Calculation of Confined Phonon Spectrum in Narrow Silicon Nanowires using the Valence Force Field Method

TL;DR

This study investigates how geometric confinement affects phonon properties in ultra-narrow silicon nanowires using the valence force field method, revealing significant variations in thermal conductance and group velocity based on orientation and size.

Contribution

It introduces a detailed computational analysis of phonon dispersion and thermal transport in silicon nanowires considering confinement and orientation effects.

Findings

Phonon dispersion and thermal conductance depend on nanowire geometry.

<110> nanowires have the highest phonon group velocity.

<111> nanowires are optimal for thermoelectric applications.

Abstract

We study the effect of confinement on the phonon properties of ultra-narrow silicon nanowires of side sizes of 1-10nm . We use the modified valence force field method to compute the phononic dispersion, and extract the density of states, the transmission function, the sound velocity, the ballistic thermal conductance and boundary scattering-limited diffusive thermal conductivity. We find that the phononic dispersion and the ballistic thermal conductance are functions of the geometrical features of the structures, i.e. the transport orientation and confinement dimension. The phonon group velocity and thermal conductance can vary by a factor of two depending on the geometrical features of the channel. The <110> nanowire has the highest group velocity and thermal conductance, whereas the <111> the lowest. The <111> channel is thus the most suitable orientation for thermoelectric devices based on Si nanowires since it also has a large power factor. Our findings could be useful in the thermal transport design of silicon-based devices for thermoelectric and thermal management applications.