The atomistic structure at interfaces and defects in crystalline materials can be investigated by transmission electron microscopy (TEM). For some time it has been possible to form phase contrast images with a resolution of 0.2 nm and better on instruments operating at practical accelerating voltages where beaminduced specimen damage is often not severe. However, the phase contrast image is hard to interpret without recourse to complex simulation and through focal series techniques [1]. In general, it is extremely difficult to invert the images to yield a model specimen structure. The effects of multiple scattering within the crystal and the phase problem (collection of image intensity, not complex amplitude) conspire to complicate analysis of the coherent phase contrast image [1, 2]. However, it has recently been shown that, using either a different illumination or detection geometry, images can be formed that are essentially incoherent and are generally much more easy to interpret; the location of atomic columns being identifiable from a raw image. In particular, a high angle annular dark-field (HAADF) detector in the scanning transmission electron microscope (STEM) has been used to acquire images at a resolution given by the size of the STEM electron probe intensity distribution [3]. This can be up to the order of 1.6 times smaller than the equivalent point resolution for STEM phase contrast images. The HAADF images additionally show a large degree of sensitivity to the average atomic number, Z, of atoms in the columns at each point in the image. For this reason the experimental technique is usually referred to a Z-contrast imaging (see, for example [4-7]). Z-contrast imaging is therefore well suited to the analysis of defects and interfaces where we desire more complete understanding of their role in determining the bulk properties of materials. Also, the information that can be obtained from a Z-contrast image, using a probe of atomic spacing dimensions, can be used to position the electron beam accurately on a structural site of interest for immediate spectroscopy via energy-dispersed X-ray (EDS) or electron energyloss (EELS) methods. In particular, with current instrumentation EELS can be performed at atomic resolution and with sufficient energy resolution to determine changes in electronic structure via fine structure analysis of core loss ionization edges in the spectrum [8]. In this paper we demonstrate the performance of STEM experiments with a sub-0.2 nm probe. Z-contrast images of high temperature superconducting Bi2Sr2Ca2Cu3O10 (BSCCO) tapes were formed and used to investigate the changes of phase that can occur at grain boundaries. Additionally, an ohmic metal contact layer of Au on GaAs was imaged at a resolution of around 0.15 nm. The changes in atomic structure were observed at different areas of morphological features at the interface.

Atomic-resolution Z-contrast imaging and electron energy loss spectroscopy combined with energy dispersive x-ray spectroscopy are used to investigate the structure-property relationships in an ideal metal–oxide–semiconductor device structure. Arsenic segregation with a very narrow profile occurring precisely at the silicide/Si interface was identified. Images show that the As is substitutional on the Si lattice sites, implying that it remains electrically active. These structural results imply desirable electronic properties for the device and are consistent with electrical measurements showing a decrease in contact resistance for these samples.

Incoherent imaging and analysis techniques in the scanning transmission electron microscope (STEM) provide the potential to map changes in structure, composition and bonding that occur at materials interfaces and defects on the fundamental atomic scale. Such comprehensive characterization capabilities permit a detailed analysis of the structure-property relationships of interfaces and defects to be performed. In this paper, we discuss the resolution limits of such techniques in the JEOL 2010F STEM/TEM operating both under standard conditions and at elevated temperatures. Examples of the use of such techniques to quantify the atomic scale defect chemistry at interfaces and defects in perovskite oxides, the growth and structure of II-VI and III-V quantum dots and the electronic structure of threading dislocations in GaN will also be presented.

Recent developments in Schottky field emission scanning transmission electron microscopes now provide the ability to map changes in structure, composition and bonding that occur at materials interfaces and defects on the fundamental atomic scale. The principles of the main techniques, Z-contrast imaging and electron energy loss spectroscopy will be described here. Additionally, recent results from the analysis of II-VI and III-V quantum dots, dislocation cores in GaN and interfaces in MOSFETs will be presented.

