Abstract
A mode-locked laser focused on the tunneling junction of a scanning tunneling microscope (STM) superimposes a microwave frequency comb with hundreds of harmonics on the DC tunneling current. Each harmonic, at an integer multiple of the laser pulse repetition frequency, sets the present state-of-the-art for narrow linewidth at its frequency to enable low-noise measurements at an average laser power of several milliwatts. Measurements of the attenuation of the harmonics, which is caused by the spreading resistance, may be used to determine the resistivity of the sample. In Scanning Frequency Comb Microscopy (SFCM) feedback control of the tip-sample distance is based on the power at the harmonics. No DC bias voltage or DC tunneling current is required and the data rate is much higher than that with an STM. Simulations of the spatial distribution of the power dissipated in the sample show the feasibility of non-destructive true sub-nm resolution in the carrier profiling of semiconductors. With no DC bias voltage and no DC tunneling current band-bending and other changes to semiconductor samples in an STM are mitigated and there is a possibility for in vivo microscopy in biology and medicine.
Highlights
We acknowledge the pioneering effort that others have made using analysis as well as numerical and experimental methods to study the spreading resistance of a grounded resistive slab having a small thin metal disk contact at its surface.1 Most of this resistance is located near the contact, as the current spreads outward from the disk, which is the reason for using the term “spreading resistance”
In the new method of Scanning Frequency Comb Microscopy (SFCM) a tunneling junction is used with a metal tip above the sample5 instead of requiring the direct contact of a probe with the sample as in Scanning Spreading Resistance Microscopy (SSRM) and Scanning Capacitance Microscopy (SCM)
We discovered that focusing a mode-locked laser on the tunneling junction of a scanning tunneling microscope (STM) generates hundreds of microwave harmonics at integer multiples of the laser pulse repetition frequency
Summary
In the new method of Scanning Frequency Comb Microscopy (SFCM) a tunneling junction is used with a metal tip above the sample instead of requiring the direct contact of a probe with the sample as in SSRM and SCM. We have discovered that feedback control of the tip-sample distance is more stable when it is based on maximizing the power of the microwave harmonics in SFCM instead of maintaining a setpoint value for the DC tunneling current as in an STM.. Conventional STM measurements with semiconductors show that the applied DC electric field (≈ 2GV/m) changes the sample including band-bending and other anomalies.14,15 These effects are mitigated in SFCM because, even though the laser creates a pulsed electric field with a peak value that is comparable to that of the continuous DC electric field in an STM, the mean value of the field is reduced by a factor of 106 due to the small duty factor of the mode-locked laser. A 10 percent increase in the sample resistivity causes the microwave power at each harmonic to decrease by 17 percent, because the maximum power occurs at a greater tip-sample distance This effect increases the sensitivity of the measurements
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