Showing papers on "Ferroelectricity published in 1996"

A ferroelectric transparent thin‐film transistor

Abstract: Operation is demonstrated of a field‐effect transistor made of transparant oxidic thin films, showing an intrinsic memory function due to the usage of a ferroelectric insulator. The device consists of a high mobility Sb‐doped n‐type SnO2 semiconductor layer, PbZr0.2Ti0.8O3 as a ferroelectric insulator, and SrRuO3 as a gate electrode, each layer prepared by pulsed laser deposition. The hysteresis behavior of the channel conductance is studied. Using gate voltage pulses of 100 μs duration and a pulse height of ±3 V, a change of a factor of two in the remnant conductance is achieved. The dependence of the conductance on the polarity of the gate pulse proves that the memory effect is driven by the ferroelectric polarization. The influence of charge trapping is also observed and discussed.

Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules

Abstract: The synthesis of a banana-shaped molecule is reported and it is found that the smectic phase which it forms is biaxial with the molecules packed in the best direction into a layer. Because of this characteristic packing, spontaneous polarization appears parallel to the layer and switches on reversal of an applied electric field. This is the first obvious example of ferroelectricity in an achiral smectic phase and is ascribed to the C2v symmetry of the molecular packing.

Phase transitions and glasslike behavior in Sr 1-x Ba x TiO 3

Abstract: A dielectric and ultrasonic study of phase transitions in ${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{TiO}}_{3}$ with x ranging from 0.0 to 1.0, is reported. Both ceramic samples and single crystals were studied. The full phase diagram is derived from dielectric measurements for ferroelectric phase transitions and from ultrasonic measurements for structural phase transition. The cubic-tetragonal ferroelectric phase transition which is of the first order in pure ${\mathrm{BaTiO}}_{3}$ (x=1.0) transforms to the second order phase transition at x close to 0.2. The transition temperature ${\mathit{T}}_{\mathit{c}}$ is a linear function of x at x values from 0.2 to 1.0. For x0.2 the transition temperature is proportional to (x-${\mathit{x}}_{\mathit{c}}$${)}^{1/2}$, where ${\mathit{x}}_{\mathit{c}}$=0.035. For x${\mathit{x}}_{\mathit{c}}$ a glasslike behavior is observed. The structural phase transition temperature ${\mathit{T}}_{\mathit{a}}$ goes down as compared to pure ${\mathrm{SrTiO}}_{3}$ when x increases and levels off near x=0.05. \textcopyright{} 1996 The American Physical Society.

The Rayleigh law in piezoelectric ceramics

Abstract: The direct longitudinal piezoelectric effect in lead zirconate titanate, barium titanate, bismuth titanate and strontium bismuth titanate ceramics was investigated with respect to the dependence on the amplitude of an alternating pressure. At low alternating pressure amplitudes, the behaviour of the piezoelectric charge and the piezoelectric coefficient may be explained in terms of the Rayleigh law originally discovered for magnetization and magnetic permeability in ferromagnetic materials. The charge versus pressure hysteresis loops measured for piezeoelectric ceramics may similarly be described as the Rayleigh loops. The results presented show that the Rayleigh law can be applied to irreversible displacement of several types of non- ferroelectric domain walls and imply universal validity of the Rayleigh law for displacement of ferromagnetic, ferroelastic and ferroelectric domain walls.

Inorganic ceramic/polymer ferroelectric composite electrets

Abstract: Ferroelectric composites are now an established alternative to conventional ferroelectric ceramic materials and to the more recently discovered ferroelectric polymers. These materials due to their unique blending of polymetric properties of mechanical flexibility, formability and low cost with high electro-active properties have been been suggested to be a viable alternative both in piezoelectric and pyroelectric transducer applications. This review is devoted to the piezoelectric and pyroelectric properties exhibited by these type of composites with a special reference to those made of ceramic particles embedded in a polymer matrix (i.e. 0-3 connectivity type composite). A review of models predicting the electro-active properties of 0-3 composites is presented together with a proposal for a new mixed connectivity cubes model to be applicable to the case of high ceramic loading and/or when the ceramic grain size incorporated in the polymer matrix is comparable to the thickness of the sample. A review of the experimental results of the piezo- and pyroelectric properties of various ferroelectric composite materials, reported by several workers, is also presented in this paper. Special reference is made to composites made from calcium modified lead titanate and poly(vinyldene fluoride-trifluorethylene) emphasizing their advantages in the poling process which is a critical phase in the process of obtaining successful electro-active 0-3 composite electrets.

