Multiple charge states of substitutional oxygen in gallium arsenide

A local-density-functional cluster method is used to calculate the structure and vibrational modes of substitutional oxygen, ${\mathrm{O}}_{\mathrm{As}}$, in gallium arsenide. We find the defect can exist in a surprisingly large number of five charge states: ${\mathrm{O}}^{\mathit{n}\mathrm{-}}$ with n=-1, 0, 1, 2, and 3. The structures of the first two of these are very different from the last three. Their calculated modes are in fair agreement with observations on O in GaP and GaAs. The first pair of defects have a midgap level, and the third has an empty upper-gap level which can become occupied in photoillumination experiments. The consequent structural change causes a lowering in vibrational mode frequency as is observed.