Long-time drift induced changes in electrical characteristics of graphene–metal contacts

Chemical vapour deposition (CVD) graphene is commonly recognized as promising 2D material for development of electronic devices. However, the long-term drift of electrical parameters still requires deeper understanding before the technological means can be selected for an individual type of the devices. In this work, the changes in the electrical resistance were investigated over long time in the planar samples based on the CVD graphene with Au and Ni contacts. The samples were arranged as arrays of the resistors on a silicon substrate covered with a 250 nm layer of thermally grown silicon dioxide. The annealing in pure argon gas flow at 573 K was used to return the electrical properties of samples to the initial state. The effects of drift and annealing were compared for the three parts of structures, namely the electrical contact, the graphene sheet and the edge of the metal film with a hanging graphene sheet. For these parts, the resistance changes were related to the strain and doping of supported and hanged parts of the graphene sheet. Raman spectroscopy and Kelvin force probe microscopy were used to characterize charge doping, strain and work function in the graphene. The drift was explained in terms of the most prominent changes in the doping, strain and work function detected within the edge zone of the contact. It was proved that the annealing significantly changed the p-type doping and work function in the graphene layer in this edge zone. The properties were almost independent of test conditions in the SiO2 supported graphene. The changes in the contact parameters produced by drift mechanisms were proved being reversible under proper annealing conditions.