Microsc. Microanal. 23, 443–448, 2017 doi:10.1017/S1431927616012563
© MICROSCOPY SOCIETY OF AMERICA 2016
Microwave Frequency Comb from a Semiconductor in a Scanning Tunneling Microscope
Mark J. Hagmann,1,2,* Dmitry A. Yarotski,3 and Marwan S. Mousa4
1Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84112, USA 2NewPath Research LLC, Salt Lake City, UT 84115, USA 3Los Alamos National Laboratory, Center for Integrated Nanotechnologies, Materials Physics and Applications Division,
Los Alamos, NM 87545, USA 4Department of Physics, Mu’tah University, Al-Karak 61710, Jordan
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 14.85GHz 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 25dB 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.
Key words: scanning tunneling microscopy, scanning probe microscopy, microwave frequency comb, spreading resistance, laser-assisted tunneling
INTRODUCTION
Yasui et al. (2011) generated microwave and terahertz frequency combs by mixing the radiation from two synchronized mode-locked ultrafast lasers in a photo- conductive diode for frequency metrology. Mixing of two frequency combs generated in a high-speed photodetector with a single mode-locked laser has been employed in distance metrology (Doloca et al., 2010). In both applications the frequency combs are generated by optical rectification. Since the 1960s, point-contact metal-insulator-metal
diodes have been used for detection and mixing at frequencies up to 520THz (Pollock et al., 1983), where detection is done through rectification in the tunneling junction. A review of laser-assisted scanning tunneling microscopy (STM) (Grafstrom, 2002) describes many studies of optical mixing and rectification in the tunneling junction of an STM. In 2011, we generated the first microwave frequency
comb (MFC) in an STM by focusing amode-locked ultrafast laser on the tunneling junction using metal samples (Hagmann et al., 2011). Measurements with a spectrum analyzer showed that the linewidth is ~1Hz (Hagmann et al., 2012b), but the apparent linewidth is a convolution of the actual power spectrum with the impulse response of the
*Corresponding author.
mhagmann@newpathresearch.com Received May 31, 2016; accepted November 18, 2016
instrument, which was 1 Hz. Later measurements, using a resolution bandwidth of 0.1 Hz, showed a linewidth of ~0.1 Hz, which sets the present state-of-the-art for a narrow linewidth microwave source (Hagmann et al., 2013). In tests with metal samples, there are hundreds of
measurable harmonics and the power at each harmonic varies inversely with the square of the frequency. Analysis suggests that optical rectification causes the tunneling junction to act as a constant current source at each harmonic up to 30 THz, where the frequency is then one-half the reciprocal of the laser pulse width (15 fs) (Hagmann et al.,
2013). Related phenomena are predicted in laser-assisted field emission (Hagmann & Mousa, 2007). Measurements show that current division between the shunting capacitance CS at the tunneling junction and the load resistance RL of the spectrum analyzer causes the power at the nth harmonic Pn as shown in the following equation:
Pn = P1
1 + 2πRLCSf1 1 + 2πRLCSnf1
ðÞ ðÞ
2 2 ; (1)
f1, the fundamental frequency = the pulse repetition frequency of the laser. Typically, CS = 6.4 pF and RL = 50Ω, so the power in the second harmonic at 150.508MHz is 94% of that at the fundamental frequency (75.254 MHz). At much higher frequencies the power at the nth harmonic varies inversely with the square of the frequency, corresponding to a decrease of 6 dB/octave.
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