GaAs(001)−c(4×4): A chemisorbed structure

We have carried out an experimental study of the As-stable $\mathrm{GaAs}\mathrm{}\left(001\right)\mathrm{}-c(4\mathrm{}\times 4)$ reconstruction using a combination of molecular-beam epitaxy with reflection electron diffraction, angle-resolved photoemission, and surface-sensitive core-level photoemission. (The measurements were performed at Laboratoire pour l'Utilisation du Rayonnement Electromagnétique, Université Paris—Sud, F-91405 Orsay, France.) Apart from the surface symmetry, the electron-diffraction data show one-dimensional disorder along the [110] direction, probably corresponding to a corrugated-sheet structure in reciprocal space. Photoemission from surface states having dangling-bond character shows a onefold periodicity and an energy dispersion of 0.65 eV along [110], while along [$\overline{1}10$] the dispersion is much smaller, but there is a doubling of the periodicity. Observations of the $\mathrm{As}3d$ and $\mathrm{Ga}3d$ core levels show a surface $\mathrm{As}3d$ component at 0.62 eV higher binding energy than the bulk component, indicative of As—As bonding. The $\mathrm{Ga}3d$ surface core-level shift is very small and the line shape is similar to that from the (2 × 4) surface. The experimental results can be understood on the basis of an As overlayer structure, for which we propose specific models derived from trigonally bonded chemisorbed As.