  In contrast to its bulk crystals, the FeSe film or layer exhibits better
superconductivity performance, which attract much interest in its fundamental
research as well as potential application. In present work, transition from
insulator to superconductor and significant enhancement of superconductivity
were achieved in the high quality (00l) oriented FeSe films via controlling the
thickness as well as the Fe/Se ratio. The highest Tc up to 15.2 K (almost 2
times higher than those of bulk crystals) and Hc2 up to 35.5 T are obtained in
our atmosphere-stable FeSe thin film with practical thickness (240 nm),
implying their great potential application in the electronic devices at high
magnetic fields. More importantly, it was found that the Fe-vacancy disorder in
FeSe films is the intrinsic factor determining the evolution of the
superconductivity, rather than thickness effect. In our non-superconducting
FeSe film with Fe/Se ratio of 1.00:1.09, insulating \b{eta}-Fe1-xSe phase with
iron-vacancy disorders is the main phase and more likely to be the parent phase
of FeSe superconducting system. Tuning the Fe-vacancy disorders via changing
the Fe/Se ratio can dramatically vary the concentration of charge carrier and
introduce proper electron doping, which finally leads to the transition from
insulator to superconductor and further enhancement in the superconductivity.
Intriguingly, our results also indicate that when the Fe/Se ratio of film is
beyond a critical value, superconducting FeSe films will become instable, and
phase separation occurs with new non-superconducting phase precipitating in the
superconducting matrix, causing the degradation in the superconductivity. The
results in present work help us to well understand the intrinsic mechanism of
superconductivity among Fe-Se superconducting system and provide a new strategy
to further pursue higher Tc in these materials.
