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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

The effect of interface oxygen content on magnetoelectric effect of epitaxial La{sub 0.7}Sr{sub 0.3}MnO{sub 3}/BaTiO{sub 3} bilayer

The epitaxial La{sub 0.7}Sr{sub 0.3}MnO{sub 3}/BaTiO{sub 3} (LSMO/BTO) bilayer films are grown on (001) oriented LaAlO{sub 3} substrate by pulsed laser deposition technique. The oxygen-rich interface is obtained through in-situ annealing process in oxygen, and the oxygen-deficient interface is obtained without the annealing process. The results show that the ferromagnetic properties of the LSMO film and the magnetoelectric effect (ME) of the bilayer films strongly correlate to the oxygen content at the interface of LSMO/BTO. The saturated magnetization and the ME voltage coefficient of the oxygen-rich bilayer film are higher than that of oxygen-deficient one. It suggests a more effective ME coupling at the LSMO/BTO interface, which are generated through not only the interface strain but also the spin polarized carriers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spontaneous emission of Bloch oscillation radiation under the competing influences of microcavity enhancement and inhomogeneous interface degradation

A theory for the spontaneous emission (SE) of terahertz radiation for a Bloch electron traversing a single energy miniband of a superlattice (SL) in a cavity, while undergoing elastic scattering is presented. The Bloch electron is accelerated under the influence of a superimposed external constant electric field and an internal inhomogeneous electric field, while radiating into a microcavity. The analysis of the SE accounts for both the spectral structure of nonharmonic miniband components and the Bloch oscillation degradation effects arising from elastic scattering due to SL interface roughness. The interface roughness effects are decomposed into contributions arising from independent planar and cross-correlated neighboring planar interfaces; parametric numerical estimates show that the cross-correlated contribution to the SE relaxation rate is relatively small, representing less than roughly 10% of the total relaxation rate. It is shown that the degradation effects from SL interface roughness can be more than compensated for by the enhancements derived from microcavity-based tuning of the emission frequency to the cavity density of states peak. The theoretical approach developed herein has general applicability beyond its use for elastic scattering due to interface roughness. As well, the results obtained in this analysis can be useful in the development of SL-based Bloch-oscillator terahertz devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Initial stages of ITO/Si interface formation: In situ x-ray photoelectron spectroscopy measurements upon magnetron sputtering and atomistic modelling using density functional theory

Initial stages of indium tin oxide (ITO) growth on a polished Si substrate upon magnetron sputtering were studied experimentally using in-situ x-ray photoelectron spectroscopy measurements. The presence of pure indium and tin, as well as Si bonded to oxygen at the ITO/Si interface were observed. The experimental observations were compared with several atomistic models of ITO/Si interfaces. A periodic model of the ITO/Si interface was constructed, giving detailed information about the local environment at the interface. Molecular dynamics based on density functional theory was performed, showing how metal-oxygen bonds are broken on behalf of silicon-oxygen bonds. These theoretical results support and provide an explanation for the present as well as previous ex-situ and in-situ experimental observations pointing to the creation of metallic In and Sn along with the growth of SiO{sub x} at the ITO/Si interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interface composition between Fe{sub 3}O{sub 4} nanoparticles and GaAs for spintronic applications

Recent interest in spintronic applications has necessitated the study of magnetic materials in contact with semiconductor substrates; importantly, the structure and composition of these interfaces can influence both device functionality and the magnetic properties. Nanoscale ferromagnet/semiconductor structures are of particular interest. In this study, the interface structure between a monolayer of ferromagnetic magnetite (Fe{sub 3}O{sub 4}) nanoparticles and a GaAs substrate was studied using cross-sectional transmission electron microscopy techniques. It was found that a continuous amorphous oxide interface layer separates the nanoparticles from the GaAs substrate, and that iron diffused into the interface layer forming a compositional gradient. Electron energy-loss near-edge fine structures of the O K absorption edge revealed that the amorphous oxide is composed of γ-Fe{sub 2}O{sub 3} directly underneath the Fe{sub 3}O{sub 4} nanoparticles, followed by a solid solution of Ga{sub 2}O{sub 3} and FeO and mostly Ga{sub 2}O{sub 3} when approaching the buckled oxide/substrate interface. Real-space density functional theory calculations of the dynamical form factor confirmed the experimental observations. The implication of the findings on the optimization of these structures for spin injection is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Atom-probe tomographic study of interfaces of Cu{sub 2}ZnSnS{sub 4} photovoltaic cells

