We analyse doping of graphene grown on SiC in two models which differ by the source of charge transfered to graphene, namely, from SiC surface and from bulk donors. For each of the two models, we find the maximum electron density induced in monolayer and bilayer graphene, which is determined by the difference between the work function for electrons in pristine graphene and donor states on/in SiC, and analyse the responsivity of graphene to the density variation by means of electrostatic gates.Comment: 3 pages; submitted to AP
We explore the robust quantization of the Hall resistance in epitaxial graphene grown on Siterminated SiC. Uniquely to this system, the dominance of quantum over classical capacitance in the charge transfer between the substrate and graphene is such that Landau levels (in particular, the one at exactly zero energy) remain completely filled over an extraordinarily broad range of magnetic fields. One important implication of this pinning of the filling factor is that the system can sustain a very high nondissipative current. This makes epitaxial graphene ideally suited for quantum resistance metrology, and we have achieved a precision of 3 parts in 10 10 in the Hall resistance quantization measurements. Graphene is believed to offer an excellent platform for QHE metrology due to the large energy separation between Landau levels (LL) resulting from the Dirac-type "massless" electrons specific for its band structure [12]. The Hall resistance quantization with an accuracy of 3 parts in 10 9 has already been established [7] in Hall-bar devices manufactured from epitaxial graphene grown on Si-terminated face of SiC (SiC/G). However, for graphene to be practically employed as an embodiment of a quantum resistance standard, it needs to satisfy further stringent requirements [11], in particular with respect to robustness over a range of temperature, magnetic field and measurement current. A high measurement current, which a device can sustain at a given temperature without dissipation, is particularly important for precision metrology as it defines the maximum attainable signalto-noise ratio.The extent of the QHE plateaux in conventional 2D electron systems is, usually, set by disorder and temperature. Disorder pins the Fermi energy in the mobility gap of the 2D system, which suppresses dissipative transport at low temperatures over a finite range of magnetic fields around the values corresponding to exactly filled LLs. These values can be calculated from the carrier density n s determined from the low-field Hall resistivity measurements and coincide with the maximum non-dissipative current, the breakdown current. Thus, the breakdown current in conventional two-dimensional semicondutors peaks very close to the field values where the filling factor ν is an even integer [11]. Though less studied experimentally, the behaviour of the breakdown current on the plateaux for the exfoliated graphene, including the ν = 2 plateau corresponding to the topologically protected N = 0 LL, looks quite similar [13].In this Brief Report we explore the robustness of the Hall resistance quantization in SiC/G. Unlike the QHE in conventional 2D systems, where the carrier density is independent of magnetic field, here specifically to SiC/G, we find that the carrier density in graphene varies with magnetic field due to the charge transfer between surface donor states in SiC and graphene. Most importantly, we find magnetic field intervals of several Tesla, where the carrier density in graphene increases linearly with the magnetic field, resulting ...
We study an epitaxial graphene monolayer with bilayer inclusions via magnetotransport measurements and scanning gate microscopy at low temperatures. We find that bilayer inclusions can be metallic or insulating depending on the initial and gated carrier density. The metallic bilayers act as equipotential shorts for edge currents, while closely spaced insulating bilayers guide the flow of electrons in the monolayer constriction, which was locally gated using a scanning gate probe.
We analyze a manifestation of the partial ordering transition of adatoms on graphene in resistivity measurements. We find that Kekule mosaic ordering of adatoms increases sheet resistance of graphene, due to a gap opening in its spectrum, and that critical fluctuations of the order parameter lead to a non-monotonic temperature dependence of resistivity, with a cusp-like minimum at T=T_c.Comment: 4 pages, 1 figur
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