Toward Switchable Photovoltaic Effect via Tailoring Mobile Oxygen Vacancies in Perovskite Oxide Films

Interface-engineered non-volatile visible-blind photodetector for in-sensor computing

Ultraviolet (UV) detection is extensively used in a variety of applications. However, the storage and processing of information after detection require multiple components, resulting in increased energy consumption and data transmission latency. In this paper, a reconfigurable UV photodetector based on CeO2/SrTiO3 heterostructures is demonstrated with in-sensor computing capabilities achieved through interface engineering. We show that the non-volatile storage capability of the device could be significantly improved by the introduction of an oxygen reservoir. A photodetector array operated as a single-layer neural network was constructed, in which edge detection and pattern recognition were realized without the need for external memory and computing units. The location and classification of corona discharges in real-world environments were also simulated and achieved an accuracy of 100%. The approach proposed here offers promising avenues and material options for creating non-volatile smart photodetectors.

Supercritical CO2-Guided Passivation Strategies for Oxygen Vacancy Modulation in LaMnO3

The development of 2D magnetic materials and the modulation of intrinsic magnetism are essential for the exploration of new materials in the field of information storage. Despite its strong ferromagnetic properties, LaMnO3 is hindered by a high number of oxygen defects, which result in a relatively short lifetime when employed in electronic memory devices. Here the successful transformation of bulk LaMnO3 into a 2D structure using supercritical carbon dioxide.is reported. This technique enables the successful modulation of the magnetic properties of the material. Interestingly, it is found that the oxygen defect is repaired, which is in sharp contrast to conventional perovskites. These promising results demonstrate the potential of using the magnetic properties of LaMnO3, which is of great importance in the context of expanding its application in electronic devices.

Ferroelectric Polarization and Oxygen Vacancy Synergistically Induced an Ultrasensitive and Fast Humidity Sensor for Multifunctional Applications

With the arrival of the Internet of Things and artificial intelligence, humidity sensors monitoring water emissions from human metabolism have attracted great attention in the fields of smart wearable devices and noncontact human-machine interaction. However, their application is seriously limited by the trade-off between the sensitivity and response speed for traditional humidity sensors. Herein, to overcome it, a self-powered high performance humidity sensor is developed on the basis of the electric-poled and oxygen vacancy-rich BiFeO3 (BFO) ferroelectric material. The synergistic effect of ferroelectric polarization and oxygen vacancy provides a strong driving force and active adsorption sites for an abundance of OH/H2O adsorption, resulting in an ultrahigh response (∼104) and ultrafast response/recovery speed (∼84/376 ms). Benefiting from its promising advantages, the wearable humidity sensor can accurately record the respiration rate/depth and recognize different human respiratory behaviors in real-time. Importantly, by utilizing the moisture from mouth-blowing and skin, the sensors are successfully applied to noncontact control of a robotic car, noncontact switch, and noncontact interface for visualization applications. This work provides an effective strategy for developing excellent humidity sensors that meet the requirement of noncontact interaction for next-generation intelligent electronics.

Photoinduced electronic and ionic effects in strontium titanate

The interaction of light with solids has been of ever-growing interest for centuries, even more so since the quest for sustainable utilization and storage of solar energy became a major task for industry and research. With SrTiO₃ being a model material for an extensive exploration of the defect chemistry of mixed conducting perovskite oxides, it has also been a vanguard in advancing the understanding of the interaction between light and the electronic and ionic structure of solids. In the course of these efforts, many phenomena occurring during or subsequent to the illumination of SrTiO₃ have been investigated. Here, we give an overview of the numerous photoinduced effects in SrTiO₃ and their inherent connection to electronic structure and defect chemistry. In more detail, advances in the fields of photoconductivity, photoluminescence, photovoltages, photochromism and photocatalysis are summarized and their underlying elemental processes are discussed. In light of recent research, this review also emphasizes the fundamental differences between illuminating SrTiO₃ either at low temperatures (<RT) or at high temperatures (>200 °C), where in addition to electronic processes, also photoionic interactions become relevant. A survey of the multitude of different processes shows that a profound and comprehensive understanding of the defect chemistry and its alteration under illumination is both vital to optimizing devices and to pushing the boundaries of research and advancing the fundamental understanding of solids.

Understanding memristive switching via in situ characterization and device modeling

Owing to their attractive application potentials in both non-volatile memory and unconventional computing, memristive devices have drawn substantial research attention in the last decade. However, major roadblocks still remain in device performance, especially concerning relatively large parameter variability and limited cycling endurance. The response of the active region in the device within and between switching cycles plays the dominating role, yet the microscopic details remain elusive. This Review summarizes recent progress in scientific understanding of the physical origins of the non-idealities and propose a synergistic approach based on in situ characterization and device modeling to investigate switching mechanism. At last, the Review offers an outlook for commercialization viability of memristive technology.

