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Initial stage of Pd adsorption on Si(111)7 × 7 surface studied by AES and EELS
Surface Science 167 (1986) 27-38 North-Holland, Amsterdam
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INITIAL S T A G E OF Pd A D S O R P T I O N ON S i ( l l l ) 7 x 7 S U R F A C E S T U D I E D BY A E S A N D E E L S S. N I S H I G A K I , T. K O M A T S U , M. A R I M O T O a n d M. S U G I H A R A
Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 440, Japan Received 8 May 1985; accepted for publication 24 October 1985
The initial stage of adsorption of Pd on a Si(lll)7x7 surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and surface workfunction change. For Pd deposition at room temperature (RT) the Si(LVV) Auger signal intensity decays in a broken linear line. The structure factor, defined as the intensity ratio of the subpeak to the main one in Si(LVV) Auger spectra, increases up to a maximum around one monolayer coverage. In EELS spectra two peaks, characteristic of Pd, appear at the completion of the first Pd layer. Pd atoms deposited on Si(lll) at RT form initially flat layers of a few monolayers height without mixing with substrate Si atoms. For Pd deposition at a moderately high temperature (MT) of about 300°C, however, the structure factor for Si(LVV) Auger spectra does not change. EELS peaks, characteristic of Si substrate, remain clearly even beyond one monolayer coverage. Pd atoms deposited at MT are unstable and easily diffuse into the bulk. We present evidences to support the "screening" model for the bond-breaking mechanism at the Pd/Si interface.
I. Introduction T h i n silicide films are n o w finding wide application as b o t h ohmic contacts a n d Schottky-barrier electrodes in silicon devices. Of the various silicides, the P d - S i system has attracted the most interest, because it is a reactive interface to form a c o m p o u n d such as PdzSi at room temperature. M u c h work has already been d o n e b y p h o t o e m i s s i o n (PES) [1-4], Auger electron (AES) [3-8], t r a n s m i s s i o n electron microscopy [3,6,7], surface extended X-ray a b s o r p t i o n fine structure [9] a n d ion scattering (ISS) [8,10,11] techniques. However, the initial stage of interface f o r m a t i o n has n o t yet explored clearly. Questions arise, whether there is or is n o t a m a x i m u m thickness before reaction a n d mixing with the substrate occurs [11]; how the initial Pd layer grows; how the elevated temperature of Si substrate influences the a d s o r p t i o n process of Pd; a n d so on. I n the present study we have used AES, electron energy loss spectroscopy (EELS) a n d surface w o r k f u n c t i o n change (A~) as m a i n tools to study the issues m e n t i o n e d above. Some first results o n initial a d s o r p t i o n of Pd o n a 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
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S. Nishigaki et al. / P d on Si( l l l ) T x 7
S i ( l l l ) 7 × 7 surface at room temperature and at a moderately high temperature will be presented. We will show that an abrupt interface without mixing of Pd with Si can be formed at the initial stage of room temperature deposition, and that Pd atoms deposited at elevated temperatures are unstable and easily diffuse into the Si substrate.
2. Experimental Silicon crystals were cut from an n-type wafer with an epitaxially-grown (111) layer (thickness = 20 #m, resistivity = 50 ~2 cm). They were clamped in a tantalum holder and mounted in a goniometer. Prior to mounting no chemical treatments were made. The crystals were cleaned by heating resistively at = l l 0 0 ° C in U H V (base pressure = 1 x 10- 10 Torr), yielding distinct low energy electron diffraction (LEED) patterns of 7 x 7 structure. No contaminants could be detected in the Auger spectra (peak-to-peak ratio C / S i less than 1 x 10 2). Every crystal was used for only one experiment of Pd deposition and never cleaned a second time. The samples were kept at room temperature (RT) and at a moderate temperature (MT) of about 300°C during Pd deposition. Pd was evaporated from a Pd foil mounted in a heated W spiral wire. The deposition rate was set to be about 0.7 x 1014 a t o m s / c m 2 per min, which was monitored by a quartz oscillator. We define unit coverage (0 = 1) as the atomic density (7.8 x 10 a4 a t o m s / c m 2) in the outermost S i ( l l l ) surface layer. For Auger and energy loss analyses a home-made retarding-field analyser with a hemispherical four-grid assembly was used. Si(LVV) Auger differential spectra have a main peak at 92 eV and a clear subpeak at 74 eV [12]. We studied on the variation not only of the peak-to-peak intensity of the former but also of the intensity ratio of the latter to the former. The second derivatives of EELS spectra were obtained by differentiating measured loss curves numerically according to Savitzky and Golay [13]. Moreover we used a 127°-cylin drical deflector analyser to monitor the low-energy threshold of secondary electron energy distribution curves during Pd deposition, which reflects the workfunction variation, Aq~.
3. Results 3.1. Pd adsorption at room temperature 3.1.1. A E S results
The Si(LVV) differential spectra were measured with the hemispherical retarding-field analyser at an incident electron energy of 1500 eV. The spectra
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Fig. 1. (a) Si(LVV) Auger peak-to-peak intensities I A (at 92 eV) versus coverage of Pd deposited at RT. (b) Structure factor, defined as I B / I A (the ratio of the subpeak to the main peak), versus Pd coverage. Coverage is measured in units of the atomic density of the topmost Si layer (7.8 × 1014 a t o m s / c m 2 ).
