Properties Of Ferroelectric Films Research Articles

Enhanced photovoltaic properties of gradient calcium-doped BiFeO3 films

The photovoltaic properties of ferroelectric films have been extensively studied due to their potential applications in the fields of photodetection, energy conversion harvesting and storage. However, the effect of the gradient distribution of oxygen vacancies on the photovoltaic properties remains unclear. Herein, we prepared BiFeO3 (BFO) and two types of gradient calcium-doped BiFeO3 (BiFeO3/Bi0.95Ca0.05FeO2.975/Bi0.90Ca0.10FeO2.950/Bi0.85Ca0.15FeO2.925: BCFO-1 and Bi0.85Ca0.15FeO2.925/Bi0.90Ca0.10FeO2.950/Bi0.95Ca0.05FeO2.975/BiFeO3: BCFO-2) films deposited on fluorine-doped tin oxide glass substrates. Piezoresponse force microscopy studies indicate the upward self-polarization phenomenon in BFO and BCFO-1 films, while the downward self-polarization phenomenon in BCFO-2 films. The J–V characteristic curves show rectifier behaviors for these films due to the configuration of oxygen vacancies. For photovoltaic response, the open circuit voltage of BCFO-1 films is more than 2 times higher than that of BFO films, and the short-circuit photocurrent densities of BCFO-1 and BCFO-2 films are increased by nearly 32 and 6 times, respectively. The direction and magnitude of photovoltaic response are possibly associated with the energy band modulation governed by self-polarization and the gradient distribution of oxygen vacancies. The work demonstrates the merits of gradient doping with lower valence cation towards enhanced photovoltaic properties with high stability for ferroelectric oxides.

Optimization of electrical and photovoltaic properties of Au-BiFeO<sub>3</sub> nanocomposite films

Ferroelectric films, are an important class of photoelectric functional material, which possess the following characteristics: the breaking of their symmetry can lead to self-polarization and this polarization state can be regulated by external stimuli. The photovoltaic properties of ferroelectric films have been extensively investigated due to their potential applications in the field of photodetection, energy conversion harvesting and nonvolatile storage. In view of the small photocurrent density and the degradation of photovoltaic property caused by the depolarization effect in ferroelectric films, it is necessary to explore an approach to improving the self-polarization phenomenon and regulating the conduction mechanism to further optimize their photovoltaic properties. Here in this work, BiFeO<sub>3</sub> (BFO) films dispersed with Au nanoparticles are deposited on FTO glass substrates by the sol-gel method to obtain the Au-BFO nanocomposite films. Moreover, the relationships between Au content (0 mol%, 0.25 mol%, 0.5 mol%, 1 mol% and 3 mol%) and microstructure, electrical and photovoltaic properties of Au-BFO nanocomposite films are investigated to determine the optimal Au content. Piezoresponse force microscopy studies show that the Au-BFO nanocomposite film with 0.5 mol% Au has the strong self-polarization phenomenon. With the increase of Au content, the conduction mechanism of the Au-BFO nanocomposite films is described by the space-charge limited current theory but not the Schottky emission model any more. The photovoltaic properties of the Au-BFO nanocomposite films first increase and then decrease. When Au content is 0.5 mol%, the Au-BFO nanocomposite film has the best photovoltaic property. The open-circuit voltage and short-circuit photocurrent density of the Au-BFO nanocomposite film with 0.5 mol% Au increase nearly 3 and 5 times counterparts of the BFO film, respectively. The photovoltaic effects of Au-BFO nanocomposite films are improved mainly by regulating the self-polarization phenomenon and conduction mechanism. This study demonstrates the merits of BFO films dispersed with Au nanoparticles, specifically, the photovoltaic properties of Au-BFO nanocomposite films are further optimized. In this work, we propose a simple and effective method to regulate the electrical and photovoltaic properties of ferroelectric films, which provides a new perspective for further understanding the photovoltaic effects of ferroelectric films.

Создание и исследования структур металл-диэлектрик--полупроводник на основе сегнетоэлектрических пленок

Abstract The effect of the synthesis temperature on the microstructure and the electrophysical properties of metal–dielectric–semiconductor structures based on ferroelectric films of the composition Ba_0.8Sr_0.2TiO_3 upon the formation of p -type silicon substrates with [100] orientation is studied. It was experimentally established that an increase in the synthesis temperature leads to an improvement in the dielectric and piezoelectric properties of ferroelectric films. The temperature stability and stability in the behavior of the capacitance–voltage characteristics of MIS structures on the number of switching cycles are shown.

