Ohmic Contacts to 2D Semiconductors through van der Waals Bonding

High contact resistances have blocked the progress of devices based on MX2 (M = Mo, W; X = S, Se, Te) 2D semiconductors. Interface states formed at MX2/metal contacts pin the Fermi level, leading to sizable Schottky barriers for p-type contacts in particular. It is shown that i) one can remove the interface states by covering the metal by a 2D layer, which is van der Waals-bonded to the MX2 layer, and ii) one can choose the buffer layer such that it yields a p-type contact with a zero Schottky barrier height. Possible buffer layers are graphene, a monolayer of h-BN, or an oxide layer with a high electron affinity, such as MoO3. The most elegant solution is a metallic M X-2 layer with a high work function. A NbS2 monolayer adsorbed on a metal yields a high work function contact, irrespective of the metal, which gives a barrierless contact to all MX2 layers.