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1. Increased versatility for carrier profiling of semiconductors by scanning frequency comb microscopy (SFCM)

   https://doi.org/10.1109/WMED.2017.7916928  

   Birch, Timothy ; Hagmann, Mark J. ( April 2017 , Increased versatility for carrier profiling of semiconductors by scanning frequency comb microscopy (SFCM))

   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 themore »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.« less
2. Microwave Frequency Comb from a Semiconductor in a Scanning Tunneling Microscope

   https://doi.org/10.1017/S1431927616012563  

   Hagmann, Mark J. ; Yarotski, Dmitry A. ; Mousa, Marwan S. ( April 2017 , Microscopy and Microanalysis)

   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 themore »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.« less
3. Possibility of carrier profiling semiconductors by terahertz spectroscopy with terahertz radiation generated in a scanning tunneling microscope

   https://doi.org/10.1109/WMED.2017.7916933  

   Coombs, Dmitrij G. ; Hagmann, Mark J. ( April 2017 , Possibility of carrier profiling semiconductors by terahertz spectroscopy with terahertz radiation generated in a scanning tunneling microscope)

   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 generationmore »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.« less
4. Electrode control methodology for a scanning tunneling microscope

   Hagmann, M. ( February 2018 , U.S. patentSearch claims & abstracts)

   A control methodology for scanning tunneling microscopy is disclosed. Instead of utilizing Integral-based control systems, the methodology utilizes a dual-control algorithm to direct relative advancement of a STM tip towards a sample. A piezo actuator and stepper motor advances an STM tip towards a sample at a given distance until measuring a current greater than or equal to a desired setpoint current. Readings of the contemporaneous step are analyzed to direct the system to change continue or change direction and also determine the size of each step. In simulations where Proportion and/or Integral control methodology was added to the algorithmmore »the stability of the feedback control is decreased. The present methodology accounts for temperature variances in the environment and also appears to clean and protect the tip electrode, prolonging its useful life.« less
5. scanning frequency comb microscopy: a new method in scanning probe microscopy

   https://doi.org/10.13140/RG.2.2.34585.72804  

   Hagmann, M. ( August 2018 , scanning frequency comb microscopy: a new method in scanning probe microscopy)

   Finer resolution with greater stability is possible using unique low-power (aW), low-noise (20 dB S/N), microwave harmonics generated within a nanoscale tip-sample junction for feedback control in place of the DC tunneling current. Please see the attached poster to be presented at the Microscopy & Microanalysis-2018 meeting in Baltimore Monday August 6th as Post-deadline poster PDP-18. Applications include true sub-nm resolution in the carrier profiling of semiconductors. This method is especially appropriate for resistive samples where the spreading resistance flattens plots of the tunneling current vs. tip-sample distance with a scanning tunneling microscope.