Electronic transport at monolayer-bilayer junctions in epitaxial graphene on SiC

Two-dimensional maps of the electronic conductance in epitaxial graphene grown on SiC were obtained by calibrated conductive atomic force microscopy. The correlation between morphological and electrical maps revealed the local conductance degradation in epitaxial graphene over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at the 1L/2L junction is a purely quantum-mechanical effect independent on the interaction with the substrate. First-principles transport calculations indicate that the weak wave-function coupling between the 1L pi/pi* bands with the respective first bands of the 2L region gives rise to a strong suppression of the conductance for energies within +/- 0.48 eV from the Dirac point. Conductance degradation at 1L/2L junctions is therefore a general issue for large area graphene with a certain fraction of inhomogeneities in the layer number, including graphene grown by chemical vapor deposition on metals.