PbxSr1−xTiO3 Research Articles

(Ba/Pb)xSr1−xTiO3 based capacitive sensor with LaNiO3 electrode for higher tunability

The high tunability range of capacitive sensors can be achieved by the use of ferroelectric materials due to its perovskite structure. Thin films provide better utilization of properties comparative to bulk but greatly affect by the surface on which it deposited. In this work thin films of BaxSr1−xTiO3 (BST) and PbxSr1−xTiO3 (PST) are developed with x = 0.3 & 0.7. Thin films without doping of Sr are also developed for comparison. The capacitors Al/Si/BST/Al and Al/Si/PST/Al are fabricated in parallel plate structure using the sol–gel processing technique with Al as metal electrodes and BST/PST as the dielectric material. A new parallel plate capacitor with the introduction of Lanthanum Nickel Oxide (LNO) thin film with the preferred (100) orientation between Si substrate and BST/PST is proposed in this work. Crystallinity and orientation of BST/PST on Si and BST/PST over LNO are observed by X-ray diffraction (XRD). The average grain size and uniformity of thin films are confirmed by scanning electron microscopy (SEM) images. The capacitances are measured at 50 mV and 70 mV using LCR meter. The highest capacitance of 79.6pF is observed for Al/Si/LNO/BST/Al. Impedance spectroscopy reveals the decrement in impedance with doping of Sr and tunability range of LNO based BST/PST capacitor is much higher compared to non-LNO capacitors. Tunability with BST material reaches 57% at frequency 2.98 kHz and the PST attains a value of 33.8% at frequency 1.44 KHz.

Pyroelectric energy harvesting and ferroelectric properties of Pb x Sr1- x TiO3 ceramics

ABSTRACT Pb x Sr1-x TiO3 (PST) (x = 0.3, 0.35 and 0.40, abbreviated as PST30, PST35 and PST40, respectively) ceramics were prepared by solid-state reaction route. Coexistence of tetragonal and cubic phase of ceramics is confirmed by X-ray diffraction pattern at room temperature (RT). Temperature-dependent dielectric permittivity εr measurements showed that Pb2+ doping increased the phase transition temperature of ferroelectric (FE) to paraelectric (PE) phase. The temperature-related polarization-electric field (P-E) measurements were performed to study the FE properties of PST ceramics. Phase transition from FE to PE was observed in P-E measurements. Furthermore, pyroelectric energy harvesting performance was evaluated based on Olsen cycle. The calculated results of pyroelectric energy harvesting density (ND ) showed that PST35 ceramic with ND = 283 kJ/m3 (∆T = 70°C, EL = 2.5 kV and EH = 40 kV) is most potential for pyroelectric energy harvesting application near RT among all samples.

Unraveling the electronic, optical, ferroelectric, and electrocaloric properties of [formula omitted] thin film: Insight from first-principles effective Hamiltonian

In this work, electronic, optical, dielectric and ferroelectric properties of PbxSr1-xTiO3 (PST) thin film are systematically investigated. This system is studied from first-principles computations using density functional theory (DFT) and effective Hamiltonian. In this regard, the influence of Pb doping on the above properties is elucidated to unravel a global description of this material. From the calculations along the higher symmetry direction, and upon substitution of Sr content, a direct band gap is obtained. Concerning the optical properties, the curves of refractive index revealed a systematic variation with composition. The results were compared with other reviews and showed a good agreement. Through a detailed analysis, investigations of several parameters are reported. However, an appropriate level of Pb doping can provide tremendous informations about this material, representing an important step toward deeper understanding for furthering knowledge, which may open the door for practical applications. This work reveals the contribution of Pb content especially on the phase transitions. The increase in the concentration of Pb shifts the critical temperature towards higher temperature and evolves the dipoles to switch for a large temperature range, resulting a large tetragonal (ferroelectric) phase and anomalous volume expansion, which in turn could improve the ferroelectric transition temperature. The dielectric constant, polarization and pyroelectric coefficient are examined as a function of the temperature. A typical hysteresis loops were clearly observed to confirm the ferroelectric-paraelectric phase transition. Besides, the electrocaloric effect (ECE) of PST thin film is also predicted with particular attention to the applied electric field and concentration x. Our calculations also revealed that PbxSr1-xTiO3 thin film exhibits a large room-temperature adiabatic temperature change (ΔT), which largely depends on the Pb content. The increase of Pb composition improves the adiabatic temperature change and enlarges its maximum at a relatively higher temperature. An enhancement regarding the ECE may potentially open the door for practical cooling device applications.

