Automated Experiments of Local Non-Linear Behavior in Ferroelectric Materials.

An automated experiment in multimodal imaging to probe structural, chemical, and functional behaviors in complex materials and elucidate the dominant physical mechanisms that control device function is developed and implemented. Here, the emergence of non-linear electromechanical responses in piezoresponse force microscopy (PFM) is explored. Non-linear responses in PFM can originate from multiple mechanisms, including intrinsic material responses often controlled by domain structure, surface topography that affects the mechanical phenomena at the tip-surface junction, and the presence of surface contaminants. Using an automated experiment to probe the origins of non-linear behavior in ferroelectric lead titanate (PTO) and ferroelectric Al0.93 B0.07 N films, it is found that PTO shows asymmetric nonlinear behavior across a/c domain walls and a broadened high nonlinear response region around c/c domain walls. In contrast, for Al0.93 B0.07 N, well-poled regions show high linear piezoelectric responses, when paired with low non-linear responses regions that are multidomain show low linear responses and high nonlinear responses. It is shown that formulating dissimilar exploration strategies in deep kernel learning as alternative hypotheses allows for establishing the preponderant physical mechanisms behind the non-linear behaviors, suggesting that automated experiments can potentially discern between competing physical mechanisms. This technique can also be extended to electron, probe, and chemical imaging.

Improving PMUT Receive Sensitivity via DC Bias and Piezoelectric Composition.

The receive sensitivity of lead zirconate titanate (PZT) piezoelectric micromachined ultrasound transducers (PMUTs) was improved by applying a DC bias during operation. The PMUT receive sensitivity is governed by the voltage piezoelectric coefficient, h31,f. With applied DC biases (up to 15 V) on a 2 µm PbZr0.52Ti0.48O3 film, e31,f increased 1.6 times, permittivity decreased by a factor of 0.6, and the voltage coefficient increased by ~2.5 times. For released PMUT devices, the ultrasound receive sensitivity improved by 2.5 times and the photoacoustic signal improved 1.9 times with 15 V applied DC bias. B-mode photoacoustic imaging experiments showed that with DC bias, the PMUT received clearer photoacoustic signals from pencil leads at 4.3 cm, compared to 3.7 cm without DC bias.

Flexible Thin-Film PZT Ultrasonic Transducers on Polyimide Substrates.

We report flexible thin-film lead zirconate titanate (PZT)-based ultrasonic transducers on polyimide substrates. The transducers are bar resonators designed to operate in the width extension mode. The active elements are 1 µm thick PZT films that were crystallized on Si substrates at 700 °C and transferred to 5 µm thick solution-cast polyimide via dissolution of an underlying release layer. Underwater pitch-catch testing between two neighboring 100 µm × 1000 µm elements showed a 0.2 mV signal at a 1.5 cm distance for a driving voltage of 5 V peak at 9.5 MHz. With the same excitation, a 33 kPa sound pressure output at a 6 mm distance and a 32% bandwidth at -6 dB were measured by hydrophone.

Comparative Solution Synthesis of Mn Doped (Na,K)NbO3 Thin Films.

(K0.5 Na0.5 )NbO3 (KNN) is a promising lead-free alternative for ferroelectric thin films such as Pb(Zr,Ti)O3 . One main drawback is its high leakage current density at high electric fields, which has been previously linked to alkali non-stoichiometry. This paper compares three acetate-based chemical solution synthesis and deposition methods for 0.5 mol % Mn-doped KNN film fabrication, using lower crystallization temperature processes in comparison to the sintering temperatures necessary for fabrication of KNN ceramics. This paper shows the crucial role of the A site homogenization step during solution synthesis in preserving alkali chemical homogeneity of Mn doped KNN films. Chemically homogeneous films show a uniform grain size of 80 nm and a leakage current density under 2.8×10-8  A cm-2 up to electric fields as high as 600 kV cm-1 , which is the highest breakdown strength reported for KNN thin films. Solution synthesis involving two-step pyrolysis resulted in films with dense, columnar microstructures, which are interesting for orientation control and enhancement of piezoelectric properties. This study reports detailed solution synthesis and deposition processes with good dielectric, ferroelectric and breakdown field properties. An optimized fabrication method that should couple low leakage current density with dense and oriented microstructures is proposed.

