Behavior Of Ferroelectric Thin Films Research Articles

Phase field modeling of domain evolution in ferroelectric materials in the context of size effects

AbstractPhase field modeling provides an efficient tool for the study of domain evolution in ferroelectric materials. Such models naturally introduce an inner length scale which represents the width of the interfaces between domains (domain walls). This inner length scale is of the order of a few unit cells, i.e. about 0.8 nm–2 nm. The focus of this contribution is on size effects in a) the switching behavior of ferroelectric thin films and b) the microstructure evolution in ferroelectric nanodots. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of strain and dead layer on the nonlinear electric-mechanical behavior of ferroelectric thin films

Strain (intrinsic and thermo) and dead layer are inevitable at the substrate–ferroelectric film interface. They significantly affects the properties of nanoscale devices. The effect of strain and dead layer on the nonlinear electric-mechanical behavior of ferroelectric thin films is investigated based on micromechanical domain-switching model. Strain enhances the Gibbs free energy of a domain, making the domains more unstable and domain switching more continuous. Due to the existence of the dead layer between the ferroelectric thin film and substrate, the dielectric constant values and the effective electric field decrease through the film, thus leading to the decrease of the remnant polarization and increase of the coercive field. Our model, in which strain and dead layer are taken into consideration, shows improved displacement and strain versus electric field loops that agree reasonably well with the experimental data. In addition, both the electric displacement hysteresis and strain butterfly loops under various stress load have also been investigated by our model.

Switching behavior of ferroelectric thin films with deadlayers

Domain switching in ferroelectric thin films with deadlayers subjected to external electric fields is investigated by a phase field model. The simulation results show that the deadlayers of ferroelectric thin films induce a strong inhomogeneous depolarization field which makes polarizations deflect from the normal direction of thin films. The locations with larger depolarization fields are nucleation centers for new domains during the polarization switching and the switching process includes two sequential 90° polarization switchings, which decrease the coercive field significantly. It is found that the thickness of the deadlayer has a significant influence on the switching behavior of the ferroelectric thin films. Both the coercive field and remanent polarization are dependent on the thickness of the deadlayer.

Mapping piezoelectric nonlinearity in the Rayleigh regime using band excitation piezoresponse force microscopy

Band excitation piezoresponse force microscopy enables local investigation of the nonlinear piezoelectric behavior of ferroelectric thin films. However, the presence of additional nonlinearity associated with the dynamic resonant response of the tip-surface junction can complicate the study of a material’s nonlinearity. Here, the relative importance of the two nonlinearity sources was examined as a function of the excitation function. It was found that in order to minimize the effects of nonlinear tip-surface interactions but achieve good signal to noise level, an optimal excitation function must be used.

Aspects of the Electrocaloric Behavior of Ferroelectric Thin Films: A Review of the Predictions of the Landau-Ginzburg Theory

A thermodynamic model is used to investigate the electrocaloric response of thin film perovskite ferroelectrics under the influence of differing electrical, thermal and mechanical boundary conditions including bias and driving field, temperature, lateral clamping and misfit strain. A comparison of the electrothermal properties of ferroelectric solid solutions comprised of BaTiO3, PbTiO3 and/or SrTiO3 illustrates the influence of composition on electrocaloric properties. Computations made for (001) textured polycrystalline BaTiO3 films on IC-friendly substrates quantify the effects of thermal stresses. The combined results provide insights concerning how the deposition temperature, substrate material and composition can be adjusted to obtain desired electrocaloric response.

Local switching behavior and electrical polarization of ferroelectric thin films under nanoindentation

The local polarization state and switching behavior of ferroelectric thin films under external mechanical loading were investigated. A nanoindentation technique has been devised to impose local mechanical stress. The results are presented for a 700 nm thick, Pb(Zr 0.30Ti 0.70)O 3 (PZT), film prepared by a sol–gel technique on a platinized Si substrate. A hysteric behavior was found in the local direct piezoelectric response of the ferroelectric thin films within the subcoercive stress range, and an enhanced piezoelectric activity is attributed to the effect of stress-induced domain-wall movement. Upon nanoindentation, voltage shift was observed in the Q–V hysteresis loops along the voltage axis, indicating an asymmetric switching behavior of the local polarization in the loaded films. The parameter of horizontal loop asymmetry increased and the Q–V loop shifted gradually to the positive voltage side as the indentation force increased. The changes in local switching behavior are suggested to result from a variation in residual stress state, asymmetric distribution of charged defects and asymmetric lattice distortion produced by the inhomogeneous indentation stress field.

Simulation of flexoelectricity effect on imprint behavior of ferroelectric thin films

The impact of flexoelectricity on the imprint behavior in ferroelectric thin films has been investigated within the framework of Landau–Khalatnikov theory, by incorporating the coupling effect between the stress gradient and polarization. It is found that the imprint phenomenon can be in part induced by flexoelectricity. In the presence of flexoelectric coupling, the compressive stress shifts the hysteresis loop to the negative electric field axis, but the tensile stress shifts it to the opposite direction, which is in good agreement with experimental result. Besides, the characteristic length of stress distribution has a significant influence on the upper part of hysteresis loop. It highlights the pressing need to avoid the stress gradient in order to prevent degradation of device performance in ferroelectric thin films.

