Point Defects in Amorphous SiO2: What Have We Learned from 30 Years of Experimentation?

Abstract

ABSTRACTPoint defects in amorphous silicon dioxide (a-SiO2) have been studied for over 30 years by numerous spectroscopic techniques, including optical absorption and emission and electron spin resonance (ESR). While all of these experimental approaches have yielded valuable insights, only ESR has afforded detailed atomic-scale structural characterizations. ESR spectra of two fundamental classes of defects in a-SiO have been knon since the 1956 work of Weeks. However, many crucial detaiis such as the 29Si and 17O hyperfine structures of these centers have been provided only within the past dozen years. From these results, it is known that the so-called E′ center comprises an unpaired electron in a dangling tetrahedral orbital of a single silicon atom at the site of an oxygen vacancy; at least three variants of the E′ center have been distinguished in a-SiO2. Two additional intrinsic defect types, the nonbridging oxygen hole center and the superoxide radical, have also been identified by ESR, these being relatively more prevalent in irradiated silicas of high and low hydroxyl contents, respectively. Recent theoretical developments have proven indispensible to the modeling of the E′ centers and have yielded valuable insights on the superoxide defects.