Mechanisms of polarization fatigue in ferroelectric PbZr_0.52Ti_0.48O₃ epitaxial thin-film capacitors

Polarization fatigue of ferroelectric capacitors has been widely discussed in terms of an interface related effect. However, responsible origin(s) and mechanism(s) behind the development of polarization fatigue have not been fully identified. In this thesis, I pinpoint the fatigue origins and mechanisms of PbZr_0.52Ti_0.48O₃ (PZT) epitaxial thin-film capacitors. It was found that the dominant origin of the early fatigue in PZT capacitors with conventional ex situ metal electrodes is the defective layer of carbon at the capacitor interface. Under field cycling, this defective layer is believed to trap electrons, subsequently hinder domain switching of the PZT layer. To avoid having this defective layer and consequently to improve the fatigue resistance of metal-electrode PZT capacitors, I suggested to use in situ deposited metal layers. The fatigue resistance of the capacitors with in situ metal electrodes is several orders of magnitude higher than those with ex situ metal electrodes. However, the capacitors with in situ metal electrodes still became strongly fatigued under prolonged field cycling. By scanning transmission electron microscopy, we observed that in the fatigued capacitor after prolonged field cycling there is a structurally degraded layer of nm in thickness, containing polycrystalline (Zr,Ti)O₂ oxides and diffused Pt grains. Therefore, the development of polarization fatigue in these capacitors was suggested to follow two main stages. First, the metal/ferroelectric interface becomes structurally degraded and decomposed by a high transient depolarization field during the domain switching under repeated field cycles, resulting in an interfacial non-ferroelectric layer in the cycled capacitors. Second, the resulting interfacial non-ferroelectric layer screens the external applied field, leading to a polarization switching suppression in the cycled capacitor. Overall, to improve the fatigue resistance of ferroelectric capacitors with metal electrodes, ones should improve the purity of the capacitor interfaces and enhance the bonding between the metal and the ferroelectric layer.