Cooling Mechanism and Structural Change of Local Regions With a Different Cooling Rate of Excimer Laser Annealed Si
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Cooling Mechanism and Structural Change of Local Regions With a Different Cooling Rate of Excimer Laser Annealed Si Byoung-Min Lee1, Baek Seok Seong2, Hak Rho Kim2, Shinji Munetoh3, and Teruaki Motooka3 1 Department of Materials Engineering, Korea University of Technology and Education, Byeongchen, Cheonan Chungnam, 330-708, Korea, Republic of 2 Neutron Physics Department, Korea Atomic Energy Research Institute, P. O. B. 105, Yuseong, Daejeon, 305-353, Korea, Republic of 3 Department of Materials Science and Engineering, Kyushu University, 744 Motooka, Fukuoka, 819-0395, Japan
ABSTRACT To investigate the cooling mechanism and the local structural changes of excimer laserannealed silicon (Si), molecular dynamics (MD) simulations were performed. Heat flow of molten Si showed a strong dependency of the local region during a natural cooling. An amorphous-to-liquid transition near an interface in the temperature range of 1600 K ~ 1800 K was expected with the results of the local diffusion coefficients calculated by integrating the velocity autocorrelation functions. It was confirmed that the structure of the interface region affected the cooling rate of the overall system. The structural properties at the various local regions after a cooling were assessed in terms of the configurational properties including the coordination and bond-angle distributions. A spontaneous nucleation of Si near a interface was observed during a natural cooling. INTRODUCTION Poly-Si fabricated at a low temperature by using an excimer laser annealing (ELA) is of great interest for large area electronics. For enhancing the electrical properties of poly-Si prepared by the excimer laser technique, the control of a cooling rate is one of the important factors since the nucleation and crystallization processes of the Si thin films is governed by a heat diffusion into the SiO2 substrates [1, 2]. Therefore, an understanding of the formation of grains and the cooling mechanism of the excimer laser-annealed Si is essential to improve the device performance. Approximately 50 ~ 100 nm thick a-Si is deposited onto the cleaned SiO2 substrates by the thin film technique, and the film is crystallized by an excimer laser irradiation. Laser crystallization is not a low temperature process because Si is heated well above 900 ℃ [3]. However, the high temperatures are only sustained for a very short time due to the short excimer laser pulse width, which typically ranges from 20 ns to 66 ns [4, 5]. The short wavelength ensures that the high laser energy will be absorbed in the Si thin films, and the Si films are rapidly heated above the melting point (1685 K) and solidify quickly with the heat flow to the unheated substrate. Due to the rapid melting and cooling processes, neighboring material and devices can maintain their property [6]. In this study, natural cooling system is applied to provide the details of the heat flow and the structural properties of Si at the different local regions. The problem of how a nucleation is
developed in the excime
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