TRANSIENT MEASUREMENTS WITH AN ULTRAFAST SCANNING TUNNELING MICROSCOPE

We use a photoconductively gated ultrafast scanning tunneling microsco pe to resolve laser-induced transients on transmission lines and photo conductors. The photoconductive switch on the tunneling probe is illum inated through a rigidly attached fiber. The use of the fiber enables us to scan across the transmission line while the change in delay time between pump beam (on the sample) and probe beam (on the probe) provi des the temporal information. The investigated photoconductor sample i s a low-temperature-grown GaAs layer placed on a sapphire substrate wi th a thin, semitransparent gold layer. In tunneling mode the probe is sensitive to laser-induced field changes in the semiconductor layer. L aser-induced transient signals of 2.2 ps widths are detected. As for t he transmission lines, the signals can be explained by a capacitive co upling across the tunneling gap.