Scanning tunneling microscopy on photoconductive semi-insulating GaAs

The increase in surface conductivity upon illumination of semi-insulating GaAs was used to enable surface-topography measurements with a scanning tunneling microscope (STM). Images were recorded in ultrahigh vacuum and in air using a HeNe laser and a halogen lamp for the generation of a photoconductive top layer. From the current-voltage characteristics of the tip-sample contact the maximum feedback-controlled tunnel current at a given voltage can be deduced. A calculation of the increase in carrier density and conductance upon illumination is given, which confirms the possibility of using STM on highly resistive photoconductive materials. Further implications and applications are discussed.