Band 97, Heft 7

The present knowledge on self-diffusion and lattice vacancies in near-stoichiometric body-centred cubic FeCo alloys is critically reviewed and related to the establishment of structural order in the alloy after quenching from elevated temperatures. The available data may be consistently explained in terms of the so-called six-jump cycle mechanism of Huntington, Elcock and McCormick, plus the additional concept of ‘pseudo-selfdiffusivity’. It is shown that this leads to an understanding of the observed combinations of high activation enthalpy and large pre-exponential factor in the Fe self-diffusivity on the one hand, and of a lower activation enthalpy and ‘normal’ pre-exponential factor in the Co self-diffusion on the other hand. For some vacancy properties numerical estimates are given. The potential of muon spin rotation measurements for filling existing gaps in our knowledge is emphasized. A critical discussion of the “500 °C anomaly” shows that the explanations attempted in the literature are unsatisfactory. It is proposed that the tetragonality of the compounds FeCo 3 and Fe 3 Co predicted by ab-initio calculations of Díaz-Ortiz et al. may lead to a metastable pattern of antiphase domains on a very small scale, the dissolution of which may cause the anomaly.

In this paper, the possibilities and intrinsic physical limits to atomic resolution electron tomography are discussed. It is shown that atomic resolution electron tomography is in principle possible and may provide atom position coordinates with sufficient precision to use as input data for theoretical ab-initio calculations. For amorphous materials which are subject to radiation damage it may require a lower voltage and the use of a C s and C c corrector. For perfect crystalline samples the projected structure can be reconstructed from exit waves using the channeling theory. Combining the reconstructions obtained from different zone axis orientations then yields the 3D atomic structure. The most difficult objects will be semicrystalline materials where strong dynamic scattering interferes with weak signals from the aperiodicity. Here we suggest to use a new technique called precession laminography. Finally, doubt is raised about the practical usability of optical sectioning by HAADF STEM.

As the dimensions of microelectronic devices continue to decrease, single transmission electron microscopy images are not able to properly represent the 3D structures when the structure’s curvature is comparable to the sample thickness. Electron tomography was used to study cylindrical vias coated with Ta-barrier layers and Cu-seed layers in 3D. Tomography reconstructions from both bright field images and high angle annular dark field images are presented. Fidelity of the reconstruction from single-axis and dual-axis tilt series is compared. Strategies for improving the fidelity of the reconstruction and making electron tomography practically applicable for device failure analysis of microelectronic industry are discussed.

Spherical and chromatic aberrations limit the resolution of conventional electron microscopes. Both defects are unavoidable in the case of static rotationally symmetric electromagnetic fields (Scherzer theorem). To compensate for these aberrations, multipole correctors or electron mirrors are required .The correction of the resolution-limiting aberrations is demonstrated for the hexapole corrector, the quadrupole-octopole corrector, and the tetrode mirror. Electron mirrors require a magnetic beam separator, which must be free of second-order aberrations. The multipole correctors are highly symmetric telescopic systems compensating for the defects of the round lenses. The hexapole corrector has the simplest structure yet eliminates only third-order spherical aberration and coma, whereas the mirror and the quadrupole-octopole (QO) corrector are able to correct the chromatic aberration as well. The QO corrector eliminates chromatic aberration by means of crossed electric and magnetic quadrupoles and the third-order spherical aberration by octopoles. At present, aberration-corrected electron microscopes obtain a resolution limit of about 1 Å.

In high-resolution off-axis electron holography, the correction of coherent aberrations allows the quantitative interpretation of the amplitude and phase of the object wave up to the information limit of the electron microscope. Since the measured object phase is directly related to the projected atomic potential for sufficiently thin samples, off-axis electron holography is expected to allow distinguishing of different elements in the reconstructed phase image (“holographic materials analysis”). This has already been verified with the example of Ga and As. However, simulations of the atomic phase shift reveal that the interpretation of the measured phase shift in terms of atomic species is generally rather complex. The findings suggest that, in some cases, the requirements as to lateral resolution and phase detection limit will be met only by electron microscopes of the next generation.

In this study we analyse diffractograms of elastically filtered images of thin amorphous foils of carbon, silicon and germanium using the weak object approximation. The use of this approximation leads to a contrast transfer function containing a phase η ( u ) depending on the spatial frequency u . Furthermore, the derivative of this phase is included in the envelope function of the contrast transfer function. The phase can be attributed to the breakdown of the first-order Born approximation leading to complex scattering amplitudes characterized by this phase η ( u ). We analyse contrast transfer characteristics to determine the phase of complex scattering amplitudes of carbon, silicon and germanium as a function of spatial frequency and to measure the two-particle structure factor of the corresponding amorphous specimens. The contrast transfer characteristics were calculated from diffractograms of focal series of elastically filtered images. The phases measured show a decay with increasing spatial frequency and additional oscillations. The results for the two-particle structure factor also decay with increasing spatial frequency and contain low local maxima. Both can be attributed to voids or inhomogeneities within the amorphous structure.

