Showing papers on "Ferroelectricity published in 1997"

Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals

Abstract: The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3 were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3, morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33’s)>2500 pC/N and subsequent strain levels up to >0.6% with minimal hysteresis were observed. Crystallographically, high strains are achieved for 〈001〉 oriented rhombohedral crystals, although 〈111〉 is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to an E-field induced phase transformation. High electromechanical coupling (k33)>90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.

Ferroelectric Field Effect Transistor Based on Epitaxial Perovskite Heterostructures

Abstract: Ferroelectric field effect devices offer the possibility of nonvolatile active memory elements. Doped rare-earth manganates, which are usually associated with colossal magnetoresistive properties, have been used as the semiconductor channel material of a prototypical epitaxial field effect device. The carrier concentration of the semiconductor channel can be "tuned" by varying the manganate stochiometry. A device with La0.7Ca0.3MnO3 as the semiconductor and PbZr0.2Ti0.8O3 as the ferroelectric gate exhibited a modulation in channel conductance of at least a factor of 3 and a retention loss of 3 percent after 45 minutes without power.

Stress and frequency dependence of the direct piezoelectric effect in ferroelectric ceramics

Abstract: It is shown that at weak alternating stress the relationship between the piezoelectrically induced charge and applied stress in ferroelectric ceramics has the same form as the Rayleigh law for magnetization versus magnetic field in ferromagnetic materials. Applicability of the Rayleigh law to the piezoelectric effect is demonstrated in detail for lead zirconate titanate (PZT) ceramics. Experimental results indicate that the dominant mechanism responsible for piezoelectric hysteresis and the dependence of the piezoelectric coefficient on the applied ac stress is the pinning of non-180 degrees domain walls. The dependence of the piezoelectric coefficient on the frequency of the driving stress is examined and is shown to be due to the frequency dispersion of both reversible and irreversible components of domain-wall displacement. Analysis of the stress dependence of the piezoelectric phase angle reveals piezoelectric hysteresis contributions that are not necessarily due to Rayleigh-type displacement of domain walls. Piezoelectric properties of a modified lead titanate composition that exhibits non-Rayleigh type behavior are examined and compared with the properties of PZT ceramics. (C) 1997 American Institute of Physics.

The dielectric response as a function of temperature and film thickness of fiber-textured (Ba,Sr)TiO3 thin films grown by chemical vapor deposition

Abstract: The temperature- and field-dependent permittivities of fiber-textured Ba0.7Sr0.3TiO3 thin films grown by liquid-source metalorganic chemical vapor deposition were investigated as a function of film thickness. These films display a nonlinear dielectric response under conditions representative of those encountered in dynamic random access memories or other integrated capacitor applications. This behavior has the exact form expected for a classical nonlinear, nonhysteretic dielectric, as described in terms of a power series expansion of the free energy in the polarization as in the Landau–Ginzburg–Devonshire approach. Curie–Weiss-like behavior is exhibited above the bulk Curie point (∼300 K), although the ferroelectric phase transition appears frustrated. Small-signal capacitance measurements of films with different thicknesses (24–160 nm) indicate that only the first term in the power series expansion varies significantly with film thickness or temperature. Possible origins for this thickness dependence are...

Relaxor based ferroelectric single crystals for electro-mechanical actuators

Abstract: The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3−PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN-PT) were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3 (PZTs), morphotropic phase boundary (MPB) compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33’s ) >2200 pC/N and subsequent strain levels up to >0.5% with minimal hysteresis were observed. Crystallographically, high strains are achieved for oriented rhombohedral crystals, though is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics could be achieved, possibly being related to an E-field induced phase transformation. High electromechanical coupling (k33) >90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.

Intrinsic dead layer effect and the performance of ferroelectric thin film capacitors

Abstract: We apply the Thomas theory of ferroelectricity to bulk and thin film perovskite ferroelectrics in the paraelectric regime above the transition temperature. From available data on bulk SrTiO3 we are able to fully determine the parameters in the Thomas theory for this material, with overall reasonable results, supporting its validity. In a new application of the Thomas theory to the surface of a thin ferroelectric film in the linear response regime, it is found that there is anticipated to be an intrinsic “dead layer effect” on the surface of a dielectric film which significantly reduces the effective dielectric constant observed in capacitor applications. Two predictions of the theory are verified from recent experimental data. An experiment is suggested to distinguish between linear and nonlinear surface effects.

