Influence of metal contacts and charge inhomogeneity on transport properties of graphene near the neutrality point

TL;DR

This paper highlights how metal contacts and charge inhomogeneity can cause significant systematic errors in measuring graphene's transport properties near the neutrality point, especially in two-probe setups.

Contribution

It demonstrates the impact of contact-induced charge inhomogeneity on transport measurements and emphasizes the need for careful experimental design.

Findings

Charge inhomogeneity affects minimal conductivity measurements.

Two-probe contact resistance varies with gate voltage.

Systematic errors are significant in common measurement setups.

Abstract

There is an increasing amount of literature concerning electronic properties of graphene close to the neutrality point. Many experiments continue using the two-probe geometry or invasive contacts or do not control samples' macroscopic homogeneity. We believe that it is helpful to point out some problems related to such measurements. By using experimental examples, we illustrate that the charge inhomogeneity induced by spurious chemical doping or metal contacts can lead to large systematic errors in assessing graphene's transport properties and, in particular, its minimal conductivity. The problems are most severe in the case of two-probe measurements where the contact resistance is found to strongly vary as a function of gate voltage.