Redistribution Of Oxygen Vacancies Research Articles

Electric-field modulated photovoltaic effect of ferroelectric double-perovskite Bi2FeMnO6 films

Ferroelectric perovskite oxide materials for photovoltaics (PV) have received considerable attention for their switchable PV responses and above-bandgap photovoltages as a type of new-generation PV device. Relatively large bandgap and low photocurrent remain major problems for their PV applications. Herein, we report the PV response of ferroelectric double-perovskite Bi2FeMnO6 (BFMO) thin films. It was found that the photocurrent density (Jsc) of a Sn:In2O3 (ITO)/BFMO/SrRuO3 (SRO) thin film device is two orders of magnitude higher than that of BiFeO3, and its power conversion efficiency is about 430 times larger than that of BiFeO3 under AM 1.5G illumination. The built-in electric field near the BFMO/SRO interface, which represents the Schottky contact, leads to the separation of photon-generated carriers. More importantly, the electric poling treatment on the BFMO device can manipulate significantly the magnitude of Jsc, which is independent of the polarization direction. This electric-field modulated PV effect in the poled BFMO device originates from the modulation of the Schottky barrier height at the BFMO/SRO interface. The redistribution of oxygen vacancies after electric poling treatment is mainly responsible for the modulation of the Schottky barrier height.

Oxygen Vacancy-Dependent Synaptic Dynamic Behavior of TiO x -Based Transparent Memristor

Transparent synaptic devices remain great prospects for a future comprehensive simulation of artificial synapses with the combination of photoelectric coupling characteristics in the neuromorphic system. In this article, one type of memristors based on a stack structure of ITO/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> /TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /TiN with high transmittance ( T > 81%) is fabricated, in which an oxygen-deficient TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> layer between TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and ITO top electrode is introduced to adjust the gradual current characteristics during the SET/RESET process. Furthermore, the devices with different thickness ratios (1:4, 1:2, 1:1, 2:1) of oxygen-deficient TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> active layers are designed to realize the adjustability of the multilevel resistance state and the analog resistive switching (RS) behavior and improve electrical performances and synaptic simulation characteristics. Meantime, the device based on optimum thickness ratio (1:2) offers excellent uniformity for 1000 cycles and better nonlinearity (0.69) at a low programming power consumption, which reveals further simulation of essential synaptic functions, including paired-pulse facilitation (PPF), and spike-timing-dependent plasticity (STDP) as well as learning-relearning characteristics. In addition, oxygen concentration gradient could be regulated by controlling the thickness ratio to drive oxygen vacancy redistribution at the interfacial layer, which are also demonstrated by high-resolution transmission electron microscopy (HRTEM) and improve electronic properties and analog modulation characteristics.

Ferroelectric, Analog Resistive Switching in Back‐End‐of‐Line Compatible TiN/HfZrO4/TiOx Junctions

Due to their compatibility with complementary metal–oxide–semiconductor technologies, hafnium‐based ferroelectric devices receive increasing interest for the fabrication of neuromorphic hardware. Herein, an analog resistive memory device is fabricated with a process developed for back‐end‐of‐line integration. A 4.5 nm‐thick HfZrO4 (HZO) layer is crystallized into the ferroelectric phase, a thickness thin enough to allow electrical conduction through the layer. A TiOx interlayer is used to create an asymmetric junction as required for transferring a polarization state change into a modification of the conductivity. Memristive functionality is obtained, both in the pristine state and after ferroelectric wake‐up, involving redistribution of oxygen vacancies in the ferroelectric layer. The resistive switching is shown to originate directly from the ferroelectric properties of the HZO layer.

Improvement in ferroelectricity and breakdown voltage of over 20-nm-thick HfxZr1−xO2/ZrO2 bilayer by atomic layer deposition

