Using gradient corrected density functional theory calculations, an investigation was made of the structure and diffusion of excess Si atoms in amorphous SiO2, as compared with their behavior in α-quartz. From first-principles calculations of their configuration, bonding and energetics, it was found that excess Si atoms could be fully incorporated into the amorphous oxide network while yielding O vacancies. The incorporation led to energy gains as high as 1.8eV, relative to the bond center state where the excess Si atom was located at a Si-O bond center. Based upon the results, a novel mechanism was proposed for Si diffusion in amorphous SiO2, in the presence of excess Si atoms, which involved the fourfold-coordinated Si2+ state creation via O vacancy diffusion and pairing, and its reconfiguration to the bond center state. The overall diffusion barrier was estimated to be 4.5 to 5.0eV; in good agreement with recent measurements. The calculated results also predicted that excess Si atoms, if they existed, could diffuse with an energy barrier of less than 3.0eV in α-quartz.

Structure and Diffusion of Excess Si Atoms in SiO2. D.Yu, G.S.Hwang, T.A.Kirichenko, S.K.Banerjee: Physical Review B, 2005, 72[20], 205204 (5pp)