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

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.