Atomic Intermixing Research Articles

Exploring the reaction mechanism in self-propagating Al/Ni multilayers by adding inert material

We present a new class of reactive materials termed “inert-mediated reactive multilayers” (IMRMs) that use inert material to decouple the effects of chemistry and maximum temperature in a reactive multilayer. Important considerations in the selection of composition and thickness for reactive and inert material in an IMRM are detailed. We then present the results from a specific set of IMRM samples that we fabricated using 23-nm-bilayer 1:1 Al:Ni reactive sections and 2:3 Cu:Ni inert sections. In these samples we observe a systematic reduction of heats of reaction, maximum reaction temperatures, and reaction propagation velocities as the volume fraction of reactive material is reduced. At the same time, metrics sensitive to the reaction mechanism and products indicate that there is little if any cross-contamination between inert and reactive material in any of the samples. This indicates that the IMRM samples all undergo the same net reaction (Al/Ni→AlNi) but at a range of different flame temperatures (roughly 1950K to 1300K). Using existing theoretical models for the relationship between flame temperature and propagation velocity, we analyze the experimental data to obtain the activation energy for the mixing process and find that this value varies significantly as the maximum reaction temperature changes. At high reaction temperatures we observe a very low activation energy (26kJ/mol) which suggests diffusion of Ni in molten Al is the rate controlling mixing mechanism in agreement with the conclusions of other studies focused on un-mediated Al/Ni reactive multilayers. However, as the reaction temperature decreases the activation energy appears to shift to much larger values implying a change in the reaction mechanism. We postulate that this shift indicates that solid products are able to form earlier in the reaction at these temperatures, impeding atomic diffusion and intermixing.

Through‐layer XPS investigations of the Si3N4/AlGaN interface

Investigations of chemical species at the dielectric/III‐N interface remain an important question in order to understand the chemical origin of the surface states, which are present at the heterostructure surface. In this work, we demonstrate a sample preparation technique to analyze the existing interface species by X‐ray photoelectron spectroscopy (XPS) through thin uniform silicon nitride (Si3N4) layers 1.4 ± 0.2 nm. We show that it is crucial that the layers are as thin as possible but as thick as necessary to avoid oxygen diffusion through the passivation. Due to the sufficient information depth provided by such layers, the photoelectrons are detected. This is an advantage over sputtering as no intermixing of atoms or cleavage of bonds occurs. We show that the deposited Si3N4 hinders oxygen diffusion through the layer to the AlGaN surface. Selective SiO2 over Si3N4 etching and XPS depth profiles indicate that ∼1 nm of the thin Si3N4 layer has formed a surface oxide. Angle‐resolved XPS measurements confirm the accumulation of oxygen at the top and bottom Si3N4 interfaces. No indication is found that interfacial oxygen is bound to any other than the group‐13 metal. In agreement with the HSAB concept, the interfacial oxygen is bound to Al rather than Ga.Scheme of the nondestructive XPS interface analysis of the dielectric/AlGaN interface.

Spin-orbit torques and spin accumulation in FePt/Pt and Co/Cu thin films from first principles: The role of impurities

Using the Boltzmann formalism based on the first principles electronic structure and scattering rates, we investigate the current-induced spin accumulation and spin-orbit torques in FePt/Pt and Co/Cu bilayers in the presence of substitutional impurities. In FePt/Pt bilayers we consider the effect of intermixing of Fe and Pt atoms in the FePt layer, and find a crucial dependence of spin accumulation and spin-orbit torques on the details of the distribution of these defects. In Co/Cu bilayers we predict that the magnitude and sign of the spin-orbit torque and spin accumulation depend very sensitively on the type of the impurities used to dope the Cu substrate. Moreover, simultaneously with impurity-driven scattering we consider the effect of an additional constant quasiparticle broadening of the states at the Fermi surface to simulate phonon scattering at room temperature, and discover that even a small broadening of the order of 25 meV can drastically influence the magnitude of the considered effects. We explain our findings based on the analysis of the complex interplay of several competing Fermi surface contributions to the spin accumulation and spin-orbit torques in these structurally and chemically non-uniform systems.

