There is growing interest in monolayer transition metal dichalcogenides (TMDs) such as MoS₂ and WSe₂ and related 2D heterostructures due to their compelling electronic and photonic properties. The majority of this work has been carried out using ultra-thin flakes exfoliated from bulk chalcogenide crystals but future device development requires the ability to synthesize large area single crystal films and heterostructures. Our research is aimed at the development of an epitaxial growth technology for layered dichalcogenides, similar to that which exists for III-V and other compound semiconductors, based on gas source chemical vapor deposition (CVD). This approach provides a high overpressure of chalcogen species needed to maintain stable growth at elevated temperature and excellent control of the precursor partial pressures to achieve layer-by-layer and heterostructure growth.

Our recent studies have focused on the epitaxial growth of WSe₂ and WS₂ monolayer films and heterostructures using metal hexacarbonyl and hydride chalcogen precursors on substrates including sapphire and hexagonal boron nitride (hBN). A multi-step precursor modulation growth method was developed to independently control nucleation density and the lateral growth rate of WSe₂ and WS₂ monolayer domains on the substrate. This approach also enables measurement of W-species surface diffusivity and domain growth rate as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Using this approach, coalesced monolayer and few-layer TMD films were obtained on sapphire substrates up to 2” in diameter at growth rates on the order of ~ 1 monolayer/hour. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire resulting from a merging of an equal mixture of 0^o and 60^o oriented domains. WSe₂ also grows epitaxially on hBN single crystal flakes but in this case the domains exhibit a predominant direction resulting in a reduced density of anti-phase boundaries. Applications and challenges of this approach in the growth of 2D heterostructures will also be discussed.