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Kim, T. H.

Publications and source records attributed to Kim, T. H..

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

In-plane quasi-single-domain BaTiO 3 via interfacial symmetry engineering

The control of the in-plane domain evolution in ferroelectric thin films is not only critical to understanding ferroelectric phenomena but also to enabling functional device fabrication. However, in-plane polarized ferroelectric thin films typically exhibit complicated multi-domain states, not desirable for optoelectronic device performance. Here we report a strategy combining interfacial symmetry engineering and anisotropic strain to design single-domain, in-plane polarized ferroelectric BaTiO 3 thin films. Theoretical calculations predict the key role of the BaTiO 3 /PrScO 3 (110) O substrate interfacial environment, where anisotropic strain, monoclinic distortions, and interfacial electrostatic potential stabilize a single-variant spontaneous polarization. A combination of scanning transmission electron microscopy, piezoresponse force microscopy, ferroelectric hysteresis loop measurements, and second harmonic generation measurements directly reveals the stabilization of the in-plane quasi-single-domain polarization state. This work offers design principles for engineering in-plane domains of ferroelectric oxide thin films, which is a prerequisite for high performance optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Isostructural metal-insulator transition in VO 2

The metal-insulator transition in correlated materials is usually coupled to a symmetry-lowering structural phase transition. This coupling not only complicates the understanding of the basic mechanism of this phenomenon but also limits the speed and endurance of prospective electronic devices. We demonstrate an isostructural, purely electronically driven metal-insulator transition in epitaxial heterostructures of an archetypal correlated material, vanadium dioxide. A combination of thin-film synthesis, structural and electrical characterizations, and theoretical modeling reveals that an interface interaction suppresses the electronic correlations without changing the crystal structure in this otherwise correlated insulator. This interaction stabilizes a nonequilibrium metallic phase and leads to an isostructural metal-insulator transition. This discovery will provide insights into phase transitions of correlated materials and may aid the design of device functionalities.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO 3 heterostructures

Exploiting multiferroic BiFeO 3 thin films in spintronic devices requires deterministic and robust control of both internal magnetoelectric coupling in BiFeO 3 , as well as exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer. Previous reports utilized approaches based on multi-step ferroelectric switching with multiple ferroelectric domains. Because domain walls can be responsible for fatigue, contain localized charges intrinsically or via defects, and present problems for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single-step switching is desirable. Here we demonstrate room temperature, deterministic and robust, exchange coupling between monodomain BiFeO 3 films and Co overlayer that is intrinsic (i.e., not dependent on domain walls). Direct coupling between BiFeO 3 antiferromagnetic order and Co magnetization is observed, with ~90° in-plane Co moment rotation upon single-step switching that is reproducible for hundreds of cycles. Finally, this has important consequences for practical, low power non-volatile magnetoelectric devices utilizing BiFeO 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2017

In-situ probing of coupled atomic restructuring and metallicity of oxide heterointerfaces induced by polar adsorbates

Microscopic understanding of the surface-controlled conductivity of the two dimensional electron gas at complex oxide interfaces is crucial for developing functional interfaces. We observe conductivity and structural modification using in-situ synchrotron surface x-ray diffraction as the surface of a model LaAlO 3 /SrTiO 3 (001) heterostructure is changed by polar adsorbates. We find that polar adsorbate-induced interfacial metallicity reduces polar distortions in the LaAlO 3 layer. First-principles density functional theory calculations show that surface dipoles introduced by polar adsorbates lead to additional charge transfer and the reduction of polar displacements in the LaAlO 3 layer, consistent with the experimental observations. Our study supports that internal structural deformations controlling functionalities can be driven without the application of direct electrical or thermal bias and offers a route to tuning interfacial properties. Furthermore, these results also highlight the important role of in-situ x-ray scattering with atomic resolution in capturing and exploring structural distortions and charge density changes caused by external perturbations such as chemical adsorption, redox reaction, and generation and/or annihilation of surface defects.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Polar Metals by Geometric Design