Characterization of thick lead zirconate titanate films fabricated using a new sol gel based process

Abstract: Lead zirconate titanate (PZT) films 60 μm in thickness have been fabricated using a new sol gel based process. PZT powders are dispersed in a sol gel matrix to form a 0–3 ceramic/ceramic composite. The dielectric properties of these films have been studied as a function of powder concentration, frequency, and temperature. The characteristic Curie point is observed at 420 °C. The ferroelectric behavior measured in terms of the remanant polarization (Pr=35 μC/cm2) and coercive field (Ec=20 kV/cm) was an improvement over values quoted for thin PZT films but lower than that of bulk ceramic. The piezoelectric properties d33 (325 pC/N) and d31 (−80 pC/N) were comparable with those of the bulk ceramic.

Local, nonvolatile electronic writing of epitaxial Pb(Zr0.52Ti0.48)O3/SrRuO3 heterostructures

Abstract: A scanning probe microscope was used to induce local, nonvolatile field effects in epitaxial, ferroelectric Pb(Zr0.52Ti0.48)O3/SrRuO3heterostructures. Field-effected regions with linewidths as small as 3500 angstroms were written by locally switching the polarization field of the Pb(Zr0.52Ti0.48)O3layer; the electronic density of the underlying metallic SrRuO3 layer was modified and the sheet resistance was changed by up to 300 ohms per square. This procedure is completely reversible and allows submicrometer electronic features to be written directly in two dimensions, with no external electrical contacts or lithographic steps required.

Ab initio study of BaTiO 3 surfaces

Abstract: We have carried out first-principles total-energy calculations of (001) surfaces of the tetragonal and cubic phases of BaTiO${}_{3}$. Both BaO-terminated (type I) and TiO${}_{2}$-terminated (type II) surfaces are considered, and the atomic configurations have been fully relaxed. We found no deep-gap surface states for any of the surfaces, in agreement with previous theoretical studies. However, the gap is reduced for the type-II surface, especially in the cubic phase. The surface relaxation energies are found to be substantial, i.e., many times larger than the bulk ferroelectric well depth. Nevertheless, the influence of the surface upon the ferroelectric order parameter is modest; we find only a small enhancement of the ferroelectricity near the surface.

Piezoelectric properties of c-axis oriented pb(zr,ti)o3 thin films

Abstract: Piezoelectric properties of the c-axis oriented Pb(Zr,Ti)O3 (PZT) thin films were investigated. The PZT films with a composition near the morphotropic phase boundary were epitaxially grown on (100)Pt-coated MgO substrates by rf-magnetron sputtering. The PZT films exhibited excellent ferroelectricity with a remanent polarization more than 50 μC/cm2. In order to examine intrinsic piezoelectric properties, cantilever structures were microfabricated with the PZT films. The piezoelectric coefficient d31 of PZT films, which were not subjected to poling treatments, was measured directly from the transverse expansion of the cantilever beams. The measurements revealed that the PZT films were naturally polarized and had a relatively large piezoelectric coefficient d31 of 100×10−12 m/V without poling.

Evidence of domain wall contribution to the dielectric permittivity in PZT thin films at sub-switching fields

Abstract: Through the use of relations analogous to that of the Rayleigh law, it is demonstrated that the ac electric field dependence of the permittivity of ferroelectric thin films can be described. It is further shown that both reversible and irreversible components of the permittivity decrease linearly with the logarithm of the frequency of the ac field. The results demonstrate that the models describing the interaction of domain walls and randomly distributed pinning centers in magnetic materials can be extended to the displacement of domain walls in ferroelectric thin films.

Planar microwave integrated phase-shifter design with high purity ferroelectric material

Abstract: Ferroelectric materials (FEM's) are very attractive because their dielectric constant can be modulated under the effect of an externally applied electric field perpendicular to the direction of propagation of a microwave signal. FEM may be particularly useful for the development of a new family of planar phase shifters which operate up to X-band. The use of FEM in the microwave frequency range has been limited in the past due to the high losses of these materials; tan /spl delta/=0.3 at 3 GHz is typical for commercial BaTiO/sub 3/ (BTO) and due to the high electric field necessary to bias the structure in order to obtain substantial dielectric constant change. In this paper, a significant reduction in material losses is demonstrated. This is achieved by using a new sol-gel technique to produce barium modified strontium titanium oxide [Ba/sub 1-x/Sr/sub x/TiO/sub 3/ (BST)], which has ferroelectric properties at room temperature. Also demonstrated is how the use of thin ceramics reduces the required bias voltage below 250 V, with almost no power consumption required to induce a change in the dielectric constant. A phase shift of 165/spl deg/ was obtained at 2.4 GHz, with an insertion loss below 3 dB by using a bias voltage of 250 V. Due to the planar geometry and light weight of the device, it can be fully integrated in planar microwave structures.

