LaAlO3 Layer Research Articles

Ammonia decomposition over La-doped Al2O3 supported Co catalyst

As the hydrogen source of carbon-free fuel cells, the key to realize the green and low-energy decomposition hydrogen production process is to select materials with excellent catalytic performance for ammonia dehydrogenation. In this study, the rare earth element lanthanum (La) was doped onto the surface of Al2O3 by the impregnation method. The aforementioned material was subsequently used as a new material for the synthesis of single metal Co catalysts that catalyzed ammonia decomposition. Various characterization techniques (XRD, HRTEM, N2 physical adsorption-desorption, ICP, XPS, SEM, H2-TPR) were employed to investigate the relationship between the structure and performance of Co/La–Al2O3 catalysts. The findings indicated that the 10Co/La(5)-Al2O3 catalyst was the most active. Under the conditions of 550 °C and 9000 mL h−1·gcat−1, the conversion of ammonia reached 90 %, and the yield of hydrogen reached 8.9 mmol·gcat−1·min−1. A perovskite-type metal oxide layer of LaAlO3 was generated on the surface of Al2O3 support after doping the appropriate amount of lanthanum (5 %), which directly affects the catalytic performance of the catalyst. The small amount of LaAlO3 optimized the strong interaction between the metal Co and the supports, thereby enhancing catalytic performance. While stabilizing the active component Co, XPS and H2-TPR also implied that the presence of La could enhance the electron transfer between the support and Co, as well as facilitate the desorption of nitrogen and hydrogen products, therefore, the low temperature activity of Co/La–Al2O3 catalyst can be improved. The ammonia decomposition efficiency of the catalyst doped with 5 % wt La was much higher than that of the catalyst without La incorporation. The simple modified support developed by us, which loads a monoatomic Co catalyst, provides a new approach for the development of high-activity transition metal catalysts.

Feld-induced modulation of two-dimensional electron gas at LaAlO3/SrTiO3 interface by polar distortion of LaAlO3

Since the discovery of two-dimensional electron gas at the LaAlO3/SrTiO3 interface, its intriguing physical properties have garnered significant interests for device applications. Yet, understanding its response to electrical stimuli remains incomplete. Our in-situ transmission electron microscopy analysis of a LaAlO3/SrTiO3 two-dimensional electron gas device under electrical bias reveals key insights. Inline electron holography visualized the field-induced modulation of two-dimensional electron gas at the interface, while electron energy loss spectroscopy showed negligible electromigration of oxygen vacancies. Instead, atom-resolved imaging indicated that electric fields trigger polar distortion in the LaAlO3 layer, affecting two-dimensional electron gas modulation. This study refutes the previously hypothesized role of oxygen vacancies, underscoring the lattice flexibility of LaAlO3 and its varied polar distortions under electric fields as central to two-dimensional electron gas dynamics. These findings open pathways for advanced oxide nanoelectronics, exploiting the interplay of polar and nonpolar distortions in LaAlO3.

Patterning and epitaxy of large-area arrays of nanoscale complex oxide epitaxial heterostructures

A combination of block copolymer (BCP) lithography and solid-phase epitaxy can be employed to form large areas, on the order of square centimeters, of a high density of epitaxial crystalline complex oxide nanostructures. We have used BCP lithography with a poly(styrene-block-methyl methacrylate) (PS-b-PMMA) copolymer to template a nanohole array either directly on an (001)-oriented SrTiO3 (STO) single crystal substrate or on a 20 nm-thick Si3N4 layer deposited on the STO substrate. BCPs with the selected compositions assembled in a cylindrical phase with 16 nm diameter PMMA cylinders and a cylinder-to-cylinder spacing of 32 nm. The substrate was modified with an energetically non-preferential polymer layer to allow for the vertical alignment of the cylinders. The PMMA cylinders were removed using a subtractive process, leaving an array of cylindrical holes. For BCPs assembled on Si3N4/STO, the pattern was transferred to the Si3N4 layer using reactive ion etching, exposing the underlying STO substrate in the nanoholes. An amorphous LaAlO3 (LAO) layer was deposited on the patterned Si3N4/STO at room temperature. The amorphous LAO epitaxially crystallized within the nanoscale-patterned holes with fully relaxed lattice parameters through solid phase epitaxy, resulting in the formation of nanoscale LAO/STO epitaxial heterostructures.

