Water-assisted multistep MOCVD for wafer-scale layer-by-layer growth of WSe2

Abstract Thin films and heterostructures of two-dimensional transition-metal dichalcogenides (TMDCs) are attractive due to their unique physical properties. However, during metal–organic chemical vapor deposition (MOCVD), the formation of parasitic bilayers (BLs) before monolayer (ML) coalescence is challenging. In this study, a novel multistep MOCVD process, including the additive H 2 O is developed. The aim is to minimize parasitic BL nucleation on WSe 2 ML domains and to grow coalesced uniform ML samples on sapphire substrates. Even after optimization, without additives, BL coverage is approximately 15 percent. We show that introducing H 2 O reduces this value to about 2 percent. H 2 O is equally effective to restrain parasitic (i.e., premature) nucleation in multilayer WSe 2 . Raman, photoluminescence, and x-ray photoelectron spectroscopy measurements neither show oxide species due to H 2 O introduction nor any compromise to the structural or optical properties. Our results provide a solid step toward the direct MOCVD of high-quality TMDC multilayers and heterostructures. Impact statement Uniform transition-metal dichalcogenide (TMDC) films of controlled thickness (i.e., layer number), and their heterostructures, potentially integrated with other two-dimensional (2D) or three-dimensional/bulk materials, promise significant progress in a wide range of applications covering beyond-CMOS electronics, optoelectronics and even neuromorphic components for next-generation artificial intelligence systems. However, for their commercialization, an industrially mature deposition technology with proven scalability, homogeneity and reproducibility is a major prerequisite. In compound semiconductor technology, metal–organic chemical vapor deposition (MOCVD) is the leading technique satisfying all of these requirements. MOCVD is also well suited to grow TMDC films of high quality, but still, quite little is known about the underlying microscopic processes of film formation. Especially, limited migration lengths of metal adatoms cause premature secondary nucleation which manifests in, for example, parasitic bilayer formation on monolayer domains and nonplanarity in 2D-2D heterostructures. In theory, microscopic processes between initial precursor decomposition and final film formation can be tuned by additives to mitigate this problem. However, just a few publications exist, and to the authors’ knowledge, none about the introduction of H 2 O to TMDC MOCVD processes. In this study, the effects of H 2 O addition to a previously optimized WSe 2 growth process are studied, and its impact on migration length are elaborated. Graphical abstract