The structural and electronic properties of cleaved silicon (111) surfaces following adsorption of silver

Applications of Surface Science 9 (1981) 190—202 North-Holland Publishing Company

THE STRUCTURALAND ELECTRONIC PROPERTIES OF CLEAVED SILICON (111) SURFACES FOLLOWING ADSORPTION OF SILVER G. LE LAY, A. CHAUVET, M. MANNEVILLE and R. KERN

2 —CNRS, Campus de Luminy, case 913, 13288 Marseille Cedex 09, France

CRMC

Received 22 June 1981 Revised manuscript received 16 September 1981

Silver overlayers for coverages ranging from zero to several monolayers are evaporated on vacuum-cleaved (111) silicon surfaces and carefully examined using low-energy electron diffraction (diffraction patterns and 1(v) curves), and Auger electron spectroscopy (condensation/desorption curves), with the aim of establishing a closer correlation between the adsorption process, the different superlattices observed (i.e. ~/7x ..J~-R(± 19°1), \/~X ~/~-R(30°), 3 X 1 and 6 X 1), the growth mechanism of the deposit on the one hand and the electronic properties of the system recently probed using photoemission yield spectroscopy on the other hand. These new results basically confirm the direct relations we had previously shown between the growth mode as monitored with electron diffraction LEED, RHEED, TED and Auger spectroscopy, and the electronic structures as investigated by low energy electron spectroscopy, but permit a deeper insight into the adsorption process at low coverage. At room temperature on the 2 x 1 cleavage structure where the silver—silicon interaction is weak, the adsorbed phase is completed at about 6/7 of a monolayer (0 ‘— 6/7) and a local arrangement of vacancies in the adlayer yields the ~ superstructure, while little effect on the silicon dangling bonds is noticed, but when silver two-dimensional islands (0 > 6/7) growing in a quasi layer fashion have covered the substrate surface. At higher temperatures three-dimensional growth of crystallites occurs after completion of the sJ~.phase whose saturation coverage increases with condensation temperatures, maxima ranging from 0 -~ 0.7 toO — 1.0 (T—’ 500°C)for different cleaves. This Si(111) sJ~—Agsurface exhibits again the same dangling bond peak as a clean 2 x 1 Si surface, despite the fact that the interaction between Ag and Si is now rather strong, as is confirmed by desorption experiments (T 600°C). We thus critically discuss the geometrical models of this ~ phase previously devised and tentatively propose a new one which accounts better for these recent results, along with models of the 3 X 1 and 6 X 1 structures observed in the course of the

desorption process.

1. Introduction The initial stages of the growth of silver films on Si(11 1) have been investigated in a large number of cases with a wide variety of techniques either from the point of view of the formation of the metal/semiconductor interface, or, more recently with respect to its electronic properties. Several studies concern the early stages of

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silver adsorption on in situ cleaved silicon (111) substrates exhibiting either the 2 X 1 cleavage structure or the 7 X 7 annealed one: the growth process of the deposit and its desorption kinetics have been followed by LEED, RHEED, AES and TEM of transfer replica by Le Lay et al. [1—3]while its electronic properties where successively probed with ELS by Derrien et al. [4], with ARUPS by McKinley et a!. [5] and lately, with photoemission yield spectroscopy by Bolmont et al. [6,7] as well as barrier height measurements [8]. The gross features of the growth mode have been established and do not depend drastically on the preparation of the clean substrate surface: LEED [2,9,10,11], RHEED [1,12], AES [2,3,11,13] experiments, in agreement with TEM [1] and UHV, SEM [14] observations ascertain that at high temperatures (T> 200°C)the silver deposits grow in a Stranski—Krastanov fashion (adlayer followed by three dimensional (3D) crystaffites formation) while at lower temperatures, and typically at RT the growth process is a two-dimensional (2D) one according to the Frank—Van der Merwe mechanism. On the contrary detailed aspects of the adsorption process may differ depending on surface preparation and silver deposition or desorption conditions. In fact a variety of superstructures have been identified by LEED or RHEED namely Si(l 11) \r3X\/~—Ag[9],Si(lll)3X 1—Ag [15],Si(111)6X 1—Ag [16],andveryrecently Si(111) ~ff X ‘~/~.R(± 19°1)—Ag [6], yet different authors disagree on the geometry of the atomic arrangement in these superlattices. Especially the superstructure is still highly controversial because of a disagreement on the coverage at completion of this structure. As the structural and electronic properties of the system are intimately connected and as fine analysis of the density of localized states during the formation of the interface are now available it was decided to reexamine with extreme care by LEED and AES the Ag/Si(1 11) system studying cleaved samples on the one hand, and several polished and annealed samples for comparison on the other hand, in the same conditions with the same equipment. The aim of this paper is to describe the last results obtained on cleaved substrates and to propose new models of the different superlattices in the light of the recent data on the electronic structure of the system.

