  Graphene-metal contact resistance is governed by both intrinsic and extrinsic
factors. Intrinsically, both the density of states bottleneck near the Dirac
point and carrier reflection at the graphene-metal interface lead to a high
contact resistance. Moreover, graphene exhibits insulating behavior for
out-of-the-plane conduction. Extrinsically, surface contamination introduced by
photoresist residue or different adsorbed species during standard lithography
processing alters graphene's intrinsic properties by uncontrolled doping and
increased scattering which results in high and inconsistent contact resistance.
Here we demonstrate a femto-second laser assisted direct patterning of graphene
microstructures that enables us to study both intrinsic and extrinsic effects
on the graphene-metal interface. We show that a clean graphene-metal interface
is not sufficient to obtain contact resistance approaching the intrinsic limit
set by the quantum resistance. We also demonstrated that unlike CVD graphene,
edge state conduction (or end-contact) is not spontaneously formed by metal
deposition in case of graphene grown on SiC(0001). We conclude that for
epitaxial graphene, intentional end-contact formation is necessary to obtain
contact resistance near the quantum contact resistance limit.
