skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.


Title: Simulation of the Frequency Comb Induced by a Periodically Excited Tunnel Junction in Silicon
In this article we use the ensemble Monte-Carlo method to study the frequency comb induced by a periodically excited tunnel junction on a semiconductor. The electron transport is modeled by solving the Boltzmann transport in p-type silicon doped with a concentration of 10 17 cm -3 . For a laser-pulse frequency of 100 MHz, we observe that, if the distance between the STM probe and the second electrode is under 1 μm and we apply a negative bias on the STM tip, the harmonics of the frequency spectrum are not reduced significantly by the electron diffusion and resistance spreading effects in the semiconductor. In this case we obtain a wide frequency comb spectrum, relatively similar to the ones measured experimentally in metals and other materials with high electron conductivity.  more » « less
Award ID(s):
1648811
PAR ID:
10066850
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Simulation of the Frequency Comb Induced by a Periodically Excited Tunnel Junction in Silicon
Page Range / eLocation ID:
1 to 4
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. We are developing a new method for the carrier profiling of semiconductors that shows promise for nm-resolution which is required at the new sub-10 nm lithography nodes. A modelocked ultrafast laser focused on the tunneling junction of a scanning tunneling microscope (STM) generates a regular sequence of pulses of minority carriers in the semiconductor. Each pulse of carriers has a width equal to the laser pulse width (e.g. 15 fs). In the frequency domain, this is a microwave frequency comb (MFC) with hundreds of measurable harmonics at integer multiples of the laser pulse repetition frequency (e.g. 74 MHz). After the minority carriers diverge rapidly into the semiconductor as a Coulomb explosion, the pulses become broader and decay, so that the MFC has less power with a spectrum limited to the first few harmonics. The frequency-dependent attenuation of the MFC is determined by the resistivity of the semiconductor at the tunneling junction so SFCM is closely related to Scanning Spreading Resistance Microscopy (SSRM). Harmonics of the MFC are measured with high speed, and high accuracy because the signal-to-noise ratio is approximately 25 dB due to their extremely narrow (sub-Hz) linewidth. Now we superimpose a low-frequency signal (e.g. 10 Hz) on either the applied bias or the voltage that is applied to the piezoelectric actuators of the STM to cause sidebands at each harmonic of the MFC which are less affected by the artifacts. 
    more » « less
  2. Summary form only given. We are developing a scanning tunneling microscope that is portable and optimized for scanning frequency comb microscopy (SFCM) as one part of our effort to complete a prototype for the carrier profiling of semiconductors by SFCM. Conventional integral or integral plus proportion feedback control of the tunneling current in a scanning tunneling microscope (STM) is satisfactory once tunneling has been established but may cause tip-crash by integral windup during coarse approach. In tip-sample contact images with atomic-resolution may be obtained but the microwave frequency comb ceases because there is no optical rectification and scanning tunneling spectroscopy also fails. We are studying a new control algorithm based on approximating the tunneling current as a polynomial in the bias voltage where the coefficients in this polynomial are not required. It is noted that hanges in the apparatus, as well as the algorithms used for feedback control in the STM, are required to optimize this instrument for measuring the microwave frequency comb. 
    more » « less
  3. Dual-comb spectroscopy (DCS) is a powerful method for the characterization of materials. It enables the rapid measurements of broad absorption spectra with high spectral resolution. However, dual-comb spectroscopy has a low duty cycle especially when used to study semiconductor materials whose excited states’ dephasing rates are much shorter than the spacing between the comb pulses. When DCS is applied to these systems, a vast majority of energy and acquisition time is wasted. Here, we demonstrate a simple approach that can aid efforts to solve this issue. In our approach, we use two frequency combs that have the same repetition frequencies and we digitally control the relative locking phase between the combs. This allows us to control the delay with very high precision and take the data rapidly and continuously only in the region where the signal is non-zero. We demonstrate the concept on a Neodymium-doped yttrium orthovanadate (Nd:YVO4) sample. We show that, we could scan the delay between the signal and local oscillator comb pulses up to ∼ 6 ps in under 150 µs. We compare an absorption spectrum of (Nd:YVO4) obtained using our approach to an absorption spectrum obtained using conventional DCS and they are in good agreement. This approach now makes DCS relevant for a wide range of materials. 
    more » « less
  4. A semiconductor carrier profiling method utilizes a scanning tunneling microscope and shielded probe with an attached spectrum analyzer to measure power loss of a microwave frequency comb generated in a tunneling junction. From this power loss and by utilizing an equivalent circuit or other model, spreading resistance may be determined and carrier density from the spreading resistance. The methodology is non-destructive of the sample and allows scanning across the surface of the sample. By not being destructive, additional analysis methods, like deconvolution, are available for use. 
    more » « less
  5. 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 μ m 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. 
    more » « less