Formation Of Vacancy Clusters Research Articles

Defect evolution in burnable absorber candidate material: Uranium diboride, UB2

The stability, diffusivity and clustering behaviour of defects in uranium diboride (UB2) was investigated in light of the potential application as a burnable absorber in nuclear fuel. UB2 was found to accommodate limited deviations from stoichiometry, which should be a consideration when manufacturing and operating the material. Self-diffusivity of both U and B was found to be sluggish (10−14 cm2/s for B and 10−19 cm2/s for U at 2000K) and highly anisotropic, with migration along the basal planes being orders of magnitude faster than c-axis migration. The anisotropy of defect migration (both interstitials and vacancies) is predicted to hinder recombination of defects produced by collision cascades, thus limiting the radiation tolerance of the material. Boron and uranium vacancies exhibit a drive to cluster. Boron vacancies in particular, which are mobile on basal planes, are predicted to cluster into strongly bound di-vacancy, which in turn are less mobile. These are then predicted to grow into larger two-dimensional vacancy clusters on the B plane, leading to anisotropic swelling. We provide an analytical expression to predict the stability of these clusters based on purely geometrical considerations. Finally, the accommodation of Li, He and Xe onto vacancy clusters was considered. Li appears to stabilise the structure upon U depletion, while the retention of He and Xe appears to rise with increasing B depletion, through the formation of vacancy clusters.

Oxygen vacancies in TiO2/SnOx coatings prepared by ball milling followed by calcination and their influence on the photocatalytic activity

We prepared TiO2/SnOx (x = 1, 2) composite coatings through ball milling followed by calcination. The composite coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), spherical aberration corrected scanning transmission microscopy (Cs-corrected STEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and other techniques. Bulk oxygen vacancies in the form of vacancy clusters rather than individual sites were directly observed by Cs-corrected STEM. With an increase in the calcination temperature from 873 K to 1073 K or holding time from five hours to ten hours, the concentration of bulk defects also increased. On the other hand, the photocatalytic activity of the as-prepared samples for degrading MB dye increased with the concentration increase of bulk defects. Sample 973 K-10 h showed the best photocatalytic activity whether under the irradiation of UV light or visible light. With a further increase in the concentration of bulk oxygen vacancies, the photocatalytic activity decreased, although surface Ti3+ defects appeared. In other words, a moderate concentration of bulk defects could benefit the enhancement of the photocatalytic activity of the as-prepared samples. Ball milling followed by calcination is an effective method to introduce defects such as oxygen vacancies.

Interplay between chemical strain, defects and ordering in Sr1-xLaxFeO3 materials

Different point defect interactions were found to govern the defect chemistry of SrFeO3-δ and La0.6Sr0.4FeO3-δ. Conventional defect structure model based on two reactions – oxygen release by oxide lattice and the charge disproportionation in Fe-sublattice – can be applied successfully to La0.6Sr0.4FeO3-δ. Successful verification of this model using the available data on the oxygen nonstoichiometry gives virtually zero standard entropy and comparatively high standard enthalpy of iron disproportionation (116.54 ± 1.14 kJ/mol). In turn, the chemical strain of La0.6Sr0.4FeO3-δ was predicted successfully using the simple dimensional model, based on the ionic radii formalism and verified defect structure model. For SrFeO3-δ, a reference set of nonstoichiometry data was chosen from among the multitudinous literature data by comparing the calculated and calorimetrically determined oxide's reduction enthalpies. Some aspects of perovskite – brownmillerite phase transition in SrFeO3-δ were discussed and the defect structure model for this oxide was proposed and then verified using the chosen data set. Introduction of the vacancy cluster formation in the defect structure model was shown to be necessary since SrFeO3-δ is highly nonstoichiometric with respect to oxygen and tends to form various ordered structures. As a consequence, chemical expansivity of SrFeO3-δ with respect to the oxygen nonstoichiometry was found to be much more complex than that of La0.6Sr0.4FeO3-δ. According to our findings, unusually high values and the anomalous character of chemical strain of SrFeO3-δ are likely to be attributed to the vacancy cluster formation (i.e. short-range ordering) and some degree of long-range vacancy ordering, respectively.

