Identification of a large amount of excess Fe in superconducting single-layer FeSe/SrTiO3 films

Single-layer FeSe films grown on ${\mathrm{SrTiO}}₃$ (STO) substrates have attracted much attention because of their record high superconducting critical temperature $T{}_C$. It is generally believed that the composition of the epitaxially grown single-layer FeSe/STO films is stoichiometric; that is, the ratio of Fe to Se is 1:1. Here we report the identification of a large amount of excess Fe in superconducting single-layer FeSe/STO films. By depositing Se onto superconducting single-layer FeSe/STO films, we find by in situ scanning tunneling microscopy the formation of second-layer FeSe islands on top of the first layer during the annealing process at a surprisingly low temperature ($\ensuremath{\sim}150{\phantom{\rule{0.28em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) which is much lower than the usual growth temperature ($\ensuremath{\sim}490{\phantom{\rule{0.28em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$). This observation is used to detect excess Fe and estimate its quantity in single-layer FeSe/STO films. The amount of excess Fe detected is at least 20%, which is surprisingly high for superconducting single-layer FeSe/STO films. The discovery of such a large amount of excess Fe should be taken into account in understanding the high-$T{}_C$ superconductivity and points to a likely route to further enhance $T{}_C$ in superconducting single-layer FeSe/STO films.