Oxide Heterointerfaces Research Articles

PL-1Precise Structural Observation and State Analysis

Functional properties found in perovskite oxides are related to the octahedral distortion as well as the constituent elements. Heteroepitaxial thin films grown on different oxide are a good platform to control their functionalities. In order to elucidate the correlation between structural distortions induced by the lattice mismatch and properties of thin films, it is required to reveal the local structure in the vicinity of the interface precisely. High-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging in aberration-corrected scanning transmission electron microscopy allows us for visualizing not only constituent cations but also oxygen atoms. It has been demonstrated that measuring atomic positions precisely from HAADF and ABF images is a good tool for analysis of structural distortions at oxide heterointerfaces [1]. Through such structural analysis we have recognized that the interfacial octahedral connection is an important factor controlling the properties of thin films. We have found that the octahedral connection can be engineered by A-site cation size [2] and the thickness of buffer layer inserted between the thin film and substrate [3].

Microscopic origin of the mobility enhancement at a spinel/perovskite oxide heterointerface revealed by photoemission spectroscopy

The spinel/perovskite heterointerface $\gamma$-Al$_2$O$_3$/SrTiO$_3$ hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO$_3$/SrTiO$_3$ by more than an order of magnitude despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and \textit{ab initio} calculations we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO$_3$ layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

Modulated Transport Behavior of Two-Dimensional Electron Gas at Ni-Doped LaAlO3/SrTiO3 Heterointerfaces.

Modulating transport behaviors of two-dimensional electron gases are of critical importance for applications of the next-generation multifunctional oxide electronics. In this study, transport behaviors of LaAlO3/SrTiO3 heterointerfaces modified through the Ni dopant and the light irradiation have been investigated. Through the Ni dopant, the resistances increase significantly and a resistance upturn phenomenon due to the Kondo effect is observed at T < 40 K. Under a 360 nm light irradiation, the interfaces exhibit a persistent photoconductivity and a suppressed Kondo effect at low temperature due to the increased mobility measured through the photo-Hall method. Moreover, the relative changes in resistance of interfaces induced by light are increased from 800 to 6600% at T = 12 K with increasing the substitution of Ni, which is discussed by the band bending and the lattice effect due to the Ni dopant. This work paves the way for better controlling the emerging properties of complex oxide heterointerfaces and would be helpful for photoelectric device applications based on all-oxides.

Magnetically Tunable Photocurrent in La0.7Sr0.3MnO3/BaSnO3 Heterojunctions

Artificially constructed oxide heterointerfaces have attracted much attention. Herein, the novel all‐perovskite p‐n heterojunction composed of a colossal magnetoresistive manganite La0.7Sr0.3 MnO3 (LSMO) and an n‐type transparent semiconducting BaSnO3 (BSO) is designed via optimizing the growth condition. This LSMO/BSO p‐n junction exhibits good rectification with a forward‐to‐reverse ratio of 275 at 1 V, high photo detection capability with a photo‐to‐dark current of 581.9 at ‐0.5 V, high ultraviolet light sensitivity with a UV (360 nm)‐to‐visible (532 nm) ratio of ∼2.4 × 103, and a significantly magneto‐tunable photocurrent with a variation ratio of ∼1.25 % under 532 nm illumination and 0.5 T magnetic field. As a result, combining synergistically the functionality of diode and magnetically tunable photo detector, the LSMO/BSO p‐n junction is a promising candidate for advanced magneto‐optoelectronic devices.

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.

(Invited) Beyond Electrostatic Effects at Oxide Hetero-Interfaces: Electrochemical Phase Change, Strong Electric Fields, and Elastic Strain

