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GaAs(110) revisited: A photoemission study
Journal of Electron Spectroscopy and Related Phenomena, 62 (1993)ES72 0368-2048/93/$06.00 0 1993 - Elsevier Science Publishers B.V., Amsterdam
Ag/GaAs(llO)
revisited:
a photoemission
study’
D.A. Evans and K. Horn* Fritz-Haber-htitut (Germany)
der Max-Planck-Gesellsckaft.
Faradayweg
4-6.0-1000
Berlin 33
(First received 9 October 1992; in final form 6 November 1992)
Abstract The Ag/GaAs(llO) system, which serves as a model system for unreactive metal-semiconductor interfaces, is re-examined by means of high resolution valence and core level photoemission. The origin of an adsorbate-induced new component on both Gaail and As3d core level lines is discussed in terms of charge transfer between overlayer and substrate atoms, or an extra-atomic screening effect. The broadening of the Ag d-band is compared with the evolution and decreasing half width of the Fermi edge with Ag deposition at low substrate temperature. These observations are discussed in relation to the search for a criterion for the onset of metallicity in metal overlayers on semiconducting substrates.
Introduction Metal overlayers on the cleavage face of zincblende semiconductors, in particular GaAs, have long served as model systems for fundamental studies of the geometric and electronic properties of metal-semiconductor contacts. The clean (110) surfaces do not undergo complex reconstructions, and the bond angle rotation relaxation has been experimentally determined. This relaxation has been shown theoretically to result in the removal of surface states from the fundamental band gap region [l], leading to the absence of Fermi level pinning on the clean surface. However, interface reactions between deposited metals and the substrate have been found to be significant for most junctions, with only a few metals yielding ideal unreactive interfaces. The Ag/GaAs(llO) system is one example of the latter, and has been studied by means of various techniques, most notably low energy electron diffraction (LEED), photoelectron spectroscopy, and scanning tunneling microscopy (STM) [2-211. In the course of a recent investigation of quantum size effects in silver islands on GaAs(ll0) [23] we have reinvestigated silver growth on cleaved GaAs(ll0) surfaces at differ‘Dedicated to the memory of John C. Fuggle. * Corresponding author.
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ent substrate temperatures. In the present paper attention is concentrated on the analysis of the Ga3d and As3d core level line shapes upon silver deposition, and the evolution of metallic behavior as manifested in the width of the Fermi level Er and the width of the Ag valence band. In view of the importance of such studies for band bending investigations, the occurrence of a surface photovoltage is also considered. The results of this study are discussed with particular emphasis on the fact that this system fulfills all requirements for an ideal, i.e. non-disruptive, metal-semiconductor interface. Experimental The experiments were performed in several ultra-high vacuum photoemission chambers, equipped with Knudsen cells for metal overlayer deposition, LEED optics, cleaving tools for substrate surface preparation, and a crystal holder which allowed sample cooling to 100K. Photoelectron spectra were recorded using light from the BESSY (Berliner-ElektronenSpeicherrings-Gesellschaft fiir Synchrotronstrahlung) storage ring in Berlin, using toroidal grating monochromators (TGMs) 4 and 6 with a photon energy range from about 10 to 120 eV. Photoelectrons were energyanalyzed and detected using a hemispherical electron energy analyzer (HA 50 from VSW, UK), with an overall resolution of about 50meV. The substrates were pre-notched bars of n-type GaAs (Wacker Chemitronic, Germany) with a carrier concentration of about 6 x 1017 cme3, which were cleaved in vacua and showed mirror-like surfaces with only a few largescale steps. Temperatures were measured with a thermocouple attached to the samples. Deposition rates were measured using a q’uartz crystal microbalance (Leybold Inficon). Results Growth
mode and substrate
core level line shape
The growth mode of thin films on solid substrates may be established from the intensity of a substrate core level or Auger emission line as a function of overlayer thickness [24]. In this study, Ga3d and As3d lines were recorded during the stepwise deposition of Ag. The spectra were normalized for photon flux and analyzed in terms of bulk and surface emission contributions. This analysis was carried out for the entire set of spectra for each coverage sequence, using identical parameters as described in the next section (Evolution of Ag valence bands and metallic character of the overlayer). The intensity of the bulk Ga3d and As3d component resulting from this procedure is plotted on a semi-logarithmic scale in Fig. 1.It is seen
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61
I--
s
5
(0
‘5 .Y
E
r z ‘ii -^s L
-As3d
-2-
$
-3-
I
0
I
5
I
10 Ag deposition (A)
15
Fig. 1. Ag deposition-induced attenuation of the bulk component of the Ga3d core level line at substrate temperatures of 300 K (0) and 100 K (O), and the As3d core level line at 100K (A).
