Dan Ricinschi

Giant ferroelectric polarization beyond 150 μC/cm2 in BiFeO3 thin film

Ferroelectric BiFeO3 thin films have been deposited on Pt/TiO2/SiO2/Si substrate by pulsed-laser deposition. From the X-ray diffraction analysis, the BiFeO3 thin film consists of perovskite single-phase, and the crystal structure shows the tetragonal structure (c/a = 1.018) with a space group P4mm. It is obtained that the BiFeO3 thin film shows a well-saturated remarkably giant saturation polarization of 158 µC/cm2 and a remanent polarization of 146 µC/cm2 for a maximum applied voltage of 20 V at 90 K. These values of polarization are largest ever-measured in ferroelectrics.

Enhancement of electrical properties in polycrystalline BiFeO3 thin films

Ferroelectric BiFeO3 thin films were grown on Pt∕TiO2∕SiO2∕Si substrates by pulsed-laser deposition. From the x-ray diffraction analysis, the BiFeO3 thin films consist of perovskite single phase, and the crystal structure shows the tetragonal structure with a space group P4mm. The BiFeO3 thin films show enhanced electrical properties with low leakage current density value of ∼10−4A∕cm2 at a maximum applied voltage of 31V. This enhanced electrical resistivity allowed the authors to obtain giant ferroelectric polarization values such as saturation polarizations of 110 and 166μC∕cm2 at room temperature and 80K, respectively.

Analysis of ferroelectric switching in finite media as a Landau-type phase transition

A detailed analysis of polarization reversal in ferroelectrics has been performed, in the framework of the Landau model for phase transitions. Some important characteristics of homogeneous switching have been emphasized and later used in studying the more general case of inhomogeneous switching. The two extremes of switching current correspond to the inflexion points of the dielectric hysteresis loop. Hysteresis loops of poled ferroelectric samples are expected to include negative-susceptibility regions, for high-frequency applied electric fields. The switching current minimum is eliminated by the experimental method used for recording the switching responses. Equivalent Landau coefficients and electric fields have been defined, in order to integrate the size effects and inhomogeneity contribution to switching of the global order parameter. We correlated the size effects on the critical parameters of the switching (the coercive field) and the ferroelectric-to-paraelectric phase transition (the Curie temperature). Polarization reversal in small-size ferroelectrics can be regarded as a diffuse phase transition, whereas its character is closer to normal for large-size samples. The size dependencies of the reversal speed and maximum current result from the size dependencies of the equivalent Landau coefficients and electric field inducing reversal.

First-order reversal curves diagrams for the characterization of ferroelectric switching

A method to describe the switching characteristics of the ferroelectrics is proposed, using the first-order reversal curve (FORC) diagrams. On these diagrams, the reversible and irreversible contributions to the ferroelectric polarization can be clearly separated. They are extremely sensitive to the changes of the hysteresis loops induced by degradation of the ferroelectric polarization, such as fatigue. Sharp in the fresh state, the FORC distribution becomes wide with its maximum shifted towards higher fields after 109 switching cycles. A strong increase of the reversible component was found in the fatigue state. With appropriate interpretation, these diagrams could be valuable as “fingerprints” of the switching characteristics of the ferroelectric systems in a particular state.

Grain Size Dependence of Switching Properties of Ferroelectric BaTiO3 Ceramics

Polarization reversal properties of BaTiO3 (BT) ferroelectric ceramics at room temperature were studied. Two types of samples, having g1=1.5 µ m and g2=70 µ m grain size, were used. The grain size effect on the transient switching current was analysed and compared with the results of two theoretical models accounting for size effects: a dipole lattice model and a finite grain size model (FGM) that uses a Kolmogorov-Avrami (K-A) type statistical approach of polarization reversal.