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2. Experiments All experiments were carried out in a multiple technique UHV chamber in a residual pressure of less than 5 X 10—10 Torr. Auger electron spectroscopy is performed with a grazing incidence gun and the four-grid LEED optics. Cleaved Si(1 11) surfaces (2 X 5 mm2) were prepared in situ as described elsewhere [2] from Si n samples (p 2—4 &2 cm). They display strong 2 X 1 LEED patterns at RT, which convert upon moderate annealing to the Si(1 11) 7 X 7 reconstruction. These surfaces show no trace of contaminants. Silver is evaporated from a Knudsen cell. Its flux is stabilized within ±0.5% and reproducible from one reopening of the vacuum ~—

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chamber to the next experiment to within ±4%. This flux is calibrated with a quartz crystal microbalance, which in turn is calibrated in an absolute fashion within ± 1% by condensing on the quartz oscifiator known silver masses from a plane source having the well-known cosine type emission profile. The frequency shift of the quartz oscifiator is known to within 1% of the thickness range checked (10— 100 A); the manufacturer (Inficon Inc., N.Y.) has also calibrated the standard quartz series within 0.05% by interferometry in the range 20—100 A. Both calibrations are thus perfectly concordant. With the stable impinging flux calibrated, variable coverage ratios (9 = 1 corresponds to one silver atom per silicon surface atom) are obtained for different evaporation times. The sticking and condensation coefficients are measured with a quadrupole mass spectrometer placed in a direct view of the sample. The polar emission profiles as a function of the coverage ratio are plotted at the working temperatures and compared with the profile obtained with that of the substrate heated to a high temperature (900°C)where the sticking probability is null. It is thus proved that up to 550°Cthe sticking coefficient is unity. We also determined by AES that no loss of matter by bulk diffusion towards to substrate (constant Ag peak to peak height versus time below the region of desorption temperatures) or by surface diffusion towards the sides of the specimen (line scans), occurs. As a consequence the silver quantity deposited is known in our experiments to within ±5.5%. The cleaved samples are held in a tantalum furnace resistively heated. The temperature is controlled within ±50 with a Pt/Pt—Rh 10% thermocouple welded on the furnace.

3. Results 3.1. Condensation at RT 3.1.1. LEED In accordance with previous results [3,6,7] we observe the gradual vanishing of all 1/7 order spots in the submonolayer coverage range when condensation is performed on a 7 X 7 surface. On the contrary when silver is evaporated on the 2 X 1 cleavage structure a sharp ~./i X ~/~-R(± 1901) pattern develops in the submonolayer range, it persists along with the fading 2 X 1 pattern up to 9 2; other spots corresponding to a silver (111) plane in parallel epitaxy, already observed at 0 0.5 become more and more intense, while the intensity of the remaining integer order spots of the Si substrate gradually vanish. A typical LEED pattern is displayed in fig. 1. 3.1.2. AES From their Auger condensation curves on Si(1 11)7 X 7 Le Lay et al. [2,3] had already shown that at RT the growth mechanism of Ag follows basically a Frank— Van der Merwe growth mode with the first adlayer completed at 0 2/3; this result

