Few Tens Of Angstroms Research Articles

Spin transfer torque in Mn3Ga -based ferrimagnetic tunnel junctions from first principles

We report on first-principles calculations of spin-transfer torque (STT) in epitaxial magnetic tunnel junctions (MTJs) based on ferrimagnetic tetragonal Mn$_3$Ga electrodes, both as analyzer in an Fe/MgO stack, and also in an analogous stack with a second Mn$_3$Ga electrode (instead of Fe) as polarizer. Solving the ballistic transport problem (NEGF + DFT) for the nonequilibrium spin density in a scattering region extended to over 7.6 nm into the Mn$_3$Ga electrode, we find long-range spatial oscillations of the STT decaying on a length scale of a few tens of angstroms, both in the linear response regime and for finite bias. The oscillatory behavior of the STT in Mn$_3$Ga is robust against variations in the stack geometry and the applied bias voltage, which may affect the phase and the amplitude of the spatial oscillation, but the wave number is only responsive to variations in the longitudinal lattice constant of Mn$_3$Ga (for fixed in-plane geometry) without being commensurate with the lattice. Our interpretation of the long-range STT oscillations is based on the bulk electronic structure of Mn$_3$Ga, taking also into account the spin-filtering properties of the MgO barrier. Comparison to a fully Mn$_3$Ga-based stack shows similar STT oscillations, but a significant enhancement of both the TMR effect at the Fermi level and the STT at the interface, due to resonant tunneling for the mirror-symmetric junction with thinner barrier (three monoatomic layers). From the calculated energy dependence of the spin-polarized transmissions at 0 V, we anticipate asymmetric or symmetric TMR as a function of the applied bias voltage for the Fe-based and the all-Mn$_3$Ga stacks, respectively, which also both exhibit a sign change below 1 V. In the latter (symmetric) case we expect a TMR peak at zero, which is larger for the thinner barriers because of a spin-polarized resonant tunneling contribution.

Experimental Observation of Crystal-Liquid Coexistence in Slit-Confined Nonpolar Fluids.

Films of carbon tetrachloride (CCl4) confined in slit geometry between two flat diamond substrates down to a few tens of Angstroms are studied by combining X-ray reflectivity with in-plane and out-of-plane X-ray scattering. The confined films form a heterogeneous structure with coexisting regions of liquid and crystalline phases. The liquid phase shows short-range ordering normal to the surfaces of the substrates. The experiments directly show the ability of the confinement to induce crystal objects, which is a long-discussed issue in the literature. The surface structure and morphology of the substrates may influence the actual realization of the crystalline phase in confinement.

Structure and Morphology of Silver Nanoparticles on the (111) Surface of Cerium Oxide

The structure of Ag nanoparticles of different size, supported on the cerium oxide (111) surface, was investigated by X-ray absorption fine structure at the Ag K-edge. The results of the data analysis in the near and extended energy range are interpreted with the help of the results obtained by X-ray photoelectron spectroscopy and scanning tunneling microscopy measurements and allow to obtain a detailed atomic scale description of the model system investigated. The Ag nanoparticles have an average size of a few tens of angstroms, which increases with increasing deposited Ag amount. The nanoparticles show a slight tendency to nucleate at the step edges between different cerium oxide layers and they have a face centered cubic structure with an Ag–Ag interatomic distance contracted by 3–4% with respect to the bulk value. The interatomic distance contraction is mainly ascribed to dimensionality induced effects, while epitaxial effects have a minor role. The presence of Ag–O bonds at the interface between the ...

Time‐of‐flight secondary ion mass spectroscopic characterization of the nitrogen profile of shallow gate oxynitride thermally grown on a silicon substrate

Silicon oxynitride has been used as a shallow gate oxide material for microelectronics and its thickness has been reduced over the years to only a few tens of angstroms due to device size scaling. The nitride distribution and density characteristic in the gate oxide thus becomes imperative for the devices. The shallow depth profiling capability using time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) has huge potential for the nitrogen characterization of the shallow gate oxide film. In this article, both positive and negative spectra of TOF‐SIMS on silicon oxynitride have been extensively studied and it was found that the silicon nitride clusters SixN− (x = 1–4) are able to represent the nitrogen profiles because their ion yields are high enough, especially for the low‐level nitride doping in the oxide, which is formed by the annealing of nitric oxide on SiO2/Si. The gate oxide thickness measured by the TOF‐SIMS profiling method using 18O or CsO profile calibration was found to correlate very well with transmission electron microscope measurement. The nitrogen concentration in the gate oxide measured using the TOF‐SIMS method was consistent with the results obtained using the dynamic SIMS method, which is currently applied to relatively thicker oxynitride films. Copyright © 2012 John Wiley & Sons, Ltd.

