Ferroelectric PbTiO3 Thin Films Research Articles

Investigation of the (micro)structural characteristics in PbTiO3 ferroelectric thin films

Ferroelectric materials have been studied over the last three decades due to their exceptional physical properties, which make them promissory for several applications, such as pyroelectric sensors, nonvolatile memories, electro-optical devices, capacitors, etc. Among them, lead titanate (PbTiO3, PT), which has a tetragonal (P4mm) perovskite structure, has been considered a promising material for infrared detector applications (in both thin films and ceramic forms) and, therefore, it has been up today under the interest of many researches from the scientific community. In fact, recent advances in the synthesis methods for obtaining PT-based materials have promoted very interesting and intriguing phenomena in their micro(structural) properties, which still remains not fully understood. In this context, this work aims the investigation of the structural properties of PbTiO3 thin films, studied through X-ray diffraction and Raman spectroscopy techniques. In particular, the influence of the synthesis parameters, such as pyrolysis temperature, has been considered.

Nonvolatile memory based on the extension–retraction of bent ferroelastic domain walls: A phase field simulation

Herein, a prototype nonvolatile bent ferroelastic domain wall (DW) memory based on extension–retraction of DWs in a top electrode/bent ferroelastic DWs/bottom electrode architecture is demonstrated and the effects of mechanical condition, electrical condition, and the material parameter on ferroelastic DWs in PbTiO3 ferroelectric thin films are studied by phase field modeling. Misfit strain can be used to drive the bend of DWs in PbTiO3 thin film, resulting in a change of ferroelastic domain size, bending degree, and conductivity. Stable and reversible switching of DWs between the extendible state with high conductivity and the retractile state with low conductivity can be realized, resulting in an apparent resistance change with a large ON/OFF ratio of >102 and an excellent retention characteristic. The extension and retraction speed, corresponding to data writing speed, can be adjusted by the electric field magnitude and distributions. The memory speed increases by 5% under a homogeneous electric field and 6% under an inhomogeneous probing electric field, after the buildup of space charges in a ferroelectric thin film, and the fastest memory speed is obtained at tip potential φ = 1.8. Moreover, polarization orientations of a and c domains separated by bent ferroelastic DWs do not affect memory performance. This paper can guide the development of new ferroelectric domain wall memory.

Understanding mechanisms of thermal expansion in complex oxide thin-films from first principles: Role of high-order phonon-strain anharmonicity

The thermal properties of materials are critically important to various technologies and are increasingly the target of materials design efforts. However, it is only relatively recent advances in first-principles computational techniques that have enabled researchers to explore the microscopic mechanisms of thermal properties, such as thermal expansion. We use the Grüneisen theory of thermal expansion in combination with density functional calculations and the quasiharmonic approximation to uncover mechanisms of thermal expansion in ferroelectric PbTiO3 thin-films. We show that although there are large changes in the elastic constants and Grüneisen parameters with both misfit strain and temperature, surprisingly the temperature evolution of the structural parameters can be modeled using a simple model of linear elasticity; the only inputs required are the misfit strain and bulk PbTiO3 elastic constants. We show that a near-cancellation between different types of anharmonicity gives rise to this behavior. Our results illustrate how different sources of anharmonicity can affect materials’ properties in different ways, even for the same material. The framework used in our study is general and illustrates how different types of anharmonicity can be systematically identified and explored.

Deconvolution of Phonon Scattering by Ferroelectric Domain Walls and Point Defects in a PbTiO3 Thin Film Deposited in a Composition-Spread Geometry.

We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO3 thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ∼25% titanium deficient to ∼20% titanium rich to be studied. By fitting the experimental results to the Debye model we deconvolute and quantify the two main phonon-scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichiometric region of the thin film ([Pb]/[Ti] ≈ 1) but also when the concentration of the cation point defects is significant (up to ∼15%). Hence, DWs in ferroelectric materials are a source of phonon scattering at least as effective as point defects. Our results demonstrate the viability and effectiveness of using reconfigurable DWs to control the thermal conductivity in solid-state devices.

