Use Of Ferroelectrics Research Articles

DFT Insight into Conductive and Magnetic Properties of Heterostructures with BaTiO3 Overlayer.

The ab initio calculations of a heterostructure based on the ferroelectric phase of barium titanate and dielectrics lanthanum manganese (LaMnO3) or silicon (Si) are presented. We analyze structures of BaTiO3/LaMnO3 and BaTiO3/Si interfaces, investigate magnetic properties and the impact of ferroelectric polarization. The use of ferroelectrics in the heterostructure plays a crucial role; in particular, ferroelectric polarization leads to the appearance of the conducting state at the interface and in the layers close to it. We show that defects (here, oxygen vacancies) incorporated into the system may change the electronic and magnetic properties of a system. Experimental results of magnetic susceptibility measurements for the Ba0.8Sr0.2TiO3/LaMnO3 heterostructure are also presented. It is shown that a correlation between the behavior of the ferromagnetic ordering and the resistance takes place. In addition, the ferromagnetic ordering at the interface of the heterostructure can be associated with the exchange interaction through current carriers that appear in high carrier concentration regions.

Narrow bandgap potassium titanate-molybdate-based d ferroelectrics

The bulk photovoltaic effect observed in ferroelectric materials can enable photovoltaic performance beyond the Shockley–Queisser limit of efficiency. This requires the use of ferroelectrics with strong polarization and low bandgap (Eg) that are typically contradictory in the common perovskite oxides ferroelectrics. Here, we use first-principles calculations to study the KNbO3–K(Ti0.5Mo0.5)O3 (KNTM) solid solutions as possible narrow-gap ferroelectric materials. KTM, the end-member of the recently discovered KNTM solid solution system, maintains a ferroelectric polarization similar to that of other K-based systems due to its d0 configuration at the B-site. The substitution of Nb in KTM reduces Eg from 2.9 of KTM to 1.83 eV for an unstrained system and 1.7 eV for a compressively strained system, while maintaining ferroelectricity. The combination of narrow Eg, strong ferroelectricity, low toxicity, and abundance of the constituent elements make Nb-substituted KTM a promising candidate material for photoferroelectric applications.

Hybrid Plasmonic-Ferroelectric Architectures for Lasing and SHG Processes at the Nanoscale.

Coherent light sources providing sub-wavelength confined modes are in ever more demand to face new challenges in a variety of disciplines. Scalability and cost-effective production of these systems are also highly desired. The use of ferroelectrics in functional optical platforms, on which plasmonic arrangements can be formed, is revealed as a simple and powerful method to develop coherent light sources with improved and novel functionalities at the nanoscale. Two types of sources with sub-diffraction spatial confinement and improved performances are presented: i) plasmon-assisted solid-state nanolasers based on the interaction between metallic nanostructures and optically active rare earth doped ferroelectric crystals and ii) nonlinear radiation sources based on quadratic frequency mixing processes that are enhanced by means of localized surface plasmon (LSP) resonances. The mechanisms responsible for the intensification of the radiation-matter interaction processes by LSP resonances are discussed in each case. The challenges, potential applications, and future perspectives of the field are highlighted.

Active electrocaloric demonstrator for direct comparison of PMN-PT bulk and multilayer samples

ABSTRACTOne potential use of ferroelectrics is as active material in electrocaloric cooling systems. These systems promise a more energy efficient cooling process than vapor compression, thermoelectric or other current cooling systems. Currently different design types of electrocaloric cooling devices are in the focus of research. In this paper, we present an electrocaloric cooling device demonstrator working as “Active Electrocaloric Regenerator” (AER) and employing relaxor ferroelectric elements as active material. The device design is such that it allows the integration of different material systems and regenerator designs as well as a broad variation of operational parameters.

Electric-field control of magnetic order above room temperature

Controlling magnetism by means of electric fields is a key issue for the future development of low-power spintronics. Progress has been made in the electrical control of magnetic anisotropy, domain structure, spin polarization or critical temperatures. However, the ability to turn on and off robust ferromagnetism at room temperature and above has remained elusive. Here we use ferroelectricity in BaTiO3 crystals to tune the sharp metamagnetic transition temperature of epitaxially grown FeRh films and electrically drive a transition between antiferromagnetic and ferromagnetic order with only a few volts, just above room temperature. The detailed analysis of the data in the light of first-principles calculations indicate that the phenomenon is mediated by both strain and field effects from the BaTiO3. Our results correspond to a magnetoelectric coupling larger than previous reports by at least one order of magnitude and open new perspectives for the use of ferroelectrics in magnetic storage and spintronics.

