Thermoelectric transport in strained Si and Si/Ge heterostructures

TL;DR

This study uses first-principles calculations to analyze how strain affects thermoelectric properties in silicon and Si/Ge heterostructures, revealing conditions for power factor enhancement and potential improvements in thermoelectric efficiency.

Contribution

It provides new insights into strain-induced enhancements of thermoelectric performance in silicon-based materials and superlattices through detailed first-principles analysis.

Findings

Power factor enhancement at low temperature and doping under specific strains.

Robust power factor at high temperature and doping with small compressive strain.

Significant suppression of cross-plane power factor in Si/Ge superlattices under hole-doping.

Abstract

The anisotropic thermoelectric transport properties of bulk silicon strained in [111]-direction were studied by detailed first-principles calculations focussing on a possible enhancement of the power factor. Electron as well as hole doping were examined in a broad doping and temperature range. At low temperature and low doping an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. For the thermoelectrically more important high temperature and high doping regime a slight enhancement of the power factor was only found under small compressive strain with the power factor overall being robust against applied strain. To extend our findings the anisotropic thermoelectric transport of an [111]-oriented Si/Ge superlattice was investigated. Here, the cross-plane power factor under hole-doping was drastically suppressed due to quantum-well effects, while under electron-doping an enhanced power factor was found. With that, we state a figure of merit of ZT$=0.2$ and ZT$=1.4$ at $T=\unit[300]{K}$ and $T=\unit[900]{K}$ for the electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in terms of band structure features.