Structural study of nickelate based heterostructures

Heterostructures consisting of SmNiO3 and NdNiO3 alternating layers with additional LaAlO3 spacer layers were grown and fully characterized by means of x-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy. A change in the orientation of the orthorhombic long-axis of the nickelate layers is observed when a single unit cell of LaAlO3 is inserted between SmNiO3 and NdNiO3, in agreement with density functional theory calculations. At the same time, the structure of the ultra-thin rhombohedral LaAlO3 layers is affected by their proximity to orthorhombic nickelate layers, with both scanning transmission electron microscopy studies and density functional theory calculations revealing a weak antipolar motion of the La-cation in the LaAlO3 layers that is not present in the bulk rhombohedral structure of this compound.

Controlling Transport Properties of 2D Electron System Hosted by SrTiO3‐Based Bilayers

Compared with the conventional 2D electron system (2DES) located between the LaAlO3 layer and SrTiO3 substrate, the LaAlO3/SrTiO3 bilayers are also capable of hosting 2DES with additional functional tunabilities. Herein, the LaAlO3 and SrTiO3 layers are epitaxially grown on the (La,Sr)(Al,Ta)O3 substrate to build a bilayer 2DES. The transport properties can be subtly tuned by the SrTiO3 layer thickness (tSTO), growth temperature (TG), and oxygen pressure (PO2), leading to various controllable electronic states. For the metallic bilayer 2DES, the low‐temperature carrier density (≈1014 cm−2) is one‐order‐of‐magnitude higher than that of the conventional 2DES (1012–1013 cm−2). Meanwhile, the bilayer 2DES exhibits the enhanced carrier mobility on increasing the carrier density, which contradicts the experimental results obtained from the conventional 2DES and suggests an important role of charge screening in the high‐carrier‐density bilayer 2DES. Moreover, the semiconducting bilayer 2DES, featured by the lower carrier density and thus less charge screening, shows the low‐field negative minimum and enhanced fourfold anisotropy in magnetoresistances. Those results infer that the majority of scattering centers shall be interfacial localized Ti3+ ions, which are screened by the high carrier density in the metallic bilayer 2DES.

Electrical characterization and extraction of activation energies of the defect states in the LaAlO3/SrTiO3 heterostructure

In this work, we study the electronic properties of defects in the LaAlO3/SrTiO3 heterostructure, which is known to host a high mobility two-dimensional electron gas (2DEG) at the interface. This 2DEG also shows photoconductance, which could be related to defects that act as deep center trapping and releasing carriers by interaction with light. This phenomenon has raised an interest for the identification of deep energy levels in the LaAlO3/SrTiO3 heterostructure. We have studied the defect state properties using electrical characterization such as capacitance–voltage (C–V), current–voltage (I–V) measurements, and deep-level transient Fourier spectroscopy (DLTFS). From C–V and I–V analyses, a hysteresis was observed indicating an effect of mobile charges in the LaAlO3. Using DLTFS, we identify three defect states located at around 0.17 eV below conduction band and at 0.23 and 0.26 eV above the valence band. These defect states were attributed to defects in SrTiO3 such as strontium vacancies or titanium vacancies. We identify a fourth defect state having an energy of about 0.69 eV below the conduction band that could be related to oxygen vacancies in LaAlO3 or in SrTiO3. In addition, the observation of an effect of the electric field with DLTFS indicated that oxygen vacancies might be involved in Fowler–Nordheim or trap-assisted tunneling through the LaAlO3 layer.