We recently showed that the Multisclice method can be used to elucidate the crystal volume contribution to a CBED pattern. Here, we demonstrate in more detail how the directions of each HOLZ line, their Laue zone order, and the specimen thickness determine the volume segment of the Bragg scattering cone of the electron beam and show that secondary rescattering from this volume must take place for deficient line formation in the (000) CBED disk. Moreover, additional prospects of the multislice method are demonstrated such as calculation of line splitting in bent crystals, hollow cone dark-field CBED, large-angle CBED, and whole diffraction patterns including Kikuchi lines.

The established technique of electron energy-loss spectrometry (EELS) in the electron microscope has surpassed the task of chemical microanalysis. Some aspects of EELS, such as low energy losses for optical studies, have a long tradition and we see them re-emerge after a long period of oblivion. Other aspects such as coherence of inelastically scattered electrons are now much better understood and can be used to advantage. Many applications of EELS can and will profit from the enormous progress in instrumentation and computing capacity. In this paper we discuss some unconventional examples of EELS beyond chemical microanalysis.

Aiming to determine the size distribution and surface area per unit mass of catalyst nanoparticles for fuel cells, we have developed a novel method of digital image processing that quantitatively determines the size of individual nano-particles from atomic-resolution transmission electron microscopy images. After introducing the basic concept of this new method, we demonstrate its efficiency using platinum nanoparticles on carbon support as a model system. Special attention is given to the error that can be introduced by estimating the surface area per unit mass from the distribution of particle diameters in the presence of asymmetry of the particle size about the mean.

It is known that sintering of powders of zinc oxide (ZnO) with small additions of iron oxide results in a ceramic with grains exhibiting a characteristic inversion domain micro-structure with planar inversion domain boundaries (IDBs) on two different habit planes. This study concentrates on a quantitative analysis, by a combination of different transmission electron microscopy methods, of those IDBs that are parallel to {0001} basal planes of the wurtzite structure of ZnO. Electron diffraction and dark-field imaging prove the nature of the inversion. High-resolution annular dark field scanning transmission electron microscopy allows measurement of the rigid body displacements across these IDBs and of the local lattice contraction related to the octahedral interstices that form the boundaries. Energy-selected imaging, electron energy-loss spectroscopy and energy-dispersive X-ray spectroscopy have been combined to determine the chemical composition of the IDBs quantitatively. It is thus shown unambiguously that every such fault consists of precisely one basal plane of octahedral interstices that are completely occupied by Fe 3+ ions and that these FeO 6 octahedra are themselves contracted along the <0001> direction. A local charge balance model explains the observations.

We have grown epitaxial LaFeO 3 thin films on different oxide substrates by molecular beam epitaxy and investigated them using transmission electron microscopy. The films reveal structural domains that strongly depend on the substrate material, substrate orientation, and the film thickness. On (001) perovskite substrates of SrTiO 3 and LaAlO 3 , the long c -axis of the orthorhombic unit cell was found always to lie in-plane parallel to one of the <100> axes of the perovskite forming 90° rotational twins with the rotation axis parallel to the [110] axis of LaFeO 3 and to the substrate normal. In contrast, MgO and MgAl 2 O 4 substrates lead to the in-plane and the out-of-plane orientation of the c -axis and give rise to smaller domains. The results are discussed with respect to the correlation between the crystallographic and the antiferromagnetic structure of LaFeO 3 .

In this work we report on investigations into the topological and chemical short-range order of binary Ti 72.3 Fe 27.7 alloy melts. In order to analyse the structure, the melts were investigated by means of neutron scattering and energy dispersive diffraction of synchrotron radiation. The combination of both diffraction techniques allowed the determination of the partial Bhatia Thornton structure factors S NN ( Q ) and S CC ( Q ). The experimental results provide evidence of an icosahedral topological short-range order prevailing in the liquid that is accompanied by a pronounced chemical short-range order such that Ti –Fe nearest neighbours are preferentially formed.

The presence of an extended structure in a Σ = 9 grain boundary of copper has been highlighted by high-resolution transmission electron microscopy. The phase, located in a grain boundary region between two different asymmetrical facets, may be described as a three dimensional phase with a hexagonal structure. A detailed analysis of the defects at the limits between the phase and the two copper crystals allows us to propose an interpretation for the occurrence of this local phase transformation.

We describe the imaging of dislocations in crystals that are composed of micron-size colloidal particles. The dislocations form in an fcc crystalline film close to the interface with a template that is used to grow these crystals. We use a simple laser diffraction microscope (LDM) to image the strain field of the dislocations on a medium-range length scale. This setup is inspired by the transmission electron microscopy technique used to image dislocations in atomic crystals. Confocal microscopy allows us to reconstruct the defect topology in the colloidal crystal in three dimensions and on the particle scale. We take advantage of the three-dimensional particle coordinates determined by confocal microscopy to simulate LDM image contrast for specific imaging conditions.

