Metal-insulator transition in SrTiO(3-x) thin films induced by frozen-out carriers

Capping-layer-mediated lattice mismatch and redox reaction in SrTiO3-based bilayers

It is well known that the traditional two-dimensional electron system (2DES) hosted by the SrTiO₃substrate can exhibit diverse electronic states by modifying the capping layer in heterostructures. However, such capping layer engineering is less studied in the SrTiO₃-layer-carried 2DES (or bilayer 2DES), which is different from the traditional one on transport properties but more applicable to the thin-film devices. Here, several SrTiO₃bilayers are fabricated by growing various crystalline and amorphous oxide capping layers on the epitaxial SrTiO₃layers. For the crystalline bilayer 2DES, the monotonical reduction on the interfacial conductance, as well as carrier mobility, is recorded on increasing the lattice mismatch between the capping layers and epitaxial SrTiO₃layer. The mobility edge raised by the interfacial disorders is highlighted in the crystalline bilayer 2DES. On the other hand, when increasing the concentration of Al with high oxygen affinity in the capping layer, the amorphous bilayer 2DES becomes more conductive accompanied by the enhanced carrier mobility but almost constant carrier density. This observation cannot be explained by the simple redox-reaction model, and the interfacial charge screening and band bending need to be considered. Moreover, when the capping oxide layers have the same chemical composition but with different forms, the crystalline 2DES with a large lattice mismatch is more insulating than its amorphous counterpart, and vice versa. Our results shed some light on understanding the different dominant role in forming the bilayer 2DES using crystalline and amorphous oxide capping layer, which may be applicable in designing other functional oxide interfaces.

Electrically and optically erasable non-volatile two-dimensional electron gas memory

The high-mobility two-dimensional electron gas (2DEG) generated at the interface between two wide-band insulators, LaAlO₃ (LAO) and SrTiO₃ (STO), is an extensively researched topic. In this study, we have successfully realized reversible switching between metallic and insulating states of the 2DEG system via the application of optical illumination and positive pulse voltage induced by the introduction of oxygen vacancies as reservoirs for electrons. The positive pulse voltage irreversibly drives the electron to the defect energy level formed by the oxygen vacancies, which leads to the formation of the insulating state. Subsequently, the metallic state can be achieved via optical illumination, which excites the trapped electron back to the 2DEG potential well. The ON/OFF state is observed to be robust with a ratio exceeding 10⁶; therefore, the interface can be used as an electrically and optically erasable non-volatile 2DEG memory.

Physical insights into the Au growth on the surface of a LaAlO3/SrTiO3 heterointerface

Au growth on the LAO/STO substrate generates an optical peak in the wavelength region of 600–1200 nm due to the interaction of the Au s and dz2 orbitals with the O pz orbital of the LAO film.

Nature of electrons from oxygen vacancies and polar catastrophe at LaAlO3/SrTiO3interfaces

The relative significance of quantum conductivity correction and magnetic nature of electrons in understanding the intriguing low-temperature resistivity minimum and negative magnetoresistance (MR) of the two-dimensional electron gas at LaAlO₃/SrTiO₃interfaces has been a long outstanding issue since its discovery. Here we report a comparative magnetotransport study on amorphous and oxygen-annealed crystalline LaAlO₃/SrTiO₃heterostructures at a relatively high-temperature range, where the orbital scattering is largely suppressed by thermal fluctuations. Despite of a predominantly negative out-of-plane MR effect for both, the magnetotransport is isotropic for amorphous LaAlO₃/SrTiO₃while strongly anisotropic and well falls into a two-dimensional quantum correction frame for annealed crystalline LaAlO₃/SrTiO₃. These results clearly indicate that a large portion of electrons from oxygen vacancies are localized at low temperatures, serving as magnetic centers, while the electrons from the polar field are only weakly localized due to constructive interference between time-reversed electron paths in the clean limit and no signature of magnetic nature is visible.

