Nanoscale Ferroelectric Domains Research Articles

Nanoscale ferroelectric domain switching and thickness scaling impact in undoped hafnium-oxide ferroelectric devices

In this work, we investigated the polarization characteristics of nanoscale undoped hafnium oxide (HfO2) ferroelectric film. The nanoscale domain switching behavior of undoped HfO2 is mainly dominated by mixed ferroelectric/nonferroelectric crystalline phases and interface domain pinning by oxygen vacancies or defect traps. Our experimental results confirm that these issues can be improved by film thickness scaling and well-controlled mechanical stress. We demonstrated a 4-nm-thick undoped HfO2 negative capacitance thin film transistor (NCFET) with an energy-efficient switch characteristic of a low overdrive voltage of -0.6 V, a steep subthreshold swing of sub-60 mV/dec and a uniform hysteresis distribution of sub-60 mV.

Bend-Induced Ferroelectric Domain Walls in α-In2Se3.

The low bending stiffness of atomic membranes from van der Waals ferroelectrics such as α-In2Se3 allow access to a regime of strong coupling between electrical polarization and mechanical deformation at extremely high strain gradients and nanoscale curvatures. Here, we investigate the atomic structure and polarization at bends in multilayer α-In2Se3 at high curvatures down to 0.3 nm utilizing atomic-resolution scanning transmission electron microscopy, density functional theory, and piezoelectric force microscopy. We find that bent α-In2Se3 produces two classes of structures: arcs, which form at bending angles below ∼33°, and kinks, which form above ∼33°. While arcs preserve the original polarization of the material, kinks contain ferroelectric domain walls that reverse the out-of-plane polarization. We show that these kinks stabilize ferroelectric domains that can be extremely small, down to 2 atoms or ∼4 Å wide at their narrowest point. Using DFT modeling and the theory of geometrically necessary disclinations, we derive conditions for the formation of kink-induced ferroelectric domain boundaries. Finally, we demonstrate direct control over the ferroelectric polarization using templated substrates to induce patterned micro- and nanoscale ferroelectric domains with alternating polarization. Our results describe the electromechanical coupling of α-In2Se3 at the highest limits of curvature and demonstrate a strategy for nanoscale ferroelectric domain patterning.

Dielectric and ferroelectric characteristics of Ba(Ti0.25Zr0.25Hf0.25Sn0.25)O3 high-entropy ceramics

Ba(Ti0.25Zr0.25Hf0.25Sn0.25)O3 high-entropy ceramics were prepared by a standard solid state reaction process, and the dielectric and ferroelectric characteristics were investigated together with the structures. Both X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis demonstrated a single-phase perovskite structure in the present ceramics. A broad dielectric peak with strong frequency dispersion feature was determined, which indicated the typical relaxor nature originating from the nanoscale ferroelectric domain structures. These resulted from the structural distortion and chemical disorder due to high-entropy, where the long-range order of ferroelectric domains was destroyed. The homogeneous microstructure led to the reduced leakage current density and significantly improved dielectric strength, which was desired for the practical applications. Compared with the similar systems of Ba(Ti1-xZrx)O3 & Ba(Ti1-xSnx)O3, the present high-entropy ceramics indicated better relaxor ferroelectric characteristics.

Stability of ferroelectric bubble domains

Nanoscale ferroelectric topologies such as vortices, antivortices, bubble patterns, etc., are stabilized in thin films by a delicate balance of both mechanical and electrical boundary conditions. A systematic understanding of the phase stability of bubble domains, particularly when the above factors act simultaneously, remains elusive. Here we present first-principle-based simulations in combination with scanning probe microscopy of ultrathin epitaxial (001) $\mathrm{Pb}{\mathrm{Zr}}_{0.4}{\mathrm{Ti}}_{0.6}{\mathrm{O}}_{3}$ heterostructures to address this gap. The simulations predict that as-grown labyrinthine domains will transform to bubbles under combinations of reduced film thickness, increased mechanical pressure, and/or improved electrical screening. These topological transitions are explained by a common fundamental mechanism. Namely, we argue that, independently of the nature of the driving force, the evolution of the domain morphology allows the system to conserve its original residual depolarization field. Thereby, the latter remains pinned to a value determined by an external or built-in electric bias. To verify our predictions, we then exploit tomographic atomic force microscopy to achieve the concurrent effect of reducing film thickness and increased mechanical stimulus. The results provide a systematic understanding of phase stability and demonstrate controlled manipulation of nanoscale ferroelectric bubble domains.

