Ohmic Behavior in Metal Contacts to n/p-Type Transition-Metal Dichalcogenides: Schottky versus Tunneling Barrier Trade-off

High contact resistance (R_C) between 3D metallic conductors and single-layer 2D semiconductors poses major challenges toward their integration in nanoscale electronic devices. While in experiments the large R_C values can be partly due to defects, ab initio simulations suggest that, even in defect-free structures, the interaction between metal and semiconductor orbitals can induce gap states that pin the Fermi level in the semiconductor band gap, increase the Schottky barrier height (SBH), and thus degrade the contact resistance. In this paper, we investigate, by using an in-house-developed ab initio transport methodology that combines density functional theory and nonequilibrium Green’s function (NEGF) transport calculations, the physical properties and electrical resistance of several options for n-type top metal contacts to monolayer MoS₂, even in the presence of buffer layers, and for p-type contacts to monolayer WSe₂. The delicate interplay between the SBH and tunneling barrier thickness is quantitatively analyzed, confirming the excellent properties of the Bi–MoS₂ system as an n-type ohmic contact. Moreover, simulation results supported by literature experiments suggest that the Au–WSe₂ system is a promising candidate for p-type ohmic contacts. Finally, our analysis also reveals that a small modulation of a few angstroms of the distance between the (semi)metal and the transition-metal dichalcogenide (TMD) leads to large variations of R_C. This could help to explain the scattering of R_C values experimentally reported in the literature because different metal deposition techniques can result in small changes of the metal-to-TMD distance besides affecting the density of possible defects.