Colour centres in vitreous silica

Abstract

Colour centres in vitreous silica are discussed in. terms of dangling bond defects on silicon and oxygen sites. Because of the large cation-anion electronegativity difference it is argued that neutral singly occupied states will be unstable, decomposing into empty positively charged silicon sp³ states and doubly occupied negatively charged oxygen 2p states. The energy levels for these states will be separate from the band edges and will give rise to features in the tail of the optical absorption edge. Following ionizing radiation, electrons and holes will be trapped at these defects converting them to neutral metastable singly occupied silicon and oxygen states. In line with current models for chalcogenide glasses, considerable distortion of the lattice is expected and this will lead to absorption bands well below the optical absorption edge. Intense radiation—particularly particle radiation—will increase the total number of defects–both charged and neutral. The absorption and luminescence data for colour centres characteristic of pure silica are analysed in terms of this model and levels for the silicon and oxygen localized states are deduced. The arrangement of states in the energy gap is found to be consistent with the stability of charged defect states. The model implies an energy gap of about 11 eV. Moreover it qualitatively describes many of the annealing and bleaching properties of the colour centres.