We have performed scanning gate microscopy (SGM) on graphene field effect transistors (GFET), using a biased metallic nanowire coated with a dielectric layer as a contact mode tip and local top gate. Electrical transport through graphene at various back gate voltages is monitored as a function of tip voltage and tip position. Near the Dirac point, the dependence of graphene resistance on tip voltage shows a significant variation with tip position. SGM imaging reveals mesoscopic domains of electron-doped and hole-doped regions. Our measurements indicate a substantial spatial fluctuation (on the order of 10^12/cm^2) in the carrier density in graphene due to extrinsic local doping. Important sources for such doping found in our samples include metal contacts, edges of graphene, structural defects, and resist residues.
@article{arxiv.1003.5404,
title = {Scanning Gate Microscopy on Graphene: Charge Inhomogeneity and Extrinsic Doping},
author = {Romaneh Jalilian and Luis A. Jauregui and Gabriel Lopez and Jifa Tian and Caleb Roecker and Mehdi M. Yazdanpanah and Robert W. Cohn and Igor Jovanovic and Yong P. Chen},
journal= {arXiv preprint arXiv:1003.5404},
year = {2011}
}