Atomic structures of GaAs(100)-(2x4) reconstructions

Atomic structures of the As-rich GaAs(100)-(2x4) reconstructions based on converged first-principles total-energy calculations are reported. All geometries are characterized by similar structural elements such as As dimers with a length of about 2.5 Angstrom, dimer vacancies, and a nearly planar configuration of the three-fold coordinated second-layer Ga atoms leading to a steepening of the dimer block. For an As coverage of theta=3/4 we find the two-dimer beta 2 phase to be energetically preferred over the three-dimer beta phase. A structure with partial replacement of As by Ga in the uppermost layer corresponding to an As coverage of theta=1/2 is found to be slightly less favourable than the alpha phase of GaAs(100). Geometry parameters are given for all structures and compared with the available experimental results.