Sequential Lateral Solidification of Ultra-Thin a-Si Films
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Sequential Lateral Solidification of Ultra-Thin a-Si Films Hans S. Cho, Dongbyum Kim, Alexander B. Limanov, Mark A. Crowder, and James S. Im, Program in Materials Science and Engineering, Department of Applied Physics and Applied Mathematics, School of Engineering and Applied Science, Columbia University, New York, NY ABSTRACT This paper demonstrates that Sequential Lateral Solidification (SLS) of Si can be carried out on films as thin as − and potentially much thinner than − 250 Å. When compared to thicker Si films, however, the SLS-processed ultra-thin films contain more twins, and successful processing requires irradiation within a narrower laser energy density range and a smaller per-pulse translation distance. The physical interpretation of these findings is formulated by analyzing the details of the microstructures observed in single-pulse-irradiation-induced Controlled Super-Lateral Growth (C-SLG) experiments. SEM and TEM analyses reveal complicated microstructural details that we interpret as originating from breakdown of epitaxial growth during lateral solidification, an effect that is detrimental to the SLS process. Based on considerations of far-fromequilibrium solidification behavior of Si, it is argued that undercooling of the solidification interface below a threshold value at which solidification no longer proceeds epitaxially − arising from reduction in interfacial recalescence during lateral solidification of ultra-thin Si films, relative to that of thicker films − is responsible for the breakdown. Based on this model, we discuss how external parameters may be adjusted so as to permit optimal crystallization of ultra-thin Si films using SLS. INTRODUCTION Pulsed-laser annealing (PLA) of thin amorphous silicon (a-Si) films deposited on glass substrates has been the focus of much interest with the growing demand for high-performance thin film transistors (TFTs). As various applications for thin Si film-based devices are developed, and as the design rule for devices decreases, it is anticipated that more interest will be directed towards films of increasingly small thickness. Deposited as a-Si or small-grained polycrystalline Si on SiO2 surfaces, these films require additional processing to improve the electronic properties of devices that would eventually be fabricated on them. Sequential Lateral Solidification (SLS) represents an effective low-temperature PLA method that can produce high quality crystalline films from amorphous and polycrystalline precursor materials [1,2]. The SLS process utilizes the Controlled Super-Lateral Growth (C-SLG) technique [3] that relies on inducing complete melting of localized regions and subsequent lateral growth from adjacent unmelted regions. Using repeated C-SLG-inducing laser irradiations, combined with systematic and coordinated spatial translations of the film relative to the beamlets, SLS makes possible the realization of various microstructures, from grain-boundary-locationcontrolled polycrystalline Si films to location-controlled single-crystal regions, on amor
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