Parallel Electron Energy Loss Research Articles

Low-dose electron energy-loss spectroscopy using electron counting direct detectors.

Since the development of parallel electron energy loss spectroscopy (EELS), charge-coupled devices (CCDs) have been the default detectors for EELS. With the recent development of electron-counting direct-detection cameras, micrographs can be acquired under very low electron doses at significantly improved signal-to-noise ratio. In spectroscopy, in particular in combination with a monochromator, the signal can be extremely weak and the detection limit is principally defined by noise introduced by the detector. Here we report the use of an electron-counting direct-detection camera for EEL spectroscopy. We studied the oxygen K edge of amorphous ice and obtained a signal noise ratio up to 10 times higher than with a conventional CCD.We report the application of electron counting to record time-resolved EEL spectra of a biological protein embedded in amorphous ice, revealing chemical changes observed in situ while exposed by the electron beam. A change in the fine structure of nitrogen K and the carbon K edges were recorded during irradiation. A concentration of 3 at% nitrogen was detected with a total electron dose of only 1.7 e-/Å2, extending the boundaries of EELS signal detection at low electron doses.

Effects of Cu and Ag additions on age-hardening behavior during multi-step aging in AlMgSi alloys

Low Cu and Ag additions (≤0.10at%) were found to strongly affect the age-hardening behavior in AlMgSi alloys with Mg+Si>1.5at%. The hardness increased during aging at 170°C and the formation of β″ precipitates was kinetically accelerated. The activation energy of the formation of the β″ phase was calculated to 127, 105, 108 and 99KJmol−1 in the base, Cu-added, Ag-added and CuAg-added alloys, respectively using the Kissinger method. The negative effect of two-step aging caused by the formation of Cluster (1) during natural aging was not overcome by the addition of microalloying elements. However, it was suppressed by the formation of Cluster (2) through a pre-aging at 100°C. Quantitative analysis of the precipitate microstructure was performed using a transmission electron microscope equipped with a parallel electron energy loss spectrometer for the determination of specimen thickness. The formation of Cluster (2) was found to increase the number density of β″ precipitates, whereas the formation of Cluster (1) decreased the number density and increased the needle length. The effects of low Cu and Ag additions in combination with multi-step aging are discussed based on microstructure observations and hardness and resistivity measurements.

Cyanoethylation of the glucans dextran and pullulan: Substitution pattern and formation of nanostructures and entrapment of magnetic nanoparticles.

Cyanoethylglucans with a degree of substitution in the range of 0.74 to 2.40 for dextran and 0.84 to 2.42 for pullulan were obtained by Michael addition of acrylonitrile to the glucans under various conditions. Products were thoroughly characterized, comprising elementary analysis, NMR and ATR–IR spectroscopy, and analysis of the substituent distribution in the glucosyl units by GC–FID and GC–MS of the constituting monosaccharide derivatives. Nanostructuring of the highly substituted cyanoethylpolysaccharides was performed by dialysis against a non-solvent. In the presence of ferromagnetic iron-oxide nanoparticles, multicore cyanoethylglucan-coated ferromagnetic nanoparticles were formed by selective entrapment. The specific interaction between cyano groups and iron could be proven. The size distribution and morphology of the nanoparticles were analyzed by dynamic light scattering (DLS), scanning electron microscopy (SEM) and energy-filtered transmission electron microscopy (EF–TEM) with parallel electron energy loss spectroscopy (PEELS).

Investigation of inter-diffusion in bilayer GeTe/SnSe phase change memory films

A metal-chalcogenide layer, SnSe, is inserted between the memory layer GeTe and the top electrode to form a phase change memory cell. The GeTe layer exhibits ovonic threshold switching at a threshold field of ~110V/μm. For subsequent implementation into applications and reliability, material inter-diffusion and sublimation are examined in bilayer phase change films of GeTe/SnSe. Transmission electron microscopy and parallel electron energy loss spectroscopy analyses reveal Sn migration to the GeTe layer, which is responsible for lowering the rhombohedral to cubic structural transformation temperature in GeTe. Incongruent sublimation of SnSe and GeTe is observed at temperatures higher than 500°C. Severe volatilization of Se results in the separation of a metallic Sn phase. The use of Al2O3 as a capping layer has been found to mitigate these effects.