Domain growth dynamics in PMN-PT ferroelectric thin films

Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) ferroelectric thin films have been widely used in many areas due to the outstanding piezoelectric and electromechanical properties. Although many studies have been operated on the properties and applications of PMN-PT films, few researches focused on the domain growth dynamics of these materials. In this paper, the domain growth dynamics of PMN-PT thin films were investigated using piezoelectric force microscopy. The shape and the size of the artificial domains were imaged and calculated to depict the relationship between the imposed pulse voltage and the domain behaviors. A linear relationship between the pulse voltage and the domain size could be obtained before the saturation of polarization. Meanwhile, the domain size and the duration show the logarithmic relationship, and such domain growth dynamics were explained according to previous theories.

Misfit strain relaxations of (101)-oriented ferroelectric PbTiO3/(La, Sr)(Al, Ta)O3thin film systems

Abstract

Giant negative electrocaloric effect induced by domain transition in the strained ferroelectric thin film

Electrocaloric effect (ECE) of ferroelectric materials is highly dependent on the domain transition with temperature. The effect of domain transition on the ECE of PbTiO3 (PTO) ferroelectric thin films subjected to different mismatch strains is investigated by a phase field model based on the time-dependent Ginzburg–Landau equation. The simulation results demonstrate that there exists a multiple-to-single domain transition induced by temperature in the PTO thin film. The domain transition occurs at a temperature that is much lower than Curie temperature, resulting in a giant negative ECE in the strained ferroelectric thin film. Furthermore, the domain transition temperature and the corresponding negative ECE can be tuned by the magnitude of mismatch strain. The present study suggests an effective way to obtain a giant negative ECE by domain transitions in the strained ferroelectric thin film.

Epitaxial integration of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (111) thin films on GaN (0002) with La0.5Sr0.5CoO3/TiO2 buffer layers

0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) ferroelectric thin films were epitaxially integrated on GaN (0002) substrates by pulsed laser deposition. La0.5Sr0.5CoO3 (LSCO)/TiO2 buffer layers are designed to decrease the lattice mismatch between PMN-PT and GaN, which promotes the epitaxial growth of pure perovskite PMN-PT (111) thin films. Meanwhile, LSCO/TiO2 heterostructures could be served as the bottom electrodes for integrated ferroelectric devices. Epitaxial relationship of the multilayer is determined to be (111)[11¯0] PMN-PT//(111)[11¯0] LSCO//(100)[001] TiO2//(0002)[112¯0] GaN. The integrated PMN-PT (111) films exhibit good ferroelectric and dielectric properties with remanent polarization of 11.3 μC/cm2, coercive field of 34.1 kV/cm at 100 Hz, dielectric constant of 2035 and dielectric tunability of 68.1% at 1 kHz, which make it a potential candidate for the applications in GaN-based integrated ferroelectric devices.

Electrical conduction on the surface of ferroelectric PbTiO3 thin film induced by electrolyte gating

We demonstrate a fairly high sheet conductance (∼1 μS) from 300 K to 10 K on the surface of ferroelectric PbTiO3 thin films in an electric double layer transistor configuration. Applying a positive gate voltage, n-type operation takes place with a high on-off ratio exceeding 105 and a high sheet electron density of 4 × 1013 cm−2. Temperature dependence of the sheet resistance changes from thermal activation-type at low gate voltage (∼3 V) to disordered two-dimensional conduction with a weak temperature dependence at high gate voltage (∼5 V). This behavior is quite different from those in BaTiO3 cases, where strong localization takes place below 100 K in electrostatically or chemically doped BaTiO3 thin films. The absence of instability to a lower symmetry crystal structure may play a role in the case of tetragonal PbTiO3.

