The ohmic contact between zinc oxide and highly oriented pyrolytic graphite

Contacts between highly oriented pyrolytic graphite (HOPG) and graphene with many important semiconductors, including silicon, are of Schottky type with significant junction energy barrier heights. Here, we show that the junction between the transparent oxide semiconductor ZnO and HOPG is ohmic in nature, but the oxygen species adsorbed to the HOPG surface at the junction cause an electron energy barrier buildup and render the device current vs. voltage characteristics rectifying. Upon a brief heat treatment in vacuum, these devices demonstrate their intrinsic ohmicity. The presented model describes the obtained experimental data and clarifies the important role of the oxygen adsorption in determining the quality of the graphene/semiconductor electrical contacts.