Sensors and Actuators B 255 (2018) 2454–2461 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo u r nal homep age: www.elsevier.com/locate/snb Research paper A high performance humidity sensor based on surface acoustic wave and graphene oxide on AlN/Si layered structure Xianhao Le a , Xiaoyi Wang a , Jintao Pang a , Yingjun Liu b , Bo Fang b , Zhen Xu b , Chao Gao b , Yang Xu c , Jin Xie a,∗ a State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou 310027, PR China b MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China c College of Information Science & Electronic Engineering, Zhejiang University, Hangzhou 310027, PR China a r t i c l e i n f o Article history: Received 16 March 2017 Received in revised form 5 September 2017 Accepted 7 September 2017 Available online 8 September 2017 Keywords: Humidity sensor Surface acoustic wave Graphene oxide AlN thin film Highly doped Si a b s t r a c t A surface acoustic wave (SAW) humidity sensor based on AlN/Si (doped) layered structure and graphene oxide (GO) sensing layer is proposed for high sensitivity and low temperature coefficient of frequency. With the GO thin film, the sensitivity of the humidity sensor is increased to 42.08 kHz/RH% when the rela- tive humidity is greater than 80%RH. The humidity sensor performs well even at both very low (≤10%RH) and very high (≥90%RH) detection range of humidity. The present sensor has low hysteresis, excellent short term repeatability and long term stability (variation less than ± 2%). The sensor also shows a fast response and short recovery time. Moreover, using the AlN/Si (doped) layered structure, the thermal stability of the sensor is significantly improved. After being covered with GO thin film, the temperature coefficient of frequency (TCF) of the sensor is further reduced to -22.1 ppm/ ◦ C, much smaller than the previously reported SAW humidity sensors. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Humidity sensors play very important roles in various fields, such as industrial production process monitoring, living environ- ment control and agricultural planting management [1–3]. The performance of the humidity sensors based on different work- ing principles [4–7] has been studied for several decades. Surface acoustic wave (SAW) humidity sensors have gained extensive attention duo to the advantages of small size, fast response and high stability [8]. Typically, the SAW humidity sensors are covered by a layer of sensing material to improve sensitivity. With the devel- opment of material technology, many sensing materials have been applied to the SAW humidity sensors, mainly polymers [9–11] and nanomaterials [8,12,13]. Graphene oxide (GO) as a carbon nano- material has large surface to volume ratio and oxygen-containing hydrophilic functional groups like carboxyl groups [14], which makes it very suitable for humidity sensing. Moreover, GO can be directly deposited on the interdigitated electrodes (IDTs) owing to its characteristic of electrical insulation [15]. Previous studies have ∗ Corresponding author. E-mail address: xiejin@zju.edu.cn (J. Xie). reported that GO humidity sensors [16,17] have high sensitivity, fast response and little hysteresis. Although the SAW based humidity sensors offer many advan- tages, they are susceptible to change of temperature [18]. It is particularly important to improve thermal stability of the SAW humidity sensors for accurate testing when ambient temperature changes. For most of the SAW humidity sensors, interdigitated elec- trodes are designed directly on bulk piezoelectric substrates such as LiNbO 3 [8], but this kinds of structure usually has a relative high temperature coefficient of frequency (TCF) [19]. Compared with the bulk SAW devices, the SAW devices with layered structures (the materials of piezoelectric layer and structural layer have opposite temperature coefficients) have lower TCF [20]. Aluminum nitride (AlN) film has recently received extensive attention due to its high acoustic wave velocity, CMOS process compatibility, good chem- ical and temperature stability [21]. SAW sensors based on AlN/Si layered structure have been proven to have good thermal stabil- ity [22], but still need to be improved for highly accurate humidity sensors. Recent studies have shown that doped Si substrate layer is bene- ficial in further reducing the temperature sensitivity of frequency of piezoelectric devices [23]. In this work, we propose a SAW humid- ity sensor based on AlN/Si layered structure, where surface of the Si layer is highly doped before deposition of AlN. The SAW humid- http://dx.doi.org/10.1016/j.snb.2017.09.038 0925-4005/© 2017 Elsevier B.V. All rights reserved.