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.

First Observation of Ferroelectricity in ∼1 nm Ultrathin Semiconducting BaTiO3 Films.

The requirements of multifunctionality in thin-film systems have led to the discovery of unique physical properties and degrees of freedom, which exist only in film forms. With progress in growth techniques, one can decrease the film thickness to the scale of a few nanometers (∼nm), where its unique physical properties are still pronounced. Among advanced ultrathin film systems, ferroelectrics have generated tremendous interest. As a prototype ferroelectric, the electrical properties of BaTiO3 (BTO) films have been extensively studied, and it has been theoretically predicted that ferroelectricity sustains down to ∼nm thick films. However, efforts toward determining the minimum thickness for ferroelectric films have been hindered by practical issues surrounding large leakage currents. In this study, we used ∼nm thick BTO films, exhibiting semiconducting characteristics, grown on a LaAlO3/SrTiO3 (LAO/STO) heterostructure. In particular, we utilized two-dimensional electron gas at the LAO/STO heterointerface as the bottom electrode in these capacitor junctions. We demonstrate that the BTO film exhibits ferroelectricity at room temperature, even when it is only ∼2 unit-cells thick, and the total thickness of the capacitor junction can be reduced to less than ∼4 nm. Observation of ferroelectricity in ultrathin semiconducting films and the resulting shrunken capacitor thickness will expand the applicability of ferroelectrics in the next generation of functional devices.

Quantized Ballistic Transport of Electrons and Electron Pairs in LaAlO3/SrTiO3 Nanowires.

SrTiO3-based heterointerfaces support quasi-two-dimensional (2D) electron systems that are analogous to III-V semiconductor heterostructures, but also possess superconducting, magnetic, spintronic, ferroelectric, and ferroelastic degrees of freedom. Despite these rich properties, the relatively low mobilities of 2D complex-oxide interfaces appear to preclude ballistic transport in 1D. Here we show that the 2D LaAlO3/SrTiO3 interface can support quantized ballistic transport of electrons and (nonsuperconducting) electron pairs within quasi-1D structures that are created using a well-established conductive atomic-force microscope (c-AFM) lithography technique. The nature of transport ranges from truly single-mode (1D) to three-dimensional (3D), depending on the applied magnetic field and gate voltage. Quantization of the lowest e2/ h plateau indicate a ballistic mean-free path lMF ∼ 20 µm, more than 2 orders of magnitude larger than for 2D LaAlO3/SrTiO3 heterostructures. Nonsuperconducting electron pairs are found to be stable in magnetic fields as high as B = 11 T and propagate ballistically with conductance quantized at 2 e2/ h. Theories of one-dimensional (1D) transport of interacting electron systems depend crucially on the sign of the electron-electron interaction, which may help explain the highly ballistic transport behavior. The 1D geometry yields new insights into the electronic structure of the LaAlO3/SrTiO3 system and offers a new platform for the study of strongly interacting 1D electronic systems.

Surface Chemically Switchable Ultraviolet Luminescence from Interfacial Two-Dimensional Electron Gas.

We report intense, narrow line-width, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron gas (2DEG) with photoexcited holes at LaAlO3/SrTiO3. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto the AlO2-terminated surface of LaAlO3, at least 2 nm away from the interface. The control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing.

Electronic and Structural Transitions of LaAlO3 /SrTiO3 Heterostructure Driven by Polar Field-Assisted Oxygen Vacancy Formation at the Surface.

The origin of 2D electron gas (2DEG) at LaAlO3 /SrTiO3 (LAO/STO) interfaces has remained highly controversial since its discovery. Various models are proposed, which include electronic reconstruction via surface-to-interface charge transfer and defect-mediated doping involving cation intermixing or oxygen vacancy (VO ) formation. It is shown that the polar field-assisted VO formation at the LAO/STO surface plays critical roles in the 2DEG formation and concurrent structural transition. Comprehensive scanning transmission electron microscopy analyses, in conjunction with density functional theory calculations, demonstrate that VO forming at the LAO/STO surface above the critical thickness (tc ) cancels the polar field by doping the interface with 2DEG. The antiferrodistortive (AFD) octahedral rotations in LAO, which are suppressed below the tc , evolve with the formation of VO above the tc . The present study reveals that local symmetry breaking and shallow donor behavior of VO induce the AFD rotations and relieve the electrical field by electron doping the oxide heterointerface.

Nanopatterning of Weak Links in Superconducting Oxide Interfaces.

