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Planar microwave resonator with electrodeposited ZnO thin film for ultraviolet detection
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A ZnO thin film is electrodeposited on the conducting strips of a planar microwave ring resonator to enable the formation of a novel sensor for ultraviolet irradiation. The fabrication of the sensor involves a low-cost process that basically utilizes a printed circuit board and an aqueous precursor solution. The resonator with no ZnO coating operates with a resonant frequency of 6.2 GHz and a quality factor of 170. The time-resolved microwave photoresponse of the sensor to UV illumination, under ambient conditions, is assessed through measurements of the resonance profile of the S21 parameter. The resonance frequency exhibited a highly sensitive downshift of ~ 6 MHz after a UV illumination time of ~ 3 min. This downshift is mostly attributed to the change in the dielectric constant of the ZnO film caused largely by the additional creation of bound charges. The usually reported long-lived and persistent post-illumination effects were not observed. The measurements of the resonance amplitude carried out at 20% and 70% relative humidities revealed average excess carrier relaxation lifetimes of 213 s and 185 s, respectively. Concomitantly, the measured resonance frequency downshift increased with increased humidity. These results highlight the difference in the interaction mechanisms of photogenerated carriers with water and oxygen molecules on the surface and grain boundaries of the ZnO film. To our knowledge, this UV irradiation sensor is the first ZnO-based sensor device implemented with planar microwave circuit technology. In addition, the capabilities demonstrated by this simple photosensing method to determine induced carrier lifetimes make it a valuable technique for in-depth investigation of the material properties.
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- Date created
- 2019-10-20
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- Type of Item
- Article (Published)