Copper Doped ZnO Thin Film for Ultraviolet Photodetector with Enhanced Photosensitivity
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Copper Doped ZnO Thin Film for Ultraviolet Photodetector with Enhanced Photosensitivity Akshta Rajan1, Kashima Arora1, Harish Kumar Yadav2, Vinay Gupta1 and Monika Tomar3 1 Department of Physics and Astrophysics, University of Delhi, Delhi, INDIA. 2 Department of Physics, St.Stephens College, University of Delhi, Delhi, INDIA. 3 Physics Department, Miranda house, University of Delhi, Delhi, INDIA. ABSTRACT Ultraviolet photoconductivity in Copper doped ZnO (Cu:ZnO) thin films synthesized by sol-gel technique is investigated. Response characteristics of Pure ZnO thin film and Cu:ZnO thin film UV photodetector with 1.3 at. wt % Cu doping biased at 5 V for UV radiation of λ = 365 nm and intensity = 24 μwatt/cm2 has been studied. Cu:ZnO UV photodetector is found to exhibit a high photocoductive gain (K = 1.5×104) with fast recovery (T90% = 23s) in comparison to pure ZnO thin film based photodetector (K = 4.9×101 and T90% = 41s). Cu2+ ions have been substituted in ZnO lattice which has been confirmed by X-ray diffraction (XRD) and Raman spectroscopy leading to lowering of dark current (Ioff ~ 1.44 nA). Upon UV illumination, more electron hole pairs are generated in the photodetector due to the high porosity and roughness of the surface of the film which favours adsorption of more oxygen on the surface of the photodetector. The photogenerated holes recombined with the trapped electrons, increasing the concentration of photogenerated electrons in the conduction band enhancing the photocurrent (Ion ~ 0.02 mA) of the Cu:ZnO photodetector. INTRODUCTION Detection of ultra violet (UV) radiations is becoming very important in various commercial, military and scientific areas, which has made ultraviolet photodetectors reasonably an interesting field. In recent years, besides GaN which is well established commercial material for detection of UV rays, ZnO in the form of nanostructures and thin films have gained interest and proved to be advantageous material for selective UV photodetection, as well as solar blind UV detectors due to wide band gap (~3.3 eV), short carrier lifetime, large exciton binding energy (~60 meV), ease of fabrication of thin films by various deposition techniques, radiation hardness, low cost synthesis etc. Photoconductivity in ZnO involves adsorption and desorption of oxygen molecules leading to the process of trapping and releasing holes at the ZnO surface [1,2]. Photoconducting property in ZnO lattice can be improved by introducing defects within the band gap by intentionally doping foreign metals such as Al, N, Te, Li and Cu at Zn lattice site [3]. Copper doping in ZnO has been reported to form acceptor states within the bandgap of ZnO resulting in higher resistance of ZnO. Moreover, doping of copper in ZnO nanowires is known for strong multiplication of photocarriers which leads to dramatically enhanced sensitivity to optical radiation over multiple spectral ranges of UV and visible regions of the electromagnetic spectrum [3,4]. In the present work, we show the feasibility of realizing a highly sensitive
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