Aggregation Of Oxygen Vacancies Research Articles

Oxygen plasma-assisted magnetron sputtering deposition of non-stoichiometric Y2O3 films: Influence of oxygen vacancies on etching resistance

Yttrium oxide (Y2O3) films are widely used to protect equipment in plasma etching, a crucial technique in semiconductor processing, due to their exceptional resistance to plasma etching. Since oxygen vacancy affects the performance of Y2O3 and may impact physical etching resistance, the internal relationship necessitates further investigation. In this article, Y2O3 films with varying oxygen vacancy concentrations are prepared by reactive magnetron sputtering with the assistance of oxygen plasma. The formation and development of oxygen vacancies in cubic Y2O3 films and their impact on the physical etching resistance were investigated. The experiment results indicate that the accumulation of oxygen vacancies causes non-stoichiometry and the etching rate is significantly reduced. Combined with density-functional theory, it is revealed that oxygen vacancy distorts the lattice, which increases covalent YO bonding and the formation energy of atoms. Moreover, the vacancy line defects resulting from diffusion and aggregation of oxygen vacancies exert a similar influence on the adjacent yttrium and oxygen layers. The enhancement of physical etching resistance is attributed to the strengthened internal YO bonds led by increasing oxygen vacancy concentration in the cubic Y2O3 films without inducing phase transition. The discovery enriches the understanding of how plasma interacts with Y2O3, and the etching mechanism.

Effects of amorphous phase and yttrium on flash sintering and microstructures of ZrO2–SiO2 ceramic nanocomposites

AbstractFlash sintering has been used to consolidate a wide range of ceramics and ceramic composites. However, flash sintering of ceramic nanocomposite with a substantial amorphous phase is rarely studied. Meanwhile, the effects of yttrium addition on the flash sintering behaviors and microstructures of ZrO2‐based ceramic nanocomposite remain unknown. Herein, ZrO2‐SiO2 crystalline‐amorphous ceramic nanocomposites (CACNs) with different contents of amorphous phase and two types of yttrium dopants were sintered by flash sintering. Results showed that the content of amorphous phase (SiO2) had significant effects on the flash sintering behavior. The CACN with 65 mol% SiO2 was nonflash sinterable due to the high electric resistivity of SiO2, whereas, the CACN with 45 mol% SiO2 can be flash sintered to high densification within 35 seconds. The flash‐sintered CACNs consisted of ZrO2 nanocrystallites distributed in an amorphous SiO2 matrix. Flash sintering enhanced the tetragonal‐to‐monoclinic phase transformation of ZrO2. In addition, influences of yttrium dopant on CACNs were also investigated. Yttrium dopants can rapidly dissolve in ZrO2 lattices and show a tetragonal phase stabilization effect during flash sintering. Firework‐like ZrO2 microcrystallites formed by dendritic growth were observed close to the cathode region, which was attributed to athermal electric field effects and the formation and aggregation of oxygen vacancies at the cathode region. The results would provide guidance for fabricating CACNs via flash sintering.

Phase-field theory study on the modulation mechanism of oxygen vacancy concentration on charged domain wall in ferroelectric thin films

This study analyzes the regulatory mechanism of oxygen vacancy concentration on tail-to-tail charged domain walls (T–T CDWs), along with the writing time, conduction current magnitude, and retention performance of through-type T–T CDWs. The research results show that the highest density and length of T–T CDWs are achieved when the oxygen vacancy concentration is 1 × 1020 cm−3. Moreover, the successful writing of through-type T–T CDWs is limited to a certain electric field range, which is controlled by oxygen vacancy concentration. An increase in the oxygen vacancy concentration leads to a decrease in the maximum and minimum threshold electric fields required for writing through-type charged domain walls. The writing time and conductivity of through-type T–T CDWs determine the information writing speed and signal strength of domain wall memories, and the oxygen vacancy concentration also plays a regulatory role in both aspects. When the oxygen vacancy concentration is 1 × 1020 cm−3, the through-type T–T CDW exhibits the fastest writing speed, requiring only 8 ns. The magnitude of the conduction current of through-type T–T CDWs is directly proportional to the oxygen vacancy concentration. The through-type T–T CDWs formed by the aggregation of oxygen vacancies exhibit excellent retention performance, making them highly promising for applications in ferroelectric domain wall memories. Our research demonstrates that oxygen vacancies have a significant regulatory effect on the morphology and current response of charged domain walls, opening up new avenues for the study of domain wall memories.

