Electronic structure of SiO2,SixGe1−xO2, and GeO2from photoemission spectroscopy

Abstract

The valence-band structure of the mixed silicate ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{O}}_{2}$ was investigated for a range of compositions by x-ray and uv photoemission spectroscopy. Structure in the valence band, which is derived from the oxygen nonbonding orbitals and from the Si-O bonding orbital, is seen to move continuously in initial energy in going from Si${\mathrm{O}}_{2}$ to Ge${\mathrm{O}}_{2}$. The width of the nonbonding bands at the top of the valence band decreases from about 3.3 eV in Si${\mathrm{O}}_{2}$ to 2.0 eV in Ge${\mathrm{O}}_{2}$ in which separate peaks are no longer resolved. The decrease in width of the nonbonding bands is correlated with an increase in the average oxygen-oxygen separation from 2.62 to 2.85 \AA{}. The results indicate that the width of the nonbonding bands is largely due to oxygen wave-function overlap. The valence bands measured by photoemission from the mixed oxide are not a superposition of Si${\mathrm{O}}_{2}$ and Ge${\mathrm{O}}_{2}$ valence bands. On the other hand, band-gap excitations, as determined by energy-loss satellites on the photoemission spectra, were found to be a superposition of those for Si${\mathrm{O}}_{2}$ and Ge${\mathrm{O}}_{2}$, indicating local conduction-band states centered about the Ge and Si sites.