6H-SiC is a wide band-gap semiconductor. In recent years, a variety of high-power, -temperature, -speed and opto-electronic devices have been produced in 6H-SiC films. The future development of SiC device technology depends on the ability to form good ohmic and Schottky contacts. Plots of the current-voltage (I-V) characteristics of Cr-B contacts on (0001) 6H-SiC revealed that they became the most ohmic-like after annealing at 1000 °C for 240 sec. and rectifying after annealing for 300 sec, therefore TEM analysis of the microstructures of as-deposited and annealed Cr-B films should help in understanding what causes both the ohmic-like and rectifying behaviors.

In this research, CrxBy film was deposited on vicinal (0001) 6H-SiC substrates by electron beam evaporation at room temperature. The intended phase was CrB2. Fig.l shows the microstructure of the as-deposited specimen. 6H-SiC appears to be a perfect crystal, but the Cr-B film had columnar polycrystalline microstructure.

Internal interfaces in materials like e.g. semiconductor heterostructures get more and more interest not only under aspects of basic research but as well under aspects of new electronic and optoelectronic devices. The interface properties often govern the device performance. Thus, the evaluation of individual heterointerfaces with respect to chemical composition and crystal structure requires characterisation techniques which offer the necessary high spatial resolution. The fine focused electron probe (< 0.3 nm at 100 keV) in a field-emission STEM (Scanning Transmission Electron Microscope) allows the application of special imaging and analytical techniques to cross-sectional specimens of semiconductor heterostructures. Qualitative information on the chemical composition is provided by atomic number (Z) contrast imaging with atomic resolution. The same fine probe can be used to analyse subnanometer areas by both spectroscopic and diffraction techniques. Quantitative compositional information is provided by electron energy-loss spectroscopy (EELS) which allows the detection of concentrations of specific elements.

Recently, we have seen that the STEM can produce 0.12-0.26nm atomic resolution image detail in Annular Dark Field (ADF) mode even at modest acceleration voltages and in partnership with EELS for bonding studies. In the semi-conductor field, we are in a good position to take advantage of this imaging capability, because several atomic plane spacings range from 0.14 - 0.26 nm in the [100], [011] and [111] projections of Si and Ge. Figure 1. shows a result for the Si [111] projection obtained in the IBM VG Microscopes HB501UX, modified to produce a 0.19nm probe at 120KeV. The image has been filtered to remove spatial distances shorter than 0.15 nm, yielding the inset power spectrum. Each bright spot in the image corresponds to a single atomic column, separated from its neighbors by 0.19 nm. At 20Mx, the atom columns show enough contrast on the screen to allow astigmatism correction. These are similar to those discussed previously.

Adhesion at ceramic/metal (C/M) interfaces often controls the macroscopic behavior of materials containing metallic and ceramic phases, and experimental studies of bonding at C/M interfaces have recently been reported. Electron energy loss spectroscopy (EELS) offers unique opportunities to examine bonding at interfaces on an atomic scale. The EELS near edge fine structure is sensitive to local atomic arrangements and thus can be used as a coordination fingerprint. Much more can be done, however, by analyzing the connection between the EELS fine structure, the underlying local electronic structure and the cohesive energy of an interface to gain a deeper understanding of the nature of the adhesion at the interface.

In this work, we apply high spatial-resolution EELS instrument to study {222} MgO/Cu interfaces produced by internal oxidation. We determine interfacial chemistry of this interface with subnanometer resolution (Fig. 1) and use EELS to directly measure the electronic states pertaining to the interface

The investigation of interfaces between NiO and Y2O3 is part of a research initiative to understand low-energy heterophase interface structures between different oxide materials. In-situ oxide composites formed by the directional solidification of pseudo-binary eutectics are used as model materials in this study because they are amenable to chemical and structural characterization at the atomic length-scale. Previously, interfaces in NiO-ZrO2(cubic) directionally solidified eutectics (DSEs) were examined by various electron imaging and spectroscopy techniques to reveal the three-dimensional interface structure.[l] The second DSE system studied, NiO-Y2O3,[2] was chosen because it is very similar in crystallography to NiO-ZrO2(cubic). The goal of this second, analogous study is not only to understand the NiO-Y2O3 interface structure but, by comparing the results to NiO-ZrO2, is to draw more general conclusions about low energy oxide-oxide interfaces. Table one compares the crystallography of the NiO-ZrO2 and the NiO-Y2O3 DSEs.

