Short range potential variations at a metal-semiconductor interface

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0038-1098/86 $3.00 + .00 Pergamon Journals Ltd.

Solid State Counnunlcatlons, Vol.60,No.10, pp.793-796, 1986. P r i n t e d i n Great B r i t a i n .

SHORT RANGE POTENTIAL VARIATIONS AT A METAL-SEMICONDUCrOR INTERFACE J. Kanski, S.P. Svensson, T.G. Andersson Department of Physics, Chalmers University of Technology, S-41296 Gtitehorg, Sweden G. Le Lay CRMCC-CNRS, Campus Luminy, Case 913, 13288 Marseille Cedex 9, France

(Received 15 September 1986 by L. Hedin)

The Au-GaAs(001) interface has been studied by UV- and X-ray photoelectron spectroscopies. By exploitingthe different probing depths in the two cases it is shown that within a layer of-10 A from the interface the electronic properties of the semiconductor are different than further away. It is also shown that this difference arises simultaneously with the appearance of a metallic surface and is most likely due to metal induced screening.

The nature of the mechanism responsible for the final stabilization of the Fermi level at a metal-semiconductor (MS) interface remains a subject of debate. In a model discussed by Tejedor et al.z and more recently by Tersoffe, the Schottky barrier formation was associated with filling of the band gap region by metal induced gap states (MIGS). The semiconductor is thus predicted to acquire (semi-) metallic properties in the vicinity of the interface. The spatial extent of this modified region is essentially determined by the decay length of metal states across the interface and is estimated to -5-10 ,/~. Via a completely different approach, emphasizing the effects of nonlocal screening, Inkson also arrived at the conclusion that the semiconductor band gap is closed near the interface3. The short range dependence of the electronic properties within the semiconductor predicted by these theories contrasts the much longer range of the coexisting dopant controlled band bending, which normally extends -1000 ~, into the semiconductor. In the present letter it is shown, for the first time, that short range variations in the electronic properties at MS interfaces can indeed be observed experimentally. The system considered here is MBE-grown n-doped (-2x1016 cm -3 Si) GaAs(001)- c(2x8) with Au overlayers deposited at room temperature. The GaAs surface was prepared such that the As(M45VV)/Ga(M23M45M45) Auger intensity ratio was -2.0. Fiirther experimental details are described elsewhere4. In view of the documented tendency of Au to form intermetallic compounds with Ga 5, one might suspect that chemical interaction would dominate the interfacial electronic properties. Indeed, as found in recent studies of Au overlayers on MBE- grown GaAs(001), the interaction is strong enough to release the outermost As layer of As- terminated surfaces4.6 [the c(4x4) and (2x8) reconstructions]. Yet, the most detailed studies of the Au/GaAs(001) system show that the interaction does not break up the first Ga layer, and the resulting interface is 67 . Alloying effects have found to be atomically abrupt, been detected by Auger electron microanalysiss only after annealing at temperatures well above 250 °C. One significant consequence of the relatively strong Au-Ga interaction in the present context is an initially two dimensional overlayer growth 6.9. This is in contrast to Ag on GaAs, in which case metal clusters are formed in the earliest growth stages 10. Let us now turn to the present results. In figure 1 we show the spectral onset energy in HelI-excited valence band photoemission for a range of Au coverages. At zero cover-

