Characterization of sapphire surfaces by electron energy-loss spectroscopy

Surface Science 273 (1992) 427-436 North-Holland

Characterization spectroscopy

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of sapphire surfaces by electron energy-loss

E. Gillet and B. Ealet Laboratoire de Microscopic& Dffractions Elecironiques, CNRS-URA, 797 Avenue EscadrilleNormandie-Niemen, 13397 MarseilleCedex 13, France Received 12 November 1991; accepted for publication 28 January 1992

By combining four surface analysis methods, AES, LEED, EELS and XPS, the stoichiometry and the crystallographic structure of cu-Al,03 surfaces can be related to specific band gap levels. A preparation process is described which permits one to obtain ordered alumina surfaces in two orientations, (0001) and (iO12). Thermal heating under ultrahigh vacuum induces stoichiometry changes. Equilibrium-reduced structures, characterized by LEED patterns and modifications in the band gap structure, are identified. It is shown that intrinsic surface states (JSS) are specific of clean stoichiometric surfaces. These levels, located at 5.7 and 4.1 eV below the conduction band minimum (CBm), are shifted and broadened on reduced surfaces decreasing the band gap value.

1. Introduction

Surface science studies have evidenced that the structure of free surface of solids determines their reactivity and bonding properties. Consequently, changes in stoichiomet~, ordering and electronic structure of sapphire surfaces are expected to play an important role on the growth, morphology and stability of metal depositceramic interfaces. In order to understand the role of surfaces in metal-ceramic interaction, various forms of alumina which are used as substrates for metal deposition, have been characterized by imbuing low-energy electron diffraction (LEED), Auger electron spectroscopy @ES), electron energy-loss spectroscopy (EELS) and Xray photoelectron spectroscopy (XPS) analysis. We have shown in previous short report that band levels generated by surface defects (e.g. dangling bonds or oxygen vacancies) are very sensitive to the surface stoichiometry and to the adsorption of metal atoms [l-4]. Thus, in the field of metal-ceramic bonding studies, the band gap structure appears as the fingerprint of alumina surfaces. In this paper, we compare the crystallographic and the electronic structure of (iO12) and (0001) sapphire surfaces following var~39-6028/92/$05.#

ious treatments (ion bombardment and annealing) which are known to cause modifications in surface ordering and stoichiometry [3,5-71. The experimental procedure is described in section 2 and results are given and discussed in sections 3 and 4.

2. Experimental

methods

2.1. Sample preparation The (Y-Al,O, single crystals oriented and polished on the ~~1~ and 6.012) faces were supplied by CRYSTAL-TEC (CEN Grenoble). The samples having approximate dimensions of 5 x 7 x 0.1 mm were strapped by tungsten wires on a molybdenum ribbon and mounted on the sample holder in an ultrahigh vacuum surface analysis chamber with a base pressure of 2.6 x 10-s Pa. The crystals were heated by radiation from the resistively-heated molybden~ ribbon and temperatures up to 2040 K can be attained. Sample temperature is assumed to be the same as that measured by a thermocouple spot-welded to the molybdenum ribbon. The error in measuring the surface sample temperature is estimated to be

0 1992 - Elsevier Science Publishers B.V. Al1 rights reserved

428

E. Gillet, B. Ealet / EELS

less than f 100°C. The temperature values given in the following are estimated temperatures. Alumina samples were cleaned under vacuum by ion bombardment using a rastered Ar+ ion beam (1 keV, 10 mA) combined with heating in oxygen pressure (750 K, 1.3 x 10e4 Pa 0,). The surface cleanliness was monitored by AES or XPS. No LEED pattern could be observed from the sapphire surfaces that are completely disordered (improperly called “amorphous surfaces”) following the polishing and smoothening processes. Thus, after the sample cleaning the correct (YAl,O, surface structures were restored by an annealing (1300 K for 1 or 2 h) followed by a rapid cooling to room temperature for LEED observation. This procedure was repeated until a 1 X 1 diffraction pattern was observed. The surface is then ready to be submitted to various treatments with the intention of introducing surface defects. The crystal surface was analyzed by AES, EELS and XPS before and after being treated. It has been shown [g-101 that ion bombardment of an oxide may reduce the surface due to preferential sputtering of oxygen anions. Annealing treatments at high temperature under vacuum can also create anion vacancies and sometimes surface reconstructions [2-6,101. In this work, similar structures have been observed

