Optimization of hydrothermally grown ZnO nanorods network towards UV-sensitive FET applications

Abstract

Semiconductor nanostructures are an emerging class of materials with great po- tential for applications in future electronic devices. The devices based on nanos- tructures are very promising for multifunctional applications with high integration density. Nanostructures of di erent materials have been investigated since quite a long time. Recently ZnO based nanostructures triggered much interest due to their exciting properties such as high surface to volume ratio, high charge carrier mobility and bio compatibility. The rst part of this work is devoted to hydrothermal synthesis of ZnO nanos- tructures and their characterization. However, hydrothermal technique is prone to defects installed inadvertently in the nanostructures during the synthesis, which as a result limits the utilization of these techniques for large scale production of elec- tronic/optoelectronic devices for their enjoyability in di erent applications. Defect states and morphologies of nanostructures were controlled by using the additives in nutrient solution, subsequently forming vertically/laterally aligned ZnO nanorods (NRs) and ZnO=ZnCr2O4 composite nanowalls. Additionally, post growth anneal- ing was used to passivate the defects and tune the structural and optical properties of ZnO nanostructures. The annealing of ZnO NRs at di erent temperatures gave insight about passivation of defects while annealing of ZnO=ZnCr2O4 nanocom- posites paved an interesting way for the formation of Cr-doped ZnO nanowalls for optoelectronic device fabrication. The second part of the work was dedicated to demonstrate the fabrication and characterization of UV sensitive eld e ect transistor (FET) based on hydrother- mally synthesized ZnO nanorods (NRs) network. FET based on vertically aligned NRs showed low on/o ratio of the drain to source current due the NRs/NRs inter- face barrier while ZnO=ZnCr2O4 composite showed poor adhesion to the substrates and discarded for FET application. Apart from interface barrier, the morphology of semiconductor near the interface and in the bulk, too becomes crucial and both these factors contribute to the current carrying e ciency of the device. Moreover, ix these factors also a ect the optical response of these devices and will be extremely signi cant from the perspective of applications such as phototransistors. Further, the laterally aligned NRs network was used to demonstrate the FET utilizing the advantage of reduced interfacial contact between NRs network. FETs based on these structures showed better performance over the vertical NRs based FETs and extracted values of various parameters such as current on/o ratio, eld e ect mo- bility of FETs were 8 102 and 4.49 cm2 V 􀀀1 s􀀀1 respectively. We have also investigated the photoresponse of the fabricated FETs under UV-illumination. Fur- thermore, to enhance the photosensitivity of the transistor, surface plasmonic e ect of metals (Au) and (Pt) NPs were utilized. The highlight of this work is the success- ful integration of surface plasmon resonance in ZnO device to improve the operation of phototransistors. This approach demonstrates a novel technique for low-cost UV sensor development. The highest photosensitivity and responsivity were of the order of 105 and 120 A/W respectively under an incident light with a wavelength 350 nm.