Ultrafast Time-Resolved Terahertz Spectroscopy of Charge Carrier Dynamics in Semiconductors

Abstract

In this thesis, we have applied time-domain THz spectroscopy (THz-TDS) and time-resolved THz spectroscopy (TRTS) techniques to study various semiconductor materials with potential applications in thermoelectrics, high-energy radiation-detection, and optoelectronics. These applications rely on the nature and the dynamics of the photoexcited states of the semiconductors. Ultrafast broadband time-resolved THz spectroscopy is a unique noncontact technique to probe the photoexcited states of such semiconductors. TRTS can unambiguously probe the nature of the charge carriers and their dynamics, phonon dynamics, carrier-phonon interactions, and determines relevant semiconductor properties such as carrier lifetime, mobility, and diffusion length. In a typical TRTS study, a photoexcited sample is probed using time-delayed broadband THz pulses. The real time photo-induced change in the complex dielectric function contains all important informations.

The TRTS study on Thallium Halide (TlX, X=Br, I) nanocrystals and bulk films suggests the possible defect-tolerant nature of this class of semiconductors, having electronic band structures very similar to the metal halide perovskites. We have also compared the charge carrier dynamics of NCs film and bulk film. Semiconductors with intrinsically ultralow thermal conductivity are of great importance in thermoelectrics. The TRTS study was carried out on such semiconductors, TlSe and InSe. Our time-resolved THz studies on TlSe and InSe unravel their carrier dynamics, which are largely governed by bulk and surface defect states. Also, the possible role of low-energy phonon modes on carrier dynamics has been investigated. The TRTS study of few-layer Black Phosphorus (FLBP) quantum dot (QD) films suggests almost wavelength-independent behaviour of carrier dynamics due to bulk-like phase formation after drop casting QD solution on a non-conducting substrate.