Control of tensile stress on inducing formation and tunability of (100) oriented Pb x Sr1−x TiO3 thin films

(100) Oriented Pb x Sr1−x TiO3 (PSTO) thin films are prepared on indium tin oxide (ITO)/glass substrates by sol–gel technique while inserting doped PbTiO3 (PTO)-inducing layer in between. The effect of tensile stress in PSTO on tunability and (100) orientation of the thin films was investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscope, and atomic force microscope, respectively. Results show that PSTO thin film deposited on doped PTO has (100) oriented structure while it is randomly oriented when deposited directly on the ITO/glass substrate. Lattice mismatch between PSTO and PTO appears, in which the in-plane lattice constant c is 0.3922–0.3924 nm in the former and 4.02–4.07 nm in the latter, respectively, contributing tensile stress in the PSTO due to different lattice constants between them. The stress in the PSTO thin film is 3.04, 3.15, 3.59 and 4.47 GPa when the doped PTOs are Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively. The orientation degrees of PSTO thin films are from 89.63, 90.31, 91.92 to 93.29 % with increasing stress of PSTO on Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively. Tunabilities of the well-oriented PSTO thin films increase in ascending order of 63 < 65 < 69 < 73 % when induced by oriented PTO layers of Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively, which is in accordance with the degree of (100) orientation appearing in the thin films. The high tunability appears in the PSTO thin film while high (100) orientation is derived from the tensile stress. It is much higher than that of randomly oriented PSTO thin film.

Effect of dispersion degree of orientation on dielectric properties of (100)-oriented PST thin film

(100) Oriented (PbxSr1−x)TiO3 (PST) thin films were prepared on indium tin oxide coated glass substrates by sol–gel technique with rapid thermal processing. The dielectric permittivity and tunability of the thin films with different dispersion degrees of orientation were investigated in detail by characterizing the full width at half maximum of their (100) peak based on rocking curves at different annealing temperatures. Influence of orientation dispersion on dielectric properties was exhibited in the tunable dielectric thin films. It shows that the dielectric constant and hence the tunability of the sol–gel derived PST thin films are improved with the decrease in the dispersion degree of orientation of the perovskite phase other than the increase in the content of crystalline phase in the thin films. The dielectric constant (capacitance) and figure of merit of the oriented thin films are 3–6 times and 1 times higher than that of randomly oriented thin film respectively.

Dielectric Properties of La2O3 Doped Composite (PbxSr1−x)TiO3 Borosilicate Glass Ceramic

Ferroelectric (PbxSr1−x)TiO3 (PST) perovskite phase has been crystallized in borosilicate glassy matrix with a suitable choice of composition and heat treatment schedule. La2O3 is a donor dopant for PST and can make it semiconducting. Dispersion of semiconducting perovskite phase in insulating glassy matrix in glass-ceramic samples may lead to the formation of space charge polarization around crystal-glass interface, leading to a high value of effective dielectric constant, εr. Therefore, with the aim of the developing glass ceramics with high dielectric constant, glasses in the system 64[(PbxSr1−x)O·TiO2]-25[2SiO2·B2O3]-5[K2O]-5[BaO]-1[La2O3] have been prepared (0.5≤x≤1). It is found that the addition of La2O3 strongly affected the crystallization and dielectric behavior of glass-ceramic with PST perovskite phase. All glass ceramic samples show a diffuse broad Curie peak in their εr versus T plots. Curie peak temperature, Tc, depends on compositions of the glass-ceramic samples as well as frequency of measurements.

High dielectric tunability of (100) oriented PbxSr1−xTiO3 thin film coordinately controlled by dipole activation and phase anisotropy

Why the dielectric tunability is maximal in dielectrics near the cubic–tetragonal phase transformation point is still unknown, although it is well accepted for tunable dielectrics till now. Here, we report dielectric tunability following a typical (100) oriented PbxSr1−xTiO3 (PST) thin film with different Pb/Sr ratio. The (100) PST thin films were prepared by sol-gel method on the terbium doped lead titanate (2%Tb-PbTiO3) inducing layer. They are tensile strained and the spontaneous polarizations (dipoles) are along the in-plain direction. Driven by the vertical DC bias, the dipoles in the (100) PST will be possibly rotated to the direction along DC bias and thus tunable permittivity appears. All these thin films show high dielectric tunability over 50%, in which the PST45(Pb0.45Sr0.55TiO3) exhibits the highest one. In this paper, the tunability is found controlled coordinately by both the anisotropy of the perovskite phase and the activation of dipole rotation. It increases with increasing the anisotropy when Pb increases based on the enlarging difference in permittivity along a and c directions, while it decreases with increasing the activation energy based on uneasy rotation of dipoles under DC bias. That is why, high dielectric tunability always exists near the cubic–tetragonal phase transformation; at this point, the activation energy is not so high and the permittivity difference keeps still a little significant.

