Strategies for Synthesis of Large-Area Epitaxial Transition Metal Dichalcogenides

Layered 2D transition metal dichalcogenides (TMDs) and their heterostructures have novel and exotic optical and electronic properties. Current research using transferred exfoliated flakes has demonstrated the rich landscape of possible properties and applications that TMDs and their heterostructures exhibit. A major challenge in harnessing the full potential of these materials is the availability of these materials uniformly over a large-scale. Scalable and adaptable synthesis strategies are necessary to obtain pristine epitaxial TMDs and heterostructures. We have developed an epitaxial growth strategy for layered dichalcogenides MX₂ (M=Mo, W and X= S, Se), based on metal organic chemical vapor deposition (MOCVD). This process uses metal hexacarbonyl and hydride chalcogen precursors to deposit monolayers in a cold-wall reactor.<br/>Epitaxial growth of nearly single crystalline TMDs like WS₂ on 2” sapphire wafers is achieved by a multi-step precursor modulation method. In case of conventional 3D substrates, nucleation density is controlled independently by precursor modulation. The lateral growth of these nuclei is promoted while suppressing additional nucleation to form coalesced monolayer films. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to sapphire. Controlling the growth temperature and chalcogen flux is crucial in establishing an epitaxial relation and achieving oriented growth. Dark-field transmission electron microscopy (DF-TEM) of transferred WS₂ monolayers show ~95% single orientation coverage with minimal bilayer and inversion domains. The key features observed during the growth of MoS₂, WS₂ and WSe₂will be discussed.<br/>MOCVD synthesis of TMDs is also investigated on 2D substrates like graphene. In this case, we investigate the effect of defects in epitaxial graphene on the growth of transition metal dichalcogenides like WS₂. The defect density in epitaxial graphene, controlled by exposure to a helium plasma for different durations, control the WS₂ nucleation density as well as nucleation sites. The results indicate that the defect generation, and subsequent sulfur incorporation is higher when the buffer layer was present between the SiC substrate the graphene layers. Another key observation was that the plasma treatment modified the WS₂ nucleation sites from step edges to terraces. This impact on the nucleation site was, however, temperature dependent. Additional details about the role of the buffer layer and the impact on the nucleation site and density will be presented.