  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. We show 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. We identify possible buffer layers
such as 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.
