Process-Induced Modulation of Domain Orientations during WS2 Epitaxy by Metal-Organic Chemical Vapor Deposition on Sapphire

Abstract

Single monolayers (MLs) of transition metal dichalcogenides (TMDs) like molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are promising semiconductors for next-generation logic devices, photodetectors, and light-emitting diodes. Applications require industry-relevant deposition techniques that form large monocrystalline TMD domains with low defect density. In this work, we study the growth of WS2 by metal-organic chemical vapor deposition on c-plane sapphire substrates at different temperatures. We investigate the growth phenomena during the initial stages before ML formation and apply the obtained knowledge to design an optimized deposition process. High deposition temperatures (1000 degrees C) yield a high degree of in-plane crystal orientation but give rise to codeposition of WS2 and tungsten, presumably due to the fast desorption of sulfur species. The preferred WS2 domain orientation can be modulated by the process: WS2 deposition at 850 degrees C followed by annealing of WS2 crystals in H2S at 1000 degrees C results mainly in step-edge guided domains, whereas crystal lattice-guided domains are predominant during subsequent deposition at 1000 degrees C. The optimized three-step process results in a preferential formation of 0 and 60 degrees oriented domains in the closed WS2 ML, with less than 2% of crystals with different orientations. These presented insights can be used to modulate and optimize the WS2 structure further toward monocrystalline monolayers.