Boundary conditions dictate frequency dependence of thermal conductivity in silicon

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 the temperature difference between low- and high-energy phonons in Si and contribute as additional non-equilibrium thermal resistance. The dependence vanishes in Al/SiO2/Si at room temperature, since the SiO2 interlayer facilitates phonon scattering and destroys thermal non-equilibrium. At 80 K, frequency dependence reemerges in Al/SiO2/Si due to reduced interfacial phonon scattering, which is not sufficient to destroy the temperature difference between low- and high-energy phonons. The frequency dependence is weakened in the Al/Si sample at 500 K, originating from the enhanced phonon scattering rate in Si. Our findings highlight the significance of boundary conditions in frequency-dependent thermal conductivity.