Mineharu Tsukada

Mechanical stress effect on imprint behavior of integrated ferroelectric capacitors

Stress-induced changes in the imprint and switching behavior of (111)-oriented Pb(Zr,Ti)O3 (PZT)-based capacitors have been studied using piezoresponse force microscopy. Visualization of polarization distribution and d33-loop measurements in individual 1×1.5-μm2 capacitors before and after stress application, generated by substrate bending, provided direct experimental evidence of stress-induced switching. Mechanical stress caused elastic switching in capacitors with the direction of the resulting polarization determined by the sign of the applied stress. In addition, stress application turned capacitors into a heavily imprinted state characterized by strongly shifted hysteresis loops and almost complete backswitching after application of the poling voltage. It is suggested that substrate bending generated a strain gradient in the PZT layer, which produced asymmetric lattice distortion with preferential polarization direction and triggered polarization switching due to the flexoelectric effect.

Top-interface-controlled switching and fatigue endurance of (Pb,La)(Zr,Ti)O3 ferroelectric capacitors

Mechanisms of polarization switching and fatigue in (Pb,La)(Zr,Ti)O3 (PLZT) films are studied by comparative analysis of degradation and leakage conduction of PLZT capacitors with Pt, SrRuO3 (SRO), and layered Pt/SRO (80/5 nm) electrodes. It is found that the asymmetrical Pt/SRO/PLZT/Pt structure exhibits a good fatigue performance in combination with low leakage like that on identically processed Pt/PLZT/Pt capacitor. This asymmetrical structure exhibits very unusual dependence of the endurance of switching polarization on the driving alternating-current (ac) electric field amplitude. Specifically, for high ac electric field amplitude it shows good switching endurance similar to SRO/PLZT/SRO capacitors, whereas for amplitude lower than 80 kV/cm a pronounced polarization fatigue similar to that of the conventional Pt/PLZT/Pt capacitors is observed. Based on the analysis of our results on fatigue and leakage conduction we conclude that: (i) In asymmetrical Pt/SRO/PLZT/Pt system the physical mechanisms of p...

Transmission electron microscopy study on the crystallization and boron distribution of CoFeB/MgO/CoFeB magnetic tunnel junctions with various capping layers

High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) were used to study the microstructural properties of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with various capping layers. Crystallization of CoFeB layers was strongly dependent on the capping materials, and was affected by B diffusion. With NiFe-cap MTJs, CoFeB crystallized from the cap interface and formed a fcc structure; on the other hand, with Ta- and Ti-cap MTJs, CoFeB crystallized from the MgO interface and formed a bcc structure. EELS analysis showed that B mainly diffused to the capping layers and rarely to the MgO layers with increasing temperature. With Ti-cap MTJs, B diffusion caused hcp-Ti crystals to form an amorphous structure and CoFeB crystallized at lower temperature.

Control of leakage conduction of high-fatigue-endurance (Pb, La)(Zr, Ti)O3 film ferroelectric capacitors with Pt/SrRuO3 electrodes

Leakage conduction of (Pb, La)(Zr, Ti)O3 (PLZT) films grown on Pt bottom electrode, with Pt and Pt/SrRuO3 (Pt/SRO) top electrodes is studied. It is found that the conduction behavior of the ferroelectric capacitors strongly varies depending on the degree of interdiffusion from the electrode into the ferroelectric material. If the diffusion is limited, the conduction properties of the Pt/SRO/PLZT/Pt capacitor are similar to that of the conventional Pt/PLZT/Pt system. For the opposite case of excessive interdiffusion, the former exhibits much higher conduction than the latter. We found that the diffusion of the top electrode material can be controlled in two ways—either by varying Pb excess in the PLZT film or by controlling the SRO layer thickness. Schottky barrier measurements show that in the case of limited diffusion the potential barrier values of SRO/PLZT and Pt/PLZT interfaces are nearly equal. Based on our results we conclude that: (a) the Schottky barrier values of Pt/PLZT and SRO/PLZT contacts are...

Spatial inhomogeneity of imprint and switching behavior in ferroelectric capacitors

Piezoresponse force microscopy has been used to perform nanoscale characterization of the spatial variations in the imprint and switching behavior of (111)-oriented Pb(Zr,Ti)O3-based capacitors on Pt electrodes. Mapping of polarization distribution in the poled capacitors as well as local d33–V loop measurements revealed a significant difference in imprint and switching behavior between the peripheral and inner parts of the capacitors. It has been found that the inner regions of the capacitors are negatively imprinted (with the preferential direction of the normal component of polarization upward) and tend to switch back after application of the positive poling voltage. On the other hand, switchable regions at the edge of the integrated capacitors generally exhibit more symmetric hysteresis behavior. Application of an ac switching voltage, contrary to what was expected, resulted in an increase of the negatively imprinted regions. The observed effect has been explained by incomplete or asymmetric switching...