Electronically reconfigurable complex oxide heterostructure freestanding membranes

In recent years, lanthanum aluminate/strontium titanate (LAO/STO) heterointerfaces have been used to create a growing family of nanoelectronic devices based on nanoscale control of LAO/STO metal-to-insulator transition. The properties of these devices are wide-ranging, but they are restricted by nature of the underlying thick STO substrate. Here, single-crystal freestanding membranes based on LAO/STO heterostructures were fabricated, which can be directly integrated with other materials via van der Waals stacking. The key properties of LAO/STO are preserved when LAO/STO membranes are formed. Conductive atomic force microscope lithography is shown to successfully create reversible patterns of nanoscale conducting regions, which survive to millikelvin temperatures. The ability to form reconfigurable conducting nanostructures on LAO/STO membranes opens opportunities to integrate a variety of nanoelectronics with silicon-based architectures and flexible, magnetic, or superconducting materials.

Photoresponsive properties at (0 0 1), (1 1 1) and (1 1 0) LaAlO3/SrTiO3 interfaces

We report the photoresponsive characteristics of (0 0 1), (1 1 0), and (1 1 1) LaAlO₃/SrTiO₃ heterointerfaces deposited at different oxygen pressures using a 360 nm light. The results show that LaAlO₃/SrTiO₃ interfaces with less oxygen vacancies exhibit a larger resistance change when illuminated by light and a slower recovery process when light is off. In addition, the (1 1 0) LaAlO₃/SrTiO₃ heterointerfaces present the smallest photoinduced change and residual photoinduced change in the resistance, which are related to the negligible polarization discontinuity at the interfaces. Our results provide a deeper insight into the photoinduced properties in the 2D electron gas system, paving the way for the design of oxide optoelectronic devices.

Erasable and recreatable two-dimensional electron gas at the heterointerface of SrTiO3 and a water-dissolvable overlayer

While benefiting greatly from electronics, our society also faces a major problem of electronic waste, which has already caused environmental pollution and adverse human health effects. Therefore, recyclability becomes a must-have feature in future electronics. Here, we demonstrate an erasable and recreatable two-dimensional electron gas (2DEG), which can be easily created and patterned by depositing a water-dissolvable overlayer of amorphous Sr₃Al₂O₆ (a-SAO) on SrTiO₃ (STO) at room temperature. The 2DEG can be repeatedly erased or recreated by depositing the a-SAO or dissolving in water, respectively. Photoluminescence results show that the 2DEG arises from the a-SAO-induced oxygen vacancy. Furthermore, by gradually depleting the 2DEG, a transition of nonlinear to linear Hall effect is observed, demonstrating an unexpected interfacial band structure. The convenience and repeatability in the creation of the water-dissolvable 2DEG with rich physics could potentially contribute to the exploration of next generation electronics, such as environment-friendly or water-soluble electronics.

Interface Engineering and Emergent Phenomena in Oxide Heterostructures

Complex oxide interfaces have mesmerized the scientific community in the last decade due to the possibility of creating tunable novel multifunctionalities, which are possible owing to the strong interaction among charge, spin, orbital, and structural degrees of freedom. Artificial interfacial modifications, which include defects, formal polarization, structural symmetry breaking, and interlayer interaction, have led to novel properties in various complex oxide heterostructures. These emergent phenomena not only serve as a platform for investigating strong electronic correlations in low-dimensional systems but also provide potentials for exploring next-generation electronic devices with high functionality. Herein, some recently developed strategies in engineering functional oxide interfaces and their emergent properties are reviewed.

