Electron Energy Loss Spectroscopy Spectroscopy Research Articles

(Invited) Advanced Characterization of III-Nitrides: From 2-D Epi-Layers to Bulk Defect Analysis

Electron Microscopy is a powerful tool for aid in the development of new compound semiconductors because it has the ability to characterize materials from the atom (aberration-corrected Transmission Electron Microscopy) to the millimeter length scale (montaged scanning electron microscopy - SEM). By including important aspects of the electron-sample interaction in a microscopy, we can couple imaging with crystallography (diffraction), chemistry (energy-dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS)) and chemical bonding (electron energy loss spectroscopy) at ultrahigh resolution. All of these techniques allow us to probe defect structure, the fine details of interfaces, and the growth mechanism in electronic materials.Recently, the concept of remote epitaxy has been introduced, with the idea of growing semiconductors through a “veil” of 2-D material – demonstrated first in GaAs/Graphene/GaAs. This facile method demonstrates the new methods for creating defect-reduced but transferable thin films, where the deposited atoms line up “through the veil” across from their substrate counterparts. This exciting development has spurred great interest in similar types of epitaxy and understanding their details at the atomic scale. It also allows for easy transfer of near-perfect films to other substrates.In this talk, I will present how EM can elucidate materials problems through several examples different types of heteroepitaxy: the first example is epitaxy of 2-dimensional forms of III-N and III-O and group III metals on SiC mediated through epitaxial graphene (EG), a technique similar to but not exactly remote epitaxy. This technique, known as confinement heteroepitaxy (CHet) is a fabrication technique for 2D heterostructure in which atoms are intercalated between defective epitaxial graphene and substrates of silicon carbide in a CVD process typically at 800 °C. The confined layers can be designed with the merit of intercalation in several combinations and has the potential to create wafer-scale van der Waals heterostructures. I will discuss the details of the SiC/EG/III-V/Graphene growth, as elucidated using TEM imaging, SEM imaging, and atomic resolution EELS.A second example is the idea of a self-assembled heteroepitaxy of CdTe on sapphire. Grown by pulsed laser deposition, we discovered using advanced EM that the CdTe spontaneously forms a van der Waals interface of Te-Te bonds, thus making these films easy to peel/transfer from the substrate. We used atomic resolution imaging, EELS spectroscopy both of composition and of the interface plasmon to demonstrate this spontaneous and unexpected coordination at the interface. This exciting example of 3D-3D epitaxy with van der Waals interface could be developed for novel semiconductor devices or in other materials systems, and shows that advanced EM techniques are breakthrough enabling tool for compound semiconductor development.

Effect of Thermally Induced Oxygen Vacancy of α-MnO2 Nanorods toward Oxygen Reduction Reaction.

MnO2 has been explored for various applications in environmental and energy aspects. However, the thermal sensitivity of the MnO2 crystal structure never been studied. As a potential cathode material for fuel cell, α-MnO2 has a higher specific activity than Pt/C based on per metals cost. In this work, the physical and electrochemical properties of α-MnO2 nanorods were explored for the first time under thermal treatment with different temperatures (300, 400, and 500 °C). Under thermal treatment, oxygen vacancies were induced. The high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) have been taken to explore oxygen vacancies of α-MnO2 materials. From EELS and X-ray photoelectron spectroscopy (XPS) analysis, the oxygen vacancies on the α-MnO2 nanorods were strengthened with the temperature increasing. The sample with 400 °C treatment exhibited the best performance toward ORR, excellent methanol tolerance and higher stability compared to commercial Pt/C in alkaline media due to its combination of preferable growth on (211) plane and moderate oxygen vacancies as well as coexistence of Mn (IV)/ Mn (III) species. It was also observed the α-MnO2 nanorods tended to become longer and thinner with increasing temperature. This work suggests that the α-MnO2 nanorods are thermal sensitive materials and their performance for ORR can be boosted under certain temperatures.