Transition metal oxide hetero-interfaces are interesting due to the distinctly different properties that can arise from their interfaces, such as memristive behavior, superconductivity, high catalytic activity and magnetism. These interfaces are the source for local heterogeneities in composition, atomic structure and electronic structure. Classically, defect redistribution is quantified at the continuum level by concurrent solution of Poisson’s equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. It is possible to inform this level of modeling with first principles calculations of band off-sets, and defect formation and segregation energies at thermodynamically relevant conditions. This approach had numerous successful implementations, including the quantification of charge transport properties at surfaces and grain boundaries. In this talk, I will discuss three phenomena that also need to be considered in a broader framework of defect structures and distributions at oxide hetero-interfaces. 1) Presence of strong electric fields that can cause polarization of defective systems and affect the defect abundance and structure. We have assessed this effect on neutral oxygen vacancies in simple binary oxides from first principles calculations. 2) Phase change under the effect of local electrostatic potential because of a change in the electrochemical potential of oxygen. We have assessed the ability to trigger phase change electrochemically in two classes of oxides, SrCoOx and VOx, and have quantified the phases and the corresponding distinctly different electronic properties by combining in operando x-ray diffraction and x-ray photoelectron and absorption spectroscopy. The results have implications both for oxide hetero-interfaces and for oxide electronic devices that aim to control properties electrically. 3) Elastic strain, that affect the stability and mobility of defects. In this recent work, we have focused on the stability of electronic defects, specifically the electron polarons versus free electrons SrTiO3, as a function of temperature and hydrostatic stress, by combining first principles calculations and quasi harmonic approximation. Our results demonstrate that it is possible to control the type of electronic defect, and so the transport properties, by means of electro-chemo-mechanics.

Dominance of Plasmonic Resonant Energy Transfer over Direct Electron Transfer in Substantially Enhanced Water Oxidation Activity of BiVO4 by Shape-Controlled Au Nanoparticles.

The performance of plasmonic Au nanostructure/metal oxide heterointerface shows great promise in enhancing photoactivity, due to its ability to confine light to the small volume inside the semiconductor and modify the interfacial electronic band structure. While the shape control of Au nanoparticles (NPs) is crucial for moderate bandgap semiconductors, because plasmonic resonance by interband excitations overlaps above the absorption edge of semiconductors, its critical role in water splitting is still not fully understood. Here, first, the plasmonic effects of shape-controlled Au NPs on bismuth vanadate (BiVO4 ) are studied, and a largely enhanced photoactivity of BiVO4 is reported by introducing the octahedral Au NPs. The octahedral Au NP/BiVO4 achieves 2.4 mA cm-2 at the 1.23 V versus reversible hydrogen electrode, which is the threefold enhancement compared to BiVO4 . It is the highest value among the previously reported plasmonic Au NPs/BiVO4 . Improved photoactivity is attributed to the localized surface plasmon resonance; direct electron transfer (DET), plasmonic resonant energy transfer (PRET). The PRET can be stressed over DET when considering the moderate bandgap semiconductor. Enhanced water oxidation induced by the shape-controlled Au NPs is applicable to moderate semiconductors, and shows a systematic study to explore new efficient plasmonic solar water splitting cells.

Conduction mechanism change with transport oxide layer thickness in oxide hetero-interface diode

An effective and facile strategy is proposed to demonstrate an engineered oxide hetero-interface of a thin film diode with a high current density and low operating voltage. The electrical characteristics of an oxide hetero-interface thin film diode are governed by two theoretical models: the space charge-limited current model and the Fowler-Nordheim (F-N) tunneling model. Interestingly, the dominant mechanism strongly depends on the insulator thickness, and the mechanism change occurs at a critical thickness. This paper shows that conduction mechanisms of oxide hetero-interface thin film diodes depend on thicknesses of transport oxide layers and that current densities of these can be exponentially increased through quantum tunneling in the diodes with the thicknesses less than 10 nm. These oxide hetero-interface diodes have great potential for low-powered transparent nanoscale applications.

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures.

A variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. However, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.

Anisotropic electrical resistance in mesoscopic LaAlO3/SrTiO3 devices with individual domain walls

The crystal structure of bulk SrTiO3(STO) transitions from cubic to tetragonal at around 105 K. Recent local scanning probe measurements of LaAlO3/SrTiO3 (LAO/STO) interfaces indicated the existence of spatially inhomogeneous electrical current paths and electrostatic potential associated with the structural domain formation in the tetragonal phase of STO. Here we report a study of temperature dependent electronic transport in combination with the polarized light microscopy of structural domains in mesoscopic LAO/STO devices. By reducing the size of the conductive interface to be comparable to that of a single tetragonal domain of STO, the anisotropy of interfacial electron conduction in relationship to the domain wall and its direction was characterized between T = 10–300 K. It was found that the four-point resistance measured with current parallel to the domain wall is larger than the resistance measured perpendicular to the domain wall. This observation is qualitatively consistent with the current diverting effect from a more conductive domain wall within the sample. Among all the samples studied, the maximum resistance ratio found is at least 10 and could be as large as 105 at T = 10 K. This electronic anisotropy may have implications on other oxide hetero-interfaces and the further understanding of electronic/magnetic phenomena found in LAO/STO.

