Revisiting the Epitaxial Growth Mechanism of 2D TMDC Single Crystals.

Epitaxial growth of 2D transition metal dichalcogenides (TMDCs) on sapphire substrates has been recognized as a pivotal method for producing wafer-scale single-crystal films. Both step-edges and symmetry of substrate surfaces have been proposed as controlling factors. However, the underlying fundamental still remains elusive. In this work, through the molybdenum disulfide (MoS) growth on C/M sapphire, it is demonstrated that controlling the sulfur evaporation rate is crucial for dictating the switch between atomic-edge guided epitaxy and van der Waals epitaxy. Low-concentration sulfur condition preserves O/Al-terminated step edges, fostering atomic-edge epitaxy, while high-concentration sulfur leads to S-terminated edges, preferring van der Waals epitaxy. These experiments reveal that on a 2 in. wafer, the van der Waals epitaxy mechanism achieves better control in MoS alignment (≈99%) compared to the step edge mechanism (<85%). These findings shed light on the nuanced role of atomic-level thermodynamics in controlling nucleation modes of TMDCs, thereby providing a pathway for the precise fabrication of single-crystal 2D materials on a wafer scale.