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DOE OSTI2024

Room-Temperature Ferroelectric Epitaxial Nanowire Arrays with Photoluminescence

The development of large-scale, high-quality ferroelectric semiconductor nanowire arrays with interesting light-emitting properties can address limitations in traditional wide-bandgap ferroelectrics, thus serving as building blocks for innovative device architectures and next-generation high-density optoelectronics. Here, we investigate the optical properties of ferroelectric CsGeX 3 (X = Br, I) halide perovskite nanowires that are epitaxially grown on muscovite mica substrates by vapor phase deposition. Detailed structural characterizations reveal an incommensurate heteroepitaxial relationship with the mica substrate. Furthermore, photoluminescence that can be tuned from yellow-green to red emissions by varying the halide composition demonstrates that these nanowire networks can serve as platforms for future optoelectronic applications. In addition, the room-temperature ferroelectricity and ferroelectric domain structures of these nanowires are characterized using second harmonic generation (SHG) polarimetry. Finally, the combination of room-temperature ferroelectricity with photoluminescence in these nanowire arrays unlocks new avenues for the design of novel multifunctional materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2025

Flux-Closure Domain Structures in Ferroelectric K 0.5 Na 0.5 NbO 3 Thin Films

Topological domain structures in ferroelectric materials have garnered increasing attention due to their intriguing physical properties and promising applications. While most existing topological structures in ferroelectric perovskite oxides originate from tetragonal or rhombohedral bulk phases, much less is understood about their counterparts in orthorhombic ferroelectrics. Here, in this work, we employ ferroelectric K 0.5 Na 0.5 NbO 3 (KNN) thin films as a model system and leverage phase-field simulations to theoretically predict the static structures and dynamic behaviors of three types of flux-closure domain configurations: in-plane (Type-I), out-of-plane (Type-II), and superdomain (Type-III) flux-closure structures. We systematically investigate the effects of finite size, misfit strains, and electrical boundary conditions on the formation and switching of these topological structures. For the Type-I structure, size reduction or small misfit strain facilitates a transition of the flux-closure pattern to polar vortices. Type-II structures emerge under open-circuit electrical boundary conditions of the film, forming at the junctions of specific domain walls with the film surface or the film–substrate interface. The formation mechanisms of these two flux-closure structures are rationalized from an energy perspective. We demonstrated switching capabilities of Type-II and Type-III structures by obtaining polarization–electric field hysteresis loops. Our simulations also reveal a reversible electric-field-induced transition between the orthorhombic and rhombohedral ferroelectric phases with a checkerboard domain pattern, during which the integrity of the flux-closure structure is preserved. These findings provide theoretical insights and practical guidance for identifying and manipulating topological structures in low-symmetry ferroelectrics, paving the way for developing energy-efficient microelectronic devices based on topological structures.

P-E loop↗

DOE OSTI2022

Intrinsic ferroelectricity in Y-doped HfO 2 thin films

Ferroelectric HfO 2 -based materials hold great potential for the widespread integration of ferroelectricity into modern electronics due to their compatibility with existing Si technology. Earlier work indicated that a nanometre grain size was crucial for the stabilization of the ferroelectric phase. This constraint, associated with a high density of structural defects, obscures an insight into the intrinsic ferroelectricity of HfO 2 -based materials. Here we demonstrate that stable and enhanced polarization can be achieved in epitaxial HfO 2 films with a high degree of structural order (crystallinity). In this work, an out-of-plane polarization value of 50 μC cm –2 has been observed at room temperature in Y-doped HfO 2 (111) epitaxial thin films, with an estimated full value of intrinsic polarization of 64 μC cm –2 , which is in close agreement with density functional theory calculations. The crystal structure of films reveals the $Pca2_1$ orthorhombic phase with small rhombohedral distortion, underlining the role of the structural constraint in stabilizing the ferroelectric phase. Our results suggest that it could be possible to exploit the intrinsic ferroelectricity of HfO 2 -based materials, optimizing their performance in device applications.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Probing ferroelectric domain structures and their switching dynamics in SrBi2Ta2O9 by in-situ electric biasing in transmission electron microscopy

