Atomic and Electronic Structure of High Purity SiC Grain Boundary
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Atomic and Electronic Structure of High Purity SiC Grain Boundary
Eriko Takuma and Hideki Ichinose Department of Materials Science, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JAPAN.
ABSTRACT Fully relaxed SiC grain boundaries were produced by the sublimation re-crystallization method. Several kinds of grain boundaries of bi- and tri-crystals were investigated applying an atomic resolution high-voltage transmission electron microscopy (ARHVTEM). The rotation axis and the rotation angle were common to frequently observed grain boundaries, which were, respectively, 70.5 degrees and . But boundary planes differed from each other. It was shown that the atomic bonding direction was continuous across the stable grain boundary.
INTRODUCTION Atomic structure of metallic grain boundary has been discussed by the aid of geometrical models [1]. Non-directional metallic cohesion allowed us to employ such a simple procedure [2]. But a kind of deviation from the geometrical model was observed at an early time in gold (112)Σ3 CSL boundary [3], in which the repulsive force between too close atomic pairs in the boundary displaced one crystal relative to the other. The CSL points in the boundary consequently disappeared. But this was a rare case in which, in addition to “geometry”, “physics” was necessary to discuss the metallic grain boundary structure. Grain boundaries of covalent bonding materials, such as silicon, germanium and SiC, explicitly required more physical consideration in order to be able to discuss the observed boundary structure [4][5][6]. It was suggested during the discussion that the number of dangling bonds strongly affected the stability of the grain boundary (the least dangling bond rule) [6]. In the present study, geometrical structure analysis was attempted on the binary SiC grain boundary considering the effect of atomic bonding. Several low energy boundaries of 6H-SiC were observed by ARHVTEM and were characterized.
EXPERIMENTAL PROCEDURE A high purity SiC bi-crystal was grown by the sublimation re-crystallization method to obtain a fully relaxed lowest energy boundary. A tri-crystal was also grown in order to analyze secondary (or thirdly) stable boundary. The bi-crystal grew very slowly without any applied external force which might prevent free formation of stable boundaries. An as-grown crystal was kept at the temperature 2500 deg. to complete further structure relaxation. Most of poly-types which might be present were not observed after the heating except for the 6H-phase [7]. A schematic illustration of the SiC bi-crystal is shown in figure 1.
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Figure 1. Schematic illustration of the specimen bi-crystal produced by the sublimation re-crystallization method.
The bi-crystal was sliced with a diamond saw in the direction perpendicular to a grain boundary. the thickness of the slice was 0.3 mm. The sliced specimen was mechanically thinned down to 0.02 mm and was finally polished using argon ion milling (GATAN PIPS model 691) for TEM observation. Fi
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