Thermoelectric Properties of Scaled Silicon Nanowires Using the sp3d5s*-SO Atomistic Tight-Binding Model and Boltzmann Transport

TL;DR

This study uses atomistic modeling and Boltzmann transport theory to analyze how silicon nanowire size and orientation affect thermoelectric properties, revealing size-dependent trade-offs between power factor and conductivity at room temperature.

Contribution

It applies a detailed atomistic tight-binding model combined with Boltzmann transport calculations to evaluate thermoelectric performance of silicon nanowires across different sizes and orientations.

Findings

Scaling nanowires below 7nm can increase the power factor due to bandstructure effects.

Enhanced phonon and surface roughness scattering at small sizes degrades conductivity.

ZT values are limited by increased scattering at very small diameters.

Abstract

As a result of suppressed phonon conduction, large improvements of the thermoelectric figure of merit, ZT, have been recently reported for nanostructures compared to the raw materials' ZT values. It has also been suggested that low dimensionality can improve a device's power factor as well, offering a further enhancement. In this work the atomistic sp3d5s*-spin-orbit-coupled tight-binding model is used to calculate the electronic structure of silicon nanowires (NWs). The linearized Boltzmann transport theory is applied, including all relevant scattering mechanisms, to calculate the electrical conductivity, the Seebeck coefficient, and the thermoelectric power factor. We examine n-type nanowires of diameters of 3nm and 12nm, in [100], [110], and [111] transport orientations at different carrier concentrations. Using experimental values for the lattice thermal conductivity in nanowires, the expected ZT value is computed. We find that at room temperature, although scaling the diameter below 7nm can be beneficial to the power factor due to banstructure changes alone, at those dimensions enhanced phonon and surface roughness scattering degrades the conductivity and reduces the power factor.