Oxide 2DEGs Research Articles

Enabling 2D Electron Gas with High Room-Temperature Electron Mobility Exceeding 100cm2 Vs-1 at a Perovskite Oxide Interface.

In perovskite oxide heterostructures, bulk functional properties coexist with emergent physical phenomena at epitaxial interfaces. Notably, charge transfer at the interface between two insulating oxide layers can lead to the formation of a 2D electron gas (2DEG) with possible applications in, e.g., high-electron-mobility transistors and ferroelectric field-effect transistors. So far, the realization of oxide 2DEGs is, however, largely limited to the interface between the single-crystal substrate and epitaxial film, preventing their deliberate placement inside a larger device architecture. Additionally, the substrate-limited quality of perovskite oxide interfaces hampers room-temperature (RT) 2DEG performance due to notoriously low electron mobility. In this work, the controlled creation of an interfacial 2DEG at the epitaxial interface between perovskite oxides BaSnO3 and LaInO3 is demonstrated with enhanced RT electron mobility values up to 119 cm2 Vs-1-the highest RT value reported so far for a perovskite oxide 2DEG. Using a combination of state-of-the-art deposition modes during oxide molecular beam epitaxy, this approach opens up another degree of freedom in optimization and in situ control of the interface between two epitaxial oxide layers away from the substrate interface. Thus this approach is expected to apply to the general class of perovskite oxide 2DEG systems and to enable their improved compatibility with novel device concepts and integration across materialsplatforms.

Visible-Light-Driven Semiconductor–Metal Transition in Electron Gas at the (100) Surface of KTaO3

Two-dimensional electron gas (2DEG) at the (100) KTaO3(KTO) surface and interfaces has attracted extensive interest because of its abundant physical properties. Here, light illumination-induced semiconductor–metal transition in the 2DEG at the KTO surface was investigated. 2DEG was formed at the surface of KTO by argon ion bombardment. The 2DEG prepared with a shorter bombardment time (300 s) exhibits semiconducting behavior in the range of 20~300 K in the dark. However, it shows a different resistance behavior, namely, a metallic state above ~55 K and a semiconducting state below ~55 K when exposed to visible light (405 nm) with a giant conductivity increase of about eight orders of magnitude at 20 K. The suppression of the semiconducting behavior is found to be more pronounced with increasing light power. After removing the illumination, the resistance cannot recover quickly, exhibiting persistent photoconductivity. More interestingly, the photoresponse of the 2DEG below 50 K was almost independent of the laser wavelength, although the photon energy is lower than the band gap of KTO. The present results provide experimental support for tuning oxide 2DEG by photoexcitation, suggesting promising applications of KTO-based 2DEG in future electronic and optoelectronic devices.

Reversible Photomodulation of Two-Dimensional Electron Gas in LaAlO3/SrTiO3 Heterostructures.

Long-lived photoinduced conductance changes in LaAlO3/SrTiO3 (LAO/STO) heterostructures enable their use in optoelectronic memory applications. However, it remains challenging to quench the persistent photoconductivity (PPC) instantly and reproducibly, which limits the reversible optoelectronic switching. Herein, we demonstrate a reversible photomodulation of two-dimensional electron gas (2DEG) in LAO/STO heterostructures with high reproducibility. By irradiating UV pulses, the 2DEG at the LAO/STO interface is gradually transformed to the PPC state. Notably, the PPC can be completely removed by water treatment when two key requirements are met: (1) the moderate oxygen deficiency in STO and (2) the minimal band edge fluctuation at the interface. Through our X-ray photoelectron spectroscopy and electrical noise analysis, we reveal that the reproducible change in the conductivity of 2DEG is directly attributed to the surface-driven electron relaxation in the STO. Our results provide a stepping-stone toward developing optically tunable memristive devices based on oxide 2DEG systems.

