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

Abstract

(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 < 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 × 2), CuInS2(001)-(1 × 1), and CuInS2(110)-(1 × 1) were observed for near-stoichiometric films (cubic indices). Based upon simulations of LEED patterns, the origin of the CuInS2/Si(111)cub-(1 × 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. Using valence 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 χ of (4.9 ± 0.15) eV was found, which is distinctly higher than previously reported χ values of CuInS2. The electron affinity decreases significantly with increasing Cu/In ratio in the film surface, down to ∼4.1 eV for [In]/([Cu] + [In])sf ∼0.3. The surface Fermi level of stoichiometric films is energetically located ∼0.8 eV above the valence band edge. © 2001 Elsevier Science B.V.