Epitaxial growth of ultrathin Bi₂Se₃ films as characterized by <i>in situ</i> angle-resolved photoemission spectroscopy

Combining molecular beam epitaxy with time- and angle-resolved photoemission spectroscopy (MBE-trARPES) provides a powerful way to investigate electronic structures of quantum materials. By using our in situ MBE-trARPES system, we measured ultrathin epitaxial films of Bi₂Se₃ grown on sapphire substrate. In its ultrathin limit, i.e. below 6 quintuple layers (QL), topological surface states are gapped due to hybridization and theory predicts the film to become a two-dimensional topological insulator (2D TI) until 3QL. From 3QL to 2QL, topological phase transition to trivial insulator is expected to occur. Previous ARPES experiments explored this phase transition in ultrathin Bi₂Se₃ films grown on bilayer graphene on SiC and Si substrate. In this experiment, we investigated various growth methods for ultrathin (2-6 QL) Bi₂Se₃ film on sapphire substrate: one-step growth, two-step growth, and growth on (Bi_0.5In_0.5)₂Se₃ buffer layer. By measuring ARPES spectra at several positions on the film, we evaluated band structure and uniformity of film. This work provides a guideline for growth of ultrathin Bi₂Se₃ on sapphire substrate for future experiments exploring topological phase transition in ultrathin Bi₂Se₃.