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-Si 3N4, c-Ge3N4, and that of the predicted double nitrides c-SiGe2N4 and c-GeS i2N4 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 sho uld 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 Mullik en effective charge and bond order. The strong covalent bonding in c-SiGe2N 4 is attributed to the optimal arrangement of the cations. The smaller Si i on occupies the tetrahedrally coordinated (8a) site and the larger Ge ion o ccupies the octahedrally coordinated (16d) site.