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.
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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.
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- Award ID(s):
- 1648811
- NSF-PAR ID:
- 10066850
- 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
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A mode-locked ultrafast laser focused on the tunneling junction of a scanning tunneling microscope (STM) superimposes harmonics of the laser pulse repetition frequency on the DC tunneling current. The power measured at each of the first 200 harmonics (up to 15 GHz) varies inversely as the square of the frequency due to stray capacitance shunting the tunneling junction. Fourier analysis suggests that in the tunneling junction the harmonics have no significant decay up to a frequency of 1/2τ ≈ 33 THz where τ = 15 fs, the laser pulse width. Two different analyses will be presented to model the generation of the frequency comb within the tunneling junction. The first is based on the observed current-voltage characteristics for the nanoscale tunneling junction. The second is a solution of the time-dependent Schrodinger equation for a modulated barrier. Both analyses indicate that optical rectification of the pulsed laser radiation in the tunneling junction causes harmonics of the pulse repetition frequency of the laser and that these harmonics may extend to terahertz frequencies. It appears that the tunneling junction may be used as a sub-nm sized source of terahertz radiation. Transmission and back scattering could not be used but loading of this source by the finite conductivity of the semiconductor would cause a loss varying inversely with the carrier density. Carrier dynamics could be measured by time-domain measurements, and time-averaged carrier profiling, but presumably with finer resolution due to the sub-nm size of the terahertz source.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1109/WMED.2017.7916934