Tunable Two‐Channel Magnetotransport in SrRuO3 Ultrathin Films Achieved by Controlling the Kinetics of Heterostructure Deposition

AbstractIn the field of oxide heterostructure engineering, there are extensive efforts to couple the various functionalities of each material. The Berry curvature‐driven magnetotransport of SrRuO3 ultrathin films is currently receiving a great deal of attention because it is extremely sensitive to the electronic structures near the Fermi surface driven by extensive physical parameters such as spin–orbit coupling and inversion symmetry breaking. Although this is beneficial in terms of heterostructure engineering, it renders transport behavior vulnerable to nanoscale inhomogeneity, resulting in artifacts called “hump anomalies.” Here, a method to tune the magnetotransport properties of SrRuO3 ultrathin films capped by LaAlO3 layers is developed. The kinetic process of pulsed laser deposition by varying the growth pressure during LaAlO3 layer deposition is systematically controlled. Furthermore, the effects of nanoscale inhomogeneity on the Berry curvature near the Fermi surface in SrRuO3 films are investigated. It is found that the high kinetic energy of the capping layer adatoms induces stoichiometric modification and nanoscale lattice deformation of the underlying SrRuO3 layer. The control of kinetics provides a way to modulate magnetization and the associated magnetotransport of the SrRuO3 layer.

Tuning the Schottky Barrier Height at the Interfaces of Metals and Mixed Conductors.

Understanding electronic and ionic transport across interfaces is crucial for designing high-performance electric devices. The adjustment of work functions is critical for band alignment at the interfaces of metals and semiconductors. However, the electronic structures at the interfaces of metals and mixed conductors, which conduct both electrons and ions, remain poorly understood. This study reveals that a Schottky barrier is present at the interface of the Nb-doped SrTiO3 metal and a LiCoO2 mixed conductor and that the interfacial resistance can be tuned by inserting an electric dipole layer. The interfacial resistance significantly decreased (by more than 5 orders of magnitude) upon the insertion of a 1 nm thick insulating LaAlO3 layer at the interface. We apply these techniques to solid-state lithium batteries and demonstrate that tuning the electronic energy band alignment by interfacial engineering is applicable to the interfaces of metals and mixed conductors. These results highlight the importance of designing positive electrode and current collector interfaces for solid-state lithium batteries with high power density.

Impacts of LaOx Doping on the Performance of ITO/Al2O3/ITO Transparent RRAM Devices

Fully transparent ITO/LaAlO3/ITO structure RRAM (resistive random access memory) devices were fabricated on glass substrate, and ITO/Al2O3/ITO structure devices were set for comparison. The electrical characteristics of the devices were analyzed by Agilent B1500A semiconductor analyzer. Compared with the ITO/Al2O3/ITO RRAM devices, the current stability, SET/RESET voltage distribution, and retention characteristic of the ITO/LaAlO3/ITO RRAM devices have been greatly improved. In the visible light range, the light transmittance of the device is about 80%, that of the LaAlO3 layer is about 95%, the on-off ratio of the device is greater than 40, and the data retention time is longer than 10,000 s. The devices have great optical and electrical properties and have huge application potential as fully transparent RRAM devices.

Two-dimensional electron systems in perovskite oxide heterostructures: Role of the polarity-induced substitutional defects

The discovery of a two-dimensional electron system (2DES) at the interfaces of perovskite oxides such as LaAlO3 and SrTiO3 has motivated enormous efforts in engineering interfacial functionalities with this type of oxide heterostructures. However, its fundamental origins are still not understood, e.g. the microscopic mechanisms of coexisting interface conductivity and magnetism. Here we report a comprehensive spectroscopic investigation of the depth profile of 2DES-relevant Ti 3d interface carriers using depth- and element-specific techniques, standing-wave excited photoemission and resonant inelastic scattering. We found that one type of Ti 3d interface carriers, which give rise to the 2DES are located within 3 unit cells from the n-type interface in the SrTiO3 layer. Unexpectedly, another type of interface carriers, which are polarity-induced Ti-on-Al antisite defects, reside in the first 3 unit cells of the opposing LaAlO3 layer (~10 {\AA}). Our findings provide a microscopic picture of how the localized and mobile Ti 3d interface carriers distribute across the interface and suggest that the 2DES and 2D magnetism at the LaAlO3/SrTiO3 interface have disparate explanations as originating from different types of interface carriers.

