Self-Poling and Ageing in PZT Thin Films With Interdigitated Electrodes - Work done at Laboratoire de Céramique (LC), EPFL
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This work details advances in thin-film lead zirconate titanate (PZT) devices with interdigitated electrodes (IDEs). They are interesting for energy harvesting applications, as they can provide small amounts of electrical energy to devices such as wireless sensors. A simple equation relating the electric field to the sample geometry of IDEs was derived in this work, and PV hysteresis loops of fabricated films were used to verify the validity of the numerical finding. A fabrication route was developed for partially covering samples with SiO2 and self-poling was observed in these samples by measuring the piezoelectric coefficient descriptive of IDEs. The self-poling observed was limited in magnitude and the origin is still not clear. Slanted PV loops were observed when measuring as-fabricated samples and the slanting was compared to the effect of an interface layer, modelled as described by A.K. Tagantsev and G. Gerra. The model gave inconsistent values for the thickness of the interface layer when comparing samples with varying electrode spacing, and the slanting is therefore thought to have other causes. The time evolution of PV loops has been observed and characterised, and possible causes are discussed. The time evolution of individual samples was found to fit well with the model by M. Grossmann et al. for charge injection across a passive layer at the electrode interface, but this model does not agree with the observed dependence on electrode spacing. Charge injection across passive regions at the grain boundaries is suggested as an alternative that would agree with the observed dependence on electrode spacing. The models by A.K. Tagantsev and M.Grossmann were made to describe devices with parallel plate electrodes, and some inconsistency with the experimental results is to be expected.