Surface bias voltages induced on a scanning-tunneling-microscope junction illuminated with laser radiation are spatially measured for both metal and semiconductor samples. A surface photovoltage of ∼0.3 eV is observed for Si(111)-(7×7), with large reductions in the vicinity of surface (subsurface) defects having midgap states. These reductions, attributed to a change in the recombination rate, have a typical surface screening distance of 15–25 Å. A small, atomically varying signal of 3–5 mV is observed on both metal and semiconductor samples and demonstrated to arise not from variation in photovoltage but from spatial variations in rectification efficiency.

• Received 12 March 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.456