Sublimation-based wafer-scale monolayer WS₂ formation via self-limited thinning of few-layer WS₂

Atomically-thin monolayer WS₂ is a promising channel material for next-generation Moore's nanoelectronics owing to its high theoretical room temperature electron mobility and immunity to short channel effect. The high photoluminescence (PL) quantum yield of the monolayer WS₂ also makes it highly promising for future high-performance optoelectronics. However, the difficulty in strictly growing monolayer WS₂, due to its non-self-limiting growth mechanism, may hinder its industrial development because of the uncontrollable growth kinetics in attaining the high uniformity in thickness and property on the wafer-scale. In this study, we report a scalable process to achieve a 4 inch wafer-scale fully-covered strictly monolayer WS₂ by applying the in situ self-limited thinning of multilayer WS₂ formed by sulfurization of WO films. Through a pulsed supply of sulfur precursor vapor under a continuous H₂ flow, the self-limited thinning process can effectively trim down the overgrown multilayer WS₂ to the monolayer limit without damaging the remaining bottom WS₂ monolayer. Density functional theory (DFT) calculations reveal that the self-limited thinning arises from the thermodynamic instability of the WS₂ top layers as opposed to a stable bottom monolayer WS₂ on sapphire above a vacuum sublimation temperature of WS₂. The self-limited thinning approach overcomes the intrinsic limitation of conventional vapor-based growth methods in preventing the 2^nd layer WS₂ domain nucleation/growth. It also offers additional advantages, such as scalability, simplicity, and possibility for batch processing, thus opening up a new avenue to develop a manufacturing-viable growth technology for the preparation of a strictly-monolayer WS₂ on the wafer-scale.