  We report a method to obtain insights into lower thermal conductivity of
\beta-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 (TDTR) to perform the
experiments, density functional theory and the Boltzmann transport equation for
thermal conductivity calculations, and the diffuse mismatch model for TBC
predictions. The experimental thermal conductivities were approximately 3 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.