consist of mainly two peaks: One is at 92 eV (peak A) and the other at 74 eV (peak B). The former has been used by other authors in determining the growth mode of Pd on S i ( l l l ) surfaces [8]. We plotted peak-to-peak intensities of peak A (IA) as a function of Pd coverage (8) in fig. la. The intensity I A decreases with # in a broken linear line, which means the monolayer growth mode (the F r a n k - V a n der Merwe mechanism). As one broken line corresponds to the growth of a single layer, no less than 3 monolayers of Pd have been formed before the occurrence of reaction or mixing with Si. The first Pd layer is completed a t 01 = 0.6, which corresponds to the atomic density of 4.7 × 1014 a t o m s / c m 2. This is similar to the case of Ag adsorption on S i ( l l l ) [14], where the Auger decay line bended at the coverage less than unity. The initial decay slope of the Auger intensity is = 43% per unit coverage. This value is again compared well with the case of A g / S i ( l l l ) system at RT, where it is about 48% from fig. 2 in ref. [14]. Next, we examined the variation of Auger spectral shape with the progress of adsorption. Since the escape depth of Si(LVV) Auger electrons is about 5 ,~ [15], its spectral shape reflects the bonding scheme of near-surface Si atoms. We define the structure factor for Si(LVV) spectra as the peak-to-peak intensity ratio of peak B (at 74 eV) to peak A (at 92 eV). Therefore the effect of
30
S. Nishigaki et al. / Pd on Si(l l l ) 7 x 7
intensity decay by decrease of escape probability due to the overlayer growth is excluded from the ratio I J l A. We show the dependence of the structure factor on the Pd coverage in fig. 1 b. The ratio I B / 1 A increases at the first stage of Pd adsorption, and then decreases gradually. The most interesting is that the Pd coverage where 1 J I A reaches a maximum corresponds very well to 0~ where the first Pd layer is completed. Theoretically, peak A is attributed to the Auger transition that both electrons, downwards and upwards, are p-like in nature near the top of the valence band [16]. Peak B corresponds, on the other hand, to the Auger transition for two s-like electrons near the bottom of the band. The initial increase of the structure factor in fig. lb, therefore, corresponds to the decrease of peak A relative to B. Beyond -~ 3 monolayers we sometimes found drastic changes of the spectral shape to indicate compound formation which has been observed by other authors [7,8]. 3.1.2. E E L S
results
Electron energy losses were measured with the same analyser as for AES. The incident electron energy was 80 eV, which is suitable for the study of surface electronic structure due to its short mean free path of ~ 5 A [15]. Second derivatives of successive EELS spectra in the course of Pd adsorption on S i ( l l l ) at RT are shown in fig. 2. Curve (a) in fig. 2 exhibits several features characteristic of the clean S i ( l l l ) 7 × 7 surface. They correspond to losses by excitation of bulk plasmon (h~0p), surface plasmon (h~0~) and single-electron excitation in the bulk (E2) and at the surface ($2 and S~) [17]. Among them the surface losses (hc0~, S2 and S~) diminish with Pd deposition and practically disappear around 0 = 0.7 where the first Pd flat layer has been completed as shown in fig. la. The loss peak by the bulk plasmon e x c i t a t i o n (h~p) is destroyed at 0 > 4. This means that the reaction of Pd with Si begins to occur by depositing Pd beyond a critical thickness. Two new peaks (P1 and p~) appear at 0 - - 0 . 7 and increase with the Pd coverage, as shown in fig. 2. The peak P~ at the energy loss of ~ 5.5 eV may be attributed to single-electron excitation: the initial states are the d-levels of adsorbed Pd, which lie at ~ 3.5 eV below E v [1], and the final states are somewhere in the conduction band of the Si substrate. The peak p~ appears at 12 eV in good agreement with a peak at 12.0 eV in photoemission data of bulk Pd by Vehse et al. [18] and with a peak at 13 eV in the imaginary part of the complex dielectric function of Pd by Robin [19]. Therefore this is explained in terms of strength in the optical transition probability function. Another peak P2 (at ~ 7.5 eV) is found clearly in the coverage range of 2 < 0 < 4. This is attributed to the energy loss by the excitation of collective oscillation (plasmon), which has previously been observed in bulk Pd by other authors [18-20]. Beyond the coverage of about 4 monolayers, the peak P2 diminishes and the strong feature by the Si bulk plasmon excitation is destroyed. We,
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therefore, find that there exists a critical Pd coverage (0 c = 3-4) beyond which the reaction of Pd with the Si substrate occurs.
3.1.3. Workfunction variation The variation of workfunction Aq~ during Pd deposition at RT was measured by detecting the low-energy threshold of secondary electron energy distribution curves. We show in fig. 3 that the workfunction increases with the Pd coverage and reaches a maximum of Aq~max-- 0.35 eV at 0--2.5. Beyond this coverage the workfunction decreases slightly and is nearly saturated with Aq,_-- 0.25 eV. The general tendency of this workfunction change agrees with the result of Rubloff et al. [4]. They also observed the initial increase followed by the plateau, which has been attributed to the workfunction of Pd2Si compound. However, there are some discrepancies between two results. Aq;~a× obtained here is lower by 0.1 eV than that of Rubloff et al. [4]. The coverage where A~ reaches the maximum iia the present experiment is about a half of that obtained by them. They observed a gentle increase beyond the plateau and attributed it to the presence of incompletely reached Pd metal at the surface of the overlayer, but we did not observe it. The difference in deposition rate for these two cases would be the main factor to influence the above behavior.
