Relaxation Of Thin Films Research Articles

Simultaneous giant strain and electrostrictive coefficient in lead-free BNT-ST-BT ergodic relaxor thin films on Pt/TiO2/SiO2/Si substrates

The acquisition of hysteresis-free and large electrostrain in lead-free piezoelectric thin films is the key issue for the development of high-performance micro actuators. The electrostrictive effect is an appropriate choice for developing hysteresis-free electrostrain. Lead-free ferroelectric compound Bi1/2Na1/2TiO3 has great potential in realizing large electrostrain response and electrostrictive coefficient. In this work, (0.8-x)Bi1/2Na1/2TiO3-0.2SrTiO3-xBaTiO3 (BNT-ST-100xBT) thin films were prepared by chemical solution deposition method. The crystal structure, morphology, dielectric properties, electrostrictive effect, dielectric nonlinearity, ferroelectric properties, and electrostrain response were systematically investigated. The most significant electrostrictive effect was obtained in the ergodic relaxor BNT-ST-6BT thin film, i.e., a giant electrostrictive coefficient of 0.21 m4/C2. Meanwhile, by increasing the applied electric field, a linearly increasing strain and the maximum value of 0.82 % were obtained in this film. This work has made an important step in the electrostrictive effect study and provides a pathway to obtain hysteresis-free electrostrain in lead-free perovskite ferroelectric thin films.

Effect of substrate clamping on evolution of properties in homovalent and heterovalent relaxor thin films

Relaxor ferroelectrics, well known for their large dielectric and piezoelectric response, are attracting growing interest in thin-film form driven by a need for miniaturized devices. Fundamental understanding and control of the performance of relaxors as thin films, however, is underdeveloped relative to studies of bulk versions. One obvious challenge is substrate clamping, which reduces the dielectric and piezoelectric response, yet systematic comparisons of the effect of substrate clamping on different relaxor materials is missing. Here, the impact of epitaxial constraint on the relaxor behavior and properties of both homovalent ${\mathrm{BaZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3}$ and heterovalent $\mathrm{Pb}{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ relaxors is studied in (001)-, (011)-, and (111)-oriented thin films. While the different orientations of $\mathrm{Pb}{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ heterostructures show little variation in dielectric permittivity, a strong orientational effect is observed in ${\mathrm{BaZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3}$ heterostructures, with (111)-oriented films exhibiting a 67% higher dielectric permittivity than (001)-oriented films, a difference that is attributed to how substrate clamping along different crystallographic axes interacts with the polar-structure nucleation and growth. Measurements of dielectric anisotropy in (001)-oriented films confirm the trend---with $\mathrm{Pb}{\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ heterostructures exhibiting just half as much anisotropy in dielectric permittivity than ${\mathrm{BaZr}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_{3}$ heterostructures. These results indicate a higher susceptibility to substrate clamping in homovalent relaxors as compared to heterovalent relaxors, implying that the random fields in the heterovalent relaxors could be advantageous for materials in such geometries and applications.

Polar state in polycrystalline BaSn0.3Ti0.7O3 thin film determined from ac- & dc-field studies

The present work reports polarization response and the effects of ac- & dc-fields on 30% Sn doped BaTiO3 polycrystalline relaxor thin films as a function of temperature. Apart from the low temperature frequency dispersion in dielectric data, a frequency independent local maxima in dielectric constant and a concomitant peak in dielectric loss at T* ∼ 245 K is observed, which is unusual of bulk relaxor systems. Below T*, dispersion in dielectric constant becomes quite evident showing signatures of non-ergodic behavior. Subsequently, the dielectric and polarization responses in ergodic (>T*) and non-ergodic (<T*) states are studied to determine the polar state in the system. The ac-field dependence of permittivity indicates the presence of domain like dynamics, predominantly in non-ergodic state. Field induced transition to ferroelectric state, both in ergodic and non-ergodic phases, is demonstrated from the dc-field studies. Further, the dc-field and polarization hysteresis studies reveal the system resembles that of anti-ferroelectric systems below T*, a possible signature for the existence of incommensurate polar regions. These experimental results put together, provides key insights in understanding the field effects of relaxor behavior in doped barium titanate systems.

