SURFACE RECONSTRUCTION GEOMETRY OF GAAS(001)-C(2X8) (2X4) BY HIGH ANGULAR RESOLUTION X-RAY PHOTOELECTRON DIFFRACTION

We have investigated the surface structure of As-stabilized GaAs(001)-c(2 x 8)/(2 x 4) by high-energy X-ray photoelectron diffraction at high angular resolution. Azimuthal-angle scans at several polar angles of both Ga3d and As3d intensities have been measured and compared to theoretical angular distributions generated by means of single scattering theory. The coordinates of As atoms in the surface layer have been systematically varied to simulate both buckled and unbuckled dimer formation in which every fourth dimer is missing. This arrangement of missing dimers leads to the basic (2 x 4) surface net that gives rise to c(2 x 8) domains. Optimal agreement between theory and experiment is achieved by modeling unbuckled dimer formation in which an in-plane displacement for surface As atoms of 0.9 +/- 0.1 angstrom and a vertical inward displacement of 0.1 +/- 0.2 angstrom are assumed. These displacements lead to bond lengths between dimer atoms and between surface As and second-layer Ga atoms of 2.2 +/- 0.2 and 2.6 +/- 0.1 angstrom, respectively. A buckled dimer geometry, such as the one deduced from angle-resolved photoemission and local-density functional calculations for Ga(001)-(2 x 1), does not lead to good agreement with experiment.