Quasi-free standing epitaxial graphene is obtained on SiC(0001) by hydrogen intercalation. The hydrogen moves between the (6 √ 3×6 √ 3)R30 • reconstructed initial carbon layer and the SiC substrate. The topmost Si atoms which for epitaxial graphene are covalently bound to this buffer layer, are now saturated by hydrogen bonds. The buffer layer is turned into a quasi-free standing graphene monolayer with its typical linear π-bands. Similarly, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation is stable in air and can be reversed by annealing to around 900 • C.

We report on the interface between graphene and 4H-SiC͑0001͒ as investigated by scanning tunneling microscopy ͑STM͒ and low energy electron diffraction ͑LEED͒. It is characterized by the so-called ͑6 ͱ 3 ϫ 6 ͱ 3͒R30°reconstruction, whose structural properties are still controversially discussed but at the same time are crucial for the controlled growth of homogeneous high-quality large-terrace graphene surfaces. We discuss the role of three observed phases with periodicities ͑6 ͱ 3 ϫ 6 ͱ 3͒R30°, ͑6 ϫ 6͒, and ͑5 ϫ 5͒. Their LEED intensity levels and spectra strongly depend on the surface preparation procedure applied. The graphitization process imprints distinct features in the STM images as well as in the LEED spectra. The latter have the potential for an easy and practicable determination of the number of graphene layers by means of LEED.

Epitaxial graphene on SiC(0001) suffers from strong intrinsic n-type doping. We demonstrate that the excess negative charge can be fully compensated by non-covalently functionalizing graphene with the strong electron acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ). Charge neutrality can be reached in monolayer graphene as shown in electron dispersion spectra from angular resolved photoemission spectroscopy (ARPES). In bilayer graphene the band gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band gap. The reduction of the charge carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy. The structural and electronic characteristics of the graphene/F4-TCNQ charge transfer complex are investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The doping effect on graphene is preserved in air and is temperature resistant up to 200• C. Furthermore, graphene non-covalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultra-high vacuum but also by wet chemistry.

To cite this version:C Riedl, C Coletti, U Starke. Structural and electronic properties of epitaxial graphene on SiC(0001): a review of growth, characterization, transfer doping and hydrogen intercalation. Journal of Physics D: Applied Physics, IOP Publishing, 2010, 43 (37) 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 carbon nanodevices. Grown epitaxially on silicon carbide (SiC) wafers, large area graphene samples appear feasible and integration in existing device technology can be envisioned. This article reviews the controlled growth of epitaxial graphene layers on SiC(0001) and the manipulation of their electronic structure. We show that epitaxial graphene on SiC grows on top of a carbon interface layer that -although it has a graphite-like atomic structure -does not display the linear π-bands typical for graphene due to a strong covalent bonding to the substrate. Only the second carbon layer on top of this interface acts like monolayer graphene. With a further carbon layer, a graphene bilayer system develops. During the growth of epitaxial graphene on SiC(0001) the number of graphene layers can be precisely controlled by monitoring the π-band structure. Experimental fingerprints for in-situ growth control could be established. However, due to the influence of the interface layer, epitaxial graphene on SiC(0001) is intrinsically n-doped and the layers have a long-range corrugation in their density of states. 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. We demonstrate methods to compensate and eliminate this structural and electronic influence of the interface. We show that the band structure of epitaxial graphene on SiC(0001) can be precisely tailored by functionalizing the graphene surface with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) molecules. Charge neutrality can be achieved for mono-and bilayer graphene. On epitaxial bilayer graphene, where a band gap opens due to the asymmetric electric field across the layers imposed by the interface, the magnitude of this band gap can be increased up to more than double of its initial value. The hole doping allows the Fermi level to shift into the energy band gap. The impact of the interface layer can be completely eliminated by decoupling the graphene from the SiC substrate by a hydrogen intercalation technique. We demonstrate that hydrogen can migrate under the interface layer and passivate the underlying SiC substrate. The interface layer alone transforms into a quasi-free standing monolayer. Epitaxial monolayer graphene turns into a decoupled bilayer. In combination with atmospheric pressure graphitization, the intercalation process allows to produce quasi-free standing epitaxial graphene on large SiC wafers and represents a highly promising route towards epitaxial graphene based nanoelectronics.Confidential: ...

Raman spectra were measured for mono-, bi-and trilayer graphene grown on SiC by solid state graphitization, whereby the number of layers was pre-assigned by angle-resolved ultraviolet photoemission spectroscopy. It was found that the only unambiguous fingerprint in Raman spectroscopy to identify the number of layers for graphene on SiC(0001) is the linewidth of the 2D (or D*) peak. The Raman spectra of epitaxial graphene show significant differences as compared to micromechanically cleaved graphene obtained from highly oriented pyrolytic graphite crystals. The G peak is found to be blue-shifted. The 2D peak does not exhibit any obvious shoulder structures but it is much broader and almost resembles a single-peak even for multilayers. Flakes of epitaxial graphene were transferred from SiC onto SiO 2 for further Raman studies. A comparison of the Raman data obtained for graphene on SiC with data for epitaxial graphene transferred to SiO 2 reveals that the G peak blue-shift is clearly due to the SiC substrate. The broadened 2D peak however stems from the graphene structure itself and not from the substrate. 2Graphene is the building block of graphite and carbon-based nanomaterials such as carbon nanotubes and fullerenes. Since the development of the micromechanical cleavage method to obtain thermodynamically stable mono-and few-layer graphene, our understanding of this purely twodimensional system has improved significantly. 1 , 2 , 3 , 4 Graphene exhibits unconventional electronic properties, 1,2,5,6 such as high and nearly equal mobilities at room temperature for both electron and hole conduction, which makes it a strong candidate for nanoelectronic circuit applications. 5,7 However, the micromechanical cleavage method is not suitable for obtaining large area graphene. For practical applications requiring large areas of graphene, full graphitization on SiC seems to be the more promising route. 8,9,10,11 Indeed, devices of epitaxial graphene on SiC have been prepared using conventional e-beam lithography and a patterning method based on O 2 plasma etching 8,9 although several problems still remain: the nature of the (6√3×6√3)R30° reconstruction at the interface between SiC and graphene is still under debate and the proper conditions for the production of large areas of homogeneous mono-, biand few-layer graphene are not well developed. 10,12Raman spectroscopy is known to be a powerful tool to determine the electronic properties of carbonbased materials and there have been several reports about Raman measurements on graphene layers which were micromechanically exfoliated from highly oriented pyrolytic graphite (HOPG) on SiO 2 substrates. 13,14,15,16,17 Recently, Ferrari et al. have demonstrated that the shape of the 2D Raman peak may serve as the fingerprint to distinguish mono-, bi-and few-layer graphene. The 2D peak stems from a double resonance electron-phonon scattering process. 14 For monolayer graphene the 2D peak can be fitted to a single Lorentzian, whereas the multiple bands in bilayers or few-layer gra...