Graphene contacts to a HfSe/SnS heterostructure.

Two-dimensional (2D) heterostructures and all-2D contacts are of high interest for electronic device applications, and the SnS/HfSe bilayer heterostructure with graphene contacts has some unique, advantageous properties. The SnS/HfSe heterostructure is interesting because of the strong intermixing of the two conduction bands and the large work function of the SnS. The band lineup of the well separated materials indicates a type II heterostructure, but the conduction band minimum of the SnS/HfSe bilayer is a coherent superposition of the orbitals from the two layers with a spectral weight of 60% on the SnS and 40% on the HfSe for AA stacking. These relative weights can be either increased or reversed by an applied vertical field. A 3×3 supercell of graphene and a 2×2 supercell of SnS/HfSe have a lattice mismatch of 0.1% and both the SnS/HfSe conduction band at M and the graphene Dirac point at K are zone-folded to Γ. Placing graphene on the SnS/HfSe bilayer results in large n-type charge transfer doping of the SnS/HfSe bilayer, on the order of 10/cm, and the charge transfer is accompanied by a negative Schottky barrier contact for electron injection from the graphene into the SnS/HfSe bilayer conduction band. Binding energies and the anti-crossing gaps of the graphene and the SnS/HfSe electronic bands both show that the coupling of graphene to the HfSe layer is significantly larger than its coupling to the SnS layer. A tunneling Hamiltonian estimate of the contact resistance of the graphene to the SnS/HfSe heterostructure predicts an excellent low-resistance contact.