Model of Preformed Hole-Pairs in c-Axis Transport in Cuprate Superconductors

Model of hole-pairs in electrical transport along ab plane in cuprate superconductors has already been proposed. It has been found to be in the shape of 3dx2–y2 orbital of an electron in an atom. This time, model of hole-pairs in transport along c-axis in cuprate superconductors is proposed. In ab-plane, hole-pairs are formed along CuO2 plane; one hole-pair covering 9 - 10 two dimensional CuO2 unit cells in 3dx2–y2 configuration. In the investigation of c-axis hole-pairs, cuprate superconductors have been sub-divided into three categories depending on the number of CuO2 planes/formula unit. There is a little different treatment for finding out the order parameter in each category. Coherence lengths along ab-planes are of the order of a few tens of Angstroms, whereas along c-axis, they are less than even their a-, b-lattice constants. In cuprates with 2 or 3 CuO2 planes, the order parameter is of 3dz2–x2 type in zx-plane with lobes along both the axes much constrained. For cuprates with a single CuO2 layer, the order parameter is of 3dx2–y2 type, but its dimensions are less than a-, b-lattice constants.

Spatial and spectral detection of protein monolayers with deterministic aperiodic arrays of metal nanoparticles

Light scattering phenomena in periodic systems have been investigated for decades in optics and photonics. Their classical description relies on Bragg scattering, which gives rise to constructive interference at specific wavelengths along well defined propagation directions, depending on illumination conditions, structural periodicity, and the refractive index of the surrounding medium. In this paper, by engineering multifrequency colorimetric responses in deterministic aperiodic arrays of nanoparticles, we demonstrate significantly enhanced sensitivity to the presence of a single protein monolayer. These structures, which can be readily fabricated by conventional Electron Beam Lithography, sustain highly complex structural resonances that enable a unique optical sensing approach beyond the traditional Bragg scattering with periodic structures. By combining conventional dark-field scattering micro-spectroscopy and simple image correlation analysis, we experimentally demonstrate that deterministic aperiodic surfaces with engineered structural color are capable of detecting, in the visible spectral range, protein layers with thickness of a few tens of Angstroms.

Metal-insulator-metal capacitors’ current instability improvement using dielectric stacks to prevent oxygen vacancies formation

Current instability in metal-oxide-semiconductor and metal-insulator-metal (MIM) capacitors has been previously reported to be a potential reliability issue. This letter intends to study a particular way to reduce these current instabilities with time in high-κ MIM capacitors. It consists in the introduction of a stable dielectric layer between the high-κ dielectric and the electrodes in order to prevent oxygen vacancy formation at interfaces. When applied to Ta2O5 capacitors, the deposition of a thin layer of Al2O3 in the range of a few tens of angstroms enables the strong reduction of current instabilities while maintaining good electrical performances.

The Frictional Properties of Thin Inorganic Halide Films on Iron Measured in Ultrahigh Vacuum

Friction coefficients are measured in ultrahigh vacuum using a tungsten carbide tribopin against thin films of sodium chloride and potassium iodide deposited onto clean iron. It is found, in accordance with previous measurements for potassium chloride on iron, that the friction coefficient falls from an initial value of ∼ 2 for the clean iron surface to a minimum value after a few tens of Angstroms of the halide have been deposited onto the surface, and remains constant for thicker films. The minimum friction coefficient is independent of applied load and therefore obeys Amontons' law. Simple theories for the effect of a low-shear-strength film suggest that the friction coefficient should depend on the shear strength of the film. This idea is tested by plotting the minimum friction coefficient versus the hardness of the film material, which is proportional to its shear strength, where a linear correlation was found. The lack of dependence of friction coefficient with film thickness for thicker films implies that ploughing forces do not contribute significantly to the friction coefficient.

