Extrinsic surface states traced by surface photovoltage in photoemission

Surface defect states are generally not directly accessable experimentally. In standard photoemission this is because of their low densities which are several orders of magnitude lower than intrinsic states. On semiconductor surfaces, however, they may induce band bending which is changing characteristically upon bias light intensity and temperature variation. We show that fitting surface photovoltage measured by photoemission to calculations considering high bias light levels and finite electric fields in the space charge layer explicitly allow for a quantitative determination of surface defect densities and the associated energy levels. The results obtained for the defect densities were corroborated by scanning tunneling microscopy investigations. For the covalent and layered semiconductor surfaces GaAs(110) and WSe2:Rb serving as model systems, we show the wide applicability and reliability of this technique.