ELECTRONIC-STRUCTURE OF HYDROGENATED AMORPHOUS-SILICON GERMANIUM ALLOYS STUDIES 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-xGex: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 EF 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 xGe 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 xGe=0.45 and 0.60.