Time-resolved terahertz spectroscopy of bulk and nanoscale semiconductors.

摘要

The nature of charge carrier dynamics and conduction in bulk and nanoscale semiconducting materials is investigated with time-resolved terahertz (THz) spectroscopy (TRTS). This powerful technique uses picosecond (10-12 s) pulses of far-infrared light to map the electrodynamic response of a photoexcited material in the 0.2 - 3 THz (1012 Hz) frequency range on ultrafast time scales.;Fundamental conduction mechanisms are investigated in dilute nitride and bismide alloys of GaAs, materials of interest for optoelectronic devices. We find that while both nitrogen and bismuth incorporation reduces the fundamental energy bandgap of GaAs, bismuth does so without deteriorating the electrical properties whereas nitrogen severely reduces the electron mobility, limiting its usefulness in future devices. This is the first measurement of electron mobility in GaAsBi, and the results should have a significant impact on the optoelectronic device community.;The inherent sensitivity of the THz pulse to the conductivity of a material, sub-picosecond resolution, and noncontact nature make time-resolved terahertz spectroscopy an ideal technique for investigating carrier capture dynamics in semiconductor nanostructures. In this work, we demonstrate how THz pulses can be used to monitor this capture process directly in both quantum dot and quantum wire structures. We further show how the THz polarization can be used to probe a photoconductive anisotropy arising from a linear ordering of both quantum wire and dot-chain systems.;Finally, we investigate how the confinement of charge carriers influences the electrodynamics of silicon films by varying the degree of structural disorder. A transition from free to localized behaviour is observed from bulk, crystalline silicon to silicon nanocrystals embedded in glass. The transition from metal-to-insulator can be observed directly as a suppression of the low frequency real conductivity, and can be explained using a model based on carrier backscattering.;We show how TRTS can be used to extract the complex conductivity of a material induced by a femtosecond pump pulse, just picoseconds after excitation. A case study of a standard III-V semiconductor, GaAs, is presented to establish a baseline for TRTS in the Ultrafast Spectroscopy lab at the University of Alberta.