We have investigated the crystal structure and superconducting properties of thin films of FeSe 0.5 Te 0.5 grown on eight different substrates. Superconductivity is not correlated with the lattice mismatch, but rather it is correlated with the degree of in-plane orientation and with the lattice parameter ratio c/a. The best superconducting properties are observed in films on MgO and LaAlO 3 (T c zero of 9.5 K). TEM observation shows that the presence or absence of the amorphous-like layer at the substrate surface plays a key role in determining the structural and superconducting properties of the grown films.* E-mail address: imai@maeda1.c.u-tokyo.ac.jpAfter the discovery of superconductivity in F-doped LaFeAsO, 1) numerous studies on iron-based superconductors have been carried out. One common iron-based superconductor is FeSe with a superconducting transition temperature T c of 8 K, 2) and the partial substitution of Te for Se raises T c to a maximum of 14 K.3)This material has the tetragonal PbO-type structure, which is the simplest structure of all the iron-based superconductors. Thus, FeSe and related materials are considered the most suitable systems to investigate how superconductivity correlates to the crystal structures.Many studies on the film growth of FeSe 1-x Te x have already been reported. 4-11)However, the question of what substrates are suitable for the growth of thin FeSe 1-x Te x films remains controversial. For example, Kumary et al. 6) reported that the T c value of the film on SrTiO 3 (STO) was higher than on LaAlO 3 (LAO). In contrast, Han et al. 7)reported an opposite result; the T c value on LAO was higher than that on STO. In addition, Bellingeri et al. thickness were fixed at 573 K and at approximately 50 nm, respectively. The crystal structure and the orientation of the films were characterized by a θ-2θ and a 4-circle X-ray diffraction (XRD) using Cu Kα radiation at room temperature. We also performed a transmission electron microscopy (TEM) observation. The electrical resistivity (ρ) was measured by a four-terminal method from 2 to 300 K. Figure 1 shows the XRD patterns of the eight films. Except for a few unidentified peaks, only the 00l reflections of a tetragonal PbO-type structure are observed, which shows that the out-of-plane alignment is excellent. It should be noted that the c-axis orientation is observed even in the film prepared on the (0001) plane in hexagonal Al 2 O 3 , as shown in Fig. 1 (h). This indicates that the FeSe 1-x Te x films intrinsically favor two-dimensional growth. The temperature dependence of ρ is summarized in Fig. 2. As can be easily seen, the eight films exhibit a variety of ρ(T)behavior. Except for the film on Al 2 O 3 , the ρ value of these films at T = 300 K is

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 *

In order to clarify the best condition for the growth of iron chalcogenide superconductor thin films, we investigated the effect of the substrate on the transport and structural properties of films. Thin films of FeSe0.5Te0.5 grown by pulsed laser deposition were characterized by DC electrical resistivity, Hall effect, X-ray diffraction measurements, and transmission electron microscopy (TEM) observation. The c-axis length of the FeSe0.5Te0.5 thin films revealed a non-systematic change with the cell constants of the substrates. In the films with poor or no superconductivity, we found the occurrence of oxygen penetration to the film and the formation of amorphous layer between the film and the substrate. The origin of the oxygen penetration and the amorphous layer was the chemical properties of substrate. From the chemical viewpoint, LaAlO3 and MgO substrates were confirmed to be appropriate to grow FeSe0.5Te0.5 films.