Investigations on hydrothermally grown Zno nanostructures towards development of UV/broadband photodetectors

Abstract

The photodetectors are one of the key components in modern technologies and are widely used in military and civil fields such as optical imaging, fire detection, optical communication, missile guidance and positioning system etc. Currently, Si-based photodetectors and photomultipliers are dominant in the market due to their extremely mature processing techniques and low cost. However, due to the lower bandgap of 1.1 eV to 1.3 eV, Si can only be used for detection in visible and near-infrared (NIR) spectral range. Additionally, with the continuous evolution of the technology, the demand for high performing UV and broadband photodetectors have been increasing in the diverse applications. Therefore, wideband metal oxide semiconductors like WO3, SnO2, NiO, ZnO, Nb2O5 Ga2O3 etc. have been explored. Due to simple and low-cost fabrication processes, biocompatibility and enriched physical, optical, electrical and chemical properties, ZnO has emerged as a promising material for sensors. It has a wide and direct bandgap of 3.37 eV at room temperature, which makes it suitable for various optoelectronic applications especially in UV region, including UV photodetectors. One dimensional (1D) and two dimensional (2D) nanostructured materials have gained a lot of interest in the past several decades due to better physical, chemical and electrical properties. Generally, the surface of a material is more sensitive to the external changes, therefore larger surface area of the nanostructures has made them more sensitive to these changes. This work is mainly focused on the development of high responsivity UV and broadband photodetectors over flexible and rigid substrates, using varieties of ZnO nanostructures obtained by simple and low-cost hydrothermal process. In the first part of work, the ZnO nanostructures growth process has been optimized towards syn thesis of complex nanostructures with higher surface area, to control the intrinsic defect density and to improve the adhesivity of nanostructures with the substrate. The obtained nanostructures have demostrated significantly high UV and broadband photoresponse, which was found to be comparable to commercially available photodetectors. In next part, the impact of high energy UV radiation over the device performance has been investigated and a suitable solution to overcome this issue has been proposed. Moreover, considering the increasing demand of flexible photodetectors for various wearable applications, highly sensitive and ultra low power, flexible UV photodetector have been synthesised. The device exhibited remarkably high photo-sensitivity of 348 and photo-responsivity 265 mA/W at just 0.1V applied bias.