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At least 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Towards an accurate description of perovskite ferroelectrics: exchange and correlation effects

Using the van der Waals density functional with C09 exchange (vdW-DF-C09), which has been applied to describing a wide range of dispersion-bound systems, we explore the physical properties of prototypical ABO 3 bulk ferroelectric oxides. Surprisingly, vdW-DF-C09 provides a superior description of experimental values for lattice constants, polarization and bulk moduli, exhibiting similar accuracy to the modified Perdew-Burke-Erzenhoff functional which was designed specifically for bulk solids (PBEsol). The relative performance of vdW-DF-C09 is strongly linked to the form of the exchange enhancement factor which, like PBEsol, tends to behave like the gradient expansion approximation for small reduced gradients. These results suggest the general-purpose nature of the class of vdW-DF functionals, with particular consequences for predicting material functionality across dense and sparse matter regimes.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Electrical Properties of Barium and Zirconium Modified NBT Ferroelectric Ceramics

Recently a new wave of interest has risen on relaxor ferroelectrics with complex perovskite structure due to their wide use in fabrication of multilayer ceramic capacitors, electrostrictive actuators, and electromechanical transducers. The polycrystalline 0.93(Bi{sub 0.5}Na{sub 0.5})Ba{sub 0.07}Z{sub 0.04}T{sub 0.96}O{sub 3}(0.07BNBZT) ceramic material, which is in the vicinity of the morphotropic phase boundary (MPB) has been prepared by using high temperature solid state reaction method. The tolerance factor has been estimated and found to be 0.815. XRD analysis revealed a rhombohedral perovskite type structure. SEM micrographs showed highly dense grains with rectangular shape. The average grain size is found to be 1.51{mu}m. Dielectric studies in the material ha indicated relaxor behaviour with diffuse phase transition. High value of {epsilon}{sub m}>1958 is found at 1kHz, Tm (phase transition temperature) 335 deg. C, The diffuseness parameter was established to be 1.60 revealing the relaxor behaviour. Further, to confirm the relaxor behaviour in the material, Vogel-Fulcher (V-F) relation has been used. Estimated V-F parameters are found to be T{sub f} = 138 deg. C, E{sub a} = 0.080 eV and {nu}{sub o} = 2.32x10{sup 8} Hz. Cole-Cole analysis has shown a non-Debye type relaxation in the system. Conductivity studies in the material obeyed the Jonscher's power law in frequency range of (45Hz-5MHz) and temperature range of (35 deg. C - 600 deg. C). The electric conduction in the system may be due to hopping/mobility/ transportation of charge carriers.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Improved Properties of Pb Based BLZT Ferroelectric Ceramics

Present report is concerning with investigation of effect of different sintering profiles on Pb based BLZT ceramics. The material powder of selected composition (Ba{sub 0.795}La{sub 0.005}Pb{sub 0.20}Ti{sub 0.90}Zr{sub 0.10}O{sub 3}) was prepared by solid state reaction route and then powder was compacted in the form of circular discs. The discs were then sintered at different temperatures (1325 deg. C for 4h, 1325 deg. C for 15min+1200 deg. C for 4h). Improved dielectric and ferroelectric properties were observed for samples sintered at 1200 deg. C. Shifting in T{sub c} to higher temperature could be related to enhanced tetragonality, which was further confirmed by X-ray diffraction analysis. All these improvements evidences that there is less Pb loss in case of modified sintering profile.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Three-terminal ferroelectric synapse device with concurrent learning function for artificial neural networks

Spike-timing-dependent synaptic plasticity (STDP) is demonstrated in a synapse device based on a ferroelectric-gate field-effect transistor (FeFET). STDP is a key of the learning functions observed in human brains, where the synaptic weight changes only depending on the spike timing of the pre- and post-neurons. The FeFET is composed of the stacked oxide materials with ZnO/Pr(Zr,Ti)O{sub 3} (PZT)/SrRuO{sub 3}. In the FeFET, the channel conductance can be altered depending on the density of electrons induced by the polarization of PZT film, which can be controlled by applying the gate voltage in a non-volatile manner. Applying a pulse gate voltage enables the multi-valued modulation of the conductance, which is expected to be caused by a change in PZT polarization. This variation depends on the height and the duration of the pulse gate voltage. Utilizing these characteristics, symmetric and asymmetric STDP learning functions are successfully implemented in the FeFET-based synapse device by applying the non-linear pulse gate voltage generated from a set of two pulses in a sampling circuit, in which the two pulses correspond to the spikes from the pre- and post-neurons. The three-terminal structure of the synapse device enables the concurrent learning, in which the weight update can be performed without canceling signal transmission among neurons, while the neural networks using the previously reported two-terminal synapse devices need to stop signal transmission for learning.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

