Vacancy Trapping Research Articles

Interaction of C with vacancy in W: A first-principles study

We provide a vacancy trapping mechanism of C in W by investigating structure, stability, and diffusion properties of C in W using a first-principles method. C easily bonds onto the internal-surface of vacancy. A monovacancy is capable of trapping as many as 4 C atoms to form CnV (n=1, 2, 3, 4) complexes. Single C atom prefers to interact with neighboring W at vacancy with the trapping energy of −1.93eV. With the C atoms added, both of them are preferred to bind with each other to form covalent-like bond despite the metallic W environment. For the CnV complexes, C2V is the major one due to its largest average trapping energy (−1.97eV). Kinetically, formation of the CnV complexes can be ascribed to the interstitial mechanism due to the lower activation energy barrier of 1.46eV for the interstitial C than 1.66eV for the vacancy.

Molecular dynamics simulation on ionic conduction process of oxygen in Ce 1− xM xO 2− x/2

Molecular dynamics simulation of CeO 2 doped with M 3+ (a trivalent cation) with ionic radii ranging from 1.019 Å (Y 3+) to 1.160 Å (La 3+) were performed to examine the effects of the dopant cation size on ionic conductivity. Interatomic potential parameters were empirically fitted with equilibrium properties and energy barriers from ab initio calculations. Vacancy trapping and edge blocking mechanisms were studied. Analysis on vacancy trapping showed that the effect was more pronounced in La- and Y-doped ceria than those doped with Gd and Sm. Analysis of the edge blocking effect showed that larger-sized dopants would limit the available pathways for vacancy hopping. The combined effects satisfactorily explained the influence of the dopant cation size on the ionic conductivity of heavily doped ceria.

Kinetic lattice Monte Carlo model for oxygen vacancy diffusion in praseodymium doped ceria: Applications to materials design

Kinetic lattice Monte Carlo (KLMC) model is developed for investigating oxygen vacancy diffusion in praseodymium-doped ceria. The current approach uses a database of activation energies for oxygen vacancy migration, calculated using first-principles, for various migration pathways in praseodymium-doped ceria. Since the first-principles calculations revealed significant vacancy–vacancy repulsion, we investigate the importance of that effect by conducting simulations with and without a repulsive interaction. Initially, as dopant concentrations increase, vacancy concentration and thus conductivity increases. However, at higher concentrations, vacancies interfere and repel one another, and dopants trap vacancies, creating a “traffic jam” that decreases conductivity, which is consistent with the experimental findings. The modeled effective activation energy for vacancy migration slightly increased with increasing dopant concentration in qualitative agreement with the experiment. The current methodology comprising a blend of first-principle calculations and KLMC model provides a very powerful fundamental tool for predicting the optimal dopant concentration in ceria related materials.

Low-Temperature Differential Scanning Calorimetry of an Al-Mg-Si Alloy

The clustering behavior at room temperature of a pure ternary Al-0.59 wt pct Mg-0.82 wt pct Si alloy was investigated by low-temperature differential scanning calorimetry (DSC). We find three clustering reactions that take place in two stages. The first two reactions are linked to each other and are completed after 1 hour. The third reaction starts around 1 hour after quenching and is completed after 2 weeks. Only the latter reaction exhibits a strong shift of the peak position of the thermal signal, indicating a change in the activation energy during aging at room temperature caused by changing solute supersaturation or increasing trapping of vacancies. The first two stages are closely linked to the known adverse effect of room-temperature preaging on the ensuing age-hardening step, since 60 to 80 pct of cluster formation is sufficient to establish the full negative effect.

