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

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.

Evaluating Broader Impacts of Nanoscale Thermal Transport Research

“…Beyond the thermal conductivity, evaluation and understanding of the interfacial thermal resistance between the adjacent layers are often a limiting factor for reducing device temperatures. Thermoreflectance characterization of metal-dielectric interfaces has demonstrated the complexity of the electron-phonon interactions at the interfaces [159]. Specifically, at these interfaces, the electrons and phonons can depart significantly from equilibrium, a situation that can be resolved in some cases using a twotemperature model [160].…”

Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization

“…The discovered phenomena imply Brillouin zone folding of acoustic phonon modes, phonon confinement, vibrational specific heat C and vibrational entropy S, which are connected to g(E) by well-known thermodynamic relations [5][6][7][8]. In addition, phononic transport, i.e., the phonon thermal conductivity κ, in nanoscale periodic structures, including multilayers, has become of particular interest [4,[9][10][11], because materials with low thermal conductivity are employed in modern technologies, e.g., in superlattice thermoelectric devices [12][13][14]. Balandin and Wang [15] predicted the effect of acoustic phonon confinement and corresponding modification of their group velocities on the thermoelectric figure of merit of quantum wells and superlattices.…”

Thermal boundary conductance across metal-nonmetal interfaces: effects of electron-phonon coupling both in metal and at interface

“…The TBC across TiN-MgO interface they calculated is in agreement with the experimental data measured by Costescu et al [7] Taking a step forward, Singh et al [23] However, there are not many works that consider three channels simultaneously and compare the contributions from different channels. Li et al [24] measured the thermal conduction in periodic Mo-Si multilayers using ω 3 method. They adopted all three channels to interpret the measured results.…”

“…A reduction of thermal conductivity is interpreted as the enhanced PP scattering at interfaces between nanoparticles and the presence of the EP coupling both inside metallic nanocrystals and at interfaces [25]. In references [13,24], all three channels are taken into account to explain their experimental results. Nevertheless, there is no general theoretical model to calculate the TBC when all channels are considered.…”