Effects of Te- and Fe-doping on the superconducting properties in FeySe1−xTex thin films

High quality FeySe1−xTex epitaxial thin films have been fabricated on TiO2-buffered SrTiO3 substrates by pulsed laser deposition technology. There is a significant composition deviation between the nominal target and the thin film. Te doping can affect the Se/Te ratio and Fe content in chemical composition. The superconducting transition temperature Tc is closely related to the chemical composition. Fe vacancies are beneficial for the FeySe1−xTex films to exhibit the higher Tc. A 3D phase diagram is given that the optimize range is x = 0.13–0.15 and y = 0.73–0.78 for FeySe1−xTex films. The anisotropic, effective pining energy, and critical current density for the Fe0.72Se0.94Te0.06, Fe0.76Se0.87Te0.13 and Fe0.91Se0.77Te0.23 films were studied in detail. The scanning transmission electron microscopy images display a regular atomic arrangement at the interfacial structure.

Combined experimental-theoretical study of electron mobility-limiting mechanisms in SrSnO3

The discovery and development of ultra-wide bandgap (UWBG) semiconductors is crucial to accelerate the adoption of renewable power sources. This necessitates an UWBG semiconductor that exhibits robust doping with high carrier mobility over a wide range of carrier concentrations. Here we demonstrate that epitaxial thin films of the perovskite oxide NdxSr1−xSnO3 (SSO) do exactly this. Nd is used as a donor to successfully modulate the carrier concentration over nearly two orders of magnitude, from 3.7 × 1018 cm−3 to 2.0 × 1020 cm−3. Despite being grown on lattice-mismatched substrates and thus having relatively high structural disorder, SSO films exhibited the highest room-temperature mobility, ~70 cm2 V−1 s−1, among all known UWBG semiconductors in the range of carrier concentrations studied. The phonon-limited mobility is calculated from first principles and supplemented with a model to treat ionized impurity and Kondo scattering. This produces excellent agreement with experiment over a wide range of temperatures and carrier concentrations, and predicts the room-temperature phonon-limited mobility to be 76–99 cm2 V−1 s−1 depending on carrier concentration. This work establishes a perovskite oxide as an emerging UWBG semiconductor candidate with potential for applications in power electronics.

Giant Wrinkles on the Surface of Epitaxial BaTiO3 Thin Films with Drastic Shrinkage during Transfer from a MgO(100) Single-Crystal Substrate to a Flexible Polyethylene Terephthalate Sheet

The transfer of ferroelectric and piezoelectric BaTiO3 epitaxial thin films from an original MgO(100) single-crystal substrate to a polyethylene terephthalate (PET) sheet has been studied to fabricate flexible epitaxial functional oxides. The outline of our previous transfer process is as follows: the epitaxial BaTiO3 thin films were deposited on the MgO(100). Then, the surface of the BaTiO3 was adhered onto a PET sheet. Finally, only the MgO(100) substrate was dissolved in a phosphoric aqueous solution, which resulted in the transfer of the epitaxial BaTiO3 thin film from the MgO(100) to a PET sheet. To establish this transfer process, our aim was to prevent any damage, such as cracks and exfoliation, during the transfer of the epitaxial functional oxides. We found that a Pt buffer layer with a ductile nature was effective for improving the quality of transferred epitaxial BaTiO3 thin films. Moreover, the epitaxial BaTiO3 thin films showed a drastic shrinkage of ca. 10%. The surfaces of the shrunk, epitaxial BaTiO3 thin films showed giant wrinkles with a micrometer-order amplitude and a 10-μm-order periodicity without any damage. The epitaxial BaTiO3 thin films with giant wrinkles, accompanied by drastic shrinkage, are similar to the thin films that are coated on a pre-stretched elastomer, which is one of the fabrication processes of stretchable devices.

Nanoscale analysis of superconducting Fe(Se,Te) epitaxial thin films and relationship with pinning properties

The process of developing superconducting materials for large scale applications is mainly oriented to optimize flux pinning and the current carrying capability. A powerful approach to investigate pinning properties is to combine high resolution imaging with transport measurements as a function of the magnetic field orientation, supported by a pinning modelling. We carry out Transmission Electron Microscopy, Electron Energy Loss Spectroscopy and critical current measurements in fields up to 16 T varying the angle between the field and c-axis of Fe(Se,Te) epitaxial thin films deposited on CaF2 substrates. We find evidence of nanoscale domains with different Te:Se stoichiometry and/or rotated and tilted axes, as well as of lattice distortions and two-dimensional defects at the grain boundaries. These elongated domains are tens of nm in size along the in-plane axes. We establish a correlation between these observed microstructural features and the pinning properties, specifically strongly enhanced pinning for the magnetic field oriented in-plane and pinning emerging at higher fields for out-of-plane direction. These features can be accounted for within a model where pinning centers are local variations of the critical temperature and local variations of the mean free path, respectively. The identification of all these growth induced defects acting as effective pinning centers may provide useful information for the optimization of Fe(Se,Te) coated conductors.

Intrinsic ferroelectricity in Y-doped HfO2 thin films

Ferroelectric HfO2-based materials hold great potential for widespread integration of ferroelectricity into modern electronics due to their robust ferroelectric properties at the nanoscale and compatibility with the existing Si technology. Earlier work indicated that the nanometer crystal grain size was crucial for stabilization of the ferroelectric phase of hafnia. This constraint caused high density of unavoidable structural defects of the HfO2-based ferroelectrics, obscuring the intrinsic ferroelectricity inherited from the crystal space group of bulk HfO2. Here, we demonstrate the intrinsic ferroelectricity in Y-doped HfO2 films of high crystallinity. Contrary to the common expectation, we show that in the 5% Y-doped HfO2 epitaxial thin films, high crystallinity enhances the spontaneous polarization up to a record-high 50 µC/cm2 value at room temperature. The high spontaneous polarization persists at reduced temperature, with polarization values consistent with our theoretical predictions, indicating the dominant contribution from the intrinsic ferroelectricity. The crystal structure of these films reveals the Pca21 orthorhombic phase with a small rhombohedral distortion, underlining the role of the anisotropic stress and strain. These results open a pathway to controlling the intrinsic ferroelectricity in the HfO2-based materials and optimizing their performance in applications.