  The transition from insulator to superconductor was achieved in the
non-superconducting FeSe ultrathin film via simple Mg coating technique in
present work. It was found that in non-superconducting FeSe ultrathin film
without Mg coating, 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. Proper Mg coating on the surface of FeSe films can lead
to Mg entering FeSe crystal lattice to fill up these Fe vacancies rather than
replace Fe. Simultaneously, additional electron doping is introduced and the
electron carrier concentration in this parent phase can be tuned, which is
responsible for the SIT and evolution of superconductivity in this system.
However, abnormal decrease of electron concentration was found in FeSe film
with excessive Mg coating, which brings about the severe degradation in
superconducting performance.
