Growth and optimization of Ag-ZnO nanostructures for development of UV photodetectors

Abstract

Currently, UV photodetectors have captured the attention of research community because of their various applications in our society. UV photodetectors can be utilized for the applications in medical as well as military defense systems such as optical imaging, flame detection, missile launching systems, fast inter-satellite communication, UV-dosimeter, etc. A number of wide bandgap nanomaterials such as TiO2, ZnO, GaN, SiC, NiO, and ZnS have been explored for UV light detection to develop photodetector with high sensitivity. Among these, ZnO has several advantages over other semiconductor nanomaterials such as low-cost fabrication, biocompatibility, chemical and thermodynamic stability at room temperature, etc. Further, ZnO has a direct bandgap of 3.37eV and a high exciton binding energy of 60 meV at room temperature which make it a potential candidate for the development of UV photodetector. The physical, chemical, and optical properties of nanomaterials strongly depend on the shape, size, and morphologies of nanostructures. Therefore, various one dimensional (1-D) and two-dimensional (2-D) ZnO nanostructures have gained interest in past years. This work is mainly focused on the development of stable and high sensitivity UV photodetector on glass substrate through a facile low-cost solution processed method. But due to surface defects and high exciton binding energy, ZnO nanostructure-based photovoltaic devices are of low efficiency because of high recombination rate. Further, the presence of mid-gap states in ZnO, responds to the visible light and therefore have less UV to visible rejection ratio and also have longer response time. To rectify these problems, we have decorated ZnO nanostructures with Ag nanoparticles. The Ag nanoparticles were deposited over ZnO nanostructures via photochemical reduction method to improve the response time and sensitivity of the device. Ag nanoparticles forms Schottky junction with the ZnO nanostructures, thereby, improves the separation of photogenerated electron-hole pairs more efficiently, which improves the photosensitivity, and rise time-fall time characteristics. In addition to that, Ag nanoparticles depleted the charge carriers near the surface which significantly decreased the dark current. The device fabricated using Ag-ZnO honeycomb nanostructures is highly selective to UV-A region with UV to visible rejection ratio at (R(330nm)/R(650nm)) 1.1x104 . In addition to that, the device has shown a stable response under UV illumination with rise time and fall time of 30 s and 65 s respectively.