Impedance Spectroscopy of Undoped BaTiO3 Ceramics

Abstract: The electrical properties of ceramic BaTiO3 were investigated by ac impedance spectroscopy over the ranges 25°-330°C and 0.03 Hz-1 MHz. Results are compared with those obtained from fixed-frequency measurements, at 1 kHz and 100 kHz. Fixed-frequency Curie-Weiss plots show deviations from linearity at temperatures well above tc. The ac measurements show that grain boundary impedances influence Curie-Weiss plots in two ways: at high temperatures, they increasingly dominate the fixed-frequency permittivities; at lower temperatures, closer to Tc, the high-frequency permittivity contains a contribution from grain boundary effects. Methods for extraction of bulk and grain boundary capacitances from permittivity and electric modulus complex plane plots are discussed. The importance of selecting the appropriate equivalent circuit to model the impedance response is stressed. A constriction impedance model for the grain boundary in BaTiO3 ceramics is proposed: the grain boundary capacitance is neither temperature-independent, nor shows Curie-Weiss behavior. The grain boundary is ferroelectric, similar to the grains, but its impedance is modified by either air gaps or high-impedance electrical inhomogeneity in the region of the necks between grains; the activation energy of the constriction grain boundary impedance differs from that of the bulk, suggesting differences in defect states or impurity levels.

Structural and dielectric studies of Pb(Mg1/3Nb2/3)O3–PbTiO3 ferroelectric solid solutions around the morphotropic boundary

Abstract: Solid solutions (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 with 0.20≤x≤0.40 were studied using impedance measurements and high resolution x‐ray powder diffraction. The dependence of the real and imaginary parts of the dielectric constant with temperature was investigated on unpoled and poled ceramics, showing several transition points. The temperature dependence of the unit‐cell parameters showed the onset of two successive spontaneous phase transitions: cubic→tetragonal →rhombohedral. A coexistence of both ferroelectric phases was evidenced in a large temperature and composition range. The dielectric and structural properties were interpreted in terms of relaxor→ferroelectric spontaneous phase transition. An intermediate behavior between a relaxor and a real ferroelectric was evidenced. A partial phase diagram was proposed including both structural and dielectric informations.

Optimization of energy storage density in ceramic capacitors

Abstract: A theoretical treatment, based on the Devonshire theory of ferroelectrics, is presented to describe the storage of electrostatic energy in ferroelectric and paraelectric materials at very high field strengths. In all cases, optimal energy density is achieved by using compositions with Curie temperatures well below the operating temperature. The theory is applied to barium - strontium titanate ceramics and optimal compositions are deduced for energy storage at given working fields. The theory is supported by experimental data showing energy densities up to 8 J at 100 kV .

Scanning nonlinear dielectric microscope

Abstract: This article describes a new scanning technique for imaging the state of spontaneous polarization of a ferroelectric material by measuring the microscopic point‐to‐point variation of its nonlinear dielectric constants. First, the theory for detecting polarization is described. Second, the technique for measuring the nonlinear dielectric response is described. Finally, using this new microscope, area scans are obtained of the polarization of poled lead zirconate titanate ceramics, a lithium niobate single crystal, and of piezoelectric thin films of the copolymer of vinylidene fluoride and trifluoroethylene.

Scanning force microscopy for the study of domain structure in ferroelectric thin films

Abstract: A piezoresponse technique based on scanning force microscopy (SFM) has been used for studying domain structure in ferroelectric thin films. Studies were performed on Pb(Zrx,Ti1−x)O3(PZT) thin films produced by a sol–gel method. The piezoresponse images of the PZT films were taken before and after inducing polarization in the films by applying a direct current voltage between the bottom electrode and the SFM tip. Polarization induced patterns were written with 20 V pulses and subsequently imaged by the SFM piezoresponse technique. The effect of the film structure on the imaging resolution of domains is discussed.

Formation and observation of 50 nm polarized domains in PbZr1−xTixO3 thin film using scanning probe microscope

Abstract: Nanometer size polarized domains were written in a PbZr1−xTixO3 (PZT) thin film using an atomic force microscope (AFM) and the relationship between the polarized domain and the grain of the film was investigated. The polarized domain was formed by applying a pulse voltage to the ferroelectric PZT thin film through a conductive AFM tip. The polarized domain structure was observed by imaging the piezoelectric‐induced surface vibration by an AFM with an ac voltage applied between the tip and the bottom electrode of a sample. The polarized domains with a diameter of 50 nm were written within a single grain.

