Role of graphene/substrate interface on the local transport properties of the two-dimensional electron gas

The electron mean free path (l(gr)) is "locally" evaluated by scanning capacitance spectroscopy on graphene obtained with different preparation methods and on different substrates, i.e., graphene exfoliated from highly oriented pyrolitic graphite (HOPG) and deposited (DG) on 4H-SiC(0001) and on SiO2 and epitaxial graphene grown on 4H-SiC (0001) (EG). l(gr) in DG on SiC was more than four times larger than in DG on SiO2. The improved mean free path is explained by the higher permittivity of SiC compared to SiO2, yielding a better dielectric screening of charged-impurities, and by the weaker coupling of graphene two-dimensional-electron-gas with surface polar phonons of SiC. On the other hand, l(gr) on EG is on average similar to 0.4 times that on DG-SiC and exhibits large variations from point to point, due to the presence of a laterally inhomogeneous positively charged layer at EG/SiC interface.