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 {}. 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.
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Kuk et al. (1990) studied this question.