This paper reviews some of the new advancements made in solving the structure of planar interfaces in a wide range of materials and interface types. The main contributions of the HREM technique are the determination of the atomic positions at the interface, the detection of additional defects such as dislocations or monoatomic steps and more recently the chemical composition when crossing the interface. It is concluded that quantitative results obtained by image processing and pattern recognition will in the future greatly improve the knowledge of interfacial structures at an atomic scale.

The geometric and electronic structure of metal adsorbates on cleaved GaAs(110) surfaces is studied with the scanning tunneling microscope. For the metals Sb and Bi, an ordered monolayer is formed, although in the case of Bi a series of misfit dislocations appear in the overlayer. In the case of Au, individual atoms are observed on the surface, forming clusters at higher metal coverage. Spectroscopic measurements reveal the presence of a state within the GaAs band gap. This state is observed near individual metal adsorbates (for Au and Sb), and near the edge of metal terraces (for Sb and Bi). The observed state is responsible for determining the position of the Fermi-level at the surface.

We first list, in catalogue form, a number of research subjects which have utilized the fieldion microscope (FIM) and atom-probe field-ion microscope (APFIM) techniques in their solution. Then we present the results of a combined transmission electron microscopy (TEM) and APFIM study of a grain boundary (GB) in a Mo-5.4 at.% Re alloy, which had been annealed in bulk form for 35 hours at 1273 K to induce Re segregation. A GB with an orientation within ≈0.4° of Σ = 9 was studied employing TEM and analyzed in detail using Bollmann's 0-Lattice theory and Frank's formula. A set of secondary GB dislocations was observed with a spacing of 11.4 nm. The APFIM measurements – on this same GB – indicate that it has a Re concentration of ≈9.8 at.%; this value is 1.75 times greater than the matrix's measured concentration of 5.6±0.9 at.% Re. Thus this research constitutes direct and quantitative experimental evidence for solute-atom segregation to a high-angle grain boundary with a relatively high degree of coincidence ( ≈Σ = 9 ). These results are consistent with our Monte Carlo simulations of high coincidence twist boundaries and a Σ = 5 tilt boundary in Pt-1.0 at.% Au alloys which show that solute-atom segregation occurs mainly to the dislocation cores. The experimental and simulated values of the enhancement factor are approximately the same.

In this paper a new method for high-resolution imaging of a crystal lattice is presented, based on high-angle electron scattering in a scanning transmission electron microscope (STEM). An electron probe of atomic dimensions is scanned over the sample and the electron flux scattered through large angles measured by an annular detector and used to form an image. The detector integrates over a large range of angles and therefore replaces the coherent phase contrast of conventional high resolution TEM with the strong atomic number or Z-contrast characteristic of high angle Rutherford scattering. These characteristics make the image entirely complementary to the conventional image, ideal for studying the atomic structure and chemistry of defects and interfaces. Examples of the high Tc superconductors, epitaxial Ge on Si, and Si1−xGex/Si strained layer superlattices are shown, and a simple approximate method of image simulation is presented.

A position sensitive detector has been added to the Oxford atom probe facility, allowing the microchemistry of field ion specimens to be analysed with excellent chemical specificity and a lateral and depth resolution of better than 0.5nm. This paper presents some recent results obtained with this equipment on the chemistry and morphology of interfaces in multi-quantum well samples, illustrating the power of the technique in obtaining very detailed information on microstructural features with dimensions less than 1nm.

A Field Ion Microscope Imaging Atom Probe (FIM-IAP) has been applied to study the atomic structure of the icosahedral phase of the Al-Mn system. A multiple twinning model for icosahedral quasicrystals could be disproved, by calculating its FI image, and comparing it with experimental images. One of the decorations of the three dimensional Penrose packing (3DPP) of which FI images were calculated does compare favourably with experimental images, as far as the relative prominence of the two and fivefold poles, and the interplanar distances along the twofold directions are concerned. In particular an atomic decoration of vertices and two sites in the interior of the thick rhombohedron, which is one of the two building blocks of the 3DPP, turned out to be in agreement with the experimental findings.