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is confirmed, it is also valid for condensation on the 2 X 1 cleavage structure but completion of the \/7 adlayer, as indicated by the break in the curves, is slightly higher i.e., probably 0 6/7 (see fig. 2). ‘-~

3.2. Condensation at high temperature (T>200°C) 3.2.1. LEED A sharp ~/~TX ~/~-R(30°) pattern rapidly develops in every case (either 2 X I or 7 X 7 initial structure); all 1/2 or 1/7 order spots have disappeared below about 9 = 2/3. 3.2.2. AES Beyond about 200°Cthe growth process turns to a Stranski—Krastanov mode [2,10] with nucleation of 3D crystallites after completion of the adsorbed phase which displays the ~ LEED pattern. However the saturation coverage, indicated by the breaks in the Auger condensation curves increases from 0 2/3 at 250°C to a higher value in the range 0.8 ~ 0 ~ I at 500°C.Among several cleaves the maximum value 0 = 1 at 500°Chas been obtained once; we note that the value 0 = lis reached more easily on polished and annealed surfaces. An illustration of this evolution is provided in fig. 3.

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3.3. Isothermal desorption experiments (T >550°C) Isothermal desorption experiments, where the remaining Ag deposit quantity was monitored versus desorption time by AES had been previously performed by Le Lay et al. [2,3]. They could reveal different kinetic steps in the desorption curves corresponding successively to the desorption, with a nearly zero-order kinetics, of the 3D crystallites, the \[3, 2D phase observed in condensation experiments and the new 2D Si(1 11) 3 X 1—Ag phase, completed at 0 = 1/3, appearing in the course of the desorption process. From a network of isotherms, the energetics i.e. half-crystal energy and atomic vibration frequency in each phase, was derived [2]. Recent RHEED experiments [16] have revealed the existence of a new structure: Si(lll) 6 X I—Ag observed upon cooling below 200°Cthe 3 X 1 structure present at the desorption temperatures (T 600°C);we present in fig. 4 what we believe to be the first LEED pattern of this structure. We note especially that almost all 1/6 order spots are sharp but show weaker intensities than those of the 1/3 order spots of the remaining 3 X 1 or ~ structures. Once formed after cooling, the 6 X 1 structure is stable: it can be stifi observed during reheating in the region of the desorption temperatures; this is contradictory to the reversibility observed at 200°C with RHEED [12]. ‘—j

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4. Discussion As already underlined, the general aspects of the growth process of Ag on Si(l 11) were already known; the new results concern mainly the adlayer formed in the submonolayer range, we will comment on its structural and electronic properties. 4.1. Structures formed at high temperatures At high temperatures the structure of the adlayer is Si(1 11) ‘./~X \/~-R(30°) Ag. The question was to elucidate the discrepancy between authors working on cleaved substrates who had determined a saturation coverage of 2 Ag atoms per 3 Si atoms in the ‘~/~ unit mesh and adopted a honeycomb model of the silver arrangement [2] (see fig. Sa) and authors working on polished samples who had reached a saturation coverage of 0 = I and proposed a trimer model of the atomic arrangement [11] (fig. 5b). This question is clarified now: it appears that the saturation coverage 0 = 1 was obtained for one cleave but the completion of the adlayer beyond 0 = 2/3 and up to 0 = I demands to jump over an activation barrier. The existence of such an activation barrier, testified by the evolution of the position of the breaks in the Auger condensation curves at increasing temperatures, was already suggested by theoretical considerations in one of our previous papers [16]. It is confirmed by the new quantum mechanical cluster calculations of Julg et al. [17] which show that for silver atoms sitting on the surface the adsorption site is centered in a valley between three adjacent silicon substrate atoms, that the most stable arrangement of silver atoms up to 0 = 2/3 is the honeycomb structure, but that extra atoms jumping over an activation barrier of —0.1 eV may sit in the center of the graphite-like hexagons of the honeycomb structure but at a slightly different height (—0.3 A) as compared to the atoms in the initial honeycomb arrangement. Thus a new stable structure may be reached with 0 = 1. The geometry of this new V~sV~ V~xV~

