Studies on the geometric and electronic structures of SiC polytypes

By using the plane-wave ultrasoft and norm-conserving pseudopotential methods of density functional theory in the local density approximation, the lattice parameters and energy band structures of Sic polytypes are, investigated. We found that the conduction-band minimum in 6H Sic lies at U (0,0.500,0.176) point along the ML line in the Brillouin zone, according to the plane-wave ultrasoft pseudopotential method. While the conduction-band, according to the norm-conserving pseudopotential method, displays discontinuity at (0,0.500,0.178) point along the ML line. The result obtained with plane-wave ultrasoft pseudopotential method does not show any marked camel-back, and its U point is closer to M (0,0.500,0) point than that with norm-conserving pseudopotential method. The lattice parameters c/p, c/pa increase slightly with increasing hexagonality in the case of the plane-wave ultrasoft pseudopotential method, The band gap and valence-band width have the same trend. The calculated total energy of 4H Sic structure is the lowest in the four structures at zero temperature. However, the Ewald energy of the 3C Sic structure is the lowest among the polytypes.