Ferroelectric dipole electrets for output power enhancement in electrostatic vibration energy harvesters

We propose a ferroelectric dipole electret composed of polarized lead zirconate titanate. Deep insight into the physics behind the parallel plate capacitor theoretically predicts that we can extract large electric field near the surface of the ferroelectric dipole electret by increasing its surface charge density and thickness. Experiment for ferroelectric dipole electret shows good agreement with the theory. The maximum output power density of electrostatic vibration energy harvesters using the ferroelectric dipole electret was 78 μW/cm3, a three-fold increase over a conventional polymer electret. Our results will pave the way for use of ferroelectrics as electrets.

Filled latex composites with high dielectric constant, based on dispersed ferroelectrics

The possibility of controlling the physicochemical properties of suspensions and the electrical properties of filled latex composites containing dispersed ferroelectrics was examined. The use of ferroelectrics as fillers allows preparation of polymeric composites with high dielectric constant ɛ and low dielectric loss, used, e.g., for the development of dielectric functional layers of electroluminescence light sources.

Analysis and design of smart electromagnetic structures

Use of ferroelectric materials to improve antenna performance is an area of active research.Applying an electric field across a ferroelectric used as the dielectric in an antenna enablestuning the antenna performance. Ferroelectrics also have coupled electro-mechanicalbehavior due to which they are sensitive to mechanical strains and fluctuations in ambienttemperature. Use of ferroelectrics in antenna structures, especially those subject tomechanical and thermal loads, requires knowledge of the phenomenological relationshipbetween the ferroelectric properties of interest (especially the dielectric permittivity) andthe external physical variables, namely electric field(s), mechanical strains andtemperature. To this end, a phenomenological model of ferroelectric materials based on theDevonshire thermodynamic theory is presented. This model is then used to obtain arelationship expressing the dependence of the dielectric permittivity on the mechanicalstrain, applied electric field and ambient temperature. The relationship is compared againstpublished experimental data and other models in the literature. A model relating theferroelectric loss tangent to the applied electric field and temperature is also discussed.Subsequently, relationships expressing the dependences of the antenna operatingfrequency and radiation efficiency on those external physical quantities are described.

Active tuning of photonic device characteristics during operation by ferroelectric domain switching

Ferroelectrics have many unusual properties. Two properties that are often exploited are first, their complex, nonlinear optical response and second, their strong nonlinear coupling between electromagnetic and mechanical fields through the domain patterns or microstructure. The former has led to the use of ferroelectrics in optical devices and the latter is used in ferroelectric sensors and actuators. We show the feasibility of using these properties together in nanoscale photonic devices. The electromechanical coupling allows us to change the domain patterns or microstructure. This in turn changes the optical characteristics. Together, these could provide photonic devices with tunable properties. We present calculations for two model devices. First, in a photonic crystal consisting of a triangular lattice of air holes in barium titanate, we find the change in the band structure when the domains are switched. The change is significant compared to the frequency spread of currently available high-quality light sources and may provide a strategy for optical switching. Second, we show that periodically poled 90° domain patterns, despite their complex geometry, do not cause dispersion or have band gaps. Hence, they may provide an alternative to the antiparallel domains that are usually used in quasiphase matching and allow for tunable higher-harmonic generation.

Imaging of out-of-plane interfacial strain in epitaxial PbTiO3∕SrTiO3 thin films

In this study, we rely on low-angle annular dark-field scanning transmission electron microscopy to probe the interface strain profile in epitaxial PbTiO3∕SrTiO3 thin-films. All samples displayed a compressively strained layer at the PbTiO3∕SrTiO3 interface, with the strain vector parallel to the polarization direction. The width of the strained layer was found to be ∼15–30Å, dependent on the electrode environment. Our findings open a perspective to use interface strain engineering in combination with control of electrostatic boundary conditions as a tool to monitor the effective interface polarization. These findings have implications for future use of ferroelectrics in electronic and mechanical devices.

