Process-Induced Modulation of Domain Orientations\nduring WS<sub>2</sub> Epitaxy by Metal–Organic Chemical Vapor\nDeposition on Sapphire

Single\nmonolayers (MLs) of transition metal dichalcogenides (TMDs)\nlike molybdenum disulfide (MoS<sub>2</sub>) and tungsten disulfide\n(WS<sub>2</sub>) are promising semiconductors for next-generation\nlogic devices, photodetectors, and light-emitting diodes. Applications\nrequire industry-relevant deposition techniques that form large monocrystalline\nTMD domains with low defect density. In this work, we study the growth\nof WS<sub>2</sub> by metal–organic chemical vapor deposition\non c-plane sapphire substrates at different temperatures. We investigate\nthe growth phenomena during the initial stages before ML formation\nand apply the obtained knowledge to design an optimized deposition\nprocess. High deposition temperatures (1000 °C) yield a high\ndegree of in-plane crystal orientation but give rise to codeposition\nof WS<sub>2</sub> and tungsten, presumably due to the fast desorption\nof sulfur species. The preferred WS<sub>2</sub> domain orientation\ncan be modulated by the process: WS<sub>2</sub> deposition at 850\n°C followed by annealing of WS<sub>2</sub> crystals in H<sub>2</sub>S at 1000 °C results mainly in step-edge guided domains,\nwhereas crystal lattice-guided domains are predominant during subsequent\ndeposition at 1000 °C. The optimized three-step process results\nin a preferential formation of 0 and 60° oriented domains in the closed WS<sub>2</sub> ML, with less than 2% of crystals with different orientations. These\npresented insights can be used to modulate and optimize the WS<sub>2</sub> structure further toward monocrystalline monolayers.