Evolution Of Defects Research Articles

Formation of Point Defects Due to Aging under Natural Conditions of Doped GaAs.

The aging dynamics of materials used to build the active part of optoelectronic devices is a topic of current interest. We studied epitaxial samples of GaAs doped with Ge and Sn up to 1×1019 cm-3, which were stored in a dry and dark environment for 26 years. Photoluminescence spectra were taken in three periods: 1995, 2001 and 2021. In the last year, time-resolved photoluminescence, Raman, and X-ray measurements were also performed to study the evolution of defects formed by the action of O2 in the samples and its correlation with the doping with Ge and Sn impurities. We found that oxygen formed oxides that gave off Ga and As atoms, leaving vacancies mainly of As. These vacancies formed complexes with the dopant impurities. The concentration of vacancies over the 26 years could be as large as 1×1018 cm-3.

Comparing the effect between room temperature and low temperature heavy ion irradiation by deep level transient spectroscopy

To promote the application of SiC devices in extreme environments, the irradiation damages of 4H-SiC Schottky barrier diodes (SBDs) irradiated with 6 MeV Au ions under room temperature (RT) and low temperature (LT) were investigated in this work. Deep level transient spectroscopy (DLTS) was used to analyze the changes in primary defects. DLTS measurements indicated the presence of six deep levels labeled EN, E1, E2, E3, E4 and E5. Notably, the peak positions for the samples irradiated under RT shifted to the right compared to those irradiated at LT with increasing fluence. This means that the deeper energy defects are produced under RT irradiation. The experimental results also demonstrated that the HI irradiation-induced defects are mainly located at interface between metal and SiC, especially the metal–semiconductor (M−S) interface edge. Therefore, these results suggest that the evolution of defects in SiC devices under LT and RT irradiation,which is helpful to provide its application in power electronics and extreme environments.

Investigation of the surface band structure and the evolution of defects in β-(AlxGa1−x)2O3

This study examines the electronic and luminescent properties of β-(AlxGa1−x)2O3 (0 ≤ x ≤ 0.42) thin films grown on (0001) sapphire using laser-MBE, with a focus on the evolution of defect energy levels and their impact on surface Fermi level pinning and luminescence. X-ray photoelectron spectroscopy (XPS) and cathodoluminescence (CL) have been employed to analyze surface band bending and defect evolution as a function of aluminum content. The results have revealed a pinned Fermi level at 3.6 eV above the valence band maximum despite the increase in the bandgap. The consequent upward band bending has been confirmed by a peak shift in the core level XPS. The defects that lead to the Fermi level pinning effect are attributed to E2*, which is related to a Ga vacancy or Ga vacancy-O vacancy complex. In addition, CL spectroscopy and depth-resolved CL have demonstrated consistent blue and ultraviolet emissions across the Al content range and a similar suppression of electron concentration on blue and ultraviolet emissions in β-(AlxGa1−x)2O3 and β-Ga2O3. Based on the observed evolution of defects with Al content, the blue band emission is attributed to electron transition in the donor–accepter pair.

Evolutions of microstructural defects in high-entropy carbide ceramics under ion irradiations at room temperature

This work studied and compared the evolutions of irradiation-induced microstructural defects in five high-entropy carbide ceramics (HECCs) at room temperature to reveal the irradiation resistances of HECCs and corresponding underlying mechanisms. The five HECCs exhibit high phase stabilities and no amorphization is observed up to 40 dpa. In-situ 800 keV Kr ion irradiation reveals similar evolutions of irradiation-induced defects. The densities of dislocation loops show non-monotonic variations, which reach the maximum at ∼ 2 dpa and then decrease due to the interactions with each other. In contrast, the sizes of dislocation loops increase monotonically with dose. (NbTaZrW)C exhibits the lowest defect densities and highest defect sizes, which is probably due to the high lattice distortion and its induced high defect recombination efficiencies. Based on the characterization results, it is deduced that the species of constituent elements play more important role in the defect behaviors than the number of constituent elements.