Addition of Ca and Sr in PLZT to improve fram® performance and reliability

Although endurance frequently receives the most attention when considering the use of ferroelectrics in memory devices, we find that retention is actually the more important concern. In particular, ferroelectric materials are prone to imprinting which is a tendency to form a preferred state. Once a preferred state is formed, memory failure can occur when trying to set the ferroelectric in the non-preferred state. In order to improve the resistance to imprint, calcium and strontium dopants in PLZT have been studied. The calcium and strontium were varied using multiple sputtering targets along with ICP to determine the composition. These films were then characterized for orientation, switching and imprint.

Thin film deposition of barium strontium titanate by metal organic decomposition technique

With increasing integration density and shrinking geometry DRAM fabrication technology has remained at the forefront of semiconductor industry. The basic unif of a DRAM is a single MOSFET with a storage capacitor. Although dimensions are being reduced there is hardly any room for reduction in the cell capacitance. This calls for methods to improve the charge storage in the miniature devices. The search for high dielectric constant materials has led to the use of ferroelectrics for charge storage. Barium strontium titanate (Ba1-xSrxTiO3) (BST) has thus found a very important place in the fabrication of DRAMs. The metal organic decomposition (MOD) proces is a techniquer for the worth of inorganic films without processing in vacuum or going through a powder step. It is a simple process that is both fast and economical, with very minimal investment. Thin films of BST were deposited on platinized silicon substrates by the spin-on method. The films were then pyrolyzed and annealed. The films obtained were characterized for their compositional and structural properties as well as dielectric constant measurements. This paper presents the method of film preparation and some of the properties of the film.

Ferroelectric sources and their application to pulsed power: a review

We present in this review an account of recent research into the use of ferroelectrics as electron beam sources for pulsed power applications. The work is reviewed according to the ferroelectric material used and the switching process employed. Most of the current research uses PLZT or PZT, which can be ferroelectric, antiferroelectric, or paraelectric depending on the stoichiometry. Switching is accomplished by the application of a fast-rising electric field to the ferroelectric material. The most commonly used materials are PLZT 2/95/5 and PLZT 8/65/35 or PZT (with no lanthanum): in the former case, the applied field switches the material from the antiferroelectric state to the ferroelectric state, and in the latter cases, around a hysteresis loop. Results have been reported with ferroelectric cathodes where current densities of up to a few hundred amperes per square centimeter have been achieved, with pulse durations of several microseconds. With shorter duration pulses and PLZT cathodes, repetition rates of up to 2 MHz have been achieved. In this paper, we focus on the results reported in the literature, and include a brief account of the physical interpretation of the data. The possible use of ferroelectric sources for pulsed power applications is indicated.

Dielectrics for Field Effect Technology

AbstractThe availability of stable MOS gate systems and high density storage capacitors is an essential requirement for the development of the field effect storage technology. For standard MOSFET gates this role has been fulfilled by SiO2 grown by thermal oxidation of the Si substrate. Improved insight into the properties of the Si/Si02 system and perfection of its growth technology will secure its role in Si MOSFET memories for the future. For charge storage application the stability requirements are less demanding. However, here SiO2 is not able to provide sufficient capacity. In this case higher dielectric constant materials (Si3N4 or Ta2O5) have to take over. Particularly attractive appears the use of ferroelectrics. These dielectric materials not only offer a high dielectric constant, but also the perspective of providing non‐volatile storage in capacitor structures.

Fatigue-free ferroelectric capacitors with platinum electrodes

A SIGNIFICANT fraction of the computer memory industry is at present involved in the manufacture of non-volatile memory devices1—that is, devices which retain information when power is interrupted. For such applications (and also for volatile memories), the use of capacitors constructed from ferroelectric thin films has stimulated much interest1. In such structures, information is stored in the polarization state of the ferroelectric material itself, which should in principle lead to lower power requirements, faster access time and potentially lower cost1. But the use of ferroelectrics is not without problems; the memories constructed to date have generally suffered from poor retention of stored information and degradation of performance ('fatigue') with use1–3. Here we describe the preparation and characterization of thin-film capacitors using ferroelectric materials from a large family of layered perovskite oxides, exemplified by SrBi2Ta2O9, SrBi2NbTaO9 and SrBi4Ta4O15. The structural flexibility of these materials allows their properties to be tailored so that many of the problems associated with previous ferroelectric memories are avoided. In particular, our capacitors do not show significant fatigue after 1012 switching cycles, and they exhibit good retention characteristics and low leakage currents even with films less than 100 nm thick.

