  Bilayer graphene (BLG) possesses a finite bandgap when a potential difference
is introduced between the two graphene layers. The potential difference is
known to be introduced by surface charge transfer. Thus, it is expected that a
finite bandgap exists at the metal contacts. The bandgap at the metal-BLG
interface can be detected by the superlinear current-voltage characteristics in
back-gate field-effect transistors, caused by carriers tunneling through the
bandgap. The superlinearity was higher in the positively gated region,
attributed to hole doping from the Cr/Au electrodes. The control experiments
using single-layer graphene (SLG) did not have a superlinearity, which is
consistent with the fact that a sizeable bandgap is not expected at the
metal-SLG interface. The opening of a bandgap at the metal-BLG interface is an
additional source of electrode-contact resistance.
