Characterization of superconducting Fe _y (Se_1−x Te _x ) thin films deposited on MgO substrates by sputtering

We have investigated the feasibility of RF sputtering for the deposition of superconducting Fe _y (Se_1−x Te _x ) (Fe-11) thin films on single-crystalline MgO substrates. Phase evolution and texture development as a function of processing conditions were studied, and the optimum conditions for high-quality films are reported. The epitaxial growth of the pure Fe-11 phase occurs in a temperature range of 300–320 °C with a critical film thickness of 50 nm. C-axis alignment is obtained in almost all processing conditions, while in-plane alignment is shown to be strongly dependent on the substrate temperature and the film thickness. When the substrate temperature exceeds 325 °C, an interlayer is formed between films and substrates which precludes epitaxial growth. Using sputtering with a single 3-element target, the deposited films tend to be enriched in Fe content and the excess Fe suppresses superconductivity, but a sharp transition at 10.2 K is found for the films with an Fe/(Se + Te) ratio close to 1.