Optical investigation of bismuth selenide (Bi2Se3) and antimony telluride (Sb2Te3).
Date
2024
Authors
Mulaye, Dorcas
Journal Title
Journal ISSN
Volume Title
Publisher
The University of Zambia
Abstract
This dissertation is devoted to the investigation of the optical properties of two topological insulators, Bismuth Selenide (Bi2Se3) and Antimony Telluride (Sb2Te3), by optical spectroscopy reflection measurements in a wide frequency range (500 – 40000 cm−1 ) and temperature (13 to 300 K) . Utilizing Kramers-Kronig analysis, complex conductivity is calculated from which other optical properties are obtained. The resistivity of both samples demonstrates metallic behavior with residual resistivity ratios of 1.8 for Bi2Se3 and 7.5 for Sb2Te3, indicating purer samples. From the study it is found that near-infrared optical conductivity for Bi2Se3 is dominated by bulk carriers and shows a linear-in-frequency increase in the frequency range from 1500 to 3000 cm−1 , reflecting transitions between bands with complex dispersion. This linearity might be interpreted as a signature of three-dimensional (bulk) Dirac bands. However, no linearity is observed in Sb2Te3. The Drude like contribution at low frequencies may be a signature of surface states contribution to the conductivity. Spectral weight analysis offers insights into the distribution of optical weight with temperature and frequency, revealing the influence of correlation effects and charge excitations on low-energy electrodynamics. Reflectivity measurements on the Bi2Se3 sample crystal reveal insights into its metallic behavior, with a plasma-like feature attributed to Dirac surface states. Direct band gaps and interband transitions characterize its optical properties, with certain transitions exhibiting temperature dependence. Conductivity analysis provides information about charge carrier behavior and scattering mechanisms, revealing the influence of electronphonon coupling and electronic band structure on charge transport. Similarly, temperature-dependent trends in reflectivity for Sb2Te3 unveil complex spectral weight transfers and interband transitions, challenging conventional Drude theory predictions. Conductivity analysis shows conductivity decreasing at low frequencies due to phonon scattering and increasing at higher frequencies accompanied by the emergence of a Near Infra-Red (NIR) band.
Description
Thesis of Master of Science in Physics.