Epitaxial Growth Following Crystal Nucleation in Laser-Quenched Si Films on SiO 2
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Epitaxial Growth Following Crystal Nucleation in Laser-Quenched Si Films on SiO2 Vernon K. Wong, A. M. Chitu, A. B. Limanov, and James S. Im Program in Materials Science and Engineering, Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA ABSTRACT We have investigated the solidified microstructure of nucleation-generated grains obtained via complete melting of Si films on SiO2 at high nucleation temperatures. This was achieved using a high-temperature-capable hot stage in conjunction with excimer laser irradiation. As predicted by the direct-growth model that considers (1) the evolution in the temperature of the solidifying interface and (2) the subsequent modes of growth (consisting of amorphous, defective, and epitaxial) as key factors, we were able to observe the appearance of “normal” grains that possess a single-crystal core area. These grains, which are in contrast to previously reported flower-shaped grains that fully make up the microstructure of the solidified films obtained via irradiation at lower preheating temperatures (and amongst which these “normal” grains emerge), indicate that epitaxial growth of nucleated crystals must have taken place within the grains. We discuss the implications of our findings regarding (1) the validity of the direct-growth model, (2) the nature of the heterogeneous nucleation mechanism, and (3) the alternative explanations and assumptions that have been previously employed in order to explain the microstructure of Si films obtained via nucleation and growth within the complete melting regime. INTRODUCTION When a thin Si film on SiO2 is melted by a laser pulse, it can undergo several possible solidification pathways depending primarily on the extent to which the film is melted [1]. For the case in which complete melting of the film is induced, the ensuing liquid-to-solid transition has been demonstrated to proceed via nucleation and growth of solids transpiring within the supercooled liquid matrix [2 – 6]. Even so, a remarkable variety of unusual microstructures have been observed. These include: (1) amorphous Si [3], (2) nearly amorphized Si consisting of densely dispersed amorphous Si annular regions separated by a region consisting of fine-grain Si [3], (3) fine-grain Si consisting of extremely small crystals [1, 3], (4) small equiaxed grains [2], and (5) flower-shaped grains consisting of an extremely defective core region and an outer region made up of relatively defect-free crystal “petals” [4, 5]. In order to account for these as well as other similarly unusual microstructures (observed in laterally solidified ultra-thin Si films [7] and in explosively crystallized amorphous Si films [8]), we have previously presented the direct-growth model [5] that identifies the solidifying interface temperature experienced by the growing solid as the critical parameter that determines the microstructure of the resulting material. When applied to nucleation-initiated solidification taking place within a completely melted Si film, the model co
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