Vacancy-related defects and the Eδ′ center in amorphous silicon dioxide: Density functional calculations

The microscopic identification of vacancy-related defects in silicon dioxide has been a major challenge. Particularly in amorphous silica, the role of vacancy clusters is still controversial. Experimental data have led to suggestions that the Eδ′ center is a four-vacancy cluster instead of a single vacancy. Here we report density functional calculations that explore the energetics and electronic structure of single vacancies and clusters of four vacancies in realistic models of amorphous silica. A total of 76 O vacancies and 38 four-vacancy clusters were examined, and their energy levels and hyperfine parameters were calculated. Results for single vacancies compare well to previous theory. A key result for four-vacancy clusters is that relaxations localize the unpaired electron preferentially on one Si atom, resulting in a strongly anisotropic electron-paramagnetic-resonance signal. Electrons at single vacancies have a more benign anisotropy which is more compatible with the observed isotropic signal.