SrTiO3 Single Crystals Research Articles

Polarization and Dielectric Properties of BiFeO3-BaTiO3 Superlattice-Structured Ferroelectric Films.

Superlattice-structured epitaxial thin films composed of Mn(5%)-doped BiFeO3 and BaTiO3 with a total thickness of 600 perovskite (ABO3) unit cells were grown on single-crystal SrTiO3 substrates by pulsed laser deposition, and their polarization and dielectric properties were investigated. When the layers of Mn-BiFeO3 and BaTiO3 have over 25 ABO3 unit cells (N), the superlattice can be regarded as a simple series connection of their individual capacitors. The superlattices with an N of 5 or less behave as a unified ferroelectric, where the BaTiO3 and Mn-BiFeO3 layers are structurally and electronically coupled. Density functional theory calculations can explain the behavior of spontaneous polarization for the superlattices in this thin regime. We propose that a superlattice formation comprising two types of perovskite layers with different crystal symmetries opens a path to novel ferroelectrics that cannot be obtained in a solid solution system.

Is Reduced Strontium Titanate a Semiconductor or a Metal?

In recent decades, the behavior of SrTiO3 upon annealing in reducing conditions has been under intense academic scrutiny. Classically, its conductivity can be described using point defect chemistry and predicting n-type or p-type semiconducting behavior depending on oxygen activity. In contrast, many examples of metallic behavior induced by thermal reduction have recently appeared in the literature, challenging this established understanding. In this study, we aim to resolve this contradiction by demonstrating that an initially insulating, as-received SrTiO3 single crystal can indeed be reduced to a metallic state, and is even stable against room temperature reoxidation. However, once the sample has been oxidized at a high temperature, subsequent reduction can no longer be used to induce metallic behavior, but semiconducting behavior in agreement with the predictions of point defect chemistry is observed. Our results indicate that the dislocation-rich surface layer plays a decisive role and that its local chemical composition can be changed depending on annealing conditions. This reveals that the prediction of the macroscopic electronic properties of SrTiO3 is a highly complex task, and not only the current temperature and oxygen activity but also the redox history play an important role.

Carrier recombination in SrTiO3 single crystals: impacts of crystal faces and Nb doping

We observed the carrier recombination in SrTiO3 single crystals with several crystal faces and Nd doping concentrations using time-resolved photoluminescence and microwave photoconductivity decay methods. The shapes of the decay curves were independent of the excited carrier concentrations, and thus the carriers recombined through the Shockley–Read–Hall and surface recombination processes. From the measurements for different crystal faces, we found that, although the surface recombination is significant for the (100), (110), and (111) faces, they have similar surface recombination velocities of ∼106 cm s−1. Therefore, as photocatalysts, the shape of the materials has a minor effect on the energy conversion efficiency. Based on the dependence of the Nb doping concentration, a high doping concentration significantly enhances the carrier recombination, whereas a moderate Nb doping induces only a slight reduction in the time constants of the carrier decay. In addition, the time constant increases with temperature, and thus moderate Nb doping will have a positive effect on the energy conversion efficiency of SrTiO3 photocatalysts at high temperatures.

Investigation on Controllable Orientation Growth of BiFeO3 Thin Films Deposited by DC Magnetron Sputtering

Nanocrystalline BiFeO3 thin films were synthesized on single crystal SrTiO3(100) substrate by the magnetron sputtering method at a mixture of Ar/O2 gas discharge, structure and morphology were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The miscut angle of 4° and 8° for substrate and the effect of deposition temperature of 650 °C, 700 °C, 750 °C and 800 °C were discussed, respectively. The other two different crystal orientations substrates of SrTiO3(110) and SrTiO3(111), different miscut angles substrate SrTiO3(100) were selected to characterize orientation growth of BiFeO3 thin films. The results showed single phase nanocrystalline BiFeO3 thin films were synthesized at 700 °C–800 °C. The best crystallinity of BiFeO3 thin films was obtained at 750 °C with the grain size of 50 nm approximately. Via optimizing controllable orientation growth through varing the miscut angle, the orientation of the film sample can be regulated by the crystal orientation of different substrates, further proved that possessed different electric domain directions with change of miscut angle for substrate.

A novel process for the preparation of Bi2Sr2CaCu2O8+δ films with smooth surface via sol-gel method

During the preparation of Bi2Sr2CaCu2O8+δ (Bi2212) superconducting thin films by sol-gel method, the complexing agent and sintering conditions can significantly influence the quality and the performance of thin film. In this study, Bi2212 thin films were prepared on single crystal SrTiO3 (001) substrate, and the phase purity, crystallinity, surface morphology and transport properties were studied particularly with different sintering temperature and time, complexing agent and its ratio to metal ions. Due to relatively low carbon content, tartaric acid (TA) as a complexing agent improved the film quality and transport properties compared with the traditional ethylenediamine tetraacetic acid (EDTA). When the complex ratios of TA to metal ions was 1:1, Bi2212 thin films exhibited good epitaxial properties, a high Bi2212 phase purity and a low surface roughness of 12.74 nm. Simultaneously, under these preparation condition, Bi2212 superconducting thin film showed a higher Tc, onset of 93.28 K.

