Migration Of Oxygen Vacancies Research Articles

Effect of temperature on polaronic transport in CeO2 thin-film.

The outstanding catalytic property of cerium oxide (CeO2) strongly depends on the polaron formation due to the oxygen vacancy (V̈O) defect and Ce4+ to Ce3+ transformation. Temperature plays an important role in the case of polaron generation in CeO2 and highly influences its electrical transport properties. Therefore, a much needed attention is required for detailed understanding of the effect of temperature on polaron formation and oxygen vacancy migration to get an idea about the improvement in the redox property of ceria. In this work, we have probed the generation of polarons in CeO2 thin-film deposited on a silicon (Si) substrate using the resonance photoemission spectroscopy (RPES) study. The RPES data show an increase in polaron density at the substrate-film interface of the thermally annealed film, indicating the formation of an interfacial Ce2O3 layer, which is, indeed, a phase change from the cubic to hexagonal structure. This leads to a modified electronic band structure, which has an impact on the capacitance-voltage (C-V) characteristics. This result nicely correlates the microscopic property of polarons and the macroscopic transport property of ceria.

The spin-state effect on the redox performance of LaCoO3 perovskites

LaCoO3 is well-known for its unique thermally induced transition of Co3+ (3d6) ions from a low spin (LS) state to a higher spin state of intermediate spin (IS) state or high spin (HS) state, due to a delicate balance between crystal-field splitting and Hund’s coupling. In this work, we try to stabilize all possible spin states in the same initial structural model and further investigate the spin-state effect on the redox performance of LaCoO3. We find that an initial model of pseudocubic with R3¯c tilting octahedra was effective in stabilizing all possible spin states. The oxygen vacancy formation values of LS > HS + LS > HS > IS indicate that the IS state is the most optimal spin state in oxygen vacancy formation, which results from a weakened Co-O bonding energy by eg orbital occupation. The oxygen migration barriers of LS > IS > HS + LS > HS show that oxygen migration is easier in the relatively loose structure and thus LS structure is the most unfavorable in oxygen migration. Combining the oxygen vacancy formation and migration results, we conclude that the higher spin states of IS and HS are more beneficial than LS in oxygen conduction. Our results confirm the influence of the spin-state effect on oxygen conduction and show that spin-state regulation is a good strategy for improving the redox performance of perovskites.

Evidence for reversible oxygen ion movement during electrical pulsing: enabler of emerging ferroelectricity in binary oxides

Ferroelectric HfO2-based materials and devices show promising potential for applications in information technology but face challenges with inadequate electrostatic control, degraded reliability, and serious variation in effective oxide thickness scaling. We demonstrate a novel interface-type switching strategy to realize ferroelectric characteristics in atomic-scale amorphous binary oxide films, which are formed in oxygen-deficient conditions by atomic layer deposition at low temperatures. This approach can avoid the shortcomings of reliability degradation and gate leakage increment in scaling polycrystalline doped HfO2-based films. Using theoretical modeling and experimental characterization, we show the following. (1) Emerging ferroelectricity exists in ultrathin oxide systems as a result of microscopic ion migration during the switching process. (2) These ferroelectric binary oxide films are governed by an interface-limited switching mechanism, which can be attributed to oxygen vacancy migration and surface defects related to electron (de)trapping. (3) Transistors featuring ultrathin amorphous dielectrics, used for non-volatile memory applications with an operating voltage reduced to ±1 V, have also been experimentally demonstrated. These findings suggest that this strategy is a promising approach to realizing next-generation complementary metal-oxide semiconductors with scalable ferroelectric materials.