Trapping of oxygen vacancy at grain boundary and its correlation with local atomic configuration and resultant excess energy in barium titanate: A systematic computational analysis

Atomic structures of [001] symmetric tilt grain boundaries (GBs) and their influences on the trapping of oxygen vacancies at GBs in barium titanate $({\text{BaTiO}}_{3})$ were analyzed using static atomistic simulation techniques. It is found that the structures are determined to minimize the deficiency in the coordination numbers of ${\text{Ti}}^{4+}$ ions and to suppress the structural distortion in the vicinity of the GBs. The excess energy of the GB is dependent on the number density of the coordination-deficient ${\text{Ti}}^{4+}$ ions, indicating that the ionic bonds between ${\text{Ti}}^{4+}$ and ${\text{O}}^{2\ensuremath{-}}$ ions are responsible for structural stabilization of GB. It is also found that the GB plays an important role in trapping oxygen vacancies, which acts as a resistance against the oxygen vacancy's diffusion. The trapping originates from the presence of irregular ${\text{O}}^{2\ensuremath{-}}$ sites, where oxygen vacancies energetically prefer to reside, influenced by coordination environment. Based on the detailed analyses on origins of GB energy and the trapping in the vicinity of GB, new physical ground that correlates GB energy and capability of oxygen-vacancy trapping are provided, enabling us to predict how much vacancies can be trapped at GBs on the atomic level by micrometer-order measurement of GB energy. Electrical degradation of the ${\text{BaTiO}}_{3}$ dielectrics used for multilayer ceramic capacitors can be prevented through controlling the characteristics of the GBs to promote oxygen-vacancy trapping at GBs in polycrystalline materials via modifying materials synthesis procedures.

Dynamics of charge trapping by electron-irradiated alumina

Electron trapping in the oxygen vacancies of various alumina samples is analyzed using x-ray and optical emissions induced by electrons. Observed x-ray intensity variations show the presence of charges in the sample. Information on the ionicity is deduced from the electron distribution around aluminum, observed by x-ray emission between 50 and 300 K. It is shown that the stable defects are oxygen vacancies with one trapped electron. Electron trapping in these defects is analyzed by cathodoluminescence as a function of the structural characters of the sample, of the temperature, and of the irradiation conditions. Conditions for weak trapping can be deduced from these experiments.

Instable Plasticity and Precipitation in AZ91D Magnesium

Instable plasticity, which is normally a consequence of interactions between diffusing solute atoms and dislocations, is observed in AZ91D magnesium at room temperature. Experimental measurements of different heat treatment of AZ91D magnesium were taken to investigate different influence on PLC effect by the presence and structural state of second phase in this study. Tensile specimens were tested at room temperature at a crosshead speed of 0.6 mm/min. The results indicate that some number of large,coherent precipitates prevents serrated flow by way of trapping vacancies, but small precipitates content do not supress it.

Analysis of Oxygen Vacancy Trapping at [001] Symmetric Tilt Grain Boundaries in Barium Titanate by Atomistic Simulations

The role of grain boundaries (GBs) in the diffusion of oxygen vacancies (VO••s) in barium titanate (BaTiO3) and its mechanism were investigated using atomistic simulation techniques. It was found that GBs trapped VO••s at specific sites in the course of the diffusion, and the excess energy reflecting structural distortion of the GB was closely related to the availability of the trapping. GBs therefore act as a resistance of the diffusion of VO••s, suggesting that electrical degradation of multilayer ceramic capacitors (MLCCs), which is derived from vacancy diffusion, enables to be additionally improved by controlling GB structures in BaTiO3-based dielectrics.

Identification of electron and hole traps in lithium tetraborate (Li2B4O7) crystals: Oxygen vacancies and lithium vacancies

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize electrons trapped by oxygen vacancies and holes trapped by lithium vacancies in lithium tetraborate (Li2B4O7) crystals. Our study includes a crystal with the natural abundances of B10 and B11 and a crystal highly enriched with B10. The as-grown crystals contain isolated oxygen vacancies, lithium vacancies, and copper impurities, all in nonparamagnetic charge states. During an irradiation at 77 K with 60 kV x-rays, doubly ionized oxygen vacancies trap electrons while singly ionized lithium vacancies and monovalent copper impurities trap holes. The vacancies return to their preirradiation charge states when the temperature of the sample is increased to approximately 90 K. Hyperfine interactions with B10 and B11 nuclei, observed between 13 and 40 K in the radiation-induced EPR and ENDOR spectra, provide models for the two vacancy-related defects. The electron trapped by an oxygen vacancy is localized primarily on only one of the two neighboring boron ions while the hole stabilized by a lithium vacancy is localized on a neighboring oxygen ion with nearly equal interactions with the two boron ions adjacent to the oxygen ion.

