Responsivity enhancement of a PtSi photodetector with graphene by the photogating effect

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 photoconductive and graphene-based photogating detection in a UV-A region

Photoconductive detectors that use intrinsic absorbent materials include a wide range of detectors. In this paper, a photoconductive detector is fabricated using a titanium dioxide (T i O 2) thin film. The mechanism of the photodetector is changed to the photogating mechanism by transferring monolayer graphene onto the T i O 2 thin film, which shows a great responsivity with a slight change in the fabrication process. Since the maximum responsivity can be obtained by applying and adjusting the gate voltage, the gate voltage is set in all experiments, and the effect of the gate voltage is investigated in both detectors. It is observed that by increasing the gate voltage, the responsivity of the photogating detector increases to 40 A/W at a gate voltage of 15 V. However, in the photoconductive detector, the increase in the gate voltage does not have a particular effect on the detector responsivity. In the photogating detector, the increase in the responsivity due to the increase in the gate voltage is attributed to applying the gate voltage to the graphene layer and not the absorber layer. The efficiency of both detectors is confirmed up to a frequency of 5 kHz.

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.