Piezoelectric Properties of PbNi1/3Nb2/3O3–PbTiO3–PbZrO3 Ceramics

The longitudinal electromechanical coupling factor (k33) was investigated as a function of PbNi1/3Nb2/3O3– PbTiO3–PbZrO3 (PNN-PT-PZ) piezoelectric ceramic compositions close to the morphotropic phase boundary (MPB). The k33 was influenced by the grain size of PNN-PT-PZ ceramics. The composition of the MPB determined by k33 is slightly different from that of the free permittivity. The maximum value of k33 in PNN-PT-PZ system attained to 0.8 on the composition of 0.5PNN-0.345PT-0.155PZ.

Unusual size effect on the polarization patterns in micron-size Pb(Zr,Ti)O3 film capacitors

Direct observation of polarization distribution at nanoscale by scanning force microscopy across partially etched Pb(Zr,Ti)O3 film ferroelectric capacitors with size 2×3 μm and preferential orientation (111) has revealed an anomalous polarization pattern characterized by polarization inversion in the center of the capacitor. This self-structured pattern is reproducible and independent on the spontaneous polarization orientation, but strongly influenced by the film texture and capacitor size. In particular, capacitors of the same texture with size 0.5×0.5 μm as well as capacitors with (100) preferential orientation of any size did not exhibit such anomalous polarization distribution. A tentative explanation consistent with the essential features of the observed polarization behavior in terms of strain-induced phase transition is discussed.

H2O vapor-induced leakage degradation of Pb(Zr,Ti)O3 thin-film capacitors with Pt and IrO2 electrodes

We have investigated the influence of H2O vapor on the leakage characteristics of Pb(Zr,Ti)O3 (PZT) thin-film capacitors with Pt and IrO2 top electrodes by conducting electrical measurements ex situ in air and in situ in an 80% N2/20% O2 atmosphere with controlled humidity ranging from <0.1% to 95.5% relative humidity at 20 °C. The results show that the leakage characteristics of PZT thin-film capacitors are strongly sensitive to H2O vapor in the atmosphere. The onset to H2O-induced leakage degradation depended on the relative humidity and on the quantity of charge passed through the capacitor. A degradation mechanism based on the electrolysis of H2O that has adsorbed on the capacitor from the atmosphere and the role of the electrode material in the degradation process are discussed.

Crossover between nucleation-controlled kinetics and domain wall motion kinetics of polarization reversal in ferroelectric films

The study of polarization reversal in (Pb,La)(Zr,Ti)O-3 thin films reveals a drastic difference between the switching kinetics observed at RT and at low temperature of 40 K. In particular, at 40 K, the switching kinetics in studied films is similar to that observed in ferroelectric single crystals. Additionally, the films with completely different switching behavior at RT show very similar switching properties at 40 K. The data analysis suggests that the polarization reversal at RT is limited mainly by nucleation of reversed domains, while at 40 K, the switching kinetics is governed by domain wall motion. It is demonstrated that the measurements of switching kinetics at low temperature can provide useful information for modeling of the important, practical case of polarization reversal at RT

Dielectric breakdown in (Pb,La)(Zr,Ti)O3 ferroelectric thin films with Pt and oxide electrodes

Dielectric breakdown of (Pb,La)(Zr,Ti)O-3(PLZT) film capacitors with Pt and SrRuO3 electrodes, in its initial or virgin state and after voltage cycling that causes polarization fatigue, is studied by constant-current measurements. It is shown that for different types of PLZT capacitors, the breakdown onset is controlled by the critical charge flown through the film (charge to breakdown) rather than the voltage applied to the capacitor. The charge to breakdown for the asymmetrical Pt/SrRuO3/PLZT/Pt capacitors is found to be much higher that that for the Pt/PLZT/Pt system. Polarization fatigue caused by bipolar voltage cycling provokes a substantial decrease of charge to breakdown. These results can be interpreted in terms of percolation model used for breakdown in SiO2, where the charge to breakdown is associated with injected-carrier-assisted creation of the critical concentration of defects required for formation of the breakdown paths. In this article we propose two possible mechanisms for interpretation of the observed variation of the PLZT capacitor breakdown performance. One mechanism is based on different contributions of PLZT film grains and grain boundaries to the total current flowing through the system. The second mechanism is related to the different kinetics of generation and relaxation of defects in the band gap of the PLZT film. The decrease of the charge to breakdown induced by polarization fatigue is explained by generation of defects during the fatigue bipolar polarization reversals