Surface properties of (1 1 1), (0 0 1), and (1 1 0)-oriented epitaxial CuInS2/Si films

(1 1 1), (0 0 1), and (1 1 0) 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(1 1 1)-(1×2), CuInS2(0 0 1)-(1×1), and CuInS2(1 1 0)-(1×1) were observed for near-stoichiometric films (cubic indices). Based upon simulations of LEED patterns, the origin of the CuInS2/Si(1 1 1)cub-(1×2) superstructure is attributed to a cation ordering of CuAu type in the film surface. For (1 1 0) oriented films, a complete faceting of the film surface into CuInS2{1 1 1} surface facets was found, independent of the surface composition. A partial {1 1 1}-faceting was obtained for near-stoichiometric CuInS2/Si(0 0 1), which, however, could be suppressed under Cu-rich growth conditions. Using valence band spectroscopy, a clear signature of the (0 0 1) 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.