Empirical selection rule of substrate materials for iron chalcogenide superconducting thin films

The crystallographic lattice constants and superconducting critical temperatures of FeSe _0:5Te _0:5 thin films grown on oxide substrates have been found to have no dependence on the in-plane lattice constants of the substrates. However, a correlation between various structural and transport properties of the films and the presence of oxygen penetration from the substrate into the film has been observed; i.e., oxygen penetration is suppressed in films with relatively high critical temperatures. Thus it is needed to identify appropriate substrates for the growth of iron chalcogenide superconducting thin films by considering the effects of the chemical properties of the substrate on the resulting structural and superconducting properties of the thin film. Upon characterization of the substrate materials used in our growth studies, the results strongly indicate that a "good" substrate has the following features: 1) its crystal structure does not have a vacancy that would permit electronegative elements to migrate, and 2) it is composed only of typical elements, in contrast to popularly used substrates that contain transition-metal elements.