Ultraviolet-induced oxygen vacancy in SrTiO3 polycrystalline

The influence of ultraviolet (UV)-induced oxygen vacancies on dynamic changes of electrical conductance in a random-oriented SrTiO3 polycrystalline and SrTiO3 single crystals with (100), (110), and (111) surfaces is reported in this study. It was discovered that a large fraction of ultraviolet-induced oxygen vacancies in the SrTiO3 polycrystalline are primarily contributed from grain boundaries, but these oxygen vacancies are less influenced to the reversible conductance changes due to UV irradiation. The dynamic change of electrical conductance in the polycrystalline is similar to what has been observed for the polar (110) and (111) oriented single crystals, offering a potential alternative for multifunctional electronic devices.

Oxygen vacancy and photoelectron enhanced flexoelectricity in perovskite SrTiO3 crystal

Photo-enhanced flexoelectricity or flexoelectricity-enhanced photovoltaic effect, named photo-flexoelectric, is an interesting topic and has application potential in photo-electro-mechanical devices. However, this effect is far from being well understood. In this work, we demonstrate the photoflexoelectric effect in perovskite-structured SrTiO3 (STO) single crystal and reveal the coupling mechanism between its photovoltaic and flexoelectric effect. Driven by the flexoelectric field, light-induced electrons can tunnel through the Schottky junction at the Au/STO interface, giving rise to enhanced flexoelectricity, i.e., photoflexoelectric effect. Thermal annealing in vacuum induces oxygen vacancies in STO and results in stronger light absorption and enlarged photoflexoelectric effect.

Characterization of W-type hexaferrite thin films prepared by chemical solution deposition

Thin films of W-type hexaferrite were grown on SrTiO3(111) single crystals using the chemical solution deposition method and metalorganic precursor solutions prepared using the modified Pechini method. The synthesis of single-phase materials required optimizing the initial stoichiometry and crystallization conditions. The results showed that a concurrent excess of alkaline earth metal and deficiency in divalent metal ion (Zn2+, Co2+) favors the synthesis of pure W hexaferrites. This off-stoichiometry suppressed the formation of an exceedingly stable spinel phase at pyrolysis temperatures. The epitaxial nature of thin films, expressed as the orientation relation (00l)W|| (111)SrTiO3 and [110]W||[112¯]SrTiO3 and hence the pronounced anisotropy of their magnetic properties as well were established by three dimensional x-ray diffraction texture analysis and magnetization measurements, respectively and enabled to identify the temperature interval in which the characteristic spin reorientation transition occurs for a specific degree of cobalt substitution.

Finite Element Simulation of Temperature Distribution in Growth Chamber for Verneuil‐Grown SrTiO3 Crystal

AbstractThe work demonstrates combustion characteristics in growth chamber for Verneuil‐grown SrTiO3 crystal with two‐tube burner. Finite element (FE) simulation is performed to analyze the temperature distribution and its influencing factors. The central oxygen flows outward along the radial direction and reacts to hydrogen surrounding crystal, and the best ratio of hydrogen and oxygen is 2.5. The ratio less than 2.5 leads to increasing the center temperature of molten cap and greater than 2.5 leads to increasing the temperature around the molten cap, which probably causes the overflow problem. The surface temperature of molten cap increases first and then decreases gradually with the decreasing nozzle aperture of oxygen. The best diameter of nozzle aperture of oxygen is 4.5 mm. The surface temperature of molten cap is mainly related to the diameter of oxygen nozzle and the flow rate of oxygen. This work opens the stable growth process and the possibility for the industrial growth of large high quality SrTiO3 single crystals.