Lead-free piezoelectric ceramics based on (Bi1/2Na1/2)TiO3-NaNbO3

Abstract: Dielectric, ferroelectric and piezoelectric pŕoperties of a BNT-based solid solution, (1-x)(Bi1/2Na1/2)TiO3-xNaNbO3, that is, Bi(1-x)/2Na(1+x)/2 Ti1-x Nb x O3 (BNTN-100x), are studied from the viewpoint of a new group of lead-free piezoelectric ceramics. The conventional ceramic fabricating technique was used to prepare BNTN ceramics. D-E hysteresis loops could be easily observed with a relatively large remanent polarization, P r, of about 25∼33 μC/cm2 and a high coercive field of about 50 kV/cm near the composition of BNT side. The electromechanical coupling factors, k p, k t, and k 33 were about 0.108, 0.424 and 0.433 for BNTN-3. BNTN ceramics seem to be a candidate for lead-free piezoelectric ceramics.

Cation disorder in ferroelectric Aurivillius phases of the typeBi2ANb2O9(A=Ba, Sr, Ca)

Abstract: The structures of the ferroelectric two-layer Aurivillius phases Bi 2 ANb 2 O 9 (A=Ba, Sr, Ca) have been refined using a combination of X-ray and neutron powder diffraction data. Bi 2 BaNb 2 O 9 is not significantly distorted from idealised symmetry and has been refined in tetragonal space group I4/mmm, a=3.9362(1) and c=25.6582(7) A. The Sr and Ca compounds have been refined in orthorhombic space group A2 1 am, with a=5.5193(3), b=5.5148(3), c=25.0857(6) A and a=5.4833(1), b=5.4423(1), c=24.8984(6) A, respectively. The orthorhombic distortion increases with decreasing A 2+ cation size and originates from bonding requirements at the perovskite A site, in agreement with previous work. However, in contrast to earlier work, we find a partial mixing of Bi and A cations on their respective sites, which increases in the order Ca

Temperature dependence of elastic, dielectric, and piezoelectric properties of “single crystalline’’ films of vinylidene fluoride trifluoroethylene copolymer

Abstract: Elastic, dielectric, and piezoelectric constant matrix elements of a “single crystalline’’ (SC) film of vinylidene fluoride trifluoroethylene copolymer, P(VDF/TrFE), in which the orthorhombic [001] and [110] axes of fully extended chain crystals are preferentially oriented parallel to the stretching axis and normal to the surface, respectively, were measured at temperatures ranging from 10 K to the Curie point (402–404 K) by using a piezoelectric resonance method. All of the electromechanical coupling factors (k31, k32, k33, k24, and k15) are larger than those of conventional lamellar crystalline films. Some of the matrix elements for a P(VDF/TrFE) single crystal are derived from the measured values of constant matrix elements for the SC film. Some features characteristic of the SC film are revealed. The SC film has a large Young’s modulus for the stretching direction (1/s11) (121 GPa at 10 K). The properties related to the molecular motions along the chain axis, such as 1/s11, shear stiffness constant c55, shear piezoelectric constant e15, etc., exhibit strong relaxations around 250 K. The origin of these relaxations in the crystalline phase is discussed.

Thin layer composite unimorph ferroelectric driver and sensor

Abstract: A method for forming ferroelectric wafers is provided A prestress layer is placed on the desired mold A ferroelectric wafer is placed on top of the prestress layer The layers are heated and then cooled, causing the ferroelectric wafer to become prestressed The prestress layer may include reinforcing material and the ferroelectric wafer may include electrodes or electrode layers may be placed on either side of the ferroelectric layer Wafers produced using this method have greatly improved output motion

Nanoscale imaging of domain dynamics and retention in ferroelectric thin films

Abstract: We report results on the direct observation of the microscopic origins of backswitching in ferroelectric thin films. The piezoelectric response generated in the film by a biased atomic force microscope tip was used to obtain static and dynamic piezoelectric images of individual grains in a polycrystalline material. We demonstrate that polarization reversal occurs under no external field (i.e., loss of remanent polarization) via a dispersive continuous-time random walk process, identified by a stretched exponential decay of the remanent polarization.

Electric field poling of flux grown KTiOPO4

Abstract: A method to periodically pole conductive ferroelectric materials has been developed. With this method we have fabricated high quality domain inverted crystals of flux grown KTiOPO4 suitable for efficient quasi-phase matched frequency conversion. Samples have been evaluated by second harmonic generation and normalized conversion efficiencies up to 6.9%/W cm2 have been obtained.

