High-resolution electron microscopy -is used to characterize the defect structure of CdTe/GaAs and ZnTe/GaAs heterojunctions before and after annealing. For as-deposited films, a variety of defects exist both in the form of perfect misfit dislocations at the interface and extended defects into the thin film. The extended defects result from dissociation of 60° dislocations and reactions between perfect and partial dislocations lying on intersecting slip planes. The annealed interfaces consist of a periodic array of perfect edge Lomer dislocations, the most efficient type of misfit dislocation for accomodating the lattice mismatch, 14.6 % for CdTe/GaAs and 8 % for ZnTe/GaAs. In both cases, the spacing between dislocations corresponds to the value predicted for completely strain-free thin fims, 31 and 54 Å for CdTe and ZnTe respectively. This paper concentrates on the different dislocation reactions which transform the interfacial structure from the as-deposited case to the annealed case.

High resolution electron microscopy has been used to study the structure of the 3C/6H interface, Σ,=3 {111}and Σ.=3 {112}grain boundaries in 3C-SiC. In SiC, as in other compound semiconductors, anti-site bonds occur in a variety of defects. These are high energy bonds comparable to that of dangling bonds. But, while dangling bonds at the grain boundaries may be eliminated by reconstruction just as in elemental semiconductors, it may not be possible to avoid anti-site bonds.These problems are discussed for the Σ=3 {112} grain boundary, where the structures proposed for Ge and Si are used as starting models for SiC.

High resolution transmission electron microscopy (HRTEM) of GaAs/AlAs hetero-structures grown by molecular beam epitaxy (MBE) is carried out in the <110> projection. It is shown that GaAs and AlAs are distinguished clearly by the difference in their lattice images at the samples thicknesses of about 15–30 nm under near Scherzer focus condition. Under these imaging conditions, very thin films consist of single monolayer AlAs are observed. Vicinal interfaces of GaAs/AlAs which were grown on (001) substrate misoriented toward [110] are also examined in the [110] projection. The interfacial structures are imaged edgeon, so that the fluctuations of terrace width, and the roughness of step-edges at these interfaces are observed on an atomic scale.

GaAs/Gal-xInxAs strained layer superlattices with well-widths of 7nm, barrier widths of 14nm and periods of 10 to 30 have been examined by HREM both in plan view and cross section. Strain release occurs mostly by dislocation generation but twins are also observed, especially at the substrate interface. High resolution images of dislocations, twins and interfaces are analyzed to elucidate mechanisms of strain release.

The theory of imaging crystallographic defects in solid specimens through the use of electron channeling contrast is reviewed and the necessary conditions for observation are deduced. It is shown that current high performance field emission scanning electron microscopes can meet these requirements and produce dislocation images from suitable materials.

It is shown how the use of a high angle annular detector in a STEM provides images which show strong compositional sensitivity and almost perfect incoherent characteristics. No contrast reversals occur with defocus or sample thickness and the contribution of an atomic string is insensitive to the distribution and strength of neighboring strings. The image is best described in real space as a simple convolution of the incident probe intensity profile with an object function sharply peaked at the atomic strings having a strength dependent on composition. How the incoherent characteristics arise is described using a Bloch wave approach and examples are shown of interfaces in semiconductor and superconductor materials.

A new strategy for atomic resolution compositional mapping at semiconductor interfaces is presented. Images can be interpreted directly to within 10% to give column-by-column compositional information with a sensitivity approaching Rutherford scattering cross sections. We apply the method to obtain the first atomic resolution images of interfacial ordering in ultrathin (SimGen)p superlattices and attribute the results to growth on (2 × 1) and (1 × 2) reconstructed surfaces.

Based upon coarsening studies of larger cavities (8 nm) in a rapidly solidified 304SS, it was determined that there should be a large number of very small cavities (1–2 nm) in these materials that are too small to observe with out-of-focus imaging techniques on a conventional TEM. Out-of-focus imaging on a high-resolution TEM could also be used but ambiguities would remain in the interpretation of the origin of the image contrast. In order to overcome these problems of ambiguity, the annular dark field (ADF) image on a VG HB501 STEM was employed. Since this signal is proportional to mass thickness and Z3/2, we expected that the change in mass thickness locally associated with a cavity would be detectable in a thin enough region. These images show a clear correlation between features in the BF through-focus series which behave as cavities and features in the ADF image. This combination of imaging modes has provided convincing evidence for existence of the 1 nm cavities.

