Spectroscopic Investigation on Contacts between Semiconducting TMDs and Semimetals with van der Waals Interface

Before two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) can replace silicon-based electronic devices, the issue of high contact resistance between the 2D semiconductors (SC) and their metallic contacts must be overcome. Here, we create epitaxial van der Waals interfaces with monolayer (ML) TMDs (MoS₂, WS₂, WSe2) and bulk semimetals (SM) (Bi, Sb, and graphite) as studied by scanning tunneling microscopy/spectroscopy (STM/S). We use barrier resonance mode to directly measure the work function and use STS mode to measure the band edges and Fermi level positions of the SC and SM individually. This combination allows us to assess the validity of Schottky – Mott (S–M) rule at different SC-SM junctions. For WS₂ on Bi, the S-M rule works perfectly, and the interface contains a very small barrier heigh (SBH) of ~ 0.1 eV, facilitating a good Ohmic contact. For MoS₂ on Bi, the S–M rule yields a negative SBH of -0.3 eV, which is not possible because the degenerate n-type doping screens out the interfacial dipolar field. Indeed, in this case we find that the Fermi level is located at ~ 0.1 eV above the conduction band minimum (CBM) where the CBM states at the K-point is also revealed using angle-resolved photoelectron spectroscopy. The case for WSe₂ on Bi is very similar to that for WS₂ on Bi. Replacing Bi by Sb, however, reveals a breakdown of S–M rule, albeit not by much, with a deviation of ~ 0.2 eV suggesting the formation of an interfacial dipole. When the semimetal is replaced by graphite, we find that the S–M rule breaks down significantly. The underlying mechanism for this break-down will be discussed.