The interstitial CiOi defect in bulk Si and Si1-xGex

The equilibrium geometries, phonon spectra, electronic structures and optical properties of the CiOi defect in bulk Si and Si1-xGex systems are calculated using the ab initio plane wave density-functional method. We find that in a Ge-doped Si crystal it is more energetically favourable for the defect to stabilize in a configuration with no Ge atoms in the first sphere. Our calculations show that the vibrational properties of the defect in the Si1-xGex alloy are similar to those in the pure Si bulk crystal and only one local vibrational mode is sensitive to the presence of the Ge substitutions. The effect of C, O, Si and Ge isotopes on the phonon spectra are also investigated and found to be in good agreement with available experimental data. It also follows from our calculations of the singlet-triplet splitting and a position of the gap state of the defect with respect to the top of the valence band that the optical energies of the CiOi defect are expected to increase due to Ge doping, which is in agreement with available experimental data.