Phonon transport in Al-rich Al_xGa_1−xN thin films

Al_xGa_1−xN with a high Al composition (x) presents significant potential for advancing next-generation high-power electronic devices. To support the thermal design of Al_xGa_1−xN-based electronics, the thermal conductivity of Al_xGa_1−xN thin films was measured as a function of Al composition, temperature, and film thickness using time-domain thermoreflectance and frequency-domain thermoreflectance techniques. The measurement results were interpreted by modeling phonon transport in Al_xGa_1−xN films using the phonon Boltzmann transport equation. Phonon properties, including frequencies, group velocities, and lifetimes, were calculated using a virtual crystal approximation, with the effects of mass-disorder scattering incorporated via the Tamura model. The measured thermal conductivity of Al_0.7Ga_0.3N is an order of magnitude lower than those for GaN and AlN, exhibits an increase followed by saturation with temperature, and shows a modest decrease with a reduction in the film thickness. The modeling results agree with the measurement results and reveal that mass-disorder scattering and phonon-boundary scattering are the dominant mechanisms that reduce the thermal conductivity of Al_xGa_1−xN thin films.