SrTiO3 Heterostructures Research Articles

Unearthing the emerging properties at buried oxide heterointerfaces: the γ-Al2O3/SrTiO3 heterostructure.

The symmetry breaking that is formed when oxide layers are combined epitaxially to form heterostructures has led to the emergence of new functionalities beyond those observed in the individual parent materials. SrTiO3-based heterostructures have played a central role in expanding the range of functional properties arising at the heterointerface and elucidating their mechanistic origin. The heterostructure formed by the epitaxial combination of spinel γ-Al2O3 and perovskite SrTiO3 constitutes a striking example with features distinct from perovskite/perovskite counterparts such as the archetypical LaAlO3/SrTiO3 heterostructure. Here, non-isomorphic epitaxial growth of γ-Al2O3 on SrTiO3 can be achieved even at room temperature with the epitaxial union of the two distinct crystal structures resulting in modification of the functional properties by the broken cationic symmetry. The heterostructure features oxygen vacancy-mediated conductivity with dynamically adjustable electron mobilities as high as 140 000 cm2 V-1 s-1 at 2 K, strain-tunable magnetism and an unsaturated linear magnetoresistance exceeding 80 000% at 15 T and 2 K. Here, we review the structural, electronic and magnetic characteristics of the γ-Al2O3/SrTiO3 heterostructure with a particular emphasis on elucidating the underlying mechanistic origins of the various properties. We further show that γ-Al2O3/SrTiO3 may break new grounds for tuning the electronic and magnetic properties through dynamic defect engineering and polarity modifications, and also for band engineering, symmetry breaking and silicon integration.

Atomic-Scale Interface Modification in Complex Oxide Heterojunctions for Near-Infrared Photodetection.

Photodetectors that detect near-infrared (NIR) light serve as important components in contemporary energy-efficient optoelectronic devices. However, detecting the low-energy photons of the NIR light has long been challenging since the ease of photoexcitation inevitably involves increasing the background current in the dark. Herein, we report the atomic-scale interface modification in SrRuO3/LaAlO3/Nb-doped SrTiO3 (SRO/LAO/Nb:STO) heterostructures for NIR photodetection. The interfacial band alignment by a polar monolayer LAO allows precise tuning of the Schottky barrier to achieve a specific energy band profile suitable for the NIR photodetection. The SRO/LAO/Nb:STO heterojunctions show a high photoresponsivity up to ∼1.1 mA/W under NIR light irradiation (λ = 850 nm), while keeping the pA-scale dark current. The increase in the responsivity by interface modification is evaluated at a maximum of 1371%. Based on the enhanced NIR photoresponsivity, as a proof of concept, we demonstrate the spatial imaging of NIR signals using a conceptual array of SRO/LAO/Nb:STO heterojunctions. In addition, the experimental-data-based simulation verifies that the array device can implement pulse-number-dependent plasticity, which is based on the characteristic persistent photoconductivity. This study suggests that atomic-scale interface modification is a facile and powerful method for optimizing the photoresponsive properties of complex-oxide-based heterojunctions.

Multimodal In‐Sensor Computing Implemented by Easily‐Fabricated Oxide‐Heterojunction Optoelectronic Synapses

AbstractThe advent of big data and the Internet of Things has created urgent demands for in‐sensor computing hardware with multimodal perception that can effectively resolve the inefficiency, high latency, and excessive energy consumption challenges faced by conventional sensory systems. Here, a simple‐structured optoelectronic synaptic device of In2O3·SnO2/Nb:SrTiO3 (ITO/NSTO) heterostructure is proposed, which vividly demonstrates in‐sensor computing and multimodal perception capabilities. First, with the ingenious synaptic responses of the device under both optical and electrical stimuli, a multimodal in‐sensor neuromorphic computing system capable of concurrently perceiving and processing visual and auditory information is constructed. Using this multimodal system to perform a human emotion recognition task, the misjudgment arising from single‐modal cognition can thereby be effectively avoided. Second, utilizing the integrated sensing and processing functions, along with the dynamic memories of the device array, a neuromorphic vision system is implemented for real‐time monitoring of moving vehicles, which displays high recognition efficiency and accuracy. This research not only provides an optoelectronic synaptic device that is low‐cost and easy to mass‐produce, but also paves the way for next‐generation in‐sensor computing that can efficiently process multimodal signals.