Large electrocaloric effect in lead-free ferroelectric Ba0.85Ca0.15Ti0.9Zr0.1O3 thin film heterostructure

A large electrocaloric effect is reported in a strain-engineered Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) thin film heterostructure driven by the near room-temperature electro-structural phase transition. An epitaxial BCZT/La0.7Sr0.3MnO3 (BCZT/LSMO) heterostructure was grown on a single-crystal SrTiO3 (100) substrate using pulsed laser deposition. In-depth x-ray diffraction and x-ray spectroscopic analyses revealed the single-crystalline nature and stoichiometric growth of the heterostructure. Both temperature dependent x-ray diffraction and dielectric measurements revealed a broad second-order-type phase transition near 430 K in the BCZT/LSMO heterostructure. From detailed theoretical analyses of the experimental data, it was confirmed that the phase transition around 430 K is second-order in nature, unlike the first-order transition observed in bulk BCZT materials. Thermodynamic analyses of polarization revealed an unprecedently large adiabatic temperature change of 13.5 K at 430 K under a field change of 1000 kV cm−1, hitherto unobserved in a lead-free material. Extremely broad adiabatic temperature change ΔT(T) curves over a wide working range of temperatures (330 K < T < 480 K) resulted in enhanced relative cooling powers, which are higher than those reported so far in most electrocaloric materials. We propose that an interfacial strain-induced enhanced tetragonal distortion of the BCZT layer gives rise to these large electrocaloric effects in the BCZT/LSMO heterostructure system. The demonstration of a large electrocaloric effect in the lead-free BCZT thin film may open up new pathways toward the design of artificial heterostructures for eco-friendly solid-state cooling applications.

Symmetry breaking in single-crystal SrTiO3 plates: EPR manifestations

Results of the electron paramagnetic resonance (EPR) studies of Fe3+ and Mn4+ impurity centers in high-quality single-crystal SrTiO3 plates with thickness are presented. Impurities served the paramagnetic probes of the crystal structure. Analysis of the collected data indicates that the symmetry of the crystal structure of thin (001) and (110) oriented strontium titanate plates above lowers in the bulk from the cubic to tetragonal. The structure of this high-temperature tetragonal state is different from that of the conventional low-temperature antiferrodistorsive phase and has not been reported for SrTiO3 before. It is shown that the effect does not originate from the residual stress; its magnitude and particular manifestations depend on plate orientation, surface quality and geometry of a sample. Peculiarities of the observed phenomenon are presented and the possible scenario of its realization is discussed.

Effect of strain and stoichiometry on the ferroelectric and pyroelectric properties of the epitaxial Pb(Zr0.2Ti0.8)O3 films deposited on Si wafers

Properties of epitaxial PbZr0.2Ti0.8O3 (PZT) films deposited on Si substrates were investigated for integration in the present CMOS technology. Polarization is downward oriented, in association with the presence of an internal electric field, and has a lower value compared to the PZT films deposited on single crystal perovskite SrTiO3 (STO) substrates (40 µC/cm2 versus 80 µC/cm2), while the dielectric constant is larger (180 versus 120). Large value for the pyroelectric coefficient was also found, 1.22 × 10−3C/m2K, as for PZT grown on single crystal STO. The macroscopic ferroelectric and pyroelectric properties appear to be affected by the structural quality and stoichiometry of the PZT film. The changes in the electric properties are an effect of the strain gradients induced by the large difference between the thermal expansion coefficients of PZT and Si substrate, leading in turn to Pb oxidation and antisite defect formation compared to PZT films deposited on STO substrates.

Dislocation-based crack initiation and propagation in single-crystal SrTiO3

Understanding the irreversible deformation (dislocation activation and crack formation) of functional oxides, based on which various advanced electronic devices are fabricated, is critical for the optimal structural design and mechanical reliability. Here, we demonstrate the dislocation-based crack initiation and propagation in a model perovskite, single-crystal SrTiO3, at small scale. Using nanoindentation tests with spherical tips and etch pit study, we identify the sequence of the irreversible deformation events occurred in single-crystal (001) SrTiO3. For a locally stressed volume that is free of pre-existing cracks, the material undergoes the following processes: (1) purely elastic deformation; (2) dislocation activation on primary slip planes that are 45° inclined to the surface; (3) dislocation activation on secondary slip planes perpendicular to the surface; (4) crack initiation by dislocation pile-up; followed by (5) concurrent crack propagation and dislocation multiplication and motion at higher loads. Specifically, we focus on the crack formation caused by the dislocation pile-up beneath the spherical indenter. We also identify the favorable crack propagation planes to be {110} when the (001) surface is indented. These findings in cubic SrTiO3 are believed to be applicable for other ceramic materials with cubic structure.

Bridging the Gap between Bulk Compression and Indentation Test on Room-Temperature Plasticity in Oxides: Case Study on SrTiO3

Dislocation-based functionalities in inorganic ceramics and semiconductors are drawing increasing attention, contrasting the conventional belief that the majority of ceramic materials are brittle at room temperature. Understanding the dislocation behavior in ceramics and advanced semiconducting materials is therefore critical for the mechanical reliability of such materials and devices designed for harvesting the dislocation-based functionalities. Here we compare the mechanical testing between indentation at nano-/microscale and bulk uniaxial deformation at macroscale and highlight the dislocation plasticity in single crystal SrTiO3, a model perovskite. The similarities and differences as well as the advantages and limitations of both testing protocols are discussed based on the experimental outcome of the crystal plasticity, with a focus on the pre-existing defect population being probed with different volumes across the length scales (“size effect”). We expect this work to pave the road for studying dislocation-based plasticity in various advanced functional ceramics and semiconductors.

