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

Marco Langer,Sabine Wurmehl,Lukas Grünewald,Sandra Kauffmann-Weiss,Kai Ackermann,Saicharan Aswartham,Sven Meyer,Jens Hänisch,Bernhard Holzapfel

doi:10.1088/1742-6596/1293/1/012014

Abstract

The competition between phase formation of BaF2 and Ba(Fe1−xCox)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−xTex 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-Tc cuprate thin films.

Highlights

CaF2 single crystalline substrates have been proven to yield high-quality thin films of Fe-based superconductors (FeSC), i.e. in terms of high critical temperature T c and high critical current density J c, grown either by pulsed laser deposition (PLD) or molecular beam epitaxy (MBE) [1]
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
Films without BaF2 precipitates show a stronger elongation of the unit cell along the c-axis compared to films which contain BaF2 precipitates. This correlation between BaF2 formation and changes in lattice parameters in case of BaF2 phase formation suppression at high growth rates supports the idea of a fluorine diffusion into the Ba122 crystal structure similar as was already discussed for FeSe1−xTex thin films grown on CaF2 [11]

Summary

Introduction

CaF2 single crystalline substrates have been proven to yield high-quality thin films of Fe-based superconductors (FeSC), i.e. in terms of high critical temperature T c and high critical current density J c, grown either by pulsed laser deposition (PLD) or molecular beam epitaxy (MBE) [1]. The growth of FeSC thin films is often accompanied by formation of secondary phases such as FeAs or other Fe-rich phases and misoriented Ba122 grains on the sample surface [6] which impedes the development of many possible applications such as planar junction technology or sub-micrometer single-photon nanowire detectors. To these precipitates usually obtained on all kind of substrates, Ba122 thin films grown on CaF2 often show the formation of BaF2 [7, 8, 9].

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Discussion

Conclusion