The ferroelectricity of metal–ferroelectric–metal capacitors with a ferroelectric HfxZr1−xO2/ZrO2 (HZO/ZO) bilayer thicker than 20 nm formed by atomic layer deposition and postdeposition annealing at 600 °C was investigated. The HZO/ZO capacitors exhibited a higher remanent polarization (2Pr = Pr+ − Pr−) and breakdown voltage (Vbd) than capacitors with a HfxZr1−xO2 (HZO) single layer. In particular, a HZO (15 nm)/ZO(10 nm) (HZ15Z10) capacitor exhibited excellent Vbd and 2Pr values of 6.7 V and 14 μC/cm2, respectively, which are much higher than those (4.3 V and 10 μC/cm2, respectively) for a HZO (15 nm) (HZ15) capacitor. The HZ15Z10 capacitor also exhibited higher endurance, a smaller wake-up effect (∼5%), and superior fatigue properties up to 108 switching cycles, compared to a HZ15 capacitor, which exhibited a large wake-up effect (∼15%) and large 2Pr degradation at 108 cycles. The wake-up behavior is attributed to domain depinning mainly related to the redistribution of oxygen vacancies and transformation from the nonferroelectric phase to the ferroelectric orthorhombic (O) phase during field cycling. The HZO/ZO bilayer grew a larger grain size with the ZO nucleation layer, which is twice as large as that of the HZO single layer, and the former structure exhibited more stable bulk-like ferroelectricity. Although the ferroelectric O phase in HZO decreased with increasing HZO thickness, the HZO/ZO structure had a larger fraction of the O phase than the HZO structure. Considering the manufacturing process of ferroelectric devices, these thick HZO/ZO bilayers are promising ferroelectric materials.

Dynamics of the spatial separation of electrons and mobile oxygen vacancies in oxide heterostructures

In the search for an oxide-based 2D electron system with a large concentration of highly mobile electrons, a promising strategy is to introduce electrons through donor doping while spatially separating electrons and donors to prevent scattering. In $\mathrm{SrTi}{\mathrm{O}}_{3}$, this can be achieved by tailoring the oxygen vacancy profile through reduction, e.g., by creating an interface with an oxygen scavenging layer. Through reduction, oxygen atoms are removed close to the interface, leaving behind oxygen vacancies in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ lattice and mobile electrons in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ conduction band. The commonly assumed picture is that the oxygen vacancies then remain confined close to the interface while the electrons leak a few nanometers into the bulk, resulting in an electron-defect separation and a highly mobile, oxide-based 2D electron system. So far it has remained unclear how the confinement and electron-defect separation develop over time. Here, we present transient finite element simulations that consider three driving forces acting on the oxygen vacancy distribution: diffusion due to the concentration gradient, drift due to the intrinsic electric field, and an oxygen vacancy trapping energy that holds oxygen vacancies at the interface. Our simulations show that at room temperature, three distinct regions are formed in $\mathrm{SrTi}{\mathrm{O}}_{3}$ within days: (1) Oxygen vacancies are partially held at the interface due to the oxygen vacancy trapping energy. (2) The accompanying positive space charge causes an oxygen vacancy depletion layer with large electron concentration and high mobility just below the interface. This electron-defect separation, indeed, leads to a highly conductive region. (3) While we are able to describe measured conductivity data with an oxygen vacancy trapping energy of $\ensuremath{-}0.2$ eV, this value does not prevent oxygen vacancy diffusion into the bulk: A diffusion front progresses into the bulk and leads to significant conductivity arising over the first micrometer within a couple of months. An enhanced oxygen vacancy trapping energy of $\ensuremath{-}0.5$ eV or below would suppress this loss of confinement, leading to a static and pronounced electron-defect separation. Consequently, our results highlight the importance of oxygen vacancy redistribution and suggest the trapping energy of oxygen vacancies at the interface as an important design parameter for oxygen-vacancy-based 2D electron systems.

Physically Transient Optic-Neural Synapse for Secure In-Sensor Computing

Herein, we demonstrate an artificial optic-neural synapse based on physically transient memristor. The physically transient optic-neural synapse with structure of W/MgO/ZnO/Mo exhibits remarkable long-term plasticity under light with various wavelengths and intensity including the red, green, and blue lights. In addition, a complete electro-thermal resistive switching model accurately simulates the migration and redistribution of oxygen vacancies in the memristor. Importantly, the long-term plasticity can disappear when the devices were immersed in deionized (DI) water, demonstrating the physically transient nature of the device. Finally, an optical convolutional neural network based on the physically transient optic-neural synapse was successfully simulated. These results show that the transient optical synapse has a great prospect for in-sensor computing and secure electronics.

