LaAlO3 Layer Research Articles

Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces.

Polar discontinuities occurring at interfaces between two materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates, or charge transfer. In the latter case, one may expect a local electronic doping of one material: one example is the two‐dimensional electron liquid (2DEL) appearing in SrTiO3 once covered by a polar LaAlO3 layer. Here, it is shown that tuning the formal polarization of a (La,Al)1− x(Sr,Ti)xO3 (LASTO:x) overlayer modifies the quantum confinement of the 2DEL in SrTiO3 and its electronic band structure. The analysis of the behavior in magnetic field of superconducting field‐effect devices reveals, in agreement with ab initio calculations and self‐consistent Poisson–Schrödinger modeling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on the interface total charge densities. These results strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO3/SrTiO3 interface and demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.

Electro- and Magnetotransport near a LaAlO3/SrTiO3 Interphase Boundary

Comparison of the channel spectra of medium energy ion scattering, visualized for LaAlO3/(001)SrTiO3 heterostructures with a thickness of the lanthanum aluminate layer of one to six unit cells, indicates that the lanthanum aluminate layer grows coherently on a TiO2-terminated surface of a strontium titanate substrate. The resistance of the interphase boundary in the heterostructure with a thickness of the LaAlO3 layer of six unit cells decreased with temperature. At T < 100 K, the positive magnetoresistance and Hall mobility of electrons increased sharply with decreasing temperature.

Thickness Dependence of Magnetic Behavior of LaAlO3/SrTiO3 Heterostructures

AbstractThickness dependence of magnetic properties of LaAlO3 (LAO)/SrTiO3 (STO) heterostructures is systematically studied. It is found that the magnetic moment is rather robust at elevated temperatures in the samples grown under ultralow PO2 of 1 × 10−8 Torr, indicating room‐temperature ferromagnetism in the LaAlO3/SrTiO3 heterostructures. Spin‐glass‐like behavior is observed in the LAO/STO heterostructures with the LAO thickness ranging from 5 to 90 unit cells (2–35.2 nm). As the LAO layer becomes thicker, the blocking temperature of spin‐glass‐like behavior decreases and magnetic moment weakens, which may be related to the reduced density of oxygen vacancies at the interfacial TiO2 layer. It is believed that the well‐controlled magnetism by tuning the LAO thickness may open an avenue for the potential application of spintronics with LaAlO3/SrTiO3 heterostructures.

Ion blocking dip shape analysis around a LaAlO3/SrTiO3 interface

We present an analysis of the widths of the blocking dips obtained in MEIS ion blocking experiments of two LaAlO3/SrTiO3 heterostructures differing in their LaAlO3 layer thicknesses. In the LaAlO3 layers, the observed blocking dips are larger than expected. This enlargement is the result of the superposition of individual dips at slightly different angular positions revealing a local disorder in the atomic alignment, i.e., layer buckling. By contrast, in the SrTiO3 substrate, just below the interface, the obtained blocking dips are thinner than expected. This thinning indicates that the blocking atoms stand at a larger distance from the scattering center than expected. This is attributed to an accumulation of Sr vacancies at the layer/substrate interface which induces lattice distortions shifting the atoms off the scattering plane.

Thickness Control of the Spin-Polarized Two-Dimensional Electron Gas in LaAlO3/BaTiO3 Superlattices

We explored the possibility of increasing the interfacial carrier quantum confinement, mobility and conductivity in the (LaAlO3)n/(BaTiO3)n superlattices by thickness regulation using the first-principles electronic structure calculations. Through constructing two different interfacial types of LaAlO3/BaTiO3 superlattices, we discovered that the LaO/TiO2 interface is preferred from cleavage energy consideration. We then studied the electronic characteristics of two-dimensional electron gas (2DEG) produced at the LaO/TiO2 interface in the LaAlO3/BaTiO3 superlattices via spin-polarized density functional theory calculations. The charge carrier density of 2DEG has a magnitude of 1014 cm−2 (larger than the traditional system LaAlO3/SrTiO3), which is mainly provided by the interfacial Ti 3dxy orbitals when the thicknesses of LaAlO3 and BaTiO3 layers are over 4.5 unit cells. We have also revealed the interfacial electronic characteristics of the LaAlO3/BaTiO3 system, by showing the completely spin-polarized 2DEG mostly confined at the superlattice interface. The interfacial charge carrier mobility and conductivity are found to be converged beyond the critical thickness. Therefore, we can regulate the interfacial confinement for the spin-polarized 2DEG and quantum transport properties in LaAlO3/BaTiO3 superlattice via controlling the thicknesses of the LaAlO3 and BaTiO3 layers.