A possible mechanism of anomalous shift and asymmetric hysteresis behavior of ferroelectric thin films

We studied theoretically the hysteresis behavior of ferroelectric thin films. The anomalous ferroelectric response is discussed by use of a bilayer model. Electrical conductivities of the films have been taken into account. To model the effects of the inhomogeneity of polarization and permittivity across the interface, the film is assumed to possess a secondary dielectric∕ferroelectric phase (a dead or passive layer) with asymmetric conductivity. This configuration is found to produce large shifting (along the field axis) and deformation of the measured hysteresis loop. This is a manifestation of the asymmetric conductivity of the material. Theoretical calculation based on this model shows that the observed phenomena of shifted and skewed hysteresis loop in ferroelectric thin films can be explained in this way.

Local Hysteresis Behavior of Ferroelectric Thin Films of Si added PbTiO3

ABSTRACTWe report ferroelectric properties and local hysteresis behavior of 2 mole percent Si added PbTiO3 thin films grown on Pt/TiO2/SiO2/Si substrate by using pulsed laser deposition technique. The ferroelectric hysteresis loop and scanning piezoresponce images obtained on these films by using AFM with conducting tip demonstrate excellent properties, which are equivalent to any other established ferroelectric material like PZT. Si segregating at the grain boundaries controls grain growth. The grain size and grain boundaries play a crucial role in determining ferroelectric hysteresis properties. The presence of Si in the matrix can be useful in tuning the properties.

A Mathematical Description of the Switching Behavior of Ferroelectric Thin Films for FRAM Applications

This paper presents a mathematical model that describes the switching behavior (Q sw as a function of applied voltage) of ferroelectric thin films tested using 4-pulse sequence (PUND) approach. The model consists of a distribution of threshold voltage (to switch the ferroelectric domains) and a linear Q sw increase in the high voltage range. It is demonstrated that this model can precisely describe the Q sw vs. applied voltage of ferroelectric thin films or integrated ferroelectric array capacitors. The physical meaning of the four parameters in the model will be discussed. Using these parameters, some integration process related effects such as interface layer and fringing effect can be quantitatively estimated. Using this model, the switching performance of ferroelectric capacitors at different temperatures can be predicted.

A Mathematical Description of the Switching Behavior of Ferroelectric Thin Films for FRAM Applications

This paper presents a mathematical model that describes the switching behavior (Q sw as a function of applied voltage) of ferroelectric thin films tested using 4-pulse sequence (PUND) approach. The model consists of a distribution of threshold voltage (to switch the ferroelectric domains) and a linear Q sw increase in the high voltage range. It is demonstrated that this model can precisely describe the Q sw vs. applied voltage of ferroelectric thin films or integrated ferroelectric array capacitors. The physical meaning of the four parameters in the model will be discussed. Using these parameters, some integration process related effects such as interface layer and fringing effect can be quantitatively estimated. Using this model, the switching performance of ferroelectric capacitors at different temperatures can be predicted.

Ferroelectric behavior in solvent cast poly(vinylidene fluoride/hexafluoropropylene) copolymer films

The ferroelectric behavior of thin films of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) random copolymers with two different HFP molar contents was investigated. Films were prepared by solvent casting using dimethyl formamide (DMF) as the solvent. The HFP molar content of the solvent cast films were 5 and 15%. The observed remanent polarization (Pr) obtained from dielectric displacement versus electric field (D–E) data for the 5 and 15% HFP copolymer samples were 80 and 50mC/m2, respectively. The Pr of these films is discussed in terms of the HFP molar content.

Ferroelectric Behavior of Thin Films of Vinylidene Fluoride/Trifluoroethylene/Hexafluoropropylene Copolymer

Ferroelectricity in vinylidene fluoride (VDF), trifluoroethylene (TrFE) and hexafluoropropylene (HFP) copolymers (P(VDF/TrFE/HFP)) with a low HFP content (lower than 11 mol%) has been investigated primarily through switching measurements using P(VDF/TrFE) as a comparison. Switching phenomena as a result of ultimately 180° rotations of individual molecular chains around their axes in crystalline regions were observed in P(VDF/TrFE/HFP) with an HFP content lower than 2.5 mol%. With increasing HFP content in P(VDF/TrFE/ HFP), the amount of reversed polarization decreases, and the dielectric permittivity at the Curie point becomes smaller. However, the switching times have very low dependence on the HFP content of P(VDF/TrFE/HFP). These results suggest that noncrystalline regions in P(VDF/TrFE/HFP) are similar to those in P(VDF/TrFE), and the disorder in crystalline regions of P(VDF/TrFE/HFP) increases compared to that in crystalline regions of P(VDF/TrFE).