Humidity responses of Love wave sensors based on (1 1 2¯ 0) ZnO/R-sapphire bilayer structures

• Love wave humidity sensors based on ZnO films are fabricated and studied.
• The nonlinear responses of the sensors to humidity are observed experimentally.
• The improved BET model and SAW propagation theory in layered media are used to explain the rsults.
• Then the thicknesses of adsorbed water layers corresponding to humidity are also evaluated.

Love wave humidity sensors are fabricated by sputtering (1 1 2¯ 0) ZnO thin films on R-sapphire substrates and lithographing interdigital transducers on the surfaces of ZnO films. The response characters of the sensors to humidity are investigated experimentally and theoretically. The experimental data show nonlinear function relations between oscillation frequency shifts of the sensors and relative humidity variations. Meanwhile, the phenomena can be explained by combining Brunauer–Emmett–Teller model for physical adsorption isotherms and surface acoustic wave propagation theory in inhomogeneous media. The experimental and theoretical results are in good agreement with each other.