It has been found that when Cu atoms are deposited on a p(2×l)-O Ag(ll0) surface, a (2×2)p2mg LEED pattern appears. The STM image has proved that zig-zag (-Cu-O-) strings grow along the [110] direction on Ag(ll0) (Fig. I)1, where a chemical reaction of (-Ag-O-) + Cu => (-Cu-O-) + Ag take place. The (-Cu-O-) strings grown on a Ag(l10) surface shows a unique reversible reaction given by an equation of (Cu)6+O2⇄ (-Cu-O-) at 570K. In this paper, we studied the bond property, the growth mechanism and electric structure of the (-Cu-O-) strings on Ag(ll0) by using XPS, HREELS and ARUPS as well as STM.

As increasing the deposition of Cu atoms on a p(2×l)-O Ag(l10) surface, the O(ls) peak in the XPS shifted from 528.2 eV to 529.9 eV(Fig. 2a). An energy loss peak at 41meV being assignable to (Ag-O) bond disappeared and a new loss peak concomitantly appeared at 35 meV in the HREELS (Fig. 2b).

The atomic structure of a symmetric 27° [001] tilt grain boundary in MgO has been determined by high-resolution Z-contrast imaging using a 300 kV VG HB603U scanning transmission electron microscope with a 1.3 Å probe. The atomic configuration in the grain boundary core is found to be considerably less open than the structures proposed earlier for similar materials.

Fig. 1 shows the Z-contrast image of the grain boundary and the projected structure derived from the image. It is clearly seen that the boundary is a shared plane of atoms with the same atomic column density as a bulk crystal {100} plane. The boundary consists of an array of separated perfect edge dislocation cores with Burgers vector b= a<100>. It is interesting to point out that the spacing between dislocation cores in the boundary is not uniform (as seen in Fig. 2). The arrangement, which follows the Fibonacci sequence, can be accurately predicted.

Many electroceramics derive their technologically useful and scientifically appealing properties through electrically active grain boundaries (GBs). Examples of such properties include traditional nonlinear current-voltage relationship (i.e. varistor behavior) and positive temperature of coefficient of resistance (PTCR effect), to recently “rediscovered” phenomena of space-charge formation across functional interfaces, including ferroeletric thin films.

In many such cases, the transport of charge across these interfaces is mediated through potential barriers which form at the core of these interfaces. For example, varistor behavior is attributed to formation of Schottky barriers at GBs which modify the transport of electrons, leading to the nonlinearity in I-V characteristics. The formation of Schottky barrier at the GB core, in turn, is attributed to the formation of space-charge across GBs. The formation of space-charge, further, is clearly attributable to decoration of GB core with charged defects with compensating opposite charge across the GB. The presence, sign, magnitude and spatial distribution of space-charge form the key to understand the Schottky barriers, thus the transport properties of interfacial systems.

The short coherence length in high-Tc superconductors (5-15Å) makes an atomic scale understanding of the electronic properties at defects and interfaces essential for device applications. This understanding is particularly relevant for grain boundaries in YBa2CU3O7-δ (YBCO), where although extensive studies have shown a clear exponential decrease in critical current with misorientation angle, the absolute value can vary by several orders of magnitude at any given misorientation angle.

Figure 1 shows Z-contrast images of an [001] low-angle tilt boundary and a 30° [001] asymmetric tilt grain boundary. An interesting feature of both of these boundaries is that there appear to be sites where two atom columns are too close together. However, the problem of like-ion repulsion can be avoided if the columns are taken to be partially occupied. Insight into the effect of this partial occupancy can be obtained through the use of bond-valence sum analysis. Here, the formal valence of an atom is made up of contributions from all of its nearest neighbors, the magnitude of which are determined by the bond length.

Small concentrations of impurities can dramatically change mechanical properties of metals and alloys since they modify bonding and cohesion at grain boundaries. In particular, impurities like B, C, P and S have received considerable attention for their effect on the mechanical properties of Fe. It is known that B and C behave as cohesive enhancers, whereas P and S tend to embrittle iron. Ab initio electronic structure calculations for supercell models of Fe crystals with B, C, P and S impurities have been performed to understand how these impurities modify the electronic states on surrounding atoms The calculations give the charge density distribution, localized densities of states (LDOS) and the total energy of the system. The angular momentum resolved LDOS, when multiplied by slowly varying matrix elements, can be directly related to the experimentally measured electron energy loss near edge structure.

The calculations have been performed using two different methods. One is the Linearized Augmented Plane Wave (LAPW) method, in which the Kohn-Sham equations of density functional theory are self-consistently solved within the Local Density Approximation (LDA) to obtain energies and charge distributions of a crystalline system.

