Migration Of Point Defects Research Articles

A Model for the Temperature Distribution in a Rolled Joint in a CANDU Reactor Exploiting the Decomposition of the β-Zr Phase

A competing-rates model is presented to account for operational changes in the metastable β-Zr phase of the Zr-2.5Nb alloy used to make CANDU reactor pressure tubes and is used to predict temperature gradients at the outlet rolled joints using the decomposition of the β-Zr phase as a proxy for temperature. High temperatures decompose the β phase by enhancing the formation of small particles of ω and α phases. Fast neutron flux causes the ω and α phases to shrink. This process is assumed to depend on the total volume of the particles, because they are comparable to, or smaller than, the size of the neutron displacement cascades. The barrier energy for thermal growth was determined to be 2.43 eV, when an Arrhenius A factor of 1013/s was assumed. The cross section for (ω+α)-phase shrinkage is 24.5 barns for Zr-2.5Nb irradiated in CANDU reactors. Assuming that the shrinkage is dominated by the migration of self-interstitial point defects, a defect production efficiency of 1.4% was found.

Gradual degradation in InAs quantum dot lasers on Si and GaAs.

Reliable quantum dot lasers on silicon are a key remaining challenge to successful integrated silicon photonics. In this work, quantum dot (QD) lasers on silicon with and without misfit dislocation trapping layers are aged for 12 000 hours and are compared to QD lasers on native GaAs aged for 8400 hours. The non-trapping-layer (TL) laser on silicon degrades heavily during this time, but much more modest gradual degradation is observed for the other two devices. Electroluminescence imaging reveals relatively uniform gradual dimming for the aged TL laser on silicon. At the same time, we find nanoscale dislocation loop defects throughout the quantum dot-based active region of all three aged lasers via electron microscopy. The Burgers vector of these loops is consistent with . We suggest that the primary source of degradation, however, is the generation and migration of point defects that substantially enhance non-radiative recombination in the active region, the visible symptom of which is the formation of dislocation loops. To prevent this, we propose that laser fabrication should be switched from deeply etched to shallow etch ridges where the active region remains intact near the mesa. Additionally, post-growth annealing and altered growth conditions in the active region should be explored to minimize the grown-in point defect density.

First-principles calculations of point defect migration mechanisms in InP

As an important second-generation semiconductor material, indium phosphide (InP) possesses excellent advantages such as a wide bandgap, high electron mobility, high photoelectric conversion efficiency, and strong radiation resistance. It is considered an excellent material for electronic devices in aerospace applications. However, point defects generated by space radiation particles in InP electronic devices can cause their electrical performance to degrade severely. In this study, first-principles calculations are employed to investigate the stable structures of point defects in InP and calculate the migration energy values of nearest-neighbor defects. Four stable structures of In vacancies and three stable structures of P vacancies are identified by constructing the stable structures of point defects in different charge states. The migration process of vacancy defects is studied, revealing that the migration energy of P vacancies is higher than that of In vacancies. Moreover, charged vacancy defects exhibit higher migration energy values than neutral vacancies. Regarding the migration process of interstitial defects, it is found that the migration energy of interstitial defects is smaller than that of vacancy defects. In the calculation of In interstitial migration process with different charge states, two different migration processes are found. Besides, during the migration calculations of P interstitial, a special intermediate state is discovered, resulting in multiple paths migrating to the nearest-neighbor position in the migration energy barrier diagram. The research results are helpful to understand the formation mechanism and migration behavior of defects in InP materials, and are important in designing and manufacturing InP devices with long-term stable operation in space environment.

