Disk- and needle-shaped grains can be seen in polycrystalline silicon (poly-Si) made by Ni-mediated crystallization of amorphous silicon (a-Si). A major parameter to give the difference of grain structure is the Ni area density on a a-Si. However, there are many other parameters such as heating rate and structure of a-Si to affect the grain structure. The use of Ni density of similar to10(13) cm(-2) on a-Si for the crystallization gives the disk-shaped grains. There is no amorphous phase in the disk-shaped grains which are composed of well-aligned needles. On the other hand, the poly-Si has some amorphous phase inside when it was crystallized into needlelike rods. It is found that the width of needles in the disk-shaped grains is smaller than that of needlelike crystallites. The Ni atoms are at the grain boundaries formed by the collisions of neighboring grains. (C) 2004 American Vacuum Society.