Role of mixed oxidant and controlled oxygen partial pressure heat treatment to regulate the dielectric properties and resistivity of SrTiO3-based CP ceramics

Sr0.99La0.01TiO3 (SLT10) colossal permittivity (CP) ceramic materials were synthesized using solid phase method and secondary heat treated using mixed oxidant 35%Bi2O3–10%Al2O3–20%MgO–25%CuO–10%SiO2 oxidant. The heat treatment atmospheres were chosen as O2, air, and Ar to correspond to different oxygen partial pressures. The dielectric properties of the Sr0.99La0.01TiO3-Percolated (SLT10-P) ceramics were investigated, and the contribution of the oxidant to the resistivity enhancement was verified by complex impedance spectroscopy and DC resistivity tests. Excellent dielectric properties were obtained for SLT10-P Air ceramic sample (16543 for dielectric permittivity and 0.016 for loss tangent), and the resistivity was increased from 2.13 × 108 Ω cm (SLT10) to 1.23 × 1011 Ω cm (SLT10-P Air). The resistance degradation mechanism of SLT10-P ceramics and oxygen vacancy migration in the process of resistance degradation were studied by using resistance monitoring device and thermally stimulated depolarization currents (TSDC) test. The introduction of oxidant optimizes the temperature stability and resistivity of SLT10 ceramics, but it will lead to the decrease of dielectric permittivity. The atmosphere of heat treatment changed the concentration of oxygen vacancy in ceramics. When SLT10-P O2 was treated in O2 atmosphere, the concentration of oxygen vacancy in ceramics decreased, which led to the decrease of dielectric permittivity and the increase of grain boundary resistance. For SLT10-P Ar ceramic, more oxygen vacancies and free electrons are induced by low oxygen partial pressure heat treatment, which enhances the dipole effect of electron pinning defects and leads to colossal dielectric permittivity, but high concentration of oxygen vacancies will affect the resistance decline. In SLT10-P Air ceramic, the coexistence of colossal permittivity and low loss tangent and high resistance is realized.

Stabilized Nickel-Rich oxide cathode by the confining effect of Nb lattice anchoring and epitaxial shielding layer

Ni-rich LiNixCoyMn1−x−yO2 cathodes attract growing attention for high energy density but still face great challenges in terms of cyclic lifespan and thermal stability issues. The structural degradation is primarily caused by the increased oxygen vacancy and lattice oxygen loss, especially at high temperatures. Here, we develop an anchoring-confining strategy to rivet lattice oxygen and inhibit the formation and migration of oxygen vacancy by constructing LiNbO3 & LiF epitaxial layer and Nb5+ lattice doping within LiNi0.92Co0.055Mn0.025O2 (NCM92) cathode. The optimized NCM-FN material exhibits excellent stability with retenion of 93 % after 200 cycles at 1C supassing the pristine one (68.7 %), conspicuous high-temperature capacity retention rate (81.6 % vs. 49.5 %) and superior reversible capacity of 165 mAh/g at 5C. In particular, it even possessed superior thermal safety pushing thermal runaway temperature to 223.65 °C. In-situ X-ray diffraction (XRD) result indicates that Nb5+ lattice substitution coupled with LiNbO3 & LiF epitaxial layer alleviates the irreversibility of H2-H3 phase transition, thus stabilizing the layered structure. Furthermore, the LiNbO3 & LiF epitaxial layer, as both a physical shielding layer and ionic conductive layer, protects interface structural integrity and simultaneously promotes Li+ diffusion. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) results demonstrate that the formation and propagation of oxygen vacancy within NCM-FN material is significantly inhibited. DFT calculation results reveal that Nb5+ introduction can increase the formation energy of oxygen vacancies and inhibit outward migration of active oxygen Oα- (α < 2) to stabilize layer structure by tuning the breaking energy barrier of nearby metal-O bonds. This work not only demonstrates the design of advanced Ni-rich layered cathodes but also underscores the importance of stabilized lattice oxygen for advanced Ni-rich cathodes.

Thermochemical and mechanical properties of Ce0.9Pr0.1O2-δ-Pr0.6Sr0.4Fe0.9Al0.1O3-δ dual-phase membrane

The fluorite-perovskite dual-phase Ce0.9Pr0.1O2-δ-Pr0.6Sr0.4Fe0.9Al0.1O3-δ (CP-PSFA) oxygen transport membrane (OTM) was developed to explore the properties and interaction of the two phases on the crystal structure, sintering behavior, thermochemical expansion, and oxygen release property, mechanical strength, and stability of the dual-phase membrane. The migration of oxygen vacancies between two phases causes the shrinking and expansion of the lattice of CP and PSFA phases, and the lattice of PSFA with higher TEC is compressed while the CP is stretched in the dual-phase membrane at the high-temperature range. The CP phase can significantly improve the mechanical properties of the dual-phase membrane, among which the flexural strength of the CP-PSFA reaches 303±42 MPa, almost twice that of PSFA membrane. Moreover, the high reducing gas resistance of the CP phase gives the CP-PSFA membrane more excellent high-temperature chemical stability. These results offer new insights into developing mixed ionic-electronic conducting dual-phase OTMs.

