Critical Role of Surface Termination of Sapphire Substrates in Crystallographic Epitaxial Growth of MoS Using Inorganic Molecular Precursors.

A highly reproducible route for the epitaxial growth of single-crystalline monolayer MoS on a C-plane sapphire substrate was developed using vapor-pressure-controllable inorganic molecular precursors MoOCl and HS. Microscopic, crystallographic, and spectroscopic analyses indicated that the epitaxial MoS film possessed outstanding electrical and optical properties, excellent homogeneity, and orientation selectivity. The systematic investigation of the effect of growth temperature on the crystallographic orientations of MoS revealed that the surface termination of the sapphire substrate with respect to the growth temperature determines the crystallographic orientation selectivity of MoS. Our results suggest that controlling the surface to form a half-Al-terminated surface is a prerequisite for the epitaxial growth of MoS on a C-plane sapphire substrate. The insights on the growth mechanism, especially the significance of substrate surface termination, obtained through this study will aid in designing efficient epitaxial growth routes for developing single-crystalline monolayer transition metal dichalcogenides.