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

TL;DR

This study demonstrates that compositionally-modulated Si1-xGex multilayers with controlled layer thicknesses can achieve cross-plane thermal conductivities below the alloy limit, aided by interdiffusion effects during growth.

Contribution

It introduces a method to reduce thermal conductivity in SiGe multilayers by exploiting interdiffusion and layer design, surpassing the alloy and perfect superlattice limits.

Findings

Thermal conductivity can be systematically lowered by adjusting layer thicknesses.

Interdiffusion during growth significantly reduces thermal conductivity.

Superlattices with sharp interfaces are less effective than modulated multilayers.

Abstract

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.