A Novel Transparent Memristor‐Type Infrared Emissivity Modulator for Multispectral Compatible Display

The regulation of infrared (IR) thermal radiation has received widespread attention; the proposals have been made for various devices that use electrochromic materials. However, the visible appearance of the mentioned devices also changes when the IR states are altered. Herein, the memristive mechanism is utilized to regulate IR emissivity and design a novel transparent memristor‐type modulator featuring an Indium tin oxide (ITO)/HfO2/ITO structure. The modulator exhibits a dynamic emissivity variation of 0.38 in the IR region of 2.5–25 μm, without compromising transparency in the visible region under the application of an electric voltage. The migration of oxygen vacancies results in the modification of interface barriers and the formation of oxygen vacancy‐enriched regions, thereby enabling the modulator to exhibit resistive switching behavior and regulate IR emissivity. An array of modulators is also assembled, demonstrating the ability to independently display visible and IR information. In this work, new possibilities are presented for various applications such as multispectral displays, energy‐efficient thermoregulation, encrypted information displays, and camouflage.

Role of Oxygen Vacancy Migration in Pyroelectric Currents of Nd2Ti2O7 Ceramics as High Temperature Sensors

Role of Oxygen Vacancy Migration in Pyroelectric Currents of Nd₂Ti₂O₇ Ceramics as High Temperature Sensors

Remarkable energy-storage density together with efficiency of above 92% in high-entropy ferroelectric ceramics

Recently, the use of “entropy engineering” to form high-entropy ceramic dielectric materials is considered to be an effective means to break through the traditional doping which modified local structures. However, the low energy storage efficiency (η) of most high-entropy ceramics cannot match their excellent energy storage density (Wrec). This work is the first to combine scheelite structure (SmTaO4) with high-entropy perovskite structure ((NaBiBaSrCa)0.2TiO3). On the one hand, the superposition of multiple structures makes the inherent lattice distortion and sluggish diffusion of high-entropy materials more obvious, thus playing a role in refining grains, reducing leakage current density, and obtaining a high breakdown electric field (Eb). On the other hand, the introduction of donor elements inhibits the generation and migration of oxygen vacancies, reducing losses to improve η and Eb. Ultimately, the goal of obtaining a remarkable Wrec (5.6 J/cm3) and η (92.2%) at 445 kV/cm was successfully achieved. This work offers a practical approach to attain superior overall performances in high-entropy ceramics.

Investigation on the mechanism and a universal structural design method for resistive switching devices

Resistive random-access memories have attracted significant attention in memory applications, while the physical mechanism behind their resistive switching behavior is still unclear. As a key issue, the migration of oxygen vacancies (VO) directly influences the performance of devices in the formation and rupture of conductive filaments (CFs). In this work, the distance of VO migration is performed as electric field dependent and affects the performance of the device. Sufficient distance of VO migration is essential for formation and rupture of CF completely, leading to, ideally, SET/RESET transition of the device. Based on this mechanism, an enhanced electric field is designed that optimizes the stability of the device significantly, which is demonstrated to be a promising approach to optimize the properties of the device. The investigation is helpful for guiding the design of device structures.

1-Selector 1-Memristor Configuration with Multifunctional a-IGZO Memristive Devices Fabricated at Room Temperature.