A Study of Dual Phase Steel Damage Evolution with Microstructure

A detailed analysis of the evolution of industrial Dual Phase (DP) steel microstructures is carried out as a function of various annealing and tempering conditions. Advanced characterization techniques such as Parallel Electron Energy Loss Spectroscopy (PEELS) in the TEM and high spatial resolution Secondary Ion Mass Spectrometry (NanoSIMS) are employed in order to provide qualitative and quantitative measurements of local carbon concentration in the martensite. For certain annealing and tempering conditions, it is observed that local variations in carbon levels have occurred inside the individual martensite islands. These carbon variations strongly influence the damage behaviour of the steel. During tensile tests, a clear dependence of the damage mode on the local martensite carbon content is observed. Better knowledge of the relationship between the microstructure evolution at the sub-grain level and the damage behaviour can facilitate the design of DP steels with improved damage resistance.

Microstructure and intergranular diffusion in exchange-coupled Sm–Co/Fe nanocomposites

We demonstrate homogenous distribution of bcc FeCo soft phase with grain size of 20–30 nm in the annealed Sm–Co/Fe bulk samples by energy filtered transmission electron microscopy (EFTEM). Quantitative Co/Fe interdiffusion measured using both energy dispersive spectroscopy (EDS) and parallel electron energy loss spectroscopy (PEELS) shows Fe60±5%Co40±5% for the magnetically soft bcc phase and Sm2(Co0.82Fe0.18)7 and Sm(Co0.80Fe0.20)5 respectively for the magnetically hard phases in these two alloy systems after optimal annealing. The graded interface develops in both samples due to the Co/Fe interchange between the hard and soft phases and the bcc soft phase was determined.

Crystallization and grain growth behavior of CoFeB and MgO layers in multilayer magnetic tunnel junctions

The relationship between crystallization, grain growth behavior, and the diffusion of B out of CoFeB has been investigated in annealed film stacks of sputtered CoFeB∣MgO using a combination of two dimensional x-ray diffraction, transmission electron microscopy, and parallel electron energy loss spectroscopy (PEELS). The analysis shows grain growth in MgO layers. It shows crystallization at approximately 350°C, and subsequent grain growth in CoFeB layers with annealing. The orientations of the grains of MgO and CoFe are definitively shown to be (002) in the out-of-plane direction. The MgO lattice is seen to have an in-plane tensile stress, while CoFe lattice is shown to have an in-plane compressive stress. CoFe grains are observed to be smaller than MgO grains, rather than being of equal size as previously understood. The physical process of B diffusion into MgO has also been investigated using PEELS and is determined that the diffusion of B through MgO is mediated through vacancies and defect states by the formation of BOx complexes.

Study of boron diffusion in MgO in CoFeB∣MgO film stacks using parallel electron energy loss spectroscopy

Boron diffusion in MgO has been investigated in annealed film stacks of sputtered CoFeB∣MgO using transmission electron microscopy and parallel electron energy loss spectroscopy. The analyses show significant B movement when the films are annealed, with the formation of BOx complexes. Characteristic diffusion lengths have been estimated in films annealed at the commonly employed temperature range of 300–400°C for the fabrication of magnetic tunnel junctions. An activation energy of 1.3eV (±0.4eV) has been extracted from these data that represent B diffusion in MgO through vacancies and defect states mediated by the formation of BOx complexes.

Metal-organic vapor phase epitaxy of crystallographically oriented MnP magnetic nanoclusters embedded in GaP(001)

Hybrid ferromagnetic-semiconductor GaP:MnP thin films were grown at 650 °C by metal-organic vapor phase epitaxy on GaP(001) using trimethylgallium, tertiarybutylphosphine, and methyl cyclopentadienyl manganese tricarbonyl (MCTMn). Overall Mn concentrations in the hybrid films, determined by Rutherford backscattering spectrometry, were found to be nearly proportional to the MCTMn precursor gas flow rate and ranged from 2 to 3.5 at. %. Cross-sectional transmission electron microscopy (TEM) analyses revealed the presence of a homogeneous distribution of 15–30 nm wide nanoclusters in a dislocation-free GaP matrix that is fully coherent with the substrate. The nanocluster facets are predominantly aligned along the (220) planes of the GaP matrix and selected-area electron diffraction patterns in TEM indicate that the nanoclusters are coherent (or semicoherent) with the single-crystal GaP matrix. The Mn:P composition ratio in the nanoclusters was determined to be 1.00±0.05 from parallel electron energy loss spectroscopy analyses. Increasing the MCTMn flow rate during film growth resulted in an increased concentration of MnP nanoclusters in the epilayer while their dimensions remained virtually unchanged. Magnetometric characterization indicates a ferromagnetic order, with a Curie temperature of about 294 K, originating from the MnP clusters.