Abstract Lead-free SrBi 2 Ta 2 O 9 (SBT) has been a promising ferroelectric material for various applications such as electronics and data storage due to its outstanding ferroelectric properties including high fatigue endurance and low leakage current. However, the atomic-scale domain structure and switching dynamics of ferroelectric SBT remain elusive. This study reveals that spontaneous polarization arises from canted bismuth-cation displacements, forming 90° and Ising-type 180° domain walls. Interestingly, topological pairs of ferroelectric vortex and antivortex connect ferroelectric boundaries where three domain walls converge. In situ electrical biasing transmission electron microscopy (TEM) reveals the dominance of 180° switching over 90°, where oxygen octahedral connectivity is protected by ferroelastic energy in the 90° domain wall. Consequently, all 180° domain walls and (anti)vortices are removed, leaving only 90° domain walls in the electrically poled states. Chemical deterioration along domain walls highlights vulnerability of SBT to ferroelectric fatigue. This study provides insight into crucial aspects for practical applications of SBT.

36 MATERIALS SCIENCE↗

DOE OSTI2026

Persistent structural distortion for anticipated improper ferroelectricity in ultrathin h-Lu 1− x Ca x MnO 3 films

Improper ferroelectricity in hexagonal rare-earth manganites (h-RMnO 3 , R = Ho–Lu, Y, Sc) arises from a geometric distortion as the primary order parameter, resilient to depolarizing fields and promising for ultrathin ferroelectric devices. However, the substrate-induced interface clamping effect, which suppresses the geometric distortion in the sub-nanometer regime, has thus far hindered the realization of two-dimensional improper ferroelectrics. This study demonstrates that doping with calcium can enhance ferroelectric structural distortion in h-LuMnO 3 thin films. Compressively strained h-Lu 1−x Ca x MnO 3 (x = 0.1, 0.2, 0.3, 0.4, 0.5) epitaxial thin films were stabilized on sapphire substrates using an h-ScFeO 3 buffer layer. We have found that the interface clamping effect is entirely overcome when the doping concentration reaches x ⩾ 0.2, establishing a potential quasi-2D ferroelectric system with a remarkably high estimated structural transition temperature of larger than 1200 K inferred indirectly from temperature-resolved reflection high-energy electron diffraction. This finding suggests a general strain engineering strategy to enhance improper ferroelectricity in hexagonal manganites.

hexagonal manganites↗

DOE OSTI2022

Chemical control of polarization in thin strained films of a multiaxial ferroelectric: Phase diagrams and polarization rotation

The emergent behaviors in thin films of a multiaxial ferroelectric (FE) due to electrochemical coupling between the rotating polarization and surface ions are explored within the framework of the 2–4 Landau-Ginzburg-Devonshire (LGD) thermodynamic potential combined with the Stephenson-Highland (SH) approach. The combined LGD-SH approach allows us to describe the electrochemical switching and rotation of a polarization vector in a multiaxial ferroelectric film covered by surface ions with a charge density defined by the oxygen pressure. In this work, we calculate phase diagrams, analyze the dependence of polarization components on the applied voltage, and discuss the peculiarities of quasistatic ferroelectric, dielectric, and piezoelectric hysteresis loops in thin strained multiaxial ferroelectric films. The nonlinear surface screening by oxygen ions makes the diagrams very different from the known diagrams of, e.g., strained BaTiO 3 films. Quite unexpectedly, we predict the appearance of ferroelectric reentrant phases. The obtained results point to the possibility to control the appearance and features of ferroelectric, dielectric, and piezoelectric hysteresis in multiaxial FE films covered with surface ions by varying their concentration via the partial oxygen pressure. The LGD-SH description of a multiaxial FE film can be further implemented within the Bayesian optimization framework, paving the way toward predictive materials optimization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Order-Disorder Ferroelectric Transition of Strained SrTiO 3

