Metalorganic Chemical Vapor Deposition of Wafer-Scale Transition Metal Dichalcogenide Monolayer and Heterostructures

Wafer-scale synthesis of transition metal dichalcogenide (TMDs) monolayers is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Metalorganic chemical vapor deposition (MOCVD) has emerged as a promising technique for scalable synthesis of TMD films as it enables growth at high temperatures and chalcogen overpressures which are beneficial for epitaxy and provides good control over precursor flux which is necessary for the synthesis of heterostructures.<br/>Our work has focused on MOCVD growth of epitaxial semiconducting TMDs (MoS₂, WS₂ and WSe₂) on 2” diameter c-plane sapphire substrates. Metal hexacarbonyls (Mo(CO)₆, W(CO)₆) and hydrides (H₂S, H₂Se) are used as precursors with H₂ as the carrier gas in cold-wall MOCVD reactor geometries. Pre-annealing the sapphire surface in H₂S or H₂Se modifies the nucleation behavior of TMDs on sapphire which is attributed to passivation of dangling bonds on the sapphire surface. Passivation of step terraces can drive nucleation to occur preferentially at step edges on sapphire leading to unidirectional domains and a concomitant reduction in mirror twin defects. In situ spectroscopic ellipsometry is demonstrated to be an effective real time monitor of TMD growth even at the sub-monolayer level which can be exploited to track surface coverage as a function of time under varying growth conditions. The ability to precisely control and modulate precursor flux during growth is used to synthesize in-plane heterostructures that enable localized exciton confinement and emission.