Field-Effect Device Using Quasi-Two-Dimensional Electron Gas in Mass-Producible Atomic-Layer-Deposited Al2O3/TiO2 Ultrathin (<10 nm) Film Heterostructures

We report the field-effect transistors using quasi-two-dimensional electron gas generated at an ultrathin (∼10 nm) Al₂O₃/TiO₂ heterostructure interface grown via atomic layer deposition (ALD) on a SiO₂/Si substrate without using a single crystal substrate. The 2DEG at the Al₂O₃/TiO₂ interface originates from oxygen vacancies generated at the surface of the TiO₂ bottom layer during ALD of the Al₂O₃ overlayer. High-density electrons (∼10¹⁴ cm^-2) are confined within a ∼2.2 nm distance from the Al₂O₃/TiO₂ interface, resulting in a high on-current of ∼12 μA/μm. The ultrathin TiO₂ bottom layer is easy to fully deplete, allowing an extremely low off-current, a high on/off current ratio over 10⁸, and a low subthreshold swing of ∼100 mV/decade. Via the implementation of ALD, a mature thin-film process can facilitate mass production as well as three-dimensional integration of the devices.

Oxygen Electromigration and Energy Band Reconstruction Induced by Electrolyte Field Effect at Oxide Interfaces

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficient interfaces between SrTiO_{3} (STO) crystals and LaAlO_{3} (LAO) overlayer through the measurements of electrical transport, x-ray absorption spectroscopy, and photoluminescence spectra. We found that oxygen vacancies (O_{vac}) were filled selectively and irreversibly after gating due to oxygen electromigration at the amorphous LAO/STO interface, resulting in a reconstruction of its interfacial band structure. Because of the filling of O_{vac}, the amorphous interface also showed an enhanced electron mobility and quantum oscillation of the conductance. Further, the filling effect could be controlled by the degree of the crystallinity of the LAO overlayer by varying the growth temperatures. Our results reveal the different effects induced by electrolyte gating, providing further clues to understand the mechanism of electrolyte gating on buried interfaces and also opening a new avenue for constructing high-mobility oxide interfaces.

A quantum-mechanical investigation of oxygen vacancies and copper doping in the orthorhombic CaSnO3 perovskite

In this study we explore the implications of oxygen vacancy formation and of copper doping in the orthorhombic CaSnO3 perovskite, by means of density functional theory, focusing on energetic and electronic properties. In particular, the electronic charge distribution is analyzed by Mulliken, Hirshfeld-I, Bader and Wannier approaches. Calculations are performed at the PBE and the PBE0 level (for doping with Cu, only PBE0), with both spin-restricted and spin-unrestricted formulations; unrestricted calculations are used for spin-polarized cases and for the naturally open-shell cases (Cu doping). An oxygen vacancy is found to have the tendency to reduce Sn neighbors by giving rise to an energy band within the energy band-gap of the pristine system, close to the valence band. At variance with what happens in the CaTiO3 perovskite (also investigated here), an oxygen vacancy in the CaSnO3 perovskite is found to lose two valence electrons and thus to be positively charged so that no F-center is formed. Regarding Cu doping, when one Sn atom is substituted by a Cu one, the most stable configuration corresponds to having the Cu atom as a first neighbor to the vacancy. These findings shed some light on the catalytic and phosphorous host properties of this perovskite.

Physics of SrTiO3-based heterostructures and nanostructures: a review

This review provides a summary of the rich physics expressed within SrTiO3-based heterostructures and nanostructures. The intended audience is researchers who are working in the field of oxides, but also those with different backgrounds (e.g., semiconductor nanostructures). After reviewing the relevant properties of SrTiO3 itself, we will then discuss the basics of SrTiO3-based heterostructures, how they can be grown, and how devices are typically fabricated. Next, we will cover the physics of these heterostructures, including their phase diagram and coupling between the various degrees of freedom. Finally, we will review the rich landscape of quantum transport phenomena, as well as the devices that elicit them.

