Properties Of Ferroelectric Thin Films Research Articles

Fine tuning epitaxial strain in ferroelectrics: PbxSr1−xTiO3 on DyScO3

Epitaxial strain can be efficiently used to modify the properties of ferroelectric thin films. From the experimental viewpoint, the challenge is to fine-tune the magnitude of the strain. We illustrate here how, by using a suitable combination of composition and substrate, the magnitude of the epitaxial strain can be controlled in a continuous manner. The phase diagram of PbxSr1−xTiO3 films grown epitaxially on (110)-DyScO3 is calculated using a Devonshire–Landau approach. A boundary between in-plane and out-of-plane oriented ferroelectric phases is predicted to take place at x≈0.8. A series of PbxSr1−xTiO3 epitaxial films grown by molecular beam epitaxy shows good agreement with the proposed phase diagram.

Drastic reduction of leakage current in ferroelectric Bi 3.15Nd 0.85Ti 3O 12 films by ionizing radiation

Bi 3.15Nd 0.85Ti 3O 12 (BNT) ferroelectric thin films were grown on Pt/Ti/SiO 2/Si substrates by a sol–gel method. The samples were exposed to different dosage of ionizing radiation. Distortions in the crystalline structure, changes in the surface roughness and decrease in the remnant polarization were observed after radiation. For films subjected to 100 Mrad radiation, decrease in the leakage current up to four orders of magnitude was also observed, which can be explained by the increased grain boundary barrier height. Our results suggest that ionizing radiation could be an effective tool in modifying the properties of ferroelectric thin film for use in non-volatile memory devices.

Interface Modifications of Lead Zirconate Titanate Thin Films

Electrical properties of ferroelectric thin films depend on the quality of not only the ferroelectric thin layer but also the ferroelectric/electrode interfaces. In the present study, attempts were made to minimize the Pb depletion at the bottom electrode/ferroelectric interface in lead zirconate titanate ferroelectric thin films by adding a PbO interfacial layer using a chemical solution deposition method. The crystallographic structure of the PZT thin films with and without PbO layer were examined by X-ray diffraction. The microstructure of the thin film samples was characterized using optical microscope and atomic force microscope.

Improved photoluminescence and ferroelectric properties of (Bi3.6Eu0.4)Ti3O12thin films via Li+doping

(Bi(3.6)Eu(0.4))Ti₃O₁₂ (BEuT) thin films with different Li+ doping contents were prepared on fused silica and Pt/Ti/SiO₂/Si substrates by chemical solution deposition, and the effects of Li+ doping contents on the photoluminescence and ferroelectric properties of the thin films were investigated in detail. The results showed that an appropriate amount of Li+ doping could effectively improve emission intensities for two characteristic Eu³+ emission transitions of ⁵D₀→⁷F₁ (594 nm) and ⁵D₀→⁷F₂ (617 nm) compared with BEuT thin films without Li doping. This photoluminescence improvement can be attributed to the dual roles of Li+ ions, one of which is that Li ions can act as co-activators which are helpful to the energy transfer from the host to the Eu³+ ions, leading to a higher quantum yield; the other is that Li ion doping can induce local distortion of crystal field surrounding the Eu³+ activator because Bi³+ and Li+ ions have different ionic radii. In addition, the Li+-doped BEuT thin films had larger remanent polarization than BEuT thin films without Li doping prepared under the same experimental conditions. These results suggest that Li+ doping is an effective way to improve photoluminescence and ferroelectric properties of the (Bi,Eu)₄Ti₃O₁₂ thin films.

Effect of Zn doping on structure and ferroelectric properties of PST thin films prepared by sol–gel method

(Pb y Sr1−y )Zn x Ti1−x O3−x thin films were prepared on ITO/glass substrate by sol–gel process using dip-coating method. The phase structure, morphology and ferroelectric property of the thin film were studied. All the thin films show the typical perovskite phase structure. Both the crystallinity and c/a ratio of the perovskite phase increases initially and then decreases gradually with doping Zn in the thin film. Ferroelectric properties of the Zn-doped PST thin films, including ferroelectric hysteresis-loop, remnant polarization and coercive force, decrease gradually with increasing Zn. And the effect of Zn on ferroelectric properties is more obvious in PST thin film with high content of Pb than that with low Pb although the high lead thin film exhibits high intrinsic ferroelectric properties.