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SHORT RANGE POTENTIAL vARIATIONS AT A METAL-SEMICONDUCTOR INTERFACE

the Ga(3d) level can still be easily extracted. In figure 1 we see that the Ga(3d) binding energy is reduced in a very similar manner as the spectral onset (but with different magnitude). In particular we note that the level remains unshifted and unbroadened up to ~IA. Also the Au(4f) levels undergo similar changes in this coverage range4. Although the Ga(3d) shifts may appear as "normal" band bending, it will next be demonstrated that this is not the case. In figure 2 we show HeR- and MgKa-excited Ga(3d) emission from clean and Au covered GaAs. All spectra were recorded with 0.3 eV analyser resolution. This is just enough to confirm the presence of the spin-orbit component.in ~ Hell-spec.wa, where,., the MgK~-spec..wa. contain a d m U o n m oroaaenmg one to me s o u r c e n n e youth (--0.7 eV). The relatively high noise level above ~20 eV in the HeII-stx~trum after Au deposition is caused by subtraction of the partially overlapping HeI-excited valence band emission. The important observation in figure 2 is that the Au induced shift appears considerably smaller in the MgKcxspectrum. It is straightforward to assign this effect to a significant difference in the probing depths: the correspon: ding mean free paths in GaAs are t l -8 7~ (HER) and -25 A (MgK a ) , i.e. the surface layer contribution is relatively larger (6ith Hell excitation. Since the spectra contain contributions from surface and bulk regions, a natural way to proceed would be to extract these components by detailed specwal shape analysis. However, apart from the fact that our noise levels are not low enough for a reliable decomposition, there is a more legitimate reason for hesitation: it cannot be assumed a priori that the spectra contain only two components (surface and bulk). For example, if the core level shifts were caused by the image charge effect, the spectra would contain a series of gradually shifted compt> nents 12 . In the study of Ag/GaAs mentioned above 10 , attempts to decompose broadened lines into surface- and bulk components did in fact fail. In view of this, we choose another approach: making use of the layered smacture of GaAs (the <100> Ga layer spacing is 2.83 A), we try to reconstruct the measured spectra. The contribution of each Ga layer is assumed to be a rigidly shifted clean surface spectrum, its relative amplitude given by an exponential attenuation involving the above mentioned electron mean free path and the distance from the interface. The use of a clean surface spectrum as the building block is not strictly correct, since this spectrum in turn contains surface and bulk components. However, for the c(2x8) reconstruction the surface contribution to the Ga(3d) line shape is very small even under the most surface sensitive conditions13 and can be neglected in this analysis. The problem is then to find the potential profile describing the shifts of the different

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Ga layers. This of course depends on which mechanism is responsible for the observed shifts. Nonlocal screening would affect layers close to the interface stronger than the more remote ones (the image potential ~l/z ). On the other hand, if there existed a reacted interface layer, the chemical shifts could well be constant throughout this region. After some tests, one of which is shown in figure 3, we found that equally good reconstructions can be achieved with different potential profiles. This is partly due to the exponential attenuation factor associated with the electron mean free path. Nevertheless, some very significant facts are established by these tests: a) independently of the potential profile, no satisfactory agreement between the measured and reconstructed spectra is found if the assumed modified region is wide. In figure 3 we see that 20 Ga layers is already considered as a wide region, b) likewise poor agreement is obtained when the affected region is assumed to be very narrow, i.e. 3 Ga layers or less, c) good reconstructions of the measured spectra are obtained simultaneously at both photon energies for an intermediate number of affected Ga layers. Assuming a linearly ramped potential as in figure 3, the best agreement is obtained with ~8 Ga layers within the modified region. Using instead a step-like potential, similar agreement is found when --4 Ga layers contribute to the shifted signal. Which is then the mechanism behind the Ga(3d) shifts? Conventional band bending can be ruled out immediately, since the band bending length scale is ~1000 JL under the present doping conditions. It is also clear that the observed shifts are not caused by preferential attenuation of the surface layer contributionl3: for the c(2x8) surface such a shift would amount to ~0.1 eV ~2 and would be expected to develop gradually between zero and one monolayer coverage. One effect which calls for careful consideration is the possibility of intefacial reactions and accompanying chemical shifts. As already mentioned, the general view based on X-ray diffractiony Auger microanalysiss and electron microscopy15 is that reactions at Au/GaAs interfaces occur only at elevated temperatures. There are additional, independent reasons to believe that chemical effects are not important in this context. Our results show that the interaction must involve more than one Ga layer, which means that interpretation in terms of chemical shifts would have to be associated with an onset of intermixing at about 1A Au coverage. However, no signs for such "critical coverage" behaviour have been found in the Auger- and X-ray photoelectron intensity studies 6.9. Another support for this con-. clusion comes from a recent study of At-implanted GaAs. Concentrating on the Ar(3p) and Ga(3d) levels, we foundt6 that after deposition of an Au overlayer the energies of the two levels were reduced by the same amounts within the experimental accuracy (better than 0.1 eV). Considering that the At-implanted surface must be somewhat disrupted, it should be at least as reactive as the MBE-grown surface. Hence we can be confident that Au induced chemical shifts are negligible also on MBE-grown GaAs. From the above it follows that the core level shifts reported here reflect changes in the intrinsic electron properties of GaAs in a near interface region. The close correlation to surface metallization (figure 2) points towards the two theories mentioned in the introduction. Regarding the effect of MIGS, numerical self consistent calculations of the ground state electron structures of M-GaAs, M-ZnSe and M-ZnS interfaces17 (M = jellium) show that the metal systematically tends to lower the potential of the outermost semiconductor atoms. As a consequence, the core levels are shifted to higher binding energies, i.e. in a direction opposite to that reported here. We are then left with the nonlocal screening model discussed by Inkson. To our knowledge there are no numerical calculations of core level binding

