Van der Waals Epitaxy of High-Quality Transition Metal Dichalcogenides on Single-Crystal Hexagonal Boron Nitride

Van der Waals (vdW) heterostructures comprising of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (h-BN) are promising building blocks for novel 2D devices. The vdW epitaxy provides a straightforward integration method for fabricating high-quality TMDs/h-BN vertical heterostructures. In this work, the vdW epitaxy of high-quality single-crystal HfSe₂ on epitaxial h-BN/sapphire substrates by chemical vapor deposition is demonstrated. The epitaxial HfSe₂ layers exhibit a uniform and atomically sharp interface with the underlying h-BN template, and the epitaxial relationship between HfSe₂ and h-BN/sapphire is determined to HfSe₂ (0001)[1 $\overline{2}$ 10]//h-BN (0001)[1 $\overline{1}$ 00]//sapphire (0001)[1 $\overline{1}$ 00]. Impressively, the full width at half maximum of the rocking curve for the epitaxial HfSe₂ layer on single-crystal h-BN is as narrow as 9.6 arcmin, indicating an extremely high degree of out-plane orientation and high crystallinity. Benefitting from the high crystalline quality of HfSe₂ epilayers and the weak interfacial scattering of HfSe₂/h-BN, the photodetector fabricated from the vdW epitaxial HfSe₂ on single-crystal h-BN shows the best performance with an on/off ratio of 1 × 10⁴ and a responsivity up to 43 mA W^-1. Furthermore, the vdW epitaxy of other TMDs such as HfS₂, ZrS₂, and ZrSe₂ is also experimentally demonstrated on single-crystal h-BN, suggesting the broad applicability of the h-BN template for the vdW epitaxy.