Dependable contact related parameter extraction in graphene-metal junctions

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.