Strain effects on oxygen vacancy energetics in KTaO3

Due to lattice mismatch between epitaxial films and substrates, in-plane strain fields are produced in the thin films, with accompanying structural distortions, and ion implantation can be used to controllably engineer the strain throughout the film. Because of the strain profile, local defect energetics are changed. In this study, the effects of in-plane strain fields on the formation and migration of oxygen vacancies in KTaO₃ are investigated using first-principles calculations. In particular, the doubly positive charged oxygen vacancy (V) is studied, which is considered to be the main charge state of the oxygen vacancy in KTaO₃. We find that the formation energies for oxygen vacancies are sensitive to in-plane strain and oxygen position. The local atomic configuration is identified, and strong relaxation of local defect structure is mainly responsible for the formation characteristics of these oxygen vacancies. Based on the computational results, formation-dependent site preferences for oxygen vacancies are expected to occur under epitaxial strain, which can result in orders of magnitude differences in equilibrium vacancy concentrations on different oxygen sites. In addition, all possible migration pathways, including intra- and inter-plane diffusions, are considered. In contrast to the strain-enhanced intra-plane diffusion, the diffusion in the direction normal to the strained plane is impeded under the epitaxial strain field. These anisotropic diffusion processes can further enhance site preferences.

Influence of In-Gap States on the Formation of Two-Dimensional Election Gas at ABO3/SrTiO3 Interfaces

We explored in-gap states (IGSs) in perovskite oxide heterojunction films. We report that IGSs in these films play a crucial role in determining the formation and properties of interfacial two-dimensional electron gas (2DEG). We report that electron trapping by IGSs opposes charge transfer from the film to the interface. The IGS in films yielded insulating interfaces with polar discontinuity and explained low interface carrier density of conducting interfaces. An ion trapping model was proposed to explain the physics of the IGSs and some experimental findings, such as the unexpected formation of 2DEG at the initially insulating LaCrO₃/SrTiO₃ interface and the influence of substitution layers on 2DEG.

Tuning the Two-Dimensional Electron Liquid at Oxide Interfaces by Buffer-Layer-Engineered Redox Reactions

Polar discontinuities and redox reactions provide alternative paths to create two-dimensional electron liquids (2DELs) at oxide interfaces. Herein, we report high mobility 2DELs at interfaces involving SrTiO₃ (STO) achieved using polar La_7/8Sr_1/8MnO₃ (LSMO) buffer layers to manipulate both polarities and redox reactions from disordered overlayers grown at room temperature. Using resonant X-ray reflectometry experiments, we quantify redox reactions from oxide overlayers on STO as well as polarity induced electronic reconstruction at epitaxial LSMO/STO interfaces. The analysis reveals how these effects can be combined in a STO/LSMO/disordered film trilayer system to yield high mobility modulation doped 2DELs, where the buffer layer undergoes a partial transformation from perovskite to brownmillerite structure. This uncovered interplay between polar discontinuities and redox reactions via buffer layers provides a new approach for the design of functional oxide interfaces.

Tailoring the Doping Mechanisms at Oxide Interfaces in Nanoscale

Here, we demonstrate the nanoscale manipulations of two types of charge transfer to the LaAlO₃/SrTiO₃ interfaces: one from surface adsorbates and another from oxygen vacancies inside LaAlO₃ films. This method can be used to produce multiple insulating and metallic interface states with distinct carrier properties that are highly stable in air. By reconfiguring the patterning and comparing interface structures formed from different doping sources, effects of extrinsic and intrinsic material characters on the transport properties can be distinguished. In particular, a multisubband to single-subband transition controlled by the structural phases in SrTiO₃ was revealed. In addition, the transient behaviors of nanostructures also provided a unique opportunity to study the nanoscale diffusions of adsorbates and oxygen vacancies in oxide heterostructures. Knowledge of such dynamic processes is important for nanodevice implementations.

