Optical properties of Bi2(Te1−xSex)3 thin films

Abstract

Using spectroscopic ellipsometry, the dielectric functions of a series of topological insulators, including Bi2Te3, Bi2Se3, and their ternary alloys, were determined. The ellipsometry measurements were obtained using an IR-spectroscopic ellipsometer, spanning a spectral range between 2000 and 35 000 nm. A standard inversion technique was used to model the ellipsometry spectra, which produced the dielectric functions of each of the topological insulator films. These dielectric functions were analyzed further to obtain characteristics such as their bandgap, carrier concentration, and effective mass. Specifically, Kramers–Kronig consistent oscillators were used to represent the Drude contribution as well as the energy gap. The authors found that the bandgap of Bi2(Te1−xSex)3 ternary alloys are larger than their binary constituents (i.e., Bi2Te3 and Bi2Se3). By exploring the temperature dependence of the bandgap, the authors deduced the electron–phonon coupling parameters for the Bi2(Te1−xSex)3 system, using the Bose–Einstein occupation distributions. Compared with Bi2Te3, the authors found that the electron–phonon coupling parameters of the ternary alloys of Bi2(Te1−xSex)3 are smaller.