Dependable contact related parameter extraction in graphene-metal junctions

TL;DR

This paper emphasizes the importance of proper protocols for extracting graphene-metal contact resistance, demonstrating that measurements in high vacuum yield more reliable results and analyzing various contact parameters across different metals.

Contribution

It introduces a standardized methodology for accurate contact resistance extraction in graphene devices, considering environmental effects and different contact metals.

Findings

High vacuum measurements improve contact resistance reliability.

Contact resistance depends on carrier density and environment.

Methodology applies to various metals like gold and copper.

Abstract

The accurate extraction and the reliable, repeatable reduction of graphene - metal contact resistance (R$_{C}$) are still open issues in graphene technology. Here, we demonstrate the importance of following clear protocols when extracting R$_{C}$ using the transfer length method (TLM). We use the example of back-gated graphene TLM structures with nickel contacts, a complementary metal oxide semiconductor compatible metal. The accurate extraction of R$_{C}$ is significantly affected by generally observable Dirac voltage shifts with increasing channel lengths in ambient conditions. R$_{C}$ is generally a function of the carrier density in graphene. Hence, the position of the Fermi level and the gate voltage impact the extraction of R$_{C}$. Measurements in high vacuum, on the other hand, result in dependable extraction of R$_{C}$ as a function of gate voltage owing to minimal spread in Dirac voltages. We further assess the accurate measurement and extraction of important parameters like contact-end resistance, transfer length, sheet resistance of graphene under the metal contact and specific contact resistivity as a function of the back-gate voltage. The presented methodology has also been applied to devices with gold and copper contacts, with similar conclusions.