  Cryogenic scanning tunneling microscopy is employed to investigate the
stoichiometry and defects of epitaxial FeSe thin films on SrTiO3(001)
substrates under various post-growth annealing conditions. Low-temperature
annealing with an excess supply of Se leads to formation of Fe vacancies and
superstructures, accompanied by a superconductivity (metal)-to-insulator
transition in FeSe films. By contrast, high-temperature annealing could
eliminate the Fe vacancies and superstructures, and thus recover the
high-temperature superconducting phase of monolayer FeSe films. We also observe
multilayer FeSe during low-temperature annealing, which is revealed to link
with Fe vacancy formation and adatom migration. Our results document very
special roles of film stoichiometry and help unravel several controversies in
the properties of monolayer FeSe films.