Oxygen Vacancy Linear Clustering in a Perovskite Oxide

Oxygen vacancies have been implicitly assumed isolated ones, and understanding oxide materials possibly containing oxygen vacancies remains elusive within the scheme of the isolated vacancies, although the oxygen vacancies have been playing a decisive role in oxide materials. Here, we report the presence of oxygen vacancy linear clusters and their orientation along a specific crystallographic direction in SrTiO₃, a representative of a perovskite oxide. The presence of the linear clusters and associated electron localization was revealed by an electronic structure represented in the increase in the Ti²⁺ valence state or corresponding Ti 3d² electronic configuration along with divacancy cluster model analysis and transport measurement. The orientation of the linear clusters along the [001] direction in perovskite SrTiO₃ was verified by further X-ray diffuse scattering analysis. Because SrTiO₃ is an archetypical perovskite oxide, the vacancy linear clustering with the specific aligned direction and electron localization can be extended to a wide variety of the perovskite oxides.

Origin and Quenching of Novel ultraviolet and blue emission in NdGaO3: Concept of Super-Hydrogenic Dopants

In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth based perovskite NdGaO₃ (NGO) and the systematic quench of the NGO photoluminescence (PL) by Ce doping. Study of room temperature PL was performed in both single-crystal and polycrystalline NGO (substrates and pellets) respectively. Several NGO pellets were prepared with varying Ce concentration and their room temperature PL was studied using 325 nm laser. It was found that the PL intensity shows a systematic quench with increasing Ce concentration. XPS measurements indicated that nearly 50% of Ce atoms are in the 4+ state. The PL quench was attributed to the novel concept of super hydrogenic dopant (SHD)”, where each Ce⁴⁺ ion contributes an electron which forms a super hydrogenic atom with an enhanced Bohr radius, due to the large dielectric constant of the host. Based on the critical Ce concentration for complete quenching this SHD radius was estimated to be within a range of 0.85 nm and 1.15 nm whereas the predicted theoretical value of SHD radius for NdGaO3 is ~1.01 nm.

Controlling Kondo-like Scattering at the SrTiO3-based Interfaces

The observation of magnetic interaction at the interface between nonmagnetic oxides has attracted much attention in recent years. In this report, we show that the Kondo-like scattering at the SrTiO3-based conducting interface is enhanced by increasing the lattice mismatch and growth oxygen pressure PO2. For the 26-unit-cell LaAlO3/SrTiO3 (LAO/STO) interface with lattice mismatch being 3.0%, the Kondo-like scattering is observed when PO2 is beyond 1 mTorr. By contrast, when the lattice mismatch is reduced to 1.0% at the (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 (LSAT/STO) interface, the metallic state is always preserved up to PO2 of 100 mTorr. The data from Hall measurement and X-ray absorption near edge structure (XANES) spectroscopy reveal that the larger amount of localized Ti(3+) ions are formed at the LAO/STO interface compared to LSAT/STO. Those localized Ti(3+) ions with unpaired electrons can be spin-polarized to scatter mobile electrons, responsible for the Kondo-like scattering observed at the LAO/STO interface.

The Effect of Polar Fluctuation and Lattice Mismatch on Carrier Mobility at Oxide Interfaces

Since the discovery of two-dimensional electron gas (2DEG) at the oxide interface of LaAlO3/SrTiO3 (LAO/STO), improving carrier mobility has become an important issue for device applications. In this paper, by using an alternate polar perovskite insulator (La0.3Sr0.7) (Al0.65Ta0.35)O3 (LSAT) for reducing lattice mismatch from 3.0% to 1.0%, the low-temperature carrier mobility has been increased 30 fold to 35,000 cm(2) V(-1) s(-1). Moreover, two critical thicknesses for the LSAT/STO (001) interface are found, one at 5 unit cells for appearance of the 2DEG and the other at 12 unit cells for a peak in the carrier mobility. By contrast, the conducting (110) and (111) LSAT/STO interfaces only show a single critical thickness of 8 unit cells. This can be explained in terms of polar fluctuation arising from LSAT chemical composition. In addition to lattice mismatch and crystal symmetry at the interface, polar fluctuation arising from composition has been identified as an important variable to be tailored at the oxide interfaces to optimize the 2DEG transport.

