Fabrication of tetragonal FeSe - FeS alloy films with high sulfur contents by alternate deposition

We report the synthesis of tetragonal $\mathrm{FeS}_x\mathrm{Se}_{1-x}$ films ($x \leq 0.78$) by pulsed-laser deposition. To fabricate the tetragonal alloy films with tetragonal FeSe and hexagonal FeS targets, we adopted an alternate deposition technique with FeSe buffer layer on MgO(001). The overall film composition is controlled by the thickness ratio of FeS / FeSe layers. The out-of-plane lattice parameter of the films follows Vegard's law, demonstrating homogeneous alloying by inter-diffusion. The sulfur solid solubility reaches $x = 0.78$ in the $\mathrm{FeS}_x\mathrm{Se}_{1-x}$ films, which is by far larger than $x \approx 0.40$ in bulk governed by the tetragonal phase instability.