on sapphire surfaces

by LEED after a reducing treatment by ion bombardment (1 keV, 10 mA) followed by an annealing under vacuum (1300 K, 1 h). Thus the present paper will be focussed on sapphire surfaces prepared by heating treatments. 2.2. LEED, AES, EELS and XPS analysis LEED, AES and EELS experiments were performed in the same chamber which was equipped with a CMA and a 4-grid LEED optics. Diffraction patterns were observed at primary energies 5 100 eV without surface charging difficulties. The CMA (Riber, 10 kV coaxial gun) was used for AES and EELS analysis with a modulation voltage of 1 V peak-to-peak. The energy resolution was better than 0.5 eV at primary beam energies below 150 eV. The instrumental broadening of the elastic peak was minimized for EELS measurements by using low primary beam energy in the 50-100 eV range. Under these conditions energy losses as low as 2.3 eV could be easily detected. In this paper, we report only on the energy losses AE which are correlated to surface states, defect levels and plasmon decays (A E < 30 eV). Auger and loss spectra were obtained under normal incidence in the first derivative mode. It appears that electron-beam damage is negligible

(a) Fig. 1. Hard-sphere models of the ideal cu-Al,O, surface for two orientations. (a) (0001); (b) (iOl2). Oxygen (white spheres); aluminum (black spheres). The 1 x 1 unit meshes are indicated and one oxygen vacancy can be seen on the two surfaces. It can be seen in fig. lb that the coordination number of Al ions, which is 5 on a 1 X 1 (iOl2) surface, becomes 4 near a vacancy.

E. Gillet, B. Ealet / EELS on sapphire surfaces

and that surface stoichiometry remains stable under the conditions currently used for AES investigations. XPS measurements were carried out in a second UHV chamber equipped with an Al Ka X-ray source (1486.6 eV) and with an hemispherical analyzer (VSW HAC 100). The resolution limit of the spectrometer is in the range of 0.6 eV for a pass energy of 25 eV. The binding energy scale is calibrated relative to the Fermi level by using gold and silver samples as reference. Nevertheless, on insulators there is a lack of Fermi edge coupling between the spectrometer and the sample and one can prefer to define the binding energies by referring the zero to the valence-band maximum (VBM). The procedure commonly used [11,12] provides a suitable position of the VBM.

3. Results 3.1. Crystallographic structure and stoichiometry of sapphire sugaces 3.1.1. LEED and AES observations After cleaning, the polished @OOl) and (1012) samples of cu-Al,O, produce only diffuse or no LEED patterns resulting from highly disturbed surfaces. AES spectra exhibit only the OKLL peak at 500 eV and a broadened peak in the energy range of the Al LVV peaks (35 and 51 eV). Surface order can be restored by heating progressively the crystal under vacuum. The 1 X 1 patterns produced by ideal @OOl) and (7012) lattices only appear after annealing at 1250 K for 2 h under vacuum or at 850 K for several hours in 1.3 x 10m4 Pa 0,. Figs. la and lb show the ball

Table 1 IA,L2,w/ZoKLuLu Auger ratio dependence of two sapphire surface orientations 0012)

@ool)

Observed structure

IAl /IO

Observed structure

0.16 0.25-0.31

1x1 J?;Txfi

1x1 2x1 and@xfi

on the structure

429

models of a-Al,O, (0001) and (iO12) planes deduced from the bulk crystallographic structure [13,14]. It can be seen that the aluminum cations of the (0001) surface are 3-fold coordinated while those of the (iO12) surface are 5-fold coordinated. The AES spectrum commonly observed on the 1 X 1 surface shows a large peak at 51 eV and another peak just visible at 60 eV that indicates two or more aluminum species existing near the surface: A13+ and Al’+ or Al+. Each species gives rise to an Lz3VV transition [15] shifted from the ideal value of 68 eV for Al0 to the value of 51 eV for A13+. We have observed that the ratio ZAIL,w/Zok,,L, of Auger peak heights for aluminum and oxygen depends on the surface structure. In table 1, it is shown that this ratio is about 0.16 for 1 x l(iO12) surface but 0.25 for 1 X l(OOO1) surface. This can be justified by assuming that the first three surface layers make up the major part of Auger signal. From this point of view, (7012) surface has Al,O, bulk stoichiometry whereas the (0001) surface seem oxygen deficient with Al,O, stoichiometry. When the annealing temperature increases (T > 1450 K) sapphire surface order changes and various LEED patterns appear. The unit-meshes that we have most frequently observed after heating treatments are listed in table 2. They correspond to equilibrium structures. During heating processes the intensities and the energies of Auger oxygen lines were monitored. It has been shown that the relative KLL electron energies and intensities are a sensitive tool in determining charge of orbitals localized at oxygen site [15-171. Then in table 3 ZKL,,L,/kL,L2, ratio and (EKLBL, - EKLILJ

Table 2 Equilibrium structures of (Y-AI~O~(iO12) and (0001) following heat treatments under vacuum 0012)

0.35

Equilibrium structure

Equilibrium structure

T (“0

1000 1400-1700

1x1 2x1 and fix$?

