Measuring voltage transients with an ultrafast scanning tunneling microscope

Abstract

We use an ultrafast scanning tunneling microscope to resolve propagating voltage transients in space and time. We demonstrate that the previously observed dependence of the transient signal amplitude on the tunneling 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 dependence is eliminated. All results can be explained with coupling through the geometrical capacitance of the tip-electrode junction. By illuminating the current-gating photoconductive switch with a rigidly attached fiber, the probe is scanned without changing the probe characteristics.

Characterization of Electronic Transients by an Ultrafast Scanning Tunneling Microscope

“…In the contact mode, the tip of the USTM has an ohmic contact with the coplanar strip lines and in non-conduct mode, a bias voltage is applied to the strip to be measured on the sample to supply a dc tunneling current. This result can be explained by the equivalent circuit of the tunneling capa-citance [3,7]. Fig.3 shows the relation between the contact mode is about 7 times smaller compared to the signal in contact mode.…”

“…Meanwhile several other groups confirmed their results [3][4][5][6][7][8][9][10]. Instead of the previously reported linear dependence of the transient signal amplitude on the dc tunneling current, a complete indepen-dence of the transient signal in non-contact mode has been observed [7]. The photocunductively gated STM is capable to resolve ultrafast electrical pulses with a temporal resolution of picosecond and subpicosecond, but the spatial resolution remains a point of contro-versy.…”

Spatio-temporal imaging of voltage pulses with an ultrafast scanning tunneling microscope

“…4 Recently, it was shown that the linear dependence of the signal amplitude on the tunneling conductance disappears if the tip is virtually grounded instead of floating. 5,6 One of the most interesting aspects of the photoconductively gated tunneling microscope is the assumed ability to combine the high spatial resolution of the scanning tunneling microscope with the high temporal resolution of optical gating techniques using femtosecond pulses. However, there has been no report on measurements with simultaneous spatial and temporal resolution until now.…”

Ultrafast scanning tunneling microscopy with 1 nm resolution

“…2,3 Subsequent work in this field has concentrated on improving the time-domain performance of ultrafast STM ͑UF-STM͒, and a number of authors have reported signal detection with time resolution on the order of 1 ps. [4][5][6][7] Demonstrating that UF-STM techniques can offer the spatial resolution inherent in the STM has proven more elusive. A theoretical and experimental analysis carried out by Groeneveld and Van Kempen 8 concluded that the detected signals in PG-STM are capacitive in origin, so that this technique should have a spatial resolution limited by the geometric capacitance of the tip to the sample surface.…”