Properties Of Ferroelectric Thin Films Research Articles

Effects of strain gradient on charge offsets and pyroelectric properties of ferroelectric thin films

The Landau-Ginzburg-Devonshire theory is used to study the effects of the strain gradient due to the epitaxial stresses in ferroelectric thin films sandwiched between two different substrates. The polarization in the film is found to be nonuniform, resulting in charge offsets and an asymmetric hysteresis response with characteristics similar to those in compositionally graded ferroelectric materials. The authors’ results suggest that the charge offset and pyroelectric effects can also be produced with effect of the strain gradient in film. In addition, such effects are found to be sensitive to an applied load.

Phase transition properties of ferroelectric thin films with two surface layers

The phase diagrams of ferroelectric thin films with two surface layers described by the transverse Ising model have been studied under the mean-field approximation. We discuss the effects of the exchange interaction and transverse field parameters on the phase diagrams. The results indicate that the phase transition properties of the phase diagrams can be greatly modified by changing the transverse Ising model parameters. In addition, the crossover features of the parameters from the ferroelectric dominant phase diagram to the paraelectric dominant phase diagram are determined for ferroelectric thin films with two surface layers.

Simulation of interface dislocations effect on polarization distribution of ferroelectric thin films

Effects of interfacial dislocations on the properties of ferroelectric thin films are investigated, using the dynamic Ginzburg–Landau equation. Our results confirm the existence of a dead layer near the film/substrate interface. Due to the combined effects of the dislocations and the near-surface eigenstrain relaxation, the ferroelectric properties of about one-third of the film volume suffers.

Diffusion processes in seeded and unseeded SBT thin films with varied stoichiometry

SrBi 2Ta 2O 9 (SBT) is a bismuth layered perovskite (BLP) with interesting ferroelectric properties for memories applications. The previous study on the synthesis of seeded and unseeded SBT thin films by the authors [G. González Aguilar, M.E.V. Costa, I.M. Miranda Salvado, J. Eur. Ceram. Soc. 25 (2005) 2331] has shown an increase of the crystallinity of the films and an improvement of the thin film ferroelectric properties when using SBT seeds. However, the detailed role of the seeds as an improver of the thin film properties has not been investigated so far. In the present work we study the role of the seeds, particularly with respect to the reactions between film and (bottom) underlying platinum electrode. The comparison of the results obtained by characterizing the seeded and unseeded thin films via Rutherford backscattering (RBS) and particle induced X-ray emission (PIXE) techniques reveals an effective modification of the substrate–thin film interface by the presence of the seeds. Moreover, the evaluation of the thin film ferroelectric properties by atomic force microscopy (AFM) shows an improvement of the local piezoelectric hysteresis loops by the seeds. These seeding effects as well as those observed in non-stoichiometric SBT thin films with different bismuth contents are used to discuss the barrier-like role of the SBT seeds against reactions between film and the platinum electrode and its contribution to the improvement of the thin film properties.

Influence of imperfect surface layers on dielectric and pyroelectric properties of ferroelectric thin film

Using the Ginzburg–Laudau–Devonshire theory and taking into account structural difference between imperfect surface layers and bulk ferroelectrics, the dielectric and pyroelectric properties of ferroelectric thin films coated with two metallic electrodes have been studied. The numerical results show that the effects of imperfect surface layers are to increase the effective dielectric susceptibility at room temperature through lowering the phase transition temperature. Both the dielectric susceptibility peak and pyroelectric coefficient peak shift to lower temperatures due to the contribution of the imperfect surface layers. The incompletely screened non-uniform depolarization field makes the susceptibility distribution more uniform.

Investigation of electromechanical properties in ferroelectric thin films using Monte Carlo simulation

Electromechanical properties of ferroelectric thin films are investigated using a two-dimensional four-state Potts model and Monte Carlo simulation. In this model, the mechanical energy density induced by strains of individual cells is included in the system Hamiltonian, in addition to the contributions from dipole-dipole and electric-field-dipole couplings. Moreover, the dipole of each individual perovskite cell is aligned to one of the four mutually perpendicular directions. Four different states of dipole orientations can be defined. The deformation of each cell is associated with its dipole orientation, resulting in two different strain states. Polarization–electric-field hysteresis loops, butterfly loops for both transverse and longitudinal strains against electric field, as well as the phase-transition temperature under different stresses and anisotropy conditions are simulated. Results are comparable to the experimental measurements.