that, over the whole coverage range of 15A, the attenuation of both Ga3d and As3d levels is linear for low temperature growth (0 and A, respectively). This growth mode has been termed “simultaneous multilayer” growth, and it has been shown that the corresponding dependence of substrate intensity on dose is exponential in shape [25]. On the basis of data for the mean free path ,I of electrons [26], the overlayer thickness calibration given in Fig. 1 has been calculated. This was found to be in good agreement with thickness
data evaluated from a quartz microbalance calibration and the known evaporation times for each dose. The kinetic energies of the Ga3d and As3d for the data in Fig. 1 are 16 and 37 eV, respectively. The value for 1 for the higher kinetic energy, derived from a functional description of many different data by Seah and Dench [26] is about 40% lower than that for the lower kinetic energy; our data for InSb [27] suggest a somewhat smaller difference (,I(16eV) = 6.0 A; A(37eV) = 4.2 A). This causes the slope for the As intensity to be somewhat shallower than that of the Ga3d line; they have been offset in Fig. 1 for clarity. Room temperature deposition leads to a much more gradual nonexponential decay of the substrate signal, as shown by the Ga3d data, also given in Fig. l(0). This is the expected behavior for island growth, which has been imaged using STM in the recent study by Trafas et al. [21]. Deposition at room temperature thus obviously leads to
island formation, and islands are also formed when a layer deposited at low temperature is warmed up, through the increased surface mobility of the adparticles. Subsequent deposition and annealing cycles lead to renewed attenuation, and after each annealing the substrate signal is slightly lower,
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indicative of the fact that the islands occupy an increasing part of the substrate surface [23]. Whereas the core level line intensities may be utilized for an evaluation of growth mode such as is displayed in Fig. 1, in reactive metal-semiconductor interfaces they find use in the investigation of reaction path and new compound formation, by an analysis of extra components which occur upon overlayer deposition. For large core level shifts such analysis is fairly straightforward, but small shifts and low intensities induce uncertainties which are not readily overcome. Moreover, such shifts have so far been mainly interpreted in terms of initial state (“chemical”) shifts, neglecting the influence of the photoionization process on the appearance of the spectra. Here we attempt to include this aspect in the interpretation of shifts on the Ga3d and As3d core levels upon Ag adsorption. Our basis is an important finding reported by Trafas et al. [21] in their STM study of this system. These authors note that Ag clusters, formed by room temperature deposition of Ag, sometimes move under the influence of the STM tip, and that the surface areas previously covered by Ag islands show no difference in the STM image when such islands have moved. This is a most direct way of demonstrating that no disruptive interaction between the islands has taken place, lending strong support to the classification of this system as an unreactive one. Any new or shifted core level line components then have to be related to a possible reversible chemisorption bond, a change of the substrate surface geometry such as an undoing of the substrate bond-angle rotation-relaxation, or to physical processes connected with the proximity of metal overlayer and semiconductor substrate surface. Consider the set of Ga3d (Fig. 2) and As3d (Fig. 3) core level emission spectra recorded for increasing coverages of Ag at low temperature. The clean surface spectra exhibit the well-known surface core level shift components [28] towards higher (Ga3d) and lower (As3d) binding energy. Even at the lowest Ag coverages, the entire peak becomes somewhat blurred, such that the extra components cannot be readily distinguished. However, extra intensity is observed on the high kinetic (low binding) energy side of the peak. This intensity increases up until about monolayer coverage (1 monolayer = 2.35 Hi, assuming close-packed fee Ag layers). Further deposition does not change the appearance of the spectra apart from an overall reduction in intensity. One representative curve-resolved spectrum, performed for a deposition of about 0.546 monolayers for both Ga3d and As3d, is shown at the top of Figs. 2 and 3. For GaSd, the line shape analysis, which requires two doublet (bulk and surface) contributions for the clean surface, now calls for three components, if all other parameters such as spin-orbit splitting, Lorentzian (lifetime) broadening and Gaussian (experimental) broadening are held constant. The new component occurs at a kinetic energy higher than that of the bulk line. The least-squares optimization
D.A. Evans, K. Horn/J.
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Ga 3d
A
l.sA
\.
Ag dTAyition 15 10.5 a
4
2:5 1.5 0.8
36
37
39 38 kinetic energy (eV)
40
Fig. 2. Set of Ga3d core level spectra (recorded at lOOK) for various Ag depositions as indicated. The top spectrum is shown along with a line shape analysis based on three contributions (see text).
routine uses the ,Marquardt algorithm, and provides, along with the parameters for the fit, a confidence interval for each parameter, corresponding to an increase of unity in the reduced chi-square if the parameter is replaced by the upper or lower boundaries marked by the confidence interval [29]. The routine thus provides a measure for the reliability of the model function. However, the need for an extra component at lower binding energy may be directly read off the spectrum through the larger wing on the low binding energy side. The surface core level component is attenuated as the Ag overlayer builds up. A line shape analysis is relatively simple for the Ga3d line, but because the decreasing surface component and the increasing new Ag-induced component occur on opposite sides of the bulk peak,
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Ag/GaAs(l 10) T P 1OOK
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As 3d eV
A
15 10.5 8 6.3 4 2.5 1.5 0.8 0.4
0
I
14
I
15
I
I
I
I
16
I?
18
19
kinetic energy (eV)
Fig. 3. Set of A&d spectra (recorded under equivalent conditions as in Fig. 2) with line shape analysis for the spectrum at 1.5A deposition.
the situation is less clear for the As3d peak. Here, the spectra up to a monolayer coverage can be readily modeled by one single surface core level contribution, but a satisfactory fit necessitates that it shifts toward the bulk emission line, and with an additional broadening. However, this shift would have to occur even for the lowest depositions. The adatoms are immobile under these conditions, so the isoIated adatoms would have to affect a large area in order to give rise to this kind of spectrum. This is in contrast with the observation of Ludeke et al. f5] that the surface core exciton persists up to high coverages, indicative of a rather localized interaction of the silver adatoms or islands with the substrate surface. Several models may be put forward in order to explain the observation of Ag-induced shifted components on the substrate core level lines. In the usual initial state model, the generation of new chemical species may be
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ruled out following the absence of surface disruption as reported in the STM experiments. Charge transfer may still occur in a chemisorption-type bond, which may be reversible. Evans et al. have recently obtained strong evidence for charge transfer in core and valence photoelectron spectra of Na/GaP( 110)and Na/GaAs(llO) [30], in which shifts in the Ga3d line were related to the occurrence of a new peak in the fundamental band gap. Similar processes may well be the cause of the new shifted component observed here, in the sense that some transfer of charge from the Ag s level occurs into the unoccupied Ga-derived surface state, which is consequently ’ pulled down in energy. No sign of the filling of such state, similar to that of Na/GaAs(llO), is observed in the valence band region, however. In an entirely different model, one may assume that the Ag layer induces a line component shifted towards lower binding (higher kinetic) energy on both Ga3d and As3d levels, and that the new lines are caused by the influence of the metal overlayer on the screening of the photon-induced core hole in the substrate core levels. This additional screening by the metal atoms will lead to the emergence of a core level line at lower binding energy on both anion and cation core levels. Thus, the deposition of Ag will, for the case of the Ga3d level, lead to a reduction of the surface core level shift (SCLS) contribution, and an increase of a new line at lower binding energy. This trend is supported by the set of Ga3d spectra shown in Fig. 2. Here, the SCLS component clearly decreases with Ag deposition, while the wing on the high kinetic energy side of the peak increases. A line shape analysis of each of the spectra of Fig. 2 and a plot of the relative intensities of SCLS and new components (not shown here) demonstrates that the decrease of the former scales with the increase of the latter. For the As3d level, a line shape analysis is rendered difficult by the fact that both SCLS and the new component postulated here occur on the high kinetic energy side of the peak. This prohibits a quantitative analysis similar to the one described for the Ga3d level, because the confidence intervals for the line shape analysis parameters become excessively large. However, even under these conditions the overall trend in the intensity of the As3d SCLS and new components matches that found for the Ga3d level. Small shifted core level line components in systems which are considered unreactive, such as indium overlayers on the (110)surfaces of the III-V semiconductors, have been observed before. However, the vital information concerning the extent of interface reactions has been missing, and thus a vague interpretation in terms of emission from “interface species” has been put forward [31]. Evolution
of Ag valence bands and metallic
Studies of silver overlayers
on GaAs(ll0)
character
of the overlayer
have been the subject of dis-
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(b)
AglGaAs( 110)
1OOK evolution of Ag valence band
Aa/GaAslllO1 vience
1 OOK
band-edge
hot61eV
Ag deposition
(4 14.5 7.6 6.2 4.2
2.4
1 -_ I
45
I
1
55 50 kinetic energy (eV)
2.4
x2
1.4
i
1.4 0.25 xs
0.05 0
I
60
55
56
58 57 kinetic energy (eV)
Fig. 4. (a) Spectra of the Ag4d band (left), for Ag depositions on GaAs(ll0). Equivalent thicknesses are given for each spectrum. (b) Spectra of the region near the Fermi edge for different equivalent thicknesses as indicated. The emergence of a metallic Fermi edge about 0.9 eV below the reference E,, which coincides with the overlayer Fermi level at saturation, is due to a surface photovoltage induced by the photoemission light source.