S. Nishigaki et al. / Pd on S i ( l l l ) 7 x 7
32
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Their deposition rate, = 1 ,~/min, was about 10 times faster than in the present experiment. Therefore excessive amount of metal Pd would have been accumulated at the surface without being reacted, as mentioned by themselves. The higher value of the workfunction at the plateau, the thicker overlayer necessary to attain the plateau and the successive increase of the workfunction beyond the plateau are understood if we assume such an effect. The present result shown in fig. 3 suggests the change of mechanism for P d / S i interface formation around 0 = 3 monolayers, namely, from the monolayer growth mode of pure Pd metal overlayers to the reaction of Pd with Si, as discussed in the next section. 3.1.4. Discussion
The AES and EELS spectra presented here demonstrate clearly that upon deposition at RT Pd atoms adsorb on S i ( l l l ) to form flat layers up to 0 = 3 before reaction of Pd with Si occurs. According to our Si(LVV) Auger decay curve, the atomic density of the first Pd layer is about 0.6 times the one of the substrate Si(111) layer. The saturation of Pd in the first layer suggests that the bonding scheme of P d - S i in the first layer is covalent rather than metallic in nature, which is in agreement with the He neutralization data of T r o m p et al. [8] and the calculation of Ihm et al. [21]. This saturated atomic density of Pd and the initial slope of Si(LVV) intensity decay are similar to those for Ag adsorption on Si(111) at RT [14]. From this similarity one may draw a picture that Pd adsorbs rather non-reactively on, not beneath, the topmost S i ( l l l ) layer [22]. We observed the slight modification of Auger spectral shape during the first layer formation, that is, the initial increase of the ratio I B / 1 A , a s shown in fig. lb. This is easily understandable by the aid of the calculation of Ihm et al. [21]. They showed that the density of states at - 2 . 5 eV below E F increases with Pd adsorption, corresponding to Pd 4d localized orbitals, whereas the density of states near the bottom of the Si band shows no marked
s. Nishigaki et al. / Pd on Si(l l l)7× 7
33
changes. Therefore the increase of the structure factor ( I J I A) may be attributed to the relative decrease of the portion A. This means that electrons near the top of the Si valence band, which is nearly p-like in nature, are partly transferred to around adsorbed Pd atoms. The saturation coverage of the first Pd overlayer, indicated by the break of the Si Auger intensity decay curve, is 01 = 0.6, which is discrepant to the value (01-- 1) by Tromp et al. [8]. We can find out no origins to cause this discrepancy except the different modes of preparation of the Si 7 × 7 surface. For the case of Ag condensation on Si(111) it is now established that the mode of preparation of the clean Si surface has some influence on the initial stages of the adsorption process to cause different values of saturation coverage of the first Ag overlayer [23]. On surfaces prepared by cleavage and annealing or by only flashing epitaxially-grown layers the saturation coverage of Ag, either at RT or at elevated temperatures of = 300°C, is around 2/3, which is definitely less than 01 = 1 obtained on cut-and-polished surfaces (by cleaning procedure of argon ion bombardment plus annealing) [15,24]. We used the former procedure in the present study on P d / S i ( l l l ) and obtained a value near 01 ---2/3. Tromp et al. prepared their surfaces by the latter method and obtained 01 = 1. Some factors on the microscopic scale (such as step density) may have influenced on the saturated atomic density of the first layer. More systematic experiments are necessary in this respect. The structure factor ( I B / I A ) of the Si(LVV) Auger spectra decreases gradually beyond 01 = 0.6. This means that the adsorption of Pd which interacts directly with the surface Si atoms taking electrons partially from them finishes at 0 a ---0.6. The condensation of Pd to form next layers may not seriously influence the bonding scheme between the Si and the first Pd layer. The gradual decrease of the ratio I B / I A can be understood if we consider that the increase of the density of states near the Fermi level caused by Pd condensation will enhance the Auger intensity near peak A. We speculate that this increase of electron density of states, which should be sp-like in character and be supplied from the Pd side, would play an important role in weakening covalent Si-Si bonds of the substrate to yield mixing and reaction of Pd with Si [241. We showed some evidences in the previous section to suggest that the Pd overlayer with the thickness of 3 - 4 monolayer was unstable and the reaction of Pd with Si occurred beyond this thickness: Occasional observation of the abrupt change of S i ( L W ) Auger spectra; the collapse of the bulk-plasmon loss peak of Si; the saturation of zlff followed by the slight decrease. According to our picture of monolayer growth followed by compound formation, UPS and A~ results by Freeouf et al. [1] can be simply understood as follows: The Pd 4d peak position in UPS would start to increase from a value for isolated Pd adsorbates on Si and reach to a value for PdzSi; the work function would also increase gradually from the clean surface value up to a value for Pd 2Si surface,
S. Nishigaki et al. / P d on S i ( l l l ) 7 × 7
34
not abruptly to the latter one. Our model on P d / S i interface formation is not contradictory to the TEM observation by Ho et al. [7], which demonstrated the epitaxial growth of Pd2Si lattice directly on the substrate Si surface. If the initial growth of a few monolayer of pure Pd is followed by diffusion of Si atoms into the Pd overlayer, consequently, an abrupt interface of Pd2Si/Si would be formed. 3.2. P d deposition at ~ 3 0 0 ° C 3.2.1. A E S results
Curve (a) in fig. 4 shows the coverage dependence of the Si(LVV) Auger peak-to-peak intensity I A. It fits neither broken linear lines nor exponential c u r v e s . I A decays more slowly than the case of RT deposition and does not approach zero even with heavier dosage of Pd. This means that substrate Si atoms are exposed to vacuum still after Pd deposition. There are several possibilities to account for the above behavior: (i) Evaporation a n d / o r surface diffusion of deposited Pd atoms. Perhaps this effect cannot be neglected at higher temperatures than RT and may be one reason to decrease Pd atoms staying on the specimen surface. (ii) The Pd2Si formation from the initial stage of Pd condensation. With this model I A would decrease and rapidly reach a constant value for Pd2Si compound surface, because the further progress of Pd2Si growth would not
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S. Nishigaki et aL / Pd on Si(111)7× 7
35
effectively° influenco 1A as the escape depth for Si(LVV) Auger electrons is only about 5 A [15]. Therefore this item cannot account for the measured decay curve of I A. (iii) Three-dimensional island formation. (iv) Diffusion of deposited Pd atoms into the bulk. These possibilities will be discussed later with other experimental results. The plots of the structure factor I B / I A versus the Pd coverage O are shown in fig. 4b. The absolute value of this ratio I B / I A cannot be compared with the one of the RT case in fig. lb, since it depends sensitively on energy-analysing conditions, such as energy-scan speed and modulation voltage. The structure factor is found to be nearly constant, which implies that the valence band structure near the Si surface does not suffer a big change due to the Pd deposition. Therefore item (ii) above is again contradictory to this experimental result. Moreover, we can exclude a model of two-dimensional Pd layer growth at the surface, different from the case of fig. lb. 3.2.2. E E L S
