Polarization Hysteresis Measurements Research Articles

Ferroelectric Al1−xBxN thin films integrated on Si

Ferroelectric Al0.93B0.07N thin films are prepared (100) Si substrates. The necessary c-axis out-of-plane orientation to observe macroscopic ferroelectric switching was achieved by implementing an initial Ar/N2 plasma treatment, followed by a thin layer of AlN to initiate the desired texture and a 150 nm W layer. The plasma treatment facilitates crystallinity enhancement of the AlN template layer, allowing for subsequent growth of highly oriented W and Al0.93B0.07N layers. The W layer exhibits random in-plane orientation and exclusive (110) out-of-plane orientation with a rocking curve width of 1.4°. When grown on these W surfaces, 175 nm thick Al0.93B0.07N films exhibit random in-plane orientation and exclusive (001) texture with rocking curve full-width-half-max values of 1.6° and RMS roughness values less than 1 nm. Polarization hysteresis measurements show robust hysteresis with coercive field values of 5.4 MV/cm and remanent polarization values of 136 μC/cm2. XPS depth profile analysis suggests that the plasma treatment converts the existing native oxide to a nitrogen rich oxynitride with approximate composition Si3O0.5N3.67. Cross-sectional TEM reveals that the oxynitride interlayer is amorphous and ∼3.4 nm thick, more than double the native oxide thickness measured by multiwavelength ellipsometry, implying that (oxy)nitride growth continues after conversion of the native oxide. This new family of ferroelectric wurtzites is interesting from an integration perspective given their chemical compatibility with mainstream semiconductors. Developing synthesis routes that promote needed texture while preserving compatible processing windows is an important step toward practical integration.

New approach for fabrication of annealing-free ferroelectric HfO2-based films at room temperature

In this work, ferroelectric properties with a coercive field of 1.6 MV/cm and a remanent polarization of 17.9 μC/cm2 were achieved in Y-doped HfO2 thin films prepared by ion beam assisted RF magnetron sputtering without any heating and annealing. Systematic grazing incidence X-ray diffraction and polarization hysteresis measurements revealed that the structural and electrical properties of the films were strongly dependent on the applied ion beam energy. A model of phase transition kinetics during deposition was introduced to explain the variations in the crystal structure and electric properties with various ion beam energies. It was found that the bombardment of the Ar ion beam provided extra energy to the sputtering particles, therefore conducive to the increased atomic diffusion and overcoming the potential barriers of the phase transition. The current processing method provides great potential for ferroelectric device applications that cannot be addressed with heating and annealing.

Indirect Electrocaloric Evaluation: Influence of Polarization Hysteresis Measurement Frequency

This study investigates the influence of applied electric field frequency for polarization–electric field (P–E) loops on indirect electrocaloric evaluation. Soft lead zirconate titanate (PZT) ceramic is used as a model material. The P–E loops under 1 and 100 Hz are presented at various temperatures with an electric field amplitude of 2.5 MV m−1. The indirect method based on the Maxwell relations is used to calculate the electrocaloric response. The result demonstrates that the electrocaloric responses based on the P–E loops measured at 1 Hz agree well with direct measurement results in the literature, but at 100 Hz, the coexistence of positive and negative electrocaloric effects (ECEs) is obtained, which has not been reported before. Domain switching is a dynamic process and rate‐dependent, which has a significant influence on the polarization behavior and the subsequent electrocaloric performance of ferroelectric materials. The result also shows that during the indirect ECE evaluation, high attention needs to be paid to the applied electric field frequency to obtain accurate and comparable electrocaloric evaluations in ferroelectric materials.

Impact of Polarization Dynamics and Charged Defects on the Electrocaloric Response of Ferroelectric Pb(Zr,Ti)O3 Ceramics

AbstractThe impact of charged point defects on the electrocaloric response of morphotropic Pb(Zr,Ti)O3 (PZT) ceramics is investigated using direct electrocaloric and polarization hysteresis measurements. The electrocaloric response determined for undoped, hard‐, and soft‐doped PZT is rationalized by considering the dipolar entropy change over polarization change and hysteresis losses. The highest electrocaloric effect is observed in poled hard PZT with the field applied along the poling direction, which is related to the reduced hysteresis losses caused by internal electric fields associated with defect complexes. The weak hysteretic dependency of polarization as function of the electric field in hard PZT also allows inducing an inverse electrocaloric effect. The experimental results are compared with model calculations that provide a physical interpretation. The results reveal how the electrocaloric response of ferroelectrics can be optimized by defect engineering.

