Quantum Hall effect in exfoliated graphene affected by charged impurities: Metrological measurements

Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-mu m-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on Si/SiO2 substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature (T < 1.3 K), the Hall resistance is measured by comparison with a GaAs-based quantum resistance standard using a cryogenic current comparator. In the low-dissipation limit, it is found quantized within 5 parts in 10(7) (one standard deviation, 1 sigma) at the expected rational fractions of the von Klitzing constant, respectively, R-K/2 and R-K/4 in the ML and BL devices. These results constitute the most accurate QHE quantization tests to date in monolayer and bilayer exfoliated graphene. It turns out that a main limitation to the quantization accuracy, which is found well above the 10(-9) accuracy usually achieved in GaAs, is the low value of the QHE breakdown current being no more than 1 mu A. The current dependence of the longitudinal conductivity investigated in the BL Hall bar shows that dissipation occurs through quasielastic inter-Landau-level scattering, assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-Landau-level transition rate and cause small breakdown currents.