Structure and diffusion of excess Si atoms in SiO2

Using gradient corrected density functional theory calculations, we have investigated the structure and diffusion of excess Si atoms in amorphous SiO2, with comparisons to their behavior in α-quartz. From the first principles calculations of their configuration, bonding, and energetics, we find that excess Si atoms can be fully incorporated into the amorphous oxide network while yielding oxygen vacancies. The incorporation turns out to gain energy as high as about 1.8eV, relative to the bond center state where the excess Si atom is located at a Si-O bond center. Based on the results, we propose a novel mechanism for Si diffusion in amorphous SiO2 in the presence of excess Si atoms, which involves the fourfold-coordinate Si2+ state creation via oxygen vacancy diffusion and pairing and its reconfiguration to the bond center state. The overall diffusion barrier is approximated to be 4.5-5.0eV, in good agreement with recent measurements. Our calculation results also predict that excess Si atoms, if they exist, may undergo diffusion with a moderate barrier of <3.0eV in α-quartz.