Piezoelectric Thin Films Research Articles

Influence of annealing ambience on structural and piezoelectric properties of single crystal LiTaO3 thin film

The crystal-ion-slicing technology is a promising technology for fabricating high-quality single crystal piezoelectric thin films. This work attempted to investigate the influence of annealing ambience on the properties of LiTaO3 thin films fabricated by crystal-ion-slicing technology. Here, two 4-inch 42° rotated Y-X propagation LiTaO3 thin films were fabricated, as well as the LiTaO3 thin films were annealed in vacuum and O2 environment, respectively. The results demonstrate that the LiTaO3 thin films annealed in O2 environment possesses more homogeneous and denser structure and fewer defects. The Zratio of the resonators fabricated on the LiTaO3 thin films annealed in O2 environment is approximately 85.3 dB and the Q-factor reaches 5107, indicating the excellent piezoelectric properties of thin films.

Quantitative Characterization of Impact‐Induced Energy Harvesting: A Case Study on Stretchable Piezoelectric Thin Films on Different Substrates

AbstractPiezoelectric materials represent a great alternative to power small electronic devices with a lower environmental impact than conventional power sources. Despite significant progress on the subject, most studies focus on energy harvesting from vibrating systems. Other publications focus on the design of the piezoelectric devices, but they are hardly comparable in terms of the amount of energy collected. In this paper a new method is presented to quantitatively characterize piezoelectric energy harvesters under a single mechanical impact using a custom Charpy‐like test bench. The method is illustrated by studying screen‐printed piezoelectric thin films of poly(vinylidenefluoride‐co‐trifluoroethylene) P(VDF‐TrFE), with different substrates acting as mechanical support. Devices printed on stretchable substrates allowing large deformation lead to overall better energy harvesting performance. However it is also demonstrated that the nature of the substrate influences the material properties of the piezoelectric film despite identical fabrication process.

Grain to grain heterogeneity in PZT thin films as probed by in situ biasing XRD

Piezoelectric thin films are particularly difficult to model at the grain scale. Moreover, this problem is sparsely experimentally documented due to the lack of adequate methods. Here, an original methodology is proposed to study the behavior of single grains during in situ biasing. Pb(Zr,Ti)O3 films have been evaluated thanks to an in situ biasing x-ray diffraction technique performed on a synchrotron source with a sub-micronic x-ray beam. Small capacitors have been biased with DC voltage between 0 and ±20 V and at each step of bias a set of spotty Debye rings have been recorded. By selecting an appropriate region of interest, the evolution of a single grain peak has been followed. The fine analysis of these peaks allows to calculate the effective piezoelectric coefficient d33,eff, the proportion of each domain variants, the tetragonality inside each grain, as well as their evolution during the electrical biasing. A higher heterogeneity in samples with morphotropic phase boundary composition is observed. This study contributes to a better understanding of the local behaviors in piezo/ferroelectric polycrystalline films and in the improvement of their performance for different applications.

Experimental and theoretical studies of effective piezoelectric coefficients of 2-2 connectivity AlN/AlScN composite piezoelectric films

Aluminum nitride (AlN) and Sc-doped AlN have become the most popular piezoelectric thin-film materials in the fields of RF and smart sensing. AlN/AlScN composite films based on 2-2 connectivity exhibit stronger interaction and better lattice matching between AlN and AlScN, resulting in improved nucleation, growth kinetics, crystal quality, piezoelectric response, and surface morphology. In this work, a method for predicting the effective coefficients of AlN/AlScN composite piezoelectric thin films is proposed and validated. This prediction method is based on first-principles calculations, the Reuss model theory, and the Mori-Tanaka (MT) method. It also conforms to the mean strain and stress theory of the micromechanical approach. The precise compliance, dielectric and piezoelectric properties (Sij, ε, dij) of AlN/Al0.8Sc0.2N composite film with different thickness ratios are calculated. Through the deflection measurement of piezoelectric cantilever with proof mass based on AlN/Al0.8Sc0.2N composite film, the experimental piezoelectric stress coefficient (d31) is calculated. The correctness of the prediction method is verified by comparing the theoretical calculation (d31* = −3.1pC/N) and experimental data (d31 = −3.4pC/N), showing excellent agreement within experimental error. Meanwhile, the voltage sensitivity of the accelerometer is significantly enhanced by the AlN/AlScN composite piezoelectric film. It is believed that this work will broaden the theoretical study and application of the AlN/AlScN composite piezoelectric film.