The interface between two wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO), hosts a quasi-two-dimensional electron gas (q2DEG), two-dimensional superconductivity, ferromagnetism, and giant Rashba spin-orbit coupling. The co-existence of two-dimensional superconductivity with gate-tunable spin-orbit coupling and multiband occupation is of particular interest for the realization of unconventional superconducting pairing. To investigate the symmetry of the superconducting order parameter, phase sensitive measurements of the Josephson effect are required. We describe an approach for the fabrication of artificial superconducting weak links at the LAO/STO interface using direct high-resolution electron beam lithography and low-energy argon ion beam irradiation. The method does not require lift-off steps or sacrificial layers. Therefore, resolution is only limited by the electron beam lithography and pattern transfer. We have realized superconducting weak links with a barrier thickness of 30-100 nm. The barrier transparency of the weak links can be controlled by the irradiation dose and further tuned by a gate voltage. Our results open up new possibilities for the realization of quantum devices in oxide interfaces.

Hot Electron Tunneling in Pt/LaAlO3/SrTiO3 Heterostructures for Enhanced Photodetection.

LaAlO3/SrTiO3 (LAO/STO) heterostructures, in which a highly mobile two-dimensional electron gas (2DEG) is formed, have great potential for optoelectronic applications. However, the inherently high density of the 2DEG hinders the observation of photo-excitation effects in oxide heterostructures. Herein, a strong photoresponse of the 2DEG in a Pt/LAO/STO heterostructure is achieved by adopting a vertical tunneling configuration. The tunneling of the 2DEG through an ultrathin LAO layer is significantly enhanced by UV light irradiation, showing a maximum photoresponsivity of ∼1.11 × 107%. The strong and reversible photoresponse is attributed to the thermionic emission of photoexcited hot electrons from the oxygen-deficient STO. Notably, the oxygen vacancy defects play a critical role in enhancing the tunneling photocurrent. Our systematic study on the hysteresis behavior and the light power dependency of the tunneling current consistently support the fact that the photoexcited hot electrons from the oxygen vacancies strongly contribute to the tunneling conduction under the UV light. This work offers valuable insights into a novel photodetection mechanism based on the 2DEG as well as into developing ultrathin optoelectronic devices based on the oxide heterostructures.

Size-Controlled Spalling of LaAlO3/SrTiO3 Micromembranes.

The ability to form freestanding oxide membranes of nanoscale thickness is of great interest for enabling material functionality and for integrating oxides in flexible electronic and photonic technologies. Recently, a route has been demonstrated for forming conducting heterostructure membranes of LaAlO3 and SrTiO3, the canonical system for oxide electronics. In this route, the epitaxial growth of LaAlO3 on SrTiO3 resulted in a strained state that relaxed by producing freestanding membranes with random sizes and locations. Here, we extend the method to enable self-formed LaAlO3/SrTiO3 micromembranes with control over membrane position, their lateral sizes from 2 to 20 µm, and with controlled transfer to other substrates of choice. This method opens up the possibility to study and use the two-dimensional electron gas in LaAlO3/SrTiO3 membranes for advanced device concepts.

Atomic Origin of Interface-Dependent Oxygen Migration by Electrochemical Gating at the LaAlO3-SrTiO3 Heterointerface.

Electrical control of material properties based on ionic liquids (IL) has seen great development and emerging applications in the field of functional oxides, mainly understood by the electrostatic and electrochemical gating mechanisms. Compared to the fast, flexible, and reproducible electrostatic gating, electrochemical gating is less controllable owing to the complex behaviors of ion migration. Here, the interface-dependent oxygen migration by electrochemical gating is resolved at the atomic scale in the LaAlO3-SrTiO3 system through ex situ IL gating experiments and on-site atomic-resolution characterization. The difference between interface structures leads to the controllable electrochemical oxygen migration by filling oxygen vacancies. The findings not only provide an atomic-scale insight into the origin of interface-dependent electrochemical gating but also demonstrate an effective way of engineering interface structure to control the electrochemical gating.

Visualizing Charge Transport and Nanoscale Electrochemistry by Hyperspectral Kelvin Probe Force Microscopy.

Charge-transport and electrochemical processes are heavily influenced by the local microstructure. Kelvin probe force microscopy (KPFM) is a widely used technique to map electrochemical potentials at the nanometer scale; however, it offers little information on local charge dynamics. Here, we implement a hyperspectral KPFM approach for spatially mapping bias-dependent charge dynamics in timescales ranging from the sub-millisecond to the second regime. As a proof of principle, we investigate the role mobile surface charges play in a three-unit-cell LaAlO3/SrTiO3 oxide heterostructure. We explore machine learning approaches to assist with visualization, pattern recognition, and interpretation of the information-rich data sets. Linear unmixing methods reveal hidden bias-dependent interfacial processes, most likely water splitting, which are essentially unnoticed by functional fitting of the dynamic response alone. Hyperspectral KPFM will be beneficial for investigating nanoscale charge transport and local reactivity in systems involving a possible combination of electronic, ionic, and electrochemical phenomena.