Evolution of extended defects in UO2 during high temperature annealing

The evolution of thermophysical characteristics of uranium oxide (UO2) under irradiation is a critical aspect of nuclear fuel performance of light water reactors. This study examines the effect of high-temperature annealing at 1000 °C - 1600 °C on defect evolution in krypton irradiated UO2 at room temperature. Transmission electron microscopy analysis reveals that the size of irradiation-induced bubbles and dislocation loops increases but their density decreases with annealing temperatures. Perfect dislocation loops with b⇀ = <110> dominate, regardless of the annealing temperature. A rate theory model for extended defect evolution is applied to conclude that current understanding of dynamical loop and bubble evolution at high-temperature annealing is governed by defect coalescence.

Tuning Chemical and Morphological Properties of Ceria Nanopowders by Mechanochemistry.

Cerium oxide powders are widely used and are of fundamental importance in catalytic pollution control and energy production due to the unique chemical properties of CeO2. Processing steps involved in catalyst preparation, such as high-temperature calcination or mechanical milling processes, can alter the morphological and chemical properties of ceria, heavily affecting its final properties. Here, we focus on the tuning of CeO2 nanopowder properties by mild- and high-energy milling processes, as the mechanochemical synthesis is gaining increasing attention as a green synthesis method for catalyst production. The textural and redox properties were analyzed by an array of techniques to follow the aggregation and comminution mechanisms induced by mechanical stresses, which are more prominent under high-energy conditions but strongly depend on the starting properties of the ceria powders. Simultaneously, the evolution of surface defects and chemical properties was followed by Raman spectroscopy and H2 reduction tests, ultimately revealing a trade-off effect between structural and redox properties induced by the mechanochemical action. The mild-energy process appears to induce the largest enhancement in surface properties while maintaining bulk properties of the starting materials, hence confirming its effectiveness for its exploitation in catalysis.

Atomic-Scale Dislocation Structure Evolution and Crystal Ordering Analysis of Melting and Crystallization Microprocesses in Laser Powder Bed Melting of γ-TiAl Alloys

Laser Powder Bed Fusion (LPBF) technology exhibits significant advantages in the manufacturing of components with high dimensional accuracy and intricate internal cavities. However, due to the inherent room-temperature brittleness and high-temperature gradient induced by the laser forming process, the LPBF fabrication of γ-TiAl alloy is often accompanied by the initiation and propagation of defects. The aim of this study is to investigate the forming process of γ-TiAl alloy by the LPBF method through molecular dynamics simulation, and to explain the microparticle arrangement and displacement evolution of the melting and crystallization processes, thus elucidating the link between the variations in the laser process parameters and defect generation during microscopic laser heating. The results show that during the melting process, the peaks of the radial distribution function (RDF) decrease rapidly or even disappear due to laser heating, and the atomic disorder is increased. Although subsequent cooling crystallization reorders the atomic arrangement, the peak value of the RDF after crystallization is still 19.3% lower than that of the original structure. By setting different laser powers (200–800 eV/ps) and scanning speeds (0.2–0.8 Å/ps), the effects of various process parameters on microforming and defect evolution are clarified. When the laser power increases from 200 to 400 eV/ps, the stable value of atomic displacement rises from 6.66 to 320.87, while it rises from 300.54 to 550.14 when the scanning speed is attenuated from 0.8 to 0.4 Å/ps, which indicates that, compared with the scanning speed, the atomic mean-square displacements are relatively more sensitive to the fluctuation of laser power. Dislocation analysis reveals that a higher laser power significantly increases the cooling rate during the forming process, which further aggravates the generation and expansion of dislocation defects.

Enhanced performance CdZnTe single crystal with few surface damages via solution based annealing

In this study, heat treatments were employed using a CdCl2 solution as the annealing medium to investigate the evolution of internal defects and photoelectric properties of CdZnTe crystals. It was observed that CdCl2 crystals formed on the surface of the annealed crystals, and their size increased exponentially with prolonged annealing time. It could be a medium for Cl transport. The liquid-phase annealing process was found to be gentle, resulting in minimal damage and the absence of large defects migrating or accumulating dislocations and less nano-defects on the crystal surface than sample annealed in vapor condition. After a 30 hour annealing, the infrared (IR) transmittance reached 55%, and the resistivity increased to 1.36×1010 Ω·cm. The presence of Cd2+ facilitated the filling of Cd vacancies (VCd) defects through diffusion reduction, while the diffusion of Cl- introduced complex defect with extra VCd, causing the Fermi level to approach the center of the band gap. Consequently, a high-resistance CdZnTe material was achieved.