High-Performance Ferroelectric Thin Film Transistors with Large Memory Window Using Epitaxial Yttrium-Doped Hafnium Zirconium Gate Oxide.

Preventing ferroelectric materials from losing their ferroelectricity over a low thickness of several nanometers is crucial in developing multifunctional nanoelectronics. Epitaxially grown 5 at. % yttrium-doped Hf0.5Zr0.5O2 (YHZO) thin films exhibit an atomically smooth surface, an ability to maintain ferroelectricity even at a thickness of 10 nm, and excellent insulating properties, making them suitable for use as gate oxides in ferroelectric thin film transistors (FeTFTs). Through the epitaxial growth of a YHZO/La0.67Sr0.33MnO3 (LSMO)/SrTiO3 (STO) heterostructure, YHZO effectively retains its ferroelectricity and orthorhombic single phase, leading to enhancing electron mobility (∼19.74 cm2 V-1 s-1) and memory window (3.7 V) in the amorphous InGaZnO4 (a-IGZO)/YHZO/LSMO/STO FeTFTs. These FeTFTs demonstrate a consistent memory function with remarkable endurance (∼106 cycles) and retention (∼104 s). Furthermore, they sustain a constant memory window even under ±6 V bias stress for 104 s and exhibit excellent stability even under ±6 V/1 ms pulse cycling for 107 cycles. For comparison, a transistor with the same structure was fabricated using epitaxial nonferroelectric LaAlO3 (LAO) and epitaxial undoped Hf0.5Zr0.5O2 (HZO) as alternatives to YHZO. This study presents a novel approach to exploit the potential of YHZO in FeTFTs, contributing to the development of next-generation logic-in-memory.

Optoelectronic Synapse Based on 2D Electron Gas in Stoichiometry-Controlled Oxide Heterostructures.

Emulating synaptic functionalities in optoelectronic devices is significant in developing artificial visual-perception systems and neuromorphic photonic computing. Persistent photoconductivity (PPC) in metal oxides provides a facile way to realize the optoelectronic synaptic devices, but the PPC performance is often limited due to the oxygen vacancy defects that release excess conduction electrons without external stimuli. Herein, a high-performance optoelectronic synapse based on the stoichiometry-controlled LaAlO3/SrTiO3 (LAO/STO) heterostructure is developed. By increasing La/Al ratio up to 1.057:1, the PPC is effectively enhanced but suppressed the background conductivity at the LAO/STO interface, achieving strong synaptic behaviors. The spectral noise analyses reveal that the synaptic behaviors are attributed to the cation-related point defects and their charge compensation mechanism near the LAO/STO interface. The short-term and long-term plasticity is demonstrated, including the paired-pulse facilitation, in the La-rich LAO/STO device upon exposure to UV light pulses. As proof of concepts, two essential synaptic functionalities, the pulse-number-dependent plasticity and the self-noise cancellation, are emulated using the 5×5 array of La-rich LAO/STO synapses. Beyond the typical oxygen deficiency control, the results show how harnessing the cation stoichiometry can be used to design oxide heterostructures for advanced optoelectronic synapses and neuromorphic applications.

Enhanced photocatalytic performance of NH2-MIL-125(Ti)/SrTiO3 heterostructures by the piezoelectric effect of SrTiO3