Direct Observation of Room-Temperature Stable Magnetism in LaAlO3/SrTiO3 Heterostructures.

Along with an unexpected conducting interface between nonmagnetic insulating perovskites LaAlO3 and SrTiO3 (LaAlO3/SrTiO3), striking interfacial magnetisms have been observed in LaAlO3/SrTiO3 heterostructures. Interestingly, the strength of the interfacial magnetic moment is found to be dependent on oxygen partial pressures during the growth process. This raises an important, fundamental question on the origin of these remarkable interfacial magnetic orderings. Here, we report a direct evidence of room-temperature stable magnetism in a LaAlO3/SrTiO3 heterostructure prepared at high oxygen partial pressure by using element-specific soft X-ray magnetic circular dichroism at both Ti L3,2 and O K edges. By combining X-ray absorption spectroscopy at both Ti L3,2 and O K edges and first-principles calculations, we qualitatively ascribe that this strong magnetic ordering with dominant interfacial Ti3+ character is due to the coexistence of LaAlO3 surface oxygen vacancies and interfacial (TiAl-AlTi) antisite defects. On the basis of this new understanding, we revisit the origin of the weak magnetism in LaAlO3/SrTiO3 heterostructures prepared at low oxygen partial pressures. Our calculations show that LaAlO3 surface oxygen vacancies are responsible for the weak magnetism at the interface. Our result provides direct evidence on the presence of room-temperature stable magnetism and a novel perspective to understand magnetic and electronic reconstructions at such strategic oxide interfaces.

Orbital Ordering of the Mobile and Localized Electrons at Oxygen-Deficient LaAlO3/SrTiO3 Interfaces.

Interfacing different transition-metal oxides opens a route to functionalizing their rich interplay of electron, spin, orbital, and lattice degrees of freedom for electronic and spintronic devices. Electronic and magnetic properties of SrTiO3-based interfaces hosting a mobile two-dimensional electron system (2DES) are strongly influenced by oxygen vacancies, which form an electronic dichotomy, where strongly correlated localized electrons in the in-gap states (IGSs) coexist with noncorrelated delocalized 2DES. Here, we use resonant soft-X-ray photoelectron spectroscopy to prove the eg character of the IGSs, as opposed to the t2g character of the 2DES in the paradigmatic LaAlO3/SrTiO3 interface. We furthermore separate the d xy and d xz/d xz orbital contributions based on deeper consideration of the resonant photoexcitation process in terms of orbital and momentum selectivity. Supported by a self-consistent combination of density functional theory and dynamical mean field theory calculations, this experiment identifies local orbital reconstruction that goes beyond the conventional eg- vs-t2g band ordering. A hallmark of oxygen-deficient LaAlO3/SrTiO3 is a significant hybridization of the eg and t2g orbitals. Our findings provide routes for tuning the electronic and magnetic properties of oxide interfaces through "defect engineering" with oxygen vacancies.

Magnetism Control by Doping in LaAlO3/SrTiO3 Heterointerfaces.

Magnetic two-dimensional electron gases at the oxide interfaces are always one of the key issues in spintronics, giving rise to intriguing magnetotransport properties. However, reports about magnetic two-dimensional electron gases remain elusive. Here, we obtain the magnetic order of LaAlO3/SrTiO3 systems by introducing magnetic dopants at the La site. The transport properties with a characteristic of metallic behavior at the interfaces are investigated. More significantly, magnetic-doped samples exhibit obvious magnetic hysteresis loops and the mobility is enhanced. Meanwhile, the photoresponsive experiments are realized by irradiating all samples with a 360 nm light. Compared to magnetism, the effects of dopants on photoresponsive and relaxation properties are negligible because the behavior originates from SrTiO3 substrates. This work paves a way for revealing and better controlling the magnetic properties of oxide heterointerfaces.

Largely Enhanced Mobility in Trilayered LaAlO3/SrTiO3/LaAlO3 Heterostructures.