Thermal stability of nanoscale ferroelectric domains by molecular dynamics modeling

Ultra-dense domain walls are increasingly important for many devices but their microscopic properties are so far not fully understood. In this study we combine atomistic and coarse-grained molecular dynamic simulations to study the domain wall stability in the prototypical ferroelectric BaTiO3. We transfer the discussion of the field-driven nucleation and motion of domain walls to thermally induced modifications of the wall without an external driving force. Our simulations show that domain wall dynamics and stability depend crucially on microscopic thermal fluctuations. Enhanced fluctuations at domain walls may result in the formation of critical nuclei for the permanent shift of the domain wall. If two domain walls are close - put in other words, when domains are small - thermal fluctuations can be sufficient to bring domain walls into contact and lead to the annihilation of small domains. This is even true well below the Curie temperature and when domain walls are initially as far apart as 6 unit cells. Such small domains are, thus, not stable and limit the maximum achievable domain wall density in nanoelectronic devices.

Nano-scale ferroelectric domain differentiation in periodically poled lithium niobate with auger electron spectroscopy

A new method for characterizing lithium niobate +/-Z ferroelectric polarization domains using Auger electron spectroscopy (AES) is presented. The domains of periodically poled samples are found to be differentiable using the peak amplitude of the Auger oxygen KLL transition, with -Z domains having a larger peak-amplitude as compared to +Z domains. The peak amplitude separation between domains is found to be dependent on the primary beam current, necessitating a balance between the insulating samples charging under the primary beam and achieving sufficient signal to noise in amplitude separation. AES amplitude-based domain characterization is demonstrated for fields of view (FOV) ranging from 1 μ m to 78 μ m. Domain spatial resolution of 91 nm is demonstrated at 1 μ m FOV.

Force‐Electric Coupling of Nanoscale Ferroelectric Domains Based on Piezoelectric Force Microscopy (PFM)

Piezoelectric and ferroelectric materials are widely used in various types of microelectronics due to their excellent mechanical and electrical coupling characteristics. In recent years, piezoelectric force microscopy has developed into a powerful tool for analyzing nanoscale ferroelectric materials. However, quantitative analysis based on PFM is difficult to properly study it. This article studies the related issues of PFM quantitative analysis. First, the relationship between the effective piezoelectric coefficient and piezoelectric coefficient of different materials is analyzed from the PFM nanometer scale to analyze the force‐electric coupling effect. The study found that the effective piezoelectric coefficient is closely related to the intrinsic electroelastic constant of the material. Secondly, the analysis of the nanoscale piezoelectric deformation of ferroelectric materials shows that under the conductive SPM probe, as the clustering with the probe increases, the in‐plane displacement first increases and then decreases, and the out‐of‐plane displacement gradually decreases. Finally, the half‐width region of the nanoscale ferroelectric response domain was analyzed by PFM. Taking the in‐plane and out‐of‐plane domains of 180° and 90° domains as examples, the relationship between the response boundary domain and the tip radius was analyzed, and the results showed that whether the PFM can effectively solve the problem of detection and analysis depends on the half‐width of the response boundary domain, and the resolution of the vertical PFM is higher than that of the lateral PFM.

Investigation on microscopic mechanisms of high-Curie temperature PMN-PH-PT piezoelectric ceramics

High-Curie temperature (TC) piezoelectric ceramics 0.15Pb(Mg1/3Nb2/3)O3-0.38PbHfO3-0.47PbTiO3 (0.15PMN-0.38PH-0.47PT) were fabricated via the citrate method, which shows pure perovskite structure. The ceramics have composition locating at the rhombohedral side around the morphotropic phase boundary (MPB), and present enhanced electrical properties as compared with those prepared by the solid-state reaction method via the columbite precursor technique. Temperature-dependent Raman spectroscopy not only proves the occurrence of the ferroelectric to paraelectric phase transition around TC, but also detects the successive phase symmetry transitions, which correlate with the polar nanoregions (PNRs) or the coexistence of multiple ferroelectric phases. Large quantities of fine stripe nanoscale ferroelectric domains are observed by piezoresponse force microscopy (PFM) in the 0.15PMN-0.38PH-0.47PT ceramics, which form the larger micron island domains. Temperature-dependent Raman spectra and PFM results indicate that the excellent dielectric, ferroelectric and piezoelectric properties of the 0.15PMN-0.38PH-0.47PT ceramics can be attributed to either the existence of the PNRs with low symmetry or the multiple-ferroelectric-phases coexistence around room temperature and the fine stripe ferroelectric domains.