Enhancement of the switchable diode effect by the surface hydroxylation of ferroelectric oxide thin films

The diode behavior of ferroelectric materials was shown to be enhanced using aqueous solution treatment of their surface. The electric current increased for both BaTiO3 and BiFeO3 nano-films after being treated with aqueous solution as compared with their as-grown films. The hydroxyl groups were confirmed to be successfully bonded on the surface, which increased the concentration of oxygen vacancies near the film surface. The rectifying diode effect can be associated with the polarization direction and the movement and redistribution of oxygen vacancies within the ferroelectric thin films. Further the enhanced diode effect for the hydroxylated film was attributed to the increased oxygen vacancies near the top electrode/film interface. The reported finding helps us understand the role of water as the most common polar molecule in the atmosphere on the ferroelectric materials that exist in many industrial devices. This will provide a low-cost strategy for improving the performance of resistive memory.

Controllable Vacancy Redistributions in Embedded Thin Films Under Concentrated Electric Fields

The redistribution and accumulation of oxygen vacancies under electric field underpins both the functionality and degradation of electrical and electrochemical devices comprising oxide layers. Interfacial accumulations of oxygen vacancies form defect dipoles, significantly reduce the breakdown strength, increase the breakdown path area, yet facilitates charge transfer reactions. Confinement of both oxygen vacancy redistributions and their mobility remains critical to preserving function in oxide systems.Here we study one of the most important, yet random transport processes: the redistribution of oxygen vacancies in binary oxides under high local electric fields. The ability to identify or predefine the shape, size, and location of vacancy distributions is critical to controlling interfacial properties of functional oxides. We achieved superior uniformity in HfO2 films by embedding highly-ordered metal nanoisland (NI) arrays to act as electric field concentrators. Embedded films exhibit significant reduction in operating voltages while displaying enhanced uniformity of resistance states. This behavior is attributed to the concentration of electric fields along Pt and Ti NIs and their interactions with the surrounding oxide film matrix environment, which induce separate and distinct filamentary formation mechanisms that affect the stability. A comparison of the density and distribution of the oxygen vacancies responsible driving filament dynamics is made via combined electrostatic force microscopy (EFM) and conductive atomic force microscopy (c-AFM) studies. Finally, the morphological evolution of conducting vacancy filaments is enabled by three-dimensional (3-D) c-AFM nanotomography and cross-sectional scanning transmission electron microscopy – energy dispersive spectroscopy (STEM-EDS) to provide direct correlations of nanoisland-oxide interactions with overall switching performance. The application of these exciting results to other vacancy-mediated electronic and electrochemical processes will also be discussed.

Hydrothermally Grown TiO2 Nanorod Array Memristors with Volatile States.

In the present study, the memristive characteristics of hydrothermally grown TiO2 nanorod arrays, particularly, the difference in the retention time of the resistance state, are investigated in dependence of the array growth temperature. A volatile behavior is observed and related to a redistribution of oxygen vacancies over time. It is shown that the retention time increases for increasing array growth temperatures from several seconds up to 20 min. The relaxation behavior is also seen in the current-voltage characteristics, which do not show the common unipolar, bipolar, or complementary switching behavior. Instead, the temporal evolution depends on the duration of the applied voltage and on the nanowire growth temperature. Therefore, electronic measurements are combined with scanning electron and scanning transmission electron microscopy, so that the amount of oxygen defect-rich grain boundaries in the upper part of the nanowires can be linked to the differences in the current-voltage behavior and retention time.

Electric fatigue of BCZT ceramics sintered in different atmospheres

Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) ceramics with excellent piezoelectric properties and environment friendliness are very potential alternatives for PbZrxTi1−xO3 (PZT) ceramics. The stability and reliability of ferroelectricity and piezoelectricity in BCZT ceramics under cyclic electric field are crucial to its long-term application. Herein, BCZT ceramics were synthesized by sol–gel method and different sintering atmospheres and the influences of sintering atmosphere on the fatigue behaviors of ferroelectric and piezoelectric properties have been investigated. XPS results confirm that the oxygen vacancy concentration of BCZT ceramics sintered in oxygen, air, and nitrogen atmosphere gradually increases. After 106 cycles, the polarization of all BCZT ceramics decreases to some extent and the fatigue resistance of polarization of BCZT ceramics sintered in the air is superior to BCZT ceramics sintered in oxygen and nitrogen atmospheres, which may result from the additional contribution of higher leakage current caused by relatively poor densification of BCZT ceramics sintered in air to polarization. The internal bias field of BCZT ceramics sintered in air, O2 and N2 atmospheres after fatigue decreases, and the reduced amplitude of internal bias field in BCZT ceramics with less oxygen vacancies sintered in air and oxygen atmospheres is obviously higher than that in BCZT ceramics with more oxygen vacancies sintered in nitrogen atmosphere, which is due to the redistribution of oxygen vacancy under the repetition of external electric field. After 106 cycles, the maximum electric field-induced strain and d33* of BCZT ceramics sintered in the air and oxygen atmospheres decrease and that of BCZT ceramics sintered in nitrogen atmosphere increases. The enhanced fatigue resistance of piezoelectric response of BCZT ceramics sintered in nitrogen atmosphere results from the interaction of (100) orientation, grain refinement, and “wake up” effect caused by the repetition of enough external electric field.

