Resistive and ballistic phonon transport in β-Ga₂O₃

The anisotropic thermal conductivity and the phonon mean free path (mfp) in monoclinic β-Ga₂O₃ single crystals and homoepitaxial films of several μm were determined using the 3ω-method in the temperature range from 10K-300 K. The measured effective thermal conductivity of both, single crystal and homoepitaxial films are in the order of 20 W/(mK) at room temperature, below 30 K it increases with a maximum of 1000 to 2000 W/(mK) and decreases with T³ below 25 K. Analysis of the phonon mfp shows a dominance of phonon-phonon-Umklapp scattering above 80 K, below which the influence of point-defect scattering is observed. Below 30 K the phonon mfp increases until it is limited by the total β-Ga₂O₃ sample size. A crossover from resistive to ballistic phonon transport is observed below 20 K and boundary effects of the total sample size become dominant. This reveals that the homoepitaxial film-substrate interface is highly phonon-transparent. The resistive and ballistic phonon transport regimes in β-Ga₂O₃ are discussed corresponding to the models of Callaway and Majumdar, respectively.