Direct growth of Ge quantum dots on a graphene/SiO2/Si structure using ion beam sputtering deposition

The growth of Ge quantum dots (QDs) using the ion beam sputtering deposition technique has been successfully conducted directly on single-layer graphene supported by SiO2/Si substrate. The results show that the morphology and size of Ge QDs on graphene can be modulated by tuning the Ge coverage. Charge transfer behavior, i.e. doping effect in graphene has been demonstrated at the interface of Ge/graphene. Compared with that of traditional Ge dots grown on Si substrate, the positions of both corresponding photoluminescence (PL) peaks of Ge QDs/graphene hybrid structure undergo a large red-shift, which can probably be attributed to the lack of atomic intermixing and the existence of surface states in this hybrid material. According to first-principles calculations, the Ge growth on the graphene should follow the so-called Volmer–Weber mode instead of the Stranski–Krastanow one which is observed generally in the traditional Ge QDs/Si system. The calculations also suggest that the interaction between Ge and graphene layer can be enhanced with the decrease of the Ge coverage. Our results may supply a prototype for fabricating novel optoelectronic devices based on a QDs/graphene hybrid nanostructure.

Analysis of quadrupole splitting of multiple Fe sites intermixed in Si(111) with Mössbauer spectroscopy

The iron silicide has various interesting phases both fundamentally and technologically, which have acquired much attention to date. Iron silicides are often fabricated on a Si substrate by a solid phase epitaxy method, and the initial stage of intermixing of iron atoms with substrate Si is of crucial importance for silicide fabrication, which remains to be clarified. Here, we have investigated the initial stage of the iron-silicide formation before crystallization with Mössbauer spectroscopy suited to characterization of magnetic and chemical properties of 57Fe atoms in materials. The sample was prepared by deposition of 57Fe of 1nm on a Si(111) surface at 450K. Conventional Mössbauer spectroscopy in the energy domain revealed presence of two iron sites with similar quadrupole splits and isomer shifts, which hampered complete analysis of this system. By combining the time-domain spectroscopy using polarized synchrotron radiation, we have separately analyzed the quadrupole splits and isomer shifts for the two iron sites. By using the theoretical simulation, furthermore, we successfully reproduced the experimentally observed time spectrum of the nuclear resonant scattering on the assumption that iron atoms randomly occupy the substitutional sites for Si at the initial stage of intermixing before crystallization of an iron silicide.

Ag on Pt(111): Changes in Electronic and CO Adsorption Properties upon PtAg/Pt(111) Monolayer Surface Alloy Formation.

The electronic and chemical (adsorption) properties of bimetallic Ag/Pt(111) surfaces and their modification upon surface alloy formation, that is, during intermixing of Ag and Pt atoms in the top atomic layer upon annealing, were studied by X-ray photoelectron spectroscopy (XPS) and, using CO as probe molecule, by temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), respectively. The surface alloys are prepared by deposition of sub-monolayer Ag amounts on a Pt(111) surface at room temperature, leading to extended Ag monolayer islands on the substrate, and subsequent annealing of these surfaces. Surface alloy formation starts at ≈600-650 K, which is evidenced by core-level shifts (CLSs) of the Ag(3d5/2 ) signal. A distinct change of the CO adsorption properties is observed when going to the intermixed PtAg surface alloys. Most prominently, we find the growth of a new desorption feature at higher temperature (≈550 K) in the TPD spectra upon surface alloy formation. This goes along with a shift of the COad -related IR bands to lower wave number. Surface alloy formation is almost completed after heating to 700 K.

Potential‐Induced Segregation Phenomena in Bimetallic PtAu Nanoparticles: An In Situ Near‐Ambient‐Pressure Photoelectron Spectroscopy Study

AbstractThe surface properties of a model membrane electrode assembly of a high‐temperature proton‐exchange‐membrane fuel cell with a nanostructured PtAu working electrode were studied under operando conditions (polarization, humidified H2 atmosphere, 150 °C) using near‐ambient‐pressure photoelectron spectroscopy (NAP–XPS). NAP–XPS proved to be a powerful tool for the in situ investigation of the changes in the chemical composition, the segregation of the two metals, and the changes in their oxidation state. Nondestructive depth profiling revealed the influence of the polarization and the gas ambient on the distribution of the two metals in the near‐surface region. The results suggest that the surface of the electrode is covered by a thin layer of gold, which is stable at potentials below the onset of surface‐oxide formation. Anodic oxidation of the PtAu electrode pulls Pt to the surface, leading to intermixing of Pt and Au atoms and the formation of an interfacial alloy.

