| Thin Si films on SiO2 that are completely melted by pulsed laser irradiation cool rapidly by thermal conduction to the substrate and eventually solidify via nucleation and growth. It is observed that a variety of microstructures can be obtained depending primarily on the degree of supercooling achieved prior to the onset of nucleation. This thesis is concerned with nucleation-initiated solidification of supercooled liquid silicon where an emphasis is placed on identifying the details of the transformation paths that dictate the resulting microstructures.; One unusual microstructure that forms the primary focus of the present thesis corresponds to what we refer to as the "flower-like" microstructure. The "flower-like" grains can be identified as being made up primarily of two distinct regions: (1) an extremely defective core region, and (2) an outer region consisting of large crystal domains that radiate outwards.; We propose a physical model to account for the formation of the "flower-like" microstructure. The model considers the evolution of the interfacial temperature experienced by the growing grains as being the key parameter that dictates the mode of growth and recognizes an unusual "defective" mode of growth that leads to the formation of fine-grained material in the early stage of the growth. The model is capable of accounting for other microstructures that have either been previously observed or newly obtained within the course of this investigation.; In particular, we were able to successfully conduct experiments at higher substrate temperatures (≥1200°C) than have previously been accomplished, and we were able to produce and analyze "normal" grains that are free of having the defective core area. This outcome is meaningful since the type of such grains is precisely what is predicted by our model, and further can be identified as being inconsistent with previously advocated suggestions.; Finally, this thesis addresses the apparent inconsistency that, we conclude, must exist between our proposed model and the "homogeneous nucleation"-based explanation, which has been presented by Stiffler et al. to account for small and equiaxed-grained microstructure that can be obtained under certain experimental conditions (This issue is addressed in Appendix B). |