We have studied point defects, linear defects as well as spatial transitions between ordered and disordered structures on silicon surfaces with atomic resolution by using scanning tunneling microscopy (STM). Point defects in the vicinity of multiple step edges as well as surface reconstructions at multiple step edges as high as 3 nm have been characterized by STM. STM images of partially disordered silicon surfaces prepared by laser and thermal annealing demonstrate the potential of STM for characterizing non-periodic surfaces on the atomic scale.

Most metals have small unit cells, and in small unit-cell materials in low-index zones the strength of the diffracted beams is often greater for lighter metals than it is for heavier metals. The reason for this is that the strong double-diffraction in heavy metals gives rise to weak Bragg diffraction. In this paper it is shown that for small unit-cell materials, light atom precipitates can be easily detected in heavy atom matrices; and that in large unit-cell materials, the breakdown in the weak-phase-object approximation in thick crystals can be used to identify atom type at defects.

This paper gives an overview of recent High Resolution Electron Microscopy (HREM) observations of transition metal carbides. The main point is to show how HREM can be used to study a sublattice of light atoms within a complex crystal. General tendencies of this approach will be outlined, which appear, to a large extent, to be common to similar possible studies of other compounds such as nitrides, borides or oxides. Specifically, attention is focused on studies of long-range ordered structures in carbides, due to non-stoichiometry accommodated by metalloid vacancies. Various examples are given for representative compounds, including V8C7, V6C, 5, Nb6C5 and W2C.

The defects which occur in polycrystalline aluminum nitride are very detrimental to the beneficial physical and electrical properties. 'Dome-like' or 'D' defects, consisting of a flat basal fault joined to a curved planar fault, are examined by high-resolution electron microscopy. The basal segments are shown to contain a thin ∼5Å layer of a second phase and are associated with a stacking-fault and an antiphase boundary. A structural model is proposed for the defect which incorporates a layer of alumina at the basal segment of the 'D' defect. The formation of thisdefect is discussed.

Knowledge about the nature of interfaces between SiC whiskers and alumina matrices is critical to understanding the mechanical behavior of these types of composites. The fracture toughness, in particular, is strongly influenced by the bond characteristics of this interface. Previous results have shown that oxidizing the whiskers in air prior to composite fabrication by vacuum hot-pressing results in composites which have lower toughness compared to those made with whiskers in the as-received condition. This investigation focusses on the effects of oxidizing and reducing surface treatments on the whiskers themselves and on the whisker-matrix interface. High resolution electron microscopy (HREM) was used to characterize the morphology and the amount of amorphous phase at the whisker-matrix interface, as well as the crystallography and topography of the interface. The present results indicate that the characteristics of the whisker/matrix interface may not be directly related to the initial whisker surface chemistry. Surprisingly, the thickest thermally-grown oxide layer resulted in the thinnest amorphous film at the SiC/Al2O3 interface.

Thin films of YAG and YAG/alumina mixtures were prepared by gelling a sol across a TEM grid. The grids were heat-treated to temperatures as high as 1550°C. The resulting ceramic thin films were observed by standard and high resolution TEM techniques. Ion-milling, carbon coating, or other specimen preparation was not necessary. The phase and microstructure evolution, as well as pore structure evolution and spheroidization of film edges could be easily observed. Abnormal grain growth was observed in yttrium-aluminum garnet (YAG) films. Lattice images were taken of the matrix, abnormal grains, and structures that evolved from the abnormal rains at higher temperatures. The grain size preceding abnormal grain growth was 20–50 nm. The abnormal grains were composed of 20–50 nm subgrains with up to several degrees of misorientation. High resolution observations were also made of small (< 0.2 μm) YAG inclusions in alumina. An orientation relationship of (111)[011]alumina//(112)[021]YAGwas observed. This orientation relationship was not observed in large inclusions.

Perovskite KNbO 3 thin films were heteroepitaxially deposited onto (100) MgO substrates. Twin domains with tetrahedral shape were typically observed. These tetrahedrons were bounded by three {211} twin planes. High resolution transmission electron microscopy (HRTEM) was employed to examine these {211} twin boundaries. Surface steps of {110}<110> type generated by dislocation slip were present on the MgO substrate. The tetrahedral twin domains originate on surface steps, then grow with a stacking fault relationship to the matrix. The strain fields of dislocations near the stacking faults slightly rotate the tetrahedral twin nuclei. This small degree of misalignment between the matrix and the twin domain result in some of the twin boundaries having amorphous boundary regions rather than coherent interfaces.

