Basal plane surfaces of hexagonal silicon carbide samples of 6H and 4H
polytype were investigated by means of low-energy electron diffractio
n (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 i
t can be determined which specific layer of the hexagonal unit cell te
rminates the surface. However, when a statistical mixture of bilayer t
erminations is present on the surface, 4H and 6H polytypes yield indis
tinguishable intensities. In a second part a surface structure analysi
s based on experimental data obtained from two 6H-samples is presented
. The surface orientation (polarity) is determined to be (0001) for bo
th samples. Different termination morphologies are found which are cle
arly distinguishable by LEED. While one of the samples shows a mixed t
ermination, 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 refine
ment of the structure analysis shows that surface dangling bonds are s
aturated by hydroxyl species. Oxygen is found to be in atop position w
ith a Si-O bond length of 1.65 Angstrom. The asymmetric bond environme
nt of the topmost Si leads to a contraction of the first bilayer (7.5%
). The subsurface geometry is bulk-like.