Optimization of PST Thin Films Grown by Sputtering and Complete Dielectric Performance Evaluation: An Alternative Material for Tunable Devices

PbxSr1-xTiO3 (PST) thin films with the ratio Pb/Sr = 40/60 were deposited on silicon substrate with Pt or LaNiO3 (LNO) bottom electrodes by radio frequency magnetron sputtering followed by a postannealing treatment. The structural and microstructural analyses were performed and a perovskite phase was obtained whatever the nature of the bottom electrodes and even at a low temperature (450°C). The optimal annealing temperature is 650°C at which the films have a dense and fine microstructure. For electrical characterization the different top electrodes (Pt and LNO) were used. In both cases, we demonstrate that PST films present excellent performances in terms of dielectric properties and in particular, the tunability and Figure of Merit. A large tunability (57% at 400 kV/cm) and low dielectric losses (1.4%) were measured at 10 kHz for Pt/PST/Pt structures postannealed at 500°C. For LNO/PST/LNO structures, postannealed at 650°C, the improved tunability (≈80%) and low loss factor (≈2%) were obtained. Prospects of PST as an alternative to BST for tunable applications with a real potentiality of monolithic integration with silicon, in terms of thermal budget, are considered.

CHARACTERISTIC OF (100)-TEXTURED (PbxSr1 −x)TiO3 THIN FILMS USED FOR DRAM AND TUNABLE DEVICE APPLICATION

ABSTRACT The (PbxSr1-x)TiO3 (PST) thin films were deposited on LaNiO3 (LNO(1 0 0))/ Pt/Ti/SiO2/Si substrates electrode by RF-magnetron sputtering using three different Pb target composition ranging from 32.5%∼37.5% and different process condition. Structural and dielectric properties of the PST thin films for tunable microwave and DRAM application were investigated. The PST thin films deposited at 400°C show higher dielectric constant than those post-annealed at 600°C because of better crystallization. The former also have lower leakage current around 10−8 A/cm2 up to applied field of 350 kv/cm, which is suitable for DRAM application. On the other hand, the post-annealed PST thin films have satisfactory tunability around 58% and figure of merit around 30, which are more suitable for microwave device application.

Ferroelectric properties of PbxSr1−xTiO3 and its compositionally graded thin films grown on the highly oriented LaNiO3 buffered Pt∕Ti∕SiO2∕Si substrates

Thin films of ferroelectric PbxSr1−xTiO3 (PST) with x=0.3–0.7 and graded composition were fabricated on LaNiO3 buffered Pt∕Ti∕SiO2∕Si substrates by a sol-gel deposition method. The thin films crystallized into a single perovskite structure and exhibited highly (100) preferred orientation after postdeposition annealing at 650°C. The grain size of PST thin films systematically decreased with the increase of Sr content. Dielectric and ferroelectric properties were investigated as a function of temperature, frequency, and dc applied field. Pb0.6Sr0.4TiO3 films showed a dominant voltage dependence of dielectric constant with a high tunability in a temperature range of 25–230°C. The compositionally graded PST thin films with x=0.3–0.6 also showed the high tunability. The graded thin films exhibited a diffused phase transition accompanied by a diffused peak in the temperature variations of dielectric constants. This kind of thin films has a potential in a fabrication of a temperature stable tunable device.

Preparation and Curie Temperatures of Nanosized (Pb,Sr)TiO3 Powders

Nanosized perovskite lead strontium titanate (PbxSr1-x)TiO3 (PST) powders were successfully prepared by a simple coprecipitation method. A lead-strontium titanyl oxalate (PSTO) precursor was first synthesized at room temperature and the precursor was then calcined at various temperatures to produce the PST powders ranging from 10 to 200 nm in size. The effects of PST particle size on Curie temperature and dielectric constant have been investigated. High-temperature X-ray diffraction results showed that the Curie temperature of Pb0.7Sr0.3TiO3 powder with a size of 10 nm decreased from 290 to 120–130°C.

