H2OandO2molecules in amorphousSiO2:Defect formation and annihilation mechanisms

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Interstitial water and oxygen molecules are ubiquitous impurities and participate in various defect formation processes in thermally grown ${\mathrm{SiO}}_{2}$ films and synthetic silica glass. Using results of first-principles calculations we report the types of defects (including different possible charge states) that ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{O}}_{2}$ molecules may form in bulk amorphous ${\mathrm{SiO}}_{2}.$ We calculate their formation energies and, in the most interesting cases, the energy barriers in order to map out the most likely defect formation scenarios. In particular, we show that water molecules may form double silanol groups (Si-OH) as well as ${\mathrm{H}}_{3}{\mathrm{O}}^{+}$ and ${\mathrm{OH}}^{\ensuremath{-}}$ ions at a low energy cost with a barrier of about 1.5 eV. The formation energies of other defects emanating from ${\mathrm{H}}_{2}\mathrm{O}$ interstitials are, however, too high to be thermally activated. We found that ${\mathrm{O}}_{2}$ molecules may form ozonyl (Si-O-O-O-Si) linkages with an energy barrier of $\ensuremath{\sim}2.4\mathrm{eV}.$ An explanation for the oxygen isotope exchange observed in thin ${\mathrm{SiO}}_{2}$ films near the Si-${\mathrm{SiO}}_{2}$ and ${\mathrm{SiO}}_{2}$-vacuum interfaces is suggested based on the energy barrier for ozonyl formation being commensurate with the ${\mathrm{O}}_{2}$ diffusion barrier close to the ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interface and the ${\mathrm{O}}_{2}$ incorporation energy from vacuum. We also explain the different creation rates of ${E}^{\ensuremath{'}}$ centers in wet and dry oxides by studying the annihilation mechanism of neutral and charged oxygen vacancies.