LaAlO3 (LAO)/SrTiO3 (STO)/LaAlO3 (LAO) heterostructures were epitaxially deposited on TiO2-terminated (100) SrTiO3 single-crystal substrates by laser molecular beam epitaxy. The electron Hall mobility of 1.2 × 104 cm2/V s at 2 K was obtained in our trilayered heterostructures grown under 1 × 10-5 Torr, which was significantly higher than that in single-layer 5 unit cells LAO (∼4 × 103 cm2/V s) epitaxially grown on (100) STO substrates under the same conditions. It is believed that the enhancement of dielectric permittivity in the polar insulating trilayer can screen the electric field, thus reducing the carrier effective mass of the two-dimensional electron gas formed at the TiO2 interfacial layer in the substrate, resulting in a largely enhanced mobility, as suggested by the first-principle calculation. Our results will pave the way for designing high-mobility oxide nanoelectronic devices based on LAO/STO heterostructures.

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.

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.

Anisotropic Transport at the LaAlO3/SrTiO3 Interface Explained by Microscopic Imaging of Channel-Flow over SrTiO3 Domains.

Oxide interfaces, including the LaAlO3/SrTiO3 interface, have been a subject of intense interest for over a decade due to their rich physics and potential as low-dimensional nanoelectronic systems. The field has reached the stage where efforts are invested in developing devices. It is critical now to understand the functionalities and limitations of such devices. Recent scanning probe measurements of the LaAlO3/SrTiO3 interface have revealed locally enhanced current flow and accumulation of charge along channels related to SrTiO3 structural domains. These observations raised a key question regarding the role these modulations play in the macroscopic properties of devices. Here we show that the microscopic picture, mapped by scanning superconducting quantum interference device, accounts for a substantial part of the macroscopically measured transport anisotropy. We compared local flux data with transport values, measured simultaneously, over various SrTiO3 domain configurations. We show a clear relation between maps of local current density over specific domain configurations and the measured anisotropy for the same device. The domains divert the direction of current flow, resulting in a direction-dependent resistance. We also show that the modulation can be significant and that in some cases up to 95% of the current is modulated over the channels. The orientation and distribution of the SrTiO3 structural domains change between different cooldowns of the same device or when electric fields are applied, affecting the device behavior. Our results, highlight the importance of substrate physics, and in particular, the role of structural domains, in controlling electronic properties of LaAlO3/SrTiO3 devices. Furthermore, these results point to new research directions, exploiting the STO domains' ability to divert or even carry current.

Subtle Interplay between Localized Magnetic Moments and Itinerant Electrons in LaAlO3/SrTiO3 Heterostructures.

Clarification of the role of magnetic ordering and scattering in two-dimensional electron gas has become increasingly important to understand the transport and magnetic behavior in the LaAlO3 (LAO)/SrTiO3 (STO) heterostructures. In this work, we report the sheet resistance of the LAO/STO heterostructures as functions of temperature, magnetic field, and field orientation. An unexpected resistance minimum was discovered at ∼10 K under a sufficiently high in-plane magnetic field. An anisotropic magnetoresistance (MR) is clearly identified, indicating the presence of magnetic scattering which may be related to the interaction between itinerant electrons and localized magnetic moments in the LaAlO3/SrTiO3 heterostructures. It is believed that the high concentration of oxygen vacancies induced by the ultralow oxygen partial pressure during the deposition process plays a predominant role in the occurrence of the anisotropic MR.

Liquid-Gated High Mobility and Quantum Oscillation of the Two-Dimensional Electron Gas at an Oxide Interface.

Electric field effect in electronic double layer transistor (EDLT) configuration with ionic liquids as the dielectric materials is a powerful means of exploring various properties in different materials. Here, we demonstrate the modulation of electrical transport properties and extremely high mobility of two-dimensional electron gas at LaAlO3/SrTiO3 (LAO/STO) interface through ionic liquid-assisted electric field effect. With a change of the gate voltages, the depletion of charge carrier and the resultant enhancement of electron mobility up to 19 380 cm(2)/(V s) are realized, leading to quantum oscillations of the conductivity at the LAO/STO interface. The present results suggest that high-mobility oxide interfaces, which exhibit quantum phenomena, could be obtained by ionic liquid-assisted field effect.

Highly sensitive gas sensor by the LaAlO3 /SrTiO3 heterostructure with Pd nanoparticle surface modulation.

The palladium nanoparticle (Pd NP)-decorated LaAlO3 /SrTiO3 (LAO/STO) heterostructure is for the first time used as a hydrogen-gas sensor with very high sensitivity and workability at room temperature. The outstanding gas-sensing properties are due to the Pd NPs' catalytic effect to different gases, resulting in charge coupling between the gas molecules and the two-dimensional electron gas (2DEG) through the Pd NPs by either a direct charge exchange or a change of the electron affinity. These results provide insight into the emerging properties at LAO/STO interfaces.