A Micromechanical Model for Damage Evolution in Thin Piezoelectric Films

Thin-film piezoelectric materials are advantageous in microelectromechanical systems (MEMS), due to large motion generation, high available energy and low power requirements. In this kind of application, thin piezoelectric films are subject to mechanical and electric cyclic loading, during which damage can accumulate and eventually lead to fracture. In the present study, continuum damage mechanics and asymptotic theory are adopted to model damage evolution in piezoelectric thin films. Our purpose is to develop a new interface model for thin piezoelectric films accounting for micro-cracking damage of the material. The methods used are matched asymptotic expansions, to develop an interface law, and the classic thermodynamic framework of continuum damage mechanics combined with Kachanov and Sevostianov’s theory of homogenization of micro-cracked media, to characterize the damaging behavior of the interface. The main finding of the paper is a soft imperfect interface model able to simulate the elastic and piezoelectric behavior of thin piezoelectric film in the presence of micro-cracking and damage evolution. The obtained interface model is expected to be a useful tool for damage evaluation in MEMS applications. As an example, an electromechanically active stack incorporating a damaging piezoelectric layer is studied. The numerical results indicate a non-linear evolution of the macroscopic response and a damage accumulation qualitatively consistent with experimental observations.

Structural‐functional unit ordering for high‐performance electron‐correlated materials

AbstractElectron‐correlated materials have been drawing ever‐increasing attention due to their fascinating physical behaviors and extensive application scenarios. In this review, a new method for material research and design (R&D), named structural‐functional unit ordering (SFU ordering), which is presented, overcomes the shortcomings—for example, the limitation of finite chemical elements and long R&D circle‐of conventional strategy and thus provides guidance for the design of these high‐performance functional materials on demand. Meanwhile, with the development of material characterization technologies, SFUs of different scales and types can be directly observed, which, moreover, regulate the corresponding orderings. The review, starts with an introduction of the profile for SFU ordering and the synergistic effect between SFUs. Then, studies on several new high‐performance electronic‐correlated materials, for example, a ferromagnetic semiconductor with local spin, ferromagnetic metals with spin topologies, ferroelectric thin films with polar topologies, piezoelectric thin films with nanopillars enclosed by charged boundaries, thermoelectric materials with local ferromagnetic nanoparticles and topotactic phase transformation with conducting nanofilaments are stated in detail one by one. The vital aspect is the breaking of local symmetry, the construction, the structure, of SFUs and their orderings existing or theoretically existing, together with the enhanced/new performance. All in all, the main comments of the review tend to the remaining challenges, promising design approaches for the SFUs, and their orderings for high‐performance functional materials.

A novel ultra-thin and high toughness piezoelectric thin film sensor based on MEMS technology

A novel ultra-thin and high toughness piezoelectric thin film sensor based on MEMS technology

Microfabricated acoustofluidic membrane acoustic waveguide actuator for highly localized in-droplet dynamic particle manipulation.

Precision manipulation techniques in microfluidics often rely on ultrasonic actuators to generate displacement and pressure fields in a liquid. However, strategies to enhance and confine the acoustofluidic forces often work against miniaturization and reproducibility in fabrication. This study presents microfabricated piezoelectric thin film membranes made via silicon diffusion for guided flexural wave generation as promising acoustofluidic actuators with low frequency, voltage, and power requirements. The guided wave propagation can be dynamically controlled to tune and confine the induced acoustofluidic radiation force and streaming. This provides for highly localized dynamic particle manipulation functionalities such as multidirectional transport, patterning, and trapping. The device combines the advantages of microfabrication and advanced acoustofluidic capabilities into a miniature "drop-and-actuate" chip that is mechanically robust and features a high degree of reproducibility for large-scale production. The membrane acoustic waveguide actuators offer a promising pathway for acoustofluidic applications such as biosensing, organoid production, and in situ analyte transport.

