arXiv subjects

Susan Trolier-McKinstry

Publications and source records attributed to Susan Trolier-McKinstry.

18 recordsLinked to original sources

arXiv2026

Ferroelectric Switching in ZnO/Zn1-xMgxO Heterostructures: Atomistic Insights into Interfacial Coupling and Layer Architecture

Ferroelectric switching in heterostructures couples composition, layer topology, temperature, and interfacial boundary conditions. ReaxFF molecular dynamics isolates these variables in ZnO/ Zn1- xMgxO/ZnO and Zn1-xMgxO/ZnO/ Zn1-xMgxO stacks. Within pristine, initially single-domain models, coupling to switchable Zn1-xMgxO reduces the applied field required to reverse ZnO by up to fivefold. Temperature generally lowers the coercive field, whereas Mg concentration produces a nonmonotonic response. At equal ZnO and Zn1-xMgxO (ZMO) proportions, structures with ZnO at the center switch at lower fields than those with ZMO at the center at all four temperatures examined, demonstrating a topology-dependent response. Layer-resolved trajectories reveal topology-dependent switching sequences with direction-dependent redistribution of normal stress near the heterointerfaces, consistent with a stress-assisted cooperative pathway. Limiting MgO-containing structures exhibit sequential multilevel switching or low-polarity trapping, depending on thickness and temperature. Fixed-charge atomistic simulations complement previous continuum descriptions by resolving structural, energetic, and local stress evolution under a common applied field. TEM and STEM-EDS observations provide experimental structural context for the modeled architectures. Together, the results establish layer topology and interfacial mechanical confinement as design variables for wurtzite ferroelectric heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Heterogeneous ferroelectricity and conductivity of oxidized BaTiO$_3$ crystals: the role of nanoscale phase segregation in the surface region

We investigate the effect of thermal oxidation on BaTiO$_3$ single crystals. Our results reveal that, even at moderate temperatures of up to 1000 °C, complex segregation mechanisms occur involving the movement of Ba-rich compounds to the upper surface, where they form inhomogeneously distributed BaO nanocrystals. As a result, deeper regions of the surface layer become depleted in Ba and become TiO$_2$-rich. A sandwich-like structure evolves in the surface layer, exhibiting a measurable electromotive force and self-polarization. This, in turn, screens the polarization and diminishes the global ferroelectric response. The conductivity is also strongly influenced by the irreversible segregation effects in the surface region. An as-received crystal can be transformed into a metallic state, while oxidation induces semiconducting properties and prevents a return to the metallic state upon subsequent reduction. Nanoscale analysis demonstrates that the conductivity is channeled to filaments forming along dislocations, whose local chemical composition changes upon reduction and oxidation even at moderate temperatures due to preferential Ba and O pipe diffusion thus determining the electric behavior of the whole sample.

cond-mat.mtrl-sci↗

arXiv2026

An Automated Magnetron Sputtering Chamber for Ferroelectric Thin Film Deposition

Optimization of next-generation materials synthesis and manufacturing processes can be accelerated by effective use of digital datasets. However, a majority of existing custom research infrastructure, including that for thin film deposition, is primarily manually operated and not compatible with this new research paradigm. Here, a template is provided for upgrading existing manual deposition chambers to enable automated and autonomous experimentation. As an example, the upgrade of an existing magnetron sputtering chamber dedicated to synthesis of wurtzite ferroelectrics is presented. Focus is placed on automation of instrumentation; system and deposition control; and synchronized and automated data collection strategies. An example use case of the system for semi-autonomous determination of process-property relationships is presented, specifically minimization of coercive field in wurtzite Al$_{1-x-y}$Sc$_x$B$_y$N thin films.

cond-mat.mtrl-sci↗

arXiv2026

Closed-loop discovery of out-of-distribution processing protocols by evolutionary search and uncertainty-aware learning