Surface structure observations by high-resolution UHV electron microscopy at the atomic level summarized. Both the profile imaging in transmission electron microscopy and superlattice imaging in reflection electron microscopy are applied to image the reconstructed structure of gold surfaces and Si(111)7×7 surfaces.

Using a UHV transmission electron microscope we have examined the initial stages of Si oxidation. Using the surface-sensitive forbidden 1/3<422> reflection on Si (111) we have imaged surface steps at various stages of oxidation, including buried Si/SiO2 interface formation.

The profile imaging technique has been used to study the oxidation of ZnTe and InP surfaces induced by in situ reaction due to the electron beam of the microscope and by ex situ heating in air. For both materials, in situ reaction with the electron beam resulted in desorption of the anion species and the formation of the metal oxide. The observation of In metal particles, and the fact that the rate of formation of In2O3 was substantially reduced by an improvement of the vacuum near the specimen region, suggested that the presence of oxygen is not involved in the desorption process. The ex situ heating of ZnTe up to 260°C in air resulted in crystals of ZnO and Te metal, generally in a layered surface region with the sequence of ZnTe/Te/ZnO. The large Te crystals usually had an epitaxial relationship with the bulk ZnTe but the small ZnO crystals had random orientations. The ex situ heating of InP to 380°C in air only gave rise to small crystals of In2o3 in random orientations.

In-situ electron microscopy is a powerful tool for the study of small particles. Since most of the interesting phenomena take place in particles smaller than ˜5 nm, high resolution is highly desirable. In-situ and high resolution conditions are difficult to achieve in a single instrument. We have combined the in-situ UHV capabilities of a modified microscope at Stanford University with the high resolution capabilities of a 200 kV and a UHV-400 kV microscopes at Xerox PARC. Examples are presented, pointing out the advantages of in-situ deposition and treatment, and post deposition ex-situ observation at atomic resolution. Advantages and limitations of this approach are discussed.

In the study of interfaces HREM is most useful when the interface is viewed edge-on while both crystals are accurately aligned along low index zone axes. The formation of such interfaces by epitaxy or topotaxy is the natural means of obtaining structures that can be usefully analyzed by HREM. Furthermore, there is intense interest in understanding the atomic structure of such interfaces in a variety of technologically important materials. This contribution addresses such structures produced by thermal decomposition, precipitation reactions and ionized-clusterbeam deposition, and reports on the structural investigation of symmetrical and asymmetrical grain boundaries, precipitate/matrix interfaces, internal defect structure of precipitates and nanocrystalline composites.

A 2×1 reconstruction has been observed at the Si/NiSi2(100) and Si/CoSi2(100) interfaces. The reconstruction has been found in both ion-implanted (mesotaxial) material and in material grown by molecular beam epitaxy (MBE). The reconstruction is apparent in HREM images obtained from <110> cross sections and in transmission electron diffraction (TED) patterns from (100) orientation samples. We propose that the reconstruction is due to a layer of dimerised silicon atoms at the interface. We conclude that the 2×1 reconstruction is a low energy state of the silicon/disilicide(100) interface.

Preliminary experimental high resolution micrographs have been obtained for a Au thin film containing a [001], Σ=5 (θ=36.5 °) twist boundary. Other twist boundary cases such as the [001], Σ=13 (θ= 22.6°) and [111] low angle (˜3°) have also been fabricated. In order to assess whether the atomistic boundary structure is being imaged, extensive computer simulations of the Σ=5 bicrystal images have also been undertaken. Structurally, several contributions to the scattering must be considered which include: both upper and lower surface layers, bulk contributions above and below the interface and the relaxed interfacial structure at the composite film midplane. The effect of specimen orientation on image features is also evaluated for dynamical diffraction conditions. Likewise, the types of image features which arise by including bulk lattice reflections or alternately including only the reflections arising from the interface and surfaces will be discussed. Several schemes for analyzing twist boundary structures flat-on or near flat-on will be presented for a wide range of microscope conditions for a 400 kV microscope.

