Paramagnetic centres associated with bonding defects in v-SiO₂

Abstract

Paramagnetic centres are produced in vitreous (v)-SiO₂ by the trapping of photogenerated electrons and holes at the intrinsic bonding defect centres associated with over- and under-coordinated oxygen atoms, C₃ ⁺ and C₁- respectively. Capture of a hole by the C₁- converts that centre to C₁ ⁰, the oxygen hole centre (OHC), whilst the capture of an electron by the C₃ ⁺ centre converts it to a C₃ ⁰ which reconstructs forming the E′ centre, a threefold coordinated neutral silicon atom T₃ ⁰. The C₃ ⁺, C₁- bonding defects occur as near neighbours in the network structure, so-called intimate valence-alternation pairs or I.V.A.P.s, so that an I.V.A.P. centre may trap either a hole or an electron, or both carriers. The unusual annealing behaviour of the OHC E.S.R. signal is shown to be related to centres which initially capture both an electron and a hole, and then convert from a spin-inactive configuration (T₃ ⁺, C_l ⁻) to the spin-active configuration (T₃ ⁺, C₁ ⁰) through the thermal release of one electron.