Influences of vacancy and doping on electronic and magnetic properties of monolayer SnS

Based on the first-principles calculations, we investigate the structural, electronic, and magnetic properties of defects in monolayer SnS. We study the formation and migration of vacancies at both Sn- and S-sites. In comparison to the S-site vacancy, our calculations show that creating a vacancy at the Sn-site requires lesser energy, indicating that the vacancy at the Sn-site is more likely to be formed in experiments with energetic particle irradiation. For the Sn-rich (S-rich) environment, the vacancy at the S-site (Sn-site) is more likely to be found than the vacancy at the Sn-site (S-site). Reducing the formation of vacancy clusters, the S vacancy remains at the position where it is created because of the high vacancy migration barrier. Both types of vacancies remain nonmagnetic. To induce magnetism in monolayer SnS, we also study the transition metal (TM = Mn, Fe, and Co) doping at the Sn-site and find a significant influence on the electronic and magnetic properties of monolayer SnS. The doping of TM changes non-magnetic monolayer SnS to magnetic one and keeps it semiconducting. Additionally, long-range ferromagnetic behavior is observed for all the doped system. Hence, doping TM atoms in monolayer SnS could be promising to realize a two-dimensional diluted magnetic semiconductor. More interestingly, all the doped TM configurations show a high spin state, which can be used in nanoscale spintronic applications such as spin-filtering devices.

Nano-sized prismatic vacancy dislocation loops and vacancy clusters in tungsten

The vacancies produced in high energy collision cascades of irradiated tungsten can form vacancy clusters or prismatic vacancy dislocation loops. Moreover, vacancy loops can easily transform into planar vacancy clusters. We investigated the formation energies of these three types of vacancy defects as a function of the number of vacancies using three embedded-atom method tungsten potentials. The most favorable defect type and vacancy loop stability was determined. For very small sizes the planar vacancy cluster is more favorable than a vacancy loop, which is unstable. The void is the most stable vacancy defect up to quite large size, after that vacancy dislocation loop is more favorable. We conclude that the vacancy dislocation loops are nevertheless hlmetastable at low temperatures as the transformation to voids would need high temperature, in contrast to previous works, which found planar vacancy clusters to have lower energy than vacancy dislocation loops.

Point defects patterning in irradiated α-zirconium: numerical study in the framework of the rate theory

ABSTRACTWe study point defects patterning in irradiated α-zirconium numerically. In our consideration, we exploit reaction-rate theory by taking into account sink density dynamics and a change in internal stress fields due to the presence of defects. Dynamics of defect populations are studied at different irradiation conditions. It is found that point defects patterning is accompanied by a formation of vacancy clusters; their morphology change is governed by irradiation temperature and damage rate. By using statistical analysis of spatially distributed vacancy clusters, it was shown that the characteristic size of these clusters is of several nanometers. Vacancy clusters' occupation densities and distributions over their sizes are studied in detail.

Deformation-related spectroscopic features in natural Type Ib-IaA diamonds from Zimmi (West African craton)

Zimmi diamonds (Sierra Leone) have 500 million year mantle residency times whose origin is best explained by rapid tectonic exhumation to shallower depths in the mantle, associated with continental collision but prior to kimberlite eruption. Here we present spectroscopic data for a new suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the link between their spectroscopic features and their unusual geological history. Cathodoluminesence (CL) imaging of these diamonds revealed irregular patterns with abundant deformation lamellae, associated with the diamonds’ tectonic exhumation. Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/− centres and H3 (NVN0). The brownish-yellow to greenish-yellow colours observed in Zimmi Ib-IaA diamonds result from visible absorption by a combination of isolated substitutional nitrogen (\( {\mathrm{N}}_{\mathrm{S}}^0 \)) and deformation-related vacancy clusters. Colour-forming centres and other spectroscopic features can all be attributed to the unique geological history of Zimmi Ib-IaA diamonds and their rapid exhumation after formation.

