SCANNING TUNNELING MICROSCOPY OF PHOTOEXCITED CARRIERS AT THE SI(001) SURFACE

A scanning tunneling microscope is used to probe changes in the surface potential created by photoexcited carriers. At small tunneling currents, the dependence of surface photovoltage on laser intensity agrees with a model based on the transport of majority carriers through the depletion layer. At larger values of the tunneling current, charging of the surface by the current modifies the surface photovoltage because of changes in band bending near the probe tip. Spatial variations in the surface photovoltage are observed on a length scale comparable to the width of the depletion layer. Variations on an atomic scale are also observed and are interpreted as an increase in surface charging at specific defects on the Si(001) surface.