Determination of local carbon content in austenite during intercritical annealing of dual phase steels by PEELS analysis

Parallel electron energy loss spectroscopy has allowed to analyse and quantify local variations in the carbon concentration of austenite islands transformed during the intercritical annealing treatment of commercial dual-phase steels. These changes in the carbon content of different austenite regions are responsible for the different volume fractions of tempered martensite, martensite and retained austenite obtained after intercritical annealing and overaging treatment. This technique reveals how carbon distribution in austenite evolves as the transformation process advances.

Direct Measurement and Interpretation of Electrostatic Potentials at 24° [001] Tilt Boundaries in Undoped and Niobium-Doped Strontium Titanate Bicrystals

Phase contrast techniques in the transmission electron microscope are used to measure electrostatic potentials at 24° [001] tilt boundaries in nominally undoped and Nb-doped SrTiO3 bicrystals. All of the boundaries are found to have lower scattering potentials than the surrounding bulk crystal. Origins for the measured changes in potential are investigated through the application of high-resolution transmission electron microscopy, diffuse dark-field imaging, energy dispersive X-ray spectroscopy, and parallel electron energy loss spectroscopy. Significantly, we show that space charge is not the dominant contribution to the potential because the magnitudes of the potentials seen cannot be explained on the basis of ionic space charge theory alone.

Mineralogical characterization and genesis of hydrothermal Mn oxides from the flank of the Juan the Fuca Ridge

Several sites of active hydrothermal flow have been found on the eastern flank of the Juan de Fuca Ridge. These sites are typically located along the edge of basaltic outcrops where sediment is thin. We present data on Mn-oxides formed on such outcrops (Zona Bare and Grinin Bare). These oxides are either black-layered crust or soft micro-concretions found in partially altered sediments. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses of Mn crusts indicate the presence of well-crystallized todorokite and birnessite encrusting detrital minerals and replacing siliceous fossil. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) analyses were used to identify amorphous and poorly crystallized Mn-rich phases in partially altered sediments and crusts. TEM of impregnated samples showed textural evidence suggesting that amorphous Mn oxides are incrusting cellular structures that could be bacteria. The valence state of Mn in these oxides was determined by parallel electron energy loss spectroscopy (PEELS). Results indicate that todorokite and birnessite have an average valence state of about 3.7 whereas the poorly crystallized Mn-rich phases have a lower valence state. These data suggest that the formation of hydrothermal Mn concretions occurs in several steps. The initial step is the adsorption or precipitation of Mn, Fe, and Si around cell-wall bacteria, extracellular polymers, and siliceous fossil remains. These mineralizations are poorly crystallized phyllomanganates, which progressively increase in size and crystallinity to give the final birnessite and todorokite products. All of these Mn-rich phases are the result of interactions between hydrothermal fluid and sediments and formed in areas where hydrothermal fluids discharge through the sediment.

Multi-dot floating-gates for nonvolatile semiconductor memories: Their ion beam synthesis and morphology

Scalability and performance of current flash memories can be improved substantially by replacing the floating polycrystalline-silicon gate by a layer of Si dots. Here, we present both experimental and theoretical studies on ion beam synthesis of multi-dot layers consisting of Si nanocrystals (NCs) embedded in the gate oxide. Former studies have suffered from the weak Z contrast between Si and SiO2 in transmission electron microscopy (TEM). This letter maps Si plasmon losses with a scanning TEM equipped with a parallel electron energy loss spectroscopy system. Kinetic Monte Carlo simulations of Si phase separation have been performed and compared with Si plasmon maps. Predicted and measured Si morphologies agree remarkably well, both change with increasing ion fluence from isolated NCs to spinodal pattern. However, the predicted fluences are lower than the experimental ones. We identify as the main reason of this discrepancy the partial oxidation of implanted Si by atmospheric humidity, which penetrates into the as-implanted SiO2.