High temperature mechanical properties of structural ceramics Si3N4 are controlled by ∼1 nm thick silicate amorphous films covering all grain boundaries. The composition of the film dictates the equilibrium film thickness resulted from a force balance at grain boundary. Many efforts arc brought to alter film chemistry and thickness, and this system offers ideal model materials to understand grain boundary and property relationship. Using a dedicated STEM (VG HB601) with high spatial resolution EELS analysis and high resolution Z-contrast imaging, various novel quantification data of the grain boundary in Si3N4 can be obtained. The methods described here can also be applied to other types of grain boundaries.

EELS profiling was performed to acquire a full spectrum from each position at a lateral increment of 1Å across a boundary in a pure Si3N4 sample with only SiO2 impurities from surface oxidation. It gives directly elemental distributions near the boundary such as Si, N and O profiles shown in Fig. 1.

A lanthanide borosilicate glass, also known as LaBS glass, is being considered for the disposition of surplus weapons plutonium and plutonium residues. The LaBS glass, based upon Löffler-type glasses, is chemically durable and can dissolve substantial amounts of plutonium as well as the neutron absorbers gadolinium and hafnium [1]. Samples of a prototype LaBS glass composition containing 10 wt. % plutonium were subjected to water vapor at 200 °C for periods of 14 to 56 days. The Argonne Vapor Hydration Test Procedure was followed [2]. This test, while not designed to replicate specific conditions that may be found in a potential geologic repository (e.g., Yucca Mountain), has been shown to accelerate alteration phase formation that is usually observed in low-temperature tests over extended time periods [2]. The surfaces of the glass samples, along with alteration phases, were examined with a transmission electron microscope (TEM) to determine the characteristic alteration products.

Using the moiré technique or the related geometrical phase technique [1,2], lattice displacement fields can be derived from high resolution micrographs by comparison with an undistorted reference lattice. A moiré image gives a magnified view of the distortions in a high resolution image, whether they are due to a distortion of the crystal lattice or result from imaging artifacts. The magnification of a moiré image is determined by the choice of the reference lattice. The closer the reference lattice is to the average lattice period in a high resolution image, the larger the magnification. To first order, both rotations and extensions of the lattice are magnified by the same factor g/Δg, where Δg is the difference between the image lattice and the reference lattice [1]. When the moiré technique is applied to interfaces, care must be taken interpret only patterns from corresponding sets of lattice planes. Thus, the technique gives a good representation of the distortions around a precipitate, but the interpretation of the moiré pattern inside a precipitate or across an interface must be made with caution.

Superconductor Quantum Interference Devices (SQUIDs), because of their extreme sensitivity to magnetic fields and radiation, have found important applications in biomagnetism, non-destructive evaluation and geophysics. One problem in the application of high-Tc SQUIDs is their noise performance. Recently, considerable progress has been made in reducing the noise. To understand the underlying mechanism, it is important to identify the microstructural origin of the junction noise.

In this work actual SQUIDs of good and poor noise performance are studied and compared by TEM. The devices were made by epitaxially growing YBa2Cu3O7-x (YBCO) films using laser ablation on 24° SrTiO3 bi-crystal substrates. The TEM samples were prepared by polishing and ion milling. The TEM observation was performed on a JEOL EM-4000EXII and a Hitachi H-9000 microscope.

Observation shows that the YBCO films and the grain boundary junctions (GB J) in the low-noise devices are in good quality. The microstructure of the films and the boundaries in the films are shown in fig. 1 and 2.

It is currently well recognized that oxides are able to accommodate deviations from stoichiometry (1) and great advances in this understanding have been achieved by using transmission electron microscopy (TEM), particularly through lattice imaging and electron diffraction techniques (2). The physical properties of non-stoichiometric oxides are strongly influenced by their exact composition and for this reason they represent a class of materials with increasing and novel properties that are put to use in, for example, oxygen sensors and high-Tc superconductors. On the other hand, in electroceramic materials, such as TiO2, grain boundary structure and chemistry are important to be characterized in detail since these variables are responsible for the electric activity.

Rutile (TiO2) can accommodate relatively large deviations from stoichiometry (TiOx with 2.0≥x≤ 1.75) by the crystallographic shear (CS) mechanism (1). The formation of CS planes is effectively a two-step process which involves the ordering of oxygen vacancies on a crystallographic plane and on their elimination by a shear of the lattice.