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\/~model is depicted in fig. Sc. Besides it has been also shown that the trimer arrangement of fig. Sb is energetically unfavourable. Moreover, we have noticed that the ~ structure is already fully developed at 0 2/3 and we could not observe any modification in the variations in the intensity of the specular spot with incident energy of the primary beam (‘oo (V) curves) measured with a spot photometer for a coverage 0 near 2/3 or near 1 [18]. This point stands in disfavour of trimer models which would leave, if such triplets pre-existed, a large part of the substrate surface uncovered or which would require a severe reconstruction beyond 0 = 2/3 to form these trimers from a honeycomb array. This interpretation is in fairly good agreement with the recent results obtained by photoemission yield spectroscopy [7] especially with the variations of the ionization energy c1 which shows a linear decrease up to 0c 0.8 followed by a saturation slightly below about 0 = 1 with increasing Ag coverage. Moreover it was emphasized that at 0~the Si(l 11) ~ X \/~—Agsurface exhibited nearly the same dangling bond peak as a perfect 2 X 1 Si(l 11) surface, a point which stands strongly in favour of a valley adsorption site and is in contradiction with a trirner model. Yet an alternative representation of this structure merits mentioning. Recent ion scattering spectroscopy experiments by Oura et a!. [191showing shadowing or blocking effects have suggested a variant of the honeycomb model where the Ag atoms instead of sitting on the Si surface would be embedded slightly below the topmost Si layer and consequently induce a considerable reconstruction of Si subsurface. New results in UPS [20] surface potential measurements [21] and LEED/ CMTA technique [22], stand in favour of the incorporation of Ag into Si. Besides the permanence of the Si dangling bond signal would be straightforwardly interpreted with such an arrangement. But the question remains, however, that no evidence of this incorporation appears in AES and moreover that the completion of the .,/~structure beyond 9 = 2/3 and up to 0 = 1, indeed not envisaged by [19], in such a geometry seems to demand an extremely severe reconstruction. The determination of the precise adsorption site of Ag, not evident at present is indeed a perequisite to solve this problem. From the position of the break in the Auger desorption curves Le Lay et al. [2] could determine the saturation coverage of the 3 X 1 phase formed during the desorption process. The value 9 1/3 led straight forwardly to the model shown in fig. 6. (For this 3 X 1 and the next 6 X 1 structure the ISS results confirm that the Ag atoms are sitting on the Si surface [23].) Yet after cooling below about 200°Ca new 6 X 1 pattern appears. Gotoh and mo [12] first observed this structure by RHEED. They remarked that all 1/6 order superlattice spots were visible in the (1 12) incidence azimuth of the primary electron beam but that some of them missed in the (110> incidence azimuth. An explanation for this peculiar behaviour in terms of double diffraction enhanced through excitation of surface waves that would permit to observe 1/6 order spots otherwise kinematically forbidden (suggested existence of twofold glide planes in the 6 X 1 structures leading to system-~-

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atic extinctions) was proposed by Ichikawa and ino [24]. They assumed a coverage of 0 = 1/3 for the 6 X 1 structure as for the 3 X 1 one and for valley adsorption sites derived the model presented in fig. 7 and concluded from the presence of twofold glide planes in the 6 X 1 structure, perpendicular to the crystal surface that a LEED study at normal incidence would confirm that the 1/6 order spots in rows of diffraction spots including the specular spot (00) should be invisible, as shown in fig. 8. Obviously this is not true as one is easily convinced by perusal of our LEED pattern of fig. 4. As a consequence it appears that all 1/6 rder spots in the 6 X 1 diffraction pattern result from simple scattering events, and no systematic extinction is revealed, that is the 6 X 1 arrangement has no peculiar symmetry element. It is thus likely that the missing spots in the RHEED patterns were only too weak to be observed. Besides, as already mentioned, the displacement of the silver atoms from their equilibrium position in the valley sites is energetically unfavourable. We thus think that the formation of the 6 X 1 structure upon cooling is due to.a reordering of the 3 X 1 phase partly depleted in the course of the desorption process. It is thus reasonable to assume that the atomic arrangement in this structure is the same as that shown for the 3 X 1 phase in fig. 6 but with one row of Ag over two missing (0 = 1/6 at saturation).

