Bipolar Photoresponse of a Graphene Field-Effect Transistor Induced by Photochemical Reactions

Graphene is an air-friendly materialthat can be easily p-dopedby oxygen; therefore, a stable, defect-free, and efficient graphenen-doping technique should be developed for achieving high performancein electronic and optoelectronic devices. In this study, we presenta unique method for n-type chemical doping of monolayer graphene grownthrough chemical vapor deposition. The doping process is thoroughlyexamined using X-ray photoelectron spectroscopy, Raman spectroscopy,and ultraviolet photoelectron spectroscopy. The findings demonstratethat the use of KBr solution is highly effective in achieving n-typedoping in monolayer graphene, offering promising prospects for itspractical application. Also, we fabricated graphene field-effect transistorsand studied their electrical properties before (pristine) and afterdoping the graphene channel with different KBr concentrations (0,0.05, 0.15, 0.20, and 0.25 M) in dark and under deep-ultraviolet (DUV)light conditions. During graphene doping in the dark environment,the charge neutrality point (CNP) shifted toward negative back gatevoltages and then saturated at 0.25 M. After photochemical dopingunder DUV light, CNP further shifted toward negative gate voltageswith improved carrier mobility at the same molar concentration of0.25 M. Additionally, the photodetectors are fabricated from pristineand doped graphene which demonstrated bipolar photoresponse, thereby,a transition of negative photocurrent to a positive photocurrent whenthe concentration of the KBr solution reached 0.20 M. Moreover, theirresponse time decreased from 8 to 3.5 s with increasing KBr concentrationfrom 0 to 0.30 M. Finally, the gate voltage-dependent broadband photoresponsivityof doped graphene (0.25 M) was investigated at different wavelengths(220, 365, 530, and 850 nm). Thus, the controlled doping-induced bidirectionalphotoresponse can provide a facile route for logic gate applications.