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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

On the structural origins of ferroelectricity in HfO{sub 2} thin films

Here, we present a structural study on the origin of ferroelectricity in Gd doped HfO{sub 2} thin films. We apply aberration corrected high-angle annular dark-field scanning transmission electron microscopy to directly determine the underlying lattice type using projected atom positions and measured lattice parameters. Furthermore, we apply nanoscale electron diffraction methods to visualize the crystal symmetry elements. Combined, the experimental results provide unambiguous evidence for the existence of a non-centrosymmetric orthorhombic phase that can support spontaneous polarization, resolving the origin of ferroelectricity in HfO{sub 2} thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Influence of interfacial coherency on ferroelectric switching of superlattice BaTiO 3 /SrTiO 3

Switching behavior of (BaTiO 3 ) 8 /(SrTiO 3 ) 4 heterostructure superlattice grown on SrTiO 3 substrate was studied by employing the phase field method. To investigate the constraint effect of the substrate on switching, three types of superlattice/substrate interface mechanical relaxation conditions were considered, i.e. the fully ommensurate, partially relaxed and fully relaxed. Our simulation results demonstrated that the hysteresis loops under the three types of constraints were very different. The interfacial coherency affects dramatically the coercivity and remanence of the superlattice films. The mechanism of the hysteresis loop varying with interfacial coherency was analyzed by the ferroelectric domain configuration and its evolution during the switching process. Finally, the hysteresis loop of fully relaxed superlattice shows application potential on ferroelectric energy storage materials.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Unusual soft mode dynamics in ferroelectric PbTiO 3 , nanowire under different mechanical boundary conditions

Here, first-principles-based atomistic simulations are used to investigate equilibrium phases and soft mode dynamics in ultrathin ferroelectric PbTiO 3 nanowire with poor surface charge compensation subjected to a wide range of mechanical boundary conditions. The presence of the depolarizing field along the nanowire's transverse directions leads to the appearance of a unique high-frequency hard phonon mode that can be used to characterize electrical boundary conditions. This mode is insensitive to the mechanical load. Hydrostatic pressure was found to significantly influence the Curie point and ferroelectric soft modes in the nanowire. Uniaxial stress applied either along axial or transverse nanowire's direction is capable of inducing polydomain flux-closure phases that have a unique ``dynamical" fingerprint. In such phases the modes that originate from the soft modes of bulk PbTiO 3 become hard, which could open a way to potential identification of such nanodomain phases. In all cases, uniaxial stress significantly increases Curie temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Time-of-flight neutron total scattering with applied electric fields: Ex situ and in situ studies of ferroelectric materials

Characterizing the structural response of functional materials (e.g., piezoelectrics and ferroelectrics) to electric fields is key for the creation of structure-property relationships. Here in this paper, we present a new sample environment and data reduction routines which allow the measurement of time-of-flight neutron total scattering during the in situ or ex situ application of high voltage (<10 kV) to a sample. Instead of utilizing the entire detector space of the diffractometer, only selected regions of detector pixels with scattering at the desired angle to the sample electric field are interrogated, which allows the generation of orientation-dependent reciprocal space patterns and real-space pair distribution functions (PDFs). We demonstrate the method using the relaxor ferroelectric Na 1/2 Bi 1/2 TiO 3 and observe lattice expansion parallel and contraction perpendicular to the electric field for both in situ or ex situ experiments, revealing the irreversible nature of the local scale structural changes to this composition. Additionally, changes in the distributions of nearest neighbor metal-oxygen bond lengths are observed, which have been difficult to observe in previously measured analogous orientation-dependent X-ray PDFs. Lastly, considerations related to sample positioning and background subtraction are discussed, and future research directions are suggested.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Correlation of cation deficiency and nanostructure to decreased magnetism in a ferroelectric BiMnO3 film

The pseudoperovskite BiMnO 3 is a multiferroic candidate, but missing details of materials properties are impeding potential technological applications. BiMnO 3 has a centrosymmetric structure that precludes ferroelectricity in bulk stoichiometric samples, while some films have reported ferroelectricity along with a decreased magnetic response. This puzzle motivated a study of one such film, deposited by pulsed laser deposition onto SrTiO 3 . Probes utilized include microscopy, diffraction, reflectometry, and X-ray absorption. These experiments in the context of the existing literature show an anomalous unit-cell volume and a (magnetic) depth profile. Then, the resulting inhomogeneous deficiency of Bi and Mn metals may stabilize a multiphase system that explains the decreased magnetism. Film nanostructure and strain effects are also considered.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Ferroelectric properties of ion-irradiated bismuth ferrite layers grown via molecular-beam epitaxy

