ATOMIC RELAXATION IN SILICON-CARBIDE POLYTYPES

The relaxed structures of the (1), (2), (23) and (3) polytypes of silicon carbide are calculated using the pseudopotential total-energy technique with norm-conserving pseudopotentials and the local density approximation to the exchange-correlation energy. A 'tension model' is proposed to account for the atomic forces and stresses of the ideal structures and the results of the detailed relaxed structures. The authors also deduce the force field due to an isolated antiphase boundary from the calculated atomic forces of the ideal structures. The energies associated with these relaxations are about 1 meV per SiC pair of atoms per antiphase boundary. In order to calculate it, they have developed a new formulation, which should be of wider use in calculating relaxation energies. They discuss the different effects of longitudinal and transverse relaxations on the stability of the polytypes, particularly (23) as a possible intermediate phase between (2) and (3).