Mechanism of Twin Formation in Excimer-laser-induced Lateral Solidification of Si Films
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Mechanism of Twin Formation in Excimer-laser-induced Lateral Solidification of Si Films G.S. Ganot1, P.C. van der Wilt1*, H.K. Effron1, B.A. Turk1†, U.J. Chung1, A.M. Chitu1, A.B. Limanov1, and James S. Im1,2 1
Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA 2 Department of Materials Science and Engineering, College of Engineering, Korea Advanced Institute of Science and Technology, Korea
*present address: Coherent GmbH, Göttingen, Germany †present address: Coherent Inc., Santa Clara, CA, USA ABSTRACT In this paper, we present experimental findings pertaining to the formation of twins in isolated single-crystal islands obtained via the dot-SLS method. Systematic characterization of the islands using EBSD reveals that Σ = 3 CSL twins constitute the predominant extended defect. Given this, the surface orientation of the seed and twinned regions are shown to be related by a 180° rotation about a axis, and the orientation of the twinned area is apparently manifested in the inverse pole figure as a reflection across the {112} zone. By considering (1) the overall pattern of the twin boundaries within the islands, (2) that solidification proceeds via a facetted mode of growth and, (3) that intragrain-defect-free regions were obtained when the dotSLS process was performed on a {100} surface-oriented seed, we suggest that these twins are most likely generated heterogeneously at the Si/SiO2 interface during the ledge nucleation stage of rapid lateral solidification of the films. INTRODUCTION The device characteristics of polycrystalline-Si-based thin film devices are determined by the microstructural quality of the material. Among the various deposition and crystallizationbased techniques for preparing polycrystalline Si thin films, a method referred to as “hybrid sequential lateral solidification (SLS)” can be recognized as being uniquely capable of providing near-SOI quality materials on SiO2 coated substrates (namely, large, location-controlled, {100} surface-oriented Si regions that are predominantly devoid of intragrain defects) [1]. One notable experimental observation made in the course of developing the dot-SLS process was that intragrain-defect-free regions were generated only when the technique was performed on precursor seeds which were {100} surface-oriented. All other regions, which were generated from non-(100)-textured seeds, were found to contain Σ = 3 CSL (coincident site lattice) twin boundaries. We suggest that this (i.e., dot-SLS of amorphous precursor films in SiO2) is an excellent opportunity to systematically investigate the formation of twins as one is provided with a fortuitous combination of the following factors. First, lateral growth proceeds radially without occlusion-led termination of the growing grains, due to the positive curvature of the solid/liquid interface. Second, as a consequence of the complex transitions taking place during the first shot
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(i.e., explosive crystallization and partial melting), one is presented with, and can subsequ
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