Origin of voltage offset and built-in polarization in in-situ sputter deposited pzt thin films

Abstract

Voltage offset and built-in polarization were investigated in in-situ reactively sputter deposited PZT thin films on Pt, as a function of composition, doping and cooling conditions. The voltage offset increases with increasing Ti content. Hot-poling treatments show that the voltage offset originates from non-mobile defects. Cooling the samples at various O₂ partial pressures leads to a change in lattice parameters and orientation, showing a ‘bulk’ effect rather than a simple ‘surface’ effect. While doping with Nb does not change the status of the films, doping with Fe increases coercive fields but removes the asymmetrical behavior of the voltage offset after hot-poling at positive and negative voltages. Annealing/cooling cycles are reversible in the suppression/creation of voltage offset and built-in polarization, respectively, excluding the idea of oxygen and lead vacancy dipolar defects. Experimental data suggest that oxygen is lost and the vacancies diffuse during cooling without being compensated to equilibrate with surface or electrode band bending, resulting in the creation of an internal bias field.     

Temperature dependence of the dielectric dispersion and ferroelectric properties of neodymium doped bismuth titanate ceramics

Nd-doped bismuth titanate Bi_{4 − x} Nd _x Ti₃O₁₂ ceramics (x = 0–1.0) were prepared by the solid state reaction method. The temperature dependence of the dielectric dispersion and ferroelectric properties were investigated. With the increase of the Nd substitution for Bi ion, the Curie temperature decreased and the corresponding dielectric constant peak broadened. In addition, the strong low-frequency dielectric dispersions were exhibited. The Nd doping decreases the temperature dependence of the ac conductivity and increases the temperature dependence of the remanent polarization, which is caused by the induced polarization by defects, such as bismuth and oxygen vacancies.     

Diffuse phase transition behavior of dysprosium doped barium titanate ceramic

The effect of dysprosium (Dy) doping on the structural and dielectric properties of barium titanate (BaTiO₃) ceramic has been studied. Dysprosium-doped barium titanate (BT) with general formula Ba_1-xDy_2x/3TiO₃ (x?=?0.0–0.1) is prepared through solid state reaction route. The XRD pattern reports the single phase tetragonal structure up to x?=?0.025 with space group of P4mm and a secondary phase is observed for higher concentration of Dy. The Raman study of Ba_1-xDy_2x/3TiO₃ (x?=?0.00, 0.01, 0.025) also reveals that the ceramic is showing tetragonal symmetry and with an increase in Dy doping the tetragonal phase is moving toward more symmetric phase i.e. cubic pm3m. The surface morphology of the sintered pellets shows a compact and homogeneous grain distribution with a decrease in grain size as the Dy content increases. The temperature and frequency dependency dielectric study of the ceramic compositions (x?=?0.00, 0.01, 0.025) are studied to show the effect of Dy ion on the A site of BaTiO₃ perovskite ceramic. The dielectric constant decreases with increase in Dy content. The dielectric diffusivity increases with increase in doping concentration. The P-E hysteresis loop confirms the ferroelectric behavior of the sample with decrease in remanent polarization and increase in coercive field.     

EFFECT OF ALKALINE EARTH SUBSTITUTION ON ELECTRICAL PROPERTIES OF SrBi4Ti4O15 CERAMICS

ABSTRACT

Ferroelectric Sr_1–xCa_xBi₄Ti₄O₁₅ (SCBT-x x = 0.00, 0.20, 0.40, 0.50, 0.70, 0.90 and 1.00) ceramic samples were prepared using the conventional solid-state reaction. The Curie temperature (T_c) increases with Ca doping. The temperature dependence of dielectric loss (tan δ) shows two dielectric anomalies at x = 0.20 while only one dielectric loss peak is found in other cases. The remnant polarization (2P_r) and coercive field (E_c) of nondoped SrBi₄Ti₄O₁₅ are about 15.7 μC/cm² and 107 kV/cm, respectively. Small amount of Ca doping increases the 2P_r, the 2P_r reaches a maximum value of 17.6 μC/cm² and E_c is increased to 113 kV/cm when x = 0.20. The piezoelectric coefficient (d₃₃) shows the maximum values of 9.0 pC/N at x = 0 and decreases almost linearly with the increase in x.     

