On the interplay between electrical conductivity and Seebeck coefficient in ultra-narrow silicon nanowires

TL;DR

This study investigates how the reduced dimensions of ultra-narrow silicon nanowires affect their electrical conductivity and Seebeck coefficient, revealing that quantum confinement and geometry significantly influence these thermoelectric properties.

Contribution

The paper provides a detailed atomistic analysis of how nanowire geometry and quantum effects alter conductivity and Seebeck coefficient, highlighting opportunities for optimizing thermoelectric performance.

Findings

Conductivity is exponentially sensitive to band edge shifts from Fermi level.

Seebeck coefficient shows linear dependence on band edge shifts.

Quantum confinement can enhance power factor by improving conductivity without reducing S.

Abstract

We analyze the effect of low dimensionality on the electrical conductivity ({\sigma}) and Seebeck coefficient (S) in ultra-narrow Si nanowires (NWs) by employing atomistic considerations for the electronic structures and linearized Boltzmann transport theory. We show that changes in the geometrical features of the NWs such as diameter and orientation, mostly affect {\sigma} and S in two ways: i) the distance of the band edges from the Fermi level ({\eta}F) changes, and ii) quantum confinement in some cases strongly affect the effective mass of the subbands, which influences the conductivity of the NWs and {\eta}F. Changes in eta_F cause exponential changes in {\sigma}, but linear changes in S. S seems to be only weakly dependent on the curvature of the bands, the strength of the scattering mechanisms, and the shape of the DOS(E) function, contrary to current view. Our results indicate that low dimensionality has a stronger influence on {\sigma} than on S due to the stronger sensitivity of {\sigma} on {\eta}F. We identify cases where bandstructure engineering through confinement can improve {\sigma} without significantly affecting S, which can result in power factor improvements.