Pr-Co permanent magnets have attracted much attention recently due to their excellent magnetic properties and low cost; these properties can be further improved by adding small amount of carbon. In this paper, we describe the mechanism of structural transformation resulting from carbon doping in PrCo2 crystals.

We identified the heavily twinned PrCo2Cx (x=0.05-0.25) as a rhombohedral phase with space group Rm. Fig.1 shows its typical morphology of the (110) and (211) primary twins and (110)(211) secondary twins together with the corresponding diffraction patterns. We note that the Bragg spots split along the [hh0]* direction, and that the splitting increases with distance from the [110]* row. The splitting (or α angle of the rhombohedral lattice) also varies with carbon concentration.

Because PrCo2 is an untwinned FCC phase (Fdm), the formation of a twin in PrCo2Cx caused by adding carbon atoms apparently is driven by a minimization in lattice strain, which also is the origin of the cubic-rhombohedral phase-transformation.

Heterophase boundaries play an important role in advanced materials since those materials often comprise different components. The properties of the materials depend strongly on the properties of the interface between the components. Thus, it is important to investigate the stability of the microstructure with respect to annealing at elevated temperatures. In this paper results will be presented on the structure and composition of the interfaces between Cu and (α -Al2O3. The interfaces were processed either by growing a thin Cu overlayer on α- Al2O3 in a molecular beam epitaxy (MBE) system or by diffusion bonding bulk crystals of the two constituents in an UHV chamber. To improve the adhesion of Cu to α -Al2O3 ultrathin Ti interlayers were deposited between Cu and α - Al2O3.

Interfaces were characterized by different transmission electron microscopy (TEM) techniques. Quantitative high-resolution electron microscopy (QHRTEM) allows the determination of the structure (coordinates of atoms) while analytical electron microscopy (AEM) allows the determination of the composition with high spatial resolution.

High-resolution electron microscopy (HREM) has been used to study the atomic-scale structure and localized relaxations at grain boundaries (GBs) in Au, Al, and MgO. The [110] tilt GBs play an important role in polycrystalline fee metals since among all of the possible GB geometries this series of misorientations as a whole contains the lowest energies, including among others the two lowest energy GBs, the (111) and (113) twins. Therefore, studies of the atomic-scale structure of [110] tilt GBs in fee metals and systematic investigations of their dependence on misorientation and GB plane is of considerable importance to materials science. [110] tilt GBs in ceramic oxides of the fee structure are also of considerable interest, since in this misorientation range polar GBs exist, i.e. GBs in which crystallographic planes that are made up of complete layers of cations or anions can join to form a GB.

Thin singlecrystalline films of (110) Au were prepared by vacuum evaporation of Au onto (110) NaCl.

HRTEM analysis and conventional TEM techniques were used to determine the nature and density of defects in AIN thin films grown by Molecular Beam Epitaxy (MBE). Extended defects, which act as phonon scattering sites, are detrimental to the thermal properties for which AIN is sought. [1] Previous research on defect structures in AIN has focused primarily on bulk materials. [2-5] This investigation extends previous studies by concentrating on defects native to AIN thin films.

The thin films for this study were grown by MBE onto annealed Al2O3 (corundum structure) substrates with a nominal (0001) surface orientation. The steps and structure of such (0001) Al2O3 surfaces have been previously characterized. [6] Films with thicknesses of 50 to 120 nm were grown at substrate temperatures between 700°C and 900°C. An RF driven source for atomic nitrogen and an aluminum effusion cell were used to generate deposition fluxes. TEM samples were prepared in plan-view and cross-section using conventional dimpling and ion milling techniques.

The determination of unknown crystal defect structures by iterative digital image matching (“quantitative HREM”) has previously been successfully applied to grain boundaries (e.g. in niobium [1]) or phase boundaries with small misfit (e.g. Nb-sapphire [2]), which offer several advantages: (i) the supercell is small leading to fast cycle times in iterative structure refinement and thus to a secure global optimum, (ii) noise-filtering is possible by translation symmetry, (iii) the interface structure is mostly planar (i.e. perfectly columnar). For dislocations and other aperiodic defects all these three issues are missing and curved atomic columns from surface strains especially cause problems [3]. Any progress in this area must therefore address (i) - (iii) separately and estimate the change of the confidence level of the retrieved structure when switching from interface-refinement to dislocation-refinement. First results of this project address questions of convergency and speed.

Since each HREM-Lab is only concerned with a limited number of materials (and elements) as well as with a few microscope-voltages only, it is advisable to calculate transmission functions for single atoms for each element and voltage a single time, and store these data files in a library.