Thermally stimulated depolarization current study on barium titanate single crystals

Thermally stimulated depolarization current (TSDC) measurements were applied to investigate BaTiO3 single crystals with <100>, <110>, and <111> orientations, and annealed under different atmospheres, respectively. The TSDC peaks related to ferroelectric domain, ferroelectric–paraelectric phase transition, TiTi′−VÖ dipole relaxation and oxygen vacancy migration were observed from the TSDC spectra. The formation of oxygen vacancy clusters in BaTiO3 single crystals leads to a high dipole activation energy. The <111> crystalline face of BaTiO3 is more stable under external field than others.

Production and aging of paramagnetic point defects in P-doped floating zone silicon irradiated with high fluence 27 MeV electrons

Enhancing the long term stable performance of silicon detectors used for monitoring the position and flux of the particle beams in high energy physics experiments requires a better knowledge of the nature, stability, and transformation properties of the radiation defects created over the operation time. We report the results of an electron spin resonance investigation in the nature, transformation, and long term stability of the irradiation paramagnetic point defects (IPPDs) produced by high fluence (2 × 1016 cm−2), high energy (27 MeV) electrons in n-type, P-doped standard floating zone silicon. We found out that both freshly irradiated and aged (i.e., stored after irradiation for 3.5 years at 250 K) samples mainly contain negatively charged tetravacancy and pentavacancy defects in the first case and tetravacancy defects in the second one. The fact that such small cluster vacancy defects have not been observed by irradiation with low energy (below 5 MeV) electrons, but were abundantly produced by irradiation with neutrons, strongly suggests the presence of the same mechanism of direct formation of small vacancy clusters by irradiation with neutrons and high energy, high fluence electrons, in agreement with theoretical predictions. Differences in the nature and annealing properties of the IPPDs observed between the 27 MeV electrons freshly irradiated, and irradiated and aged samples were attributed to the presence of a high concentration of divacancies in the freshly irradiated samples, defects which transform during storage at 250 K through diffusion and recombination processes.

Irradiation evolution of Cu precipitates in Fe1.0Cu alloy studied by positron annihilation spectroscopy

The Fe1.0Cu alloy was irradiated by 2 MeV Fe ions to a peak dose of 0.06dpa, 0.45dpa and 1.35dpa, respectively. The irradiation evolution of Cu precipitates in Fe1.0Cu alloy was studied at various depths by positron annihilation spectroscopy. The values of the S parameters directly indicate the formation of vacancy clusters. The S-W relationship showed that CunVm complexes (n > m) dominated the microstructure in the track region. Vacancy clusters and CunVm complexes (n < m) were the main defect types in the damage region. There finding were supported by the results of the performed theoretical calculations. The coincidence Doppler broadening showed that the Cu precipitates formed easily at a low irradiation dose, and that the irradiation-induced Cu precipitation was enhanced by the increase of the irradiation dose in the track region. We showed that the Cu precipitates finally reach a steady-state morphology under multiple active processes.

Clustering and segregation of small vacancy clusters near tungsten (0 0 1) surface

Abstract Nanoporous metals have been shown to exhibit radiation-tolerance due to the trapping of the defects by the surface. However, the behavior of vacancy clusters near the surface is not clear which involves the competition between the self-trapping and segregation of small vacancy clusters (Vn) nearby the surface. In this study, we investigated the energetic and kinetic properties of small vacancy clusters near tungsten (0 0 1) surface by combining molecular statics (MS) calculations and object Kinetic Monte Carlo (OKMC) simulations. Results show that vacancies could be clustered with the reduced formation energy and migration energy of the single vacancy around a cluster as the respective energetic and kinetic driving forces. The small cluster has a migration energy barrier comparable to that for the single vacancy; the migration energy barriers for V1–5 and V7 are 1.80, 1.94, 2.17, 2.78, 3.12 and 3.11 eV, respectively. Clusters and become unstable near surface (0 0 1) and tend to dissociate into the surface. At the operation temperature of 1000 K, the single vacancy, V2, 2 V 3 V3 and V4 were observed to segregate to the surface within a time of one hour. Meanwhile, larger clusters survived near the surface, which could serve as nucleating center for voids near the surface. Our results suggest that under a low radiation dose, surface (0 0 1) could act as a sink for small vacancy clusters, alleviating defect accumulation in the material under a low radiation dose. We also obtained several empirical expressions for the vacancy cluster formation energy, binding energy, and trapping radius as a function of the number of vacancies in the cluster.

