Possibility of carrier profiling semiconductors by terahertz spectroscopy with terahertz radiation generated in a scanning tunneling microscope

Abstract

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 Schrödinger 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.