First-principles study of the effects of biaxial tensile strain and size confinement on phonon transport in <i>β</i> -Ga2O3

β-Ga2O3 is a promising ultrawide-bandgap semiconductor for high-power and optoelectronic applications; yet, its intrinsically low thermal conductivity poses challenges for heat dissipation. To clarify how strain and size effects govern phonon transport, we investigate bulk β-Ga2O3 under biaxial tensile strain and finite-size confinement using first-principles calculations and the phonon Boltzmann transport equation. We show that biaxial tensile strain softens acoustic and optical phonon branches near the Γ point and can slightly increase the group velocity of long-wavelength acoustic modes at low strain. However, biaxial tensile strain ultimately reduces lattice thermal conductivity, consistent with enhanced intrinsic anharmonic phonon–phonon scattering, which generally shortens phonon mean free paths, even in the infinite-size limit. Phonon accumulation spectra reveal that heat conduction is dominated by low-frequency modes below 3 THz with mean free paths below 100 nm, and that biaxial strain alters how phonons with different mean free paths contribute to heat transport, showing non-monotonic behavior at low strain but an overall shift toward shorter mean free paths at higher strain. When boundary scattering is introduced (L = 1 μm), long-mean free path phonons are further suppressed, leading to stronger size sensitivity in strained systems. Mode-dependent Grüneisen parameters confirm that strain-enhanced anharmonicity is concentrated in the low- and mid-frequency acoustic modes that control heat transport, providing a microscopic origin for the observed mean free path redistribution.