Characterization of ferroelectric lead zirconate titanate films by scanning force microscopy

Abstract: Scanning force microscopy (SFM) has been used for the determination of friction, phase transformation, piezoelectric behavior (in the contact mode), polarization state, and dielectric constant (in the noncontact mode) of nanometer regions of lead zirconate titanate (PZT) films. The use of the SFM tip in the contact mode, to polarize different nanoregions of the PZT film and to apply an oscillating field thereon, led to effective piezoelectric coefficients and piezoelectric loops. The measured effective piezoelectric coefficient was shown to depend appreciably on both the tip contact force and the quality of the tip-to-film electrical contact. In the noncontact mode, application of an ac signal (with a frequency ω) across the tip—PZT film—electrode system produced an oscillation of the tip at frequencies ω (fundamental or first harmonic) and 2ω (second harmonic). The signals at ω and 2ω were related to the state of polarization and the dielectric constant of the PZT film, respectively. Analysis of the comb...

Lead-Free Piezoelectric Ceramics of (Bi1/2Na1/2)TiO3–1/2(Bi2O3·Sc2O3) System

Abstract: Bismuth sodium titanate, (Bi1/2Na1/2)TiO3 (BNT)-based solid solution, (1- x)(Bi1/2Na1/2)TiO3– x1/2(Bi2O3Sc2O3) [BNST-100 x], was studied in order to determine its dielectric, ferroelectric and piezoelectric properties as a new group of lead-free piezoelectric ceramics. Measurements of dielectric and piezoelectric properties show that the BNST-100 x ceramics of composition near BNT composition have relatively low free permittivity, e33 T/e0 (=431) and high electromechanical coupling factor, k33 (=0.42). These ceramics are good candidates for use as lead-free piezoelectric ceramics.

Thickness dependence of the switching voltage in all-oxide ferroelectric thin-film capacitors prepared by pulsed laser deposition

Abstract: Thin-film ferroelectric capacitors consisting of PbZr0.53Ti0.47O3 sandwiched between La0.5Sr0.5CoO3 electrodes have been deposited using pulsed laser deposition. The combination of oxidic perovskite-type materials results in capacitors with a coercive field (Ec) which is comparable with values for bulk ceramics. Textured thin-film capacitors with a columnar microstructure show lower switching voltages than epitaxial films. No thickness dependence of Ec and a good endurance up to 1011 cycles have been observed for epitaxial as well as textured capacitors with oxidic electrodes. In contrast, capacitors with a metallic top electrode show an increase of Ec with decreasing thickness of the ferroelectric layer. We show that charge injection can explain the experimentally observed increase of Ec with decreasing ferroelectric layer thickness. An overview is given of the growth conditions needed for PbZr0.53Ti0.47O3 films, because the precise stoichiometry is of the utmost importance for the capacitor quality.

Ferroelectric dielectric for integrated circuit applications at microwave frequencies

Abstract: A ferroelectric dielectric for microwave applications is provided including a polycrystalline perovskite phase of lead zirconate titanate dielectric material. Small grain size material is provided by a low temperature process, by a rapid thermal annealing process. A layer of amorphous ferroelectric precursor material is deposited and annealed in an oxygen containing atmosphere in the presence of water vapour, preferably with the addition of a few percent of ozone, and at a temperature of less than 500° C. Advantageously, the method provides for formation of a ferroelectric material including lead zirconate titanate with a grain size less than 20 nm, with low film stress, high dielectric constant and low leakage current, which has excellent ferroelectric characteristics up to 10 GHz. This material has applications for capacitors, as filters, decoupling, coupling, and bypass elements and also for high frequency surface acoustic wave devices.

Electric field induced phase transition of antiferroelectric lead lanthanum zirconate titanate stannate ceramics

Abstract: The electric field induced phase transition behavior of lead lanthanum zirconate titanate stannate (PLZTS) ceramics was investigated. PLZTS undergoes a tetragonal antiferroelectric (AFETet) to rhombohedral ferroelectric (FERh) phase transition with the application of an electric field. The volume increase associated with this antiferroelectric (AFE)–ferroelectric (FE) phase transition plays an important role with respect to actuator applications. This volume increase involves an increase in both transverse and longitudinal strains. The E field at which the transverse strain increases is accompanied by an abrupt jump in polarization. The longitudinal strain, however, lags behind this polarization jump exhibiting a slight decrease at the onset of phase switching. This decoupling was related to the preferentially oriented AFE domain configuration, with its tetragonal c-axis perpendicular to the applied electric field. It is suggested that phase switching involves multiple steps involving both structural tran...

