Neutral O vacancies in amorphous (a-) SiO2 and GeO2 as well as bulk a-SiO2 and a-GeO2 configurations were 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 were constructed by quenching using classical molecular-dynamics method and subsequent relaxation using the first-principles method. Obtained configurations and electronic properties were in good agreement with experiments of a-SiO2 and a-GeO2, and the gross features of the densities of states of these glasses were similar to each other. The highest valence band was the O-2p non-bonding band and the lowest conduction band consisted 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 induced peculiar differences in the densities of states. And the band gap of a-GeO2 was much smaller than that of a-SiO2. The O-deficient centers were formed by removing identical O atoms from the Si-O-Si and Ge-O-Ge networks. One occupied defect state was generated in the band-gap. The Ge-Ge bond at the Ge- O-deficient centers was shorter than the Si-Si bond at the Si- O-deficient centers in the present configurations. The larger network flexibility and the lesser electrostatic repulsion in a-GeO2 than in a-SiO2 should cause the shorter Ge-Ge bond length at the Ge- O-deficient centers, which resulted in the lower occupied defect level in the gap in a-GeO2 and the much lower O-deficient center formation energy in a-GeO2. This smaller formation energy indicated much more O-deficient centers 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 existed in Ge-doped a-SiO2.

First-Principles Study of Neutral Oxygen Vacancies in Amorphous Silica and Germania. T.Tamura, G.H.Lu, R.Yamamoto, M.Kohyama: Physical Review B, 2004, 69[19], 195204 (10pp)