cussion in the long-standing debate about the causes of band bending induced by metal over-layers [ll]. Here the transition from single adatoms to a continuous metal overlayer, and its evolution towards metallic character, has been the subject of controversy [16]. The width of the group of Ag d bands has been used as a criterion for metallicity [ll], with the implication that a width equal to the bands in bulk Ag metal signals metallicity. In a simple model of metal atom interaction, the width of the d band will scale with orbital overlap and the evolution of metallic behavior [32]. However, another direct measure of metallicity can be obtained from the width of the developing metallic Fermi level of the overlayer. The high flux on the TGM 6 wiggler-undulator beamline enables us to follow its evolution with excellent statistics and high resolution, and provides a suitable testing ground for a comparison which so far has not been reported in the literature. Here we compare the d band width with the width of the Fermi level as the overlayer thickness increases. Photoemission data for the d band region and the Fermi edge are shown in Fig. 4 for various Ag depositions at 100K. Up to the third spectrum, GaAs valence band features may still be distinguished along with a single and fairly symmetric Ag4d peak. At higher depositions, this level starts to broaden, fully evolving into the silver d band at the highest coverage. The full width
D.A. Evans, K. Horn/J_ Electron
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(4 low temperature
Ag/GaAs(l 10)
o__________________________ ___--- ________--_ LgT k-----O
$
g
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0 =
_
Ag deposition (A)
(t ig/GaAs( 1 IO)
I
I
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low temperature deposition
I
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10 Ag deposition (A}
I
I
15
Fig. 5. (a) Shift of the Fermi level (o), bulk components of the Ga3d level (0) and the As&l level (A) as a function of equivalent thicknesses of Ag as indicated. The shift of the core levels is found to coincide with that of the Fermi level once it is developed well enough for evaluation. (b) Plot of Ag d band width (+), and the width of the Fermi level (o), as a function of equivalent Ag thickness at lOOK, extracted from the raw data of Fig. 4.
at half maximum (FWHM) of the whole group of peaks which form the Ag d band is taken as a measure of band width. The emergence of the separate peaks which constitute this band serve to further confirm the development of metallic character. These peak width data are plotted (+) in Fig. 5(b). The top of the GaAs valence band, shown on the right-hand side of Fig. 4, exhibits the bulk and surface band emissions of GaAs for the clean surface, and superimposed on these are the evolving emission from the top of the silver s-p band (fourth and fifth spectra). It is at this stage that an analysis of the Fermi level width may start. The Fermi level emission was
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modeled by the Fermi function, taking account of the substrate temperature. Also included is a Gaussian broadening term, which accounts for both experimental resolution, and any extraneous broadening caused by the emergence of metallic character. An additional third-order polynomial accounts for the shape of the spectrum beyond the Fermi edge. The Gaussian broadening is plotted (a) as a function of deposition in Fig. 5(b); the error bars indicate the confidence intervals resulting from the fit routine. Although the connection between both Ag d band width and Fermi level width and metalhcity can only be qualitatively discussed, the trends in both curves indicate that the data follow a very similar trend with coverage. A saturation in Ag d band width and Fermi level width is attained at about 10 A in Fig. 5(b), i.e. about 4 monolayers of close-packed Ag, where 1 monolayer = 2.35A. One puzzling feature in Fig. 4 is the emergence (at a coverage of about 4 A) of the Fermi level about 0.9 eV below the system reference level, which coincides with the Fermi level emission from the topmost spectrum. This is related to the occurrence of a surface photovoltage induced by the light used for photoexcitation [33]. The subsequent decrease of this shift has been sucessfully interpretedon the basis of model calculations as due to a leakage resistancein the metal overlayerwhich short circuitsthe surfacephotovoltage [34]. From the relatively high barrier height of Ag/GaAs(llO), @t = 0.86 eV, complete band flattening is expected at 100K in this medium-doped material. Mao et al. [35] have investigated this effect at low coverages using photoemissionand Kelvin probe measurements.They have demonstratedthat the position of the Fermi level close to the conduction band minimum is an artifact of the photoemission technique, and that EF is in fact located somewhere near the final pinning position even at low coverages. Here we extendthis study to a regime from about 1-15 A layer thickness. A comparison of the Ga3d, the As3d peak and the Fermi level in Fig. 5(a) shows how these two features move in parallel, indicating the extent of the surface photovoltage, and its breakdown due to a leakage current at an overlayer thickness of about IO A. These data again demonstrate the artifact nature of the so-called “metallization step” in Fermi level movement, and suggest, when considered together with the Kelvin probe data of Mao et al., that little movement of EF occurs beyond the very first stages of metal deposition. Discussion Among the investigations of metal-semiconductor interfaces, the Ag/ GaAs(ll0) system has been extensively studied by means of surface sensitive methods. Bolmont et al. [3,4] used LEED and Auger electron spectroscopy (AES) for the determination of overlayer morphology and geometric structure. They found that the attenuation of the GaAs substrate signal at
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room temperature was nonexponential with metal deposition, indicating island growth as confirmed by the bulk Ga3d emission peak intensity data presented in Fig. 1. Their analysis of the LEED pattern showed that these islands had a triangular cross section, exposing the (111)faces, with an orientation of (441) along the substrate surface normal. Ludeke et al. [5], in an extensive core and valence level study of room temperature growth, noted the absence of chemically shifted core level line components, thus classifying this system as an unreactive one. They found that the Fermi level movement upon Ag deposition took place over a wide coverage range, saturating beyond 20 monolayers (ML) deposition. In fact, because the barrier heights for Ag on n- and p-type material did not add up to the band gap even at these depositions, one may conclude that saturation had not yet occurred. The Fermi level movement persisting to high coverages was regarded by the authors as evidence against the validity of “chemical” or defect models for the explanation of band bending and barrier heights. Cao et al. [12,14], in a photoemission study that involved metal deposition at various substrate temperatures, discussed the average size of metal clusters, and the criteria for the development of metallic character in the overlayer. They demonstrated island growth by the fact that, at room temperature, the Ag4d band had almost the bulk metal value