MEASURING VOLTAGE TRANSIENTS WITH AN ULTRAFAST SCANNING TUNNELING MICROSCOPE

We use an ultrafast scanning tunneling microscope to resolve propagati ng voltage transients in space and time. We demonstrate that the previ ously observed dependence of the transient signal amplitude on the tun neling resistance was only caused by the electrical sampling circuit. With a modified circuit, where the tunneling tip is directly connected to the current amplifier of the scanning tunneling microscope, this d ependence is eliminated. Ail results can be explained with coupling th rough the geometrical capacitance of the tip-electrode junction. By il luminating the current-gating photoconductive switch with a rigidly at tached fiber, the probe is scanned without changing the probe characte ristics. (C) 1997 American Institute of Physics.