Characterization of single-atom catalysts by EELS and EDX spectroscopy

Fitted with a field emission source, aberration-corrected optics and an energy-dispersive X-ray detector of large solid angle, a modern analytical TEM can generate a current density high enough to chemically identify a single metal atom within a fraction of a second, if the atom remains stationary within the electron probe. However, atom motion will occur if the atomic binding energy is too low, the specimen temperature too high, or the electron accelerating voltage above a certain threshold. We discuss such motion in terms of thermal diffusion, beam-induced sputtering and beam-assisted surface migration. Calculations based on a Rutherford-scattering approximation suggest that when atomic displacement is possible, it drastically reduces the analytical signal and signal/noise ratio. For certain elements, electron energy-loss spectroscopy (EELS) provides a higher detectability than energy-dispersive X-ray (EDX) but suffers from the same problem of atomic displacement.

Effect of Electron Beam Irradiation on Polymers

Electron energy loss spectroscopy (EELS) in combination with transmission electron microscopy (TEM) is widely used for chemical state analysis of variety of chemical compounds. High beam sensitivity of substances like polymers hinders the possibility of exploring in-depth analysis provided through the high spatially resolved EELS spectroscopy. In this study, the electron beam irradiation damage on polymers were analyzed with varying dose of electron beams. The stability of the polymers under electron beam exposure depends on the chemical structure on the polymers. In this study the polymers with and without phenyl groups namely Polycarbonate, Polyethylene terephthalate, Polystyrene, Styrene Maleic Anhydride and Polymethylmethacrylate are selected for the comparative degradation study. Effect of varying the electron dose on the stability of polymers were monitored by recording the low-loss EELS spectrum in π to π* transition and (π+σ) to (π+σ)* transition region.

From a physicist's toy to an indispensable analytical tool in many fields of science: A personal view of the leading contribution of Ondrej Krivanek to the spectacular successes of EELS spectroscopy in the electron microscope

This contribution aims at reporting, from the subjective point of view of a witness based in Orsay, the fundamental role of Ondrej Krivanek in the spectacular emergence of EELS (Electron Energy-Loss Spectroscopy) as a key tool in analytical electron microscopy. In this regard, he has successively designed and built while he was at Gatan, serial EELS spectrometers, parallel EELS spectrometers and post-column energy filters which have been fitted to many different (S)TEM columns installed around the world. More recently the implementation of monochromators on the NION dedicated STEM together with the realization and performance of aberration correctors (which lie out of the scope of the present review), have placed the most advanced instrumental tool in the hands of continuously increasing populations of users in many domains of materials science and in life sciences. Furthermore, the impact of Ondrej Krivanek has spread widely beyond his technical achievements into that of a highly respected organizer of workshops, bringing together at regular intervals, all the experts from around the world and building up a real community of scientists.

Self-Formed Mn Oxide Barrier on SiOCH for Nanoscale Copper Interconnect by Metal Organic Chemical Vapor Deposition of Mn

Both the formation of self-formed barrier (SFB) of Mn oxide on porous low dielectric constant (low-k) SiOCH trench and Cu filling (MOCVD) in order were carried out by in-situ metal organic chemical vapor deposition to make Cu interconnect. Oxygen-plasma pretreatment of the low-k dielectrics surface enhanced the uniformity of deposited Mn layers as well as the formation of SFB prior to Cu MOCVD. X-ray photoelectron spectroscopy confirmed the presence of amorphous MnOx and MnSi(y)Oz layers in SFB. Electron energy loss spectroscopy (EELS) measurements of cross-sectional samples of the deposited layers on porous low-k blanket and trench surfaces enabled in-depth analysis of the elemental composition of the Cu/SFB multilayers with high spatial resolution. The effectiveness of the SFB layer in protecting Cu diffusion into the low-k layer was proven by EELS and energy dispersive X-ray spectroscopy (EDX) analyses. The Mn L3/L2 intensity ratio from EELS data enabled us to identify the possible compounds of the SFB layers including MnSiO3 near the dielectric surface, Mn2O3 near the Cu layer, and Mn3O4 at the intermediate region.

Aberration-corrected transmission electron microscopy analyses of GaAs/Si interfaces in wafer-bonded multi-junction solar cells

Abstract Aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) investigations have been applied to investigate the structure and composition fluctuations near interfaces in wafer-bonded multi-junction solar cells. Multi-junction solar cells are of particular interest since efficiencies well above 40% have been obtained for concentrator solar cells which are based on III-V compound semiconductors. In this methodologically oriented investigation, we explore the potential of combining aberration-corrected high-angle annular dark-field STEM imaging (HAADF-STEM) with spectroscopic techniques, such as EELS and energy-dispersive X-ray spectroscopy (EDXS), and with high-resolution transmission electron microscopy (HR-TEM), in order to analyze the effects of fast atom beam (FAB) and ion beam bombardment (IB) activation treatments on the structure and composition of bonding interfaces of wafer-bonded solar cells on Si substrates. Investigations using STEM/EELS are able to measure quantitatively and with high precision the widths and the fluctuations in element distributions within amorphous interface layers of nanometer extensions, including those of light elements. Such measurements allow the control of the activation treatments and thus support assessing electrical conductivity phenomena connected with impurity and dopant distributions near interfaces for optimized performance of the solar cells.