Fatigue mechanisms in thin film PZT memory materials

The recent evolution of high-quality thin films of the PZT family has brought closer the use of ferroelectrics in non-volatile memory applications. While the inherent properties of the films are more than acceptable for most memory applications, the reduction in switched charge under repeated ferroelectric switching remains a problem. This so-called fatigue leads to changes of phonon frequencies and Raman cross sections in PbZr0.48Ti0.52O3 (PZT). Simultaneously, a characteristic IR absorption band near 4000 cm-1 shifts towards higher energies. This absorption signal is discussed in terms of polaronic states related to carriers generated by oxygen loss from the samples. It will be shown that the fatigue in PZT can be related to an increase of the local lattice distortion and a formation of defects both caused by oxygen loss from the samples.

The use of ferroelectrics for acceleration of charged particles

Several mechanisms are described which lead to fast polarization changes in ferroelectric materials. The resulting surface charges may ultimately generate electric field spikes on the order of 1 GV/m. The fast response of ferroelectric materials to phase transitions under the influence of external electrical and/or mechanical stress allows the design of structures which emit and accelerate charged-particle beams. >

Ferroelectric materials for thermal IR detectors

The physics of pyroelectric infra red detectors as it applies to the selection of detector materials is briefly reviewed and the properties of a number of the more commonly used materials are compared. The use of ferroelectrics in the region of the Curie temperature as dielectric bolometers with an applied bias field is assessed and the properties of barium strontium titanate and lead magnesium niobate for use in this mode are reported and their potential compared with the conventional pyroelectrics.

Adsorptive properties of semiconducting thin NiO and TiO2 films combined with an oppositely polarized ferroelectric support

Semiconducting oxides of NiO and TiO2 have been deposited as 30–50 nm thin films on either the positive (+) or negative (–) polar LiNbO3 ferroelectric surface, and the polarization field effects of the support upon the adsorptive properties of the combined oxides have been studied. Thermal desorption spectra show that the activation energy for desorption, Ed, varies as a function of the direction of polarization depending on the ferroelectric support, the effects of which are reversed for p- and n-type semiconducting oxides; Ed for O2 desorption was 102 kJ mol–1 for NiO/(+) and 157 kJ mol–1 for NiO/(–), whereas Ed for H2 desorption was 51 kJ mol–1 for TiO2/(+) and 37 kJ mol–1 for TiO2/(–). When hydrogen was adsorbed on TiO2/LiNbO3 surfaces, the changes in the surface electrical conductivity were larger by a factor of 40 for the positive than for the negative support, which was opposite to the polarization-direction effects observed previously for O2 adsorption on NiO/LiNbO3 surfaces. The different adsorptive properties are accounted for in terms of the effects of band-bending induced in thin semiconducting oxides in contact with the polar ferroelectric surfaces, and it is suggested that the use of ferroelectrics as a support is effective in modifying the adsorptive properties of semiconducting oxide surfaces.

Current developments in piezoelectric applications of ferroelectrics

Abstract The major use of ferroelectric materials continues to be in piezoelectric devices. Furthermore virtually all piezoelectric applications, with the exception of the very large business in quartz filters and oscillators, involve ferroelectric materials, mostly in ceramic form. The use of ferroelectrics in piezoelectric devices is reviewed with emphasis on the newer developments, particularly those with business potential. Features of the ferroelectric material which are desirable for the application are discussed and areas of possible improvement noted. The largest application areas have remained essentially the same for the last few years, even though there has been considerable technical improvement. They are electric wave filters, fluid loaded transducers for sonar and ultrasonic cleaning, phonograph cartridges, force transducers and high voltage ignition devices, Of these only electric wave filter devices are discussed in detail here; the others are reviewed briefly. Among newer applications whi...