Two-dimensional (2D) transition metal dichalcogenides (TMDs) comprise a compelling class of new materials with intriguing properties that arise from their ultra-thin form, thickness-dependent bandstructure, large exciton binding energies and unusual spin and valley polarization physics. The majority of the research that has been carried out thus far employs ultra-thin TMD flakes exfoliated from bulk 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 TMDs and related layered chalcogenides based on gas source/metalorganic chemical vapor deposition (CVD/MOCVD) to enable monolayer growth over large substrate areas and layer-by-layer growth of heterostructures.

Our recent studies have focused on the epitaxial growth of WSe₂ and (Mo,W)S₂ monolayer films, alloys 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 TMD domains on the substrate. This approach also enables an estimation of metal-species surface diffusivity and domain growth rate as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Using the multi-step growth process, 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 0^o and 60^o oriented domains. Room temperature photoluminescence (PL) measurements demonstrate strong emission peaks in the range of ~1.6 eV for WSe₂ and ~2.0 eV for WS₂ monolayers with lateral variations that may arise from residual strain in the films. (Mo,W)S₂ alloy growth using simultaneous flows of Mo and W hexacarbonyl precursors reveals evidence of atomic scale ordering of Mo and W atoms within the monolayer. Growth and characterization of MoS₂/WS₂ and WSe₂/WS₂ vertical heterostructures will also be discussed.