Y. K. Lee

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.

Mechanisms for retention loss in ferroelectric Pt/Pb(Zr0.4Ti0.6)O3/Pt capacitors

The switching current profiles of ferroelectric Pt/Pb(Zr0.4Ti0.6)O3/Pt capacitors were investigated after high-temperature baking to elucidate the mechanisms for retention loss. In the same-state retention, a decrease in the peak value of switching current and increase in the switching time were observed. These changes in the switching characteristics were attributed to the growth of an internal field. By comparing with the switching characteristics of a virgin capacitor and using the Merz equations, we estimated quantitatively the magnitude of the internal field. In the opposite-state retention, backswitching of polarization, triggered by the internal field, was found to be the main cause of the retention loss.

Polarization dynamics and retention loss in fatigued PbZr0.4Ti0.6O3 ferroelectric capacitors

Short-time retention loss behaviors were investigated for fatigued Pt/PbZr0.4Ti0.6O3/Pt capacitors. In the short-time regime of t<1 s, fatigued capacitors showed a significant loss in retained polarization, which could be well described by a power-law function. This behavior was interpreted in terms of a superposition of polarization relaxations with a relaxation time distribution. The effects of the pulse voltage on the relaxation time distribution suggested that the retention loss should be activated by a depolarization field. As the fatigue stress was applied, the retention loss became worse. This effect can be explained in terms of the increase of the depolarization field, possibly due to the growth of an interfacial passive layer.

Hydrogen-induced degradation in ferroelectric Bi3.25La0.75Ti3O12

Effects of forming gas annealing (FGA) were investigated on ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films fabricated by a chemical solution deposition. With the FGA up to 350 °C, the Pt/BLT/Pt capacitors showed good ferroelectric characteristics without significant degradation. As the FGA temperature was increased, a decomposition of the BLT powder sample was observed by using thermogravimetric analysis. By comparing the time-dependent weight loss of BLT with that of Bi4Ti3O12 during FGA, La doping was found to significantly impede the decomposition rate. The decomposition, especially in the (Bi2O2)2+ layers, was discussed as a hydrogen-induced degradation mechanism in the Bi-layered perovskite ferroelectrics.

Mechanism of low temperature hydrogen-annealing-induced degradation in Pb(Zr0.4Ti0.6)O3 capacitors

Changes in the polarization–field hysteresis loop were systematically investigated for Pb(Zr0.4Ti0.6)O3 capacitors after forming gas annealing at 200 °C. Voltage shift in hysteresis was strongly dependent on the polarization states and ascribed to an asymmetric distribution of defect charges and pinned defect dipoles. Field recovery of the imprinted capacitors and increase in coercive field after the recovery were discussed in conjunction with reversible defect dipoles. From the relaxation of the voltage shift with an activation energy of 0.21 eV, it is inferred that charge trapping may be the main cause of the voltage shift and the subsequent degradation of the capacitors by pinning the polarization and defect dipoles.

First Playback of High-Definition Video Contents from Super-Resolution Near-Field Structure Optical Disc

A super-resolution near-field structure (super-RENS) read-only-memory disc with an In–Sb thin film as a super-resolution active layer corresponding to a 50 Gbyte capacity per layer was fabricated. The carrier-to-noise ratio at a pit length of 75 nm exceeded 40 dB and a bit error rate (bER) of the order of 10-5 satisfying the 3.0×10-4 criterion was obtained. To the best of our knowledge, this is the first report on the successful and seamless playback performance of a high-definition video content from the super-RENS disc at a data transfer rate of either 36 or 72 Mbps. The bER results of fixed and adaptive partial response maximum likelihood detections are described.

Microstructural and optical analysis of superresolution phenomena due to Ge2Sb2Te5 thin films at blue light regime

Superresolution (SR) phenomena due to Ge2Sb2Te5 films were examined by combined analysis of the transmission electron microscopy (TEM) microstructures of the laser-irradiated films and the results from dynamic and static tests using blue lasers. A new finding was made that comprises a complementary case of the classical SR readout by Ge2Sb2Te5 film; an amorphous band instead of a closed aperture of melt in the crystalline background forms behind a moving laser but still produces a high SR signal. A complete carrier-to-noise-ratio curve of a SR-read-only memory employing Ge2Sb2Te5 may be derived from a nonlinear optical effect, specifically thermally assisted saturable absorption.

Mechanism of Charge Retention Loss in Ferroelectric Pt/Pb(Zr,Ti)O3/Pt Capacitors and Its Relation to Fatigue and Imprint

Charge retention loss was investigated for Pt/Pb(Zr0.4Ti0.6)O3/Pt capacitors in short- (t < 1 s) and long-time (t > 1 s) regimes. The short-time retention loss behaviors of fatigued capacitors were well described by a power-law function and analyzed in terms of a superposition of polarization relaxation with a distribution of relaxation time. We showed that depolarization field governs the retention loss in the short-time regime. Fatigue-induced defects, probably the interfacial passive layers, seemed to increase the depolarization field. In the long-time regime, the retention loss is shown to be related to the internal field developed parallel to the polarization direction. Particularly in “opposite-state” retention, the internal field caused polarization backswitching resulting in a severe degradation of retention characteristics. The internal field developed during retention was estimated by using Merz equations of polarization switching. Defects dynamics under electric field was discussed in conjunction with the recovery process of imprint.

Stability enhancement of super-RENS high temperature readout signal

We report the readout stability improvement results of super-resolution near field structure (Super-RENS) write-once read-many (WORM) disk at a blue laser optical system. (Laser wavelength 405nm, numerical aperture 0.85) By using diffusion barrier structure (GeSbTe sandwiched by GeN) and high transition temperature recording material (BaTiO3), material diffusion of phase change layer and recording mark degradation were greatly improved during high power (Pr=2.0mW) readout process up to 1X105 times.

Hydrogen-Induced Degradation in Ferroelectric Bi 3.25 La 0.75 Ti 3 O 12 and PbZr 0.4 Ti 0.6 O 3

Effects of forming gas annealing (FGA) on ferroelectric Bi 3.25 La 0.75 Ti 3 O 12 (BLT) and PbZr 0.4 Ti 0.6 O 3 (PZT) capacitors were investigated as a function of FGA temperature. For low temperature FGA below 350°C, BLT capacitors showed a quite high resistance against hydrogen-induced damages compared to PZT capacitor. For poled capacitors before FGA, PZT capacitors showed a severe imprint failure while BLT capacitors did not show significant change. The difference in degradation at the low temperature FGA seemed to be due to the degree of polarization pinning. On the other hand, for high temperature FGA, both BLT and PZT capacitors showed severe degradation behaviors. Changes in the shape of hysteresis loops, as the degradation proceeded, were thought to reveal quite different degradation mechanisms.