Full Electroresistance Modulation in a Mixed-Phase Metallic Alloy

We report a giant, ∼22%, electroresistance modulation for a metallic alloy above room temperature. It is achieved by a small electric field of 2  kV/cm via piezoelectric strain-mediated magnetoelectric coupling and the resulting magnetic phase transition in epitaxial FeRh/BaTiO_{3} heterostructures. This work presents detailed experimental evidence for an isothermal magnetic phase transition driven by tetragonality modulation in FeRh thin films, which is in contrast to the large volume expansion in the conventional temperature-driven magnetic phase transition in FeRh. Moreover, all the experimental results in this work illustrate FeRh as a mixed-phase model system well similar to phase-separated colossal magnetoresistance systems with phase instability therein.

Giant conductivity switching of LaAlO3/SrTiO3 heterointerfaces governed by surface protonation

Complex-oxide interfaces host a diversity of phenomena not present in traditional semiconductor heterostructures. Despite intense interest, many basic questions remain about the mechanisms that give rise to interfacial conductivity and the role of surface chemistry in dictating these properties. Here we demonstrate a fully reversible >4 order of magnitude conductance change at LaAlO3/SrTiO3 (LAO/STO) interfaces, regulated by LAO surface protonation. Nominally conductive interfaces are rendered insulating by solvent immersion, which deprotonates the hydroxylated LAO surface; interface conductivity is restored by exposure to light, which induces reprotonation via photocatalytic oxidation of adsorbed water. The proposed mechanisms are supported by a coordinated series of electrical measurements, optical/solvent exposures, and X-ray photoelectron spectroscopy. This intimate connection between LAO surface chemistry and LAO/STO interface physics bears far-reaching implications for reconfigurable oxide nanoelectronics and raises the possibility of novel applications in which electronic properties of these materials can be locally tuned using synthetic chemistry.

Epitaxial Growth of Intermetallic MnPt Films on Oxides and Large Exchange Bias

High-quality epitaxial growth of inter-metallic MnPt films on oxides is achieved, with potential for multiferroic heterostructure applications. Antisite-stabilized spin-flipping induces ferromagnetism in MnPt films, although it is robustly antiferromagnetic in bulk. Moreover, highly ordered antiferromagnetic MnPt films exhibit superiorly large exchange coupling with a ferromagnetic layer.

The effect of oxygen vacancies on water wettability of transition metal based SrTiO3 and rare-earth based Lu2O3

The effect of oxygen vacancy on water wettability of different oxide surfaces are studied and different type of interface interaction is found in 3d and 4f based oxide surfaces..

Transport properties of the n-type SrTiO3/LaAlO3 interface

The thermoelectric properties of the (001) n-type 6.5STO/1.5LAO interface were investigated by means of the all-electron full-potential method based on the semi-classical Boltzmann theory.

Visible-light-accelerated oxygen vacancy migration in strontium titanate

Strontium titanate is a model transition metal oxide that exhibits versatile properties of special interest for both fundamental and applied researches. There is evidence that most of the attractive properties of SrTiO3 are closely associated with oxygen vacancies. Tuning the kinetics of oxygen vacancies is then highly desired. Here we reported on a dramatic tuning of the electro-migration of oxygen vacancies by visible light illumination. It is found that, through depressing activation energy for vacancy diffusion, light illumination remarkably accelerates oxygen vacancies even at room temperature. This effect provides a feasible approach towards the modulation of the anionic processes. The principle proved here can be extended to other perovskite oxides, finding a wide application in oxide electronics.

Interface mediated semiconducting to metallic like transition in ultrathin Bi2Se3 films on (100) SrTiO3 grown by molecular beam epitaxy

Compressive strains due to the structural phase transition of SrTiO₃ substrate facilitate semiconductor to metal like transition in Bi₂Se₃ thin films.

Effects of microstructures on the compression behavior and phase transition routine of In2O3 nanocubes under high pressures

In addition to size and morphology, the microstructure may play an important role and induce differences in the compression and phase transition behaviors of nanomaterials under high pressures.