The electronic structure of pure edge threading dislocations in metalorganic vapor phase epitaxy grown wurtzite GaN thin films has been studied directly by atomic resolution Z-contrast imaging and electron energy loss spectroscopy in a scanning transmission electron microscope. Dislocation cores in n-type samples grown in N-rich conditions show no evidence for the high concentration of Ga vacancies predicted by previous theoretical calculations. Nitrogen K-edge spectra collected from edge dislocation cores show a sudden and significant increase in the intensity of the first fine-structure peak immediately above the edge onset compared to the bulk spectra. The origin of this increase is discussed.

The evolution of self-assembled CdSe quantum dots deposited on (and subsequently capped by) ZnSe was investigated on a series of samples grown by molecular beam epitaxy, with CdSe coverages from 0.5 to 2.6 monolayers. The samples were investigated by cross-sectional scanning transmission electron microscopy, as well as macro- and microphotoluminescence. The results clearly indicated a coexistence of 2D ZnCdSe platelets and 3D islands, showing clearly that the platelets act as precursors for the formation of the 3D islands.

Although internal interfaces have long been known to dominate the performance of many materials systems critical to modern technology, there has yet been little incontrovertible evidence pointing to the fundamental origin of the structure-property relationships. However, the recent development of direct imaging and analysis techniques in the scanning transmission electron microscope (STEM) has provided a new experimental pathway to obtain information on the local atomic structure, chemical composition and bonding at interfaces on the fundamental atomic scale. This is precisely the information that is required to unravel the complexities of interfaces and opens up a new paradigm for investigating the structure-property relationships at internal interfaces. In this paper we discuss the practical aspects of the experimental STEM techniques and demonstrate the resolution possible in current commercially available instrumentation. The application of these techniques to the study of internal interfaces is highlighted by a discussion of the analysis of homophase interfaces in SrTiO 3, Bi 2Sr2Ca2Cu3O10 and YBa2Cu3O7-δ and the heterophase interfaces between GaAs and Au, and between Si and molecular beam epitaxy (MBE) grown II-VI semiconductors.

In recent years, GaN and its alloys have been the subject of an intense global research effort to develop its optoelectronic properties in the blue-green region of the spectrum. Of particular interest has been the fact that despite a high density of threading dislocations, on the order of 10 8 to 10 10 per cm 2 , thin film devices retain their ability to emit light. The origin of this behavior remains unclear, and it has even been suggested that reducing the number of defects by employing different growth techniques does not necessarily increase the crystal's lasing abilities. As research is now aiming to grow GaN on silicon (silicon has a large lattice-mismatch to GaN) in order to develop an inexpensive LED technology, dislocations are likely to remain a major issue. It is therefore essential that we develop a fundamental understanding of the electronic structure of these defects, in order to determine their effect on the properties.

The direct interpretability of atomic resolution Z-contrast images obtained from a scanning transmission electron microscope (STEM) makes this imaging technique particularly powerful for the analysis of interfaces and defects in semiconductor materials and devices. In this paper, the principles of the technique are outlined and representative examples of its use are presented. In particular, we show the use of Z-contrast imaging to determine the polarity of a CdTe film grown on a Si substrate, the atomic structures of stacking faults and threading dislocation cores in GaN, and the atomistic structure of an ohmic metal/semiconductor contact of Au/GaAs.

A complete understanding of the complexities behind the structure-property relationships at materials interfaces requires the structure, composition and bonding to be characterized on the fundamental atomic scale. This level of characterization is beyond the scope of a single imaging or microanalysis technique and so to solve practical interface problems, correlation between multiple techniques must be achieved. Here we describe recent advances in the JEOL 201 OF 200kV field-emission STEM/TEM that now allow atomic resolution imaging and analysis to be obtained in both TEM and STEM mode and discuss two applications of these techniques.

The optical arrangement of the scanning transmission electron microscope (STEM) allows formation of incoherent images by use of a large annular detector. Here we show this capability in the imaging of defects in GaN and the interfacial region of an Au/GaAs ohmic contact. A resolution of around 0.15 nm is attained. Such Z-contrast images show strong atomic number contrast and allow the probe to be positioned accurately at the defect or interface for the purpose of performing high spatial resolution electron energy-loss spectroscopy (EELS).