Order-disorder antiphase domains (APD's) were directly imaged with HRTEM. Ultra small APD's ranging from 10 to 30 atomic spacings were observed. The origin of these APD's was either the surface steps of {100} <1 10> type on MgO substrates or the random nucleation of anions at either of two equivalent sites on the MgO substrate.

High-resolution electron microscopic examination of alkoxide thin-film gels has been employed to study the drying and aging of thin-film Pb(Zr0.45Ti0.55)O2 (OR) 2 as it evolves from the gel to the ferroelectric perovskite product. The initially amorphous film is observed to aggregate to form ordered islands with inter-metal distances close to that of the final structure embedded in a highly disordered matrix. When these aggregates are at the surface they grow and decompose more rapidly to the perovskite product. The progress to a polycrystalline final state is followed and recorded in real time and in time-lapse at high resolution.

Isothermal and isochronal annealing of alpha-recoil damage was performed on the mineral titanite, CaTiSiO5. Crushed-grain samples were heated for 21 hrs, at 100°C intervals from 300°-700°C. HRTEM imaging of alpha-recoil collision cascades (amorphized regions), after each temperature interval, showed that there was no change in the abundance or appearance of the cascades at 300°C, but an apparent decrease in cascade size occurs at 400°C (from 3.8 nm to 2.4 nm diameter). A decrease in the size of collision cascades may indicate repair of interstitial defects at cascade boundaries (where interstitials dominate), while the cascade cores remain amorphous (cores acquire more extensive knock-on damage). At 500°C, the number of collision cascades observed decreases by greater than 90%, indicating major annealing of the amorphous cascade cores, and precipitates appear on titanite grain surfaces. At 600°C, changes do not occur. At 700°C, both the cascades and precipitates are absent; the alpha-recoil damage is fully annealed.

The results of image simulations on partially amorphous microlite (Ca2Ta2O7, pyrochlore structure) are presented. Results indicate that HRTEM images are not sensitive to the position of amorphous layers within a crystalline matrix. In addition, it is observed that the limit of detection of amorphous material within a crystalline matrix is dependent upon the total thickness of the sample. In thin crystals (<150 Å), up to 75 volume percent crystalline material can give rise to aperiodic images, yet the addition of a small amount of crystalline material (80 volume percent crystalline) produces a periodic image. Images calculated for isolated spheres of amorphous material distributed within crystalline microlite suggest that isolated domains of amorphization are observable at sample thicknesses less than three times the diameter of the feature. The image contrast of amorphized domains is enhanced by imaging at defocus settings significantly different than Scherzer focus. These results indicate that interpretation of HRTEM images of partially amorphized crystalline materials should be undertaken with caution, and estimates of the volume of damage considered only qualitative.

HREM studies of the low-pressure form of Nb3O7F, obtained by thermal decomposition of NbO2F or by reacting stoichiometric amounts of NbO2F and Nb2O5, confirmed that the crystals are built up of a shear-type structure in which slabs of ReO3 type, infinite in two dimensions and three octahedra wide, separate the shear planes. Most of the crystals were well-ordered, although faults could occasionally be detected. However, some crystals were found to be intergrown with a structure closely related to the high-pressure form of Nb3O7F. The latter structure is composed of wavy rows of edge-sharing NbX7 pentagonal bipyramids, joined to each other by NbX6 octahedra. The connection between the two atomic arrangements can be understood in terms of the common c-axes and a slight distortion of the polyhedra along the boundary.

Electron-beam-induced surface reactions and rearrangements have been observed for a number of oxides including MgO, TiO2, SrTiO3 and SnO2 using conventional and ultra-high-vacuum high-resolution electron microscopes. Electron irradiation of TiO2 resulted in a variety of effects including reduction, re-oxidation and amorphization. SnO2 was observed to form facets readily during high-resolution imaging but it was stable against reduction except when exposed to extreme current densities which caused the formation of metallic tin crystals. Electron irradiation of MgO under UHV conditions resulted in the formation of facetted pits. The surfaces of SrTiO3 were stable for moderate electron dose rates but rapidly amorphized when irradiated at extreme current densities.