Through-Thickness Thermal Conductivity of Chemical Vapor Deposited Bismuth Telluride Thin Films

There has been a growing interest in solid-state thermoelectric (TE) materials over the past few decades due to their efficient waste heat harvesting. Thermoelectric figure of merit (ZT), a dimensionless parameter commonly used to assess efficiency, is inversely related to thermal conductivity of material. In most TE materials, thermal transport is generally governed by phonons and ZT is thus engineered by controlling lattice thermal transport, without degrading electron transport. Phonon scattering at boundaries is a common approach to limit lattice thermal conductivity. Confining phonons to nanosized structures/films nullify their contribution to thermal transport thereby decreasing the materials thermal conductivity. Knowledge of phonon mean free path is critical for evaluating the thermal conductivity reduction using phonon scattering. Several 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. Among the novel TE materials, bismuth telluride (Bi₂Te₃) has shown remarkable improvements in the ZT value, much higher than the conventional materials. The electronic properties for Bi₂Te₃ 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 measurement and challenges in depositing high quality Bi₂Te₃ thin films. In this regard, the present work is geared towards understanding the room temperature thermal transport in Bi₂Te₃ thin films. High-quality Bi₂Te₃ thin films have been prepared using chemical vapor deposition on a silicon substrate. Thickness dependent thermal conductivity has been studied with the aim of assessing phonon mean free paths in the material. The through-thickness thermal conductivities of Bi₂Te₃ 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 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-principle based predictions. This result sheds light on potential thermal conductivity reduction in Bi₂Te₃ thin films as thermoelectric materials.