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FULLY DIGITAL - NO PUBLIC : Phd Defence Minh Thanh Do | Mechanisms of polarization fatigue in ferroelectric PbZr0.52Ti0.48O3 epitaxial thin-film capacitors

Mechanisms of polarization fatigue in ferroelectric PbZr0.52Ti0.48O3 epitaxial thin-film capacitors

Due to the COVID-19 crisis measures the PhD defence of Minh Thanh Do will take place online without the presence of an audience.

The PhD defence can be followed by a live stream.

Minh Thanh Do is a PhD student in the research group Inorganic Materials Science (IMS). His supervisors are A.J.H.M. Rijnders and G. Koster from the Faculty of Science and Technology (TNW).

Ferroelectric materials have been used in many applications such as non-volatile memories and micro-electromechanical systems. However, these devices are often instable after a prolonged period of working under drive AC voltage. This issue is due to the so-called polarization fatigue, i.e. the loss of polarization of integrated ferroelectric capacitors upon repeated field cycling. Understanding on the fatigue behavior of ferroelectric capacitors might provide practical solutions to enhance the stability of ferroelectric-based devices. This project therefore aims to gain further insight into the mechanism(s) of how ferroelectric capacitors become fatigued under electric field cycling.

A large body of data on various aspects of the polarization fatigue, such as fatigue onset, field cycling dependences, electrode-material dependence, ferroelectric and structural properties of fatigued capacitors, has been reported in literature (chapter 2). However, models for mechanisms of fatigue still remain controversial. This is because the films investigated in previous studies, commonly fabricated by chemical solution deposition, contain many different kinds of defects. Therefore, in such polycrystalline capacitors, the development of polarization fatigue, a defect-originated phenomenon, is concurrently attributed to multiple unknown scenarios. Moreover, the traditional way to interpret the fatigue is based on the graphic profile of the remnant polarization as function of the field cycle number. This sometimes leads to incorrect interpretations and the deduced mechanism models therefore lack of supporting evidence. Based on these discussions, we have selected PbZr0.52Ti0.48O3 (PZT) epitaxial thin-film capacitors, grown on a SrRuO3-buffered SrTiO3 substrate by pulse laser deposition (PLD), as the benchmark sample for investigation. PZT is the most commonly used ferroelectrics with a high polarization value, and by PLD, we are able to control and engineer well the microstructure of the capacitor.

First, we identified the main origin of fatigue in metal-electrode capacitors by investigating the dependence of the fatigue behavior on the top-interface structure (chapter 3). To engineer the top interface, we varied the material (conductive oxide such as SrRuO3 vs metal such as Pt, Au) and the fabrication procedure (in situ vs ex situ) of the top electrode. The fatigue behaviour was found to be directly related to the microstructure of the top interface. Capacitors with an ex situ deposited metal top electrode showed an as-grown 1.5-nm thick defective layer at the top interface and became rapidly fatigued after only 103-104 field cycles. Capacitors with in situ deposited metal top electrodes show clean top interfaces without any defective layer and became fatigued much later, after about 107-108 field cycles. Capacitors with both SrRuO3 electrodes show atomically sharp epitaxial interfaces and were free of fatigue for a least 109 cycles. The defective layer at the ex situ metal/PZT interfaces contains carbon contaminants, which arise from the exposure of the PZT surface to ambient atmosphere. In-situ removal of this defective layer significantly enhances the fatigue resistance of the capacitor. These results clearly indicate that an as-grown interfacial dielectric defective layer is the major origin of polarization fatigue in ferroelectric capacitors with conventional ex situ metal electrodes.

We then answered how an as-grown 1.5-nm thick interfacial defective layer can cause rapid fatigue in the capacitors with ex situ deposited metal electrodes (chapter 4). To answer the question, we characterized the fatigue-profile dependence on field cycling conditions, the domain switching, and the interface structure of the Pt-sputtered/PZT/SRO capacitor (with Pt electrode made by ex situ sputtering) during field cycling. The polarization fatigue was observed to be practically independent of the field cycling conditions and to depend only on the number of field cycle. The fatigued capacitors become switchable again under sufficiently high external field (much higher than needed to switch the pristine capacitor), but the switching has become asymmetric between the two polarization directions. However, both capacitor interfaces stay structurally unmodified under field cycling. Guided by these findings, we proposed that electrons were injected through the interface under the depolarization field and then trapped in the defective layer, subsequently inhibited the nucleation of domains at this interface, leading to the polarization fatigue in the device.

