Ab initio calculations of structural and electronic properties of 6H-SiC(0001) surfaces

We report ab initio calculations of structural and electronic properties of hexagonal 6H-SiC(0001) surfaces. The calculations have been carried out self-consistently within a local density approximation employing supercell geometries, smooth norm-conserving pseudopotentials in separable form and Gaussian orbital basis sets. In a systematic study, we have investigated the relaxation, as well as (root 3 x root 3)R30 degrees reconstructions of both Si- and C-terminated substrate surfaces. We have optimized eight structural models for the reconstructed surfaces with Si or C adatoms and Si or C trimers adsorbed in threefold-symmetric T-4 or H-3 positions. In the case of the Si-terminated substrate surface, our results favor Si adatoms in T, sites as the optimal configuration which is compatible with structure data from experiment. For the C-terminated substrate surface, our results indicate that none of the investigated root 3 x root 3 adatom or trimer configurations is the optimal surface structure. We present the electronic structure of the relaxed and of some representative reconstructed surfaces and discuss our results in comparison with other theoretical results and with experimental data from the literature.