H2O and O2 molecules in amorphous SiO2:: Defect formation and annihilation mechanisms -: art. no. 195206

Interstitial water and oxygen molecules are ubiquitous impurities and participate in various defect formation processes in thermally grown SiO2 films and synthetic silica glass. Using results of first-principles calculations we report the types of defects (including different possible charge states) that H2O and O-2 molecules may form in bulk amorphous SiO2. 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 H3O+ and OH- ions at a low energy cost with a barrier of about 1.5 eV. The formation energies of other defects emanating from H2O interstitials are, however, too high to be thermally activated. We found that O-2 molecules may form ozonyl (Si-O-O-O-Si) linkages with an energy barrier of similar to2.4 eV. An explanation for the oxygen isotope exchange observed in thin SiO2 films near the Si-SiO2 and SiO2-vacuum interfaces is suggested based on the energy barrier for ozonyl formation being commensurate with the O-2 diffusion barrier close to the Si/SiO2 interface and the O-2 incorporation energy from vacuum. We also explain the different creation rates of E-' centers in wet and dry oxides by studying the annihilation mechanism of neutral and charged oxygen vacancies.