Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire.

Xin Mu,Pu Zhang,Albert C To,Tengfei Luo,Lili Wang,Xueming Yang

doi:10.1038/srep16697

Abstract

Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics.

Highlights

Due to the emergence of effective fabrication techniques, Si/Ge-based superlattice nanowires (SNWs) have become very important candidates for thermoelectric applications[34]
Considering that the electrical transport properties of very thin superlattice nanowire do not change significantly compared to perfectly smooth NW, which is evidenced by recent ab-initio calculations[32,33], this remarkable thermal conductivity reduction can be translated into almost an order of magnitude enhancement in the ZT coefficient
Through molecular dynamics (MD) simulations we find that a large portion of the thermal conductivity in regular Si/Ge SNW is contributed by coherent phonons

Summary

Introduction

Due to the emergence of effective fabrication techniques, Si/Ge-based superlattice nanowires (SNWs) have become very important candidates for thermoelectric applications[34] It is well-known that the thermal conductivity of superlattice can be lower than its alloy counterparts due to the phonon scattering at superlattice interfaces and the reduction of phonon group velocity due to the formation of mini-bands in the phonon dispersion relation[28,35,36,37,38,39]. That it is understood that coherent phonons can contribute to the thermal transport in superlattice, strategies to scatter these coherent phonons or even prevent their formation will be valuable to further lowering thermal conductivity beyond those achieved using regular SNWs. Hierarchical structuring, which has been applied in thermoelectrics and photonics[20,45], can be an effective way to further lower the thermal conductivity of Si/Ge SNW. The findings in this work can be very instrumental to improving the thermoelectric properties of Si/Ge-based SNW

Methods

Results

Conclusion