In this study, we calculate the $T=300\mathrm{K}$ scattering and thermoelectric transport properties of rhombohedral GeTe using first-principles modeling. The room-temperature phase of GeTe has a layered structure, with cross-plane and in-plane directions oriented parallel and perpendicular to [111], respectively. Based on rigorous electron-phonon scattering, our transport calculations reveal unusual anisotropic properties; $n$-type GeTe has a cross-plane electrical conductivity that is roughly $3\times$ larger than in plane. $p$-type GeTe, however, displays opposite anisotropy with in-plane conducting roughly $2\times$ more than cross plane, as is expected in quasi-two-dimensional materials. The power factor shows the same anisotropy as the electrical conductivity, since the Seebeck coefficient is relatively isotropic. Interestingly, cross-plane $n$-GeTe shows the largest mobility and power factor approaching $500{\mathrm{cm}}^2/\mathrm{V}$-s and $32\mu \mathrm{W}/\mathrm{cm}\text{−}{\mathrm{K}}^2$, respectively. The thermoelectric figure of merit, $zT$, is enhanced as a result of this unusual anisotropy in $n$-GeTe since the lattice thermal conductivity is minimized along cross plane. This decouples the preferred transport directions of electrons and phonons, leading to a threefold increase in $zT$ along cross plane compared to in plane. The $n$-type anisotropy results from high-velocity electron states formed by Ge $p$ orbitals that span across the interstitial region. This surprising behavior, that would allow the preferential conduction direction to be controlled by doping, could be observed in other quasi-two-dimensional materials and exploited to achieve higher-performance thermoelectrics.

• Received 13 April 2019
• Revised 5 June 2019

DOI:https://doi.org/10.1103/PhysRevB.100.075201