Dielectric aging of Mn and V-codoped BaTiO3 was investigated. The increase of V concentration had little influence on aging, whereas that of Mn increased it. Thermally stimulated depolarization current (TSDC) of low Mn concentration specimen showed one peak, whereas Mn-increased specimen showed two peaks. The first and second peak is supposed to be caused by the phase transition of the undoped core region and the defect dipole of Mn such as MnTi″-VO•• or MnTi′-VO••, respectively. High TSDC associated with the defect dipole of Mn and significant aging rate experimentally demonstrates that the dielectric aging is controlled by the defect dipole.

Thermally stimulated depolarization current analysis for the dielectric aging of Mn and V-codoped BaTiO3 multi layer ceramic capacitor

The discovery of ferroelectricity in polycrystalline thin films of doped HfO2 has reignited the expectations of developing competitive ferroelectric non-volatile memory devices. To date, it is widely accepted that the performance of HfO2-based ferroelectric devices during their life cycle is critically dependent on the presence of point defects as well as structural phase polymorphism, which mainly originates from defects either. The purpose of this review article is to overview the impact of defects in ferroelectric HfO2 on its functional properties and the resulting performance of memory devices. Starting from the brief summary of defects in classical perovskite ferroelectrics, we then introduce the known types of point defects in dielectric HfO2 thin films. Further, we discuss main analytical techniques used to characterize the concentration and distribution of defects in doped ferroelectric HfO2 thin films as well as at their interfaces with electrodes. The main part of the review is devoted to the recent experimental studies reporting the impact of defects in ferroelectric HfO2 structures on the performance of different memory devices. We end up with the summary and perspectives of HfO2-based ferroelectric competitive non-volatile memory devices.

Defects in ferroelectric HfO2.

Thermally stimulated currents (TSCs) are measured in amorphous barium titanate thin films deposited by the rf sputtering technique The TSC global curve is composed of three overlapping peaks in the 0–200°C temperature range At 50°C, a first TSC peak is observed that can be related to a shallow-trap level A second peak due to a dipolar polarization process is observed at 95°C Finally, a third peak appears at 140°C that is ascribed to the oxygen vacancy motion and their accumulation at electrodes The different peaks constituting the global TSC spectrum are separately studied by the fractional polarization technique in order to analyze their fine structures and to determine their activation energies

Thermally stimulated currents in amorphous barium titanate thin films deposited by rf magnetron sputtering

Two series of experiments were performed to study the experimental conditions for the formation of {111} twins and related microstructures in barium strontium titanate ((Ba, Sr)TiO3). In the first series, the phase equilibria in the BaTiO3–SrTiO3–TiO2 system were determined. XRD and WDS analysis, done in the BaTiO3-rich region, of 45(Ba,Sr)TiO3–10TiO2 samples annealed at 1250°C for 200 h in air showed that (Ba,Sr)TiO3 was in equilibrium with Ba6Ti17O40 (B6T17) and Ba4Ti13O30 phases with strontium solubility (Sr/(Ba + Sr)) of ∼0.02 and 0.20, respectively. In the second series the microstructures of samples consisting of a mixture of (Ba,Sr)TiO3 and 2.0 mol% TiO2, were observed after sintering at 1250°C for 100 h in air. {111} twins formed only in the samples with faceted B6T17 second phase particles, similar to the case of BaTiO3. In these samples, abnormal grain growth occurred in the presence of the {111} twins. In contrast, no {111} twins formed and no abnormal grain growth occurred in the samples containing second phase particles other than B6T17. With an increased substitution of strontium for barium, the aspect ratio of abnormal grains containing {111} twin lamellae was reduced. This result was attributed to a reduction in the relative stability of the {111} planes with the strontium substitution.

{111} Twin Formation and Abnormal Grain Growth in Barium Strontium Titanate

I (Current)-V (voltage) characteristics of Mn-doped BaTiO3 multilayer ceramic capacitor were investigated. The increase of Mn concentration reduced the slope of current increase with increasing dc field and enhanced the dc field to the abrupt increase of current and thereby breakdown. Ohmic, Schottky, and Poole-Frenkel type conduction was successively observed with the increase of dc field in all specimens. The region of Child's law did not appear before breakdown, which suggests space charge limited (SCL) conduction with field enhanced trap barrier lowering. A peak of thermally stimulated depolarization current (TSDC) beyond 300 °C associated with trapped space charges appeared, which increased by the increase of Mn concentration. In addition, the peak intensity increased to a maximum value and then decreased again with increasing polarization dc field in TSDC measurement. Its activation energies were around 1.9 eV for all specimens, which is very close to the energy level for the ionization of Mn3+/Mn2+...

Correlation between I (current)-V (voltage) characteristics and thermally stimulated depolarization current of Mn-doped BaTiO3 multilayer ceramic capacitor

A Thermally Stimulated Current in paraelectric barium-strontium titanate ceramics doped with iron up to 1% was studied in the range above the room temperature. Two TSC peaks, A and B, was observed: The peak A lied around at 90°C, the activation energy being 0.68 eV, whereas the peak B was found around at 130°C with the activation energy of 0.97 eV and regarded as due to migration of oxygen vacancies. These results suggest that the asymmetric hysteresis property in iron-doped ferroelectric ceramics may be brought about from the same charges.

http://iopscience.iop.org/article/10.7567/JJAPS.26S2.46/pdf

Hidetoshi Tsucniya

Papers

TSC Study on Fe-Doped Barium-Strontium Titanate Ceramics