Probing phonon–surface interaction by wave-packet simulation: Effect of roughness and morphology

Abstract

One way to reduce the lattice thermal conductivity of solids is to induce additional phonon–surface scattering through nanostructures. However, the way in which phonons interact with surfaces, especially at the atomic level, is not well understood at present. In this work, we perform two-dimensional atomistic wave-packet simulations to investigate angular-resolved phonon reflection at a surface. Different surface morphologies, including smooth surfaces, periodically rough surfaces, and surfaces with amorphous coatings, are considered. For a smooth surface, mode conversion can occur after reflection, with the resulting wave-packet energy distribution depending on the surface condition and the polarization of the incident phonon. At a periodically rough surface, the reflected wave-packet distribution does not follow the well-known Ziman model but shows a nonmonotonic dependence on the depth of the surface roughness. When an amorphous layer is attached to a smooth surface, the incident wave packet is absorbed by the amorphous region and is then reflected diffusively at the surface. Our results show that the commonly adopted specular-diffusive model is insufficient to describe phonon reflection at a periodically rough surface and that an amorphous layer can induce strong diffusive reflection. This work provides a comprehensive analysis of phonon reflection at different types of surfaces, which is important for better understanding of thermal transport in various nanostructures.