Various morphologies of the ω phase are observed in the α and the β phases of group-IV elements and their alloys where formation of this phase is induced by various means, namely, quenching from the β-phase field, isothermal annealing, under high static as well as dynamic pressure, and irradiation. The presence of various morphologies, such as ellipsoidal, cuboidal, granular or plate shapes, under different conditions is demonstrated. It is shown that various factors, including shuffling of atoms, shearing of atomic planes, long-range diffusion of solute atoms, change in volume during transformation, etc., influence the formation of different morphologies of the ω phase. The underlying mechanisms in each case are discussed. Ordered ω phases, which exist as equilibrium phases in various alloy systems, are shown as a case of superimposition of the concentration wave on the displacement wave.

(Sr 1− x Ca x ) 33 Bi 24+ δ Al 48 O 141+3 δ /2 fullerenoid solid solutions have been synthesized and the effect of partial substitution of Sr by Ca has been characterized by (scanning) transmission electron microscopy, applying different imaging methods. Most of the defects commonly observed in face centered cubic compounds, have also been observed in (Sr 1− x Ca x ) 33 Bi 24+ δ Al 48 O 141+3 δ /2 . Based on purely geometrical and topological models, structural presentations for the coherent twin boundaries and stacking faults have been constructed on the basis of complex spherical “Al 84 O 210 ” units. The results are compared to defects observed in the crystallite fullerite C 60 .

Many complex metallic alloys are known to form in aluminium-based systems containing transition metals like Cu, Pd, Fe or Cr, the most famous one being the stable icosahedral quasicrystal discovered in the Al –Cu –Fe system. Although covered by a thin native oxide layer, adhesion of water onto the complex compounds is very different from that onto the oxide or onto oxidised aluminium. We show here how this atypical behaviour is related to the structural complexity of the compound. We then produce data that allows us to estimate the actual surface energy of the same compounds, a property that is also a fingerprint of structural complexity in Al-based intermetallics.

The phase formation of a copper-mold-cast Zr 60 Ti 2 Nb 6 Cu 14 Ni 9 Al 9 alloy has been investigated upon cooling from the melt as well as upon annealing of as-cast specimens. The different states of the samples are characterized by X-ray diffraction, optical and electron microscopy, and differential scanning calorimetry. The cooling rate as realized upon copper-mold casting leads to micrometer-sized quasicrystals, which are embedded in a glassy phase. The thermal stability Δ T x of the supercooled liquid state of the glassy phase that forms near to the wall of the copper mold, differs from that of the glassy matrix in the center of the rod due to different compositions of the glassy phase. This is a consequence of the change in local cooling conditions, which affects the phase formation upon solidification as well as the subsequent transformation behavior of the alloy upon constant-rate heating.

A systematic high-resolution electron microscopy study of a special type of two-dimensional defects, phason planes, has been carried out in the structurally complex metallic alloys of Al –Pd –Mn and Al –Ni –Rh. The structural characteristics of the phason planes in these two alloys consist of pentagon-banana pairs of polygons but with a different arrangement. Located on the vertex of the polygons are pseudo-Mackay atomic clusters. The detailed atomic structures of the phason planes in both ξ′- and ξ-phases of the two alloys have been determined by means of high-resolution electron microscopy and theoretical simulations. The possible ways of displacement of the phason planes in the complex alloys are discussed.

Silicon nanowires can be successfully grown by applying the vapor – liquid – solid process. In the case of the commonly used chemical vapor deposition technique, a Si containing gas/precursor is cracked at Au droplets acting as seeds. Si adatoms are subsequently dissolved in the liquid metal. Due to a supersaturation within this droplet, Si precipitates predominantly at the liquid – solid interface – a nanowire grows. A different situation occurs if nanowires are grown by molecular beam epitaxy via the vapor– liquid – solid mechanism. The difference consists, for example, of the role of the metal seed, the morphology of the nanowires and their aspect ratio. In particular, surface diffusion including the metal used as well as Si, strongly influences the growth process. This article describes molecular beam epitaxy growth experiments of Si nanowires under ultra-high vacuum conditions and compares the results with other growth techniques.

