Defects in oxide glasses

An insight into the present understanding of point defects in the simplest and the most radiation-resistant oxide glass, glassy silicon dioxide (silica) is presented. The defects and their generation processes in glassy and α-quartz forms of silicon dioxide are significantly different. The only defect, confirmed to be similar in both materials, is oxygen vacancy. In silica, additional defects of dangling bond type are generated from precursor sites formed by strained Si-O bonds, and by modifier ions. The optical absorption spectra of silica are dominated by paramagnetic dangling bond type defects: silicon dangling bond (“E′-center”) and oxygen dangling bond (“non-bridging oxygen hole center, NBOHC”). Radiation-induced interstitial oxygen atoms exist in peroxy linkage (Si-O-O-Si) form, they can react with oxygen dangling bonds to create peroxy radicals or dimerize into interstitial O2 molecules. Hydrogen doping helps to reduce the defect concentration, however, creates new precursors in the form of hydroxyl groups and may stimulate O vacancy generation. Doping by fluorine reduces the number of strained Si-O bonds and results in glass, which has higher vacuum ultraviolet transparency and higher resistance to excimer laser light than pure silica. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)