On the growth of Co-doped BaFe₂As₂ thin films on CaF₂

The competition between phase formation of BaF₂ and Ba(Fe_1−xCo_x)₂As₂ on CaF₂ single crystals has been analysed. Ba(Fe_0.92Co_0.08)₂As₂ thin films have been deposited by pulsed laser deposition. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies have revealed that the formation of secondary phases and misorientations as well as the growth modes of the Ba(Fe_0.92Co_0.08)₂As₂ thin films strongly depend on the growth rate. At high growth rates, formation of BaF₂ is suppressed. The dependency of the Ba(Fe_0.92Co_0.08)₂As₂ lattice parameters supports the idea of fluorine diffusion into the crystal structure upon suppression of BaF₂ formation similar as was proposed for FeSe_1−xTe_x thin films on CaF₂. Furthermore, a growth mode transition from a layer growth mechanism to a three-dimensional growth mode at high supersaturation has been found, suggesting similarities between the growth mechanism of iron-based superconductors and high-T_c cuprate thin films.