Atomic configuration, stabilizing mechanism, and impurity vibrations of carbon-oxygen complexes in crystalline silicon

We have investigated the atomic configuration, stabilizing mechanism, and impurity vibrations of the carbon-oxygen complexes in silicon using norm-conserving pseudopotentials with the supercell method. We have found that a configuration in which an oxygen atom occupies a second-neighbor bond-interstitial site of a substitutional carbon atom without a direct C-O bond (CO-2 configuration) is more stable than the configuration in which an oxygen atom occupies a first-neighbor bond-interstitial site with a direct C-O bond (CO-1 configuration). The calculated total-energy reduction in the formation of the complex of the CO-2 configuration is 1.17 eV. Lattice-relaxation is essential to this stability. The previously observed infrared-absorption lines at 589, 640, 690, and 1104 cm-1 due to a C-O complex are well explained with the carbon- or oxygen-localized impurity vibrational modes calculated for the CO-2 configuration. Based on the comparison between the calculated and observed impurity vibrational energies, a type of configuration is suggested to explain another set of absorption lines at 716, 725, 744, and 1052 cm-1.