Epitaxial Lead Zirconate Titanate Research Articles

Time-resolved piezoelectric response in relaxor ferroelectric (Pb0.88La0.12)(Zr0.52Ti0.48)O3 thin films

The domain switching dynamics in a relaxor ferroelectric lanthanum-modified lead zirconate titanate thin film with 12 mol. % La was investigated by time-resolved x-ray diffraction. While most frequently epitaxial thin films are investigated, the present work reports results on a polycrystalline thin film. Asymmetric butterfly loops of the strain as a function of the applied electric field evidenced a built-in electric field oriented toward the thin film–substrate interface. The piezoelectric coefficient d33 (in the film reference frame) diminishes with the increasing frequency of an applied AC electric field. From the strain transient during the application of positive-up negative-down voltage pulse sequences with frequencies of up to 100 kHz, characteristic times of the order of 100–200 ns were determined for these relaxor ferroelectric thin films. While switching times ranging from the picosecond to the millisecond range are reported in the literature for different materials, these characteristic switching times are comparable to epitaxial lead zirconate titanate thin films of various compositions despite the polycrystallinity of the present thin film. However, the evidenced built-in electric field significantly influences the switching behavior for different polarities.

Electromechanical Manipulation of Topological Defects to Yield Giant Piezoelectric Response in Epitaxial Lead Zirconate Titanate Bilayers on Silicon

AbstractThe combination of topological defect mobility and exploitation of the boundary conditions in bilayer heterostructures presents an enormous opportunity for enhancing actuation responses from modest external stimuli in electromechanical applications. Tetragonal PbZr0.3Ti0.7O3 and underlying rhombohedral PbZr0.54Ti0.46O3 ferroelectric bilayers are epitaxially grown on silicon substrates. The topmost layer shows crystallographic symmetry lower than a typical tetragonal structure but is arranged in‐plane with a mosaic pattern of superdomains and ferroelastic a1/a2 nanodomains within. The domain pattern also shows a series of vortex/anti‐vortex pairs induced in the pseudo‐tetragonal film, which can be manipulated to either migrate across the film or annihilate one another in the presence of an external electrical bias. The mobility of defect pairs and domain walls, through reduced substrate clamping by the underlying rhombohedral film, is the driving force for endowing the bilayer with a ≈45% increased effective d33 response compared to a rhombohedral single layer on silicon. Furthermore, a square in‐plane polarization loop (≈32 µC cm–2) provides further evidence of the mobility and large‐scale polarization switching. An improvement of this magnitude demonstrates the opportunity to implement bilayer technology while exploiting a functional silicon substrate for enhanced, industry‐ready, smart material applications.

Feasibility study of ultrasonic biometrics based on finger vessel imaging by epitaxial-PZT/Si piezoelectric micromachined ultrasonic transducer

In this study, we demonstrated a potential usability of a piezoelectric micromachined ultrasonic transducer (pMUT) for a high-performance body-coupled ultrasonic imager based on an epitaxial lead zirconate titanate (Epi-PZT) thin film. A finger vessel biometrics was proposed as one of its applications. At first, transmitting abilities and receiving sensitivities of the single-element pMUT based on Epitaxial-PZT (Epi-PZT) and polycrystalline-PZT (Poly-PZT) were evaluated as a fundamental study. It was experimentally verified that the Epi-PZT was a preferable material for a higher performance pMUT. Then, the linear array driven by a commercially-available pulser board was constructed, and successfully formed a focused ultrasonic beam. Finally, we demonstrated the ultrasonic imaging of a polydimethylsiloxane-based phantom by using the Epi-PZT pMUT array. We believe this achievement is a great step to realize the finger vessel biometric by pMUT.

Modulation of Pb chemical state of epitaxial lead zirconate titanate thin films under high energy irradiation

The chemical states of epitaxial PbZrxTi1-xO3 films were investigated by an X-ray photoelectron spectroscopy as a function of the gamma-ray doses. An anomalous behaviour was observed in Pb4f states, and a core level of Pb4f shifts towards a higher binding energy at 50 kGy and towards a lower binding energy at 200 kGy. The behaviour can be explained by a radiation induced reduction of PbO to metallic Pb. The metal-insulator-metal electrodes were fabricated by lithography, and the current-voltage characteristics were measured. A negative differential resistance (NDR) was observed in the leakage currents at room temperature. A higher current and disappearance of NDR characteristics were found in the 200 kGy irradiated samples, which further confirms the presence of metallic Pb.

