Polarization In Ferroelectric Materials Research Articles

Modeling the polarization in ferroelectric materials: a novel analytical approach

We propose for the first time a fully analytical formulation of the polarization in ferroelectric materials that takes into account history effects. Our approach is based on the Preisach theory of the hysteresis loops. A symmetric exponential decay distribution of the dipoles thresholds has been introduced. Consequently, an exact analytical and continuous solution of the polarization could be derived, taking the electric field history into consideration. Experimental data on PZT capacitors show relevant agreement with the model for both saturated and minor loops. This confirms the validity of this approach, which consequently represents a very interesting candidate for efficient ferroelectric compact models to be used either in memory or in analog design applications.

Polarization Reversal Anti-parallel to the Applied Electric Field Observed Using a Scanning Nonlinear Dielectric Microscopy

ABSTRACTIt is well known that spontaneous polarization of ferroelectric material is an intrinsic property applied for nonvolatile memory devices. Poling direction can be reversed in a nanometer size area using the conductive cantilever of a scanning probe microscope. In order to detect nanodots patterns, scanning nonlinear dielectric microscope (SNDM) is superior to piezore-sponse microscope in terms of resolution. In this paper, a real-time measuring method of a poling direction is proposed. Using this method, the domain reversal process was observed and an unexpected phenomenon was found, namely, that the poling directions were aligned antiparallel to the poling electric field. This antiparallel poling reversal took place when the film thickness was more than 350 nm in the case of lithium tantalate. At present, the reason and mechanism of the antiparallel poling reversal are uncertain, although it might be related to the concentrated electric field near the cantilever tip.

A Novel Approach to Modeling the Polarization in Ferroelectric Materials

We propose a new empirical model of the polarization in ferroelectric materials, which is aimed to be used in electrical simulation of analog ferroelectric-based devices. A first attempt to model ferroelectric capacitors was derived by Miller et. al. [1], based on an hyperbolic tangent function that failed to predict internal closed loops. Furthermore, a different model, based on the counting of a set of dipoles, was proposed by Jiang et. al. [2]. This model can describe closed loops but its discrete structure prevents its use in applications that require continuous functions. Based on the approach proposed by Jiang , we developed a new model which is continuous and correctly predicts internal closed loops. Our model assumes a particular distribution of the electrical critical fields, owing to derive a simulation-efficient analytical expression of the polarization as a function of the electrical field and ferroelectric history that can be used in electrical simulators.

A Novel Approach to Modeling the Polarization in Ferroelectric Materials

We propose a new empirical model of the polarization in ferroelectric materials, which is aimed to be used in electrical simulation of analog ferroelectric-based devices. A first attempt to model ferroelectric capacitors was derived by Miller et. al. [1], based on an hyperbolic tangent function that failed to predict internal closed loops. Furthermore, a different model, based on the counting of a set of dipoles, was proposed by Jiang et. al. [2]. This model can describe closed loops but its discrete structure prevents its use in applications that require continuous functions. Based on the approach proposed by Jiang , we developed a new model which is continuous and correctly predicts internal closed loops. Our model assumes a particular distribution of the electrical critical fields, owing to derive a simulation-efficient analytical expression of the polarization as a function of the electrical field and ferroelectric history that can be used in electrical simulators.

Comparison between standard and chain-type FRAM architectures

A standard 1T/1C and chain-type ferroelectric memory architecture are presented. The standard memory cell consists of a transistor connected in series to a ferroelectric capacitor while the chain-type cell connects these elements in parallel. Based on the different memory cells, two different arrays have been designed, simulated, and integrated on a single test chip in a 0.35μm process (not presented in this work). They are compared in regard to area, performance, and reliability. It is shown that chain FRAM has the potential to be superior to standard FRAM concerning area because of its innovative cell configuration which reduces the average number of lines per row from two to almost one and therefore removing the connectivity drawback required for bipolar polarization of ferroelectric materials compared to unipolar dielectric polarization. However, this obviously comes at the cost of limited performance and increased design complexity.