Gate Tuning of Synaptic Functions Based on Oxygen Vacancy Distribution Control in Four-Terminal TiO2\u2212x Memristive Devices

Recent developments in artificial intelligence technology has facilitated advances in neuromorphic computing. Electrical elements mimicking the role of synapses are crucial building blocks for neuromorphic computers. Although various types of two-terminal memristive devices have emerged in the mainstream of synaptic devices, a hetero-synaptic artificial synapse, i.e., one with modulatable plasticity induced by multiple connections of synapses, is intriguing. Here, a synaptic device with tunable synapse plasticity is presented that is based on a simple four-terminal rutile TiO2−x single-crystal memristor. In this device, the oxygen vacancy distribution in TiO2−x and the associated bulk carrier conduction can be used to control the resistance of the device. There are two diagonally arranged pairs of electrodes with distinct functions: one for the read/write operation, the other for the gating operation. This arrangement enables precise control of the oxygen vacancy distribution. Microscopic analysis of the Ti valence states in the device reveals the origin of resistance switching phenomena to be an electrically driven redistribution of oxygen vacancies with no changes in crystal structure. Tuning protocols for the write and the gate voltage applications enable high precision control of resistance, or synaptic plasticity, paving the way for the manipulation of learning efficiency through neuromorphic devices.

Room temperature electric field control of magnetic properties for the α-Fe2O3/Fe3O4 composite structure

Abstract The raw nano α-Fe2O3 powders consisting of nanorods with 50–100 nm in length and 8–14 nm in width exhibit superparamagnetism. The vacuum annealing of the raw nano α-Fe2O3 at 400 °C for 3 h leads to ferromagnetic-like behaviors at room temperature (RT), due to the appearance of ferrimagnetic Fe3O4. The α-Fe2O3/Fe3O4 composite structure has an antiferromagnetic α-Fe2O3 layer and a ferrimagnetic Fe3O4 layer contains a certain amount of Fe2+ and oxygen vacancies. In addition, nano α-Fe2O3 annealed in H2 at 300 °C for 3 h completely become Fe3O4. Some magnetic properties of the α-Fe2O3/Fe3O4, such as magnetization, magnetic remanence (Mr), coercivity (Hc) and exchange bias (EB), can be modulated almost reversibly and non-volatilely by applied external RT electric field without loop current. However, for both the raw nano α-Fe2O3 and the Fe3O4, these magnetic properties cannot be modulated by applied external RT electric field. The electric field control of magnetic properties for the α-Fe2O3/Fe3O4 should be due to the mechanism of phase transition induced by migration and redistribution of oxygen vacancies within the interface of α-Fe2O3/Fe3O4.

Field-Driven Hopping Transport of Oxygen Vacancies in Memristive Oxide Switches with Interface-Mediated Resistive Switching

The dynamics of mobile oxygen vacancies depend on electric field and temperature, and this is key to controlling interfacial resistive switching in BiFeO${}_{3}$ memristive devices, which are interesting especially for neuromorphic computing. The authors use impedance spectroscopy and quasistatic state measurements to reveal the dynamics of resistance changes in such devices, and relate these changes to the redistribution of oxygen vacancies via modeling. This work will also impact the use of other oxides with mobile oxygen vacancies in similar devices.

