Toward Ultrafast Response and Recovery Characteristics Through ZnO Multilayer Thin Films in Surface Acoustic Wave UV Sensor

We present a novel surface acoustic wave (SAW)-based ultraviolet (UV) sensor with an extremely fast response and recovery time of less than 1 s, achieving the best record in SAW-based UV sensor technology. A 40-nm-thick multilayered ZnO thin film was employed as the sensing material within the cavity of the two-port SAW delay-line, providing optimal mass loading to prevent SAW damping, minimizing recombination-generation (R-G) centers, and reduced surface defects around sensing material. The evaluated sensitivity of the sensor to UV light ( $\lambda ~\sim ~365$ nm) was 15787 ppm (mW/cm2) $^{-{1}}$ , with a minimum detection limit of 11.5 nW/cm2, marking the highest values reported in SAW-based UV sensors. The multilayered thin film (ML-TF) was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). The analysis confirmed that oxygen vacancies and surface defects were significantly reduced in the ML-TF, while the ZnO grain size was greatly increased, leading to an enhanced UV response. Additionally, the study revealed that the sensor responses largely depend on the UV wavelength illuminated and the incident angles of the UV light, due to the time required for band-to-band transition, which is the fastest process. The sensor interface electronics were developed to visually observe frequency shifts on a PC monitor in real time. The system consists of an oscillator, mixer, low-pass filter (LPF), comparator, field-programmable gate array (FPGA), and PC. A reference SAW device identical to the SAW sensor with ZnO was included in the interface electronics to compensate for environmental perturbations, such as temperature and humidity.