Graphene: a new electronics material

Graphene is a 2-dimensional network of carbon atoms. These carbon atoms are bound within the plane by strong bonds into a honeycomb array comprised of six-membered rings. By stacking of these layers on top of each other, the well known 3-dimensional graphite crystal is formed. Thus, graphen is nothing else than a single graphite layer. The quasi-1-dimensional carbon nanotubes are formed by rolling up graphene sheets. By adding five-membered rings it is possible to form the quasi-0-dimensional fullerenes. The most prominent one is the Buckminsterfullerene C₆₀ (bucky ball), which looks like a football (soccer ball for the Yanks).

The electronic properties of graphene are most interesting. The electronic structure of graphene has been known from theory for a long time and was frequently used to describe the properties of carbon nanotubes. Recent progress in the preparation of graphene and thin stacks of graphene (so-called few layer graphene, FLG) has put graphene into the focus of many research activities worldwide. Thereby different methods are currently used to obtain graphene. Among them are mechanical or chemical exfoliation from graphite and epitaxial growth on silicon carbide or transition metal surfaces.

Graphene differs extremely from conventional 2D electron gas systems created in semiconductor heterostructures. The reason is the linear dispersion relation E(k) of the charge carriers in the vicinity of the K-point of the hexagonal Brillouin zone, where the bonding π-band meets the anti-bonding π*-band. The band structure thus has the form of a double cone, which formally is equivalent to the dispersion relation of rest-mass-less particles. The symmetry of the lattice requires a two-component wave funktion, similar to particles known from relativistic quantum mechanics. The charge carriers and their behavior are described by Diracs equation for mass-less fermions. This has several interesting consequences. An example is the unusual Landau level spectrum when the system is subject to a magnetic field, and the resulting new half-integer quantum Hall effect. Other interesting properties of charge carriers in graphene are their scattering and interference phenomena. Graphene thus provides new and interesting physics to study both experimentally and theoretically.

Another interesting property of graphene is its high charge carrier mobility, for which values of 10.000 cm²/Vs, in some cases even 200.000 cm²/Vs were reported. Graphene is thus a candidate for high frequency electronic devices. But other applications such as ultrasensitive gas detectors, spintronics, or quantum computing were proposed as well.

Several groups in Erlangen are now working on this interesting material. The research carried out concerns different aspects such as growth of graphene, its electronic properties and its modification (e.g. doping, band gap engineering), and processing of graphene into electronic devices.