Two-dimensional simulation of ferroelectric memory cells

An approach to increase the capabilities of integrated circuit nonvolatile memory is to take advantage of the hysteresis in the polarization of ferroelectric materials. For a rigorous analysis of the resulting devices, a suitable model for the ferroelectric effects has been developed. We present this model and show the results of its implementation into a device simulator. Although this model was designed especially for analysis of ferroelectric materials, it is also applicable to magnetic hysteresis phenomena.

Sputter Synthesis of Ferroelectric Films and Heterostructures

Ferroelectric films can display a wide range of dielectric, ferroelectric, piezoelectric, electrostrictive, and pyroelectric properties. The potential utilization of these properties in a new generation of devices has driven the intensive studies on the synthesis, characterization, and determination of processing-microstructure-property relationships of ferroelectric thin films during the last five years. In addition there has been an increased drive for integrating ferroelectric film-based heterostructures with different substrate materials to demonstrate numerous devices that exploit the dielectric, ferroelectric, piezoelectric, electrostrictive, and pyroelectric properties of these materials. For example the high dielectric permittivities of perovskite-type materials can be advantageously used in dynamic random-access memories (DRAMs), while the large values of switchable remanent polarization of ferroelectric materials are suitable for nonvolatile ferroelectric random-access memories (NVFRAMs).Various vapor-phase deposition techniques such as plasma and ion-beam sputter deposition (PSD and IBSD, respectively), pulsed laser-ablation deposition (PLAD), electron-beam or oven-induced evaporation for molecular-beam epitaxy (MBE), and chemical vapor deposition (CVD) have been applied to produce ferroelectric films and layered heterostructures. See References 4–7 for recent reviews. However, work is still necessary to optimize the techniques to produce device-quality films on large semiconductor substrates in a way that is fully compatible with existing semiconductor process technology. Therefore research efforts should be focused on the optimization of suitable process methods and on the investigation of processing-composition-microstructure property relationships. These efforts are the focus of this article with emphasis on PSD and IBSD techniques.

Mechanism(s) for the Suppression of the Switchable Polarization in PZT and BaTiO3

ABSTRACTSwitchable polarization can be significantly suppressed in ferroelectric (FE) materials by optical, thermal, and electrical processes. The thermal process can occur by either annealing the FE in a reducing environment or by heating it in air to 100°C while impressing a bias near the switching threshold. The optical process occurs while biasing the FE near the switching threshold and illuminating with bandgap light. And the electrical suppression effect occurs by subjecting the FE to repeated polarization reversals. Using electron paramagnetic resonance, polarization-vol tage measurements, and charge injection scenarios, we have been able to elucidate both electronic and ionic trapping effects that lead to a suppression in the amount of switchable polarization in FE materials. The relative roles of electronic and ionic effects in the same material can depend on the stress condition. For instance, in oxidized BaTiO3 crystals, optical and thermal suppressions occur by electronic domain pinning; electrical fatigue in the BaTiO3 crystals also appears to involve electronic charge trapping, however, it is suggested that these electronic traps are further stabilized by nearby ionic defects. In sol-gel PZT thin films with either Pt, RuO2, or La-Sr-Co-O electrodes it appears that the polarization suppression induced by electrical fatigue, a temperature/bias combination, or a light/bias combination are all primarily due to the trapping of electronic charge carriers to first order.

Study of the spatial distribution of the polarization in ferroelectric ceramics by means of low frequency sinusoidal thermal waves

Abstract A new method for the determination of the distribution of polarization in ferroelectric materials has been developed. It is an intermediate between that of Low Frequency Thermal Waves and the one known as LIMM. This method allows measurements at frequencies in the range of 10−4 Hz to 200 Hz. Experimental results on three different kinds of ferroelectric materials are shown: singlecrystal, ceramic and polymer. This technique can be easily implemented at any laboratory.

Fast polarization changes in ferroelectrics and their application in accelerators

Several mechanisms are described which change the polarization in ferroelectric material. Provided the change is too rapid for the related surface charges to be screened or neutralized, the high charge density can lead to strong electric fields. The fields may possibly be used for emission and acceleration of electrons. First results of experiments are reported in which fast spontaneous polarization changes by reversal or phase transition have been demonstrated. Electrons of 25 keV energy have been observed, emitted from triglycine sulfate (TGS) crystals during phase transition while being slowly heated across the Curie temperature. With fast polarization changes, electron beams of even higher energy and density are expected.