Thin Films of Bismuth-Telluride-Based Alloys

Abstract

In this chapter, thermoelectric thin films of bismuth-telluride-(Bi2Te3)-based alloys are presented. Bi2Te3-based alloys have been developed in the 1950s and are promising materials owing to the highest thermoelectric performances near 300 K. These alloys originally exhibit good thermoelectric characteristics such as high carrier mobilities and low lattice thermal conductivities. The conduction type of Bi2Te3 can be tuned by adding dopants or other elements. In addition, this material satisfies all of the requirements for a thermoelectric material, such as a simple control of the carrier concentration and simplicity to obtain an optimal carrier concentration. Typical alloys are bismuth antimony telluride ((Bi1-xSbx)2Te3) (p-type) and bismuth selenium telluride (Bi2(Se1-xTex)3) (n-type). In general, the thermoelectric performance of a material is evaluated by the dimensionless figure of merit, ZT = σS2T/κ, where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity. This material system is the only thermoelectric material system whose ZT is larger than 1.0 near room temperature. In this regard, various studies have been performed on crystal materials (single-crystal, polycrystal), powdered sintered materials (cold-press sintered, hot-pressed, spark plasma sintered), thin-film materials, etc. The thin-film methods provided several favorable features compared to the bulk methods. For example, it is possible to reduce the device size and manufacturing cost. In addition, the thin-film technology can easily incorporate nanoscale effects such as a reduction in thermal conductivity due to phonon scattering and increase in Seebeck coefficient due to the reduced dimensions. The structural and thermoelectric properties of the thin films differ according to the deposition method. We present the characteristics of each deposition method. The deposited thin films are used for the development of thin-film thermoelectric generators. The structures and performances of the thin-film generators are presented. It is to be noted that all the thermoelectric and transport properties in this chapter were measured at room temperature.