Synthesis and characterization of ZnO nanostructures for ultraviolet (UV) light sensing application
Qazi Muhammad, Humayun
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Nanotechnology has strong influence over many known technologies with plenty of advantages, such as low-cost and larger surface-area-to-volume ratio compared with their bulk counterpart. Among II-IV semiconductor materials zinc oxide (ZnO) is an ntype semiconductor with band gap energy of 3.37 eV and having large exciton binding energy of ~ 60 meV. ZnO and its alloys have vast device applications mainly in manufacturing of light emitting diodes (LEDs), solar cells, optical waveguides and Ultraviolet (UV) photodetectors. Ultraviolet (UV) photodetectors are widely used in various commercial and military applications, especially to secure space-to-space communications, pollution monitoring, water sterilization, flame sensing, and early missile plume detection. In contrast to gallium nitride (GaN), ZnO has a highest electron saturation velocity thus, photodetectors equipped with ZnO can perform at a maximum operation speed. The objective of research is to deposit ZnO thin film and ZnO nanorods by sol-gel method at selective area of microgap electrodes spacing and characterization for ultraviolet (UV) sensing application. Therefore the Zerogap structure of butterfly topology was designed by AutoCAD software, and to achieve the better resolution during photo masking process the design was transferred to commercial chrome glass photomask. All the area selective deposited ZnO based nanosensors were further tested for ultraviolet (UV) sensing application. On exposure of ultraviolet (UV) light the current gains, response/recovery times, repeatability, sensitivity, reproductivity and responsivity of the fabricated ZnO based microgap electrodes sensors displayed the promising application for UV light detection. Moreover the signal detection at low operating voltage (1 V) revealed that fabricated sensors can be used for miniaturized devices with low power consumption. The surface morphologies structural, optical and electrical properties of the synthesized nanostructures ZnO were characterized using SEM, XRD, and sourcemeter respectively. To study the doping effect on ZnO nanostructures finally, tin (Sn) was selected, and successfully synthesized on glass substrate by low temperature sol-gel hydrothermal growth process. The as synthesized Sn-doped ZnO nanorods were post annealed at three different temperatures and investigated the effect of post-annealing temperatures on structural, optical, electrical and photoresponse properties by using Xray diffraction, UV-Vis spectroscopy, I-V and i-t measurements. The crystallinity and c-axis orientation of Sn-doped ZnO nanorods were increased with annealing temperatures. As post-annealing temperature increased the Sn-doped ZnO nanorods showed noticeable variations having agglomerated and spherical shape at surface morphology than those at a lower post-annealing temperature; this result indicates that the samples are highly crystalline in nature. The optical bandgap energy of Sn-doped ZnO nanorods decreased as annealing temperature increases. Electrical characteristics reveal the effect of annealing temperature on resistivity and photoresponse properties of Sn-doped ZnO nanorods. Hence, the proposed Herve and Vandamme model and the improved ultraviolet (UV) photoresponse of post-annealed samples are applicable in optoelectronic device applications.