Interatomic force fields for zirconium based on the embedded atom method and the tabulated Gaussian Approximation Potential

The accuracy of interatomic interaction potentials – also known as force fields – is the main factor determining the physical soundness of classical molecular dynamics (MD) simulations. Here, we present a multi-objective framework to generate embedded-atom-method (EAM) force fields using ab initio data. The EAM force fields were tuned via particle swarm optimization to capture the non-linear association between atomic structures and system energies. Using this framework, 95 standard EAM force fields for zirconium were developed and 45 physical features for each developed force field were tracked. Principal component analysis (PCA) was performed to provide insights into the compromises that must be made when generating EAM force fields. Of note, by assigning large fitting weights to generalized stacking fault energy (GSFE) surfaces, there exist EAM force fields with properly positioned minima on prismatic GSFE surfaces and containing no spurious minima in basal GSFE surfaces. However, while standard EAM force fields achieved this without explicitly taking the angular dependence of atomic interactions into account, they led to a severe mismatch between other important physical properties and benchmarks. Hence, we also constructed two machine-learned tabulated Gaussian approximation potentials (tabGAP) with an additional three-body term, which successfully tackled the aforementioned issue and exhibit acceptable prediction accuracy across many physical properties (lattice parameters, elastic properties, dimer potential energies, melting temperatures, phase stability, point defect formation energies, point defect migration energies, and free surface energies) of Zr. Remarkably, its computational efficiency is only 6 times slower than standard EAM force fields.

A Set of Moment Tensor Potentials for Zirconium with Increasing Complexity.

Machine learning force fields (MLFFs) are an increasingly popular choice for atomistic simulations due to their high fidelity and improvable nature. Here we propose a hybrid small-cell approach that combines attributes of both offline and active learning to systematically expand a quantum-mechanical (QM) database while constructing MLFFs with increasing model complexity. Our MLFFs employ the moment tensor potential formalism. During this process, we quantitatively assessed the structural properties, elastic properties, dimer potential energies, melting temperatures, phase stability, point defect formation energies, point defect migration energies, free surface energies, and generalized stacking fault (GSF) energies of Zr as predicted by our MLFFs. Unsurprisingly, the model complexity has a positive correlation with prediction accuracy. We also find that the MLFFs were able to predict the properties of out-of-sample configurations without directly including these specific configurations in the training dataset. Additionally, we generated 100 MLFFs of high complexity (1513 parameters each) that reached different local optima during training. Their predictions cluster around the benchmark DFT values, but subtle physical features such as the location of local minima on the GSF energy surface are washed out by statistical noise.

Regularities of Changes in the Physico-Mechanical and Corrosion Properties of Reinforcement Steel Grade 23Kh2G2T

The purpose of the work was to study of the regularities of changes in the physico-mechanical and corrosion properties of reinforcing steel 23Kh2G2T in hot-rolled and heat-treated (furnace tempering) states.Methods. The study of the physical and mechanical properties and resistance (long-term corrosion resistance) of bar reinforcement to stress corrosion cracking (SCC) was carried out on experimental melts of steel grade 23Kh2G2T. The choice of melts was made in such a way that their chemical composition corresponded, if possible, to the upper, middle and lower levels of the grade composition.To create experimental conditions as close as possible to operational ones, the tests were carried out in a boiling solution of nitrates (60% Ca(NO3)2 + 5% NH4NO3 + 35% H2O) at a temperature of 110°С and operating voltages σe = (0,1–0,8 )σВ. To elucidate the features of transformations occurring in steel during tempering, the temperature dependences of internal friction were taken from samples tempered at different temperatures.Processing of the results of the time dependence of the amplitude-independent internal friction was carried out according to the theory of Granato, Hikata, Lucke, which describes the kinetics of the return of internal friction due to the migration of point defects to dislocations.Results. It has been established that by controlling the chemical composition and technological regimes for the production of steel 23Kh2G2T, it is possible not only to sharply increase its resistance to cracking, but also to obtain a guaranteed complex of mechanical and corrosion properties. It was revealed that reinforcement made of 23Kh2G2T steel should be produced with mandatory tempering. The greatest resistance to stress corrosion cracking with practically unchanged strength for reinforcement made of steel 23Kh2G2T is provided by 2-hour tempering in the temperature range of 350…400°С.Conclusion. The results obtained can be used to establish patterns of behavior of various types of ingot, powder and composite materials with high dispersion in phase and structural components in various conditions and states.