Antiferroelectric‐ferroelectric phase transition in lead‐free AgNbO3 ceramics for energy storage applications

AbstractThe high‐energy storage density reported in lead‐free AgNbO3 ceramics makes it a fascinating material for energy storage applications. The phase transition process of AgNbO3 ceramics plays an important role in its properties and dominates the temperature and electric field dependent behavior. In this work, the phase transition behavior of AgNbO3 ceramics was investigated by polarization hysteresis and dielectric tunability measurements. It is revealed that the ferrielectric (FIE) phase at room temperature possesses both ferroelectric (FE)‐like and antiferroelectric (AFE)‐like dielectric responses prior to the critical AFE‐FE transition point. A recoverable energy storage density of 2 J/cm3 was achieved at 150 kV/cm due to the AFE‐FE transition. Based on a modified Laudau phenomenological theory, the stabilities among the AFE, FE and FIE phases are discussed, laying a foundation for further optimization of the dielectric properties of AgNbO3.

Large electric field-induced strain and large improvement in energy density of bismuth sodium potassium titanate-based piezoelectric ceramics

Ceramic solid solutions based on (1-x)Bi0.5(Na0.84K0.16)0.5TiO3-xBa(Nb0.01Ti0.99)O3 or (1-x)BNKT-xBNbT) (where x = 0, 0.01, 0.03, 0.05 and 0.07 mol fraction) were investigated to demonstrate the improvement of electrical properties as compared to the unmodified BNKT ceramic. The dielectric, ferroelectric, piezoelectric and electric field-induced strain properties were investigated as a function of composition and temperature. All ceramics presented a single phase perovskite structure. X-ray diffraction analysis revealed a transition from co-existing rhombohedral-tetragonal phases to a single tetragonal phase for compositions with x > 0.03. The maximum dielectric constant tended to increase with increasing BNbT content and the dielectric spectra for all compositions exhibited a broad maximum around Td and Tm. The polarization hysteresis measurement indicated a disruption in the long range ferroelectric order at the critical composition of x = 0.03 along with a significant enhancement in the electric field-induced strains (Smax = 0.41%) with a large normalized strain coefficient (d*33 = Smax/Emax) of 683 pm/V. The x = 0.03 composition also presented a high energy density (0.67 J/cm3) as compared with the unmodified BNKT composition (0.25 J/cm3). These results suggest that the (1-x)BNKT-xBNbT solid solution is a promising lead-free piezoelectric material candidate.

Flexible PZT thin film transferred on polymer substrate

Highly flexible lead zirconate titanate, Pb(Zr,Ti)O3 (PZT), thin films have been realized by an all chemical process on a plastic substrate. The procedure is composed of three steps: the first one is the PZT deposition on aluminium thin substrate, the second step corresponds to the bonding PZT thin film to a polymer layer and the final step is the aluminium substrate etching. Diffractions and microscopy techniques were used to check the quality of the new PZT/polymer composite structure. Polarization hysteresis and permittivity measurements were also performed to determine the electrical characteristics of the composite. These results demonstrate that the recently developed PZT/polymer thin films are very attractive for both bending actuation and sensing as well as for low frequency vibrating energy harvesting applications.

Lead-Free Antiferroelectric Silver Niobate Tantalate with High Energy Storage Performance.

Antiferroelectric materials that display double ferroelectric hysteresis loops are receiving increasing attention for their superior energy storage density compared to their ferroelectric counterparts. Despite the good properties obtained in antiferroelectric La-doped Pb(Zr,Ti)O3 -based ceramics, lead-free alternatives are highly desired due to the environmental concerns, and AgNbO3 has been highlighted as a ferrielectric/antiferroelectric perovskite for energy storage applications. Enhanced energy storage performance, with recoverable energy density of 4.2 J cm-3 and high thermal stability of the energy storage density (with minimal variation of ≤±5%) over 20-120 °C, can be achieved in Ta-modified AgNbO3 ceramics. It is revealed that the incorporation of Ta to the Nb site can enhance the antiferroelectricity because of the reduced polarizability of B-site cations, which is confirmed by the polarization hysteresis, dielectric tunability, and selected-area electron diffraction measurements. Additionally, Ta addition in AgNbO3 leads to decreased grain size and increased bulk density, increasing the dielectric breakdown strength, up to 240 kV cm-1 versus 175 kV cm-1 for the pure counterpart, together with the enhanced antiferroelectricity, accounting for the high energy storage density.