1000

1x1

1700

&lXa

IA, /IO

0.25-0.28

(0001)

T (“0

R45”

E. Gil& B. E&et / EELS olt sapphire surfaces

430

Table 3 Ranges of the intensity ratio and of the energy difference AE(KLL) between OKL,,L,, and OKL,L,, transitions Auger OKLL line variation IKJ.+z7 /I&L?.? 3.8-3.2 3.0-2.4

A E(KLL) (eV)

1

Reduction level Class a Class b

17-20 22-24

The results are obtained on several surfaces of different orientations and following different heat treatments under vacuum. We have made a compilation of these results giving only the energy difference following the range of the intensity ratio.

values are reported. In accordance with analysis of Ascarelli and Moretti [171, the above results indicate that oxygen ionicity decreases when surface is reduced. 3.2. Electronic structure of sapphire surfaces

To interpret electron energy-loss spectra due to interband transitions, it is necessary to know where possible initial 1eveIs are Iocated. Therefore XPS spectra were measured from (0001) and 6012) surfaces before and after annealing treatments. The results are listed in table 4. Binding energy values of Al2s, A12p, 0 Is core levels and valence-band features are in agreement with previous studies [11,12,18]. It is interesting to notice that features of valence-band density of states are similar for different forms of alumina surfaces and orientations: short or long ordered, stoichiometric or reduced, (0001) or (?012) oriented; sim-

Table 4 Binding-energy values of the core levels obtained on ordered (u-AIz03 6012) and (0001) surfaces (the energy vafues are referenced to the VBM and are given in eV) Core level

Binding energy

0 2p anti~nding 0 2p bonding Al 3s 02s A12p Al 2s 01s

3.5 7.6 10.0 20.6 71.1 116.1 530.0

35 30 25 20 15 10

5

0

AE (ev) Fig. 2. Example of electron energy-loss spectrum obtained for E, = 150 eV with a poor resolution in first-derivative mode for a (iOX 1 x 1 sapphire surface. The spectrum is dominated by three peaks at 5.3, 13.2 and 23.3 eV.

ilariy the peaks of aluminum core levels keep the same width and do not shift when the surface composition changes. 3.2.2. Electron energy- fess spectra Bulk and surface transitions could be distinguished by using primary beam energies in the 50-250 eV range. Fig. 2 shows typical EELS structures obtained from (Y-Al,O,(iO12) for a primary beam energy of 150 eV in first-derivative mode. The loss spectrum is dominated by three peaks at 5.3, 13.2 and 23.3 eV, respectively. The first two structures are found to be surface sensitive features. In contrast, the 23.3 eV peak appeared to be insensitive to the surface structure. Using a lower-energy beam which optimizes the resolution of energy-loss spectra, one finds that energy-loss structures depend on surface orientation and stoichiometry. For each of the two orientations, one can clearly identify three classes of electronic structures, respectively given by - perfect 1 x 1 clean stoichiometric surfaces (class a); - reduced and ordered surfaces giving equilibrium LEED patterns (class b); - amorphous or ion-bombarded disordered surfaces (class c).

E. Gillet, B. Ealet / EELS on sapphire surfaces

Only EELS structures which can be correlated to well-defined surfaces, i.e. class-a and class-b surfaces, are considered here. In figs. 3a-3d typical loss curves from the surfaces 6012) and (0001) are compared. It is seen that the main broad peak of fig. 2 exhibits a fine structure. In table 5

431

the energy losses which have been recorded following the analysis of five samples for each class are listed. They correspond to structures which are separated by more than 0.5 eV (resolution of the instrument) and which are observed at least on three samples of the same class.

II1

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I

I

t30

20

10

0

20

10

15

AE (0’4)

AE

WI (W

(4

20

10

0

AE (W

NJ) Fig. 3. Electron energy-loss spectra obtained for E, = 100 eV in first-derivative mode. These spectra point out the differences between the electronic structures of various sapphire surfaces: 6012) class-a and -b curves (a) and (b); @OOl)class-a and -b curves Cc)and (d).

