Surface properties of (111), (001), and (110)-oriented epitaxial CuInS2/Si films

(111), (001) and (110) oriented CuInS2 films were grown epitaxially on H-terminated Si substrates by molecular beam epitaxy. The epilayers had surface compositions in the range 0.27 < [In]/([Cu] + [In])(sf) < 0.63. Their electronic and atomic surface structure were investigated by means of photoelectron spectroscopy and low energy electron diffraction (LEED). LEED pattern symmetries of CuInS2(111)-(1 x 2). CuInS2(001)-(1 x 1), and CuInS2(110)-(1 x 1) were observed for near-stoichiometric films (cubic indices). Based upon simulations of LEED patterns, the origin of the CuInS2/Si(111)(cub)-(1 x 2) superstructure is attributed to a cation ordering of CuAu type in the film surface. For (110) oriented films, a complete faceting of the film surface into CuInS2(111) surface facets was found, independent of the surface composition. A partial (111)-faceting was obtained for near-stoichiometric CuInS2/Si(001), which, however, could be suppressed under Cu-rich growth conditions. Usingvalence band spectroscopy, a clear signature of the (001) surface was detected and is interpreted as due to a surface resonance. Ionization energies and surface Fermi energies of differently oriented CuInS2 epilayers were found to be non orientation dependent. For stoichiometric and Cu-poor CuInS2 an electron affinity X of (4.9 +/- 0.15) eV was found, which is distinctly higher than previously reported X values of CuInS. The electron affinity decreases significantly with increasing Cu/In ratio in the film surface, down to similar to4.1 eV for [In]/([Cu] + [In])(sf) similar to 0.3. The surface Fermi level of stoichiometric films is energetically located similar to0.8 eV above the valence band edge. (C) 2001 Elsevier Science B.V. All rights reserved.