Serving as neuromorphic hardware accelerators, memristors play a crucial role in large-scale neuromorphic computing. Herein, two-terminal memristors utilizing amorphous indium-gallium-zinc oxide (a-IGZO) are fabricated through room-temperature sputtering. The electrical characteristics of these memristors are effectively modulated by varying the oxygen flow during the deposition process. The optimized a-IGZO memristor, fabricated under 3 sccm oxygen flow, presents a 5 × 103 ratio between its high- and low-resistance states, which can be maintained over 1 × 104 s with minimal degradation. Meanwhile, desirable properties such as electroforming-free and self-compliance, crucial for low-energy consumption, are also obtained in the a-IGZO memristor. Moreover, analog conductance switching is observed, demonstrating an interface-type behavior, as evidenced by its device-size-dependent performance. The coexistence of negative differential resistance with analog switching is attributed to the migration of oxygen vacancies and the trapping/detrapping of charges. Furthermore, the device demonstrates optical storage capabilities by exploiting the optical properties of a-IGZO, which can stably operate for up to 50 sweep cycles. Various synaptic functions have been demonstrated, including paired-pulse facilitation and spike-timing-dependent plasticity. These functionalities contribute to a simulated recognition accuracy of 90% for handwritten digits. Importantly, a one-selector one-memristor (1S1M) architecture is successfully constructed at room temperature by integrating a-IGZO memristor on a TaOx-based selector. This architecture exhibits a 107 on/off ratio, demonstrating its potential to suppress sneak currents among adjacent units in a memristor crossbar.

Enhanced Computational Study with Experimental Correlation on I-V Characteristics of Tantalum Oxide (TaOx) Memristor Devices in a 1T1R Configuration.

Memristors, non-volatile switching memory platform, has recently attracted significant interest, offering unique potential to enable the realization of human brain-like neuromorphic computing efficiency. Memristors also demonstrate excellent temperature tolerance, long-term durability, and high tunability with nanosecond pulses, making them highly attractive for neuromorphic computing applications. To better understand the material processing, microstructure, and property relationship of switching mechanisms in memristor devices, computational methodologies, and tools are developed to predict the I-V characteristics of memristor devices based on tantalum oxide (TaOx) resistive random-access memory (ReRAM) integrated with an n-channel metal-oxide-semiconductor (NMOS) transistor. A multiphysics model based on coupled partial differential equations for electrical and thermal transport phenomena is solved for the high- and low-resistance states during the formation, growth, and destruction of a conducting filament through SET and RESET stages. These stages effectively represent the migration of oxygen vacancies within an oxide exchange layer. A series of parametric studies and energy minimization calculations are conducted to determine probable ranges for key material and model parameters accounting for the experimental data. The computational model successfully predicted the measured I-V curves across various gate voltages applied to the NMOS transistor in the one transistor one resistance (1T1R) configuration.

Si/CuO Heterojunction-Based Photomemristor for Reconfigurable, Non-Volatile, and Self-Powered In-Sensor Computing.

In-sensor computing has attracted considerable interest as a solution for overcoming the energy efficiency and response time limitations of the traditional von Neumann architecture. Recently, emerging memristors based on transition-metal oxides (TMOs) have attracted attention as promising candidates for in-memory computing owing to their tunable conductance, high speed, and low operational energy. However, the poor photoresponse of TMOs presents challenges for integrating sensing and processing units into a single device. This integration is crucial for eliminating the need for a sensor/processor interface and achieving energy-efficient in-sensor computing systems. In this study, a Si/CuO heterojunction-based photomemristor is proposed that combines the reversible resistive switching behavior of CuO with the appropriate optical absorption bandgap of the Si substrate. The proposed photomemristor demonstrates a simultaneous reconfigurable, non-volatile, and self-powered photoresponse, producing a microampere-level photocurrent at zero bias. The controlled migration of oxygen vacancies in CuO result in distinct energy-band bending at the interface, enabling multiple levels of photoresponsivity. Additionally, the device exhibits high stability and ultrafast response speed to the built-in electric field. Furthermore, the prototype photomemristor can be trained to emulate the attention-driven nature of the human visual system, indicating the tremendous potential of TMO-based photomemristors as hardware foundations for in-sensor computing.

One Stone, Two Birds: Using High Electric Fields to Enhance the Mobility and the Concentration of Point Defects in Ion-Conducting Solids.

