Influence of GeP precipitates on the thermoelectric properties of P-type GeTe and Ge0.9−xPxSb0.1Te compounds

Abstract

Germanium telluride (GeTe) is a very well known IV–VI group semiconducting material with the advantageous property of showing metallic conduction, which materializes from its superior carrier concentration (n) (high number of Ge vacancies). A systematic investigation into the thermoelectric properties (TEP) of GeTe was reported by way of carrier concentration (n) engineering. The present investigation focuses on studying the effects of doping (antimony – Sb) and co-doping (phosphorus – P) on the TEP of GeTe. In order to understand the system, we have prepared p-type GeTe and Ge_0.9−xP_xSb_0.1Te (x = 0, 0.01, 0.03, or 0.05) samples via a non-equilibrium solid state melt quenching (MQ) process, followed by hot press consolidation. Temperature dependent synchrotron X-ray diffraction studies reveal a phase transition from rhombohedral to simple cubic in the Ge_0.9−xP_xSb_0.1Te system at 573 K, which is clearly reflected in the TEP. Further high resolution transmission electron microscopy (HRTEM) studies reveal the pseudo-cubic nature of the sample. However, powder X-ray diffraction (PXRD) and field emission scanning electron microscopy (FESEM) images and energy dispersive X-ray spectroscopy (EDX) studies confirm the presence of germanium phosphide (GeP) in all P-doped samples. The presence of a secondary phase and point defects (Sb & P) enhanced the additional scattering effects in the system, which influenced the Seebeck coefficient and thermal conductivity of GeTe. A significant enhancement in the Seebeck coefficient (S) to ∼225 μV K⁻¹ and a drastic reduction in thermal conductivity (κ) to ∼1.2 W mK⁻¹ effectively enhanced the figure-of-merit (ZT) to ∼1.72 at 773 K for Ge_0.87P_0.03Sb_0.1Te, which is a ∼3 fold increase for GeTe. Finally, P co-doped Ge_0.9Sb_0.1Te demonstrates an enhancement in ZT, making it a good candidate material for power generation applications.