Coherent Multilayers Research Articles

Coherent Multilayer Photonic Nanoantenna Array with off-Aperture Phase Adjustment

Coherent Multilayer Photonic Nanoantenna Array with off-Aperture Phase Adjustment

A Coherent Multilayer Simulator of Radargrams Acquired by Radar Sounder Instruments

Reliable electromagnetic simulators are of prime importance for the design of radar sounder instruments and for supporting the subsequent analysis of their data. In this paper, we present a coherent, facet method-based simulator that can compute radar echoes from the subsurface of a target area with an arbitrary number of geological layers, thus going beyond the surface-only or the two-layer descriptions so far implemented in coherent ray-tracing radar sounder simulators. Propagation of fields throughout the subsurface is computed according to Snell’s law following a ray-tracing approach. For each ray interacting with the surface, be it a direct reflection or a refracted ray coming from the subsurface, the phase contribution of each facet is calculated through the linear phase approximation, while the total field received at the antenna is computed using Huygen’s principle. Validation simulations have been performed against the radar data of lunar and martian areas characterized by a multilayer nature and collected by the Lunar Radar Sounder instrument of JAXA’s Kaguya lunar probe and the Shallow Radar instrument onboard NASA’s Mars Reconnaissance Orbiter, respectively. Results confirm the effectiveness of the proposed simulator.

Adhesion energy and related plastic deformation mechanism of Cu/Ru nanostructured multilayer film

Sputter-deposited Cu/Ru nanolaminate composites with individual layer thickness as small as 1.5 nm were tested by nanoindentation probing to initiate and drive delamination. Focused ion beam (FIB) observations show the layer buckling and interface delamination with lateral crack along the interface between the multilayer and substrate. The buckling behaviors, dependent on the critical length scale, are rationalized in the light of the repeating structural unit in the coherent multilayer. In addition, due to the presence of the interfaces and constraints between the hetero-layers, the condition for plastic dissipation in multilayers shifts significantly from those of single layer films, therefore a modified energy-dissipative model has been employed to obtain the quantitative adhesion energy for Cu/Ru multilayer, which agrees well with previous reports on the Cu-based films adherent on rigid substrates. The present result provides meaningful adhesion energy estimates and helps to understand the underlying deformation mechanism in metallic multilayers.

Solution processed ZnO homogeneous quasisuperlattice materials

Heterogeneous multilayered oxide channel materials have enabled low temperature, high mobility thin film transistor technology by solution processing. The authors report the growth and characterization of solution-based, highly uniform and c-axis orientated zinc oxide (ZnO) single and multilayered thin films. Quasisuperlattice (QSL) metal oxide thin films are deposited by spin-coating and the structural, morphological, optical, electronic, and crystallographic properties are investigated. In this work, the authors show that uniform, coherent multilayers of ZnO can be produced from liquid precursors using an iterative coating-drying technique that shows epitaxial-like growth on SiO2, at a maximum temperature of 300 °C in air. As QSL films are grown with a greater number of constituent layers, the crystal growth direction changes from m-plane to c-plane, confirmed by x-ray and electron diffraction. The film surface is smooth for all QSLs with root mean square roughness <0.14 nm. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) of electronic defects in the QSL structure show a dependence of defect emission on the QSL thickness, and PL mapping demonstrates that the defect signature is consistent across the QSL film in each case. XPS and valence-band analysis shown a remarkably consistent surface composition and electronic structure during the annealing process developed here.

Synthesis and characterization of Ti/Al reactive multilayer films with various molar ratios

This study describes the fabrication and characterization of the bimetallic Ti/Al reactive multilayer thin films. Reactive films with different molar ratios of Ti and Al (mTi/nAl, m=1; n=1, 2, 3) were fabricated by using Magnetron Sputter Ion Plating (MSIP) method. Differential scanning calorimetry (DSC) was used to analyze the phase change temperature and heat flow. The heat of formation of different phases in Ti-Al system was also calculated by using thermodynamic simulation. Here, reaction heat depends on types of phases form. Microstructural features of the as-deposited Ti/Al reactive multilayer films show columnar microstructures with coherent multilayers. After an electrical ignition, all foils exhibit self-propagating reaction. The experimental result shows that 1Ti/1Al reactive foils have higher reaction velocity of 2.6±0.6m/s compared to other molar ratios at 9V electrical ignition. Here, 1Ti/1Al and 1Ti/2Al reactive films show only steady reaction propagation, whereas 1Ti/3Al films exhibit both steady and unsteady reaction propagation. The microstructural characterization exhibits two types of diffusion modes of reactants mixing in the periodic layers and grain boundary, which assure even faster diffusion during self-propagating reaction.

