Phase transitions and domain structures in thin ferroelectric films

Recent advances in technology made available high quality thin ferroelectric films and ferroelectric–paraelectric multilayers. But understanding of the properties of these systems is far from being complete. In particular, it is not clear why various anomalies observed at phase transitions here are different from those in high quality bulk systems. The aim of the paper is to discuss some specific features of ferroelectric phase transitions in thin films and multilayers which are taken into account only partially by the theory. The discussion is limited to revealing the character of ferroelectric state forming just after the transition, i.e. if it is single-domain or multi-domain, if it is single-phase or two-phase. First, thin films with electrodes and on a substrate are discussed. The role of non-ideality of electrodes in realization of one of the first pairs of possibilities is emphasized. A more recent idea is that even for ideal electrodes a domain formation is possible when some conditions on the electrode–ferroelectric interface and the material constants of the material are met. This seems to be quite possible for the considered systems. Clamping by the substrate may lead to formation of a two-phase state which is practically unexplored for very thin films on substrates. Second, ferroelectric–paraelectric multilayers are considered which are more challenging for theoretical study than the thin films. Indeed, despite periodicity in the composition the domain structures are almost never periodic along the multilayer. The non-ideality of electrodes seems to lead to practical impossibility of single-domain ferroelectric phase transition in the multilayers of the considered type.     

Phase transitions and low-frequency dielectric dispersion in ferroelectric Langmuir-Blodgett films of the copolymer vinylidene fluoride with trifluoroethylene

The dielectric properties of multilayer ferroelectric Langmuir-Blodgett films based on the copolymer vinylidene fluoride with trifluoroethylene with 70/30 composition are investigated. Good agreement with theoretical models on the basis of the phenomenological Landau-Ginzburg approach is demonstrated for the first time for ultrathin films. Expressions describing the temperature variation of the permittivity in the temperature range of hysteresis and giving quantitative agreement with experimental data are obtained. It is shown that the Langmuir-Blodgett films are conducting. This conductivity does not depend on the frequency of the field. The results are explained by the fact that the motion of charge in the films is not bounded by domain walls. The jumps observed in the frequency dispersion at volume and low-temperature (surface) phase transitions are explained by a sharp increase in the relaxation times at the transition into the ferroelectric state.     

Interface effects in ferroelectric PbTiO3 ultrathin films on a paraelectric substrate

Interface effects on the ferroelectric behavior of PbTiO3 ultrathin films deposited on a SrTiO3 substrate are investigated using an interatomic potential approach with parameters fitted to first-principles calculations. We find that the correlation of atomic displacements across the film-substrate interface is crucial for the stabilization of the ferroelectric state in films a few unit cells thick. We show that the minimum film thickness for the appearance of a spontaneous polarized domain state is not an intrinsic property of the ferroelectric film but depends on the polarizability of the paraelectric substrate. We also observe that the substrate displays an induced polarization with an unusual oscillatory behavior.     

Stress-induced phase transition in ferroelectric domain walls of BaTiO3

The seminal paper by Zhirnov (1958 Zh. Eksp. Teor. Fiz. 35 1175-80) explained why the structure of domain walls in ferroelectrics and ferromagnets is drastically different. Here we show that the antiparallel ferroelectric walls in rhombohedral ferroelectric BaTiO(3) can be switched between the Ising-like state (typical for ferroelectrics) and a Bloch-like state (unusual for ferroelectric walls but typical for magnetic ones). Phase-field simulations using a Ginzburg-Landau-Devonshire model suggest that this symmetry-breaking transition can be induced by a compressive epitaxial stress. The strain-tunable chiral properties of these domain walls promise a range of novel phenomena in epitaxial ferroelectric thin films.     

Switching time dispersion and retention of bistable states in Langmuir-Blodgett ferroelectric films

Polarization switching and retention of each of the two polarized states in Langmuir—Blodgett (LB) ferroelectric films are studied using nonlinear dielectric spectroscopy. It is found that polarized states can be preserved for a long time, but the polarization dynamics in 10–40-nm-thick LB films is characterized by a considerable switching-time dispersion. In addition, ferroelectric LB films exhibit clearly manifested asymmetry of switching to states with opposite directions of polarization. To explain the experimental results, a polarization-switching model is proposed that takes into account the energy of interaction of a ferroelectric polymer with boundary surfaces. The effect of inhomogeneity of the LB film structure on the ferroelectric switching dynamics is also discussed.     

Self-polarization and migratory polarization in thin lead zirconate-titanate films

This paper reports on a study of the electrical properties of 0.7–1-μm-thick textured PZT ferroelectric films prepared by rf magnetron sputtering of a PbZr_0.54Ti_0.46O₃ target which additionally contained 10 mol % lead oxide. Such films are shown to feature a combination of a self-polarized state and migratory polarization. The totality of the data obtained suggest that the films had n-type conduction. As shown by the laser beam modulation technique, the polarization was distributed nonuniformly in depth, with most of the poled state localized near the lower interface of the thin-film ferroelectric capacitor. The mechanism underlying the onset of this self-polarization is related to the charging of the lower interface of the structure by electrons, which occurs during the cooling following the high-temperature treatment of the PZT film, and to poling of the bulk of the film by the charged interface. This mechanism of the self-polarization of ferroelectric films is believed to have a universal character. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 4, 2002, pp. 739–744. Original Russian Text Copyright ? 2002 by Pronin, Kaptelov, Tarakanov, Shaplygina, Afanas’ev, Pankrashkin.     

