Paramagnetic Si dangling-bond defects in amorphous SiO2 were modelled by using a generalized-gradient density-functional approach. By creating single O vacancies in a periodic model of amorphous SiO2, several model structures were first generated in which the core of the defect consisted of a threefold coordinated Si atom carrying a dangling bond. These model structures were then fully relaxed and the hyperfine parameters calculated. It was found that the hyperfine parameters of such model defects, in both the neutral and positive charge states, reproduced those characteristic of the Ef, in accord with experimental observations for amorphous SiO2. By eliminating a second O atom in the nearest-neighbor shell of these defect centers, model defects were then generated in which the Si atom carrying the dangling bond forms bonds with two O atoms and one Si atom. In this defect, the spin density was found to delocalize over the Si-Si dimer bond, giving rise to two important hyperfine interactions. These properties match the characteristics of the hyperfine spectrum measured for the S center. The results were complemented by the calculation of hyperfine interactions for small cluster models which serve the threefold purpose of comparing different electronic-structure schemes for the calculation of hyperfine interactions, estimating the size of core-polarization effects, and determining the reliability of cluster approximations used in the literature.

First-Principles Modeling of Paramagnetic Si Dangling-Bond Defects in Amorphous SiO2. A.Stirling, A.Pasquarello: Physical Review B, 2002, 66[24], 245201 (11pp)