Capacitors with in situ deposited metal electrodes outperform with several orders of magnitude in cycle number fatigue the capacitors with conventional ex situ deposited metal electrodes. However, such capacitors still become strongly fatigued after prolonged field cycling. What is the mechanism(s) behind this fatigue behavior? We provided the answer based on investigations into domain switching and interface microstructure changes of the Pt-inPLD/PZT/SRO structure (fabricated by in situ pulsed laser deposition) at different cycling states (chapter 5). The Pt-inPLD/PZT interface is found to structurally degrade during field cycling, forming a non-ferroelectric, degraded layer. This structural decomposition is thought to be caused by the depolarization field during the polarization switching under repeated field cycles. The interfacial degraded PZT layer then screens the external field, consequently suppresses polarization switching of the cycled capacitor, resulting in the observed fatigue effect. Consequently, to further improve the fatigue resistance of the sample, one needs to protect the interfacial ferroelectric lattice against the repeated depolarization field by inserting a conducting oxide or adhesion layer to improve the adhesion/structural connectivity between the ferroelectric and the metal electrode.

The capacitors with ex situ metal electrodes become rapidly fatigued but then ferroelectrically ‘revive’ again by the so-called rejuvenation process. Investigations into this effect will gives indications how one rejuvenates and reuses the capacitors after the fatigue. We observed that the rejuvenation of fatigued Pt-sputtered/PZT/SRO capacitors strongly depends on the DC-bias/field cycling application conditions. These data lead to a proposition that trapped electrons at the Pt/PZT interface in the fatigued capacitor are compensated by the slow accumulation of oxygen vacancies, which migrate under the applied field. The rejuvenated capacitors are observed to fatigue again after 107-108 cycles, which means a 104 times larger fatigue resistance than the as-grown capacitors. In this second-fatigued state, the capacitor shows a relatively thick PZT-degraded layer at the top interface. This indicates that the fatigue mechanism of the rejuvenated capacitor is similar to that in the in-situ metal-electrode capacitor, which is sequence of decomposition of the interfacial PZT lattice by the switching depolarization field and the field screening effect causing the polarization switching suppression in the cycled capacitor. Details are presented in chapter 6.

Last but not least, we extended some investigations into the fatigue behaviour of SrRuO3/PbZrxTi1-xO3/SrRuO3 capacitors with respect to the effect of the Zr/Ti ratio and the harsh cycling conditions on the fatigue (chapter 7). SrRuO3/PbZrxTi1-xO3/SrRuO3 capacitors showed fatigue dependently on the Zr/Ti ratio: PZT with Zr/Ti of 20/80 shows strong temporary fatigue while the ones with 52/48 and 80/20 ratio show no fatigue. We proposed that the presence of high-energy 90° domain walls between in-plane oriented a-domains and out-of-plane oriented c-domains, is the origin of fatigue in the PZT-20/80. Under field cycling, the domains are repeatedly switched, giving rise to the formation of transient charged 90° domain walls, which subsequently hinder further polarization switching of the domains. Further we showed that SrRuO3/PbZrxTi1-xO3/SrRuO3 capacitors with Zr/Ti of 52/58 are promising for application because they are almost free of fatigue under field cycling with various field amplitude and operating temperature. However, if the conditions become too harsh the capacitor will show catastrophic electric breakdown.

Overall, this thesis provides a wide range of experimental data on the ferroelectric and structural behaviours of PZT capacitors, giving direct evidence for understanding the development of fatigue in these samples. Based on these insights, two efficient ways for enhancing the fatigue resistance of metal-electrode ferroelectric capacitors are suggested: first, improve the purity of the metal/ferroelectric interface by using in situ electrode fabrications or in situ ferroelectric surface cleaning and second, strengthen the bonding between electrodes and ferroelectrics by using conducting oxides and/or adhesion layers.