Crystallization processes of amorphous Si during excimer laser annealing in complete-melting and near-complete-melting c
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Crystallization processes of amorphous Si during excimer laser annealing in completemelting and near-complete-melting conditions: A molecular dynamics study Tomohiko Ogata, Takanori Mitani, Shinji Munetoh, and Teruaki Motooka Department of Materials Science and Engineering, Kyushu University, 744 Motooka, Fukuoka, 819-0395, Japan ABSTRACT We investigated crystallization processes of amorphous Si (a-Si) during the excimer laser annealing in the complete-melting and near-complete-melting conditions by using molecular dynamics simulations. The initial a-Si configuration was prepared by quenching liquid Si (l-Si) in a MD cell with a size of 50×50×150Å3 composed of 18666 atoms. KrF excimer laser (wavelength: 248nm) annealing processes of a-Si were calculated by taking account of the change in the optical constant upon melting during a laser pulse shot with the intensity 2
I o exp[− (t − t 0 ) 2 σ ] ( I o : laser fluence, t : irradiation time). The refractive indices of a-Si and lSi were set at n+ik=1.0+3.0i and n+ik=1.8+3.0i, respectively. The simulated results well reproduced the observed melting rate and the near-complete-melting and complete-melting conditions were obtained for I o = 160mJ/cm2 and 180mJ/cm2, respectively. It was found that larger grains were obtained in the near-complete-melting condition. Our MD simulations also suggest that nucleation occurs first in a-Si and subsequent crystallization proceeds toward l-Si in the near-complete-melting case.
INTRODUCTION High-quality polycrystalline Si (poly-Si) is required to realize "System-in-display". Excimer laser annealing is an effective technique to obtain high-quality polycrystalline silicon (poly-Si) from amorphous silicon (a-Si) thin film deposited on glass or plastic substrates. Hatano et al. previously reported the relationship between grain sizes and excimer laser fluences [1]. In the lower fluence region, the grain size increases with increasing laser fluence. This irradiation condition gives rise to "partial-melting" of a-Si. On the other hand the grain size rapidly decreases with increasing laser fluence in the higher fluence region. This condition gives rise to "complete-melting" of a-Si. The transition region, which gives rise to "near-complete-melting" of a-Si, exists between "partial-melting" condition and "complete-melting" condition. We previously reported the mechanism of nucleation in the various laser fluences by MD simulations [2, 3]. Under the near-complete-melting condition, nucleation occurred predominantly in the unmelted a-Si region during the laser irradiation and then the crystal growth proceeds toward liquid Si region. In this paper, we performed the MD simulations of the nucleation and crystal growth processes during excimer laser annealing by taking account of reflection and absorption of a-Si and l-Si layers upon melting. We visualized the nucleation and crystal growth processes under each melting condition and discussed why the large grain can be obtained under the nearcomplete-melting condition.
50Å
a-Si 150Å
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