results
Electron energy loss spectra are shown in fig. 5 as a function of Pd coverage O deposited at a substrate temperature of 300°C. There appears many features in comparison with the case of RT deposition (fig. 2). The bulk-plasmon loss peak (ht~p) persists even under 5.8 monolayer deposition, implying that Si
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36
S. Nishigaki et al. / Pd on Si(l l l)7× 7
lattice is not destroyed by the exposure to Pd. Four peaks from clean Si ( E 2, S2, hw~ and S3 peaks) still appear clearly at around one monolayer deposition, and some of them even at 2 monolayers. This is in contrast to the RT behavior that they disappear after completion of the first Pd layer (01 -- 0.6). Among peaks originating from the condensed Pd, P~ is not found in the spectra even with heavier dosages of Pd. This means that the metallic Pd is not formed, and that item (iii) in the latest section is denied for the M T deposition. The only possibility to account for the above findings is the model of diffusion of Pd atoms into the bulk. With this picture we can speculate that the concentration of Pd would increase gradually in the three-dimensional region near the surface. The Si-substrate lattice, however, would not be destroyed. Then the energy loss peaks from clean Si is expected to remain clearly even with heavy dosages of Pd, while the loss peak P~ not to appear. 3.2.3. Discussion There have been no reports which deal with P d / S i interface formation at elevated temperatures above RT. The behavior of Pd atoms on heated Si(111) surface has, therefore, been unknown. The present experiments demonstrate the instability of Pd atoms deposited on S i ( l l l ) at MT. We induce the model that Pd atoms diffuse into the Si substrate at ~ 300°C without breaking the Si-lattice structure. This can surely explain the findings by AES that I A does not approach zero and that the structure factor I B / I A changes scarcely. It can also account for the EELS results that the loss peaks from clean Si can survive and that the loss peak ~ does not appear. Our picture is consistent with a recent result by Ratherford backscattering [11]. It showed that a thin Pd film ( -- 3 monolayers) deposited at R T diffused rapidly into the Si substrate when heated at ~ 500°C. The instability of the Pd overlayer which is thinner than a critical Pd film thickness has been mentioned [11]. The constancy of the ratio I B / I A and the stability of the loss peaks from the Si against Pd deposition mean that the diffusion of metal atoms into the substrate does not play a main role in breaking Si-Si bonds of the substrate. There have been two different models on the breaking mechanism of Si-Si bonds with the aid of metal atoms from outside. One is the "interstitial" model [25] and the other the "screening" model [11,26]. According to the former, covalent Si bonds would be broken by in-diffusion of metal atoms and arranging them around each Si atom. For P d / S i system, however, the in-diffusion of Pd does not destroy Si-Si bonds, as shown in the present experiment. The latter postulates electronic screening of Coulomb interaction, responsible for the covalent bonding of Si crystal, due to mobile free electrons in the deposited metal overlayer. With this model a metal film of a few monolayer thickness is thought to be necessary to promote the reaction of the deposited metal with the substrate. The present results, both at R T and at MT, are in favor of the screening model.
S. Nishigaki et al. / Pd on Si(111)7× 7
37
4. Conclusion Summarizing the AES, EELS and Aq) results, the following picture emerges for Pd/Si interface formation at RT: Pd overlayer grows on the S i ( l l l ) surface in the monolayer growth mode up to about 3 monolayers; the atomic density of each Pd layer increases from 0 a --- 0.6 towards that of Pd metal; Pd atoms in the first overlayer are bonded with surface Si atoms covalently; when the overlayer grows thicker than 3 - 4 monolayers, it becomes unstable and starts to react with Si. For the behavior of Pd atoms deposited at MT we get the following picture: Pd atoms incident on the Si surface at MT do not stay at surface sites stably but diffuse into the substrate; the lattice structure of Si is not easily destroyed by the in-diffusion of Pd. We remark in conclusion that the initial stage of P d / S i interface formation proceeds in the monolayer growth mode, the Frank-Van der Merwe type, followed by the low-temperature reaction of Pd with Si beyond the critical Pd-layer thickness.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
J.L. Freeouf, G.W. Rubloff, P.S. Ho and T.S. Kuan, Phys. Rev. Letters 43 (1979) 1836. J.L. Freeouf, J. Vacuum Sci. Technol. 18 (1981) 910. P.E. Schmid, P.S. Ho, H. FSII and G.W. Rubloff, J. Vacuum Sci. Technol. 18 (1981) 937. G.W. Rubloff, P.S. Ho, J.L. Freeouf and J.E. Lewis, Phys. Rev. B15 (1981) 4183. G.Y. Robinson, Appl. Phys. Letters 25 (1974) 158. P.S. Ho, T.Y. Tan, J.E. Lewis and G.W. Rubloff, J. Vacuum Sci. Technol. 16 (1979) 1120. P.S. Ho, P.E. Schmid and H. FSII, Phys. Rev. Letters 46 (1981) 782. R.M. Tromp, E.J. van Loenen, M. Iwami, R.G. Smeenk and F.W. Saris, Surface Sci. 124 (1983) 1. J. St~hr and R. Jaeger, J. Vacuum Sci. Technol. 21 (1982) 619. R.M. Tromp, E.J. van Loenen, M. Iwami, R.G. Smeenk and F.W. Saris, Thin Solid Films 93 (1982) 151. A. Hiraki, Japan. J. Appl. Phys. 22 (1983) 549. C.C. Chang, Surface Sci. 25 (1971) 53. A. Savitzky and M.J.E. Golay, Anal. Chem. 36 (1964) 1627. S. Nishigaki, K. Takao, T. Yamada, M. Arirnoto and T. Komatsu, Surface Sci. 158 (1985) 473. M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2. P.J. Feibelman, E.J. McGuire and K.C. Pandey, Phys. Rev. B15 (1977) 2202. J.E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. R.C. Vehse, J.L. Stanford and E.T. Arakawa, Phys. Rev. Letters 19 (1967) 1041. S. Robin, in: Optical Properties and Electronic Structure of Metals and Alloys, Ed. F. Abel6s (North-Holland, Amsterdam, 1966) p. 202. R.C. Vehse and E.T. Arakawa, Bull. Am. Phys. Soc. 11 (1966) 830. J. lhm, M.C. Cohen and J.R. Cherikowsky, Phys. Rev. B22 (1980) 4610.
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S. Nishigaki et al. / Pd on S i ( l l l ) 7 X 7
[22] For Ag adsorption on Si(l 11) at elevated temperatures there exists a controversy whether Ag is embedded beneath the topmost Si layer or not. For room temperature deposition of Ag on Si(lll), however, many authors seem to be in agreement on the point that Ag adsorbs at sites above the topmost Si layer. See discussions and references in ref. [4]. [23] G. Le Lay, Surface Sci. 132 (1983) 169, and references therein. [24] For the A u / S i system it is established that substrate Si-Si bonds are cut by an active influence of mobile electrons in condensed Au overlayers. See ref. [26]. [25] K.N. Tu, Appl. Phys. Letters 27 (1975) 221. [26] A. HirakL J. Electrochem. Soc. 127 (1980) 2662.
