On the growth of Co-doped BaFe₂As₂thin films on CaF₂

Abstract

The competition between phase formation of BaF2 and Ba(Fe1−x Co x )2As2 on CaF2 single crystals has been analysed. Ba(Fe0.92Co0.08)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(Fe0.92Co0.08)2As2 thin films strongly depend on the growth rate. At high growth rates, formation of BaF2 is suppressed. The dependency of the Ba(Fe0.92Co0.08)2As2 lattice parameters supports the idea of fluorine diffusion into the crystal structure upon suppression of BaF2 formation similar as was proposed for FeSe1−x Te 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.

Structural and chemical properties of superconducting Co-doped BaFe2As2 thin films grown on CaF2

“…Overall, the measured c/2 values are all larger than the literature value of c/2 = 0.65 nm for bulk Ba122 (ICSD code 169555, dashed red line). This is also observed in x-ray diffraction measurements [22] and indicates an enlargement of the Ba122 unit cell in c-direction due to compressive in-plane strain (Poisson effect, see also [5]) for these thin films. For sample/slow an enlargement of the out-of-plane (c) lattice parameter by 0.8% and a decrease of 2.0% for the in-plane (a/b) lattice parameters was measured from electron diffraction patterns (appendix figure A3).…”

“…Regarding the influence of fabrication parameters, considerable differences in thinfilm quality and superconducting properties are observed in Ba122 films when varying the deposition rate [21,22]. For the Ba122 films investigated in this work, the formation of Ferich precipitates and BaF 2 was observed by x-ray diffraction measurements [22]. BaF 2 formation could be fully suppressed at higher deposition rates, which, however, was accompanied by a reduction of T c .…”

“…Furthermore, F diffusion into IBS is possible when grown on CaF 2 [18][19][20]. Regarding the influence of fabrication parameters, considerable differences in thinfilm quality and superconducting properties are observed in Ba122 films when varying the deposition rate [21,22]. For the Ba122 films investigated in this work, the formation of Ferich precipitates and BaF 2 was observed by x-ray diffraction measurements [22].…”

“…Figure 2(c) shows the possibility of in-situ oxidation of CaF 2 and BaF 2 [33,34] during dose-intensive EDXS and EELS measurements. Even though the secondary phase BaF 2 was identified by Fourier-transform (FT) analysis (figure 2(c), BaF 2 structure ICSD code 64717) and x-ray diffraction measurements [22], elemental maps obtained by STEM-EDXS or STEM-EELS often show an O signal instead of F as depicted in figure 2(c). A rapid oxidation process during spectrum collection explains the O content in the BaF 2 region.…”

Fe-based superconducting thin films—preparation and tuning of superconducting properties

“…A similar behaviour was also found for the Ba(Fe 1-x Ni x ) 2 As 2 system [135]. In a recent study, Langer et al [286] nevertheless argue that thermal mismatch between CaF 2 and Ba(Fe 1-x Co x ) 2 As 2 alone cannot explain the observed change in lattice parameters. Full understanding of the film growth on CaF 2 should regard the strong tendency of F to diffuse into the film (as also observed for FeSe 0.5 Te 0.5 films by Ichinose et al [100]), therefore include the formation of foreign phases and misoriented components as well as a possible incorporation of F into the crystal structure.…”