Achieving high carrier mobility and thermoelectric performance in nearly twin-free rhombohedral GeTe (00l) films

GeTe-based thermoelectric (TE) films have garnered significant attentions due to their promising TE performance near room temperature. However, it is challenging to further optimizing the TE performance due to the inferior carrier mobility (μ) and the excessively high hole density (p). Herein, we developed a novel method based on molecular beam epitaxy (MBE) technique to successfully fabricate nearly twin-free GeTe (00l) films via incorporating Bi2Te3 buffer layers to alleviate epitaxial strain. Consequently, μ was significantly enhanced. Additionally, through comprehensively investigating the processing conditions, we found that substrate temperature and Te/GeTe flux ratio can shape intrinsic atomic defects and further decrease p. With the optimal synthesis and processing conditions, the GeTe film achieves optimized p of 3.44×1020 cm-3 and a high μ of 73.31 cm2V-1s-1, which lead to the highest room-temperature power factor of 2.67 mWm-1K-2, outperforming the values of other GeTe films. This work provides important guidance on fabricating twin-free GeTe films and on further improving their TE performance.