Capturing the signature of single atoms with the tiny probe of a STEM

With their first scanning transmission electron microscope (STEM), Albert Crewe and his collaborators have succeeded 40 years ago in bringing to reality a dream for all electron microscopists, to see individual atoms. In the derivation of Crewe's pioneering work, the present review describes various historical and present steps, involving continuous instrumental and methodological developments as well as the preparation of suitable specimens. They have lead to the identification of individual atoms by electron energy-loss spectroscopy (EELS) and to the demonstration of atom-by-atom spectroscopy. Beyond these spectacular successes which open wide fields of use, most recent technical achievements, such as the introduction of monochromators on the incident electron beam or of optical spectrometers for recording spectra (in the visible as well as in the X-ray domain), will undoubtedly lead to refine the accessible signature of single atoms and molecules.

Study by AES, EELS Spectroscopy of electron Irradiation on InP and InPO4/InP in comparison with Monte Carlo simulation

We give the great interest to characterise the InP and InPO4/InP submitted to electron beam irradiation owing to the Auger Electron Spectroscopy (AES) associated to both methods Electron Energy Loss Spectroscopy (EELS). The incident electron produces breaking of (In-P) chemical bonds. The electron beam even acts to stimulate oxidation of InP surface involving on the top layers. Other, the oxide InPO4 developed on InP does appear very sensitive to the irradiation due to electron beam shown by the monitoring of EELS spectra recorded versus the irradiated times of the surface. There appears a new oxide thought to be In2O3. We give the simulation methods Casino (Carlo simulation of electron trajectory in solids) for determination with accuracy the loss energy of backscattered electrons and compared with reports results have been obtained with EELS Spectroscopy. These techniques of spectroscopy alone do not be able to verify the affected depth during interaction process. So, using this simulation method, we determine the interaction of electrons in the matter.

STUDY BY AES AND EELS OF InP, InSb, InPO4 AND InxGa1-xAs SUBMITTED TO ELECTRON IRRADIATION

The surface of materials plays an important role in their technological applications. In the interest to study the stability of materials and their behavior, we irradiate them by the electrons by using the electron spectroscopy such as the Auger electron spectroscopy (AES) and the electron energy loss spectroscopy (EELS). These methods have proved their good sensitivity to study material surfaces. In this paper, we give some results about the effect of the electron beam irradiating the compounds InP , InSb , InPO4 and In x Ga 1-x As . The III–V semiconductors InP and InSb seem to be sensitive to the electron irradiation. This breaks the chemical bonds between the element III and V which leads to an oxidation process at the surface. The AES and EELS spectroscopy are also used to characterize the oxide InPO4 whose thickness is about 10 Å grown on the substrate InP (100). The irradiation of the system InPO4/InP(100) by the electron beam of 5 keV energy leads to a structural change of the surface, so that there is breaking of chemical bonds between indium and phosphorus (In–P) and formation of new oxide other than InPO4 . In this study we show an important result concerning the effect of the electron beam on the compound In x Ga 1-x As by varying the parameter x to obtain In0.2Ga0.8As and In0.53Ga0.47As . It appears that the electron beam affects In0.2Ga0.8As too much in comparison with In0.53Ga0.47As . In the case of the irradiation of In0.2Ga0.8As , there is breaking of chemical bonds between indium and GaAs leading to formation of indium oxide associated to GaAs .

Aberration-corrected scanning transmission electron microscopy of semiconductors

The scanning transmission electron microscope (STEM) has been able to image individual heavy atoms in a light matrix for some time. It is now able to do much more: it can resolve individual atoms as light as boron in monolayer materials; image atomic columns as light as hydrogen, identify the chemical type of individual isolated atoms from the intensity of their annular dark field (ADF) image and by electron energy loss spectroscopy (EELS); and map elemental composition at atomic resolution by EELS and energy-dispersive X-ray spectroscopy (EDXS). It can even map electronic states, also by EELS, at atomic resolution. The instrumentation developments that have made this level of performance possible are reviewed, and examples of applications to semiconductors and oxides are shown.

