CARRIER INJECTION AND SCANNING-TUNNELING-MICROSCOPY AT THE SI(111)-2X1 SURFACE

Surface photovoltage at the Si(111)-2 X 1 surface is measured using a scanning tunneling microscope by simultaneously modulating the sample bias and the intensity of illumination from a He-Ne laser. In the limit of small tunneling current, the dependence of surface photovoltage on light intensity measured for p- and n-type Si gives the Fermi level at the surface, 0.46+/-0.03 eV. Tunneling out of the surface injects bulk carriers that reduce band bending at the surface of n-type Si. Tunneling into the surface reduces the surface photovoltage on p-type Si and increases the photovoltage on n type demonstrating that the tunneling current changes the surface potential by charging of surface electronic states or through bulk spreading resistance. Spatial variations in the surface photovoltage measured at positive sample bias and large tunneling current are observed at surface defects.