Low-frequency piezoelectric-transducer applications of ZnO film

It is revealed that a high-resistivity ZnO film highly oriented with the c axis perpendicular to a metal substrate surface can be used as a low-frequency piezoelectric transducer which generates and detects compressional strain or stress perpendicular to the c axis, i.e., perpendicular to the film thickness through the piezoelectric tensor component e311 or d311. For example, tuning-bar filters and a flat tuning-fork filter are successfully composed of ZnO films and Elinvar-alloy substrates whose resonant outputs are obtained even at frequencies lower than 100 kHz, and a piezoelectric microphone is composed of a ZnO film and a Ti membrane substrate.