Transfer-Free Electrical Insulation of Epitaxial Graphene

In the last years the scientific interest on graphene, the two dimensional arrangement of carbon atoms just one atom thick, has grown enormously due to the remarkable properties of this material. In particular, the very high carrier mobility at room temperature, tolerance to high temperature and inertness make graphene the most promising candidate for future nanoelectronics. Several manufacturing methods have been developed to produce graphene layers of various dimensions and quality. However, exfoliation-based techniques produce small flakes or graphene of poor quality whereas large-scale growth on metal substrates requires the transfer of graphene on an insulating support in order to guarantee the conduction through graphene.
We have developed a novel transfer-free method to electrically insulate epitaxial graphene from the metal substrate it is grown-on. This is achieved by growing an insulating SiO₂ layer of the desired thickness directly under the epitaxial graphene layer, through a stepwise reaction between intercalated silicon and oxygen. Firstly, epitaxial graphene is grown on a Ru(0001) crystal surface. The graphene layer is then exposed to silicon that intercalates below graphene and forms a silicide with the metal. Finally, by exposure of the surface to molecular oxygen, the metal silicide is oxidized to form an insulating SiO₂ layer that separates graphene from the metal. We have also shown that in this system the transport is dominated by graphene and not by the underlying metal by performing lateral transport measurements.
All the processes taking place under the graphene layer have been followed by high-energy-resolution X-ray photoelectron spectroscopy experiments performed at the SuperESCA beamline. The starting point is the clean Ru(0001) surface (Fig. 1a). After graphene growth, the C 1s spectrum shows two contributions from graphene regions weakly (C1) and strongly (C2) interacting with the Ru substrate. The subsequent exposure to silicon at 720 K causes the complete conversion of the C2 component into the narrow component C1 (Fig. 1c). At this point, as shown in Fig. 2, the Si 2p spectrum exhibits two doublet components, Si1 and Si2, likely due to the formation of Ru silicide (Fig. 1b). The surface is then exposed to molecular oxygen at a temperature of 640 K. The oxygen intercalates below graphene, leading to the progressive silicide oxidation, as evidenced by the evolution of the Si 2p spectrum which transforms into a broad peak at ~103 eV, indicative of Si in a SiO₂ environment. The C 1s intensity converts into the single and narrow C3 peak, interpreted as graphene supported on SiO₂.