10 MHz Thin-Film PZT-Based Flexible PMUT Array: Finite Element Design and Characterization.

Piezoelectric micromachined ultrasound transducers (PMUT) incorporating lead zirconate titanate PbZr0.52Ti0.48O3 (PZT) thin films were investigated for miniaturized high-frequency ultrasound systems. A recently developed process to remove a PMUT from an underlying silicon (Si) substrate has enabled curved arrays to be readily formed. This research aimed to improve the design of flexible PMUT arrays using PZFlex, a finite element method software package. A 10 MHz PMUT 2D array working in 3-1 mode was designed. A circular unit-cell was structured from the top, with concentric layers of platinum (Pt)/PZT/Pt/titanium (Ti) on a polyimide (PI) substrate. Pulse-echo and spectral response analyses predicted a center frequency of 10 MHz and bandwidth of 87% under water load and air backing. A 2D array, consisting of the 256 (16 × 16) unit-cells, was created and characterized in terms of pulse-echo and spectral responses, surface displacement profiles, crosstalk, and beam profiles. The 2D array showed: decreased bandwidth due to protracted oscillation decay and guided wave effects; mechanical focal length at 2.9 mm; 3.7 mm depth of field for -6 dB; and -55.6 dB crosstalk. Finite element-based virtual prototyping identified figures of merit-center frequency, bandwidth, depth of field, and crosstalk-that could be optimized to design robust, flexible PMUT arrays.

Dynamical Magnetic Field Accompanying the Motion of Ferroelectric Domain Walls.

The recently proposed dynamical multiferroic effect describes the generation of magnetization from temporally varying electric polarization. Here, we show that the effect can lead to a magnetic field at moving ferroelectric domain walls, where the rearrangement of ions corresponds to a rotation of ferroelectric polarization in time. We develop an expression for the dynamical magnetic field, and calculate the relevant parameters for the example of 90° and 180° domain walls, as well as for polar skyrmions, in BaTiO_{3}, using a combination of density functional theory and phenomenological modeling. We find that the magnetic field reaches the order of several µT at the center of the wall, and we propose two experiments to measure the effect with nitrogen-vacancy center magnetometry.

Design and fabrication of prototype piezoelectric adjustable X-ray mirrors.

Lynx, a next generation X-ray observatory concept currently under study, requires lightweight, high spatial resolution X-ray mirrors. Here we detail the development and fabrication of one of the candidate technologies for Lynx, piezoelectric adjustable X-ray optics. These X-ray mirrors are thin glass shell mirrors with Cr/Ir X-ray reflective coatings on the mirror side and piezoelectric thin film actuators on the actuator side. Magnetron sputtering was used to deposit metal electrodes and metal-oxide piezoelectric layers. The piezoelectric (Pb0.995(Zr0.52Ti0.48)0.99Nb0.01O3) was divided into 112 independent piezoelectric actuators, with 100% yield achieved. We discuss the fabrication procedure, residual thermal stresses and tuning of the Cr/Ir coating stress for the purposes of stress balancing.

Fast Magnetic Domain-Wall Motion in a Ring-Shaped Nanowire Driven by a Voltage.

Magnetic domain-wall motion driven by a voltage dissipates much less heat than by a current, but none of the existing reports have achieved speeds exceeding 100 m/s. Here phase-field and finite-element simulations were combined to study the dynamics of strain-mediated voltage-driven magnetic domain-wall motion in curved nanowires. Using a ring-shaped, rough-edged magnetic nanowire on top of a piezoelectric disk, we demonstrate a fast voltage-driven magnetic domain-wall motion with average velocity up to 550 m/s, which is comparable to current-driven wall velocity. An analytical theory is derived to describe the strain dependence of average magnetic domain-wall velocity. Moreover, one 180° domain-wall cycle around the ring dissipates an ultrasmall amount of heat, as small as 0.2 fJ, approximately 3 orders of magnitude smaller than those in current-driven cases. These findings suggest a new route toward developing high-speed, low-power-dissipation domain-wall spintronics.

Room-temperature voltage tunable phonon thermal conductivity via reconfigurable interfaces in ferroelectric thin films.

Dynamic control of thermal transport in solid-state systems is a transformative capability with the promise to propel technologies including phononic logic, thermal management, and energy harvesting. A solid-state solution to rapidly manipulate phonons has escaped the scientific community. We demonstrate active and reversible tuning of thermal conductivity by manipulating the nanoscale ferroelastic domain structure of a Pb(Zr0.3Ti0.7)O3 film with applied electric fields. With subsecond response times, the room-temperature thermal conductivity was modulated by 11%.

