Optimisation of processing conditions during CVD growth of 2D WS₂ films from a chloride precursor

Monolayer tungsten disulphide (WS₂) is a direct band gap semiconductor which holds promise for a wide range of optoelectronic applications. The large-area growth of WS₂ has previously been successfully achieved using a W(CO)₆ precursor, however, this is flammable and a potent source of carbon monoxide (CO) upon decomposition. To address this issue, we have developed a process for the wafer-scale growth of monolayer WS₂ from a tungsten hexachloride (WCl₆) precursor in a commercial cold-wall CVD reactor. In comparison to W(CO)₆, WCl₆ is less toxic and less reactive and so lends itself better to the large-scale CVD growth of 2D layers. We demonstrate that a post-growth H₂S anneal can lead to a dramatic improvement in the optical quality of our films as confirmed by photoluminescence (PL) and Raman measurements. Optimised films exhibit PL exciton emission peaks with full width at half maximum of 51 ± 2 meV, comparable to other state-of-the-art methods. We demonstrate that our WS₂ films can be readily transferred from the sapphire growth substrate to a Si/SiO₂ target substrate with no detectable degradation in quality using a polystyrene support layer. Our approach represents a promising step towards the industrial-scale fabrication of p-n junctions, photodetectors and transistors based on monolayer WS₂.