Piezoelectric and ferroelectric properties of lead-free (1-x)(Na1−yKy)(Nb1−zSbz)O3-xBaTiO3 solid solution

The solid solutions of lead-free (1-x)(Na1-yKy)(Nb1-zSbz)O3-xBaTiO3 (with x=0.1, 0.2; y=0.03, 0.05; z=0.05, 0.1) (abbreviated as (1-x)NKNS-xBT) ceramics have been synthesized using conventional solid-state reaction method. The results of X-ray diffraction analysis show that all the grown specimens of NKNS display typical perovskite structure. With BaTiO3 (BT) addition, a structural phase transition from tetragonal to cubic structure has been observed. The structural parameters of (1-x)NKNS-xBT powders were determined by profile refinements based on the analysis of X-ray powder diffraction. The charge density distributions of the prepared samples have been investigated by observed structure factors to understand the chemical bonding nature of (1-x)NKNS-xBT powders. The optical absorption of the ceramics has been investigated using UV–visible spectrophotometer. Scanning electron microscopic (SEM) measurements were performed to study the surface morphology of the prepared solid solutions. The elemental compositions of the (1-x)NKNS-xBT samples were analyzed by energy-dispersive X-ray (EDS) spectrometer. The dielectric constant versus temperature plots of the solid solutions exhibit ferroelectric to paraelectric phase transition, which is dependent on the BaTiO3 content. The ferroelectric nature of the samples has been determined through polarization and electric field hysteresis measurements.

Formation of (111) orientation-controlled ferroelectric orthorhombic HfO2 thin films from solid phase via annealing

0.07YO1.5-0.93HfO2 (YHO7) films were prepared on various substrates by pulse laser deposition at room temperature and subsequent heat treatment to enable a solid phase reaction. (111)-oriented 10 wt. % Sn-doped In2O3(ITO)//(111) yttria-stabilized zirconia, (111)Pt/TiOx/SiO2/(001)Si substrates, and (111)ITO/(111)Pt/TiOx/SiO2/(001)Si substrates were employed for film growth. In this study, X-ray diffraction measurements including θ–2θ measurements, reciprocal space mappings, and pole figure measurements were used to study the films. The film on (111)ITO//(111)yttria-stabilized zirconia was an (111)-orientated epitaxial film with ferroelectric orthorhombic phase; the film on (111)ITO/(111)Pt/TiOx/SiO2/(001)Si was an (111)-oriented uniaxial textured film with ferroelectric orthorhombic phase; and no preferred orientation was observed for the film on the (111)Pt/TiOx/SiO2/(001)Si substrate, which does not contain ITO. Polarization–hysteresis measurements confirmed that the films on ITO covered substrates had saturated ferroelectric hysteresis loops. A remanent polarization (Pr) of 9.6 and 10.8 μC/cm2 and coercive fields (Ec) of 1.9 and 2.0 MV/cm were obtained for the (111)-oriented epitaxial and uniaxial textured YHO7 films, respectively. These results demonstrate that the (111)-oriented ITO bottom electrodes play a key role in controlling the orientation and ferroelectricity of the phase formation of the solid films deposited at room temperature.

Ferrielectric phase in the (Bi1/2Na1/2)TiO3–Ba(Mg1/3Nb2/3)O3 system

We have investigated the crystal structure of (1 − x)(Bi1/2Na1/2)TiO3–xBa(Mg1/3Nb2/3)O3 (BNT–BMN) by high-resolution neutron powder diffraction (NPD) analysis. Our NPD study combined with polarization hysteresis measurements for ceramics reveals that a boundary between rhombohedral R3c and tetragonal P4bm phases exists at x = 4–5%. The Rietveld analysis of the NPD data shows that the crystal structure with x = 10% has a single-phase tetragonal structure in space group P4bm, as has been reported for BNT–7%BaTiO3. The P4bm structure exhibits antiparallel displacements of A- and B-site cations, which results in a small spontaneous polarization (Ps) of 13.3 µC/cm2 (estimated from the refined structural parameters). The results obtained for BNT–10%BMN ceramics strongly suggest ferrielectric features; the application of electric fields induces a transition from the P4bm phase with a small Ps to other ferroelectric phases with a large Ps.

Heavy Mn-doping effect on spontaneous polarization in ferroelectric BiFeO3 thin films

Mn doping effect on the crystal structure and spontaneous polarization (Ps) in ferroelectric BiFeO3 (BFO) thin films are experimentally and theoretically investigated. Polarization hysteresis measurements for BiFe1−xMnxO3 (BFMO) films grown on (100) SrTiO3 substrates with SrRuO3 electrodes indicate that remanent polarization (Pr) remains unchanged in the range of 0 ≤ x ≤ 0.1. As a result of the density-functional theory (DFT) calculations for the rhombohedral, tetragonal, and monoclinic phases of BFO and BFMO (x = 0.5), we find that the monoclinic phase with compressive strain from the substrates yields the most reasonable Ps. The DFT calculations also indicate that heavy doping of up to 50% Mn does not lead to a considerable deterioration in Ps, which is consistent with the experiments. Our investigations demonstrate that the ferroelectric polarization in the BFMO system can be retained for a wide range of Mn content either in bulk or thin-film form.