E. Gillet, B. Ealet / EELS on sapphire surfaces

432

4. Interpretation 4.1. Crystallographic structures of reduced sapphire surfaces It is now established that ordered structures which appear after heating oxides under vacuum are due to reduced metal oxide layers [2,5,6,18]. The increase of ZA,L2,vv/ZoKL,,L, Auger ratio with annealing temperature (table 1) confirms this interpretation.

(a)

On (0001) surfaces, the observed LEED patterns can be explained by coincidence between the unit cell of a suboxide overlayer with the unit cell of the substrate. Following previous studies [5-71, the m x m diffraction pattern can be interpreted by a cubic suboxide phase (Al,0 or AlO) epitaxially grown on the substrate (see fig. 4~). A similar interpretation is suggested to explain LEED patterns observed, after reducing treatments, on (0001) Fe,O, faces [lo]. On the (1012) faces reduction proceeds by formation of oxygen vacancies each four, three,

(b)

Fig. 4. Equilibrium surface structures obtained by reducing treatments, on a (1012) surface by an ordering of oxygen vacancies along the “zig-zag” rows: (a) fi x &R45”, (b) 2 x 1; on an (0001) surface by surface reconstruction: Cc) on the left side part of the stoichiometric surface is represented, on the right side the reduced surface has been reconstructed following the Somorjai model [l].

E. Gilkt, B. Eakt / EELS on sapphire surfaces

+5.4 +l.O WI

-1.3

.

433

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-

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0.0

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_

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-

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-29.0

Fig. 5. Energy-loss spectra interpretation by interband-transition mechanism following a one-electron model. Transition values correspond to energy losses of table 5 with an accuracy of 0.3 eV. Dotted lines represent the defect states in the band gap. These states are marked by a broadening for the reduced surfaces (b and d): (a) (0001) class a; (b) @OOl) class b; (c) (1012) class a; (d) (iO12) class b.

E. Gillet, 5. E.&t

434

/ EELS on sapphire surfaces

Table 5 Energy-loss values observed in various sapphire surfaces: (a) stoichiometric 1 x 1 surfaces; (b) reduced and reconstructed surfaces Energy loss feV)

Orientation

(ioi2) a 2.3 3.2 4.0 4.6 5.3 7.4 x.7 9.5 10.5 12.2 13.2 14.1 20.9 22.2 23.3 24.4

iuQO1) -b

a

b

.

l

. l

l l

0

l

*

. 0

l

l

l

l

*

l

l

l

e * 0

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l

l

0 e

l

l

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l

0

. . l

l

0

l

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or two ions along the [02211 oxygen rows. We have yet interpreted the equilibrium LEED patterns by ordered arrangements of oxygen vacancies along [0221] oxygen row and a second direction [2]. Figs. 4a and 4b show examples of such ordered structures which give rise to 2 X 1 and fi x fi LEED patterns and correspond to the same surface stoichiometry. The number of oxygen vacancies is around 1012/cm2. These structures are obtained by formation of one oxygen vacancy each two meshes. It must be pointed out that they retain the symmetry of the underlying substrate and can be generated by changing the coordination number from 5 to 4 for only some of the cations. The reduction process on (0001) surfaces is quite different: it happens by a surface reconstruction which changes the hexagonal unit mesh to a square one as the Al coordination number increases from 3 to 4.

For high primary beam energy (EP > 200 eV> collective electron excitations are predominant. In the energy-loss spectra, they correspond to

