In order to obtain an improved understanding of the fundamental microscopic effects of hole-trapping by O-vacancy sites in the amorphous oxide, ab initio Hartree-Fock calculations were made of the structures and energies of model SiO2 clusters. Three different precursor clusters were used in the calculations: The first was a 15-atom cluster without rings, the second was a 39-atom cluster which contained four B-atom (3-membered) rings, and the third was an 87-atom cluster with four 12-atom (6-membered) rings. The results suggested that the formation energy of VO in the neutral and positive charge states depended upon the starting size and geometry of the precursor. Microscopic structural changes, mainly network relaxation due to hole-trapping by VO0, depended strongly upon the initial local structure around the vacancy. A neutral vacancy tended to form a Si-Si dimer bond, regardless of the network structure. Hole-trapping at VO in a relatively rigid network which contained 6-atom (3-membered) fused rings resulted in a small but symmetrical relaxation (elongation) of the Si-Si bond at the vacancy site. When the network contained more flexible structures, such as 12-atom (6-membered) rings adjacent to VO, plus sufficient asymmetry, trapping of a hole caused an asymmetrical relaxation of the 2 adjacent Si atoms. The asymmetrical relaxation, in the present calculations, proceeded without a barrier. The formation energies of VO0 and VO+ decreased with flexibility and asymmetry of the oxide network.

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

A.C.Pineda, S.P.Karna: Journal of Physical Chemistry A, 2000, 104[20], 4699-703