Landau theory-based analysis of grain-size dependence of ferroelectric-to-paraelectric phase transition and its thermal hysteresis in barium titanate ceramics

Ferroelectric barium titanate ceramics with grain sizes in the range (0.5, 20) µm were used for a comparative study of grain-size influence on the ferro-para phase transition and its thermal hysteresis. The investigations were performed by recording the temperature dependence of the dielectric constant, heat capacity and pyrocharge. The experimental results have been analysed in the framework of Landau-Devonshire theory. The grain-size dependent Landau coefficients of the free energy have been derived from our experimental data and used to compute the grain-size dependence of the theoretical transition interval related to the thermal hysteresis. The good qualitative agreement between theory and experiment recommends the size dependent Landau coefficients as a way to improve the modelling of the size dependent ferro-para phase transition in ceramics.

Improved Retention Characteristics of Metal-Ferroelectric-Insulator-Semiconductor Structure Using a Post-Oxygen-Annealing Treatment

An oxygen-annealing process after SrBi2Ta2O9 (SBT) ferroelectric film deposition has effectively improved retention properties of the metal-ferroelectric-insulator-semiconductor (MFIS) structure for FET-type ferroelectric nonvolatile memory. The annealing treatment 1) reduces the surface roughness of the SBT film, 2) improves crystallinity, 3) effectively decreases the leakage current through the MFM structure, 4) increases the barrier height at the metal-ferroelectric interface, 5) decreases the carrier trap density in SBT film, and 6) improves the retained polarization of SBT film itself. Finally, the retention time of the memorized state in the MFIS diode was successfully increased to 2×104 s or more by the postannealing treatment.

Relationships between macroscopic polarization hysteresis and local piezoresponse of fatigued Pb(Zr, Ti)O3 films within a landau theory-based lattice model

Characterization of the fatigued state in Pb(Zr,Ti)O3 (PZT) films has been carried out by nanoscale piezoelectric displacements measured simultaneously with the macroscopic polarization hysteresis. For explaining the fatigue peculiarities of PZT films prepared by sol-gel and rf sputtering, a Landau theory-type model based on formation of degraded regions in a ferroelectric lattice has been used. The relevance of local piezoelectric loops for the macroscopic polarization hysteresis has been assessed and the possibility of recovering the latter as a weighted convolution of the former has been shown to depend on the film quality.

Ferroelectric and structural properties of stress-constrained and stress-relaxed polycrystalline BiFeO3 thin films

The stress influence of the structural and ferroelectric properties of polycrystalline BiFeO3 (BFO) thin films has been investigated using a membrane substrate for relaxing stress. Reciprocal space mapping (RSM) measurement has been performed to confirm the stress dependence of the crystal structure of polycrystalline BFO thin films on the Pt (200 nm)/TiO2 (50 nm)/SiO2 (600 nm)/Si (625 μm) substrate (stress-constrained BFO film) and the Pt (200 nm)/TiO2 (50 nm)/SiO2 (600 nm)/Si (15 μm) membrane substrate (stress-relaxed BFO film). The BFO thin films prepared by pulsed laser deposition were polycrystalline and mainly exhibit a texture with (001) and (110) plane orientations. From the RSM results, the crystal structure of the (001)-oriented domain changes from Pm monoclinic to Cm monoclinic or to R3c rhombohedral due to stress relaxation. Moreover, at room temperature as well as at 150 K, remanent polarization (Pr) increases and double coercive field (2Ec) decreases (in the latter case from 88 to 94 μC/cm2 ...

Density functional theory and experimental study of the electronic structure and transport properties of La, V, Nb, and Ta doped SrTiO3

The electronic structure and transport properties of donor doped SrTiO3 are studied using density functional theory with spin-orbit coupling and conductivity, Hall, and Seebeck effect measurements over a wide temperature range (100 K to 600 K). Split-off energies ΔSO are tunable through the dopant SO interaction strength and concentration varying from 28.1 meV for pure STO to 70.93 meV for SrTi0.5Nb0.5O3. At lower carrier concentrations and temperatures, SO coupling has a marked effect on both the filling dependence of the density-of-states mass as well as the temperature dependence of the Seebeck coefficient, with quantitative theoretical predictions based on DFT calculations that include the SO interaction in closer agreement to the experimental data. Moreover, the results suggest that the predictive power of the current theory is not unlimited, with less accuracy for the calculated S predicting the magnitude of the experimental S data at lower dopant concentrations than for degenerately doped systems. ...