ELECTRON-STATES IN A NEARLY IDEAL RANDOM-NETWORK MODEL OF AMORPHOUS SIO2 GLASS

A large continuous random-network model with 1296 atoms and periodic b oundary conditions has been constructed for amorphous SiO2 glass. The atoms in this model are all fully coordinated with an overall small bo nd length and bond angle distortions. The calculated pair distribution function is in close agreement with the diffraction data. Based on th is model, a first-principles calculation of the electron states is per formed and the resulting wave functions are analyzed. Subtle differenc es in the density of states with the crystalline SiO2 are found. The c alculated density of states are in good agreement with x-ray emission data and show the importance of Si 3d orbitals. The distributions of e ffective charges on Si and O atoms are studied in relation to the shea -range order in the glass. It is found that O atoms with a Si-O-Si bri dging angle of less than 120 degrees have smaller effective charges an d can be identified as quasidefective centers that are responsible for the two-level tunneling at low temperature. It is also shown that loc alized 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 val ence band is obtained. A similar mobility edge for the conduction band cannot be located because of the much less localized nature of the st ates.