LEED STRUCTURE DETERMINATION OF HEXAGONAL ALPHA-SIC SURFACES

Basal plane surfaces of hexagonal silicon carbide samples of 6H and 4H polytype were investigated by means of low-energy electron diffraction (LEED). In a first part it is demonstrated by test calculations how diffraction spot intensities are dependent on the stacking sequence of the first few bilayers. It is shown that for an ideally flat sample it can be determined which specific layer of the hexagonal unit cell terminates the surface. However, when a statistical mixture of bilayer terminations is present on the surface, 4H and 6H polytypes yield indistinguishable intensities. In a second part a surface structure analysis based on experimental data obtained from two 6H-samples is presented. The surface orientation (polarity) is determined to be (0001) for both samples. Different termination morphologies are found which are clearly distinguishable by LEED. While one of the samples shows a mixed termination, the other exhibits a preferential termination with linear stacking of the first three bilayers involving typical step heights of three bilayer spacings or multiples of that. For this sample a refinement of the structure analysis shows that surface dangling bonds are saturated by hydroxyl species. Oxygen is found to be in atop position with a Si-O bond length of 1.65 Angstrom. The asymmetric bond environment of the topmost Si leads to a contraction of the first bilayer (7.5%). The subsurface geometry is bulk-like.