Improvement in the Photocurrent to Dark Current Contrast Ratio of ZnO/Au Schottky Photodetector with PMMA Microlens Arrays

Abstract

A method is proposed to improve the photocurrent to dark current contrast ratio for the ZnO nanowire/Au Schottky barrier diode fabricated using dielectrophoresis to align the nanowires. Because the leakage current would lead to a large background noise, a poly͑methyl methacrylate͒ ͑PMMA͒ layer is first employed as the passivation layer to cap on the surface of the ZnO nanowire Schottky diode; furthermore, the microlens array ͑MLA͒ structures are added to improve the light trapping effect and to magnify the probability of the photons to be absorbed by ZnO nanowires. After depositing the PMMA capping layer, the leakage current can be reduced by more than 10 3 times, whereas the contrast ratio can be increased from 3.4 ϫ 10 to 6.7 ϫ 10 2 at Ϫ1.0 V after the MLA structures are added on the diode.Zinc oxide is one of the most promising materials for fabricating high sensitivity UV detectors because it has a wide direct bandgap ͑3.37 eV at room temperature͒ and a high resistance to high energy radiation, 1 making it a candidate for space application. ZnO has given rise to a great interest in recent years, including the study of diverse synthetic methods, various processing technologies, and fabrication of versatile functional devices. The devices based on ZnO nanowires ͑NWs͒, such as optically pumped nanolaser, light emitting diode, and transistor, have been demonstrated due to the combination of particular structural and semiconducting properties of ZnO nanomaterials. There are different types of semiconductor photodectors: photoconductors, Schottky barrier, metal-insulatorsemiconductor structures, and p-n and p-i-n photodiodes. The detectors that are mentioned above are all operated in the process that the photogenerated carriers interact with the external circuit to provide the output signal through carrier transport. Photoconductors are fabricated by depositing electrodes with ohmic contacts at both ends of a semiconductor slab. For a sample with a long lifetime and short electrode spacing, the gain can be substantially greater than unity. However, the photoconductors show a very nonlinear response with illuminating power, and a very slow nonexponential transient response. 2,3 The metal-insulator-semiconductor structure is designed to obtain a high breakdown voltage and large barrier height which could improve the response. 4 The photodetectors based on the Schottky junction are desirable due to their ability to operate at high switch speeds. Furthermore, to improve the contrast ratio ͑ratio of photo to dark current͒ of the photodiodes, the diodes are often operated in reverse bias to reduce the probability of recombination of the photogenerated electron-hole pairs. The width of the depletion region can be optimized by tuning the reverse bias. For high frequency operation, the depletion region must be kept thin to reduce the transit time. To increase the quantum efficiency, the depletion layer must be sufficiently thick to allow a large fraction of incident light to be absorbed. Because the charge distribution of a...

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