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We have investigated the disorder of epitaxial graphene close to the chargeneutrality point (CNP) by various methods: i) at room temperature, by analyzingthe dependence of the resistivity on the Hall coefficient ; ii) by fitting thetemperature dependence of the Hall coefficient down to liquid heliumtemperature; iii) by fitting the magnetoresistances at low temperature. Allmethods converge to give a disorder amplitude of $(20 \pm 10)$ meV. Because ofthis relatively low disorder, close to the CNP, at low temperature, the sampleresistivity does not exhibit the standard value $\simeq h/4e^2$ but diverges.Moreover, the magnetoresistance curves have a unique ambipolar behavior, whichhas been systematically observed for all studied samples. This is a signatureof both asymmetry in the density of states and in-plane charge transfer. Themicroscopic origin of this behavior cannot be unambiguously determined.However, we propose a model in which the SiC substrate steps qualitativelyexplain the ambipolar behavior.

Magnetic-field driven ambipolar quantum Hall effect in epitaxial graphene close to the charge neutrality point