Experimental observation of ballistic to diffusive transition in AlN thin films

TL;DR

This study experimentally observes the transition from ballistic to diffusive thermal transport in AlN thin films, revealing how phonon scattering mechanisms change with film thickness and impacting nanoscale heat conduction understanding.

Contribution

It provides the first experimental evidence of ballistic to diffusive transition in AlN thin films across a wide thickness range, highlighting phonon behavior changes.

Findings

Ballistic transport characterized by constant thermal resistance with thickness.

Phonon lifetimes increase significantly in the diffusive regime.

Optical phonon lifetimes remain nearly constant after transition.

Abstract

Bulk AlN possesses high thermal conductivity 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. The lifetime of the optical phonons increases by more than an order of magnitude in the diffusive regime, however, remains nearly constant afterwards. Our study is important for understanding the details of nano and microscale thermal transport in a highly conductive material.