Non-volatile electric control of spin-orbit torques in an oxide two-dimensional electron gas

Spin-orbit torques (SOTs) have opened a novel way to manipulate the magnetization using in-plane current, with a great potential for the development of fast and low power information technologies. It has been recently shown that two-dimensional electron gases (2DEGs) appearing at oxide interfaces provide a highly efficient spin-to-charge current interconversion. The ability to manipulate 2DEGs using gate voltages could offer a degree of freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for spin-orbitronics, in which the sign and amplitude of SOTs at a given current are fixed by the stack structure. Here, we report the non-volatile electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We demonstrate that the 2DEG is controlled using a back-gate electric-field, providing two remanent and switchable states, with a large resistance contrast of 1064%. The SOTs can then be controlled electrically in a non-volatile way, both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO heterostructures with large perpendicular magnetization further validates the compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT oscillators, skyrmion and domain-wall-based devices, and magnonic circuits.

High-photoresponsivity heterojunction based on MoTe2/2D electron gas at the LaAlO3/SrTiO3 interface

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are ideal elements for many optoelectronic devices owing to their outstanding optoelectrical performance under visible and infrared light. Heterostructures composed of TMDs and other non-TMD materials may exhibit rich properties. In this study, a high-performance heterojunction based on 2D MoTe2 and 2D electron gas (2DEG) at the LaAlO3/SrTiO3 interface was fabricated. The device exhibits good current rectification properties with a high rectification ratio exceeding 103 and a low leakage current (∼1 nA at −6 V bias). Moreover, a high photoresponsivity of ∼800 A W−1 and a large specific detectivity of 4 × 1012 Jones at 405 nm were also obtained at room temperature. Heterostructures based on 2D TMDs and oxide 2DEG are expected to become essential elements in multifunctional microdevices and optoelectronic devices.

Circular Photogalvanic Effect in Oxide Two-Dimensional Electron Gases.

Two-dimensional electron gases (2DEGs) at the LaAlO_{3}/SrTiO_{3} interface have attracted wide interest, and some exotic phenomena are observed, including 2D superconductivity, 2D magnetism, and diverse effects associated with Rashba spin-orbit coupling. Despite the intensive investigations, however, there are still hidden aspects that remain unexplored. For the first time, here we report on the circular photogalvanic effect (CPGE) for the oxide 2DEG. Spin polarized electrons are selectively excited by circular polarized light from the in-gap states of SrTiO_{3} to 2DEG and are converted into electric current via the mechanism of spin-momentum locking arising from Rashba spin-orbit coupling. Moreover, the CPGE can be effectively modified by the density and distribution of oxygen vacancies. This Letter presents an effective approach to generate and manipulate the spin polarized current, paving the way toward oxide spintronics.

From Low-Field Sondheimer Oscillations to High-Field Very Large and Linear Magnetoresistance in a SrTiO3-Based Two-Dimensional Electron Gas.

Quantum materials harbor a cornucopia of exotic transport phenomena challenging our understanding of condensed matter. Among these, a giant, nonsaturating linear magnetoresistance (MR) has been reported in various systems, from Weyl semimetals to topological insulators. Its origin is often ascribed to unusual band structure effects, but it may also be caused by extrinsic sample disorder. Here, we report a very large linear MR in a SrTiO3 two-dimensional electron gas and, by combining transport measurements with electron spectromicroscopy, show that it is caused by nanoscale inhomogeneities that are self-organized during sample growth. Our data also reveal semiclassical Sondheimer oscillations arising from interferences between helicoidal electron trajectories, from which we determine the 2DEG thickness. Our results bring insight into the origin of linear MR in quantum materials, expand the range of functionalities of oxide 2DEGs, and suggest exciting routes to explore the interaction of linear MR with features like Rashba spin-orbit coupling.

Room temperature large photoresponse in p-n heterojunction composed of WSe2 and 2D electron gas at LaAlO3/KTaO3 interface

The heterostructures combining 2D layered material and two-dimensional electron gas (2DEG) at the surface or interfaces of complex oxides may exhibit abundant and interesting physical properties. In this paper, we report the development and photoelectrical properties of a p-n heterojunction composed of a p-type 2D layered material WSe2 and n-type 2DEG at the LaAlO3/KTaO3 interface. The WSe2 flake was first fabricated by mechanical exfoliation and then integrated with 2DEG at the LaAlO3/KTaO3 interface obtained using pulsed laser deposition. The WSe2/2DEG heterostructure shows good rectifying behavior with a low leakage current at room temperature. Additionally, compared with oxide 2DEG, the WSe2/2DEG heterostructure exhibits greatly enhanced visible-light photoresponse. The results imply that the 2D heterostructures based on disparate material systems can be realized and demonstrate potential applications in nanoscale electronics and optoelectronics, which are of great scientific and technological significances.