Visible-light-enhanced gating effect at the LaAlO₃/SrTiO₃ interface

Electrostatic gating field and light illumination are two widely used stimuli for semiconductor devices. Via capacitive effect, a gate field modifies the carrier density of the devices, while illumination generates extra carriers by exciting trapped electrons. Here we report an unusual illumination-enhanced gating effect in a two-dimensional electron gas at the LaAlO3/SrTiO3 interface, which has been the focus of emergent phenomena exploration. We find that light illumination decreases, rather than increases, the carrier density of the gas when the interface is negatively gated through the SrTiO3 layer, and the density drop can be 20 times as large as that caused by the conventional capacitive effect. This effect is further found to stem from an illumination-accelerated interface polarization, an originally extremely slow process. This unusual effect provides a promising controlling of the correlated oxide electronics in which a much larger gating capacity is demanding due to their intrinsic larger carrier density.

Oxygen vacancy clustering and pseudogap behaviour at the LaAlO₃/SrTiO₃ interface

The two-dimensional electron gas at the LaAlO3/SrTiO3 interface promises to add a new dimension to emerging electronic devices due to its high degree of tunability. Defects in the form of oxygen vacancies in titanate surfaces and interfaces, on the other hand, play a key role in the emergence of the ordered states and their tunability at the interface. On the basis of an effective model, we study the influence of oxygen vacancies on the superconductivity and ferromagnetism at the LaAlO3/SrTiO3 interface. Using the Bogoliubov-de Gennes formulation in conjunction with Monte Carlo simulation, we find a clustering of the oxygen vacancies at the interface that favours the formation of coexisting ferromagnetic puddles spatially separated from the superconductivity. We also find a carrier freeze-out at low temperatures, observed experimentally in a wide variety of samples. A sufficiently large amount of oxygen vacancies leads to pseudogap-like behaviour in the superconducting state.

Neutral defects in SrTiO3 studied with screened hybrid density functional theory

The properties of neutral defects in SrTiO3 are calculated using the screened hybrid density functional of Heyd, Scuseria, and Ernzerhof. The formation energies, the crystal field splittings affecting the SrTiO3 band structure, and the relaxation geometries around each defect are discussed. Oxygen vacancies introduced in SrTiO3 are found to cause a small tetragonal elongation of the lattice along the z-axis. The resulting conduction band minimum electron effective masses deviate from the bulk values and support the proposal of enhanced electron mobility along the direction of the compressive strain. The locations of the various defect bands within the SrTiO3 gap are estimated without introducing any post hoc corrections, thus allowing a more reliable comparison with experiment.

Creation and control of two-dimensional electron gas using Al-based amorphous oxides/SrTiO₃ heterostructures grown by atomic layer deposition

The formation of a two-dimensional electron gas (2-DEG) using SrTiO(3) (STO)-based heterostructures provides promising opportunities in oxide electronics. We realized the formation of 2-DEG using several amorphous layers grown by the atomic layer deposition (ALD) technique at 300 °C which is a process compatible with mass production and thereby can provide the realization of potential applications. We found that the amorphous LaAlO(3) (LAO) layer grown by the ALD process can generate 2-DEG (∼1 × 10(13)/cm(2)) with an electron mobility of 4-5 cm(2)/V·s. A much higher electron mobility was observed at lower temperatures. More remarkably, amorphous YAlO(3) (YAO) and Al(2)O(3) layers, which are not polar-perovskite-structured oxides, can create 2-DEG as well. 2-DEG was created by means of the important role of trimethylaluminum, Me(3)Al, as a reducing agent for STO during LAO and YAO ALD as well as the Al(2)O(3) ALD process at 300 °C. The deposited oxide layer also plays an essential role as a catalyst that enables Me(3)Al to reduce the STO. The electrons were localized very near to the STO surface, and the source of carriers was explained based on the oxygen vacancies generated in the STO substrate.