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We describe epitaxial Ge/Si multilayers with cross-plane thermalconductivities which can be systematically reduced to exceptionally low values,as compared both with bulk and thin-film SiGe alloys of the same averageconcentration, by simply changing the thicknesses of the constituent layers. Abinitio calculations reveal that partial interdiffusion of Ge into the Sispacers, which naturally results from Ge segregation during growth, plays adeterminant role, lowering the thermal conductivity below what could beachieved without interdiffusion (perfect superlattice), or with totalinterdiffusion (alloy limit). This phenomenon is similar to the one previouslyobserved in alloys with embedded nanoparticles, and it stresses the importanceof combining alloy and nanosized scatterers simultaneously to minimize thermalconductivity. Our calculations thus suggest that superlattices with sharpinterfaces, which are commonly sought but difficult to realize, are worse thancompositionally-modulated Si1-xGex multilayers in the search for materials withultralow thermal conductivities.

Compositionally-modulated Si1-xGex multilayers with cross-plane thermal conductivity below the thin-film alloy limit