Design of a novel correlative reflection electron microscope for in-situ real-time chemical analysis

A novel instrument that integrates reflection high energy electron diffraction (RHEED), electron energy loss spectroscopy (EELS), and imaging is designed and simulated. Since it can correlate the structural, elemental, and spatial information of the same surface region via the simultaneously acquired patterns of RHEED, EELS, and energy-filtered electron microscopy, it is named correlative reflection electron microscopy (c-REM). Our simulation demonstrates that the spatial resolution of this c-REM is lower than 50 nm, which meets the requirements for in-situ monitoring the structural and chemical evolution of surface in advanced material.     

Size and density control of silicon oxide nanowires by rapid thermal annealing and their growth mechanism

In this work, we demonstrate a fast approach to grow SiO₂ nanowires by rapid thermal annealing (RTA). The material characteristics of SiO₂ nanowires are investigated by field emission scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field (HAADF) imaging, electron energy loss spectroscopy (EELS), and energy-filtered TEM (EFTEM). The HAADF images show that the wire tip is predominantly composed of Pt with brighter contrast, while the elemental mappings in EFTEM and EELS spectra reveal that the wire consists of Si and O elements. The SiO₂ nanowires are amorphous with featureless contrast in HRTEM images after RTA at 900°C. Furthermore, the nanowire length and diameter are found to be dependent on the initial Pt film thickness. It is suggested that a high SiO₂ growth rate of >1 μm/min can be achieved by RTA, showing a promising way to enable large-area fabrication of nanowires.     

Cr掺杂锐钛矿型TiO2（001）薄膜中Cr原子的直接观测

Imaging the doping elements is critical for understanding the photocatalytic activity of doped TiO₂ thin film. But it is still a challenge to characterize the interactions between the dopants and the TiO₂ lattice at the atomic level. Here, we use high angle annular dark-field/annular bright-field scanning transmission electron microscope (HAADF/ABF-STEM) combined with electron energy loss spectroscopy (EELS) to directly image the individual Cr atoms doped in anatase TiO₂(001) thin film from [100] direction. The Cr dopants, which are clearly imaged through the atomic-resolution EELS mappings while can not be seen by HADDF/ABF-STEM, occupy both the substitutional sites of Ti atoms and the interstitial sites of TiO₂ matrix. Most of them preferentially locate at the substitutional sites of Ti atoms. These results provide the direct evidence for the doping structure of Cr-doped A-TiO₂ thin film at the atomic level and also prove the EELS mapping is an excellent technique for characterizing the doped materials.     

Comparison of the electronic structure of a thermoelectric skutterudite before and after adding rattlers: an electron energy loss study

Skutterudites, with rattler atoms introduced in voids in the crystal unit cell, are promising thermoelectric materials. We modify the binary skutterudite with atomic content Co(8)P(24) in the cubic crystal unit cell by adding La as rattlers in all available voids and replacing Co by Fe to maintain charge balance, resulting in La(2)Fe(8)P(24). The intention is to leave the electronic structure unaltered while decreasing the thermal conductivity due to the presence of the rattlers. We compare the electronic structure of these two compounds by studying the L-edges of P and of the transition elements Co and Fe using electron energy loss spectroscopy (EELS). Our studies of the transition metal white lines show that the 3d electron count is similar for Co and Fe in these compounds. As elemental Fe has one electron less than Co, this supports the notion that each La atom donates three electrons. The L-edges of P in these two skutterudites are quite similar, signalling only minor differences in electronic structure. This is in reasonable agreement with density functional theory (DFT) calculations, and with our multiple scattering FEFF calculations of the near edge structure. However, our experimental plasmon energies and dielectric functions deviate considerably from predictions based on DFT calculations.     

Nano-scale copper oxidation on leadframe surface

Copper oxidation studies were carried out by means of field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) techniques. The growth of copper oxide occurs as a copper surface comes in an oxygen containing environment. The reaction sequence leading to oxidation of the copper surface is generally accepted to be oxygen chemisorption, nucleation and growth of the surface oxide and bulk oxide growth. HRTEM examination of the cross section of the oxidized copper sample revealed the interface region in between the copper and copper oxide. At high oxidation temperature, formation of micro-voids and separations were observed along this interface region. Poor adhesion at this interface region due to micro-voids and separation were found to be the root cause of delamination issue. EELS analysis determined that for regions with intact interface the oxidation system is Cu/CuO/Cu₂O/CuO, however, in regions containing micro-voids or separation it is found to be Cu/Cu₂O/CuO.     

