Effect of Initial Bulk Material Composition on Thermoelectric Properties of Bi₂Te₃ Thin Films

Abstract

V₂VI₃ compounds and solid solutions based on them are known to be the best low-temperature thermoelectric (TE) materials. The predicted possibility of enhancement of the TE figure of merit in two-dimensional (2D) structures has stimulated studies of the properties of these materials in the thin-film state. The goal of the present work is to study the dependences of the Seebeck coefficient S, electrical conductivity σ, Hall coefficient R _H, charge carrier mobility μ _H, and TE power factor P = S ² σ of Bi₂Te₃ thin films on the composition of the initial bulk material used for preparing them. Thin films with thickness d = 200 nm to 250 nm were grown by thermal evaporation in vacuum of stoichiometric Bi₂Te₃ crystals (60.0 at.% Te) and of crystals with 62.8 at.% Te onto glass substrates at temperatures T _S of 320 K to 500 K. It was established that the conductivity type of the initial material is reproduced in films fairly well. For both materials, an increase in T _S leads to an increase in the thin-film structural perfection, better correspondence between the film composition and that of the initial material, and increase in S, R _H, μ _H, σ, and P. The room-temperature maximum values of P for the films grown from crystals with 60.0 at.% and 62.8 at.% Te are P = 7.5 × 10⁻⁴ W/K² m and 35 × 10⁻⁴ W/K² m, respectively. Thus, by using Bi₂Te₃ crystals with different stoichiometry as initial materials, one can control the conductivity type and TE parameters of the films, applying a simple and low-cost method of thermal evaporation from a single source.