Effect of Hole Trapping on the Microscopic Structure of Oxygen Vacancy Sites in a-SiO2

In order to develop an improved fundamental understanding of the microscopic effects of hole trapping by oxygen vacancy sites (VO) in amorphous a-SiO2, we have performed ab initio Hartree-Fock calculations of the structure and energy of model silicon dioxide clusters. Three different precursor clusters were employed in these calculations: (A) a 15 atom cluster without rings; (B) a 39 atom cluster containing four 6-atom (3-membered) rings; and (C) an 87 atom cluster with four 12-atom (6-membered) rings. For clusters A and B, a double zeta plus polarization (DZP) basis set was used. For cluster C, a minimal (STO-3G) basis set was employed. Our results suggest that the energy of formation, ∆Ef of VO in the neutral (VO) and positive (VO) charge states depends upon the starting size and geometry of the precursor. Similarly, microscopic structural changes, primarily network relaxation, due to hole trapping by VO strongly depend on the initial local structure around the vacancy. A neutral vacancy, VO, tends to form a Si—Si dimer bond regardless of the network structure. Similarly, hole trapping at VO in a relatively rigid network containing 6-atom (3-membered) fused rings results in a small, but symmetric relaxation (i.e., elongation) of the Si—Si bond at the vacancy site. When the network contains more flexible structures, such as 12-atom (6-membered) rings adjacent to VO and sufficient asymmetry, trapping of a hole causes an asymmetric relaxation of the two adjacent Si atoms. The asymmetric relaxation in our calculation proceeds without a barrier. The value of ∆Ef for VO and VO decreases with the flexibility and asymmetry in the oxide network.