Unipolar Fatigue Research Articles

Bipolar and unipolar electrical fatigue in ferroelectric lead zirconate titanate thin films: An experimental comparison study

By performing standard positive-up-negative-down, hysteresis-loop and dielectric measurements on the ferroelectric lead zirconate titanate thin-film capacitors subject to bipolar/unipolar electrical cycling, we show that unipolar fatigue is evident though still less severe than bipolar fatigue conducted at the same voltage. That has been attributed to a series of periodic events of polarization backswitching (driven by the residual depolarization field) and switching (driven by the residual applied field) during unipolar electrical cycling, and explained using the LPD-SICI model (LPD-SICI stands for local phase decomposition caused by switching-induced charge injection). The dielectric results have been used to estimate the effective thickness di of the fatigue-induced degraded (pyrochlorelike) interfacial layer after bipolar/unipolar fatigue, which has not been done so far to our best knowledge. The fact that di is still much less than the film thickness even after the most severe bipolar fatigue strongly suggests that polarization fatigue in ferroelectrics is an interface effect, not a bulk one.

Unipolar and bipolar fatigue in antiferroelectric lead zirconate thin films and evidences for switching-induced charge injection inducing fatigue

We show that unipolar fatigue does occur in antiferroelectric capacitors, confirming the predictions of a previous work [Appl. Phys. Lett. 94, 072901 (2009)]. We also show that unipolar fatigue in antiferroelectrics is less severe than bipolar fatigue if the driving field is of the same magnitude. This phenomenon has been attributed to the switching-induced charge injection, the main cause for polarization fatigue in ferroelectric and antiferroelectric materials. Other evidences for polarization fatigue caused by the switching-induced charge injection from the nearby electrode rather than the charge injection during the stable/quasistable leakage current stage are also discussed.

Fatigue of Lead Zirconate Titanate Ceramics. I: Unipolar and DC Loading

In actuator applications, a degradation of the ferroelectric material occurs both under unipolar cycling as well as DC loads. The developing material state is characterized by measurements of large and small signal unipolar as well as bipolar hysteresis loops. While the unipolar loops indicate a small decrease in strain and polarization amplitudes, the bipolar loops unveil the development of an offset field, an offset polarization, and an asymmetry in the hysteresis of strain and dielectric constant. A mechanism for unipolar fatigue is suggested to explain the observed changes in material properties with cyclic loading.

Fatigue of Lead Zirconate Titanate Ceramics II: Sesquipolar Loading

Piezoelectric actuators generally are driven with unipolar electric load cycles. Although the obtainable strain is increased by small excursions into the negative field regime, this type of load cycle is rarely considered, as its long‐time reliability has been questioned. Here, we investigate the degradation of lead zirconate titanate during cycling between high positive and low negative electric fields. Measurements of the large and small signal parameters are used to quantify changes of the material. The fatigue behavior shifts from one best described with existing models of unipolar fatigue to bipolar‐like fatigue with increasing field amplitude.

Fatigue Characteristics of PZT Thin Films Deposited by ECR-PECVD

Fatigue characteristics of lead zirconate titanate (PZT) films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) were investigated. The fatigue characteristics were investigated with respect to PZT film thickness, domain structure, fatigue pulse height, temperature, electrode materials and electrode configurations. The used top and bottom electrode materials were Pt and <TEX>$RuO_2$</TEX>. In the fatigue characteristics with fatigue pulse height and PZT film thickness, the fatigue rates are independent of the applied fatigue pulse height at the electric field regions to saturate the P-E hysteresis and polarization <TEX>$(P^*,\;P^A)$</TEX> characteristics. The unipolar and bipolar fatigue characteristics of PZT capacitors with four different electrode configurations <TEX>$(Pt//Pt,\;Pt//RuO_2,\;RuO_2//Pt,\;and\;RuO_2//RuO_2)$</TEX> were also investigated. The polarization-shifts during the unipolar fatigue and the temperature dependence of fatigue rate suggest that the migration of charged defects should not be expected in our CVD-PZT films. It seems that the polarization degradations are attributed to the formation of charged defects only at the Pt/PZT interface during the domain switching. The charged defects pin the domain wall at the vicinity of Pt/PZT interface. When the top and bottom electrode configurations are of asymmetric <TEX>$(Pt//RuO_2,\;RuO_2//Pt)$</TEX>, the internal fields can be generated by the difference of charged defect densities between top and bottom interfaces.

Unipolar fatigue of ferroelectric lead–zirconate–titanate

In multilayer actuators the ferroelectric material is known to degrade under bipolar and to some degree under unipolar electric loading. Fatigue effects due to unipolar cycling of a commercial bulk lead–zirconate–titanate up to 4·108 cycles at 2 Ec are demonstrated. Unipolar as well as bipolar polarisation and strain hysteresis loops are measured after fatigue. Bipolar measurements after cycling show a small decrease of the remnant polarisation and an increasing asymmetry of the strain hysteresis with cycle number, which is explained by an offset polarisation. This offset polarisation is reduced under application of bipolar fields. A small offset-field is also observed. Etch grooves known to occur after bipolar fatigue are not found after unipolar fatigue. The results reveal that different mechanisms are responsible for bipolar and unipolar fatigue.

Stability of defects in lead–zirconate–titanate after unipolar fatigue

Bipolar fatigue in ferroelectric lead–zirconate–titanate was previously shown to induce stable pinning centers for ferroelectric domain walls resulting in a significant loss in switchable polarization, but only fairly weak offsets were induced. The corresponding scenario after unipolar fatigue is reversed inducing fairly stable offsets while hardly any loss in domain mobility is encountered. The charged defects inducing the offsets are suggested to be located at grain boundaries, while pinning centers reducing domain wall mobility predominantly reside within the grain.