Direct Epitaxy of SnSe2/SnSe Hetero-Bilayer with a Type-III Band Gap Alignment

Van der Waals (vdW) heterostructures formed by stacking two distinct semiconductor monolayers have gained increasing research interest because of the various predicted and realized exotic phenomena that are absent in the corresponding monolayers. However, constructing such a vdW hetero-bilayer is very challenging and mostly relies on top-down mechanical methods. Here, we report a direct growth of an SnSe2/SnSe hetero-bilayer by using molecular beam epitaxy (MBE), in which elaborate interface engineering is the key to success. Scanning tunneling microscopy (STM) characterization demonstrated the well-defined and uniform moiré patterns, indicating an atomic-scale clean and uniform SnSe2/SnSe interface. In combination with first-principles density functional theory (DFT) calculations, we further unveiled a type-III band gap alignment between the SnSe2 and SnSe monolayers. This work provides a new method for building vertical SnSe2/SnSe hetero-bilayers and a novel platform for exploring functional devices based on the type-III band alignment.