Applying piezoelectric effect on modulating charge transport behavior was considered as one efficient strategy to make feasible charge separation in photocatalysts towards boosting hydrogen evolution performance. Here, with employing corona-poling treatment, SrTiO3 is converted to be a piezoelectric component in the constructed type II NH2-MIL-125(Ti)/SrTiO3 heterojunction. Since the piezoelectric field could tilt the band bending to a degree to be convenient for the charge transfer from SrTiO3 to NH2-MIL-125(Ti), it is also favorable to offer additional active sites on SrTiO3 to form dual reduction sites for the following piezo-photocatalytic reaction. Both the ameliorated surficial polarization and charge transfer trajectory in NH2-MIL-125(Ti)/SrTiO3 type II heterojunction are devoted to the boosted hydrogen production rate for NH2-MIL-125(Ti)/SrTiO3-0.1 (NM/STO-0.1), that is up to 1562.3 μmol h−1 g−1, 1.79 times higher than that of unpolarized NM/STO-0.1 (871.6 μmol h−1 g−1) and 2.35 times higher than that of NH2-MIL-125(Ti) (663.8 μmol h−1 g−1), respectively. This research reported initially the assistant of SrTiO3 with piezoelectric feature for enhanced performance photocatalytic hydrogen evolution, which could be one prominent strategy to achieve the reasonable design of efficient photocatalytic system.

SrTiO3 Based Two Dimensional Nanoplatelets for Low-Cost Solar Hydrogen Generation: Materials Beyond the State-of-the-Art

SrTiO3 is an important multifunctional perovskite, that can possess versatile characteristics such as photocatalytic behaviour, ferroelectricity, high electronic conductivity and others. In the context of solar hydrogen (H2) generation, Domen’s group has investigated SrTiO3 for almost two decades and finally have been able to achieve quantum efficiencies approaching unity [1] and large-scale demonstrations [2]. However, the achieved solar to hydrogen (STH) efficiencies remain well under 1%. One of the key bottlenecks limiting the photocatalytic efficiencies of SrTiO3 is its wide bandgap that limits its visible light absorption capability. Several strategies have been used to reduce the bandgap of SrTiO3 to improve visible light absorption but it can only be improved up to a certain limit. Nevertheless, there are other challenges as well in the form of higher recombination rates and an alternate way to further improve the photocatalytic rates is to employ strategies that can inhibit the recombination of photogenerated charges. Our group has been investigating the effect of Al doping on the morphological as well as chemical properties of SrTiO3 to gain fundamental understanding behind the improvement in photocatalytic rates. Our recent report explains that Al ions in SrTiO3 lattice act as hole trapping sites, which helps in suppressing charge recombination [3].Furthermore, to go beyond the current state-of-the-art, our group has been working on forming 2D SrTiO3 heterostructures. The 2D platelets facilitate efficient charge separation as their low thickness enables fast charge transport to the surface, where they are utilized for redox reactions. 2D area also offers large number of active sites and the possibility to form 2D/2D heterostructure with maximum face contact between two semiconductors. However, SrTiO3 does not have a thermodynamic tendency to grow in 2D morphology. Our group has designed hydrothermal conditions to form 2D SrTiO3-based platelets (Fig. 1) through topochemical transformation reaction at a much lower temperature of 200⁰C [4]. Without any support from noble-metal doping or cocatalysts, the epitaxially grown SrTiO3 heterostructural platelets showed stable and 15 times higher photocatalytic H2 production than the commercial SrTiO3 nanopowders. Our recent study [5] elucidates the role of supersaturation conditions on controlling the morphology of these 2D SrTiO3 nano-platelets and formulates their precise growth mechanism. By optimizing the synthetic procedure, the photocatalytic performance can be tuned to achieve much higher STH efficiencies. The presented synthesis strategy provides guiding principles and ideas for designing other defined 2D semiconductors under hydrothermal conditions, eventually going beyond the current state-of-the-art.

Oxygen vacancy induced conductivity in Sr n+1Ti n O3n+1/SrTiO3 heterostructures

SrTiO3-based heterostructures have attracted much attention due to the abundant properties compared to single components. Here, we fabricate oxide heterostructure of layed perovskite/perovskite Sr n+1Ti n O3n+1/SrTiO3 and investigate the n value-dependent and thickness-dependent conductivity. X-ray diffraction peaks and reflective high energy electron diffraction indicate good film quality. For films of n=6, the heterostructures are conductive, and the conductivity is better for thicker film. On the contrary, heterostructures with films of n=1, 2, 3, 4, ∞ are insulating. Conductive atomic force microscopy results and surface conductivity tests manifest that the oxygen vacancy induced conductive layer exists near the interface between film and substrate. This work provides feasible method to modulate the transport properties of functional transition metal oxides.