Piezoelectric micromachined ultrasound transducer (PMUT) arrays for integrated sensing, actuation and imaging.

Many applications of ultrasound for sensing, actuation and imaging require miniaturized and low power transducers and transducer arrays integrated with electronic systems. Piezoelectric micromachined ultrasound transducers (PMUTs), diaphragm-like thin film flexural transducers typically formed on silicon substrates, are a potential solution for integrated transducer arrays. This paper presents an overview of the current development status of PMUTs and a discussion of their suitability for miniaturized and integrated devices. The thin film piezoelectric materials required to functionalize these devices are discussed, followed by the microfabrication techniques used to create PMUT elements and the constraints the fabrication imposes on device design. Approaches for electrical interconnection and integration with on-chip electronics are discussed. Electrical and acoustic measurements from fabricated PMUT arrays with up to 320 diaphragm elements are presented. The PMUTs are shown to be broadband devices with an operating frequency which is tunable by tailoring the lateral dimensions of the flexural membrane or the thicknesses of the constituent layers. Finally, the outlook for future development of PMUT technology and the potential applications made feasible by integrated PMUT devices are discussed.

Sputter deposition of PZT piezoelectric films on thin glass substrates for adjustable x-ray optics.

Piezoelectric PbZr(0.52)Ti(0.48)O(3) (PZT) thin films deposited on thin glass substrates have been proposed for adjustable optics in future x-ray telescopes. The light weight of these x-ray optics enables large collecting areas, while the capability to correct mirror figure errors with the PZT thin film will allow much higher imaging resolution than possible with conventional lightweight optics. However, the low strain temperature and flexible nature of the thin glass complicate the use of chemical-solution deposition due to warping of the substrate at typical crystallization temperatures for the PZT. RF magnetron sputtering enabled preparation of PZT films with thicknesses up to 3 µm on Schott D263 glass substrates with much less deformation. X-ray diffraction analysis indicated that the films crystallized with the perovskite phase and showed no indication of secondary phases. Films with 1 cm(2) electrodes exhibited relative permittivity values near 1100 and loss tangents below 0.05. In addition, the remanent polarization was 26 µC/cm(2) with coercive fields of 33 kV/cm. The transverse piezoelectric coefficient was as high as -6.1±0.6 C/m(2). To assess influence functions for the x-ray optics application, the piezoelectrically induced deflection of individual cells was measured and compared with finite-element-analysis calculations. The good agreement between the results suggests that actuation of PZT thin films can control mirror figure errors to a precision of about 5 nm, allowing sub-arcsecond imaging.

Tuning Nb Solubility, Electrical Properties, and Imprint through PbO Stoichiometry in PZT Films.

Lead zirconate titanate (PZT) films with high Nb concentrations (6-13 mol%) were grown by chemical solution deposition. In concentrations up to 8 mol% Nb, the films self-compensate the stoichiometry; single phase films were grown from precursor solutions with 10 mol% PbO excess. Higher Nb concentrations induced multi-phase films unless the amount of excess PbO in the precursor solution was reduced. Phase pure perovskite films were grown with 13 mol% excess Nb with the addition of 6 mol% PbO. Charge compensation was achieved by creating lead vacancies when decreasing excess PbO level; using Kroger-Vink notation, NbTi• are ionically compensated by VPb″ to maintain charge neutrality in heavily Nb-doped PZT films. With Nb doping, films showed suppressed {100} orientation, the Curie temperature decreased, and the maximum in the relative permittivity at the phase transition broadened. The dielectric and piezoelectric properties were dramatically degraded due to increased quantity of the non-polar pyrochlore phase in multi-phase films; εr reduced from 1360 ± 8 to 940 ± 6, and the remanent d33,f value decreased from 112 to 42 pm/V when increasing the Nb concentration from 6 to 13 mol%. Property deterioration was corrected by decreasing the PbO level to 6 mol%; phase pure perovskite films were attained. εr and the remanent d33,f increased to 1330 ± 9 and 106 ± 4 pm/V, respectively. There was no discernable difference in the level of self-imprint in phase pure PZT films with Nb doping. However, the magnitude of the internal field after thermal poling at 150 °C increased significantly; the level of imprint was 30 kV/cm and 11.5 kV/cm in phase pure 6 mol% and 13 mol% Nb-doped films, respectively. The absence of mobile VO••, coupled with the immobile VPb″ in 13 mol% Nb-doped PZT films, leads to lower internal field formation upon thermal poling. For 6 mol% Nb-doped PZT films, the internal field formation was primarily governed by (1) the alignment of (VPb″-VO•• )x and (2) the injection and subsequent electron trapping by Ti4+. For 13 mol% Nb-doped PZT films, hole migration between VPb″ controlled internal field formation upon thermal poling.

