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

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 sublinear 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 sublinearity 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.

Electronic doping and scattering by transition metals on graphene

“…Generally, the relative work function ͑WF͒ between the TM and the graphene is believed to be an important factor for determining the charge transfer, 11 i.e., graphene will be p doped ͑n doped͒ if the TM's WF is larger ͑smaller͒ than graphene. Recently, density-functional calculations predict the presence of a strong interfacial dipole that promotes the n-type doping of graphene.…”

“…[6][7][8] Because of their importance for graphene-based electronics and the investigation of novel phenomena, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] there have been extensive theoretical studies. [1][2][3][4][5][6][7][8]13,14 In contrast, the experimental exploration of TM/graphene systems is much more limited.…”

Quantum interference noise near the Dirac point in graphene

“…1(b). Electrical measurements were done in a 4-probe geometry, using external voltage probes [22] as shown on the schematic in Fig. 1(c).…”

Effective contact model for geometry-independent conductance calculations in graphene

“…In current and potential applications relying on the electronic degrees of freedom, graphene devices must be contacted to a metallic lead. Therefore understanding how the contact itself impacts the performance of the device is of critical importance, both fundamentally and on a more applied level [18][19][20][21].In quantum transport calculations in the context of graphene there are a variety of commonly used and accepted models for a contact that meet the requirements mentioned above. Common to nearly all these models is the fact that the contact geometry eventually converges to a ballistic semi-infinite ribbon at some distance from the contact/graphene interface.…”

“…In current and potential applications relying on the electronic degrees of freedom, graphene devices must be contacted to a metallic lead. Therefore understanding how the contact itself impacts the performance of the device is of critical importance, both fundamentally and on a more applied level [18][19][20][21].…”