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We have measured the lifetimes of two zone-center longitudinal acousticphonon modes, at 320 GHz and 640 GHz respectively, in a 14 nm GaAs / 2 nm AlAssuperlattice structure. By comparing measurements at 296 K and 79 K we separatethe intrinsic contribution to phonon lifetime determined by phonon-phononscattering from the extrinsic contribution due to defects and interfaceroughness. At 296 K, the 320-GHz phonon lifetime has approximately equalcontributions from intrinsic and extrinsic scattering, whilst at 640 GHz it isdominated by extrinsic effects. These measurements are compared withfirst-principles lattice dynamics calculations of intrinsic and extrinsicscattering rates in the superlattice. The calculated room-temperature intrinsiclifetime of longitudinal phonons at 320 GHz is in agreement with theexperimentally measured value of 0.9 ns. The model correctly predicts thetransition from predominantly intrinsic to predominantly extrinsic scattering;however the predicted transition occurs at higher frequencies. Our analysisindicates that the 'interfacial atomic disorder' model is not entirely adequateand that the observed frequency dependence of the extrinsic scattering rate islikely to be determined by a finite correlation length of interface roughness.

Intrinsic to extrinsic phonon lifetime transition in a GaAs-AlAs superlattice