Ferroelectric and piezoelectric properties of tungsten doped CaBi4Ti4O15 ceramics

Tungsten doped CaBi₄Ti₄O₁₅ (CBT) ceramics was prepared by solid-state reaction method. A pure single phase of layer-structured ferroelectric with m?=?4 was formed when x?≤?0.025. The effect of W doping on dielectric, ferroelectric and piezoelectric properties was investigated. W⁶⁺ doping increased the temperature stability of dielectric constant and decreased dielectric loss. W⁶⁺ doping decreased the remanent polarization, while the coercive field decreased as well, as a result, the piezoelectric constant was increased. AC conductivity measurement showed that W⁶⁺ doping increased the conductivity of CBT ceramics and showed n-type conducting mechanism. W⁶⁺ doped ceramics has smaller activation energy due to many defects introduced in the crystal lattice.     

Characterization of lead cation-incorporated strontium bismuth tantalate ferroelectrics

Abstract

The synthesis and characterization of lead cation-incorporated strontium bismuth tantalate ferroelectrics were investigated. Monophasic layered perovskites were successfully prepared, and the formation of solid solutions of lead-doped SrBi₂Ta₂O₉ was corroborated. The value of lattice parameter c monotonicly increased with an increase in the lead content. The sinterability of the formed powder was poor; however, adding excess bismuth oxide markedly facilitated the sintering process. After sintering, the specimens exhibited strong preferred c-axis orientation. The degree of c-axis orientation significantly varied with the sintering temperature and the doping amount of lead species. SEM analysis indicated that the variation in the morphology of the grains was related to the degree of c-axis preferred orientation. The lead-doped specimens exhibited ferroelectric properties with well saturated P-E loops. Both the remanent polarization and the coercive field increased with the doping amount of lead cations.     

Improvement of the electrical properties of PZT thin films using TiO2 buffer layer

Abstract

Thin TiO₂ layers were sputter-deposited on Pt/Ti/SiO₂/Si wafers, as buffer layers for PZT thin film capacitors. It was found that TiO2 buffers of less than 4-nm-thickness could assist in obtaining highly uniform PZT thin films with no second phase. The leakage current behaviors of the PZT based capacitor are improved, while retaining the ferroelectric properties of PZT thin films such as remanent polarization and coercive field. In addition, the uniform distribution of oxygen in PZT on TiO₂/Pt indicates that the TiO₂ buffer layer act as a barrier for lead-platinum reaction, as well as for oxygen diffusion.     

Investigation of magnetic and ferroelectric properties along with the magnetoelectric coupling behavior for asserting a room temperature bi-phase composite as multiferroics

Various composites of nominal composition (1-x)Ba_0.95Ca_0.05TiO₃?+?xLi_0.1Cu_0.1Co_0.1Zn_0.6Fe_2.1O₄ have been prepared and studied thoroughly. X-ray diffraction and Rietveld refinement confirmed the presence of Ba_0.95Ca_0.05TiO₃ and Li_0.1Cu_0.1Co_0.1Zn_0.6Fe_2.1O₄ phases in the composites. The microstructures have been investigated by field emission scanning electron microscopy. Temperature dependent dielectric constant shows two peaks, one is at 150 °C and another at 270 °C for x?=?0.10 composite which resembles the characteristic ferroelectric and ferromagnetic transition peaks. A gradual progression of ferro-para electric transition towards room temperature is observed with doping. The non-Debye type dipole relaxations have been found. The linearity in the log(σ_AC) vs. log(ω) plots indicate that conduction is due to small polaron hopping. The real part of initial permeability increases with growing ferrite concentration but the cut-off frequency decreases. The magnetic property is also enhanced with doping content. The typical ferroelectric hysteresis loops have also seen with the addition of Li_0.1Cu_0.1Co_0.1Zn_0.6Fe_2.1O₄ up to x?=?0.40. The impedance values are found to decreases in the Nyquist plots. The magnetoelectric voltage coefficient is obtained 287?×?10³ V/mT for x?=?0.15 at room temprrature. We found both ferromagnetic and ferroelectric hysteresis loops at room temperature. So, it confirms that the composites exhibit room temperature multiferroicity. This type of composites offers variety of opportunity for multifunctional devices application like hetero-structured read / write memory devices, switching devices and magnetic field sensing devices.

