Plasmonic Enhancement of Optical Absorption of UV Radiation in ZnO Thin Film Based Ultraviolet Photodetectors
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Plasmonic Enhancement of Optical Absorption of UV Radiation in ZnO Thin Film Based Ultraviolet Photodetectors Akshta Rajan1, Ayushi Paliwal1, Vinay Gupta1 and Monika Tomar2 1 Department of Physics and Astrophysics, University of Delhi, Delhi, INDIA. 2 Physics Department, Miranda house, University of Delhi, Delhi, INDIA. ABSTRACT Ultraviolet (UV) photoconductivity in pure ZnO thin films and metal (Ag, Au, Pt) nanoparticles (NPs) dispersed on ZnO thin films based UV photodetectors biased at 5 V for ultra violet radiation of λ = 365 nm and intensity = 24 µwatt/cm2 has been studied. All the three metal (Ag, Au, Pt) NPs synthesized by Polyol process when dispersed on the surface of 100 nm thin ZnO film results in enhanced photoconductive gain (K) in comparison to pure ZnO (3.1×103). An increase of about an order in K has been obtained in the case of Ag NPs/ZnO and Au NPs/ZnO UV photodetectors ( K = 6.9×104 and 5.3×104 respectively). On the other hand, Pt NPs enhance K by about two orders (5.0×105). Such an enhanced photoconductive gain has been achieved due to the lowering of dark current after dispersing the metal NPs on the surface of ZnO and increased photocurrent upon UV illumination. This may be attributed to the plasmon propagating property in metal NPs which enhances the light trapping through optical absorption in ZnO thin film surface (high photo current). INTRODUCTION Emerging field like plasmonics gives a very broad area for research, its application for silicon based thin film solar cells and photodetectors has been a recent interest as evident from increasing number of publications over the last couple of years in the same area. Structuring of metals exhibiting plasmonic propagating properties on the subwavelength scale in the form of nanostructures/nanoparticles can leads to coupling of light to surface plasmons. Metal nanoparticles (Au, Ag) can exhibit strong localized surface plasmon resonances (LSPRs) at UV, visible and near infrared (NIR) wavelengths. In recent years, metal nanoparticles have been shown to improve the performance of photo- diodes, Si based photodetectors, solar cells and LEDs [1,2]. Detection of ultra violet (UV) radiations is becoming very important in various commercial, military and scientific arenas, which has made ultraviolet photodetectors reasonably an interesting field. Zinc oxide (ZnO) possessing wurtzite crystallographic structure having a wide band gap value (~ 3.3 eV) is a promising candidate for ultraviolet (UV) optoelectronic applications, with UV lasers, light emitting diodes and photoconductors being reported in various nanostructural forms like thin films, nanoparticles, nanowires etc. Incorporation of metal in the form of overlayer and dopant in ZnO thin film based photodetectors have been reported to be useful for better UV photoresponse characteristics [3]. However, NPs has not been exploited for the UV photodetectors. In the present study, over layer of spherically shaped, small size metal nanoparticles (NPs) of different metals including silver (Ag), gold (Au) and
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