Enhanced multiferroic properties of relaxor bulk and thin film Na0.5Bi0.5TiO[formula omitted] by ion doping: Theoretical study

We have calculated the spontaneous polarization Ps of ferroelectric Na0.5Bi0.5TiO3 (NBT) thin films. Ps increases with decreasing film thickness due to size and surface effects. It increases with increasing Dy and Nd ion doping and decreases by Er and Ho ion doping due to the different strain caused by the difference in the doping and host ion radii. Transition metal ion doping leads to appearance of ferromagnetism, i.e. of multiferroic properties at room temperature. The temperature dependence of the dielectric constant ϵ′ shows anomalies at both temperatures, Td and Tm. ϵ′ decreases with increasing magnetic field. The critical exponents γ, β and δ are observed for bulk and thin film NBT. The value of γ = 1.55 confirms the relaxor behavior of NBT.

Effect of polarization fatigue on the electrocaloric effect of relaxor Pb0.92La0.08Zr0.65Ti0.35O3 thin film

The electrocaloric effect represents an adiabatic temperature change or isothermal entropy change upon the application or removal of an external electric field. However, the effect of the repetitive bipolar/unipolar electric field on the electrocaloric effect in ferroelectrics is not well understood. In this work, the electrocaloric effect after both bipolar and unipolar electrical cycling in Pb0.92La0.08Zr0.65Ti0.35O3 relaxor thin films has been indirectly evaluated using the Maxwell relationship. It was found that at room temperature, the electrocaloric temperature change ΔT under higher electric fields decreases gradually from a positive value to a negative one with the increase in the bipolar/unipolar fatigue cycle number. Intriguingly, around the Curie temperature, the positive ΔT of the film after bipolar/unipolar cycling was found to decrease under low or intermediate electric fields, while it remains almost unchanged under high electric fields. This behavior is attributed to the pinning effect of defects caused by phase decomposition during both bipolar and unipolar fatigue. Note that the increase in polarization by temperature induced domain depinning close to room temperature after the fatigue process is manifested by using the Maxwell relation indirectly. Our work further reveals the origin of both the electrocaloric effect and ferroelectric electrical fatigue and is important for the applications of practical electrocaloric refrigerators.

Low-temperature-poling awakened high dielectric breakdown strength and outstanding improvement of discharge energy density of (Pb,La)(Zr,Sn,Ti)O3 relaxor thin film

Abstract Ferroelectric thin films possessing high dielectric breakdown strength (DBS) are attractive materials employed to satisfy the requirements of miniaturization and integration of electronic components, especially for dielectric capacitors with large discharge energy density (W). In this work, it is demonstrated for the first time that high DBS in ferroelectric thin film can be awakened by a low-temperature-poling method. Mn-doped Pb0.97La0.02(Zr0.905Sn0.015Ti0.08)O3 (PLZST) relaxor thin films were prepared by using a sol-gel method, as when poled at near liquid nitrogen temperature, its DBS and W at room temperature are greatly enhanced (nearly doubled) from 1286 kV/cm to 2000 kV/cm, and from 16.6 J/cm3 to 31.2 J/cm3, respectively. The high ordering of defect dipoles during the low-temperature-poling process is responsible for the great enhancement of the awakened DBS and the outstanding improvement of the W. It concludes that the low-temperature-poling method can provide a new alternative strategy to strength the DBS and thus to improve the electrical performances of ferroelectric materials for the applications required strong electric fields in many fields, especially for dielectric energy storage, electrocaloric cooling, and energy harvesting, etc.

Finite-size effects in lead scandium tantalate relaxor thin films

Relaxor ferroelectrics exhibit large electromechanical effects in response to external stimuli, making them promising materials for a variety of energy-harvesting, sensing, and actuating applications. An open question in the field, however, is how the polarization response evolves when the sample size approaches the nanometer length scale of polar correlations. Here, the evolution of polarization response as a function of film thickness is studied in epitaxial thin films of the relaxor lead scandium tantalate, demonstrating a large suppression of polarization response at reduced thickness.

Lead-free relaxor thin films with huge energy density and low loss for high temperature applications

We report record energy storage density (>80 J cm−3) in Pb-free relaxor ferroelectrics based on Mn-doped BiFeO3–BaTiO3 thin films. Rapid interval deposition was used to impose layer-by-layer growth improving crystallinity and lowering unwanted defects concentration. The growth and Mn doping produced an order of magnitude lower leakage, with strongly reduced dielectric loss (from room temperature to >300 °C, and 100 Hz to 1 MHz), e.g. by a factor of 5 at 225 °C and 25 kHz. At room temperature (RT), the dielectric breakdown strength increased by a factor of 1.5 to >3000 kV cm−1 while the dielectric constant remained flat, at ~1000 from RT to 350 °C. The films perform better than competing materials (e.g. PZT and SrTiO3-based) while being Pb-free and while operating up to 350 °C, which SrTiO3-based systems do not. Our work gives considerable promise for high energy and power density capacitors for harsh environments.