Memristor as the fourth basic element of electric circuits has drawn substantial attention for developing future computing technologies. Sun et al. report the progress and the challenges facing researchers on understanding memristive switching, and advocate continuous studies using a synergistic approach.

Conduction mechanism and switchable photovoltaic effect in (1 1 1) oriented BiFe0.95Mn0.05O3 thin film

Epitaxial 200 nm BiFe_0.95Mn_0.05O₃ (BFO) film was grown by pulsed laser deposition (PLD) on (1 1 1) oriented SrTiO₃ substrate buffered with a 50 nm thick SrRuO₃ electrode. The BFO thin film shows a rhombohedral structure and a large remnant polarization of Pr  =  104 µC cm^-2. By comparing I(V) characteristics with different conduction models we reveal the presence of both bulk limited Poole-Frenkel and Schottky interface mechanisms and each one dominates in a specific range of temperature. At room temperature (RT) and under 10 mW laser illumination, the as grown BFO film presents short-circuit current density (J _sc) and open circuit voltage (V _oc) of 2.25 mA cm^-2 and  -0.55 V respectively. This PV effect can be switched by applying positive voltage pulses higher than the coercive field. For low temperatures a large V _oc value of about  -4.5 V (-225 kV cm^-1) is observed which suggests a bulk non-centrosymmetric origin of the PV response.

Manipulation of Oxygen Vacancy for High Photovoltaic Output in Bismuth Ferrite Films

Very recently, the ferroelectric photovoltaic property of bismuth ferrite (BiFeO₃, BFO) has attracted much attention. However, the physical mechanisms for its anomalous photovoltaic effect and switchable photovoltaic effect are still largely unclear. Herein, a novel design was proposed to realize a high photovoltaic output in BiFeO₃ films by manipulating its oxygen vacancy concentration through the alteration of the Bi content. Subsequent results and analysis manifested that the highest photovoltaic output was achieved in Bi_1.05FeO₃ films, differing 1000 times from that of Bi_0.95FeO₃ films. Simultaneously, the origin of photovoltaic effect in all BiFeO₃ films was suggested as the bulk photovoltaic mechanism instead of the Schottky effect. Moreover, oxygen vacancy migration should be the dominant factor determining the switchable photovoltaic effect rather than the ferroelectric polarization. A switchable Schottky-to-Ohmic interfacial contact model was proposed to illustrate the observed switchable photovoltaic or diodelike effect. Therefore, the present work may open a new way to realize the high power output and controllable photovoltaic switching behavior for the photovoltaic applications of BiFeO₃ compounds.

Synaptic memory devices from CoO/Nb:SrTiO3 junction

Non-volatile memristors are promising for future hardware-based neurocomputation application because they are capable of emulating biological synaptic functions. Various material strategies have been studied to pursue better device performance, such as lower energy cost, better biological plausibility, etc. In this work, we show a novel design for non-volatile memristor based on CoO/Nb:SrTiO3 heterojunction. We found the memristor intrinsically exhibited resistivity switching behaviours, which can be ascribed to the migration of oxygen vacancies and charge trapping and detrapping at the heterojunction interface. The carrier trapping/detrapping level can be finely adjusted by regulating voltage amplitudes. Gradual conductance modulation can therefore be realized by using proper voltage pulse stimulations. And the spike-timing-dependent plasticity, an important Hebbian learning rule, has been implemented in the device. Our results indicate the possibility of achieving artificial synapses with CoO/Nb:SrTiO3 heterojunction. Compared with filamentary type of the synaptic device, our device has the potential to reduce energy consumption, realize large-scale neuromorphic system and work more reliably, since no structural distortion occurs.

Ferroelectrics with a controlled oxygen-vacancy distribution by design

Controlling and manipulating defects in materials provides an extra degree of freedom not only for enhancing physical properties but also for introducing additional functionalities. In ferroelectric oxides, an accumulation of point defects at specific boundaries often deteriorates a polarization-switching capability, but on the one hand, delivers interface-driven phenomena. At present, it remains challenging to control oxygen vacancies at will to achieve a desirable defect structure. Here, we report a practical route to designing oxygen-vacancy distributions by exploiting the interaction with transition-metal dopants. Our thin-film experiments combined with ab-initio theoretical calculations for BiFeO₃ demonstrate that isovalent dopants such as Mn³⁺ with a partly or fully electron-occupied e_g state can trap oxygen vacancies, leading to a robust polarization switching. Our approach to controlling oxygen vacancy distributions by harnessing the vacancy-trapping capability of isovalent transition-metal cations will realize the full potential of switchable polarization in ferroelectric perovskite oxides.