Interaction of Stacking Faults with point/extended defects in Fe-He irradiated 6H-SiC

The study explored the microstructure evolution of 6H-SiC that underwent sequential iron and helium ion irradiation with energies of 2.5 MeV and 500 keV, respectively, at room temperature, followed by annealing at 1500°C for two hours. Following irradiation, the entire damaged layer underwent amorphization. However, during subsequent annealing, epitaxial recrystallization took place, resulting in the formation of defected polycrystalline 6H-SiC characterized by the presence of Fe-rich clusters, cavities, and stacking faults. Fe-rich cavities were found to predominantly form at the edges of the stacking faults, as revealed by XTEM. The interaction of microstructural defects is further investigated via first-principles calculations. The periphery of the stacking faults has been identified as the primary location for the emergence of vacancy clusters, serving as favorable sites for the accumulation of point defects, including Fe atoms. This behavior can be attributed to the combined effects of mechanical and electronic energy relaxation mechanisms. Mechanically, the presence of stacking faults allows for the release of elastic energy that had been stored at the boundary. Electronically, the energy relaxation arises from the saturation of C- and Si-dangling bonds. Both of these processes contribute to the observed behavior, highlighting the intricate interplay between mechanical and electronic factors in the system. The low point defect migration energy barriers in the vicinity of the stacking faults promise high recombination, which can limit cavity growth and enhance radiation resistance. The study not only offers valuable insights into the mechanism of cavity/stacking faults interaction, contributing to a better understanding of radiation damage in 6H-SiC but also demonstrates that 6H-SiC material containing stacking faults could serve as a viable alternative to 3C-SiC for nuclear application.

Tunable charge injection from graphene to halide perovskite induced by reversible ion segregation

Migration of point defects can result in various performance instabilities in organometal halide perovskite devices. In particular, the segregation of charged defects can influence the electron transport, but the correlation between ionic and electronic transports remains elusive. In this work, we use a graphene/methylammonium lead iodide capacitor to probe the effect of ion segregation on the charge injection behavior at the graphene/perovskite junction. In our samples, the ion segregation in the perovskite film is induced by a controllable electric field applied through an electrostatic gate. At the same time, the charge injection from graphene to the perovskite is probed by monitoring changes in the graphene conductivity. It is found that a positive (negative) defect accumulation layer near the graphene/perovskite interface can facilitate electron (hole) injection from the graphene electrode to the perovskite. Because the polarity of the defect accumulation layer is switchable by changing the direction of the applied electric field, the graphene/perovskite interface behaves like a Schottky junction with a tunable charge injection barrier height. This behavior should be generally found in other metal/halide perovskite interfaces as well.

Electric Field Manipulation of Defects and Schottky Barrier Control inside ZnO Nanowires.

We directly measure the three-dimensional movement of intrinsic point defects driven by applied electric fields inside ZnO nano- and micro-wire metal-semiconductor-metal device structures. Using depth- and spatially resolved cathodoluminescence spectroscopy (CLS) in situ to map the spatial distributions of local defect densities with increasing applied bias, we drive the reversible conversion of metal-ZnO contacts from rectifying to Ohmic and back. These results demonstrate how defect movements systematically determine Ohmic and Schottky barriers to ZnO nano- and microwires and how they can account for the widely reported instability in nanowire transport. Exceeding a characteristic threshold voltage, in situ CLS reveals a current-induced thermal runaway that drives the radial diffusion of defects toward the nanowire free surface, causing VO defects to accumulate at the metal-semiconductor interfaces. In situ post- vs pre-breakdown CLS reveal micrometer-scale wire asperities, which X-ray photoelectron spectroscopy (XPS) finds to have highly oxygen-deficient surface layers that can be attributed to the migration of preexisting VO species. These findings show the importance of in-operando intrinsic point-defect migration during nanoscale electric field measurements in general. This work also demonstrates a novel method for ZnO nanowire refinement and processing.