Improving the ionic conductivity of outstanding, composition-optimized crystalline electrolytes is a major challenge. Achieving increases of orders of magnitude requires, conceivably, highly nonlinear effects. One known possibility is the use of high electric fields to increase point-defect mobility. In this study, we investigate quantitatively a second possibility that high electric fields can increase substantially point-defect concentrations. As a model system, we take a pyrochlore oxide (La2Zr2O7) for its combination of structural vacancies and dominant anti-Frenkel disorder; we perform molecular-dynamics simulations with many-body potentials as a function of temperature and applied electric field. Results within the linear regime yield the activation enthalpies and entropies of oxygen-vacancy and oxygen-interstitial migration, and from three independent methods, the enthalpy and entropy of anti-Frenkel disorder. Transport data for the nonlinear regime are consistent with field-enhanced defect concentrations and defect mobilities. A route for separating the two effects is shown, and an analytical expression for the quantitative prediction of the field-dependent anti-Frenkel equilibrium constant is derived. In summary, we demonstrate that the one stone of a nonlinear driving force can be used to hit two birds of defect behavior.

Thermal activation energy on electrical degradation process in BaTiO3 based multilayer ceramic capacitors for lifetime reliability

For a high capacitance and high lifetime reliability of multilayer ceramic capacitors for automotive applications, the activation energy on thermal activation process can typically be calculated by using Arrhenius based Prokopowicz–Vaskas equation as a method for lifetime prediction. In this study, it is clearly observed that the activation energy shows to be constant in the range of ~ 1.5 eV for the prototype MLCCs, higher than the activation energy values of ~ 1.0 eV related to the motion or diffusion of oxygen vacancies reported in the previous literature. The activation energy value of ~ 1.5 eV for three prototype MLCCs is close to a half the energy band gap (Eg/2 ≈ 1.6 eV) of BaTiO3 obtained from specific environment, where oxygen vacancies are stabilized by external containment such as the effect of rare earth oxide additives. Due to an obvious difference in activation energy values, it difficult to explain the conduction mechanism for failure by only oxygen vacancy migration. Therefore, the concepts of electronic processes and oxygen vacancy should be considered together to understand conduction mechanism for failure of BaTiO3-based MLCCs in thermal activation processes. It can be useful as an indicator for future MLCC development with high lifetime reliability.

P‐N Junction‐Driven Abnormal Electric Field Distribution in the Degraded Multilayer Ceramic Capacitors

Local electric field distribution in the dielectric layer of BaTiO3‐based multilayer ceramic capacitors (MLCCs) is investigated by Kelvin probe force microscopy before and after highly accelerated life test (HALT) degradation combined with the energy band diagram. An unusual electric field concentration phenomenon is directly visualized near the HALT cathode region in the degraded MLCCs while a reverse voltage is applied. Such abnormal behavior is ascribed to the migration of oxygen vacancies within the dielectric layer during the HALT, leading to the formation of a P‐N junction structure and further a heightened barrier under a reverse bias. As a result, a P‐N junctional model is proposed for understanding local failure mechanism of the degraded MLCCs, which enrich the insights into the insulation resistance degradation and the reliability of MLCCs.

Ni-doping effects on formation and migration of oxygen vacancies in SrFe1-xNixO3-δ oxygen carriers.

Ni is a promising B-site doping element capable of improving the oxygen carrier performance of SrFeO3 perovskite. In this work, the effect of Ni doping on the formation and migration of oxygen vacancies in SrFe1-xNixO3-δ (x = 0, 0.0625, 0.125, 0.1875, and 0.25) is investigated using density functional theory calculations. Our results show that the oxygen vacancies formed from Ni-O-Fe chains exhibit lower formation energy (Ef) compared to those from Fe-O-Fe chains in each doping system. Additionally, Ef generally decreases with an increase of Ni content. This Ni-promoted formation of VO is attributed to three factors: weakened Ni-O bonding, the closure of O-2p states to the Fermi level by Ni-O hybridization, and Ni3+ decreasing the positive charges to be compensated by VO formation. Due to these multiple advantages, a modest Ni doping of x = 0.25 can induce a higher PO2 and a lower T comparted to the relatively larger Co doping of x = 0.5, thermodynamically. Kinetically, Ni-doping appears to be a disadvantage as it hinders oxygen migration, due to a higher oxygen migration barrier through SrSrNi compared to the SrSrFe pathway. However, the overall oxygen ion conduction would not be significantly influenced by hopping through a nearby pathway of SrSrFe with a low migration barrier in a system doped with a small amount of Ni. In a word, a small amount of Ni doping has an advantage over Co doping in terms of enhancing the oxygen carrier performance of the parent SrFeO3 system.

