SCANNING TUNNELING MICROSCOPY AT MICROWAVE-FREQUENCIES

We have applied to a scanning tunneling microscope (STM) in addition to the usual DC bias voltage an RF voltage at a frequency f of about 300 MHz, and observed the frequency components of the tip current at the second (2f) and third harmonic (3f) of the fundamental frequency f. In order to enhance the detection sensitivity at the higher harmonic frequencies the tip-substrate tunneling gap was part of a re-entrant cavity tuned either to 2f or 3f. We studied the generation of harmonics using the STM on surfaces of HOPG graphite, gold and semiconducting WSe2 under atmospheric conditions. The origin of the observed generation of higher harmonics turned out to be either the nonlinear current-voltage characteristic of the tunneling gap as we observed on HOPG substrates, or in the case of semiconducting WSe2, a strong distortion of the RF displacement current due to the field-induced change of the space-charge capacitance near the semiconductor surface. Using the generation of harmonics we have been able to image the space-charge density on a semiconductor surface with 15 nm resolution. The experiments reported here are of importance also to the question of whether a microwave STM can be used for the investigation of insulating substrates.