Cs bonding at the Cs/GaAs(110) interface

Evaporation of a Cs overlayer in the submonolayer and monolayer regime on a cleaved GaAs(110) surface has been studied by core-level photoemission spectroscopy. The experimental results show three different adsorption regimes deduced from the lineshape of the Ga 3d, As 3d and Cs 4d core levels. These spectra, in fact, result from the overlap of different contributions that have been identified as a function of the Cs coverage: (a) at low Cs deposition a prevalent Cs-As binding and a small percentage of Cs-Ga bonds are observed; (b) at intermediate coverage a strong increase of Cs-Ga bonds, with a reduction of the Cs-As feature, is attributed to disruption of the covalent pristine Ga-As bonds and Ga out-diffusion; and (c) at the highest Cs coverage, the relative intensity of the Cs-Ga component goes to saturation, whereas the initial Cs-As intensity is strongly reduced and a high density of As dangling bonds is restored. The nonlocal polarization of the interface with a quite limited charge transfer is detected though the low-energy shift of the core levels. The importance of these results is emphasized with respect to the Cs promoted oxidation of the semiconductor.