for the spinorbit splitting and FWHM even for the lowest depositions, whereas at 80 K these parameters developed towards the bulk values only gradually with deposition_ They questioned the value of d band width and spin-orbit splitting as a measure of metallicity with reference to cluster photoemission data by Wertheim et al. [36]. Spicer and Cao noted the importance of determining the shape and position of the overlayer Fermi level, and challenged [16] the view of Stiles and Kahn [ll]who had correlated a saturation of band bending with the appearance of metallicity as inferred from the d band width. The connection between band bending and metallicity, and the so-called metallization step in band bending on n-type samples in particular, has meanwhile been found to be an artifact of the photoemission process [33,35,37] as discussed in conjunction with the observation of a surface photovoltage in the data of Figs. 2 and 4 above. However, the proper criterion for the occurrence of metallicity is still a matter of interest, in view of reports that metallicity may not be reached even in micron-size clusters [38]. In this context observations of overlayer morphology on the development of the Ag band width and Fermi level width are important. In the 100 K data of Figs. 4 and 5, both parameters saturate at around 10 A. In contrast, in the room temperature study of Ludeke et al. [5] emission from the Fermi level occurs at about l/4 monolayer (0.6& whereas the bulk Ag d band line width and shape is only achieved at about 10 monolayers deposition. The emergence of EF at much lower coverages than the bulk d band would imply that the crystalline structure
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of islands is different from that of bulk Ag. Systematic studies concerning this matter have not been reported, however, and the LEED and STM observations do not show evidence for structural changes with coverage. At low substrate temperature, however, larger areas of coherent metal deposits are presumably formed, such that Ag d band width and Fermi level width saturate at the same deposition. The observed disparity of Fermi level and Ag d band evolution at room temperature, and their dependence on deposition temperature, in any case lends support to the criticism of Spicer and Cao [16] against taking the Ag band width as a criterion for overlayer metallicity. Turning to the core level line shape analysis of Ag-covered GaAs(ll0) as shown in Figs. 2 and 3, we note that “chemically shifted” core level lines provide one of the few experimental means to study reactions at interfaces on a submonolayer scale, and this technique has been used extensively for the analysis of reactions at metal-semiconductor interfaces. In contrast to studies of adsorbed layers on metal surfaces by core level photoemission pioneered by Fuggle and coworkers (see, for example, ref. 39), in which final state effects are ubiquitous [40], the interpretation of line shifts induced by adsorbates and overlayers has been mostly based on a straightforward initial state picture. This seems to find justification from a comparison of line shifts with charge transfer data based on the concept of electronegativity [41]. The present system, being an “unreactive” system, nevertheless does not allow an unambiguous interpretation of the peak shifts in terms of either initial state or final state effects, because disruptive interaction with new chemical compound formation is not required for the occurrence of chemical shifts. It is well known in adsorption studies on metal substrates that reversible chemisorption may induce core level binding energy shifts on the substrate lines. The problems in interpreting small shifts on the substrate core lines upon metal deposition also apply to other unreactive systems such as In/GaAs(llO). Aristov et al. [42] have in fact attributed a small shifted component on the In4d and Sb4d level in Ag/InSb( 110) to screening by the metal overlayer. The fact that the extra peaks are seen to persist even beyond the monolayer coverage (Figs. 2 and 3) can also be explained in terms of both initial and final state effects. They decrease in intensity along with the rest of the core level emission. This should be the case whether they are due to screening, which will persist in an increasing metal layer, or due to chemical interaction, which again is strongest at the interface. The influence of final state screening on the kinetic energy of an electron core-ionized from a semiconductor in a metal-semiconductor interface has recently been considered by Karlsson et al. [43], who found from wavevector-dependent screening calculations that shifts on the order of 0.2eV should occur because of this process. The calculated peak shift exhibits a strong dependence on the distance of the core ionized atom from the
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interface, which is only seen to level off at about 152OA. In the context of the present experiments this would imply a broadening rather than a separate component due to the presence of the metal overlayer. This is not borne out by the data. The Gaussian broadening in the contributions to the Ga3d line is fairly large (0.4 eV) but the line shape can be modeled well with only one additional component (as shown at the top of Fig. 2), with a shift of about 0.35 eV towards lower binding energies; a distribution of several additional peaks is not required. Acknowledgement This work was supported by the Bundesministerium Technologie under grant 05 490 FXB.
fiir Forschung und
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Materials, The Physical and Chemical Basis of Heterogenous Catalysis, Vol. 5, Elsevier, Amsterdam, 1968. 23 D.A. Evans, M. Alonso, R. Cimino and K. Horn, in preparation. 24 L.C. Feldman and J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, NorthHolland, Amsterdam, 1986. 26 G. Argile and G.E. Rhead, Surf. Sci. Rep., 10 (1989) 277. 26 M.P. Seah and W. Dench, Surf. Interface Anal. 1 (1979) 2. 27 V.Hinkel, L. Sorba and K.‘Horn, Surf. Sci., 194 (1988) 597. 28 D.E. Eastman, T.C. Chiang, J. Knapp and F.J. Himpsel, Phys. Rev. Lett., 45 (1980) 656. 29 P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGrawHill, New York, 1968. 30 D.A. Evans, G.J. Lapeyre and K. Horn, in preparation. 31 Th. Chass~, M. Alonso, R. Cimino, W. Theis, W. Braun and K. Horn, Appl. Surf. Sci., in press. 32 R. Baetzold, Surf. Sci., 106 (1981) 243. 33 M. Alonso, R. Cimino and K. Horn, Phys. Rev. Lett., 64 (1990) 1947. 34 D.A. Evans, T.P. Chen, Th. Chassb and K. Horn, Surf. Sci., 233 (1992) 56-58. 35 D. Mao, A. Kahn, M. Marsi and G. Margaritondo, Phys. Rev. B, 43 (1990) 3228. 36 G.K. Wertheim, S.B. DiCenzo and D.N.E. Buchanan, Phye. Rev. B, 33 (1985) 5364. 37 M. Hecht, Phys. Rev. B, 41 (1999) 7918. 38 P. Marquardt and G. Nimtz, Adv. Solid State Phys. 29 (1989) 317, Wieweg, Braunschweig. 39 J-C. Fuggle and D. Menzel, Surf. Sci., 79 (1979) 1. 40 J-C!. Fuggle, E. Umbach, D. Menzel, K. Wandelt and CR. Brundle, Solid State Commun. 27 (1978) 65. 41 F.J. Himpsel, B.S. Meyerson, F.R. McFeely, J.F. Morar, A. Taleb-Ibrahimi and J.A. Yarmoff, in M. Campagna and R. Rosei (Eds.), International School of Physics “Enrico Fermi”, North-Holland, Amsterdam, 1990. 42 V. Yu. Aristov, M. Bertolo, P. Althainz and K. Jacobi, Surf. Sci., 281 (1993) 74. 43 K. Karlsson, 0. Nyqvist and J. Kanski, Phys. Rev. Lett., 67 (1991) 236.
