Scanning Gate Microscopy on Graphene: Charge Inhomogeneity and Extrinsic Doping

TL;DR

This study uses scanning gate microscopy to visualize charge inhomogeneity and extrinsic doping effects in graphene, revealing mesoscopic electron and hole domains caused by local impurities and structural features.

Contribution

It demonstrates the application of SGM to map charge fluctuations and identify sources of extrinsic doping in graphene devices.

Findings

Significant spatial carrier density fluctuations (~10^12/cm^2) detected.

Identification of metal contacts, edges, defects, and residues as doping sources.

Visualization of mesoscopic electron and hole domains.

Abstract

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.