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2009 CARBON ALLOTROPESObservation of the half-integer quantum Hall effect in epitaxial grapheneGraphene, a monolayer of sp 2 -bond carbon atoms, has recentlyattracted considerable interest due to its extraordinaryelectronic properties such as high charge carrier mobilitiesand a novel quantum Hall signature. These properties,that were first observed in exfoliated graphene, are adirect consequence of: (i) the linear band structure and (ii)the pseudo-spin, arising from the two sub-lattices. However,we use an epitaxial process to grow graphene monolayerson the Si-face of a semi-insulating silicon carbidesubstrate. The epitaxy and the sample fabrication is describedin [Emtsev et al. Nature Materials 8, 203 (2009)].As we can grow epitaxial graphene with a high and reproduciblequality (transport properties vary only about30% from sample to sample [figure 11(a) and (b)]) wewant to address an important question; are the quasirelativisticproperties of free-standing graphene also presentin graphene on SiC? Theory predicts distinct magnetotransportproperties for monolayer, bi-layer and multi-layergraphene that are nicely reproduced in exfoliated samples.In order to clarify if these predictions also hold for epitaxialgraphene, we have measured magneto-transport up to 28 Tin two different systems.The as prepared Hall bars show Shubnikov-de Haas (SdH)oscillations in the resistance R xx and emerging plateausin the Hall resistance R xy [see figure 12(a)]. The valuesof these plateaus, that are precursors to the quantumHall effect (QHE), follow the scheme of monolayergraphene: R xy = e 2 /((4n + 2)h) where n is an integer. Ifthe SdH extrema n are plotted against the inverse field1/B a linear dependence can be recognized [see inset infigure 12(b)]. The axis intercepts of 0 (1/2) for minima(maxima) yield a Berry phase of π as predicted for singlelayer graphene and confirmed in exfoliated samples.To probe the most interesting point of the band structure,the charge neutrality point, we evaporated ≈ 5 Å oftetrafluorotetracyanoquinodimethane (F4-TCNQ). This resultsin a reduced carrier density of n ≈ 5 × 10 11 cm −2 . Insuch a sample the half-integer QHE is visible [figure 12(b)].An evaluation of the SdH oscillations (barely visible below4 T) with the above mentioned procedure further confirmsthe picture of unperturbed graphene.We conclude that the close contact to the SiC substrate doesnot change the magneto-transport properties noticeable andepitaxial graphene reproduces the unique features observedin exfoliated graphene, while remaining a system which iscertainly more suitable for large scale production.Figure 11: (a) Histograms of the charge carrier density n, and (b)mobility µ, of samples with different geometries and sizes from5 mm to 400 nm. Some Hall bars are patterned on atomicallyflat terraces of the SiC substrate and therefore consist of perfectmonolayer graphene. (c) The step edges are clearly visible in theelectron micrograph.Figure 12: (a) R xx and R xy in a Hall bar on a single substrate terrace[cf. figure 11(c)]. SdH oscillations and plateaus in the Hallresistance are clearly visible. (b) The same quantities in a samplegated close to charge neutrality with F4-TCNQ. Half-integerQHE is visible above 7 T. The inset shows the evaluation of theSdH oscillations [open (closed) symbols: minima (maxima)]. Thelines are best fits to the data [black: Hall bar on a single substrateterrace, red: Hall bar covering several substrat terraces,blue: F4-TCNQ covered (plotted against 0.1/B for clarity)] thatyield a Berry phase of π as expected for monolayer graphene.D.K. MaudeJ. Jobst, D. Waldmann, H.B. Weber (Applied Physics Department, University of Erlangen-Nürnberg)11