UV Sensor Based on Surface Acoustic Waves in ZnO/Fused Silica

• Main Authors: Benetti, M. (Author), Buzzin, A. (Author), Caliendo, C. (Author), Cannatà, D. (Author), de Cesare, G. (Author), Grossi, F. (Author), Verona, E. (Author)
• Format: Article
• Language: English
• Published: MDPI 2023
• Subjects: Acoustic surface wave devices; Acoustic waves; acoustoelectric effect; Electric potential; Energy gap; fused silica; Fused silica; Fused silica substrates; Harsh environment; II-VI semiconductors; Incident light; Light absorption; Oxide films; Oxide layer; photoconductivity; Photoconductivity; Piezoelectricity; Radio frequency sputtering; SAW; Scanning electron microscopy; Sensor response; sensors; Sputtering; Sputtering techniques; Substrates; Surface acoustic waves; UV light; UV sensor; UV-light; Wave propagation; Zinc oxide; Zinc oxide thin films; Zinc sulfide; ZnO
• Online Access: View Fulltext in Publisher; View in Scopus

 Zinc oxide (ZnO) thin films have been grown by radio frequency sputtering technique on fused silica substrates. Optical and morphological characteristics of as-grown ZnO samples were measured by various techniques; an X-ray diffraction spectrum showed that the films exhibited hexagonal wurtzite structure and were c-axis-oriented normal to the substrate surface. Scanning electron microscopy images showed the dense columnar structure of the ZnO layers, and light absorption measurements allowed us to estimate the penetration depth of the optical radiation in the 200 to 480 nm wavelength range and the ZnO band-gap. ZnO layers were used as a basic material for surface acoustic wave (SAW) delay lines consisting of two Al interdigitated transducers (IDTs) photolithographically implemented on the surface of the piezoelectric layer. The Rayleigh wave propagation characteristics were tested in darkness and under incident UV light illumination from the top surface of the ZnO layer and from the fused silica/ZnO interface. The sensor response, i.e., the wave velocity shift due to the acoustoelectric interaction between the photogenerated charge carriers and the electric potential associated with the acoustic wave, was measured for different UV power densities. The reversibility and repeatability of the sensor responses were assessed. The time response of the UV sensor showed a rise time and a recovery time of about 10 and 13 s, respectively, and a sensitivity of about 318 and 341 ppm/(mW/cm2) for top and bottom illumination, respectively. The ZnO/fused silica-based SAW UV sensors can be interrogated across the fused silica substrate thanks to its optical transparency in the UV range. The backlighting interrogation can find applications in harsh environments, as it prevents the sensing photoconductive layer from aggressive environmental effects or from any damage caused by cleaning the surface from dust which could deteriorate the sensor’s performance. Moreover, since the SAW sensors, by their operating principle, are suitable for wireless reading via radio signals, the ZnO/fused-silica-based sensors have the potential to be the first choice for UV sensing in harsh environments. © 2023 by the authors.