Unipolar recovery of dielectric breakdown in fully silicided high-κ gate stack devices and its reliability implications

We report observations of unipolar recovery of dielectric breakdown in fully silicided NiSi-gate HfSiON–SiOx bilayer dielectric based high-κ metal-insulator-semiconductor (MIS) gate stack, analogous to resistive switching in metal-insulator-metal (MIM) nonvolatile memory devices. The dependence of the recovery voltage on breakdown hardness and filament location is analyzed and the physics behind MIS recovery, governed by joule heating induced oxygen vacancy trap passivation, is explained using failure analysis and statistical investigations. The observed MIS recovery phenomenon can be a tool to design for reliability in novel metal gate high-κ gate stacks.

Trapping of oxygen vacancies in the twin walls of perovskite

The binding and pairwise interaction of oxygen vacancies in the ferroelastic (100) twin walls of the orthorhombic phase of ${\text{CaTiO}}_{3}$ perovskite $(Pbnm)$ have been investigated by numerical simulations using empirical force fields. An oxygen vacancy finds it energetically favorable to reside inside a twin wall, particularly when bridging two titanium ions located in the twin-wall plane. In such case, the binding energy of the vacancy to the wall is $0.7\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{eV}$, the error bar reflecting variability within two different force fields. A different disposition of the vacancy in the wall sees its binding energy reduced by a factor of 2. This implies that depending on the relative time scales for twin-wall motion and for oxygen-vacancy diffusion, anelastic motion of twin walls can display two different energy dissipation mechanisms associated to these point defects. The strongest interactions among oxygen vacancies in a twin wall are due to short-range repulsion so that no defect clusters were found. The vacancy-wall binding energy is found to increase substantially with pressure.

Investigating behaviours of hydrogen in a tungsten grain boundary by first principles: from dissolution and diffusion to a trapping mechanism

We have investigated the dissolution, segregation and diffusion of hydrogen (H) in a tungsten (W) grain boundary (GB) using a first-principles method in order to understand the GB trapping mechanism of H. Optimal charge density plays an essential role in such a GB trapping mechanism. Dissolution and segregation of H are directly associated with the optimal charge density, which can be reflected by the H solution and segregation energy sequence for the different interstitial sites. To occupy the optimal-charge-density site, H can be easily trapped by the W GB with the solution and segregation energy of −0.23 eV and −1.11 eV, respectively. Kinetically, such a trapping is easier to realize due to the much lower diffusion barrier of 0.13–0.16 eV from the bulk to the GB in comparison with the segregation energy, suggesting that it is quite difficult for the trapped H to escape out of the GB. However, the GB can hold no more than 2 H atoms because the isosurface of optimal charge density almost disappears with the second H atom in, leading to the conclusion that H2 molecule and thus H bubble cannot form in the W GB. Taking into account the lower vacancy formation energy in the GB as compared with the bulk, we propose that the experimentally observed H bubble formation in the W GB should be via a vacancy trapping mechanism.