Metalorganic chemical vapor deposition of ferroelectric SrBi2Ta2O9 thin films

Abstract: Ferroelectric layered‐oxides SrBi2Ta2O9 thin films were prepared on Pt coated Si wafers and single‐crystal sapphire by metalorganic chemical vapor deposition (MOCVD). The films were specular and crack‐free and showed complete crystallization at temperatures between 650 and 700 °C. Good ferroelectric properties were obtained for a 200 nm thick film with Pt electrodes: 2Pr and Ec were about 8.3 μC/cm2 and 60 kV/cm, respectively. The leakage currents were as low as 8×10−9 A/cm2 at 150 kV/cm. The films also showed fatigue‐free characteristics: no fatigue was observed up to 1.4×1010 switching cycles. These high quality MOCVD films make high‐intensity (≳1 Mbit) nonvolatile memory devices possible.

Analysis of the Dependence of Ferroelectric Properties of Strontium Bismuth Tantalate (SBT) Thin Films on the Composition and Process Temperature

Abstract: Ferroelectric properties, crystal structure and microstructures were examined for various Sr/Bi/Ta atomic ratio strontium bismuth tantalate (SBT) films prepared by metalorganic decomposition at 700 and 800° C. The 20% Sr-deficient and 10% Bi-excess (0.8/2.2/2) composition showed maximum remanent polarization (P r) values for both 700 and 800° C crystallization temperatures. From TEM analysis, the P r dependence on composition variation around the stoichiometric 1/2/2 composition was related to grain size and volume of voids. The effect of postannealing after Pt top electrode fabrication was also studied. On the films prepared at 800° C, postannealing markedly reduced the capacitor shorting rate. This was attributed to recrystallization of the Pt top layer, based on SEM analysis of the Pt layer and the Pt/SBT interface.

Defect-dipole alignment and tetragonal strain in ferroelectrics

Abstract: We show the alignment of defect dipoles along the direction of the spontaneous polarization in polycrystalline Pb(Zr,Ti)O3 and BaTiO3 ferroelectric ceramics using electron paramagnetic resonance (EPR). The alignment is demonstrated via orientation dependent paramagnetic centers in the polycrystalline materials and computer modeling of the EPR line shapes. It is shown that defect dipoles can become aligned by oxygen vacancy motion in the octahedron about a negatively charged center for the oxygen vacancy‐related dipole complexes or by defect displacement and domain realignment in the lattice for isolated defect centers. We find that the alignment is not observed in nonferroelectric materials such as SrTiO3, and is destroyed in ferroelectric materials by heating above the Curie temperature. These observations suggest an interplay between distortion in the unit cell and the ability to align defect dipoles, as is the case more generally for ferroelectric dipole alignment. We also directly observe aligned intr...

Qualitative model for the fatigue‐free behavior of SrBi2Ta2O9

Abstract: SrBi2Ta2O9 (SBT) thin films are known to exhibit no polarization fatigue with electric field cycling. However, we have discovered that optical illumination combined with a bias voltage near the switching threshold can result in significant (≳90%) suppression of the switchable polarization of SBT thin film capacitors. A similar effect has also been reported for Pb(ZrxTi1−x)O3 (PZT) capacitors. However, it is found that electric field cycling of the optically fatigued SBT capacitors results in near‐complete recovery of the suppressed polarization. In contrast, electric field cycling of optically fatigued PZT capacitors does not result in any polarization recovery. These results suggest that optical fatigue in both SBT and PZT capacitors results from pinning of domain walls due to trapping of the photogenerated carriers at domain boundaries, whereas the recovery exhibited by SBT thin films indicates that the domain walls are more weakly pinned in SBT than in PZT thin films. Consequently, the fatigue‐free beh...

Multilayered ferroelectric composite waveguides

Abstract: A multi-layer Ferroelectric composite wave guide in which the effective dielectric constant of the waveguide can be reduced while maintaining tunability. The waveguide is constructed of high and low dielectric constant layers. The multi-layer waveguide is comprised of bias plates which are perpendicular to the laminate direction on direction of film deposition to maintain tunability in the structure.