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4.2. Structure formed at room temperature

Upon RT adsorption two different behaviours of the adlayer are noticed. When the initial substrate structure is Si(1 11)7 X 7 the adlayer is disordered (vanishing of all 1/7 order spots); it is probably built up with small domains of the honeycomb structure that are too small to scatter coherently (size of the domains inferior to the coherence length) as the knee at 0 2/3 in the Auger condensation curves indicate. On the contrary, when silver is deposited on a freshly cleaved surface showing the 2 X 1 pattern, a new ~ structure with sharp 1/7 order spots is observed. From the break in the Auger condensation curves a coverage at saturation of 9 6/7 is likely. Alternatively we may consider that this structure is due to an ordered array of advacancies in a hypothetical 1 X 1 structure, and propose the arrangement shown in fig. 9. To be ordered these vacancies must interact somehow. Allan [25] had studied the ordering of surface defects (advacancies or adatoms) in a triangular lattice. In an extended range lattice gas model, where pairwise interactions~between defects up to third neighbours were considered, a ~/i structure was foignd as a

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ground state for a defect concentration of 1/7 withJ1 ~ 0,J2 ~ 0,J3 ~ O;J~is the kth neighbour interaction and the energy is given by E = ~k~k~k for interacting defects (the zero of the energy is chosen for non interacting defects), ~k being the total number of the kth pairs. Despite the fact that the pairwise interactions assumed could be a serious limitation of the lattice gas model, the ~ structure of Ag on Si(1 11) seems to be a nice illustration of this theory. Nevertheless the question is not solved why this structure forms on the cleavage structure of Si and not on Si(l 11)7 X 7. The reason is probably that the 2 X 1 reconstruction of Si(l 11) which is metastable can more easily transform to a 1 X 1 structure than the 7 X 7 stable one. Besides when looking at the pattern of fig. 1 one notices that in fact three different superstructures appear superimposed in the LEED pattern. The 2 X 1 pattern observed which persists far beyond 0 = 6/7 can only arise from regions of the substrate surface uncovered by silver atoms. In fact an order array of silver atoms with a ‘../7 X \/~unit mesh over a 2 X 1 underlying surface would display a LEED pattern corresponding to the coincidence mesh, that is 2\/~X 2\/~ and not ‘~/~ X \/~. This justifies the assumption of the conversion of the 2 X 1 to the 1 X 1 Si structure on the area covered by the silver adlayer. Now, if at 0 = 6/7 a large area of the substrate, capable of showing a strong LEED pattern, is still uncovered by the silver deposit one may wonder where the corresponding deposited silver quantity has gone and question the reliability of the saturation coverage 9 = 6/7 if one considers possible diffusion from the virgin zones to the regions showing the ‘~/Jstructure. However this seems ruled out by the simultaneous observation of the third pattern due to (111) Ag two-dimensional islands in parallel epitaxy. These 2D islands which must be a few hundred of A wide to contribute to a LEED pattern concentrate locally a high number of Ag atoms but leave around a large depleted zone which maintains its 2 X 1 structure. Now it is tempting to attribute this heterogeneity on the cleaved surface to the cleavage steps which may have a certain influence. In fact, it has been shown that the 2 X 1 reconstruction is in some way pinned by the cleavage steps [26]. Thus the ~ might develop preferably on the flat terraces in the most planar regions while 2D Ag islands would nucleate and grow in the vicinity of the steps in the most perturbed zones; unluckily at the present moment we have no direct means of visualizing such a situation; indeed REM has been recently, successfully applied to the visualization of 2D phases of Ag and Au on polished and annealed Si(l 11) surfaces [27] but the experimental conditions prohibit the use of cleaved substrates. If one retains this general description one understands that the surface state band on the clean cleaved 2 X 1 surface remains almost unchanged up to the completion of the structure [6]: in fact a large part of the substrate remains uncovered and even on the regions covered by the silver atoms in the \/~°T arrangement their localization in valley site has no drastic influence on the Si dangling bonds. ~

