Lead-free Piezoelectric Thin Films Research Articles

Integration of epitaxial LiNbO3 thin films with silicon technology

Development of bulk acoustic wave filters with ultra-wide pass bands and operating at high frequencies for 5th and 6th generation telecommunication applications and micro-scale actuators, energy harvesters and sensors requires lead-free piezoelectric thin films with high electromechanical coupling and compatible with Si technology. In this paper, the epitaxial growth of 36°Y-X and 30°X-Y LiNbO3 films by direct liquid injection chemical vapour deposition on Si substrates by using epitaxial SrTiO3 layers, grown by molecular beam epitaxy, has been demonstrated. The stability of the interfaces and chemical interactions between SrTiO3, LiNbO3 and Si were studied experimentally and by thermodynamical calculations. The experimental conditions for pure 36°Y-X orientation growth have been optimized. The piezoelectricity of epitaxial 36°Y-X LiNbO3/SrTiO3/Si films was confirmed by means of piezoelectric force microscopy measurements and the ferroelectric domain inversion was attained at 85 kV.cm−1 as expected for the nearly stoichiometric LiNbO3. According to the theoretical calculations, 36°Y-X LiNbO3 films on Si could offer an electromechanical coupling of 24.4% for thickness extension excitation of bulk acoustic waves and a comparable figure of merit of actuators and vibrational energy harvesters to that of standard PbZr1−x Ti x O3 films.

Large Electromechanical Response in a Polycrystalline Alkali-Deficient (K,Na)NbO3 Thin Film on Silicon.

The demand for large electromechanical performance in lead-free polycrystalline piezoelectric thin films is driven by the need for compact, high-performance microelectromechanical systems (MEMS) based devices operating at low voltages. Here we significantly enhance the electromechanical response in a polycrystalline lead-free oxide thin film by utilizing lattice-defect-induced structural inhomogeneities. Unlike prior observations in mismatched epitaxial films with limited low-frequency enhancements, we achieve large electromechanical strain in a polycrystalline (K,Na)NbO3 film integrated on silicon. This is achieved by inducing self-assembled Nb-rich planar faults with a nonstoichiometric composition. The film exhibits an effective piezoelectric coefficient of 565 pm V-1 at 1 kHz, surpassing those of lead-based counterparts. Notably, lattice defect growth is substrate-independent, and the large electromechanical response is extended to even higher frequencies in a polycrystalline film. Improved properties arise from unique lattice defect morphology and frequency-dependent relaxation behavior, offering a new route to remarkable electromechanical response in polycrystalline thin films.

Macro and nanoscale properties of (001)-oriented Bi0.5Na0.5TiO3 lead-free piezoelectric thin films grown by sputtering on LaNiO3/Si substrates

In this paper, the effect of the post-annealing temperature on the structural and electrical properties at macro and nanoscale of Sodium Bismuth Titanate - Bi0.5Na0.5TiO3 (BNT) films is reported. Stoichiometric BNT thin films were deposited by ex-situ radio frequency (rf) sputtering at substrate temperature of 200 °C. The as-deposited films were amorphous and post-annealing treatment was necessary to crystallize the film in the perovskite phase. 400-nm-thick stoichiometric BNT films were annealed at various temperatures from 400 °C up to 700 °C. The BNT film annealed at 400 °C remains amorphous. Film starts to crystallize at 450 °C and then the crystallization increases with the post-annealing temperature to reach an optimal at 650 °C without any secondary phase. Dielectric, ferroelectric and piezoelectric properties were also improved with the increasing of the post-annealing temperature. The piezoelectric coefficient d33eff, measured at macroscale, reaches a maximum value of 57 pm/V for 400 nm-thick film post-annealed at 650 °C. On nanoscale, measurements performed by piezoresponse force microscopy are in perfect concordance with the performances obtained at macroscale. The efficient polarization reversal for domains and their retention when locally manipulated are observed, combined to a strong piezo-activity.