Practical spatial resolution of electron energy loss spectroscopy in aberration corrected scanning transmission electron microscopy

The resolution of electron energy loss spectroscopy (EELS) is limited by delocalization of inelastic electron scattering rather than probe size in an aberration corrected scanning transmission electron microscope (STEM). In this study, we present an experimental quantification of EELS spatial resolution using chemically modulated 2×(LaMnO(3))/2×(SrTiO(3)) and 2×(SrVO(3))/2×(SrTiO(3)) superlattices by measuring the full width at half maxima (FWHM) of integrated Ti M(2,3), Ti L(2,3), V L(2,3), Mn L(2,3), La N(4,5), La N(2,3) La M(4,5) and Sr L(3) edges over the superlattices. The EELS signals recorded using large collection angles are peaked at atomic columns. The FWHM of the EELS profile, obtained by curve-fitting, reveals a systematic trend with the energy loss for the Ti, V, and Mn edges. However, the experimental FWHM of the Sr and La edges deviates significantly from the observed experimental tendency.     

Study of changes in composition and EELS ionization edges upon Ni4Ti3 precipitation in a NiTi alloy

Concentration gradients surrounding Ni4Ti3 precipitates grown by appropriate annealing in a Ni51Ti49 B2 austenite matrix are investigated by electron energy loss spectroscopy (EELS) and energy filtered transmission electron microscopy (EFTEM). Concentration gradients of approximately 1.0-2.0 at.% in Ni within the surrounding B2 matrix can be detected by both EELS and EFTEM, revealing a Ni depleted zone in the matrix. Besides the concentration gradients, the EELS integrated cross-section of the Ni L(2,3) edges for the Ni-depleted region increased slightly, when compared with a matrix region away from the precipitate and not depleted in Ni.     

Electron spectroscopy of chemically synthesized ZnS clusters

Thin films of ionic compounds of ZnS clusters were measured by electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy (UPS). A size effect was observed in the valence plasmon energy measured by EELS from which the coherence length of the plasmon excitation can be estimated. The difference between the lowest excitations observed in UPS and EELS can be explained by the final state charging effect of a single cluster ion in UPS, which strongly depends upon the nominal charges of the clusters.     

Initial oxidation process of Mg films characterized by AES,EELS, and UPS

Initial oxidation process of Mg films was investigated by Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy (UPS). A polycrystalline Mg film was exposed to oxygen at room temperature. An inspection of EELS reveals that the initial oxidation process could be classified according to the curve profile in the low energy loss region. The oxide formation is initiated after oxygen exposure of 3 L (Langmuir).     

Depth profiling of melting and metallization in Si(111) and Si(001) surfaces

An original approach for measuring the depth profile of melting and metallization of the Si(111) and Si(001) surfaces is proposed and applied. The different probing depths of the Auger electron and electron energy loss (EELS) spectroscopies are exploited to study the number of molten and metallic layers within 5-30 ? from the surface up to about 1650 K. Melting is limited to 3 atomic layers in Si(001) in the range 1400-1650 K while the number of molten layers grows much faster (5 layers at about 1500 K) in Si(111) as also indicated by the L(3)-edge shift observed by EELS. The relationship between melting and metallization is briefly discussed.     

Inelastic electron scattering observation using energy filtered transmission electron microscopy for silicon -- germanium nanostructures imaging

This paper presents a new technique using energy filtered TEM (EFTEM) for inelastic electron scattering contrast imaging of Germanium distribution in Si-SiGe nanostructures. Comparing electron energy loss spectra (EELS) obtained in both SiGe and Si single crystals, we found a spectrum area strongly sensitive to the presence of Ge in the range [50-100 eV]. In this energy loss window, EELS spectrum shows a smooth steeply shaped background strongly depending on Ge concentration. Germanium mapping inside SiGe can thus be performed through imaging of the EELS background slope variation, obtained by processing the ratio of two energy filtered TEM images, respectively, acquired at 90 and 60 eV. This technique gives contrasted images strongly similar to those obtained using STEM Z-contrast, but presenting some advantages: elastic interaction (diffraction) is eliminated, and contrast is insensitive to polycrystalline grains orientation or specimen thickness. Moreover, since the extracted signal is a spectral signature (inelastic energy loss) we demonstrate that it can be used for observation and quantification of Ge concentration depth profile of SiGe buried layers.     