Angular dependence of Raman spectrum for Pb(Zr,Ti)O3 epitaxial films

Herein, we investigate the angular dependence of the Raman spectra of (001)/(100)-oriented tetragonal and (111)/-oriented rhombohedral epitaxial lead zirconate titanate (PZT) films. The A1 and E modes have the same angular dependence for the (111)/-oriented rhombohedral epitaxial PZT film, but not for the (001)/(100)-oriented tetragonal epitaxial PZT film. These results agree well with theoretical predictions. On the basis of the fact that the angular dependence of the phonon mode in PZT films is linked to the crystal orientation and structure, the angular dependence of the Raman modes can be used to investigate the orientation and structure of PZT films.

Research Update: Enhanced energy storage density and energy efficiency of epitaxial Pb0.9La0.1(Zr0.52Ti0.48)O3 relaxor-ferroelectric thin-films deposited on silicon by pulsed laser deposition

Pb0.9La0.1(Zr0.52Ti0.48)O3 (PLZT) relaxor-ferroelectric thin films were grown on SrRuO3/SrTiO3/Si substrates by pulsed laser deposition. A large recoverable storage density (Ureco) of 13.7 J/cm3 together with a high energy efficiency (η) of 88.2% under an applied electric field of 1000 kV/cm and at 1 kHz frequency was obtained in 300-nm-thick epitaxial PLZT thin films. These high values are due to the slim and asymmetric hysteresis loop when compared to the values in the reference undoped epitaxial lead zirconate titanate Pb(Zr0.52Ti0.48)O3 ferroelectric thin films (Ureco = 9.2 J/cm3 and η = 56.4%) which have a high remanent polarization and a small shift in the hysteresis loop, under the same electric field.

Shift of morphotropic phase boundary in high-performance [111]-oriented epitaxial Pb (Zr, Ti) O3 thin films

High-performance epitaxial niobium-doped lead zirconate titanate thin films [Pb(ZrxTi1−x)0.98Nb0.02O3] with a fixed thickness were deposited on [111]-cut single-crystalline SrTiO3 substrates by a sol-gel process as a function of Zr/Ti ratio ranging from 20/80 to 80/20. An obvious shift of morphotropic phase boundary (MPB) was observed in the [111]-oriented epitaxial films. Deviating from the Zr/Ti ratio of 52/48 in bulk materials, the MPB composition in the as-deposited films was found to move to a lower Zr/Ti ratio between 30/70 and 40/60, which agrees with the theoretical analysis on the basis of lattice mismatch between films and substrates and is further confirmed by the XRD patterns and Raman spectrum as well as ferroelectric test. Furthermore, a superior remnant polarization (Pr) value among all the compositions was obtained at a Zr/Ti ratio of 30/70, indicating that the extrinsic ferroelectric polarization near the new morphotropic phase boundary will lead to outstanding dielectric and piezoelectric performance in the epitaxial PZT thin film system.

Spontaneous polarization estimation from the soft mode in strain-free epitaxial polar axis-oriented Pb(Zr,Ti)O3 thick films with tetragonal symmetry

The ferroelectric property, crystal structure, and A1(1TO) soft mode of almost strain-free epitaxial polar axis-oriented tetragonal lead zirconate titanate thick films grown on CaF2 substrates were investigated by changing the Zr/(Zr+Ti) ratio and the temperature. The square of spontaneous polarization (Ps), Ps2, monotonically increased as tetragonal distortion, (c/a)−1, increased where a and c are the lattice parameters of the films and as the square of frequency in A1(1TO), ω2[A1(1TO)], increased. Additionally, the relationships of Ps2, (c/a−1), and ω2[A1(1TO)] were experimentally obtained, which confirmed the Ps value obtained from (c/a−1) and ω[A1(1TO)] without direct polarization measurements.

Misfit strain–film thickness phase diagrams and related electromechanical properties of epitaxial ultra-thin lead zirconate titanate films

The phase stability of ultra-thin (0 0 1) oriented ferroelectric PbZr 1– x Ti x O 3 (PZT) epitaxial thin films as a function of the film composition, film thickness, and the misfit strain is analyzed using a non-linear Landau–Ginzburg–Devonshire thermodynamic model taking into account the electrical and mechanical boundary conditions. The theoretical formalism incorporates the role of the depolarization field as well as the possibility of the relaxation of in-plane strains via the formation of microstructural features such as misfit dislocations at the growth temperature and ferroelastic polydomain patterns below the paraelectric–ferroelectric phase transformation temperature. Film thickness–misfit strain phase diagrams are developed for PZT films with four different compositions ( x = 1, 0.9, 0.8 and 0.7) as a function of the film thickness. The results show that the so-called rotational r-phase appears in a very narrow range of misfit strain and thickness of the film. Furthermore, the in-plane and out-of-plane dielectric permittivities ε 11 and ε 33, as well as the out-of-plane piezoelectric coefficients d 33 for the PZT thin films, are computed as a function of misfit strain, taking into account substrate-induced clamping. The model reveals that previously predicted ultrahigh piezoelectric coefficients due to misfit-strain-induced phase transitions are practically achievable only in an extremely narrow range of film thickness, composition and misfit strain parameter space. We also show that the dielectric and piezoelectric properties of epitaxial ferroelectric films can be tailored through strain engineering and microstructural optimization.

