PHOTOVOLTAGE ON SILICON SURFACES MEASURED BY SCANNING TUNNELING MICROSCOPY

Surface bias voltages induced on a scanning tunnel microscope (STM) junction illuminated with laser radiation are spatially resolved for silicon surfaces. Surface photovoltages of approximately 0.3 V for Si(111)-(7 x 7) and < 0.1 V for Si(001)-(2 x 1) are observed with large reductions in the vicinity of surface/subsurface defects associated with midgap states. These reductions, attributed to a variation in the recombination rate, have typical surface screening lengths that are less than those found in the bulk. A small, atomically varying signal of 3-5 mV is also observed and is due to spatial variations in rectification efficiency rather than photovoltage.