Bandstructure manipulation of epitaxial graphene on SiC(0001) by molecular doping and hydrogen intercalation
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Bandstructure manipulation of epitaxial graphene on SiC(0001) by molecular doping and hydrogen intercalation Ulrich Starke Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany ABSTRACT Graphene, a monoatomic layer of graphite hosts a two-dimensional electron gas system with large electron mobilities which makes it a prospective candidate for future nanocarbon devices. Grown epitaxially on silicon carbide (SiC) wafers, large area graphene samples appear feasible and integration in existing device technology can be envisioned. A precise control of the number of graphene layers and growth of large homogeneous graphene samples can be achieved. However, as-grown epitaxial graphene on SiC is electron doped due to the influence of the reconstructed interface layer present between graphene and SiC. Covalent bonds between SiC and the first carbon layer grown on top induce a dipole layer which induces charges into the graphene. As a result, the Dirac point energy where the π-bands cross is shifted away from the Fermi energy, so that the ambipolar properties of graphene cannot be exploited. How this effect can be overcome by a precise control and manipulation of the electronic structure of the π-bands is demonstrated by two methods. On the one hand, transfer doping of the epitaxial graphene surfaces with the strong acceptor molecule tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) allows for a fine tuning of the doping level. Charge neutrality can be achieved for mono- and bilayer graphene. On bilayer samples the magnitude of the existing bandgap can be increased up to more than double of its initial value. On the other hand, the impact of the SiC-graphene interface can be completely eliminated by annealing the samples in molecular hydrogen. The hydrogen atoms migrate under the graphene layers, intercalate between the SiC substrate and the interface layer and bind to the Si atoms of the SiC(0001) surface. Thus the interface layer, decoupled from the SiC substrate, is turned into a quasi-free standing graphene monolayer. Similarly, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation process represents a highly promising route towards epitaxial graphene based nanoelectronics. INTRODUCTION Graphene can be viewed as a single-atom thick layer pulled out of graphite. It is one of the most promising materials for future nano-scaled carbon electronics for developments beyond the Si CMOS era [1]. Its unconventional two-dimensional electron gas properties have been discovered primarily using graphene flakes obtained from mechanical exfoliation from highlyoriented pyrolytic graphite [2,3]. However, these flakes have to be searched for and manipulated individually which makes up-scaling towards wafer production cumbersome. Other methods of graphene production have been based on a chemical derivation [4,5] which, however, cannot provide high-quality single crystalline material, or by a catalytic reaction on metal surfaces [6-8] which, unpractically, yields the graph
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