Study of tips in an ultrafast scanning tunneling microscope

Ultrafast measurement system for transient electrical signals using a scanning tunneling microscope has been developed. The key of the system is a probe integrated with a low-temperature grown GaAs photoconductive switch that is used as a sampler of transient signals generated by ultrashort laser pulses with another photoconductive switch. The tunneling tip is attached to a coplanar strip transmission line with an integrated photoconductive switch. The probe fabrication process and tip characteristics have been reported here. A topographic STM image scanned with such a probe on a gold sample on Si substrate is given. A transient signal with 1.2 ps pulse width in tunneling mode and 2.0 ps in contact mode were observed with the probe.