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.

Figure 1 Graphene formation and Si evaporation on Ru(0001): XPS of the C 1s and Ru 3d core levels. (a) Core level spectra measured on the clean Ru(0001). The components S, B’ and B correspond to first layer, second layer and bulk Ru atoms, respectively. (b) GR on Ru(0001) shows two C 1s peaks for carbon strongly (C2) and weakly (C1) interacting with the Ru surface. A C-induced component appears in the Ru core level, marked as S’. (c) Intercalation of Si that alloys with the metal forming Ru silicide, giving rise to two Ru 3d5/2 peaks (Ru1 and Ru2) and a single C 1s peak C1. The central inset displays a 2D plot of the fast XPS spectra measured while evaporating Si on GR, showing the decay of C2 and the rise of the C1 component.

Figure 2: Oxidation of the Si intercalated graphene/Ru(0001) interface following O 1s, C 1s, and Si 2p core level spectra. Before oxidation the bottom Si 2p spectrum shows two doublets Si1 and Si2 due to Ru-Si bonds in the silicide phase. During oxidation the silicide decomposes and oxygen binds exclusively to Si forming SiO2 as witnessed by the development of the Si 2p and O 1s core level spectra.

The transport measurements show that the recorded resistance has a behavior typical of a two dimensional system and its absolute value has the expected order of magnitude for weakly doped graphene (~1000 Ω) which is five orders of magnitude higher than that expected for the clean ruthenium surface.
The demonstrated process combines the advantages of high-quality large-scale graphene growth with a non-conducting substrate. These results are expected to provide new insight for fundamental studies on graphene, and to open new perspectives for the advancement of next-generation graphene-based devices.

This research was conducted by the following research team:

  • Silvano Lizzit, Paolo Lacovig, Matteo Dalmiglio, Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
  • Rosanna  Larciprete, CNR-Istituto dei Sistemi Complessi, Roma, Italy
  • Fabrizio Orlando, Alessandro Baraldi, Physics Department, University of Trieste, IOM-CNR TASC Laboratory and CENMAT, Trieste, Italy
  • Lauge Gammelgaard, Capres A/S, 2800 Kgs. Lyngby, Denmark
  • Lucas Barreto, Marco Bianchi, Edward Perkins, Philip Hofmann, Department of Physics and Astronomy, Interdisciplinary Nanoscience Center, Aarhus University, Aarhus, Denmark


Silvano Lizzit, Rosanna Larciprete, Paolo Lacovig, Matteo Dalmiglio, Fabrizio Orlando, Alessandro Baraldi, Lauge Gammelgaard, Lucas Barreto, Marco Bianchi, Edward Perkins, and Philip Hofmann, "Transfer-Free Electrical Insulation of Epitaxial Graphene from its Metal Substrate",
Nano Letters 12, 4503 (2012); DOI: 10.1021/nl301614j.
Commented in the Research Highlights of Nature Nanotechnology, Graphene: Silica in between, Nature Nanotech. 7, 613 (2012).

Last Updated on Friday, 14 December 2012 14:17