There has been a growing interest in solid-state thermoelectric (TE) materials over the past few decades due to their applications in harvesting industrial waste heat. TE figure of merit (ZT), a dimensionless parameter commonly used to assess the efficiency for TE energy conversion, is inversely related to the thermal conductivity of material. In most TE materials, thermal transport is mainly contributed by phonons and the ZT can be largely improved by suppressing the lattice thermal transport, without degrading the electron transport. Along this line, phonon scattering at boundaries/interfaces is a common approach to dramatically reduce the lattice thermal conductivity. Confining phonons to nanosized structures/films nullifies their contribution to the thermal transport thereby decreasing the materials thermal conductivity. Knowledge of spectral phonon mean free paths is critical for evaluating the thermal conductivity reduction using classical phonon size effects. Numerous reports have been published that demonstrate lowering of thermal conductivity in conventional TE materials. Despite the thermal conductivity reduction, the ZT for existing TE materials is still subpar to be considered for solid-state TE power harvesting devices. Using the nanostructuring approach, bismuth telluride (Bi~2~Te~3~) has shown remarkable improvements in the ZT value. The electronic properties for Bi~2~Te~3~ are comparatively straightforward to measure and have been extensively reported but current understanding of thermal properties is still lacking. This is partly due to limited reports on accurate thermal conductivity measurements and challenges in preparing high-quality single-crystal Bi~2~Te~3~ thin films. In this regard, the present work is geared towards understanding the room temperature thermal transport in Bi~2~Te~3~ thin films. High-quality Bi~2~Te~3~ thin films have been prepared using chemical vapor deposition on a SiO2/Si substrate. Thickness-dependent thermal conductivity has been studied with the aim of assessing spectral phonon mean free paths in the material. The through-thickness thermal conductivities of Bi~2~Te~3~ thin films with thicknesses ranging from 20 nm – 300 nm has been experimentally measured using the time domain thermoreflectance (TDTR) method, which is a widely accepted and accurate method for thermal conductivity characterization of thin films. The thickness of the films has been confirmed using the atomic force microscopy. TDTR experiments have been performed at two different modulation frequencies to decouple the effect of the interfacial conductance. The measured room-temperature thermal conductivity values stay close to the bulk value of 0.45 W m^-1^ K^-1^ for all the thin films, suggesting that mean free path of most phonons is smaller than 20 nm, which agrees well with the first-principles predictions. This result sheds light on potential thermal conductivity reduction in Bi~2~Te~3~ thin films as TE materials.


Through-Thickness Thermal Conductivity of Bi2Te3 Thin Films Grown by Chemical Vapor Deposition

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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