Oxygen and dioxygen centers in Si and Ge: Density-functional calculations

Ab initio density-functional calculations using Gaussian orbitals are carried out on large Si and Ge supercells containing oxygen defects. The formation energies, local vibrational modes, and diffusion or reorientation energies of Oᵢ, O₂ᵢ, VO, VOH, and VO₂ are investigated. The piezospectroscopic tensors for Oᵢ, VO, and VO₂ are also evaluated. The vibrational modes of Oᵢ in Si are consistent with the view that the defect has effective D3d symmetry at low hydrostatic pressures but adopts a buckled structure for large pressures. The anomalous temperature dependence of the modes of O₂ᵢ is attributed to an increased buckling of Si-O-Si when the lattice contracts. The diffusion energy of the dimer is around 0.8 eV lower than that of Oᵢ in Si and 0.6 eV in Ge. The dimer is stable against VO or VO₂ formation and the latter defect has modes close to the reported 894-cm^-1 band. The reorientation energies for O and H in VO and VOH defects are found to be a few tenths of an eV and are greater when the defect has trapped an electron.