Highly sensitive layered ZnO/LiNbO3 SAW device with InOx selective layer for NO2 and H2 gas sensing

Layered surface acoustic wave (SAW) devices for the monitoring of NO2 and H2 in synthetic air have been fabricated on XZ LiNbO3 with a 1.2 μm ZnO guiding layer. To increase selectivity and sensitivity, InOx layers of thickness 40 and 200 nm were employed. The sensor's performance was analyzed in terms of frequency shift as a function of different gas concentrations. The sensors were tested over a range of operating temperatures between 100 and 273 °C. A large response magnitude with fast response and recovery time was observed. Positive frequency shifts of 91 kHz for 2.125 ppm of NO2 and negative frequency shifts of 319 kHz for 1% of H2 in synthetic air are presented; demonstrating the high sensitivity of the layered SAW structure with the DC sputtered InOx thin film. The surface of the layered SAW structure was studied by atomic force microscopy (AFM) before and after the deposition of the InOx selective layer. The AFM analysis demonstrates that the InOx films deposited on ZnO, the guiding layer, resulted in an increase in surface area due to the highly uniform nanostructured surface morphology of InOx.