Harnessing Conductive Oxide Interfaces for Resistive Random-Access Memories

Two-dimensional electron gases (2DEGs) can be formed at some oxide interfaces, providing a fertile ground for creating extraordinary physical properties. These properties can be exploited in various novel electronic devices such as transistors, gas sensors, and spintronic devices. Recently several works have demonstrated the application of 2DEGs for resistive random-access memories (RRAMs). We briefly review the basics of oxide 2DEGs, emphasizing scalability and maturity and describing a recent trend of progression from epitaxial oxide interfaces (such as LaAlO3/SrTiO3) to simple and highly scalable amorphous-polycrystalline systems (e.g., Al2O3/TiO2). We critically describe and compare recent RRAM devices based on these systems and highlight the possible advantages and potential of 2DEGs systems for RRAM applications. We consider the immediate challenges to revolve around scaling from one device to large arrays, where further progress with series resistance reduction and fabrication techniques needs to be made. We conclude by laying out some of the opportunities presented by 2DEGs based RRAM, including increased tunability and design flexibility, which could, in turn, provide advantages for multi-level capabilities.

Switchable two-dimensional electron gas based on ferroelectric Ca: SrTiO3

Two-dimensional electron gases (2DEGs) can form at the surface of oxides and semiconductors or in carefully designed quantum wells and interfaces. Depending on the shape of the confining potential, 2DEGs may experience a finite electric field, which gives rise to relativistic effects such as the Rashba spin-orbit coupling. Although the amplitude of this electric field can be modulated by an external gate voltage, which in turn tunes the 2DEG carrier density, sheet resistance and other related properties, this modulation is volatile. Here, we report the design of a ''ferroelectric'' 2DEG whose transport properties can be electrostatically switched in a non-volatile way. We generate a 2DEG by depositing a thin Al layer onto a SrTiO$_3$ single crystal in which 1 percent of Sr is substituted by Ca to make it ferroelectric. Signatures of the ferroelectric phase transition at 25 K are visible in the Raman response and in the temperature dependences of the carrier density and sheet resistance that shows a hysteretic dependence on electric field as a consequence of ferroelectricity. We suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of ferromagnet-free spintronic architectures.

Modulated in-plane carrier distribution of oxide two-dimensional electron gas systems by light assisted electrostatic gating

The oxide two-dimensional electron gases (2DEGs) have been intensively studied for both fundamental and applied research over the past decade. It is known that the electrostatic and optical gating can both effectively modulate the transport behavior of 2DEG. However, the in-plane carrier distribution under the light assisted electrostatic gating is scarcely studied. In this study, we systematically investigate this problem in the 3d and 5d oxide-based 2DEGs samples, and observe strong lateral photovoltaic effect (LPE) in both of the 2DEGs. Remarkably, the lateral voltage is strongly dependent on the gating electrical field especially in the 5d oxide-based 2DEGs and its signs are absolutely reversed as the bias voltage becomes negative. Through systematic research, it is found that the leak current caused by light assisted electrostatic gating, strongly affects the in-plane carrier distribution. These results have significance for fundamental research and device application based on the photovoltaic effect of oxide 2DEGs.

