Defect-Controlled Nucleation and Orientation of WSe on hBN: A Route to Single-Crystal Epitaxial Monolayers.

A defect-controlled approach for the nucleation and epitaxial growth of WSe on hBN is demonstrated. The WSe domains exhibit a preferred orientation of over 95%, leading to a reduced density of inversion domain boundaries (IDBs) upon coalescence. First-principles calculations and experimental studies as a function of growth conditions and substrate pretreatment confirm that WSe nucleation density and orientation are controlled by the hBN surface defect density rather than thermodynamic factors. Detailed transmission electron microscopy analysis provides support for the role of single-atom vacancies on the hBN surface that trap W atoms and break surface symmetry leading to a reduced formation energy for one orientation of WSe domains. Through careful control of nucleation and extended lateral growth time, fully coalesced WSe monolayer films on hBN were achieved. Low-temperature photoluminescence (PL) measurements and transport measurements of back-gated field-effect transistor devices fabricated on WSe/hBN films show improved optical and electrical properties compared to films grown on sapphire under similar conditions. Our results reveal an important nucleation mechanism for the epitaxial growth of van der Waals heterostructures and demonstrate hBN as a superior substrate for single-crystal transition-metal dichalcogenide (TMD) films, resulting in a reduced density of IDBs and improved properties. The results motivate further efforts focused on the development of single crystal hBN substrates and epilayers for synthesis of wafer-scale single crystal TMD films.