Ferroelectric polarization reversal tuned by magnetic field in a ferroelectric BiFeO3/Nb-doped SrTiO3 heterojunction

Interfacial resistive switching of a ferroelectric semiconductor heterojunction is highly advantageous for the newly developed ferroelectric memristors. Moreover, the interfacial state in the ferroelectric semiconductor heterojunction can be gradually modified by polarization reversal, which may give rise to continuously tunable resistive switching behavior. In this work, the interfacial state of a ferroelectric BiFeO₃/Nb-doped SrTiO₃ junction was modulated by ferroelectric polarization reversal. The dynamics of surface screening charges on the BiFeO₃ layer was also investigated by surface potential measurements, and the decay of the surface potential could be speeded up by the magnetic field. Moreover, ferroelectric polarization reversal of the BiFeO₃ layer was tuned by the magnetic field. This finding could provide a method to enhance the ferroelectric and electrical properties of ferroelectric BiFeO₃ films by tuning the magnetic field.     

Retention characteristics of Au/Bi3.25La0.75Ti3O12/Si metal-ferro- electric-semiconductor structure

Lanthanum-substituted bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂) (BLT) thin films were deposited on p-type Si(100) substrates using a chemical solution deposition process. The ferroelectric and dielectric properties of the films with an Au/BLT/Si structure were investigated. It was found that retention behaviors of the capacitors after polling with a negative and positive voltage were very different. The capacitor at an accumulation state exhibited a better retention characteristic than that at a depletion state. A rapid loss of memory for the capacitor at depletion state was found and attributed to the depolarization fields inside the ferroelectric film. It is proposed that the interaction between injected charges and ferroelectric polarization plays a role in the retention properties of the MFS capacitors.     

Electric-field-induced spin flop in BiFeO3 single crystals at room temperature

Bismuth ferrite, BiFeO3, is the only known room-temperature magnetic ferroelectric material. We demonstrate here, using neutron scattering measurements in high quality single crystals, that the antiferromagnetic and ferroelectric order parameters are intimately coupled. Initially in a single ferroelectric state, our crystals have a canted antiferromagnetic structure describing a unique cycloid. Under electrical poling, polarization reorientation induces a spin flop. We argue here that the coupling between the two orders may be stronger in the bulk than in thin films where the cycloid is absent.     

Effect of annealing on the self-poled state in thin ferroelectric films

It is shown that high-temperature treatment of self-poled lead zirconate-titanate films containing an excess of lead oxide, followed by prolonged storage at room temperature, results in a charge redistribution in the near-electrode regions of ferroelectric films. Such heat treatment destroys, as a rule, the self-poled state and removes the dielectric nonuniformity. A model of a thin-film ferroelectric capacitor is proposed which makes it possible to reproduce variations in the P-V hysteresis loop shape and capacity-voltage (C-V) characteristics, as well as in the frequency-dependent pyroelectric response (LIMM). The effect of the interface and grain boundaries on the onset of the self-poled state, its variation, and destruction is discussed in terms of the proposed model.     

Depolarization field and properties of thin ferroelectric films with inclusion of the electrode effect

The influence of metallic electrodes on the properties of thin ferroelectric films is considered in the framework of the Ginzburg-Landau phenomenological theory. The contribution of the electrodes with different screening lengths l _s of carriers in the electrode material is included in the free-energy functional. The critical temperature T _cl , the critical thickness of the film, and the critical screening length of the electrode at which the ferroelectric phase transforms into the paraelectric phase are calculated. The Euler-Lagrange equation for the polarization P is solved by the direct variational method. The results demonstrate that the film properties can be calculated by minimizing the free energy, which has a standard form but involves the coefficient of the term P². This coefficient depends not only on the temperature but also on the film thickness, the surface and correlation effects, and the electrode characteristics. The calculations of the polarization, the dielectric susceptibility, the pyroelectric coefficient, and the depolarization field show that the ferroelectric state of the film can be destroyed using electrodes from a material whose screening length exceeds a critical value. This means that the electrodes being in operation can induce a transition from the ferroelectric phase to the paraelectric phase. The quantitative criteria obtained indicate that the phase state and properties of thin ferroelectric films can be controlled by choosing the appropriate electrode material.     

Flexoelectricity induced increase of critical thickness in epitaxial ferroelectric thin films

Flexoelectricity describes the coupling between polarization and strain/stress gradients in insulating crystals. In this paper, using the Landau–Ginsburg–Devonshire phenomenological approach, we found that flexoelectricity could increase the theoretical critical thickness in epitaxial BaTiO₃ thin films, below which the switchable spontaneous polarization vanishes. This increase is remarkable in tensile films while trivial in compressive films due to the electrostriction caused decrease of potential barrier, which can be easily destroyed by the flexoelectricity, between the ferroelectric state and the paraelectric state in tensile films. In addition, the films are still in a uni-polar state even below the critical thickness due to the flexoelectric effect.     

