Relaxation of Grain Boundaries in Au {110} Mazed Bicrystal Thin Films Observed by HREM
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Relaxation of Grain Boundaries in Au {110} Mazed Bicrystal Thin Films Observed by HREM Tamara Radetic and Ulrich Dahmen National Center for Electron Microscopy, Lawrence Berkeley National Laboratory Berkeley, CA 94720, U.S.A. ABSTRACT Thin films of gold can be grown on {001} Ge single crystal substrates in two equivalent {110} orientation variants, related to each other by a 90° rotation about the surface normal. The morphology of the films is that of a mazed bicrystal, a polycrystalline film with many randomly distributed columnar grains in only two orientations. All grain boundaries are of the type Σ99 and display pure tilt character. In this work, we report on observations of the structural relaxation of these grain boundaries, with special emphasis on their characteristic behavior at the intersection with free surfaces and their evolution during thermal annealing.
INTRODUCTION In the last decade, a major focus of high resolution electron microscopy (HREM) research has been the study of the atomic structure of the grain boundaries. Coupled with extended simulations and modeling, this research has contributed to considerable advances in our understanding of the structure and properties of grain boundaries. For example, contrary to previous views, it has been established that some asymmetrical grain boundaries have lower energy than symmetrical ones [1-3]. Experimental observations have shown that grain boundaries tend to relax via a number of different mechanisms, as discussed by Merkle [4]. In addition to the well-known mechanisms of relaxation - rigid body translation and rearrangement of atoms at the core of the interface - a second possible relaxation mode is the emission of partial dislocations. This mode leads to the formation of extended boundaries, and modeling has confirmed the stability of such boundaries [4-6]. When the extended defect structure forms a slab of material with a different crystal structure or orientation, the boundary is dissociated. Grain boundary dissociation has been observed in a number of fcc or diamond cubic materials with low stacking fault energy [7-11]. In thin films, in addition to the structure of the grain boundaries, an understanding of the behavior of grain boundaries at the intersection with the free surface is of importance. However, with the exception of grain boundary grooving, which has long been recognized as a mechanism of relaxation [12], there is a paucity of studies of the grain boundary relaxation modes at the intersection with a free surface. In the present work, the intersections of grain boundaries with the free surface are characterized in gold thin films with the mazed bicrystal structure. Mazed bicrystal films consist of large number of grains in only two orientations [13,14]. For the case of {110} mazed bicrystals the two grain orientations are related to each other by 90° rotation around the common axis such that all grain boundaries are of the Σ99 type [15]. Since all grain boundaries are perpendicular to the surface, i.e. parallel to the rotation axis,