Ag/GaAs(llO)
revisited:
a photoemission
study’
D.A. Evans and K. Horn* Fritz-Haber-htitut (Germany)
der Max-Planck-Gesellsckaft.
Faradayweg
4-6.0-1000
Berlin 33
(First received 9 October 1992; in final form 6 November 1992)
Abstract The Ag/GaAs(llO) system, which serves as a model system for unreactive metal-semiconductor interfaces, is re-examined by means of high resolution valence and core level photoemission. The origin of an adsorbate-induced new component on both Gaail and As3d core level lines is discussed in terms of charge transfer between overlayer and substrate atoms, or an extra-atomic screening effect. The broadening of the Ag d-band is compared with the evolution and decreasing half width of the Fermi edge with Ag deposition at low substrate temperature. These observations are discussed in relation to the search for a criterion for the onset of metallicity in metal overlayers on semiconducting substrates.
Introduction Metal overlayers on the cleavage face of zincblende semiconductors, in particular GaAs, have long served as model systems for fundamental studies of the geometric and electronic properties of metal-semiconductor contacts. The clean (110) surfaces do not undergo complex reconstructions, and the bond angle rotation relaxation has been experimentally determined. This relaxation has been shown theoretically to result in the removal of surface states from the fundamental band gap region [l], leading to the absence of Fermi level pinning on the clean surface. However, interface reactions between deposited metals and the substrate have been found to be significant for most junctions, with only a few metals yielding ideal unreactive interfaces. The Ag/GaAs(llO) system is one example of the latter, and has been studied by means of various techniques, most notably low energy electron diffraction (LEED), photoelectron spectroscopy, and scanning tunneling microscopy (STM) [2-211. In the course of a recent investigation of quantum size effects in silver islands on GaAs(ll0) [23] we have reinvestigated silver growth on cleaved GaAs(ll0) surfaces at differ‘Dedicated to the memory of John C. Fuggle. * Corresponding author.
D.A. Evans, K. Horn/J. Electron
60
Spectrosc.
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ent substrate temperatures. In the present paper attention is concentrated on the analysis of the Ga3d and As3d core level line shapes upon silver deposition, and the evolution of metallic behavior as manifested in the width of the Fermi level Er and the width of the Ag valence band. In view of the importance of such studies for band bending investigations, the occurrence of a surface photovoltage is also considered. The results of this study are discussed with particular emphasis on the fact that this system fulfills all requirements for an ideal, i.e. non-disruptive, metal-semiconductor interface. Experimental The experiments were performed in several ultra-high vacuum photoemission chambers, equipped with Knudsen cells for metal overlayer deposition, LEED optics, cleaving tools for substrate surface preparation, and a crystal holder which allowed sample cooling to 100K. Photoelectron spectra were recorded using light from the BESSY (Berliner-ElektronenSpeicherrings-Gesellschaft fiir Synchrotronstrahlung) storage ring in Berlin, using toroidal grating monochromators (TGMs) 4 and 6 with a photon energy range from about 10 to 120 eV. Photoelectrons were energyanalyzed and detected using a hemispherical electron energy analyzer (HA 50 from VSW, UK), with an overall resolution of about 50meV. The substrates were pre-notched bars of n-type GaAs (Wacker Chemitronic, Germany) with a carrier concentration of about 6 x 1017 cme3, which were cleaved in vacua and showed mirror-like surfaces with only a few largescale steps. Temperatures were measured with a thermocouple attached to the samples. Deposition rates were measured using a q’uartz crystal microbalance (Leybold Inficon). Results Growth
mode and substrate
core level line shape
The growth mode of thin films on solid substrates may be established from the intensity of a substrate core level or Auger emission line as a function of overlayer thickness [24]. In this study, Ga3d and As3d lines were recorded during the stepwise deposition of Ag. The spectra were normalized for photon flux and analyzed in terms of bulk and surface emission contributions. This analysis was carried out for the entire set of spectra for each coverage sequence, using identical parameters as described in the next section (Evolution of Ag valence bands and metallic character of the overlayer). The intensity of the bulk Ga3d and As3d component resulting from this procedure is plotted on a semi-logarithmic scale in Fig. 1.It is seen
D.A
Evans, K. Horn/J.
Electron
Spectrosc.
Reid.
Phenom.
62 (1993) 59-72
61
I--
s
5
(0
‘5 .Y
E
r z ‘ii -^s L
-As3d
-2-
$
-3-
I
0
I
5
I
10 Ag deposition (A)
15
Fig. 1. Ag deposition-induced attenuation of the bulk component of the Ga3d core level line at substrate temperatures of 300 K (0) and 100 K (O), and the As3d core level line at 100K (A).
that, over the whole coverage range of 15A, the attenuation of both Ga3d and As3d levels is linear for low temperature growth (0 and A, respectively). This growth mode has been termed “simultaneous multilayer” growth, and it has been shown that the corresponding dependence of substrate intensity on dose is exponential in shape [25]. On the basis of data for the mean free path ,I of electrons [26], the overlayer thickness calibration given in Fig. 1 has been calculated. This was found to be in good agreement with thickness
data evaluated from a quartz microbalance calibration and the known evaporation times for each dose. The kinetic energies of the Ga3d and As3d for the data in Fig. 1 are 16 and 37 eV, respectively. The value for 1 for the higher kinetic energy, derived from a functional description of many different data by Seah and Dench [26] is about 40% lower than that for the lower kinetic energy; our data for InSb [27] suggest a somewhat smaller difference (,I(16eV) = 6.0 A; A(37eV) = 4.2 A). This causes the slope for the As intensity to be somewhat shallower than that of the Ga3d line; they have been offset in Fig. 1 for clarity. Room temperature deposition leads to a much more gradual nonexponential decay of the substrate signal, as shown by the Ga3d data, also given in Fig. l(0). This is the expected behavior for island growth, which has been imaged using STM in the recent study by Trafas et al. [21]. Deposition at room temperature thus obviously leads to
island formation, and islands are also formed when a layer deposited at low temperature is warmed up, through the increased surface mobility of the adparticles. Subsequent deposition and annealing cycles lead to renewed attenuation, and after each annealing the substrate signal is slightly lower,
62
D.A. Evans, K. HornfJ.
