title: Microwave Frequency Comb from a Semiconductor in a Scanning Tunneling Microscope.
abstract: Quasi-periodic excitation of the tunneling junction in a scanning tunneling microscope, by a mode-locked ultrafast laser, superimposes a regular sequence of 15 fs pulses on the DC tunneling current. In the frequency domain, this is a frequency comb with harmonics at integer multiples of the laser pulse repetition frequency. With a gold sample the 200th harmonic at 14.85 GHz has a signal-to-noise ratio of 25 dB, and the power at each harmonic varies inversely with the square of the frequency. Now we report the first measurements with a semiconductor where the laser photon energy must be less than the bandgap energy of the semiconductor; the microwave frequency comb must be measured within 200 μm of the tunneling junction; and the microwave power is 25 dB below that with a metal sample and falls off more rapidly at the higher harmonics. Our results suggest that the measured attenuation of the microwave harmonics is sensitive to the semiconductor spreading resistance within 1 nm of the tunneling junction. This approach may enable sub-nanometer carrier profiling of semiconductors without requiring the diamond nanoprobes in scanning spreading resistance microscopy.
authors.affiliation: 1Department of Electrical and Computer Engineering,University of Utah,Salt Lake City,UT 84112,USA.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Hagmann
authors.affiliation: 3Los Alamos National Laboratory,Center for Integrated Nanotechnologies,Materials Physics and Applications Division,Los Alamos,NM 87545,USA.
authors.forename: Dmitry A
authors.initials: DA
authors.lastname: Yarotski
authors.affiliation: 4Department of Physics,Mu'tah University,Al-Karak 61710,Jordan.
authors.forename: Marwan S
authors.initials: MS
authors.lastname: Mousa
keywords: laser-assisted tunneling; microwave frequency comb; scanning probe microscopy; scanning tunneling microscopy; spreading resistance