Logarithmic frequency dependence of the piezoelectric effect due to pinning of ferroelectric-ferroelastic domain walls

Abstract: Pinning of the ferroelastic-ferroelectric domain walls on randomly distributed defects in ferroelectric ceramics leads to a field dependence of the piezoelectric coefficient that is analogous to the Rayleigh law for magnetic susceptibility It is shown in this paper that the piezoelectric coefficient of a lead zirconate titanate ferroelastic-ferroelectric system depends linearly on the logarithm of the frequency of the field Both the reversible and irreversible components of the piezoelectric coefficient are found to be frequency dependent A similar type of frequency dependence due to domain-wall pinning has been predicted for the magnetic susceptibility in disordered ferromagnets and dilute antiferromagnets The presented results offer experimental evidence that the theoretical approach developed for domain-wall pinning effects in magnetic materials is generally valid for pinning processes in all ferroic systems, ferromagnetics, ferroelectrics, and ferroelastics

Electrode dependence of hydrogen-induced degradation in ferroelectric Pb(Zr,Ti)O3 and SrBi2Ta2O9 thin films

Abstract: Forming gas annealing causes changes in the remanent polarization (Pr), coercive field (Ec), and leakage current (I) in both PZT [Pb(Zr,Ti)O3] and SBT (SrBi2Ta2O9) samples with a variety of top electrode materials (Pt, Au, Ag, Cu, Ni, and In2O3), and the degree of degradation depends strongly on the top electrode material. These results may be explained by a model that is based on the catalytic activities of the top electrode to dissociate hydrogen molecules into hydrogen atoms, with the latter subsequently migrating into PZT or SBT films to cause oxygen deficiency and its associated property degradation. This model can be expanded to explain the recovery phenomenon resulting from oxygen annealing, which also depends on the catalytic activity of the top electrode to produce atomic oxygen from molecular oxygen.

Slater model applied to polarization graded ferroelectrics

Abstract: Ferroelectric thin films of BaxSr1−xTiO3 with compositional gradients normal to the growth surface have been formed by the successive deposition and annealing of films having step-variable Ba to Sr ratios. By suitably tailoring the magnitude and sense of the gradient in Ba to Sr ratio, directional potentials can be built into the structures yielding a new, but controllable, hysteresis phenomenon. Slater’s empirical model for ferroelectric materials has been extended to also describe thin films with polarization gradients normal to the growth surface, i.e., graded ferroelectric devices. This model accounts for several aspects of these structures, including: the broadness of the permittivity plots with temperature, the formation of a spontaneous potential upon oscillatory field excitation, offsets in the hysteresis graphs along the displacement axis with directions which are gradient dependent, and the electric field dependence of that offset.

Scanning force microscopy of domain structure in ferroelectric thin films: Imaging and control

Abstract: Scanning force microscopy (SFM) has been used to perform nanoscale studies of domain structures and switching behaviour of (PZT) thin films. An SFM piezoresponse mode, based on the detection of the piezoelectric vibration of a ferroelectric sample, was shown to be suitable for high resolution imaging of ferroelectric domains in thin films. The lower limit of the piezoresponse mode imaging resolution depends on the radius of the probing tip and is estimated to be of the order of several nanometers. The effect of the film microstructure on the imaging resolution is discussed. The ability of effective control of domains as small as 50 nm by means of SFM has been demonstrated. It is shown that SFM can be used in the investigation of electrical degradation effects in ferroelectric thin films. Formation of regions with unswitchable polarization as a result of fatigue, within grains of submicron size, was experimentally observed.

Control of the growth and domain structure of epitaxial SrRuO3 thin films by vicinal (001) SrTiO3 substrates

Abstract: We report the effect of both miscut angle (α) and miscut direction (β) of vicinal substrates on the epitaxial growth and domain structure of isotropic metallic oxide SrRuO3 thin films. The thin films have been grown on vicinal (001) SrTiO3 substrates with α up to 4.1° and β up to 37° away from the in-plane [010] axis. Single-crystal epitaxial (110)o SrRuO3 thin films were obtained on vicinal SrTiO3 substrates with a large miscut angle (α=1.9°, 2.1°, and 4.1°) and miscut direction close to the [010] axis. Decreasing the substrate miscut angle or aligning the miscut direction close to the [110] axis (β=45°) resulted in an increase of 90° domains in the plane. The films grown on vicinal substrates displayed a significant improvement in crystalline quality and in-plane epitaxial alignment as compared to the films grown on exact (001) SrTiO3 substrates. Atomic force microscopy revealed that the growth mechanism changed from two-dimensional nucleation to step flow growth as the miscut angle increased.