The heterophase interfaces between the CdS buffer layer and the Cu{sub 2}ZnSnS{sub 4} (CZTS) absorption layers are one of the main factors affecting photovoltaic performance of CZTS cells. We have studied the compositional distributions at heterophase interfaces in CZTS cells using three-dimensional atom-probe tomography. The results demonstrate: (a) diffusion of Cd into the CZTS layer; (b) segregation of Zn at the CdS/CZTS interface; and (c) a change of oxygen and hydrogen concentrations in the CdS layer depending on the heat treatment. Annealing at 573 K after deposition of CdS improves the photovoltaic properties of CZTS cells probably because of the formation of a heterophase epitaxial junction at the CdS/CZTS interface. Conversely, segregation of Zn at the CdS/CZTS interface after annealing at a higher temperature deteriorates the photovoltaic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effective interaction between a colloid and a soft interface near criticality

Within mean-field theory we determine the universal scaling function for the effective force acting on a single colloid located near the interface between two coexisting liquid phases of a binary liquid mixture close to its critical consolute point. This is the first study of critical Casimir forces emerging from the confinement of a fluctuating medium by at least one shape responsive, soft interface, instead of by rigid walls only as studied previously. For this specific system, our semi-analytical calculation illustrates that knowledge of the colloid-induced, deformed shape of the interface allows one to accurately describe the effective interaction potential between the colloid and the interface. Moreover, our analysis demonstrates that the critical Casimir force involving a deformable interface is accurately described by a universal scaling function, the shape of which differs from that one for rigid walls.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Quantization of interface currents

At the interface of two two-dimensional quantum systems, there may exist interface currents similar to edge currents in quantum Hall systems. It is proved that these interface currents are macroscopically quantized by an integer that is given by the difference of the Chern numbers of the two systems. It is also argued that at the interface between two time-reversal invariant systems with half-integer spin, one of which is trivial and the other non-trivial, there are dissipationless spin-polarized interface currents.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

A comparison of molecular dynamics and diffuse interface model predictions of Lennard-Jones fluid evaporation

The unsteady evaporation of a thin planar liquid film is studied by molecular dynamics simulations of Lennard-Jones fluid. The obtained results are compared with the predictions of a diffuse interface model in which capillary Korteweg contributions are added to hydrodynamic equations, in order to obtain a unified description of the liquid bulk, liquid-vapor interface and vapor region. Particular care has been taken in constructing a diffuse interface model matching the thermodynamic and transport properties of the Lennard-Jones fluid. The comparison of diffuse interface model and molecular dynamics results shows that, although good agreement is obtained in equilibrium conditions, remarkable deviations of diffuse interface model predictions from the reference molecular dynamics results are observed in the simulation of liquid film evaporation. It is also observed that molecular dynamics results are in good agreement with preliminary results obtained from a composite model which describes the liquid film by a standard hydrodynamic model and the vapor by the Boltzmann equation. The two mathematical model models are connected by kinetic boundary conditions assuming unit evaporation coefficient.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Stable finite element method of eight-band k·p model without spurious solutions and numerical study of interfaces in heterostructures

