Low contact resistance metals for graphene based devices

In order to search a guideline to prepare low-resistance ohmic contacts to graphene by depositing a single metal element, the contact resistance (R-C) is measured by a transmission line method (TLM) for a variety of metals (Ti, Ag, Co, Cr, Fe, Ni. and Pd) contacting to the graphene channel. To obtain the precise R-C value, we fabricate a defined rectangular graphene channel and a TLM pattern with uniform interface area and channel width. The R-C value as small as 700 +/- 500 Omega mu m for Ti contact is obtained, which is smaller than the value reported previously. In addition, we find that the R-C is not strongly related to the metal work function and is significantly affected by the microstructure of the metals. We conclude that the chemical cleaning and the control of the microstructure of the metal films are essential for preparing the low-resistance ohmic contact to achieve the direct contact between the metal and the graphene. (C) 2012 Elsevier B.V. All rights reserved.