Phase-Field Modeling of Domain Structure Energetics and Evolution in Ferroelectric Thin Films

A computational model developed based on the phase-field approach is used to model domain structures in ferroelectric thin films and to quantify the effects of strain and applied electric field on the microstructural evolution, and on the induced dielectric, electrostrictive, and piezoelectric film properties. Theoretically predicted vortex-like polydomain and experimentally observed bidomain and monodomain film morphologies are modeled using the continuum phase-field approach. A nonlinear finite element method is used to solve the boundary value problems relevant to ferroelectric thin films. The computed results agree with the Kittel law for specific ranges of film strain. Simulations that track the domain structure evolution and compute ferroelectric thin film properties given the film dimensions and the imposed electromechanical boundary conditions are also reported.

Bi<sub>4</sub>Ti<sub>3</sub>O<sub>12</sub> and Bi<sub>3.25</sub>La<sub>0.75</sub>Ti<sub>3</sub>O<sub>12</sub> Thin Films Prepared by RF Magnetron Sputtering

Bi4Ti3O12 (BTO) and Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films were deposited on Pt/Si substrates by RF magnetron sputtering with Bi4Ti3O12 (BTO) and Bi3.25La0.75Ti3O12 (BLT) targets with 50-mm diameter and 5-mm thickness. The microstructure and ferroelectric properties of thin films were investigated. The grain growth behavior and ferroelectric properties such as remanent polarization were different in these two kinds of film, the effects of La doping in the BLT thin film were very obvious.

10.1007/s11451-008-3013-9

The effect of various metal electrodes on the properties of thin ferroelectric films is considered using the phenomenological Ginzburg-Landau theory. The electric field produced by charges on electrodes is taken into account (with allowance for the screening of the charges in the metals) in the free energy functional and in the Euler-Lagrange equation for the film polarization. This equation is solved using the variational method, and the free energy functional is reduced to the conventional free energy with a renormalized coefficient of P 2. This coefficient is dependent on the properties and thickness of the film and the properties of the electrode. Therefore, the physical characteristics of the size effect can be found by merely substituting the renormalized coefficient into the usual formulas from phenomenological theory. The calculations are shown to be in good agreement with the experimental data for Pt, Ir, IrO2, and SrRuO3.

Enhanced electrical properties of ferroelectric thin films with electric field induced domain control

Abstract (0 0 1) oriented Pb(Zr0.5Ti0.5)O3 (PZT) thin films with high piezoelectric constant were deposited by the electric field-assisted annealing (EFA-A) of alkoxide-derived precursor thin films. So far, selective orientation control of (0 0 1) domain and (1 0 0) domain is very difficult, especially for the chemical solution deposition (CSD). We tried an electric field induced domain control to improve the electrical properties with CSD. An electric field of 10 kV/cm has been applied during an annealing. The high (0 0 1) domain ratio of 75.6% was obtained from the deconvolution of (0 0 2) and (2 0 0) X-ray diffraction peaks. The PZT thin films showed very high piezoelectric constant of 352 pm/V. This shows electric field induced domain control is very effective to enhance the electrical properties of CSD-derived PZT thin films.

Monte Carlo Investigation of Ferroelectric Properties in Thin Films

In this work, the ferroelectric hysteresis in thin films was investigated with Monte Carlo simulation using the DIFFOUR Hamiltonian and the Metropolis algorithm. The field frequency, the filed amplitude and the thickness dependence of hysteresis properties were found. For instance, at high enough field, the area increases, maintains and reduces with increasing frequency. However, with increasing the film’s thickness, the area increases over the whole considered frequencies, and the frequency at maximum area shifts to a lower frequency due to the stronger ferroelectric coupling in thicker films. This strongly indicates the thickness dependence of the ferroelectric hysteresis. In addition, the power law scaling relation between the hysteresis properties and input parameters, which are the film’s thickness and the field parameters, were proposed. The scaling exponents were compared and discussed with previous investigations on thin ferroelectric films.