Improved piezoelectric performance via orientation regulation in novel BNT-BT-SBT thin film

Lead-free piezoelectric thin films are urgently needed in microelectronic and microelectromechanical systems. In this work, (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-x(Sr0.7Bi0.2□0.1)TiO3 (abbreviated as BNT-BT-xSBT) piezoelectric thin films were prepared successfully. The optimal piezoelectric performance can be achieved when x = 0.02 at morphotropic phase boundary (MPB). Further, to explore the impacts of orientation control on the properties of thin films, (100), (110), and (111)-oriented BNT-BT-0.02SBT thin films were prepared on Nb-SrTiO3 single crystal. The (110) preferred orientation thin film can achieve a high inverse piezoelectric coefficient (d33*) of 170 pm/V, which has a great improvement contrasted with non-oriented films. From the perspective of engineering domain configuration, the high d33* of (110)-oriented thin film can be characterized that the ferroelectric domain can get a complete flip in the direction of non-easily polarized axes. In short, orientation regulation is of great importance in improving the piezoelectric properties of thin films and has important applications in microelectronic sensing.

Domain structure and optical band gap of MnO2 doped (K,Na)NbO3-LiTaO3 thin films

In this paper, potassium sodium niobate lead-free piezoelectric thin films were prepared by sol-gel method. The electric domain structure and optical band gap of the thin films were regulated by doping Mn. It is found that Mn doping increases the grain size and density of KNNLT piezoelectric film surface. In addition, Mn doping can regulate the electric domain of thin films. Increasing Mn doping concentration can regulate the size and distribution of electric domain, and the ratio of 71° electric domain increases with the increase of doping concentration. The results show that the optical band gap decreases gradually with the increase of doping amount when the annealing temperature is 650 °C. The lowest optical band gap reaches 3.426 eV, and is lower than the undoped KNNLT films.

Nano-Interfacial Layer Effects on Dielectric and Piezoelectric Characteristics in Chemical Solution-Derived (K,Na)NbO3 Thin Films

Since non-piezoelectric interfacial layers even at the nanoscale significantly affect the performance of lead-free piezoelectric thin films, the quantitative characterization of property changes of thin films due to interfacial layers is of great importance and should be precisely undertaken for piezoelectric microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) devices. In contrast to widely accepted concepts for interfacial layer thickness estimation based on the existing series capacitor model, we find that the interfacial layer thickness at the top and the bottom interfaces is clearly different in chemical solution deposition (CSD)-derived (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thin films. Interestingly, the thickness of the bottom interface increases linearly with increasing the thin film thickness, while the thickness of the top interface is constant regardless of the thin film thickness. In this work, nano-interfacial layer effects of CSD-derived KNMN thin films are theoretically and experimentally addressed in a combinatorial way using a modified series capacitor model. The gained information is used to envisage the origins and the mechanisms of non-piezoelectric interfacial layers and associated dielectric and ferroelectric properties of KNMN thin films. Our research connects macroscopic properties with the microscopic origins and is greatly facilitated by separating intrinsic and extrinsic contributions to phenomenological behaviors, as well as engineering interface-related properties of the films. We believe these studies to be crucial for the further development and applications of KNN-based lead-free piezoelectric devices, and will also open the door to future studies on other lead-free piezoelectric material systems for practical MEMS and NEMS applications.

Giant Piezoelectric Coefficient in Lead-Free BiFe0.975Ti0.025O3–CaTiO3 Solid Solution Thin Films

Searching for lead-free piezoelectric thin films with a high piezoelectric coefficient is beneficial for applications in microelectromechanical systems. The solid solution method is verified as an effective route to largely improve the piezoelectric performance. Herein, we report the piezoelectric properties in polycrystalline (1 – x)BiFe0.975Ti0.025O3–xCaTiO3 thin films. A giant effective piezoelectric coefficient of 542 pm/V has been achieved in 0.76BiFe0.975Ti0.025O3–0.24CaTiO3 thin films because of the enhanced dielectric constant as well as remanent polarization, which originated from the coexistence of rhombohedral and pseudo-cubic phases and enhanced growth kinetics. The results indicate a type of lead-free piezoelectric thin film based on BiFeO3 with the effective piezoelectric coefficient larger than 500 pm/V.

All-Inorganic Flexible (K, Na)NbO3-Based Lead-Free Piezoelectric Thin Films Spin-Coated on Metallic Foils.

