Molecular Beam Epitaxy Growth And Superconductivity Of Single Crystalline FeSe Films On SrTiO₃（001）

The study of two-dimensional electronic properties requires the preparation of single-crystalline thin films with atomically-flat surface over a macroscopic scale. In this thesis, first, the SrTiO3(001) substrates are treated by deionized water boiling, HCl etching and thermal annealing under oxygen flux so that a specific TiO2-terminated STO(001) surface with well-defined step-terrace structure is achieved. Then, large-scale single-crystalline FeSe thin films are grown on SrTiO3(001) substrate using molecular beam epitaxy (MBE). Finally, the superconductivity of one unit-cell FeSe films is studied by transport measurements combined with scanning tunneling microscopy (STM). The thesis mainly contains the two parts:(1)Through deionized water boiling, HCl etching and thermal annealing under oxygen flux, we successfully obtain the specific TiO2-terminated STO(001) surface with well-defined step-terrace structure. We find that there are two key steps during pretreatment of substrates, HCl etching and the control of the oxygen flux. Unlike the conventional methods, here10%HCl, instead of HF, is used to etch the STO after boiling in deionized water. STO substrates etched by HCl are more suitable to grow FeSe films. The oxygen flux during thermal annealing can affect the distribution of SrO and TiO2surfaces. When the value of the oxygen flux is38.5ml/min, we can get the specific TiO2-terminated STO(001) surface with well-defined step-terrace structure.(2)We have successfully prepared large-scale single-crystalline FeSe thin films on TiO2-terminated SrTiO3(001) substrates. The Fe and Se fluxes and substrate temperature have to be controlled delicately during the molecular beam epitaxy growth of single unit-cell FeSe films on SrTiO3(001) substrates. The post-growth annealing is also critical, which can remove extra Se adatoms, and more importantly facilitate the necessary electron transfer for superconductivity transition. We have demonstrated the high temperature superconductivity in one unit-cell (UC) thick FeSe film. Transport measurement reveals that one unit-cell FeSe films on insulating SrTiO3(001) substrate have a Tc above29K. The enhancement of Tc is due to the heterojunction interface of FeSe films and STO substrate. It’s a kind of interface superconductivity. Finally, we use top gate and back gate to adjust the carrier concentration of the FeSe films and the Tc has become a little higher.