Unusual Fermi-level pinning and Ohmic contact engineering for Janus TMD heterojunctions from an electronegativity perspective

Using density functional theory calculations, we systematically investigated the fat band structures and Schottky barriers of 72 van der Waals heterojunctions (vdWHs) formed by combining 1T-phase transition metal dichalcogenides (TMDs) (MX2, M = Mo, W; X = S, Se, Te) with 2H-phase Janus TMDs (MXY, M = Mo, W; X, Y = S, Se, Te). Unlike conventional TMDs, Janus TMDs lack mirror symmetry and exhibit a built-in in-plane dipole due to their asymmetric atomic configuration. A larger electronegativity gradient across the Janus TMDs leads to enhanced charge redistribution at the vdWH interface and a stronger in-plane dipole, resulting in unusual Fermi-level pinning (FLP). The calculated pinning factors for 1T-TMDs/MoSTe and 1T-TMDs/WSTe vdWHs are particularly low, reflecting a strong FLP effect. Despite the wide variation in work functions among the 1T-TMDs, their Fermi levels are consistently pinned near the band edges of MoSTe and WSTe, enabling quasi-Ohmic or Ohmic p-type and n-type contacts. Furthermore, the vdWHs exhibit low tunneling-specific resistivity, confirming their potential for ultra-low contact resistance applications in next-generation single-layer transistors.