Systematic ab initio investigation of the elastic modulus in quaternary transition metal nitride alloys and their coherent multilayers

We give a comprehensive overview of the elastic properties of cubic quaternary transition metal nitride alloys and coherent nitride multilayers for design of wear resistant hard coatings. The elastic stiffness constants of the alloys are calculated using the special quasirandom structure method. For multilayers with sharp interfaces we prove the applicability of a linear-elasticity approximation and show that it can be used with success instead of performing direct computationally demanding ab initio calculations. We explore the trends and the potential of multicomponent alloying in engineering the strength and ductility of both, quaternary alloys and their multilayers. We investigate X(1−x−y)TixAlyN alloys where X is Zr, Hf, V, Nb or Ta, and present an analysis based on increasing x. We show that with increasing Ti content ductility can increase in each alloy. Elastic isotropy is observed only in (Zr,Hf,V)(1−x−y)TixAlyN alloys in the middle of the compositional triangle, otherwise a high Young's modulus is observed along [001]. We predict that coherent TiN/X(1−x−y)TixAlyN and ZrN/X(1−x−y)TixAlyN alloy multilayers with the [111] interfacial direction show increasing ductility with increasing x, while the multilayers with the [001] orientation become more brittle. We show that the Young's moduli variation in the parent bulk quaternary nitride alloy provide a reliable descriptor to screen the Young's modulus of coherent multilayers in high-throughput calculations.

Designing Novel Metallic Multilayer Nanocomposites Through Atomic Engineering of Interfaces – Influence of Heat of Mixing

Metallic multilayered nanocomposites have observed varying flow strength and indentation moduli values for the same type of interface. The enhanced mechanical properties from these nanocomposites have been attributed to their interfaces. The heat of mixing of the interface determines how much the two different layers have an affinity with each other and the resultant amount of shearing of the interface or transmission of dislocations through the interface. Nanoindentation of thin films on substrate technique is performed on five different metallic multilayer systems with a combination of FCC/FCC, BCC/BCC, and FCC/BCC interfaces to understand the effect of heat of mixing on its mechanical properties of hardness and indentation modulus of the films. In situ scanning probe microscopy (SPM) was done before and after each indentation. From the analysis, it is found that the heat of mixing has significant influence only in the hardness or flow strength values of the FCC/BCC multilayered nanocomposites due to dislocation plasticity mechanisms where the interface shear strength decreases with the change of heat of mixing from negative to positive values. The difference in the mechanical properties of FCC/FCC and BCC/BCC coherent multilayers are mainly due to either their lattice misfit or shear moduli differences, not the heat of mixing.

Enhanced radiation tolerance in immiscible Cu/Fe multilayers with coherent and incoherent layer interfaces

Abstract

Effect of incidence angle and polarization on the optimized layer structure of organic solar cells

We theoretically and experimentally investigate the effect of the incidence angle and polarization of sunlight on the optimized layer structure of organic solar cells (OSCs) to obtain the best absorption efficiency in a realistic deployment situation. We use the generalized transfer matrix method with respect to the incidence angle and polarization, which can calculate the spatial distribution of the optical power dissipation in both the incoherent glass substrate and the coherent multilayer without using any indirect correction factor. The angular dependence of the short-circuit current, the open-circuit voltage, and the output electric power is calculated and compared with the experimental results. Using the simulation parameters matched with the experimental results, we calculate the generation energy density per day by considering the variation of the incidence angle during daytime and determine the optimized thickness of the active region for maximum absorption efficiency. We show that the optimized active-region thickness based on the generation energy density per day is different from that determined from the variation of the short-circuit current density in only normal incidence, which is generally used for optimizing device structure of OSCs.