We systematically investigate the role of defects, introduced by varying synthesis conditions and by carrying out ion irradiation treatments, on the structural and ferroelectric properties of commensurately strained bismuth ferrite Bi x Fe 2-x O 3 layers grown on SrRuO 3 -coated DyScO 3 (110) o substrates using adsorption-controlled ozone molecular-beam epitaxy. Our findings highlight ion irradiation as an effective approach for reducing through-layer electrical leakage, a necessary condition for the development of reliable ferroelectrics-based electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2020

A new era in ferroelectrics

Topological structures in ferroic materials have drawn great interest in recent years due to the richness of the underlying physics and the potential for applications in next generation electronics. Recent advances in atomically precise thin-film materials synthesis and characterization of structural/physical phenomena at unprecedented length/energy/time scales have enabled us to study exotic phases and their associated physics [Rößler et al., Nature 442, 797 (2006); S. Das, Nature 568, 368 (2019); Yadav et al., Nature 530, 198 (2016); and Stoica et al., Nat. Mater. 18, 377 (2019)]. It is appropriate that, in the second century of ferroelectrics, some dramatic discoveries are propelling the field into directions heretofore unimaginable. In this review, we explore the recent progress in ferroelectric-oxide superlattices in which researchers can control structure and physical properties through the application of epitaxial strain, layer thickness, temperature, electric field, etc. We provide a discussion of exotic topological structures (e.g., closure domains, vortices, polar skyrmions, and other exotic phases) and associated functionalities in ferroelectric/paraelectric superlattices. We conclude with a brief overview of and prospects for how the field may evolve in the coming years.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Robust low-temperature (350 °C) ferroelectric Hf 0.5 Zr 0.5 O 2 fabricated using anhydrous H 2 O 2 as the ALD oxidant

Here, in this Letter, the robust ferroelectric properties of low-temperature (350 °C) Hf 0.5 Zr 0.5 O 2 (HZO) films are investigated. We demonstrate that the lower crystallization temperature of HZO films originates from a densified film deposition with an anhydrous H 2 O 2 oxidant in the atomic layer deposition process. As a consequence of this densification, H 2 O 2 -based HZO films showed completely crystallinity with fewer defects at a lower annealing temperature of 350 °C. This reduction in the crystallization temperature additionally suppresses the oxidation of TiN electrodes, thereby improving device reliability. The low-temperature crystallization process produces an H 2 O 2 -based HZO capacitor with a high remanent polarization ( P r ), reduced leakage current, high breakdown voltage, and better endurance. Furthermore, while an O 3 -based HZO capacitor requires wake-up cycling to achieve stable P r , the H 2 O 2 -based HZO capacitor demonstrates a significantly reduced wake-up nature. Anhydrous H 2 O 2 oxidant enables the fabrication of a more reliable ferroelectric HZO device using a low process thermal budget (350 °C).

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ferroelectric AlBN films by molecular beam epitaxy

We report the properties of molecular beam epitaxy deposited AlBN thin films on a recently developed epitaxial nitride metal electrode, Nb2N. While a control AlN thin film exhibits standard capacitive behavior, distinct ferroelectric switching is observed in the AlBN films with increasing Boron mole fraction. The measured remnant polarization Pr∼15μC/cm2 and coercive field Ec∼ 1.45 MV/cm in these films are smaller than those recently reported on films deposited by sputtering, due to incomplete wake-up, limited by current leakage. Because AlBN preserves the ultrawide energy bandgap of AlN compared to other nitride hi-K dielectrics and ferroelectrics, and it can be epitaxially integrated with GaN and AlN semiconductors, its development will enable several opportunities for unique electronic, photonic, and memory devices.