Many materials and chemical systems exhibit history-dependent responses, where functional outcomes are governed not only by final-state variables but by the time-dependent sequence of fields, temperatures, or chemical potentials applied during operation. Discovering new processing protocols is therefore a high-dimensional search problem in which the control variable is an entire waveform or sample history, and conventional strategies either remain confined to conservative interpolative families or become prohibitively measurement intensive. Here, a closed-loop workflow is introduced that couples evolutionary search over a compact waveform representation with uncertainty-aware deep kernel learning to generate, rank, and experimentally validate candidate protocols. Applied to ferroelectric thin films, with the scanning-probe tip-bias waveform as the protocol and the nonlinear electromechanical response as the reward, the workflow discovers waveform families that enhance nonlinearity by de-aging the film. Spatially resolved before/after measurements show that the best-performing waveforms selectively activate pre-existing, weakly pinned domain-wall segments, whereas the worst drive long-range irreversible switching. This framework reframes protocol tuning as out-of-distribution discovery, generalizable to synthesis and annealing trajectories, battery formation protocols, and other high-dimensional control problems.

cond-mat.mtrl-sci↗

arXiv2025

Indirect Tunneling Enabled Spontaneous Time-Reversal Symmetry Breaking and Josephson Diode Effect in TiN/Al$_2$O$_3$/Hf$_{0.8}$Zr$_{0.2}$O$_2$/Nb tunnel junctions

Josephson diode (JD) effect found in Josephson tunnel junctions (JTJs) has attracted a great deal of attention due to its importance for developing superconducting circuitry based quantum technologies. So far, the highly desirable electrical control of the JD effect has not been demonstrated in any JTJ prepared by techniques used in semiconductor industry. We report the fabrication of JTJs featuring a composite tunnel barrier of Al$_2$O$_3$ and Hf$_{\mathrm{0.8}}$Zr$_\mathrm{0.2}$O$_2$ prepared by complementary-metal-oxide-semiconductor (CMOS) compatible atomic layer deposition (ALD). These JTJs were found to show the JD effect in nominally zero magnetic fields with the nonreciprocity controllable using an electric training current, yielding a surprisingly large diode efficiency not achieved previously. The quasiparticle tunneling, through which the Josephson coupling in a JTJ is established, was found to show no nonreciprocity. We attribute these observations to the simultaneous presence of positive and negative Josephson couplings, with the latter originating from indirect tunneling. The resulted spontaneous time-reversal symmetry breaking and the double-minima washboard potential for the ensemble averaged phase difference in the resistively and capacitively shunted junction (RCSJ) model are shown to fully account for the experimentally observed JD effect.

cond-mat.supr-con↗

arXiv2025

Material-Limited Switching in Nanoscale Ferroelectrics

The ferroelectric switching speed has been experimentally obfuscated by the interaction between the measurement circuit and the ferroelectric switching itself. This has prohibited the observation of real material responses at nanosecond timescales and lower. Here, fundamental polarization switching speeds in ferroelectric materials with the perovskite, fluorite, and wurtzite structures are reported. Upon lateral scaling of island capacitors from micron to nanoscales, a clear transition from circuit-limited switching to a material-limited switching regime is observed. In La$_{0.15}$Bi$_{0.85}$FeO$_{3}$ capacitors, switching is as fast as ~150 ps, the fastest switching time reported. For polycrystalline Hf$_{0.5}$Zr$_{0.5}$O$_{2}$ capacitors, a fundamental switching limit of ~210 ps is observed. Switching times for Al$_{0.92}$B$_{0.08}$N are near 20 ns, limited by the coercive and breakdown electric fields. The activation field, instantaneous pseudo-resistivity, and energy-delay are reported in this material-limited regime. Lastly, a criterion for reaching the material-limited regime is provided. This regime enables observation of intrinsic material properties and favorable scaling trends for high-performance computing.

