Low carrier density epitaxial graphene devices on SiC.

The transport characteristics of graphene devices with low n- or p-type carrier density (∼10(10) -10(11) cm(-2) ), fabricated using a new process that results in minimal organic surface residues, are reported. The p-type molecular doping responsible for the low carrier densities is initiated by aqua regia. The resulting devices exhibit highly developed ν = 2 quantized Hall resistance plateaus at magnetic field strengths of less than 4 T.

Localization and electron-electron interactions in few-layer epitaxial graphene.

This paper presents a study of the quantum corrections caused by electron-electron interactions and localization to the conductivity in few-layer epitaxial graphene, in which the carriers responsible for transport are massive. The results demonstrate that the diffusive model, which can generally provide good insights into the magnetotransport of two-dimensional systems in conventional semiconductor structures, is applicable to few-layer epitaxial graphene when the unique properties of graphene on the substrate, such as intervalley scattering, are taken into account. It is suggested that magnetic-field-dependent electron-electron interactions and Kondo physics are required for obtaining a thorough understanding of magnetotransport in few-layer epitaxial graphene.

Insulator-quantum Hall transitionin monolayer epitaxial graphene.

We report on magneto-transport measurements on low-density, large-area monolayer epitaxial graphene devices grown on SiC. We observe temperature (T)-independent crossing points in the longitudinal resistivity ρxx, which are signatures of the insulator-quantum Hall (I-QH) transition, in all three devices. Upon converting the raw data into longitudinal and Hall conductivities σxx and σxy, in the most disordered device, we observed T-driven flow diagram approximated by the semi-circle law as well as the T-independent point in σxy near e2/h. We discuss our experimental results in the context of the evolution of the zero-energy Landau level at low magnetic fields B. We also compare the observed strongly insulating behaviour with metallic behaviour and the absence of the I-QH transition in graphene on SiO2 prepared by mechanical exfoliation.

Hot carriers in epitaxial graphene sheets with and without hydrogen intercalation: role of substrate coupling.

The development of graphene electronic devices produced by industry relies on efficient control of heat transfer from the graphene sheet to its environment. In nanoscale devices, heat is one of the major obstacles to the operation of such devices at high frequencies. Here we have studied the transport of hot carriers in epitaxial graphene sheets on 6H-SiC (0001) substrates with and without hydrogen intercalation by driving the device into the non-equilibrium regime. Interestingly, we have demonstrated that the energy relaxation time of the device without hydrogen intercalation is two orders of magnitude shorter than that with hydrogen intercalation, suggesting application of epitaxial graphene in high-frequency devices which require outstanding heat exchange with an outside cooling source.

Dirac fermion heating, current scaling, and direct insulator-quantum Hall transition in multilayer epitaxial graphene.

We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (TDF) at various driving currents. At zero field, it is found that TDF ∝ I≈1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity ρxx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in ρxx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.