The formation and migration of point defects and the thermodynamic behavior of TiD2 by first-principles calculations

TiD2 has important applications in nuclear materials, for example neutron generator. Therefore, it is of great importance to study the evolution of the defects in TiD2 during the service. In this work, first-principles calculations are performed to study the formation and migration behaviors of point defects in TiD2, as well as the phase stability of TiD2. The formation energies of Ti vacancy (VTi) and D vacancy (VD) in TiD2 are 2.34 and 0.73 eV, respectively. The migration barriers of VD are 0.80 and 0.66 eV for the migration path within ab plane and bc plane, respectively, which is much lower than the those of the interstitial D atoms. So, the predominant diffusion mechanism of a D atom in TiD2 is the vacancy hopping mechanism. Furthermore, we find that VTi will trap interstitial D atoms and D vacancies. At last, the thermal stability of TiD2 is investigated and we find that interstitial D atoms and D vacancies are formed during the molecular dynamics simulations at 1200 K. This work presents a microscopic picture for the formation and transformation of the point defects in TiD2.

Thermokinetics of point defects in α-Fe2O3

Point defect formation and migration in oxides governs a wide range of phenomena from corrosion kinetics and radiation damage evolution to electronic properties. In this study, we examine the thermodynamics and kinetics of anion and cation point defects using density functional theory in hematite (-Fe2O3), an important iron oxide highly relevant in both corrosion of steels and water-splitting applications. These calculations indicate that the migration barriers for point defects can vary significantly with charge state, particularly for cation interstitials. Additionally, we find multiple possible migration pathways for many of the point defects in this material, related to the low symmetry of the corundum crystal structure. The possible percolation paths are examined, using the barriers to determine the magnitude and anisotropy of long-range diffusion. Our findings suggest highly anisotropic mass transport in hematite, favoring diffusion along the c-axis of the crystal. In addition, we have considered the point defect formation energetics using the largest Fe2O3 supercell reported to date.

Collective Control of Potential-Constrained Oxygen Vacancies in Oxide Heterostructures for Gradual Resistive Switching.

Filamentary resistive switching in oxides is one of the key strategies for developing next-generation non-volatile memory devices. However, despite numerous advantages, their practical applications in neuromorphic computing are still limited due to non-uniform and indeterministic switching behavior. Given the inherent stochasticity of point defect migration, the pursuit of reliable switching likely demands an innovative approach. Herein, a collective control of oxygen vacancies is introduced in LaAlO3 /SrTiO3 (LAO/STO) heterostructures to achieve reliable and gradual resistive switching. By exploiting an electrostatic potential constraint in ultrathin LAO/STO heterostructures, the formation of conducting filaments is suppressed, but instead precisely control the concentration of oxygen vacancies. Since the conductance of the LAO/STO device is governed by the ensemble concentration of oxygen vacancies, not their individual probabilistic migrations, the resistive switching is more uniform and deterministic compared to conventional filamentary devices. It provides direct evidence for the collective control of oxygen vacancies by spectral noise analysis and modeling by Monte-Carlo simulation. As a proof of concept, the significantly-improved analog switching performance of the filament-free LAO/STO devices is demonstrated, revealing potential for neuromorphic applications. The results establish an approach to store information by point defect concentration, akin to biological ionic channels, for enhancing switching characteristics of oxide materials.

