High-precision determination of atomic positions in 4H- and 6H-SiC crystals

The atomic structure of the two SIC polytypes, 4H- and 6H-SiC, differs by the stacking sequences of the Si-C bilayers in the [0001] direction. Conforming with the space group symmetry, the atoms are in principle allowed to move freely in [0001]-direction. The bonding tetrahedra are pressed or stretched (atomic relaxations). These atomic relaxations are determined by means of high-precision X-ray diffraction measurements and ab initio calculations. The calculations were performed in the framework of pseudopotential-density-functional theory in the local density approximation. For the experimental determination of these atomic relaxations an analysis 'quasiforbidden' reflections is possible. A comparison of the measured data with those for calculated geometries is given. We find a good agreement between the calculated and the experimental data. (C) 1999 Elsevier Science S.A. All rights reserved.