Length‑Dependent Electron-Phonon Nonequilibrium Thermal Resistance in Metal-Insulator Superlattices

When heat flows across a metal-insulator interface, it must be transferred between electrons and phonons at a certain length scale that depends on the electron-phonon coupling characteristics. This nonequilibrium between electrons and phonons gives rise to thermal resistance, in addition to the native resistance owing to interface scattering. The electron-phonon nonequilibrium effect on heat conduction can become particularly significant in nanostructures with distances between metal and insulator interfaces smaller than or comparable to the nonequilibrium length scale. A metal-insulator superlattice is an ideal structure for magnifying and investigating the electron-phonon nonequilibrium effect because the interface distance can be tuned at the nanoscale. In this study, the thermal conductivities of metal-MgO superlattices were measured using time-domain thermoreflectance (TDTR) method and analyzed using a two-temperature model (TTM). Two types of superlattices with different metals, gold silicon (AuSi) and tantalum (Ta), with relatively weak and strong electron-phonon coupling, respectively, were adopted, and the metal layer thickness was varied from 3 to 15 nm while maintaining a constant total interface density. Consequently, the thermal conductivity of the AuSi-MgO superlattice significantly decreased with increasing metal layer thickness, whereas that of Ta-MgO remained invariant, reflecting the stronger electron-phonon nonequilibrium effect in the former weaker coupling case. Fitting the measurement results with the TTM quantifies the thermal resistance owing to the electron-phonon nonequilibrium effect and its length scale.