Fig. 1. Monolayer film growth and characterization.

a Schematic of the MOCVD system with a cold-wall horizontal reactor. b 2″ sapphire wafer with MOCVD grown MoS₂ and WS₂. AFM images of c MoS₂ and d WS₂ at the center and edge of the respective wafers. Towards the center of the MoS₂ film, few bilayers are seen. In-plane XRD ϕ-scan of e MoS₂ and f WS₂ on sapphire (α-Al₂O₃), showing the epitaxial relationship between the monolayers and the sapphire substrate. Raman map of the A_1g peak position for g MoS₂ and h WS₂ films transferred from the growth substrate onto the device fabrication substrate with 50 nm ALD Al₂O₃. Low variation in the peak position is observed for both MoS₂ and WS₂ with an average of ≈403.5 cm⁻¹ and ≈417 cm⁻¹, respectively. PL peak map of i MoS₂ with an average of ≈1.85 eV and j WS₂ with an average of ≈1.97 eV, confirm monolayer films. PL is a characteristic of monolayer film owing to indirect to direct bandgap transition.