Transfer characteristics and contact resistance in Ni- and Ti-contacted graphene-based field-effect transistors

Abstract

We produced graphene-based field-effect transistors by contacting mono- and bi-layer graphene by sputtering Ni or Ti as metal electrodes. We performed electrical characterization of the devices by measuring their transfer and output characteristics. We clearly observed the presence of a double-dip feature in the conductance curve for Ni-contacted transistors, and we explain it in terms of charge transfer and graphene doping under the metal contacts. We also studied the contact resistance between the graphene and the metal electrodes with larger values of ~30 kΩμm(2) recorded for Ti contacts. Importantly, we prove that the contact resistance is modulated by the back-gate voltage.

Contact Resistance and Channel Conductance of Graphene Field-Effect Transistors under Low-Energy Electron Irradiation

“…According to the TLM, we can extract the specific contact resistivity ${\rho }_c$ by evaluating (for the general situation of irregular shaped flakes) the intercept of a plot of R eff vs. L [30], with L the separation between the two electrodes, and $R_{\mathrm{eff}}=R{\left(\frac{1}{W_1{d}_1}+\frac{1}{W_2{d}_2}\right)}^{-1}$ where W i and d i (for i = 1, 2 ) indicate width and length of each contact, respectively. From the linear fitting of R eff vs. L (see Figure 1c), we find ${\rho }_c=19\pm 2 \mathrm{normalk}\mathrm{sans-serif\Omega }·{\mathrm{sans-serif\mu }\mathrm{normalm}}^2$, an intermediate value compared with previously reported values of 7 kΩ·μm 2 for Ni and 30 kΩ·μm 2 for Ti [31]. …”

Contact resistance and mobility in back-gate graphene transistors

“…The different slope of the two branches corresponds to the hole mobility (~--V s 150 cm 2 1 1 ) higher than the electron one (~--V s 100 cm 2 1 1 ), while the slight shift of the Dirac point to positive V gs indicates a low p-type carrier concentration due to adsorbates and process residues such as PMMA, not removed by the 1 mbar vacuum and 400 K annealing [59,84]. The holeelectron asymmetry is due to both unbalanced carrier injection from metal contacts and graphene interaction with the SiO 2 dielectric [84][85][86][87][88][89].…”

“…Despite the several doping techniques available to tune the graphene conductivity and boost the performance of graphene transistors, a major problem remains the suppression of device on-current caused by the graphene/contact resistance [56]. Indeed, ohmic and low resistance contacts are important figures of merit for high frequency devices and the realization of stable and low-resistance contacts is still under intensive study [57][58][59][60][61]. The variation of the contact resistance, R , C is attributed to many different causes, related to graphene growth and number of layers, metal type and deposition process, quality of the metal graphene/interface, measurement conditions, etc.…”

The role of contact resistance in graphene field-effect devices

“…Several works to increase DOS in graphene and to decrease contact resistance by using different metals, surface treatments or innovative device architecture have been reported [108][109][110][111][112][113][114][115]. Some results are summarized in Table 1.…”