Ab initio studies on the beta-SiC(001)-(5x2) surface

The (5 x 2) reconstruction of the beta-SiC(001) surface is investigated emp loying the ab initio pseudopotential method with a Gaussian basis set. By c omparing the grand-canonical potential for a number of conceivable structur al models, we find a two-adlayer asymmetric-dimer model to be the most stab le. This model is similar to the respective one suggested previously for th e (3 x 2) surface. In this model, two adlayers of Si atoms, both with cover ages less than one, adsorb on the clean Si-terminated surface. The lower of these adlayers contains 3/5 of a monolayer and the upper contains 1/5 of a monolayer of Si adatoms so that the total coverage by Si adatoms is 4/5. B y minimizing the total energy, we find that asymmetric dimers are formed in both adlayers. They give rise to a rich spectrum of surface bands, which a re presented and discussed according to their nature and physical origin. C harge densities are discussed in relation to scanning tunneling microscopy (STM) data. The calculated filled-state STM image shows a bright oval-shape d spot in each (5 x 2) unit cell, which is consistent with the experimental observation. Height profiles along high-symmetry lines are also calculated .