First principles calculation of substitutional oxygen and oxygen-hydrogen complex in gallium arsenide

We report first-principles investigation of the electronic structure, atomic geometry and formation energy of the isolated substitutional oxygen impurity (O_As) and the oxygen-hydrogen pair complex (O_As-H) in GaAs. Our results for O_As in neutral charge state, show an on-site defect with an A₁ midgap level occupied with one electron. The O_As^-2 charge state shows an off-center displacement of 0.7 Å on the < 111 > direction associated to a JT distorion. The calculated formation energy reveal a negative-U behavior for the doubly negative charge state. For the neutral complex O_As-H, we calculated the total energy and atomic configuration for seven different positions of hydrogen atom with oxygen in the same substitutional site. The lowest energy configuration obtained from our calculations has hydrogen near a bond-center site of O_As-Ga bond, with O_As-H bond length of 1.88 Å and H-Ga of 1.54 Å. The angle between these bonds is found to be 160°. The hydrogen passivation of the impurity level is observed.