Surface photovoltage of Ag on Si(111)-7×7 by scanning tunneling microscopy

Using a scanning tunneling microscope and light from a He-Ne laser, we have measured the surface photovoltage of the Si(111)-7×7 surface with coverages of Ag up to 1 monolayer. The data agree with a model for the photovoltage based on the recombination of photoexcited minority carriers with majority carriers thermally excited over the surface Schottky barrier. The surface Fermi-level positions derived from these data are consistent for n- and p-type Si decreasing from 0.60 eV above the valence-band maximum for the clean surface to 0.40 eV near 1 monolayer coverage. Although our photovoltage data are spatially resolved on an atomic scale, we have not observed any spatial variation in the photovoltage on the clean surface or on surfaces partially covered by Ag islands. This can be understood on the basis of the finite surface conductivity of the Si(111)-7×7 reconstruction.