In this study, a 3.5-GHz solidly mounted resonator (SMR) was developed by doping scandium in aluminum nitride to form AlScN as the piezoelectric thin film. Molybdenum (Mo) of 449 nm thickness and silicon dioxide (SiO₂) of 371 nm thickness were used as the high and low acoustic impedance films, respectively, which were alternately stacked on a silicon substrate to form a Bragg reflector. Then, an alloy target with atomic ratio of 15% Sc was adopted to deposit the piezoelectric AlScN thin film on the Bragg reflector, using a radio frequency magnetron sputtering system. The characteristics of the c-axis orientation of the AlScN thin films were optimized by adjusting sputtering parameters as sputtering power of 250 W, sputtering pressure of 20 mTorr, nitrogen gas ratio of 20%, and substrate temperature of 300 °C. Finally, a metal top electrode was coated to form a resonator. The X-ray diffraction (XRD) analysis showed that the diffraction peak angles of the AlScN film shifted towards lower angles in each crystal phase, compared to those of AlN film. The energy dispersive X-ray spectrometer (EDX) analysis showed that the percentage of scandium atom in the film is about 4.5%, regardless of the sputtering conditions. The fabricated resonator exhibited a resonance frequency of 3.46 GHz, which was a small deviation from the preset resonance frequency of 3.5 GHz. The insertion loss of −10.92 dB and the electromechanical coupling coefficient of 2.24% were obtained. As compared to the AlN-based device, the AlScN-based resonator exhibited an improved electromechanical coupling coefficient by about two times.

An ammonia sensor based on a delay-line surface acoustic wave (SAW) device is developed in this study by coating the delay line area of the device with a nano-structured molybdenum disulfide (MoS₂) sensitive material. A SAW device of 122 MHz was designed and fabricated with a pair of interdigital transducers (IDTs) defined on a 128° y-cut LiNbO₃ substrate using photolithography technologies, and the aluminum IDT electrodes were deposited by a DC magnetron sputtering system. By adjusting the pH values of precursor solutions, molybdenum disulfide (MoS₂) nanospheres were prepared with various structures using a hydrothermal method. Finally, an NH₃ gas sensor with high sensitivity of 4878 Hz/ppm, operating at room temperature, was successfully obtained. The excellent sensitivity performance may be due to the efficient adsorption of NH₃ gas molecules on the surfaces of the nanoflower-like MoS₂, which has a larger specific surface area and provides more active sites, and results in a larger change in the resonant frequency of the device due to the mass loading effect.