Achieving Extraordinary Power Factors in GeTe Epitaxial Films through Carrier Transport Engineering

GeTe-based films have attracted tremendous attention from the thermoelectric community owing to their excellent thermoelectric performance. It is vital to reduce the hole density and maintain a high carrier mobility for GeTe films; however, this remains a significant challenge. To overcome this drawback, we succeeded in fabricating high-crystalline quality GeTe-based films and remarkably improve their electrical properties using molecular beam epitaxy under a low substrate temperature and optimized Te/GeTe flux ratios. The Bi₂Te₃/GeTe double-layer buffer facilitated the reliable fabrication of high-quality GeTe films. The hole density and carrier mobility were synergistically optimized under a relatively low substrate temperature of 503 K and Te/GeTe flux ratio of 0.25/1 that suppress the formation of Ge vacancies, as well as a trace amount of Sb₂Te₃ incorporation that introduces Sb_Te substitutional defects. The best (GeTe)₂₄/(Sb₂Te₃)_0.25 film acquires a very low hole density of 2.57 × 10²⁰ cm^-3 and, simultaneously, a high carrier mobility of 96.53 cm² V^-1 s^-1, which leads to an extraordinary power factor of 3.36 mW m^-1 K^-2 at room temperature as well as an average power factor of 4.15 mW m^-1K^-2 within 300-475 K, outperforming the values of GeTe from previous reports. This work provides valuable insights for fabricating high-performance GeTe-based films to promote their future applications near room temperature.