Understanding the mechanism of diffuse phonon scattering at disordered surfaces by atomistic wave-packet investigation

Phonon surface scattering is of great importance for understanding thermal transport in nanostructured materials and has been widely utilized to tailor thermal properties. However, the current understanding of phonon surface scattering is largely based on the Ziman's formula which was derived at the continuum limit of the scalar wave equation and, thus, ignoring the atomistic information of the surface. In this work, we applied the atomistic phonon wave-packet simulations to study the impact of surface amorphization and surface roughness on phonon reflection. We found that for both types of surfaces, the energies of the specularly reflected components decrease with increasing wavelength. However, the underlying reflection mechanisms are different. For the amorphous surface, the energy of the incident wave packet will first enter the amorphous layer before being partially and diffusely scattered in this layer. The local density and elastic moduli fluctuations as well as the localized vibrational modes are reasons for the diffuse scattering in the amorphous region, which affects the short wavelength phonons more severely. For the rough surface, the diffuse reflection is the combined effect from surface irregularity, surface Rayleigh waves, and spatially localized modes induced by the surface roughness. The extent of diffuse reflection at the rough surface can be tuned by engineering the surface boundary condition.