Ferroelectric Exchange Bias Affects Interfacial Electronic States.

In polar oxide interfaces phenomena such as superconductivity, magnetism, 1D conductivity, and quantum Hall states can emerge at the polar discontinuity. Combining controllable ferroelectricity at such interfaces can affect the superconducting properties and sheds light on the mutual effects between the polar oxide and the ferroelectric oxide. Here, the interface between the polar oxide LaAlO3 and the ferroelectric Ca-doped SrTiO3 is studied by means of electrical transport combined with local imaging of the current flow with the use of scanning a superconducting quantum interference device (SQUID). Anomalous behavior of the interface resistivity is observed at low temperatures. The scanning SQUID maps of the current flow suggest that this behavior originates from an intrinsic bias induced by the polar LaAlO3 layer. Such intrinsic bias combined with ferroelectricity can constrain the possible structural domain tiling near the interface. The use of this intrinsic bias is recommended as a method of controlling and tuning the initial state of ferroelectric materials by the design of the polar structure. The hysteretic dependence of the normal and the superconducting state properties on gate voltage can be utilized in multifaceted controllable memory devices.

Role of point and line defects on the electronic structure of LaAlO3/SrTiO3 interfaces

Realization of heterostructures containing multiple two-dimensional electron liquids requires a fine control of the fabrication process. Here, we report a structural and spectroscopy study of LaAlO3/SrTiO3/LaAlO3 trilayers grown on the SrTiO3 substrate by pulsed-laser deposition. Scanning transmission electron microscopy with the help of ab initio calculations reveals that antisite defects associated with oxygen vacancies are primarily present in the SrTiO3 film (STO-f) close to the p-type interface (STO-f/LaAlO3), while oxygen vacancies prevail close to the top n-type interface (LaAlO3/STO-f). At the same interface, misfit dislocations relax the tensile strain of the top LaAlO3 layer. Combining x-ray absorption spectroscopy, x-ray linear dichroism, resonant photoemission spectroscopy, and electron energy loss spectroscopy, we observe that the 3d orbital reconstruction at the interface between LaAlO3 and the SrTiO3 substrate is confined over a few interfacial Ti planes while, at the top n-type interface (LaAlO3/STO-f), the absence of a dichroic signal can be related to the blurring of the interfacial orbital reconstruction due to the heterogeneity of defects.

Absence of superconductivity in Nd0.8Sr0.2NiOx thin films without chemical reduction

The recently reported superconductivity 9-15 K in Nd0.8Sr0.2NiO2/SrTiO3 heterostructures that were fabricated by a soft-chemical topotactic reduction approach based on precursor Nd0.8Sr0.2NiO3 thin films deposited on SrTiO3 substrates, has excited an immediate surge of research interest. To explore an alternative physical path instead of chemical reduction for realizing superconductivity in this compound, using pulsed laser deposition, we systematically fabricated 63 Nd0.8Sr0.2NiOx (NSNO) thin films at a wide range of oxygen partial pressures on various different oxide substrates. Transport measurements did not find any signature of superconductivity in all the 63 thin-film samples. With reducing the oxygen content in the NSNO films by lowering the deposition oxygen pressure, the NSNO films are getting more resistive and finally become insulating. Furthermore, we tried to cap a 20-nm-thick amorphous LaAlO3 layer on a Nd0.8Sr0.2NiO3 thin film deposited at a high oxygen pressure of 150 mTorr to create oxygen vacancies on its surface and did not succeed in higher conductivity either. Our experimental results together with the recent report on the absence of superconductivity in synthesized bulk Nd0.8Sr0.2NiO2 crystals suggest that the chemical reduction approach could be unique for yielding superconductivity in NSNO/SrTiO3 heterostructures. However, SrTiO3 substrates could be reduced to generate oxygen vacancies during the chemical reduction process as well, which may thus partially contribute to conductivity.