Long-Term Oxidation of Zirconium Alloy in Simulated Nuclear Reactor Primary Coolant—Experiments and Modeling

Oxidation of Zr-1%Nb fuel cladding alloy in simulated primary coolant of a pressurized water nuclear reactor is followed by in-situ electrochemical impedance spectroscopy. In-depth composition and thickness of the oxide are estimated by ex-situ analytical techniques. A kinetic model of the oxidation process featuring interfacial reactions of metal oxidation and water reduction, as well as electron and ion transport through the oxide governed by diffusion-migration, is parameterized by quantitative comparison to impedance data. The effects of compressive stress on diffusion and ionic space charge on migration of ionic point defects are introduced to rationalize the dependence of transport parameters on thickness (or oxidation time). The influence of ex-situ and in-situ hydrogen charging on kinetic and transport parameters is also studied.

Ferroelectric Orthorhombic ZrO2 Thin Films Achieved Through Nanosecond Laser Annealing.

A new approach for the stabilization of the ferroelectric orthorhombic ZrO2 films is demonstrated through nanosecond laser annealing (NLA) of as-deposited Si/SiOx /W(14nm)/ZrO2 (8nm)/W(22nm), grown by ion beam sputtering at low temperatures. The NLA process optimization is guided by COMSOL multiphysics simulations. The films annealed under the optimized conditions reveal the presence of the orthorhombic phase, as confirmed by X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. Macroscopic polarization-electric field hysteresis loops show ferroelectric behavior, with saturation polarization of 12.8µC cm-2 , remnant polarization of 12.7µCcm-2 and coercive field of 1.2MVcm-1 . The films exhibit a wake-up effect that is attributed to the migration of point defects, such as oxygen vacancies, and/or a transition from nonferroelectric (monoclinic and tetragonal phase) to the ferroelectric orthorhombic phase. The capacitors demonstrate a stable polarization with an endurance of 6.0× 105 cycles, demonstrating the potential of the NLA process for the fabrication of ferroelectric memory devices with high polarization, low coercive field, and high cycling stability.

Microstructural Effects on Irradiation Creep of Reactor Core Materials

The processes that control irradiation creep are dependent on the temperature and the rate of production of freely migrating point defects, affecting both the microstructure and the mechanisms of mass transport. Because of the experimental difficulties in studying irradiation creep, many different hypothetical models have been developed that either favour a dislocation slip or a mass transport mechanism. Irradiation creep mechanisms and models that are dependent on the microstructure, which are either fully or partially mechanistic in nature, are described and discussed in terms of their ability to account for the in-reactor creep behaviour of various nuclear reactor core materials. A rate theory model for creep of Zr-2.5Nb pressure tubing in CANDU reactors incorporating the as-fabricated microstructure has been developed that gives good agreement with measurements for tubes manufactured by different fabrication routes having very different microstructures. One can therefore conclude that for Zr-alloys at temperatures < 300 °C and stresses < 150 MPa, diffusional mass transport is the dominant creep mechanism. The most important microstructural parameter controlling irradiation creep for these conditions is the grain structure. Austenitic alloys follow similar microstructural dependencies as Zr-alloys, but up to higher temperature and stress ranges. The exception is that dislocation slip is dominant in austenitic alloys at temperatures < 100 °C because there are few barriers to dislocation slip at these low temperatures, which is linked to the enhanced recombination of irradiation-induced point defects.

Elastic Strain Relaxation of Phase Boundary of α' Nanoscale Phase Mediated via the Point Defects Loop under Normal Strain.

Irradiation-induced point defects and applied stress affect the concentration distribution and morphology evolution of the nanophase in Fe-Cr based alloys; the aggregation of point defects and the nanoscale precipitates can intensify the hardness and embrittlement of the alloy. The influence of normal strain on the coevolution of point defects and the Cr-enriched α' nanophase are studied in Fe-35 at.% Cr alloy by utilizing the multi-phase-field simulation. The clustering of point defects and the splitting of nanoscale particles are clearly presented under normal strain. The defects loop formed at the α/α' phase interface relaxes the coherent strain between the α/α' phases, reducing the elongation of the Cr-enriched α' phase under the normal strains. Furthermore, the point defects enhance the concentration clustering of the α' phase, and this is more obvious under the compressive strain at high temperature. The larger normal strain can induce the splitting of an α' nanoparticle with the nonequilibrium concentration in the early precipitation stage. The clustering and migration of point defects provide the diffusion channels of Cr atoms to accelerate the phase separation. The interaction of point defect with the solution atom clusters under normal strain provides an atomic scale view on the microstructure evolution under external stress.