Domain configuration changes under electric field-induced antiferroelectric-ferroelectric phase transitions in NaNbO3-based ceramics

We recently developed a feasible crystal chemistry strategy to stabilize the antiferroelectricity in NaNbO3 through a chemical substitution to decrease the tolerance factor and increase the average electronegativity of the system [Shimizu et al., Dalton Trans. 44, 10763 (2015) and Guo et al., J. Appl. Phys. 117, 214103 (2015)]. Two novel lead-free antiferroelectric (AFE) solid solutions, (1-x)NaNbO3-xCaZrO3 and (1-x)NaNbO3-xSrZrO3, have been found to exhibit the double polarization hysteresis typical of a reversible AFE ↔ ferroelectric (FE) phase transition. In this study, as demonstrated by (1-x)NaNbO3-xCaZrO3 system, the influence of chemical modification and electrical poling on the AFE/FE phase stability was investigated, primarily focusing on the microstructural and crystallographic evolutions. Together with the macroscopic polarization hysteresis measurements, a well-demonstrated structure-property relationship was presented. It was found that the CaZrO3 substitution into NaNbO3 can effectively destabilize the FE Q phase and correspondingly lead to a spontaneous reverting to AFE P phase. In contrast to the reversible AFE ↔ FE phase transition, the domain morphology evolution exhibits irreversible nature with a growing process of the orientational domains after applying electric field. Moreover, a multiple-zone axes electron diffraction map of P and Q phases has been summarized and is believed to be an efficient diagram to determine the AFE/FE nature of the NaNbO3-based systems.

Ferroelectric and ferromagnetic properties in BaTiO3 thin films on Si (100)

In this paper, we report on the epitaxial integration of room temperature lead-free ferroelectric BaTiO3 thin (∼1050 nm) films on Si (100) substrates by pulsed laser deposition technique through a domain matching epitaxy paradigm. We employed MgO and TiN as buffer layers to create BaTiO3/SrRuO3/MgO/TiN/Si (100) heterostructures. C-axis oriented and cube-on-cube epitaxial BaTiO3 is formed on Si (100) as evidenced by the in-plane and out-of-plane x-ray diffraction, and transmission electron microscopy. X-ray photoemission spectroscopic measurements show that Ti is in 4(+) state. Polarization hysteresis measurements together with Raman spectroscopy and temperature-dependent x-ray diffraction confirm the room temperature ferroelectric nature of BaTiO3. Furthermore, laser irradiation of BaTiO3 thin film is found to induce ferromagnetic-like behavior but affects adversely the ferroelectric characteristics. Laser irradiation induced ferromagnetic properties seem to originate from the creation of oxygen vacancies, whereas the pristine BaTiO3 shows diamagnetic behavior, as expected. This work has opened up the route for the integration of room temperature lead-free ferroelectric functional oxides on a silicon platform.

Polarization properties and crystal structures of ferroelectric (Ba,Ca)TiO3 single crystals

We have investigated the spontaneous polarization (P s ) of Ba 1-x Ca x TiO 3 (BCT) by polarization hysteresis measurements using single crystals and by density functional theory (DFT) calculations. Single crystals of BCT (x = 0.07) were grown by a top-seeded solution growth (TSSG) method. The polarization measurements show that the crystals (x = 0.07) have a P s of 26.0 μC/cm2, which is slightly small compared with BaTiO 3 (27.3 μC/cm2). Our DFT calculations based on a supercell approach show that Ca atoms are markedly displaced cooperatively with the adjacent Ti atoms along the P s direction. It is suggested that the CaTiO 3-like octahedral rotation is constructed in the BCT supercell around the Ca atoms, which is the origin of the smaller P s observed for the BCT crystals.

Polarization Switching Ability Dependent on Multidomain Topology in a Uniaxial Organic Ferroelectric

The switching of electric polarization induced by electric fields, a fundamental functionality of ferroelectrics, is closely associated with the motions of the domain walls that separate regions with distinct polarization directions. Therefore, understanding domain-walls dynamics is of essential importance for advancing ferroelectric applications. In this Letter, we show that the topology of the multidomain structure can have an intrinsic impact on the degree of switchable polarization. Using a combination of polarization hysteresis measurements and piezoresponse force microscopy on a uniaxial organic ferroelectric, α-6,6'-dimethyl-2,2'-bipyridinium chloranilate, we found that the head-to-head (or tail-to-tail) charged domain walls are strongly pinned and thus impede the switching process; in contrast, if the charged domain walls are replaced with electrically neutral antiparallel domain walls, bulk polarization switching is achieved. Our findings suggest that manipulation of the multidomain topology can potentially control the switchable polarization.

