Oxygen doped SiC nanocrystals: first principles study of the optical properties.

We have studied a typical spherical SiC nanocrystal with a diameter of 1.2 nm (Si43C44H76) using linear combination of atomic orbitals in combination with pesudopotential density functional calculation. The role of fluorine and oxygen impurities was investigated on the electronic and optical properties of the Si43C44H76 nanocrystal. Total energy calculations show that the fluorine doped Si43C44H76 nanocrystals are unstable. Oxygen doped Si43C44H76 have different binding energies in various substitutional and interstitial defects. The maximum binding energy of the oxygen at carbon substitutional defect is about -0.5 eV and at interstitial defect is -0.18 eV. The HOMO-LUMO gap of the pure Si43C44H76 is about 6.71 eV and after doping with oxygen changes on the order of 0.1 eV. Our studies show that the refractive index of the doped Si43C44H76 nanocrystal significantly dispersed in comparison with pure SiC nanocrystal especially at the range of 6 to 8 eV.