Ab-initio study of oxygen vacancies in alpha-quartz
  Extrinsic levels, formation energies, and relaxation geometries are calculated ab initio for oxygen vacancies in alpha-quartz SiO2. The vacancy is found to be thermodynamically stable in the charge states Q=+3, Q=0, Q=--2, and Q=-3. The charged states are stabilized by large and asymmetric distortions near the vacancy site. Concurrently, Franck-Condon shifts for absorption and recombination related to these states are found to be strongly asymmetric. In undoped quartz, the ground state of the vacancy is the neutral charge state, while for moderate p-type and n-type doping, the +3 and -3 states are favored, respectively, over a wide Fermi level window. Optical transitions related to the vacancy are predicted at around 3 eV and 6.5 eV (absorption) and 2.5 to 3.0 eV (emission), depending on the charge state of the ground state. 
C. M. Carbonaro, V. Fiorentini, S. Massidda (Cagliari)