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Electronic and optical properties of the cubic spinel phase of c − Si 3 N 4 , <mml:math xmlns…

2001; American Physical Society; Volume: 63; Issue: 24 Linguagem: Inglês

10.1103/physrevb.63.245110

1095-3795

W. Y. Ching, Shang‐Di Mo, Lizhi Ouyang,

Inorganic Chemistry and Materials

The electronic and optical properties of the new cubic spinel nitrides $c\ensuremath{-}{\mathrm{Si}}_{3}{\mathrm{N}}_{4},$ $c\ensuremath{-}{\mathrm{Ge}}_{3}{\mathrm{N}}_{4},$ and that of the predicted double nitrides $c\ensuremath{-}{\mathrm{SiGe}}_{2}{\mathrm{N}}_{4}$ and $c\ensuremath{-}{\mathrm{GeSi}}_{2}{\mathrm{N}}_{4}$ 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 $c\ensuremath{-}{\mathrm{SiGe}}_{2}{\mathrm{N}}_{4}$ should be a stable compound while $c\ensuremath{-}{\mathrm{GeSi}}_{2}{\mathrm{N}}_{4}$ could be metastable. The compound $c\ensuremath{-}{\mathrm{SiGe}}_{2}{\mathrm{N}}_{4}$ 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 $c\ensuremath{-}{\mathrm{SiGe}}_{2}{\mathrm{N}}_{4}$ 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.