Interactions between hydrogen impurities and vacancies in Mg and Al: A comparative analysis based on density functional theory

Using first-principles methods we have studied the interactions between hydrogen impurities and vacancies in hcp Mg and fcc Al. We find that single vacancies can, in principle, host up to 9 H atoms in Mg and 10 in Al, not 12 as recently reported in the case of Al. The difference between our results and the results in previous work is attributed to a more appropriate definition of the trapping energy of hydrogen impurities in vacancies. The concentration of hydrogen-vacancy complexes depends on the amount of hydrogen dissolved in the metal, which in turn is dictated by the hydrogen chemical potential ${\ensuremath{\mu}}_{\text{H}}$. We evaluated the concentration of all relevant hydrogen-vacancy complexes as a function of ${\ensuremath{\mu}}_{\text{H}}$, corresponding to different H loading conditions---ranging from low pressures to high pressures of ${\text{H}}_{2}$ gas, up to hydrogen plasma conditions. Our analysis reveals fundamental differences in the characteristics of the hydrogen-vacancy interaction between Mg and Al. In the case of Al, up to 15% of H atoms are trapped in single vacancies in the form of H-vacancy complexes even for very low values of ${\ensuremath{\mu}}_{\text{H}}$. The trapping effect slows down the diffusion of H atoms in Al by more than an order of magnitude. While interactions between vacancies and single hydrogen atoms are therefore clearly important, interactions with multiple H atoms and related mechanisms (such as hydrogen-induced superabundant vacancy formation) are predicted to occur in Al only at very high values of ${\ensuremath{\mu}}_{\text{H}}$. In the case of Mg, the effects of H trapping in single vacancies are negligible for low values of ${\ensuremath{\mu}}_{\text{H}}$ due to the relatively low formation energy of isolated interstitial H. However, vacancies containing multiple H atoms and related mechanisms such as hydrogen-induced superabundant vacancy formation are predicted to occur in Mg at much lower values of ${\ensuremath{\mu}}_{\text{H}}$ than in Al. We estimate that, at room temperature, the critical pressure of an ${\text{H}}_{2}$ gas to induce hydrogen-enhanced (superabundant) vacancy formation is $\ensuremath{\sim}1\text{ }\text{GPa}$ in Mg and $\ensuremath{\sim}10\text{ }\text{GPa}$ in Al.

Vacancy trapping by solute atoms during quenching in Cu-based dilute alloys studied by positron annihilation spectroscopy

Frozen-in vacancies and the recovery have been investigated in some Cu-based dilute alloys by using positron annihilation lifetime spectroscopy. Cu-0.5at%Sb, Cu-0.5at%Sn and Cu-0.5at%In dilute bulk alloys were quenched to ice water from 1223 K. A pure-Cu specimen was also quenched from the same temperature. As a result, no frozen-in vacancies have been detected in as-quenched pure-Cu specimen. On the other hand, as-quenched Cu-0.5at%Sb alloy contained frozen-in thermal equilibrium vacancies with concentration of 3 × 10−5. Furthermore, these frozen-in vacancies in Cu-0.5at%Sb alloy were stable until 473 K, and began to migrate at 523 K. Finally, the Cu-Sb alloy were recovered to the fully annealed state at 823 K. This thermal stability clearly implies some interaction exists between a vacancy and Sb atom and due to the interaction, thermal equilibrium vacancies are trapped by Sb atoms during quenching.

Effect of hydrogen ion/electron dual-beam irradiation on microstructural damage of a 12Cr-ODS ferrite steel

The effect of hydrogen ion/electron dual-beam irradiation on the microstructural evolution of a new 12Cr-ODS ferrite steel made by chemical soaking method (CSM) was investigated. The results showed that the dislocations were introduced at the initial stage of irradiation and then developed into dislocation networks. The void swellings after the irradiation to a dose of 15 dpa were less than 0.15%. The interface between the dispersed oxide particle and the matrix became irregular due to the irradiation; while the macroscopic size change of the oxide particle was not recognized, thus suggesting that the steel had a good resistance to irradiation during the dual-beam irradiations between 623 K and 823 K. The formation of voids with small mean size and high number density was closely concerned with hydrogen which would assist the void nucleation as a result of hydrogen ion trapping vacancies during the irradiation.