Charge-transfer in B-site-depleted NdGaO3/SrTiO3 heterostructures

Cation stoichiometry has been identified as a major key in establishing 2-dimensional electron gases (2DEGs) in oxide heterostructures. Here, we discuss a 2DEG formation scenario in B-site deficient perovskite/perovskite heterostructures, which previously were predicted to show insulating behavior. We elaborate an ionic picture based on oxygen-vacancy-buffered B-site vacancy defects in the polar oxide layer that yields a continuous transition from 2DEG formation to less conducting interfaces to insulating interfaces with increasing B-site deficiency. Experimentally, a corresponding modulation of charge transfer across NdGaO3/SrTiO3 interfaces is inferred using hard x-ray photoelectron spectroscopy analysis and transport experiments. With increasing B-site deficiency, we observe a decrease of the interfacial Ti3+ core level contribution, indicating a reduced charge transfer at the interface. This result is corroborated by temperature-dependent transport measurements, revealing increased low temperature resistance, with a dominant influence of a reduced electron density in the Ga-depleted sample. We consider a redistribution of oxygen vacancies in the B-site deficient polar oxide layer to explain the alleviated interface reconstruction, adding a new perspective on potential built-up in polar-oxide thin films.

Oxygen Exchange Processes between Oxide Memristive Devices and Water Molecules.

Resistive switching based on transition metal oxide memristive devices is suspected to be caused by the electric-field-driven motion and internal redistribution of oxygen vacancies. Deriving the detailed mechanistic picture of the switching process is complicated, however, by the frequently observed influence of the surrounding atmosphere. Specifically, the presence or absence of water vapor in the atmosphere has a strong impact on the switching properties, but the redox reactions between water and the active layer have yet to be clarified. To investigate the role of oxygen and water species during resistive switching in greater detail, isotope labeling experiments in a N2 /H218 O tracer gas atmosphere combined with time-of-flight secondary-ion mass spectrometry are used. It is explicitly demonstrated that during the RESET operation in resistive switching SrTiO3 -based memristive devices, oxygen is incorporated directly from water molecules or oxygen molecules into the active layer. In humid atmospheres, the reaction pathway via water molecules predominates. These findings clearly resolve the role of humidity as both oxidizing agent and source of protonic defects during the RESET operation.

In-situ TEM study of the aging micromechanisms in a BaTiO3-based lead-free piezoelectric ceramic

Aging and fatigue are the two main concerns regarding the performance reliability of piezoelectric ceramics. Compared with fatigue, less efforts have been made towards clarifying the micromechanisms of aging. In this report, we employ electric field in-situ transmission electron microscopy (TEM) to directly visualize the domain structure evolution during fatigue and the subsequent aging process in the 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–BCT) polycrystalline ceramic. The macroscopic aging behaviors, including the development of internal bias field (Ebias) and the degradation in switchable polarization (2Pr), are correlated with the microscopic domain wall clamping and domain disruption resulted from the redistribution of oxygen vacancies driven by depolarization field.

Polarized neutron reflectivity studies on epitaxial BiFeO3/La0.7Sr0.3MnO3 heterostructure integrated with Si (100)

This work reports polarized neutron reflectivity (PNR) measurements performed using the Magnetism Reflectometer at Oak Ridge National Laboratory on epitaxial BiFeO3(BFO)/La0.7Sr0.3MnO3(LSMO)/SrTiO3(STO)/MgO/TiN heterostructure deposited on Si (100) substrates. By measuring the angular dependence of neutrons reflected from the sample, PNR can provide insights on interface magnetic spin structure, chemical composition and magnetic depth profiles with a nanometer resolution. Our first analysis of nuclear scattering length density (NSLD) and magnetic scattering length density (MSLD) depth profiles measured at 4 K have successfully reproduced most of the expected features of this heterostructure, such as the NSLD for the Si, TiN, MgO, STO, LSMO layers and remanent magnetization (2.28μB/Mn) of bulk LSMO. However, the SLD of the BFO is decreased by about 30% from the expected value. When 5 V was applied across the BFO/LSMO interface, we found that the magnetic moment of the LSMO layer could be varied by about 15-20% at 6 K. Several mechanisms such as redistribution of oxygen vacancies, interface strain, charge screening and valence state change at the interface could be at play. Work is in progress to gain an improved in-depth understanding of these effects using MOKE and STEM-Z interface specific measurements.

