Ultrahigh zT from low-dimensional-like electronic structure realized in bulk GeTe

Te-Hsien Wang^1*, V.K. Ranganayakulu^2,3,4, Cheng-Lung Chen⁵, Angus Huang⁶, Min-Nan Ou³, Horng-Tay Jeng^3,6,7, Kuei-Hsien Chen⁸, Wen-Hsien Li⁹, G. Jeffrey Snyder¹⁰

¹Department of Physics, National Chung Hsing University, Taichung, Taiwan
²Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan
³Institute of Physics, Academia Sinica, Taipei, Taiwan
⁴Taiwan International Graduate Program, Academia Sinica, Taipei, Taiwan
⁵Bachelor Program in Semiconductor Materials and Fabrication, Ming Chi University of Technology, New Taipei City, Taiwan
⁶Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁷Physics Division, National Center for Theoretical Sciences, Hsinchu, Taiwan
⁸Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
⁹Department of Physics, National Central University, Jhongli, Taiwan
¹⁰Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, USA
¹¹Graduate Institute of Applied Physics, National Chengchi University, Taipei, Taiwan

* Presenter:Te-Hsien Wang, email:thwang@phys.nchu.edu.tw

In this research, we discover low-dimensional-like electronic structures in Sb and Bi co-doped GeTe. The low-dimensional-like electronic structures provide exceptionally high effective valley degeneracy leading to the high power factor. It also causes strong electron-phonon interaction leading to a nearly temperature-independent lattice thermal conductivity lower than the amorphous limit, 0.45 W/m-K, in our single-crystal specimens. As a result, the record-breaking value of zT ≈ 2.7 in GeTe-based materials is achieved. The weak temperature dependence of the lattice thermal conductivity indicates the phonon-phonon scattering should be only a minor effect. The defects observed from HRTEM images also cannot explain such an extremely low lattice thermal conductivity. Using inelastic neutron scattering (INS) measurements on Sb and Bi co-doped GeTe crystals (Ge_0.86Sb_0.08Bi_0.06)Te, we discovered a Kohn anomaly in the phonon dispersion indicating a strong electron-phonon coupling therein. Such a strong electron-phonon coupling is further confirmed by the quantitative agreement between the soften phonon wavevectors observed in the INS measurement and the electronic nesting vectors from DFT calculations. The novel thermoelectric effect from the low-dimensional-like electronic structure realized in 3D (bulk) materials investigated in this research provides a practical guide to searching for more promising thermoelectric materials.

Keywords: thermoelectric , electronic band convergency, electron-phonon coupling, germanium telluride, inelastic neutron scattering