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 through 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.