J. Y. Jo

Polarization Relaxation Induced by a Depolarization Field in Ultrathin Ferroelectric BaTiO~3 Capacitors

Time-dependent polarization relaxation behavior induced by a depolarization field E(d) was investigated on high-quality ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors. The E(d) values were determined experimentally from an applied external field to stop the net polarization relaxation. These values agree with those from the electrostatic calculations, demonstrating that a large E(d) inside the ultrathin ferroelectric layer could cause severe polarization relaxation. For numerous ferroelectric devices of capacitor configuration, this effect will set a stricter size limit than the critical thickness issue.

Observation of inhomogeneous domain nucleation in epitaxial Pb(Zr,Ti)O3 capacitors

We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O3 capacitors under a modified piezoresponse force microscope. We obtained domain evolution images during polarization switching process and observed that domain nucleation occurs at particular sites. This inhomogeneous nucleation process should play an important role in an early stage of switching and under a high electric field. We found that the number of nuclei is linearly proportional to log(switching time), suggesting a broad distribution of activation energies for nucleation. The nucleation sites for a positive bias differ from those for a negative bias, indicating that most nucleation sites are located at the ferroelectric/electrode interfaces.

Ferroelectric properties of SrRuO3∕BaTiO3∕SrRuO3 ultrathin film capacitors free from passive layers

Structural studies on ultrathin SrRuO3∕BaTiO3∕SrRuO3 capacitors, with BaTiO3 thicknesses of between 5nm and 30nm, show well-defined interfaces between ferroelectric BaTiO3 and electrode SrRuO3 layers. In these capacitors, we cannot observe any extrinsic electrical effects due to either the formation of an insulating interfacial passive layer or passive-layer-induced charge injection. Such high-quality interfaces result in very good fatigue endurance, even for the 5nm thick BaTiO3 capacitor.Structural studies on ultrathin SrRuO3∕BaTiO3∕SrRuO3 capacitors, with BaTiO3 thicknesses of between 5nm and 30nm, show well-defined interfaces between ferroelectric BaTiO3 and electrode SrRuO3 layers. In these capacitors, we cannot observe any extrinsic electrical effects due to either the formation of an insulating interfacial passive layer or passive-layer-induced charge injection. Such high-quality interfaces result in very good fatigue endurance, even for the 5nm thick BaTiO3 capacitor.

Polarization retention in Pb(Zr0.4Ti0.6)O3 capacitors with IrO2 top electrodes

The retention characteristics of Ir/IrO2/Pb(Zr0.4Ti0.6)O3(PZT)/Pt/IrO2 capacitors were investigated by measuring the switching and nonswitching polarizations, the switching current profiles, and the P–V hysteresis loops. The retention losses of Ir/IrO2/PZT/Pt/IrO2 capacitors were compared with those of the Pt/PZT/Pt capacitors, and a significant improvement of an opposite-state retention was observed. It was found that the growth and the relaxation behaviors of the internal field are quite similar for both capacitors, but that the polarization backswitching becomes significantly smaller for the PZT capacitors with the IrO2 top electrodes. The difference in polarization backswitching might originate from the nature of the ferroelectric/electrode interfaces, possibly suppressing the nucleation of opposite domains at the interfaces.

Coercive fields in ultrathin BaTiO3 capacitors

Thickness-dependence of coercive field (EC) was investigated in ultrathin BaTiO3 capacitors with thicknesses (d) between 30 and 5.0nm. The EC appears nearly independent of d below 15nm, and decreases slowly as d increases above 15nm. This behavior can be explained not by effects of interfacial passive layers or strain relaxation, but by domain nuclei formation models. Based on domain nuclei formation models, the observed EC behavior is explainable via a quantitative level. A crossover of domain shape from a half-prolate spheroid to a cylinder is also suggested at d∼15nm, exhibiting good agreement with experimental results.

Thick Pb(Zr,Ti)O3 film without substrate

In order to fabricate thick PbZrxTi1−xO3 (PZT) films for microelectromechanical system applications, the authors introduce a concept of freestanding film without a substrate. PZT films with a thickness of up to 20μm were deposited on a very thin Pt layer without a substrate by the rf-magnetron sputtering method using a single oxide target. The Pt layer (thickness <1μm) was obtained by sputtering the Pt on a Si substrate with a carbon layer between them, and subsequently removing the carbon layer by oxidation in air at 400°C. Piezoelectric properties of the film were comparable to those of bulk PZT as a result of the removal of clamping effect of the substrate.

Mechanical strength and biocompatibility of ultrafine-grained commercial purity titanium.

The effect of grain refinement of commercial purity titanium by equal channel angular pressing (ECAP) on its mechanical performance and bone tissue regeneration is reported. In vivo studies conducted on New Zealand white rabbits did not show an enhancement of biocompatibility of ECAP-modified titanium found earlier by in vitro testing. However, the observed combination of outstanding mechanical properties achieved by ECAP without a loss of biocompatibility suggests that this is a very promising processing route to bioimplant manufacturing. The study thus supports the expectation that commercial purity titanium modified by ECAP can be seen as an excellent candidate material for bone implants suitable for replacing conventional titanium alloy implants.

Retention properties of fully integrated (Bi,La)4Ti3O12 capacitors and their lateral size effects

We investigated the retention characteristics of (Bi,La)4Ti3O12 (BLT) capacitors and their lateral size effects in a fully integrated device structure. Unlike the commonly used Pb(Zr,Ti)O3 capacitors for ferroelectric random access memories (FeRAMs), which have poor opposite-state retention characteristics, BLT capacitors showed very stable characteristics in both the same- and the opposite-state retention tests. These good retention properties were closely related to the small amount of imprint in the BLT capacitors. In addition, the retention characteristics of BLT capacitors showed no practical degradation due to the size reduction, down to 0.49×0.64μm2, which could be used for highly integrated FeRAMs of 32MB density.

Role of IrO2 Electrode in Reducing the Retention Loss of Ir/IrO2/Pb(Zr,Ti)O3/Ir Capacitors

Oxide electrodes are known to significantly improve reliability problems, such as fatigue and retention, in Pb(Zr, Ti)O3(PZT)-based ferroelectric capacitors. To understand the roles of the oxide electrodes on the opposite-state retention, we investigated the isotope tracer experiments and hysteresis measurements on Ir/IrO2/PZT/Ir and Ir/PZT/Ir capacitors before and after the retention tests. The depth profile of isotopic 18O in the Ir/IrO2/PZT/Ir capacitor, measured by the second ion mass spectroscopy, shows little changes in the 18O and 16O distributions at the IrO2/PZT interface. In addition, the hysteresis measurements showed that the internal field created by the retention tests should be nearly the same for both capacitors, indicating that the IrO2 layer should not play an important role in compensating the interface defect charges (possibly, the oxygen vacancies).