Analysis of superconducting FeSe thin films deposited by a sputtering technique

Superconducting FeSe thin films have been deposited onto (001)-oriented single-crystalline MgO substrates by conventional radio-frequency magnetron co-sputtering of a FeSe target and a pure Se target. The composition of the films is within the homogeneity range of the tetragonal β phase close to stoichiometry. Films with a minimum residual resistivity and a maximum superconducting transition temperature can be prepared within a narrow substrate temperature range from 480 to 510 °C. In contrast to the equilibrium phase diagram the β phase surprisingly exists in thin films deposited at substrate temperatures as high as 680 °C. The polycrystalline tetragonal films grow with a preferred c-axis texture and a tiny admixture of misaligned grains. The temperature-dependent resistivity reveals a low-temperature linear part and a high-temperature part with a pronounced negative curvature. The origin of the non-Fermi liquid behaviour and the underlying electron scattering mechanisms are still unknown so far. (paper)