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

Ab initio density-functional calculations using Gaussian orbitals are carri ed out on large Si and Ge supercells containing oxygen defects. The formati on energies, local vibrational modes, and diffusion or reorientation energi es of O-i, O-2i, VO, VOH, and VO2 are investigated. The piezospectroscopic tensors for O-i, VO, and VO2 are also evaluated. The vibrational modes of O -i in Si are consistent with the view that the defect has effective D-3d sy mmetry at low hydrostatic pressures but adopts a buckled structure for larg e pressures. The anomalous temperature dependence of the modes of O-2i is a ttributed to an increased buckling of Si-O-Si when the lattice contracts. T he diffusion energy bf the dimer is around 0.8 eV lower than that of O-i in Si and 0.6 eV in Ge. The dimer is stable against VO or VO2 formation and t he latter defect has modes close to the reported 894-cm(-1) band. The reori entation energies for O and H in VO and VOH defects are found to be a few t enths of an eV and are greater when the defect has trapped an electron.