Giant Piezoelectric Effect Induced by Porosity in Inclined ZnO Thin Films

Piezoceramics have been the common choice for sensing applications, however their integration with other electronics is limited due to their large size. Also, environmental concerns have been limiting the use of ceramics due to their lead content, toxic for humans. Piezoelectric crystalline thin films have arisen as alternative materials offering compatibility with miniaturization and integrated processes, though their piezoelectric properties are low (d(33) < 15 pC N-1) compared to ceramics (d(33) > 300 pC N-1), hindering their applicability. Several methods have been studied aiming to improve the piezoelectric output of thin films, including doping or the less investigated inclusion of porosity. However, they all require complex techniques that increase the cost. In this work, a giant electromechanical d(33) coefficient of 115.6 pC N-1 has been obtained inducing porosity in inclined ZnO thin films via oblique angle deposition, which is beyond record values reported for doped ZnO thin films and an order of magnitude higher than standard ZnO thin films (11.6 pC N-1). Morphology, composition, crystal structure, porosity, and piezoelectricity are reported for standard and inclined films. Finite element simulations have been carried out to investigate the performance of the piezoelectric-enhanced thin films in an ultrasonic pulse-echo setup.