Vacuum/volume 41/numbers 4-6/pages 1021 to 1024/1990 O042-207X/90S3.00 + .00 Printed in Great Britain Q 1990 Pergamon Press plc A photoemission study of the Bi-lnP(110) interface C Stephens(1,2), D R T Zahn*(3), R Ciminot(4), W Braun(5), K Fives(2) and I T McGovern(2), (1) Physikalisches Institut der Universitat, Robert-Mayer-Str. 2-4, D- 6000 Frankfurt am Main, FRG. (2) Department of Pure and Appfied Physics, Trinity College, Dublin 2, Republic of Ireland. (3) Physics Department, University of Wales College of Cardiff, Cardiff CF1 3TH, UK. (4) Fritz-Haber-lnstitut der MPG, Faradayweg 4-6, D- 1000 Berlin 33, West Germany. (5) BESSY, Lentzeallee 100, D-1000 Berlin 33, West Germany The room temperature adsorption of bismuth on clean cleaved p-type InP(110) surfaces has been studied with soft X-ray photoelectron spectroscopy in the coverage range 0.01-60 monolayers (ML ): Line shape analysis of the overlayer (Bi 5d) and substrate (In 4d) core level emission reveals that the interface is abrupt. Bismuth is preferentially adsorbed at a single substrate site within the first layer. However, strong clustering takes place above this coverage. The evolution of the band bending indicates that different mechanisms may be important in the high and low coverage regimes. At low coverages the imperfections in the first bismuth layer cause high band bending which is reduced as the first layer nears completion. Above 20 ML the semi-metallisation of the overlayer leads to pinning in accordance with the metal induced gap state model. Introduction In most cases the interface between a metal and a semiconduc- tor exhibits a chemical interaction between the overlayer and the substrate. This increases the difficulty in assessing the factors which determine the formation of the Schottky barrier (SB). However, antimony has been shown to form an abrupt interface with an epitaxial first layer on GaAs(ll0) ~.2 and InP(ll0)3-s and these interfaces may lead to a better under- standing of the SB formation. The possibility arises that other group V elements will also form similarly ideal interfaces with III-V semiconductors and enlarge the data set. For this reason bismuth has recently attracted the interest of many re- searchers 6 8. We have previously studied the Bi-InP(ll0) interface using X-ray photoelectron spectroscopy (XPS) and Raman tech- niquesg: XPS indicated that the interface is abrupt and a complex coverage dependence of the band bending was evalu- ated from electric field induced Raman spectroscopy (EFIRS). In this paper we present a further study of the interface, exploiting the advantages of soft X-ray photoelectron spec- troscopy (SXPS) with synchrotron radiation. Experiment SXPS experiments were performed at the TGM 2 beamline of the electron storage ring in Berlin (BESSY). Overlayer (Bi 5d) and substrate (In 4d) core level photoemission spectra were *Permanent address: Institut ffir Festk6rperphysik, Technische Univer- sitfit, Hardenbergstr. 36, D-1000 Berlin 12, West Germany. §Present address: ISM-CNR, Via E. Fermi 38, 00044 Frascati, Italy. taken at normal emission using a Vacuum Generators ADES 400 angle-resolving spectrometer. The analyser had an angular acceptance of approximately 4 ° . The combined resolution (light and electrons) was 0.2 eV at the photon energy used, 45 eV. The resolution was measured from the Fermi edge of a clean gold foil in electrical contact with the sample. Clean InP(ll0) surfaces were prepared by cleaving aligned pre-notched bars at pressures better than 1 x 10-~° mbar. The samples were Zn-doped p-type (p ~ 1.1 x 1017cm-3; MCP Ltd). Bismuth was evaporated from a Knudsen cell (W.A. Technology Ltd) at pressures better than 2 x 10-1° mbar. The evaporation rates were measured using a quartz crystal balance mounted close to the cell and this was calibrated for a large thickness against a similar balance mounted at the sample position. A nominal thickness scale in monolayers (ML) which assumes layer-by-layer growth and unity sticking coefficient both on samples and balances was established (1 ML = 2.9 ~). Stable evaporation rates between 0.02-0.2 ML min -~ on the sample were achieved. Results In Figures 1 and 2 we present sets of selected photoemission spectra of the In 4d and Bi 4d regions for increasing bismuth coverage. These spectra have been selected to illustrate the salient points in the evolution of the interface. Also shown are the results from a core level fitting routine which has been applied to the spectra. This analysis allows each spectrum to be fitted to as many as four gaussian-broadened Lorentzian dou- blets with the option of introducing Doniac-Sunjic (DS) asym- metry. Figure 1 shows In 4d spectra which have been fitted by two 1021