Long mean free paths of room-temperature THz acoustic phonons in a high thermal conductivity material

Abstract

We report measurements of room-temperature mean free paths of long-lived THz acoustic phonons in wurtzite GaN. Longitudinal phonon wave packets are excited and probed by femtosecond laser pulses in two InGaN-GaN multiple quantum well structures separated by a GaN layer. By measuring the temperature dependence of the phonon attenuation in the range 80--300 K we isolate the intrinsic phonon mean free path at 300 K which is found to be 5.3 and 3.5 \ensuremath{\mu}m at 1.06 and 1.43 THz, respectively. The measurements are found to be in good agreement with ab initio calculations which show that the main channel of the acoustic phonon decay is a three-phonon scattering process involving the acoustic phonon and two high-frequency optical phonons. Our results indicate that the contribution of low-THz acoustic phonons to thermal transport in GaN is relatively smaller than in Si; thus finite size effects are expected to be less important in GaN.