Unit Cells Of SrTiO3 Research Articles

Structural, electrical, and magnetic properties of Ce and Fe doped SrTiO3

Here, we report on the structural, vibrational phonon, electrical, and magnetic properties of undoped strontium titanate SrTiO3, Ce doped Sr1−xCexTiO3, and (Ce, Fe) co-doped Sr1−xCexTi1−yFeyO3 samples synthesized through solid state reaction route. The Rietveld refined powder x-ray diffraction analysis confirmed the cubic Pm-3m phase in our as-synthesized samples. We observed grain size reduction in SrTiO3 from scanning electron micrographs due to the incorporation of Ce and Fe dopants. The sample purity in terms of chemical species identification has been confirmed from energy-dispersive x-ray spectroscopy. The characteristic phonon modes in our samples are identified using room temperature Raman spectroscopy and benchmarked against existing relevant experimental observations. The incorporation of Ce and Fe as substitutional dopants in SrTiO3 unit cell was confirmed from the absence of absorption at 480, 555, 580, and 1635 cm−1 band in Fourier transform infrared spectra. The 3% Ce doping in Sr0.97Ce0.03TiO3 sample may have induced ferroelectric order, whereas the undoped SrTiO3 (STO) revealed lossy paraelectric nature. In the case of (Ce = 3%, Fe = 10%) co-doped Sr0.97Ce0.03Ti0.90Fe0.10O3 sample, we observed ferromagnetic hysteresis with orders of magnitude enhancement in remnant magnetization and coercivity as compared to undoped STO sample. This long range robust ferromagnetic order may have originated from F-center mediated magnetic interaction.

Engineering the stoichiometry of a TiO2-rich SrTiO3(001) surface

We investigate the stoichiometry of a TiO2-rich SrTiO3(001) surface by depositing less than one monolayer of SrO under typical epitaxy conditions. We find that this deposition yields atomically flat islands on the step-and-terrace surface. All these islands have a height of exactly one full SrTiO3 unit cell, instead of the expected half unit cell. Reflection high-energy electron diffraction indicates that a reconstruction occurs on the bare surface, which disappears with SrO deposition. To explain these results, we propose a simple model with two key elements: (i) a TiO2 double layer must intrinsically be present on the as-prepared SrTiO3(001) surface, and (ii) with deposited SrO, this top layer rearranges itself to form islands with a height of one unit cell.

Control of magnetic anisotropy by orbital hybridization with charge transfer in (La0.67Sr0.33MnO3)n/(SrTiO3)n superlattice

The chemical asymmetry at the hetero-structure interface offers an effective opportunity to design desirable electronic structures by controlling charge transfer and orbital hybridization across the interface. However, controlling the hetero-interface remains a daunting task. Here, we report the modulation of interfacial coupling of (La0.67Sr0.33MnO3)n/(SrTiO3)n superlattices by manipulating the periodic thickness with n unit cells of SrTiO3 and n unit cells of La0.67Sr0.33MnO3 with a fixed thickness of ~120 unit cells. The easy axis of magnetic anisotropy rotates ~45° towards the out-of-plane direction from n = 10 to n = 2 at reduced temperature TRe = T/TS = 0.87, where TS is the temperature at the onset of magnetization. Transmission electron microscopy reveals an enlarged tetragonal ratio >1 with breaking of volume conservation around the (La0.67Sr0.33MnO3)n/(SrTiO3)n interface and electronic charge transfer from Mn to Ti 3d orbitals across the interface. Orbital hybridization accompanying the charge transfer results in preferred occupancy of {3d}_{3z^2-r^2} orbitals at the interface and induces a stronger electronic hopping integral and interfacial magnetic anisotropy along the out-of-plane direction, which contributes to the rotation towards the out-of-plane direction of an effective magnetic easy axis for n = 2. We demonstrate that interfacial orbital hybridization with charge transfer in the superlattice of strongly correlated oxides may be a promising approach to tailor electronic and magnetic properties in device applications.

Electron microscopy with high accuracy and precision at atomic resolution: In-situ observation of a dielectric crystal under electric field

Measuring atomic positions in-situ under an external electric field can provide important insights into the structure-property relationship of electronic materials. In this paper, we demonstrate picometer level accuracy and precision of atomic positions in single-crystalline SrTiO3 under an electric field through annular dark-field scanning transmission electron microscopy. By carrying out electrical biasing in-situ electron microscopy at the atomic scale, the lattice constant was measured with a precision of 9.0 pm under an electric field of ±0.57 kV/cm. In addition, the Ti position in the SrTiO3 unit cell was measured with an accuracy of 20.0 pm at a confidence level of greater than 93%. This opens up a possibility of characterizing functional electronic devices at atomic resolution under operative conditions.