Anisotropic Surface Effect on Electronic Structures and Electrochemical Properties of LiCoO2

Increasing industrial needs for rechargeable batteries delivering higher power has encouraged advanced research on nanosized electrochemically active compounds. Although nanosized electrode materials are expected to possess much higher power output due to short diffusion length, the nanosize effects on electrodes have not been understood well to date. Here, we report that nanosized LiCoO2 as suggested as electrode material for Li-ion rechargeable batteries has anisotropic surface properties affecting electronic structures, which is evidenced by electron energy loss spectroscopy (EELS). EELS spectroscopy reveals the reduced valence state of the cobalt near the surface, especially along the stacking direction of CoO2 layers. The observed anisotropic surface property explains the nanosize effect on the electrochemical properties.

Production of amorphous carbon by plasma immersion ion implantation of polymers

The surface of poly ether ether ketone (PEEK) has been treated with plasma immersion ion implantation using hydrogen ions. Nanoindentation and transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) studies of the modified surface show that the modified layer is harder, denser and has a greater elastic recovery than the unmodified material. The electronic structure of the material is characterized by low-loss EELS and UV-Visible spectroscopy, which show that the surface molecular structure is strongly disrupted by the ion implantation. The changes observed are consistent with the modification creating a carbon-rich amorphous material containing a sp 2 rich network structure.

Plasmon as a tool for in situ evaluation of physical properties for carbon materials

Various carbon materials were studied using electron energy loss spectroscopy (EELS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The aim was to relate the information that can be extracted from EELS (plasmon energy values) to physical properties in order to obtain a new, simple and in situ tool that would allow the properties of carbon materials to be deduced routinely from EELS. The principle of the method is expected to be valid for any carbon material whatever the morphology (fibre, film), texture (anisotropic, isotropic), nanotexture, structure (turbostratic, graphitised) and also whatever the way the materials are processed. Provided the EELS spectroscopy is performed using appropriate acquisition parameters, the only limitation revealed so far is about the carbon material type, which has to derive from thermoplastic precursors. Reasons for both relations and limitations are discussed.

Surface enhanced Raman scattering as a probe of adsorbate–substrate charge-transfer excitations

The chemical mechanism of surface enhanced Raman scattering (SERS) is used as a probe of adsorbate–substrate charge-transfer excitations on atomically smooth, single crystal copper surfaces in ultra-high vacuum. It is demonstrated that SERS and charge-transfer excitations are supported on atomically smooth surfaces in contradiction to earlier consensus. The combination of SERS with electron energy loss spectroscopy (EELS) provides a great deal of insight into the nature of these excitations. It is found that these excitations are sensitive probes of local electronic structure at metal surfaces. The spatial extent, transition moment, and excited state potential energy surfaces of these excitations are probed using SERS, EELS and molecular spectroscopy simulations.

N-type doping in CdO ceramics: a study by EELS and photoemission spectroscopy

Ceramic samples of yttrium- and indium-doped CdO (Cd1 − xYxO with 0 ≤ x ≤ 0.035 and Cd1 − xInxO with 0 ≤ x ≤ 0.023) have been studied by electron energy loss spectroscopy (EELS) and ultraviolet and X-ray photoemission spectroscopy (UPS and XPS). Both In and Y act as efficient n-type dopants, although the carrier concentration is higher in In-doped material. The maximum surface plasmon loss energy in EELS is 0.66 eV in Y-doped samples and 0.72 eV for in-doped samples. Effective masses (m∗) increase with increasing carrier concentration N and obey an approximately linear variation of the form m∗ = m∗o + cN, where c is a constant. UPS shows a well-defined conduction band feature for doped samples. A shift of the valence band edge towards high binding energy due to chemical doping was observed in He(I) UPS, which is confirmed by shifts of O 2p valence band features and Cd 4d core level peaks in high resolution XPS. However, the shifts are less than expected from the increased widths of the occupied part of the conduction band. This is due to shrinkage of the bandgap with doping. XPS demonstrates that there is pronounced surface segregation of dopant atoms. However, the segregated dopant atoms do not act as donor centres.