Gauss's law dictates that the net electric field inside a conductor in electrostatic equilibrium is zero by effective charge screening; free carriers within a metal eliminate internal dipoles that may arise owing to asymmetric charge distributions(1). Quantum physics supports this view(2), demonstrating that delocalized electrons make a static macroscopic polarization, an ill-defined quantity in metals(3)-it is exceedingly unusual to find a polar metal that exhibits long-range ordered dipoles owing to cooperative atomic displacements aligned from dipolar interactions as in insulating phases(4). Here we describe the quantum mechanical design and experimental realization of room-temperature polar metals in thin-film ANiO(3) perovskite nickelates using a strategy based on atomic-scale control of inversion-preserving (centric) displacements(5). We predict with ab initio calculations that cooperative polar A cation displacements are geometrically stabilized with a non-equilibrium amplitude and tilt pattern of the corner-connected NiO6 octahedra-the structural signatures of perovskites-owing to geometric constraints imposed by the underlying substrate. Heteroepitaxial thin-films grown on LaAlO3 (111) substrates fulfil the design principles. We achieve both a conducting polar monoclinic oxide that is inaccessible in compositionally identical films grown on (001) substrates, and observe a hidden, previously unreported(6-10), non-equilibrium structure in thin-film geometries. We expect that the geometric stabilization approach will provide novel avenues for realizing new multifunctional materials with unusual coexisting properties.

Kim, T. H.↗
DOE OSTI2016

Visualization of dielectric constant-electric field-temperature phase maps for imprinted relaxor ferroelectric thin films

The dielectric phase transition behavior of imprinted lead magnesium niobate–lead titanate relaxor ferroelectric thin films was mapped as a function of temperature and dc bias. To compensate for the presence of internal fields, an external electric bias was applied while measuring dielectric responses. The constructed three-dimensional dielectric maps provide insight into the dielectric behaviors of relaxor ferroelectric films as well as the temperature stability of the imprint. The transition temperature and diffuseness of the dielectric response correlate with crystallographic disorder resulting from strain and defects in the films grown on strontium titanate and silicon substrates; the latter was shown to induce a greater degree of disorder in the film as well as a dielectric response lower in magnitude and more diffuse in nature over the same temperature region. Strong and stable imprint was exhibited in both films and can be utilized to enhance the operational stability of piezoelectric devices through domain self-poling.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter

Ultrashort flashes of THz light with low photon energies of a few meV, but strong electric or magnetic field transients have recently been employed to prepare various fascinating nonequilibrium states in matter. Here we present a new class of sources based on superradiant enhancement of radiation from relativistic electron bunches in a compact electron accelerator that we believe will revolutionize experiments in this field. Our prototype source generates high-field THz pulses at unprecedented quasi-continuous-wave repetition rates up to the MHz regime. We demonstrate parameters that exceed state-of-the-art laser-based sources by more than 2 orders of magnitude. The peak fields and the repetition rates are highly scalable and once fully operational this type of sources will routinely provide 1 MV/cm electric fields and 0.3 T magnetic fields at repetition rates of few 100 kHz. In conclusion, we benchmark the unique properties by performing a resonant coherent THz control experiment with few 10 fs resolution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Room temperature Ultraviolet B emission from InAlGaN films synthesized by plasma-assisted molecular beam epitaxy

Thin films of the wide bandgap quaternary semiconductor In{sub x}Al{sub y}Ga{sub (1−x−y)}N with low In (x = 0.01–0.05) and high Al composition (y = 0.40–0.49) were synthesized on GaN templates by plasma-assisted molecular beam epitaxy. High-resolution X-ray diffraction was used to correlate the strain accommodation of the films to composition. Room temperature ultraviolet B (280 nm–320 nm) photoluminescence intensity increased with increasing In composition, while the Stokes shift remained relatively constant. The data suggest a competition between radiative and non-radiative recombination occurs for carriers, respectively, localized at centers produced by In incorporation and at dislocations produced by strain relaxation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Observation of three crystalline layers in hydrothermally grown BiFeO{sub 3} thick films