Flexible piezoelectric thin films are raising interest in energy harvesting and wearable electronics, although their direct fabrication is challenging in the selection of substrates and thermal processing. In this work, we developed direct fabrication of flexible lead-free (K, Na)NbO3 (KNN)-based piezoelectric films on commercially available metallic foils by sol-gel processing. Stainless steel and platinum foils are selected as flexible substrates because of their good thermal stability, robust flexibility, and cost-efficiency. The sol-gel-processed KNN-based thin films on both of the metallic foils show good flexibility, with the bending radii reaching ±3 mm. The flexible thin films grown on stainless steel and platinum foils present high breakdown electric fields that reach 1760 and 2530 kV/cm, respectively, resulting from the fine-grained dense structure, limited leakage current density, and suppressed mobility of charged carriers. Improved effective piezoelectric coefficient d33,eff* (75.4 pm/V) with a slight decrease after bending was obtained in the flexible thin films on Pt when compared to their rigid counterparts. The flexible lead-free piezoelectric thin films with combined high breakdown electric fields and piezoelectric and energy storage properties may pave the way for integrating KNN-based multifunctional thin films into flexible electronics.

Construction of multi-domain coexistence enhanced piezoelectric properties of Bi0.5Na0.5TiO3-based thin films

Although the multi-phase coexistence makes Bi0.5Na0.5TiO3-based piezoelectric thin films possess stronger piezoelectric properties and more spacious application prospects in electronic devices, the domain reversal mechanism of Bi0.5Na0.5TiO3-based thin films cannot be accurately understood due to the size effect. In this study, the relationship between domain structure and piezoelectric properties of the (0.94-x)Bi0.5Na0.5TiO3-0.06BaTiO3-xBi(Fe0.95Mn0.03Ti0.02)O3 thin films are studied by using visualization technology PFM, structure and electrical properties characterizations. The results show that the addition of Bi(Fe0.95Mn0.03Ti0.02)O3 creates a long-range ordered/short-range disordered nanodomain coexisting structure. This kind of coexisting domain structure can realize the long-range reversal driven by disordered nanodomains under the external electric field, reduce the potential barrier and the hysteresis, and significantly enhance the piezoelectric properties of the thin films. Under the same conditions, the piezoelectric properties of the 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 thin films are enhanced nearly 2.3 times. This provides a reference for exploring the physical mechanism of high performance lead-free piezoelectric thin films.

Nanointerfacial Layer Effect on Dielectric and Piezoelectric Responses in Chemical Solution-Derived Lead-Free Alkaline Niobate-Based Thin Films.

Since nonpiezoelectric interfacial layers even at the nanoscale significantly affect the performance of lead-free piezoelectric thin films, the quantitative characterization of property changes of thin films due to interfacial layers is of great importance and should be precisely undertaken for piezoelectric microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS) devices. In contrast to widely accepted concepts for interfacial layer thickness estimation based on the existing series capacitor model, we find that the interfacial layer thickness at the top and the bottom interfaces is clearly different in chemical solution deposition (CSD)-derived (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thin films. Interestingly, the thickness of the bottom interface increases linearly with increasing thin-film thickness, while the thickness of the top interface is constant regardless of the thin-film thickness. In this work, nanointerfacial layer effects of CSD-derived KNMN thin films are theoretically and experimentally addressed in a combinatorial way using a modified series capacitor model. The obtained information is used to envisage the origins and the mechanisms of nonpiezoelectric interfacial layers and associated dielectric and ferroelectric properties of KNMN thin films. Our research connects macroscopic properties with microscopic origins and is greatly facilitated by separating intrinsic and extrinsic contributions to phenomenological behaviors, as well as engineering interface-related properties of the films. We believe these studies to be crucial for the further development and applications of KNN-based lead-free piezoelectric devices, which also open the door to future studies on other lead-free piezoelectric material systems for practical MEMS and NEMS applications.

