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-SiO2, we hav e 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-a tom cluster with four 12-atom (6-membered) rings. For clusters A and B, a d ouble-zeta plus polarization (DZP) basis set was used. For cluster C, a min imal (STO-3G) basis set was employed. Our results suggest that the energy o f formation, Delta E-f of V-O in the neutral (V-O(0)) and positive (V-O(+1) ) charge stares depends on the starting size and geometry of the precursor, Similarly, microscopic structural changes, primarily network relaxation, d ue to hole trapping by V-O(0) strongly depend on the initial local structur e around the vacancy. A neutral vacancy, V-O(0), tends to form a Si-Si dime r bond regardless of the network structure. Similarly, hole trapping at V-O in a relatively rigid network containing 6-atom (3-membered) fused rings r esults in a small, but symmetric relaxation (i.e., elongation) of the Si-Si bond at the vacancy site. When the network contains more flexible structur es, such as 12-atom (6-membered) rings adjacent to V-O and sufficient asymm etry, trapping of a hole causes an asymmetric relaxation of the two adjacen t Si atoms, The asymmetric relaxation in our calculation proceeds without a barrier. The value of Delta E-f for V-O(0) and V-O(+1) decreases with the flexibility and asymmetry in the oxide network.