Frequency dependent electrocaloric effect in Nb-doped PZST relaxor thin film with the coexistence of tetragonal antiferroelectric and rhombohedral ferroelectric phases

Electrocaloric refrigeration material with good comprehensive performance such as large temperature change (ΔT), coefficient of performance (COP) and electrocaloric coefficient (ΔT/ΔE), is very attractive in the design of commercial refrigeration devices. Here, a large positive (ΔT ∼ 12.3 K with ΔS ∼ 13.6 J K−1kg−1 at 293 K) at 10 kHz and negative EC effects (ΔT ∼ - 5.8 K with ΔS ∼ - 4.5 J K−1kg−1 at ∼ 425 K) at 100 Hz are obtained in the Nb-doped Pb0.99(Zr0.65Sn0.3Ti0.05)0.98O3 antiferroelectric thin film by using a sol-gel route. Moreover, the large positive and negative EC effects are sensitive and dependent to the applied frequencies. As the applied frequency increases, the positive EC effect intends to increase. However, the negative EC effect intends to decrease as the applied frequency increases. This interesting phenomenon is ascribed to the co-coupling between the phase-transition (nanoscale tetragonal transferring into rhombohedral along the direction which is vertical to the surface of the thin film, namely the [111]) & defect dipoles. Moreover, the COP (3.37) and ΔT/ΔE (0.01 K⋅cmkV−1) are also comparable to those of previous research works. These breakthroughs and findings enable the Nb-doped PZST thin film an ideal candidate for application of next-generation solid-state cooling devices.

Thermal strain induced large electrocaloric effect of relaxor thin film on LaNiO3/Pt composite electrode with the coexistence of nanoscale antiferroelectric and ferroelectric phases in a broad temperature range

Ferroelectric/antiferroelectric thin/thick films with large electrocaloric (EC) effect in a broad operational temperature range are very attractive in solid-state cooling devices. We demonstrated that a large positive electrocaloric (EC) effect (maximum ΔT ~ 20.7 K) in a broad temperature range (~ 110 K) was realized in Pb0.97La0.02(Zr0.65Sn0.3Ti0.05)O3 (PLZST) relaxor antiferroelectric (AFE) thin film prepared using a sol-gel method. The large positive EC effect may be ascribed to the in-plane residual thermal tensile stress during the layer-by-layer annealing process, and the high-quality film structure owing to the utilization of the LaNiO3/Pt composite bottom electrode. The broad EC temperature range may be ascribed to the great dielectric relaxor dispersion around the dielectric peak because of the coexistence of nanoscale multiple FE and AFE phases. Moreover, a large pyroelectric energy density (6.10 Jcm−3) was harvested by using an Olsen cycle, which is much larger than those (usually less than 10− Jcm−3) obtained by using direct thermal-electrical, Stirling and Carnot cycles, etc. These breakthroughs enable the PLZST thin film an attractive multifunctional material for applications in modern solid-state cooling and energy harvesting.

Relaxor properties of barium titanate crystals grown by Remeika method

Barium titanate (BaTiO3, BT) crystals have been grown by the Remeika method using both the regular KF and mixed KF-NaF (0.6-0.4) solvents. Typical acute angle “butterfly wing” BT crystals have been obtained, and they were characterized using x-ray diffraction, scanning electron microscopy (including energy dispersive spectroscopy), conventional dielectric and acoustic emission methods. A typical wing has a triangular plate shape which is up to 0.5mm thick with a 10–15mm2 area. The plate has a (001) habit and an atomically smooth outer surface. Both K+ and F− solvent ions are incorporated as dopants into the crystal lattice during growth substituting for Ba2+ and O2− ions respectively. The dopants' distribution is found to be inhomogeneous, their content being almost an order of magnitude higher (up to 2mol%) at out surface of the plate relatively to the bulk. A few μm thick surface layer is formed where a multidomain ferroelectric net is confined between two≤1µm thick dopant-rich surfaces. The layer as a whole possess relaxor ferroelectric properties, which is apparent from the appearance of additional broad maxima, Tm, in the temperature dependence of the dielectric permittivity around the ferroelectric phase transition. Intense acoustic emission responses detected at temperatures corresponding to the Tm values allow to observe the Tm shift to lower temperatures at higher frequencies, or dispersion, typical for relaxor ferroelectrics. The outer surface of the BT wing can thus serve as a relaxor thin film for various electronic application, such as capacitors, or as a substrate for BT-based multiferroic structure. Crystals grown from KF-NaF fluxes contain sodium atoms as an additional impurity, but the crystal yield is much smaller, and while the ferroelectric transition peak is diffuse it does not show any sign of dispersion typical for relaxor behavior.

