Phonon impedance matching: minimizing interfacial thermal resistance of thin films

Carlos A.Polanco, J. Zhang and A. W. Ghosh , Bulletin of the American Physical Society , 59 :1 (2014).
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The challenge to minimize interfacial thermal resistance is to allow a broad band spectrum of phonons, with non-linear dispersion and well defined translational and rotational symmetries, to cross the interface. We explain how to minimize this resistance using a frequency dependent broadening matrix that generalizes the notion of acoustic impedance to the whole phonon spectrum including symmetries. We show how to match two given materials by joining them with a single atomic layer, with a multilayer material and with a graded superlattice. Atomic layer matching requires a layer with a mass close to the arithmetic mean (or spring constant close to the harmonic mean) to favor high frequency phonon transmission. For multilayer matching, we want a material with a broadening close to the geometric mean to maximize transmission peaks. For graded superlattices, a continuous sequence of geometric means translates to an exponentially varying broadening that generates a wide-band antireflection coating for both the coherent and incoherent limits. Our results are supported by first principles calculations of thermal conductance for GaAs/GaxAl1−xAs/AlAs thin films using the Non-Equilibrium Greens Function formalism coupled with Density Functional Perturbation Theory.