Superconducting property and Fe valence state of FeSe thick films grown from high temperature solution

Thick FeSe films (1–2 μm) were grown from high temperature solution with SeSn as the flux. Electron backscatter diffraction confirmed the films of tetragonal β phase with high crystallinity. Superconducting transition was observed by magnetic measurements, with the onset Tc of 6.1 K for the as-grown films and rising to 6.9 K after post-growth annealing at 400 °C, which was still 1.5 K lower than the sintered powder samples. X-ray photoelectron spectroscopy showed that the Fe 2p3/2 binding energy in the FeSe compound was composed of two peaks at 707.8 eV and 706.6 eV, respectively. The former was close to the value of Fe in polarized ionic bonds, while the later had the typical value in metallic bondings. The ratio of the two bondings was 1.56 and 1.94 for the films and sintered powders, respectively. The critical temperature may have some correlation with the ratio of the two bondings. A lower average Fe valence was probably the cause for the lower Tc observed in thick films.

► Thick FeSe films (1–2 μm) of pure β-phase were grown from high temperature solution with SeSn as flux.
► Electron backscatter diffraction showed the films of high crystallinity.
► Superconducting transition was observed with onset Tc of 6.1 K for as-grown films and rising to 6.9 K after post-growth annealing at 400 °C.
► X-ray photoelectron spectroscopy showed that Fe had two valence states in FeSe and their ratio may have some correlation with the critical temperature.