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INITIAL S T A G E OF Pd A D S O R P T I O N ON S i ( l l l ) 7 x 7 S U R F A C E S T U D I E D BY A E S A N D E E L S S. N I S H I G A K I , T. K O M A T S U , M. A R I M O T O a n d M. S U G I H A R A
Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 440, Japan Received 8 May 1985; accepted for publication 24 October 1985
The initial stage of adsorption of Pd on a Si(lll)7x7 surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and surface workfunction change. For Pd deposition at room temperature (RT) the Si(LVV) Auger signal intensity decays in a broken linear line. The structure factor, defined as the intensity ratio of the subpeak to the main one in Si(LVV) Auger spectra, increases up to a maximum around one monolayer coverage. In EELS spectra two peaks, characteristic of Pd, appear at the completion of the first Pd layer. Pd atoms deposited on Si(lll) at RT form initially flat layers of a few monolayers height without mixing with substrate Si atoms. For Pd deposition at a moderately high temperature (MT) of about 300°C, however, the structure factor for Si(LVV) Auger spectra does not change. EELS peaks, characteristic of Si substrate, remain clearly even beyond one monolayer coverage. Pd atoms deposited at MT are unstable and easily diffuse into the bulk. We present evidences to support the "screening" model for the bond-breaking mechanism at the Pd/Si interface.
I. Introduction T h i n silicide films are n o w finding wide application as b o t h ohmic contacts a n d Schottky-barrier electrodes in silicon devices. Of the various silicides, the P d - S i system has attracted the most interest, because it is a reactive interface to form a c o m p o u n d such as PdzSi at room temperature. M u c h work has already been d o n e b y p h o t o e m i s s i o n (PES) [1-4], Auger electron (AES) [3-8], t r a n s m i s s i o n electron microscopy [3,6,7], surface extended X-ray a b s o r p t i o n fine structure [9] a n d ion scattering (ISS) [8,10,11] techniques. However, the initial stage of interface f o r m a t i o n has n o t yet explored clearly. Questions arise, whether there is or is n o t a m a x i m u m thickness before reaction a n d mixing with the substrate occurs [11]; how the initial Pd layer grows; how the elevated temperature of Si substrate influences the a d s o r p t i o n process of Pd; a n d so on. I n the present study we have used AES, electron energy loss spectroscopy (EELS) a n d surface w o r k f u n c t i o n change (A~) as m a i n tools to study the issues m e n t i o n e d above. Some first results o n initial a d s o r p t i o n of Pd o n a 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
28
S. Nishigaki et al. / P d on Si( l l l ) T x 7
S i ( l l l ) 7 × 7 surface at room temperature and at a moderately high temperature will be presented. We will show that an abrupt interface without mixing of Pd with Si can be formed at the initial stage of room temperature deposition, and that Pd atoms deposited at elevated temperatures are unstable and easily diffuse into the Si substrate.
2. Experimental Silicon crystals were cut from an n-type wafer with an epitaxially-grown (111) layer (thickness = 20 #m, resistivity = 50 ~2 cm). They were clamped in a tantalum holder and mounted in a goniometer. Prior to mounting no chemical treatments were made. The crystals were cleaned by heating resistively at = l l 0 0 ° C in U H V (base pressure = 1 x 10- 10 Torr), yielding distinct low energy electron diffraction (LEED) patterns of 7 x 7 structure. No contaminants could be detected in the Auger spectra (peak-to-peak ratio C / S i less than 1 x 10 2). Every crystal was used for only one experiment of Pd deposition and never cleaned a second time. The samples were kept at room temperature (RT) and at a moderate temperature (MT) of about 300°C during Pd deposition. Pd was evaporated from a Pd foil mounted in a heated W spiral wire. The deposition rate was set to be about 0.7 x 1014 a t o m s / c m 2 per min, which was monitored by a quartz oscillator. We define unit coverage (0 = 1) as the atomic density (7.8 x 10 a4 a t o m s / c m 2) in the outermost S i ( l l l ) surface layer. For Auger and energy loss analyses a home-made retarding-field analyser with a hemispherical four-grid assembly was used. Si(LVV) Auger differential spectra have a main peak at 92 eV and a clear subpeak at 74 eV [12]. We studied on the variation not only of the peak-to-peak intensity of the former but also of the intensity ratio of the latter to the former. The second derivatives of EELS spectra were obtained by differentiating measured loss curves numerically according to Savitzky and Golay [13]. Moreover we used a 127°-cylin drical deflector analyser to monitor the low-energy threshold of secondary electron energy distribution curves during Pd deposition, which reflects the workfunction variation, Aq~.
3. Results 3.1. Pd adsorption at room temperature 3.1.1. A E S results
The Si(LVV) differential spectra were measured with the hemispherical retarding-field analyser at an incident electron energy of 1500 eV. The spectra
S. Nishigaki et aL / Pd on S i ( l l l ) 7 x 7
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29
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Fig. 1. (a) Si(LVV) Auger peak-to-peak intensities I A (at 92 eV) versus coverage of Pd deposited at RT. (b) Structure factor, defined as I B / I A (the ratio of the subpeak to the main peak), versus Pd coverage. Coverage is measured in units of the atomic density of the topmost Si layer (7.8 × 1014 a t o m s / c m 2 ).