SrTiO 3 is an incipient ferroelectric that is believed to exhibit a prototype displacive, soft mode ferroelectric transition when subjected to mechanical stress or alloying. We use high-angle annular dark field imaging in scanning transmission electron microscopy to reveal local polar regions in the room temperature, paraelectric phase of strained SrTiO 3 films, which undergo a ferroelectric transition at low temperatures. These films contain nanometer-sized domains in which the Ti columns are displaced. In contrast, these nanodomains are absent in unstrained films, which do not become ferroelectric. Here, the results show that the ferroelectric transition of strained SrTiO 3 is an order-disorder transition. We discuss the impact of the results on the nature of the ferroelectric transition of SrTiO 3 .

36 MATERIALS SCIENCE↗

DOE OSTI2025

First-principles effective Hamiltonian for finite-temperature modeling of nonperovskite ferroelectrics

First-principles-based effective Hamiltonian techniques have been widely employed for over three decades to investigate ferroelectricity and related phenomena in perovskite materials. These techniques offer high accuracy, transferability, compatibility with various finite-temperature algorithms, computational efficiency, and ease in incorporating interactions with external fields. They have been adapted to study diverse phenomena, ranging from topological dipole patterns in ferroelectric nanostructures to multicaloric effects. In this work, we develop an effective Hamiltonian for the nonperovskite ferroelectric HfO 2 (hafnia). Applying this methodology to explore the finite-temperature and finite-electric-field properties of ferroelectric hafnia revealed (1) exceptionally large intrinsic coercive fields, an order of magnitude higher than those observed in perovskite ferroelectrics; (2) their atomistic origin; and (3) the existence of a regime where the relationship between the coercive field and the energy barrier for polarization reversal is counterintuitive. Here, these developments could accelerate progress both in methodological advancements for simulating ferroics and in the atomistic understanding of a broad range of ferroelectrics.

Electric polarization↗

DOE OSTI2020

Structure and Dynamics of Domains in Ferroelectric Nanostructures – Phase-Field Modeling (Final Technical Report)

Domain pattern formation is one of the most common phenomena in nature and is a topic of immense interest in many fields ranging from materials science and physics to chemistry and biology. This DOE sponsored research project explored the basic science concerning the thermodynamic stability of mesoscale polarization domain patterns and their temporal evolution mechanisms during formation and subsequent switching in ferroelectric nanostructures and heterostructures. The project employed the computational phase-field method in combination of microelasticity and electrostatic theories. The research was carried out in close collaboration with a number of experimental groups who used High Resolution Transmission Electron Microscopy (HRTEM), in situ TEM with Scanning Probe Microscopy (SPM), or Piezoresponse Force Microscopy (PFM) to characterize the domain structures and dynamics of ferroelectric thin films and heterostructures and who grow high-quality ferroelectric and multiferroic thin films using advanced growth techniques such as Molecular Beam Epitaxy (MBE), Pulsed Laser Deposition (PLLD), and sputtering. The research efforts of the project help establish the phase-field method as the most powerful method for understanding and predicting domain structures in ferroelectric thin films and nanostructures. The findings of the project led to the basic understanding of stability and switching mechanisms of ferroelectric domains under different mechanical boundary conditions and under either homogeneous capacitor configurations or local fields using metallic probes as electrodes and the emergence of charged domain walls during domain switching. The project predicted the spatial length scales, temperature ranges, and electromechanical conditions for different polar states in heterostructures and guided the discovery of both transient and stable novel polarization states containing vortex lattices in oxide superlattices. The project resulted in 134 journal publications and 6 PhD theses with all the PhD graduates currently working in either academia or industry within the United States. The basic understanding on the stability of mesoscale polar states and pattern evolution achieved by the project improved our ability to control and engineer properties of ferroelectric thin films and heterostructures for potential applications in nanoscale electronic devices.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Nanoscale Ferroelectric Control of Novel Electronic States in Layered Two-Dimensional Materials (Final Report)