Determination of the Steady State Leakage Current in Structures with Ferroelectric Ceramic Films

Steady state leakage currents have been investigated in capacitor structures with ferroelectric solgel films of lead zirconate titanate (PZT) formed on silicon substrates with a lower Pt electrode. It is established that Pt/PZT/Hg structures, regardless of the PZT film thickness, are characterized by the presence of a rectifying contact similar to p–n junction. The steady state leakage current in the forward direction increases with a decrease in the film thickness and is determined by the ferroelectric bulk conductivity.     

Nanodynamics of ferroelectric ultrathin films

The nanodynamics of ferroelectric ultrathin films made of PbTi(0.6)Zr(0.4)TiO(3) alloy is explored via the use of a first-principles-based technique. Our atomistic simulations predict that the nanostripe domains which constitute the ground state of ferroelectric ultrathin films under most electric boundary conditions oscillate under a driving ac field. Furthermore, we find that the atomically thin wall, or nanowall, that separates the nanodomains with different polarization directions behaves as an elastic object and has a mass associated with it. The nanowall mass is size-dependent and gives rise to a unique size-driven transition from resonance to relaxational dynamics in ultrathin films. A general theory of nanodynamics in such films is developed and used to explain all computational findings. In addition, we find an unusual dynamical coupling between nanodomains and mechanical deformations that could potentially be used in ultrasensitive electromechanical nanosensors.     

Study of Depolarization Field Influence on Ferroelectric Films Within Transverse Ising Model

An improved transverse Ising model is proposed by taking the depolarization field effect into account. Within the framework of mean-field theory we investigate the behavior of the ferroelectric thin film. Our results show that the influence of the depolarization field is to flatten the spontaneous polarization profile and make the films more homogeneous, which is consistent with Ginzburg-Landau theory. This fact shows that this model can be taken as an effective model to deal with the ferroelectric film and can be further extended to refer to quantum effect. The competition between quantum effect and depolarization field induces some interesting phenomena on ferroelectric thin films.     

Magnetic Skyrmions and Phase Transitions in Antiferromagnetic/Ferroelectric Bilayers

JETP Letters - We study in this paper the ground state and the properties of a skyrmions in magnetoelectric films, namely antiferromagnetic/ferroelectric superlattices in a support by steepest...     

A novel fabrication method for stoichiometric strontium bismuth tantalate thin films for memory devices

A novel method of processing ferroelectric thin films to eliminate excess bismuth is reported. Rapid thermal annealing initiates the crystallization of bismuth layer- structured ferroelectric thin films. Subsequent crystallization annealing for longer periods improves the crystallinity of the thin films. During annealing bismuth is known to diffuse to the surface and into the electrode, which is deleterious to the device performance. Forming-gas annealing after the rapid thermal annealing (prior to the crystallization annealing) reduces the bismuth diffusion into the electrode significantly. The suppression of bismuth diffusion into the electrode and the elimination of excess bismuth in the films improves and stabilizes the electrical properties of the capacitors, in particular their electronic conduction.     

Au/PZT/BIT/p-Si异质结的制备与性能研究

采用脉冲激光沉积(PLD)工艺,制备了以Bi₄Ti₃O₁₂(BIT)为过渡阻挡层的Au/PZT/BIT/p-Si异质结.研究了BIT铁电层对Pb(Zr_0.52Ti_0.48)O₃(PZT)薄膜晶相结构、铁电及介电性能的影响,对Au/PZT/BIT/p-Si异质结的导电机制进行了讨论.氧气氛530℃淀积的PZT为多晶铁电薄膜,与直接淀积在Si基片上相比,加入BIT铁电层后PZT铁 关键词： 铁电薄膜 异质结构 脉冲激光沉积(PLD)     

General features of the intrinsic ferroelectric coercive field

The value of the intrinsic ferroelectric coercive field is obtained independently, from general energy considerations and from the predictions of several models of the ferroelectric state. All predictions yield a value of the order of the depolarization field, which is equal to the spontaneous polarization divided by the dielectric permittivity, and are consistent with the recent measurements of the intrinsic ferroelectric coercive field in ultrathin Langmuir-Blodgett films of copolymers of polyvinylidene fluoride with trifluoroethylene. Prior studies succeeded only in measuring the much smaller extrinsic coercive fields, which are limited by nucleation processes and domain motion.     

Efficiency of thermoelectric conversion in ferroelectric film capacitive structures

Thermal heating/cooling conditions for metal-insulator-metal structures based on barium strontium titanate ferroelectric films are studied by numerical methods with the aim of their application in capacitive thermoelectric converters. A correlation between the thermal and capacitive properties of thin-film ferroelectric capacitors is considered. The time of the temperature response and the rate of variation of the capacitive properties of the metal-insulator-metal structures are determined by analyzing the dynamics of thermal processes. Thermophysical calculations are carried out that take into consideration the real electrical properties of barium strontium titanate ferroelectric films and allow estimation of thermal modulation parameters and the efficiency of capacitive thermoelectric converters on their basis.