We describe epitaxial Ge/Si multilayers with cross-plane thermal conductivities which can be systematically reduced to exceptionally low values, as compared both with bulk and thin-film SiGe alloys of the same average concentration, by simply changing the thicknesses of the constituent layers. Ab initio calculations reveal that partial interdiffusion of Ge into the Si spacers, which naturally results from Ge segregation during growth, plays a determinant role, lowering the thermal conductivity below what could be achieved without interdiffusion (perfect superlattice), or with total interdiffusion (alloy limit). This phenomenon is similar to the one previously observed in alloys with embedded nanoparticles, and it stresses the importance of combining alloy and nanosized scatterers simultaneously to minimize thermal conductivity. Our calculations thus suggest that superlattices with sharp interfaces, which are commonly sought but difficult to realize, are worse than compositionally-modulated Si1-xGex multilayers in the search for materials with ultralow thermal conductivities.
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