Dielectric SrTiO3 Research Articles

Side-jump scattering enhanced spin Hall effect in SrTiO3-implanted Pt

A spin Hall effect (SHE) enables the electrical generation and detection of spin currents for promising applications in spintronics, but heavy metals with low spin Hall angle θSH limit the development of SHE devices. In this work, we have introduced dielectric oxide material SrTiO3 into Pt by magnetron sputtering and measured the θSH on the NiCo/Pt1–x(STO)x heterostructure through spin-torque ferromagnetic resonance. Our results demonstrate that the maximum spin Hall angle in Pt0.98(STO)0.02 is 0.121 ± 0.003, which is approximately twice that of pure Pt (0.064 ± 0.003). Moreover, theoretical analysis has revealed that the spin Hall angle arises from a complementary interplay between intrinsic and extrinsic mechanisms, namely, the strong spin–orbit coupling in Pt for the intrinsic mechanism and side-jump scattering caused by scalar potential and lattice expansion at dielectric impurities for the extrinsic mechanism. This interplay significantly contributes to enhancing the spin Hall angle. This work demonstrates an effective strategy for fabricating high-performance spin Hall materials with low resistivity, large spin Hall angle, and excellent compatibility with semiconductor processes in low-power spin-torque devices.

Enhanced energy storage performance of (Pb, La)(Zr, Ti)O3/SrTiO3 multilayer films via a synergistic strategy

Dielectric film capacitors possess a larger dielectric breakdown field than that of their bulk ceramic counterparts, thus showing a better energy density. Herein, a synergistic strategy has been employed to enhance the energy storage density of (Pb, La)(Zr, Ti)O3/SrTiO3 multilayer films through combing the effects of atomic doping, heat treatment, and multilayer stacking. La partially substituting Pb improves relaxor characteristics of Pb(Zr, Ti)O3. A dense amorphous microstructure results in a relatively high breakdown electric field at a low annealing temperature. Additionally, the insertion of dielectric layer SrTiO3 into (Pb, La)(Zr, Ti)O3 suppresses the carriers transport due to the interface role and further improves the insulating strength and breakdown strength. Thus, a high recoverable energy density of 67.1 J/cm3 is achieved at an ultrahigh breakdown electric field of 7.45 MV/cm. Such a synergistic multiscale approach in this work is universally effective to improve the energy storage performance of dielectric multilayer films.

Coexistence of surface oxygen vacancy and interface conducting states in LaAlO3/SrTiO3 revealed by grazing-angle resonant soft x-ray scattering

Oxide heterostructures have shown rich physics phenomena, particularly in the conjunction of exotic insulator–metal transition (IMT) at the interface between polar insulator LaAlO3 and non-polar insulator SrTiO3 (LaAlO3/SrTiO3). The polarization catastrophe model has suggested an electronic reconstruction, yielding to metallicity at both the interface and surface. Another scenario is the occurrence of surface oxygen vacancy at LaAlO3 (surface-Ov), which has predicted surface-to-interface charge transfer, yielding metallic interface but insulating surface. To clarify the origin of IMT, one should probe surface-Ov and the associated electronic structures at both the surface and the buried interface simultaneously. Here, using grazing-angle resonant soft x-ray scattering (GA-RSXS) supported with first-principles calculations, we reveal the co-existence of the surface-Ov state and the interface conducting state only in conducting LaAlO3/SrTiO3 (001) films. Interestingly, both the surface-Ov state and the interface conducting state are absent for the insulating film. As a function of Ov density, while the surface-Ov state is responsible for the IMT, the spatial charge distribution is found responsible for a transition from two-dimensional-like to three-dimensional-like conductivity accompanied by spectral weight transfer, revealing the importance of electronic correlation. Our results show the importance of surface-Ov in determining interface properties and provide a new strategy in utilizing GA-RSXS to directly probe the surface and buried interface electronic properties in complex oxide heterostructures.

