Growth of ultrathin Bi2Se3 films by molecular beam epitaxy

[Formula: see text] is a widely studied 3D topological insulator having potential applications in optics, electronics, and spintronics. When the thickness of these films decreases to less than approximately 6 nm, the top and bottom surface states couple, resulting in the opening of a small gap at the Dirac point. In the 2D limit, [Formula: see text] may exhibit quantum spin Hall states. However, growing coalesced ultrathin [Formula: see text] films with a controllable thickness and typical triangular domain morphology in the few nanometer range is challenging. Here, we explore the growth of [Formula: see text] films having thicknesses down to 4 nm on sapphire substrates using molecular beam epitaxy that were then characterized with Hall measurements, atomic force microscopy, and Raman imaging. We find that substrate pretreatment—growing and decomposing a few layers of [Formula: see text] before the actual deposition—is critical to obtaining a completely coalesced film. In addition, higher growth rates and lower substrate temperatures led to improvement in surface roughness, in contrast to what is observed for conventional epitaxy. Overall, coalesced ultrathin [Formula: see text] films with lower surface roughness enable thickness-dependent studies across the transition from a 3D-topological insulator to one with gapped surface states in the 2D regime.