Experimental observation of ballistic to diffusive transition in phonon thermal transport of AlN thin films

Ultrawide bandgap semiconductor aluminum nitride (AlN) possesses high thermal conductivity in bulk form due to long phonon mean-free-paths, high group velocity, and long lifetimes. However, the thermal transport scenario becomes very different in a thin AlN film due to phonon-defect and phonon-boundary scattering. Herein, we report experimental observation of ballistic to diffusive transition in a series of AlN thin films (1.6-2440 nm) grown on sapphire substrates. The ballistic transport is characterized by constant thermal resistance as a function of film thickness due to phonon scattering by defects and boundaries. In this transport regime, phonons possess very small group velocities and lifetimes. A gradual increase in the optical phonon lifetime is observed in the diffusive regime. Our study will aid the incorporation of highly conductive thin film materials in the semiconductor production line for efficient thermal management.