Degradation Mechanisms in Ferroelectric and High-Permittivity Perovskites
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derstand them and to develop methods of eliminating or mitigating their effects. By combining results from studies on thin films with ones on ceramics and single crystals, a consistent picture of the mechanisms involved in these degradation processes is emerging. In this article, we discuss these degradation mechanisms with particular emphasis on the interaction between ferroelectric domains and charge trapping and the role of oxygen vacancies and associated defect dipoles. Ferroelectric Fatigue The loss of switchable polarization with repeated polarization reversals that characterizes ferroelectric fatigue in thin films is illustrated in Figure 1 for the case of a PZT (40/60) thin film with Pt electrodes. The loss of switchable polarization is due to pinning of domain walls, which inhibits switching of the domains. A variety of mechanisms for domain-wall pinning have been proposed, including pinning due to electronic-charge trapping 12 or by oxygen vacancies.91113 While it is impossible to distinguish between these mechanisms from purely electrical measurements, significant insight has been provided through studies of photoinduced fatigue and restoration and through the use of electron-paramagnetic-resonance (EPR) spectroscopy. Specifically it was shown that a loss of switchable polarization could be induced
not only in PZT thin films but also in (Pb,La)(Zr,Ti)O3 ceramics and BaTiO3 crystals by illuminating them with bandgap light, which excites electronhole pairs, while applying a dc bias just below the switching threshold.14 In addition it was demonstrated that the switchable polarization of both optically suppressed14 and electrically fatigued12 Pt/PZT/Pt capacitors could be essentially restored to its initial value by fully poling the capacitor while illuminating it, as also illustrated in Figure 1 for the specific case of electrical fatigue. Together these results indicate that pinning of domains by charge trapping at internal domain boundaries, which is depicted schematically in the inset of Figure 1, is a primary fatigue mechanism. As this picture suggests, it is the polarization (i.e., bound charge) discontinuity at domain boundaries that acts as the driving force for charge trapping. In optical restoration, the photoexcited carriers recombine with the trapped charge, allowing the dc bias to reorient the previously locked domains. To elucidate the role of defects on charge trapping in perovskites, electrical and optical fatigue in BaTiO3 single crystals has been studied since these crystals are also ideally suited to detailed EPR studies of defect states. Consistent with the idea that fatigue is due to charge trapping is the observation that fatigue induces changes in the oxidation states of isolated impurity point defects, such as Fe and Pt, which are clearly due to electronic-charge capture.8 Furthermore the fatigue-induced Pt3+ centers in BaTiO3 were found to be much more stable than those optically generated in unfatigued samples. This result suggests that the simple fatigue-induced process involv
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