Local ferroelectric polarization switching driven by nanoscale distortions in thermoelectric {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}}

A remarkable decrease in the lattice thermal conductivity and enhancement of thermoelectric figure of merit were recently observed in rock-salt cubic SnTe, when doped with germanium (Ge). Primarily, based on theoretical analysis, the decrease in lattice thermal conductivity was attributed to local ferroelectric fluctuations induced softening of the optical phonons which may strongly scatter the heat carrying acoustic phonons. Although the previous structural analysis indicated that the local ferroelectric transition temperature would be near room temperature in {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}}, a direct evidence of local ferroelectricity remained elusive. Here we report a direct evidence of local nanoscale ferroelectric domains and their switching in {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}} using piezoeresponse force microscopy(PFM) and switching spectroscopy over a range of temperatures near the room temperature. From temperature dependent (250–300 K) synchrotron X-ray pair distribution function (PDF) analysis, we show the presence of local off-centering distortion of Ge along the rhombohedral direction in global cubic {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}}. The length scale of the {text {Ge}}^{2+} off-centering is 0.25–0.10 Å near the room temperatures (250–300 K). This local emphatic behaviour of cation is the cause for the observed local ferroelectric instability, thereby low lattice thermal conductivity in {text {Sn}}_{0.7}{text {Ge}}_{0.3}{text {Te}}.

Crystal Structure and Ferroelectric Evidence of BaZnSi3 O8 , a Low-Permittivity Microwave Dielectric Ceramic.

BaZnSi3 O8 ceramic was prepared by the conventional solid-state method and sintered at 1100 °C. XRD and synchrotron Rietveld refinement analyses revealed the BaZnSi3 O8 ceramic presented a monoclinic structure with a space group of P21 /a (No.14), which is reported for the first time. The BaZnSi3 O8 ceramic presented a weak ferroelectricity, which was confirmed by the P-E loop and the 90° nanoscale ferroelectric domain. Although ϵr -T displayed two ϵr abnormal peaks at 400 °C and 460 °C, the Curie temperature (Tc ) was located at 460 °C according to the dielectric loss and Curie-Weiss law. Moreover, the BaZnSi3 O8 ceramic exhibited optimized microwave dielectric properties with ϵr =6.55, Q×f=52400 GHz, and τf =-24.5 ppm/°C. Hence, the BaZnSi3 O8 ceramic in the ternary BaO-ZnO-SiO2 system possessed both weak ferroelectricity and microwave dielectric properties. These results are expected to break the technical barrier of ferroelectric phase shifter applications in microwave and even millimeter-wave frequency bands.

Image Distortion When Detecting Nano-Scale Ferroelectric Domains with Piezoelectric Force Microscope Based on Depth Image Segmentation

Image Distortion When Detecting Nano-Scale Ferroelectric Domains with Piezoelectric Force Microscope Based on Depth Image Segmentation

Ferroelastic‐Domain‐Assisted Mechanical Switching of Ferroelectric Domains in Pb(Zr,Ti)O3 Thin Films

AbstractA recent breakthrough in mechanical polarization switching provides a valuable handle to achieve nanoscale ferroelectric domain control. This flexoelectric switching is usually observed in ultrathin films (≈10 nm or less in thickness), where a large strain gradient is possible. However, from the point of view of device applications, it will be more attractive to achieve mechanical domain switching in thicker films. Here, it is experimentally demonstrated that by introducing ferroelastic a‐domains in PbZr0.1Ti0.9O3 films the potential barrier against 180° c‐domain switching can be greatly decreased, enabling mechanical ferroelectric domain switching in 50 nm thick films by applying a loading force from an atomic force microscope tip. Moreover, these a‐domains are stable in the c‐domain matrix without further mechanical stressing. This makes it possible to create nanoscale domain wall circuitry. These results shed light on the mechanism of domain switching in ferroelectric thin films and may facilitate the design of mechanically controlled novel ferroelectric devices.

Investigation of polarization switching processes in 0.8PMN-0.2PT single crystal in a temperature range from 100 K to 300 K

Ferroelectric hysteresis loops were measured and the temperature evolution of the coercive field traced in [110]-oriented 0.8Pb(Mb1/3Nb2/3)O3–0.2PbTiO3 single crystal in the temperature range from 120 to 300 K at frequencies from 2 to 50 Hz. It was found that the coercive field rised with decreasing temperature in power-law manner at all frequencies. Temperature-dependent hysteresis model was used for approximation of acquired coercive field-temperature dependences. It was shown that nanoscale ferroelectric domains could be created at PMN-20PT surface by voltage pulses applied between the tip of atomic force microscope and bottom electrode at temperatures as low as 100 K.

High Thermoelectric Performance Achieved in GeTe–Bi2Te3 Pseudo‐Binary via Van der Waals Gap‐Induced Hierarchical Ferroelectric Domain Structure

AbstractGeTe is an interesting material presenting both spontaneous polarization (ferroelectrics) and outstanding electrical conductivity (ideal for thermoelectrics). Pristine GeTe exhibits classic 71° and 109° submicron ferroelectric domains, and near unity thermoelectric figure of merit ZT at 773 K. In this work, it is demonstrated that Bi2Te3 alloying in GeTe lattice can introduce vast Ge vacancies which can further evolve into nanoscale van der Waals gaps upon proper heat treatment, and that these vacancy gaps can induce 180° nanoscale ferroelectric domain boundaries. These microstructures eventually become a hierarchical ferroelectric domain structure, with size varying from submicron to nanoscale and polarization from 71°, 109° to 180°. The establishment of hierarchical ferroelectric domain structure, together with the nanoscale Ge vacancy van der Waals gaps, has profound effects on the electrical and thermal transport properties, resulting in a striking peak thermoelectric ZT ≈ 2.4 at 773 K. These findings might provide an alternative conception for thermoelectric optimization via microstructure modulation.