Stoichiometric growth of SrTiO3 films via Bayesian optimization with adaptive prior mean

Perovskite insulator SrTiO3 (STO) is expected to be applied to the next generation of electronic and photonic devices as high-k capacitors and photocatalysts. However, reproducible growth of highly insulating stoichiometric (STO) films remains challenging due to the difficulty of precise stoichiometry control in perovskite oxide films. Here, to grow stoichiometric (STO) thin films by fine-tuning multiple growth conditions, we developed a new Bayesian optimization (BO)-based machine learning method that encourages exploration of the search space by varying the prior mean to get out of suboptimal growth condition parameters. Using simulated data, we demonstrate the efficacy of the new BO method, which reproducibly reaches the global best conditions. With the BO method implemented in machine-learning-assisted molecular beam epitaxy (ML-MBE), a highly insulating stoichiometric (STO) film with no absorption in the bandgap was developed in only 44 MBE growth runs. The proposed algorithm provides an efficient experimental design platform that is not as dependent on the experience of individual researchers and will accelerate not only oxide electronics but also various material syntheses.

Effect of Light and Electrostatic Gate at Oxide Interface LaFeO3–SrTiO3 at Room Temperature

AbstractElectrostatic gating and light illumination are two widely used stimuli for semiconductor devices. Two‐dimensional electron gas (2DEG) at oxide heterostructures has shown potential in optoelectronics due to its high optical response and gate tuning property. The appearance of high photoconductivity and persistent photoconductivity at the oxide heterostructure of two insulators LaFeO3 and SrTiO3 is shown here. The photoconductivity has been further tuned using positive and negative back gating. A large change in conductivity has been achieved under the simultaneous application of light and electrostatic gating. A few measurement protocols that manifest possible applications of this interface as memory and switching devices are implemented.

Synthesis of freestanding perovskite oxide thin films by using brownmillerite SrCoO2.5 as a sacrificial layer

Freestanding perovskite thin films display many unprecedented properties and exhibit the potential to be easily integrated on other non-oxide substrates or layers. In this work, we demonstrated a pathway to synthesis freestanding perovskite oxide thin films by using brownmillerite SrCoO2.5 as a sacrificial layer. Four representative freestanding perovskite oxide films, e.g., ferromagnetic SrRuO3, La0.7Sr0.3MnO3, dielectric SrTiO3, and ferroelectric Pb(Zr0.2Ti0.8)O3, were produced by etching SrCoO2.5 in Fe(NO3)3 weak acidic solution at room temperature. A 80 nm SrRuO3, which served as an H+ conduction channel, was deposited as a bottom layer of SrCoO2.5 to trigger a quick dissolution for the exfoliation of SrTiO3 and Pb(Zr0.2Ti0.8)O3 poor H+ conductor. Their crystal structure and physical properties were well retained in transferred films. Our work demonstrated the wide applicability of SrCoO2.5 as a sacrificial layer on the synthesis of freestanding perovskite oxide thin films.

Energy Storage Properties of Sol-Gel-Processed SrTiO3 Films.

Dielectric films with a high energy storage density and a large breakdown strength are promising material candidates for pulsed power electrical and electronic applications. Perovskite-type dielectric SrTiO3 (STO) has demonstrated interesting properties desirable for capacitive energy storage, including a high dielectric constant, a wide bandgap and a size-induced paraelectric-to-ferroelectric transition. To pave a way toward large-scale production, STO film capacitors were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by the sol-gel method in this paper, and their electrical properties including the energy storage performance were studied as a function of the annealing temperature in the postgrowth rapid thermal annealing (RTA) process. The appearance of a ferroelectric phase at a high annealing temperature of 750 °C was revealed by X-ray diffraction and electrical characterizations (ferroelectric P-E loop). However, this high dielectric constant phase came at the cost of a low breakdown strength and a large hysteresis loss, which are not desirable for the energy storage application. On the other hand, when the RTA process was performed at a low temperature of 550 °C, a poorly crystallized perovskite phase together with a substantial amount of impurity phases appeared, resulting in a low breakdown strength as well as a very low dielectric constant. It is revealed that the best energy storage performance, which corresponds to a large breakdown strength and a medium dielectric constant, is achieved in STO films annealed at 650 °C, which showed a large energy density of 55 J/cm3 and an outstanding energy efficiency of 94.7% (@ 6.5 MV/cm). These findings lay out the foundation for processing high-quality STO film capacitors via the manufacturing-friendly sol-gel method.