PKA Energy dependence of defect evolution in ion irradiated Fe-15Ni-15Cr alloy – a successive cascade study using molecular dynamics simulation

Molecular dynamics has been used to carry out successive cascade simulations on model alloy Fe-Ni-Cr for three distinct primary knock-on atom (PKA) energies to elucidate the earlier experimentally obtained results of ion irradiation studies on SS316 and its Ti-modified version. The PKA energies used for the simulations were calculated using the stopping and range of ions in matter (SRIM-2013) to represent the irradiation of SS316 using 145 MeV Ne, 35 MeV He and 315 keV Ar ions. The simulation has been carried out successively to emulate the progress of irradiation and obtain results as a function of dose. The results for each PKA energy have been analysed to obtain the evolution of defect clusters in terms of their size, different types of dislocations, stacking fault probabilities, etc., in all three cases. The simulation results have been compared with our earlier experimental studies.

Influence of transmutation-induced Re/Os content on defect evolution in neutron-irradiated W

The microstructure of neutron irradiated tungsten (0.1 dpa - 0.8dpa at Tirr = 1200 °C,) was analyzed by transmission electron microscopy applying high-sensitivity energy dispersive X-ray spectroscopy, which enables the visualization of the distribution of transmutation induced Re and Os. The study demonstrates the dose-dependent evolution of radiation-induced defects such as voids and dislocation loops as well as segregation behavior of Re and Os at these defects. Re segregation around voids occurs at the lowest damage dose of 0.1 dpa, while their enrichment on dislocation loops was detected at 0.2 dpa. Further generation of Re and Os leads to the formation of the χ-phase ((Re,Os)3 W) precipitates with a needle shape on voids and on dislocation loops. The formation of Os-rich nuclei within Re clouds around loops or voids is crucial for the onset of precipitate formation. It could be shown that the subsequent formation of precipitates on dislocation loops causes the disappearance of the loops. Based on these results, a modification of a well-known defect formation diagram in W has been proposed as a function of the Re/Os content. This would provide a comparison of defect evolution in W irradiated in reactors with different transmutation rates. It would also allow a prediction of the microstructural evolution in W under fusion conditions.

Impacts of local chemical ordering on the primary radiation damage in Cr–Fe–Ni multi-principal element alloys

Face-centered cubic (FCC) single-phase multi-principal element alloys (MPEAs) have demonstrated promising irradiation resistance. The generation and evolution of irradiation defects are integral aspects to comprehend, as they offer valuable insights into the radiation resistance of materials. In this work, we use molecular dynamics simulations to investigate the primary radiation damage of CrFeNi FCC MPEAs, while focusing on the impact of local chemical ordering (LCO) on the generation and evolution of defects. Our simulation results reveal that the LCO reduces the probability of defect production, consequently mitigating the maximum number of defects. Furthermore, the LCO increases defect migration barriers, thereby impeding the migration of interstitial atoms and leading to an increased number of residual defects subsequent to primary damage. However, the dispersed distribution of defects throughout space facilitates the uniform nucleation of dislocations in subsequent irradiation cascades. Additionally, the increased migration energy acts as a barrier to the movement of defect clusters, impeding their growth and thereby enhancing the radiation resistance of MPEAs. Our findings underscore the potential for enhancing the radiation resistance of MPEAs through the deliberate tailoring of LCOs.

The influence of chemical and technological parameters of materials in the design of objects in industrial design on the example of state facilities

AbstractThe purpose of this study is to generalise the thermodynamic model describing the fragmentation of metals or alloys at integrated project delivery, and to study the features and conditions for the formation of limit (stationary) structures of various types. Thus, a Project Delivery is constructed in the two‐defect model approximation with dislocation density and grain boundaries taken into account, which establishes the conditions for the formation of limit (stationary) structures of different types. In addition, the article studies in detail the evolution of the main structural defects and their interaction during the implementation of the stationary regime, and also establishes the dependence of the type of limit structure formed on the values of landslide deformation and the initial state of the material. It is found that the change of states of the system is of solid‐state phase transformation nature. It follows from the methods of description that the limit (stationary) structure is not immutable, but represents a dynamic equilibrium of the processes of generation and annihilation of structural defects.