Electric field induced migration of native point defects in Ga2O3 devices

While the properties of β-Ga2O3 continue to be extensively studied for high-power applications, the effects of strong electric fields on the Ga2O3 microstructure and, in particular, the impact of electrically active native point defects have been relatively unexplored. We used cathodoluminescence point spectra and hyperspectral imaging to explore possible nanoscale movements of electrically charged defects in Ga2O3 vertical trench power diodes and observed the spatial rearrangement of optically active defects under strong reverse bias. These observations suggest an unequal migration of donor-related defects in β-Ga2O3 due to the applied electric field. The atomic rearrangement and possible local doping changes under extreme electric fields in β-Ga2O3 demonstrate the potential impact of nanoscale device geometry in other high-power semiconductor devices.

How fast do defects migrate in halide perovskites: insights from on-the-fly machine-learned force fields.

The migration of defects plays an important role in the stability of halide perovskites. It is challenging to study defect migration with experiments or conventional computer simulations. The former lacks an atomic-scale resolution and the latter suffers from short simulation times or a lack of accuracy. Here, we demonstrate that machine-learned force fields, trained with an on-the-fly active learning scheme against accurate density functional theory calculations, allow us to probe the differences in the dynamical behaviour of halide interstitials and halide vacancies in two closely related compositions CsPbI3 and CsPbBr3. We find that interstitials migrate faster than vacancies, due to the shorter migration paths of interstitials. Both types of defects migrate faster in CsPbI3 than in CsPbBr3. We attribute this to the less compact packing of the ions in CsPbI3, which results in a larger motion of the ions and thus more frequent defect migration jumps.

Point defect properties of the VCrMnFe0.33 multi-principal alloy from first-principles calculations

We performed first-principles calculations to explore the point defect formation and migration properties and the corresponding chemical environment effects for a bcc VCrMnFe 0.33 multi-principal alloy. The results indicate that the vacancy formation energies vary with local environment, and vacancies favor Mn-rich surroundings. The exchange between vacancies and V atoms presents lower energy barriers due to the large atomic size of V. Moreover, interstitial calculations found that [110] direction dumbbells take up more than 93%, suggesting a slow defect diffusion. The large V interstitials can substitute neighboring atoms and leave the squeezed atom forming dumbbells with its surroundings. Our investigation indicates that this low-activation multi-principal alloy may exhibit promising irradiation resistance due to its special defect properties. All data generated or analyzed during this study are included in this published article.

Identification of the Kirkendall effect as a mechanism responsible for thermal decomposition of the InGaN/GaN MQWs system

A drop in the efficiency of light-emitting diodes based on InGaN/GaN QWs known as the ‘green gap’ has been studied intensively over the past dozen years. Several factors were revealed to contribute to its origin, such as random fluctuations in the indium concentration or the diffusion of point defects during the growth of QWs. The aim of this paper is to demonstrate that the Kirkendall effect can be the mechanism responsible for the thermal decomposition of InGaN/GaN MQWs structures, contributing to the green gap problem. By applying density functional theory, harmonic approximation, and harmonic transition state theory, we calculated the heights of the migration energy barriers of In and Ga atoms diffusing in alloys (), the vibrational frequencies of alloys in the presence of migrating point defects, the temperature dependencies of the defect migration energy barriers and diffusion coefficients of Ga and In atoms migrating in alloys. We demonstrated the presence of unbalanced diffusion rates of In and Ga atoms at the interfaces and finally explained the experimentally observed mechanism of void formation at the interfaces by means of the Kirkendall effect.