In this DOE Early Career project “Nanoscale Ferroelectric Control of Novel Electronic States in Layered Two-Dimensional Materials,” the PI’s group has combined ferroelectric field effect with nanoscale domain imaging and writing to design the electronic and optical properties of two-dimensional (2D) van der Waals materials, including graphene and transition metal dichalcogenides MoS2 and ReS2. The van der Waals materials have been prepared into field effect transistor (FET) devices with ferroelectric gates. Through domain patterning in a ferroelectric polymer PVDF-TrEF top-gate via conductive atomic force microscopy, the team has created programmable Schottky junctions in monolayer MoS 2 , where both barrier height and I-V rectifying polarity can be reconfigured. The transport anisotropy of monolayer to few-layer ReS 2 has been mapped out by defining the entire channel into an insulating state and creating nanoscale conducting paths along different directions through domain writing in the ferroelectric top-gate. The result shows that the conductivity along and perpendicular to the Re-chain can differ by >5.5x10 4 . Theoretical modeling points to the band origin of the transport anomaly and reveals the emergence of a flat band in few-layer ReS 2 . The interfacial epitaxial relation between ReS 2 and PVDF-TrFE further promotes the formation of close-packed, highly ordered PVDF-TrFE nanowires with width of 35 nm and 10 nm. Nonvolatile modulation of quantum Hall effect has been achieved in graphene FETs with a ferroelectric oxide Ba 0.4 Sr 0.6 TiO 3 back-gate. Scattering from the remote surface optical phonon in Ba 0.4 Sr 0.6 TiO 3 limits the room temperature mobility of graphene to be about 3x10 4 cm 2 /Vs. Steep-slope switching has been achieved in MoS 2 FETs back-gated by polycrystalline Pb(Zr,Ti)O 3 , which signals a static-state negative capacitance mode without involving an additional dielectric layer. Piezoresponse force microscopy studies show that the sub-threshold swing can be well correlated with the domain wall density in Pb(Zr,Ti)O 3 . The team also observes an unconventional filtering effect of the second harmonic generation response at the MoS 2 /Pb(Zr,Ti)O 3 heterointerface, which can be accounted for by the alignment between one of the polar axes of MoS 2 and the chiral dipole rotation at the surface of domain wall in Pb(Zr,Ti)O 3 . The research supported by this DOE grant has significantly advanced the fundamental understanding and functional design of ferroelectric/2D van der Waals heterostructures for their implementation towards energy applications.

36 MATERIALS SCIENCE↗

DOE OSTI2023

A Landau-Devonshire Approach of Elastic Strain Effects on Ferroelectric Al1-xScxN

Landau-Devonshire thermodynamic modeling enables the phenomenological description of ferroelectrics. 1 The approach has been validated across a wide range of ferroelectric materials from the first ferroelectric discovered, Rochelle Salt, to recent HfO2-based materials. 2-4 Extension to what is now referred to as the Landau-Ginsburg-Devonshire model enables rigorous investigation of polarization boundaries and reorientation, phase transformations, and domain formation/evolution. We present a thermodynamic analysis of the recently discovered nitride ferroelectric materials 5-7 using the classic Landau-Devonshire approach. The electrostrictive and dielectric stiffness coefficients of Al1-xScxN with wurtzite structure (6mm) are determined using a free energy density function assuming a hexagonal parent phase (6/mmm), with the first order phase transition based on the dielectric stiffness relationships (Fig. 1). The results of this analysis show that the strain sensitivity of the energy barrier is one order of magnitude larger than that of the spontaneous polarization in these wurtzite ferroelectrics, yet both are less sensitive to strain compared to classic perovskite ferroelectrics (Fig. 2). These analysis results reported here explain experimentally reported sensitivity of coercive field to elastic strain/stress in Al1-xScxN films, and would enable further thermodynamic analysis via phase field simulation and related methods.

CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS↗

DOE OSTI2022

Thin‐Film Ferroelectrics

Abstract Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial‐thin‐film‐based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro‐, meso‐, and macroscopic length scales. This review traces the evolution of ferroelectric thin‐film research through the early days developing understanding of the roles of size and strain on ferroelectrics to the present day, where such understanding is used to create complex hierarchical domain structures, novel polar topologies, and controlled chemical and defect profiles. The extension of epitaxial techniques, coupled with advances in high‐throughput simulations, now stands to accelerate the discovery and study of new ferroelectric materials. Coming hand‐in‐hand with these new materials is new understanding and control of ferroelectric functionalities. Today, researchers are actively working to apply these lessons in a number of applications, including novel memory and logic architectures, as well as a host of energy conversion devices.

Fernandez, Abel↗

DOE OSTI2024

Highly Responsive Polar Vortices in All‐Ferroelectric Heterostructures

Abstract The discovery of polar vortices and skyrmions in ferroelectric‐dielectric superlattices [such as (PbTiO 3 ) n /(SrTiO 3 ) n ] has ushered in an era of novel dipolar topologies and corresponding emergent phenomena. The key to creating such emergent features has generally been considered to be related to counterpoising strongly polar and non‐polar materials thus creating the appropriate boundary conditions. This limits the utility these materials can have, however, by rendering (effectively) half of the structure unresponsive to applied stimuli. Here, using advanced thin‐film deposition and an array of characterization and simulation approaches, polar vortices are realized in all‐ferroelectric trilayers, multilayers, and superlattices built from the fundamental building block of (PbTiO 3 ) n /(Pb x Sr 1− x TiO 3 ) n wherein in‐plane ferroelectric polarization in the Pb x Sr 1− x TiO 3 provides the appropriate boundary conditions. These superlattices exhibit substantially enhanced electromechanical and ferroelectric responses in the out‐of‐plane direction that arise from the ability of the polarization in both layers to rotate to the out‐of‐plane direction under field. In the in‐plane direction, the layers are found to be strongly coupled during switching and when heterostructured with ferroelectric‐dielectric building blocks, it is possible to produce multistate switching. This approach expands the realm of systems supporting emergent dipolar texture formation and does so with entirely ferroelectric materials thus greatly improving their responses.

Kavle, Pravin↗

DOE OSTI2021

Unique Features of Polarization in Ferroelectric Ionic Conductors

Abstract Ferroelectrics that are also ionic conductors offer possibilities for novel applications with high tunability, especially if the same atomic species causes both phenomena. In particular, at temperatures just below the Curie temperature, polarized states may be sustainable as the mobile species is driven in a controlled way over the energy barrier that governs ionic conduction, resulting in unique control of the polarization. This possibility is recently demonstrated in CuInP 2 S 6 , a layered ferroelectric ionic conductor in which Cu ions cause both ferroelectricity and ionic conduction. Here, it is shown that the commonly used approach to calculate the polarization of evolving atomic configurations in ferroelectrics using the modern theory of polarization, namely concerted (synchronous) migration of the displacing ions, is not well suited to describe the polarization evolution as the Cu ions cross the van der Waals gaps. An asynchronous Cu‐migration scheme is introduced, which reflects the physical process by which Cu ions migrate, resolves the difficulties, and describes the polarization evolution both for normal ferroelectric switching and for transitions across the van der Waals gaps, providing a single framework to discuss ferroelectric ionic conductors.

