Effect of Deviation from Stoichiometry on Thermoelectric Properties of Bi2Te3 Polycrystals and Thin Films in the Temperature Range 77-300 K

Abstract

Bi2Te3 semiconductor compound and Bi2Te3-based solid solutions are presently among the best lowtemperature thermoelectric materials. One of the methods of controlling the conductivity type and properties of Bi2Te3 is changing the stoichiometry of this compound. Earlier, we have obtained the room-temperature dependences of mechanical and thermoelectric properties of Bi2Te3 polycrystals on the degree of deviation from stoichiometry. The goal of this work is to investigate the behavior of such dependences at other temperatures. Bismuth telluride polycrystals with compositions in the range of 59.6-67.5 at. % Te were obtained, and for all the crystals the Seebeck coefficient, the Hall coefficient, electrical conductivity and charge carrier mobility were measured in the temperature range 77-300 K. On the basis of the temperature dependences, the isotherms of kinetic coefficients were plotted. It was found that similar to the room-temperature isotherms, the isotherms at lower temperatures were non-monotonic: they exhibited inversion of the conductivity sign between 60.5 and 61.0 at. % Te and extrema near 60.0 and 63.0 at. % Te. The experimental data are interpreted taking into account changes in the band and defect structures of Bi2Te3 under varying stoichiometry. The obtained results make it possible to control thermoelectric properties of Bi2Te3 polycrystals in the temperature range 77-300 K by changing the degree of deviation from stoichiometry.