Effect of oxygen vacancy defect on leakage current of PbTiO3 ferroelectric thin film

Ferroelectric (FE) materials have been extensively applied to the multifunctional electronic devices, particularly the FE memories due to their excellent physical properties. The FE memory is a kind of nonvolatile memory device, and it could overcome the shortcomings of the traditional memory. But the development of the FE memory is very slow due to the FE failure problem. However, with the continuous decrease of the thickness of FE thin film, when it reaches microns or nanometers in magnitude, the leakage current is the main cause of the FE failure of FE thin film. The leakage current of FE thin film is directly related to whether the FE memory is applicable, and it has been the hot spot of scientific researches. There are still a lot of factors influencing the FE memory leakage current except for the thickness of the film, such as interface, processing temperature, defect, domain wall, etc. Of these factors, the defect and domain wall are the most common and the most probable. In this paper, the first-principle calculation method through combining the density function theory with the nonequilibrium Green's function is used to systematically study the influence of oxygen vacancy defect on the leakage current of the FE thin film. The doping with four kinds of Cu, Al, V, and Fe cations is used to regulate and control the leakage current of the FE thin PbTiO3 film caused by the oxygen vacancy defects. We investigate the leakage current induced by oxygen vacancies in PbTiO3 films, and the doped PbTiO3 thin FE films having oxygen vacancies. It is found that Fe and Al doping will increase the leakage current of oxygen vacancy defects of FE thin films, while the Cu and V doping significantly reduce the leakage current of oxygen vacancy defects of FE thin films. This is because the Cu and V doping have obvious pinning effect on oxygen vacancy defect. In addition, we find that the oxygen vacancies are pinned by Cu and V atoms due to the fact that the formation energy of oxygen vacancies can be remarkably reduced. So Cu and V doping in PbTiO3 not only induce the leakage current but also improve the fatigue resistance of the FE thin film induced by oxygen vacancies. Moreover, since the ionic radius of V is closer to the ionic radius of Ti than the ionic radius of Cu, V is easier to implement doping to suppress the leakage current caused by the oxygen vacancy defects. These conclusions are of important theoretical significance and application value for improving the performance of FE thin films and their FE memories.

Nonvolatile ferroelectric memory based on PbTiO3 gated single-layer MoS2 field-effect transistor

We fabricated ferroelectric non-volatile random access memory (FeRAM) based on a field effect transistor (FET) consisting of a monolayer MoS2 channel and a ferroelectric PbTiO3 (PTO) thin film of gate insulator. An epitaxial PTO thin film was deposited on a Nb-doped SrTiO3 (Nb:STO) substrate via pulsed laser deposition. A monolayer MoS2 sheet was exfoliated from a bulk crystal and transferred to the surface of the PTO/Nb:STO. Structural and surface properties of the PTO thin film were characterized by X-ray diffraction and atomic force microscopy, respectively. Raman spectroscopy analysis was performed to identify the single-layer MoS2 sheet on the PTO/Nb:STO. We obtained mobility value (327 cm2/V·s) of the MoS2 channel at room temperature. The MoS2-PTO FeRAM FET showed a wide memory window with 17 kΩ of resistance variation which was attributed to high remnant polarization of the epitaxially grown PTO thin film. According to the fatigue resistance test for the FeRAM FET, however, the resistance states gradually varied during the switching cycles of 109.

The influence of the crystallization temperature on the reliability of PbTiO3 thin films prepared by chemical solution deposition

The preparation of quality ferroelectric PbTiO3 (PT) thin films at the lowest possible temperature preserving its functional properties is necessary for their integration in microdevices. The crystallization below, close or above the para-ferrolectric transition temperature of the PbTiO3 must produce an important effect on the microstructure, texture and residual stress state of the films and on functional properties. In this paper, the Chemical Solution Deposition method has been used to prepare PbTiO3 films at different temperatures around that of the ferroelectric to paraelectric phase transition. The films were analyzed by X-ray diffraction, Optical and Scanning Electron Microscopy, and their microstructure, texture and residual stress correlated with their functional properties. The results show that these PT films prepared with crystallization temperatures close or below the phase transition develop a favorable microstructure and texture that lead to high remnant and saturation polarization values, making them good candidates for their integration in microdevices.