Electron S’ctrosc.
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62 (I993) 5$72
indicative of the fact that the islands occupy an increasing part of the substrate surface [23]. Whereas the core level line intensities may be utilized for an evaluation of growth mode such as is displayed in Fig. 1, in reactive metal-semiconductor interfaces they find use in the investigation of reaction path and new compound formation, by an analysis of extra components which occur upon overlayer deposition. For large core level shifts such analysis is fairly straightforward, but small shifts and low intensities induce uncertainties which are not readily overcome. Moreover, such shifts have so far been mainly interpreted in terms of initial state (“chemical”) shifts, neglecting the influence of the photoionization process on the appearance of the spectra. Here we attempt to include this aspect in the interpretation of shifts on the Ga3d and As3d core levels upon Ag adsorption. Our basis is an important finding reported by Trafas et al. [21] in their STM study of this system. These authors note that Ag clusters, formed by room temperature deposition of Ag, sometimes move under the influence of the STM tip, and that the surface areas previously covered by Ag islands show no difference in the STM image when such islands have moved. This is a most direct way of demonstrating that no disruptive interaction between the islands has taken place, lending strong support to the classification of this system as an unreactive one. Any new or shifted core level line components then have to be related to a possible reversible chemisorption bond, a change of the substrate surface geometry such as an undoing of the substrate bond-angle rotation-relaxation, or to physical processes connected with the proximity of metal overlayer and semiconductor substrate surface. Consider the set of Ga3d (Fig. 2) and As3d (Fig. 3) core level emission spectra recorded for increasing coverages of Ag at low temperature. The clean surface spectra exhibit the well-known surface core level shift components [28] towards higher (Ga3d) and lower (As3d) binding energy. Even at the lowest Ag coverages, the entire peak becomes somewhat blurred, such that the extra components cannot be readily distinguished. However, extra intensity is observed on the high kinetic (low binding) energy side of the peak. This intensity increases up until about monolayer coverage (1 monolayer = 2.35 Hi, assuming close-packed fee Ag layers). Further deposition does not change the appearance of the spectra apart from an overall reduction in intensity. One representative curve-resolved spectrum, performed for a deposition of about 0.546 monolayers for both Ga3d and As3d, is shown at the top of Figs. 2 and 3. For GaSd, the line shape analysis, which requires two doublet (bulk and surface) contributions for the clean surface, now calls for three components, if all other parameters such as spin-orbit splitting, Lorentzian (lifetime) broadening and Gaussian (experimental) broadening are held constant. The new component occurs at a kinetic energy higher than that of the bulk line. The least-squares optimization
D.A. Evans, K. Horn/J.
Electron
Ag/GaAs(110) T= 100K
Spctrosc.
Relat. Phenom.
62 (1993) Z-h?2
Ga 3d
A
l.sA
\.
Ag dTAyition 15 10.5 a
4
2:5 1.5 0.8
36
37
39 38 kinetic energy (eV)
40
Fig. 2. Set of Ga3d core level spectra (recorded at lOOK) for various Ag depositions as indicated. The top spectrum is shown along with a line shape analysis based on three contributions (see text).
routine uses the ,Marquardt algorithm, and provides, along with the parameters for the fit, a confidence interval for each parameter, corresponding to an increase of unity in the reduced chi-square if the parameter is replaced by the upper or lower boundaries marked by the confidence interval [29]. The routine thus provides a measure for the reliability of the model function. However, the need for an extra component at lower binding energy may be directly read off the spectrum through the larger wing on the low binding energy side. The surface core level component is attenuated as the Ag overlayer builds up. A line shape analysis is relatively simple for the Ga3d line, but because the decreasing surface component and the increasing new Ag-induced component occur on opposite sides of the bulk peak,
D.A. Evans, K. Horn/J. Electron Spectrosc.
64
Ag/GaAs(l 10) T P 1OOK
ho-61
Relat. P/worn.62(1993) 59-72
As 3d eV
A
15 10.5 8 6.3 4 2.5 1.5 0.8 0.4
0
I
14
I
15
I
I
I
I
16
I?
18
19
kinetic energy (eV)
Fig. 3. Set of A&d spectra (recorded under equivalent conditions as in Fig. 2) with line shape analysis for the spectrum at 1.5A deposition.
the situation is less clear for the As3d peak. Here, the spectra up to a monolayer coverage can be readily modeled by one single surface core level contribution, but a satisfactory fit necessitates that it shifts toward the bulk emission line, and with an additional broadening. However, this shift would have to occur even for the lowest depositions. The adatoms are immobile under these conditions, so the isoIated adatoms would have to affect a large area in order to give rise to this kind of spectrum. This is in contrast with the observation of Ludeke et al. f5] that the surface core exciton persists up to high coverages, indicative of a rather localized interaction of the silver adatoms or islands with the substrate surface. Several models may be put forward in order to explain the observation of Ag-induced shifted components on the substrate core level lines. In the usual initial state model, the generation of new chemical species may be
D.A. Evans, K. Horn/J. Electron Spectrosc. Relat. Phenom. 62 (1993) 59-72
65
ruled out following the absence of surface disruption as reported in the STM experiments. Charge transfer may still occur in a chemisorption-type bond, which may be reversible. Evans et al. have recently obtained strong evidence for charge transfer in core and valence photoelectron spectra of Na/GaP( 110)and Na/GaAs(llO) [30], in which shifts in the Ga3d line were related to the occurrence of a new peak in the fundamental band gap. Similar processes may well be the cause of the new shifted component observed here, in the sense that some transfer of charge from the Ag s level occurs into the unoccupied Ga-derived surface state, which is consequently ’ pulled down in energy. No sign of the filling of such state, similar to that of Na/GaAs(llO), is observed in the valence band region, however. In an entirely different model, one may assume that the Ag layer induces a line component shifted towards lower binding (higher kinetic) energy on both Ga3d and As3d levels, and that the new lines are caused by the influence of the metal overlayer on the screening of the photon-induced core hole in the substrate core levels. This additional screening by the metal atoms will lead to the emergence of a core level line at lower binding energy on both anion and cation core levels. Thus, the deposition of Ag will, for the case of the Ga3d level, lead to a reduction of the surface core level shift (SCLS) contribution, and an increase of a new line at lower binding energy. This trend is supported by the set of Ga3d spectra shown in Fig. 2. Here, the SCLS component clearly decreases with Ag deposition, while the wing on the high kinetic energy side of the peak increases. A line shape analysis of each of the spectra of Fig. 2 and a plot of the relative intensities of SCLS and new components (not shown here) demonstrates that the decrease of the former scales with the increase of the latter. For the As3d level, a line shape analysis is rendered difficult by the fact that both SCLS and the new component postulated here occur on the high kinetic energy side of the peak. This prohibits a quantitative analysis similar to the one described for the Ga3d level, because the confidence intervals for the line shape analysis parameters become excessively large. However, even under these conditions the overall trend in the intensity of the As3d SCLS and new components matches that found for the Ga3d level. Small shifted core level line components in systems which are considered unreactive, such as indium overlayers on the (110)surfaces of the III-V semiconductors, have been observed before. However, the vital information concerning the extent of interface reactions has been missing, and thus a vague interpretation in terms of emission from “interface species” has been put forward [31]. Evolution
of Ag valence bands and metallic
Studies of silver overlayers
on GaAs(ll0)
character
of the overlayer
have been the subject of dis-
D.A. Evans, K. Horn/J.
