Electronic and optical properties of the cubic spinel phase of c-Si3N4, c-Ge3N4, c-SiGe2N4, and c-GeSi2N4 -: art. no. 245110

The electronic and optical properties of the new cubic spinel nitrides c-Si3N4, c-Ge3N4, and that of the predicted double nitrides c-SiGe2N4 and c-GeSi2N4 are studied by a first-principles method. They are all semiconductors with band gaps between 1.85 and 3.45 eV and a bulk modulus between 258 and 280 GPa. From the total-energy calculations, it is shown that c-SiGe2N4 should be a stable compound while c-GeSi2N4 could be metastable. The compound c-SiGe2N4 is of particular interest because of a favorable direct band gap of 1.85 eV and a conduction-band effective mass of 0.49. The crystal has a very strong covalent bonding character as revealed by the calculated Mulliken effective charge and bond order. The strong covalent bonding in c-SiGe2N4 is attributed to the optimal arrangement of the cations. The smaller Si ion occupies the tetrahedrally coordinated (8a) site and the larger Ge ion occupies the octahedrally coordinated (16d) site.