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

To develop an improved fundamental understanding of the microscopic effects of hole trapping by oxygen vacancy sites (V O ) in amorphous a -SiO 2, 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-ζ 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, Δ E f of V O in the neutral (V O 0 ) and positive (V O +1 ) charge states depends on the starting size and geometry of the precursor. Similarly, microscopic structural changes, primarily network relaxation, due to hole trapping by V O 0 strongly depend on the initial local structure around the vacancy. A neutral vacancy, V O 0, tends to form a Si−Si dimer bond regardless of the network structure. Similarly, hole trapping at V O 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 V O 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 Δ E f for V O 0 and V O +1 decreases with the flexibility and asymmetry in the oxide network.