Electrically modulated photoresponse and optically modulated electroresistance in a ferroelectric heterostructure with PbZr0.2Ti0.8O3 barriers

Ferroelectric heterostructures hold great promise for developing multifunctional memristors and optoelectronic devices. In this study, we report a ferroelectrically modulated photoresponse and optically modulated electroresistance behaviors in the Pt/PbZr0.2Ti0.8O3(PZT)/Nb-doped SrTiO3 (NSTO) heterostructure. The short-circuit photocurrent rises from 28 nA (after poling at +5 V) to 345 nA (after poling at −5 V) when illuminated with 360 nm of 10 mW·cm−2, exhibiting a massive photocurrent variation ratio of 1230%. This result can be attributed to the modulation of the ferroelectric polarization on the built-in field at the PZT/NSTO interface, which impacts the separation of photogenerated carriers. Furthermore, the heterostructure has a large high/low resistance ratio of 6 × 105%, which decreases to 2 × 104% when illuminated with 360 nm light. This finding is attributed to ferroelectric polarization and light illumination modulating the barrier height and width. Overall, this research suggests a promising strategy for developing self-powered heterojunction photodetectors and multifunctional memory devices.

Unusual Electrical Transport Characteristic of the SrSnO3/Nb-Doped SrTiO3 Heterostructure

Unusual Electrical Transport Characteristic of the SrSnO3/Nb-Doped SrTiO3 Heterostructure

Ultrasmall Polar Skyrmions and Merons in SrTiO3 Heterostructures by Polaron Engineering.

Topological objects with skyrmionic textures in ferroelectrics, i.e., polar skyrmions, are promising technological paradigms in next-generation electronic devices. While breakthrough discoveries of stable polar skyrmions approximately ten nanometers in size have been very recently witnessed in complex systems, such a nontrivial topological order in ferroelectrics inevitably disappears below the ferroelectric critical size of several nanometers. Herein, we propose a strategy to overcome this limitation and achieve ultrasmall and isolated polar skyrmions by engineering excess-electron polarons in otherwise nonferroelectric SrTiO3 heterostructures. Our first-principle calculations demonstrate that a polaron localized at a SrTiO3 surface induces a Neel-type polar skyrmion as small as 1.8 nanometers attributed to the effect of atomic-scale surface roughness. Furthermore, we show that this polar topological structure is tunable by the choice of heterostructures and by the mechanical approach, which undergoes a phase transition to a meron state in the twisted boundary and to an antiskyrmion state in the surface with external shear strain, respectively. Such ultraminiaturization of skyrmions and their transitions unexpectedly unravels the formula of ultrasmall topological orders originating from the interplay between an electron polaron and structural symmetry breaking, which is completely different from the common mechanism of geometric confinement for larger-scale skyrmions. Our results not only provide a mechanism for the exploration of polar skyrmions and their rich topological transitions but also hold potential for ultrahigh-density memories.

Collective Control of Potential-Constrained Oxygen Vacancies in Oxide Heterostructures for Gradual Resistive Switching.

Filamentary resistive switching in oxides is one of the key strategies for developing next-generation non-volatile memory devices. However, despite numerous advantages, their practical applications in neuromorphic computing are still limited due to non-uniform and indeterministic switching behavior. Given the inherent stochasticity of point defect migration, the pursuit of reliable switching likely demands an innovative approach. Herein, a collective control of oxygen vacancies is introduced in LaAlO3 /SrTiO3 (LAO/STO) heterostructures to achieve reliable and gradual resistive switching. By exploiting an electrostatic potential constraint in ultrathin LAO/STO heterostructures, the formation of conducting filaments is suppressed, but instead precisely control the concentration of oxygen vacancies. Since the conductance of the LAO/STO device is governed by the ensemble concentration of oxygen vacancies, not their individual probabilistic migrations, the resistive switching is more uniform and deterministic compared to conventional filamentary devices. It provides direct evidence for the collective control of oxygen vacancies by spectral noise analysis and modeling by Monte-Carlo simulation. As a proof of concept, the significantly-improved analog switching performance of the filament-free LAO/STO devices is demonstrated, revealing potential for neuromorphic applications. The results establish an approach to store information by point defect concentration, akin to biological ionic channels, for enhancing switching characteristics of oxide materials.

