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The low-temperature optical properties of a SrTiO3 crystal doped with 0.7% wt. Nb and a pure SrTiO3 reference crystal were studied using spectroscopic ellipsometry. The optical constants and dielectric functions were obtained in the spectral range of 0.8–8.8 eV at temperatures from 10 to 300 K, and the optical conductivity was obtained in the spectral range of 0.03–1 eV at 300 K. Analysis of the optical conductivity spectra in the infrared spectral range confirmed the presence of free electrons and additional absorption hump between 0.1 and 0.4 eV (806 and 3226 cm−1) in doped SrTiO3 and optical phonons in both doped and undoped SrTiO3. The performed analysis of optical absorption spectra in the range 0.8–8.8 eV revealed frustration of the indirect bandgap and an increase in the direct bandgap energy by ∼0.03 eV in Nb-doped SrTiO3, compared to undoped SrTiO3 over the whole temperature range. The energies of the peaks' maxima of the dielectric function spectra did not significantly differ for doped and pure SrTiO3. For both Nb-doped and pure SrTiO3, temperature dependence of the direct bandgap energy and that of the index of refraction showed inflection between 100 and 150 K, which may be considered evidence of an antiferrodistortive phase transition from a cubic to a tetragonal structure. [ABSTRACT FROM AUTHOR]

Technologically important high-permittivity dielectrics are often achieved using cationic engineering of ABO3-type perovskite para(ferro)electrics. Here, we experimentally and theoretically explore the potential of less conventional anionic engineering in ABO3 dielectrics. We demonstrate that in an archetypal representative SrTiO3, nitrogen substitution can occur on two distinct oxygen atomic sites, reduce crystal symmetry, and lead to significant changes in the patterns and frequencies of lattice vibrations. These phonon transformations diminish permittivity, whereas contribution from nitrogen-induced nanoregions can raise it. The effects of nitrogen are found to be especially strong in epitaxial films. We anticipate that the revealed phenomena may be relevant for a broad class of high-permittivity perovskite oxides. [ABSTRACT FROM AUTHOR]

In ABO3 perovskite oxide ferroelectrics, electrical conductivity ranges from insulator- to superconductor-type and is virtually critical for all applications of these materials. Compared to bulk ceramics and crystals, ferroelectric thin films can enable advanced control of the conductivity. Here, small-polaron hopping conductivity was evidenced and examined in various pulsed-laser-deposited films of ferroelectric BaTiO3 and reference films of SrTiO3. For this, AC conductivity was studied in a broad range of temperatures and frequencies for films sandwiched between the bottom and top electrodes. In the BaTiO3 films, with increasing temperature, a significant increase in activation energy for small-polaron hopping was found and ascribed to strong electron–phonon coupling and complex lattice oscillations therein. Plain relations of the activation energy to microstructure, composition, or phase transitions were lacking, which corroborated the critical role of phonons. Additionally, a phonon-less transport was detected. It was anticipated that owing to strong electron–phonon coupling, rich phonon ensembles, and coexistence of phonon-stimulated and phonon-less processes, the small-polaron conductivity can heavily vary in ferroelectric films that necessitates further studies. [ABSTRACT FROM AUTHOR]

Control of lattice strain in epitaxial films of ABO3 perovskite oxides is crucial for modern understanding and applications of these scientifically and technologically important materials. Here, we show that oxygen vacancies have unique impacts on lattice strain in such films. We suggest that in the presence of substrate-imposed misfit in epitaxial or highly oriented films, the crystallographic alignment of anisotropic elastic dipole tensors of oxygen vacancies is energetically favorable. The dipolar alignment leads to an enhanced above-misfit magnitude of maximal lattice strain and to increased inhomogeneous strain or strain gradients. The vacancy-induced remarkably strong elastic effects are experimentally validated by varying the misfit strain and oxygen content in thin films of perovskite niobate (ANbO3) and titanate (ATiO3) ferroelectrics. It is anticipated that elastic effects of oxygen vacancies are relevant for controlling strain in epitaxial films of a broad range of functional oxides. [ABSTRACT FROM AUTHOR]

The high-temperature paraelectric state of thin-film (Ba,Sr)Ti[O.sub.3] (BSTO) with different composition and thickness grown on MgO or [Al.sub.2][O.sub.3] is analyzed. The high-temperature dielectric hysteresis in found to be of a relaxational nature and the proper paraelectric state is observed by Curie-Weiss behavior.

Ferroelectric thin films of perovskite PbLu0.5Nb0.5O3-PbTiO3 were studied.