Co-sputtering yttrium into hafnium oxide thin films to produce ferroelectric properties

Thin film capacitors were fabricated by sputtering TiN-Y doped HfO{sub 2}-TiN stacks on silicon substrates. Yttrium was incorporated into the HfO{sub 2} layers by simultaneously sputtering from Y{sub 2}O{sub 3} and HfO{sub 2} sources. Electric polarization and relative permittivity measurements yield distinct ferroelectric properties as a result of low yttrium dopant concentrations in the range of 0.9-1.9 mol. %. Grazing incidence x-ray diffraction measurements show the formation of an orthorhombic phase in this range. Compared to atomic layer deposition films, the highest remanent polarization and the highest relative permittivity were obtained at significantly lower doping concentrations in these sputtered films.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Extension of thickness-dependent dielectric breakdown law on adiabatically compressed ferroelectric materials

It is experimentally found that the E{sub b}(d) = {gamma} {center_dot} d{sup -{xi}} law describing the thickness-dependent breakdown electric field for solid dielectrics at ambient conditions can be extended for dielectrics in other thermodynamic states. It follows from the experimental results reported herein that the breakdown field, E{sub b}(d), of Pb(Zr{sub 0.95}Ti{sub 0.05})O{sub 3} (PZT 95/5) and Pb(Zr{sub 0.52}Ti{sub 0.48})O{sub 3} (PZT 52/48) ferroelectrics subjected to explosive adiabatic compression obeys the above-mentioned law in a wide range of voltages, up to 150 kV.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Low temperature dielectric relaxation and charged defects in ferroelectric thin films

We report a dielectric relaxation in BaTiO{sub 3}-based ferroelectric thin films of different composition and with several growth modes: sputtering (with and without magnetron) and sol-gel. The relaxation was observed at cryogenic temperatures (T < 100 K) for frequencies from 100 Hz up to 10 MHz. This relaxation activation energy is always lower than 200 meV and is very similar to the relaxation that we reported in the parent bulk perovskites. Based on our Electron Paramagnetic Resonance (EPR) investigation, we ascribe this dielectric relaxation to the hopping of electrons among Ti{sup 3+}-V(O) charged defects. Being dependent on the growth process and on the amount of oxygen vacancies, this relaxation can be a useful probe of defects in actual integrated capacitors with no need for specific shaping.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Interface control of surface photochemical reactivity in ultrathin epitaxial ferroelectric films

Asymmetrical electrical boundary conditions in (001)-oriented Pb(Zr{sub 0.2}TiO{sub 0.8})O{sub 3} (PZT) epitaxial ultrathin ferroelectric films are exploited to control surface photochemical reactivity determined by the sign of the surface polarization charge. It is shown that the preferential orientation of polarization in the as-grown PZT layer can be manipulated by choosing an appropriate type of bottom electrode material. PZT films deposited on the SrRuO{sub 3} electrodes exhibit preferential upward polarization (C{sup +}) whilst the same films grown on the (La,Sr)CoO{sub 3}-electrodes are polarized downward (C{sup -}). Photochemical activity of the PZT surfaces with different surface polarization charges has been tested by studying deposition of silver nanoparticles from AgNO{sub 3} solution under UV irradiation. PZT surfaces with preferential C{sup +} orientation possess a more active surface for metal reduction than their C{sup -} counterparts, evidenced by large differences in the concentration of deposited silver nanoparticles. This effect is attributed to band bending at the bottom interface which varies depending on the difference in work functions of PZT and electrode materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Pyroelectric field assisted ion migration induced by ultraviolet laser irradiation and its impact on ferroelectric domain inversion in lithium niobate crystals

The impact of UV laser irradiation on the distribution of lithium ions in ferroelectric lithium niobate single crystals has been numerically modelled. Strongly absorbed UV radiation at wavelengths of 244–305 nm produces steep temperature gradients which cause lithium ions to migrate and result in a local variation of the lithium concentration. In addition to the diffusion, here the pyroelectric effect is also taken into account which predicts a complex distribution of lithium concentration along the c-axis of the crystal: two separated lithium deficient regions on the surface and in depth. The modelling on the local lithium concentration and the subsequent variation of the coercive field are used to explain experimental results on the domain inversion of such UV treated lithium niobate crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Direct writing of ferroelectric domains on strontium barium niobate crystals using focused ultraviolet laser light

