The stability of a GaAs(001)-(2 × 4) surface with Si adatoms

The stability of GaAs(001)-(2 × 4)β2 surface with Si adatoms is theoretically investigated using the ab initio pseudopotential method. The calculated results imply that stable lattice sites for Si adatoms strongly depend on the adatom coverage. At lower coverage θ = 0.125, Si adatoms stably reside in the empty Ga sites in the missing dimer trenches. This is consistent with recent STM observations at low coverages. On the other hand, the surface with Si dimers on the upper As dimers are the most stable at higher coverage θ = 0.25. The coverage dependence of adsorption site preferences is qualitatively discussed in terms of energy band structure and excess energy of hetero interatomic bonds such as AsAs, SiAs and GaSi.