Field-induced polarization response and energy storage behavior of lead-free BNT-BKT-SZ films

Lead-free piezoelectric thin films of composition (1-x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3-0.03SrZrO3 (x = 15, 17.5, 20 and 22.5%) have been created using the sol-gel spin coating approach over a Pt/Ti/SiO2/Si substrate. The X-ray diffraction profiles suggest that a remarkable transition from tetragonal to rhombohedral-phase passing by a morphotropic phase boundary (MPB) occurs at x values from 17.5 to 22.5%. Additionally, the surface morphology, piezoelectric response, ferroelectric behavior, and energy storage properties were also investigated. Piezoresponse force microscopy (PFM) depicted a strong piezoelectric and ferroelectric behavior at a very localized size; at x = 17.5%. The piezoelectric behavior was enhanced and reached values of the recoverable energy storage density (Jrec) and the energy storage efficiency (η) of 3.27 J/cm3, and 39.9%, respectively. Such materials are anticipated to play an important role in lead-free devices.

Low-temperature crystalline lead-free piezoelectric thin films grown on 2D perovskite nanosheet for flexible electronic device applications

A monolayer of Sr2Nb3O10 (SNO) is deposited on the Pt/Ti/SiO2/Si (Pt-Si) or Pt/Ti/polyimide (Pt-PI) substrate by using the Langmuir-Blodgett method and employed as a seed-layer for the growth of a crystalline (Na1−xKx)NbO3 (NKN) film at 350 °C. The crystalline NKN film is grown along the [001] direction on the SNO/Pt-Si (or SNO/Pt-PI) substrate. Due to the presence of oxygen vacancies in the SNO seed-layer, the NKN film exhibits low ferroelectric properties and large leakage current. To ameliorate these properties, the SNO/Pt-Si substrate is annealed in a 50 Torr oxygen atmosphere at 300 °C, which removes the oxygen vacancies. Consequently, the NKN film deposited on this substrate exhibits promising electrical properties, namely a dielectric constant of 278, dissipation factor of 1.7%, a piezoelectric constant of 175 pmV−1, and a leakage current density of 6.47 × 10−7 Acm−2 at −0.2 MV·cm−1. Similar electrical properties are obtained from the NKN film grown on the flexible SNO/Pt-PI substrate at 350 °C. Hence, the NKN films grown on the SNO seed-layer at 350 °C can be applied to electronic devices with flexible polymer substrates.

High electromechanical strain and enhanced temperature characteristics in lead-free (Na,Bi)TiO3\u2013BaTiO3 thin films on Si substrates

Here, we demonstrate the high electromechanical strain and enhanced temperature characteristics in the c-axis-oriented lead-free (Na,Bi)TiO3–BaTiO3 (NBT–BT) polycrystalline thin film prepared on Si substrates by rf magnetron sputtering. The effective transverse piezoelectric coefficient, e31*, estimated from the electromechanical strain measured under high electric field, reaches a high level of −12.5 C/m2, and is comparable to those of conventional Pb(Zr,Ti)O3 films. In-situ X-ray diffraction measurement and electron diffraction analysis revealed the electromechanical strain of the NBT–BT film to originate predominantly in elongation of the tetragonal (P4bm) crystal lattice in the c-axis direction. In addition to the large e31*, the NBT–BT film exhibits enhanced permittivity maximum temperature, Tm, of ~400 °C and no depolarization below Tm, as compared to bulk NBT–BT having Tm ≈ 300 °C and a depolarization temperature of ~100 °C. We conclude that the enhancement of temperature characteristics is associated with the distorted P4bm crystal lattice formed by deposition-induced stress and defects. We believe that the present study paves the way for practical applications of lead-free piezoelectric thin films in electromechanical devices.