A Lagrange-Hermite finite element method for the eight-band k·p model is developed. We demonstrate that besides the incompletion of k·p basis functions, the ill representation of first-order derivatives can also bend the conduction band structure down and lead to the highly oscillatory solutions. Our method simultaneously solves these two problems and achieves robust stability and high accuracy in real-space numerical calculation. The more physical asymmetric operator ordering is employed and the connection problem in abrupt interface is resolved by using an approximately abrupt interface. The situation of smooth interface used to explain the discrepancies between experiment and simulation of abrupt interface is also calculated by our method, and the result suggests that the influence of the interface smoothing should be considered in the short period superlattices or quantum structures of the narrow well.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Modeling small-signal response of GaN-based metal-insulator-semiconductor high electron mobility transistor gate stack in spill-over regime: Effect of barrier resistance and interface states

We provide theoretical and simulation analysis of the small signal response of SiO{sub 2}/AlGaN/GaN metal insulator semiconductor (MIS) capacitors from depletion to spill over region, where the AlGaN/SiO{sub 2} interface is accumulated with free electrons. A lumped element model of the gate stack, including the response of traps at the III-N/dielectric interface, is proposed and represented in terms of equivalent parallel capacitance, C{sub p}, and conductance, G{sub p}. C{sub p} -voltage and G{sub p} -voltage dependences are modelled taking into account bias dependent AlGaN barrier dynamic resistance R{sub br} and the effective channel resistance. In particular, in the spill-over region, the drop of C{sub p} with the frequency increase can be explained even without taking into account the response of interface traps, solely by considering the intrinsic response of the gate stack (i.e., no trap effects) and the decrease of R{sub br} with the applied forward bias. Furthermore, we show the limitations of the conductance method for the evaluation of the density of interface traps, D{sub it}, from the G{sub p}/ω vs. angular frequency ω curves. A peak in G{sub p}/ω vs. ω occurs even without traps, merely due to the intrinsic frequency response of gate stack. Moreover, the amplitude of the G{sub p}/ω vs. ω peak saturates at high D{sub it}, which can lead to underestimation of D{sub it}. Understanding the complex interplay between the intrinsic gate stack response and the effect of interface traps is relevant for the development of normally on and normally off MIS high electron mobility transistors with stable threshold voltage.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Symmetry-dependent interfacial reconstruction to compensate polar discontinuity at perovskite oxide interfaces (LaAlO{sub 3}/SrTiO{sub 3} and LaAlO{sub 3}/CaTiO{sub 3})

We report the crystal symmetry-dependency of the interfacial reconstruction to relieve the polar discontinuity at the complex oxide heterointerfaces. We chose LaAlO{sub 3}/SrTiO{sub 3} and LaAlO{sub 3}/CaTiO{sub 3} interfaces as model systems, where the neutral TiO{sub 2} and the positive LaO{sup +} layers form the polar discontinuity at the interface with TiO{sub 2}-terminated (001) cubic SrTiO{sub 3} and orthorhombic CaTiO{sub 3}. Using scanning transmission electron microscopy, we observed that the interlayer distance abnormally increased at the interface. We performed the first-principles calculations to understand the detailed atomic displacement at the interfaces having no oxygen vacancy and intermixing. Our results show that cations were reconstructed in different ways depending on the crystal symmetry through the octahedral tilts and atomic displacements to compensate the polar discontinuity at the interfaces. Our results imply that the interfacial reconstructions have to be considered along with the ionic compensation (intermixing) and electronic compensation (two dimensional electron gas) to fully understand the interfacial phenomena.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Surface thermodynamics of planar, cylindrical, and spherical vapour-liquid interfaces of water

