Modelling of paramagnetic trivalent silicon defect centres in amorphous silica and at Si SiO2 interfaces

We review the structural and hyperfine properties of Si dangling bond defects occurring in amorphous SiO₂ and at various Si-SiO₂ interfaces. These defects have in common a singly occupied orbital on a trivalent Si centre. We first briefly summarize the most important methodologies for calculating hyperfine parameters, pointing out their advantages and drawbacks. The properties of the defect centres composed of {\mathrm {Si}}{\equiv } {\mathrm {Si}}_n{\mathrm

{O}}_{3-n} kernels, with n varying from 0 to 3, are then discussed in a systematic manner. We present three important factors affecting the hybrid state of the unpaired dangling bond: the local geometry around the defect centre, the electronegativity of the first-neighbour atoms, and the polarization effect due to the oxide environment around the dangling bond. We demonstrate that the cage polarization effect significantly increases the Fermi contact term of the defect Si atom, and discuss the relevant implications. We also quantify the interaction between the dangling orbital and the oxygen atoms belonging to the oxide cage by focusing on the ¹⁷O hyperfine couplings. For various defects in amorphous SiO₂ and at Si-SiO₂ interfaces, we discuss assignments to {\mathrm {Si}}{\equiv } {\mathrm {Si}}_n{\mathrm

{O}}_{3-n} structural units in relation to available experimental data. In particular, we address the charge state of the E_γ^' centre, the peculiar hyperfine properties of the S centre, and the atomic structures of the three P_b-type centres.