Microstructure and Coarsening in Mazed Bicrystal Films of Au Grown on Ge Substrates
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Microstructure and Coarsening in Mazed Bicrystal Films of Au Grown on Ge Substrates Tamara Radetic and Ulrich Dahmen National Center for Electron Microscopy , Lawrence Berkeley National Laboratory 1 Cyclotron Road, Berkeley, CA 94 720, U.S.A. ABSTRACT Thin films of gold (Au) were grown on single crystal germanium (Ge) or silicon (Si) substrates using physical vapor deposition (PVD). The resulting microstructure was that of a mazed bicrystal in which two equivalent grain orientations, related to each other by a 90° rotation, are arranged in a morphology of irregularly shaped, convoluted grains. Quantitative morphological analysis showed a strong dependence of grain shape on size, with larger grains being more convoluted and smaller grains more compact. The evolution of grain size, anisotropy and shape during heating in the temperature range from 300-340 °C was studied by in-situ transmission electron microscopy (TEM). INTRODUCTION Recently, a method based on a PVD has been developed to grow metal thin films with a different number of orientation variants on a single crystal substrate (Ge, Si) [1,2,3]. Depending on the epitaxial relationship of the film and substrate as well as the conditions of deposition it was possible to grow single crystal, bicrystal, tricrystal and polycrystalline thin metal films. In this work {110} mazed Au bicrystal films have been grown on {100} Ge and Si substrates. The epitaxial relationship between film and substrate is: {110}Au || {001}Ge, Si and < 1 10 > Au || < 1 10 > Ge, Si
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The 2-fold symmetry axis of the film is parallel to the 4-fold symmetry axis of the substrate, resulting in two equivalent orientation variants in the film. The microstructure of the mazed bicrystal film consists of a number of intertwined, irregularly shaped grains in only two orientations, with a 90ÛPLVRULHQWDWLRQEHWZHHQJUDLQV)URPWKHRULHQWDWLRQUHODWLRQVKLSEHWZHHQ variants it follows that all grain boundaries are of the Σ99 type, but due to the convoluted shape of the grains, the inclination (i.e. the orientation of the grain boundary plane) is variable. Previous studies [3] showed that grain boundaries in the film are perpendicular to the substrate plane, and the rotation axis that relates orientation variants is parallel to the grain boundaries. Hence all boundaries are pure tilt in character. Due to capillary forces acting on a curved interface, there is a tendency of boundaries to straighten themselves in order to minimize curvature and decrease their area. In polycrystalline materials, triple junctions and other geometrical constraints can act as pinning sites during grain growth. In the mazed Au bicrystal there are no such constraints, with the exception of occasional microtwins, which occur randomly due to the low stacking fault energy of Au. The majority of grains have both convex and concave interfaces, and the parameter “average grain size” alone is not sufficient to characterize the morphological evolution. Additional stereological parameters are used to describe the grain shape as a fun
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