Epitaxial Growth of FeSe_0.5Te_0.5Thin Films on CaF₂Substrates with High Critical Current Density

Abstract

In-situ epitaxial growth of FeSe 0.5 Te 0.5 thin films is demonstrated on a nonoxide substrate CaF 2 .Structural analysis reveals that compressive stress is moderately added to 36-nm-thick FeSe 0.5 Te 0.5 , which pushes up the critical temperature to above 15 K, showing higher values than that of bulk crystals. The critical current density at T = 4.5 K reaches 5.9 × 10 4 Acm −2 at µ 0 H = 10 T, and 4.2 × 10 4 Acm −2 at µ 0 H = 14 T. These results indicate that fluoride substrates have high potential for the growth of iron-based superconductors in comparison with popular oxide substrates.Since the discovery of iron-based superconductors, 1) much effort has been devoted to establish a thin-film growth technique of these compounds. 2-8) At an early stage, iron-chalcogenide superconductors 9) had been considered rather inappropriate for practical applications simply because of their low critical temperature (T c ). However, pressure-effect studies demonstrate that the potential T c of iron-chalcogenide superconductors is as high as 37 K, 10, 11) which motivated us to begin the study of thin-film growth of FeSe 1−x Te x . Through many reports, [12][13][14][15][16] one problem has gradually emerged; There is a close correlation between T c and the structure of the films grown epitaxially, but their lattice parameters are influenced by too many growth parameters, and the lattice parameters of the substrate material are not a dominant factor. [17][18][19] In other words, FeSe 1−x Te x can be grown epitaxially on a single-crystalline substrate, but the lattice parameters cannot be designed in accordance with those of the substrates. One of the possible reasons that we have previously proposed for this is oxygen contamination from oxide substrates, 18,19) and we actually confirmed the presence of oxygen at the interface of FeSe 1−x Te x grown on YSZ and LaSrAlO 4 . In order to avoid this problem and further improve superconducting properties, it is important to investigate the film growth *

Comparative Review on Thin Film Growth of Iron-Based Superconductors

“…However, because superconducting FeTe films can have both longer and shorter a-axis length [124], the mechanism seems not simple like the strain scenario by Han et al [122], whereas the superconducting properties of FeTe may be largely affected by the lattice strain, suggested by thermal expansion measurements [126]. Physical properties of the superconducting FeTe are similar to those of FeSe 1−x Te x : (1) strain largely affects the T c values [126]; (2) importance of both n-and p-type carriers for the superconductivity, suggested by the Hall effect [40,127,128]; (3) huge B c2 (> 30 T) [122,123,129]. These may suggest that the mechanism of the superconductivity is the same for both FeTe and FeSe 1−x Te x .…”

“…On the other hand, Naito et al reported that the SmFeAsO (Sm1111) film on CaF 2 substrate with a cap layer of SmF 3 fabricated by MBE showed a superconducting transition at ∼56 K which was slightly higher than T c of SmFeAs(O,F) bulk samples [38,39]. Although CaF 2 was firstly used by Tsukada et al as a substrate material in the film growth of FeSe 1−x Te x [40], which will be discussed later, the enhancement of T c also in Sm1111 film on CaF 2 [38] shows that CaF 2 is efficient for the thin film growth of other iron-based superconductors. In these reports, the extra cap layer is necessary to obtain a superconducting 1111 film, which prevents an application of 1111 film for a sandwich-type junction [36,38].…”

“…These studies were performed for obtaining the high J c or fabricating junctions and devices, and there were no reports on the growth of Co-doped Ba122 films on oxide substrates with T c higher than ∼26 K which was the highest T c of Co-doped Ba122 bulk samples [61,62]. Iida et al fabricated the Co-doped Ba122 films on fluoride AEF 2 (AE = Ca, Sr, Ba) substrates [63], which had been used in the film growth of 11- [40] and 1111-systems [38]. The in-plane lattice parameter of Co-doped Ba122 film on CaF 2 is smaller than those of Co-doped Ba122 bulk samples [61,62], which indicates that the film received the in-plane compressive strain.…”

“…Tsukada et al focused on non-oxide substrates, and grew FeSe 0.5 Te 0.5 films on CaF 2 [40]. The grown films have shorter a-axis length compared with oxide substrates, and T c increases to above 15 K, higher than bulk value.…”

“…However, in thin films of 1111, 122, and 11, the highest T c is commonly obtained in the case of CaF 2 substrates. Tsukada et al firstly reported the effectiveness of the CaF 2 substrate in the 11 film fabrication [40], and then the fabrication of the film with high T c were reported in other systems [31,41,63]. Here, we define the lattice misfit,…”