Multi-peak emission of In2O3 induced by oxygen vacancy aggregation

Oxygen vacancy is crucial to the optical properties in In2O3, however, the single oxygen vacancy model fails to explain the observed multi-peak emission in the experiment. Herein, we have theoretically investigated the diversity of oxygen vacancy distribution, revealing the relationship between the defect configurations and the optical properties. Combining the first-principles calculations and bayesian regularized artificial neural networks, we demonstrate that the structural stability can be remarkably enhanced by multi-oxygen vacancy aggregation, which will evolve with the defect concentration and temperature. Notably, our results indicate that the single oxygen vacancy will induce the emission peaks centered at 1.35 eV, while multi-peak emission near 2.35 eV will be attributed to the distribution of aggregated double oxygen vacancies. Our findings provide a comprehensive understanding of multi-peak emission observed in In2O3, and the rules of the vacancy distribution may be extended for other metal oxides to modulate the optical properties in practice.

Gradual conductance modulation by defect reorganization in amorphous oxide memristors.

Amorphous oxides show great prospects in revolutionizing memristors benefiting from their abundant non-stoichiometric composition. However, an in-depth investigation of the memristive characteristics in amorphous oxides is inadequate and the resistive switching mechanism is still controversial. In this study, aiming to clearly understand the gradual conductance modulation that is deeply bound to the evolution of defects-mainly oxygen vacancies, forming-free memristors based on amorphous ZnAlSnO are fabricated, which exhibit high reproducibility with an initial low-resistance state. Pulse depression reveals the logarithmic-exponential mixed relaxation during RESET owing to the diffusion of oxygen vacancies in orthogonal directions. The remnants of conductive filaments formed through aggregation of oxygen vacancies induced by high-electric-field are identified using ex situ TEM. Especially, the conductance of the filament, including the remnant filament, is larger than that of the hopping conductive channel derived from the diffusion of oxygen vacancies. The Fermi level in the conduction band rationalizes the decay of the high resistance state. Rare oxidation-migration of Au occurs upon device failure, resulting in numerous gold nanoclusters in the functional layer. These comprehensive revelations on the reorganization of oxygen vacancies could provide original ideas for the design of memristors.

Hopping and clustering of oxygen vacancies in BaTiO3−δ and the influence of the off-centred Ti atoms

The diffusion and aggregation of oxygen vacancies (VO) in perovskites are still poorly understood, even though they are involved in a wide range of applications and phenomena, from solid-state electrolytes for fuel cells to ferroelectric fatigue. In strontium titanate it has been possible, by measuring the complex elastic modulus, to identify peaks in the elastic energy loss versus temperature due to hopping of isolated and paired VO and evaluate all the relevant kinetic and thermodynamic parameters. We present similar experiments in barium titanate, where the ferroelectric transition at TC = 400 K partially hides the anelastic relaxation processes due to VO. The introduction of VO, however, depresses TC, and it has been possible to lower it enough to reveal all the relaxation processes due to free and clustered VO. The resulting anelastic spectra are similar to those of SrTiO3−δ but there are also important differences. In BaTiO3−δ the anisotropy of the elastic dipole of the isolated VO is about three times larger, the anelastic relaxation peaks markedly shift to lower temperature with doping, the activation energy for the diffusion of the isolated VO is 0.72 eV, larger than 0.60 eV in SrTiO3, while that for the pair reorientation is smaller, 0.86 eV compared to 0.97 eV. All these observations are explained by taking into account that, unlike in SrTiO3, Ti is dynamically disordered over eight off-centre positions. A strong indication in this sense comes from the temperature dependence of Young’s modulus, with anharmonic stiffening perfectly linear in temperature down to 200 K in SrTiO3, and with anomalous softening already below 750 K in BaTiO3.