Влияние температуры синтеза на микроструктуру и электрофизические свойства пленок BST 80/20

The paper shows the effect of synthesis temperature on the microstructure and electrophysical properties of ferroelectric films of composition Ba0.8Sr0.2TiO3 when formed on silicon substrates with a platinum sublayer. On the basis of electrophysical and topographical measurements, a conclusion is made about the effect of the temperature of synthesis of ferroelectric films on their properties.

Планарные емкостные структуры на основе сегнетоэлектрических пленок титаната-станната бария на сапфире для сверхвысокочастотных применений

The structural properties of ferroelectric films of barium titanate-stannate on the sapphire substrates and the microwave characteristics of planar capacitors on their base are investigated. It was found that the composition of the gas ambient during the deposition of the film has a significant effect on the crystal structure, the phase composition of the films and their electrical characteristics. A low level of dielectric loss of planar capacitors with high tunability based on films of barium titanate stannate in the frequency range of 2-60 GHz was demonstrated.

Ink-jet printed BNT thin films with improved ferroelectric properties via annealing in wet air

In this work, an ink-jet printing process based on the sol-gel route was applied to prepare lead-free ferroelectric Na0.5Bi0.5TiO3(BNT) thin films for the first time. Dense and crack-free films with perovskite structure were obtained from a modified precursor solution through multiple printing and pyrolysis processes. The ferroelectric, dielectric and electrical properties were significantly affected by the annealing temperature and atmosphere. The film annealed at 670 °C in wet air showed a high remnant polarization of 24.7 μC/cm2 with a low coercive field of 263 kV/cm, the dielectric constant and loss were 185 and 0.1 at 10 kHz, respectively. It was found that wet air was an alternative to reduce oxygen vacancies and enhance properties of ferroelectric films, which can be explained by the defect chemical reaction between water and oxygen vacancies. X-ray photoelectron spectroscopy(XPS) confirmed the decrease of oxygen vacancies after annealing with water presence, with a formation of Ohmic conduction mechanism dominated by charged hydroxyl groups.

Domain Wall Motion in Perovskite Ferroelectrics Studied by the Nudged Elastic Band Method

Because of its unique physical properties, domain wall (DW) in ferroelectrics not only plays a key role in the electrical properties of ferroelectric film but also has shown tremendous prospect in the DW-based memory and logic devices. However, the motion mechanism of ferroelectric DW, which is of great significance for the application of ferroelectric films, has not been clearly understood. In this paper, the 180° DW motions in BaTiO3 (BTO) and PbTiO3 (PTO) were studied by using the nudged elastic band method based on the density functional theory. The evolutions of atomic structure, local polarization, and energy of the system for the DW motion process between the two adjacent equilibrium positions were systematically revealed. The DW migration across the oxygen vacancy was also simulated, and the corresponding potential well for the DW motion was obtained. It was also found that the DW motion barrier could be significantly influenced by the in-plane strain. The “activation field” deduced from the energ...

Enhancement of Polarization in Ferroelectric Films via the Incorporation of Gold Nanoparticles.

Ferroelectric thin films have been extremely studied for many applications such as nonvolatile memories, super capacitors, and solar cells. For these devices, improving the polarization properties of ferroelectric thin films is of great significance to their performance. Here, Au–lead zirconate titanate (PZT) nanocomposite thin films were prepared by a simple one-step chemical solution deposition (CSD) method on silicon substrates, and the effects of Au concentration on the ferroelectric properties were investigated. The experimental results show that the remanent polarization of the Au–PZT films with 1.2 mol % Au is about 80 μC/cm2, which is 50% higher than that of the pure PZT thin films. On the basis of the analysis of chemical valences, the enhanced polarization properties can be ascribed to the interaction between Au nanoparticles (Au NPs) and PZT at the Au–PZT interfaces. Our results demonstrate that the incorporation of an appropriate amount of Au NPs is an effective way to enhance the polarization properties of ferroelectric films. The Au–PZT nanocomposite thin films with excellent polarization properties on silicon substrates are expected to be widely used in integrated ferroelectric devices.

Dielectric and Optical Properties of Strontium–Barium Niobate Films in the Range of 0.2–1.3 THz

The knowledge of optical and dielectric properties of ferroelectric films, in particular, strontium–barium niobate films, in the terahertz spectral range is needed to use these films as a basis of active elements and structures for detection and control of terahertz radiation. The properties of strontium–barium niobate films with x = 0.5 grown on oriented sapphire substrates with a deposited electrode are studied by the method of terahertz time-domain spectroscopy in the spectral range of 0.2–1.3 THz. It is found that strontium–barium niobate films can be used to develop devices for detection and control of terahertz radiation.

