Electronic structure of Si${\mathrm{O}}_2$, ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}{\mathrm{O}}_2$, and Ge${\mathrm{O}}_2$ from photoemission spectroscopy

The valence-band structure of the mixed silicate ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}{\mathrm{O}}_2$ 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 Å. 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.