T. K. Song

Polarization switching kinetics of epitaxial Pb(Zr0.4Ti0.6)O3 thin films

The polarization switching kinetics of epitaxial Pb(Zr0.4Ti0.6)O3 thin films were investigated by measuring write pulse-width dependences of switched polarization ΔP(t) under various applied electric fields. It was found that ΔP(t) follows the predictions of the Kolmogorov–Avrami–Ishibashi model quite well. However, the detailed behaviors of ΔP(t) were different for low and high electric field regions, which separated out around 220kV∕cm. The coercive field also showed different frequency dependences depending on the region. These differences were attributed to changes in the polarization switching kinetics. A possible origin of the switching kinetics changes is also discussed.

ac Dynamics of Ferroelectric Domains from an Investigation of the Frequency Dependence of Hysteresis Loops

We investigated nonequilibrium domain wall dynamics under an ac field by measuring the hystere- sis loops of epitaxial ferroelectric capacitors at various frequencies and temperatures. Polarization switching is induced mostly by thermally activated creep motion at lower frequencies, and by vis- cous ow motion at higher frequencies. The dynamic crossover between the creep and ow regimes unveils two frequency-dependent scaling regions of hysteresis loops. Based on these findings, we constructed a dynamic phase diagram for hysteretic ferroelectric domain dynamics in the presence of ac fields.

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.

FERROELECTRIC SWITCHING DYNAMICS AND PULSE-SWITCHING POLARIZATION MEASUREMENTS

ABSTRACT Pulse switching properties were studied in polycrystalline Pb(Zr,Ti)O3 capacitors to check a recently proposed nucleation limited switching (NLS) model, where very wide switching was observed. The backing-switching responses (P b) at the end of writing pulses were studied to understand the role of writing pulse together with switching polarization (Δ P). The P b+Δ P, with high enough write voltage, is almost independent of writing pulse width (t w), even though Δ P is observed to increase with increasing t w. From these results, it is concluded that the NLS dynamics are attributed to the switching and poling processes with external fields around coercive fields.

Hydrogen Effects on the Electrical Conductivity of Pulsed Laser Deposited ZnO Thin Films

ABSTRACT Well c-axis oriented 320 nm and 160 nm thick ZnO thin films were deposited with pulsed laser deposition on Pt/Ti/SiO2/Si(100) substrates. Diode characteristics of Au/ZnO/Pt in electrical conductivity were observed. The conductivity in forward bias was sensitive to the hydrogen annealing but almost insensitive in reverse bias. The conductivity of 320 nm thick film in forward bias increased from 6.1 × 10−5 Ω−1 cm−1 in as-deposited film to 3.3 × 10−3 Ω−1 cm−1 in hydrogen annealed one at 200°C. The observed dielectric constant 70 was much higher than that of ceramic ZnO. Dielectric anomaly at about 1 kHz was observed and attributed to the space charge.

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.

Structure of Ferroelectric (Ba,Sr)TiO3 Thin Films for Tunable Microwave Devices

Ferroelectric (Ba,Sr)TiO3 films have been deposited on (001) MgO single crystals by a pulsed laser deposition with oxygen background while heating the substrates. Deposited BST films exhibit epitaxial growth along (001), which are confirmed by x-ray diffraction measurement. Structure of (Ba,Sr)TiO3 along in-plane and surface normal direction have been investigated and found to have a tetragonal distortion depend on the deposition conditions, such as oxygen pressure. Lattice parameter decreases with increasing oxygen pressure, and tetragonallity (c/a) changes from 1.005 to 0.997 as oxygen pressure increase. Interestingly, energy gap measured by FTIR decreases with decreasing oxygen pressure until it reach a certain oxygen pressure, then increases again with increasing oxygen pressure. Furthermore, microwave properties of devices measured by a HP 8510C vector network analyzer from 0.045–20 GHz suggest that the least distorted films exhibit a larger dielectric constant changes with dc bias field.

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.

Pulse width dependent polarizations of ferroelectric (Pb, La)(Zr, Ti)O3 thin film capacitors

Abstract We report the measurements of pulse polarizations of (Pb, La)(Zr, Ti)O3 ferroelectric capacitors as a function of pulse width. Pulse polarizations were measured from the switching current responses in the pulse width range from 1 μs to 1 s. The relation between pulse polarizations(switched(P*) and non-switched polarization(P⁁)) and hysteresis loop parameters was studied. In the case of all write/read pulse widths are same, P* increased but P⁁ decreased with increasing pulse width. These results are explained by poling effects and confirmed by measuring pulse width dependencies with different patterns of pulse trains.

Charge Retention Loss and Its Mechanism of (Bi, La)4Ti3O12 Capacitors

We investigated the retention properties of BLT capacitors fabricated by a chemical solution deposition with Pt electrodes. BLT capacitors showed an opposite-state retention loss much better than that of PZT capacitor. On the other hand, BLT capacitors showed a large polarization loss of the same-state retention within 1 hour baking, while the corresponding loss of PZT was negligible. The hysteresis loops after different baking time showed a negligible imprint and a large relaxation of polarization. These behaviors were explained by a high resistance against imprint and a polarization relaxation possibly due to damaged interfacial layers developed by etching process of the BLT capacitors.