应力对铁电薄膜电滞回线及蝶形曲线的影响

应用与时间有关的Ginzburg- Landau方程(time dependent Ginzburg-Landau,简称TDGL方程),在考虑表面效应的条件下分析了应力对外延铁电薄膜铁电性能的影响.计算结果显示剩余极化强度值随着压应力的增加而增加,随着张应力的增加而减小.场致应变值随着压应力的增加而增加,随着张应力的增加而减小.这种变化趋势与实验结果是一致的.考虑表面效应计算得到的剩余极化强度值小于不考虑表面效应时计算得到的数值(当外推长度＞0时).     

Preparation of ferroelectric BaNb2O6 by the urea method

A precipitate was formed when an aqueous solution of BaCl₂, NbF₅ and urea in required ratio (the ratio of total metal cations to the urea is five) is heated on a water bath at 100 °C. This precipitate on decomposition at 600 °C yielded the nanocrystallites of BaNb₂O₆ (BN), as confirmed by X-ray diffraction study (XRD). This is a much lower temperature as compared to that prepared by traditional solid state method (1000° C) as reported for the formation of BaNb₂O₆. Transmission electron microscope (TEM) investigations revealed that the average particle size is 85 nm for the calcined powders. The room temperature dielectric constant at 1 kHz is found to be 900. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Coprecipitation method for the preparation of nanocrystalline ferroelectric CaBi2Ta2O9

A simple coprecipitation technique had been successfully applied for the preparation of pure ultrafine single-phase CaBi₂Ta₂O₉ (CBT). Ammonium hydroxide and ammonium oxalate were used to precipitate Ca²⁺, Bi³⁺ and Ta⁵⁺ cations simultaneously. No pyrochlore phase was found while heating powder at 800 °C and pure CaBi₂Ta₂O₉ phase was found to be formed by XRD. Particle size and morphology was studied by transmission electron microscopy (TEM). The room temperature dielectric constant at 1 kHz is 100. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Co-precipitation technique for the preparation of nanocrystalline ferroelectric SrBi2Ta2O9

A simple co-precipitation technique had been successfully applied for the preparation of pure ultrafine single phase SrBi₂Ta₂O₉. Ammonium hydroxide was used to precipitate Sr²⁺, Bi³⁺ and Ta⁵⁺ cations simultaneously. No pyrochlore phase was found while heating powder at 800 °C and pure SrBi₂Ta₂O₉ phase was found to be formed by X-ray diffraction. Particle size and morphology was studied by scanning electron spectroscopy. Ferroelectric hysteresis loop parameters of these samples were also studied.     

Reflection of light by domain walls in the uniaxial ferroelectric Sn2P2S6

The occurrence of light scattering during light transmission is described for some orientations of an incident laser beam in polydomain crystals of the uniaxial ferroelectric tin thiohypodiphosphate. This scattering is probably caused by the presence of a reflecting layer near the 180° domain walls. A reflecting layer may be formed as a result of the appearance of charged nonparallel domain walls. Pis’ma Zh. Tekh. Fiz. 23, 45–50 (August 26, 1997)     

Characterization of coprecipitated ferroelectric Bi2 VO5·5

Fine powders of orthorhombic bismuth vanadate (Bi₂ VO_5·5) have been synthesized by coprecipitation method. Powder X-ray diffraction and electron microscopic techniques have been used to characterize these samples. The formation of the monophasic Bi₂ VO_5·5 was confirmed. The compacted powders sintered at 1070K have been characterized for their dielectric properties as a function of both temperature (300–900 K) and frequency (100 Hz–10 MHz) and found to be superior to those obtained by the conventional solid-state reaction route.     

Enhancement of ferroelectric and piezoelectric characteristics in europium substituted SrBi2Ta2O9 ferroelectric ceramics

Europium substituted samples of compositions Sr_1 − xEu_xBi₂Ta₂O₉ were synthesized by solid-state reaction method. The prepared samples were characterized for their structural and electrical properties. X-ray analysis confirms the formation of the single-phase layered perovskite structure. The microstructural studies reveal that the average grain size increases with increase in Eu content. An increase in remanent polarization and d₃₃ values with increasing concentration of europium has been observed. The maximum 2P_r ~ 14 μC/cm² is observed in the sample with x = 0.20. The observed results have been discussed in terms of contribution from the cation vacancies introduced into the lattice structure due to donor doping.     

