Tuning lattice thermal conductance by porosity control in ultra-scaled Si and Ge nanowires

TL;DR

This study investigates how porosity in ultra-scaled silicon and germanium nanowires reduces their thermal conductance, which could enhance thermoelectric device efficiency by controlling phonon transport.

Contribution

The paper introduces a modified valence force field model to predict phonon dispersion and thermal conductance in porous Si and Ge nanowires, revealing anisotropic effects and phonon localization.

Findings

Porous Si and Ge nanowires show approximately 30% reduction in thermal conductance.

Porosity induces anisotropic reduction, with [111] orientation showing the maximum decrease.

Phonon localization explains the reduction in thermal conductance due to porosity.

Abstract

Porous nanowires (NWs) with tunable thermal conductance are examined as a candidate for thermoelectric (TE) devices with high efficiency (ZT). Thermal conductance of porous Si and Ge NWs is calculated using the complete phonon dispersion obtained from a modified valence force field (MVFF) model. The presence of holes in the wires break the crystal symmetry which leads to the reduction in ballistic thermal conductance ($\sigma_{l}$). $[100]$ Si and Ge NWs show similar percentage reduction in $\sigma_{l}$ for the same amount of porosity. A 4nm $\times$ 4nm Si (Ge) NW shows $\sim$ 30% (29%) reduction in $\sigma_{l}$ for a hole of radius 0.8nm. The model predicts an anisotropic reduction in $\sigma_{l}$ in SiNWs, with $[111]$ showing maximum reduction followed by $[100]$ and $[110]$ for a similar hole radius. The reduction in $\sigma_{l}$ is attributed to phonon localization and anisotropic mode reduction.