     

Influence of electric field on the ultrasound velocity in PZT ceramics

The paper deals with the influence of the electric bias field on the ultrasound velocity of various samples of Pb(Zr _x Ti_1−x )O₃ (PZT) ceramics. The ultrasonic velocities were measured on commercial types of PZT ceramics as APC 841, APC 850, and APC 856, at room temperature. The comparison of the ultrasound velocities dependence on electric bias field was made for poled/unpoled soft ceramics. The ultrasonic pulse-echo technique was used in our experiment. The ultrasonic system is based on Matec Instruments, Inc. modules. The time domain response was recorded and time of flight between echoes was directly measured by digital oscilloscope, type Agilent 54622D. The high bias field was applied on disk samples by the high voltage amplifier type Trek 610D, using a special setup and sample holder. The sound velocity was found to change drastically near the coercive field for a PZT ceramics, where the velocity of longitudinal waves decreases with an increasing field while the velocity of shear waves increases, which is caused by the change of the elastic anisotropy under influence of the depolarization field. The consequent change of the piezoelectric contribution to effective elastic constant decreases the velocity of longitudinal waves and at the same time increases the velocity of shear waves around the coercive field.     

FERROELECTRIC,PIEZOELECTRIC AND DIELECTRIC PROPERTIES OF Nb MODIFIED Bi4Ti3O12-SrBi4Ti4O15 INTERGROWTH

ABSTRACT

Doping and constructing intergrowth are found to be an effectual approach to modify the electrical properties of bismuth layer-structured ferroelectrics. In this work, ferroelectric, piezoelectric and dielectric properties of Nb-doped Bi₄Ti₃O₁₂-SrBi₄Ti₄O₁₅ (BTN-SBTN-x) intergrowth ceramics were investigated. The ferroelectric property of Bi₄Ti₃O₁₂-SrBi₄Ti₄O₁₅ was improved by niobium doping. The remanent polarization (2P _r ) increases at first, then decreases with the increase of niobium content, while the coercive field changes little with Nb-doping. As niobium content is 0.06, the 2P _r maximizes at a value of 34.9 μC/cm², which is increased by about 75% in comparison with that of Bi₄Ti₃O₁₂-SrBi₄Ti₄O₁₅ (2P _r = 20.0 μC/cm²). The Curie temperature of the samples varies hardly upon Nb-doping, which indicates that the good thermal stability of Bi₄Ti₃O₁₂-SrBi₄Ti₄O₁₅ is not deteriorated after Nb addition. Piezoelectric coefficient d₃₃ was increased from 9.3 pC·N^?1 to 18.6 pC·N^?1 due to the enlargement of 2P _r .     

Fatigue and dielectric properties of undoped and Ce doped PZT thin films

Abstract

Ce doping improves the ferroelectric properties of sol-gel derived PZT thin films by facilitating easier domain wall movement. It also decreases the leakage current densities by reducing the concentration of free carriers through a decrease in concentration of Pb and O vacancies. Ce-PZT films retain good dielectric dispersion characteristics since the concentration of defects and defect dipoles are reduced. Ce doping dramatically improves the fatigue resistance of PZT thin films. We have studied the frequency dependence of fatigue behavior and shown that the loss of polarization due to fatigue follows a universal scaling behavior with N/f², where N is the number of the switching cycles and f is the frequency. The origin of the scaling is attributed to the drift of oxygen vacancies, which is the rate limiting process in the growth of the interface layer responsible for fatigue. Empirical fits for both undoped and cerium doped samples show that switchable polarization follows a stretched exponential decay with time or N/f. Cerium doping is believed to improve fatigue resistance by impeding the motion of oxygen vacancies.     