Strain on ferroelectric thin films

Strain engineering aims to take advantage of the stress field imposed by substrates on thin films. It requires an understanding of the consequences of stress fields on the physical properties of the deposited materials. This is achieved in ferroelectric thin films through the use of misfit-strain phase diagrams that show the stability regions for the possible phases. These encompass bulk phases as well as new ones exhibiting symmetries that are not present in the bulk. For the solid solution lead zirconate–lead titanate, Pb(Zr1−xTix)O3, monoclinic phases found in the bulk morphotropic phase boundary region and associated to concentrations exhibiting the highest properties can be stabilized on a wider range of composition in thin films. In addition, phases of lower symmetry can be stabilized through the use of anisotropic biaxial stress fields, generated by orthorhombic substrates for example. Another crucial aspect of the influence of biaxial stress fields is the generation of domain structures. Theoretical tools as well as experimental verifications have provided much insight on the underlying physics. We, therefore, present here an overview of the influence of both iso- and anisotropic biaxial stress fields on the structures and properties of ferroelectric thin films exemplified on Pb(Zr1−xTix)O3.

Effect of uniaxial stress on the electromechanical properties in ferroelectric thin films under combined loadings

The electromechanical properties of ferroelectric thin films under an alternating electric field and a static uniaxial compressive stress are investigated using the modified planar four-state Potts model. To implement the electromechanical properties and the coupling of the electrical and mechanical response, the mechanical energy density as well as the energy due to anisotropic switching between a-domain and c-domain are incorporated in the Hamiltonian. Besides, there are two contributions to the strain at each cell: eigenstrain and elastic strain. Our simulation results show that the longitudinal strain-electric field butterfly loop shifts downward along strain axis and that for the transverse strain shifts upward as the stress magnitude is increased. Moreover, the polarization-electric field hysteresis loop becomes a double-loop under a large compressive stress. The piezoelectric coefficient increases with the stress magnitude and reaches a maximum value at a critical stress level. It then gradually decreases to a small value at large stress magnitudes. Our results qualitatively agree with experimental ones.

Preparation and investigation of ferroelectric Pb(Zr0.53Ti0.47)O3 by modified Pechini method

This paper describes a new method, namely modified Pechini for one, for the preparation of nano-sized ferroelectric Pb(Zr0.53Ti0.47)O3 (PZT (53/47)) from low cost precursors. With this method, PZT (53/47) was prepared by microwave-assisted dissolving a pre-calcinated mixture of component oxides in dilute HNO3 for a short time and precipitating in NH4OH solution (pH = 9–10). On the other hand, the ferroelectric thin films PZT (53/47) on Al substrates were also prepared. The films and the ceramic powders of PZT (53/47) were obtained with high quality in terms of structure and microstructure. The ferroelectric properties of thin films have also been investigated.

POLARIZATION PROPERTIES OF FERROELECTRIC THIN FILMS UNDER DIFFERENT BOUNDARY CONDITIONS

By taking into account two types of boundary conditions (BC), free boundary condition (FBC) and zero boundary condition (ZBC), an improved transverse Ising model with the consideration of surface transition layers (STL) is used to describe the polarization properties of ferroelectric thin films in the framework of the mean field approximation. Functions representing the intra-layer and inter-layer couplings are introduced to characterize the BC and STL, which reflect the structure variation of film surface. Comparing the results obtained by employing FBC and ZBC, some effects of BC are derived in the model, which demonstrated that the BC play important roles on the properties of ferroelectric thin films. It is shown that the effect of ZBC can extend to deeper film than that of FBC. When different BC are adopted, competition between the fields (depolarization field and transverse field) and BC induces some different and interesting phenomena. Some theoretical results in this paper are found to be in reasonable accordance with experimental data and have some application importance.

