Effects of Confinement and Orientation on the Thermoelectric Power Factor of Silicon Nanowires

TL;DR

This study uses atomistic modeling and Boltzmann transport theory to analyze how confinement and orientation affect the thermoelectric power factor of silicon nanowires, revealing size-dependent effects on conductivity and power factor.

Contribution

It provides a comprehensive analysis of thermoelectric properties of silicon nanowires considering size, orientation, and doping, highlighting the dominant role of conductivity changes due to confinement.

Findings

Confinement effects on power factor are significant below 7nm diameter.

Conductivity changes, not Seebeck coefficient, mainly drive power factor variations.

Certain confinement conditions can enhance power factor by 2-3 times.

Abstract

It is suggested that low dimensionality can improve the thermoelectric (TE) power factor of a device, offering an enhancement of the ZT figure of merit. In this work the atomistic sp3d5s*-spin-orbit-coupled tight-binding model and the linearized Boltzmann transport theory is applied to calculate the room temperature electrical conductivity, Seebeck coefficient, and power factor of narrow 1D silicon nanowires (NWs). We present a comprehensive analysis of the thermoelectric coefficients of n-type and p-type NWs of diameters from 12nm down to 3nm, in [100], [110], and [111] transport orientations at different carrier concentrations. We find that the length scale at which the influence of confinement on the power factor can be observed is at diameters below 7nm. We show that contrary to the current view, the effect of confinement and geometry on the power factor originates mostly from changes in the conductivity which is strongly affected, rather than the Seebeck coefficient. In general, enhanced scattering at these diameter scales strongly degrades the conductivity and power factor of the device. We identify cases, however, for which confinement largely improves the channel's conductivity, resulting in ~2-3X power factor improvements. Our results may provide guidance in the design of efficient low dimensional thermoelectric devices.