Growth and electrical properties of Pb(Zr,Ti)O3 thin films by sputtering using an alloy target

Abstract

Pb(Zr_x,Ti_1?x)O₃ thin films were deposited on Pt/SiO₂/Si substrates by the rf magnetron sputtering using an alloy target consisting of Zr-Ti alloy and Pb metal. The dependence of electrical properties on film thickness and sputtering gas pressure was investigated. The dielectric constant and the remanent polarization decreased and the coercive field increased with the decrease of the film thickness. In the dependence of gas pressure, the relative dielectric constant of the film with only a perovskite phase were in the range of 235–280, which were higher than those of the film with only a pyrochlore phase, 20. The asymmetry of hysteresis loops increased with the decrease of the gas pressure.     

Dielectric,ferroelectric, and piezoelectric properties of lead-free Ba0.95Ca0.05Ti1-xZrxO3 ceramics

Abstract

Ba_0.95Ca_0.05Ti_1-xZr_xO₃ (BCTZO) ceramics were prepared by a solid state reaction method. The samples were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and X-ray absorption near edge structure (XANES). The ceramics exhibit a pure perovskite structure. The average grain size gradually decreases with increasing Zr concentration. XANES results indicate that the intensities of pre-edge peaks dropped with increasing Zr concentration. The BCTZO ceramic of x?=?0.05 has the optimum electrical properties with the maximum dielectric constant (ε'_m), remanent polarization (2P_r), coercive electric field (2E_c) and piezoelectric charge constant (d₃₃) of 7,244, 12.54 (μC/cm²), 5.29 (kV/cm) and 288 (pC/N), respectively.     

Phase transition and electrical studies of wolframium doped SrBi2Ta2O9 ferroelectric ceramics

In this study, crystalline structure, dielectric and impedance properties of SrBi₂Ta₂O₉ (SBT) - based ferroelectric ceramics have been investigated with the substitution of wolframium/tungsten (W) onto the tantalum site. Wolframium doped SrBi₂(W _x Ta_{1 − x} )₂O₉ (0.0 ≤ x ≤ 0.20) ceramics were synthesized by solid state reaction method. The X-ray diffractogram analysis revealed that the substitution formed a single phase layered perovskite structure for the doping content up to x ≤ 0.05. The dielectric measurements as a function of temperature show an increase in Curie temperature (T _c ) over the composition range of x = 0.05 to 0.20. The W^{6 +} substitution in perovskite-like units results in a sharp dielectric anomaly at the ferroelectric phase transition. Furthermore, the dielectric constant at their respective Curie temperature increases with wolframium doping. Both enhanced Curie temperatures and dielectric constants at the Curie points indicate an increase in polarizability, which could be attributed to the increased “rattling space” due to the incorporation of the smaller tungsten cations. The dielectric loss reduces significantly with tungsten addition. AC impedance properties vis-à-vis wolframium content has also been studied.     

Room temperature multiferroic properties of Mn doped La2Ti2O7

Polycrystalline La₂Ti_2-xMn_xO₇ (LTMO) (x?=?0?0.16) ceramics fabricated by the conventional solid-state reaction are utilized to investigate the effect of Mn doping on the ferroelectric (FE) and ferromagnetism (FM) systematically. Both the FE and FM properties strongly depend on the doping level of Mn atoms, which directly induces a structural phase transition from monoclinic to orthorhombic. Compared with pure La₂Ti₂O₇ (LTO), remnant polarization decreases sharply with the doping level at first, and slightly increases when x?≥?0.08, which is proposed to result from the evolution oxygen vacancies. All the doped samples exhibit weak FM property at room temperature, until the maximum can be observed for x?=?0.08. Mn³⁺-Mn⁴⁺ double-exchange and anti-FM Mn³⁺-Mn³⁺ and Mn⁴⁺-Mn⁴⁺ super-exchange interactions are responsible for the different ferromagnetic behavior of LTMO.     