Some metals share an elusive property with silicon (and other semiconductors): they may exhibit strong self-induced current oscillations during anodic dissolution in electrochemical experiments. While this feature, as well as related features concerning self-organization at reactive solid-liquid interfaces, is still not well understood, the so-called “current-burst model” of the authors succeeded in reproducing many effects quantitatively that have been observed at the Si electrode. The current-burst model assumes that current flow through the electrode on a nm scale is inhomogeneous in both time and space; a single current-burst is a stochastic event. Current oscillations in time and space result from interactions in space or time of single current-bursts. The paper outlines the basics of the model and gives results of Monte Carlo simulations concerning stable and damped oscillations for the current and, as a new feature, for the voltage. With the current-burst model a kind of “nano”-electrochemistry is introduced; its strengths, weaknesses, and possible implications for other electrochemical phenomena and for other materials are briefly discussed.

Ferromagnetic Fe and Fe 3 Si films are grown on GaAs(001) substrates by molecular beam epitaxy. This review article summarizes the important aspects of the heteroepitaxial growth and the associated structural properties of the Fe/GaAs and Fe 3 Si/GaAs heterostructures. The magnetic anisotropy of the thin Fe epilayers is analyzed and correlated to the interface structure, which is determined by high-resolution transmission electron microscopy. The structural and electrical properties of Fe 3 Si alloys depend on the exact stoichiometry and the long-range atomic ordering that is controlled by the growth conditions during the molecular beam epitaxy. The interface reactivity and the thermal stability are critically discussed for both heterosystems.

The adaptation of titanium/titanium nitride barriers which are commonly used for tungsten-filled contacts to aluminum-filled contacts has encountered some limitations. A main issue is the decomposition of titanium nitride manufactured by physical vapour deposition through reaction with aluminum. In this investigation two different concepts for barrier improvement have been pursued: 1. stuffing of a conventional barrier by a rapid thermal process in an oxygen-rich atmosphere and, 2. use of a barrier manufactured by chemical vapour deposition. The aim of the transmission electron microscopic analysis was to explain different electrical breakdown characteristics during thermal stressing of chips with these barriers. Energy-filtered transmission electron microscopy was used to monitor elemental distributions down to nanometer scale. Both concepts are found to improve barriers for aluminumfilled contacts.

During transmission electron microscopy (TEM) investigations of Al alloys, defects caused by the electron irradiation can occur. Since the image contrast of these irradiation defects is similar to that of early stages of precipitates, care is needed to avoid confusion. In the present paper the formation and the coarsening of radiation damage defects were studied by in situ TEM in both, pure Al and Al alloys. High-resolution (HR)TEM images show on an atomic level that the radiation defects are extrinsic Frank loops (some converted into unfaulted prismatic loops); they are distributed homogeneously within the TEM foil but inhomogeneously on the four {111} planes. Applying HRTEM imaging conditions, the minimum electron energy causing defects is found to be as low as 110 keV using a [110] beam direction. The results for pure Al are very similar to those of the Al alloys. Therefore, during HRTEM studies (using accelerating voltages > 100 kV) the formation of radiation defects seems inevitable; they can be distinguished from the early stages of precipitates if they lie on different planes.

Using a Mo/Si multilayer stack, capabilities and limits of high-resolution transmission electron microscopy with respect to thin film analysis are pointed out. Image processing is shown to provide non-subjective criteria for the determination of layer thicknesses. In comparison to measurements by X-ray reflectometry, an integrating method capable of determining multilayer periodicities, preparation and observation artefacts in transmission electron microscopy is discussed. Special emphasis is placed on cleavage as a powerful method for TEM sample preparation, its advantages and disadvantages.

The structure of the fiber –matrix interface region of carbon-fiber/carbon-matrix (C/C)-composites, which is important for the mechanical properties of a composite, was studied by transmission electron microscopy. The pyrolytic carbon matrix was obtained by chemical vapor infiltration of carbon fiber preforms with different fiber architecture and fiber types. While the texture of the carbon matrix can be controlled by the infiltration conditions, the carbon texture at the fiber –matrix interface differs from the bulk matrix and cannot be determined by the infiltration parameters. We often observe a high-textured layer with a thickness up to 100 nm at the interface prior to the formation of the bulk matrix with a lower texture. The fiber –matrix interface properties of C/C-composites are compared with the interface of pyrolytic carbon layers deposited on planar Si-, BN- and cordierite-substrates. In contrast to BN-substrates, thin high-textured pyrolytic layers are observed at the interface of carbon on planar Si- and cordierite-substrates where strong bonding between pyrolytic carbon and the substrate is achieved by the formation of Si-C bonds. Stress-induced ordering is discussed as a mechanism for the formation of the high-textured interface layers.

Microstructure as well as micro-mechanical and tribological properties of hard surface layers formed on Timetal 834 alloy during its nitridation or oxynitridation under glow discharge were investigated. The tests show that both layers exhibited higher microhardness and Young’s modulus as well as much better wear resistance than the untreated alloy. The microstructure investigations, performed by light microscopy and analytical transmission electron microscopy methods, show that improved micro-mechanical and tribological properties of surface-treated alloy are related to the presence of a multiphase micro/nanostructure containing several sublayers with different morphologies and composition.