Nanostructured heterophase thin films of lead zirconate titanate

The results of a comprehensive study of electrophysical and photoelectric properties of capacitor structures are analyzed within the proposed model of Pb(ZrTi)O₃ (PZT) films with an excess lead content, which is based on the presence of heterophase intergrain boundaries. It is shown that aging of thin-film capacitor structures is accompanied by a significant increase in the oxygen content in submicron PZT films, as well as by the modification of elemental and phase compositions of the interfaces. It is confirmed experimentally that a decrease in the switching charge in the aged PZT films is due to the oxygen sorption at heterophase crystallite boundaries containing lead oxide and to pinning of the polarization in regions adjacent to the charged boundaries. It is demonstrated that the current-voltage characteristics of the capacitor structures are described in terms of the mechanisms of space-charge-limited currents.     

Fatigue of lead titanate and lead zirconate titanate thin films with various values of coercive and internal bias fields

The dependence of the residue polarization on the number of switching cycles in thin ferroelectric films with various thicknesses and on substrates differing by values of the coercive and internal bias fields was experimentally studied. A model describing the features of the fatigue effect in these films is proposed.     

Stochastic ferroelectric switching of lead zirconate titanate thin films

We investigate the repolarization phenomenon in a ferroelectric film. Our ferroelectric sample was lead zirconate titanate (PZT) obtained by sol-gel synthesis and deposited by spin coating on ITO/glass substrate. A series of repolarizations were induced in the ferroelectric film by applying a triangular wave and the current peaks related to the switchings of the ferroelectric domains were acquired for statistical analyses. It is shown that the dynamics and statistics of polarization switchings are well simulated by a simple mean-field model in which a double-well, asymmetric potential is included to describe the asymmetry at the PZT-ITO interface.     

Internal field and self-polarization in lead zirconate titanate thin films

It has been shown that 300-nm-thick polycrystalline films of lead zirconate titanate (PZT), the compositions of which correspond to the region of morphotropic phase boundary, undergo anomalous changes in the composition and the microstructure as the annealing temperature increases. This causes substantial variations in the dielectric and piezoelectric responses, the spontaneous polarization and the internal field value. It has been demonstrated that the self-polarization varies nonmonotonically with increasing the internal field and is characterized by a threshold effect. The role of excess lead oxide and its influence on the formation of the self-polarization and the change in the effective permittivity of the ferroelectric layer has been discussed.     

Mechanical properties of sol-gel derived lead zirconate titanate thin films by nanoindentation

Lead zirconate titanate (PZT) thin films are deposited on platinized silicon substrate by sol-gel process. The crystal structure and surface morphology of PZT thin films are characterized by X-ray diffraction and atomic force microscopy. Depth-sensing nanoindentation system is used to measure mechanical characteristics of PZT thin films. X-ray diffraction analyses confirm the single-phase perovskite structures of all PZT thin films. Nanoindentation measurements reveal that the indentation modulus and hardness of PZT thin films are related with the grain size and crystalline orientation. The increases of the indentation modulus and hardness with grain size are observed, indicating the reverse Hall-Petch effect. Furthermore, the indentation modulus of (1 1 1)-oriented PZT thin film is higher than those of (1 0 0)- and random-oriented films. The consistency between experimental data and numerical results of the effective indentation moduli for fiber-textured PZT thin films using Voigt-Reuss-Hill model is obtained.     

Evolution of the switching current during the accumulation of fatigue in lead titanate and lead zirconate titanate thin films

The fatigue processes caused by repeated cyclic switching of polarization in lead titanate and lead zirconate titanate thin films on silicon substrates by applying rectangular pulse and sine wave fields are investigated. Upon an increase in the cycling number, an increase in the coercive and internal bias fields is observed, along with a reduction in the switching current maximum, the switching time, and the switchable polarization. At the same time, no change in the mobility of domain boundaries is observed.     

Crystallization of lead zirconate titanate films by laser annealing

The structure of lead zirconate titanate Pb(Zr _x Ti_{1 ? x} )O₃ thin films grown by chemical solution deposition on Si-SiO₂-Ti-Pt substrates has been studied using transmission electron microscopy and energy-dispersive analysis. Films crystallization has been performed using laser annealing. It has been found that, in contrast to isothermal annealing where nucleation on the platinum layer dominates, crystallization and growth of spherical perovskite crystals occur in the film bulk. The perovskite phase crystal size increases from 10 to 120 nm with increasing laser beam energy.     

