We analyze Ge-related defects in Ge-doped SiO_2 using first-principles density functional techniques. Ge is incorporated at the level of ~ 1 mol % and above. The growth conditions of Ge:SiO_2 naturally set up oxygen deficiency, with vacancy concentration increasing by a factor 10^5 over undoped SiO_2, and O vacancies binding strongly to Ge impurities. All the centers considered exhibit potentially EPR-active states, candidates for the identification of the Ge(n) centers. Substitutional Ge produces an apparent gap shrinking via its extrinsic levels.
@article{arxiv.cond-mat/0202497,
title = {Stability of Ge-related point defects and complexes in Ge-doped SiO_2},
author = {C. M. Carbonaro and V. Fiorentini and F. Bernardini},
journal= {arXiv preprint arXiv:cond-mat/0202497},
year = {2009}
}