A novel ultra-thin and high toughness piezoelectric thin film sensor based on MEMS technology

A novel ultra-thin and high toughness piezoelectric thin film sensor based on MEMS technology

Thin Film Piezoelectric Nanogenerator Based on (100)-Oriented Nanocrystalline AlN Grown by Pulsed Laser Deposition at Room Temperature.

In wearable or implantable biomedical devices that typically rely on battery power for diagnostics or operation, the development of flexible piezoelectric nanogenerators (NGs) that enable mechanical-to-electrical energy harvesting is finding promising applications. Here, we present the construction of a flexible piezoelectric nanogenerator using a thin film of room temperature deposited nanocrystalline aluminium nitride (AlN). On a thin layer of aluminium (Al), the AlN thin film was grown using pulsed laser deposition (PLD). The room temperature grown AlN film was composed of crystalline columnar grains oriented in the (100)-direction, as revealed in images from transmission electron microscopy (TEM) and X-ray diffraction (XRD). Fundamental characterization of the AlN thin film by piezoresponse force microscopy (PFM) indicated that its electro-mechanical energy conversion metrics were comparable to those of c-axis oriented AlN and zinc oxide (ZnO) thin films. Additionally, the AlN-based flexible piezoelectric NG was encapsulated in polyimide to further strengthen its mechanical robustness and protect it from some corrosive chemicals.

Piezoelectric Micromachined Ultrasound Transducer Technology: Recent Advances and Applications

The objective of this article is to review the recent advancement in piezoelectric micromachined ultrasound transducer (PMUT) technology and the associated piezoelectric materials, device fabrication and characterization, as well as applications. PMUT has been an active research topic since the late 1990s because of the ultrasound application needs of low cost large 2D arrays, and the promising progresses on piezoelectric thin films, semiconductors, and micro/nano-electromechanical system technology. However, the industrial and medical applications of PMUTs have not been very significant until the recent success of PMUT based fingerprint sensing, which inspired growing interests in PMUT research and development. In this paper, recent advances of piezoelectric materials for PMUTs are reviewed first by analyzing the material properties and their suitability for PMUTs. PMUT structures and the associated micromachining processes are next reviewed with a focus on the complementary metal oxide semiconductor compatibility. PMUT prototypes and their applications over the last decade are then summarized to show the development trend of PMUTs. Finally, the prospective future of PMUTs is discussed as well as the challenges on piezoelectric materials, micro/nanofabrication and device integration.

PET/ZnO@MXene-Based Flexible Fabrics with Dual Piezoelectric Functions of Compression and Tension.

The traditional self-supported piezoelectric thin films prepared by filtration methods are limited in practical applications due to their poor tensile properties. The strategy of using flexible polyethylene terephthalate (PET) fabric as the flexible substrate is beneficial to enhancing the flexibility and stretchability of the flexible device, thus extending the applications of pressure sensors. In this work, a novel wearable pressure sensor is prepared, of which uniform and dense ZnO nanoarray-coated PET fabrics are covered by a two-dimensional MXene nanosheet. The ternary structure incorporates the advantages of the three components including the superior piezoelectric properties of ZnO nanorod arrays, the excellent flexibility of the PET substrate, and the outstanding conductivity of MXene, resulting in a novel wearable sensor with excellent pressure-sensitive properties. The PET/ZnO@MXene pressure sensor exhibits excellent sensing performance (S = 53.22 kPa-1), fast response/recovery speeds (150 ms and 100 ms), and superior flexural stability (over 30 cycles at 5% strain). The composite fabric also shows high sensitivity in both motion monitoring and physiological signal detection (e.g., device bending, elbow bending, finger bending, wrist pulse peaks, and sound signal discrimination). These findings provide insight into composite fabric-based pressure-sensitive materials, demonstrating the great significance and promising prospects in the field of flexible pressure sensing.