A template-free synthesis of mesoporous SrTiO3 single crystals

Well-defined mesoporous SrTiO3 single crystals were fabricated via a template-free method, undergoing a reversed crystal growth, which crystallize from the external and then proceed gradually inwards.

Релаксация поляризации при статическом прямом флексоэлектрическом эффекте в монокристалле SrTiO-=SUB=-3-=/SUB=-

The results of a study of the dielectric polarization induced by spherical bending of thin single-crystal SrTiO3 plates are presented, and the flexoelectric coefficients are estimated. The processes of relaxation of inhomogeneous deformation and induced polarization are investigated.

Structural, vibrational, and magnetic properties of self-assembled CoPt nanoalloys embedded in SrTiO3

Pulsed laser deposition was used to synthesize vertically aligned nanocomposites (VAN), consisting of ultrathin CoPt nanocolumns embedded in a single crystal SrTiO3 thin film. Combining highresolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations, we provide an in-depth analysis of the structural and vibrational properties of the CoPt phase over a wide range of concentrations. Despite growing the samples at high temperatures (> 600 • C), we find that the CoPt nanoalloys are characterized by a high degree of structural disorder and complete absence of chemical ordering, even after additional sample annealing. Unexpectedly, for a Co concentration exceeding c(Co) 0.55, pole figure measurements unravel a transition from a highly textured short range disordered fcc phase to a disordered hcp phase with c-axis oriented out of plane. These findings, further confirmed by magnetometry measurements, illustrate the profound impact of vertical epitaxy on the structural and thermodynamic properties of bimetallic CoPt alloys.

Nanocomposites with Three-Dimensional Architecture and Impact on Photovoltaic Effect.

We demonstrate the synthesis of self-assembled three-dimensional nanocomposite thin films consisting of NiO nanocolumns in an layered Aurivillius phase matrix. The structures were grown on single-crystal SrTiO3 substrates via pulsed laser deposition (PLD) with single ceramic (PbTiO3)x(BiNi2/3Nb1/3O3)1-x targets. The nanocolumns, which are about 10 nm in diameter each, extend over the entire film thickness of up to 225 nm. We reveal the difference in electrical conduction properties of the nanocolumns and the surrounding matrix on the nanoscale via conductive atomic force microscopy. The nanocomposite thin films exhibit improved photovoltaic performance compared to both pure PbTiO3 and homogeneous Aurivillius phase thin films.

Strain-Engineered Metal-to-Insulator Transition and Orbital Polarization in Nickelate Superlattices Integrated on Silicon.

Epitaxial growth of SrTiO3 (STO) on silicon greatly accelerates the monolithic integration of multifunctional oxides into the mainstream semiconductor electronics. However, oxide superlattices (SLs), the birthplace of many exciting discoveries, remain largely unexplored on silicon. In this work, LaNiO3 /LaFeO3 SLs are synthesized on STO-buffered silicon (Si/STO) and STO single-crystal substrates, and their electronic properties are compared using dc transport and X-ray absorption spectroscopy. Both sets of SLs show a similar thickness-driven metal-to-insulator transition, albeit with resistivity and transition temperature modified by the different amounts of strain. In particular, the large tensile strain promotes a pronounced Ni orbital polarization for the SL grown on Si/STO, comparable to that reported for LaNiO3 SL epitaxially strained to DyScO3 substrate. Those results illustrate the ability to integrate oxide SLs on silicon with structure and property approaching their counterparts grown on STO single crystal, and also open up new prospects of strain engineering in functional oxides based on the Si platform.

Conceptual Framework for Dislocation-Modified Conductivity in Oxide Ceramics Deconvoluting Mesoscopic Structure, Core, and Space Charge Exemplified for SrTiO3.

The introduction of dislocations is a recently proposed strategy to tailor the functional and especially the electrical properties of ceramics. While several works confirm a clear impact of dislocations on electrical conductivity, some studies raise concern in particular when expanding to dislocation arrangements beyond a geometrically tractable bicrystal interface. Moreover, the lack of a complete classification on pertinent dislocation characteristics complicates a systematic discussion and hampers the design of dislocation-modified electrical conductivity. We proceed by mechanically introducing dislocations with three different mesoscopic structures into the model material single-crystal SrTiO3 and extensively characterizing them from both a mechanical as well as an electrical perspective. As a final result, a deconvolution of mesoscopic structure, core structure, and space charge enables us to obtain the complete picture of the effect of dislocations on functional properties, focusing here on electric properties.