Structural and electronic properties of cubic, 2H, 4H, and 6HSiC

We study the structural and electronic properties of various polytypes of SiC through self-consistent ab initio pseudopotential calculations. For the wurtzite (2H), 4H, and 6H structures, the equilibrium lattice constants and bulk moduli are very similar to those for the cubic structure. The energies calculated for the polytypes considered here are very close to within 4.3 meV/atom, which may explain the polytypism of SiC. The 4H structure is found to be lowest in energy because of the attractive interactions between the alternating cubic and hexagonal stacking layers, while the wurtzite structure is most unstable among the polytypes. We find the asymmetric charge distribution for a Si-C bond to be on the boundary separating the zinc-blende and wurtzite phases, which should be related to the polytypism of SiC. In the hexagonal polytypes, the M conduction-band energy increases, while that of the K point decreases as the hexagonal close packing becomes more prominent. Thus, the conduction-band minimum state located at the X point for cubic SiC changes to the M point, and then to the K point for the 2H structure. For the cubic structure, the density of states near the conduction-band edge increases slowly with energy, while it shows very rapidly increasing behavior for the 6H polytype because its conduction-band edge states are flattened due to the band folding and the energy-increasing behavior of the M state when the hexagonal-close-packing nature is enhanced.