Two-Dimensional Crystal Grain Size Tuning in WS2 Atomic Layer Deposition: An Insight in the Nucleation Mechanism

When two-dimensional (2D) group-VI transition metal dichalcogenides such as tungsten disulfide (WS2) are grown by atomic layer deposition (ALD) for atomic growth control at low deposition temperatures (<= 450 degrees C), they often suffer from a nanocrystalline grain structure limiting the carrier mobility. The crystallinity and monolayer thickness control during ALD of 2D materials is determined by the nucleation mechanism, which is currently not well understood. Here, we propose a qualitative model for the WS2 nucleation behavior on dielectric surfaces during plasma-enhanced (PE-) ALD using tungsten hexafluoride (WF6), dihydrogen (H-2) plasma and dihydrogen sulfide (H2S) based on analyses of the morphology of the WS2 crystals. The WS2 crystal grain size increases from similar to 20 to 200 nm by lowering the nucleation density. This is achieved by lowering the precursor adsorption rate on the starting surface using an inherently less reactive starting surface, by decreasing the H-2 plasma reactivity, and by enhancing the mobility of the adsorbed species at higher deposition temperature. Since silicon dioxide (SiO2) is less reactive than aluminum oxide (Al2O3), and diffusion and crystal ripening is enhanced at higher deposition temperature, WS2 nucleates in an anisotropic island-like growth mode with preferential lateral growth from the WS2 crystal edges. This work emphasizes that increasing the crystal grain size while controlling the basal plane orientation is possible during ALD at low deposition temperatures, based on insight in the nucleation behavior, which is key to advance the field of ALD of 2D materials. Moreover, this work demonstrates the conformal deposition on three-dimensional (3D) structures, with WS2 retaining the basal plane orientation along topographic structures.