Graphene is a novel material consisting of a single layer of regularly arranged carbon atoms. Its special properties will presumably lead to the development of faster, smaller and more flexible electronic components and will allow applications in many fields, for example, transistors, touch-screen displays, smart sensors, batteries, super capacitors, solar cells as well as bio markers and sensors for medical technology.
A monolayer of carbon atoms in a "honeycomb" lattice.
Furthermore, graphene shows a quantum Hall effect which is of great interest for metrology. The physical properties of graphene (in particular the electronic band structure) strongly differ from the properties of conventional GaAs heterostructures. Therefore, the quantum Hall resistance of graphene differs in detail from that of GaAs heterostructures. One the one hand, graphene is capable of carrying larger currents. On the other hand, graphene has the potential to show a precise quantum Hall effect already at more moderate temperatures (≥4 K) and more moderate magnetic fields (≤4 T). This would clearly simplify the measuring technique and the effort of the experimental set-up.
Due to this reason, PTB participates in international research programs in which graphene devices are manufactured and characterised. Moreover, Working Group 2.53 fabricates high-quality, epitaxially grown graphene on SiC substrates. From this material, structured quantum Hall devices are fabricated by means of modern lithographic techniques.
Photograph of a graphene-based quantum Hall device with lithographic gold contacts. Both the SiC substrate and the graphene are transparent. Therefore, the Hallbar is indicated by a dashed line.
In our working group, the dc and ac properties of these devices are characterised. This includes the ac quantum Hall resistance, the magneto capacitance and the dissipation factor, as well as the electronic noise. The ac properties of graphene-based quantum Hall resistances are very promising and will be optimised by an improved device design. The long-term aim is a simpler and more user-friendly resistance and impedance standard which also could be applied in calibrations service labs and maybe even in some industry desiring correspondingly low uncertainties.