Mobility Modulation and Suppression of Defect Formation in Two-Dimensional Electron Systems by Charge-Transfer Management

Electron mobility is one of the most-debated key attributes of low-dimensional electron systems emerging at complex oxide heterointerfaces. However, a common understanding of how electron mobility can be optimized in these systems has not been achieved so far. Here, we discuss a novel approach for achieving a systematic increase in electron mobility in polar/nonpolar perovskite interfaces by suppressing the thermodynamically required defect formation at the nanoscale. We discuss the transport properties of electron gases established at interfaces between SrTiO3 and various polar perovskites [LaAlO3, NdGaO3, and (La,Sr)(Al,Ta)O3], allowing for the individual variation of epitaxial strain and charge transfer among these epitaxial interfaces. As we show, the reduced charge transfer at (La,Sr)(Al,Ta)O3/SrTiO3 interfaces yields a systematic increase in electron mobility, while the reduced epitaxial strain has only minor impact. As thermodynamic continuum simulations suggest, the charge transfer across these interfaces affects both the spatial distribution of electrons and the background distribution of ionic defects, acting as major scatter centers within the potential well. Easing charge transfer in (La,Sr)(Al,Ta)O3/SrTiO3 yields an enlarged spatial separation of mobile charge carriers and scattering centers, as well as a reduced driving force for the formation of ionic defects at the nanoscale. Our results suggest a general recipe for achieving electron enhancements at oxide heterostructure interfaces and provide new perspectives for atomistic understanding of electron scattering in these systems.

Heterojunction oxide thin-film transistors with unprecedented electron mobility grown from solution.

Thin-film transistors made of solution-processed metal oxide semiconductors hold great promise for application in the emerging sector of large-area electronics. However, further advancement of the technology is hindered by limitations associated with the extrinsic electron transport properties of the often defect-prone oxides. We overcome this limitation by replacing the single-layer semiconductor channel with a low-dimensional, solution-grown In2O3/ZnO heterojunction. We find that In2O3/ZnO transistors exhibit band-like electron transport, with mobility values significantly higher than single-layer In2O3 and ZnO devices by a factor of 2 to 100. This marked improvement is shown to originate from the presence of free electrons confined on the plane of the atomically sharp heterointerface induced by the large conduction band offset between In2O3 and ZnO. Our finding underscores engineering of solution-grown metal oxide heterointerfaces as an alternative strategy to thin-film transistor development and has the potential for widespread technological applications.

Multiple Supersonic Phase Fronts Launched at a Complex-Oxide Heterointerface.

Selective optical excitation of a substrate lattice can drive phase changes across heterointerfaces. This phenomenon is a nonequilibrium analogue of static strain control in heterostructures and may lead to new applications in optically controlled phase change devices. Here, we make use of time-resolved nonresonant and resonant x-ray diffraction to clarify the underlying physics and to separate different microscopic degrees of freedom in space and time. We measure the dynamics of the lattice and that of the charge disproportionation in NdNiO_{3}, when an insulator-metal transition is driven by coherent lattice distortions in the LaAlO_{3} substrate. We find that charge redistribution propagates at supersonic speeds from the interface into the NdNiO_{3} film, followed by a sonic lattice wave. When combined with measurements of magnetic disordering and of the metal-insulator transition, these results establish a hierarchy of events for ultrafast control at complex-oxide heterointerfaces.

On the mobility of carriers at semi-coherent oxide heterointerfaces.

In the quest to develop new materials with enhanced ionic conductivity for battery and fuel cell applications, nano-structured oxides have attracted attention. Experimental reports indicate that oxide heterointerfaces can lead to enhanced ionic conductivity, but these same reports cannot elucidate the origin of this enhancement, often vaguely referring to pipe diffusion at misfit dislocations as a potential explanation. However, this highlights the need to understand the role of misfit dislocation structure at semi-coherent oxide heterointerfaces in modifying carrier mobilities. Here, we use atomistic and kinetic Monte Carlo (KMC) simulations to develop a model of oxygen vacancy migration at SrTiO3/MgO interfaces, chosen because the misfit dislocation structure can be modified by changing the termination chemistry. We use atomistic simulations to determine the energetics of oxygen vacancies at both SrO and TiO2 terminated interfaces, which are then used as the basis of the KMC simulations. While this model is approximate (as revealed by select nudged elastic band calculations), it highlights the role of the misfit dislocation structure in modifying the oxygen vacancy dynamics. We find that oxygen vacancy mobility is significantly reduced at either interface, with slight differences at each interface due to the differing misfit dislocation structure. We conclude that if such semi-coherent oxide heterointerfaces induce enhanced ionic conductivity, it is not a consequence of higher carrier mobility.

