Enfeebled oxygen bonding and metastability in GaP:O

Using the self-consistent Green's-function method, the self-consistent supercell technique, and a modified valence force model, we study the bonding of oxygen to the surrounding gallium atoms in the point defect GaP:O. Norm-conserving Hamann-Schluter-Chiang nonlocal pseudopotentials are used in the self-consistent calculations. Total ground-state energies are obtained for the neutral state of this defect. We calculate a Ga—O bond which is surprisingly weak compared to that inferred from data on the GaO molecule and the ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ crystal where, as in GaP:O, the oxygen atom is surrounded by four cations. A simple tight-binding model of the electronic structure accounts for the charge distribution found in the Green's-function calculation and makes plausible both the weakness of the bond and its marked softening with electron capture. The calculations imply that capture of a second electron to give ${\mathrm{O}}^{-}$ may weaken the Ga—O bond so much that, as suggested originally by Kukimoto and Henry, a completely different bonding configuration is more stable. The model does suggest that the original symmetric bonding configuration, although not necessarily the lowest-energy one, is, nonetheless stable against small displacements, i.e., metastable. We cite experimental evidence which supports its existence.