The importance of the image forces and dielectric environment in modeling contacts to two-dimensional materials
Madhuchhanda Brahma, Maarten L. Van de Put, Edward Chen, Massimo V. Fischetti, William G. Vandenberghe
Abstract
Abstract The performance of transistors based on two-dimensional (2D) materials is affected largely by the contact resistance due to high Schottky barriers at the metal-2D-material interface. In this work, we incorporate the effect of surrounding dielectrics and image-force barrier-lowering in calculating the resistance of Schottky edge-contacts between a metal and a transition-metal dichalcogenide (TMD) thin layer. The electrostatic potential is computed by solving the Poisson equation numerically. The transmission probability is computed using the Wentzel–Kramers–Brillouin (WKB) approximation using the full-band density of states obtained from density functional theory (DFT). The effect of the image force is obtained analytically using the Coulomb kernel of a point charge with boundary conditions appropriate to the geometry we have considered. We find that the image-force barrier-lowering (IFBL) in edge-contacts is determined mainly by the dielectric permittivity of the surrounding oxide. We find that low- κ surrounding dielectrics are crucial for obtaining low resistance monolayer-TMD edge-contacts. Our results show metal-to-n(p)-type MoS 2 (WSe 2 ) edge-contacts with SiO 2 as top and bottom insulators, a doping concentration > 1 × 10 13 cm −2 and a metal work-function < 5.1 eV ( > 4.6 eV) result in a contact resistance as low as 50 Ω ⋅ μm.