Evidence of the role of contacts on the observed electron-hole asymmetry in graphene

TL;DR

This study investigates how different contact geometries and metal interfaces influence electron-hole asymmetry in graphene's electrical conductance, revealing that contact-induced charge pinning causes the asymmetry and affects conductance behavior.

Contribution

It demonstrates that contact geometry and metal interface properties are key factors in electron-hole asymmetry in graphene, providing new insights into transport measurements.

Findings

External probes show symmetric conductance with linear density dependence.

Invasive probes reveal asymmetry and sub-linear conductance.

Contact-induced charge pinning causes the observed asymmetry.

Abstract

We perform electrical transport measurements in graphene with several sample geometries. In particular, we design ``invasive'' probes crossing the whole graphene sheet as well as ``external'' probes connected through graphene side arms. The four-probe conductance measured between external probes varies linearly with charge density and is symmetric between electron and hole types of carriers. In contrast measurements with invasive probes give a strong electron-hole asymmetry and a sub-linear conductance as a function of density. By comparing various geometries and types of contact metal, we show that these two observations are due to transport properties of the metal/graphene interface. The asymmetry originates from the pinning of the charge density below the metal, which thereby forms a p-n or p-p junction depending on the polarity of the carriers in the bulk graphene sheet. Our results also explain part of the sub-linearity observed in conductance as a function of density in a large number of experiments on graphene, which has generally been attributed to short-range scattering only.