Photoresponse of the AlN-Based SAW Device on Polymeric and Silicon Substrates

This paper shows the optical photoresponse in the IR-Vis-UV range of a AlN-based piezoelectric surface acoustic wave (SAW) delay-line device. The piezoelectric aluminum nitride (AlN) thin film has been sputtered on both silicon rigid substrate and flexible polyethylene naphthalate (PEN) substrate. Both devices have been investigated in their electroacoustic response, by measuring the transfer function S
₂₁
and by laser Doppler vibrometer characterization. The silicon based SAW devices, stimulated by the IR-Vis-UV light, are strongly affected in the out-of-band insertion loss due to the photovoltaic effect. A mathematical model has been implemented to correlate the out-of-band loss with the material's electrical admittance change. In contrast PEN based SAW devices, due to the polymeric nature of the substrate, did not show any variation in the out-of-band loss. Moreover, when exposed to UV light, a frequency downshift of the Rayleigh and Lamb resonances modes have been observed in all the devices, due screening of the photoinduced electrons in the AlN piezoelectric layer which induces an acoustic wave velocity reduction. To the best of our knowledge, this is the first photoresponse study exploiting SAW in the range IR-Vis-UV, suggesting a new detection mode of UV light by a flexible AlN based SAW device. Further development of these devices can lead to a new class of light sensors from UV to IR, based on remote SAW devices.