Influence of 90° charged domain walls on the electrocaloric effect in PbTiO3 ferroelectric thin films

The electrocaloric (EC) effect in PbTiO3 ferroelectric thin films (FETFs) with an array of partially compensated head-to-head (HH) and tail-to-tail (TT) 90° charged domain walls (CDWs) has been studied by using a phase field method. The calculation results indicate that the magnitude of the adiabatic temperature change ΔT increases with the diminution of the charge density (ρ*), in which ρ* is provided by charged defects or band bending. In particular, a negative EC effect caused by the direction difference between the applied electric field and the dipole is found near the HH and TT domain walls when ρ* > 2.0. The EC strength |ΔT|/|ΔE| is 0.020 K cm kV−1 with the positive ΔT = 2.03 K at the external dimensionless electric field Eb*=0.08 (ΔE = 104 kV cm−1). These results indicate that the CDWs can provide an efficient way to adjust the EC effect of FETFs for refrigeration applications at room temperature.

Misfit Strain Relaxation of Ferroelectric PbTiO3/LaAlO3 (111) Thin Film System.

Ferroelectric thin films grown on high index substrates show unusual structural and switching dynamics due to their special strain states. Understanding the misfit relaxation behavior is crucial to facilitate the high index thin film growth with improved quality. In this paper, ferroelectric PbTiO3 thin films were grown on LaAlO3 (111) substrates by pulsed laser deposition technique. The microstructures were investigated by combinations of conventional and aberration-corrected transmission electron microscopy. Diffraction contrast analysis and high resolution imaging reveal that high density interfacial dislocations were distributed at the interfaces. These dislocations have mixed character with Burgers vectors of a <110> and line directions of <112>. The edge components of the dislocations, with the Burgers vectors parallel to the interface, accommodate the lattice mismatch and are the main contributor to the misfit relaxation of this system. The formation mechanism of these dislocations is proposed and discussed to elucidate the novel mismatch relaxation behavior of <111> oriented perovskite films.

Instability criterion for ferroelectrics under mechanical/electric multi-fields: Ginzburg-Landau theory based modeling

Ferroelectric materials interact with not only electric fields, but also with mechanical stress/strain through intriguing cross-coupling between ferroelectric polarization and ferroelastic strain. Such mechanical and/or electric multi-field interactions allow symmetry breaking of the rotationally invariant switching field and cause a variety of complicated instability phenomena in ferroelectric systems, e.g., super switching of in-plane ferroelectric nanodomains in strained thin films, labile and ultrafast switching of ferroelastic nanodomains, and ferroelectric polarization reversal via successive ferroelastic transitions. To systematically understand the nature of instabilities in ferroelectrics, here, we propose an analytical method based on Ginzburg-Landau theory to enable rigorous description of any type of instability in arbitrary morphologies and complex microstructures under a finite electric field and/or mechanical loading. The present theory yields, as an instability criterion, the condition that the minimum eigenvalue of the Hessian matrix of potential energy with respect to displacements, electrical potential, and polarization vectors must be zero. In addition, the corresponding eigenvector represents the polarization behavior at the onset of instability, which is successfully validated by application of the criterion to domain switching and successive ferroelastic transitions in PbTiO3 ferroelectric thin film under electrical and mechanical excitation, respectively. This approach thus provides a novel insight into the cause of instability in ferroelectrics. In addition, the proposed criterion is scale-independent, which enables elucidation of the nature of various types of instability in arbitrary ferroelectric systems so that complicated instability issues in practical situations can be addressed.

Inhomogeneity of phase transition in lead titanate thin films formed by magnetron layer-by-layer deposition

The formation of deposited ferroelectric perovskite PbTiO3 thin films using layer-by-layer reactive magnetron sputtering was investigated in this work. Deposition rates of each layer (PbO and TiO2) for 7.2 and 9.2 nm/min, respectively, were chosen there. Silicon (110) substrate with the heater was moved periodically and parallel to the magnetron cathodes during deposition. The heater temperature (380 °C–700 °C) has influence on the stoichiometry of thin films. Perovskite phase of lead titanate was not obtained without post annealing. The reasons for this are discussed in the work. The results of the structure of thin layers deposited on silicon substrates at 380 °C and annealed for one hour at 700 °C in air have shown that a pure perovskite phase of PbTiO3 is formed there.

