Nanoscale origin of thermal anisotropy in monoclinic \texorpdfstring{β-Ga2O3}{beta-Ga2O3}
Abstract
Monoclinic β-Ga2O3 is a promising ultra-wide-bandgap semiconductor for power electronics, but its low and anisotropic thermal conductivity limits heat removal, making it a compelling model system for nanoscale thermal transport studies. Here, we demonstrate a pronounced nanoscale enhancement of thermal anisotropy and uncover the underlying phonon mechanism by combining complementary thermal and acoustic spectroscopy techniques with first-principles calculations. Anisotropic thermoreflectance thermometry determines the full thermal-conductivity tensor from angular measurements within the (100), (010), and (001) crystallographic planes. Angle-resolved Brillouin light scattering reveals that low-frequency acoustic phonons exhibit only weak velocity anisotropy, insufficient to account for the measured thermal anisotropy. Frequency-resolved ab initio Green--Kubo calculations show that the bulk anisotropy pattern emerges only when the accumulated conductivity includes phonon modes above 4 THz, whereas the sub 3 THz acoustic contributions are nearly isotropic. Extreme-ultraviolet transient thermal grating spectroscopy reveals that reducing the grating period to 28 nm strongly enhances the in-plane thermal anisotropy, most markedly in the (001) plane, where it reaches nearly twice its macroscopic value. Reconstructed and calculated mean-free-path (MFP) accumulation functions link this enhancement to the stronger directional contrast of short-MFP phonons. Together, these results identify high-frequency, short-MFP phonons as the dominant origin of thermal anisotropy in β-Ga2O3, while low-frequency phonons attenuate the directional contrast at macroscopic scales.
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BibTeX
@misc{xu2026nanoscale,
title = {{Nanoscale origin of thermal anisotropy in monoclinic \texorpdfstring$β$-Ga$_2$O$_3$beta-Ga2O3}},
author = {Kai Xu and Shuo Zhao and Luca Sung-Min Choi and Nils Bernhardt and Pouria Emtenani and Felix Nippert and Moritz Meißner and Hans Tornatzky and Oliver Bierwagen and Piero Mazzolini and Riccardo Mincigrucci and Laura Foglia and Danny Fainozzi and Filippo Bencivenga and Zbigniew Galazka and George Fytas and Bartlomiej Graczykowski and Riccardo Rurali and Matthias Scheffler and Christian Carbogno and Juan Sebastian Reparaz and Markus R. Wagner},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.31976},
eprint = {2610.31976},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.31976},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Nanoscale origin of thermal anisotropy in monoclinic \texorpdfstring{β-Ga_2O_3}{beta-Ga2O3}
AU - Kai Xu
AU - Shuo Zhao
AU - Luca Sung-Min Choi
AU - Nils Bernhardt
AU - Pouria Emtenani
AU - Felix Nippert
AU - Moritz Meißner
AU - Hans Tornatzky
AU - Oliver Bierwagen
AU - Piero Mazzolini
AU - Riccardo Mincigrucci
AU - Laura Foglia
AU - Danny Fainozzi
AU - Filippo Bencivenga
AU - Zbigniew Galazka
AU - George Fytas
AU - Bartlomiej Graczykowski
AU - Riccardo Rurali
AU - Matthias Scheffler
AU - Christian Carbogno
AU - Juan Sebastian Reparaz
AU - Markus R. Wagner
PY - 2026
DA - 2026/10/22
PB - arXiv
DO - 10.48550/arxiv.2610.31976
UR - https://arxiv.org/abs/2610.31976
AB - Monoclinic β-Ga_2O_3 is a promising ultra-wide-bandgap semiconductor for power electronics, but its low and anisotropic thermal conductivity limits heat removal, making it a compelling model system for nanoscale thermal transport studies. Here, we demonstrate a pronounced nanoscale enhancement of thermal anisotropy and uncover the underlying phonon mechanism by combining complementary thermal and acoustic spectroscopy techniques with first-principles calculations. Anisotropic thermoreflectance thermometry determines the full thermal-conductivity tensor from angular measurements within the (100), (010), and (001) crystallographic planes. Angle-resolved Brillouin light scattering reveals that low-frequency acoustic phonons exhibit only weak velocity anisotropy, insufficient to account for the measured thermal anisotropy. Frequency-resolved ab initio Green--Kubo calculations show that the bulk anisotropy pattern emerges only when the accumulated conductivity includes phonon modes above 4 THz, whereas the sub 3 THz acoustic contributions are nearly isotropic. Extreme-ultraviolet transient thermal grating spectroscopy reveals that reducing the grating period to 28 nm strongly enhances the in-plane thermal anisotropy, most markedly in the (001) plane, where it reaches nearly twice its macroscopic value. Reconstructed and calculated mean-free-path (MFP) accumulation functions link this enhancement to the stronger directional contrast of short-MFP phonons. Together, these results identify high-frequency, short-MFP phonons as the dominant origin of thermal anisotropy in β-Ga_2O_3, while low-frequency phonons attenuate the directional contrast at macroscopic scales.
ER -