Temperature-dependent Poole–Frenkel transport properties of the SrSnO3/Nb-doped SrTiO3 heterostructure

All-perovskite oxide heterostructure of SrSnO3/Nb-doped SrTiO3 was fabricated by using the pulsed laser deposition method. Unusual transport properties of the interface between SrSnO3 and Nb-doped SrTiO3 have been investigated at temperatures from 100 to 300 K. A diodelike rectifying behavior has been demonstrated by the temperature-dependent current–voltage (IV) measurements. The forward current showed typical IV characteristics of p–n junctions or Schottky diodes and was perfectly fitted using the thermionic emission model. At the reverse bias, however, the temperature-dependent IV curves developed in the opposite direction, indicating the tunneling effects on the interface. The Poole–Frenkel emission was used to explain this electrical transport mechanism under the reverse voltages.

Enhanced gain and detectivity of unipolar barrier solar blind avalanche photodetector via lattice and band engineering

Ga2O3-based solar blind avalanche photodetectors exhibit low voltage operation, optical filter-free and monolithic integration of photodetector arrays, and therefore they are promising to be an alternative to the bulky and fragile photomultiplier tubes for weak signal detection in deep-ultraviolet region. Here, by deliberate lattice and band engineering, we construct an n-Barrier-n unipolar barrier avalanche photodetector consisting of β-Ga2O3/MgO/Nb:SrTiO3 heterostructure, in which the enlarged conduction band offsets fortify the reverse breakdown and suppress the dark current while the negligible valance band offsets faciliate minority carrier flow across the heterojunction. The developed devices exhibit record-high avalanche gain up to 5.9 × 105 and detectivity of 2.33 × 1016 Jones among the reported wafer-scale grown Ga2O3-based photodetectors, which are even comparable to the commercial photomultiplier tubes. These findings provide insights into precise manipulation of band alignment in avalanche photodetectors, and also offer exciting opportunities for further developing high-performance Ga2O3-based electronics and optoelectronics.

Perspectives on oxide heterostructures - the curious case of γ-Al2O3/SrTiO3.

The heterostructure formed by depositing nanoscale thin films of spinel γ-Al2O3 on perovskite SrTiO3 exhibits a range of exciting properties including room temperature epitaxial growth, high electron mobility, strain-tunable magnetic order, and a symmetry-related reordering of the conduction bands. In comparison to the benchmark LaAlO3/SrTiO3 heterostructure, the γ-Al2O3/SrTiO3 heterostructure has been more sparsely investigated, which leaves plenty of room for scientific and technological discoveries. In this perspective article, I describe the key findings of the γ-Al2O3/SrTiO3 heterostructure and propose five directions for future research: (1) an exploration of novel phenomena emerging when relaxing the conventional epitaxial constraint of matching crystal structures across the interface, (2) a dynamic switching of a strong polarization through nanoscale electromigration of aluminum vacancies, (3) autonomous and forced enhancement of the electron mobility via oxygen vacancy diffusion, (4) writing and erasing of magnetic and conducting nanolines using ferroelastic domain walls, and (5) a multiferroic state formed by combining ferroelectricity, ferromagnetism, and ferroelasticity. The proposed research directions may shed light on both fundamental aspects of the heterostructure and pave the way for applications within green energy devices and nanoelectronics.

Polarized neutron reflectometry study on the modulation of resistance and magnetism in resistive switching cobalt ferrite thin films

In this work, the resistive switching and electrical-control of magnetization in Pt/CoFe2O4/Nb:SrTiO3 heterostructures have been investigated. The films exhibit a classic bipolar resistive switching effect with a maximum switch ratio of about 5 × 103 and good anti-fatigue performance. Associated with resistive switching, the saturated magnetization of the thin film at high resistance state is found to be larger than that at low resistance state. Meanwhile, polarized neutron reflectivity of the thin film under different resistance states was in situ measured. The results reveal that the interfacial migration of oxygen vacancies driven by an applied electric field plays an important role in the modulation of resistive and magnetism of CoFe2O4 resistive switching devices.

