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. 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. Here, we investigate the epitaxial growth kinetics of the MoS₂ on a-plane, m-plane, and c-plane sapphire substrates under identical metal-organic chemical vapor deposition (MOCVD) growth conditions and substrate pretreatment. Our results reveal a strong surface-guided epitaxy on c-plane sapphire, leading to highly aligned MoS₂ domains with high coverage. In contrast, weaker interfacial interactions on a-plane and m-plane sapphire result in smaller, randomly oriented domains with lower density. Importantly, first-principles calculations indicate that the c-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 c-plane sapphire exhibit outstanding electronic properties, including an average mobility of 25.8 cm^{2·V−1·s−1} and an on/off ratio exceeding 10⁵. 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.