Evolution of precipitates, in particular cruciform and cuboid particles, during simulated direct charging of thin slab cast vanadium microalloyed steels

A study has been undertaken of four vanadium based steels which have been processed by a simulated direct charging route with processing parameters typical of thin slab casting, where the cast product has a thickness of 50 to 80 mm (in this study 50 mm) and is fed directly to a furnace to equalise the microstructure prior to rolling. In the direct charging process, cooling rates are faster, equalisation times shorter, and the amount of deformation introduced during rolling less than in conventional practice. Samples in this study were quenched after casting, after equalisation, after the fourth rolling pass, and after coiling, to follow the evolution of microstructure. The mechanical and toughness properties and the microstructural features might be expected to differ from equivalent steels which have undergone conventional processing. The four low carbon steels (~0.06 wt-%) which were studied contained 0.1 wt-%V (V – N), 0.1 wt-%V and 0.010 wt-%Ti (V – Ti), 0.1 wt-%V and 0.03 wt-%Nb (V – Nb), and 0.1 wt-%V, 0.03 wt-%Nb and 0.007 wt-%Ti (V – Nb – Ti). steels V – N and V – Ti contained around 0.02 wt-% N, while the other two contained about 0.01 wt-%N. The as cast steels were heated at three equalising temperatures of 1050°C, 1100°C, or 1200°C and held for 30 – 60 min before rolling. Optical microscopy and analytical electron microscopy, including parallel electron energy loss spectroscopy (PEELS), were used to characterise the precipitates. In the as cast condition, dendrites and plates were found. Cuboid particles were seen at this stage in steel V – Ti, but they appeared only in the other steels after equalisation. In addition, in the final product of all the steels, fine particles were seen, but it was only in the two titanium steels that cruciform precipitates were present. PEELS analysis showed that the dendrites, plates, cuboids, cruciforms, and fine precipitates were essentially nitrides. The two Ti steels had better toughness than the other steels but inferior lower yield stress values. This was thought to be, in part, due to the formation of cruciform precipitates in austenite, thereby removing nitrogen and the microalloying elements, which would have been expected to precipitate in ferrite as dispersion hardening particles.

Ultrastructure of the calcium-sequestering gastrodermal cell in the hydroid Hydractinia symbiolongicarpus (Cnidaria, Hydrozoa).

Large, free-floating crystals of calcium carbonate occur in vacuoles of gastrodermal cells of the hydroid Hydractinia symbiolongicarpus. Here, morphological details about the process by which these cells accumulate and sequester calcium are provided by a cytochemical method designed to demonstrate calcium at the ultrastructural level. Electron-dense material presumably indicative of the presence of calcium was EGTA-sensitive and was shown by parallel electron energy loss spectroscopy (EELS) and energy spectroscopic imaging (ESI) to contain calcium. Calcium occurred in only one cell type, the endodermally derived gastrodermal cell. In these cells, the electron-dense material appeared first as a fine precipitate in the cytosol and nucleus and later as larger deposits and aggregates in the vacuole. During the life cycle, gastrodermal cells of the uninduced planula and the planula during metamorphic induction sequestered calcium. In primary polyps and polyps from established colonies, gastrodermal cells sequestered calcium, but the endodermal secretory cells did not. Our observations support the hypothesis that gastrodermal cells function as a physiological sink for calcium that enters the organism in conjunction with calcium-requiring processes such as motility, secretion, and metamorphosis.

The Effects of Vanadium, Niobium, Titanium and Zirconium on the Microstructure and Mechanical Properties of Thin Slab Cast Steels

The evolution of precipitation and microstructure during a simulation of the thin slab direct rolling process, in six vanadium based, low carbon, steels with V, V-N, V-Ti-N, V-Nb, V-Nb-Ti and V-Zr additions was studied by optical microscopy, analytical transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and parallel electron energy loss spectroscopy (PEELS). Tensile properties and Charpy vee-notch toughness of the final strip were also determined. The effects of microalloying additions and processing conditions, including equalisation temperature (1 200°C, 1 100°C and 1 050°C) and end water cool temperature, on the austenite and ferrite grain sizes, as well as the type and composition of the precipitates, were determined. The relationship between the microstructure and the properties in the steels was also ascertained.

Electron spectroscopy of nanodiamond surface states

Electronic states of nanodiamond (ND) were investigated by PEELS, XPS and CKVV Auger spectra. Parallel electron energy loss spectra (PEELS) show that the electrons inside of ND particles are sp 3 hybridized but there is a surface layer containing distinct hybridized states. The CKVV Auger spectra imply that the HOMO of the ND surface has a shift of 2.5 eV from natural diamond levels of σ p up to the Fermi level. Hydrogen (H) treatment of natural diamond surface produces a chemical state indistinguishable from that of ND surfaces using CKVV. The ND electronic structure forms σ s 1σ p 2π 1 surface states without overlapping of π-levels. Surface electronic states, including surface plasmons, as well as phonon-related electronic states of the ND surface are also interesting and may also be important for field emission mechanisms from the nanostructured diamond surface.

