Ultrathin Ferroelectric Films Research Articles

Non-volatile memory characteristics of epitaxially grown PVDF-TrFE thin films and their printed micropattern application

Highly ordered poly(vinylidene fluoride- co-trifluoroethylene) (PVDF-TrFE) ultrathin films epitaxially grown on friction-transferred polytetrafluoroethylene (PTFE) surface were incorporated in the Metal-Ferroelectric-Metal (MFM) and Metal-Ferroelectric-Insulator-Semiconductor (MFIS) memory structure. The non-volatile memory properties in epitaxially ordered ferroelectric films were characterized with polarization and capacitance hysteresis curves at low voltage sweep of ±12 V and ±5 V in each stacking structure of MFM and MFIS, respectively. Furthermore, we present the facile micro- and nano-patterning method of fabricating MFM arrays including highly ordered PVDF-TrFE films by microimprinting. Ultrathin ferroelectric polymer films grown by epitaxy were microimprinted with a silver coated polydimethylesiloxane (PDMS) mold at 170 °C with excellent quality and the simultaneous transfer of silver electrodes on the imprinted PVDF-TrFE enabled us to fabricate the arrays of MFM capacitors in which ferroelectricity in imprinted region was well-maintained after patterning process at high temperature above 170 °C with good thermal stability.

Direct spectroscopic evidence of charge reversal at the Pb(Zr0.2Ti0.8)O3/La0.7Sr0.3MnO3heterointerface

At the heterointerface of a top ferroelectric Pb(Zr${}_{0.2}$Ti${}_{0.8}$)O${}_{3}$ (PZT) ultrathin film and a bottom La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$ (LSMO) electrode, we used continuous synchrotron-radiation photoelectron spectroscopy to probe in situ and demonstrated that the interfacial charges are reversible and their affected valence-band barrier height becomes modulated upon switching the polarization in the top layer. By monitoring the core-level shifting of the buried LSMO layer under continuous illumination of synchrotron radiation, we directly observed a temporal screening of polarization induced by the photon-generated carriers in the top PZT layer. This dynamic characterization of the core-level shifting of the buried layer demonstrates an effective method to probe the electric conduction and ferroelectric polarization of an ultrathin ferroelectric oxide thin film.

Quantum Effect on SrRuO3/BaTiO3/SrRuO3 Ferroelectric Ultrathin Film Capacitor

We have carried out a detailed investigation on the quantum effect on SrRuO3/BaTiO3/SrRuO3 ferroelectric ultrathin film capacitors fully strained with a SrTiO3 substrate. We employ the transverse field Ising model, taking into account the incomplete charge compensation of the realistic SrRuO3 electrode and the misfit strain imposed by the SrTiO3 substrate in the Hamiltonian, to quantitatively explain the experimental observation in the literature. It is found that quantum mechanism under the influence of the incomplete charge compensation of the electrode plays an important role in determining the ferroelectric properties of BaTiO3 ultrathin films between two metallic electrodes.

Spin‐coated organic ferroelectric films for non‐volatile memories

AbstractFor low cost memory devices like ferroelectric field‐effect transistors (FeFETs) or cross bar arrays, organic materials are important, due to their processability from solutions at room temperature. The copolymer Poly[vinylidene fluoride trifluoroethylene] (P[VDF/TrFE]) is a candidate for a possible application as a ferroelectric component in such structures. In order to improve the performance of FeFETs with P[VDF/TrFE], we investigate metal–ferroelectric–insulator–semiconductor (MFIS) capacitors. Implementing a method proposed by Miller and McWorther, we simulate the capacitance‐voltage (C–V) characteristics of MFIS stacks and the hysteresis due to the ferroelectric switching of the copolymer. We vary the parameters thickness and permittivity of the insulator as well as the as the thickness of the ferroelectric layer and predict in that way conditions where a saturation of the polarization can be reached. We also report on retention and cycle endurance measurements. Furthermore, we show measurements of the dipole alignment of ultrathin ferroelectric copolymer films, down to 10 nm thickness. No ferroelectric threshold was found, even for this thickness, if an adapted system of electrodes is used. Here, we introduce the near edge X‐ray absorption fine structure (NEXAFS) method as a new tool for the characterization of ferroelectric dipole switching.

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.