Here we demonstrate sub- 1.5 Å resolution in compositionally sensitive high-angle annular dark-field (HAADF) (“Z-contrast”) imaging. For the first time this has been achieved on a 200 kV field-emission transmission electron microscope (FEGTEM), the JEOL JEM-2010F. With a Gatan imaging filter, this type of instrument is then capable of both analytical imaging and electron energy-loss spectroscopy at similar spatial resolution as in the 300 kV dedicated STEM.The Z-contrast imaging technique has a spatial resolution given by the size of the electron probe. When used to image periodic specimens and their defects, the effective incoherent nature of the Z-contrast method leads to higher resolution for given lens Cs, higher sensitivity to atomic number and easier qualitative image interpretation than in HRTEM.In practice, the ability to form a small (atomic resolution) probe depends on the brightness of the electron source, and achieving low enough levels of mechanical and electrical instabilities that otherwise incoherently broaden the probe.

In the scanning transmission electron microscope (STEM), the spatial resolution of experimental images and spectra is determined by the size and stability of the electron probe. Atomic resolution, of 0.2 nm and under, is possible if all experimental parameters influencing probe formation are carefully optimized. Here, the formation and alignment of the STEM probe using electron Ronchigrams is described. Practical examples of probe formation, Z-contrast imaging and electron energy-loss spectroscopy (EELS) are demonstrated on a Schottky field emission, JEOL JEM-2010F microscope. Single crystal Si [1 1 0] images were used for resolution testing and showed that probe sizes of under 0.14 nm are obtainable. A 36.5 degrees Sigma 5 tilt grain boundary in nominally iron doped SrTiO3 was imaged incoherently and analyzed with EELS, using this probe. (C) 1999 Elsevier Science B.V. All rights reserved.

The experimental techniques of Z-contrast imaging and electron energy-loss spectroscopy can yield a wealth of information on the atomic and electronic structures of defects and interfaces. The spatial resolution of these techniques depends on the size and stability of the electron probe. Here we demonstrate the characterisation and optimisation of small probe performance in the 200 kV FEGTEM, equipped with a Schottky field-emission electron source. Probe sizes of around 1.5 Angstrom are obtained.

The collection of atomic resolution Z-contrast images, using an annular darkfield detector, has until recently been exclusively performed using the dedicated scanning transmission electron microscope (STEM). Here, preliminary results demonstrate the capability of performing this technique with a 200 kV FEGTEM, featuring a Schottky-emission electron gun. As in the dedicated STEM, the microscope's spatial resolution limit, for both Z-contrast imaging and chemical micro-analysis experiments, depends on the objective lens spherical aberration and the source brightness. Images from a number of materials indicate sub-2 Angstrom Z-contrast resolution. In addition, the instrument exhibits probe stability sufficient to observe 1.6 Angstrom lattice fringes in coherent convergent beam electron diffraction (CBED) patterns. Using an electron Ronchigram, a spherical aberration coefficient of 0.5 mm has been measured in STEM alignment for one particular pole-piece. Therefore, higher spatial resolution than that observed here may be possible with a further optimized instrument.

Here we demonstrate sub- 2Å resolution, in compositionally sensitive Z-contrast imaging, for a 200 kV field-emission transmission electron microscope (FEGTEM). This method has shown great promise for determining the atomic structure of interfaces in such materials as ceramics, semiconductors and high-T c superconductors. The Z-contrast technique involves focusing an electron probe at a specimen surface (which is oriented with a zone-axis along the beam direction). The transmitted signal, at an annular detector, is displayed on a VDU, rastered synchronously with the probe. Coherent, contrast reversal effects are suppressed in this geometry; images approximate to a convolution of the probe intensity profile with the square of the projected specimen potential. The Z 2 dependence means that images can, with care, be interpreted as chemically sensitive maps of the projected structure. Furthermore, electron energy loss spectroscopy (EELS) can be performed simultaneously with Z-contrast imaging to yield additional compositional information.