Interface control by homoepitaxial growth in pulsed laser deposited iron chalcogenide thin films

Low-cost architecture for iron-based coated conductors

The design of iron-based coated conductors (IBS-CC) with a simplified architecture is possible thanks to the material properties that allow for milder requirements on the template crystalline quality. With respect to the state-of-the-art multilayered layout, it is possible to use a single buffer layer that remains necessary for protection and to promote the oriented growth of the superconducting film. In this work, Fe(Se,Te) films are grown via pulsed laser deposition (PLD) on commercial tapes using a single, chemically deposited, CeO2-based buffer layer, and interesting properties are obtained. In detail, the preparation and characterization of the buffer layer is presented, along with the detailed analysis of the Fe(Se,Te) current transport properties. The samples show superconducting transitions with T c 0 around 12 K and critical current densities of ∼0.1 MA cm -2 at 4.2 K at zero field. These results show that the design of a low-cost IBS-CC with a single chemical buffer layer is possible.

High-performance Fe(Se,Te) films on chemical CeO2-based buffer layers

The fabrication of a Fe-based coated conductor (CC) becomes possible when Fe(Se,Te) is grown as an epitaxial film on a metallic oriented substrate. Thanks to the material's low structural anisotropy, less strict requirements on the template microstructure allow for the design of a simplified CC architecture with respect to the REBCO multi-layered layout. This design, though, still requires a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template. In this work, Fe(Se,Te) films are grown on chemically-deposited, CeO₂-based buffer layers via pulsed laser deposition, and excellent properties are obtained when a Fe(Se,Te) seed layer is used. Among all the employed characterization techniques, transmission electron microscopy proved essential to determine the actual effect of the seed layer on the final film properties. Also, systematic investigation of the full current transport properties J(θ, H, T) is carried out: Fe(Se,Te) samples are obtained with sharp superconducting transitions around 16 K and critical current densities exceeding 1 MA cm^-2 at 4.2 K in self-field. The in-field and angular behavior of the sample are in line with data from the literature. These results are the demonstration of the feasibility of a Fe-based CC, with all the relative advantages concerning process simplification and cost reduction.

A Mini Review on Thin Film Superconductors

Thin superconducting films have been a significant part of superconductivity research for more than six decades. They have had a significant impact on the existing consensus on the microscopic and macroscopic nature of the superconducting state. Thin-film superconductors have properties that are very different and superior to bulk material. Amongst the various classification criteria, thin-film superconductors can be classified into Fe based thin-film superconductors, layered titanium compound thin-film superconductors, intercalation compounds of layered and cage-like structures, and other thin-film superconductors that do not fall into these groups. There are various techniques of manufacturing thin films, which include atomic layer deposition (ALD), chemical vapour deposition (CVD), physical vapour deposition (PVD), molecular beam epitaxy (MBE), sputtering, electron beam evaporation, laser ablation, cathodic arc, and pulsed laser deposition (PLD). Thin film technology offers a lucrative scheme of creating engineered surfaces and opens a wide exploration of prospects to modify material properties for specific applications, such as those that depend on surfaces. This review paper reports on the different types and groups of superconductors, fabrication of thin-film superconductors by MBE, PLD, and ALD, their applications, and various challenges faced by superconductor technologies. Amongst all the thin film manufacturing techniques, more focus is put on the fabrication of thin film superconductors by atomic layer deposition because of the growing popularity the process has gained in the past decade.

