Strain and charge carrier coupling in epitaxial graphene

We report a striking coupling between strain and carrier concentration variations at micrometer scale in single-layer graphene grown on silicon carbide (SiC) (0001). The in-plane compressive strain (up to 0.4%) and carrier concentration are probed using Raman spectroscopy. We show that the large strain inhomogeneities in graphene initiate at the growth stage and develop further by strain relaxation along the mismatched symmetry axes of the graphene and the underlying substrate. The strain relaxation is accompanied by a locally larger electron concentration, suggesting that charge transfer reduces the strain energy in the overall system. Our work establishes the strain and doping variations as coupled, intrinsic properties of epitaxial graphene growth on SiC(0001).