The two-dimensional spatial distribution of the composition of the deposit was experimentally studied and analyzed by using the sorption approach. Results show a good agreement between the experimental and sorption analysis, and suggest a determining role of the plume-substrate interaction in the deposition process.

Pulsed laser deposition was used in forming highly-oriented perovskite films of PbMg1/3Nb2/3-PbTiO3 and PbSc1/2Nb1/2O3-PbTiO3 on La0.5Sr0.5CoO3/MgO (100). The resulting films have ambient polarization, dielectric permitivitty and temperatures of the dielectric peaks similar to that of bulk ceramics. However, the width of the transition in the films was found to be larger than in the bulk material. The small grain size influence the physical properties of the films and not the film interfaces.

Research was conducted to examine experimentally the deposition rate, elemental composition and the spatial distribution of the deposition rate and of the composition in the deposit during room temperature laser ablation of multicomponent targets of PbZr0.65Ti0.35O3, Bi2Sr2CuO6 and CuInSe2. A number of discrepancies were revealed by a comparison of the obtained experimental data with previous results. Findings suggest that the discrepancies are associated with the assumption in the modeling of unit sticking probability of the species.

The electronic properties, including bandgap and conductivity, are critical for nearly all applications of multifunctional perovskite oxide ferroelectrics. Here we analysed possibility to induce semiconductor behaviour in these materials, which are basically insulators, by replacement of several percent of oxygen atoms with nitrogen, hydrogen, or vacancies. We explored this approach for one of the best studied members of the large family of ABO3 perovskite ferroelectrics — strontium titanate (SrTiO3). The atomic and electronic structure of defects were theoretically investigated using the large-scale first-principles calculations for both bulk crystal and thin films. The results of calculations were experimentally verified by studies of the optical properties at photon energies from 25 meV to 8.8 eV for in-situ prepared thin films. It was demonstrated that substitutions and vacancies prefer locations at surfaces or phase boundaries over those inside crystallites. At the same time, local states in the bandgap can be produced by vacancies located both inside the crystals and at the surface, but by nitrogen substitution only inside crystals. Wide-bandgap insulator phases were evidenced for all defects. Compared to pure SrTiO3 films, bandgap widening due to defects was theoretically predicted and experimentally detected. [ABSTRACT FROM AUTHOR]

Cutting-edge acousto-optic devices require optically transparent thin films, which possess a high index of refraction and large elasto-optic coefficients. For the wide near-infrared to ultraviolet spectral region, the mainstream technology employs lithium niobate crystals, which interferes with the vital demands of global sustainability. Here, we demonstrate unprecedented elasto-optic properties in thin films of sustainable and environmentally friendly strontium titanate [SrTiO3 (STO)]. Compared to cubic STO, a nearly twofold increase in elasto-optic coefficients is achieved in epitaxial tetragonal STO films, which concurrently exhibit excellent transparency and a high index of refraction at wavelengths from 400 to 1700 nm. The room-temperature non-polar state is evidenced by the thermo-optical behavior of the films. The obtained enhancement is related to the tetragonal antiferrodistortive phase of STO. It is suggested that such films can form a platform for future sustainable acousto-optic materials. [ABSTRACT FROM AUTHOR]

Experiments were conducted to determine the growth rate and composition of films deposited by laser ablation of Pb(Zr(sub 0.65)Ti(sub 0.35))O3, with respect to the laser fluence and ambient gas pressure. Among the findings are an increase in the film growth rate in vacuum as laser fluence increases and a similar behavior of the film growth rate with the addition of either oxygen or argon. No changes were noticed in the spatial distribution of the film growth rate in a gas.

Single-crystal epitaxial films of technologically important and scientifically intriguing multifunctional ABO3 perovskite-type metal oxides are essential for advanced applications and understanding of these materials. In such films, a film-substrate misfit strain enables unprecedented crystal phases and unique properties that are not available in their bulk counterparts. However, the prerequisite growth of strained epitaxial films is fundamentally restricted by misfit relaxation. Here we demonstrate that introduction of a small oxygen deficiency concurrently stabilizes epitaxy and increases lattice strain in thin films of archetypal perovskite oxide SrTiO3. By combining experimental and theoretical methods, we found that lattice distortions around oxygen vacancies lead to anisotropic local stresses, which interact with the misfit strain in epitaxial films. Consequently, specific crystallographic alignments of the stresses are energetically favorable and can facilitate epitaxial growth of strained films. Because anisotropic oxygen-vacancy stresses are inherent to perovskite-type and many other oxides, we anticipate that the disclosed phenomenon of epitaxial stabilization by oxygen vacancies is relevant for a very broad range of functional oxides. [ABSTRACT FROM AUTHOR]