O'Hara, Andrew↗

DOE OSTI2020

Revealing Antiferroelectric Switching and Ferroelectric Wakeup in Hafnia by Advanced Piezoresponse Force Microscopy

Hafnium oxide (HfO 2 )-based ferroelectrics offer remarkable promise for memory and logic devices in view of their compatibility with traditional silicon complementary metal oxide semiconductor (CMOS) technology, high switchable polarization, good endurance, and thickness scalability. These factors have led to a steep rise in the level of research on HfO 2 over the past number of years. While measurements on capacitors are promising for understanding macroscopic effects, many open questions regarding the emergence of ferroelectricity and electric field cycling behaviors remain. Continued progress requires information regarding the nanoscale ferroelectric behaviors on the bare surface (i.e., without encapsulation), which is notably absent. To overcome this barrier, we have applied complementary modes of piezoresponse force microscopy with the goal of directly and quantitatively sensing nanoscale ferroelectric behaviors in bare HfO 2 thin films. Our results on 8 nm Si-doped HfO 2 reveal nanoscale domains of local remnant polarization states exhibiting a weak piezoelectric coupling (deff) in the range 0.6–1.5 pm/V. While we observed localized enhancement of deff during progressive stressing of the bare HfO 2 thin film, we did not detect stable polarization switching which is a prerequisite of ferroelectric switching. We find that this result could be explained using polarization switching spectroscopy which revealed antiferroelectric-like switching in the form of pinched hysteresis loops as well as increasing remnant response with repeated cycling. As such, our results offer a promising route for material scientists who want to explore the nanoscale origins of antiferroelectricity and ferroelectric wakeup in HfO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Tuning the interfacial spin-orbit coupling with ferroelectricity

Detection and manipulation of spin current lie in the core of spintronics. Here we report an active control of a net spin Hall angle, θ SHE (net), in Pt at an interface with a ferroelectric material PZT (PbZr 0.2 Ti 0.8 O 3 ), using its ferroelectric polarization. The spin Hall angle in the ultra-thin Pt layer is measured using the inverse spin Hall effect with a pulsed tunneling current from a ferromagnetic La 0.67 Sr 0.33 MnO 3 electrode. The effect of the ferroelectric polarization on θ SHE (net) is enhanced when the thickness of the Pt layer is reduced. When the Pt layer is thinner than 6 nm, switching the ferroelectric polarization even changes the sign of θ SHE (net). This is attributed to the reversed polarity of the spin Hall angle in the 1 st -layer Pt at the PZT/Pt interface when the ferroelectric polarization is inverted, as supported by the first-principles calculations. These findings suggest a route for designing future energy efficient spin-orbitronic devices using ferroelectric control.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

A combinatory ferroelectric compound bridging simple ABO 3 and A-site-ordered quadruple perovskite

The simple ABO 3 and A-site-ordered AA' 3 B 4 O 12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg 3 Ti 4 O 12 , the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg 2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg 3 Ti 4 O 12 . Thus PbHg 3 Ti 4 O 12 appears to be a combinatory link from simple ABO 3 perovskites to A-site-ordered AA' 3 Ti 4 O 12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA' 3 B 4 O 12 -type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.

36 MATERIALS SCIENCE↗

DOE OSTI2022

The role of lattice dynamics in ferroelectric switching

Reducing the switching energy of ferroelectric thin films remains an important goal in the pursuit of ultralow-power ferroelectric memory and logic devices. Here, we elucidate the fundamental role of lattice dynamics in ferroelectric switching by studying both freestanding bismuth ferrite (BiFeO 3 ) membranes and films clamped to a substrate. We observe a distinct evolution of the ferroelectric domain pattern, from striped, 71° ferroelastic domains (spacing of ~100 nm) in clamped BiFeO 3 films, to large (10’s of micrometers) 180° domains in freestanding films. By removing the constraints imposed by mechanical clamping from the substrate, we can realize a ~40% reduction of the switching voltage and a consequent ~60% improvement in the switching speed. Our findings highlight the importance of a dynamic clamping process occurring during switching, which impacts strain, ferroelectric, and ferrodistortive order parameters and plays a critical role in setting the energetics and dynamics of ferroelectric switching.

36 MATERIALS SCIENCE↗