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(Color online) (a) Sketch of the device. (b) Simulation of the doping profile for nitrogen atoms introduced by implantation. Simulation was performed by the I2SiC in-house software from CNM Barcelona. The implantation energy is 160 keV. The maximum concentration of about 3 × 1018 cm−3 occurs at ∼200 nm below the surface. (c) Schematic of the graphene and SiC conduction bands at T ∼ 50 K. (d) Temperature dependence of the resistivity for one device. Below the inflexion point at T ∼ 100 K (arrow), the graphene becomes more conductive than the implanted substrate. The sudden increase of the resistivity at T < 20 K is due to weak localization. The inset shows an optical view of the device (graphene dimension 20 μm× 6 μm).
(Color online) (a) Longitudinal (solid line) and transverse (dashed line) resistivities ρxx and ρxy as a function of the gate voltage, at T = 46 K. The charge neutrality point is observed at Vg ∼ 2 V. (b) Estimated carrier concentration from the Hall measurements. The central divergence does not correspond to a homogeneous concentration, but rather reveals the coexistence of electrons and holes puddles. (c) σ(ns ) curve at 46 K, where ns is calculated from the Hall resistivity of (a). (d) Hall resistivity ρxy as a function of the magnetic field up to 28 T, at a temperature of 77 K and for different gate voltages from −18 to 18 V. A transition from holes to electrons is observed around Vg = 0 V. (e) Longitudinal resistivity at various positive Vg , evidencing the shift of the Shubnikov-deHaas peaks (negative Vg are not plotted for clarity).
(Color online) Longitudinal and transverse resistivities ρxx and ρxy , at different carrier concentrations, are represented by black and gray (black and red) solid lines respectively. (a) Electrons, showing a quantization between 7.5 and 28 T; (b) electrons, higher concentration, where the plateau at ∼4 kΩ (ν ∼ 6) is visible in the transverse resistivity; (c) holes, with a collapse of the plateau at ∼12.8 kΩ (ν ∼ 2) above B = 14 T.
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