There is growing interest in the properties and applications of the family of layered metal dichalcogenides, MX2 (X=Se, S), which includes transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 and group IV dichalcogenides particularly SnSe2 and SnS2. While significant scientific advances have been made using monolayer and few-layer flakes exfoliated from bulk chalcogenide crystals, 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 II-VI compound semiconductors, based on gas source chemical vapor deposition (CVD) and metalorganic CVD (MOCVD) in cold-wall reactor geometries. This approach provides excellent control of the precursor partial pressure and reduced pre-deposition upstream of the substrate thereby enabling control over nucleation density, lateral growth rate and film composition for the layer-by-layer growth of 2D films and heterostructures.
Our recent studies have focused on the epitaxial growth of WSe2 and WS2 monolayer films and vertical heterostructures using metal hexacarbonyl and hydride chalcogen precursors on substrates including sapphire, SiC, epitaxial graphene and hexagonal boron nitride. A multi-step precursor modulation growth method was developed to control the nucleation density, size, orientation and the lateral growth rate of monolayer domains on the substrate. 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 predominantly 0o and 60o oriented domains. Epitaxial growth of SnS2, SnSe2 and Sn(S,Se)2 alloy films on epitaxial graphene have also been demonstrated using evaporated sources. The gas source CVD method also provides a means to study and quantify surface diffusivities and lateral growth rates of domains as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Applications and challenges of this approach in the growth of 2D heterostructures will also be discussed.