Structural and chemical properties of superconducting Co-doped BaFe$_2$As$_2$ thin films grown on CaF$_2$

Thin films of Co-doped BaFe$_2$As$_2$ of similar thickness (~40 nm) were grown with different growth rates (0.4 Å s$^{-1}$ and 0.9 Å s$^{-1}$) by pulsed laser deposition on CaF$_2$(001) substrates. Analytical transmission electron microscopy (TEM) was applied to analyze the microstructure and secondary phases. The formation of BaF$_2$ and a high concentration of planar defects (mainly stacking faults) are observed for the sample grown at a low rate. A higher growth rate results in high-quality epitaxial films with only few antiphase boundaries. A higher $T_\text{c}$ was measured for the sample grown at a low growth rate, which is attributed to the difference in strain state induced by the high concentration of defects. Large crystalline Fe precipitates are observed in both samples. Chemical analysis shows a pronounced O and slight F content at the planar defects which highlights the role of O in defect formation. Electron-beam-induced irradiation damage during TEM measurements is observed and discussed.