Ultrathin Ferroelectric Films Research Articles

Effects of two relaxation processes on the dielectric spectra and conductivity of ultrathin ferroelectric polymer films

Based on the study of dielectric dispersion for 2-nm-thick Langmuir films made of an poly(vinylidenefluoride-trifluoroethylene) P(VDF-TrFE) copolymer, we demonstrated the existence of two simultaneously developing relaxation processes. It was shown that Langmuir-Blodgett films are conducting.

Phase transitions in ultrathin ferroelectric polymer films and nanocrystals

The dielectric dispersion of nanocrystals of vinylidene fluoride—trifluoroethylene copolymer of 70/30 composition was investigated. A phase transition was revealed in ferroelectric nanocrystals at T = 100°C. A wide temperature hysteresis was observed, indicating a first-order phase transition.

Use of the solvent chemistry for the control of the critical thickness of PbTiO3 ultrathin films

The preparation of high-quality ferroelectric PbTiO3-based ultrathin films by chemical solution deposition, using a diol-based sol-gel method, has proved to be successful. However, there is a critical thickness below which the films break up into isolated structures. According to previous studies, above a certain grain size to thickness ratio a microstructural instability occurs and the coatings are no longer continuous. We explore the use of the solvent chemistry to control this phenomenon, as an alternative to the more conventional variation of the crystallization parameters. The use of diols with short C chain lengths leads to films with smaller grain sizes, whose critical thicknesses are lower. A reduction from 40 to 15 nm is achieved by reducing the number of C of the diol used from 5 to 2. A critical value of G/t < 5.0 is necessary to obtain continuous ultrathin films with the processing conditions used.

Diffuse phase transitions in ferroelectric ultrathin films from first principles

A first-principles-derived scheme is developed and used to investigate the diffuse character of the paraelectric-to-ferroelectric transition in ultrathin films made of ${\text{BaTiO}}_{3}$, and to reveal its dependency on mechanical and electrical boundary conditions, as well as on thickness. It is found that such diffuse character does not require the presence of defects to occur, unlike commonly believed, but rather can originate from surface-induced dipolar inhomogeneities in defect-free films.

Dependence of Curie temperature on the thickness of an ultrathin ferroelectric film

The thickness dependency of the Curie temperature in stress-free $\text{Pb}({\text{Zr}}_{0.5}{\text{Ti}}_{0.5}){\text{O}}_{3}$ ultrathin films under open-circuit conditions is revealed from the computation of some nontrivial statistical quantities (such as fourth-order cumulants), via a first-principles-based technique. For thicknesses above $16\text{ }\text{\AA{}}$, this dependency follows the usual finite-size scaling law with a critical exponent that is consistent with the one associated with the three-dimensional-random-Ising universality class. On the other hand, the Curie temperature-versus-film's thickness curve deviates from this scaling law below $12\text{ }\text{\AA{}}$ while being rather well described by an empirical equation down to $8\text{ }\text{\AA{}}$.

Size dependent domain configuration and electric field driven evolution in ultrathin ferroelectric films: A phase field investigation

Size dependent domain configuration and its evolution under an external electric field are investigated for ultrathin ferroelectric films using an unconventional phase field method. The simulation reveals a series of domain configurations at different thicknesses, including zigzag patterns with eight variants or four variants coexisting, a vortex pattern with four variants coexisting, and a stripe pattern with two variants coexisting. When the film thickness falls below a critical value of 2.8 nm, the polarization vanishes, indicating the suppression of ferroelectricity. The evolution of domain configuration under an alternating electric field is also investigated, and the reduction in remnant polarization and coercive field with respect to decreasing thickness is observed.

Giant tunnel electroresistance with PbTiO3 ferroelectric tunnel barriers

The persistency of ferroelectricity in ultrathin films allows their use as tunnel barriers. Ferroelectric tunnel junctions are used to explore the tunneling electroresistance effect—a change in the electrical resistance associated with polarization reversal in the ferroelectric barrier layer—resulting from the interplay between ferroelectricity and quantum-mechanical tunneling. Here, we use piezoresponse force microscopy and conductive-tip atomic force microscopy at room temperature to demonstrate the resistive readout of the polarization state through its influence on the tunnel current in PbTiO3 ultrathin ferroelectric films. The tunnel electroresistance reaches values of 50 000% through a 3.6 nm PbTiO3 film.