Cation ordering in epitaxial lead zirconate titanate films

Electron diffraction and atom location by channeling enhanced microanalysis were used to show that epitaxial PbZr0.2Ti0.8O3 films grown on (001) SrTiO3 substrates by pulsed laser deposition exhibit long-range order on the tetravalent cation sublattice parallel to the film/substrate interface. This ordering gives two distinct tetravalent cation sites, one Zr lean and one Zr rich, and results in a superlattice with a tetragonal unit cell with lattice parameters a0≈√2aPZT and c0≈aPZT. Since such ordered states are inherently unstable in homovalent perovskite solutions, it is inferred that the ordering arises in response to the lattice misfit and could constitute an additional relaxation mode.

Stress Relaxation by Cation Ordering in Epitaxial Lead Zirconate Titanate Films

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

High sensitivity ultrasonic sensor for hydrophone applications, using an epitaxial Pb(Zr,Ti)O3 film grown on SrRuO3/Pt/γ-Al2O3/Si

A piezoelectric ultrasonic sensor has been fabricated using an epitaxial lead zirconate titanate, Pb(Zrx, Ti1−x)O3 (PZT) thin film, grown on an epitaxial SrRuO3/Pt/γ-Al2O3/Si substrate. The 3-μm thick PZT film was prepared by the sol–gel method. This sensor structure was very stable during the fabrication process after preparation of the PZT. The fabricated sensor offers a reception sensitivity of up to −243dB re 1V/μPa in the frequency range from 1 to 15MHz, which means that it has higher sensitivity than that of a conventional polyvinylidene fluoride (PVDF) hydrophone. The pressure field of a focused ultrasonic transducer was determined using this ultrasonic sensor. The measured pressure peak was in good agreement with the results derived from calculations. These results suggest that the fabricated ultrasonic sensor can be used as a high sensitivity hydrophone device.

Quantitative mapping of switching behavior in piezoresponse force microscopy

The application of ferroelectric materials for nonvolatile memory and ferroelectric data storage necessitates quantitative studies of local switching characteristics and their relationship to material microstructure and defects. Switching spectroscopy piezoresponse force microscopy (SS-PFM) is developed as a quantitative tool for real-space imaging of imprint, coercive bias, remanent and saturation responses, and domain nucleation voltage on the nanoscale. Examples of SS-PFM implementation, data analysis, and data visualization are presented for epitaxial lead zirconate titanate (PZT) thin films and polycrystalline PZT ceramics. Several common artifacts related to the measurement method, environmental factors, and instrument settings are analyzed.

Crystal orientation dependence of the electro-optic effect in epitaxial lanthanum-modified lead zirconate titanate films

Lanthanum-modified lead zirconate titanate epitaxial films with (100), (101), and (111) orientations were grown on (100), (101), and (111) niobium lending conductivity strontium titanate using chemical solution deposition, respectively. We investigated the changes in the refractive index induced by the electric field in these films using the prism coupling method. Birefringence was observed in the (101)- and (111)-oriented epitaxial films induced by the electric field. However, no birefringence was observed in the (100) epitaxial film. This crystal orientation dependence of the electrooptic effect can be explained by the rotation of the refractive indicatrix that accompanies the switching of the polar clusters.

Epitaxial Lead Zirconate Titanate Nanocrystals Obtained by a Self-Patterning Method

Epitaxial lead zirconate titanate nanoislands were obtained by a self-patterning method using the instability of ultrathin films during high-temperature treatments. After high-temperature annealing, the as-deposited film breaks into islands with a narrow size distribution. Nanostructures, as small as 20 nm lateral size with a height of 9 nm, were epitaxially grown on Nb-doped (001) SrTiO3 substrates. The formation of nanostructures was investigated as a function of the initial film thickness and crystallisation condition. Single-crystal nanoislands were studied by scanning and high-resolution transmission electron microscopy, atomic force microscopy and X-ray diffraction. Ferroelectric switching of several individual islands was investigated by piezoresponse force microscopy.

