First-principles study of the atomic reconstructions and energies of Ga- and As-stabilized GaAs(100) surfaces

We have carried out ab initio total-energy density-functional calculations to study the reconstructions of GaAs (100) surfaces as a function of Ga and As surface coverage. Equilibrium atomic geometries and energies for Ga- and As-stabilized 11, 21, 12, and 22 surfaces consisting of various combinations of dimers and vacancies were determined. Dimerization of Ga (As) surface atoms is calculated to lower the energy by 1. 7 eV (0. 7 eV) per dimer and to lead to the most stable atomic configurations. For half-monolayer coverages, relaxation energies are very large, and nondimerized structures are only slightly (0. 03--0. 05 eV per 11 cell) higher in energy. Asymmetric dimers were tested for As surfaces and found to be higher in energy than symmetric dimers. The stability of surfaces in equilibrium with Ga and As sources is considered and it is shown that the chemical potentials are restricted within limits set by the free energies of the elemental bulk phases of Ga and As. Ab initio calculations of these bulk energies at T=0 K determine the limiting chemical potentials and also the heat of formation, which we find to be 0. 73 eV per GaAs pair, compared with the experimental value of 0. 74 eV. Our calculations indicate that with excess bulk As available, a full monolayer coverage of As is energetically more favorable than a half-monolayer coverage, whereas with excess Ga available, the surface energy of full and half-monolayer coverages are nearly the same. To examine the effects of larger unit-cell dimensions on total energies, we rely on results from tight-binding calculations. For half-monolayer coverages, 24 unit cells are found to have a significantly lower energy than 22 cells not because of a greater lattice relaxation but because of orbital rehybridization effects which are not possible in a smaller cell. The results of the ab initio and tight-binding calculations indicate that the optimal surface coverage for Ga- and As-terminated surfaces is less than a full monolayer.