Mixed valence as a necessary criteria for quasi-two dimensional electron gas in oxide hetero-interfaces

Abstract The origin of quasi-two dimensional electron gas at the interface of polar-nonpolar oxide hetero-structure, such as LaAlO3/SrTiO3, is debated over electronic reconstruction and defects/disorder models. Common to these models is the partial valence transformation of substrate Ti ions from its equilibrium 4 + state to an itinerant 3 + state. Given that the Hf ions have a lower ionization potential than Ti due to the 4 f orbital screening, one would expect a hetero-interface conductivity in the polar-nonpolar LaAlO3/SrHfO3 system as well. However, our first principles calculations show the converse. Unlike the Ti 3 + − Ti 4 + valence transition which occur at a nominal energy cost, the barrier energy associated with its isoelectronic Hf 3 + − Hf 4 + counterpart is very high, hence suppressing the formation of quasi-two dimensional electron gas at LaAlO3/SrHfO3 hetero-interface. These calculations, therefore, emphasize on the propensity of mixed valence at the interface as a necessary condition for an oxide hetero-structure to exihibit quasi two-dimensional electron gas.

Thermodynamic Ground States of Complex Oxide Heterointerfaces.

The formation mechanism of 2-dimensional electron gases (2DEGs) at heterointerfaces between nominally insulating oxides is addressed with a thermodynamical approach. We provide a comprehensive analysis of the thermodynamic ground states of various 2DEG systems directly probed in high temperature equilibrium conductivity measurements. We unambiguously identify two distinct classes of oxide heterostructures: For epitaxial perovskite/perovskite heterointerfaces (LaAlO3/SrTiO3, NdGaO3/SrTiO3, and (La,Sr)(Al,Ta)O3/SrTiO3), we find the 2DEG formation being based on charge transfer into the interface, stabilized by the electric field in the space charge region. In contrast, for amorphous LaAlO3/SrTiO3 and epitaxial γ-Al2O3/SrTiO3 heterostructures, the 2DEG formation mainly relies on the formation and accumulation of oxygen vacancies. This class of 2DEG structures exhibits an unstable interface reconstruction associated with a quenched nonequilibrium state.

Unidirectional Oxide Hetero-Interface Thin-Film Diode with Improved Electrical Current

Thin film technology is a basic technology that is used various electronic device applications. It is becoming increasingly important in commerce and research such as TFT LCD, Semiconductor, solid state circuitry and chemical experiments. Many company and research institute are increasingly interested in low-voltage device, particularly in the commercial areas of new memory and LEDs. This research has focused on making high performance and low power diodes with oxide hetero-interface structure. We introduce a new method that increases on-current of unidirectional oxide hetero-interface thin-film diode at insulator thickness control. The unidirectional oxide hetero-interface thin-film diode consists of the metal anode, electron-transporting oxide(ETO), electron-injecting oxide(EIO), metal cathode (MEEM) structure. Previously reported MEEM diodes have a vulnerability which is low on-current density. So we reduced electron-transporting oxide (Y2O3) thickness below 10nm, gained high current density. Electrical characteristics of improved MEEM diode are 2X10-2A/cm2 (previous is lower than 3X10-6A/Cm2) on-current density measured at 2V. This MEEM diode was fabricated on 2.5cmX 2.5cm P++Si substrates. Next, a 3~10 nm thick Y2O3layer was deposited by atomic layer deposition (ALD). . Then, the deposition of the ZnO followed using radio frequency (RF) magnetron sputtering from ZnO target with an Ar working pressure of 10m torr and 25 sccm. The RF power was 91 W. We used Rapid Thermal Annealing (RTA) to anneal under atmospheric pressure at 350 ℃ for 1 minute. After RTA, a 100 nm thick aluminum layer was deposited as a top electrode thermal evaporator. Principle of MEEM diodes is as follows. Applying positive voltage at metal anode, electrons were injected to EIO from metal cathode, because the junction of metal cathode and EIO is ohmic-like. Then electrons were injected to ETO from EIO, because bandgap of ETO and EIO deceases due to interface dipole. Finally, electrons were passed to metal anode from ETO as space charge limited current (SCLC) modelling. On the contrary to this, applying negative voltage at metal anode, electrons were couldn`t pass to ETO from metal anode due to large band gap between metal and insulator. Figure 1-(b) shows J-V characteristics of diodes by various insulator thicknesses. The more insulator thickness decreases, the more on-current density increase. An equation for the SCLC in an insulator can be developed as shown below, [Equation (1)] where εiis the permittivity of the oxide, μ is the mobility, θ is the ratio of free and shallow trapped charge, and d is thickness of the oxide. An equation for the DT Tunneling in an insulator can be developed as shown below, [Equation (2)] where m* is the electron effective mass in the oxide, h is Planck’s constant, and ΦB is the junction barrier height, κ is the relative dielectric constant of the oxide layer and tox,eqis the equivalent oxide thickness(EOT). The current density characteristics of thicker insulators over to 10 nm and the simulated SCLC current fitting curves are as shown in figure 1-(b)(Blue line), while the simulated DT(Direct Tunneling) current for thinner insulators down to 10 nm are shown in figure 1-(b)(Red line). To use a MEEM diode in many applications, current density of the diode should be high and operating voltage should be low. However, the current of the existing MEEM diode was absurdly insufficient. In order to increase the current density, the electrical characterization was conducted on different insulator thickness. Especially when the thickness of the insulator is reduced to less than 10 nm, the current increases exponentially. Our fitting model of the results was changed DT model into SCLC model. As a result, current density is increase in a million times. Hence, improved MEEM diodes were confirmed to available as various applications such as crossbar array 1D1R RRAM device. Figure 1