Atomic variations in digital alloy InGaP/InGaAlP multiple quantum wells due to thermal treatment

High-resolution transmission electron microscopy (HRTEM) images showed that the lattice distortion of as-grown InGaP/InGaAlP multiple quantum wells (MQWs) appeared to be due to the small thickness of the alloyed layers, and that their distortion was relaxed owing to the high atomic mobility. High-angle annular dark-field scanning transmission electron microscopy images demonstrated that the chemical intermixing of Ga and Al atoms between the InAlP and InGaP alloy layers due to thermal annealing relaxed the stress of the InGaAlP layer. The atomic arrangements of the as-grown and annealed MQWs are described on the basis of the experimental results.

Atomic intermixing and interface roughness in short-period InAs/GaSb superlattices for infrared photodetectors

A set of advanced characterization methods, including high-resolution X-ray diffraction (measurements and simulations), cross-sectional scanning tunneling microscopy, and high-angle annular dark-field scanning transmission electron microscopy is applied to quantify the interface roughness and atomic intermixing (in both cation and anion sub-lattices) in short period (6–7 nm) InAs/GaSb superlattices intended for mid-wavelength (M) and long-wavelength (L) infrared detectors. The undesired atomic intermixing and interface roughness in the L-samples were found to be considerably lower than in the M-samples. In all specimens, anion intermixing is much higher than that in the cation sub-lattice. Possible origins of these findings are discussed.

Thin film annealing and alloying of a Au/Cu two-layer system studied by positron annihilation spectroscopy

Abstract The high temperature behavior of a Au(d = 180 nm)/Cu(d = 480 nm) two-layer system was studied by Positron Annihilation Spectroscopy (PAS). Therefor defect sensitive and element specific Coincident Doppler Broadening Spectroscopy (CDBS) with a monoenergetic positron beam was applied. In situ CDBS during isothermal tempering at 633 K for 7 h enabled the observation of both annealing of the Au film and alloying of Au and Cu. The changes in the recorded spectra could be separated into two stages: In the first stage, annealing in the Au film was detected, whereas in the second stage intermixing of Au and Cu atoms was observed. The intermixing zone was found to be homogeneous up to a depth of more than 180 nm. In addition, ab initio calculations were performed in order to determine the composition of the alloyed film, which was found to be Au0.7Cu0.3.

Atomic and electronic structure of GaP/Si(111), GaP/Si(110), and GaP/Si(113) interfaces and superlattices studied by density functional theory

The atomic structure of GaP(111)/Si(111), GaP(110)/Si(110), and GaP(113)/Si(113) heterointerfaces was studied by ab initio calculations employing the density functional theory (DFT). Relative formation energies were computed for the interface layers allowing for atomic intermixing. The application of the electron-counting model, a construction principle used for surface reconstructions, to the case of the GaP(111)/Si(111) interfaces leads to electronic compensation at the heterovalent interfaces and to a reduction of the interface formation energy. The specific equilibrium (111) interface reconstruction can be tuned by changing the chemical potential. In particular, the GaP(111)A/Si(111) interface was found to be abrupt and uncompensated under P-rich conditions, whereas it is compensated under Ga-rich conditions. The GaP(111)B/Si(111) interface was found to be compensated. Contrary to the (111) interfaces, stoichiometric abrupt interfaces were found to be the most energetically favorable for the GaP(110)/Si(110) and the GaP(113)/Si(113) interfaces. These interfaces do not reconstruct. Although both interfaces are compensated, the GaP(113)/Si(113) superlattice exhibits a polarization field, in contrast to the (110) superlattice.

Mechanism of magnetization enhancement at CoO/permalloy interfaces

We investigate the magnetic properties of CoO(111)/permalloy(111) interfaces by ab-initio calculations. We employ a (5 × 5)CoO/(6 × 6)permalloy supercell, to satisfy the ∼5:6 ratio of the lattice constants, and optimize the atomic density near the interface. While experimentally the interface magnetization is 14% higher than in the bulk, we find for the purely O/Co-terminated interface a decrease/increase by 140%/40%, which enables insight into the real interface with partial O deficiency and atomic intermixing. Intermixing between Fe and Ni significantly lowers the total energy, which promotes Fe accumulation at the interface. Since Co-O bonds are energetically favorable, O diffusion into the permalloy is suppressed.