Influence of lateral size on dielectric properties of ferroelectric thin films with structure transition zones

By taking into account structural transition zones near the lateral and thickness direction edges, this paper uses a modified transverse Ising model to study dielectric properties of a finite size ferroelectric thin film in the framework of the mean-field approximation. The results indicate that the influence of the lateral size on the dielectric susceptibility cannot be neglected and lateral structural transition zones could be a crucial factor that improves the mean susceptibility of the fixed size film.

The effects of surface transition layers on the phase diagrams and the pyroelectric properties of ferroelectric thin films

AbstractBy taking into account surface transition layers STLs and single surface transition layer SSLs (introduced for comparison), the phase‐transition and pyroelectric properties of ferroelectric thin films described by the transverse Ising model (TIM) are discussed in the framework of the effective‐field theory, based on the probability distribution technique. We discuss an N ‐layer film of simple cubic symmetry with nearest‐neighbor exchange interaction in which the exchange‐interaction strength is assumed to be different from the bulk values in surface layers. We derive and illustrate expressions for the phase‐transition and pyroelectric profiles. We demonstrate that the phase transition depends more strongly on intra‐layer interaction than inter‐layer interaction and the ferroelectricity of a film will disappear at a critical STL thickness if the intra‐layer and inter‐layer interactions are weak. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

STRAINED FERROELECTRIC THIN FILMS

Using thermodynamics approach, the present work analytically studies the effect of mismatch strains on the material properties of ferroelectric thin films. A one-dimensional model is first developed to illustrate the physical picture and the procedure of the theoretical approach. Then, the effect of non-equal mismatch strains is investigated by using the same theoretical approach.

PHASE TRANSITION PROPERTIES OF FERROELECTRIC THIN FILM WITH ONE DISTINCT INSERTING-LAYER

The mean-field expressions derived from the transverse Ising model are used to investigate the second-order ferroelectric phase transition. With this theory, the properties of ferroelectric thin films formed by inserting a monolayer of material B into material A in the middle was studied. Firstly, a recursive equation for the phase transition properties of the ferroelectric thin film with one distinct inserting-layer in the middle was obtained. Next, the effect of the exchange interaction and transverse field parameters of materials A and B on the phase diagrams was investigated. The results show that with the modification of the parameter values of the inserting-layer B, the properties of the phase transition, the crossover value of the transverse field, the polarizations of different layers, and the critical temperature, change sensitively.

Polarization properties of ferroelectric thin films on transverse ising model

AbstractBy taking into account surface transition layers (STL), an improved transverse ising model (TIM) with a depolarization field effect also considered is used to describe the polarization properties of ferroelectric thin films in the framework of the mean‐field approximation. Functions representing the intralayer and interlayer couplings are introduced to characterize STL, which makes the model more realistic compared to previous treatment of surface layers using uniform surface exchange interactions and a transverse field. By comparison with the results obtained from the traditional treatments for the thin films using only the single surface layer (SSL), some new results are derived by adding the STL in the model. The results also show that the effect of the transverse field is notable when tunneling is essential. The influence of the depolarization field is to flatten the spontaneous polarization profile and make the polarization distribution of films more homogeneous, which is consistent with the results obtained from the Ginzburg–Landau–Devonshire (GLD) theory. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Surface Transition Layers on Phase Transformation and Pyroelectric Properties of Ferroelectric Thin Film

Taking into account surface transition layers (STLs), we study the phase transformation and pyroelectric properties of ferroelectric thin films by employing the transverse Ising model (TIM) in the framework of the mean field approximation. The distribution functions representing the intra-layer and inter-layer couplings between the two nearest neighbour pseudo-spins are introduced to characterize STLs. Compared with the results obtained by the traditional treatments for the thin films using only the single surface transition layer (SSL), it is shown that the STL model reflects a more realistic and comprehensive situation of films. The effects of various parameters on the phase transformation properties have shown that STL can make the Curie temperature of the film higher or lower than that of the corresponding bulk material, and the thickness of STL is a key factor influencing the film properties. For a film with definite thickness, there exists a critical STL thickness at which ferroelectricity will disappear when the intra-layer and inter-layer interactions are weak.