Hydrogen-assisted step-edge nucleation of MoSe₂ monolayers on sapphire substrates

The fabrication of large-area single crystalline monolayer transition metal dichalcogenides (TMDs) is essential for a range of electric and optoelectronic applications. Chemical vapor deposition (CVD) is a promising method to achieve this goal by employing orientation control or alignment along the crystalline lattice of the substrate such as sapphire. On the other hand, a fundamental understanding of the aligned-growth mechanism of TMDs is limited. In this report, we show that the controlled introduction of H₂ during the CVD growth of MoSe₂ plays a vital role in the step-edge aligned nucleation on a c-sapphire (0001) substrate. In particular, the MoSe₂ domains nucleate along the [1120] step-edge orientation by flowing H₂ subsequent to pure Ar. Systematic studies, including the H₂ introduction time, flow rate, and substrate temperature, suggest that the step-edge aligned nucleation of MoSe₂ can be controlled by the hydrogen concentration on the sapphire substrate. These results offer important insights into controlling the epitaxial growth of 2D materials on a crystalline substrate.