Excimer-Laser-Induced Melting and Solidification of PECVD a-Si films under Partial-Melting Conditions
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Excimer-Laser-Induced Melting and Solidification of PECVD a-Si films under PartialMelting Conditions Q. Hu1, Catherine S. Lee1, 2, T. Li1, Y. Deng1, U.J. Chung1, A. B. Limanov1, A. M. Chitu1, M.O. Thompson3 and James S. Im1, 2 1 Program in Materials Science and Engineering, 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, Daejeon, Korea 3
Department of Materials Science and Engineering, College of Engineering, Cornell University, Ithaca, NY, USA ABSTRACT This paper reports on new experimental findings and conclusions regarding the pulsed-laserinduced melting-and-solidification behavior of PECVD a-Si films. The experimental findings reveal that, within the partial-melting regime, these a-Si films can melt and solidify in ways that are distinct from, and more complex than, those encountered in microcrystalline-cluster-rich LPCVD 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 remelting of the defective regions and grain boundaries (while the beam is still on) are identified as being some of the fundamental factors that can participate and affect how these PECVD films melt and solidify. INTRODUCTION Fundamentally, melting of a-Si corresponds to one of the simplest examples of phase transitions that can transpire between two metastable phases in condensed systems. The melting of a metastable solid phase inevitably requires rapid heating (as can, for instance, be accomplished through excimer laser irradiation), since slow heating of the material would kinetically lead to solid-phase transformation of the metastable solid phase into a more thermodynamically stable solid phase. In general, irradiating amorphous or polycrystalline Si films on SiO2 using a short-duration laser pulse can lead to several distinct melting and solidification scenarios. It is well established that the process can be characterized in terms of two major regimes (i.e., low-energydensity/partial-melting regime and high-energy-density/complete-melting regime) [1] and a subregime (i.e., near-complete-melting/super-lateral-growth regime) [2]. In this paper, we focus our attention on investigating the details associated with melting and solidification of dehydrogenated PECVD a-Si films irradiated within the low-energy-density/partial-melting
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regime. Whereas the behavior of microcrystalline-cluster-containing LPCVD a-Si films within the partial-melting regime was characterized and explained (in terms of the early and rapid microcrystal-triggered explosive crystallization of the films followed sequentially b
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