Beating the Thermal Conductivity Alloy Limit Using Long-Period Compositionally Graded Si_1–xGe_x Superlattices

Abstract

Superlattices with scattering mechanisms at multiple length scales efficiently scatter phonons at all relevant wavelengths and provide a convenient route to reduce thermal transport. Here, we show, both experimentally and by atomistic simulations, that SiGe superlattices with well-established compositional gradients and a sufficient number of interfaces exhibit extremely low thermal conductivity. Our results reveal that the thermal conductivity of long-period (30−50 nm) superlattices with thicknesses below 200 nm is still thicknessdependent and higher than that of the corresponding alloy thin film. Increasing the number of periods up to 16 has a strong impact on heat propagation, leading to thermal conductivity values below the thin-film alloy limit. Lattice dynamics calculations confirm that the reduced thermal conductivity stems from the simultaneous effects of mass scattering, graded interface scattering, and coherent interference from the lattice periodicity. This study provides a significant step forward in understanding the role of compositional gradients in heat transport across nanostructures. The strategy of employing long-period graded superlattices with extremely low thermal conductivities has great potential for micro-and nano-thermoelectric generation and cooling of Si-based devices.

A Review on Fundamentals, Design and Optimization to High ZT of Thermoelectric Materials for Application to Thermoelectric Technology

“…For instance, the thermal conductivity of the thin film was reported below the alloy limit using long-period compositionally graded Si 1-x Ge x superlattices. 158 The phonon thermal conductivity was reduced in n-type PbTe-PbSe nanostructured thin films grown by atomic layer deposition 159 . Huang et al 160 reduced the thermal conductivity in a silicon thin film using nanocone.…”