Electronic subgap levels due to point and surface defects in silica glass and α-quartz

We present a theoretical study with focus on the transparency of silica glass (a-SiO2) in the ultraviolet light spectrum. All imperfections in the crystal structure can cause electronic subgap levels and thus increase the absorption of light. Therefore, we investigate a wide variety of defects and their respective electronic levels, like intrinsic point defects, defects connected to H2, O2 or H2O, and (inner) surfaces. Our calculations show that most subgap levels originating from stretched bonds, silane (SiH) or silanol (SiOH) groups concentrate in a range of 0–2 eV around the band edges. For an energy of ∼
3.5 eV, like that of lasers in the controlled fusion ignition at the National Ignition Facility (NIF), these subgap levels do not cause absorption of such photons due to the large band gap ≥
8 eV of a-SiO2. However, E’-centers (≡
Si•
), non bridging oxygen hole centers (NBOHC, ≡
Si–O•
) and peroxy linkages (POL, ≡
Si–O–O–Si≡
) may act as possible source of absorption. The addition of hydrogen can reduce the number of those defect levels in the middle of the band gap by shifting them to the band edges.