Theoretical studies of the initial stages of Zn adsorption on GaAs(001)-(2X4)

Using a first-principles pseudopotential technique, we have investigated fo ur possible models for the adsorption of Zn on the GaAs(001)-beta2(2 x 4) s urface. In agreement with the experimental studies of Ohtake and co-workers , our total energy calculations suggest that Zn atoms adsorb preferentially in trench (third-layer) sites, All structures are characterized by the bre aking of the As dimer and the subsequent formation of two mixed dimers. Des pite the formation of mixed dimers, all other structural features for the f our models studied (i.e., minimum interplanar distance and remaining As dim er bond lengths) retain the characteristics of the free-surface structure. We observed that ail four studied models present a very similar band struct ure, but while the binding energies and dispersion of the surface states ar e very similar to that observed for the free surface, their orbital charact ers depend on the model considered. From a theoretical scanning-tunneling-m icroscopy (STM) image simulation, we have shown that Zn adsorption in the t op layer leads to a clearly identifiable change in the brightness. On the o ther hand, Zn adsorption in the trench results in small changes in the brig htness in the trench region, which is not easily detectable in a common STM experiment.