Atomic layer chemical vapor deposition of ZrO2-based dielectric films: Nanostructure and nanochemistry

A 4 nm layer of ZrOx (targeted x∼2) was deposited on an interfacial layer (IL) of native oxide (SiO, t∼1.2 nm) surface on 200 mm Si wafers by a manufacturable atomic layer chemical vapor deposition technique at 300 °C. Some as-deposited layers were subjected to a postdeposition, rapid thermal annealing at 700 °C for 5 min in flowing oxygen at atmospheric pressure. The experimental x-ray diffraction, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and high-resolution parallel electron energy loss spectroscopy results showed that a multiphase and heterogeneous structure evolved, which we call the Zr–O/IL/Si stack. The as-deposited Zr-O layer was amorphous ZrO2-rich Zr silicate containing about 15% by volume of embedded ZrO2 nanocrystals, which transformed to a glass nanoceramic (with over 90% by volume of predominantly tetragonal-ZrO2 (t-ZrO2) and monoclinic-ZrO2 (m−ZrO2) nanocrystals) upon annealing. The formation of disordered amorphous regions within some of the nanocrystals, as well as crystalline regions with defects, probably gave rise to lattice strains and deformations. The interfacial layer (IL) was partitioned into an upper SiO2-rich Zr silicate and the lower SiOx. The latter was substoichiometric and the average oxidation state increased from Si0.86+ in SiO0.43 (as-deposited) to Si1.32+ in SiO0.66 (annealed). This high oxygen deficiency in SiOx was indicative of the low mobility of oxidizing specie in the Zr–O layer. The stacks were characterized for their dielectric properties in the Pt/{Zr–O/IL}/Si metal oxide-semiconductor capacitor (MOSCAP) configuration. The measured equivalent oxide thickness (EOT) was not consistent with the calculated EOT using a bilayer model of ZrO2 and SiO2, and the capacitance in accumulation (and therefore, EOT and kZr−O) was frequency dispersive, trends well documented in literature. This behavior is qualitatively explained in terms of the multilayer nanostructure and nanochemistry that evolves.

High-Resolution Analytical Electron Microscopy Investigation of Metastable Tetragonal Phase Stabilization in Undoped, Sol-Gel Derived Zirconia Nanoceramics

ABSTRACTThe mechanisms underlying stabilization of the metastable tetragonal (t)-phase in sol-gel derived, nanocrystalline ZrO2 were studied by high-resolution analytical electron microscopy, utilizing parallel electron-energy loss (PEEL) and energy-dispersive X-ray spectroscopies. The powders were synthesized by hydrolysis of Zr (IV) n-propoxide at ratios of molar concentration of water to Zr n-propoxide, R=5 and 60, respectively, followed by calcination at 400°C. Dense particles of the as-precipitated ZrO2 (R=5) revealed 4–11 nm-sized nanocrystals embedded in the amorphous matrix that may serve as nuclei for the t-phase during calcination. The calcined particles consist of 10–100 nm–sized t-crystals. For as-precipitated ZrO2 (R=60), week aggregates (50–100 nm) of largely amorphous 4–20 nm-sized particles after calcination yield a mixture of t-and monoclinic (m-) nanocrystals. PEELS fingerprints of the band structure with the intensity threshold matching the expected position of a direct bandgap at 4–5 eV allow to differentiate between the amorphous and nanocrystalline ZrO2. Stabilization of t-phase (R=5) with sizes up to 16 times larger than reported earlier is likely due to strain-induced confinement from surrounding growing grains, which suppress the volume expansion associated with the martensitic t-m transformation. For R=60, loose nanoparticle agglomerates cannot suppress the transformation. In this case, the t-phase may be partially stabilized due to a crystal size effect and /or to the presence of m-phase.

Oxide Reduction in Advanced Metal Stacks for Microelectronic Applications

ABSTRACTAluminum and copper are widely used for microelectronic interconnect applications. Interfacial oxides can cause device performance degradation and failure by significantly increasing electrical resistance. Interfacial oxide layers found in Al/Ta and Ta/Cu metal stacks were studied using Transmission Electron Microscopy (TEM) combined with Energy Dispersive Spectroscopy (EDS) and Parallel Electron Energy Loss Spectroscopy (PEELS). The analysis indicates that the observed interfacial oxide layers, Al2O3 and mainly Ta2O5, result from spontaneous reductions of Ta oxide and Cu oxide, respectively. Thermodynamics enables interpretation of the results.