Domain inversion in ferroelectric MgO:LiNbO3 by applying electric fields

Abstract: An antiparallel ferroelectric domain in LiNbO3 doped with 5 mol % MgO (MgO:LN) was inverted at room temperature by applying a step‐like electric field. The lowest electric field was about one fifth of that for undoped LN, but a high field compared to that for BaTiO3 was found to be necessary from the analysis of the switching current. The long switching time (about 102 s under an electric field of 4.45 kV/mm) enabled us to observe the domain growth process in MgO:LN. The needle‐shaped nuclei grew into larger domains by nucleation on the existing domain wall, and the wall of the coalesced domain moved forward slowly. The process was similar to the domain reversal model proposed for BaTiO3 under low electric fields.

First-principles investigation of 180° domain walls in BaTi O 3

Abstract: We present a first-principles study of 180\ifmmode^\circ\else\textdegree\fi{} ferroelectric domain walls in tetragonal barium titanate. The theory is based on an effective Hamiltonian that has previously been determined from first-principles ultrasoft-pseudopotential calculations. Statistical properties are investigated using Monte Carlo simulations. We compute the domain-wall energy, free energy, and thickness, analyze the behavior of the ferroelectric order parameter in the interior of the domain wall, and study its spatial fluctuations. An abrupt reversal of the polarization is found, unlike the gradual rotation typical of the ferromagnetic case.

Dielectric Activity and Ferroelectricity in Piezoelectric Semiconductor Li-Doped ZnO

Abstract: Temperature dependences of dielectric constants, specific heat and D–E hysteresis loops of Li-doped zinc oxide ceramics were investigated. A new dielectric anomaly was found at 330 K in Zn1-xLixO with x=0.17. A cusp-like anomaly was also found in specific heat. A ferroelectric D–E hysteresis loop was successfully observed for the first time. These observations suggest that replacement of host Zn ions by small Li ions induces a ferroelectric phase in the wurtzite-type ZnO piezoelectric semiconductor. This material is a candidate for ferroelectric thin films in integrated ferroelectric devices.

Dielectric properties and phase transitions of Ba(Ti1-xSnx)O3 solid solution

Abstract: In Ba(Ti1-x Snx )O3 solid solution, for Sn concentration x≤0.1, the permittivity follows the Curie-Weiss law above the Curie point T c. The increase in the peak value of the permittivity at T c is due to the change in ferroelectric transition from first order to second order with increasing x. For x>0.1, some deviations from the Curie-Weiss law are observed in a narrow temperature region above T c. The diffuse phase transition was observed. For x>0.2, the Curie-Weiss law does not hold over a wide temperature range. The dielectric dispersion was observed in the radio-frequency range (10 kHz–10 MHz). With increasing Sn concentration, the change in the phase transition from a normal ferroelectric transition to relaxor through the diffuse phase transition was observed.

Size-related ferroelectric-domain-structure transition in a polycrystalline PbTiO 3 thin film

Abstract: We studied the size dependence of the ferroelectric domain structure in unique free-standing ${\mathrm{PbTiO}}_{3}$ thin film, composed of grains 60--1000 nm in size, with transmission-electron microscopy. With such samples, we showed that the apparent dependence of electrical properties on the thickness of polycrystalline thin films stems from their grain-size dependence. We found that a domain-structure transition, from predominantly multidomain to predominantly single domain occurs at a grain size of \ensuremath{\sim}150 nm (corresponding to a thickness 200--300 nm). Based on these experimental results, the drastic change of coercive field and dielectric permittivity below a thickness 200--300 nm was reasonably explained as resulting from a domain-structure transition. \textcopyright{} 1996 The American Physical Society.

Electronically graded multilayer ferroelectric composites

Abstract: A laminated ceramic ferroelectric material has adjacent layers of Barium ontium Titanate (BSTO)--oxide composite stacked in order of descending oxide content and sized in thickness to produce a generally equal capacitance across each layer, resulting in a material having a graded dielectric constant for use in phased array antenna, for example. The oxides are from the metallic oxide group consisting of alumina, zirconia and magnesia and the layers are produced by tape casting.

