Contact modulation in NbX₂-HfY₂ (X = Se, Te; Y = S, Se) van der Waals heterostructures: Schottky-to-Ohmic transition

Extensive efforts have been directed toward exploring novel metal-semiconductor (MS) interfaces and engineering their contact properties to minimize contact resistance. The electronic structure and interfacial properties of van der Waals heterostructures (vdWHs) composed of metallic NbX₂ (X = Se, Te) and semiconducting HfY₂ (Y = S, Se) monolayers are systematically investigated via first-principles calculations. Binding energy analysis, phonon dispersion spectra, and Born mechanical stability criteria are used to verify the structural integrity, thermodynamic stability, and experimental feasibility of these materials. Ab Initio Molecular Dynamics (AIMD) simulations also confirm the thermal stability of the NbX₂-HfY₂ vdWHs at 300 K, with no significant structural reconstruction or bond breakage observed over a 4 ps timescale. These investigations reveal that the vdWHs preserve the intrinsic electronic characteristics of their constituent layers due to weak vdW coupling, while benefiting from the high metallic conductivity of the NbX₂ layer. At equilibrium, the interfaces predominantly form n-type Schottky contact, where a significant interfacial dipole potential modulates the band alignment. These heterostructures exhibit highly tunable behavior; specifically, mechanical deformation triggers a transition from Schottky to Ohmic contacts, facilitating high-efficiency carrier injection. This transition is driven by enhanced orbital hybridization between Nb-d and Hf-d states near the Fermi level. While HfS₂-based systems retain a robust n-type character, the HfSe₂ counterparts display higher sensitivity to band shifting under strain. These findings provide a comprehensive theoretical framework for utilizing NbX₂-HfY₂ (X= Se, Te; Y = S, Se) MS contacts in next-generation, low-resistance 2D field-effect transistors and flexible nanoelectronics.