The growth scale and kinetics of WS2 monolayers under varying H2 concentration

The optical and electronic properties of tungsten disulfide monolayers (WS2) have been extensively studied in the last few years, yet growth techniques for WS2 remain behind other transition metal dichalcogenides (TMDCs) such as MoS2. Here we demonstrate chemical vapor deposition (CVD) growth of continuous monolayer WS2 films on mm(2) scales and elucidate effects related to hydrogen (H2) gas concentration during growth. WS2 crystals were grown by reduction and sulfurization of WO3 using H2 gas and sulfur evaporated from solid sulfur powder. Several different growth formations (in-plane shapes) were observed depending on the concentration of H2. Characterization using atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed etching of the SiO2 substrate at low concentrations of H2 and in the presence of an Ar carrier gas. We attribute this to insufficient reduction of WO3 during growth. High H2 concentrations resulted in etching of the grown WS2 crystals after growth. The two dimensional X-ray diffraction (2D XRD) pattern demonstrates that the monolayer WS2 was grown with the (004) plane normal to the substrate, showing that the WS2 conforms to the growth substrate.