PHYSICAL REVIEW B 84, 125449 (2011) Large-area homogeneous quasifree standing epitaxial graphene on SiC(0001): Electronic and structural characterization S. Forti, 1,* K. V. Emtsev, 1 C. Coletti, 1 A. A. Zakharov, 2 C. Riedl, 1 and U. Starke 1,† 1 Max-Planck-Institut f¨ ur Festk¨ orperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany 2 MAXLab, Lund University, P.O. Box 118, Lund, S-22100, Sweden (Received 12 July 2011; revised manuscript received 31 August 2011; published 30 September 2011) The growth of epitaxial graphene on SiC has been identified as one of the most promising techniques to produce graphene for electronic applications. In this paper, we present a systematic study of the electronic and structural properties of large-area quasifree standing epitaxial monolayer graphene grown on top of the SiC(0001) surface. For this purpose, we combine the thermal treatment of SiC in Ar atmosphere to achieve a homogeneous coverage of the surface with the hydrogen intercalation process, which leads to the removal of the interaction between the substrate and the carbon layer. The band structure in the vicinity of the K point is measured using high-resolution angle-resolved photoelectron spectroscopy. A detailed analysis of the quasiparticle dynamics reveals a renormalization of the band velocity estimated to about 3% at energies around 200 meV below the Fermi level, which mainly originates from electron-phonon interaction. Further analysis of the momentum distribution curves leads to the formulation of a model for the doping reduction in such a system in the course of sample annealing above 650 ◦ C. The uniformity and homogeneity of the graphene is demonstrated by means of low-energy electron microscopy (LEEM). Microphotoelectron spectroscopy data confirm the high structural quality and homogeneity of the quasifree standing graphene. Using LEEM and scanning tunneling microscopy, we demonstrate that the hydrogen desorption at elevated temperatures of approximately 750 ◦ C sets in on the graphene terraces rather than via the step edges. DOI: 10.1103/PhysRevB.84.125449 PACS number(s): 73.22.Pr, 73.20.At I. INTRODUCTION Graphene is a single atomic layer consisting of sp 2 - hybridized carbon atoms arranged in a honeycomb lattice. 1 It possesses a special kind of electronic structure owing to the linear dispersion relation of its π bands, which are formed by the p z orbitals of the carbon atoms. 2 In graphene, the π and π ∗ bands cross without a band gap at the K point of the Brillouin zone. The region around the K point where the electronic dispersion remains linear is usually denominated as the Dirac cone. The crossing point is noted as the Dirac point and its energy as the Dirac energy (E D ). In pristine, undoped graphene, the Fermi energy (E F ) coincides with E D as sketched in Fig. 1(a), implying that graphene is a semimetal with vanishing charge-carrier density. When the Fermi energy shifts above or below E D , graphene develops either electron or hole conductivity as sketched in Figs. 1(b) and 1(c), respectively. Due to these remarkable electronic prop- erties, graphene displays some peculiar quantum-mechanical effects, 3 and it has been identified as a promising candidate for future nanoelectronics applications. 4 Nonetheless, the most common technique used in fundamental research for graphene production, namely, mechanical exfoliation from graphite single crystals, 5–7 can provide flakes just a few tens of μm in size. Several other techniques to synthesize graphene have been demonstrated, such as the growth of graphene on metal substrates 8–10 or graphene obtained by chemical exfoliation of graphite. 11 However, the former case necessarily requires the transfer onto an insulating substrate, while graphene obtained by the latter method yields only small crystallites. Rather recently, attempts have been reported to grow graphene by chemical vapor deposition (CVD) on an insulating substrate. 12 In contrast to other methods, epitaxial graphene grown on silicon carbide 13 (SiC) can be produced directly on a semi-insulating substrate and, hence, does not require any transfer. Moreover, large single-crystal SiC wafers can be readily processed to obtain graphene using existing indus- trial processing technology. Synthesis of graphene on SiC is achieved by a solid-state decomposition reaction taking place on the surface of SiC upon annealing at elevated temperatures. Under such conditions, Si atoms sublimate from the surface, leaving excessive carbon behind. Decomposition of approximately three SiC bilayers is necessary to crystallize a single layer of graphene on SiC. On the SiC(0001) surface, the first graphenelike carbon layer forms a large unit-cell super- structure with a (6 √ 3 × 6 √ 3)R30 ◦ periodicity. 14,15 Although having the same geometrical atomic arrangement as graphene, this layer strongly interacts with the substrate via p z orbitals, so that the π bands typical of graphene can not develop yet. 16 The layer is, therefore, electronically inactive and usually called buffer layer or zero-layer graphene (ZLG). Further annealing at higher temperatures leads to additional accumulation of carbon on the surface, which nucleates into a true monolayer of graphene (MLG) on top of the ZLG. The MLG is electrically active and possesses a typical graphene band structure. 16 Due to charge transfer from the interface, the MLG shows a strong n doping of the order of 10 13 cm −2 as sketched in Fig. 1(b). In the case of strongly doped MLG, renormalization of the energy dispersion due to many-particle interactions was observed in the vicinity of both the Dirac point and the Fermi level (see also Sec. III). 17 Our group has recently demonstrated that the ZLG can be decoupled from the SiC substrate by hydrogen intercalation. 18 The effect of this method is to remove the covalent interaction between ZLG and substrate. Consequently, the ZLG is turned into quasifree standing monolayer graphene (QFMLG). It was also shown that the process works for few-layer graphene as 125449-1 1098-0121/2011/84(12)/125449(10) ©2011 American Physical Society