Layer-by-layer epitaxial growth of transition-metal dichalcogenide thin films and heterostructures on sapphire by molecular-beam epitaxy.

Bottom up synthesis of transition-metal dichalcogenide (TMDC) thin film is of significant importance in particular for future advanced researches on TMDC-based heterostructures. The most-frequently-used method is chemical-vapor deposition enabling wafer-scale growth of monolayer TMDCs, but growing epitaxial thin films of TMDCs with in-plane orientation in millimeter scale is still challenging. We have recently succeeded in establishing a versatile route to layer-by-layer epitaxial growth of millimeter-scale TMDC thin films on insulating sapphire substrates by molecular-beam epitaxy (MBE)^[1]. The growth recipe is broadly applicable to various TMDCs, opening a door to fabrication of van der Waals heterostructures with various types of TMDCs toward novel properties and functionalities. In the presentation, we will introduce our growth recipe and discuss epitaxial relationships between TMDC thin films and sapphire substrates revealed by in-plane x-ray diffraction measurements. We will also show transport properties of those MBE-grown TMDC epitaxial thin films and heterostructures. [1] M. Nakano et al., Nano Lett. 17, 5595 (2017).