Resistive switching and photovoltaic response characteristics for the BaTiO3/Nb:SrTiO3 heterostructure

Recently, perovskite compounds with ABX3 structures have been attracting increasing attention because of their excellent properties and their potential for applications in fields such as optoelectronics and memory devices. In this Letter, we introduce a BaTiO3/Nb:SrTiO3 (BTO/NSTO) heterostructure that displays both resistive switching and photovoltaic response characteristics. As a dual-function device, our heterostructure device not only exhibits bipolar resistive switching behavior with a switching ratio of up to 103 without any forming process but also shows a tunable photovoltage effect with an open-circuit voltage (Voc) of approximately 0.38 V. In addition, a high-resistance state and a low-resistance state of the device can be modulated by light illumination. This photo-modulation mechanism is revealed and shows that resistive switching can be attributed to migration of photogenerated carriers and charge trapping/detrapping caused by ferroelectric polarization reversal, which changes depletion layer characteristics at the BTO/NSTO interface. This work may help to provide an understanding of multifunctional characteristics of the BTO/NSTO heterostructure and will pave the way toward practical applications of this heterostructure in memory and optoelectronic devices.

KTaO3 -The New Kid on the Spintronics Block.

Long after the heady days of high-temperature superconductivity, the oxides came back into the limelight in 2004 with the discovery of the 2D electron gas (2DEG) in SrTiO3 (STO) and several heterostructures based on it. Not only do these materials exhibit interesting physics, but they have also opened up new vistas in oxide electronics and spintronics. However, much of the attention has recently shifted to KTaO3 (KTO), a material with all the "good" properties of STO (simple cubic structure, high mobility, etc.) but with the additional advantage of a much larger spin-orbit coupling. In this state-of-the-art review of the fascinating world of KTO, it is attempted to cover the remarkable progress made, particularly in the last five years. Certain unsolved issues are also indicated, while suggesting future research directions as well as potential applications. The range of physical phenomena associated with the 2DEG trapped at the interfaces of KTO-based heterostructures include spin polarization, superconductivity, quantum oscillations in the magnetoresistance, spin-polarized electron transport, persistent photocurrent, Rashba effect, topological Hall effect, and inverse Edelstein Effect. It is aimed to discuss, on a single platform, the various fabrication techniques, the exciting physical properties and future application possibilities of this family of materials.

A bifunctional catalyst based on a carbon quantum dots/mesoporous SrTiO3 heterostructure for cascade photoelectrochemical nitrogen reduction

An ultrathin carbon quantum dots modified hydrogenated mesoporous SrTiO3 heterostructure (CQDs/STO) has been developed as a bifunctional catalyst for enhanced photoelectrochemical nitrogen reduction, resulting in a ∼50% increase in the ammonia yield.

Understanding and modulation of resistive switching behaviors in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 multilayer junctions

The insertion layer in the ferroelectric multilayer junctions plays a key role in regulating the energy band structure of interface and their resistive switching behavior. Here, PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 (PZT/LSMO/NSTO) heterostructures with different LSMO thicknesses were prepared by pulsed laser deposition and their ferroelectric properties and resistive switching behaviors were investigated. It is found that the ferroelectric properties of the heterostructures almost do not change with the increasing in the thickness of LSMO, while the resistive switching behaviors are closely related to the LSMO thickness, and the maximum switching ratio of about 103 can be achieved in the PZT/LSMO/NSTO heterostructure with the LSMO thickness of 18 nm. By analyzing the I-V curves and energy band structures, it can be concluded that the resistive switching behaviors depend on the competition between the tunability of depletion layer width and the ability of ferroelectric filed effect. These results provide insights to understand ferroelectric resistive switching in ferroelectric heterostructures, and demonstrate an effective way to improve resistive switching performance by adjusting the band structure through the appropriate thickness of the insertion layer.