SrTiO3 Films Research Articles

Effect of deposition temperature on ultra-low voltage resistive switching behavior of Fe-doped SrTiO3 films

The effect of deposition temperature on the microstructures and resistive switching properties of Fe-doped SrTiO3 (Fe-STO) films deposited via magnetron sputtering has been investigated. The as-deposited Fe-STO films change from amorphous to polycrystalline when the deposition temperature increases to 600 °C, but 800 °C-deposited Fe-STO films exhibit cracked surface morphologies with Sr-rich nanosheet segregation. Fe-STO films deposited at ≤600 °C exhibit reversible bipolar resistive switching behaviors with ultra-low switching voltages of <±0.6 V, while 450 °C-deposited Fe-STO films retain an ON/OFF resistance ratio of ∼105 after more than 2500 endurance cycles and 600 °C-deposited Fe-STO films exhibit three different resistive switching patterns in sequence. Fe-assisted oxygen-vacancy conductive filaments are responsible for the ultra-low voltage resistive switching behaviors of Fe-STO films.

Epitaxial optimization of atomically smooth Sr3Al2O6 for freestanding perovskite films by molecular beam epitaxy

The epitaxial crystal quality of strontium aluminate (Sr3Al2O6) films under various Sr/Al ratios were systematically investigated by reactive molecular beam epitaxy. Efficient guiding rules for real-time optimization are summarized that a four-fold reconstructed reflection high-energy electron diffraction (RHEED) pattern and 4 periods of RHEED oscillations coinciding in the growth of one unit cell of Sr3Al2O6 is the key signature for the optimal growth condition. Following above rules, atomically smooth Sr3Al2O6 and freestanding SrTiO3 films with a full width at half maximum less than 0.03∘ (mainly limited by the substrates) were synthesized. The high-crystalline quality of freestanding SrTiO3 and atomically smooth interface between SrTiO3 film and Sr3Al2O6 were highlighted by the appearance of well-defined fringes from X-ray diffraction data and well-organized atomic distribution from electron microscopy. The epitaxial optimization of Sr3Al2O6 buffer layer with atomic flatness and high-crystalline quality will shed light on the synthesis of ultrathin freestanding oxide perovskite films, paving the way to the exploration of incorporating strongly correlated properties in conventional semiconductors for a generation of multifunctional electronic devices.

Toward Multifunctional Electronics: Flexible NBT-Based Film with a Large Electrocaloric Effect and High Energy Storage Property.

Advances in smart and wearable devices are driving innovations in multifunctional flexible materials at a tremendous pace. Here, drawing support from the unique flexible fluorophlogopite mica platform, we present a promising all-inorganic bendable Mn-modified 0.65(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-0.35SrTiO3 (NBBST) film with dual use in electrocaloric (EC) refrigeration and energy storage via a cost-effective transfer-free process. An appreciable room-temperature EC effect with adiabatic temperature change of 12 K and isothermal entropy of 18 J K-1 kg-1 was realized in the NBBST film, which benefits from the large change in dipolar ordering near depolarization temperature. Also, the film exhibits a broad operating temperature span over 25 °C because of its relaxor feature. Most importantly, the film can maintain a high EC performance either under bending deformation at 5 mm radius or after undergoing 104 bending-unbending cycles. Meanwhile, the flexible NBBST film possesses good energy storage property with a recoverable energy density of 56 J cm-3 and an efficiency of 66%. This is the first example of a lead-free all-inorganic multifunctional film capacitor toward the flexible EC refrigeration and energy storage devices. This work shows bright prospects in the emerging flexible e-market.

