Electron states in a nearly ideal random-network model of amorphous SiO2 glass.

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A large continuous random-network model with 1296 atoms and periodic boundary conditions has been constructed for amorphous ${\mathrm{SiO}}_{2}$ glass. The atoms in this model are all fully coordinated with an overall small bond length and bond angle distortions. The calculated pair distribution function is in close agreement with the diffraction data. Based on this model, a first-principles calculation of the electron states is performed and the resulting wave functions are analyzed. Subtle differences in the density of states with the crystalline ${\mathrm{SiO}}_{2}$ are found. The calculated density of states are in good agreement with x-ray emission data and show the importance of Si 3d orbitals. The distributions of effective charges on Si and O atoms are studied in relation to the short-range order in the glass. It is found that O atoms with a Si-O-Si bridging angle of less than 120\ifmmode^\circ\else\textdegree\fi{} have smaller effective charges and can be identified as quasidefective centers that are responsible for the two-level tunneling at low temperature. It is also shown that localized states at the top of the band are induced by the elongation of the Si-O bond and those at the bottom of the band are related to atoms with shortened bonds. A mobility edge of 0.2 eV at the top of the valence band is obtained. A similar mobility edge for the conduction band cannot be located because of the much less localized nature of the states. \textcopyright{} 1996 The American Physical Society.