The test-area (TA) perturbation approach has been gaining popularity as a methodology for the direct computation of the interfacial tension in molecular simulation. Though originally implemented for planar interfaces, the TA approach has also been used to analyze the interfacial properties of curved liquid interfaces. Here, we provide an interpretation of the TA method taking the view that it corresponds to the change in free energy under a transformation of the spatial metric for an affine distortion. By expressing the change in configurational energy of a molecular configuration as a Taylor expansion in the distortion parameter, compact relations are derived for the interfacial tension and its energetic and entropic components for three different geometries: planar, cylindrical, and spherical fluid interfaces. While the tensions of the planar and cylindrical geometries are characterized by first-order changes in the energy, that of the spherical interface depends on second-order contributions. We show that a greater statistical uncertainty is to be expected when calculating the thermodynamic properties of a spherical interface than for the planar and cylindrical cases, and the evaluation of the separate entropic and energetic contributions poses a greater computational challenge than the tension itself. The methodology is employed to determine the vapour-liquid interfacial tension of TIP4P/2005 water at 293 K by molecular dynamics simulation for planar, cylindrical, and spherical geometries. A weak peak in the curvature dependence of the tension is observed in the case of cylindrical threads of condensed liquid at a radius of about 8 Å, below which the tension is found to decrease again. In the case of spherical drops, a marked decrease in the tension from the planar limit is found for radii below ∼ 15 Å; there is no indication of a maximum in the tension with increasing curvature. The vapour-liquid interfacial tension tends towards the planar limit for large system sizes for both the cylindrical and spherical cases. Estimates of the entropic and energetic contributions are also evaluated for the planar and cylindrical geometries and their magnitudes are in line with the expectations of our simple analysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Contribution of individual interfaces in the MgO/Co/Pd trilayer to perpendicular magnetic anisotropy upon annealing

The contribution of each interface of the MgO/Co/Pd trilayer to the perpendicular magnetic anisotropy (PMA) was studied by changing chemical and crystalline structures through annealing. We found that volumetric anisotropy in the MgO/Co/Pd trilayer was significantly increased due to enhanced magnetoelastic anisotropy caused by stress built up most likely at the MgO/Co interface during annealing. When the trilayer was annealed at 400 °C, the alloy formation at the Co/Pd interface additionally increased the volumetric anisotropy. Our x-ray magnetic circular dichroism study supported that those structural modifications led to an increase in the orbital moment through spin-orbit coupling (SOC) along the film normal two times larger than that of the as-deposited trilayer, thereby enhancing PMA greatly. Our experimental results prove that the Co/Pd interface, rather than the MgO/Co interface, plays an essential role in inducing strong PMA in the trilayer. The precise investigation of annealing effect on both volumetric and interfacial anisotropies can provide a methodological solution to improve the SOC of the trilayer that can serve as the core unit of spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electron-phonon coupling and thermal conductance at a metal-semiconductor interface: First-principles analysis

The mechanism of heat transfer and the contribution of electron-phonon coupling to thermal conductance of a metal-semiconductor interface remains unclear in the present literature. We report ab initio simulations of a technologically important titanium silicide (metal)–silicon (semiconductor) interface to estimate the Schottky barrier height, and the strength of electron-phonon and phonon-phonon heat transfer across the interface. The electron and phonon dispersion relations of TiSi{sub 2} with C49 structure and the TiSi{sub 2}-Si interface are obtained using first-principles calculations within the density functional theory framework. These are used to estimate electron-phonon linewidths and the associated Eliashberg function that quantifies coupling. We show that the coupling strength of electrons with interfacial phonon modes is of the same order of magnitude as coupling of electrons to phonon modes in the bulk metal, and its contribution to electron-phonon interfacial conductance is comparable to the harmonic phonon-phonon conductance across the interface.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Hydrogen accumulation as the origin of delamination at the a-carbon/SiO{sub 2} interface

This work reports the characterization of the interface amorphous carbon (a-C)/SiO{sub 2} by neutron and X-ray reflectometry. Neutrons have shown the existence of an intermediate layer (IL) between the a-C and the SiO{sub 2} layers that was not evidenced by XRR. This IL has been associated with the accumulation of H inside the SiO{sub 2} layer near the interface with the a-C. The characteristics of this layer, in particular, its H-concentration and thickness, seem to be correlated with the weakness of this interface. A plot of the molecular weight as a function of the mass density for the SiO{sub 2} and the IL layers graphically demonstrates the risk of delamination of each sample. The combination of NR and XRR is shown to be a powerful technique in the characterization of layers and interfaces used in the micro/nanoelectronics industry. The same approach can be extended to other interfaces of interest involving hydrogen.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Controlling the interface charge density in GaN-based metal-oxide-semiconductor heterostructures by plasma oxidation of metal layers