Effect of Helium Ion Irradiation on the Tunneling Behavior in Niobium/Aluminum–Aluminum Oxide/Niobium Josephson Junctions

The study of the effects of high-energy particle irradiation on trilayer Josephson junctions has allowed us to investigate the influence of defects and interfacial intermixing on the junction electrical characteristics. In this paper, we investigated the influence of 2-MeV helium ion irradiation with doses up to 5.25 × 1016/cm2 on the tunneling behavior of Nb/Al-AlOx/Nb Josephson junctions. The modeling of the irradiation-induced chemical changes, combined with transmission-electron-microscopy characterization, indicates that over 2 nm of atomic intermixing occurs at all interfaces. Surprisingly, the quality of the tunneling characteristics is not significantly degraded, although some properties were affected: 1) The onset of the quasi-particle current characteristic of the electrodes' sum gap, Δ1 + Δ2, decreased from 2.8 to 2.6 mV; 2) the temperature onset of the Josephson current dropped from 9.2 K to 9.0 K; and 3) the normal resistance, Rn, increased by <; ~ 20%. The dependence of the Josephson current on the magnetic field was not significantly altered. Modeling using proximity theory indicated that the irradiation-induced changes in the current-voltage (I -V) characteristics are likely due to an increased drop in the order parameter near the barrier interface as a result of a reduced mean free path in the Al proximity layer from Nb and O contamination.

Quantification of atomic intermixing in short-period InAs/GaSb superlattices for infrared photodetectors

Cross-sectional scanning tunneling microscopy (XSTM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are applied for characterizing the MBE-grown short-period InAs/GaSb superlattices for mid- and long-wavelength infrared detection. The focus of this study is on atomic intermixing in close proximity to the interfaces, which is a key issue for device performance. HAADF-STEM permits visualization of the anion-cation dumbbells in individual sub-layers and is even capable to resolve anions and cations separated by 0.15 nm within a dumbbell. On this basis, the interfacial width is extracted with high accuracy. XSTM technique allows us direct counting of certain kinds of atoms within atomic rows, which sheds light on compositional fluctuations and isovalent atomic intermixing. Both methods reveal rather abrupt interfaces with InAs-on-GaSb interfaces being slightly wider than GaSb-on-InAs interfaces. According to HAADF-STEM, in the set of investigated samples, the 90%-to-10% interface width is confined within 2.6–4.4 monolayers (0.8–1.3 nm). In all the samples studied, XSTM analysis showed some residual concentration (on the level of a few atomic percent) of Sb atoms in the middle of the InAs sub-layers, as a result of As/Sb substitutions during growth. The level of intermixing is diminishing with increasing the thickness of individual InAs layers.

Magnetic anisotropy and thermal stability study of perpendicular Co/Ni multilayers

The perpendicular magnetic anisotropy (PMA) and thermal stability of Ta (3.0 nm)/Cu (tCu)/[Co (0.29 nm)/Ni (0.65 nm)]N/Ta (3.0 nm) multilayers are investigated with various Cu thicknesses (tCu) and Co/Ni periodic numbers (N). A relatively thick Cu seed layer and a suitable Co/Ni repetition number benefit the PMA with larger coercivity and remanence ratio. But the magnetic hysteresis loop for the structure with N in excess of 10 exhibits a bow-tie shape due to the formation of multi-domains, which may cause a significant drop of the remanence ratio. The post-annealing experiments show that the thermal stability of PMA increases with increasing N. In addition to the intermixing of Co and Ni atoms at interfaces, we consider that the diffusion of nonmagnetic atoms into the magnetic layers is also responsible for the PMA decay with increasing annealing temperature.

Atomic resolution mapping of interfacial intermixing and segregation in InAs/GaSb superlattices: A correlative study

We combine quantitative analyses of Z-contrast images with composition analyses employing atom probe tomography (APT) correlatively to provide a quantitative measurement of atomic scale interfacial intermixing in an InAs/GaSb superlattice (SL). Contributions from GaSb and InAs in the Z-contrast images are separated using an improved image processing technique. Correlation with high resolution APT composition analyses permits an examination of interfacial segregation of both cations and anions and their incorporation in the short period InAs/GaSb SL. Results revealed short, intermediate, and long-range intermixing of In, Ga, and Sb during molecular beam epitaxial growth and their distribution in the SL.

