We have studied the characterization of thin-film silicon formed by high-speed zone-melting recrystallization (ZMR) process. The dependence of crystal quality and solidification-front morphologies on scanning speed of melting heater was investigated by in situ observation of the molten-zone. It is found that solidification-front morphologies become unstable with quickening scanning speed and defect density increased extremely for a relatively thick poly-Si layer. On the contrary, by thinning the poly-Si layer, the solidification-front morphologies were kept stable at even higher scanning speeds and defect density was also kept low. In such ZMR films with low defect density, however, there are two types of subboundaries with and without branches. Furthermore, solar cells with simple structures were fabricated to evaluate the crystal quality of ZMR films. Characteristics of solar cells were strongly affected by the crystal quality of ZMR-Si films. Thus, it was clarified that stable solidification-front morphologies obtained by thinning the poly-Si layer and subboundaries without branches are suitable to obtain a high-quality active layer. Consequently, the crystal quality of ZMR-Si films formed by high-speed ZMR process for a relatively thin poly-Si layer of 0.5 mu m thickness and high scanning speed of 3.0 mm/s is comparable to that formed at a low scanning speed of 0.1 mm/s.