Thermostable ferroelectric capacitors based on graded films of barium strontium titanate

The influence of the pressure of working gas during the ion–plasma sputtering on properties of deposited ferroelectric barium strontium titanate coatings has been experimentally studied. Variations in the of pressure of the working gas during deposition allows the component composition of the deposited layer to be changed, which leads to the diffusion of the phase transition and the improvement of temperature stability of properties of ferroelectric film. The gradation of layers has an impact on the temperature of the dielectric permittivity maximum, the shape of the dependence of the capacity on temperature, and the capacitance–voltage characteristics of the capacitor structures.

Ferroelectric HfO2 thin film testing and whole wafer mapping with non‐contact corona‐Kelvin metrology

We present results of a non‐contact, real‐time feedback corona‐Kelvin characterization of the ferroelectric properties of Si:HfO2 thin films. Measurements were performed on free dielectric surfaces. The method uses corona charging pulses coupled with Kelvin probe‐measured surface voltage and gives non‐contact QV and CV characteristics. Analogous to voltage biasing of metal‐insulator‐metal (MIM) capacitors, large corona charge biasing is found to provide an effective means for poling of the ferroelectric films. Subsequent small increment corona charging is used to acquire hysteresis loops and determine the coercive field and permittivity characteristics. Negative and positive coercive fields of about ±1.2 MV/cm for the 3.5 mol% Si film and ±1.0 MV/cm for the 4.6 mol% Si film were determined from the hysteresis loops. These findings were in agreement with MIM capacitor measurements on sister wafers. Non‐ferroelectric behavior was also confirmed for the 11.3 mol% Si film. Another strength of the corona‐Kelvin method is the ability to perform full wafer mapping to assess spatial non‐uniformities of important dielectric properties. Results of this work indicate a radial symmetry of ferroelectric film properties, such as the permittivity at the coercive field, that are consistent with processing conditions during the crystallization anneal.

Interface effect on leakage current of ferroelectric film

The miniaturization trend of ferroelectric memory puts forward higher requirements on the quality of ferroelectric thin films. One of the most important factors which restrict the use of ferroelectric thin films is the leakage current. The electrode and substrate show significant effects on the film leakage, in which the interface has been considered to play an important role. In order to study the interface effect on the leakage current, the transport of electrode/PbTiO3(PTO)/electrode ferroelectric capacitor was calculated by first principle method. Taking consideration of Pt, LaNiO3 electrodes and the PbO- or TiO2-terminated PTO thin film, we constructed four capacitor structures. The results indicated the current depends not only on the electrode types but also on the interface termination of film. On the whole, PTO thin film with oxide electrode has the greater current than that with metal electrode. Besides, PTO thin film with TiO2-terminated interface shows a larger leakage current than that with PbO-terminated interface. The interface barriers for the four ferroelectric capacitors were also investigated to analyze the intrinsic mechanism of interface effects. It was founded that there is an increase of leakage current following the decrease of the potential barrier and current of PTO thin film obeys tunneling emission conduction mechanism. It suggests that the interface barrier is the important factor which determines leakage and can be used to tune the electrical properties of ferroelectric films.

Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms

Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO3, KNbO3 (KNO), and NaNbO3 (NNO) with a thickness of 10–30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO3 (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.

Tuning and optimizing properties of ferroelectric films grown on a single substrate: A first-principles-based study

A first-principles-based effective Hamiltonian is used to reveal the effects of the growth direction on properties of films made of the prototype of ferroelectric materials (namely, ${\mathrm{BaTiO}}_{3}$) and grown on one of the most common and available substrates (that is, ${\mathrm{SrTiO}}_{3}$). Continuously varying the growth direction from the pseudocubic [001] to [110] direction, via the high-symmetry [111] direction, results in (i) an original temperature-versus-growth angle phase diagram possessing different structural states, (ii) a tuning of the Curie temperature ${T}_{c}$ by more than 450 K, (iii) a minimal value of ${T}_{c}$ being near room temperature, and (iv) optimization of important physical responses at room temperature, such as dielectric and piezoelectric coefficients, for some specific growth directions. The origins of these features are also revealed, including the finding of a law correlating ${T}_{c}$ with the growth direction.

Pyroelectric and dielectric properties of ferroelectric films with interposed dielectric buffer layers

The dielectric and pyroelectric properties of c-domain ferroelectric films with linear dielectric buffer layers were investigated theoretically. Computations were carried out for multilayers consisting of PbZr0.2Ti0.8O3 with Al2O3, SiO2, Si3N4, HfO2, and TiO2 buffers on metalized Si. It is shown that the dielectric and pyroelectric properties of such multilayers can be increased by the presence of the buffer compared to ferroelectric monolayers. Calculations for PbZr0.2Ti0.8O3 films with 1% Al2O3 interposed between electrodes on Si show that the dielectric and pyroelectric coefficients are 310 and 0.070 μC cm−2 °C−1, respectively. Both values are higher than the intrinsic response of PbZr0.2Ti0.8O3 monolayer on Si.

