Surprisingly high in-plane thermoelectric performance in a-axis-oriented epitaxial SnSe thin films

Abstract SnSe has recently drawn extensive attention as a promising thermoelectric (TE) material since the discovery of record high figure of merit (zT, ∼2.6) in the monocrystalline bulk. However, thin films with remarkable TE performance are highly desired for microscale devices used as semiconductor coolers or power wireless sensors. Studies on the preparation and the TE properties of SnSe epitaxial thin films are scarce to date. Herein, a-axis-oriented SnSe epitaxial films were successfully prepared via the pulsed laser deposition technology. The thin films exhibited a relatively high power factor (PF) of ∼472 μW⋅m−1 K−2 at 600 K along the in-plane direction perpendicular to the a-axis after the optimization of Sn vacancy concentration. The out-of-plane thermal conductivities of the thin films decreased by 33% compared with that of the single-crystalline bulk. The in-plane thermal conductivities were conservatively estimated using the proportion between a and b axes in the single-crystalline bulk. An ultrahigh estimated-zT value (approximately 1.2 at 600 K) was achieved along the in-plane direction of SnSe thin films. The superior thermal stability of the high-quality samples was also investigated via an ex situ annealing process, and the corresponding PF and estimated-zT remained at high values (445 μW⋅m−1 K−2 and 1.1, respectively). The surprisingly high TE properties and the rational preparation solution in this study will greatly contribute to the promotion of the prospective practical application of SnSe thin films.