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

The electron mean free path $(lgr)$ 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). $lgr$ 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, $lgr$ on EG is on average $∼0.4$ 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.