Synergistically enhanced electrical transport properties of SrTiO3via Fermi level regulation and modulation doping

The thermoelectric performance of the insulator SrTiO3 was improved through synergistically enhanced electrical transport properties via Fermi level regulation and modulation doping.

Resonant tunneling driven metal-insulator transition in double quantum-well structures of strongly correlated oxide

The metal-insulator transition (MIT), a fascinating phenomenon occurring in some strongly correlated materials, is of central interest in modern condensed-matter physics. Controlling the MIT by external stimuli is a key technological goal for applications in future electronic devices. However, the standard control by means of the field effect, which works extremely well for semiconductor transistors, faces severe difficulties when applied to the MIT. Hence, a radically different approach is needed. Here, we report an MIT induced by resonant tunneling (RT) in double quantum well (QW) structures of strongly correlated oxides. In our structures, two layers of the strongly correlated conductive oxide SrVO3 (SVO) sandwich a barrier layer of the band insulator SrTiO3. The top QW is a marginal Mott-insulating SVO layer, while the bottom QW is a metallic SVO layer. Angle-resolved photoemission spectroscopy experiments reveal that the top QW layer becomes metallized when the thickness of the tunneling barrier layer is reduced. An analysis based on band structure calculations indicates that RT between the quantized states of the double QW induces the MIT. Our work opens avenues for realizing the Mott-transistor based on the wave-function engineering of strongly correlated electrons.

Advances in strain engineering on oxide heteroepitaxy

Advances in strain engineering on oxide heteroepitaxy

Hard x-ray photoemission spectroscopy of LaVO3/SrTiO3 : Band alignment and electronic reconstruction

The heterostructure consisting of the Mott insulator LaVO$_3$ and the band insulator SrTiO$_3$ is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of LaVO$_3$ match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the non-polar/polar interface when integrated with SrTiO$_3$, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition is scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of soft and hard X-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the electrical properties, band bending, and band alignment of LaVO$_3$/SrTiO$_3$ heterostructures. We find a critical LaVO$_3$ thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3$d$ states at the interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as the driving mechanism for the formation of the metallic interface in LaVO$_3$/SrTiO$_3$.

Controlling the magnetoelectric coupling in Ni/BiFe0.95Mn0.05O3 with a dielectric spacer

In artificial multiferroic heterostructures combining ferromagnetic transition metal films and bismuth ferrite layers the electric field control of the magnetic state is generally achieved through interface exchange coupling. Here we report a reversible and non-volatile magnetoelectric (ME) effect in Ni/ BiFe0.95Mn0.05O3 bilayers originating primarily from a strain-transfer mechanism. The non-volatility of the ME effect is explained by an asymmetric piezoelectric strain response of the ferroelectric BiFe0.95Mn0.05O3 film with respect to the applied electric field, yielding two different states at remanence. We further show that the piezoelectric properties and the ferroelectric domain structure of the BiFe0.95Mn0.05O3 film can be controlled by tuning the screening of the polarization charge at the interface through an interleaved dielectric SrTiO3 spacer. As the dielectric spacer thickness is decreased, the strain-mediated ME response evolves from the typical butterfly-like hysteresis loop towards a rectangular shape, thereby making such artificial multiferroics suitable for non-volatile memory applications.

