Evidence of air-induced surface transformation of atomic step-engineered sapphire in relation to epitaxial growth of 2D semiconductors

Engineering sapphire substrates with specific surface characteristics is crucial for the epitaxial growth of high-quality wafer-scale transition metal dichalcogenides, essential for integration with semiconductor industry processes. Here, we report that atomic-step-engineered sapphire surfaces undergo structural and chemical changes upon air exposure, which may be associated with surface hydrolysis and the formation of aluminum (oxy)hydroxides as revealed by a self-developed charge-contrast enhanced X-ray photoelectron spectroscopy technique. We suggest these species transform into oxygen-deficient Al2O3-x under typical growth conditions, associated with disrupted domain alignment. We further demonstrate that ultraviolet light irradiation in air appears to mitigate this degradation, restoring surface stoichiometry and promoting epitaxial alignment. The grown monolayer WS₂ films exhibit high crystalline quality, good uniformity, and low defect density. Statistical analysis of 100 field-effect transistors shows a device yield >95% and a mobility variation