strong plasmon loss peaks. Previous studies have found the sapphire bulk plasmon in the 22-25 eV energy range [12]. Thus, in table 5 each of the energy losses observed at 22.2, 23.3 and 24.4 eV can be interpreted by a plasmon excitation. However, the 24.4 eV loss vanishes on the (‘iOl2) class-b face and the 22.2 eV appears on the (iOl2) class-b face only. As the 23.3 eV loss has a more bulk character, we assign this value to the sapphire plasmon energy. This value is lower than the plasmon value (24-25 eV) determined with high-energy electron beams (Z-10 keV). This result can be easily explained by the fact that with E, = 150 eV only few layers under the surface participate in plasmon excitation. Thus the 23.3 eV plasmon observed is emitted by a material which is quite different from bulk stoichiometric a-alumina. If we assume that near the surface (reduced or not reduced) the ionicity of AI-O bond decreases, it is reasonable to think that the plasma frequency must be changed also. Energy-loss features that are not explained by plasmon loss events are commonly analyzed in terms of electronic excitations from valence or core state to unoccupied levels in the conduction band. The XPS data of table 4 provide energy positions of occupied initial states in the valence band, while final-empty-state energies are calculated by adding the loss energy to the initial-state energy. The transition diagrams shown in figs. 5a-5d for (1012) and (0001) faces have been obtained with an accuracy of 0.3 eV by averaging the data. For A E higher than the band gap value E, = 8.7 eV, only the transitions involving high density-of-state levels which are consistent with previous results [12,19] have been considered. For lower energy-loss structures (AE lower than the band gap value), final states lie in the gap. It has already been shown that such levels are introduced by defects [19]. As one can see in fig. 5, each surface is characterized by its own gap structure. We shall discuss this behaviour with regard to c~staIlographic orientation and chemical composition of surfaces. For clean (0001) and tiOl2) class-a surfaces (figs. 5a and 5c, respectively) energy-loss peaks at 3.2, 13.2 and 10.5 eV are associated with final states located at 5.7 and 1.3 eV below the con-

E. Gillet,l3. Ealet / EELS on sapphiresurjiices

duction band minimum (CBm), respectively. Calculations made by Ciraci and Batra [19] for an (0001) unreconstructed face yield two empty band-gap states produced by s + pz Al dangling bonds at 1.0 and 5.7 eV below CBm. It can be seen in figs. 5b and 5d that energy losses related to the 1.3 eV level vanish on heat-treated surfaces when energy losses associated with the 5.7 eV level remain. The same behaviour is also observed for the 3.4 and 4.1 eV levels below CBm on (0001) and (iO12) faces, respectively. Thus, the 1.3 eV level is attributed to an excitonic state as suggested by Olivier and Poirier [ 111. Other levels in the gap are intrinsic surface-state bands (ISSB): 5.7 and 4.1 eV levels are ISSB of sapphire (1012) class-a surfaces, 5.7 and 3.4 eV levels are ISS of (0001) class-a surfaces. Ciraci and Batra [19] have shown that band surface states are produced mainly by dangling bonds of surface Al atoms. As a result, the location of ISSB is found to be strongly dependent on the effective charge of surface Al ions which is closely related to the AI-O distance and hence to the nature of Al-O bond. The analysis of oxygen KLL lines has revealed (table 3, section 3.1) that near the surface (reduced or not reduced) the Al-O bond becomes more covalent, and therefore EELS results have been explained in this frame. It can be seen in fig. 1 that oxidation state of the surface Al atoms varies from Al”” on (0001) to Al’+ on (iO12). Thus, one may expect that on (iO1211 x 1 surfaces Al-O bonds are more covalent and that ISSB positions are lower than on (0001) 1 x 1 faces. As a matter of fact, we observe a shift from 3.4 to 4.1 eV for the higher ISSB, the 5.7 eV level remaining at the same position, whatever the crystallographic orientation may be. This behaviour suggests that the first ISSB is produced by surface Al atoms that could explain the sensitivity of this level to surface Al oxidation states. The second ISSB (5.7 eV> would be derived with a higher participation of Al and 0 atoms below the surface which have a charge comparable to that of the bulk, whatever the surface orientation is. Upon reducing treatment other energy losses appear for both orientations and they are interpreted by transitions from the upper-valence-band

435

level to empty states in the gap at 4.9 and 6.5 eV below CBm in the (7012) class-b face and 4.6 eV below CBm in the (0001) class-b face. These levels can also be correlated to change of coordination number of Al cations on reduced ordered surfaces (section 4.1). Features of the EELS spectra (fig. 3) seem to indicate that sample heating produces an enlargement of ISSB rather than new levels. This observation means that more layers under the surface participate in electron transfers. It must be noted that on (iOl2) faces the two ISS levels are broadened because on this surface the ordering of oxygen vacancies decreases the coordination of most of the Al ions in the second and the third layer below surface. On (0001) faces, a new phase (Al,0 or AlO) appears by reduction with a cubic structure that does not change drastically Al coordination in the first layers and thus the 5.7 eV level remains unchanged. In turn, Al-O bond is quite different to that of the 1 X 1(0001) face. The formation of an ordered surface (ideal or reduced) creates Al dangling bonds which cause a decrease of Al-O bond ionicity. As a result, the surface band gap reduces to 3.0 or 2.2 eV (fig. 5) depending on the orientation and reduction state of the surface. Such a tendency to lower f$ with decreasing ionicity has been emphasized by Hayes and Stoneham 1201. These observations are also in agreement with resistivity measurements performed on bulk samples which show that alumina becomes a semiconductor after reducing treaments [21].

