Effect of fabrication process on contact resistance and channel in graphene field effect transistors.

Contact resistance, as one of the main parameters that limits the performance of graphene-based transistors, is highly dependent on the metal-graphene contact fabrication processes. These processes are investigated and the corresponding resistances are measured based on the transfer length method (TLM). In fabrication processes, when annealing is done on chemical vapor deposition (CVD)-grown graphene samples that are transferred onto SiO2/Si substrates, the adhesion of graphene to the substrate is improved, and poly methyl methacrylate (PMMA) residues are also reduced. When the metal deposition layer is first applied to the graphene, and then, the photolithography process is performed to define the electrodes and graphene sheet, the graphene-metal contact resistance is better than that in other methods due to the removal of photoresist residues. In fact, by changing the sequence of the fabrication process steps, the direct contact between photoresist and graphene surface can be prevented. Thus, the contact resistance is reduced and conductivity increases, and in this way, the performance of graphene transistor improves. The results show that the fabrication process has a noticeable effect on the transistor properties such as contact resistance, channel sheet resistance, and conductivity.| Here, by using the annealing process and changing the order of photolithography processes, a contact resistance of 470 Ω µm is obtained for Ni-graphene contact, which is relatively favorable.

Experimental comparison between Nb2O5- and TiO2-based photoconductive and photogating GFET UV detector.

In the present study, by adding graphene to a photoconductive photodetector with a niobium pentoxide (Nb2O5) absorber layer and exploiting the photogating effect, the responsivity of the photodetector is significantly improved. In this photodetector, the Nb2O5 layer detects the light, and the graphene improves the responsivity based on the photogating effect. The photocurrent and the percentage ratio of the photocurrent to dark current of the Nb2O5 photogating photodetector are compared with those of the corresponding photoconductive photodetector. Also, the Nb2O5 photoconductive and photogating photodetectors are compared with titanium dioxide (TiO2) photoconductive and photogating photodetectors in terms of responsivity at different applied (drain-source) voltages and gate voltages. The results show that the Nb2O5 photodetectors have better figures of merit (FOMs) in comparison with the TiO2 ones.