Oxygen vacancies in perovskite oxide piezoelectrics

Abstract The excellent electro-mechanical properties of perovskite oxide ferroelectrics make these materials major piezoelectrics. Oxygen vacancies are believed to easily form, migrate, and strongly affect ferroelectric behavior and, consequently, the piezoelectric performance of these materials and devices based thereon. Mobile oxygen vacancies were proposed to explain high-temperature chemical reactions half a century ago. Today the chemistry-enabled concept of mobile oxygen vacancies has been extrapolated to arbitrary physical conditions and numerous effects and is widely accepted. Here, this popular concept is questioned. The concept is shown to conflict with our modern physical understanding of ferroelectrics. Basic electronic processes known from mature semiconductor physics are demonstrated to explain the key observations that are groundlessly ascribed to mobile oxygen vacancies. The concept of mobile oxygen vacancies is concluded to be misleading

Proceedings of the 16th International Student Seminar “Microwave and optical applications of novel phenomena and technologies”, June 8–9, Oulu, Finland

Abstract The present volume contains 9 selected papers from the 16th International Student Seminar “Microwave and optical applications of novel phenomena and technologies”, June 8–9, Oulu, Finland. The authors are young researchers and post-graduate students. Their works reflect the variety of phenomena, models, technologies, and materials currently studied and employed for high-frequency applications in microwave to optical range. Especial emphasis is made on design and miniaturization of microwave components and circuits

Elasto-optic behaviour in epitaxial films of perovskite oxide ferroelectrics

Abstract Large variations of refractive index in the visible spectral range are obtained in epitaxial perovskite oxide ferroelectric films experiencing lattice strain. The strain is imposed by substrates, on top of which the films are grown. The optical constants are determined using the spectroscopic ellipsometry. As a reference and for comparison, also prototype single crystals are inspected. The variations in refraction are related to the lattice strain in the films. Elasto-optic coefficient is formally estimated using the out-of-plane lattice elongation or shrinkage in the films compared to bulk. The obtained elasto-optic coefficients exceed significantly those previously reported for ferroelectric materials

Large negative photoresistivity in amorphous NdNiO₃ film

Abstract A significant decrease in resistivity by 55% under blue lighting with ~0.4 J·mm⁻² energy density is demonstrated in amorphous film of metal-insulator NdNiO₃ at room temperature. This large negative photoresistivity contrasts with a small positive photoresistivity of 8% in epitaxial NdNiO₃ film under the same illumination conditions. The magnitude of the photoresistivity rises with the increasing power density or decreasing wavelength of light. By combining the analysis of the observed photoresistive effect with optical absorption and the resistivity of the films as a function of temperature, it is shown that photo-stimulated heating determines the photoresistivity in both types of films. Because amorphous films can be easily grown on a wide range of substrates, the demonstrated large photo(thermo)resistivity in such films is attractive for potential applications, e.g., thermal photodetectors and thermistors

Low-temperature NIR-VUV optical constants of (001) LaAlO3 crystal

Abstract The optical constants and dielectric function of (001) LaAlO3 crystal were investigated at low temperatures down to 10 K in the NIR-VUV spectral range (photon energies 0.8–8.8 eV). Reflection variable angle spectroscopic ellipsometry and transmission spectroscopy were applied. Interband transitions were examined using the Tauc plots and the critical-point analysis. At room temperature, the indirect bandgap of 5.6 ± 0.01 eV and the lowest-energy direct transition at 7.2 ± 0.03 eV were detected. On cooling to 10 K, a blueshift of ∼0.2 eV and ∼0.1 eV was observed for the indirect and direct transitions, respectively. In the transparency spectral range, the index of refraction was found to be nearly temperature-independent and vary with photon energy from 2.0 (1 eV) to 2.5 (5.5 eV). It was suggested that the excellent thermal stability of the index of refraction may be related to the revealed thermally stable interband transitions. The results are of importance for modeling and design of modern optical devices

Perovskite ferroelectric tuned by thermal strain

Abstract Modern environmental and sustainability issues as well as the growing demand for applications in the life sciences and medicine put special requirements to the chemical composition of many functional materials. To achieve desired performance within these requirements, innovative approaches are needed. In this work, we experimentally demonstrate that thermal strain can effectively tune the crystal structure and versatile properties of relatively thick films of environmentally friendly, biocompatible, and low-cost perovskite ferroelectric barium titanate. The strain arises during post-deposition cooling due to a mismatch between the thermal expansion coefficients of the films and the substrate materials. The strain-induced in-plane polarization enables excellent performance of bottom-to-top barium titanate capacitors akin to that of exemplary lead-containing relaxor ferroelectrics. Our work shows that controlling thermal strain can help tailor response functions in a straightforward manner

PLD prepared bioactive BaTiO₃ films on TiNb implants

Abstract BaTiO₃ (BTO) layers were deposited by pulsed laser deposition (PLD) on TiNb, Pt/TiNb, Si (100), and fused silica substrates using various deposition conditions. Polycrystalline BTO with sizes of crystallites in the range from 90 nm to 160 nm was obtained at elevated substrate temperatures of (600 °C–700 °C). With increasing deposition temperature above 700 °C the formation of unwanted rutile phase prevented the growth of perovskite ferroelectric BTO. Concurrently, with decreasing substrate temperature below 500 °C, amorphous films were formed. Post-deposition annealing of the amorphous deposits allowed obtaining perovskite BTO. Using a very thin Pt interlayer between the BTO films and TiNb substrate enabled high-temperature growth of preferentially oriented BTO. Raman spectroscopy and electrical characterization indicated polar ferroelectric behaviour of the BTO films