Adding RF capabilities to a mK-STM for capacitance measurements

Scanning tunneling microscopy (STM) typically operates at a low bandwidth of a few kHz. There are several reasons to expand the bandwidth into the radio frequency (RF) range such as noise measurements, electron spin resonance (ESR), scanning on insulators, or measuring the tip-sample capacitance. This capability has been achieved by many research groups in different systems and environments. Conventionally, the low frequency tunneling signal is separated from a superimposed RF signal and the STM tip acts as a near field antenna.

Our goal is to measure voltage dependent changes in tip-sample capacitance due to the charge occupation of an induced quantum dot in semiconducting materials using our existing mK STM system. To this end, we’ve added a superconducting inductor of ~300 nH in series with the STM tip. The inductor, in combination with the tip stray capacitance - which includes the tip sample capacitance - forms a tank circuit resonator. Therefore, upon sweeping the tip-sample bias, a shift in the resonance frequency (currently ~300 MHz) corresponds to a change in capacitance. The resonator is probed by reflectometry using a directional coupler, bias-Tee and a cryogenic amplifier. With these additions, we can currently achieve the detection of capacitance changes on the order of < 1aF. Here, the sensitivity depends on the resonator Q-factor as well as the overall stray capacitance. Future incremental changes of the setup are expected to increase sensitivity and RF bandwidth. We will present our current setup and initial measurements illuminating the prospects of the system.