consist of mainly two peaks: One is at 92 eV (peak A) and the other at 74 eV (peak B). The former has been used by other authors in determining the growth mode of Pd on S i ( l l l ) surfaces [8]. We plotted peak-to-peak intensities of peak A (IA) as a function of Pd coverage (8) in fig. la. The intensity I A decreases with # in a broken linear line, which means the monolayer growth mode (the F r a n k - V a n der Merwe mechanism). As one broken line corresponds to the growth of a single layer, no less than 3 monolayers of Pd have been formed before the occurrence of reaction or mixing with Si. The first Pd layer is completed a t 01 = 0.6, which corresponds to the atomic density of 4.7 × 1014 a t o m s / c m 2. This is similar to the case of Ag adsorption on S i ( l l l ) [14], where the Auger decay line bended at the coverage less than unity. The initial decay slope of the Auger intensity is = 43% per unit coverage. This value is again compared well with the case of A g / S i ( l l l ) system at RT, where it is about 48% from fig. 2 in ref. [14]. Next, we examined the variation of Auger spectral shape with the progress of adsorption. Since the escape depth of Si(LVV) Auger electrons is about 5 ,~ [15], its spectral shape reflects the bonding scheme of near-surface Si atoms. We define the structure factor for Si(LVV) spectra as the peak-to-peak intensity ratio of peak B (at 74 eV) to peak A (at 92 eV). Therefore the effect of
30
S. Nishigaki et al. / Pd on Si(l l l ) 7 x 7
intensity decay by decrease of escape probability due to the overlayer growth is excluded from the ratio I J l A. We show the dependence of the structure factor on the Pd coverage in fig. 1 b. The ratio I B / 1 A increases at the first stage of Pd adsorption, and then decreases gradually. The most interesting is that the Pd coverage where 1 J I A reaches a maximum corresponds very well to 0~ where the first Pd layer is completed. Theoretically, peak A is attributed to the Auger transition that both electrons, downwards and upwards, are p-like in nature near the top of the valence band [16]. Peak B corresponds, on the other hand, to the Auger transition for two s-like electrons near the bottom of the band. The initial increase of the structure factor in fig. lb, therefore, corresponds to the decrease of peak A relative to B. Beyond -~ 3 monolayers we sometimes found drastic changes of the spectral shape to indicate compound formation which has been observed by other authors [7,8]. 3.1.2. E E L S
results
Electron energy losses were measured with the same analyser as for AES. The incident electron energy was 80 eV, which is suitable for the study of surface electronic structure due to its short mean free path of ~ 5 A [15]. Second derivatives of successive EELS spectra in the course of Pd adsorption on S i ( l l l ) at RT are shown in fig. 2. Curve (a) in fig. 2 exhibits several features characteristic of the clean S i ( l l l ) 7 × 7 surface. They correspond to losses by excitation of bulk plasmon (h~0p), surface plasmon (h~0~) and single-electron excitation in the bulk (E2) and at the surface ($2 and S~) [17]. Among them the surface losses (hc0~, S2 and S~) diminish with Pd deposition and practically disappear around 0 = 0.7 where the first Pd flat layer has been completed as shown in fig. la. The loss peak by the bulk plasmon e x c i t a t i o n (h~p) is destroyed at 0 > 4. This means that the reaction of Pd with Si begins to occur by depositing Pd beyond a critical thickness. Two new peaks (P1 and p~) appear at 0 - - 0 . 7 and increase with the Pd coverage, as shown in fig. 2. The peak P~ at the energy loss of ~ 5.5 eV may be attributed to single-electron excitation: the initial states are the d-levels of adsorbed Pd, which lie at ~ 3.5 eV below E v [1], and the final states are somewhere in the conduction band of the Si substrate. The peak p~ appears at 12 eV in good agreement with a peak at 12.0 eV in photoemission data of bulk Pd by Vehse et al. [18] and with a peak at 13 eV in the imaginary part of the complex dielectric function of Pd by Robin [19]. Therefore this is explained in terms of strength in the optical transition probability function. Another peak P2 (at ~ 7.5 eV) is found clearly in the coverage range of 2 < 0 < 4. This is attributed to the energy loss by the excitation of collective oscillation (plasmon), which has previously been observed in bulk Pd by other authors [18-20]. Beyond the coverage of about 4 monolayers, the peak P2 diminishes and the strong feature by the Si bulk plasmon excitation is destroyed. We,
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therefore, find that there exists a critical Pd coverage (0 c = 3-4) beyond which the reaction of Pd with the Si substrate occurs.
3.1.3. Workfunction variation The variation of workfunction Aq~ during Pd deposition at RT was measured by detecting the low-energy threshold of secondary electron energy distribution curves. We show in fig. 3 that the workfunction increases with the Pd coverage and reaches a maximum of Aq~max-- 0.35 eV at 0--2.5. Beyond this coverage the workfunction decreases slightly and is nearly saturated with Aq,_-- 0.25 eV. The general tendency of this workfunction change agrees with the result of Rubloff et al. [4]. They also observed the initial increase followed by the plateau, which has been attributed to the workfunction of Pd2Si compound. However, there are some discrepancies between two results. Aq;~a× obtained here is lower by 0.1 eV than that of Rubloff et al. [4]. The coverage where A~ reaches the maximum iia the present experiment is about a half of that obtained by them. They observed a gentle increase beyond the plateau and attributed it to the presence of incompletely reached Pd metal at the surface of the overlayer, but we did not observe it. The difference in deposition rate for these two cases would be the main factor to influence the above behavior.