Mechanisms of Oxygen Vacancy Aggregation in SiO2 and HfO2

Dielectric oxide films in electronic devices undergo significant structural changes during device operation under bias. These changes are usually attributed to aggregation of oxygen vacancies resulting in formation of oxygen depleted regions and conductive filaments. However, neutral oxygen vacancies have high diffusion barriers in ionic oxides and their interaction and propensity for aggregation are still poorly understood. In this paper we briefly review the existing data on static configurations of neutral dimers and trimers of oxygen vacancies in technologically relevant SiO$_2$ and HfO$_2$ and then provide new results on the structure and properties of these defects in amorphous SiO$_2$ and HfO$_2$. These results demonstrate weak interaction between neutral O vacancies, which does not explain their quick aggregation. We propose that trapping of electrons, injected from an electrode, by the vacancies may result in creation of new neutral vacancies in the vicinity of pre-existing vacancies. We describe this mechanism in a-SiO$_2$ and demonstrate that this process becomes more efficient as the vacancy clusters grow larger.

Formation of structural defects and strain in electrodegraded Fe‐doped SrTiO3 crystals due to oxygen vacancy migration

AbstractWe report on our investigation of structural defect and strain formation in electrodegraded reduced and oxidized, Fe‐doped SrTiO3 (Fe:STO) single crystals using optical second harmonic generation (SHG) and confocal Raman spectroscopy. SHG and Raman spectra reveal structural and electrochemical inhomogeneity resulting from the formation of Fe4+/oxygen ion and Fe3+/oxygen vacancy aggregation sites along the degraded anode and cathode interfaces, respectively. We show that mixed Fe3+/Fe4+ states and structural strain gradients are generated across the color fronts. These results, as well as oxygen sublattice differences between the anodic and cathodic bulk, present the color front as an interface between two dominant oxygen bonding distortions. The strain near the color front shows a strong dependence on oxygen vacancy concentration and diffusion within the crystals. Our characterization of structural and electrochemical changes due to electric field‐induced strain and oxygen vacancy migration advances knowledge of electrodegradation in perovskite‐based titanate single crystals.

Retention Characteristics of Five‐Layered Aurivillus Films With Large Polarization

The value of 2Pr (twice the polarization at zero electric field) in the five‐layered Aurivillus film Bi6Fe2Ti3O18 is greater than 50 μC cm−2, which is almost an order of magnitude higher than that of the bulk (4 μC cm−2). Moreover, the well‐defined hysteresis loop of this film can persist at the measurement frequency of 100 kHz, indicating the ultra‐fast switching speed of such a domain. The frequency dependence of the coercive field obeys a power‐law form, , indicating that its domain switching kinetics are influenced by the motion of the domain wall. The polarization of the Bi6Fe2Ti3O18 thin film remains essentially constant at a waiting time of up to 2 × 104 s, and is independent of the applied electric field. The switchable polarization loss is 32% after 9 × 1010 read/write switching cycles at a frequency of 1 MHz. This change in the switchable polarization is discussed in terms of the aggregation of oxygen vacancies in the film.

Modeling resistive switching materials and devices across scales

Resistance switching devices based on electrochemical processes have attractive significant attention in the field of nanoelectronics due to the possibility of switching in nanosecond timescales, miniaturization to tens of nanometer and multi-bit storage. Their deceptively simple structures (metal-insulator-metal stack) hide a set of complex, coupled, processes that govern their operation, from electrochemical reactions at interfaces, diffusion and aggregation of ionic species, to electron and hole trapping and Joule heating. A combination of experiments and modeling efforts are contributing to a fundamental understanding of these devices, and progress towards a predictive understanding of their operation is opening the possibility for the rational optimization. In this paper we review recent progress in modeling resistive switching devices at multiple scales; we briefly describe simulation tools appropriate at each scale and the key insight that has been derived from them. Starting with ab initio electronic structure simulations that provide an understanding of the mechanisms of operation of valence change devices pointing to the importance of the aggregation of oxygen vacancies in resistance switching and how dopants affect performance. At slightly larger scales we describe reactive molecular dynamics simulations of the operation of electrochemical metallization cells. Here the dynamical simulations provide an atomic picture of the mechanisms behind the electrochemical formation and stabilization of conductive metallic filaments that provide a low-resistance path for electronic conduction. Kinetic Monte Carlo simulations are one step higher in the multiscale ladder and enable larger scale simulations and longer times, enabling, for example, the study of variability in switching speed and resistance. Finally, we discuss physics-based simulations that accurately capture subtleties of device behavior and that can be incorporated in circuit simulations.

