Low thermal conductivity is essential for efficient operation of thermoelectric/thermionic power generation devices. There have been several attempts to design materials with low thermal conductivity without sacrificing electrical transport. These approaches utilized different mechanisms of phonon scattering, such as acoustic impedance mismatch of the adjacent layers in superlattices or defect scattering of phonons etc [1, 2]. However, each of these approaches scatter phonons only in a particular region of the phonon spectrum. In this paper we present experimental results of the thermal conductivity of epitaxially grown superlattices engineered to take advantage of the various scattering mechanisms to scatter phonons over the entire phonon spectrum.

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