As illustrated in FIG. 6, a plurality of grains 134a may be formed in the polysilicon layer 134. When the solid amorphous silicon layer 132 is irradiated with the laser beam 240, the amorphous silicon layer 132 may absorb heat to change to a liquid state, and then may release heat to change to a solid state again. In this case, the grain 134a may be formed by growing a crystal from the crystal seed. When there is a difference in the cooling rate in the process of the amorphous silicon layer 132 changing from the liquid state to the solid state, the grain 134a may grow from a region having a fast cooling rate toward a region having a slow cooling rate, so that a grain boundary 134b may be formed in the region having the slow cooling rate.

A projection may be formed at the grain boundary 134b on a surface of the polysilicon layer 134. As the amorphous silicon layer 132 melted by the laser beam 240 is recrystallized around the grain 134a, the projection may be formed at the grain boundary 134b. The projection may protrude upward from the surface of the polysilicon layer 134, and may have a sharp end shape.

A root-mean-square (RMS) value of a surface roughness of the polysilicon layer 134 may be about 4 nm or less. In this case, an RMS value of thicknesses of the projections formed on the surface of the polysilicon layer 134 may be about 4 nm or less.