Substrate clamping effects on irreversible domain wall dynamics in lead zirconate titanate thin films

The role of long-range strain interactions on domain wall dynamics is explored through macroscopic and local measurements of nonlinear behavior in mechanically clamped and released polycrystalline lead zirconate-titanate (PZT) films. Released films show a dramatic change in the global dielectric nonlinearity and its frequency dependence as a function of mechanical clamping. Furthermore, we observe a transition from strong clustering of the nonlinear response for the clamped case to almost uniform nonlinearity for the released film. This behavior is ascribed to increased mobility of domain walls. These results suggest the dominant role of collective strain interactions mediated by the local and global mechanical boundary conditions on the domain wall dynamics. The work presented in this Letter demonstrates that measurements on clamped films may considerably underestimate the piezoelectric coefficients and coupling constants of released structures used in microelectromechanical systems, energy harvesting systems, and microrobots.     

Effects of microstructure on the properties of ferroelectric lead zirconate titanate (PZT) thin films

Lead zirconate titanate (PZT) thin films were prepared with pulsed laser deposition and sol–gel techniques. The PZT films fabricated by these two techniques have similar randomly oriented single perovskite phases, but the film derived from the pulsed laser deposition exhibits a more compact and flat morphology. The dielectric, ferroelectric, and piezoelectric properties of the two kinds of films are comparatively characterized and discussed. It is observed that a denser microstructure would lead to a significantly higher dielectric constant and remanent polarization and a much lower coercive electric field, but only a relatively slight enhancement on the piezoelectric constant. The film with a looser microstructure could have a substantially higher piezoelectric voltage constant g₃₃ due to the much lower dielectric constant. Our results and discussion provide a better understanding of the relationship between the microstructure and the film properties, which is essential in order to tailor the microstructure and hence determine the performance aiming at a specific application. PACS 68.55.Jk; 77.22.-d; 77.65.-j     

Determination of the nanoscale dielectric properties in ferroelectric lead zirconate titanate (PZT) thin films

We report on nanoscale experiments with <100 nm lateral resolution being able to differentiate the effective dielectric polarisation P_z, deposited charge density σ, surface dielectric constant ε_surface, its voltage dependence ε_surface(U), as well as the built-in electric bias voltage U_int in ferroelectric lead zirconate titanate (PZT) thin films. This is possible by combining piezoresponse force microscopy (PFM) and pull-off force spectroscopy (PFS), both methods based on scanning force microscopy (SFM). The differentiation becomes possible since both P_z and σ contribute additively in PFS, while they are subtractive in PFM, hence allowing the two contributions to be separated. ε_surface can be quantified by means of the experimental PFS data and the calculated effective penetration depth of PFM developed in a finite element modelling. Finally, U_int and ε_surface(U) are derived by an absolute matching of the P_z values measured by PFM and PFS.Our nanoscale results obtained on PZT thin films reveal values for the above specified quantities that have the same order of magnitude as those obtained from macroscopic measurements reflecting the integral response using large electrode areas. However, we stress that the data reported here reveal physical properties deduced on the nanometer scale. Furthermore, they are recorded during one single experimental investigation, using one single set-up only.     

Influence of oxygen partial pressure on the composition and orientation of strontium-doped lead zirconate titanate thin films

The influence of oxygen partial pressure during the deposition of piezoelectric strontium-doped lead zirconate titanate thin films is reported. The thin films have been deposited by RF magnetron sputtering in an atmosphere of high purity argon and oxygen (in the ratio of 9:1), on platinum-coated silicon substrates (heated to 650 degrees C). The influence of oxygen partial pressure is studied to understand the manner in which the stoichiometry of the thin films is modified, and to understand the influence of stoichiometry on the perovskite orientation. This article reports on the results obtained from films deposited at oxygen partial pressures of 1-5 mTorr. The thin films have been studied using a combination of X-ray photoelectron spectroscopy (XPS), glancing angle X-ray diffraction (GA-XRD), and atomic force microscopy (AFM). XPS analysis highlights the marked influence of variations in oxygen pressure during sputtering, observed by variations in oxygen concentration in the thin films, and in some cases by the undesirable decrease in lead concentration in the thin films. GA-XRD is used to study the relative variations in perovskite peak intensities, and has been used to determine the deposition conditions to attain the optimal combination of stoichiometry and orientation. AFM scans show the marked influence of the oxygen partial pressure on the film morphology.     

Influence of high-temperature annealing on the orientation of the unipolarity vector in lead zirconate titanate thin films

The factors responsible for the change in the orientation of the natural unipolarity vector due to heating to the Curie temperature of a Pt/PZT/Pt thin-film capacitor (PZT—lead zirconate titanate) formed on a TiO₂/SiO₂/Si substrate have been considered. Lead zirconate titanate thin layers containing a small excess of lead oxide have been formed ex situ using high-frequency magnetron sputtering with a variation in the annealing temperature (crystallization of the perovskite phase) in the range from 580 to 650°C. It has been assumed that the reorientation of the unipolarity vector in the PZT layer is caused by the change in the mechanism of crystallization of the perovskite phase with an increase in the annealing temperature.     