The observation of III–V multilayered semiconductors on 900 cleaved wedges is an interesting alternative to the conventional TEM observation on cross-sectioned samples. It offers a fast specimen preparation, the absence of ion irradiation damage or preferential etching, and a well controlled thickness across the sample. HREM observations are used to derive the layer thickness down to the atomic level. Image simulations were calculated with EMS programs on wedges described by a supercell.

InSb films grown directly on (100) GaAs substrates by MBE have been examined in the transmission electron microscope. High quality, epitaxial layers were deposited despite the 14.6% lattice mismatch between film and substrate. Nearly all of the misfit strain has been accommodated by a square array of a/2{011} edge-type misfit dislocations spaced on average 29Å apart. These defects are proposed to be spontaneously generated in the epitaxial layer almost as soon as it begins to grow, and are favoured over 60° type dislocations because they are more efficient at relieving misfit strain while allowing more coherent interface area to form. The films that have been produced have low defect densities (ie. threading dislocations and microtwins) considering the large lattice mismatch in this system. Antimony precipitates have been noted in some films, but can be totally eliminated by careful control of the In/Sb flux ratio. Finally we have observed that loss of Sb from the specimen surface leads to a gradual degradation of the InSb during TEM observation, leading to the appearance of surface indium or indium oxide.

I review the current results of our group about high-resolution electron microscopy of the icosahedral quasicrystals, in particular, in rapidly solidified Al-Mn-Si and conventionally solidified Al-Fe-Cu alloys, by using 200 kV and 1 MV microscopes which have resolutions of .23 and .16 nm, respectively. High-resolution lattice images of Al-Mn-Si taken with the 200 kV microscope show characteristic features of a model of the three-dimensional Penrose tiling, and also show the existence of quenched phason strains and edge-type dislocations in the Al-Mn-Si and as-casted Al-Fe-Cu alloys. After annealing the stable Al-Fe-Cu icosahedral phase at 1118 K for 48 hr, quenched phason strains are almost relaxed and nearly perfect icosahedral symmetry is formed. A high-resolution structure image of Al-Mn-Si taken with the 1 MV microscope clearly shows an arrangement of the Mackay icosahedral atomic clusters.

Indium precipitation in Si after ion implantation and rapid thermal annealing was studied using plan-view and cross-sectional TEM and SIMS analysis. The precipitates were found to be single crystalline with the bulk In body-centered tetragonal lattice based on periodicity data in two dimensions. The orientation relationship between the precipitate and Si lattice has been derived to be:

The structure of the Si(111)-CaF2 interface has been determined with Medium Energy Ion Scattering and High Resolution Transmission Electron Microscopy. Methods to determine this interface structure with HRTEM are discussed.

In this talk we will discuss the use of high resolution transmission electron microscopy (HRTEM) in the study of high Tc superconducting oxides. HRTEM has played an important part in the characterization of microstructure in YBa2Cu3O7−x, and in structure refinement of mixed phase Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O compounds. It is unlikely that HRTEM will contribute to any great extent to the understanding of why these materials are superconductors. But HRTEM will continue to make vital contributions to the studying and understanding of defect structures (such as grain boundaries and planar defects) which interfere with the flow of supercurrents.

The superconducting phases of Tl2 Ba2 CaCu2O8 (2212) and Tl2Ba2Ca2Cu3O10 (2223) have been identified by means of electron diffraction analysis and high resolution electron microscopy. The 2212 phase is essentially perfect, with the structure previously determined by x-ray studies but with a different c/a ratio, while the 2223 phase shows structural variations which may be due to oxygen vacancies. In addition, superstructures based on intergrowths of 2212 and 2223 and the block structures Tl2Ba2CanCun+1O2n+6(n=1,3,4,5) intergrown in 2223 were observed.