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.