Phonon Dominated Heat Conduction Normal to Mo/Si Multilayers with Period below 10 nm

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Abstract
Thermal conduction in periodic multilayer composites can be strongly influenced by nonequilibrium electron–phonon scattering for periods shorter than the relevant free paths. Here we argue that two additional mechanisms—quasiballistic phonon transport normal to the metal film and inelastic electron-interface scattering—can also impact conduction in metal/dielectric multilayers with a period below 10 nm. Measurements use the 3ω method with six different bridge widths down to 50 nm to extract the in- and cross-plane effective conductivities of Mo/Si (2.8 nm/4.1 nm) multilayers, yielding 15.4 and 1.2 W/mK, respectively. The cross-plane thermal resistance is lower than can be predicted considering volume and interface scattering but is consistent with a new model built around a film-normal length scale for phonon–electron energy conversion in the metal. We introduce a criterion for the transition from electron to phonon dominated heat conduction in metal films bounded by dielectrics.

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