Effect of Substrate Temperature on Morphological, Structural, and Optical Properties of Doped Layer on SiO2-on-Silicon and Si3N4-on-Silicon Substrate

A high concentration of Er³⁺ without clustering issues is essential in an Er-doped waveguide amplifier as it is needed to produce a high gain and low noise signal. Ultrafast laser plasma doping is a technique that facilitates the blending of femtosecond laser-produced plasma from an Er-doped TeO₂ glass with a substrate to form a high Er³⁺ concentration layer. The influence of substrate temperature on the morphological, structural, and optical properties was studied and reported in this paper. Analysis of the doped substrates using scanning electron microscopy (SEM) confirmed that temperatures up to approximately 400 °C are insufficient for the incoming plasma plume to modify the strong covalent bonds of silica (SiO₂), and the doping process could not take place. The higher temperature used caused the materials from Er-doped tellurite glass to diffuse deeper (except Te with smaller concentration) into silica, which created a thicker film. SEM images showed that Er-doped tellurite glass was successfully diffused in the Si₃N₄. However, the doping was not as homogeneous as in silica.

Chemical Composition Control at the Substrate Interface as the Key for FeSe Thin-Film Growth

The strong fascination exerted by the binary compound of FeSe demands reliable engineering protocols and more effective approaches toward inducing superconductivity in FeSe thin films. Our study addresses the peculiarities in pulsed laser deposition that determine FeSe thin-film growth and focuses on the film/substrate interface, which has only been considered hypothetically in the past literature. The FeSe/MgO interface has been assumed (1) to be clean and (2) to obey lattice-matching epitaxy. Our studies reveal that both assumptions are misleading and demonstrate the tendency for domain-matching epitaxial growth, which accompanies the problem of chemical heterogeneity. We propose that homogenization of the film/substrate interface by an Fe buffer can improve the control of stoichiometry and nanostrain in a way that favors superconductivity even in ultrathin FeSe films. We will also show that on a chemically homogenized FeSe/Fe interface, the control of film texture with preparation conditions is still possible.

Growth of High-Quality Superconducting FeSe0.5Te0.5 Films on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 and Electric-Field Modulation of Superconductivity

Heterostructures composed of superconductor and ferroelectrics (SC/FE) are very important for manipulating the superconducting property and applications. However, growth of high-quality superconducting iron chalcogenide films is challenging because of their volatility and FE substrate with rough surface and large lattice mismatch. Here, we report a two-step growth approach to get high-quality FeSe_0.5Te_0.5 (FST) films on FE Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ with large lattice mismatch, which show superconductivity at only around 10 nm. Through a systematic study of structural and electric transport properties of samples with different thicknesses, a mechanism to grow high-quality FST is discovered. Moreover, electric-field-induced remarkable change of T_c (superconducting transition temperature) is demonstrated in a 20 nm FST film. This work paves the way to grow high-quality films which contain volatile element and have large lattice mismatch with the substrate. It is also helpful for manipulating the superconducting property in SC/FE heterostructures.

Origin of the emergence of higher T c than bulk in iron chalcogenide thin films

Fabrication of epitaxial FeSe_xTe_1−x thin films using pulsed laser deposition (PLD) enables improving their superconducting transition temperature (T_c) by more than ~40% than their bulk T_c. Intriguingly, T_c enhancement in FeSe_xTe_1−x thin films has been observed on various substrates and with different Se content, x. To date, various mechanisms for T_c enhancement have been reported, but they remain controversial in universally explaining the T_c improvement in the FeSe_xTe_1−x films. In this report, we demonstrate that the controversies over the mechanism of T_c enhancement are due to the abnormal changes in the chalcogen ratio (Se:Te) during the film growth and that the previously reported T_c enhancement in FeSe_0.5Te_0.5 thin films is caused by a remarkable increase of Se content. Although our FeSe_xTe_1−x thin films were fabricated via PLD using a Fe_0.94Se_0.45Te_0.55 target, the precisely measured composition indicates a Se-rich FeSe_xTe_1−x (0.6 < x < 0.8) as ascertained through accurate compositional analysis by both wavelength dispersive spectroscopy (WDS) and Rutherford backscattering spectrometry (RBS). We suggest that the origin of the abnormal composition change is the difference in the thermodynamic properties of ternary FeSe_xTe_1−x, based on first principle calculations.