Sapphire facet engineering for van der Waals epitaxy of 2D semiconductors by MOCVD

The growth of atomically thin transition metal dichalcogenide (TMDC) films via van der Waals (vdW) epitaxy offers a promising route to overcome the stringent lattice-matching constraints of conventional heteroepitaxy. However, the substrate effect remains critical and complex, influencing the crystallinity, orientation, as well as device performance of the TMDC films.&nbsp;Sapphire is widely used for TMDC epitaxy due to its atomic flatness and chemical stability; a universal understanding of how its crystallographic planes determine epitaxial behavior—separating the inherent substrate effect from other processing variables—is still lacking.&nbsp;Here, we investigate the epitaxial growth kinetics of the MoS₂ on <em>a</em>-plane, <em>m</em>-plane, and <em>c</em>-plane sapphire substrates under identical metal-organic chemical vapor deposition (MOCVD) growth conditions and substrate pretreatment.&nbsp;Our results reveal a strong surface-guided epitaxy on <em>c</em>-plane sapphire, leading to highly aligned MoS₂ domains with high coverage. In contrast, weaker interfacial interactions on <em>a</em>-plane and <em>m</em>-plane sapphire result in smaller, randomly oriented domains with lower density. Importantly, first-principles calculations indicate that the <em>c</em>-plane sapphire substrate has the highest surface adsorption energy and interlayer charge density, demonstrating strong coupling characteristics. Furthermore, electrical characterizations further demonstrate that the MoS₂ films on <em>c</em>-plane sapphire exhibit outstanding electronic properties, including an average mobility of 25.8 cm² V^-1 S^-1 and an on/off ratio exceeding 10⁵.&nbsp;This study elucidates the inherent influence of the sapphire's crystallographic planes, providing general insights into substrate-guided vdW epitaxy and a reliable strategy for wafer-scale single-crystal TMDC growth.