  The Hall coefficient (R_H) is investigated along the in-plane direction for
carefully prepared La_2-x_Sr_x_CuO_4_ thin films from x=0.08 to x=0.40. It is
found that R_H becomes almost temperature independent around room temperature
for x >= 0.24, and its sign smoothly changes from positive to negative between
x = 0.28 and 0.32, showing contrasting behavior to previously reported results.
This result indicates that the carrier can be doped smoothly across the hole-
to electron-dominant regions. According to the sign change of R_H,
superconductivity disappears for x >= 0.32 films, which suggests that a
positive R_H is one of the necessary conditions for superconductivity in p-type
cuprates.
