Contact resistivity and current flow path at metal/graphene contact

Abstract

The contact properties between metal and graphene were examined. The electrical measurement on a multiprobe device with different contact areas revealed that the current flow preferentially entered graphene at the edge of the contact metal. The analysis using the cross-bridge Kelvin structure (CBK) suggested that a transition from the edge conduction to area conduction occurred for a contact length shorter than the transfer length of ~1 μm. The contact resistivity for Ni was measured as ~5×10 -6 Ωcm 2 using the CBK. A simple calculation suggests that a contact resistivity less than 10 -9 Ωcm 2 is required for miniaturized graphene field effect transistors.Graphene-based devices are promising candidates for future high-speed field effect transistors (FETs). An increase in the on/off current ratio (I on /I off ) is one of the critical issues to realize the graphene FETs. Although the contact properties are important in terms of an increase in I on , only a small number of experiments [1][2][3][4][5][6] have addressed this matter compared to the bandgap engineering for a decrease in I off . 7,8 In fact, an ohmic contact is obtained without any difficulty due to the lack of a bandgap, but it is concerned that a very small density of states (DOS) for graphene might suppress the current injection from the metal to graphene. Recently, we reported that the contact resistivity for a typical Cr/Au electrode was high and that it varied by several orders of magnitude. It has been suggested that the contact resistivity might significantly mask the outstanding performance of the monolayer graphene channel. 5,6 Although a lower contact resistivity was reported for a Ti/Au electrode, it was described in the units of either Ωμm or Ωμm 2 , 1,2,4 because the current flow path at the graphene/metal contact was not revealed. Furthermore, the actual contact resistivity required for FET applications has not yet been discussed. In this study, we first reveal the current flow path at the graphene/metal contact by using a multiprobe device with different contact areas. Then, the contact resistivities required for the miniaturized graphene FETs are quantitatively assessed based on the contact resistivity obtained experimentally by the cross-bridge Kelvin (CBK) method. Finally, the graphene/metal contact is discussed from the viewpoint of metal work function of contact metals employed.Graphite thin films were mechanically exfoliated from Kish graphite onto 90 nm SiO 2 /p + -Si substrates. The number of layers was determined by the optical contrast and Raman spectroscopy. 9 Electron-beam lithography was utilized to pattern electrical contacts onto graphene. The contact metals Cr/Au (~10/20 nm), Ti/Au (~10/20 nm), and Ni (~25 nm) were thermally evaporated on the resist-patterned graphene in a chamber with a background pressure of 10 -5 Pa and were subjected to the lift-off process in warm acetone. To remove the resist residual, graphene devices were annealed in a H 2 -Ar mixture at 300°C for 1 hour. The electrical measurements were p...

Atomic-scale model for the contact resistance of the nickel-graphene interface

“…One of the outstanding questions is the magnitude of the contact resistance between graphene and metal electrodes since a high contact resistance will limit the performance of field-effect transistors. 1 There have been several experimental investigations of the contact resistance of the metal-graphene interface using four-or two-probe measurements [2][3][4][5][6] and the transfer length method, 3,7 however, currently there is no clear consensus on the value and the dependence of the contact resistance on contact area, temperature, and applied gate potential. Thus, there is a need for complementary theoretical studies, which can give insight about the physical mechanism at play at the metal-graphene interface.…”

Carrier density modulation in graphene underneath Ni electrode

“…Previously, we determined the transfer length d T ( / , where  C is the specific contact resistivity and R ch is the sheet resistance of graphene underneath the metal electrode) as ~0.5-1 m at the graphene/Ni contact region. 4 This is the effective length for the current injection at the graphene/Ni interface, as shown in Fig. 1.…”

“…11 Experimentally, the band structure of graphene grown on a Ni substrate has been confirmed to differ from that of the pristine graphene. 12 Therefore, the Fermi level of graphene is assumed to be fixed at the graphene/metal interface as mentioned above, especially for chemisorption metals.However, in the conventional back-gate graphene FET device, a dependence of the contact resistivity at the Ni/graphene contact on V G has been observed, 4 suggesting that the carrier density in the graphene underneath the metal could be modulated by V G , even though the graphene electrically contacts the metal. 13 This carrier density modulation was first proposed for Ti/Pd/Au contacts because the drain current at the DP increases with increasing V G , even for a graphene FET device in which the entire channel region is completely covered by the topgate.…”

“…1 Contact resistance, however, limits the device performance of graphene FETs. [2][3][4][5] To improve the contact resistance, a rigorous understanding of the graphene/metal interface is crucial.…”

“…1 Contact resistance, however, limits the device performance of graphene FETs. [2][3][4][5] To improve the contact resistance, a rigorous understanding of the graphene/metal interface is crucial.Because of its monatomic, two-dimensional structure, graphene is highly sensitive to its environment. In the contact region, the properties of graphene are influenced by the contacting metal.…”