Free-Ion Yield and Electron Mobility in Liquid Hydrocarbons: A Consistent Correlation

A correlation is sought between the free-ion yield and electron mobility in liquid hydrocarbons in terms of the elastic and the inelastic scattering mean free paths of epithermal (≤0.2 eV) electrons. These determine the thermalization distance distribution and consequently the free-ion yield. The thermal quasi-free electron mobility, μqf, can also be obtained from the same cross sections. Finally, the effective mobility is derived from μqf using the electron trap concentration and binding energy, thereby establishing a relationship between free-ion yield and mobility. Thus, given the input data for trapping and the elastic and inelastic cross sections, both the free-ion yield and the effective mobility may be obtained from the interactions of epithermal electrons. In very low mobility liquids (μ < 0.1 cm2 v-1 s-1), transport is governed by trapping and detrapping rates, being relatively independent of μqf. As found by Jay-Gerin et al. (Can. J. Chem. 1993 , 71, 287) the free-ion yield in such cases is virtually independent of mobility, a phenomenon which is naturally explained in the quasiballistic model of the author (Chem. Phys. Lett. 1993, 207, 245 and 1995, 233, 167) but not in the usual trapping model. For low mobility liquids, the elastic mean free path, L, is found to be ca. 1−5 A and there is a probability of ∼0.1−0.3 inelastic scatterings per elastic mean free path. These values increase progressively with mobility, where, in the high mobility cases, L is on the order of a few tens of angstroms and inelastic collisions outnumber elastic ones by a factor of ∼2−4. The situation is reminiscent of liquefied rare gases.

Convenient calibration of FTIR peak ‘size’ for thin organic/polymer films

AbstractThin films of organic/polymer material can be fabricated as self‐assembled monolayers and by dip‐coating, spin‐coating, etc. for use in microelectronics, particularly in applications in the BioChip area, molecular electronics, microfluidics and cell‐phone fuel cells. In any analysis the inevitable question is ‘How much’ (not how tall, but how many atoms per square centimeter). A unique feature of organic/polymer films is the molecular vibrations of the organic groups; these can be measured with Fourier transform infrared spectroscopy (FTIR) and we will use this method for quantitation. Using weight gain measurements, we provide the methodology for determining the amount of material in a thin film using transmission FTIR. We provide the calibration factors for CH2, C=O, and N–H bonds using Nylon‐6 as an example. Films as thin as 200 Å are used in this study and we estimate that, under certain conditions, films as thin as a few tens of angstroms could be measured. The calibration factors that we provide can be used directly for films where these functional groups are in similar chemical environments. Where the chemical environments are significantly different, we have suggested a method for determining the appropriate calibration factors. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Enhanced interfacial roughness in metallic multilayers prepared by pulsed cathodic arc deposition

The interfacial roughness of Fe/C multilayers with individual layer thicknesses from a few angstroms to a few tens of angstroms, fabricated using a pulsed cathodic arc, have been studied by cross-sectional transmission electron microscopy (XTEM) and atomic force microscopy (AFM). Very thin perfect 100-period Fe/C multilayers were prepared with flat and smooth interfaces in the sub-nanometer range by a high degree of ionization of the pulsed plasma.

Inductively coupled plasma etching of III–V semiconductors in BCl 3-based chemistries : II. InP, InGaAs, InGaAsP, InAs and AlInAs

A parametric study of etch rates and surface morphologies of In-containing compound semiconductors (InP, InGaAs, InGaAsP, InAs and AlInAs) obtained by BCl 3-based Inductively Coupled Plasmas (ICP) is reported. Etch rates in the range 1500–3000 Å/min are obtained for all the materials at moderate source powers (500 W), with the rates being a strong function of discharge composition, rf chuck power and pressure. Typical root-mean-square surface roughness of ∼5 nm were obtained for InP, which is worse than the values obtained for Ga-based materials under the same conditions (∼1 nm). The near surface of etched samples is typically slightly deficient in the group V element, but the depth of this deficiency is small (a few tens of angstroms).

Surface discharge characteristics of MgO thin films prepared by RF reactive magnetron sputtering

Abstract This paper describes the surface glow-discharge effect of MgO thin films prepared by reactive radio-frequency planar magnetron sputtering on the dielectric layer of an alternating-current plasma display panel. By introducing an MgO coating on the dielectric the discharge voltage decreases sharply, although the thickness is only a few tens of Angstroms. The lowest discharge voltage is obtained for the sample prepared at a 30% O2 content in an O2+Ar gas mixture and at a sputtering gas pressure of about 5 mTorr. Moreover, high transparency (∼95%) is also obtained under the same experimental conditions. The samples prepared show more sputter-resistant properties than samples prepared by the electron-beam method and no cracks are observed on the surface after post-deposition annealing.