Sol-gel processing and nanoscale characterization of (Bi0.5Na0.5)TiO3-SrTiO3 lead-free piezoelectric thin films

(1-x)(Bi0.5Na0.5)TiO3-xSrTiO3 (BNT-xST) (0 ≤ x ≤ 0.4) thin films were fabricated using a sol-gel technique on Pt(111)/Ti/SiO2/Si(100) substrates, which were investigated by piezoresponse force microscopy (PFM) and Raman spectroscopy. The composition-induced phase transition was analyzed by acquiring structural variations and the domain distribution on a local scale. Raman spectra showed phonon anomalies with peak broadening and shifting when increasing SrTiO3 (ST) concentrations were used. Changes in the domain morphology with changes in the composition were observed, and grains smaller than 0.5µm were observed at lower concentrations of x = 0–0.25, while larger grains appeared with increasing ST contents. The switching spectroscopy PFM (SS-PFM) results supported a ferroelectric (FE) to relaxor ferroelectric (RFE) phase transition at approximately x ≈ 0.3 by means of analyzing the parameters as a function of the composition including the piezoresponse parameters of hysteresis loops (Dmax, Drem) and amplitude butterfly loops (Stotal, Sneg). Hence, these results demonstrated that the composition-driven FE to RFE phase transition behavior, which is consistent with the localized response behavior, is dependent on the ST content in bulk BNT-xST ceramics.

Recoverable Self-Polarization in Lead-Free Bismuth Sodium Titanate Piezoelectric Thin Films.

Bismuth sodium titanate, Bi0.5Na0.5TiO3 (BNT), is a promising lead-free ferroelectric material. However, its potential applications have not been fully explored, mainly because of the complex domain structure arising from its intricate phase transitions. A deep and thorough study of its domain structure and polarization switching behavior will greatly help with understanding the polarization nature and with promoting future applications. In this work, we demonstrate that BNT polycrystalline films possess a highly ordered out-of-plane polarization (self-polarization) and randomly oriented in-plane polarizations. Interestingly, the inherent nature of polarization in the BNT films does not allow for the nonvolatile domain writing, as the switched polarization spontaneously and rapidly reverses to the initial orientation state once the external poling electric field is removed, making the self-polarization recoverable. Such a stable self-polarization vanishes gradually with temperature increasing over 150 °C but starts to recover to its initial state upon cooling down to 250 °C, and entirely recovers once the temperature is reduced to below 200 °C. Such interesting properties of BNT films are attributed to the combined effects of the free charges at the Pt electrode, (detected) cation vacancies at the oxide/Pt interface and the defects in oxide lattices. Our results make a step closer to fully understand the nature of polarization and related piezoelectricity in BNT. Such films with recoverable self-polarization are of great interest for applications as sensors, actuators, and transducers that can operate particularly under high temperatures and high electric field conditions.

Preparation of 0.055BaZrO3–0.935(K0.45Na0.5Li0.05)NbO3–0.01(Bi0.5Na0.5)TiO3 piezoelectric thin films from precursor solution by Pechini method

Lead-free piezoelectric thin films of 0.055BaZrO3–0.935(K[Formula: see text]Na[Formula: see text]Li[Formula: see text])NbO3–0.01(Bi[Formula: see text]Na[Formula: see text])TiO3 (BZ–KNLN–BNT) were fabricated on (100)-LaNiO3/SiO2/Si substrates. The Pechini (polymeric precursor) method was carried out to prepare the complicated multi-component precursor solution. This method was suited for controlling the complex components accurately. The films crystallized at 700[Formula: see text]C showed a (100)pc-oriented perovskite structure, whose grain size was about 200[Formula: see text]nm and thickness was about 700[Formula: see text]nm. The Curie temperature [Formula: see text]c of the films was 292[Formula: see text]C that was near to that of the bulk ceramics. The preparation method used in this work provided a possibility for the application of multi-component lead-free piezoelectric films.