Nanoscale domain patterns in ultrathin polymer ferroelectric films

High-resolution studies of domain configurations in Langmuir–Blodgett films offerroelectric polymer poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE),have been carried out by means of piezoresponse force microscopy (PFM).Changes in film thickness and morphology cause significant variations inpolarization patterns. In continuous films and nanomesas with relatively lowthickness/grain aspectratio (<1/10), the relationship between the average domain size and thicknessfollows the Kittel law. Nanomesas with high aspect ratio (>1/5) exhibit significant deviations from this law, suggesting additional surface-energy-relatedmechanisms affecting the domain patterns. Polarization reversal within a single crystallitehas been demonstrated and local switching parameters (coercive voltage and remnantpiezoresponse) have been measured by monitoring the local hysteresis loops. Reliablecontrol of polarization at the sub-grain level demonstrates a possibility of studyingthe mechanism of the intrinsic switching behavior down to the molecular scale.

The Inlaid Al2O3 Tunnel Switch for Ultrathin Ferroelectric Films

The Inlaid Al₂O₃ Tunnel Switch for Ultrathin Ferroelectric Films

Preparation and Characterization of Ferroelectric Composite Ultrathin Film by Electrostatic Self-assembly

The ferroelectric ultrathin multilayer of PDDA/PVDF-BT were fabricated on the pretreated quartz substrates between cationic electrolyte of poly (diallyldimethyl ammonium chloride) (PDDA) and anion reagent by electrostatic self-assembly method. Negatively charged poly (vinylidene fluoride) (PVDF) dispersed with inorganic BaTiO3 nano-granules in organic solvent was using as anion reagent. The thickness of the 12-layer ferroelectric composite ultrathin films characterized by quartz crystal microbalance (QCM) is about 82nm,and there exists a nearly linear relationship between the thickness and layer number of ultrathin films. The results of SEM and XRD show that the composite ultrathin films are deposited on the quartz surface smoothly and densely with uniform distribution of BaTiO3 nano-granules,and excellent crystalline performance. The hysteresis loop of ferroelectric composite ultrathin films shows that the electric polarization of the composite ultrathin films is increased to 3μC/cm2.

Photovoltaic mechanisms in ferroelectric thin films with the effects of the electrodes and interfaces

It was discovered that the dielectric constant of the electrodes substantially determines the photovoltaic output in (Pb0.97La0.03)(Zr0.52Ti0.48)O3 ferroelectric thin films. With the screening charges distributed extensively away from the electrode interfaces, the use of the electrodes with a high dielectric constant gives rise to dramatically enhanced magnitude of photocurrent in photovoltaic thin films, and extremely high photovoltaic efficiency is theoretically predicted to be possible in ferroelectric ultrathin films or nanostructures.

Absence of Critical Thickness in an Ultrathin Improper Ferroelectric Film

We study the ferroelectric stability and surface structural properties of an oxygen-terminated hexagonal YMnO3 ultrathin film using density functional theory. Under an open circuit boundary condition, the ferroelectric state with the spontaneous polarization normal to the (0001) surface is found to be metastable in a single domain state despite the presence of a depolarizing field. Our results highlight that improper ferroelectric ultrathin films can have rather unique properties that are distinct from those of very thin films of ordinary ferroelectrics.

Size effect on SrRuO 3/BaTiO 3/SrRuO 3 ferroelectric ultrathin film capacitor

We have carried out a detailed investigation on the size effect on SrRuO 3/BaTiO 3/SrRuO 3 ferroelectric ultrathin film capacitors with film thickness fully strained with a SrTiO 3 substrate. We employ the transverse field Ising model, taking into account the incomplete charge compensation of the realistic SrRuO 3 electrode and the misfit strain imposed by the SrTiO 3 substrate in the Hamiltonian, to quantitatively explain the experimental observation in the literature. It is found that BaTiO 3 ultrathin films between two metallic electrodes lose their ferroelectric properties below a critical thickness of about 4.17 nm due to the enhancement of the quantum effect under the influence of the incomplete charge compensation of the electrode.

Polarization switching in epitaxial films of BaTiO3: A molecular dynamics study

We use molecular dynamics simulations with a first-principles model Hamiltonian to study polarization switching in ultrathin epitaxial films of BaTiO3 sandwiched by ideal electrodes as a function of temperature and epitaxial strain. We find that the coercive fields of polarization switching reduce with tensile epitaxial strain and as temperature increases up to the transition, and depend sensitively on the nature of the epitaxial constraint. Our results should be directly relevant to the design of high frequency ferroelectric random access memories made with ultrathin epitaxial ferroelectric films.

