High-Quality FeTe_1-xSe_x Monolayer Films on SrTiO_3(001) Substrates Grown by Molecular Beam Epitaxy

We report the growth process of FeTe_1-xSe_x (0\le x \le 1) monolayer films on SrTiO_3 (STO) substrates through molecular beam epitaxy and discuss the possible ways to improve the film quality. By exploring the parameters of substrate treatment, growth control and post growth annealing, we successfully obtain a series of FeTe_1-xSe_x monolayer films. In the whole growth process, we find the significance of the temperature control through surface roughness monitored by the reflection high-energy electron diffraction and scanning tunneling microscopy. We obtain the best quality of FeSe monolayer films with the STO substrate treated at T=900–950^\circ\!C before growth, the FeSe deposited at T=310^\circ\!C during growth and annealed at T=380^\circ\!C after growth. For FeTe_1-xSe_x (x < 1), both the growth temperature and annealing temperature decrease to T=260^\circ\!C. According to the angle-resolved photoemission spectroscopy measurements, the superconductivity of the FeTe_1-xSe_x film is robust and insensitive to Se concentration. All the above are instructive for further investigations of the superconductivity in FeTe_1-xSe_x films.