Initial phases of CuInS2–Si heteroepitaxy

Abstract

The heterojunction formation between silicon Si(111) and copper indium disulfide CuInS2 is investigated using ultraviolet photoelectron spectroscopy (UPS)/x-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Thin layers of CuInS2 films were deposited on hydrogen terminated Si(111) surfaces by molecular beam epitaxy (MBE). The film growth process starts from a partially sulfurized Si surface with a nominal composition of SiS0.8±0.1 and a thickness of 2.5±0.8 Å. The dependence of XPS measured substrate and film peak intensities and LEED patterns on deposition time indicates a disturbed layer-by-layer growth mode. In addition, a change in film stoichiometry from Cu rich to In rich is observed at the beginning of the film growth. The analysis of the Si 2p core level shows that the Si substrate is disturbed to a depth of about 7 Å beyond the initial hydrogenated surface. The hydrogenated surface exhibits a band bending of 0.4 eV. Upon S exposure, the band bending increases to 0.64 eV due to the donor character of S in Si and the electron affinity χ changes by 0.54 eV which can be attributed to dipole formation by adsorbed S. The MBE growth procedure results in a 2 nm thick film showing the valence band structure of CuInS2 with a total change in χ to about 0.7 eV on the completed junction. The band discontinuities of the Si/intermixed phase/2 nm CuInS2 structure were determined as ΔEL=0.35±0.05 eV and ΔEV=0.06±0.05 eV.