Nonvolatile Control of Magnetocaloric Operating Temperature by Low Voltage

The limited operating temperature is the main obstacle for the practical applications of magnetic refrigeration. In this work, the voltage control of magnetocaloric effect (MCE) is investigated in a La_0.7Sr_0.3MnO₃ (LSMO)/CeO₂/Pt device. Different from the conventional method of volatile manipulating MCE by large-voltage-induced strain, nonvolatile manipulation of magnetocaloric operating temperature with good stability is realized in the LSMO film by applying low voltages of less than 2.3 V. The experimental results demonstrate that the magnetic entropy change peak temperature for the LSMO film can be extended from 302 to 312 K by voltage. This nonvolatile effect can be well-understood with the resistive switching mechanism and has potential in promoting microscale refrigeration technology.

Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼10⁴ V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (E_g). Here, we present a promising FPV based on hexagonal YbFeO₃ (h-YbFO) thin-film heterostructure by exploiting its narrow E_g. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al₂O₃ device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces E_g substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.

Controllable Photovoltaic Effect of Microarray Derived from Epitaxial Tetragonal BiFeO3 Films

Recently, the ferroelectric photovoltaic (FePV) effect has attracted great interest due to its potential in developing optoelectronic devices such as solar cell and electric-optical sensors. It is important for actual applications to realize a controllable photovoltaic process in ferroelectric-based materials. In this work, we prepared well-ordered microarrays based on epitaxially tetragonal BiFeO₃ (T-BFO) films by the pulsed laser deposition technique. The polarization-dependent photocurrent image was directly observed by a conductive atomic force microscope under ultraviolet illumination. By choosing a suitable buffer electrode layer and controlling the ferroelectric polarization in the T-BFO layer, we realized the manipulation of the photovoltaic process. Moreover, based on the analysis of the band structure, we revealed the mechanism of manipulating the photovoltaic process and attributed it to the competition between two key factors, i.e., the internal electric field caused by energy band alignments at interfaces and the depolarization field induced by the ferroelectric polarization in T-BFO. This work is very meaningful for deeply understanding the photovoltaic process of BiFeO₃-based devices at the microscale and provides us a feasible avenue for developing data storage or logic switching microdevices based on the FePV effect.

Origin of abnormal structural transformation in a (BiPb)FeO3/SrRuO3/SrTiO3 hetero-structure probed by Rutherford backscattering

Scientific efforts are growing to understand artificial BiFeO₃/SrRuO₃/SrTiO₃-heterostructures, wherein an altered environment at each interface, caused by epitaxial strains, broken symmetry, off-stoichiometry and charge transfer, can generate a rich spectrum of exotic properties. Herein, (BiPb)FeO₃/SrRuO₃/SrTiO₃-heterostructures were sputtered with various top (BiPb)FeO₃-layers at different growth temperatures (T_g). Strain relaxation at each interface changes with T_g and generates an additional peak alongside with (BiPb)FeO₃ at a high T_g of 700 °C. Rutherford backscattering (RBS) was employed to understand this unusual behavior as to whether it is a mixture of two phases, layer splitting or inter-diffusion of elements. Surprisingly, complete overlapping of random and aligned RBS spectra from the sample with T_g = 700 °C indicates the presence of a large amount of defects/distortions at the interfaces. The RBS compositional analysis gives clear evidence of Fe and Ru vacancies to an extent that the structural integrity may not be maintained. This abnormal condition can be explained by the inter-diffusion of Pb and Bi elements into whole films and even into the top layer of the SrTiO₃ substrate, which compensates for these vacancies by substitutional replacement and is responsible for the generation of the additional SrTi(BiPb)O₃—peak. Below T_c^SrRuO₃, the magnetic properties change significantly with T_g.

Oxygen Vacancies Control Transition of Resistive Switching Mode in Single-Crystal TiO2 Memory Device

Epitaxial TiO₂ thin films were grown by radio-frequency magnetron sputtering on conductive Nb-SrTiO₃ substrates. X-ray photoelectron spectroscopy reveals that the oxygen vacancies inside the TiO₂ films can be dramatically reduced by postannealing treatment under an oxygen atmosphere. The decreasing concentration of oxygen vacancies modifies the resistive switching (RS) mechanism from a valence change mode to a electrochemical metallization mode, resulting in a high switching ratio (≥10⁵), a small electronic leakage current in the high-resistance (≥10⁹ Ω) state, and a highly controlled quantized conductance (QC) in the low-resistance state. These results allow for understanding the relationship between different RS mechanisms as well as the QC for multilevel data storage application.