Epitaxial integration of ferroelectric and conductive perovskites on silicon

BaTiO3 (BTO) and LaxSr1 − xTiO3 (x ≤ 0.15) perovskite heterostructures are deposited epitaxially on SrTiO3 (STO)-buffered Si(001) via atomic layer deposition (ALD) to explore the formation of a quantum metal layer between a ferroelectric film and silicon. X-ray diffraction and scanning transmission electron microscopy show the crystallinity of the heterostructure deposited by ALD. After postdeposition annealing of the La-doped STO film in ultrahigh vacuum at 600 °C for 5 min, x-ray photoelectron spectra show the lack of La-dopant activation when the film is deposited on 10 nm-thick BTO. The same postdeposition annealing condition activates the La-dopant when LaxSr1 − xTiO3 films are deposited on STO-buffered Si(001) surfaces consisting of 2.8 nm of STO(001) on Si(001). Annealing of LaxSr1 − xTiO3 films sandwiched between BTO and STO-buffered Si(001) layers in air at temperatures ≤350 °C preserves the La-dopant activation. Piezoresponse force microscopy demonstrates the ferroelectric behavior of BTO films grown on LaxSr1 − xTiO3 surfaces. Sheet resistance and capacitance-voltage measurements further demonstrate the conductivity of the LaxSr1 − xTiO3 films sandwiched between the BTO film and the Si(001) substrate.

Engineering nanoscale polarization at the SrTiO3/Ge interface

The ability to manipulate electric polarization at reduced dimension has recently generated intense research interests. Here, we report the engineering of interfacial ferroelectricity by the monolithic integration of a single crystalline SrTiO3 (STO) film on a germanium Ge(001) substrate. A combination of nonlinear optical measurements, microstructural analyses, and phase-field simulations provide evidences of room-temperature ferroelectricity in the epitaxial STO ultrathin film. We show that the collective atomic displacements of Ti near the STO/Ge interface are responsible for the generation of an in-plane ferroelectric polarization. An a1/a2 domain pattern with a stable net polarization along the [110] in-plane direction is revealed.

In situ anion-doped epitaxial strontium titanate films.

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.

Investigation of Rh–titanate (ATiO3) interactions on high-surface-area perovskite thin films prepared by atomic layer deposition

Thin, ∼1 nm films of CaTiO3, SrTiO3, and BaTiO3 were deposited onto MgAl2O4 by Atomic Layer Deposition (ALD) and studied as catalyst supports for Rh.

Atomic Scale Understanding of the Epitaxy of Perovskite Oxides on Flexible Mica Substrate

AbstractThe excellent functionalities of perovskite oxides and the growing demands for flexible devices lead to great interests on epitaxial growth of functional oxide films on flexible mica substrates. Understanding the film epitaxy on the substrate with a very different crystal structure is a key issue for the optimization of the film growth and hence properties. Such understanding largely depends on knowing the atomic structure of the interfaces between the films and the substrates. Here, the interface between the epitaxial films of SrTiO3 on the fluorophlogopite mica substrate is studied in detail. Two types of interfaces, clean or with secondary phase, exist in this system, leading to two types of crystallographic orientation relationships. Atomic‐resolution scanning transmission electron microscopy images reveal that at the clean interface the (111) Sr–O3 atomic plane of SrTiO3 interacts with the (001) (SiAl)2–O3 plane of mica. This interface structure and thus the epitaxy of the film are understood in light of the strong similarity of the oxygen sublattices in these two atomic planes. First‐principles calculations demonstrate strong bonding of the atoms at the interface, which is also corroborated by the observation of misfit dislocations at the interfaces.

Aluminum doping for optimization of ultrathin and high-k dielectric layer based on SrTiO3

An ultrathin SrTiO3 dielectric layer is optimized through Al doping to solve the problems existing in development of ultra-high-k oxide MOS capacitors. Through post-deposition annealing, Al doping induces changes in the electronic structure of SrTiO3, thereby effectively reducing leakage current to <10−8 A/cm2 at 0.5 MV/cm but maintains good capacitance values (ε > 80) of ultrathin SrTiO3 MOS capacitors.Strontium titanate (SrTiO3) is a high-k material but its bandgap is smaller than that of other oxide dielectrics (e.g., SiO2, Al2O3). Consequently, an ultrathin SrTiO3 film may have a high tunneling leakage current, which is not suitable for capacitor-based applications. To improve the performance of metal–oxide–semiconductor (MOS) capacitors using SrTiO3, an approach based on homogenous and uniform aluminum doping to SrTiO3 through co-sputtering is introduced. The bandgap of a pristine SrTiO3 film showed an increase of 0.5 eV after Al doping. Furthermore, Al doping decreased the leakage current of SrTiO3/Si-based MOS capacitors by more than five orders of magnitude (at the level of nanoampere per square centimeter). Importantly, a dielectric constant of 81.3 and equivalent oxide thickness less than 5 Å were achieved in an 8-nm-thick Al-doped SrTiO3 film owing to changes in its crystal structure and conduction band edge electronic structure. Thus, the obtained data show the effectiveness of the proposed approach for solving the problems existing in the development of ultra-high-k oxide MOS capacitors.

