Sensors and Actuators A 193 (2013) 87–94 Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical jo u rn al hom epage: www.elsevier.com/locate/sna Love mode surface acoustic wave ultraviolet sensor using ZnO films deposited on 36 ◦ Y-cut LiTaO 3 Hua-Feng Pang a,b , Yong-Qing Fu b,∗ , Zhi-Jie Li a , Yifan Li c , Jin-Yi Ma d , Frank Placido b , Anthony J. Walton c , Xiao-Tao Zu a,∗ a School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, PR China b Thin Film Centre, Scottish Universities Physics Alliance (SUPA), University of the West of Scotland, Paisley PA1 2BE, UK c Institute for Integrated Micro and Nano Systems, Scottish Microelectronics Centre, School of Engineering, University of Edinburgh, Edinburgh EH9 3JF, UK d Sichuan Institute of Piezoelectric and Acousto-optic Technology, Chongqing 400060, PR China a r t i c l e i n f o Article history: Received 21 August 2012 Received in revised form 8 January 2013 Accepted 9 January 2013 Available online xxx Keywords: Ultraviolet sensor Love mode SAW ZnO film Photoconductivity Acoustic-electric interaction a b s t r a c t Love mode surface acoustic wave (SAW) ultraviolet (UV) sensors were fabricated through sputtering ZnO films on 36 ◦ Y-cut LiTaO 3 substrate. Crystalline structure, morphology and photoluminescence (PL) of the sputtered ZnO films were characterized using X-ray diffraction, atomic force microscopy and fluorescence spectrometer. The PL spectra revealed that different defects in the ZnO films were dependent on the oxygen partial pressure during deposition. UV sensing measurements showed that at a power density of 350 W cm -2 , the amplitude of the Love mode SAW UV sensor decreased up to -6.4 dB with a frequency shift of ∼150 kHz under a 254 nm illumination. When this device was illuminated with 365 nm UV light at 570 W cm -2 , the amplitude of the transmission signal decreased only -2.5 dB without any significant frequency shift. A frequency hopping effect during the downshift and recovery periods was identified due to the simultaneous interplays between the variations of the acoustic velocity and attenuation during the acoustic-electric interaction. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Recently, ZnO-based surface acoustic wave (SAW) devices have attracted much interest in ultraviolet (UV) sensor, gas sensor, biosensor and microfluidics [1–5]. Semiconducting ZnO films with a wide bandgap of ∼3.3 eV exhibit a good photoconductivity, and thus have been used to fabricate Schottky photodiodes, field-effect transistors, and SAW resonators for UV sensing applications [6–10]. ZnO-based SAW UV sensor possesses advantages of good passivity, low-cost, high reliability and good reproducibility [11]. ZnO films serving as a photo-conducting layer can be deposited on the SAW propagating path, and a good photo-response to the slight change in the acoustic-electric interaction arises from the variation of the sheet resistivity and carrier concentration in the ZnO film [12]. Changes in the velocity and amplitude of the SAW signal can be exploited to monitor the physical variation of the sensing layer, surface and interface of the multi-layers among sensing layers and piezoelectric substrates influenced by the wavelength and power density of the UV light. ∗ Corresponding authors. E-mail addresses: richard.fu@uws.ac.uk (Y.-Q. Fu), xtzu@uestc.edu.cn (X.-T. Zu). For SAW UV sensing, the Rayleigh-mode SAW devices can be used to detect the acoustic-electric interaction, and the sensing mechanism have been explained [11,13–18]. Typical sensing lay- ers such as GaN film or ZnO film have been deposited on the SAW devices and a frequency downshift of 60 kHz has been reported based on a GaN/sapphire SAW oscillator with a fre- quency of 200 MHz under UV irradiation in the wavelength range of 330–400 nm [13]. For another example, a 200 nm-thick ZnO film was sputter-deposited onto a LiNbO 3 SAW device operated at 37 MHz, which exhibited a large frequency shift of 170 kHz when illuminated using 365 nm UV light with an intensity of 40 mW cm -2 [14]. Kumar et al. deposited a 71 nm-thick ZnO film on a LiNbO 3 SAW filter with an operating frequency of 37 MHz, which could detect a low-level intensity of 450 nW cm -2 for the 365 nm UV light [15]. Furthermore, nanocystalline ZnO films with vari- ous morphologies of nanoparticles, nanorods and nanowires have been grown on the Rayleigh mode SAW devices to improve their photoconductivity [10,16–19]. Besides the fundamental Rayleigh mode, the Sezawa mode SAW has also been adopted to analyze the acoustic-electric effect [1,11]. Emanetoglu et al. introduced a Mg:ZnO layer in the ZnO/Mg:ZnO/ZnO/Si structure for the ZnO SAW filter, which exhibited a good sensitivity to a power den- sity of 810 W cm -2 when operated using the Sezawa mode SAW of 711.3 MHz [1]. Wei et al. demonstrated that a large frequency 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.01.016