Efficient Carrier Tunneling and Weak Fermi-Level Pinning Enabled by Intrinsic Covalent-like Quasi-bonding Interactions in van der Waals Metal–Semiconductor Junctions

Efficient electrical contacts in two-dimensional metal–semiconductor junctions (2D MSJs) are crucial for the continued scaling of 2D field-effect transistors. While ultraclean van der Waals (vdW) contacts, known for their weak Fermi-level pinning, are highly promising, their performance is often limited by extra contact resistance from the vdW-gap-induced tunnel barrier. Here, using first-principles calculations, we propose a strategy that achieves the simultaneous realization of high carrier tunneling efficiency and weak Fermi-level pinning in 2D MSJs within the vdW interaction regime without the need for external interfacial engineering. Using a ferroelectric PtBi 2 monolayer as the metal electrode and various transition-metal dichalcogenides (TMDs) as semiconductors, we identify an intrinsic, weakly covalent-like quasi-bonding mechanism enabled by the out-of-plane Bi p z orbitals that cross the Fermi level. These inherent covalent-like vdW interactions at the PtBi 2 /TMD interfaces simultaneously promote efficient carrier tunneling and weak Fermi-level pinning. Furthermore, the Schottky barrier height can be tuned via ferroelectric polarization of the PtBi 2 monolayer. Owing to this unique interfacial coupling effect, pristine PtBi 2 / M S 2 ( M = Mo, W) and strained PtBi 2 /MSe 2 junctions are n-type ohmic contacts with contact resistances below 100 Ω·μm at a carrier density of 3 × 1013 cm –2 . Meanwhile, the Te-interfaced PtBi 2 /WSTe junction can be tuned into a p-type ohmic contact under combined strain and electric field, achieving an ultralow resistance of 73.28–77.63 Ω·μm at the same carrier density. This work underscores the critical role of the inherent orbital characteristics of metal electrodes in interfacial coupling with 2D semiconductors, offering a key descriptor for selecting contact metals to obtain high-performance vdW 2D MSJs with an optimal balance between carrier tunneling efficiency and Fermi-level pinning.