3D CORE-LEVEL SHIFTS AT SE GAAS(110)

We determine 3d surface core-level shifts (SCLS) for clean and Se-depo sited GaAs(110) surfaces by means of ab initio pseudopotential calcula tions. The experimental findings for the clean GaAs(110) surface can b e described within the initial-state picture. If relaxation effects ar e taken into account we find a distinct overestimation of the SCLS for both As and Ga. We conclude that final-state effects play only a mino r role due to the dynamics of the photoemission process for the GaAs(1 10) surface. The validity of the initial-state picture for III-V(110) surfaces is further corroborated by calculations on InP and GaP. In or der to clarify the surface chemistry of the Se/GaAs(110) interface we calculate the 3d core level shifts of Ga, As and Se for four structura l models of an exchange-reacted GaAs(110) surface. Only one of these m odels gives rise to a reasonable agreement between calculated and meas ured shifts. Our calculations support a geometry where each surface As atom is substituted by Se and one further Se binds to the surface Ga atom.