Discharge Evolution of Metal Protrusion Defects Observed at High Time Resolution

Discharge Evolution of Metal Protrusion Defects Observed at High Time Resolution

Simulation of defect evolution in tungsten during annealing by developing a vacancy and interstitial-type defect evolution model

In this work, a Vacancy and Interstitial-Type Defect Evolution (VITDE) model is developed to study the annealing and clustering behaviors of the irradiation-induced defect in tungsten (W). The post annealing of W-ion damaging at 300 K (called post-damaging annealing) and simultaneous annealing of W-ion damaging (called dynamic annealing) are simulated. In the post-damaging annealing case, the W is damaged by 20 MeV W-ion irradiation for 1h at 300 K, then the damaged W is annealed for 1h via increasing the temperature to 1000 K. In the dynamic annealing case, the defect evolution under simultaneous damaging and annealing is simulated using the same parameters as Case 1. The simulation results indicate the dynamic annealing case has lower induced defects concentration than the post-irradiation annealing case, which is in qualitative agreement with the experiment. The diffusion of interstitial, vacancy, di-interstitial and di-vacancy, and the cluster properties in the defect annealing are investigated and larger size Vn clusters are observed in dynamic annealing case than in post-damaging annealing case. The dependences of the defect evolution on material temperature (T) and grain size are revealed. By increasing the T, the defect concentration is decreased, which reproduces and explains the experimental result. The Vn trap amount with the reduction of grain size for two cases show a larger difference at T = 400 K, observing the Vn trap amount is unchanged for post-damaging annealing, whereas it is significantly increased by dynamic annealing.

Flash sintering BaTiO3-0.01MgO-0.01Nb2O5-xBi2O3 ceramics with superior dielectric temperature stability via defect engineering

In this paper, Bi2O3 doped BaTiO3-0.01MgO-0.01Nb2O5 ceramics are designed to increase dielectric constant, reduce dielectric loss and improve temperature dielectric stability through inducing defects, and are prepared by flash sintering method to inhibit the volatilization of Bi element and ensure the element ratio. The microstructure, polarization mechanisms, dielectric properties and conduction mechanism of the ceramics are studied. The doping of Bi2O3 can enhance the relaxation behavior and dielectric properties. The defects evolution in BaTiO3-0.01MgO-0.01Nb2O-0.01Bi2O3 ceramics are studied. The results show that the BaTiO3 ceramics doped with different amounts of Bi2O3 can form defect dipoles {[BiBa∙‐MgTi″‐BiBa∙]×, [NbTi∙‐VBa″‐NbTi∙]×, [MgTi″‐VO∙∙]×} and oxygen vacancy, which enhance the short-range transition within the grain and inhibit the movement of free electrons at the grain boundary. The ceramics have excellent room temperature properties, high dielectric constant (εr = 2894) and low dielectric loss (tanδ < 0.03). The ceramics have superior temperature stability and satisfying the EIA X8R (Δεr/ε25 ≤ ±15 %, over the temperature range from −55 °C to 150 °C). It is found that flash sintering can effectively guarantee the stoichiometric concentration of bismuth element and obtain good properties of ceramics. This study demonstrates the potential of Bi2O3 doped BaTiO3-based ceramics synthetized by flash sintering as high-performance electronic device materials.

Design and Application of a Miniature Pneumatic Bellows Loading Device for In-Situ Tensile Testing inside the Scanning Electron Microscope

A miniature pneumatic bellows device was designed for in-situ testing inside the scanning electron microscope. The device uses a pneumatic actuator to load the specimen and displacement is directly monitored with a linear variable differential transformer sensor. Its application allowed the direct monitoring of microstructural and defect evolution in materials at the micro scale. The data produced by this testing device, in combination with measurements from micro digital image correlation, were successfully used to model the crystal plasticity of a martensitic/bainitic steel at the microstructural length scale.

Evolution of point defects in mechanical cracks of fused silica after CO2 laser melting

The traditional polishing method inevitably results in subsurface cracks in the fused silica, which seriously degrades their ultraviolet laser damage resistance. CO2 laser irradiation can melt these cracks and improve their laser induced damage threshold (LIDT). Photoluminescence spectrum and SEM-FIB were employed to investigate the changes in the material microstructure at the crack location with CO2 laser melting. The density of the oxygen-deficient centers of type II (ODC II) defects decreases, while the density of the non-bridging oxygen hole center (NBOHC) defects increases after high-temperature melting. The reason for this change is related to the dihydroxylation reaction and the participation of environmental oxygen in the defect type conversion. The reduction of ODC II defects is most likely the reason for the improvement of LIDT.