A Review on Piezoelectric Energy Harvesting: Materials, Methods, and Circuits

Abstract Piezoelectric microelectromechanical systems (PiezoMEMS) are attractive for developing next generation self-powered microsystems. PiezoMEMS promises to eliminate the costly assembly for microsensors/microsystems and provide various mechanisms for recharging the batteries, thereby, moving us closer towards batteryless wireless sensors systems and networks. In order to achieve practical implementation of this technology, a fully assembled energy harvester on the order of a quarter size dollar coin (diameter=24.26 mm, thickness=1.75 mm) should be able to generate about 100 μW continuous power from low frequency ambient vibrations (below 100 Hz). This paper reviews the state-of-the-art in microscale piezoelectric energy harvesting, summarizing key metrics such as power density and bandwidth of reported structures at low frequency input. This paper also describes the recent advancements in piezoelectric materials and resonator structures. Epitaxial growth and grain texturing of piezoelectric materials is being developed to achieve much higher energy conversion efficiency. For embedded medical systems, lead-free piezoelectric thin films are being developed and MEMS processes for these new classes of materials are being investigated. Non-linear resonating beams for wide bandwidth resonance are also reviewed as they would enable wide bandwidth and low frequency operation of energy harvesters. Particle/granule spray deposition techniques such as aerosol-deposition (AD) and granule spray in vacuum (GSV) are being matured to realize the meso-scale structures in a rapid manner. Another important element of an energy harvester is a power management circuit, which should maximize the net energy harvested. Towards this objective, it is essential for the power management circuit of a small-scale energy harvester to dissipate minimal power, and thus it requires special circuit design techniques and a simple maximum power point tracking scheme. Overall, the progress made by the research and industrial community has brought the energy harvesting technology closer to the practical applications in near future.

Temperature and composition dependent phase transitions of lead-free piezoelectric (Bi0.5Na0.5)TiO3-BaTiO3 thin films.

Extensive studies on (Bi0.5Na0.5)TiO3-BaTiO3 piezoceramics have aroused growing interest in developing high-performance piezoelectric films based on this promising lead-free piezoelectric system. High quality (1 - x%) (Bi0.5Na0.5)TiO3-x% BaTiO3 (x = 0-15) thin films were synthesized on Pt(111)/Ti/SiO2/Si(100) substrates via a sol-gel method. The phase transitions as functions of composition and temperature were investigated by X-ray diffraction (XRD) and temperature-dependent Raman spectra analysis. A morphotropic phase boundary (MPB) located at 5 ≤ x ≤ 10 was found in the films, being identical to that in bulk materials. Additionally, complicated structural changes with temperature in a wide composition range were observed, which correspond to the evolution from a low-symmetry ferroelectric phase or a relaxor phase to high-symmetry relaxor phases. The ferroelectricity and piezoelectricity were also systematically studied with an emphasis on the influence of composition and phase structure. A phase diagram focusing on the above two kinds of phase transitions has been proposed in this work.

LEAD-FREE BNT PIEZOELECTRIC THIN FILMS BY SOL-GEL METHOD.

The objective of this work is to synthesize lead-free piezoelectric thin films by the Sol-Gel method:bismuth sodium titanate (Bi0.5Na0.5TiO3 or BNT).BNT films were annealed with the rapid thermal process (RTP).The film treated at 700°C is dense and well crystallized in the perovskite phase. The first results of electricalcharacterizations showed promising dielectric, ferroelectric and piezoelectric performance. At 12 kHz, thedielectric constant and losses are 430 and 0.07, respectively. Ferroelectric hysteresis measurements indicated aremanent polarization of 10μC/cm2, associated with a coercive field of 70 kV/cm. The piezoelectric properties ofBNT films were studied with a laser Doppler interferometer: a piezoelectric coefficient (d33effmax) of 42 pm/V was measured.

新規無鉛圧電材料の状態図に関する第一原理計算

Ferroelectric thin films are using the actuators and sensors in medical devices. Therefore, developments of biocompatible novel lead-free piezoelectric materials are required. In previous study, novel lead-free piezoelectric materials, which were solitary crystals ABO3 and complex crystals (A,A’)BO3 and A(B,B’)O3 of perovskite-type oxide, were discovered by employing first principle calculation based on density functional theory (DFT). In this paper, to support the fabrication of novel lead-free piezoelectric thin films, the phase diagram was predicted through first principle calculation. Firstly, stable crystal structure of perovskite oxides was evaluated. Secondly, total energy, spontaneous polarization, elastic compliance and electrostrictive coefficient were calculated from crystal structures. Thirdly, the free energy function with consideration of misfit strain was approximated from total energy and spontaneous polarization of asymmetric crystal structures. Finally, the stable thin film phase with minimum energy was investigated by employing the approximated free energy function. The developed prediction procedure was applied to existing materials PZT and novel lead-free piezoelectric materials. Especially, the effect of material composition on phase diagram was discussed.