Effect of an Incoherent Glass Substrate on the Absorption Efficiency of Organic Solar Cells at Oblique Incidence Analyzed by the Transfer Matrix Method with a Glass Factor

We theoretically investigate the effect of the incoherent glass substrate on the absorption efficiency of organic solar cells (OSCs) at oblique incidence. The light propagation in the mixed incoherent glass substrate and coherent multilayer is calculated based on the transfer matrix method with a simple correction factor, which is derived at oblique incidence by taking multiple reflections inside the glass substrate. The validity of the proposed method is demonstrated by comparing the calculation results with those obtained by the generalized transfer matrix method. We investigate how the multiple reflections within the incoherent glass substrate affect the light absorption efficiency of thin-film OSCs, depending on the incidence angle and polarization.

Effects of coherency stress and vacancy sources/sinks on interdiffusion across coherent multilayer interfaces – Part I: Theory

A phase field model corresponding to vacancy-mediated interdiffusion in coherent multilayers with completely miscible constituents is developed to explore the effects of several factors on interdiffusion across coherent multilayer interfaces, such as: (1) the dependence of diffusion potentials and mobilities on coherency stress; (2) the dependence of diffusion potentials and mobilities on composition; (3) the elastic constant inhomogeneity resulting from a inhomogeneous composition distribution; and (4) the properties of vacancy sources/sinks. The Gibbs free energy of the system consists of chemical and elastic energies. The gradient energy is neglected as the multilayers under consideration can be chemically well approximated by an ideal substitutional solution model. Elastic energy is a function of the stress-free strain and inhomogeneous elastic moduli distributions, while the stress is solved by anisotropic phase field microelasticity theory. The diffusion potentials are obtained straightforwardly as functional derivatives of the free energy with respect to composition and are in keeping with previous derivations that involved many mathematical manipulations or quite advanced theories. The diffusion mobilities are affected by the stress through modification of the vacancy formation and migration energies. Two limiting cases of vacancy sources/sinks are taken into account: ideal vacancy sources/sinks are uniformly and densely distributed, or not present at all, so the vacancy concentration is in equilibrium all the times, as determined by the local stress and composition in the former case, but deviates from the equilibrium concentration in the latter. The model can be conveniently extended to consider the non-ideal activity of vacancy sources/sinks by introducing a general kinetic relation for the vacancy creation rate.

Optical Modeling and Analysis of Organic Solar Cells with Coherent Multilayers and Incoherent Glass Substrate Using Generalized Transfer Matrix Method

We present optical modeling and physical analysis results of thin-film organic solar cells (OSCs) based on a generalized transfer matrix method, which can calculate, with a simple matrix form, the mixed coherent and incoherent interaction of an incoherent glass substrate with other coherent layers. The spatial distribution of the electric field intensity, power density, and power dissipation are calculated in both coherent and incoherent layers with respect to the optical spacer thickness. By decomposing the power density and power dissipation into forward-propagating, backward-propagating, and their interference components, we demonstrate that the dependence of the spacer thickness on the total device reflectance plays an important role in determining the light absorption efficiency of the OSC.

Optical Modeling and Analysis of Organic Solar Cells with Coherent Multilayers and Incoherent Glass Substrate Using Generalized Transfer Matrix Method

We present optical modeling and physical analysis results of thin-film organic solar cells (OSCs) based on a generalized transfer matrix method, which can calculate, with a simple matrix form, the mixed coherent and incoherent interaction of an incoherent glass substrate with other coherent layers. The spatial distribution of the electric field intensity, power density, and power dissipation are calculated in both coherent and incoherent layers with respect to the optical spacer thickness. By decomposing the power density and power dissipation into forward-propagating, backward-propagating, and their interference components, we demonstrate that the dependence of the spacer thickness on the total device reflectance plays an important role in determining the light absorption efficiency of the OSC.

Study of composition modulation in Cu/Co(Cu) multilayers electrodeposited using pulse trains

Composition modulation in copper/cobalt (copper) multilayers grown by a pulse train based deposition technique has been studied using grazing angle X-ray diffraction and X-ray photoelectron spectroscopy techniques. Higher pulse frequency employed during pulse electrodeposition, has been shown to yield a mixed copper/cobalt phase. Formation of coherent multilayers exhibiting antiphase oscillation of the Cu/Co composition profile has been demonstrated using pulse frequency (duty cycles) of 200 Hz (50%) and 400 Hz (80%) for copper and cobalt electrodeposition, respectively.