We report ferroelectric domain inversion in strontium barium niobate (SBN) single crystals by irradiating the surface locally with a strongly focused ultraviolet (UV) laser beam. The generated domains are investigated using piezoresponse force microscopy. We propose a simple model that allows predicting the domain width as a function of the irradiation intensity, which indeed applies for both SBN and LiNbO{sub 3}. Evidently, though fundamentally different, the domain structure of both SBN and LiNbO{sub 3} can be engineered through similar UV irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Minimum tetragonality in PbTiO3/BaTiO3 ferroelectric superlattices

PbTiO3/BaTiO3 ferroelectric superlattices commensurately constrained by a SrTiO3 substrate are studied using both phenomenological theory and phase field simulations. It is found that the spontaneous polarization of the superlattice does not vary significantly with volume fraction until PbTiO3 becomes the majority of the superlattice. A minimum tetragonality exists at room temperature when the PbTiO3 volume fraction is around 90%, resulted from its different sensitivity to epitaxial strains and the electrostatic coupling of the two polarized layers in this PbTiO3/BaTiO3 superlattice.

and phase field simulations↗
DOE OSTI2013

Wake-up effects in Si-doped hafnium oxide ferroelectric thin films

Hafnium oxide based ferroelectric thin films have shown potential as a promising alternative material for non-volatile memory applications. This work reports the switching stability of a Si-doped HfO{sub 2} film under bipolar pulsed-field operation. High field cycling causes a “wake-up” in virgin “pinched” polarization hysteresis loops, demonstrated by an enhancement in remanent polarization and a shift of negative coercive voltage. The rate of wake-up is accelerated by either reducing the frequency or increasing the amplitude of the cycling field. We suggest de-pinning of domains due to reduction of the defect concentration at bottom electrode interface as origin of the wake-up.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Laser-induced breakdown and damage generation by nonlinear frequency conversion in ferroelectric crystals: Experiment and theory

Using our experimental data for ns pulsed second harmonic generation (SHG) by periodically poled stoichiometric LiTaO{sub 3} (PPSLT) crystals, we consider in detail the mechanism underlying laser-induced damage in ferroelectric crystals. This mechanism involves generation and heating of free electrons, providing an effective kinetic pathway for electric breakdown and crystal damage in ns pulsed operation via combined two-photon absorption (TPA) and induced pyroelectric field. In particular, a temperature increase in the lattice of ≈1 K induced initially by ns SHG and TPA at the rear of operating PPSLT crystal is found to induce a gradient of spontaneous polarization generating a pyroelectric field of ≈10 kV/cm, accelerating free electrons generated by TPA to an energy of ≈10 eV, followed by impact ionization and crystal damage. Under the damage threshold for ns operation, the impact ionization does not lead to the avalanche-like increase of free electron density, in contrast to the case of shorter ps and fs pulses. However, the total number of collisions by free electrons, ≈10{sup 18} cm{sup −3} (generated during the pulse and accelerated to the energy of ≈10 eV), can produce widespread structural defects, which by entrapping electrons dramatically increase linear absorption for both harmonics in subsequent pulses, creating a positive feedback for crystal lattice heating, pyroelectric field and crystal damage. Under pulse repetition, defect generation starting from the rear of the crystal can propagate towards its center and front side producing damage tracks along the laser beam and stopping SHG. Theoretical analysis leads to numerical estimates and analytical approximation for the threshold laser fluence for onset of this damage mechanism, which agree well with our (i) experiments for the input 1064 nm radiation in 6.8 kHz pulsed SHG by PPSLT crystal, (ii) pulsed low frequency 532 nm radiation transmission experiments, and also (iii) with the data published for other nonlinear crystals and operated wavelengths.

36 MATERIALS SCIENCE↗
DOE OSTI2013

In situ self-assembled organic interface layers for the controlled growth of oligothiophene thin films on ferroelectric Pb(Zr{sub 0.2}Ti{sub 0.8})O{sub 3}

We introduce an in situ vacuum procedure for the optimal preparation and analysis of self-assembled monolayers (SAMs) as used in organic molecular electronics on ferroelectric lead zirconate titanate (PZT) substrates. Excellent ordering of oligothiophene semiconductor layers is heavily promoted through the presence of an interfacial bi-functional SAM layer that binds to both the oxidic PZT surface and the organic semiconductor molecules. The described method can be extended to other material combinations, featuring a variety of substrate materials and molecular functionalities.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Enhanced ferroelectric properties and thermal stability of nonstoichiometric 0.92(Na{sub 0.5}Bi{sub 0.5})TiO{sub 3}-0.08(K{sub 0.5}Bi{sub 0.5})TiO{sub 3} single crystals

