  We performed systematic studies on the transport properties of FeSe thin
films with controlled degrees of in-plane lattice strain, including both
tensile and compressive strains. The superconducting transition temperature,
$T_{\mathrm c}$, increases up to 12 K for films with compressive strain while
the superconductivity disappears for films with large tensile strains. On the
other hand, the structural (nematic) transition temperature, $T_{\mathrm s}$,
slightly decreases as the in-plane strain is more compressive. This suggests
that the structural transition can be extinguished by a smaller amount of Te
substitution for films with more compressive strain, which may lead to higher
$T_{\mathrm c}$ in FeSe$_{1-x}$Te$_x$. It was also found that the carrier
densities evaluated via transport properties increase as the in-plane strain
becomes more compressive. A clear correlation between $T_{\mathrm c}$ and the
carrier densities suggests that it is essential to increase carrier densities
for the $T_{\mathrm c}$ enhancement of iron chalcogenides.