Chemical polishing method of GaAs specimens for transmission electron microscopy

A practical method for transmission electron microscopy specimen preparation of GaAs-based materials with quantum dot structures is presented to show that high-quality image observations in high-resolution transmission electron microscopy (HRTEM) can be effectively obtained. Specimens were prepared in plan-view and cross-section using ion milling, followed by two-steps chemical fine polishing with an ammonia solution (NH₄OH) and a dilute H₂SO₄ solution. Measurements of electron energy loss spectroscopy (EELS) and atomic force microscopy (AFM) proved that clean and flat specimens can be obtained without chemical residues. HRTEM images show that the amorphous regions of carbon and GaAs can be significantly reduced to enhance the contrast of lattice images of GaAs-based quantum structure.     

A quantitative analysis of a multi-phase polycrystalline cubic boron nitride tool material using DualEELS

This paper presents a quantitative analysis of a polycrystalline cubic boron nitride tool material by electron energy-loss spectroscopy spectrum imaging acquired in dual range mode. Having both the low-loss and core-loss regions acquired nearly simultaneously provides the advantage of accurate corrections for thickness effects and thus the possibility to perform quantification calculations. This has resulted in extracted bonding maps with areal (atoms/nm(2)) or volumetric (atoms/nm(3)) densities. Spectroscopic signatures in the low-loss and core-loss energy ranges, of the elements (Al, B, C, N, Ti and O) present in the existing phases, were studied and used when extracting the element specific bonding maps by the multiple linear least squares fitting procedure. Variations of elemental concentrations across the investigated area were determined, despite of phase overlap in the beam direction or energy overlaps in the EELS spectrum. Moreover, the surface oxidation of Ti(C,N) and AlN as well as the amorphisation of α-Al(2)O(3) is discussed.     

Energy-filtered TEM imaging and EELS study of ODS particles and argon-filled cavities in ferritic-martensitic steels

Oxide-dispersion-strengthened (ODS) ferritic-martensitic steels with yttrium oxide (Y(2)O(3)) have been produced by mechanical alloying and hot isostatic pressing for use as advanced material in fusion power reactors. Argon gas, usually widely used as inert gas during mechanical alloying, was surprisingly detected in the nanodispersion-strengthened materials. Energy-filtered transmission electron microscopy (EFTEM) and electron energy loss spectroscopy (EELS) led to the following results: (i) chemical composition of ODS particles, (ii) voids with typical diameters of 1-6 nm are formed in the matrix, (iii) these voids are filled with Ar gas, and (iv) the high-density nanosized ODS particles serve as trapping centers for the Ar bubbles. The Ar L(3,2) energy loss edge at 245 eV as well as the absorption features of the ODS particle elements were identified in the EELS spectrum. The energy resolution in the EEL spectrum of about 1.0 eV allows to identify the electronic structure of the ODS particles.     

Quantitative interpretation of EELS and REELS spectra

A critical analysis of the present day Electron Energy Loss Spectroscopy (EELS) data interpretation methods has been done. The necessity for the consideration of a target as a multilayered structure with different inelastic energy loss cross sections in the surface and the bulk layers has been shown to be a reality both for the transmission EELS and the reflection EELS (REELS). A method to reconstruct inelastic energy loss cross sections in various target layers from the experimental data has been presented. Essential qualitative and quantitative dependence of the path length distribution function for reflected electrons as a function of scattering angle has been revealed. The tested method for extraction of the information from REELS experiments with angular resolution has been presented.Received: 9 October 2003, Published online: 19 February 2004PACS: 34.80.-i Electron scattering - 34.50.Bw Energy loss and stopping power - 25.30.Fj Inelastic electron scattering to continuum     

Cerenkov losses: a limit for bandgap determination and Kramers-Kronig analysis

Measuring low energy losses in semiconductors and insulators with high spatial resolution becomes attractive with the increasing availability of modern transmission electron microscopes (TEMs) equipped with monochromators, C(s) correctors and energy filters. In this paper, we demonstrate that Cerenkov losses pose a limit for the interpretation of low energy loss spectra (EELS) in terms of interband transistions and bandgap determination for many materials. If the velocity of a charged particle in a medium exceeds the velocity of light, photons are emitted leading to a corresponding energy loss of a few electronvolt. Since these losses are strong for energies below the onset of interband transitions, they change the apparent loss function of semiconductors and insulators, with the risk of erroneous interpretation of spectra. We measured low energy losses of Si and GaAs with a monochromated TEM demonstrating the effect of sample thickness on Cerenkov losses. Angle resolved EELS and energy filtered diffraction patterns (taken without a monochromator) show the extremely narrow angular distribution of Cerenkov losses. The latter experiment provides a method that allows to decide whether Cerenkov radiation masks the very low loss signal in EELS.     