S. Nishigaki et al. / Pd on S i ( l l l ) 7 x 7
32
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Their deposition rate, = 1 ,~/min, was about 10 times faster than in the present experiment. Therefore excessive amount of metal Pd would have been accumulated at the surface without being reacted, as mentioned by themselves. The higher value of the workfunction at the plateau, the thicker overlayer necessary to attain the plateau and the successive increase of the workfunction beyond the plateau are understood if we assume such an effect. The present result shown in fig. 3 suggests the change of mechanism for P d / S i interface formation around 0 = 3 monolayers, namely, from the monolayer growth mode of pure Pd metal overlayers to the reaction of Pd with Si, as discussed in the next section. 3.1.4. Discussion
The AES and EELS spectra presented here demonstrate clearly that upon deposition at RT Pd atoms adsorb on S i ( l l l ) to form flat layers up to 0 = 3 before reaction of Pd with Si occurs. According to our Si(LVV) Auger decay curve, the atomic density of the first Pd layer is about 0.6 times the one of the substrate Si(111) layer. The saturation of Pd in the first layer suggests that the bonding scheme of P d - S i in the first layer is covalent rather than metallic in nature, which is in agreement with the He neutralization data of T r o m p et al. [8] and the calculation of Ihm et al. [21]. This saturated atomic density of Pd and the initial slope of Si(LVV) intensity decay are similar to those for Ag adsorption on Si(111) at RT [14]. From this similarity one may draw a picture that Pd adsorbs rather non-reactively on, not beneath, the topmost S i ( l l l ) layer [22]. We observed the slight modification of Auger spectral shape during the first layer formation, that is, the initial increase of the ratio I B / 1 A , a s shown in fig. lb. This is easily understandable by the aid of the calculation of Ihm et al. [21]. They showed that the density of states at - 2 . 5 eV below E F increases with Pd adsorption, corresponding to Pd 4d localized orbitals, whereas the density of states near the bottom of the Si band shows no marked
s. Nishigaki et al. / Pd on Si(l l l)7× 7
33
changes. Therefore the increase of the structure factor ( I J I A) may be attributed to the relative decrease of the portion A. This means that electrons near the top of the Si valence band, which is nearly p-like in nature, are partly transferred to around adsorbed Pd atoms. The saturation coverage of the first Pd overlayer, indicated by the break of the Si Auger intensity decay curve, is 01 = 0.6, which is discrepant to the value (01-- 1) by Tromp et al. [8]. We can find out no origins to cause this discrepancy except the different modes of preparation of the Si 7 × 7 surface. For the case of Ag condensation on Si(111) it is now established that the mode of preparation of the clean Si surface has some influence on the initial stages of the adsorption process to cause different values of saturation coverage of the first Ag overlayer [23]. On surfaces prepared by cleavage and annealing or by only flashing epitaxially-grown layers the saturation coverage of Ag, either at RT or at elevated temperatures of = 300°C, is around 2/3, which is definitely less than 01 = 1 obtained on cut-and-polished surfaces (by cleaning procedure of argon ion bombardment plus annealing) [15,24]. We used the former procedure in the present study on P d / S i ( l l l ) and obtained a value near 01 ---2/3. Tromp et al. prepared their surfaces by the latter method and obtained 01 = 1. Some factors on the microscopic scale (such as step density) may have influenced on the saturated atomic density of the first layer. More systematic experiments are necessary in this respect. The structure factor ( I B / I A ) of the Si(LVV) Auger spectra decreases gradually beyond 01 = 0.6. This means that the adsorption of Pd which interacts directly with the surface Si atoms taking electrons partially from them finishes at 0 a ---0.6. The condensation of Pd to form next layers may not seriously influence the bonding scheme between the Si and the first Pd layer. The gradual decrease of the ratio I B / I A can be understood if we consider that the increase of the density of states near the Fermi level caused by Pd condensation will enhance the Auger intensity near peak A. We speculate that this increase of electron density of states, which should be sp-like in character and be supplied from the Pd side, would play an important role in weakening covalent Si-Si bonds of the substrate to yield mixing and reaction of Pd with Si [241. We showed some evidences in the previous section to suggest that the Pd overlayer with the thickness of 3 - 4 monolayer was unstable and the reaction of Pd with Si occurred beyond this thickness: Occasional observation of the abrupt change of S i ( L W ) Auger spectra; the collapse of the bulk-plasmon loss peak of Si; the saturation of zlff followed by the slight decrease. According to our picture of monolayer growth followed by compound formation, UPS and A~ results by Freeouf et al. [1] can be simply understood as follows: The Pd 4d peak position in UPS would start to increase from a value for isolated Pd adsorbates on Si and reach to a value for PdzSi; the work function would also increase gradually from the clean surface value up to a value for Pd 2Si surface,
S. Nishigaki et al. / P d on S i ( l l l ) 7 × 7
34
not abruptly to the latter one. Our model on P d / S i interface formation is not contradictory to the TEM observation by Ho et al. [7], which demonstrated the epitaxial growth of Pd2Si lattice directly on the substrate Si surface. If the initial growth of a few monolayer of pure Pd is followed by diffusion of Si atoms into the Pd overlayer, consequently, an abrupt interface of Pd2Si/Si would be formed. 3.2. P d deposition at ~ 3 0 0 ° C 3.2.1. A E S results
Curve (a) in fig. 4 shows the coverage dependence of the Si(LVV) Auger peak-to-peak intensity I A. It fits neither broken linear lines nor exponential c u r v e s . I A decays more slowly than the case of RT deposition and does not approach zero even with heavier dosage of Pd. This means that substrate Si atoms are exposed to vacuum still after Pd deposition. There are several possibilities to account for the above behavior: (i) Evaporation a n d / o r surface diffusion of deposited Pd atoms. Perhaps this effect cannot be neglected at higher temperatures than RT and may be one reason to decrease Pd atoms staying on the specimen surface. (ii) The Pd2Si formation from the initial stage of Pd condensation. With this model I A would decrease and rapidly reach a constant value for Pd2Si compound surface, because the further progress of Pd2Si growth would not
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S. Nishigaki et aL / Pd on Si(111)7× 7
35
effectively° influenco 1A as the escape depth for Si(LVV) Auger electrons is only about 5 A [15]. Therefore this item cannot account for the measured decay curve of I A. (iii) Three-dimensional island formation. (iv) Diffusion of deposited Pd atoms into the bulk. These possibilities will be discussed later with other experimental results. The plots of the structure factor I B / I A versus the Pd coverage O are shown in fig. 4b. The absolute value of this ratio I B / I A cannot be compared with the one of the RT case in fig. lb, since it depends sensitively on energy-analysing conditions, such as energy-scan speed and modulation voltage. The structure factor is found to be nearly constant, which implies that the valence band structure near the Si surface does not suffer a big change due to the Pd deposition. Therefore item (ii) above is again contradictory to this experimental result. Moreover, we can exclude a model of two-dimensional Pd layer growth at the surface, different from the case of fig. lb. 3.2.2. E E L S
