Sublimation-based wafer-scale monolayer WS 2 formation via self-limited thinning of few-layer WS 2

Atomically-thin monolayer WS 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 WS also makes it highly promising for future high-performance optoelectronics. However, the difficulty in strictly growing monolayer WS , 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 WS by applying the self-limited thinning of multilayer WS formed by sulfurization of WO films. Through a pulsed supply of sulfur precursor vapor under a continuous H flow, the self-limited thinning process can effectively trim down the overgrown multilayer WS to the monolayer limit without damaging the remaining bottom WS monolayer. Density functional theory (DFT) calculations reveal that the self-limited thinning arises from the thermodynamic instability of the WS top layers as opposed to a stable bottom monolayer WS on sapphire above a vacuum sublimation temperature of WS . The self-limited thinning approach overcomes the intrinsic limitation of conventional vapor-based growth methods in preventing the 2 layer WS 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 WS on the wafer-scale.