STRUCTURE OF GAAS(001) SURFACES - THE ROLE OF ELECTROSTATIC INTERACTIONS

We report first-principles total-energy calculations for the GaAs(001) surface. Our results indicate that the 2 x 4 reconstruction correspon ds to the beta2(2 X 4) structure, which exhibits two As dimers in the top layer and a third As dimer in the third layer. This structure has a lower surface energy than the beta(2 x 4) model, which has three As dimers in the top layer. We also find that a model recently proposed b y Skala et al. [Phys. Rev. B 48, 9138 (1993)] for the structure of the Ga-rich 4 x 2 phase is energetically unfavorable. From our results we conclude that electrostatic interactions between the charged building blocks of polar semiconductor surfaces play an important role in dete rmining the equilibrium structure. We introduce a simple model for est imating these interactions.