Accumulation-Type Ohmic van der Waals Contacts to Nearly Intrinsic WSe₂Nanosheet-Based Channels: Implications for Field-Effect Transistors

We report the fabrication of ohmic van der Waals (vdW) contacts to nearly intrinsic WSe2 nanosheet-based channels in field-effect transistors (FETs) using degenerately p-doped MoS2 (p+-MoS2) as a contact metal. We demonstrate that accumulation-type ohmic contacts and the high device performance are achievable without electrostatically gating the drain/source contact regions despite the nearly intrinsic nature of WSe2. Back-gated WSe2 FETs with p+-MoS2 bottom contacts (which screen the back-gate electric field in the drain/source regions) exhibit linear output characteristics, a high on/off ratio of 108, and a high two-terminal field-effect mobility up to ∼200 cm2 V-1 s-1 at room temperature. Our theoretical modeling reveals that the p+-MoS2/WSe2 vdW junction behaves like a metal/semiconductor ohmic contact signified by a vanishingly thin space-charge region of ∼1 nm on the p+-MoS2 side and a substantial accumulation layer of free holes on the WSe2 side, which is further verified by additional temperature-dependent and dual-gated measurements of WSe2 FETs. We attribute the formation of accumulation-type ohmic contacts free of a Schottky barrier to the near absence of Fermi-level pinning at the vdW interface and the work function of p+-MoS2 being larger than the ionization energy of WSe2. This study represents an important step toward achieving low-resistance ohmic contacts to two-dimensional (2D) semiconductors by eliminating the Fermi-level pinning effects, which is expected to have significant implications for next-generation 2D semiconductor-based nanoelectronics.