Quantized conductance in a one-dimensional ballistic oxide nanodevice

The electric-field effect control of two-dimensional electron gases (2-DEGs) has allowed nanoscale electron quantum transport to be explored in semiconductors. Structures based on transition metal oxides have electronic states that favour the emergence of novel quantum orders that are absent in conventional semiconductors and the 2-DEG formed at a LaAlO3/SrTiO3 interface—a structure in which superconductivity and spin–orbit coupling can coexist—is a promising platform to develop devices for spintronics and topological electronics. However, field-effect control of the properties of this interface at the nanoscale remains challenging. Here we show that a quantum point contact can be formed in a LaAlO3/SrTiO3 interface through electrostatic confinement of the 2-DEG using a split gate. Our device exhibits a quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels. Under a magnetic field, the direct observation of the Zeeman splitting between spin-polarized bands allows the determination of the Lande g-factor, whose value differs strongly from that of the free electrons. Through source–drain voltage measurements, we also performed a spectroscopic investigation of the 3d energy levels inside the quantum point contact. The LaAlO3/SrTiO3 quantum point contact could potentially be used as a spectrometer to probe Majorana states in an oxide 2-DEG. A quantum point contact formed in the two-dimensional electron gas of a LaAlO3/SrTiO3 interface exhibits quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels.

Electrolyte gate controlled metal-insulator transitions of the CaZrO3/SrTiO3 heterointerface

Two-dimensional electron gas (2DEG) at a complex oxide interface shows an extraordinary spectrum of intriguing phenomena and functionality. Another oxide 2DEG was recently created via strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO3/SrTiO3 (CZO/STO). Herein, we report an effective way to tune the CZO/STO interface via ionic liquid (IL) electrolyte gating. An unexpected metal-insulator transition of the interfacial 2DEG occurs readily with the immersion of the sample in an IL even before the gate voltage is applied. This suggests the presence of intrinsic polarization of CZO, which could act as a negative bias. The carrier density is found to be suppressed and shows a temperature-independent behavior after electrolyte gating which also resulted in higher electron mobility. These results suggest that the oxygen vacancies are annihilated via oxygen electromigration to the interface induced by electrolyte gating. The effective tunability by IL gating shed more light on the mechanism of electrolyte gating on the buried heterointerface.

Progress and Perspectives of Atomically Engineered Perovskite Oxide Interfaces for Electronics and Electrocatalysts

AbstractThe 2D electron gases (2DEGs) at the surfaces and interfaces of SrTiO3‐based homo‐ and heterostructures provide new opportunities for electronics and spintronics. Herein, the recent progresses in the creation of advanced material systems of conducting oxide interfaces with engineered electronic reconstructions and redox reactions, as well as their characterization by a nondestructive resonant X‐ray reflectivity technique, are summarized. Moreover, the development of modulation‐doped high‐mobility oxide 2DEGs and the magnetic‐proximity‐induced spin‐polarized 2DEGs at oxide interfaces are also discussed. Finally, the perspectives on design of conducting oxide interfaces for a new generation of quantum devices and mixed electronic and ionic conducting electrocatalysts are addressed. Atomically engineered oxide interfaces will represent a unique family of quantum materials for future information and energy technologies.

Two-dimensional electron gas oxide remote doping of Si(001)

We demonstrate the integration of a two-dimensional electron gas (2DEG) oxide structure composed of $\mathrm{LaTi}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (LTO/STO) on undoped Si(001) that possesses the attractive attributes of high charge density from the oxide and high mobility of silicon. Key to this approach is modification of the oxygen content at the STO-Si interface, which tunes the band alignment and induces electron carriers to move from the oxide 2DEG to form a 2DEG in the silicon substrate. As a consequence, the overall mobility of the heterostructure increases by two orders of magnitude compared to that in the oxide 2DEG to up to $100\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$ at room temperature, with a carrier density of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$. This approach can be applied to technologies that require both high carrier and high carrier mobility for applications in plasmonics and high power electronics.

Observation of Out-of-Plane Spin Texture in a SrTiO_{3}(111) Two-Dimensional Electron Gas.

We explore the second order bilinear magnetoelectric resistance (BMER) effect in the d-electron-based two-dimensional electron gas (2DEG) at the SrTiO_{3}(111) surface. We find evidence of a spin-split band structure with the archetypal spin-momentum locking of the Rashba effect for the in-plane component. Under an out-of-plane magnetic field, we find a BMER signal that breaks the sixfold symmetry of the electronic dispersion, which is a fingerprint for the presence of a momentum-dependent out-of-plane spin component. Relativistic electronic structure calculations reproduce this spin texture and indicate that the out-of-plane component is a ubiquitous property of oxide 2DEGs arising from strong crystal field effects. We further show that the BMER response of the SrTiO_{3}(111) 2DEG is tunable and unexpectedly large.

