First-principles study of neutral oxygen vacancies in amorphous silica and germania

Neutral oxygen vacancies in amorphous (a-) SiO₂ and GeO₂ as well as bulk a-SiO₂ and a-GeO₂ configurations have been investigated by using the first-principles pseudopotential method in order to elucidate the mechanism of the photorefractive effect of Ge-doped SiO₂. Amorphous configurations of SiO₂ and GeO₂ have been constructed by quenching using classical molecular-dynamics method and subsequent relaxation using the first-principles method. Obtained configurations and electronic properties are in good agreement with experiments of a-SiO₂ and a-GeO₂, and the gross features of the densities of states (DOS's) of these glasses are similar to each other. The highest valence band is the O-2p nonbonding band and the lowest conduction band consists of Si or Ge orbitals. However, the structural differences between the a-SiO₂ and a-GeO₂ configurations such as the larger O-O distance in a-GeO₂ and the relatively shorter Ge-Ge distance in a-GeO₂ induce the peculiar differences in the DOS's. And the band gap of a-GeO₂ is much smaller than that of a-SiO₂. The oxygen deficient centers (ODC's) are formed by removing identical oxygen atoms from the Si-O-Si and Ge-O-Ge networks. One occupied defect state is generated in the band gap. The Ge-Ge bond at the Ge-ODC is shorter than the Si-Si bond at the Si-ODC in the present configurations. The larger network flexibility and the less electrostatic repulsion in a-GeO₂ than in a-SiO₂ should cause the shorter Ge-Ge bond length at the Ge-ODC, which results in the lower occupied defect level in the gap in a-GeO₂ and the much lower ODC formation energy in a-GeO₂. This smaller formation energy indicates much more ODC's in a-GeO₂ than in a-SiO₂. Therefore more E^' centers may be generated in Ge-doped a-SiO₂ than in pure a-SiO₂ if a-GeO₂ clusters exist in Ge-doped a-SiO₂. Furthermore, we have discussed the electron trapping defects.