Direct Evidence for the Role of Oxygen Vacancy Configuration for Oxide-Based Resistive Switching Devices

The miniaturization of standard silicon-based memory technologies reaches physical limits in terms of size and power dissipation. Novel memory and computing architectures based on resistive switches are promising for future electronics beyond Von Neumann computing architecture. In oxide-based resistive switches, the memristance is associated with the movement of ionic carriers through oxygen vacancies in an oxide film at high electric field strength via structural defect modifications that are still poorly understood. Here, we newly employ an oxide solid solution by systematic extrinsic doping to probe the role of oxygen vacancy configuration either set as free, immobile or clustered to directly answer the fundamental question how the mobility of oxygen vacancies affects the switching characteristics of the devices at high electric fields. Till date strategies to tailor systematically oxygen vacancy concentration and configuration for oxide-based resistive switches are still to be explored. For this, we design resistive switching devices of Ce1-xGdxO2-y as a novel resistive switching oxide constituent. The active oxide layer of the devices, consisting of a 500 nm thick CeO2 film with Gd3+ doping concentrations of 3 to 30 mol% deposited by Pulsed Laser Deposition (PLD), is sandwiched between micro-fabricated Pt-crosspoint electrodes. Cyclovoltammetry measurements show a strong dependence of the ROFF/RON -resistance ratio on the effective gadolinia concentration per bit. At low gadolinia doping concentration < 10 mol%, a purely capacitive behaviour prevails. By increasing up to 20 mol% a significant hystereis in the current-voltage profile is measured with resistance ROFF/RON -ratios up to 100. Turning to high gadolinia concentrations from 20 to 30 mol%, decreases the hysteretic current-voltage profile. To analyse the structural changes related to the oxygen vacancy concentration and configuration, changes opposed by the extrinsic gadolinia dopant in the ceria lattice of the resistive switching devices, we turn to Raman Spectroscopy. From these results we conclude that for moderate gadolinium concentrations up to 20 mol% the concentration of mobile oxygen vacancies steadily increases and is available as "free" carriers for the resistive switches; in this state the oxide is in a fluorite structure type. Further increase in doping, for which we see a reduction of switching characteristics, leads to the formation of oxygen vacancy clusters in the structure, which are simply as a defect configuration "immobile". For this state we confirm a phase change to a more bixbyite structure. Unequivocally the experiments together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol% Gd3+ doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching. A clear correlation between memristance and oxygen vacancies’ mobility was seen in this defect chemical model experiment on gadolinium doped ceria device structures. Extrinsic doping allows to tune the configuration, concentration and mobility of the oxygen vacancies in a much more systematic and controlled manner than what can be achieved through oxygen-deficient deposition or annealing steps as being the state-of-the-art in resistive switches. The research demonstrates guidelines for engineering of the oxide`s solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the memristance for future nonvolatile memory and logic applications.

Cr-induced fast vacancy cluster formation and high Ni diffusion in concentrated Ni-Fe-Cr alloys

We have performed molecular dynamics (MD) simulations on pure Ni, Ni-Cr, Ni-Fe and Ni-Fe-Cr single phase concentrated solid solution systems to elucidate the kinetics of point defects and vacancy cluster formation. We find that diffusion-based vacancy clustering leads to the formation of stacking fault tetrahedra (SFT) in all of these systems. The presence of Cr leads to faster SFT formation and high Ni diffusion in Ni-Cr binary systems than in pure Ni and Ni-Fe binary systems. The same trend is observed for the ternary systems that have high Cr composition. This Cr-induced effect is due to the low Cr migration barrier that first induces vacancy diffusion and later leads to faster clustering and SFT formation. We find that the fast Ni diffusion is also Cr-induced, where the binding of two vacancies provides much lower migration barrier pathways for Ni diffusion. Due to this ‘Cr-effect’, the vacancy-vacancy pair distribution shows higher peaks in Ni-Cr and Ni-Fe-Cr systems than in pure Ni and Ni-Fe binary systems. The low migration barriers of Cr compared to Ni and Fe are confirmed by density functional theory calculations.

Diffusion and interactions of point defects in hard-sphere crystals.

