Optically active defects in SiO2:: The nonbridging oxygen center and the interstitial OH molecule -: art. no. 075203

Irradiation of high-OH containing amorphous SiO2 is known to introduce a broad absorption band at 4.8 eV and an associated sharp photoluminescence band in the visible red domain, at 1.9 eV. Both absorption and luminescence bands have been unambigously attributed to the nonbridging oxygen (NBO) center, a common defect in irradiated SiO2. Using results of first-principles calculations we show that the NBO center and the interstitial OH molecule (OHi) may both be responsible for the observed absorption and luminescence bands. Although the absorption spectra of the two defects are very similar, their luminescence spectra are different, but overlapping. We found that the NBO center has a sharp luminescence line centered around 1.8 eV in the visible red, whereas OHi molecules have a wide emission spectrum ranging from the infrared (0.8 eV) to visible red (1.8 eV). Investigation of the results show that the difference in the emission spectra is due to the different extent of atomic and electronic relaxations around the two defects: in the case of the NBO, site-independent electronic relaxations are responsible for the defect emitting a photon only at a particular energy, whereas in the case of OHi molecules an interplay between site-dependent atomic and (therefore site-dependent) electronic relaxations result in a wide emission spectrum. A further, intriguing result is that both defects show similar vibrational and polarization properties, therefore their contribution to the red luminescence band of irradiated amorphous SiO2 may not have been decoupled in previous studies.