  The initial steps of MBE growth of GaAs on beta 2-reconstructed GaAs(001) are
investigated by performing total energy and electronic structure calculations
using density functional theory and a repeated slab model of the surface. We
study the interaction and clustering of adsorbed Ga atoms and the adsorption of
As_2 molecules onto Ga atom clusters adsorbed on the surface. The stable nuclei
consist of bound pairs of Ga adatoms, which originate either from dimerization
or from an indirect interaction mediated through the substrate reconstruction.
As_2 adsorption is found to be strongly exothermic on sites with a square array
of four Ga dangling bonds. Comparing two scenarios where the first As_2 gets
incorporated in the incomplete surface layer, or alternatively in a new added
layer, we find the first scenario to be preferable. In summary, the
calculations suggest that nucleation of a new atomic layer is most likely on
top of those surface regions where a partial filling of trenches in the surface
has occurred before.
