  Two-dimensional (2D) tin monoxide (SnO) has attracted much attention owing to
its distinctive electronic and optical properties, which render itself suitable
as a channel material in field effect transistors (FETs). However, upon contact
with metals for such applications, the Fermi level pinning effect may occur,
where states are induced in its band gap by the metal, hindering its intrinsic
semiconducting properties. We propose the insertion of graphene at the contact
interface to alleviate the metal-induced gap states. By using gold (Au) as the
electrode material and monolayer SnO (mSnO) as the channel material, the
geometry, bonding strength, charge transfer and tunnel barriers of charges, and
electronic properties including the work function, band structure, density of
states, and Schottky barriers are thoroughly investigated using
first-principles calculations for the structures with and without graphene to
reveal the contact behaviours and Fermi level depinning mechanism. It has been
demonstrated that strong covalent bonding is formed between gold and mSnO,
while the graphene interlayer forms weak van der Waals interaction with both
materials, which minimises the perturbance to the band structure of mSnO. The
effects of out-of-plane compression are also analysed to assess the performance
of the contact under mechanical deformation, and a feasible fabrication route
for the heterostructure with graphene is proposed. This work systematically
explores the properties of the Au-mSnO contact for applications in FETs and
provides thorough guidance for future exploitation of 2D materials in various
electronic applications and for selection of buffer layers to improve
metal-semiconductor contact.
