Surface phonons limit heat conduction in thin films

TL;DR

This study reveals that surface-localized phonons play a crucial role in limiting heat conduction in silicon thin films, with anharmonic coupling and low-frequency surface phonons significantly suppressing in-plane thermal conductivity.

Contribution

It provides the first detailed analysis of surface phonons' impact on heat conduction in sub-10-nm silicon thin films using anharmonic lattice dynamics calculations.

Findings

Surface phonons significantly suppress in-plane heat conduction.

Anharmonic coupling between surface and internal phonons is a key mechanism.

Low-frequency surface phonons contribute notably to thermal conductivity reduction.

Abstract

Understanding microscopic heat conduction in thin films is important for nano/micro heat transfer and thermal management for advanced electronics. As the thickness of thin films is comparable to or shorter than a phonon wavelength, phonon dispersion relations and transport properties are significantly modulated, which should be taken into account for heat conduction in thin films. Although phonon confinement and depletion effects have been considered, it should be emphasized that surface-localized phonons (surface phonons) arise whose influence on heat conduction may not be negligible due to the high surface-to-volume ratio. However, the role of surface phonons in heat conduction has received little attention thus far. In the present work, we performed anharmonic lattice dynamics calculations to investigate the thickness and temperature dependence of in-plane thermal conductivity of silicon thin films with sub-10-nm thickness in terms of surface phonons. Through systematic analysis of the influences of surface phonons, we found that anharmonic coupling between surface and internal phonons localized in thin films significantly suppresses overall in-plane heat conduction in thin films. We also discovered that specific low-frequency surface phonons significantly contribute to surface--internal phonon scattering and heat conduction suppression. Our findings are beneficial for the thermal management of electronics and phononic devices and may lead to surface phonon engineering for thermal conductivity control.