Synthesis of wafer-scale monolayer MoS₂ on sapphire: Unlocking the influence of key growth parameters

Large-scale synthesis of high-quality two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS₂), is a prerequisite for their lab-to-fab transition. It is crucial to systematically explore and understand the influence of key synthetic conditions on the nucleation, uniformity, and quality of MoS₂ wafers. Here, we report the epitaxial growth of high-quality and uniform monolayer MoS₂ films on 2-in c-plane sapphire by chemical vapor deposition (CVD) method under optimized growth conditions (0–1 mg NaCl, adequate S/Mo ratio, and the addition of 0–1 sccm O₂). We systematically explore the influence of critical synthetic conditions on the nucleation, and stitching of MoS₂ domains over the wafer scale, including the dosage of the alkali metal salt NaCl additive, the evaporation temperature of MoO₃, the distance between MoO₃ and the substrate, and the flow rate of O₂. Among them, the dosage of NaCl and the S/Mo ratio have important influences on the quality and film coverage of MoS₂, while the flow rate of O₂ plays a key role in controlling the nucleation density and domain size. We further discovered that a-plane sapphire could easily guide the unidirectional growth of MoS₂ without the need for other specific synthetic conditions compared with c-plane and m-plane sapphire. The field-effect transistors (FETs) fabricated from the full-coverage films show an average and the highest mobilities of 28.5 and around 45 cm²·V⁻¹·s⁻¹, respectively.