Structures of quasi-freestanding ultra-thin silicon films deposited on chemically inert surfaces

Abstract

Silicon thin films were deposited on a sapphire and a highly oriented pyrolytic graphite (HOPG), which have atomically flat and chemically inert surfaces. The electronic and geometrical structures of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and polarization-dependent X-ray absorption fine structure (XAFS). It was found that the silicon K-edge XAFS spectra for ultra-thin silicon films thinner than 0.2 monolayer exhibited two distinct resonance peaks which were not observed for bulk silicon. The peaks were assigned to the resonance excitations from the Si 1s into the valence unoccupied orbitals with π∗ and σ∗ characters. The average tilted angle of the π∗ orbitals was determined by the polarization dependencies of the peak intensities. It was demonstrated that direction of a part of the π∗ orbitals in silicon film is perpendicular to the surface. These results support the existence of quasi-freestanding single-layered silicon films with sp2 configuration.