We report the observation of three different crystalline layers in hydrothermally grown BiFeO{sub 3} (BFO) thick films on SrRuO{sub 3}/SrTiO{sub 3} substrates. High-resolution X-ray diffraction and transmission electron microcopy results suggest that compressively strained, partially relaxed epitaxial layers, and a mixture of polycrystalline and amorphous BFO layers, were successively formed from the bottom to the top of the films. The resistance and capacitance of the mixed layer were significantly lower than those of the epitaxial layers. The atomic concentrations of Bi and Fe in the mixed layer were fluctuating for each point. Based on the observed three crystalline layers, we have discussed the growth mechanism and the leakage current of hydrothermally grown BFO thick films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Room temperature photoluminescence from In{sub x}Al{sub (1−x)}N films deposited by plasma-assisted molecular beam epitaxy

InAlN films deposited by plasma-assisted molecular beam epitaxy exhibited a lateral composition modulation characterized by 10–12 nm diameter, honeycomb-shaped, columnar domains with Al-rich cores and In-rich boundaries. To ascertain the effect of this microstructure on its optical properties, room temperature absorption and photoluminescence characteristics of In{sub x}Al{sub (1−x)}N were comparatively investigated for indium compositions ranging from x = 0.092 to 0.235, including x = 0.166 lattice matched to GaN. The Stokes shift of the emission was significantly greater than reported for films grown by metalorganic chemical vapor deposition, possibly due to the phase separation in these nanocolumnar domains. The room temperature photoluminescence also provided evidence of carrier transfer from the InAlN film to the GaN template.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Creation of a two-dimensional electron gas and conductivity switching of nanowires at the LaAlO{sub 3}/SrTiO{sub 3} interface grown by 90{sup o} off-axis sputtering

Two-dimensional electron gas (2DEG) formed at the interface between two oxide band-insulators LaAlO{sub 3} and SrTiO{sub 3} raises the possibility to develop oxide nanoelectronics. Here, we report the creation of a 2DEG at the LaAlO{sub 3}/SrTiO{sub 3} heterointerfaces grown by 90° off-axis sputtering which allows uniform films over a large area. The electrical transport properties of the LaAlO{sub 3}/SrTiO{sub 3} heterointerface are similar to those grown by pulsed laser deposition. We also demonstrate room-temperature conductive probe-based switching of quasi-one-dimensional structures. This work demonstrates that a scalable growth process can be used to create the two-dimensional electron gas system at oxide heterointerfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Multilevel Data Storage Memory Using Deterministic Polarization Control

Multilevel non-volatile memory for high-density date storage is achieved by using the deterministic control of ferroelectric polarization. In a real ferroelectric thin-film system, eight stable and reproducible polarization states are realized (i.e., 3-bit data storage) by adjusting the displacement current. This approach can be used to triple or quadruple the memory density, even at existing feature scales.

36 MATERIALS SCIENCE↗
NASA NTRS2005

In-situ High-energy X-ray Diffraction Study of the Local Structure of Supercooled Liquid Si

While changes in the coordination number for liquid silicon upon supercooling, signaling an underlying liquid-liquid phase transition, have been predicted, x-ray and neutron measurements have produced conflicting reports. In particular some studies have found an increase in the first shell coordination as temperature decreases in the supercooled regime, while others have reported increases in the coordination number with decreasing temperature. Employing the technique of electrostatic levitation coupled with high energy x-ray diffraction (125 keV), and rapid data acquisition (100ms collection times) using an area detector, we have obtained high quality structural data more deeply into the supercooled regime than has been possible before. No change in coordination number is observed in this temperature region, calling into question previous experimental claims of structural evidence for the existence of a liquid-liquid phase transition.

Lee, G. W.↗