Two-step molecular beam epitaxy growth of superconducting monolayer FeSe film on SrTiO 3

The monolayer FeSe film grown on SrTiO₃ has attracted tremendous interest due to its remarkable interface-enhanced high-temperature superconductivity and emergent quantum phenomena. However, achieving controlled growth of monolayer FeSe on the SrTiO₃ substrate remains challenging. Here, utilizing an in situ molecular beam epitaxy-scanning tunneling microscopy and spectroscopy combined system, we develop a method to successfully grow monolayer FeSe films on both conductive and insulating SrTiO₃ substrates through a two-step deposition approach that involves the deposition of Fe clusters and subsequent selenization. By applying postannealing, we further tune the crystallinity and superconducting properties of the as-grown film, yielding an in situ scanning tunneling spectroscopy gap of {~}15.7 meV and an ex situ macroscopic transition temperature of {~}37.2 K. This work provides a valuable route for the controlled growth of interface-enhanced high-temperature superconductors and could be branched out into growing other similar compound films.