Mobile oxygen vacancies are increasingly widely believed to be responsible for electrical conductivity in perovskite oxide ferroelectrics. Here, this hypothesis is debated. The small-signal conductivity is investigated in oxygen-deficient films of barium titanate, where oxygen vacancies are epitaxially clamped and immobile. The observed behavior of conductivity as a function of temperature and frequency evidences pure electronic processes. Importantly, it is shown that these processes mimic motion of oxygen vacancies, which are immobile. It is also demonstrated that under the applied dc electric field, the electronic processes lead to such effects as coloration and degradation, which before were plausibly ascribed to migration of oxygen vacancies. Finally, it is concluded that the hypothesis of mobile oxygen vacancies is misleading. [ABSTRACT FROM AUTHOR]

Misfit strains arising from a film–substrate mismatch can induce novel phases and properties in the epitaxial films of perovskite oxides. Here we employ yet another effect, namely, strain-assisted formation of oxygen vacancies. We demonstrate the misfit-promoted presence of oxygen vacancies and related substitutional incorporation of anion dopants in the epitaxial films of archetypal perovskite oxide SrTiO3. Both the oxygen vacancies and hydrogen or nitrogen dopants are introduced in situ during the pulsed-laser deposition of the films using compressive substrates. The films exhibit peculiar chemical expansion and optical properties, which are consistent with substitutional anion doping. [ABSTRACT FROM AUTHOR]

Diverse effects of lattice strain on the optical properties in the near-infrared to vacuum ultraviolet spectral range are experimentally revealed in cube-on-cube-type epitaxial perovskite SrTiO3 films grown on compressive substrate. Compared to the reference crystal, the tetragonal antiferrodistortive film exhibits spectral blueshifts, which are consistent with the theoretically predicted bandgap widening. In addition to this strain-induced interband effect, the peculiar near-edge absorption and smearing of interband transitions are found. It is shown that Fröhlich-type electron–phonon coupling can be enhanced by strain and lead to this behavior. It is suggested that electron–phonon interactions can play an important role in optical properties of ferroelectric films and deserve further studies. [ABSTRACT FROM AUTHOR]

Multiple strong impacts of anion doping Λ (Λ = oxygen vacancy, nitrogen, or hydrogen) on the optical properties in the near-infrared to vacuum ultraviolet spectral range are experimentally revealed in cube-on-cube-type epitaxial perovskite BaTiO3−δΛδ films (δ ≤ 0.01) grown on (001)SrTiO3 substrates. Compared to the reference undoped films and bulk crystal, the anion-doped films exhibit massive changes of the index of refraction in the transparency range Δn of ∼0.1 to 0.2, a profound bandgap widening ΔEg of ∼0.3 eV to 0.4 eV, a dopant-specific near-edge absorption, and doping-induced interband transitions. Both the conduction-band uplift and the changes in the index of refraction are ascribed to the internal stress and electric field, which result from epitaxially controlled dopant locations in the [Ba–O](001) planes. The dopant-induced in-gap states are responsible for the near-edge absorption, whereas the dopant-induced conduction-band states are suggested to modify the interband transitions and the intrinsic elasto- and electro-optic coefficients. [ABSTRACT FROM AUTHOR]

NaNbO3:SrTiO3 interface nanolayers are studied by combining first-principles modeling and experimental approach. The nature of NaNbO3:SrTiO3 interface is investigated using density-functional theory calculations. Interface parameters are experimentally estimated using analysis of the dielectric response of SrTiO3 and NaNbO3 epitaxial films and NaNbO3:SrTiO3 superlattices. It is shown that NaNbO3:SrTiO3 interfaces can be approximated by dielectric nanolayers with thickness 1-2 nm and with temperature independent permittivity 25-55. Mechanism responsible for the nanolayer formation is the ionic charge mismatch between constituents. [ABSTRACT FROM AUTHOR]