Atomic-scale polarization switching in wurtzite ferroelectrics.

Ferroelectric wurtzites have the potential to revolutionize modern microelectronics because they are easily integrated with multiple mainstream semiconductor platforms. However, the electric fields required to reverse their polarization direction and unlock electronic and optical functions need substantial reduction for operational compatibility with complementary metal-oxide semiconductor (CMOS) electronics. To understand this process, we observed and quantified real-time polarization switching of a representative ferroelectric wurtzite (Al0.94B0.06N) at the atomic scale with scanning transmission electron microscopy. The analysis revealed a polarization reversal model in which puckered aluminum/boron nitride rings in the wurtzite basal planes gradually flatten and adopt a transient nonpolar geometry. Independent first-principles simulations reveal the details and energetics of the reversal process through an antipolar phase. This model and local mechanistic understanding are a critical initial step for property engineering efforts in this emerging material class.

Anomalously abrupt switching of wurtzite-structured ferroelectrics: simultaneous non-linear nucleation and growth model.

Ferroelectric polarization switching is one common example of a process that occurs via nucleation and growth, and understanding switching kinetics is crucial for applications such as ferroelectric memory. Here we describe and interpret anomalous switching dynamics in the wurtzite-structured nitride thin film ferroelectrics Al0.7Sc0.3N and Al0.94B0.06N using a general model that can be directly applied to other abrupt transitions that proceed via nucleation and growth. When substantial growth and impingement occur while nucleation rate is increasing, such as in these wurtzite-structured ferroelectrics under high electric fields, abrupt polarization reversal leads to very large Avrami coefficients (e.g., n = 11), inspiring an extension of the KAI (Kolmogorov-Avrami-Ishibashi) model. We apply this extended model to two related but distinct scenarios that crossover between (typical) behavior described by sequential nucleation and growth and a more abrupt transition arising from significant growth prior to peak nucleation rate. This work therefore provides a more complete description of general nucleation and growth kinetics applicable to any system while specifically addressing the anomalously abrupt polarization reversal behavior in new wurtzite-structured ferroelectrics.

Correction: Anomalously abrupt switching of wurtzite-structured ferroelectrics: simultaneous non-linear nucleation and growth model.

Correction for 'Anomalously abrupt switching of wurtzite-structured ferroelectrics: simultaneous non-linear nucleation and growth model' by Keisuke Yazawa et al., Mater. Horiz., 2023, https://doi.org/10.1039/D3MH00365E.

32 Element Piezoelectric Micromachined Ultrasound Transducer (PMUT) Phased Array for Neuromodulation.

Interest in utilizing ultrasound (US) transducers for non-invasive neuromodulation treatment, including for low intensity transcranial focused ultrasound stimulation (tFUS), has grown rapidly. The most widely demonstrated US transducers for tFUS are either bulk piezoelectric transducers or capacitive micromachine transducers (CMUT) which require high voltage excitation to operate. In order to advance the development of the US transducers towards small, portable devices for safe tFUS at large scale, a low voltage array of US transducers with beam focusing and steering capability is of interest. This work presents the design methodology, fabrication, and characterization of 32-element phased array piezoelectric micromachined ultrasound transducers (PMUT) using 1.5 µm thick Pb(Zr0.52 Ti0.48)O3 films doped with 2 mol% Nb. The electrode/piezoelectric/electrode stack was deposited on a silicon on insulator (SOI) wafer with a 2 µm silicon device layer that serves as the passive elastic layer for bending-mode vibration. The fabricated 32-element PMUT has a central frequency at 1.4 MHz. Ultrasound beam focusing and steering (through beamforming) was demonstrated where the array was driven with 14.6 V square unipolar pulses. The PMUT generated a maximum peak-to-peak focused acoustic pressure output of 0.44 MPa at a focal distance of 20 mm with a 9.2 mm and 1 mm axial and lateral resolution, respectively. The maximum pressure is equivalent to a spatial-peak pulse-average intensity of 1.29 W/cm2, which is suitable for tFUS application.

Development of Polymer-Ceramic-Metal Graded Acoustic Matching Layers via Cold Sintering.

