UV sensors based on the propagation of the fundamental and third harmonic Rayleigh waves in ZnO/fused silica

c-axis oriented piezoelectric ZnO layers were grown by rf reactive magnetron sputtering technique onto fused silica substrates. The propagation of the fundamental mode, the Rayleigh wave, and its harmonics (third and ninth) was excited by metal interdigitated transducers (80 μm wavelength) photolithographically implemented onto the free surface of the ZnO layer (4 μm thick). The acoustoelectric effect, the waves velocity changes induced by the UV light adsorption in the photoconductive ZnO layer, was experimentally investigated at some UV power densities 365 nm. The ZnO/fused silica substrates were illuminated from the top surface or from the back surface of the ZnO layer (through the fused silica substrate) for UV power values up to about 0.45 W. The sensors sensitivities, the relative velocity shift per unit UV power density, were calculated for the fundamental and third harmonic modes for top illumination (1044 and 2305 ppm/(mWcm-2) and for back illumination (1084 and 2488 ppm/(mWcm-2). The third harmonic wave, whose resonant frequency is about three times the resonant frequency of the fundamental mode, has sensitivity larger than that of the latter. Both the two sensors tested under back illumination have a sensitivity larger than that measured under front illumination. The ninth harmonic mode as well was tested for UV sensing at 365 nm: it is sensitive to UV but its sensitivity could not be measured with good accuracy due to the high insertion loss of the mode (equal to - 80 dB) which was much larger than that of the fundamental and third harmonic mode (equal to -62 and -50 dB).