The structure, properties, and dynamics of oxygen vacancies in amorphous SiO2

Analysis of a random population of oxygen vacancies in several amorphous SiO₂ structures, using first-principles calculations, shows that a distribution of defect states exists due to different local structure. The results also reveal a new structure of an E(γ⁵)\´, a five-fold coordinated puckered Si atom, which assists in explaining reverse-bias switching behavior, and the subsequent reduction of E_γ\´ defects. The distribution of energy levels in the amorphous SiO₂ energy gap is consistent with experimental data on both shallow and deep hole traps. The relative frequency of the occurrence for the different hole traps is determined by examining the local geometries of all the possible oxygen vacancy sites in the amorphous structures. Most oxygen vacancies in amorphous SiO₂ are found to form dimer defects upon hole capture, with the remainder almost evenly divided between "puckered" defects that allow dipole formation upon electron capture (E(γ⁴)\´) and those that do not (E(γ⁵)\´).