Surface properties of Cd 2SnO 4 ceramics doped with both In and Sb: a study by EELS and photoemission spectroscopy

Ceramic samples of Cd 2SnO 4 doped with 2% In plus 2% or 5% Sb have been studied by electron energy-loss spectroscopy (EELS) and X-ray photoemission spectroscopy and ultraviolet (XPS and UPS). The carrier concentrations derived from surface plasmon energies for these materials are low in comparison with those for purely Sb-doped samples, but high in comparison with 2% In-doped Cd 2SnO 4. The intensities of the Sb 3d peaks in Cd 2SnO 4 doped with both In and Sb are lower than those in samples doped only with Sb. This provides direct evidence that In(III) ions substitute on both Cd(II) and Sn(IV) cation sites. UPS shows well-defined conduction bands.

Surface properties of indium-doped Cd 2SnO 4 ceramics studied by EELS and photoemission spectroscopy

Ceramic samples of indium-doped dicadmium stannate (Cd 2SnO 4) have been studied over the doping range 0.5–2.0 at% In by electron energy loss spectroscopy (EELS) and ultraviolet and X-ray photoemission spectroscopy (UPS and XPS). The samples were prepared from SnO 2 and Cd 1− x In x O, which was itself made from CdO and In 2O 3. The maximum surface plasmon energy is around 0.450 eV. The carrier concentration derived from the plasmon energy is always lower than that estimated from the bulk dopant concentration. The atomic percentages of both cadmium and tin on the surface before annealing, determined from XPS, are lower than those in undoped Cd 2SnO 4, giving evidence for the substitution of In 3+ ions on both Cd 2+ and Sn 4+ sites. XPS shows a pronounced decrease in the surface concentration of segregated indium after annealing under UHV conditions. He(I) UPS shows a well defined conduction band feature with a higher intensity than that for undoped samples. It is evident that the lower edge of the valence band shifts towards higher binding energy as the doping level increases.

Inelastic electron scattering and magnetic collective response of mesoscopic carbon structures.

We obtain analytically the self-consistent quasiparticle energy (QPE) spectra associated with certain fullerenes, e.g., ${\mathrm{C}}_{50}$, ${\mathrm{C}}_{60}$, ${\mathrm{C}}_{70}$, and the graphitic nanotubules. A random-phase approximation-level formalism for calculating the collective electronic excitations is then developed using the above QPE spectra as input. Multipole excitations are obtained for ${\mathrm{C}}_{60}$ and related quasispherical fullerenes. In the presence of a strong external magnetic field applied along the axis of the nanotubule, an extra peak in the de Haas--van Alphen oscillations predicted previously manifests itself as a discontinuity in the highest magnetoplasmon dispersion curve for the nanotubules in the infrared regime. The magnetic field breaks the degeneracy of the subbands such that each intersubband excitation branch is split into four distinct branches. An approximate plasmon dispersion relation in the high frequency regime is also derived. Furthermore, the inelastic electron scattering cross section characterized by the density-density correlation function is calculated. Our results are in good agreement with experiments on photoionization, gas phase electron-energy loss spectroscopy (EELS), transmission EELS, and high-resolution electron-energy loss spectroscopy.

Improved detection limits with parallel-detection EELS

Parallel-detection electron energy-loss spectrometers (PEELS) have improved the detection limits of electron energy-loss spectroscopy (EELS) so much that established comparisons between EELS and energy-dispersive X-ray spectroscopy (EDXS) no longer apply. We have therefore decided to reinvestigate the comparative advantages of EELS as a microanalytical technique.Two types of detection limits are of interest: minimum detectable mass (MDM), and minimum detectable mass fraction (MMF). MDM has been improved by parallel-detection EELS so much that single atoms of elements with favorable cross-sections have become detectable. Since detecting a single atom requires a probe with at least 0.1 nA of current and a diameter of no more than 1 nm, this level of performance needs an electron microscope equipped with an efficient field-emission gun (FEG). On the other hand, because the ratio of the signal from a uniformly dispersed element to the signal from the matrix does not depend on the size of the probed area, MMF depends on the total available probe current rather than the probe current density. This means that MMF should be better for microscopes equipped with thermionic guns than for ones equipped with FEGs.