Study of structure, microstructure and giant magnetoresistance in nanogranular FeCuAg thin films with wide concentration range

Structural, microstructural, morphology and magnetoresistive properties of FexCuyAgz granular thin films exhibiting giant magnetoresistance were investigated. Several series of samples with wide concentration range were prepared by DC magnetron sputtering. The binary FeCu and FeAg systems are the end components of the ternary FexCuyAgz system. The compositions of the samples were determined by energy-dispersive X-ray analysis. X-ray diffraction patterns showed Ag(111) and Cu(111) peaks and the Fe peaks were not observed. The d-spacings calculated from the Ag(111) and Cu(111) peaks are smaller and greater than the standard bulk value for Ag and Cu, respectively, indicating a partial intermixing of Fe, Cu and Ag atoms. The electron diffraction patterns obtained on selected samples showed many Cu and Ag reflections and they also did not show any reflection from Fe. The electron micrographs show the granular structure of these films with typical granule size of 4–5nm. The surface image of the FeAg films observed using a scanning electron microscope showed the presence of droplet-like Ag particles on the film surface. On the other hand the FexCuy and FexCuyAgz films are smooth without any Ag particles on the film surface. Surface morphology of some samples studied by atomic force microscopy showed that the films are smooth and their typical surface roughness is 0.5–2.0nm. Magnetoresistance measurements on the samples up to 20kOe showed typical superparamagnetic behaviour. We have obtained a maximum giant magnetoresistance ratio of 3.8% in these films measured at 300K for an in-plane magnetic field of 20kOe indicating an increase in the value of magnetoresistance due to Ag substitution in the FeCu films. Percolation limit for the Fe grains has also been obtained from the magnetoresistance data.

Strain induced intermixing of Ge atoms in Si epitaxial layer on Ge(111)

The relationship between strain and effective mass of the √3 × √3-Ag structure surface metallic state, when formed on strained Si(111) layers on Ge(111), has been investigated by angle resolved photoelectron spectroscopy at various film thicknesses. Si layer lattice spacing expands and effective mass decreases at film thickness greater than 2 bilayers. This result is inconsistent with a previous study showing increasing effective mass with tensile strain for the √3 × √3-Ag structure. Ge-3d core level photoelectron spectra confirm that this disparity is caused by the intermixing of Ge atoms in the Si layer. The relationship between effective mass and strain is useful for gauging intermixing, and the effective mass of surface metallic states is useful for gauging nanoscale strain.

Arsenic Irradiation Induced Atomic Interdiffusion of In&lt;sub&gt;x&lt;/sub&gt;Ga&lt;sub&gt;1-x&lt;/sub&gt;As/InP Quantum Well Structures

The atomic intermixing of In x Ga 1-x As/InP quantum well structures induced by arsenic irradiation has been investigated using photoluminescence (PL) measurement. The ion doses used were varried from 5x10 11 As/cm 2 to 1x10 13 As/cm 2 . Also, the irradiation temperature were carried out from -200 o C to 300 o C. The samples were annealed under Argon flow in rapid thermal annealer (RTA) at 750 o C for 60 second. The photoluminescence result showed that there was increase in energy shift at lower doses. However, the energy shift was saturated at higher dose. At elevated temperature implantation showed that the energy shift did not change significantly for all the samples (LM, TS, CS). In additon to this, the higher energy shift was observed in the Compressive Strain (CS) samples, but the lower of the energy shift was experienced in the Tensile Strain (TS) samples. Received: 06 September 2012; Revised: 27 November 2012; Accepted: 03 December 2012

Crystal growth of platinum–ruthenium bimetallic nanocrystallites and their methanol electrooxidation activity

This study investigates how the nucleation sequences of Pt and Ru crystallites affect the structure evolutions along with the methanol electrooxidation activity of bimetallic PtRu nanoparticles (BiNPs). We found that the BiNPs crystallites would transform from cubic to tetrakaidecahedron crystals on increasing the Ru content from 0 to 54 at%. This reduces the particle size and the size distributions during the nucleation-growth of the crystal. In the meantime, the unfilled d state and the heteroatomic intermixing of Pt atoms along with the MOR reactivity of the BiNPs will be enhanced. By increasing Ru >54 at%, the BiNPs would transform into an ellipsoid-like crystallite with a Ru rich core. This is because the Pt anions would replace the metallic Ru atoms and form a discrete sub-nano Pt nuclei at the NPs surface. Consequently, the surface MOR activity of the BiNPs is substantially suppressed.