Physics↗
DOE OSTI2024

Nonvolatile memory cells from hafnium zirconium oxide ferroelectric tunnel junctions using Nb and NbN electrodes

Ferroelectric tunnel junctions (FTJs) utilizing hafnium zirconium oxide (HZO) have attracted interest as non-volatile memory for microelectronics due to ease of integration into back-end-of-line (BEOL) complementary metal oxide semiconductor fabrication. This work examines asymmetric electrode NbN/HZO/Nb devices with 7 nm thick HZO as FTJs in a memory structure, with an output resistance that can be controlled by read and write voltages. The individual FTJs are measured to have a tunneling electroresistance of 10 during the read state without significant filament conduction formation and reasonable ferroelectric performance. Endurance and remanent polarizations of up to 10 5 cycles and 20 μC/cm 2 , respectively, are measured and are shown to be dependent on the cycling voltage. Electrical measurements demonstrate how magnitude of the write pulse can modulate the high state resistance and the read pulse influences both resistance values as well as separation of resistance states. Then, by using two opposite switching FTJ devices in series, a programmable nonvolatile resistor divider is demonstrated. Measurements of these two FTJ unit memory cells show wide applicability to a BEOL microfabrication process for a re-readable, rewritable, and nonvolatile memory cell.

42 ENGINEERING↗
DOE OSTI2014

Phase transitions and thermal-stress-induced structural changes in a ferroelectric Pb (Zr 0.80 Ti 0. 20 )O 3 single crystal

Zirconium-rich lead-zirconate-titanate (PZT) Pb(Zr 0.80 Ti 0.20 )O 3 single crystal is studied by polarized-Raman scattering experiments as a function of temperature. We show that the crystal possesses complex domain boundaries which show no sign of instability even 200 K above the ferroelectric-to-paraelectric phase transition. The boundary stabilized the domains which had the same orientation after cyclic heating and cooling up to 773 K. The domains transformed normally to the cubic paraelectric phase, but returned to their original state after cooling. Physical mechanism behind the reversible behavior is related to the strain between the boundary region and domain, which forces the domain to grow similarly after each heating and cooling cycle. Electron backscattering diffraction measurements show that macroscopic domains, lengths scale in hundreds of microns, are divided into regions whose Pb-displacements are not correlated. The results are notably interesting for non-volatile memory development as it implies that the original ferroelectric state can be restored after the material has been transformed to a paraelectric phase. Incoherent Pbdisplacements play a role for polarization switching.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Compression mechanisms of ferroelectric PbTiO 3 via high pressure neutron scattering

Switchable atomic displacements generate electric dipole moments in ferroelectric materials utilized in many contemporary devices. Lead titanate, a perovskite oxide with formula PbTiO 3 , has been referred to as a textbook example of a prototype displacive ferroelectric and is a testing platform of widely used models of piezoelectric response of complex solid-solutions. PbTiO 3 has been addressed by experimental and computational studies, often with apparently conflicting conclusions. To date, hydrostatic pressure experiments have been interpreted in terms of a model in which the dipole moments gradually diminish with increasing pressure until a transition to a cubic phase, characterized by a zero average dipole moment, occurs. Additionally, the model unrealistically assumes an even compression of the crystal. Here we show by high-pressure neutron powder diffraction measurements that a fast and slow shrinkage of 12-oxygen cages around Pb and octahedra around Ti, respectively, takes place. A phase diagram consolidating earlier and present results is given.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Ferroelectric Domain Scaling and Switching in Ultrathin BiFeO3 Films Deposited on Vicinal Substrates

We report on electrically switchable polarization and ferroelectric domain scaling over a thickness range of 5-100 nm in BiFeO{sub 3} films deposited on [110] vicinal substrates. The BiFeO{sub 3} films of variable thickness were deposited with SrRuO{sub 3} bottom layer using the pulsed laser deposition technique. The domains are engineered into preferentially oriented patterns due to substrate vicinality along the [110] direction. The domain width scales closely with the square root of film thickness, in agreement with the Landau-Lifschitz-Kittel (LLK) law. Switching spectroscopy piezo-response force microscopy provides clear evidence for the ferroelectric switching behavior in all the films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Search for ferroelectricity in fluoroperovskites: comparison between LiNiF 3 and NaNiF 3