Fabrication of continuous ultrathin ferroelectric films by chemical solution deposition methods

The integration of ferroelectrics in nanodevices requires firstly the preparation of high-quality ultrathin films. Chemical solution deposition is considered a rapid and cost-effective technique for preparing high-quality oxide films, but one that has traditionally been regarded as unsuitable, or at least challenging, for fabricating films with good properties and thickness below 100 nm. In the present work we explore the deposition of highly diluted solutions of pure and Ca-modified lead titanates to prepare ultrathin ferroelectric films, the thickness of which is controlled by the concentration of the precursor solution. The results show that we are able to obtain single crystalline phase continuous films down to 18 nm thickness, one of the lowest reported using these methods. Below that thickness, the films start to be discontinuous, which is attributed to a microstructural instability that can be controlled by an adequate tailoring of the processing conditions. The effect of the reduction of thickness on the piezoelectric behavior is studied by piezoresponse force microscopy. The results indicate that films retain a significant piezoelectric activity regardless of their low thickness, which is promising for their eventual integration in nanodevices, for example, as transducer elements in nanoelectromechanical systems.

High efficient photovoltaics in nanoscaled ferroelectric thin films

Photovoltaic effect in ferroelectric thin films with thickness below 100nm was investigated through both theoretical and experimental approaches. Unprecedented high photovoltaic power conversion efficiency around ∼0.28% was achieved with epitaxial (Pb0.97La0.03)(Zr0.52Ti0.48)O3 ferroelectric thin films, which is about 2 orders of magnitude higher than the reported in literature for ferroelectrics. Theoretical analysis indicated that efficiency can be further significantly improved by reducing the thickness in nanoscale. Extremely high efficient bulk photovoltaic effect is predicted in high quality ferroelectric ultrathin films.

Multiscale kinetic model for polarization switching in ferroelectric polymer thin films

Polarization switching in ferroelectric polymers is studied using a multiscale framework. A continuum Landau-Ginzburg-Devonshire model for a first-order phase transition is parametrized for ideal all trans chains of P(VDF-TrFE) (70:30) copolymers using data obtained from molecular-dynamics (MD) simulations. Thermal fluctuations and kinetics are accounted for by using a time-dependent Ginzburg-Landau model where the length and time scales, as well as the thermal noise amplitude, are also set from MD simulations. This method is used to investigate the nature of polarization switching in ferroelectric polymers and to test recent claims that ultrathin ferroelectric polymer films undergo intrinsic switching. Our simulations show that for a defect-free system, domain nucleation due to thermal fluctuations prevents homogeneous switching of the polarization, even at very small thicknesses. However, this nucleation does not substantially decrease the coercive field compared to the intrinsic value.

Phase transitions in ultra-thin ferroelectric films and fine period multilayers

Phase transition behaviour in ferroelectric materials is strongly dependent on boundary conditions, both electrical and mechanical. In this article, we cover several topics. Beginning with thin films, we look at the suppression of ferroelectricity as film thickness is reduced, covering two situations, the first in which the polarisation decreases and eventually is suppressed, and the second where the system forms domains in order to sustain ferroelectricity. The focus then moves to superlattices, highlighting the way that electrostatic coupling dominates the behaviour of superlattices, but revealing that for very thin layers interfacial effects can communicate through this electrostatic coupling to produce new kinds of macroscopic ferroelectric behaviour.

Ferroelectric Polymer Nanotubes with Large Dielectric Constants for Potential All‐Organic Electronic Devices

AbstractTo fabricate ferroelectric ultrathin polymer films with large dielectric constants for potential all‐organic electronic devices, ferroelectric polymer nanotubes and a composite of the nanotubes with a dispersed organic semiconductor have been fabricated by template‐assisted methods. The ferroelectricity drops markedly in spin‐coated ultrathin films less than 100 nm thick, whereas P(VDF‐TrFE) nanotubes with a wall thickness of a few ten nm sustain ferroelectricity. The composite nanotubes exhibit a giant dielectric constant as a result of significantly enhanced interface polarization between the nanosized fillers and the polymer matrix. They could be of practical use in supercapacitors, optoelectronic devices, and sensors.magnified image

Dependence of polarization on epitaxial strain in ferroelectric ultrathin films from first principles

First-principles-based methods are used to determine the response of polarization to epitaxial strain in films made of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ (BT) and $\mathrm{Pb}({\mathrm{Zr}}_{0.5}{\mathrm{Ti}}_{0.5}){\mathrm{O}}_{3}$ (PZT). Unlike in BT films, the strength of this response as well as its sign dramatically depend on the film's thickness and electrical boundary conditions in PZT films. A phenomenological model provides a rationale for these findings.