Influence of semiconducting electrodes on properties of thin ferroelectric films

AbstractThe influence of semiconducting electrodes on the properties of thin ferroelectric films is considered within the framework of the phenomenological Ginzburg–Landau theory. The contribution of the electric field produced by charges in the electrodes allowing for the screening length of the carriers is included in the functional of the free energy and so in the Euler–Lagrange equation for the film's polarization. Application of the variational method to the solution of this equation allows the transformation of the free energy functional into a conventional type of free energy with renormalized coefficients. The obtained dependence of the coefficients on the film thickness, temperature, electrodes, carrier screening length, and other parameters make it possible to calculate the dependence of the properties on these parameters by conventional minimization of the free energy. The semiconducting carrier screening length was shown to influence strongly the film's properties and critical parameters of the thickness‐induced phase transition. At some fixed temperature and thickness of the film electrode an induced phase transition from the ferroelectric to the paraelectric phase is shown to exist when the screening length of the carriers is larger than some critical value. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Device Fabrication and Optimisation for Josephson Broadband Spectroscopy of Ferroelectric Thin Films

Conventional methods of detecting dielectric properties of ferroelectric thin films like microwave techniques, are difficult to operate in a frequency range higher than around 50 GHz. Optical methods based on measurement of refractive indices of materials operate in the THz range. In this work, use of a simple and convenient technique to investigate the dielectric properties of ferroelectric thin films in the millimetre wave-band (50–1000 GHz) is discussed. Here we report measurements of ϵ R and the tunability of ϵ R of thin films of STO.

Enhanced electrical properties of ferroelectric thin films by ultraviolet radiation

Ferroelectric films in the paraelectric phase exhibit two undesirable properties: hysteresis in the voltage-capacitance characteristics and a significant relaxation time of the capacitance. Our experiments show that suppression of both of these is achieved by using UV radiation with wavelengths corresponding to the material forbidden gap. Experimentally we also observed UV radiation induced modulation of thin film permittivity without an applied electric field. The observed phenomena are believed to have the same origin: UV light generates nonequilibrium charge carriers that screen out local electric field induced by defects and interfaces inside ferroelectric thin films and change films effective dielectric properties.

Pulsed laser deposition of perovskite relaxor ferroelectric thin films

Structural, dielectric and ferroelectric properties of thin films of La-doped lead zirconate titanate (PLZT) and sodium bismuth titanate–barium titanate (NBT–BT) perovskite relaxor ferroelectric have been investigated. PLZT films were deposited on Pt/Si substrates in oxygen atmosphere by pulsed laser deposition (PLD) and radio frequency (RF) discharge-assisted PLD, using sintered targets with different La content and Zr/Ti ratio, near or at the boundary relaxor ferroelectric. The films are polycrystalline with perovskite cubic or slightly rhombohedral structure. A slim ferroelectric hysteresis loop, typical for relaxors, has been measured for all film sets. Dielectric characterization shows a large value of capacitance tunability and low dielectric loss. However, common problems related to lead diffusion into the metallic electrode layer do not allow one to obtain high capacitance values, due to the formation of an interface layer with low dielectric constant. Lead-free NBT–BT thin films have been deposited on single crystal (1 0 0)-MgO substrates starting from targets with composition at the morphotropic phase boundary between rhombohedral and tetragonal phase. Films deposited by PLD are polycrystalline perovskite with a slight (1 0 0) orientation. Capacitance measurements were performed using interdigital metallic electrodes deposited on the film's top surface and showed high relative dielectric constant, on the order of 1300.

Strain gradients in epitaxial ferroelectrics

X-ray analysis of ferroelectric thin layers of Ba1/2Sr1/2TiO3 with different thickness reveals the presence of internal strain gradients across the film thickness and allows us to propose a functional form for the internal strain profile. We use this to calculate the direct influence of strain gradient, through flexoelectric coupling, on the degradation of the ferroelectric properties of thin films with decreasing thickness, in excellent agreement with the observed behaviour. This work highlights the link between strain relaxation and strain gradients in epitaxial films, and shows the pressing need to avoid strain gradients in order to obtain thin ferroelectrics with bulk-like properties.

Exact and Variational Treatment of Ferroelectric Thin Films with Different Materials of Electrodes

We trace the influence of metallic electrodes on the properties of thin ferroelectric films. This has been done in the framework of the phenomenological Landau theory. We present both exact analytical solution of the problem in terms of Weierstrass elliptic functions and the variational solution. We calculate polarization, dielectric susceptibility, pyroelectric coefficient and depolarization field. We have shown that the ferroelectric phase of the film can be destroyed by the choice of material of electrodes with screening length exceeding some critical value. This demonstrates that we can control the physical properties of thin ferroelectric films by the proper choice of the electrode material.

