Decoupling Acoustoelectric and Thermal Effects of Ultraviolet Responses for Acoustic Wave Sensing Mechanisms

Ultraviolet (UV) detectors based on surface acoustic wave (SAW) technology offer unique advantages of remote or wireless operation capability with a potential of zero power consumption. Frequency shift of SAW devices induced by UV irradiation is mainly caused by acoustoelectric and thermal effects, although mass-loading and photo-capacitive effects might also have minor influences. Currently, the individual contribution from either acoustoelectric effect or thermal effect for UV sensing has not been thoroughly studied. In this work, we systematically investigated the contribution of acoustoelectric and thermal effects at different UV light intensities based on a LiNbO3 SAW device with its wavelength of $20 \mu \text{m}$ , and decoupled their individual contributions to the overall UV responses for acoustic wave sensing. We found that for the LiNbO3 SAW device without any surface treatment, the frequency shift was mainly caused by the thermal accumulation induced by UV radiation. Whereas for the LiNbO3 SAW devices coated with zinc oxide nanowires (ZnO NWs), the frequency shift is caused by the combined acoustoelectric and thermal effects. We found that the frequency shift caused by acoustic electric effect is dominant at a low UV intensity (< 30 mW/cm $^{{2}}{)}$ . Whereas with the UV light intensity larger than 90 mW/cm2, the photogenerated carriers generated by ZnO NWs became saturated, and then the thermal effect became dominant. This study clarifies the nature of individual influences of these two factors on the dominant mechanisms of the SAW-based UV sensing under different conditions.