Models for the frequency dependence of coercive field and the size dependence of remanent polarization in ferroelectric thin films

Abstract

We have confirmed the theory of Ishibashi and Orihara for the frequency dependence of coercive fields in ferroelectric thin films.SrBi₂NbTaO₉, SrBi₂Ta₂O₉, and PbZr_1-x Ti _x O₃ (PZT) data from DeVilbis et al. reveal a power-law dependence of form E_c (f) = Bf^d/α , where d is the dimensionality of domains (ca. unity in uniaxial ferroelectrics such as SBT and SBNT) and α is approximately 6, in agreement with theoretical predictions. Models are also presented for the polarization and fatigue data from NEC of micron-sized thin-film strontium bismuth tantalate capacitors, emphasizing behaviour at unsaturated voltages.     

(Zr/Ti) ratio effect on RF magnetron sputtered lead titanate zirconate films

Abstract

Sputtered Pb(Zr_xTi_1?x)O₃ thin films with various (Zr/Ti) compositions ranging from 15/85 to 70/30 were grown and characterised in terms of structural and electrical properties. PZT thin films, with 0.7–0.8μm thickness, were deposited on Si/SiO₂/Ti/Pt by sputtering followed by conventional annealing. The sputtering conditions were the same for all the compositions. There were no apparent differences in crystallographic orientation as a function of Zr/Ti and the films a-lattice constant evolution is not exactly identical to the one of bulk ceramics. The permittivity increases when the Zr concentration increases and decreases just after the mor-photropic composition i.e. Zr-rich films. The ferroelectric properties are very sensitive to the Zr/Ti ratio. For example, the coercive field increases when the Ti concentration in the film increases.     

Device effects of various Zr/Ti ratios of PZT thin-films prepared by sol-gel method

Abstract

PZT, PbZr_xTI_1?xO₃, thin-films with various Zr/Ti ratios, 100/0 (lead zirconate) to 0/100 (lead titanate), were prepared by the sol-gel method. Basic electric properties, dielectric constant, tan σ, P-E hysteresis curve, switching properties were measured respectively as a function of composition. Dielectric constant indicated a specific peak value (～1100) around the morphotropic phase boundary between tetragonal and rhombohedral phase. Satisfactory low-voltage saturated hysteresis curves were observed for the compositions of PZT(90/10) through PZT(20/80). The remanent polarization and the coercive field increased as the titanium content decreased. The result of X-ray measurement showed that the lattice constants of thin-Film PZT are different from bulk ceramics for compatible compositionsS. The boundary, on which the c/a ratio must be 1 (rhombohedral phase), was slightly shifted to PbTiO₃ side. This distortion in crystal structure is considered to be due to the thin-film effect, which the lattice mismatch between the platinum substrate and the PZT layer, restricted the ions to position into proper cites.     

Effect of Rare Earth Europium Substitution on the Microstructure,Dielectric, Ferroelectric and Pyroelectric Properties of PZT Ceramics

Polycrystalline samples of Europium modified lead Zirconate titanate (Pb_{1 – x} Eu_x)(Zr_0.55Ti_0.45)_{(1 – x/4)}O₃ with x = 0.00, 0.02, 0.04, 0.06 has been prepared by mixed oxide (MO) method at sintering temperature of 1250C. The structural characterization of the samples investigated by X-ray diffraction technique exhibit tetragonal structure. PEZT (2/55/45) ceramics show single perovskite phase. Scanning electron micrographs depict uniform, densely packed structure. Dielectric, Pyroelectric and Ferroelectric studies have been performed and are reported and discussed in this paper. PEZT (2/55/45) show good current responsivity (F_i), voltage responsivity (F_v) and detectivity (F_d), in comparison to other compositions under study. Moreover its ferroelectric properties (high remanent polarization with low coercive field) make it suitable material for nonvolatile memory applications.     

Control of the ferroelectric and electrical properties of Nd substituted bismuth titanate ceramics

Nd-doped bismuth titanate Bi_4?x Nd _x Ti₃O₁₂ ceramics were prepared by a solid state reaction method. The effects of Nd doping on ferroelectric and electrical properties were investigated. With Nd substitution for Bi ion, a BNdT single-phase with Bi-layered perovskite was confirmed. The Nd doping decreased the dissipation factor (tanδ) and the dc conductivity, and increased the dielectric constant and the remanent polarization. A small Nd doping on Bi₄Ti₃O₁₂ leads to a higher remanent polarization and improves the ferroelectric properties.