Boundary conditions dictate frequency dependence of thermal conductivity in silicon

TL;DR

This study reveals that boundary conditions at interfaces significantly influence the frequency dependence of thermal conductivity in silicon, with non-equilibrium phonon effects observable under specific interface and temperature conditions.

Contribution

It demonstrates how boundary conditions and interfacial properties dictate the frequency dependence of silicon's thermal transport, highlighting the role of non-equilibrium phonons.

Findings

Frequency dependence appears in Al/Si with sharp interfaces.

Interfacial scattering destroys frequency dependence at room temperature.

Reduced phonon scattering at low temperature reestablishes frequency dependence.

Abstract

Non-Fourier thermal transports have drawn significant attention for decades. Among them, the frequency dependent thermal conductivity has been extensively explored by pump-probe techniques, such as time-domain thermoreflectance, which is employed to probe the spectra of phonon mean free paths. However, previous studies on silicon have not exhibited apparent frequency dependence despite its broad phonon distribution. Here, we report the frequency dependent thermal transport in Al/Si with an atomically sharp interface, where the matched Debye temperatures preserve non-equilibrium between low- and high-energy phonons in Si. The dependence vanishes in Al/SiO$_2$/Si at room temperature, since the SiO$_2$ interlayer facilitates phonon scattering and destroys thermal non-equilibrium. At 80 K, frequency dependence reemerges in Al/SiO$_2$/Si, due to reduced interfacial phonon scattering. Our findings highlight the significance of boundary conditions in frequency dependent thermal conductivity.