Large Energy Storage Density and High Thermal Stability in a Highly Textured (111)-Oriented Pb0.8Ba0.2ZrO3 Relaxor Thin Film with the Coexistence of Antiferroelectric and Ferroelectric Phases.

A highly textured (111)-oriented Pb0.8Ba0.2ZrO3 (PBZ) relaxor thin film with the coexistence of antiferroelectric (AFE) and ferroelectric (FE) phases was prepared on a Pt/TiOx/SiO2/Si(100) substrate by using a sol-gel method. A large recoverable energy storage density of 40.18 J/cm(3) along with an efficiency of 64.1% was achieved at room temperature. Over a wide temperature range of 250 K (from room temperature to 523 K), the variation of the energy density is within 5%, indicating a high thermal stability. The high energy storage performance was endowed by a large dielectric breakdown strength, great relaxor dispersion, highly textured orientation, and the coexistence of FE and AFE phases. The PBZ thin film is believed to be an attractive material for applications in energy storage systems over a wide temperature range.

Giant Electric Energy Density in Epitaxial Lead‐Free Thin Films with Coexistence of Ferroelectrics and Antiferroelectrics

Ferroelectrics/antiferroelectrics with high dielectric breakdown strength have the potential to store a great amount of electrical energy, attractive for many modern applications in electronic devices and systems. Here, it is demonstrated that a giant electric energy density (154 J cm−3, three times the highest value of lead‐based systems and five times the value of the best dielectric/ferroelectric polymer), together with the excellent fatigue‐free property, good thermal stability, and high efficiency, is realized in pulsed laser deposited (Bi1/2Na1/2)0.9118La0.02Ba0.0582(Ti0.97Zr0.03)O3 (BNLBTZ) epitaxial lead‐free relaxor thin films with the coexistence of ferroelectric (FE) and antiferroelectric (AFE) phases. This is endowed by high epitaxial quality, great relaxor dispersion, and the coexistence of the FE/AFE phases near the morphotropic phase boundary. The giant energy storage effect of the BNLBTZ lead‐free relaxor thin films may make a great impact on the modern energy storage technology.

High energy-storage performance of 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 relaxor ferroelectric thin films prepared by RF magnetron sputtering

0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 (PMN-PT 90/10) relaxor ferroelectric thin films with different thicknesses were deposited on the LaNiO3/Si (100) by the radio-frequency (RF) magnetron sputtering technique. The effects of thickness and deposition temperature on the microstructure, dielectric properties and the energy-storage performance of the thin films were investigated in detail. X-ray diffraction spectra indicated that the thin films had crystallized into a pure perovskite phase with a (100)-preferred orientation after annealed at 700°C. Moreover, all the PMN-PT 90/10 thin films showed the uniform and crack-free surface microstructure. As a result, a larger recoverable energy density of 31.3J/cm3 was achieved in the 750-nm-thick film under 2640kV/cm at room temperature. Thus, PMN-PT 90/10 relaxor thin films are the promising candidate for energy-storage capacitor application.

Strain-induced long range ferroelectric order and linear electro-optic effect in epitaxial relaxor thin films

Relaxor ferroelectrics have neither long range ferroelectric order nor structural transformation down to the lowest temperatures, and display isotropic optical properties like quadratic electro-optic effect. However, if an anisotropy is forced through an external agent, like electric field or uniaxial strain, a ferroelectric and structural long range order can be induced in these materials. Here, we show that epitaxial strain in relaxor ferroelectric thin films can be employed to induce a linear electro-optic effect, opening the path to new strain-controlled electro-optic materials. Epitaxial thin films with Pb1-3x/2LaxZr0.2Ti0.8O3 (x = 0.22) composition grown by pulsed laser deposition on (001) SrRuO3/SrTiO3 single crystal heterostructures become tetragonal below the susceptibility peak, which occurs at a temperature 140 K higher as compared to bulk. These films show piezoelectric properties and almost linear electro-optic behaviour.

