Unusual Thermoelectric Anisotropy in Rhombohedral Germanium Telluride

Abstract

Developing high performance thermoelectrics to convert waste heat into useful electrical power is an important challenge that has the potential to impact our energy future. This requires exploring and discovering new materials with enhanced electronic and thermal (phonon) transport properties. Advances in computational modeling now allow the thermoelectric characteristics of materials to be calculated fully predictively, and thus represents a powerful tool to be partnered with experimental efforts.In this talk, I will present our recent findings on rhombohedral GeTe, a layered quasi-2D material predicted to have unusual anisotropic transport properties [1]. Our approach combines density functional theory with the Boltzmann transport equation, to extract the detailed electron and phonon dispersions along with the electron-phonon scattering rates, in order to predict the thermoelectric transport coefficients. Our results show that the electrons preferentially conduct along the cross-plane direction, while the holes favor the in-plane direction. Interestingly, this feature enhances the thermoelectric figure-of-merit ZT for n-type GeTe, since the conductivity (and power factor) is maximized perpendicular to the atomic layers while the lattice (phonon) thermal conductivity is minimized - leading to a three-fold increase in ZT. The cross-plane power factor and mobility in n-GeTe reach roughly 32 μW/cm-K2 and 500 cm2/V-s, respectively. An analysis of the band structure reveals that the large cross-plane transport originates from high-velocity conduction states, formed by the Ge p-orbitals that span across the interstitial region. These findings illustrate how the dominant electron and phonon transport directions are effectively decoupled in n-GeTe; a feature that can potentially be found in other materials to achieve enhancements in thermoelectric performance.[1] V. Askarpour and J. Maassen, “Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe”, Phys. Rev. B 100, 075201 (2019). Figure 1