The Role of Processing Variables in the Flux Growth of Lead Zinc Niobate-Lead Titanate Relaxor Ferroelectric Single Crystals

Abstract: Relaxor ferroelectric single crystals of Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) are of interest as high performance transducers due to their very large piezoelectric coupling and dielectric properties. A high temperature flux solution method was used to grow (1- x)PZN-( x)PT, where x= 0.0, 0.1 and 0.15 single crystals. Processing conditions were optimized to increase the size and yield of the perovskite crystals, including variation of the flux to composition ratio, cooling rate, soak time and soak temperature. The crystals varied in size from 0.01 cm to 1.5 cm on an edge, and in color from opaque to brown due to the changes in processing conditions. The crystals were characterized by XRD, dielectric constant and dielectric loss measurements. As the content of PT increased the transition from the paraelectric to the ferroelectric phase approached first order behavior and the crystal structure transformed from rhombohedral to tetragonal. This structure transition caused the lattice constant along the c-axis to elongate as the c/a ratio increased. At room temperature, the dielectric constants for the PZN-PT compositions along the [111] or [001] axes were as great as 5000 and the dielectric losses were as low as 0.01.

Ceramic ferroelectric composite material - BSTO-ZrO2 +B

Abstract: A novel ceramic ferroelectric material having a low dielectric constant, low loss and high tunability. The material is a composite comprising Barium Strontium Titanate (BSTO) and a ceramic material having a low dielectric constant. The preferred composite is represented by Ba1-xSrxTiO3-ZrO2, wherein x is greater than 0.00, but less than or equal to 0.70, and wherein the percent weight ratio between Ba1-xSrxTiO3 and ZrO2 ranges from approximately 99%-40% and 1%-60%, respectively. The novel materials possess superior electronic properties; and they may be employed in various antenna systems.

Fabrication and Electrical Properties of Lead Zirconate Titanate Thick Films

Abstract: Thick films of lead zirconate titanate of the morphotropic phase boundary composition, Pb(Zr0.52Ti0.48)O3, have been fabricated on platinum-buffered silicon using a modified sol–gel spin-coating technique. Crack-free films of 12-μm thickness can be uniformly deposited on 3-in.-diameter wafers with high yield and properties comparable to those of bulk ceramics. The thickness dependence of film structure and the dielectric, ferroelectric, and piezoelectric properties have been characterized over the thickness range of 1–12 μm. A strong (100) texture develops as film thickness increases above 5 μm; the films were marked by saturation values of longitudinal piezoelectric coefficient d33, 340 pC/N; remanent polarization, 27 μC/cm2; and dielectric permittivity, 1450. PZT films in this thickness range are extremely well-suited to application as electromechanical transduction media in silicon-based microelectromechanical systems (MEMS).

Imprint in Ferroelectric Capacitors.

Abstract: We show that voltage offsets in the polarization-voltage characteristics of Pb(Zr, Ti)O3 capacitors can lead to imprint in ferroelectric memory devices. The thermal-induced voltage shifts (internal bias field) are in part attributed to the role of oxygen vacancy-related defect dipoles throughout the film. In support of this, it is found that donor doping at the Ti(Zr) sites reduces the thermally-induced voltage shifts. The stress-induced voltage shifts are found to be dependent on the Zr/Ti cation ratio. This compositional dependence is explained by considering the role of deep bulk Ti3+ centers and/or a compositional dependent oxygen vacancy density.

A random field theory based model for ferroelectric relaxors

Abstract: A model for ferroelectric relaxors such as PMN, PSN and PLZT giving a quantitative description of their properties and phase diagrams is proposed within the framework of the random field theory. In this model, the relaxors are considered as systems with random sites and orientations of electric dipoles, lattice vacancies, antisite ions and other defects as well as impurities embedded into the paraelectric phase, which is proposed to be the `host' lattice for these materials. The calculations of the temperature which corresponds to the transition from the paraelectric to the ferroelectric phase is carried out as a function of the concentration of lattice defects (point charges and dilatational centres). On the basis of these calculations, the peculiarities of the ferroelectric relaxor phase diagram are discussed. The main features of the phase transition sequence when decreasing the temperature in relaxors with constant dipole and defect concentrations are described. The Cross superparaelectric model and Burns temperature have been shown to appear in a natural way in the proposed model. A comparison between calculated and experimental data has been made for the model ferroelectric relaxor PLZT x/65/35. Fairly good agreements between calculated and measured and critical concentrations of lanthanum have been obtained from the model.

Method of making ferrolectric thin film composites

Abstract: A method of making ferroelectric thin film composites includes the step ofroviding a barium strontium titanate material with an additive magnesia-based and forming a thin film ferroelectric composite using pulsed laser deposition. The method forms a thin film composite having enhanced electronic properties.