Heat conduction below diffusive limit in amorphous superlattice structures

Advanced means to further reduce the thermal conductivity of dense amorphous materials by nanostructuring is important for thermal management in future electronic and optical devices. While the works so far have realized the reduction by scattering propagons, here, we demonstrate that nanostructures can in addition inhibit transport of diffusons, by measuring the thermal conductivity of amorphous-silicon/amorphous-silica superlattices with a periodicity ranging from 4.5 nm to 29.3 nm at room temperature. The measurements show that thermal conductivity of the superlattice with periodicity below 9.5 nm is significantly below the amorphous diffusive limit (similar to 1 W/m K). The measured thermal conductivity can be reproduced using a combination of the phonon-gas kinetics model and Allen-Feldmann theory without any fitting parameters, with the interfacial transmittance of propagons and diffusons obtained by atomistic Green's function. In addition to the known reduction of propagon transport, the effective mean free paths of diffusons are reduced from that of the bulk (1-3 nm) to several angstroms or even atomistic by extreme boundary scattering at the interfaces in the superlattice, giving rise to a significant tunability of the amorphous thermal conductivity using truly nanoscale structures. The influence of interface scattering on the diffusion transport can be effectively described by the Boltzmann transport equation, and in this sense, propagons and diffusons do not fundamentally differ. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.