Experimental Evidence of Suppression of Subterahertz Phonons and Thermal Conductivity in GaAs/AlAs Superlattices Due to Extrinsic Scattering Processes

Abstract

We report on the lifetimes of zone-center longitudinal phonon modes in GaAs/AlAs superlattices with varying period thicknesses. By measuring the lifetimes at 78 K and room temperature, we separate the contributions of intrinsic and extrinsic contributions to the lifetime of a 223 GHz longitudinal phonon mode in a 12 nm × 12 nm period superlattice. The intrinsic lifetime that is determined by phonon–phonon scattering of the 223 GHz mode falls in a transition region between the Akhiezer relaxation and the anharmonic three-phonon scattering regimes. While the lifetime of the 223 GHz mode has contributions from both intrinsic and extrinsic processes at room temperature for the 12 nm × 12 nm period superlattice, lifetimes of phonon modes at 376 and 737 GHz for the 6 nm × 6 nm and 3 nm × 3 nm superlattices, respectively, are dominated by extrinsic contributions. This suggests that in thin period GaAs/AlAs superlattices the lifetimes of subterahertz acoustic phonons that could contribute substantially to the total thermal conductivity of the superlattices are mainly determined from extrinsic rather than phonon–phonon scattering processes. Along with lifetime measurements, we also report on the thermal conductivity of GaAs/AlAs superlattices with periods ranging from 12 nm × 12 nm to 1 nm × 1 nm, from 78 to 300 K as measured by time domain thermoreflectance. For all structures, thermal conductivities are independent of temperature and show a monotonic decrease in thermal conductivity with decreasing period thicknesses for the entire temperature range studied in this work, which suggests that scattering of vibrations at the internal boundaries rather than the sample boundaries dictates the overall thermal transport across these superlattices.