Dynamics of Melt-mediated Crystallization of Amorphous Silicon Films
- Dynamics of Melt-mediated Crystallization of Amorphous Silicon Films
- Hu, Qiongying
- Thesis Advisor(s):
- Im, James Sungbin
- Permanent URL:
- Ph.D., Columbia University.
- This thesis reports on the new experimental findings and the corresponding conclusions that were made regarding the pulsed-laser-induced melting-and-solidification behavior of a-Si films. In particular, it focuses on investigating the melt-mediated crystallization details that are associated with the a-Si films, which presumably do not contain preexisting microcrystal clusters (as for instance can be formed via high-dose ion-irradiation of Si wafers and PECVD deposition of a-Si films). Whereas the behavior of microcrystalline-cluster-containing a-Si films within the partial-melting regime was well characterized and accounted for , a more intrinsic, and, therefore, more fundamentally important situation involving microcrystalline-cluster-deficient a-Si films in the partial melting regime has yet to be definitively resolved. The present thesis addresses this unsatisfactory situation. The samples used in this work consisted of 50nm to 200nm dehydrogenated PECVD a-Si films (with or without additional ion irradiation of the films) on SiO2-coated glass and quartz substrate. Single-shot irradiation experiments using an excimer-laser-based system were conducted at various pulse durations (30ns-Gaussian-profile beam to 250ns beam obtained via an optical pulse duration extender) and at various energy densities. Extensive in situ transformation analysis was performed using both front-side and back-side transient reflectance measurements also microstructural characterization of the irradiated films was conducted using TEM and AFM. The experimental findings obtained in this investigation reveal that these a-Si films can melt and solidify in ways that are quite distinct, more varied, and highly complex compared to those encountered in microcrystalline-cluster-rich a-Si films. Specifically: (1) spatially dispersed and temporally stochastic nucleation of crystalline solids occurring relatively effectively at the moving liquid-amorphous interface, (2) very defective crystal growth that leads to the formation of fine-grained Si proceeding, at least initially after the nucleation, at a sufficiently rapidly moving crystal solidification front, and (3) the propensity for local preferential re-melting of the defective regions and grain boundaries (while the beam is still on) are some of the fundamental factors that can participate and affect how these films melt and solidify. We discuss, by providing an extensive and critical review of the relevant papers, how the present conclusions are fundamentally distinct from those that have been made by the previous investigators in the field. The implications of these findings on the conventional ELA (i.e., excimer-laser annealing) method as well as the possibility of developing partial-melting-regime-based ultra-high-throughput crystallization methods are also discussed.
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