Early stages of cesium adsorption on the As-rich c(2x8) reconstruction of GaAs(001) : adsorption sites and Cs-induced chemical bonds

We investigate the adsorption of Cs on the As-rich $c(2\times 8)/(2\times 4)$ reconstruction of GaAs(001) at low coverages using a combination of theoretical and experimental techniques. DFT-LDA total-energy calculations and X-ray diffraction experiments find only minimal Cs-induced surface relaxation and identify three preferential adsorption sites within the partially disordered overlayer. These sites are, in order of decreasing occupation probability, the arsenic dimer bridge D site, the gallium dangling bond T$_{2}^{\prime }$ site and the arsenic T$_{3}$ trench site. Detailed analysis of the wavefunctions and electronic charge densities allows us to clarify the bonding mechanisms at the three sites.\ At the gallium site, the bonding is strongly ionic and involves significant charge transfer to a new Cs-induced state reminiscent of the $p_{z}$ orbital of the gallium atom in the $sp^{_{2}}$ configuration. In sharp contrast, at the arsenic sites, the charge transfer is minimal and the bonding rather occurs through mixing with a relatively delocalized state of the clean surface. The ionization energy decreases are estimated and compared for the three sites.