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 the 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.