Thermoelectric properties of the bismuth telluride nanowires in the constant-relaxation-time approximation

TL;DR

This study investigates the thermoelectric properties of bismuth telluride nanowires, highlighting how quantum confinement and growth direction influence efficiency metrics like the Seebeck coefficient and figure of merit across various temperatures.

Contribution

It provides a detailed analysis of electronic structure and thermoelectric performance of bismuth telluride nanowires considering anisotropic effective mass and confinement effects, which was not comprehensively explored before.

Findings

Carrier confinement enhances the figure of merit ZT.

Maximum ZT values increase with decreasing nanowire thickness.

Confinement effects are more pronounced in the [015] growth direction.

Abstract

Electronic structure of bismuth telluride nanowires with the growth directions [110] and [015] is studied in the framework of anisotropic effective mass method using the parabolic band approximation. The components of the electron and hole effective mass tensor for six valleys are calculated for both growth directions. For a square nanowire, in the temperature range from 77 K to 500 K, the dependence of the Seebeck coefficient, the electron thermal and electrical conductivity as well as the figure of merit ZT on the nanowire thickness and on the excess hole concentration are investigated in the constant-relaxation-time approximation. The carrier confinement is shown to play essential role for square nanowires with thickness less than 30 nm. The confinement decreases both the carrier concentration and the thermal conductivity but increases the maximum value of Seebeck coefficient in contrast to the excess holes (impurities). The confinement effect is stronger for the direction [015] than for the direction [110] due to the carrier mass difference for these directions. The carrier confinement increases maximum value of ZT and shifts it towards high temperatures. For the p-type bismuth telluride nanowires with growth direction [110], the maximum value of the figure of merit is equal to 1.3, 1.6, and 2.8, correspondingly, at temperatures 310 K, 390 K, 480 K and the nanowire thicknesses 30 nm, 15 nm, and 7 nm. At the room temperature, the figure of merit equals 1.2, 1.3, and 1.7, respectively.