ULTRAFAST TIME RESOLUTION IN SCANNED PROBE MICROSCOPES - SURFACE PHOTOVOLTAGE ON SI(111)-(7X7)

The speed limitations conventionally encountered in scanning tunneling microscopy, scanning capacitance microscopy, and atomic force microscopy result from the external electronics and are not inherent to the techniques themselves. Ultrafast time resolution can be achieved through the use of correlation methods. We demonstrate the application of time-resolved optical correlation techniques to scanned probe microscopy by probing the relaxation of photo-excited carriers at the Si(111)-(7 x 7) surface on the nanosecond and picosecond time scales using scanning tunneling and scanning capacitance microscopy measurements of the surface photovoltage. The observed temporal response demonstrates that the voltage arises from photovoltaic effects and does not arise from direct optical rectification in the tunnel junction gap.