Oxygen deficient centers in silica: optical properties within many-body perturbation theory

The electronic and optical properties of neutral oxygen vacancies, also called oxygen deficient centers (ODC(I)s), have been investigated in pure and germanium doped silica (both amorphous and alpha-quartz) through first-principles calculations. By means of density functional theory and many-body perturbation theory (GW approximation and the solution of the Bethe-Salpeter equation), we obtain the atomic and electronic structures as well as the optical absorption spectra of pure and Ge-doped silica in the presence of ODCs (SiODC(I) s and GeODC(I) s); our study allows us to interpret and explain the very nature of the optical features in experimental absorption spectra. The theoretical optical absorption signatures of these defects show excellent agreement with experiments for the SiODC(I) s, i.e. two absorption bands arise around 7.6 eV due to transitions between the defect levels. Our theoretical results also explain the experimental difficulty in measuring the GeODC(I) absorption band in Ge-doped silica, which was in fact tentatively assigned to a broad and very weak absorption signature, located between 7.5 and 8.5 eV. The influence of Ge-doping induced disorder on the nature of the defect-related optical transitions is discussed. We find that even if the atomic and electronic structures of SiODC(I) and GeODC(I) defects are relatively similar, the slight network distortion induced by the presence of the Ge atom, together with the increase in the Ge-Si bond asymmetry, completely changes the nature of the optical absorption edge.