Anisotropic thermoelectric properties of GeTe single crystals

Abstract

Semiconductor materials with layered structures usually have excellent thermoelectric properties and exhibit pronounced anisotropy such as Bi₂Te₃, SnSe, etc. The layered-like nature of GeTe has been studied by theoretical calculations, but it lacks sufficient experimental evidence. Herein, we study the crystal structure of GeTe single crystals by synchrotron radiation and atomic resolution scanning transmission electron microscopy. Lattice distortion along the [111]_PC direction is found to be the main reason for the layered-like structure exhibited by r-GeTe. Meanwhile, the structure-related anisotropy for thermal expansion coefficients and thermal conductivities of r-GeTe single crystals is reported. Since the layered-like structure induces noticeable scattering of phonons, GeTe single crystals exhibit an intrinsic low lattice thermal conductivity in the [111]_PC direction compared with the ⊥[111]_PC direction and polycrystals. All of these are based on chemical bonding and the crystal structure, which emphasizes the key role they play in material properties. Our work led to a deeper understanding of the relationships between the crystal structure and thermal/electrical transport properties of GeTe families, which also provides a guide for the further investigation of GeTe-based materials.