Structural, microstructure, dielectric relaxation, and AC conduction studies of perovskite SrSnO3 and Ruddlesden-Popper oxide Sr2SnO4.

Here, we report a comparison study on the synthesis and characterization of perovskite SrSnO3 (SSO) and Sr2SnO4 (S2SO). Rietveld refinement studies were performed on both prepared samples and suggest that they crystallized in cubic (SSO) and tetragonal (S2SO) structures. Fourier-transform infrared (FTIR) and Raman spectroscopy studies supported the XRD observations. Improved dielectric parameters were observed for S2SO over SSO due to differences in dislocation density, larger crystallite size, and denser microstructure. The electrical conduction and relaxation processes followed the Arrhenius type in both samples through the migration of oxygen vacancies via the Sn-site and the transfer of electrons between the Sn sites in two different temperature regions. These processes in the samples occurred via correlated barrier hopping (CBH) in SSO and the non-overlapping of small-polaron tunnelling (NSPT) in S2SO. The conduction and relaxation processes had similar sources of charge carriers but differed in the concentration and mobility of charge carriers. The presented materials can be utilized for dielectric capacitors, sensors, and mixed ionic and electronic conductor-based electrodes in IT-SOFC applications.

Simulation of the resistance switching performance and synaptic behavior of TiO2-based RRAM devices with CoFe2O4 insertion layers.

The electrothermal coupling model of Pt/CoFe2O4/TiO2/TiN devices was established to study their resistive switching characteristics and basic biological synaptic properties in our research. The processes of set and reset are simulated, and the distribution of the temperature, the electric field and the concentration of oxygen vacancies in the dielectric layer are obtained. The switching performance of the TiO2-based device is significantly improved after the CoFe2O4 layer is inserted, with the switching voltage, working current and power consumption being reduced, while the switching ratio is increased. By changing the thermal conductivity of the top electrode, the rupture position of the conductive filament can be controlled. The I-V characteristics of the Pt/CoFe2O4/TiO2/TiN device during the reset and set processes are fitted linearly in logarithmic coordinates, and the ohmic conduction mechanism or the space-limited charge conduction mechanism is mainly satisfied in the high and low resistance states. Finally, the application of dual-layer devices on biological synapses is studied, and the basic biological characteristics of enhancement, inhibition and paired pulse promotion are simulated successfully. In addition, the redox reaction induced by oxygen vacancy migration also promotes the formation and rupture of the conductive filament. Results of the study show this ferrite material as an insertion layer in a resistive random-access memory structure that offers potential for future information storage and bioneuromorphic computation devices.

Theoretical approach to defect-induced magnetism in oxygen-deficient γ-Ga2O3 films

Gallium oxide (Ga2O3) is an emerging material with several known polymorphs that exhibit many interesting properties for applications. In this work is theoretically studied and discussed the unconventional ferromagnetism observed in oxygen-deficient thin films of Ga2O3-x (0.42 < x < 0.60) grown on sapphire substrates. The formation of the magnetic moments is demonstrated in good agreement with experimental magnetization measurements using first-principles calculations based on the density functional theory performed for a standard crystal structure of γ-Ga2O3 with oxygen vacancies in different configurations. The random distribution of pairs of oxygen vacancies and rearrangements of them within the spinel-like cubic structure containing tetrahedral and octahedral sites for increasing temperatures are consistent with long-range persistent non-collinear magnetic order up to temperatures that may be exceed the temperature at which full conversion of γ-Ga2O3 to β-Ga2O3 takes place. Our present results also reveal that magnetic irreversibility observed from 10 K to room temperature are possibly determined by the intrinsic disorder in the structure of tetrahedral and octahedral sites that allows numerous possible paths for the migration of oxygen vacancies with low values of activation energies and migration barriers, as indicated by the previously published reports.