Soft x-ray core level photoemission study of the Cs/InP interface formation

The room temperature formation of the Cs/InP interface has been studied with soft x-ray core level photoemission spectroscopy. The line shape analysis of the Cs 4d core level indicates that at submonolayer coverages Cs is bonded at both In and P sites. This is deduced from a large (1 to 0.55 eV) coverage-dependent splitting of the Cs 4d into two doublets. The broadening at coverages close to a monolayer is consistent with the growth of an out registry overlayer as observed before for the Cs/GaAs interface. The energy shift of the substrate core level spectra indicates that the interface Fermi level stabilizes at about 0.8 eV above the valence band maximum which corresponds to relatively large (small) band bending for n-type (p-type) InP(110). This result is unexpected due to the small work function of Cs. The line shape and intensity analysis of the spectra indicates the formation of a laterally uniform overlayer and the lack of any noticeable chemical reaction.