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

• Published in: CrystEngComm Vol. 2; no. 41; pp. 6449 - 6457
• Main Authors: Rajeev Gandhi, J, Nehru, Raja, Chen, Sheng-Ming, Sankar, Raman, Bayikadi, Khasim Saheb, Sureshkumar, Palanivel, Chen, Kuei-Hsien, Chen, Li-Chyong
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 2018
• Subjects: Antimony; Carrier density; Diffraction; Doping; Electric power generation; Energy dispersive X ray spectroscopy; Field emission microscopy; Germanium; Heat conductivity; Heat transfer; Intermetallic compounds; Materials selection; Microscopy; Phase transitions; Phosphides; Point defects; Precipitates; Scanning electron microscopy; Seebeck effect; Synchrotron radiation; Tellurides; Temperature dependence; Thermal conductivity; Thermoelectricity; Transmission electron microscopy; X ray powder diffraction; X-ray diffraction
• ISSN: 1466-8033
• DOI: 10.1039/c8ce01134f

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− x P x Sb 0.1 Te ( 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− x P x Sb 0.1 Te 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 0.87 P 0.03 Sb 0.1 Te, which is a ∼3 fold increase for GeTe. Finally, P co-doped Ge 0.9 Sb 0.1 Te demonstrates an enhancement in ZT, making it a good candidate material for power generation applications. The incorporation of P in GST forms the secondary GeP rich phase. The presence of secondary phase and point defects (Sb and P) enhanced the additional scattering effects in the system.