| AVS 60th International Symposium and Exhibition | |
| Graphene and Other 2D Materials Focus Topic | Friday Sessions |
| Session GR+EM+MS+NS+SP-FrM |
| Session: | 2D Materials: Device Physics & Applications |
| Presenter: | A. Dahal, University of South Florida |
| Authors: | A. Dahal, University of South Florida M. Batzill, University of South Florida R. Addou, University of South Florida H. Coy-Diaz, University of South Florida J. Lallo, University of South Florida |
| Correspondent: | Click to Email |
The contact resistance between graphene and metals critically affects device operations. In most realistic graphene device structures, graphene is supported on an insulating substrate and metals are deposited on top of graphene to make electrical contacts. Thus we should evaluate the contact resistance of a metal/graphene/dielectric sandwich structure rather than just a metal/graphene interface. A critical component for evaluating the contact resistance is the Fermi-level shift in graphene underneath the metal contact. We show that this Fermi-level shift relative to the graphene’s Dirac point, can be measured from C-1s core level x-ray photoelectron spectroscopy (XPS). In XPS of solids the binding energy is referenced to the Fermi-level, consequently measurement of C-1s core level of graphene allows us, in the absence of chemical shifts, to determine the Fermi-level of graphene at metal/graphene interfaces as well as at metal/graphene/dielectric interfaces. We show that the Fermi-level shift for metal/graphene interfaces of the weakly interacting metals such as Pt, Ir, Al, and Cu agrees well with previously reported DFT calculations(ref.1). However, the Fermi-level shift of graphene is strongly altered if graphene is sandwiched between a metal and a dielectric oxide. This behavior can be explained by a modified Schottky contact model. In metal/graphene/oxide sandwich structure, metal is replaced by a graphene/metal heterostructure and thus charges in the Schottky contact will be located on the graphene. A simple capacitor model for graphene/oxide interfaces predicts the difference in charge doping for graphene on a metal compared to graphene sandwiched between a metal and dielectric as ΔEF ≈ 0.2×(Φmetal – Φdielectric),in good agreement with our measurements.
Key words: Interface, Doping, Fermi-level, Spectroscopy, Schottky contact
Ref(1) G. Giovannetti, P. A. Khomyakov, G. Brocks, V. M. Karpan, J. van den Brink, and P. J. Kelly, Physical Review Letters 101, 026803 (2008).