Ultrafast spectroscopy studies of topological insulators and transparent conductive oxides

Abstract

Ultrafast optical spectroscopy, which is based on the development of the femtosecond laser, is an excellent tool for studying the electronic and vibrational properties of many solid-state systems. In this thesis, ultrafast optical pump-optical probe (OPOP) technique and terahertz time-domain spectroscopy (THz-TDS) have been used to study topological insulators (TIs) and transparent conductive oxides (TCOs). As a TI, Bi1.5Sb0.5Te1.8Se1.2 (BSTS) is studied by OPOP in my Ph.D. work. The dynamics of carriers and phonons in BSTS at temperature from 10-300 K are revealed. A fast relaxation process of carriers has been found, which is the relaxation of electrons from higher energy level to the bottom of conduction band. Such process could be described by the two-temperature model, which is based on the electron- phonon interaction. Moreover, the temperature-dependent behavior of coherent optical phonons could be explained by the anharmonic effect. Furthermore, the optical penetration depth is obtained from the study of coherent acoustic phonon. Meanwhile, the validity of two-temperature model in BSTS makes it a good tool to study similar materials. THz-TDS studies of two TCOs, Ta-doped TiO2 and amorphous In2O3 (a-In2O3), will be introduced in this thesis. (1) In the study Ta-doped TiO2, conductivities of three samples with different doping concentration from 10-300 K were obtained. According to the temperature-dependence of THz conductivity, the nature of carriers in Ta-doped TiO2 is identified as interacting polaron gas, rather than non-interacting electrons. The electron-phonon interaction in these samples is found to be highly dependent on the doping concentration. Furthermore, the results indicate that the high conductivity of Ta-doped TiO2 does not just originate from the high carrier density, but also its low electron-phonon coupling. (2) In the research of a-In2O3, the conductivities of three samples grown at different temperatures are obtained. According to the Drude model fitting of the conductivities, carrier density and mobility are obtained, which show good electronic performance that comparable with crystalline In2O3 . Moreover, the mobility and carrier density of a-In2 O3 is highly dependent on the growth temperature. Finally, in a-In2O3, the mobility obtained from AC measurement is found to be different from Hall effect (a DC measurement), which might come from its amorphous nature.