The effects of strain relaxation on the dielectric properties of epitaxial ferroelectric Pb(Zr0.2Ti0.8)TiO3 thin films

We study the effects of strain relaxation on the dielectric properties of epitaxial 40 nm Pb(Zr0.2Ti0.8)TiO3 (PZT) films. A significant increase in the defect and dislocation density due to strain relaxation is observed in PZT films with tetragonality c/a &amp;lt; 1.07 grown on SrTiO3 (001) substrates, which results in significant frequency dispersion of the dielectric constant and strong Rayleigh type behavior in those samples. This combined structural-electrical study provides a framework for investigating strain relaxation in thin films and can provide useful insights into the mechanisms of fatigue in ferroelectric materials.

Gamma-ray irradiation effects on electrical properties of ferroelectric PbTiO3 and Pb(Zr0.52Ti0.48)O3 thin films

We investigated gamma-ray irradiation effects on electrical properties of ferroelectric PbTiO3 (PTO) and Pb(Zr0.52Ti0.48)O3 (PZT) thin films. PTO and PZT thin films were prepared on the Pt/Ti/SiO2/Si substrates by using a sol–gel method with a spin-coating process. The prepared PTO and PZT thin films were subjected to gamma radiation with various total doses from 0kGy to 300kGy. There were no noticeable morphological and structural changes in PTO and PZT films before and after irradiation. As a total dose increases up to 300kGy, a larger degradation behavior of electrical properties was observed in the PZT films rather than in the PTO films. About 35% of remanent polarization value decreased for the PZT films, while just 10% of that decreased for the PTO films. Dielectric constant of PZT films decreased much from 850 to 580, but that of the PTO films decreased just a little from 400 to 340. This degradation behavior of polarization and dielectric properties can be explained by the pinning of domain walls by some radiation-induced defects. These results suggest that the PTO films have higher radiation hardness properties than the PZT films.

Giant electrocaloric effect of PbTiO3 thin film tuned in a wide temperature range by the anisotropic misfit strain

The influence of anisotropic in-plane strains on the electrocaloric effect (ECE) in PbTiO3 (PT) epitaxial ferroelectric thin films is investigated by using a Landau-Devonshire thermodynamic theory. The calculation results show that the anisotropic strain can tune the ECE of PT ferroelectric thin films to obtain a large adiabatic temperature change in a wide temperature range which is attributed to the shift of c-phase boundary of PT thin films under the anisotropic strains with an external electric field. These results indicate that the anisotropic strain can provide an efficient way to adjust the ECE of ferroelectric thin films to refrigerate in a wide temperature range.

Effects of ferroelectric/metal interface on the electric properties of PMN-PT thin films epitaxially grown on Si substrates

Lead magnesium niobate-lead titanate 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-PT) ferroelectric thin films were deposited on SrRuO3 (SRO), SrTiO3, and TiN-buffered Si substrates by pulsed laser deposition. X-ray diffraction θ–2θ and Phi scans reveal that PMN-PT films were epitaxially grown on Si substrates. Pt, Al, and Gd metals were employed as top electrodes to investigate the ferroelectric and dielectric properties of these metal/PMN-PT/SRO capacitors. It was found that the coercive field (E C) of the Gd(or Al)/PMN-PT/SRO capacitor is 4.5 times larger than that of the Pt/PMN-PT/SRO capacitor while the permittivity for the former is only ~27 % of that for the latter, which is analyzed using the model for metal-ferroelectric-metal heterostructures with Schottky contacts. Compared with the Pt/PMN-PT/SRO capacitor, the higher E C and lower permittivity of the Gd(or Al)/PMN-PT/SRO capacitor are attributed to the stronger space charge field at the Gd(or Al)/PMN-PT interface. The capacitance–voltage characteristics of the metal/PMN-PT/SRO capacitors were also discussed.