5. Conclusions We have shown that to each alumina surface ordering and stoichiometry corresponds a specific electronic structure in the gap. We have identified two intrinsic surface states (ISS) on monocrystal surfaces. Their energies differ slightly depending on the two surfaces: on (iO12) 1 x 1 surfaces ISS are located at 5.7 and 4.1 eV below CBm and on (0001) 1 x 1 at 5.7 and 3.4 eV below CBm. These states are empty. We identified the lower ISS (5.7 eV) with a flat band state generated by Al and 0 atoms

E. Gillet, B. Ealet / EELS on sapphire surfaces

436

located in layers below the surface. The other ISS are characteristic of surface Al atoms and as a result are very sensitive to the surface structure. These states are produced by rehybridization of oxygen and aluminium orbitals inducing more covalent Al-O bonds. When surfaces are reduced, the electronic states delocalize in a domain located below the surface, giving rise to bands in the gap. Appearence of band states in the gap decreases the value of E, from 8.7 eV in the bulk to 3.0 or 2.2 eV in surface layers. States which appear in the gap are given by an hybridization of AI and 0 dangling orbitals and depend on sample treatment: the more reduced the surface is, the smaller the band gap. This phenomenon is more effective on 6012) than on (0001) surface. We may explain this effect by the different behaviour of these surfaces under reducing treatments. It can be expected that alumina surfaces with low band gap (2-3 eV> will have a behaviour quite different from high band gap surfaces (8.7 eV> when metal deposition is carried out. In this area, studies are in progress in our laboratory.

Acknowledgements This work is supported by the DRET. The authors thank htf. Long and B. Repetti for their technical assistance.

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[2] E. Gillet, C. Legressus and M.F. Gillet, J. Chim. Phys. 84 (1987) 167. [3] E. Gillet, Le Vide les Couches Minces “Surfaces, Materiaux, Gomposants” 243 (1988) 113. 141 E. Gillet, Proc. 2nd Int. Meeting on Surface Physics, Oran, AlgCrie, Eds. B. Kmelifa, S. Sahraoui and A. Kadri, Editions de la Fact&C des Sciences d’Oran (19881 Q. 49. [S] T.M. French and G.A. Somorjai, J. Phys. Chem. 74 (197012489. 161 CC. Chang, J. Vat. Sci. Technol. 8 (1971) 500. [7] M. Gautier, J.P. Duraud, L. Pham Van and M.J. Guittet, Surf. Sci. 250 (1991) 71. [8] T.J. Chuang, C.R. Brundle and K. Wandelt, Thin Solid Films 53 (1978) 19. [9] W. GGpeI, J.A. Anderson, D. Franked, M. Jaehnig, K. Phillips, J.A. Schafer and G. Rocker, Surf. Sci. 139 (1984) 133. [lo] R.J. Lad and V.E. Henrich, Surf. Sci. 193 (19881 81. [ll] J. Olivier and R. Poirier, Surf. Sci. 105 (1981) 347. 1121 W.J. Gignac, R.S. Williams and S.P. Kowalczyk, Phys. Rev. B 32 (19851 1237. f13j R.W.G. Wyckoff, in: Crystal Structures II, 2nd ed. (Wiley, New York, 1964). [14] V.E. Henrich, Rep. Prog. Phys. 48 (1985) 1481. 1151 R. Weissmann and K. Miiller, Surf. Sci. Rep. 105 (19811 251. [16] S. Evans, Chem. Phys. Lett. 23 (1973) 134. [17] P. Ascarelli and G, Moretti, Surf. Interface Anal. 7 (198518. [18] A. Balzarotti and A. Bianconi, Phys. Status Solidi 76 (1976) 689. [19] S. Ciraci and I.P. Batra, Phys. Rev. B 28 (1985) 982. 1201 W. Hayes and A.M. Stoneham, in: Defects and Defect Processes in Non-Metallic Solids (Wiley, New York, 1985) p. 4. 1211 B. Arghiro~uIos, F. Juillet, M. Prettre and S. Teichner, CRAS Sciance du 9.1159, p. 1895.