Effects of rapid thermal annealing on surface acoustic wave ultraviolet sensors using ZnO nanorods grown on AlN/Si struc
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Effects of rapid thermal annealing on surface acoustic wave ultraviolet sensors using ZnO nanorods grown on AlN/Si structures Duy-Thach Phan & Gwiy-Sang Chung
Received: 24 August 2012 / Accepted: 14 January 2013 / Published online: 25 January 2013 # Springer Science+Business Media New York 2013
Abstract In this study, we demonstrate a high sensitivity of surface acoustic wave (SAW) ultraviolet (UV) sensor based on ZnO nanorods (NRs) grown on an aluminum nitride (AlN)/silicon (Si) layered structure. The one-dimensional ZnO NRs act as a high-UV sensing material due to their large surface-to-volume ratio. The fabrication of SAW UV sensor is entirely compatible with micro/nano electromechanical (M/NEMS) process with conventional lithography and synthesized ZnO NRs by hydrothermal method at low temperature. The rapid thermal annealing (RTA) process effectively improved the optical properties of ZnO NRs and the sensitivity of the SAW UV sensors. The resulting SAW UV sensors responded to various UV light intensities, and the RTA-processed samples showed high sensitivity. The SAW UV sensor after RTA treatment at 600 °C showed the highest sensitivity with a 130 kHz frequency shift at a UV light intensity of at 0.6 mW/cm2, a 5-fold increase in sensitivity compare with as-grown sample. Keywords SAW . UV sensor . ZnO nanorods . AlN thin film . Rapid thermal annealing
1 Introduction Zinc oxide (ZnO) is a unique material which exhibits both semiconductor and piezoelectric properties. ZnO is sensitive to ultraviolet light due to optical properties such as a wide band gap of 3.4 eV and a large exciton binding energy of 60 meV at room temperature [1]. The strong photoresponse D.-T. Phan : G.-S. Chung (*) School of Electrical Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 680-749, Republic of Korea e-mail: [email protected] URL: http://home.ulsan.ac.k/gschung
of ZnO thin films makes this a promising material for optical and optoelectric applications. ZnO has been widely used for ultraviolet (UV) detectors such as metalsemiconductor-metal phototransistors, photodiodes, and surface acoustic wave (SAW)-based photodetectors. Among these devices, SAW devices have advantages which allow for remote wireless operation and have a strong potential for use as passive sensors [2–5]. Lattice mismatch between ZnO and its growth substrate has a significant effect on the optical properties of ZnO [6, 7]. Hence many researchers have investigated the properties of ZnO growth substrates including sapphire [8], quartz [3] and lithium niobate [9] to improve the sensitivity of ZnO film SAW UV sensors with some interesting results. The electromechanical coupling coefficient (K2) of piezoelectric substrates also enhances the sensitivity of SAW UV sensors [8, 9]. However, these substrates are not compatible with micro/nano electromechanical (M/NEMS) systems and are difficult to integrate with other electronic elements on the same silicon (Si) substrate. Polycrystalline aluminum nitride (AlN) is a good candidate to form ZnO nanorods (NRs)
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