Characterization of sol-gel-prepared magnesium-modified lead titanate thin films

Pb_1-xMg_xTiO₃ (PMT) thin films (x=0–0.08) were deposited onto Pt/SiO₂/Si substrates by diol-based sol-gel processing. The influence of various processing methods on the characteristics of the thin films was studied by varying the magnesium contents and heating temperatures (500–800 °C) of the samples. In the present study, the relative dielectric constant (_r) of the PMT thin films increased from 25.8 to 91.8 as the magnesium content increased. The coercive field (E_c) and the remanent polarization (P_r) decreased as the magnesium content increased, but the dielectric loss factor (cos) and the pyroelectric coefficient (p) increased. The PMT thin films with x=0.06 exhibited the largest figures of merit for voltage responsivity and specific detectivity. These results indicate that the PMT thin film with x=0.06 was most suitable for application to high-performance pyroelectric thin-film detectors. PACS 81.20.Fw; 77.84.Dy; 77.70.+a     

Preparation and properties of lead titanate thin ferroelectric films

A new technique is proposed for preparing lead titanate ferroelectric films. The technique involves a solid-phase reaction in an oxygen environment between titanium and lead layers deposited on a substrate. The thicknesses of the component films being deposited are chosen based on the stoichiometric ratio in the compound to be synthesized. The composition and structure of the film obtained are checked by x-ray phase analysis. The films exhibit a dielectric hysteresis loop and a temperature dependence of the permittivity characteristic of ferroelectrics. A study has been made of the temperature and thickness dependences of the film coercive field. They also are shown to follow a pattern typical of ferroelectrics.     

Influence of strontium concentration on the structural,morphological, and electrical properties of lead zirconate titanate thin films

We report the strontium-doping effect on polycrystalline lead zirconate titanate thin films. The thin films were prepared by the polymeric precursor method and their structural, morphological, and electrical properties were studied. The Pb_1-xSr_xZr_0.3Ti_0.7 (PSZT) thin films with strontium concentration x between 0.10 and 0.30 were heated at 700 °C for 2 h for the crystallization. The structural phase evolution, as a function of the Sr content, was followed using micro-Raman spectroscopy, specular reflectance Fourier transform infrared spectroscopy, and X-ray diffraction (XRD). With the addition of strontium to PSZT, the broadening of the Raman peaks increases. This broadening indicates a structural change of PSZT from the tetragonal phase to a pseudocubic phase. A higher dielectric constant value, as a consequence of the higher strontium concentration, was observed. This increase may indicate the presence of a phase transition from tetragonal to pseudocubic at room temperature, which corroborates the XRD and Raman-spectra analyses, and can lead us to suppose a possible decrease of the Curie temperature. The results for the remanent polarization and the coercive field, as a function of the strontium content, showed a possible phase transformation from ferroelectric to paraelectric. PACS 61.10.Nz; 68.37.Ps; 77.55.+f; 78.30.-j; 81.15.-z     

Dielectrophoretic assembly of lead zirconate titanate microtubes

Lead zirconate titanate (PZT) microtubes were assembled onto pre-patterned substrates using dielectrophoresis of a colloidal suspension of microtubes dispersed in isopropyl alcohol. High aspect ratio tubes with an outer diameter of 2 μm and length of about 30 μm were prepared by vacuum infiltration of mesoporous silicon templates. An interdigitated electrode structure with gap sizes ranging from 2 μm to 15 μm was patterned on a silicon substrate via conventional lithography. This allowed a non-uniform alternating electric field to be generated. The influence of the electrode gap along with the effects of the waveform, amplitude and frequency of an applied signal on the dielectrophoretic assembly of PZT microtubes was investigated. A square wave signal of 5 and 10 Hz was found to be the most effective in assembling the microtubes on a 12 μm electrode gap. The results show potential for the dielectrophoretic technique in realizing integrated 3D devices using the high aspect ratio piezoelectric tube structures as building blocks.     

Dielectric properties of lead zirconate titanate films synthesized through oxidation of metal layers

Films of the composition Pb(Zr _x Ti_1?x )O₃ are prepared by magnetron sputtering of metal layers onto titanium substrates with subsequent heat treatment in an oxygen atmosphere. The electrical properties of the samples prepared are investigated using impedance spectroscopy in the frequency range from 10² to 5 × 10⁵ Hz at temperatures of 300–750 K. The temperature dependences of the permittivity and the dielectric loss tangent at different frequencies exhibit a behavior typical of ferroelectrics and indicate the occurrence of a ferroelectric phase transition at temperatures close to T = 663 K. Analysis of the imaginary part of the electric modulus has revealed two possible relaxation mechanisms. The activation energy for dc electrical conduction in the paraelectric phase is determined.     

Aftereffect relaxation processes in lead zirconate titanate

Relaxation processes in lead zirconate titanate after switching-off perturbations of different physical nature have been considered. The model of the process, which accounts for the partial depolarization during perturbations, the formation of charged F-centers during released charge trapping, and their relaxation, has been proposed.