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 t he stacking sequences of the Si-C bilayers in the [0001] direction. Conform ing with the space group symmetry, the atoms are in principle allowed to mo ve freely in [0001]-direction. The bonding tetrahedra are pressed or stretc hed (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-densi ty-functional theory in the local density approximation. For the experiment al determination of these atomic relaxations an analysis 'quasiforbidden' r eflections is possible. A comparison of the measured data with those for ca lculated geometries is given. We find a good agreement between the calculat ed and the experimental data. (C) 1999 Elsevier Science S.A. All rights res erved.