Microscopic analysis of the chemical reaction between Fe(Te, Se) thin films and underlying CaF2

To understand the chemical reaction at the interface of Fe(Te, Se) and CaF2 materials, we used transmission electron microscopy to analyze five types of Fe(Te, Se) superconducting thin films prepared on different types of substrate: two on a CaF2 substrate, one on a CaF2 substrate with a CaF2 buffer layer, one on a CaF2 substrate with an FeSe buffer layer, and one on a LaAlO3 substrate with a CaF2 buffer layer. Energy-dispersive x-ray spectroscopy analysis revealed a potential interdiffusion between fluorine and selenium that plays a significant role in the variation of the lattice parameters. We conclude that the lattice parameters of the Fe(Te, Se) thin films are primarily determined by the chemical substitution of anions, while the lattice mismatch plays a secondary role.