Two-dimensional semiconductor HfSe₂ and MoSe₂/HfSe₂ van der Waals heterostructures by molecular beam epitaxy

Using molecular beam epitaxy, atomically thin 2D semiconductor HfSe₂ and MoSe₂/HfSe₂ van der Waals heterostructures are grown on AlN(0001)/Si(111) substrates. Details of the electronic band structure of HfSe₂ are imaged by in-situ angle resolved photoelectron spectroscopy indicating a high quality epitaxial layer. High-resolution surface tunneling microscopy supported by first principles calculations provides evidence of an ordered Se adlayer, which may be responsible for a reduction of the measured workfunction of HfSe₂ compared to theoretical predictions. The latter reduction minimizes the workfunction difference between the HfSe₂ and MoSe₂ layers resulting in a small valence band offset of only 0.13 eV at the MoSe₂/HfSe₂ heterointerface and a weak type II band alignment.