Piezoelectricity in ferroelectric thin films: Domain and stress issues

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Abstract
As ferroelectric thin films are investigated as alternative sensors and actuators for microelectromechanical systems, it is becoming important to understand which mechanisms control the magnitude of the observed piezoelectric properties. It is well known that in bulk soft lead zirconate titanate actuators, over half the room temperature response is in fact associated with domain wall contributions to the properties. However, recent studies on bulk ceramics have demonstrated that the complexity of the domain structure, and the mobility of the twin walls depend on the grain size. This leads to appreciable degradation in the dielectric and piezoelectric properties for grain sizes below a micron. This has significant consequences in thin film actuators since a lateral grain size of one micron is often the upper limit for the observed grain size. In addition, since the pertinent domain walls are ferroelastic as well as ferroelectric, the degree of stress imposed on the film by the substrate can also clamp the observed piezoelectric response. To investigate the importance of these factors, controlled stress levels were imposed on several types of ferroelectric thin films while the dielectric and electromechanical properties were measured. It was found that for undoped sol-gel lead zirconate titanate thin films, the extrinsic contributions to the dielectric and electromechanical properties make very modest contributions to the film response. No significant enhancement in the properties was observed even when the film was brought through the zero global stress condition. Comparable results were obtained from laser ablated films grown from hard and soft PZT targets. Finally, a similar lack of twin wall mobility was observed in atomic force microscopy experiments. The consequences of this, as well as several alternative methods to increase the available piezoelectric coefficients and achievable strains in ferroic films will be presented.