Diffusion and aggregation of oxygen vacancies in amorphous silica

Using density functional theory (DFT) calculations, we investigated oxygen vacancy diffusion and aggregation in relation to dielectric breakdown in amorphous silicon dioxide (a-SiO2). Our calculations indicate the existence of favourable sites for the formation of vacancy dimers and trimers in the amorphous network with maximum binding energies of approximately 0.13 eV and 0.18 eV, respectively. However, an average energy barrier height for neutral vacancy diffusion is found to be about 4.6 eV, rendering this process unfeasible. At Fermi level positions above 6.4 eV with respect to the top of the valence band, oxygen vacancies can trap up to two extra electrons. Average barriers for the diffusion of negative and double negatively charged vacancies are found to be 2.7 eV and 2.0 eV, respectively. These barriers are higher than or comparable to thermal ionization energies of extra electrons from oxygen vacancies into the conduction band of a-SiO2. In addition, we discuss the competing pathways for electron trapping in oxygen deficient a-SiO2 caused by the existence of intrinsic electron traps and oxygen vacancies. These results provide new insights into the role of oxygen vacancies in degradation and dielectric breakdown in amorphous silicon oxides.

Coexistence of two types of metal filaments in oxide memristors

One generally considers the conducting filament in ZnO-based valence change memristors (VCMs) as an aggregation of oxygen vacancies. Recently, the transmission electron microscopy observation showed the filament is composed of a Zn-dominated ZnOx. In this study, careful analysis of the temperature dependence of the ON state resistance demonstrates that the formation/rupture of a Zn filament is responsible for the resistive switching in ZnO VCMs. Cu/ZnO/Pt memristive devices can be operated in both VCM and ECM (electrochemical metallization memristor) modes by forming different metal filaments including Cu, Zn and a coexistence of these two filaments. The device operation can be reversibly switched between ECM and VCM modes. The dual mode operation capability of Cu/ZnO/Pt provides a wide choice of select devices for constructing memristive crossbar architectures.

Polarization fatigue of Pr and Mn co-substituted BiFeO3 thin films

Polarization fatigue of (Bi0.85Pr0.15)(Fe0.95Mn0.05)O3 (BPFMO) films is studied as functions of switching fields and frequencies. Poor fatigue resistance is observed at low switching frequencies and small cycling fields. An increase of dielectric constant is observed in accompany with the suppression of switchable polarization. Both the switchable polarization and the dielectric constant can be restored to their original values simultaneously by high electric field cycling. These characteristics are discussed in terms of migration and aggregation of oxygen vacancies on domain walls to block domain switching. The results support the domain wall pinning scenario for polarization degradation in BPFMO films.

An initial model for the RIED effect

A simple model based on electron acceleration in the conduction band giving rise to an increased F + oxygen vacancy lifetime provides an explanation for several radiation induced electrical degradation (RIED) associated observations in Al 2O 3. The increased F + radioluminescence noted during RIED is a direct consequence of the lifetime increase. The model predicts the observed electric field threshold for RIED, and an increase in the field threshold with increasing impurity content. RIED for RF electric fields is also explained. In addition the lifetime increase provides an explanation for the enhanced oxygen vacancy aggregation including colloid and gamma alumina production observed under RIED conditions.

Ferroelectric properties of Bi3.25La0.75Ti3O12 thin films prepared by chemical solution deposition

