Th. Seyller

Raman spectra of epitaxial graphene on SiC(0001)

We present Raman spectra of epitaxial graphene layers grown on 63×63 reconstructed silicon carbide surfaces during annealing at elevated temperature. In contrast to exfoliated graphene a significant phonon hardening is observed. We ascribe that phonon hardening to a minor part to the known electron transfer from the substrate to the epitaxial layer, and mainly to mechanical strain that builds up when the sample is cooled down after annealing. Due to the larger thermal expansion coefficient of silicon carbide compared to the in-plane expansion coefficient of graphite this strain is compressive at room temperature.

Local work function measurements of epitaxial graphene

The work function difference between single layer and bilayer graphene grown epitaxially on 6H-SiC(0001) has been determined to be 135±9 meV by means of the Kelvin probe force microscopy. Bilayer films are found to increase the work function as compared to single layer films. This method allows an unambiguous distinction between interface layer, single layer, and bilayer graphene. In combination with high-resolution topographic imaging, the complex step structure of epitaxial graphene on SiC can be resolved with respect to substrate and graphene layer steps.

Intrinsic Terahertz Plasmons and Magnetoplasmons in Large Scale Monolayer Graphene

We show that in graphene epitaxially grown on SiC the Drude absorption is transformed into a strong terahertz plasmonic peak due to natural nanoscale inhomogeneities, such as substrate terraces and wrinkles. The excitation of the plasmon modifies dramatically the magneto-optical response and in particular the Faraday rotation. This makes graphene a unique playground for plasmon-controlled magneto-optical phenomena thanks to a cyclotron mass 2 orders of magnitude smaller than in conventional plasmonic materials such as noble metals.

The quasi-free-standing nature of graphene on H-saturated SiC(0001)

We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001) which was prepared by intercalation of hydrogen under the buffer layer. Using infrared absorption spectroscopy, we prove that the SiC(0001) surface is saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer layer into graphene which exhibits a slight tensile strain and short range defects. The layers are hole doped (p = 5.0 − 6.5 × 1012 cm−2) with a carrier mobility of 3100 cm2/Vs at room temperature. Compared to graphene on the buffer layer, a strongly reduced temperature dependence of the mobility is observed for graphene on H-terminated SiC(0001) which justifies the term “quasi-free-standing.”

Schottky barrier between 6H-SiC and graphite: Implications for metal/SiC contact formation

Using photoelectron spectroscopy we have determined the Schottky barrier between 6H-SiC(0001) and graphite layers grown by solid state graphitization. For n-type 6H-SiC(0001) we find a low Schottky barrier of ϕbn=0.3±0.1eV. For p-type SiC(0001) a rather large value of ϕbp=2.7±0.1eV was determined. It is proposed that these extreme values are likely to have an impact on the electrical behavior of metal/SiC contacts subjected to postdeposition anneals.

Electronic and chemical passivation of hexagonal 6H–SiC surfaces by hydrogen termination

Hydrogenation of 6H–SiC (0001) and (0001) is achieved by high-temperature hydrogen treatment. Both surfaces show a low-energy electron diffraction pattern representative of unreconstructed surfaces of extremely high crystallographic order. On SiC(0001), hydrogenation is confirmed by the observation of sharp Si–H stretching modes. The absence of surface band bending for n- and p-type samples is indicative of electronically passivated surfaces with densities of charged surface states in the gap of below 7×1010 cm−2 for p-type and 1.7×1012 cm−2 for n- type samples, respectively. Even after two days in air, the surfaces show no sign of surface oxide in x-ray photoelectron spectroscopy.

Growth and electronic structure of boron-doped graphene

The doping of graphene to tune its electronic properties is essential for its further use in carbon-based electronics. Adapting strategies from classical silicon-based semiconductor technology, we use the incorporation of heteroatoms in the 2D graphene network as a straightforward way to achieve this goal. Here, we report on the synthesis of boron-doped graphene on Ni(111) in a chemical vapor deposition process of triethylborane on the one hand and by segregation of boron from the bulk of the substrate crystal on the other hand. The chemical environment of boron was determined by x-ray photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy was used to analyze the impact on the band structure. Doping with boron leads to a shift of the graphene bands to lower binding energies. The shift depends on the doping concentration and for a doping level of 0.3 ML a shift of up to 1.2 eV is observed. The experimental results are in agreement with density-functional calculations. Furthermore, our calculations suggest that doping with boron leads to graphene preferentially adsorbed in the top-fcc geometry, since the boron atoms in the graphene lattice are then adsorbed at substrate fcc-hollow sites. The smaller distance of boron atoms incorporated into graphene compared to graphene carbon atoms leads to a bending of the doped graphene sheet in the vicinity of the boron atoms. By comparing calculations of doped and undoped graphene on Ni(111), as well as the respective freestanding cases, we are able to distinguish between the effects that doping and adsorption have on the band structure of graphene. Both doping and bonding to the surface result in opposing shifts on the graphene bands.

Strong phonon-plasmon coupled modes in the graphene/silicon carbide heterosystem

We report on strong coupling of the charge carrier plasmon

Multicomponent magneto-optical conductivity of multilayer graphene on SiC

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Tuning the charge carriers in epitaxial graphene on SiC(0001) from electron to hole via molecular doping with C60F48

in graphene with the surface optical phonon