Thermal conductivity of (Zr,W)N/ScN metal/semiconductor multilayers and superlattices

The cross-plane thermal conductivities of metal/semiconductor multilayers and epitaxial superlattices have been measured as a function of period by time-domain thermoreflectance at room temperature. (001)-oriented ZrN (metal)/ScN (semiconductor) multilayers and (Zr,W)N/ScN epitaxial superlattices with the rocksalt crystal structure were grown on (001)MgO substrates by reactive magnetron sputtering. A distinct minimum in thermal conductivity at a period of ∼6 nm is observed for ZrN/ScN multilayers. The minimum thermal conductivity of 5.25 W/m K is a factor of ∼2.7 smaller than the mean of the thermal conductivities (including only the lattice contributions) of the values measured for films of the constituent materials, and approximately equal to the lattice component of the thermal conductivity of a Zr0.65 Sc0.35 N alloy film (∼5 W/m K). Alloying the ZrN layers with WNx reduces the lattice mismatch, yielding epitaxial (Zr,W)N/ScN superlattices. The addition of WNx also reduces the thermal conductivity to ∼2 W/m K, a value that is sufficiently low to suggest promise for these materials as solid-state thermionic generators.