Probing the phonon surface interaction by wave packet simulation: effect of roughness and morphology

TL;DR

This study uses atomistic wave packet simulations to analyze how phonons interact with different surface morphologies, revealing complex reflection behaviors that influence thermal transport in nanostructures.

Contribution

It provides detailed atomic-level insights into phonon-surface interactions, challenging existing models and highlighting the effects of surface roughness and amorphous layers.

Findings

Phonon reflection at smooth surfaces involves mode conversion and depends on surface condition.

Reflected wave distribution at rough surfaces deviates from Ziman's model with nonmonotonic behavior.

Amorphous layers cause diffusive phonon reflection and absorption.

Abstract

One way to reduce the lattice thermal conductivity of solids is to induce additional phonon surface scattering through nanostructures. However, how phonons interact with boundaries, especially at the atomic level, is not well understood. In this work, we performed two-dimensional atomistic wave packet simulations to investigate the phonon surface interaction. Emphasis has been given to the angular-resolved phonon reflection at smooth, periodically rough, and amorphous surfaces. We found that the acoustic phonon reflection at a smooth surface is not simply specular. Mode conversion can occur after reflection, and the detailed energy distribution after reflection will dependent on surface condition and polarization of incident phonon. At periodically rough surfaces, the reflected wave packet distribution does not follow the well-known Ziman's model, but shows a nonmonotonic dependence on the depth of surface roughness. When an amorphous layer is attached to the surface, the incident wave packet will be absorbed by the amorphous region, and results in quite diffusive reflection. Our results clearly show that the commonly used specular-diffusive model is not enough to describe the phonon reflection at a periodically rough surface, while an amorphous layer can induce strong diffusive reflection. This work provides a careful analysis of phonon reflection at a surface with different morphology, which is important to a better understanding of thermal transport in various nanostructures.