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  1. Summary form only given, as follows. We have described a method to generate a microwave frequency comb (MFC) which has hundreds of measurable harmonics in the tunneling junction of a scanning tunneling microscope with a metal sample electrode. With semiconductor samples the harmonics have an attenuation that varies inversely with the local carrier density at the tunneling junction. Three methods for carrier profiling that are based on the MFC, and a fourth method where terahertz radiation is generated within the tunneling junction, are already implemented virtually in the prototype. Parallel and deterministic operation of two or more of these methodsmore »with simulations is made possible by basing this system on a field-programmable gate array (FPGA). Thus, different types of information about the semiconductor could be obtained in a fast and efficient manner with optimization and analysis in real time. The unique combination of simulations and measurement tools in a single instrument will facilitate maintenance and debugging as well as the optimization and characterization of each component and the full system. User-friendly LabVIEW software will be used with subpanel and tab control to access and combine the various functions. At present, in the development stage, each component that will later be attached to the FPGA is simulated but the physical parts may be switched in and out with the simulated components.« less
  2. 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
  3. 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 spectroscopymore »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.« less
  4. 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 effectsmore »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.« less
  5. 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
  6. 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
  7. The first two harmonics of a microwave frequency comb (MFC) were measured at a probe which must be within 1 mm of the tunneling junction at the surface of a semiconductor as the sample electrode in a scanning tunneling microscope. The MFC was generated using a passively mode-locked Ti:Sapphire laser with GaN, but lasers with lower photon energy would be required with silicon. The attenuation of the MFC is primarily caused by the spreading resistance in a sub-nm spot at the tunneling junction. Thus, the measured attenuation could be used to determine the carrier density at this spot as anmore »extension of scanning spreading resistance microscopy (SSRM). We anticipate that this effect will enable new nondestructive methods for sub-nm carrier profiling of semiconductors.« less