Colossal positive magnetoresistance in surface-passivated oxygen-deficient strontium titanite.

Modulation of resistance by an external magnetic field, i.e. magnetoresistance effect, has been a long-lived theme of research due to both fundamental science and device applications. Here we report colossal positive magnetoresistance (CPMR) (>30,000% at a temperature of 2 K and a magnetic field of 9 T) discovered in degenerate semiconducting strontium titanite (SrTiO3) single crystals capped with ultrathin SrTiO3/LaAlO3 bilayers. The low-pressure high-temperature homoepitaxial growth of several unit cells of SrTiO3 introduces oxygen vacancies and high-mobility carriers in the bulk SrTiO3, and the three-unit-cell LaAlO3 capping layer passivates the surface and improves carrier mobility by suppressing surface-defect-related scattering. The coexistence of multiple types of carriers and inhomogeneous transport lead to the emergence of CPMR. This unit-cell-level surface engineering approach is promising to be generalized to others oxides, and to realize devices with high-mobility carriers and interesting magnetoelectronic properties.

Defect separation in strontium titanate: Formation of a polar phase

Stoichiometric perovskite-type strontium titanate acts as an insulator because of its wide electronic band gap and has therefore great potential as high-k dielectric and storage material in memory applications. Degradation phenomena of insulating properties of transition metal oxides occur during long time voltage application. From the defect chemistry point of view the question arises how mobile species react on an external electric field and which impact the redistribution has on the stability of the crystal structure. Here, we discuss near-surface reversible structural changes in SrTiO3 single crystals caused by oxygen vacancy redistribution in an external electric field. We present in-situ X-ray diffraction during and after electroformation. Several reflections are monitored and show a tetragonal elongation of the cubic unit cell. Raman investigations were carried out to verify that the expansion involves a transition from the centrosymmetric to a less symmetric structure. Regarding a whole formation cycle, two different time scales occur: a slow one during the increase of the lattice constant and a fast one after switching off the electric field. Based on the experimental data we suggest a model containing the formation of a polar SrTiO3 unit cell stabilized by the electric field, which is referred to as migration-induced field-stabilized polar phase [1] at room temperature. As expected by a non-centrosymmetric crystal structure, pyroelectric properties will be presented in conjunction with temperature modulated electroformation cycles. Furthermore, we show that intrinsic defect separation establishes a non-equilibrium accompanied by an electromotive force. A comprehensive thermodynamic deduction in terms of theoretical energy and entropy calculations indicates an exergonic electrochemical reaction after the electric field is switched off. Based on that driving force the experimental and theoretical proof of concept of a solid-state SrTiO3 battery is reported.

Two‐dimensional thermoelectric Seebeck coefficient of SrTiO3‐based superlattices

AbstractThis review provides the origin of the unusually large thermoelectric Seebeck coefficient |S | of a two‐dimensional electron gas confined within a unit cell layer thickness (∼0.4 nm) of a SrTi0.8Nb0.2O3 layer of artificial superlattices of SrTiO3/SrTi0.8Nb0.2O3 [H. Ohta et al., Nature Mater. 6, 129 (2007)]. The |S |2D values of the [(SrTiO3)17/(SrTi0.8Nb0.2O3)y ]20 superlattice increase proportional to y–0.5, and reach 290 μV K–1 (y = 1) at room temperature, which is ∼5 times larger than that of the SrTi0.8Nb0.2O3 bulk (|S |3D = 61 μV K–1), proving that the density of states in the ground state for SrTiO3 increases in inverse proportion to y. The critical barrier thickness for quantum electron confinement is also clarified to be 6.25 nm (16 unit cells of SrTiO3). Significant structural changes are not observed in the superlattice after annealing at 900 K in a vacuum. The value of |S |2D of the superlattice gradually increases with temperature and reaches 450 μV K–1 at 900 K, which is ∼3 times larger than that of bulk SrTi0.8Nb0.2O3. These observations provide clear evidence that the [(SrTiO3)17/(SrTi0.8Nb0.2O3)1]20 superlattice is stable and exhibits a giant |S | even at high temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical study on the reaction mechanism of Sr,Ti,O reactions in early growth of SrTiO3 thin films

It is very important to learn the microscopic reaction and particle states of Sr,Ti,O atoms which are evaporated by laser molecular beam epitaxy in the early growth of SrTiO3 thin films. The preferential reaction processes of Sr,Ti,O atoms were investigated by generalized gradient approximation of density functional theory at the PW91/DNP level. The reaction mechanisms of SrO,TiO2 and SrTiO3 molecules were calculated and analyzed by highest occupied and lowest unoccupied molecular orbital theory, and several possible configurations of SrTiO3 molecules were calculated. We obtained stable configurations of SrTiO3 molecule, of which the geometric eigenvalue is similar with the SrTiO3 unit cell. These results indicate that the SrO,TiO2 and SrTiO3 molecules are the primary particle states in the early growth of SrTiO3 thin film.