Weathering and polymerization of tektites: An x‐ray photoelectron spectroscopy (XPS) investigation

Abstract— Tektites are natural glasses formed from terrestrial material that was melted and displaced by the impact of an extraterrestrial body. The surface and near‐surface compositions of tektite glass results from fractionation during impact and ejection, and/or postsolidification weathering. The first goal of this study was to characterise the surface and near‐surface (in the order of tens of angstroms) chemical composition of two tektites by x‐ray photoelectron spectroscopy (XPS), and to estimate the importance of weathering vs. fractionation during flying. In order to separate the chemical modification due to weathering from that due to fractionation during ballistic flight, we studied two samples from the Australasian tektite strewn field. One of them was collected in a hot desert area (Nullarbor Plain, Australia) and the other, in a humid climate (Thailand). Our study reveals the presence of well‐developed leached layers in both tektites. In the Australian tektite, Si is depleted in the topmost layers (a few tens of angstroms). A more complex chemical zoning is defined in the tektite from Thailand. These leached layers are comparable to those observed in weathered glasses, and therefore we conclude that weathering is responsible for the chemical composition of the surface and near‐surface compositions. The second goal was to investigate the chemical environment of O, N and C in the glass. The O peak was resolved into two bridging O components (Si‐O‐Si and Al‐O‐Si) that are comparable to O environments in artificial glasses. The binding energy of the C1s electron is typical for C‐C and C‐H bonds in hydrocarbons; minor organic acid components are also present. Nitrogen is only observed on the surface of the Thailand tektite. The binding energy of N1s is comparable to that of ammonia, and the surface enrichment in N is interpreted as due to sorption related to interactions between glass and fluid buffered by the organic material in the soil.

Cation Ordering in Synthetic Layered Double Hydroxides

AbstractA combined powder X-ray diffraction (XRD) and X-ray absorption (XAS) study of Fe(III) cation ordering within pyroaurite is described. It is concluded that there is no correlation between Fe(III) cation positions over distances of a few tens of angstroms, but that there is a very high level of local ordering, involving the absence of Fe(III)-Fe(III) neighbors. These observations are rationalized in terms of a significant frequency of lattice defects in the form of cation vacancies or Mg for Fe(III) substitutions. These results are expected to be generalizable to other M(II)/M(III) layered double hydroxides (LDHs), but are in contrast to the long-range cation ordering observed in Li/Al LDHs. This raises the interesting possibility of differing properties and stabilities based on the degree of cation ordering.

High-resolution solid state 13C n.m.r. study of the interpolymer interaction, morphology and chain dynamics of the poly(acrylic acid)/poly(ethylene oxide) complex

The interpolymer interaction, morphology and chain dynamics of the poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO) complex are examined by using 13C CP/MAS n.m.r. methods. By analysing 13C CP/MAS spectra of the complex we conclude that there exist three hydrogen bonding forms for the carboxyl group of PAA, namely: (1) the complex form, interpolymer hydrogen bonding between PEO molecules; (2) the dimeric form, intrapolymer hydrogen bonding within PAA molecules; and (3) the free form, no particular form of hydrogen bonding. The morphology of the complex is investigated by the 1H spin-lattice relaxation time in the laboratory frame ( T 1), and that in the rotating frame ( T 1 ρ ). We found that the domain sizes of the three hydrogen bonding forms of PAA are less than a few tens of angstroms, and the PAA/PEO complex is miscible on a molecular level. Further, temperature dependence of 13C linewidth ( T 2) is examined to study effects of complexation on the molecular motion of the component polymers. The temperatures at which the maximum linewidths are observed for the main chain carbon of PEO and PAA in the PAA/PEO complex are 310 and 362 K, respectively. This indicates that the motional heterogeneity is present in spite of a single T g for the PAA/PEO complex. Further, we discuss the structural difference between the poly(methacrylic acid)/PEO complex and the PAA/PEO complex.

