Two-dimensional metals thickness scaling effect on electrical contact in metal–semiconductor junctions: Carrier transport and ultrafast dynamics study

Two-dimensional (2D) van der Waals metal–semiconductor junctions (MSJs) with low-resistance contacts have great potential for designing high-performance electronic and optoelectronic devices. However, due to the quantum confinement effect, 2D metals exhibit layer-dependent conductivity that inevitably influences the contact properties of 2D MSJs, the underlying mechanism behind this effect is unclear. Herein, taking multilayer graphene and MoS2 as examples, we systematically studied the effect of 2D metal layer number on the MSJs by non-equilibrium Green's function and non-adiabatic molecular dynamics methods. Compared with the trilayer-graphene/MoS2 (Gr/MoS2) MSJ (5.0 × 104 KΩ µm), the contact resistance of the monolayer-Gr/MoS2 MSJ (7.6 × 103 KΩ µm) is reduced by one order of magnitude. Under a 0.6 V bias voltage, the reduced contact resistance results in the current increasing from 0.1 to 60 nA. The superior transport performance of monolayer-Gr/MoS2 MSJ derives from the reduction in the Schottky barrier of the MSJ as the number of graphene layers decreases, whereas the tunneling barrier remains nearly constant. Meanwhile, monolayer-Gr/MoS2 MSJ exhibits high photogenerated carrier gain, which is attributed to the ultrafast transfer (388 fs) of photogenerated electrons and long carrier lifetime (71 ns), resulting in superior optoelectronic performance. Our study presents a layer-number engineering strategy for optimizing contact properties in 2D MSJs.