66 (a)
Electron
Spectrosc.
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62 (1993) 5%72
(b)
AglGaAs( 110)
1OOK evolution of Ag valence band
Aa/GaAslllO1 vience
1 OOK
band-edge
hot61eV
Ag deposition
(4 14.5 7.6 6.2 4.2
2.4
1 -_ I
45
I
1
55 50 kinetic energy (eV)
2.4
x2
1.4
i
1.4 0.25 xs
0.05 0
I
60
55
56
58 57 kinetic energy (eV)
Fig. 4. (a) Spectra of the Ag4d band (left), for Ag depositions on GaAs(ll0). Equivalent thicknesses are given for each spectrum. (b) Spectra of the region near the Fermi edge for different equivalent thicknesses as indicated. The emergence of a metallic Fermi edge about 0.9 eV below the reference E,, which coincides with the overlayer Fermi level at saturation, is due to a surface photovoltage induced by the photoemission light source.
cussion in the long-standing debate about the causes of band bending induced by metal over-layers [ll]. Here the transition from single adatoms to a continuous metal overlayer, and its evolution towards metallic character, has been the subject of controversy [16]. The width of the group of Ag d bands has been used as a criterion for metallicity [ll], with the implication that a width equal to the bands in bulk Ag metal signals metallicity. In a simple model of metal atom interaction, the width of the d band will scale with orbital overlap and the evolution of metallic behavior [32]. However, another direct measure of metallicity can be obtained from the width of the developing metallic Fermi level of the overlayer. The high flux on the TGM 6 wiggler-undulator beamline enables us to follow its evolution with excellent statistics and high resolution, and provides a suitable testing ground for a comparison which so far has not been reported in the literature. Here we compare the d band width with the width of the Fermi level as the overlayer thickness increases. Photoemission data for the d band region and the Fermi edge are shown in Fig. 4 for various Ag depositions at 100K. Up to the third spectrum, GaAs valence band features may still be distinguished along with a single and fairly symmetric Ag4d peak. At higher depositions, this level starts to broaden, fully evolving into the silver d band at the highest coverage. The full width
D.A. Evans, K. Horn/J_ Electron
Spectrosc.
Relut. Phenom.
62 (1993) 59-72
67
(4 low temperature
Ag/GaAs(l 10)
o__________________________ ___--- ________--_ LgT k-----O
$
g
F .$ c 5
a
8 ‘CI - 0.4 z
“p
8 5 .p -0.4-
1
lE, -OS-
&fifJ
F
q Ga 3db”lk * *’ whlk
-
0.8
0 =
_
Ag deposition (A)
(t ig/GaAs( 1 IO)
I
I
5
low temperature deposition
I
I
10 Ag deposition (A}
I
I
15
Fig. 5. (a) Shift of the Fermi level (o), bulk components of the Ga3d level (0) and the As&l level (A) as a function of equivalent thicknesses of Ag as indicated. The shift of the core levels is found to coincide with that of the Fermi level once it is developed well enough for evaluation. (b) Plot of Ag d band width (+), and the width of the Fermi level (o), as a function of equivalent Ag thickness at lOOK, extracted from the raw data of Fig. 4.
at half maximum (FWHM) of the whole group of peaks which form the Ag d band is taken as a measure of band width. The emergence of the separate peaks which constitute this band serve to further confirm the development of metallic character. These peak width data are plotted (+) in Fig. 5(b). The top of the GaAs valence band, shown on the right-hand side of Fig. 4, exhibits the bulk and surface band emissions of GaAs for the clean surface, and superimposed on these are the evolving emission from the top of the silver s-p band (fourth and fifth spectra). It is at this stage that an analysis of the Fermi level width may start. The Fermi level emission was
68
D.A. Evans, K. Horn/J. Electron Spectrosc. Relat. Phenom. 62 (1993) 59-72
modeled by the Fermi function, taking account of the substrate temperature. Also included is a Gaussian broadening term, which accounts for both experimental resolution, and any extraneous broadening caused by the emergence of metallic character. An additional third-order polynomial accounts for the shape of the spectrum beyond the Fermi edge. The Gaussian broadening is plotted (a) as a function of deposition in Fig. 5(b); the error bars indicate the confidence intervals resulting from the fit routine. Although the connection between both Ag d band width and Fermi level width and metalhcity can only be qualitatively discussed, the trends in both curves indicate that the data follow a very similar trend with coverage. A saturation in Ag d band width and Fermi level width is attained at about 10 A in Fig. 5(b), i.e. about 4 monolayers of close-packed Ag, where 1 monolayer = 2.35A. One puzzling feature in Fig. 4 is the emergence (at a coverage of about 4 A) of the Fermi level about 0.9 eV below the system reference level, which coincides with the Fermi level emission from the topmost spectrum. This is related to the occurrence of a surface photovoltage induced by the light used for photoexcitation [33]. The subsequent decrease of this shift has been sucessfully interpretedon the basis of model calculations as due to a leakage resistancein the metal overlayerwhich short circuitsthe surfacephotovoltage [34]. From the relatively high barrier height of Ag/GaAs(llO), @t = 0.86 eV, complete band flattening is expected at 100K in this medium-doped material. Mao et al. [35] have investigated this effect at low coverages using photoemissionand Kelvin probe measurements.They have demonstratedthat the position of the Fermi level close to the conduction band minimum is an artifact of the photoemission technique, and that EF is in fact located somewhere near the final pinning position even at low coverages. Here we extendthis study to a regime from about 1-15 A layer thickness. A comparison of the Ga3d, the As3d peak and the Fermi level in Fig. 5(a) shows how these two features move in parallel, indicating the extent of the surface photovoltage, and its breakdown due to a leakage current at an overlayer thickness of about IO A. These data again demonstrate the artifact nature of the so-called “metallization step” in Fermi level movement, and suggest, when considered together with the Kelvin probe data of Mao et al., that little movement of EF occurs beyond the very first stages of metal deposition. Discussion Among the investigations of metal-semiconductor interfaces, the Ag/ GaAs(ll0) system has been extensively studied by means of surface sensitive methods. Bolmont et al. [3,4] used LEED and Auger electron spectroscopy (AES) for the determination of overlayer morphology and geometric structure. They found that the attenuation of the GaAs substrate signal at
D.A. Evans, K. Horn/J. Electron Spectrosc. Relat. Phenom. 62 (1993) 59-72
69
