  Molecular-beam epitaxy (MBE) provides a simple but powerful way to synthesize
large-area high-quality thin films and heterostructures of a wide variety of
materials including accomplished group III-V and II-VI semiconductors as well
as newly-developing oxides and chalcogenides, leading to major discoveries in
condensed-matter physics. For two-dimensional (2D) materials, however, main
fabrication routes have been mechanical exfoliation and chemical-vapor
deposition by making good use of weak van der Waals bonding nature between
neighboring layers, and MBE growth of 2D materials, in particular on insulating
substrates for transport measurements, has been limited despite its fundamental
importance for future advanced research. Here we report layer-by-layer
epitaxial growth of scalable transition-metal dichalocogenide (TMDC) thin films
on insulating substrates by MBE, and demonstrate ambipolar transistor
operation. The proposed growth protocol is broadly applicable to other TMDC,
providing a key milestone toward fabrication of van der Waals heterostructures
with various 2D materials for novel properties and functionalities.
