Solving the Structure of Interfaces by High Resolution Electron Microscopy
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SOLVING THE STRUCTURE OF INTERFACES BY HIGH RESOLUTION ELECTRON MICROSCOPY ALAIN BOURRET Service de Physique,S. CEN-Grenoble, 85 X, 38041 Grenoble C~dex, France
ABSTRACT 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. INTRODUCTION Numerous observations of interfaces between two similar or dissimilar materials with high resolution electron microscopy (HREM) have been reported in the last few years. Most of the results were obtained on semiconductor materials : grain boundaries, silicon-silicide interfaces, multilayers, quantum wells, semiconductor-metal interfaces were the configurations on which HREM has given significant contributions (for recent reviews, see [1] and [2]). However other metallic or composite systems were recently observed [3]. For instance the product of chemical reactions at surfaces or metallic interfaces as well as metal oxides have been characterized (Cu/Au [4] ; MgOAl [5] ; Cu/O [6]). Similarly the interfaces between a precipitate and the matrix have been observed in few systems such as Al/Ge [7], Al-Cu-Li alloy [8], superalloys [9]. With the improved resolution of modern microscopes (down to 0.16 nm) the possibilities have been greatly extended : more than one zone-axis can be looked at, opening the way to true 3-D HREM and more compact structures can be studied. Most of these observations have been treated qualitatively, the interpretation ranging from a direct read-out of the structure to a visual comparison with computer simulated images of a small number of atomic models. These qualitative approaches were sufficient to give some insight into the overall characteristics of interfaces. One can deduce for instance i) the periodicities at the interface, ii) the coherency or the loss of coherency with or without interfacial dislocations iii) the rotation of the material iv) the presence of steps or facets as small as a monolayer, v) the presence of additionnal extrinsic defects such as dislocations or impurities seggregated at the interface. However more subtle details are also accessible although they are more demanding for the technique. For instance the following items are of remarkable interest : i) determination of the atomic position at the interface particularly when large distortions of the bonds (or change of the coordination number) are likely to occur. This is the case for grain boundaries separating two disoriented but similar crystals, for epitaxial layers at vicinal surfaces, or for interfaces between two very different structures - ii) de
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