results
Electron energy loss spectra are shown in fig. 5 as a function of Pd coverage O deposited at a substrate temperature of 300°C. There appears many features in comparison with the case of RT deposition (fig. 2). The bulk-plasmon loss peak (ht~p) persists even under 5.8 monolayer deposition, implying that Si
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36
S. Nishigaki et al. / Pd on Si(l l l)7× 7
lattice is not destroyed by the exposure to Pd. Four peaks from clean Si ( E 2, S2, hw~ and S3 peaks) still appear clearly at around one monolayer deposition, and some of them even at 2 monolayers. This is in contrast to the RT behavior that they disappear after completion of the first Pd layer (01 -- 0.6). Among peaks originating from the condensed Pd, P~ is not found in the spectra even with heavier dosages of Pd. This means that the metallic Pd is not formed, and that item (iii) in the latest section is denied for the M T deposition. The only possibility to account for the above findings is the model of diffusion of Pd atoms into the bulk. With this picture we can speculate that the concentration of Pd would increase gradually in the three-dimensional region near the surface. The Si-substrate lattice, however, would not be destroyed. Then the energy loss peaks from clean Si is expected to remain clearly even with heavy dosages of Pd, while the loss peak P~ not to appear. 3.2.3. Discussion There have been no reports which deal with P d / S i interface formation at elevated temperatures above RT. The behavior of Pd atoms on heated Si(111) surface has, therefore, been unknown. The present experiments demonstrate the instability of Pd atoms deposited on S i ( l l l ) at MT. We induce the model that Pd atoms diffuse into the Si substrate at ~ 300°C without breaking the Si-lattice structure. This can surely explain the findings by AES that I A does not approach zero and that the structure factor I B / I A changes scarcely. It can also account for the EELS results that the loss peaks from clean Si can survive and that the loss peak ~ does not appear. Our picture is consistent with a recent result by Ratherford backscattering [11]. It showed that a thin Pd film ( -- 3 monolayers) deposited at R T diffused rapidly into the Si substrate when heated at ~ 500°C. The instability of the Pd overlayer which is thinner than a critical Pd film thickness has been mentioned [11]. The constancy of the ratio I B / I A and the stability of the loss peaks from the Si against Pd deposition mean that the diffusion of metal atoms into the substrate does not play a main role in breaking Si-Si bonds of the substrate. There have been two different models on the breaking mechanism of Si-Si bonds with the aid of metal atoms from outside. One is the "interstitial" model [25] and the other the "screening" model [11,26]. According to the former, covalent Si bonds would be broken by in-diffusion of metal atoms and arranging them around each Si atom. For P d / S i system, however, the in-diffusion of Pd does not destroy Si-Si bonds, as shown in the present experiment. The latter postulates electronic screening of Coulomb interaction, responsible for the covalent bonding of Si crystal, due to mobile free electrons in the deposited metal overlayer. With this model a metal film of a few monolayer thickness is thought to be necessary to promote the reaction of the deposited metal with the substrate. The present results, both at R T and at MT, are in favor of the screening model.
S. Nishigaki et al. / Pd on Si(111)7× 7
37
4. Conclusion Summarizing the AES, EELS and Aq) results, the following picture emerges for Pd/Si interface formation at RT: Pd overlayer grows on the S i ( l l l ) surface in the monolayer growth mode up to about 3 monolayers; the atomic density of each Pd layer increases from 0 a --- 0.6 towards that of Pd metal; Pd atoms in the first overlayer are bonded with surface Si atoms covalently; when the overlayer grows thicker than 3 - 4 monolayers, it becomes unstable and starts to react with Si. For the behavior of Pd atoms deposited at MT we get the following picture: Pd atoms incident on the Si surface at MT do not stay at surface sites stably but diffuse into the substrate; the lattice structure of Si is not easily destroyed by the in-diffusion of Pd. We remark in conclusion that the initial stage of P d / S i interface formation proceeds in the monolayer growth mode, the Frank-Van der Merwe type, followed by the low-temperature reaction of Pd with Si beyond the critical Pd-layer thickness.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
J.L. Freeouf, G.W. Rubloff, P.S. Ho and T.S. Kuan, Phys. Rev. Letters 43 (1979) 1836. J.L. Freeouf, J. Vacuum Sci. Technol. 18 (1981) 910. P.E. Schmid, P.S. Ho, H. FSII and G.W. Rubloff, J. Vacuum Sci. Technol. 18 (1981) 937. G.W. Rubloff, P.S. Ho, J.L. Freeouf and J.E. Lewis, Phys. Rev. B15 (1981) 4183. G.Y. Robinson, Appl. Phys. Letters 25 (1974) 158. P.S. Ho, T.Y. Tan, J.E. Lewis and G.W. Rubloff, J. Vacuum Sci. Technol. 16 (1979) 1120. P.S. Ho, P.E. Schmid and H. FSII, Phys. Rev. Letters 46 (1981) 782. R.M. Tromp, E.J. van Loenen, M. Iwami, R.G. Smeenk and F.W. Saris, Surface Sci. 124 (1983) 1. J. St~hr and R. Jaeger, J. Vacuum Sci. Technol. 21 (1982) 619. R.M. Tromp, E.J. van Loenen, M. Iwami, R.G. Smeenk and F.W. Saris, Thin Solid Films 93 (1982) 151. A. Hiraki, Japan. J. Appl. Phys. 22 (1983) 549. C.C. Chang, Surface Sci. 25 (1971) 53. A. Savitzky and M.J.E. Golay, Anal. Chem. 36 (1964) 1627. S. Nishigaki, K. Takao, T. Yamada, M. Arirnoto and T. Komatsu, Surface Sci. 158 (1985) 473. M.P. Seah and W.A. Dench, Surface Interface Anal. 1 (1979) 2. P.J. Feibelman, E.J. McGuire and K.C. Pandey, Phys. Rev. B15 (1977) 2202. J.E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. R.C. Vehse, J.L. Stanford and E.T. Arakawa, Phys. Rev. Letters 19 (1967) 1041. S. Robin, in: Optical Properties and Electronic Structure of Metals and Alloys, Ed. F. Abel6s (North-Holland, Amsterdam, 1966) p. 202. R.C. Vehse and E.T. Arakawa, Bull. Am. Phys. Soc. 11 (1966) 830. J. lhm, M.C. Cohen and J.R. Cherikowsky, Phys. Rev. B22 (1980) 4610.
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S. Nishigaki et al. / Pd on S i ( l l l ) 7 X 7
[22] For Ag adsorption on Si(l 11) at elevated temperatures there exists a controversy whether Ag is embedded beneath the topmost Si layer or not. For room temperature deposition of Ag on Si(lll), however, many authors seem to be in agreement on the point that Ag adsorbs at sites above the topmost Si layer. See discussions and references in ref. [4]. [23] G. Le Lay, Surface Sci. 132 (1983) 169, and references therein. [24] For the A u / S i system it is established that substrate Si-Si bonds are cut by an active influence of mobile electrons in condensed Au overlayers. See ref. [26]. [25] K.N. Tu, Appl. Phys. Letters 27 (1975) 221. [26] A. HirakL J. Electrochem. Soc. 127 (1980) 2662.