Growth of LaAlO3 on silicon via an ultrathin SrTiO3 buffer layer by molecular-beam epitaxy

(001)-oriented LaAlO3 films were grown epitaxially on (001) Si substrates utilizing an ultrathin 5 unit-cell-thick SrTiO3 buffer layer. The SrTiO3 layer was grown at ∼250 °C and annealed in vacuum at 550 °C, following an epitaxy-by-periodic-annealing procedure. Upon this buffer layer, the LaAlO3 layer was then grown by codeposition at 580 °C. The rocking curve of the as-grown LaAlO3 film exhibits a full width at half maximum value as small as 0.02°. Atomic force microscopy shows that the surface of the LaAlO3 film has a root-mean-square roughness of 1.3 Å. Scanning transmission electron microscopy reveals that the LaAlO3/SrTiO3 interface and the SrTiO3/Si interfaces are sharp. This high crystalline quality, twin-free, epitaxial LaAlO3 on SrTiO3 on silicon could be relevant to integrating oxides with multiple functionalities on silicon.

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO3 Overlayers.

Interfaces between complex oxides constitute a unique playground for two-dimensional electron systems (2DESs), where superconductivity and magnetism can arise from combinations of bulk insulators. The 2DES at the LaAlO3/SrTiO3 interface is one of the most studied in this regard, and its origin is determined by the polar field in LaAlO3 as well as by the presence of point defects, like oxygen vacancies and intermixed cations. These defects usually reside in the conduction channel and are responsible for a decrease of the electronic mobility. In this work, we use an amorphous WO3 overlayer to obtain a high-mobility 2DES in WO3/LaAlO3/SrTiO3 heterostructures. The studied system shows a sharp insulator-to-metal transition as a function of both LaAlO3 and WO3 layer thickness. Low-temperature magnetotransport reveals a strong magnetoresistance reaching 900% at 10 T and 1.5 K, the presence of multiple conduction channels with carrier mobility up to 80 000 cm2 V-1 s-1, and quantum oscillations of conductance.

Room-Temperature Giant Charge-to-Spin Conversion at the SrTiO3-LaAlO3 Oxide Interface.

The two-dimensional electron gas (2DEG) formed at the interface between SrTiO3 (STO) and LaAlO3 (LAO) insulating layer is supposed to possess strong Rashba spin-orbit coupling. To date, the inverse Edelstein effect (i.e., spin-to-charge conversion) in the 2DEG layer is reported. However, the direct effect of charge-to-spin conversion, an essential ingredient for spintronic devices in a current-induced spin-orbit torque scheme, has not been demonstrated yet. Here we show, for the first time, a highly efficient spin generation with the efficiency of ∼6.3 in the STO/LAO/CoFeB structure at room temperature by using spin torque ferromagnetic resonance. In addition, we suggest that the spin transmission through the LAO layer at a high temperature range is attributed to the inelastic tunneling via localized states in the LAO band gap. Our findings may lead to potential applications in the oxide insulator based spintronic devices.

Effect of oxygen content of the LaAlO3 layer on the synaptic behavior of Pt/LaAlO3/Nb-doped SrTiO3 memristors for neuromorphic applications

We report the effect of the oxygen content of the LaAlO3 layer on the synaptic behavior in the Pt/LaAlO3/Nb-doped SrTiO3 memristor for neuromorphic applications. As the oxygen-content decreases, the current becomes larger and the spike time-dependent plasticity (STDP) becomes less sensitive to the time difference between pre- and post-synaptic spike voltage. In addition, the conduction mechanism, which was found to be a combination of thermionic and Poole-Frenkel emissions, and the effect of oxygen content are explained in association with the oxygen vacancy in the LaAlO3 layer. The trade-off between large current and efficient STDP can be controlled by the oxygen content. Furthermore, the results of extracting the synaptic strength-based model parameters indicate that the Pt/LaAlO3/Nb-doped SrTiO3 shows the efficient STDP characteristics in comparison to previously reported memristor materials.

Highly Mobile Two-Dimensional Electron Gases with a Strong Gating Effect at the Amorphous LaAlO3/KTaO3 Interface.

