Transient Measurements With A Photoconductively Gated Scanning Tunneling Microscope

Abstract

For the combination of high spatial resolution with high temporal resolution we use a photoconductively gated scanning tunneling microscope (PGSTM). The instrument resolves voltage transients propagating on a coplanar waveguide (CPW) in space and time. In addition, we demonstrate the detection of laser induced field changes in a photoconductor. The setup, shown in Fig. I , differs in two aspects from previously reported designs.[ 1, 21 The tunneling tip is attached to a coplanar strip line providing direct electrical access to the tip. In addition, a fiber supplying the probe beam, is directly connected to the photoconductive (PC) switch which ensures an unconstrained positioning of the tip without variations in the illumination of the PC switch. The 100-fs laser source is split into a pump beam generating an electrical pulse on the sample and a probe beam gating the probe PC switch with a delay. Sample and probe substrates are low temperature grown (LT) GaAs and the PC switch response time is measured to be less than 1 ps. The current preamp controlling the feedback loop is connected to the tunneling tip through one electrode of the coplanar strip line. The other electrode is connected to an external preamp measuring the gated signal. Due to this modification we do not observe the previously reported linear dependence of the transient signal amplitude on the tunneling conductance.[ 13 The amplitude is independent of the tunneling resistance and decreases by a factor of four in a distance of 50 pm above the surface. All results can be explained by coupling through the geometrical capacitance of the tipelectrode gap. The instrument is used to map out the mode structure of voltage transients on transmission lines. Figure 2 shows a gray-scale contour plot of a pulse on a CPW generated