Engineering Schottky-to-Ohmic contact transition for 2D metal–semiconductor junctions

An Ohmic contact is critical for achieving 2D material-based high performance electronic devices. Unfortunately, the formation of an intrinsic Ohmic contact for 2D materials is difficult; thus, current studies mostly stay in the Schottky regime. In this work, density functional calculations are performed for work function engineering for metal–semiconductor junctions involving 2D H-WSe2 and 2D metals of MX2 (M = Ti, V, Nb, Ta, Mo, and W and X = S and Se). We unambiguously identify a Schottky-to-Ohmic contact transition boundary, beyond which p-type Ohmic contacts are demonstrated to be stable. We show that the Fermi level pinning effect is relatively weak in the Schottky region, while similar pinning-like behavior is strong in the Ohmic region, creating a discontinuity near the contact transition boundary. The observed deviation from the ideal Schottky–Mott limit is directly related to the charge redistribution and interface dipole-induced potential step, reflected by metal work function modification due to contact formation. Our work not only provides a strategy to identify effective Ohmic contacts but also offers insights for prospection into the fundamental electronic properties of van der Waals-based heterojunctions.