Static and dynamic neutron X-ray scattering

Interfaces made up of amphiphiles have typical thicknesses of up to a few tens of angstroms and structures with characteristic lengths of a few hundred angstroms, for example micelles and microemutsions. These dimensions make amphiphilic interfaces especially well suited to small angle neutron and X-ray scattering. After the recent progress achieved in structural determinations the way is now open to extend studies to the dynamics of these systems. A clear trend towards polymer science is emerging, where research on block copolymers represents a natural extension to the work on amphiphiles.

GOOD ABUNDANCES FROM BAD SPECTRA: II. APPLICATION AND A NEW STELLAR COLOR-TEMPERATURE CALIBRATION

ABSTRACT Stellar spectra derived from current multiple-object fiber-fed spectroscopic radial-velocity surveys, of the type feasible with, among other examples, AUTOFIB, 2dF, HYDRA, NESSIE, and the Sloan survey, differ significantly from those traditionally used for determination of stellar abundances. The spectra tend to be of moderate resolution (around 1 A) and signal-to-noise ratio (around 10-20 per resolution element), and cannot usually have reliable continuum shapes determined over wavelength ranges in excess of a few tens of Angstroms. Nonetheless, with care and a calibration of stellar effective temperature from photometry, independent of the spectroscopy, reliable iron abundances can be derived. We have developed techniques to extract true iron abundances and surface gravities from low signal-to-noise ratio, intermediate resolution spectra of G-type stars in the 4000-5000A wavelength region. The theoretical basis and calibration using synthetic spectra are described in detail in another paper (Jones, Gilmore and Wyse, 1995). The practical application of these techniques to observational data, which requires some modification from the ideal case of synthetic data, is given in the present paper. An externally-derived estimate of stellar effective temperature is required in order to constrain parameter space sufficiently; a new derivation of the V-I effective temperature relation is thus an integral part of the analysis presented here. We have derived this relationship from analysis of available relevant data for metal-poor G dwarfs, the first such calibration. We test and calibrate our techniques by analysis of spectra of the twilight sky, of member stars of the cluster M67, and of a set of field stars of known metallicity. We show that this method, combined with our new color-temperature calibration, can provide true iron abundances, with an uncertainty of less than 0.2 dex over the range of metallicty found in the Galactic thick and thin disks, from spectra obtained with fiber-fed spectrographs.

Optical approaches to the determination of composition of semiconductor alloys during epitaxy

Realization of sample-driven closed-loop feedback control of the epitaxial growth of semiconductor alloys requires that the fluctuations of the composition of the most recently deposited material be determined over an outermost region that is ideally no more than several angstroms thick. This information is encoded in the dielectric function /spl epsiv///sub 0/ of the region. The standard approach for obtaining /spl epsiv//sub 0/ is to mathematically subdivide the sample into thin layers and analyze them sequentially outward from the substrate by solving the Fresnel equations. However, the feed-forward nature of this approach leads to instabilities if thicknesses are reduced below a few tens of angstroms, a value too large for practical control applications. Derivative methods avoid these instabilities by allowing /spl epsiv//sub 0/ to be determined for dynamic processes involving deposition or etching even if nothing is known about the underlying sample structure. The limited accuracy of earlier derivative approaches based on the exponential-spiral form of interference expressions for optically thick films has been improved to acceptable levels with the recently developed virtual-interface (V-I) approach. Here, a background of the field is provided, the various methods of obtaining /spl epsiv//sub 0/ from the perspective of control applications are discussed, and V-I theory is used to obtain analytic expressions describing the accuracy of the earlier derivative methods. For ellipsometric measurements the best combination of speed and accuracy is obtained with the virtual-substrate approximation. For situations where higher accuracy is needed, a hybrid V-I-Fresnel method is proposed that retains the stability of derivative approaches yet is exact.

Ab initio theory of dislocation interactions: From close-range spontaneous annihilation to the long-range continuum limit.

Parallel supercomputer technology now permits ab initio studies of systems of sufficient size to explore the interactions among dislocations in a solid. This study shows that the silicon shuffle-set (110) screw dislocation is stable against spontaneous dissociation, provides an ab initio value for the dislocation core energy, demonstrates a dislocation-antidislocation interaction approaching the classical limit within a few tens of angstroms, and reveals a pathway for the spontaneous mutual annihilation of a dislocation dipole of the type that occurs when a Frank-Read source emits a dislocation loop.