Phonon mode contributions to thermal conductivity of pristine and defective β-Ga₂O₃

β-Ga2O3 is emerging as a promising semiconductor for high-power high-frequency electronics. The low thermal conductivity of pristine β-Ga2O3 and the presence of defects, which can further suppress the thermal conductivity, will result in critical challenges for the performance and reliability of β-Ga2O3-based devices. We use first-principles density functional theory (DFT) along with the Boltzmann transport equation (BTE) to predict the phonon transport properties of pristine and defective β-Ga2O3. Our predictions of anisotropic thermal conductivity are in good agreement with the experimental results. We find that the low-frequency optical phonon modes make a significant contribution to the thermal conductivity compared to the acoustic modes, especially in the [010] direction because of the non-negligible group velocities of the low-frequency optical branches. To better understand the influence of defects on the phonon transport mechanism, we investigate the thermal conductivity of β-Ga2O3 with 1-2% oxygen or gallium vacancies considering the defect-induced phonon scatterings. We observe that the Ga vacancies lead to a larger suppression in the thermal conductivity than in those with O vacancies. Furthermore, we find that the vacancies have more influence on the optical modes than on acoustic modes, which suppress the contribution of optical modes to the thermal conductivity. The results from this work will help us understand the mechanism of phonon transport considering the influence of defects and provide insights for the future design of β-Ga2O3-based electronic devices.