Defects in amorphous SiO2: reactions, dynamics and optical properties
This thesis focuses on two main topics: the reactions and dynamics of water and oxygen interstitials and the optical properties and associated relaxation mechanisms of two selected defects in amorphous SiO2. The theoretical approach is based on first-principles density-functional theory using ultrasoft pseudopotentials, the generalized gradient approximation and supercells. We show that H2O and O2 molecules are viable entities in the voids of the amorphous SiO2 structure. H2O molecules diffuse through such voids by a novel "reactive cut" mechanism in which the molecule breaks up at an oxygen site forming adjacent silanol groups and reforms on the other side of the void to continue its migration. Besides the formation of silanol groups, the formation of H3O+ and OH- complexes is possible by a moderate barrier. O2 molecules may form ozonyl (Si-O-O-O-Si) linkages with a low energy barrier explaining the observed oxygen isotope exchange at the interfaces of SiO2 films. Finally, we point out that the different concentrations of charged oxygen vacancies (E' centers) in heavily irradiated wet and dry oxides are due to the different nature and energy barrier of their annihilation mechanism. We identify and describe a novel phenomenon that occurs only in the solid state: Stokes shifts caused by slow electronic relaxations. We demonstrate the nature of this phenomenon on two examples: the non-bridging oxygen center and the interstitial hydroxyl group (OHi) in SiO2. Once these defects are excited, a deep hole is created in the valence band that "bubbles up" slowly and nonradiatively to the SiO2 valence band edge. We analyze this process in detail, showing the participating electronic states and the accompanying charge reconstruction inside the SiO2 valence band. Once this hole is at the top of the valence band, we show that the ensuing photoluminescence energies are similar, although the luminescence energy is site-dependent in the case of the OHi defect. We suggest that both defects may contribute to the experimentally observed red photoluminescence in irradiated wet oxides.