Using computer simulations, we study the diffusion, interactions, and strain fields of point defects in a face-centered-cubic crystal of hard spheres. We show that the vacancy diffusion decreases rapidly as the density is increased, while the interstitial diffusion exhibits a much weaker density-dependence. Additionally, we predict the free-energy barriers associated with vacancy hopping and find that the increasing height of the free-energy barrier is solely responsible for the slowing down of vacancy diffusion. Moreover, we find that the shape of the barriers is independent of the density. The interactions between vacancies are shown to be weakly attractive and short-ranged, while the interactions between interstitials are found to be strongly attractive and are felt over long distances. As such, we find that vacancies do not form vacancy clusters, while interstitials do form long-lived interstitial clusters. Considering the strain field of vacancies and interstitials, we argue that vacancies will hardly feel each other, as they do not substantially perturb the crystal, and as such exhibit weak interactions. Two interstitials, on the other hand, interact with each other over long distances and start to interact (attractively) when their strain fields start to overlap.

Structure and stability of defective silicene on Ag(001) and Ag(111) substrates: A computer experiment

The structure and stability of a two-layer defective silicene on Ag(001) and Ag(111) substrates have been investigated using the molecular dynamics method. The transformation of the radial distribution function of silicene due to the formation of monovacancies, divacancies, trivacancies, and hexavacancies is reduced primarily to a decrease in the intensity of the peaks and the disappearance of the “shoulder” in the second peak. With the passage of time, multivacancies can undergo coalescence with each other and the fragmentation into smaller vacancies, as well as form vacancy clusters. According to the geometric criterion, the Ag(001) substrate provides a higher stability of a perfect two-layer silicene. It has been found, however, that the defective silicene on this substrate has a lower energy only when it contains monovacancies and divacancies. A change in the size of defects leads to a change in the energy priority when choosing between the Ag(001) and Ag(111) substrates. The motion of a lithium ion inside an extended channel between two silicene sheets results in a further disordering of the defective structure of the silicene, during which the strongest stresses in the silicene are generated by forces directed perpendicular to the external electric field. These forces dominate in the silicene channel, the wall of which is supported by the Ag(001) or Ag(111) substrate.

Interaction of vacancies on the Cu(001) surface

The results of quantum chemical DFT calculations of energy barriers during the diffusion of vacancies on the Cu(001) surface at increased concentrations of vacant sites were described. The formation and destruction of dimers, trimers, and configuration of four vacancies were considered. For vacancies located at the neighboring sites of the surface lattice, there is effective attraction, which promotes the formation of vacancy clusters.

Embedded Si nanoclusters in α-alumina synthesized by ion implantation: An investigation using depth dependent Doppler broadening spectroscopy

Embedded Si nanoclusters in optically transparent host materials have shown potential applications in opto-electronic devices. Ion implantation followed by annealing at higher temperatures is considered a promising way to produce embedded nanoclusters in variety of host materials. In the present work, α-alumina crystals were irradiated with Si ion beam (50 keV) with varying total dose (1 × 1016, 5 × 1016 and 1 × 1017 ions/cm2). The ion implantation and vacancy defect profiles in alumina have been calculated using computer code SRIM. The irradiated samples were annealed at 500 and 1000 °C in a reducing atmosphere for 30 min. Depth dependent Doppler broadening measurements using high purity Germanium (HPGe) detector coupled to a slow positron beam were carried out on as-irradiated as well as annealed samples. The line shape (S- and W-) parameters showing contribution from valence (low momentum) and core (high momentum) electrons were evaluated from the positron implantation energy dependent Doppler broadened spectra. The S-E profiles of as irradiated samples indicated the formation of open volume defects in the damaged region. On annealing, S-E profiles are modified in the damaged region that is attributed to the formation of vacancy clusters and Si nanoclusters as confirmed through S-W correlation plots. The S-E profiles have been fitted using variable energy positron fit (VEPFIT) to evaluate the characteristic S-parameter corresponding to the damaged region. It is observed that ∼90% of implanted positrons in the damaged region are confined to the Si nanoclusters due to their high affinity toward Si as compared to host material, α-alumina. The present study confirms that positrons are confined in embedded nanoclusters and acts as a self seeking probe for their characterization.