Local symmetry breaking in PbxSr1−xTiO3 ceramics and composites studied by Raman spectroscopy

Local symmetry breaking in PbxSr1−xTiO3 (PST) solid solutions was revealed by Raman spectroscopy below and above the ferroelectric phase-transition temperature. In the paraelectric phase, local symmetry breaking associated with the polar clusters localized mostly at grain boundaries in ceramic samples leads to the activation of the polar modes far above the dielectric maximum. This effect was found remarkably enhanced in the PST:MgO composites due to the compositional inhomogeneity of the grain boundaries and the intergrain stresses in the composite. In the ferroelectric phase, the degeneracy of the E(TO1) soft mode was lifted due to translational symmetry breaking in the solid solutions with x=0.2 and 0.3, where the substitution of Sr by Pb replaces partially the ionic bonding between the A cations and the oxygens by a stronger covalent bonding.

Direct experimental evidence of hybridization of Pb states with O2p states in ferroelectric perovskite oxides

This work presents the OK- and TiL3,2-edge x-ray absorption near-edge structure (XANES) spectra of PbxSr1−xTiO3 (PxSTO) and BaxSr1−xTiO3 (BxSTO) compounds with various Pb and Ba concentrations. The result provides direct evidence that the Pb–O bonding strongly affects O2p–Ti3d hybridization in the TiO6 octahedron of PxSTO. In contrast, the Ba–O bonding does not substantially affect O2p–Ti3d hybridization in BxSTO. The TiL3-edge XANES spectra show the splitting of the eg band for PxSTO with x≳0.5, which provides an evidence of Pb-induced tetragonal distortion in the TiO6 octahedron. In contrast, eg band splitting is absent in BxSTO.

Switching Properties of PST Ferroelectric Films for Memory Applications using Conductive Oxide LSCO Electrodes

ABSTRACTWe have investigated the switching properties of ferroelectric lead strontium titanate (Pbx Sr1-x)TiO3 (PST) capacitors using epitaxial and polycrystalline La0.5Sr0.5CoO3 (LSCO) electrodes to evaluate its potential for non-volatile memory applications (FRAM'S). The electrical performance of the PST based capacitors grown on epitaxial and polycrystalline LSCO substrates were evaluated through Polarization-Voltage (P-V) and Fatigue measurements. The [001] preferentially oriented PST ferroelectric capacitor did not show a decrease in polarization up to 108 switching cycles at an applied voltage of 4 volts and a frequency of 100 kHz. Polycrystalline PST films, on the other hand were severely fatigued. It is proposed that the good performance of the PST capacitors can be attributed to the high degree of orientation of the PST films in the [001] direction induced by the epitaxial LSCO film. However, after 1010 switching cycles in the fatigue tests, the decay of the non-volatile polarization of these films was about 18% of its initial value. We use a simple model to describe two major microscopic scenarios for the suppression of polarization at the electrodes interfaces. This model is supported by Auger Electron Spectroscopy to determine the bulk and interfacial characteristics. The depth distribution of the elements of the PST film is nearly uniform throughout the volume of the film; however the relative concentrations of Pb and O were abnormally distributed on the surface. It is concluded that, in spite of the good performance due to the favorable conditions for the growth of the PST films provided by the textured LSCO, higher quality interfaces and better control of composition is necessary to improve on fatigue.

Structural and Electrical Investigations of Ferroelectric Lead Strontium Titanate Thin Films and Ceramics

ABSTRACTLead strontium titanate (PbxSr1-xTiO3) (x=0.3–1.0) ceramic targets were prepared by the conventional powder-processing method. Thin films of these compositions were deposited on platinized silicon substrates by pulsed laser deposition technique. X-ray diffraction studies of the ceramic targets showed that the lattice structure changes from tetragonal to cubic phase with the increase of Sr content in PbTiO3. Raman spectroscopic studies of PbxSr1-xTiO3(PST) ceramics and thin films showed that the soft mode decreases to lower frequency and finally disappear at around 60–70 at% Sr content, which confirms the tetragonal to cubic phase transition at room temperature. Dielectric constant measured for PST thin films was in the range of 900–1500 at 1 MHz, with maximum value obtained for PST30 thin film. The loss tangents at room temperature were in the range of 0.07–0.1 for PST thin films with different compositions.