Local Fermi-level pinning at a single adatom (Cs) or vacancy (As) on a GaAs(110) surface

Local surface photovoltage imaging with a scanning tunneling microscope is used to demonstrate two types of local Fermi-level pinning next to a single adsorbate or defect, taking advantage of the tip-induced band bending due to the lack of gap states at clean GaAs(110) surfaces. The pinning energy at a single cesium adatom location was determined to be 0.56±0.04eV above the valence-band maximum (VBM), while for As vacancy, the Fermi level cannot go below the top of a filled surface state, 0.62±0.04eV above the VBM.