Relaxor ferroelectric PbMg1/3Ta2/3O3 ceramics and thin films were investigated by means of broad-band dielectric, time-domain terahertz (THz), and Fourier-transform infrared (IR) spectroscopy in the frequency range 100 Hz–90 THz at temperatures 100–490 K; the THz and IR spectra were studied from 20 to 900 K. A diffused and strongly temperature dependent peak in the complex permittivity is caused by a dielectric relaxation due to the dynamics of polar clusters. The relaxation appears below the Burns temperature Td in the THz range; it slows down on cooling through the microwave and MHz range and anomalously broadens. The shortest and longest relaxation times of the distribution of relaxation times follow Arrhenius and Vogel–Fulcher laws, respectively. The degree of the B-site order has only a small influence on the parameters of the dielectric relaxation and almost no influence on the phonon parameters. Below Tm≅180 K the distribution of relaxation frequencies becomes broader than our experimental spectral range, and frequency independent dielectric losses develop below 100 GHz in the spectra. Although the macroscopic crystal structure is cubic, IR spectra give evidence about the lower local symmetry, which can be assigned to the presence of the polar clusters below Td. Infrared spectra above Td still reveal more modes than predicted by selection rules in the paraelectric phase of the Fm3m space group so that we suggest selection rules which take into account chemical inhomogeneity in the β″-perovskite sublattice. [ABSTRACT FROM AUTHOR]

Epitaxial multilayers and superlattices of Ba0.8Sr0.2TiO3 and Ba0.4Sr0.6TiO3 were grown by pulsed laser deposition on MgO substrates using a La0.5Sr0.5CoO3 bottom electrode. The epitaxial relations, crystal perfection, and strains were studied by x-ray diffraction at room temperature. Pseudomorphic growth of superlattices was demonstrated for the period as large as 62 unit cells. The obtained in-plane strain was consistent with that theoretically expected. Low-frequency functionality was found to depend on strain, period, and amount and quality of interfaces. [ABSTRACT FROM AUTHOR]

Vertical capacitor heterostructures of (Ba,Sr)TiO3 thin films with different Ba:Sr ratios, microstructures, and thicknesses were grown by in situ pulsed laser deposition using MgO or Al2O3 substrates and La0.5Sr0.5CoO3 or Pt electrodes. The low-frequency dielectric response was experimentally studied as a function of frequency, temperature, and applied field. High-temperature Curie-Weiss behavior, temperature evolution of the dynamic dielectric nonlinearity, and dielectric hysteresis were observed and analyzed. The observed high-temperature relaxorlike state and relaxational nature of dielectric hysteresis are discussed. [ABSTRACT FROM AUTHOR]

Vertical capacitor heterostructures of (Ba,Sr)TiO3 thin films with different Ba:Sr ratios, microstructures, and thicknesses were grown by in situ pulsed laser deposition using MgO or Al2O3 substrates and La0.5Sr0.5CoO3 or Pt electrodes. The low-frequency dielectric response was experimentally studied as a function of frequency, temperature, and applied field. High-temperature Curie-Weiss behavior, temperature evolution of the dynamic dielectric nonlinearity, and dielectric hysteresis were observed and analyzed. The observed high-temperature relaxorlike state and relaxational nature of dielectric hysteresis are discussed. [ABSTRACT FROM AUTHOR]

Epitaxial heterostructures of pure perovskite relaxor ferroelectric PbMg1/3Nb2/3O3 thin films were grown by pulsed laser ablation varying the temperature of deposition. Dielectric response of the films was studied as a function of frequency, temperature, amplitude of ac electric field, and magnitude of dc electric field. The temperatures of dielectric maxima and the freezing temperatures determined in different regimes were analyzed with respect to the microstructure of the films. Different nonmonotonic tendencies in the behavior of the characteristic temperatures were found. The shift of the electric-field–temperature phase diagram of PbMg1/3Nb2/3O3 involving the sequence of transformations between paraelectriclike, glasslike, and ferroelectriclike states was suggested to be responsible for this. The shift of the phase diagram was found to be a monotonic function of the in-plane strain and it was ascribed to the internal electric field. [ABSTRACT FROM AUTHOR]

Epitaxial heterostructures of pure perovskite relaxor ferroelectric PbMg1/3Nb2/3O3 thin films were grown by pulsed laser ablation varying the temperature of deposition. Dielectric response of the films was studied as a function of frequency, temperature, amplitude of ac electric field, and magnitude of dc electric field. The temperatures of dielectric maxima and the freezing temperatures determined in different regimes were analyzed with respect to the microstructure of the films. Different nonmonotonic tendencies in the behavior of the characteristic temperatures were found. The shift of the electric-field–temperature phase diagram of PbMg1/3Nb2/3O3 involving the sequence of transformations between paraelectriclike, glasslike, and ferroelectriclike states was suggested to be responsible for this. The shift of the phase diagram was found to be a monotonic function of the in-plane strain and it was ascribed to the internal electric field. [ABSTRACT FROM AUTHOR]