Evolution of domains and thermal hysteresis effects in the pyrochlore-structure ferroelectric Cd2Nb2O7

An analysis of the thermally induced domain behavior in Cd₂Nb₂O₇ under cooling and heating in the dark has revealed that below T _c the coexistence of ferroelectric-ferroelastic domains of different types is accompanied by irreversible changes in the domains of the previous phase (T _c <T<T _s ). Interaction of domains with dislocations and other lattice defects causes thermal hysteresis in the phase transition region near T _s and T _c typical of the “superheated” ferroic state. Pis’ma Zh. Tekh. Fiz. 23, 64–69 (December 26, 1997)     

Normal ferroelectric to ferroelectric relaxor conversion in fluorinated polymers and the relaxor dynamics

To elucidate the molecular origin of the polarization dynamics in the ferroelectric relaxor poly(vinylidene fluoride—trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer, a broadband dielectric study was carried out in the frequency range from 0.01 Hz to 10 MHz and temperatures from −150°C to 120°C for the terpolymer and a normal ferroelectric P(VDF-TrFE) copolymer. The relaxation processes were also studied using dynamic mechanical analysis. It was shown that in the terpolymer, which was completely converted to a ferroelectric relaxor, there is no sign of the relaxation process associated with the ferroelectric-paraelectric transition which occurs in the P(VDF-TrFE) copolymer. In the copolymer, three additional relaxation processes have been observed. It was found that the relaxation process β_a, which was commonly believed to be associated with the glass transition in the amorphous phase, in fact, contains significant contribution from chain segment motions such as domain boundary motions in the crystalline region. In the temperature range studied, the terpolymer exhibits the latter three relaxation processes with the one (termed β_r) near the temperature range of β_a significantly enhanced. This is consistent with the observation that in conversion from the normal ferroelectric to a ferroelectric relaxor, the macro-polar domains are replaced by nano-polar-clusters and the boundary motions as well as the reorientation of these nano-clusters generate the high dielectric response. The experimental data also reveal a broad relaxation time distribution related for the β_r process whose distribution width increases with reduced temperature, reflecting the molecular level heterogeneity in the crystalline phase due to the random introduction of the CFE monomer in the otherwise ordered macro-polar domains. The random interaction among the nano-clusters as well as the presence of the random fields produces ferroelectric relaxor behavior in the terpolymer.     

Device modeling of ferroelectric memory field-effect transistor for the application of ferroelectric random access memory

An improved theoretical analysis on the electrical characteristics of ferroelectric memory field-effect transistor (FeMFET) is given. First, we propose a new analytical expression for the polarization versus electric field (P-E) for the ferroelectric material. It is determined by one parameter and explicitly includes both the saturated and nonsaturated hysteresis loops. Using this expression, we then examine the operational properties for two practical devices such as the metal-ferroelectric-insulator-semiconductor field-effect transistor (MFIS-FET) and metal-ferroelectric-metal-insulator-semiconductor field-effect transistor (MFMIS-FET) as well. A double integral also has been used, in order to include the possible effects due to the nonuniform field and charge distribution along the channel of the device, to calculate the drain current of FeMFET. By using the relevant material parameters close to the (Bi, La)/sub 4/Ti/sub 3/O/sub 12/ (BIT) system, accurate analyses on the capacitors and FeMFET's at various applied biases are made. We also address the issues of depolarization field and retention time about such a device.     

Grain size effects on the dielectric properties of ferroelectric Bi2VO5.5 ceramics

Dielectric properties and microstructural characteristics of ferroelectric bismuth vanadate (Bi₂VO_5.5) ceramics exhibiting grain sizes of 7, 10, 20 and 25 m have been studied. Microstructural studies indicate the presence of ferroelectric 90° domain patterns on the surface as well as in the bulk of the coarse-grained ceramics. The dielectric constant and the loss tangent both at room temperature and in the vicinity of the Curie temperature have been found to increase with increasing grain size. The Curie temperature (725 K) is found to shift slightly (by about 7 K) towards higher temperatures as the grain size increases (7–25 m). The magnitude of the dielectric anomaly around 725 K is found to be higher for coarse-grained ceramics. The dielectric constant and the loss have been found to decrease with increase in frequency (1–100 kHz) for all the ceramics studied. The increase in dielectric constant with increasing grain size is attributed to a decrease in thickness of the relatively more insulating grain boundary layer.     

Electron emission from ferroelectric thin films enhanced by the presence of 90 degree ferroelectric domains

In this work, a ferroelectric domain-enhanced electron emission mechanism is proposed. The polarization distribution near 90 degrees domain walls is calculated by solving a set of second order differential equations, including the Poisson's one and equations derived from an expansion of the free energy Phi(P) in power series of the polarization according to the Devonshire-Landau-Ginzburg theory. Domain walls intersecting the emitting surface cause sufficient electric fields and lower the potential barrier for electron emission. This induces centers of enhanced electron emission. Relaxing domain walls were found to excite trapped excess electrons in front of the wall.     

