Electronic structure of hydrogenated amorphous silicon-germanium alloys studied by X-ray photoelectron spectroscopy and soft-X-ray spectroscopy

The authors present a study of Si 2p and Ge 3d core levels and of the valence Si 3p and conduction Si p and Ge s,d states by, respectively, X-ray-induced photoelectron spectroscopy and soft X-ray emission and absorption spectroscopy, for hydrogenated amorphous silicon-germanium a-Si 1-x Ge x :H alloys with 0.16<x<0.60 prepared by the glow-discharge technique. As the Ge concentration increases, the core level distributions are gradually broadened; they attribute this effect to an increase in the local disorder by alloying. The low-binding-energy edge of the valence p distribution is shifted towards E F while the bottom of the conduction band remains at the same energy position. Thus they clearly show that the decrease in the optical gap as x Ge increases is only due to a shift of the valence band edge. At the bottom of the conduction band, Si p and Ge s,d empty states are totally mixed; a spreading of the edge is observed as the Ge concentration increases. Eventually, the X-ray emission of a core excitonic state is observed as a faint structure in the Si K beta spectra, for x Ge =0.45 and 0.60.