Gated Si nanowires for large thermoelectric power factors

TL;DR

This study demonstrates that electrostatic gating significantly enhances the thermoelectric power factor in silicon nanowires by improving mobility and bandstructure, with the greatest benefits observed in smaller diameters and specific crystallographic orientations.

Contribution

It introduces atomistic simulations showing gating can boost power factors in Si nanowires up to 5 times, especially in smaller diameters and certain orientations, surpassing doped channels.

Findings

Gated nanowires exhibit ~5x higher power factor than doped channels.

Surface roughness scattering does not significantly degrade transport in gated structures.

The advantage of gating persists up to ~40nm diameter nanowires.

Abstract

We investigate the effect of electrostatic gating on the thermoelectric power factor of p-type Si nanowires (NWs) of up to 20nm in diameter in the [100], [110] and [111] crystallographic transport orientations. We use atomistic tight-binding simulations for the calculation of the NW electronic structure, coupled to linearized Boltzmann transport equation for the calculation of the thermoelectric coefficients. We show that gated NW structures can provide ~5x larger thermoelectric power factor compared to doped channels, attributed to their high hole phonon-limited mobility, as well as gating induced bandstructure modifications which further improve mobility. Despite the fact that gating shifts the charge carriers near the NW surface, surface roughness scattering is not strong enough to degrade the transport properties of the accumulated hole layer. The highest power factor is achieved for the [111] NW, followed by the [110], and finally by the [100] NW. As the NW diameter increases, the advantage of the gated channel is reduced. We show, however, that even at 20nm diameters, (the largest ones that we were able to simulate), a ~3x higher power factor for gated channels is observed. Our simulations suggest that the advantage of gating could still be present in NWs with diameters of up to ~40nm.