Temperature-Driven Twin Structure Formation and Electronic Structure of Epitaxially Grown Mg₃Sb₂ Films on Mismatched...

Mg₃Sb₂-based compounds are one type of important room-temperature thermoelectric materials and the appropriate candidate of type-II nodal line semimetals. In Mg₃Sb₂-based films, compelling research topics such as dimensionality reduction and topological states rely on the controllable preparation of films with high crystallinity, which remains a big challenge. In this work, high quality Mg₃Sb₂ films are successfully grown on mismatched substrates of sapphire (000l), while the temperature-driven twin structure evolution and characteristics of the electronic structure are revealed in the as-grown Mg₃Sb₂ films by in situ and ex situ measurements. The transition of layer-to-island growth of Mg₃Sb₂ films is kinetically controlled by increasing the substrate temperature (T_sub), which is accompanied with the rational manipulation of twin structure and epitaxial strains. Twin-free structure could be acquired in the Mg₃Sb₂ film grown at a low T_sub of 573 K, while the formation of twin structure is significantly promoted by elevating the T_sub and annealing, in close relation to the processes of strain relaxation and enhanced mass transfer. Measurements of scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) elucidate the intrinsic p-type conduction of Mg₃Sb₂ films and a bulk band gap of ~0.89 eV, and a prominent Fermi level downshift of ~0.2 eV could be achieved by controlling the film growth parameters. As elucidated in this work, the effective manipulation of the epitaxial strains, twin structure and Fermi level is instructive and beneficial for the further exploration and optimization of thermoelectric and topological properties of Mg₃Sb₂-based films.