Initial microstructure and temperature dependence of irradiation defects evolution in tungsten

Using the object kinetic Monte Carlo (OKMC) method and the sink strength model, we systematically investigated the influence of the initial microstructure and temperature on the aggregation, migration, removal, and resulting size distribution of defects emerging in tungsten (W) under neutron irradiation. It is found that the probability of defect absorption at grain boundaries is generally higher than that at dislocations. This suggests that the grain size (2 ∼ 20 μm) has a stronger effect on the evolution of irradiation defects compared to dislocation density (1010 m−2 ∼ 1013 m−2). As the grain size increases, the number of residual defects decreases but the size distribution of vacancies is basically unchanged. Besides initial microstructure, the evolution of radiation damage also depends on the irradiation temperature. The increase in temperature induces an initial increase in the number density of voids, and then followed by subsequent decreases, corresponding to the peak density at 800–1000 K. Furthermore, the defect evolution in irradiated W with different initial microstructures and temperatures is simulated, and the calculated results are compared with recent experiments. These results help us to understand the role of the initial microstructure of the material in the evolution of the irradiation defects.

First-principles study of diffusion of intrinsic point defects in tungsten borides

The evolution of point defects is crucial in determining the radiation resistance of materials in a fusion environment. A comprehensive investigation of diffusion mechanisms for intrinsic point defects in six stoichiometric tungsten borides (WxBy), including W2B, WB, WB2, W2B5, WB3 and WB4, has been performed using first-principles density functional theory (DFT) calculations based on the W boride with a minimum formation enthalpy for each involved stoichiometry. The calculations indicate that boron point defects are more mobile than W point defects in each W boride. The activation energy of the W vacancy (VW) diffusion in W borides increases with B content. In general, with increasing B content, the activation energy for one-dimensional (1D) diffusion of B vacancy (VB) slightly decreases, while the activation energy for 3D diffusion of VB does not show a clear trend with B content. VW in B-rich WxBy (y/x > 1) enhances the mobility of adjacent B lattice atoms. In W2B, WB and WB2 compounds, complex configurations of W interstitials (IW), which involve B displacement, are more energetically stable than single W atoms at interstitial sites; while no or less stable complexes of W interstitials are observed in W2B5, WB3 or WB4. The activation energy for 1D/2D diffusion of IW in B-rich compounds increases with B content, and the 3D diffusion of IW in WB3 has the largest activation energy. The activation energy for 1D/2D diffusion of B interstitials (IB) generally increases with B content; however, the activation energy for 3D diffusion of IB is dependent on specific W boride composites. This study is helpful in building a database related to the diffusion behavior of defects in stoichiometric W borides. It is important to note pre-existing, as well as irradiation-induced, defects in non-stoichiometric WxBy will impact the diffusion kinetics of all intrinsic species, which needs further investigation.

The evolution of H+ implantation induced defects and the different cleavage behaviors under different thermal excitation in 4H-SiC during Crystal-Ion-Slicing technology

The Silicon Carbide (SiC) composite substrate fabricated by crystal-ion-slicing (CIS) technology can be an excellent material platform to realize a variety of microelectronic device functions. The difference in the surface morphology of the exfoliated films under different thermal excitation conditions was found in research. In order to explore the different cleavage behaviors resulting the differences, the basic models of defect evolution during the fabrication of SiC film by CIS technique is established based on AFM, Raman, XRD and TEM material characterizations, which can be divided into detachment, aggregation, growth, and concatenation stages. Combining the molecular dynamics simulation results and defect evolution models, the difference of the cleavage behavior leading to different surface topography under different anneal thermal excitations can be attributed to the different H2 bubble nucleation rate. The work in this paper investigates the fundamental physicochemical phenomena of defect evolution and crystal cleavage in CIS technology, and guides the improvement of heat treatment process, which is beneficial to achieve neat cleavage for the transfer of SiC single crystal film and to realize available heterogeneous integration of SiC single crystal film.