cond-mat.mtrl-sci↗

arXiv2025

Quantitative Nonlinear Optical Polarimetry with High Spatial Resolution

Nonlinear optical microscopy such as in the optical second-harmonic generation (SHG) modality has become a popular tool today for probing materials in the physical and biological sciences. While imaging and spectroscopy are widely used in the microscopy mode, nonlinear polarimetry, which can shed light on materials' symmetry and microstructure, is relatively underdeveloped. This is partly because quantitative analytical modeling of the optical SHG response for anisotropic crystals and films largely assumes low-numerical aperture (NA) focusing of light, where the plane-wave approximation is sufficient. Tight focusing provides unique benefits in revealing out-of-plane polarization responses, which cannot be detected by near-plane-wave illumination at normal incidence. Here, we outline a method for quantitatively analyzing SHG polarimetry measurements obtained under high-NA focusing within a microscope geometry. Experiments and simulations of a variety of standard samples, from single crystals to thin films, are in good agreement, including measured and simulated spatial SHG maps of ferroelectric domains. A solution to the inverse problem is demonstrated, where the spatial distribution of an SHG tensor with unknown tensor coefficient magnitudes is determined by experimentally measured polarimetry. The ability to extract the out-of-plane component of the nonlinear polarization in normal incidence is demonstrated, which can be valuable for high-resolution polarimetry of 2D materials, thin films, heterostructures, and uniaxial crystals with a strong out-of-plane response. Copyright 2025 Optica Publishing Group. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved. https://doi.org/10.1364/OPTICA.559060

cond-mat.mtrl-sci↗

arXiv2025

Nitrogen Vacancies Induce Fatigue in Ferroelectric $\mathrm{Al_{0.93}B_{0.07}N}$

Wurtzite ferroelectrics (e.g., $\mathrm{Al_{0.93}B_{0.07}N}$) are being explored for high-temperature and emerging near-, or in-compute, memory architectures due to the material advantages offered by their large remanent polarization and robust chemical stability. Despite these advantages, current $\mathrm{Al_{0.93}B_{0.07}N}$ devices do not have sufficient endurance lifetime to meet roadmap targets. To identify the defects responsible for this limited endurance, a combination of electronic measurements and optical spectroscopies characterized the evolution of defect states within $\mathrm{Al_{0.93}B_{0.07}N}$ with cycling. Ultrathin ($\sim$10 nm) metal contacts were used to optically probe regions subject to ferroelectric switching; photoluminescence spectroscopy identified the emergence of a transition near 2.1 eV whose intensity scaled with the non-switching polarization quantified via positive-up negative-down (PUND) measurements. Accompanying thermally stimulated depolarization current (TSDC) and modulus spectroscopy measurements also observed the strengthening of a state near 2.1 eV. The origin of this feature is ascribed to transitions between a nitrogen vacancy and another defect deeper in the bandgap. Recognizing that the impurity concentration is largely fixed, strengthening of this transition indicates an increase in the number of nitrogen vacancies. Switching, therefore, creates vacancies in $\mathrm{Al_{0.93}B_{0.07}N}$ likely due to hot-atom damage induced by the aggressive fields necessary to switch wurtzite materials that ultimately limits endurance.

cond-mat.mtrl-sci↗

arXiv2025

Mixed Finite Element Analysis of Flexoelectric Response: Exploring Unit Cell Stacking and Strain Gradient Modulation

Flexoelectricity, a coupling between strain gradients and electric polarization, has attracted significant interest due to its critical role in enhanced effects at small scales and its applicability across a diverse range of materials. Modeling flexoelectricity is challenging, especially in 3D, due to the higher-order equations involved, which require continuity conditions that limit the use of standard finite element methods. This study employs a modified mixed finite element formulation, specifically designed to account for spurious oscillations that can hinder convergence, particularly in large-scale problems solved with iterative solvers. A numerical analysis has been conducted to evaluate the effectiveness of stacking individual flexoelectric unit cells to achieve an enhanced overall flexoelectric response. The study also seeks to determine the feasibility of locally modifying the strain gradient to induce localized enhancements in the flexoelectric effect.

