Efficient Carrier Tunneling and Weak Fermi-Level Pinning Enabled by Intrinsic Covalent-like Quasi-bonding...

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 PtBi₂ 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 PtBi₂/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 PtBi₂ monolayer. Owing to this unique interfacial coupling effect, pristine PtBi₂/MS₂ (M = Mo, W) and strained PtBi₂/MSe₂ 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 PtBi₂/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.