Dependable Contact Related Parameter Extraction in Graphene–Metal Junctions

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.