Effect of metal contact on electron transport and its removal in graphene field effect devices

Effect of metal contact on the transfer characteristics in graphene field effect devices is studied for several metal species. We find that two Dirac points appear in the transfer characteristics: one corresponding to the graphene portion underneath/near the contact and the other to the graphene portion far from the contact. The difference in the gate voltages of the these Dirac points correlates with the work function of the contact metal, clearly showing that the additional Dirac point originates from the carrier doping from the contact metal. This carrier doping effect is successfully removed by inserting multilayer graphene between metal and graphene, without significantly increasing the contact resistance.