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

L Grünewald,J Hänisch,B Holzapfel,M Langer,D Nerz,D Gerthsen,S Meyer

doi:10.1088/1361-6668/abcecf

L Grünewald, J Hänisch + Show 5 more

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https://doi.org/10.1088/1361-6668/abcecf

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Abstract

Thin films of Co-doped BaFe2As2 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 CaF2(001) substrates. Analytical transmission electron microscopy (TEM) was applied to analyze the microstructure and secondary phases. The formation of BaF2 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 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.

Highlights

Iron-based superconductors (IBSs) have received much interest in the scientific community since their discovery in 2006 [1, 2]
The formation of BaF2 clearly indicates a reaction of Ba122 with CaF2 during pulsed laser deposition (PLD), the degree of which depends on deposition rate and substrate temperature
To gain more insight into these aspects, we investigated the microstructural and chemical properties of two Ba122 thin films on CaF2 deposited with different rates by analytical and high-resolution scanning transmission electron microscopy (STEM) in combination with energy-dispersive x-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS)

Summary

Introduction

Iron-based superconductors (IBSs) have received much interest in the scientific community since their discovery in 2006 [1, 2]. CaF2 recently gained much interest as a substrate for Ba122 due to generally higher Tc of the superconducting thin films compared with other substrates [5] This observation is usually explained by the large mismatch of thermal expansion coefficients between CaF2 and Ba122 leading to biaxial strain upon cooling, which enhances Tc. the effects of strain on the superconducting properties can be investigated in thin-film systems by using different substrates with varying lattice mismatch with respect to Ba122 [7, 8]. The effects of strain on the superconducting properties can be investigated in thin-film systems by using different substrates with varying lattice mismatch with respect to Ba122 [7, 8] Despite this scientific progress, the mechanisms of Ba122 growth on CaF2 via pulsed laser deposition (PLD) are still not fully understood due to the influence of various fabrication parameters and possible interactions with the (heated) substrate [9,10,11]. The formation of BaF2 clearly indicates a reaction of Ba122 with CaF2 during PLD, the degree of which depends on deposition rate and substrate temperature

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