Metal oxide-decorated AlN surface acoustic wave sensors for rapid room-temperature detection of NH₃ and NO₂

Surface acoustic wave (SAW) gas sensors based on aluminum nitride (AlN) offer fast response and stable operation at room temperature; however, their gas sensitivity is limited by the chemically inert nature of pristine AlN surfaces. In this study, metal oxide–decorated AlN SAW sensors are developed for rapid and selective detection of ammonia (NH3) and nitrogen dioxide (NO2) under ambient conditions. AlN-based SAW resonators were fabricated on sapphire substrates using epitaxial AlN thin films and optimized interdigital transducer structures to achieve low insertion loss and high frequency stability. To enhance gas–surface interactions, p-type copper oxide (CuO) and n-type zinc oxide (ZnO) sensing layers were selectively deposited onto the SAW propagation path, forming metal oxide/AlN heterojunctions. The CuO/AlN SAW sensor exhibited pronounced and reversible frequency shifts in response to NH3 at room temperature, demonstrating rapid response and improved sensitivity compared to bare AlN devices. For NO2 detection, the ZnO/AlN SAW sensor showed enhanced sensitivity and faster response under ultraviolet illumination, attributed to photo-induced carrier modulation at the ZnO/AlN interface. The sensing mechanisms were investigated through band alignment analysis, revealing that interfacial charge redistribution and conductivity modulation play dominant roles in the observed SAW responses rather than simple mass loading effects. These results demonstrate that metal oxide functionalization effectively overcomes the intrinsic limitations of bare AlN SAW sensors. The proposed oxide-decorated AlN SAW platform provides practical design guidelines for room-temperature gas sensors and shows strong potential for real-time environmental monitoring and industrial safety applications.