Phase-Dependent Phonon Heat Transport in Nanoscale Gallium Oxide Thin Films.

Different phases of Ga 2 O 3 have been regarded as superior platforms for making new-generation high-performance electronic devices. However, understanding of thermal transport in different phases of nanoscale Ga 2 O 3 thin-films remains challenging, owing to the lack of phonon transport models and systematic experimental investigations. Here, thermal conductivity (TC) and thermal boundary conductance (TBC) of the ( 1 ¯ 010 ) $( {\bar 1010} )$ α-, ( 2 ¯ 01 ) $( {\bar 201} )\;$ β-, and (001) κ-Ga 2 O 3 thin films on sapphire are investigated. At ≈80 nm, the measured TC of α (8.8 W m -1 K -1 ) is ≈1.8 times and ≈3.0 times larger than that of β and κ, respectively, consistent with model based on density functional theory (DFT), whereas the model reveals a similar TC for the bulk α- and β-Ga 2 O 3 . The observed phase- and size-dependence of TC is discussed thoroughly with phonon transport properties such as phonon mean free path and group velocity. The measured TBC at Ga 2 O 3 /sapphire interface is analyzed with diffuse mismatch model using DFT-derived full phonon dispersion relation. Phonon spectral distribution of density of states, transmission coefficients, and group velocity are studied to understand the phase-dependence of TBC. This study provides insight into the fundamental phonon transport mechanism in Ga 2 O 3 thin films and paves the way for improved thermal management of high-power Ga 2 O 3 -based devices.