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Conclusion

In short, despite the fact that the adlayer may present different structures, according to coverage and temperature and also despite the fact that it is strongly bound to the Si substrate (binding energies of about 3 eV as determined by the isothermal desorption experiments of Le Lay et a!. [2]), it has no strong effects on the Si dangling bonds. Moreover, as the Ag—Ag distances in these structures (at least 3.82 A) is markedly larger than the bulk Ag interatomic distance (2.88 A) it is thus not surprising to obtain at RT and high temperatures nearly the same electron loss spectra in the submonolayer range [4] and to observe a quasi atomic like emission from the Ag d states in photoemission experiments [5,11,28,29]. Abulk like emission is only obtained at RT when about two dense (111) silver planes are lying on the substrate.

References [11G. Le Lay, G. Quentel, J.P. Faurie and A. Masson, Thin Solid Films 35 (1976) 273; 35 (1976) 283. [2] G. Le Lay, M. Manneville and R. Kern, Surface Sci. 72 (1978) 405. [3] R. Kern, G. Le Lay and J.J. Métois, in: Current Topics in Materials Science, ed. E. Kaldis, [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]

Vol. 3 (North-Holland, Amsterdam, 1979) p. 131. J. Derrien, G. Le Lay and F. Salvan, J. Phys. Lett. 39 (1978) L 287. A. McKinley, R.H. Williams and A.W. Parke, I. Phys. C12 (1979) 2447. D. Bolmont, P. Chen, C.A. Sebenne and F. Proix, Phys. Rev., to be published. D. Bolmont, P. Chen and C.A. Sebenne, J. Phys. C, to be published. J.D. van Otterloo, Surface Sd. 104 (1981) L 205. K. Spiegel, Surface Sci. 7 (1967) 125. E. Bauer and H. Poppa, Thin Solid Films 12 (1972) 167. F. Wehking, H. Beckermann and R. Niedermayer, Surface Sci. 71(1978) 364. Y. Gotoh and S. mo, Japan I. App!. Phys. 17 (1978) 2097. M. Housley, R. Heckingbottom andC.i. Todd, Surface Sci. 68 (1977) 179. J.A. Venables, J. Derrien and A.P. Jansen, Surface Sci. 95 (1980) 411. E. Bauer, in: Techniques of Metals Research, ed. R.F. Bunshah, VoL 2 (Interscience, New York, 1969) part II. V. Barone, G. Del Re, G. Le Lay and R. Kern, Surface Sci. 99(1980)223. A. Juig and A. Aliouche, Surface Sci., to be published. A. Chauvet, Thesis, Université Aix—Marseilie III (1980), unpublished. T. Hanawa, M. Saito, F. Shoji and K. Oura, in: Proc. 4th ICSS, Cannes, 1980. G. Rossi, I. Abbati, L. Braicovich, I. Lindau and W.E. Spicer, Surface Sci. Letters, to be published. K. Oura, T. Taminaga and T. Hanawa, Solid State Commun. 37 (1981) 523. Y. Terada, T. Yoshizuka, K. Oura and T. Hanawa, Japan J. App!. Phys. Lett., to be published. K. Oura, private T. Ichikawa and S. Ino, Surface Sci. 97 (1980) 489. G. Allan, in: Proc. 7th Intern. Vacuum Congr. and Intern. Conf. Solid Surfaces (Vienna, 1977) 611.

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[26] P.P. Auer and W. Mdnch, Surface Sci. 80 (1979) 45. [27] K. Yagi, N. Osakabe, Y. Tanishiro and G. Honjo, in: Proc. 4th Intern. Conf. on Solid Surfaces and 3rd European Conf. Surface Sci., eds. D.A. Degras and M. Costa, suppl. Le Vide, Les Couches Minces 201 (1980) 1007. [28] J.P. Gaspard, J. Derrien, A. Cros and F. Salvan, Surface Sci. 99 (1980) 183. [29] G. Dufour, J.M. Mariot, A. Masson and H. Roulet, J. Phys. C, to be published.