Conductivity enhancement in La-doped Sr2IrO4 thin film by charge transfer

In this paper, the conductivity of La-doped Sr2IrO4 thin films is enhanced by covering amorphous LaAlO3. Electrons of the order of 1020 cm−3 can be transferred into La-doped Sr2IrO4 thin film from the top amorphous LaAlO3 layer. This value is close to that of the largest polarization induced by the field effect. The conductivity of La-doped Sr2IrO4 thin film with a fixed thickness will be enhanced as the thickness of the amorphous LaAlO3 film increases until which reaches a critical value. For a certain thickness of amorphous LaAlO3, the thinner the La-doped Sr2IrO4 film, the more significant the resistance reduction. Furthermore, the conductivity and carrier density can be controlled by varying the growth oxygen pressure of amorphous LaAlO3 layer. This work provides a permanent and controllable method to enhance the conductivity of La-doped Sr2IrO4 film without changing the conduction mechanism and magnetic properties and facilitates subsequent research on the intrinsic properties of La-doped Sr2IrO4.

Correlated Lattice Instability and Emergent Charged Domain Walls at Oxide Heterointerfaces

AbstractCharged domain walls provide possibilities in effectively manipulating electrons at nanoscales for developing next‐generation electronic devices. Here, using the atom‐resolved imaging and spectroscopy on LaAlO3/SrTiO3//NdGaO3 heterostructures, the evolution of correlated lattice instability and charged domain walls is visualized crossing the conducting LaAlO3/SrTiO3 heterointerface. When increasing the SrTiO3 layer thickness to 20 unit cells and above, both LaAlO3 and SrTiO3 layers begin to exhibit measurable polar displacements to form a tail‐to‐tail charged domain wall at the LaAlO3/SrTiO3 interface, resulting in the charged redistribution within the 2‐nm‐thick SrTiO3 layer close to the LaAlO3/SrTiO3 interface. The mobile charges in different heterostructures can be estimated by summing up Ti3+ concentrations in the conducting channel, which is sandwiched by SrTiO3 layers with interdiffusion and/or oxygen octahedral rotations. Those estimated mobile charges are quantitatively consistent with results from Hall measurements. The results not only shed light on complex oxide heterointerfaces, but also pave a new path to nanoscale charge engineering.

Ferroelectric Tunnel Junctions Enhanced by a Polar Oxide Barrier Layer

Ferroelectric tunnel junctions (FTJs) have recently aroused significant interest due to the interesting physics controlling their properties and potential application in nonvolatile memory devices. In this work, we propose a new concept to design high-performance FTJs based on ferroelectric/polar-oxide composite barriers. Using density functional theory calculations, we model electronic and transport properties of LaNiO3/PbTiO3/LaAlO3/LaNiO3 tunnel junctions and demonstrate that an ultrathin polar LaAlO3(001) layer strongly enhances their performance. We predict a tunneling electroresistance (TER) effect in these FTJs with an OFF/ON resistance ratio exceeding a factor of 104 and ON state resistance as low as about 1 kΩμm2. Such an enhanced performance is driven by the ionic charge at the PbTiO3/LaAlO3 interface, which significantly increases transmission across the FTJ when the ferroelectric polarization of PbTiO3 is pointing against the intrinsic electric field produced by this ionic charge. This is due to the formation of a two-dimensional (2D) electron or hole gas, depending on the LaAlO3 termination being (LaO)+ or (AlO2)-, respectively, which is formed to screen the polarization charge of the nonuniform polarization state. This 2D electron (hole) gas can be switched ON and OFF by the reversal of ferroelectric polarization, resulting in the giant TER effect. The proposed design suggests a new direction for creating FTJs with a stable and reversible ferroelectric polarization, a sizable TER effect, and a low-resistance-area product, as required for memory applications.