Leakage current characteristics of SrTiO3/LaNiO3/Ba0.67Sr0.33TiO3/SrTiO3 heterostructure thin films

There is an urgent demand to explore new approaches of improving the electric breakdown strength of the ferroelectric thin-film capacitor without the degradation of the high dielectric constant. In this work, LaNiO3/Ba0.67Sr0.33TiO3 (LNO/BST) thin film and SrTiO3(STO)/LNO/BST/SrTiO3 (STO) were prepared by using radio frequency (RF) magnetron sputtering technique and ultrathin STO insulator layers were inserted between the LNO/BST and metal electrodes (Au or Pt) to improve the electric breakdown strength and the leakage current of the LNO/BST thin film. X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations revealed that these multilayer thin films show compact, smooth and uniform morphologies. The leakage current density of STO/LNO/BST/STO thin film was decreased by one order of magnitude and breakdown strength of STO/LNO/BST/STO thin film was enhanced from 0.72 to 1.26 MV·cm−1 compared with that of LNO/BST thin film. Moreover, the dielectric constant of the STO/LNO/BST/STO thin film keeps at the same level as that of LNO/BST thin film, and dielectric loss of the STO/LNO/BST/STO thin film was decreased slightly compared with that of LNO/BST thin film.

Colossal flexoresistance in dielectrics

Dielectrics have long been considered as unsuitable for pure electrical switches; under weak electric fields, they show extremely low conductivity, whereas under strong fields, they suffer from irreversible damage. Here, we show that flexoelectricity enables damage-free exposure of dielectrics to strong electric fields, leading to reversible switching between electrical states—insulating and conducting. Applying strain gradients with an atomic force microscope tip polarizes an ultrathin film of an archetypal dielectric SrTiO3 via flexoelectricity, which in turn generates non-destructive, strong electrostatic fields. When the applied strain gradient exceeds a certain value, SrTiO3 suddenly becomes highly conductive, yielding at least around a 108-fold decrease in room-temperature resistivity. We explain this phenomenon, which we call the colossal flexoresistance, based on the abrupt increase in the tunneling conductance of ultrathin SrTiO3 under strain gradients. Our work extends the scope of electrical control in solids, and inspires further exploration of dielectric responses to strong electromechanical fields.

Growing a LaAlO3/SrTiO3 heterostructure on Ca2Nb3O10 nanosheets

The two-dimensional electron liquid which forms between the band insulators LaAlO3 (LAO) and SrTiO3 (STO) is a promising component for oxide electronics, but the requirement of using single crystal SrTiO3 substrates for the growth limits its applications in terms of device fabrication. It is therefore important to find ways to deposit these materials on other substrates, preferably Si, or Si-based, in order to facilitate integration with existing technology. Interesting candidates are micron-sized nanosheets of Ca2Nb3O10 which can be used as seed layers for perovskite materials on any substrate. We have used low-energy electron microscopy (LEEM) with in-situ pulsed laser deposition to study the subsequent growth of STO and LAO on such flakes which were deposited on Si. We can follow the morphology and crystallinity of the layers during growth, as well as fingerprint their electronic properties with angle resolved reflected electron spectroscopy. We find that STO layers, deposited on the nanosheets, can be made crystalline and flat; that LAO can be grown in a layer-by-layer fashion; and that the full heterostructure shows the signature of the formation of a conducting interface.

Magnetism and Conductivity Along Structural Domain Walls of SrTiO3

The interface between the oxide insulators LaAlO3 and SrTiO3 (LAO/STO) hosts a two-dimensional electron gas. The combination of interfacial conductivity and superconductivity at ultra-low temperatures with the physical phenomena of the oxide parent materials has fueled extensive research in the field since its discovery in 2004. Scanning superconducting quantum interference device (SQUID) measurements have shown that structural domain walls, formed below 105 K, modulate the current flow at the interface and recently revealed weak magnetic signals along the same domain structure. Here we use scanning SQUID to investigate the temperature dependence of different electronic properties of the LAO/STO interface. We find correlation between magnetism and conductivity, which are both spatially modulated on the domain structure. This data suggests a possible relation between the populations of electrons participating in each order.

