**Localization of vibrational modes leads to reduced thermal conductivity
of amorphous heterostructures**

A Giri and BF Donovan and PE Hopkins, PHYSICAL REVIEW MATERIALS, 2, 056002 (2018).

DOI: 10.1103/PhysRevMaterials.2.056002

We investigate the vibrational heat transfer mechanisms in amorphous
Stillinger-Weber silicon and germanium-based alloys and heterostructures
via equilibrium and nonequilibrium molecular dynamics simulations along
with lattice dynamics calculations. We find that similar to crystalline
alloys, amorphous alloys demonstrate large size effects in thermal
conductivity, while layering the constituent materials into superlattice
structures leads to length-independent thermal conductivities. The
thermal conductivity of an amorphous SixGe1_(x) alloy reduces by as much
as similar to 53% compared to the thermal conductivity of amorphous
silicon; compared to the larger reduction in crystalline phases due to
alloying, we show that compositional disorder rather than structural
disorder has a larger impact on the thermal conductivity reduction. Our
thermal conductivity predictions for a-Si/a-Ge superlattices suggest
that the alloy limit in amorphous SiGe-based structures can be surpassed
with interface densities above similar to 0.35 nm(-1). We attribute the
larger reduction in thermal conductivity of layered Si/Ge
heterostructures to greater localization of modes at and around the
cutoff frequency of the softer layer as demonstrated via lattice
dynamics calculations and diffusivities of individual eigenmodes
calculated according to the Allen-Feldman theory **P. B. Allen and J. L.
Feldman, Phys.Rev.B 48, 12581 (1993)** for our amorphous SiGe-based
alloys and superlattice structures.

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