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films were prepared on platinum coated silicon substrate by chemical solution deposition. The layered-perovskite phase was obtained by rapid thermal annealing the spin-on films at 650 or 700 °C for 180 s. Scanning electron micrographs showed uniform surfaces composed of spherical grains. The grain size increased with increasing annealing temperature. The remanent polarization and coercive field of 650 °C annealed film were 12.3 μC/cm2 and 48.9 kV/cm, respectively, and those of 700 °C annealed films were 18.2 μC/cm2 and 51.1 kV/cm. BLT thin films showed little polarization fatigue under 250 kV/cm bipolar cycling at 50 kHz, while fatigue properties deteriorated with decreasing cycling field and frequency. At various frequencies from 1 Hz to 50 kHz, nonvolatile polarization Pnv showed nearly no degradation over an initial period of cycling, then decayed logarithmically with switching cycles. The onset of logarithmic decay of Pnv was found to increase linearly with cycling frequency. The cycling field dependence of fatigue characteristics was interpreted in terms of competition of charge trapping which blocks the domain switching and field-assisted detrapping of charged defects which set the locked domains free. Frequency dependent fatigue properties were discussed on the basis of drift and aggregation of oxygen vacancies.

Enhanced oxygen vacancy aggregation and colloid production in Al 2O 3

Optical luminescence and absorption measurements on Al 2O 3 (sapphire) electron irradiated at temperatures between 200 and 270°C with and without an applied electric field have been used to identify an oxygen vacancy aggregation process leading to the formation of aluminium colloids. This process, which is only observed when an electric field is applied during irradiation, occurs within the volume and is considered as a possible precursor for RIED. The results help to clarify the observed similarity between RIED and colloid production, and help to provide an explanation for the observation of gamma alumina in sapphire which has suffered RIED.

On the grain boundary and defect structure of nanocrystalline titanium oxide

Abstract Transmission electron microscopy was used to study the crystal size distribution, grain-boundary disorder and defect structure in nanocrystalline titanium oxide prepared by reactive sputtering. The average grain diameter determined by measurements on dark-field micrographs was approximately 15 nm. Evidence of both ordered and disordered grain-boundary regions was found, and planar defects observed in grain interiors were identified as (011) deformation twins. Crystallographic shear defects that can occur as a result of aggregation of oxygen vacancies in understoichiometric titanium oxide were rarely present. Dislocations, with the exception of those necessary to accommodate misfit, were not seen. The limitations of high-resolution electron microscopy for investigation of grain-boundary structures in nanocrystalline materials are discussed.

Nanometer-scale patterning of high-T c superconductors for Josephson junction-based digital circuits

A straightforward method for nanometer-scale patterning of high-Tc superconductor thin films is discussed. The technique combines direct-write electron beam lithography with well-controlled aqueous etches and is applied to the fabrication of Josephson junction nanobridges in high-quality, epitaxial thin-film YBa2Cu3O7. We present the results of our studies of the dimensions, yield, uniformity, and mechanism of the junctions along with the performance of a representative digital circuit based on these junctions. Direct current junction parameter statistics measured at 77 K show critical currents of 27.5 μA±13% for a sample set of 220 junctions. The Josephson behavior of the nanobridge is believed to arise from the aggregation of oxygen vacancies in the nanometer-scale bridge.

Micro-Raman spectroscopy of electromigration-induced oxygen vacancy aggregation in YBa2Cu3O7−δ

We describe the results of micro-Raman spectroscopy and optical microscopy studies of basal-plane chain-oxygen vacancy motion in YBa2Cu3O7−δ thin films under the influence of a high current bias near 300 K. Above a threshold level this bias causes vacancy aggregation and then long-range displacement, leading to an enhancement of oxygen order in the region of highest current density and the complex accumulation of oxygen vacancies in the region where the electromigration force is near the threshold level.

Magnetic exchange interactions in perovskite solid solutions. Part 6. Iron-57 and tin-119 Mössbauer spectra of SrFe1 –xSnxO3 –y(0 ⩽x⩽ 0.7)

The oxygen-deficient perovskite phase SrFe1 –xSnxO3 –y(0 ⩽x⩽ 0.7) has been studied using 57Fe and 119Sn Mossbauer spectroscopy and X-ray powder diffraction. The oxygen content of each sample depends on its thermal history, and magnetic ordering occurs at low temperatures. The aggregation of oxygen vacancies to produce layers of tetrahedrally co-ordinated Fe3+ cations which occurs for x= 0 appears to be suppressed by the tin substitution. The supertransferred hyperfine field interaction at tin reveals a continuous distribution of fields, and the spectroscopic data are consistent with a delocalised-electron behaviour in which the more usual near-neighbour interactions are obscured by longer-range effects.