Impact of pulse poling on static and dynamic ferroelastic-domain contributions in tetragonal Pb(Ti, Zr)O3 films determined by in-situ x–ray diffraction analysis

The effects of bipolar pulse poling on the ferroelastic domain structure and their contribution to the electrical and piezoelectric properties of Pb(Ti0.7Zr0.3)O3 films are investigated. Micro x-ray diffraction measurements clearly show that the volume fraction of the c-domain increases irreversibly as the poling field is increased, leading to changes in the remanent polarization, dielectric constant, and piezoelectric coefficient. Theoretical estimations well explain the changes of remanent polarization and dielectric constant, but the increase in piezoelectric coefficient is much larger than the theoretical estimation. In-situ x-ray diffraction analysis under an electric field reveals that this disagreement is due to the unexpected activation of the ferroelastic domain wall motion. Our results provide new insight into the poling effect on the electric and piezoelectric properties of ferroelectric films.

Physical properties of ferroelectric film with continuous space charge distribution

We present a theory of ferroelectric phase transitions in a film with a homogeneously distributed space charge leading to the so-called built-in polarization. On the base of our complete solution of the problem of continuous space charge distribution in a ferroelectric film, we calculate the ferroelectric phase transition temperature , spatial polarization distribution in paraelectric and ferroelectric phases as well as hysteresis loop in the ferroelectric phase. We have shown that the presence of continuous space charge in a film inhibits the ferroelectricity in it: it lowers the ferroelectric phase transition temperature and coercive field. Our estimates of these differences show that they can be substantial and should be taken into account for ferroelectric thin films with built-in polarization.

Enhancement of ferroelectricity and large dielectric strength of Pb(Zr0.52Ti0.48)O3 thin films on SrTiO3 substrate with a SrRuO3 buffer

Epitaxial PbZr 0.52 Ti 0.48 O 3 (PZT) films with perfectly c-axis oriented tetragonal phase were deposited on SrTiO 3 (STO) substrates using a SrRuO 3 (SRO) buffer layer by pulsed laser deposition (PLD) method. Ferroelectric behavior of PZT on STO substrates along with an improved remnant polarization (2P r ) of 118 μC/cm2 and a low coercive field (E c ) of 193 kV/cm at 15 V were observed at room temperature indicating that the SRO/STO substrates with small lattice misfit can make some contributions to enhance PZT film's ferroelectric properties. Moreover, the capacitance characteristics of the PZT thin films were detected, a large dielectric constant of 1476, the charging and discharging characteristics determine the large dielectric strength of the PZT thin film capacitors.

Monte Carlo Investigation of Defect-Driven Ferroelectric Phase-Transition in Two Dimensional Defected Film

In this work, the hysteresis loops of ferroelectric film with defects were investigated with Monte Carlo simulation and the modified Heisenberg model (discrete vector model) with DIFFOUR type interaction. The effect of localized vacancy defects on the electrical properties of ferroelectric film was studied. The defected ferroelectric film was simulated on the two-dimensional lattice with periodic boundary conditions. From results, at a given electric field, the hysteresis loop area was found to depend on the defect concentration of system. The higher defect concentration, the smaller hysteresis loop was obtained, which resulted from weaker ferroelectric interaction.

Intrinsic and quantitative effects of in-plane strain on ferroelectric properties of Mn-doped BiFeO3 epitaxial films by in situ inducing strain in substrates

We report in situ manipulation of the in-plane strain εxx(BFMO) and coercive field EC(BFMO) of BiFe0.95Mn0.05O3 (BFMO) films epitaxially grown on La0.7Sr0.3MnO3 film buffered 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) substrates. PMN-PT poling-induced strain is effectively transferred to BiFe0.95Mn0.05O3 films and enhances εxx(BFMO) and EC(BFMO), with a gauge factor (ΔEC(BFMO)/EC(BFMO))/(δεxx) ∼−25 and −326 for the BFMO(001) and BFMO(111) films, respectively. Based on the strain dependence of EC(BFMO), we established a quantitative relationship between EC(BFMO) and εxx(BFMO). Using ferroelastic strain of PMN-PT, we achieved reversible and non-volatile modulation of strain and EC(BFMO) of BFMO films, providing an approach for non-volatile and reversible turning of strain and physical properties of ferroelectric films.