Two-dimensional electron gas (2DEG) at the perovskite oxide interface exhibits a lot of exotic properties, presenting a promising platform for the exploration of emergent phenomena. While most of the previous works focused on SrTiO3-based 2DEG, here we report on the fabrication of high-quality 2DEGs by growing an amorphous LaAlO3 layer on a (001)-orientated KTaO3 substrate, which is a 5d metal oxide with a polar surface, at a high temperature that is usually adopted for crystalline LaAlO3. Metallic 2DEGs with a Hall mobility as high as ∼2150 cm2/(V s) and a sheet carrier density as low as 2 × 1012 cm-2 are obtained. For the first time, the gating effect on the transport process is studied, and its influence on spin relaxation and inelastic and elastic scattering is determined. Remarkably, the spin relaxation time can be strongly tuned by a back gate. It is reduced by a factor of ∼69 while the gate voltage is swept from -25 to +100 V. The mechanism that dominates the spin relaxation is elucidated.

In-situ probing of coupled atomic restructuring and metallicity of oxide heterointerfaces induced by polar adsorbates

Microscopic understanding of the surface-controlled conductivity of the two dimensional electron gas at complex oxide interfaces is crucial for developing functional interfaces. We observe conductivity and structural modification using in-situ synchrotron surface x-ray diffraction as the surface of a model LaAlO3/SrTiO3 (001) heterostructure is changed by polar adsorbates. We find that polar adsorbate-induced interfacial metallicity reduces polar distortions in the LaAlO3 layer. First-principles density functional theory calculations show that surface dipoles introduced by polar adsorbates lead to additional charge transfer and the reduction of polar displacements in the LaAlO3 layer, consistent with the experimental observations. Our study supports that internal structural deformations controlling functionalities can be driven without the application of direct electrical or thermal bias and offers a route to tuning interfacial properties. These results also highlight the important role of in-situ x-ray scattering with atomic resolution in capturing and exploring structural distortions and charge density changes caused by external perturbations such as chemical adsorption, redox reaction, and generation and/or annihilation of surface defects.

Enhancing the barrier height in oxide Schottky junctions using interface dipoles

We demonstrate a 0.37 eV increase in the n-type Schottky barrier height at the Pt/TiO2-terminated SrTiO3 (001) interface by insertion of an ultrathin insulator AgTaO3. While a conventional metal-insulator-semiconductor model predicts a reduction in the barrier height with insulator thickness, the observed increase originates from the two polar surface layers, (AgO)– and (TaO2)+ in AgTaO3 (001), forming an electrostatic dipole directed towards the metal, increasing the barrier height. Based on directly measured optical and dielectric properties of high quality AgTaO3 thin films, we determine the interface energy band diagram including the interface dipole. The capability to exploit the strong ionic charges of oxide surfaces to manipulate interface band alignments beyond bulk values greatly expands the design strategies for tailoring functional oxide interfaces. Up to now, the use of an ultrathin LaAlO3 layer could only decrease the barrier height, but now bidirectional control is possible.

(Invited) Inorganic Solid-State Solar Cells Using Plasmon-Induced Charge Separation