Ferroelectric properties of an epitaxial lead zirconate titanate thin film deposited by a hydrothermal method below the Curie temperature

Deposition of a hydrothermal method has a number of advantages; low deposition temperature, high-purity, deposition on a three-dimensional structure, and a large thickness. The present paper investigates the improvement of an epitaxial lead zirconate titanate (PZT) thin film deposited via hydrothermal synthesis. The critical parameter contributing to the film morphology was revealed to be the holding position of the reaction solution during deposition. In addition to this improvement, the SrRuO3 bottom electrode was confirmed to be able to withstand high alkali concentrations during the hydrothermal reaction. A hydrothermal synthesis time of 24 h yielded a PZT film with a thickness of 500 nm on SrRuO3/SrTiO3 (100) at 150 °C. The remanent polarization 2Pr and coercive electric field were 38.4 μC/cm2 and 21.8 kV/cm, respectively.

Ferroelectric property of an epitaxial lead zirconate titanate thin film deposited by a hydrothermal method

Deposition of thin films via hydrothermal method has various advantages: low deposition temperature, high purity, deposition on a three-dimensional structure,and a large thickness. Although an epitaxial lead zirconate titanate (PZT) thin-film deposition has been reported, the ferroelectric measurement has not been conducted due to the peel-off morphology of the film. The current paper investigates the improvement of an epitaxial PZT thin film deposited via a hydrothermal method. By adjusting the position at which the substrate was suspended in the solution, smooth morphology surface was successfully obtained. As a bottom electrode, a 200-nm SrRuO3 thin film was deposited on SrTiO3 single crystals, and the PZT thin film was deposited on SrRuO3. The remanent polarization 2Pr and coercive electric field for PZT on SrRuO3/SrTiO3 (001) were 17.1 μC/cm2 and 36 kV/cm, respectively, and those of PZT on SrRuO3/SrTiO3 (111) were 32.7 μC/cm2 and 59 kV/cm, respectively. The reason for large imprint electrical field, 91 kV/cm and 40 kV/cm for each film, was unclear at this stage, although it is associated with self–alignment poling direction. This self–alignment poling direction was confirmed via scanning nonlinear dielectric microscopy and is thought to have been related to the deposition mechanisms.

Deep level transient spectroscopy of lead zirconate titanate grown on La0.5Sr0.5CoO3/LaAlO3 substrate

Epitaxial lead zirconate titanate thin films were grown by pulsed laser deposition method on LaAlO3 substrate using La0.5Sr0.5CoO3 (LSCO) as the bottom electrode. Two different top electrode systems were studied: a single platinum layer and a platinum/LSCO bi-layer. The as-grown, post-annealed and fatigued samples were subsequently investigated via polarization-electric field hysteresis loops measurement and deep level transient spectroscopy. It was found that polarization fatigue occurs only for films with single platinum layer top electrode but not with Pt/LSCO electrode bi-layer ones. Hole traps with activation energy 0.209 eV were detected in the fatigued samples with single Platinum top electrode. This is the first observation of fatigue-related deep levels in PZT films and provides new insight into the switching fatigue mechanisms.

In Situ Epitaxial Growth of Lead Zirconate Titanate Films by Bias Sputtering at High RF Power

High-quality epitaxial lead zirconate titanate (PZT) films with high growth rate were successfully grown by bias sputtering on (001) MgO single crystal substrates using a composite ceramic target. The composite ceramic target composed of a PbO pellet on the center of Pb1.2(Zr0.4Ti0.6)O3 ceramics was necessary to stably obtain perovskite-phase PZT films under high RF power by sputtering. At high RF power, the application of DC bias voltages to the growing PZT films increased the growth rate and changed the Ti/Zr composition. In particular, the optimum negative bias voltage considerably improved the crystallinity and surface morphology of the films compared with the conventional sputtering process. The optimum negative bias voltage plays a role in suppressing the bombardments of the energetic negative oxygen ions on growing films and also in making low-energy positive ions assist help in crystallization.

Deep level transient spectroscopy of lead zirconate titanate grown on La 0.5Sr 0.5CoO 3/LaAlO 3 substrate

Epitaxial lead zirconate titanate thin films were grown by pulsed laser deposition method on LaAlO 3 substrate using La 0.5Sr 0.5CoO 3 (LSCO) as the bottom electrode. Two different top electrode systems were studied: a single platinum layer and a platinum/LSCO bi-layer. The as-grown, post-annealed and fatigued samples were subsequently investigated via polarization-electric field hysteresis loops measurement and deep level transient spectroscopy. It was found that polarization fatigue occurs only for films with single platinum layer top electrode but not with Pt/LSCO electrode bi-layer ones. Hole traps with activation energy 0.209 eV were detected in the fatigued samples with single Platinum top electrode. This is the first observation of fatigue-related deep levels in PZT films and provides new insight into the switching fatigue mechanisms.