Nanoscale EELS analysis of dielectric function and bandgap properties in gaN and related materials

The low loss region of an EEL spectrum (<50 eV) contains information about excitations of outer shell electrons and thus the electronic structure of a specimen which determines its optical properties. In this work, dedicated electron energy loss spectroscopy (EELS) methods for the experimental acquisition and analysis of spectra are described, which give improved information about the electronic structure near the bandgap region at a spatial resolution in the range of nanometers. For this purpose, we made use of a cold field emission scanning transmission electron microscope (STEM) equipped with a dedicated EELS system. This device provides a subnanometer electron probe and offers an energy resolution of 0.35 eV. Application of suitable deconvolution routines for removal of the zero loss peak extracts information on the bandgap region while the Kramers-Kronig transformation deduces the dielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap materials for the case of III-nitride compounds, which are currently the most promising material for applications on optoelectronic devices working in the blue and ultraviolet spectral range. The obtained results are in excellent agreement with experimental measurements by synchrotron ellipsometry and theoretical studies. The potential of the superior spatial resolution of EELS in a STEM is demonstrated by the analysis of dielectric properties of individual layers of heterostructures and individual defects within wurtzite GaN.     

Inorganic WS<Subscript>2</Subscript> nanotubes revealed atom by atom using ultra-high-resolution transmission electron microscopy

The characterization of nanostructures to the atomic dimensions becomes more important, as devices based on a single particle are being produced. In particular, inorganic nanotubes were shown to host interesting properties making them excellent candidates for various devices. The WS₂ nanotubes outperform the bulk in their mechanical properties offering numerous applications especially as part of high strength nanocomposites. In contrast, their electrical properties are less remarkable. The structure–function relationship can be investigated by aberration-corrected high-resolution transmission electron microscopy (HRTEM), which enables the insight into their atomic structure as well as performing spectroscopic measurements down to the atomic scale. In the present work, the deciphering of atomic structure and the chiral angle of the different shells in a multiwall WS2 nanotube is demonstrated. In certain cases, the helicity of the structure can also be deduced. Finally, first electron energy loss spectra (EELS) of a single tube are presented, acquired by a new acquisition technique that allows for high spatial resolution (denoted StripeSTEM). The measured band gap values correspond with the values found in literature for thin films, obtained by spectroscopic techniques, and are higher than the values resulting from STM measurements.     

Shells on nanowires detected by analytical TEM

Nanostructures in the form of nanowires or filled nanotubes and nanoparticles covered by shells are of great interest in materials science. They allow the creation of new materials with tailored new properties. For the characterisation of these structures and their shells by means of analytical transmission electron microscopy (TEM), especially by energy dispersive X-ray spectroscopy (EDXS), and electron energy loss spectroscopy (EELS), the accurate analysis of linescan intensity profiles is necessary. A mathematical model is described, which is suitable for this analysis. It considers the finite electron beam size, the beam convergence, and the beam broadening within the specimen. It is shown that the beam size influences the measured result of core radius and shell thickness. On the other hand, the influence of the beam broadening within the specimen is negligible. At EELS, the specimen thickness must be smaller than the mean free path for inelastic scattering. Otherwise, artifacts of the signal profile of a nanowire can pretend a nanotube.     

Composition of the “GaAs” quantum dot,grown by droplet epitaxy

Self-assembled strain-free quantum dot (QD) structures were grown on AlGaAs surface by the droplet epitaxal method. The QDs were developed from pure Ga droplets under As pressure. The QDs were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both techniques show that the QDs are very uniform in size and their distribution on the surface is also homogeneous. The high resolution cross-sectional TEM investigation shows perfect lattice matching between the QD and the substrate, and also the faceting of the side walls of QD can be identified exactly by lattice planes. Analytical TEM (elemental mapping by EELS) unambiguously identifies the presence of Al in the QD.