Influence of Highly Non-Equilibrium Electron-Phonon Transport on Heat Conduction across Metal-Insulator Superlattice

Nanostructuring is often utilized to improve figure-of-merit of thermoelectric materials by reducing thermal conductivity (TC). There, thermal boundary conductance at nanostructure interfaces is important, and population of heat carriers can be highly non-equilibrium. One strategy to further reduce TC is to utilize metal–semiconductor interfaces, such as in metal-semiconductor superlattice with the electron-phonon scattering mean free path being comparable or longer than the nanostructure length-scale. There, the nonequilibrium interfacial regions are expected to make phonons the dominant heat carrier even in the metal part, which should contribute to the reduction of TC. In this study, we take metal-insulator(MgO) superlattice as a model case and measure the TC by the time-domain thermoreflectance method. We prepare two kinds of samples with different metal layers (AuSi or Ta) with relative difference in the electron-phonon coupling strength. As a result, for both kinds of samples, TC of superlattice was reduced as the unit-layer of the superlattice was made thinner. While the reduction of TC is expected due to the increase in the number of interfaces, the different trends of reduction between AuSi and Ta samples suggest roles of non-equilibrium phonon-electron transport.