Tuning the superconducting transition of SrTiO3-based 2DEGs with light

The resistivity of the two dimensional electron gas that forms at the interface of strontium titanate with various oxides is sensitive to irradiation from visible light. In this letter, we present data on the interface between the bandgap insulators LaAlO3 (LAO) and SrTiO3 (STO). We operate a light emitting diode at temperatures below 1 K and utilize it to irradiate the LAO/STO interface at ultralow temperatures. On irradiation, the resistance of this system is lowered continuously by a factor of five and the resistance change is persistent at low temperatures as long as the sample is kept in the dark. This makes the characterization of transport properties in different resistive states over extended time periods possible. Our pristine sample gets superconductive below 265 mK. The transition temperature Tc shifts downward on the persistent photo-induced lowering of the resistance. The persistent photoconductance can be completely reverted by heating the structure above 10 K, in which case, Tc takes on its original value. Thus, very similar to field-effect induced changes of the electron density, irradiation at low temperatures offers a versatile tuning knob for the superconducting state of STO-based interfaces which in addition has the advantage to be nonvolatile.

Thermoelectric phase diagram of the SrTiO3-LaTiO3 solid-solution system through a metal to Mott insulator transition

Systematic clarification of thermoelectric properties of a solid-solution system is essentially important, especially if the materials have different crystal and electronic structures. Here, we report the thermoelectric phase diagram of the full-range Sr1 − xLaxTiO3 solid solutions composed of a band insulator SrTiO3 (cubic perovskite) and a Mott insulator LaTiO3 (distorted perovskite), which shows the room temperature thermoelectric power factor changing pattern with the La substitution (x) in the SrTiO3 system. We measured the room temperature thermoelectric properties of Sr1 − xLaxTiO3 (0.01 ≤ x ≤ 1) epitaxial films grown by the pulsed laser deposition and found that there are two thermoelectric phase boundaries at x ∼ 0.5 and x ∼ 0.7 in the Sr1 − xLaxTiO3 system. The former is attributed to the lattice distortion from cubic to orthorhombic variation, whereas the latter is due to the electronic phase transition from metal to Mott insulator. The thermoelectric phase diagram obtained in our study will be fruitful for the future design of n-type thermoelectric materials based on the electron-doped SrTiO3.

Structural and electronic properties of heterointerface composed of non-polar oxides: SrTiO3 and ferroelectric BaTiO3

We investigate the structural and electronic properties of heterostructure composed of two insulators: ferroelectric BaTiO3 and dielectric SrTiO3, by means of ab initio calculations. Both components of the heterostructure are non-polar in the simple ionic limit. We consider three types of BaTiO3 polarization: non-polar, dipoles towards the interface, and dipoles towards the surface. For the non-polar heterostructure, we obtain that system remains an insulator up to eight BaTiO3 overlayers. However, we show that polarization in the BTO slab induces conductivity. We analyze an impact of ferroelectric polarization onto conducting properties, as well as a possibility of switchable and controllable metal-insulator transition.

Cladding effects on Wave propagation in Dielectric Slab waveguide.

Microwave with frequencies (12-18) GHz propagation in dielectric material SrTiO3 (refractive index n1=2.41) with two different cladding, air(n2=1) and polyacrylate(n2=1.5), were investigated. Better results had been found with cladding polyacrylate including increasing in the group velocity vg for the fundamental mode by nearly 0.2%. Behavior of electromagnetic wave propagation along z-direction for different cladding was investigated graphically and numerically with the aid of MATLAB program. The graphically and numerically results are coincide. The modes of propagation have been analyzed and all parameters have been given. The transmission of microwave depends on the slab thickness and the frequencies applied and has been found to varied from 75% to 100%. The reflection amplitude inside the slab does not affected when the angle of the incident wave is less than the critical angle, while it increased rapidly as the angle increased higher than the critical angle.