Piezoelectric properties of epitaxial Pb(Zr,Ti)O3 thin films grown on Si substrates by the sol–gel method

In this study, we investigated the piezoelectric characteristics of the sol-gel Pb(Zr0.52Ti0.48)O3 (PZT) films deposited on two different types of substrates. Epitaxial PZT piezoelectric thin films were grown on ZrO2-buffered Si substrates, while polycrystalline PZT thin films were grown on conventional Pt/Ti/Si substrates by the sol–gel method. X-ray diffraction measurements revealed a cube-on-cube epitaxy on the buffered Si substrate. The piezoelectric coefficient (e31,f) of the epitaxial PZT thin films was investigated by measuring the tip displacement of PZT cantilevers, and the |e31,f | values were in the range of 8.9–12 C/m2. The voltage dependence of the |e31,f | values was more constant than that of the polycrystalline sol–gel PZT/Si cantilever. The epitaxial sol–gel PZT films thus exhibited good piezoelectric properties for applications in microelectromechanical systems devices.

Piezoelectric Micromachined Ultrasonic Transducers (PMUTs): Performance Metrics, Advancements, and Applications

With the development of technology, systems gravitate towards increasing in their complexity, miniaturization, and level of automation. Amongst these systems, ultrasonic devices have adhered to this trend of advancement. Ultrasonic systems require transducers to generate and sense ultrasonic signals. These transducers heavily impact the system's performance. Advancements in microelectromechanical systems have led to the development of micromachined ultrasonic transducers (MUTs), which are utilized in miniaturized ultrasound systems. Piezoelectric micromachined ultrasonic transducers (PMUTs) exhibit higher capacitance and lower electrical impedance, which enhances the transducer's sensitivity by minimizing the effect of parasitic capacitance and facilitating their integration with low-voltage electronics. PMUTs utilize high-yield batch microfabrication with the use of thin piezoelectric films. The deposition of thin piezoelectric material compatible with complementary metal-oxide semiconductors (CMOS) has opened novel avenues for the development of miniaturized compact systems with the same substrate for application and control electronics. PMUTs offer a wide variety of applications, including medical imaging, fingerprint sensing, range-finding, energy harvesting, and intrabody and underwater communication links. This paper reviews the current research and recent advancements on PMUTs and their applications. This paper investigates in detail the important transduction metrics and critical design parameters for high-performance PMUTs. Piezoelectric materials and microfabrication processes utilized to manufacture PMUTs are discussed. Promising PMUT applications and outlook on future advancements are presented.

Lorentz-force gyrator based on AlScN piezoelectric thin film

This paper reports a chip-scale radio frequency Lorentz-force gyrator based on an aluminum scandium nitride (Al0.7Sc0.3N) thin film. The two-port gyrator, which is essentially a lateral overtone bulk acoustic resonator, consists of a planar coil for Lorentz-force transduction and two top-bottom electrode pairs for piezoelectric transduction. The non-reciprocity is generated by the phase transition in the Lorentz-force coupling when an external vertical magnetic field is applied. The Lorentz-force gyrators based on both AlN and Al0.7Sc0.3N thin films demonstrate good non-reciprocity, i.e., the 180° phase difference, at approximately 517 and 388 MHz, respectively. Thanks to larger piezoelectric constants, the Al0.7Sc0.3N gyrator demonstrates easier impedance matching and a wider fractional bandwidth of 6.3% at a magnetic field of 1.65 T compared to 1.3% for an AlN device. Finally, an isolator consisting of the Lorentz-force gyrator and a shunt resistor is demonstrated over 35 dB of isolation and flat unidirectional transmission.

In situ characterisation for studying nucleation and growth of nanostructured materials and thin films during liquid-based synthesis

Knowledge about the nucleation, growth, and formation mechanisms during materials synthesis using sol-gel and solution-based methods is important to design a material with desired properties. We used aqueous chemical synthesis as an environmentally friendly and highly flexible route to tailored and reproducible synthesis of oxide nanomaterials and thin films. For studies of hydrothermal synthesis an in situ cell using synchrotron X-ray diffraction was used to investigate the formation mechanisms of SrxBa1-xNb2O6 piezoelectrics. Aqueous chemical solution deposition of phase pure oriented piezoelectric thin films demands strong control of processing parameters. An in situ cell for synchrotron X-ray diffraction studies of the annealing and crystallisation steps during aqueous chemical solution deposition was used to understand the nucleation and crystallisation of Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT). We discuss how the knowledge about nucleation and growth obtained by in situ characterisation can be used to design the optimal procedure for fabrication of oxide materials with desired properties.Graphical abstract

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.