Domain Geometry Engineering in Thin Films: A Quasi-Halbach Array

Domain geometry engineering is one of the most essential application aspects of ferroic materials. Up to now domain geometries have been considered represented by 180° domain walls in samples of infinite size. Here a new possibility is addressed theoretically: periodic domain systems based on 90° domain pairs in a thin film. This case is realistic, as shown by bending plate-like samples of ferroelectric/ferroelastic materials. A symmetry-based analysis is presented of macroscopic properties of plate-like samples in which a 90° domain pattern similar to the Halbach array has been produced. The conclusions can lead to new application related properties of ferroelectric thin films.

Depolarization Field in Thin Ferroelectric Films With Account of Semiconductor Electrodes

Within the framework of the phenomenological Ginzburg-Landau theory influence of semiconductor electrodes on the properties of thin ferroelectric films is considered. The contribution of the semiconductor electrodes with different Debye screening length of carriers is included in the functional of free energy. The influence of highly doped semiconductor electrodes on the depolarization field and the film properties was shown to be great.

Ferroelectric Thin Film Properties: Peculiarities Related to Mismatch-Induced Polarization

The ferroelectric thin film properties were calculated in phenomenological theory framework. Surface energy that defined boundary conditions for Euler-Lagrange differential equation was written as surface tension energy. The latter was expressed via surface polarization and tension tensor related to mismatch of a substrate and a film lattice constants and thermal expansion coefficients. The calculations of the film polarization distribution, temperature, thickness and external electric field dependence and hysteresis loops as well as average dielectric susceptibility dependence on temperature and film thickness have been performed allowing for mismatch-induced polarization Pm. The obtained asymmetry of hysteresis loop makes it possible to suppose that the self-polarization phenomenon recently observed in some films is related to mismatch effect. The large enough mismatch effect (|Pm|/PS > 1) could be the physical reason of ferroelectric phase conservation in ultrathin film. The possibility to observe the peculiarities of the films properties temperature and thickness dependencies related to mismatch effect is discussed.

Theoretical Predictions for the Intrinsic Converse Longitudinal Piezoelectric Constants of Lead Zirconate Titanate Epitaxial Films

The orientation dependences of the converse longitudinal piezoelectric coefficient d33,f are calculated for epitaxial Pb(ZrxTi1–x)O3 films with different compositions. This calculation showed that the substrate clamping plays an important role on determining the piezoelectric properties of ferroelectric thin films, and thus may serve as an example illustrating why physical properties of thin films are different from bulk materials.

Effect of anisotropic in-plane strains on phase states and dielectric properties of epitaxial ferroelectric thin films

A nonlinear thermodynamic theory is used to predict the equilibrium polarization states and dielectric properties of ferroelectric thin films grown on dissimilar substrates which induce anisotropic strains in the film plane. The “misfit strain-temperature” phase diagrams are constructed for single-domain PbTiO3 and Pb0.35Sr0.65TiO3 films on orthorhombic substrates. It is shown that the in-plane strain anisotropy may lead to the appearance of new phases which do not form in films grown on cubic substrates. The strain-induced dielectric anisotropy in the film plane is also calculated and compared with the anisotropy observed in Pb0.35Sr0.65TiO3 films deposited on NdGaO3.

Techniques for microwave measurements of ferroelectric thin films and their associated error and limitations

This paper examines the problem of evaluating the microwave properties of thin ferroelectric films patterned as planar capacitors. Two types of microwave measurements of ferroelectric thin films are considered: reflection- and resonance-type measurements. Algorithms are presented for evaluation of capacitance–permittivity and dielectric loss. Using sensitivity analysis, the error and limitations associated with these measurements are estimated. The end result is a series of formulae that use the network analyser's measurement data to calculate the capacitance–permittivity, the dielectric loss and the associated error.

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.

Impact of stress on the thermodynamic properties of ferroelectric films within the transverse Ising model

The thermodynamic properties of ferroelectric thin films are investigated within the framework of the transverse Ising model. A two-dimensional in-plane stress is introduced into the Hamiltonian of the system, and is supposed exponentially decreasing from the interface between the substrate and the film to the surface of the film. It is demonstrated that the compressive stress is benefitial to the polarization and shifts the Curie temperature to higher temperatures, but the tensile stress has the inverse influence on the Curie temperature and polarization. Besides, it is also shown that the diffusive length greatly affects the thermodynamic properties of the film.