High permittivity 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 relaxor thin films for high-value, wide-temperature capacitor applications

High permittivity 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 (PMNT) relaxor thin films with a nearly pure perovskite structure as well as a dense and uniform microstructure have been prepared on Pb(Zr,Ti)O3-buffered platinized silicon substrates by sol-gel method. Interestingly, the PMNT thin film exhibits high dielectric permittivity, εr ∼ 1200, and high dielectric tunability, ∼70% under a moderate E = 333 kV/cm, over a wide temperature range. These results are explained in terms of a relaxor behavior of the PMNT film. Moreover, the leakage current density of the PMNT thin film is reasonably low, roughly 5.2 × 10−6 A/cm2 at an electric field intensity of 400 kV/cm. As the dc electric field increases, the leakage current mechanism is transformed from ohmic law to Fowler–Nordheim tunneling mechanism. All the results obtained indicate that the PMNT relaxor thin film is a good candidate for high-value, wide-temperature capacitor applications.

Studies on Field Dependent Domain Structures in Multi-Grained 0.85PbMg1/3Nb2/3O3–0.15PbTiO3 Thin Films by Scanning Force Microscopy

0.85PbMg1/3Nb2/3O3–0.15PbTiO3 (0.85PMN–0.15PT) ferroelectric relaxor thin films have been deposited on La0.5Sr0.5CoO3/(111) Pt/TiO2/SiO2/Si by pulsed laser ablation by varying the oxygen partial pressures from 50 mTorr to 400 mTorr. The X-ray diffraction pattern reveals a pyrochlore free polycrystalline film. The grain morphology of the deposited films was studied using scanning electron microscopy and was found to be affected by oxygen pressure. By employing dynamic contact-electrostatic force microscopy we found that the distribution of polar nanoregions is majorly affected by oxygen pressure. Finally, the electric field induced switching in these films is discussed in terms of domain wall pinning.

Effect of oxygen pressure on the grain and domain structure of polycrystalline 0.85PbMg1/3Nb2/3O3–0.15PbTiO3 thin films studied by scanning probe microscopy

0.85PbMg1/3Nb2/3O3–0.15PbTiO3 ferroelectric–relaxor thin films have been deposited on La0.5Sr0.5CoO3/(1 1 1) Pt/TiO2/SiO2/Si by pulsed laser ablation at various oxygen partial pressures in the range 0.05 to 0.4 Torr. All the films have a rhombohedral perovskite structure. The grain morphology and orientation are drastically affected by the oxygen pressure, studied by x-ray diffraction and scanning electron microscopy. The domain structure investigations by dynamic contact electrostatic force microscopy have revealed that the distribution of polar nanoregions and their dynamics is influenced by the grain morphology, orientation and more importantly, oxygen vacancies. The correlation length extracted from autocorrelation function images has shown that the polarization disorder decreases with oxygen pressure up to 0.3 Torr. The presence of polarized domains and their electric field induced switching is discussed in terms of internal bias field and domain wall pinning. Film deposited at 0.4 Torr presents a curious case with unique triangular grain morphology and large polarization disorder.

Attenuation of the depolarizing field in a thin film model relaxor

The behavior of the polarization in a microscopic statistical model for a thin film relaxor placed between two metallic electrodes is studied by numerical simulations. Depolarization fields different from zero, due to a non-perfect compensation of surface charges at the metallic electrodes, are taken into account. Different thicknesses and different values of the compositional charge disorder density for the relaxor are considered. Depolarization field is found to be extremely attenuated for large values of the number density of charge carriers in the relaxor. In such a case, field attenuation allows for the existence of a homogeneous ferroelectric ground state.

Crystal defects and cation ordering domains in epitaxial PbSc 0.5Ta 0.5O 3 relaxor ferroelectric thin films investigated by high -resolution transmission electron microscopy

Epitaxial thin films of the relaxor ferroelectric PbSc 0.5Ta 0.5O 3 (PST) were grown by pulsed laser deposition on an SrTiO 3 substrate with an SrRuO 3 buffer layer and investigated by diffraction contrast imaging and high-resolution transmission electron microscopy (TEM) in cross-section and plan-view. Crystal defects, viz. misfit dislocations, π stacking faults and cation ordering domains, have been characterized and the mechanism of their formation is discussed. The state of the structural disorder in PST relaxor thin films is characterized by the high density of π stacking faults and the rather small size (<10 nm) of the cation ordering domains, and is therefore markedly distinct from the state of the disorder in bulk relaxor PST. Polar nanoregions, supposed to be essential for explaining the relaxor properties, could not be detected using TEM, possibly due to their high fluctuation frequency. The dielectric constant of the relaxor PST thin films is about an order of magnitude smaller than that of bulk relaxor PST, which is attributed to the large density of π stacking faults in the thin films.