Systematic study of the physical origin of ferromagnetism in CeO2−δ nanoparticles

We have carried out a Schrieffer-Wolff transformation on a general tight-binding Hamiltonian and obtained a $4f$-one-band effective Hubbard Hamiltonian to study the physical origin of ferromagnetism in ${\mathrm{CeO}}_{2\ensuremath{-}\ensuremath{\delta}}$ nanoparticle systems. For a low temperature regime and low concentrations of oxygen vacancies, isolated vacancies have previously been showed to form on the ${100}$ and ${110}$ surfaces and our studies indicate these will be in singlet and triplet states, respectively. This is sustained by a superexchange interaction between the $4f$ electrons of the two cerium atoms, which are the nearest neighbors of the vacancy, and ferromagnetism and antiferromagnetism can coexist. Moreover, increasing the vacancy concentration we found that pairs of vacancies, which have been previously shown to form on the ${111}$ surfaces, produce Nagaoka ferromagnetism and isolated vacancies in the bulk produce an antiferromagnetic sign. Furthermore, further oxygen vacancy increases are previously known to favor the formation of oxygen vacancy clusters. In this case, our results showed a weakening of the magnetic correlations with respect to temperature. Thus, at a fixed temperature, the magnetic moment is reduced when the concentration of vacancies is increased, which is in agreement with experimental results reported in the literature. Interestingly, at a room-temperature regime, the antiferromagnetic order is destroyed and only the ferromagnetic couplings, produced mainly by isolated vacancies on the ${110}$ surfaces, survive. Finally, as temperature is increased further, the paramagnetic behavior of $4f$ electrons dominates.

Off-stoichiometric defect clustering in irradiated oxides

A cluster dynamics model describing the formation of vacancy and interstitial clusters in irradiated oxides has been developed. The model, which tracks the composition of the oxide matrix and the defect clusters, was applied to the early stage formation of voids and dislocation loops in UO2, and the effects of irradiation temperature and dose rate on the evolution of their densities and composition was investigated. The results show that Frenkel defects dominate the nucleation process in irradiated UO2. The results also show that oxygen vacancies drive vacancy clustering while the migration energy of uranium vacancies is a rate-limiting factor for the nucleation and growth of voids. In a stoichiometric UO2 under irradiation, off-stoichiometric vacancy clusters exist with a higher concentration of hyper-stoichiometric clusters. Similarly, off-stoichiometric interstitial clusters form with a higher concentration of hyper-stoichiometric clusters. The UO2 matrix was found to be hyper-stoichiometric due to the accumulation of uranium vacancies.

High energy oxygen irradiation-induced defects in Fe-doped semi-insulating indium phosphide by positron annihilation technique

Positron annihilation technique is applied to study the recovery of radiation-induced defects in 140 MeV oxygen (O[Formula: see text]) irradiated Fe-doped semi-insulating indium phosphide during annealing over a temperature region of 25[Formula: see text]C–650[Formula: see text]C. Lifetime spectra of the irradiated sample are fitted with three lifetime components. Trapping model analysis is used to characterize defect states corresponding to the de-convoluted lifetime values. After irradiation, the observed average lifetime of positron [Formula: see text] ps at room temperature is higher than the bulk lifetime by 21 ps which reveals the presence of radiation-induced defects in the material. A decrease in [Formula: see text] occurs during room temperature 25[Formula: see text]C to 200[Formula: see text]C indicating the dissociation of higher order defects, might be due to positron trapping in acceptor-type of defects ([Formula: see text]). A reverse annealing stage is found at temperature range of 250[Formula: see text]C–425[Formula: see text]C for [Formula: see text]-parameter probably due to the migration of vacancies and the formation of vacancy clusters. Increase in [Formula: see text]-parameter from 325[Formula: see text]C to 425[Formula: see text]C indicates the change in the nature of predominant positron trapping sites. Beyond 425[Formula: see text]C, [Formula: see text], [Formula: see text]-parameter and [Formula: see text]-parameter starts decreasing and around 650[Formula: see text]C, [Formula: see text] and [Formula: see text]-parameter approached almost the bulk value showing the annealing out of radiation-induced defects.