Tuning lattice thermal conductance by porosity control in ultrascaled Si and Ge nanowires

Abstract

Porous nanowires (NWs) with tunable thermal conductance are examined as a candidate for thermoelectric devices with high efficiency. Thermal conductance (σl) of porous NWs is calculated using the phonon dispersion obtained from a modified valence force field model. Porosity in the NWs break the crystal symmetry leading to the reduction in ballistic σl. [100] Si and Ge NWs show similar percentage reductions in σl for the same amount of porosity. The model predicts an anisotropic reduction in σl in SiNWs, with [111] showing the maximum reduction followed by [100] and [110] for a similar hole radius. The reduction in σl is attributed to phonon localization and anisotropic mode reduction.