In recent years, investigating and engineering the oxide-semiconductor interface in GaN-based devices has come into focus. This has been driven by a large effort to increase the gate robustness and to obtain enhancement mode transistors. Since it has been shown that deep interface states act as fixed interface charge in the typical transistor operating regime, it appears desirable to intentionally incorporate negative interface charge, and thus, to allow for a positive shift in threshold voltage of transistors to realise enhancement mode behaviour. A rather new approach to obtain such negative charge is the plasma-oxidation of thin metal layers. In this study, we present transmission electron microscopy and energy dispersive X-ray spectroscopy analysis as well as electrical data for Al-, Ti-, and Zr-based thin oxide films on a GaN-based heterostructure. It is shown that the plasma-oxidised layers have a polycrystalline morphology. An interfacial amorphous oxide layer is only detectable in the case of Zr. In addition, all films exhibit net negative charge with varying densities. The Zr layer is providing a negative interface charge density of more than 1 × 10{sup 13 }cm{sup –2} allowing to considerably shift the threshold voltage to more positive values.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Accurate characterization and understanding of interface trap density trends between atomic layer deposited dielectrics and AlGaN/GaN with bonding constraint theory

Many dielectrics have been proposed for the gate stack or passivation of AlGaN/GaN based metal oxide semiconductor heterojunction field effect transistors, to reduce gate leakage and current collapse, both for power and RF applications. Atomic Layer Deposition (ALD) is preferred for dielectric deposition as it provides uniform, conformal, and high quality films with precise monolayer control of film thickness. Identification of the optimum ALD dielectric for the gate stack or passivation requires a critical investigation of traps created at the dielectric/AlGaN interface. In this work, a pulsed-IV traps characterization method has been used for accurate characterization of interface traps with a variety of ALD dielectrics. High-k dielectrics (HfO{sub 2}, HfAlO, and Al{sub 2}O{sub 3}) are found to host a high density of interface traps with AlGaN. In contrast, ALD SiO{sub 2} shows the lowest interface trap density (<2 × 10{sup 12 }cm{sup −2}) after annealing above 600 °C in N{sub 2} for 60 s. The trend in observed trap densities is subsequently explained with bonding constraint theory, which predicts a high density of interface traps due to a higher coordination state and bond strain in high-k dielectrics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Molecular dynamics study on condensation/evaporation coefficients of chain molecules at liquid–vapor interface

The structure and thermodynamic properties of the liquid–vapor interface are of fundamental interest for numerous technological implications. For simple molecules, e.g., argon and water, the molecular condensation/evaporation behavior depends strongly on their translational motion and the system temperature. Existing molecular dynamics (MD) results are consistent with the theoretical predictions based on the assumption that the liquid and vapor states in the vicinity of the liquid–vapor interface are isotropic. Additionally, similar molecular condensation/evaporation characteristics have been found for long-chain molecules, e.g., dodecane. It is unclear, however, whether the isotropic assumption is valid and whether the molecular orientation or the chain length of the molecules affects the condensation/evaporation behavior at the liquid–vapor interface. In this study, MD simulations were performed to study the molecular condensation/evaporation behavior of the straight-chain alkanes, i.e., butane, octane, and dodecane, at the liquid–vapor interface, and the effects of the molecular orientation and chain length were investigated in equilibrium systems. The results showed that the condensation/evaporation behavior of chain molecules primarily depends on the molecular translational energy and the surface temperature and is independent of the molecular chain length. Furthermore, the orientation at the liquid–vapor interface was disordered when the surface temperature was sufficiently higher than the triple point and had no significant effect on the molecular condensation/evaporation behavior. The validity of the isotropic assumption was confirmed, and we conclude that the condensation/evaporation coefficients can be predicted by the liquid-to-vapor translational length ratio, even for chain molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