Vibration Energy Conversion Power Supply Based on the Piezoelectric Thin Film Planar Array.

Vibration energy harvesting has received much attention as a new type of power solution for low-power micro/nano-devices. However, VEH (vibration energy harvester) based on PVDF (polyvinylidene fluoride) piezoelectric materials have a low output power and energy conversation efficiency due to the relatively low piezoelectric constant, coupling coefficient, and dielectric constant. For this reason, we design a vibration energy conversion power supply, which consists of a VEH with a PVDF piezoelectric thin film planar array vibration structure and an energy harvesting circuit for regulating the electric energy of multiple sources. Furthermore, our solution was validated by simulations of structural dynamics in COMSOL and equivalent circuits model in Multisim. From the circuitry simulation results, the output current and the charging period increase and decrease, doubling, respectively, for each doubling of the number of array groups of films. Moreover, the solid mechanics simulation results show that the planar array structure makes the phase and amplitude of the input vibration waves as consistent as possible so that the same theoretical enhancement effect of the circuitry model is achieved. An identical experimental test was implemented with vibration conditions of 75 Hz-2.198 g. The fabricated harvester quickly charged the 22 V-0.022 F ultracapacitor bank to 5 V in 24 min. The maximum open circuit voltage and output power, respectively, were 10.4 V and 0.304 mW. This maximum charging power was 11.69 times higher than that of a single film. This special power supply can replace batteries to power low-power electronics deployed in vibrating environments, thus reducing the maintenance costs of equipment and environmental pollution rates.

In-situ damage self-monitoring of fiber-reinforced composite by integrating self-powered ZnO nanowires decorated carbon fabric

The tradeoff between damage self-sensing abilities and mechanical properties of composites is still a challenge for incorporating functional materials as potential sensing components. In this work, multifunctional carbon fabric coated with piezoelectric zinc oxide nanowires (ZnO NWs) was integrated into a carbon fiber reinforced composite as simultaneously a self-powered damage sensing component and mechanical reinforcement. The embedded ZnO NWs endowed the composite with in-situ self-sensing of damage. The sensing characteristics corresponding to the composite damage were validated by a well-established acoustic emission method. The results showed comparable performances to those obtained by common damage detection techniques. Additionally, the introduction of ZnO NWs had a positive impact on the mechanical properties of the host composite, increasing tensile and flexural strengths by 7.4% and 4.8%, respectively. These values were significantly higher than those of the interleaved strong piezoelectric PVDF thin films, leading to severe degradation in both tensile strength (−28.6%) and flexural strength (−82.4%). The enhanced mechanical properties may be ascribed to the interfacial mechanical interlocking and the increased bond area induced by the penetration of stiff ZnO NWs into the resin matrix. Overall, the excellent online and in-situ damage self-monitoring abilities with inherent structural benefits make ZnO NWs-based sensing scheme promising for broad applications in the field of structural health monitoring.

Resistance degradation in sputtered sodium potassium niobate thin films and its relationship to point defects

The reliability of piezoelectric thin films is crucial for piezoelectric micro-electromechanical system applications. The understanding of resistance degradation in piezoelectric thin films requires knowledge about point defects. Here, we show the resistance degradation mechanism in the lead-free alternative sodium potassium niobate (KNN) thin films and the relationship to point defects in both field-up and field-down polarities. The conduction mechanism of KNN thin films is found to be Schottky-limited. Furthermore, a reduction in Schottky barrier height accompanies the resistance degradation resulting from interfacial accumulation of additional charged defects. We use thermally stimulated depolarization current measurements and charge-based deep level transient spectroscopy to characterize the defects in KNN thin films. Our results show that oxygen vacancies accumulate at the interface in field-up polarity, and multiple defects accumulate in field-down polarity, potentially oxygen vacancies and holes trapped by potassium vacancies. We use wafer deposition variation to create samples with different film properties. Measurement results from these samples correlate resistance degradation with the defect concentration. We find the natural logarithm of leakage current to be linearly proportional to the defect concentration to the power of 0.25. The demonstrated analysis provides a precise and meaningful process assessment for optimizing KNN thin films.