Experimental signature of bandgap opening in bilayer graphene at metal contacts

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.