Modified transverse Ising model for the dielectric properties of SrTiO3 films and interfaces

The transverse Ising model (TIM), with pseudospins representing the lattice polarization, is often used as a simple description of ferroelectric materials. However, we demonstrate that the TIM, as it is usually formulated, provides an incorrect description of SrTiO3 films and interfaces because of its inadequate treatment of spatial inhomogeneity. We correct this deficiency by adding a pseudospin anisotropy to the model. We demonstrate the physical need for this term by comparison of the TIM to a typical Landau–Ginzburg–Devonshire model. We then demonstrate the physical consequences of the modification for two model systems: a ferroelectric thin film, and a metallic LaAlO3/SrTiO3 interface. We show that, in both cases, the modified TIM has a substantially different polarization profile than the conventional TIM. In particular, at low temperatures the formation of quantized states at LaAlO3/SrTiO3 interfaces only occurs in the modified TIM.

Non-synchronized rotation of layered spin configurations in La0.825Sr0.175MnO3 /SrTiO3 film

Magnetic properties of single perovskite-structure epitaxial film are reported to have layered distribution recently. However, the different responses of spin configurations in individual layers to the variations of temperature and strain state, especially the subtle response of layered film due to structural phase transformation of SrTiO3 (STO) substrate around 105 K, have not been revealed completely. Drastic drop and concomitant abnormal increase of remnant magnetic moments (REM) and coercivity of low-doped La1-xSrxMnO3 (x = 0.175) /SrTiO3 film were observed around 105 K only in the in-plane direction, and layered magnetic structures were further inferred based on investigation of microstructure, strain distribution and chemical inhomogeneity. Abnormal change of magnetic properties around 105 K was discussed by a three-layer model, in which softer ferromagnetic layer was supposed to form in the middle layer of strain-induced layered structure, and the spin configuration of middle layer underwent non-synchronous transformation relative to other parts of the film, which may be attributed to the change of in-plane strain inside the film around 105 K. Our work reveals the variation of individual magnetic layers with the increase of temperature through utilizing the remnant magnetic field in measuring system, and the abrupt reversal of spin configurations at the intermediate layer can also serve the design of novel spintronics devices in the future.

Preparation and Photocatalytic Properties of Graphene/SrTiO&lt;sub&gt;3&lt;/sub&gt; Thin Film Catalyst

A novel graphene-bridged SrTiO3 (STO) thin film loaded on the glass was fabricated using a facile sol-gel method followed by the dip-coating and spin-coating method. As-prepared STO film and GO/STO film were tested by XRD, SEM to identify the composition and surface morphology. The as-prepared thin film catalyst was employed to degrade methylene blue (MB) in water under near ultraviolet ray irradiation. Compared to pure STO, GO/STO prepared by dip-coating method displayed good photocatalytic degradation efficiency with 58% removal of MB (60 mL, 5mg/L) in 6.0 h, the GO/STO prepared by spin-coating method displayed more superior degradation efficiency with 94% removal of MB (60 mL,5mg/L) in 6.0 h. The influences of MB volume and the different methods were also investigated in details. The enhanced photocatalytic activities could be attributed to the suppression of charge recombination, high specific surface area and great adsorption capability of GO/STO. This work provides a promising approach to construct novel film GO/STO with high stability and photodegradation efficiency that can be applied in efficient treatment pollutants in wastewater.