Strain actuation of barium titanate single crystals under electromechanical loading in the non-polar [110] direction

The switching characteristics of BaTiO3 single crystals subjected to uniaxial electromechanical loading in the non-polar [110] direction at room temperature and 55 ° C were investigated. Polarization and strain hysteresis measurements in the [110] direction revealed that under the combination of large in-plane switching coercivities and a strong out-of-plane depolarization field (induced by the shape irregularity and large unshielded surfaces of the crystal sample), strain-inducing 90° in-plane to out-of-plane (or vice versa) switching was the dominant switching behavior even in the absence of mechanical bias. By lowering the in-plane switching coercivities, which was achieved by increasing the loading temperature from room temperature to 55 ° C, a contrasting non-strain-inducing switching behavior characterized by 90° and/or 180° switches between the in-plane variants was observed instead. The contrasting switching behavior at two different temperatures indicates that apart from the bias stress magnitude, the combined effect of the depolarization field and switching coercivity is another critical factor governing the strain actuation of BaTiO3 single crystals. The [110] electromechanical loading responses reveal the possibility of increasing the strain capacity of BaTiO3 single crystals by introducing accompanying depolarization fields which promote strain-inducing 90° switching. Such an approach could be potentially useful when bias stress-activated ferroelastic switching is not attainable.

Pulsed Laser Deposition and Ferroelectric Characterization of Nanostructured Perovskite Lead Zirconate Titanate (52/48) Thin Films

Perovskite lead zirconate titanate nanostructured (PZT) thin films with Zr/Ti ratio of 52/48 were deposited on Pt/TiO2/SiO2/Si(100) substrate using pulsed laser deposition method. A metal/ ferroelectric/metal (MFM) structure was used for ferroelectric property measurements, formed by depositing gold electrode on top of the film. A Nd:YAG UV laser having a wavelength of 355 nm and an energy fluence of -2.7 J/cm2 was used to deposit the film. The film was deposited on platinum (Pt) coated silicon substrate at the substrate temperature of 600 degrees C and the base vacuum of 10(-6) mbar. The scanning electron microscopy (SEM) images revealed well-crystallized films with a fine microstructure and an average grain size of - 50 nm. The ferroelectric properties of the film were studied and the results were discussed. The voltage dependent Polarization versus Electric field hysteresis measurements of PZT (52/48) pellet showed a well-defined hysteresis loop with a fairly high remnant polarization (P(r)) and low coercive field (E(c)).

Structure and properties of La-modified Na0.5Bi0.5TiO3 at ambient and elevated temperatures

The crystal structure and property changes of sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) piezoelectric ceramics are reported as a function of La modification (0.5–2.0 at. %) and increasing temperature using high resolution x-ray diffraction, permittivity, depolarization, and polarization and strain hysteresis measurements. La substitution is found to decrease the depolarization temperature of NBT (e.g., 1.5 at. % La substitution lowers the depolarization temperature by 60 °C relative to the unmodified composition) with little impact on the room temperature polarization and strain hysteresis. The room temperature structures of the various NBT compositions were modeled using a mixture of the monoclinic Cc space group and the cubic Pm3¯m phase, where the Pm3¯m phase is used to model local regions in the material which do not obey the long range Cc space group. With increasing La substitution, the lattice parameter distortions associated with the Cc phase approached that of the prototypical cubic unit cell and the fraction of the Pm3¯m phase increased. The relationship between these crystallographic changes and the depolarization behavior of La-modified NBT is discussed.

THE EFFECT OF TEMPERATURE ON SYNTHESIS AND CHARACTERIZATION OF BMN–BASE PIEZOELECTRIC THIN FILM PREPARED BY SPRAY PYROLYSIS

The 0.65Ba(Mg1/3,Nb2/3)O3-0.35BaTiO3 thin films was deposited on glass/aluminum substrates by using spray pyrolysis method. The effect of annealing temperature on perovskite phase and crystalline structure analysis of the BMN-BT films was performed on an X-ray diffraction (XRD). Surface morphology, roughness and particle size of the BMN-BT thin films was characterized with atomic force microscopy (AFM) and scanning electronic microscopy (SEM). Polarization hysteresis measurements was performed at different temperatures. From the P–E hysteresis loops one can observe the best temperature that led to improved properties of dielectric constant, polarization and piezoelectric coefficients were 600°C.