On The Mechanism of Nucleation in Pulsed-laser Quenched Si Films on SiO 2
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On the Mechanism of Nucleation in Pulsed-Laser Quenched Si Films on SiO2 Y. Deng, Q. Hu, U. J. Chung, A. M. Chitu, A. B. Limanov, and James S. Im Program in Materials Science and Engineering, Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, U.S.A. ABSTRACT We have investigated the solid nucleation mechanism in laser-quenched Si films on SiO2. Previously neglected experimental steps, consisting of BHF-etching and irradiation in vacuum, were implemented to reduce potential extrinsic influences. The resulting experimental findings and computational analysis lead us to conclude that solid nucleation consistently takes place heterogeneously at, and only at, the bottom liquid Si-SiO2 interface. INTRODUCTION Irradiating thin Si films on SiO2 using a short-duration laser pulse can lead to various melting and solidification scenarios [1]. For those cases in which the incident laser energy density exceeds the critical value needed to induce full melting of the films (i.e., above the complete melting threshold (CMT)), the films are found to eventually transform, after being rapidly quenched to temperatures well below the equilibrium point, via nucleation and growth of solids [2]. The precise mechanism through which solid nucleation takes place within the deeply supercooled elemental liquid phase constitutes one of several fundamentally meaningful topics encountered within the field. Additionally, the topic can also be identified as being technologically relevant in that the presence of nucleated grains are found to substantially degrade the microstructural quality of the excimer-laser-crystallized polycrystalline Si films that are presently utilized for fabricating low-temperature polycrystalline Si films-based thin-films transistors [3]. Up until now, it has been predominantly viewed as being the case within the community [2, 4, 5] that homogeneous nucleation corresponds to the “intrinsic” mechanism of nucleation involving “pure” Si films on “clean” SiO2 (i.e., the nucleation mechanism manifested in those cases for which various extrinsic factors and influences, such as chemical contaminants and/or surface layers and reactions, have been eliminated). This paper focuses on reexamining this particular topic regarding the mechanism of nucleation in pulsed-laser-quenched Si films on SiO2. Our present investigation reveals preliminary findings that, when steps are taken to ensure that clean Si films are irradiated in vacuum, heterogeneous nucleation of solids proceeding at the bottom interface between liquid Si and SiO2 corresponds to the singularly prevalent mechanism of nucleation. EXPERIMENT The samples used in the current study consisted of dehydrogenated amorphous or polycrystalline Si films (prepared by PECVD as well as LPCVD) on SiO2-layer covered glass, quartz, or Si-wafer substrates. The thickness of the Si films ranged from 50 nm to 300 nm. An excimer-laser-based system (308 nm wavelength, ~30 ns FWHM) was employed to irradiate the
films at various energy dens
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