Ferroelectric domain engineering of lithium niobate single crystal confined in glass

Abstract

Architecture of nanoscale ferroelectric domains in GaMo4S8

Local-probe imaging of the ferroelectric domain structure and auxiliary bulk pyroelectric measurements were conducted at low temperatures with the aim to clarify the essential aspects of the orbitally driven phase transition in GaMo4S8, a lacunar spinel crystal that can be viewed as a spin-hole analogue of its GaV4S8 counterpart. We employed multiple scanning probe techniques combined with symmetry and mechanical compatibility analysis to uncover the hierarchical domain structures, developing on the 10-100 nm scale. The identified domain architecture involves a plethora of ferroelectric domain boundaries and junctions, including primary and secondary domain walls in both electrically neutral and charged configurations, and topological line defects transforming neutral secondary walls into two oppositely charged ones.

Far-infrared reflectivity spectra of nanotwinned GaV4Se8

ABSTRACT GaV Se is one of the lacunar spinel materials able to host magnetic skyrmionic textures. Earlier investigations at temperatures below its Jahn–Teller ferroelectric phase transition disclosed a nanoscale ferroelectric domain separated by 109 domain walls. Here, we have investigated the impact of this nanoscale twinning on polar phonon modes. We report dielectric and reflectivity spectra of GaV Se crystal in the phonon frequency region, derived from the ab initio predicted phonon frequencies, eigenvectors and Born-effective charges. Reflectivity spectra of the nanotwinned crystal, calculated using exact expressions for the effective macroscopic dielectric response of a perfect regular twin, are compared with the predicted reflectivity spectra of single-domain crystal. A characteristic spectral feature caused by the nanoscale twinning has been identified. While this feature is not a well-defined geometric resonance, it may still be accessible in experiments.

Selective control of multiple ferroelectric switching pathways using a trailing flexoelectric field.

Flexoelectricity is an electromechanical coupling between electrical polarization and a strain gradient 1 that enables mechanical manipulation of polarization without applying an electrical bias2,3. Recently, flexoelectricity was directly demonstrated by mechanically switching the out-of-plane polarization of a uniaxial system with a scanning probe microscope tip3,4. However, the successful application of flexoelectricity in low-symmetry multiaxial ferroelectrics and therefore active manipulation of multiple domains via flexoelectricity have not yet been achieved. Here, we demonstrate that the symmetry-breaking flexoelectricity offers a powerful route for the selective control of multiple domain switching pathways in multiaxial ferroelectric materials. Specifically, we use a trailing flexoelectric field that is created by the motion of a mechanically loaded scanning probe microscope tip. By controlling the SPM scan direction, we can deterministically select either stable 71° ferroelastic switching or 180° ferroelectric switching in a multiferroic magnetoelectric BiFeO3 thin film. Phase-field simulations reveal that the amplified in-plane trailing flexoelectric field is essential for this domain engineering. Moreover, we show that mechanically switched domains have a good retention property. This work opens a new avenue for the deterministic selection of nanoscale ferroelectric domains in low-symmetry materials for non-volatile magnetoelectric devices and multilevel data storage.

Nanoscale Bubble Domains and Topological Transitions in Ultrathin Ferroelectric Films.

Observation of a new type of nanoscale ferroelectric domains, termed as "bubble domains"-laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix-is reported. The bubble domains appear in ultrathin epitaxial PbZr0.2 Ti0.8 O3 /SrTiO3 /PbZr0.2 Ti0.8 O3 ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel-Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

Nonvolatile ferroelectric domain wall memory.

Ferroelectric domain walls are atomically sharp topological defects that separate regions of uniform polarization. The discovery of electrical conductivity in specific types of walls gave rise to "domain wall nanoelectronics," a technology in which the wall (rather than the domain) stores information. This paradigm shift critically hinges on precise nanoengineering of reconfigurable domain walls. Using specially designed nanofabricated electrodes and scanning probe techniques, we demonstrate a prototype nonvolatile ferroelectric domain wall memory, scalable to below 100 nm, whose binary state is defined by the existence or absence of conductive walls. The device can be read out nondestructively at moderate voltages (<3 V), exhibits relatively high OFF-ON ratios (~103) with excellent endurance and retention characteristics, and has multilevel data storage capacity. Our work thus constitutes an important step toward integrated nanoscale ferroelectric domain wall memory devices.