physics.comp-ph↗

arXiv2024

Reward based optimization of resonance-enhanced piezoresponse spectroscopy

Dynamic spectroscopies in Scanning Probe Microscopy (SPM) are critical for probing material properties, such as force interactions, mechanical properties, polarization switching, and electrochemical reactions and ionic dynamics. However, the practical implementation of these measurements is constrained by the need to balance imaging time and data quality. Signal to noise requirements favor long acquisition times and high frequencies to improve signal fidelity. However, these are limited on the low end by contact resonant frequency and photodiode sensitivity, and on the high end by the time needed to acquire high-resolution spectra, or the propensity for samples degradation under high field excitation over long times. The interdependence of key parameters such as instrument settings, acquisition times, and sampling rates makes manual tuning labor-intensive and highly dependent on user expertise, often yielding operator-dependent results. These limitations are prominent in techniques like Dual Amplitude Resonance Tracking (DART) in Piezoresponse Force Microscopy (PFM) that utilize multiple concurrent feedback loops for topography and resonance frequency tracking. Here, a reward-driven workflow is proposed that automates the tuning process, adapting experimental conditions in real time to optimize data quality. This approach significantly reduces the complexity and time required for manual adjustments and can be extended to other SPM spectroscopic methods, enhancing overall efficiency and reproducibility.

cond-mat.mtrl-sci↗

arXiv2024

Integration of Scanning Probe Microscope with High-Performance Computing: fixed-policy and reward-driven workflows implementation

The rapid development of computation power and machine learning algorithms has paved the way for automating scientific discovery with a scanning probe microscope (SPM). The key elements towards operationalization of automated SPM are the interface to enable SPM control from Python codes, availability of high computing power, and development of workflows for scientific discovery. Here we build a Python interface library that enables controlling an SPM from either a local computer or a remote high-performance computer (HPC), which satisfies the high computation power need of machine learning algorithms in autonomous workflows. We further introduce a general platform to abstract the operations of SPM in scientific discovery into fixed-policy or reward-driven workflows. Our work provides a full infrastructure to build automated SPM workflows for both routine operations and autonomous scientific discovery with machine learning.

cond-mat.mtrl-sci↗

arXiv2024

Electrical Degradation in Dielectric and Piezoelectric Oxides: Review of Defect Chemistry and Associated Characterization Techniques

The properties of dielectric and piezoelectric oxides are determined by their processing history, crystal structure, chemical composition, microstructure, dopants (or defect) distribution, and defect kinetics. These materials are essential in a diverse range of applications including aerospace, medical, military, transportation, power engineering, and communication, where they are used as ceramic discs, thick and thin films, multilayer devices, etc. Significant advances in understanding the materials, processing, properties, and reliability of these materials have led to their widespread use in consumer electronics, military, and aerospace applications. This review delves into electrical degradation in dielectrics and piezoelectrics, focusing on defect chemistry and key characterization techniques. It also provides a detailed discussion of various spectroscopic, microscopic, and electronic characterization techniques essential for analyzing defects and degradation mechanisms.

cond-mat.mtrl-sci↗

arXiv2023

The interplay between ferroelectricity and electrochemical reactivity on the surface of binary ferroelectric Al$_x$B$_{1-x}$N

Polarization dynamics and domain structure evolution in ferroelectric Al$_{0.93}$B$_{0.07}$N are studied using piezoresponse force microscopy and spectroscopies in ambient and controlled atmosphere environments. The application of negative unipolar, and bipolar first-order reverse curve (FORC) waveforms leads to a protrusion-like feature on the Al$_{0.93}$B$_{0.07}$N surface and reduction of electromechanical response due to electrochemical reactivity. A surface change is also observed on the application of fast alternating current bias. At the same time, the application of positive biases does not lead to surface changes. Comparatively in a controlled glove box atmosphere, stable polarization patterns can be observed, with minuscule changes in surface morphology. This surface morphology change is not isolated to applying biases to free surface, a similar topographical change is also observed at the electrode edges when cycling a capacitor in ambient environment. The study suggests that surface electrochemical reactivity may have a significant impact on the functionality of this material in the ambient environment. However, even in the controlled atmosphere, the participation of the surface ions in polarization switching phenomena and ionic compensation is possible.

cond-mat.mtrl-sci↗

arXiv2023

Large Enhancements in Optical and Piezoelectric Properties in Ferroelectric Zn1-xMgxO Thin Films through Engineering Electronic and Ionic Anharmonicities

