ZnO Nanorods Morphology Control in Hydrothermal Method and their Humidity Sensing Enhancement on AlN SAW Device

A surface acoustic wave (SAW) humidity sensor which utilizes ZnO nanorods on top of AlN/c-plane sapphire aims to achieve high sensitivity. The nanorod's morphology, aspect ratio (L/W), and vertical directivity could be controlled by adjusting the concentration of solutions during the hydrothermal process. With a concentration of 0.003 moles, the ZnO nanorods exhibited relatively low rod density and uniform rod-height with highly vertical-alignment. By incorporating such ZnO nanorods on AlN SAW, the sensitivity of the humidity sensor reaches 3.77 kHz/RH% at a relative humidity level of 90 %RH. The ZnO nanorods structure provides a 2.43 times improvement in sensitivity. Additionally, with the ZnO nanorods, the AlN SAW humidity sensor exhibits a fast recovery, which recovery time reduced to 46% of the one without nanorods.