[Introduction] Metallic nanostructures which show localized surface plasmon resonances have received considerable attention as a light harvesting optical antenna for light-energy conversion systems such as solar cells as well as artificial photosynthesis [1-5]. Recently, we have successfully fabricated a plasmonic photoelectric conversion device using a gold nanostructured titanium dioxide photoelectrode with a nickel oxide (NiO) thin film as a hole transport layer [6]. However, the conversion efficiency was still low. To overcome this problem, we investigated the modulation of the diffusion potential of the junction to control the cell voltage and rectification characteristics. Yajima et al. reported that an interface dipole in an atomic scale strongly affects the rectification characteristics [7]. In this study, we fabricated plasmonic solar cells composed of n-strontium titanate (SrTiO3), gold nanoparticles (Au-NPs) and p-NiO. Also, we inserted a unit cell (uc) of lanthanum aluminate (LaAlO3) as an interface dipole between SrTiO3 and NiO to modulate the diffusion potential. [Experimental] We prepared the plasmonic solar cells as the following procedure. At first, a 0.05wt% niobium-doped SrTiO3 single crystal (100) was treated with HF and NH4F to expose the TiO2 surface of SrTiO3 (t-SrTiO3). Au-NPs were fabricated on the t-SrTiO3 by the deposition of the gold thin film by a pulsed laser deposition (PLD) and subsequent annealing of the substrate at 800°C. After that, the following different cells were prepared. (i) NiO was deposited on Au-NPs/t-SrTiO3 with a thickness of 100 nm by PLD (NiO/Au-NPs/t-SrTiO3); (ii) 100 nm thick NiO was deposited after the deposition of 1uc of LaAlO3 (NiO/LaAlO3/Au-NPs/t-SrTiO3); (iii) 100 nm thick NiO was deposited after the deposition of 1uc of SrO and 1uc of LaAlO3 (NiO/LaAlO3/SrO/Au-NPs/t-SrTiO3). A gold thin film with a thickness of 4 nm was deposited by the helicon sputtering as an electrode. Photoelectric properties of the plasmonic solar cell were measured by the solar simulator (AM 1.5G). [Results and Discussion] First of all, we evaluated current-voltage characteristics of cells under the irradiation of AM 1.5G. The open circuit voltages were dramatically changed by the insert of the LaAlO3 layer and showed large values in the order of (ii)>(i)>(iii). Additionally, the cell (ii) showed most clear rectification characteristics. Furthermore, the cell (ii) showed the largest solar energy conversion efficiency. These results indicate that the diffusion potential was increased in the case of cell (ii) and decreased in the case of (iii). It is considered that the difference is derived from the deposition order of LaAlO3 layer. It was reported that LaAlO3 was deposited in the order of LaO+ and AlO2 - on the TiO2 surface of SrTiO3 [7]. In contrast, LaAlO3 was deposited in the order of AlO2 - and LaO+ on the SrO surface of SrTiO3. As a result, the opposite dipole direction had an opposite effect on the diffusion potential. We also measured action spectra of incident photon-to-current efficiency to evaluate the contribution of the plasmon to photoelectric conversion. As a result, all cells showed a visible light responsivity in the region of plasmon resonance, indicating plasmon-induced charge separation works well even in the presence of the interface dipole.

Effect of Sr doping on electronic structure of La1-xSrxMnO3/LaAlO3/SrTiO3 heterointerface

In the past decades, the interface between two oxides LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention since a quasi-two-dimensional electron gas (q2DEG) at the interface was observed. It is generally believed that polar discontinuity at polar/non-polar oxide interface is responsible for the emergence of q2DEG at the interface. Recently, how to modulate the q2DEG at the interface is becoming a new research focus. Capping other oxide thin layer on LAO layer is one of alternative approaches to controlling the generation of q2DEG at interface. However the mechanism or origin for tuning q2DEG at capped LAO/STO interface has not yet completely understood. Using the first-principles calculations within the density functional theory, the electronic properties of La1-xSrxMnO3-capped LaAlO3/SrTiO3 heterointerfaces with different doping concentrations of Sr atoms are investigated. The system is composed of four layers of La1-xSrxMnO3 (LSMO), three layers of LAO and four layers of STO, denoted as 4LSMO/3LAO/4STO. The interface is normal to the[001] direction of cubic phase, namely (La1-xSrxO) layer and (MnO2) layer appear alternately at LSMO, and (LaO) layer and (AlO2) layer appear alternately at LAO. In the absence of LSMO layers, q2DEG does not appear at the LAO/STO interface. It is found that the electronic structure of 4LSMO/3LAO/4STO can be tuned significantly by capping LSMO layers. For concentration of doped Sr atoms less than 1/3, a q2DEG at LAO/STO interface is observed. In this case, a strong polarization existing in LSMO, together with the polarization in LAO, forces the electrons to be redistributed, thus inducing the q2DEG at LAO/STO interface. With the increase of the concentration of Sr atoms, the polarization in LSMO becomes weaker and weaker. When the concentration is higher than 1/3, the polaried electric field fails to make the electrons redistributed, thus the q2DEG disappears from interface.#br#Another interesting feature of the present work relates to the distribution of Sr atoms in LSMO. It is found that the electronic structure of 4LSMO/3LAO/4STO changes little with respect to the distribution of Sr atoms in LSMO. The system does not undergo the conductor-to-insulator transition for Sr atoms doping at different sites as long as the concentration of Sr does not change. The reason could be understood as follows. The LSMO layer is in a metallic state, the extra electrons, which are generated due to substituting La with Sr, will be delocalized rather than localized at each doped Sr atom. It is reasonable to expect that the electronic structure of the system should be less sensitive to the specific doping site of Sr in LSMO.

