Rational Manipulation of Epitaxial Strains Enabled Valence Band Convergence and High Thermoelectric Performances in Mg₃Sb₂ Films

Abstract Strain engineering is demonstrated to effectively regulate the functionality of materials, such as thermoelectric, ferroelectric, and photovoltaic properties. As the straightforward approach of strain engineering, epitaxial strain is usually proposed for rationally manipulating the electronic structure and performances of thermoelectric materials, but has rarely been verified experimentally. In this study, tunable and large epitaxial strains are demonstrated, as well as the resulting valence band convergence can be achieved in the Mg 3 Sb 2 epi‐films with the choice of substrates. The large epitaxial strains up to 8% in Mg 3 Sb 2 films represent one of the most striking results in strain engineering. The angle‐resolved photoemission spectroscopy measurements and the theoretical calculations reveal the vital role of epitaxial strain in tuning the crystal field splitting and the band structure of Mg 3 Sb 2 . Benefiting from the appropriate manipulation of the crystal field effect via in‐plane compressive strain, the valence band convergence is unambiguously discovered in the strained Mg 3 Sb 2 film grown on InP(111) substrate. As a result, a state‐of‐the‐art thermoelectric power factor of 0.94 mWm −1 K −2 is achieved in the strain‐engineered Mg 3 Sb 2 film, well exceeding that of the strain‐relaxed Mg 3 Sb 2 . The work paves the way for effectively manipulating epitaxial strain and band convergence for Mg 3 Sb 2 and other thermoelectric films.