Nearly Perfect Crystal Orientation of Nanocrystalline Bismuth Telluride Thin Films Deposited by Pressure-Gradient Sputtering and Their Thermal Transport Properties

Bismuth telluride (Bi2Te3) is a thermoelectric material that exhibits excellent thermoelectric properties primarily because of its low thermal conductivity. The ideal structure of Bi2Te3 contains nanocrystals with a high crystal orientation. However, achieving both nanocrystallization and a high crystal orientation is challenging. Furthermore, experimental analyses of thermal transport properties, namely the sound velocity, lattice thermal conductivity, and phonon mean free path (MFP) are limited. In this study, Bi2Te3 thin films were deposited using pressure-gradient sputtering (PGS), and their thermal transport properties were determined. These films exhibited a crystallite size of 23.0 nm and an F value of 0.97, indicating a nearly perfect crystal orientation. The average sound velocity of 2046 m/s, in-plane lattice thermal conductivity of 0.66 W/(m·K), and phonon MFP of 0.37 nm were determined using nanoindentation, the 3ω method, and a combination of both of these methods, respectively. The dimensionless figures of merit of the Bi2Te3 thin films were 1.3 × 10−1 and 1.0 × 10−1 in the in-plane and cross-plane directions, respectively. The PGS system is useful for the fabrication of high quality thermoelectric materials, and the analysis method that combines the 3ω method and nanoindentation provides a detailed estimation of their thermal transport properties.