Epitaxial heterostructures of ordered perovskite PbSc[sub 0.5]Nb[sub 0.5]O[sub 3] thin films were grown by pulsed laser deposition using MgO and LaAlO[sub 3] single-crystal substrates, and La[sub 0.5]Sr[sub 0.5]CoO[sub 3] bottom electrode layers. The small-signal dielectric response of the heterostructures was measured in a broad range of temperatures (290-625 K) and frequencies (10²- 10[sup 6] Hz). The contribution of the film-electrode interfaces was evaluated, and the true properties of the films were analyzed. Both relaxor and normal ferroelectric types of behavior were identified in the films. The evolution of behavior and shift of dielectric maxima were found to correlate with the temperature of deposition and the type of the substrate. This was suggested to refer to the microstructural factors such as in-plane strain, grain size, and misfit dislocations. [ABSTRACT FROM AUTHOR]

Dielectric response of relaxor PbMg1/3Nb2/3O3 thin films is experimentally studied and analyzed. Based on the analysis of the dielectric properties, a qualitative strain-electric-field-temperature phase diagram of relaxor thin films is suggested and discussed.

Focuses on the experimental study of the dielectric properties of pulsed laser deposited ferroelectric lead lanthanum zirconium titanium (PLZT) oxide thin films. How the experiment was conducted; Composition and microstructure of PLZT films; Ferroelectric hysteresis of the study; Dielectric permittivity and loss tangent of the study; Conclusions.

Allying epitaxial strain and synthesis conditions allows for the introduction of specific point defects in perovskite oxide films. In ferroelectric films, such defects lead to essential polar and electronic properties, which can enable advanced applications. Here, to elucidate the nature of the defects, optical constants are investigated in the spectral range of 0.7–8.8 eV in epitaxial ferroelectric BaTiO3 films, which are synthesized under different conditions. It is demonstrated that oxygen-vacancy-related defects are responsible for a peculiar transition below the bandgap at ∼2.7–2.9 eV and significant blueshifts of ∼0.3–0.4 eV of the gap and the main interband transitions. These observations suggest that the defects are dipolar complexes comprising titanium cations and oxygen vacancies (Ti3+–VO). [ABSTRACT FROM AUTHOR]

In ferroelectric superlattices (SLs), the short-range interactions and intermixing between contacting dissimilar materials can lead to the formation of interfacial nanolayers with physical properties different from those of both constituents. We demonstrate theoretically that such nanolayers can give rise to a sizable dependence of the SL permittivity on the period even in the paraelectric regime. The results of dielectric measurements performed for coherent Ba0.8Sr0.2TiO3-Ba0.4Sr0.6TiO3 SLs are qualitatively consistent with the theoretical predictions. At the same time, the experimental data indicate that other factors also contribute to the variation of the permittivity with the period in ferroelectric superlattices and multilayers. [ABSTRACT FROM AUTHOR]

Epitaxial films and heterostructures of perovskite oxides attract tremendous scientific interest because of the unique phenomena therein. Especially important is the epitaxial growth of films subjected to substrate-induced misfit strain. We show here that in contrast to conventional misfit-controlled epitaxy, chemical bonds determine the crystal stability and strain in epitaxial films of sodium niobate on different cubic substrates. Strain relaxation in sodium niobate is independent of misfit magnitude and proceeds through perovskite-specific tilting of oxygen octahedra in addition to common defect formation. The observed structural relaxation evidences a major role of a large internal strain that originates from chemical bonds in the perovskite cell. The effect of chemical bonds on film strain is anticipated to also control the epitaxy of other perovskite oxides and related compounds. [ABSTRACT FROM AUTHOR]

Single-crystal-type epitaxial films of perovskite oxide ferroelectrics are attractive for integrated photonic applications because of the remarkable optical properties and effects in ferroelectrics. The properties of the films may be influenced by epitaxial strain arising from the film-substrate mismatch. Here, dramatic strain-induced changes of the absorption and refraction are experimentally detected by spectroscopic ellipsometry in epitaxial films of archetypical ferroelectric PbTiO3. Comparison of the properties of a tensile-strained film with those of reference films and crystals reveals that epitaxial tension produces blueshifts of the primary above-bandgap absorption peaks by 1 eV and a decrease in the refractive index by 0.5 in the transparent spectral range. The obtained quadratic electrooptic and effective elastooptic coefficients exceed the bulk values by orders of magnitude. The experimental observations prove that epitaxy is a powerful tool for engineering unprecedented optical properties that may enable future photonics innovations. [ABSTRACT FROM AUTHOR]