Leakage Current Control of SrTiO3 Thin Films through Al Doping at the Interface between Dielectric and Electrode Layers via Atomic Layer Deposition

Al is doped to a SrTiO3 (STO) thin film grown by atomic layer deposition (ALD) to decrease the leakage current. One ALD cycle of Al2O3 is processed either at the top or at the bottom of the STO films, and the electrical properties are compared with those of the undoped STO films. The ≈3.5 nm‐thick seed layer is first deposited and crystallized through rapid thermal annealing, and from 5 to 20 nm‐thick main layers are subsequently grown for in situ crystallization of the main layer. When an Al2O3 deposition cycle is inserted below the first seed layer, the crystallization of the STO films is disturbed, and the bulk dielectric constant degrades from 149 (undoped STO) to 71. When the Al2O3 deposition cycle is performed after the STO main layer growth, however, the dielectric constant degradation is minimized (120). In both cases, the leakage current decreases 20 times compared with the undoped STO case because of the conduction band offset increase. Consequently, the equivalent oxide thickness and physical oxide thickness decrease from 0.71 to 0.63 nm and from 10.3 to 8.6 nm, respectively, through Al doping on the top of the STO films with a sufficiently low leakage current (&lt;10−7 A cm−2).

Dichotomy of the photo-induced 2-dimensional electron gas on SrTiO3 surface terminations

Oxide materials are important candidates for the next generation of electronics due to a wide array of desired properties, which they can exhibit alone or when combined with other materials. While SrTiO3 (STO) is often considered a prototypical oxide, it, too, hosts a wide array of unusual properties, including a 2-dimensional electron gas (2DEG), which can form at the surface when exposed to ultraviolet (UV) light. Using layer-by-layer growth of high-quality STO films, we show that the 2DEG only forms with the SrO termination and not with the TiO2 termination, contrary to expectation. This dichotomy of the observed angle-resolved photoemission spectroscopy (ARPES) spectra is similarly seen in BaTiO3 (BTO), in which the 2DEG is only observed for BaO-terminated films. These results will allow for a deeper understanding and better control of the electronic structure of titanate films, substrates, and heterostructures.

Response of the Dielectric Parameters of (110)SrTiO3 Films to the Formation of Ferroelectric Domains in Their Volume

Three-layer SrRuO3/SrTiO3/SrRuO3 heterostructures were grown by laser evaporation on (110)LaAlO3 substrates. Photolithography and ion etching are used to form plane-parallel film capacitors, in which a layer of strontium titanate is placed between two film electrodes of strontium ruthenate. Data on the structure and orientation of the intermediate SrTiO3 layer in the grown heterostructures were obtained. The variation of the dielectric constant and dielectric loss of the SrTiO3 intermediate layer upon varying the temperature and intensity of an external electric field was studied.

Synaptic-like conductivity and plasticity in epitaxially strained SrTiO3 films

In this work, we use epitaxial strain and an asymmetric electrode design to engineer the conductivity of SrTiO3 thin films in order to use them as active components in planar artificial synaptic devices. First, the tensile strain imposed by the rare-earth scandate substrate on epitaxial grown SrTiO3 films results in a significant increase of the conductivity of the SrTiO3. Second, a further enhancement of the conductivity is obtained by the use of Ti/Pt electrodes. Finally, the asymmetric electrode design consisting of a flat and a tapered electrode ensures the asymmetric response and plasticity of electronic synapse. The modifications of the conductivity are explained in terms of changes in the density and mobility of oxygen vacancies. The resulting electronic synapses (e-synapse) show memristor behavior and the plasticity of the signal, which are both essential characteristics of a synapse. Similar to the synaptic long-term and short-term potentiation/depression, our SrTiO3 e-synapses show two different types of plasticity, a fast process associated with the ionic dipole formation (relaxation time in the 100 ps regime) and a slow process defined by the mobility of oxygen vacancies (relaxation time of several seconds).