The Role of Oxygen Vacancy Mobility at the Oxide Bulk and Electrode Interfaces for Ceria-Based Memristive Devices

Resistive switching is a recently extensively studied phenomenon for use in non-volatile memories as promising building blocks of future electronics. In such devices transition metal oxides are operated at high electric field strengths to initiate non-linear and hysteretic current-voltage relations that can be addressed in their various resistance states by voltage pulse amplitude or polarity. Despite the known importance of oxygen vacancy defects in metal-oxide memristive devices, systematic strategies on tuning their concentration and mobility in the transition metal oxide are rare. In this work, we utilized the fluorite-type ceria-gadolinia binary system Ce1-xGdxO2-x/2 (GDC) to tune the oxygen vacancy concentration in order to correlate structure and defect chemistry to the memristive properties. The mixed conduction of GDC is well investigated at high temperature, and it is possible to extrinsically dope over the wide range of 3 to 30 mol% Gd; however, has only recently been reported for ambient and high electric fields1. Therefore, we can tune the oxygen vacancy concentration, migration and association energy by the acceptor dopant as a model system to directly investigate the influence on resistive switching. For this purpose Pt-GDC-Pt sandwich structures were fabricated as resistive switching devices with cross-bar electrodes. Based on hysteretic current-voltage profiles, a maximum in resistive switching (with ROFF/RON up to 100) is found for the best ion conducting2 20 mol% Gd-doped CeO2. In-operando Raman spectroscopy revealed a homogeneous reduction of the ceria film during electroforming, together with a strong increase in conductivity. Additional high energy XPS measurements we observe the electrostatic potential drop at the electrode-oxide interface. These experimental results directly prove that the electronic charge carriers generated by the formation and redistribution of oxygen vacancies at high electric fields is the origin of memristance in doped ceria films, and that the switching happens in a rather homogeneous manner. The importance of oxygen vacancy migration in our model is further verified by the doping study. Doping levels of 10-20 mol% resulted in good switching properties, whereas for low (<10 mol%) and high (>20 mol%) concentrations no resistive switching is observed. In both cases the number of mobile oxygen vacancies is not sufficient, either because of the low extrinsic doping level, or because of the formation of bixbyite-like domains with ordered or clustered "immobile" oxygen vacancies. This clustering is observed by the appearance of new vibrational modes in Raman spectroscopy due to the reduced symmetry in a defective crystal. Excitingly, one can study based on the findings the direct implication of "free" oxygen vacancies at low and high mobilites and also of "trapped" clustered vacancies on both, resistive switching and also local structural symmetry breaks3. In conclusion, we observed a clear correlation of oxygen vacancy mobility and switching characteristics. We suggest that a high concentration of mobile oxyen vacancies is of general importance when designing oxide materials for future computing applications.

Switchable diode effect in oxygen vacancy-modulated SrTiO3 single crystal

SrTiO3 (STO) single crystal wafer was annealed in vacuum, and co-planar metal–insulator–metal structure of Pt/Ti/STO/Ti/Pt were formed by sputtering Pt/Ti electrodes onto the surface of STO after annealing. The forming-free resistive switching behavior with self-compliance property was observed in the sample. The sample showed switchable diode effect, which is explained by a simple model that redistribution of oxygen vacancies (OVs) under the external electric field results in the formation of n–n+ junction or n+–n junction (n donated n-type semiconductor; n+ donated heavily doped n-type semiconductor). The self-compliance property is also interpreted by the formation of n–n+/n+–n junction caused by the migration of the OVs under the electric field.

Distinguishing Oxygen Vacancy Electromigration and Conductive Filament Formation in TiO2 Resistance Switching Using Liquid Electrolyte Contacts.

Resistance switching in TiO2 and many other transition metal oxide resistive random access memory materials is believed to involve the assembly and breaking of interacting oxygen vacancy filaments via the combined effects of field-driven ion migration and local electronic conduction leading to Joule heating. These complex processes are very difficult to study directly in part because the filaments form between metallic electrode layers that block their observation by most characterization techniques. By replacing the top electrode layer in a metal-insulator-metal memory structure with easily removable liquid electrolytes, either an ionic liquid (IL) with high resistance contact or a conductive aqueous electrolyte, we probe field-driven oxygen vacancy redistribution in TiO2 thin films under conditions that either suppress or promote Joule heating. Oxygen isotope exchange experiments indicate that exchange of oxygen ions between TiO2 and the IL is facile at room temperature. Oxygen loss significantly increases the conductivity of the TiO2 films; however, filament formation is not observed after IL gating alone. Replacing the IL with a more conductive aqueous electrolyte contact and biasing does produce electroformed conductive filaments, consistent with a requirement for Joule heating to enhance the vacancy concentration and mobility at specific locations in the film.