A. J. McGibbon

Direct determination of grain boundary atomic structure in SrTiO3

An atomic structure model for a 25� [001] symmetric tilt grain boundary in SrTiO3 has been determined directly from experimental data with the use of high-resolution Z-contrast imaging coupled with electron energy loss spectroscopy. The derived model of the grain boundary was refined by bond-valence sum calculations and reveals candidate sites for dopant atoms in the boundary plane. These results show how the combined techniques can be used to deduce the atomic structure of defects and interfaces without recourse to preconceived structural models or image simulations.

Direct Observation of Dislocation Core Structures in CdTe/GaAs(001)

A strategy is presented for determining sublattice polarity at defects in compound semiconductors. Core structures of 60-degree and Lomer dislocations in the CdTe/GaAs(001) system have been obtained by the application of maximum-entropy analysis to Z-contrast images (Z is atomic number) obtained in a 300-kilovolt scanning transmission electron microscope. Sixty-degree dislocations were observed to be of the glide type, whereas in the case of Lomer dislocations, both a symmetric (Hornstra-like) core and an unexpected asymmetric structure made up of a fourfold ring were seen.

The atomic structure of asymmetric [001] tilt boundaries in SrTiO3

Abstract The atomic structures of 25° and 45° asymmetric [001] tilt boundaries in SrTiO3 have been determined experimentally using the unique combination of high-resolution Z-contrast imaging and simultaneous electron-energy-loss spectroscopy in the scanning transmission electron microscope. These complementary techniques permit the direct location of the cation columns at the boundary to an accuracy of 0.2A and the coordination of the O atoms to be investigated with atomic resolution. Using bond-valence sum calculations, the structural and electronic results were combined to produce models for the grain-boundary structures. These calculations highlighted the presence of half-occupied columns at both grain boundaries as an important feature for reducing the grain-boundary energy associated with these ionic materials. In addition, the grain-boundary structure units identified in the experimental images were used to predict structures for other misorientations by the application of the principles of continu...

Observation of structural units at symmetric [001] tilt boundaries in SrTiO3

Incoherent Z-contrast imaging in the scanning transmission electron microscope allows atom column positions to be deduced directly from the experimental image, including locations where the column separation is less than the resolution limit. Maximum entropy analysis applied to the incoherent image locates the high-Z columns to an accuracy of ±0.2 Å. Oxygen coordination at the boundary plane can be deduced by high spatial resolution electron energy loss spectroscopy, and approximate column positions determined by simple bond-valence sum calculations. Observations of 25° (Σ=85), 36° (Σ=5) and 67° (Σ=13) [001] symmetric tilt grain boundaries in SrTiO3 bicrystals show that “half columns” are a ubiquitous feature of grain boundary structural units. The observed structural units can be combined to produce structural models for symmetric tilt boundaries over a 0–90° range. The Σ=17 (410), Σ=5 (310), and Σ=5 (210) are found to be favored boundaries and the structures of all the other tilt boundaries are comprised of these units combined with Σ=1 (100) and Σ=1 (110) structural units. All the proposed boundary models show continuity of grain boundary structure over the entire misorientation range. The Σ=17 (410) structural unit is asymmetric which induces microfacetting on all boundaries less than the Σ=5, 36.87° misorientation.

Atomic-resolution imaging and spectroscopy of semiconductor interfaces

Incoherent Z-contrast imaging uses a high-angle annular detector to collect only highly local, incoherently generated scattering with the result that images become dependent on intensities, not phases. No model structures are required for a first-order structure determination, and the images remain intuitively interpretable even at interfaces. Under suitable conditions, incoherently generated inelastic scattering may be collected simultaneously with a large-aperture axial spectrometer, and, by using the Z-contrast image to locate the scanning transmission electron microscope (STEM) probe over selected atomic columns, can provide an atomic-resolution chemical analysis. This is demonstrated with reference to an epitaxial CoSi2/Si(100) interface, achieving a 2.7 Å spatial resolution. Recent insights into the growth and relaxation of strained Si-Ge epitaxial films are described, highlighting the role of stress concentrations, and contrasting the case of a free surface with that of a surface constrained by an oxide layer.

Direct determination of grain boundary atomic structure in SrTiO{sub 3}

In the electroceramic SrTiO{sub 3} the grain boundary atomic structure governs a variety of electrical properties such as non-linear I-V characteristics. An understanding of this atomic structure-property relationship for individual grain boundaries requires a technique which probes both composition and chemical bonding on an atomic scale. Atomic structure models for tilt boundaries in SrTiO{sub 3} bicrystals have been determined directly from experimental data, by combining high-resolution Z-contrast imagine to locate the cation columns at the boundary, with simultaneous electron energy loss spectroscopy to examine light element coordination at atomic resolution. In this paper we compare and contrast the grain boundary structure models of symmetric and asymmetric boundaries in SrTiO{sub 3}.

Structural characterization of semiconductor heterostructures by atomic resolution Z-contrast imaging at 300kV

By applying the technique of Z-contrast imaging to the study of a GaAs/AlGaAs multilayer using a newly developed 300kV scanning transmission electron microscope, we show that it is possible to directly observe the interlocking group III and group V sub-lattices on a column-by-column level. In addition to the direct observation of structural polarity in the [110] orientation, we show that, by using a maximum entropy approach to image processing, the experimentally acquired data can provide direct information on interface structures at atomic resolution.

Atomic scale structure and chemistry of interfaces by Z-contrast imaging and electron energy loss spectroscopy in the STEM

The macroscopic properties of many materials are controlled by the structure and chemistry at grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted directly without the need for preconceived atomic structure models. Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale. The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface. In this paper we use the complimentary techniques of high resolution Z-contrast imaging and PEELS to investigate the atomic structure and chemistry of a 25{degree} symmetric tilt boundary in a bicrystal of the electroceramic SrTiO{sub 3}.