Effects of Addition of Si and Sb on the Microstructure and Thermoelectric Properties of GeTe

The effects of the addition of Si and Sb on the microstructure and thermoelectric properties of GeTe were investigated in this study. Pure GeTe showed a typical herringbone structure that consisted of regularly aligned domains with alternating bright and dark contrast areas. When Sb atoms were added to the GeTe, a clean herringbone structure was not formed. Instead, many sharp lines that are likely narrow domains were observed, and their formation can be ascribed to the cubic stabilizing nature of Sb in GeTe. The co-addition of Si and Sb to GeTe resulted in a breakdown of the herringbone structure and formation of SiGe and Si precipitates. Even though Si and Ge can form a complete solid solution, the solubility of Si in GeTe remained low. The addition of Sb to GeTe significantly reduced the electrical conductivity due to the aliovalent donor nature of Sb. Its thermal conductivity was also very low due to the mass fluctuation in the solid solution and narrow domain structure. Therefore, the best figure-of-merit (ZT) of 0.67 at 570 K was obtained. However, Ge_1−x−ySb_xSi_yTe compounds with both Si and Sb addition showed a relatively low ZT value.