Irradiation of high-OH containing amorphous SiO2 was known to introduce a broad absorption band at 4.8eV and an associated sharp photoluminescence band in the visible red domain, at 1.9eV. Both absorption and luminescence bands were unambiguously attributed to the non-bridging O center, a common defect in irradiated SiO2. Using results of first-principles calculations it was shown that the non-bridging O 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 were very similar, their luminescence spectra were different, but overlapping. It was found that the non-bridging O center had a sharp luminescence line centered around 1.8eV in the visible red, whereas OHi molecules have a wide emission spectrum ranging from the infrared (0.8eV) to visible red (1.8eV). Investigation of the results showed that the difference in the emission spectra was due to the different extent of atomic and electronic relaxations around the two defects: in the case of the non-bridging O, site-independent electronic relaxations were 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 was that both defects exhibited similar vibrational and polarization properties, therefore their contribution to the red luminescence band of irradiated amorphous SiO2 may not were decoupled in previous studies.

Optically Active Defects in SiO2 - the Non-Bridging Oxygen Center and the Interstitial OH Molecule. T.Bakos, S.N.Rashkeev, S.T.Pantelides: Physical Review B, 2004, 70[7], 075203 (9pp)