Multifunctionality as a paradigm requires materials exhibiting multiple superior properties. Integrating second-order optical nonlinearity and large bandgap with piezoelectricity could, for example, enable broadband, strain-tunable photonics. Though very different phenomena at distinct frequencies, both second-order optical nonlinearity and piezoelectricity are third-rank polar tensors present only in acentric crystal structures. However, simultaneously enhancing both phenomena is highly challenging since it involves competing effects with tradeoffs. Recently, a large switchable ferroelectric polarization of ~ 80 uC cm-2 was reported in Zn1-xMgxO films. Here, ferroelectric Zn1-xMgxO is demonstrated to be a platform that hosts simultaneously a 30% increase in the electronic bandgap, a 50% enhancement in the second harmonic generation coefficients, and a near 200% improvement in the piezoelectric coefficients over pure ZnO. These enhancements are shown to be due to a 400% increase in the electronic anharmonicity and a ~200% decrease in the ionic anharmonicity with Mg substitution. Precisely controllable periodic ferroelectric domain gratings are demonstrated down to 800 nm domain width, enabling ultraviolet quasi-phase-matched optical harmonic generation as well as domain-engineered piezoelectric devices.

cond-mat.mtrl-sci↗

arXiv2023

Anomalously Abrupt Switching of Ferroelectric Wurtzites

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 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 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 more complete description of general nucleation and growth kinetics applicable to any system while specifically addressing both the anomalously abrupt polarization reversal behavior in new wurtzite ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2022

Unique Ferroelectric Fatigue Behavior and Exceptional High Temperature Retention in Al0.93B0.07N Films

This paper reports the fatigue and retention behavior for Al1-xBxN thin films, a member of the novel family of wurtzite ferroelectrics, with an emphasis on the role of capacitor architecture. By modifying the capacitor architecture, and thus thermal and electrical boundary conditions, we create insight regarding the relative importance of intrinsic and extrinsic contributors to the degradation tendencies. Our experiments suggest that bipolar cycling of metal (Pt/W)/Al0.93B0.07N/W/Al2O3 film stacks first induced wake-up, then a region of constant switchable polarization. On additional cycling, the film leakage current increased, and then films underwent dielectric breakdown. For unpatterned first generation Al0.93B0.07N films with 100 nm thick Pt top electrodes survive ~104 bipolar cycles, whereas films with 1000 nm W top electrodes survive ~10^5 cycles before thermal dielectric breakdown. Sentaurus modeling was used to design an SU8 field plate which improved the performance to ~10^6 fatigue cycles. It was found that the thermal failures during fatigue were not due to surface flashover events but were associated with hard breakdown events in the dielectric. The films showed excellent retention of the stored polarization state. As expected, data retention was slightly inferior in the opposite state (OS) measurements. However, it is noted that even after 3.6x10^6 sec (1000 hr). at 200°C, the OS signal margin still exceeded 200 uC/cm2. The predicted OS retention is 82% after 10 years baking at 200oC.

physics.app-ph↗

arXiv2022

Automated Experiments of Local Non-linear Behavior in Ferroelectric Materials

We develop and implement 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. 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, potentially, the presence of surface contaminants. Using an automated experiment to probe the origins of non-linear behavior in model ferroelectric lead titanate (PTO) and ferroelectric Al0.93B0.07N films, it was found that PTO showed asymmetric nonlinear behavior across a/c domain walls and a broadened high nonlinear response region around c/c domain walls. In contrast, for Al0.93B0.07N, well-poled regions showed high linear piezoelectric responses paired with low non-linear responses and regions that were multidomain indicated low linear responses and high nonlinear responses. We show 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 this approach automated experiments can potentially discern between competing physical mechanisms. This technique can also be extended to electron, probe, and chemical imaging.

cond-mat.mtrl-sci↗

arXiv2020

Electric field induced metallic behavior in thin crystals of ferroelectric α-In2Se3

Ferroelectric semiconductor field effect transistors (FeSmFETs), which employ ferroelectric semiconducting thin crystals of α-In2Se3 as the channel material as opposed to the gate dielectric in conventional ferroelectric FETs (FeFETs) were prepared and measured from room to the liquid-helium temperatures. These FeSmFETs were found to yield evidence for the reorientation of the electrical polarization and an electric field induced metallic state in α-In2Se3. Our findings suggest that FeSmFETs can serve as a platform for the fundamental study of ferroelectric metals as well as the exploration of the integration of data storage and logic operations in the same device.

cond-mat.mtrl-sci↗