Investigation of Cu and Co multilayer deposition in aqueous ambient

Electrochemical preparation of multilayers of Cu/Co(Cu) has been investigated using cyclic voltammetry and in situ atomic force microscopy. Potentiostatic pulse deposition of Copper and Cobalt has been shown to reduce the surface roughness of the electrodeposits. Multilayers of Cu/Co(Cu) have been grown electrochemically using pulse deposition technique. Formation of coherent multilayers has been demonstrated using grazing angle X-ray diffraction studies.

Effects of coherency on diffusivity in multilayers

In coherent multilayers, the remote spacing of vacancy sources and sinks indicates that Darken’s equations no longer apply. Under these circumstances, after a transient, the diffusivity should drop to that of the slower diffusing component. The magnitude of this effect in comparison with other effects that can retard diffusivity such as sink efficiency and internal stress fields, is discussed.

Monte Carlo Study of the Evolution of the Phase Morphology in Coherent Metallic Multilayers with Immiscible Components

Monte Carlo Study of the Evolution of the Phase Morphology in Coherent Metallic Multilayers with Immiscible Components

Monte Carlo Investigation of Thermal Stability and Morphology Development of Coherent Multilayers at Elevated Temperatures

Monte Carlo Investigation of Thermal Stability and Morphology Development of Coherent Multilayers at Elevated Temperatures

Monte Carlo investigation of the thermal stability of coherent multilayers

Institut fu¨r Werkstoffwissenschaft, Technische Universita¨t Dresden, Hallwachsstrasse 3, D-01062Dresden, Germany(Received June 8, 2000)(Accepted June 20, 2000)Keywords: Multilayers; Alloys; Annealing; Thermal stability; Computer simulationIntroductionNanoscale metallic multilayers possess outstanding physical properties for various promising applica-tions. In order to optimize these properties, as-deposited multilayers are often thermally treated atelevated temperatures. A thermal loading of multilayers can also occur under processing and operatingconditions. The present paper addresses the question of the thermal stability of A/B layer stacks withthin individual layers of only a few atomic layers. The components A and B are supposed to beimmiscible. Nanoscale Co/Cu multilayers, exhibiting the giant magnetoresistance, are a typical exam-ple. Because of the small lattice misfit of about 2%, the fcc-Co layers and the Cu layers are coherent.As a first step to the understanding of the thermal decomposition of such multilayer films, weconsidered a coherent A/B layer stack with B-layer thickness of 4 to 6 monolayers (ML) andconsiderably thicker A-layers. To investigate the thermally activated rearrangements of atoms in thisstructure, Monte Carlo (MC) simulations have been performed. As initial state, an A/B layer stack withideally planar A/B interfaces was assumed. This assumption has most relevance to single crystalsuperlattices, but also to polycrystalline multilayers with lateral grain size which is large compared tothe multilayer period. The aim of the simulations was to elucidate the initiation mechanism of thethermally induced layer decomposition as well as the subsequent development of the phase morphol-ogy.Monte Carlo AlgorithmThe present simulations were performed on a fcc lattice occupied by A- and B-atoms as well as onevacancy. The chemical binding between atoms has been described by nearest neighbor pair interactionse

On the size dependence of the critical point of nanoscale interstitial solid solutions

It is shown that a size dependence of the critical temperature Tc of the miscibility gap in nanocrystalline and nanoscale particle interstitial solid solutions results from stress owing to elastic interaction of the bulk with layers of material at the grain boundaries or surfaces which have a small solute susceptibility (i.e. a weak dependence of the concentration on the chemical potential) at the phase transition. When the volume fraction occupied by the interfacial layers is not too large, then the changes in Tc and in the critical concentration xc can be predicted on the basis of a series expansion of the solute chemical potential in the bulk about the critical point. The model can be extended to free-standing thin films and coherent multilayers. The dependence of the pressure on the hydrogen concentration in the crystal lattice of nanocrystalline palladium-hydrogen is measured on the basis of X-ray diffraction data. The result agrees with the predictions of the theory.