Enhancement of Critical Current Density by Establishing a YBa2Cu3O7−x/LaAlO3/YBa2Cu3O7−x Quasi-Trilayer Architecture Using the Sol-Gel Method

We developed a solution-derived method to establish a YBa2Cu3O7−x/LaAlO3/YBa2Cu3O7−x quasi-trilayer architecture. Using the method, nano-scale pinning sites were induced into the quasi-trilayer architecture and yielded an apparent improvement in the in-field critical current density (Jc) of high-quality YBa2Cu3O7−x (YBCO). The improvement in the in-field Jc of the films was closely related to the thickness of the LaAlO3 (LAO) interlayer. In this paper it is demonstrated that when the nominal interlayer thickness approximates 20 nm, which is slightly higher than the roughness of the YBa2Cu3O7−x surface, the LaAlO3 interlayer is discontinuous due to synchromesh-like growth of the LaAlO3 layer on relatively rough YBa2Cu3O7−x surface resulting from the mobility of the solution. Nanoscale defects, such as particles, some amorphous phases, and especially their concomitant lattice defects (such as stacking faults and plane buckling) arise in YBa2Cu3O7−x layers. These nanoscale defects could play a role in flux pinning and thus enhancing Jc. The effective non-vacuum solution to induce vortex pinning into YBa2Cu3O7−x films could be a reference for the further design of an optimal pinning landscape for higher Jc.

Tuning LaAlO3 lattice structure by growth rate at the picometer scale in LaAlO3/SrTiO3 heterostructures

Here, we report an abnormal lattice reconstruction of LaAlO3 in the LaAlO3/SrTiO3 two-dimensional electron liquid heterostructure using the layer-by-layer pulsed laser deposition. We found that the lattice structure change of the top LaAlO3 layer is at the picometer level and is mainly controlled by the oxygen vacancies with an abnormal dependence on the oxygen background pressure. The lattice of the LaAlO3 film expands with more oxygen vacancies grown with high oxygen pressure and collapses with less oxygen vacancies grown with low oxygen pressure. The in situ monitoring of reflective high energy electron diffraction shows the lattice change coincident to the layer-by-layer deposition period change, revealing a non-equilibrium correlation between the lattice and the deposition environment, instead of the commonly assumed equilibrium correlation.

Distortion Analysis of Ir- and Co-doped LaAlO3/SrTiO3 (001) Interfaces by Hard X-ray Photoelectron Diffraction

The structural distortions at LaAlO3/SrTiO3 (001) interfaces doped with Ir or Co have been studied by hard X-ray photoelectron diffraction (HXPD). In this work, HXPD at a photon energy of 7940 eV was used for studying the buried interfaces. The magnitude of lattice strain perpendicular to the interface in the LaAlO3 layer of Ir- or Co-doped LaAlO3/SrTiO3 heterostructure was estimated from forward focusing peak shift. The largest strain was found in a 3% Ir-doped sample while the strain was negligibly small in a 5% Ir-doped sample due to strong Ir interdiffusion. A similar decrease of the out-of-plane strain at higher doping concentration was not seen in Co-doped samples. This difference is attributed to the different ionic radii of Co and Ir.

Two-dimensional electron gas at manganite buffered LaAlO3/SrTiO3 (001) interface by spin coating chemical methods

High mobility spin-polarized two-dimensional electron gas (2DEG) is crucially important for spintronic applications. Here, we report our investigations on the 2DEG fabricated by spin coating a LaAlO3 layer on a (001) SrTiO3 substrate with a La2/3Sr1/3MnO3 buffer layer. When the buffer layer is below 3 uc, the 2DGE is highly mobile. Corresponding to the layer thicknesses of 0, 1, and 2 uc, the Hall mobilities are ∼24 000 cm2/V s, ∼28 000 cm2/V s, and ∼59 600 cm2/V s at 2 K. In contrast, the 2DEG with a buffer layer of 3 uc shows a relatively low mobility (∼3000 cm2/V s). However, an anomalous Hall effect was observed in this 2DEG below 20 K, indicating a long range ferromagnetic order. This work demonstrates the great potential of the chemical method in gaining high quality spin-polarized 2DEGs at the LaAlO3/SrTiO3 interface.