ZnO-Sn Bilayer Ultraviolet (UV) Photon Detector with Improved Responsitivity
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0957-K07-14
ZnO-Sn Bilayer Ultraviolet (UV) Photon Detector with Improved Responsitivity Harish Kumar Yadav, K. Sreenivas, and Vinay Gupta Department of Physics and Astrophysics, University of Delhi, Delhi, 110007, India ABSTRACT The influence of the integration of ultra-thin layer of different metals (Al, Cu, Sn, Te, Pb and Au) with the c-axis oriented ZnO thin film on the ultraviolet (UV) photoresponse is investigated. The transfer of electrons from the metal layer to the semiconductor at the interface compensate the surface states which otherwise takes electrons from the interior of ZnO layer and thereby increases the conductivity under UV illumination. The Sn/ZnO sample exhibits a responsivity of the order of 8.5 KV/W at a low UV intensity of 140 µW/cm2 (λ = 365 nm) with a fast rise and fall time of 105 and 400 ms respectively.
INTRODUCTION Zinc oxide (ZnO), a wide band gap (Eg = 3.3 eV) semiconductor, is found to be an ideal candidate for the fabrication of high-speed, inexpensive, and radiation hard ultraviolet (UV) photon detectors [1-4]. Recently, UV photoresponse from ZnO thin films and nanostructures has generated a keen interest for selective UV (375 nm) photodetection [1,2,5,6]. The detection range can be extended from UV-A (320-400 nm) through UV-B (280-320 nm) to UV-C (200280 nm) region of solar spectrum by doping ZnO with magnesium (Mg) [7]. Several groups reported the effect of crystallographic orientation, surface microstructure, and thickness on the photoresponse characteristics of the ZnO [8-10] and the increased conductivity was ascribed to the adsorption or desorption of oxygen neutrals on the surface or at the grain boundaries. Passivation or compensation of these surface states were reported to influence significantly the UV photoresponse characteristics of ZnO thin film. Short term annealing in hydrogen ambient was shown to increase the photoconductivity of the ZnO sample under UV illumination [10] and was attributed to the compensation of surface states, due to oxygen vacancies, by the electrons. In another study, Zhang et al. reported a significant decrease in the response time of the rf sputtered ZnO thin film by doping the surface layer with nitrogen acceptors [11]. The strongly bonded nitrogen was supposed to stabilize the surface of the ZnO film by filling the oxygen vacancies on the surface. Recently, Moazzami et al. showed that passivating the surface of ZnO thin film deposited by plasma-assisted molecular beam epitaxy with SiO2 significantly reduces both the photoconductive signal and the photoresponse time constants [5]. Doping ZnO films with N or Te is also reported to enhance the photoresponse characteristics [2]. However, no systematic attempt has been made to study the thin over-layer of different metals on the photoresponse characteristics of the ZnO thin films. In this letter the photoresponse characteristics of the ZnO thin films integrated with ultra thin layer of different metals is shown to depend on the chemistry of the ZnO with the metal layer at the interface.
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