Bipolar resistive switching characteristics of amorphous SrTiO3 thin films prepared by the sol-gel process

ABSTRACTAmorphous SrTiO3 thin films grown on fluorine-doped tin oxide (FTO) glass were fabricated via the sol-gel route and coating process. The composition and chemical state of the thin films were studied by X-ray photoelectron spectroscopy. A high switching ratio and good endurance were demonstrated in the Au/amorphous SrTiO3/FTO/glass memory cells, with an ability to achieve a ratio of high and low resistance (Roff/Ron) of 102. A stable switching voltage and uniform resistance states could be identified, moreover, using standard Weibull distribution. The results showed that Ohmic and space charge limited conduction mechanisms coexisted in the amorphous SrTiO3 thin films. Ohmic conduction dominated in the initial high- and low-resistance state, but the space charge limited conduction mechanism was dominant in the later high-resistance field. The resistance switching effect in the device was explained by the formation and rupture of oxygen vacancies interrelated with filaments. These amorphous SrTiO3 films have potential resistive memory applications.

Freestanding crystalline oxide perovskites down to the monolayer limit.

Two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides reveal the electronic phases that emerge when a bulk crystal is reduced to a monolayer1-4. Transition-metal oxide perovskites host a variety of correlated electronic phases5-12, so similar behaviour in monolayer materials based on transition-metal oxide perovskites would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored. Here we report the fabrication of freestanding perovskite films with high crystalline quality almost down to a single unit cell. Using a recently developed method based on water-soluble Sr3Al2O6 as the sacrificial buffer layer13,14 we synthesize freestanding SrTiO3 and BiFeO3 ultrathin films by reactive molecular beam epitaxy and transfer them to diverse substrates, in particular crystalline silicon wafers and holey carbon films. We find that freestanding BiFeO3 films exhibit unexpected and giant tetragonality and polarization when approaching the 2D limit. Our results demonstrate the absence of a critical thickness for stabilizing the crystalline order in the freestanding ultrathin oxide films. The ability to synthesize and transfer crystalline freestanding perovskite films without any thickness limitation onto any desired substrate creates opportunities for research into 2D correlated phases and interfacial phenomena that have not previously been technically possible.

RF magnetron sputter deposition and electrical properties of La and Y doped SrTiO3 epitaxial films

La and Y doped SrTiO3 (STO) thin films were grown on (100) oriented pure STO single crystal substrates by the RF magnetron sputter technique. The La/Y doped layer is intended for use as bottom electrode for epitaxial growth of various functional oxide films. The results showed that one of the crucial deposition parameters to achieve the epitaxial growth of La/Y doped STO films with a pure phase and good electrical conductivity was sputter power, which must exceed 120 W. The conductivity of the grown films increased with the increase in sputter power and the La0.1Sr0.9TiO3 (LSTO) films deposited at 200 W had the highest conductivity of 292 S/cm. This was ascribed to the increased percentage of Ti3+ ions in the sputtered films, which was confirmed by the X-ray photoelectron spectroscopy. LSTO films deposited at 130 W were (100) oriented but the increased sputter power promoted the growth of a second texture, i.e. (110). On the other hand, Y0.08Sr0.92TiO3 (YSTO) films were able to keep the unique (100) texture in the films deposited over a range of sputter powers up to 200 W, although the conductivity of YSTO was lower, which was 98.0 S/cm for the films sputtered at 200 W.

Unraveling the Origin and Mechanism of Nanofilament Formation in Polycrystalline SrTiO3 Resistive Switching Memories.

Three central themes in the study of the phenomenon of resistive switching are the nature of the conducting phase, why it forms, and how it forms. In this study, the answers to all three questions are provided by performing switching experiments in situ in a transmission electron microscope on thin films of the model system polycrystalline SrTiO3 . On the basis of high-resolution transmission electron microscopy, electron-energy-loss spectroscopy and in situ current-voltage measurements, the conducting phase is identified to be SrTi11 O20 . This phase is only observed at specific grain boundaries, and a Ruddlesden-Popper phase, Sr3 Ti2 O7 , is typically observed adjacent to the conducting phase. These results allow not only the proposal that filament formation in this system has a thermodynamic origin-it is driven by electrochemical polarization and the local oxygen activity in the film decreasing below a critical value-but also the deduction of a phase diagram for strongly reduced SrTiO3 . Furthermore, why many conducting filaments are nucleated at one electrode but only one filament wins the race to the opposite electrode is also explained. The work thus provides detailed insights into the origin and mechanisms of filament generation and rupture.