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energies taking these effects into account. Estimates of the image potential energy, which represents the classical limit of Inkson's model, do however give shifts of the same magnitude and direction as those found in our experiments is. While this circumstance is very suggestive, it is that ~,-Jditionalexperiments are required to establish the influence of metal induced screening in the context of Schottky barriers. Among the already reported results which can be understood within this framework, we would like to mention the "anomalous" behaviour of the Fermi level vinninR in p-type GaAs(ll0) with Ag and Au overlayers"14.t9. gince the metal induced screening is independent of doping parameters, the resulting "band bending" should be the same for n- and p-type material, as found experimentally. It should also be noted that in a rather different context, namely in studies of noble gas layers on metal surfaces 12, the nonlocal metal screening has been proven to induce core level shifts similar to those reported here.

Since the discussed effect is connected with the excited state, we must finally ask how relevant it can be to the Schottky barrier problem. The valence and conduction states in semiconductors arc certainly less localized than core states and should therefore be less affected by final state effects. However, according to recent studies of conduction bands in Si and Ge the excitonic lowering of optical transition energies at critical points can be of the same magnitude as the core exciton energy2°. As the charge transport threshold at a MS interface involves states at the bottom (top) of the conduction (valence) band, the screening effect may after all be crucial for the Schottky barrier height. In conclusion, we have shown that the core level shifts presented here cannot be explained in terms of conventional band bending or chemical interaction.The derived spatial range, as well as the close correlation to surface metallization indicate that image potential effects are operative. We

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SHORT RANGE POTENTIAL VARIATIONS AT A METAL-SEMICONDUCTOR INTERFACE

wish to emphasize that these effects are not claimed to be generally the most important ones for the Fermi level pinning at all MS interfaces, but it is clear that they cannot be ignored in a realistic description of a Schottky harder.

Acknowledgements

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we wish to thank the Swedish Natural Science Research Council and the Swedish Board for Technical Development for irmancial support of this project. -

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12. T.-C. Chiang, G. Kaindl and T. Mandel, Phys. Rev. B33, 695 (1986) 13. A.D. Kamani, P.Chiaradia, H.W. Sang Jr., P. Zurcber and R.S. Bauer, Phys. Rev.B31, 2146 (1985) 14. R. Ludeke, Surface Science 168, 290 (1986) 15. C.L. Bauer, Surface Science 168, 395 (1986) 16. J. Kanski, T.G.Andersson and J. Westin, Proceedings of the 18th International Conference on the Physics of Semiconductors, Stockholm, 1986 17 S. Louis, J.R. Chelikowsky and M.L. Cohen, Phys. Rev.B15, 2154 (1977) 18. K. Karlsson, O. Nyqvist and G. Wendin, to be published 19. W.G. Petro, I.A.Babalola, P.Skeath, C.Y. Su, I. Hino and W.E. Spicer, J. Vac. Sci.Technol. 21, 585 (1982) 20. D. Straub, L. Ley and F.J. Himpsel, Phys. Rev.B33 2607 (1986)

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