Enhanced Seebeck coefficient of quantum-confined electrons in SrTiO3∕SrTi0.8Nb0.2O3 superlattices

We report two-dimensional Seebeck coefficients (∣S∣2D) of [(SrTiO3)x∕(SrTi0.8Nb0.2O3)y]20 (x=1–60, y=1–20) superlattices, which were grown on the (100) face of insulating LaAlO3 substrates to clarify the origin of the giant ∣S∣2D values of the SrTiO3 superlattices [H. Ohta et al., Nat. Mater. 6, 129 (2007)]. The ∣S∣2D values of the [(SrTiO3)17∕(SrTi0.8Nb0.2O3)y]20 superlattices increased proportionally to y−0.5 and reached 320μVK−1 (y=1), which is approximately five times larger than that of the SrTi0.8Nb0.2O3 bulk (∣S∣3D=61μVK−1). The slope of the log∣S∣2D-logy plots was −0.5, proving that the density of states in the ground state for SrTiO3 increases inversely proportionally to y. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25nm (16 unit cells of SrTiO3).

X-ray absorption fine-structure determination of interfacial polarization in SrTiO3 thin films grown on Si(001)

Polarization-dependent x-ray absorption fine structure together with x-ray diffraction have been used to study the local structure in SrTiO3 thin films grown on Si(001). Our data indicate that an interfacial polarization of the SrTiO3 layer by the Si substrate results in a tetragonal distortion of the SrTiO3 unit cell.

Effect of Ultra-Thin SrTiO3 Seed Layer on the Microwave Surface Resistance of YBa2Cu3O7 Films Deposited on (1 0 0) MgO

RHEED controlled ultra-thin buffer layers of SrTiO3 have been deposited on (1 0 0) MgO by pulsed laser deposition to grow YBa2Cu3O7 films for microwave applications. A buffer layer with a thickness from 5 to 40 unit cells of SrTiO3 is sufficient to expand to more than 60°C the range of deposition temperatures for which a low microwave surface resistance [Rs(10 GHz, 77 K)<0.5 mΩ] is obtained. The Rs values are as low as those obtained on LaAlO3 substrates, furthermore they present a slightly lower magnetic field dependency. The XRD Φ-scans show that the SrTiO3 seed layer induces an oriented epitaxial growth with the [1 0 0] axis of YBCO parallel to the one of MgO over this broadened range of deposition temperatures. This seed layer promoting effect is not observed with other oxides such as Ba0.15Sr0.85TiO3, CeO2, Ce1−x La x O2.

Microstructure and microwave dielectric properties of epitaxial SrTiO3 films on LaAlO3 substrates

Sr-deficient and stoichiometric epitaxial (001) SrTiO3 films, deposited on (110)rhombohedral LaAlO3 substrates by rf magnetron sputtering, have been characterized using high-resolution transmission electron microscopy. A subsequent heat treatment in oxygen had a positive influence on the dielectric properties. Sr-deficiency had a large negative impact on the microwave dielectric constant of the films. These changes were correlated to changes in lattice parameters. In all samples, at the film/substrate interface, were misfit dislocations present. The residual elastic strain compressed the SrTiO3 unit cell in the substrate surface plane and expanded it an equal amount in the [001] direction. X-ray diffraction revealed that the tetragonal distortion, due to the mismatch strain, was concentrated to a narrow region closest to the film/substrate interface.

Microstructure of SrTiO3 Thin Films as Single Layer and Incorporated in Yba2Cu3O7-x/SrTiO3 Multilayers

AbstractEpitaxial single layer (001) SrTiO3 films and an epitaxial Yba2Cu3O7-x/SrTiO3 multilayer were dc and rf sputtered on (110)rhombohedral LaAIO3 substrates. The microstructure of the films was characterised using transmission electron microscopy. The single layer SrTiO3 films exhibited different columnar morphologies. The column boundaries were due to the lattice mismatch between film and substrate. The boundaries were associated with interfacial dislocations at the film/substrate interface, where the dislocations relaxed the strain in the a, b plane. The columns consisted of individual subgrains. These subgrains were misoriented with respect to each other, with different in-plane orientations and different tilts of the (001) planes. The subgrain boundaries were antiphase or tilt boundaries.The individual layers of the Yba2Cu3O7-x/SrTiO3 multilayer were relatively uniform. A distortion of the SrTiO3 unit cell of 0.9% in the ‘001’ direction and a Sr/Ti ratio of 0.62±0.04 was observed, both in correspondence with the single layer SrTiO3 films. Areas with different tilt of the (001)-planes were also present, within each individual SrTiO3 layer.