Microstructure and Electrical Properties of Novel piezo-optrodes Based on Thin-Film Piezoelectric Aluminium Nitride for Sensing

Thin-film piezoelectric materials are currently employed in micro- and nanodevices for energy harvesting and mechanical sensing. The deposition of these functional layers, however, is quite challenging onto non-rigid/non-flat substrates, such as optical fibers (OFs). Besides the recent novel applications of OFs as probes for biosensing and bioactuation, the possibility to combine them with piezoelectric thin films and metallic electrodes can pave the way for the employment of novel opto-electro-mechanical sensors (e.g., waveguides, optical phase modulators, tunable filters, energy harvesters or biosensors). In this work the deposition of a thin-film piezoelectric wurtzite-phase Aluminium Nitride (AlN), sandwiched between molybdenum (Mo) electrodes, on the curved lateral surface of an optical fiber with polymeric cladding, is reported for the first time, without the need of an orientation-promoting interlayer. The material surface properties and morphology are characterized by microscopy techniques. High orientation is demonstrated by SEM, PFM and X-ray diffraction analysis on a flat polymeric control, with a resulting piezoelectric coefficient (d33) of ~5.4 pm/V, while the surface roughness Rms measured by AFM is 9 ÷ 16 nm. The output mechanical sensing capability of the resulting AlN-based piezo-optrode is investigated through mechanical buckling tests: the peak-to-peak voltage for weakly impulsive loads increases with increasing relative displacements (up to 30%), in the range of 20 ÷ 35 mV. Impedance spectroscopy frequency sweeps (10 kHz-1 MHz, 1 V) demonstrate a sensor capacitance of ~8 pF, with an electrical Q factor as high as 150. The electrical response in the long-term period (two months) revealed good reliability and durability.