Numerical analysis of ferroelectric/ferroelastic domain switching in ferroelectric ceramics

A numerical approach predicting the behavior of ferroelectric ceramics under electric field and mechanical loading is proposed in this paper. In the model, macroscopic properties of ferroelectric ceramics are determined by microscopic structures. Ferroelectric ceramics are seen to be composed of many domains with different orientations, and domain switching is the source of the nonlinear constitutive behavior of the ferroelectric ceramics. Numerical calculations based on the model were carried out, and the computational results are compared with the experimental results, which shows the two sets of results consist with each other. The calculation approach can provide a guidance for the ceramics component design.     

Influence of accidental impurities and defects on the low-frequency dielectric relaxation in the ferroelectric Cd2Nb2O7

It is shown that accidental impurities and structural defects (cation vacancies) formed in the (CdO₈) ⁿ⁻² sublattice during synthesis enhance the relaxation properties of the system near T _Curie and suppress the ferroelectric phase transition. Pis’ma Zh. Tekh. Fiz. 24, 36–44 (September 12, 1998)     

Preparation of nanocrystalline ferroelectric BaNb2O6 by citrate gel method

A gel was formed when a aqueous solution of BaCl₂, NbF₅ and citric acid in stoichiometric ratio is heated on a water bath. This gel on decomposition at 600°C yielded the nano crystallites of BaNb₂O₆, as confirmed by X-ray diffraction study (XRD). This is a much lower temperature as compared to that prepared by traditional solid state method (1000°C) as reported for the formation of BaNb₂O₆. Transmission electron microscopic (TEM) investigations revealed that the average particle size is 50 nm for the calcined powders. The room temperature dielectric constant at 1 kHz is found to be 1000. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Lattice dynamics of layered ferroelectric semiconductor compound TlGaSe2

We present the first-principles calculation of the lattice dynamics of the TlGaSe₂ ternary semiconductor having highly anisotropic crystal structure. Calculations have been performed using open-source code ABINIT on the basis of the density functional perturbation theory within the plane-wave pseudopotential approach. Results on the frequencies of phonon modes in the centre of Brilloin zone and the dispersion of transverse shear acoustic branch of the phonon spectra agree well with the experimental data on Raman scattering, infrared reflectivity and ultrasound wave propagation in TlGaSe₂. The calculated and experimental temperature dependencies of heat capacity are in a good agreement up to the room temperature. Along the layer, the low-frequency acoustic branch displays the bending wave behavior which is characteristic of the layer crystal structures.     

Nanoscale ferroelectric and piezoelectric properties of Sb2S3 nanowire arrays

We report the first observation of piezoelectricity and ferroelectricity in individual Sb(2)S(3) nanowires embedded in anodic alumina templates. Switching spectroscopy-piezoresponse force microscopy (SS-PFM) measurements demonstrate that individual, c-axis-oriented Sb(2)S(3) nanowires exhibit ferroelectric as well as piezoelectric switching behavior. Sb(2)S(3) nanowires with nominal diameters of 200 and 100 nm showed d(33(eff)) values around 2 pm V(-1), while the piezo coefficient obtained for 50 nm diameter nanowires was relatively low at around 0.8 pm V(-1). A spontaneous polarization (P(s)) of approximately 1.8 μC cm(-2) was observed in the 200 and 100 nm Sb(2)S(3) nanowires, which is a 100% enhancement when compared to bulk Sb(2)S(3) and is probably due to the defect-free, single-crystalline nature of the nanowires synthesized. The 180° ferroelectric monodomains observed in Sb(2)S(3) nanowires were due to uniform polarization alignment along the polar c-axis.     

Science and technology of ferroelectric films and heterostructures for non-volatile ferroelectric memories

We present in this article a review of the status of thin film ferroelectric materials for nonvolatile memories. Key materials issues relevant to the integration of these materials on Si wafers are discussed. The effect of film microstructure and electrode defect chemistry on the ferroelectric properties relevant to a high density nonvolatile memory technology are discussed. The second part of this review focuses on approaches to integrate these capacitor structures on a filled poly-Si plug which is a critical requirement for a high density memory technology. Finally, the use of novel surface probes to study and understand broadband polarization dynamics in ferroelectric thin films is also presented.     

Magnetic ferroelectric materials

In the form of a succinct overview the structure and symmetry requirements of magnetic ferroelectrics are discussed. Boracites are the best-studied examples and have phases being simultaneously ferroelectric, ferromagnetic and ferroelastic. One of the salient features of such materials is the obligatory occurrence of the linear and bilinear magnetoelectric effects. They represent an invaluable auxiliary information for magnetic symmetry determination by neutron diffraction. Owing to the complexity of property combinations, work with single crystals and polarized light microscopy is obligatory. Key references of the field are given.