Band offsets at amorphous-crystalline Al2O3–SrTiO3 oxide interfaces

2D electron gases (2DEGs) formed at oxide interfaces provide a rich testbed for fundamental physics and device applications. While the discussion of the physical origins of this phenomenon continues, the recent discovery of oxide 2DEGs at non-epitaxial interfaces between amorphous and crystalline oxides provides useful insight into this debate. Furthermore, using amorphous oxides offers a low-cost route towards realizing 2DEGs for device applications. In this work, the band offsets of a simple model system of an amorphous-crystalline oxide interface are investigated. The model system consists of amorphous Al2O3 grown on single-crystalline (001) SrTiO3. X-ray photoelectron spectroscopy is employed to study the chemical states, bandgap, and band offsets at the interface. The density of ionic defects near the interface is found to be below the detection limit, and the interface is found to be insulating. Analysis of the relative band structure yields significant interfacial barriers, exceeding 1.05 eV for holes and 2.0 eV for electrons. The barrier for holes is considerably larger than what is known for related material systems, outlining the promise of using amorphous Al2O3 as an effective and simple insulator, an important building block for oxide-based field effect devices.

Oxide heterostructures for high density 2D electron gases on GaAs

2D electron gases (2DEGs) that form at oxide interfaces provide a rich testbed of phenomena for condensed matter research, with emerging implementations in devices. Integrating such oxide systems with semiconductors advances these interesting phenomena toward technological applications. This approach further opens prospects of new functionalities arising from the potential to couple the 2DEG carriers with the semiconductor. In this work, we demonstrate the first integration of oxide 2DEGs with a direct bandgap III-V semiconductor. The growth and structural characteristics of (001) GdTiO3-SrTiO3 (GTO–STO) heterostructures on (001) GaAs are described. Analysis of the magnetotransport data yields a high electron density of ∼2 × 1014 cm−2 per GTO–STO interface, and points to the oxide interface as the source of the carriers. The effect of structure and defects on the transport properties is discussed within the context of the growth conditions and their limitations. These results provide a route for integrating oxide 2DEGs and other functional oxides with GaAs toward future device prospects and integration schemes.

Correction to Creation of High Mobility Two-Dimensional Electron Gases via Strain Induced Polarization at an Otherwise Nonpolar Complex Oxide Interface.

The discovery of two-dimensional electron gases (2DEGs) in SrTiO3-based heterostructures provides new opportunities for nanoelectronics. Herein, we create a new type of oxide 2DEG by the epitaxial-strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO3/SrTiO3. Remarkably, this heterointerface is atomically sharp and exhibits a high electron mobility exceeding 60 000 cm2 V–1 s–1 at low temperatures. The 2DEG carrier density exhibits a critical dependence on the film thickness, in good agreement with the polarization induced 2DEG scheme.

Light induced suppression of Kondo effect at amorphous LaAlO3/SrTiO3 interface

We report photoelectric properties of two-dimensional electron gas (2DEG) at an amorphous LaAlO3/SrTiO3 interface. Under visible light illumination (650 nm), an enhancement of electric conductivity is observed over the temperature range from 2 to 300 K. Particularly, a resistance upturn appearing below 25 K, which is further proved to from the Kondo effect, is suppressed by the 650 nm visible light. From the results of light-assisted Hall measurements, light irradiation increases the carrier mobility rather than carrier density in the Kondo regime. It is suggested that light induces the decoherence effect of localized spin states, hence the electron scattering is weakened and the carrier mobility is improved accordingly. Moreover, the enhancement of electrical conductivity by visible light verifies that in-gap states located in the SrTiO3 side of the interface play an important role in the electrical transport of the amorphous SrTiO3-based oxide 2DEG system. Our results provide deeper insight into the photoinduced effects in the 2DEG system, paving the way for the design of optoelectronic devices based on oxides.