room temperature was nonexponential with metal deposition, indicating island growth as confirmed by the bulk Ga3d emission peak intensity data presented in Fig. 1. Their analysis of the LEED pattern showed that these islands had a triangular cross section, exposing the (111)faces, with an orientation of (441) along the substrate surface normal. Ludeke et al. [5], in an extensive core and valence level study of room temperature growth, noted the absence of chemically shifted core level line components, thus classifying this system as an unreactive one. They found that the Fermi level movement upon Ag deposition took place over a wide coverage range, saturating beyond 20 monolayers (ML) deposition. In fact, because the barrier heights for Ag on n- and p-type material did not add up to the band gap even at these depositions, one may conclude that saturation had not yet occurred. The Fermi level movement persisting to high coverages was regarded by the authors as evidence against the validity of “chemical” or defect models for the explanation of band bending and barrier heights. Cao et al. [12,14], in a photoemission study that involved metal deposition at various substrate temperatures, discussed the average size of metal clusters, and the criteria for the development of metallic character in the overlayer. They demonstrated island growth by the fact that, at room temperature, the Ag4d band had almost the bulk metal value for the spinorbit splitting and FWHM even for the lowest depositions, whereas at 80 K these parameters developed towards the bulk values only gradually with deposition_ They questioned the value of d band width and spin-orbit splitting as a measure of metallicity with reference to cluster photoemission data by Wertheim et al. [36]. Spicer and Cao noted the importance of determining the shape and position of the overlayer Fermi level, and challenged [16] the view of Stiles and Kahn [ll]who had correlated a saturation of band bending with the appearance of metallicity as inferred from the d band width. The connection between band bending and metallicity, and the so-called metallization step in band bending on n-type samples in particular, has meanwhile been found to be an artifact of the photoemission process [33,35,37] as discussed in conjunction with the observation of a surface photovoltage in the data of Figs. 2 and 4 above. However, the proper criterion for the occurrence of metallicity is still a matter of interest, in view of reports that metallicity may not be reached even in micron-size clusters [38]. In this context observations of overlayer morphology on the development of the Ag band width and Fermi level width are important. In the 100 K data of Figs. 4 and 5, both parameters saturate at around 10 A. In contrast, in the room temperature study of Ludeke et al. [5] emission from the Fermi level occurs at about l/4 monolayer (0.6& whereas the bulk Ag d band line width and shape is only achieved at about 10 monolayers deposition. The emergence of EF at much lower coverages than the bulk d band would imply that the crystalline structure
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62 (1993) 5%72
of islands is different from that of bulk Ag. Systematic studies concerning this matter have not been reported, however, and the LEED and STM observations do not show evidence for structural changes with coverage. At low substrate temperature, however, larger areas of coherent metal deposits are presumably formed, such that Ag d band width and Fermi level width saturate at the same deposition. The observed disparity of Fermi level and Ag d band evolution at room temperature, and their dependence on deposition temperature, in any case lends support to the criticism of Spicer and Cao [16] against taking the Ag band width as a criterion for overlayer metallicity. Turning to the core level line shape analysis of Ag-covered GaAs(ll0) as shown in Figs. 2 and 3, we note that “chemically shifted” core level lines provide one of the few experimental means to study reactions at interfaces on a submonolayer scale, and this technique has been used extensively for the analysis of reactions at metal-semiconductor interfaces. In contrast to studies of adsorbed layers on metal surfaces by core level photoemission pioneered by Fuggle and coworkers (see, for example, ref. 39), in which final state effects are ubiquitous [40], the interpretation of line shifts induced by adsorbates and overlayers has been mostly based on a straightforward initial state picture. This seems to find justification from a comparison of line shifts with charge transfer data based on the concept of electronegativity [41]. The present system, being an “unreactive” system, nevertheless does not allow an unambiguous interpretation of the peak shifts in terms of either initial state or final state effects, because disruptive interaction with new chemical compound formation is not required for the occurrence of chemical shifts. It is well known in adsorption studies on metal substrates that reversible chemisorption may induce core level binding energy shifts on the substrate lines. The problems in interpreting small shifts on the substrate core lines upon metal deposition also apply to other unreactive systems such as In/GaAs(llO). Aristov et al. [42] have in fact attributed a small shifted component on the In4d and Sb4d level in Ag/InSb( 110) to screening by the metal overlayer. The fact that the extra peaks are seen to persist even beyond the monolayer coverage (Figs. 2 and 3) can also be explained in terms of both initial and final state effects. They decrease in intensity along with the rest of the core level emission. This should be the case whether they are due to screening, which will persist in an increasing metal layer, or due to chemical interaction, which again is strongest at the interface. The influence of final state screening on the kinetic energy of an electron core-ionized from a semiconductor in a metal-semiconductor interface has recently been considered by Karlsson et al. [43], who found from wavevector-dependent screening calculations that shifts on the order of 0.2eV should occur because of this process. The calculated peak shift exhibits a strong dependence on the distance of the core ionized atom from the
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interface, which is only seen to level off at about 152OA. In the context of the present experiments this would imply a broadening rather than a separate component due to the presence of the metal overlayer. This is not borne out by the data. The Gaussian broadening in the contributions to the Ga3d line is fairly large (0.4 eV) but the line shape can be modeled well with only one additional component (as shown at the top of Fig. 2), with a shift of about 0.35 eV towards lower binding energies; a distribution of several additional peaks is not required. Acknowledgement This work was supported by the Bundesministerium Technologie under grant 05 490 FXB.
fiir Forschung und
References 1
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