Nucleation-Initiated Solidification of Thin Si Films
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Nucleation-Initiated Solidification of Thin Si Films S. Hazair, P. C. van der Wilt, Y. Deng, U.-J. Chung, A. B. Limanov, and James S. Im Program of Materials Science, Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, 10027
ABSTRACT Thin Si films on SiO2 that are completely melted by pulsed laser irradiation cool rapidly and eventually solidify via nucleation and growth of solids. It has been observed that a variety of solidified microstructures can be obtained, depending primarily (but not exclusively) on the degree of supercooling achieved prior to the onset of nucleation. This paper focuses on investigating one particular and unusual polycrystalline microstructure that consists of “flowerlike” grains, the interiors of which can be described as being made up of two distinct regions: (1) an extremely defective core region consisting of fine-grained material, and (2) an outer region consisting of relatively defect-free crystal “petals” that radiate outwards. After considering the microstructural details and experimental behavior of the microstructure, we have formulated a growth-based physical model to account for the formation of the microstructure. The model is found to be also capable of accounting for other complex and unusual microstructures obtained via nucleation and growth in the complete melting regime. INTRODUCTION Pulsed-laser-induced melting and solidification of thin Si films on SiO2 can allow for investigating various fundamental aspects of rapid solidification. Several solidification scenarios are possible depending primarily upon the extent to which the film is melted [1]. For the case in which complete melting is induced, the ensuing transformation has been demonstrated to take place primarily via nucleation and growth of solids [2-4]. The situation is notable in that it is possible to substantially vary the degree of supercooling that takes place prior to the onset of nucleation by controlling various experimental parameters and details (in particular the substrate preheating temperature [3, 4]). The early moments following nucleation can be an opportune period for studying rapidsolidification-related phenomena as the degrees of interfacial undercooling that are encountered here can correspond to some of the deepest values that can be experimentally induced; such an extreme far-from-equilibrium interfacial environment may subsequently yield new and unexpected modes of transformation pathways. Not surprisingly, it has been the case that laserquenched Si films with various unusual microstructures have been obtained. The resultant microstructures obtained within the complete melting regime include Si films that consist of (1) a fully amorphized layer (a-Si) [3, 5], (2) a heterogeneous mixture comprised of amorphous and fine-grained polycrystalline regions [3], (3) fine grains (fg-Si) [1, 3, 6], (4) small equiaxed grains (seg-Si) [2], and (5) large “flower-like” grains (flg-Si) with complex and unusual intra-grain microstructural details [7, 8].
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