To investigate theoptoelectronic response in fabricated structures we irradiated the samples withthe same fs laser at lower photon fluxes. Experiments with fs laser reveal thephotocurrent generation in both pristine and modified structures. Thetwo-step mechanism of photocurrent dynamics after laser pump was observed. Thefirst step is related to intrinsic charges relaxation after hot carriers generationin graphene. The average time of hot carriers cooling is about 50 ps and in ourcase we observed only consequences of this process (the minimal resolution ofthe multimeter is 1 ms, the total time of irradiation was ~ 5 ms). The second part of the decay we associate with chargecarriers trapping at the graphene-SiO interface.
The photocurrent, generated in the channel during thelaser treatment, is strongly related to the graphene width. Narrowgraphene ribbons provide higher response to laser irradiation. The narrowgraphene channel has higher body effect in response to light irradiationcompare to wide ribbons where contact effect dominates. Additional data onphotocurrent measurements with fs laser can be found.Optoelectronic measurements withcontinuous wave 532 nm laser were performed to investigate the origin of thegenerated photocurrent. The laser was focused to a ~ 0.
7 m spot and the laserpower was controlled by neutral density filter in the range of 0.1 W – 0.5 mW.We compared the laser power dependence of generated photocurrent in pristineand functionalized areas.
Figure shows the typical power dependence of thephotocurrent for pristine and functionalized with twodifferent photon fluxes GFETs. The photocurrent for pristine graphene is foundto saturate at 2×10 W cm and the saturation limit forfunctionalized graphene goes beyond 1×10 W cm, whichis rather high. The non-unity exponent may be as a result of complexprocesses of electron-hole generation, trapping, and recombination, implying carrier transport and electron trapping inthe processed graphene photodetector.
The calculated photoresponsivity for GFETs varies from 0.3 to 100 mA W-1 and highly depends on powerdensity and applied voltages V and V. These valuesare comparable or exceed the results from similar works for single-layergraphene photodetectors.Additionally, photocurrentsignals were measured while laser light at 100 W scanned the channel.A photocurrent as large as 7 nA was measured directly at and close to the functionalizedarea. As expected, no significant photocurrent was observed for pristinegraphene areas.
The photocurrent generation area is wider than the spatial regionof the junctions, which indicates that the excited carriers stayed hot acrossthe GFET channel and contributed to the photocurrent.The shift of the Gband, as discussed previously, and shown in , can be quantitativelytranslated into a charge density. We found that the fslaser treatment of single layer graphene increases the charge density of up to1×10 cm. The significant change in holeconcentration at the edges of irradiated region defines sharp p-p heterojunctions.