The Effect of Germanium Doping on the Production of Carbon–Related Defects in Electron-Irradiated Czochralski Silicon

We present infrared (IR) spectroscopy measurements on carbon-rich, germanium-doped Czochralski-grown (Cz-Si) subjected to irradiation with 2 MeV electrons. The study is focused on the effect of germanium doping on the production of carbon-related defects CiCs, CiOi and CiOi(SiI). For carbon concentrations [Cs] up to 11017 cm-3 the production of the defects increases with the increase of Ge content, for [Ge] up to 11020 cm-3. However, for carbon concentrations around 21017 cm-3 the production of these defects shows a decrease for samples with [Ge]=21020 cm-3 in comparison with those of [Ge]=21019 cm-3. The results are discussed taking into account the effect of germanium on the annihilation of vacancies and self-interstitials in the course of irradiations. In the first case, due to the temporary trapping of vacancies by Ge atoms in the course of irradiation, more self-interstitials are available for the production of carbon interstitials (Cs+ SiI Ci), leading finally to an increase of the carbon-related defects. In the second case, and for [Ge] of the order of ~1020 cm-3 or higher, Ge atoms tend to form large clusters. These clusters attract primary defects facilitating their annihilation on them. As a result, the availability of self-interstitials decreases, which finally leads to a decrease of the carbon-related defects.

Temperature and dose dependence of defect complex formation with ion implanted Mn/Fe in ZnO

57Fe Mössbauer spectroscopy following ion implantation of radioactive 57Mn+ ( T1/2=85.4 s) has been applied to study the formation of Fe/Mn implantation-induced defects in ZnO at temperatures between 319 and 390 K. The formation of ferric iron–vacancy complexes is found to depend strongly on the implanted dose and to be faster and more efficient at higher temperatures. The results at these temperatures suggest the mobility of the Zn vacancy, together with vacancy trapping at the substitutional Mn/Fe impurities are responsible for the formation of Fe–VZn complexes.

Electrical conductivity measurements of periclase under high pressure and high temperature

Abstract The electrical conductivity of sintered polycrystalline periclase has been measured at 20 GPa and temperatures up to 1700 K with a KAWAI-type multi-anvil apparatus. Arrhenius plot of the electrical conductivity shows two linear regions, implying a change of charge transport mechanism with temperature in this experiment. At temperatures below 700 K small polaron conduction dominates, with an activation energy of 0.2 eV. At higher temperatures, the activation energy is determined to be 1.18 eV, which is much higher than 0.2 eV at low temperature. Our observations suggest the high-temperature conduction is attributed to a large polaron process with magnesium vacancy trapping hole on oxygen.

Temperature dependence of dislocation bias factors in metals

Recently, it has been shown that edge dislocations are not stable sinks for vacancies. Trapping and detrapping of vacancies occur as a thermally-activated process. In this paper, the temperature dependence of vacancy absorption coefficient of edge dislocations under irradiation was calculated by using rate equations in Fe and Ni. The temperature dependence was almost the same in both Fe and Ni and did not depend on the damage rate between 10−10dpa/s and 10−6dpa/s. At low temperatures such as room temperature, the coefficient was low and with increasing irradiation temperatures, it had a peak (500K) and decreased.

A kelvin probe force microscopy of charged indentation-induced dislocation structures inKBr

The incipient stages of plasticity in KBr single crystals have been examined in ultra-highvacuum by means of atomic force microscopy and Kelvin probe force microscopy (KPFM).Conducting diamond-coated tips have been used to both indent the crystals and image theresulting plastic deformation. KPFM reveals that edge dislocations intersecting the surfacecarry a negative charge similarly to kinks in surface steps, while screw dislocations show nocontrast. The charges are attributed to trapped cation vacancies which compensatethe charge of divalent impurities. Furthermore, the site of indentation has beenfound to carry a large positive charge. Weak topographic features extending in the⟨110⟩ direction from the indentation are identified by atomic-resolution imagingto be pairs of edge dislocations of opposite sign, separated by a distancesimilar to the indenter radius. They indicate the glide of two parallel{110} planes perpendicular to the surface, a process which allows for a slice of KBr to be pushedaway from the indentation site.