Bi deficient, Mn doped 0.92(Na{sub 0.5}Bi{sub 0.5})TiO{sub 3}-0.08(K{sub 0.5}Bi{sub 0.5})TiO{sub 3} single crystals were grown by carefully controlled top-seeded solution growth method. Local structures were investigated by transmission electron microscopy. The site occupation and valence state of manganese were characterized by electron paramagnetic resonance spectrum. The leakage current density in the as-grown single crystals is effectively depressed. The introduced defect complexes suppress the temperature induced phase transformation, increasing the depolarization temperature (165 °C) and thermal stability of ferroelectric properties.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Upward ferroelectric self-poling in (001) oriented PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} epitaxial films with compressive strain

Upward self-poling phenomenon was observed in PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} ferroelectric films which were grown on (001) SrTiO{sub 3} substrate with either p-type La{sub 0.7}Sr{sub 0.3}MnO{sub 3} or n-type SrRuO{sub 3} buffered layer, or on n-type (001) Nb-SrTiO{sub 3} substrate. Both upward self-poling and epitaxial strain are strong in the super-thin PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} epitaxial films, while they become weak and finally disappear in thick epitaxial films or thin epitaxial films with high-density oxygen vacancies. Besides, both of them disappear in PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} polycrystalline films on Pt/TiO{sub 2}/SiO{sub 2}/Si substrate. Therefore, the main origin of upward self-poling is epitaxial strain rather than p-n/Schottky junction or charged vacancies in PbZr{sub 0.2}Ti{sub 0.8}O{sub 3} epitaxial films here.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Enhanced ferroelectric and dielectric properties of (111)-oriented highly cation-ordered PbSc{sub 0.5}Ta{sub 0.5}O{sub 3} thin films

Cation-ordered (111)-oriented epitaxial PbSc{sub 0.5}Ta{sub 0.5}O{sub 3} (PST) thin films were deposited by pulsed laser deposition on SrRuO{sub 3}-electroded SrTiO{sub 3} (111) substrates at three different temperatures of 525 °C, 550 °C, and 575 °C. All the films were well crystalline and (111)-oriented at all the three growth temperatures; however, the films deposited at the temperatures other than 550 °C exhibited the presence of a pyrochlore phase. X-ray diffraction analysis and transmission electron microscopy measurements revealed that the films were epitaxial and highly cation-ordered. In comparison to (001)-oriented PST films, (111)-oriented films on SrRuO{sub 3}/SrTiO{sub 3} (111) exhibited enhanced ferroelectric and dielectric properties with a broad size distribution of cation-ordered domains (5–100 nm). At a measurement temperature of 100 K, the remnant polarization of PST (111) films is almost √3 times larger than the remnant polarization observed for (001)-oriented PST films, which is attributed to the (111) orientation of the films, as the spontaneous polarization in PST lies close to the [111] direction. The observed dielectric constant and loss at 1 kHz were around 1145 and 0.11, respectively. The dielectric constant is thus almost three times higher than for previously reported (001)-oriented PST thin films, most probably due to the enhancement in cation-ordering.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structures, electrical properties, and leakage current behaviors of un-doped and Mn-doped lead-free ferroelectric K{sub 0.5}Na{sub 0.5}NbO{sub 3} films

Lead-free ferroelectric un-doped and doped K{sub 0.5}Na{sub 0.5}NbO{sub 3} (KNN) films with different amounts of manganese (Mn) were prepared by a chemical solution deposition method. The thicknesses of all films are about 1.6 μm. Their phase, microstructure, leakage current behavior, and electrical properties were investigated. With increasing the amounts of Mn, the crystallinity became worse. Fortunately, the electrical properties were improved due to the decreased leakage current density after Mn-doping. The study on leakage behaviors shows that the dominant conduction mechanism at low electric field in the un-doped KNN film is ohmic mode and that at high electric field is space-charge-limited and Pool-Frenkel emission. After Mn doping, the dominant conduction mechanism at high electric field of KNN films changed single space-charge-limited. However, the introduction of higher amount of Mn into the KNN film would lead to a changed conduction mechanism from space-charge-limited to ohmic mode. Consequently, there exists an optimal amount of Mn doping of 2.0 mol. %. The 2.0 mol. % Mn doped KNN film shows the lowest leakage current density and the best electrical properties. With the secondary ion mass spectroscopies and x-ray photoelectron spectroscopy analyses, the homogeneous distribution in the KNN films and entrance of Mn element in the lattice of KNN perovskite structure were also confirmed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