Pulsed-laser crystallized poly-Si films typically have small grain size less than 100nm. This is because the solidification velocity during excimer laser annealing is extremely high. However, recent studies demonstrate that large-grain size poly-Si film can be obtained at specific energy and shot de...
Pulsed-laser crystallized poly-Si films typically have small grain size less than 100nm. This is because the solidification velocity during excimer laser annealing is extremely high. However, recent studies demonstrate that large-grain size poly-Si film can be obtained at specific energy and shot density regime. In the present work, the microstructure and crystallization mechanism of excimer laser(308 nm, 45 nsec) crystallized poly-Si film(60nm), initially deposited as a-Si(:H) on $SiO_2$ substrate by LPCVD or PECVD, have been investigated and compared in respect of the energy fluence and numbers of pulse irradiations. For single pulse irradiation experiment, it was found that there exist very narrow energy density window (less than 50 mJ/$cm^2$ ) for lateral growth silicon films. This result is almost independant of the hydrogen content and substrate conditions. In this experiment, the distribution of grain size shows distinct differences for 2 different energy regimes. For low energy density regime, the bimodal size distribution appeares. i.e. A population of fine grains and a population of large grains co-exist. This implies that there occures heterogeneous or abnormal grain growth in addition to the dominant homogeneous grain growth. For higher energy density regime, the bimodal distribution shifts to the right-hand side - the larger grain size direction. This implies that although the melt-front approach the Si/$SiO_2$ interface, the heterogeneous nature of grain distribution remains unchanged. However, when the energy density exceeds a critical point, the grain size becomes very small and the grain size distribution is reduced to monomodal one. Above results imply that some nuclei seeds which act as a sources of heterogeneous nucleation were completely destroyed for very high energy fluences. At the critical point, where the average grain size reaches the maximum, the average grain diameter is 2 or 3 times larger than the film thickness. For multiple pulse irradiation experiment, it was found that there exists very narrow window of secondary grain growth energy density regime. At this regime the average grain size increases up to 20 or 50 times larger than the film thickness, as shot density increases. In this super lateral growth regime, it was observed that the XRD peak of (111) orientation preferentially increases comparing to the single pulse irradiation cases. This phenomenon seems due to the effect of surface free differences which is the main driving force of secondary grain growth of solid-state regrowth. However, this melting and solidification regrowth mechanism shows very high degrees of surface roughness comparing to the solid-state regrowth. This is due to the ridges at the grain boundaries, of which orgin seems far different from those of SPC film. Finally, SPC poly-Si film was irradiated by the multiple pulse of different energy fluence. For low energy density regime, the defects in the grains were gradually disappeared as energy fluence increases. However, the twin boundaries which exist at the middle of the dendrites survive for relatively high energy density regime. This twin boundaries disappear when the energy fluence reaches the second grain growth regime. This implies that at the secondary grain growth regime, the grains are almost fully melted by the laser irradiation. With these pulsed-laser irradiated super-laterally grown silicon thin films, we may have excellent TFT's, which can be constructed on the low price glass.
Pulsed-laser crystallized poly-Si films typically have small grain size less than 100nm. This is because the solidification velocity during excimer laser annealing is extremely high. However, recent studies demonstrate that large-grain size poly-Si film can be obtained at specific energy and shot density regime. In the present work, the microstructure and crystallization mechanism of excimer laser(308 nm, 45 nsec) crystallized poly-Si film(60nm), initially deposited as a-Si(:H) on $SiO_2$ substrate by LPCVD or PECVD, have been investigated and compared in respect of the energy fluence and numbers of pulse irradiations. For single pulse irradiation experiment, it was found that there exist very narrow energy density window (less than 50 mJ/$cm^2$ ) for lateral growth silicon films. This result is almost independant of the hydrogen content and substrate conditions. In this experiment, the distribution of grain size shows distinct differences for 2 different energy regimes. For low energy density regime, the bimodal size distribution appeares. i.e. A population of fine grains and a population of large grains co-exist. This implies that there occures heterogeneous or abnormal grain growth in addition to the dominant homogeneous grain growth. For higher energy density regime, the bimodal distribution shifts to the right-hand side - the larger grain size direction. This implies that although the melt-front approach the Si/$SiO_2$ interface, the heterogeneous nature of grain distribution remains unchanged. However, when the energy density exceeds a critical point, the grain size becomes very small and the grain size distribution is reduced to monomodal one. Above results imply that some nuclei seeds which act as a sources of heterogeneous nucleation were completely destroyed for very high energy fluences. At the critical point, where the average grain size reaches the maximum, the average grain diameter is 2 or 3 times larger than the film thickness. For multiple pulse irradiation experiment, it was found that there exists very narrow window of secondary grain growth energy density regime. At this regime the average grain size increases up to 20 or 50 times larger than the film thickness, as shot density increases. In this super lateral growth regime, it was observed that the XRD peak of (111) orientation preferentially increases comparing to the single pulse irradiation cases. This phenomenon seems due to the effect of surface free differences which is the main driving force of secondary grain growth of solid-state regrowth. However, this melting and solidification regrowth mechanism shows very high degrees of surface roughness comparing to the solid-state regrowth. This is due to the ridges at the grain boundaries, of which orgin seems far different from those of SPC film. Finally, SPC poly-Si film was irradiated by the multiple pulse of different energy fluence. For low energy density regime, the defects in the grains were gradually disappeared as energy fluence increases. However, the twin boundaries which exist at the middle of the dendrites survive for relatively high energy density regime. This twin boundaries disappear when the energy fluence reaches the second grain growth regime. This implies that at the secondary grain growth regime, the grains are almost fully melted by the laser irradiation. With these pulsed-laser irradiated super-laterally grown silicon thin films, we may have excellent TFT's, which can be constructed on the low price glass.
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