Quantization of Hall Resistance at the Metallic Interface between an Oxide Insulator and SrTiO_{3}.

The two-dimensional metal forming at the interface between an oxide insulator and SrTiO_{3} provides new opportunities for oxide electronics. However, the quantum Hall effect, one of the most fascinating effects of electrons confined in two dimensions, remains underexplored at these complex oxide heterointerfaces. Here, we report the experimental observation of quantized Hall resistance in a SrTiO_{3} heterointerface based on the modulation-doped amorphous-LaAlO_{3}/SrTiO_{3} heterostructure, which exhibits both high electron mobility exceeding 10,000 cm^{2}/V s and low carrier density on the order of ∼10^{12} cm^{-2}. Along with unambiguous Shubnikov-de Haas oscillations, the spacing of the quantized Hall resistance suggests that the interface is comprised of a single quantum well with ten parallel conducting two-dimensional sub-bands. This provides new insight into the electronic structure of conducting oxide interfaces and represents an important step towards designing and understanding advanced oxide devices.

Epitaxial strain and its relaxation at the LaAlO3/SrTiO3 interface

A series of LaAlO3 thin films with different thicknesses were deposited by pulsed laser deposition at temperatures from 720 °C to 800 °C. The results from grazing incidence x-ray diffraction and reciprocal space mapping indicate that a thin layer of LaAlO3 adjacent to the SrTiO3 substrate remains almost coherently strained to the substrate, while the top layer starts to relax quickly above a certain critical thickness, followed by a gradual relaxation at larger film thickness when they are grown at lower temperatures. The atomic force microscopy results show that the fast relaxation is accompanied by the formation of cracks on the film surface. This can be ascribed to the larger energy release rate when compared with the resistance of LaAlO3 to cracking, according to calculations from the Griffith fracture theory. For films grown at 720 °C, a drop in sheet resistance by two orders of magnitude is observed when the top layer starts to relax, indicating a relationship between the strain and the conductivity of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface. The strain engineered by growth temperature provides a useful tool for the manipulation of the electronic properties of oxide heterointerfaces.

Unidirectional Oxide Hetero-Interface Thin-Film Diode with Improved Electrical Current

In this paper, we propose new structure of unidirectional oxide hetero-interface thin-film diode for enhanced on-current density with low operating voltage by inserting ultrathin insulator layer. It is figured out that the governing mechanism of thin film diode transfers from the space-charge-limited-conduction (SCLC) model to Fowler–Nordheim tunneling (FNT) model depending on the insulator thickness of below 10 nm. The current density of the ultrathin insulator film (3 nm thick) diode is 104 times higher than the diode with 30 nm thick insulator. Consequently, the simple engineering of insulator layer thickness enables to reach the high current density and low operating voltage. Furthermore, the ultrathin insulator film diode has strong potential for nano-scale applications such as resistive-random-access-memory (ReRAM) selectors and micro-electro-mechanical-systems (MEMS).