Direct measurement of oxygen stoichiometry in perovskite films

We present a direct method to measure the oxygen stoichiometry in an oxide film with an accuracy of about 2%. It is based on a combination of 18O annealing and high mass resolution secondary ion mass spectroscopy. Calibration has been done on a LaNiO3 film whose electrical properties dependence on oxygen stoichiometry are well documented. The method is illustrated with a series of LaNiO3 films grown on SrTiO3 substrates prepared with different oxygen stoichiometries. The large influence of the surface state on oxygen exchange is evidenced in films grown on different substrate orientations or coated with a thin layer of LaAlO3. Oxygen surface exchange and bulk diffusion is then discussed for both LaNiO3 and SrVO3 films.

Confinement-driven metal-insulator transition and polarity-controlled conductivity of epitaxial LaNiO3/LaAlO3 (111) superlattices

Recently, topological conductivity has been predicted theoretically in LaNiO3(111)-based superlattices. Here we report high-quality epitaxial LaNiO3/LaAlO3 superlattices on (111)-oriented SrTiO3 and LaAlO3 single crystals. For both substrates a metal-insulator transition with decreasing number of LaNiO3 monolayers is found. While the electrical transport is dominated by two-dimensional variable range hopping for superlattices grown on polar mismatched SrTiO3(111), it switches to a thermally activated single gap behavior on polar matched LaAlO3(111). The gap energy of the polar double-layer LaNiO3 superlattices can be tuned via the thickness of the insulating LaAlO3 layers.

Tuning carrier density at complex oxide interface with metallic overlayer

We have systematically investigated the electronic transport properties of the LaAlO3/SrTiO3 interfaces with several different metal capping layers. The sheet carrier density can be tuned in a wide range by the metallic overlayer without changing the carrier mobility. The sheet carrier density variation is found to be linearly dependent on the size of metal work function. This behavior is explained by the mechanism of the charge transfer between the oxide interface and the metal overlayer across the LaAlO3 layer. Our results confirm the existence of a built-in electric field in LaAlO3 film with an estimated value of 67.7 eV/Å. Since the metallic overlayer is essential for devices, the present phenomena must be considered for future applications.

Charge Transfer to LaAlO3/SrTiO3 Interfaces Controlled by Surface Water Adsorption and Proton Hopping

Electronic properties of low dimensional systems are particularly sensitive to surface adsorbates. Clear understanding of such phenomena can lead to highly effective and nondestructive material engineering techniques. In this work, water adsorption at the surface of LaAlO3/SrTiO3 heterostructures is systematically studied. The saturation of surface dangling bonds by spontaneous water chemisorptions is found to be a main enabler of the formation of the interface 2D electron gas. In particular, when imbalanced distributions of water based ions, namely protons and hydroxyls, are generated, interface electron doping or depletion becomes surface adsorbates dominant and independent of the LaAlO3 layer thickness. The investigations also reveal the importance of hydrogen bonding through molecular water layers, which provides an energetically feasible pathway for manipulating the surface‐bond protons and thus the interface electrical characteristics.

Layer-Resolved Cation Diffusion and Stoichiometry at the LaAlO3/SrTiO3 Heterointerface Probed by X-ray Photoemission Experiments and Site Occupancy Modeling.

The layer-resolved cation occupancy for different conducting and insulating interfaces of LaAlO3 (LAO) thin films on SrTiO3 (STO) has been determined by angle-resoled X-ray photoelectron spectroscopy (AR-XPS). Three STO interfaces with LAO have been considered, namely, a conducting interface with a 5 unit cell (u.c.) LAO layer, an insulating interface with a 5 u.c. LAO layer, and an insulating interface with a 3 u.c. LAO layer. Considering inelastic and elastic scattering processes in the transport approximation, the core-level signal attenuation has been modeled on the basis of Monte Carlo calculations of the electron trajectories across the heterostructures. Different effects involving cation stoichiometry and diffusion through the interface have been considered to interpret data. Beyond a mere abrupt interface modeling, the LaAlO3/SrTiO3 heterojunction is shown to host cation diffusion processes within 3-4 unit cells in the bulk layer, along with a clear Sr substoichiometry, an issue so far virtually neglected in the analysis of these systems. The present results show the capability of the AR-XPS modeling to explore element-sensitive properties at the oxide interfaces, matching and completing the information that can be provided by probes based on electron microscopy or X-ray scattering.

Control of magnetism in heterostructures

Magnetism The interface between two different materials in a heterostructure can exhibit properties unique to either of the two materials alone. A well-known example is a conducting gas that forms when LaAlO3 is grown on SrTiO3, but only if the LaAlO3 layer is at least four unit cells thick. Wang et