MOCVD and Device Technology of 2D-TMDC Layers and Heterostructures

Two-dimensional transition metal dichalcogenides (2D-TMDC) are attractive materials for various applications such as integration in next-generation highly scaled CMOS technology, neuromorphic computing, (opto)electronic devices and advanced sensors. However, for a successful industrial implementation, reproducible and scalable fabrication processes are required. This includes 2D material synthesis, device processing, and finally circuit integration. Regarding material deposition, MOCVD (metal-organic chemical vapor deposition) is established as a technologically mature method with excellent homogeneity, reproducibility and yield in high-volume fabrication of compound semiconductors on large wafer diameters. Developing MOCVD of 2D-TMDC requires dedicated reactor technology, in-situ monitoring techniques and elaborate deposition processes. Especially for the latter, an in-depth understanding of nucleation and growth mechanisms is a prerequisite. Another advantage of MOCVD is the option to successively deposit 2D-2D heterostructures in a continuous process, avoiding interface contaminations related to mechanical heterostructure stacking.<br/>In many implementation scenarios, 2D integration requires a transfer of 2D films or film stacks from their growth substrates to target wafers. With respect to the atomic thickness and the “surface-only” characteristics of 2D layers, transfer and processing steps demand an even more careful consideration than known for classical bulk semiconductor structures. This especially relates to processing chemicals and all substances coming into contact with the 2D layers.<br/>In this presentation, results of MOCVD process development of 2D materials from the (Mo,W)(S, Se)₂ family will be shown. The focus will be on growth kinetics and layer morphology. The different stages of growth starting with film nucleation, continued to lateral growth as well as finally layer coalescence and cool-down stabilization will be discussed. Critical aspects and the interplay between initial nucleation density, monolayer growth time/chalcogen precursor consumption and parasitic bilayer formation before coalescence of the first monolayer will be highlighted. Migration enhancement by reducing metal precursor supply towards coalescence is one approach to suppress premature bilayer formation, especially for W-containing materials. Limits of the parameter window set by the metal adatom migration length and parasitic carbon deposition will be addressed. Using this approach, 2D-2D heterostructures will be presented and compared to counterparts realized by mechanical stacking of monolayers.<br/>Material characterization relies on spectroscopic in-situ reflectance measurements, SEM (scanning electron microscopy), AFM (atomic force microscopy), Raman, photoluminescence (PL) and X-ray photoemission spectroscopy (XPS). Additionally, to especially access e. g. field-effect transistor performance metrics, electrical devices have to be fabricated. In this talk, selected results from transistors and memristors processed on our layers will be presented. This topic includes details on wet and dry transfer of 2D TMDC layers onto target substrates, lithography processing, etching, metallization and device characteristics. Furthermore, first correlations between characteristics of the MOCVD processes and device metrics will be presented.<br/>Finally, an outlook will be given including strategies to improve layer properties (e. g. by additives promoting metal adatom migration), processing steps and finally device performance.