What can limit the quantum Hall effect quantization in graphene?

We report on investigations about limitations of the Hall resistance quantization accuracy (4×10^-7) measured in Hall bars based on monolayer and bilayer exfoliated graphene deposited on Si/SiO₂ substrates (best results in such devices). Transport measurements at low magnetic field (including quantum corrections) reveal that charge carrier scattering is mainly caused by charged impurities. In the QHE regime, dissipation occurs through quasi-elastic inter-Landau level (LL) scattering assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-LL transitions and cause the low breakdown currents (≈1μA) observed in the narrow (<;4μm) samples considered.