., both should be involved in the oxidation. It seems that this involvement could concern the precursor state of oxidation (molecular adsorption), as at small oxygen exposures in SXPS spectra an appreciable amount of shifted As was not found. On the other side, for Ga a shifted component was observed at exposures as small as 104L 0,. From the UPS peaks 1 and 3, conclusions as to the specific bonding to surface Ga or As could not be obtained. On account of the varying amounts of surface As and Ga,, including the case of oxidised Ga alone (after annealing), peaks 1 and 3 should only give information on the bonding of the adsorbate (oxygen) atoms themselves.

85 ACKNOWLEDGEMENTS

The authors express their sincere thanks to Prof. Dr. H. Berger for kindly supporting these investigations, to Dr. L. Diiweritz and Dr. S. Dorshchand for preparing the surfaces by gas phase etching, to Dr. L. Daweritz for numerous discussions on crystal surface structure problems and to Dr. H.-J. Miissig from the Department of Electron and Surface Physics, Technical University Dresden, for supporting the LEED investigations with his apparatus. APPENDIX

1

Estimation of arsenic coverage Assuming that the SXPS As/Ga ratios observed for different surfaces under varying preparation conditions are caused mainly by composition changes within the uppermost surface layer, the As coverage, oAs, can be estimated from a layer model of the crystal according to procedures described in [ll, lo]. Layer spacings for the different surfaces amount to (a/4) 4 for (llO), a/4 for (OOl), and alternating a/(4$) and 3a/(4$) for the (lll)Ga and (iii)As surfaces (a lattice constant). Following Seah and Dench [31], the attenuation lengths of photoelectrons were calculated from the formula for elements, and for hv = 151.4eV, values of 0.44 and 0.48nm for As3d and Ga3d photoelectrons were obtained. For the average escape angle of 60“ from the normal used in our experiments these values must be bisected. Fitting the measured peak areas using a least squares against the As/Ga emission ratios, interpolating procedure, one gets the desired tIAsvalues. The following values for three of the sputtered and annealed surfaces were taken from the literature as fixed entrance values 0,, for this procedure: eAs = 0.2 for (001) annealed at 770 K which showed a (4 x 1) structure [21 to 231; eAs = 0 for (iii)As annealed at 670 K and 0,, = 0.5 for (iii)As annealed at 770 K [ll]. For the ratio q of sensitivity factors for 3 d photoemission (with Zr Ml excitation) from As and Ga levels an optimum value of 1.07 was obtained, close to the supposed value q = 1 [32]. The error boundaries of the estimated 0,, values are approximately + 0.1. The results are not quite right as the reconstructed (001) surfaces may be rough [24] and composition changes for sputtered and annealed surfaces may go beyond the uppermost surface layer. The results and problems will be discussed in more detail at a later date [33]. APPENDIX

2

Oxide coverage Oxide coverage oxide components

was estimated using the same method as in Appendix 1. The were assumed to weaken the emission from the substrate

86

components and thus the oxide layer thicknesses could be calculated. For the calculations, oxidation in monolayers was assumed. This again is not quite right, as even for cleaved (110) surfaces, bulk oxidation was proposed to occur at defects [34]. Thus, the values in Fig. 7(b) are only rough estimations. As the oxide components grow linearly with the measured 0 2p peak area, the amount of dissociated (atomic) oxygen is proportional to the oxide components. Supposing Ga,O, and As,O, as the main oxidation products according to the chemical shifts observed, and using [2], atomic oxygen coverage was calculated in Fig. 8 as 1.5 times (oxide coverage).

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