A family of three phase, polymer-ceramic-metal (Poly-cer-met) electrically conducting composites was developed via cold sintering for acoustic matching application in medical ultrasound transducers. A range of acoustic impedance ( Z ) between MRayl with low attenuation (<3.5 dB/mm, measured at 10 MHz) was achieved in composites of zinc oxide, silver, and in thermoplastic polymers like Ultem polyetherimide (PEI) or polytetrafluoroethylene (PTFE) at sintering pressure less than 50 MPa and temperature of 150 °C. Densities exceeding 95% were achieved, with resistivities less than 1 Ω -cm. The acoustic velocity was homogeneous across the part (variations <5%). The acoustic velocities exceeded 2500 m/s for Z above 12 MRayl. The experimentally measured acoustic impedance of ZnO/Ag/PEI composites was observed to be in close agreement with the theoretical logarithmic model developed for different volume fractions of individual phases at the percolation limit for Ag. Thus, the acoustic properties of this family of matching layers (MLs) can be predicted to a good approximation before experimental realization. Additionally, a non-conducting low Z (5 MRayl MRayl) with acoustic velocities exceeding 2000 m/s was achieved using hydrozincite as the ceramic component. Scaling of the composites to 2'' diameter was demonstrated. A -6 dB bandwidth greater than 85% was measured for a three ML ultrasound transducer, fabricated using a single cold sintered layer ( Z = 19 MRayl) and two other commercial layers in the stack. Finally, a co-cold sintered graded prototype consisting of three tape-casted formulations corresponding to Z = 5 , 9, and 19 MRayl, while still retaining the correct distributions of the components was demonstrated.

Residual Stress and Ferroelastic Domain Reorientation in Declamped {001} Pb(Zr0.3Ti0.7)O3 Films.

Ferroelectric films are often constrained by their substrates and subject to scaling effects, including suppressed dielectric permittivity. In this work, the thickness dependence of intrinsic and extrinsic contributions to the dielectric properties was elucidated. A novel approach to quantitatively deconstruct the relative permittivity into three contributions (intrinsic, reversible extrinsic, and irreversible extrinsic) was developed using a combination of X-ray diffraction (XRD) and Rayleigh analysis. In situ synchrotron XRD was used to understand the influence of residual stress and substrate clamping on the domain state, ferroelastic domain reorientation, and electric field-induced strain. For tetragonal {001} textured Pb0.99(Zr0.3Ti0.7)0.98Nb0.02O3 thin films clamped to an Si substrate, a thickness-dependent in-plane tensile stress developed during processing, which dictates the domain distribution over a thickness range of 0.27- [Formula: see text]. However, after the films were partially declamped from the substrate and annealed, the residual stress was alleviated. As a result, the thickness dependence of the volume fraction of c -domains largely disappeared, and the out-of-plane lattice spacings ( d ) for both a - and c -domains increased. The volume fraction of c -domains was used to calculate the intrinsic relative permittivity. The reversible Rayleigh coefficient was then used to separate the intrinsic and reversible extrinsic contributions. The reversible extrinsic response accounted for ~50% of the overall relative permittivity (measured at 50 Hz and alternating current (ac) field of 0.5·Ec ) and was thickness dependent even after poling and upon release.

Thermal Conductivity of Aluminum Scandium Nitride for 5G Mobile Applications and Beyond.

Radio frequency (RF) microelectromechanical systems (MEMS) based on Al1-xScxN are replacing AlN-based devices because of their higher achievable bandwidths, suitable for the fifth-generation (5G) mobile network. However, overheating of Al1-xScxN film bulk acoustic resonators (FBARs) used in RF MEMS filters limits power handling and thus the phone's ability to operate in an increasingly congested RF environment while maintaining its maximum data transmission rate. In this work, the ramifications of tailoring of the piezoelectric response and microstructure of Al1-xScxN films on the thermal transport have been studied. The thermal conductivity of Al1-xScxN films (3-8 W m-1 K-1) grown by reactive sputter deposition was found to be orders of magnitude lower than that for c-axis-textured AlN films due to alloying effects. The film thickness dependence of the thermal conductivity suggests that higher frequency FBAR structures may suffer from limited power handling due to exacerbated overheating concerns. The reduction of the abnormally oriented grain (AOG) density was found to have a modest effect on the measured thermal conductivity. However, the use of low AOG density films resulted in lower insertion loss and thus less power dissipated within the resonator, which will lead to an overall enhancement of the device thermal performance.