Measurements and numerical calculations of thermal conductivity to evaluate the quality of β-gallium oxide thin films grown on sapphire and silicon carbide by molecular beam epitaxy

We report a method to obtain insight into lower thermal conductivity of β-Ga2O3 thin films grown by molecular beam epitaxy (MBE) on c-plane sapphire and 4H-SiC substrates. We compare experimental values against the numerical predictions to decipher the effect of boundary scattering and defects in thin-films. We used time domain thermoreflectance to perform the experiments, density functional theory and the Boltzmann transport equation for thermal conductivity calculations, and the diffuse mismatch model for thermal boundary conductance predictions. The experimental thermal conductivities were approximately three times smaller than those calculated for perfect Ga2O3 crystals of similar size. When considering the presence of grain boundaries, gallium and oxygen vacancies, and stacking faults in the calculations, the crystals that present around 1% of gallium vacancies and a density of stacking faults of 106 faults/cm were the ones whose thermal conductivities were closer to the experimental results. Our analysis suggests the level of different types of defects present in the Ga2O3 crystal that could be used to improve the quality of MBE-grown samples by reducing these defects and, thereby, produce materials with higher thermal conductivities.