Employing first-principles calculations, we have investigated the possible existence of the ferroelectric instability in the G-type antiferromagnetic NaNiF 3 and LiNiF 3 fluoroperovskites. The behavior of the unstable modes, at the cubic high-symmetry structure, is studied as a function of pressure. This study shows the vibrational landscape and define the conditions to drive the ferroelectricity in these materials, where the A-site dominates the polar instabilities.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Low-temperature processed beta-phase In 2 Se 3 ferroelectric semiconductor thin film transistors

As scaling becomes increasingly difficult, there is growing interest in vertical or three-dimensional stacking of transistors and especially memory. Ferroelectric semiconductor field effect transistors can be key enablers to improve energy efficiency and overall chip and memory performance. In this work, low-temperature processed, back-end-of-the-line compatible transistors were demonstrated by depositing a layered chalcogenide ferroelectric semiconductor, beta-phase In 2 Se 3 , at temperature as low as 400 °C. Top gate n-channel In 2 Se 3 thin film transistors were fabricated with field-effect mobility ∼1 cm 2 V −1 s −1 , and simple polarization switching based memory results are presented.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Propagation of priors for more accurate and efficient spectroscopic functional fits and their application to ferroelectric hysteresis

Multi-dimensional spectral-imaging is a mainstay of the scanning probe and electron microscopies, micro-Raman, and various forms of chemical imaging. In many cases, individual spectra can be fit to a specific functional form, with the model parameter maps, providing direct insight into material properties. Since spectra are often acquired across a spatial grid of points, spatially adjacent spectra are likely to be similar to one another; yet, this fact is almost never used when considering parameter estimation for functional fits. On datasets tried here, we show that by utilizing proximal information, whether it be in the spatial or spectral domains, it is possible to improve the reliability and increase the speed of such functional fits by ~2-3x, as compared to random priors. We explore and compare three distinct new methods: (1) spatially averaging neighborhood spectra, and propagating priors based on functional fits to the averaged case, (2) hierarchical clustering-based methods where spectra are grouped hierarchically based on response, with the priors propagated progressively down the hierarchy, and (3) regular clustering without hierarchical methods with priors propagated from fits to cluster means. Our results highlight that utilizing spatial and spectral neighborhood information is often critical for accurate parameter estimation in noisy environments, which we show for ferroelectric hysteresis loops acquired on a prototypical PbTiO3 thin film with piezoresponse spectroscopy. This method is general and applicable to any spatially measured spectra where functional forms are available. Examples include exploring the superconducting gap with tunneling spectroscopy, using the Dynes formula, or current-voltage curve fits in conductive atomic force microscopy mapping. Here we explore the problem for ferroelectric hysteresis, which, given its large parameter space, constitutes a more difficult task than, for example, fitting current-voltage curves with a Schottky emission formula.

42 ENGINEERING↗
DOE OSTI2019

Lattice dynamics of the hybrid improper ferroelectrics (Ca, Sr) 3 Ti 2 O 7

The structure and lattice dynamics of the hybrid improper ferroelectric compound Ca 3 - x Sr x Ti 2 O 7 (for x = 0 , 0.6 , and 0.9) have been studied with a combination of diffraction, inelastic scattering experiments, Raman spectroscopy, and calorimetry measurements, as well as first-principles simulations. Using inelastic neutron scattering, we have measured the phonon density of states (DOS) for x = 0.9 , which revealed a strong broadening but little change in phonon energies on heating from 10 K to 728 K across the ferroelectric phase transition temperature, T F E . Using inelastic x-ray scattering, the momentum-resolved phonon dispersions were measured from 80 K to 950 K on a single crystal (for x = 0.6 ), and also revealed a strong phonon broadening but a small energy shift for acoustic modes on heating across T F E . Additionally, our Raman measurements (for x = 0.6 ) showed robust rotational and oxygen breathing modes but soft tilt modes, consistent with previous measurements on similar compounds. Our density functional calculations achieve good agreement with both the phonon DOS and dispersions measured. We did not observe any unusual quadratic dispersion for c -polarized transverse acoustic modes, at odds with a recently predicted quasi-2D character, for either undoped ( x = 0 ) or doped ( x = 0.6 ) compounds. 0.6) compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Emergent spin-1 Haldane gap and ferroelectricity in a frustrated spin- 1 2 ladder

We report experimental and theoretical evidence that Rb 2 Cu 2 Mo 3 O 12 has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-12 chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a unique class of ferroelectricity induced by a vector spin chirality order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