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 photoem ission spectroscopy. The experimental results show three different ads orption 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 dif ferent contributions that have been identified as a function of the Cs coverage: (a) at low Cs deposition a prevalent Cs-As binding and a sm all percentage of Cs-Ga bonds are observed; (b) at intermediate covera ge a strong increase of Cs-Ga bonds, with a reduction of the Cs-As fea ture, is attributed to disruption of the covalent pristine Ga-As bonds and Ga out-diffusion; and (c) at the highest Cs coverage, the relativ e intensity of the Cs-Ga component goes to saturation, whereas the ini tial Cs-As intensity is strongly reduced and a high density of As dang ling bonds is restored. The nonlocal polarization of the interface wit h a quite limited charge transfer is detected though the low-energy sh ift of the core levels. The importance of these results is emphasized with respect to the Cs promoted oxidation of the semiconductor.